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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
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METHANE CONVERSION VIA METHANOL
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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
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
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,
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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 (<20 k c a l / m o l e f o r CHq), monoenergetic ( A E / E
8).
< 12%) CH4 molecules ( r e f .
The most p r o b a b l e k i n e t i c energy o f t h e CH4 molecules i n t h e beam and t h e
54
spread of t h e e n e r g i e s a r e determined by a t i m e - o f - f l i g h t u t i l i z i n g t h e chopper, ( C ) , shutter,
(B),
t e c h n i q u e ( r e f s . 4,s)
mounted i n t h e d i f f e r e n t i a l stage.
c o n t r o l s t h e exposure o f t h e beam on t h e c r y s t a l ,
A N i ( l l 1 ) s i n g l e c r y s t a l , o r i e n t e d t o w i t h i n 0.2'
An e l e c t r o n i c (M).
o f t h e (111) plane, i s
mounted on a m a n i p u l a t o r which r o t a t e s i t 360' around an a x i s p a r a l l e l t o i t s s u r f a c e and t r a n s l a t e s i t i n t h r e e m u t u a l l y p e r p e n d i c u l a r d i r e c t i o n s w h i l e m a i n t a i n i n g t h e u l t r a h i g h vacuum c o n d i t i o n s i n t h e main chamber. can be c o o l e d t o 8
The c r y s t a l
K by c o n t a c t w i t h a l i q u i d He r e s e r v o i r and can be heated
A chromel-constantan thermocouple i s spot-welded t o t h e c r y s t a l
t o 1400 K.
f o r t e m p e r a t u r e measurements.
The procedure f o r c l e a n i n g t h e c r y s t a l by A r i o n
s p u t t e r i n g , o x i d a t i o n and r e d u c t i o n has been d i s c u s s e d p r e v i o u s l y ( r e f . 5).
Fig. 1. Schematic o f t h e apparatus.
E
F G
ij
RESULTS and DISCUSSION Evidence f o r t h e Mechanism The a b s o l u t e p r o b a b i l i t y f o r 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 on N i ( l l 1 ) i s p l o t t e d versus t h e normal component o f t h e k i n e t i c energy of t h e i n c i d e n t methane m o l e c u l e i n Fig. 2.
The d i s s o c i a t i o n p r o b a b i l i t y was
measured by m o n i t o r i n g t h e amount o f d e p o s i t e d carbon by Auger e l e c t r o n spectroscopy.
Since methane does n o t adsorb m o l e c u l a r l y a t t h e s u r f a c e tempera-
t u r e o f 475 K a t which t h e s e measurements were c a r r i e d o u t ( r e f . 9), t h e carbon Auger f e a t u r e r e s u l t s o n l y f r o m t h e methane t h a t has d i s s o c i a t i v e l y chemisorbed.
The carbon Auger s i g n a l i s c a l i b r a t e d f o r a b s o l u t e carbon
coverage and t h e a b s o l u t e f l u x o f t h e i n c i d e n t methane beam i s determined from procedures o u t l i n e d i n d e t a i l p r e v i o u s l y ( r e f s . 6b,10).
The a b s o l u t e
d i s s o c i a t i o n p r o b a b i l i t y p l o t t e d i n Fig. 2 i s t h e r a t i o o f t h e a b s o l u t e number o f adsorbed carbon atoms p e r u n i t area t o t h e a b s o l u t e number o f methane m o l e c u l e s p e r u n i t area i n c i d e n t on t h e s u r f a c e .
55
ANGLE OF INCIDENCE
0-
lb
10-3
10-4
=
0
10-5
10 11 12 13 14 15 16 17 18 EL
F i g . 2. A b s o l u t e d i s s o c i a t i o n p r o b a b i l i t y of C H 4 and CD4 vs.
El.
EiCOS2BilKuVmole
Fig.3. D i s s o c i a t i o n p r o b a b i l i t y o f C H 4 vs. El f o r i n c i d e n t angles 5' t o 30" away from normal angle.
F o r e n e r g i e s l e s s t h a n about 12 kcal/mole, a b l e by Auger spectroscopy.
I
I
1s 20
no carbon d e p o s i t i o n i s d e t e c t -
Therefore, a d i s s o c i a t i o n p r o b a b i l i t y o f 10-5
r e p r e s e n t s t h e s e n s i t i v i t y l i m i t f o r t h e d e t e c t i o n o f carbon.
Above 12 k c a l /
mole, carbon i s observed t o d e p o s i t on t h e surface. As t h e energy i s i n c r e a s e d b y 5 k c a l / m o l e t o 17 kcal/mole, 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 i n c r e a s e s by two o r d e r s o f magnitude. 6-8 measurements. slope, A9.n
The e r r o r b a r s a r e 95% c o n f i d e n c e l i m i t s o f a s e r i e s of
The l i n e i s a l i n e a r l e a s t squares f i t t o t h e d a t a and i t s
( d i s s o c i a t i o n probability)/AEl,
i s equal t o 0.8t0.1
kcal'hole.
The d i s s o c i a t i o n p r o b a b i l i t i e s p l o t t e d i n Fig. 2 a r e measured as a funct i o n o f t h e k i n e t i c energy o f t h e methane beam i n c i d e n t on t h e s u r f a c e a t t h e normal angle.
The d i s s o c i a t i o n p r o b a b i l i t i e s have a l s o been measured a t a
f i x e d k i n e t i c energy as a f u n c t i o n o f t h e i n c i d e n t a n g l e o f t h e beam on t h e s u r f a c e i n o r d e r t o v a r y t h e normal component o f t h e k i n e t i c energy. These r e s u l t s 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 i e s a r e p l o t t e d i n Fig. 3 versus t h e normal component o f t h e t r a n s l a t i o n a l energy.
These measurements f a l l along-
s i d e t h e d o t t e d l i n e which i s t h e l i n e a r l e a s t squares f i t t o t h e d a t a i n F i g . 2.
T h i s o b s e r v a t i o n means t h a t 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 c o r r e l a t e s
e x p o n e n t i a l l y not w i t h t h e t o t a l energy o f t h e i n c i d e n t molecule b u t 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 energy.
Therefore, i t i s c l e a r
f r o m t h e d a t a i n Figs. 2 and 3 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 o f methane and o n l y t h e normal component o f t h e m o l e c u l e ' s i n c i d e n t energy i s e f f e c t i v e i n overcoming t h i s b a r r i e r . We should now l i k e t o understand t h e o r i g i n o f t h e b a r r i e r t o d i s s o c i a t i o n
56
and why o n l y t h e normal component o f t h e m o l e c u l e ' s i n c i d e n t energy i s e f f e c tive.
An i n t r i g u i n g way t o t h i n k about t h e e f f e c t o f t r a n s l a t i o n a l energy i s
t o c o n s i d e r i t t o cause t h e CH4 m o l e c u l e t o " s p l a t t e r " a g a i n s t t h e surface. Upon c o n s i d e r a t i o n o f t h e e n e r g e t i c requirements, i t i s reasonable t h a t t h e methane m o l e c u l e be d i s t o r t e d from i t s t e t r a h e d r a l c o n f i g u r a t i o n i n o r d e r f o r C-H bond cleavage t o occur.
For s u f f i c i e n t energy r e l e a s e t o break t h e 100
k c a l / m o l e C-H bond, a 63 k c a l / m o l e N i - H bond and a 40 k c a l / m o l e N i - C bond must be formed.
To a f l a t N i s u r f a c e w i t h no p r o t r u s i o n s , t h e methane molecule ap-
pears s p h e r i c a l and i s o t r o p i c . The hydrogen atoms e f f e c t i v e l y bury t h e carbon atom, p r e v e n t i n g 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 N i s u r f a c e and t h e carbon atom from o c c u r r i n g .
A s l o w l y moving methane molecule i n c i d e n t on
t h e s u r f a c e i n t e r a c t s p r i m a r i l y t h r o u g h t h e s h i e l d i n g hydrogen atoms.
No C-H
bond cleavage occurs because t h e carbon atom cannot move i n c l o s e t o t h e N i surface t o i n t e r a c t strongly.
However, as t h e t r a n s l a t i o n a l energy o f t h e
i n c i d e n t m o l e c u l e i s increased, t h e methane molecule begins t o s u f f e r substant i a l d e f o r m a t i o n upon c o l l i s i o n w i t h t h e s u r f a c e due t o i t s i n c r e a s i n g impact. T h i s d e f o r m a t i o n serves t o push t h e hydrogen atoms o u t from between t h e s u r f a c e and t h e carbon atom, t h e r e b y exposing t h e carbon atom t o t h e N i surface. I n t h i s c o n f i g u r a t i o n , b o t h a N i - C bond and a N i - H bond can be formed, breaki n g a C-H bond.
Only t h e normal component o f t h e t r a n s l a t i o n a l energy i s e f -
f e c t i v e i n p r o m o t i n g d i s s o c i a t i o n because d e f o r m a t i o n can a r i s e o n l y from t h e m o t i o n p e r p e n d i c u l a r t o t h e surface.
The p a r a l l e l v e l o c i t y component of t h e
m o l e c u l e ' s m o t i o n encounters no r e p u l s i v e i n t e r a c t i o n from which c o n v e r s i o n o f t r a n s l a t i o n a l t o v i b r a t i o n a l m o t i o n can t a k e place. T h i s p i c t u r e f o r t h e e f f e c t o f t r a n s l a t i o n a l energy on t h e p r o b a b i l i t y o f 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 d e p i c t s t h e b a r r i e r as t h e energy r e q u i r e d t o deform t h e methane molecule t o achieve s u f f i c i e n t i n t e r a c t i o n between t h e carbon atom and t h e N i surface.
I f t h e d e f o r m a t i o n energy i s t h e s o l e c o n t r i -
b u t i o n t o t h e b a r r i e r , t h e n t h e energy o f t h e b a r r i e r can be c a l c u l a t e d f r o m t h e known f o r c e f i e l d o f t h e methane molecule ( r e f . 11).
Assume t h a t t h e
t r a n s i t i o n s t a t e t h a t l e a d s t o d i s s o c i a t i o n i s a pyramidal c o n f i g u r a t i o n o f t h e methane molecule where t h r e e hydrogen atoms and t h e carbon atom a r e coplanar.
When a methane m o l e c u l e w i t h t h r e e o f i t s hydrogens d i r e c t e d toward
t h e s u r f a c e h i t s t h e surface, t h e t h r e e hydrogen atoms a r e pushed i n t o one p l a n e w i t h t h e carbon atom, c l e a r l y exposing t h e carbon atom t o t h e surface. The energy r e q u i r e d t o d i s t o r t t h e t e t r a h e d r a l methane m o l e c u l e i n t o t h i s p y r a m i d a l c o n f i g u r a t i o n i s 16.3 k c a l / m o l e c a l c u l a t e d f r o m t h e harmonic f o r c e field.
The a b s o l u t e v a l u e o f t h i s c a l c u l a t e d d e f o r m a t i o n energy i s s i g n i f i -
c a n t because i t l i e s i n t h e range o f t r a n s l a t i o n a l e n e r g i e s over which probab i l i t i e s f o r d i s s o c i a t i v e chemisorption are large.
This observation lends
s u p p o r t t o t h e i d e a t h a t t h e t r a n s l a t i o n a l energy l e a d s t o d e f o r m a t i o n o f t h e
57
m o l e c u l e and t h a t t h e pyramidal c o n f i g u r a t i o n o f t h e m o l e c u l e i s t h e geometry o f t h e t r a n s i t i o n s t a t e t h a t leads t o dissociation.
A more complete
c a l c u l a t i o n which c o r r e l a t e s t h e e x p o n e n t i a l dependence o f 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 on energy w i t h t h e amount o f d e f o r m a t i o n t h a t i s achieved i n t h i s p y r a m i d a l c o n f i g u r a t i o n a t each i n c i d e n t energy has been c a r r i e d o u t and i s d i s c u s s e d elsewhere ( r e f s . 6b,10,12).
I n s h o r t , t h e e x p o n e n t i a l dependence o f
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 on energy i s c o r r e c t l y p r e d i c t e d f r o m t h i s model. T h e r e f o r e , t h e energy r e q u i r e d t o d i s t o r t t h e molecule i s a major p a r t o f t h e b a r r i e r t o d i s s o c i a t i o n o f CH4 on a N i c a t a l y s t . However, i f t r a n s l a t i o n a l energy i s e f f e c t i v e l y c o n v e r t e d t o v i b r a t i o n a l energy upon impact on t h e surface, t h e n v i b r a t i o n a l energy should be e q u a l l y e f f e c t i v e i n surmounting t h e b a r r i e r ,
Therefore, experiments designed t o
probe t h e e f f e c t o f v i b r a t i o n a l energy have a l s o been c a r r i e d out.
The degree
o f v i b r a t i o n a l e x c i t a t i o n o f t h e i n c i d e n t methane m o l e c u l e i s i n c r e a s e d by r a i s i n g t h e t e m p e r a t u r e o f t h e n o z z l e from which t h e methane-helium m i x t u r e i s expanded.
Since v i b r a t i o n a l energy r e l a x a t i o n i n t h e expansion i s i n e f f i c i e n t
due t o t h e wide energy spacings between t h e v i b r a t i o n a l energy l e v e l s , t h e v i b r a t i o n a l energy d i s t r i b u t i o n o f t h e methane molecules i s i n e q u i l i b r i u m w i t h t h e n o z z l e a t i t s temperature.
The f r a c t i o n o f methane molecules i n each
v i b r a t i o n a l mode i s e a s i l y c a l c u l a t e d .
The average 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 i s t h e sum o f t h e p r o d u c t s o f t h e f r a c t i o n o f molecules i n each mode t i m e s t h e energy o f t h a t mode.
Most o f t h e v i b r a t i o n a l energy i s
c o n t a i n e d i n t h e w4 o r u m b r e l l a mode. However, i n c r e a s i n g t h e n o z z l e t e m p e r a t u r e a l s o r a i s e s t h e t r a n s l a t i o n a l energy o f t h e methane molecule.
I n o r d e r t o probe t h e e f f e c t o f v i b r a t i o n a l
energy on t h e d i s s o c i a t i o n o f methane, t h e t r a n s l a t i o n a l energy must be maint a i n e d a t a c o n s t a n t v a l u e as t h e v i b r a t i o n a l energy i s v a r i e d .
T h i s i s ac-
complished by a d j u s t i n g t h e average mass o f t h e gas m i x t u r e ( v a r y i n g t h e r a t i o o f methane t o He) t o compensate f o r t h e change i n n o z z l e temperature.
F o r ex-
ample, expansion o f a 1%m i x t u r e o f CH4 i n He f r o m a n o z z l e a t 769 K and exp a n s i o n o f 5% CH4 i n He f r o m a n o z z l e a t 829 K produce beams w i t h i d e n t i c a l k i n e t i c e n e r g i e s o f 16.6 kcal/mole, b u t t h e average v i b r a t i o n a l e n e r g i e s a r e
1.26 k c a l / m o l e and 1.45 kcal/mole.
Thus, by m o n i t o r i n g t h e d i f f e r e n c e i n 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 i e s between two beams o f equal t r a n s l a t i o n a l energies, t h e e f f e c t i v e n e s s o f v i b r a t i o n a l energy i n overcoming t h e 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 can be assessed. i n Table 1.
The r e s u l t s o f t h i s experiment a r e shown
A l s o shown i s t h e r a t i o o f t h e change i n t h e n a t u r a l l o g o f 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 w i t h r e s p e c t t o t h e change i n t h e average v i b r a t i o n a l energy.
T h i s q u a n t i t y i s a measure o f t h e e f f e c t i v e n e s s o f v i b r a t i o n a l energy
and has an average v a l u e o f about 4 k c a l - 1 mole,
The d i f f i c u l t n a t u r e of
t h e s e experiments p r e v e n t e d us from c a r r y i n g o u t more t h a n f o u r experiments.
58
TABLE 1 Average v i b r a t i o n a l energies, d i s s o c i a t i o n p r o b a b i l i t i e s and e f f e c t i v e n e s s o f v i b r a t i o n a l energy f o r methane w i t h a f i x e d k i n e t i c energy. ET ( k c a l /mol e )
5% CH4/He 1%CH4/He
-
EVIB ( k c a l /mol e )
TN
(K)
1.45 1.26
829 769
16.6 16.6
.
DISSOCIATION Alln ( D I SS. PROB ) PROBABILITY A ~ IB V 4.4 2.0
10-3 10-3
4.1
T h e r e f o r e , t h e very small changes i n v i b r a t i o n a l energy t h a t can be achieved and t h e l a c k o f a s t a t i s t i c a l a n a l y s i s c a s t s some doubt on t h e accuracy o f t h i s number. Regardless, by comparison o f t h e e f f e c t i v e n e s s q u a n t i t y f o r v i b r a t i o n a l energy t o t h a t o b t a i n e d p r e v i o u s l y f o r t h e normal component o f t h e t r a n s l a t i o n a l energy,
2
1 kcal'hole
( s l o p e o f l i n e i n Fig. 2 ) , i t i s c l e a r t h a t
v i b r a t i o n a l energy i s a t l e a s t as e f f e c t i v e as t r a n s l a t i o n a l energy. V i b r a t i o n a l energy may a l s o prove, w i t h more s o p h i s t i c a t e d t e c h n i q u e s i n t h e f u t u r e , t o be even more e f f e c t i v e t h a n t r a n s l a t i o n a l energy because o f t h e l o n g c o l l i s i o n t i m e o f t h e m o l e c u l e compared t o t h e v i b r a t i o n a l p e r i o d .
That i s , i n t h e
t i m e t h a t t h e methane m o l e c u l e i s i n t h e near v i c i n i t y o f t h e surface, i t has undergone t h r e e o r f o u r v i b r a t i o n a l p e r i o d s o f t h e v 4 o r u m b r e l l a mode, exposi n g t h e carbon atom t o t h e N i s u r f a c e t h r e e o r f o u r times.
The i m p o r t a n t p o i n t
here, however, i s t h a t v i b r a t i o n a l energy i n t h e v 4 mode and t r a n s l a t i o n a l energy a r e c o m p l e t e l y i n t e r c h a n g e a b l e because t h e y b o t h l e a d t o t h e same m o t i o n o f t h e n u c l e i over t h e r e a c t i o n c o o r d i n a t e . The 12 k c a l / m o l e o f energy necess a r y t o a c h i e v e a minimum o b s e r v a b l e d i s s o c i a t i o n p r o b a b i l i t y o f 10-5 can be s u p p l i e d as t r a n s l a t i o n a l energy, v i b r a t i o n a l energy o r a c o m b i n a t i o n o f both. A l t h o u g h d e f o r m a t i o n o f t h e m o l e c u l e p l a y s t h e e s s e n t i a l r o l e i n how t h e m o l e c u l e overcomes t h e b a r r i e r t o d i s s o c i a t i o n , i t i s n o t t h e complete p i c t u r e o f t h e d i s s o c i a t i o n mechanism.
The remainder o f t h e p i c t u r e comes i n t o view
upon e x a m i n a t i o n o f 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 i e s versus t r a n s l a t i o n a l energy measured f o r CD4.
These measurements a r e a l s o shown i n Fig. 2.
The e r r o r b a r s
on t h e CD4 d a t a r e p r e s e n t 95% c o n f i d e n c e l i m i t s o f t h r e e measurements.
The
beam o f CD4 i s normal t o t h e c r y s t a l i n t h e s e measurements and t h e temperature o f t h e s u r f a c e i s 475 K.
As can be seen i n Fig. 2, t h e d i s s o c i a t i o n proba-
b i l i t y f o r CD4 i s below t h e d e t e c t i o n l i m i t o f carbon f o r i n c i d e n t k i n e t i c e n e r g i e s l e s s t h a n 14 kcal/mole.
Above 14 kcal/mole,
b i 1 it y o f CD4 increases exponenti a1 ly.
t h e d i s s o c i a t i o n proba-
However, t h e d i s s o c i a t i o n probabi 1 it y
of CD4 i s always a t l e a s t a f a c t o r o f 8 below t h a t o f CH4.
59
The l a r g e magnitude o f t h e k i n e t i c i s o t o p e e f f e c t i s a f l a g f o r t h e occurrence o f quantum mechanical t u n n e l i n g because t h e k i n e t i c i s o t o p e e f f e c t expected as t h e r e s u l t o f t h e d i f f e r e n c e i n z e r o p o i n t e n e r g i e s i s much smaller.
E s t i m a t i o n o f t h e k i n e t i c i s o t o p e e f f e c t based on t h e z e r o p o i n t
energy d i f f e r e n c e s r e q u i r e s knowledge o f t h e geometry o f t h e t r a n s i t i o n s t a t e and v i b r a t i o n a l f r e q u e n c i e s o f t h e molecule a t t h e t r a n s i t i o n s t a t e .
These
q u a n t i t i e s a r e unknown b u t t h e y can be e s t i m a t e d based on t h e i n f o r m a t i o n o b t a i n e d i n t h i s study.
Since v i b r a t i o n a l energy appears t o be s l i g h t l y more
e f f e c t i v e t h a n t r a n s l a t i o n a l energy 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 , i t i s T h i s means
l i k e l y t h a t t h e b a r r i e r i s p o s i t i o n e d i n t h e e x i t channel ( r e f . 1 3 ) . t h a t t h e t r a n s i t i o n s t a t e resembles t h e p r o d u c t molecule, CH3.
Since t h e
v i b r a t i o n a l s p e c t r a o f CH3 and CD3 have been measured i n t h i s study and a r e d i s c u s s e d below, t h e f r e q u e n c i e s o f t h e i r v i b r a t i o n s a r e known.
The k i n e t i c
i s o t o p e e f f e c t can t h e n be c a l c u l a t e d on t h e assumption t h a t t h e t r a n s i t i o n s t a t e i s e s s e n t i a l l y an adsorbed CH3 species,
A t a t e m p e r a t u r e o f 640 K,
which i s t h e l o w e s t n o z z l e temperature used i n t h e s e expansions, a k i n e t i c i s o t o p e e f f e c t o f 4.8
i s calculated.
This value f o r t h e k i n e t i c isotope
e f f e c t i s t o o small compared t o t h e observed value.
It i s u n l i k e l y t h a t t h e
magnitude o f t h i s i s o t o p e e f f e c t can be e x p l a i n e d based s o l e l y on t h e d i f f e r e n c e i n z e r o p o i n t energies. Therefore, t u n n e l i n g o f t h e l i g h t hydrogen atom i s i n d i c a t e d .
This i s a
reasonable mechanism because i n o r d e r f o r t h e C-H bond t o break, t h e r e must be m o t i o n o f t h e n u c l e i a l o n g t h e C-H c o o r d i n a t e once t h e methane molecule i s s u f f i c i e n t l y deformed.
It i s p o s s i b l e f o r t h e l i g h t hydrogen atom t o t u n n e l
t h r o u g h t h e r e m a i n i n g b a r r i e r b e f o r e t h e m o l e c u l e i s deformed completely.
The
d i s s o c i a 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 k i n e t i c energy i s t h e n a convol u t i o n o f t h e t u n n e l i n g p r o b a b i l i t y w i t h t h e degree o f d e f o r m a t i o n a t t a i n e d by t h e m o l e c u l e a t each energy. The mechanism f o r t h e d i s s o c i a t i o n o f methane can now be summarized as f o l lows.
As t h e methane m o l e c u l e approaches t h e s u r f a c e more c l o s e l y t h a n t h e
e q u i l i b r i u m d i s t a n c e f o r m o l e c u l a r methane adsorbed on t h e surface, i t e x p e r i ences a r e p u l s i v e i n t e r a c t i o n which can be overcome by i n c r e a s i n g t h e normal component o f t h e t r a n s l a t i o n a l energy o r t h e v i b r a t i o n a l energy. T h i s i n c r e a s e d 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 r e s u l t s i n d e f o r m a t i o n o f t h e molecule, t h e r e b y moving t h e hydrogen atoms o u t o f t h e way o f t h e N i - C a t t r a c t i v e i n t e r action.
However, t h e m o l e c u l e does n o t need t o be deformed c o m p l e t e l y because
a t some d i s t a n c e t h e b a r r i e r becomes s u f f i c i e n t l y narrow f o r t h e l i g h t hydrogen atom t o t u n n e l t h r o u g h t o t h e p r o d u c t regime.
T h i s p i c t u r e f o r t h e dynamics 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 process p r o v i d e s t h e most c o n s i s t e n t e x p l a n a t i o n f o r t h e o r i g i n o f t h e a p p r o x i m a t e l y equal e f f e c t i v e n e s s 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 towards d i s s o c i a t i o n , t h e exponenti a1 dependence of t h e
60
d i s s o c i a t i o n p r o b a b i l i t y on energy and t h e l a r g e k i n e t i c i s o t o p e e f f e c t . E f f e c t of Potassium on 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 Chemisorption o f C H q Potassium i s used as a dopant on c a t a l y s t s f o r t h e methanation r e a c t i o n and ammonia synthesis.
I t s purpose i s t o increase t h e r a t e o f t h e reaction.
Potassium i s a l s o used on t h e steam reforming c a t a l y s t , n o t as a promotor b u t as a dopant t h a t i n h i b i t s c a t a l y s t d e a c t i v a t i o n by coke formation ( r e f . 1).
It
i s reasonable t h a t t h e r o l e of potassium as a promotor o f r e a c t i o n r a t e s i s t o lower some b a r r i e r t o bond d i s s o c i a t i o n .
Since molecular beam techniques
a f f o r d a convenient means o f measuring changes i n b a r r i e r heights as w e l l as i n shapes o f t h e b a r r i e r through measurements o f 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 versus energy, t h e p o s s i b l e e f f e c t o f potassium on t h e d i s s o c i a t i o n o f CH4 i s investigated. The potassium i s deposited on t h e s u r f a c e from a beam o f K+ i o n s evaporated from a z e o l i t e g e t t e r source and d i r e c t e d toward t h e c r y s t a l . Potassium coverages, monitored by t h e i n t e n s i t y o f t h e potassium Auger feature, up t o 0.1 monolayer are i n v e s t i g a t e d .
The procedures 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
measurements a r e as described previously.
We f i n d t h a t t h e r e i s no e f f e c t o f
potassium on 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 o f methane and t h e r e f o r e no e f f e c t on t h e r a t e o f t h e steam reforming r e a c t i o n i s expected.
This r e s u l t i s
c o n s i s t e n t w i t h t h e use o f potassium as an i n h i b i t o r o f c a t a l y s t d e a c t i v a t i o n r a t h e r than a r a t e promotor o f t h e commercial steam reforming reaction. The Chemistry and Stabi 1it y o f Adsorbed Methyl Radicals As demonstrated by t h e r e s u l t s presented above, t h e p r o b a b i l i t y o f d i s s o c i a t i v e chemisorption can be r e a d i l y probed by measuring t h e e x t e n t o f carbon However, a complete p i c t u r e o f t h e
d e p o s i t i o n by Auger e l e c t r o n spectroscopy.
d i s s o c i a t i v e adsorption process r e q u i r e s t h a t t h e product o f t h e d i s s o c i a t i v e
For example, although t h e d i s c u s s i o n has assumed t h a t a s i n g l e C-H bond cleaves upon d i s s o c i a t i o n , chemisorption event be s p e c t r o s c o p i c a l l y i d e n t i f i e d .
no evidence f o r t h i s has been presented.
I n order t o i d e n t i f y chemically t h e
product o f t h e d i s s o c i a t i v e chemisorption event, we have measured t h e 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 spectrum f o r methane deposited on t h e N i ( l l 1 ) s u r f a c e a t 140 K w i t h an i n c i d e n t energy o f 17 kcal/mole. shown i n Fig. 4a.
The spectrum i s
A low surface temperature i s chosen i n order t o t r a p t h e
nascent product of t h e d i s s o c i a t i v e chemisorption and n o t a thermal decomposit i o n product.
The temperature o f t h e s u r f a c e has no e f f e c t on 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 chemisorption s i n c e t h e d i s s o c i a t i o n occurs immediately upon impact o f t h e molecule on t h e surface. Three d i s t i n c t loss features a t 370 cm-l, observed.
1220 cm-l and 2660 cm-l a r e
By comparison t o t h e v i b r a t i o n a l frequencies o f metal a l k y l compounds
61
( r e f . 14) and t o t h e i n t e n s i t i e s and number of l o s s f e a t u r e s f o r NH3 adsorbed
ENERGY (CM-'1
Fig. 5. HREELS a f t e r h e a t i n g CH3(a) t o 300 K.
ENERGY
on N i ( l l 1 ) ( r e f .
Fig. 4. HREELS a f t e r d e p o s i t i o n o f CH4 a t 17 k c a l / m o l e on N i ( l l 1 ) a t 140 K. ( a ) s p e c u l a r a n g l e ( b ) 8' f r o m s p e c u l a r angle.
(wovenumbers)
15), t h e s e l o s s f e a t u r e s a r e assigned t o t h e Ni-CH3 s t r e t c h i n g
mode, t h e symmetric d e f o r m a t i o n ( u m b r e l l a ) mode and t h e u n r e s o l v e d symmetric and asymmetric C-H s t r e t c h i n g modes, r e s p e c t i v e l y .
V e r i f i c a t i o n o f these
assignments i s made by comparing t h e s e s p e c t r a t o t h o s e measured f o r t h e i s o t o p i c a l l y s u b s t i t u t e d methane, CD4.
The C-H s t r e t c h i n g mode i s observed t o
s h i f t from 2660 cm-l t o 1990 cm-1 f o r CD3 and t h e symmetric d e f o r m a t i o n o r u m b r e l l a mode s h i f t s f r o m 1220 cm-1 t o 940 cm-l.
The N i - C s t r e t c h i n g mode does
n o t s h i f t w i t h i n t h e r e s o l u t i o n o f t h i s measurement, as expected.
The f a c t
t h a t a l l modes except t h e N i - C mode a t 370 cm-1 s h i f t downward i n frequency by a f a c t o r o f about 1.3
i n d i c a t e t h a t no carbon-carbon bond f o r m a t i o n has
occurred. The frequency o f t h e C-H s t r e t c h observed h e r e i s v e r y l o w f o r a "normal" C-H mode.
These l o w frequency o r " s o f t " C-H modes i n d i c a t e a s t r o n g i n t e r -
a c t i o n between t h e hydrogens and t h e s u r r o u n d i n g N i atoms.
T h i s C-H-Ni
inter-
a c t i o n has been p r e d i c t e d f o r adsorbed methyl r a d i c a l s by extended H'irckel molec u l a r o r b i t a l c a l c u l a t i o n s ( r e f . 16).
These s o f t C-H modes have a l s o been
observed p r e v i o u s l y f o r l a r g e r hydrocarbons adsorbed on t r a n s i t i o n m e t a l s which have many hydrogens i n c l o s e p r o x i m i t y t o t h e metal s u r f a c e where t h e y can i n t e r a c t strongly (ref.
17).
These t h r e e - c e n t e r ,
been observed i n organometall ic compounds ( r e f .
t w o - e l e c t r o n bonds have a l s o 18).
Presumably, t h i s
geometric c o n f i g u r a t i o n leads r e a d i l y t o dissociation. Adsorbed methyl r a d i c a l s have more v i b r a t i o n a l modes t h a n appear i n Fig. 4a.
62
The o t h e r modes a r e e x c i t e d by a n o n - d i p o l a r t r a n s i t i o n mechanism and t h e r e f o r e a r e weak i n i n t e n s i t y and a r e hidden under t h e s t r o n g d i p o l a r s c a t t e r i n g t h a t occurs a t t h e s p e c u l a r angle. However, t h e s e modes o f weak i n t e n s i t y can be d i s c r i m i n a t e d from t h e s t r o n g d i p o l a r s c a t t e r i n g by r o t a t i n g t h e energy a n a l y z e r f o r t h e s c a t t e r e d e l e c t r o n s away from t h e s p e c u l a r angle. Fig. 4b shows a spectrum measured 8' m i s s i n g modes a r e now d e t e c t a b l e .
away f r o m t h e s p e c u l a r a n g l e where t h e
The u n r e s o l v e d emission from 450-900 cm-l
a r i s e s f r o m t h e t o r s i o n and r o c k i n g modes as w e l l as f r o m some o f t h e f r u s t r a t e d t r a n s l a t i o n a l modes. The f e a t u r e a t 1360 cm-1 i s assigned t o t h e degene r a t e d e f o r m a t i o n (H-C-H bend) mode o f CH3.
The l o s s f e a t u r e a t 2690 cm-1 i s
n a r r o w e r and h i g h e r i n frequency t h a n t h a t measured a t t h e s p e c u l a r a n g l e because i t now r e p r e s e n t s a s i n g l e mode, t h e asymmetric C-H s t r e t c h i n g mode. The i n t e n s i t y o f t h e l o w e r frequency, d i p o l e a c t i v e , symmetric C-H s t r e t c h i n g mode i s s u f f i c i e n t l y suppressed a t t h i s o f f - s p e c u l a r a n g l e t o a l l o w t h e asymm e t r i c mode t o be w e l l - r e s o l v e d .
Therefore, 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 i s i d e n t i f i e d as an adsorbed methyl r a d i c a l and an adsorbed
Loss f e a t u r e s due t o t h e N i - H s t r e t c h i n g modes a r e n o t
hydrogen atom.
observed because o f a v e r y small t r a n s i t i o n moment d i p o l e .
The absence o f
N i - H l o s s f e a t u r e s was v e r i f i e d by measuring a v i b r a t i o n a l spectrum o f a N i ( l l 1 ) s u r f a c e a t 300 K t h a t had been exposed t o H2. The h i g h t r a n s l a t i o n a l energy a t t a i n a b l e by m o l e c u l a r beam t e c h n i q u e s prov i d e s a c o n v e n i e n t method f o r t h e s y n t h e s i s o f adsorbed methyl r a d i c a l s . D e s p i t e t h e importance o f methyl r a d i c a l s as proposed i n t e r m e d i a t e species i n mechanisms f o r h i g h e r hydrocarbon f o r m a t i o n , adsorbed methyl r a d i c a l s have n o t been p r e v i o u s l y i d e n t i f i e d s p e c t r o s c o p i c a l l y .
We have s t u d i e d t h e s t a b i l i t y o f
methyl r a d i c a l s by r e c o r d i n g t h e v i b r a t i o n a l spectrum as a f u n c t i o n o f s u r f a c e temperature.
Fig. 5 shows a v i b r a t i o n a l spectrum measured a f t e r r a i s i n g t h e
s u r f a c e t e m p e r a t u r e from 140 t o 300 K, a l t h o u g h t h e same spectrum i s observed a f t e r h e a t i n g t o 200 K. T h i s spectrum i s c l e a r l y no l o n g e r t h a t o f a methyl radical.
I n s t e a d , i t i s a spectrum o f a p a r t i a l l y r e h y b r i d i z e d C2H2 fragment.
T h i s spectrum i s i d e n t i c a l t o t h e spectrum o b t a i n e d a f t e r a d s o r p t i o n o f molecu l a r a c e t y l e n e on t h e N i ( l l 1 ) s u r f a c e ( r e f . 19).
Presumably, t h e methyl
r a d i c a l s d i s s o c i a t e i n t o CH2 i n t e r m e d i a t e s p e c i e s between 150-200 K.
These
CH2 s p e c i e s t h e n recombine t o form C2H4, which i s u n s t a b l e r e l a t i v e t o C2H2 a t t h e s e temperatures. hydrogen atoms.
The C2H4 s p e c i e s t h e n d i s s o c i a t e i n t o C2H2 and adsorbed
I n t e r e s t i n g l y , r a i s i n g t h e t e m p e r a t u r e t o 200 K o f a s u r f a c e
on which m o l e c u l a r e t h y l e n e i s adsorbed r e s u l t s i n t h e same spectrum o f a C2H2 s p e c i e s ( r e f . 19).
Recent s u p p o r t f o r t h i s m e c h a n i s t i c r o u t e t o C2H2
f o r m a t i o n i s p r o v i d e d by a p r e l i m i n a r y o b s e r v a t i o n o f t h e v i b r a t i o n a l spectrum of a CH2 s p e c i e s i n t h e t e m p e r a t u r e range between 150-200 K ( r e f . 20). The C2H2 s p e c i e s a r e produced from m o l e c u l a r a c e t y l e n e , e t h y l e n e o r
€9
64
bypassed s i m p l y by r a i s i n g t h e energy o f t h e i n c i d e n t molecule.
We have used
t h i s t r i c k t o s y n t h e s i z e and s p e c t r o s c o p i c a l l y i d e n t i f y an adsorbed methyl r a d i c a l f o r t h e f i r s t t i m e i n low pressure, u l t r a h i g h vacuum c o n d i t i o n s .
We
can now a l s o c a r r y out h i g h p r e s s u r e r e a c t i o n s a t low p r e s s u r e where t h e e n t i r e arsenal o f s u r f a c e science techniques can be a p p l i e d t o study p r a c t i c a l l y i m p o r t a n t heterogeneous c a t a l y t i c r e a c t i o n s .
A NEW MECHANISM FOR D I S S O C I A T I V E CHEMISORPTION: COLLISION-INDUCED DISSOCIATION OF ADSORBED CH4 OR CHEMISTRY WITH A HAMMER We have j u s t demonstrated t h a t t h e b a r r i e r t o t h e d i s s o c i a t i o n o f methane
on a n i c k e l c a t a l y s t i s l a r g e l y t h e energy r e q u i r e d t o deform t h e molecule. I f t h i s i s c o r r e c t , t h e n a CH4 molecule, physisorbed on a n i c k e l s u r f a c e and
h i t by a hammer, c o u l d be pounded i n t o t h e shape t h a t l e a d s t o d i s s o c i a t i o n . We have shown t h a t t h i s mechanism f o r d i s s o c i a t i o n does indeed occur by bombarding a monolayer o f CH4, physisorbed on t h e N i ( l l 1 ) s u r f a c e a t 46 K, w i t h an A r atom beam ( r e f . 22).
The impact o f t h e A r atom deforms t h e
m o l e c u l a r l y adsorbed methane molecule i n t o t h e c o n f i g u r a t i o n f o r t h e t r a n s i t i o n s t a t e t h a t leads t o d i s s o c i a t i o n . These experiments a r e performed i n t h e apparatus p r e v i o u s l y described. The N i ( l l 1 ) c r y s t a l i s m a i n t a i n e d a t 46 K i n an ambient atmosphere o f 1 W 6 t o r r o f methane.
Under t h e s e c o n d i t i o n s , a monolayer o f m o l e c u l a r l y adsorbed methane
i s p r e s e n t ( r e f . 9).
A monoenergetic A r atom beam, produced by t h e h i g h
p r e s s u r e expansion o f a 1%m i x t u r e o f A r i n He from a n o z z l e a t 800 K, i m pinges on t h e methane covered s u r f a c e a t normal i n c i d e n c e f o r about 1 min. The ambient CH4 i s t h e n pumped away and t h e c r y s t a l i s heated t o 74 K t o remove any m o l e c u l a r l y adsorbed CH4.
R a i s i n g t h e c r y s t a l temperature t o 74 K
r e s u l t s o n l y i n d e s o r p t i o n o f m o l e c u l a r methane; no t h e r m a l l y induced d i s s o c i a t i o n o f methane occurs.
The d e r i v a t i v e carbon Auger s i g n a l
p r o p o r t i o n a l t o t h e amount o f d i s s o c i a t i o n ,
i s t h e n recorded.
, which
is
The procedure
i s repeated on a c l e a n s u r f a c e f o r o t h e r i n c i d e n t angles up t o 45" from normal.
A p l o t o f t h e carbon Auger s i g n a l , n o r m a l i z e d f o r exposure, versus
t h e normal component o f t h e A r atom t r a n s l a t i o n a l energy i s shown i n F i g . 8. For t r a n s l a t i o n a l e n e r g i e s where t h e normal component i s l e s s t h a n 18 k c a l h o l e , no d i s s o c i a t i o n i s observed above t h e d e t e c t i o n l i m i t o f carbon. As t h e normal component o f t h e energy o f t h e i n c i d e n t A r i s i n c r e a s e d t o 36 k c a l h o l e , 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 o n induced by t h e A r atoms i n c r e a s e s e x p o n e n t i a l l y by almost two o r d e r s o f magnitude. We have observed another c o l l i s i o n induced process, t h e c o l l i s i o n induced d e s o r p t i o n o f m o l e c u l a r methane ( r e f . 9).
The simultaneous occurrence o f
t h i s process, which i s r o u g h l y an o r d e r o f magnitude more p r o b a b l e t h a n d i s s o c i a t i o n , n e c e s s i t a t e s t h a t t h e f l u x o f methane i n c i d e n t on t h e s u r f a c e
65
t
0
15
.
NORMAL
20
D
25
D
XI
l 35
n
I
40
1.
\
KINETIC ENERGY (kcol/nold
V)
n U
Fig. 8. R e l a t i v e d i s s o c i a t i o n p r o b a b i 1 it y o f p h y s i sorbed CH4 vs E, o f Ar. Fig. 9. ( a ) HREELS b e f o r e A r atom bombardment ( b ) a f t e r bombardment
I. 0. 0.
ENERGY
LOSS (CM-')
be an o r d e r o f magnitude l a r g e r t h a n t h e A r f l u x i n o r d e r t o m a i n t a i n a monolayer coverage o f methane.
When t h e c r y s t a l t e m p e r a t u r e i s m a i n t a i n e d a t
74 K, which i s above t h e d e s o r p t i o n t e m p e r a t u r e o f m o l e c u l a r methane, no carbon d e p o s i t i o n i s observed.
This eliminates t h e possible contributions t o t h e
carbon s i g n a l from t h e d i s s o c i a t i o n o f e n e r g e t i c gas phase methane molecules (refs.
3,lO)
produced by c o l l i s i o n s w i t h h i g h energy A r atoms.
The p r o d u c t o f t h e 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 v e c h e m i s o r p t i o n event i s i d e n t i f i e d by 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.
Fig. 9a
shows t h e v i b r a t i o n a l spectrum o f a monolayer o f methane a t 46 K b e f o r e bombardment w i t h Ar.
The v i b r a t i o n a l f r e q u e n c i e s a r e u n p e r t u r b e d f r o m t h e gas
phase values w i t h i n t h e r e s o l u t i o n o f t h i s t e c h n i q u e (t20 cm-1).
The l o s s
observed a t 1305 cm-l i s assigned t o t h e u 4 mode, t h e l o s s a t 1550 cm-l t o t h e v 2 mode and t h e l o s s e s a t 2895 cm-1 and 3015 cm-l t o t h e v l and v 3 modes, respectively.
Fig. 9b shows t h e v i b r a t i o n a l spectrum a f t e r exposure o f t h e
methane monolayer a t 46 K t o a beam o f A r atoms w i t h a t r a n s l a t i o n a l energy o f 36 kcal/mole.
T h i s spectrum has been assigned p r e v i o u s l y t o an adsorbed
methyl radical. The p r e s e n t o b s e r v a t i o n supports t h e c o n c l u s i o n t h a t t h e 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 o f methane a r i s e s l a r g e l y from t h e energy r e q u i r e d t o deform t h e m o l e c u l e so t h a t t h e 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 atom and t h e N i s u r f a c e becomes s u f f i c i e n t l y l a r g e t o r e s u l t i n N i - C bond f o r m a t i o n .
66
The i m p l i c a t i o n s o f 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 v e c h e m i s o r p t i o n o f adsorbates f o r h i g h p r e s s u r e c a t a l y s i s a r e p o t e n t i a l l y immense, because i n a h i g h p r e s s u r e
environment , t h e adsorbate-covered c a t a l y s t i s c o n t i n u a l l y bombarded by a l a r g e f l u x of h i g h energy molecules.
Therefore, no r e a c t i o n mechanism f o r h e t e r o -
geneous c a t a l y s i s under h i g h p r e s s u r e c o n d i t i o n s can now be c o n s i d e r e d c o r r e c t w i t h o u t a s s e s s i n g t h e importance o f c o l l i s i o n - i n d u c e d c h e m i s t r y as a major s t e p i n t h e r e a c t i o n mechanism.
Collision-induced d i s s o c i a t i v e chemisorption o f
adsorbates i s l i k e l y another c o n t r i b u t o r t o t h e o r i g i n o f t h e p r e s s u r e gap i n t h e r e a c t i v i t y o f heterogeneous c a t a l y t i c r e a c t i o n s . T h i s work i s supported by t h e N a t i o n a l Science Foundation (CHE-8508734) and t h e S y n t h e t i c Fuels Center o f t h e Energy Lab a t M I T .
STC thanks t h e A.P.
Sloan
F o u n d a t i o n f o r a f e l l o w s h i p and t h e C a m i l l e and Henry D r e y f u s Foundation f o r a Teacher-Scholar g r a n t . REFERENCES
Rostrup-Nielsen, J.R. Anderson and M. Boudart (Eds.) , Vol. 5, S p r i n g e r Verlag, B e r l i n , 1984, p. 1. 2 F.C. Schouten, O.L.J. Gijzeman and G.A. Bootsma, B u l l . SOC. Chim. Belg., 88 (1979) 541; Surf. Sci., 87 (1979) 1. 3 M.B. Lee, Q.Y. Yang, S.L. Tang and S.T. Ceyer, J. Chem. Phys., 85 (1986)
1 J.R.
1693. 4 S.T. Ceyer, J.D.
Hines, J.
5
84 (1986)
Beckerle, M.B. Lee, S.L. Tang, Q.Y. Yang and M.A. Vac. Sci. Tech., A7 (1987) 000. S.L. Tang, J.D. Beckerle, M.B. Lee and S.T. Ceyer, J. Chem. Phys.,
6488.
6 ( a ) S.L. 7
Tang, Ph.D. Thesis, Massachusetts I n s t i t u t e o f Technology, 1985; ( b ) M.B. Lee, Ph.D. Thesis, Massachusetts I n s t i t u t e o f Technology, 1987. S.L. Tang, M.B. Lee, Q.Y. Yang and J.D. Beckerle, J. Chem. Phys., 84 (1986)
1876. 8 N. Abuaf, J.B. Anderson, R.P. Andres, J.B. Fenn and D.G.H. Marsden, Science, 155 (1967) 997. 9 J.D. Beckerle, Q.Y. Yang, A.D. Johnson and S.T. Ceyer, t o be published. 10 M.B. Lee, Q.Y. Yang and S.T. Ceyer, J. Chem. Phys. , 88 (1987) 000. 11 G. Herzberg, M o l e c u l a r Spectra and M o l e c u l a r S t r u c t u r e 11. I n f r a r e d and Raman S p e c t r a o f Polyatomic Molecules, van Nostrand Reinhold, New York,
1945. 12 Q.Y. Yang, Ph.D. Thesis, Massachusetts I n s t i t u t e o f Technology, 1988. 13 J.C. P o l a n y i and W.H. Wong, J. Chem. Phys., 51 (1969) 1439. 14 K. Nakamoto, I n f r a r e d and Raman Spectra o f I n o r g a n i c and C o o r d i n a t i o n Compounds, Wiley, New York, 1978. 15 G.B. F i s h e r and G.E. M i t c h e l l . J. E l e c t r o n . SDec. Rel. Phenom., 29 (1983) 253. 16 R.M. Gavin, Jr., J. R e u t t and E.L. M e u t t e r t i e s , Proc. N a t l . Acad. Sci., 78 (1981) 3981. 17 J.E. Demuth, H. Ibach and S. Lehwald, Phys. Rev. Lett., 40 (1978) 1044; F.M. Hoffmann and T.H. Upton, J. Phys. Chem., 88 (1984) 6209. 18 M. B r o o k h a r t and M.L.H. Green, J. Organomet. Chem., 250 (1983) 395. 19 H. I b a c h and D.L. M i l l s , E l e c t r o n Energy Loss Spectroscopy and Surface V i b r a t i o n s , Academic, New York, 1982. 20 Q.Y. Yang, M.B. Lee and S.T. Ceyer, i n p r e p a r a t i o n . 21 J.E. Demuth and H. Ibach, S u r f a c e Sci., 78 (1978) L238. 22 J.D. Beckerle, Q.Y. Yang, A.D. Johnson and S.T. Ceyer, J. Chem. Phys., 87 (1987) 000.
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
67
METHANE REFORMING BY CARBON DIOXIDE AND STEAM OVER SUPPORTED Pd, Pt, AND Rh CATALYSTS
M. MASAI, H. KADO, A. MIYAKE, S. NISHIYAMA, and S. TSURUYA Faculty of Engineering, Kobe University, Rokkodai, Nada, Kobe 657, Japan ABSTRACT Methane reforming reaction by C02 and H20 over supported Pd, Pt, and Rh catalysts has been studied. The Pd/y-AlzO~, Ptly-AlpO3, Rh/~-AlzOj and Pd/MgO-SiOz showed high activity in the CH4 reforming by COz. The adsorption experiments of NH3 and pyridine showed that the v-Al203 and MgO-Si02 supports used here retain a large amount of Lewis acidity,The dispersion of Pd on yA1203 and MgO-Si02 was found to be higher than on Si02 and MgO. These results may show that Lewis acid sites make the dispersion of Pd high. INTRODUCTION CH4, a major constituent of natural gases, can be an important carbon resource; however, CH4 is a less valuable substance because of its low reactivity. It is important to utilize this stable substance by reaction with other stable and less valuable substances such as COz and H20. Reactions between CH4 and CO2 or H20 lead to CO and Hz 111. The mixture of CO and Hz is a raw material for synthesis o f alcohols, carbonylation reactions, and the Fischer-Tropsch synthesis [2]. In this work, we studied basically CH4 reforming by COz and H20 over Pd, Pt, and Rh supported on several oxide carriers, and the main object was to stress the effect of acidbase properties of the carriers on the activity of the above metals for the CH4 reforming. EXPERIMENTAL Catalyst Dreuaration The oxide supports studied here were SiOz (silica gel grade 62 obtained from Davison Chemical) , two kinds of y-AlpO3 [AlzO3(I): Merck’s y-Al~O3and AlzO3(II): JRC-ALO-4 (Japan Reference Catalysts)], Ti02-1/2H20 (31(from Kyushu Refractories Co., Ltd.) whose XRD pattern is shown in Fig. 1, MgO (from Nakarai Chem., Ltd.), and MgO-Si02, NaY (Linde SK 40), and NaZSM-5 (prepared by the method reported in patent I41). The MgO-Si0z was prepared by impregnating SiOz with an aqueous solution of MgC12 (from Nakarai Chem.,
68
8 0
zo .
I
c
v)
0,
f
8 2 9 (dcg)
FIGURE 1 FIGURE 2
pt/iI,o,
PdlAIzOj(I)0
V
f .-
.o
8,
RhlAI;O,
pdnioz
:I I
I
10’
I
PdlNayOPd/M& 0 pd/SQ
0 ;
PdlNaZSM-5
n
!
CH4 conversion in CH4
.
+
H 2 0(%)
(left) XRD pattern of Ti02+1/2H20 using Ni-filtered Cu Ka. (right) The catalytic activities of CH4 + C02 and CH4 + H20. The contact time was 224 g-cat*min/mol. Window: The change of conversion rate with time-on-stream over Pd/A1203 (I) at 22.4 g-cat*min/mol of contact time. - - _ -: Xeq for CH4 + COP taking CO + H2O $ COP + H2 into account in calculating Xeq. ------* Xeq for CH4 + H20 taking CO + H20 f C02 + H2 into account in calculating Xeq.
.
Ltd.) followed by calcination in flowing air at 773 K for 3 h. Pd was supported on the oxide carriers by impregnating it with an aqueous solution of PdC12, followed by calcination in flowing air and reduction in flowing H2 each at 773 K for 6 h; Pt/A1203 and Rh/A1203 were supplied by Kawaken Fine Chemicals Co. The concentrations of these metals were 5 wt% of the carriers. Reaction procedure CH4 + C02 2CO + 2H2 and CH4 + H20 * CO + 3H2 reaction were carried out in a tubular flow reactor made of pyrex glass at 773 K and normal pressure. The reactant gases were CHq/CO2/N2(carrier gas)=1/1/2 and CHq/H20/N2=1/1/2 both in molar ratio. The products were analyzed by gas chromatography. Adsorution measurement Adsorption of NH3, C02, and HE on the carriers and the supported Pd catalysts was carried out in conventional static system. The supported Pd catalysts were reduced by 13.3 kPa of H2 at 773 K for 1 h and evacuated under Pa just before the adsorption experiments in the apparatus. The amounts of adsorbed gases were measured at 13.3 kPa for NH3 and Cop, and 1.33 kPa for H2 at r.t. The IR spectra of adsorbed pyridine on the carriers and the supported Pd catalysts were measured with an Analect fx-6200 spectrometer. The ESR spectra of adsorbed phenothiazine (TDPA) on several Pt catalysts were -L
69
Amount of adsorbed NH3 (Wollg)
FIGURE 3 FIGURE 4
Amount of adsorbed C 0 2 (urnol/g)
(left) Correlation between activity and amount o f adsorbed NH3. The numbers in the parentheses next to "MgO-Si02" indicate the loading o f MgO in wt%. The contact time was 22.4 g-cat-min/mol. (right) Correlation between activity and amount o f adsorbed CO2.
measured at r.t. and 100-MHz modulation by using a JES-ME-3X spectrometer. TDPA was introduced to the carriers and the reduced Pt catalysts. RESULTS AND DISCUSSION Figure 2 shows the activities of the catalysts at stationary state during the CH4 + C02 and CH4 + H20 reactions at 773 K. The change o f conversion rate with time-on-stream is also shown in Fig. 2. Little deactivation was observed. The dashed lines in the figure indicate the conversions at equilibrium (Xeq in the following). All the catalysts studied The y-Al203-supported here almost reached Xeq in CH4 + H20 (Fig. 2). catalysts and the Ti02-1/2H20-supported Pd showed high activity and PdlSiOp, Pd/MgO, and Pd/NaZSM-5 showed only low activity in the CH4 + C02 reaction. A clear effect of carriers is observed in the CH4 + C02 reaction. Acid DroDertv and catalytic activity in CH4 + COP reaction To elucidate the effect of carriers in the CH4 + C02 reaction, the effect of their acid-base property was studied on the supported Pd catalysts. Figure 3 shows the correlation between the activity and the amount of adsorbed NH3 on the carriers. Al2O3( I)* indicates Na+-scarce Al2O3(I). Figure 3 indicates that the activity increased with adsorbed NH3 on the
70
Wave number (cm-1)
Pd dispersion (HIPd)
FIGURE 5 FIGURE 6
(left) Correlation between activity and Pd dispersion. The contact time was 22.4 g-cat.min/mol. (right)IR spectra o f pyridine adsorbed on AlzO3(I), MgO-Si02, and their supported Pd catalysts.
TABLE 1
ESR signal o f phenothiazine adsorbed on the carriers.
Evacuated carriers and catalysts
ESR signal
+
y-Al203 supported Pt
n.d.
Ti 02*1/2H20 supported Pt
n.d.
+
+
si02 SUDDOrted Pt
+: detected, n.d.:
-I-
not detected
71
acidic carriers. The amount of adsorbed C02 showed only an obscure correlation with the activity as shown in Fig. 4. Figure 5 shows the correlation between the activity and Pd dispersion measured by the amount o f adsorbed hydrogen on catalysts. It indicates that the activity increased with Pd dispersion. These results are also divided into two groups, MgO-Si02 and the others, and show that finely dispersed Pd can be obtained by using carriers having large amounts of acid and that high activity would be observed over Pd on these acidic carriers. Figure 6 shows the IR spectra of adsorbed pyridine on Pd/A1203(I), AlzO3(I) , Pd/MgO-SiOZ and MgO-SiOz. It indicates that the absorption bands assignable to pyridine adsorbed on Lewis acid sites weakened with increasing Pd loading from 0 to 7 wt% Pd on Al203(I), and similar results were observed on Pd/MgO-SiOz. Pd particles would be located on Lewis acid sites. Similar results were obtained by adsorbed cation radicals o f TDPA on the carriers and the Pt-supported catalysts (Table 1). The adsorbed cation radicals show Lewis acid sites. The ESR signal of the cation radical was not detected on the Pt-loaded catalysts. Lewis acid sites may decrease considerably by Pt loading The finely dispersed Pd or Pt particles may be located on Lewis acid sites, and they may show high activity for CH4 + C02 reaction.
.
REFERENCES Ill J. R. Rostrup-Nielsen, "CATALYSIS", Vol 5 (ed. J. R. Anderson and M. Boudart) , Chapter 1, pp. 1, Springer-Verlag, (1984). [2] C. N. Satterfield, "HETEROGENEOUS CATALYSIS IN PRACTICE', Chapter 10, pp. 280, McGraw-Hi 11, (1980). 131 By courtesy of Professor Hiroaki YANAGIDA, The University of Tokyo 141 R. J. Argauer and G. R. Landolt, US Patent 3,702,886 (1972).
.
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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
SYNGAS FOR C,-CHEMISTRY.
73
LIMITS OF THE STEAM REFORMING PROCESS
JENS ROSTRUP-NIELSEN Hal d o r Topsoe A / S ,
Nymoel l e v e j 55, 2800 Lyngby
(Denmark)
ABSTRACT B e t t e r u n d e r s t a n d i n g o f t h e steam r e f o r m i n g a l l o w s b e t t e r u t i l i s a t i o n o f t h e process f o r manufacture o f CO-rich syngases. The paper summarizes t h e l i m i t s and d i s c u s s e s concepts as C02-reforming, p r e c o n v e r s i o n and a u t o t h e r m a l reforming. CHARACTERISTICS OF STEAM REFORMING 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 f o r t h e manufacture o f syngases. The r e f o r m i n g r e a c t i o n i s s t r o n g l y endothermic: CH4 -+ H20
=
CO + 3H2 (-AHig8 = 206 kJ/mol)
I n o r d e r t o s u p p l y t h e h e a t f o r t h e o v e r a l l endothermic r e a c t i o n , t h e c a t a l y s t i s loaded i n t o a number o f h i g h a l l o y tubes p l a c e d i n s i d e a f u r n a c e , t h e t u b u l a r reformer. The maximum a l l o w e d s t r e s s v a l u e i n t h e t u b e s i s s t r o n g l y i n f l u e n c e d by t h e maximum t u b e w a l l temperature and by t h e maximum h e a t f l u x . Even a s l i g h t i n c r e a s e i n t h e maximum tube w a l l temperature may r e s u l t i n a s e r i o u s d e c l i n e o.f t h e expected t u b e l i f e . The steam r e f o r m i n g c a t a l y s t i s n o r m a l l y based on n i c k e l . The p r o p e r t i e s a r e d i c t a t e d by t h e severe o p e r a t i n g c o n d i t i o n s . The a c t i v i t y depends on t h e n i c k e l s u r f a c e a r e a and p a r t i c l e s i z e . The shape s h o u l d be o p t i m i z e d t o a c h i e v e maximum a c t i v i t y w i t h minimum i n c r e a s e i n p r e s s u r e drop. CONSTRAINTS I N DESIGN The r e f o r m i n g r e a c t i o n i s a complex i n t e r a c t i o n o f h e a t t r a n s f e r , r e a c t i o n k i n e t i c s and mechanical c o n s t r a i n t s . Strong, a x i a l and r a d i a l temperature g r a d i e n t s a r e p r e s e n t i n t h e c a t a l y s t bed. The b a l a n c e between h e a t i n p u t t h r o u g h t h e r e f o r m e r tubes and t h e h e a t consumption i n t h e endothermic r e f o r m i n g r e a c t i o n i s t h e c e n t r a l problem i n steam reforming. I t can be shown ( r e f . 1 ) t h a t t h e l i m i t s i n t h e h e a t t r a n s f e r r a t e s a r e d i c -
t a t e d f r o m mechanical and m e t a l l u r g i c a l c o n s t r a i n t s r a t h e r t h a n f r o m t h e c a t a -
l y s t a c t i v i t y . P r e s e n t c a t a l y s t s may work a t h e a t f l u x e s t w i c e t h e p r e s e n t des i g n p r a c t i c e . However, i f t h e c a t a l y s t i s d e a c t i v a t e d , o v e r h e a t i n g o f tubes may r e s u l t . Hydrocarbons show d i f f e r e n t r e a c t i v i t y f o r t h e r e f o r m i n g r e a c t i o n . The 1 i m i t a t i o n s i n converting feedstocks a r e r e l a t e d t o t h e d e s u l f u r i z a t i o n step r a t h e r than f i n a l b o i l i n g p o i n t . When p r o p e r l y d e s u l f u r i z e d ( < 0.1 w t ppm S ) , gas o i l c o u l d be c o m p l e t e l y c o n v e r t e d i n t o C1-components ( r e f . 2 ) . Steam r e f o r m i n g i n v o l v e s t h e r i s k f o r carbon f o r m a t i o n by t h e decomposition o f methane and o t h e r hydrocarbons o r by t h e Boudouard r e a c t i o n ( r e f . 1 ) . Carbon f o r m a t i o n may l e a d t o breakdown o f t h e c a t a l y s t , and t h e b u i l d - u p o f carbon d e p o s i t s and degraded c a t a l y s t may cause p a r t i a l o r t o t a l blockage o f some t u b e s r e s u l t i n g i n development o f " h o t s p o t s " o r h o t tubes. The uneven f l o w d i s t r i b u t i o n w i l l cause a s e l f - a c c e l e r a t i n g s i t u a t i o n w i t h f u r t h e r o v e r h e a t i n g o f t h e h o t tubes. T h e r e f o r e , carbon f o r m a t i o n cannot be t o l e r a t e d i n t u b u l a r r e f o r m e r s . The i m p o r t a n t problem i s whether o r n o t carbon i s formed and n o t t h e r a t e a t which i t i s formed. I t i s i m p o r t a n t t o e x p l o r e t h e l i m i t s o f t h e steam r e f o r m i n g r e a c t i o n . When
t h e s e a r e known, i t i s p o s s i b l e t o e s t a b l i s h s a f e d e s i g n f o r optimum c o n d i t i o n s . F o r r e f o r m i n g o f methane, k i n e t i c s a l l o w methane t o decompose i n t o carbon i n s t e a d o f r e a c t i n g w i t h steam even i f thermodynamics p r e d i c t no carbon format i o n . However, w i t h p r o p e r r e f o r m e r design, i n d u s t r i a l o p e r a t i o n a t H20/CH4 = 1.3 i s p o s s i b l e , whereas thermodynamic l i m i t s a r e around H20/CH4 = 1.0 depending on p r e s s u r e ( r e f s . 1-2).
preconverter
waste heat channel
tubular reformer
F i g . 1. T y p i c a l arrangement o f a p r e c o n v e r t e r .
-
1.1
75 H i g h e r hydrocarbons r e s u l t more e a s i l y i n carbon f o r m a t i o n , and s p e c i a l c a t a l y s t s a r e r e q u i r e d f o r r e f o r m i n g o f naphtha and s i m i l a r f e e d s t o c k s . Operat i o n a t H20/C = 2-3 i s p o s s i b l e depending on f e e d s t o c k c h a r a c t e r i s t i c s , c a t a l y s t and reformer design. By i n s t a l l i n g an a d i a b a t i c p r e c o n v e r t e r as shown i n F i g . 1 , steam t o carbon r a t i o s as l o w as 1.0 have been demonstrated f o r naphtha ( r e f . 2). The p r e c o n v e r t e r a l l o w s t h e use of a h i g h p r e h e a t temperature i n t h e t u b u l a r reformer w i t h i n h e r e n t s a v i n g s i n r e f o r m e r d u t y . I t can a l s o s o l v e t h e problem o f u n c o n t r o l l e d amounts o f h i g h e r hydrocarbons i n n a t u r a l gas. SYNGAS FOR C1-CHEMISTRY.
C02-REFORMING
The i n c r e a s i n g i n t e r e s t f o r C1-chemistry has c r e a t e d a need f o r syngases w i t h l o w H2/C0 r a t i o . T h i s can be o b t a i n e d by adding C02 t o t h e r e f o r m e r f e e d stock:
CH4 + C02 = 2CO + 2H2
1
(2)
2
3
F i g . 2. E q u i l i b r i u m c h a r t . Thermodynamic carbon l i m i t . Aged c a t a l y s t , CH4 = C + 2H2, 2CO = C + C02. 400-10OO0C, l i n e s show H2/C0 r a t i o i n t h e reformer e x i t gas.
6 b a r abs. The d o t t e d
76
The H/C r a t i o i n eq. ( 2 ) i s 1, whereas t h e r a t i o i n eq. ( 1 ) i s 3. The use
co2
of
i n s t e a d o f steam r e p r e s e n t s no change i n o v e r a l l r e a c t i o n k i n e t i c s
( r e f . 2 ) . However, t h e presence o f C02 i n t h e feedstock r e s u l t s i n more c r i t i c a l c o n d i t i o n s f o r carbon because o f l o w e r H/C r a t i o . The r a t i o o f H2/C0 i n t h e r e f o r m e r e x i t gas can be e s t i m a t e d by thermodynamic c a l c u l a t i o n s knowing t h e a t o m i c r a t i o O / C and H / C i n t h e f e e d stream, and t h e p r e s s u r e and temperat u r e a t t h e r e f o r m e r e x i t . The r e s u l t s of t h e c a l c u l a t i o n s a r e p r e s e n t e d i n t h e e q u i l i b r i u m c h a r t ( r e f . 3 ) shown i n F i g . 2 f o r a g i v e n p r e s s u r e and temperature. The s e l e c t i o n of process parameters i s l i m i t e d by t h e p o t e n t i a l f o r carbon formation.
The c u r v e shows t h e thermodynamic carbon l i m i t c o n s i d e r i n g t h e
d e v i a t i o n of t h e carbon s t r u c t u r e f r o m i d e a l g r a p h i t e observed on c a t a l y s t s (ref.
1 ) . F o r O / C and H/C r a t i o s below t h e v a l u e s i n d i c a t e d by t h e curve,
t h e r e i s thermodynamic p o t e n t i a l f o r t h e f o r m a t i o n o f carbon. Hence, t h e p o s i t i o n o f t h e carbon l i m i t c u r v e depends on t h e t y p e o f c a t a l y s t .
Passivated Reforming) - p r i n c i p l e ( r e f . 3 ) a l l o w s operaThe SPARG ( S u l f u r -
t i o n below t h e carbon l i m i t curve. I t was demonstrated ( r e f . 4 ) t h a t carbonf r e e o p e r a t i o n c o u l d be o b t a i n e d above a c e r t a i n s u l f u r coverage a t c o n d i t i o n s which would o t h e r w i s e r e s u l t i n carbon f o r m a t i o n . S u l f u r p a s s i v a t e d r e f o r m i n g as p r a c t i c e d i n t h e Topsoe SPARG process ( r e f . 5 ) s o l v e s t h e problem o f c a r bon f o r m a t i o n by "ensemble c o n t r o l " which means t h a t t h e s i t e s f o r carbon f o r m a t i o n a r e b l o c k e d w h i l e s u f f i c i e n t s i t e s f o r t h e r e f o r m i n g r e a c t i o n s a r e maint a i n e d . T h i s e f f e c t i s o b t a i n e d by adding s u l f u r t o t h e process feed. I t i s o b v i o u s t h a t o p e r a t i o n on t h e l e f t s i d e o f t h e carbon l i m i t c u r v e i n
F i g . 2 r e s u l t s i n more economic c o n d i t i o n s ( l o w e r steam and C 0 2 - a d d i t i o n f o r a g i v e n H2/C0 r a t i o ) . O p e r a t i o n on m i x t u r e s o f C02 and methane w i t h o u t steam i s a l s o p o s s i b l e . The c o n v e n t i o n a l processes a r e l i m i t e d by t h e carbon l i m i t c u r v e . C o n d i t i o n s f o r t h e SPARG process which have been demonstrated i n a f u l l s i z e monotube process d e m o n s t r a t i o n u n i t a r e l i s t e d i n T a b l e 1. TABLE 1 SPARG t e s t s i n f u l l - s i z e monotube r e f o r m e r Duration,
H/C H20/CH4
630 1.15 6.30 1.15
200 1.27 3.40 0.96
1720 1.32 3.71 0.91
5x200 1.15 3.30 0.73
300 1.07 2.97 0.50
300 1.43 1.14 0
C02/CH4
0
0.64
0.59
0.77
0.80
2.5
1-6 950
6 900
5.6 880
1 925
1 935
9.0 900
o/c
P, T,
exit, exit
hrs
bar
F i g . 3. Flow scheme f o r SPARG process. F i g . 3 i l l u s t r a t e s a case where C02 i s a v a i l a b l e as excess C02 f r o m o t h e r sources. I f so, i t i s p o s s i b l e t o produce t h e r i g h t amount o f H2 and CO d i r e c t l y i.e.
no excess H2 i s produced w i t h f u e l v a l u e o n l y , which i s t h e case
i n an o r d i n a r y steam r e f o r m i n g case w i t h no LO2. W i t h t h e r e q u i r e m e n t f o r a d i r e c t p r o d u c t i o n o f , e.g.,
a H 2 / C 0 = 1.0 gas,
t h e SPARG process shows t o be s u p e r i o r t o c o n v e n t i o n a l r e f o r m i n g . H2/C0 = 1 can be o b t a i n e d w i t h C02/CH4 = 1.5 and H20/CH4
=
0.7.
Without s u l f u r present,
o p e r a t i o n i s p o s s i b l e w i t h b o t h r a t i o s b e i n g ca. 2, which r e s u l t s i n a l a r g e r r e f o r m e r . On t h e o t h e r hand, t h e CH4 leakage a t g i v e n p r e s s u r e i s s m a l l e r , w h i c h i s advantageous i f t h e c o l d box i s r e p l a c e d by o t h e r s e p a r a t i o n systems. Autothermal r e f o r m i n g w i t h oxygen and steam ( r e f . 6 ) i s an a l t e r n a t i v e t o t h e SPARG process. By combined p a r t i a l o x i d a t i o n and steam r e f o r m i n g o f t h e hydrocarbon, i t i s p o s s i b l e t o achieve l o w H 2 / C 0 r a t i o s w i t h o u t t h e a d d i t i o n o f LO2. The SPARG process i s p r e f e r r e d when cheap C02 i s a v a i l a b l e , whereas a u t o thermal r e f o r m i n g r e q u i r e s cheap oxygen t o be c o m p e t i t i v e . Two-step r e f o r m i n g ( r e f . 6 ) i.e.,
t u b u l a r r e f o r m i n g w i t h a u t o t h e r m a l r e f o r m i n g as a second s t e p ,
may be a f a v o r a b l e s o l u t i o n i n some cases, f o r i n s t a n c e , f o r t h e TIGAS process ( r e f . 7). CONCLUSION
Reforming w i t h optimum c o m b i n a t i o n o f o x i d i z i n g agents, steam, carbon d i o x i d e , and oxygen, i s a v e r y e f f e c t i v e r o u t e t o syngas when knowing t h e l i m i t s o f t h e technology. T h i s makes t h e i n d i r e c t c o n v e r s i o n o f methane a s t r o n g a l t e r n a t i v e t o t h e new d i r e c t c o n v e r s i o n processes.
REFERENCES
1 R o s t r u p - N i e l s e n , J.R.,
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 $ Technology, S p r i n g e r , B e r l i n , 1983, V o l . 5, Chapter 1 . 2 Rostrup-Nielsen, J.R. and T s t t r u p , P.B., i n Symposium on Science o f C a t a l y s i s and I t s A p p l i c a t i o n i n I n d u s t r y , FPDIL, S i n d r i 1979, p. 380. 3 Rostrup-Nielsen, J.R., J. C a t a l . 85, 1984, 31. 4 D i b b e r n , H.C., Olesen, P., R o s t r u p - N i e l s e n , J.R., T o t t r u p , P.B., UdenHydrocarbon Process. 65 ( I ) , 1986, 71. gaard, N.R., 5 Rostrup-Nielsen, J.R., Proc. 1 0 t h N o r t h Amer. C a t a l . SOC., San Diego, May 1987, E l s e v i e r ( i n p r e s s ) . 6 D y b k j c r , I . and Hansen, J.B., Chem. Econ. Eng. Review, 17 ( 5 ) , 1985, 13. 7 Topp-Jsrgensen, J., i n Proc. Symposium on Methane Conversion, Auckland, A p r i l 1987, E l s e v i e r ( i n p r e s s ) .
D.M. Bibby, C.D. Chang, R.F.Howe and S. Yurchak (Editors), Methane Conversion 1988Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
79
THE OPTIMAL REACTION ASSEMBLAGE IN ?HE STEAEI REFORMING OF METHANE TO PRODUCE SYNGAS ZOU RENJUN
Hebie Academy of Sciences, Shijiazhuang (People's Republic of China) ABSTRACT The steam reforming of methane to produce syngas includes nine basic reactions. The global equilibrium yield (GEY) and the potential maximal equilibrium yield (PMEY) can be calculated by a new method proposed by this paper. The optimal reaction assemblage (ORA) can be constructed by an optimization method. Where hydrogen is the desired product, the ORA is composed of reactions (1) and (2) in the nine-reaction system. The observed peak yield (OPY) of hydrogen equals 2.410 (mole fraction), its PMEY and GEY equal 3.341 (mle fraction). If carbon monoxide is the desired product, the ORA includes only reaction (1) in the nine-reaction system and the OPY, GEY and PMEY of carbon monoxide equal 0.344, 0.2767 and 0.9789 (mole fraction) respectively. Construction of the ORA is the key and fundamental way to improve the yield of a desired product. In this paper, the method of constructing the ORA is developed. INI'RODUCTION
The author has recently proposed the new concepts of global equilibrium yield (GEY) and potential m a x i m equilibrium yield (FMEY) of a q l e x reaction system (1). The GEY is the equilibrium yield of desired products calculated by conventional methods. The FMEY is the thermodynamic constraint. The OPY is the observed peak yield of desired products. lhe GEY could be exceeded by the OPY with thermodynamic constraints giving the PMEY. These concepts give information on potential and possible optimal yields and thereby show how to improve production. The reaction system of steam reforming of methane to produce syngas is a very q l e x one, in which the relationship among GEY-OPY-PMEY is also present. It will be analysed and discussed in this paper. THE REACTION SYSTEM OF STEAM REFORMING OF METHANE AND ITS OPTIMAL REACTION ASSEMBLAGE The steam reforming of methane takes place on a catalyst under the following reaction conditions: T = 873 K, P = 0.1 MPa (1 atm), m l e ratio of H20 : CH4 equals 5:l ( 3 ) . lhis system consists of nine basic reactions as below:
80
Kp(973K) CH4 H20
+ H20 = 3H2 + CO = H2 +
+ CO
0.5112
CO2
2.186
CH4 = 2H2 + C H20 + C = H2 2co = c02 + 2CH4 = C2H6
c
+
2.194 CO
+ H2
1-120 = H2 + &02
co
=
c + $402 c + 02
0.233 0.0868 8.35 x 10-5 1.12 x 10-12 1.49 x 10-l2
c02 =
1.08
10-23
From the viewpoint of chemical equilibrium, reactions ( 6 ) t o ( 9 ) i n the
above system can be neglected because of the smaller Kp.
Reactions (3)
-
(5)
can also be neglected since they are unable to compete against reactions (1) and ( 2 ) when the m l e r a t i o of H2 t o CH4 is very large. Reactions (1) and ( 2 ) are treated by set theory according t o the procedure proposed i n a previous paper ( 2 ) . The calculated r e s u l t s are l i s t e d i n Table 1. TABLE 1.
The calculated results of the reaction assemblage of steam reforming of methane set
Aj
A.
involved reaction
YG.j
set equivalent
subordination of reaction
GEY YG
OPY yo
FNEY YM
81
DISCUSSION 'he Relationship between GEY- OPY- PMEY
It can be seen from the calculated results that if H2 is the desired product, there is the following relationship:
If CO is the desired product, the relationship is as shown below:
So PMEY cannot be exceeded by OPY but GEY may be exceeded by the latter. This viewpoint has been proved.
The number of reactions suboLdinated to the optimal reaction assemblage M If H2 is the desired product then R1 and R2 are subordinates of the optimal reaction assemblage MI i.e.
If CO is the desired product then only R1 is a subordinate of the optimal reaction assemblage M I i.e.
If a co-product, such as H2 + CO, is the desired product then the demanded ratio of co-products determines the construction of the optimal reaction assemblage. Thereby, the correct strategy to decide which reactions must be accelerated and which have to be inhibited will be adopted comnercially. The set-subordination of reaction It can be seen that to gain a clear idea of the set-subordination of every reaction is a key for inproving production. Let us take reaction (4) for example. Superficially, it is favourable to product H2 and CO, so that it lcoks as if the reaction is subordinated to the optimal reaction assemblage M. But this view is not correct. Although C form CO via reaction (4), the C itself comes from CH4 via reaction (3). Clearly 1 mole of CH4 forms 1 mole of H2 and 1 mole of CO, through C, via reactions (3) and (4), whereas 1 mole of CH4 forms 3 moles of H2 and 1 mole of (1). ~ h u sreaction (4) is not the m s t efficient for producing
co via reaction
82
H2 and CO and the value of the GEY of the reaction set involving reaction (4) will reduce accordingly. Thereby only the reactions belonging to the optimal reaction assemblage can be promoted, and other reactions m s t be inhibited. This is an imprtant idea for increasing the production. CONCLUSION The reaction system of steam reforming of rnethane to produce syngas consists of nine basic reactions. If H2 is the desired product, the optimal reaction assemblage involves reactions (1) and (2), i.e. M = A1 = {Ri , R2} YO(H2) = 2.410 < YG(H2)
=
YM(H2) = 3.341 (mol.fr.)
If CO is the desired product then the optimal reaction assemblage involves only reaction (1) i.e.
The value of PMEY gives the information on the maximal limit of yield of a desired product. The construction of an optimal reaction assemblage is the fundamental way to improve the yield of desired products. "CIATURE a - the number of set Aj A - symbol of reaction set GEY - global equilibrium yield Kp - chemical equilibrium constant in terms of partial pressure M - symbol of optimal reaction assemblage OPY - observed peak yield P - pressure PMEY -potential maximum equilibrium yield R - symbol of reaction as the element of the set T - temperature YG , Yo I YM - parameter of GEY, OPY and PMEY, respectively. LITERATURE 1. Zou Renjun, Journal de Chimie Physique, 83[11 (1986) 1-6. 2. Zou Renjun, Proceedings of 2nd International Chemical Reaction Engineering Conference (ICREC-2) Pune, India, April, 1987. 3. Zou Renjun et al., Reaction Engineering in Basic organic Chemical Industry, Chemical Industry Press, Beijing, 1981.
D.M. Bihhy, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
83
THE INFLUENCE OF THE SUPPORT ON THE EFFECTIVE ORDER OF THE STEAM REFORMING REACTION
A.S. AL-UBAID, S.S.E.H. ELNASHAIE, M.E.E. ABBASHAR Chemical Reaction Engineering Group (CREG) Chemical Engineering Department, C o l l ege o f Engi neeri ng King Saud U n i v e r s i t y , P.O. Box 800, Riyadh 11421 Saudi Arabia
-
ABSTRACT The k i n e t i c s o f the c a t a l y t i c steam reforming o f methane over n i c k e l on supports w i t h d i f f e r e n t a c i d i t i e s have been investigated. The e f f e c t i v e order o f the steam reforming r e a c t i o n w i t h respect t o steam was found t o be p o s i t i v e f o r the more a c i d i c support (Y-Zeolite) and negative f o r the less a c i d i c support. These empirical r e a c t i o n orders are elucidated using LangmuirHinshelwood (Hougen-Watson) K i n e t i c s and the concept o f competition between d i f f e r e n t reactants i n a bimolecular r e a c t i o n f o r the same a c t i v e s i t e s , which gives r i s e t o a non-monotonic dependence o f the r a t e o f r e a c t i o n upon the r e a c t a n t concentrations. This approach suggests a possible rigorous explanat i o n f o r t h i s phenomenon and ways o f r e s o l v i n g t h i s c o n t r a d i c t i o n experimental-. l y and t h e o r e t i c a l l y
.
INTRODUCTION The k i n e t i c s o f steam reforming have been studied extensively [ l - 3 1 . However a general r a t e expression which f i t s the various experimental f i n d i n g s and t i e s together a l l the r a t e equations reported i n l i t e r a t u r e has n o t been found yet.
The approach used by Froment e t a1.[4]
seems t o be the most promising.
Obviously a c r i t i c a l evaluation o f the k i n e t i c expression obtained by Froment e t al.[ 41 i s beyond the scope o f t h i s s h o r t paper. The Chemical Reaction Engineering Group (CREG) a t t h e Chemical Engineering Department, King Saud U n i v e r s i t y has undertaken a t h r e e years research p r o j e c t financed by King Abdul Aziz City f o r Science and Technology (KACST) t o i n v e s t i gate the k i n e t i c s and r e a c t o r modelling f o r the steam reforming o f methane. I n t h i s e a r l y paper o f the p r o j e c t we present some o f the k i n e t i c r e s u l t s obtained using d i f f e r e n t supports and we suggest the possible explanations f o r t h e existence o f p o s i t i v e and negative e f f e c t i v e order o f the r e a c t i o n w i t h respect t o steam.
Research i s underway t o f i n d o u t the c o r r e c t answer among
t h e postulated p o s s i b i l i t i e s . Langmuir-Hinshelwood
Our k i n e t i c i n v e s t i g a t i o n i s based upon the
(Hougen-Watson) k i n e t i c s and the concept o f competition
between the reactants (methane and steam) f o r the same a c t i v e sites, g i v i n g a non-monotonic r a t e o f r e a c t i o n dependence upon r e a c t a n t concentrations.
84
EXPERIMENTAL Activity measurements were conducted in a flow reactor w i t h a gas feed section, steam generator, and gas analysis section [ 5 ] . The gas feed section included metering valves, a manifold and flow-meters to measure the flow r a t e of each gas stream. Steam was generated in a heated s t a i n l e s s steel tube (1 i n 0.D) f i l l e d w i t h rashig rings (1/8 i n ) constantly supplied w i t h water by a f l u i d metering pump. The reactor consisted of a quartz tube (12 i n . long, 1/2 i n diameter) and was heated by a 1200 w a t t furnace with a feed back temperature controller. The c a t a l y s t was placed i n powder form on a porous f r i t , and i t s temperature was measured by a chrome1 Alumel thermocouple inserted into a thermowell. The reactor inlet and o u t l e t compositions were measured by an on l i n e gas chromatograph (G.C.) equipped w i t h an 18 f t long, 1/8 i n 0.D column f i l l e d w i t h 80/100 Porapak S and operated a t 115OC u s i n g He as a c a r r i e r gas. An ice bath, placed sampling valve, condensed the steam and between the reactor e x i t and the G.C. removed the water. Prior t o each experiment, 0.5 or 1 gm of catalyst were pretreated in situ w i t h 80 cc/min of a i r a t 4OOOC f o r 1 h r followed by Hydrogen reduction a t the same conditions for 12 hrs. After reduction, hydrogen was replaced by He and the reactor temperature was raised t o i t s desired value. Helium was passed through the steam generator heated t o 200°C, and a f t e r a constant flow r a t e had been attained the steam-He flow was mixed w i t h methane and the composition of the i n l e t stream was measured. The feed was then introduced into the reactor, and the composition of the gas leaving the reactor was measured by the GC as a function of time-on-stream. Three types of catalysts were used, Ni/Na-Y Zeolite [6], N i / N i A120, [5] and Ni/(Ni, Ca) Al,O,(nickel on nickel calcium aluminate)[5]. RESULTS T h e r e s u l t s obtained indicated t h a t f o r Ni/Na-Y Zeolite catalyst ( t h e most acidic support) f o r the range of variable studied, the effective order of the reaction is zero w i t h respect t o methane, positive w i t h respect t o steam and negative w i t h respect t o hydrogen. For Ni/Ni A120,,, the effective order is positive w i t h respect to steam and methane and negative w i t h respect t o hydrogen. However f o r the catalyst w i t h the l e a s t acidic support, Ni/(Ni, Ca) A1204, the effective orders of the reactions are: positive w i t h respect to methane, negative w i t h respect to steam ( b u t to l e s s extent) and negative w i t h respect t o hydrogen. Table 1 summarises these results. T h i s change of the sign of the effective order of reaction w i t h respect t o
85
steam w i t h the change of the support i s a very important and interesting phenomenon. We will concentrate our attention on the discussion of t h i s phenomenon t o p u t forward our plan for experimental investigation of i t . TABLE 1 Kinetic Parameters of Steam Reforming of Methane i n this Study ( T = 450-550°C, 1 atm)
Catalyst
E KCal /mol
Rate Determining Step
N i /Y -2eol i t e
11.2
Desorpti on (bifunctional i ty
N i / N i Aluminate
16.5
N i / ( N i , Ca) A1 umi nate
21.7
(Spi nel 1
.
r (gmol e/a h r ) k [H20]1.2
1 + K[H2]1'22 k[CH4] 0'24 [ H20]o'28 1+ K
Surface reaction (competition)
k[CHq]0'62 [H20]1'05 "H201
+
K [H21232
DISCUSSION Analysis of these results using the Langmir-Hinshelwood (Hougen-Watson) approach gives a sounder physical foundation than power law kinetics. On the basis of the r a t e expression derived by Langmir-Hinshelwood approach an empirical power law f i t t i n g of the same r a t e equation will show a changing e f f e c t i v e order of the reaction. For bimolecular reactions, when the two reactants are adsorbed on different active s i t e s (Mechanism I (Table 211, the r a t e of reaction dependence upon the concentration of e i t h e r of the reactants will have the form shown i n Figure la. The effective order f o r t h i s case changes from 1 to 0 w i t h o u t ever changing into a negative value. However, when the two reactants are competing for the same type of active s i t e s , then the r a t e of reaction dependence upon the concentration of e i t h e r of the reactants will be a non-monotonic function, giving r i s e to the possibility of negative effective reaction orders w i t h respect t o the reactants concentration i n a certain region of the parameters, as shown i n Figure l b . When these principles are applied t o the results reported i n this paper f o r the effective order of the reaction w i t h respect to steam concentration we obtain the following p o s s i b i l i t i e s :
86
TABLE 2 Proposed Mechanism Mechanism I (Dual S i t e s ) [ 6 ] H20
+
SgH2O
CH, + M$C.M
-
S
+ 2H,
C.M + H2O.S$HCO.M + 1/2 H, + S HC0.M + H2O.SeHCOO.M + H, + S
slow
HC0O.M +CO,
+ 1/2
H2
+ M
Mechanism I1 ( S i n g l e S i t e ) :
Cb +
M S C H , .M + H2 $0 + MS0.M + H, CH, .M + 0.M sloyC0.M
C0.M + O.M+CO,
+ 2M
+ H, +
M
where S i s t h e support s i t e and M i s t h e metal s i t e
Fig. 1 Monotonic and non-monotonic dependence of the r a t e o f r e a c t i o n on r e a c t a n t concentrations.
87
1.
With regard t o the c a t a l y s t w i t h the m r e a c i d i c support, whichgives
negative e f f e c t i v e order f o r steam, there i s the p o s s i b i l i t y t h a t t h e r e i s no region
o f parameters w i t h negative e f f e c t i v e order and thus the reactants
should f o l l o w mechanism I r e f . [6], w i t h no competition f o r the same a c t i v e sites. However there i s also the p o s s i b i l i t y t h a t non-monotonic dependence does e x i s t ( w i t h negative e f f e c t i v e order) i n a region o f concentration which i s n o t explored yet. 2. The negative e f f e c t i v e r e a c t i o n order w i t h respect t o steam, which i s observed f o r the l e a s t a c i d i c support, very strongly suggests a non-monotonic
dependence o f the r a t e o f r e a c t i o n upon reactant
concentration, thus type I 1
mechanism [5] i s m s t probable w i t h competition between steam and methane f o r t h e same a c t i v e s i t e s w i t h p o s i t i v e order i n c e r t a i n regions and negative order i n the other regions. These p o s s i b i l i t i e s nust be resolved through
further
i n v e s t i g a t i o r s o f the
dependence o f the r a t e o f r e a c t i o n upon steam and methane concentrations over a very wide range o f r e a c t i o n conditions f o r d i f f e r e n t supports w i t h varying degrees o f a c i d i t y . supported by
These i n v e s t i g a t i o n s w i l l be p a r t o f
CREG
Research P r o j e c t
KACST.
REFERENCES
1 James P. Van Hook, Catal. Rev. Sci. Eng. 21(1), 1980, 1-51. 2 J.R. Rostrup-Nielsen, Steam Reforming Catalysts, Copenhagen, Danish Technic a l Press, 1975. 3 S. El-Nashaie, M.A. Soliman and A.S. Al-Ubaid, I n t e r . "AMSE" Conference f o r Modelling and Simulation, Cairo, Egypt, March 1987. 4 Jianguo Xu and G.F. Froment To be published. 5 A.S. Al-Ubaid, The Second National Meeting o f Chemists, Riyadh, 7-9 March, 1987. 6 A.S. Al-Ubaid, and E.E. Wolf, Accepted f o r p u b l i c a t i o n i n Applied Catalysis.
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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
STEAM REFORMING:
89
KINETICS, CATALYST DEACTIVATION AND REACTOR DESIGN
S.S.E.H. ELNASHAIE, A.S. AL-UBAID and M.A. SOLIMAN Chemical Reaction Engineering Group (CREG) Chemical Engineering Department, Col lege o f Engineering King Saud U n i v e r s i t y , P.O. Box 800, Riyadh 11421 Saudi Arabia
-
ABSTRACT A c r i t i c a l review o f steam reforming 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 reformer modelling has been c a r r i e d o u t by CREG p r i o r t o i t s embarking on the research p r o j e c t financed by KACST. This review was c a r r i e d o u t i n o r d e r t o determine the most c r i t i c a l p o i n t s t h a t need f u r t h e r research. The conclusions drawn from t h i s c r i t i c a l i n v e s t i g a t i o n o f the steam reforming process are summarized i n t h i s s h o r t communication. Model equations f o r reactions i n steam reforming are a l s o given.
INTRODUCTION Steam reforming o f hydrocarbons, e s p e c i a l l y natural gas, has become the most important and cheapest source f o r the production o f hydrogen [1,2]. The production o f hydrogen i s the essential step i n the production o f ammonia and methanol [ 3 ] as w e l l as f o r t h e reduction o f i r o n ores [ 4 , 5 ] and f o r use i n f u e l c e l l s [ 61
.
Because o f the special demands on t h e q u a l i t y o f t h e hydrogen
produced f o r some processes, t h e need t o b u i l d l a r g e production units and energy saving considerations, the demand has increased f o r a low steam t o methane feed r a t i o f o r steam reformers.
T h i s demand poses a number o f challen-
ges w i t h regard t o the k i n e t i c s as w e l l as t h e r e a c t o r design and construction. This i s because the decrease o f the steam t o methane r a t i o n o t only d e c r e a s e s t h e r a t e o f r e a c t i o n and the e q u i l i b r i u m conversion, b u t i t a l s o d r a s t i c a l l y a f f e c t s the carbon formation which deactivates the C a t a l y s t [ 7 ] . A t a high steam t o methane r a t i o another d e a c t i v a t i o n mechanism p r e v a i l s , t h a t i s the r e o x i d a t i o n o f N i t o N i O which i s then rereduced by the hydrogen formed during steam reforming. The Chemical Reaction Engineering Group (CREG) a t t h e Chemical Engineering Department o f the College o f Engineering, King Saud U n i v e r s i t y has undertaken a t h r e e years p r o j e c t financed by King Abdul Aziz City f o r Science and Technology (KACST) i n order t o c o n t r i b u t e t o the i n t e r n a t i o n a l e f f o r t s i n t h e f i e l d o f c a t a l y s t Droduction. k i n e t i c s o f the reaction.
r e a c t o r modelling and design o f
90
Before embarking on the research p r o j e c t CREG has undertaken a thorough i n v e s t i g a t i o n and c r i t i c a l evaluation o f the l i t e r a t u r e i n order t o p o i n t o u t t h e most c r i t i c a l research p o i n t s t h a t need t o be t a c k l e d f o r t h i s extensively i n v e s t i g a t e d process. ces
[ 831.
The d e t a i 1s o f t h i s i n v e s t i g a t i o n are given i n referen-
Summary o f the conclusions reached from t h i s i n v e s t i g a t i o n and a
c r i t i c a l review o f the l i t e r a t u r e i s given i n the next section. RESULTS AND DISCUSSION Many p o i n t s have been r a i s e d i n the c r i t i c a l review paper [ 8 ] , however t h e f o l l o w i n g p o i n t s are o f special importance f o r f u r t h e r research on the subject: 1-
Producing more a c t i v e c a t a l y s t s i s s t i l l an important o b j e c t i v e i n order
t o be able t o operate a t lower tube w a l l temperature f o r the same heat f l u x and t o b u i l d more compact steam reformers. 2- I t i s important t o develop more general k i n e t i c r a t e expressions f o r the steam reforming r e a c t i o n i n order t o account f o r the dependence o f t h e r a t e o f r e a c t i o n on the p a r t i a l pressure o f the various components over the whole range o f operating conditions.
Special emphasis should be given t o the phenomenon o f
i n t e r a c t i o n between chemi s o r p t i o n and the surface reactions.
The possi b i 1it y
o f non-monotonic dependence o f the r a t e o f r e a c t i o n upon the concentration o f some o f the reactants should be i n v e s t i g a t e d t o e l u c i d a t e the observations o f d i f f e r e n t authors o f negative and p o s i t i v e e f f e c t i v e order o f the r e a c t i o n s w i t h respect t o steam [ 81.
3-
The heat t r a n s f e r l i m i t a t i o n t o the c a t a l y s t bed i s a serious problem i n
t h e design o f the steam reformer;
t h i s l i m i t a t i o n increases the s i z e o f the
reformer and prevents the operation from gaining the f u l l b e n e f i t from improvi n g the c a t a l y s t a c t i v i t y .
This problem should be thought about by considering
a strong preheating o f the reactants p r i o r t o t h e i r i n t r o d u c t i n i n t o the r e f o r mer tube ( a d i a b a t i c preheating). The other more promising, b u t possibly m r e d i f f i c u l t , r o u t e i s by considering the use o f f l u i d i z a t i o n o f the c a t a l y s t i n s i d e the reformer tubes which w i l l enhance the r a t e o f heat t r a n s f e r considerably.
4-
The problem o f the k i n e t i c s o f coke formation i s a very important espe-
c i a l l y w i t h the increasing demand f o r the use of low steam t o methane r a t i o s
[lo].
K i n e t i c r a t e expressions f o r the coke formation need t o be developed. These r a t e equations should give the r a t e o f coke formation i n terms of the p a r t i a l pressure o f the various components and n o t only i n terms o f the carbon d e p o s i t i o n and t i m e ;
i t should a l s o take i n t o consideration pore blockage as
w e l l as a c t i v e s i t e coverage by coke. 5- With t i g h t e r design o f reformers being sought, t h e problem o f N i reoxida t i o n a t the entrance t o the reformer c a t a l y s t tubes should be t a c k l e d and t h e
91
suggestion o f Froment [ l l ] t o r e c y c l e p a r t o f the hydrogen t o the entrance of t h e reformer c a t a l y s t tubes should be i n v e s t i g a t e d . 6-
The r o l e o f the support i n c a t a l y z i n g the r e a c t i o n s needs t o be i n v e s t i -
gated i n some d e t a i l . 7- A comprehensive model f o r the steam reformer should be developed.
This
model should take i n t o c o n s i d e r a t i o n t h e c h a r a c t e r i s t i c s o f the combustion chamber as w e l l as the d e t a i l s o f the processes t a k i n g place i n t h e c a t a l y s t tubes: steam reforming reaction, coke formation, t i o n o f NiO,
r e o x i d a t i o n o f N i and rereduc-
mass and heat t r a n s f e r between t h e c a t a l y s t p e l l e t s and t h e bulk
gas ( b o t h e x t e r n a l and i n t r a p a r t i c l e ) , heat t r a n s f e r between t h e c a t a l y s t tubes and t h e combustion chamber
... etc.
A TWO-DIMENSIONAL HETEROGENEOUS MODEL FOR THE STEAM REFORMER A two-dimensional
heterogeneous m d e l i s developed f o r the c a t a l y s t tubes,
w i t h o u t c a t a l y s t deactivation,
t a k i n g i n t o c o n s i d e r a t i o n t h e steam reforming
reactions,
CH,, + H&I Co + tLp
+ 2H$ +LO2
CH,,
R,,
CO
+CO,
\ , R,
+ 3H,
(1) (2)
+ H, + 4H2
are r a t e s o f r e a c t i o n
(3) f o r r e a c t i o n s ( l 1 , (21, ( 3 ) r e s p e c t i v e l y .
The two-dimensional heterogeneous model equations takes t h e form,
and
t o g e t h e r w i t h t h e h e a t balance equation,
92
w i t h the boundary conditions: a t II = 0
,
CA = CAFy CB = CBFy Cc = CcFy C,
atr=O
,
$ = L acL = ac - = - acD = o
a t r = R t y
where Ci
ar
ar
ar
acA
acB
acC
acD
ar
ar
ar
ar
A e r a T -- - U(T
- Tw)
-=-=-=-=
= CDFy T = TF
aT ar 0
,
i s t h e c o n c e n t r a t i o n o f the component i ( K g m l e / m 3 ) , CiF
i s the feed
c o n c e n t r a t i o n (Kg m o l e / d ) , C i s t h e s p e c i f i c heat o f t h e gas (KJ/Kg.K), Deri P i s t h e e f f e c t i v e r a d i a l d i f f u s i v i t y o f t h e component i ( d / h r ) , R i s the a x i a l l e n g t h coordinate (m), r i s the r a d i a l length coordinate (m), q i s the superf i c i a l velocity, (Kg/d
(m3 gas/m2 bed h r ) ,
pb
i s the bulk density o f t h e c a t a l y s t
1, R j i s t h e r a t e o f r e a c t i o n f o r r e a c t i o n
j (Kg mole/Kg c a t a l y s t . h r ) ,
T i s t h e c a t a l y s t temperature (K) (assuming the c a t a l y s t and the gas tempera-
t u r e a r e equal), Aer
i s the e f f e c t i v e r a d i a l c o n d u c t i v i t y o f the bed (KJ/m.hr.
n j i s the effectiveness f a c t o r f o r r e a c t i o n j ( t o be computed from the d i f f u s i o n r e a c t i o n equation a t each p o i n t i n the r e a c t o r ) , (-AHj) i s the heat
K),
o f r e a c t i o n f o r r e a c t i o n j (KJ/Kg mole), R t i s the c a t a l y s t tube radius, m, U i s the o v e r a l l heat t r a n s f e r c o e f f i c i e n t i n 1KJId.K)
and Tw i s the w a l l
temperature (K) which i s determined through the coupling between the above model equations f o r the c a t a l y s t tube and the model equations f o r the combusti o n chamber ( t h e furnace).
A d i s t r i b u t e d parameter model f o r t h e combustion
chamber i s being developed by (CREG) f o r both top f i r e d and s i d e f i r e d f u r naces. The c a t a l y s t tube p a r t i a l d i f f e r e n t i a l equations are reduced t o a s e t o f ordinary d i f f e r e n t i a l equations using the orthogonal c o l l o c a t i o n method. t h e combustion chamber the governing d i f f e r e n t i a l
For
equations describing t h e
temperature d i s t r i b u t i o n i n the furnace are t o be a l s o solved, u s i n g t h e c o l l o c a t i o n method.
93
REFERENCES
1 James P. van Hook, "Methane-Steam Reforming", Catal. Rev. Sci. Eng. 21(1), 1-51, 1980. 2 Jens R. Rostrup-Nielsen, "Catalytic-Steam Reforming", I n C a t a l y s i s Science and Technology, Vol. 5, p. 1 (J.R. Anderson and M. Boudart ( e d i t o r s ) , Springer Verlag, B e r l i n ) , 1984. Buvida, C.J., Hydrocarbon Processing, 58 (91, p. 197, 1979. 3 Czuppon, T.A.; 4 Rose, F.; Stahl Eisen, 95, p. 1012, 1975. 5 Manely, J., Metals B u l l . , p. 49, 1981. 6 Dybkjaer, I.Chem. Econ. Eng. Review, 13, 17, 1981. 7 Mukherjee, D.K. and Ghosal, S,R., Chem. Age o f I n d i a , Vol. 32, No. 5, 1981. 8 Elnashaie, S.S.E.H., Al-Ubaid, A.S., Soliman, M.A. and Adris, A.M., "AMSE" I n t e r n a t i o n a l Conference on Modelling and Simulation, Cairo, March 1987. 9 Al-Ubaid, A.S. , Soliman, M.A. , Elnashaie, S.S.E.H. , Progress Report presented t o King Abdul Aziz City f o r Science and Technology, KACST, P r o j e c t No. AR-7-19, March 1987. 10 Paloumbis, S. and Petersen, E.E., Chem. Reaction Engng., American Chem. Sci. No. 38, p. 489-494, 1982. 11 Froment, G.F., Lecture given a t the Chem. Engng. Dept., King Saud Univers i t y , Feb. 1987.
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I)5
D.M. Bihby. C.D. Chang, R.F. Howe and S.Yurchak (Editors),Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CATALYTIC PROCESSESFOR METHANOLSYNTHESIS - ESTABLISHEDAND FVTURE
M.S. WAINWRIGHT
of
School
Chemical
Engineering
and Industrial Chemistry, The University 2033 N.S.W., Australia.
o f New South Wales, P.O. Box 1, Kensington,
ABSTRACT Recent research into improved catalysts, catalytic processes and reactor designs for methanol synthesis is reviewed. Raney Cu-ZnO catalysts prepared by caustic leaching of Cu-Al-Zn alloys and copper based catalysts produced from Cu-Th and Cu-Ce alloys show potential for replacing coprecipitated Cu-ZnO-A1 0 catalysts in existing plants. Among the novel reactor configurations being 2 3 proposed, the Gas-Solid-Solid Trickle Flow Reactor shows great potential. Preliminary studies of the low temperature slurry-phase process being developed at Brookhaven National Laboratories suggest that it is the most promising of the three liquid-phase methanol synthesis processes reviewed. Direct catalytic oxidation of methane is seen as being a considerable way from commercialisation.
INTRODUCTION Current methanol synthesis introduced
copper-based catalysts,
zinc chrornite introduced
catalysts,
the first
1).
(ref.
industrial
The plants
introduction
result
catalytic
by I.C.I.
of the synthesis
significant
catalysts
than
since BASF process in 1923
required
of 20-30 MPa for methanol synthesis.
was largely
temperatures
of
The as a result
gas which enabled the catalyst
of
to
The advantage of the copper catalyst
deactivation.
enabled operating
temperatures
to be reduced to ca 250'C with the
of 5-10 MPa could be used.
that pressures
more active
methanol synthesis
of the copper-based catalyst
operate without
back to 1966 when I.C.I.
that were significantly
using the zinc chromite
improved desulphurisation it
relates
which had been used almost exclusively
ca 350°C and hence pressures
was that
technology
This involved
significant
energy savings. The introduction its
licensees
market.
of the copper-based catalyst
to obtain a large
It effectively
which had enjoyed a large the high pressure
section
eliminated
process,
technology
of the methanol plant
from the field
share of the U.S.A. This was a result
enabled I.C.I. construction
the Vulcan Cincinnati
methanol plant
and
construction
Company for
of the fact that whereas Vulcan
96
had an effective zinc chromite catalyst, it had not developed a copper-based catalyst. In the two decades since I.C.I. introduced the low-pressure methanol process others have gained increasing shares of the market. Major competitors include Lurgi, Mitsubishi Gas Chemicals and Haldor Topsoe. All have their own variations of the co-precipitatedCu-ZnO catalysts. Recently, Alberta Gas Chemicals has constructed several 1200 tonne/day plants : one at Medicine Hat, Alberta and the other at Taranaki near the Synfuels methanol-to-gasolineplant in New Zealand. The Alberta Gas Chemicals plants use catalysts under licence. Recent developments in low pressure methanol synthesis processes result from improvements in energy efficiency obtained in various parts of the plant including converter, separator, and heat exchange operations. There is also the on-going development of improved co-precipitated catalysts. However, there have been no major advancements to rival that achieved by I.C.I. in the mid 1960's. This paper reviews research and development in several areas of methanol synthesis technology. Alternatives to the co-precipitatedCu-ZnO-A10 and 2 3 Cu-ZnO-Cr0 catalysts are considered first. Novel processes for syngas 2 3 conversion are then reviewed, and the paper ends with a discussion of direct conversion of methane to methanol by partial oxidation of natural gas. ALTERNATIVES TO GO-PRECIPITATED CU-ZnO CATALYSTS FOR SYNGAS CONVERSION IN CONVENTIONAL METHANOL SYNTHESIS PLANTS This section of the review considers several recent developments in catalysts for low to medium pressure methanol synthesis that could be used in existing converters. These include the Raney copper-zinc catalysts, which have similar compositions and properties to co-precipitated catalysts, thorium-copper and cesium-copper intermetallics and supported noble metals Raney Comer-Zinc Catalysts Based on the high capacity of Raney copper to adsorb carbon monoxide (ref. 2) whilst having weak hydrogenation activity, it was first proposed in 1978
that Raney copper might show some promise as a methanol synthesis catalyst (ref. 3).
The early work by Marsden et a1 (ref. 4) showed that methanol could
be produced under conditions of industrial practice using catalysts prepared by selectively leaching aluminium and zinc from particles of Cu-Zn-A1 alloys in caustic solutions at temperatures around 50'C.
This work, which formed the
basis of patents for catalyst preparation (ref. 5) and methanol synthesis (ref.
6), showed that the porous copper sponge, which contained residual ZnO and A1203, had activity for methanol production comparable to that achieved using a commercial co-precipitated catalyst (C79.4 from United Catalysts, Inc., Kentucky, U.S.A).
Later studies revealed that the '279.4 catalyst had
97
significantly less activity than state-of-the-artco-precipitatedcatalysts. The work of Marsden et a1 (ref. 4 ) and later studies (refs. 7 , 8 ) were concerned with optimizing the alloy composition. The earlier research showed that high levels (10-20wt
%)
of zinc in the precursor alloy were necessary to enable
re-precipitationof Zn(OH)2
in the grains of porous copper which were produced
by dissolving A1 and Zn from the alloy. Friedrich et a1 made several fundamental studies of the leaching process which led to a better understanding of the morphological development of the leached copper structures (ref. 9 ) and the kinetics of the leaching reactions and the relationship to the phase composition of the alloys (ref. 10). In two other papers (refs. 11,12) they characterised the surface of fully leached and partially leached alloy particles using physical and chemical adsorption and endeavoured to relate catalyst activities to surface area and pore morphology. The early work (refs. 4 , 5 , 7 , 9 - 1 2 )used alloys which were based on substituting Zn for copper in alloys containing 50 wt
%
Al. In that way alloys
ranging from CuAl2 (no zinc) to alloys of composition Cu-30 wt %; Zn-20 wt % , A1-50 wt % were investigated. Whilst these alloys containing such high levels of aluminium later proved not to be optimal they were valuable in leading to an understanding of the leaching process and to the design of improved catalysts. In particular, studies of morphological development (ref. 9 ) and leaching kinetics (ref. 10) used metallographic techniques such as X-ray diffraction, electron microprobe analysis and microscopy to examine partially leached alloy pieces.
These studies showed that the alloys contained a Cu(2n) A12 phase which
was in the form of grains that were surrounded by a second major phase, which was a Al-Zn solid solution. The former leached to produce porous copper whilst the latter leached completely leaving void space. A major observation, important to improved catalyst development, was that the level of zinc present in the porous copper was several times greater than the amount present in the original Cu(Zn)A12,
taking into account the dissolution of A1 from the grain.
This revealed that some of the Zn, which had dissolved from the intergranular phase had re-precipitatedas Zn(0H)
2
on the surface of the porous copper,
thereby accounting for the high activity of these catalysts in methanol synthesis. The studies also showed that the grains of copper which were around 40 pm in diameter had pore diameters of the order of 30 run diameter and that
the voids between the grains had diameters of around 2 or 3 pm.
Despite the
rather porous nature of the sponge catalyst material, the particles appeared to be quite robust. A study by Bridgewater et a1 (ref. 8 ) revealed that alloys containing 50 wt %
A1 were far from optimal and that superior catalysts could be prepared using
alloys with compositions around 47 wt
%
Cu, 39 wt
%
A1 and 15 wt
%
Zn.
Catalysts prepared from these alloys had both greater activity and greater
98
mechanical strength than those prepared from the higher A1 alloys. Therefore more recent work has concentrated on the use of an alloy containing 43 wt Cu, 39 wt
%
A1 and 18 wt % Zn.
%
Bridgewater et a1 (ref. 13) compared the
performance of catalysts prepared by leaching alloy particles in 20 wt
%
aqueous NaOH at 50°C with the same size particles (0.5 to 0.7 nun dia) of the C79.4 co-precipitated catalyst, which had been used in previous studies (ref. 4)l.
It was concluded that despite the markedly different methods of catalyst
preparation, the same active species are involved in methanol synthesis on both types of catalyst and that the active species involve metallic copper. In both of the studies by Bridgewater et a1 (ref. 8,13) it was suggested that ZnO produced as Zn(OH)2
during the leaching process is responsible for the high
levels of promotion of the activity of Raney copper for methanol synthesis. Furthermore, evidence is presented (ref. 8) for a synergistic effect on activity between the Raney Cu and ZnO re-precipitatedduring the extraction. Because of commercial pressures to produce catalysts with geometries similar to pelleted co-precipitatedcatalysts used in industrial converters, recent studies have concentrated on pellets of alloy that are 3.6 mm x 5.4 mm dia. Curry-Hyde et a1 (ref. 14) prepared catalysts by leaching pellets of an alloy containing 43.2 wt
%
Cu, 39.0 wt
% A1
and 17.8 wt
concentrations of caustic solutions (5 to 20 wt 1°C to 50'Cl.
%)
%
Zn in various at different temperatures
They showed that the surface area of the leached product
decreased with increasing leach depth, with the decreases being more pronounced at higher leach temperatures. Specific catalytic activities increased with leach depth until they reached a maximum and then decreased. Thus an optimal leach depth was obtained. The observed maximum in activity with leach depth can be related to two effects. The first of these is the re-arrangement of the copper leading to reduced surface area per unit volume of Raney copper caused by long exposures of leached material in caustic solution (ref. 15).
The other
is the secondary dissolution of Zn(OH)2 from the porous copper at the longer times of leaching. The outcome of the previous work has been that a new approach to the development of Cu-ZnO methanol synthesis catalysts has been devised (ref. 16). In this new procedure the alloy is activated by leaching the alloy in caustic solutions containing relatively high concentrations of zinc in the form of sodium zincate. In this way the surface of the porous copper can be more readily promoted by deposition of Zn(0H) 2 during the leaching process. The result of this improved activation technique is that catalysts up to 100% more active than state-of-the-artco-precipitatedcatalysts can be produced. INTERMETALLIC CATALYSTS Recent research at the U.S. Bureau of Mines has investigated the use of
99
intermetallic compounds, notably binary copper-thorium alloys, for the synthesis of methanol from synthesis gas containing CO and H2 (ref. 17).
The
catalyst preparation involved alloying the two metals by arc-melting in an inert (helium) atmosphere. The alloy samples were activated by first oxidizing to convert the thorium to thoria and then reduced in hydrogen or synthesis gas to convert any copper oxide to metallic copper. Various oxidizing media were used including air, air/water, H /water or pure C02. The catalysts were
2
evaluated using a syngas containing 16 H2:C0 at 6 MPa pressure and 31000 GHSV. Catalysts prepared from alloys over a wide range of compositions (Table 1) had similar activities and surface areas. Comparative testing with a co-precipitated Tho /5 Cu and an industrial Cu-Zn0-A1203catalyst (United 2 Catalysts Inc. C79.4) showed that the catalysts prepared from the binary alloys were around an order of magnitude more active. The catalysts were quite selective for methanol synthesis at low temperatures where methane accounted for around 2% of the product at 240°C. However, the extent of methanation increased with temperature. The results for these catalysts are very promising. However, the test conditions employed were not favourable for a coprecipitated Cu-ZnO-A10 such 2 3 as the C79.4 since it is well known that C02 in the synthesis gas is essential to achieve significant activity with this type of catalyst (ref. 18). Therefore, the comparisons were not made on a basis that resulted in maximum activities in the various catalyst systems. TABLE 1 Conversions to methanol over catalysts produced
from copper-thorium alloys
activated in air at ambient conditions (from ref.17). Reaction conditions: 16 H2/CO: GHSV 31000 h-':
Catalyst
6 MPa: 250°C
Conversion (2)
Th2cu Th cup Th cu Th02/i?C$ co- precipit a t e C74.9-Cu-Zn0-A1203
27.9 38.2 40.7 20.4 42.0
45.3
35.9 39.5 1.3 2.7
Daly (ref. 19) has used a ThCu alloy sample provided by the Bureau of Mines 6
to further investigate the mechanism of methanol synthesis over this novel catalyst system. Kinetic experiments conducted at 240°C revealed that the reaction is first order in Hg and zero order in CO which is to be expected
100
since copper strongly adsorbs CO, and H2 adsorption is weak. It was found that when syngas containing 2.4 H /CO was used there was no deactivation. However, 2 when the ratio was decreased to 0.8 H2/GO, a linear decrease in activity was observed over 100 hours on stream. Similar deactivation in lean H2/CO mixtures had been observed by Baglin et a1 (ref. 17). The rate of deactivation was reduced by including C02 in syngas. The apparent stability of catalysts
produced from the Th-Cux alloys when using synthesis gases containing no GO2 is attributed to a steady-state condition between coke deposition and coke removal by methanation (ref. 19). The activity of these catalysts in methanol synthesis i is derived from finely divided copper species (ref. 17), thought to be Cu (ref. 19), interdispersed on thoria. Another interesting development using intermetallics has been described in the patent literature (ref. 20). Alloys of copper and oxidisable rare earth metals such as cerium and lanthanum were prepared by melting mixtures of the powders of the pure metals. Additives such as aluminium and palladium were investigated. The alloys were crushed and screened to obtain 0.6 to 0.85 mm particles that were suitable for laboratory testing under typical methanol synthesis conditions. Typical results obtained are presented in Table 2. TABLE 2 Methanol yields over catalysts produced from copper-cerium and copper-lanthanum alloys (from ref. 20). Reaction conditions : 72H2/28C0 : 5MPa. Alloy Composition (wt %)
GHSV
Temp.
Time on Stream (h)
Methanol in Product Gas ( % v/v)
Cu/Ce 50/50
20,000 40,000
150 250
100 40
1.0 2.6
Cu/Ce/Al 45/45/10
20,000 40,000
200 250
100 280
3.2 7.0
Cu/Ia./Al 45/45/10
20,000 20,000
200 250
100 280
4.0 1.0
Cu/Ce/Pd 45/50/5
40,000
250
2.0
The results in Table 2 show that these catalysts are active for methanol synthesis. It was shown in the patent that they are also highly selective with the non-methanol organics content of the product being less than 1 wt % . The results in Table 2 show that the addition of A1 to the alloys results in increased activity and life. They also suggest that cerium is preferred to lanthanum as the rare earth metal. After reaction the catalysts were examined and were found to consist of finely dispersed metallic copper in cerium oxide. As such they are similar to those produced from Cu-Th alloys (refs. 17,19).
101
As with the Cu-Th alloys (refs. 17, 19) the conditions of pre-treatment of the alloys were investigated. Treatment with air gave high initial activity but poor stability whilst hydrogen pretreatment resulted in lower initial activity but greater catalyst life. Pretreatment using an inert gas (N ) led to 2 intermediate behaviour. The performance of these novel catalysts was compared with that of a conventional copper-zinc-aluminamethanol synthesis catalyst under the same conditions (250°C, 40,000 h-',
72H2/28 CO, 5 MPa).
The methanol yield was only
8.0 g mol/l/hr which was more than an order of magnitude less than that obtained with the catalyst produced from the Cu-Ce-A1alloy. The methanol yields obtained using the Cu-Ce catalysts and the 72H /28 CO syngas were comparable to those 2 obtained using conventional catalysts with C02 present in the feed. It is claimed (ref. 21) that the advantage of these intermetallic Cu catalysts based on thorium and cerium is that since no C02 is necessary to maintain activity, provided the H /CO ratio is high enough, they can be used in 2 a low pressure process generating anhydrous methanol directly. Noble Metal Catalvsts Methanol synthesis over noble metal catalysts has been pursued in a number of laboratories. However, the research is very much of academic interest since the catalysts have poor selectivity for methanol synthesis (ref. 22).
The
activity and selectivity of the noble metal catalysts is influenced by both the metal and the support (ref. 22).
Metals that have been investigated include Rh
(refs. 22,23), Pd (refs. 22,25), Ir (ref. 22) and Pt (refs. 22-26).
A wide
range of support materials have also been investigated including MgO, Ce02, Ti02, A1203, La 0 and Si02 (refs. 22,26). The fact that many of these 2 3 catalysts produce higher alcohols along with methanol has been seen as a disadvantage in methanol synthesis, However, it would be a distinct advantage in "methyl fuel" production providing activities were high enough to justify the use of such expensive metals. ALTERNATIVE PROCESSES FOR CONVERTING SYNTHESIS GAS TO METHANOL Several processing alternatives have been proposed for converting synthesis gas to methanol.
The main incentives are reduced energy costs due to the
ability to operate at lower temperatures, lower pressures or both. The most notable of these alternatives (in terms of recent interest) have been the alkyl formate process (ref. 27) and the Chem Systems three-phase reactor approach (ref. 28).
A very recent development is the use of a gas-solid-solid
trickle flow reactor,which it is proposed can be retrofitted in conventional low pressure methanol synthesis plants (ref. 29). be reviewed in turn.
These three alternatives will
102
Alkvl Formate Process The synthesis of methanol via an alkyl formate was proposed as early as 1919 (ref. 30).
In this process an alcohol is first carbonylated to its formate
ester according to: CO
+ ROH
+
ROCHO
(1)
and the formate is subsequently hydrogenated to produce the parent alcohol and methanol : ROCHO
+
2H2
+
ROH
+
CH30H
The catalyst for reaction 1 is an alcoholate formed by
(2)
dissolving an alkali
metal, such as sodium, in the starting alcohol. The hydrogenolysis reaction (eq.2) is carried out in either the gas or liquid phase using copper-based (preferably copper chromite) catalysts. It has been claimed (ref. 31) that a process incorporating both reactions 1 and 2 can be conducted at around 110°C and 0.5 MPa in a single bubble column reactor using both dissolved sodium and copper chromite catalysts. In his patent application (ref. 27) Onsager used much higher pressures ( 6 to 8 MPa) and a wide range of temperatures (90-170°C) when conducting the reaction in a micro autoclave reactor (volume 120 cm3) . Detailed research in the author's laboratory has been concerned with all aspects of the alkyl formate process.
Initial attention focused on the possibility of using a parent alcohol other than methanol. Tonner et a1 (ref. 32) studied the carbonylation of a wide range of alcohols using Li, Na and K as catalysts at temperatures in the range 30-90°C and a pressure of 4 MPa. The carbonylation reaction was very selective for all alcohols with the only byproduct being sodium formate which was produced by reaction between NaOH and the formate: NaOH
+
ROCHO
+
HCOONa
+
ROH
(3)
The NaOH in the mixture was thought to be formed by reaction between the alkali catalyst and traces of water. The higher alcohols (for example sec. and tert. butanol) were significantly more active than methanol in formate synthesis. It was proposed that the carbonylation reaction occurs by a two-stage mechanism in which an alcoholate ion reacts with CO to form a complex which, in turn, reacts with the alcohol to produce the formate and regenerate the ion. The hydrogenolysis step has been studied in both the gas and liquid phases. Evans et a1 (ref. 33) studied the hydrogenolysis of alkyl formates over a
103
copper chromite catalyst at atmospheric pressure and temperatures in the range of 120 to 230°C. In contrast to the carbonylation reaction, the rate of
hydrogenolysis decreased with increasing carbon number in the alkyl group. Thus any perceived advantage from using an alcohol of higher molecular weight as the parent alcohol, to be recycled in the process, was lost. Furthermore, the study showed that methanol produced via reaction 2 reacts with the original alkyl formate to produce methyl formate and the original alcohol. The transesterification reaction:
CH30H
+
ROCHO
ROH
+
CH30CHO
(4)
exhibits a rapid equilibrium, the position of which is little influenced by temperature. Completion of the overall hydrogenolysis is partially determined by the rate of hydrogenolysis of methyl formate produced by reaction 4 .
The
hydrogenolysis of the higher formates also showed traces of other alkyl alkanoates. For example, ethyl acetate was produced in the case of ethyl formate hydrogenolysis. The outcome of the investigation was to suggest that methanol is the preferred starting alcohol. The gas-phase hydrogenolysis of methyl formate over a copper chromite catalyst (ref. 33) at atmospheric pressure revealed that the decarbonylation of methyl formate according to: CH30CHO 2 CO
+
CH30H
(5)
became significant at temperatures above 120'C. Monti et a1 (ref. 34) studied the kinetics of the vapour-phase hydrogenolysis of methyl formate over copper-on-silicacatalysts in a recycle reactor at temperatures from 120°C to 190°C and atmospheric pressure.
Carbon
monoxide produced by the decarbonylation of methyl formate inhibited the hydrogenolysis reaction. At low CO concentrations the inhibition was due to competitive adsorption of CO on copper. However, at higher CO concentrations (greater than 1-2%) a continuous loss in activity was observed. An insitu infra-red spectroscopic study (ref. 35) showed that methyl formate will partially displace adsorbed CO but not vice versa. The lower rates observed in the presence of CO were attributed to CO displacing Hg from the surface. The lower H concentrations on the surface also reduced the rate of hydrogenation 2 of a formaldehyde intermediate leading to its deposition as a polymer and continuous deactivation of the catalyst. These studies (refs. 33-35) showed that an atmospheric pressure process for methyl formate hydrogenolysis, whilst feasible at moderate temperature, cannot be used in a two-step methanol synthesis via methyl formate due to poisoning of the catalyst by CO produced in
104
the decarbonylation reaction. The liquid-phasehydrogenolysis of methyl formate has been studied over copper chromite catalysts (refs. 36,37). Sorum and Onsager (ref. 36) found that under their conditions (140-185°C and 3.8-10 MPa) all the copper chromite catalysts investigated were both highly active and selective for methanol production. Monti et a1 (ref. 37) used one of the catalysts (Girdler G - 8 9 copper chromite) studied by Sorum and Onsager (ref. 36) and, whilst similar high selectivity was obtained, the activity was more than an order of magnitude less under the same conditions. Monti et a1 (ref. 37) also investigated the influence of CO in the liquid-phasehydrogenolysis. As in the case of the vapour-phase hydrogenolysis (refs. 32,34,35)inhibition caused by CO adsorption was apparent with an order of -0.32 compared to -0.16 for the gas phase reaction. The continuous deactivation that was observed in the gas phase due to polymer fouling the catalyst was not observed in the liquid phase studies. The results of the experiments conducted in the presence of CO for both vapour-phase and liquid-phase reactions have important implications for the methyl formate route to methanol. The liquid-phasehydrogenolysis study (ref. 37) and the earlier carbonylation work (ref. 32) suggest that it is not feasible to conduct methanol synthesis via methyl formate in a single reactor. Both reactions require high pressures in order to obtain useful rates of reaction at moderate temperatures and the high CO pressures would severely limit the hydrogenolysis reaction. There seems little possibility of the alkyl formate process being introduced in either a single or two-step basis. The perceived advantage of operating at low pressures (ref. 31) is not realistic since there is no gain to be made by operating at pressures below that of steam reforming (3 to 4 MPa). If a two step process was to be used it would be a necessity to separate CO and H2 and this would introduce significant costs. Further cost in syngas preparation would be incurred due to the requirement to remove H20 and C02 to ppm levels in order to reduce the loss of the CH 0 Na catalyst in the carbonylation step. 3 Chem Svstems Three-Phase Methanol Synthesis Process Chem Systems Inc. has been developing a three phase reaction system for methanol synthesis since the mid 1970’s (ref. 28).
The original concept
incorporated a liquid-fluidized-bedreactor. This research, which was funded by the Electric Power Research Institute, used particles of a heterogeneous catalyst, obtained by crushing pellets of a commercial Cu-Zn0-A1203 type catalyst, which was fluidized by a circulating inert hydrocarbon liquid such as a mineral oil. One of the major benefits of the process over conventional synthesis is claimed (ref. 28) to be excellent temperature control of the reactions so that higher per pass conversions can be achieved, thereby reducing
105
the recycle gas flow and the energy required for recompression. Another advantage is claimed to be the improved recovery of the heat of reaction as high pressure steam in a simple manner. In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitatedCu-Zn0-A1203catalysts will have the desired characteristics, particularly mechanical strength. The ICI patents (ref. 20) describe the use of intermetallics (similar to those in Tabel 2 above) in liquid-phase methanol synthesis. The reaction was conducted at 70°C and 5 MPa using a syngas of 67H2/33 CO which was contacted with a sample of ground catalyst suspended in 10 times its mass of octane. Although no details of activity are given it is claimed that high methanol yields were produced under those conditions. Therefore, the copper-based intermetallic catalysts may have application in the Chem Systems process. Brookhaven National Laboratorv (BNL) Process Recently (ref. 39) researchers at the Brookhaven National Laboratory (BNL) have described a low temperature liquid-phase process for methanol synthesis. The catalyst consists of a complex reducing agent with a structure of NaH-RONa-M(OAc)2 where the metal can be Ni, Pd or Co and R is a low molecular weight (C1 to C 6 ) alkyl group. The preferred catalyst is one in which M Ni and R
- tertiary amyl.
-
typical BNL catalyst was prepared by adding a stirred suspension of NaH and tertiary amyl alcohol in tetrahydrofuran to anhydrous nickel acetate under an inert atomosphere. An excess of tertiary amyl alcohol A
was added to neutralise any excess NaH. The resultant catalyst was a non-pyrophoric black suspension. Methanol synthesis was carried out using the BNL catalyst which was transferred to a batch reactor under an inert (argon) atmosphere. The stoichiometric 2H2/CO syngas was used at a pressure of around 2MPa and a temperature of 100 " C . A reaction rate of lo-' turnovers per second (molecules of methanol produced per molecule of catalyst per second) was observed. The selectivity to methanol was high (96%) with the only by-product being methyl formate ( 4 % ) . This low temperature (80-120°C) slurry-phase BNL process therefore shows considerable promise for producing essentially anhydrous
106
methanol at very mild reaction conditions. Gas-Solid-SolidTrickleflow Reactor A method for improving the performance of conventional methanol synthesis has recently been described (ref. 29). In this process the product methanol is selectively adsorbed on an amorphous low-alumina cracking catalyst which trickle flows over a fixed bed containing a commercial Cu-ZnO-A10 methanol 2 3 synthesis catalyst in the form of 5 x 5 nun cylindrical pellets and inert Raschig rings. In this way the methanol is removed from the reactant stream (unconverted syngas) thereby enabling the reaction to proceed to up to 100% conversion since the reaction is no longer under equilibrium control. The concept of insitu product removal in order to increase yields in equilibrium limited reactions has been proposed before this work. However, the gas-solidsolid trickleflow converter for methanol synthesis represents one of the few practical demonstrations of such systems and the results of the study show that the process has great promise. However, there may be some problems in engineering the feeding and removal of adsorbent from the converter. If these practical problems can be overcome the process would result in significant energy savings over existing processes which have high energy requirements due to recompression and high recycle rates PARTIAL OXIDATION OF METHANE OR NATURAL GAS The direct oxidation of methane has long been conceived as the most desirable route for methanol production from natural gas since it avoids the highly energy intensive steam reforming reaction. Currently there is considerable renewed interest in this route with a large number of research groups world-wide looking at novel catalysts. Foster (ref. 40) has made an extensive review of the literature on direct catalytic oxidation of methane to methanol up to 1985. The most promising results to date, from a vast array of catalysts and oxidants tested, have used molybdena-based catalysts (refs. 41,42) with nitrous oxide as oxidant. There is little doubt that the potential exists for methanol production by direct catalytic partial oxidation (ref. 43). However, vast improvements in catalysts and reactor designs will be needed
so
that cheap oxidants such as air or oxygen can be used. Until that time, conventional technology such as that introduced by I.C.I. in the mid 1960's will be the major method of methanol production. CONCLUSIONS Since the o i l crisis in the early 197O's, there has been considerable research activity, both government and industry sponsored, towards improved processes for the production of methanol for use as a fuel, fuel blendstock or feedstock (in the Mobil process).
Despite this intense research, the I.C.I. low-pressure
107
methanol synthesis process that was introduced in 1966 appears to have no immediate rivals. The most likely improvements to be made in the short term would involve catalysts with vastly improved performance over those currently in use. The most promising of these appear to be copper based and produced as alloys and subsequently activated. Thus, the Raney Cu-Zn and the Cu-Tho 2' Cu-CeO show potential to replace conventional coprecipitated Cu-ZnO-A10 2 2 3 catalysts in converters in existing plants. The Gas-Solid-SolidTrickle Flow Reactor appears to offer considerable potential for reduced energy costs in conventional methanol plants. If the claims made by the inventors are realised, it would be a simple matter to retrofit plants by replacing existing converters with reactors using this novel contacting method. The ultimate goal in methanol production will be achieved if satisfactory catalysts and reactor technologies can be developed for efficient direct catalytic oxidation of methane or natural gas. ACKNOWLEDGEMENT Support for Methanol Synthesis research involving Raney copper-zinc catalysts and alkyl formates was provided under the National Energy Research Development and Demonstration Program administered by the Commonwealth Department of National Development.
REFERENCES 1 Badische Anilin and Soda Fabrik (BASF) Ger. Pat. Nos. 415, 686; 441, 4333; 462,837 (1923). 2 M.S. Wainwright and R.B. Anderson, J. Catal., 64 (1980) 124. 3 M.S. Wainwright, Proc. Alcohol Fuels, Sydney, Aug.9-11, 1978 (8) 1-5. 4 W.L. Marsden, M.S. Wainwright and J.B. Friedrich, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 551. 5 M.S. Wainwright, W.L. Marsden and J.B. Friedrich, U.S. Patent No. 4,349,464 (1982). 6 M.S. Wainwright, W.L. Marsden and J.B. Friedrich, U.S. Patent No. 4,366,260 (1982). 7 J.B. Friedrich, M.S. Wainwright and D.J. Young, Chem. Eng. Comm., 14 (1982) 1279. 8 A.J. Bridgewater, M.S. Wainwright, D.J. Young and J.P. Orchard, Appl. Catal. 7 (1983) 369. 9 J.B. Friedrich, D.J. Young and M.S. Wainwright, J. Electrochem. SOC., 128 (1981) 1840. 10 J.B. Friedrich, D.J. Young and M.S. Wainwright, J. Electrochem. SOC., 128 (1981) 1845. 11 J.B. Friedrich, M.S. Wainwright and D.J. Young, J. Catal., 80 (1983) 1. 12 J.B. Friedrich, D.J. Young and M.S. Wainwright, J. Catal., 80 (1983) 14. 13 A.J. Bridgewater, M.S. Wainwright and D.J. Young, Appl. Catal., 28 (1986) 241. 14 H.E. Curry-Hyde,D.J. Young and M.S. Wainwright, Appl. Catal., 29 (1987) 31. 15 A.D. Tomsett, H.E. Curry-Hyde,M.S. Wainwright, D.J. Young and A.J. Bridgewater, Appl. Catal., (1987) in press.
108
16 H.E. Curry-Hyde,M.S. Wainwright and D.J. Young, Appl. Catal., (1987) in press. 17 E.G. Baglin, B.G. Atkinson and L.J. Nicks, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 87. 18 R.G. Herman, K. Klier, G.W. Simmons, B.P. Finn and J.B. Bulko, J. Catal., 56 (1979) 407. 19 F.P. Daly, J. Catal., 89 (1984) 131. 20 G.D. Short and J.R. Jennings, Jap. Patent 59,116,238,Euro.Patent 117,944 (1984, to I.C.I.). 21 J.R. Jennings and M.V. Twigg, in J.R. Jennings (editor) Critical Reports on Applied Chemistry Selected Developments in Catalysis, Blackwell Sci. Pub. London 1985, Vo. 12, p.126. 22 M. Ichikawa, Bull. Chem. SOC. Jap., 51 (1978) 2268. 23 E.K. Poels, P.J. Mangnus, J.V. Welzen and V. Ponec, Proc. 8th Int. Gong. Catal., 2 (1984) 59. 24 J.P. Hinderman, A. Kiennmann, A. Chakor-Alami and R. Kieffer, Proc. 8th Int. Gong. Catal., 2 (1984) 163. 25 R. Hicks, Y. Oi Jie and A.T. Bell, 8th North. Amer. Meet. of Catal., SOC., May 1-4,1983, Philadelphia Paper B21. 26 M. Meriaudeau, M. Dufaux and C. Naccache, Proc. 8th Int. Gong. Catal., 2 (1984) 185. 27 O.T. Onsager, Int. Patent Appl., No.WO 84/00360 (1984). 28 M.S. Sherwin and M.E. Frank, Hydrocarbon Processing, 55 (11) (1976) 122. 29 K.R. Westerterp and M. Kuczynski, Hydrocarbon Processing, 65 (11) (1986) 80. 30 J.A. Christiansen, U.S. Patent No. 1,302,011(1919). 31 Petrole Informations, 34 (1982) 13 May. 32 S.P. Tonner, D.L. Trimm, M.S. Wainwright and N.W. Cant, J. Mol. Catal., 18 (1983) 215. 33 J.W. Evans, P.S. Casey, M.S. Wainwright, D.L. Trimm and N.W. Cant, Appl. Catal., 7 (1983) 31. 34 D.M. Monti, M.S. Wainwright, D.L. Trimm and N.W. Cant, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 397. 35 D.M. Monti, N.W. Cant, D.L. Trimm and M.S. Wainwright, J. Catal., 100 (1986) 17. 36 P.O. Sorum and O.T. Onsager, Proc. 8th Int. Gong. Catal., 2 (1984) 223. 37 D.M. Monti, M.A. Kohler, M.S. Wainwright, D.L. Trimm and N.W. Cant, Appl. Catal., 22 (1986) 123. 38 M.E. Frank, Proc. 15th Intersoc. Energy Conv. Eng. Conf., Seattle, Aug.18-22, 1980, ~1567-1572. 39 R.A. Sapienza, W.A. Slegeir, T.W. O'Hare and D. Mahajan, U.S. Patent Nos. 4,614,749 and 4,616,946 (1986). 40 N.R. Foster, Appl. Catal., 19 (1985) 1. 41 D.A. Dowden and G.T. Walker, Brit. Patent No. 1,244,001,(1971). 42 R.S. Liu, M. Iwamoto and J.H. Lunsford, J. Chem. SOC. Chem. Commun., 1982 (1) 78. 43 H.F. Liu, R.S. Liu, K.Y Liew, R.E. Johnson and J.H. Lunsford, J. Am. Chem. SOC., 106 (1984) 4117. 44 J.H. Edwards and N.R. Foster, Fuel Sci. and Tech. Internat.,4 (1986) 365.
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
109
MECHANISM OF MBTHANOL AND HIGHER OXYGENATE SYNTHESIS I(.
Klier, with R. G. Herman, J. G. Nunan, K. J. Smith, C. E. Bogdan, C.-W.
Young, and J. G. Santiesteban Department of Chemistry and Zettlemoyer Center for Surface Studies Lehigh University Bethlehem, Pennsylvania 18015 U.S.A. ABSTRACT The mechanism of methanol synthesis is discussed with account taken of reported isotope labeling and chemical trapping experiments. Recent studies of alkali promotion of methanol synthesis over the Cu/ZnO catalyst revealed the ion specificity Cs>Rb>K>Na,Li. Alkali are also essential for the development of methanol synthesis activity in the recently discovered alkali/MoS2 heterogeneous catalysts and reported Cut-based homogeneous catalysts. The catalysts are bifunctional and when the synthesis takes place from CO/H2, the basic component activates CO and the Cu/ZnO, MoS2, or Cut soluble complex activates hydrogen. Over the ICI Cu/ZnO/A1203 catalyst, 14C labeling experiments show that C02 is hydrogenated preferentially under the low pressure and high H2/(COtCO2) ratio industrial conditions. Side products are oxygenates, such a s higher alcohols, aldehydes, ketones, esters, and ethers, and hydrocarbons in varying amounts. The product distribution is determined by the catalyst used. The synthesis patterns have been successfully modeled over the Cu/ZnO, Cu/ZnO/Cr2%, Ce/Cu/ZnO, Cs/Cu/ZnO/Cr2Og, Cs/MoS2, Cs/Co/MoS2 and K/Co/MoS2 catalysts based on a few fundamental mechanistic steps involving Cn--Un+l linear growth, Cn--Un+l (nb2) 8-addition, and methyl ester forming reactions. Differences in the mechanisms of linear growth and in the ratio of rates of the linear growth to the 8-addition result in two substantially different distributions: over the Cu/ZnO-based catalysts, 2-methyl-1-propanol and 1-propanol are the dominant Czt products, while over the alkali/(Co)/MoSg catalyst ethanol prevails among the C2+ oxygenates. While methanol synthesis may be operated with selectivity &99%, particularly over the (Cs)/Cu/ZnO catalyst, the higher alcohol content may be increased up to 40% of oxygenated products over the alkali-doped copper-based catalysts and to 80% over the alkali/Co/MoS2 catalyst by the choice of reaction conditions. The kinetic models presented here and in the quoted literature permit the prediction of various oxygenated side product compositions a t different synthesis conditions from the temperature coefficients of the kinetic constants for linear growth, @-addition, and methyl ester forming reactions. INTRODUCTION Methanol synthesis Occurs by the reactions CO
+ 2Hz
# C H S H , A H ~ O O K= -100.46 kJ/mol and
(1)
A G ~ O O K= +45.36 kJ/mol
C02
+ 3H2 @ CHQOH + H20, AH&OK = -61.59 kJ/mol and A G ~ O O K= +61.80 kJ/mol
Simultaneously Occurring with catalytic methanol synthesis is the water gas
(2)
110
shift (WGS) reaction co + H20 e C02 + H2, AH&)OK Although
= -38.7 kJ/mol and
(3)
= -16.5 kJ/mol
AG&K
the current industrial methanol
synthesis is highly
selective to
methanol, side products are formed in quantities determined by the specific catalyst used and by the reaction conditions.
These side reactions involve
methyl formate synthesis 2CO + 2H2
--$
HCOOCH3,
(4)
the synthesis of saturated higher alcohols nCO + 2nH2 + CnHzn+lOH + (n-l)H20
(5)
and their dehydrogenation products aldehydes and ketones, the synthesis of higher, primarily methyl, esters, (n+l)CO + 2nHZ + Cn-1H2n-1COOCH3
+ (n-1)H20J
(6)
the formation of dimethylether (DME), 2CH3OH + CH3OCH3 + H20,
(7)
and the formation of higher ethers and hydrocarbons.
A t extreme reaction
conditions, acetals, ketals, and carboxylic acids are also significant products over less active older generation catalysts such as ZnO and Cr203 but not over the copper-based and alkali/MoSz catalysts discussed in the present paper. Reactions (5)
-
(7) combined with the WGS reaction (3) yield alcohols,
esters, and DME with C02, in excess to water, as co-product. generating
reactions
(4)
-
(6) follow regular
patterns,
The side-product and
the
1-3)
and
alkali-MoS2 (ref. 4) catalyst within a wide range of reaction conditions.
The
distributions DME-forming
have
been
modeled
for
the
copper-baaed
(refs.
product
reaction (7) can be minimized by suppressing acidity of the
catalyst or maximized by enhancing acidity of the catalyst (refs. 5, 6). Although DME and methanol are the primary feedstocks for the ZSM-5 catalyzed MTG o r MTO process (ref. 7), higher alcohols that can be produced over methanol catalysts by reactions (5), o r esters by reactions ( 6 ) , have been demonstrated to be suitable feedstocks for conversion to aromatic gasoline or olefins over the ZSM class of acid catalysts (ref. 8 ) .
Hence, crude methanol
containing from <1X to a large fraction of higher oxygenates may be used in the MTG and MTO processes. The catalysts used in alcohol synthesis hold the key to selectivity for methanol, for higher oxygenates, and to the control of hydrocarbon formation. Of interest are the mechanisms and the structure-function relationships in the catalysis of the C-H bond formation in reactions (1) and (2), C-C bond formation in reaction (5), and C-0 bond formation in reactions (4), (6) and (7), as well as of reactions utilizing the synthesis intermediates as building blocks for organic syntheses such as amine (refs. 9-11) and aldol (refs. 12-14) syntheses.
Further,
the mechanistic roles of Co;! and w a t e r a r e of importance to understanding the
111
kinetic behavior of oxygenate synthesis, particularly of methanol synthesis (1) and (2) in a variety of reaction conditions, with the WGS reaction (3) occurring simultaneously. METHANOL SYNTHESIS AND THE C-H BOND FORMATION
After the discovery of a number of methanol catalysts based on oxides, salts and metals by Patart in 1921 (ref. 15), commercialization of the ZnO/Cr2Og catalysts by BASF in the 1920’s (ref. 16), and systematic studies of the binary Cu/ZnO catalysts by FrBhlich and coworkers in the late 1920’s (ref. 17), the new generation low pressure (
steam
reforming,. methanol
synthesis,
and
the
MTG
process.
the process for methanol synthesis is well established
and the
catalysts well developed and performing over years of service, mechanistic work still continues in an effort to understand the reaction and to rationalize the complex kinetic behavior of reactions (1)
-
(3).
The early, primarily kinetic studies of methanol synthesis have been reviewed by Natta in 1955 (ref. 20), the mechanistic work by Kung in 1980 (ref. 21) and the characteristics of the Cu/ZnO/Mx+
catalysts along with the
available mechanistic information by the author in 1982 (ref. 22). Subsequently, a large number of papers emerged that indicate that the mechanism and kinetics of methanol synthesis a r e complex, may not be identical for different catalysts, and vary considerably with reaction conditions. The reactions that result in the first C-H bond formation have been proposed to be CO +
fi + HCOe
(8)
formyl
CO + O@ + HCOOe
(9)
formate
C02 + fi + HCOOe formate
(10)
Formyl may be formed directly from CO/H2 or by hydrogenation of the formate. Both the formate and formyl may be hydrogenated to methoxide HC@
+ 2H2 + CH30° + Hfl
which is then hydrogenated or hydrolyzed to form methanol.
(11) Formate and
methoxide are readily detected under reaction conditions by I R spectroscopy (refs. 23-25) and formyl has been reported to form on co-adsorption of CO and H2 over the Cu/ZnO catalysts (ref. 26). Initially, a hydroxycarbene route CO
+ H2
+ HtOH
hydroxycarbene
(12)
was postulated (ref. 27), but later it was pointed out that the catalyst would
have
to
lower
the
200
kJ/mol
thermodynamic
barrier
of
hydroxycarbene
formation for this path to be effective (ref. 22). The proportion with which the different mechanisms operate has been attempted to be resolved with the help of labeled compounds.
Takeuchi and
Katzer (ref. 27) used a mixture of 13C16O and 12C180 that produced 13CH3160H and
but not
12CH3180H,
catalyst.
13CH3180H and
12CH3160H,
methanol over Rh/TiOZ
This result favors the formyl path (8) and rules out the formate paths
(9) and (10) for the Rh/Ti02 catalyst under the conditions employed.
However,
the Cu/ZnO catalysts promote a rapid scrambling of 13C160 and 12C180 that is accelerated by preadsorbed w a t e r (ref. 28).
This isotope flow is consistent with
a reversible course of the formate mechanism (9).
To establish the kinetic role
of w a t e r in methanol synthesis via route (9), Vedage et al. (ref. 28) injected D 2 0 into the CO/&
mixture to obtain methanol singly deuterated on the CIQ group, Quantitative evaluation of the isotope flow led to the conclusion
CHzDO(H,D).
that reaction (9) accounted for a t least 65% of the methanol synthesis from CO/Hz
+
HzO, again under the conditions employed in ref. 28.
Evidence for path
as the primary reactant has been obtained by hydrogenating 12CO/14CO2 and 14CO/12CO2 mixtures to methanol (refs. 29-31). that
(10)
utilizes
CO2
For example, with 12CO/14CO2 mixtures, the 14C label appeared in the product methanol for a large range of CO/COz ratios, and a quantitative analysis of 14C as a function of the flow rate of the reactants over the catalyst led to the conclusion that CO2 hydrogenation is the exclusive primary path to methanol under the industrial conditions (temperature 25OOC, pressure 40-50 atmospheres, and GHSV range of 10,000 catalyst
(ref.
31).
In
-
a
120,000 hr-I), utilized with the I C I Cu/ZnO/A1203
prior
paper
(ref.
32),
the
first step of
hydrogenation was proposed to be the formate forming reaction (10). based on the evidence utilizing the
13C16O
+
12Cl80
mixtures,
14CO2/12CO and 12CO2/14CO mixtures as reactants, paths (8)
-
DzO,
C02
Thus, and
(10) are all
feasible but their dominance will be dictated by the catalyst and the reaction conditions. Reaction (9) is well-known to occur under mild conditions even in aqueous solutions of alkali hydroxides (ref. 33).
The details of this reaction have
recently been investigated by reaction path calculations (refs. 34, 35) with the result that a facile nucleophilic attack of CO
Hoe + CO + H
8
m
(1) is followed by a n activated hydrogen transfer 8
H - O Z + HCOOe (1)
(11)
113
a s represented
in Figure 1, where T is the transition
state.
The stable
structures of the metalloformate (I), formate (TI), and the transition state (T)
I\
a r e shown in Figure 1.
O -100
-
. 0
E
co+oHo
,
\
-200 -300
W
z
\
-/
\
/
\
\
0T.C
t
0.Y
-400 -
-500 -600
\
/
\
-
\
/
\
7 Y
r
/
\
\
0-H
\
1 REACTION COORDINATE
Fig. 1. MNDO energy diagram for the reaction of carbon monoxide with hydroxide to form formate. The reaction represented by equation (9) has been documented by Bogdan
(ref. 36) using the Cu/ZnO and CsOH-doped Cu/ZnO catalysts.
The I R spectrum
of the formate formed from a surface hydroxyl and CO on the Cu/ZnO catalyst is shown in Figure 2a and that of formate on CsOH/Cu/ZnO catalyst in Figure 2b. A formate specifically bonded
to the Cs+ ions is documented by the
comparison of the spectrum in Fig. 2h with reference spectra of HCOOCs (ref.
36).
The facile formation of surface HCOOCs from CsOH and CO led the author
and
coworkers
to
the
probing
of
CsOII/Cu/ZnO
and
later
HCOOCs/Cu/ZnO
catalysts for methanol synthesis (ref. 37) and the WGS reaction (ref. 38).
The
promotion by C s of the Cu/ZnO catalyst for methanol is shown in Figure 3. Data a r e given here for methanol formation from CO
+
H2, but the
simultaneous promotion of the WGS reaction in the additional presence of If20 has been documented in ref. 38.
Further, the promotion of the Cu/ZnO catalysts
for methanol is ion specific a s Cs>Rb>K>Na,L.i(ref. 39), in the same order as the basic strength of the counterion of the surface alkali cation such as O H .
The
methanol activity dependence on the concentration of the alkali surface dopant shown in Figure 3 has been explained a s follows.
The catalyst is bifunctional
and contains a basic component (e.g. CsOH) that enhances activation of CO by
114
NT
2753*(660
3.00
3500
4000
,
,
3000
2500
,
I
2000
1500
Wavenumbere (cm-’)
Pig. 2. Infrared spectra of a ) Cu/ZnO = 5/95 mol% and b) Cs/Cu/ZnO (50% surface coverage with cesium) obtained a t 20WC and ambient pressure after carrying out methanol syntheds for 2 h r a t 50 atm with H2/CO = 0.50 synthesis gas.
=.
Reduced
8 = Calcined
300’
- doped -
\
doped
I
Nominal Cs Conc. Mol.%
Fig. 3. Yield of methanol a s a function of cesium loading of the binary (Cu/ZnO = 30/70) catalyst obtained a t 250OC and ‘75 atm with H2/CO = 2.33 synthesis gas a t GHSV = 6120 t(STP)/kg catal/hr. reaction (9) and a hydrogenation component (Cu/ZnO) that activates hydrogen for the conversion HCOOe + cH3Oe.
The maximum methanol yield is obtained
when the CO and €I2 activating components are balanced. A similar pattern is obtained with the alkali/MoSp catalyst a s exemplified in
Figure 4.
The steady state activities for methanol from CO/H2 displayed in
115
I
I
I 300
T
I
a. 275OC
0
”
n
I
0
I
10
I
I
20
I
\
I
30 0
10
20
30
CSOOCH LOADING ON MoS, CATALYST, w t %
Fig. 4. Yields of methanol ( m ) , ethanol (A), total alcohols (O), and hydrocarbons (0)a s a function of cesium loading of the MoS2 catalyst obtained a t a) 275OC and b) 295°C and 81.6 atm with H2/CO = 0.96 synthesis gas a t GHSV = 7750 1 (STP)/kg catal/hr. Figure 4 have been obtained in the author’s laboratory (ref. 40) following the announcement by Dow Chemical scientists of their discovery of this new alcohol synthesis catalyst (ref. 41).
Only methanol, ethanol, and hydrocarbon yields a r e
shown although the catalyst also makes appreciable amounts of higher alcohols. The catalyst requires simultaneous presence af the alkali component and the MoS2 component for developing alcohol synthesis activity.
Consistent with the
picture obtained with the Cs/Cu/ZnO catalysts, the Cs/MoS2 catalyst appears to be a combination of basic (CsOH) and hydrogenation (MoS2) components.
The
amount of the alkali compound necessary to develop the maximum activity is significantly larger than that in the Cs/Cu/ZnO catalyet because the alkali compounds agglomerate into ca. 20 nm particles in contact with the low energy non-polar
MoS2 surface (ref. 42), while they a r e molecularly dispersed in a
submonolayer on the polar Cu/ZnO surface (ref. 43). A
methanol
further example of a bifunctional base-hydrogenation has
recently
been
reported
by
Union
Carbide
(ref.
catalyst for 44).
Their
homogeneous catalyst consists of a Cut compound and an alkali methoxide, and the hydrogenation component is believed to be the copper hydride CuH.
The
alkali methoxide may then serve as a base that activates CO by a nucleophilic attack analogous to reaction (9)
CH3@
+ CO
-+
CH30COe (111)
116
followed by hydrogenation of the metallocarboxylate 111. In summary, several new successful synthesis catalysts for methanol synthesis from CO and H2 appear to be bifunctional and consist of a basic component and a hydrogenation component.
The Cu/ZnO/A1203 catalysts appear
to hydrogenate preferentially CO2 under industrial conditions. C-H
(8) -
forming reactions
(10) have
catalysts and s e t s of conditions. mechanistic
One major remaining task is to translate the
input into kinetic equations that
synthesis reactions (1) of
conditions and
-
describe the behavior of t h e
(3) in a wide range of conditions.
CO/H2
All three initial
been found plausible f o r different
For a limited range
synthesis gas only, methanol synthesis has been
modeled as a function of surface C s concentration for the Cs/Cu/ZnO Cs/Cu/ZnO/Cr203 catalysts (ref. 45).
and
The differential equation describing the
cesium concentration dependence of the synthesis is + ko'WeHZn(l-eCs) 1 ( 1 W K e q ) S (16) 2 Find the theoretical curves obtained by the best fit to the experimental methanol
= (kl@CseH ( 1 % ~ )
synthesis rates a t 250OC and 75 atmospheres a t Hz/CO Figure 5.
=
2.33/1 a r e shown in
The key term in equation (16) proportional to BCs(l-BCe)
reflects the
bifunctionality of the catalyst, the rate of activation of CO being proportional to eCs and that of hydrogen to the free Cu/ZnO surface through
2.oq
(l-ecs).
I 0 : CslCulZnOlCr.0,
: CslCulZnO
X "
1.5
0.015
0.03 0.045
C s %/(rn'/g
0.06
0.075
1
of catalyst)
Fig. 5. Correlation of specific methanol activity as a function of normalized cesium surface concentration of Cs/Cu/ZnO (a) and Cs/Cu/ZnO/Cr~03 ( 0 ) catalysts tested at 250°C and 75 atm with Hz/CO = 2.33 synthesis gas at GHSV = 6120 (unsupported) and 10,000 (Cr203-supported) t(STP)/kg catal/hr.
117
THE ALDEHYDIC C1 INTERMEDWTE The synthesis patterns of
higher
alcohols, esters, and amines a r e
consistent with reactions of an aldehydic C1 intermediate as the building block for the
c-c,
C-0, and C-N bond forming reaction.
Formy1 HCO has already been mentioned.
Other forms of an aldehydic
intermediate that have been proposed include n-bonded formaldehyde, its H. C ,which, i f bonded t o a isomer hydroxycarbene, and “dioxymethylene“ H,C,o-, cation(s) is an anion of hydrated formaldehyde H z C ( 0 H ) z .
-
IR spectra in the 2700
3000 cm-1 region have been interpreted a s vibrational transitions of the CH2
group of dioxymethylene (ref. 46) or adsorbed formaldehyde (ref. 47), but the evidence for hydroxycarbene is lacking. A number of chemical trapping reactions provide support for the aldehydic
C 1 intermediate.
RlRZNH
+
CO/H2
Vedage e t al. (ref. 11) utilized the reaction
--$
RiRzNCH3
-
+ H20
(17)
in which the CH3 group of the product amine RlRzNCH3 w a s synthesized via
RlRZNH amine-Ci aldehyde coupling. Deluzarche e t al. (ref. 48) used methyl iodide to trap formyl with the result CH3CHo ( + @&), CH3I + CO/Hz (18) and Young e t al. (ref. 49) used various alcohols and ketones, e.g. a 3
a 3
(32 I + CHOH
I
a 3
CHzOH
I
CH3 \ ,C=O a 3
+ CO/& -+
I
(19)
I
a 2
a 2 I
CHS
to demonstrate that the addition of the C 1 intermediate formed from CO/Hz, occurred preferentially in the fl position of the Cn alcohol or ketone.
Such a
reaction is typical of aldol condensation followed by hydrogenation, with some specific features regarding oxygen retention that a r e discussed in detail below. The high rates with which all of these reactions occur over the copper-based catalysts
under
the
synthesis
conditions
indicate
that
intermediate is a kinetically important reactive species.
the
C1
aldehydic
However, there a r e
differences in the extent of the aldol-type reactions and other pathways for the C-C bond formation over different catalysts as is demonstrated next. C-C BOND FORMING REACTIONS
These reactions give rise to C2+ alcohols, aldehydes, and ketones.
Four
different types of catalysts, Cu/ZnO, Cs/Cu/ZnO, Cs/MoS2, and Cs(K)/Co/MoSz,
118
will be compared here.
Over the copper-based catalysts the C2+ oxygenates a r e
favored by low Hz/CO (1-0.5) ratios and high temperatures (>280OC)(refs. 1, 39). The main products aside from methanol a r e ethanol, I-propanol, and 2-methyli-propanol (isobutanol), and the alkali dopants enhance the rates of the chain growth.
Over the alkali/MoSZ catalysts, C2” oxygenate synthesis has been
demonstrated (ref. 41) a t Hz/CO
1 and temperatures of 250-330°C
to yield
mainly C2+ linear alcohols, and t h e presence of cobalt in the catalyst has been found to greatly enhance t h e methanol hornologation C1-C2
(ref. 50).
Isotope
and mechanistic studies have been conducted with these catalyst systems in the author’s laboratory on the Cl+Cz,
Cz--’C3,
C 3 4 C 4 , and C4+C5
reaction
steps with the following results:
clz2Czr
-
(i) Injection of 13CH30N into t h e CO/H2 synthesis gas stream yields doubly
labeled ethanol over the Cu/ZnO and Cs/Cu/ZnO catalysts (ref. 51) I3CH30H + CO/H2
(Cs)/Cu/ZnO
13CH313CH20H
This w a s interpreted a s the c1-C~
(.t
(21)
CO/Hz).
step occurring by coupling of two C1
aldehydic species by a mechnnism similar to that proposed by Fox et al. (ref. 52).
This outcome (21) rules out any
12C0
insertion
mechanism such as
-
proposed before (refs. 39, 53-55).
(ii) Injection of 13CH30H yields only P-labeled ethanol over the Cs/MoS2 and Cs/Co/MoS2 catalysts (ref. 56), a s represented by equation (22). 13CH30H + l2C0/H2
Cs/(Co)/MoSz
13CH312CH20H
(22)
This outcome (22) is opposite to that (21) observed over the copper-based catalysts and indicates a CO insertion path for linear alcohol growth over the MoS2 catalysts.
This path i s enhanced by the presence of cobalt and accounts
for t h e dominance of linear alcohols over alkali/MoS2 catalysts. (2-2:
Injection of C H Q ~ ~ C H Z Oyields H different isotopic 1-propanols over t h e Cu/ZnO, Cs/Cu/ZnO, and Cs/MoS2 catalysts. (i)
Over Cs/Cu/ZnO catalysts a t high temperatures, path (23) occurs
selectively (refs. 14, 57).
This outcome (23) is consistent with aldol-type &addition with oxygen retention reversal (ref. 14), a s shown in reaction sequence (24).
CH213CHOe + H2CO
-----)
[eOCH2CH213CHO] + % C H Z C H ~ ~ ~ C H ~
*I
@-addition
HOCH$H213CH3 The retention of
the anionic oxygen in the [-OCH2CH13CHO] intermediate is
specific to the C s promoter which prevents the dehydration of the alcoholate oxygen and favors hydrogenation of constitutes
a
reversal
of
the
the free 13CHO group.
normal
aldol
synthesis
Such a path
pattern
in
which
C B ~ C H Z ~ ~ C Hpropanol ~OH would be formed in the presence of hydrogen. (ii) Over alkali/MoS2 and alkali/Co/MoS2 catalysts, the C p C 3 s t e p occurs
-
by the same type linear growth through CO insertion a s in the C1-2
step a s
evidenced by the isotope reaction 13CH3CH2OH + CO/H2
13CH3CHzCHzOH
Cs/(Co)/MoS2
(25)
C . m A :
(i) Over the Cu/ZnO and Cs/Cu/ZnO catalysts, injection of I-propanol yields
dominantly 2-methyl-I-propanol
and 1-butanol a s a minor product, and the C s
promoter enhances t h e rate of t h e @-branching (ref. 49),
minor The dominant 8-addition
to form 2-methyl-3-propanol
occurs via a mechanistic
path analogous to (24) a s indicated by t h e 1% isotope experiments of Nunan et al.
(ref.
57).
This aldol path
with
oxygen retention
reversal
is further
corroborated by t h e outcome of 2-propanol injection into the synthesis gas (ref. 49) which results in the dominance of 1-butanol in the C4 product,
I
CH3
I CH3 enolate
(ii) Over t h e alkali-MoS2 and alkali/Co/MoS2 catalysts, the C F C 4 growth
step occurs mainly b y linear CO insertion, giving rise to the dominance of I-butanol in the C4 product (ref. 56).
120
C . 4_-Y?-C& The patterns of steps C1-4
continue over the different catalysts a s
shown above with the exception that 2-methyl-1-propanol any C5 products over the copper based catalysts.
does not give rise to
The high rate of &addition
a t C3 and the termination of t h e synthesis a t the branched C4 alcohol a r e the major factors determining the high selectivity for 2-methyl-I-propanol. C-0 BOND FORMING REACTIONS New C-0 bonds a r e formed in the CO/H2 synthesis when CO is converted to
C 0 2 by the WGS reaction (3) and in the synthesis of esters. be discussed here.
Only the latter will
Primarily methyl esters are formed, and they a r e significant.
side products over the (Cs)/Cu/ZnO catalysts but not over the alkali/(Co)/MoS2 catalysts.
The mechanism for methyl ester formation has been suggested (ref.
39) to occur via a coupling of a Cn aldehyde with a C1 aldehyde by the Cannizzaro reaction or by a nucleophilic attack of a Cn aldehyde by methoxide (Tischenko reaction).
The exception is the formation of methyl formate that
occurs via a nucleophilic attack of CO by adsorbed methoxide CH30* + CO + cH30
n "
-C - 0
_.)
IP
HCOOCHQ
(28)
The source of protons for reaction (28) could be water, surface hydroxyls, or alcohols.
-
The mechanism of reaction (28) is consistent with the location of the
isotopic label in the reaction l3CH30H + 12CO/H2
H12C0013CH3
(29)
which clearly traces the carbonyl carbon of methyl formate to CO and t h e methyl carbon to methanol (ref. 51).
Theoretical calculations similar to that for
the formate reaction (14) show that the nucleophilic attack (28) is facile but the subsequent methyl transfer that would form the acetate anion CH3COOel an isomer of t h e metallocarboxylate cH3080 of reaction [28], would have to go over
a large energy barrier.
The calculated energy
levels of
the
stable and
transient states of the pathway (28) followed by methyl transfer a r e shown in Figure 6. Unlike in the hydrogen transfer in HCOOe formation by reaction (14) the methyl transfer in Figure 6 is rendered difficult by t h e large barrier, and this explains the observation that the acetate formation is not effective as a C-C bond forming step.
On the other hand, the CH30°
C-0 bond forming step.
+
CO reaction is an effective
Nunan e t al. (ref. 51) further noted that the methyl
formate concentration in the product is below the equilibrium of the reaction CH30H t CO @ HCOOCH3 mi
tnmn-ratiiraa nf 250-2AIWr! and annrnarhaa -niiilihriiim
nt. t.nrnnnrnt.iirnn
(30) >%?IWr!.
121
higher methyl esters, explains the different rates with which methyl formate and higher methyl e s t e r s a r e formed, as will be demonstrated by the kinetic model for t h e overall Cn oxygenate synthesis pattern.
-100
t
W
z
\ /
/
\
/ /
\
>
0 LT
/
\ \
-300
-/
\
\
/
\ \
e
W
/
CH3OCO
-400
\
\-
C H C 0 Oe
-500 REACTION COORDINATE
Fig. 6. MNDO energy diagram for the reaction of carbon monoxide with methoxide to form methyl formate. KINETIC
MODEL
Cs/Cu/ZnO/C-%
FOR
OXYGENATE
SYNTHESIS
OVER
THE
Cs/Cu/ZnO
AND
CATALYSTS
The C2+ oxygenate synthesis has been succesBfully modeled for the Cs/Cu/ZnO/(Cr203) catalysts by Smith et al. (refs. 2, 3) following the pioneering work on modeling of isobutanol synthesis over K/Cu/ZnO/A1203 Smith and Anderson (ref. 1). and
ester synthesis and
catalysts by
The scheme used to account for the c 1 - C ~ alcohol
based
on
the
mechanistic
steps outlined
in
the
preceding section on the C-C and C-0 bond formation is presented in Figure 7. The model has been applied, and the rate constants 1 , @, determined, in the temperature
range of
220-33OOC a t pressures
atmospheres with synthesis gas having H2/CO
and %
of 41-75
ratios 0.5 to 2.3 for Cu/ZnO,
Cs/Cu/ZnO, and Cs/Cu/ZnO/Cr203 catalysts (refs. 2, 3).
A comparison of the
calculated and observed oxygenate product composition over the Cs/Cu/ZnO
=
0.4/30/70 catalyst at 310% is given in Figure 8. The mechanistic features stemming from the rapid aldol-type @-addition, the slow C1-+C2 aldehyde coupling step, and t h e lack of growth of the @-branched alcohols give rise to t h e selectivity pattern in which ethanol is at minimum and
122
COIH? a
-
.1
HCOOC 0 C-OH C-COOC C-C-COOC C-C-C-COOC C-C-C-C-COOC
ao' a,'
ao'
a 4
C-C-OH C-C-C-OH
1l
-
C-C-C-C-OH
I'
C-C-C-C-C-OH
1'
P1'
PI'
P1'
F:
C-C-C-OH
C C-C-C-C-OH
7
C-C-C-C-C-OH
C - C- C - C- C- C - 0 H Fig. 7. Scheme of carbon chain growth to form linear alcohols, branched alcohols, and methyl esters over alkali-promoted Cu/ZnO-based catalysts.
Fig. 8. Predicted oxygenate yields, based upon the reaction scheme shown in Fig. 7 but also including C2 (82) and C3 (83) in addition to C1 (81') carbon addition, compared to measured yields obtained with a 0.4% CsOOCH/Cu/ZnO catalyst at 310OC and 75 atm with H2/CO 0.45 synthesis gas at GHSV = 3265 #(STP)/kg catal/hr. Estimated kinetic parameter values are 8 = 0.254, 81 = 3.299, 81' = 1.138, 82 = 0.121, 83 = 0.152, a = 0.007, and ao' 0.221.
123
methanol
plus
2-methyl-1-propanol
is a t
maximum.
Methyl formate is in
equilibrium (30) with methanol and all the remaining methyl esters a r e accounted for by a single value of KINETIC MODEL
FOR
0~0.
OXYGENATE
SYNTHESIS OVER
THE ALKALI/MoS2
AND
ALKALI/Co/MoS2 CATALYSTS Smith et al. (ref. 4) have modeled the Czt oxygenate synthesis over the MoSz-based
catalysts taking into account the mechanistic features that the
linear growth I #-branching.
now proceeds by CO insertion and is more efficient than
The main kinetic feature is that the C1-C2
s t e p is faster than
(na2) steps particularly in the presence of cobalt in the catalyst.
Cn-vn+i
This kinetic pattern results in product. distribution that can be maximized at ethanol, opposite to that obtained in the synthesis over the copper based catalysts. CONCLUSIONS Methanol and higher oxygenate syntheses follow different mechanistic and kinetic patterns over the various catalysts discussed here.
Each such pattern
is regular, however, and can be modeled with a few kinetic parameters based on fundamental mechanistic steps involved in the C-H, C-C, and C-0 bond forming reactions.
Alkali
co-catalysts
play
an
important
role
by
promoting
base-catalyzed reaction steps that appear important not only in the formation of Czt oxygenates but also in methanol synthesis.
The rapid 8-addition over the
(Cs)/Cu/ZnO catalysts gives rise to the dominance of 2-methyl-1-propanol 1-propanol
in
the
Czt
coproducts of
methanol
synthesis.
Cobalt
and
greatly
enhances the rate of methanol homologation over alkali/MoS2 catalysts by a CO insertion mechanism, and the rapid linear growth particularly a t the (21-2 step over the alkali/Co/MoS~ catalysts gives rise to the dominance of ethanol in the Czt products.
Aldehydic intermediates appear significant in all alcohol and
ester forming reactions over the (Ce)/Cu/ZnO catalysts, while CO insertion appears
to
be
particularly
important
in
the
growth
pattern
over
the
dkali/(Co)/MoS2 catalysts. ACKNOWLEDGEMENTS This work was supported in part by U.S. Department of Energy Contracts DE-FG22-83PC60786,
DE-AC22-84PC70021,
and
National Science Foundation Grant INT-8612603.
DE-AC22-85PC80014
and
by
the
124
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125
G. Thomas, Ann. Chim., S (1951) 367. K. Klier, D. Zeroka, and D. Bybell, 189th National Meeting of the American Chemical Society, Miami Beach, FL Abstract NO. COLL-0033 (April 1985). 35. C. E. Bogdan et al., to be submitted. C. E. Bogdan, Ph.D. Dissertation, Department of Chemistry, Lehigh Uni36. versity, Bethlehem, PA (1988). 37. J. G. Nunan, K. Klier, C. W. Young, P. R. Himelfarb, and R. G. Herman, J. Chem. Soc., Chem. Commun., (1986) 193. 5 (1986) 38. K. Klier, C. W. Young, and J. G. Nunan, Ind. Eng. Chem., Fundam., 2 36. 39. G. A. Vedage, P. B. Himelfarb, G. W. Simmons, and K. Klier, ACS Symp. Ser., 279 (1985)-295. ier. ~~, R. G. Herman, G. W. Simmons. C. E. Lyman. and J. G. Santiesteban, 40. K-Kl_--"Direct Synthesis of Alcohol Fuels. over Molybdenum-based Catalysts," Quarterly Technical Progress Report DOE/PC/80014-3 to U.S. DOE/PETC (July 1986). G. A. Cochran, M. M. Conway, C. B. Murchison, B. W. Pynnonen, G. J. 41. Quarderer, R. F. Stevens, R. A. Stowe, and E. D. Weihl, Intern. Chem. Congr. Pac. Basin SOC., Honolulu, HI, Abstr. No. 03G35 (Dec. 1984). 42. K. Klier, R. G. Herman, G. W. Simmons, M. Najbar, and J. G. Santiesteban, "Direct Synthesis of Alcohol Fuels over Molybdenum-based Catalysts," Quarterly Technical Progress Report DOE/PC/80014-5 to U.S. DOE/PETC (March 1987). P. B. Himelfarb, Ph.D. Dissertation, Department of Materials Science and 43. Engineering, Lehigh University, Bethlehem, PA (1986). 44. B. D. Dombek , Final Technical Report DE-AC22-84PC70022 from Union Carbide Corp. to the U.S. Department of Energy (Jan. 1987). 45. C. W. Young, Ph.D. Dissertation, Department of Chemical Engineering, Lehigh University, Bethlehem, PA (1987). 46. J. C. Lavelley, J. Lamotte, G. Busca, and V. Lorenzelli, J. Chem. SOC., Chem. Commun., (1985) 1006. 47. J. F. Edwards and G. L. Schrader, J. Phys. Chem., 88 (1984) 5620; arid J. Catal., 94 (1985) 175. 48. A. Deluzarche, J. P. Hindermann, and R. Kieffer, Tetrahedron Lett., (1978) 2787. 49. C. W. Young, R. G. Herman, and K. Klier, to be published. 50. G. J. Quarderer, G. A. Cochran, R. R. Stevens, and C. B. Murchison, Eur. Patent Appl. 85109213.0 (July 23, 1985); assigned t o Dow Chemical Co. 51. J. G. Nunan, C. E. Bogdan, K. Klier, K. J. Smith, C. W. Young, and R. G. Herman, J. Catal., to be submitted. 52. J. R. Fox, F. A. Pesa, and B. S. Curetolo, J. Catal., 90 (1984) 127. 53. F. Fischer, Ind. Eng. Chem., 11 (1925) 576, and Conversion of Coal into O;ls, Van Nostrand, New York, 1925, p. 251. 54. G. N a t t a , U. Colombo, and I. Pasquon, in P. H. Emmett (Ed.), Catalysis, Vol. V, Reinhold, New York, 1957, p. 131. 55. T. J. Mazanec, J. Catal., 98 (1986) 115. J. G. Santiesteban, C. E. Bogdan, R. G. Herman, and K. Klier, Proc. 9th 56. Intern. Congr. Catal., to be published. 57. J. G. Nunan, C. E. Bogdan, K. Klier, K, J. Smith, C, W. Young, and R. G. Herman, J. Catal., t o be submitted. 33. 34.
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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
127
MECHANISM OF HYDROCARBON FORMATION FROM METHANOL
CLARENCE 0. CHANG M o b i l Research and Development C o r p o r a t i o n , C e n t r a l Research L a b o r a t o r y , P. 0. Box 1025, P r i n c e t o n , New J e r s e y 08540, USA
Hydrocarbon f o r m a t i o n f r o m methanol has been i n t e n s i v e l y i n v e s t i g a t e d i n t h e p a s t decade s i n c e t h e f i r s t r e p o r t s f r o m M o b i l [I] describing t h e conversion o f methanol t o a r o m a t i c g a s o l i n e u s i n g z e o l i t e c a t a l y s t s .
The g e n e r a l r e a c t i o n
pathway was e l u c i d a t e d i n t h i s e a r l y work and i s r e p r e s e n t e d by t h e sequence:
where [ C H z ] = average f o r m u l a o f a p a r a f f i n - a r o m a t i c m i x t u r e . Methanol dehydrates i n i t i a l l y t o an e q u i l i b r i u m m i x t u r e w i t h d i m e t h y l e t h e r (DME) and w a t e r .
F u r t h e r d e h y d r a t i o n a f f o r d s l i g h t o l e f i n s , which a r e t r a n s -
formed t o a r o m a t i c s and p a r a f f i n s i n t h e f i n a l s t e p . The mechanisms o f a c i d - c a t a l y z e d DME f o r m a t i o n f r o m methanol and a r o m a t i z a t i o n of o l e f i n s were w i d e l y i n v e s t i g a t e d i n t h e y e a r s b e f o r e t h e d i s c o v e r y o f t h e methanol-to-gasol i n e r e a c t i o n .
There i s a consensus t h a t t h e i n t e r m e d i a t e
i n DME f o r m a t i o n f r o m methanol o v e r s o l i d a c i d c a t a l y s t s i s a p r o t o n a t e d s u r Aromaface methoxyl, which i s s u b j e c t t o n u c l e o p h i l i c a t t a c k by methanol [2]. t i z a t i o n o f o l e f i n s i s b e l i e v e d t o proceed a l o n g c l a s s i c a l carbenium pathways, w i t h c o n c u r r e n t hydrogen t r a n s f e r [ 3 ] .
The mechanism o f t h e c r u c i a l s t e p o f
i n i t i a l C-C bond f o r m a t i o n f r o m MeOH/DME i s an u n s o l v e d problem, however, and i s t h e s u b j e c t o f ongoing c o n t r o v e r s y .
A t l a s t t a l l y , t h e r e were some two dozen
mechanistic proposals i n t h e l i t e r a t u r e .
It i s n o t possible here t o present a
comprehensive r e v i e w o f t h e e n t i r e f i e l d .
However, a number o f common themes
can be i d e n t i f i e d .
T h i s commonality i s d i s c u s s e d and t h e concepts c u r r e n t l y i n
vogue a r e c r i t i c a l l y reviewed.
Another i s s u e , whether e t h y l e n e i s t h e " f i r s t "
o l e f i n , has been w i d e l y debated [2],
b u t i s beyond t h e scope o f t h i s survey.
Proposed mechanisms f o r t h e i n i t i a l C - C bond f o r m a t i o n may be b r o a d l y c l a s s i f i e d as c a r b e n i c [lb,10], d i a t e s [4,5],
c a r b o c a t i o n i c , i n v o l v i n g h y p e r v a l e n t carbon i n t e r m e -
oxonium y l i d e [6,7],
and f r e e r a d i c a l [8,9],
while recognizing
t h a t t h e c l a s s i f i c a t i o n i s somewhat a r b i t r a r y s i n c e many mechanisms embody several o f these categories.
128
Carbenic mechanisms.
Venuto and L a n d i s
[lo]
were t h e f i r s t t o address t h e
q u e s t i o n o f mechanism o f hydrocarbon f o r m a t i o n f r o m methanol o v e r z e o l i t e s , i n t h i s case z e o l i t e X [ l l ] .
These workers proposed a scheme i n v o l v i n g a - e l i m i n a -
t i o n o f w a t e r and p o l y m e r i z a t i o n o f t h e r e s u l t a n t methylcarbenes t o o l e f i n s . Swabb and Gates [12],
e l a b o r a t i n g on Venuto-Landis,
proposed t h a t c o n c e r t e d
a c t i o n o f a c i d and b a s i c s i t e s i n t h e z e o l i t e ( m o r d e n i t e ) f a c i l i t a t e s a - e l i m i n a t i o n o f w a t e r f r o m methanol.
A c c o r d i n g t o S a l v a d o r and K l a d n i g [13],
carbenes
a r e g e n e r a t e d t h r o u g h decomposition of s u r f a c e methoxyls ( a - e l i m i n a t i o n o f s i l a n o l ) formed i n i t i a l l y upon c h e m i s o r p t i o n o f methanol on t h e z e o l i t e (zeol i t e Y).
Hydrocarbons a r e assumed t o form, i n t h e l a t t e r two schemes, a l s o by
carbene p o l y m e r i z a t i o n . Chang and S i l v e s t r i [ l b ] ,
r e p o r t i n g t h e c o n v e r s i o n o f methanol t o h y d r o -
carbons o v e r z e o l i t e ZSM-5, a l s o f a v o r e d a carbene mechanism f o r i n i t i a l C-C bond f o r m a t i o n .
However, t h e c o m p l i c i t y o f f r e e carbenes was c o n s i d e r e d un-
l i k e l y f r o m an e n e r g e t i c s v i e w p o i n t , and a c o n c e r t e d mechanism was proposed f o r carbene g e n e r a t i o n w i t h c o n c u r r e n t sp3 i n s e r t i o n i n t o MeOH o r DME.
Later, the
p o s s i b i l i t y o f a s e q u e n t i a l mechanism was c o n s i d e r e d [14]. Thus f a r , evidence i n f a v o r o f a carbene i n t e r m e d i a c y has been i n d i r e c t . Chang and Chu [ I 5 1 a t t e m p t e d t o t r a p t h e r e a c t i v e C 1 i n t e r m e d i a t e f r o m 1% MeOH decomposition o v e r ZSM-5 u s i n g u n l a b e l l e d propane.
A n a l y s i s o f p r o d u c t isomer
and i s o t o p e d i s t r i b u t i o n s gave i n d i c a t i o n o f t h e p o s s i b l e n a t u r e o f t h e i n t e r mediate.
In MeOH c o n v e r s i o n o v e r ZSM-5, t h e p r o d u c t p a r a f f i n s have c h a r a c t e r -
i s t i c a l l y h i g h iso-to-normal r a t i o s [Ib].
Chang and Chu observed t h a t when MeOH
was r e a c t e d i n t h e presence o f h e l i u m d i l u e n t , t h e i / n r a t i o o f p r o d u c t butanes was a b o u t 4, b u t upon r e p l a c i n g h e l i u m w i t h propane ( u n d e r c o n d i t i o n s where n e a t propane was u n r e a c t i v e ) t h e i / n r a t i o dropped t o 1, n e a r t h e thermodynamic e q u i l i b r i u m value.
Upon adding I 3 C MeOH (90% I 3 C ,
10% I 2 C ) t o t h e feed, t h e amount
o f s i n g l y l a b e l l e d butanes was 30-45 t i m e s g r e a t e r t h a n expected f r o m random distribution,
i n d i c a t i n g t h e m e t h y l a t i o n o f propane by MeOH.
Furthermore,
i/n
r a t i o o f butanes decreased w i t h d e c r e a s i n g I 3 C s u b s t i t u t i o n ( F i g . 1) f r o m -2.8 t o near u n i t y f o r s i n g l y - l a b e l l e d butanes.
These r e s u l t s were t a k e n as e v i d e n c e
t h a t t h e r e a c t i v e C 1 i n t e r m e d i a t e was c a r b e n i c and t h e mode o f a t t a c k on propane was sp3 i n s e r t i o n i n t o C-H bonds.
Such carbene i n s e r t i o n s a r e i n d i s c r i m i n a t e i n
s e l e c t i v i t y [16], r e s u l t i n g i n a s t a t i s t i c a l d i s t r i b u t i o n o f isomers ( i / n = 0.75 f o r butanes).
M e t h y l a t i o n by a c a r b o c a t i o n i c C 1 i n t e r m e d i a t e would, by con-
t r a s t , r e s u l t i n h i g h i s o b u t a n e s e l e c t i v i t y [17]. T h i s i n t e r p r e t a t i o n was c h a l l e n g e d by van H o o f f [ 1 8 ] , who proposed t h a t l o w i / n r a t i o s , and e s p e c i a l l y t h e presence o f doubly l a b e l l e d butanes i n t h e Chang-Chu experiment, c o u l d be e x p l a i n e d by h y d r i d e t r a n s f e r between propane and b u t y l c a t i o n s .
I f h y d r i d e t r a n s f e r were s l o w e r t o t - b u t y l c a t i o n t h a n t o
t h e secondary c a t i o n , l o w i / n butane r a t i o s would r e s u l t .
Hydride t r a n s f e r
129
n=o
1
2
13
3
4
Cn
F i g . 1. Butanes f r o m i n t e r a c t i o n o f 13CH30H w i t h C3Hg o v e r HZSM-5. ( a ) 13C d i s t r i b u t i o n i n butanes. Comparison a g a i n s t t h e random d i s t r i b u t i o n ( b r o k e n l i n e ) f o r a 90% 1% + 10% 1% m i x t u r e . ( b ) Iso/norrnal r a t i o o f 1 % - l a b e l e d butanes. would a l s o a c t i v a t e propane f o r C-C s c i s s i o n , l e a d i n g t o d o u b l y l a b e l l e d butanes.
Chang and Chu c o u n t e r e d t h i s argument by showing t h a t once propane
e n t e r s i n t o t h e main r e a c t i o n network i n t h i s manner, t h e butane i / n r a t i o s
w i l l be c o n s t a n t due t o complete i s o t o p e s c r a m b l i n g , and independent on t h e degree o f 1% s u b s t i t u t i o n , c o n t r a r y t o e x p e r i m e n t a l o b s e r v a t i o n [19].
1 -
13CH30H - C i ( i / n - l ) ,
etc. -
----+ci(
i/n-4),
etc.
van H o o f f ' s p r o p o s a l a l s o f a i l s t o account f o r t h e h i g h i / n r a t i o s observed i n r e a c t i o n o f methanol a l o n e . Lee and Wu [20] conducted a model s t u d y u s i n g diazomethane, which was r e a c t ed o v e r v a r i o u s s o l i d s i n c l u d i n g ZSM-5, s u r f a c e - s t a b i l i z a t i o n o f carbene.
w i t h t h e aim o f o b s e r v i n g p o s s i b l e
A l t h o u g h o l e f i n s were formed even w i t h
" i n e r t " m a t e r i a l s such as Vycor, hydrocarbon y i e l d s were seen t o i n c r e a s e subs t a n t i a l l y i n t h e presence of a c i d i c s u r f a c e s .
They proposed t h e f o r m a t i o n o f
a methyloxonium s p e c i e s v i a i n t e r a c t i o n w i t h an a c i d s i t e . T h i s s p e c i e s r e a c t s w i t h excess CH2N2, f o r m i n o an e t h v l o x o n i u m s p e c i e s by C-H i n s e r t i o n , B - E l i m i n a t i o n a f f o r d s ethylene.
130 H
I
0
O " 0
-Nz H
I
0
0
CH,N,
CH,CH,
The p r o p o s a l of Lee and Wu was c r i t i c i z e d by Olah e t a1 [21] who were unable t o induce t h e i n s e r t i o n o f s i n g l e t methylene ( g e n e r a t e d by CH2N2 p h o t o l y s i s ) i n t o The a1 k y l a t i o n o f CH2N2 by a1 k y l c a t i o n s o r
trimethyloxonium t e t r a f l u o r o b o r a t e .
t h e i r p r e c u r s o r s , on t h e o t h e r hand, i s known.
-
A c c o r d i n g l y , Olah and co-workers
proposed an a l t e r n a t e i n t e r p r e t a t i o n where a l a b i l i z e d " i n c i p i e n t methyl c a t i o n " on t h e a c i d s i t e r e a c t e d w i t h CH2N2 t o f o r m e t h y l e n e .
CH2N2
Ht
CH3N2
t
CH2N2
>CH3CH2N2
t
t
N2
t
N2
-H+ CH2 = CH2
E t h y l e n e c o u l d a l s o be formed by d i m e r i z a t i o n o f s i n g l e t methylene, a l t h o u g h t h e p r o b a b i l i t y would be l o w due t o t h e h i g h r e a c t i v i t y o f such s p e c i e s . The carbene mechanism was f u r t h e r c r i t i c i z e d by Olah [21] on thermochemical grounds, c i t i n g t h e h i g h e n d o t h e r m i c i t y o f methylene f o r m a t i o n f r o m methanol (AH = t349.3 k J / m o l ) .
T h i s argument d i s c o u n t s , however, t h e i n f l u e n c e o f t h e
c a t a l y s t l o c a l environment, e.g. z e o l i t e c r y s t a l f i e l d energy.
D r e n t h [22],
u s i n g ab i n i t i o methods (MIND03), s i m u l a t e d t h e z e o l i t e a c i d s i t e u s i n g HOA10, t
HOF, H30 as models o f a c i d e l e c t r o n a c c e p t o r s o f i n c r e a s i n g s t r e n g t h , w i t h two h y d r o x y l a n i o n s as e l e c t r o n donors, and examined t h e i n t e r a c t i o n o f CH2 w i t h t h i s assemblage ( F i g . 2 ) . t
a c i d , H30 proton.
,will
Upon energy m i n i m i z a t i o n , i t i s found t h a t t h e s t r o n g
p r o t o n a t e CH2, w h i l e t h e weaker a c i d s t e n d t o r e t a i n t h e i r
C a l c u l a t e d s t a b i l i z a t i o n e n e r g i e s i n t h e range o f 255-314 kJ/mol show
t h a t s u r f a c e - s t a b i l i z e d CH2 i s n o t an unreasonable i n t e r m e d i a t e f o r hydrocarbon f o r m a t i o n f r o m methanol. C a r b o c a t i o n i c mechanisms. c a r b o c a t i o n i c mechanisms.
Ono and M o r i [4],
and o t h e r s [5, 231, f a v o r e d
A c c o r d i n g t o Ono and M o r i , s u r f a c e methoxyls such as
an A l - b r i d g e d s i l y l m e t h y l e t h e r may f u n c t i o n as f r e e methyl c a t i o n s , which adds t o t h e C-H o f DME t o f o r m a p e n t a v a l e n t carbonium t r a n s i t i o n s t a t e . completes t h e r e a c t i o n , a n e t e l e c t r o p h i l i c s u b s t i t u t i o n a t a o-bond,
Proton l o s s analogous
131 H
I
f
t R'
R'
1
0
I
t-R21
Optimized bonding energy 01 CH, X and bond lengths (In pml ACld
R,9Opm
HOAIO HOF
R,
I€
R,a
IP
190 190
275 270 240
255 267
105
284 313
isn
H-n+
Varying R,
R,
IkJmol '1
(kJrnol
115 132
485
1
"Value 01 R In Ihe CH, complex W, andR,&IhisTable R,oplimized bolhInlhecomplexandInlhe CH,-free model
rvarialion 01 R, does not yield a meaningful 1E since in the CH, free model the proton 8s lranslerred lo one 01 the OH groups
F i g . 2.
Z e o l i t e model f o r ab i n i t i o m o l e c u l a r o r b i t a l c a l c u l a t i o n s .
t o t h e " s u p e r a c i d mechanism" o f Olah [17] f o r a l k a n e a l k y l a t i o n i n HF-SbF5.
However, as p o i n t e d o u t by Olah [24],
t h e a n a l o g y w i t h alkanes i s n o t v a l i d ,
s i n c e m e t h y l a t i o n o f DME g i v e s t r i m e t h y l o x o n i u m i o n .
Ono and M o r i observed no e f f e c t o f H C l a d d i t i o n , and concluded t h a t b a s i c s i t e s a r e n o t i n v o l v e d s i n c e t h e y would be poisoned by HC1. One may i n q u i r e whether t h e C-H bond o f methanol o r DME i s s u f f i c i e n t l y n u c l e o p h i l i c t o undergo s u b s t i t u t i o n as proposed by Ono and M o r i . The r e a c t i o n o f methylcarbenium i o n w i t h methanol was examined by Smith and F u t r e l l [25] u s i n g i o n c y c l o t r o n resonance (ICR) techniques.
H y d r i d e a b s t r a c t i o n t o f o r m methane
and Cti20Ht i s t h e predominant r e a c t i o n (85-90%).
CH;
CH:
CH;
+ CH,OH
+ CH,OH
-
CH3
I +
CH3-0-H
-
+ CH3OH
[CHZOH]+
+ CH,
(80%)
AH = -255 kJimol
CH;OH,
+ CH,
(7%) AH = 84 kJ/mol
[CHzOH]+
+ CH,
(5-10%)
132
I n t e r e s t i n g l y , proton t r a n s f e r from the methyl c a t i o n t o methanol, forming methylene, occurs t o a s i g n i f i c a n t e x t e n t ( 7 % ) . C2 species were n o t detected. These I C R r e s u l t s c a s t some doubt on mechanisms invoking d i r e c t methyl c a t i o n a t t a c k a t C-H o f the methoxyl group. Oxonium y l i d e mechanisms.
A t present, the mechanism s t i m u l a t i n g the
greatest debate has been one invoking oxonium y l i d e s as intermediates. van den Berg e t a1 [6] postulated t h a t DME i n t e r a c t s w i t h a Br6nsted a c i d s i t e t o form a dimethyloxonium ion, which reacts f u r t h e r w i t h another DME t o form a trimethyloxonium (TMO) ion.
The TMO i o n i s then deprotonated by a basic s i t e
t o form a dimethyloxonium methylide species, which undergoes a Stevens r e arrangement t o g i v e methylethyloxonium ion.
no si’
0 0 0 \AI/
\sI
+cH,ocn,
SI’
cp 0
n,c/a‘n 0 0 0 ‘A,’ \sI
MeOEt i s formed upon 8-el imination. cn, cn, I I
-
n,c
+CH,OCH,
0 B +/3\8
/O*\.
n,c
0
/
(
+
‘H
-
H,c----,!W H’
Olah 171 proposed a b i f u n c t i o n a l acid-base catalyzed condensation where MeOH and DME r e a c t i n t h e presence o f a c a t a l y s t (e.g.
W03 on A1203) t o form TMO ion,
which i s a l s o deprotonated t o t h e dimethyloxonium methylide.
However, Olah con-
cludes on t h e basis o f l a b e l l i n g experiments [26] t h a t t h e y l i d e undergoes bimolecular methylation t o g i v e e t h y l dimethyloxonium ion, r a t h e r than i n t r a molecular rearrangement. Three issues need t o be addressed i n connection w i t h oxonium y l i d e mechanisms.
The f i r s t question concerns t h e existence o f oxonium y l i d e s .
Whereas
sulfonium, phosphonium, and ammonium y l i d e s are w e l l known, oxonium y l i d e s have n o t been i s o l a t e d . Secondly, ifthey e x i s t , w i l l they undergo Stevens rearrangement, o r decompose v i a other routes? F i n a l l y , i s the z e o l i t e conjugate base s u f f i c i e n t l y basic t o a b s t r a c t a proton from oxonium ions t o form an y l i d e ? Rimmelin e t a1 [27] t r e a t e d TMOtSbC16- w i t h the strong hindered base,
2,2,6,6-tetramethylpiperidyl-li thium (TMPLi) and obtained MeOEt as w e l l as C2-C4
133
*CH@H
-HzO
catl CH,OCH,
CH,OCH,
CH3\
/
+
CH3,
0
cat.
, CH3
+
0 -cat
CH3
1
CH,OCH,
CH,
\d/
CH~CH,
I
+
%=cH,
hydrocarbons (Table 1).
H\+ CH',
0-cat.
Olah [7]
The presence o f E t O E t was a t t r i b u t e d t o solvent impur-
i t y and the authors were c a r e f u l t o r u l e o u t any p o s s i b i l i t y o f e t h y l exchange
from t h i s source (Experiment 3, Table 1 ) . TMO i o n appears, therefore, a t l e a s t p l a u s i b l e as an intermediate i n MeOH conversion t o hydrocarbon. A Stevens-type rearrangement may be operative, however, a carbene mechanism cannot be r u l e d o u t since t h e y l i d e may be regarded as a solvated carbene. Reaction of TMO+SbC16- with TMPLi Table 1. Composition of the head space gases
Experiment l(a) 30minatroomtemperalure l(b) 30minal70"C 2 Oxoniumsall TMPH 30 min at 70°C 3 Oxoniumsall EtOEl 30 min at 70°C 4 Oxoniumsalt + 2 6-di-tbutylpyridine30minat 70°C
+ +
CH, 05 20
_ _
_
CH ,, 05 20
_ _
_
C2H6 CH ,, 05 05 20 50
_ _
_ _
_
-
mol% MeCl 150 150
MeOMe 700 320
220
700
50 530
C,H,
EtOMe 06 250
EtOEt
-
_
100
-
-
85 0
470
-
_
-
10
30
110 120
60
Another p o s s i b i l i t y apparently overlooked by the authors i s the well-known B-elimination o f E t O E t i n t h e presence o f strong base t o y i e l d ethylene and Exchange w i t h DME, a l s o formed, would a f f o r d MeOEt and LiOMe. LiOEt [28]. Rimmelin also used t - b u t y l l l i t h i u m instead o f TMPLi. I n t h i s case, isopentane r a t h e r than MeOEt was formed, showing the c r i t i c a l i t y o f a s t e r i c a l l y hindered base f o r y l i d e formation.
134
CH3
I ,q
CH3
CH3
+ C,HS-Li+-
C,H,,+CH,-0-CH,
SbCI,CH3-O-CH2-CH3
Olah approached t h e problem by a t t e m p t i n g t o s y n t h e s i z e t h e y l i d e d i r e c t l y , v i a three routes:
d e p r o t o n a t i o n o f TMO i o n , d e s i l y l a t i o n o f d i m e t h y l ( ( t r i -
m e t h y l s i l y l )methyl )oxonium i o n , and r e a c t i o n o f photolytically-generated m e t h y l carbene w i t h DME [26,29]. CH3 CH3
I
-0+
CH, -H
CH3
+
CH3
I
-0+
:CH2
CH3
c-CH3OCH3
-
CH3
I
CH30-
+
F-
CH~SI(CH~)~
-(CH,)SiF
CH,
I
CH3-0-CH3
+
I n n e i t h e r r e a c t i o n was t h e y l i d e i s o l a t e d , b u t i t s e x i s t e n c e was i n f e r r e d 13 f r o m p r o d u c t i s o t o p e d i s t r i b u t i o n s . TMO+BF4- ( C o r D l a b e l l e d ) was m i x e d w i t h NaH i n a f l a s k and t h e mixed s o l i d s heated w i t h a f l a m e u n t i l t h e h i g h l y exothermic r e a c t i o n i n i t i a t e d .
A n a l y s i s o f t h e head gas shows t h e presence o f
e t h y l e n e (0.3-2.5%) and ethane (0.3-2.1%), among o t h e r p r o d u c t s (Tables 2 and 3 ) . A l t h o u g h h y d r i d e m e t h y l a t i o n t o CH4 and TMO decomposition t o DME were t h e
q I
m a j o r r e a c t i o n s , i t was concluded from t h e i s o t o p e d i s t r i b u t i o n s , p r i m a r i l y 13C i n e t h y l e n e , t h a t d e p r o t o n a t i o n o f TMO i o n t o dimethyloxonium methyl i d e , which i s t h e n m e t h y l a t e d i n t e r m o l e c u l a r l y by excess TMO i o n , had occurred. CH30CH3 t CH,
1
BF4-NaBF4
-
\A/
C H CH3 3\6F
(CH,),OBF, + -
442
CH30CH3
+ CH3F + BF,
CH3
-(CH&O 3 t
2
I
-
CH,
\&/
CH3
I
BF4
THz CH3
+$
N
CH,OCH,CH,
CH30H
+ CH30CH3
g
:Stevens rearrangement
i
=CH, + CH30CH3
CH3-CH3
CH,
+ [CH,OCH,CH,]
+ CH2 =CH,
-
CH,=
CH,
+ CH30H
The d e s i l y l a t i o n o f Me3SiCH20+ (CD3)CH3BF4- was c a r r i e d o u t by r e a c t i o n w i t h CsF i n a manner s i m i l a r t o t h e p r e v i o u s NaH experiment. Among t h e p r o d u c t s ( T a b l e 4 ) were e t h y l e n e ( l % ) , E t F (0.5%), and MeOEt (1.6%).
To e x p l a i n t h e s e
p r o d u c t s , a r e a c t i o n scheme was proposed where dimethyloxonium methyl i d e
135
Table 2 Reaction of 13CH30+(CH3)2BF4-a with NaH isotopic distribution, % products
yield, %b
'3C,
methane MeF ethylene ethane
58.4 4.6 2.5 2.1 32.4
60.3
39.7
55.0d c 41 .O
36.9
8.1
51.3
7.7
Me,O
13c1
C
13c*
a13CH3 group 90% labeled. bunlabeled trimethyloxonium ion with BF,-, PF,-, and SbCI,- as counter ion aave similar Droduct distributions. CNot 2etermined. ' dCalculatedforintermolecularpath:49.013C0, 42.0 13C,, 9.0 13C2.Calculated for intramolecular path: 36.6 13C0,63.4 13C1.
Table 3 Reaction of (CD3)3OtBFQa with NaH products
isotopic distribution, O h
yield,
do
YO
d2
d,
d?
dd ~
methane MeF ethylene ethane Me,O
9.1 16.8 0.3 0.3 73.5
-84
b 5.5 b
16.4
18.3
26.5
d5
dfi
7.0
93.0
~~~
-16 33.3
alsotopic purity 99%. bNot determined.
generated by F-induced d e s i l y l a t i o n i s transmethylated by CH3 and CD3 t o g i v e dimethylethyloxonium ions.
CD,OCH,
These are cleaved by F- t o l a b e l l e d MeOEt and E t F .
+ FCH2SiMe,
CH,
CD3
+
(CH,),Si(Et)F CH3F
IF-
+ CD3F + CD3OCH3 +
CH30CH,CD, CD,CH,F
+ CD,OCH,CD,
+
+ CH,
CH,F
IF-
+ CD,F + CD30CH,
CH,0CH2CH3
+
+ CD,OCH,CH, t
=CD2
CH,CH2F
+ CH,=CH,
136
The observed i s o t o p e d i s t r i b u t i o n o f t h e MeOEt (Table 4 ) appears t o r u l e o u t t h e Stevens rearrangement.
The presence of Me3Si ( E t ) F ( 4 . 1 % ) was a t t r i b u t e d t o
Me3SiCH2F decomposition, a l t h o u g h t h e r e i s c o n t r a r y evidence i n t h e l i t e r a t u r e
1381. The two r e a c t i o n schemes proposed above a r e b o l d a t t e m p t s t o r a t i o n a l i z e t h e m u l t i p l i c i t y o f p r o d u c t s generated under a u t o t h e r m a l c o n d i t i o n s . I n t e r p r e t a t i o n i s f u r t h e r confounded by e x t r e m e l y l o w y i e l d s of key l a b e l l e d p r i m a r y p r o d u c t s , e.g. e t h y l e n e (0.3%, c o n t a i n i n g 33.3X d4, Table 3 ) , E t F (0.5%, Table 4 ) , and NeOEt (1.6%, Table 4): The presence o f s i g n i f i c a n t amounts o f t p r o d u c t s such as CD30CD3 (9.4%) and CH30CH3 ( 7 . 8 % ) f r o m Me3SiCH20(CD3)CH3BFi ( T a b l e 4) i s i n c o n s i s t e n t w i t h proposed mechanism and i n d i c a t e s t h a t secondary r e a c t i o n s were i m p o r t a n t and s h o u l d n o t have been n e g l e c t e d .
Intramolecular
H-D exchange ( v i d e i n f r a ) i n t h e dimethyloxonium m e t h y l i d e would i n t r o d u c e a d d i -
t i o n a l uncertainty.
F i n a l l y , t i l e l a r g e d i s c r e p a n c y i n methane y i e l d (58.4 vs
9.1%) f r o m t h e 13C l a b e l l e d vs t h e d e u t e r a t e d TMO s a l t cannot be s a t i s f a c t o r i l y explained. Table 4 Reaction of Me,SiCH,O+(CD,)CH,BF,-d products
yield, % b
methane MeF
12.1d
ethylene EtF
0.5
Me,O MeOEt
Me,SiF Me,Si(Et)F Me,SiCH,F Me,SiCH,OMe
O.ld
1.Od
55.9 1.6
3.4
4.1 9.2 12.1
with CsF
isotopic distribution,c%
e
60.6 CD3F 39.4 CH,F
e
48.4 CD,CH,F 51 .6CH,CH,F 16.8 CDiOCb, 69.2CD,0CH3 14.0 CH,OCH, 29.0 CD3CH,0CD,' 27.0 CD3CH,0CH3 22.7 CH,CH,OCD, 21.3CHjCH;OCHj no D no D no D
9
aCD3 group 99% labeled.
*Three experimentswith the unlabeledoxoniumsalt showedsimilar product distributions. CSimilar isotopic distributions were obtained in two other runs. dValues obtained from analysis of gas phase and converted under the assumption mol % MeF = mol % Me,SiCH,OMe. eNot determined. Calculated distribution for Stevens rearrangement: 16.8:34.6:34.6:14.0. gMS data did not allow differentiation between Me,SiCH,OCD, and Me3SiCH20CH,.
Olah and co-workers [29] a l s o examined t h e p h o t o l y t i c r e a c t i o n o f CH2N2 w i t h d i a l k y l e t h e r s , a r e a c t i o n f i r s t d e s c r i b e d b y Meerwein e t a1 [30]. According t o Huisgen 1311 t h i s r e a c t i o n proceeds v i a i n i t i a l y l i d e f o r m a t i o n , f o l l o w e d by a Stevens t y p e rearrangement.
T h i s was d i s p u t e d by Franzen and
137
F i k e n t s c h e r [32] who found, u s i n g 14CH2, t h a t E t O P r f r o m E t O E t was l a b e l l e d i n t h e y - p o s i t i o n r a t h e r t h a n t h e a - p o s i t i o n expected f r o m an y l i d e r e a r r a n g e n e n t .
a
P
-Y
A t t h e same t i m e , MeOEt and e t h y l e n e were a l s o formed, which c o u l d be r a t i o n a l i z e d by B - e l i m i n a t i o n f r o m a methyleneoxonium y l i d e i n t e r m e d i a t e . r
CH?.
CHL
,
7
w -
L
No l a b e l l i n g s t u d i e s were conducted t o c o n f i r m t h i s p o s s i b i l i t y .
However, i n
a n o t h e r experiment, 14C-methoxy l a b e l l e d MeOBu was r e a c t e d w i t h 12CH2.
In this
case t h e l a b e l was r e t a i n e d on t h e methoxy carbon. '4CH30-C4H,
+ :CH2
-14CH30-CH-C3H,
I
CH3
+ Isomers I f an y l i d e mechanism were o p e r a t i v e , s c r a m b l i n g would r e s u l t . Olah used CD2N2 as w e l l as CH2N2 w i t h u n l a b e l l e d e t h e r s (MeOEt, MeO-n-Pr, E t O E t , EtO-n-Pry and THF), p h o t o l y z i n g ca. 10 mol% e t h e r e a l s o l u t i o n s o f t h e
diazomethane.
Conversions were k e p t t o ca. 1% t o m i n i m i z e secondary r e a c t i o n s .
R e a c t i o n w i t h MeOEt y i e l d e d MeOPr, E t O E t as w e l l as t h e cleavage p r o d u c t DME. The i s o t o p e c o n t e n t i n t h e p r o d u c t MeOPr was s i m i l a r t o t h a t o f t h e s t a r t i n g diazomethane, v e r i f y i n g t h a t p r i m a r y p r o d u c t s were observed.
Based on an
a n a l y s i s o f p r o d u c t i s o t o p e d i s t r i b u t i o n s , t h e f o l l o w i n g scheme was proposed t o d e s c r i b e t h e t r a n s m e t h y l a t i o n process y i e l d i n g DME:
r
CH3-O-Et
-L ' J 4 ICD2
CD,
CH3-Y-Et
1
+ CH30CD2H
dCH2=CH,
CD30D
CD3
CH,-O-CD,
I
+ EtOCH, + E t O C D 3 e H 3 - O+-
I
+
bCD3 CH3-O-CH3
CH3
CD3
CH3
CH30Et Et -CH.j-0-Et
I
I
+ C H r Y - E t + Et -0Et +
+ Et-
p3 OCH3
+ Et
-OCD3 +
CH3OCD3
+ CD3OCD3 +
Et,O
138
M e t h y l e t h y l o x o n i u m m e t h y l i d e , formed v i a methylene a d d i t i o n t o MeOEt, e i t h e r decomposes t o e t h y l e n e + DME-d2, o r i s p r o t o n a t e d ( d e u t e r a t e d ) by CD30D ( o r D20) i m p u r i t i e s t o g i v e an oxonium s a l t , which t h e n undergoes r a p i d methyl t r a n s -
f e r s t o g i v e dQ, d3, and d6-DME.
S i g n i f i c a n t amounts o f d2, d3, and d -DFlE a r e Q indeed observed (Tables 5 and 6 ) , c o n s i s t e n t w i t h t h e proposed mechanism.
E t h y l e n e i s h e a v i l y d4- and d 3 - s u b s t i t u t e d i n d i c a t i n g t h a t i t i s formed m a i n l y f r o m CD2N2 i t s e l f , and n o t t h r o u g h i n t r a m o l e c u l a r B - e l i m i n a t i o n f r o m t h e y l i d e . T h i s i s taken as evidence a g a i n s t a Stevens rearrangement as t h e predominant pathway
.
Table 5 Deuterium Distributionain Percent in Dimethyl Etherb from the Reaction of Ethers with :CDP startingether
d6
d5 d4
4 4 4
do
MeOEt
MeO-n-Pr
5.3 6.7 6.7 12.6 38.5 6.3 23.9
6.5 14.6 25.2 37.8 6.1 9.8
Et,O
EtO-n-Pr
THF
10.4 18.2 36.4 24.7 8.2
15.4 39.8 33.7 6.2 3.5 1.4
3.5
2.1
24.4 41.1 11.4 10.8 8.8
acalculated from mass spectra (assuming no isotope effect). bDetected in the reaction mixture afler photolysis. The starting ethers and yields of Me,O are as follows: MeOEt, 0.0613 mmol; MeO-n-Pr, 0.0117 mmol; Et,O, 0.0059 mmol; EtO-n-Pr, 0.0081 mmol; THF, 0.0026 mmol.
Table 6 Deuterium Distribution in Percent in Ethylenea Found in the Reaction of Ethers with :CD2 startina ether
d4
d3
4 dl
dn
MeOEt
MeO-n-Pr
73.5 18.1 3.3 1.9 3.2
76.4 18.7 1.7 1.3 1.9
Et,O 70.1 20.7 2.2 1.2 5.86
EtO-n-Pr
THF
76.4 17.2 1.7 1.o 3.7
64.6 26.0 6.1 1.4 1.9
aTrapped as 1,2-dibromoethane.The starting ethers and yields of ethylene are as follows: MeOEt,0.0092 mmol; MeO-n-Pr,0.008mmol; Et,O. 0.0016 mmol; EtO-n-Pr, 0.003 mmol; THF, 0.0045 mmol. bOnly 0.0001 mmol of unlabeled ethylene, compared to 0.0054 mmol of MeOEt, is formed in the reaction.
I n c o n t r a s t , i s o t o p e d i s t r i b u t i o n s f r o m t h e THF e x p e r i m e n t a r e i n c o n s i s t e n t w i t h t h e proposed mechanism.
The THF-oxonium m e t h y l i d e would be expected e i t h e r
t o d e p r o t o n a t e t h e r i n g and/or undergo p r o t o n a t i o n i t s e l f t o g i v e t h e MeTHFoxonium s a l t .
139
I?
I
I n t h i s case t h e s t a r t i n g e t h e r has no m e t h y l t o exchange so t h a t decomp o s i t i o n o f t h e s a l t s h o u l d g i v e DME-d6 (and d5, r e f l e c t i n g i s o t o p i c p u r i t y ) exclusively.
However i n t h e observed i s o t o p e d i s t r i b u t i o n s ( T a b l e 5 ) , d6 i s
t h e s m a l l e s t component ( 3 . 5 % ) and d5 i s absent.
The d a t a i n d i c a t e e x t e n s i v e
H-D s c r a m b l i n g even a t t h e l o w (ca. 1 % ) c o n v e r s i o n s .
Tetrahydropyran (0.4%), formed presumably v i a e i t h e r a Stevens rearrangement o r a c a r b e n o i d r o u t e was a1 so observed. These experiments, w h i l e p r o v i d i n g i m p o r t a n t new i n s i g h t s , cannot be cons i d e r e d d e f i n i t i v e i n answering t h e y l i d e q u e s t i o n i n view o f t h e i r s h o r t comings, some o f which a r e d e t a i l e d above.
Assuming n e v e r t h e l e s s t h a t oxonium
y l i d e s do e x i s t and a r e i m p l i c a t e d i n C-C bond f o r m a t i o n , t h e r e remains t h e q u e s t i o n whether an a c i d i c z e o l i t e can f u n c t i o n as a s t r o n g , hindered, nonn u c l e o p h i l i c base i n y l i d e g e n e r a t i o n f r o m a1 k y l o x o n i u m p r e c u r s o r s . Hunter and Hutchings [33] approached t h i s i s s u e by examining t h e r e a c t i o n o f TMOfSbC16- w i t h 1 i t h i u m t e t r a i s o p r o p o x i d e ,
which t h e y regarded as a reason-
a b l e model o f t h e z e o l i t e b a s i c s i t e , w i t h i s o p r o p o x y l r e p l a c i n g s i l o x y l groups. No MeOEt was d e t e c t e d , b u t o n l y p r o d u c t s o f i s o p r o p o x i d e decomposition, o r n u c l e o p h i l i c c h l o r i d e and i s o p r o p o x i d e a t t a c k . (CH,),O+SbCI;
+ LiAI(OPi),-
PiOMe
27%
+P
h 31%
+ PlOH 42%
(7% conversion to ~ b ~ e )
They concluded t h a t t h e A1-0 bond i s more n u c l e o p h i l i c t h a n b a s i c . D e v e l o p i n g t h e s e concepts f u r t h e r , Hunter and H u t c h i n g s [34] s t u d i e d t h e r e a c t i o n o f v a r i o u s m e t h y l a t i n g agents MeX (X = Oti, I , OS03Me) o v e r HZSM-5 and NaZSM-5 ( T a b l e 7 ) .
The f a c t t h a t hydrocarbons a r e formed f r o m d i m e t h y l s u l f a t e
o v e r b o t h ti- and NaZSM-5 i s c o m p e l l i n g evidence a g a i n s t TMO i o n as a c e n t r a l i n t e r m e d i a t e , s i n c e t h e oxygen i n d i m e t h y l s u l f a t e i s t o o weakly n u c l e o p h i l i c t o f o r m an oxonium i o n .
I t i s i n t e r e s t i n g t o n o t e t h a t as t h e r e a c t i v i t y o f t h e
m e t h y l a t i n g agent d e c l i n e s , more and more methane and e t h y l e n e a r e formed, c o n s i s t e n t w i t h t h e v i e w t h a t t h e s e a r e p r i m a r y p r o d u c t s o f MeOH c o n v e r s i o n [35].
140
Table 7 Decomposition of Various Methylating Agents over ZSM-5
Catalyst H-ZSM-5 H-ZSM-5
H-ZSM-5
T.’C
Timeon iine. mtn
0.15 015 015
250 250 250
15 60 100
7.1 16.7 226
0.075 0.075 0.075 0.075
250 250 250 250
15 60
0.8 0.8
250 250
Methylating agent W.H.S.V. MeOH Me,SO,
Mela
Total conversion. %
Product selectivity,mole% CH, C2H4 CH ,, PC, 4.0 1.1 09
3.9 14.1 24.3
4.1 56 6.3
92.0 79.2 68.5
230
0.12 3.3 12.9 21.2
81.5 20.5 2.5 1.5
18.5 44.9 28.9 45.2
0 28.2 11.2 15.1
0 6.4 57.4 38.2
60 100
002 0.13
66.1 7.0
33.9 44.1
0 32.5
0 16.4
100
Na-ZSM-5
MeOHb
0.075
300
200
0.001
64.4
35.6
0
0
Na-ZSM-5
Me,SO,
0.075 0075
250 300
90 130
0.06 2.0
52.4 48.1
47.6 51.9
0 Trace
0 0
Na-ZSM-5
Mei
0.1 0.1
250 300
15 90
0.05 0.07
66.0 70.0
34.0 30.0
0 0
0 0
aNo conversion ObSeNed after 15 min.
NO conversionobserved for 160 min at 250 ‘C.
Hunter and Hutchings propose t h a t a surface-bound methoxyl i s i n i t i a l l y formed, which i s deprotonated t o a surface-bound y l i d e , i s o e l e c t r o n i c w i t h surfacebound carbene [ZO, 351, which i n t u r n engages i n C-C bond formation by C-H attack. The nature o f the basic s i t e f o r deprotonation was n o t specified. CH 2
Surface incorporated yiide
Surface carbene
H e l l r i n g and co-workers [36] r e c e n t l y addressed the b a s i c i t y question by synthesizing TMO-ZSM-5 and monitoring i t s decomposition by means o f 13C magic angle spinning NMR (MASNMR).
TMO-ZSM-5 was synthesized by i o n exchange o f
TMOtBF4- i n nitromethane w i t h HZSM-5 a t low temperature (T<253 K) using Schlenk techniques.
The r e s u l t a n t spectrum (Fig. 3a) shows t h a t TMO-ZSM-5 has a carbon s h i f t a t 80 ppm, i d e n t i c a l t o o t h e r TMO s a l t s . Upon warming t o room temperature and “spinning” overnight, the sample slowly decomposed t o methyl ZSM-5 and
physisorbed DME (Fig. 3b & 3c), confirming t h a t the conjugate base o f HZSM-5 i s n u c l e o p h i l i c r a t h e r than basic toward t h e methylating agent. The formation o f methyl ZSM-5 from MeOH and HZSM-5 a t elevated temperatures (T>473 K) was r e c e n t l y observed by Forester e t a1 [37] using i n s i t u FTIR.
The
appearance o f any AlOMe species coincided w i t h t h e onset o f hydrocarbon formation.
Most s i g n i f i c a n t l y the methyl z e o l i t e e x h i b i t e d the c a p a b i l i t y t o methy-
l a t e ethylene, propylene and cyclopentene.
Benzene was a l s o methylated, i n
accord w i t h e a r l i e r r e s u l t s o f Ono and Mori [4].
TMO i o n could n o t be detected.
14
""'""1'"[""1""'""1""1""1""1""1""'""Im 10000
PPM 10000
000
5000
5000
000
PPM 10000
5000
000
PPM
Figure 3 50 1MHz CP-MAS ')C-NMR spectra 01 lrimelhyloxoniumion m ZSM-5 clen to right) 4 2OP M , 9 2OP M and 11 20P.M SpecIra Show the Slow decomposilion Of TMO.ZSM.5 l o = 79pDm) and appearance of rnelhyl.ZSM.5 [ h = S l p p m )
Free r a d i c a l methanisms.
There has l a t e l y been renewed i n t e r e s t i n the
p o s s i b i l i t y t h a t f r e e r a d i c a l s may play a r o l e i n the conversion o f methanol t o hydrocarbons.
Zatorski and Krzyzanowski [8] had e a r l i e r proposed a r a d i c a l
mechanism f o r hydrocarbon formation from methanol over natural mordenite.
+ OH
CH30H -CH, 2CH3 +C2H.5 CH, CH,
+ CZH6 +CH4 + CH& + CH,CH2 -C3H,
2CH3CHp -C4H1, C2H4 + H
CH3CH2-
However, no experimental evidence was provided.
More r e c e n t l y , Clarke e t a1
[9] detected f r e e r a d i c a l s i n the r e a c t i o n o f DME over HZSM-5 using the spin t r a p p i n g reagent a-phenyl -N-t-butyl n i trone, w i t h ESR analysis.
These workers
suggested t h a t r a d i c a l s are formed i n i t i a l l y by i n t e r a c t i o n o f DME w i t h paramagnetic centers ( s o l i d - s t a t e defects) i n the z e o l i t e and t h a t C-C bond format i o n r e s u l t s from d i r e c t coupling o f r a d i c a l s . Choukroun e t a1 [39] reported t h a t r a d i c a l d i m e r i z a t i o n o f DME by (FS03)2 i n FS03H gives dimethoxyethane exclusively, however, t h i s product has n o t been detected i n DME r e a c t i o n over zeolites.
The p o s s i b i l i t y o f carbene generation v i a r a d i c a l s c i s s i o n was also
recognized by Clarke e t a1
.
-
S. + CH30CH3 2 CH2-O-CH3 S-H or
CH,-O-CH,
S-H
+ .CH2WH,
CH,-O-[CH,]2-O-CH,
-S.
-
:CH2 + CH,-O-CH,
:CH,
+ H. + GCH,
-CH,CH2-O-CH3
142
The p r o p o s a l o f C l a r k e e t a1 e l i m i n a t e s t h e r e q u i r e m e n t o f s t r o n g l y b a s i c s i t e s f o r p r o t o n removal f r o m C-H,
a m a j o r drawback o f most schemes.
Assuming t h a t r a d i c a l s can be generated by t h e r m o l y s i s c f s u r f a c e m e t h o x y l s , and t h a t t h e r a d i c a l s c i s s i o n of C l a r k e e t a1 occurs, t h e f o l l o w i n g i n i t i a t i o n mechanism would be p l a u s i b l e :
CHIOH CH,OZ
+ ZOH+ -0Z -.CH,OZ
CH,OZ
+ H,O
+ ZOH
I t i s a p p a r e n t t h a t much r e s o u r c e f u l , i m a g i n a t i v e e x p e r i m e n t a t i o n has been
done t o r e s o l v e t h e q u e s t i o n o f C-C bond f o r m a t i o n f r o m methanol.
Although t h e
answer remains e l u s i v e , t h e s e experiments t e l l us a t l e a s t what i s p r o b a b l y
not
i n v o l v e d i n t h e bond f o r m a t i o n , p a r t i c u l a r l y i n t h e presence of z e o l i t e c a t a lysts.
The Stevens r e a r r a n g e r e n t o f oxonium y l i d e can be r u l e d o u t , as w e l l as t h e c a r b o c a t i o n i c r o u t e i n v o k i n p h y F e r v a l e n t carbon t r a n s i t i o n s t a t e s . Not excluded a r e surface-bound s p e c i e s such as carbenoids and y l i d e s .
Again t h e r e
seems t o be a consensus t h a t s u r f a c e methoxyls a r e p r e c u r s o r s t o t h e s e r e a c t i v e Cl
i n t e r m e d i a t e s , which seems somehow t o be "coming f u l l c i r c l e " , s i n c e s u r f a c e
methoxyls were e a r l y shown t o be i n t e r m e d i a t e s i n t h e f o r m a t i o n o f DME, which i s i t s e l f an i n t e r m e d i a t e i n hydrocarbon f o r m a t i o n . F i n a l l y , i f t h e f r e e r a d i c a l character o f t h e i n i t i a t i o n step proves c o r r e c t , t h e i m p l i c a t i o n s t o z e o l i t e c a t a l y s i s w i l l be f a r - r e a c h i n g . REFERENCES
a) S. L. M e i s e l , J . P. McCullough, C. H. L e c h t h a l e r and P. B. Weisz, Chemtech, 1976, 6, 86; b) C. 0. Chang and A. J. S i l v e s t r i , J. C a t a l . , 1977, 47, 249. 2 C. D. Chang, Hydrocarbons f r o m Methanol, M. Dekker, New York, 1983, ch.5. 3 J . A. Rabo (Ed.), Z e o l i t e Chemistry and C a t a l y s i s , ACS Monograph 171, h e r . Chem. SOC., Washington, DC, 1976, p.437. 4 Y. On0 and T. M o r i , J . Chem. S O C . , Faraday Trans.1, 1981, 77, 2209. 5 J . 3. Nagy, J. P. G i l s o n and E. G. Derouane, J . Mol C a t a l . , 1979, 5, 393. 6 J . P. van den Berg, J . P. W o l t h u i z e n and J . H. C. van H o o f f , Proc. 5 t h I n t . Conf. on Z e o l i t e s , L. V. Rees (Ed.), Heyden, London, 1980, p.649. 7 G. A. Olah, Pure Appl Chen., 1931 , 53, 201. 8 M. Z a t o r s k i and S. Krzyzanowski, Acta Phys. Chem., 1978, 29, 347. 3 J . K. A. C l a r k e , R. Darcy, B. F. Hegarty, E. O'Donoghue, V . Amir-Ebrahimi and J. J. Rooney, J . Chem. SOC., Chem. Commun., 1986, 425. 1
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10 11 12 13 14 15 16 17 1.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
P. E. Venuto and P. S. Landis, Adv. Catal., 1968, 18, 259. A. B. Schwartz and J . C i r i c , r e p o r t e d i n Ref. 10. F. A. Swabb and B. C. Gates, I n d . Eng. Chem., Fundam., 1972, 11, 540. P. Salvador and W. Kladnig, J . Chem. SOC., Faraday Trans.1, 1977, 1153. C. D. Chang, J . Catal., 1981, 69, 244. C. D. Chang and C. T-W. Chu, J . Catal., 1982, 74, 203. W. Kirmse, Carbene Chemistry, Academic Press, New York, 1971. G. A. Olah, G. Klopman and R. H. Schlosberg, J . Amer. Chem. SOC., 1969, 91, 3261. J . H. C. van Hooff, J . Catal., 19S3, 79, 242. C. D. Chang and C. T-W. Chu, J. Catal. , 1983, 79, 244. C. S. Lee and M. M. Wu, J. Chem. SOC., Chem. Commun., 1985, 250. G. A. Olah, G. K. Surya Prakash, R. bl. E l l i s and J . A. Olah, J . Chem. SOC., Chem. Commun., 1986, 9. W. Drenth, W. T. M. Andriessen and F. 5. van D u i j n e v e l d t , J . Mol. C a t a l . , 1983, 21, 291. D. Kagi, J . Catal., 1981, 69, 242. G. A. Olah, H. Doggweiler, J . D. Felberg, S. F r o h l i c h , M. J . Grdina, R. Karpeles, T. Keumi, S. Inaba, W. M. I p , K. Lammertsma, G. Salem and D. C. Tabor, J . Amer. Chem. SOC., 1984, 106, 2143. R. D. Smith and J . H. F u t r e l l , Chem. Phys. L e t t . , 1976, 41, 64. 6 . A. Olah, H. Doggweiler and J . D. Felberg, J . Org. Chem., 1984, 49, 2172. P. Rimmelin, H. Taghavi and J . Sommer, J . Chem. SOC., Chem. Commun., 1984, 1210. a) A. Maercker and W. Theysohn, Justus L i e b i g s Ann. Chem. , 1971 , 747, 70. b ) R. A. E l l i s o n , R. G r i f f i n and F. N. Kotsonis, J . Grganometal. Chem., 1972, 36, 209. G. A. Olah, H. Doggweiler and J . D. Felberg, J. Org. Chem., 1984, 49, 2116. H. Meerwein, H. Rathjen and H. Werner, Chem. Ber., 1942, 75,1610. R. Huisgen, Angew. Chem., 1955, 67, 439. V. Franzen and L. Fikentscher, L i e b i g s Ann. Chem., 1958, 617, 1. R. Hunger and G. J . Hutchings, J . Chem. SOC., Chem. Commun., 1985, 886. R. Hunger and G. J . Hutchings, J. Chem. SOC., Chem. Commun., 1985, 1643. C. T-W. Chu and C. D. Chang, J . Catal., 1984, 86, 297. S. D. H e l l r i n g and C. D. Chang, r e p o r t e d a t 2 1 s t ACS S t a t e - o f - t h e - A r t Symp., Methanol as a Raw M a t e r i a l f o r Fuels and Chemicals, Marco Is., F l o r i d a , June 1986. T. R. Forester, S-T. Wong and R. F. Howe, J . Chem. SOC., Chem. Commun., 1986, 1611. E. S. Alexander, R. N. Haszeldine, M. J . Newlande and A. E. Tipping, J. Chem. SOC., A, 1970, 13, 2285. H. Choukroun, D. Brunel and A. Germain, J . Chem. SOC., Chem. Commun., 1986, 6.
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D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion
145
1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
I S O T O P I C AND MECHANISTIC S T U D I E S OF PETHANOL CONVERSION
T. MOLE C S I R O D i v i s i o n o f V a t e r i a l s Science and Technology, Locked Rag 33, C l a y t o n , V i c t o r i a 3168, A u s t r a l i a .
ARSTRACT O l i g o m e r i z a t i o n o f o l e f i n s and t h e i r homologation by methanol ( o r methanold e r i v e d s p e c i e s ) appear t o he e s s e n t i a l f e a t u r e s o f methanol c o n v e r s i o n o v e r ZSP-5 z e o l i t e . A s e l f - c o n s i s t e n t - i n t e r p r e t a t i o n o f t h e e n t i r e process i s p o s s i b l e i n terms o f Rrfinsted-acid ZSM-5 z e o l i t e c a t a l y s i n g p r o t o n t r a n s f e r and m e t h y l a t i o n r e a c t i o n s , and t h e f o r m a t i o n o f carbenium i o n s , and t h e i r v a r i o u s o l i g o m e r i z a t i o n , c r a c k i n g , rearrangement and h y d r i d e - t r a n s f e r r e a c t i o n s .
INTROOllCTION Conversion o f methanol t o hydrocarbons has been reviewed by Chang ( r e f . 1). a)
It i n v o l v e s :
establishment o f t h e methanolldimethyl ether e q u i l ihrium.
2CH30H h)
*
CH30CH3
+ H20
formation o f o l e f i n s .
nCH30H >-
CnHzn
+ nHzO
c ) c y c l i z a t i o n and h y d r i d e anion t r a n s f e r r e a c t i o n s , which c o n v e r t o l e f i n s t o c y c l o a l kanes, and t o a1 kanes p l u s aromatic hydrocarbons. CnHzn >-
CyClO-C
H n 2n
’ *
> 3cn H ~ n + 2 4CnH2n -
’ ‘nH2n-6
Most m e c h a n i s t i c i n t e r e s t a t t a c h e s t o t h e f o r m a t i o n o f o l e f i n s .
Even t h e
f o r m a t i o n of e t h y l e n e from one d i m e t h y l e t h e r m o l e c u l e i n v o l v e s breakage o f two carbon-oxygen and two carbon-hydrogen bonds and f o r m a t i o n o f a carbon-carbon d o u b l e bond.
The s t o i c h i o m e t r i c simp1 i c i t y o b v i o u s l y masks r e l a t i v e l y complex
r e a c t i o n mechanisms.
146
M o b i l ' s E M - 5 z e o l i t e ( r e f . 2 ) appears t o be by f a r t h e b e s t c a t a l y s t f o r c o n v e r s i o n o f methanol t o hydrocarbons.
The a c t i v i t y o f t h e z e o l i t e , i n t h e
p r o t o n form, i s a consequence o f i t s Rrdnsted a c i d i t y .
I t s s e l e c t i v i t y and
freedom from r a p i d c o k i n g i s due t o t h e l o c a t i o n o f t h e Rr4nsted a c i d s i t e s w i t h i n channels o f ca. 0.6 nn diameter.
There a r e two s e t s o f i n t e r s e c t i n g
channels, e s s e n t i a l l y p a r a l l e l t o t h e a- and h-axes o f t h e c r y s t a l .
The
channel dimensions c o n f e r s h a p e - s e l e c t i v i t y on t h e z e o l i t e , d e t e r m i n i n g what reagants can g a i n access t o t h e a c i d s i t e s , what p r o d u c t s and t r a n s i t i o n s t a t e s can be formed, and what p r o d u c t s can d i f f u s e from t h e s i t e s
-
rather than
remain and undergo f u r t h e r r e a c t i o n ( r e f . 3,4). A l u m i n o s i l i c a t e z e o l i t e s have a n i o n i c l a t t i c e s based on 2 - c o o r d i n a t e oxygen and 4 - c o o r d i n a t e aluminium and s i l i c o n atoms. t o a formal n e g a t i v e charge.
Each aluminium atom corresponds
One can r e p r e s e n t t h e a n i o n i c l a t t i c e by t h e
e m p i r i c a l formula, xSi02.A102-.
I n t h e case o f ZSM-5 z e o l i t e , x i s l a r g e
-
m o s t l y > 20 and p o s s i b l y >>20. Consequently one expects t h e aluminium atoms o f t h e l a t t i c e t o be w e l l separated and t o r e p r e s e n t u n i f o r m l y s t r o n g a c i d s i t e s . The aluminium atoms a r e n o t c o n f i n e d t o t h e i n t e r n a l channels o f t h e zeolite,
A small f r a c t i o n a r e a t t h e s u r f a c e o f t h e z e o l i t e c r y s t a l s .
Thus
t h e z e o l i t e most commonly shows s h a p e - s e l e c t i v e a c t i v i t y due t o t h e s i t e s i n t h e channels, b u t can show n o n - s h a p e - s e l e c t i v e a c t i v i t y due t o t h e s i t e s on t h e e x t e r n a l s u r f a c e , p a r t i c u l a r l y i f a f a c i l e r e a c t i o n i s observed and i f t h e channels o f t h e z e o l i t e s a r e hlocked. The a n i o n i c l a t t i c e o f a z e o l i t e accommodates charge-halancing c a t i o n s and n e u t r a l molecules.
Most commonly t h e charge b a l a n c i n g c a t i o n i s Na+ and t h e
n e u t r a l m o l e c u l e i s water.
However, when t h e p r o t o n form of a z e o l i t e i s used
as a c a t a l y s t , t h e n e u t r a l molecules i n c l u d e o r g a n i c molecules and t h e c a t i o n s i n c l u d e carbo-cations.
It i s w o r t h n o t i n g t h a t t h e a n i o n i c l a t t i c e may n o t
o n l y accommodate c a r b o - c a t i o n s b u t a l s o hond them c o v a l e n t l y , p a r t i c u l a r l y if Thus, c o v a l e n t honding o f t h e CH3+
t h e carbo-cation i s o f low s t a b i l i t y .
m o i e t y t o t h e l a t t i c e oxygen o f ZSV-5 z e o l i t e ( a s S i - 6 C H 3 - S i
o r Al-bCH3-Si)
is
now w e l l e s t a b l i s h e d ( r e f . 5). OLIGOMERIZATION AND CRACKING OF OLEFINS The b e h a v i o u r o f o l e f i n s over ZSP-5 z e o l i t e i s c e n t r a l t o an u n d e r s t a n d i n g o f methanol conversion, so we b e g i n w i t h a d i s c u s s i o n o f t h i s t o p i c and r e t u r n l a t e r t o t h e q u e s t i o n o f how methanol i s c o n v e r t e d t o o l e f i n s . O l e f i n s undergo r a p i d p r o t o n a t i o n and o l i g o m e r i z a t i o n over ZSM-5 z e o l i t e .
147
Furthermore, t h e carbenium i o n i n t e r m e d i a t e s r e a d i l y undergo rearrangements. Therefore,
t h e p r o d u c t s a r e n o t n e c e s s a r i l y t h e h i g h l y branched p r o d u c t s
expected from k i n e t i c a l l y c o n t r o l l e d carbenium i o n o l i g o m e r i z a t i o n ,
h u t can be
l e s s h i g h l y branched. The above r e a c t i o n s o c c u r r a p i d l y when o l e f i n s a r e sorbed by d r y ZSM-5 z e o l i t e , even a t ambient temperatures ( r e f .
6,7).
Subsequently t h e channels
became b l o c k e d by o l i g o m e r s , and f u r t h e r o l i g o m e r i z a t i o n i s c o n f i n e d t o t h e s u r f a c e o f t h e z e o l i t e and i s n o t s h a p e - s e l e c t i v e . t o H-ZSM-5 z e o l i t e a t ca.200°C/20
So, when p r o p y l e n e i s f e d
bar, t h e p r o p y l e n e o l i g o m e r r e s u l t i n g i s
h i g h l y branched and q u i t e comparable t o p r o p y l e n e polymer g a s o l i n e formed 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 ( r e f . 8).
I f one poisons t h e e x t e r n a l
surface, the z e o l i t e i s i n e f f e c t i v e f o r p r a c t i c a l oligomerization a t 200OC ( r e f . 8).
The s h a p e - s e l e c t i v e p r o p e r t i e s o f ZSM-5 z e o l i t e can o n l y be u t i l i z e d
f o r p r a c t i c a l p r o p y l e n e o l i g o m e r i z a t i o n a t temperatures i n excess o f 300OC. When f a c i l e o l i g o m e r i z a t i o n and c r a c k i n g do o c c u r o v e r ZSM-5 z e o l i t e , t h e o v e r a l l r e s u l t i s e s t a b l i s h m e n t o f an o l e f i n q u a s i - e q u i l i b r i u m ( r e f . 9).
There i s some c o n t r o v e r s y ( r e f . 10) about t h e e x t e n t t o which e t h y l e n e p a r t i c i p a t e s i n t h e quasi-equilibrium,
since ethylene i s t h e l e a s t r e a c t i v e
o l e f i n i n c a t i o n i c o l i g o m e r i z a t i o n and t h e l e a s t r e a d i l y o b t a i n e d by a carbenium-ion c r a c k i n g mechanism. However, e q u i l i b r i u m between o l e f i n s o f 3+ carbon atoms i s almost c e r t a i n l y e s t a b l i s h e d under c o n d i t i o n s o f methanol c o n v e r s i o n ( r e f . 11). o l e f i n s observed a r e p r o d u c t s o f t h e q u a s i - e q u i l i b r i u m ,
Thus t h e
and t h e i r r e l a t i v e
amounts a r e determined by t h e p o s i t i o n o f e q u i l i b r i u m and r a t e s o f d i f f u s i o n . One expects i n c r e a s e i n p a r t i a l p r e s s u r e t o f a v o u r h i g h e r o l e f i n s and i n c r e a s e i n temperature t o favour lower o l e f i n s .
It i s c l e a r l y n o t p o s s i b l e f o r
methanol c o n v e r s i o n t o g i v e p r o p y l e n e s e l e c t i v e l y o r butenes s e l e c t i v e l y . E q u a l l y c l e a r l y , i t i s d i f f i c u l t t o o b t a i n evidence on t h e mechanism of methanol c o n v e r s i o n from l a b e l l i n g experiments i f carbon and hydrogen a r e scrambled r a p i d l y i n t h e o l e f i n i c p r o d u c t s . Under r e l a t i v e l y m i l d c o n d i t i o n s o f methanol c o n v e r s i o n one can be reasonably sure t h a t ethylene i s not p a r t i c i p a t i n g f u l l y i n t h e quasiequilibrium.
Such c o n d i t i o n s a r e t h o s e o f l o w methanol conversion, o r when
aqueous methanol undergoes c o n v e r s i o n a t about 300OC.
Then i t i s p o s s i b l e t o
148
a t t a c h some s i g n i f i c a n c e t o an e x a m i n a t i o n o f carbon and hydrogen l a b e l s i n t h e ethylene product. It has o n l y r e c e n t l y been recognised, by Espinoza ( r e f . 12), t h a t
o l i g o m e r i z a t i o n o f o l e f i n i c p r o d u c t s may 1 i m i t c o n v e r s i o n o f methanol b y b l o c k i n g t h e z e o l i t e channels, w i t h t h e r e s u l t t h a t s i g n i f i c a n t c o n v e r s i o n cannot occur below a l i m i t i n g t e m p e r a t u r e around 280°C i n a r e a c t o r o p e r a t i n g around 1 b a r pressure. AUTOCATALYSIS OF METHANOL CONVERSION There have been numerous r e p o r t s ( r e f . 13-16) t h a t methanol c o n v e r s i o n has a u t o c a t a l y t i c c h a r a c t e r and i s a c c e l e r a t e d by added o l e f i n ( o r C3+ a l c o h o l ) . Espinoza’s r e s u l t s i n d i c a t i n g t h a t o l e f i n o l i g o m e r s b l o c k t h e z e o l i t e channels c l o u d t h e evidence on a u t o c a t a l y s i s .
They p r o b a b l y e x p l a i n why added o l e f i n s
( o r a l c o h o l s ) a c c e l e r a t e c o n v e r s i o n o v e r o n l y a narrow temperature range below t h a t a t which complete c o n v e r s i o n would o c c u r i n t h e absence o f added o l e f i n . Aromatic hydrocarbons (benzene , t o 1 uene and p-xylene) a l s o e x e r t a
c o c a t a l y t i c e f f e c t on t h e c o n v e r s i o n o f aqueous methanol, which i s l i m i t e d t o a n a r r o w t e m p e r a t u r e range below t h a t a t which c o n v e r s i o n i s complete w i t h o u t t h e added aromatic.
The e f f e c t i s o f a m e c h a n i s t i c r a t h e r t h a n physico-chemical
n a t u r e s i n c e e t h y l e n e w i t h some 13C-label i s o h t a i n e d from u n l a b e l l e d methanol i n t h e presence o f 13C-labelled benzene ( r e f . 17,18). OLEFIN HOMOLOGATION Many a u t h o r s (see, f o r example r e f . 1,16) have suggested t h a t c o n v e r s i o n o f methanol m a i n l y proceeds by homologation o f o l e f i n i c p r o d u c t by an e l e c t r o p h i l i c m e t h y l a t i o n mechanism.
The m e t h y l a t i n g s p e c i e s m i g h t be
p r o t o n a t e d methanol, p r o t o n a t e d d i m e t h y l e t h e r , o r methyl groups a t t a c h e d t o t h e z e o l i t e l a t t i c e (CH3-6Zeol
).
Ry way o f i l l u s t r a t i o n , p r o t o n a t e d methanol
i s d e p i c t e d as t h e m e t h y l a t i n g s p e c i e s i n t h e f o l l o w i n g equations. Homologation:
repeated n t i m e s :
CmHh
f o l l o w e d by c r a c k i n g :
-
’
+-
+ CH30H2
CmHh
> Cm+1H2m+3
t nCH30H
Cm+nH2mt2n
>>-
t
H20
Cmtn H2mt2n t nH20
C H m 2m
’ ‘nH2n
has t h e o v e r a l l r e s u l t o f c o n v e r t i n g n molecules o f methanol i n t o a Cn o l e f i n nCH30H >-
+ nHpO C H n 2n
149 D i r e c t evidence f o r homologation i s n o t r e a d i l y a v a i l a b l e because r e a c t i o n s between o l e f i n
molecules a r e so much more f a c i l e t h a n homologation.
Rehrsing e t al.
( r e f . 19; see a l s o r e f . 20) o b t a i n e d C7 hydrocarbons s i n g l y
Recently
l a b e l l e d w i t h 13C by r e a c t i o n o f 1 3 C - l a b e l l e d methanol w i t h hexenes under c o n d i t i o n s where hexene o l i g o m e r i s a t i o n i s i n c o m p l e t e and methanol c o n v e r s i o n i s low.
Table 1 shows a t y p i c a l r e s u l t f r a n one o f t h e s e experiments.
Single
1 3 C - l a b e l l i n g i s even more pronounced i n t h e C13 p r o d u c t s ( f r o m hexene d i m e r and 13C-methanol)
t h a n i n t h e C7 products.
X S i n g l e 13C-label
Products (average y i e l d over 2h.) 2-methylpentane 3-methyl pentane n-hexane
hexanes (52CX y i e l d )
(26%) (21%) (5%)
3,4-dimethyl pentane (0.2%) 2-methyl hexane (0.9%) 2,3-dimethylhexane (t0.lX) 3-methyl hexane (0.8%) n-heptane (0.2%)
heptanes (2CX y i e l d )
C hydrocarbons (&% y i e l d )
n a t u r a l ahundance ditto ditto 55 57 n o t determined 53
38
m a i n l y C12 hydrocarbons
Thus i t i s p o s s i b l e t o r e p r e s e n t t h e f o r m a t i o n o f o l e f i n s from methanol as i n F i g u r e 1.
Most observers would agree t h i s i s t h e main r o u t e t o o l e f i n s .
The sorbed o l e f i n s which undergo homologation a r e p r o b a b l y o f h i g h e r m o l e c u l a r weight t h a n t h e desorbed ( p r o d u c t ) o l e f i n s . roles
-
Sorbed o l e f i n s p l a y two opposing
b l o c k i n g t h e z e o l i t e channels, e s p e c i a l l y a t lower temperature, and
a u t o c a t a l y s i ng methanol c o n v e r s i o n , es p e c i a1 l y a t h i g h e r temperature. The o l e f i n s produced hy t h e homologation mechanism a r e t h o s e t y p i c a l o f a c i d c a t a l y s e d c r a c k i n g r e a c t i o n s , i.e.
C3+ o l e f i n s .
E t h y l e n e i s produced i n
amounts c o r r e s p o n d i n g t o o l e f i n n e a r - e q u i l i b r i u m o n l y a t h i g h e r temperature, p a r t i c u l a r l y when feed r a t e s a r e l o w and c o n t a c t t i m e s a r e long, e.g. x
g. methanol/g.
c a t a l y s t l h r (ref.
21).
40OOC; 5
150
CH30H PCH3OH
i CH30+Zeol (+H20)
* CH30CH&+H@)
-I
homologation Sorbed C,,olefin
1
Sorbed C,,+1olefin
Desorbed olefins (mainly C3+)
F i g . 1.
Homologation mechanism f o r methanol conversion.
I N I T I A L STEPS I N METHANOL CONVERSION AND AN ALTERNATIVE HOMOLOGATION MECHANISM A small amount o f methane (ca. 1C%) i s formed i n methanol c o n v e r s i o n , and appears t o be one o f t h e f i r s t p r o d u c t s formed ( r e f .
11).
When a small amount
o f methanol i s sorbed o n t o ZSM-5 z e o l i t e , t h e l a t t i c e i s m e t h y l a t e d ( r e f . 5). Subsequent temperature-programmed d e s o r p t i o n g i v e s d i m e t h y l e t h e r and desorbed methanol f i r s t , t h e n ( a t 2 5 0 - 3 0 0 ° C )
methane ( s t a b l e ) and formaldehyde
( u n s t a b l e ) , and f i n a l l y a r o m a t i c p r o d u c t s ( r e f . 22-23). The f o r m a t i o n o f methane may be a s s o c i a t e d w i t h i n i t i a t i o n o f t h e homol o g a t i o n reaction.
The mechanism o f methane f o r m a t i o n i s unknown (perhaps o f
n a t u r e ; r e f . 24).
free-radical
What i s c l e a r i s t h a t t h e C-H bonds o f methanol
a r e l a b i l e a t t h e 250-3OO0C temperatures a t which methanol c o n v e r s i o n begins. We have i n v e s t i g a t e d t h e c o n v e r s i o n o f methanol/D20 feeds o v e r ZSM-5 z e o l i t e ( r e f . 16,25)
and looked p a r t i c u l a r l y a t t h e i n c o r p o r a t i o n o f d e u t e r i u m i n t o t h e
dimethyl ether. n o t otherwise.
Deuterium was i n c o r p o r a t e d i f o l e f i n f o r m a t i o n occurred, b u t Furthermore a d d i t i o n o f small amounts o f propanol, t o
a c c e l e r a t e conversion, a l s o a c c e l e r a t e d d e u t e r a t i o n o f t h e d i m e t h y l e t h e r . We sought t o i n t e r p r e t t h e r e s u l t s i n terms o f t h e Brdnsted a c i d i t y of t h e c a t a l y s t , D20 b e i n g a source o f D,'
and a common r a t e - d e t e r m i n i n g s t e p l e a d i n g
t o a high-energy i n t e r m e d i a t e , which c o u l d e i t h e r he d e u t e r a t e d i n t h e m e t h y l group o r r e s u l t i n f o r m a t i o n o f carbon-carbon bonds.
F i g u r e 2 shows how t h e
151
r e s u l t s may he i n t e r p r e t e d
.
The i n t e r p r e t a t i o n which we o r i g i n a l l y f a v o u r e d
i n v o l v e s an oxonium y l i d e i n t e r l n e d i a t e ( r e f . 26) which can he d e u t e r a t e d o r undergo a Stevens rearrangement t o a methyl a l k y l e t h e r and t h u s g i v e C2+ o l e f i n s ( r e f . 26).
F i g . 2.
However t h i s i s not a u n i q u e e x p l a n a t i o n .
Oxonium y l i d e mechanism f o r d e u t e r a t i o n and carbon-carbon bond formation.
F i g u r e 3 shows an a l t e r n a t i v e i n t e r p r e t a t i o n i n which p r o t o n t r a n s f e r from a l a t t i c e - h o u n d methyl group t o an o l e f i n l e a d s t o d e u t e r a t i o n o f t h e m e t h y l group (hence o f d i m e t h y l e t h e r ) o r t o homologation o f t h e o l e f i n .
I n t h i s case, however, homologation i s q u i t e d i f f e r e n t t o t h e e l e c t r o p h i l i c m e t h y l a t i o n o f o l e f i n d i s c u s s e d above. m e t h y l i d e group by a l k y l c a t i o n .
I n s t e a d i t comprises a l k y l a t i o n o f a
I f t h i s a l t e r n a t i v e homologation mechanism i s
o p e r a t i v e i t may n o t be c o n f i n e d t o i n i t i a l o l e f i n f o r m a t i o n b u t may compete w i t h t h e p r e s e n t l y accepted homologation mechanism i n f o r m i n g o l e f i n s more generally. I f t h e m e t h y l i d e group i s a l k y l a t e d by p r o t o n a t e d methanol o r an e q u i v a l e n t
w e t h y l a t i n g species, t h e n an e t h o x y l m o i e t y r e s u l t s .
T h i s dehydrates t o
e t h y l e n e , which i s a prominent p r o d u c t a t l o w methanol conversion, i.e.
when
152
F i g . 3.
A l t e r n a t i v e homologation mechanism.
t h e c o n c e n t r a t i o n s o f methanol and d i m e t h y l e t h e r a r e f a r g r e a t e r t h a n t h o s e o f product o l e f i n s
H ~ O + - C H ~+
-cH~-o+(->
CH~-CH~-O+<.->
CYCLOALKANES AND AROMATIC PRODlICTS Under m i l d r e a c t i o n c o n d i t i o n s and p a r t i c u l a r l y o v e r z e o l i t e o f l o w a1 uminium c o n t e n t a t h i g h temperature, t h e p r o d u c t s o f methanol c o n v e r s i o n a r e olefins.
Subsequent, l e s s f a c i l e r e a c t i o n s c o n v e r t t h e sorbed o l e f i n s i n t o
c y c l o a l kanes, and t o benzenoid hydrocarbons and a1 kanes.
These subsequent
r e a c t i o n s i n v o l v e h y d r i d e anion t r a n s f e r r e a c t i o n s and c y c l i z a t i o n r e a c t i o n s o f l a r g e r sorbed o l e f i n s ( r e f . 27).
F i g u r e 4 shows t h e p r o d u c t sequence.
H i g h e r temperature f a v o u r s h y d r i d e t r a n s f e r r e a c t i o n s , b u t a l s o s h i f t s t h e o l e f i n e q u i l i h r i u n towards l i g h t e r o l e f i n s , away from t h e h e a v i e r sorbed o l e f i n s which can l e a d t o c y c l i z e d hydrocarbons.
Thus t h e c o m b i n a t i o n o f h i g h
t e m p e r a t u r e ( 5 0 O O C ) and low A l / S i r a t i o
the z e o l i t e favours exclusive
w3)i n
o l e f i n f o r m a t i o n ( r e f . 28). Figure 5 depicts t h e r o l e o f hydride t r a n s f e r reactions i n the formation o f c y c l i c products.
Formation o f an u n s a t u r a t e d carbenium i o n by h y d r i d e a n i o n
t r a n s f e r from an o l e f i n t o a carbenium i o n can l e a d t o a c y c l o a l k a n e (most
153
commonly a s u b s t i t u t e d c y c l o p e n t a n e ) o r t o a henzenoid hydrocarbon. i o n rearangements, i n c l u d i n g s k e l e t a l 6 * 5
r i n g rearrangements almost The o v e r a l l
c e r t a i n l y occur f r e e l y d u r i n g t h e h y d r i d e t r a n s f e r sequence. r e a c t i o n sequences may be summarised as
Carhenium
-
Cycloalkanes and alkanes a r e n o t e n t i r e l y u n r e a c t i v e o v e r ZSM-5 z e o l i t e . The s m a l l e r , s i m p l e r o f them t e n d t o desorb complex ones t e n d t o be r e t a i n e d and cracked.
as products.
The l a r g e r , more
Thus t h e p r o d u c t s found a r e n o t
a good guide t o t h e s i z e s o f t h e sorbed molecules undergoing c y c l i z a t i o n and hyd r id e t r a n s f e r
I
.
n CHsOH
I
+ "/2 CHaOCHs
Sorbed Olefins
-
+ "/2
H20
Olefin Products
Cycloalkanes, Alkanes + Benzenoid Hydrocarbons
F i g . 4.
I
Products o f methanol conversion.
Xylenes and t r i m e t h y l b e n z e n e s a r e t h e main aromatic products.
These a r e
p r o b a b l y secondary a r o m a t i c products, formed from more h i g h l y s u b s t i t u t e d p r i m a r y products, which undergo l o s s o f a l k y l groups o f
> 2 carbon atoms b y
proto-deal kal k y l a t i o n before desorption.
EFFECT OF AROMATIC ON METHANOL CONVERSION Added benzene, t o l u e n e o r p-xylene a c c e l e r a t e s c o n v e r s i o n o f aqueous methanol a t ca.300".
The e f f e c t i s s m a l l , b u t has a m e c h a n i s t i c o r i g i n r a t h e r
t h a n a physico-chemical one, since, i f "C-labelled
henzene o r t o l u e n e i s used,
154
some of t h e 13C-label produced ( r e f .
17,18).
i s foiind i n t h e e t h y l e n e and o t h e r l i g h t hydrocarbons Furthermore, n o t o n l y t h e a r o m a t i c p r o t o n s h u t a l s o t h e
methyl p r o t o n s o f p-xylene a r e d e i i t e r a t e d by P+l over ZSM-5 zeol i t e a t temperatures >33OoC.
One i s , t h e r e f o r e , tempted t o seek a m e c h a n i s t i c
c o n n e c t i o n between t h e l a h i l i t y o f t h e methyl p r o t o n s and t h e f o r m a t i o n o f carbon-carbon bonds i n t h e presence o f a r o v a t i c s such as p-xylene.
Figure 6
shows a m e c h a n i s t i c scheme, whereby p r o t o n a t i o n / d e p r o t o n a t i o n r e a c t i o n s of p-xylene can l e a d t o s i d e - c h a i n m e t h y l a t i o n t o p - e t h y l t o l u e n e and t h e n t o f o r m a t i o n o f t o l u e n e and e t h y l e n e .
How t h e I 3 C - l a b e l o f t h e benzene r i n g i s
i n c o r p o r a t e d i n t o o l e f i n s and alkanes i s s t i l l a m a t t e r f o r s p e c u l a t i o n ( r e f . 29).
+
CYCIO- CnH2n-1
F i g . 5.
loss of 3 protons hvdride transfe7 to 2 carbenium ions
Cn H2n-6
H y d r i d e anion t r a n s f e r and c y c l i z a t i o n .
CONCLllS I O N It i s now g e n e r a l l y agreed t h a t homologation o f o l e f i n s i s t h e main f e a t u r e
o f c o n v e r s i o n o f methanol t o o l e f i n .
However, b o t h t h e phenomenon and t h e
e v i d e n c e f o r i t a r e c o n s i d e r a h l y c o m p l i c a t e d by 01 e f i n o l i g o m e r i z a t i o n . Homologation has g e n e r a l l y been t h o u g h t t o proceed hy e l e c t r o p h i l i c m e t h y l a t i o n
o f o l e f i n s , b u t an a l t e r n a t i v e mechanism e x i s t s , as d i s c u s s e d above and by Hutchings e t a l .
(ref.
30).
155
F o r m a t i o n of c y c l o a l kanes and a1 kanes and a r o m a t i c hydrocarhons i n v o l v e s There i s evidence t h a t aromatic hydrocarhons
h y d r i d e anion t r a n s f e r r e a c t i o n s .
can p l a y a r o l e i n i n i t i a t i n g conversion. A s e l f - c o n s i s t e n t e x p l a n a t i o n o f a l l t h e c h e m i s t r y can be developed on t h e b a s i s of Rrbnsted a c i d i t y o f t h e z e o l i t e , p r o t o n t r a n s f e r and e l e c t r o p h i l i c m e t h y l a t i o n r e a c t i o n s , and t h e w e l l known rearrangement, o l i g o m e r i z a t i o n and c r a c k i n g r e a c t i o n s o f carbenium i o n s .
-
L
F i g . 6.
C
H
s
V CH2
hence deuteration
H2
Reactions o f a r y l m e t h y l groups.
REFERENCES 1 2 3
4 5 6 7
8
C.D. Chang, C a t a l . Rev.-Sci. Eng., 25 (1983) 1. R.J. Argauer and G.R. L a n d o l t , U.S. Pat. 3,702,886 (1972). P.R. Weisz, i n T. Sayama and K . Tanabe (Ed.) Proc. 7 t h I n t e r n a t i o n a l Conf. on C a t a l y s i s , E l s e v i e r , Amsterdam, 1981, p. 3. S.M. Csicsery, i n J.A. Rabo (Ed.) A.C.S. Monograph 171, " Z e o l i t e Chemistry and C a t a l y s i s " , 1976, p. 680. T.R. F o r e s t e r , S-T. Wang and R.F. Howe, J. Chem. SOC. Chem. Comm. (1986) 1611. Clague, G.R. Hays, R. H u i s and J.P. van den Berg, J.P. Wolthuizen, A.D.H. J.H.C. van Hooff, J. Catal 80 (1983) 130. J.P. van den Berg, J.P. Wolthuizen and J.H.C. van H o o f f , J. C a t a l . 80 (1983) 139. K.G. W i l s h i e r , P. Smart, R. Western, T. Mole and T. Rehrsing, Appl. Catal., (1987) " i n press".
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156
9
W.E. Garwoood, Prepr. Div. Pet. Amer. Chem. SOC., 27(2) (1982) p. 563; W.E. Garwood i n G.0. Stucky and F.G. Owyer (Ed.) " I n t r a z e o l i t e Chemistry", A.C.S. Symposium S e r i e s 218, 1983, p. 383. T. Mole, J. Catal. 103 (1987) 524; R.M. Dessau, i h i d . 526. 10 11 W.O. Haag, R.M. Lago and P.G. Rodewald, J. Molec. Catal., 17 (1982) 161. 1% R.L. Espinoza, Appl. Catal., 26 (1986) 203. 13 N.Y. Chen and W.J. Regan, J. Catal 59 (1979) 123. Y. Ono, E. Imai and T. Mori 2. Phys. Chem. n.f., 115 (1979) 99. 14 15 Y. Ono and T. Wori, J. Chem. SOC. Faraday I , 77 (1981) 2209. 16 T.Mole, J. Catal., 84 (1983) 423. 17 T. Mole, J.A. W h i t e s i d e and D. Seddon, J. Catal., 82 (1983) 261. 18 T. Mole, G, B e t t and D. Seddon, J. Catal., 84 (1983) 435. 19 T. Rehrsing, T. Mole, P. Smart and R.J. Western, J. Catal., 102 (1986) 151. 20 R.W. nessau and R.R. La P i e r r e , J. Catal., 78 (1982) 136. 21 R.P. Dessau, J. Catal., 99 (1986) 111. 22 J. Novakova, L. Kubelkova, K. Habersberger and 2. D o l e j s e k , J. Chem. SOC. Faraday I, 80 (1984) 1457. L. Kubelkova, J. Novakova, and P. J i r u , i n P.A. Jacobs (Ed.), S t u d i e s i n 23 S u r f a c e Science and C a t a l y s i s , 18, E l s e v i e r , Amsterdam, 1984, p. 217. 24 J.K.A. Clarke, R. Darcy, B.F. Hegarty, E. O'Donoghue, V. Amir-Ebrahimi and J.J Rooney, J. Chem. SOC. Chem. Comm., 425 (1986) 425. 25 T. Mole, and J.A. Whiteside, J. Catal., 75 (1982) 284. J.P. van den Perg, J.P. Wolthuizen and J.H.C. van Hooff, i n L.V.C. Rees e t 26 a l . (Ed.) "Proceedings 5 t h Conf. on Z e o l i t e s , " Hayden, London, 1980, p. 649. J.C. Vedrine, P. D e j a i f v e , E.D. Garbowski and E.G. Derouane, i n R. I m e l i k 27 e t a l . (Ed.), 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 , 5, E l s e v i e r , Amsterdam, 1980, p. 29. C.D. Chang, C.T-W. Chu, and R.F. Socha, J. C a t a l . 86 (1984) 289. 28 29 H. Pines, J. Catal. 93 (1985) 205; T. Mole and D. Seddon, i h i d . 207. G.J. Hutchings, F. G o t t s c h a l k , W.V.M. H a l l and R. Hunter, J. Chem. SOC. 30 Faraday I, 83 (1987) 571.
.,
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
METHANOL TO GASOLINE:
157
SPECTROSCOPIC STUDIES OF CHEMISTRY AND CATALYST
R.F. HOWE Chemistry Department, University of Auckland, Private Bag, Auckland (New Zeal and) ABSTRACT A review is given of the use of spectroscopic techniques to investigate the MTG process. Examples are quoted of catalyst characterization, investigation of the first carbon-carbon bond formation, alkene oligomerization and catalyst deactivation through coke formation. 1. INTRODUCTION
This paper will review by means of selected examples the information that can be obtained from spectroscopic studies o f the ZSM-5 zeolite catalyst and the many different reactions occurring during the conversion of methanol to gasoline. With a process as chemically complex as MTG it is hardly necessary to emphasize that all possible means of investigation must be employed to achieve a complete understanding of all aspects of the process at the molecular level. Spectroscopic studies do not replace but rather complement the traditional methods for catalyst characterization and determination of reaction mechanisms by for example analysis of reaction products and use of isotopic tracers. The following questions can in principle be addressed with spectroscopy: ( 1 ) Zeolite synthesis; what are the mechanisms of ZSM-5 synthesis and how do they influence the quality of the catalyst synthesized? (2) Catalyst characterization; what are the structure and composition of the zeolite, and what is the configuration o f the active site for methanol conversion? (3) How do meth’anol and dimethylether interact with the active sites i.e. what species are present in the catalyst in the initial stages of methanol conversion? (4) What are the subsequent reaction pathways leading to the final alkane, alkene and aromatic products? (5) What causes catalyst deactivation? This question concerns both the temporary deactivation associated with coke formation, which can be reversed by oxidative regeneration, and the permanent deactivation which occurs after repeated deactivation-regeneration cycles. The spectroscopic methods which have been most usefully employed to date are nuclear magnetic resonance (NMR) and infrared spectroscopy (nowadays usually Fourier transform infrared spectroscopy, FTIR) Other methods which have been less widely used are ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Two monographs provide general
.
background i n f o r m a t i o n on t h e s e methods ( r e f s . 1,2),
and t h e a p p l i c a t i o n o f NMR
s p e c t r o s c o p y t o z e o l i t e c a t a l y s t s i n p a r t i c u l a r has r e c e n t l y been reviewed i n d e p t h b y Thomas and K l i n o w s k i ( r e f . 3 ) . A complete l i t e r a t u r e s u r v e y i s n o t f e a s i b l e i n t h e space a v a i l a b l e f o r t h e
p r e s e n t a r t i c l e , b u t an a t t e m p t w i l l be made t o i l l u s t r a t e r e c e n t p r o g r e s s i n answering t h e above q u e s t i o n s u s i n g s p e c t r o s c o p i c methods.
2. SYNTHESIS OF ZSM-5 NMR spectroscopy i s i n p r i n c i p l e a p o w e r f u l t e c h n i q u e f o r examining t h e mechanisms b y which z e o l i t e s f o r m under hydrothermal s y n t h e s i s c o n d i t i o n s , s i n c e b o t h 2 9 S i and 2 7 A l a r e observable.
The problem can be d i v i d e d i n t o t h r e e areas:
what a r e t h e S i and A1 s p e c i e s i n i t i a l l y p r e s e n t i n s o l u t i o n (and i n p a r t i c u l a r a r e a l u m i n o s i l i c a t e species and secondary b u i l d i n g u n i t s a l r e a d y p r e s e n t ) , how do t h e s e combine i n t h e g e l phase, and f i n a l l y what o c c u r s d u r i n g c r y s t a l l i zation?
I n t h e case o f ZSM-5 t h e r o l e o f t h e o r g a n i c base i n ' t e m p l a t i n g ' t h e
s t r u c t u r e i s a f u r t h e r key q u e s t i o n .
The a r t i c l e o f Thomas and K l i n o w s k i ( r e f .
3 ) s h o u l d be c o n s u l t e d f o r examples o f r e c e n t s t u d i e s a d d r e s s i n g t h e s e p o i n t s .
3. CHARACTERIZATION OF ZSM-5 The c h a r a c t e r i z a t i o n o f ZSM-5 can be d i s c u s s e d a t two l e v e l s :
the structure
and c o m p o s i t i o n o f t h e u n i t c e l l (and p o s s i b l e v a r i a t i o n s t h r o u g h o u t t h e c r y s t a l ) , and t h e n t h e n a t u r e and c o n f i g u r a t i o n o f t h e a c t i v e s i t e ( s ) w i t h i n each u n i t c e l l . S o l i d s t a t e NMR spectroscopy (
27
A 1 and 2 9 S i ) has now been w i d e l y a p p l i e d t o
s t u d y t h e s t r u c t u r e s o f z e o l i t e l a t t i c e s ( r e f . 3).
I t was r e c o g n i z e d i n e a r l y
s t u d i e s t h a t t h e p r e c i s e p o s i t i o n o f t h e 2 9 S i resonance depended on t h e number
o f n e i g h b o u r i n g A1 atoms ( w h i c h can v a r y between 0 and 4) ( r e f . 4).
The *'Si
spectrum o f h i g h A1 c o n t e n t z e o l i t e s can t h u s show up t o 5 d i f f e r e n t peaks c o r r e s p o n d i n g t o 5 d i f f e r e n t p o s s i b l e S i environments.
ZSM-5 z e o l i t e s c o n t a i n
however l e s s t h a n ca. 6 A1 p e r u n i t c e l l ( S i : A l v a r i e s between 15 and i n f i n i t y ) ,
so t h a t t h e 2 9 S i spectrum u s u a l l y c o n s i s t s o f a s i n g l e peak a t t h e chemical s h i f t c h a r a c t e r i s t i c o f S i w i t h no A1 neighbours.
I t has been found more
r e c e n t l y however t h a t i n v e r y l o w A1 c o n t e n t ZSM-5 t h e *'Si considerable f i n e structure.
F o r example, F y f e
gal. ( r e f .
spectrum can show 5 ) have r e p o r t e d a
spectrum o f a h i g h l y s i l i c e o u s ZSM-5 ( p r e p a r e d by d e a l u m i n a t i o n w i t h HC1) cont a i n i n g a t l e a s t 21 d i s t i n c t peaks, which a r e a t t r i b u t a b l e t o d i f f e r e n t c r y s t a l lographically inequivalent s i t e s i n the z e o l i t e u n i t c e l l .
Although i t i s n o t
y e t p o s s i b l e t o a s s i g n i n d i v i d u a l peaks t o p a r t i c u l a r s i l i c o n s i t e s , t h i s r e s u l t has c l e a r l y demonstrated t h e w e a l t h o f s t r u c t u r a l i n f o r m a t i o n t h a t i s now a v a i l a b l e f r o m v e r y h i g h r e s o l u t i o n "Si
NMR s t u d i e s .
These and o t h e r a u t h o r s
( r e f . 6 ) have shown a l s o t h a t p e r t u r b a t i o n s o f t h e z e o l i t e l a t t i c e caused b y
159
adsorbed organic molecules or even variations in temperature can be followed by monitoring changes in the "Si spectrum. 27Al NMR spectra of ZSM-5 do not show the same high resolution as "Si spectra because of the quadrupole moment of the "A1 nucleus (the second order quadrupole effects which largely determine the line width are inversely proportional to magnetic field strength, so that the best spectra are obtained at the highest possible magnetic fields) (ref. 3). The most widespread use of 27Al NMR to date has been in determining the relative concentrations of tetrahedral and octahedral A1 in the zeolite, since the signals of tetrahedrally and octahedrally coordinated A1 can be clearly separated, even at relatively low resolution. In crystalline ZSM-5 containing no impurities, all A1 should be tetrahedrally coordinated in the zeolite lattice; thus the absence of an octahedral A1 signal can be taken as an indication of a pure well-crystalline sample. Haag -et al. (ref. 7) have shown that there is a direct proportionality between the catalytic activity for hexane cracking and the intensity of the tetrahedral A1 signal in well-crystalline homogenous ZSM-5 samples, indicating that all A1 in such samples is catalytically active. Infrared spectroscopy can be used to monitor the crystallinity of ZSM-5 preparations through observation of bands due to vibrations of the zeolite lattice. This method relies on the fact that bands at 1226 cm-' and 548 cm-' in the spectrum of ZSY-5 appear to be characteristic o f the ZSM-5 structure ("structure sensitive", in the terminology o f Flanigen (ref. 8 ) ) , while others at 1090, 800 and 460 cm-I occur in both crystalline and amorphous aluminosilicates. Flanigen (ref. 8) attributed all of these bands to symmetric and asymmetric stretching vibrations of 0-T-0 groups, where T is either Si or Al. More recently Thomas g al. (ref. 9) have proposed on the basis of frequency shifts observed when other tetrahedral elements are incorporated (e.g. Ga or Fe) that the vibrational modes are better described a Si-0-Si or Si-0-T stretches. Regardless of the exact form of these modes, their use in an empirical manner to define the presence of crystalline ZSM-5 is well established. For example, Jacobs (ref. 10) has shown that crystalline ZSM-5 is formed at an earlier stage in the synthesis than that indicated by X-ray diffraction; this difference is due to the fact that X-ray 0 diffraction is sensitive to crystallinity on the scale o f hundreds of A (tens of unit cells), whereas the structure sensitive infrared lattice bands will appear as soon as single unit cells are formed. The decomposition of the ZSM-5 lattice at high temperatures can also be followed by monitoring the structure sensitive infrared bands. Figure 1 shows a set of spectra from the recent paper of Tallon and Buckley (ref. 11) in which the kinetics of ZSM-5 decomposition were investigated by using FTIR spectroscopy to determine the crystallinity of samples subjected to isothermal annealing. The decomposition of Al-free ZSM-5 (silicalite) was found to follow first order
160
1ooo.c
I\
4
Q)
U
E
d
n
L 0
ul
n
a
1600
I 1200
I 800
I ) 600
Wave number ( crn-') Fig. 1.
Infrared spectra o f ZSM-5 after heating at 1000°C (reproduced with permission from ref. 11).
Fig. 2.
Infrared spectra o f NH4 ZSM-5 heated in flowing nitrogen (a) 25°C; (b) 100°C; (c) 200°C; (d) 500°C; (e) 400°C.
161
k i n e t i c s , whereas i n ' A 1 c o n t a i n i n g samples second o r d e r k i n e t i c s were observed. These a u t h o r s suggested t h a t i n t h e l a t t e r case decomposition i s i n i t i a t e d b y m i g r a t i o n and p a i r w i s e a s s o c i a t i o n of A1 ( i . e . t h e r m a l l y a c t i v a t e d v i o l a t i o n o f Loewenstein's r u l e ) . The q u e s t i o n o f u n i f o r m i t y o f c o m p o s i t i o n w i t h i n a ZSM-5 sample has aroused wide i n t e r e s t . XPS i s a s u r f a c e s e n s i t i v0 e t e c h n i q u e which can d e t e r m i n e t h e Si:A1 r a t i o o f t h e o u t e r l a y e r s (ca. 10 A) o f t h e z e o l i t e c r y s t a l s . The e x t e n t t o which t h e s u r f a c e c o m p o s i t i o n d i f f e r s f r o m t h e b u l k c o m p o s i t i o n appears t o depend on p r e p a r a t i o n c o n d i t i o n s , and a l l t h r e e p o s s i b l e s i t u a t i o n s ( s i l i c o n r i c h s u r f a c e , s i l i c o n d e f i c i e n t s u r f a c e and s u r f a c e c o m p o s i t i o n equal t o b u l k c o m p o s i t i o n ) have been r e p o r t e d ( r e f s . 12-14).
V a r i a t i o n s i n aluminium
d i s t r i b u t i o n have a l s o been probed by h i g h r e s o l u t i o n scanning e l e c t r o n m i c r o scopy ( r e f . 15) and energy d i s p e r s i v e X-ray a n a l y s i s ( r e f . 1 6 ) . C h a r a c t e r i z a t i o n o f t h e a c i d s i t e s w i t h i n ZSM-5 can be performed w i t h b o t h NMR and i n f r a r e d s p e c t r o s c o p i e s .
F i g u r e 2 shows f o r example a s e t o f i n f r a r e d
s p e c t r a measured i n - s i t u d u r i n g t h e a c t i v a t i o n o f an ammonium exchanged ZSM-5 b y 1
h e a t i n g i n f l o w i n g n i t r o g e n . Loss o f b o t h adsorbed H20 (bands a t ca. 3400 cm1 1 and 1640 cm- ) and NH; (band a t 1460 cm- ) o c c u r s a t about 2OO0C, and t h e c o m p l e t e l y a c t i v a t e d z e o l i t e shows two k i n d s o f v(0H) bands a t 3740 and 3610 -1 cm I t i s now w e l l e s t a b l i s h e d t h a t t h e 3610 cm-' band i s due t o a c i d i c
.
The i n t e n s i t y o f t h i s band i s p r o p o r t i o n a l t o
p r o t o n s w i t h i n t h e z e o l i t e pores.
t h e A1 c o n t e n t of t h e z e o l i t e ( r e f . 17), and t i t r a t i o n o f t h e a c i d i c p r o t o n s The 3740 cm-'
w i t h bases causes removal o f t h i s band.
band has been a t t r i b u t e d
t o e i t h e r amorphous i m p u r i t i e s o r s i l a n o l groups t e r m i n a t i n g t h e z e o l i t e c r y s t a l l a t t i c e ( r e f . 17).
We have found t h a t t h i s band i s p e r t u r b e d b y m o l e c u l e s which
a r e t o o l a r g e t o e n t e r t h e z e o l i t e p o r e s a t room t e m p e r a t u r e (e.g.
mesitylene),
and t h a t t h e e x t e r n a l s i l a n o l groups a r e i n c a p a b l e o f p r o t o n a t i n g bases such as p y r i d i n e i.e.
are not strongly acidic.
Two k i n d s o f h y d r o x y l groups have a l s o
been d e t e c t e d i n ZSM-5 by h i g h r e s o l u t i o n 'H NMR spectroscopy ( r e f s . 18,19). (Recent i n f r a r e d e v i d e n c e has been p r e s e n t e d f o r o t h e r t y p e s o f h y d r o x y l groups i n v e r y l o w A1 c o n t e n t ZSM-5) ( r e f . 20). The s t r u c t u r e o f t h e i n t e r n a l a c i d s i t e i n ZSM-5 can be i n f e r r e d f r o m t h e s i m i l a r i t y o f i n f r a r e d and NMR d a t a t o t h o s e o f t h e a c i d i c p r o t o n s i n z e o l i t e HY, where n e u t r o n d i f f r a c t i o n s t u d i e s ( r e f . 21) have shown t h a t t h e p r o t o n i s
l o c a t e d on an o x i d e i o n b r i d g i n g between s i l i c o n and aluminium: H
Si
b
'
'A1
To a c h i e v e f o r m a l charge b a l a n c e t h i s s t r u c t u r e i s u s u a l l y w r i t t e n w i t h a
162
0
C n I
m t
1700
Fig. 3.
Infrared spectra of pyridine chemisorbed in: (b),
1300
(a), fresh HZSM-5;
HZSM-5 containing 2.25 wt % coke from methanol conversion.
163
p o s i t i v e charge on t h e oxygen and a n e g a t i v e charge on t h e aluminium, a l t h o u g h such a d e s c r i p t i o n i s undoubtedly an o v e r s i m p l i f i c a t i o n o f t h e a c t u a l charge distribution. The a c t i v e a c i d s i t e s i n ZSM-5 can a l s o be c h a r a c t e r i z e d s p e c t r o s c o p i c a l l y i n a l e s s d i r e c t manner b y o b s e r v i n g t h e i r i n t e r a c t i o n w i t h b a s i c molecules.
For
example, t h e i n f r a r e d spectrum o f chemisorbed p y r i d i n e shows bands c h a r a c t e r i s t i c of t h e p y r i d i n i u m c a t i o n formed b y r e a c t i o n w i t h Brdnsted ( p r o t o n i c ) a c i d s i t e s , and p y r i d i n e c o o r d i n a t e d t o Lewis a c i d s i t e s ( c o o r d i n a t i v e l y u n s a t u r a t e d A13+).
F i g u r e 3 ( a ) shows t h e spectrum o f p y r i d i n e chemisorbed i n a f r e s h l y
a c t i v a t e d ZSM-5, compared w i t h t h e c o r r e s p o n d i n g spectrum i n F i g u r e 3 ( b ) o f p y r i d i n e i n t h e same z e o l i t e p a r t i a l l y coked a f t e r methanol c o n v e r s i o n ( r e f . 22).
The f r e s h z e o l i t e c o n t a i n s v e r y few Lewis a c i d s i t e s ;
t h e 1454 cm-'
band
o f p y r i d i n e c o o r d i n a t e d t o A13+ i s b a r e l y d e t e c t e d ( t h i s i s a g e n e r a l f e a t u r e o f w e l l - c r y s t a l l i n e ZSM-5 p r e p a r a t i o n s which have n o t been s u b j e c t e d t o any dealumination treatment ( r e f . 17)). cation;
The 1545 cm-'
band i s due t o t h e p y r i d i n i u m
with appropriate normalization o f i n t e n s i t i e s the loss o f acid sites
caused by coke d e p o s i t i o n d u r i n g methanol c o n v e r s i o n can be f o l l o w e d b y monit o r i n g t h e i n t e n s i t y o f t h i s band.
One d i f f i c u l t y w i t h p y r i d i n e as a probe i s
t h a t n o t a l l o f t h e a c i d s i t e s may be counted ( r e f . 23).
The ZSM-5 example i n
F i g u r e 3 c o n t a i n e d 2.7 A1 p e r u n i t c e l l , b u t o n l y 2.2 p y r i d i n e m o l e c u l e s p e r u n i t c e l l were chemisorbed ( a s determined b y t e m p e r a t u r e programmed d e s o r p t i o n ) . S m a l l e r probe m o l e c u l e s such as NH3 d o n ' t s u f f e r t h i s disadvantage;
t h e number
of chemisorbed NH3 m o l e c u l e s i s equal t o t h e number o f A1 p e r u n i t c e l l (and can be f o l l o w e d by o b s e r v i n g an i n f r a r e d band a t 1460 cm-'
due t o NH4+).
We have
a l s o observed a s t e r i c d i s t i n c t i o n between NH3 and p y r i d i n e i n p a r t i a l l y coked samples;
f o r example, no p y r i d i n e c h e m i s o r p t i o n c o u l d be d e t e c t e d i n ZSM-5
c o n t a i n i n g > 10 w t % coke, whereas a s m a l l number o f a c i d s i t e s a c c e s s i b l e t o
NH3 was d e t e c t e d a t coke l e v e l s up t o 23 w t X ( r e f . 22). NMR spectroscopy o f adsorbed probe m o l e c u l e s can a l s o be used t o c h a r a c t e r i z e
a c i d s i t e s i n z e o l i t e s , a l t h o u g h few such s t u d i e s have y e t been r e p o r t e d f o r ZSM-5.
The d i s t i n c t i o n between B r d n s t e d and Lewis a c i d s i t e s seems t o b e
p a r t i c u l a r l y c l e a r i n t h e case o f I 5 N NMR s p e c t r a o f adsorbed bases such as ammonia and p y r i d i n e ( r e f . 2 4), and t h e 31P s p e c t r a o f phosphine probes a l s o show promise ( r e f . 2 5 ) . 4. INTERACTION OF METHANOL AND DIMETHYLETHER WITH ZSM-5
As d i s c u s s e d b y o t h e r a u t h o r s i n t h i s volume, t h e r e a c t i o n s which i n i t i a t e t h e c o n v e r s i o n o f methanol t o g a s o l i n e b y f o r m i n g t h e f i r s t carbon-carbon bonds have a t t r a c t e d c o n s i d e r a b l e i n t e r e s t and s p e c u l a t i o n .
I t s h o u l d i n p r i n c i p l e be
p o s s i b l e t o i d e n t i f y s p e c t r o s c o p i c a l l y t h e s p e c i e s adsorbed i n t h e z e o l i t e a t t h i s p o i n t and hence deduce l i k e l y r e a c t i o n pathways, a l t h o u g h i t s h o u l d be
164
Fig. 4.
Infrared spectra o f :
(a), methanol injected into HZSM-5 (3.6 A1
per unit cell) at 2 5 0 O C ;
(b),
containing 6 A1 per unit cell;
same experiment with HZSM-5 (c),
HZSM-5 (6 A 1 per unit cell) at 2 5 0 O C .
dimethylether injected into
165
emphasized t h a t a c t u a l r e a c t i o n i n t e r m e d i a t e s a r e u n l i k e l y t o be d e t e c t e d spectroscopically. The f i r s t i n f r a r e d s p e c t r a o f methanol i n t e r a c t i n g w i t h ZSM-5 were r e p o r t e d b y Ono and Mori ( r e f . 26), who d e s c r i b e d t h e f o r m a t i o n o f methoxy groups f r o m CD30H and showed t h a t d e s o r p t i o n o f t h e methoxy groups on h e a t i n g was accompanied by C-D bond cleavage.
NMR evidence f o r methoxy group f o r m a t i o n has
L a t e r i n f r a r e d s p e c t r a have a l s o shown t h e f o r m a t i o n
been p r e s e n t e d ( r e f . 27).
of methoxy groups and t h e subsequent appearance of complex s p e c t r a due t o adsorbed r e a c t i o n p r o d u c t s ( r e f s . 28-31). I n a l l o f these p r e v i o u s s p e c t r o s c o p i c s t u d i e s s p e c t r a were n o t r e c o r d e d under r e a c t i o n c o n d i t i o n s .
W i t h a r e a c t i o n sequence as complex as t h a t i n MTG
s p e c t r a r e c o r d e d a t room temperature a f t e r h e a t i n g t h e z e o l i t e i n t h e presence o f methanol a r e l i k e l y t o c o n t a i n c o n t r i b u t i o n s f r o m s t r o n g l y adsorbed r e a c t i o n p r o d u c t s which may obscure t h e species i n i t i a l l y formed.
Although these c o n t r i -
b u t i o n s can be m i n i m i z e d b y h i g h t e m p e r a t u r e d e s o r p t i o n p r i o r t o measuring s p e c t r a , t h i s may a l s o p e r t u r b t h e adsorbed s p e c i e s formed i n i t i a l l y . I n - s i t u measurement o f NMR s p e c t r a a t MTG temperatures under h i g h r e s o l u t i o n c o n d i t i o n s (magic a n g l e s p i n n i n g ) i s a t e c h n i c a l l y d i f f i c u l t experiment which has n o t so f a r been r e p o r t e d f o r ZSM-5.
I t i s however r e l a t i v e l y s t r a i g h t -
f o r w a r d t o measure i n f r a r e d s p e c t r a under r e a c t i o n c o n d i t i o n s . d e s c r i b e d ( r e f s . 32,33)
We have r e c e n t l y
an i n - s i t u FTIR s t u d y o f methanol and d i m e t h y l e t h e r i n
ZSM-5 i n which t h e i n f r a r e d c e l l f u n c t i o n s as a p u l s e m i c r o r e a c t o r ;
i n such a
c e l l s p e c t r a can be r e c o r d e d when p u l s e s o f methanol o r o t h e r r e a c t a n t s f i r s t encounter a f r e s h c a t a l y s t a t v a r i o u s temperatures, m i n i m i z i n g i n t e r f e r e n c e f r o m adsorbed r e a c t i o n p r o d u c t s , w h i l e a t t h e same t i m e a n a l y z i n g desorbed r e a c t i o n p r o d u c t s downstream f r o m t h e c e l l . P u l s e s o f methanol i n j e c t e d i n t o ZSM-5 between 100°C and 200°C undergo v e r y l i t t l e r e a c t i o n w i t h t h e a c i d i c p r o t o n s , b u t do r e a c t w i t h e x t e r n a l s i l a n o l groups ( t h e 3720 cm-’
band i n F i g u r e 2) t o f o r m a methoxy species w i t h
v i b r a t i o n a l f r e q u e n c i e s v e r y c l o s e t o t h o s e formed by c h e m i s o r p t i o n o f methanol on s i l i c a g e l ( r e f . 34).
No hydrocarbon p r o d u c t s a r e d e t e c t e d downstream i n
t h i s temperature range.
I n j e c t i o n o f methanol a t 25OOC however does produce
hydrocarbons ( l i g h t a l k e n e s and alkanes) and a second t y p e o f methoxy s p e c i e s which i s formed f r o m r e a c t i o n w i t h t h e i n t e r n a l a c i d i c p r o t o n s .
Figure 4(a)
shows f o r example an expansion o f t h e v(CH) r e g i o n o f a spectrum o b t a i n e d a f t e r i n j e c t i o n o f a p u l s e o f methanol i n t o ZSM-5 c o n t a i n i n g 3.6 A1 p e r u n i t c e l l a t 250OC;
t h i s c o n t a i n s c o n t r i b u t i o n s f r o m b o t h t y p e s o f methoxy s p e c i e s ( t h e
s p e c i e s formed f r o m t h e s i l a n o l groups a t 2959 cm-’ second a t 2980 and 2868 cm-’.
and 2855 cm-’,
and t h e
The r e l a t i v e c o n c e n t r a t i o n o f t h e second methoxy
s p e c i e s was found t o be p r o p o r t i o n a l t o t h e A1 c o n t e n t o f t h e z e o l i t e ;
the
spectrum i n F i g u r e 4 ( b ) i s t h e same experiment w i t h a ZSM-5 c o n t a i n i n g 6 A1 p e r
166
unit cell. D i m e t h y l e t h e r i n j e c t e d i n t o f r e s h ZSM-5 between 1 0 0 ° C and 2OOOC i s p r o t o n a t e d b y t h e i n t e r n a l a c i d s i t e s ( t h e i n f r a r e d spectrum shows bands c o r r e s p o n d i n g t o v(CH),
v ( 0 H ) and & ( O H ) v i b r a t i o n s o f CH30HCCH3),
detected.
b u t no hydrocarbon p r o d u c t s a r e
A t 2 0 0 ° C and above i n j e c t i o n o f d i m e t h y l e t h e r g i v e s t h e second
methoxy s p e c i e s formed f r o m methanol ( F i g u r e 4 ( c ) ) , and hydrocarbon p r o d u c t s a r e f i r s t d e t e c t e d (propene i s t h e m a j o r c o n s t i t u e n t ) . The second methoxy s p e c i e s was found t o f u n c t i o n as a m e t h y l a t i n g agent. I n j e c t i o n o f a p u l s e o f benzene f o r example a t 25OOC i n t o ZSM-5 p r e v i o u s l y exposed t o methanol o r d i m e t h y l e t h e r causes a decrease i n t h e c o n c e n t r a t i o n o f t h e second methoxy s p e c i e s and t h e appearance o f t o l u e n e as a p r o d u c t downstream f r o m t h e c e l l (benzene a l o n e does n o t r e a c t o v e r ZSM-5 a t t h i s temperature).
S i m i l a r m e t h y l a t i o n was observed w i t h ethene ( f o r m i n g propene).
With
o t h e r alkenes t h e r e s u l t s were l e s s c l e a r c u t , s i n c e t h e s e formed o l i g o m e r i z a t i o n p r o d u c t s i n t h e absence o f methoxy species, b u t a decrease i n t h e i n f r a r e d bands o f t h e second methoxy s p e c i e s c o u l d s t i l l be d e t e c t e d when propene, c y c l o p e n t e n e and I - h e x e n e were i n j e c t e d i n t o z e o l i t e s p r e t r e a t e d w i t h methanol o r d i m e t h y l ether. T h i s m e t h y l a t i n g f u n c t i o n o f t h e second methoxy species, and t h e c o r r e l a t i o n between i t s f o r m a t i o n and t h e f i r s t appearance o f hydrocarbon p r o d u c t s on i n j e c t i o n o f methanol o r d i m e t h y l e t h e r suggests s t r o n g l y t h a t i t p l a y s a p i v o t a l r o l e i n t h e f o r m a t i o n o f t h e f i r s t carbon-carbon bonds.
----
I t s formation from
dimethylether occurs v i a t h e protonated ether: C H ~ O C H+~ ZOH C H ~ O H + C H zo~
----
C H ~ O H + C H zo~
Z O C H ~+ C H ~ O H
I n t h e case o f methanol, a s i m i l a r i n t e r m e d i a c y o f p r o t o n a t e d methanol may be suggested, a l t h o u g h we have n o t d e t e c t e d t h i s s p e c i e s s p e c t r o s c o p i c a l l y i n t h e i n - s i t u p u l s e experiments ( t h e l o w e r p r o t o n a f f i n i t y o f methanol may mean t h a t i t i s p r o t o n a t e d o n l y above 200°C, and i m m e d i a t e l y e l i m i n a t e s H 2 0 ) .
The r e a c t i v e methoxy s p e c i e s formed f r o m methanol and d i m e t h y l e t h e r may be r e g a r d e d as an i n c i p i e n t methyl carbenium i o n i n t h e same way t h a t t h e a c i d i c h y d r o x y l group f r o m which i t i s formed i s an i n c i p i e n t p r o t o n , o r as a m e t h y l oxonium c a t i o n
167
T h i s assignment i s c l o s e t o t h e o r i g i n a l s u g g e s t i o n o f Ono and M o r i ( r e f . 2 6 ) , t h a t methyl carbenium i o n s i n i t i a t e carbon-carbon bond f o r m a t i o n .
There seems
l i t t l e s u p p o r t however f o r t h e i r proposed m e t h y l a t t a c k on t h e carbon o f methanol. M e t h y l a t i o n o f t h e oxygen w i l l f o r m p r o t o n a t e d d i m e t h y l e t h e r Y H ~ + ~ It C H ~ O H
+
CH~OH+CH~
whereas m e t h y l a t i o n o f d i m e t h y l e t h e r w i l l produce t h e t r i m e t h y l o x o n i u m c a t i o n , "CH3'"
+ CH30CH3
+
(CH3)30t
which c o u l d i n t u r n undergo t h e subsequent d e p r o t o n a t i o n and y l i d e f o r m a t i o n proposed by van den Berg ( r e f . 35) and Olah ( r e f . 36).
A t no t i m e have we
d e t e c t e d s p e c t r o s c o p i c a l l y t h e t r i m e t h y l o x o n i u m c a t i o n i n ZSM-5 d u r i n g methanol o r d i m e t h y l e t h e r c o n v e r s i o n ( t h e v i b r a t i o n a l f r e q u e n c i e s o f an a u t h e n t i c t r i methyloxonium c a t i o n a r e a v a i l a b l e f o r comparison ( r e f . 3 7 ) ) , b u t such s p e c i e s (and t h e subsequently formed y l i d e s ) may have l i f e t i m e s t o o s h o r t t o a l l o w detection.
The r e c e n t l y proposed s u r f a c e bound oxonium m e t h y l i d e s p e c i e s ( r e f .
38) i s another a l t e r n a t i v e which cannot be d i s c o u n t e d .
Nevertheless, t h e
s p e c t r o s c o p i c o b s e r v a t i o n o f a r e a c t i v e methoxy s p e c i e s p o i n t s s t r o n g l y towards a c a t i o n i c (carbenium o r oxonium) mechanism f o r f o r m a t i o n o f t h e f i r s t carboncarbon bonds.
5. ALKENE OLIGOMERIZATION Once t h e f i r s t alkenes have been formed f r o m methanol o r d i m e t h y l e t h e r , t h e r e a r e two r e a c t i o n pathways l e a d i n g t o h i g h e r hydrocarbons:
methylation o f
alkenes, which w i l l i n c r e a s e t h e carbon c h a i n l e n g t h one carbon a t a time, and oligomerization,
which w i l l g i v e a more d r a m a t i c i n c r e a s e i n c h a i n l e n g t h .
S p e c t r o s c o p i c e v i d e n c e f o r t h e m e t h y l a t i o n pathway was d e s c r i b e d above;
methy-
l a t i o n has a l s o been shown t o occur w i t h h i g h e r alkenes by Mole, u s i n g i s o t o p i c t r a c e r s ( r e f . 39). Alkene o l i g o m e r i z a t i o n o c c u r s r e a d i l y i n ZSM-5 i n t h e absence o f methanol. I 3 C NMR spectroscopy has been used by van den Berg
gt. ( r e f .
t h e l o w temperature o l i g o m e r i z a t i o n o f l i g h t alkenes.
40) t o f o l l o w
F i g u r e 5 shows f o r
example NMR s p e c t r a o f ethene adsorbed a t 100°C, propene, i s o b u t e n e and 2m e t h y l - I - b u t e n e adsorbed a t room temperature.
The s i m i l a r i t y o f t h e s p e c t r a
suggests t h a t t h e s p e c i e s formed f r o m t h e d i f f e r e n t alkenes have s i m i l a r structures;
t h e most i n t e n s e peak was a t t r i b u t e d t o CH2 groups i n a l i n e a r
hydrocarbon chain, w h i l e t h e s m a l l e r peaks a t l o w e r chemical s h i f t s were t a k e n as evidence f o r b r a n c h i n g o f t h e hydrocarbon c h a i n (e.g.
CH3 groups), which was
suggested t o o c c u r 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 . I n - s i t u i n f r a r e d s p e c t r o s c o p i c s t u d i e s have c o n f i r m e d t h e g e n e r a l NMR c o n c l u s i o n t h a t s i m i l a r s p e c i e s a r e formed f r o m d i f f e r e n t alkenes, b u t show f u r t h e r t h a t o l i g o m e r i z a t i o n i s o c c u r r i n g w i t h i n t h e ZSM-5 p o r e s ( r e f s . 41,42).
168
C.
0
100 -ppm
b.
Fig. 5.
I3C NMR spectra of: (b),
I
0
-pPm
d.
(a),
propene at 25OC;
100
t
ethene in HZSM-5 heated to 100°C;
(c),
isobutene at 25OC;
(d), Z-methyl-
butene-I at 25OC (reproduced with permission from ref. 40).
169
F i g u r e 6 shows f o r example a s e t o f s p e c t r a o b t a i n e d a f t e r i n j e c t i n g a p u l s e o f ethene o n t o ZSM-5 a t v a r i o u s temperatures.
The h i g h e s t c o n c e n t r a t i o n o f ad-
sorbed o l i g o m e r i s o b t a i n e d a t 100°C, and t h i s i s a s s o c i a t e d w i t h complete removal o f t h e a c i d i c h y d r o x y l band a t 3610 crn-I, w i t h o u t any p e r t u r b a t i o n o f 1 t h e e x t e r n a l h y d r o x y l band a t 3720 cmF i g u r e 7 shows an expansion o f t h e 1 v(CH) r e g i o n around 3000 cm- ; t h i s c o n s i s t s o f a t l e a s t 5 o v e r l a p p i n g bands
.
CH2 and CH groups, as shown.
due t o CH3,
The r e l a t i v e i n t e n s i t i e s o f t h e s e 5
bands h a r d l y changed w i t h t h e amount o f ethene i n j e c t e d o r w i t h t e m p e r a t u r e (below 30OOC).
Furthermore, almost i d e n t i c a l s p e c t r a were o b t a i n e d f r o m
propene, butenes, pentenes and hexenes.
o f CH3,
D e t e r m i n a t i o n o f t h e r e l a t i v e numbers
CH2 and CH groups i n t h i s common oligorner species r e q u i r e s c a l i b r a t i o n
o f t h e i n f r a r e d e x t i n c t i o n c o e f f i c i e n t s , which i s n o t y e t f e a s i b l e , b u t comparison w i t h model compounds suggests t h a t t h e o l i g o m e r s h o u l d be d e s c r i b e d as a s l i g h t l y branched hydrocarbon chain, c e r t a i n l y much l e s s branched t h a n t h e c o r r e s p o n d i n g s p e c i e s formed i n l a r g e r p o r e z e o l i t e s (e.g.
HY).
Formation o f
t h e o l i g o m e r f r o m ethene occurs v i a i n i t i a l p r o t o n a t i o n t o f o r m a carbenium c a t i o n which must v e r y q u i c k l y r e a c t w i t h f u r t h e r ethene t o f o r m t h e o l i g o m e r + etc + ZOH + C2H4 ZO- C2H5 + ZO- CnH2n+l
.
-+
I n j e c t i o n o f ethene a t 15OOC o r above produces a new i n f r a r e d band a t about
1500 cm-'
( F i g u r e 6), which i s i n t h e f r e q u e n c y range c h a r a c t e r i s t i c o f
p a r t i a l l y u n s a t u r a t e d C-C bonds.
Alkanes a r e d e t e c t e d i n t h e gas phase above
z e o l i t e s heated i n t h e presence o f ethene, and an i n t e n s e a b s o r p t i o n band appears i n t h e u l t r a v i o l e t - v i s i b l e spectrum o f
ZSM-5 on h e a t i n g i n ethene t o
100°C a t 280 nm which has been a t t r i b u t e d t o an a l l y l carbenium c a t i o n ( r e f s . 43, 4 4 ) : ( F i g u r e 8 )
(11
R--cH--c~--cH--R~
F o r m a t i o n o f such a s p e c i e s f r o m alkenes r e q u i r e s h y d r i d e a b s t r a c t i o n s i n c e ZSM-
5 does n o t n o r m a l l y c o n t a i n Lewis a c i d s i t e s , t h i s must i n v o l v e a l k y l carbenium cations:
++
R
R-CH=CH-CH2-R'
-+
RH
+
(I)
F o r m a t i o n o f I i s t h u s t h e f i r s t s t e p i n t h e hydrogen t r a n s f e r process t h a t i s necessary t o produce a r o m a t i c s (and alkanes) f r o m alkenes.
The 1500 cm-'
i n f r a r e d band may be a s s o c i a t e d w i t h ( I ) , a l t h o u g h t h a t has n o t y e t been proved. The u v - v i s i b l e s p e c t r a o f ethene i n ZSM-5 change i n a complex manner on h e a t i n g t o h i g h e r temperatures.
Vedrine
G. ( r e f .
43) have suggested t h a t
p r o t o n a b s t r a c t i o n f r o m t h e a l l y l c a t i o n would y i e l d a d i e n e which c o u l d undergo f u r t h e r hydride a b s t r a c t i o n t o form a d i e n y l cation.
Such c a t i o n s t y p i c a l l y
absorb a t ca. 400 nm, and a band a t t h i s wavelength can be d e t e c t e d i n t h e spectrum o f adsorbed ethene a f t e r h e a t i n g t o 200OC.
C y c l i z a t i o n o f f o r example
170
d
2 m
d
Fig. 6.
I n f r a r e d s p e c t r a o f ethene i n j e c t e d i n t o HZSM-5 a t v a r i o u s temperatures.
t
3iL
Fig. 7.
3021.6
2955.2
2888.8
2822.4
2756
D e c o n v o l u t i o n o f v(CH) r e g i o n f o r ethene i n HZSM-5 a t 100°C.
171
'i"-'i 190
820 nm
C H 2 4
820 nm
190
Fig. 8.
U V - v i s i b l e s p e c t r a o f ethene
(b),
100°C;
(c),
200OC;
in HZSM-5 a t : (d),
(a),
25OC;
225°C f o r 20 hours;
(e), 325OC.
172
t h e p e n t a d i e n y l c a t i o n would y i e l d a c y c l o p e n t e n y l c a t i o n which c o u l d i s o m e r i z e t o c y c l o h e x e n y l ; f u r t h e r hydrogen t r a n s f e r r e a c t i o n s would produce u l t i m a t e l y a r o m a t i c s . A l t h o u g h v a r i o u s bands i n t h e u v - v i s i b l e s p e c t r a above 400 nm may be t e n t a t i v e l y assigned t o c y c l i c species, more d e t a i l e d s p e c t r o s c o p i c s t u d i e s i n which a c o m b i n a t i o n o f methods ( i n f r a r e d , u v - v i s i b l e and NMR s p e c t r o s c o p i e s ) a r e b r o u g h t t o bear upon i d e n t i c a l samples under i n - s i t u c o n d i t i o n s a r e needed b e f o r e a complete m o l e c u l a r d e s c r i p t i o n o f t h e a r o m a t i z a t i o n process can be given.
O t h e r alkenes (propene, butenes, pentenes and hexenes) g i v e v e r y s i m i l a r
u v - v i s i b l e s p e c t r a t o t h o s e d e s c r i b e d f o r ethene, w i t h t h e i m p o r t a n t d i f f e r e n c e t h a t l o w e r temperatures a r e needed t h a n f o r ethene t o observe t h e v a r i o u s t r a n s formations.
I t i s c l e a r t h a t i n o v e r a l l terms o l i g o m e r i z a t i o n proceeds v i a
i n i t i a l p r o t o n a t i o n and a l k y l c a t i o n f o r m a t i o n , f o l l o w e d by hydrogen t r a n s f e r r e a c t i o n s and e v e n t u a l c y c l i z a t i o n and a r o m a t i z a t i o n . D u r i n g methanol c o n v e r s i o n t h e o l i g o m e r i z a t i o n c h e m i s t r y d e s c r i b e d above w i l l be o c c u r r i n g i n p a r a l l e l w i t h m e t h y l a t i o n . t h e m e t h y l a t i o n r o u t e dominates;
Mole ( r e f . 45) has suggested t h a t
f u r t h e r i n - s i t u spectroscopic studies o f t h e
t y p e d e s c r i b e d above a r e needed t o c o n f i r m t h i s p o i n t b y o b s e r v a t i o n o f s p e c i e s adsorbed on t h e c a t a l y s t s u r f a c e .
I t i s c e r t a i n l y c l e a r t h a t t h e low
t e m p e r a t u r e o l i g o m e r formed f r o m alkenes w i l l n o t s u r v i v e a t methanol c o n v e r s i o n temperatures, b e i n g c r a c k e d and/or undergoing hydrogen t r a n s f e r and m e t h y l a t i o n reactions. 6. COKE FORMATION DURING MTG S p e c t r o s c o p i c methods have a l s o been u s e f u l l y a p p l i e d t o t h e problem o f coke f o r m a t i o n d u r i n g MTG, a d d r e s s i n g q u e s t i o n s such as:
what i s t h e chemical con-
s t i t u t i o n o f t h e coke, where i s i t formed, how i s i t formed and how can i t be removed?
I 3 C NMR s p e c t r a o f coked z e o l i t e s show a complex p a t t e r n o f s i g n a l s
due t o a r o m a t i c and a l k y l carbons, and some oxygenates ( r e f s . 46,47),
suggesting
t h a t m e t h y l aromatics, p r o b a b l y p o l y n u c l e a r , a r e a m a j o r coke c o n s t i t u e n t (coke i n t h i s c o n t e x t i s d e f i n e d as a l l m a t e r i a l r e m a i n i n g i n t h e z e o l i t e a f t e r f l u s h i n g w i t h n i t r o g e n a t MTG t e m p e r a t u r e s ) .
The i n f r a r e d s p e c t r a o f coked ZSM-
5 a l s o show c l e a r l y t h e presence o f m e t h y l a r o m a t i c s e.g.
v(CH) bands o f
a r o m a t i c r i n g s above 3000 cm-',
o f m e t h y l groups a t t a c h e d t o a r o m a t i c r i n g s , and v(CC) bands o f a r o m a t i c r i n g s (e.g. 1510 cm- 1 ). The i n f r a r e d s p e c t r a c a n n o t be accounted f o r i n terms o f a s i n g l e coke "molecule", b u t must be due t o a m i x t u r e o f m e t h y l a t e d p o l y n u c l e a r a r o m a t i c s which a r e t o o l a r g e t o escape f r o m t h e z e o l i t e pores, w i t h p o s s i b l y some s m a l l e r molecules t r a p p e d w i t h i n t h e pores. The s p e c t r a a l s o show evidence f o r a more g r a p h i t i c m a t e r i a l ;
a marked i n c r e a s e
i n absorbance a t t h e h i g h f r e q u e n c y end o f t h e i n f r a r e d spectrum i s p a r t o f a broad e l e c t r o n i c a b s o r b t i o n band i n t h e n e a r i n f r a r e d r e g i o n which i s a l s o seen i n u v - v i s i b l e s p e c t r a o f coked z e o l i t e s .
173
The q u e s t i o n o f where coke f o r m a t i o n o c c u r s has evoked some d i s c u s s i o n i n t h e l i t e r a t u r e . E a r l y suggestions were t h a t coke d e p o s i t i o n 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 , t h e r e b y b l o c k i n g p o r e s ( r e f . 48). An i n c r e a s i n g body o f e v i d e n c e now p o i n t s however t o i n t e r n a l coke formation,
at least i n i t i a l l y .
A
p a r t i c u l a r l y c o n v i n c i n g s p e c t r o s c o p i c d e m o n s t r a t i o n i s t h e r e c e n t XPS s t u d y of
B i b b y Cfi. ( r e f . 49). XPS i s a s u r f a c e s e n s i t i v e t e c h n i q u e which can d e t e c t 0 s p e c i e s o n l y i n t h e outermost 10-20 A o f z e o l i t e c r y s t a l s . A t l o w coke l e v e l s ,
t h e carbon t o s i l i c o n r a t i o measured by XPS i s c l o s e t o t h a t c a l c u l a t e d ( f r o m t h e b u l k chemical c o m p o s i t i o n ) f o r a u n i f o r m d i s t r i b u t i o n o f carbon t h r o u g h o u t t h e z e o l i t e i.e. interior.
t h e s u r f a c e carbon c o n c e n t r a t i o n i s no h i g h e r t h a n t h a t i n t h e
Only a t h i g h coke l e v e l s does t h e C : S i r a t i o measured b y XPS i n c r e a s e
d r a m a t i c a l l y , i n d i c a t i n g coke d e p o s i t i o n on t h e e x t e r n a l s u r f a c e .
The p o i n t a t
which t h i s occurs corresponds t o c e s s a t i o n o f hydrocarbon p r o d u c t i o n (ca. 15% coke i n t h e p a r t i c u l a r c a t a l y s t s t u d i e d ) .
The s p e c t r o s c o p i c measurements o f bases adsorbed i n coked z e o l i t e s r e f e r r e d t o above a l s o s u p p o r t t h i s p i c t u r e o f i n t e r n a l coke f o r m a t i o n ; t h e r e i s an i n i t i a l s t e e p d r o p i n a c i d s i t e concen-
t r a t i o n w i t h i n c r e a s i n g coke c o n t e n t , f o l l o w e d b y a more g r a d u a l d e c l i n e a t h i g h e r coke l e v e l s ( r e f . 22). I n - s i t u FTIR s t u d i e s o f coke p y r o l y s i s and o x i d a t i o n a r e d e s c r i b e d elsewhere i n t h i s volume ( r e f . 50).
P y r o l y s i s (heating i n f l o w i n g nitrogen) evolves
m o s t l y methane and hydrogen, due t o d e m e t h y l a t i o n and g r a p h i t i z a t i o n o f t h e a l k y l a r o m a t i c coke molecules, and a c i d s i t e s a r e n o t r e s t o r e d .
Heating i n
f l o w i n g a i r , on t h e o t h e r hand, o x i d i z e s t h e coke t o COP and H20, and t h e c h e m i s t r y o f t h i s process has been m o n i t o r e d b y FTIR. A s i n g l e r e g e n e r a t i o n o f a coked z e o l i t e b y o x i d a t i o n r e s t o r e s a l m o s t a l l o f t h e o r i g i n a l a c i d s i t e s (depending on t h e e x t e n t o f c o k i n g ) .
Published c a t a l y s t
performance d a t a ( r e f . 51) i n d i c a t e t h a t r e p e a t e d c o k i n g and r e g e n e r a t i o n c y c l e s l e a d e v e n t u a l l y t o a d e c l i n e i n a c t i v i t y , presumably due t o steam-induced dealurnination o f t h e z e o l i t e .
Steam d e a l u m i n a t i o n o f ZSM-5 has been s t u d i e d by 2 7 A l
NMR s p e c t r o s c o p y ( r e f . 52) b u t n o t under MTG c o n d i t i o n s .
7. CONCLUDING REMARKS Undoubtedly, t h e l a r g e s t c h a l l e n g e r e m a i n i n g i n t h e a r e a o f s p e c t r o s c o p i c s t u d i e s of MTG i s t h e problem o f i d e n t i f y i n g t h e adsorbed s p e c i e s p r e s e n t i n t h e c a t a l y s t under s t e a d y s t a t e MTG c o n d i t i o n s , and t h e r e a c t i o n s which t h e y undergo. The key t o s o l v i n g t h i s problem w i l l be t h e simultaneous a p p l i c a t i o n o f s e v e r a l s p e c t r o s c o p i c t e c h n i q u e s i n an i n - s i t u manner. T h i s i s a l r e a d y f e a s i b l e w i t h FTIR and u v - v i s i b l e s p e c t r o s c o p i e s , as d e s c r i b e d above; and t h e t e c h n i c a l d i f f i c u l t i e s o f measuring NMR s p e c t r a a t e l e v a t e d t e m p e r a t u r e s s h o u l d a l s o be s o l u b l e . The rewards o f such s t u d i e s w i l l be an u n d e r s t a n d i n g n o t o n l y o f r e a c t i o n pathways and p r o d u c t d i s t r i b u t i o n s i n t h e u s u a l MTG process, b u t
174
a l s o o f why and how p r o d u c t d i s t r i b u t i o n s v a r y when t h e c a t a l y s t i s m o d i f i e d i n v a r i o u s ways, o r when t h e r e a c t i o n c o n d i t i o n s a r e v a r i e d , as i n processes such as MTO o r MOGD. ACKNOWLEDGEMENTS The work d e s c r i b e d here c a r r i e d o u t a t t h e U n i v e r s i t y o f Auckland was performed b y r e s e a r c h s t u d e n t s T.R.
F o r e s t e r , S.T.
supported i n p a r t b y a r e s e a r c h c o n t r a c t f r o m D S I R .
Wong and
G.D.
McLellan, and
An on-going c o l l a b o r a t i o n
w i t h C h e m i s t r y D i v i s i o n D S I R i n t h i s area i s a l s o g r a t e f u l l y acknowledged. REFERENCES
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W.N. D e l g l a s s , G. H a l l e r , R. K e l l e r m a n and J.H. Lunsford, "Spectroscopy i n Heterogeneous C a t a l y s i s " , Academic Press, 1979. J.R. Anderson and K.C. P r a t t , " I n t r o d u c t i o n t o C h a r a c t e r i z a t i o n and T e s t i n g o f C a t a l y s t s " , Academic Press, 1985. J.M. Thomas and J. K l i n o w s k i , Advan. Catal., 1985, 33, 199. J. K l i n o w s k i , S. Ramdas and J.M. Thomas, J. Chem. SOC., Faraday Trans. 11, 1983, 78, 1025. C.A. Fyfe, J.H. O ' B r i e n and H. S t r o b l , Nature, 1987, 326, 281. D.G. Hay, H. Jaeger and G.W. West, J. Phys. Chem., 1985, 89, 1070. W.O. Haag, R.M. Lago and P.B. Weisz, Nature, 1984, 309, 589. E.M. F l a n i g e n , H. Khatami and H.A. Szymanski, Advan. Chem., 1971, 101, 201. R. Szostak and T.L. Thomas, J. Catal., 1986, 101, 549. P.A. Jacobs, E.G. Derouane and J. Weitkamp, J. Chem. SOC., Chem. Commun., 1981 , 591. J.L. T a l l o n and R.G. Buckley, J. Phys. Chem., 1987, 91, 1469. S.L. Suib, E.D. S t u c k y and R. B l a t t n e r , J. Catal., 1980, 65, 174. E.G. Derouane, J.P. G i l s o n , Z. Gabelica, C. Mousty-Desbuquoit, J. V e r b i s t , J. Catal., 1981, 71, 447. G. Debras, A. Gourgue, J.B. Nagy and G. De C l i p p e t e i r , Z e o l i t e s , 1985, 5, 369. J.M. Thomas and G.R. M i l l w a r d , J. Chem. SOC., Chem. Commun., 1982, 1383. R. Von Ballmoos and W.M. Meier, Nature, 1981, 289, 782. P.A. Jacobs and R. Von Ballmoos, J. Phys. Chem., 1982, 86, 3050. K. S c h o l l e , W.S. Veeman, J.G. P o s t and J.H.C. van Hoof, Z e o l i t e s , 1983, 3, 214. K. S c h o l l e , A.P.M. Kentgens, W.S. Veeman, P. Frenken and G.P.M. van d e r Velden, J. Phys. Chem., 1984, 88, 5. G.L. Woolery, L.B. Alemany, R.M. Dessau and A.W. Chester, Z e o l i t e s , 1986, 6, 14. V. Bosack, S. Beran and Z. J i r a k , J. Phys. Chem., 1981, 85, 3856. G.D. McLellan, R.F. Howe, L.M. P a r k e r and D.M. Bibby, J. Catal., 1986, 99, 486. A. Auroux, V. B o l i s , P. Wierzchowski, P.C. G r a v e l l e and J.C. Vedrine, J. Chem. SOC., Faraday Trans. I , 1979, 75, 2544. J.F. Haw, I.S. Chuang, B.L. Hawkins and G.E. M a c i e l , 3. Am. Chem. SOC., 1983, 105, 7206. J.H. Lunsford, W.P. R o t h w e l l and W. Shen, J. Am. Chem. SOC., 1985, 107, 1540. Y. Ono and T. Mori, J. Chem. SOC., Faraday Trans. I , 1981, 77, 2209. E.G. Derouane, J.P. G i l s o n and J.B. Nagy, Z e o l i t e s , 1982, 2, 42. L. Kubelkova, J. Novakova and P. J i r u i n " S t r u c t u r e and R e a c t i v i t y o f Modif i e d Z e o l i t e s " (P.A. Jacobs e t al., eds), E l s e v i e r , Amsterdam, 1984, p. 217. J. Novakova, L. Kubelkova a n d Z T D o l e j s e k , J. Catal., 1986, 97, 277. M. Sayed and R.P. Cooney, Aust. 3. Chem., 1982, 35, 2483.
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J. Haber, J. Komorek-Hlodzik and T. Romotowski, Z e o l i t e s , 1982, 2, 179. T.R. F o r e s t e r , S.T. Wong and R.F. Howe, J. Chem., SOC. Chem. Commun., 1986, 1611. T.R. F o r e s t e r and R.F. Howe, J. Am. Chem. SOC., i n p r e s s (1987). B.A. Morrow, J. Chem. SOC., Faraday Trans. I , 1974, 70, 1527. J.P. van den Berg, J. Wolthuizen and J. van Hoof, i n "Proceedings o f 5 t h I n t e r n a t i o n a l Conference on Z e o l i t e s " (L.V.C. Rees ed.), Heyden, London, 1980, p. 649. G.A. Olah, Pure A p p l i e d Chem., 1981, 53, 201. R.F. Howe and M.J. T a y l o r , Spectrochim. Acta, 1987, 43A, 73. G.J. Hutchings, F. G o t t s c h a l k , M.V.M. H a l l and R. Hunter, J. Chem. SOC., Faraday Trans. I , 1987, 83, 571. T. Behrsing, T. Mole, P. Smart and R.J. Western, J. Catal., 1986, 102, 151. J.P. van den Berg, J.P. Wolthuizen, A.D.H. Clague, G.R. Hays, R. H u i s and J.H.C. van Hoof, 3. Catal., 1983, 80, 130. A. Ghosh and R.A. Kydd, J. Catal., 1986, 100, 185. S.T. Wong and R.F. Howe, t o be p u b l i s h e d . J.C. Vedrine, P. D e j a i f v e , E.D. Garbowski and E.G. Derouane, i n " C a t a l y s i s by Z e o l i t e s " (6. I m e l i k e t al., eds.), E l s e v i e r , Amsterdam, 1980, p . 29. G.D. M c L e l l a n and R.F. H G e F t o be p u b l i s h e d . T. Mole, t h i s volume, p D.M. B i b b y and R. Meinhold, t o be p u b l i s h e d . L. C a r l t o n , R.G. Copperthwaite, G.J. Hutchings and E.C. Reynhardt, J. Chem. SOC., Chem. Commun., 1986, 1008. P. D e j a i f v e , A. Auroux, P. G r a v e l l e , J.C. Vedrine, Z. G a b e l i c a and E.G. Derouane, J. Catal., 1981, 70, 123. D.M. Bibby, A.E. Hughes and B. Sexton, J. Catal., i n p r e s s (1987). G.D. McLellan, D.M. B i b b y and R.F. Howe, t h i s volume, p S. Yurchak, t h i s volume, p C.A. Fyfe, G.C. Gobbi and G.J. Kennedy, J. Phys. Chem., 1984, 88, 3248.
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177
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
A RE-EXAMINATION OF EVIDENCE FOR CARBENE (CH2:) AS AN INTERMEDIATE I N THE CONVERSION OF METHANOL TO GASOLINE.
D.V.
DASS, R.W.
MARTIN, A.L.
THE EFFECT OF ADDED PROPANE
ODELfand G.W. QUINN
Urey Radiochemical Laboratory, 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 1
ABSTRACT Evidence i n favour o f carbene intermediacy i n the conversion o f methanol t o gasoline over H-ZSM-5 i n v o l v i n g a decrease i n t h e abundance r a t i o o f isobutane t o t h a t o f n-butane, observed on adding propane t o the feed, has been reviewed. The lowering o f t h e i / n r a t i o may be accounted f o r i n p a r t by conversion o f propane y i e l d i n g butanes, predominantly n-butane. An a d d i t i o n a l e f f e c t i s the e x a l t a t i o n o f i / n r a t i o on adding helium as d i l u e n t i n the c o n t r o l sample. A t l e a s t the greater p a r t o f the i / n lowering e f f e c t may be a t t r i b u t e d t o these two causes. INTRODUCTION Methylation i s an important r e a c t i o n i n causing carbon chain growth d u r i n g the conversion o f MeOH t o gasoline over the z e o l i t e c a t a l y s t H-ZSM-5.
The
nature o f the methylating agent i s uncertain, the choice l y i n g between ( i ) carbenium i o n CH3
,
i
(ii
carbene CH2:,
and ( i i i 1 methyl oxonium ion.
Chang and Chu ( r e f . 1 ) added propane ( 3 moles) t o converting methanol and found t h a t the y i e l d r a t i o o f i-butane t o n-butane, i/n, changed from 3.8 t o 1.1 on t h i s addition.
Helium was used instead o f propane i n the c o n t r o l run.
They
argue t h a t carbene methylates by an i n s e r t i o n r e a c t i o n i n which i t attacks C-H l i n k s a t random so t h a t propane, having 6 primary C-H l i n k s and 2 secondary C-H l i n k s , should y i e l d i / n = 2/6 o r 1/3.
The f a c t t h a t i / n i a l l s on a d d i t i o n o f
propane i s then due t o methylation by carbene. Support f o r t h i s view has been o f f e r e d from studies using 13C methanol (90 % 13C) which y i e l d e d butanes having from 1 t o 4 atoms o f 13C.
The
i n t e r p r e t a t i o n o f t h i s evidence i s complex and has been challenged ( r e f . 2) and answered ( r e f . 3 )
One aspect t h a t has n o t been discussed i s as f o l l o w s :
I n the methylation o f propane by 13C (90 % ) methanol, the r e s u l t i n g butanes must t h e o r e t i c a l l y show 13C012C4,
10 % and 13C112C3,
90 %. The observed values
were 8 % and 11 %, respectively, f o r i-butane and 11 % and 1 7 % f o r n-butane. A d i f f e r e n t route i n v o l v i n g 12C, which can o n l y come from propane, must
production than does t h e m e t h y l a t i o n o f propane.
To m a i n t a i n t h e r e q u i r e d 9 : l
r a t i o f o r b o t h isomers, t h e maximum y i e l d o f 13C012C4
t h a t c o u l d a r i s e from
simple m e t h y l a t i o n i s 11/9 = 1.2 % f o r i-butane and 17/9 = 1.9 % f o r n-butane. Hence i t would appear t h a t ca 98 % o f l 3 C O l 2 C 4 butanes must come from propane a1 one. Chang and Chu claim, however, t h a t propane i s " v i r t u a l l y i n e r t " and s u f f e r s ca 0.3 % conversion only.
We have shown ( r e f . 4) t h a t propane s u f f e r s h i g h e r
conversion y i e l d i n g butanes. EXPERIMENTAL The equipment used has been described ( r e f s . 5-6).
Three ZSM-5 type
c a t a l y s t s were used: # 1 Si/A1 36.3
donated by DSIR NZ
#2 Si/A1 27.0* donated by Mobil synthesised according t o p a t e n t U.S. #4,257,885,
#3 Si/A1 16.0
supplied
by D r R. Howe, Auckland U n i v e r s i t y . RESULTS and DISCUSSION Product y i e l d s f o r conversion o f pure propane over t h e 3 c a t a l y s t s a r e i n Table 1 where i / n values f o r butanes a r e seen t o be (1.
Product y i e l d s from
conversion o f MeOH/He (1:3 moles) and o f MeOH/propane (1:3 moles) a r e i n Table 2. TABLE 1 Cracking and conversion o f propane over t h r e e H-ZSM-5 C a t a l y s t s a t 380 "C. Catalyst
Propane MHSV,
C1
C;
-
C i
Products ( w t % ) Ci
-
Cj
i-C4
n-C4
i/n
hr-'. #1 #3 #2 #2
0.55 0.55 0.55 1.65
%
Conv. 0.73 1.15 0.66 0.29
<0.1 1.37 0.52 0.27
0.69 0.48 0.52 0.38
97.02 91.27 96.34 97.53
1.56 0.43 0.45 0.53
0 1.90 0.46 0.30
0 3.36 1.04 0.68
-
0.57 0.44 0.44
2.98 8.73 3.66 2.47
New c a t a l y s t was found t o g i v e lower i / n r a t i o from c o n v e r t i n g methanol than c a t a l y s t t h a t had already converted a q u a n t i t y o f methanol, making i t e s s e n t i a l t o conduct comparative experiments c l o s e together i n the c a t a l y s t ' s h istory.
A l l t h r e e c a t a l y s t s gave 100 % conversion o f methanol t o gasoline b u t a l l
* Mean o f 5 analyses u s i n g E.D.A.X., M.A.S.N.M.R., X.R.F., which we a r e indebted t o D r R.F. Howe and h i s students.
A.A.S.
& X.P.S.
for
179
TABLE 2 Comparison o f m e t h a n o l / h e l i u m and methanol/propane c o n v e r s i o n s o v e r t h r e e H-ZSM-5 c a t a l y s t sa ye Catalyst
#1
Feeds
#3
#2
#2
MeOH/
MeOH/
MeOH/
MeOH/
MeOH/
MeOH/
MeOH/
MeOH/
He
Prop.
He
Prop.
He
Prop.
He
Prop.
0.44
0.44
0.44
0.44 1.65
0.44 1.65 1:3
0.55
0.98 (517) 0.56 (2971 7.13 (3760)
1.64 (1044) 1.35 (867) 5.85 (3726)
-~
MeOH r a t e b prop, He r a t e b Molar r a t i o Prods. (wt%)Cs Methane Ethane
E t h y 1 ene Propane P r o p y l ene
0.55
0.52 (291) 0.34 (191) 8.03 (4503)
-
0.55
1:l
0.67 (365) 0.51 (276) 6.99 (3789)
-
0.44
0.55
0.55
1:l
2.07 (934) 2.73 (1229) 3.26 (1471)
-
4.35 (2234) 5.92 (3039) 3.56 (1824)
-
-
0.44
1.14 (542) 1.00 (474) 4.98 (2359)
-
0.44
0.55
1:l
1.97 (1010) 2.17 (1108) 4.45 (2289)
6.93 6.61 3.55 10.74 10.10 15.24 16.44 2.68 (8552) (8909) (1208) (1823) (5668) (6433) (3281) (3395) 9.12 12.84 17.09 19.63 16.72 17.17 i-Butane 8.11 12.70 (4550) (6881) (4112) (6586) (9014) (12499) (7921) (8827) 3.48 8.38 5.90 10.33 2.52 4.93 11.16 n-Butane 1.48 (830) (1366) (2220) (5728) (1835) (5335) (2793) (5311) C5-7 a l i p h s & ) 66.28 60.17 75.21 58.62 60.02 53.04 63.34 57.31 c6-10 aroms. )(37221) (32680) (33893) (30073) (31664) (33772) (30007) (29456) i/n ratio
5.48
5.04
1.15
1.85
4.91
2.34
2.83
1.66
Note: aTemperature i s 380 'C; bRate i s MHSV i n h r - l ; CResults a r e n o r m a l i s e d on propane f r e e b a s i s ; dRaw i n t e g r a t e d peak a r e a s i n a r b i t r a r y u n i t s shown i n parentheses; e c o n v e r s i o n o f methanol i s 100 % i n a l l cases. were n o t e q u a l l y e f f e c t i v e i n showing r e d u c t i o n o f i / n on a d d i t i o n o f propane t o feed.
H-ZSM-5 #1 shows 8 % r e d u c t i o n , #2 shows 41.0 % r e d u c t i o n and #3
shows 37.8 % r e d u c t i o n .
The c o n v e r s i o n o f methanol t o g a s o l i n e i s t h u s
independent o f t h e r e a c t i o n t h a t causes l o w e r i n g o f i / n . Pure propane o v e r #1 c a t a l y s t g i v e s z e r o y i e l d o f butanes, o v e r #2 c a t a l y s t
1.5
% butanes and o v e r
#3 c a t a l y s t 5.25 % butanes a t propane MHSV
= 0.55 hr-1.
Pure propane o v e r c a t a l y s t #2 g i v e s 0.98 % butanes a t MHSV = 1.65 h r - l . Conversion percentages a r e a t l e a s t one o r d e r o f magnitude g r e a t e r t h a n i n e a r l i e r r e p o r t s ( r e f . 1) W h i l e a d d i t i o n o f propane t o c o n v e r t i n g methanol r a i s e s y i e l d s o f butanes i n a l l cases, t h e i / n r a t i o was l o w e r e d o n l y i n t h e case o f #2 and #3 c a t a l y s t s , w h i c h a l s o c o n v e r t propane t o butanes
-
p r e d o m i n a n t l y n-butanes.
Conversion of
propane c l e a r l y c o n t r i b u t e s t o t h e l o w e r i n g o f i / n on these c a t a l y s t s . Whether t h e whole o f t h e i / n l o w e r i n g e f f e c t i s caused by such c o n v e r s i o n i s more d i f f i c u l t t o e s t a b l i s h .
An e s t i m a t e o f t h e r e l a t i v e c o n t r i b u t i o n o f
180
methanol conversion and propane conversion has been made using the f a c t t h a t the l a t t e r leads t o an enhanced y i e l d o f methane (Table 11. The r i s e i n methane y i e l d i s used as a measure o f propane conversion.
Assuming t h a t these
two conversions account f o r a l l t h e butanes formed, we estimate the i / n r a t i o expected.
Results i n Table 3.
TABLE 3 Calculated i / n values assuming no methylation.
Methanol and propane
conversion proceeding independently i n r a t i o i n d i c a t e d by methane y i e l d s . Catalyst
MeOH/He
MeOH/Propane
#3
i/n
1:l 1.85
#2 i/n
1:l 2.84
2.19 ( c a l c . )
#2 i/n
1:3 4.91
3.10 ( c a l c . )
1:l
1:3
1.66 (obs.)
2.34 (obs.)
I n each case the c a l c u l a t e d lowering o f i / n i s smaller than the observed, b u t i n the case where 3 moles o f propane were used, lowering i s 70 % o f t h a t observed. E f f e c t o f d i l u e n t gas Helium was used ( r e f . 1 ) as " i n e r t d i l u e n t " i n place o f propane i n the "methanol-only"
runs.
I t was assumed t h a t , as helium i s i n e r t , i t does n o t
a l t e r the course o f the run and does n o t a f f e c t i / n r a t i o .
Table 4 shows
experimental studies o f d i l u e n t gases. TABLE 4 E f f e c t o f d i l u e n t gas on conversion o f methanol.
370 "C. D i 1 uent
-
He
N2
CH4 propane
MHSV Methanol (hr-1)
(3 moles) II ,I 'I
0.44 0.44 0.44 0.44 0.44
i / n Ratios over H-ZSM-5 #2 a t
i/n
4.2 6.6
5.4 5.3 4.0
Helium a d d i t i o n r a i s e s t h e i / n r a t i o from about 5.4 t o 6.6. Had t h e i n v e s t i g a t o r s used methane (shown t o be i n e r t i n t h i s system) o r
181
n i t r o g e n as " i n e r t d i l u e n t " t h e propane e f f e c t would have been s m a l l e r by about 50 % and t h e e f f e c t c o u l d be w h o l l y accounted f o r i n terms o f c o n v e r s i o n of
propane, as e s t i m a t e d above. While t h e c h o i c e between carbene, carbenium i o n , and methyl oxonium i o n as m e t h y l a t i o n i n t e r m e d i a t e s s t i l l remains open, i t i s c l e a r t h a t
the lowering o f
i / n butane r a t i o s h o u l d n o t be c i t e d as e v i d e n c e i n f a v o u r o f carbene. ACKNOWLEDGEMENTS We thank D r R. Howe f o r h e l p f u l d i s c u s s i o n , D r D. Bibby o f t h e Chemistry D i v i s i o n , DSIR, f o r a g i f t o f H-ZSM-5 c a t a l y s t , M r A. Wernham f o r d e s i g n and c o n s t r u c t i o n o f t h e c o n v e r t o r s and m o d i f y i n g GLC equipment, Entomology D i v i s i o n , DSIR, f o r l o a n o f GLC equipment, Auckland V a l v e and F i t t i n g Co. L t d . and A l l t e c h A s s o c i a t e s (N.Z.)
f o r g i f t s o f equipment, N.Z.
Committee f o r a s a l a r y g r a n t ( f o r D.V.D.), N.Z.
N.Z.
Energy Research and Development U n i v e r s i t y G r a n t s Committee and
L o t t e r y D i s t r i b u t i o n Committee f o r S c i e n t i f i c Research f o r equipment
grants. REFERENCES
1 2 3 4 5
C.D. Chang J.H.C. van C.D. Chang A.L. O d e l l , D.V. Dass,
6
D.V.
410-415.
and C.T-W. Chu, J . Catal., 74 (1982) 203-206. Hoof, J. Catal., 79 (1983) 242-243. and C.T-W. Chu, J . Catal., 79 (1983) 244-245. Chem. i n NZ, 50(1) (1986) 7. R.W. M a r t i n , A.L. O d e l l and A.J. Wernham, J. Chrom.,
Dass, R.W.
M a r t i n , A.L.
Odell, J. Catal.
387 (1987)
I n Press. (1987).
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183
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
HYDROCARBON FORMATION FROM METHANOL U S I N G W03/A1203 AN0 ZEOLITE ZSM-5 CATALYST: A M E C H A N I S T I C STUDY
R Hunter, W P i c k 1 and L Jansen van Rensburg Department of Chemistry, U n i v e r s i t y o f t h e Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, 2001, South A f r i c a
G J Hutchings,
ABSTRACT A m e c h a n i s t i c s t u d y o f i n j t i a l C-C bond f o r m a t i o n i n t h e methanol c o n v e r s i o n r e a c t i o n i s r e p o r t e d . R e a c t i o n o f methanol w i t h added hydrogen u s i n g WO /A1 0 and H-ZSM-5 c a t a l y s t s does n o t s i g n i f i c a n t l y a l t e r t h e p r o d u c t d i s t r ? b u t ? o i f r o m t h a t o f normal methanol c o n v e r s i o n . T h i s i s c o n s i d e r e d t o be c l e a r evidence a g a i n s t t h e i n v o l v e m e n t o f a gas phase methylene i n t e r m e d i a t e . I n a d d i t i o n t h e b e h a v i o u r o f t h e methoxymethyl r a d i c a l i n t h e gas phase has been s t u d i e d and t h e r e s u l t s demonstrate t h a t t h i s species i s a l s o n o t i n v o l v e d as an i n t e r m e d i a t e i n i n i t i a l C - C bond f o r m a t i o n .
INTROOUCTION I n view o f t h e t e c h n o l o g i c a l importance o f t h e methanol t o g a s o l i n e r e a c t i o n i t i s s u r p r i s i n g t h a t t h e mechanism by which i n i t i a l C-C bond f o r m a t i o n o c c u r s
remains unresolved. E l u c i d a t i o n o f t h i s mechanism s h o u l d enable t h e d e s i g n o f improved c a t a l y s t s e i t h e r w i t h r e s p e c t t o a c t i v i t y o r s e l e c t i v i t y . m e c h a n i s t i c p r o p o s a l s have been made,
and t h e most p l a u s a b l e p o s s i b i l i t i e s
i n c l u d e ( a ) a one s t e p g e n e r a t i o n o f carbene i n t e r m e d i a t e v i a and subsequent i n s e r t i o n i n t o t h e C-H generation
of
a
trimethyloxonium
A number o f
ion
bond o f methanol ylide
intermediate
rearrangement o r i n t e r m o l e c u l a r r e a c t i o n ( r e f s . 3, 41,
CL
(refs. and
elimination 1,
Z),
(b)
subsequent
( c ) two s t e p f o r m a t i o n
o f a s u r f a c e bonded m e t h y l oxonium i n t e r m e d i a t e ' ( i s o e l e c t r c n i c w i t h a carbene i n t e r m e d i a t e ) , ( r e f s . 5, 6 ) and ( d ) i n v o l v e m e n t o f a gas phase r a d i c a l r e a c t i o n (refs. 7
-
9 ) i n w h i c h t h e methoxymethylene r a d i c a l o r a gas phase carbene
species are c e n t r a l intermediates.
In p r e v i o u s s t u d i e s we have shown u s i n g
model m e t h y l a t i o n s t u d i e s t h a t t h e t r i m e t h y l o x o n i u m i o n y l i d e i s an u n l i k e l y i n t e r m e d i a t e f o r C-C t h a t a one stage
bond f o r m a t i o n .
In a d d i t i o n , t h e s e s t u d i e s a l s o showed
g e n e r a t i o n o f a carbene s p e c i e s c o u l d n o t account e a s i l y f o r
t h e v a r i a t i o n s observed f o r methane p r o d u c t i o n . I n t h i s paper we use f u r t h e r model s t u d i e s t o p r o v i d e c l e a r evidence a g a i n s t t h e i n v o l v e m e n t o f e i t h e r a gas phase r a d i c a l o r carbene i n t e r m e d i a t e .
TABLE 1
Experiment
Reagent
Catalyst
Temp OC
WHSVa h-1
Conversion %
b Hydrocarbon S e l e c t i v i t y (% by mass) CH4 C2H4 C2H6 C3H6 C3H8 C4 C5+
1
MeOH
W03/AC203
400
0.008
97.5
31.7
16.8
1.9
17.4
tr
7.3
24.9
4.5
1.8
2
d Me0H/H2
WQ3/Ae20,.
400
0.008
96.3
29.0
18.2
2.8
20.5
tr
8.6
20.9
e
-
3
MeOH
WQ3/AC203
350
0.008
38.4
28.3
13.6
0.4
24.4
tr
13.3
20.0
0.7
0.7
W03/Ae203
350
0.008
38.4
24.5
14.5
0.5
27.0
tr
15.2
18.3
e
2.2
d
4
Me0H/H2
5
MeOH
H-ZSM-5
370
0.051
100
0.3
9.9
0.1
38.0
tr
20.0
31.7
0.02
6
Me0H/H2d
H-ZSM-5
370
0.051
100
0.3
11.2
0.1
35.5
tr
21.0
31.6
e -
a
"
L
g MeOH/g c a t a l y s t / h .
based only on hydrocarbon p r o d u c t s . based on t o t a l e x i t g a s a n a l y s i s . 9 . 8 mole % H2 added t o c a r r i e r g a s . n o t determined, p r e s e n t i n f e e d .
185
EXPERIMENTAL 10% W03/A1203 was prepared as p r e v i o u s l y d e s c r i b e d ( r e f . 1 0 ) and H-ZSM-5 was prepared by t h e method o f Howden ( r e f . 1 1 ) . C a t a l y s t s were d r i e d a t 450°C f o r 2h i n d r y c a r r i e r gas i n s i t u i n t h e m i c r o r e a c t o r b e f o r e b e i n g r e a c t e d w i t h methanol which was f e d t o t h e r e a c t o r w i t h d r y argon c a r r i e r gas.
Reactions
w i t h added hydrogen were e f f e c t e d by r e p l a c i n g t h e argon c a r r i e r gas f o r a 10% H
2 i n n i t r o g e n c a r r i e r gas. Products were analysed u s i n g gas chromatograplv,
RESULTS AND D I S C U S S I O N
__ Investigation
o f t h e involvement o f a gas phase carbene i n t e r m e d i a t e
The r e a c t i o n o f methanol over t h e c a t a l y s t W03/A1203 ( T a b l e 1 ) shows t h a t t h e major hydrocarbon p r o d u c t i s t y p i c a l l y methane as has been n o t e d p r e v i o u s l y by Olah ( r e f . 1 0 ) . I n a d d i t i o n , comparable hydrogen y i e l d s a r e a l s o observed with t h i s catalyst.
I n c o n t r a s t t h e r e a c t i o n o f methanol o v e r H-ZSM-5 always
g i v e s low methane and hydrogen y i e l d s a t c o n v e r s i o n s i n Table 1 ) .
> 1% ( t y p i c a l d a t a shown
To t e s t i f t h e methane i s generated f o r e i t h e r c a t a l y s t
r e a c t i o n o f an i n t e r m e d i a t e w i t h hydrogen t h e same r e a c t i o n s were c a r r i e d o u t w i t h 10 mole % hydrogen added t o t h e c a r r i e r gas. The r e s u l t s ( T a b l e 1 ) c l e a r l y i n d i c a t e t h a t no i n c r e a s e i n methane was observed i n any o f t h e s e experiments. From t h e s e r e s u l t s i t can be concluded t h a t a f r e e methylene i n t e r m e d i a t e i s n o t i n v o l v e d s i n c e an i n c r e a s e i n methane y i e l d would have been expected based on t h e known r e a c t i o n between methylene and hydrogen ( r e f . 1 2 ) . I t i s t h e r e f o r e c l e a r t h a t g e n e r a t i o n o f a gas phase m e t h y l e n e f r o m t h e .CH2-O-CH3
radical
(mechanism 3 f i g u r e 1 ) i s n o t a p l a u s i b l e mechanism f o r C-C bond f o r m a t i o n .
I.
S o
+
CHgO-CH3-S-H
*CH,-O- CH3
-
2.
2 *CH2-O-CH3-
3.
*CH2-O-CH3-:CH, CH3-O-CH3:CH,
+
+ *CH,-O-CH, H-ZSM-5
C2H50* -C2H50H,
C2H,
CH3-O-(CH2)yO-CH3
+ *O-CH,
CH3CH2-O-CH3
F i g . 1 M e c h a n i s t i c p r o p o s a l s f o r C-C bond f o r m a t i o n u s i n g a r a d i c a l pathway.
186
Table: 2 Reaction o f dimethyl ether w i t h dibenzoyl peroxide dibenzoyl peroxide % by mass
Temperature "C 25
Product S e l e c t i v i t y MeO(CH2I20Me
0.2
50
0.5 5.3 3.9 2.3
100 100 100 100 100
2oa
25 50 80 140 170
0.3 0.3 1.6 7.1 2.0
lob
100 250
2.1 2.3
1oa
a b
MeOMe Conversion %
80 140 200
EtOH
C2H4
--T--T0 0 0 0
0 0 0 0
100 100 100 100 100
0 0 0 0 0
0 0 0 0 0
100 100
0 0
0 0
d i b e n z o y l p e r o x i d e s u p p o r t e d on c e l i t e . d i b e n z o y l p e r o x i d e supported on celite/Na-ZSM-5.
Model s t u d i e s o f t h e methoxymethyl ~-
radical
Dimethyl e t h e r was r e a c t e d o v e r a r a d i c a l i n i t i a t o r ,
peroxide supported a c e l i t e ,
at
dibenzoyl
l i v e d under
p e r o x i d e was
short
various
temperatures
pre-dried dibenzoyl (Table 2).
Although
these c o n d i t i o n s s u f f i c i e n t
r e a c t i v e i n t h e t i m e s c a l e o f t h e experiments,
t y p i c a l l y 5 min.
was
Under a l l
c o n d i t i o n s o n l y dimethoxyethane was observed as a d i m e r i s a t i o n p r o d u c t showing t h a t t h e methoxymethyl
radical
absence o f o t h e r p r o d u c t s , e.g.
was
generated
under
these conditions.
The
alkenes o r e t h a n o l , d i s c o u n t s t h e methoxymethyl
r a d i c a l fragmentation/recombination pathway (mechanism 1, F i g . 1 ) . To i n v e s t i g a t e
whether mechanism 2 ( F i g . 11, i n which dimethoxyethane i s
suggested as t h e c r u c i a l hydrocarbon precusor, r e a c t e d o v e r H-ZSM-5 ( l g ) a t 250 and 300°C.
i s v i a b l e , dimethoxyethane was
The p r o d u c t s analysed by o f f l i n e
gas chromatography c o n s i s t e d o f methanol and d i m e t h y l e t h e r (hydrocarbons were o n l y observed i n t r a c e amounts) a t t o t a l c o n v e r s i o n s o f 12
-
20% r e s p e c t i v e l y
f o r t h e t e m p e r a t u r e s s t u d i e d . F o r comparable r e a c t i o n c o n d i t i o n s d i m e t h y l e t h e r gave c o n v e r s i o n s t o hydrocarbons o f 20% and 90%. These r e s u l t s show t h a t dimethoxyethane,
generated f r o m t h e d i m e r i s a t i o n o f
methoxymethyl r a d i c a l s , i s n o t a s u i t a b l e ethene p r e c u s o r and hence mechanism 2 ( F i g . 1 ) can a l s o be d i s c o u n t e d as a p l a u s i b l e p o s s i b i l i t y .
18i
CONCLUSIONS The r e s u l t s o f t h i s s t u d y u s i n g a d d i t i o n o f hydrogen o r model s t u d i e s o f t h e methoxymethal r a d i c a l g i v e c l e a r evidence a g a i n s t t h e involvement phase carbene o r methoxymethyl
radical
intermediates
of
a gas
i n t h e mechanisms
of
i n i t i a l C - C bond f o r m a t i o n i n t h e methanol t o g a s o l i n e c o n v e r s i o n r e a c t i o n . ACKNOWLEDGEMENT We thank t h e U n i v e r s i t y o f t h e W i t w a t e r s r a n d and t h e FRD, C S I R ,
Pretoria f o r
f i n a n c i a1 a s s i stence. REFERENCES
1. 2. 3. 4. 5. 6. 7.
8. 9.
10.
11. 12.
C.D. Chang and A.J. S i l v e s t r i , J. C a t a l . , 47, (1977) 249. C.S. Lee and M.M. Wu, J Chem. SOC., Chem. Commun., (1985) 250. J.P. van den Berg, J.P. Wolthuizen, and J.H.C. van Hoof, i n ' P r o c . 5 t h I n t . Conf. Z e o l i t e s , ' ed. L.V.C. Rees, Heydon, London, 1980 p649. G.A. Olah, Pure A p p l i e d Chem., 53, (1981) 201. R. Hunter and G.J. Hutchings, J. Chem. SOC., Chem. Commun., (19851, 1643. (1986) 1006. G.J. Hutchings, M.V.M. H a l l , F. G o t t s c h a l k and R. Hunter, J. Chem. SOC., Faraday Trans. 1 , 83, (1987) 571. H. Choukroun, D. Brunel and A. Germain, J. Chem. SOC., Chem. Commun., (1986) 6. J.K.A. C l a r k e , R. Darcy, B.F. Hegarty, E O'Donoghue, V. Amir-Ebrahim and J.J. Rooney, J. Chem. SOC., Chem. Commun., (1986) 425. P. Rimmelin, A. Brenner, K. F i s c h e r and 3. Sommer, J . Chem. SOC., (1986) 1497. G.A. Olah, A. Doggweiler, J.D. F e l b e r g , S. F r o h l i c h , M.J. Grdina, R. Karpeles, T. Keumi, S. Inaba, W.M. I p , K. Lammertsma, G . Salem and D. Tabar, J. Am. Chem. SOC., 106, (1984) 2143. M.G. Howden, C S I R R e p o r t C. Eng. (CSIR, P r e t o r i a , South A f r i c a , 1982) 413. W. Kirmse, i n 'Carbene C h e m i s t r y ' Academic Press, New York, 1971.
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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
189
ON THE MECHANISM OF HYDROCARBON FORMATION FROM METHANOL OVER PROTONATED ZEOLITES
STEIN KOLBOE Department of Chemistry, University of Oslo P . O.
Box 1 0 3 3 . Blindern, N-0315 Oslo 3 . Norway
ABSTRACT A new mechanism for the formation of hydrocarbons from methanol over proton-
ated zeolites invoking adsorbed carbenium ions as an essential part of the catalytically active site is proposed. The mechanism is based on new experimental results which can not be explained from previously proposed mechanisms.
INTRODUCTION Since i t was first reported in 1976 that protonated ZSM-5 zeolites are excellent catalysts for conversion of methanol (and many other oxygenated compounds ) into hydrocarbons in the %- C10 range the catalyst and the reactions have been intensely studied. Several aspects of the reactions leading to hydrocarbon formation from methanol or dimethyl ether over H-ZSM-5 or other protonated zeolites still remain unclear. In particular the first G C bond formation has been debated , and several mechanisms proposed (ref. 1). The mechanisms proposed in the literature fail to take into account the very high proton affinity of many of the alkenes formed (ref. 2 ) . After a short time the catalyst will contain carbenium ions rather than protonated oxygenates. Experiments to be reported below have shown that at temperatures below ca. 25OoC a fresh H-ZSM-5 is essentially inactive for hydrocarbon formation until adsorbed hydrocarbons have formed. These novel observations seem to necessitate modifications of mechanisms previously proposed in the literature. A modified carbene type mechanism invoking a push pull action appears to be consistent with the experimentally observed behav iour. EXPERIMENTAL BASIS FOR NEW MECHANISM 1. An H-ZSM-5 catalyst which has been heated to 500 OC in air does not form hydrocarbons when methanol is passed over i t at e.g. 220 OC (But it is very active for DME ( dimethyl ether ) formation, so methanol , DME, and water are virtually equilibrated. )
.
190
2. Addition of propene or butene to the reaction system gives a catalyst which produces hydrocarbons from methanol, i.e. makes the catalyst C-C active.
Once the catalyst has become C C active i t remains active also after flushing the catalyst with carrier gas for 10-20 h at 220-250 OC. If methanol is then added to the reaction system, a high activity is at once observed. Figure 1. 3.
When the catalyst is in the C-C active state the rate of hydrocarbon formation is not increased when e.g. butene is added. When the feed rate is changed, - so that the conversion is changed the reaction rate ( mole product per second ) does not change. i.e. in this case there is no indication of
-
autocata1ysi s
.
4. The TPD curves ( methanol exposed catalysts ) from the C C active catalyst are very different from the TPD curves from the C C inactive catalyst (methanol exposed 1. Figure 2. After TPD to
>
450 OC the catalyst becomes almost inac-
tive. It can be regenerated as above to its former activity level. 5.
The C-C active catalyst behaves normally when the temperature is changed. (Normal Arrhenius behaviour.) The energy of activatlon for hydrocarbon formation is 135 kJ/mol. Abnormal behaviour is reported in the literature (ref. 3).
It is known from mass spectrometry that the proton affinity (gas phase ) of many alkenes is much higher than that of water, methanol or DME (ref. 2). so there will hardly be any protonated water, methanol or DME in the zeolite when there are alkenes present , but there will be carbeniwn ions. Table 1. 6.
FIGURE 1
TABLE 1 PROTON AFFINITIES (PA1 [ F r o m P.Voge1 (211 PA Rel. t o H 2 0 Compound kJ/mol kJ/mol NH3 846 134 CHjNH2 884 172 H 9 712 0 762 (CH3) 20 800 C2H5OH 795 CH2=CH2 672 CHjCH=CH2 755 tCH3) 2C=CH2 810 Ph-CH=CHrr 834 Ph-C(CHj)=CH2 859 147
CH~OH
a
-
m
o
o 0
C
U
w 0
m
-
Figure 1. olromatograms showing that presence of alkenes is not necessary, once the catalyst has been activated. ( 0.lg H-ZSM-5, methanol pressure 40 mbar, carrier 10 mllmin) A: System 36 min on stream after 5OO0C in 0 B: Activation carried out. C: After 10h flushing with N at 22OoC. D: %e catalyst is active imediately after admission of metfanol, as shown by the effluent samples analyzed 3 and 7 min later. (There is a time lag about 3 min before admitted reactant is appearing in the effluent.)
191
L
m
C-C a c t i v e c a t a l y s t f l u s h e d w i t h 10 ml/mln N a t 224OC f o r 52 h b g f o r e TPD experiment. 702 a r o m a t i c s l x y l e n e s ) i n desorbate.
n
i n a c t i v e cata1ySt f l u s h e d With 10 ml/min N~ a t 224OC f o r 2 h b e f o r e TPD - e x p e r i m e n t . Less than 10 X a r o m a t i c s I n desorbate.
m
c 3
d
m
z 2 L
L
:m:
CONDITIONS: TPD : 1.5 K/min 180 mg c a t a l y s t 10 ml/min N2.
1
L
260
-m L
n
U
:
Run 2
m
n
m
u m n
Run'
4
2 5 . 4J d
L
m
.
- C-C
+
300
400
1
. \ . E,=135 kJImO1
m
$
L
m
0
4
l.B
2.0
2.:
2.2
Ternperature/'C
Figure 2. TPD curves fran C C active and inactive H-ZSM-5. (Same sample.)
Figure 3 . Activation energy for ethene formation over C-C active H-ZSI-5.
NEW REACTION MECHANISM The mechanisms proposed in the literature for the hydrocarbon formation (ref.1) are hardly consistent with the observations outlined above. A reformulated carbene theory appears, however, to be in agreement with experiment. The following mechanism is proposed: The catalytic entity is a dual site consisting of a carbenium ion locked up in the zeolite close to an aluminium ion on T-site, and an induced basic site on one of the oxide ions surrounding the A1 ion. This provides a dual acidlbase site which is necessary for the carbene mechanism. The absence of basic sites on fresh H-ZSM-5 has been a major argument for rejecting the carbene mechanism. A formal description of the mechanism is given in schemes 1 and 2. How does basicity arise?
This is best discussed in terms of crystal field theory. The zeolite may
be viewed as an ionic compound. It may be derived from a SiOz crystal where some Si4+ ions have been replaced by A13+. At these lattice points a 4+ charge is then replaced by a 3+ -charge so these lattice points formally acquire a negative ch,arge. This is compensated by an extra-framework positive charge nearby, which usually is a metal ion or a proton. The compensation could also be obtained by replacing an 02- by an 01-, The formation of a large carbenium ion moves the proton away from the negative lattice charge it should be compensating. Charge compensation can then be obtained by removal of an electron from one of the surrounding oxide
192
>-(-
-
Y
When is activation not needed ? The larger part of experiments reported in the literature was carried out at temperatures well above 300°C, and then
H
there was no need to activate the catalyst. Also at these temperatures the primary reactions are establishment of a near equilibrium, H20, H30+, CH30H. SCHEME 1
1'
L 0 E
'
CH~OH;.
(cH~)~o (, ~ ~ 3 ) 2 m (a3)30+, +, and C H 3 ' . The predominant adsorbed species is
'
'
'
1 .
\ P
2
Y
3
d
u
\
0.8.
- 8
1 -
-55.1.
0.8'
5 W
SI,.l.
pNth.nO,-
5s
100 mbeP
EL
C
d
u c u
:r::. 100 :1r:, ma4:.E" .tl",t'
I
0.
do
Go
.do
5bo
w, 82'
S
a :
- 4
-M* 100
.
2
m-.
-
Ancillary experiments have shown that the activating process is not due to impurities by e.g. higher alcohols (ref. 4). AISO at temperatures above 3OO0C an
2
5* m
initial increase in hydrocarbon forming activity can be observed, but essentially steady state conditions are then
193
RESULTS ON OTHER PROTONATED ZEOLITES Experiments which have been carried out indicate that the results discussed above which have all been obtained on H-ZSM-5 zeolites are actually valid for all protonated zeolites. Experiments have so far been carried out on protonated ZSM-11, erionite , mordenite, faujasite (Y-zeolite), offretite, ferrierite and L-zeolite (ref. 4). Till now, all the the investigated zeolites have given fully concordant results. While they all exhibit the same behaviour as regards activation, their resistance to deactivation and their product spectrum are in many cases dramatically different. Offretite for instance deactivates almost completely after a couple of hours with high activity, and the dominant hydrocarbon, by far, is methane (ref. 4). Not surprisingly, the zeolites which deactivate fastest appear to be the ones with a one dimensional, or quasi one dimensional, pore system. Furthermore i t appears that there is a high correlation between fast deactivation and a mainly methane producing catalyst. The high selectivity for methane over some zeolites may indicate that there are parallel pathways leading to either methylene ( :CH2) or to carbenium ( CH3') intermediates. The methylene intermediate would then be the dominant one in zeolites like H-ZSM-5 which produce almost no methane, whereas carbenium could be rather dominating in a zeolite like offretite. The similarity in behaviour in all other respects indicates a mechanism along very similar lines in the two exemplified zeolites. A preliminary report on some of the ideas expressed here has recently been given (ref. 5). EXPERIMENTAL The experiments were carried out in a conventional glas flow microreactor system Typical catalyst loadings have been 30 to 300 mg , and carrier gas flow has been 5 to 50 ml/min, typically 15. The TPD experiments were carried out using gas flow 10 ml/min and temperature rise 1.5 K/min. Reaction products and (TPD) desorbate were analysed by gas chromatography using Bentone and Porapak N colurns.
REFERENCES 1 A review covering most of the literature up to 1983 has been given by C. D. &ang, Catal.Rev. -Sci.Eng.-25 (1983) 1. 2 P. Vogel, Carbocation aemistry (Studies in Organic Chemistry; 21). Elsevier. Amsterdam 1985. p. 74. 3 Y. ()no, E. Imai and T. Mori, Z.*Phys. aem.,N.F. 115 (1979) 99. 4 S. Kolboe et al., In preparation. 5 S. Kolboe, Acta C3an. Scand. A40 (1986) 711.
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11.M.Hibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors 1, M e t h n n ~C'onccwion
0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands
FORMATION OF p-XYLENE FROM METHANOL OVER H-ZSM-5
E r i k UNNEBERG and S t e i n KOLBOE Department o f Chemistry, The U n i v e r s i t y o f Oslo, P.O. Box 1033 B l i n d e r n , 0315 Oslo 3, Norway.
ABSTRACT D i f f e r e n t H-ZSM-5 z e o l i t e s were synthesized, c h a r a c t e r i z e d b y v a r i o u s methods and u t i l i z e d i n t h e MTG-reaction. The aim o f t h e c a t a l y t i c experiments was t o produce p - x y l e n e f r o m methanol. Several r e a c t i o n c c o n d i t i o n s were a p p l i e d , and t h e p r o d u c t d i s t r i b u t i o n s i n d i c a t e d t h a t px y l e n e i s formed n e a r l y e x c l u s i v e l y w i t h i n t h e p o r e s as a p r i m a r y a r o m a t i c p r o d u c t . The f o r m a t i o n mechanisms o f o t h e r a r o m a t i c s a r e discussed. INTRODUCTION Aromatic g a s o l i n e i s now produced f r o m methanol i n MOBIL's MTG-process, which i s c a t a l y z e d b y t h e z e o l i t e H-ZSM-5.
Much a t t e n t i o n has been p a i d t o
u n d e r s t a n d i n g t h e m e c h a n i s t i c pathway(s) o f t h e chemical r e a c t i o n s t h a t t a k e p l a c e . A l t h o u g h t h e r e i s s t i l l some c o n t r o v e r s y a b o u t t h e d e t a i l s , t h e main s t e p s a r e g e n e r a l l y accepted:
-HZO -Hzo (CH30H* CH30CH3)* C2-C5-alkenes--)alkanes
a n d aromatics
The x y l e n e c o n t e n t o f t h e a r o m a t i c hydrocarbons i s h i g h , and when nons e l e c t i v e H-ZSM-5 c a t a l y s t s a r e employed t h e x y l e n e isomers a r e produced i n t h e i r thermodynamic e q u i l i b r i u m (o:m:p = 1:2:1). P-xylene i s t h e most v a l u a b l e x y l e n e i s o m e r due t o i t s i m p o r t a n c e f o r t h e p r o d u c t i o n o f t e r e p h t h a l i c a c i d , f o r w h i c h t h e r e i s a demand i n t h e polymer i n d u s t r y . Because o f t h e c o m p l e x i t y o f s e p a r a t i n g t h e c l o s e - b o i l i n g components i n t h e C8-aromatic f r a c t i o n , i t i s o f g r e a t i n t e r e s t t o produce px y l e n e s e l e c t i v e l y . I t has been r e p o r t e d t h a t o v e r m o d i f i e d H-ZSM-5 c a t a l y s t s p - x y l e n e can be produced i n g r e a t excess o f i t s thermodynamic e q u i l i b r i u m v a l u e i n v a r i o u s s h a p e - s e l e c t i v e r e a c t i o n s ( r e f . 1). We w i l l show t h a t enhanced p a r a - s e l e c t i v i t y can be achieved even o v e r u n m o d i f i e d H-ZSM-5 c a t a l y s t s i n t h e methanol r e a c t i o n : ( 3 n t 8 ) CH30H+
CH3
CH3 + 3 C,H2n+2
+ (3n+8) H20
195
196
EXPERIMENTAL The syntheses o f t h e ZSM-5 z e o l i t e s were based on two d i f f e r e n t p a t e n t s ( r e f s . 2-3).
I n two o f t h e s y n t h e s i s m i x t u r e s sodium i o n s were, however,
l a r g e l y r e p l a c e d b y potassium i o n s ( t a b l e 1). P r o t o n a t i o n was achieved b y ion-exchange w i t h an aqueous 1 M s o l u t i o n o f ammonium-nitrate f o l l o w e d b y c a l c i n a t i o n f o r 8 h o u r s a t 500
OC.
I n a d d i t i o n , one H-ZSM-5 c a t a l y s t was
k i n d l y s u p p l i e d f r o m MOBIL. The z e o l i t e s were c h a r a c t e r i z e d b y x - r a y d i f f r a c t i o n , scanning e l e c t r o n microscopy, pore-volume measurements (BET) and elemental a n a l y s i s (ICP). C a t a l y t i c experiments were performed a t atmospheric p r e s s u r e b y f e e d i n g methanol d i l u t e d w i t h n i t r o g e n ( 1 : 5 ) i n t o a c o n t i n u o u s f l o w q u a r t z m i c r o r e a c t o r c o n t a i n i n g 25-300 mg H-ZSM-5. t e m p e r a t u r e was v a r i e d f r o m 245 t o 400
OC
The r e a c t i o n
and t h e p r o d u c t s were analyzed b y
gas-chromatography. RESULTS AND D I S C U S S I O N The x - r a y d i f f r a c t o g r a m s showed t h a t a l l z e o l i t e s were h i g h l y c r y s t a l l i n e ZSM-5 and t h e p o r e volumes were a l s o found t o be c o n s i s t e n t w i t h t h a t o f c a t a l y t i c a l l y a c t i v e ZSM-5 r e p o r t e d e a r l i e r (BET-surface g r e a t e r t h a n 300 m2/g), O t h e r c h a r a c t e r i z a t i o n r e s u l t s a r e p r e s e n t e d i n t a b l e 1. I t was observed t h a t t h e c r y s t a l s became l a r g e r and more u n i f o r m when a s u b s t a n t i a l p a r t o f t h e Na-ions were r e p l a c e d b y K-ions i n t h e s y n t h e s i s m i x t u r e . These o b s e r v a t i o n s a r e i n accordance w i t h e a r l i e r o b s e r v a t i o n s ( r e f . 4). U n i f o r m and l a r g e c r y s t a l s p r o v e t o be i m p o r t a n t f o r t h e s h a p e - s e l e c t i v i t y . TABLE 1. S y n t h e s i s and c h a r a c t e r i z a t i o n o f H-ZSM-5 z e o l i t e s . Designation S y n t h e s i s method Molar K/Na-ratio i n the synthesis mixture Molar S i / A l - r a t i o i n t h e z e o l i t e (ICP) Crystal size, ym
H-ZSM-5(M) H-ZSM-5(34) Obtained Based on f r o m MOBIL ref. 3
-
<
34 0.5
H-ZSM-5(10) Based on ref. 2
H-ZSM-5(40) Based on ref. 3
0
3.8
4.5
34 2-3
10 20-30
40 60-40
Depending on e x p e r i m e n t a l c o n d i t i o n s and t y p e o f c a t a l y s t , t h e a r o m a t i c s c o n s t i t u t e d up t o 40 w t % o f t h e hydrocarbon p r o d u c t . A t y p i c a l v a l u e o f t h e x y l e n e c o n t e n t i n t h e a r o m a t i c f r a c t i o n was a b o u t 2/3 b y w e i g h t . Some e x p e r i m e n t a l r e s u l t s where t h e a c t i v i t y and s e l e c t i v i t y l e v e l s no more depended on t h e t i m e on stream a r e l i s t e d i n t a b l e 2. The p a r a - s e l e c t i v i t y i s v i s u a l i z e d i n f i g u r e 1 b y some chromatograms o f t h e C8-aromatic f r a c t i o n .
197
The p a r a - s e l e c t i v i t y o f t h e e t h y l - t o l u e n e s was v e r y c l o s e t o t h e p a r a s e l e c t i v i t y o f t h e x y l e n e s and t h e d i e t h y l - b e n z e n e s (DEB) a l s o seemed t o g i v e s i m i l a r isomer d i s t r i b u t i o n . However, t h e DEB-content i n t h e p r o d u c t was n o t s u f f i c i e n t t o a l l o w a q u a n t i t a t i v e analysis. TABLE 2. S e l e c t e d r e s u l t s f r o m t h e methanol c o n v e r s i o n ( a f t e r t h e i n i t i a l p e r i o d ) . Ca t a l ys t Temperature, WHSV, h - l
H-ZSM-5(M) OC
H-ZSM-5( 10)
H-ZSM-5(40)
399
290
H-ZSM-5(34) 375
338
342
340
369
1.1
5.8
0.65
0.23
1.7
0.32
W t % aromatics
26
15
12
27
9
7
4
W t % xylenes i n aromatics
47
50
73
65
67
75
76
24.0 12.9 49.5 32.8 26.6 54.3 A . B
15.0 34.9 50.0
3.2 10.9 85.9 C
0.5 1.6 97.9
0.6 1.9 97.5
0.8 1.9 97.3
Xyl ene ortho distrimeta bution, % para Chromatogram ( f i g . 1)
-
D
0.52
-
-
Several parameters a f f e c t t h e p a r a s e l e c t i v i t y . I n general, t h e para-
PX 1
y i e l d i s h i g h when secondary r e a c t i o n s a r e n o t allowed t o take place at, o r c l o s e t o , t h e o u t e r z e o l i t e surface. H i g h space v e l o c i t y was f o u n d t o favour a nearly exclusive formation o f t h e para-isomer ( f i g u r e Z ) , and t h e p a r a - s e l e c t i v i t y was a l s o improved o v e r each z e o l i t e a s t h e r e a c t i o n t e m p e r a t u r e was decreased. We t h e r e f o r e suggest t h a t o n l y t h e paraisomers a r e formed w i t h i n t h e p o r e s
~~~~
i n i t i a l l y due t o 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 . Secondary i s o m e r i z a t i o n r e a c t i o n s w i l l , however, l e a d t o t h e appearance o f a l l t h r e e isomers i n t h e product. This proposal i s supported b y t h e f a c t t h a t t h e p a r a - y i e l d increased w i t h increasing s i z e o f the z e o l i t e
€8
A
8
C
D
Examples of chromatograms (of different attenuations) showing various ~ r a - s e l e c t i v i t i e spxs ' Mx and aX = p-, m and o-xylene, 5 = e thy1-benzene.
Figure
c r y s t a l s . When t h e e x t e n t o f i s o m e r i z a t i o n was low, e.g. t h e m-/o-xylene
when t h e p a r a - s e l e c t i v i t y was a t a h i g h l e v e l ,
r a t i o was measured t o b e g r e a t e r t h a n t h e thermodynamic
198
e q u i l i b r i u m r a t i o . As has been p o i n t e d o u t e a r l i e r ( r e f . 5), t h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e r e p o r t e d m e c h a n i s t i c sequence o f t h e i s o m e r i z a t i o n o f p-xylene: p + m + o
( r e f . 6). %Xgene isomer
F i g u r e 2 . The dependence on " c o n t a c t t i m e " f o r d i f f e r e n t H-ZSM-5 z e o l i t e s .
F i g u r e 3. The x y l e n e d i s t r i b u t i o n vs. t h e t i m e on stream. H-fSM-5(10), 400 OC, WHSV = 0.12 h -
.
The x y l e n e isomer d i s t r i b u t i o n was found t o depend on t h e t i m e on stream i n t h e f i r s t p e r i o d o f t h e c a t a l y t i c experiment ( f i g u r e 3 ) . T h i s e f f e c t was l e s s pronounced a t l o w t e m p e r a t u r e o r h i g h space v e l o c i t y and can be e x p l a i n e d b y t h e assumption t h a t n o n - s e l e c t i v e r e a c t i o n s o c c u r a t t h e a c t i v e s i t e s a t t h e o u t e r z e o l i t e s u r f a c e . As t h e s e a c t i v e s i t e s become p a r t l y o r c o m p l e t e l y d e a c t i v a t e d t h e p a r a - s e l e c t i v i t y i n c r e a s e s . The o b s e r v a t i o n t h a t t h e i n c r e a s e o f t h e para-amount w i t h i n c r e a s i n g t i m e on stream d i d n o t a f f e c t t h e t o t a l x y l e n e y i e l d i s c o n s i d e r e d t o be f u r t h e r e v i d e n c e t h a t i s o m e r i z a t i o n reactions take place. The o n l y a r o m a t i c components t h a t appeared a t r e a c t i o n temperatures below
300
OC
were t o l u e n e and p-xylene.
I n t h e case o f t h e s m a l l - c r y s t a l l i n e H-ZSM-
5(M), however, some m- and o - x y l e n e were p r e s e n t i n t h e p r o d u c t m i x t u r e even OC (WHSV = 6 h-'). T h i s can be e x p l a i n e d b y x y l e n e i s o m e r i z a t i o n a t
a t 245
t h e o u t e r z e o l i t e s u r f a c e . A t c o n d i t i o n s where t h e p a r a - s e l e c t i v i t y was h i g h (more t h a n 90% p a r a ) , t h e amount o f p - e t h y l - t o l u e n e
(PET) i n t h e p r o d u c t were
one o r d e r o f magnitude g r e a t e r t h a n t h a t o f a n y o t h e r C9-component, i t was low, t h e r a t i o 1,2,4-trimethyl-benzene
about 1 O : l .
b u t when
(124TMB) : PET was f o u n d t o be
These e x p e r i m e n t a l f a c t s i n d i c a t e t h a t 124TMB i s m a i n l y formed b y
secondary x y l e n e a l k y l a t i o n w i t h methanol. Toluene, p-xylene,
PET and perhaps
ethyl-benzene a r e more l i k e l y t o be t h e p r i m a r y a r o m a t i c p r o d u c t s formed i n t h e MTG-reaction. To c o n f i r m t h i s s u g g e s t i o n t h e m o l a r p r o d u c t r a t i o s EB/PX,
199
T/PX and PET/PX were measured a t d i f f e r e n t space v e l o c i t i e s ( r e a c t i o n temperature = 370 OC). F i g u r e 4 shows t h a t , over a h i g h l y para-selective c a t a l y s t , these r a t i o s v a r i e d o n l y s l i g h t l y w i t h space v e l o c i t y , and c o u l d be e x t r a p o l a t e d t o non-zero v a l u e s a t z e r o " c o n t a c t t i m e " . The s t e r i c s i m i l a r i t y o f t h e s e components m i g h t be due t o equal o r c l o s e l y r e 1 a t e d f o r m a t i o n mechanisms.
-
+
p.-., 1
"
i
I
EWPI
w ;
332
01
a2
wnsv-1 101
F i g u r e 4. P r o d u c t r a t i o s , H-ZSM-5(40). CONCLUSION W i t h o u t m o d i f y i n g t h e c a t a l y s t , p - x y l e n e can be produced i n g r e a t excess o f t h e thermodynamic e q u i l i b r i u m d i s t r i b u t i o n . The p a r a - s e l e c t i v i t y was enhanced when l a r g e H-ZSM-5 c r y s t a l s were employed. R e a c t i o n parameters t h a t reduced t h e degree o f r e a c t i o n s t a k i n g p l a c e a t t h e o u t e r z e o l i t e s u r f a c e l e d t o an i n c r e a s e d p a r a - y i e l d .
M- and o - x y l e n e a r e formed m a i n l y f r o m p-
xylene, which seems t o be a p r i m a r y a r o m a t i c p r o d u c t , as do o t h e r mono- o r para-a1 k y l arenes. REFERENCES
1. 2. 3. 4. 5. 6.
N.Y. Chen, W.W. Kaeding and F.G. Dwyer, J. Am. Chem. SOC., 101 (1979) 6783. A.J. Argauer and G.R. L a n d o l t , U.S. P a t e n t no. 3702886, 1972. C.J. Plank, E.J. R o s i n s k i and A.B. Schwarz, B r i t i s h P a t e n t no.1402981, 1973. Z. Gabelica, E.G. Derouane and N. Blom, A. C. S. Symp. Ser., 248 (1984) 219. E. Unneberg and S. Kolboe, Chemistry Express, 1 (1986) 745. L.B. Young, S.A. B u t t e r and W.W. Kaeding, J. C a t a l . , 76 (1982) 418.
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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
201
FURTLER STUDIES ON THE PROCESS OF PIETHANOL CCNVERSIGN TO OLFFINS Guoquan Chen, J u a n L i a n g , Qingxia Wang, Guangyu Cai, S u q i n Zhao, Hongyuan L i e t a i . D a l i a n I n s t . of Chem. P h y s . , Academia S i n i c a , D a l i a n , China ABSTRACT Conversion of methanol t o o l e f i n s is c o n s i d e r e d more f a v o r a b l e a t h i g h e r t e m p e r a t u r e . But a t s u c h c o n d i t i o n , c o n v e n t i o n a l p e n t a s i l z e o l i t e even w i t h moderate Si/A1 r a t i o would be g r a d u a i i y d e t e r i o r a t e d w i t h i n c r e a s i n g of time on s t r e a m . The i n i t i a i symptom of agi n g i s r e f i e c t e d on t h e s h i f t i n g of hydrocarbons d i s t r i b u t i o n . The most s i g n i f i c a n t c r i t e r i o n i s t h e d e c l i n a t i o n of t h e y i e l d of e t h ene. Aging is comprised of r e v e r s i b l e t y p e , a t t r i b u t a b l e t o coking and i r r e v e r s i b l e t y p e , t o t h e d i m i n i s h i n g o f a c i d i c s i t e s due t o d e a l u m i n a t i o n . C a t a l y s t s w i t h r a t h e r s a t i s f a c t o r y s t a b i l i t y and s e l e c t i v i t y have been F r e p a r e d and t e s t e d up t o bench s c a l e . INTRODUCTION The p e l i m i n a r y r e s u l t s of o u r r e s e a r c h on methanol c o n v e r s i o n t o o l e f i n s encouraged us t o c a r r y o u t t h e work f u r t h e r f o r e v a l u -
a t i n g i t s f e a s i b i l i t y . Before t h e r e c e n t expanding o f propene mark e t , s e l e c t i v i t y f o r h i g h e t h e n e was b e i n g pursued. F o r t h i s purp o s e , a h i g h e r r e a c t i o n t e m p e r a t u r e is c o n s i d e r e d t o be more f a v o r a b l e ( r e f s . 1 - 2 ) . I n f a c t a n e t h e n e s e l e c t i v i t y of 37% h a s been a t t a i n e d a t r e a c t i o n t e m p e r a t u r e of 550°C i n o u r e a r l i e r work But a t s u c h a t e m p e r a t u r e l e v e l , a c o n v e n t i o n a l modified p e n t a s i l zeol i t e c a t a l y s t was found a l m o s t c o m p l e t e l y d e a c t i v a t e d w i t h i n two h o u r s due t o coking. Furthermore t h e r e a c t i v i t y of c a t a i y s t was found u n a b l e t o r e s t o r e t o i t s o r i g i n a l l e v e l t h r o u g h l o n g e r time on s t r e a m and more t i m e s of o x i d a t i v e r e g e n e r a t i o n . Thus t h i s problem was b e i n g t a c k l e d and s i g n i f i c a n t p r o g r e s s has been made.
.
IBTHOD P e n t a s i l t y p e z e o l i t e c a t a l y s t s a r e s y n t h e s i z e d and modified s i milar t o t h e p r o c e d u r e s i n ( r e f . 2 ) , b u t some o f them were admixed w i t h s u i t a b l e b i n d i n g m a t e r i a l . A c i d i t y was c h a r a c t e r i z e d by NH 3TPD and I R s F e c t r a and s u r f a c e p r o p e r t y by ESCA as i n ( r e f s . 2 - 3 ) . C a t a l y t i c r e a c t i o n was c a r r i e d o u t i n c o n t i n u o u s flowtype s y s t e m s w i t h 2-30 g and more o f c a t a l y s t and r e g e n e r a t i o n by o x i d a t i o n a t a b o u t 53G°C.
202
RESULTS AND DISCLSS I C N F u r t h e r e v a l u a t i o n of t h e modified p e n t a s i l z e o l i t e c a t a l y s t s ( i ) A summary of o u r e a r l i e r r e s u l t s i s shown i n Table 1 . The modified e r i o n i t e / o f f r e t i t e c a t a l y s t s e x h i b i t e d w i t h h i g h e r s e l e c t i v i t y f o r e t h e n e and propene, b u t t h e y were d e a c t i v a t e d v e r y r a p i d l y by c o k i n g ; w h i l e t h o s e of p e n t a s i l t y p e , w i t h b e t t e r s t a b i l i t y ( r e f . 2 ) . F u r t h e r e f f o r t has been devoted t o t h a t modified p e n t a s i l z e o l i t e w i t h h i g h e r e t h e n e s e l e c t i v i t y , t y p e HZ-29-P. The e f f e c t of t e m p e r a t u r e on t h e d e a c t i v a t i o n o f t h i s t y p e of c a t a l y s t showed t h a t no obvious change i n r e a c t i v i t y was observed a t 475OC w i t h i n 20 h o u r s , w h i l e a t 55OoC t h e change became v e r y pronounced a f t e r two h o u r s on s t r e a m . TABLE 1 Conversion of methanol t o o l e f i n s on d i f f e r e n t z e o l i t e s
Type of z e o l i t e
Pentasil
HZ-29 Hz-29-P 79-2 79-2Y,g React. Temp. O C 548 553 547 506 bHSV l / h KO 4.2 3.8 7.6 P!eOH Conv. yo 1oc 100 100 100 Prod. D s t r . ' wt% DME 0 1.4 0 0.5 HCS 88.6 79.9 82.8 95.6 HCS. D i s t r . wt% ke t h a n e 1.8 7.3 8.5 1.7 Ethene 26.8 17.7 22.5 37.6 Propene 32.5 21.9 61.8 21.1 Butenes 6.5 7.1 2.4 15.3 c + 27.2 1 2 . 0 29.6 1.4 C2-Z4 C l e f i n s 5G.7 76.4 51.1 94.9 heOH c t g . 70% of w a t e r b y w t .
9
Erionite/offretite HE ZnHE HSh Sb-2
410 4.2
400 4.1 100 6 8
406 - 4 0 1 3.6 4.9 100 100
2.5 4.5 1.0 1.1 77.C 68.2 95.6 96.1 4.6 40.7 20.3 7.6 0.6 68.7
8.4 59.2 21.8 C.6
-
3.8 30.8 15.0 3.3
-
3.7 52.8 15.9 6.G
-
81.6 49.1 74.6
-
e x c l u d i n g of w a t e r and unconv.Ne0H
( i i ) Improvement of t h e HZ-29-P t y p e z e o l i t e c a t a l y s t s By s t r i c t l y c o n t r o l l i n g t h e c o n d i t i o n s of s y n t h e s i s , m o d i f i c a t i o n and p r e t r e a t m e n t t o o b t a i n c a t a l y s t s w i t h a p p r o p r i a t e c r y s t a l l i n i t y , c r y s t a l s i z e , c o m p o s i t i o n a i d a c i d i t y p a t t e r n and s o on, we have made a remarkable p r o g r e s s i n t h e improvement of t h e s t a b i l i t y of t h e c a t a l y s t s . The d u r a t i o n on s t r e a m w i t h o u t MeGH o r DIW b r e a k t h r o u g h e x t e n d i n g t o 30 h o u r s as shown i n Pig.1. ( i i i ) Comparision of z e o l i t e c a t a l y s t s s y n t h e s i z e d w i t h d i f f e r e n t b a s e s . The 25CG and 520b s e r i e s of z e o l i t e s were s y n t h e s i z e d w i t h i n o r g a n i c and o r g a n i c b a s e s r e s p e c t i v e l y and modified i n t h e same way. C a t a l y t i c e v a l u a t i o n were c a r r i e d o u t w i t h samples b o t h i n form of whole z e o l i t e and t h a t admixed w i t h b i n d i n g m a t e r i a l .
203
The r e s u l t s a r e shown i n Table 2 s n d F i g . 2 . The c a t a l y s t o f 5 2 G O s e r i e s p o s s e s s e d o u t s t a n d i n g s t a b i l i t y . One o f t h e samples w i t h b i n d i n g m a t e r i a l (5253) can m a i n t a i n a n e t h e n e s e l e c t i v i t y o f 3099 o v e r 80 hours w i t h o u t b r e a k t h r o u g h . The s u p e r i o r i t y of t h e l a t t e r o v e r t h e former may be a t t r i b u t e d t o i t s h i g h e r Si/A1 r a t i o , b e t t e r c r y s t a l l i n i t y and o t h e r c h a r a c t e r i s t i c s .
Time
rig.
-7.
on
Atream ( h )
1. S t a b i l i t y T e s t o f Z e o l i t e C a t a l y s t HZ-2910
TxBU 2 Comparision of performance o f 25OC & 5200 s e r i e s z e o l i t e c a t a l y s t s Catalysts No.of r e g e n e r a t i o n , R e a c t . temp. C WHSV l/h MeOH Conv. % Prod. U i s t r i b t n . w t % DPIE hCS HCS. D i s t r i b t n . w t % he t h a n e Ethene E r o pene
2500 Z e o l i t e 5200 Z e o l i t e No b i n d e r w. b i n d e r No b i n d e r w . b i n d e r 0 3 0 2 0 2 G 2 550 5 5 0 5 5 C 550 550 5 5 0 549 550 5.3 5.0 4.1 4.8 4.9 5.3 4.3 6.8 100 100 10C 1OC 10G 1OC 106 1GO
0 0 99.3 99.1
2.2 36.6 39.3 2.5 c5+ C2- C O l e f i n s 86.9 4 Iv.eOH c t g . 70% of w a t e r by
0 98.7
2.5 2.5 34.6 37.2 39.3 34.6 3.3 6.2 84.6 81.5 wt.
G
O
G
O
c
5.1 38.7 33.2 5.8 80.C
1.7 37.7 40.6 2.8 88.3
40.6 41.1 1.3 90.6
1.8
1.9 43.2 38.3
1.5 42.4 41.1
98.9 99.2
93.4 99.1
99.3
1.0
0.5
91.4
93.9
( i v ) F u r t h e r improvement o f t h e 5200 s e r i e s c a t a l y s t s Though a c a t a l y s t sample 5253 has g i v e n a much b e t t e r performa n c e , however, i t was s t i l l d e a c t i v a t e d s l o w l y as t h e t i m e on
204
1
2
t =-
2500 Regenerdced
2500
5 200
50
(h)
T. 0 . S.
F i g . 2 . Comparision o f S t a b i l i t y o f 250C and 5200 S e r i e s C a t h l y s t s o f hydrocarbons d i s t r i b u t i o n i n t h e product. The most s i g n i f i c a n t c r i t e r i o n is the d e c l i n a t i o n o f t h e y i e l d o f ethene with a F a r t i a l
compensation of i n c r e a s i n g i n propene and b u t e n e s , t h u s keeping t h e o l e f i n s r e l a t i v e l y i n v a r i a n t . Another n o t i c e a b l e changes a r e t h e i n c r e a s i n g of C + f r a c t i o n and methane, t h e l a t t e r presumably 5 c l o s e l y r e l a t e d t o coking. A s g e n e r a l l y acknowledged, d e a c t i v a t i o n of z e o l i t e c a t a l y s t may be comprised o f r e v e r s i b l e type due t o coki n g and i r r e v e r s i b l e , t o t h e d i m i n i s h i n g of a c i d i c s i t e s by hgdrothermal dealumination. The mechanistic c o n s i d e r a t i o n o f d e a c t i v a t i o n and i t s c o r r e l a t i o n w i t h t h e r e s u l t o f d i a g n o s t i c a n a l y s i s o f a n aged c a t a l y s t has been r e c e n t l y r e p o r t e d ( r e f . 4 ) . F i g s 3-4 and Table 3 a r e t h e r e s u l t s of a n improved c a t a l y s t which has s u s t a i n e d 83 o x i d a t i v e r e g e n e r a t i o n s and over 1000 hours on s t r e a m , g i v i n g a n averaged y i e l d of t o t a l hydrocarbons 99.3, C2-C4 o l e f i n s 89.7, e t h ene 31.5 and propene 44.4%,designated as 5263. C2-C4
a. Fresh Catalyst "5263" b. After 1000 h Test I
I I
150
3000 C
400
I
500
600
~I
Fig.3 NH3-TPD s p e c t . of cat.5263 Fig.4.
2000 1800 1600 1400 Cm-1
PyrdrIR s p e c t . o f cat.5263
205
TABLE 3
ESCI.. r e s u l t s of c a t a l y s t s 11525311and 11526311 Catalysts
T.O.S.(hr)
5 L53 5263
243 1L 2 9
R e l d t i v e v u l u e s f o r aged and f r e s h c a t a l y s t si/d (.&-tom of modified compd)/(Si+Al) 0.45 2.39 G.54 1.05
F u r t h e r t e s t i n g s of t h e improved c a t a l y s t 5263 fin a d d i t i o n a l s t a b i l i t y t e s t was c a r r i e d o u t i n a n e n l a r g e d l a b o r a t o r y r e a c t o r w i t h 3Cg of whole p e l l e t c a t a l y s t , which i n 81G h o u r s on s t r e a m and b e i n g s u b j e c t e d t o 3 8 r e g e n e r a t i o n s w i t h 24 h o u r s p e r c y c l e , gave a n averaged y i e l d s of t o t a l hydrocarbons 99.3 C2-C o l e f i n s 88.6, e t h e n e 31.9 and Fropene 42.4%. B e s i d e s , some 4 i n v e s t i g a t i o n s on t h e e f f e c t of p r o c e s s v a r i a b l e s were conducted i n a bench s c a i e u n i t , as shown i n F i g . 5. R e s u l t s c o n s i s t e n t w i t h t h o s e mentioned above have been r e p r o d u c e d . I n c o n c l u s i o n , t h e s e s t u d i e s may, more o r l e s s , p r o v i d e some c u e s f o r a p e l i r n i n a r y f e a s i b i l i t y e v a l u a t i o n of s u c h p r o c e s s .
F i g . 5. Flow diagram of t h e bench s c a l e u n i t . ( 1 . Feed pumps; 2. P r e h e a t e r ; 3 , 4 . R e a c t o r s ; 5. C o o l e r ; 6 . S e p a r a t o r ; 7 . G a s s a m p l e r ) HCKN G vv’ LEDGEbiENT Coworkers: Y.H.Yang, r..L.Ying, Z.Z. Wang, L.G. G u o , S.Y. L i , J.W. Zhang, S . Zhu, Y.S. Liu, R.M. S h i , P. J i , Y.H. Wang, J.S. Lo e t a i
REFERENCES
-
1 C.D. Chang, C a t a l . Rev. S c i . & Eng. 26 ( 3 & 4 ) ) 1984, 324 2 G. Q . Chen, J . L i a n g e t a l , Proc. CHN-JPN-USA Syrnp. Hetero. Cat. D a l i a n , China, 1982, A1 OC 3 G. Y. C a i , G. Q. Chen e t a l , Z e o l i t e s , H . D r z a j ( e d i t o r ) 1985, E l s e v i e r S c i . Publ. B.V., 319-327. 4 G.Q. Chen, J. Liang e t a l , Proc. 7 t h I n t . Zeoi. Conf., Tokyo, 1986,9C7
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D.M. Bihhy, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conversion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
207
REACTIONS OF METHANOL AND TOLUENE OVER MOLYBDENUM ZEOLITES
M.M.
HUANG’ and R.F.
HOWE2
I
Department o f Modern Chemistry, U n i v e r s i t y o f Science and Technology o f China, H e f e i , Anhui ( P e o p l e ’ s R e p u b l i c o f China) ‘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) ABSTRACT The c a t a l y t i c p r o p e r t i e s o f z e o l i t e s ZSM-5, m o r d e n i t e and Y e i t h e r i o n exchanged o r impregnated w i t h molybdenum f o r t h e c o n v e r s i o n o f methanol and t o l u e n e have been i n v e s t i g a t e d . The d i s t r i b u t i o n o f p r o d u c t s and t h e change i n c a t a l y t i c p r o p e r t i e s w i l l be accounted f o r i n terms o f a c i d i t y and t h e l o c a t i o n o f molybdenum i n z e o l i t e s . INTRODUCTION Recently, i n t e r e s t has grown i n t h e m o d i f i c a t i o n o f z e o l i t e s t o e f f e c t t h e c o n v e r s i o n o f methanol ( r e f s . 1-4) o r t o l u e n e ( r e f . 1,5) hydrocarbons.
i n t o s p e c i f i c types o f
The i n t r o d u c t i o n o f m o d i f i e r s , such as P and B, i n c r e a s e d t h e
y i e l d o f o l e f i n s i n t h e f o r m e r case and t h e r e l a t i v e d i s t r i b u t i o n o f p a r a a r o m a t i c s i n b o t h cases.
However, t h e fundamental q u e s t i o n remains open as t o
whether t h e r o l e o f m o d i f i c a t i o n c o n s i s t s o n l y o f b l o c k i n g t h e channels and t h u s c r e a t i n g d i f f u s i o n a l hindrances, o r a l s o i n a l t e r i n g t h e c o n c e n t r a t i o n and strength o f acid sites.
I n t h i s work, M o - z e o l i t e s (ZSM-5, m o r d e n i t e and Y ) were
used t o i n v e s t i g a t e t h e m o d i f i c a t i o n e f f e c t o f Mo on t h e s e r e a c t i o n s . EXPERIMENTAL I o n exchanged Mo-ZSM-5 and Mo-mordenite were prepared i n an aqueous s o l u t i o n h a v i n g a pH below 1.0 and u s i n g Mo02C12 as t h e s t a r t i n g m a t e r i a l .
Impregnated
M o - z e o l i t e s were prepared by s i m i l a r method except a t h i g h pH which was c o n t r o l l e d by a d d i t i o n o f p y r i d i n e .
A detailed description o f the preparation
and c h a r a c t e r i z a t i o n o f t h e c a t a l y s t w i l l be r e p o r t e d elsewhere ( r e f . 6). Methanol c o n v e r s i o n was c a r r i e d o u t u s i n g a c o n v e n t i o n a l f l o w r e a c t o r a t 370°C and a WHSV = 20 h r - l . WHSV = 0.7 h r - l .
Toluene c o n v e r s i o n was c a r r i e d o u t a t 5OOOC and a
Toluene vapour was c a r r i e d t h r o u g h t h e c a t a l y s t bed u s i n g H2
o r N2 as v e c t o r gas.
208
RESULTS
Methanol conversion Methanol conversion was carried out over Mo-ZSM-5 and a series of reference samples. Under the standard conditions (5 minutes exposure to methanol at 37OoC and WHSV = 20 hr-I) the Mo exchanged ZSM-5 (0.2 wt % Mo) gave a signfficantly higher yield of aliphatics than H-ZSM-5 (82% cf with 74%), although the total hydrocarbon yield was the same. An Mo impregnated ZSM-5 (7.5 wt % Mo) gave 100% conversion to aliphatics, although in this case the total hydrocarbon yield was reduced to 70% of that over H-ZSM-5. Significant differences were also found in the relative distributions of aliphatic molecules (Fig. 1 ) . The yield of alkenes was enhanced relative to that of the corresponding alkanes over Moexchanged ZSM-5 (this was a general result under other reaction conditions also). A similar trend was found over partially pyridine poisoned H-ZSM-5, although in this case the conversion was reduced. The Mo impregnated zeolite gave methane as the only major product. Enhancement of the methane yield (although less pronounced) was also observed for a physical mixture of Moo3 and H-ZSM-5 (7.5 Wt %).
24601
1w
X
80
M
C(
c*
c;
cj
c;
qJ 4
Figure 1 Relative yield in methanol conversion
d X
I
1. '
Mo exchanged ZSM-):Reduced in H at 450 C for 1 hr (the fresh sample had almost same product yield and dfstribution)
Mo impregnated,ZSM-5:Calcined in air at 600 C for 3 hrs 0 Partly poisoned H-ZSM-5:Pyridine was adsorbed a t 120 C followed by evacuation at 400 C for 30 minutes 0 Physical mixture of H-ZSM-5 and Moo3: 7.5 wtZ Mo
209
The distribution among aromatic products was also altered by the presence o f molybdenum (Fig. 1 ) . The relative yields of para and meta isomers o f xylenes and ethyltoluenes were increased over Mo exchanged ZSM-5 and over the physical mixture o f Moo3 and H-ZSM-5 (no aromatics were detected over the impregnated catalyst), but this effect was not found for the partially poisoned catalyst. Toluene conversion All the Mo-zeolites were tested using the toluene conversion reaction. In most cases the carrier gas was hydrogen, then the conversion of toluene mainly consisted of disproportionation, hydrodealkylation and hydrocracking. The disproportionation of toluene was the dominant reaction on Mo exchanged ZSM-5 and mordenite (0.4 wt % Mo). Compared with H-ZSM-5 and H-mordenite, the Mo exchanged samples had a much better resistance to deactivation, and this efftect was more pronounced on Mo mordenite, especially when H2 was used as carrier gas. The variation in the distribution of xylene isomers with the conversion is plotted in Fig. 2. The Mo zeolites enhanced the relative yield of para + meta xylenes. In contrast, the hydrodealkylation and hydrocracking became pronounced on all Mo impregnated zeolites (Table 1 ) . For example, benzene became the only aromatic product after 2 hours of reaction on Mo impregnated ZSM-5.
4
3.8 0 \
3.6
0 \
E + n
E + n 3.4
3.2 3
II
I
8
* \
I4
\
H ZSN-5
\
I
13
18 Conversion
23
(%I
28
2
4
6
8
Conversion
(%I
Figure 2 Xylene distribution on no exchanged zeolites
10
DISCUSSION
A f t e r e v a c u a t i o n a t 4OOOC t h e p y r i d i n e r e m a i n i n g on poisoned H-ZSM-5 can o n l y be h e l d by t h e s t r o n g a c i d s i t e s , t h u s t h e s i m i l a r i t y between Mo-exchanged and p y r i d i n e poisoned ZSM-5 s t r o n g l y suggests e l i m i n a t i o n o f t h e s t r o n g a c i d s i t e s i n t h e Mo exchanged ZSM-5,
and t h i s i s t h e main reason f o r t h e i n c r e a s e i n t h e
y i e l d o f a l i p h a t i c s , l a r g e l y t h e l i g h t alkenes.
The e l i m i n a t i o n o f s t r o n g a c i d
s i t e s was a l s o p o s t u l a t e d by Ratnasamy ( r e f . 3 ) f o r Mo-ZSM-5 prepared f r o m ammonium paramolybdate and c o n f i r m e d by TPD.
The C2-C4
o l e f i n s are i n t e r -
mediates i n t h e c o n v e r s i o n o f methanol and t h e f o r m a t i o n o f a r o m a t i c s occurs a t strong acid s i t e s ( r e f . 7), t h e r e f o r e t h e deactivation o f strong acid s i t e s w i l l r e s u l t i n h i g h e r y i e l d s o f l i g h t alkenes.
The same reason i s a p p r o p r i a t e f o r
t h e s l o w d e a c t i v a t i o n o f t o l u e n e c o n v e r s i o n on Mo exchanged z e o l i t e s , because t h e s t r o n g a c i d s i t e s a r e r e s p o n s i b l e f o r r a p i d coke f o r m a t i o n which i s t h e main cause o f d e a c t i v a t i o n .
The weak a c i d i t y has been a l s o used t o e x p l a i n ( r e f . 8 )
t h e b e t t e r l i f e t i m e and t h e l o w e r coke f o r m a t i o n o f a l k a l i n e e a r t h m e t a l exchanged m o r d e n i t e i n t h e benzene a l k y l a t i o n r e a c t i o n .
I n t h i s work, however,
t h e f u n c t i o n o f Mo was n o t c o n f i n e d t o t h e r e d u c t i o n i n a c i d i t y , t h e remarkable r e s i s t a n c e t o t h e d e a c t i v a t i o n i n t h e presence o f H2 a l s o suggested a hydrog e n a t i o n f u n c t i o n o f Mo, s i n c e coke f o r m a t i o n i s a g r a d u a l dehydrogenation process ( r e f . 9 ) .
T h i s f u n c t i o n o f Ma became v e r y apparent on t h e Mo
impregnated samples, t h a t i s , t h e h y d r o d e a l k y l a t i o n and h y d r o c r a c k i n g r e a c t i o n s dominated o v e r t h e d i s p r o p o r t i o n a t i o n r e a c t i o n .
The h i g h y i e l d o f methane i n
t h e methanol c o n v e r s i o n on Mo impregnated ZSM-5 can a l s o be e x p l a i n e d b y a h y d r o c r a c k i n g r o u t e , which was c a t a l y z e d by Mo and t h e t o t a l process c o u l d be as 2CH30H
-+
CH4 + 2H20 + [C].
The r e l a t i v e d i s t r i b u t i o n o f p a r a + meta a r o m a t i c s i n methanol c o n v e r s i o n was i n c r e a s e d o v e r Mo exchanged ZSM-5 b u t n o t on t h e p y r i d i n e poisoned sample.
The
same i n c r e a s e t r e n d was a l s o observed i n t h e d i s p r o p o r t i o n a t i o n r e a c t i o n o v e r Mo exchanged z e o l i t e s ( F i g . 3), t h u s a r e a s o n a b l e e x p l a n a t i o n i s t h e presence o f i n t e r n a l Mo.
Both methanol and t o l u e n e c o n v e r s i o n s performed on z e o l i t e s a r e
m o l e c u l a r shape s e l e c t i v e processes.
I n t e r n a l Mo w i l l c r e a t e d i f f u s i o n a l
h i n d r a n c e s which w i l l f a v o u r t h e f o r m a t i o n o f p a r a a r o m a t i c s ( p r o d u c t s e l e c t iv i ty)
.
I n summary, we suggest t h a t t h e changes i n a c i d i t y a r e r e s p o n s i b l e f o r t h e s e l e c t i v e f o r m a t i o n o f l i g h t o l e f i n s , whereas t h e g e o m e t r i c l o c a t i o n o f M o was more i m p o r t a n t f o r t h e s e l e c t i v e f o r m a t i o n o f p a r a a r o m a t i c s i n b o t h methanol and t o l u e n e c o n v e r s i o n r e a c t i o n s .
The e x i s t e n c e o f Mo i n t r o d u c e d a hydro-
g e n a t i o n f u n c t i o n e i t h e r w i t h i n t h e z e o l i t e channels o r on t h e e x t e r n a l s u r f a c e . We g r a t e f u l l y acknowledge t h e p r o v i s i o n o f equipment and a d v i c e f o r t h e methanol c o n v e r s i o n experiments b y P r o f e s s o r A.L.
Ode11 and D r
D. Dass.
211
Table
1 : Toluene conversion* on Pa impregnated z e o l i t e
Sample
*t
Conversion
(%I
Mo -Y (5.0 wt:
t A t 10 min
{A CX
CB
E
A t 60 n i n
EA CX CB
E
A t 120 n i n
FA CX
Reaction conditions: T = 500% CB
**
~ 0 )
U-mrdeni t e ( 5 . 3 ws UO)
U-2 SM-5 (7.5 wt% MI
13.7 13.7 42.2 57.8
3.4 2.4 12.5 87.5
19.7 19.7 14.2 85.8
13.6 11.9
9.1 7.3
61.3 46.7
25.0 75.0
15.1 84.9
1.4 98.6
10.8 9.1
8.0 6.4
93.2 42.0
20.1
23.2
0
19.9
76.8
100
-
c a r r i e r gas = H2 YHSV = 0.7 h r - * c a t a l y s t wt.= 50 mg F toluene conversion C, conversion t o arcinatics
cx
w l e f r a c t i o n o f Xylene i n aromatics C8
w l e f r a c t i o n o f benzene i n aranatics
REFERENCES
1
W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and S.A. B u t t e r , J. Catal., 67 (1981), 159-174. 2 J.C. Vedrine, A. Auroux, P. D e j a i f r e , V. Ducarme, H. Hoser and S. Zhou, J. Catal., 73 (1982), 147-160. I. Balakrishnan and P. Ratnasamy, i n "Chem. Uses Molybdenum, Proc. I n t . 3 Conf., 4 t h 1982", H.F. B a r r y and P.C.H. M i t c h e l l , Ed., Climax Molybdenum Co. Mich., p. 331. 4 H. Okado, T. Sano, H. S h o j i , K. Kawamura, H. Hagiwara and H. Takaya, Proc. 7 t h I n t e r n . Conf. on Z e o l i t e s , Tokyo, 1986, p. 339. W.W. Kaeding, C. Chu, L.B. Young and S.B. B u t t e r , J. Catal., 69 (1981), 3925 398. 6 M. Huang and R.F. Howe, t o be published. P. D e j a i f r e , J.C. Vedrine, V. R o l i s and E.G. Derouane, J. Catal., 63 (1980), 7 331-345. 8 H.G. Karge and J. Ladebeck, i n " C a t a l y s i s by Z e o l i t e s " , B. I m e l i k e t al., Ed., E l s e v i e r Sci. Pub. Co., Amsterdam, 1980, p. 151. 9 D.G. Blackmond, J.G. Goodwin, Jr., and J.E. Lester, J. Catal., 78 (1982), 34-43. 10 E.G. Derouane, i n " Z e o l i t e : Science and Technology", F.R. R i b e i r o , A.E. Rodrigues, L.D. Rollmann and C. Naccache, Ed., NATO AS1 S e r i e s E, 1984, No. 80, p. 347.
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213
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
THE SELECTIVITY OF CATALYSTS COWOSED OF V205 SUPPORTED OM Zr02-Y203 M I X E D O X I D E S FOR METHANOL OXIDATION
J.G.
van Ommen, P.J.
G e l l i n g s and J.R.H.
Ross
F a c u l t y o f Chemical Technology, U n i v e r s i t y o f Twente, P.O. Enschede (The N e t h e r l a n d s )
Box 217,
7500 AE
ABSTRACT V 0 monolayer c a t a l y s t s were prepared on Z r O and Z r O doped w i t h Y 0 by 2 2 3 t w o ie?hods. The coverages o b t a i n e d a r e o n l y h a l f a monolayer and d i d n o t depend on t h e p r e p a r a t i o n method o r t y p e o f support. The s e l e c t i v i t y f o r o x i d a t i o n o f methanol o v e r t h e s e V 0 c a t a l y s t s changes f r o m a predominance o f formaldehyde t o a predominance o f r$e$hyl fomate when t h e s u p p o r t i s doped w i t h Y203, independent o f t h e amount o f Y203. INTRODUCTION The s e l e c t i v e o x i d a t i o n o f methanol t o g i v e formaldehyde i s i n p r a c t i c e performed i n two d i f f e r e n t processes, one u s i n g m e t a l l i c s i l v e r , t h e o t h e r u s i n g i r o n molybdate as c a t a l y s t . Vanadium o x i d e has been shown t o be a good s e l e c t i v e c a t a l y s t i n a v a r i e t y o f o x i d a t i o n processes ( r e f s . 1-2) and we have p r e v i o u s l y shown t h a t i t i s a l s o s e l e c t i v e f o r methanol o x i d a t i o n ( r e f s . 3-5):
when t h e
V 0 i s a p p l i e d as a v e r y t h i n l a y e r (monolayer) on d i f f e r e n t s u p p o r t s : t h e 2 5 s u p p o r t can have a s i g n i f i c a n t i n f l u e n c e on 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 t h e s e monolayer c a t a l y s t s , as was shown by Roozeboom ( r e f . 6). paper ( r e f . 5).
I n a previous
i t was shown t h a t b o t h t h e t y p e o f s u p p o r t (A1203 o r Ti02) and
t h e c r y s t a l s t r u c t u r e o f t h e Ti02 have an i n f l u e n c e on t h e s e l e c t i v i t y o f t h e c a t a l y s t f o r t h e p r o d u c t i o n o f formaldehyde:
i n general,
production o f t h e
formaldehyde i n c r e a s e s w i t h a decrease i n t h e r e d u c i b i l i t y o f t h e vanadia. T h i s 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 c a t a l y t i c a c t i v i t y and s e l e c t i v i t y o f vanadia can a l s o b e a f f e c t e d by d o p i n g t h e s u p p o r t b y i o n s w i t h a l o w e r valency. I n t h e p r e s e n t work, we r e p o r t on t h e e f f e c t on 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 V205/Zr02 c a t a l y s t s b r o u g h t about b y d o p i n g a Zr02 s u p p o r t w i t h Y203 (0-17 molX).
EX P E R IF1E MTA L P r e p a r a t i o n o f t h e Support The z i r c o n i u m o x i d e s u p p o r t s doped w i t h y t t r i u m o x i d e were prepared by h y d r o l y z i n g w i t h w a t e r a m i x t u r e o f z i r c o n i u m and y t t r i u m a l l t o x i d e s ( r e f .
7),
214
f o l l o w e d b y c a l c i n a t i o n f o r t w o h o u r s a t 650
OC.
Preparation o f the Catalyst The c a t a l y s t s were p r e p a r e d b y a d s o r p t i o n u s i n g one o f t h e f o l l o w i n g solutions: ( a ) an aqueous s o l u t i o n o f NH4V03 ( 8 . 5 mmol 1 - l ) a c i d i f i e d w i t h HN03 t o pH=4 (ref.
6 ) (method 1, see t a b l e s ) : ( b ) a s o l u t i o n o f V O ( A C A C ) ~ ( 4 . 3 mmol 1-')
in
t o l u e n e p r e p a r e d and k e p t i n t h e absence o f a i r and m o i s t u r e (method 2, seetables) ( r e f . 3).
S o l u t i o n ( a ) o r (b),
m o i s t u r e , was passed t h r o u g h 22 h o u r s a t 2 2
OC.
t h e l a t t e r i n t h e absence o f a i r and
a bed o f t h e s u p p o r t p a r t i c l e s (0.3
The r e s u l t a n t m a t e r i a l was t h e n d r i e d a t 110
and c a l c i n e d a t 450
OC
OC
-
0.6 mm) f o r f o r two hours
f o r two hours.
A n a l y s i s and A c t i v i t y Measurements The c o n t e n t s o f vanadium,
t h e s u r f a c e a r e a s and t h e c r y s t a l s t r u c t u r e s were
d e t e r m i n e d as d e s c r i b e d p r e v i o u s l y ( r e f .
5).
RESULTS Table 1 g i v e s t h e composition o f t h e supports, t h e i r s u r f a c e areas, t h e s u r f a c e a r e a s o f t h e c a t a l y s t s and t h e i r vanadium c o n t e n t s .
I t c a n b e seen t h a t
u s e o f s o l u t i o n ( a ) g i v e s an i n c r e a s e i n s u r f a c e a r e a w h i l e t h e u s e o f s o l u t i o n ( b ) c a u s e s t h e s u r f a c e a r e a t o r e m a i n a l m o s t unchanged. The c o v e r a g e s a r e a l l o f t h e same o r d e r o f m a g n i t u d e and o n l y a b o u t h a l f o f a m o n o l a y e r c o v e r a g e ( r e f . 3 ) i s r e a c h e d . The c o v e r a g e s a c h i e v e d a r e a p p a r e n t l y n o t dependent on t h e amount o f y t t r i u m p r e s e n t i n t h e ziconium. TABLE 1 C h a r a c t e r i z a t i o n o f C a t a l y s t s and S u p p o r t s Method o f
Composition
Support
Catalyst
Preparation
o f Support-"
.''
2 -1 m .gr
2 -1 m .gr
1 2 1 1
zy, ZYn ZYU
ZYL
2 ZY;; 1 "1 8 2 ZY 1 complete monolayer
Y- A
e.g.
ZY5= 5% YOlw5
57 57 84 108 108 93 93 87
66 57 98 119 108 103 95 36
at.V/nm
2
2.4 3.2 2.6 2.5 2.2 3.8 2.9 3.7 5.95
i n Zr02
From t h e r e s u l t s o f T a b l e 2,
i t appears
,
d e p e n d i n g o n t h e amount o f y t t r i u m
oxide present i n zirconium oxide, t h a t t h e c r y s t a l s t r u c t u r e o f t h e support changes f r o m m o n o c l i n i c ( p u r e Zr02 = ZY,)
t o p u r e c u b i c ZY18.
The p r e s e n c e o f
h a l f o f a m o n o l a y e r o f V205 appears, f o r t h e f i r s t t w o samples, t o h e l p t h e
215
s u p p o r t t o a t t a i n a c r y s t a l s t r ! i c t u r e n e a r e r t o t h e thermodynamically more s t a b l e s t r u c t u r e p r e d i c t e d f r o m t h e phase d i a g r a m (see Table 2). TABLE 2 X-ray D i f f r a c t i o n o f C a t a l y s t s and Supports Composition o f Support
Support
P It C C C
zyo
ZY ZY ;o
1-
ZY
8
c
Catalyst
M
b1
S t a b l e Phases According t o Phase Diaqram
P I M t c M + C C
+c C C
M = m o n c l i n i c ; C = Cubic ( f l u o r i t e ) The r e s u l t s f o r methanol o x i d a t i o n on t h e s e s u p p o r t s and f o r t h e c a t a l y s t s prepared f r o m t h e s e s u p p o r t s a r e shown i n Tables 3 and 4 r e s p e c t i v e l y . The r e s u l t s a r e p r e s e n t e d i n terms o f T s t a r t ( t h e t e m p e r a t u r e f o r 2% MeOH c o n v e r s i o n ) , T50 ( t h e t e m p e r a t u r e f o r 50% FleOH c o n v e r s i o n ) o r T(CH20)rnax ( t h e t e m p e r a t u r e t o g i v e t h e maximum y i e l d o f CH20);
the selectivities a t the last
t e m p e r a t u r e a r e a l s o given. The s u p p o r t s a l o n e ( T a b l e 3 ) produce m o s t l y CO and
C02 and o n l y v e r y s m a l l amounts o f CH 0 and CH30CH3. A l t h o u g h t h e r e a r e s m a l l
2 changes i n t h e a c t i v i t i e s , t h e r e i s no c l e a r dependence on t h e amount o f y t t r i u m oxide present. TABLE 3 Methanol O x i d a t i o n o v e r Zr02/Y203 Supports
ZY
245 275 245
364 365 380 360 340
T s t a r t = 2% c o n v e r s i o n ; T 50% = 50%conversion TABLE 4 Methanol O x i d a t i o n o v e r V205 on Zr02/Y203 Supports Method o f preparation 1 2 1
Catalyst V205 on support
ZY ZYE ZY10
zyl 0
2 1
1 ;8
y24
T s t a r t T(CH20)max Conv% S e l e c t i v i t y % OC
150 150 150 150 140 150 150 2 50
OC
230 225 240 230 235 235 245 400
MeOH 86 85 84 81
78
89 75 93
CH,O
CHOOCH, 48 56 31 30 30 30 33
11
41 38 53 63 63 56 57 2
S e l e c t i v i t y = moles o f FleOH c o n v e r t e d t o p r o d u c t i / m o l e s o f MeOH c o n v e r t e d
216
The r e s u l t s f o r t h e V205 monolayer c a t a l y s t s g i v e n i n Table 4 show, w i t h t h e e x c e p t i o n o f t h a t supported on Y203. t h a t t h e r e i s no g r e a t d i f f e r e n c e i n a c t i v i t y o f t h e supported V205 w i t h r e s p e c t t o T s t a r t o r T(CH20)max.
However,
t h e r e i s a c l e a r d i f f e r e n c e i n t h e s e l e c t i v i t y . When t h e V205 i s supported on m a t e r i a l s c o n t a i n i n g Y203, t h e r e i s a remarkable i n c r e a s e i n s e l e c t i v i t y t o methyl f o r m a t e compared t o t h a t w i t h t h e V205/Zr02 c a t a l y s t . The method b y w h i c h t h e monolayer o f V 0
2 5 was p r e p a r e d has no i n f l u e n c e . The V205/Y20,
c a t a l y s t was
found t o r e q u i r e much h i g h e r temperatures and t o be r e l a t i v e l y v e r y u n s e l e c t i v e .
DISCUSSION The r e s u l t s i n Table 1 show t h a t t h e c a t a l y s t s prepared w i t h NH,V03 VO(AcAC)
or
2 on doped Zr02 b o t h g i v e t h e same coverages o f t h e support. The amount
o f V205 a t t a i n e d i s o n l y h a l f t h a t r e q u i r e d f o r a monolayer, t h i s p r o b a b l y b e i n g caused b y t h e f a c t t h a t Zr02 c o n t a i n s fewer
OH
groups t h a n does T i 0 2 ( r e f . 3):
i t would seem t h a t t h e number o f OH groups does n o t depend on t h e amount o f Y203 i n the ZrO
2 s u p p o r t . The X-ray d i f f r a c t i o n p a t t e r n s show t h a t t h e s u p p o r t s have
d i f f e r e n t c r y s t a l s t r u c t u r e s , depending on t h e amount o f Y203 p r e s e n t . The most s t a b l e c r y s t a l phase i s n o t a t t a i n e d . However, i f a (sub-)monolayer
o f V205 i s
p r e s e n t , r e c r y s t a l l i z a t i o n o f t h e s u p p o r t o c c u r s more e a s i l y , as i s shown by t h e f a c t t h a t ZYo and ZY5 have a t t a i n e d a s t r u t u r e as p r e d i c t e d f r o m t h e phase diagram. W h i l e a l l t h e c a t a l y s t s have a l m o s t t h e same coverages o f V205, t h e r e i s a remarkable d i f f e r e n c e i n s e l e c t i v i t y between t h o s e on t h e p u r e s u p p o r t s and t h o s e on y t t r i u m - d o p e d supports. These d i f f e r e n c e s i n c a t a l y t i c b e h a v i o u r a r e most l i k e l y caused by d i f f e r e n t i n t e r a c t i o n s between t h e s u p p o r t and t h e a c t i v e phase. C o n s i d e r i n g f i r s t t h e r e s u l t s f o r methanol o x i d a t i o n o v e r t h e s u p p o r t s alone, we f i n d t h a t t h e y a l l have n e a r l y t h e same a c t i v i t y and s e l e c t i v i t y :
Y 0
i s n o t d r a m a t i c a l y d i f f e r e n t i n behaviour. However t h e V 0
even
containing
2 5 2 3 c a t a l y s t s show a remarkably d i f f e r e n c e i n s e l e c t i v i t y : t h e V205/Y203 c a t a l y s t i s a l s o l e s s a c t i v e . Those V205 c a t a l y s t s c o n t a i n i n g Y203 i n t h e Zr02 s u p p o r t produce much more m e t h y l f o r m a t e t h a n do t h o s e c o n s i s t i n g o f V 0
2 5 on p u r e Zr02;
i t i s n o t i m p o r t a n t how much Y 0 i s p r e s e n t . The d i f f e r e n c e i n c r y s t a l 2 3 s t r u c t u r e between ZY5 and ZY,o does n o t seem t o b e o f g r e a t importance. One
e x p l a n a t i o n o f t h e s e o b s e r v a t i o n s c o u l d be t h a t t h e r e i s a s u r f a c e enrichment o f t h e Zr02 by Y203 ( r e f . 8 ) i n such a way t h a t t h e s u r f a c e o f t h e r e s u l t a n t s u p p o r t has a l m o s t t h e same composition,
independent o f t h e b u l k c o n c e n t r a t i o n
o f Y203. The independence of t h e r e s u l t s o f methanol o x i d a t i o n on t h e n a t u r e o f t h e s u p p o r t i s i n agreement w i t h t h i s suggestion. The i n f l u e n c e o f t h e Z Y s u p p o r t s on t h e change o f s e l e c t i v i t y o f t h e V205 f o r t h e o x i d a t i o n o f methanol t o m e t h y l f o r m a t e r a t h e r t h a n t o formaldehyde c o u l d be caused by an i n c r e a s e i n t h e p r o p o r t i o n o f t h e c a t i o n vacancies i n t h e support,
217
c a u s i n g an e l e c t r o n d e f i c i e n c y i n t h e c a t a l y t i c a l l y a c t i v e V205. T h i s r e s u l t s i n a s t r o n g e r a d s o r p t i o n o f methanol and t h e o x i d a t i o n p r o d u c t s on t h e c a t a l y t i c s u r f a c e , t h u s l e a d i n g t o f u r t h e r o x i d a t i o n o f t h e methanol t o methyl formate. The use o f a p u r e Y203 s u p p o r t i n c r e a s e s t h e c a p a c i t y o f t h e V205 f o r t h e t o t a l o x i d a t i o n o f methanol t o CO and C02. T h i s c a t a l y s t a l s o has a l o w e r BET s u r f a c e area: i t i s t h u s u n l i k e l y t h a t d i f f e r e n c e s i n s u r f a c e area can account f o r t h e differences i n selectivity. IJe may conclude t h a t t h e s e l e c t i v i t y f o r t h e o x i d a t i o n o f methanol o v e r V205 monolayer c a t a l y s t s changes from g i v i n g p r e d o m i n a n t l y formaldehyde as p r o d u c t t o p r e d o m i n a n t l y methyl f o r m a t e when t h e Zr02 s u p p o r t i s doped w i t h Y203. O p t i m a l i z a t i o n o f t h i s c a t a l y s t system m i g h t l e a d t o a c a t a l y s t which can produce e x c l u s i v e l y methyl formate. ACKNOWLEDGEMENTS lde thank G.J.H.
A l t e n a f o r p e r f o r m i n g p a r t o f t h e e x p e r i m e n t a l work, M.A.C.G.
van de Graaf f o r t h e p r e p a r a t i o n o f t h e supports, A.J.
van Hengstum and H. Bosch
f o r valuable discussions. REFER E MC E S 1. A. B i e l a n s k i and J . Haber, C a t a l . Rev. S c i . Eng., 19 (1979) 1. 2. D.J. Hucknall, i n S e l e c t i v e O x i d a t i o n o f Hydrocarbons, Academic Press,
London, (1974) 3. J.G. van Ommen, K. Hoving, H. Bosch. A.J. van Hengstum and P.J. G e l l i n g s , Z. Phys. Chem. N.F.. 134 (1983) 99. 4. A.J. van Hengstum, J.G. van Ommen. H. Bosch and P.J. G e l l i n g s , Appl. Catal., 5 (1983) 207. 5. A.J. van Hengstum, J.G. van Ommen, H.Bosch and P.J. G e l l i n g s , i n Proc. 8 t h I n t . Congr. Catal., B e r l i n (1984). V e r l a g Chemie Weinheim (1984). vo1.4,
p.297.
6. F. Roozeboom, P.P. C o r d i n g l y and P.J. G e l l i n g s , J. Catal., 68 (1981) 464. 7. M.J. Verkerk, P.J. F l i d d e l h u i s and A.J. Burggraaf, S o l i d S t a t e I o n i c s , 6 (1982) 159. 8. L. Winnubst, Thesis, Twente U n i v e r s i t y o f Technology, 1984.
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219
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
ACTIVE S P E C I E S AND MECHANISM FOR M I X E D ALCOHOL SYNTHESIS OVER
SILICA-SUPPORTED
MOLYBDENUM CATALYSTS
T. TATSUMI, A.
MURAMATSU. K.
YOKOTA and H. TOMINAGA
Department o f S y n t h e t i c Chemistry, Tokyo, Hongo, Tokyo 113 (Japan)
F a c u l t y o f Engineering,
The U n i v e r s i t y o f
ABSTRACT
The presence o f m e t a l l i c iilo and MOO on Si02 was found t o be a p r e r e q u i s i t e t h e development o f a c t i v i t y and s e f e c t i v i t y f o r a l c o h o l s y n t h e s i s f r o m COThe g r a d u a l i n c r e a s e i n a l c o h o l p r o d u c t i o n d u r i n g t h e r e a c t i o n i s H2' a s c r i b e d t o t h e f o r m a t i o n o f CO-reduction induced d e f e c t s on Mo02: Experiments performed by adding o l e f i n s , t o CO-H2 r e v e a l e d t h a t CO i n s e r t i o n i n t o a metala l k y l l i k e bond c o n s t i t u t e s t h e r e a c t i o n pathway t o a l c o h o l s . The r o l e o f K and C 1 i n t h e p r o m o t i o n o f a l c o h o l p r o d u c t i o n i s discussed. to
INTRODUCTION Since
A l c o h o l s a r e o f g r e a t promise as octane improver i n a u t o m o t i v e f u e l . blending
o f MeOH i n t o g a s o l i n e r a i s e s t h e problem o f
volatility additives
and
lowering
in
calorific
and t h e i r s y n t h e s i s f r o m CO-H2
value,
phase
higher
separation,
alcohols
are
i s o f special interest.
o f r e p o r t s on t h e c a t a l y s t s f o r t h e s y n t h e s i s o f mixed h i g h e r
(refs.
1.2).
synthesis
is
catalysts
well-known t h a t a p p r o p r i a t e m o d i f i c a t i o n s o f and
also
of
the
reaction
p r o d u c t i o n o f h i g h e r a l c o h o l s t o g e t h e r w i t h MeOH.
conditions
better
There a r e a
number
it
high
alcohols the
result
MeOH
in
the
The a l c o h o l s e l e c t i v i t y
of
c o n v e n t i o n a l Fischer-Tropsch c a t a l y s t s can be improved b y a d d i t i o n o f promoters or
nitridation(Fe).
Institute
catalysts
which
appear
alcohols,
consisting
c a t a l y s t s (Co) ( r e f . Molybdenum
of
Francais
prospective
du
Petrole
MeOH s y n t h e s i s c a t a l y s t s
(Cu)
however,
series
and
of
of
light
Fischer-Tropsch
have l o n g been r e c o g n i z e d as b e i n g e f f e c t i v e f o r 4,5).
the
I n our previous
t h e supported molybdenum c a t a l y s t s were found t o be a c t i v e i n
t h e s y n t h e s i s o f mixed a l c o h o l s , and a d d i t i v e ( r e f .
a
mixture
3).
catalysts
Fischer-Tropsch s y n t h e s i s o f l i g h t hydrocarbons ( r e f s . study,
claimed
f o r t h e synthesis o f
6-8).
amounted t o 420 g kg-cat-'
which was s i g n i f i c a n t l y i n f l u e n c e d by s u p p o r t
The space t i m e y i e l d o v e r 20 w t % Mo-1.63 h-l a t 573 K.
5.0 MPa.
w t % K/Si02
The aim o f t h i s paper i s t o
c l a r i f y t h e a c t i v e s p e c i e s f o r a l c o h o l s and hydrocarbons o f t h e
Si02-supported
Mo c a t a l y s t s f o r mixed a l c o h o l s y n t h e s i s and t o pursue t h e pathway t o them from
CO
and
He.
It should be n o t e d t h a t Dow and Union Carbide have
published
a
220 n u m b e r o f p a t e n t s o f t h e c a t a l y s t s based o n MoS2, p r o m o t e d b y SoS a n d a l k a l i metal s a l t s f o r higher alcohol synthesis (ref.
9).
EXPERIMENTAL C a t a l y s t s were prepared by i m p r e g n a t i n g F u j i - D a v i s o n I D s i l i c a g e l w i t h an aqueous s o l u t i o n o f (NH&Mo7Oz4.
For KC1-promoted
catalysts,
s i l i c a g e l was
f o l l o w e d b y a i r c a l c i n a t i o n a t 6 7 3 K,
i m p r e g n a t e d w i t h KC1 f i r s t ,
and t h e
r e s u l t a n t m o d i f i e d s i l i c a g e l was s u b j e c t e d t o i m p r e g n a t i o n w i t h molybdenum, u n l e s s o t h e r w i s e noted.
The s t a n d a r d way o f p r e t r e a t m e n t i s as f o l l o w s :
i m p r e g n a t e s w e r e d r i e d a t 393 K o v e r n i g h t and t r e a t e d i n He a t 673 and t h e c a t a l y s t s w e r e r e d u c e d b y H2 a t 773 K f o r 1 2 h.
K
the
for 1 h
Catalyst performance
was t e s t e d b y u s i n g a s t a i n l e s s s t e e l t u b u l a r r e a c t o r c o n t a i n i n g 1 g o f c a t a l y s t i n a f l o w system.
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 :
p r e s s u r e (H2/C0 = l),1.6 MPa; t e m p e r a t u r e , 573 c a t a l y s t h/mol.
K;
c o n t a c t time(W/F),
1 0 g-
Analyses o f t h e p r o d u c t s were based on gas chromatographs u s i n g
f i v e columns as p r e v i o u s l y d e s c r i b e d ( r e f .
10).
Detailed analysis o f the
hydrocarbon and a l c o h o l p r o d u c t s was p e r f o r m e d on a gas chromatograph equipped w i t h a 50 m c h e m i c a l l y bonded OV-1 f u s e d s i l i c a c a p i l l a r y column.
Each p r o d u c t
was i d e n t i f i e d mass s p e c t r o m e t r i c a l l y by a Shimadzu QP-1000 mass spectrometer. The c a t a l y t i c s u r f a c e a r e a s o f r e d u c e d Mo c a t a l y s t s w e r e m e a s u r e d b y O2 a d s o r p t i o n a t 1 9 5 K.
A f t e r m e a s u r i n g t h e 02 a d s o r p t i o n ,
s u b j e c t e d t o oxygen t i t r a t i o n .
e a c h s a m p l e was
D e t a i l s h a v e been d e s c r i b e d i n r e f s . 7 and 8.
X-ray p o w d e r d i f f r a c t i o n m e a s u r e m e n t s w e r e p e r f o r m e d u s i n g a R i g a k u D e n k i d i f f r a c t o m e t e r RU-POOA w i t h CuK,
radiation.
RESULTS AND DISCUSSION A c t i v e species f o r synthesis o f alcohols (i)E f f e c t o f r e d u c t i o n t e m p e r a t u r e . I n f l u e n c e o f r e d u c t i o n t e m p e r a t u r e on
t h e s t e a d y - s t a t e a c t i v i t i e s a r e shown i n T a b l e 1 f o r t h e s e r i e s o f 5 w t % MoThe c a t a l y s t r e d u c e d a t 6 7 3 K e x h i b i t e d l o w C O
0.81 w t % K / S i 0 2 c a t a l y s t s . conversion.
As t h e r e d u c t i o n t e m p e r a t u r e was r a i s e d ,
was increased.
hydrocarbon p r o d u c t i o n
The a c t i v i t y and s e l e c t i v i t y f o r a l c o h o l
s y n t h e s i s reached a
maximum f o r t h e c a t a l y s t reduced a t 773 K. Table 1 a l s o l i s t s amount r a y d i f f r a c t o g r a m s (XRD). reduction.
phase observed by
a n d a p p a r e n t o x i d a t i o n number o f t h e Mo,
X-
after
02 uptake f o r t h e c a t a l y s t reduced a t 773 K was s i m i l a r t o t h a t f o r
t h e c a t a l y s t reduced a t 673 K. 773 K,
o f O2 a d s o r p t i o n uptake,
The h i g h e r
a c t i v i t y o f t h e c a t a l y s t reduced a t
seems t o be r e l a t e d t o t h e h i g h e r m e t a l l i c Mo c o n t e n t .
However,
r e d u c t i o n a t 8 7 3 K r e s u l t e d i n t h e formation o f l a r g e a m o u n t o f m e t a l l i c Mo, l e a d i n g t o t h e c a t a l y s t producing hydrocarbons predominantly. agreement
w i t h t h e e f f e c t o f Mo p r e c u r s o r s ( r e f .
This i s i n
7) and t h e o r d e r o f
221 TABLE 1 I n f l u e n c e of r e d u c t i o n t e m p e r a t u r e on c a t a l y t i c p e r f o r m a n c e s a a n d s u r f a c e c h a r a c t e r i s t i c s o f 5 w t % Mo-0.81
w t % K/Si02 Catalysts.
Alcohol Space-time y i e l d XRD phase 0 Apparent s e l e c t i v i t y b A l c o h o l s HCC a f t e r r e d u c t i o n d u p t a i e e oxidati n (C-atom%) ( g / k g - c a t h) Moo2 Mo metal (umol/g-cat) number
Reduction temperature
?
(K)
22
673 173 873
2.6 17 14
30 18
5.3 25 34
s
n.d.
S
W
vw
86 79
3.4
2.0 -
-
S
aReaction c o n d i t i o n s : 573 K, 1.6 MPa (H2/CO = 1). W/F = 10 g-cat h/mol. bCO - f r e e basis. CHy%rocarbons. dn.d.:not detected: vw: v e r y weak: w: weak: s : strong. eMeasured a t 195 K. fEstimated f r o m O2 consumption from 195 t o 773 K.
8).
impregnation (ref.
i n t h a t e x t e n s i v e r e d u c t i o n o f Mo g i v e s a n a c t i v e
I t i s suggested t h a t t h e p r e s e n c e o f b o t h
hydrocarbon-forming catalyst.
m e t a l l i c Mo and Moo2 b e f o r e r e a c t i o n i s r e q u i r e d t o e f f e c t t h e p r o d u c t i o n o f a l c o h o l s and
t h a t t h e r e i s an a p p r o p r i a t e c o n t e n t o f m e t a l l i c Mo.
(ii)A c t i v i t y
7).
chanqe
w i t h t i m e on s t r e a m . As d e s c r i b e d p r e v i o u s l y ( r e f .
t h e a c t i v i t y f o r a l c o h o l s f o r m a t i o n i n c r e a s e d w i t h t i m e on stream.
This
i m p l i e s t h a t a c t i v e s p e c i e s f o r a l c o h o l p r o d u c t i o n were formed d u r i n g t h e CO-Hz reaction.
The change i n t h e space-time y i e l d o f a l c o h o l s , w t % K / S i 0 2 a t 523 K,
o v e r 1 0 w t % Mo-1.63
hydrocarbons and CO2
1.6 MPa a r e i l l u s t r a t e d i n F i g .
1.
The y i e l d o f C02 was s i g n i f i c a n t l y h i g h a t t h e i n i t i a l s t a g e o f t h e r e a c t i o n and decreased t o r e a c h a n e a r l y s t e a d y - s t a t e
a f t e r ca. 15 h on stream.
On t h e
o t h e r hand, t h e y i e l d o f a l c o h o l s was e x t r e m e l y l o w i n i t i a l l y a n d i n c r e a s e d r e m a r k a b l y w i t h t i m e on stream.
T h i s i s c o n t r a s t e d w i t h t h e t i m e dependence o f
t h e y i e l d o f hydrocarbons. As i s u s u a l l y t h e c a s e f o r t h e Mo c a t a l y s t s ( r e f s . 4.5).
almost a l l o f the
o x y g e n l e f t t h e r e a c t o r a s C02(eqns. 3 and 4) r a t h e r t h a n H20(eqns. under t h e c o n d i t i o n s eqn.
5
employed
was e s t i m a t e d a t 96%.
i n t h i s study
the
1 and 2):
e q u i l i b r i u m conversion o f
T h e r e was a c o i n c i d e n c e b e t w e e n t h e t i m e s
r e q u i r e d t o reach t h e steady s t a t e o f a l c o h o l s y n t h e s i s and C02 f o r m a t i o n , when t h e y i e l d o f CO2 n CO n CO 2n CO
+ +
+
(2n-l)CO n CO
+
2n H2
came t o correspond t o t h e y i e l d o f a l c o h o l s and hydrocarbons, -(CH2)n-
+
n H20
2n H2
4 CH3(CH2)n-10H
+
n H2
-(CH2)n-
n C02
+
+
( n + l ) H2 4CH3(CH2)n-10H
n H20
-3
n C02
+
n HZ
(n-1)
+
H20
(n-1)
CO2
222
10 20 30 40 Time on stream (h 1
0
50
Fig. 1. A c t i v i t y change w i t h t i m e on s t r e a m o v e r 10 w t % Mo-1.63 w t % K /Si02 a t 523 K. 1.6 MPa ( H z / C O = 1). W/F = 10 g - c a t h/mol.
the
n o t i c e a b l y exceeded t h e y i e l d s o f a l c o h o l s and h y d r o c a r b o n s .
The
3 and
I n t h e i n i t i a l stage o f t h e reaction,
r e d u c t i o n o f Moo2 b y CO (eqn. n Mo
+ 2
M002
+
20 h,
+
C O - 3 MonC
CO
(6)
CO2
(7)
t r e a t m e n t . A c o n t r o l e x p e r i m e n t was c a r r i e d o u t i n w h i c h t h e 5 w t % -
l e f t u n d e r H2 s t r e a m a t 773 K u n t i l CH4 f o r m a t i o n ceased, reaction.
pretreatment
A c t i v i t y change w i t h t i m e o n s t r e a m
conditions,
as
shown i n F i g .
2.
The i n i t i a l r a t e o f
i s much f a s t e r o v e r t h e C O - t r e a t e d c a t a l y s t t h a n o v e r
resulting
from
H2
r e d u c t i o n alone.
Over t h e former,
the
the rate
s y n t h e s i s i n c r e a s e d b y o n l y 50% a f t e r 2 h t o r e a c h a s t e a d y s t a t e , the
l a t t e r t h e r a t e i n c r e a s e d b y a f a c t o r o f 20 a f t e r 40 h.
for
alcohol
synthesis
appear
t o be formed on t h e
for
and s u b j e c t e d
was d e p e n d e n t on
synthesis
treatment.
or
7).
w t % K / S i 0 2 c a t a l y s t r e d u c e d i n H2 was f u r t h e r t r e a t e d i n CO a t 523 K
t o CO-H2
catalyst
the
alcohol catalyst
of
alcohol
while
Therefore during
over sites
the
co
The amount o f C02 p r o d u c e d f r o m t h e CO t r e a t m e n t was 117% based o n
t h e l o a d e d Mo. a
6)
C02
+
-+M002-~
(iii) CO
0.81
F i g . 2. Change i n a l c o h o l p r o d u c t i o n w i t h t i m e o n s t r e a m o v e r 5 w t % Mo0.81 w t % / S i 0 2 a t 573 K, 1.6 MPa (Hp/ CO = l),W/F = 1 0 g - c a t h/mol. E f f e c t o f CO t r e a t m e n t .
o f e x c e s s C02 s h o u l d be a s c r i b e d t o Boudouard r e a c t i o n (eqn.
formation
50
however,
based o n eqns. y i e l d o f CO2
4.
10 20 30 40 Time on stream (h )
0
The CH4 f o r m e d b y s u c c e s s i v e H2 t r e a t m e n t w?s o n l y 46% so t h a t
s i g n i f i c a n t p o r t i o n o f Moo2 seems t o be r e d u c e d t o f o r m d e f e c t s a c c o r d i n g t o
eqn.
7.
Amount
o f O2 a d s o r p t i o n u p t a k e o f t h e C O - t r e a t e d c a t a l y s t
and
the
223
catalyst
resulting
from
H2
reduction
alone
was
46
79 mmol/g-cat,
and
Thus i t i s s p e c u l a t e d t h a t t h e lower a c t i v i t y o f t h e
respectively.
would be accounted f o r by t h e l o w e r d i s p e r s i o n o f reduced
catalyst
CO-treated molybdenum
oxides. Since Mo c a r b i d e s a r e r e p o r t e d t o be a c t i v e f o r methanation ( r e f .
one
co-
wonder t h a t t h e enhancement o f a l c o h o l s y n t h e s i s a c t i v i t y d u r i n g t h e
might
H2
ll),
reaction
initial
and a l s o b y t h e CO-treatment
activity
for
i s due t o
the
carburization.
t h e hydrocarbon f o r m a t i o n was g r e a t l y
f a c t o r o f 3 a f t e r c a r b u r i z a t i o n by n-butane and H2. was
increased
by
a
On t h e o t h e r hand, g r a d u a l
increase
in
alcohol
findings
it
i s considered t h a t the increase i n alcohol production d u r i n g
CO-Hz
production a c t i v i t y
The
still
observed.
From
these the
r e a c t i o n i s a s c r i b e d t o t h e f o r m a t i o n o f CO-reduction induced d e f e c t s
on
M002. E f f e c t o f potassium ( i ) C a t a l y t i c performance a t 573 K. The e f f e c t o f K c o n t e n t on a c t i v i t i e s o f
20 w t % Mo/Si02 i s shown i n Fig. and
hydrocarbon
yield i n particular.
w i t h KC1 c o n t e n t and reached 70 o f alcohols,
The a d d i t i o n o f KC1 reduced CO c o n v e r s i o n
3.
The s e l e c t i v i t y o f a l c o h o l s a t K/Mo = 0.4.
wt%
was a t t a i n e d a t K/Mo = 0.2.
however,
The v a l u e o f
u p t a k e and phase observed by XRD a r e a l s o p r e s e n t e d i n Fig.
Mo
of
can
dispersion
of
estimated from
O2
uptake.
(wt%)",
I
r-
I 20
I n the
f r e s h l y reduced c a t a l y s t was 51%.
Carbon-atom selectivity (%)
K
0
be
I 40
]
I 60
]
I I
I 80
,
,
100
co
conv.
(%I
37
1.63
increased
A maximum o f t h e y i e l d
absence
3. of
assuming O/Mo
O2
adsorption
The d i s p e r s i o n KC1, ratio
the
Mo
to
be
0 Alcohol XRD upt&e STY MOO2 MO (pmol/g) (g/kg.h) 532 1.4 vw vs
144
22
w
vs
2.44
5.8
163
20
ms
s
3.25
3.4
126
13
s
m
ygenates The e f f e c t of K c o n t e n t on a c t i v i t y o f 20 w t % Mo/Si02 a t 573 K, F i g . 3. MPa (H2/C0 = l ) , W/F = 1 0 g-cat h/mol. O2 u p t a k e was measured a t 195 K.
1.6
224
TABLE 2
Isomer d i s t r i b u t i o n i n product a l c o h o l s and alkanes over Mo(20 wt%)-K(1.63 w t % ) ISiO,? Carbon number
6
Isomer s e l e c t i v i ty(mol%) Alcohols A1 kanes
CH3CH(CH3)CH20H 32
(CH ) CH
C2H5CH(CH3)CH20H 46
C2H5CH(CH3)2 8
3;
CH3(CH2 )SOH 49 n-C3H7CH(CH3)CH20H 33 ( C H ) CHCH20H 5g2
a573 K, 1.6 MPa, H2/CO g-cat. h rno1-l'
=
-
453
493
533
Temperature (K)
573
Fig. 4. Temperature dependence o f y i e l d o v e r S i O supported 20 w t % Mo a t 1.6 MPa (H2,%0 = 1). W/F = 10 gc a t h/mol.
1, W/F = 10
0CO-H2
[7CO-H2-C2H4(ca.l%) 200 OC, 1.6 MPa, W/F= 10 g-cat: hr/mol .
Fig. 5. I n f l u e n c e o f a d d i t i o n o f ethene (1.2%) t o CO-H2 on product d i s t r i b u t i o n over 20 w t % Mo/SiOe and 20 w t % Mo-1.63 wt%/SiO2.
225 unity.
The d i s p e r s i o n was s i g n i f i c a n t l y r e d u c e d b y a d d i t i o n o f KC1 and was
v i r t u a l l y c o n s t a n t beyond K/Mo 2 0.2.
The XRD o b s e r v a t i o n s o f t h e reduced 20
w t % Mo/Si02 c a t a l y s t i d e n t i f i e d m e t a l l i c
Mo as t h e m a j o r phase.
With increase
i n t h e K/Mo r a t i o t h e i n t e n s i t y o f Moo2 i n c r e a s e d a t t h e e x p e n s e o f m e t a l l i c Mo.
T h i s i s i n agreement w i t h t h e o b s e r v a t i o n by Kantschewa e t al.
(ref.
12),
who found t h a t t h e a d d i t i o n o f K decreased t h e r e d u c i b i l i t y o f t h e Mo supported Thus t h e K p e r f o r m s a s i g n i f i c a n t f u n c t i o n :
on A1203.
i t retards the reduction
o f Mo, r e s u l t i n g i n i n c r e a s e i n t h e p r o d u c t i o n o f a l c o h o l s .
The decrease i n CO
c o n v e r s i o n by a d d i t i o n o f K should r e s u l t f r o m t h e decrease i n Mo d i s p e r s i o n and a l s o t h e decrease i n t h e c o n t e n t o f m e t a l l i c Mo. (ii)Temperature dependence.
was
m a r k e d l y dependent
Surprisingly,
reaction
temperature,
as
shown
i n Fig.
4.
t h e unpromoted 20 w t % Mo/Si02 c a t a l y s t gave much more a l c o h o l s
t h a n t h e 20 w t % Mo-1.63 catalyst,
on
The p r o d u c t i v i t y o f a l c o h o l s and hydrocarbons
w t % K/SiO2 a t l o w temperatures.
Over t h e unpromoted
w i t h r a i s i n g t h e temperature t h e y i e l d o f hydrocarbons sharply
increased a t t h e c o s t o f t h e y i e l d o f alcohols:
t h e maximum y i e l d o f a l c o h o l s
I n t h e presence o f K t h e y i e l d o f a l c o h o l s increased
was o b t a i n e d a t 573K.
w i t h i n c r e a s i n g t h e t e m p e r a t u r e u p t o 523 K,
although t h e i r s e l e c t i v i t y
monotonously decreased. (iii)I s o m e r d i s t r i b u t i o n i n a l c o h o l s .
Table 2
shows t h e i s o m e r
d i s t r i b u t i o n i n a l c o h o l s and alkanes i n t h e c4-c6 range o b t a i n e d w i t h 20 w t % w t % K / S i O 2 a t 573 K.
Mo-l.63 ones,
particularly,
position.
The a l c o h o l s c o n s i s t s e x c l u s i v e l y o f p r i m a r y
i s o m e r s w i t h s t r a i g h t c h a i n and b r a n c h i n g i n t h e
2-
T h i s i s o m e r d i s t r i b u t i o n suggests t h a t t h e r e i s a s t r o n g resemblance
between a l c o h o l f o r m a t i o n and 0 x 0 r e a c t i o n o f o l e f i n s ( r e f s . (iv) Addition o f olefins.
p r o d u c t o b t a i n e d w i t h unpromoted and K(1.63
K i s e x h i b i t e d i n F i g . 5.
13, 14).
The e f f e c t o f a d d i t i o n o f ethene t o CO-H2
wt%)-promoted 20 w t % Mo/Si02 a t 473
Upon t h e a d d i t i o n o f e t h e n e t o t h e CO-H2,
enhancement o f t h e y i e l d o f 1-propanol
on t h e a great
was observed f o r b o t h o f t h e c a t a l y s t s .
It can be seen t h a t t h e r a t e o f h y d r o g e n a t i o n o f ethene t o ethane decreased b y a f a c t o r o f 3.5 f o l l o w i n g K p r o m o t i o n w h i l e t h e r a t e o f f o r m a t i o n o f 1-propanol o n l y s l i g h t l y decreased.
I t has been f o u n d t h a t t h e b u t a n o l f r o m p r o p e n e
c a r b o n y l a t i o n was v e r y s i m i l a r i n t h e i s o m e r d i s t r i b u t i o n t o t h a t f r o m CO-Hz alone (ref.
15).
These f i n d i n g s i m p l y t h a t a l c o h o l f o r m a t i o n f r o m CO-HZ
proceeded v i a t h e same i n t e r m e d i a t e as t h e o l e f i n c a r b o n y l a t i o n ; i n t o a metal-alkyl
CO i n s e r t e d
bond t o f o r m a n a c y l s p e c i e s w h i c h was s u b s e q u e n t l y
hydrogenated t o t h e a l c o h o l product. I n CO h y d r o g e n a t i o n t o g i v e h i g h e r hydrocarbons and a l c o h o l s ,
the growing
a l k y l c h a i n on t h e c a t a l y s t s u r f a c e s h o u l d have, i n p r i n c i p l e , t h e f o l l o w i n g reaction possibilities,
namely,
(a) CO i n s e r t i o n t o g i v e p r e c u r s o r s l e a d i n g t o
C2toxygenates, (b)@-H a b s t r a c t i o n t o g i v e alkene, (c) h y d r o g e n o l y s i s o f t h e
226
0,
-5
I
I
9 i% ;::i
1-
metal-alkyl addition
bond t o g i v e alkane and ( d ) of
methylene
l o n g e r a l k y l groups.
4 '
I
K= 0
propanol
units
giving
The r a t i o o f
1
o f ethene may g i v e a c l u e t o t h e
i
o f t h e r a t e ( a ) t o (c).
I
4
in
1-
t o ethane d u r i n g t h e a d d i t i o n
5
Fig.
resulted
in
rate
(a)
of
ratio
I t can be seen
t h a t the addition
of
l e s s suppression
in
than
in
that
KC1 the
of
(c).
Temperature dependence o f t h e r a t i o
the s e l e c t i v i t y F i g . 6. Temperature dependence o f t h e r a t e o f c a r b o n y l a t i o n t o hydrog e n a t i o n o f ethene o v e r S i O s u p p o r t e d 20 w t % MO c a t a l y s Z s . 1.6 MPa ( H2/CO=1, ethene=l .2%).
Mo/SiOZ
at
h i g h temperatures ( F i g .
p r o m o t i o n o f (c).
t o 1-propanol o v e r t h e
unpromoted 20 w t % Mo/Si02 c a t a l y s t comparable
Mo-1.63
Over t h e Mo-K/SiOZ,
was
t o t h a t observed f o r 20 w t % wt% K/sio2 at lower
temperatures,
declined steeply
it
Hence n e g l i g i b l e a l c o h o l f o r m a t i o n f r o m CO-Hz
t h e t e m p e r a t u r e was r a i s e d .
of
4) would be accounted
for
by
as over
excessive
t h e r o u t e ( c ) c o u l d be suppressed enough
a t h i g h temperatures t o g i v e a c o n s i d e r a b l e amount o f a l c o h o l s . ( v ) R o l e o f potassium. As has been e l a b o r a t e d above, t h e K added r e t a r d s t h e reduction
of
However, also
Mo,
affects
dependence and
leading
t o the increase i n the s e l e c t i v i t y
for
alcohols.
K not o n l y increases t h e a c t i v e s i t e s f o r alcohol formation
the
t h e hydrocarbon-forming s i t e s .
Difference o f
the
o f carbonylation/hydrogenation s e l e c t i v i t y between
unpromoted
catalysts
but
temperature
the
K-promoted
suggests t h a t t h e a d d i t i o n a l r o l e o f K i s
to
slow
competitive
hydrogenation t o form alkane e f f e c t i v e l y a t h i g h
particular.
No s i g n i f i c a n t p r o m o t i o n o f t h e c a r b o n y l a t i o n r a t e i n h e r e n t i n t h e
a d d i t i o n o f K was observed. at
low
temperatures,
temperatures
Low a c t i v i t y f o r a l c o h o l s y n t h e s i s o v e r Mo-K/Si02
compared t o t h a t o v e r Mo/Si02 c o u l d be due t o
content
of
m e t a l l i c Mo,
through
the
promotion
which m i g h t be a l s o r e q u i s i t e f o r a l c o h o l of
in
CO d i s s o c i a t i o n
(involved
in
the
low
formation
formation
C2t
of
a l c o h o l s ) and/or a c t i v a t i o n o f m o l e c u l a r H2. Mechanism f o r b r a n c h i n g Table 2 shows t h a t f r o m CO/H2 a t 573 K, hydrocarbons were obtained,
of
l o w branched/ s t r a i g h t r a t i o s o f t h e The d i f f e r e n c e
compared t o t h o s e o f t h e a l c o h o l s .
at
t h e s e l e c t i v i t y t o branched p r o d u c t s i n d i c a t e s t h a t a l c o h o l s a r e formed
sites
different
from t h e f o r m a t i o n o f hydrocarbons.
It i s
speculated
a l c o h o l p r e c u r s o r s a r e formed on r a t h e r e l e c t r o n - d e f i c i e n t s i t e s
(M'
that
= MOO*-,),
227
p 3 RCHM'
4H3 RCHCM'
>
co
Ib
H2
>-
p 3 RCHCH20H
which would f a v o u r secondary a l k y l groups (eqn.
8) ( r e f .
CO i n s e r t i o n t o (C2H5)2CH-M'
formed t h r o u g h f u r t h e r
C3H7CH(CH3)-M'.
Presumably
mechanism:
groups
alkyl
CO
molecular
on
MOO^-^.
C2+
were
oxygenates
formed
were
16).
0-TI
9
is
formed
via
on m e t a l l i c Mo(M) and
a
on
S i m i l a r d u a l - s i t e mechanism has been
electron-rich
metallic
sites.
The
to
proposed
17).
a for
The pathway should
alkali
c o n s i d e r a b l y lowered t h e s e l e c t i v i t y t o branched p r o d u c t s ( r e f . by t h e r e s u l t s o f Vedage e t a l . ( r e f .
dual-site
migrated
supposed t o be i n f r e q u e n t s i n c e secondary a l k y l groups
disadvantageous
branched
by assuming
i n t e r c o n v e r s i o n o f n-
f o r m a t i o n o f C2 oxygenates over supported Rh c a t a l y s t s ( r e f . eqn.
T h i s mechanism
( T a b l e 2),
can a l s o account f o r t h e f o r m a t i o n o f 2-ethyl-1-butanol
be
addition
15). c o n t r a s t e d
18), who found t h a t f o r m a t i o n o f 2-methyl
1 - a l c o h o l s o v e r Cu/ZnO was enhanced by a l k a l i a d d i t i o n and proposed a
mechanism f o r b r a n c h i n g i n v o l v i n g a l d o l condensation o f aldehyde p r e c u r s o r s . Effect o f chlorine
It
would be i m p o r t a n t t o e x p l a i n why KC1 and K2CO3 d i f f e r so much i n
propensities
to
e f f e c t t h e formation o f alcohols ( r e f .
coimpregnated w i t h K2CO3 and Mo s a l t s ,
noticeable. of
CH3Cl
or
6).
their
Si02 was
promotion o f a l c o h o l p r o d u c t i o n was n o t
To c l a r i f y t h e r o l e o f C1 i n a l c o h o l synthesis, CH2C12
When
was added d u r i n g t h e r e a c t i o n o v e r 20
low concentration wt%
Mo-1.63
wt%
TABLE 3 Y i e l d o f a l c o h o l s and hydrocarbons w i t h o u t o r w i t h c h l o r i n a t e d methane as an a d d i t i v e a .
Product (C-mmol/kg
h)
Hydrocarbons Alcohols
Additive none CH3C1 CH2C12
140
256
81
40
346
10
a C a t a l y s t : 20 w t % Mo-1.63 w t % K/Si02. R e a c t i o n c o n d i t i o n s : 523 K, 1.6 MPa ( H /CO - 1, a d d i t i v e = 0.6-0.8%). WfF = ;O g-cat h/mol.
Time on stream (h ) F i g . 7. E f f e c t o f i m p r e g n a t i o n method on a c t i v i t y change o v e r 10 w t % Mo-1.63 w t % K / S i O , 1.6 MPa (H*/CO = 1). W/F = I o g-cat i / m o l .
228
K/Si02.
As shown i n Table 3, a d d i t i o n o f b o t h c h l o r i n a t e d methanes r e s u l t e d i n
severe r e t a r d a t i o n o f a l c o h o l formation.
This e f f e c t i s o n l y temporary,
for
a f t e r 5 h t h e p r o d u c t i o n o f a l c o h o l s reached t h e same v a l u e as b e f o r e addition. The p r o d u c t i o n o f h y d r o c a r b o n s was l a r g e l y i n c r e a s e d b y a d d i t i o n o f CH2C12. w h i c h c o u l d be e x p l a i n e d by t h e enhancement o f t h e c o n c e n t r a t i o n o f C H x s p e c i e s on t h e s u r f a c e by t h e e x t e r n a l supply.
On t h e o t h e r hand,
no
enhancement o f p r o d u c t i o n o f h y d r o c a r b o n s was o b s e r v e d w i t h CH3C1 as an additive. I t has a l s o been f o u n d t h a t > 80% o f t h e C1 added as t h e c o u n t e r a n i o n o f
t h e K was removed d u r i n g t h e r e d u c t i o n .
Thus i t i s h a r d t o c o n s i d e r t h a t C1
plays a d e f i n i t e r o l e i n the promotion o f production o f alcohols during the reaction.
A c t u a l l y we have found t h a t t h e a l c o h o l s e l e c t i v i t y can be s i m i l a r l y
improved by impregnating SiO2 w i t h K2CO3 f i r s t (Fig. 7).
The change o f a l c o h o l
s e l e c t i v i t y w i t h t i m e on stream proved v e r y s e n s i t i v e t o impregnation method f o r K ~ C O S Thus the remarkable e f f e c t o f KC1 on i n c r e a s i n g a l c o h o l p r o d u c t i o n compared t o K2CO3, when added b y c o i m p r e g n a t i o n ,
m i g h t be a s c r i b e d t o i t s
g r e a t e r a b i l i t y t o p r e v e n t Mo species f r o m i n t e r a c t i n g w i t h S i O p
REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
G. N a t t a , U. Colombo and I. Pasquon. i n P.H. Emmett ( E d i t o r ) , C a t a l y s i s , Vol. V, Reinhold, New York, 1957, pp.131-174. K. K l i e r , i n W.R. M o s e r ( E d i t o r ) , C a t a l y s i s o f Organic Reactions, Marcel Dekker. New York, 1981, pp.195-218. A. S u g i e r and E. Freund, U.S. P a t e n t 4122110 (1978). J.F. S c h u l t z , F.S. Karn and R.B. Anderson, U.S. Bureau o f M i n e s Report, 6974 (1967) 1-20. C.B. Murchison and D.A. Murdick, Hydrocarbon Process., 60 (1981) 159-164. T. Tatsumi, A. Muramatsu and H. Tominaga, Chem. Lett.,(1984) 685-688. T. Tatsumi, A. Muramatsu, T. Fukunaga and H. Tominaga, Polyhedron, 5 (1986) 2 5 7-260. T. T a t s u m i , A. Muramatsu and H. Tominaga, J. Catal., 101 (1986) 553-556. G.J. Q u a r d e r e r , EPO 0119609 (1984); N.E. K i n k a d e r , E P O 0149255, 0149256 (1985); R.R. Stevens, EPO 0172431 (1986). T. Tatsumi, A. Muramatsu and H. Tominaga. Appl. Catal., 27 (1986) 69-82. L. L e c l e r c q . K. Imura. S. Yoshida, T. Barbee and M. B o u d a r t . i n B. Delmon e t 1978, a l . ( E d i t o r s ) , P r e p a r a t i o n o f C a t a l y s t s 11, E l s e v i e r , Amsterdam, pp. 627-639. M. Kantschewa, F. Delannay, H. J e z i o r o w s k i , E. Delgado, S. Eder, G. E r t l and H. Knonzinger, J. Catal., 87 (1984) 482-496. R.L. P r u e t t , Adv. Organomet. Chem., 17 (1979) 1-60. B. C o r n i l s , i n J. F a l b e ( E d i t o r ) , New S y n t h e s i s w i t h Carbon Monoxide, Springer, B e r l i n , 1980, pp.1-242. A. Muramatsu, K. Yokota, T. T a t s u m i and H. Tominaga. J. Mol. Catal., i n press. K. Tamao, Y. Kiso. K. S u m i t a n i and M. Kumada, J. Am. Chem. Soc., 94 (1972) 9268-9269. F.G.A. van den Berg, J.H.E. G l e z e r and W.M.H. S a c h t l e r , J. Catal., 93 (1985) 340-352. G.A. Vedage. P. H i m e l f a r b , G.W. Simmons and K. K l i e r , ACS Symp. Ser., 279 (1985) 295-312.
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ALCOHOLS FROM METHANE
H. Dotsch, C.B.
von der Decken, H. Fedders and B. Hohlein
I n s t i t u t e o f Reactor Components, Nuclear Research Centre J u l i c h , 5170 J u l i c h , Federal Republic o f Germany
ABSTRACT Among t h e v a r i o u s s y n t h e t i c f u e l s t h a t can be used e i t h e r as blending stocks, as s u b s t i t u t e f o r crude o i l d e r i v e d g a s o l i n e o r as f u e l f o r D i e s e l engines a l c o h o l s d e r i v e d from methane a r e r e c e i v i n g s p e c i a l a t t e n t i o n . The I n s t i t u t e o f Reactor Components a t t h e Nuclear Research Centre J u l i c h has s t a r t e d a research program concerning methane and a l c o h o l s . The examination o f t h e i r p o t e n t i a l as a s y n t h e t i c l i q u i d energy c a r r i e r w i t h regard t o economical p r o d u c t i o n and e c o l o g i c a l l y b e n e f i c i a l combustion behaviour i s t h e aim o f t h i s programme.
INTRODUCTION
The conversion o f methane t o methanol and h i g h e r a l c o h o l s i s g a i n i n g more and more i n t e r e s t w i t h respect t o t h e p r o d u c t i o n o f chemicals and f u e l s . As n a t u r a l gas w i l l become t h e predominant f o s s i l f u e l , and crude o i l w i l l have passed i t s maximum market share a t t h e beginning o f n e x t century ( r e f s . l,Z), t h e demand f o r chemicals and f u e l s d e r i v e d from n a t u r a l gas w i l l increase. Alcohols can be used e i t h e r as a chemical feedstock o r as a l i q u i d secondary energy c a r r i e r , depending on t h e s e l e c t i v i t y a t which t h e y can be produced. The work a t o u r i n s t i t u t e focusses on t h e p r o d u c t i o n and use o f a l c o h o l s as s y n t h e t i c l i q u i d energy c a r r i e r s ( r e f . 3 ) , because o f their
- environmental and
-
economical p o t e n t i a l .
229
230 It i s t h e r e f o r e t h e aim o f t h i s a r t i c l e t o r e p o r t about t h e a c t i v i t i e s i n
t h e " E n e r g i e - A l k o h o l Programm" a t KFA J u l i c h .
WORK PROGRAMME
F i g . 1 Conversion r o u t e s t o be s t u d i e d a t KFA J u l i c h
F i g u r e 1 shows t h e main c o n v e r s i o n r o u t e s which we a r e l o o k i n g f o r i n o u r l a b o r a t o r y a n d / o r p i l o t p l a n t work. The system o f l o n g - d i s t a n c e n u c l e a r energy t r a n s p o r t covers t h e s t e p s
@
and
@
and has k e n t h e m a j o r f i e l d
o f s t u d y i n t h e p a s t t e n y e a r s ( r e f . 4 ) . The c o u p l i n g o f h e a t and power g e n e r a t i o n by t h e h i g h t e m p e r a t u r e n u c l e a r r e a c t o r l e d t o a h i g h l y e f f i c i -
231 e n t h e a t t r a n s p o r t system. T h i s system combines t h e endothermic methane/ steam-reforming
process a t t h e n u c l e a r h e a t s o u r c e w i t h t h e e x o t h e r m i c
n e t h a n a t i o n r e a c t i o n t o f o r m superheated steam o r h o t w a t e r a t t h e consumer abroad. The t r a n s p o r t of h e a t v i a s y n t h e s i s gas a t ambient t e m p e r a t u r e g i v e s r i s e t o no h e a t l o s s e s i n t h e c l o s e d c y c l e . T h i s system has been s u c c e s s f u l l y demonstrated i n lam p i l o t p l a n t s w i t h 1 MW and 10 MW c a p a c i t y . Steps
@
and
@
d e s c r i b e t h e way f r o m methane t o methanol a n d / o r h i g h e r
a l c o h o l s . D e s p i t e t o d a y ' s l o w crude o i l p r i c e s s y n t h e t i c l i q u i d f u e l s have a great potential: i n i m p r o v i n g m i n o r q u a l i t y c o n v e n t i o n a l f u e l s by b l e n d i n g and
i n meeting low emission l e v e l s . The process s t a r t s w i t h t h e above mentioned methane/steam-reforming u n i t ( i n t h i s case w i t h a C02 s u p p l y t o m o d i f y t h e H2/C0 r a t i o ) f o l l o w e d up by a L u r g i - t y p e s y n t h e s i s p l a n t w i t h a 4 t / d c a p a c i t y ( r e f . 5 ) . T h i s p i l o t p l a n t has been s e t i n o p e r a t i o n about one y e a r ago w i t h c o n v e n t i o n a l methanol s y n t h e s i s , and i t i s f o r e s e e n t o s w i t c h t o a l c o h o l m i x t u r e s i n t h e c o u r s e o f t h i s y e a r . The p r o d u c t s w i l l be t e s t e d i n bench t e s t engines and s t a t i o n a r y b u r n e r systems t o q u a n t i f y t h e i r e n v i r o n m e n t a l p o t e n t i a l . Step
@
i s o f i n t e r e s t when t a k i n g a l c o h o l s as a t r a n s p o r t o r s t o r a g e medium f o r
energy. T h i s r o u t e i s t h e u s e f u l b a s i s o f t h e H y t a n o l process and has been d e s c r i b e d elsewhere ( r e f . 6 ) . It i s used t o c o v e r peak l o a d s i n t h e West B e r l i n gas s u p p l y system. The a l c o h o l r e f o r m i n g process i s s t u d i e d i n a s m a l l l a b o r a t o r y s c a l e p l a n t and produces s y n t h e s i s gases f o r l a b o r a t o r y purposes. Although f a r f r o m economical o p e r a t i o n t h e most a t t r a c t i v e way i s t h e direct route
@
f r o m methane t o methanol by p a r t i a l o x i d a t i o n . The CPO
process ( C o n t r o l l e d P a r t i a l O x i d a t i o n ) p r e s e n t e d by H u n t e r e t a l . ( r e f . 7) seems t o be q u i t e f a v o u r a b l e w i t h methanol s e l e c t i v i t i e s up t o 80 % and
232
methane conversion of about 8 %, b u t l i t t l e i s ye t known about
it. Our
i n t e r e s t i n t h i s f i e l d focusses on t h e c a t a l y t i c combustion of methane and t h e s t u d y o f i t s i n t er med i at es .
10OYo-
Methanol Ref.
@
Conversion per pass Methanol Synth.@
Selective
Oxidation
a
8
0 100 Yo
Selectivity Fig. 2
C a t a l y s t performance i n t h e methane/alcohol-system
I n Fig. 2 t h e conversion per pass i s p l o t t e d a g a i n s t t h e s e l e c t i v i t y of t h e c a t a l y t i c systems used. It g i v es a good i n d i c a t i o n where research work i n c a t a l y s i s and process engineering i s worthwile. While t h e nuclear energy t r a n s p o r t system (NFE), r o u t e s
@)
and
@ , as
well a s methanol
233
reforming
4
, a r e c l o s e t o optimum, t h e e f f i c i e n c y o f methanol syn-
t h e s i s can be improved by d e v e l o p i n g c a t a l y s t s which work a t l o w e r temper a t u r e s where t h e thermodynamics a r e more f a v o u r a b l e . An i n c r e a s e i n c o n v e r s i o n p e r pass would reduce s u b s t a n t i a l l y t h e amount o f r e c y c l e gas. I n t h e production o f a l c o h o l mixtures
3
, t h e study o f s e l e c t i v i t y i s t h e
S u b j e c t o f r e s e a r c h , whereas i n s e l e c t i v e o x i d a t i o n o f methane
5
, much
r e s e a r c h i s needed t o improve b o t h c o n v e r s i o n and s e l e c t i v i t y .
EXPERIMENTAL STUDIES
Gas
Fig. 3
Methanii
P i l o t p l a n t f o r t h e s y n t h e s i s o f a l c o h o l s f r o m methane v i a s y n t h e s i s gas
As mentioned above, a p i l o t p l a n t f o r methane/steam/carbon d i o x i d e r e f o r m i n g has gone i n t o o p e r a t i o n about one y e a r ago and w i l l be b r i e f l y d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n . The h e l i u m h e a t e d r e f o r m e r p l a n t ,
234
c a l l e d EVA I, i s used f o r s y n t h e s i s gas p r o d u c t i o n . As can be seen i n F i g . 3, t h e s y n t h e s i s gas i s o b t a i n e d f r o m n a t u r a l gas, carbon d i o x i d e and w a t e r by means o f c a t a l y t i c r e f o r m i n g . The EVA
I
p l a n t used f o r t h i s purpose i s
equipped w i t h a c o r r e s p o n d i n g t u b u l a r r e a c t o r (steam r e f o r m e r ) which i s ext e r n a l l y h e a t e d w i t h h o t h e l i u m t o cover t h e h e a t r e q u i r e m e n t s o f t h e r e a c t i o n . T h i s demonstrates t h e p o s s i b i l i t i e s o f u s i n g n u c l e a r generated h e a t f o r t h e endothermic r e f o r m i n g p r o c e s s and shows t h a t t h e energy r e q u i r e ments needed f o r c o n v e r s i o n can be covered by u s i n g n u c l e a r energy. The r e former p r o d u c t gas a t 850
OC
i s c o o l e d t o ambient t e m p e r a t u r e a f t e r r e -
forming, t h e r e s i d u a l w a t e r b e i n g p r e c i p i t a t e d as condensate. The C02 c o n t e n t o f t h e s y n t h e s i s gas i n t h e a l c o h o l s y n t h e s i s l o o p can be reduced t o t h e r e q u i r e d low v a l u e s w i t h t h e a i d o f C02 s e p a r a t i o n . The H2/C0 r a t i o i n t h e s y n t h e s i s gas can be v a r i e d between 5 and 1 by r e c y c l i n g t h e s e p a r a t e d carbon d i o x i d e back t o t h e steam r e f o r m e r and by t h e a d d i t i o n o f a f u r t h e r C02 source. The s y n t h e s i s p l a n t i s designed f o r t h e p r o d u c t i o n o f approx. 4000 l i t r e s o f raw a l c o h o l p e r day. F o r t h i s purpose t h e r e f o r m e r p r o v i d e s up t o
3
600 Nm / h s y n t h e s i s gas which can be compressed f r o m 20 b a r t o a maximum o f 160 b a r . The s y n t h e s i s gas i s mixed w i t h r e c y c l i n g gas i n a r a t i o o f 1/1t o 1 / 5 and p r e h e a t e d i n a f e e d / r e a c t o r e f f l u e n t h e a t exchanger. The r e a c t i o n t u b e s , where t h e exothermic a l c o h o l s y n t h e s i s r e a c t i o n t a k e s p l a c e , a r e e x t e r n a l l y c o o l e d by b o i l i n g w a t e r , e n s u r i n g n e a r l y i s o t h e r m a l r e a c t i o n c o n d i t i o n s . The p r o d u c t gas passes i n a r e v e r s e f l o w t h r o u g h t h e f e e d / r e a c t o r e f f l u e n t h e a t exchanger and r e l e a s e s h e a t t o t h e f e e d . F u r t h e r c o o l i n g l e a d s t o condensation o f t h e p r o d u c t s , namely a l c o h o l s and w a t e r . The p r o c e s s f l o w s a r e analyzed by gas chromatography and mass s p e c t r o m e t r y .
235
Experiments o n a l a b o r a t o r y s c a l e w i l l s e r v e t o t e s t d i f f e r e n t c a t a l y t i c systems f o r t h e s y n t h e s i s o f a l c o h o l s , namely Cu/Zn-type and Cu/Cot y p e c a t a l y s t s . T h e r e f o r e , r e c y c l e r e a c t o r s w i l l be used i n t h e l a b o r a t o r y t o e v a l u a t e c a t a l y s t performance i n terms o f a c t i v i t y and s e l e c t i v i t y .
ALCOHOL USE
D i f f e r e n t a l c o h o l c o m p o s i t i o n s a r e necessary f o r d i f f e r e n t a p p l i c a t i o n s . The f i e l d s o f a l c o h o l use and t h e i r a t t r a c t i v e n e s s a r e l i s t e d i n F i g . 4. Possible a p p l i c a t i o n s are: as a medium f o r l o n g d i s t a n c e n a t u r a l gas t r a n s p o r t
. as a medium f o r energy s t o r a g e
.
as domestic f u e l as a b l e n d i n D i e s e l - and spark i g n i t i o n - e n g i n e s as a s u b s t i t u t e o f D i e s e l and g a s o l i n e
As disadvantages compared t o c o n v e n t i o n a l f u e l s o r s y n t h e t i c l i q u i d hydrocarbons, t h e l o w h e a t i n g v a l u e has t o be mentioned as w e l l as t h e need t o adapt combustion d e v i c e s ( m a t e r i a l s , f u e l s u p p l y systems). On t h e o t h e r hand, t h e e n v i r o n m e n t a l p o t e n t i a l i s o b v i o u s and t h e f l e x i b i l i t y o f t h e s y n t h e s i s process w i t h r e s p e c t t o p r o d u c t c o m p o s i t i o n f o r d i f f e r e n t a p p l i c a t i o n f i e l d s seems t o be p r o m i s i n g .
236
FIELDS O F ALCOHOL USE - main targets transport medium : "liquified synthesis gos" (CH,OH )
- tronsport economics (nat. storage
- noturol gas
gas)
(
/
/
-
SOXCH,OH+
higher olc. )
- lead substitute Diesel blend
( CHJOH and or higher olc. )
~
- reduction in HC-emission - no sulfur compounds
- reduction in NO,-emission
[
y
gasoline substitute N 90XCH,OH+ light HC's)
yUSER1-y
- reduction in HC-emission - supply reliability
- reduction in NOx-emission Fig. 4
- reduction of soot emission - reduction in NO,-emission
- reduction in HC-emission
- supply
reliability
Alcohols a s energy c a r r i e r s
SUMMARY
It i s t h e aim of t h i s review t o present t h e work on methane conversion
t o s y n t h e t i c f u e l s a t t h e Nuclear Research Centre i n J u l i c h , Fed. Rep. of Germany. The research on alcohol f u e l s i s t h e l o g i c a l continuation of t h e s u c c e s s f u l work t h a t has been c a r r i e d out i n t h e long d i s t a n c e nuclear energy t r a n s p o r t system (methane reforming-methanation). The main goals may be summarized a s follows:
. -
improvement of methanol production e f f i c i e n c y study of s e l e c t i v i t y i n al co h o l s y n t he sis q u a n t i f i c a t i o n of t h e environmental p o t e n t i a l of a lc ohol use.
237
REFERENCES 1
HAFELE, W., i n W. Terhorst (Ed.), E r d o l und Erdgas i n der Kernforschungsanlage J u l i c h , Jul-Conf-58, 1986, p.6.
2
RUNGE, H.C., i n W. Terhorst (Ed.), E r d o l und Erdgas i n der Kernforschungsanlage J u l i c h , Jul-Conf-58, 1986, p.78.
3
VON DER DECKEN, C.B., Energy-Alcohols, Production and A p p l i c a t i o n o f a s y n t h e t i c l i q u i d Energy C a r r i e r , Jul-Spez-392, 1987.
4
HOHLEIN, B., R. MENZER and J. RANGE, High Temperature Methanation i n t h e long-distance nuclear Energy Transport System, Applied C a t a l y s i s , 1 (1981) 125.
5
VON DER DECKEN, C.B., H. FEDDERS and 8. HOHLEIN, Energol, P i l o t a n l a g e f u r d i e Synthese von Energiealkohol, V I I n t . Symp. on Alc. Fuels Technol., Ottawa, May 21 - 25, 1984.
6
RESTIN, K . , P . A . JURKAT and H. HILLER, Peaking Gas from Methanol, 1 6 t h World Gas Conference, Munich, June 24 - 27, 1985.
7
HUNTER, N . R . , H.D. GESSER, L.A. MORTON, P.S. YARLAGADOA and 0. Fung. The d i r e c t conversion o f n a t u r a l gas t o a l c o h o l s , V I I I n t . Symp. on Alc. Fuels Technol., P a r i s , Oct. 20 - 23, 1986.
8
FEDDERS, H. and E. Riensche, Untersuchung von Reformerrohren i n EVA I, HTR-Komponenten i n S c h r i f t e n r e i h e " E n e r g i e p o l i t i k i n NordrheinWestfalen", Bd.16, M i n i s t e r i u m fur W i r t s c h a f t des Landes NordrheinWestfalen, Dusseldorf (FRG) , 1984.
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D.M. Bibby,C.D.Chang,R.F. Howe and S.Yurchak (Editors),Methane Conuersion 0 1988 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
239
IMPROVEUENTS TO BANM COPPER KETHANOL SyNTHesIS CATALYSTS THROUGH ZINC
.
IMPREGNATION. 111 ACTIVITY TESTING* H. Edward Curry-Hyde,Mark S. Wainwright and David J. Young School of Chemical Engineering and Industrial Chemistry, The University of New South Wales,P.O. Box 1, Kensington, N.S.W.,2033, Australia *Parts I. Electron microprobe analysis, and 11. Optimisation of surface area, submitted to Applied Catalysis.
ABSTRACT Raney Cu-Zn catalysts prepared from a Cu-39 wt% A1-17.8 wt% Zn alloy and a Cu-47 wt% A 1 alloy by leaching in 0.62 M Na-zincate/6.1 M NaOH have been tested for activity in methanol synthesis. Comparison is made with Raney Cu-Zn catalysts prepared by NaOH leaching and to a commercial coprTcipitated catalyst. The tests, conducted at 220°C, 4500 kPa and GHSV of 36000 h , show the zincate leached catalysts to be up to twice as active as the other catalysts. INTRODUCTION Marsden et al. [l] were the first to show that an active low temperature methanol synthesis catalyst can be produced by alkali leaching small particles of a Cu-Al-Zn alloy
.
Other studies,usingsmall particles, have investigated the
effect of alloy composition on catalyst activity [2-41.Recently [5] ,an alloy of optimal composition was used to study the preparation and characteristics of the catalyst in pellets form. The long times required to leach the large particles was found to have two significant effects on the nature of the catalyst. The first decreased the overall pellet surface area after long periods of leaching. The second caused a decrease in the specific activity of the catalyst. The decrease in pellet surface area resulted from physical rearrangement of copper crystallites in the porous copper whilst the alloy core was still being leached [ 6 ] . The decreased activity resulted from decreases in zinc oxide concentrations on the copper surface [7] caused by secondary leaching effects that become significant at long leach times [7]. Recently it was shown that Raney copper has some activity for methanol synthesis [7,8].However, ZnO deposition on the surface is essential to a highly active copper surfaces [7]. Previous investigations sought to improve the ZnO content in the leached Cu structures by changing the Zn content of the Al-Cu-Zn alloy [2-41.This paper describes a leaching/precipitation technique which produces ZnO promoted Raney Cu (RCD) and ZnO enriched Raney Cu-Zn (RCZD)
240
catalysts in pellet form. Comparisons are made with conventional Raney copper-zinc catalysts and commercial coprecipitated Cu-Zn0-Al2O3catalysts. ExPERmAL
Pellets of CUAl2 alloy (53 wt. wt.
%
Cu, 39 wt.
%
Al, 17.8wt.
%
Cu, 47 wt.
% Zn),
%
Al) and Cu-Al-Zn alloy (43.2
(3.8 mm x 5.4 mm dia), were leached in
large excesses of two different leach solutions, 6.1 M sodium hydroxide and 0.62 M zincate in 6.1 M sodium hydroxide. Leaching was terminated by washing the pellets in distilled water to a pH of 7. The catalyst preparation procedure is described in greater detail elsewhere [7]. To simplify references to the four Raney catalysts Table 1 identifies them according to the precursor alloys and leach conditions used. TABLE 1
A summary of alloys and leach solutions used to prepare the Raney catalysts. The labels reference each catalyst catalyst i n the text.
~~
Alloy CUA12 CUA12 CUAl Cu-A$- Zn Cu-Al-Zn
Temp. (K)
Leach Solution 6.1 M NaOH 0.62 M Na-zincate/NaOH 0.62 M Na-zincate/NaOH 6.1 M NaOH 0.62 M Na-zincate/NaOH
274 274 303 303 303
Catalyst Raney Raney Raney Raney Raney
Cu Cu-(Zn doped) Cu-(Zn doped) Cu-Zn Cu-Zn (Zn doped
Label RC RCDl RCD2 RCZ RCZD
After washing, 5 cm3 of moist Raney catalyst pellets and 5 cm3 of unleached inactive alloy pellets (as diluent) were loaded into a reactor 38 mm internal diaiameter and 150 mm long. The catalyst was dried for 1 hour at 90°C in a flow of 100% H2 by immersing the reactor in an oil bath, and the temperature was raised to 160°C for a further hour. The reactor was then transferred to a molten salt bath at 220°C under pure H2 for approximately 1 hour and the catalyst was then ready for activity testing as no further water was produced during the latter part of this period. Two commercial coprecipitated Cu-Zn0-A1203catalysts
(X and Y) were used for comparative testing. They were supplied in oxidic form and reduction was carried out by slowly ramping the temperature to 250°C over 18 hours and gradually increasing the amount of H
in a H /N mixture from 0.5% H 2 2 2 2 to pure H2 over the same period. Activities of all catalysts for methanol
synthesis were measured at 220'C
and 4500 kPa. The syngas mixture which
contained 5 f 0.2 % CO and 4 f 0.2 % CO in H was used at a GHSV (NTP) of 2 2 -1 36,000 h . These conditions closely represent industrial practice. Measurement of the surface areas of the catalysts was described in part I1 of this series of publications.
241
RESULTS AND DISCUSSION Figure l(a) shows specific activity (obtained by dividing activity per unit mass by the BET surface area) for alloys leached to different depths. The surface areas of the catalysts used to calculate the catalyst specific activities are reported in part I1 of this series of publications. The specific activity of catalyst RCDl is higher than catalyst RC and it increases with leach depth whereas the activity of catalyst RC remains approximately constant. Previous studies [7,8] concluded that the continuous copper surface of Raney Copper is only mildly active and that zinc deposits on the surface promote hydrogenation of carbon oxides through secondary interaction with the adsorbed species [7]. Composition characterisation studies 171, using an electron microprobe, on these catalysts showed that the change in activity is caused by increases in ZnO loading on the surface and/or decreases in A1203 residuals which are in contact with the porous copper. Compared to one of the the coprecipitated catalyst (designated X in Figure l(a) and (b) and hereafter in the text) catalyst RC has very low activity. However catalyst RCD has higher specific activity at leach depths greater than 0 . 3 nun, as a result of accumulating ZnO on the Cu-A1 0 catalyst surface [7]. 2 3 The specific activity of catalyst RCZD (Figure l(b)) increases significantly compared with RCD2 which shows a minor increase and RCZ which passes through a maximum at a leach depth of ca. 0.8 mm. Figure l(b) shows significantly improved specific activity for the catalyst RCZD over catalyst RCZ (prepared from the same alloy) at leach depths greater than 0.7 mm. The increased activity RCZD must be attributed to the effect of leaching a zinc containing alloy in a 4
at 27L
52
=
K
c1
E
1 A
c
1.8-
0
U .-
E 0.6 -
Y-
vl
o
E
CAT X
>
z" 1.2-
-
__---
--
---.-
2 .-w c
I
I
0.4
I/ I
.
5.-a U
8
0.2
Leoc hont Allor 8 Zinrote Cu-Al-Zn o No OH Cu-Al-Zn Zincote Cu-Al
J
\ E
\
r4
-g
101
X
ul I
0.6
I
0.8 Leach depth fmm)
1
1
0
I
0.3
I
I
I
0.9 1.2 Leach depth Imm)
0.6
I
1.5
Figure 1. Specific methanol activity as a function of lea59 depth in pellets. Methanol yield determined at a space velocity of 36 000 h .(a) Catalysts: (o)RCDl, (a)RC,(CAT X) commercial catalyst. (b)Catalysts: (r)RCZD, (o)RCDZ, (o)RCZ. Raney catalyst preparation conditions described in Table 1.
242
zincate-rich leach solution.This was confirmed by composition characterisation, using an electron microprobe [7], which showed that catalyst RCZD had the highest ZnO and lowest A1203 loadings. The highest specific activity for the RCZD catalyst ca. 5 times greater than that X (the industrial coprecipitated Cu-ZnO-A10 catalyst). This shows that the ZnO deposited,from the zincate 2 3 solution, together with residual ZnO from the Cu-Al-Zn alloy, results in significantly higher densities of active sites in the Raney Cu-ZnO-A10 2 3 catalyst than achieved by conventional techniques used to make coprecipitated Cu-ZnO-A10 catalysts. It is interesting to note that the RCZ catalyst (Figure 2 3 l(b)) which has ZnO in the leached structure resulting solely from ZnO deposited from Zn contained in the precursor Cu-Al-Znalloy, has similar specific activity to the RCD catalyst which contains ZnO resulting solely from deposition from the zincate/NaOH solution. This suggests that similar promotion of the copper surface can be achieved from zinc species originating in the alloy or leachant [71. RCZD catalysts were found [7] to have higher surface areas than the RCZ catalysts as a result of a slower leach rate the sodium zincate [7]. The improvements in specific activity and surface area of the RCZD catalyst result in significantly improved methanol yields compared to the RCZ catalyst (Figure
2). The best RCZ catalyst and RCD catalyst and the industrial coprecipitated Cu-Zn0-A1203catalyst (CAT X) all have similar methanol yields, and the RCZD catalyst has a yield of approximately twice that of the industrial coprecipitated catalyst. Table 2 summarises the best methanol yields achieved using the different techniques for preparing the Raney Cu-Zn0-A1203catalysts,
.
Leochont Alloy-
$ 3.5\ E
m
Zintote
n NoOH
Zincote
CU-AI -Zn Cu-AI-Zn CU-AI
-2 2.8-
Figure 2. The methanol yields for Raney catalysts produced by leaching , and an industrial coprecipitated catalyst (CAT X) tested for comparison. Catalysts: (m) RCZD, ( 0 ) RCZ, ( 0 ) RCD. Raney catalyst preparation conditions descibed in Table 1.
E
W c
c .+ 0
21-
-
Ha 1.1,U
-
n 0
f
0.7-
P 0
I
0.2
I
0.1,
I
0.6
Vol. frac. leached
I
0.8
1
243
and methanol yields for industrial catalysts tested under the same conditions. Included in Table 2 are two ICI coprecipitated catalysts reported in the patent literature [9,10]which were tested under similar conditions. These comparisons show that the Raney Cu-Zn0-A1203catalyst (RCZD) is superior to the two commercial catalysts tested and the ICI catalysts quoted in the patents. TABLE 2.
Highest yields for Raney catalysts (produced at different leaching conditions) compared to coprecipitated catalysts tested under the same conditions and other catalysts reported in the literature. Catalyst Preparation RCZD RCZD RCD RCD RCZ CAT X CAT X CAT Y ICI ICI
I I IIa IIb I11 IV IV IV IV IV
Preparation I I1 I11 IV
Temperature ("C)
Pressure (atm)
220 220 220 220 220 220 220 220 250 226
45 45 45 45 45 45 45 45 50 50
Space veloclty (h- ) 36000 15000 36000 36000 12000 36000 15000 15000 40000 12000
Yield (kg/l/h) 1.12 0.80 0.64 0.61 0.60 0.60 0.44 0.45 0.5 0.7
Referrence
5 5 9 10
Cu-Al-Zn leached in 6.1 M NaOH/0.62 M Na-zincate,at 303K; Cu-A12 leached in 6.1 M NaOH/0.62 M Na-zincate,(a)274K (b)303K; Cu-Al-Zn leaching 6.1 M NaOH, at 274K Coprecipitation to form Cu-ZnO-A10 (commercial) 2 3
AQ("T
Support was provided under the National Energy, Research and Development Program administered>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.
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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
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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 <10
50% 90%
EP
Existent Gum, mg/100 ml Sulfur, Nitrogen, ppm Composition, wt% n-Paraf f ins i-Paraf f ins Olef ins Naphthenes Ar omatics Durene
(115) (210) (330)
(400)
4.6 41.6 9.5 9.2 35.1 1.84
Several vehicle driveability tests using trained drivers were conducted with MTG gasoline (ref. 17). These tests included studies with New Zealand-type cars, six 1981-model US cars, and 34 cars.
The latter test was a consumer-type test and included 23
US, 6 Japanese, and 5 European cars.
The tests were conducted
under a wide range of ambient conditions.
For example, with the
New Zealand-type cars, the tests were conducted in all-weather chassis dynamometer rooms at temperatures ranging from -18 to 32OC
271 ( 0 90"F), which represents the temperature range expected for New Zealand. In all tests, the performance of MTG gasoline was equivalent to that of conventional gasolines of similar
volatility. CONCLUDING REMARKS Bench-scale studies defined conditions f o r successful operation of the MTG process.
The long-term performance of the
DME and ZSM-5 catalysts was demonstrated.
Inherent gasoline
selectivity was found to be insensitive to catalyst age.
The raw
gasoline produced is of high quality, and its only unusual aspect is that is contains a much higher concentration of durene than
conventional gasolines. Scale-up of the process to a size of 4 B/D was an unqualified success. Product yields, gasoline quality, and general reactor behavior were essentially identical to those of the bench unit. The scale-up produced one benefit: longer ZSM-5 cycle lengths, which are attributed to a lower rate of coke formation.
Extensive
studies better defined the durene level required to assure acceptable gasoline quality from all aspects.
The
HGT process was
developed to achieve reduced d'urene levels in finished MTG gasoline without exacting significant penalties in gasoline yield or quality.
Waste water treating studies determined that
conventional biological treating methods give acceptable effluent discharge quality. The information developed in the bench- and 4 was used to design the first commercial
B/D unit studies
MTG installation, located
in Motunui, Kew Zealand.
The commercial plant represents a scaleup factor of at least 3000 over the 4 B/D unit. This factor is much higher than usually recommended (ref. 2 0 ) . A s superb methanol conversion efficiency is mandatory, additional studies were conducted to ensure uniform catalyst loading and an even distribution of reactants in the catalyst bed.
The excellent
performance of the commercial MTG unit (ref. 21) attests to the success of these efforts. ACKNOWLEDGEMENT The author acknowledges the significant contributions of all the people at Mobil's Paulsboro Laboratory who were involved with development of the fixed-bed MTG and HGT processes, the number of
212
whom is too large to mention individually.
Figures 8 and 12 are
reprinted by permission of Reference 8 . REFERENCES 1 Meisel, S.L.,J.P. McCullough, C.H. Lechthaler, P.B. Weisz, CHEMTECH, 6 (1976) 86. 2 Chang, C.D., and A.J. Silvestri, J. Catal., 47 (1977) 249. 3 Wise, J.J., and A.J. Silvestri, Oil Gas J., 74 (1976) 140. 4 Voltz, S.E., and J.J. Wise, Development Studies on Conversion of Methanol and Related Oxygenates to Gasoline, Final Report, ERDA Contract No. E(49-18)-1773, November 1976. 5 Yurchak, S., S.E. Voltz, and J.P. Warner, Ind. Eng. Chem. Process Des. Dev., 18 (1979) 527. 6 Lee, W., S. Yurchak, N. Daviduk, and J. Maziuk, A Fixed-Bed Process for the Conversion of Methanol to Gasoline, 1980 NPRA Annual Meeting, New Orleans, Mar. 25, 1980. 7 Liederman, D., S. Yurchak, J.C.W. Kuo, and W. Lee, Mobil Methanol-to-Gasoline Process, 15th Intersociety Energy Conversion Engineering Conference, Seattle, Aug. 18-22, 1980. 8 K a m , A.Y., M. Schreiner, and S. Yurchak, in R. A. Meyers (Editor), Handbook of Synfuels Technology, McGraw-Hill, New York, 1984, Chapter 2-3. 9 Penick, J.E., W. Lee, and J. Maziuk, in J. Wei and C. Georgakis (Editors), Chemical Reaction Engineering - Plenary Lectures, ACS Symposium Series 226, ACS, Washington, 1983, p p . 19-48. 10 Kokotailo, G . T . ,S.L. Lawton, D.H. Olson, and W.M. Meir, Nature, 272 (1978) 437. 11 Chang, C.D., J.C.W. Kuo, W.H. Lang, S.M. Jacob, J.J. Wise, and A.J. Silvestri, Ind. Eng. Chem. Process Des. Dev., 17 (1978) 255. 12 Chang, C.D., Catal. Rev. - Sci. Eng., 25 (1983) 1. 13 Chen, N . Y . , and W.J. Reagan, J. Catal., 59 (1979) 123. 14 Anderson, D.H.,and A.V. Sapre, Simulation of Heat Effects in Laboratory Adiabatic Reactors, presented at 1986 Annual AIChE Meeting, Miami Beach, November 2-7, 1986. 15 Rogerson, P.L.,in R. A. Meyers (Editor), Handbook of Synfuels Technology, McGraw-Hill, New York, 1984, Chapter 22. 16 Schipper, P.H.,and F.J. Krambeck, A Reactor Desigr. Simulation with Reversible and Irreversible Catalyst Deactivation, presented at 9th International Symposium on Chemical Reaction Engineering (ISCRE-9), Philadelphia, May 18-21, 1986. 17 Fitch, F.B., and W. Lee, Methanol-to-Gasoline, An Alternative Route to High Quality Gasoline, SAE International Pacific Conference, Honolulu, Kov. 16-19, 1981. 18 Silvestri, A.J., Mobil Methanol-to-Gasoline Process, 181st ACS National Meeting, Atlanta, Mar. 29 - Apr. 3, 1981. 19 Sullivan, R.F., C.J. Egan, G.E. Langlois, and R.P. Sieg, J. Amer. Chem. SOC., 83 (1961) 1156. 20 Bisio, A., in A. Bisio and R. L. Kabel (Editors), Scale Up of Chemical Processes, John Wiley, New York, 1986, Chapter 1. 21 Allum, KZG, and D.M. Turnbull, Start-up of the World’s First Gas-to-Gasoline Plant, presented at ACS State-of-the-Art Symposium on Methanol as a Raw Material for Fuels and Chemicals, Marco Island, Florida, June 15-18, 1986.
273
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
CONVERSION OF METHANOL TO LIQUID FUELS BY THE FLUID BED Mobil PROCESS ( A commercial concept) Grimmer, N. T h i a g a r a j a n , and E . N i t s c h k e UHDE GmbH, F r i e d r i c h - U h d e - S t r a s s e 15, Dortmund, F. R. Germany
H. R.
ABSTRACT From 1981 t o 1984 M o b i l Research + Development C o r p o r a t i o n , Union Rheini s c h e Braunkohlen K r a f t s t o f f AG and UHDE GmbH have o p e r a t e d a demons t r a t i o n p l a n t f o r t h e F l u i d Bed M o b i l Process. The advantage o f t h e process l i e s i n t h e d e s i r e d s e l e c t i v i t y o f hydrocarbons i n t h e p r o d u c t . Besides p r o d u c i n g h i g h q u a l i t y g a s o l i n e (MTG o p e r a t i o n ) process c o n d i t i o n s can be v a r i e d t o produce o l e f i n s (MTO o p e r a t i o n ) , w h i c h can be c o n v e r t e d t o g a s o l i n e and d i e s e l . On t h e b a s i s o f t h e r e s u l t s g a i n e d a t Wesseling, a commercial s t u d y was performed, which d e a l s w i t h f e a s i b i l i t y o f g a s o l i n e f r o m c o a l and n a t u r a l gas.
INTRODUCTION The c o n v e r s i o n o f n a t u r a l gas i n t o l i q u i d t r a n s p o r t a t i o n f u e l i s a w e l l proven t e c h n o l o g y t o d a y . There a r e two b a s i c t e c h n o l o g i e s a v a i l a b l e ; t h e Fischer-Tropsch
s y n t h e s i s and t h e methanol r o u t e developed by M o b i l Re-
search and Development C o r p o r a t i o n , USA. The F i s c h e r - T r o p s c h s y n t h e s i s i s n o t v e r y s e l e c t i v e . The sist
predominantly
of
products
s t r a i g h t c h a i n m o l e c u l e s which a r e n o t v e r y f a -
v o u r a b l e f o r h i g h octane g a s o l i n e . F u r t h e r r e f i n i n g i s r e q u i r e d tain
a
premium
to
at-
g a s o l i n e q u a l i t y . The p r o c e s s produces a wide range o f
b y - p r o d u c t s w h i c h have t o be worked up and hence i n c r e a s e s ment c o s t .
con-
the
invest-
274
The
methanol
route
is
highly
s e l e c t i v e towards p r o d u c t i o n o f l i q u i d
t r a n s p o r t a t i o n f u e l s . O n l y a v e r y s m a l l amount C,,
are
produced.
The
process
M o b i l Research and Development Corp. (MRDC), produced
does
of
hydrocarbons
beyond
uses a z e o l i t e c a t a l y s t , developed b y USA.
The
final
gasoline
n o t need f u r t h e r r e f i n i n g and a t t a i n s t h e q u a l i t y o f un-
l e a d e d premium g a s o l i n e . The w o r l d ' s f i r s t commercial s y n f u e l p l a n t the
production
for
o f g a s o l i n e f r o m n a t u r a l gas v i a methanol has been con-
s t r u c t e d i n New Zealand and went s u c c e s s f u l l y onstream i n l a t e 1985. The c a p a c i t y i s 570,000
tonnes o f g a s o l i n e p e r y e a r . The MTG r e a c t i o n system
i s an a d i a b a t i c f i x e d bed v e r s i o n . The development o f a f l u i d bed r e a c t i o n system advantages
in
which
offers
potential
terms of y i e l d , q u a l i t y , h e a t r e c o v e r y and l o w e r i n v e s t -
ment c o s t has been completed and t h e system i s now
commercially
avail-
able. Uhde has been d i r e c t l y i n v o l v e d i n t h e design, e n g i n e e r i n g and o p e r a t i o n of
a f l u i d - b e d d e m o n s t r a t i o n p l a n t , j o i n t l y developed b y MRDC, URBK and
Uhde and p a r t l y f i n a n c e d b y t h e American and plant
has
successfully
German
Governments.
This
demonstrated t h e performance o f t h i s f l u i d bed
r e a c t i o n system f o r MTG and MTO (methanol t o o l e f i n s ) t e c h n o l o g y . Uhde's commercial experience, g a t h e r e d f r o m s e v e r a l f l u i d bed EDC u n i t s lene
dichloride)
provides
(ethy-
a s o l i d background f o r t h e d e s i g n and e n g i -
n e e r i n g o f commercial f l u i d - b e d r e a c t i o n systems. The economics o f commercial-scale p l a n t s a r e demonstrated i n t h i s for
paper
two s e l e c t e d c a p a c i t i e s (2060 tonnes/day and 6180 tonnes/day o f un-
l e a d e d premium g a s o l i n e f r o m n a t u r a l gas v i a methanol). DEVELOPMENT OF MTG / MTO FLUID-BED TECHNOLOGY The f l u i d - b e d r e a c t o r i s an i d e a l and v e r y e f f i c i e n t system f o r t h e conv e r s i o n o f methanol t o hydrocarbons. The hydrodynamics o f a f l u i d - b e d system f u l l y s a t i s f i e s t h e process quirement.
I n comparison w i t h o t h e r r e a c t i o n systems, t h e f l u i d - b e d has
t h e f o l l o w i n g advantages:
-
re-
c a t a l y s t usage i s e f f i c i e n t good m i x i n g p r o v i d e s t e m p e r a t u r e u n i f o r m i t y a c r o s s t h e bed
275
-
t h e e x c e l l e n t heat t r a n s f e r p r o p e r t i e s o f a t u r b u l e n t bed p e r m i t d i r e c t
-
The r e g e n e r a t i o n o f t h e c a t a l y s t i s c o n t i n u o u s . T h i s p e r m i t s t h e c a t a -
steam g e n e r a t i o n ( s t e a m c o i l s immersed i n t h e r e a c t o r dense bed) l y s t a c t i v i t y t o be h e l d c o n s t a n t .
- The s p e c i f i c t h r o u g h p u t i n a f l u i d - b e d s y s t e m i s h i g h e r . The f l u i d - b e d d e m o n s t r a t i o n p l a n t was l o c a t e d a t t h e URBK f a c i l i t i e s Wesseling,
FRG
in
( F i g . 1) and was o p e r a t e d f r o m December 1982 t o t h e end
o f 1985. The d e m o n s t r a t i o n p l a n t r e s u l t s have been w i d e l y
published.
A
summary o f p l a n t o p e r a t i o n and o p e r a t i o n a l r e s u l t s a r e p r e s e n t e d b e l o w : Demonstration P l a n t Operation Operation period: 3 years Onstream t i m e : 12.200 h o u r s S e r v i c e f a c t o r t o t a l : o v e r 60 % Service f a c t o r d u r i n g schedule t e s t runs: 99 % T o t a l methanol processed: 8.930 m t
MTG O p e r a t i o n Pressure: 2.7 t o 4.5 b a r T e m p e r a t u r e : 380
OC
t o 430
O
C
M e t h a n o l f e e d r a t e : 500 t o 1 . 0 5 0 k g / h Methanol c o n v e r s i o n i s complete. Long t e r m p r o d u c t i o n r u n s c o n f i r m e d t h e h i g h y i e l d and t h e h i g h q u a l i t y o f the
u n l e a d e d g a s o l i n e . The p r o d u c t q u a l i t y meets t h e premium s t a n d a r d f o r
unleaded g a s o l i n e .
MTO O p e r a t i o n Pressure: 2.2 t o 3.5 b a r T e m p e r a t u r e : 470
C t o 515
OC
M e t h a n o l f e e d r a t e : 570 t o 620 kg/h Methanol c o n v e r s i o n i s complete. A t s t e a d y s t a t e c o n d i t i o n s t h e o l e f i n e y i e l d was more t h a n 60 %.
276
DemonslralionPlant at URBK. Wesseling. FRG
Ni
Y
Fls.
Feed
v+Fluid Bed principle
Fig.2
1
211
Summary o f results
- Regeneration of Catalyst Continuous regeneration and stable circulation of the catalyst weresuccessfully demonstrated. With continuous regeneration the selected catalyst activity and the product slate is held constant.
- Heat removal and Temperature Profile Heat removal was successfully demonstrated. The isothermal profile in the reactor could be maintained easily.
-
Liquid Injection of Methanol Liquid injection of methanol (up to 50 % of feed as liquid) was successfully demonstrated.
- Liquid injection permits the steam balance to be tailored a s required.
-
Catalyst Recovery System Conventional equipment used in fluid-bed reactors was incorporated and successfully demonstrated. Flexibility of Operation Operational flexibility in terms o f selected catalyst activity and partial load operation (down to 50 %) was successfully demonstrated.
- Mechanical Integrity Long-term material testing was performed to confirm material selection for equipment , catalyst transfer piping and control valves. Scale-up ( F i g . 2) On the basis of the results obtained and the experience gathered from the operation o f the demonstration plant, Uhde has developed a reaction system design which can be installed as a single train unit up t o processing capacity of 2500 tonnes/day of methanol. This corresponds to a liquid fuel production of about 1030 tonnes/day.
278
FIXED BED-ADIABATIC RECYCLE
n
REACTOR PROD.
t
COMPR.
120 BAR
REACTOR (1+1)
-
COOLANT
REACTOR
(N+l)
(N+1
1 REACTOR r
FIXED BED-TUBULAR
FLUIDIZED BED SINGLE STAGE
MTG Reaction Systems
Olefine recycle
-
PRODUCT
f
Fig. 3
Light gas
I
MTG: Process configuration
rmpane
Fig.4
279
FEATURES OF FLUID-BED MTG ( F i g . 3)
-
The y i e l d o f g a s o l i n e i n c l u d i n g a l k y l a t e i s a t l e a s t 7 . 5 % h i g h e r t h a n i n a f i x e d bed.
-
F l u i d bed g a s o l i n e meets a s p e c i f i c a t i o n o f unleaded premium g a s o l i n e . Durene c o n t e n t i s l o w e r t h a n i n a f i x e d bed. The performance o f t h e f l u i d - b e d i n u t i l i z i n g t h e h e a t o f r e a c t i o n i s b y f a r s u p e r i o r t o t h a t o f t h e f i x e d bed. Steam g e n e r a t i o n i s d i r e c t . The
-
120 b a r steam g e n e r a t e d i s 0.8 t o n n e d t o n n e o f methanol processed. Continuous r e g e n e r a t i o n i n t h e f l u i d - b e d m a i n t a i n s c o n s t a n t p r o d u c t composition. L i q u i d i n j e c t i o n , a unique f e a t u r e i n t h e f l u i d - b e d ,
provides the f l e x i -
b i l i t y t o t a i l o r t h e steam b a l a n c e as p e r r e q u i r e m e n t . S p e c i f i c i n v e s t m e n t c o s t i s l o w e r t h a n t h a t o f t h e f i x e d bed ( h i g h e r s p e c i f i c t h r o u g h p u t , l e s s h e a t t r a n s f e r area, no r e c y c l e compression)
DESIGN OF COMMERCIAL FLUID-BED MTG AND MTO PROCESSES Uhde has developed commercial schemes f o r t h e p r o d u c t i o n o f l i q u i d motor f u e l s v i a methanol.
-
t h e p r o d u c t i o n o f MTG g a s o l i n e t h e p r o d u c t i o n o f o l e f i n s (MTO) which serve as base f e e d s t o c k f w r t h e c o - p r o d u c t i o n o f g a s o l i n e and d i e s e l (MOGD)
MTG Process ( F i g . 4 ) The process t e c h n o l o g y a s s o c i a t e d w i t h t h e MTG p l a n t i s as f o l l o w s : methanol d i s t i l l a t i o n
Uhde
MTG r e a c t i o n system
M o b i l / Uhde
gas f r a c t i o n a t i o n
Uhde
HF a l k y l a t i o n
p r o p r i e t a r y process
280 heavy g a s o l i n e t r e a t m e n t
Mobi 1
g a s o l i n e b l e n d i n g and
Uhde
product storage Crude methanol from t h e methanol s y n t h e s i s w i l l be d i s t i l l e d i n remove
the
water
order
to
c o n t e n t and hence improve t h e o v e r a l l process economy.
The d i s t i l l e d methanol i s t h e n c o n v e r t e d t o hydrocarbons i n t h e
fluid-bed
r e a c t i o n system. The hydrocarbon p r o d u c t s a r e processed i n a gas f r a c t i o n ation
unit
t o produce C,
p l u s hydrocarbons, a l k y l a t i o n f e e d (C,/C,
t i o n ) and o l e f i n r e c y c l e . The C 3 / C 4
frac-
f r a c t i o n i s processed i n t h e HF
alky-
l a t i o n u n i t t o produce a l k y l a t e . Heavy
gasoline
treatment w i l l be r e q u i r e d o n l y i f a f u r t h e r reduction o f
t h e durene c o n t e n t i s necessary. Durene produced i n t h e f l u i d - b e d is
process
no more t h a n 5 w e i g h t %. V e h i c l e t e s t s w i t h t h i s g a s o l i n e conducted i n
t h e Federal R e p u b l i c o f Germany p r e s e n t e d no problems. The C 5 p l u s f r a c t i o n , a l k y l a t e and n-butane a r e b l e n d e d t o
produce
leaded h i g h octane gasoline. P r o d u c t i o n and Consumption F i g u r e s MTG p r o d u c t i o n and consumption f i g u r e s a r e shown below:
TABLE 1 P r o d u c t ( p e r t o n n e o f methanol processed): Gasoline
(kg)
412
Propane
(kg)
18
L i g h t Gas* ( k g )
9
*
L i g h t gas i s s e n t t o gas p r o d u c t i o n u n i t - f e e d f o r steam r e f o r m e r
un-
281
TABLE 2 MTG Consumption F i g u r e s ( p e r t o n n e o f methanol processed)
HGT u n i t
Yes
No
Consumption 15 b a r steam (Kg)
158
98
5 b a r steam (Kg)
160
168
E l e c t r i c energy (KWh)
18.4
Demin. w a t e r (Kg) Cooling water Nitrogen
(m3)
(Nm3)
17.4
200
250
20
20
46
46
MTO PROCESS ( F i g . 5) The process t e c h n o l o g y a s s o c i a t e d w i t h t h e p r o d u c t i o n o f g a s o l i n e and d i e s e l f r o m methanol i s as f o l l o w s : M o b i l / Uhde
MTO u n i t gas f r a c t i o n a t i o n
Uhde
MOGD-uni t
Mobi 1
HDT-un it
p r o p r i e t a r y process
alkylation unit
p r o p r i e t a r y process
l i q u i d f u e l b l e n d i n g system
Uhde
The
MTO
unit
(methanol
to
olefins)
i n c o m b i n a t i o n w i t h t h e MOGD u n i t
( M o b i l o l e f i n s t o g a s o l i n e and d i s t i l l a t e ) co-produces g a s o l i n e tillate.
and
dis-
The d i s t i l l a t e f r a c t i o n can be used e i h t e r e n t i r e l y as d i e s e l o r
s p l i t i n t o d i e s e l and j e t f u e l . The f l e x i b i l i t y o f t h e combined MTO / MOGD process p e r m i t s p l a n t o p e r a t i o n t o v a r y t h e r a t i o o f g a s o l i n e
to
distil-
late. Methanol
is
c o n v e r t e d t o hydrocarbons i n t h e MTO u n i t . The m a j o r p a r t o f
t h e hydrocarbons c o n s i s t s o f o l e f i n s . The hydrocarbon p r o d u c t s a r e p r o c e s s e d i n t h e gas produce
an
ethylene
recycle
fractionation
unit
to
s t r e a m t o t h e MTO r e a c t o r , MOGD f e e d and a
heavy a r o m a t i c f r a c t i o n which can b e b l e n d e d t o e i t h e r g a s o l i n e o r d i s t i l late.
282
I Methanol
MTO '[luid-bed-
r
Fractionation
MOGD
1 mzl3El
-
Fraction- Gasoline Gasoline ation -blending
Raw d i s t i l l a t e q HDT
I
HZ from front e n d
,
Lidit s a s Prop a n e
I
I I
Gasoline product
t
Diesel product
MTO/MOGD Process configuration
Fig. 5
I
Gas to Gasoline (GTG) via methanol
Fig.6
,
,
283
L i g h t o l e f i n s a r e o l i g o r n e r i z e d i n t h e MOGD u n i t t o coproduce g a s o l i n e
and
d i s t i l l a t e . The d i s t i l l a t e i s h y d r o t r e a t e d (HTD u n i t ) t o produce t h e f i n a l d i s t i l l a t e product. The
alkylation
unit
included
i n t h e scheme serves t o u t i l i z e t h e C,/C,
f r a c t i o n d e r i v e d from t h e MOGD u n i t . A l k y l a t e i s b l e n d e d t o g a s o l i n e . P r o d u c t i o n and consumption f i g u r e s a r e shown below: TABLE 3 P r o d u c t i o n F i g u r e s ( p e r t o n n e o f methanol processed) L i g h t gas
(kg)
20
LPG
(kg)
20
Gasoline
(kg)
198
Distillate
(kg)
198
TABLE 4 Consumption F i g u r e s ( p e r t o n n e o f methanol processed) Steam, 15 b a r , s a t u r a t e d ( k g ) 10 E l e c t r i c energy (KWh) Cool i n g w a t e r
(rn3)
Demin. w a t e r ( k g )
30 12 50
Hydrogen ( k g )
4
N i t r o g e n ( Nm3)
45
The f l e x i b i l i t y o f t h e process p e r m i t s v a r i a t i o n o f t h e g a s o l i n e t o d i e s e l r a t i o . As a t y p i c a l example t h e s e l e c t e d g a s o l i n e t o d i e s e l r a t i o i s 1 : l . THE COMMERCIAL CONCEPT The w o r l d w i d e d i s c o v e r y o f l a r g e gas f i e l d s and t h e d e c l i n i n g d i s c o v e r y o f new o i l f i e l d s may l e a d i n t h e d i r e c t i o n t o produce g a s o l i n e and d i s t i l l a t e f r o m n a t u r a l gas up t o t h e l a t e 1990s. O i l - p r o d u c i n g companies a r e a l s o f o r c e d t o search f o r and produce o i l i n more d i s t a n t and remote areas w h i c h w i l l add on c o s t w h i c h c o u l d n o t b e j u s t i f i e d by p r e s e n t c r u d e o i l p r i c e s . The p r o d u c t i o n o f l i q u i d t r a n s p o r t f u e l s f r o m n a t u r a l
284
gas w i l l a l s o c r e a t e a market independence o f c o u n t r i e s , w h i c h do n o t have t h e i r own c r u d e o i l r e s o u r c e s . A commercial concept on t h e f l u i d - b e d v e r s i o n s h a l l demonstrate t h e f e a s i b i l i t y o f a GTG p l a n t . The c o u n t r y ' s economies o f s c a l e w i l l i n f l u e n c e t h e p r o d u c t i o n c a p a c i t y o f t h e GTG p l a n t . The concept i s based on two p l a n t c a p a c i t i e s :
-
2060 t o n n e s / d a y o f premium u n l e a d e d g a s o l i n e 6180 t o n n e s / d a y o f premium u n l e a d e d g a s o l i n e
The p l a n t s w i l l be g r a s s - r o o t i n s t a l l a t i o n s , i n c l u d i n g s i g n i f i c a n t s t r u c t u r e f o r e f f i c i e n t performance and l i f e s u p p o r t a c t i v i t i e s
infra-
necessary
f o r successful operation o f the u n i t s . The economic v i a b i l i t y o f a l a r g e s y n f u e l p l a n t l i e s i n such f a c t o r s as:
-
l o w - c o s t gas s u p p l y s c a l e and r e l i a b i l i t y o f o p e r a t i o n s
- c o o p e r a t i o n between Government and p r i v a t e s e c t o r organizations
- f i n a n c i n g methods
-
management s k i l l s a b i l i t y t o be an e n v i r o n m e n t a l l y a c c e p t a b l e n e i g h b o u r while minimizing cost i n achieving t h a t objective. efficient infrastructure productive labour
The
importance
of
t h e methods o f f i n a n c i n g t h e p r o j e c t i s emphasized by
t h e f a c t t h a t s y n f u e l p l a n t s a r e c a p i t a l - i n t e n s i v e p r o j e c t s and once b u i l t o p e r a t i o n c o s t s ( e x c l u s i v e o f c a p i t a l burdens) a r e l a r g e l y gas maintenance,
supplies
and
feedstock,
l a b o u r w h i c h s h o u l d change w i t h t h e t i m e as a
f u n c t i o n o f i n f l a t i o n . The c a p i t a l e s t i m a t e o f t h e p l a n t u n i t s a r e budgeta r y e s t i m a t e t y p e s based on P a c i f i c b a s i n c o n d i t i o n s , l a t e 86. units
such
as
The
plant
p r o d u c t i o n o f s y n t h e s i s gas v i a steam r e f o r m i n g , methanol
s y n t h e s i s , methanol d i s t i l l a t i o n , gas f r a c t i o n a t i o n , H F - a l k y l a t i o n , gasoline
treatment
and
gasoline
w e l l - p r o v e n modules w h i c h can fluid-bed
MTG
module
is
the
be
blending estimated
only
from
as-built
plants.
The
new d e s i g n component i n t h e s y n f u e l
p l a n t . Percentage-wise t h i s i s o n l y a p p r o x . u n i t s costs.
heavy
and s t o r a g e a r e c o m m e r c i a l l y
10
percent
of
the
process
285 Scope o f t h e GTG P l a n t s The MTG p l a n t s w i l l comprise t h e process s e c t i o n and o f f - s i t e s as shown i n t h e b l o c k flow-diagram,
F i g . 6 . The p l a n t u n i t s w i l l be as f o l l o w s :
o Syngas g e n e r a t i o n
-
n a t u r a l gas compression and d e s u l p h u r i z a t i o n
u n i t 01
steam r e f o r m i n g and h e a t r e c o v e r y
u n i t 02
o Methanol p r o d u c t i o n
- syngas compression
u n i t 20
methanol s y n t h e s i s
u n i t 21
methanol d i s t i l l a t i o n
u n i t 22
-
o Gasoline production
- MTG r e a c t i o n system
-
u n i t 30
reactor effluent cooling
u n i t 31
gas f r a c t i o n a t i o n
u n i t 32
heavy g a s o l i n e t r e a t m e n t
u n i t 33
HF a l k y l a t i o n
u n i t 34
o Storage f a c i l i t i e s
-
methanol i n t e r m e d i a t e s t o r a g e
u n i t 60
hydrocarbon s t o r a g e and g a s o l i n e b l e n d i n g
u n i t 61
MTG c a t a l y s t s t o r a g e
u n i t 35
o U t i l i t i e s systems
-
-
-
b o i l e r feed water treatment
u n i t 51
steam system
u n i t 52
p l a n t a i r and i n s t r u m e n t a i r
u n i t 53
c o o l i n g w a t e r system
u n i t 54
blow-down and f l a r e
u n i t 55
e l e c t r i c a l system
u n i t 56
f i r e - p r o t e c t i o n system
u n i t 57
s u r f a c e w a t e r system
u n i t 70
Scope o f E s t i m a t e ( F i g . 7) The accuracy o f t h e budget e s t i m a t e c o s t f o r t h e e n t i r e complex w i t h i n t h e range o f +/- 20 %.
is
286
0 2,060 Tonnes/Day GTG Plant
using Fluid Bed reaction system
0 6,180 Tonnes/Day GTG Plant
using Fluid Bed reaction system
I
2,900Mill. DM 1,610 Mill. US$
Investment Costs
I I
Fig.7
m
Lay-out plan of a 2,060 tonnes/day Fluid Bed GTG plant
Fig'8
m?r;n
t
-
Lay-out plan of a 6.180 tonnes/dav Fluid Bed GTG dant
-
Fig. g
I’ MTG Module 1,030 tonnes/day
m.10
288 The f o l l o w i n g i t e m s a r e i n c l u d e d i n t h e c o s t e s t i m a t e :
-
-
p r o j e c t management and p r o j e c t c o n t r o l process engineering
-
d e t a i l e d e n g i n e e r i n g and procurement
-
construction
-
-
-
equipment and m a t e r i a l d e l i v e r e d t o s i t e pre-commissioning and mechanical t e s t s commissioning spare p a r t s temporary s i t e f a c i l i t i e s contingencies l i c e n c e fees.
The
site
area
( l a n d p u r c h a s i n g and p r e p a r a t i o n c o s t ) i s n o t i n c l u d e d i n
t h e c o s t estimate.
Plant Lay-Out and Battery Limits The p l a n t b a t t e r y l i m i t s a r e shown i n t h e g e n e r a l l a y - o u t p l a n s , Fig.
9
and
Fig.
Fig.
8,
10. B u i l d i n g s such as workshop, warehouse and a d m i n i s -
t r a t i o n b u i l d i n g s are included. Product loading
facilities
for
gasoline
and LPG a r e n o t i n c l u d e d w i t h i n t h e b a t t e r y l i m i t s . The
correlation
close t o 1 (0.92). processing
units,
f a c t o r between t h e two c a p a c i t i e s shown i n t h i s paper i s Except f o r t h e methanol
distillation
and
down-stream
a l l o t h e r u n i t s a r e m u l t i - t r a i n systems. Syngas gener-
a t i o n c o v e r s 40 % o f t h e i n v e s t m e n t c o s t s o f t h e p r o c e s s i n g u n i t . Uhde now
is
i n t h e process o f d e v e l o p i n g a new t y p e o f r e f o r m e r , w h i c h would p e r -
m i t a s i n g l e t r a i n syngas g e n e r a t i o n c a p a c i t y e q u i v a l e n t t o t h e p r o d u c t i o n
o f 2060 tonnes/day o f g a s o l i n e . The s e l e c t e d s i n g l e t r a i n c a p a c i t y f o r MTG r e a c t i o n system i s e q u i v a l e n t t o t h e p r o d u c t i o n o f 1030 tonnes/day o f gaso l i n e ; i . e . t o process 2500 tonnes/day o f methanol i n one t r a i n . The diame t e r o f such a r e a c t o r i s approx. 6.5 m. L a r g e r d i a m e t e r f l u i d - b e d v e s s e l s ( i . e . 9 m) a r e known f r o m FCC t e c h n o l o g y . These developments a r e bound reduce
the
to
s p e c i f i c i n v e s t m e n t c o s t even a t r e l a t i v e l y s m a l l e r p l a n t ca-
p a c i t y , say 2060 tonnes o f g a s o l i n e p e r day.
289
Item
Unit Consumption
Unit Cost
Raw materials
Cost per tonne of unleaded premium gasoline
DM (US$)
-Natural gas
71 GJ
-catalyst Utilities -El. Power Labour -Production -Overheads Capital rel. costs -Maintenance -Interest -Depreciation
130 kWh
-
DM
1.8'
(I.-) 128.-
3.6'
(20)
256.-
0.08 (0.04) (42,800.-) 77.000.-
200 Personnel 80% of Prod. labour
a 3% of Inv. cost
@ 4.9%
01 Inv. cost"
10% of Inv. cost
') DM or US $/GJ
Production Cost
") Financing 100% interest 8 % repayment 10 years
Unit Cost
Unit Consumption
71 GJ
11.-
23.-
17.-
17.-
47.. 76.156.-
47.. 76.156.-
483.-
611.-
(270.- US E)
(340.- US 5)
Table 5
DM
1.8'
(I..) 128.-
3.6'
(2.4
DM
258.25.11.-
0.08 (0.04) (42,800.-) 77,000.-
130 kWh
550 Personnel 80% of Prod. labour
20.-
16.-
(33% of Inv. cost
I
@ 4.9% of Inv. cost" 10% 01inv. cost
I
*) DM or US S/GJ
") Financing 100% interest 8% repayment 10 years
m
11.-
23.-
Cost per tonne of unleaded premium gasoline
DM (US$)
Raw materials
Production Cost
25.-
II*I".yY
-Natural gas
9
25.-
Production Cost for a 2,060 tonneslday GTG Plant
Item
-catalyst Utilities -El. Power Labour -Production -Overheads Capital rel. costs -Maintenance -Interest -0epreclatlon
DM
I
41.68.138.-
41.68.138.-
447.-
575.-
(250.- US $)
(320.- US 5)
Production Cost for a 6,180 tonneslday GTG Plant
Table 6
290
L
-
2,060 tonneslday GTG plant
Production Cost 1.8
-Gas
2.7
6,180 tonneslday 3.6GTG plant
(1) (2) (3) price DMIGJ (US $/GJ)
Production Cost of Gasoline vs. gas price
Fig.ll
291 Coal t o G a s o l i n e Uhde has a l s o s t u d i e d commercial-scale p l a n t s f o r t h e
production
of
un-
l e a d e d premium g a s o l i n e from c o a l , i . e . u s i n g o u r i n d u s t r i a l l y proven gas-
i f i e r s o r t h e second g e n e r a t i o n ; t h e H i g h Temperature W i n k l e r g a s i f i e r f o r lignite
and t h e advanced Texaco g a s i f i e r f o r h a r d c o a l . Even a t l a r g e ca-
p a c i t i e s these coal-to-gasoline
p l a n t s v i a methanol
will
cost
twice
as
much as t h e e q u i v a l e n t GTG p l a n t s . CONCLUSION ( T a b l e 5 & 6 and F i g . 11) o
A t a gas p r i c e o f 1 U W G J and a GTG p l a n t o f 6,180 tonnes/day capaci t y , one l i t r e o f unleaded premium g a s o l i n e w i l l c o s t 0.19 US$ o r
0 . 3 4 DM. o
The 2,060 tonnes/day GTG p l a n t i s a l r e a d y f e a s i b l e t a k i n g a gas p r i c e o f 1 US$/GJ and an average p r o d u c t i o n c o s t o f g a s o l i n e f r o m c r u d e o i l o f 500 DM/tonne (280 US$/tonne) i n t o account.
o
A p l a n t t h r e e ( 3 ) t i m e s l a r g e r can r e a c h t h e same r e s u l t a t a gas p r i c e o f 1.5 US$/GJ o r 2 . 7 DM/GJ.
o
F u r t h e r development, e s p e c i a l l y i n t h e f i e l d o f syngas g e n e r a t i o n , i s expected t o c o n t r i b u t e t o f u r t h e r r e d u c t i o n o f t h e s p e c i f i c i n v e s t m e n t c o s t o f gas-to-gasoline
o
plants.
W i t h comparable energy p r i c e s and an e q u i v a l e n t p l a n t c a p a c i t y , t h e p r o d u c t i o n o f g a s o l i n e v i a methanol f r o m c o a l i s a b o u t 70
X
more expen-
sive. o
The f l u i d - b e d t e c h n o l o g y w h i c h i s now r e a d y f o r c o m m e r c i a l i z a t i o n provi'des a l l advantages i n terms o f i n c r e a s e d p r o d u c t y i e l d , b e t t e r q u a l i t y and a v e r y e f f i c i e n t h e a t r e c o v e r y .
o
It i s reasonable t o expect t h a t t h e c u r r e n t t r e n d o f i n c r e a s i n g o i l
p r i c e s w i l l p r o v i d e t h e i n c e n t i v e t o produce l i q u i d f u e l s v i a methanol i n t h e n e a r f u t u r e . The s e l e c t i v i t y o f MOBIL's Z e o l i t e C a t a l y s t and t h e e f f i c i e n t performance o f t h e f l u i d - b e d system w i l l c e r t a i n l y h e l p t o achieve t h i s goal.
This page intentionally left blank
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
TOPSOE INTEGRATED GASOLINE SYNTHESIS
-
THE TIGAS PROCESS
J. TOW-JORGENSEN Haldor Tops@e A/S, Nym0llevej 55, 2800 Lyngby, Denmark
ABSTRACT A new low investment process for the conversion of natural gas to gasoline is presented. The process incorporates the MTG process into a combined MeOH and dimethylether synthesis. The conversion of synthesis gas to gasoline is carried out in one single synthesis loop. The process has been demonstrated in a 1 MTPD gasoline process demonstration unit. INTRODUCTION Future synthetic fuel plants are most likely to be built in remote areas where the price of natural gas is very low and not related to the price of gasoline.
For plants of this kind a low investment is essential, as the in-
vestment related costs will constitute a high proportion of the cost of production due to the low energy price. Consequently, further development of the MTG process as it is realised in New Zealand should aim at a reduction of the investment. The TIGAS process represents such an effort.
In the TIGAS process the two process steps, MeOH
synthesis and the MTG process, are integrated into one single synthesis loop without isolation of MeOH as an intermediate (ref. 11, (Fig. 1). In the integrated loop the first step is a new process for the synthesis of oxygenates developed by Haldor Tops0e A/S.
The second step is similar to
Mobil's MTG process (refs. 2-3) operated at conditions selected to achieve optimum operation of the integrated loop.
STEAM
OXYGEN
SYNTHESIS
OXYGENATE
GASOLINE
SYNTHESIS
SYNTHESIS
GAS PRODUCTION
Fig. 1. Gasoline fran natural gas w i t h the TIGAS process.
293
294
THE INTEGRATED CONCEPT The gasoline synthesis involves three sequential steps (Fig. 2 ) .
Thus,
when a process is to be designed, two different approaches can be used. Each step in the synthesis can be treated either as a rather independent plant, or process integration can be attempted. Typically, with a sequential approach, costly operations energy and investment
-
-
both in terms of
are required when going from one process to another.
For example, with the conventional fixed-bed MTG scheme the synthesis gas has to be compressed from 15-20 bar to 50-100 bar before entering the MeOH loop and the already condensed raw liquid MeOH from the MeOH plant has to be re-evaporated and partly dehydrated to DME before entering the MTG loop.
NATURAL GAS
-L
SYNGAS PRODUCTION 15-20bars
b
METHANOL SYNTHESIS 50-100 bars
GASOLINE 15-25 bars
Fig. 2. Idatural gas i s converted t o gasoline i n 3 sequential steps.
Integration of the sequential process steps offers the elimination of such interconnecting unit operations. Furthermore, integration of the MeOH loop and the MTG loop reduces the number of recycle operations from two (one in each loop) to one. The basic problem of process integration is that the syntheses involved are preferably carried out at very different pressures, synthesis gas production at 15-20 bar, MeOH synthesis at 50-100 bar, and the MTG process at 15-25 bar. The aim of the process development work on the integrated gasoline synthe-
sis has been to arrive at a process scheme in which all three steps of the conversion of natural gas to gasoline are conducted at the same pressure level.
This means that operating conditions and catalysts should be modified
so that the low pressure processes can operate at relatively high pressure, and efforts should be made to reduce the necessary operating pressure of the MeOH synthesis.
295
EXPERIMENTAL WORK The exploratory work on the integrated gasoline synthesis was commenced in the late seventies in bench-scale reactors.
After successful results process
studies and ageing tests were carried out in a 15 kg gasoline per day pilot plant. On the basis of the results obtained in this pilot, it was decided to build a process demonstration unit, a 1 MTPD, gasoline pilot plant.
The 1 MTPD pi-
lot was started up in early 1984 (Fig. 3 ) . The experimental programme lasted three years and was terminated in January 1987.
From 1984 to 1987 the demonstration plant has been in operation for
more than 10,000 hours.
The first part of the programme was devoted to the
optimisation and testing of different process schemes and reactor designs, whereas the last 2 , 5 0 0 hours were used to demonstrate the integrated process in one
straight run at constant operating conditions.
The demonstration
plant and the experimental programme have been supported by the Commission of the European Communities. The HZSM-5 catalyst for the demonstration plant was supplied by Mobil.
Fig. 3. The 1 MCPD process damnstration u n i t located in Houston, Texas.
296
SYNTHESIS GAS PRODUCTION If synthesis gas for the TIGAS loop is provided simply by steam reforming of natural gas, the maximum operating pressure will be approx. 20 bar. Operation at higher pressure will not be possible due to an increase in the amount of unconverted methane out of the reformer. Operating the steam reformer at high pressures without a substantial CH4 leakage would require extreme exit temperatures from the reformer, which, for mechanical reasons, is not feasible. If the pressure of the steam reformer is as low as 20 bar, synthesis gas compression is necessary because the MeOH process cannot operate at this low pressure. Using only steam reforming, the synthesis gas will contain a surplus of hydrogen in relation to the MeOH stoichiometry in the synthesis gas.
This sur-
plus hydrogen has to be purged from the loop. A process design which enables the front-end to be operated at high pressure
-
thus approaching the pressure of the second step of the process, the
MeOH synthesis - is the two-step reforming with a tubular steam reformer combined with autothermal reforming using oxygen in a secondary reforming step (Fig. 4).
In this case, the maximum possible operating pressure is consider-
ed to be approx. 50 bars.
Again, the limiting factor is the methane leakage
which increases with higher pressure. Autothermal reforming with oxygen is not a new technology. Already in the 195Os, Haldor Tops0e A/S and Societe Belge de 1'Azote et des Produits
Chimiques de Marly, SBA, Belgium, developed an autothermal reforming process, the Topsoe-SBA Process, which permits the use of a wide variety of light hydrocarbon feedstocks
-
from methane and refinery off-gases to LPG and light
naphtha. This technology has been used for the production of ammonia (fired with air or enriched air) and for the production of hydrogen or MeOH (fired with oxygen).
About 15 commercial plants worldwide are presently using the
Topsoe-SBA process (ref. 4).
Steam Air
+
Primary Reformer
--
Oxygen Plant
Oxygen
Secondary ' Synthesis Gas Reformer
b
Fig. 4. Synthesis gas production w i t h two stage reforrrting.
297
For the secondary reforming step an oxygen plant will be required. Oxygen plants are expensive, but for larger gasoline plants, or MeOH plants, the investment costs of oxygen production are more than set off by the reduced costs
of the much smaller primary reformer operating at the less severe conditions needed for a combined front-end. With this concept it is also possible to produce a gas which corresponds to the MeOH stoichiometry hence avoiding a surplus of hydrogen. OXYGENATE SYNTHESIS The simplest way of integrating MeOH synthesis with gasoline synthesis is to operate both processes at the pressure of a conventional MeOH plant, i.e. 50- 100 bar.
But operating the MTG Process at 50-100 bar
preferred pressure range
-
is not without problems.
-
far away from the
The product distribution
is negatively affected and the irreversible steam deactivation of the zeolite is increased, etc. The loop pressure for a MeOH plant is too high to avoid synthesis gas compression, even when using a combined front-end, unless a substantial methane leakage from the secondary reformer is allowed. It would be favourable if the integrated synthesis was carried out at a pressure lower than that required for a conventional MeOH plant. The minimum loop pressure for a MeOH synthesis is determined by the thermodynamics of the reactions taking place. 2n2
+ co:
n20 +
cH30n
co $
H~
+ co2
However, in the integrated gasoline synthesis there is no reason to restrict the formation of oxygenates to MeOH, as the HZSM-5 catalyst employed in the third step of the integrated gasoline synthesis can convert not only MeOH but a wide range of other oxygenates to hydrocarbons as well (ref. 5). By using a multifunctional catalyst system, allowing not only (1) and (2) but also, e.g., the dehydration of MeOH to dimethylether (DME) and water, Z C H ~ O H~
C3oHw 3
+
n20
(3)
the chemical equilibrium is changed in favour of the formation of oxygenates, MeOH and DME.
The result is that the conversion of synthesis gas to oxygen-
ates can be carried out at a substantially lower pressure (Fig. 5).
298
t
% 100 Conversion
75
~
50
-
25
-
0
Composition 70 ~ 0 1 % CO 5 VOl% con 5 vo190 lnerts 20 vol%
1
20
I
40
I
60
I
80
I
Bars Pressure
100 0
Fig. 5. Conversion of synthesis gas versus pressure at 250 C for MeOH synthesis alone and for combined MeOH/DME synthesis. The reduced operating pressure of the oxygenate synthesis allowed by the introduction of the DME reaction means that the synthesis gas production and the oxygenate process can be operated at the same pressure, thus eliminating the need for synthesis gas compression. Further, the significant reduction of the loop pressure minimises the problems caused by high operating pressure in the MTG process. GASOLINE SYNTHESIS The third step in the conversion of natural gas to gasoline is the conversion of oxygenates to hydrocarbons over the zeolite, HZSM-5.
When this syn-
thesis is integrated into the oxygenate process, it will not only have to be operated at a pressure higher than is normally preferred, but the gas composition in the reactor will also differ significantly from conventional MTG conditions.
Especially the hydrogen content will be high due to the presence of
unconverted synthesis gas.
The operating conditions imposed on the gasoline
synthesis by the integration will influence both the deactivation patterns and product distribution. EFFECTS OF HYDROGEN IN THE INTEGRATED LOOP In the MTG process the gasoline reactors have to be operated in a cyclic way as the HZSM-5 ages due to coking. When the reaction zone has moved through the catalyst bed and MeOH or DME is found in the reactor effluent, the reactor will be taken off stream for reseneration (ref. 6 ) .
The cvcle lensth at a
299
The integration of the oxygenate and MTG processes or, more precisely, the presence of large amounts of hydrogen strongly influences the rate of deactivation. Coking is inhibited by high partial pressures of hydrogen, most likely through hydrogenation of the olefinic coke precursors. The hydrogen partial pressure in the loop is a function of how much hydrogen in relation to the MeOH stoichiometry is present in the gas from the synthesis gas production.
The hydrogen content in a gas can be expressed by the
so-called module.
- co 2 = H2
Module
co + co2
If the module is equal to 2.0, the gas contains just the stoichiometric amount of hydrogen for MeOH production.
A module lower than 2 indicates a de-
ficiency in hydrogen, and a value higher than 2 reflects a surplus of hydrogen. The two-stage reforming concept enables the production of a synthesis gas with the desired module. Lower coking rates caused by high partial pressures of hydrogen result in longer cycle.lengths. By adjusting the front-end design in the integrated synthesis, the hydrogen content can be controlled and with that the cycle length (Fig. 6).
4 3 500
5001L----J 01.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
Module in Gas from Front-end
Fig. 6. Influence of synthesis gas c a n p s i t i o n on cycle length of zeolite (HZSM-5) catalyst. The cycle length is &pressed as the m u n t of hW3-I equivalents (DME = 2 MeoH) processed'durm a cycle by 1 kg of catalyst.
If the integrated process is operated with a cycle length of approx. 6 months, the plant can be shut down for regeneration every 6 months and the existing equipment in the loop used for regeneration. In this way multiple reactors and a separate regeneration system for the gasoline synthesis can be avoided, and the investment costs for the plant further reduced. The hydrogen content in the loop does not only influence the rate of deactivation of the HZSM-5 catalyst, but also has a significant effect on the oxygenate catalyst volume, the recycle flow rate, and the product distribution. The oxygenate catalyst volume is also related to the module of the gas from the synthesis gas production (Fig. 7). obtained at high modules.
Mininum oxygenate catalyst volumes are
The reason is simply that the reaction rate of MeOH
formation depends on the partial pressure of hydrogen (ref. 7 ) .
"2 1
--
Relative 0.8Catalyst volume 0.6--
0.4.-
0.2-
Module in Gas from Front-end
Fig. 7. Influence of synthesis gas composition on oxygenate catalyst volume.
A low level of hydrogen in the oxygenate reactor shifts the equilibrium conversion in a negative direction. To compensate for a lower conversion, the recycle must be increased to maintain the carbon efficiency in the loop. A high module, on the other hand, will also give a higher recycle flow due to the surplus of hydrogen. The recycle flow reaches its minimum at a module close to 2 (Fig. 8 ) .
30 I
1.1--
Relative Recycle
1--
Flow 0.9-
0 . 8 ~
So far, the process parameters have benefited from high or medium hydrogen
levels. For the product distribution, however, a low hydrogen pressure is advantageous.
The selectivity to gasoline increases slightly and the content of
aromatics and with that the octane number of the product will be higher with lower hydrogen pressures. The proper choice of hydrogen level in the Integrated Process will be a result of a balance between cycle length, catalyst volume, recycle flow and selectivity. The process design of a particular TIGAS plant will therefore depend on specific requirements such as the specifications for the gasoline product, whether the gasoline is going to be blended in a pool, whether LPG is a salable product and at which price, and the extent to which low investment is more important than low energy consumption. OLEFINES The olefinic content in the product from the integrated synthesis is low, resulting in a stable gasoline product (Table 1).
The reason for this is that
in the integrated loop the recycle stream from the high pressure gasoline separator will have to pass the oxygenate catalyst. Being a copper-based catalyst, it quantitatively hydrogenates any unsaturated compounds present in the recycle gas.
302
TABLE 1
Stability of Raw Gasoline
Existent Gum Potential Gum Bromine Number Olefines
1 mg/100 ml
5 mg/100 ml 3.4 g Br2/100 g
vol%
0.5
DURENE A problem caused by high pressure operation of the gasoline synthesis can
be durene formation. Durene (1,2,4,5-tetramethylbenzene), being an unwanted component in the gasoline with a high melting point of 79OC can in excessive amounts
-
-
if present
cause problems in the engine carburetor system. The
formation of durene is generally believed to be favoured by increased pressure (refs. 8-9). To minimise the durene formation caused by high pressure operation, the loop pressure in the integrated process has been reduced by the introduction
of a combined MeOH/DME synthesis.
Fig. 9 shows the durene content in a typi-
cal cycle from the 1 MTPD plant.
Also shown is the durene content in the C5+
fraction assuming that equilibrium between the tetramethylbenzenes has been established by isomerisation.
In the integrated synthesis such an isomerisa-
tion step can be carried out within the loop.
8.. 7f
Durene Weight
6-
%
5-
in C 5 +
4-
++
~
+
+*
**
*
+++'**+ ++++**,
+ +++++d.
+ +
+
+
*l
t +
I
21
__
+
Dumne in C5+ Fraction
+ Equilibrium Dumw in C5+Fraction
0 .
Hours on Stream
Fig. 9. Durene content i n the C5+ fraction during a typical cycle in the 1 M P D process d m n s t r a t i o n unit.
303 94
'
1
*
Research Octane
+
Motor Octane
9
No. No.
*
90
I
89
*
* f X
85
I
*
+
*
+
+ +
+
*
I
*
02
**
*
+
+
+
I
+
Low
High
I
Hydrogen Partial Pressure in TIGAS Loop
Fig. 10. Octane No.'s of the gasoline product f m the TIGAS process as a function of the h y m e n partial pressure in the synthesis loop. OCTANE NUMBERS The octane number of the gasoline product from the TIGAS process will depend on the process layout. Especially the hydrogen level in the loop will influence the octane number (Fig. 12).
The highest research octane numbers
observed have been a few numbers lower than those obtained in the MTG process (ref. 9 ) , whereas the motor octane numbers are similar. The reduced octane number, which is the result of the low olefinic content in the gasoline, is the penalty to be paid for the benefits obtained by integration. The size of the penalty will depend on the end use of the product from a TIGAS plant, the octane requirements at the specific location and the extent to which octane improvers are available and acceptable. GASOLINE YIELD Characteristic of the integrated synthesis is the fact that the selectivity to gasoline in the gasoline reactor does not change much during a cycle (Fig. 11).
A constant selectivity to C5+ close to 80 wt% of the hydrocarbons form-
ed is maintained even during the first cycle. Whereas the selectivity expresses the efficiency in one of the 3 synthesis steps, the gasoline yield expresses the overall efficiency of the process and is the important figure for process evaluation. as the gasoline product in weight
%
The gasoline yield is defined
of feed plus fuel (100% CH4).
The yield of
the TIGAS process together with consumption figures are given in Table 2.
304
90.-
Weight X
ao--
of Produced Hydrocarbons
. . _...
.
... .
I
.
..
.
70-
50
0
500
-~ ~
--
1000
1500
2000
Hours on Stream
Fig. 11. Selectivity to C4+ and C5+ hydrocarbons i n the gasoline reactor during a cycle i n the 1 MTPD process demonstration unit. The LPG by-product is given separately.
Depending on the product value at
the specific plant location, the LPG can be sold or used as replacement for natural gas.
The consumption figure comprises all the energy requirements for
a TIGAS plant including electrical power generation and energy for compressors.
The figures refer to a standard case operating at medium hydrogen pres-
sure.
If the gas feed is not 100% CH4 the figures will change significantly.
TABLE 2 Consumption figures, yield and thermal efficiency for a typical TIGAS Plant Consumption Feed + Fuel as 100% CH 4 6
3
Production: (From 1x10 Nm 11 RVP Gasoline LPG
3
2417 Nm /ton Gasoline
CH4)
Gasoline Yield: 11 RVP Gasoline of Feed
+
Thermal Efficiency (LHV basis)
Fuel
403.7 metric ton 59.6 metric ton 56.5 wt% 55
305
CONCLUSION The purpose of the process development work on the integrated gasoline synthesis was to modify the three process steps, synthesis gas production, oxygenate synthesis and the MTG process, in order to be able to operate all steps at the same pressure and the last two steps in one single synthesis loop. By selecting combined steam reforming and autothermal reforming for the synthesis gas production, and by using a multifunctional catalyst system, producing a mixture of oxygenates instead of only MeOH, the front-end and the oxygenate synthesis can operate at the same relatively low pressure, which eliminates the need for synthesis gas compression.
Furthermore, a combined
front-end is more economical for synthetic gasoline plants as they are likely to be large plants. When the MTG Process is integrated into the oxygenate synthesis, producing both MeOH and DME, the operating conditions for the process are relatively mild due to the low pressure.
The operation of the MTG process and the oxy-
genate synthesis in one loop will then only call for minor changes in the MTG process. Process flexibility and simplicity are main features of the Integrated Gasoline Synthesis.
Yield and quality, cycle length etc. can be adjusted to
ensure a desired balance between capital investment and operating costs. thesis gas compression can be avoided.
This means low investment.
Syn-
Further,
the exclusion of the synthesis gas compressor means higher reliability. The TIGAS process is a demonstrated process after more than 10,000 hours of operation in the 1 MTPD plant. REFERENCES 1 Topp-Jujrgensen, J., Rostrup-Nielsen, J., Oil & Gas Journal 84, 20 (1986) 68. 2 Chang, C.D., Kuo, J.C.W., Lang, W.H., Jacob, S.M., Wise, J.J., and Silvestri, A.J., Ind. Eng. Chem. Process Des. Dev., 17, 3 (1978) 255. 3 Lee, W., Maziuk, J., Weekman Jr., V.M., and Yurchak, S., "Large Chemical Plants", Elsevier, Amsterdam (1979) 171. 4 Dybkjsr, I., Hansen, J.B., CEER, 17 (5) (1985) 188. 5 Chang, C.D., Lang, W.H., Smith, R.L., Catal, J., 56 (1979) 169. 6 Yurchak, S., Voltz, S.E., and Warner, J.P., Ind. Eng. Chem. Process Des. Dev. 18, (1979) 527. 7 Hansen, J.B., H@jlund-Nielsen, P.E., Dybkjar, I., "Synthesis of Methanol on Cu-based Catalysts". Paper presented at AIChE 1981 Annual Meeting, New Orleans, Nov., 1981. 8 Keim, K.H., Maziuk, J., Tdnnesmann, A., Erddl und Kohle, Erdgas, Petrochemie, 3 7 (1984) 558. 9 Voltz, S.E., Wise, J.J., "Development Studies on Conversion of Methanol and Related Oxygenates to Gasoline", Final Report, ERDA Contract No. E(49-18) 1773, NoV. 1976.
-
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307
D.M. Bibby, C.D.Chang,R.F. Howe and S.Yurchak (Editors), Methane Conuersion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
GASOLIKE AND DISTILLATE
FUELS FROM METHANOL
A . A . AVIDAN
Mobil Research and Development Corporation, Paulsboro Research Laboratory , Paulsboro, New Jersey 08066 (USA)
ABSTRACT
Gasoline and distillate fuels can be produced from methanol by combining two Mobil processes: Methanol to olefins (MTO) and Mobil olefins to gasoline and distillate (MOGD). Both processes use the medium pore zeolite ZSM-5 catalyst. The combined process offers gasoline and distillate in various proportions, as well as light olefinic byproducts if needed. Liquid fuel yields of up to 95 weight percent of hydrocarbons can be obtained. Flexibility, high yields and excellent product quality make this combined process an attractive alternative for producing a wide range of hydrocarbon products from natural gas or coal. The MTO process was successfully demonstrated in a 100 B/D fluid-bed semi-works unit in Germany. The MOGD process was demonstrated in fixed-bed reactors in a Mobil refinery.
INTRODUCTION The production of high-quality gasoline and distillate fuels from natural gas via methanol is described in this paper.
Various
schemes are discussed, with references to potential improvements in methanol production.
The use of methanol as a transportation
fuel is beyond the scope of this communication. However, the addition of ZSM-5 technology is only a small part of the cost of a complete gas-, or coal-to-fuels plant.
It adds flexibility to the
plant, and produces a wide range of products:
high-octane
gasoline, distillate fuels such as jet fuel, light olefins for petrochemicals, etc.
The relative mix of these products can be
varied over a wide range, and upon a short notice.
The choice of
medium-pore ZSM-5 zeolite as MTO catalyst over smaller pore catalysts is described. In an MTO-MOGD combination ZSM-5 yields a higher valuable gasoline plus distillate yield. Scale up and commercial experience with both processes, product yields and qualities are then described. Gasoline and distillate fuels can be produced via methanol as illustrated schematically in Fig. 1.
Synthesis gas can be
manufactured from natural gas by steam reforming, or from coal by partial oxidation.
Since synthesis gas manufacture, and methanol
synthesis, represent the largest cost in synthetic fuels production, considerable effort has been dedicated to improving
308
?hcse pro(esses.
In a recent c-ommercial design of a coal-to-
gasoline plant (ref. I), the cost of the fluid-bed MTG section was
-
calculated to be o n l y 13% of the total plant cost.
Coal
Gasification
MTG
Natural Gas
Mobil Methanol lo Gasoline Synthesis
MTO Mobil
Natural Gas Liquids
Gasoline
MOGD Mobil
I [
Gasoline
I
Fig. 1. Gasoline and distillate production via methanol and Mobil’s ZSM-5 technology The partial oxidation of natural gas has been considered as an alternative to steam reforming for many years.
While natural
gas partial oxidation was practiced commercially for ammonia manufacture over 30 years ago (ref. 2), it’s use for methanol synthesis remains to be demonstrated commercially. Autothermic reforming (ref. 3 ) , and various other improvements, such as low energy distillation (ref. 4 ) , demonstrate that methanol production can be improved in the short term. In addition, new and novel methanol production processes are being developed, and the patent literature is very active.
Many
of the contributions to this conference describe the forefront of these technologies.
Liquid-phase methanol synthesis via the Chem
Systems process has been demonstrated in a large semi-works unit since 1984 (ref. 5 ) .
And a new methanol production process which
uses air partial oxidation has been proposed by Brookhaven National Laboratory (ref. 6). These, and other developments may ultimately lead to considerable savings in methanol production from natural gas. Such savings will greatly enhance the utilization of natural gas for liquid fuels production (ref. 7).
Considerable effort is also
309
being expended in alternative tcchnologi e s , such as direct conversion of methane to hydrocarbons, improved Fischer-Tropsch synthesis, etc. METHANOL-TO-OLEFINS (MTO) Olefins can be produced from methanol, in varying concentrations, over many catalysts. is extensive.
Again, the patent literature
The most promising ones seem to be zeolites and
other molecular sieve catalysts. are listed in Table 1 .
Some of the most prominent ones
Experimental data for many of these
catalysts were summarized before (refs. 8-10). MTO catalysts can be divided into t w o major groups:
"small pore", such as erionite,
and ZSM-34, and "medium pore", such as ZSM-5. Various combinations and modifications of catalysts are also known.
A
particularly interesting example is phosphorous modified ZSM-5 (ref. 1 1 ) .
TABLE 1 Major Methanol-to-Olefins Catalysts "Small Pore" Number of Zeolite Oxygen Atoms/Ring Typical Catalysts
"Medium Pore"
8
10
erionite, zeolite TI chabazite,
ZK-5
ZSM-5,
modified
ZSM-5,
ZSM-34, SAPO-34 Typical Yields, wtX C,= C,= Total Olefins
cs+
Coke
-
55 50 95
0.5 -
5
25 20 50 1
5 20 50 25
15
- 25
-40 - 80 - 60
less than 0.5
The major differences between the two classes of catalysts seem to be the result of shape selectivity.
The smaller pore
catalysts typically yield mostly C,- products, high ethene yields and up to 95 wt% olefins yields.
The medium pore catalysts, such
310
as
ZSM-5,typically give
hydrocarbons.
a lower
olefins yield and more C,+ The C5+ fraction is comprised of an aromatic-
olefinic gasoline with Research octane numbers of 95 - 105. Coke yields for the small pore catalysts are reported to be an order of magnitude higher than the medium pore one, with several notable exceptions
-
for example, SAP0 catalysts (ref. 12).
Catalyst
modifications and process conditions such as the use of diluents (water, nitrogen, light gases, etc.), temperature, and pressure, can vary yields considerably, as the ranges in Table 1 indicate. The choice of an MTO catalyst would depend on the required product slate, and such practical considerations as catalyst stability, cost, etc.
We chose ZSM-5 for gasoline and distillate
fuels production from methanol.
ZSM-5 has been demonstrated
commercially in a variety of refining processes.
It exhibits
exceptional stability, and can be used over a wide range of operating conditions.
In addition, ZSM-5 gives high olefin yields
(up to approximately 80 wt% of hydrocarbons, if required), with low ethene, and low light saturate yields. The major byproduct is high-octane gasoline. The low ethene yield is important as ethene is not the preferred feed to the MOGD process (ref. 13). High ethene yields lower distillate selectivity because of significantly higher MOGD processing temperature.
This higher
temperature results in shifting the equilibrium to produce more gasoline.
The high gasoline and distillate yield, and the
reasonable split between them (typically 50/50), combined with
ZSM-5’s commercial experience, has made it the choice MTO catalyst in an MTO-MOGD plant.
Low light saturate yields over ZSM-5 contribute to high overall thermal efficiency by minimizing recycle of methane, ethane and propane to the steam reformer.
Gasoline and distillate
yields of up to 95 wt% of hydrocarbons were demonstrated by this approach.
These high yields are responsible for achieving a high
thermal efficiency in MTO (up to 95%).
This efficiency is higher
than currently demonstrated commercial technologies, such as Fischer-Tropsch (Table 2 ) , or the use of smaller pore MTO catalysts, even when higher olefins yields are reported.
311
TABLE 2 A Comparisgn Between Typical
MTD and Fischer-Tropsch Yields
F-T
MTO
Cl c2 c3 c4
C2= C3= C4= Gasoline C,-Cll Diesel C12-C, Heavy Product C18+ Water Soluble Oxygenates
Total Light Saturates (C1-C3") Total Light Olefins (C2=-C4=)
_ Product ___
Product
1.4 0.3 2.3 3.9 5.0 31.7 19.6
10.0 4.0 2.0 2.0 4.0 12.0 9.0
35.5
40.0
7.0
-
4.0
-
6.0 --
0.3 100.0
100.0
16.0 25.0
4.0
56.3
The overall path of methanol conversion to hydrocarbons over
ZSM-5 is illustrated in Fig. 2. Methanol and dimethyl ether (DME) form olefins, which are then converted to naphthenes, aromatics, and paraffins.
Olefins initially react by oligomerization and
methylation, and at increasing conversion olefins distribution is governed by kinetics.
This effect, and the effects of process
variables were summarized by Chang (ref. 1 4 ) .
The directional
effects of process and catalyst variables on the MTO reaction are summarized in Table 3.
TABLE 3 Directional Effects of Catalvst and Process Variables on the MTO Reaction Variabl e Higher Catalyst: Alpha Activity Si/A1 Coke Level Higher Process: Temperature Pressure Diluent
WHSV
Methanol Conversion
Olef ins Yield
UP
Down
UP UP
Down
Down Down
Down Down
UP UP UP
UP
UP
312
70 -
' '
' "'"
60 -
s 50-
11
\
-
I
' ' ""'I
Methanol A
o#
Dimethyl Ether,'
/
,
' ' ' ""'I
1
_____ ---- - ----o-
0-0-
'
1 ' 1 '
Water
Fig. 2. Methanol to hydrocarbon reaction path (ref. 8) Generally, catalyst and process variables which increase methanol conversion, decrease olefins yield. Process variables such as the use of an inert diluent may increase the spread between olefins formation and aromatization reactions (Fig. 2 ) . Raising temperature increases methanol conversion and light olefins yield up to a point.
However, higher temperatures have
the disadvantage of increasing coke and light saturate yields,
-
which lowers overall process efficiency.
"Reaction Index" Propane/ Propene Ratio Is Proportlonal to C I 6, In the Schematic Path
PCH,OH
CH,OCH,
f
Llght Oleflns
11
+
H,O
C5+ Olefins
t
Paraffins Cycloparafflns Aromatics
+
H,O
Schematic
313
Since the measurement of on-line catalyst activity is difficult, we found it convenient to follow an on-line "reaction index" (RI), which is a selectivity ratio. The complex MTO reaction scheme can be presented schematically as A
+B
j
where A represents methanol and DME, B - olefins, and C aromatics and paraffins (Fig. 3). One particularly useful the propane/propene ratio.
C,
RI is
Propene is the primary light olefin
and propane represents paraffins. The propane/propene RI can be easily monitored by an on-line GC. We found that hydrocarbon selectivities correlate well with
RI. For fixed hydrodynamics, it
also correlates well with methanol conversion.
The use of an
RI can also be useful in screening for
selectivity differences.
In many cases, the aeolite catalyst is
modified by the insertion of various elements, the Si/A1 ratio is changed, etc. In cases where catalyst shape selectivity is not considerably affected, the modification only changes catalyst activity in the same way that steaming or coking would.
The
apparent selectivity change is then only a function of activity. We propose
RI as an indicator of catalyst activity to determine
whether catalyst or process modifications really change inherent
MTO selectivity, or merely change selectivity because of an activity change.
SCALEUP OF THE MTO PROCESS The conversion of methanol to olefins over ZSM-5 was discovered by Mobil scientists in the 1970's, together with the similar process of conversion of methanol to gasoline.
Initial
process development was in small-scale reactors (ref. 15). Interest in this process has increased following the successful
MTG in the 100 B/D demonstration plant in Germany. The 100 B/D MTG project was supported by the USA
demonstration of fluid-bed
Department of Energy, the German Federal Ministry for Research and Technology
(BMFT), and the three industrial participants:
Mobil Research and Development Corporation (MRDC), Uhde GmbH, and Union Rheinische Braunkohlen Kraffstoff AG
(URBK).
MRDC provided
technological guidance to the project, supplied the catalyst and process design basis and was operations advisor. engineering agent, and manager.
Uhde was the
URBK was the operating agent and project
314
MTO #as first scaled up in MRDC’s 4 B/D fluid-bed pilot plant in Paulsboro, New Jersey. Following successful completion
B/D MTG project, the project was extended, and the plant modified to demonstrate MTO (refs. 16, 17). The plant is
of the 100
shown schematically in Fig. 4 . Methanol is converted in a turbulent fluid bed reactor with typical conversions exceeding 99.9%.
The products are recovered in a simple gas plant.
Coked I
catalyst is continuously withdrawn from the reactor, and the coke is burned in a fluid-bed regenerator.
Coke yield and catalyst
circulation are an order of magnitude lower than in fluid catalytic cracking.
Methanol b
Fig. 4 . 100 B/D MTO demonstration plant Isothermal temperature control in the fluid-bed reactor was easily maintained under all process conditions investigated.
The
temperature gradients in the catalyst bed did not exceed 5°C even at mean temperature gradients of 200 to 3OOOC between the catalyst bed and the heat transfer medium.
The plant accumulated
17 months-on-stream of MTG/MTO operation, including 5 months at
MTO conditions. MTO operation started with sensitivity studies to determine the effects of temperature and pressure on selectivity. to n r o v i d e a
Deactivation periods to vary catalyst activity and cornoarison w i t h t h e 4 B / D o i l o t n l a n t were oerformed
315
stable steady state conditions were re--established. A successful ethene recycle simulation was included in the second steady-state run. The dependence of 100 B/D MTO product selectivity on RI is shown in Fig. 5. JIigher olefin yields were
propane/propene
obtained at low propane/propene ratios.
Olefins selectivity
increased by about 7% when the reaction temperature was raised from 470°C to 515OC.
Most of this increase was due to higher
light olefin yields.
90
-
80
-
C; Paraffins
c,'
I
Paraffins
Yield, Wt.
0
.2
.6
.4
.8
1
Propane I Propene RI
Fig. 5. Typical 100
BPD MTO product distribution
The two steady-state runs were performed at a constant methanol conversion (above 99.9%) and coke level.
Catalyst
makeup rate was lower than 0 . 5 wt% of catalyst inventory/day. Olefin yields higher than 60 wt% can be achieved at lower pressures, and by the use of diluents. possible in the modified 100
Both options were not
B/D MTG plant, but were demonstrated
in the smaller pilot plants. Scaleup of the fluid-bed MTO process has been successfully demonstrated in the 100
B/D plant.
Results obtained in this unit
are in close agreement with those obtained in the 4 plant under the same conditions.
B/D pilot
Total olefin yields for the 100
B/D demonstration plant were similar to those obtained in the 4 B/D unit (Fig. S ) , and methanol breakthrough occurred at approximately the same propane/propene RI for both units (Fig.
316
7). The conversion efficiency of the 100 B/D plant was better than the 4 B/D unit, which in turn was better than the benchscale reactor.
Such improvements in fluid-bed efficiency as its
size increased were also observed for the MTG process. 75 70
c2
-
c7
Olefins Yield, Wt.%
-
55 50 45
-
40
-
35 0.1
I
I
I
I
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Propane / Propene RI
Fig. 6. Olefins yield scales up 99,85
99,90
Methanol Conversion % 99,95
00%
0:l
0.15
0.2
0.25
0.3
0.35
Propane / Propene RI
Fig. 7. Methanol conversion scales up
MOGD The MOGD process reacts light olefins to gasoline and distillate products (refs. 13, 18).
I n this process, gasoline and
distillate selectivity is greater than 95% of the olefins in the feed, and gasoline/distillate Product ratios range from 0 . 2 to
317
>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 <5 48 92 105 120 135 178
TABLE 6 MTO-MOGD Typical Distillate Properties Total Disti 1late Volume % Density, g/cc
CFPP, "C
Freeze Pt, "C Flash Pt, "C Cetane No. Smoke Pt, MM Aromatics, vol% Viscosity, cs 4 0 ° C Sulfur, ppm Distilltion, D86, "C 10% 30% 50% 70% 90%
EP
100 0.792 -50 - 60 60 50 25 4 2.70 50 237 247 258 275 316 359
Jet
Diesel Fue 1
Fuel 30 0.774
70 0.800 -30
50
100 52
- 60 -
25 5 -
205 218 226 231 235 238
-
3.1
258 261 270 284 302 338
Many other options are available within an MTO-MOGD plant in addition to changing the distillate-to-gasoline ratio, and the splits between the various distillate cuts.
Some of the light
olefins can be separated before the MOGD unit and used as alkylate feed if iso-butane is available, or as a feed to a petrochemical plant.
Other possible uses may include alcohols and ethers for
gasoline additive production, since methanol, water and olefins are readily available.
The exceptional flexibility of both
MTO
and MOGD makes switching between the various options easy, and possible even on a frequent basis.
Methanol Olefins
Fmdlonatlon
canmhn
- Mobil Olefins
to
Gasoline and Dlstillate
FractlonaHon
Gasoline
Dlstillate Hydrotreater
-Diesel
Fig. 11. MTO/MOGD process schematic
ACKNOWLEDGMENT The author wishes to acknowledge the many contributions to
MTO and
MOGD development made by his colleagues at Mobil's Paulsboro Laboratory.
REFERENCES Avidan, A.A., M. Edwards, W. Loeffler, H.-H. Gierlich, N. Thiagarajan, and E. Nitschke , "The Fluid-Bed MTG Process", 21st State-Of-The-Art Symp., Div. of Ind. and Eng. Chem., ACS, Marco Island, Florida, June 15-18, 1986. Bakemeir, H., T. Huberich, R. Krabetz, W. Liebe and M. Schunck, Ammonia, in Ullmann's Ency. of Ind. Chem., W . Gerhartz, exec. ed., Vol. A2, VCH Verlagssellschaft, Weinheim, 1985, page 143. Supp, E., Hyd. Proc., Vol. 63, July 1984, page 34-c. Thiagarajan, N., H. Ilgner, G. Heck, and I. Lienerth, Hyd. Proc., Vol 63, March 1984, page 89. Tsao, T. R., and E. C. Heydorn, "Liquid Phase Methanol PDU Results", Fifth DOE Indirect Liquefaction Contractors' Meeting Proceedings, published by the Pittsburgh Energy Technology Center, December 2-5, 1985. O'Hare, T. E., R. S. Sapienza, D. Mahajan, and G. T. Skaperdas, "Low Temperature Methanol Processft,21st State-ofthe-Art Symp., Div. of Ind. and Eng. Chem., ACS, Marco Island, Florida, June 15-18, 1986. Dautzenberg, F. M . , R. L. Garten, and G. Klingman, 21st State-of-the-Art Symp., Div. of Ind. and Eng. Chem., ACS., Marco Island, Florida, June 15-18, 1986.
323
8
Chang, C.D., W.H. Lang, and A.J. Silvestri, U.S. Patent
9
Liu, L., R.G. Tobias, K. McLaughlin, and R.G. Anthony, in
4,062,905.
R.G. Herman (Editor), Catalytic Conversions of Synthesis Gas and Alcohols to Chemicals, Plenum Press, New York, 1983, page 32.
Fleckenstein, T., K. Belendorff, and F. Fetting, Ger. Chem. Eng. 9 (1986) page 346. 11 Kaeding, W.W. and S . A . Butter, J. Catal., 61 (1980) page 155. 12 Kaiser, S., "Silicoaluninophosphate Molecular Sieves: C -C, Olef ins Formation", 21st State-Of-The-Art Symp. , Div. of Ind. and Eng. Chem., ACS., Marco Island, Florida, June 15-18, 10
1986.
13
Tabak, S.A. and F.J. Krambeck, Hyd. Proc., Vol. 64, September
14
Chang, C.D., Catal. Rev.-Sci. Eng., 26 (3&4), (1984) page
15
Socha, R.F., C.D. Chang, R.M. Gould, S.E. Kane and A.A. Avidan, in D.R. Fahey (Editor), Industrial Chemicals via C, Processes, ACS Symp. Ser. 328, Washington, D.C., 1987, Chap.
16
17 18
(1985) page 72.
323.
3. Soto, J.L., and A.A. Avidan, "Status of the 100 BPD Fluid-Bed Methanol-to-Olefins Demonstration Plant", DOE/FE Indirect Liquefaction Contractors' Review Meeting, Houston, Texas, December 2-5, 1985. Keim, K.-H., F.J. Krambeck, J. Mazuik and A . Tonnesmann, Erdoel & Kohle Erdgas Petrochemie, Vol. 103, Feb. (1987), page 82. Tabak, S.A., F.J. Krambeck, and W.E. Garwood, AIChEJ., 32 (1986) page 9.
Quann, R.J., L.A. Green, S.A. Tabak, and F.J. Krambeck, "Chemistry of Olefin Oligemerization Over ZSM-5 Catalyst", AIChE Spring Nat. Meet., New Orleans, April 6 - 10, 1986. 20 Tabak, S.A., A.A. Avidan, and F.J. Krambeck, "MTO-MOGD Process", 21st State-of-the-Art Symp., Div. of Ind. and Eng. Chem., ACS, Marco Island, Florida, June 15-18, 1986. 19
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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
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SYNTHETIC GASOLINE CCMKNENTS SUITABLE FOR CHEMICAL FEEDSTOCKS Ian J MILLER Carina Chemical Laboratories Ltd, FO Box 30366, Lower Hutt (New Zealand) ABSTRACT A comprehensive survey is given of the current and potential uses of component chemicals in the hydrocarbon product of the M E process at Motunui. The utilisation of many of these components may depend significantly on how the product of major interest, i.e. durene, is isolated. IXlrene itself comprises ca. 50% of the heavy gasoline stream: its potential uses as well as those of its oxidation product pyromellitic acid and derivatives thereof, are reviewed in detail, as is available information on other durene derivatives such as durohydroquinone. INTROWCTION Synthetic gasoline is defined in this paper as the mix of hydrocarbons. I produced by the Mobil process, i.e. obtained by passing methanol over the zeolite catalyst ZSM-5 at elevated temperatures and pressures. Some practical economic restrictions must be considered, if only to reduce the scope of the paper to a reasonable limit. Many feedstocks are readily available from modern petroleum refineries, and it is reasonable to infer that such feedstocks generally will be considerably cheaper than those obtained from synthetic gasoline. For the present we concentrate on those components which can be separated m r e effectively from synthetic gasoline than from elsewhere in the oil industry. The ease of separation depends partly on how m c h of the c-nent is present, and what its boiling point is in relation to that of the remainder. Naturally it is easier to isolate components from either end of the overall boiling range than from the middle. The overall composition of the synthetic gasoline has been discussed elsewhere in this conference; for the present it is noted that the composition depends on how the plant is operated. In this work the composition data used are largely from independent studies based on catalysts which are not necessarily identical with corrmercial catalysts, and which may be used under somewhat different conditions. Such data should be considered solely as a guideline.
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The hydrocarbon products of the plant falls naturally into four fractions. The first fraction consists of "light-gas",e.g. methane, ethane, carbon dioxide, hydrogen, etc. These amprise approximately 1-2% of the output and has no major potential use as a chemical feedstock. The second fraction (>20% of the output) consists of the "readily-condensible"gases, e.g. propane, n-butane and isobutane; this can be regarded as the LFG fraction. The third fraction consists of approximately 33-45% of aliphatic liquid hydrocarbons and the fourth fraction can be arbitrarily divided into light and heavy aromatic hydrocarbons. DISCUSSION Tne LFG Fraction The 1pg fraction can be used as a chemical feedstock; the gases can be converted to propylene, butene, butadiene and isobutene and these, following separation, are feedstocks for the production of a number of industrial chemicals. mere are, of course, a number of decisions to be taken before such use could be advocated, the main one being the necessity to assign a value to the lpg as a resource. Clearly natural lpg offers a competitive source of feedstock. However, the composition of the synthetic 1pg differs from that of the natural 1 ~ ;in particular, the synthetic 1~ is richer in isobutane. One obvious use for isobutane in a synthetic fuels scenario is to dehydrogenate the isobutane to isobutylene, and react it with methanol (already manufactured on site) to produce methyltertiarybutyl ether (MTBE), which can then be used as an octane enhancer(1). This use is particularly attractive as there should be a ready market for the MIBE. If this choice was adopted, the remaining 1pg fraction could be used to rrrake other chemicals, or it could be recycled back through the plant. Care will be required in selecting other conpnents from the lpg fraction as many of the obvious products from propene and butene are already in oversupply throughout the world. Aliphatic Liquid Hydrocarbon Rraction This fraction is the largest fraction produced, and as such its use tends to dominate the remaining opportunities. If this fraction is sold as gasoline the octane m s t be enhanced. Currently the aromatic fraction fulfils this function. In principle the aliphatic fraction can be cracked to a m t i c hydrocarbons; hcwever, a major difficulty with any strategy of synthesising
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aromatics is that a very large amount of ethylene/ propylene will be formed from the C-5 component, and will depress the overall aromatic yield. These alkenes could be oligomerised to give higher molecular weight hydrocarbons, but by this stage the level of processing would make such derived products quite uneconomic. The inevitable conclusion is that the use of this fraction is limited to gasoline production only. Light Aromatic Hydrocarbon Fracth These are benzene, toluene and xylenes, which are some of the basic feedstocks of the chemical industry. The products that can readily be made from them can be found in most industrial chemical texts. The synthetic gasoline has certain advantages, e.g. the yield of p-xylene (the feedstock for terephthalic acid) is enhanced relative to that rmrmally found in petrochemical feedstocks but the overwhelming requirement for octane in the synthetic gasoline stream may make it economically unattractive to attempt to remove these components. Heavy Aromatic Fraction This consists of the C9+ aromatics and consists mainly of trimethyl benzenes (and in particular, the 1,2,4-isomerr pseudocumene); methyl ethyl benzene (with predaninance to the para isomer); propyl benzenes; dimethyl ethyl benzene; lr2,4,5-tetramethylbenzene (durene); lr2,3,5-tetramethyl benzene (iscdurene); lr2,3,4-tetramethyl benzene (prehnitene); miscellaneous other methylated, ethylated, propylated benzenes; methylated indans; methylated tetralins; methylated napthalenes; and a variety of other minor components. In the Motunui plant the synthetic gasoline product passes through a distillation splitter; the fraction nominally boiling above 16OOC is termed heavy gasoline and contains most of the above components. As this is separated from the remaining gasoline, and comprises approximately 14% of the production, it is an attractive source of feedstock. While a number of components of the heavy gasoline have existing, or potential, markets, durene which ccenprises of ca. 50% of the heavy gasoline is potentially the most valuable. Pseudccumene is an item of cOmnerce, and amprises approximately 15% of the heavy gasoline fraction. Its major use is for oxidation to trimellitic acid or its anhydride. These compounds have a number of uses, but the major ones are to form temperature resistant plasticizers for pvc (e.g. as the trioctyl ester), to form high performance plastics (e.g. the polyamide-imides) r
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and to introduce additional functionality, e.g. for cross-linking in polyesters, to convey solubility to alkyds, etc (2). 1,44iethyl benzene is approximately 2% of the heavy gasoline fraction, and possibly may be worth recovering. In principle, this could be converted to p-divinyl benzene which could be used for cross-linking and forming gels with radical-initiated polymers. mere is considerable literature on the use of p-divinyl benzene, but the details are outside the scope of this paper. lr2-dimethyl-4-ethylbenzene comprises approximately 8% of the heavy gasoline, and could be recovered. It could also be oxidised to trimellitic acid, and hence could provide additional feedstock to pseudsocumene. It is possible to take advantage of the additional ethyl group, however, to make dimethyl styrene and dimethyl acetophenone and to build up from these molecules. Of these, 3,4-dimethyl styrene would appear to be the m s t promising compound. Currently 41nethyl styrene is manufactured for polymer production; the additional methyl groups in the 3,4-dimethyl styrene lower the volatility of the monomer (which eases manufacturing problems for thermoset polymers), should confer a higher glass transition point to the polymer, and would confer other different and possibly useful, properties to polymers. In principle, isodurene and prehnitene could be recovered from the heavy gasoline, where they occur in about 3% and 1% levels respectively, However, it is not clear that they can be recovered m r e efficiently here than from reformates produced elsewhere. Their derivatives are not corranercially utilized at present and it is unlikely that they could be mmercialised from this source. There could be up to four different methylated indans, three methylated naphthalenes, and 9 methylated tetralins present in the heavy gasoline when lpg has been used as a recycle gas. The indans and tetralins have no obvious c m r c i a l value on their m, but they could be dehydrogenated to give indenes and naphthalenes (the naphthalenes presumably being the same as those already found in the heavy gasoline). The indenes m y have some value as raw materials for resins, and also possibly as raw materials for specific chemicals with biological activity. However, without knowing their specific structure it is difficult to give further details. Among the naphthalenes the major potential raw materials would be 21nethyl naphthalene (the major naphthalene produced in the absence of lpg recycle) and 2,6-dimethyl naphthalene. 21nethyl naphthalene is the raw material for menadione, and for naphthalene-2-carbxylic acid. While the acid has no known comnercial use (although patents have been taken out covering some potential Uses), menadione is produced ccPrmercially, and has a number of well documented uses, mainly
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arising from its bacteriocidal and fungicidal properties ( 2 ) . It has anticoagulant properties, and is vitamin K3. The m s t apparently desirable of the minor components is the 2,6-dimethyl naphthalene which can be used to make naphthalene-2,6-dicarboxylic acid, which in turn can be used to make special polyesters ( 3 ) . Following oxidation of the alkylated naphthalenes (the alkyl groups do not necessarily have to be methyl, but the substitution pattern is important) the resulting dicarboxylic acids should be readily marketable. However, since the overall yields of naphthalene derivatives in the synthetic gasoline are low, this fraction is unlikely to be of c m r c i a l value in the near term. This is unfortunate, because these compounds are also unsuitable for gasoline as their boiling points lie outside the required specification. The utilization of any of these cmponents may depend significantly on how the compound of major interest (i.e. durene) is isolated. Unless substantial markets for durene are found, it is unlikely to be economic to carry out the careful fractionation necessary to recover the m r e minor components. To sununarize: apart from durene, the materials most likely to be extracted from the heavy aromatic fraction at this time are pseudocumene and 1,2-dimethyl-4-ethyl benzene; for conversion to trimellitic acid and 3,4-dimethyl styrene respectively. Since trimellitic acid will largely compete with pyromellitic acid produced from durene (see below) the two options mnpete for a relatively limited market. Since the extraction costs of pseudocumene from synthetic gasoline do not appear to be significantly laver than extraction from reformate, it is not a highly attractive development under current market conditions. It is also unlikely that the naphthalenes will be extracted unless the high boiling ends of the heavy gasoline were to be obtained separately. Production of naphthalenes would be increased if the 1pg fraction were not recycled through the zeolite beds. Ihrene turene, or 1,2,4,5-tetramethyl benzene, is an attractive chemical feedstock. It is a solid at roan temperature with a melting point of 79OC and comprises over 50% of the heavy gasoline. It can be isolated with reasonable purity and fair efficiency simply by allawing the heavy gasoline to cool, when it crystallises in high yield.
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The lr2,4,5-tetrasubstitution pattern confers a symnetrically opposed pair of dipoles on the molecule which, in turn, W s e s enhanced crystallinity and accounts for its high melting point compared to the other tetramethyl benzenes prehnitene and isodurene (MP -6OC and -24OC respectively(4)). One use of durene and its derivatives, therefore, is to increase the crystallinity of polymers. For example, consider the glass transition points of polyamides; if a polyamide is made from a diamine and, say, sebacic acid, durene diamine raises the glass transition temperature of the polymer more than 4OoC above that produced by lr4-diaminobenzene( 5 ) . A second feature of this substitution pattern is that it permits two pairs of adjacent functional groups. mi-and tetrafunctional ring systems allow polymer crosslinking, and durene derivatives are as useful for this as are other mlecules. However, the ability to allow the functionality to react as two linear pairs is unusual and allows the production of stepladder polymers, where the polymer chain has alternate single and double linkages, as well as ladder polymers where the polymer chain has double linkages. These polymers have a high temperature resistance because in the ladder parts of the molecule, two bonds rmst be broken for chain scission. In stepladder polymers, there are segments with only one bond to be broken; however, provided care is taken in selecting the nature of such a bond, considerable thermal stability can still be incorporated. ?hus one of the main potential uses of durene derivatives is that they may be corrponents of polymers which perform very well at elevated temperatures. Pyromellitates mrene can be oxidised to the tetracarboxylic acid (pyromellitic acid) which can in turn be dehydrated to pyromellitic dianhydride. There are three oxidation routes which have been applied commercially, and a fourth which has been advanced to a stage where commercialization is possible. These routes are: Vapour phase oxidation. mrene and excess air is passed over (a) V205 on an inert support at a temperature of 35OOC or relatively poor (approximately 40%), but as with phthalic anhydride production, the addition of further prmters, co-catalysts, etc., can raise the.yield quite acceptably to the 60-70% range(6-9). Partially oxidised durene products such as methyl trimellitic acid and dimethyl phthalic acids can be also produced. (b) Oxidation by nitric acid. This process can produce the pyramellitic acid in approximately 80% yield (10).
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Oxidation by chrcinates(l1). The yield of pyromellitic acid is high, and the chrcmates can be recycled. However, while the reaction conditions are the mildest of all the options, quinone byprcduct formation m y be more of a problem with this route. (d) Air oxidation in acetic anhydride. At elevated temperatures and pressures, in the presence of catalyst such as cobalt or manganese ions, durene can be oxidised using air. This reaction is similar to the process used to manufacture terephthalic acid, but the additional methyl groups on the ring require m r e extreme reaction conditions(l2). The o p t i m method for the oxidation step depends on the price of durene. If it is not too high, the lower-yielding vapour phase oxidation is to be preferred because of cheaper plant and operating costs. There are a number of potential uses for pyromellitic acid and its derivatives. These include: (i) Epoxy curing. Fyromellitic dianhydride (PMCA) can be added to the expoxies in the anhydride curing process. Its use has distinct advantages: it gives a higher temperature resistance to the cured epoxy, and also permits large castings, as the reaction exotherm is lowered. On the other hand, PMW is an unpleasant material to handle, and the market may be restricted for this reason. The basic information on PMW curing was outlined in a book as early as 1957(13). While PMDA gives excellent temperature resistance to an epoxy, the additional cross-linking conveys some degree of brittleness. Consequently, a compromise in which PMDA takes up only part of the anhydride component of the curing agent is often recommended. Thus when 25-65% PMW was incorporated as the curing agent, a casting lost only 2.26% by weight when heated to 200°C for 500 hrs(l4). Similarly, a curing agent with 64% PMW and 36% succinic anhydride gave an epoxy casting with good mechanical properties, including the ability to take repeated thermal shocks from -55°C to 100°C without cracking(l5). To s m r i z e , there is technical literature which suggests that there are advantages in using PMDA for curing epoxy castings, especially in large castings or in castings which must sustain wide ranges of thermal conditions. However, the volume likely to be used for this purpose may not be large, particularly in the short term. (ii) Polyesters. It is difficult to introduce PMDA effectively to thermoplastic polyesters, because cross-linking tends to result in brittleness. Further, if simple linear condensation can occur, e.g. when a glycol is reacted with PMM in the cold, the resultant polymer is susceptible to hydrolysis. Nevertheless, polyesters can be used for moulding purposes although only a very (c)
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limited amount of FMDA may be used. A m r e practical use may be in saturated themset polyester muldings(l7) and m r e hprtantly in unsaturated polyesters. Inclusion of small amounts of PMDA is claimed to impart shrink resistance to muldings(l8), higher impact strength and improved high temperature properties(l9). One could reasonably infer that they would also convey increased fire resistance. This improvement in properties need not necessarily result in market sales since unsaturated plyesters are not high performance plastics, and are generally sold in the lowest price markets. However, the cost of adding a few percent of low cost PMDA should be small and there may be m significant price disadvantage. (iii) Coatings. Powder coatings are frequently based on saturated polyester themsets or epoxies. Patents have been taken out claiming enhanced properties for these coatings when they are cured with the inclusion of a few per cent of FMDA(20-23). The benefits include better impact resistance of the coat and lower baking temperatures. There have been a limited nunber of patents taken out for the use of PMW in coatings based on epoxies and polyesters. However, there is no clear indication as to the benefits of incorporating PMDA. It can be inferred from general data, that increased heat resistance of the coating would result, as would increased hardness and -act strength. Finally, it may be possible to formulate polyimide coatings. Since some plyimides have very similar coefficients of expansion to those of metals, and since plyimides can be made to adhere hell to metals, their general use in metals coatings should be investigated. Polyimide varnishes are also currently manufactured for use in electrical insulation and the required specifications can be obtained from the major companies in the field. Another use of PMDA is in the formulation of water soluble alkyd resins. If an alkyd is condensed with, say, a 10% anhydride deficiency, and FMDA is added in the 10% mlar equivalent, an alkyd resin is produced containing free carboxylic acid groups. Wine salts of these would allow the manufacture of a water soluble alkyd which, it has been claimed(24), has useful properties. One problem that might be anticipated, and which is not discussed in reference 24, is that such esters would be expected to be m r e susceptible to hydrolysis, and hence the paint might have a shorter life. (iv) Adhesives. The adhesives market is very specialized, and while a number of patents have been taken out on the incorporation of PMDA into adhesives, no real advantages have been shown. A significant demand for PMDA for this use is unlikely. One of the m r e interesting claims was that an imide/ester/urethane polymer could be obtained by condensing PMDA with caprolactam and reacting this
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with TDI(25); the resultant adhesive had good peel strength to 265OC and adhered well to both polyimides and copper metal. This latter point is worth noting; both copper and polyimides are difficult to find adhesives for, and the introduction of hide linkages into adhesives will assist in such bonding. Indeed, hide containing adhesives appear to.adhere well to many metals. PMDA has also been claimed to improve the quality of polyester based adhesives(26), acrylics(27), and epoxies( 29). fie inclusion of PMDA into thermoplastic and copolyester rubber, with alkaline earth filler, is claimed to make an excellent hot-melt adhesive(30). (v) General uses of Fyromellitic Acid derivatives. Tne tetrasodium salt of pymellitic acid acts as a detergent builder(31), in part through the action of the adjacent carboxylic acid groups which chelate polyvalent metal ions. However, while there is a desire to reduce the use of phosphates in detergents for environmental reasons, zeolites can be a cheap alternative. Since pyromellitic acid sequesters heavy metals, it can also be used as an antiscaling agent(32) and as a corrosion inhibitor(33); but whether it has sufficient benefits for market applications has yet to be shown. An unusual proposed use is to dissolve kidney stones(34). A related property is that pyromellitic acid will precipitate lanthanide ions from solutions containing a number of divalent cations such as Mg, Ni, Mn, etc.(35). As a consequence, there may be possible applications for large scale purification of the rare earths. Fyromellitic derivatives can also be used to prepare phthalccyanins, which have been proposed as pigments(36-38) and as oxidation catalysts(38,39). The use of PMDA in place of phthalic anhydride leads to the formation of large sheet-like mlecules, or phthalccyanins with external carboxylic groups. In the latter case, the sodium salts are quite water soluble, which may be of importance for homogeneous catalytic applications. Apart from that, however, they offer few advantages and are a little more difficult to make. However, phthalocyanins have been advocated for use in high performance greases, and in this context, the sheet-like structures of the RlDA phthalocyanins may be of benefit. (vi) Fyromellitate esters. Cne of the major uses for trimellitate esters is as low mbility plasticizers, particularly in the case of the trioctyl esters. The increased molecular weight decreases the mobility and volatility with respect to the corresponding phthalates, and as a consequence a number of potential uses exist, e.g. in electrical wiring insulation. The pyromellitate esters are also good low mobility plasticizers, and have scme advantages over the trimellitates (e.g. a still lower mobility) and some disadvantages (e.g. the esters are m r e viscous, and gelling is more
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difficult). Phase diagrams have been published(40), and the plasticizing efficiency of the pyromellitate ester in polyvinylchloride (pvc) is found to be roughly equivalent to that of the trimellitate(41), giving a high performance and excellent electrical properties. It was also claimed that the esters acted as heat stabilizers for pvc. There are clearly market opportunities for these esters. Unusual uses of pyromellitate esters with pvc include the formation of vinyl foams(42), particularly a semi-rigid foam for construction purposes(43) and the formation of vinyl p d e r coatings(44). The advantages of these are not clear, as vinyls can be readily foamed anyway. The advocacy for the use in construction foam was based on increased fire resistance, but much improved fire resistance would be obtainable from imide foams. A further possible use is in the field of synthetic lubricants. The most likely use for the pyromellitate esters are as viscosity improvers(45), as the esters have quite high viscosities. They cannot be used at high temperatures, hmever, as at elevated temperatures they pyrolyse to form PMW. There is an alternative use for PMW in high temperature greases where pyrolysis is prevented by the formation of diimides, e.g. by reacting PMDA with p-aminobenzoic acid(46) and subsequently forming esters. Extreme pressure greases have also been claimed with the use of diimide derivatives(47). (vii) F'yromellitic diimide. A potential use for PMDI lies in its ability to act as a methane suppressor during anaerobic fermentation with increased yields of short-chain carboxylic acids. This would allow an increase in the efficiency of food usage by minants(48). The value of this is unknown, but it is being pursued. An alternative use is as an epoxy curing agent, to produce high temperature epoxy mulding compounds(49). An advantage is that FMDI is m c h less noxious to handle than is mMW. More intriguing is the possibility of using Cr(II1) as a catalytic curing agent when it has been claimed(50) that a one-pot epoxy formulation, with a shelf-life of six mnths, could be fomlated as a mulding powder. (viii) Polyimides. There are several methods of forming hide bonds(2): if an anhydride is reacted with an alcohol, an ester is obtained, and if this ester, or the diacid, is reacted with an mine which is sufficiently basic, an amine salt is formed, which, when heated, gives an hide; if an anhydride is reacted with an amine an amic acid is formed, which, on heating, gives an imide; if an anhydride is heated with an isocyanate, an imide is formed. When difunctional mlecules are used, polymers result. There is an extensive literature on polyimides; a useful introduction can be found in one review(51).
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Polyimides are step-ladder polymers, i.e. they have single-strand and double-strand linkages. The polymer chains are only as strong as their weakest link, and hence it is the single strand, i.e. the diamine, which limits the performance of the pyromellitimides. The softening point of the polyimide depends on chain flexibility which is inversely dependent on chain length. At least nine carbon a t m appear to be required for the resultant polyimide to be mouldable below its decomposition temperature. The resultant polymers retain toughness for up to 25 hours in air at 175OC. The polyimides made from arornatic diamines are mre stable but the decreased flexibility of an aromatic ring raises the glass transition temrature (Tg) of the polymer quite significantly, and no Tg can be measured for, say, the polyimide based on phenylene diamine. Thus flexibilizing units must be introduced, but these tend to raise the price of the diamine, and decrease the temperature stability. A ccenprmise mst be reached, and the best known polypyromellitimide is Kaptonflespel made by du Pont; this is based on oxydianiline. It is difficult to assess the consequences of introducing relatively low cost pyromellitic dianhydride on the world polymer market. Diamines such as oxydianiline are not cheap, and the processing conditions required for Vespel are so severe that the polymer is unlikely to find general use. 'Ihe challenge, therefore, is to find a diamine which has sufficient heat stability to give the desired polymer properties but which can be processed by conventional means. There are, of course, other desirable properties besides temperature resistance, e.g. solvent resistance, creep resistance, strength, lubricity, in all of which polyimides show good to outstanding performance characteristics. The properties can be adjusted by various means, and, for example, heat resistance can be sacrificed for fire resistance. For aircraft fittings in particular polymers with lcw snoke generation properties would be attractive, arid the fact that the polymers were useless following a fire is imnaterial. (ix) Other Durene Derivatives. While a very large amount of information is available regarding the possible uses of pyromellitic acid and its dianhydride, m c h less is available on other durene derivatives. There are a number of derivatives which have been made, which could be regarded either as curiosities or as comnodity chemicals with a very limited use to date. The mononitro derivative, the mnoamine and the phenol have been known for soqe time, but no importance appears to have been attached to them. The mnonitroso derivative can be used as a spin trap, and, in a m r e general sense, may have value as a catalyst in certain free radical reactions. However, in general little work has been done on these derivatives, possibly in part because of the difficulty in synthesising them.
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Similarly, it is possible to react the side chains. Obvious derivates include the tetra-kis-chloromethyl, brmmthyl and hydroxymethyl derivates. In principle, the aminomethyl derivatives could be made, and these can be cyclised to the isoindolines. Apart from being chemical intermediates, little value can be placed on them, although the diisoindolines can be used to make step-ladder polymers; unfortunately the current literature suggests that the resultant polymers have no advantage over polyimides, and, in addition, the starting material is far more difficult to synthesise(52). A further chemical which could be made is durcquinone. Uses which have been proposed for this include the use of the mnoxim as a fungicide, particularly to protect seeds prior to planting(53). mrcquinone also form a complex with Ni(OI(54) and with Ni(0) and cyclooctadiene(55), either of these complexes being claimed as catalysts for polymerizing unsaturated compounds such as acrylonitrile or phenylacetylene. IXroquinone m y also have value in electrochrmic devices(56,57); however, the requirements for this use are relatively stringent, namely it must have an oxidation/reduction potential within an acceptable range and it m s t be able to be cycled tens of thousands of times. The advantage of duroquinone is its lack of available reaction sites. Clearly the tetrachloro derivative would also be a possibility. !hroquinone may have a similar potential in electrical accumulators. As with benzoquinone, duroquinone has potential uses in photography. Patents have been taken out for its use in photochromic responsive elements( 58), for persulphate silver bleach baths( 59), photographic recording materials(60), colour diffusion transfer photographic films(61), silver halide-free photographic recording materials based on a photoreduction agent forming a redox pair with Co(II1) and with various colour recording layers(62-64), a photographic recording device(65), a stabilizer for light sensitive dispersion based on tellurium(66), use in high contrast photographic images(67) and in an organic holographic recording material(68). Durohydroquinone can be used to make polyester fibres(691, and, of course, could be partially incorporated into other polyesters if it was shown to be desirable. Low concentrations of durohydroquinone have been claimed to have insecticidal properties(70), and the compound is claimed to be non-toxic. Also, durohydroquinone can be oxidised with air to form hydrogen peroxide, and, in the presence of As(OEt)3 it is a powerful epoxidizing agent for alkenes(71). Since the duroquinone is reducible back to the hydroquinone, the material can be considered to act in a similar way to a catalyst. Durene diamine can be readily made by reducing dinitrodurene by standard methods. Possible uses include forming a green-black ink suitable for thermcgraphic copying or inpact printing(72), and inclusion in pesticides(73)
337
and in bis-maleimide antifouling agents(74). However, as might be expected, the m s t likely use would appear to be as an inclusion in aramid polymers. Ararnides have a 4OoC higher melting point with durene diarnine than with p-phenylene diamine(5). Furthemre, durene diamine may be able to be produced at a price equivalent to or lower than that of p-phenylene diamine. This arises because the linear substitution is not generally simple in nitration reactions; for durene, however, there is no other option. There is an alternative method of mking functional derivatives for polymerization. Ihrene can be condensed with formaldehyde in the presence of hydrochloric acid and zinc chloride to give the bis-chloromethyl durene(75). This can be converted into a number of derivatives from which polymers can be made, e.g. durene-l,44icarboxylic acid for polyamides(76), the diacetic acid for polyesters(77,78) or for polyamides(79), the diisocyanatomethyl derivative for polyureas and polyurethanes(80,81), the dimethanol derivative for polyurethanes(82) and for epoxies(831, while the bis chloromethyl derivative In each has also been proposed for making polyethers (with bisphenol A)(84). case, the attraction from the durene derivative has been the introduction of higher melting points and inproved softening properties. None of these polymers have been comnercialized, possibly in part because of the difficulty of obtaining durene at suitable.prices. PRDPOSED DEVELOPMENTS
I first raised publicly the possibility that durene could be mmercially exploited in October 1982, while I was employed at the DSIR Chemistry Division. Following further discussion the New Zealand Government called for proposals as to how the resource could be developed. The first set of such proposals was received in May, 1984, but with the change of Government which followed, IX) decisions were taken. A sequence of further activity took place and at the close of 1987, a reconmendation was placed before the Government that a Joint Venture between ICI New Zealand Ltd and Applied Chemistry Ltd be allowed to develop the extraction of durene and, following a feasibility study, construct a plant of at least 5000 tpa capacity to produce pyranellitic dianhydride fran durene( 85) In the final sutmission to the C r m , Applied Chemistry Ltd additionally prcmised to set up a caopany to manufacture as fine chemicals other durene derivatives, so that the full range of such derivatives would be available for further developnent work. It was further undertaken that such derivatives would be produced at a sufficient rate that market requirements would be met, subject, of course, to having the necessary quantity of durene and the time to
.
meet such requirements. It is intended that any of the derivatives noted in
338
section 9 will be mnnnercially available within a reasonable time-scale. Whether chemicals other than those derived from durene will k manufactured is unclear at this time; durene is the most attractive raw material in terms of availability and ease of extraction/purification, and as its products have saw almost unique properties, it is by far the easiest to cmrcialise. The economic availability of m n y of the other components in the synthetic gasoline will probably depend on the method used to extract the durene. REFERENCES 1. 2.
Chem.Eng.News, 1979, 9. Hydrocarbon Processing, May 1986, 36. Kirk-Othmer, Encyclopaedia of Chemical Technology, 3rd Ed. Wiley-Interscience. 3. T. Shima, S. Yamashiro, N. Yoshimra, T. Kobayashi, T. Kuratsuji and H. Inata. Brit.Pat. 1,325,107, 1971. Physical Data, Unless otherwise referenced, is from Handbook of Chemistry 4. and Physics, CRC Press. 5. D.C. Pease, U.S. Pat. 3,197,443 (1965). A. di Cio, A. Neri and F. bveri, La Chimica e L'Industria 56 (1974) 8. 6. Brit. Pat. 1,081,287 (1964). 7. R. Yokoyama U.S. Pat. 3,721,683 (1973). 8. R.M. Cahen, H.R. Debus and J.M.J.G. Andre, U.S. Pat. 3,926,852 (1975). 9. 10. A.G. Akhmetov, S.Z. AKhmetova, N.M. Reksheneva and T.M. Khannanov, Neftekhirniya (1970) 879-82. (Chem. 4bs 74 88316). (1971) 1 4 3 7 11. D.E. Drayer, Hydrocarb. Proc. 12. T. Horie, K. Yashida, H. Hori and Y. Katsuyama. Cer. Offen 2,112,009 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
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L. Horner and K. Miesel, Werkst. Korros. 2 (1978) 654. P. Leskovar. U.K. Pat. App. 2,091,998 (1982). W. Brzyska and 2. Kmalewicz-Malinskwa. ChemAnal. 26 (1981) 755. B.N. Achar, G.M. Fohlen and J.A. T a r k e r . J.Polym.Sci. Polym. Chem.Ed.
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W.C. Drinkard and J.C. B a i l a r . J.Am.Chem.Soc. (1959) 4795. F r . Cemande, 1,577,512 (1969). A.M. Brownstein and D.L. Kerr. U.S. Pat. 4,028,423 (1977). A.A. Tager. Acta Polym. (1983) 489. I. Nano, M. Tolan and C. Coserio. Mater.Plast. 2 (1975) 138. R.A. park, polym. Plast.Technol.Eng. 5 (1976) 157. !7.M. Nielson. Ger. Offen. 2,311,235 (1973). A. Y a m t o , K. Osura, K. Ueda and T. Kawahima. Jap. Kokai 79 36,360
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(Chem.Abs 1 5 8 , 8 7 8 ) . A.K. Lazarus and M.H. Knapp. Ger. Offen 2,305,919 (1973). Jap. Kokai 81, 139,592 (Chem.Abs 96 88,299). R.K. Smith %d C.S. Popoff. U.S. Pat. 3,280,144 (1966). 8.0. Lin J.Agric.Food and Chem. (1982) 1236. C. Schwarzer. U.S. 3,346,665 (1967). R. S t e e l e and H. Weyland. Ger. Offen 2,525,248 (1975). C.E. S r q . J.Polym.Sci..Macraml.%v. (1976) 161-208. J.T. S t a p l e r and J. Bornstein. U.S. Pat. 3,959,232 (1976), J.Het.Chem. (1973) 983. Neth. Appl. 6,413,711 (1965). G.W. Schrauzer and H. Tnynet. Ger. o f f e n 1,154,474 (1963). G.W. Schrauzer and H. Tnynet. Ger. Offen 1,168,903 (1964). S. Kondo and N. Yoshiike. Jap. Kokai 80, 17,115. S. Kondo and N. Yoshiike. Jap. Kokai 41,473. H.A. Brown. U.S. Pat. 3,486,899 (1965). C.R. van d e r V c o r n and R.G. \?illis. Ger. Offen 2,141,199 (1972). D.S. Bailey e t a l . Ger. Offen 2,437,382 (1975). M. Miyakawa. Ger. Offen 2,501,597 (1975). A. Adin and J.C. Fleming. Ger. Offen 2,516,270 (1975). A. Adin and J.C. Fleming. U.S. Pat. 4,195,998 (1980). A. Adin and J.C. Fleming. U.S. Pat. 4,201,588 (1980). J.C. Fleming and J.W. Manthy. Ger. Offen 2,437,382 (1975). J.V. Mitchell and W.E. Nixon. Ger. Offen 2,841,418 (1979). H. Mifune Ger. Offen 2,758,765 (1979). A. B l m et a l . U.S. Pat. 4,084,970 (1978). E.C.A. Schwarz. U.S. Pat. 3,374,202 (1968). I Ichikawa and H. B u r u t a . Jap. Kokai 107,126. (Chem.Abs g 1259). N.A. Clinton. Eur. Pat. Appl. E P 9,262 (1980). T.L. Cairns and E.G. McGeer. U.S. Pat. 3,140,308 (1964). G. Rsato and J . A . Pankovich. Ger. Offen 2,739,215 (1978). K. Mori and S. Matsui. Jap. Kokai 77 148,621 (Chern.Abs 165,499). W.G. de P i e r r i and H.W. Earhart. U Z . Pat. 2,964,573 (1960). A.B. Conciatori and R.W. Stackman. U.S. Pat. 4,035,437 (1977). J.F. Knifton and R.M. Suggit. U.S. Pat. 4,016, 194 (1977). E.C.A. Schwarz. U.S. Pat. 3,354,124 (1967). J.S. Ridgeway. U.S. Pat. 3,426,000 (1969). A. Mohajer. Ger. Offen 2,118,591. M.F. Brooks and W.G. Healey. B r i t . Pat. 1,284,340 (1972). Neth. Appl. 6,501,872 (1966). French Bmande 1,403,326 (1965). J.R. Caldwell. U.S. Pat. 3,075,949 (1963). Europ.Chem.News. Jan 5/12 1987, p.18.
st.
30
11
B,
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2
9
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ALTERNATIVE ROUTES TO METHANE CONVERSION
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D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors). Methane Convers~on 0 1988 Elsevier Science Publishers H V., Amsterdam - Printed i n The Netherlands
343
FEASIBILITY OF ETHYLENE SYNTHESIS V I A O X I D A T I V E COUPLING OF METHANE
M. M. BHASIN Research & Development Department, I n d u s t r i a l Chemicals D i v i s i o n , Union Carbtde Corp., P. 0. Box 8361, South C h a r l e s t o n , WV 25303
ABSTRACT Shortages o f p r i m e f e e d s t o c k s f o r e t h y l e n e manufacture have spawned numerous a t t e m p t s t o use a l t e r n a t e raw m a t e r i a l s . Methane i s one such raw m a t e r i a l t h a t i s t h e most abundant component o f n a t u r a l gas, u s u a l l y c o m p r i s i n g up t o -90 mole % of t h e hydrocarbon f r a c t i o n . Thus, methane r e p r e s e n t s a c o n s i d e r a b l y more abundant source f o r e t h y l e n e than ethane/propane, t h e two most w i d e l y used raw m a t e r i a l s .
Dehydrogenative c o u p l i n g o f methane r e q u i r e s temperatures i n excess o f 800°C f o r p r a c t i c a l c o n v e r s i o n s a l o n g w i t h a h i g h endothermic h e a t o f 53 K c a l / g mol - q u i t e d i f f i c u l t t o g e n e r a t e a t temperatures o f >800°C. O x i d a t i v e c o u p l i n g removes t h e thermodynamic b a r r i e r o f s t r a i g h t dehydrogenative c o u p l i n g and t h e h e a t - o f - r e a c t i o n problem. The p r i m a r y o b j e c t i v e o f t h e e a r l y Union Carbide s t u d y was t o uncover s e l e c t i v e c a t a l y s t s f o r c o u p l i n g o f methane t o f o r m C ~ ' S , and p r i n c i p a l l y e t h y l e n e . S y n t h e s i s o f e t h y l e n e (and some ethane) has been demonstrated 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 o f methane a t atmospheric p r e s s u r e and temperatures o f 500-1000°C. A g r e a t number o f m e t a l o x i d e s , supported on an alpha-alumina s u p p o r t , have been screened f o r a c t i v i t y and s e l e c t i v i t y i n t h e normal, c o n c u r r e n t f e e d i n g mode o f r e a c t a n t s , as w e l l as, i n t h e s e q u e n t i a l o r f e e d p r o g r a m i n g mode. The c o n c u r r e n t f e e d i n g experiments gave l o w s e l e c t i v i t i e s o f 0-2091, w h i l e f e e d programming experiments gave s e l e c t i v i t i e s o f -50%. I n t h e l a t t e r case, however, a s t a i n l e s s s t e e l r e a c t o r became c a t a l y t i c a l l y a c t i v e f o r b u r n i n g o f methane t o carbon o x i d e s . Although a s t a i n l e s s s t e e l r e a c t o r was used i n most o f t h e work r e p o r t e d here, a q u a r t z r e a c t o r was found t o be i n e r t . The m o s t a c t i v e c a t a l y s t s f o r C2-formation were t h e oxides o f Mn, Pb, Sn, Sb, B i , T1, and Cd, w h i l e L i , Mg, Zn, T i , Zr, Mo, Fe, Cr, W , Cu, Ag, P t , Ce, V , B and A 1 showed l i t t l e o r no a c t i v i t y . The l o w C2-formlng a c t i v i t y o f P t and Ce however, may be due t o t h e secondary b u r n i n g o f C2s on t h e s t a i n l e s s s t e e l r e a c t o r w a l l s . T h l s a l s o makes i t d i f f i c u l t t o q u a n t i f y t h e C2-forming s e l e c t i v i t y o f t h e a c t t v e m e t a l o x i d e s . The a c t i v e m e t a l s seem t o e x h i b i t a common c h a r a c t e r i s t i c : t h e y can c y c l e between a t l e a s t two o x i d a t i o n s t a t e s . A l t h o u g h t h e r e a r e d i f f e r e n c e s i n s e l e c t i v i t i e s i n C 2 - f o r m a t i o n and carbon o x i d e s f o r m a t i o n . no c o r r e l a t i o n seems t o e x i s t w i t h t h e free-energy changes i n t h e o x i d a t i o n s t a t e s . A p o s s i b l e mechanism f o r C2-formation f r o m methane i s proposed. There has been numerous o t h e r r e p o r t s r e c e n t l y ( f o r example, s e v e r a l p a t e n t s i s s u e d t o Jones, Leonard. and Sofranko i n U.S. 4,443,644, -645, -647, -648, -649) o f t h e s y n t h e s i s o f C2t hydrocarbons f r o m methane over Mn3O4. Sb2O3, PbO, 1 9 0 3 , and Ge02. Samarium o x i d e "as i s " , o r a f t e r a l k a l i m o d i f i c a t i o n has been r e p o r t e d by Otsuka, e t . a l . t o be an
344
even more a c t i v e c a t a l y s t t h a n t h e p r e v i o u s l y known. most a c t i v e m e t a l o x i d e s . However, a l l c a t a l y s t systems r e p o r t e d t o - d a t e r e q u i r e e i t h e r f r e q u e n t r e g e n e r a t i o n s o r an unsteady s t a t e o p e r a t i o n . The f e a s i b i l i t y o f an o x i d a t i v e c o u p l i n g process s h a l l depend on: ( i ) h i g h e r a c t i v i t y / s e l e c t i v i t y c a t a l y s t t o p e r m i t a lower temperature o p e r a t j o n i n t h e range o f 400-6OO0C. Such a c t i v e c a t a l y s t s h a l l a l s o m i n i m i z e p o t e n t i a l l o s s o f c a t a l y t i c a l l y a c t i v e m e t a l s and m e t a l o x i d e s . ( l i ) s c a l e a b i l i t y o f unsteady s t a t e o p e r a t i o n - though i t has a p o t e n t i a l advantage i n s e p a r a t i n g t h e a i r f r o m t h e hydrocarbon e f f l u e n t . These h u r d l e s must be overcome b e f o r e t h i s c o n c e p t u a l l y a t t r a c t i v e r o u t e t o e t h y l e n e can be c o n s i d e r e d a v i a b l e a l t e r n a t i v e t o c o n v e n t i o n a l r o u t e s .
INTRODUCTION
As e a r l y as 1969 a r e s e a r c h program was undertaken i n Union Carbide t o i n v e s t i g a t e t h e f e a s i b i l i t y of u s i n g methane ( f r o m abundant n a t u r a l gas) as a raw m a t e r i a l f o r e t h y l e n e manufacture.
Methane's h i g h m o l e c u l a r s t a b i l i t y
compared t o o t h e r a l i p h a t i c s (ethane/propane
-
t h e commonly used raw
m a t e r i a l s ) , however, makes i t s use i n e t h y l e n e p r o d u c t i o n d i f f i c u l t and no s i g n i f i c a n t amount o f e t h y l e n e i s produced c o m n e r c i a l l y f r o m methane today. T h i s e a r l y Union Carbide work was l a t e r p u b l i s h e d i n J o u r n a l o f C a t a l y s i s i n 1982.(1)
I n t e r e s t t n t h e d i r e c t o x i d a t i v e c o u p l i n g o f methane t o produce
e t h y l e n e (and/or m i x t u r e o f o l e f t n s ) has r e c e n t l y been h e i g h t e n e d because o f t h e f e a s i b i l i t y o f a p o t e n t i a l economically a t t r a c t i v e r o u t e f o r f u r t h e r c o n v e r s i o n s t o e i t h e r g a s o l i n e , d i s t i l l a t e s o r o t h e r chemicals e i t h e r i n a s i n g l e converter o r i n a two-converter-in-series s e p a r a t i o n (2,3)
(Fig. 1 ) .
w i t h o u t an i n t e r m e d i a t e
Such d i r e c t c a t a l y t i c methane conversions a r e t h e
p r e f e r r e d r o u t e s t o e t h y l e n e (and propylene, e t c . ) and g a s o l i n e o r d l s t i l l a t e s t h a n t h o s e i n v o l v i n g t h e i n t e r m e d i a t e p r o d u c t i o n s o f e i t h e r s y n t h e s i s gas ( C O
+ H2) o r methanol(2.3)
F i g . 1.
(Fig. 1).
The s y n t h e s i s gas and methanol r o u t e s have
Methane Conversion Pathways.
.
F T.
___a SYN.
CRUDE
345
been extensively covered In many lectures presented at the New Zealand Methane Conversion Symposium. In this paper, I would like to cover these topics on the subject o f direct methane conversion: 1. Early Union Carbide Work. 2. Review and hlghlights o f other recent work. 3. The Problems, Challenges and Opportunities Ahead.
DISCUSSION 1. Early Union Carbide Work An early look into the thermodynamics of straight dehydrogenative coupling of methane:
revealed that practical levels o f equilibrium conversion to C2 products would requlre temperatures in excess of 800°C ( 1 ) . Another problem is that high endothermic heat is expensive t o generate and transfer to the gas at high temperatures.
-
Oxidatlve coupling,
CH4
02
C2H2, C2H4, C2Hb + H20
would appear t o solve the thermodynamic-barrler and heat-of-reaction problems, but there is no indication that a highly selective reaction would result without the use of a catalyst (1). The primary objective of the early Union Carbide work was t o uncover selective catalysts for the oxidative coupling of methane to form C 2 s and principally ethylene. A major problem in studying catalytic oxidative coupling is the presen e of competing gas-phase, non-catalyzed or catalyzed total combustion of the products and the methane feed. To minimize such non-selective reactions, the reactor was operated cyclically, i.e., methane and air were fed one-at-a- i me across the catalyst with short purging flows of nitrogen in between. A diagram of a typtcal feed program is shown in Figure 2. Feeding in this
:I46
Fashion does impose another c r i t e r i o n on t h e c a t a l y s t :
I t must have t h e
a b i l i t y t o r e t a i n one r e a c t a n t u n t i l t h e o t h e r r e a c t a n t i s f e d .
The
LF[m
c o n v e r s i o n and s e l e c t i v i t y under steady s t a t e and f e e d program mode were rneas ured and shown t o be c l e a r l y super o r f o r t h e feed program descr bed previous y (1). Figure 3
: (3
I
3
3 r
3
However, a diagram o f t h e r e a c t i o n system
N2
a44
, 10,
30
I'S
given
n
Line-out
N,
W
(L
Li
GO
,lo,
60
Fig. 2
T y p i c a l Feed Program
Fig. 3
Schematic Diagram o f C a t a l y s t T e s t i n g Reactor
Feeds t o t h e r e a c t o r ( a i r , n i t r o g e n , and methane) were f e d c o n t i n u o u s l y t h r o u g h r o t a m e t e r s and n e e d l e v a l v e s f o r f l o w c o n t r o l .
The gases t h e n passed
t h r o u g h three-way s o l e n o i d v a l v e s a c t i v a t e d by t i m e r s t o g i v e d e s i r e d f e e d program.
A t y p i c a l " r u n " c o n s i s t i n g o f t h e sequence o f N2, A i r .
CH4. N2, e t c . i s shown i n F i g u r e 2.
N2,
The r e a c t o r e f f l u e n t was p i p e d
d i r e c t l y t o a General E l e c t r i c monopole f a s t - s c a n n i n g mass spectrometer i n l e t t o monitor t h e products.
Most c a t a l y s t s were prepared u s i n g s t a n d a r d vacuum i m p r e g n a t i o n techniques using n i t r a t e s a l t s i n d i s t i l l e d water.
T y p i c a l l y , the alpha-alumina
(LA-4102) impregnated c a r r i e r was d r i e d i n steps a t 60°C/30 minutes and 110°C f o r 2-3 hours.
F i n a l h e a t t r e a t m e n t was a t
- 700°C
f o r 1-3 h o u r s .
An e a r l y d e t e r m i n a t i o n of t h e b e n e f i t s of c y c l l c compared t o steady f l o w o p e r a t i o n was made u s i n g t h e 1 / 2 - i n c h diameter r e a c t o r w i t h a c a t a l y s t c o n s i s t i n g o f l e a d o x i d e supported on =-alumina ( T a b l e 1 ) .
Flows were such
TABLE 1 Comparison o f C 2 Y i e l d s Ouring Feed-Programming and Steady-Flow Modes o f Operation* Feed Programming Catalyst**
Steady Flow
PbO on N o r t o n LA 4102 alumina
Feed Rates. gm m o l / h r 1.6
CH4 N2 Air
0.8 1.b
1.6
0.8 1.6
Feed Proqram. sec. 20 5 20 5 -1 0
CH4 N2 - B l a n k i n g Pulse A ir N2 B l a n k i n g Pulse Maximum C2 Per Pass Y i e l d
None
-1
*These t e s t s were conducted i n 1/2" schedule 40 p i p e w i t h a 1/8" a x i a l thermowell. Reactor p r e s s u r e was 6 p s i g and temperature ranged between 800-1 000°C. * * 4 7 m l o f c a t a l y s t was d i l u t e d w i t h 47 cc of b a r e s u p p o r t i n t h e s e t e s t s .
t h a t t h e same numbers o f moles p e r hour o f t h e t h r e e feeds were added i n each experiment.
Thus, b o t h t h e gas-phase r e s i d e n c e t i m e s o f methane i n t h e
r e a c t o r and t h e average moles o f methane f e d p e r u n i t t i m e p e r u n i t o f c a t a l y s t were t h e same i n b o t h experiments.
The p a r t i a l p r e s s u r e o f methane
i n t h e c y c l i c experiment was h i g h e r -- a s i t u a t i o n which. i f a n y t h i n g , one c o u l d s p e c u l a t e l e a d i n g t o a lower C2 s e l e c t i v i t y . y i e l d o f about one-order-of-magnitude compared t o s t e a d y - f l o w o p e r a t i o n .
An i n c r e a s e i n t h e C2s
was observed w i t h c y c l i c o p e r a t i o n The m a j o r f r a c t i o n o f t h e C2s formed was
e t h y l e n e , w i t h s m a l l amounts o f ethane (and o n l y t r a c e s o f a c e t y l e n e ) a l s o b e i n g formed.
348
E a r l y t e s t s made, g e n e r a l l y i n t h e temperature range o f about 600 t o 1000°C, (Table 2) showed t h a t c o b a l t , manganese, cadmium oxides and p o s s i b l y zinc oxide possess some C2-forming a c t i v i t y .
I r o n , n i c k e l ' a n d s i l v e r were
TABLE 2 C2-Forming Tendencies f o r Various Catalysts* Impregnating Salt
Max. Y i e l d t o
2 s .
** x
Temp. a t Max. Y i e l d ,
1 1 3 1 2 5 4
OC
1000 900 950 81 0 825
*Support was Norton LA4102 alumina. **Includes small amount o f C2H6. e s s e n t i a l l y i n a c t i v e since t h e i r a c t i v i t y was comparable t o t h a t o f a non-catalyzed r e a c t i o n from several bare supports i n c l u d i n g F i l t r o s , Johns-Hanville 6-20 f i r e b r i c k , and Norton LA956 and LA4102.
Other c a t a l y s t s
which promoted more C2 formation than t h e b a s e l i n e Included the oxides o f t i n , antimony, bismuth. lead, t h a l l i u m , antimony. and perhaps l i t h i u m and boron.
A l l o f these t e s t s were made w i t h t h e l / 4 . - i n c h r e a c t o r and w i t h
c y c l i c feeding.
Flows o f each stream were 0.2 gm mole/hr, and t h e c y c l i c
feeding schedule was t h a t o f F i g u r e 2. However, some bare supports d i d seem t o promote t h e formation o f Cog. This r a i s e d the suspicion, which was confirmed, t h a t t h e s t a i n l e s s s t e e l r e a c t o r w a l l indeed was a c t i v e i n t h e COP formation above 800°C (1). However, q u a r t z r e a c t o r was i n e r t t o methane combustion (1).
Hence, f o r
t h i s reason, no q u a n t i t a t i v e comparison/conclusion can be drawn from the data presented here regarding t h e s e l e c t i v l t y t o C2s.
However, semiq u a n t i t a t i v e l y t h e more a c t i v e metal oxides were o f Hn, Cd, Sn, T1, P t , Ce,
349
Pb, and B i (Table 3).
The low C2-forming a c t i v i t y o f P t and Ce, however,
TABLE 3 More A c t i v e Elements f o r C2 Formation
No.
E 1ement Blank (Support) Mn Cd Sn T1 Pt Ce Pb Bi
c2s 0.2 5 4.0 0.4
3.0
<0.1 <0.1 2.0 0.4
Methane Conversion t o
COP-
Total
1.6 6.0* 6.0* -6.0
1 .B 11* 1o* 0.4 6.6 8.0 5.0 4.6 4.4
3.6 8.0 5.0 2.6 4.0
*Estimated may be due t o the secondary burning o f C2s on t h e s t e e l r e a c t o r w a l l s . The l e s s a c t i v e metal oxides were those o f Mo, Cu, Sb, L i , and Mg (Table 4) of these Sb and Mg oxldes e x h i b i t e d higher C2 a c t i v i t i e s than the blank r e a c t o r .
TABLE 4 Less A c t i v e Elements f o r Cp Formation
No.
E 1ement Blank (Support) Mo cu Sb Li Mg
1 2
3 4
6
!& 0.2
<0.1 n.m. 1.5 0.2 0.8
Methane Converslon t o
4 2 -
Total
1.6 4.0 -4.0 2.0
1.8 4.0 4.0
1.2
2.0
3.0
3.5 3.2
P e r i o d i c a l l y d u r i n g these t e s t s , an a n a l y s i s was made f o r hydrogen. amounts found were much l e s s than would be expected f o r dehydrogenative coupling.
The
350
Nature o f Catalysts I n F i g u r e 4 i s shown a p e r i o d i c t a b l e l i s t i n g t h e c a t a l y s t s i n v e s t i g a t e d and an i n d i c a t i o n as t o whether o r n o t t h e y produced g r e a t e r o r l e s s e r amounts o f C2s t h a n t h e b a r e s u p p o r t s .
IA
I n s p e c t i o n o f t h i s f i g u r e shows
0 = No octivity obove thot of bore support I = Possibly smoll octivity obove thoi of
2 = Cleor activity obove thot of bors “LOW
V I llA
Melting hletok’
Ce
0
F i g . 4.
A c t i v e and I n a c t i v e C a t a l y s t s f o r C2 Formatton.
t h a t t h e g r e a t e s t r e g i o n o f a c t i v i t y corresponds t o t h e s o - c a l l e d l o w - - m e l t i n g m e t a l s o f I I I A . I V A and VA and Hn o f group V I I B and Cd f r o m
116.
The s u r p r i s i n g f a c t i s t h a t a p p a r e n t l y some c a t a l y t i c a c t i v i t y i s
possessed by q u i t e a number o f m a t e r i a l s . A f u r t h e r unusual f a c t i s t h a t t h e amount o f oxygen t r a n s f e r r e d t o t h e
c a t a l y s t s and subsequently t o t h e c o u p l i n g r e a c t i o n d u r i n g c y c l i c f e e d i n g i s one-two o r d e r s o f magnitude l a r g e r than can be accounted f o r by simple monolayer coverage o f t h e c a t a l y s t by oxygen ( 1 ) . l h e e x p l a n a t i o n f o r t h i s s i t u a t i o n seems t o l i e i n h y p o t h e s i z i n g t h a t oxygen can d i f f u s e i n and o u t o f t h e b u l k ( s u b - s u r f a c e a t o m i c l a y e r s ) o f t h e c a t a l y s t . presumably o x i d i z i n g t h e metal t o a h i g h e r v a l e n c e s t a t e . d u r i n g t h e a i r - f l o w p a r t of t h e c y c l e .
Then d u r i n g t h e methane-flow p a r t o f t h e
c y c l e , t h e oxygen can d i f f u s e o u t t o t h e t h e c a t a l y s t s u r f a c e and r e a c t w i t h
351
t h e methane,
The s t o i c h i o m e t r y o f t h i s l a t t e r s t e p would be:
Oxide -,
1 / 2 C2H4
H20
t
t
CH4 + M e t a l
Reduced M e t a l Oxide
[31
One t e s t o f t h i s h y p o t h e s i s can be s u p p l i e d by d e t e r m i n i n g i f r e a c t i o n s o f t h e n a t u r e o f e q u a t i o n 3 a r e thermodynamically a l l o w a b l e f o r t h e a c t i v e catalysts.
Free e n e r g i e s f o r these r e a c t i o n s were c a l c u l a t e d f r o m l i t e r a t u r e
d a t a ( 4 ) and a r e l i s t e d i n F i g u r e 5.
The a c t i v i t i e s o f b o t h t h e o x i d i z e d and
0
Y W
(3
2
a
r V
>
nw
(3
2 W
W W
2 D
a a
n 2
a
f
F i g . 5.
Standard F r e e Energy Changes f o r The R e a c t i o n CHq + M e t a l
o x i d e d 1 / 2 C2H4 + H20
t
Reduced M e t a l Oxide.
reduced metals were assumed t o be one, i n l i e u o f d e t a i l e d phase I n f o r m a t i o n . F i g u r e 5 i n d i c a t e s t h a t most o f t h e c a t a l y s t s showing o x i d a t i v e c o u p l i n g a c t i v i t y have one o r more n e g a t i v e ( f a v o r a b l e ) changes f o r e q u a t i o n 3.
or n e a r - z e r o f r e e energy
Thus i t appears p o s s i b l e t h a t t h e c y c l i n g o f t h e
352
o x i d a t i o n states o f metals was occuring d u r i n g the c y c l i c r e a c t i o n .
-
A
p o s s i b l e mechanism f o r the C 2 s formation may i n v o l v e these steps: 2CH4
t
Mt(2n)On
'
CH3CH3ads
+(2n) On
M
t(2n-2)
-
On-1
CH2
t
'
OHads
M t(2n-2)0n
""3ads
t
H20
Where metal oxide, Mt(2n) reduces t o Ht ( 2 n - 2 ) and o x i d i z e s t o M +( 2n) t o a depth o f about 10-20 l a y e r s . C a t a l y s t S e l e c t i v i t y and A c t i v i t y I n s p e c t i o n o f t h e C2 and carbon d i o x i d e y i e l d s o f the various c a t a l y s t s shows d e f i n i t e d i f f e r e n c e s i n t h e r a t i o s o f these two species.
Unfortunately,
t h e f i n d i n g subsequent t o these t e s t s t h a t the source o f much o f t h e carbon oxides was associated w i t h t h e metal r e a c t o r w a l l makes i t d i f f i c u l t t o determine i n h e r e n t c a t a l y s t s e l e c t i v i t i e s .
S u b t r a c t i o n o f the amount o f
carbon oxides formed w i t h j u s t bare support i n t h e r e a c t o r i s n o t warranted, since t h e r a t e o f o x i d a t i o n o f C2s compared t o methane i s n o t known. Based on t o t a l conversion o f CH4 ( t o C2s and COP), approximate rankings o f t h e more a c t i v e elements and t h e l e s s a c t i v e elements a r e tabulated i n Tables 3 and 4, r e s p e c t i v e l y .
C e r t a i n l y , manganese, lead, t h a l l i u m , cadmium,
and antimony appear t o e x h i b i t h i g h s e l e c t i v i t i e s and good a c t i v i t i e s . Process Considerations
I t i s c l e a r t h a t t h e c a t a l y s t s t e s t e d here do n o t produce methane conversions h i g h enough t o be o f comnercial i n t e r e s t . 25 percent o r more t o C2s would be required.
A conversion o f perhaps
Furthermore several o f t h e
metals o r metal oxides have appreciable vapor pressures above 800'C. Nevertheless, t h e abundance o f m a t e r i a l s which e x h i b i t e d o x i d a t i v e c o u p l i n g a c t i v i t y suggests t h a t f u r t h e r research i s warranted.
Also, t h e c y c l i c
feeding technique. besides producing a s u b s t a n t i a l e f f i c i e n c y increase t o
353
coupled products, provldes a second important economic advantage.
By p r o p e r l y
d i v e r t i n g the r e a c t o r e f f l u e n t stream the a i r used t o o x l d l z e t h e c a t a l y s t can be vented, w h i l e t h e hydrocarbon e f f l u e n t can be sent t o a recovery u n i t . Such d i v e r s i o n e l i m i n a t e s most o f t h e n i t r o g e n from the recovery u n i t , thereby g r e a t l y reducing t h e recovery cost o f ethylene. Conc 1us ions The c y c l i c feedlng o f a i r and methane produced C2s ( p r i m a r i l y ethylene) i n t h e presence o f a number o f metal oxldes. Most o f these metals l i e i n the low-melting metal p a r t o f the p e r i o d i c t a b l e , i.e.,
groups I I I A , I V A and VA.
The amount o f oxygen t r a n s f e r r e d t o t h e c a t a l y s t s and then t o t h e methane was many times l a r g e r than t h a t which could have been adsorbed on t h e c a t a l y s t surface, i n d i c a t i n g t h a t oxygen must have d i f f u s e d i n t o and out o f t h e b u l k of the c a t a l y s t o r a t l e a s t , 10-20 atomic l a y e r below the surface.
Supporting
t h i s c o n t e n t i o n i s t h e f a c t t h a t a l l o f t h e most h i g h l y a c t i v e c a t a l y s t s could s t o r e and release oxygen by c y c l i n g between a t l e a s t two valence states. 2.
Review and H i g h l i g h t s o f Other Recent Work
(I) The E a r l y Exxon Work.
M i t c h e l l and Waghorne o f Exxon i n
1979 ( 5 ) converted methane over m a t e r i a l s termed by them as " C a t a l y s t s Reagents I' By t h i s term and t h e manner o f t h e i r experiments, c a t a l y t i c c ompo s it ons were used up as reagents i n r e a c t i o n w i t h methane a t temperatures o f 702OC.
To a c t i v a t e methane, they employed a combination o f Gp V I I I noble
metal, a group V I B metal oxide (alone o r as a mixture)-capable o f being reduced t o a lower v a l e n t oxide, and Gp IIA metal ( l i k e barium) composited w i t h a s p i n e l coated i n o r g a n i c oxide l i k e gamna-alumina.
An example o f such a
c a t a l y s t - r e a g e n t would be a HgA1204 s p i n e l blocked alumina c o n t a i n i n g 6.9% Ba, 5% C r ( o r V, U o r W) and 0.1-0.3% P t .
T y p i c a l conversions were 40-50%
w i t h a broad carbon number products ranging from 4-5% C2H4. 0.1-2% C2H6 w i t h much l a r g e r amounts o f other aromatic hydrocarbons and coke.
The residence times were long (30 minutes) and t h e c a t a l y s t - r e a g e n t s were consumed and/or poisoned by products. c o n d i t i o n s were exemplified.
A v a r i e t y o f compositions and
By t h e use o f P t t o a c t i v a t e methane, these
Exxon workers showed t h a t CH4 can be converted t o a l a r g e number o f a l i p h a t i c and aromatic hydrocarbons, a small f r a c t i o n o f t h e products being ethylene p l u s ethane.
(ii) Baerns. e t . a l .
Baerns, e t . a l . ( R u h r - U n i v e r s i t a t Bochurn, FHG).
I n 1983-86
( 6 ) s t u d i e d s e v e r a l l e a d o x i d e c a t a l y s t s supported on
ydmma-alumina, s i l i c a and s i l i c a - a l u m i n a supports - some m o d i f i e d by a l k a l i treatment.
The c o u p l i n g r e a c t i o n was c a r r i e d o u t i n a continuous feed mode.
The c a t a l y s t o f c h o i c e , PbO, was indeed one o f t h e s e v e r a l most s e l e c t i v e c a t a l y s t s found i n t h e Union Carbide work ( 1 ) .
I t was c l e a r l y shown by Baerns, e t . a l . preferred f o r high s e l e c t i v i t i e s t o C2s.
t h a t l o w a c i d i t y supports a r e
Thus, s i l i c a - a l u m i n a gave t h e
l o w e s t s e l e c t i v i t y f o l l o w e d by gamma-alumina, w i t h s i l i c a and t h e a l k a l i treated silica-aluminas being the best i n s e l e c t i v i t y .
I n fact, the acidic
supports possess some methane c o n v e r s i o n a c t i v i t y o f t h e i r own b u t g i v e poor selectivities (6).
C o n c e n t r a t i o n s o f PbO, u p t o -4 wt.91,
a r e t h o u g h t t o be
c o v e r i n g t h e a c t i v e s u r f a c e a c i d i t y w i t h a consequent i n c r e a s e in C2-selectivity.
These r e s u l t s , i n g e n e r a l , c o n f i r m our assumptions and a
b a s i s f o r t h e s e l e c t i o n of alpha-alumina as t h e s u p p o r t f o r a l l m e t a l o x i d e s evaluated ( 1 ) . acidity.
Alpha-aluminas a r e known t o possess e x t r e m e l y l o w - t o - - n i l
A mechanism f o r t h e l o s s o f s e l e c t i v i t y was suggested t o occur v i a
an a t t a c k o f m e t h y l carbonium i o n s by n e g a t i v e l y charged s u r f a c e oxygen species l e a d i n g f i r s t t o a methoxy s u r f a c e species and t h e n t o methanol and/or formaldehyde which a r e f i n a l l y o x i d i z e d t o C02 + H20 ( o r v i a CO + H 2 ) . Such n o n - s e l e c t i v e combustion o f CH4 would l i k e l y occur on a c i d i c s i t e s w h i l e CH3 s u r f a c e species may be o x i d i z e d on d i s s o c i a t i v e l y adsorbed oxygen atoms and/or m o l e c u l a r oxygen.
I n a s i m i l a r manner t h e c o u p l i n g p r o d u c t s
C2H6 and C2H4 may be n o n - s e l e c t i v e l y o x i d i z e d .
( i i i ) ARC0 WORK ( C . 1984-87).
A. Jones. J. J. Leonard and J. A . Sofranko
-
Jones, Leonard and Sofranko (7a. 7b and 7 c ) screened a l a r g e number
o f m e t a l o x i d e s supported on s i l i c a .
Gamma alumina s u p p o r t was found t o be
u n d e s i r a b l e as i t r e a c t e d w i t h m e t a l o x i d e s t o f o r m t h e m e t a l a l u m i n a t e s . Manganese, indium, germanium, antimony, t i n , bismuth and l e a d o x i d e s were found t o be e f f e c t i v e c o u p l i n g agents, g i v i n g 10-50% s e l e c t i v i t y t o hydrocarbons.
Reactions were done i n c y c l i c redox mode i n which o x i d i z e d
c a t a l y s t was r e a c t e d w i t h methane i n t h e absence o f oxygen f o r s e v e r a l minutes t o form c o u p l i n g p r o d u c t s and t h e reduced c a t a l y s t was o x i d i z e d w i t h a i r i n a separate step.
I n a d d i t i o n t o t h e C 2 s , f o r m a t i o n o f C3+ hydrocarbons
i n c l u d i n g benzene t o l u e n e , and coke were observed.
The most a c t i v e c o u p l i n g
agents found were same as t h o s e found I n t h e Union Carbide work ( 1 ) . except f o r i n d i u m and germanium which were n o t s t u d i e d .
However, i n d i u m and
germanium b e l o n g t o t h e same p e r i o d i c t a b l e group as t h a l l i u m (Gp tin/antimony(Gp I V A ) ,
IIIA) and
r e s p e c t i v e l y , which were found t o be a c t i v e c o u p l i n g
agents i n t h e Union Carbide work ( 1 ) .
355
Recently t h e ARCO workers have e v a l u a t e d a l k a l i m o d i f i e d manganese/silica supported c a t a l y s t and found them t o be more s e l e c t i v e than t h e unmodified o x i d e c a t a l y s t s ( 7 b ) .
Thus, sodium pyrophosphate (5%) doping
of manganese on s i l i c a c a t a l y s t improved s e l e c t i v i t y by 10-2oX and p r o v l d e d t h e a d d i t i o n a l advantage o f i n c r e a s i n g c a t a l y s t l i f e .
XRD analyses o f t h e
c a t a l y s t s r e v e a l e d t h a t t h e improved s e l e c t i v i t y c a t a l y s t s were formed when t h e c r y s t a l phase b r a u n i t e , Mn7S1Ol2,
was enhanced.
The ARCO work has
a l s o i n v e s t i g a t e d t h e continuous oxygen cofeed process approach.
In
comparison t o t h e c y c l i c redox process, t h e c o n t i n u o u s cofeed process has been shown t o g i v e s i m i l a r c o n v e r s i o n / s e l e c t i v i t y r e l a t i o n s h i p and p r o d u c t d i s t r i b u t i o n though t h e c o n v e r s i o n / s e l e c t i v i t y i s somewhat l o w e r .
Thus, t h e
ARC0 workers suggest t h a t methyl r a d i c a l s formed by r e a c t i o n w i t h t h e s o l i d m e t a l o x i d e undergo hydrocarbon b u i l d i n g ( v i a c o u p l i n g and o l l g o m e r i z a t i o n ) i n t h e gas phase, w h i l e t h e dehydrogenation o f p a r a f f i n s t o o l e f i n s and t h e d e s t r u c t i v e o x i d a t i o n ( t o CO/CO,)
occur on t h e s u r f a c e o f c a t a l y s t .
mechanism i s s i m i l a r t o t h a t proposed by L u n s f o r d ( 8 ) .
This
T y p i c a l l y , a t 25%
methane c o n v e r s i o n C2+ s e l e c t i v i t i e s o f 70-75% have been o b t a i n e d by t h e ARCO workers ( 7 c ) .
(iv) 1985-87).
Otsuka and Coworkers (Tokyo I n s t i t u t e o f Technology, JapanL
Over 30 m e t a l o x i d e s were e v a l u a t e d f o r methane c o u p l i n g a c t i v i t y
which i n c l u d e d s e v e r a l o f t h e more a c t i v e m e t a l o x l d e s discussed e a r l i e r ( 9 ) . Rare e a r t h metal o x i d e s showed h i g h e r C 2 - s e l e c t i v i t i e s t h a n t h e o t h e r a c t i v e metal oxides.
Samarium o x i d e was shown t o be t h e most a c t i v e and s e l e c t i v e
c a t a l y s t f o r C2 f o r m a t i o n . v e l o c i t y o f 60-30 hr-’
(9a).
T h i s e a r l y work was done a t f a i r l y l o w space However, i n a l a t e r s t u d y a t h i g h e r space
v e l o c i t y , samarium o x i d e was found t o be o n e - t o - t h r e e o r d e r s o f magnitude more r e a c t i v e t h a n most o t h e r metal oxides ( 9 b ) .
Otsuka and coworkers have a l s o
shown b e n e f i c i a l e f f e c t s o f a l k a l i and a l k a l i h a l i d e promoters w i t h v a r i o u s metal oxide c a t a l y s t s .
I n a d d i t i o n , combinations o f a c o u p l i n g c a t a l y s t w i t h
Ga-ZSM-5 were shown t o g i v e h i g h e r s e l e c t i v i t y / y i e l d s o f a r o m a t i c hydrocarbons (9c). Several o t h e r l a b o r a t o r i e s i n Japan a r e becoming i n c r e a s i n g l y i n v o l v e d i n t h e d i r e c t methane c o n v e r s i o n c a t a l y s i s .
Several p o s t e r papers
and papers a t t h i s symposium a r e a good evidence i n s u p p o r t o f t h i s observation. (v)
Roos. et.al..
N e t h e r l a n d s (1986) and Doval. et.al..
Argentina.
I n c r e a s e d r e s e a r c h a c t i v i t y has been n o t i c e d i n s e v e r a l l a b o r a t o r i e s around t h e w o r l d ( 3 , 5, 6 , 7, 8, 9. 10, 1 1 ) .
However, t i m e c o n s t r a i n t s does n o t
356
p e r m i t a more d e t a i l e d d i s c u s s i o n o f a l l those s t u d i e s . Lunsford, e t . a l .
I n addition,
( 8 ) have done an extensive study o f t h e nature o f c a t a l y t i c
c o u p l i n g s i t e as w e l l as t h a t o f t h e r e a c t i o n mechanism.
3.
The Problems, Challenges and O m o r t u n i t i e s Ahead
I n summary, though 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 methane t o form ethylene/ethane has now been amply demonstrated I n many research l a b o r a t o r i e s worldwide, t h e commercial f e a s i b i l i t y o f such a process depends c r i t i c a l l y on:
(i) developing more a c t i v e (and s e l e c t i v e ) c a t a l y s t t h a t w i l l p e r m i t o p e r a t i o n of a steady s t a t e ( o r cofeed) r e a c t i o n a t 400-600°C.
(ii)long term c a t a l y s t s t a b i l i t y . and/or (ill) s c a l e a b i l i t y o f unsteady s t a t e operation. The author i s o p t i m i s t i c t h a t w i t h the continued worldwide e f f o r t s i n t h e d i r e c t methane conversion area a comnercial process w i l l become t e c h n i c a l l y f e a s i b l e i n t h e 1990s.
ACKNOWLEDGMENT The author thanks the I n d u s t r i a l Chemicals D i v i s i o n o f Union Carbide Corporation f o r t h e permission t o p u b l i s h t h i s work.
I n a d d i t i o n . he would
l i k e t o thank Or. George E. K e l l e r f o r h i s c o l l a b o r a t i o n and f u t u r i s t i c v i s i o n t h a t made t h e e a r l y Union Carbide work possible.
REFERENCES
6. E. K e l l e r and M. M. Bhasln, J. Catalysis, 73. (1982) 9-19. F. M. Oautzenberg and 6. Klingman. Paper Presented a t t h e ACS D i v i s i o n o f I & EC 21st S t a t e - o f - t h e A r t Symposium; -"Methanol As Raw M a t e r i a l f o r Fuels and Chemicals", Marco I s l a n d , F l o r i d a , June 15-18, 1986. C. A. Jones, J. J. Leonard and J. A. Sofranko, Energy and Fuels, 'I. 1987, 12-16. D. R. S t u l l , e t . a l . , JANAF "Thermochemlcal Tables," 2nd ed., N a t i o n a l Bureau o f Standards, Washington, DC. 1971. H. L. M i t c h e l l , 111. R. H. Waghorne, U. S. Patent 4,172.810, October 30, 1979 and 4,205,194, May 27, 1980. W. 8 y t y n and M. Baerns, Applied C a t a l y s i s , 28. (1986) 199-207. J. A. Sofranko, J. J. Leonard and C. A. Jones, J. Catalysis. 103 (1987) 302-310 and U.S. Patents 4,443,644, 645. 647. 648 and 649 ( A p r i l 17, 1984) C. A. Jones, J. J. Leonard and J. A. Sofranko, J. Catalysis. 103 (1987) 311-319. C. A. Jones, J. J. Leonard and J. A. Sofranko, Energy and Fuels, 1 (1987) 12-16 and U.S. Patents 4,443,644, 645, 647, 648 and 649 ( A p r i l 17, 1984).
.
357
8
CH. H. L i n , K. 0. Campbell and J. H. Lunsford. J. Phys. Chem., 90 (1986)
9(a) (b)
K. Jinno and A. Horikawa, J. C a t a l y s i s , 100 (1986) 353-359. K . Otsuka and T. Komatsu, Chem. L e t t e r s (Chem. SOC. Japan) (1987) 483-484. K. Otsuka and T. Komatsu, Chem. L e t t e r s (Chem. SOC. Japan) (1986) 1955-1 958. J. A. Roos, A. G. Bakker, H. Bosch, J. G. van Omen and J. R. H. Ross, P r e p r l n t , Europ. Workshop on New Developments i n S e l e c t i v e Oxidation. Louvain-La-Neuve, March 17-18, 1986. H. J. F . Doval, 0. A. Scelza and A. A. Castro, P r e p r i n t , Tenth Iberoamerlcan Symposium on C a t a l y s l s , Herida, Venezula, J u l y 6-12, 1986.
(c) 10 11
534-531.
K . Otsuka,
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:),%I. Hihhy, C D. Chang, R.F. Xowe and S.Yiirchak (Editors).:rlcthanc, (.'on[vrsion 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
DIRECT CONVERSION OF METHANE TO METHANOL AND HIGHER HYDROCARBONS Jack H. Lunsford Department o f Chemistry, Texas A M University College Station, Texas
77843
ABSTRACT For the production of both oxygenates and higher hydrocarbons the initial step in the activation of CH4 over metal oxide catalysts appears to be the formation of methyl radicals. Although several types of surface oxygen may be effective in abstracting H from CH4 at elevated temperatures, there is considerable experimental evidence to support the role of 0 - ions in this reaction. These ions may be formed by a one-electron transfer to N20, they may be generated by photolysis of a metal oxide surface (e.g. Mo03/Si02), or they may result from substitutional impurities (e.g. Li' in MgO), The resulting methyl radicals may either react by reductive addition to form methoxide ions or they may desorb into the gas phase. Under reacting conditions the methoxide ions subsequently may decompose to HCHO or react with H20 to form CH30H. The gas phase coupling of methyl radicals yields C2H6, which may be dehydrogenated to C2H4. I n all cases yields of the desired partial oxidation products are limited by both heterogeneous and homogeneous secondary reactions. Generally, a surface which can activate methane also can activate intermediates for further oxidation. INTRODUCTION One approach to the conversion of methane to more useful chemicals and fuels involves its catalytic oxidation. By suitably choosing the catalyst and oxidant it is possible to direct the reaction to oxygenated products (HCHO and CH30H) or hydrocarbons (mainly C2H4 and C2H6).
Unfortunately, high selectivities to
methanol and formaldehyde have been achieved only at low conversion levels and only using nitrous oxide as the oxidant [Refs. 1, 21.
For example, over
molybdena supported on silica at 1.9% conversion the selectivities to CH30H and HCHO were 20% and 80%, but at 6 % conversion the respective selectivities became
8% and 49%. Moreover, N20 is not a commercially acceptable oxidant, except perhaps when it is a by-product of an existing process, such as the production of adipic acid. Considerably more progress has been made in the oxidative coupling of methane to form ethane and ethylene (C2 products).
Since the early work of Keller and
Bhasin [Ref. 3 1 , the steady-state yields of C2 products have improved to a level of about 20%. Among the more effective catalysts are the Group IIA oxides which
360
have been promoted with the appropriate Group IA ions [Refs. 4-61 and certain members of the lanthanide oxide series which also have been promoted with Group I A ions [Ref. 7 , 81. Among these, La203 and Sm2O3 are the most active and selective. Recently, Li-promoted ZnO has been reported to give yields of C2 products up to 25%. with 80% ethylene in the Cq fraction[Ref. 9 1 . In the pulse
mode, where CH4 and 02 are not simultaneously present over the catalyst, supported manganese oxide, also modified 'by alkali metal ions, is active and selective for Cg formation [Ref. 101. Although the partial oxidation of methane is of great technological importance, it also is of fundamental interest. One must be concerned with the mode by which methane is activated, which includes the nature of the active site, and with the mechanism through which the products are formed. As we shall subsequently see, the mechanism for oxidative dimerization may not entirely be heterogeneous, thus gas phase chain mechanisms may be important. In considering the mechanism and the mode of activation a question arises concerning the a possible intrinsic upper limit for C2 yield, at least when one is operating in a single pass flow reactor. Have we reached that upper limit at about 25% C2 yield? In considering the mechanisms both for the formation of oxygenates and for the formation of C2 compounds there is considerable evidence which suggests that methyl radicals are a primary intermediate. This paper will focus on these CH3' radicals - their mode of formation and their subsequent reactions. As one might expect they are highly reactive and sometimes elusive species; however, under favorable conditions it is possible to obtain direct evidence for their role in the oxidation reactions. FORMATION OF METHYL RADICALS In the Gas Phase: The initiation step in the classic mechanism for the pyrolysis of ethane involves the formation of methyl radicals, but of more relevance to catalysis by metal oxides is the role of 0- ions in generating these radicals. Bohme and Fehsenfeld [Ref. 111 have shown that the reaction CH4
+
0-
-+
CH3'
+-
OH-
is exothermic and that it occurs in the gas phase with a high effeciency. At 25°C approximately 8% of the collisions between CH4 and 0- result in the abstraction of a hydrogen atom. It also is significant that the analogous reaction occurs with ethane, but in this case the efficiency for reaction is considerably greater (- 50%).
361
On Mo/Si02
By impregnating silica (Cab-0-Sil)with ammonium heptamolybdate and heating the sample to 600°C for 24 h it is possible to obtain a dispersed form of Moo3 on the surface which we will represent as Mov1/Si02.
It was first demonstrated
by Lipatkina et al. [Ref. 121, and more recently by our group [Ref. 11 that W irradiation of Mov1/Si02 in the presence of CH4 at -196°C results in the formation of CH3' radicals on the surface. At the wavelengths of radiation employed (254 run) no methyl radicals were formed on the pure Si02. The methyl radicals are believed to be produced by the reactions MoV102MoVO'
hu +
iCH4
MoVO-+
MoVOH- + CH3
The epr spectrum of the methyl radicals thus formed is evident in Fig. lb. Similarly, the spectrum of Mov may be partially seen as a high field shoulder. Since there must be a separation between the paramagnetic .species of eq. 3 (Mov and CH3') it is likely that the methyl radicals have moved away from their center of formation and are located on the silica surface.
,
20G
,
Fig. 1. EPR spectra of methyl radicals: (a) after reaction of CH4 with 0- on Mo/Si02; (b) after W irradiation of oxidized Mo/SiOp in the presence of CH4. Reactions were carried out and spectra recorded with the sample at -196°C. [Ref. 11. Of more significance to the thermal catalytic mechanism is the observation that 0 ' ions, formed by reacting Mov with N20, also react with CH4 to yield CH3'
-
-
radicals. Two lines of this four-line spectrum are apparent in Fig. la, along 2.020 and gu 2.004. These radicals were
with the residual 0- spectrum at gl
fgrmed within minutes after the CH4 was admitted to the sample a t -196°C. which suggests that the surface analog of reaction 1 ha; almost no barrier; i.e. EaCt
*
As expected the CH3' radicals also rapidly reacted upon warming the sample.
0.
Surface-Generated Gas-Phase Radicals At higher temperatures certain metal oxides, including all of those which are effective in the oxidative dimerization reaction, have the ability to generate CH3' radicals on the surface. Moreover, these radicals emanate to the gas phase.
By employing a matrix isolation system in tandem with a hot catalyst bed
it has been possible to detect these radicals by esr and to relate their
formation to the state of the catalyst [Refs. 13, 1 4 1 .
This technique has
provided valuable information on the active forms of oxygen which are responsible for the generation of radicals. For example, the observation that Li-doped MgO greatly enhanced the formation of methyl radicals resulted in the conclusion that centers of the type [Li'O-]
were responsible for the hydrogen
atom abstraction [Ref 131. One can immediately recognize that the reactive
0-
ion is part of this center. Actually these centers are mainly in the bulk of the crystallites and they react with the surface via hole migration [Ref. 151.
It is the surface O - s ions which react with methane. This is not the only reactive form of oxygen, as La203 also is effect in generating CH3' radicals, but there is no evidence for
ions on this material
0-
[Ref. 161. Rather, the epr spectrum of a surface 02- has been detected after the oxide was quenched from high temperatures [Ref. 1 7 1 .
The superoxide ion is
relatively unreactive at low temperatures; however, it may become active at the high temperatures where the oxidative dimerization occurs. As indicated by results of Fig. 2, the members of the lanthanide oxide series
vary considerably in their ability to generate gas phase methyl radicals [Ref. 141.
Moreover, for a given oxide the method of preparation has a marked effect
on the specific activity for radical formation. For example, hydrothermal treatment of SmpO3, followed by activation at 600"C, resulted in a material which was very effective in generating CH3' radicals, in agreement with the overall catalytic activity observed by Otsuka et al. [Ref. 181. When the same series of oxides was activated at 700°C, the sequence of activity for CH3' radical formation became La203 > SmgO3 = Nd2O3; whereas, the order of C2 yield for these oxides was NdgO3 > La203 = Sm2O3. When one considers the entire series it becomes evident that certain of the
oxides, such as Ce2O3, are quite ineffective in the formation of gas phase radicals. In fact, Ce2O3 will scavenge methyl radicals by reacting with them.
Cerium oxide has multiple oxidation states which may be involved in these reactions (see below).
Likewise, Ce2O-j is an inactive and nonselective catalyst
for the oxidative coupling o f meEhane.
I
(12.11
Ce
Nd
Eu
Tb -Ho
Tm
Lu
Fig. 2. Relative rates of CH3' radical production per m2 for original ( . . . ) and hydrothermally (-) treated lanthanide oxides. Results on the rate of CH4 conversion obtained by Otsuka et al. [Ref. 181. [Ref. 141. REACTIONS OF METHYL RADICALS On Mo/Si02: Infrared results suggest that methyl radicals react with Mov102- by a reductive addition reaction to form surface methoxide ions, Mo"OCH3- [Ref.11. Earlier Kochi [Ref.l9] had demonstrated the reductive addition of alkyl radicals to transition metal ions. As depicted in Fig. 3b, the spectrum of methoxide ions was observed following adsorption of CH4 on a sample which had previously been exposed to N20 in order to form 0 - centers. A similar spectrum was observed (Fig. 3c) following the addition of CH30H to the sample, although in
364
this case the methoxide ions were mainly on the silica. Methoxide ions serve as a convenient intermediate for the formation of HCHO by thermal decomposition or for the formation of CH30H by reaction with water. Sleight and co-workers [Ref.20] have convincingly demonstrated that methoxide ions are intermediates in the partial oxidation of methanol to formaldehyde over Moo3 catalysts. In the partial oxidation of CH4 over Mo/Si02 catalysts we suggested that formaldehyde was formed by the direct decomposition of methoxide ions [Ref.l] and the subsequent work of Kahn and Somorjai [Ref.2] on this catalyst provided experimental evidence.
2928
'I
,2857
'
Fig. 3. Infrared spectra of methoxide ions on Mo/SiOp; (a) background after reduction of catalyst in CO, followed by adsorption of N20 and evacuation; (b) after subsequent adsorption of CH4 and evacuation; (c) after adsorption of CH30H and evacuation. [Ref. 11. Alkoxide ions, including those in the compound MoO(OCH3)4, readily react with water to form methanol [Ref.l]. Thus, the presence of water in the reactor system results in improved selectivities to methanol. The methanol yield, however, was never very great, as Mo/Si02 also is a good catalyst for converting methanol to formaldehyde [Ref.21]. For the latter reaction a catalyst typically operates at ca. 200'C; whereas, for methane oxidation temperatures on the order of 580'C are required. Thus, the problem of secondary oxidation reactions, including the conversion of HCHO to CO and Cop, becomes apparent when an active catalyst such as Mo/Si02 is employed. The reductive addition of methyl radicals to Mov102-, followed by the subsequent reactions of methoxide ions suggests a catalytic cycle which is depicted in Scheme I. All of the steps in this cycle would be rapid except for dehydroxylation. In addition to this selective cycle one must also consider the possibility of a two-electron reaction with N20 to form oxide ions. A molecule of CH4 would then have to be used to reduce the oxide in a typical Mars-van Krevelen mechanism (Ref.221. The two-electron transfer could be minimized, in
365
principle, by dispersing the Mo in a mononuclear manner on the surface, and by avoiding the reduction of this molybdenum to the IV oxidation state. On Li/MeO and Related Oxides: Relatively little is known concerning the reactions of radicals with alkaline earth oxides; however, Smith and Tench [Ref.23] reported that H. reacts with MgO to form Fs centers (electrons trapped at anion vacancies) and OH- ions. This is similar to the reductive addition reaction described above, and by analogy one Scheme I
8MOV1+4O2-+CH4
2MoV+ N20
-
-
EMoV+C02+2H20 2 MoV' + 0 2 - + N 2
/ \
0
might expect that an alkyl radical would react to form a trapped electron and an alkoxide ion. We have used this concept to describe the formation of ethylene following the reaction of ethane with 0- ions on MgO [Ref.24]. Similarly, it is anticipated that CH3' radicals would react with the MgO surface, even on the Li/MgO catalyst, to form methoxide ions, but unlike the ethoxide ion, the methoxide ion cannot form an alkene by giving up another hydrogen to the surface. Rather, at the elevated temperatures of the oxidative dimerization reaction, the surface methoxide ions decompose to form CO [Refs.4,24]. Thus, in contrast to the case of Mo/Si02 the reaction of methyl radicals with Li/MgO and other combinations of Group IA/IIA oxides results in
complete oxidation. Consequently, C2 selectivity is inversely related to surface area, as emphasized in a recent report by Aika and co-workers [Ref.25]. The relationship between activity, selectivity, surface area and [Na'O-] centers is depicted in Fig. 4 , which was obtained over a Na/CaO catalyst under continuous flow conditions [Ref.6], Clearly, both the activity and C2 selectivity increased, whereas the surface area decreased up to a NazCO3 content of 15 wt% Na.
This behavior is evidence for the formation of specific active
sites ([Na'O-]
centers) on a surface which generally is nonselective for the
I
0
10
20
Na Content ( % )
Fig. 4 . Effect of Na concentration on [Na'O-] concentration ( ) , surface area (0), the amount of CH4 converted (m, total; a , to C2; V , to C1) over Na/CaO: 1.0 g catalyst, 300 torr of CH4, 20 torr of 02, flow rate = 0.92 mL sec-l, T = 675°C [Ref. 61. formation of ethane and ethylene. As the Na2C03 content is increased (>15% Na) the active sites become covered with the carbonate phase and the ability of the catalyst to generate CH3' radicals is decreased. Finally, the material corresponding to 43% Na is pure Na2C03, and one may see that it is neither very active nor selective for the oxidative dimerization reaction.
In the Gas Phase: The high-pressure limit for the coupling of methyl radicals occurs at
a
few
torr total pressure. and the rate constant is only a weak function of temperature. Assuming a collision cross section of 54
A
', at 700°C 6% of all
collisions between CH3' radicals result in the formation of C2H6. This high reaction efficiency makes possible the favorable C2 selectivities which have been observed over several metal oxide catalysts As noted previously, the generation of gas phase methyl radicals generally parallels the overall yield of C2 products obtained with a particular catalyst or class of catalysts. Although these correlations indicate the importance of
CHj' radicals in the oxidative dimerization of CH4, they do not prove that the coupling reactions occur mainiy in the gas phase. The matrix-isolation electron spin resonance (MIESR) system, however, has been adapted to give more quantitative results under catalytic conditions, and the results confirm that the flux of radicals emanating from the surface is sufficient to account for >LO% of the C2 products.
The value of 40% is a lower limit for the gas phase
coupling reaccion sincc it does not take into account radical coupling reactions which mighc occur in the void volume of the catalyst particle, before the radicals are sampled by a probe. Thus the reaction
is a critical step in the formation of C2 products. In addition other gas phase radical reactions occur which generally result in complete oxidation. One such chain branching mechanism is [Ref.6]
A critical feature of th-s mechanism involves the chhir, branching in reaccion 5. One should note that the formation of branching reagent CH302H requires the presence of C2H6 which has a smaller bond strength than CH4.
The role of ethane
is evident in the dara of Fig. 5 which shows that over a Na/CaO catalyst at 625" C the CH4 conversion goes through a maximum at an 02 partial pressure of ca. 50
torr. klen the ethane was depleted the CH4 conversion decreased to 7.5 torr,
368
but when 3 torr C2Hg was separately added to the system at 185 torr 02 the amount of CH4 converted increased to 8.4 torr, which was greater than the maximum of 8.1 torr in Fig. 5. We estimate that under these conditions approximately 15% of the methane is consumed via the gas phase reactions described in reactions 2-7. Another important element of the gas phase reaction is the equilibrium described by reaction 2 and the subsequent reactions of CH3O2' to form CO and
C02. The equilibrium has been measured in the gas phase, and as might be expected, higher temperatures favor the dissociation of CH3O2' radicals [Refs.
26,271. The consequence of this phenomenon is depicted in the data of Fig. 6 which were obtained using La203 as the catalyst [Ref.l6]. When the CH4 conversion was limited by the availability of 02, it was observed that a
-0
I
1.o
r
0.5
e
-z
A 300 400
200
Fig. 5. Effect of oxygen pressure on the CH4 converted and [Na+O-]concentration over 2.0 g of 15 % Na/CaO at 625°C: 0 , total; A , to C2; , to C1; 0,to C2H4; , [Na+O-]. 300 torr of CH4 was used at a flow rate of 0 . 9 2 mL sec [Ref. 61. decrease in C1 selectivity was accompanied by an equal increase in C2 selectivity as the temperature was increased. Likewise, the 02 partial pressure influences the CH3O2' concentration via reaction 2. This is clearly evident in the results of Fig. 5. Oxygen is required to generate [M'O-]
centers and to replenish the oxygen which is lost
from the lattice as water; hence, both the activity and C2 selectivity increased
up to about 50 torr. But above this pressure, oxygen incorporation into the lattice is no longer rate limiting and the adverse effects of 02 via CH3O2' become paramount. In order to circumvent this problem pulse reactors have been employed
so
that the hydrocarbons and 0 2 are not in contact with one another
[Ref.lO]. This approach, o f course, improves the selectivity but limits the rate of CHq conversion. Whereas the discussion has focused mainly on the role of 02 in the activation of CH4, it also should be noted that at temperatures 3
700°C oxygen promotes the complete oxidation of the desired products, C2H4 and C2H6. LIMITATIONS ON C2 YIELD As noted previously, C2 yields of about 20% have now been achieved over a
500
600
Temperature("Q
700
lo
Fig. 6 . Methane conversion and selectivity variations as a function of reaction temperature: 0,CH4 conversion; , C1 selectivity; 0 , C2H4 selectivity, A , C2H6 selectivity. A reactant mixture of 56 torr of CH4 and 17 torr of 02 was fed over 1.0 g of La203 at a flow rate of 0.42 mL s-l. [Ref. 161. number of catalysts, and this suggests that there may be an intrinsic limitation in the process for the oxidative dimerization of methane.
Clearly, the
formation of methyl radicals requires a highly reactive form of surface oxygen
-
one that could also attack ethane and even ethylene by hydrogen atom abstraction. We have demonstrated that 0- on MgO reacts readily with ethane and
370
ethylene at low temperatures [Refs. 24,281. Moreover, ethyl radicals are generated from ethane over Li/MgO, but the corresponding vinyl radicals have not been observed in the gas phase [Ref.29]. It seems that the vinyl radicals undergo a series of surface reactions which ultimately result in C02 [Ref.29]. The ethyl radicals, by contrast, provide a pathway for the formation of ethylene via the reaction
In a conventional catalytic system one might therefore expect that as the partial pressures of ethane and ethylene begin to approach that of methane, the product molecules will begin to compete effectively for the active center. At least with Li/MgO and Na/CaO it appears that the secondary reactions with C2H6 are not deleterious for selectivity but reactions with C2H4 are nonselective [Refs. 4,6]. The unpromoted Lag03 is even more active for the complete oxidation of C2 products [Ref.l6]. Because of the differences in C-H bond energies of CH4 (Dc-H
> 108 kcal/mol) CH4 and C&,
- 105 kcal/mol),
C2H6 (Dc-H = 98 kcal/mol) and C2H4 (Dc-H
one can imagine an active center which is capable of attacking
but not C2H4. If this were achieved, it should be possible to
obtain ethylene selectivities which are limited only by undesirable gas phase radical reactions.
SUMMARY Most of the current results on the selective oxidation of methane over metal oxide catalysts may be interpreted in terms of methyl radical chemistry. These radicals may either react with the oxides themselves to form methoxide ions or they may enter the gas phase. The methoxide ions on supported molybdena decompose to form formaldehyde or they react with water to yield methanol. On the basic oxides methoxide ions result in complete oxidation. Those radicals which enter the gas phase undergo typical free radical chemistry which includes coupling reactions to give ethane and chain branching reactions to give nonselective oxidation products.
Secondary surface reactions, particularly with ethylene, also may result in complete oxidation. If further improvements in
yields of partial oxidation products are to be achieved, ways must be found to more efficiently utilize the methyl radicals, both with respect to surface reactions and to gas phase reactions. In addition, if ethylene is the desired product, catalysts must be fine-tuned to the point where they will activate methane, but not ethylene.
371
ACKNOWLEDGMENT The research carried out at Texas A&M University has been supported, in part, by the Division of Basic Energy Sciences, Department of Energy. The author also acknowledges the contributions of recent graduate students and research associates including K.D. Campbell, D.J. Driscoll, T. Ito, C.-H.Lin, J.-X.Wang and H. Zhang. REFERENCES 1 H.-F. Liu, R.-S. Liu, K.Y. Liew, R.E. Johnson and J.H. Lunsford, 3. Am. Chem. S O C . , 106 (1984) 4117. 2 M.M. Khan and G.A. Somorjai, J . Catal., 91 (1985) 263. 3 G.E. Keller and M.M. Bhasin, J . Catal., 73 (1982) 9. 4 T. Ito and J.H. Lunsford, Nature (London), 314 (1985) 721; T. Ito; J.-X. Wang, C.-H. Lin and J.H. Lunsford, J . Am. Chem. SOC. 107 (1985) 5062. 5 C.-H. Lin, J.-X. Wang, T. Ito and J.H. Lunsford, J. Am. Chem. S O C . , in press. 6 C.-H. Lin, J.-X. Wang and J.H. Lunsford, submitted to J. Phys. Chem. 7 K. Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lett., (1986) 467. 8 C.-H. Lin, Ph.D. Dissertation, Texas A M University, 1987. 9 I. Matsuura, Y. Utsumi, M. Nakai and T. Doi, Chem. Lett., (1986) 1981. 10 C.A. Jones, J.J. Leonard and J.A. Sofranko, J. Catal. 103 (1987) 311. 11 D.K. Bohme and F.C. Fehsenfeld, Can. J. Chem., 47 (1969) 2717. 12 N.I. Lipatkina, V.A. Shvets and V.B. Kazansky, Kinet. Katal., 19 (1978) 979. 13 D.J. Driscoll, W. Martir, J.-X. Wang and J.H. Lunsford, J. Am. Chem. SOC., 107 (1985) 58. 14 K.D. Campbell, H. Zhang and J.H. Lunsford, submitted to J. Phys. Chem. 15 J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986) 5883. 16 C.-H. Lin, K.D. Campbell, J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986) 534. 17 J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986) 3890. 18 K. Otsuka, K. Jinno, A. Morikawa, J. Catal., 100 (1986) 353. 19 J.K. Kochi, J. Am. Chem. S O C . , 85 (1963) 1958. 20 W.-H. Cheng, V. Chowdhry, A. Ferretti, L.E. Firment, R.P. Groff, C.J. Machiels, E.M. McCarron, F. Ohuchi, R.H. Staley and A.W. Sleight, in B.L. Shapiro (Ed.) Heterogeneous Catalysis, Texas A&M Univ. Press, College Station, TX, 1984, pp. 165-181. 21 T.-J.Yang and J.H. Lunsford, J. Catal., 103 (1987) 55. 22 P. Mars and D. van Krevelen, Chem. Eng. Sci., Suppl. 3 (1954) 41. 23 D.R. Smith and A.J. Tench, J. Chem. SOC. Chem. Conunun., (1968) 1113. 24 K. Aika and J.H. Lunsford, J. Phys. Chem., 81 (1977) 1393. 25 T. Moriyama, N. Takasaki, E. Iwamatsu and K. Aika, Chem. Lett., (1986) 1165; E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Chem. S O C . Chem. Commun., (1987) 19. 26 L.A. Khachatryan, O.M. Niazyan, A.A. Mantashyan, V.I. Vedneev and M.A. Teitel'boim, Int. J. Chem. Kinet., 14 (1982) 1231. 27 I.R. Slagle and D. Gutman, J. Am. Chem. SOC., 107 (1985) 5342. 28 K. Aika and J.H. Lunsford, J. Phys. Chem., 82 (1978) 1974. 29 D.J. Driscoll and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415.
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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
373
OXIDATIVE COUPLING OF METHANE OVER PROMOTED MAGNESIUM O X I D E CATALYSTS; RELATION BETWEEN ACTIVITY AND S P E C I F I C SURFACE AREA
E. IWAMATSU, T. MORIYAMA, N. TAKASAKI, and K. AIKA Research L a b o r a t o r y o f Resources U t i l i z a t i o n , Tokyo I n s t i t u t e o f Technology, 4259 Nagatsuta, M i d o r i - k u , Yokohama 227 (Japan)
ABSTRACT O x i d a t i v e c o u p l i n g o f methane was s t u d i e d o v e r promote9 My0 c a t a l y s t s a t 1023 K. An a l k a l i doped c a t a l y s t w i t h a s u r f a c e a r e a o f 1-20 m g i s t h e most a c t i v e f o r C -compounds (C H6 + C H ) p r o d u c t i o n . When more t h a n 2 mol% o f a l k a l i m e t a l i s doged, t h e s u r f a z e a r e a z i t decreased and C -compounds y i e l d i s increased. On t h e o t h e r hand, when 0.2 molX o f any k i n d o f f;e$allis doped t h e MgO c a t a l y s t has an i n c r e a s e d and a c o n s t a n t s u r f a c e a r e a (200 m g ), which enables us t o compare t h e e f f e c t o f dopant. G e n e r a l l y , an sp-element ( a l k a l i m e t a l , a l k a l i n e e a r t h m e t a l , 38 and 4B group) promotes t h e C -compounds f o r m a t i o n , whereas a t r a n s i t i o n - e l e m e n t (5A, 6A, 8 t h group) r e t a r g s i t . Besides t h e k i n d o f promoter, a s p e c i f i c s u r f a c e a r e a was found t o be an i m p o r t a n t f a c t o r o f c o n t r o l l i n g C compounds y i e l d . I t i s d i s c u s s e d t h a t t h i s phenomenon i s r e l a t e d t o tne natu6e o f a heterogeneous-homogeneous mechanism.
-
INTRODUCTION Recently, o x i d a t i v e c o u p l i n g o f methane has been s t u d i e d i n o r d e r t o u t i l i z e n a t u r a l gas as a chemical carbon source. V a r i o u s m a t e r i a l s ( r e f . 1-12) have been r e p o r t e d t o be e f f e c t i v e c a t a l y s t s . However, r o l e s o f t h e c a t a l y s t s have n o t been s t u d i e d w e l l . I t o , L u n s f o r d e t a l . have r e p o r t e d t h a t an a c t i v e c a t a l y s t has a r a d i c a l c e n t e r ( f o r example (Li+O-])
under t h e r e a c t i o n c o n d i t i o n ( r e f . 3-
a, b, c ) . A l t h o u g h t h i s must be an e s s e n t i a l f a c t o r , i t seems d i f f i c u l t t o e x p l a i n t h e e f f e c t i v e n e s s o f such a v a r i e t y o f c a t a l y s t m a t e r i a l s b y one f a c t o r . I n t h i s s t u d y we t r i e d t o a b s t r a c t t h e i m p o r t a n t f a c t o r s t o determine t h e apparent a c t i v i t y and s e l e c t i v i t y o f t h i s r e a c t i o n b y u s i n g MgO c a t a l y s t s doped w i t h various metal oxides. EXPERIMENTAL The r e a c t i o n was performed by u s i n g a c o n v e n t i o n a l f l o w r e a c t o r ( 8 mm 0.d.) a t temperatures between 673 t o 1073 K. CH4, a i r , and He were charged w i t h f l o w 1 1 r a t e s o f 1.5 m l min-' (4.02 mmol h- ), 3.75 m l min-' and 50 m l min- , r e s p e c t i v e l y . A r a t i o o f CH4/02 was 2.0
5
0.1 (2CH4 + O2 = C2H4 + 2H20).
The
s e l e c t i v i t y and y i e l d a r e d e f i n e d as ( 2 x moles C2-compounds produced)/(moles CH4 r e a c t e d ) and ( 2 x moles C2-compounds produced)/(moles CH4 i n t h e f e e d ) ,
314
respectively. Metal nitrates were added to MgO (Soekawa Chemical Co., 99.75%) in water, then the samples were dried, pelletted and weighed. Amount of promoter is expressed by mol% of metal against 100 mol% of MgO. Pure MgO sample is also made through soaking with water and pelletting. 2 g of a sample was evacuated or treated in He flow at 773 K for 1 h and then at 1073 K for 2 h and used for the reaction. Some of them were calcined at 1273 K for 2 h in a separate oven by using a crucible made of MgO. Surface area was measured by BET method using N2 after the heat treatment. SEM pictures were taken by using Hitachi SEM S-800.
0.2
Reaction
temp. I K
Fig. 1. Reactants and products at the outlet of a reactor with MgO (29) catalys) at various reaction temperature. Flow rate; CH4/air/He = 1.5/3.75/50 ml min-
.
RESULTS Reaction Profile Fig. 1 shows a composition of outlet gas by the reaction over unpromoted MgO as a function of the reaction temperature. CH4 begins to react with O2 forming C02 at about 773 K. An amount of H20 produced was not measured. Small amount of CO is produced, which is neglected in this figure. C2-hydrocarbons are observable at 873 K and increase up to 1073 K. Other products such as propane (ref. 13) are negligibly few. Carbon balance of 100 2 5% is obtained for every run in half an hour. All the other promoted catalysts showed similar reaction profiles. However, a temperature giving 5% of COP yield, T(5%C02), ranges 690 to 940 K and a temperature giving 1% of C2-compounds yield, T(1%C2), ranges 850 to 1023 K depending on the catalyst. Since T(5%C02) is always lower by 0 to 200
375
and C 2 2 compounds seem t o be d i f f e r e n t a t l o w e r temperatures. C2-compounds y i e l d ( o r degree t h a n T(1%C2) f o r t h e same c a t a l y s t , t h e r e a c t i o n p a t h s giving CO
s e l e c t i v i t y ) a t 1023 K i s h i g h over a c a t a l y s t whose T(1%C2) i s c l o s e t o T(5%C02), w h i l e i t i s low over a c a t a l y s t whose T(1%C2) i s much h i g h e r t h a n T(5%C02) as i s shown i n F i g . 2 . Over Co and Ag doped MgO, C02 f o r m a t i o n i s p r e v a i l i n g a t a l o w e r temperature o f t h e r e a c t i o n . On t h e c o n t r a r y , COP and C2cornpound:are f o r m i n g a t t h e same t e m p e r a t u r e o v e r a l k a l i doped MgO as i s shown i n F i g . 2. Since C2-compounds y i e l d reaches maximum a t 1023 K o r 1073 K o v e r catalysts
,
most
t h e a c t i v i t y i s compared by r e s u l t s a t 1023 K i n t h i s study. A t 1023
K, O2 c o n v e r s i o n i s about 95% and CH4 c o n v e r s i o n i s i n 25 t o 35% o v e r any
c a t a l y s t , which p r o v i d e s r o u g h l y a c o n s t a n t r e a c t i o n c o n d i t i o n f o r any c a t a l y s t . Under t h i s c o n d i t i o n an a c t i v i t y i s n o t based on a d i f f e r e n t i a l r e a c t o r mode b u t based on an i n t e g r a l r e a c t o r mode. C2-compounds y i e l d a t 1023 K o v e r MgO depends v e r y 1i t t l e
on space
v e l o c i t y ( 4 - f o ~v a r i a t i o n )
,
Only t r a c e o f products
(0.06% y i e l d o f C02 and 0.03% y i e l d o f C2-compounds) a r e observed i n a b l a n k r u n a t 1023
K, which means gas phase r e a c t i o n s a r e n e g l e c t e d under t h e s e c o n d i t i o n s .
OK
L
0
F i g . 2. C -compounds y i e l d ( % ) f o r a r e a c t i o n a t 1023 K as a f u n c t i o n o f T(I%C )-T(5%20 ) . T(I%C ) * Temperature t o g i v e 1% y i e l d o f C2-compounds. T ( 5 % d 2 ) ; Tempzrature t 8 i i v e 5% y i e l d o f COP. R e l a t i o n between a c t i v i t y a t 1023 K and s p e c i f i c s u r f a c e a r e a The a u t h o r s have n o t found any o u t s t a n d i n g s p e c i f i c i t y o f element among a l k a l i promoters f o r
p r o d u c t i o n o f C2-compounds. V a r i o u s f a c t o r s have been
a b s t r a c t e d and compared w i t h t h e a c t i v i t y o f C2-compounds f o r m a t i o n . I t was found t h a t a c a t a l y s t w i t h low s p e c i f i c s u r f a c e area g e n e r a l l y gave a h i g h C2-
376
compounds y i e l d , e s p e c i a l l y when i t i s below 100 m2g-'
( F i g . 3 ) . Such a tendency
i s observed i n a r e l a t i o n between C2-compounds " s e l e c t i v i t i e s " and s p e c i f i c 2 -1 . s u r f a c e area, t o o . S p e c i f i c s u r f a c e area o f MgO (70 in g ) i s once i n c r e a s e d by doping o f 0.2% any a l k a l i m e t a l . However, i t decreases w i t h adding more of it t h a n 2%. C2-compounds y i e l d i s h i g h f o r a c a t a l y s t w i t h h i g h a l k a l i c o n t e n t as
+
i s seen i n F i g . 3. Among a l k a l i doped MgO c a t a l y s t t e s t e d here, 15 mol% Na -MgO which has
low s u r f a c e a r e a
2 m2g-'
g i v e s t h e h i g h e s t C2-compounds y i e l d :
19.5% a t 1023 K and 22.4% a t 1073 K. These values correspond t o t h e r e p o r t e d t
v a l u e o v e r 7 w t % L i -MgO ( r e f . 3 - b ) . The c o r r e s p o n d i n g s e l e c t i v i t i e s f o r C 2 compounds a r e 54.3% a t 1023 K and 56.8% a t 1073 K. Under CH4 r i c h c o n d i t i o n (CH4/02 = 43/1),
t h e C2-compounds y i e l d and s e l e c t i v i t y a r e 6.4% and 88.9% a t
1023 K, 6.7% and 90.8% a t 1073 K, r e s p e c t i v e l y .
These v a l u e s a r e comparable t o
r e p o r t e d s e l e c t i v i t i e s under s i m i l a r c o n d i t i o n s : 93% o v e r Sm203 ( r e f . 4-a) and 89.2% o v e r La203 ( r e f . 3-d) . E t h y l e n e s e l e c t i v i t y among C2-compounds i s p l o t t e d as a f u n c t i o n o f s p e c i f i c 2 -1 s u r f a c e a r e a i n F i g . 4. F o r a sample w i t h l o w e r s u r f a c e area t h a n 100 m g e t h y l e n e s e l e c t i v i t y seems t o i n c r e a s e a l i t t l e . However, f o r a sample w i t h t h e area above 100 m2g-'
t h e s e l e c t i v i t y i s almost c o n s t a n t (60 _t 10%). A r e a c t i o n
o f C2H6 t o C2H4 i s n o t much r e l a t e d w i t h t h e s p e c i f i c s u r f a c e area. F i g . 5 shows a r e l a t i o n between s p e c i f i c s u r f a c e area and C02 y i e l d . A c a t a l y s t w i t h a h i g h e r s p e c i f i c s u r f a c e a r e a g i v e s a h i g h e r C02 y i e l d , which i s in c o n t r a s t t c C2-compounds f o r m a t i o n . However, t h i s tendency i s n o t remarkable because O2 c o n v e r s i o n i s as h i g h as about 95%.
BET
surface area lm2g-1
F i g . 3. Percentage C -compounds ( C H + C H ) y i e l d as a f u c t i o n o f s p e c i f i c s u r f a c e area. R e a c t i o n Z o n d i t i o n s ; CH2/8 = $/?, sample w t . = 29, temp. = 1023 K. Doped m e t a l c o n t e n t s a r e shown b y tymgols i n d i c a t e d i n t h e f i g u r e . None; unpromoted MgO c a l c i n e d a t 1073 K. None(1273K); unpromoted MgO c a l c i n e d a t 1273 K.
311
-4 IN
I
10 010
Na
V
A A
v
15 %
20 010
F i g . 4. S e l e c t i v i t y (%) o f e t h y l e n e among C2-compounds as a f u n c t i o n o f s p e c i f i c s u r f a c e a r e a under t h e same c o n d i t i o n s as F i g . 3.
01 0
100
200
BET surface area I m2g-1
300
I
4 00
F i g . 5. C02 y i e l d (%) as a f u n c t i o n o f s p e c i f i c s u r f a c e a r e a under t h e same c o n d i t i o n s as F i g . 3.
E f f e c t o f doped element S i n c e a s p e c i f i c s u r f a c e a r e a was f o u n d t o be one o f i m p o r t a n t f a c t o r s f o r C2 f o r m a t i o n , an e f f e c t o f dopant s h o u l d be compared b y u s i n g samples w i t h t h e same c o n t e n t and s u r f a c e area. I t i s seen f r o m F i g . 3 t h a t s p e c i f i c s u r f a c e a r e a o f MgO i n c r e a s e d f r o m 70 t o about 200 m2g-'
when 0.2% o f element a r e added. Thus,
i n F i g . 6, we compare t h e C2-compounds y i e l d s f o r 20 samples w i t h d i f f e r e n t elements whose s u r f a c e areas a r e almost c o n s t a n t . Elements on a b s c i s s a a r e arranged a l o n g t h e p e r i o d i c t a b l e . A l k a l i m e t a l s ( I A ) , Ba, and Pb seem t o be
378
most e f f e c t i v e , which a r e f o l l o w e d by group 2A (Ca, S r ) , 3A (Y, L a ) , 4A (Zr), N i and 28 ( Z n ) . Since t h e e f f e c t o f s p e c i f i c s u r f a c e a r e a i s n o t remarkable f o r t h e sample w i t h area h i g h e r t h a n 70 m2g-'
( F i g . 3 ) , i t i s p o s s i b l e t o compare t h e
C2-compounds y i e l d over a p u r e MgO (70 m2g-')
w i t h t h a t o v e r doped MgO. I t i s
concluded t h a t t h e above a d d i t i v e s a r e e f f e c t i v e o r a t l e a s t n o n - r e t a r d i n g ( r e f . 1 4 ) . O t h e r elements such as t r a n s i t i o n m e t a l s ( f r o m 5A t o 1B except N i ) seem t o r e t a r d C 2 - f o r m a t i o n o r t o promote complete o x i d a t i o n . Among 1A group any d e f i n i t e t r e n d i s n o t observed when doped more t h a n 2% ( F i g . 3 ) , a l t h o u g h heavy elements seem t o be e f f e c t i v e al; 0.2%
(Fig. 6).
N
E
;
2.
200
0
2
9-
1 0 0 !j In
I-
Li Na K Sr Ba; Y LajZr j V i C r M F e C o Ni iCuiZnjAl IniPb 1A : 2 A : 3 A gAI5A16A;IA: 8 r182B: 3 8 148
0
w m
F i g . 6. E f f e c t o f added m e t a l element on C -compounds y i e l d a t 1023 K ( l e f t S i e c i f i c s u r f a c e areas, w h i c h a r e o r d i n a t e ) o v e r 0.2% m e t a l doped MgO (29). almost c o n s t a n t , a r e a l s o shown i n t h e r i g h t o r d i n a t e .
T a b l e 1.
Effect of sintering
Sample (2 9)
Calcined temp. (K)
Surface arpa-1 (mg 1
MgO
1073 1273 1073 1273 1073 1273 1073 1273
70 17 21 2 7 66 10 85 7
0.2%Na+-MgO 2%K+-Mg0 2%Cs+-MgO
a R e a c t i o n d a t a a t 1073 K.
C select. a? 1023 K
(%I
11.7 26.1 22.2 30.0 26.6a 35. 7a 14.9 30.1
C yield a? 1023 K
(%I
4.4 9.0 7.4 9.8 8.3a 13.2a 5.6 8.2
379
Reaction
s i n t e r e d sample
S p e c i f i c s u r f a c e area o f MgO i s decreased from 70 t o 17 m2g-'
by a
c a l c i n a t i o n a t 1273 K f o r 2 h w i t h o u t doping. C2-compounds y i e l d increases from
+ o ) and i n Table 1. The e f f e c t o f + s i n t e r i n g was a l s o s t u d i e d f o r t h e promoted MgO. The r e s u l t s f o r 0.2% Na -MgO,
4.4 t o 9.0% as i s shown i n F i g . 3 ( t
2% K+-MgO, and 2% Cs -MgO are shown i n Table 1. S i n t e r i n g phenomena a r e seen on SEM p i c t u r e s (Fig. 7). For any case, a sample which has been c a l c i n e d a t 1273 K has a lower s p e c i f i c s u r f a c e area and g i v e s h i g h e r C2-compounds y i e l d . I t i s proved t h a t s p e c i f i c s u r f a c e area i s one o f t h e most important f a c t o r s which c o n t r o l t h e C2-compounds y i e l d .
DISCUSSIONS Promoter e f f e c t I t was found here t h a t t h e s p e c i f i c s u r f a c e area was one o f t h e important
f a c t o r t o g i v e h i g h C2-compounds y i e l d over promoted MgO c a t a l y s t s ( F i g . 3 and Table 1 ) . However, no work has discussed t h e e f f e c t o f s p e c i f i c s u r f a c e area except a s h o r t comment by I t o , Lunsford e t a l . ( r e f . 3-c) and r e c e n t r e p o r t ( r e f . 12-b) among many works ( r e f s . 1-12). Our work suggests t h a t o t h e r than k i n e t i c c o n d i t i o n s and c a t a l y s t weight t h e s p e c i f i c s u r f a c e area should be k e p t constant, i f p o s s i b l e , when t h e a c t i v i t i e s are compared. These c o n d i t i o n s are f u l f i l l e d i n Fig. 6, where a l k a l i i o n s a r e found t o promote t h e C2-compounds formation. However, t h e e f f e c t i s n o t large. 4A, 46 a r e a l s o
Elements o f ZA, 28, 3A, 3B,
useful t o some e x t e n t . T r a n s i t i o n metals (5A t o 16) except N i
are n o t e f f e c t i v e promorers as have been found f o r Mn02 c a t a l y s t ( r e f . 15). F o r a l k a l i promoters, t h e r e i s no such t r e n d i n a c t i v i t y as t o i n c r e a s e o r decrease i n accordance w i t h atomic weight. 15% Na+-MgO happens t o be most e f f e c t i v e among t
t h e MgO c a t a l y s t s t e s t e d here (Fig. 3). S u p e r i o r i t y o f Na
on Mn02 c a t a l y s t has
been found i n a p a t e n t ( r e f . 15), although L i has been r e p o r t e d t o be s p e c i f i c on MgO ( r e f . 3-a,
b, c ) . A l k a l i o r o t h e r element i s i n f e r r e d t o cause a
s t r u c t u r a l change o f MgO which would i n c r e a s e c o n c e n t r a t i o n o f any r a d i c a l c e n t e r on MgO i n o r d e r t o a b s t r a c t hydrogen f r o m CH4. R e l a t i o n between C2-compounds y i e l d and s p e c i f i c s u r f a c e area
I t i s known thaTMgO s i n t e r s w i t h c a l c i n a t i o n ( r e f . 16) and t h a t many oxides s i n t e r when doped w i t h a l k a l i metals ( r e f . 17, 18). The s p e c i f i c s u r f a c e area of MgO has been r e p o r t e d t o be decreased f r o m 49.9 t o 1.5 m2g-' o f Na+ ( r e f . 18) o r f r o m 60 t o 9 m2g-'
by doping 7.4 w t %
b y doping 7 w t % o f L i + ( r e f . 3-b).
I n c l u d i n g our r e s u l t s , ample a d d i t i o n o f a l k a l i t o MgO decreases t h e s p e c i f i c s u r f a c e area and increases C2-compounds formation. The r e l a t i o n between t h e s p e c i f i c s u r f a c e area and C2-compounds a c t i v i t y found over MgO system can be extended t o o t h e r systems. Although any comnents have n o t been done b y Hinsen e t al.,
t h e s p e c i f i c s u r f a c e area decreases from 87
380
381
t o 15 m‘g-’
when w t . % o f PbO i s i n c r e a s e d f r o m 6.5 t o 36 on A1203 s u p p o r t ,
whereas t h e C2-compounds s e l e c t i v i t y i n c r e a s e s f r o m 12.8 t o 56.2% ( r e f . 2 ) . Other e f f e c t i v e c a t a l y s t s a l s o have s m a l l s u r f a c e areas such as 2 m2g-’ f o r LaA103, 2 m2g-’ f o r Sm203 ( r e f s . 4-a, 6 ) , and 1 rn2g-’ f o r 1.7% Na+-10% MnOx/Si02 ( r e f . 12-b).
I n t h i s way, a c a t a l y s t which i s e f f e c t i v e f o r C2-compounds
p r o d u c t i o n has a s m a l l s p e c i f i c s u r f a c e area o f about 1-20 m2g-’. R e l a t i o n between s p e c i f i c s u r f a c e area and r e a c t i o n mechanism Supports w i t h s m a l l s p e c i f i c s u r f a c e area a r e r e p o r t e d t o be e f f e c t i v e f o r a f o r m a t i o n o f e t h y l e n e o x i d e over s i l v e r c a t a l y s t s ( r e f . 1 9 ) . T h i s case may be due t o s u p p r e s s i o n o f a complete o x i d a t i o n . However, i n t h i s r e a c t i o n oxygen consumption i s almost complete and C02 f o r m a t i o n does n o t i n c r e a s e r e m a r k a b l y w i t h t h e s u r f a c e a r e a ( F i g . 5 ) . Moreover, t h e C2-compounds y i e l d does n o t much depend upon t h e space v e l o c i t y . Thus, t h e l o w C2-compounds y i e l d a t h i g h s p e c i f i c s u r f a c e a r e a does n o t seem t o be due t o o n l y a f u r t h e r o x i d a t i o n o f C2 p r o d u c t s ( p a t h 6 ) . In accordance w i t h t h i s , ethane t o e t h y l e n e process ( p a t h 5) i s n o t promoted b y t h e i n c r e a s e o f a s p e c i f i c s u r f a c e a r e a ( F i g . 4 ) .
i -
S i n c e m e t h y l r a d i c a l s a r e observed o v e r L i -MgO ( r e f . 3-c) o r La203 ( r e f . 3d), t h e i n i t i a l s t e p s h o u l d be t h e m e t h y l r a d i c a l f o r m a t i o n ( p a t h 2 ) . I t i s c o n s i d e r e d t h a t methyl radicals can be oxidized t o CO 2 ( p a t h 3) o r or combined t o g i v e C2H6 ( p a t h 4 ) . S i n c e
co2
i s formed a t l o w e r
t e m p e r a t u r e w i t h o u t accompanying C2-compound as a r e shown i n F i g s . 1 and 2 , a d i r e c t p a t h 1 c o u l d be p r e s e n t e s p e c i a l l y o v e r t r a n s i t i o n m e t a l doped MgO. I t i s assumed t h a t p a t h e s 1, 2 , and 3 proceed o v e r a c a t a l y s t s u r f a c e (heterogeneous), w h i l e p a t h 4 proceeds i n t h e gas phase (homogeneous). The f o r m e r pathes should be improved b y a h i g h s p e c i f i c s u r f a c e area, w h i l e p a t h 4 s h o u l d n o t . Thus, t h e s u i t a b l e s p e c i f i c s u r f a c e a r e a would g i v e t h e maximum C2 y i e l d among a s e r i e s o f c a t a l y s t s depending on t h e k i n e t i c c o n d i t i o n s .
In t h e case o f MgO system, t h e
s u i t a b l e area would be so s m a l l as below 20 m2g-’.
I f t h e above mechanism i s
a p p l i c a b l e , t h i s r e a c t i o n s h o u l d be c a l l e d a heterogeneous-homogeneous r e a c t i o n ( r e f s . 20, 21). I n t h i s case, s u r f a c e morp.hology, which determines a r e l a t i v e r a t i o o f space and s u r f a c e , s h o u l d be i m p o r t a n t .
382
REFERENCES
1 G. E. K e l l e r and M. M. Bhasin, J. Catal., 73 (1982) 9-19. 2 W. Hinsen, W. B y t y n and M. Baerns, Proc. 8 t h I n t e r n . Congr. Catal., B e r l i n , 1984, 111-p.581-592. 3 ( a ) T. I t o and J. H. L u n s f o r d , N a t u r e (London), 314 (1985) 721-722; ( b ) T. Ito, J. -X. Wang, C. -H. L i n and J. H. L u n s f o r d , J. Am. Chem. SOC., 107, (1985) 5062-5068; ( c ) J. D r i s c o l l , W. M a r t i r , J. -X. Wang and J. H. L u n s f o r d , J. Am. Chem. SOC., 107 (1985) 58-63; ( d ) C. -H. L i n , K. D. Campbell, J. -X. Wang and J. H. Lunsford, J. Phys. Chem., 90 (1986) 534-537. 4 ( a ) K. Otsuka, K. J i n n o and A. Morikawa, Chem. L e t t . , (1985) 499-500; ( b ) K. Otsuka, Q. L i u , M. Hatano and A. Morikawa, Chem. L e t t . , (1986) 467-468; ( c ) K. Otsuka, Q. L i u and A. Morikawa, J. Chem. SOC. Chem. Commun., (1986) 586587; ( d ) K. Otsuka, Q. L i u , M. Hatano and A. Morikawa, Chem. L e t t . , (1986) 903-906; ( e ) K. Otsuka and T. Komatsu, Chem. L e t t . , (1987) 483-484; ( f ) K. Otsuka and T. Komatsu, J. Chem. SOC. Chem. Commun., (1987) 388-389; (9) K. Otsuka, K. J i n n o and A. Morikawa, J. Catal., 100 (1986) 353-359. 5 T. Moriyama, N. Takasaki, E. Iwamatsu and K. A i k a , Chem. L e t t . , (1986) 11651168; ( b ) K. Aika, T. Moriyama, N. Takasaki and E. Iwamatsu, J. Chem. S O C . Chem. Commun., (1986) 1210-1211; ( c ) E. Iwamatsu, T. Moriyama, N. Takasaki and K. Aika, J. Chem. SOC. Chem. Commun., (1987) 19-20. 6 H. I m a i and T. Tagawa, J. Chem. SOC. Chem. Commun., (1986) 52-53. ' 7 I.Matsuura, Y. Utsumi, M. Nakai, T. Doi, Chem. L e t t . , (1986) 1981-1984. 8 K. Asami, S. Hashimoto, T. Shikada, K. F u j i m o t o and H. Tominaga, Chem. L e t t . , (1986) 1233-1236. 9 I . T. A l i Emesh and Y. Amenomiya, J. Phys Chem., 90 (1986) 4785-4789. 10 N. Yamagata, K. Tanaka, S. Sakai, S. Okazaki, Chem. L e t t . , (1987) 81-82. 11 S. Imamura, M. I k e h a t a and S. I s h i d a , Chem. Exp., 2 (1987) 49-52. 12 ( a ) J. A. Sofranko, 3. J. Leonard and C. A. Jones, J. C a t a l . , 103 (1987) 302-310; ( b ) C. A. Jones, J. J. Leonard and J. A. Sofranko, J. Catal., 103 (1987) 311-319. 13 J. B. Kimble, J. H. K o l t s , AIChE S p r i n g Meeting, New Orleans, (1986).2 14 O t h e r p u r e MgO samples; f i r s t one (Soekawa Chemical Co2,-q9.9%, 157 m g ) and second one (Kamishima Chemical Co., 99.999%, 163 m g ) g i v e C compounds y i e l d o f 5.3 and 4.6%, r e s p e c t i v e l g a3 1023 K. These v a l z e s a r e about t h e same as t h a t o v e r t h e sample (70 m g- ) i n t h e t e x t . 15 US p a t e n t s 4,499,322; 4,499,323 (1985). 16 P. J. Anderson and D. T. L i v e y , Powder Metal., 7 (1961) 189-203. 17 C. Mougey, J. F r a n c o i s - R o s s e t t i and B. I m e l i k , "The S t r u c t u r e and P r o p e r t i e s of Porous M a t e r i a l s . " , Academic Press, New York, 1958, pp 266-292. 18 T. Matsuda, Z. Minami, Y. S h i b a t a , S. Nagano, H. M i u r a and K. Sugiyama, J. Chem. SOC. Farad. Trans., 1 , 82 (1986) 1357-1366. 19 F. Wolf and H. Goetze, Chem. Tech. ( B e r l i n ) , 14 (1962) 600-606. 20 V . V . Shalya, M. G. K u l i n i c h and M. V . Polyakov,' K i n e t i k a i K a t a l i z , 5 (1964) 916-919. 21 V . P. Latyshev and N. I. Popova, K i n e t i k a i K a t a l i z , 8 (1967) 73-78.
383
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
SYNTHESIS
OF
C2H4 BY PARTIAL OXIDATION OF CH4 OVER TRANSITION METAL O X I D E S WITH
ALKALI-CHLORIDES
K i y o s h i OTSUKA, Masaharu HATANO
and Takayuki KOMATSU
Department o f Chemical Engineering, Tokyo I n s t i t u t e o f Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
ABSTRACT The c a t a l y t i c s y n t h e s i s o f C2H4 d i r e c t l y f r o m CHq has been examined i n t h e presence of oxygen f o r many m e t a l o x i d e s c o n t a i n i n g a l k a l i m e t a l s a l t s .
Addi-
t i o n o f a l k a l i h a l i d e s o n t o m e t a l o x i d e s depressed t h e c a t a l y t i c a c t i v i t i e s o f t h e h o s t o x i d e s i n deep o x i d a t i o n o f CH4, s y n t h e s i s o f C2H4 f r o m CHQ
but favorably
enhanced t h e d i r e c t
Among t h e c a t a l y s t s tested, t h e o x i d e s o f Mn and
N i w i t h L i C l produced e t h y l e n e w i t h h i g h s e l e c t i v i t y (ca.
27%).
60%) and y i e l d (ca.
The r o l e o f a l k a l i h a l i d e 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 C2H4 was
examined f o r LiC1-added NiO.
It i s speculated t h a t the a l k a l i metals incor-
p o r a t e d i n t o h o s t m e t a l o x i d e s may i n h i b i t t h e a c t i v e c e n t e r s f o r deep oxidat i o n o f hydrocarbons.
The r o l e o f h a l i d e s i s t o g e n e r a t e c h l o r i n e atoms
t h r o u g h t h e f o r m a t i o n o f CH3C1.
The c h l o r i n e atoms may c a t a l y z e dehydrogena-
t i o n o f C2H6 i n t o C2H4 i n t h e gas phase. INTRODUCTION O x i d a t i v e c o u p l i n g o f methane i n t o ethane and e t h y l e n e (C2-compounds) has a t t r a c t e d much a t t e n t i o n s i n c e t h e p i o n e e r i n g work r e p o r t e d by K e l l e r and Bhasin ( r e f s .
1-10).
p r e f e r e n c e t o C2HQ
Usually,
t h e C2-compounds formed c o n s i s t o f C2H6 i n
The h i g h C ~ H , J : C ~ H r~a t i o i n t h e C2-products
i s o f great
advantage because no f u r t h e r process t o c o n v e r t C2H6 t o C2H4 i s r e q u i r e d . Thus, t h e f i r s t o b j e c t i n t h i s work i s t o l o o k f o r t h e c a t a l y s t s w h i c h synthes i z e C2H4 d i r e c t l y f r o m CH4 w i t h h i g h one-pass c o n v e r s i o n and y i e l d .
The
second o b j e c t i s t o d i s c u s s t h e reasons 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 C ~ H Q EXPERIMENTAL Powder m e t a l o x i d e s were used as t h e h o s t o x i d e s f o r a l k a l i m e t a l a d d i tives.
The o x i d e s w i t h s a l t s o f a l k a l i m e t a l s ( c h l o r i d e s ,
n i t r a t e s , carbo-
nates, etc.) were p r e p a r e d by i m p r e g n a t i o n method u s i n g aqueous s o l u t i o n s o f the salts.
The c o n t e n t o f t h e a l k a l i m e t a l was a d j u s t e d t o 20 mol% f o r each
384
catalyst.
The c a t a l y s t s were c a l c i n e d i n a i r a t 700
OC
f o r 2 h r b e f o r e use.
The t e s t o f t h e c a t a l y s t s was c a r r i e d o u t u s i n g a f i x e d - b e d r e a c t o r w i t h a c o n v e n t i o n a l gas f l o w system a t atmospheric p r e s s u r e u s i n g h e l i u m as a c a r r i e r The 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 u n l e s s o t h e r w i s e s t a t e d : T=750
gas. OC,
P(CH4)=0.05
atm,
P(CH4)/P(02)=2/1,
The c o n v e r s i o n o f CH4,
gl-'h.
w e i g h t o f catalyst=1.00
g.
and W/F=0.167
t h e s e l e c t i v i t i e s and t h e y i e l d s o f t h e pro-
d u c t s were c a l c u l a t e d on t h e b a s i s o f carbon numbers o f t h e CH4 reacted. RESULTS AND DISCUSSION
I.
C2-compounds.
E f f e c t o f A l k a l i S a l t s on t h e Y i e l d and S e l e c t i v i t y
Enhancing e f f e c t o f a l k a l i m e t a l s on o x i d a t i v e c o u p l i n g o f methane has been r e p o r t e d f i r s t by Hinsen e t al.
2) f o r PbO/A120?
(ref.
The p r o m o t i n g
e f f e c t o f a l k a l i m e t a l s has a l s o been observed f o r MgO ( r e f .
3).
6). Sm2O3 ( r e f . 4b), ZnO ( r e f . 8)
Among t h e a l k a l i
s a l t a d d i t i v e s tested,
and B i 2 0 3 / A 1 2 0 3 ( r e f . 9).
BaC03 ( r e f .
we have found t h a t l i t h i u m c h l o r i d e e x e r t s t h e most
f a v o r a b l e e f f e c t on t h e r e a c t i o n o v e r Sin203 s i n c e i t enhanced n o t o n l y t h e y i e l d o f C2 compounds b u t a l s o t h e r a t i o o f C2tt4/C2H6. o f L i C l was observed f o r many m e t a l o x i d e s (EueO3,
This favorable e f f e c t
LaZO3, Ce02, MgO, CaO, SiO2,
A1203, Z r O p Nb2O5, Ga2O3, Ge02, In203, Sn02. PbO. BipO3, Ti02, Mn02, FezOg, Co304, NiO, CuO
and ZnO).
V2O5,
Cr2O3,
Among t h e o x i d e s t e s t e d , t h e enhancing
e f f e c t o f L i C l on t h e y i e l d o f e t h y l e n e was most marked f o r t h e o x i d e s o f t h e f i r s t s e r i e s t r a n s i t i o n elements. 11.
E f f e c t o f A l k a l i C h l o r i d e s added t o T r a n s i t i o n M e t a l Oxides.
T a b l e 1 shows t h e r e s u l t s observed f o r t h e m e t a l o x i d e s o f t h e f i r s t s e r i e s t r a n s i t i o n elements w i t h L i C l a t a t i m e on s t r e a m 20 min.
The c a t a -
l y s t s were prepared w i t h L i C l added t o TiO2, Cr2O3, MnO2, FezO-3, C03O4. CuO, and ZnO as t h e s t a r t i n g oxides.
NiO,
The m e t a l o x i d e s w i t h o u t L i C l c a t a l y z e d
o n l y deep o x i d a t i o n o f CH4 p r o d u c i n g CO and COP
However, as can be seen i n
Table 1, h i g h c o n v e r s i o n o f CH4 and h i g h C2-yield were observed f o r t h e o x i d e s o f Ti,
Mn, Co,
N i , Cu, and Zn i n t h e presence o f LiC1.
f o r LiCl/Mn-oxide
The + y i e l d
observed
The C p - y i e l d f o r t h e L i C l / N i -
was f a i r l y h i g h (30.6%).
o x i d e i n c r e a s e d t o 28.9% when W/F was i n c r e a s e d t o 0.34 gl-'h.
The s t r i k i n g
e f f e c t o f L i C l on t h e y i e l d o f C2H4 i s shown on t h e l a s t column o f Table 1. The percentages o f C2H4 i n t h e C2-compounds t h e LiC1-added o x i d e s o f T i ,
Mn, Co, N i .
produced were h i g h e r t h a n 90% f o r
and Cu.
T a b l e 2 shows t h e e f f e c t s o f NaC1, CsC1, and v a r i o u s l i t h i u m - s a l t s on t h e r e a c t i o n o v e r NiO.
I t i s obvious t h a t a l k a l i - h a l i d e s such as LiC1.
NaCl e x e r t f a v o r a b l e e f f e c t on t h e s e l e c t i v e f o r m a t i o n o f C2Hk
L i B r and
Moreover, i t
s h o u l d be n o t e d t h a t a d d i t i o n o f any k i n d s o f a l k a l i m e t a l s a l t s improved
385
TABLE 1.
S y n t h e s i s o f CPH4 f r o m CH4 o v e r t h e L i C l / t r a n s i t i o n m e t a l o x i d e s
% CH4 conversion
Catalyst
L i C1 / T i - o x i d e L i C1 /Cr-oxide L i C1 /Mn-oxide L i C1/Fe-oxi de L i C1 /Co-oxide LiCl/Ni-oxide L i C1 /Cu-ox ide L i C1 /Zn-oxide
TABLE 2. Catalyst
17.6 26.1 47.3 22.2 48. o 25.9 49.5 22.0
% 02 conversion 40 a5 95 92 91 57
% C2Hq selectivity 77.1 0 59.4 0' 32.1 56.2 20.6 42.6
ao 49
% C2H6 selectivity 5.7 0 5.3 0 3.4 15.6' 0 26.3
% C
yieed
% C2H yiel!
14.6 0 30.6 0 17.0 18.6 10.2 15.2
13.6 0 28.1 0 15.4 14.5 10.2 9.4
E f f e c t s o f A l k a l i S a l t s on O x i d a t i v e Coupling o f CH4 o v e r Ni-oxide.
% CH4 conversion
NiO NaCl /Ni-oxide CsCl /Ni-oxide L i C1 /Ni-oxi de L i Br/Ni-oxide LiF/Ni-oxide L i OH/Ni -ox ide LiNO /Ni-oxide L i 2Cd3/Ni - o x i de
29.9 17.9 9.3 25.9 35.1 20.5 25.8 26.1 25.9
% 02 conversion a1 44 16 57 94 90 68 75 70
% C2H6 selectivity 0 20.2 26.3 56.2 43.4 12.9 23.4 23. a 26.5
0 14.5 36.8 15.6 2.9 12.9 27.9 32.0 22.4
% C
vieed
% C2H viel!
0 6.2 5.9 18.6 16.2 5.3 13.2 14.6 12.7
0 3.6 2.5 14.5 15.2 2.6 6.0 6.2 6.9
-3
Li CL I Ni 0
0.2 -2
\
A
c
0 .4-
U
1
en
-0
d
U
-0
0.1
g re
0 c
c
3
E a 200 40 0 Time on stream / min
600
O 800
Fig. 1. The changes i n t h e C2H4/C2H6 r a t i o and t h e r e l a t i v e c o n c e n t r a t i o n o f CH3C1 w i t h t i m e on s t r e a m o f r e a c t a n t s .
386
r e m a r k a b l y t h e f o r m a t i o n o f C2-compounds because o n l y deep o x i d a t i o n o c c u r r e d o v e r N i O i n t h e absence o f a l k a l i metals.
111. L i f e Time o f LiC1-added C a t a l y s t s . As d e s c r i b e d e a r l i e r , LiC1-added t r a n s i t i o n m e t a l o x i d e s have h i g h c a t a l y t i c a c t i v i t y i n c o n v e r s i o n o f CH4 d i r e c t l y t o CzH4.
However, t h e d e a c t i v a t i o n
o f c a t a l y s t s was observed f o r most o f t h e LiC1-added t r a n s i t i o n m e t a l oxides. The d e a c t i v a t i o n f o r t h e s y n t h e s i s o f C2H4 o c c u r r e d suddenly a t a t i m e on s t r e a m between 2 and 3 hours f o r t h e c a t a l y s t s w i t h 20 mol% LiC1.
The d e a c t i -
v a t i o n commenced e a r l i e r and o c c u r r e d f a s t e r as t h e c o n t e n t of L i C l was decreased. Fig. 1 shows t h e change i n t h e C2H4/C2H6 r a t i o a c c o r d i n g t o t i m e on s t r e a m f o r t h e C2-compounds produced o v e r L i C l ( 2 0 mol%)/Ni-oxide.
Analysis o f t h e
e f f l u e n t gas by mass-spectrometry showed t h e f o r m a t i o n o f CH3C1 d u r i n g reacThe r e l a t i v e amount o f t h e CH3C1 was a l s o p l o t t e d i n Fig.
tion.
1.
The
s i m i l a r t r e n d observed f o r t h e t w o curves s t r o n g l y suggests t h a t t h e CH3C1 p l a y s an i m p o r t a n t r o l e i n t h e s e l e c t i v e s y n t h e s i s o f C2H4.
IV.
The Roles o f A l k a l i M e t a l s and Halogens.
As can be seen i n Tables 1 and 2. a d d i t i o n o f any k i n d o f a l k a l i m e t a l s a l t s o n t o m e t a l o x i d e depressed t h e o r i g i n a l a c t i v i t y o f t h e o x i d e s f o r deep o x i d a t i o n o f CH4,
b u t t h e a d d i t i v e s f a v o r a b l y enhanced o r f u r n i s h e d t h e c a t a l y -
t i c a c t i v i t y f o r t h e o x i d a t i v e c o u p l i n g o f CHQ
T h i s f a v o r a b l e e f f e c t can be
a s c r i b e d t o a l k a l i m e t a l s because t h e e f f e c t was n o t c o n f i n e d t o a s p e c i a l a l k a l i metal salt.
A l k a l i m e t a l s i n c o r p o r a t e d i n t o t h e h o s t m e t a l o x i d e s may
i n h i b i t t h e a c i d c e n t e r s o r o x i d a t i o n c e n t e r s which a r e a c t i v e i n deep o x i d a t i o n o f hydrocarbons.
Moreover, t h e a l k a l i m e t a l s must c r e a t e b a s i c oxygen
s p e c i e s w h i c h can e a s i l y a b s t r a c t H f r o m CH4 g i v i n g CH3' r a d i c a l s .
A t h i g h temperatures, t h e c h l o r i n e r a d i c a l s a r e known t o c a t a l y z e t h e f o r m a t i o n s o f C2Hq f r o m CH4 i n t h e gas phase ( r e f .
11).
The o b s e r v a t i o n t h a t
o n l y a l k a l i h a l i d e s c o u l d enhance t h e c o n v e r s i o n of.CH4 s e l e c t i v e l y t o C2H4 suggests t h a t halogen r a d i c a l s s u p p l i e d f r o m t h e h a l i d e s m i g h t c o n t r i b u t e i n t h e reaction.
The r e s u l t s i n Fig.
s e l e c t i v e s y n t h e s i s o f C2Hp
1 suggested t h e r o l e o f CH3C1 f o r t h e
We have observed t h a t CH3Cl c a t a l y z e s dehyd-
r o g e n a t i o n o f C2H6 i n t o C2H4 i n t h e gas phase a t t h e t e m p e r a t u r e s above 650 The t e n t a t i v e mechanism e x p l a i n i n g t h e a c t i o n o f c h l o r i n e r a d i c a l ( f o r example) i s w r i t t e n below:
OC.
385
The CH3C1 formed i n eq. 2 must l i b e r a t e a c h l o r i n e atom i n eq. 3.
The C2H6
formed i n o x i d a t i v e c o u p l i n g o f CH4 o v e r t h e a l k a l i metal-doped s u r f a c e (eq. 1 ) Chlorine would be c o n v e r t e d q u i c k l y t o C2H4 a c c o r d i n g t o r e a c t i o n s 4 and 5. atom i s r e g e n e r a t e d t h r o u g h r e a c t i o n s 6 and 7.
Thus, t h e c h l o r i n e atoms sup-
p l i e d by a l k a l i c h l o r i d e s may c a t a l y z e t h e s y n t h e s i s o f C2H4 f r o m C2H6 d u r i n g t h e . s t a y o f f l o w i n g gases i n t h e h o t zone o f t h e r e a c t o r .
REFERENCES
G.E. K e l l e r and M.M. B h a s i n , J. C a t a l . , 73, ( 1 9 8 2 ) 9. W. H i n s e n , W. B y t y n and M. Baerns, i n Proc. 8 t h I n t . Congr. C a t a l . , 3, ( 1 984) 581. L i n and J.H. L u n s f o r d . J. Am. Chem. SOC., 107, 3 T. I t o , J.-X. Wang, C.-H. ( 1 9 8 5 ) 5062. 4 ( a ) K. O t s u k a , K. J i n n o and A. M o r i k a w a , Chem. Lett..1985, 499; ( b ) K. O t s u k a , Q. L i u , M. H a t a n o and A. M o r i k a w a , i b i d . , 1986, 467; ( c ) K. O t s u k a , Q. L i u , M. H a t a n o and A. M o r i k a w a , i b i d . , 1986, 903; ( d ) K. O t s u k a , Q. L i u and A. M o r i k a w a . J. Chem. SOC., Chem. Commun., 1986, 586; ( e ) K. O t s u k a , Q. L i u and A. M o r i k a w a , I n o r g . Chim. A c t a , 118, (1986) L23; ( f ) K. O t s u k a , K. J i n n o and A. M o r i k a w a , J. Catal., 100, (1986) 353; ( 9 ) K. O t s u k a a n d K. J i n n o , I n o r g . Chim. A c t a , 121, (1986) 237. 5 H. I m a i and T. Tagawa, J. Chem. SOC., Chem. Commun., 1986, 52. 6 T. M o r i y a m a , N. T a k a s a k i , E. I w a m a t s u and K. A i k a , Chem. L e t t . , 1986, 1165. 7 K. Asami, S. Hashimoto, T. Shikada, K. F u j i m o t o and H. Tominaga, Chem. L e t t . , 1986, 1233. 8 I. M a t s u u r a , Y. U t s u m i , N. N a k a i , and T. Doi. Chem. L e t t . , 1986, 1981. 9 I.T.A. Emesh and Y.,Amenomiya, J. Phys. Chem., 90, (1986) 4785. 10 N. Yamagata, K. Tanaka, S. S a s a k i . and S. O k a z a k i , Chem. L e t t . , 1987, 81. 11 M. Weissman and S.W. Benson, I n t e r n . J. Chem. K i n e t . , 16, ( 1 9 8 4 ) 307. 1 2
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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
389
CATALYTIC OXIDATION OF METHANE OVER AP04-5 AhQ METAL - DOPED AP04-5 John L. GARNETT, Eric M. KENNEDY,Mervyn A. LONG , Chit THAN and Ashley J. WATSON School of Chemistry , University of N.S.W. , Kensington , N.S.W. , 2033 , Australia. ABSTRACT The catalytic oxidation of methane over APo4-5 and metal - doped A1P04-5 has been studied over a range of temperatures (500 - 9Oo'C) and 0 2 : CH4 ratios .The major products of the reaction were ethane , ethene , C02 and 1320. The product distribution was found to be dependent on both temperature and 0 2 : CH4 . Doping of AlP04-5 with various metals ( Pt , Pb , V , Ag , Co , Cu ) altered the product distribution , with PbAlP04-5 the most reactive and selective supported metal catalyst for methane oxidation. INTRODUCTION The oxidative dehydrogenation of methane to ethane and ethene (CH4 + 0 2 ----> C2's + H20 ) shows great potential for methane utilization if the selectivity is controlled
t. The use of
a catalyst to control the kinetics of the
reaction has been examined, with some success. Otsuka et a1 (1) reported the catalysis of methane oxidation over 30 metal oxides, including many rare earth oxides. Very high selectivities were reported ( for Sm2O3 ), within a range of reactivities*. Keller & Bhasin (2) reported the oxidation of methane over 23 metal oxides. Selectivitiesof up to 50% were achieved. The more efficient catalysts were the oxides of Pb,Bi, Sb, Sn, TI, Mn & Cd . Lithium doped MgO has been reported by Ho and
Lunsford (3) to catalyse methane oxidation with high selectivity and high reactivity .Imai & Tagawa (4) reported high selectivities and reactivities using the mixed metal oxide LaAlO3. Pb02 supported on Si% was examined by Hinsen & Baerns (S), and high selectivities were reported. Anderson and Tsai (6) studied the partial oxidation of nethane using N20 and 0 2 as oxidants. With 0 2 as oxidant, all catalysts produced no appreciable yield of either ethene or ethane, CO and C02 being the only products detected. In 1982, Wilson et al(10) reported the discovery of the aluminophosphate(AlPO4-X) molecular sieves. These are considered to be composed of Al02- and P02+ units, resulting in no nett framework charge over the crystal. Thus, unlike zeolites, AlPOs do not ( at least ideally ) contain intracrystallineacid sites. Here we report the results of methane oxidation catalysed by metals supported on AlP04-5. The metals supported were Pt, Ag, Pb, Cu, Co & V.
In this context the selectivity is defmed as the yield of C2 hydrocarbons (rather than other oxygenated products such as C02 ) expressed as a percentage of methane reacted
*
Reactivity is defined as the yield of all products of the methane oxidation as a percentage of the initial methane
concentration
390
EXPERIMENTAL
- see Figure 1 catalyst
thermocouple
Figure 1-Schematic of the reactor Purified LH4 , u2 & N2 were red tnrough pressure control valves and needle valves. The gas mixture was then fed into a small ( 4mm i.d. ) quartz tube reactor. The reactor contained between 0.1 and 0.2 g of catalyst, and was situated inside another section of quartz tubing (12mm Ld.) which was wound with nichrome wire and insulated. The catalyst temperature was monitored by a thermocouple placed between the two quartz tubes. The products of the conversion were analysed by a Varian 3700 gas chromatograph fitted with TCD and FID detectors. The output of both detectors was fed into an apple
112
computer for integration and data storage. Prenara AlPO4-5 was prepared as per the literature (1 1). X-ray powder patterns of the as synthesised A p 0 4 - 5 was virtually identical to that reported by Bennett et al(12). Cu, Co, Pb and Ag loaded AlP04-5 was prepared by the "incipient wetness technique", impregnating the catalyst with an aqueous solution of the corresponding metal nitrate, followed by drying at 100 'C for 24 hours. The metal AlPO was then calcined at 350 'C in air. Pt AlPo4-5 was prepared as above, except that the salt used was Na2PtCl4. V AlPo4-5 was prepared from an aqueous solution of VCl3. Metal content in all catalysts was 3-5 wt%.
RESULTS The reactivity and selectivity of A P 0 4 - 5 and the metal doped AlPOs are presented in figures 2 to 7. The effect of temperature and 02: CH4 ratio on the oxidative coupling of methane has been examined in this study. In all experiments, the major products of the methane oxidation were COz, ethene, ethane, and H20. Propane, CO and formaldehyde were found occasionally in trace amounts.
391
Figures 2 to 8 , me effect of %02 and Temperature on the PYtia1 oxidation Of melhvle caulysed by APO4 - 5 and metal-loaded AlPO4 -5
1 1
reactivity
t
selectivity
I
Figure 2 - AIPO, -5
t
reactiwty
I
t I
1
se’er Figure 4 - V A l p 0 4 - 5
reaciiviy
t
Figure 5 - CU AIPO,
-5
reactivity
1
1
t
““ciii‘“ Figure 8 - Ag A l p 0 4
-
Temperature scale, 500 10 9Oo’C ; Oxygen concenuadon scale, 0 to 25 70
392
Catalvtic Activity (1). NPO4-5. At temperatures below 700 'C ,the reactivity (figure 2) of A1po4-5 is very low
( <1%),
even at high oxygen concenaations. From 700 - 850 'C there is a substantial increase in reactivity , with maximum activity ( 25%) at high oxygen concentration. Even at low [ 0 2 ] , there is a substantial reaction ( 2 - 3 4 ) , Selectivity generally decreases as [ 0 2 ] increases, as shown in figure 2(b). At high conversions, selectivities of 10% is achieved while at low reactivity, selectivity of >50% can be attained. Above 850 'C, reactivity tends to decrease or stabilize, and selectivity also decreases. (2). Metal AIPO's. The effect of metal loading on APO's in the conversion of methane is presented in
figures 3 to 8. Pt and V AlPOs showed greatly increased reactivity ( figures 3 and 4 respectively). The selectivity, however, decreased dramatically, with Pt A p 0 4 - 5 producing only trace amounts of C2's at any temperature or 0 2 : CH4 ratio. Selectivity of V AlP04-5 was very poor, although C2's were produced in small quantities at low 0 2 : CH4 ratios. The results of Cu U 0 4 - 5 oxidation are presented in figure 5. Very little change is observed in
either selectivity or reactivity, suggesting the metal loading did not affect the overall product distribution. Co AlP04-5
( figure
6 ), does appear to differ from A P 0 4 - 5 in that a slight increase in reactivity was observed.
Although this change is quite small ( 2% overall) it is probable that the metal affects the methane oxidation conversion. The metals which enhanced both selectivity and reactivity were Pb and Ag. Pb A P 0 4 - 5 ( figure 7 ) increased selectivity by approximately 10% overall, while the reactivity of Pb A P 0 4 - 5 increased slightly ( 5%). A: AlP04-5 ( figure 8 ) affected the reactivity of the CH4 oxidation resulting in an increased selectivity, while reactivity was possibly increased by 2%. A summary of the effect of each metal on both selectivity and reactivity is presented in tablel.
TABLE 1 Effect of metal loading on ALPo4-5 on the partial dxidation of CH4
Metal
Selectivity
Reactivity
Pt
large decrease
large increase
V
large decrease
increase
cu
little change
little change
little change
slight increase
Ag
increase
slight increase
Pb
increase
increase
co
393
DISCUSSION The catalytic activity of AP04-5 for methane oxidation is not readily expected, due to the support's absence of Bronsted acid sites (13). One explanation of the activity is presented, by analogy with studies of amorphous AlPO4. Campelo et al(14) studied the acid and base sites of amorphous AlPO4 and reported a correlation between surface basicity of the solid and their activity for catalysis. Moffat et al(15) have shown from quantum chemical calculations that Lewis acid and base sites exist in amorphous AIpO4. The importance of basic properties of the catalyst was postulated ( l ) , where the catalysts most active for methane oxidation were the metal oxides with some basic or
amphoteric properties . These observations would suggest that basic sites are necessary for oxidative coupling of
methane , and these active sites may be present in porous aluminophosphates. The nature of the site is, however, not clear. The metaYAPO4-5 catalysts examined in these studies all catalyse methane oxidation, the metal affecting the product distribution to varying degrees. Pt and V APOs both show high reactivity for reaction but greatly decreased selectivity. Ag and Pb
Alp0
enhanced both selectivity and reactivity of the oxidation while Co and Cu AlPO had
little effect on the activity of the AlP04-5 oxidation of methane. Studies are being undertaken to measure the catalytic activity of other metals supported on AlP04-5. Of particular interest will be a study of the use of ALeo's to support the active metal oxide catalysts reported elsewhere (1-5). ACKNOWLEDGEMENTS The authors wish to thank The Ausealian Institute of Nuclear Science and Engineering and The Australian Research Grants Scheme for their support. REFERENCES Otsuka, K . , Jinno, K. , Morikawa, A. , J.Cata1, 100 , 1986, pp. 354. Keller, G. E. , Bhasin, M. M. , J.Cata1, 73, 1982, pp. 9. Ho, T., Lunsford, J. H . , Nature, 314, 1985, pp. 721. Imai, H . , Tagawa, T . , J. Chem. SOC.,Chem. Commun.. 1986, pp. 52. 5 . Hinsen,W., Baerns, M. , Chem. Zfg. 107, 1983, pp. 223. 6. Anderson, J. R . , Tsai, P. , Appl. Catal. 1 9 , 1985, pp. 141. 7. Takahashi, N. , Saito , M. , Nagumo, M and Mijin, A. , Zeolites, 6, 1986, pp. 412. 8. Rudham, R . , Sanders, M. K. , J.Catal., 2 7 , 1972, pp. 287. 9. Cullis, C. F., Keene , D. E . , Trimm, D. L . , J.Catnl. , 1 9 , 1970, pp. 378. 10. Wilson, S.T., Lok ,B .M., Messina, C .A., Cannan, T.R., Flanigen, E. M. J. Am. Chem. SOC. , 104, 1982, pp.1,146. 11. US. Patent #4,310,440. 12. Bennett, J. M., Dytrych, W J., Pluth, J .J ., Richardson,Jr , J.Smith, J. V. Zeolites. 6 , 1986, pp. 349. 13. Barrer, R .M. , Pure & Appl. Chem., 5 8 , 1986, pp. 1317. 14. Campelo,J. M., Garcia, A . , Luna ,D., Marinas ,J.M. , J. Catal., 102, 1986, pp.299. 15. Moffat, J.B., Vetrivel, R., Viswanathan, B., 3 0 , J.Mol.Caral., 1985, pp171. 1. 2. 3. 4.
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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
395
THE PRODUCTION OF LIQUID FUELS VIA THE CATALYTIC OXIDATIVE COUPLING OF METHANE
J.H. EDWARDS and R.J. TYLER C S I R O D i v i s i o n o f F o s s i l F u e l s , P.O.
Box 136, N o r t h Ryde, NSW 2113 ( A u s t r a l i a )
ABSTRACT The o x i d a t i v e coup1 i n g o f methane o v e r a l i t h i u m - p r o m o t e d magnesium o x i d e c a t a l y s t was s t u d i e d i n a small f i x e d - b e d r e a c t o r a t 770°C. Methane c o n v e r s i o n i n c r e a s e d w i t h oxygen l e v e l i n t h e f e e d gas and gas c o n t a c t t i m e b u t a t t h e expense o f reduced s e l e c t i v i t y t o C2+ hydrocarbons. Nonetheless, w i t h a f e e d gas c o n t a i n i n g 9.4 v o l % oxygen, 11%methane c o n v e r s i o n and 76% hydrocarbon s e l e c t i v i t y were achieved. The h i g h e x o t h e r m i c i t y o f t h e o x i d a t i v e c o u p l i n g r e a c t i o n s would r e q u i r e a l a r g e - s c a l e f i x e d - b e d r e a c t o r t o be o p e r a t e d as a m u l t i - b e d u n i t w i t h staged oxygen a d d i t i o n . A l t e r n a t i v e l y , i f t h e process i s conducted i n a s i n g l e s t e p somc form o f f l u i d i s e d - b e d r e a c t o r i s necessary. INTRODUCTION A u s t r a l i a has l a r g e , uncommitted r e s e r v e s o f n a t u r a l gas and t h e r e i s t h u s c o n s i d e r a b l e i n c e n t i v e f o r c o n v e r t i n g methane, t h e m a j o r component o f n a t u r a l gas, i n t o t r a n s p o r t t u e l s t o supplement t h e p r e d i c t e d s h o r t f a l l i n domestic crude o i l p r o d u c t i o n i n t h e 1990's. E x i s t i n g r o u t e s f o r methane c o n v e r s i o n ( F i s c h e r - T r o p s c h and M o b i l methanolt o - g a s o l i n e processes) r e q u i r e , as t h e f i r s t s t e p , t h e p r o d u c t i o n o f s y n t h e s i s gas (CO and H2) f r o m methane by t h e c o s t l y and i n e f f i c i e n t process o f steam reforming.
These r o u t e s a r e nonetheless accepted t e c h n o l o g i e s s i n c e t o d a t e i t
has n o t been p o s s i b l e t o achieve l a r g e - s c a l e c o n v e r s i o n o f methane d i r e c t l y t o h i g h e r hydrocarbons. R e c e n t l y I t o and L u n s f o r d Cref.11 have shown t h a t ethane and e t h y ene can be o b t a i n e d d i r e c t l y f r o m methane by o x i d a t i v e c o u p l i n g (OXCO) u s i n g a promoted magnesium o x i d e c a t a l y s t .
ithium-
C S I R O , i n c o l l a b o r a t i o n w i t h The Broken
H i l l P r o p r i e t a r y Company L i m i t e d (BHP), i s c o n d u c t i n g r e s e a r c h on ox d a t i v e
c o u p l i n g t o determine t h e f e a s i b i l i t y o f u s i n g t h i s t e c h n i q u e i n t h e commercial p r o d u c t i o n o f l i q u i d f u e l s from n a t u r a l gas. T h i s paper o u t l i n e s t h e OXCO c h e m i s t r y and process concept and p r e s e n t s methane c o n v e r s i o n and p r o d u c t s e l e c t i v i t y d a t a o b t a i n e d i n a s m a l l - s c a l e fixed-bed reactor.
The i m p l i c a t i o n s o f t h e s e r e s u l t s f o r t h e d e s i g n o f a
l a r g e - s c a l e OXCO r e a c t o r a r e t h e n b r i e f l y discussed.
396
Xl HAKE OXCO CHEl4ISTRY AluD PROCESS CONCEPT When methane anti oxygen a r t ' passed o v e r a l i t h i u m - p r o m o t e d magnesium o x i d e
catalyst
dt
7SU-8Ou"C
s i g n i f i c a n t y i e l d s o f ethane and e t h y l e n e a r e o b t a i n e d .
The i n i t i a l s t e p i s t h o u g h t by I t o e t a l . Cref.21 t o be t h e f o r m a t i o n o f methyl r a d i c d l s by hydrogen a b s t r a c t i o n froiii methane a t thermal l y - g e n e r a t e d Li'Os i t e s on t h e c a t a l y s t .
Ethane i s forrned by t h e r e - c o m b i n a t i o n o f tiro niethyl
r a d i c a l s w h i l s t e t h y l t n e i s d e r i v e o frorii ethane by a f u r t h e r p a r t i a l o x i d a t i o n r e a c t i o n o r by p y r o l y s i s . sites.
Oxygen i s r e q u i r e d t o r e g e n e r a t e t h e Li'O-
active
The d e s i r e d r e a c t i o n s can be summarised t h u s : AH250C
(kJ mol-'1
C2H6
+
C2H4 + H2
-177
(1)
+137
(3)
Undesired o x i d a t i o n o f methane t o carbon o x i d e s and w a t e r a l s o o c c u r s : CH4
+
1.502
+
CO + 2H20 ( g )
(4)
-520
The conceptual process f o r t h e p r o d u c t i o n o f l i q u i d f u e l s from methane by t h e OXCO r o u t e i s shown i n F i g . 1.
I n a d d i t i o n t o t h e OXCO r e a c t o r i t i n c l u d e s
f a c i l i t i e s f o r removal o f r e a c t i o n b y p r o d u c t s , r e c o v e r y o f t h e C2+ hydrocarbons and t h e p r o d u c t i o n o f l i q u i d f u e l s by e t h y l e n e o l i g o m e r i s a t i o n .
'
co2
H20
oxco
ML -v
REACTOR
GAS TREATMENT
-
C2+ RECOVERY
+
AIR SEPARAl"
c2+ CONVERSION
LlOUlD FUELS
*
1
F i g . 1.
Process c o n c e p t for methane c o n v e r s i o n by o x i d a t i v e c o u p l i n g .
397
(Iwing t o chemical c o n s t r a i n t s , t h e methane c o n v e r s i o n p r i o r t o C2+ hyorocarbon r e c o v e r y i s l i k e l y t o be r e l a t i v e l y l o w (10-30%).
It w i l l
t h e r e f o r e be necessary t o r e c y c l e t h e unconverted methane t o t h e OXCO r e a c t o r ana consequently oxysen r a t h e r t h a n a i r w i l l most l i k e l y be r e q u i r e d as o x i d a n t t o avoici a i l u t i n g t h e uriconvertea methane w i t h l a r g e q u a n t i t i e s o f n i t r o g e n . EXPERINENTAL RESULTS The 0XCU r e d c t i o n s a r e b e i n g s t u a i e a u s i n g s e v e r a l l a b o r a r o r y - s c a l e r e a c t o r s t o deterinine t h e i n f l u e n c e o f process v a r i a b l e s sucn as oxygen l e v e l , r e s i d e n c e t i m e , temperature ana p r e s s u r e on methane c o n v e r s i o n ana s e l e c t i v i t y t o proaucts.
Iklethane c o n v e r s i o n i s d e f i n e a as t h e percentage o f i n p u t methane
c o n v e r t e a t o t o t a l p r o d u c t s , ancl s e l e c t i v i t y as t h e amount o f methane c o n v e r t e d t o a p a r t i c u l a r p r o d u c t expressed as a percentage o f t h e t o t a l methane converted.
The r e s u l t s presentea here were o b t a i n e a i n a f i x e d - b e d r e a c t o r
(20 mil i . a .
x 39 mm; 17.3 g c a t a l y s t ) which was o p e r a t e d a t 77OoC and
atmospheric p r e s s u r e . lblethdne c o n v e r s i o n , oxygen consumption and p r o d u c t s e l e c t i v i t i e s a r e shown as f u n c t i o n s o f t h e pseudo-contact t i m e (W/F) f o r two oxygen l e v e l s i n F i g s . 2a
(1.1 volX 0 2 ) and 2b (9.4 volX 0 2 ) .
The methane c o n t e n t o f t h e f e e d gas was 90
v o l % w i t h any b a l a n c e b e i n g n i t r o g e n . a i v i a e d by t h e f e e d gas f l o w r a t e (ml s-'
W/F i s t h e w e i g h t o f c a t a l y s t ( g l a t o p e r a t i n g c o n d i t i o n s ) and i s equal
t o t h e nominal gas r e s i d e n c e t i m e ( s ) when t h e b u l k d e n s i t y o f t h e c a t a l y s t i s
1
~"~-3. W i t h a r e a c t a n t f e e d gas c o n t a i n i n g 1.1 v o l % 02, methane c o n v e r s i o n ( F i g .
2a) i n i t i a l l y i n c r e a s e d w i t h i n c r e a s i n g W/F b u t l e v e l l e d o u t a t 3% and t h e oxygen consuinption approached 100% a t W/F = 1.4.
A notable r e s u l t i s the high
s e l e c t i v i t y t o C2+ hydrocarbons (94% a t W/F = 0.3) which d i d n o t d e c l i n e w i t h i n c r e a s e i n W/F.
The C2+ hydrocarbons were p r e d o m i n a n t l y ethane and e t h y l e n e ,
w i t h p r o p y l e n e b e i n g t h e m a j o r component o f t h e C3+ f r a c t i o n .
The s e l e c t i v i t y
t o carbon o x i d e s ( m a i n l y C02) was l o w ana e f f e c t i v e l y independent o f W/F.
An
i m p o r t a n t b e n e f i t g a i n e d by i n c r e a s i n g W/F was t h e s h i f t i n t h e e t h y l e n e / e t h a n e d i s t r i b u t i o n i n favour o f the desired product, ethylene ( t h e r a t i o increased f r o m 0.21 a t W/F = 0.3 t o 0.63 a t W/F = 2.0). W i t h a f e e d gas c o n t a i n i n g 9.4 v o l % O2 (Fig.2b1,
W/F had s i m i l a r , b u t more
pronounced, e f f e c t s on t h e r e a c t o r performance t o t h o s e a t t h e l o w e r oxygen l e v e l (Fig.2a).
Over t h e range o f W/F v a l u e s s t u d i e d (0.25-1.551,
t h e methane
c o n v e r s i o n more t h a n doubled, r i s i n g f r o m 5 t o 11%, as t h e oxygen consumption i n c r e a s e d f r o m 27 t o 78%.
A l t h o u g h t h e d e c l i n e i n C2+ hydrocarbon s e l e c t i v i t y
w i t h i n c r e a s i n g W/F i s s i g n i f i c a n t , t h e s e l e c t i v i t y o f 76% a t W/F = 1.55 i s s t i l l o f practical significance.
The l o s s i n C2+ hydrocarbon s e l e c t i v i t y i s
398
-
2 z
-
75
g 5 a
v)
-
50
z
8
N
- 25 1
0
0
TOTAL HC's
80 C2'S
60-
3
F
&O-
v)
20 -
-co
OO
1.0 WIF(g ml-ls)
,
2.0
F i g . 2. I n f l u e n c e o f pseudo-contact t i m e on methane c o n v e r s i o n , oxygen consurnption and p r o d u c t s e l e c t i v i t i e s a t 770°C. ( a ) Feed gas 1.1%02. 90% CH4 ( b ) Feed gas 9.4% 02, 90% CH4. r e f l e c t e d i n a c o r r e s p o n a i n g i n c r e a s e i n t h e f o r m a t i o n o f carbon o x i d e s , w i t h C02 a g a i n b e i n g t h e m a j o r component.
A t t h e h i g h e r oxygen l e v e l i n t h e f e e d gas (where 100% oxygen consumption i s n o t a c h i e v e d ) , W/F had a marked e f f e c t on t h e e t h y l e n e / e t h a n e s e l e c t i v i t y r a t i o , i n c r e a s i n g i t from 0.56 t o 1.5.
The s h i f t towards e t h y l e n e a t h i g h W/F
i s presumably t h e r e s u l t o f r e a c t i o n s r e p r e s e n t e d o v e r a l l by t h e p a r t i a l o x i d a t i o n o f ethane ( r e a c t i o n 2 ) .
However, i t s h o u l d be n o t e d t h a t i n s i m i l a r
experiments w i t h t h i s r e a c t o r , s u b s t a n t i a l amounts o f hydrogen (up t o 20% o f t h e hydrogen r e l e a s e d by t h e c o n v e r t e d methane) have been measured i n t h e p r o d u c t gas, i n d i c a t i n g t h a t t h e ethane p y r o l y s i s r e a c t i o n ( r e a c t i o n 3 ) may a l s o be o c c u r r i n g t o a s i g n i f i c a n t e x t e n t . The r e s u l t s i n F i g . 2 i n d i c a t e t h a t t h e oxygen l e v e l i n t h e f e e d gas i s an i m p o r t a n t v a r i a b l e i n t h e performance o f t h e f i x e d - b e d r e a c t o r .
This i s
f u r t h e r demonstrated i n F i g . 3 which shows, a t a f i x e d W/F v a l u e o f 1.5,
the
e f f e c t o f oxygen l e v e l (between 1.1 and 9.4 v o l % 02) on methane c o n v e r s i o n , oxygen consumption and s e l e c t i v i t y .
The i n c r e a s e d methane c o n v e r s i o n and
e t h y l e n e / e t h a n e r a t i o achieved a t t h e h i g h e r oxygen l e v e l s a r e o b t a i n e d a t t h e
399
8
15
Y
lo
B
2
8
*
5 5 0 100
-
80
2 60 >
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
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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.
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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 (<850°C), CHI, conversions (100C reacted/C in methane) of <5X were observed. Increasing the reaction temperature resulted in an increase of activity. A t 940°C the free r a d i c a l gaseous phase reaction was dominant. A 5OZ methane conversion with 65% s e l e c t i v i t y t o CO (lOO*C i n CO/C reacted) was observed. 2. Activity of various Li loaded MgO catalysts (loo* Li/Li + M g , w t basis) Blank &catalyst ,(I resulted in 19% conversion of methane with 38% selecti v i t y t o C2. 'Ihe presence of Li resulted i n a s i m i f i c a n t increase i n selectivi t y t o C2 (>80%), (Fig. l), i n agreement with previous wxkers (ref. 3). A t higher Li loading the overall a c t i v i t y decreased due t o the loss of surface area from Li$03 sintering, but the selectivity t o C2 remained stable a t around 80%. me catalyst i s activated by a Ii loading of 0.1 wt%. 3. Effect of different lithiun source on a c t i v i t y of 6% Li/MgO catalyst As
illustrated i n Fig.
2 doping with L j - 2 ~ 0 , o~ Lj-OH salts resulted in catalyst with similar a c t i v i t y with -20% methane conversion and -70% C 2 select-
411 80
,
1
A1203 I-
K
B
3
rn
Ti02
4 con. c2 C2H4
. .
Si02
6
Ca(OH)2
MgO
L12CO3 LlOH
LlCi Li2SO4
0
20
40
60
80
100
UNIT IN PERCENTAGE
LITHIUMSALT
Fig. 2. CtL, conversion and C2 selectivity of Li/MgO catalyst systems a t 800" C, as a function of Li salt.
Fig. 3. CH, conversion and C, s e l e c t i v i t y of wt% Li on different support.
i v i t y . Catalysts derived from L i C l salts on MgO a l s o have high conversion, however the s e l e c t i v i t y t o C2 was lower. Li2S0, derived Eatalyst systems had
low a c t i v i t y , while IiF derived catalyst systems were inactive. 4. Activity of various lithiun-doped supports The a c t i v i t y of w ( O H ) 2 and of QO based catalysts is similar, as s h m i n Fig. 3, with 20-25% conversion and with -7CE C2 selectivity. had a s l i g h t l y lower a c t i v i t y (-l5%), while the SiO, and Ti02 based catalysts have low a c t i v i t y (<5%) a t 800°C. Although the y A1203 based catalyst has high a c t i v i t y (21%) the s e l e c t i v i t y t o C2 i s only half that of the QO based catalyst. 5. Effect of doping 6% Li/MgO with a transition metal The effects of doping the Li/MgO catalyst with 2 mol % transition metal rela t i v e t o lithiun (100%(M
+
Li) m o l basis) is sham i n Fig. 4. 'Ihe presence of
t h e transition metal generally, with chromium being the exception, increased
In terms of C2 s e l e c t i v i t y manganese, iron and cobalt were most effective. A tenperature dependent study showed that the I% doped catalyst had comparable a c t i v i t y t o an undoped Li/NgO catalyst a t 50°C lower tenperature. 'Ihe effect of C o on a c t i v i t y of k / L i M g O catalyst was also determined. A s the p a r t i a l pressure of CO i n the reactant mixture was increased t h e conversion of methane decreased and the s e l e c t i v i t y t o C2 increased (Fig. t h e CHI,
conversion a t 800°C.
5).
DISCUSSION The present study has sham t h a t the e f f e c t of doping MgO and A 1 2 0 3 materi a l s with Ii is t o increase the s e l e c t i v i t y t o
C2
while not significantly
412 100
Zn
g W
A
cu
wfJJ
i!J
4 I-
NI
260
B U
W
z c0 2 t Fe v) z a Mn K I-
w
a
z t 5
Cr
40 20
Blank
.
0
20
40
60
80
0
100
l
10
.
,
.
20
l
.
30
,
40
.
,
50
.
60
PARTIAL PRESSURE OF CO (Torr)
UNIT IN PERCENTAGE Fig. 4. C% conversion and C2 s e l e c t i v i t y of M/Li/MgO for various dopants a t 800°C.
Fig. 5. Dependence of CH, conversion and C ' 2 s e l e c t i v i t y on the p a r t i a l pressure of CO (100 T 02' 190 T (X,) a t 800°C using h / I . i / M g O catalyst.
affecting the methane conversion. Loadings of 0.1 wt% (Fig. 1) are sufficient t o enforce the selectivity t o Cp. "he Selectivity t o CO over fresh chemical was found t o increase with the acidity of the surface (Fig. 6). These surfaces are likely t o generate intermediate species which preferentially oxidise t o carbon oxide. The data i n Fig. 2 and Fig. 6 a l s o support the view that acidic sites are not primarily responsible for the cleavage of H-CH3 band of methane t o form methyl radicals since the GL, conversion is higher i n general for the mre basic oxides. The presence of transition metal oxide on the surface increases the the concentration of surface oxygen and hence the oxidative reaction is f a c i l i t ated. Since chromiun oxide is a very good oxidation catalyst it r e s u l t s i n the f u l l oxidation of
the hydrocarbons
t o carbon dioxide
(Fig.
4) although
conversion i s low. Other experiments not reported here shawed that the presence of 2 mole % Mn was found t o optimise the performance of 6% Li/MgO. When CO was present i n the reactant gas it w a s preferentially oxidised t o C Q . As the p a r t i a l pressure of CO was increased methane conversion decreases while selectivity t o C2 increases but no significant difference i n product distribution was observed. This result indicates that the oxidation of CO only competes for
surface oxygen sites with
the oxidative dehydrogenation of
methane. Iess oxygen is available for the full oxidation of product and hence t h e s e l e c t i v i t y t o C2 increased.
413
A12031S102
-I
a 0 r
A1203 TI02
H %CH4con.
0
20
40
60
80
UNIT IN PERCENTAGE
Fig. 6. Ctlr, conversion and C, selectivity of fresh undoped chemical compounds. CONCLUSION
The oxidative dehydrogenation reactions over these catalysts are similar t o t h e gas phase r e s u l t of shock tube experiments determined by Skinner e t al. (ref. 6).
This observation supports the fact that the recombination reactions
of methyl radicals i n the gaseous phase are an important source of ethane and t h a t the ethene is a secondary product derived from ethane. This secondary reaction proceeds i n the gaseous phase a s well as the catalyst surface. The major r o l e of the MgO surface is t o produce the methyl radical efficiently. The active sites for cleaving the H-CH3 band should be moderated by Li t o enforce selectivity. In addition t o gas phase oxidatim, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for
C2
acidic materials. RWERL?"ES S k i l l , E.F. Westm Jr. and G.C. Sinke, The Chemical Thermodynamics of Organic Conpounds, IJiley, New York, 1969. G.E. Keller and M.H. Basin, J. Catal., 73 (1982) 9-19. T. Ito, J.X. Wang, C.H. Lin and J.H. Zllnsford, J. Am. C h a . Soc., 107 (1985) 5062-5068. E Iwamatsu, T. briyama, N. Takasaki, and K. Aita, J. Chem. Soc., Chern. C O ~ ,. (1987) 19-20. K. Aika, T. b r i y a n a , N. Takasaki and E. Iwamatsu, J. Chem. Soc., Chem. Corn., (1986) 1210-1211. G.B. Skinner and R.A. Ruehrwein, J. Phys. Chm., 63(2) (1959) 1736-1742.
1 D.R. 2 3 4 5 6
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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
415
EFFECT 9F O3 VERSUS O2 AS OXIJANTS FOR NETHAPIE
G.J. H u t c h i n g s ' , I4.S.
Scurrell
2
and J.R.
Woodhouse
'Department o f Chemistry, U n i v e r s i t y c f Avenue, Johannesburs, 2001 South A f r i c a .
the
1 Witwatersrand, 1 Jan Smuts
P 0 Box 395, P r e t o r i a , 0001 South A f r i c a
'NICER-CSIR,
ABSTRACT A comparative s t u d y o f ozone and oxygen as o x i d a n t s f o r methane c o n v e r s i o n i s r e p o r t e d u s i n g 5% Li/FlgO, MgO and Y A t 0, as c a t a l y s t s . Ozone i s found t o be a more a c t i v e o x i d i s i n g agent a t t4C0"C2 and i s more s e l e c t i v e th,an oxygen w i t h a l l c a t a l y s t s . The p r o d u c t s under t h e s e c o n d i t i o n s a r e o n l y hydrogen, carbon d i o x i d e and carbon monoxide ( w a t e r was n o t d e t e c t a b l e ) and t h e a d d i t i o n o f water vapour (methane/water = 1 mole r a t i o ) d i d n o t s i g n i f i c a n t l y a f f e c t p r o d u c t s e l e c t i v i t y . A t h i g h e r t e m p e r a t u r e s ozone and oxygen gave s i m i l a r a c t i v i t i e s and s e l e c t i v i t i e s b u t t h e c a t a l y s t s s t e a d i l y l o s t a c t i v i t y due t o s i n t e r i n g / r e c r y s t a l 1i z a t i o n . INTRODUCTION
The o x i d a t i v e c o u p l i n g o f methane t o f o r m ethene and ethane i s c u r r e n t l y t h e s u b j e c t o f c o n s i d e r a b l e r e s e a r c h i n t e r e s t . Several m e t a l o x i d e c a t a l y s t s (1,Z) have been i d e n t i f i e d as a c t i v e and s e l e c t i v e f o r t h i s r e a c t i o n .
Particular
a t t e n t i o n has been g i v e n t o MgG based c a t a l y s t s promoted by a l k a l i m e t a l s (3-5) and r a r e e a r t h o x i d e s (6-8). These c a t a l y s t s have g i v e n C2 s e l e c t i v i t i e s o f up to
5 70
mole % u s i n g m o l e c u l a r oxygen as o x i d a n t .
A t p r e s e n t most work has
been d i r e c t e d a t t h e i d e n t i f i c a t i o n o f s e l e c t i v e c a t a l y s t s w i t h m o l e c u l a r oxygen;
the
use
of
alternative
oxidising
i n f o r m a t i o n has r e c e i v e d l i m i t e d a t t e n t i o n .
agents
to
obtain
mechanistic
The i d e n t i f i c a t i o n o f s e l e c t i v e
oxygen species f o r e i t h e r gas phase o r s u r f a c e r e a c t i o n s o f methane i s o f crucial
importance i n t h e d e s i g n o f s u i t a b l e c a t a l y s t s f o r t h i s r e a c t i o n .
Lunsford
(4)
subsequently
has
observed
dimerise
to
that
methyl
ethane,
radicals
ethene
are
being
formed
formed
which by
could
oxidative
dehydrogenation. More r e c e n t l y (9) s t u d i e s o f N-H and C-H a c t i v a t i o n u s i n g N20 2s
o x i d a n t have i n d i c a t e d t h a t b o t h methylene and methyl s p e c i e s a r e formed and
O F s ) has been p o s t u l a t e d as t h e s e l e c t i v e o x i d i s i n g species.
E a r l i e r studies
( 1 0 ) have shown t h a t ozone could, by v i r t u e o f i t s enhanced o x i d i s i n g a c t i v i t y , be a c t i v e f o r methane o x i d a t i o n a t t e m p e r a t u r e s l o w e r t h a n ,those observed w i t h molecular
oxygen
( t y p i c a l l y > 500°C).
Recent
studies
(11 )
i n d i c a t e d t h a t Ozone d i d n o t demonstrate enhanced a c t i v i t y ,
have,
however,
but definitive
416
c o m p a r a t i v e s t u d i e s have n o t y e t been r e p o r t e d .
I n t h i s paper we p r e s e n t a
comparative s t u d y o f ozone and oxygen as o x i d a n t s f o r methane c o n v e r s i o n u s i n g 57; Li/ClgO, MgO and v A e 2 0 3 as c a t a l y s t s .
EX P E R IF1ENT AL
tlg0 was prepared as p r e v i o u s l y d e s c r i b e d
( 1 2 ) and 5%Li/Mg0. was p r e p a r e d
by t h e method o f L u n s f o r d ( 4 ) . Y AP203 (Strem) was used as s u p p l i e d . C a t a l y s t s (450"C, l h , were p e l l e t e d (0.5 mm p a r t i c l e s ) and p r e t r e a t e d w i t h O 2 GHSV
=
4 5 0 h - I ) p r i o r t o c a t a l y s t t e s t i n g . Methane o x i d a t i o n was c a r r i e d o u t a t
85 kPa u s i n g an a l l s i l i c a m i c r o r e a c t o r . determined
u s i n g on
l i n e gas
I n l e t and e x i t gas c o m p o s i t i o n s were
chromatography
with
a
thermal
conductivity
d e t e c t o r . L i q u i d p r o d u c t s were condensed i m m e d i a t e l y downstream o f t h e r e a c t o r and analysed u s i n g o f f l i n e gas chromatography f i t t e d w i t h a c a p i l l a r y column and f l a m e i o n i s a t i o n d e t e c t o r .
S u r f a c e areas were determined u s i n g n i t r o g e n
a d s o r p t i o n a c c o r d i n g t o t h e BET method. TABLE 1
Conversion o f methane o v e r Li/MgO and MgO
Catalyst
Temp "C
T o t a l Reagent GHSV h-l
Li/MgO
215 310 400
114 114 114
215 305 400
114 114 114
200 300 400 500 700
692 692 692 692 692
200 300 400 500 700
545 545 545 545 545
MgO
400
732
MgO
400
732
Li/MgO Li/MgO
Li/MgO
03/02a I
F12 ,I
'36'2 I, I, I,
FI2 I, I,
I1
O3l02 O2
0.4 0.4 0.4
0.01 1.4 2.4
0 0.06 0.34
0.2 0.2 0.2
0.3 0.3 0.3 0.3 0.3
0.005b 0.006 0.14 tr 0.75 0.91 4.1 6.9'
0 0 0 0.26 0.83 1.68 0.60
0.3 0.3 0.3 0.3 0.3
0 0 0.11 4.1d 7.0
0 0 0.88 1.98 0.5
0.3
0.38
1.29
0.3
0.09
0.53
0 - 5.6 - 25.1
13.0
13.1
-
15.9 22.5 54.8 21.8 23.3
-
-
47.1 56.7 20.6 21.3
-
100 94.4 74.9 -
-
100
-
100 60.9 23.2 59.2 18.4 26.8 18.4 37.9 4.0 -
-
52.9 25.5 17.8 41.2 3.9
56.3
43.7
54.8
65.2
-
/O = 5 mole % ozone i n oxygen; t r a c e o f p r o d u c t s observed; 60% C2 3 2 s e l e c t i v i t y based on moles o f CH4 converted; 58% C2 s e l e c t i v i t y based on
a 0
moles of CH4 c o n v e r t e d .
417
TABLE 2
Conversion o f methane over v A e 2 0 3
Temperature “C
Total GHSV
Reagent
h-l 200 300 400
200 200 200
200 300
120 120
200 300 400 500
580 580 580 580
200 300 400
250 250 250
Reagent/
H2/C02
CH4
Product S e l e c t i v i t y a mole %
CH4 mole r a t i o
conv mole %
0.4 0.4 0.4
0.6 0.7 1 .o
0.4 0.4
0 0.04
0
0
100
0.3 0.3 0.3 0.3
0.37 0.86 1.15 4.6
0.06 0.29 0.64 0.65
3.9 10.0 20.1 32.0
0.3 0.3 0.3
0 tr 0.15
69.2 34.3 32.7 49.4 -
1 .o
03!02b I
F12 03!02b
a no C 2 p r o d u c t s observed;
mole r a t i o
0.17 0.31
H2
co2
0 14.7 23.8
100 85.3 76.2
100 50
co
26.9 55.7 46.4 18.5
50
03/02 = 5 mole % ozone on oxygen
RESULTS AND DISCUSSION Methane o x i d a t i o n was c a r r i e d o u t u s i n g oxygen and ozoneloxygen m i x t u r e s
( 5 mole % O3 i n 0 2 ) under a range o f r e a c t i o n c o n d i t i o n s w i t h Li/MgO,
MgO and
Y At203 as c a t a l y s t s and t h e r e s u l t s a r e shown i n Tables 1 and 2.
Results
o b t a i n e d w i t h Li/MgO a t 700°C a r e i n good agreement w i t h t h o s e p r e v i o u s l y observed w i t h t h i s c a t a l y s t and t y p i c a l l y C2 s e l e c t i v i t i e s o f c a 60 mole % were observed under t h e s e c o n d i t i o n s . C2 p r o d u c t s were observed o n l y a t t e m p e r a t u r e s >/
500°C.
A t t e m p e r a t u r e s > 400°C t h e p r o d u c t d i s t r i b u t i o n s and a c t i v i t i e s o f
MgO and Li/MgO showed no a p p r e c i a b l e d i f f e r e n c e s f o r t h e O2 and 03/02 o x i d a n t s . It i s c l e a r t h a t a t t h e s e e l e v a t e d t e m p e r a t u r e s ozone i s p a r t i c u l a r l y u n s t a b l e
and r a p i d l y forms oxygen and hence no e f f e c t would be expected. A t temperatures
<
400°C r a d i c a l f o r m a t i o n d i r e c t l y f r o m ozone,
b u t n o t f r o m oxygen,
can be
expected. I t i s c l e a r t h a t f o r a l l c a t a l y s t s ozone i s a more r e a c t i v e o x i d a n t t h a n oxygen a t t h e s e temperatures. I n p a r t i c u l a r ozone i s an a c t i v e o x i d a n t i n t h e temperature r a n g e 200
-
300°C whereas oxygen becomes a c t i v e o n l y a t
400°C. The p r o d u c t s observed w i t h b o t h oxygen and ozone a t < 500°C c o n s i s t o f
H2, c02 and CO ( w a t e r was n o t d e t e c t a b l e ) a t h i g h space v e l o c i t i e s ( > 5 0 0 h - l ) , and o n l y H2 and C02 a t l o w space v e l o c i t i e s ( < 2 0 0 h - l ) ,
i n d i c a t i n g t h a t CO i s
b e i n g s e q u e n t i a l l y o x i d i s e d t o C02. F o r a l l c a t a l y s t s a t
< 400°C ozone i s a
more s e l e c t i v e o x i d a n t t h a n i s oxygen when s e l e c t i v i t y i s expressed i n terms o f t h e H2/C02 mole r a t i o . The p r o d u c t i o n o f h i g h hydrogen s e l e c t i v i t i e s r e q u i r e s f u r t h e r Hydrogen has been observed p r e v i o u s l y i n t h e n o n - c a t a l y t i c
gas phase
comment. reaction
418 (13).
The presence o f
s i g n i f i c a n t hydrogen y i e l d s w i t h Li/MgO
a t 700°C
is
i n d i c a t i v e t h a t a major p a r t o f t h e r e a c t i o n o c c u r s i n t h e gas phase which i s i n agreement w i t h L u n s f o r d ' s p r e v i o u s o b s e r v a t i o n ( 4 ) t h a t t h e r o l e o f t h e surface
is
to
generate
gas
phase
methyl
radicals.
The
hydrogen
can
be
considered t o a r i s e from t h e f o l l o w i n g r a d i c a l r e a c t i o n s . ( 1 3 ) .
:
1
c&a
2.8 2.6
-
2.4
-
2.2
Ln
0 <
c
c
Y x
z
N
u >
, > N
1.2' 0
"
"
.5
1.0
'
TIME
8
1.5
I
2.0
I
L
I
2. 5
I
3.0
I
3.5
ON LINE /h AT 7 0 0 ~
F i g . 1 . Loss o f C 2 y i e l d w i t h t i m e on l i n e ; open symbols oxygen, c l o s e d symbols ozoneloxygen
.
A t 700°C ( F i g u r e 1 ) p r o d u c t y i e l d s were f o u n d
to
vary
significantly
with
r e a c t i o n t i m e and i n p a r t i c u l a r t h e y i e l d s o f C2 p r o d u c t s decreased. The d a t a c l e a r l y show t h a t a t t h i s t e m p e r a t u r e ozone and oxygen behave s i m i l a r l y oxidants.
The o v e r a l l
C2
s e l e c t i v i t y d i d n o t v a r y w i t h t i m e on
remained c o n s t a n t a t c a 59 mole X .
The e f f e c t
as
l i n e and
i s t h e r e f o r e due t o a s t e a d y
d e c l i n e i n c a t a l y s t a c t i v i t y which we a t t r i b u t e t o loss o f s u r f a c e a r e a o f t h e Li/MgO c a t a l y s t on use (due t o s i n t e r i n g ) . T h i s was c o n f i r m e d by measurement o f t h e BET s u r f a c e areas of t h e d i s c h a r g e d c a t a l y s t s (c a 1.3 m2 / g ) r e l a t i v e t o t h e 2 f r e s h c a t a l y s t (E60 m / g ) . I n an a t t e m p t t o improve t h e s e l e c t i v i t y o f t h e p r o d u c t s o f l o w t e m p e r a t u r e ozone o x i d a t i o n ,
experiments were c a r r i e d o u t w i t h a d d i t i o n o f w a t e r vapour
(CH4/H20 = 1 : l moie r a t i o ) . A t 400°C t h e a d d i t i o n o f steam had l i t t l ' e e f f e c t on
419
t h e c a t a l y s t a c t i v i t y o r p r o d u c t s e l e c t i v i t i e s f o r LO2, CO and H2. A t 700°C t h e a d d i t i o n o f steam enhanced t h e f o r m a t i o n o f C02 as c o u l d be expected f r o m r a d i c a l r e a c t i o n ( i i i ) . No methanol was d e t e c t e d i n t h e l i q u i d p r o d u c t s w i t h o r w i t h o u t steam a d d i t i o n . CONCLUSIONS Ozone i s a more a c t i v e o x i d i s i n g agent t h a n i s oxygen a t
< 400°C
with
Li/MgO, MgO and Y At203 c a t a l y s t s . Under t h e s e c o n d i t i o n s t h e p r o d u c t s c o n s i s t o n l y o f H2, product
C02 and CO and a d d i t i o n o f H20 does n o t s i g n i f i c a n t l y a f f e c t
selectivity.
Catalysts
rapidly
decline
in
activity
at
higher
temperature and t h i s e f f e c t i s a t t r i b u t e d t o l o s s o f s u r f a c e area t h r o u g h
sintering/recrystallization. REFERENCES 1 2 3 4
G.E. K e l l e r and M.M. Bhasin, J. C a t a l . , 73 (1982) 9. I.T.A. Emesh and Y. Amenomiya, J . Phys. Chem., 90 (1986) 4785. T. I t o and J.H. Lunsford, N a t u r e (London), 314 (1985) 721. 07 J . D r i s c o l l , W. M a r t i r , J-X Wanq and J.H. Lunsford, J . Am. Chem. SOC., _. (1985) 58. 5 T. Moriyama, N . Takasaki, E. Iwamatsu and K - I Aika, Chem. L e t t . , (1986) 1165. 6 K. Otsuka, K. J i n n o and A. Morikawa, J. C a t a l . , 100 (1986) 353. 7 K. Otsuka, Q. L i u , M. Hatano and A. Morikawa, Chem. L e t t . , (1986) 467. 8 K. Otsuka, Q. L i u , M. Hatano and A. Morikawa, Chem. L e t t . , (1986) 903. 9 C.T. Au and M.W. Roberts, Faraday Symp. Chem. SOC., 21 (1986) paper 9. 10 N.R. F o s t e r , App. Cat., 19 (1985) 1. 11 V. Ya. V o l ' f s o n , S.A. S o l o v ' e v , A.F. Sadak and V.M. Vlasenko, Zh. P r i k l , Khim., 57 (1984) 1821. 12 M. Boudart, A. D e l b o u i l l e , J.A. Dumesic, S. Khammouma and H. Topsoe, J. Catal., 37 (1975) 486. 13 H.D. Gesser, N.R. Hunter and C.B. Prakash, Chem. Rev., 85 (1985) 235.
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D.M. Bibby,C.D. Chang,R.F. Howe and S.Yurchak (Editors),Methane Conuersion 8 1988 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
42 1
THE OLIGOMERIZATION OF OLEFINS DERIVED FROM PARTIAL METHANE OXIDATION
V.W.L.CHIN l, A.F. MASTERS l, M.VENDER l, and R.J.TYLER lDepartment of Inorganic Chemistry, University of Sydney, N.S.W., 2 0 0 6 , AUSTRALIA 2CSIR0 Division of Fossil Fuels, North Ryde, N.S.W., 2113, AUSTRALIA
ABSTRACT . Catalysts derived from nickel complexes containing dithio-B-diketonate and phosphine ligands, and having the general formula Ni[R1 C(S)CR2C(S)R31(PL1L2L3)X ( R J ~Lj = alkyl or aryl, X = halide) are shown to be active and selective in the oligomerization of butenes at room temperature and under autogenous pressure in toluene and chlorobenzene. Conversions of 93% and octene selectivities in excess of 90% are reported. The monomer concentration as a function of time is described by the formula ln(monomer) = A exp(-Bt) + C. No deleterious effect is detected with hydrogen as a co-feed, carbon monoxide is a catalyst inhibitor and dienes are oligomerized by the catalyst system.
INTRODUCTION Olefin oligomerization is a pivotal process in the catalytic transformations of indigenous resources. In the present context, several routes can be used to generate olefins from methane. The many reactions of the olefin double bond can then lead to both fuels and specialty chemicals. Catalytic olefin oligomerization is perhaps the simplest of these reactions. Until recently, the available technologies were energy intensive and variously suffered such disadvantages as low catalyst activity, poor conversions, relatively severe operating conditions, poor selectivity, short catalyst lifetimes, etc..
However,
the recently commercialized Dimersol process (ref. 1 ) and Shell's higher olefins process (ref. 2 ) both owe their sucesses in surmounting some of these difficulties to the use of active homogeneous nickel-based catalysts. We have been interested in surveying the potential of homogeneously catalyzed olefin oligomerization as a process option downstream of methane conversion technologies. Homogeneous catalytic systems have the potential advantages of very high product selectivities and low energy inputs. However, they are possibly more sensitive to poisons than say, solid acid catalysts. The compounds, Ni(R2-R1sacR3sac)(PL1L2L3)X (I; Rj = alkyl or aryl, Lj = alkyl or aryl, X = halogen) have recently been shown to generate novel olefin oligomerization catalysts.(refs. 3-4) These compounds, when activated by a
422
suitable co-catalyst, form highly active, highly selective and long lived olefin oligomerization catalyst systems. The activity and selectivity can be controlled by the choice of substituents, Rj and Lj. These operational advantages make this system appropriate to survey the use of homogeneous systems in the present context.
For experimental convenience, previous studies on the catalysts derived from the compounds (I) have concentrated on propylene as a substrate at subambient temperatures. These conditions were selected to obtain managable activities from this extremely highly active system. We sought to investigate the catalyst longevity, stability and performance at ambient temperature and moderate pressures, and to explore the effects of likely co-feeds from potential upstream methane conversions.
In this context, butenes
are convenient and appropriate substrates for such a survey. In particular, the use of butenes in catalyst lifetime and stability studies avoids the high costs and potential hazards of large reagent volumes which attend the use of the more reactive lower olefins. Moreover, butenes are likely to be derived either directly or indirectly (e.g., from ethylene) from the conversion of methane. Accordingly, we have investigated the performance of catalysts derived from the compounds (I) in the oligomerization of butenes at about ambient temperature and pressure and in the presence of other co-feeds. EXPERIMENTAL The Ni(R2-R1 sacR3sac)(PL1L2L3)X compounds were synthesised as described previously.(ref. 5 )
Diethylaluminiumchloride,butenes and other reagent gases
were used as received from Merk and from CIC Australia, respectively. The reactor was a Fischer and Porter aerosol bottle fitted with a thermocouple and connected to a manometer and a vacuum / gas manifold.
Products were analysed
using a Hewlett Packard 5890 gas chromatograph fitted with an SGE 25m BP1 capillary column and an FID. Pressure / composition curves were calibrated by comparison with gas chromatographic analyses. The high activities associated with such homogeneous catalytic systems can make catalytic experiments extremely sensitive to the method of testing. The following procedure gives highly reproducible results. Butenes are condensed into the weighed evacuated reactor vessel.
Excess butenes are distilled off to
423
leave a charge of 7.50 f 0.01 gm.
The reactor is cooled to -15OC and an
accurately weighed aliquot of approximately 10 em3 of a 1.5 mg ml-l solution of the nickel compound is added by injection. reweighed and connected to a manometer.
The reactor is warmed to 22
f
1 OC,
After the temperature / pressure
equilibrium is established, 0 . i ml of the co-catalyst is added at the surface of the vigorously stirred solution. The reaction is stopped after a known time interval. To stop the reaction, the reactor is weighed, a known mass of water is added, excess butenes are vented, the reactor weighed, and the products analyzed by gas chromatography. A conversion profile was derived from a series of such experiments of differing durations. RESULTS The catalytic activity has been shown to depend on the nature of the substituents Rj and Lj in (I).(refs.
3-41
A highly active catalyst system and a
less active catalyst system, based on Ni(sacsac)(PBus)C1
H, L1 L1
=
=
L2
L2 =
=
L3
=
Bu, X
C,H,, L3
=
=
C1) and Ni(sacsac)(PPhzMe)C1
CH3, X
=
(I; R1
(I; R 1
=
=
R3
R3 =
=
CH,, R2
CH3, R2
=
=
H,
C1) respectively, were chosen for study. The
reaction, and in particular the catalyst activation, are quite exothermic (AHo
= 43 kJmole-l of butene).
A temperature rise of up to 7OC can be observed
in the first few minutes after activation, even when the reactor is held in a very large temperature controlled bath.
Rapid magnetic stirring appeared to be
adequate in these systems. No differences in catalytic performance were observed when a vigorous mechanical shaker was used. Contemporary commercial homogeneous nickel based catalytic systems utilize high concentrations of olefin.
We therefore investigated the Ni(sacsac)(PBu3)Cl
derived system in toluene (12% w/w solvent).
The solution was apparently
homogeneous before activation, but formed a multiphase system after activation.
A significant amount of an organometallic nickel compound is apparently precipitated. Results were somewhat variable, but the amount of monomer present (g) as a function of time (h) followed the equation ln(monomer)t
=
0.836 exp(-O.O225t)
+
1.633
over a period of 48 hours with an index of determination (R2) of 0.904. After 48 hours of operation, a conversion of 56% with 93% selectivity to octenes was observed.
The highest octene selectivities (97%) were observed at low
conversions, suggesting that co-dimerization of octenes and butenes becomes more significant at higher conversions. The apparent initial conversion rate of 1300 mole butenes / mole Ni / hour is likely to be a gross underestimate as it is based upon total added nickel. When the initial butene concentration was lowered to 41% w/w butene in
424
chlorobenzene, the catalyst system derived from Ni(sacsac)(PPhZCHs)Cl formed apparently homogeneous solutions for the duration of the experiments. The amount of monomer present (g) as a function of time (h) followed the equation ln(monomer)t
=
1.383 exp(-0.0243t)
+
0.547
with R2
=
0.951
After 48 hours a conversion of 63% was obtained with an octene selectivity of 97.3%. A t low conversions, the octene selectivity was 99.8$. With this system, the conversions after the same reaction time were higher with high olefin concentrations in chlorobenzene (15% w/w solvent) than in toluene (12% w/w solvent) and higher still in the more dilute olefin solutions in chlorobenzene (39% w/w solvent). The catalytic system derived from Ni(sacsac)(PBus)C1 was also tested in chlorobenzene (approximately 40% w/w solvent). The solutions were apparently homogeneous for the duration of the experiments. The amount of monomer present (g) as a function of time (h) followed the equation ln(monomer)t
=
2.674 exp(-0.0815t) -0.9976
This is illlustrated in the Figure.
with R2
=
0.990
A conversion of 93% with an octene
selectivity of 91.0% was observed after 25 hours. It is possible to compare the effects of co-feeds on the catalysis by reference to these mathematical descriptions of the catalyst's performance.
No effect on the activity of the catalyst was observed when the system was maintained under hydrogen at the initial butene partial pressure.
Nor was olefin
hydrogenation detected under these conditions. When oligomerization was attempted under an atmosphere of carbon monoxide in the same manner, there was an immediate and characteristic colour change on addition of the co-catalyst. and a complete loss of activity. Carbon monoxide appears to react with the catalyst rather than with the nickel containing precursor. Thus, no colour change was observed when a Ni(sacsac)(PBu,)Cl butene solution was pressurised with carbon monoxide. Alternatively,
/
pressurising a reacting solution with carbon monoxide resulted in the same colour change and a complete loss of activity. The catalyst system will rapidly oligomerize dienes.(ref. 6) The products depend on the experimental conditions. Near quantitative conversions of butadiene have been achieved. DISCUSSION The present results demonstrate that the cataly,stsystem derived from the compounds (I) operates quite satisfactorily at temperatures of at least 30°C and
425
-
y = 2.6744~ 0.9976 R-squared: 0.9900
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Transform of time
at pressures up to 2OOkPa. The catalyst is quite long lived under these conditions, and near quantitative conversions of olefins can be achieved. Extremely high dimerization selectivities are obtained. Not unexpectedly, co-oligomerization of butenes and octenes does become important at the low butene / high octene concentrations of long conversions. A solvent dependence of the activity was demonstrated earlier,(refs. 3-41 and was also observed in the present study. This may be due to catalyst solubility. The butene concentration as a function of time closely followed the general empirical relationship ln(monomer)t = A exp(-Bt) + C. We ascribe no mechanistic interpretation to this observation, but note, however, that similar formulae have been derived from models of olefin polymerization reaction mechanisms. ACKNOWLEDGEMENTS Generous support from the University of Sydney Research Grants Scheme, the Potter Foundation, the %SIRO/University of Sydney Collaborative Research Grants Scheme, the Australian Research Grants Scheme and the National Energy Research Development and Demonstration Council is gratefully acknowledged. REFERENCES 1 Y .Chauvin, J.F.Gaillard, D.V.Quang and J.W. Andrews, Chem. Ind. (London),
2 3 4 5 6
(19741, 375. E.R.Freitas and C.R.Gum, Chem.Eng.Prog., 75, (19791, 73. K.J.Cavel1 and A.F.Masters, J.Chem.Res., (1983), 72. A.F.Masters and K.J.Cavel1, U.S. Pat. 4,533,651 to CSIRO. J.P.Fackler, Jr., and A.F.Masters. 1norg.Chim. Acta, 2, (1980), 1 1 1 . A.F.Masters and M.Vender in preparation.
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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
THE O X I D A T I V E COUPLING J.A. ROOS, S.J. and J.R.H.ROSS
OF
427
FIETHANE: CATALYST REQUIREMENTS AND PROCESS C O N D I T I O N S
KORF, A.G.
DE BRUIJN, J.G. VAN OMMEN
BAKKER, N.A.
F a c u l t y o f Chemical Technology, Enschede, The N e t h e r l a n d s
University
of
PO
Twente.
Box
217.
7500AE
ABSTRACT Good methane c o u p l i n g c a t a l y s t s a r e b a s i c m a t e r i a l s which cannot p r o v i d e l a r g e amounts o f l a t t i c e oxygen and which c o n t a i n e l e c t r o n i c o r s t r u c t u r a l d e f e c t s on t h e surface. Using h i g h gas v e l o c i t i e s i n a T i x e d bed r e a c t o r r e s u l t s i n improved c o n v e r s i o n s and s e l e c t i v i t i e s . The improvement i n s e l e c t i v i t y i s e x p l a i n e d by a b e t t e r p l u g - f l o w b e h a v i o u r o f t h e r e a c t o r . INTRODUCTION The o x i d a t i v e
coupling
of
methane
to
i n t e r e s t i n g a l t e r n a t i v e t o t h e conventional
give use
ethane of
s y n t h e s i s gas and d e r i v e d products. A number o f r e p o r t s on appeared r e c e n t l y ( r e f s .
1-5).
For example, I t o e t a l .
t h a t lithium-doped magnesium o x i d e i s an a c t i v e suggest t h a t t h e Li'
and
ions j u s t f i t i n tho matrix o f
and
methane
ethylene as
this
(refs. host
is
an
source
of
reaction catalyst: lattice
d i v a l e n t oxide, c r e a t i n g h o l e s and p r o v i d i n g t h e a c t i v e s i t e s f o r t h e They have argued t h a t t h e mechanism i n v o l v e s t h e f o r m a t i o n
of
which can combine t o g i v e ethane o r r e a c t f u r t h e r t o g i v e
the
p r o d u c t s , CO and C02. Otsuka e t a l .
have
1,2) have shown
selective the
a
of
they the
reaction.
methyl total
radicals oxidation
( r e f . 3 ) have shown t h a t Sm203 and Dy203 a r e
a l s o i n t e r e s t i n g c a t a l y s t s f o r t h e r e a c t i o n : t h e y have r e p o r t e d s e l e c t i v i t i e s o f up t o 93% b u t t h e s e were achieved u s i n g a v e r y d i l u t e r e a c t o r f e e d w i t h CH,/02
r a t i o . Hinsen e t a l .
but
we
have
t h a t t h i s t y p e o f c a t a l y s t i s l e s s s t a b l e t h a n Li20/Mg0 o r
aims: t o f i n d improved c a t a l y s t s by f i r s t g a i n i n g a b e t t e r u n d e r s t a n d i n g o f way i n which e x i s t i n g proven c a t a l y s t s f u n c t i o n : and t o flow-rate
and
composition,
establish and
reactor
geometry ( r e f . 6 ) ) under which t o t e s t t h e c a t a l y s t s . I n a d d i t i o n t h e behaviour o f supported lead oxide
materials
o x i d e under t h e same process c o n d i t i o n s ( r e f . 5).
with
the
shown
promoted
Sm 0 c a t a l y s t s . A more d e t a i l e d l i t e r a t u r e s u r v e y i s g i v e n i n r e f . 5. 2 3 Our i n v e s t i g a t i o n s o f t h e methane c o u p l i n g r e a c t i o n have a t p r e s e n t t w o
process c o n d i t i o n s (gas
high
( r e f . 4) have shown t h a t PbO supported on alumina i s
a l s o an a c t i v e and s e l e c t i v e c a t a l y s t f o r methane c o u p l i n g ( r e f s . 5,6)
a
main the
optimum
design
and
to
comparing
lithium-doped
magnesium
we have s t u d i e d t h e r a r e - e a r t h
o x i d e s , p a y i n g p a r t i c u l a r a t t e n t i o n t o t h e i r s y n t h e s i s and p r e t r e a t m e n t and t h e
428 e f f e c t o f doping t h e s e m a t e r i a l s w i t h i o n s o f
group
examined i n some d e t a i l t h e e f f e c t o f p r e p a r a t i o n f o r Li-doped MgO c a t a l y s t s ( r e f .
I
11.
or
conditions
We
have
and
also
pretreatment
6).
I n t h i s paper, we o u t l i n e a number o f o u r g e n e r a l
conclusions,
giving
r e q u i r e m e n t s f o r t h e c a t a l y s t and f o r t h e r e a c t o r system t o ensure
as
some
high
as
p o s s i b l e a c o n c e n t r a t i o n o f t h e d e s i r e d C2 p r o d u c t s i n t h e r e a c t o r e f f l u e n t . Experimental The m a j o r i t y o f t h e c a t a l y s t s used i n t h e work r e p o r t e d h e r e MgO o r ZnO and were promoted by L i o r Ba i o n s . i m p r e g n a t i o n u s i n g an a p p r o p r i a t e a l k a l i - m e t a l samples a r e r e p o r t e d elsewhere ( r e f s .
5.6).
They
were
s a l t . The
were
based
synthesised preparation
on
by of
wet other
A l l t h e samples were c a l c i n e d i n a i r
a t 85OoC p r i o r t o b e i n g t e s t e d . The r e a c t i o n system made use o f q u a r t z fixed-bed
5).
r e a c t o r s and gss a n a l y s i s was c a r r i e d o u t w i t h gas chromatography ( r e f .
The
c a t a l y s t ( p a r t i c l e - s i z e s f r o m 0.3 - 0.6 mm) was d i l u t e d w i t h t h e same w e i g h t q u a r t z p a r t i c l e s o f t h e same s i z e . The process c o n d i t i o n s used
are
given
t h e r e s u l t s . One s e t o f d a t a were o b t a i n e d w i t h a r e c y c l e r e a c t o r : f o r n e t gas f l o w o f 0.13 cm3s-l was used w i t h a r e c y c l e r a t i o o f 30
of with
this,
a
(i.e.
the
gas
al.
(ref.
4)
f l o w t h r o u g h t h e r e a c t o r was 4 cm3s-’). RESULTS AND DISCUSSION Cata 1ys t Requ irements (i)C a t a l y s t A c i d i t y .
I t has p r e v i o u s l y been argued by Hinsen e t
t h a t a l l good o x i d a t i v e c o u p l i n g c a t a l y s t s a r e n o n - a c i d i c a c i d i c c a t a l y s t r e s u l t s i n low s e l e c t i v i t y .
solids
and
that
an
They showed t h a t t h e s e l e c t i v i t y
of
a c a t a l y s t composed o f PbO supported on Y-A1203 was improved by n e u t r a l i s i n g t h e a c i d i c s i t e s o f t h e alumina
by
impregnating
it
with
an
alkali
(Na).
concluded t h a t a c i d i c s u r f a c e s g i v e r i s e t o n o n - s e l e c t i v e o x i d a t i o n .
They
They
also
catalyst
with
more t h a n a monolayer coverage o f PbO. Using t h e same s o r t o f c a t a l y s t , we
have
found t h a t t h e s u r f a c e c o u l d be n e u t r a l i s e d by i m p r e g n a t i n g
the
shown t h a t PbO i s l o s t d u r i n g use and t h a t b a r e alumina i s t h u s
exposed
t h e gas phase: t h i s r e s u l t s i n a d r a m a t i c l o s s o f s e l e c t i v i t y as a t i m e on stream i f t h e s u p p o r t i s
not
experiments have shown t h a t t h e use
treated of
a
with
support
r e s u l t s i n a l o w e r l o s s o f s e l e c t i v i t y i n experiments t h a n 180 h).
alkali
(ref.
neutralised of
long
to
to
function
of
5).
with
Recent Na
duration
ions (more
However, i t was shown by e l e m e n t a l a n a l y s i s t h a t t h e c a t a l y s t s t i l l
l o s t r e l a t i v e l y h i g h q u a n t i t i e s o f lead, t h e c o n t e n t d r o p p i n g f r o m 18 w t % t o 2.2 w t % o v e r t h i s p e r i o d . The r e m a i n i n g l e a d and t h e Na’
i o n s a r e s t i l l a b l e t o mask
t h e a c i d i c s i t e s o f t h e alumina and t h e t o t a l o x i d a t i o n r e a c t i o n Baerns has r e c e n t l y r e p o r t e d s i m i l a r c o n c l u s i o n s ( r e f . 7).
is
minimised.
429 We have a l s o c a r r i e d o u t a l i m i t e d number magnesium-exchanged material
C2
to
of
preliminary
ZSM5-type m a t e r i a l ; we found t h a t t h e
products
was
relatively
low
but
experiments
with
selectivity
still
s e l e c t i v i t y g r a d u a l l y improved w i t h use. p o s s i b l y by g r a d u a l
of
a
this
significant.
The
poisoning
the
of
a c i d i c s i t e s . I t i s i n t e r e s t i n g t o n o t e t h a t Anderson and T s a i ( r e f . 8) r e c e n t l y r e p o r t e d t h a t ZSM-5 gave t o t a l o x i d a t i o n ; t h i s o b s e r v a t i o n l e a s t i n p a r t , due t o r e a c t i o n on t h e s t a i n l e s s - s t e e l (ii)S u r f a c e D e f e c t S t r u c t u r e .
may
have
been,
at
r e a c t i o n v e s s e l used.
I t appears f r o m t h e work o f
Ito
et
al.
(refs.
1,2) t h a t o x i d a t i v e c o u p l i n g c a t a l y s t s must c o n t a i n s u r f a c e d e f e c t s i n o r d e r a c t i v a t e methane; t h e s e a u t h o r s argued t h a t a (Li'O-)
Li-doped
MgO
catalyst
to
contains
c e n t r e s c a p a b l e o f s u b s t r a c t i n g a hydrogen r a d i c a l f r o m methane t o
methyl r a d i c a l s and t h a t t h e s e
react
d iscussed above. A p a r t f r o m these
further
to
" e l e c t r o n ic t l
give
C2
the
defects,
products,
"structural
(independent o f charge imbalance) may a l s o be formed on t h e s u r f a c e adding Ba2+ i o n s ; t h e Ba2+ i o n ( r a d i u s 0.134 nm) i s i o n ( r a d i u s 0.066 nm) and can t h e r e f o r e according t o
Nunan e t a l .
not
( r e f . 9), t h i s
fit
the
in
oxygen
results
MgO
for
the
act
the
by
Mg2+
lattice
s i t e s f o r methane c o u p l i n g . To t e s t t h i s idea, we t e s t e d c a t a l y s t s MgO doped by BaO and compared t h e r e s u l t s w i t h t h o s e
MgO
of
species
c o o r d i n a t i o n . These low-coordinated oxygen s p e c i e s may a l s o a c t
as
defects
"
much l a r g e r t h a n into
give
and,
of
as
low
active
composed
Li20/Mg0
of
system
The r e s u l t s a r e shown i n Table 1; t h e c o n v e r s i o n s
under t h e same c o n d i t i o n s .
of
) and oxygen a r e g i v e n t o g e t h e r w i t h t h e c o n c e n t r a t i o n s o f p r o d u c t s Ol formed (mol%), t h e s e l e c t i v i t y t o C2 p r o d u c t s (S2) and t h e y i e l d o f t h e s e
methane (
p r o d u c t s ( Y 2 = a l x S2).
The d a t a f o r t h e two promoted samples are r e p o r t e d f o r
t h e optimum y i e l d s . It can be seen t h a t p r o m o t i o n b y improves t h e
activity
and
selectivity
of
the
both
Lit
catalysts
and
as
Ba2'
ions
compared
with
unpromoted MgO. L i t h i u m i s s t i l l t h e more e f f e c t i v e promoter. Table 1 C a t a l y t i c experiments w i t h doped MgO; c o m p o s i t i o g gf f e e d gas: 67% CH4. 7% 02, 26% N2; c a t a l y s t weight: 93 mg; f l o w r a t e 0.42 cm s
.
C a t a l y s t A n a l y s i s T/OC MgO Li/MgO-B Ba/MgO
Conv./%
CH4
wt%
Concentrations/mol%
O2
C2H4
C2H6
C02
Selectivity/%
s2
y2
-
780 830
6.9 100 7.1 100
0.35 0.30
0.32 0.43
2.54 2.46
0.72
0.81
29 31
2.0 2.2
3.1
780
13.3 100
1.89
1.36
1.76
0.69
73
9.7
19.5
830
11.1
1.89
0.52
2.00
0.63
64
7.2
96
(iii)C a t a l y s t R e d u c i b i l i t y . The performance o f an o x i d a t i v e
c a t a l y s t can s u p p l y oxygen, (TPR).
Yield/%
CO
appears ( r e f . 6 ) t o show an optimum as a f u n c t i o n o f t h e as
measured
by
temperature
Some r e s u l t s which i l l u s t r a t e t h i s p o i n t a r e g i v e n
clear thbt
--
__
coupling
extent
to
programmed in
Table
catalyst
which
the
reduction 2.
It
is
430 a c a t a l y s t capable o f s u p p l y i n g a l a r g e q u a n t i t y o f oxygen (Tb407) g i v e s a degree o f t o t a l o x i d a t i o n . A l l t h e e f f e c t i v e c a t a l y s t systems (e.g. c a t a l y s t shown) were found t o between 650
-
850
OC,
give
a
relatively
small
amount
t h e normal range o f process temperatures:
of
Li20/Zn0 c a t a l y s t , however,
showed no r e d u c t i o n under t h e s e
was found t o be a r e l a t i v e l y i n a c t i v e m a t e r i a l .
reduction
the
oxygen removed f o r t h e Li20/Mg0 sample was 0.4% o f t h e t o t a l oxygen
high
t h e Li20/Mg0 amount
of
content.
A
conditions
and
T h i s l e a d s us t o c o n c l u d e
it that
t h e a c t i v e (and s e l e c t i v e ) s i t e s on an e f f e c t i v e o x i d a t i v e c o u p l i n g c a t a l y s t a r e a s s o c i a t e d w i t h s u r f a c e oxygen
species
which
are
labile
under
the
process
conditions. Table 2 R e s u l t s f o r c a t a l y s t b e h a v i o u r a t t h e t e m p e r a t u r e g i v i n g t h e maximum y i e l d f o r d i f f e r e n t c a t a l y s t s , and t h e amount of oxygen removable f r o m t h e c a t a l y s t i n t h e temperature range 65O-85O0C (Od): o t h e r c o n d i t i o n s as i n T a b l e 1
-
Sp/%
Y2/%
2.26
41.9
2.9
0
1.73
0.68
72.6
9.5
97
2.85
0.47
32.0
2.3
972
Catalyst
T/OC
Li/ZnO
780
6.8
66
0.56
0.40
0.39
Li/Mg0
780
13.1
100
1.86
1.33
780
7.3
100
0.48
0.30
Tb407
Conversion/% Concentrations/mol%
CH4
O2 C2H4
C2H6
C02
CO
Od/pmol g-'
Requirements o f Process C o n d i t i o n s (i)L i n e a r Gas V e l o c i t y .
R e s u l t s i l l u s t r a t i n g t h e e f f e c t o f v a r i a t i o n o f gas
flow
at
constant
contact
t i m e w i t h t h e c a t a l y s t as w e l l as back-mixing (see f o l l o w i n g s e c t i o n ) a r e f o r two
different
Li20/Mg0
catalyst
samples
in
Tables
3
and
shown
4.
In
the
experiments shown i n Table 3, t h e f l o w t h r o u g h t h e r e a c t o r was v a r i e d w h i l e b o t h t h e r a t i o o f c a t a l y s t w e i g h t t o gas f l o w r a t e (W/F) were k e p t c o n s t a n t : t h e t a b l e g i v e s t h e gas f l o w (F),
and
the
the
reactor
catalyst
diameter
and t h e s u p e r f i c i a l gas v e l o c i t y ( U ) as w e l l as c o n v e r s i o n r e s u l t s . The
results
conversion
r e p o r t e d were o b t a i n e d a t 725OC: a t t h i s temperature, t h e oxygen
w e l l below 100% and so t h e r a t e o f c o n v e r s i o n o f methane i s n o t l i m i t e d a v a i l a b i l i t y o f oxygen as i s t h e case a t
the
higher
temperatures
optimum conversions. The r e s u l t s o f Table 3 show t h a t b o t h
the
the s e l e c t i v i t y are increased considerably i f t h e s u p e r f i c i a l
(W)
weight
by
which
conversion gas
velocity
is the
give and is
i n c r e a s e d even though t h e c o n t a c t t i m e i n t h e c a t a l y s t bed i s k e p t c o n s t a n t . The reason f o r t h i s improvement i s n o t y e t f u l l y understood.
I t may have been caused
by a l o w e r i n g o f t h e e x t e r n a l mass t r a n s f e r r e s i s t a n c e a t h i g h e r gas v e l o c i t y o r by a b e t t e r gas d i s t r i b u t i o n ( l e s s c h a n n e l i n g ) i n t h e c a t a l y s t F u r t h e r work t o d i s t i n g u i s h t h e s e e f f e c t s i s i n progress.
bed
(ref.
10).
431 Table 3 C a t a l y t i c ex eriments w i t h Li/MgO-A f o r d i f f e r e n t values of F a t constant W/F: T = 725 gC; U = l i n e a r gas v e l o c i t y : o t h e r c o n d i t i o n s as Table 1.
W /g
/cmF3 s -1
U-l /ms
Conversion/% CH4 O2
Concent r a t ions/moI% C2H4 C2H6 C02 CO
S2/%
V,l%
0.21
0.046
0.04
3.2322.3
0.180.520.510.25
65
2.1
0.42
0.092
0.05
3.22
19.3
0.13
0.61
0.55
0.13
59
2.2
0.84
0.184
0.18
4.07
23.3
0.19
0.82
0.56
0.15
74
3.0
1.68
0.368
0.35
5.19
32.8
0.22
1 - 0 7 0.71
0.19
74
3.9
3.36
0.736
0.71
5.97
38.7
0.27
1.22
0.81
0.20
75
4.4
6.72
1.472
1.42
9.94
60.1
0.72
1.79
1.36
0.27
75
7.5
Table 4 C a t a l y t i c experiments w i t h Li/MgO-B; T = 78OOC: f o r experiments 1 10% 02,and 40% N2; f o r experiment 3, 2, t h e gas composition was 50% CH compositions were as i n Table 1: %r d e f i n i t i o n of N, see t e x t . /cm s
W
N
U
and the *
S2/% V,/%
Conversion/% Concentrations/% CH4 02 C2H4 C2H6 C02 CO
1
0.42
0.092
4-5
0.09
22.5
81
2.50
0.52
3.31
1.90
54
12.1
2
0.13
0.092
1
0.90
14.0
75
0.94
0.48
2.72
1.43
41
5.7
3
3.36
0.736 10-16
0.75
18.5
100
2.41
2.63
1.99
0.35
81
15.0
~~~
~
~~~
~~~~~
~
(ii)Back-mixing o f Products.
I t appears t h a t t h e degree o f
backmixing
in
the
r e a c t o r should be minimised. Table 4 shows r e s u l t s f o r a s i n g l e t y p e o f L i O/MgO 2 c a t a l y s t i n two d i f f e r e n t types of r e a c t o r : a single-pass packed-bed r e a c t o r (experiments 1 and 3) and a r e c i r c u l a t i o n r e a c t o r (experiment 2).
Experiment
g i v e s r e s u l t s obtained i n t h e single-pass r e a c t o r w i t h t h e same gas as t h a t used f o r experiment 2. The q u a n t i t y N i s a measure o f o f t h e gas f l o w p a t t e r n i n t h e r e a c t o r s which was
estimated
the by
1
composition non-ideality
measuring
the
residence t i m e d i s t r i b u t i o n o f t h e r e a c t o r a t t h e d e s i r e d gas f l o w u s i n g a p u l s e o f N2 i n a He flow, t h e r e a c t o r temperature b e i n g
7OO0C
(ref.
6).
can
N
thought o f asbeing t h e number o f c o n t i n u o u s l y s t i r r e d i d e a l tank r e a c t o r s g i v e n s i z e which make up t h e r e a c t o r : t h e h i g h e r t h e value o f N. r e a c t o r approaches p l u g
flow
conditions.
The
recirculation
experiment 2 behaves as an i d e a l l y s t i r r e d t a n k r e a c t o r (N=l):
the system
it i s
be
of
more
a the
used seen
in
that
t h i s r e a c t o r g i v e s poorer y i e l d s of C2 products even though t h e gas v e l o c i t y
is
h i g h e r than i n t h e e q u i v a l e n t experiment (1) i n t h e continuous f l o w system. This i s probably caused by t h e f a c t t h a t t h e C2 products a r e r e l a t i v e l y u n s t a b l e i n
432
an atmosphere c o n t a i n i n g oxygen a t h i g h t e m p e r a t u r e s and t h a t b a c k m i x i n g o f
C2 p r o d u c t s t o r e g i o n s o f h i g h p a r t i a l p r e s s u r e o f oxygen t h u s l e a d s t o o x i d a t i o n and hence l o w e r s e l e c t i v i t y .
We c o n c l u d e t h a t u s e o f
a
the
further
stirred
tank
r e a c t o r i s t h u s u n d e s i r a b l e due t o t h e c o m p l e t e b a c k - m i x i n g a c h i e v e d . E x p e r i m e n t 3 was c a r r i e d o u t t o see w h e t h e r a p p l i c a t i o n o f t h e s e i d e a s g i v e a f u r t h e r improvement o f t h e y i e l d o f C2 catalyst.
hydrocarbons
using
The t o t a l c o n c e n t r a t i o n o f C2 p r o d u c t s o b t a i n e d i s , t o o u r
an
would
improved knowledge,
t h e h i g h e s t r e p o r t e d u n t i l now f o r 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. ACKNOWLEDGEMENTS S.J.K. thanks t h e Dutch Foundation f o r f i n a n c i a l s u p p o r t . We a l s o t h a n k G. A l t e n a f o r Z o u t Chemie f o r f i n a n c i a l a s s i s t a n c e .
S c i e n t i f i c Research (ZWO) f o r t e c h n i c a l a s s i s t a n c e and Akzo
REFERENCES
1. T. I t o and J.H. L u n s f o r d , N a t u r e (London), 314 ( 1 9 8 5 ) 721. 2. T. I t o , J-X. Wang, C-H. L i n and J.H. Lunsford, J . Amer.
Chem. SOC., 107 (1985) 5062. 3. K. Otsuka, K. J i n n o and A. Morikawa, J. C a t a l . , 100 (1986) 353-359. 4. W. H i n s e n , W. B y t y n and M. Bearns, Proc. 8 t h I n t . Congr; C a t a l . 3 (1984) 581. Catal. 5. J.A. Roos. A,G, Bakker, H. Bosch, J.G. v a n Ommen and J.R.H. Ross, Today, 1 (1987) 133. Biermann, J.G. van Ommen and 6. J.A. Roos, S.J. K o r f , N.A. d e B r u i j n , J.J.P. J.R.H. Ross. t o b e p u b l i s h e d . 7. M. Baerns, C a t a l . Today, i n p r e s s . 8. J.R. Anderson and P. T s a i , Appl. C a t a l . , 1 9 ( 1 9 8 5 ) 141. 9. J. Nunan, J. Cunningham, A.M. Deane. E.A. Colbourn, and W.C. Mackrodt, A d s o r p t i o n and C a t a l y s i s o n O x i d e S u r f a c e s , S u r f a c e S c i e n c e and C a t a l y s i s S e r i e s , E l s e v i e r (Amsterdam), 21 (1985) 83. 10. D. K u n i i and M. Suzuki, I n t . J. H e a t Mass T r a n s f e r , 10 (1967) 845.
D.M. Bibby,C.D. Chang, R.F.H o w e and S. Yurchak (Editors),Methane ConLersion @> 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.
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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
85 65 62 v
60
50
45 (Z)2) 10 (20) 60 c5 50 40 10
-
> 1000
-
-1
5
65 20
50
(85)3)
70 15
<5 <5 <5
-
30
1 21deal hydrocracking of heavy hydrocarbons 01igomerization of C3/C4-01 efins 31sobutane/C3-C4-olef ins-a1kyl ati on
INTERMEDIATES OF CHAIN GROWTH Chemisorbed intermediates of the Fischer Tropsch synthesis (reactants: species 1,2, 3; oxygen containing intermediates: species 6, L, g, 2, lo and hydrocarbon intermediates: species 11,2, 13,2, 5)are pictured in Fig. 3. C, species are the compounds 2, 3, 4,6, 1,8, II-, 12,13. According to the model of Anderson (refs. 3-5) and the early work of Herington (ref. 6) a C1 species is added to the growing chain. This was thought to be species 5 (ref. 4). Even through investigations with labelled compounds by Emmett and coworkers (ref. 7) this compound has never been observed. Chemisorbed CO (1) has been suggested as C, species for chain prolongation in analogy t o the homo-
461
0
._ CO.H2
H
111 C
1
2
H I
H OH
\/
H
H
I
H
C
\/ C
11
2
C
3
4
H
8 H H H
\I/ C
g
H
geneous CO i n s e r t i o n o f t h e hydroformy-
O
l a t i o n by S t e r n b e r g and Wender ( r e f . 8 )
I
5 R
R
c=o
'l-0
I
c-0
7
H
C
\/
c=o
C
0 II
9
R \
,
10
R
&
H
MAIN PRODUCIS
%H3
fi
ICHb) R'-CH:CHz R-CH-0 R-CH2-OH
F i g . 3. Chemisorbed i n t e r m e d i a t e s o f F i s c h e r Tropsch s y n t h e s i s (observed and p o s t u l a t e d )
Roginsky ( r e f . 9 ) and P i c h l e r and Schulz ( r e f . 1 0 ) . P a r t o f t h e s y n t h e s i s p r o d u c t i s f r e q u e n t l y o b t a i n e d as primary alcohols. This i n d i c a t e s t h a t t h e oxygen a t t h e c h a i n end i s b e i n g i n t r o d u c e d v i a C O i n s e r t i o n and i t has s u r v i v e d i n t h i s p o s i t i o n . I t has a l s o been shown by P i c h l e r , Schulz and Rao ( r e f . 11) and Schulz and A c h t s n i t ( r e f . 1 2 ) t h a t e t h y l e n can be i n c o r p o r a t e d as a l i n e a r b u i l d i n g b l o c k i n t o growing c h a i n s d u r i n g F i s c h e r Tropsch synthesis w i t h a c o b a l t c a t a l y s t .
Treatment o f F i s c h e r Tropsch c a t a l y s t s w i t h CO l e a d s t o t h e d e p o s i t i o n o f carbon ( s p e c i e s 4 ) a c c o r d i n g t o t h e e q u a t i o n
2
co
co2
+
c
T h i s s u r f a c e carbon i s v e r y r e a c t i v e and can be i n c o r p o r a t e d i n t o growing hydrocarbon c h a i n s d u r i n g a subsequent F i s c h e r Tropsch s y n t h e s i s ( r e f s . 13, 14). Through t r e a t m e n t w i t h hydrogen i t i s t r a n s f o r m e d t o methane ( r e f . 15). I t i s concluded t h a t s u r f a c e carbon can b e t h e b u i l d i n g u n i t o f a l i p h a t i c c h a i n s d u r i n g F i s c h e r Tropsch s y n t h e s i s . However, t h e q u e s t i o n remains t o be answered i f t h e C1 i s added t o t h e growing c h a i n as s p e c i e s
4,11 o r 12. The
existence
o f a CH2 s p e c i e s on t h e c a t a l y s t s u r f a c e d u r i n g CO h y d r o g e n a t i o n has been observed by B e l l ( r e f . 16) t h r o u g h i t s t r a p p i n g w i t h cyclohexene. The main r e a c t i o n s o f p r o d u c t f o r m a t i o n ( c h a i n d e s o r p t i o n ) a c c o r d i n g t o ( r e f . 1 ) have been a l r e a d y v i s u a l i z e d above: t h e d i s s o c i a t i v e r e a c t i o n o f an a l k y l s p e c i e s (1.5 ) t o y i e l d t h e a - o l e f i n o r i t s a s s o c i a t i v e r e a c t i o n w i t h hydrogen ( 1 ) t o y i e l d t h e p a r a f f i n . These c h e m i - d e s o r p t i o n r e a c t i o n s a r e t h e slow s t e p s o f t h e F i s c h e r Tropsch mechanism. The p a r a f f i n c h e m i - d e s o r p t i o n i s e.g.
a b o u t f o u r t i m e s slower t h a n t h e o l e f i n c h e m i - d e s o r p t i o n ( r e f s . 1,Z)
and
the value o f t h e r a t e constant o f product desorption ( p a r a f f i n p l u s o l e f i n ) i s e.g.
o n l y 1 / 5 t h t o l / l O t h o f t h e r a t e c o n s t a n t o f c h a i n p r o l o n g a t i o n . These
c o n s t r a i n t s a r e most e s s e n t i a l f o r e s t a b l i s h i n g a F i s c h e r Tropsch system. CO CONSUMPTION AND PRODUCT FORMATION RATES
K i n e t i c s o f t h e F i s c h e r Tropsch s y n t h e s i s i n terms o f t h e CO consumption r a t e have been f r e q u e n t l y s t u d i e d (e.g.
r e f . 17). T h i s r a t e g e n e r a l l y i n c r e a s e s
462
w i t h i n c r e a s i n g hydrogen p a r t i a l p r e s s u r e and decreases r a t e o f COconsumption
w i t h i n c r e a s i n g CO p a r t i a l
r a t e o f organic product f o r m a t i o n on a carbon b a s i s
I
pressure. The r a t e o f f o r m a t i o n o f o r g a n i c compounds on a m o l a r
I
r a t e o f product r a t e o f product average f o r m a t i o n on a = f o r m a t i o n on a Xcarbon carbon b a s i s molar basis number
c a r b o n b a s i s i s t h e n equal t o t h e r a t e o f CO consumption on a m o l a r carbon b a s i s ( e q u a t i o n
dt
dt
molar r a t e o f t h e compound "i'I f o r m a t i on
=
molar r a t e o f molar f r a c t i o n a l l compounds X o f compound "i" f o r m a t i on among a l l product compounds
(41
( 1 ) i n F i g . 4 ) . I t i s assumed here
t h a t no C02 i s b e i n g
formed i n t h e system. I n o r d e r t o o b t a i n t h e molar r a t e o f p r o d u c t f o r m a t i o n , t h e average
I n d i v i d u a l compound formation r a t e s a r e thus obtained v i a measuring (1) the molar product f r a c t i o n o f compound "i"( 2 ) t h e average carbon number o f a l l product compounds and ( 3 ) the CO-consumption r a t e . Molar p r o duct f r a c t i o n s a r e thus e q u i v a l e n t t o r e l a t i v e molar r a t e o f formation
carbon number o f t h e r e a c t i o n p r o d u c t has t o be determined. The m o l a r r a t e o f p r o d u c t f o r m a t i o n is. t h e n equal t o t h e r a t e o f p r o d u c t f o r m a t i o n on a
F i g . 4. D e f i n i t i o n o f r e l a t i v e m o l a r f o r m a t i o n r a t e s o f i n d i v i d u a l compounds.
m o l a r carbon b a s i s d i v i d e d by t h e average carbon number o f t h e product (equation (2) i n
F i g . 4 ) . Now we can d i s c r i m i n a t e CO consumption f o r c h a i n s t a r t and c h a i n growth. When t h e average carbon number o f t h e p r o d u c t i s e.g.
NC = 5, t h e n
1 / 5 t h o f t h e CO consumption r a t e i s a t t r i b u t e d t o c h a i n i n i t i a t i o n and 4 / 5 t h t o c h a i n growth. The m o l a r r a t e o f f o r m a t i o n o f one i n d i v i d u a l compound "i" i s obt a i n e d as t h e p r o d u c t o f f o r m a t i o n r a t e o f a l l t h e p r o d u c t moles and " F r I' t h e i m o l a r f r a c t i o n o f compound "i" ( e q u a t i o n ( 3 ) i n F i g . 4). A c c o r d i n g t o e q u a t i o n ( 4 ) t h e sum o f moles o f a l l o r g a n i c p r o d u c t compounds i s n o r m a l i z e d t o t h e v a l u e o f 1.00. pound "Fri"
The mole
f r a c t i o n o f one regarded com-
has t h e n t h e meaning o f t h e r e l a t i v e m o l a r f o r m a t i o n r a t e o f com-
pound "i". With t h e h e l p o f e q u a t i o n (3) i t i s e a s i l y t r a n s f o r m e d i n t o t h e a b s o l u t f o r m a t i o n r a t e o f t h e compound. T a k i n g i n t o account t h e d e f i n i t i o n o f e q u a t i o n ( 4 ) t h e k i n e t i c model o f c h a i n growth i s developed below on t h e b a s i s o f r e l a t i v e m o l a r f o r m a t i o n r a t e s o f compounds, t h e v a l u e s o f which a r e equal t o t h e r e s p e c t i v e molar p r o d u c t f r a c t i o n . GENERAL KINETIC SCHEME OF C H A I N GROWTH
A s i m p l e p i c t u r e o f F i s c h e r Tropsch c h a i n growth i s g i v e n i n F i g . 5. The m o l e c u l e c h a i n o f n carbon atoms ( s u r f a c e s p e c i e s "Sp,")
i s prolonged t o a
c h a i n w i t h n + l carbon atoms ( s u r f a c e species 'oSpn+l'') t h e carbon atoms n,
463
Fig. 5. 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 , Chain pro1 ongation by 1 "CH2" - according t o t h e equation
Surface of catalyst ,;rowing chain surface species "Spn" I Cn- C n T C
i;C&+
w c2- c1
n
1 CO + 2 H2
2 H2
Surface species
-
1 -CH2-
+ 1 H20
"
F i g . 6. Scheme o f s u r f a c e p o l y merization w i t h s t r a i g h t chain and branched growth and 3 s o r t s o f products ( p a r a f f i n s , o l e f i n s , oxygenates) sat = saturated ( p a r a f f i n ) 01 = o l e f i n ox = oxygenates ( a l c o h o l and aldehyde) b r = branched = s u r f a c e species w i t h 'Pn+I (n-1) n + l C-atoms and a methyl brand-ing i n p o s i t i o n (n-1)
r e s p e c t i v e l y n+l, b e i n g attached t o t h e surface. The general r e a c t i o n p o s s i b i l i t i e s o f a s u r f a c e species a r e s p e c i f i e d i n Fig. 6. Every arrow i n d i c a t e s a r e a c t i o n step. Species "Sp," species I'Spn-l''
v i a t h e growth r e a c t i o n ''gn-l''.
f u r t h e r l i n e a r l y through s t e p "gn" t o y i e l d species ''SP,+~"
o f a methyl branched species "Spn+,
I'
i s formed from
The species "Sp,"
can grow
o r under f o r m a t i o n
v i a t h e growth r e a c t i o n "gnYbr".
tl' t o 'I a r e i n d i c a t e d as 'Idn P ,a y i e l d the p a r a f f i n p r o d u c t molecule "Prn,sat"y as "dnyOl" t o y i e l d t h e a - o l e f i n
The chemi-desorption r e a c t i o n s o f species "S product "Pr,
" and as ''dn,ox'' t o form t h e oxygenated products t'Prn,ox'' YO1 (these a r e a l i n e a r aldehyde and a l i n e a r primary a l c o h o l , which a r e t h e main
oxygen c o n t a i n i n g p r o d u c t compounds). I n a general k i n e t i c model o f t h e F i s c h e r Tropsch system a t l e a s t these kinds o f r e a c t i o n s have t o be taken i n t o account. Regarding t h e r e a c t i o n p o s s i b i l i t i e s which a r e shown i n Fig. 6, then i n F i g . 7 t h e k i n e t i c scheme o f c h a i n growth and p r o d u c t f o r m a t i o n when s t a r t i n g w i t h an i n i t i a l C, species "Sp,"
i s developed up t o t h e carbon number 7.
I n t h e e a r l y scheme o f Anderson ( r e f . 3 ) branching i s a d m i t t e d w i t h a c a r h number independent branching r a t e constant. Only one s o r t o f p r o d u c t i s v i s u a l i z e d and t h e c h a i n propagation r a t e constants a r e thought t o be carbon number independent. A r e c e n t model o f Wojciechowski and T a y l o r ( r e f . 18) i n troduces a p a r t i c u l a r s t e p o f p o s t branching c h a i n p r o l o n g a t i o n , t h e r a t e
464
F i g . 7. K i n e t i c scheme 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 r e g a r d i n g c h a i n b r a n c h i n g and p r o d u c t d e s o r p t i o n as p a r a f f i n s , o l e f i n s and a l c o h o l s ( f o r m a t i o n o f d i m e t h y l branched c h a i n s o m i t t e d ) . c o n s t a n t o f which i s t a k e n t o be d i f f e r e n t f r o m normal c h a i n growth. The model o f Wojciechowski and T a y l o r d e f i n e s s e v e r a l a b s t r a c t k i n e t i c parameters and w i t h t h e h e l p o f d i f f i c u l t mathematics a procedure f o r c a l c u l a t i n g p r o d u c t d i s t r i b u t i o n s i s b e i n g developed. I n t h e o r i g i n a l paper o f P i c h l e r , Schulz and E l s t n e r ( r e f . 19) t h e m o l a r p r o d u c t c o m p o s i t i o n i s developed on t h e b a s i s o f r e a c t i o n p r o b a b i l i t i e s o f t h e s u r f a c e species. I t was found, t h a t t h e 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
i s carbon number dependent p a r t i c u l a r l y i n t h e range o f small carbon numbers. I n v e s t i g a t i o n s o f Schulz, Rosch and Gokcebay ( r e f . 20) showed t h e r a t e c o n s t a n t o f c h a i n b r a n c h i n g t o be s t r o n g l y p r e s s u r e and t e m p e r a t u r e dependent and a l s o a f u n c t i o n o f carbon number ( r e f . 18). B a s i c a l l y , e v e r y a r r o w i n F i g . 7 i n d i c a t e s a r e a c t i o n s t e p w i t h i t s i n d i v i d u a l r a t e c o n s t a n t . Then t h e p r o d u c t c o m p o s i t i o n c o u l d be c a l c u l a t e d o n l y w i t h t h e h e l p o f a g r e a t number o f p a r a meters. The concept o f a s u r f a c e p o l y m e r i z a t i o n , however, i m p l i e s t h a t t h e r a t e c o n s t a n t s o f r e p e a t e d l y o c c u r i n g r e a c t i o n s t e p s a r e t h e same f o r s p e c i e s w i t h d i f f e r e n t carbon number and t h i s s i m p l i f i e s any model d r a s t i c a l l y . The concept o f t h i s work i s t o f i r s t d e f i n e t h e i d e a l model o f t h e non t r i v i a l s u r f a c e p o l y m e r i z a t i o n , t o compare t h i s w i t h measured p r o d u c t d i s t r i b u t i o n s and t o i n t r o d u c e s t e p w i s e a d d i t i o n a l assumptions i n o r d e r t o adopt t h e model t o a c t u a l p r o d u c t d i s t r i b u t i o n s . I n t h e f o l l o w i n g c h a p t e r t h e i d e a l model i s b e i n g developed. IDEAL MODEL OF THE
NON TRIVIAL SURFACE POLYMERIZATION
The k i n e t i c scheme o f t h e i d e a l F i s c h e r Tropsch p o l y m e r i z a t i o n model i s g i v e n i n F i g . 8. Chain growth i s p o s s i b l e o n l y w i t h one t y p e o f r e a c t i o n s t e p and t h i s i s assumed t o be independent o f c h a i n l e n g t h . Only one s o r t o f
465
Fig.8. Ideal Fischer Tropsch surface polymerization scheme Sp = surface species Pr = product d = desorption n = number of C atoms g = growth The arrow "r0" indicates the formation rate of "species 1". This is equal (for the steady state of reaction) to the sum of all product formation rates. Normalization of the sum of all product formation rates to the value "one" leads to relative rates of product formation which are equal to probabilities of product formation. The kinetic model can therefore be developed on the basis of "probabilities" of individual reaction steps. products is allowed. The mathematic formulation of the model is presented in Fig. 9. According to scheme ( 1 ) in Fig. 9 the rate of formation of surface species "Sp," is equal to its consumption rates through chain growth or product chemidesorption (equation (2)). This scheme imp1 ies stationarity of the system. Assuming first order kinetics of the reaction rates with respect to the surface concentrations o f the surface species of chain growth, equation ( 3 ) is obtained. Regarding the quotient of the reaction rates of chain prolongation to chain termination in equation ( 4 ) it is noticed that relative rates of the reaction possibilities of one surface species are equal to the relative rate constants of the reactions and they are also equal to the ratio of probabilities for the species "Sp," to react in the one or the other direction. The relation between reaction probabilities and rate constants is given in equation (7). It has to be acknowledged that the sum of all reaction probabilities of one species is equal to one (equation ( 6 ) ) . The number of all product moles is defined as one (equation ( 5 ) ) . With these simple definitions the product distribution is easily expressed in terms of probabilities of chain growth and chain termination (equation (8)): The number of moles of Product with "n" carbon atoms "M," is obtained as the product of (n-1) times multiplying the chain propagation probability with once the probability of chain termination. This formula had been earlier given and used by Pichler, Schulz and Elstner (ref. 19) in order t o calculate chain prolongation and chain termination rates as a function o f carbon number from experimental product distributions. When knowing the mole fraction of one product compound, its absolute rate o f formation
466
ASSUMPTIONS FOR THE IDEAL MODEL
(1) CHAINS GROW ONLY LINEARLY (2) ALL SORTS OF RATE CONSTANTS ARE CARBON NUMBER I N DEPENDENT
(3) COMPOUNDS OF SAME CARBON NUMBER ARE TREATED AS ONE PRODUCT
( 4 ) FIRST ORDER KINETICS WITH RESPECT TO THE CONCENTRA-
-
TION OF SURFACE INTERMEDIATES
'
Trdn
I -
Scheme:
hSp,
F i g . 9 . I d e a l model o f F i s c h e r Tropsch s u r f a c e p o l y mer iza t ion ( r = reaction rate relative t o r o = 1.00, g = growth, d = d e s o r p t i o n , Sp = s u r f a c e s p e c i e s , n = carbon number, k = r a t e konstant, p = p r o b a b i l i t y , M = moles o f product). may be c a l c u l a t e d w i t h t h e
rgn
h e l p o f equation ( 3 ) i n F i g . 4 ( F r i and Mni
being
i d e n t i c a l l y defined). For the i d e a l surface p o l y m e r i z a t i o n scheme t h e i=m
!he normalization .L Mi = 1 and IZ1 pg, Pdn = 1.00 ore generolly valid +
(5) 16)
molar product d i s t r i b u t i o n i s now d e s c r i b e d w i t h o n l y one parameter: t h e c h a i n g r o w t h p r o b a b i l i t y "p
'I.
9 The l o g a r i t h m i c f o r m o f
e q u a t i o n ( 8 ) as w r i t t e n i n lg M, = ( n - 1 )
Ig(pg,)
lg(l-pgn)
(9)
F i g . 9 as t h e e q u a t i o n ( 9 ) i s a l i n e a r r e l a t i o n between l o g Mn and carbon number w i t h t h e s l o p e l o g pgn. P l o t s o f l o g a r i t h m i c molar F i s c h e r
Tropsch p r o d u c t d i s t r i b u t i o n s as a f u n c t i o n o f carbon number have been o r i g i n a F l y developed and i n t e r p r e t e d by R. B. Anderson. They a r e p a r t i c u l a r l y u s e f u l
f o r e v a l u a t i n g t h e k i n e t i c s o f a F i s c h e r Tropsch system. I n t h e p i c t u r e o f t h e F i s c h e r Tropsch c o n v e r s i o n as a non t r i v i a l p o l y m e r i z a t i o n t y p e r e a c t i o n t h e p r o b a b i l i t y o f p r o l o n g a t i o n o f t h e c h a i n o f t h e chemisorbed s p e c i e s i s t h e k i n e t i c parameter which d e f i n e s t h e p r o d u c t c o m p o s i t i o n . On t h e o t h e r hand,
i f a l i n e a r r e l a t i o n s h i p f o r l o g Mn i n dependence o f p r o d u c t carbon number i s o b t a i n e d , t h e assumptions f o r t h e i d e a l non t r i v i a l s u r f a c e p o l y m e r i z a t i o n a r e j u s t i f i e d . I t has been quoted i n l i t e r a t u r e t h a t a "CH2" s h o u l d be t h e s p e c i e s which adds t o t h e growing c h a i n , becausea l i n e a r l o g a r i t h m i c d i s t r i b u t i o n had a l s o been
o b t a i n e d f o r polymethylene
from
decomposition
of
diazo-
methane ( r e f . 21). However, t h i s c o n c l u s i o n c a n n o t be v a l i d . The f i n d i n g o n l y i n d i c a t e s t h a t i n b o t h cases a p o l y m e r i z a t i o n model can be a p p l i e d b u t n o t t h a t t h e s p e c i e s o f c h a i n growth i s t h e same. A c c o r d i n g l y , when t h e F i s c h e r Tropsch p r o d u c t d i s t r i b u t i o n can be m o d e l l e d i n t h e same way as a p r o d u c t d i s t r i b u t i o n f r o m s i m p l e homogeneous p o l y m e r i z a t i o n ( S c h u l z - F l o r y t y p e ( r e f .22))
467
t h i s a g a i n o n l y proves t h a t t h e same model assumptions a r e v a l i d . T h e case o f t h e F i s c h e r Tropsch system i s b e i n g termed h e r e as "non t r i v i a l " because t h e "monomer" f o r c h a i n growth i s o n l y b e i n q formed i n a c o m p l i c a t e d s e t o f heterogensously c a t a l y z e d r e a c t i o n s . However, a g a i n t h e v a l i d i t y o f t h e model g i v e s no i n f o r m a t i o n about t h e sequence o f r e a c t i o n s which s u p p l y t h e monomer. EXPERIMENTAL FISCHER TROPSCH DISTRIBUTIONS AS COMPARED WITH THE IDEAL MODEL C a l c u l a t e d i d e a l p r o d u c t d i s t r i b u t i o n s as d e f i n e d by t h e c h a i n p r o l o n g a t i o n probabilities p
J!
= 0.6:
0.8 and 0.9 a r e shown i n t h e upper p a r t o f F i g . 1 0
0.7;
on a m o l a r c a r b o n b a s i s , a m o l a r p r o d u c t b a s i s a n d o n a m o l a r l o a a r i t b m i c o r o d u c t b a s i s . O b v i o u s l y t h e Anderson p l o t , t h e l o g a r i t h m i c molar p r o d u c t r e p r e s e n t a t i o n i s s u i t e d b e s t t o observe d e v i a t i o n s f r o m i d e a l i t y because t h e i d e a l d i s t r i b u t i o n i s j u s t a s t r a i g h t l i n e . The e x p e r i m e n t a l d i s t r i b u t i o n s shown i n t h e l o w e r p a r t o f F i g . 10 f o l l o w t h e i d e a l model f a i r l y : D e v i a t i o n s a r e ( 1 ) t h e
CALCULATED PRODUCT DISTRIBUTIONS:
u
a
a
pg,
.
n-1
0.9
5
n
L
+ u 3
Mn =
0
2
6
10 14 18
0
a
6 10 14 18
2
0
CL
cL
-3
2
6
10 14 18
EXPERIMENTAL PRODUCT DISTRIBUTIONS
0 c3
LL,
0
50
s 40
d 30 IT
20 10
2
6
10 14 18
0
6 10 14 18
2
C A R B O N N U M B E R , Nc
- 3 L2 r r6 r r10r r14r r18r
F i g . 10. C a l c u l a t e d and e x p e r i m e n t a l FT p r o d u c t d i s t r i b u t i o n s
A 100 Co 0 100 Fe 0 100 Co
-
1 8 Tho2
Mn -
-
100 k i e s e l g . ,
187 OC, 1 b a r , H2/C0 = 8, 85 h-',
688 T i
6 K20, 3OO0C, 11 b a r , H2/C0 = 1.8, 500 h-',
530
100 A e r o s i l , 225
OC,
26 bar, H2/CO=1.7,
250 h
( r e f . 12)
468
a d d i t i o n a l f o r m a t i o n o f methane and ( 2 ) m i n o r f o r m a t i o n o f compounds C 2 (and Thus i t i s concluded, t h a t t h e i d e a l model approximates r e a l product,
C-).
d y s t r i b u t i o n s q u i t e c l o s e l y i n s e v e r a l cases. I n Table 2 a 3rd d e v i a t i o n f r o m t h e i d e a l system i s mentioned: t h e change o f t h e s l o p e o f t h e p r o d u c t d i s t r i b u t i o n c u r v e i n t h e Anderson r e p r e s e n t a t i o n i n t h e ranae o f carbon number o f about C I 3 .
T h i s d e v i a t i o n f r o m i d e a l i t y w i l l be discussed i n d e t a i l
i n t h e f o l l o w i n g c h a p t e r . F u r t h e r d e v i a t i o n s w i l l o n l y be t r e a t e d i n f u t u r e p u b l i c a t i o n s because o f t h e p r e s e n t space l i m i t a t i o n s f o r t h i s p u b l i c a t i o n . TABLE 2 D e v i a t i o n s from i d e a l i t y o f FT-product d i s t r i b u t i o n s
(1) A d d i t i o n a l methane f o r m a t i o n
C2 (and C 3 )
(2)
Minor formation o f
(3)
Chanqe o f s l o p e o f t h e d i s t r i b u t i o n c u r v e i n t h e R.B. Anderson-Dlot i n t h e range c12 - c i 4
'
TWO P R O P A G A T I O N PROBABILITY D I S T R I BUT1ONS
Anderson F i s c h e r Tropsch p r o d u c t d i s t r i b u t i o n graphs f r e q u e n t l y s b o w a c h a n q e i n s l o p e o f t h e d i s t r i b u t i o n curves i n t h e range C l 0 t o C I 4 as demonstrated i n F i g . 11. I t has been proposed by Gaube e t a1 ( r e f . 24) and S a t t e r f i e l d e t a1 ( r e f , 25) t h a t these d i s t r i b u t i o n s r e s u l t f r o m t h e s u p e r i m p o s i t i o n o f two i n A EGIEEOR U. COOPER
IG FARBEN Pg, 0.78
I
= -2 0
6
2
10
14
18
2
6
10
14
18
2
6
CARBON NUMBER OF PROOUCT FRACTION Nc
10
14
18
F i g . 11. Experimental R.B. Anderson p l o t s o f FT-product d i s t r i b u t i o n s showing t h e C14 change i n c h a i n growth r a t e c o n s t a n t i n t h e range C l l
-
-
-
0 100 Fe 5.2 CuO 8 K20, 300 O C , 7.1 bar, H2/C0 = 1, 0 Fe (2% K2C03), 240 O C , 10 bar, H2/C0 = 1.1,
100 Fe 100 Co -
A 100 Fe A 100 Fe B
0
110 Mn, 300 O C , 7 . 1 b a r H2/C0 = 1, 1 K2C03 - 2 A1203/Ca0, 220 OC, 10 bar, H2/C0 = 1.25, 716 Mn, 283 O C , 12.4 b a r , H2/C0 = 1.19, 9 Tho2 - 9 Mg - 100 A e r o s i l , 175 %, 9 b a r , H2/C0 = 1.9,
( r e f . 23) ( r e f . 24) ( r e f . 23) (ref.
5)
( r e f . 25) ( r e f . 26)
469
dependent p a r t i a l d i s t r i b u t i o n s which a r e formed on d i f f e r e n t c a t a l y s t s i t e s w i t h d i f f e r e n t chain propagation r a t e constants. I n order t o w o v e t h i s s u g g e s t i o n t h e above developed i d e a l model i s used below t o c a l c u l a t e bimodal p r o d u c t d i s t r i b u t i o n s . The corresDonding k i n e t i c scheme i s shown i n F i g . 12. The c a l c u l a t e d c u r v e s a r e p r e s e n t e d i n F i g . 13. I n t h e upper p a r t of t h e f i g u r e
fd" Pr1
sp,+
SP,+
9
91
' a .
f d l
spn+
-
Prn
Pr 1
sp,,,+
-
Prn
fdl
!l
sp2+
9
91 AND 911 CORRESPOND
***
To
911
spn,*+
DIFFERENT
9
F i g . 12. K i n e t i c scheme 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 assuming two d i f f e r e n t c a t a l y s t s i t e s . 3 diagrams a r e g i v e n f o r t h r e e d i f f e r e n t c o n t r i b u t i o n s o f t h e two o v e r l a p p i n g p r o d u c t s . When comparing these c a l c u l a t e d c u r v e s w i t h t h e ex-
p e r i m e n t a l Ones i n F i g . 1, t h e f o l l o w CONSTANTS AT THE ASSUMED TWO DIFFERENT CAT- ing differences are obvious: ( , ) the ALYST S I T E S I AND I 1 p o i n t o f i n t e r s e c t i o n s h i f t s over a wide range f r o m ca. C3 t o C,9
and ( 2 )
t h e r e i s no d i s t i n c t p o i n t o f change o f t h e s l o p e o f t h e d i s t r i b u t i o n c u r v e s . There i s o n l y a g r a d u a l change i n s l o p e o v e r a ranqe o f c3. 10 carbon numbers t o be n o t i c e d . I t i s concluded t h a t t h e i d e a o f a bimodal two d i f f e r e n t c a t a l y s t s i t e d i s t r i b u t i o n i s n o t supported by t h e model c a l c u l a t i o n s .
I n t b e lower p a r t
o f F i g . 13 t h r e e f u r t h e r c a l c u l a t e d diaqrams a r e shown. I n t h i s case t h e p o i n t o f i n t e r a c t i o n o f tkie two d i s t r i b u t i o n s i s a r b i t r a r i l y k e p t c o n s t a n t a t t h e carbon number 13. However, t h e range o f carbon number i n which t h e s l o p e cbanges i s t o o b r o a d as compared w i t h t h e e x p e r i m e n t a l curves and t h e r e i s n3 argument f r o m t h e two s i t e model, why t h e change i n s l o p e s h o u l d always appear around t h e carbon number 13. Another p r o p o s a l f o r e x p l a i n i n g t h e two s l o p e d i s t r i b u t i o n s i s v e r y c o n s i s t e n t w i t h t h e p e c u l i a r i t i e s o f t h e F i s c h e r Tropsch system: The p r o d u c t s o f F i s c h e r Tropsch s y n t h e s i s do u s u a l l y p r o v i d e a l i q u i d phase and a gaseous phase under r e a c t i o n c o n d i t i o n s . T h e gaseous compounds l e a v e t h e r e a c t o r n o r m a l l y w i t h i n a few seconds. The l i q u i d does need a day o r more u n t i l i t e l u t e s f r o m t h e c a t a l y s t bed. S o l u b i l i t y o f p a r a f f i n i c hydrocarbon vapours i n a p a r a f f i n i c h y d r o c a r -
bon l i q u i d i n c r e a s e s by a f a c t o r o f about 2 f o r each carbon number o f t h e p r o d u c t ( r e f . 2 7 ) . Thus i t needs o n l y an i n c r e a s e o f a v e r y few carbon numbers o f t h e p r o d u c t m o l e c u l e s t o have them l e a v i n g t h e r e a c t o r m a i n l y w i t h t h e gas phase o r w i t h t h e l i q u i d phase. W i t h i n c r e a s i n g r e s i d e n c e t i m e i n t h e r e a c t o r t h e chance of r e a d s o r p t i o n i n c r e a s e s and c o r r e s p o n d i n g l y t h e p r o b a b i l i t y o f c h a i n p r o l o n g a t i o n i n c r e a s e s . The k i n e t i c scheme o f t h i s model i s shown i n F i g . 14. T h i s model i s v e r y c o n s i s t e n t w i t h t h e e x p e r i m e n t a l d i s t r i b u t i o n s .
470
2 CENTER MODEL (MN
=
M,
I
+
MNII
=
l,OO);(P,
I
=
0,65: P,
I1
=
0,851
Mnl-0.396
c
-
u z a
--pg ,II = 0 7
0 0
0
-1
-
-2-
0,
0
-3
4
I
5
lb
I
15
l
20
l
25
CARBON NUMBER OF PRODUCT FRACTION Nc
F i g . 13. C a l c u l a t e d RB-Anderson p l o t s o f FT-product d i s t r i b u t i o n a c c o r d i n g t o t h e 2 c e n t e r model ( q u o t e d by S a t t e r f i e l d ( r e f . 25) and Gaube ( r e f . 24)
Prn, sat
ds\
Prn,ol j o t
S P n - n SPn
9' SPn+l
valid inthe
range n
F i g . 14. K i n e t i c scheme 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 r e cognizing o l e f i n readsorption a t carbon number above n*. (above "n*" t h e p r o d u c t s a r e m a i n l y l i q u i d s under r e a c t i o n c o n d i t i o n s ; n * i s u s u a l l y c l o s e t o carbon number 1 3 ) .
471
CONCLUSION The F i s c h e r Tropsch k i n e t i c s o f p r o d u c t f o r m a t i o n s a r e b e s t u n d e r s t o o d as
a non t r i v i a l s u r f a c e p o l y m e r i z a t i o n . The b a s i c k i n e t i c i n t e r r e l a t i o n s a r e w e l l d e s c r i b e d by an i d e a l model w i t h carbon number independent p r o b a b i l i t y o f c h a i n p r o p a g a t i o n . The p e c u l a r i t i e s o f r e a l F i s c h e r Tropsch systems a r e then d e s c r i b e d as d e v i a t i o n s f r o m t h e i d e a l model. I n t h i s paper (because o f space l i m i t a t i o n s ) o n l y t h e Anderson two s l o p e d i s t r i b u t i o n s a r e d i s c u s s e d and exp l a i n e d by a
r e a d s o r p t i o n e x t e n s i o n o f t h e i d e a l model. The f u l l model i n -
c l u d i n g c h a i n b r a n c h i n g and f o r m a t i o n o f o l e f i n s , a l c o h o l s and aldehydes i s b e i n g p u b l i s h e d s h o r t l y ( r e f . 28). ACKNOWLEDGEMENT F i n a n c i a l s u p p o r t o f p a r t s o f t h i s work t h r o u g h t h e Bund2sministerium f i r Forschung und Technologie i s g r a t e f u l l y acknowledged. REFERENCES
6 7 8 9. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 25 27 28
H. Schulz, C 1 Mol. Chem., 1 (1985) 231 H. Schulz and H. Gokcebay, Proc. N i n t h Conference on C a t a l y s i s o f Organic Reactions, C h a r l e s t o n , S.C., USA, A p r i l (1982) R. A. F r i e d e l and R. B. Anderson, J. Am. Chem. SOC., 72 (1950) 1212, 2307 H. H. S t o r c h , PI. Golumbic and R. B. Anderson, The F i s c h e r Tropsch and R e l a t e d S y n t h e s i s , Wiley, New York, 1951 R. B. Anderson, i n C a t a l y s i s I V , P. H. Emmett ( E d i t o r ) , R e i n h o l d , New York, 1951 E. F. G. H e r i n g t o n , Chem. and Ind., (1946) 347 J.T. Kummer, T.W. DeWitt and P.H. Emmett, J . em. Chem. SOC., 70 (1948) 3632 H.W. S t e r n b e r g and I. Wender, Proc. I n t . Conf. on C o o r d i n a t i o n Chem., London, (1959) 54 S.Z. Roginsky, Proc. I n t . Congr. C a t a l y s i s , Qrd, Amsterdam, 2 (1965) 939 H. P i c h l e r and H. Schulz, Chem. I n g . Techn., 42 (1970) 1162 H. P i c h l e r , H. ScLlulz and B.R. Rao, L i e b i g s Ann. Chem., 61 (1968) 719 H. Schulz and H.D. A c h t s n i t , R e v i s t a Portuguesa Quimica, L i s b o a , 19 (1377) 31 .. 7 . P. B i l o e n , J.N. H e l l e and W.M.H. S a c h t l e r , J. C a t a l . , 58 (1979) 95 Y. A r a k i and V. J. Ponec, J. C a t a l . , 44 (1976) 439 J.A. Rabo, A.P. R i s c h and M.L. Poutsma, J. Catal., 53 (1978) 295 A.T. B e l l , C a t a l . Rev. S c i . Engl, 23 (1981) 203 W.Z. B r o t z , Elektrochem., 5 (1949) 301 P.D. T a y l o r and B.W. Wojciechowski, Can. J. Chem. Eng., 61 (1983) 98 H. P i c h l e r , p . Schulz and M. E l s t n e r , Brennstoffchem., 48 (1967) 78 H. Schulz, S. Rosch and H. Gokcebay, Proc. Symposium Coal Phoenix o f t h e 80s. 6 4 t h Annual C I C Conference, H a l i f a x , Canada, A.M. Taweel ( E d i t o r ) Can. SOC. Chem. Eng. Ottawa (1982) R.C. Brady and R. P e t t i t , J. Am. Chem. SOC., 102 (1980) 6181 P.J. F l o r y , J. Am. Chem. SOC., 58 (1936) 1877 N.O. E g i e b o r and W.C. Cooper, Appl. C a t a l . , 14 (1985) 323 L. Konig and J. Gaube, Chem. I n g . Tech., 55 (1983) 14 G.A. H u f f and C.N. S a t t e r f i e l d , J. C a t a l . , 85 (1984) 370 S. Rosch, D i s s e r t a t i o n , U n i v e r s i t a t K a r l s r u h e , 1980 A.B. L i t t l e w o o d , Gas Chromatography, Academic Press, New York, 2nd ed.,1370 H. Schulz, K. Beck and E. E r i c h , 1986, u n p u b l i s h e d r e s u l t s
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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
THE SHELL MIDDLE DISTILLATE SYNTHESIS PROCESS M.J.
v . d . Burgt and C . J .
van Leeuwen
S h e l l I n t e r n a t i o n a l e Petroleum Maatschappij (SIPM) B.V., (The N e t h e r l a n d s )
The Hague
and J.J. d e l ' h i c o and S.T.
Sie
K o n i n k l i j k e / S h e l l Laboratorium, Amsterdam (The N e t h e r l a n d s ) P r e s e n t e r : I. Maxwell
Abstract A d e s c r i p t i o n i s g i v e n of t h e S h e l l Middle D i s t i l l a t e S y n t h e s i s (SMDS) p r o c e s s . I n t h i s two-stage p r o c e s s , which h a s been developed s p e c i f i c a l l y f o r t h e p r o d u c t i o n of middle d i s t i l l a t e s , a l i q u i d p r o d u c t i s o b t a i n e d which c o n s i s t s t y p i c a l l y of n a p h t h a , k e r o s i n e and g a s o i l i n t h e r a t i o s 15:25:60 t o 25:50:25. Both t h e k e r o s i n e and t h e g a s o i l have e x c e l l e n t b l e n d i n g p r o p e r t i e s : t h e smoke p o i n t of t h e k e r o s i n e can be o v e r 45 mm and t h e g a s o i l h a s a c e t a n e number i n e x c e s s of 70. S t a r t i n g from n a t u r a l g a s , a t h e r m a l e f f i c i e n c y of 60% f o r a s t a n d - a l o n e p l a n t can be a c h i e v e d by u s i n g S h e l l t e c h n o l o g y f o r b o t h syngas manufacture and middle d i s t i l l a t e s s y n t h e s i s . Apart from t h e s y n t h e s i s p e r se, s p e c i a l a t t e n t i o n i s p a i d t o t h e p r o d u c t i o n of t h e s y n t h e s i s g a s w i t h i t s consequences f o r t h e o v e r a l l p r o c e s s e f f i c i e n c y and t h e impact on t h e environment. Over t h e p a s t few y e a r s s u b s t a n t i a l c o s t r e d u c t i o n s have been a c h i e v e d from developments i n c a t a l y s t R&D and from o p t i m i z i n g t h e r e a c t o r s i z e , p r o c e s s l i n e - u p and manufacture of s y n t h e s i s gas. Furthermore a s i m p l e Syncrude mode h a s been developed i n a d d i t i o n t o t h e SMDS k e r o s i n e and g a s o i l modes.
16/0623
473
474
1.
INTRODTCTION For maiiy countries, the long-term supply of automotive fuels could well depend on the ability to produce them from resources other than crude oil. Since the 1,ite 1 9 4 0 s Royal Dutch/Shell companies have been carrying out research a n d development work on hydrocarbon synthesis for the conversion o f various raw materials, such as coal and natural gas, into liquid transpoitation fuels. When crude from the Middle East became increasingly available, some reduction in this work occurred, but interest revived in the early 1970s. Much attention was then paid to developing processes for the indirect liquefaction of coal (coal gasification followed by synthesis), but the high level of investment required for coal conversion remains a major constraint on its commercial application. On the other hand, the conversion of natural gas into liquid hydrocarbons is seen as a more attractive alternative, at least in the medium term, since the specific capital expenditure, expressed in capital investment per barrel of distillate produced per calendar day, is significantly lower than when starting from coal.
2.
NATURAL GAS AS FEEDSTOCK FOR HYDROCARBON SYNTHESIS The following main factors favour the production of synthetic liquid hydrocarbons, notably transport fuels, from natural gas : (i)
Remote and/or small gas fields cannot support the substantial investments required for a gas pipeline system or LNG production facilities; as a consequence, the alternative value of the gas is very l o w .
(ii)
Synthetic fuels which are localiy produced from indigenous natural gas may carry a premium owing to the high costs for the importation of transportation fuels.
(iii) The inherent high quality of e.g. synthetic middle distillates makes it possible to use them as quality improvers in combination with conventional components. (iv)
Energy strategy, e.g. national economic self-sufficiency also play a role.
Furthermore, the use of an alternative such as Compressed Natural Gas (CNG) for transportation fuel is very limited and requires considerable capital expenditure. In many countries there is a growing interest in the production of diesel fuel for automotive use. Production of diesel fuel has the added advantage, because of its chemical composition, that it can be produced from natural gas in a high yield (see Section 3 below).
475
3.
SYKTAESIS GAS NASUFACTURE FROM SATURA!. GAS The production o f syngas before middle distillate synthesis proper is an important process step, not o n l y from a technical point o f view but also from an economic standpoint : some two thirds of the process capital cost (if natural gas is the feedstock) relates tu syngas manuiacturing units. When starting from natural p a s , the most common conversion via steam reforming will produce a synthesis gas with a H,/CO ratio of at least 3 , unless the natural gas used as a feed Qlready contahs carbon dioxide or an expensive C 0 2 recycle is incorporated (Table 1 ) . A s the H / C O consumption ratio for high-quality middle 2 distillates and also for methanol is about 2 and for aromatics about 1 . 6 , it is clear that the combination of steam methane reforming (SMR) with these synthesis processes will always result in surp1.u~ hydrogen production (see Table 2 ) . The most common solution is to burn the surplus hydrogen in the S M R furnace, which implies that the hydrogen leaves the process as steam via a stack. Although theoretically the process is water-balanced, in practice schemes based solely on SMR a s a syngas manufacturing process will require substantial amounts of make-up water. A gas with an H / C O ratio of about 2 can be produced very competi2
tively by non-catalytic autothermal partial oxidation, for example the Shell Gasification Process (SGP) (see Table 1). Such gas is particularly suitable for the production of high-quality middle distillates which are (iso)paraffinic in nature. For the manufacture of these products little or no adjustment of the H /CO ratio is 2 required, consequently giving a high overall process efficiency. I n the case of paraffinic middle distillate production there is a net formation of water, as illustrated by the following reactions :
CH4 +
t
O2
CO
+
2 H2
(-CH2
-)
+
H20
For the conversion of natural gases, which consist predominantly of methane and some higher hydrocarbons, into synthesis gas, the partial oxidation route is therefore a highly competitive one in combination with hydrocarbon synthesis.
476 Table 1
SYN’THESTS GAS
PRODUCTION FROM NATURAL GAS
Chemical Reaction
H /CO ratio in practice 2
A . Partial oxidation (Shell Gasification
cr catalytic partial oxidation) 2 CH4
€3.
+
O2 =
+
2 CO
4 H2
Steam reforming CH4
+
H 0 2
=
+
CO
3 H,
i
Water gas shift reaction CO
+
H20 = CO 2
+
H
2 Table 2
THEORETICAL PROCESS EFFICIENCIES based on Lower Heating Value (LHV)
Product
Methane to
Syngas to product (optimum ratio) 80
84*
*
2.33
ao
82
CH30H
Optimum ratio syngas H,/CO 2.25
80
80**
2.17
78
80
2.06
80
84
2.00
79
83
2.00
65
81
1.50
68
80
1.67
80
83
2.00
Thermal efficiency for making propane from methane : LHV of C-H, Der mole 3 6 -
3 times LHV of CH4 per mole x 100%
**
Thermal efficiency for making n-hexane from syngas with an optimum H2/C0 ratio of 2.17 : LHV of C6H14 per mole LHV of 6 moles CO
+ LHV
of 13 moles H2
+
13 H2
=
Reaction : 6 CO
C H 6 14
-+
H2°
x 100%
4.
THE SXDS COKCEPT
The S h e l l Middle D i s t i l h t e S y n t h e s i s (SMIIS) p r o c e s s i s a two-stage p r o c e s s , which c o n v e r t s S y n t h e s i s p a s , i . e . a mixtur.e o f iiydrogen a n d carbon monoxide, i n t o middle d i s t i l l a t e s . The f i r s t s t a g e , Heavy P a r a f f i n Syr.thesis (HI'S), c o n v e r t s hydrogen and carbon monoxide i n t o heavy p a r a f f i n s by t h e Fischer-Tropsch p r o c e s s . The p r o d u c t d i s t r i b u t i o n i s i n accordance w i t h S c h u l t z - F l o r y , the p o l y m e r i z a t i o n k i n e t i c s , which i s c h a r a c t e r i z e d by p r o b a b i l i t y of c h a i n growth. The p r o p r i e t a r y c a t a l y s t h a s been developed f o r SMDS w i t h a h i g h , i . e . a h i g h s e l e c t i v i t y towards h e a v i e r p r o d u c t s , i n c l u d i n g heavy wax, a n d , a s a consequence, w i t h a low y i e l d of p r o d u c t s i n t h e g a s and g a s o l i n e range ( s e e Fig. 1 ) . It i s e v i d e n t t h a t t h e above approach w a s followed t o a r r i v e a t a h i g h s e l e c t i v i t y towards middle d i s t i l l a t e s , t h e p r e r e q u i s i t e b e i n g a second s t a g e which can c o n v e r t t h e heavy wax f r a c t i o n i n t h e HPS e f f l u e n t v e r y s e l e c t i v e l y i n t o middle d i s t i l l a t e s , t h e Heavy P a r a f f i n Conversion (HPC) s t a g e ( s e e Fig. 2 ) . I n t h e HPC t h e waxy p r o d u c t of t h e HPS i s hydro-isomerized and hydrocracked t o g i v e t h e maximum y i e l d o f middle d i s t i l l a t e s . A commercial S h e l l c a t a l y s t i s used i n a t r i c k l e flow r e a c t o r similar t o a g a s o i l h y d r o d e s u l p h u r i z e r .
The HPC f e e d s t o c k i s mixed w i t h r e c y c l e g a s c o n t a i n i n g hydrogen and i s p r e h e a t e d t o r e a c t o r t e m p e r a t u r e i n a f e e d / p r o d u c t h e a t exchanger and a f u r n a c e . In t h e r e a c t o r t h e c h a r g e m i x t u r e i s c o n t a c t e d w i t h a f i x e d bed of hydroconversion c a t a l y s t under mild c o n d i t i o n s . The HPC can be o p e r a t e d a t a low c o n v e r s i o n p e r p a s s t o maximize k e r o s i n e and g a s o i l p r o d u c t i o n . The h i g h p u r i t y hydrogen r e q u i r e d f o r t h e second s t a g e may be d e r i v e d d i r e c t l y from t h e SMR u n i t o r from s y n t h e s i s g a s by e . g . membrane s e p a r a t i o n .
4.1
INTEGRATION OF A SYNTHESIS COMPLEX
A complex t o s y n t h e s i z e l i q u i d hydrocarbons r e q u i r e s l a r g e energy streams b o t h f o r t h e manufacture o f t h e s y n t h e s i s g a s and f o r t h e s y n t h e s i s o p e r a t i o n i t s e l f . To combine h i g h e f f i c i e n c y w i t h minimum c a p i t a l i n v e s t m e n t t h e elements of t h e complex must n o t o n l y be e f f i c i e n t i n t h e m s e l v e s b u t a l s o be w e l l matched t o one a n o t h e r .
-
using the Shell GasifiSynthesis gas manufacturing technology c a t i o n P r o c e s s f o r p a r t i a l o x i d a t i o n of n a t u r a l g a s i n c o n j u n c t i o n w i t h steam r e f o r m i n g - and SMDS can be r e a d i l y combined i n a complex of h i g h t h e r m a l e f f i c i e n c y f o r t h e p r o d u c t i o n of middle d i s t i l l a t e t r a n s p o r t f u e l s ( s e e Fig. 3 ) . The hydrogen/carbon monoxide r a t i o a c h i e v e d i n t h e g a s i f i c a t i o n p r o c e s s a l l o w s a h i g h u t i l i z a t i o n of s y n t h e s i s g a s i n t h e SMDS p r o c e s s . Moreover, t h e u t i l i t i e s can be i n t e g r a t e d i n s u c h a way t h a t a l l t h e i r r e q u i r e m e n t s - i n c l u d i n g t h o s e f o r a i r s e p a r a t i o n - are i n t e r n a l l y g e n e r a t e d from t h e complex's w a s t e h e a t streams. I f commercially a t t r a c t i v e o u t l e t s are a v a i l a b l e f o r hydrogen a n d / o r steam a n d / o r e l e c t r i c i t y , t h e s e energy s o u r c e s could b e produced i n e x c e s s of t h e complex's r e q u i r e m e n t s , thus r a i s i n g t h e o v e r a l l thermal e f f i c i e n c y .
418
//, Figure 1
100
80
60
%W
I C1 *C2 " E L
/
GAS'
/
40
20
0
0.80
0.75
0.90
0.85
Co (CLASSICAL) Fo (CLASSICAL)
4
0.95
a 3 PROBABILITY OF CHAIN GROWTH e
NEW CATALYSTS _____--_---___------
Figure 2
SHELL MIDDLE DISTILLATE SYNTHESIS SIMPLIFIED FLOW SCHEME
-
r-
FUEL GAS INCLUDING LPG
SYNGAS
FLASH
Hp
]
HPS : WAVY PARAFFIN SYNTHESIS HPC : HEAVY PARAFFIN CONVERSION
wc
H D
TOPS/NAP(ITIIA IGA O N S
~
~
~
~
479 Figure 3
-
SMDS SYNGAS MANUFACTURE
SYNTHESIS
CONVERSION
TES
1 CH4 + - 0 2 2
w
CO + 2H2 w (CH2-)+ H20 HEAVY PARAFFINS
Figure 4
TRANSPORT FUEL APPLICATIONS HIGH QUALITY
GAS OIL KEROSINE
KEROSINE
CETANE NR.: SMOKE POINT (mm) :
> 70 45
TOPS/
GAS OIL
TYPICAL GAS OIL MODE
TYPICAL KEROSINE MODE
480
5.
SMDS PRODUCT QUALITY The low-density products manufactured in the SMDS process are predominantly paraffinic and free from impurities such as nitrogen and sulphur. Both the kerosine and gas oil have excellent combustion properties (smoke point and cetane number), and their cold-flow characteristics meet all relevant specifications even the stringent freezing point requirements of aviation turbine kerosine. They also make excellent blending components for upgrading low-quality stock that would otherwise have to be used in fuel oil. The excellent quality of the products was proved in extensive engine tests.
-
Because of its paraffinic nature, the light fraction (tops/naphtha) makes an excellent chemical feedstock. It is also suitable for use as a gasoline blending component, though catalytic conversion will be required to increase its octane number. The two-stage concept of the SMDS process provides considerable flexibility with regard to the product slate; i.e. product yie1.d~can be varied as illustrated in Fig. 4 , whilst all relevant product specifications can be met or will be exceeded (Table 3). Alternative outlets for SMDS products are, for instance, as paraffinic solvents and as feed material for XHVI (extra high viscosity index) luboils. 5.1
SYNCRUDE MODE For some applications it may be attractive to convert the synthesis gas into a synthetic crude which can be commingled with conventional crude oil. This minimizes the need for separate product storage and transportation systems. Compared to SMDS, this simplified process employs a different catalyst in the synthesis stage and does not include a Heavy Paraffinic Conversion stage for producing the finished middle distillate fractions. The syncrude product is a broad boiling range of hydrocarbons (Table 4 ) , and the relative amounts of individual products can be varied by adjusting the reaction conditions. Alternatively, syncrude products can be processed in existing refineries into finished transportation fuels.
6.
RESEARCH AND DEVELOPMENT A number of catalysts have been examined and optimized, leading to the development of proprietary Shell catalysts both for the selective production of heavy wax in the first stage of the SMDS process or for the direct production of syncrude. For the hydroconversion of heavy wax a totally different catalyst has been developed. Process research has been carried out in bench-scale units. This work has also continued in larger-scale pilot plants, using the newly developed catalysts. Studies in large pilot plants were initiated to investigate the process configuration, to help in the development of the reactor and to prove the process concept. These studies have provided the necessary process design data. The largest of the pilot plants comprised of fully integrated units, including syngas manufacturing and featuring multitubular reactors with a capacity of 2 and 35 bbl/day respectively. Because of the catalyst's long life, a water-cooled, tubular fixedbed type reactor, which can easily be scaled-up to commercial capacity, was selected for the first-stage synthesis (Fig. 3 ) .
481 Table 3 SMDS PRODUCT QUALITY SMDS
Product Quality Gas o i l
Kerosine
Specification
-
Cetane Number Cloud point, "C
-10
40 to 50 -10 to +20
-
Smoke point, mm Freezing point, OC
45 -47
19 -47
70
Tops/Naphtha similar to SR Material
Table 4 SYNCRUDE PRODUCT MODE None 40
Sulphur, nitrogen Pourpoint, "C Typical component range, X wt Naphtha Middle distillate Wax
25 57 18
n-paraffins i-paraffins Olefins Aromatics
84 1 15
Nil
Table 5 COST INDICATIONS DISTILLATE SYNTHESIS
Devel-opedsite, industrialized country
Developing site, developing country
US $ Mill-ion Capex € o r n 10 000 b/d SMDS f n c i l j t y
Feedstock
300
10 MM HTU/bbl at $0.5/KM BTll
F i x e d and other variable costs
$h/bbl.
600
$5/bbl
482 E x t e n s i v e s t u d i e s have a l s o been made of t h e s c a l e - u p t o v e r y l a r g e r e a c t o r s of 6-8 ni d i a m e t e r . Moreover, t h e l i n e - u p of t h e r e a c t o r s i n t h e s y n t h e s i s s e c t i o n h a s been o p t i m i z e d , l e a d i n g t o s u b s t a n t i a l l y i n c r e a s e d middle d i s t i l l a t e s e l e c t i v i t i e s .
7.
ENVIRONMENTAL ASPECTS
The p r o d u c t i o n of s y n t h e t i c l i q u i d hydrocarbons from n a t u r a l g a s h a s a low e n v i r o n m e n t a l impact. A l l waste w a t e r l e a v i n g t h e complex would f i r s t be t r e a t e d t o a l l o w d i s c h a r g e i n t o a r i v e r . A s a g e n e r a l p r i n c i p l e , t h e re-use of p r o c e s s w a t e r and c o n d e n s a t e would be d e s i g n e d t o minimize d i s c h a r g e of w a s t e w a t e r . The o n l y n o n - s o l i d e f f l u e n t s would b e c l e a n w a t e r produced i n t h e p r o c e s s and s u l p h u r - f r e e f l u e g a s e s . A s f o r s o l i d w a s t e , s p e n t c a t a l y s t can c o n c e i v a b l y be r e t u r n e d t o t h e c a t a l y s t m a n u f a c t u r e r f o r m e t a l s recovery. Owing t o t h e e x c e p t i o n a l q u a l i t y of b o t h k e r o s i n e and g a s o i l p r o d u c t s , t h e e m i s s i o n of harmful e x h a u s t p r o d u c t s i s f u r t h e r reduced. 8.
ECONOMICS
C a p i t a l and o p e r a t i n g c o s t s f o r s y n f u e l complexes a r e h i g h l y dependent on l o c a t i o n . Some f i g u r e s a r e g i v e n below which a r e based on S h e l l ' s economic a s s e s s m e n t s . It w a s c a l c u l a t e d t h a t t h e c a p i t a l c o s t ( b a s i s 1986) f o r a 10,000 b b l / d a y p l a n t b u i l t on a developed s i t e i n a n i n d u s t r i a l i z e d c o u n t r y would b e around US $300 m i l l i o n and f o r t h e same p l a n t b u i l t on a d e v e l o p i n g s i t e i n a remote a n d / o r undeveloped l o c a t i o n of t h e o r d e r of US $600 m i l l i o n ( s e e Table 5 ) . For n a t u r a l g a s a t US $0.5/MM BTU, t h e f e e d s t o c k c o s t element i n t h e p r o d u c t i s about US $ 5 / b b l . The t o t a l f i x e d and v a r i a b l e o p e r a t i n g c o s t s a r e e s t i m a t e d a t a n o t h e r US $ 6 / b b l . The t o t a l r e q u i r e d s e l l i n g p r i c e w i l l depend on f i s c a l r e g i m e s , d e b t / e q u i t y r a t i o , t y p e of l o a n s , c o r p o r a t e r e t u r n r e q u i r e m e n t s , t h e premium f o r t h e v e r y good q u a l i t y of t h e p r o d u c t s e t c .
It h a s been n o t e d t h a t , of t h e t o t a l p r o c e s s c a p i t a l c o s t , more t h a n 50% i s r e q u i r e d f o r syngas manufacture, i n d i c a t i n g t h a t improvements i n t h i s f i e l d a r e j u s t as i m p o r t a n t as t h o s e i n t h e s y n t h e s i s p r o p e r .
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
483
DIRECT CONVERSION OF METHANE TO LIQUID HYDROCARBONS THROUGH CHLOROCARBON INTERMEDIATES Charles E. Taylor, Richard P. Noceti, and Richard R. Schehl Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
ABSTRACT The chemical activation of methane and its subsequent conversion to oxygenates or higher hydrocarbons have been the objects of intensive research in the past several years. At the Pittsburgh Energy Technology Center, a novel combination of two existing process concepts has been examined and appears capable of producing higher hydrocarbons from methane with high yield and selectivity. Methane, oxygen, and hydrogen chloride are reacted over an oxyhydrochlorination catalyst in the first stage to produce methyl chloride and water. In the second stage, the methyl chloride is converted to higher hydrocarbons, namely paraffins, olefins, aromatics, and cycloparaffins, over a zeolite, such as ZSM-5. In the process concept described, the final hydrocarbon mixture is largely in the gasoline (Cb-Cl0) boiling range.
I NTROOUCTION There is strong incentive for the development of technology that will enable the conversion of light hydrocarbon gas produced in the indirect liquefaction of coal to liquid products in an efficient manner. State-ofthe-art processing requires that 1 ight hydrocarbons produced during coal gasification or Fischer-Tropsch synthesis be reformed to hydrogen. and carbon monoxide and recycled to the synthesis units. A direct methane conversion process should result in less expensive and more efficient utilization of this by-product gas. In addition, current estimates of worldwide natural gas resouces, coupled with demographic patterns, indicate that a large fraction of this carbon source is located in areas far from the marketplace. In most instances, efficient modes for the delivery of this resource to the world energy market are not available. For this reason, much research has recently been directed toward the development of less complex or more efficient chemical processes than those currently available to convert natural gas to liquid fuels amenable to transport. Advanced technologies now under consideration for the conversion of methane to more useful materials involve halogenation [ 11 , oxychlorination [ Z ] , oxidation, including oxidative coupling and metal oxide reactions 131, reaction 4 t h Superacids 141, and various other methods (51.
484
Reforming of methane to synthesis gas for subsequent conversion to methanol or production of gasoline and diesel fuels from methanol via Mobil technology is commercially available. With the announcement of the discovery o f a novel family of medium-pore (5-6 R ) , shape-selective zeolites by Mobil in the early 1970's, an entirely new avenue was opened for the production of fuels and chemicals from synthesis gas. The most publicized member of this family, ZSM-5, converts alcohols and olefins to gasoline in high yield without rapid coking [6]. The unique structural characteristics of this zeolite not only inhibit the formation of carbonaceous deposits within the catalyst pores but also restrict the formation of hydrocarbons greater than about Cia. Fortuitously, this corresponds very closely to the end point o f the gasoline boiling range. Although later patents from Mobil claimed that ZSM-5 would convert other monosubstituted methanes to higher hydrocarbons, methanol was primarily the feedstock of interest. In 1982, Ione et al. (71 reported that the conversion products of monosubstituted methanes over zeolites were independent of the substituent and depended only on the reaction conditions for a given catalyst. If methane can be selectively monofunctionalized in one step, then a facile route for its conversion to higher hydrocarbons would present itself. In work performed at Allied Chemical Corporation, Pieters et al. [8] and Conner et al. [91 reported the selective functionalization of methane by reaction with oxygen and hydrogen chloride over a supported oxyhydrochlorination catalyst to give chloromethanes. The advantage of this catalytic chemistry are significant. Conditions are mild, and conversion and product distribution are functions of feed stoichiometry and temperature. The use of hydrogen chloride to produce chloromethane suggested a two-step process, outlined in Figure 1, in which the step involving the conversion of chloromethane to hydrocarbons provides hydrogen chloride for methane chlorination. Hydrogen chloride and chlorine are essentially placeholders in the methyl and methylene synthons. This is the basis of the two-stage concept proposed by the Pittsburgh Energy Technology Center. EXPERIMENTAL Both oxyhydrochlorination of methane and chloromethane oligomerization studies were conducted in essentially the same microreactor system. All the reactants were introduced at pressures slightly above one atmosphere from gas cylinders, and flow rates were controlled by a Brooks four-channel mass-flow controller. The feed stream for the oxyhydrochlorination reaction was sampled before and after an experimental run, while the product stream was continuously sampled on-line during the run to obtain a mass balance. A quadrupole mass spectrometer was used to analyze the feed and product
485
CUCl
HCI
CHiCI
gasoline Fig. 1. Cyclic Pathway for the Conversion of Methane to Gasoline. streams. The product o f the oligomerization reaction was collected at dry ice temperatures and analyzed on a Hewlett-Packard 5880 capillary column gas chromatograph equipped with a Hewlett-Packard 5970 mass-selective detector. The catalysts were contained between quartz wool plugs inside a horizontal quartz tube reactor (1-cm-i.d. by 35-cm-long). The reactor was heated with a split tube furnace, and the temperatures were controlled by a feedback controller. The reactants were preheated to 175OC before entering the catalyst zone. The temperature o f the product stream was maintained at 150OC to prevent condensation in the sample chamber or capillary inlet to the mass spectrometer. The oxyhydrochlorination catalyst was prepared according to the procedure of Conner et al. (91. The material was prepared in nonaqueous solvents by successive impregnation of metal chloride salts onto a silica support. A saturated solution of copper (I) chloride in acetonitrile was sorbed into fumed silica (325 m2/gram). The acetonitrile was then removed under reduced pressure. A solution o f potassium chloride and lanthanum chloride in formic acid was added to the cuprous chloride coated silica. The formic acid was removed under reduced pressure to produce the layered oxyhydrochlorination catalyst. The weight composition o f the final catalyst was 41.7% CuC1, 37.5% SiOz, 11.5% KC1, and 9.4% LaC13. The catalyst could be activated in a stream of hydrogen chloride at 300oC for ten minutes. The ZSM-5 catalyst was obtained from the Mobil Oil Corporation in the ammonium form with a silica-to-alumina ratio of 70:l. The ammonium form was converted to the hydrogen form by calcining in air at 538oC for 16 hours. The residence time of the oxyhydrochlorination reaction was changed by varying the flow o f the reactants and inert gas (nitrogen) for a fixed
486
quantity o f catalyst. The reaction temperature was typically maintained at 342oC, and the total pressure was about one atmosphere. The reaction conditions for the conversion o f methyl chloride were typical of those reported for methanol conversion, i .e., reaction temperature of 350OC, LHSV=l, one atmosphere pressure, and one gram of catalyst. RESULTS The conversion of methane to chloromethane has been observed under various reaction conditions. The data at constant temperature and pressure, summarized in Table I, display several trends. Note that methane conversion and polychlorination both decrease with space velocity. The production of
TABLE I. Conversion of Methane Over an Oxyhydrochlorination Catalyst Space Velocity, GHSV Temperature, OC Pressure, Atm. CH4 Conversion, Mole % Carbon Product Distribution, Mole % CH 3 C1 CHzC12 CHCl3 cc14
HCOOH
co co 2
47 331 1 42.7
94 330 1 22.3
156 331 1 18.4
59.6 27.9 4.6 0.01 0.8 0 7.1
74.8 22.7 1.5 0.01 0.3
85.1 10.7 0.6 0.01 3.1
0 0.8
0 0.6
'Feed stream consisted of 40% CHr , 40% HC1 , and 20% 02.
carbon dioxide and formic acid also varied with space velocity. As the space velocity decreased, the amount of carbon dioxide produced increased, while the amount of formic acid decreased. Carbon monoxide was not detected in the product stream. The reaction exhibited a marked temperature dependence. The highest level o f methane conversion occurred at a temperature of approximately 345OC. This temperature is slightly below the eutectic melting point of the supported catalyst. If the catalyst temperature is allowed to exceed about 35OoC, a decrease in conversion is observed, probably owing to loss of active surface
487
area. The oxyhydrochlorination catalyst is also susceptible to deactivation by exposure to oxygen in the absence of methane, at temperatures greater than 1OOOC. Reactivation requires passing hydrogen over the catalyst at temperatures between 280OC and 300OC. The catalyst is stable in air at ambient temperatures but is hygroscopic. Surface moisture is indicated by a color change from brown to green. The water can be removed by heating above 100°C under an inert atmosphere, but the activity of a catalyst that has been hydrated i s less than that o f one that has not. The conversion of chloromethane over ZSM-5 to gasoline-range hydrocarbons occurred under conditions comparable to those for the conversion of methanol. The reaction was typically conducted at constant temperature, whereas conversions and product distributions were determined as functions o f space velocity or catalyst time-on-stream. The mass-selective detector allowed identification of most of the components in the liquid samples. Generally, the products contain ten carbons or less, and a large fraction of the products are aromatic. The ZSM-5 catalyst was stable under extended exposure to chloromethanes. Figure 2 illustrates the catalyst activity after nearly 700 hours o f exposure to chloromethane, during which time the catalyst had been oxidatively regenerated to remove coke that had been deposited on the catalyst. +
c
k Q)
100.0--$
I
-
I
I
I
I
I
I
I
1.04WHSV 350 "C I Atmosphere
-
Z a
99.2-
-
g 0
98.8-
-
z
L
0 W
>
-
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.
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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
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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.
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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
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.
-
-
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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
mixtures were a 1000 p p mix of C, N2
t o C6 normal paraffins diluted i n
(Alltech) and a special gas mixture of 6% CH,,, 31% C02, 38% CO and 25%
(CIG). Screening was carried at at atmospheric pressure and various tanperatures i n a single pass fixed catalyst bed reactor. For Fischer-Tropsch screening, the flow rates of Ar, CO and H2 were 8 mlfmin, 1 mlfmin and 2 ml/min respectively. The sample s i z e was approximately 0.25 g. A l l catalysts were pre-reduced i n
H2
s i t u at 400°C for 2 hours i n a 5 mlfmin H2 f l m . The samples were packed between quartz mol plugs. A l l samples were dried i n vacuo before weighing. For Temperature Programmed Reduction Studies (TPR) the heating rate used w a s 0.2"Cfs.
'he H2 and Ar flow rates used were 0.48 mlfmin and 7.1 mlfmin
respectively. Where calcined catalysts were used, the sample s i z e w a s approximately 40 mg, whereas the TPR profiles of activated catalysts were obtained using 0.25 g samples. The activation procedure used w a s as described for I T screening. f i e tanperature range was from room tanperature t o 800°C. For Temperature Progarmned Carbiding Studies (TEC) CO was substituted for H 2 . The CO
and Ar flaw r a t e s used were 0.47 ml/min and 3.9 ml/min respectively. The heating r a t e w a s O.l"C/s. 'he sample s i z e w a s approximately 0.25 g. Each sample w a s activated as described for I T .screening. Calibration of the TFC runs was performed on a Stanton Redcroft TG750 t h m b a l a n c e . A 12.5 mg sample of Co/TiO2 was reduced i n flawing H2 a t 400°C and then carbided i n pure CO at a heating rate of 5"Cfmin. Dispersions were measured using a modified Mettler BE22 balance.
Pressed
sanples of approximately 30 mg were oxidised a t 200°C under 100 mbar of 0 2 for 12 hours followed by reduction under 100 mbar of H2 at 400°C for 12 hours before exposure at rocan temperature t o CO. In calculatirg dispersions it w a s assmed t h a t one atan of CO associated with one surface metal atom. RESULTS
The w% loadings of the calcined supported metal catalysts are given i n Table 1 along with the BET surface areas, the % dispersion measurements and quantitative TPR and TFC data. f i e data is ordered i n terms of increasing
511
TABLE 1 Characteristics of Cobalt Supported Catalysts
Catalyst
Wt%% Coa Surface area (m2IgP
Co/Ti02 Co/SiOZ co /Mgo Co /A12 0 3 Co/SiO2 40A Co/z eoli t e
10.9 8.08 8.18 7.88 8.12 7.77
5.8 6.0 48.9 114.9 410.4 418.5
% Disper-
% Co
sionc
0.043 0 0.82 3.5 4.8 5.7
reduced I I1 111 65 9 5 20 20 25 45 40 35 35
-
150°C TPC peak relative arease
-
-
20(i)
25 25 25(i)
100 90
-
60
-
aAtomic adsorption determination. b€ET model N2 measurement. CCO gavimetric uptake measurement. dTPR study. T P C study. lInconplete reduction a t 800°C. surface area. The cobalt dispersion increases as the surface area increases, although there i s no simple correlation between the two variables. ?he Co/A1203 catalyst has the largest dispersion per unit surface area of support.
0
100 200 300 400 500 600 700 800 0
100 200 300 400 500 600 700 800
temperature ("C)
Fig. 1. Teqerature Programred Reduction Profiles for some cobalt-supported catalysts previously calcined a t 400°C i n a i r for 24 hrs. & t e different scales are used for the two sets of results. The TPR profiles of the s i x calcined catalysts along with the profile for unsuPPorted a 3 0 4 are sham i n Fig. 1. The distribution of cobalt species varied depending on the support. Essentially some combination of three rain reduction phases, designated I (-420"C), I1 (-520°C) and I11 (>62O0C) occurred
512
on the catalysts. They are due t o the reduction of bulk Co,O,, &I*+ i n 2 f i n tetrahedral sites ("surface spinel") respectively. octahedral sites and Co The bulk oxide reduction proceeds by two stages CO~O,, > - Ia Coo >- Ib Co. The area of the second peak, I b , should be three times the area of the f i r s t peak, I a , when the stoichimetry of the reaction i s considered. In addition, some evidence for a fourth phase (-809°C)
due t o the reduction of cobalt aluminate
(refs. 1,2) was evident on Co/A1203. TPK profiles were also run on samples which w e r e pre-reduced i n s i t u for 2
h r a t 400°C with a H2 f l m r a t e of 5 ml/min. In general, cobalt associated with the reduction phases I and I1 was pre-reduced, while that associated with phase I11 reduction did not reduce. However, i n addition, a substantial metal support interaction occurred t o give new oxidic species especially on the low surface area Si02 and Ti02 supports. TFC experiments revealed four main peaks centred a t 1 5 0 ° C , 300"C, 400°C and
500°C
on the cobalt catalysts with Ti02 and high and lm surface area
Si02 supports. With A1203 only a lm temperature peak (-175°C) was observed, with no significant peaks a t higher tanperatwe. No evidence of CO consmption
was obtained using the NO or zeolite supports. 'Ihe four TFC peaks have been assigned t o dissociative chemisorption of CO (150"C), formation of bvlk carbide
(300OC)
and two coking phases
(400°C
and 500°C).
These observations are
generally consistent with themgravimetric carbiding studies
(ref.
3) on
related Fe catalysts. The
activities
of
the
cobalt
supported catalyst
for
Fischer-Tropsch
synthesis a r e sumnarised i n Fig. 2 i n term of yields of C 1 t o C3 hydrocarbons. Only data a t 250°C and 325°C are presented although other temperatures have been examined. The cobalt on QO, on the zeolite support, and on the blank
L*
supports alone, w e r e inactive. 'Ihe same groupings occurred with respect t o CO
t* ,
consunption i n the TFC runs.
%
25 U
CO/S~OZ ~oA Co/A1203
~~
conioz ColsiOZ
.
, 250;c
,
.
325' C
2 5
2 G
Co/SiOZ 40A ColA1203 co~i02 ColsiOZ
0
4
8 12 CONVERSION I%]
16
20
Fig. 2. Activity of various cobalt supported catalysts for Fischer-Tropsch synthesis a t 25" and 325°C.
513 DISCUSSION
Temperature F'rograrmned Reduction and Carbiding Results By comparison with the TPR profile for Co304 (Fig. la) i t is concluded that on the low surface area Si02 (Fig. lb) and on the Ti02 support (Fig. lc) the supported cobalt oxide a f t e r calcination i s predominantly Co304.
On alumina (Fig. Id) reduction of a l l three types of cobalt is evident with phase 111 reduction, caused by C02+ i n tetrahedral sites, being dominant. Diffuse reflectance spectroscopy was used t o confirm the presence of tetrahedral Co2+. C h high surface area s i l i c a with a 40A pore size, denoted A (Fig. l e ) , the two st%es of Co304 reduction, resulting i n phases Ia 40 and Ib, a r e w e l l separated i n temperature. Approximately half the cobalt is i n SiOp
octahedral s i t e s , resulting i n substantial phase I1 reduction, and a minimal a m n t in tetrahedral sites. h i n s i t u X-ray photoelectron spectroscopy study was used t o show that Co3+ w as reduced t o Co2+ (corresponding t o phase Ia) a f t e r treatment of the Co/Si02 40A catalyst a t 325°C for 5 min with l00 m b a r H2.
Ib).
Reduction a t 350°C for 10 min saw the appearance of COO (reduction phase Residual Co2+ was present (phases I1 and 111) which w e r e not totally
reduced even a f t e r H2 exposure for I2 hrs a t 380°C. Similar high temperature reduction peaks have been reported by others (refs. 4,5). Assignment of the peaks as being due t o the reduction of cobalt s i l i c a t e s t o Coo and Sio is not quantitatively consistent with the evidence obtained i n the present work. Instead the surface C02+ networks reduce t o Coo without reduction of the
.
support The Co/zeolite catalyst (Fig. I f ) w a s reduced only t o the extent of -60 wt% a t 800°C. 'Be unreduced cobalt is a t t r i h t e d t o tetrahedral Co2+ species (phase 111) since they exhibited the characteristic intense blue colour of tetrahedral Co2+ ions. 'Ihe cobalt on mgnesiun oxide (Fig. 1g) also underwent reduction t o only -6oWt% up t o 800°C. 'Ihe s h i f t of the phase I1 and 111 peaks t o higher temperatures is evidence of a stronger metal-support interaction for these c a t a l y s t systems. R o reduction phases for cobalt on magnesiun oxide have been previously reported (ref. 6). h e y were assigned t o the bm stage reduction of Co301, rather than d i s t i n c t Co2+ species. The pre-reduced TPR results are consistent with Coo derived from
Co304
combining with the Si02 or T i 0 2 supports t o form cobalt s i l i c a t e s (e.g. Co2Si04) or cobalt titanates (e.g. COTi03) respectively. Quantitative analysis of the peak from 520°C indicates that -35 per cent of the Co originally present a s C03Q formed Co2SiO4 and -10 per cent formed CoTiO,. 'Ihe balance was reduced d i r e c t l y t o Coo i n the pre-reduction step. These two catalysts had the least favourable i n i t i a l dispersion (Table 1). Some evidence was also obtained for t h e formation of cobalt aluminate (C0A1204) from COO. 'Ihis is consistent with previous work (ref. 7).
514
Chly high surface area catalysts exhibited the hi& temperature TPR peaks characteristic of dispersed Co2+ interactive species. Such materials are mre l i k e l y t o have available octahedral and tetrahedral defect sites on or near the surface for the Co2+ ions. h e chemical nature of the support was not always a primary consideration, as instanced by the results for hi& and low surface 2+ area s i l i c a samples. It is these Co interactive species which give r i s e t o well dispersed metal clusters. On the basis of the temperature prograrmned reduction and carbide studies with T i 0 2 and hi& and lm surface area SiO, supports it is evident t h a t there was a direct correlation between the amount of Coo derived from phase I reduction of bulk Cojoi, and the amount of dissociative CO chemisorption (peak a t l5O0C), carbide formation (peak a t 300°C) and coke formation (peaks at 400 and 500°C) (ref. 8).
Fi scher-Tr opsch Activity Only the catalysts which exhibited Fischer-Tropsch a c t i v i t y consumed CO i n TFC runs. h i s suggests that carbiding and FT polymerisation proceed v i a a c m n i n t e m d i a t e formed by dissociative CO chemisorption. A l l active FT
catalysts examined promoted dissociative CO chemisorption below 200°C but none of these catalysts showed FT activity below 250°C. h i s is consistent with a C Y , stepwise polymerisation being the r a t e determining step, as advocated by many workers (ref. 9).
The amount of metal available on the catalyst surface following b 3 0 s reduction rather than its dispersion determines the extent of CO chemisorpt i o n , and so the presence or absence of FT activity. Dissociative chemisorption, carbide formation and coke formation appear therefore t o have been dynamic processes with migration of Coo atom occurring as progressive carburization of m e y l clusters occurred. hus metal dispersion, as measured by CO absorption at room temperature, is of questionable u t i l i t y i n terms of a
description of FT a c t i v i t y since the Coo atoms are s t a t i c . The enhanced methane yields at temperatures >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.
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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
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.
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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
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 (<151n*g-~),
r o u t e s t o h i g h e r s u r f a c e area m a t e r i a l s have a l s o been developed (3). Ruthenium c a r b o n y l , Ru3(CO)1zY r a t h e r t h a n RuCl3 was s e l e c t e d as t h e m e t a l p r e c u r s o r , w i t h a view t o m a x i m i s i n g t h e d i s p e r s i o n and m i n i m i s i n g s u r f a c e c o n t a m i n a t i o n by anions.
O x i d a t i o n - r e d u c t i o n c y c l e s have a l s o been r e p o r t e d t o
i n c r e a s e d i s p e r s i o n o f ruthenium and s e l e c t i v i t y t o h i g h e r hydrocarbons and, i n p a r t i c u l a r , t o o l e f i n s (4,5). r e p o r t e d here.
Some p r e l i m i n a r y r e s u l t s o f o u r s t u d i e s a r e
530 EXPERIMENTAL ( i ) Materials. s o l - g e l method.
Lanthana and y t t e r b i a were prepared from t h e i r h y d r o x i d e s by a C e r i a and holmia were prepared v i a i n t e r m e d i a t e h i c a r b o n a t e s
( 3 ) . Ruthenium carbonyl , Ru3(C0)12, was adsorbed from n-heptane s o l u t i o n under a n i t r o g e n atmosphere o n t o t h e oxide, p r e t r e a t e d by c a l c i n i n g a t 873K. (ii)
C h a r a c t e r i s a t i o n Procedures.
Temperature programmed r e d u c t i o n (TPR)
p r o f i l e s were o b t a i n e d i n 3% H7/N2 on 50mg samples o f t h e supported c a r b o n y l s . The e x i t stream was m o n i t o r e d b o t h by TCD, f o r H2 uptake, and FID, f o r hydrocarbon f o r m a t i o n .
Comparative d i s p e r s i o n and r u t h e n i u m p a r t i c l e s i z e s were
o b t a i n e d by hydrogen c h e m i s o r p t i o n i n a dynamic system.
Specimens were heated
i n 3%H2/N2 a t 623K f o r 2 h r ; t h e system was t h e n swept w i t h n i t r o g e n b e f o r e b e i n g c o o l e d t o 473K, whereupon 3% H2/N2 was f l o w e d t h r o u g h f o r t h e a d s o r p t i o n measurement.
The e f f e c t on r u t h e n i u m d i s p e r s i o n o f o x i d a t i o n f o r l h r i n 1%
02/He a t 623K, f o l l o w e d by r e - r e d u c t i o n , was a l s o o b t a i n e d i n t h i s way. (iii)
Fischer-Tropsch Synthesis.
A down-flow m i c r o r e a c t o r , t o g e t h e r w i t h i t s
a s s o c i a t e d a n a l y s i s and c o n t r o l f a c i l i t i e s has been d e s c r i b e d ( 6 ) .
Prior t o
r e a c t i o n each c a t a l y s t was reduced i n f l o w i n g hydrogen a t 623K f o r 2hr.
For
r e d i s p e r s i o n experiments, r e d u c t i o n was f o l l o w e d by o x i d a t i o n a t 623K f o r l h r i n
1%OplHe, and a r e p e a t r e d u c t i o n step. RESULTS AND DISCUSSION (i)
Catalyst Characterisation.
A d s o r p t i o n of R u ~ ( C O )f r~o m ~ n-heptane s o l u t i o n
was found t o be v e r y poor o n t o e i t h e r commercial o r o t h e r low area RE0 b u t t o o c c u r q u i t e r e a d i l y o n t o t h e h i g h area m a t e r i a l s r e p o r t e d here ( T a b l e 1).
Their
s u r f a c e s a p p a r e n t l y d i f f e r n o t o n l y i n h a v i n g h i g h e r areas b u t a l s o i n t h e n a t u r e o f t h e species exposed.
S i m i l a r a b n o r m a l l y h i g h s u r f a c e energy has been
n o t e d f o r h i g h area MgO (8). TABLE 1 ADSORPTION OF NITROGEN AND HYDROGEN ON RUTHENIUM/REO ~
S~pport
Np BETl m g-
Ru *%a
La203
54
1.8
Ce02
170
5.0
60
4.5 1.8
Ho203 Yb203
45
1.0
~~
Pretreatb
r
r-0-r
r
r-0-r
r
r
r-0-r
r
r-0-r
p
g-
rfoles H2 cat.
Dispersion H/Ru
19.3 33.0 211.0 169.0 88.5 40.0 39.8 17.5 20.7
0.21 0.37 0.84 0.67 0.39 0.45 0.44 0.35 0.41
d nmc 5.3 3.0 1.6 2.0 3.4 3.0 3.0 3.8 3.2
a ) m e t a l l o a d i n g s determined f r o m TPR areas a f t e r 1%OplHe, l h r , 623K, based on z x i d i s e d 1% b) r = Leduced 3% Hp/N2, 623K, 2hr; r o r = Ru(IV) --> 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
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ZEOLITES AND OTHER CATALYSTS
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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*<36; 3*<5 2,2-di-~e-C4;2,3-di-Me-C5 2,2,3-tri-Me-C4; 3-Et-Cg 2,4-di+e-C6; 2,2,4-tri+e-C5
(Me = methyl) (Et = ethyl)
Dianin's canpound therefore has a potential for selecting particular isomers from Cg to C8 paraffin mixtures. SPATIAL CHARACTERISTICS OF MICROPORES IN 3-D FFWlEWFXS
In zeolites, porosils and AlpO's of k n m structures we can be rather clear about windm dimensions, shapes, volumes and connectivities of pores - in contrast with microporous but amorphous sorbents. Because of the grwing diversification of porous crystals of knwn structures it may be timely to review spatial characteristics of their pores. The pores are polyhedral; their faces are polygonal and, where shared with other polyhedra, may provide windms through which molecule diffusion can occur.
543
The Polygonal Windows. The polygonal faces of polyhedra are formed of linked TO4 tetrahedra, 4,5,6,8,10 or 12 in number. Inner peripheries of each polygon are always lined by oxygens equal in number to the tetrahedra forming the polygonal ring or window. 4- and 5- rings have free diameters mch too smll for molecules to pass through them. 6- ring windows are permeable only by small molecules (eg H20). Thus the inportant windows are 8-, 10- and 12 rings. Free dimensions of windows and connectivities of channel system are illustrated for some zeolites and zeosils in Table 1. The examples chosen are grouped, sanewhat arbitrarily, into narrow, intermediate and wide port sieves. Arbitrariness arises inter alia because
(i ) molecule diffusivities increase exponentially with temperature; (ii) window dimensions m y be distorted by the guest molecules; (iii) there m y be cations in or adjacent to windows which can obstruct the diffusing molecules; (iv) there m y be detrital material within the crystals; and (v) there m y be crystal defects such as stacking faults. It is well established that cation exchange can decisively alter molecule sieving capabilities of zeolites. Stacking faults can be equally decisive, eg. ideal cancrinite hydrate and gmelinite would be wide port sieves (Table 1). In fact stacking faults turn the cancrinite into a non-sorbent and the qlinite into a narrow port sorbent. Estimates by Flanigen et al. [ 131 of free dimensions of windows in some AlPO's are given in Table 2. These estimates do not allow for the variations in shapes of windows, as seen in Table 2 for zeolites and zeosils. The values nevertheless show that windows in AlpO's have about the same range in free dimensions as in zeolites.
544
TABLE 1
Free dimensions of windows in some zeolites and zeosils (a) Narrow port Dimensions(a ) Window Crystal Levynite 8-ring 3.3 x 5.3 Chabazite 8-ring 3.6 x 3.7 8-ring Erionite 3.6 x 5.2 8-ring Zeolite ZK-5 3.9 Double 8-ring Paulingite 3.9 Zeolite RHO Double 8-ring 3.9 x 5.1 Laurnontite 10-ring 4.0 x 5.6 Linde A 8-ring 4.2 8-ring Lkca-dodecasil 3R 4.5 Crystal (b) Intermediate port Dachiardite Ferrierite ZSM-5 (and Silicalite I) ZSM-11 (and Silicalite 11) (c) Wide p r t ZSM-12 Cancrinite O f f retite Mordeni te melinite Zeolite L Mazzite Faujasite * 3-D for
Window 10-ring 8-ring 10-ring 8-ring 10-ring 10-ring 10-ring
12-ring 12-ring 12-ring 8-ring 12-ring 12-ring 8-ring 12-ring 12-ring 12-ring small molecules only.
Channel system 2-D 3-D 3-D 3-D 3-D 3-D 1-D 3-D 2-D
Dimensions(A) Channel system 3.7 3.4 4.3 3.4 5.1 5.4 5.1
x 6.7 x 4.8 x 5.5 x 4.8 x 5.6 x 5.6 x 5.5
5.7 x 6.1 6.2
6.4 3.2 x 5.2 6.7 x 7.0 7.0 3.6 x 3.9 7.1 7.4 7.4
2-D 2-D 3-D 3-D
1-D 1-D 1-D or 3-D* 1-D 1-D or 3-D* 1-D 1-D 3-D
545
TABLE 2 Approximate free dimensions of windows in sane AlpO's All% number Window Structure type Windows dimension( A ) (a) Narrow port 14 -4 Novel -4 33 Novel Novel 39 -4 8-ring Erionite 17 4.3 18 Novel 4.3 26 Novel 4.3 34, 44 & 47 Chabazite-type 8-ring 4.3 35 Levynite 8-ring 4.3 Linde A 8-ring 42 4.3 ( b ) Intermediate port 11 10-ring Novel -6 Novel 31 -6.5 43 Novel -6 (c) Wide port 40 Novel -7 46 Novel -7 Novel 12-ring 5 -8 Novel -8 36 Faujasite 12-ring -8 37
Channel system
-
3-D
-
3-D 2-D 3-D 1-D -
-
-
1-D
-
3-D
polyhedral Cavities. Because the T-atoms in zeolite, porosil and Alp0 fraaneworks are embedded in tetrahedra of oxygens, all channels and pore walls ideally present only oxygen surfaces. In zeolites these surfaces may be studied with the cations needed for electrical neutrality. Table 3 illustrates the types, and free dimensions and volmes, of polyhedral cavities 'found among zeolites and porosils. The free volumes 'are estimated by approximating the cavity interiors as spheroids, ellipsoids or short cylinders, as most appropriate. There is an 8-fold increase in the pore volume in going from 12-hedra to 26-hedra. The cavities are described as indicated for the 14-hedron in sodalite hydrate for example. "here are eight 6-ring and six 4-ring faces, so the polyhedron is described as 6846. A given zeolite or prosil normally contains m r e than one cavity tw, as the table indicates, and many structures can be built by appropriate stacking of these cages, with certain shared faces, so as to fill all space. As an illustration Fig. 3 [ 121
546
shows the three kinds of cage found in deca-ddecasil 3R. This porous crystalline silica could be of interest as an organophilic mlecular sieve with its 8-ring windcws -4.5 A in free diameter (Table 1). Sometimes the channels show no clear division into cavities linked through shared windows. Thus Fig. 4[201 shows the almost smoothly cylindrical channels in AlFG-5 ( W I ) and those in ideal cancrinite ( C A N ) . TABLE 3 cages found in 3-D 4-connected nets. Cavity Cavity Approx: internal Approx. free faces type dmnsions ( A ) volume (A31 Some
-
NOMSil Paulingite ;zeolite RHO
9 10
4158 4882
10 11
435661 4665
12
512
5.7
100
12 14
435663 4668
5.7 6.6
100 150
14 14 16 17 17 18 19 20 20 20
51262 496283 51264 496583 46611 41286 435126183
5.8 x 5.8 x 6.0 x 7.4 x 7.5 9.0 x 7.0 x 7.7 x 6.4 x 10.8 x 6.6
7.3 6.4
140 170 220 240 170
4126286 58612 51268 4126586 4126886
11 x 6.5 x 6.5 9.1 x 8.8 x 7.0
350 370 290
11.2 x 7.7 x 7.7 15 x 6.3 x 6.3 11.4
350 470 780
23 26 26
3.9 ( PAU ) 3.9 x 5.1 (RHO)
-
-
4.7 x 3.5 x 3.5
30
7.7 7.4
-
Present in
-
860
Bca-ddecasil 3R. Cancrinite;zeolite L lcscd; erionite; offretite;AlFG-17. Melanophlcgite ; dcdecasils 1H & 3C;ZSM-39; deca-ddecasil 3R Dodecasil 1H. Soda1ite;faujasite; zeolite A; AlFO's 42 and 37. Melanophlcqite. Gmelinite; offretite; mazzite Dodecasil X; 2%-39 Levynite; A1-35. Losod. Paulingite; ZK-5. Deca-dcdecasil 3R. Chabazite; AlEO-34. Nonasi1. Dodecasil 1H. Erionite, AlEO-17. Zeolites A, ZK-5 and RHO; AlEO-42. Faujasite; AlpO-37.
547
14'5'%' 8'1
Fig.3 The three kinds of cage found in deca-dcdecasil 3R[ 121.
CAN
Fig.4 Channels in ideal AlpO4-5 ( M I ) and cancrinite (CAN) showing the nearly uniform cross-section in each [ 201. Each type of channel is based on a different framework topology. Other straight channels have free dimensions which vary along their length, e.g. zeolite L where the channel diameter varies between 7.1 A (Table 1) and -11 A , the periodicity being 7 . 5 A . In mrdenite the channels are lined on opposite sides by pockets with 8-ring windows. The pockets are large enough to acccmdate small molecules. These and other tubular channels have been considered by Gramlich-Meier and Meier [ 211.
Total Porosities. Tne total p r e volume of a microporous crystal can usually be estimated from the amounts of water (polar sorbents) or N 2 (polar or non-polar sorbents) which fill the pores. With polar sorbents isotherms of water are very rectangular at roan temperature; with all sorbents this is true of N2 at liquid nitrogen temperatures. The flat tops of the isotherms then permit estimates of saturation uptakes. From these a u n t s , if one assumes that the co-volumes of H20 and N2 are the same as in their liquids at the respective experimental temperatures, the pore volumes per g. or per cm3 of crystal can be found. Total pore volumes can also be estimated from tne calculated free volume of each cavity and the number of cavities per g . or per cm3 of crystal. Table 4 gives, for zeolites, approximate total pore volumes derived from ideal compositions quoted by Meier and Olson 221. Nitrogen monolayer equivalent areas per cm3 of crystal are also derived, assuming a co-volume of N 2 at-195W of 55.2 x 10-24 cm3 per mlecule, and a co-area of 16.2 x 10-16 cm2. The porosities and areas in Table 4 place the zeolites among high capacity sorbents. TABLE 4 Porosities and N2 monolayer equivalent areas of some zeolites. Porosity Zeolite Zeolite Area Porosity ( cm3/cm3 (m2/cm3 ( cm3/cm3 ) Merlinoite 0.185 Analcime 0.36 Mazzite Mordenite 0.26 880 0.37 Zeolite RHO 0.26 Ferrierite 880 0.39 Dachiardite Stilbite 0.26 880 0.39 Zeolite L Levynite 0.26 970 0.42 Cancrinite hydrate Zeolite A 0.33 1130 0.43 m e 1inite 0.33 1130 0.44 TMA-E(AB) 0.34 1140 Zeolite ZK-5 Sodalite hydrate 0.45 1140 0.47 Faujasite Heulandite 0.34 Chabazite Erionite 1190 0.35 0.48 Offretite Paulingite 1230 0.36 0.48
Area (m2/cm3) 1230 1270 1330 1330 1430 1470 1490 1530 1610 1650 1650
The extent to which the total pore volumes can be utilised depends not only upon molecule sieving but also on the packing efficiency of the guest in the pores. In analcime only mall polar molecules (H20 or NH3) can enter because the interstices which make up the pore volume are too small and the windows are very narrow. Even for H20 and NH3 rather high temperatures are needed for equilibration in a reasonable time, and N2 areas calculated from water uptake are meaningless because N2 is not sorbed at -195OC. At high
549
pressures and temperatures Ar and Kr can enter outgassed sodalite hydrate [ 231, but only one such molecule (their equilibrium diameters being 3.83 and 3.94) can be fitted into a cavity of free diameter -6.68, (Table 3 ) so that packing is inefficient in that free space is wasted. The same is true of these molecules in the 512 and 51262 polyhedral cavities in certain porosils. In such cases N2 monolayer equivalent areas derived from saturation uptakes of water or from total porosities will be exaggerated. Table 5 gives total approximate porosities for porosils [ 103 and AlFO's [13]. For the prosils these have been derived from the cavity volumes of Table 3. For both categories of porous crystal the porosities are canparable with those of zeolites. TABLE 5 Porosities of some AlFO's and porosils. (a) AlPO's Porosity Porosity AlFO number AlFO Number (m3/d ( m3/9 ) 34 (Chabazite) 0.3 0.16 11 1.
31
0.17 0.19
35 (Levynitel 42 (Zeolite A )
0.3
14 41
0.22
43 (Gisnondine)
0.3
26
0.23
5
0.31
33
36 40
0.31
39
0.23 0.23
20 (Sodalite)
0.24
37 (Faujasite)
0.35
17 (Erionite)
0.28
18
0.35
46 (Chabazite) (b) Porosils porosil
0.3
0.33
0.28
porosity
bnsity*
( cm3/cm3
( g/cm3
0.36 Nonasil 0.43 Ceca-dodecasil 3R Melanophlcqite 0.44 tbdecasil 1H 0.46 0.46 Dodecasil X * Calculated for porosil free of
Porosity (cm3/g 1
1.911
0.19
1.755
0.25
1.902
0.23
1.838
0.25
1.853
0.25
guest.
Cluster sizes represented as the numbers of molecules saturating a cavity are shown in Table 6 for several of the cavities having the free dimensions given in Table 3.
These values depend upon packing efficiency in the pores,
550
and are based upon experimental measurements.
'Ihe c r y s t a l s r e p r e s e n t d i f f e r e n t
d e g r e e s of i s o l a t i o n of each c l u s t e r frcm its neighbours.
I s o l a t i o n is
v i r t u a l l y c a n p l e t e f o r molecules i n s d a l i t e cages (where t h e 6-ring windows have f r e e diameters of o n l y -2.2
A).
I s o l a t i o n is also r a t h e r c a n p l e t e i n
c h a b a z i t e and Linde z e o l i t e A i n which 8-ring windckrls j o i n i n g cages have the I n f a u j a s i t e t h e windckrls between 26-hedra are
f r e e diameters given i n Table 1. 12-rings o f f r e e diameters -7.4
A so t h a t t h e r e may be d i r e c t c o n t a c t s between
c l u s t e r s , g i v i n g a connected i n t r a c r y s t a l l i n e f l u i d . TABLE 6 Cluster s i z e s a t saturation of c a v i t i e s i n several zeolites
Sodalite
Chabaz ite
Zeolite A
hydrate
(20-hedra)
( 26-hedra
4H20
12-14 H20
-29 H20( 25+4) ( a )
1Ar
-7.7 NH3
19-20 NH3
1Kr
-6 Ar, 'NzI 02
14-16
-4.9 CH3NH2
-15 H2S
-7.5
-4.3 CH3C12
-12 CH30H
-6.5 SF6
-3.1 CH2C12
-10 s o 2
-5.8 C2F6
[ 241.
Faujasite
type I )
(26-hedrar type 11)
(14-hedra)
-2 I 2
Art
-32 H20(28+4) ( a ) 17-19 A r r N 2 1 02
N2r 02
-9 c02 -5.5 12 -4 n-C4H10
-7.8 CF4 I2
- 5.6 cyclopentane - 5.4 benzene
-4.6
toluene
-4.5 n-CgH12 -4.1 cyclohexane -4.1 p e r f l u o r c c y c l o b u t a n e -4.1 C2F4C12 -3.5 n47H16 -3.4 C3F8 -2.9 n 4 4 F 1 0 -2.8
iSHgH18
( a ) fie f o u r water molecules are i n the 14-hedral sodalite cages also p r e s e n t i n t h e s e crystals.
551
Metal atom clusters in the 26-hedra could (as with water mlecules) contain many metal atoms. ~n example is the fourteen atom k36+8&l+cluster in y-irradiated zeolite %-A[ 251. Saturation is achieved for mercury sorbed into silver-exchanged faujasites and other zeolites [ 261. The Pg+ is reduced to Ag atoms and then at an approximate critical pressure of mercury vapour there is nucleation of mercury clusters which fill all the pore volume as the pressure of Hg vapour increases further. Mercury-zeolite systems are the only ones in which sorption isotherms have been investigated quantitatively. However other metal atintroduced into zeolites (by ion exchange and reduction, or as metal carbonyls and their decomposition) all show, on heating, a strong tendency to form clusters by migration of a t m , which can aggregate both within and cutside the crystals. CATIONS IN ZEOLITES Since cations are also present in the same zeolite channels and cavities as the guest mlecules, their location and distribution are significant. In zeolite structures there are generally several different types of cation sub-lattices which usually provide more sites than are needed for neutralising the total framework charge. Accordingly cations and cation vacancies distribute themselves among the available kinds of cation sub-lattice according to the differing site preferences of the cations. In highly aluminous zeolites (eg Si/Al=l) the anionic framework charge can be considered to be equally distributed Over all anionic oxygens, so that there is in principle an equal probability of occupation of each site on a given type of cation sub-lattice. On the other hand, in highly siliceous zeolites like 2%-5, the A102 units of the framework are so far apart that charge cannot be considered to be shared equally m n g anionic oxygens. Sites on a given kind of cation sub-lattice do not now have equal probabilities of occupation. Indeed only those near an A102 could have a cation occupant. Cations and framework oxygens between them create powerful local electrostatic fields and field gradients, which interact with molecular dipoles and quadruples respectively [ 271. This makes aluminous zeolites very selective for dipolar molecules like water or annnonia, and selective for quadrupolar species like CO2 or N2, and is the basis of zeolite desiccants or of their use in processes such as air separation. CONCLUDING REMARKS In porous 3-D, I-connected nets which form the frameworks of crystals the dimensions and geometry of the micropores and channels are defined as accurately, as are the positions of the framework atoms. This makes porous crystals of particular interest as d e l s in theoretical calculations of heats
552
of s o r p t i o n and s t a t i s t i c a l thermodynamic t r e a t m e n t s of s o r p t i o n and ion exchange equilibrium.
A t t h e same time, it is p r i m a r i l y lattice d e f e c t s , such
as Bronsted and L e w i s a c i d sites, which are t h e basis f o r t h e remarkable s u c c e s s e s of z e o l i t e c a t a l y s t s . REFERENCES 1 2 3 4 5 6
7 8 9 10 11 12 13
14 15
16
17 18 19 20 21 22 23 24 25 26 27
R.M. Barrer, B.P. (1942) 548905; U.S.P. (1942) 2,306.610. R.M. Barrer, J. Soc. Chem. Ind., 64, 1945, 130 and 133. R.M. Barrer and L. Belchetz, J. Soc. Chem. Ind., 64, 1945, 131. R.M. B a r r e r and D.M. Mckcd, Trans. Faraday Soc., 51, 1955, 1290. R.M. Barrer, J. I n c l u s i o n Phencmena, 4, 1986, '109. D.E.W. Vaughan and R.J. Ussier, Proc. 5 t h I n t . Conf. on Zeolites, Ed. L.V.C. Rees, Heyden, 1980 p. 94. R.M. Lewis, K.C. G t t and R.A. van Santen, U.S.P. (1985) 4,510,257. For r e f e r e n c e s see R.M. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves" Academic P r e s s , 1978 p. 20. R.M. Barrer, B.P. (1946) 574,911. F. Liebau " S t r u c t u r a l Chemistry of S i l i c a t e s " Springer, 1985 pp. 156 and 240. D.M. Bibby and M.R. mle, Nature, 317, 1985, 157. H. Gies, 2. Krist., 175, 1986, 93. E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson, i n "New Developments i n Zeolite Science and Technology", (Eds) Y. Murakami, A. Iijima and J.W. Ward, Kcdansha, E l s e v i e r , 1986, p. 103. J . M . Bennett, W.J. Dytrych, J.J. Pluth, J.W. Richardson and J.V. Smith, Zeolites, 6, 1986, 349. S.A. A l l i s o n and R.M. Barrer. J. Chem. Soc., A, 1969, 1968. P. Cartraud, A. C o i n t o t and A. Renaud, J. Chem. Soc. Faraday 1, 77, 1981, 1561. G. Boxhoorn, J. Moolhuysen, J.G.F. Coolegem and R.A. van Santen, Chem. Corm., 1985, 1305. R.M. Barrer and V.H. Shanson, Chem. Corn., 1976, 333. A. Goldup and G.W. Smith, Separation Sci. 6, 1971, 791. W.M. Meier, i n "New Lkvelopments i n Zeolite Science and Technology", Eds. Y. Murakami, A. Iiyima and J.W. Ward, m a n s h a , E l s e v i e r , 1986 p. 13. R. Gramlich-Meier and W.M. Meier, J. S o l i d S t a t e Chem., 44, 1982, 41. W.M. Meier and D.H. Olson "Atlas of Zeolite S t r u c t u r e Types", Druck and Verlag A.G., 1978. R.M. Barrer and D.E.W. Vaughan, J. Phys. Chem. S o l i d s , 32, 1971, 731. R.M. Barrer i n "Non-Stoichimetric Compounds", Ed. L. Mandelcorn Academic P r e s s , 1964, p. 393. J.R. Morton and K.F. Preston, Zeolites, 7, 1987, 1. Ref. 8, p. 394 e t Seq. R.M. Barrer, J. Coll. and I n t e r f a c e Sci., 21, 1966, 415.
D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane ConL,ersion 1988 Eisevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
553
ALUMINOPHOSPHATES AS POSSIBLE ALTERNATIVES T3 . ZEOLITES N.B. MILESTONE and N.J. TAPP Chemistry Division, CGIR, Private Bag, Petone (New Zealand) ABSTRACT
The discovery of a microporous oxide system based not on silicates h t on aluminophosphates has proved very exciting and promises great potential for tailoring catalysts. These new materials are formed by hydrothermal treatment of a reactive aluminophosphate gel, in the presence of an organic base which acts as a structure directing species. Using several tenplating agents 31 different microporous structures have been currently reported. Many of these have analogues in zeolite chemistry h t several are less thermally stable structures and are unusual in having 5-cmrdinate aluminium. A wide range of elements can be substituted into the f r m o r k for either A1 or P. These include transition metals such as Co, Zn, Mn or Fe which substitute for Al, and elements such as Sir Ge or Ti which seem to substitute for P. Both these types of substitution produce Bronsted acid sites so that these materials can be used as catalysts in a similar way to zeolites. Methods of characterising and quantifying the nlrmber of acid sites are described. Since the acid strength of the substituted alumincphosphates depends on both the substituting atom and the structure, a wider range of catalysts is available than with the zeolites. So far, there have been few reports of particular catalytic uses for these materials h t the list can be expected to grcw rapidly as the potential of these new ampmxls is explored. This paper reviews some of the characteristics of both the substituted and unsubstituted alumincphosphates together with their potential uses as catalysts and mlecular sieves. INTRoIxlCTIoN Aluminophosphate analogues of the varicus fohns of silica are well k n m . The quartz analogue, berlinite, exhibits the typical a-6 transitions (ref. 1) and the cristobalite and tridymite analogues behave in a similar way. In these + structures the building units are tetrahedra of A102 and PO2 which are isoelectronic with Si02. The average distance between the Al and P centres is approximately the same as that between the Si atam in silicates so the unit cells of the A~FQ analogues have similar dimensions to those in the silicas. The synthesis of microporous form of silica (e.g. silicalite (ref. 2 ) and silicalite-2 (ref. 3)) suggested that microporous aluminophosphates might also be stable. If these could be formed and substituted, then materials with properties similar to thcse of zeolites auld be prepared. If these were stable, they could provide alternative or conplementary materials to zeolites which have become invaluable in the petrochemical industry.
554
In 1982, workers from Union Carbide (ref. 4) announced the synthesis of a new microporous oxide system based on the aluminophosphates. The synthesis of this system resulted from the application of an organic base to an aluminophosphate gel follmed by hydrothermal treatment. The resulting aluminopbosphate structures have indeed been shmn capable of ismrphous substitution (refs 5-81 and the range of structures and substituting elements continues to grm at a mch faster rate than that for the conventional silicate systems. The substituted aluminophosphates have been sham to be catalytic and could provide a conplementary series of shape selective catalysts. The original patent (ref. 4) listed 15 new structures to which the acronym ~ 1 ~ 0 4 -was n applied where n denoted a particular structure type. Since then a further 16 structures have been added, sme of which seem to require substitution to form (ref. 8). A rnmber of zeolite analogues have keen identified but only those with even numbered rings in the structure can occur t since these can contain the alternating A105 and Po2 units. A1FQ4-5 and AlFO4-11 are two of the novel structures found but they are not related to ZSM-5 or ZSM-11 (ref. 4). Not all structures are thermally stable as some collapse to a condensed phase on heating. The structures of s m of the thermally unstable species have been determined (ref. 9,lO) and found to contain 5-coordinated Al, an unusual state for Al. Other structures have long basal spacings indicative of a sheet like structure and we have found that s m can be intercalated (ref. 11). It is the substitution within the framework which has provided so rmch interest, since the range of structures and substituted elements seems a l m t unlimited, providing materials with a wide range properties. SYNTHESIS
The various structures are synthesized by hydrothermal treatment of an 1:l aluminophosphate gel in the presence of an organic base. We have found that the way the gel is fontled and treated is important to obtain good yields of clean crystals (ref. 11). Crystalline alumina phases have not proved satisfactory as a suitable aluminium scurce since a good gel is not f o n d on reaction with phosphoric acid. Large amounts of alkali cations inhibit microporousproduct formation so phosphate salts are not suitable as phosphorous sources. We have found that the m s t satisfactory gel is formed by reacting a poorly crystalline alumina hydrate such as bayerite or boehmite with phosphoric acid and mixing until a hmogeneous gel is formed. "he gel when initially formed is acidic but addition of mine raises the pH. The formation of a gel in an acid medium has distinct advantages if substitution into the framework is required. Most mtal salts are insoluble in the highly alkaline media used for zeolite synthesis and so little substitution has been effected.
555
Only recently, by u t i l i z i n g an i n i t i a l acid pH step, have suitable f e r r o s i l i c a t e s of the ZSM-type been successfully synthesized (ref. 1 2 ) . The temperature and t i m e of the hydrothermal treatment together with the actual organic base used seem to determine the structure t h a t is f o m d . The two structure types t h a t we have concentrated on are the A1FQ-5 and 11 forms, the s t r u c t u r e s of which are shown i n Figs 1 and 2.
Both contain a
combination of 4- and 6membered rings with a unidimensional channel system bounded by 12 tetrahedral u n i t s i n AlpO4-5 (ref. 13) and 10 i n AlpO4-11 (ref. 14).
The 4- and 6membered rings are also present i n m t a v a r i s c i t e .
We
have shown (ref. 11) t h a t t h i s compound is f o m d as an intermediate i n the synthesis of AlpO4-ll, A1P04-5 and AlP04-20.
Fig 1. Structure of AlpO4-5
Fig 2.
Structure of AlPO4-11
SUBSTITUTION The f i r s t element reported t o be substituted i n the aluminophcephate system was S i (ref. 51, a natural choice since it has an ionic s i z e intermediate to A 1 and P.
I t appears from the chemical analyses and properties
t h a t some, i f not a l l , of the S i s u b s t i t u t e s f o r P within the framework. Substitution of Si02 f o r a PO; u n i t creates a negatively charged site within the framework which nust be balanced by a cation. acid site is created.
I f this is a proton then an
The formal charge is spread over 4 at-
less
electronegative than S i so the acid strength is generally less than t h a t obtained by A l s u b s t i t u t i o n i n a silicate framework (ref. 7). Since the i n i t i a l work (ref. 51, a wide range of ions have been reported as being substituted, ranging i n valencies f r a n +1t o +5 and having widely d i f f e r i n g electronegativities (Table 1). Flanigen
gt. (ref. e ) reported the
substitution of 13 elements i n t o the Alm4 structures, but several o t h e r elements have a l s o been reported (refs. 6-7).
I n order to e a s i l y describe the
556
various substituted f o m , Union Carbide workers have created a number of acronyms (ref. 8) such as SAEQ-n to describe the Si-substituted structures or MeAPO-n where divalent metals such as Co, Zn, Mg or Mn have been substituted. Generally, the latter produce acid sites with slightly higher acid strengths than Sir but the actual acid strength depends on the electronegativity differences between the substituting ions and surrounding phosphorus atoms. TABLE 1 Elements reportedly substituted in A l p 0 4 structures Ionic Radius (nm) (Pauling) 68 35 66 80 63 74 78 93
Element
Charge
Li Be Mg Mn co Zn Fe Sn
+1 +2
+2
Fe Ga B As
+3 +3 +3 +3
62
23 58
1.96 1.81 2.04 2.18
Si Ti Zr Ge Sn
+4 +4 +4 +4 +4
41 61 72 53 69
1.90 1.54 1.33 2.01 1.96
As
+5 +5
50 54
2.18 1.63
V
+2
+2 +2 +2 +2
64
Electronegativity (Pauling) 0.98 1.57 1.31 1.55 1.88 1.65 1.83 1.80
Reference
(a) oxidation state of substituting ion not defined The range of structures into which any one ion will substitute is not well determined, but most will substitute into the AlP04-5 structure. The AlP04-34 structure (the chabazite analme) also appears to be able to easily accomnodate different atoms. Some structures have only been reported if substitution is effected e.g. M-36 and MeApo-47 (ref. 8). Recently, we have been able to produce an unsubstituted AlP04-41 by carefully controlling synthesis conditions (ref. 11). Until then this had been reported only as the Si substituted form. (ref. 5 ) . It is therefore likely that further substitutions and structures will eventually be prepared. Substitution is not limited to only one element at a time but m n y two three and even four element substituted structures have been characterized (ref. 8). With so mny substitutions it is not clear what the parent elements
557
might be. Some of the substituting elements such as L i appear to require the presence of other substituting ions before they w i l l e n t e r the framework ( r e f . 8 ) but proof of substitution has y e t to be established. In a l l the work reported so f a r , the actual amount of substitution achieved has not been independently characterized.
Bulk chemical analyses and energy
dispersive X-ray analysis have keen used to characterize the amount of substitution but these do not t a l l y with expected chemical properties such as ion exchange. Certainly chemical analysis suggests t h a t the s u b s t i t u t i n g element has keen incorporated in the framework but we have not been able t o measure systematic changes in l a t t i c e dimensions by XRD. Pyke st. reported ( r e f . 7 ) changes in the spacings f o r sore of the species they prepared but w e have noticed s i m i l a r s h i f t s in different preparations of the same Mp04. Aluminium substitution in ZSM-5 z e o l i t e structures makes a masurable systematic difference in l a t t i c e spacings (ref. 16).
This suggests t h a t the
actual level of substitution might be quite lowla f a c t substantiated by other properties.
In the case of S i , paired substitution where two S i a t m replace
both an A 1 and a P atcm may be occurring so t h a t the average spacing w i l l not change greatly.
Alternatively the element may s u b s t i t u t e i n a site where the
l a t t i c e changes a r e l i k e l y t o be small and not readily observed.
Clearly a lot
more work is needed i n t h i s area. PROPERTIES L i k e the h i g h s i l i c a zeolites, the microporous aluminophosphates are prepared as precursors which nust be calcined t o remve the occluded tenplating
The s t r u c t u r e s are f o r the mst p a r t s u f f i c i e n t l y s t a b l e ( t o be calcined) although substitution lowers t h e s t a b i l i t y and collapse t o the
molecules.
condensed fornti can occur. W e have found by IWR t h a t t h e amine is held i n a protonated form. The counter anion is l i k e l y to be PO43- or OH- passibly held i n the 6membered channels. In a number of the substituted species the framework charge, based on bulk chemical analysis, is not zero and it has been suggested t h a t protons or hydroxide ions may reside in the pores (ref. 17). For the precursors, the charge imbalance can be met by the tenplating molecule.
Elsewhere (ref. 18) we
have sham t h a t f o r AlFO4-ll there are typically t w o amines per unit-cell.
By observing what happens on removal of t h i s tenplating amine we can obtain some important information a b u t the f o m t i o n of the acid sites. In AlFO4-ll t h e channel size is such t h a t the amine w i l l desorb without
deccmposition and t h i s can be monitored by E / M S ( r e f . 16).
The weight loss
f o r the unsubstituted species is sirrple (Fig. 31, occurring i n three steps.
Mass s p e c t r m t r y shcxrled t h a t sorbed water is lost f i r s t , followed by loss of the m i n e and then a f i n a l loss of water which w e suspect is from the
558
6-membered channels (ref. 18). On substitution however, the weight loss occurs in several steps, (Fig. 4) and mass spectrometry is needed to determine what is happening. The mine is still mainly lost at 2OOOC h t tails badly. The high temperature loss is seen to be due to m n i a and alkene (Fig. 5). This is very similar to the pattern noted by Parker st. (ref. 19) in a study of the decomposition of ZSM-5 precursors. The mine, protonated by the acid site, breaks down by a Hofmann type elimination providing confirmation of the formation of B&nsted acid sites upon substitution.
- TG
__
Tempemtm / c Unsubstttuted ALPOL-11 Precursor
Fig 3.
1
OTG
Substituted ALPOL- 11 Precursor
Fig 4. Thertrcgravimetric traces of substituted AlP04-ll precursor
Thermogravimetric traces of unsubstituted AlP04-ll precursor
Acid sites/g vs %Si
1
I
I t dg 2
800
/
-
, /
/
/
/
/ / //
,
m
1enMratun/ .c Substituted ALPOL-11 Precursor
Fig 5.
Mass spectrograph of weight loss of substituted AlP04-ll precursor.
0
2
4 9. Si/AI+P+Si
6
Fig 6. Number of acid sites produced on Si substitution.
The unsubstituted AlP04's have sane properties similar to those of the pure silica ZSM species, showing little catalytic activity. However, unlike the silicalites they are hydrophilic, adsorbing water in preference to organic
559
molecules. When pore volums were measured by N2 adsorption in a dynamic system, we found the AlP04's bound trace amounts of water, drastically effecting N2 adsorption. The water is readily removed by heating but clearly the framework has a strong attraction for water. In fact in UPo4-17, (the erionite analogue) the interaction with water is so strong that some of the Al is converted into octahedral coordination (ref. 20). In the substituted AlFO4's we have endeavoured to measure the number of acid sites using aqueous ion exchange with m n i u m ions. Hcwever, the results obtained this way have not proved quantitative and we have had to resort to alternative mans. Adsorption of gaseous m n i a at 15OOC has also not proved very satisfactory. It would appear that water adsorption occurs in a different way to that in zeolites and this affects properties such as ion exchange. We have used temperature-prcgrmd &sorption of n-propylamine as a way of identifying acid sites. The mine held at an acid site desorbs as alkene and m n i a and can be easily masured by integration. In SAFQ-5 the number of acid sites produced equates with the number of Si atoms substituted up to 1% substitution (Fig. 6). At higher levels of Si substitution the experimntal results fall away from the theoretical line. This is unlike the case in ZSM-5 zeolites where up to 4.5% A1 substitution produced an acid site (ref. 21). The size of the propylamine molecule is such that it should fit at least 4 molecules/unit cell which would allcw masuremnt of 4 acid sites per unit cell i.e. 8% Si substitution unless the Si atoms were clumped. Xu Qinhua -et al. (ref. 22) found similar results using pyridine as the probe molecule. They shcwed that at 9% Si substitution only 25% of the Si produced acid sites. This suggests the Si may be substituting in pairs. NMR studies of AlW4's have not been widely reported. In those reprted to date (refs. 17,23,24) the A1 and P are clearly sham to lie in tetrahedral coordination although the Al peak is assymetrical suggesting some interaction with species held in the channels (ref. 23). In their S-5 study, Appleyard -et al. (ref. 24) are cautious in their interpretation of the single Si peak. Although within the tetrahedral range the chemical shift is unusual. Initial assignment of the Si peaks for zeolite A was wrong because of an unusual shift (ref. 35) so Appleyard & &. (ref. 34) reason that Si could substitute in pairs and still produce the single resonance. while Si can be envisaged as substituting in pairs it is difficult to conceive this happening for Co or other divalent ions. Clearly, further work is needed to define what is happening during attempted substitution.
560
CATALYSIS Amorphous aluminophosphates have been used a s c a t a l y s t s or catalyst supports f o r a number of years. Canpelo and h i s coworkers have shown t h a t they can be promoted by a l k a l i metals or F- ions (Ref. 25-26), but they behave l i k e
the amorphous aluminosilicates in b i n g nonselective.
The unsubstituted
alumincphcsphates have e s s e n t i a l l y no c a t a l y t i c a c t i v i t y although they do dehydrate methanol to dimethyl e t h e r (ref. 11).
They can, however, be used a s
c a t a l y s t supportsr and Coughlin and Rabo (ref. 27) have considered impregnated ~ 1 P 0 4 ' sas supports f o r Fischer-Tropsch catalysts. A m a s u r e of likely use f o r these materials can be ascertained from
consideration of acid strength and Union Carbide have reported butane cracking a c t i v i t y as a measure of this.
In the list presented by Flanigen
&t.
(ref. 8 ) it is apparent t h a t the strength depends on the structure, the s u b s t i t u t i n g ion and a l s o it would appear, the degree of substitution. Although this list is incolnplete, the values found f o r the substituted Alm4's typically lie below those of zeolites. The f u l l range of reactions f o r which these m t e r i a l s can be used has y e t
t o be identified. There has been a flourish of new patents recently so it m y not be long before scale-up r e s u l t s ( a t l e a s t to p i l o t p l a n t ) appear. Table 2 provides a list of some of the laboratory-scale reactions so f a r reported. TABLE 2
Exaitples of c a t a l y t i c reactions studied
Type of reaction
Reference
A r m t i s a t i o n of a l i p h a t i c s Cumene cracking tkwaxing Hydrocarbon cracking Hydrocarbons from methanol Hydrocarbons from heterocompounds Hydrogenation of o l e f i n s Processing of crude o i l Small o l e f i n conversion Xylene isomerization Hydrogenation of o l e f i n s Fischer-Tropsch synthesis
28 22 29 30 t h i s work 31 28 32 33, 34 7, 22 28 27
W e have been investigating the E t h a n o l to hydrocarbon reaction using Co and
S i substituted AlP04-5 and -11. I t is c l e a r t h a t shape s e l e c t i v i t y plays an important role in what products are formed. approximately
In substituted AlFQ4-5,
40% of the hydrocarbon product is hexamethylbenzene with large
amunts of pentamethylbenzene also being produced. ?his d i s t r i b u t i o n is s i m i l a r t o t h a t obtained using ZSM-12 which has approximtely the same channel dimnsions, althcugh mre liquids are formed using the zeolite.
In substituted
561
AlFO4-11 the products contain durene as the largest species but with large amounts of di- and tri- methylbenzenes also present. It appears that the substituted AlP04's are very good methylating agents. Xu Qinhua &. (ref.22) found that cracking and xylene isomerization over SAPD-5 occurred at much higher tenperatures than required for ZSM-5. This is amsistant with the lower acid strength of the substituted AlFQ4's. It has been found that toluene can be readily mthylated to xylene without the normal disproportionation reaction to benzene occurring (ref. 34). Thus the AlFQ4's would appear to have good potential as alkylation catalysts without concurrent cracking or ismrization which normally occurs over zeolites. while extensive testing has yet to be carried out, we have found that the SAW'S typically last as long as the ZSM-5's before they deactivate by coking. CaApo's hmever seem to coke up m r e readily h t both materials can be readily regenerated by heating in air. cmcLusIoN The discovery of the new range of microporous aluminophosphates provides the potential for further advances in the field of shape selective catalysis. It appears that the acid strength of the catalyst can be tailored over a wide range by selecting the appropriate substituting ion. While superficially these materials behave similarly to zeolites, there are a number of properties such as water adsorption and ion exchange which are significantly different. There is still uncertainty as to h m mch of the substituting ion actually enters the framework and suitable ways of masuring this have yet to be devised. Clearly a great deal of work remains to realize the full potential of these new materials. REFERENCES 1. A. Dietzel and H.J. Poegel. Die Naturwis. 40, (1953), 604. 2. E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. M e n , R.L.,Patton, R.M. Kirchner and J.V. Smith. Nature, (19781, 512. 3. D.M. Bibby, N.B. Milestone and L.P. Aldridge. Nature 280, (1979), 664-5. 4. S.T. Wilson, B.M. Lok, C.A. MeSSiM, T.R. Cannan and E.M. Flanigen. Intrazeolite Chemistry, ACS Synp Series 218. pp 79-106, (1983). U.S. Patent 4,310,440 (1982). 5. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen. J. Am. Chem. Soc. 106,6092-93, (1984). U.S. Patent 4,440,871. 6. N.J. Tapp, N.B. Milestone and L.J. Wright. J. Chem. Soc. Chem. Ccmnn (1985), 1801-1803. 7. D.R. Pyke, P. Whitney and H. Houghton. Applied Catalysis 18, (1985), 177-190. UK Pat App 2,155,916 (1985). 8. E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson in Y. Murakami, A. Iijima and J.W. Ward (Eds) New Develpments in Zeolite Science and Technology. Proc. 7th Internat. Zeolite Conf., Elsevier, 1986, 103. 9. J.B. Parise and C.S. Cay. Acta Cryst. C41, (19851, 515-520. 10. J.B. Parise. Inorg. Chem. 24, (19851, 4312-4316.
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11. N.J. Tapp, N.B. Milestone and D.M. Bibby. "Submitted for publication", 1987. 12. R. Szostak, N. Nair and T.L. Thomas. J.Chem. SOC., Faraday Trans I (19871,83, 487. 13. J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth and J.V. Smithin, G.D. Stucky and F.G. LkYyer (Eds) Intrazeolite Chemistry, ACS Symp. Serie'; 218 (1983) 109. 14. J.M. Bennett, J.W. Richardson, J.J. Pluth and J.V. Smith, Zeolites 2 (1987) 160-162. 15. N.B. Milestone and N.J. Tapp, personal conmnication. 16. D.M. Bibby, L.P. Aldridge and N.B. Milestone. J. Catal. 2,(19811, 373-4. 17. I.P. Appleyard, R.K. Harris and F.R. Fitch. Zeolites 6, (19861, 428-431. 18. N.J. Tapp and N.B. Milestone. Poster presented at this conference. 19. L.M. Parker, D.M. Bibby and J.E. Patterson. Zeolites 4, (1984) 168. 1984, 6135-39. 20. C.S. Blackwell and R.L. Patton. J. Phys. Chem. 21. D.H. Olsen, W.O. Haag and R.M. Lago. J. Catal. 61, (19801, 390-396. 22. Xu Qinhua, Yan Aizhen, Bao Shulin and Xu Kaijun ifi Y. Murakami, A. Iijima and J.W. Ward (Eds) New Developments in Zeolite Science and Technology, Proc. 7th Int. Zeolite Conference, 1986, Elsevier, 835. 23. D. Muller, E. Jalin, B. Fahlke, G. Ladwig and U. Haubenrisser. Zeolites 2, (1985) 53-56. 24. IIP. Appleyard, R.K. Harris and F.R. Fitch. Chem. Letters (19851, 1747-1750. 25. J.M. Canpelo, A. Garcia, J.M. Gutierrez, D. Luma and J.M. Marinas. J. Coll. Interface Sci. 95, (1983), 544-550. 26. J.M. Carplo, A. Garcia, D. Luma and J.M. Marinas. J. Catal. 102, (19861, 299-308. 27. P.K. Goughlin and J.A. Rabo. US Patent 4,556,645 (1985). 28. D.C. Garsker and B.M. Lok. US Patent 4,499,315 (1985). 29. F.G. Gortsemer and R.J. Pellet. Europ. Pat. App. 195, 329 (1986). 30. R.J. Pellet, P.K. Loughlin M.T. Staniuliz, G.N. Long and J.A. Rabo. Int. Pat. Pub. VD 86/032218 (1986). 31. S.W. Kaiser. US Patent 4,524,234 (1985). 32. G.N. Long, R.J. Pellet, J.A. Rabo. Europ. Pat. App. 0124119 (1984). 33. S.W. Kaiser, Europ. Pat. App. 0142,156 (1985). 34. R.J. Pellet, G.N. Long and J.A. Rabo in Y. Murakami, A. Iijima and J.W. Ward (Eds) New Developments in Zeolite Science and Technology, Elsevier, Tokyo, 1987, p.843. 35. J.M. Bennett, C.S. Blackwell and D.E. Cox. J. Phys. Chem. 87, (19831, 3783-90.
e,
I
563
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
THE CONVERSION OF METHANOL TO HYDROCARBONS AND THE OXIDATION OF METHANE ON HETEROPOLY OXOMETALATES
J.B. M o f f a t Departrnent o f Chemistry and Guelph Waterloo Center f o r Graduate Work i n Cherni s t r y , U n i v e r s i t y o f Waterloo, Waterloo, O n t a r i o , Canada, N2L 3G1
INTRODUCTION The announcement i n 1972 o f t h e d i s c o v e r y o f a new a c i d i c s o l i d w i t h r e p r o d u c i b l e m i c r o p o r e s t r u c t u r e generated a new wave o f i n t e r e s t i n z e o l i t i c m a t e r i a l s ( r e f . 1). The d e m o n s t r a t i o n o f t h e c a t a l y t i c e f f i c i e n c y o f t h e ZSM family o f z e o l i t e s i n t h e c o n v e r s i o n o f methanol t o g a s o l i n e - r a n g e
hydrocarbons was p a r t i c u l a r l y t i m e l y i n view o f t h e w o r l d w i d e concern o v e r t h e continued a v a i l a b i l i t y o f o i l supplies.
There f o l l o w e d a b u r s t o f a c t i v i t y
r e l a t i n g t o t h e s t r u c t u r e o f t h e ZSM c a t a l y s t s , t h e i r shape s e l e c t i v e properties, t h e i r acidic characteristics, t h e i r c a t a l y t i c a c t i v i t i e s i n a v a r i e t y o f r e a c t i o n s and t h e mechanism o f t h e methanol c o n v e r s i o n p r o c e s s ( r e f s . 2-4).
E f f o r t s i n t h i s a r e a have c o n t i n u e d .
I n t e r e s t i n a r e l a t e d process, t h a t o f t h e p a r t i a l o x i d a t i o n o f methane, has a l s o h e i g h t e n e d i n r e c e n t y e a r s ( r e f s . 5-7).
T h i s has r e s u l t e d f r o m two
d r i v i n g f o r c e s , a fundamental i n t e r e s t i n t h e a c t i v a t i o n o f t h e C-H bond and a renewed awareness o f t h e p o t e n t i a l o f p l e n t i f u l s u p p l i e s o f n a t u r a l gas as a s u b s t i t u t e f o r o i l - d e r i v e d energy sources and chemical p r e c u r s o r s ( r e f . 8 ) . The a p p l i c a t i o n o f heterogeneous c a t a l y s t s t o t h e methane c o n v e r s i o n process has r e c e n t l y been examined by L u n s f o r d ( r e f . 9) w i t h MgO and Mo/SiO,,
Somorjai
( r e f . 10) w i t h Mo/SiOp and V / S i O p and S o f r a n k o ( r e f . 11), t h e l a t t e r o f whom has examined v a r i o u s s u p p o r t e d o x i d e s f o r t h e o x i d a t i v e c o u p l i n g o f methane. SURFACE AND STRUCTURAL PROPERTIES OF HETEROPOLY OXOMETALATES I n t h i s l a b o r a t o r y t h e c o n v e r s i o n o f methanol ( r e f s . 12-16) and t h e p a r t i a l o x i d a t i o n o f methane ( r e f . 17) have been s t u d i e d on h e t e r o p o l y oxometalates, p a r t i c u l a r l y t h o s e w i t h Keggin s t r u c t u r e . s o l i d s w i t h h i g h m o l e c u l a r weight,
These a r e i o n i c
c a g e l i k e anions and c a t i o n s w h i c h may b e
chosen f r o m a wide v a r i e t y o f p o s s i b i l i t i e s , r a n g i n g f r o m t h e p r o t o n t o charged o r g a n i c species.
564
The a n i o n c o n t a i n s an atom such as phosphorus a t i t s c e n t r e t o which a r e bonded f o u r oxygen atoins arranged t e t r a h e d r a l l y around t h e c e n t r a l atom ( F i g . 1). The c e n t r a l t e t r a hedron i s enveloped by t w e l v e o c t a h e d r a w i t h oxygen atoms a t t h e i r v e r t i c e s and a p e r i p h e r a l i n e t a l atom such as t u n g s t e n a t t h e i r centres.
The o c t a h e d r a share t h e i r oxygen
atoms w i t n each o t h e r and w i t h t h e c e n t r a l tetrahedron.
The oxygen atoms o f t h e a n i o n
may be d i v i d e d i n t o t h r e e types, one b r i d g i n g
0
t h e c e n t r a l atom and t h e p e r i p h e r a l m e t a l atoms, a second b r i d g i n g two o f t h e l a t t e r and a t h i r d bonded s o l e l y t o each p e r i p h e r a l inetal atom and p r o t r u d i n g f r o m t h e anion.
F i g . 1.
H e t e r o p o l y oxometalate anion o f Keggi n s t r u c t u r e .
A l t h o u g h c r y s t a l l o g r a p h i c d a t a a r e f r e q u e n t l y l a c k i n g f o r t h e secondary s t r u c t u r e o f h e t e r o p o l y oxometalates, i n f o r m a t i o n i s a v a i l a b l e t h r o u g h X-ray which e x i s t s and n e u t r o n d i f f r a c t i o n f o r 1 2 - t u n g s t o p h o s p h o r i c a c i d (H P, W 1$140) i n c u b i c (Pn3m) form.
I n t h i s s o l i d a c i d t h e p r o t o n s a r e each surrounded by
f o u r w a t e r molecules, b u t because o f a t w o - f o l d t h e r m a l d i s o r d e r , a r e hydrogen-bonded t o o n l y two o f t h e l a t t e r a t a g i v e n t i m e ( F i g . 2) ( r e f . 18). The c u b i c arrangement o f anions, p r o t o n s and w a t e r m o l e c u l e s i s shown i n F i g .
3 ( r e f . 18).
F i g . 2.
(H502)' i n H3PW1fl4o0nH20 ( r e f . 18).
F i g . 3.
Anion-Cation arrangement i n H3PW120,,,-nH.&l ( r e f . 18).
565 I t i s o f i n t e r e s t t o compare and c o n t r a s t t h e p r o p e r t i e s o f h e t e r o p o l y
oxometalates w i t h t h o s e o f z e o l i t e s .
U n l i k e t h e f a u j a s i t e s t h e c a g e l i k e anion
o f t h e h e t e r o p o l y o x o m e t a l a t e s i s impermeable t o molecules.
I n addition the
s u r f a c e areas o f t h e h e t e r o p o l y a c i d s a r e small, 10 m2/g o r l e s s .
However as
w i l l be d i s c u s s e d l a t e r , i t can b e shown t h a t p o l a r m o l e c u l e s can p e n e t r a t e i n t o t h e bulk s t r u c t u r e o f t h e heteropoly oxometalates with concomitant changes i n t h e l a t t i c e c o n s t a n t s .
Consequently t h e s e s o l i d s d i s p l a y a
t h r e e - d i m e n s i o n a l f l e x i b i l i t y n o t t o o d i s s i m i l a r f r o m t h a t observed w i t h 1anellar solids.
Hence t h e t e r m i n t e r c a l a t i o n appears e n t i r e l y a p p r o p r i a t e
In
h e r e s i n c e t h e s t r u c t u r a l f e a t u r e s o f t h e h o s t a r e a l s o maintained. addition,
a l t h o u g h t h e a c i d forms o f t h e h e t e r o p o l y o x o m e t a l a t e s a r e low
s u r f a c e area s o l i d s , work i n t h i s l a b o r a t o r y has shown t h a t c e r t a i n o f t h e s a l t s w i t h monovalent c a t i o n s may b e produced i n i n t r i n s i c a l l y m i c r o p o r o u s forin ( r e f . 19-22).
A l t h o u g h t h i s w i l l b e d i s c u s s e d i n more d e t a i l
subsequently, i t i s w o r t h n o t i n g h e r e t h a t , u n l i k e t h e z e o l i t i c s o l i d s , t h e o r i g i n o f t h e m i o c r o p o r o s i t y appears t o l i e i n t h e passages between t h e anions (ref. 22).
To f a c i l i t a t e subsequent d i s c u s s i o n a b r i e f r e v i e w of c e r t a i n p r o p e r t i e s of t h e heteropoly oxometalates i s essential.
S e m i e m p i r i c a l quantun mechanical
techniques, such as t h e extended Huckel method, when a p p l i e d t o s t r u c t u r a l f r a g m e n t s which s i m u l a t e t h e h e t e r o p o l y anions, have shown t h a t t h e c h a r g e on t h e t e r m i n a l o r o u t e r oxygen atoms o f t h e a n i o n c o n t a i n i n g P and W i s a p p r o x i m a t e l y equal t o t h a t c o n t a i n i n g S i and W ( F i g . 4) ( r e f . 23).
y
__ ----__ 0
1.0
z
n
8
o
Charges on and p a r t i t i o n e d bond e n e r g i e s f o r t e r m i n a l oxygen atoms.
( a ) PW ,( b ) SiW ,( c ) PMo
10
z
F i g . 4.
Q
b
C
In
.
566
c o n t r a s t t h e charge on t h e same t e r m i n a l oxygen atoms o f a n i o n s c o n t a i n i n g
P
It1 t h e absence o f water i t would be a n t i c i p a t e d t h a t , i n t h e a c i d i c forms, t h e
and Mo i s s i g n i f i c a n t l y l a r g e r t h a n found w i t h t h e f o r m e r species.
p r o t o n s would be s u b j e c t l a r g e l y t o coulombic i n t e r a c t i o n s w i t h t h e t e r m i n a l oxygen atoms o f t h e anion.
Consequently, t h e m o b i l i t y o f t h e p r o t o n and hence
t h e a c i d i t y would b e expected t o b e i n v e r s e l y r e l a t e d t o t h e magnitude o f t h e n e g a t i v e charge on t h e t e r m i n a l oxygen atoms.
The r e s u l t s of t h e c a l c u l a t i o n s
suggest t h a t t h e PW and SiW s o l i d s s h o u l d d i s p l a y h i g h e r Bronsted a c i d i c s t r e n g t h s t h a n t h a t o f t h e PMo h e t e r o p o l y oxometalates.
Evidently the
p e r i p h e r a l metal element p l a y s a v i t a l r o l e i n d e t e r m i n i n g t h e a c i d i c p r o p e r t i e s o f these solids. The EXH c a l c u l a t i o n s a l s o show t h a t t h e energy o f t h e bond between t h e p e r i p h e r a l rnetal atoms and t h e t e r m i n a l oxygen atoms depends on t h e n a t u r e o f t h e foriner species.
With t u n g s t e n t h e b i n d i n g energy o f t h e ox.ygen i s
c o n s i d e r a b l y h i g h e r t h a n t h a t f o u n d w i t h molybdenum.
The t e r m i n a l oxygen
atoms i n t h e molybdenum h e t e r o p o l y o x o m e t a l a t e s a r e t h u s p r e d i c t e d t o b e more 1a b i l e t h a n t h o s e i n t h e i r t u n g s t e n c o u n t e r p a r t s . Temperature programmed d e s o r p t i o n s t u d i e s o f 1 2 - t u n g s t o p h o s p h o r i c
(HPW),
1 2 - t u n g s t o s i l i c i c (HSiW) and 12-molybdophosphoric (HPMo) a c i d s show t h a t two major and one mino;
peaks emerge, a l l due t o w a t e r ( F i g . 5).
The w a t e r
. 8 t-
I
773
I
I
573
I
1
373
TEMP/?(
F i g . 5.
Temperature-programed d e s o r p t i o n p r o f i l e s f o r HPW, HSiW and HPMo.
w h i c h desorbs a t l o w e r t e m p e r a t u r e s has d e s o r p t i o n e n e r g i e s o f a p p r o x i m a t e l y 10-20 K c a l / m o l e , o f t h e o r d e r expected for hydrogen bonding.
I t i s concluded
567
t h a t such w a t e r e x i s t e d i n m o l e c u l a r f o r m on and i n t h e h e t e r o p o l y acids, p r o b a b l y hydrogen bonded t o t h e p r o t o n s .
The peaks which appear a t h i g h e r
t e m p e r a t u r e s a r e a s s o c i a t e d w i t h e n e r g i e s f r o m 35-100 K c a l / m o l e and t h e r e f o r e p r o b a b l y r e s u l t f r o m t h e a s s o c i a t i v e d e s o r p t i o n o f water. w a t e r p r o d u c i n g t h e s e TPD peaks have been e s t i m a t e d as 1.4,
The q u a n t i t i e s o f
1.5 and 1.9 w a t e r
m o l e c u l e s p e r anion f o r HPW, HPMo and HSiW, r e s p e c t i v e l y , a p p r o x i m a t e l y c o r r e s p o n d i n g t o t h e number o f p r o t o n s p r e s e n t i n each o f t h e h e t e r o p o l y acids.
I t may t h u s be concluded t h a t t h e water y i e l d i n g t h e h i g h e r
t e m p e r a t u r e TPD peaks r e s u l t s f r o m t h e a b s t r a c t i o n o f ( t e r m i n a l ) oxygen atoms f r o m t h e anions b y t h e p r o t o n s .
Furthermore, t h e emergence o f t h e h i g h e r
t e m p e r a t u r e peaks f o r HPMo a t a s i g n i f i c a n t l y l o w e r t e m p e r a t u r e t h a n t h o s e f o r HPW and H S i W appears t o b e c o n s i s t e n t w i t h t h e EXH p r e d i c t i o n o f t h e h i g h e r
l a b i l i t y o f t h e t e r m i n a l oxygen atoms i n t h e molybdenum-containing anions. O b s e r v a t i o n s f r o m t h e temperature-programmed d e s o r p t i o n experiments suggest t h a t molecules, e.g.
water, may be t r a n s p o r t e d t h r o u g h t h e b u l k
s t r u c t u r e o f t h e h e t e r o p o l y oxometalates.
F u r t h e r evidence f o r such a
p o s s i b i l i t y as w e l l as f o r t h e t h e r m a l s t a b i l i t y o f t h e h e t e r o p o l y oxometal a t e s may b e o b t a i n e d f r o m p h o t o a c o u s t i c (PAS) FTIR spectroscopy
I
wee.
Fig. 6 .
:
3288.
29d8.
lrde.
s d e . tn-1
PAS FTIH s p e c t r a f o r HPW a f t e r h e a t i n g t o v a r i o u s temperatures.
( r e f s . 19, 24-27).
F o r 1 2 - t u n g s t o p h o s p h o r i c a c i d t h e P A S FTIK s p e c t r a ( F i g .
6) show an envelope o f f i v e o r s i x bands between '800 and 1100 cm-' which a r e
568
4 k -
Fig. 7.
3 h - -
2&6--
1dkO '
&CM"
PAS F T I K s p e c t r a o f HPW and sorbed NH,.
c h a r a c t e r i s t i c o f t h e Keggin s t r u c t u r e .
I n p a r t i c u l a r tvJo bands a t
a p p r o x i m a t e l y 980 and 1080 cm-' may be a t t r i b u t e d t o t h e W-0 ( t e r m i n a l ) s t r e t c h i n g v i b r a t i o n and t h e t r i p l y degenerate asymmetric s t r e t c h i n g v i b r a t i o n of the central
PO4
tetrahedron,
respectively.
Although a d i m i n u t i o n i n t h e
i n t e n s i t y o f bands between 1200-4200 cm-' i s e v i d e n t , i t i s apparent t h a t t h e Kegyin s t r u c t u r e i s r e t a i n e d a t 450OC. Exposure o f 1 2 - t u n g s t o p h o s p h o r i c a c i d a t 15OOC t o a l i q u o t s o f gaseous ammonia produces PAS
F T I K s p e c t r a (Fig. 7) d i s p l a y i n g t h e c h a r a c t e r i s t i c bands
f o r t h e NH4+ i o n a t 3200 and 1420 cm-'
( r e f s . 19, 24-25).
A f t e r approximately
t h r e e m o l e c u l e s o f NH3 p e r a n i o n have been t a k e n up by t h e s o l i d a c i d no f u r t h e r change i n t h e amount s o r b e d i s observed.
A comparison o f t h e
r e s u l t i n g s p e c t r a w i t h t h a t f o r t h e ammonium s a l t p r e p a r e d froin an aqueous s o l u t i o n o f ammonium carbonate and t h e p a r e n t a c i d r e v e a l s a c l o s e similarity.
E v i d e n t l y t h e m o l e c u l e s o f gaseous NH, have p e n e t r a t e d t o t h e
b u l k o f t h e s t r u c t u r e and i n t e r a c t e d s t o i c h i o i n e t r i c a l l y t o produce ammoni urn i o n s and t h e e q u i v a l e n t o f t h e ammonium s a l t .
X-ray d i f f r a c t o r n e t r y o f t h e
a c i d w i t h t h r e e sorbed NH, p e r a n i o n c o n f i r m s t h e r e t e n t i o n o f t h e c u b i c s t r u c t u r e and t h e s h i f t o f t h e l a t t i c e c o n s t a n t froin t h e v a l u e o f 12.11A found f o r t h e p a r e n t a c i d t o t h a t o f 11.71Aas
e x p e c t e d f o r t h e ammonium s a l t .
It
569
i s o f i n t e r e s t t o note p a r e n t h e t i c a l l y t h a t l i t t l e o r no evidence was observed f o r t h e presence of Lewis a c i d i t y .
Somewhat s i m i l a r observations were found
w i t h pyridine, although d i f f e r e n c e s were noted, apparently due t o the formation o f t h e d i p y r i d i n i u m i o n (Py2H)'
(ref. 2 6 ) .
COINVERSION OF METHANOL ON HETEROPOLY OXOMETALATES
As noted e a r l i e r t h e conversion o f inethanol t o hydrocarbons has been studied on various heteropoly oxometalates ( r e f s . 12-16), '
0 - 0
Fig. 8.
0
30
°
F
100
W/F
1
XK)
On 12-tungsto-
4
1000
Conversion o f methanol on HPW.
-
30
100
300 W/ F
IOOO-
phosphoric a c i d t h e s e l e c t i v i t y t o hydrocarbons other than methane i s found t o depend on r e a c t i o n temperature, and residence t i m e as w e l l as pretreatment conditions.
Typical r e s u l t s are shown i n Fig. 8 f o r a r e a c t i o n temperature o f
It i s c l e a r t h a t a t low 352'C and pretreatment i n helium a t 350 and 450'C. residence times t h e predominant product i s dimethyl ether (DME) w h i l e a t high
residence times hydrocarbons l a r g e r than methane are evident.
Substitution o f
dimethyl ether f o r methanol y i e l d s product d i s t r i b u t i o n s s i m i l a r t o t h a t obtained w i t h methanol, p r o v i d i n g f u r t h e r evidence f o r t h e p a r t i c i p a t i o n o f t h e ether i n t h e conversion process (Fig. 9).
It was also found t h a t
e q u i l i b r i u m q u a n t i t i e s o f DME can be obtained over t h e sodium s a l t o f HPW, but r e l a t i v e l y small q u a n t i t i e s o f hydrocarbons, suggesting t h a t l e s s a c i d i c s i t e s may be s u f f i c i e n t t o catalyze t h e formation o f t h e ether. Various m e t a l l i c s a l t s o f HPW were a l s o t e s t e d f o r t h e i r a c t i v i t i e s i n t h e conversion o f methanol (refs. 13, 15). The c a t a l y t i c p r o p e r t i e s o f these s a l t s were found t o vary considerably w i t h t h e n a t u r e o f t h e cation. Semiempirical c a l c u l a t i o n s again provided a c o r r e l a t i o n between t h e magnitude o f t h e charge on t h e terminal oxygen atoms and t h e y i e l d o f hydrocarbons l a r g e r than methane.
PAS FTIR work subsequent t o t h i s has shown t h a t t h e
m e t a l l i c s a l t s are frequently nonstoichiometric, t h e charge balance being
570
F i g . 9.
W/F
WIF
Comparison o f product d i s t r i b u t i o n s from methanol and d imet hy l ether on HPW.
established by r e s i d u a l protons ( r e f s . 19, 24-26).
This provides f u r t h e r
support f o r t h e importance o f Bronsted a c i d i c s i t e s i n t h e conversion of methanol and a t the same time i s consistent w i t h t h e r e s u l t s o f t h e EXH c a l c u l a t i o n s which predicted t h a t t h e Bronsted a c i d i c strength should be i n v e r s e l y r e 1 at d t o t h e magnitude o f charge on t h e terminal oxygen atoms o f t h e anion. O f p a r t i c u ar i n t e r e s t are t h e r e s u l t s obtained from the conversion o f
methanol on t h e ammonium s a l t o f HPW ( r e f s . 14-15).
With t h i s s o l i d t h e y i e l d
o f hyd r o c ar bo ns i s enhanced as compared w i t h t h a t obtained on the parent acid, b u t i n a d d i t i o n the products w i t h t h e former are l a r g e l y a l i p h a t i c as compared w i t h t h e o l e f i n i c species from t h e l a t t e r c a t a l y s t (Fig. 10). This observation i m p l i e s t h a t t h e ammonium s a l t i s a c t i v e i n c a t a l y z i n g t h e hydride
x)
t r a n s f e r process, somewhat reminiscent o f t h a t p r e v i o u s l y observed w i t h many z e o l i t e s i n which micropores are present. The experimental data and appropriate c a l c u l a t i o n s from measurement o f t h e adsorpt i o n o f n i t r o g e n a t 78 K on ammonium 12tungstophosphate have confirmed t h e presence o f a microporous s t r u c t u r e ( r e f s . 20, 22). Subsequent work w i t h s a l t s o f 12-tungstophosphoric acid prepared from a number o f monovalent cations has revealed t h a t those o f pot assi um and cesi um are a1 so m i croporous w h i l e those o f t h e sodium, methylanmonium and t e t r a n e t h y l ammonium cations show no evidence o f m i c r o p o r o s i t y ( r e f s . 20, 22). S i m i l a r r e s u l t s have been obtained f o r t h e monovalent s a l t s o f 12-molybdophosphoric, 1 2 - t u n g s t o s i l i c i c , and
300 325 350 375 400 REACTION TEMPERATURE 1.C)
Fig. 10. Comparison of t h e product d i s t r i b u t i o n s from t h e conversion of methanol on HPW and NHsPW.
571
1 2 - t u n g s t o a r s e n i c a c i d s ( r e f s . 20-22).
It i s i m p o r t a n t t o n o t e t h a t t h e
inicropores i n t h e h e t e r o p o l y oxometalates a r e n o t u n i f o r m i n s i z e as found i n t h e z e o l i t e s , b u t have r a d i i which a r e a p p r o x i m a t e l y 2-38, above and below t h e average.
The dependence o f t h e m i c r o p o r e volume on t h e c a t i o n d i a m e t e r f o r
t h e s a l t s o f t u n g s t o p h o s p h o r i c a c i d i s d e p i c t e d i n F i g . 11 t o g e t h e r w i t h t h e r a t i o o f t h e i n t e n s i t i e s of t h e [110] XRD r e f l e c t i o n t o t h a t i n t h e [222] Na+
I
H+ I
L
K+
&:IN$ I
Me,NH:
Cs'
I
1
MeN$
70
0.5
CATION DIAMETER (nm)
F i g . 11. plane.
M i c r o p o r e volume and XRD [ l l O ] / [ 2 2 2 ] monovalent s a l t s o f HPW.
i n t e n s i t y r a t i o f o r various
It i s apparent t h a t , a t l e a s t approximately,
t h e I ( l l O ) / I ( 2 2 2 ) r a t i o are r e c i p r o c a l l y r e l a t e d .
t h e m i c r o p o r e volume and
A s i m p l e i o n p a c k i n g model
o f t h e secondary s t r u c t u r e f o r t h e h e t e r o p o l y o x o m e t a l a t e s a l t s shows t h e presence o f i n t e r s t i t i a l v o i d s which appear t o b e separated f r o m one another b y t h e t e r m i n a l oxygen atoms o f t h e a n i o n s and a r e a l i g n e d w i t h each o t h e r i n d i r e c t i o n s p a r a l l e l w i t h o r normal t o t h e [110] p l a n e o f t h e c r y s t a l .
In
a d d i t i o n i t has been shown ( r e f . 20) t h a t t h e c u b i c l a t t i c e c o n s t a n t o f t h e water-insoluble s a l t s increases w i t h increasing c a t i o n size.
The a d d i t i o n a l
s e p a r a t i o n o f t h e anions would r e s u l t i n w i d e n i n g o f t h e i n t e r s t i t i a l voids, changes i n t h e a n i o n i c bonding p a t t e r n , and r e o r i e n t a t i o n o f t h e anions w i t h i n t h e secondary s t r u c t u r e .
Thus t h e t e r m i n a l oxygen atoms o f t h e anions may be
s h i f t e d so t h a t t h e y no l o n g e r s e p a r a t e t h e i n t e r s t i t i a l v o i d s , hence removing electron density from the [ l l O ]
plane, d e c r e a s i n g t h e c o r r e s p o n d i n g XRD
i n t e n s i t y and f o r m i n g c o n t i n u o u s channels r u n n i n g b o t h p a r a l l e l w i t h and normal t o t h i s p l ane. PAS FTIR s t u d i e s have p r o v i d e d f u r t h e r i n f o r m a t i o n on t h e mechanism of methanol c o n v e r s i o n on t h e h e t e r o p o l y o x o m e t a l a t e s ( r e f s . 27, 28).
After
p r e e v a c u a t i o n o f 1 2 - t u n g s t o p h o s p h o r i c a c i d a t 350"C, methanol i s r a p i d l y sorbed a t 25'C u n t i l a l i m i t of a p p r o x i m a t e l y 6-8 molecules/KU i s reached.
On
572
e v a c u a t i o n a t 25'C t h e q u a n t i t y o f sorbed methanol decreases r a p i d l y t o a p p r o x i m a t e l y 3 molecules p e r KU o r one m o l e c u l e o f methanol p e r p r o t o n .
The
P A S spectrum o f t h e s o l i d a c i d and sorbed methanol i s s i g n i f i c a n t l y d i f f e r e n t
f r o m t h a t o f t h e o r i g i n a l acid.
The s p e c t r a f o r sorbed CH$H
have been i n t e r p r e t e d as r e s u l t i n g f r o m p r o t o n a t e d methanol.
and sorbed C D g H W i t h sorbed
C D P H a broad band c e n t r e d a t 1460 cm-' and a broad s h o u l d e r a t 1360 cm-' have
been assigned t o t h e a s y n m e t r i c and s y n m e t r i c modes, r e s p e c t i v e l y , o f t h e COH g r o u p i n g i n CD$H2+
c o r r e s p o n d i n g t o t h o s e a t 1525 and 1430 cm-' i n CHPH;.
On s t e p w i s e h e a t i n g o f t h e a c i d w i t h sorbed methanol t h e PAS FTIR s p e c t r a show t h e development o f two sharp bands, one a t 1453 cm-
a t t r i b u t e d t o t h e CH
symmetric d e f o r m a t i o n i n t h e C H $ groups and t h e second a t 1022 cm-l, i d e n t i f i e d as t h a t due t o t h e C-0 s t r e t c h i n WOCH, ( F i g . 1 2 ) .
1
F i g . 12.
I
E f f e c t o f s t e p w i s e h e a t i n g i n vacuo on spectrum o f " i r r e v e r s i b l y ( b ) 70'C, ( c ) llO"C, ( d ) 150"C, ( e ) sorbed" CH$H on HPW. ( a ) 50'C, e f f e c t o f d o s i n g ( d ) w i t h excess methanol a t 25°C and e v a c u a t i o n a t 25°C. I n s e t peak o b t a i n e d b y s u b t r a c t i o n p f spectrum o f preevacuated acid, n o r m a l i z e d a t 1080 cm-
.
573 The observations j u s t described p r o v i d e s t r o n g evidence f o r t h e f o r m a t i o n o f protonated methanol on HPW a t room temperature and t h e subsequent m e t h y l a t i o n o f t h e anion, presumably a t t h e t e r m i n a l oxygen atoms. Further h e a t i n g o f t h e aforementioned methyl ated c a t a l y s t ( t o 200°C) produced bands a t 1490, 1387, 1366 and 1331 c h a r a c t e r i s t i c o f i s o - b u t y l and/or i s o p r o p y l w h i l e those bands r e p r e s e n t i n g CH 3 diminished.
The observations i n
t h i s work appear t o be c o n s i s t e n t w i t h e i t h e r t h e oniun y l i d e mechanivn ( r e f s . 29-31) o r t h e carbene mechanism ( r e f s . 32-33). model i s appealing f o r i t s mechanistic s i m p l i c i t y .
However, t h e carbene
Although :CH2 i s e v i d e n t l y
a h i g h l y e n e r g e t i c intermediate, r e c e n t t h e o r e t i c a l c a l c u l a t i o n s ( r e f . 34) show t h a t methylene may be s t a b i l i z e d by t h e charged environment w i t h i n t h e framework of t h e ZSM-5 z e o l i t e .
This model also accommodates t h e f a c t t h a t
dimethyl e t h e r produces a s i m i l a r product d i s t r i b u t i o n t o t h a t observed w i t h methanol ( r e f . 1 2 ) .
THE PARTIAL O X I D A T I O N OF METHANE ON HETEROPOLY OXOMETALATES The o x i d a t i o n o f methane has also been s t u d i e d w i t h heteropoly oxometalates as heterogeneous c a t a l y s t s ( r e f . 17).
While a number o f these
s o l i d s was examined t h e h i g h e s t a c t i v i t y and s e l e c t i v i t y were found w i t h 12-molybdophosphoric a c i d supported on s i l i c a w i t h N $
as oxidant.
S u b s t i t u t i o n o f tungsten f o r molybdenum as p e r i p h e r a l metal element produces a r e d u c t i o n i n b o t h t h e a c t i v i t y and s e l e c t i v i t y .
This appears t o be c o n s i s t e n t
w i t h t h e r e s u l t s o f t h e EXH c a l c u l a t i o n s r e f e r r e d t o e a r l i e r which showed t h a t t h e l a b i l i t y o f t h e t e r m i n a l oxygen atoms o f t h e anion i s h i g h e r i n t h e molybdenum-containing species than i n those c o n t a i n i n g tungsten.
Pretreatment
o f t h e HPMo/SiO2 c a t a l y s t i n a reducing atmosphere increases t h e a c t i v i t y o f t h e c a t a l y s t w h i l e t h e s e l e c t i v i t y i s r e l a t i v e l y unchanged.
It i s important
t o n o t e t h a t t h e increase i n conversion i s n o t temporary b u t appears t o remain r e l a t i v e l y unchanged f o r l o n g periods o f time.
Whereas i n t h e TPD experiments
1.5 water molecules per KU were desorbed w i t h HPMo, as noted e a r l i e r , temperature progranmed r e d u c t i o n produced 8.0 water molecules per anion.
It
t h u s appears t h a t t h e added hydrogen i n t h e l a t t e r experiment i s capable o f augmenting t h e protons i n s t r i p p i n g oxygen atoms from t h e h e t e r o p o l y anion. I n t h e conversion o f methane pretreatment i n a reducing atmosphere may remove a number o f t e r m i n a l oxygen atoms from each heteropoly anion,
apparently
e s t a b l i s h i n g s i t e s f o r t h e r e v e r s i b l e replacement and consumption o f oxygen. The oxidants, N20 o r 02, are o f p a r t i c u l a r importance.
While t h e former
produces formaldehyde, t h e amounts o f which decrease w i t h i n c r e a s i n g temperature and w i t h decreasing residence time, t h e l a t t e r produces no p a r t i a l
514
H3PYo12040 LMDING 5
0
10
Za
( Wt % )
30
40
I
,+/'
. . . . . . . . , . .
Fig. 13. Effect o f the HPtMo loading of the support on t h e production r a t e of the different products o f t h e CH, + N @ reaction a t 843 K. Reaction conditions: CHI, (67%) N20 (33%), W = 0.5 g, F = 30 mLminSmbols: ( X IN2. ( + I t o t a l
'.
575 o x i d a t i o n products.
D i f f e r e n t oxygen s p e c i e s o r d i f f e r e n t amounts o f t h e
same a c t i v e oxygen s p e c i e s may b e generated b y t h e r e a c t i o n of t h e s e two o x i d a n t s w i t h t h e HPMo. S t u d i e s of t h e e f f e c t s o f l o a d i n g and t h e t e m p e r a t u r e and d u r a t i o n o f c a l c i n a t i o n have demonstrated t h a t t h e K e g g i n U n i t i s p r i m a r i l y r e s p o n s i b l e f o r t h e c a t a l y t i c a c t i v i t y i n t h e p a r t i a l o x i d a t i o n o f methane ( F i g s . 13-14). The l i n e a r i n c r e a s e i n t h e r a t e o f f o r m a t i o n o f v a r i o u s p r o d u c t s w i t h i n c r e a s e i n l o a d i n g c l e a r l y demonstrates t h a t t h e s u p p o r t e d m a t e r i a l s a r e t h e a c t i v e species (Fig. 13).
A t t e m p e r a t u r e s i n excess o f 650°C t h e s e l e c t i v i t i e s of
i r t e d c a t a l y s t HPMo/Si02 c o n v e r t t o t h o s e expected f o r S i 0 2 ( F i g . 14), A/
A\
F i g . 14.
-A-
CALCINATION TEMPERATURE ( K)
TIME OF CALCINATION ( b u r r )
E f f e c t o f t h e t e m p e r a t u r e o f c a l c i n a t i o n d u r i n g 1 6 h o u r s ( l e f t ) and o f t h e t i m e o f c a l c i n a t i o n a t 823 K under a i r ( r i g h t ) on t h e CH, c o n v e r s i o n , s e l e c t i v i t y and Mo l o a d i n g of t h e 23 HPMo C a t a l y s t . R e a c t i o n q o n d i t i o n s : CH,, (67%) N $ I (33%) TR = 843, W = 0.5 q, F = 30 mlmin-
w h j l e t h e i n f r a r e d s p e c t r a show t h a t t h e c o n c e n t r a t i o n o f Keggin U n i t s i s decreasing (Fig. 15).
Consequently i t i s concluded t h a t t h e anions o f Keggin
s t r u c t u r e a r e t h e a c t i v e s p e c i e s i n t h e process.
The o b s e r v a t i o n s on t h e
e f f e c t o f c a l c i n a t i o n t e m p e r a t u r e a l s o show t h a t t h e elemental c o m p o s i t i o n possessed by t h e h e t e r o p o l y compound i s n o t s u f f i c i e n t t o guarantee c a t a l y t i c s e l e c t i v i t y i n t h e o x i d a t i o n o f methane.
F i g . 15.
I n f r a r e d s p e c t r a o f a c e t o n i t r i l e s o l u t i o n a f t e r washing of t h e f o l l o w i n g s u p p o r t e d HPMo samples c a l c i n e d i n d i f f e r e n t c o n d i t i o n s . ( a ) 1.16 HPMo, 350°C, 2h., b) 11.1 HPMo, 350°C1 Zh., t h e n 20.1 HPMo 16h., d) 450"C, 16h., e ) 550 C, 16h., f ) 640°C, s a n p l e c ) 350'C, 16h., g) 640°C, 16h., f o l l o w e d b y a t e s t a t 57OoC, lOh., h ) 730°C, 16h.. The p o s i t i o n s o f t h e bands o f t h e b u l k H & ' M O ~ $ ~a r~e reported a t t h e bottom o f t h e f i g u r e .
As i s e v i d e n t f r o m F i g . 1 3 t h e optimum l o a d i n g o f HPMo on S i 0 2 i s a p p r o x i m a t e l y 120 microinole KU p e r g r a n o f s u p p o r t which i s e q u i v a l e n t t o a p p r o x i m a t e l y 1000 A2/KU, t h e cross-sectional
as compared w i t h t h e v a l u e o f 100 A2 e s t i m a t e d f o r
a r e a of a K e g g i n U n i t .
Consequently i t i s h i g h l y p r o b a b l e
t h a t each KU i s i s o l a t e d on t h e s u p p o r t s u r f a c e .
511
The i n f r a r e d s p e c t r a a l s o demonstrate t h a t t h e suDported species r e t a i n t h e Keggin s t r u c t u r e f o r temperatures as h i g h as 700°C, i n c o n t r a s t t o t h e o b s e r v a t i o n t h a t b u l k HPMo has apparently l o s t a s u b s t a n t i a l p o r t i o n o f i t s Keggin s t r u c t u r e by 450°C.
The S i O p support i s e v i d e n t l y p l a y i n g a dual r o l e
i n t h e present experiments, t h a t o f i n c r e a s i n g t h e o p e r a t i v e s u r f a c e area of t h e HPMo as w e l l as p r o v i d i n g an enhancement o f t h e thermal s t a b i l i t y o f t h e supported h e t e r o p o l y oxometal ate. The f o r e g o i n g b r i e f d e s c r i p t i o n o f t h e c a t a l y t i c p r o p e r t i e s , and physical,
s t r u c t u r a l and s u r f a c e c h e m i s t r y o f h e t e r o p o l y oxometalates
demonstrates t h e m u l t i f u n c t i o n a l c a p a b i l i t i e s o f these s o l i d s i n v a r i o u s processes.
While i n some r e s p e c t s s i m i l a r i n t h e i r p r o p e r t i e s and behaviour The p r o p e r t i e s o f
t o z e o l i t e s some n o t a b l e d i f f e r e n c e s are evident.
h e t e r o p o l y oxometal a t e s e v i d e n t l y depend predoini n a n t l y b u t n o t e n t i r e l y on t h e n a t u r e o f t h e c e n t r a l and p e r i p h e r a l metal elements o f t h e anion, b u t t h e c o n t r i b u t i o n o f t h e c a t i o n cannot be neglected.
Nevertheless, t h e r e i s
convincing evidence t h a t t h e t e r m i n a l oxygen atoms o f t h e anion p l a y a c e n t r a l r o l e i n t h e mechanism o f any c a t a l y t i c process i n which h e t e r o p o l y oxometalates p a r t i c i p a t e , and i n p a r t i c u l a r i n t h e conversion o f methanol t o hydrocarbons and t h e p a r t i a l o x i d a t i o n o f methane. ACKNOWLEDGEMENTS The f i n a n c i a l support o f t h e N a t u r a l Sciences and Engineering Research Council o f Canada, Energy, Mines and Resources, Canada, and I m p e r i a l O i l L i m i t e d i s g r a t e f u l l y acknowledged.
The permission o f Academic Press, Inc.
,
t o reproduce f i g u r e s i s much appreciated. REFERENCES
1 2 3 4 5 6 7
a 9
10
11 12 13
R.J. Argauer and G.R. Landolt, U.S. Patent 3,702,886 (November 14, 1972). E.G. Derouane, Stud. Surf. Sci. Catal., 5 (1980) 5. C.D. Chang, Catal. Rev.-Sci. Eng., 25 (1983) 1. N.Y. Chen and W.E. Garwood, Catal. Rev.-Sci. Eng., 28 (1986) 185. H.D. Gesser, N.R. Hunter and C.B. Prakash, Chem. Rev., 85 (1985) 235. N.R. Foster, Appl. Catal., 19 (1985) 1. R. P i t c h a i and K. K l i e r , Catal. Rev*.-Sci. Eng. 28 (1986) 13. C.A. Jones, J.J. Leonard and J.A. Sofranko, J. Energy and Fuels, 1 (1987) 12. D.J. D r i s c o l l and J.H. Lunsford, 3. Phys. Chem. 89 (1985) 4415, and r e f e r e n c e s contained t h e r e i n . K.J. Zhen, M.M. Khan, C.H. Mak, K.B.Lewis and G.A. Somorjai, J. Catal., 94 (1985) 501. J.A. Sofranko, J.J. Leonard and C.A. Jones, J. Catal., 103 (1987) 302; J. Catal., 103 (1987) 311. H. Hayashi and J.B. Floffat, J. Catal.,.77 (1982) 473. H. Hayashi and J.B. Moffat, J. Catal., 81 (1983) 61.
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24 25 26 27 28 29 30 31 32 33 34
H. Hayashi and 3.6. Moffat, I n C a t a l y t i c Conversion o f Synthesis Gas and A l c o h o l s t o Chemicals ( E d i t e d by R.G. Herman), p. 395. Plenum, New York (1984) J.B. McMonagle and J.B. # o f f a t , J. Catal 91 (1985) 132. S. Kasztelan and J.6. d o f f a t , J. Catal.,'!in press). G.M. Brown, M.R. Noe-Spirlet, W.R. Busing and H.A. Levy, Acta Cryst. 833 (1977) 1038. J.G. H i g h f i e l d , B.K. Hodnett, J.B. McMonagle and J.B. Moffat, Proceedings 8t'h 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. 5, P. 611, Verlag-Chernie, Wei nheim, 1984. 3.b. IJlcivlonagle and J.B. Moffat, 3. C o l l o i d I n t e r f a c e Sci., 101 (1984) 479. D.B. Taylor, J.B. McMonagle and J.6. Moffat, J. C o l l o i d I n t e r f a c e Sci., 108 (1985) 278. ( a ) J.B. Moffat, Polyhedron, 5 (1986) 261; ( b ) G.B. McGarvey and J.B. M o f f a t ( t o be p u b l i s h e d ) . J.B. Woffat, J. 1401. Catal., 26 (1984) 385; Proceedings, 9 t h Iberoamerican Symposi urn on C a t a l y s i s , Lisbon, 1984; C a t a l y s i s by Acids and Bases, 6. I m e l i k , C. Naccache, G. Courdurier, Y. Ben T a a r i t and J.C. Vedrine (Eds.), P. 17, E l s e v i e r , Amsterdam, 1985; C a t a l y s i s on t h e Energy Scene, Studies i n Surface Science and C a t a l y s i s , S. K a l i a g u i n e and A. Mahay (Eds.), Vol. 19, P. 165, E l s e v i e r , Amsterdam, 1984. J.G. H i g h f i e l d and 3.8. Moffat, J. Catal., 88 (1984) 177. J.B. M o f f a t and J.G. H i g h f i e l d , C a t a l y s i s on t h e Energy Scene, Studies i n S u r f a c e Science and C a t a l y s i s , S. K a l i a g u i n e and A. Mahay (Eds.), Vol. 19, P. 77, E l s e v i e r , Amsterdam, 1984. J.G. H ' i g h f i e l d and J.B. Moffat, J. Catal., 89 (1984) 185. J.G. H i g h f i e l d and 3.8. Moffat, J. Catal., 95 (1985) 108. J.G. H i g h f i e l d and 3.8. Moffat, J. Catal., 98 (1986) 245. J.P. Van den Berg, J.P. Wolthuizen and J.H.C. Van Hoof, Proceedings, 5 t h Conference on Z e o l i t e s , Naples, I t a l y , 1980, P. 649. G.A. Olah, Pure Appl. Chem., 53 (1981) 201. T. Mole, J. Catal., 84 (1983) 423. C.D. Chang and A.J. S i l v e s t r i , 3. Catal., 47 (1977) 249. F.A. Swabb and B.C. Gates, Ind. Eng. Chem. Fundam., 11 (1972) 540. W. Drenth, W.T.N. Andriessen and F.B. Van Duyneveldt, J. Mol. Catal., 2 1 (1983) 2Y1.
.
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
579
INVESTIGATION OF ACIDIC PROPERTIES OF H-ZEOLITES AS A FUNCTION OF Si/Al RATIO K. SEGAWA,+ M. SAKAGUCHI and Y.
KURUSU
Department o f Chemistry, F a c u l t y o f Science and Technology, Sophia U n i v e r s i t y , 7-1 Kioi-cho, Chi yoda-ku, Tokyo 102( Japan)
ABSTRACT A c i d i c p r o p e r t i e s o f f o u r d i f f e r e n t t y p e s o f H - z e o l i t e c a t a l y s t s were A c i d i c s i t e s o f these z e o l i t e s s t u d i e d : H-Y, La-Y, H-M(mordenite) and H-ZSM5. have been c h a r a c t e r i z e d by TPD and high-temperature c a l o r i m e t r y o f N H 3 , and b y I R spectroscopy. The comparison c o n s i s t e n t l y a f f o r d e d t h e f o l l o w i n sequence The 2 9 S i - and "Al-MASNMR f o r t h e a c i d s t r e n g t h : H-M > 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.
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
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.
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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
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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.
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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.
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
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
n-a1 kane S i n g l y Branched Doubly Branched T r i p l y Branched
Sample B a t 571K
c6
0.9
Sample A a t 557K
c5
n-a1 kane S i n g l y Branched Doubly Branched T r i p l y Branched
-
-
-
-
-
-
-
-
-
-
-
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.
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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.
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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 (
loss m u n t e d to between 11%and 12% of the f i n a l weight.
Fig.2(a) Thermgravimetric and ( b ) Differential thermgravimetric r e s u l t s from AlFO4-ll precursors containing d i f f e r e n t mines.
I
0
100
200
330
400
I
500
600
I
700
TernperaturevC
The dtg r e s u l t s suggested t h a t there were a t least two overlapping weight
losses in the 100-3OO0C region. Td/ms enabled these overlapping weight losses t o be resolved and provided a mans of identifying the species as they desorbed from the sample. Figures 3a-d s h m the td/ms results.
Plotted in each case is m/z = 18 which
corresponds to water, and the appropriate m/z f o r the m i n e under observation. The amine e s s e n t i a l l y desorbs intact.
The high temperature water peak (k) is
due to dehydroxylation of the kaolinite internal standard, as no water is desorbed f m the AlFO4-ll i n t h i s region. The d n e is observed to &sorb over a large temperature range w i t h a t a i l up to approximately 400OC. The m u n t of water desorbed from the AlFO4-11 samples w a s determined by numerical integration of the peaks, and from the knmn water content of the
642
kh
11 I!
a
b
C
-
water Ei2NH
I J
-
Fig.3 Td/m results from AlFQ4-11 precursors containing different dnes. (a) Di-iso-propylamine. (b) Di-n-propylamine. (c) n-Butylethylamine. (d) Di-ethylamine, k =water from kaolinite dehydroxlyation. Details of peaks i, ii, and iii are given in Table 2. kaolinite. Subtraction of this calculated weight from the total weight loss yielded the m u n t of d n e that was present in the precursor. These results are tabulated in Table 1, as are the concentrations of d n e mlecules per channel-unit as determined by conventional chemical analysis performed on a Perkin Elmer Model 240 (JHN Analysert. The results from both methods agree within experimental error. The samples of AlW4-ll produced using i-Pr2NH and n-Pr2NH have one mine per channelunit, while that using Et2NH has greater than one and that using n-BuEtNH has less than one. It was found that the yield of AlpO4-11 was considerably lmer in the n-BuEtNH preparation, which suggests that this amine YAnalysed by the Microanalytical Lab0ratory, mago University, FQ Box 56, mnedin, New Zealand.
643
is not as good a t tenplating the AlpO4-11 s t r u c t u r e as a r e the other m i n e s studied. This may be a consequence of the non-central position of the N-atcm i n t h i s amine. Consistent with t h i s is t h e f a c t t h a t primary m i n e s and n-butylmethylamine do not d i r e c t the formation of AlpO4-11 e i t h e r ( r e f . 3). TABLE 1 Concentration of amine molecules per channel-unit mine
m i n e ndecules
Der
td/m i-Pr2NH n-Pr2NH n-BuEtNH Et2NH
channel-unit Chemical analysis
1 . 0 3 0.06 1.0@ 0.06 0.8@ 0.07 1.2e0.09
1.00~0.10 0.9WO. 10 0.7W0.11 1.06t 0.10
The td/m experiment also allows a d e t a i l e d analysis of the process of water desorption.
I n a l l cases, three losses are observed.
Values of Tmxr the
temperature of each peak maximum, are l i s t e d i n Table 2, together with the temperatures a t which the m a x i m m u n t of m i n e is desorbed.
These
temperatures are s l i g h t l y lmer than those observed by tg, due to the lower pressure of the M/m system. TABLE 2 -sorption
temperatures a t peak maxima (Tmx) of water and m i n e
from AlP04-U precursors Tmax ("C) by td/m
Structure-directing species
(i) i-Pr2NH n-Pr2NH n-BuEtNH Et2NH
88 60 60 58
Water (ii)
(iii)
sh 210 168 205 172 218 163 213 s h = shculder
hi ne 205 173 177 164
Given t h e AlpO4-11 s t r u c t u r e (figure l ) , a possible assignment to these three losses is t h a t peak ( i ) is due to water on the external c r y s t a l surfaces while peak ( i i )which desorbs w i t h the m i n e is l i k e l y to be water frcm the large channels. Finally peak (iii) could be due to water frcm t h e sixmembered rings as it is too large to e n t e r the fourmembered rings (ref.6).
644
The temperature at which t h e m i n e s desorb is a measure of the ease with The smallest
which they are remved from the AlFO4-ll channel system.
molecule, Et2NH, desorbs easiest while t h e b l k y i-Pr2NH r e q u i r e s t h e h i g h e s t temperature. CONCLUSION Td/m enables rapid determination of t h e m u n t of s t r u c t u r e - d i r e c t i n g s p e c i e s remaining i n t h e ~ 1 ~ 4 - 1precursors. 1
This mthcd g i v e s similar
r e s u l t s t o those from conventional chemical a n a l y s i s ht, i n a d d i t i o n provides information as to t h e d i s t r i b u t i o n of water w i t h i n the s t r u c t u r e and on t h e
ease of removal of t h e m i n e . REFERENCES
1 S.T Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J.Amer.Chem.Soc. 104, (19821, 1146-1149. 2 B.M. Lok, T.R. Cannan, C.A. Messina, Zeolites, 2, (19831, 282-291. 3 N.J. Tapp, N.B. Milestone, D.M. Bibby, S u h i t t e d t o Zeolites, 1987. 4 L.M. Parker, J.E. P a t t e r s o n , Chemistry Division Report N o CD2330, Department of S c i e n t i f i c and I n d u s t r i a l Research, New Zealand. 5 J.M. Bennett, J.V. Smith, 2. Kristall. 171, (1985), 65-68 6 D.W. Breck, ieolite Molecular Sieves, W z y and Sons, New York, 1974, pp.65 and 636.
COMM ERClALlSATlON OF THE GAS-TO-GASOLIN E PROCESS
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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
641
Joseph D. Korchnak, Davy M c K e e corporation, P. 0. Drawer 5000, 4715 South Florida Avenue, Lakeland, Florida 33813 USA
AEsTRAcr
In this paper the various routes to methanol synthesis are sumrarized and ccmpared with a new catalytic @a1 oxidation route. Also ampared are quench, steam raising and tube amled converters, along with comparative energy and econanic sumnaries of the various routes.
INTRoDucrON Methanol is synthesised frm a gaseous mixture of hydrogen and carbon oxides ccmnonly called synthesis gas (syngas). The syngas can be prcducd from several sources, such as coal, heavy residues, biomass, natural gas or associated hydrocarbons. Hwever, since the gasification of coal, heavy residues, or biomass is not yet econunical, the most c a m m feedstocks for methanol plants will be natural gas or associated hydrocarbons. This trend in feedstocks is likely to continue for at least the next couple of decades. The object of this paper is to examine the most prcmising synthesis-based processes for metham1 production which are: Route 1
Conventional Steam Reforming
Route
Steam reforming followed by a semmkry oxygen blown reactor (Autotherml)
2
Route 3
Partial Oxidation (POX)
Route 4
catalytic Partial oxidation
(CFO)
SIESPHESIS GAS murEs Route 1: Conventional Steam Reforming Utilizmg a natural gas feedstock for methanolsynthesis, the conventional steam refonring route yields "excessive hydrogen" as follows:
0 Davy McKee Corporation 1987
SYNGAS -HEAT RECOVERY
COMPRESSION IOPTIONALI
METHANOL SYNTHESIS
DISTILLATION
PRODUCT
HI RECOVERY IOPTIONALI STEAM STEAM
[PO REACTOR
SYNGAS HEAT RECOVERY
FIGURE 1 : ROUTES TO METHANOL
t METHANOL SYNTHESIS
'
OlSTlLLAllON
PRODUCT
a
649 CH4 CO
--->
+ H20
--->
+ H20
00 +
C02 CH4
2H2
+
+
H2 H20
0
---> ---> --->
CO
+ 3H2
a 2
+
H2
( Shift)
(Methanol)
(3H30H
00
(Reforming)
+ H20
CH30H + H2
( Reverse
Skift)
(Overall)
This excessive hydrcqen is usually purged from the methanol loop and burned in the reforming furnace to provide heat for the endotherroic reforming reaction. Note that if CO2 is present or added to the natural gas, a synthesis gas can result as follaws: CH4
+
H20
+ 1/3 CO2 --->
CO + 3H2 + 1/3 CO2 --->
Steam Reforming In this paper, considered.
cO2
11/3 CH3OH
+ 1/3
H20
(6)
Methanol Synthesis addition from an external source will not be
The basic process steps in Route 1 are shown in Fig. la, and consist of the follming:
1. Natural gas steam reforming 2. Cornpression 3. Methanol Synthesis 4. Distillation 5. Steam Generation Route 2:
Steam Reforming Plus m e n secondary The conventional steam reforming of natural gas is supplemented by the addition of a secondary oxygen-blm reactor where oxygen and by-pass natural gas are added to enhane the yield of syngas. A "stoichimtric" syngas is prcduced, and the methanol loop purge gas is usually used as fuel in the primary steam reformx, which is approximately a quarter of the size of that in Route 1. The overall reactions in Route 2 are those previously indicated along with the follming canbustion reactions of the autothema1 reactor.
650
The basic process steps in Route 2 are shm i n Fig. lb, and consist of the following basic steps:
1. Natural gas steam reforming
5.
Oxygen b l m secondary reactor Cmpression Methanol Synthesis Distillation
6.
Steam Generation
2. 3. 4.
Route 3:
P a r t i a l Oxidation (POX)
P a r t i a l oxidation of
natural gas in a gasifier, such a s a Texaco o r
Shell generator, using oxygen produces a "carbon rich" syngas. I n order t o adjust the hydrcgen t o carbon r a t i o of the syngas t o that required for
methanol synthesis,
it is necessary to remove carbon.
It is removed either
in the form of carbon dioxide by shifting c a r h n monoxide ' and then remxring
it in an acid gas remxral system such as MDm, o r by parixally removing CU and a 2 by pressure swing adsorption (PSA)
.
The basic process steps in Route 3 are s h m i n Fig. lc, and consist of the following: 1. p a r t i a l oxidation Generator
2. 3.
€IT s h i f t
4.
5.
ccmpression (optional) Methanol Synthesis
6.
Distillation
I.
Steam Generation
Carbon Reimval/X2 Recovery
Route 4: catalytic P a r t i a l oxidation (ao) catalytic partial oxidation is a process which produces a "stmichicmetric" methanol synthesis gas by reading natural gas w i t h n 1 -i steam and oxygen i n a catalytic vessel with or without reawered hydrogen.
65 1 The
basic process
steps in Route 4 , catalytic partial oxidation, are
shown i n Fig. Id, and consist of the following:
1. Catalytic Fixed Bed R e a c t o r
canpression (optional1
2. 3.
Methanol Synthesis
4.
Hydrogen R e c o V e r y (optional)
5.
Distillation
6.
Steam Generation
METHANOL SToIaIcMETRY
The various routes
to methanol synthesis gas a l l prcduce hydrogen and
carbon oxides in d i f f e r e n t ratios. It must be emphasized that high purity syngas is essential f o r the methanol synthesis catalyst, which is easily poisoned o r deactivated by camponents such as chlorides, sulfur and heavy metals, i.e., mercury. Also, large quantities of inerts, namely, nitrogen and methane, which have no e f f e c t on the methanol catalyst, can reduce the overall carbon efficiency which w i l l increase the amount of syngas required per u n i t make of methanol. The stoichianetric syngas ratio requir-t f o r the methanol process is :
H2 2co + 3 a 2 Table
=
1
summrizes
1
(9)
the
typical
relationship
between
syngas s t o i c h i m t r y for methanol made w i t h a n a t u r a l g a s f e e d s t o d c t h a t h a s a H2/C molar ratio of 1.5 to 2.0. TABLE1
Syngas Ccmposi t i o n f m t h e V a r i o u s Routes v i a a Natural G a s Feedstock
m a s Production Steam Reforming Stm. Ref. + 0 seccaadary Partial oxi&ion catalytic P a r t i a l oxidation
E@ 2.9 2.5 1.6 1.8
H2/(2C0
- 7.0
- 3.0 - 2.0 - 3.5
1.3 0.96 0.7 0.96
+
3C02)
- 1.5 - 1.04 - 0.9 - 1.04
f “ H RICH“ S‘uNGAS
7
PURGE
L C R U O E METHANOL
I . C. I . OUENCH CONVERTER WARM-SHOT LOOP
I
I CllJFNIl
ONVFRER COLO-
H f f J IN
SYNGAS PURGE
CW
c zc /u
-tlK:kOL
;TEAM
ENERATING METHANOL CONVERTEF LOOP
F I G IRI 2 METtANOL LOOP DIAGRAMS
653
Note: Adjusting syngas carpsitions for suitability in various processes is often challenging and is most likely dictatd by economics of the units involved. r n W L SYNTHESIS
New technologies for methanol synthesis are presently being developxi. These new systens, which have been tested in bench or demonstration units, use either l m temperature liquid-gas phase or solid-gas phasemethanol reaction techniques. Hawever, the suitability for comnercialization of these new technologies is uncertain due to either questionable ec0ncmical advantages or insufficient developnent; therefore, t h i s paper will address the conventional methanol synthesis loop. I.C.I. Wann-Shot Quench Loop Fig. 2 shms the I.C.I. warm-shot methanol synthesis loop. The adiabatic methanol reactor has mltiple catalyst beds which are quenchd with warm reactant gas that control the methanol converter's temperature profile and methanol outlet concentration as portrayed in Fig. 3.. In t h i s adiabatic quench rdctor methanol loop scheme, the main features are: 1. Simple vessel structure 2. Ease of catalyst loading and unloading 3. Large single reactor capacity with capabilities over 3000 tons of methanol per day 4. CaTlplex heat recovery 5. High rate of recycle gas 6. Numerous control loops 7. High catalyst volume
I.C.I. Cold-Shot Quench Loop Fig. 2 shows I.C.I. cold-shot quench converter methanol loop which operates similar to the warm-shot loop. This adiabatic type quench reactor with its associated e q u i m t is used when the syngas entering the methanol loop is stoichianetric or slightly carbon rich. It has the same adiabatic reactor profile as sham in Fig. 3 . The main features of the cold-shot reactor loop are:
654
1. Simple vessel structure 2.
Ease of catalyst lcadmg and unloading
3. 4.
Large single reactor capacity with capabilities over 3000 tons of methanol per day Simplified heat recovery
5.
Low rate of recycle gas
6.
Numerous control loops Low grade heat recovery
7.
Steam Generating Methanol Loop The object of t h i s type of methanol l a p is to recover the heat of the methanol synthesis reaction by raising nominal 2750 K Pa (400 psig) steam, as sham i n Fig. 2. There are presently two ccormercially available variations of the steam-
raising converters, normally called "tubular reactors". The f i r s t variation has tubes packed with methanol catalyst which areextexnallycooledby boiler feedwater on the shell, a s i n a s h e l l and tube heat exchanger. The second variation has a spiral-coiled tubular heat exchanger within a converter shell, as in cryogenic heat exchangers. The catalyst bed is placed within the converter shell, and the boiler feedwater is i n the tubes. Bath types of tubular reactors are isothermal in operation and have a temperature vs. methanol concentration profile similar to that sham i n Fig. 3. The main features of the steam raising methanol loop are: 1. Simplified heat recovery
2. 3.
4.
5. 6.
7.
2750 K Pa (400 psig) steam raising capabilities Law r a t e of recycle gas Single reactor capacity of approximately 1500-2000 tons of
methanol per Canplex construction of a pressure vessel (tubesheet or pitch arrangement w i t h large pressure differential) Difficulties i n maintenance and inspection Law catalyst volume
Tube cooled converter (TCc) Methanol Loop
The concept of the new tubecooled converter is a simple gas t o gas exchanger with no high differential pressure tubesheets. The reactant gas is fed into the bottun of the TCcwhereit i s d i r e c t e d u p a r d a n d preheated t o reaction temperatures through multiple tubes. Upon exitmg the tubes, the reactant gases are channeled downward into the catalyst which is heat
655 10.0
9.0 8.0 7 .O %
Methanol
6.0
5.0 4 .O
3.0 2 .o
1 .o 0
100
200
300
400
TEMPERATURE DEG C
10.0
-1
9 -0
8.0 7.0 %
6.0
-
TYPE REACFOB
-
Methanol
5 .O 4 .O
3.0 2.0
1 .o
0
Fig. 3.
Reaction
REACTOR PROFILE
f - 7
I
I I I I
I
SYNGAS REACTOR
I I I
IOL
I I
roR
I
STEAM CONDlNSER
RAISING
RECOVERY
m
I PRODUCT METHANOL
I
--Jq-i
R E F I N I N G COLUMN
COOLING SINGLE STAGE COMPRESSION
CIRCULATOR
HEAVY ENDS RECYCLE
REBOILE
R RECYCLE
(2) M E T H A N O L S Y N T H E S I S
FIGURE 4 :
NEW METHANOL PLANT
_ _ _ _ -13)_ _M E_T H_A N_O L_D I_S T-I L_L A_T I O_N _ _
I
J
657
packed in the space between the tubes. The gas flaws in the tubes and the catalyst bed are counter-current, as sham in Fig. 2. The temperature profile vs. methanol concentration is quasi-isothermal and is very favorable in terms of Eeaction rates and conversion. The main features of the tube cooled converter are: 1. Mechanical soundness with nunium differential pressure and no tubesheet construction problems. 2. Ease of catalyst loading and unloading. 3 . Single reactor capacity with capabilities of over 2000 tons per day of methanol. 4. L a rate of recycle gas. 5. Simplified heat recovery with capability of heating boiler feedwater for generation of high pressure steam, or external generation of steam at the 2750 K Pa (400 psig) level. 6. Minimum control loops. 7. Lcky catalyst volume.
P m s s SELEXYION After camparing the various options to methanol production, a new methanol plant design evolved which utilizes catalytic oxidation syngas gemation and the tube cooled methanol converter. This new route w a s selected by Davy as the mstadvantageousprocessintermsof ecOlnaRics, simplicity, reliability and operability, see Fig. 4. New Methanol Plant Features 1. Stable Syngas Catalyst - Sulfur is not a specific poison. 2.
Lcw Catalyst Volume Requirement - For canparison in a 2000 ton/day methanol plant, the approximate reactor size equivalences are:
-
Item
SteamReformer 02
Route 1 640 tubes None
Route 2 160 tubes 1x3.7 m Dia.
Route 3
None 2x3.0 m Dia.
New Route 4
None 1x<2.5 m Dia.
Gas
IKET
.TOP OF CATALYST
BED
CATALYST
CERFVIIC BFILLS
CFITRLYSi DISCWIRW
I C I CONWRTER
-I .
I ms
INLET
k S OUTLET
TUBE COOLED CONVERTER
FIGURE 5: I. C. I. QUENCH CONVERTER VS TUBE COOLED CONVERTER-
659
-
Expectancy greater than one year.
3.
Catalyst Life
4.
Feedstock Rangeability Excellent
such a s No.
oils,
2
-
Hydrccarbons
ranging
from heavy
fuel o i l , t o l i g h t natural gas, with o r without
hydrogen, are suitable feedstocks. 5.
L m Oxy9en Consumption - As much a s 30% savings i n 02 can be realized compared t o pox reactors (Route 3 ) . 02 consumption is comparable t o secondary reforming are :
(Route 2 ) .
1
2
The approximate oXy9en C o n s ~ t i o n s
3
4
Route
Fig. 6 . S/C
6.
Oxygen Consumption Ratio
-
Syngas catalyst can operate with very law steam to
carbon ratio, and no extemally generated steam is required f o r the new
process route to methanol. 7.
Good
Temperature and Pressure Rangeability
- Atmspheric
to 8250 K Pa
(1200 psig) with operating temperature to 105OOc.
8.
No B u r n e r s Required
9.
Simplicity of Design
10. Flexibility
-
- Catalyst provides required equilibrium. - Fixed bed with
s e l f &stxibution.
Can operate a t varying temperatures, pressures and steam
to carbon ratios because catalyst is not carbon limited.
660
11. The approximate boiler feed w a t e r ( E W ) make-up for the optional routes is approximtely:
I0
.2
.4
0.6
0.8
Fig. 7. Boiler Feedwater Requiremnts The 1w COnsLrmption of BFW make-up in Route 4 allaws econanical use of
seawater f o r methanol plants, requred. 12. Law Energy consumption
-
i.e.,
IH)
fresh ground water would be
A "grass-roots" methanol p h t with its
own
oxygen plant, a l l u t i l i t i e s , o f f s i t e s , and its m p e r generation can achieve the f o l l w i n g total energy usage.
Route Fig. 8.
Energy Requiremerrts
NOTE:
An additional reduction i n energy usage of approximately
1 W/ton plant.
of methanol can be achieved with a battery limits
661 Other
Features
With the new apprcach t o mthanol plant designs, it became apparent
a small, economical, packaged methanol plant using a PSA oxygen unit would be suitable f o r production of up t o 100 ton per day of f u e l grade methanol. These small methanol plants would require a single level steam system and have the energy usage of approxhately 31 GJ/metric ton methanol. that
Overall Cost Canparison
Placing a l l optional routes on the same basic U.S.
Gulf Coast price,
ccmparative capital expenditures f o r "grass-roots" 2000 ton per day single tram methanol plants are: 160
140
120 Millioy U.S.DOllars
100
80
60 40 20
0 1
2
Route Fig. 9.
Grass Route Capital Cost
3
4
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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
FROM MOLECULES
663
TO CONCRETE AND STEEL
J. Z. BEM Manager o f Engineering, B e c h t e l , I n c . , San F r a n c i s c o , C a l i f o r n i a , U.S.A.
ABSTRACT The New Zealand Gas t o G a s o l i n e p r o j e c t i l l u s t r a t e s t h e e f f o r t r e q u i r e d t o t r a n s l a t e r e s e a r c h and l a b o r a t o r y d a t a t o a p l a n t o f c o n c r e t e and s t e e l . The p r o j e c t was engineered by t h r e e major c o n t r a c t o r s i n t h e USA and o t h e r s i n New Zealand. P o r t i o n s o f t h e p l a n t were preassembled i n Japan and shipped i n l a r g e complete segments t o New Zealand. The p l a n t was designed and b u i l t t o s t r i c t environmental r e q u i r e m e n t s which were reviewed and approved i n accordance w i t h t h e laws o f New Zealand. The c o o r d i n a t i o n and c o o p e r a t i o n between t h e many e n t i t i e s i n v o l v e d i n t h e p r o j e c t p r e s e n t e d some u n i q u e c h a l l e n g e s i n p r o j e c t management.
INTRODUCTION The Gas t o G a s o l i n e p l a n t i n New Zealand i s t h e f i r s t commercial f a c i l i t y b u i l t t o c o n v e r t n a t u r a l gas t o g a s o l i n e .
It i s a s t r a t e g i c a l l y and
e c o n o m i c a l l y s i g n i f i c a n t p l a n t s u p p l y i n g a p p r o x i m a t e l y one t h i r d o f t h e t o t a l t r a n s p o r t a t i o n f u e l demand i n New Zealand. The d e s i g n and c o n s t r u c t i o n o f t h e f a c i l i t i e s p r e s e n t e d u n i q u e c h a l l e n g e s
i n e n g i n e e r i n g , l o g i s t i c s and c o n s t r u c t i o n , c o n s i d e r i n g t h e d i s t a n c e f r o m New Zealand o f t h e sources o f a major p o r t i o n o f t h e process equipment and t h e r e l a t i v e lack o f skilqed construction labour required t o b u i l d t h e p l a n t w i t h i n a reasonable schedule. The key t e c h n i c a l i s s u e s were: 1.
The s t r i n g e n t environmental r e q u i r e m e n t s
2.
The preassembly o f p o r t i o n s o f t h e p l a n t
3.
The c o o r d i n a t i o n o f d e s i g n and c o n s t r u c t i o n e f f o r t by numerous contractors
The h i s t o r y o f t h e p r o j e c t i l l u s t r a t e s t h e method and t h e e f f o r t r e q u i r e d i n t h e c o n v e r s i o n o f l a b o r a t o r y d a t a t o a p h y s i c a l p l a n t made o f c o n c r e t e and steel.
664
D E S C R I P T I O N OF THE PLANT The p l a n t i s l o c a t e d on t h e c o a s t a p p r o x i n a t e l v 26 kms n o r t h e a s t o f New Plymouth.
I t i s a grass r o o t s p l a n t ; t h a t i s , i t was c o n s t r u c t e d where no
p r e v i o u s f a c i l i t i e s e x i s t e d , and i t i s s e l f - s u f f i c i e n t w i t h t h e e x c e p t i o n o f e l e c t r i c power supp1,y.
A s i m p l i f i e d b l o c k f l o w diagran! o f t h e f a c i l i t y i s shown i n F i g u r e 1. The p r o c e s s i n g area o f t h e p l a n t c o n s i s t s o f t h r e e p r o c e s s u n i t s :
two
methanol p l a n t s and t h e methanol t o g a s o l i n e (MTG) c o n v e r s i o n p l a n t .
The
methanol u n i t s c o n v e r t n a t u r a l gas f r o m t h e Maui P l a t f o r m t o c r u d e methanol u s i n g t h e I C I l o w p r e s s u r e s y n t h e s i s process.
2200 tonnes o f methanol p e r day.
Each methanol p l a n t can produce
The methanol i s f e d d i r e c t l y t o t h e MTG u n i t
where i t i s c a t a l y t i c a l l y c o n v e r t e d t o g a s o l i n e and small amounts o f LPG. The p l a n t produces 2300 c u b i c meters (14500 b l s l o f premium grade g a s o l i n e b l e n d i n g stock.
No h e a v i e r t h a n g a s o l i n e hydrocarbons a r e produced.
The
t h r e e process u n i t s a r e s u p p o r t e d by a complete range o f s u p p o r t f a c i l i t i e s as shown on F i g . 1.
These s u p p o r t f a c i l i t i e s a r e :
The w a t e r t r e a t m e n t p l a n t which t r e a t s w a t e r f r o m t h e W a i t a r a R i v e r f o r u s e as b o i l e r feedwater, c o o l i n g w a t e r make-up, and p o t a b l e water, The l i q u i d e f f l u e n t p l a n t which i s a t e r t i a r y t r e a t m n t f a c i 1 i t . y t h a t r e m v e s process wastes f r o m t h e p l a n t l i q u i d d i s c h a r g e , The c o o l i n g w a t e r system composed o f an e i g h t e e n - c e l l ,
induced-draft
c o o l i n g tower, and pumping f a c i l i t i e s , and O t h e r s u p p o r t f a c i l i t i e s such as u t i l i t y and i n s t r u m e n t a i r systems, steam g e n e r a t i o n , f l a r e r e l i e f s y s t e m , maintenance, f i r e f i g h t i n g , warehouse and a d m i n i s t r a t i o n b u i l d i n g s . The completed p l a n t i s shown i n F i g s . 2 and 3.
WATER 1,270 M3/HR + WATER TREATMENT 5,600 GPM
COOLING WATER
INSTRUMENT
PLANT
P R
..
0 C E
s s
FUEL GAS A hATURALGAS 155,000 M3/HR 131MMSCFD
BFw
I
STEAM 550 TONNESIHR 1,300,000 LBS/HR
1
TREATMENT
+
AUXILIARY BOILER
STEAM SYSTEM
MTG REACTOR
FUEL GAS
METHANOL
METHANOL SYNTHESIS
MTG
I+
MTG DISTILLATION
I
HIGH VAPOR PRESSURE GASOL IN€
STORAGE BULLETS
*
HVY HGTFUNITS 4 GASOLINE STORAGE
LIGHT GASOLINE
LIGHT GASOLINE STORAGE
2
{3
1
BLENDING
- 1
* HEAVY GASOLINE TREATMENT FACILITY
Fig. 1 SIMPLIFIED BLOCK FLOW DIAGRAM
ADDITIVES
GASOLINE 14,500 BPD
666
667
HISTORY OF THE PROJECT I n c o n t r a s t t o t h e photographs of t h e completed p l a n t , F i g . 4 .shows t h e p l a n t s i t e i n 1980 p r i o r t o s t a r t o f c o n s t r u c t i o n . Between t h e s e two views o f t h e s i t e i s a t i m e i n t e r v a l o f f i v e y e a r s and t h e e f f o r t s of thousands o f engineers, designers, buyers, e x p e d i t e r s , i n s p e c t o r s , c o n s t r u c t i o n s u p e r v i s o r s , foremen, c r a f t s m e n and o t h e r s on f o u r c o n t i n e n t s and i n seven c o u n t r i e s . An o v e r a l l schedule o f t h e p r o j e c t i s shown i n F i g . 5. t o o k f i v e y e a r s t o engineer, p r o c u r e and c o n s t r u c t .
The t o t a l p r o j e c t
T h i s does n o t i n c l u d e
t h e t i m e r e q u i r e d t o develop t h e MTG process by M o b i l Corporation,
or the
f e a s i b i l i t y s t u d i e s performed by t h e New Zealand Government. P r e l i m i n a r y e n g i n e e r i n g was s t a r t e d i n May 1980 w i t h t h e award o f t h e p r o j e c t s e r v i c e s c o n t r a c t t o B e c h t e l , Inc. o f San Francisco, C a l i f o r n i a . S h o r t l y a f t e r w a r d s , two more c o n t r a c t o r s were selected:
Davy-McKee o f
Lakeland, F l o r i d a , f o r t h e d e s i g n o f t h e two methanol p l a n t s and Foster-Wheeler o f L i v i n g s t o n , New Jersey, f o r c o n c e p t u a l and b a s i c d e s i g n o f t h e MTG p l a n t .
I
1980
I WED CONST.
=SITE SELECTION ENVIRONMENTAL IMPACT REPORT
TRIBUNAL
=ENGINEERING
*STUDIES, PROCESS DESIGN *DETAIL DESIGN
W PROCUREMENT PREASSEMBLI
*FABRICATION
c-
I
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T
CONSTRUCTION
PR ECOMMISSIONING
Fig. 5 GTG PROJECT SCHEDULE
m CW
669
An e a r l y a c t i v i t y on t h e p r o j e c t was t h e s e l e c t i o n o f a s i t e o u t o f t h e f o r t y - t h r e e s i t e s p r e v i o u s l y surveyed and deemed s u i t a b l e .
A composite team
o f Mobi 1 and B e c h t e l p e r s o n n e l supported by New Zealand c o n s u l t a n t s narrowed t h e s e l e c t i o n f i r s t t o t h r e e s i t e s , t h e n t o one by August 1980. f o r s i t e s e l e c t i o n were many and v a r i e d .
The c r i t e r i a
The most s i g n i f i c a n t were: p r o x i m i t y
t o t h e n a t u r a l gas p i p e l i n e , t o a r e l i a b l e source o f water, t o a c e n t r e o f p o p u l a t i o n which wou I d p r o v i d e t h e necessary i n f r a s t r u c t u r e i n s u p p o r t o f p l a n t operation,
and t o t h e P o r t o f T a r a n a k i f o r s h i p p i n g t h e g a s o l i n e p r o d u c t
and f o r reasonable t r a n s p o r t a t i o n access f o r preassemblies.
A c l e a r and l e v e l
s i t e t o m i n i m i z e c i v i l works was needed as w e l l as a l o c a t i o n t h a t would n o t have an o v e r w h e l m i n g l y n e g a t i v e e n v i r o n m e n t a l impact on t h e area. Once t h e s i t e had been s e l e c t e d , t h e work on t h e Environmental Impact Report (EIR) began.
P r e l i m i n a r y e n g i n e e r i n g s t u d i e s and surveys c o n t i n u e d
u n t i l December 1980 when t h e E I R was submitted.
The E I R addressed t h e i s s u e s
o f o v e r a l l impact o f t h e p l a n t on t h e s u r r o u n d i n g area w i t h s p e c i f i c r e f e r e n c e t o t h e p r e s e r v a t i o n o f M a o r i h i s t o r i c s i t e s w h i c h were l o c a t e d a d j a c e n t t o t h e site.
The EIR was f i l e d under t h e N a t i o n a l Development A c t on a f a s t t r a c k
schedule.
The E I R T r i b u n a l h e a r i n g s t o o k p l a c e i n New Plymouth i n t h e t h i r d
q u a r t e r o f 1981,and t h e p e r m i t t o c o n s t r u c t was g i v e n i n March 1982. I n a d d i t i o n t o p r o v i d i n g r e q u i r e d d a t a t o t h e EIK,the
c o n t r a c t o r s had
s t a r t e d e n g i n e e r i n g a t t h e end o f t h e t h i r d u u a r t e r o f 1980. By t h e second q u a r t e r o f 1981 c e r t a i n long d e l i v e r y i t e m s o f m a j o r equipment were p l a c e d on order w i t h suppliers.
The d u r a t i o n o f t h e e n g i n e e r i n g e f f o r t was 2-1/2 years.
A key element t h r o u g h o u t t h e p r o j e c t was t h e d e c i s i o n t o preassemble p o r t i o n s o f t h e p l a n t o u t s i d e New Zealand and s h i p them i n c o m p l e t e l y p r e f a b r i c a t e d assemblies f o r i n s t a l l a t i o n a t t h e s i t e . C o n s t r u c t i o n s t a r t e d i n March 1982 w i t h s i t e c l e a r i n g and grading. work c o n t i n u e d f o r about a year.
The n a t u r e o f t h e soi1,which
Civil
has a low
b e a r i n g s t r e n g t h r e s u l t e d i n e x t e n s i v e u s e o f p i l i n g under equipment foundations.
A t o t a l o f 7000 p i l e s were i n s t a l l e d .
Equipment i n s t a l l a t i o n
was s t a r t e d i n 1983; t h e f i r s t preassemblies a r r i v e d a t t h e j o b s i t e i n August 1983; The l a s t i n February 1984.
The p l a n t was m e c h a n i c a l l y complete,ready
f o r s t a r t - u p i n J u l y 1985,and was t u r n e d o v e r t o t h e New Zealand S y n t h e t i c Fuels Corporation.
The p l a n t was on stream and o p e r a t i o n a l by t h e end o f 1985.
PROJECT ORGANIZATION The o r g a n i z a t i o n o f t h e p r o j e c t was s e t up t o support t h e t h r e e a c t i v i t i e s o f e n g i n e e r i n g , procurement and c o n s t r u c t i o n .
Although B e c h t e l , I n c . had t h e
o v e r a l l r e s p o n s i b i l i t y as p r o j e c t s e r v i c e s c o n t r a c t o r , p l a n t were engineered and p r o c u r e d by o t h e r s .
major p o r t i o n s o f t h e
, ,4 ,
MOB1L RESEARCH & DEVELOPMENT
BECHTEL PROCESS SERVICES CONTRACTOR
METHANOL
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OFFSITES/UTI LIT1ES
INFRASTRUCTURE NEWZEALAND MINISTRY OF WORKS
DAVY Mc K EE
.PROCESS DESIGN 0 DETAIL DESIGN PROCUREMENT EQUIPMENT
DESIGN PROCUREMENT 0 CONSTRUCTION
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0 DESIGN 0 PROCUREMENT
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Fig. 6 CONTRACTOR ASSIGNMENTS
NEWZEALAND CONTRACTORS I
1
BUILDINGS 0 SOILS 0
671 F i g u r e 6 shows t h e v a r i o u s c o n t r a c t o r assignments.
These were made on a
p l a n t area b a s i s ; t h a t i s , p o r t i o n s of t h e p l a n t were t o t a l l y assigned t o d i f f e r e n t c o n t r a c t o r s f o r complete d e s i g n i n a l l e n g i n e e r i n g d i s c i p l i n e s . P a r t s o f t h e p l a n t and s u p p o r t f a c i l i t i e s i n c l u d i n g p l a n t b u i l d i n g s p l u s landscaping were engineered o r designed i n New Zealand.
F a c i l i t i e s outside
t h e c o n f i n e s o f t h e p l a n t such as roads, b r i d g e s , p i p e l i n e s , t h e w a t e r s u p p l y system, t h e p r o d u c t t a n k f a m i n New Plymouth and t h e e f f l u e n t l i n e t o W a i t a r a were a l s o engineered i n New Zealand.
Most o f t h e e n v i r o n m e n t a l work
a s s o c i a t e d w i t h t h e E I R was performed by New Zealand c o n s u l t a n t s .
All
s u r v e y i n g and s o i l i n v e s t i g a t i o n s were done by New Zealand f i r m s i n New Z e a l and. The c o o r d i n a t i o n o f a number o f c o n t r a c t o r s thousands o f m i l e s a p a r t was a m a j o r e f f o r t n e c e s s i t a t i n g many meetings by t h o s e i n v o l v e d and t h e development o f p r e c i s e procedures d e l i n e a t i n g t h e i n t e r f a c e s .
The payout i n
t h i s c l o s e a t t e n t i o n t o c o o r d i n a t i o n was e s s e n t i a l l y m i n i m a l i n t e r f a c e problems d u r i n g c o n s t r u c t i o n . Procurement was handled i n two ways: m a j o r equipment was purchased by i n d i v i d u a l c o n t r a c t o r s w h i l e non-engineered items, such as p i p i n g , s t e e l , e l e c t r i c a l cable, e t c e t e r a , were purchased by Bechtel. equipment was on a worldwide b a s i s .
Procurement o f
The s o u r c i n g o f equipment was d i c t a t e d
by t h e f i n a n c i n g arrangements f o r t h e p r o j e c t .
S i g n i f i c a n t numbers o f major
equipment and non-engineered items were s u p p l i e d by Japanese companies. The p r o j e c t o r g a n i z a t i o n was f l e x i b l e and i t was adapted t o t h e needs and requirements o f t h e p r o j e c t .
For a c e r t a i n p e r i o d d u r i n g t h e p r o j e c t , t h e
c e n t e r o f g r a v i t y o f t h e a c t i v i t i e s was i n Japan.
These a c t i v i t i e s i n c l u d e d
i n a d d i t i o n t o preassemblies, t h e s u p p l y o f equipment, s u p p l y o f non-engineered b u l k i t e m s and t h e p r e f a b r i c a t i o n o f a p o r t i o n o f t h e p i p i n g . The a b i l i t y t o a d j u s t t h e o r g a n i z a t i o n i n response t o t h e needs o f t h e p r o j e c t r e f l e c t s f a v o u r a b l y on t h e c a l i b e r and r e s i l i e n c e o f t h e p e r s o n n e l who were a b l e t o move homes and f a m i l i e s f r o m U.S.A. Zealand.
t o Japan t o New
A key t o t h e success o f t h e p r o j e c t was t h e c o o p e r a t i o n between t h e
main e n g i n e e r i n g c o n t r a c t o r s Davy-McKee and B e c h t e l , and t h e c l o s e l i a i s o n w i t h t h e owners, M o b i l and New Zealand S y n t h e t i c F u e l s C o r p o r a t i o n (NZSFC). T h i s was ensured by r e p r e s e n t a t i v e s o f M o b i l , NZSFC, Davy-McKee and B e c h t e l b e i n g l o c a t e d i n each o t h e r s o f f i c e s , t h e preassembly y a r d and t h e construction site. C o n s t r u c t i o n management was under t h e o v e r a l l s u p e r v i s i o n o f B e c h t e l . Zealand n a t i o n a l s comprised a p p r o x i m a t e l y h a l f o f t h e management i n t h e f i e l d ; t h e o t h e r h a l f was r e c r u i t e d f r o m USA, A u s t r a l i a , Canada and t h e U n i t e d Kingdom.
C o n s t r u c t i o n was done on a s u b c o n t r a c t b a s i s , managed by
New
672
B e c h t e l , performed by New Zealand c o n s t r u c t i o n companies.
A t o t a l o f 49
c o n s t r u c t i o n s u b c o n t r a c t s were issued. The preassembly y a r d was l o c a t e d i n t h e H i t a c h i - Z o s e n y a r d i n A r i a k e , Japan.
T h i s y a r d manufactured 76 s e p a r a t e preassemblies which were shipped
t o New Zealand as complete u n i t s . ENGINEERING APPROACH A l l t h e e n g i n e e r i n g c o n t r a c t o r s f o l l o w e d e s s e n t i a l l y t h e same approach t o
the project.
E n g i n e e r i n g was done on a t a s k f o r c e b a s i s where p e r s o n n e l were
assigned f u l l t i m e t o t h e p r o j e c t on an as needed b a s i s .
Most e n g i n e e r i n g
management and s u p e r v i s o r y p e r s o n n e l were on t h e p r o j e c t t h r o u g h o u t t h e d u r a t i o n o f t h e engineering e f f o r t . The sequence o f e n g i n e e r i n g was adapted t o t h e s p e c i a l r e q u i r e m e n t s o f t h e project.
I n i t i a l l y , process d e s i g n was completed f o r t h e t h r e e process u n i t s
and t h e process i n s t r u m e n t diagrams were developed.
The s u p p o r t f a c i l i t i e s
were scoped i n s u f f i c i e n t d e t a i l t o develop t h e o v e r a l l s i t e p l a n and t o d e t e r m i n e t h e r e a l e s t a t e r e q u i r e d f o r s i t e s e l e c t i o n and t h e p r e p a r a t i o n o f t h e EIR.
A f t e r t h e E I R was submitted, d e t a i l e d d e s i g n s t a r t e d w i t h t h e
p r e p a r a t i o n o f equipment m a t e r i a l r e q u i s i t i o n s f o r f a b r i c a t i o n and d e l i v e r y o f long d e l i v e r y i t e m s such as compressors,
l a r g e vessels, e t c .
A l l process u n i t s and some o f t h e o f f - p l o t p l a n t s were modeled on t h e
The models were t o s c a l e (3/8" t o 1'-0 o r 1:33) made o u t o f
design f l o o r .
p l a s t i c and showed i n d e t a i l a l l equipment, p i p i n g , s t r u c t u r a l s t e e l , i n s t r u m e n t a t i o n and most o f t h e e l e c t r i c a l items.
A p o r t i o n o f t h e MTG model i s shown i n F i g . 7.
The use o f p l a s t i c models
enables e n g i n e e r i n g c o n t r a c t o r s t o e l i m i n a t e t h e p r e p a r a t i o n o f t h e complex o r t h o g r a p h i c p i p i n g drawings.
It a l s o f a c i l i t a t e s t h e r e v i e w and a p p r o v a l o f
t h e p l a n t l a y o u t by t h e owners and m i n i m i z e s e r r o r s caused by i n t e r f e r e n c e between p i p i n g , s t r u c t u r a l s t e e l and concrete. Drawings of f o u n d a t i o n s , underground sewers and p i p i n g , and e l e c t r i c a l d i s t r i b u t i o n were prepared c o n v e n t i o n a l l y .
Computer aided d e s i g n and drawing
(CADD) a l s o was used.
Computers were e x t e n s i v e l y used i n e n g i n e e r i n g c a l c u l a t i o n s , m a t e r i a1 c o n t r o l and p r o j e c t p r o g r e s s m o n i t o r i n g . The d e c i s i o n t o preassemble p a r t o f t h e p l a n t i n Japan n e c e s s i t a t e d d e v i a t i o n s f r o m t h e normal sequence o f e n g i n e e r i n g design.
Instead o f a
p r o g r e s s i o n o f d e s i g n u s u a l l y f o l l o w e d i n process p l a n t s f r o m process d e s i g n t o layout, t o p i p i n g r o u t i n g , t o s t r u c t u r a l design, t o f o u n d a t i o n design, t o e l e c t r i c a l and i n s t r u m e n t a t i o n , a l l t h e s e f u n c t i o n s had t o be performed s i m u l t a n e o u s l y , l e a d i n g t o some r e d e s i g n and an i n c r e a s e o f engineeringmanhours.
D u p l i c a t e s c a l e models o f t h e preassemblies were b u i It
and s e n t t o t h e preassembly y a r d w h i l e t h e complete p l a n t models were s e n t t o t h e c o n s t r u c t i o n s i t e i n New Zealand. PROJECT HIGHLIGHTS T h i s p r o j e c t has u n i q u e f e a t u r e s d i f f e r e n t f r o m a n y t h i n g encountered The f o l l o w i n g have had a m a j o r e f f e c t on t h e e x e c u t i o n o f t h e
previously. project. Preassembly
The d e c i s i o n t o preassemble p a r t s o f t h e p l a n t was made at' t h e o u t s e t o f t h e project.
It was d r i v e n by t h e p r o j e c t i o n o f t h e numbers o f s k i l l e d
c r a f t s m e n r e q u i r e d t o b u i l d t h i s p l a n t i n New Zealand on a r e a s o n a b l e schedule.
S t u d i e s showed t h a t t i m e l y c o m p l e t i o n was n o t p o s s i b l e g i v e n t h e
a v a i l a b l e s k i l l e d manpower i n New Zealand u n l e s s preassemblies were used.
As
soon as t h e e n g i n e e r i n g had progressed t o t h e s t a g e a t which p l a n t f a c i l i t i e s had been i d e n t i f i e d and scoped, a s t u d y was s t a r t e d t o i d e n t i f y t h e preassemblies. preassemblies,
Emphasis was p l a c e d on t h e l a b o u r c o n t e n t o f t h e t h a t i s p r i o r i t y was g i v e n t o preassembling p o r t i o n s o f t h e
p l a n t with a h i g h s k i l l e d l a b o u r component i n p i p i n g , e l e c t r i c a l and
instrument in s t a1 1a t ion.
674
The p r e a s s e m b l y d e s i g n was i n t e g r a t e d w i t h t h e d e s i g n o f t h e non-preassembled p o r t i o n s o f t h e p l a n t so t h a t no d i f f e r e n c e s between t h e m c a n be d i s c e r n e d .
F o r example, t h e p r e a s s e m b l y s u p p o r t s k i d s a r e l o c a t e d
b e l o w g r a d e and c o v e r e d by p a v i n g . Simultaneously,
t w o o t h e r s t u d i e s were u n d e r t a k e n : l o c a t i o n o f a s u i t a b l e
p r e a s s e m b l y y a r d , and t h e method o f t r a n s p o r t a t i o n f r o m t h e y a r d t o New Z e a l a n d as w e l l as t r a n s p o r t a t i o n o f t h e p r e a s s e m b l i e s w i t h i n New Z e a l a n d t o the site. Japan.
The s e l e c t i o n o f p r e a s s e m b l y s i t e s was q u i c k l y n a r r o w e d down t o
F i n a l l y t h e Hitachi-Zosen yard i n Ariake,
Japan was s e l e c t e d .
An
e x t e n s i v e s t u d y was made o f t h e v a r i o u s methods a v a i l a b l e t o t r a n s p o r t u n u s u a l l y l a r g e and heavy l o a d s f r o m Japan t o New Z e a l a n d . A l t e r n a t i v e s c o n s i d e r e d were r o l l o n / r o l l o f f b a r g e s and s h i p s ,
lift
o n / l i f t o f f s h i p s and c o n v e n t i o n a l f r e i g h t e r s .
The method o f l a n d i n g
p r e a s s e m b l i e s f r o m t h e s h i p s was i n v e s t i g a t e d .
A dedicated barge harbour
t h a t was t o b e c o n s t r u c t e d n e a r t h e p l a n t s i t e was r e j e c t e d e a r l y i n f a v o u r
675
o f t h e P o r t of Taranaki i n New Plymouth because o f t h e p r e v a i l i n g adverse sea
conditions.
The l o c a t i o n o f t h e harbour determined t h e s i z e and w e i g h t o f
t h e preassemblies which had t o be conveyed from t h e p o r t t h r o u g h t h e s t r e e t s o f New Plymouth and e x i s t i n g highways t o t h e s i t e . F i g . 8 shows a preassembly b e i n g t r a n s p o r t e d .
The maximum w e i g h t o f any
preassembly was l i m i t e d t o 600 tonnes w i t h a m x i r m m s i z e o f 15 meters w i d t h and 33 treters l e n g t h .
These dimensions were governed by t h e l i m i t a t i o n s i n
s i z e o f New Plymouth s t r e e t s and b r i d g e s .
A t o t a l of 76 preassernblies
w e i g h i n g 15,000 tonnes was shipped f r o m Japan t o New Zealand r e q u i r i n g t h i r t e e n sea voyages,
T h i s r e p r e s e n t e d a p p r o x i m a t e l y 25% o f t h e work c o n t e n t
o f t h e p l a n t as measured by manhours. Soil donditions The area o f T a r a n a k i s u r r o u n d i n g M t . Egmont i s o v e r l a i d by v o l c a n i c ash which does n o t p r o v i d e good b e a r i n g s t r e n g t h f o r equipment f o u n d a t i o n s .
As a
r e s u l t a l l major f o u n d t a t i o n s a r e supported by p i l e s , a t o t a l o f a p p r o x i m a t e l y 7000 b e i n g i n s t a l 1ed. Early i n the s o i l investigation o f the site, the potential o f l i q u e f a c t i o n o f t h e s o i l d u r i n g an earthquake because o f a h i g h w a t e r t a b l e was i d e n t i f i e d .
It was concluded t h a t t h e s i t e c o u l d be s t a b i l i z e d by a
program o f d e w a t e r i n g t o l o w e r t h e w a t e r l e v e l . w e l l s was i n s t a l l e d .
A s e r i e s o f eductor water
These a r e c u r r e n t l y o p e r a t i n g t o m a i n t a i n t h e w a t e r
t a b l e a t a s a f e l e v e l below grade. Environmental C o n s i d e r a t i o n s The l o c a t i o n o f t h e p l a n t i n a n o n - i n d u s t r i a l p r i l r a r i l y r u r a l a r e a necessitated c a r e f u l a t t e n t i o n t o e n v i r o n w n t a l issues.
Extreme c a r e was
t a k e n d u r i n g a l l stages o f d e s i g n and c o n s t r u c t i o n t o m i n i a i z e t h e impact o f t h e p l a n t on t h e s u r r o u n d i n g s .
The impact o f 1500 t o 2000 c o n s t r u c t i o n
w o r k e r s on t h e a r e a was considered; h o u s i n g and bus t r a n s p o r t a t i o n t o t h e s i t e f r o m New Plymouth were p r o v i d e d . s u r r o u n d i n g l a n d owners.
Good r e l a t i o n s were m a i n t a i n e d w i t h
The l o c a t i o n o f such a l a r g e f a c i l i t y i n a r u r a l
area caused some m i s g i v i n g s and apprehension. community was m a i n t a i n e d . operating noise l e v e l s .
Good i n f o r m a t i o n f l o w t o t h e
P a r t i c u l a r a t t e n t i o n was g i v e n t o r e d u c i n g t h e The l a y o u t o f t h e p l a n t i s such t h a t p o t e n t i a l l y
n o i s y equipment i s l o c a t e d as f a r away f r o m t h e p l a n t b o u n d a r i e s as possible.
Large r o t a t i n g equipment i s p l a c e d i n sound i n s u l a t e d e n c l o s u r e s .
The c o o l i n g t o w e r and a i r c o o l e r f a n s a r e l i m i t e d i n speed. f u r n a c e s have m u f f l e r s .
Burners on
The n o i s e l e v e l a t t h e f e n c e l i n e i s l i m i t e d t o 45
676
d e c i b e l s i n comparison t o t h e normal l i m i t o f 55 d e c i b e l s i n comparable plants.
The v i s u a l impact o f t h e p l a n t f r o m t h e s u r r o u n d i n g area i s l i m i t e d
by e x t e n s i v e landscaping. E f f l u e n t Treatment The p l a n t generates c o n s i d e r a b l e q u a n t i t i e s o f w a t e r c o n t a i n i n g u n d e s i r a b l e compounds which have t o be removed p r i o r t o discharge.
The
e f f l u e n t i s t r e a t e d i n a t e r t i a r y b i o l o g i c a l treatment p l a n t p r i o r t o d i s c h a r g e w i t h o t h e r e f f l u e n t f r o m t h e n e a r l y town o f W a i t a r a i n t o t h e sea. The d e s i g n o f t h e t r e a t m e n t p l a n t was g i v e n g r e a t c a r e and a t t e n t i o n because o f t h e concern r e g a r d i n g t h e r e l e a s e o f p o l l u t a n t s i n t o t h e sea so as n o t t o a f f e c t t h e h a r v e s t i n g o f f i l t e r feeders. Coordi n a t i o n The e x c e l l e n t c o o r d i n a t i o n between t h e v a r i o u s e n t i t i e s i n v o l v e d w i t h t h e p r o j e c t was t h e m a j o r c o n t r i b u t i o n t o t h e success and t i m e l y c o m p l e t i o n o f the project.
Considerable e f f o r t was expended t o ensure good communication
between c o n t r a c t o r s , t h e Japanese preassembly yard, equipment s u p p l i e r s and t h e s i t e i n New Zealand.
M o b i l and New Zealand S y n t h e t i c F u e l s C o r p o r a t i o n
m a i n t a i n e d teams o f p e r s o n n e l i n a l l c o n t r a c t o r s ' o f f i c e s t h r o u g h o u t t h e design e f f o r t .
Additionally, Bechtel maintained engineering representatives
i n Davy-McKee o f f i c e s i n Lakeland and i n t h e preassembly y a r d i n Japan.
The
preassembly y a r d management team was r e c r u i t e d m a i n l y f r o m p e r s o n n e l who had been on t h e p r o j e c t t h r o u g h o u t t h e d e s i g n e f f o r t . r e v i e w e d t o ensure c o m p a t i b i l i t y o f design.
Key d e s i g n documents were
Frequent meetings were h e l d t o
i d e n t i f y i n t e r f a c e problems and t o r e s o l v e d e s i g n issues.
677
TABLE 1
P r o j e c t ManHour Data 1) Total Manhou r s
Peak
( M i l l ions)
TYP e Design O f f i c e s
Manpower
F i e l d Manual
2 8
F i e l d Nonmanual
3
600
P/A Yard
2
1,700
Total
700
2,000
15
TABLE 2 P r o j e c t S i g n i f i c a n t Q u a n t i t i e s1 ) Major Equipment Col umns/Vessel s/Tanks
143 ea
Exchanger
133 ea
F i r e d Heaters
25 ea
Pumps
171 ea
Compressorss Total
16 ea 488
Bulks Aboveground Pipe
180,000 M
Underground Pipe
15,000 M
Steel ( T o t a l ) Wire and Cable
10,600 MT 738,000 M
Cable Tray
56,000 M
Concrete (Found. and S t r u c t . )
58,000 M3
Excavations
403,000 M3
678
Tables 1 and 2 i l l u s t r a t e t h e s i g n i f i c a n t q u a n t i t i e s in t h e p r o j e c t . Table 1 i l l u s t r a t e s t h e level of e f f o r t in manhours t o engineer, procure a n d construct the p l a n t . The design o f f i c e manhours include engineering, procurement and other home o f f i c e services f o r t h e major c o n t r a c t o r s : Bechtel, Davy-McKee and Foster Wheeler. By f a r t h e s a j o r e f f o r t was by Bechtel i n San Francisco where the s t a f f reached 400-500 personnel a t t h e peak. The wanhours shown do not include t h e e f f o r t expended by Mobil, New Zealand Synthetic Fuel Corporation, t h e New Zealand Government o r t h e many equiprent a n d waterial suppl i e r s . Table 2 shows t h e quantity of t h e major perrranent materials i n s t a l l e d in t h e plant including materials used in construction in New Zealand and in t h e preassembly yard in Japan.
OVERALL ASSESSMENT The pooject was a success as weasured by t h e c r i t e r i a of being under budget and on schedule. I t was unique in t h a t i t was a f i r s t of a kind p l a n t f o r t h e commercial conversion of natural gas t o gasoline. This enables New
Zealand t o be l e s s dependent on imported petroleum products. As w i t h any p r o j e c t , DrOblemS arose, were addressed i m e d i a t e l y and tackled vigorously; a t no time were they allowed t o a f f e c t t h e c o w l e t i o n o f t h e plant. The construction of t h e plant in New Zealand in a non-industrial area presented unusual d'hallenges which were met w i t h c r e a t i v e s o l u t i o n s . The cooperation o f so many diverse e n t i t i e s located t h r o u g h o u t t h e world i s a n exawple of t h e best of international r e l a t i o n s .
REFERENCE 1.
Bechtel, Inc., Gas t o Gasoline F a c i l i t i e s , Project Historical Report Job 141 97
D.M. Bibby, C:D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Concersion
679
1988Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE FIRST FIXED-BED METHANOL-TO-GASOLINE (MTG) PLANT: DESIGN AND SCALE-UP CONSIDERATIONS D. E. KROHN AND M. G. MELCONIAN MOBIL RESEARCH AND DEVELOPMENT CORPORATION ENGINEERING DEPARTMENT PRINCETON, NEW JERSEY 08540 U.S.A ABSTRACT The f i r s t commercial a p p l i c a t i o n o f t h e M o b i l Methanol-to-Gasoline (MTG) process i s now i n o p e r a t i o n f o r over a year i n t h e Gas-to-Gasoline (GTG) Comp l e x i n New Zealand. The unique c a t a l y s t and r e a c t i o n mechanism impose import a n t design c o n s t r a i n t s . The paper discusses t h e scale-up c o n s i d e r a t i o n s i n t h e design o f t h e fixed-bed r e a c t o r system. Design philosophy and s e l e c t i o n o f equipment t o meet t h e s t i p u l a t e d process and o p e r a t i n g o b j e c t i v e s a r e reviewed. Such unique designs f o r t h i s p l a n t ' s e f f l u e n t heat exchanger and t h e u t i l i z a t i o n o f computer dynamic s i m u l a t i o n for design c o n t r o l w i l l be highlighted. INTRODUCTION New Zealand's d e c i s i o n t o a l l o c a t e a p o r t i o n o f t h e i r n a t u r a l gas reserves
for conversion t o g a s o l i n e was based on a comprehensive e v a l u a t i o n o f a l t e r n a t i v e uses and a v a i l a b l e technologies.
As a r e s u l t o f these studies, i t was
concluded t h a t u s i n g t h e M o b i l process for t h e conversion o f n a t u r a l gas t o gasoline provided t h e most e f f i c i e n t method. To p r o t e c t t h i s decision, t h e New Zealand government s t i p u l a t e d unusual and s t r i n g e n t requirements on t h e performance o f t h e process.
I n response,
M o b i l Research and Development Corporation b u i l t and conducted p i l o t p l a n t s t u d i e s t o demonstrate t h e process and t h e g a s o l i n e q u a l i t y f o r commercial applications.
Thus, our challenge was t o design and b u i l d a l a r g e commercial
p l a n t , from t h e l a b o r a t o r y and p i l o t p l a n t scale, which would have t h e operat i n g and mechanical r e l i a b i l i t y t o achieve t h e c o n t r a c t e d g a s o l i n e p r o d u c t i o n a t t h e s t i p u l a t e d process conversion e f f i c i e n c y . The f i r s t commercial a p p l i c a t i o n o f t h e M o b i l Methanol-to-Gasoline
(MTG)
process has now been i n o p e r a t i o n f o r over a year and a h a l f i n t h e Gas-toGasoline (GTG) p l a n t i n New Zealand.
The c a t a l y s t used i n t h e process and t h e
r e a c t i o n mechanism impose important design c o n s t r a i n t s .
T h i s paper discusses
t h e approach used f o r t h e scale-up and design of t h e fixed-bed MTG r e a c t o r system.
Design philosophy and s e l e c t i o n o f equipment t o meet t h e s t i p u l a t e d
process and o p e r a t i n g o b j e c t i v e s a r e a l s o reviewed.
680
PROCESS DESCRIPTION I n t h e MTG process, methanol i s q u a n t i t a t i v e l y converted t o hydrocarbon
and water over a shape-selective z e o l i t e with t h e unique s t r u c t u r e o f M o b i l p r o p r i e t a r y ZSM-5 c a t a l y s t .
The conversion o f methanol proceeds a t r e l a t i v e l y
m i l d c o n d i t i o n s f o l l o w i n g a r e a c t i o n p a t h w e l l discussed i n t h e l i t e r a t u r e ( r e f . 1). The hydrocarbons formed a r e p r i m a r i l y i n t h e gasoline b o i l i n g range s u i t a b l e f o r use as h i g h q u a l i t y automotive f u e l . The MTG process i s one o f s e v e r a l processes based on ZSM-5 c a t a l y s t s which a r e c u r r e n t l y l i c e n s e d by M o b i l t o t h e petroleum and petrochemical i n d u s t r y . Over 25 commercial p l a n t s have been commissioned u s i n g these c a t a l y s t s .
A l l
o f these a p p l i c a t i o n s use fixed-bed r e a c t o r s and, very i m p o r t a n t l y , a l l were scaled-up from bench-scale p i l o t p l a n t data ( r e f . 2).
The major advantage o f
fixed-bed technology i s t h a t i t can be r e l a t i v e l y simple and r e q u i r e s minimum scale-up s t u d i e s .
T h i s successful scale-up experience with ZSM-5 c a t a l y s t was
an i m p o r t a n t c o n s i d e r a t i o n i n t h e MTG process development f o r t h e New Zealand plant. The b a s i c process f l o w o f t h e MTG r e a c t i o n s e c t i o n i s shown i n F i g . 1. Conversion o f methanol t o g a s o l i n e occurs i n two steps.
F i r s t , t h e methanol
i s vaporized and i s p a r t l y dehydrated t o an e q u i l i b r i u m m i x t u r e o f d i m e t h y l ether, methanol and water over an alumina c a t a l y s t i n a dehydration r e a c t i o n . About 15% o f t h e r e a c t i o n heat i s released i n t h i s f i r s t step.
The e q u i l i b -
rium m i x t u r e i s then combined w i t h r e c y c l e gas o f l i g h t hydrocarbon r e a c t i o n products and passed t o a conversion r e a c t o r where t h e second s e t o f r e a c t i o n s take p l a c e over ZSM-5 c a t a l y s t t o form gasoline.
The major and s u b s t a n t i a l
amount o f t h e r e a c t i o n heat (85%) i s released i n t h e Conversion Reactor where t h e heat i s removed by t h e r e c y c l e gas.
Reactor temperature i s c o n t r o l l e d t o
l i m i t t h e temperature r i s e i n t h e c a t a l y s t bed.
Hot r e a c t o r e f f l u e n t i s used
t o preheat t h e r e c y c l e gas and t o vaporize t h e methanol feed t o t h e DME reactor.
The g a s o l i n e i s separated from t h e r e c y c l e gas and water formed and sent
t o f r a c t i o n a t i o n , treatment and b l e n d i n g i n t o f i n i s h e d stock.
68 1
Dehydration Reactor
Conversion
Reactors ( 5 )
3 :
Separator
I Waste Water
flGURE 1
NEW ZEALAND MTG PLANT REACTION SECTION PROCESS FLOW DIAGRAM
DESIGN CONSIDERATIONS
The unique c a t a l y s t and reaction mechanism i n t h e proc ss imp0 important design considerations:
two
f i r s t , the aging characteristics o f the
ZSM-5 catalyst and second, management o f the highly exothermic heat o f the reaction. Under MTG reaction conditions, the c a t a l y s t undergoes two types o f aging which contribute t o gradual loss i n c a t a l y s t a c t i v i t y .
The f i r s t i s a rever-
s i b l e loss r e s u l t i n g from the coke formed on the catalyst as a reaction byproduct.
This type o f deactivation i s t y p i c a l i n c a t a l y t i c processes.
Coke i s
removed by "burning" with a i r during cabalyst regeneration.
The second type
o f deactivation i s due t o the steam formed i n the reaction.
However, w i t h
proper selection o f reaction temperature and water p a r t i a l pressure t h i s type
o f aging i s minimized.
682
The r e a c t i o n occurs over a r e l a t i v e l y small zone i n the c a t a l y s t bed.
As
the r e a c t i o n moves down the c a t a l y s t bed, coke deposits deactivate the f r o n t p a r t o f the bed.
The reaction continues down the bed u n t i l a s u b s t a n t i a l p a r t
o f the c a t a l y s t i s deactivated and unconverted methanol "breakthrough" i s detected i n the reactor e f f l u e n t stream.
Use o f s u f f i c i e n t c a t a l y s t permits
reactor onstream periods, o r cycles, s u f f i c i e n t l y long t o avoid excessive regenerations.
To enable t h i s t o be done onstream, m u l t i p l e reactors are pro-
vided and operated i n p a r a l l e l on a c y c l i c mode.
The New Zealand p l a n t i s
designed t o operate w i t h four reactors onstream, w i t h a f i f t h reactor i n regeneration. Reactor Design
As the major design o b j e c t i v e was t o ensure t h a t the New Zealand p l a n t duplicates the p i l o t p l a n t performance, the design o f these vapor-phase react o r s focused closely on achieving the same uniform mass v e l o c i t i e s across the c a t a l y s t bed as experienced i n the p i l o t p l a n t reactor.
This i s c r i t i c a l t o
avoid non-uniform r e a c t i o n w i t h r e s u l t i n g premature methanol breakthrough and shortened c y c l e length.
C l a s s i c a l chemical engineering p r i n c i p l e s coupled
with engineering judgement were used t o e s t a b l i s h reactor vessel diameter and c a t a l y s t bed depth f o r proper flow d i s t r i b u t i o n and reasonable pressure drop. As the operating conditions f o r the New Zealand reactor were designed t o be
the same as i n the p i l o t p l a n t , by making the depth o f the c a t a l y s t beds ident i c a l , the mass v e l o c i t i e s and thus a l l process variables become i d e n t i c a l . Hence, d i r e c t and simple scale-up i s achieved. Other fixed-bed reactor systems were considered but r e j e c t e d f o r reasons o f design complexity, s i z e and operating considerations.
The tubular, heat-
exchange reactor has several a t t r a c t i v e features f o r heat removal and good temperature c o n t r o l .
However, the l i m i t a t i o n i n s i z e o f a s i n g l e reactor, the
c a r e f u l mechanical design required t o withstand t h e process and regeneration cycles, and t h e operating problems w i t h a s u i t a b l e c o o l i n g medium, precluded the use o f the tabular, heat-exchange reactor.
S i m i l a r l y , the exacting
requirements f o r uniform r e a c t i o n and c a t a l y s t bed depth t o avoid premature methanol breakthrough eliminated r a d i a l reactors from consideration.
683
Considerable attention was given to the design o f the reactor internals including the inlet distributor, outlet collector and thermocouple locations as shown in Fig. 2. Laboratory simulation and testing of catalyst characteris tics helped to develop catalyst bed support criteria and loading procedures to ensure uniform packing and bed density for good flow distribution. Inlet
Catalyst Bed
Outlet
FIGURE 2
NEW ZEALAND MTG PLANT CONVERSION REACTOR
The decision for the number o f reactor exchanger.trains was based on equipment size and operating considerations. Thus, by specifying the maximum acceptable diameter f o r the reactor vessel, the number of reactors was established. A staggered reactor operating sequence was developed with allowance f o r reactor switching and regeneration period for the aged catalyst at "end of life" cycles as illustrated in Fig. 3. Regeneration is conducted in separate facilities designed t o carry out the catalyst regeneration steps in a reasonable period. Regeneration and switching of reactors are sequenced and rnonitored with the aid o f a programmable controller.
684
r
I
I
I
1
I I
.5
0 flGURE 3
I I
I
1
Cycle Length
1.5
2
NEW ZEALAND MTG PLANT REACTOR OPERATING SEQUENCE
Reaction Heat U t i l i z a t i o n The heat o f reaction o f methanol, i f uncontrolled, would r e s u l t i n an adiabatic temperature r i s e o f about 650OC.
The primary consideration i n
designing the reactor system i s , therefore, management o f the reaction heat. One common method t o do t h i s i n fixed-bed reactors i s t o d i l u t e the reactant stream by recycling reaction product gas which w i l l provide the mass t o absorb the heat o f reaction (Fig. 1). As i t i s w e l l known, the costs associated with recycle operations could be substantial i f the reaction i s strongly exothermic.
Special consideration was given t o establishing the recycle r a t i o i n
t h i s f i r s t - o f - k i n d MTG design f o r New Zealand.
Furthermore, considerable
engineering judgement and design e f f o r t were undertaken t o ensure t h a t the heat recovery i s e f f i c i e n t and yet operationally r e l i a b l e . Design studies had shown t h a t t o recover the high heat content i n the reactor e f f l u e n t stream e f f i c i e n t l y , i t i s desirable t o s p l i t the e f f l u e n t streams from each reactor i n t o two parts, with the major p a r t going t o preheat the recycle gas t o t h a t reactor.
A small f u e l - f i r e d heater i s provided i n
each reactor t r a i n t o t r i m the temperature o f the recycle gas t o t h a t reactor.
The excess hot e f f l u e n t streams from a l l the reactors are combined and
used t o preheat and vaporize the methanol feed t o the dehydration reactor. Although the process design i s nearly i n heat balance, a discrete quantity o f
685
excess hot e f f l u e n t i s used f o r generating steam, thus providing a Itflywheel" f o r process c o n t r o l f l e x i b i l i t y .
I t should be noted, however, t h a t t h i s large
p o r t i o n o f the heat i n the reactor e f f l u e n t i s recovered i n the cold recycle gas i n special l a r g e s h e l l and tube heat exchangers.
These l a r g e exchangers
received special a t t e n t i o n i n terms o f thermal efficiency, layout and construction. The exchangers are s i n g l e pass, l a r g e diameter u n i t s with long tubes as shown i n Fig. 4.
Special a t t e n t i o n was given t o the s h e l l and tube side
design with f a b r i c a t i o n d e t a i l s t o minimize maldistribution and bypassing.
In
order t o maximize heat recovery and a t t a i n the high thermal e f f i c i e n c y desired, these exchangers were designed f o r closer temperature approach (pinch) than the general accepted practice i n industry.
Actual performance proved t o
be b e t t e r than past experience with s i m i l a r designs i n comparable services.
I t i s noteworthy t o mention that the performance o f these exchangers i s very sensitive t o the flow r a t i o between tube and s h e l l sides which i n turn a f f e c t s the temperature difference a t the "pinch".
Total
5 Trains
Description:
flGURE 4
Duty: 206 MMBTU/Hr./Train TEMA Type: CET 2 Exchangers in Seriesnrain Diam: 94 Inches Total Length: 83 Ft. 360,000 L bs ./S he1I Weight: Tube Material: Type 304LSS
N.Z. MTG PLANT REACTOR EFFLUENT/ RECYCLE GAS EXCHANGERS
Special a t t e n t i o n was given t o the ease i n inspection and maintenance o f these large exchangers. following:
For example, some o f the features include the
686
a.
The combination o f double hubbed stationary tubesheet welded t o s h e l l and a welded f l o a t i n g head r e s u l t e d i n reduced s h e l l diameter, minimum number o f flanges and allowed the use o f an i n t e r n a l expansion j o i n t i n the pipe connection between f l o a t i n g head and shell cover.
b.
A double-flanged spool piece was used a t the f l o a t i n g end e f f l u e n t p i p i n g
t o access the f l o a t i n g tubesheet f o r inspection and maintenance without removing the s h e l l cover. c.
Rollers were i n s t a l l e d t o f a c i l i t a t e removal o f t h e very l a r g e 50 t o n bundles.
Gas C i r c u l a t i o n The l a r g e steam turbine-driven recycle compressor i s c r i t i c a l f o r the r e l i a b l e operation and c o n t r o l o f the plant.
I t was recognized t h a t a primary
concern must be t o make t h i s v i t a l element simple. g a l compressor
6f
A single barrel centrifu-
a s i z e and type t h a t i s f a m i l i a r t o us was selected so t h a t
a s i n g l e u n i t only needed t o be i n s t a l l e d .
All other elements o f the machine
have been designed t o be e i t h e r replaced o r serviced w i t h the u n i t on-line o r w i t h a minimum down time. very quickly.
Bearings and seals are items t h a t can be replaced
Other items such as o i l f i l t e r s can be changed independently.
The driveq was kept simple and takes the form o f a multistage condensing turbine i n which considerable e f f o r t was made t o provide for on-line maintenance.
The main t r i p and t h r o t t l e valves can be exercised without d i s t u r b i n g
the operation and the main condenser, which can be cleaned on the waterside without shut down o f the machine.
This i s very u s e f u l t o combat f o u l i n g from
marine growth i n the cooling system which o f necessity must be open t o the atmosphere. S i m i l a r d e t a i l e d design and scale-up considerations were extended t o the other sections o f the MTG p l a n t and r e s u l t e d i n meeting i t s s t a r t u p and operat i n g goals.
681 Control System V e r i f i c a t i o n Generally, during a p l a n t ' s design phase there are e x i s t i n g s i m i l a r process u n i t s whose c o n t r o l performance has been f i e l d tested.
However, t h i s
type o f data was not available, as we were designing the f i r s t commercial MTG p l a n t integrated w i t h two world-scale methanol plants.
An important design t o o l i s the use o f "Dynamic Simulation".
This compu-
t e r modeling technique was used t o v e r i f y t h a t the c o r i t r o l systems were ade-
quate f o r the purchased equipment (such as pumps, compressors, c o n t r o l valves , heat exchangers and pipe s i z i n g ) and were i n f a c t going t o meet our stated c o n t r o l objective. By way o f background, Fig. 5 i l l u s t r a t e s i n broad terms how t o use a dynamic simulation program.
Parameters such as size, e f f i c i e n c y , operational mode,
set p o i n t s and gain are established.
I n i t i a l conditions f o r pressure and tem-
perature are developed t o represent important steady s t a t e conditions.
Then
f o r c i n g functions such as feed-rate and operational changes o r equipment f a i l u r e are defined f o r each upset c o n d i t i o n o f i n t e r e s t .
A l l these data
become i n p u t t o a customized mathematical model u t i l i z i n g d i f f e r e n t i a l equat i o n s t o represent the dynamics o f the process.
The r e s u l t s produced are many
t i m e h i s t o r i e s o f the design system response t o each o f the upset conditions w i t h output obtained v i a e i t h e r p r i n t e d tables, graphs, o r CRT displays.
Parameters Size; Efficiency; Operation Mode; Set. Points; Gains ~
initial Conditions Pressures & Tern peratures
* *
r
I
1
Dynamic Simulation Model (Computer)
I
I
Forcing Functions Feed Rates Changes Operational Changes Equipment Failures flGURE 5
DYNAMIC SlMUlATlON
f
Time History of System . , .
- Printed Tables - Graphs - CRT Displays
688
There are important steps i n using dynamic simulation methodology r e l a t i v e t o any p r o j e c t as o u t l i n e d i n Fig. 6.
During the problem d e f i n i t i o n step a
scope i s developed by using a computer simulation o f the process and i n s t r u mentation diagrams (P&ID), and prepare a statement o f the study objectives, the l a t t e r being r e f i n e d by discussions between simulation s p e c i a l i s t s and engineering personnel from the p r o j e c t .
Then the mathematical model i s b u i l t
using small models, each representing one i t e m from the simulation P&ID.
A
v i t a l f u n c t i o n occurs during the r e p o r t s and discussions w i t h other engineers t o assess the p l a n t ' s o p e r a b i l i t y , how robust was t h e c o n t r o l scheme, and i t s safety during major p l a n t upsets.
A f t e r these discussions, the simulation
engineers perform more dynamic experiments i n an attempt t o resolve stated problems r e s u l t i n g from s h i f t s i n operational mode o r upset conditions.
What
f i n a l l y r e s u l t s i s a blend o f knowledge and experience between a l l engineering d i s c i p l i n e s and the simulation engineer t o uncover design e r r o r s t h a t are, a t t h i s stage i n the p r o j e c t , e a s i l y resolved a t minimum cost.
Our experience
now c l e a r l y shows t h a t f a s t answers are also a v a i l a b l e t o a host o f "what i f " questions t h a t arose during the design and construction o f t h i s p l a n t .
In
addition, t h i s program was able t o accurately answer newly-trained operators' questions on how t h e i r c o n t r o l systems performed under s p e c i f i c upsets.
Problem Definition Build and Test Computer Model Using Formula Equations and Math Model Report and Discuss with Process and Project Engineers Execute Pre-Planned Dynamic Experiments Resolve Identified Problems Additional Work to Resolve "What If" Questions Final Report and Archive Model
FIGURE 6
DYNAMIC SIMULATION METHODOLOGY
These models were used t o provide c o n t r o l loop evaluation o f the e n t i r e p l a n t ' s steam generation system, the steam t o carbon r a t i o c o n t r o l i n methanol plants, f u e l gas c o n t r o l system and i s o l a t e d equipment c o n t r o l review (refs.
3 , 4).
689
We have attempted t o h i g h l i g h t t h e c o n f i d e n t a t t i t u d e t h a t p r e v a i l e d and e x c e l l e n t r e s u l t s achieved i n u t i l i z i n g these v a r i o u s design techniques d u r i n g t h e GTG p r o j e c t .
Our l i m i t e d time has allowed us t o h i g h l i g h t o n l y some o f
t h e major design considerations, but h o p e f u l l y you now have a b e t t e r understanding o f some o f t h e v a r i a b l e s and unknowns t h a t have been overcome i n t h e p a s t s i x years so t h a t New Zealand's Gas t o Gasoline P l a n t i s a worthy accomplishment. REFERENCES
1. Chang, C.O., 2.
Penick, J. E.,
and A. J. S i l v e s t r i , J. C a t a l , 47 (1977) 249. W. Lee, and J. Maziuk, i n J. Wei and C. Georgakis
( E d i t o r s ) , Chemical Reaction Engineering
-
Plenary Lectures, ACS Symposium
S e r i e s 226, ACS, Washington, 1983, pp. 19-48.
3.
Womack, J. W.,
O i l & Gas Journal, A p r i l 7 , (1986), 66.
4.
Womack, J. W . ,
Modeling, I d e n t i f i c a t i o n and C o n t r o l Journal (Norway), No.
4 , V o l 6 , 1986, pg. 201-216.
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D.M.Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editon),Methane Concersion
691
0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
OPERRTlON O r T H E WORLD'S FIRST GRS - T O - G A S O L I N E PLRNT
K . G . Rllum and A . R . Williams Nciw Zealand Synthcttic Fuels New Zealand
Corporation,
New
Plymouth,
ABSTRRCT T h e w o r l d ' s f i r s t gas-to-gasoline plant, w h i c h e m p l o y s Dauy M c K e e / I C I t e c h n o l o g y t o c o n v e r t n a t u r a l g a s t o m e t h a n o l and n e w M o b i l t e c h n o l o g y t o c o n v e r t m e t h a n o l t o g a s o l i n e , w a s c o m m i s s i o n e d i n O c t o b e r 1 9 8 5 o n t i m e and w i t h i n b u d g e t . S i n c e start-up, t h e p l a n t has o p e r a t e d s a t i s f a c t o r i l y a n d a high on-stream f a c t o r has been achieued. Plant throughputs, product yields and p r o d u c t q u a l i t i e s haue all been as a n t i c i p a t e d i n t h e d e s i g n . B y R p r i l 1987. i n e x c e s s o f 2 m i l l i o n t o n n e s o f m e t h a n o l and 7 7 0 , 0 0 0 t o n n e s o f g a s o l i n e had been produced. This paper describes the plant and reuiews operation t o date. INTRODUCTION an
N e w Z e a l a n d has a economy which is
small widely dispersed population with very d e p e n d e n t o n a g r i c u l t u r e . R s a
c o n s e q u e n c e t h e t o t a l energy d e m a n d i s d o m i n a t e d by t h e requirement f o r transport fuels. Howeuer, although N e w Zealand has l a r g e r e s e r u e s o f n a t u r a l g a s , i t has r e l a t i v e l y m e a g r e l i q u i d energy r e s e r u e s f r o m w h i c h t o p r o d u c e t h e s e t r a n s p o r t fuels. T o rectify this imbalance and t o reduce our dependence o n i m p o r t e d o i l , t h e G o u e r n m e n t d e c i d e d i n 1 9 8 2 t o proceed w i t h t h e c o n s t r u c t i o n o f t h e w o r l d ' s f i r s t gas-to-gasoline c o m p l e x u s i n g Dauy M c K e e / I C I t e c h n o l o g y t o p r o d u c e m e t h a n o l and n e w M o b i l t e c h n o l o g y t o c o n u e r t t h e m e t h a n o l t o g a s o l i n e . P h y s i c a l c o n s t r u c t i o n o f t h e p l a n t c o m m e n c e d i n M a r c h 1 9 8 2 and w a s c o m p l e t e d o n t i m e ( m e c h a n i c a l c o m p l e t i o n i n J u l y 1985). F u r t h e r m o r e t h e p r o j e c t w a s c o m p l e t e d f o r USL1218M. s o m e 17% under
the
estimated
budget
of
US31475M
(in
dollars
of
the
day). T h e f i r s t g a s o l i n e w a s p r o d u c e d o n 17 O c t o b e r 1 9 8 5 and t h e name plate capacity was achieued o n 27th December 1985.
692
This
paper w i l l describe
date,
detailed
A
commissioning "Start-Up Allum
and
of
and
start-up
the World's lurnbull
M
0
the
process
description may
and
be
reuiew operation
the
of
found
i n
-
a
paper
entitled
F i r s t Gas--to.-Gasol.irie P l a n t " prcsented
i n
to
precommissioning,
the
ACS
by
K G
Symposium
i n
F l o r i d a June 1986. CONTKFICTUAL ASPECTS The c o m p l e x Fuels
Company's Zealand
(NZSFC)
shareholding
Government
been d e s i g n e d t/a
i s owned a n d r u n b y t h e New Z e a l a n d S y n t h e t i c
Corporation
of
and
feedstock
currently dioxide be
a
to
52.5
75/25
between
Corporation.
PJ/a
The
third
of
the
the
the
New
plant
has
o f n a t u r a l gas t o
one
blend
of
Maui
and
Kapuni
5%
and
44%
approximately The
dioxide
varied
gasoline
diuided O i l
operation,
570,000
New Z e a l a n d
RON o f 9 2 a n d a RUP o f 8 2 4 m b a r ( 1 2 p s i ) .
respectiuely. carbon
16-17%
can
i s
contain
tolling
being
(approximately
demand) w i t h a c l e a r The
a
Mobil
to' conuert
gasoline
as
and,
resulting
(design as
a
be p r o d u c e d by
blend
ualue
result
typically
the
contract
which carbon
contains The
13.4% C 0 2 ) .
=
NZSFC m u s t
gases
volume
mix
quantity
be r e c a l c u l a t e d .
of
NZSFC
i s p a i d a p r o c e s s i n g f e e b y t h e Crown t o c o v e r c o s t s and d e b t servicing
and
discounted
cash f l o w r a t e o f
risk
of
to
prouide
(adjusted
16%
processing fee t o
for
shareholders inflation).
be r e a l i s e d ,
with
a
tax
paid
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 However
uery s t r i n g e n t
for
the
full
performance and
t h r o u g h p u t c r i t e r i a must be a c h i e v e d . Under required
the to
processing
predict
f o r uarying feedstock dspen
Process
the
agreement operating
compositions.
Simulator which
has
with
the
efficiency
Crown, of
the
NZSFC
is
complex
This
i s achieued using t h e
been
jointly
NZSFC a n d Dauy McKee f o r t h i s p u r p o s e .
developed
by
693 PROCESS
Dt
SCRIPTlON
fit d e s i g n c o n d i t i o n s , a p p r o x i m a t e l y 155,000 Nm3/hr of n a t u r a l g a s i s c o n u e r t e d t o 66'72 1n3/d o f c r u d e m e t h a n o l ( e q u i u a l e n t t o 4400 t/d pure m e t h a n o l ) a s a 83/17% m e t h a n o l / water mixture in two of the w o r l d ' s largest methanol plants. 3 T h e c r u d e m e t h a n o l i s t h e n c o n u e r t e d t o 2 3 0 0 rn /d (1680 t/d) of g a s o l i n e u i a t h e Mobj.1 M T G p r o c e s s . T h e o n l y p r o d u c t f r o m t h e p l a n t i s a p r e m i u m m o t o r gasoline! b l e n d s t o c k w i t h a clear RON o f 9 2 . l h i s pipeline and is
p r o d u c t i s t r a n s f e r r e d t o P o r t T a r a n a k i by e i t h e r s h i p p e d t o t h e r e f i n e r y at M a r s d e n
Point f o r blending markets.
and
distribution or
exported
to overseas
i ) M e t h a n o l P l a n t s ( S e e F i g u r e 1). T h e m e t h a n o l f a c i l i t y consists of two i d e n t i c a l 2200 t/d units d e s i g n e d by D a u y - M c K e e . Each e x p o r t s c r u d e m e t h a n o l , HP s t e a m ( 1 0 5 barg and 4 8 2 O C ) and M P s t e a m (28 barg a n d 3 3 O o C ) to t h e r e s t o f the complex. M a u i and K a p u n i f e e d s t o c k s a r e b l e n d e d and s u p p l i e d by t h e C r o w n t o t h e c o m p l e x a t 4 5 b a r g . T h e g a s i s let-down t o 3 2 barg and s t r earn. a
split
into
two
feedstock
streams
and
a
fuel
gas
Indigeneous natural gas is virtually sulphur-free but, as precaution, the feedstock t o each reformer is pretreated
o u e r t w o z i n c o x i d e beds a t 3 5 O o C . T h e g a s i s t h e n s a t u r a t e d by counter-current c o n t a c t w i t h hot w a t e r and a d d i t i o n a l M P s t e a m a d d e d t o r a i s e t h e steam-to-carbon r a t i o t o 3 : l . T h e stearnlgas m i x t u r e i s s u p e r h e a t e d t o 4 8 2 O C a t 20 barg a g a i n s t r e f o r m e d g a s a n d passed t o t h e r e f o r m e r f u r n a c e . T h e f u r n a c e s a r e o f a c o n v e n t i o n a l Dauy M c K e e down-fired d e s i g n c o n t a i n i n g 680 t u b e s ( K H R 3 5 C a l l o y ) a r r a n g e d i n 10 r o w s and heated by 220 J Z i n k b u r n e r s w h i c h s u p p l y a t o t a l o f a p p r o x i m a t e l y 3 5 0 MH.
T h e reformed g a s p r o d u c e d i s f i r s t c o o l e d t o 5 5 O o C i n t w o r e f o r m e d g a s b o i l e r s ( g e n e r a t i n g H P steam), f u r t h e r c o o l e d i n t h e f e e d s t o c k s u p e r h e a t e r a n d BFW h e a t e r a n d f i n a l l y c o o l e d t o 3 5 O C a t 17 b a r g i n c o o l i n g w a t e r e x c h a n g e r s .
694
695
Reformed gas is compressed t o 100 barg i n a 27 MW split, three-stage compressor and injected i n t o t h e converter loop at t h e suction o f the 5 MW circulating compressor. Converter circulating gas plus make-up gas is heated and fed to a six-bed converter (ZnO/CuO/A1203 catalyst). Part o f the gas is fed t o the inlet o f the reactor at 210-240°C and the balance i s fed, a t a lower temperature, t o the lower beds as quench gas. Reactor effluent i s cooled both against reactor feed and saturator water feed before passing t o the methanol condensers. The crude methanol i s then rundown t o off-site tankage and the M T G unit. The reformer furnaces are designed t o supply the HP and MP steam requirements o f the Complex and thereby obviate the necessity f o r a separate field boiler. T h e f l u e gas ducts o f the reformer furnace are thus principally employed to generate HP steam. Flue gds at about 1000°C first generates HP steam in a radiant-shield boiler (this protects the downstream superheater from radiant heat impingement) and then passes to a two stage superheater and f l u e g a s boiler. Finally the f l u e gas heats combustion a i r i n a twin rotating a i r preheater before being discharged t o atmosphere at 108°C via a doublesuction ID f a n . Upstream o f the radiant shield boiler 3 0 L down-fired auxiliary burners are installed capable of generating u p t o 80 t/h o f additional HP steam. The complex steam demand and steam supply a r e balanced by adjusting auxiliary firing and the dampers in the bypass around the flue-gas boiler. T h e HP steam is principally used t o drive the t w o syngas compressors and the M T G circulating compressor. M P superheated steam i s obtained by extraction f r o m t h e syngas turbines and, i f necessary, by direct letdown f r o m HP steam. MP steam is used t o drive a number o f turbines (eg three 2.3 M W cooling water pumps) and a l s o i n reformer feed. LP steam discharged from t h e back-pressure turbines i s returned t o the methanol units for use in the BFW deaerator and as interstage motive steam f o r the converter circulator turbines.
696
I
8-
697
Approximately 60 t/h o f HP steam and 30 t/h of M P steam a r e exported from each methanol unit t o the rest o f the complex. ii) Methanol-to Gasoline (MTG) Unit (see Figure 2). The process, w h i c h is based o n t h e Mobil proprietary catalyst ZSM-5, employs conventional fixed-bed reactors and standard engineering technology. The overall reaction converts methanol to hydrocarbons in the gasoline boiling range. T h e hydrocarbon product, which is obtained in high yield, contains predominantly isoparaffins, aromatics and olefins and, a s a consequence, has a high octane number. The reaction is very exothermic (normal heat o f reaction = 1.74 MJ/kg) and heat must be removed t o control reaction conditions. This is facilitated by dividing the reaction i n t o steps. I n the first, crude methanol feed is vaporized and partially dehydrated over a special alumina catalyst t o a n equilibrium mixture o f methanol, dimethyl ether and water. This reaction takes place a t a reactor inlet temperature o f 310-320°C and 2 6 barg and releases 15-20% of the overall heat o f reaction. This step i s controlled by chemical equilibrium and, as such, is inherently stable. The DME reactor effluent is split i n t o f o u r parallel streams, mixed w i t h heated recycle gas and passed into four parallel conversion reactors containing ZSM-5 catalyst, where t h e conversion t o hydrocarbons and water is completed and the remaining heat o f reaction released. Recycle gas is used t o limit the temperature rise across the reactors. f-iue conversion reactors a r e installed o f which only four are o n stream. In service, coke builds u p in the catalyst pores causing a loss o f activity. T h e fifth reactor i s thus either i n regeneration mode o r o n standby.
698
The
conversion
individually effluent and by
at
to
reactor
the recycle
adjusting the
recycle
gas
typically
temperatures
adjusting
the
are flow
gasireactor e f f l u e n t
controlled of
reactor
h e a t exchangers
temperature d i f f e r e n c e across t h e f i r e d heaters.
trim
18-22
inlet by
350-366OC
barg.
Reactor
Excess reactor
inlet
pressures
effluent,
are
superfluous t o
i n the r e c y c l e gas/reactor
t h a t r e q u i r e d t o h e a t r e c y c l e gas
e f f l u e n t exchangers i s used t o p r e h e a t ,
v a p o r i z e and s u p e r h e a t
t h e m e t h a n o l f e e d t o t h e DME r e a c t o r . The h e a t f l e x i b i l i t y i n the
excess
reactor
effluent
system i s
r e t a i n e d by
utilising
some o f t h e r e a c t o r e f f l u e n t t o g e n e r a t e MP steam i n a b o i l e r . Steam
generation
i s
adjusted
to
balance
process
heat
requirements. Excess together
reactor
with
effluent
reactor
exchangers i s t h e n f u r t h e r bank
of
where
water
gas.
phase,
coolers
liquid
i s
the
passed
the
trace quantities to
recycle
gas
of
a
effluent
large
compressor
preheat
recycle
the
h y d r o c a r b o n and w a t e r
passed
suction
the
to
by
a
heat
15 b a r g i n a
separate.
separator The w a t e r
o f oxygenated The
organic
gas
phase
C O and C02) i s r e t u r n e d
split-suction,
driven
system
gas
product
treatment.
( m o s t l y l i g h t h y d r o c a r b o n s , hydrogen, to
feed
from
c o o l e d t o 25-35OC a t
and
which co n ta i n s
compounds
from
effluent
single
35 MW
discharge
condensing
steam
turbine. The
liquid
hydrocarbon
mainly gasoline b o i l i n g hydrogen,
carbon
meeting
specifications.
and
light
Gases
gasoline)
contains
as
dissolved
hydrocarbons
t h e non-hydrocarbons,
o f t h e C 4 hydrocarbons gasoline
(raw
r a n g e m a t e r i a l as w e l l
dioxide
Essentially a l l o f
product
(Cl-C4).
C1, C2, C3 and p a r t
a r e removed by d i s t i l l a t i o n t o produce
the
required
volatility
i n c l u d i n g methane,
a r e removed i n a d e - e t h a n i s e r .
ethane
The o f f - g a s e s
and and
RUP
some Cj
tggether w i t h a
p u r g e gas s t r e a m f r o m t h e p r o d u c t s e p a r a t o r a r e scrubbed i n a sponge
absorber
(to
p a s s i n g t o f u e l gas.
retain
any
gasoline
components)
before
The l i q u i d p r o d u c t f r o m t h e d e - e t h a n i s e r
i s t h e n passed t o a s t a b i l i z e r where C3 and p a r t o f t h e C 4
699
components are removed overhead (to f u e l gas). A high uclpour pressure ( H U P ) gasoline blendstock containing C4/C5 components is withdrawn as a sidestream. Stabilized gasoline is then passed to a gasoline splitter w h e r e it is separated i n t o light and heavy gasoline fractions. Each stream is cooled and sent t o storage. M T G gasoline contains a relatively high proportion o f 1,2,4,5--tetramethyl benzene (durene). It i s desirable to reduce the durene content t o less than 2% t o prevent driveability problems (similar to carburettor icing) associated with the high melting point (79OC) o f durene. The durene i s thus concentrated into t h e heavy gasoline fraction i n the splitter and then subjected t o a mild hydrofinishing process over a proprietary Mobil catalyst i n the HGT reactor. Here durene undergoes isomerisation, disproportionation and demethylation i n the presence o f hydrogen a t 220-270°C and 30-40 barg. The product is obtained in nearly quantitive yield w i t h virtually unaltered RON but w i t h a greatly reduced durene content. iii) Off-sites and Utilities. T h e complex has t h e usual range o f off-site and utilities encountered i n modern refineries and chemical works including water clarification and demineralisation. instrument and plant a i r supply, liquid nitrogen generation and storage, a n off-site MP steam start-up boiler, HP and LP flares, a firewater system, w a s t e w a t e r treatment and facilities f o r blending and storing products. The Crown i s responsible f o r the infra-structure and supplies fresh w a t e r and natural gas t o t h e complex and receives product and treated w a s t e water f r o m the complex. Electricity i s supplied by t h e Taranaki Electric Power Board. Demineralised water i s used primarily t o generate HP and MP steam. Recovered condensate f r o m both the condensing turbines and t h e reformer process i s recycled t o minimise water consumption. lpproximately 140 t/d of water i s generated when t h e crude methanol i s converted t o gasoline i n t h e MTG unit. This w a t e r is contaminated w i t h trace quantities o f
oxygenates (primdrily methanol, acetone, methyl ethyl ketone and acetic acid). This water, together with other minor, potentially contaminated streams are purified in a conventional tertiary treatment system comprising trickle filters, aeration basins and a clarifier. The biological solids generated are handled in a digester and rotary vacuum filter. In order to minimise the environmental impact of the plant, clarified w a s t e water is used as make-up for the cooling water system. The overall thermal efficiency o f the plant is of the order o f 53% and l o w grade w a s t e heat from, for example, the M T G f i n a l product coolers, the reformed gas coolers and the methanol condensers, is discharged via a large circulating cooling water system. fit design throughputs approximately 7 2 5 MW o f heat are discharged via 18 induced-draft cooling cells. 3 Cooling water circulates at 34,000 m /h and has a n inventory 3 of 13,000m . The presence o f large stainless steel heat exchangers necessitates t h e very stringent quality control of the circulating water, particularly as regards chlorides and organic solids, and this also impinges o n the operation o f the effluent treatment plant. Blowdown f r o m the cooling tower is discharged t o the infrastructure. OPERFl TION
T h e f i r s t natural gas w a s brought o n site i n May 1985 after extensive precommissioning effort by Synfuels, Mobil and Contract staff. A phased hand o v e r o f plant began i n the Utilities area i n November 1984. By August 1985 catalyst loading and refractory dry-out procedures. w e r e underway. T h e first methanol unit w a s brought o n stream o n 12th October 1985 and achieved design rate within t w o days o f initial production. T h e first gasoline w a s produced o n 17th October. T h e second methanol unit w a s commissioned o n 12th December. Subsequently two additional M T C reactors w e r e streamed and the complex w a s up to 100% of design capacity by December 27th 1985. O t h e r milestones w e r e as follows:-
701
500.000 Tonnes Methanol 1,000,000Tonnes Methanol
111 3/86 17/ 7/86
500,000 Torincs Gasolinr.
1 1 / 10/86
i) Methanol Plant Operation. The operation and the performance of the Methanol units is well understood and has been precisely predicted from previous commercial experience. Since start- up on 12th October 1985 approximately two million tonnes of methanol had been produced by early March 1987. Figures 3 and 4 show methanol production and stream factor. Methanol production can be seen as steady and relatively trouble free by examining the respective plant production and stream times for the 18 months October 1985 to March 1987. To achieve contract requirements, a 94% streamtime is required at design conditions. Average streamtime for Methanol 01 in 1986 has been 97.9% with average production of 2180 tonnes/day. Average stream time f o r Methanol 02 in 1986 has been 96.6 "x with average production of 2191 tonneslday. Figures 3 and 4 indicate steady operation. As in all operating plants, the representation on a monthly basis has a good averaging effect and Operations and Maintenance are pursuing problems o n a daily basis. Howeuer, problems in the Methanol plants however cannot be considered as isolated to a Methanol Plant owing to the complexity of the thermal integration. Steam raising is by excess heat removal from the reforming train, thus any upset t o a Methanol plant can effect a domino reaction through the Complex. With Methanol plants now i n operation f o r some 18 months, a n unanticipated deposition of carbon o n the reformer catalyst has progressively developed. Fls a consequence, it has proved necessary to shutdown the plants t o steam the catalyst and this requirement i s causing concern. The natural gas feed stock i s quality controlled to d e w point and calorific value specifications. Evidence indicates that small concentrations
MONTHLY
100
Voduction ('000's t e ) Stream Factor ( W )
80 60 40 20
Production
+ Stream Factor
100
MONTHLY Production ('000's t e ) Stream Factor ( 9 6 )
80
Synfuel
Production
+ Stream Factor
FIGURE 4
I 0 w
704
o f heavy hydrocarbons (C5+) are present i n the natural gas. Their presence i n the reformer causes t h e formation o f carbon resulting in lower activity, hence the need for higher temperatures t o meet reaction requirements. Steaming o f the reformer i n part reverses this process and allows processing t o continue without undue detriment t o reformer tube life. Later this year both Methanol plants are t o have major turnarounds. During this time the reformer catalyst i s t o be replaced w i t h a split load o f 50% alkali-promoted reforming catalyst (on top) and 50% normal reforming catalyst. The new catalyst i s more resistant t o the heavy hydrocarbon i n the feed gas. Another project currently underway, the installation o f hydrogen recycle facilities, w i l l also reduce this problem by maintaining the top part o f the catalyst in a fully reduced state. ii) Methanol Gasoline Plant. T h e MTG plant is a n excellent example o f the ability o f engineers t o successfully scale u p a Plant from a small pilot plant ( 5 0 0 kg/d to 1700 tld). Production, yields, product qualities and catalyst performance have been a s anticipated from pilot plant data. By bpril 1987 one train has begun cycle 9 and no major deviations from anticipated performance have been experienced. Regenerations are straightforward and product qualities and properties are such. that synthetic petrol i s produced at design rates without variation. T o d a t e approx 7 7 0 , 0 0 0 tonnes o f gasoline have been produced. Typical gasoline quality is shown i n Table 1 . TABLE 1 GASOLINE QUALITY fiverage Density (kg/m3 at 15OC) RUP (psia) RON MON Durene Content (% wt) Induction Period (minutes)
730 12.5 92.2 82.6 2.0 325
Range
728 12.192.082.21.74260 -
733 13.2 92.5 83.0 2.29 370
705
&tillation % Evap a t 7OoC % Evap a t 100°C % Evap a t i8oOc
End P o i n t
31.5 53.2 94.9 204.5
OC
(fiverage r e s u l t s d u r i n g J a n u a r y of g a s o l i n e )
-
29.551.594.0196 -
34.5 55.5 96.5 209
F e b r u a r y 1987 f o r 9 7 , 0 0 0 t o n n e s
The p r o d u c t i o n p r o f i l e ( f i g 5 ) mirrors t h e m e t h a n o l p l a n t production curves. V a r i a t i o n s have been u s u a l l y t h e r e s u l t o f t h e loss of m e t h a n o l p r o d u c t i o n . The MTG p l a n t h a s t h e c a p a c i t y t o make u p f o r l o s t p r o d u c t i o n so i n c i d e n t s i n t h i s p l a n t a r e masked u n l e s s t h e y a r e major. R e s e a r c h a n d p i l o t s c a l e work p r e d i c t e d t h a t , as t h e c a t a l y s t a g e d , t h e y i e l d w o u l d r i s e . T h i s h a s b e e n m i r r o r e d by a c t u a l p l a n t y i e l d s ( F i g 5 ) . O v e r a l l y i e l d s f o r t h e Complex
a r e shown i n F i g u r e 6 . C u r r e n t o p e r a t i n g c r i t e r i a i n v o l v e f e e d f l o w programming from t h e b e g i n n i n g o f c y c l e u n t i l t h e e n d of c y c l e ( i n d i c a t e d by m e t h a n o l b r e a k t h r o u g h ) . Ea rl y i n t h e l i f e o f t h e p l a n t i t was f o u n d t h a t t h e c y c l e l e n g t h c o u l d b e e x t e n d e d by a technique termed catalyst rejuvenation. Operation today i n v o l v e s r e j u v e n a t i o n a f t e r a n a v e r a g e o f 30 d a y s o n s t r e a m . A c t i v i t y of t h e c a t a l y s t i s p a r t i a l l y r e s t o r e d by t h i s t e c h n i q u e a n d t h e t r a i n may b e r e s t r e a m e d a t h i g h e r r a t e s . The c y c l e i s e x t e n d e d by a n a v e r a g e o f 1 8 d a y s . MTC c a t a l y s t l i f e i s r e p r e s e n t e d by t h e number of d a y s o n stream f o r p r o g r e s s i v e c y c l e s ( F i g 7 ) . T h i s g r a p h of t h e a c t u a l c y c l e times a c h i e v e d , f o l l o w s a s i m i l a r p a t t e r n t o t h e a g i n g r a t e s p r e d i c t e d by p i l o t p l a n t t r i a l s . O p e r a t i o n of t h e MTC P l a n t h a s b e e n i n a c c o r d a n c e w i t h v e r y c l e a r l y d e f i n e d p r o c e s s c o n s t r a i n t s . T h e s e w e r e s e t by M o b i l t o a c t as o p e r a t i n g p a r a m e t e r s so t h a t p e r f o r m a n c e g u a r a n t e e s c o u l d b e met. The g u a r a n t e e s a r e b e i n g met a n d t h e r e a c t o r s a r e n e a r i n g e n d of l i f e . W i t h y i e l d s h i g h a t e n d of l i f e , t h e way i s o p e n f o r c a r e f u l o p t i m i s a t i o n t o e x t e n d l i f e as l o n g as p o s s i b l e .
706
L 0
V
4 [rJ
LL
E
m Q) L 4 v,
*
C
0
s V =I
L
0
U
a.
t
LA
w @ i 3 LL.
s
I,
I
I
I
0
v 50
0 0
Days
a
Stream
0
On
0
0 D
E0l
0
I
v
25
0
I
2
I
4
0
TRAIN 1
V
TRAIN 2
A
TRAIN 3
0
TRAIN 4
CI
TRAIN 5
I
6
-
709
The redctor effluent comprises gasoline, water and g a s . The water is the primdry feed to the cffluenl treatment plant. Detailed studies hdue shown that i t will be feasible t o return the waste water to the reformer saturators thereby re-using the waste water and recovering the oxygenates. Is a consequence the load o n the demineralisation unit and the effluent treatment system will be reduced and the overall efficiency o f the plant marginally improved. Detailed design of this enhancement has now been completed and installation is i n progress. Heat f o r vapourising and superheating the methanol feed for the DME reactor is generated f r o m M T G reactor effluent. Experience obtained during start-up, upset conditions and during restarts has indicated that this operation is very tight. I fired heater i s being installed between the superheater and the DME reactor. This will greatly increase the heat flexibility o f the feed system, reduce the loading o n t h e M T C feed trim heaters and result i n a n overall energy saving. Distillation offgas feeds the complex f u e l gas system. Instabilities i n operation o f this section have caused considerable problems for the Methanol reformers, the primary users o f fuelgas. O n one occasion the reformers w e r e fed with liquid and not gas. To protect the Methanol plants a n offgas KO drum and associated control system has been installed. iii) Utilities and Off-Sites. Water i s supplied t o the plant f r o m t h e Waitara River. T h e w i d e fluctuations i n water quality (eg turbidity) and the unusual properties o f the water (eg very l o w alkalinity) necessitate very close control o f the operating parameters i n order t o achieve satisfactory water quality. Operation o f the demineralisation plant has required close attention since start-up. Mechanical failure of resin vessel internals has occurred and o n occasions limited t h e availability o f demineralised water supply.
710
Operation o f the cooling water system has been the focus of considerable attention. T o minimise the quantity o f was'te water discharged from the Complex, t h e water f r o m the effluent treatment plant i s used as make-up t o the cooling towers. This has resulted in a unique set o f operating conditions and has presented a challenge t o both NZSFC staff and chemical suppliers to provide a chemical programme t o control corrosion and deposition. For environmental reasons, a phosphate programme, containing no heavy metals, has been employed. T h e presence of a number o f large stainless steel heat exchangers has placed a n additional constraint o n the circulatory water quality control i n that chloride levels must be maintained at 100 ppm or less. Chlorides originate from the r a w water, the primary coagulant (poly aluminium chloride), and chlorine gas injection (for biological control). Measures t o reduce the chlorides haue included the use o f chlorine dioxide rather than chlorine for biological control and partial replacement o f polyaluminium chloride with an organic polymer. T h e effluent plant has the capacity t o treat a C O D loading o f 25,000 kglday. equivalent t o that generated by a population o f 200,000 people. T h e primary waste i s water discharged from the M T G Plant, w i t h contaminated stormwater being also processed. The waste characteristics allow f o r easy biological treatment but as i s the nature o f this type o f treatment, constant treated waste water quality requires constant feed quality. Plant modifications involving tankage and pondage have been made to ensure that feed variations are kept t o a mimimum. CONCLUSION The world's first gas to gasoline plant w a s completed o n time and under budget. The first year's operation is a testament to the hard work and dedication o f a large number of people. The success has been illustrated by the production achieved. the stream times and efficiencies. Emphasis i n our second year o f production centres around t h e first Turnaround and maximising on-stream throughput and yield.
711 R b b...r.... e u .i. a 1.. s ............ ..ri. ..o ..n ..........
p e t: a j o IJ Ic!
l o n n e s pc!r annurri r e s e a r c h o c t a n e number
Reid u a p o u r prc!ssurc! normal. c u b i c r r i e t e r s p e r h o u r
h g h pressure
rried i u l ~ ip r e s s u r e
l o w prc!ssure b a r gauge b o : i l e r fc!ed w a t e r .induced d r a f t h e a u y g a s o l i ne .t:r e a t e r c al.CJ r -i.f i c u a l u e
knock o u t c h c! nii. c a I CIx y g e ri cl e rria n d
rriotor o c t a n e nurnber
This Page Intentionally Left Blank
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
TN VALUE OF
COlguTER
713
SIMLATION
TO THE PROCESS INxlSTRIES
H J Weake G A Robertson
New Zealand Synthetic Fuels Corporation Limited New Zealand Synthetic Fuels Cornoration Limited
Abstract: This paper h i g h l i g h t s the value of computer simulations during design, commissioning and o p e r a t i q phases of the NZ Synfuel Gas-to-Gasoline Plant.
1.0 INlRODUCTION
New Zealand S y n t k t i c Fuels Corporation Ltd (Synfuel) operates a 1650 tonne p e r day gas-to-gasoline f a c i l i t y a t Motunui, Taranaki. The f a c i l i t y c o n s i s t s of two i d e n t i c a l 2200 tonnes p e r day I C I licensed methanol p l a n t s cocpled t o a s i n g l e stream Mobil designed methanol-to-gasoline p l a n t . Synfuel decided well before construction began i n 1982 t o develop a s t r o n g t e c h n i c a l base i n New Zealand. This involved: (i) The cost of developing an advanced ASPEN steady s t a t e simulation o f tt-e Methanol p l a n t s i n conjunction with Davy McKee (ii) T h e p a r t i c i p a t i o n i n the development of the dynamic steam simulation using ACSL with Bechtel & Mobil (iii) The p a r t i c i p a t i o n i n the development of the dynamic MTG simulation using ACSL with Mobil (iv) The development i n the various MTG steady s t a t e simulations using the Mobil Proprietary Programs i n conjunction with Mobil ( QUIKBAL )
T h i s paper examines the important role simulations have played i n t h e s t a r t u p and operation of t h e GTG complex and the follow-on e f f e c t s which have been a c h i e e d . Developed below i s a chronological progression of t h e major uses the simulations have seen over the p a s t 5 years.
714
Figure 1
NEW ZEALAND MTG UNIT REACTION SECTION PROCESS FLOW DIALRAM
NEW ZEALAND RECYCLE GAS HEATER
MTG UNIT
Figure 2
EXCHANGER M A X I M U M STEAM GENERATION I h E - 3 3 1 3
NO STEAM G E NERAT I0 N I N E-0313
0
,
50
60
70
80
90
100
HEAT EXCHANGE PERFORMANCE, % DESIGN DUTY
SlhB!LlSiR D P E M l lNi GU!OEiINfS
110
Figure 3
715
2.0 SIMULATIONS AS PREDICTIVE TOM FOR PLANT SENSITIVITY/ORRABILITY
The steady s t a t e simulations (QUIKBAL and ASPEN) comprise a number o f f l a s h e s ; exchangers, splits, compressors, towers and t a i l o r 4 a d e f o r t r a n subroutines which a r e s t r u c t u r e d i n s e q u e n t i a l modular form. The output includes m a t e r i a l and heat balances, exchanger heat curves
with various physical p r o p e r t i e s , compressor performance c a l c u l a t i o n s and any a d d i t i o n a l output desired through the use o f f o r t r a n subroutines. These models were used t o p r e d i c t the plant s e n s i t i v i t y . MTG The work i n the MTG u n i t covered two main a r e a s , namely the Reaction and
D i s t i l l a t i o n Sections. The r e a c t i o n s e c t i o n of the MTG u n i t comprises 5 p a r a l l e l t r a i n s , each converting methanol to g a s o l i n e across the Mobil ZSM-5 c a t a l y s t i n the presence of 9 p a r t s molar of i n e r t recycle gas. Excess h e a t generated from the above r e a c t i o n is u t i l i s e d by heating the methanol feed to t h e unit. The h e a t balance s e n s i t i v i t y o f the r e a c t i o n s e c t i o n of the MTG u n i t was e s t a b l i s h e d by determining the e f f e c t o f varying recycle g a d e f f l u e n t exchanger performance a t maximum and minimum s u r p l u s heat for c o n t r o l requirements (see f i g u r e 2 ) .
The d i s t i l l a t i o n s e c t i o n of the MTG u n i t c o n s i s t s of a deethaniser,
s t a b i l i s e r , gasoline splitter and lean oillsponge absorber c i r c u i t f o r recovering gasoline from the deethaniser o f f q a s and product s e p a r a t o r off-gas. The gasoline s p l i t t e r s e p a r a t e s s t a b i l i s e r bottoms i n t o l i g h t g a s o l i n e and heavy gasoline containing about 5OWT% durene. This is used as the feedstock f o r the Heavy g a s o l i n e t r e a t i n g (I-GT) u n i t .
716
For each of the t h r e e main towers, namely the deethaniser, s t a b i l i s e r
and gasoline s p l i t t e r , d e t a i l e d operating guidelines were developed t o optimise gasoline recovery before the towers were comnissioned (see f i g u r e 3 f o r operating guidelines f o r the s t a b i l i s e r ) . Several key areas of concern such a s gasoline splitter temperature control s e n s i t i v i t y were highlighted p r i o r t o startup. Before MTG s t a r t u p the d i s t i l l a t i o n plant was comnissioned using Maui condensate. Steady s t a t e simulation models were r e a d i l y adapted t o r e f l e c t the d i f f e r e n t feedstock w i t h guidelines developed f o r d i s t i l l a t i o n plant operation. Methanol P l a n t s With t h e methanol plants, the ASPEN steady s t a t e simulation was used t o i n v e s t i g a t e the performance of the u n i t s w i t h changing feedgas composition (H/C r a t i o and gas c a l o r i f i c valve) p r i o r t o startup. Since Synfuels processing fee f o r turning natural g a s i n t o gasoline is set by the y i e l d obtained, t h e ASP04 simulator provides an important b a s i s f o r f e e determination under gas Lpset conditions. 3.0 CONTROL SYSTEM DESIGN
A t an e a r l y s t a g e i n the p r o j e c t it was recognised t h a t the complex contained many corrplicated and i n t e r a c t i n g c o n t r o l systems (Pre 1981). It was then decided t o dynamically model some of t h e more critical control systems t o ensure s a f e s t a r t and operability. The t h r e e major systems looked a t by Bechtel/MRDC/Synfuel wen?: (Ref 1) (1) Corrplex steam system (2) MTG r e a c t o r control system (3) Cooling water system The purpose of modelling these systems and i n p a r t i c u l a r (1) and (2) was t o e s t a b l i s h the control philosophy, r a t h e r than an i n d e p t h evaluation of c o n t r o l system tuning. One of the l a r g e s t simulations developed was the steam system.
717
Because the steam system encompassed t h e e n t i r e complex, it was necessary t o develop a control system which s a t i s f i e d both pressure and flow requirements a t every point i n the complex under steady s t a t e and t r a n s i e n t conditions. This was p a r t i c u l a r l y important f o r the methanol p l a n t s w h e r e strict control is required t o avoia damage t o the steam reformers. To this end a dynamic simulation program was developed t o model the hi@ (103.4 Barg), rnediun (27.6 Barg) and low (3.5 Barg) pressure headers and t h e associated steam generators and users (figure 4).
Considerable work was done p r i o r t o commissioning on the MTG reactor c o n t r o l system and the cooling water system. Simulation code was written f o r the MTG control t o model the h e a t h a s s t r a n s f e r e f f e c t s during reactor switching and various t r i p scenarios. As a r e s u l t no s e r i o u s c o n t r o l p b l e r n s have been experienced i n the MTG plant. T h e simulations presented here, while not leading t o an in-depth analysis of the control systems, allowed deficiencies t o be highlighted and a r e a s for improvement t o be investigated. 0 STARTUP PROCEDURE DEVELOPMENT
Dewlopment of t h e MTG s t a r t u p procedure was c a r r i e d out w i t h extensive use of steady s t a t e simulation models. The MTG u n i t reaction s e c t i o n was simulated from the point where the
recycle gas compressor was c i r c u l a t i n g a mixture o f cold nitrogen and n a t u r a l gas. Using the anticipated method of s t a r t u p the sequence o f events was broken down i n t o di s t i nct quasi steady s t a t e s t e p s a s follows: 1. Cold recycle gas c i r c u l a t i o n 2. hit heat-Llp t o normal operating conditions 3. Preparation o f the u n i t p r i o r t o feed introduction
Introduction of methanol t o MTG u n i t I n i t i a l methanol vaporisation Elimination of s t a r t q gas t o the DME reactor Feed i n c r e a s e t o design r a t e
4.
5. 6.
7.
A t each of these conditions complete heat and MSS balances and equipment performance evaluations could be carputed. With t h e use of engineering judgement and p r a c t i c a l considerations a p r e c i s e procedure
could be formulated. The use of computer simulation h i m l i g h t e d some p o t e n t i a l problems with tk proposed method of s t a r t u p , namely: (i)
(ii)
Two phase l i q u i d slugging would occur between the methanol preheater and methanol f l a s h drum f o r which the piping was not designed. There may have been uncontrolled methanol flow t o tk DME r e a c t o r u n t i l the l i q u i d l e v e l i n the methanol f l a s h drum could be e s t a b l i s h e d
Plant modifications were t h e r e f o r e implemented p r i o r t o startw. Based on these modifications and information gained from the models sirrulating t h e o r i g i n a l method of s t a r t u p the new a n t i c i p a t e d method o f s t a r t y , was devised and m u l t i p l e q u a s i steady s t a t e Conditions developed a s follows: 1. Cold recycle g a s c i r c u l a t i o n 2. Methanol f i l l i n g of f l a s h drum and preheater 3. Conrnissioning methanol recovery system t o normal operating conditions 4. Unit heat5. F i r s t r e a c t o r preparation f o r methanol feed 6 . Introduction o f methanol t o DME r e a c t o r 8.
Heat c o n t r o l t o vaporisers, preheaters Line-out first r e a c t o r t o design conditions
9.
Introduce feed t o other r e a c t o r s a s necessary
7.
719
5.0 MRGENCY/lRIP ROCEDURES
A f t e r the development o f the steam system dynamic simulation, done
to
investigate
equipnent/plant t r i p s .
the
response
of
steam
system
work was
following
Since the e n t i r e complex steam system a t the H p
l e v e l was simply a number o f l a r g e r steam generators supplying only 3 steam turbines, the t r i p o f one machine steam pressure transients.
or a plant would lead t o severe
The simulation was used t o develop a series
o f operator procedures following any t r i p scenario. The
simulation
development
of
work an
which operator
was
done
training
was
also
the
simulator.
basis
This
for
the
allowed
the
operators t o gain experience before working with the r e a l plant, t o i t s dynamic responses and also t o f a r n i l i a r i s e them with the d i s t r i b u t e d c o n t r o l system (DCS).
6.0 COMPARISON BETWEEN ACTUAL STARTUP AND SIMJLATION
Startup o f the MTG unit i n October 1985 e s s e n t i a l l y followed the procedures developed f r o m the simulation with the exception t h a t not a l l t r a i n s were used t o contribute excess e f f l u e n t t o vaporise the i n i t i a l methanol feed. The f i r s t methanol was vaporised and reacted t o produce hydrocarbons i n the f i r s t conversion reactor as anticipated.
However, the high pmpane
production due t o h i g h i n i t i a l c a t a l y s t a c t i v i t y was not simulated.
The
a c t u a l unit heat balance was therefore not as predicted with the r e s u l t that
the
expected.
process
fired
heater duty
was
significantly
higher
than
The s t a r t y , heater which was i n service t o provide additional
methanol superheat upstream o f the DME reactor during startup was not taken o f f - l i n e towards the end o f startup as predicted.
I
1
Figure 4
4 N 0
721
2707
Q.rZ!% ............................
$ 600L
.-+
500-
I
2 400-
1
NATURAL
0
10
@Synfuel
20
30
40 50 60 70 Controller Output
80
90
100
U
Figure 5
Extensive test runs were implemented on the MTG u n i t during steady s t a t e operation shsequent t o starty, t o resolve 'apparent' data discrepancies. These test runs revealed t h a t the estimated enthalpy required t o superheat methanol frun t h e dew point t o 315OC d i f f e r e d from a c t u a l by approximately 30%. However, the t o t a l estimated enthalpy required t o preheat, vaporise and sLperheat methanol was confirmed by a c t u a l plant data. The high i n i t i a l propane make and the highly non-ideal behaviour of methanol above t h e c r i t i c a l temperature accounted f o r the major deviation between simulated s t a r t l p conditions and a c t u a l startup.
7.0 STARTUP TROUBLESHIOTING One of the most important uses of simulation work i n this project was
the work done during the startlp.
From e a r l y January 1986 t o April t h a t year a number of complete complex t r i p s occurred due to c o n t r o l i n s t a b i l i t y problems with the natural gas feedstock system and the methanol p l a n t steam-carbon c o n t r o l loops. The first problem which was evident during s t a r t w was t h a t the steam reformer steam t o carbon c o n t r o l s (S/C) were inherently unstable and t h a t minor Lpsets would l e a d t o p l a n t t r i p s due t o low s/c ratio. While the control system i s complex, by%modelling using dynamic simulation, it was easy t o determine t h a t one of the control loops was unstable due t o the discrete sampling nature o f the control system and the chosen tuning parameters. (Ref 2)
722 The second major troubleshooting use of simulation during s t a r t u p was on
the n a t u r a l g a s feedstock c o n t r o l system. After t h e t r i p of one of the methanol p l a n t s , Severe t r a n s i e n t s i n the feedstock system caused t h e second methanol p l a n t t o t r i p due t o overpressure l i f t i n g PSV's an3 t r i p p i n g on low steam t o carbon r a t i o . T h i s led t o a complex t r i p and a downtime of days. The problem was found t o be i n t h e n a t u r a l g a s letdown c o n t r o l s (see f i g u r e 5 ) . Simple a s this c o n t r o l loop appears, i t caused a number of t r i p s during s t a r t l p . T h e problem i n t h i s case was t h a t the two valves caused t h e combined l i f t curve f o r the loop t o be non-linear. Under t r i p circumstances the
c o n t r o l a c t i o n was from a p o i n t of low valve g a i n causing the pressure c o n t r o l response t o be slow (See f i g u r e 5) leading t o overpressure. While the two problems discussed above appear t o be sinple, dynamic simulation was necessary f o r their s o l u t i o n f o r two reasons. F i r s t l y i t allowed the s a f e evaluation of any hypothesis as to the cause of the problems (ie d i d not jeopardise plant operation) and secondly provided the necessary evidence t o j u s t i f y any changes which were t o be made. 8.0 APPLICATION OF SIMULATIONS FOR OPTIMISING UNIT EFFICIENCY
I n MTG, the steady s t a t e simulation models were tuned t o a c t u a l p l a n t performance by a d j u s t i n g compositions, flows and heat exchanger fouling resistances. Based on the 'tuned' models, equipment performance could be evaluated and compared with design. T h e models could then be r e a d i l y adapted t o simulate an a d d i t i o n a l process f i r e d h e a t e r ( t o unload the methanol superheater) and t o e v a l u a t e i t ' s impact on the o v e r a l l MTG u n i t performance. Multiple c a s e s t u d i e s were performed t o determine optimm heater size with regard t o maximising heat recovery and providing u n i t heat balance f l e x i b i l i t y . The, a d d i t i o n a l heat balance f l e x i b i l i t y allows t h e 294-5 c a t a l y s t o u t l e t temperature t o be reduced hence maximising g a s o l i n e yield. 9.0 UNIT MONITORING
Complex-wide the c u r r e n t scope of monitoring is t o weekly c a l c u l a t e a l l heat exchangerlboiler performance, c a t a l y s t a c t i v i t y , compressor/turbine
723
performance and reformer a c t i v i t y . T h i s i s done by automated t r a n s f e r of a l l d a t a from the process computer (Fox-1A) t o the technical corrputer (IBM 4361 o r IBM PC). Pmcess c a l c u l a t i o n s a r e then implemented on the I B M mainframe o r PC. T h e flow-on e f f e c t s of a l l t h i s work is t h a t it i s now possible t o immediately detect any p l a n t i n e f f i c i e n c i e s and t o be a b l e t o optimise p l a n t performance based on c a t a l y s t a c t i v i t y and equipment performance. 10.0
FUTURE SIMULATION DEVELOPMENT
an MTG ASPEN steady s t a t e simulation is being progressed with the aim of replacing the Mobil proprietory steady s t a t e program t o enable more extensive modelling c a p a b i l i t y . It is envisaged t h a t conversion r e a c t o r models could be developed u s i q empirical r e l a t i o n s h i p s based on s t a t i s t i c a l analyses of l a r g e amounts of p l a n t data. These models could then be used t o optimise y i e l d s and thermal e f f i c i e n c y using modern optimising techniques. Further the MTG ASFEN model could be linked t o the e x i s t i n g methanol ASPEN model t o optimise the GTG complex production and e f f i c i e n c y . The development of
Future work i n methanol w i l l involve accurate t u n i q of the models developed so f a r t o allow f u r t h e r optimisation of the u n i t . As well as t h i s the simulations w i l l provide some o f the process design c a l c u l a t i o n s f o r expansion s t u d i e s (eg d i r e c t i n j e c t i o n of COP i n t o the methanol loop) and case s t u d i e s i n t o d i f f e r e n t operating modes of the p l a n t s .
11.0
CONCLUSIONS This paper has examined the chronological development o f computer simulations a t the SYNFUEL gas-to-gasoline complex from e a r l y design s t a g e s through t o present day commercial operation. The usefulness of these models was i n p a r t r e f l e c t e d by t h e smooth s t a r t l q of the complex and the small number of s i g n i f i c a n t engineering problems encountered s i n c e then. The p r e d i c t i v e c a p a b i l i t y o f t h e models enabled major p l a n t problems t o be i d e n t i f i e d and remedied p r i o r t o the complex being comissioned. Gnergercy t r i p procedrres and c o n t r o l systems were developed with the a i d of these models.
724
Detailed s e n s i t i v i t y analyses were implemented on various sections o f the plant t o provide guidelines f o r operation. The MTG u n i t s t a r t u p procedures were developed u s i m multiple quasi-steady s t a t e steps. Follow-on work involved development o f plant optimisation techniques and f u r t h e r expansion studies. It is hoped t h a t this paper has given some i n s i g h t i n t o the value computer simulations have been t o SYNFUEL i n the past and w i l l continue t o be t o process engineering i n the future.
REFEREKES
12.0
-
1. J W Womack O i l & Gas Journal, 7 a p r i l 1986 2. W B Earl and G A Robertson, Paper presented a t IPENZ Conference 1987
31534
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CSIIiO D i v i s i o n of Energy Chemistry P r i v a t e Bag Menai, NSW 2234
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Carina Chemicals P.O. Box 30366 Lower H u t t
NEM ZEALAND
Chemistry Department U n i v e r s i t y of Waterloo
Waterloo Ontario, N2L 3GI
CANADA
CSIIiO Locked Bag 33 Clayton Victoria 3168
AUSTRALIA
O i l & Gas I n d u s t r i e s M i n i s t r y of Energy P r i v a t e Bag Wellington
NEW ZEALAND
Chemistry Department U n i v e r s i t y of Tasmania GPO Box 2 5 X Hobart
AUSTRALIA
New Zealand S y n t h e t i c Fuels P r i v a t e Bag New Plymouth
NEW ZEALAND
Chemistry Department U n i v e r s i t y of Auckland P r i v a t e Bag Auckland
NEW ZEALAND
732 O’LEARY B.
Department of Resources Development
SGIO ATRIUM
170 S t George Terrace Perth, W.A. 6000
AUSTRALIA
I n d u s t r i a l Processing Division, E I R P r i v a t e Bag Petone
NEW ZEALAND
University of Tokyo Hongo 7-3-1 Tokyo, 113
JAPAN
Tokyo I n s t i t u t e of Technology Department of Chemical Engineering Ookayam, Meguro-ku Tokyo 152
JAPAN
Chemistry Division, E I R P r i v a t e Bag Petone
NEW ZEALAND
P e t r a l g a s Chemicals “2 Ltd P r i v a t e Bag New Plymouth
NEW ZEALAND
RATNASAMY P.
National Chemical Laboratory Pune 411 008
INDIA
R E W D M.
Swinburne I n s t i t u t e P.O. Box 218 Hawthorn, Victoria 3122
AUSTRALIA
New Zealand S y n t h e t i c Fuels P r i v a t e Bag New Plymouth
NJiW ZEALAND
Ministry of Energy P r i v a t e Bag Wellington
NEW ZEALAND
New Zealand S y n t h e t i c Fuels P r i v a t e Bag New Plymouth
NEW ZEALAND
Chemistry Division, S I R P r i v a t e Bag Petone
NEW ZEALAND
University of mente P.O. Box 217 Roan CT 1719, 7500 AE Enschede
NEIHERLANDS
O’MALLEY J.
OMATA K.
OTSUKA
K.
PARKER L.M.
PEARSON R.P.
REES I.F.
RICHARSON J.B.
ROBERTSON G.A.
ROGERS D.
Roo6 J.A.
ROSTRUPNIELSEN J.
SCHHEHL R.R.
Haldor TopA/S Nymoellevej 55 DK-2800, Lyngby
US Department of Energy P.O. Box 10940, Pittsburgh, PA 15236
D U.S.A.
733 SCHULZ H.
Engler-Bunte-Institut Universitat Karlruhe Kaiserstr. 12. 75 Karlsruhe
WEST GERMANY
SCURRELL M.S.
National Institute for Chemical Eng. Res. CSIR P.O. Box 395 Pretoria 0001 SOCPTH AFRICA
SEDD3N D.
Broken Hill Pty Co Ltd P.O. Box 264 Clayton Victoria 3168
AUSTRALIA
Department of Chemistry Sophia University 7-1 Kioi-cho Chiyoda-ku Tokyo 102
JAPAN
BP Gas International Britannic House Moor Lane London EC2Y 9BU
ENGLAND
New Zealand Synthetic Fuels Private Bag New Plputh
N!W ZEALAND
BP Research Centre Chertsey Road Sunburry Middlesex
ENGLAND
University of Florida Gainesville Florida 32611
U.S.A.
Statoil Postuttar, N-7000 Trondheim
NQRWAY
Chemistry Division, S I R Private Bag Petone
NEW ZEALAND
University of Tckyo Hongo Tokyo, 113
m A N
CSIRO Division of Energy Chemistry Private Bag 7 Menai, NEW 2234
AUSTRALIA
SEGAWA
K.
SHEPHERD P.
SMITH A.
SMITH D.J.H.
SMITH P.
SOLBAKKEN A.
TAPP N.J.
TATSMI T.
TAYLOR J.C.
TOPPJOIGENSEN J. Haldor Topsce A/S Nymolle Vev 55 2800 Lingby
734
TRIMM D.L.
TURS\IEY T.W.
TYLER R.J.
School of Chemical Engineering University of New South Wales P.O. Box 1 Kensington, NSW 2033
AUSTRALIA
CSIIEO, Division Mat. Science & Technology
Locked Bag 33 Clayton, Victoria 3168
AUSTRALIA
CSIRO, Division of Fossil Fuels P.O. Box 136 North Ryde, NSW 2113
AUSTRALIA
UNNEBEEIL;E.
Department of Chemistry University of Oslo P.O. Box 1033 0315 Oslo 3
VAN ClVlMEN J.G.
University of TWente P.O. Box 217 Room CT 1763 7500 AE Enschede
NETEIERLANDS
SAECE/ACS P.O. Box 31274 Braaimfontein 2017
SOUTH AFRICA
School of Chemical Engineering University of New South Wales P.O. Box 1 Kensington, NSW 2033
AUSTRALIA
Department of Chemistry University of New South Wales P.O. Box 1 Kensington, NSW 2033
AUSTRALIA
New Zealand Synthetic Fuels Private Bag New Plymouth
NEW ZEALAND
Broken Hill Pty Co Ltd P.O. Box 264 Clayton 3168 Victoria
AUSTRALIA
New Zealand Synthetic Fuels Private Bag New Plymouth
NEW
Curtin University of Technology School of Applied Chemistry Kent Street Bentley , Western. Australia
AUSTRALIA
VLOK K.
WINWRIGHT M.S.
WATSON A.J.
WEAKE H.J.
WHITE N.
WILLLAMS A.R.
WILLIX R.
WILSHIER K.G.
CSIFO, Division of Material Science Locked Bag 33 Nomnby Road Cla Vi&oria ton 3168
&
ZEALAND
Technology AUSTRALIA
735
WDHOUSE J.R.
YAMAMURA M.
YURCHAK
ZOU R.
s.
University of the Witwatersrand 1 Jan Smuts Avenue Johannesburg 2001
SOUTH AFRICA
Japan Petroleum Exploration Co 3-5-5 Midorigaoka Hamuramachi Nishitam Tokyo 190-11
JAPAN
Mobil Research & Development Corporation Billingsport Road Paulsboro, N J 08066
U.S.A.
Hebei Academy of Sciences Shijiazhuang
THE PEOPLE'S REPUBLIC OF CHINA
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737
AUTHOR INDEX
Abbashar, M.E.E. 83 Aika, K. 373 691 Allum, K.G. Al-Ubaid, A.S. 83, 89 Asami, K. 403 I Avidan, A.A. 307 Baker, B.G. 497 Bakker, A.G. 427 537 Barrer, R.M. B a r t l e t t , J.R. 609 Baumann Ofstad, E. 491 Beck, K. 457 Beckerle, J.D. 51 Bem, J.Z. 663 Bhasin, M.M. 343 Bibby, D.M. 609, 633 109 Bogdan, C.E. Bruce, L. 529 B r u i j n , N.A. de 427 Buckley, R.G. 597 Burgt, M.J. van der 473 Cai, G. 201 Cavell, K.J. 509, 523 Ceyer, S.T. 51 127 Chang, C.D. 201 Chen, G. Chin, V.W.L. 421 Clare, J.F. 597 Clark,.N.J. 497 Cooks, M. 433 Cooney, R.P. 609 Curry-Hyde, H.E. 239 Dass, D.V. 177 De B r u i j n , N.A. 427 229 Decken, C.B. von der Del'Amico, 3.5. 473 229 DBtsch, H. Dry, M.E. 447 Edwards, J.H. 395 Elnashaie, S.S.E.H. 83, 89 Erich, E. 457 Fedders, H. 229 Fujimoto, K. 245, 403 Garnett, J.L. 389 Gellings, P.J. 213 Grimmer, H.R. 273 Guangyao, 0. 627 Halvorsen, S. 491 Hardin, S. 529 Hashimoto, S. 403 383 Hatano, M. 109 Herman, R.G. Hoang, M. 529
229 Hohlein, B. Howe, R.F. 157, 207, 503, 615, 633 Huang, M.M. 207 Hunter, R. 183 Hutchings, G.J. 183, 415 I t o h , H. 517 Iwamatsu, E. 373 Jackson, P.J. 439 Jansen van Rensburg, L. 183 Jens, K.-J. 491 Johns, J.R. 615 Johnson, A.D. 51 Kado, H. 67 Kennedy, E.M. 383 Kikuchi , E. 517 K l i e r , K. 109 Kolboe, S. 189, 195 Komatsu, T. 383 Korchnak, J.D. 647 Korf, S.J. 427 Krohn, D.E. 679 Kurusu, Y. 579 Langdon, A.G. 603 Larkins, F.P. 409, 509 51 Lee, M.B. Leeuwen, C.J. van 473 201 L i , H. Liang, J. 201 389 Long, M.A. Lunsford, J.H. 359 633 McLellan, G.D. Maiden, C.J. 1 Martin, R.W. 177 Masai, M. 67 Masters, A.F. 421 Maxwell, I. 473 245 Mazaki, H. Meisel, S.L. 17 Melconian, M.G. 679 Milestone, N.B. 553, 639 M i l l e r , I.J.325 Miyake, A. 67 M o f f a t t , J.B. 563 Mole, T. 145 Moriyama, T. 373 Moroney, L.M. 603 219 Muramatsu, A. Nishiyama, S. 67 Nitschke, E. 273 Noceti, R.P. 483 Nordin, M.R. 409 109 Nunan, J.G. Odell, A.L. 177
738
Ofstad, E. Baumann 491 245 Omata, K. Ommen, J.G. van 213, 427 383 Otsuka, K. Parker, L.M. 589 183 P i c k l , W. Quinn, G.W. 177 79 Renjun, 2. Rensburg, L. Jansen van 183 509 Ridd, M.J. Robertson, G.A. 713 Roe, G.M. 509 Roos, J.A. 427 Ross, J.R.H. 213, 427 Rostrup-Nielsen, J. 73 579 Sakaguchi, M. Santiesteban, J.G. 109 Schehl, R.R. 483 Schulz, H. 457 415, 433 S c u r r e l l , M.S. Segawa, K. 579 Shanmugan, S. 603 S i e , S.T. 473 Smith, K.J. 109 Soliman, M.A. 89 373 Takasaki, N. T a l l o n , J.L. 597 Tapp, N.J. 553, 639 Tatsumi, T. 219 T a y l o r , C.E. 483 389 Than, C. T h i a g a r a j a n , N. 273
219, 245, 403 Tominaga, H. Topp-Jfirgensen , J. 293 T r i m m , D.L. 39 67 Tsuruya, S. Turney, T. 529 T y l e r , R.J. 395, 421 Unneberg, E. 195 Van d e r B u r g t , M.J. 473 Van Leeuwen, C.J. 473 213, 427 Van Ommen, J.G. Van Rensburg, L. Jansen 183 Vender, M. 421 Von d e r Decken, C.B. 229 Wainwright, M.S. 95, 239 201 Wang, Q. Watson, A.J. 389 713 Weake, H.J. White, N. 439 691 W i l l i a m s , A.R. W i l s h i e r , K.G. 621 Woodhouse, J.R. 415 245 Yagita, H. Yang, Q.Y. 51 219 Yokota, K. Yong, Y.-S. 503 Young, C.-W. 109 239 Young, D.J. Yulong, Z. 627 251 Yurchak, S. Zhao, S. 201 627 Zhi, 2 .
139
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve,Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1 Preparation of Catalysts 1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17, 1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15, 1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29-October 3, 1980 edited by M. LazniEka Volume 10 Adsorption a t the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouquerol and K.S.W. Sing Volume 1 1 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-1 6, 1982 edited by B. Irnelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jirir and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Volume 14 Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
740
Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts Ill. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an lntenational Conference, Prague, July 9-1 3, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. JirPI, V.B. Kazanskyand G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Driaj, S. HoEevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the InternationalSymposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations a t Surfaces 1985. Proceedings of the Fourth InternationalConference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Volume 27 Catalytic Hydrogenation edited by L. Cerveng Volume 28 N e w Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijirna and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 30 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 3 2 Thin Metal Films and Gas Chemisorption edited by P. Wissmann
741
Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 34 Catalyst Deactivation 1987.Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment Volume 35 Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite MaterialsScience. Proceedings of an International Symposium, Nieuwpoort, Semptember 13-1 7, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff
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