Hydrotreating Catalysts: Preparation Characterization and Performance : Proceedings of the Annual International Aiche Meeting, Washington, Dc, Novem (Studies in Surface Science and Catalysis)
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Studies in Surface Science and Catalysis 50 HYDROTREATING CATALYSTS Preparation, Characterization and Performance
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis Advisory Editors:B. Delmon and J.T. Yates Vol. 50
HYDROTREAT1NG CATALYSTS Preparation, Characterization and Performance Proceedingsof the Annual International AlChE Meeting, Washington, DC, November 27-December 2,1988 Editors
M.L. Occelli Union Oil Company of California, Science and Technology Division, 3 7 6 South Valencia A venue, Brea, CA 9262 1, U.S.A. and
R.G. Anthony Chemical Engineering Department, Texas A&M University, College Station, TX 77843, U.S.A.
ELSEVIER
Amsterdam - Oxford - New York - Tokyo
1989
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U S A .
L i b r a r y o f Congress C a t a l o g i n g - I n - P u b l i c a t i o n
Data
Amekican I n s t i t u t e of C h e m i c a l E n g i n e a r s . M e e t i n g ( 1 9 8 8 : W a s h i n g t o n , D.C.) H y d r o t r e a t i n g c a t a l y s t s : p r e p a r a t i o n , c h a r a c t e r i z a t l o n , and p e r f o r m a n c e : p r o c e e d i n g s o f t h e Annual I n t e r n a t i o n a l AIChE M e e t i n g , Washington, DC. November 27-December 2. 1 9 8 8 / e d i t o r s , M.L. O c c e l l i and R.G. Anthony. p. cm. ( S t u d i e s i n s u r f a c e s c i e n c e and c a t a l y s t s ; 5 0 ) Bibliography: p. Includes index. ISBN 0-444-88032-1 (U.S.) 2. P e t r o l e u m - - R e f i n i n g 1. H y d r o t r e a t i n g catalysts--Congresses. 3. C r a c k i n g process--Congresses. I. O c c e l l i . M a r i o -Congresses. L.. 194211. Anthony, R a y f o r d G. ( R a y f o r d G a i n a s ) . 1935III. T i t i e . I V . Serles. TP690.4.A54 1988 89- 1 6 7 8 8 665.5'33--d~20 CIP
--
.
.
ISBN 0-444-88032-1 (Vol. 50) ISBN 0-444-4 180 1 -6 (Series)
0 Elsevier Science Publishers B.V., 1989 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./ Physical Sciences & Engineering 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 publisher. No responsibility is assumed by the Publisher for any injury and/or damage t o 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. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Netherlands
S t r u c t u r e / f u n c t i o n r e l a t i o n s i n t r a n s i t i o n metal s u l f i d e c a t a l y s t s R.R.
C h i a n e l l i and M. Daage
............................................
1
S t a c k i n g o f molybdenum d i s u l f i d e l a y e r s i n h y d r o t r e a t i n g c a t a l y s t s R.C.
Ryan, R.A.
Kemp, J.A. Smegal, D.R.
Denley and G.E. S p i n n l e r
.......
21
Chevrel phase HDS c a t a l y s t s : s t r u c t u r a l and c o m p o s i t i o n a l r e l a t i o n s h i p s t o catalytic activity G.L. Schrader and M.E.
Ekman
...........................................
41
I n f l u e n c e o f t h e s u p p o r t and t h e s u l p h i d a t i o n temperature on t h e c a t a l y t i c p r o p e r t i e s o f molybdenum s u l p h i d e i n p y r i d i n e h y d r o g e n a t i o n and p i p e r i d i n e hydrodeni t r o g e n a t i o n J.L. P o r t e f a i x , M. C a t t e n o t , J.A. Dalmon and C. Mauchausse
.............
67
The e f f e c t o f phosphate on t h e h y d r o d e n i t r o g e n a t i o n a c t i v i t y and s e l e c t i v i t y o f alumina-supported s u l f i d e d Mo, N i and Ni-Mo c a t a l y s t s S. E i j s b o u t s , L. van G r u i j t h u i j s e n , J .
I n f l u e n c e o f p r e p a r a t i o n on t h e morphology and m i c r o s t r u c t u r e o f c o b a l t molybdenum s u l p h i d e s G. Diaz, F . Pedraza, S. Fuentes
and hydrogen s u l p h i d e on h y d r o d e n i t r o g e n a t i o n
o f q u i n o l i n e o v e r a s u l p h i d e d Ni0-Mo03/A1203 c a t a l y s t C. Moreau, L. Bekakra, A. Messalhi,
J.L. O l i v e and P. Geneste
.......... 107
Search f o r s i m p l e model compounds t o s i m u l a t e t h e i n h i b i t i o n o f hydrodeni t r o g e n a t i o n r e a c t i o n s by asphal tenes C. Moreau, L. Bekakra, R. Durand, N. Zmimita and P. Geneste
............
115
VI The v e r s a t i l e r o l e o f n i c k e l i n Ni-MoS2/A1203 h y d r o t r e a t i n g c a t a l y s t s as shown by t h e use o f probe molecules J.P.
Bonnelle, A. Wambeke, A. Kherbeche, R. Hubaut, L. J a l o w i e c k i ,
S. Kasztelan and J. G r i m b l o t
...........................................
123
A h i s t o r y o f the development o f high-metals h y d r o t r e a t i n g c a t a l y s t s . The use o f c r y s t a l l o g r a p h i c concepts i n c a t a l y s t design
S t r u c t u r e s o f b i m e t a l l i c c a t a l y s t s (Pt/Sn) on S i 0 2
133
A1203 supports:
NEXAFS and EXAFS d i a g n o s t i c s
N-S. Chiu, W-H. Lee, Y - X i L i , S.H. Bauer and B.H. Davis
................ 147
Mossbauer study o f the s u l f i d a t i o n of h y d r o d e s u l f u r i z a t i o n c a t a l y s t s : s o - c a l l e d "Co-Mo-S" phase observed i n carbon-supported Co and Co-Mo sulfide catalysts M.W.J.
Craje, E. Gerkema, V.H.J.
de Beer and A.M.
van der Kraan
........ 165
A new approach f o r s t u d y i n g t h e a c i d s t r e n g t h d i s t r i b u t i o n i n h y d r o t r e a t i n g c a t a l y s t s by d i f f e r e n t i a1 scanning c a l o r i m e t r y
A.K.
Aboul-Gheit and A.M.
Summan
.......................................
181
Supported Co-Mo t h i n f i l m s u l p h i d e c a t a l y s t s f o r h y d r o d e s u l p h u r i z a t i o n .
1. XPS s t u d i e s o f the e f f e c t s o f r e a c t a n t pressure N.S.
McIntyre, T.C. Chan, P.A.
Spevack and J.R. Brown
..................
187
Adsorption, r e a c t i o n and d e s o r p t i o n r a t e constants i n heterogeneous c a t a l y s i s , measured simultaneously by gas chromatography
N.A.
Katsanos and J. Kapolos
...........................................
211
A m i n i a t u r e o n - l i n e c l o s e d - c y c l e r e a c t o r f o r X-ray p h o t o e l e c t r o n spectroscopy s t u d i e s o f h y d r o d e s u l p h u r i z a t i o n r e a c t i o n s
P.A.
Spevack, L.L. Coatsworth, N.S.
M c I n t y r e , I. Schmidt and J.R. Brown
229
C a t a l y t i c p r o p e r t i e s i n h y d r o t r e a t i n g r e a c t i o n s o f ruthenium s u l p h i d e s on
Y z e o l i t e s : i n f l u e n c e o f t h e support a c i d i t y S. Gobolos, M. Breysse, M. Cattenot, T. Decamp,
J.L.
P o r t e f a i x and M. V r i n a t
M. L a c r o i x ,
...........................................
243
VII Upgrading o f coprocessed naphtha by h y d r o t r e a t i n g M.V.C.
Sekhar and P.M.
Rahimi
..........................................
251
Improved h y d r o c r a c k i n g performance by combining c o n v e n t i o n a l h y d r o t r e a t i n g and z e o l i t i c c a t a l y s t s i n s t a c k e d bed r e a c t o r s
A.A.
Esener and I . E .
Maxwell
...........................................
263
The m i c r o b i a l upgrading of model heavy o i l s
S t u d i e s i n Surface 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 )
.... 293
This Page Intentionally Left Blank
IX
The c a t a l y t i c c r a c k i n g o f p e t r o l e u m f r a c t i o n s i s b e l i e v e d t o have begun i n 1936 when E. Houdry observed t h a t r a c i n g c a r s ’ performance c o u l d be g r e a t l y improved by u s i n g h i g h - o c t a n e g a s o l i n e o b t a i n e d f rom c r a c k i n g heavy
p et ro l e u m
fractions
montmorillonites o r halloysites.
over
packed
beds
of
acid
treated
C o l l a b o r a t i o n between Mobil O i l and E .
Houdry l e a d t o t h e development o f t h e f i r s t 2,000
bbl/day
c a t a l y t i c c r a c k i n g u n i t a t M o b i l ’ s Paulsboro R e f i n e r y . b b l l d a y was i n o p e r a t i o n i n t h e U.S.A.
commercial
By 1940,
100,000
and t h i s c a p a b i l i t y t o produce h i g h
grade a v i a t i o n f l u i d s c o n t r i b u t e d t o v i c t o r y i n World War 11. H y d r o t r e a t i n g became t h en an i n t e g r a l p a r t o f o i l r e f i n i n g and i t s import ance has continuously increased over t h e years. U n t i l f a i r l y r e c e n t l y , t h e p r e p a r a t i o n o f h y d r o t r e a t i n g c a t a l y s t s has been t h ought by many t o be a l a s t b a s t i o n o f alchemy.
The c o m p l e x i t y o f
c a t a l y s t d e s i g n p r e c l u d e d p r e p a r a t i o n o f commercially u s e f u l m a t e r i a l s f rom a knowledge o f t h e physicochemical p r o p e r t i e s o f t h e s o l i d s used. Although methods o f procedures have been developed t o p r e p a r e t h e v a r i o u s hydrotreating
(HDM,
HDN,
HDS,
etc)
catalysts,
a
large
number
of
experiments and t e s t s was r e q u i r e d t o d e velop new o r improved p r o d u c t s . Today, t h e i n c r e a s i n g a v a i l a b i l i t y and a p p l i c a t i o n o f modern charact e r i z a t i o n t e c h n i q u e s such as l a s e r Raman spectroscopy, (MASNMR),
X-ray
photoelectron
spectroscopy
(XPS),
s o l i d s t a t e NMR
and extended X-ray
a n a l y s i s o f f i n e s t r u c t u r e s (EXAFS), t o g e t h e r w i t h t r a n s m i s s i o n and scann i n g e l e c t r o n microscopy (TEM and SEM) p r o v i d e v a l u a b l e guidance i n t o t h e m o s t l y e m p i r i c a l approach t o c a t a l y s t d e sign.
I t was t h e i n t e n t o f t h i s
X symposium
to
characterization
examine
the
techniques
contribution have made t o
that the
all
these
scientific
novel
design
and
understanding o f h y d r o t r e a t i n g c a t a l y s t s . The
editors
express
their
appreciation
to
the
authors
i n d i v i d u a l chapters,
t o o u r c o l l e a g u e s t h a t s e r v e d as r e f e r e e s ,
American
of
Institute
Chemical
Engineers
(Fuels
and
of
the
t o the
Petrochemical
D i v i s i o n , Area 16a, Petroleum) and t o The C a t a l y s i s S o c i e t y f o r s p o n s o r i n g t h i s I n t e r n a t i o n a l Symposium.
I n p a r t i c u l a r t h e E d i t o r s want t o e x p r e s s
t h e i r a p p r e c i a t i o n t o P r o f e s s o r Henry McGee, M e e t i n g Program Chairman, f o r h i s c o o p e r a t i o n and u s e f u l s u g g e s t i o n s and f o r f e a t u r i n g t h i s symposium a t t h e November 29-December 3, 1988 Annual AIChE m e e t i n g .
We a l s o e x p r e s s
o u r a p p r e c i a t i o n t o Sony Oyekan, Chairman o f t h e P e t r o l e u m Subcommittee, for his
support
and t o M s .
G.
Smith o f
Unocal
for
her
invaluable
s e c r e t a r i a1 he1 p .
Unocal C o r p o r a t i o n , P.O. Box 76, Brea, CA 92621, U.S.A.
M.L. OCCELLI
Chemical E n g i n e e r i n g Department, Texas A&M U n i v e r s i t y , C o l l e g e S t a t i o n , TX 77843-3122, U.S.A.
R . G . ANTHONY
1
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STRUCTURE/FUNCTION RELATIONS IN TRANSITION METAL SULFIDE CATALYSTS R. R. CHIANELLI and M. DAAGE Corporate Research Laboratories, Exxon Research and Engineering Co., Route 22E, Annandale, NJ 08801 ABSTRACT Transition Metal Sulfide based catalysts have been industrially important for over sixty years in hydrodesulfurization, hydrodenitrogenation and hydrogenation reactions which occur during petroleum hydroprocessing and hydrofinishing. Renewed interest in this class of materials centers around recent developments in alcohol synthesis from CO/H2. The useful properties of these catalysts arise from their sulfur tolerance and from their anisotropic structure properties. This paper explores recent progress in understanding the relation between the structures of these catalyst (electronic, chemical and geometric) and their ability to catalyze important reactions. INTRODUCTION Hydroprocessing catalysts based upon the transition metal sulfides (TMS) have been widely used for over 60 years. Catalysts such as Co/Mo/A1203 and Ni/Mo/AlzOg are currently found in every refinery in,the world. They find their application primarily in removal of sulfur (hydrodesulfurization), removal of nitrogen (hydrodenitrogenation) and product quality improvement (hydrogenation) of petroleum-based’feedstocks. These hydrotreated feedstocks become primary components in fuel, lubrication and petrochemical based products (ref. 1).
Prior to World War 11, interest in these catalysts originated in
their activity in hydrogenation of coal liquids to clean products.
Because
they were sulfide based catalysts, they were able to maintain high activity even in the presence of considerable amounts of sulfur in the coal liquids. This ability is one of the most useful properties of TMS based catalysts.
It
is this property which assures that TMS based catalysts will be used far into the future to convert heavy petroleum, coal and shale oil based feedstocks as cleaner feedstocks diminish. In this early period it was quickly discovered that Co, Ni, Mo and W sulfides and their mixtures were the most active and least expensive of the TMS (ref. 2).
Originally, these catalysts were used in an unsupported form and
many examples exist of successful processes based on unsupported catalysts. In fact, unsupported catalysts were used for high activity special applications well into the sixties. A careful reading of the voluminous TMS literature forces the conclusion that the modern ~ 1 2 0 3supported catalyst evolved for reasons of cost (effective use of metals) and ease of production and handling (pyrophoricity and storage stability).
In other words, there is no fundamental
2
catalytic property that A1203 adds to the system as was previously believed (ref. 3). Therefore, the catalytic properties of the TMS can be completely understood in terms of the active unsupported sulfide phases. TMS will continue to be important far into the future because of their high activity, selectivity and stability in the presence of sulfur containing feedstocks. As clean petroleum feedstock supplies dwindle, we are required to process larger quantities of "dirtier" feeds containing larger amounts of sulfur, nitrogen and metals.
In order to meet these requirements in the
future, a new generation of TMS based catalysts will be needed which have higher activities, greater selectivity to desired products and greater resistance to poisons. However, though TMS are well known for their hydrotreating applications, they are less well known for the great versatility that they exhibit.
For example, TMS catalysts have been commercially utilized for re-
forming of sour feedstocks and many other types of reactions catalyzed by TMS have been reported (ref. 1). of olefins by TMS (ref.4).
Another recent example is the selective oxidation Perhaps the most interesting example is the
discovery that the TMS catalyze the reaction of CO and H2 to alcohols (ref. 5). This reaction which may be very important in the future assures a continuing and growing interest in TMS based catalysts. Because of the current and future importance of TMS based catalysts much effort has been put into trying to understand the fundamental basis for their activity and selectivity (ref. 6).
Considerable progress has been realized in
the past ten years, but many questions remain to be answered. Periodic effects which describe the ability of the simple TMS to catalyze various hydrotreating reactions form the underpinning for any fundamental understanding of these catalysts. First measured for unsupported TMS catalyzing the HDS (hydrodesulfurization) reactions. A typical "volcano plot" between the HDS activity and the periodic position emerged which showed that the Group VIII TMS (Ru, Rh, Ir) were the most active HDS catalysts (ref. 7).
Os,
Subsequently, similar trends
were reported on carbon and oxide supports for HDS (refs. 8 , 9 ) .
Recently,
almost the same trends have been reported for HDN and hydrogenation reactions (refs. 10,11,12,13). These trends are of fundamental importance because they dramatically emphasizes the importance of the 4d and 5d electrons in catalyzing these reactions. In this respect, the TMS resemble the pure noble metal catalyzed hydrogenolysis trends reported by Sinfelt (ref. 14).
It may be
further added that all o f the above periodic trends involve hydrogen as a common factor and therefore it is interesting to note that exchange current densities for electrolytic hydrogen evolution follows a similar periodic trend (ref. 15).
A theoretical foundation for understanding these periodic trends is
to be found in the calculated bulk electronic structures of the first and second row TMS (ref. 16).
It was shown that a relation exists between the
3
calculated bulk electronic structure of the TMS and their activity as HDS catalysts. Several electronic factors appeared to be related to catalytic activity. These are the orbital occupation of the HOMO, the degrees of covalency of the metal-sulfur bond, and the metal-sulfur covalent bond strength. These factors were incorporated into an activity parameter (A2). This activity parameter was shown to correlate with the HDS of DBT (dibenzothiophene) activity and more recently to a heavy gas oil conversion (ref. 17 and total heteroatom removal in a mixture of DBT and quinoline (ref. 12). These results, while re-emphasizing the importance of the 4d and 5d electrons also enabled the authors to explain the promotional effect of the first row transition series in the same electronic terms as described below (ref. 18). PROMOTED SYSTEMS It is well known and of great industrial importance that the addition of a second transition metal such as Co or Ni to a binary sulfide such as MoS2 or US2 can give rise to an enhancement of HDS activity. This enhancement can be quite pronounced. Tenfold increases of activity over the activity of the unpromoted sulfide are not uncommon. Industrially this effect is exploited in the common Co/Mo/Al2Og and Ni/Mo/A1203 catalysts. These systems have been studied intensively for many years but progress toward understanding the origin of the promotion effect has been slow until quite recently. A vast majority of
the studies done in this area have dealt with supported systems. Recent work has demonstrated with a fair degree of certainty that the active Mo component of the supported catalyst is found in a MoS2-like structure, although the degree of dispersion and stoichiometry are still debated.
It is clear however,
from very early work and current work that unsupported MoS2 exhibits the promotional effect with respect to Co and Ni.
Thus, a question arises
as
to
whether both the supported and unsupported phases exhibit the same promotion effect coming from a common mechanism.
The principle of simplicity seems to
demand an answer in the affirmative. Early work on WS2 by Voorhoeve, et. al. (ref. 19) and on MoS2 by Farragher, et.al., (ref. 20) laid the ground work for the study of the promotional effect from a solid state point of view. This work pointed to the importance of the edge planes of the layered compounds MoS2 and WS2 in the promotion by Ni and Co. Co
Both groups of workers attributed promotion to "pseudo-intercalation''of
or Ni at the edges of the layered compounds. The term "pseudo-intercalat-
ion" refers to the idea that MoS2 only fully intercalates Co at high temperatures forming the relatively inactive phase CohMoS2. However, at catalytic temperatures Co intercalates near the MoS2 edges thus, "pseudo-intercalation". Though, pseudo-intercalation has been shown to exist, the essential point of this work is that Co is located near the edge of MoS2 and that promotion occurs
4
via charge transfer from Co to Mo. This basic idea remains in use today supported by theoretical considerations (ref. 16). When the promoter metal is in large concentration, a second phase containing the promoter metal phase separates from the MoS2. This second phase is Cogs8 for Co and Ni3S2 for Ni.
It was the presence of this second phase which lead
to another early explanation of promotion, the idea of "contact synergy" (ref. 21).
In fact, the separate phase Cogs8 and MoS2 can be ground together and the
resultant mixture exhibits the promotional effect. Following this idea, Ni/Mo, Co/Mo, C O D and N i p can be said to behave as "synergic pairs" incorporating the idea that the members of these pairs "work together or cooperate". In the case of Co/Mo it was envisioned that Cogs8 in close contact with MoS2 would cooperate, the Cogs8 activating H2 and the MoS2 providing the sulfur vacancies for binding of the sulfur bearing molecules (ref. 22).
The problem with this
particular interpretation of "contact synergy" is that both Cogs8 and MoS2 seem to be equally effective at activating H2 and in desulfurizing sulfur bearing molecules (ref. 7 ) .
Although this specific idea may be incorrect, it has been
noted that the "synergic systems" are related to the simple binary sulfides through average heats of formation (ref. 23).
This work suggests that the
synergic systems behave at their surface as if they are hypothetical "pseudobinary" systems having average properties of their two components. It would appear that both ideas "pseudo-intercalation''and "contact snyergy", as well as many other theories are consistent with the generalized picture shown in figure 1. If the Co concentration is low, Co is located near the MoS2 surface in some position. As the Co concentration increases, Co surface segregates as suggested by Phillips and Fote (ref. 24).
At larger Co
concentration, Cogs8 begins to phase separate but always in contact with some MoS2. Thus, the particular descriptions of the Macro structural aspects of Co promotion are dependent on Co concentration and on MoS2 dispersion which controls the level of Co concentration at which phase separation occurs. A
\ interface ("zone of contact")
Figure 1: Schematic representation of CogS8/MoS2 "zone of contact" on interface.
5
similar picture for Ni/Mo has been discussed by Garreau, et. al. (ref. 25). Current ideas of promotion in this picture all focus on the specific structure of the Co/Mo/S atoms in the "zone of contact" or interface which is indicated in figure 1. Many competing candidates exists for this specific structure. All of these models, whether supported or unsupported, suffer from the same problem, i.e.; lack of conclusive evidence regarding the degree of dispersion of the MoS2 and/or knowledge of precise Co concentration at the interface of the MoS2 surface. It is this same problem which is at the source of the confusion in the literature regarding the nature of promotion. There are two basic concepts: firstly, "electronic promotion" meaning that Co and Mo act together to create sites or vacancies which are more active than sites on either components (pseudo-binary). The second concept, "structural promotion", states that Co/Mo interaction increases the dispersion of either phase, thus increasing activity. In the later idea, either the dispersion of MoS2 is increased or MoS2 is dispersing a very active form of Co.
Some authors believe
Electronic promotion : Co, Ni
0
Electronic poison : Cu Figure 2: Schematic representation of Electronic promotion by Co and Ni or electronic poisoning by Cu.
6
that both electronic and structural promotion ideas are necessary to explain all results as discussed below. Regardless, of which mechanism is correct, there is general (but not unanimous) agreement that interaction between the Mo(W) 4d electrons and Co(Ni) 3d electrons are required for promotion, This interaction has been theoretically described using model catalyst calculations and experimental trends indicated in figure 2 (ref. 18).
The measured HDS activities show that only Co
and Ni serve as effective promoters, while Fe and Zn are neutral and Cu functions as a poison. The calculated electronic structure of the model cluster models of these promoted catalyst systems indicates that Co and Ni have the ability to formally reduce Mo in these systems, while Cu has the ability to formally oxidize Mo.
None of the other 3d metals has this ability. The number
of 3d electrons which Co, Ni or Cu contributes to the cluster and the energies of their 3d orbitals relative to the Mo 4d orbitals make these metals unique when combined with Mo.
Thus, promotion occurs with formal reduction of Mo and
poisoning with oxidation of Mo.
These results are consistent with the earlier
identification of electronic factors which are related to the HDS activity of the binary sulfides, i.e.; the covalent contribution to the metal-sulfur bond strength and the metal d orbital occupations. For the promoted MoS2 catalysts, both of these factors are affected by the presence of a 3d metal promoter or poison, although the dominant effect of a promoter is the increase in the number of "d" electrons formally associated with Mo.
Though the increase in
the number of electrons on Mo appears to be the dominant electronic factor influencing the HDS activity, there is an accompanying change in the metalsulfur covalent bond strength. In the Co/Mo system, the formal transfer of an electron from Co to Mo involves an electron transfer from a Co-S antibonding orbital to a Mo-S antibonding orbital. This results in a weakening of the Mo-S bonds and a strengthening of the Co-S bonds relative to the metal-sulfur bonds in the binary sulfides. A sulfur shared between Mo and Co (figure 3) would be expected to behave much like a sulfur in a binary sulfide having some intermediate metal-sulfur bond strength. Thus, for systems where such electron transfers occur, it is reasonable to see a correlation between average heats of formation and activity as mentioned earlier. Several microscopic structural models of promotion have been presented in the literature which usually attempt to locate a specific Co/Mo/S or Ni/Mo/S near the edge of MoS2 or WS2.
Ratnasamy, et. al. (ref. 26); Voorhoeve, et. al.
(ref. 19); Farragher, et. al. (ref. 20), and others have all suggested different locations for the Co or Ni. Precise information regarding the structure of this "promoting unit" has been very difficult to obtain primarily due to lack of specific probes for the catalytically active phase of the promoters. In-situ Mossbauer emission spectroscopy (MES) combined with activity measurements related activity (HDS) to the intensity of a unique
7
Figure 3 :
Schematic representation of "Electronic Averaging" of promoted site.
Mossbauer component. The Co atoms giving rise to this component were associated with Mo in the MoS2 and termed the "Co-Mo-S" phase (ref. 27,28). The Co-Mo-Sphase was considered to be the most catalytically significant phase present. A model of the Co-Mo-S phase was presented which had Co atoms at the edges of very small MoS2 crystallites. The size of the MoS2 crystallites was determined by in-situ EXAFS studies (ref. 29).
However, it seems clear that
the normal method for determining crystallite size using EXAFS cannot be used in the case of highly anisotropic materials such as MoS2.
Attenuation of the
second Mo-Mo shell is mostly due to disorder in the layers and cannot be interpreted as crystallite size. Thus, the crystallite size is probably much larger than the Co-Mo-S model indicates. This also explains why a Co-Mo interaction has not been seen by EXAFS as the model predicts. has recently been shown that
a
Furthermore, it
very similar Co MES spectrum can be produced by
a Co/C catalysts containing no MoS2 (ref. 30).
Some authors have attributed
the promotion effect as due to Co only based upon extrapolated high activity of low loaded Co catalysts (ref. 31).
However, it is difficult to believe in view
of the overwhelming evidence in the literature of the importance of the 4d and 5d electrons that these results will hold up with time. This is especially true in view of the fact that much evidence exists for Co and Mo in the sulfided phases have about the same intrinsic activity. Furthermore, though the meaning of the Co MES spectra published by Topsae, et. al. may currently be unclear, it appears to be a valuable tool in studying these systems though it may not uniquely determine the structural properties of the promoted sites. Ledoux, et. al., have introduced the use of an even more specific probe, Co
NMR, to the problem of Co promotion (ref. 3 2 ) .
This technique clearly
distinguishes between four types of Co present in Co/Mo catalysts. Two which only occur at high Co loadings are typical of the Co found in C o g S 8 .
Two new
types are found which because of their NMR properties are called "distorted tetrahedral Co" and rapid octahedral Co".
Ledoux, et. al., have presented a
model which assigns the "rapid octahedral Co" the role of "gluing" the active "distorted Co" to the MoS2 edges.
I n a second paper, Ledoux, et. al., propose
8
that the promotion effect is a combination of "electronic" and "structural" effects.
In their model, small crystallites of MoS2 are stabilized by electron
transfer from Co or Ni.
Pure MoS2 deactivates rapidly in the presence of
reaction thiophene but the deactivation is reduced by the presence of the stabilizing promoter which keeps the Mo in MoS2 in the + 4 state preventing oxidation to the +5 state during reaction. These ideas are in agreement with the electronic theory discussed above because the +5 state would be less effective than the +4 state for activity. Following this idea, the authors have suggested some very specific models with dispersion information coming from selected TEM micrographs. The Co NMR results and the edge stabilizing role of the promoter metal clearly advance our understanding of these systems, but more convincing evidence of the Co and MoS2 dispersion are required for total acceptance of these models. Structural disorder in MoS2(WS2) based catalysts is a major problem in our ability to accurately measure dispersion by traditional physical means in these catalysts (ref. 3 4 ) .
This disorder must be understood and taken into account
for proper interpretation of the physical characteristics of these catalysts. MoS2 prepared at temperatures and conditions which are typical for catalytic preparations usually forms in the "rag structure" (ref. 35).
This structure
consists of several stacked, but highly fold and disordered, MoS2 layers and is a consequence of rapid growth during preparation and the anisotropy of the structure; the layers grow very rapidly in two dimensions but only slowly in the c or stack direction. The resulting "rags" can be several thousand angstroms across but only 20 to 30
A thick. Because of this structure, x-ray
analysis of these materials can be very misleading. The random stacking, combined with the folding, makes it impossible to extract the crystallite or particle size dimension by x-ray line broadening analysis. Large rags or small rags may have the same order length in the MoS2 layer plane, as determined by line broadening analysis, but vastly different particle sizes, edge areas and therefore, catalytic activity (ref. 3 6 ) .
Thus, as opposed to isotropic
systems, x-ray diffraction data is only marginally useful in interpreting catalytic properties of these anisotropic systems. Unfortunately, the same problem appears to exist when examining supported or unsupported catalysts using EXAFS. Basic studies have not been performed which would permit EXAFS analysis to distinguish between particle size effects and disorder effects as described above.
RECENT STUDIES OF MoS2 "EDGE PLANES" The above section illustrates that much progress has been made in understanding the properties of TMS based catalysts, but it also points out some of the basic problems preventing further progress.
The major impediment to
9
further progress arises from the nature of MoS2 itself, its highly anisotropic structure. In this section, we report some of our recent progress in understanding how the anisotropic crystal structure of MoS2 is fundamentally related to its catalytic properties. The structural anisotropy of MoS2(WS2) is a consequence of the chemical bonding. Within one layer, the structure can be viewed as a two-dimensional macromolecule. Each metal is bound to six sulfur atoms and each sulfur atom is bound to three metal atoms.
Because the sulfur
is s o tightly bound, its interaction with the next layer of sulfur above it is extremely weak. This creates the "van der Waals" gap which is the main feature of interest in regard to intercalation and lubricity properties (ref. 37). Thus, although the basal planes (002 have been the general focus of studies in the vast intercalation literature, the "edge" planes (100) of the layered TMS become the focus of catalytic studies. The potential importance of MoS2 edge planes in hydrotreating catalysts has long been recognized and some of the evidence for this has been cited above. A good example of further evidence for the reactivity of the edge planes in MoS2 can be found in the linear correlation between 02 chemisorption and the HDS of dibenzothiophene (ref. 3 8 ) .
In general, HDS activity does not correlate to N2
BET surface area measurement. This is because the basal plane area contributes to the total surface area but not to the catalytic activity. Therefore, MoS2 catalysts made by a variety of preparative methods will have widely different edge to basal plane ratios and only 02 chemisorption will give a good correlation to activity. If the preparative method is constant, however, the basal plane area can be proportional to the edge area and a good correlation between total surface area and activity can be obtained (ref. 39). A basic problem with 02 chemisorption arises from the fact that 02 chemi-
sorbs corrosively, i.e., monolayer coverages at the edges is not achieved unless very mild conditions are used.
If mild conditions are not used,
oxidation occurs deeper into the bulk and the number of 02 adsorbed is in general only proportional to the number of edge sites (ref. 40). Furthermore, the presence of the promoter phase further complicates 02 chemisorption studies and there is no general agreement as to its utility for supported catalysts. However, the technique has been widely applied, most recently to the supported WS2 system using mild (low temperature) conditions (ref. 41). We may conclude at this writing that the most quantitatively detailed models of these catalyst systems come from a combination of activity data and chemisorption data. The recent geometric model of Kasztelan, et al., is a good example (refs. 42,43). Using a geometrical model based on assumed shapes of MoS2 crystallites, these authors were able to fit chemisorption and activity data for both promoted and unpromoted supported MoS2 and WS2 sites.
In their model, small slabs of MoS2
consist of basal, edge or corner sites. By fitting activity curves with
10
different shapes and numbers of these sites, the authors concluded that hexagonal or rhombobedral crystallites of single layers of about 10-20 A gave the best fit. Furthermore, they concluded that the edge sites were the active sites, that promotion occurred through enhancement of the quality of the sites and this promotion factor was calculated as being a factor of 4 . 4 ->
5.2.
Again, this procedure leads to a model which fits well with the edge-decoration model but does not give a detailed picture of the "promoted" sites. In order for this to be accomplished more detailed physical chemical and theoretical work is needed.
4
2
0
z
- 2
v
h
F Q)
- 4
- 6
-a
Density of states
Figure 4 :
Schematic representation of density of states in MoS2
The effect of 02 on the dz2 tail states of MoS2 was described in a recent publication (ref. 4 4 ) .
In this work UPS studies showed the existence of
surface states above the dz2 band near the Fermi level [figure 4 1 .
Further-
more, these tail states were reversibly quenched with 9OOL 02 but irreversibly quenched at 1 at. of 02. The irreversible quenching occurred with an
11
accompanying appearance of bulk oxide states in the UPS spectrum. This result not only demonstrates the problems with 02 chemisorption but also shows the relation between the bulk electronic states of MoS2 and the active surface states. The dz2 orbitals are the highest occupied molecular orbitals of MoS2 and the crucial catalytic electronic states lie just above them arising from the surface termination of the bulk states. The previously presented calculated bulk electronic trends and their correlation to activity may now be understood in terms of the bulk electronic structure providing an "electronic support" for the catalytically important surface electrons. In a recent paper the optical properties of these "tail-states''were examined catalytically and optically (ref. 4 5 ) .
The optical properties of MoS2
powder and platelets were measured in the near infrared using photothermal deflection spectroscopy (PDS).
PDS is a technique well suited for the
measurement of the optical properties of black highly adsorbing catalytic powders because it is insensitive to optical scattering (ref. 4 6 ) .
The
adsorption as measured by this technique for crystalline samples is shown in figure 5 . A = 1 -
This adsorption A can be related to the adsorption coefficient a by
where d is the average sample thickness.
The absolute value of the adsorption
is known for all samples because they can be normalized to the strongly adsorb-
ing excitonic region were al>>l.
The spectra of small (1.7pm diameter) and
large (36pm diameter) platelets are compared against the spectrum of a single crystal of MoS2 in figure 5.
The adsorption for the single crystal begins
strongly at 1.2 eV and increases toward higher energy due to the indirect bandgap (ref. 4 7 ) .
The flat lower energy adsorption in the single crystal is
from defects in the material and varies strongly from sample to sample.
Photon Energy (eV)
Figure 5 :
PDS measured spectra of MoS2 single crystal and microcrystallite
platelets. Also included are calculated positions for MoS2 defects occurring on edge planes after ref. 4 8 .
12
The spectrum of the large platelets is seen to be very similar to that of the single crystal, except that the defect adsorption below 1.2 eV is an order of magnitude higher. The striking similarity of the spectrum of the single crystal and the large platelets between 1 . 3 and 1.6 eV shows that the large platelets are indeed single crystals with an average thickness of 5 k lpm because the magnitude of this absorption agrees with that of the 5 pm thick single crystal. The absorption spectrum of the smaller platelets is also shown as the upper curve in figure 5.
In this case, the spectrum must be corrected
for the difference in thickness by normalizing the spectrum at 1.5 eV. The low-energy adsorption due to defects is an order of magnitude greater in the small platelets than it is in the large ones. From these data it is evident that the optical adsorption observed below 1.2 eV in the platelets is due to the exposed edges planes. This is because SEM studies of the small and large platelets revealed that the small platelets have a greater edge plane area per gram than that of the large micro platelets. In fact, a statistical study of micrographs of these samples showed that the "edge sets" density of the small platelets was 6.1 x 1017 sites/gm and that of the large platelets was 7.2 x 1016 sites/gm (ref. 45).
"Dangling bonds",
vacancies, or other similar surface defects would be expected to have electronic states in midgap and thus increase the optical adsorption in this From the known density of edge sites (N) the average optical
region.
absorption (G) of a single edge site can be calculated by A - N o yielding 6.1 x 1017 cm2 for the small platelets and 8.4 x 1017 cm2 for the large platelets. The agreement between these two numbers is excellent and shows that the low-energy absorption is indeed proportional to the edge area. The catalytic activity of the microplatelets could be determined directly (ref. 45).
The HDS of dibenzothiophene (DBT) was measured. Biphenyl was the
only product observed with no hydrogenation occurring. Conversion of DBT with time yielded a straight line below 15% conversion and the slope of this line an HDS rate
- 4.8 x 1OI6 molec/g-s was determined at 350°C and
450 p.s.i. H2.
From this and the density of edge sites determined above a turnover frequency of 7.9 x 10-2 molec/edge site-s was determined. This calculation assumes that each exposed Mo atom is catalytically active; it is, of course, possible that only a fraction of the edge sites is active in which case the appropriate turnover number would be higher. Nevertheless, we believe that this is the only turnover number for MoS2 which has been determined without an ambiguity in the edge plane dispersion. Because of this, this number becomes the basis for further studies. The above result has been extended to MoS2 unsupported powder where because of disorder knowledge of edge area has been limited to oxygen chemisorption
13
studies. A series of powders was prepared by decomposing (NHq)2MoSq at different temperatures from 350°C to 900°C. The optical spectra of these samples showed a strong broad adsorption tail below the band-to-band absorption which is dependent on the anneal temperature. This adsorption is very similar to that observed from edge plane defects in the platelets with a slight difference in shape due to disorder. The catalytic activity of these powder for the HDS of DBT was measured and a linear correlation between the activity and the adsorbance was observed. Assuming that the absorption cross section is the same in both materials, the turnover frequency calculated from the slope of the Absorption/Activity plot was 3 x 10-2 molc/edge sites. This value is approximately two times lower than that obtained from the platelets, an agreement which is reasonable given uncertainties in the size and density of the disordered materials. It is also possible that disorder induces sites which, while counted by the PDS method, are not catalytically as effective or as accessible as those on well ordered materials. The similarity in turnover frequency between the disordered and micro crystalline materials indicates that the active sites for desulfurization in each are similar and are located on the edge surfaces. Such defects which are catalytically active, would generally be expected to have energy level lying between the conduction and valence bands and thus absorb photons with below bandgap energies. This is indeed the behavior observed,and the -10-16-cm2 cross section observed is typical of such defects. In fact, a recent set of Xo calculations which modeled different types of sulfur vacancies which could occur at MoS2 edges; showed that allowed optical transitions for these defects, fall into the observed energy ranges below 1.2 eV (ref. 48). These results suggest that sulfur vacancies are responsible for the optical absorptions measured for the edge planes. It is also noted that for a similar set of samples a turnover frequency of 1.2 x 10-2 molec/site-s using 02 chemisorption was obtained (figure 6).
Again
this emphasizes that more molecules of 02 are chemisorbed per active site due to bulk oxidation. Furthermore, it was noted that the turnover number for the platelets was based on production of biphenyl only.
In the powders as much as
50% cyclohexyl-benzenewas produced indicating multiple sites. Therefore, at this writing we feel that the highest turnover number on geometrically well determined material producing a single product is the most reliable measurement.
This work presumably can be extended to supported catalysts as well.
However, the extension to promoted systems is not quite as straight forward because the presence of Co or Ni modifies the semi-conducting properties of MoS2 confusing the interpretation of the measured optical spectra. The above studies were performed on conventionally prepared microcrysalline materials. These materials are difficult to study because they have relatively low edge area because growth occurs primarily in the direction parallel to the
14
0 0 0 capacity
edge site density
I
5
15
10
HDS rate ( lo1
rnolecules/g/s
Figure 6:
1
10
Thickness
I
Figure 7:
l - r - d I _ L 1 - l - l - l L r
I
(Iim)
I
I
15
layers. A well-ordered edge surface is difficult to create by cutting or polishing because the layers fold and break irregularly. However, we recently reported a new way of preparing chemically reactive surfaces by using lithographic fabrication methods (ref. 4 9 ) .
Single crystals of MoS2 prepared in
this way have a surface that consists primarily of edge planes which allows exceptional control of the surface morphology. These microstructures are also ideal for fundamental studies of edge surface properties described above.
PDS studies of samples of MoS2 prepared in this manner are shown in figure 7.
In the figure the single crystal spectrum is shown and above it a textured
I
I
I
Textured
I
Mo Defect
.**
*. ..
Mo
Flat
312 ( 3 d ) 512
I .
I
238.0 Figure 8 :
I
235.0
I
I
232.0 229.0 Binding Energy (eV)
I
226.0
Mo 3d core levels of a textured and a flat crystal.
223.0
16
sample from the same crystal. Again, we see that creating edge plane creates the same defect absorption below 1 . 2 eV described above. The edge defects were also observed in x-ray photo emission spectroscopy. Figure 8 shows the Mo 3d core levels of a textured and flat crystal. The textured crystal was treated in H2/H2S at 350°C to reduce and resulfide the surface. The edge surface spectrum is considerably broader than the spectrum of the basal surface and is also shifted to lower energy. The spectrum of the textured sample can be resolved into two peaks as shown in the figure. The spectrum for the textured sample has an additional component that is shifted 0 . 8 eV to lower energy. These two components gave a good fit to the entire spectrum and showed that the edge defects contain Mo that is reduced relative to the bulk. The shift of 0 . 8 eV is about that expected for reduction of M o + ~to Mo+3 in sulfide compounds.
UPS measurements also showed that the Fermi level shifted 0.8 eV closer to the valence band upon texturing. Because this shift is nearly as large as the band gap (1.2 eV) the Fermi level of the edge surface must be within -0.3 eV of the valence band maximum. This implies that most of the edge surface defects within the band gap would be unoccupied, and that optical transitions would involve the excitation of electrons out of the valence band into the defect level. Such transitions would lead to the monotonic increase in absorption with photon energy and absorption cross section observed.
SUMMARY In this article we have presented a brief review of the status of our current fundamental understanding of the TMS based catalysts which will play an increasingly important role in the petroleum, synthetic fuels and chemical industries. The fundamental origins of the catalytic properties of the TMS are completely contained in the unsupported active TMS phases with the support playing a secondary role in enhancing properties required for industrial application. Foundation knowledge for understanding the fundamental properties of the TMS is found in the periodic trends for HDS, HDN and hydrogenation reactions and theoretical electronic trends for the simple TMS. These trends emphasize the importance of the 4d and 5d electrons in the most active catalysts. These trends also form a basis for understanding promotion as arising from the same source; i.e., optimization of the maximum number of 4d and 5d electrons. A question which remains to be answered regarding the periodic effects is the role of H2
since all currently known trends have
similar shapes for different reactions which have H2 in common. All recent models o f the promoted system involve Ni or
Co
somewhere near the
edge of MoS2 or WS2 phase separating at higher concentrations of Ni or Co to the corresponding sulfides of Ni or Co with MoS2 or WS2.
The precise model of
17
the Co or Ni near the edge of MoS2 or US2 is still controversial. No one model giving the precise structure of the active phase is consistent with all experimental evidence found in the literature. This situation exists because there is lack of conclusive evidence regarding the degree of dispersion of the MoS2 or the Co concentration at the MoS2 edges. However, promotion requires interaction between Mo(W) 4d electrons and Co(Ni) 3d electrons which results in net charge transfer and an increase in the number of 4d electrons in the highest occupied 4d orbitals of the MoS2. This results in either an active site which is more active than are unpromoted site (electronic promotion) or stabilization of more active sites (structural promotion).
It is also possible
that both mechanisms result but the weight of evidence seems to suggest that electronic promotion is dominant. However, detailed quantitative evidence regarding the dispersion is required to settle the question. Disorder in these systems represents a major stumbling block in determining dispersion in these anisotropic systems. Nevertheless, Co MES and Co NMR are yielding new information about these systems and new insite into the role of promotors. Complete understanding of the catalytic properties of MoS2 (WS2) require more knowledge of the "edge planes" which terminate the anisotropic layers and are the location of the catalytically active "sites". Physical "edge plane based" derived from chemisorption and activity measurements exist which fit observed data well. But again, absence of absolute knowledge of MoS2 dispersion tends to lead to models which greatly underestimate crystallite sizes of MoS2. The bulk electronic structure of MoS2 is related to the catalytically active surface states. The dz2 orbitals are the highest occupied molecular orbitals of MoS2 in the +4 state; the catalytic states, created by edge termination, lie just above them and are probably in the +3 state when operating in a catalytic environment. Optical and electron spectroscopic techniques directly measure these defect states and catalytic measurements on geometrically well determine catalysts yield an "HDS edge plane turnover number'' which does not suffer from an ambiguity in dispersion. This turnover number now enables MoS2 dispersion to be determined in all unpromoted MoS2 catalysts. This result should lead to more precise models of promoted MoS2 catalysts in the future.
REFERENCES 1 0. Weisser and S . Landa, Sulfide Catalysts: Their Properties and Applications, Pergamon, Oxford, 1973. 2 E. E. Donati, Adv. Catal., 8, 39 (1956). 3 G . C. A. Schuit and B. G . Gates, AIChE J., 19, 417 (1973). 4 A . L. Farragher and G . W. J. Heimerixx, 4th International Conference on the Chemistry and Uses of Molybdenum, August 1982, Golden Colorado, P. 432.
18
5 C. B. Murchison, M. M. Conway, R. R. Stevens, G. J . Quarderer, Proceedings 9th International Congress on Catalysis, u2, 626-633 (1988). 6 R. R. Chianelli, Catal. Rev.-Sci. Eng., 26 (3&4), 361-393 (1984). 7 T. A. Pecoraro and R. R. Chianelli, J. Catal., 67, 430 (1981). 8 J . P. R. Vissers, C. K. Groot, E. M. Vanoers, V. H. J. deBeer and R. Prins, Bull. SOC. Chim. Belg., 93, 813 (1984). 9 M. J . Ledoux, 0. Michaux, G. Agostini and P. Pannisod, J . Cata . , 102, 275 (1986). 10 S. Eijsbouts, V. H. J. deBeer, R. Prins, J . Cat., J.09,217-220 (1988). 11 M. Ledoux, et. al., Personal Communication, accepted J. Cat., 988. 12 M. Daage and R. R. Chianelli, Catal. Letts, in press. 13 J . P. Bonnelle, et al., Personal Communication, (1987). 14 J . Sinfelt, Proe. Solid State Chem. lO(2), 55 (1975). 15 M. M. Jaksic, J . Molecular Catalysis, 38, 161-202 (1986). 16 S . Harris and R. R. Chianelli, J. Cat., 86, 400-412 (1984). 17 M. Ternan, J. Cat., 104,256-257, (1987). 18 S . Harris and R. R. Chianelli, J . C a t . , 98, 17-31 (1986). 19 R. J . H. Voorhoeve and J . C. M. Stuiver, J . Cat., 23, 228 (1971). 2 3 , 243 (1971). 20 A. L. Farragher and P. Cossee, Proceedings, 5th International Congress on Catalysis, Palm Beach, 1972. (J. W. Hightower, Ed.), P 1301, North-Holland, Amsterdam, (1973). 21 B. Delmon, Proceedings, 3rd International Conference on the Chemistry and Uses of Molybdenum, 73-85, Ann Arbor, Michigan, Aug. 19-23, 1979. 22 D. Pirotte, J. M. Zabala, P. Grange and B. Delmon, Bull. SOC. Chim. Belg., 90, 1239, (1979). 23 R. R. Chianelli, T. A. Pecoraro, T. R. Halbert, W. -H. Pan and E. I. Stiefel, J. Cat., &, 226 (1984). 24 R. W. Phillips and A. A. Fote, J. Cat., 41, 168 (1976). 25 F. B. Garreau, H. Toulhoat, S . Kasztelan and R. Paulus, Polyhedron, Vo. 5. ~211-217(1986). 26 P. Ratnasamy and S. Sivasanker, Catal. Rev-Sci. Eng. 2 , 401 (1980). 27 C. Wivel, R. Candia, B. S . Clausen and H. Topsee, J . C a t . , 68,453 (1981). 28 H. Topsee, B. S . Clauson, R. Candia, C. Wivel and S . Merup, J . Cat., 68, 433 (1981). 29 (a) B. S . Clausen, H. Topsee, B. Lengeler and R. Candia, in "EXAFS and Near Edge Structure 111" (K. 0. Hodgson, B. Hedman and J. E. Penner-Hahn,Eds.), Springer Proceedings in Physics 2 , Springer-Verlag,Berlin, Heidelberg, 181 (1984). (b) B. S. Clausen, H. Topsse, B. Lengeler and R. Candia, in "EXAFS and Near Edge Structure 111" J . Cat. 98, 32;51 (1986) 30 A. M. Van der Kraan, M. W. J. Craje, E. Gerbema and W. L. T. M. Ramselaar, Applied C a t . 3,L7 (1988). 31 J. P. R. Vissers, V. H. J. de Beer and R. Prins, J . Chem. SOC. Faraday Trans 1 , 82, 2145 (1987). 32 M. J . Ledoux, 0. Michaux, G. Agostini and P. Panissod, J . C a t l . , 102, 275 (1986). 33 M. J . Ledoux, G. Maire, S . Hantzer and 0. Michaux, Proceedings 9th International Congress on Catalysis, -, 74-81 (1988). 34 R. R. Chianelli, Solid State Chemistry in Catalysis, ACS Symposium Series 2 7 9 , 13, ~221-234(1985). 35 R. R. Chianelli, E. B. Prestridge, T. A. Pecoraro, J. P. DeNeufville, Science, 203, 1105 (1979). 36 R. R. Chianelli, International Reviews in Physical Chemistry, 2, 127-165 (1987). 37 M. S . Whittingham and A. J . Jacobson, eds, in: Intercalation Chemistry (Academic Press, New York, 1982). 38 S . J . Tauster, T. A. Pecoraro and R. R. Chianelli, J . Cacal., 63, 515 (1980) . 39 R. Fretz, M. Breysse, M. Lacroix and M. Vrinat, in: Second Workshop on Hydrotreating Catalysts (Louvain, la Neuve, Oct. 1984).
19
40 R. R. Chianelli, A. F. Ruppert, S . K. Behal, B. H. Kear, A. Wold and R. Kershaw, J . C a t a l . 2 , 56 (1985). 41 N. K. Nag, K. Sai Prasada Ras, K. V. R. Chary, B. Rama Rao and V. S. Subrahmanyam, Applied C a t . , 4, 165-176 (1988). 42 S . Kasztelan, H. Toulhoat, J. Grimblot and J . P. Bonnelle, Bull. SOC. Chem. Belg., 89, 807 (1984). 43 S . Kasztelan, H. Toulhoat, J. Grimblot and J. P. Bonnelle, Applied C a t a l . , 13, 127 (1984). 44 K. S . Liang, G . J. Hughes and R. R. Chianelli, J . Vac. Sci. Tech. A2(2), 991-994 (1984). 45 C. B. Roxlo, M. Daage, A. F . Ruppert and R. R. Chianelli, J . C a t a l . , 100, 176-184 (1986). 46 W. B. Jackson, N. M. Amer, A . C. Baccara and D. Fournier, Appl. Opt., 2oW, 1333 (1981). 47 A . M. Goldberg, A. R. Beal, F . A. Levy and E. A. Davis, Phil. Mag., 2 , 367 (1975). 48 J. Horsley, personal communication. 49 C. B. Roxlo, H . W. Deckman, J. Gland, S . D. Cameron and R. R. Chianelli, SCIENCE, 235, 1629-1631 (1987).
This Page Intentionally Left Blank
M.1,. Occelli and R.(i. Anthony (Editors),Advances in Hydrotreating Catalysts 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
63
STACKING OF MOLYBDENUM DISULFIDE LAYERS IN HYDROTREATING CATALYSTS
R. C. RYAN, R. A. KEMP, J. A. SMEGAL, 0. R. DENLEY and G. E. SPINNLER Shell Development Company, P. 0. Box 1380, Houston, Texas 77251 ABSTRACT Over the past several years there has been an intensive effort reported in the open literature addressing the nature of the active site(s) in supported hydroprocessing catalysts. Generally, these catalysts are either nickel- or cobalt-promoted molybdenum disulfide supported on an alumina carrier. Several different theories describing the promotion effect of nickel or cobalt have been proposed, each to some extent excluding the other theories. We have recently prepared a number of alumina based Ni/Mo and Co/Mo catalysts designed to aid us in understanding the roles played by both the molybdenum disulfide and the promoter metal. In addition, we were also interested in examining the effect of phosphorus addition to these catalysts. These catalysts have been examined by X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscdpy (HRTEM). Our results show that the primary difference between nickel promoted and cobalt promoted catalysts after sulfidation is the extent of stacking of the resulting molybdenum disulfide layers. Nickel promoted catalysts, generally used industrially to remove nitrogen from crude oil feedstocks, have molybdenum disulfide stacks of 5-6 layers while cobalt promoted catalysts, most useful industrially for sulfur removal, appear to contain molybdenum disulfide primarily as monolayers. If phosphorus i s present in the impregnating solution then the number of stacks increases for both nickel- and cobalt-promoted catalysts while the length of the molybdenum disulfide crystallite decreases. A reason for this difference in behavior can be attributed to the preference of the nickel and the cobalt to be interlayer coordinated and tetrahedrally coordinated, respectively, when sulfided under hydrotreating conditions. INTRODUCTION In the area of hydroprocessing significant advances have been made over the years in catalyst activity and stability. Improvements have primarily focused on optimization of carrier properties and metal loading but the basic components of these catalysts have remained the same for over 40 years. These catalysts are either nickel- or cobalt- promoted molybdenum or tungsten sulfide systems supported on an alumina carrier. While the catalysts have received wide commercial acceptance there is still a lack of fundamental understanding into the nature of the active site in hydroprocessing catalysts. Over the last 20 years no less than six
21
22
theories have been advanced in the literature to account for the catalytic activity of the MoS2 and WS2 based catalysts. Four of these have received the greatest attention. The monolayer model was first proposed by Lipsch, eta1 in 1969[11. They dealt with the oxide form of Co/Mo catalysts and concluded that the molybdenum is dispersed in a monolayer as Moo3 over the alumina surface while the cobalt is distributed throughout the support as cobalt aluminate. Further refinement of the model was provided by Sonnemans, eta1[2-6] while Schuitt etal. introduced the concept of the epitaxial character of the monolayer[7]. The role of the promoter (Co or Ni) was investigated by Cimino, eta1 who found that in agreement with the model a certain portion of the promoter cations pentrates some distance into the support, Co preferring tetrahedral sites, and Ni, octahedral sitesI8l. The pseudointercalation model was the next to be proposed, this by Voorhoeve, etal[9] and later modified by Farragher, etal[lO-111. It starts from the layer structure of MoS2 or WS2 where the metal cations occupy trigonal prismatic sites. The cationic sites between successive sulfur layers being alternately all empty or all filled. Because of the symmetry of the trigonal prismatic crystal intercalation between the empty sulfur layers should not occur. However, this model proposes that intercalation. of the promoter atoms occurs only between the edges of the molybdenum or tungsten disulfide crystals. Evidence was presented, primarily by electron spin resonance (ESR), that nickel resides in the van der Waals gaps between the WS2 layers. The observed ESR signal was assigned to W3+. A correlation was found between ESR signal strength and the rate of hydrogenation of benzene but not with the much faster cyclohexene hydrogenation. This indicates that two different sites are responsible for aromatic and olefin hydrogenation. It should be noted that the VoorhoeveFarragher-Cossee model considers the carrier only as a diluent and useful for dispersing the active sites. A later model attempting to explain the promotion effect of Group VIII metals on the Group VI metal sulfides has been termed the "contact synergy" or "remote control" model [ 12-14]. In this model the "synergistic" effect is a result of the mere contact of the Group VIII sulfide (e.g., CogSB) with the Group VI sulfide (e.g., MoS2). Oelmon has proposed that the interaction might be electron transfer at the junction or perhaps hydrogen atom spillover from one phase to the other. Also in the contact synergy model, which incorporates very thorough structural data on the sulfides, the influence of the carrier is not considered essential. The most recent work in the area of Co/Mo catalysts has been the Co-Mo-S model, developed in a series of papers by Topsoe, eta1[15-16]. As in
23
the pseudointercalation model Topsoe suggests that the cobalt is located at the edge of a molybdenum disulfide crystallite, however, there is no stacking of MoS2 to form multiple layers as suggested earlier by Voorhoeve. A further refinement of this model has been proposed recently where two different Co-Mo-S phases were identified. For alumina-supported Co-Mo catalysts high temperature sulfiding studies have revealed the existence of a "low-temperature" (Type I) and a "high-temperature' (Type 11) Co-Mo-S structure[l71. A number of analytical techniques were used to characterize these sulfide phases. These methods include X-ray photoelectron spectroscopy ( X P S ) , infrared spectroscopy (IR), highresolution transmission electron microscopy (HRTEM), Mossbauer emission spectroscopy (MES), and extended X-ray absorption fine structure (EXAFS). However, much of the primary evidence for the Co-Mo-S phases is based on MES which is unfortunately not useful for nickel promoted catalysts. The vast majority of the previous work in the literature is concerned only with desulfurization activity of sulfided cobalt-molybdenum catalysts and these studies have primarily used model feeds to determine catalyst activity. These catalysts have been synthesized by a wide range of methods. Some of the studies have centered on the use o f unsupported catalysts (prepared by comaceration or homogeneous sulfide precipitation techniques) while others have been concerned with catalysts prepared by impregnating alumina carriers, either in single or multiple steps. Other supports such as silica[l81 and carbon[l9-211 have also been studied. With this wide variety o f synthetic methods that have been used in the past it is not surprising that different catalytic activities were found and various conclusions about the active site drawn. It was of interest to us to examine in a fundamental sense the optimal catalysts we have - Co/Mo and Ni/Mo catalysts prepared using a single impregnation step on an alumina carrier. Since Co/Mo catalysts are generally used for hydrodesulfurization (HDS) and Ni/Mo catalysts are often used for hydrodenitrification (HDN) [ 2 2 ] we were interested in any structural differences we might observe in the two types of catalysts. Although there is interest in understanding the nature of the active site in hydroprocessing catalysts there is also a need to develop more active commercial catalysts. One area of interest, common to all catalysts, is the use of promoters. For the Co/Mo and Ni/Mo based systems a wide variety of promoters have been claimed such as phosphorus[23-241, silicon[251, and titanium[26]. Of these phosphorus is of most importance because of its use in a number of commercial catalysts. Our interest in phosphorus containing catalysts stems not only from its ability to stabilize high metal content
24
solutions but also its promotion effect on alumina based Co/Mo and Ni/Mo systems for the HDN reaction. This paper summarizes our analyses of impregnated Ni/Mo and Co/Mo catalysts using HRTEM and XPS. The effect of phosphorus as a promoter in these systems is also examined. An attempt is made to explain the observed differences in stacking of molybdenum disulfide layers found in the catalysts. Similarities and differences with other theories will also be provided. EXPERIMENTAL All catalysts were prepared by conventional, one-step impregnations of gamma-alumina extrudates. The alumina carrier had a surface area of 260 m2/gm, a water pore volume of 0.77 cc/gm, and a compacted bulk density of 0.56 gm/cc. For the phosphorus containing catalysts nickel or cobalt nitrate, ammonium heptamolybdate, and phosphoric acid were used while the non-phosphorus systems used the appropriate metal carbonate and ammonium dimolybdate dissolved in aqueous ammonium hydroxide. The desired metal salts were dissolved in a volume of water essentially equivalent to the total pore volume of the support. Impregnation of the carrier was followed by drying in air at 120°C for two hours and calcination for two hours at 482°C. Each catalyst is identified with its molar metal ratio calculated as [(Ni or Co)/Mo, Table 11. The total moles of metal was kept constant at 1.87 mmol/gm of catalyst. TABLE 1 Chemical Properties for Impregnated Catalysts Catalyst Co/Mo(O. 37) Co/Mo/P(0.37) Mo(O.00) Ni/Mo/P(O. 10) Ni/Mo/P(0.37) Ni /Mo/P( 0.50) Mo/P (0.00) N i /Mo (0.37)
%w
Ni
____ 1.0 3.0 4.5
---3.0
%w
co
%w Mo
%w
P
13.0 13.0 17.9
----
16.2
3.2
13.0 10.5 17.9 13.0
3.2 3.2 3.2
3.2
__-_
Mol ar Surface Ratio Area ( d / g m ) ~-
0.37 0.37 0.00 0.10 0.37 0.50 0.00 0.37
zoo 157 189 139 157 166 134 200
The XPS results were obtained on a VG ESCALAB M k I I instrument. Catalysts were sulfided in a 5% HzS/H2 gas stream for one hour at 350OC. After cooling, the samples are sealed while under flowing H2 and then transferred to an Argon-filled glove box. All samples were ground in the glove box, mounted on a sample stage and transferred to the spectrometer under
25 A r . The observed XPS s p e c t r a l i n t e n s i t i e s were corrected w i t h e m p i r i c a l
s e n s i t i v i t y f a c t o r s obtained from bulk reference compounds such as MoS2, Mo03, Ni3S2, NiMoOq, and CoAl2O4. The XPS b i n d i n g energies were determined using the A1 2s reference l i n e a t 119.8 eV. The HRTEM d a t a were c o l l e c t e d on a P h i l i p s 430T instrument u s i n g s u l f i d e d samples. The s u l f i d e d samples were ground i n acetone under atmospheric c o n d i t i o n s and suspended on Cu-mesh g r i d s covered w i t h a holey-carbon support f i l m . RESULTS
xps One technique t h a t has been s u c c e s s f u l l y used t o c h a r a c t e r i z e Co-Mo/A1203 c a t a l y s t s i s XPS[27]. Therefore,
i t was o f i n t e r e s t t o use t h i s
technique t o c h a r a c t e r i z e both Ni/Mo and Co/Mo systems. A summary o f t h e
XPS b i n d i n g energy data i s found i n Table 2.
TABLE 2 XPS Binding Energies(eV) f o r S u l f ided Catalysts* Co/Mo Metal R a t i o Element Line N i o r Co 2 ~ 3 1 2 Mo 3~312 p 3P s 2P 0 1s c 1s
Co/Mo/P
0.37
0.37
780.2 395.9
780.4 396.0 135.1 162.7 532.4 285.4
---
162.6 532.3 285.3
Mo
N i /Mo/P
0.00
0.10
0.37
0.50
---
855.4 395.9 134.9 162.5 532.1 285.0
855.1 395.9 135.0 162.7 532.7 285.2
855.1 395.8 135.0 162.5 532.3 285.2
395.9
---
162.5 532.1 285.0
Mo/P
Ni/Mo
0.00
0.37
--395.7 135.0 162.5 532.4 285.2
855-6 395.8
---
162.6 NA NA
*Reference Line i s A l ( 2 s ) a t 119.8 eV
The b i n d i n g energies f o r s u l f u r , carbon, and c o b a l t agree w i t h published values f o r s i m i l a r alumina supported Co/Mo c a t a l y s t s [ 2 7 1 . For t h e two c o b a l t preparations t h e r e i s l i t t l e change i n t h e b i n d i n g energies f o r t h e components i n d i c a t i n g o n l y minor v a r i a t i o n i n t h e e l e c t r o n i c environment o f t h e Co o r Mo due t o t h e i n c l u s i o n o f phosphorus. This was n o t t h e case f o r t h e n i c k e l c a t a l y s t s . The a d d i t i o n o f phosphorus caused a decrease i n t h e N i b i n d i n g energy. This can be seen by comparing t h e N i 2 ~ 3 1 2b i n d i n g energy f o r NilMo(0.37)
w i t h Ni/Mo/P(0.37).
This difference
o f 0.5eV i s approximately what has been seen p r e v i o u s l y f o r a s i m i l a r series of catalysts[28]. The use o f XPS f o r measurement o f p a r t i c l e s i z e / d i s p e r s i o n o f supp o r t e d metal c a t a l y s t s has been r e p o r t e d i n t h e l i t e r a t u r e [ 2 9 - 3 1 ] . For any
26
technique to be considered for determining dispersion it should probe the catalyst surface, be sensitive to the metal and discriminate against substrate material. These requirements can be met by using the XPS technique since it only detects atoms found in the "outer layers" of a material and it can discriminate among elements. This is because XPS sensitivity decreases exponentially with the decay length set by the electron mean free path, A , which ordinarily is in the range of 0.5-2 nm. The effect of this surface sensitivity is that, when metal oxide crystallites of size, d, supported on a substrate are analyzed, the measured intensity increases roughly as the total crystallite surface area exposed. By considering the total metal concentration (crystallite interior plus exterior), simple calculations can yield a relative dispersion, 0, and, if a particle geometry is assumed, a crystallite size, d. It should be noted that practical considerations require the concentrations to be measured as fractions of the total surface concentration or relative to the substrate rather than in absolute units. Because the value of A is only estimated, there is less uncertainty in the determination of dispersion than in the crystallite size, which is more sensitive to A . Therefore, we have concentrated our attention on 0. The XPS intensity that is measured is a function of the total surface concentration. When comparing a series o f catalysts it is important to take into account various effects that could cause the observed surface concentration as measured by XPS to change. Three of these effects are: 1) changes in metal dispersion, 2) changes in bulk loading, and 3) changes in available substrate surface area. Bulk loading is expressed as a bulk atomic ratio, BAR, which is the ratio of the number of moles of an element relative to the number o f moles of aluminum in a sample. By using these values with the surface atom ratios, SAR, from XPS we can separate the effects of bulk loading and dispersion. It is possible to simply take the ratio of ratios, 0 = SAR/BAR, and thereby eliminate the variations that will be seen in the XPS intensity, but which are merely due to variations in the catalyst loading for a fixed dispersion. This figure of merit can readily be seen to contain the effects of specific surface area. To correct for this and also the fact that the measurement is not strictly confined to the surface and goes to a characteristic depth, h (the electron mean free path) the dispersion, 0, can be normalized to predicted maximum value of dispersion[321. The expression for dispersion, 0, can be found in eqn. 1. 0
=
[SAR/BARI [tanh(T/2)/(T/2)]
21
where T = ( 2 / p S o h ) is the effective thickness of the alumina support relative to the electron escape depth, p is its skeletal density (3.01 g/cc), and SO is the specific surface area. For this study the electron mean free path is assumed to be the same for all the materials and is given the value of 1.43 nm as determined from A1203 data(331. The results of this analysis for the oxide catalysts are found in Table 3.
TABLE 3 XPS Analysis-Metal Dispersions of Oxide and Sulfide Catalysts and Percent Molybdenum Oxide in Sulfided Catalysts Co/Mo
N i /Mo/P
Mo
Co/Mo/P ~
Metal Ratio
0.37 Co Mo
Mo/P Ni/Mo
~
0.37 0.00 Co Mo Mo
0.10
0.37
Ni Mo N i Mo
0.50
Ni Mo
0.00 Mo
0.37 N i Mo
DISPERSION Oxide
.64 .65
.31 .61
.64 .39 .50 .46 .57 .49 .58 .52 .64 .65
Sulfide
.56 .59
.24 .53
.53 .62 .42
%Moo3 in Sulfide
23
22
20
20
.62 .56 .48 .48 .20 20
20
26
.63 .61 23
The dispersion results for molybdenum only catalyst, Mo(O.OO), indicates a fairly well dispersed system. The addition of phosphorus to this Mo only catalyst, Mo/P(O.OO), significantly decreases the Mo dispersion while with the addition of nickel or cobalt to the catalyst the molybdenum dispersion stays constant [compare Ni/Mo(0.37) and Co/Mo (0.37) with Mo(O.OO), Table 31. The effect of changing the amount of nickel promoter was also examined. As the nickel metal loading was increased from 1.0%~ to 3.0%~ there was an increase in nickel dispersion from 0.39 to 0.46 and an increase in molybdenum dispersion from 0.50 to 0.57 (Table 3). A further increase in the nickel loading to 4.5%~ [Ni/Mo/P(O.SO), Table 31 does not change the Mo dispersion and only slightly increases the Ni dispersion when compared to the Ni/Mo/P (0.37) catalyst with 3.0%~ Ni loading. The addition of phosphorus to the cobalt catalyst also resulted in a decrease in both the cobalt and molybdenum dispersions. It was also of interest to examine the metal dispersion of the sulfided catalysts and to compare the results to the oxide systems. Although all catalysts were sulfided with 5% H2S/H2 at 350°C for one hour a close examination of the Mo 3d5/2 and 3p3/2 peaks revealed that a minor amount o f oxidized molybdenum species was present. An example of this can be seen in the Mo 3 ~ 3 1 2peak for Mo(O.00) (Fig. 1). This oxidized Mo species represents approximately 20% of the molybdenum signal. This
28
oxidized molybdenum may be Moo3 and was found in all samples in amounts of 20-26% of the total molybdenum species (Table 3). Sulfiding procedures using times of up to 17 hours decreased the oxide component to only approximately 14%. No attempts were made to sulfide at higher temperatures than 350°C. The dispersion results for the sulfided catalysts are summarized in Table 3. A comparison of the cobalt and molybdenum dispersion results for the oxide systems (Table 3) with the sulfide catalysts reveals that there is a loss of Co and Mo dispersion upon sulfiding. In some cases this decrease is quite dramatic as is the case for Mo/P(O.OO) where the molybdenum dispersion decreases from 0.52 to 0.20 in the sulfide form. For the nonphosphorus containing catalysts a similar pattern is found in the Ni dispersion as was found for Co and Mo and that is a decrease in the metal dispersion upon sulfidation (compare dispersions for the respective oxide and sulfide catalysts, Table 3). This is not the case with the nickel plus phosphorus promoted catalysts, the Ni/Mo/P(O.lO) and Ni/Mo/P(0.37) increase significantly in nickel dispersion while the Ni/Mo/P(0.50) catalyst remains essentially constant. 8000
6000
4000
2000
Binding Energy, eV
Fig. 1. XPS Mo 3 ~ 3 1 2spectra for sulfided Mo(O.00)
catalyst.
29
HRTEM The HRTEM data for the sulfided hydrotreating catalysts show several interesting features. In the samples where MoS2 lattice fringes were observed, they corresponded to the approximately 0.62 nm separation of the basal planes of the molybdenite structure. Imaging calculations show that when the MoS2 basal planes are oriented parallel to the electron beam (and at approximate defocus) each spacing from dark fringe to dark fringe on the micrograph corresponds to a structural layer of MoS2. In general, the molybdenum-only or the Ni/Mo based catalysts show stacking of the resulting MoS2 layers. A typical HRTEM micrograph of the molybdenum-only [(Mo(O.OO))] sample is shown in Fig. 2. Immediately apparent are the long, multiple curving layers of MoS2 that can be seen as dark fringes representing the basal plane. However, careful analysis of the HRTEM images also reveals that in addition to the MoS2 images there are two other distinct types of particles. Identification of these materials was
Fig. 2. High resolution TEM micrograph of sulfided Mo(O.00) catalyst revealing a blocky Moo3 particle surrounded by 0.62nm MoS2 fringes. The MoS2 layers form a continuous covering around the particle. A layer dislocation where one layer terminates is arrowed.
attempted using Energy Dispersive X-ray (EDX) analysis and Electron Energy Loss Spectroscopy (EELS). One material that appeared "spongy" was easily
30
identified as the alumina support. Another "blocky-type" particle consisted of a crystalline molybdenum oxide type material. The use of Selected Area Diffraction (SAD) and Convergent Beam Electron Diffraction (CBED) was not successful in identifying this compound. The "blocky-type" molybdenum oxide material was coated with several layers of MoS2. Tilting experiments in the TEM indicate that the layers of MoS2 completely surround the particles. This same observation was made recently with carbon supported Co/Mo catalysts where after a partial sulfidation the particles exhibit sulfide layering around residual oxide particles[34]. Upon addition o f nickel and/or phosphorus to the catalyst there is substantial reduction of the length in the lateral direction of the MoS2 crystallite when compared to the Mo(O.00) case. This can be seen in figure 3, a HRTEM micrograph of sample Ni/Mo/P(0.37). The MoS2 fringes were readily observable in the HRTEM images. It appears that the MoS2 packetsts consist of a few more layers than in the Mo(O.00) sample but the packets of sheets are not continuous and exhibit numerous dislocations.
fig. 3. High resolution TEM micrograph of sulfided Ni/Mo/P(0.37) catalyst revealing multiple (5-10) layers o f MoS2. Dark fringes represent basal planes of MoS2.'
31
Interestingly, comparison of the Mo/P(O.OO) sample with no metal promoter with the Mo only sample [Mo(O.OO)l also shows a shortening of the MoS2 crystallite in the lateral direction. This is also the case with the nickel catalyst prepared without phosphorus, Ni/Mo(0.37). The situation is entirely different when cobalt is used as a promoter metal. The Co/Mo(0.37) catalyst can be used as an example (Fig. 4). The sulfided catalyst shows no tendency to form MoS2 stacks. The HRTEM micrograph shows only single layer MoS2 crystallites and not the multi layered structures seen in the Ni systems.
Fig. 4. High resolution TEM micrograph of sulfided Co/Mo/P(0.37) Arrows indicate single layers of MoS2.
catalyst.
In addition to examining the effect of changing the promoter from Ni to Co it was also of interest to study the combination of Co and P promoters. One catalyst, CoiMolP(O.37) , was prepared in the identical manner to its N i analog except cobalt nitrate was substituted for nickel nitrate. A HRTEM micrograph of Co/Mo/P(0.37) is shown in Fig. 5. It is apparent that the MoS2 microstructure is very similar to that o f Co/Mo(0.37), the other Co/Mo catalyst, and not to the Ni/Mo/P(0.37) or other Ni/Mo catalysts. However, close inspection does seem to indicate that there are more stacks of MoS2 in Co/Mo/P(0.37) than in Co/Mo(0.37).
32
These stacks are generally only two, perhaps three, layers thick and are significantly shorter in the lateral direction than those seen in the Ni/Mo cases.
Fig. 5. High resolution TEM micrograph of sulfided Co/Mo/P(0.37) catalyst revealing multiple (2-3) layers o f MoS2. Dark fringes represent basal planes o f MoS2.
DISCUSSION The XPS and HRTEM results presented here point out that significant differences in the crystallite morphology of MoS2 are obtained depending on the promoter that is used. For this series only nickel, cobalt, and phosphorus promoters were considered and only one alumina support was used. The differences that were noted were primarily in crystallite size in the lateral as well as in the vertical direction (number of stacks). We feel that these morphological changes can be explained by recognizing that the various promoters have vastly different chemical and structural requirements. These differences can be used to not only explain our data but also to unify previous theories on the active site in hydrotreating catalysts. The most simple system to examine is the molybdenum only, Mo(O.OO), case. Based on the value of the dispersion for the oxide, and further supported by the multilayers exhibited in HRTEM for the sulfided form for
33
which the dispersion is not too greatly different, one can conclude that monolayer coverage is not obtained in this catalyst. This is in contrast to what would be expected by the monolayer model[ll but the result should not be not surprising. Because this catalyst was prepared by pore volume impregnation it is reasonable to assume that the high metal loading of 26.9%~ Moo3 has been reacted with the surface by two different mechanisms. A portion of the molybdenum has chemically ion-exchanged onto the alumina surface while part of the molybdenum has been deposited during drying and calcining of the catalyst. This would result in Moo3 existing as "clumps" containing multiple layers. Upon sulfiding there are further changes in the surface morphology. Both the XPS and HRTEM data show that only a Portion of the surface is covered by MoS2; the MoS2 exists as a layered structure with 5 to 10 MoS2 layers, and approximately 20% of the Mo oxide is not sulfided with a standard sulfiding procedure. While these results are interesting their significance can only be appreciated by contrasting them to the systems promoted with Ni, Co, or P. Since most of the work in the literature has been concerned with cobalt promoted molybdenum disulfide catalysts we will focus on these cases next. The XPS and HRTEM results again present a complimentary picture for the Co/Mo(0.37) system. Only single layers of MoS2 are observed in the HRTEM and the XPS data indicate that the molybdenum is more dispersed upon sulfiding than is found in the molybdenum only case. This result is in agreement with the Co-Mo-S model proposed by Topsoe where the key feature is the location of promoter cobalt atoms along the edges of a monolayer of MoS2[15]. Topsoe has presented indirect evidence for this based on a variety of techniques such as EXAFS, MES, and IR. Direct evidence has recently been published[351 that shows the edge intercalation of cobalt in large crystals of the Co-Mo-S phase. The technique used in this study was analytical electron microscopy (AEM). Further evidence for this is found in an examination of the difference in binding energies between the cobalt 2p and sulfur 2p peaks. In an early study[26] by Topsoe and co-workers they found that a difference of 617.0 eV confirmed the assignment of cobalt being in a Co-Mo-S phase and not in a cogs8 phase which had a binding energy difference of 616.2 eV. For the Co/Mo(0.37) catalyst of this study the difference was 617.5 eV, similar to that found for Co-Mo-S phase. Since at least a portion of the promoter cobalt is associated with the molybdenum oxide in the calcined catalystl361, it is reasonable to predict that as the Mo oxide begins to sulfide the cobalt coordinates at the edges and prevents any further epitaxial crystal growth. This is also supported by the XPS results that indicate that the decrease in Mo dispersion that
34
is seen upon sulfiding i s less with catalysts containing cobalt. It has previously been shown that addition of cobalt to molybdenum based catalysts spreads out the the molybdenum oxide on the surface(371, thus creating a situation where vertical growth of stacks would not be favored. If this hypothesis of edge coordination is correct then there are two possible geometries for the cobalt atom. A simple substitution for one of the molybdenum atoms would place the cobalt in a trigonal prismatic coordination site because bulk molybdenum atoms in MoS2 are coordinated to six sulfur atoms. The cobalt atoms are not likely to adopt this unfavorable configuration but would probably adopt either an octahedral or tetrahedral coordination. The preferred form in sulfided species is tetrahedral. This can be seen by examining the form of cobalt sulfide, CogSg, that is stable under hydrotreating conditions. In this species eight of the nine cobalt atoms are tetrahedrally-~oordinated[38~.This tendency for cobalt to be tetrahedrally-coordinated explains why cobalt would have a tendency to intercalate at the edges rather than substitute for molybdenum in MoS2 and thus limit crystal growth near cobalt. A representation of what this active phase might look like on the alumina surface is presented in Fig. 6.
Fig. 6. Illustration of sulfided Co/Mo catalyst supported on alumina showing the cobalt tetrahedrally coordinated at the edge of a single layer of MoS2.
35
Strong evidence has been presented by Topsoe c o r r e l a t i n g these s i n g l e slabs o f Co promoted MoS2 as the a c t i v e s i t e s f o r d e s u l f u r i z a t i o n [ l 5 ] . While Co/Mo c a t a l y s t s are used commercially f o r t h e s e l e c t i v e removal o f s u l f u r species from a v a r i e t y of feedstocks these type c a t a l y s t s account f o r l e s s than 30% o f t h e h y d r o t r e a t i n g c a t a l y s t market. O f more i n t e r e s t commercially are c a t a l y s t s t h a t can combine s u l f u r and n i t r o g e n removal along w i t h aromatics s a t u r a t i o n . The primary c a t a l y s t s used f o r these h i g h s e v e r i t y a p p l i c a t i o n s are based on Ni/Mo. Because Ni/Mo and Co/Mo c a t a l y s t s are used f o r d i f f e r e n t a p p l i c a t i o n s and are n o t interchangeable
i t seems reasonable t h a t each type w i l l have d i f f e r e n t a c t i v e s i t e s . The Ni/Mo r e s u l t s w i l l be discussed next.
As p r e v i o u s l y mentioned t h e n i c k e l promoted c a t a l y s t s have stacks o f MoS2 w i t h as many as 4-8 l a y e r s i n c o n t r a s t t o t h e Co/Mo(0.37) where o n l y s i n g l e l a y e r s o f MoS2 were observed.
catalyst
It i s interesting t o
speculate why t h e n i c k e l promoted c a t a l y s t would f a v o r s t a c k i n g o f MoS2 l a y e r s and could t h i s be r e l a t e d t o c a t a l y s t a c t i v i t y d i f f e r e n c e s . Previous work w i t h a r e l a t e d system, Ni/WS2, by Voorhoeve,
e t a l . 19-11]
proposed t h a t t h e l o c a t i o n o f t h e n i c k e l promoter was a t t h e edge between s u l f i d e l a y e r s o f stacks o f WS2. They a l s o found t h a t t h e h e i g h t o f t h e WS2 c r y s t a l l i t e increases from 3.8 nm t o 5.1 nm upon a d d i t i o n o f n i c k e l i n an alumina supported c a t a l y s t and from 28 nm t o 58 nm i n an unsupported case. While MoS2 w i l l c e r t a i n l y form stacks w i t h o u t t h e a i d o f a promoter atom such as n i c k e l perhaps t h e n i c k e l can a c t as a s t a b i l i z e r f o r t h e molybdenum stacks. The edge i n t e r c a l a t e d o r pseudointercalated n i c k e l would be i n a d i f f e r e n t c o o r d i n a t i o n geometry i n these m u l t i - s t a c k e d systems from t h a t o f in-plane Mo s u b s t i t u t i o n a l s i t e s . While i t has p r e v i o u s l y been argued t h a t an octahedral c o o r d i n a t i o n f o r c o b a l t i s n o t favored under h y d r o t r e a t i n g c o n d i t i o n s t h e s i t u a t i o n w i t h n i c k e l i s l e s s c l e a r . No s t a b i l i t y arguments e x i s t which preclude n i c k e l from becoming octahedral i n a n i c k e l s u l f i d e and i n f a c t t h e h i g h temperature phase o f
N i S contains octahedral n i c k e l [391, although another s t a b l e s u l f i d e d n i c k e l species, Ni3S2, contains t e t r a h e d r a l n i c k e l . Since s i n g l e l a y e r s o f MoS2 are known t o be s t a b l e on alumina t h e i n t e r c a l a t i o n o f n i c k e l must cause a s l i g h t s t a b i l i z a t i o n o f t h e m u l t i - l a y e r e d s t r u c t u r e . Thermodynamic arguments p u t f o r t h by Furimsky have i n d i c a t e d t h a t n i c k e l should i n t e r c a l a t e between adjacent s u l f i d e l a y e r s b e t t e r than c o b a l t [ 4 0 ] . I f one simp1 i s t i c a l l y considers t h e c r y s t a l f i e l d s t a b i l i z a t i o n energies of d7 and d8 metal ions i t can be shown t h a t t h e d7 case (Co2+) i s s i m i l a r i n energetics i n both octahedral and t e t r a h e d r a l geometries w h i l e t h e d8 case (Ni2+) overwhelming f a v o r s the octahedral geometry over t e t r a h e d r a l
36
coordination[41]. However, edge intercalation of nickel between MoS2 layers would probably not be in strict octahedral geometry because the nickel would not be surrounded by six sulfur atoms. Arguments have been put forth in this paper that suggest single layers of MoS2 promoted with cobalt are active and selective catalysts for desulfurization of petroleum feedstocks while multi-layered structures containing nickel are active for aromatics hydrogenation and denitrification. In addition to possibly functioning as a structural promoter in MoS2 stacks another role of nickel could be electronic in nature. It was suggested by Voorhoeve, eta1 that these edge-intercalated nickel atoms (in octahedral geometry) can cause electron delocalization into the Mo(W)S2 slabs to form Mo(W)3+ ions, the site believed to be active for hydrogenation o f benzene[9-11]. With increased stacking the number of possible Mo(W)3+ sites increases as well , creating more hydrogenation sites and, hence, more active HDN catalysts. It has long been known that hydrogenation plays a more critical role in HDN than HDS. Involvement of nickel in a monolayer system such as is produced using cobalt could also create the Mo(W)3+ sites but it is difficult to hydrogenate an aromatic ring in this configuration. Benzene hydrogenation is believed to involve a -bonded complex to the Mo(W)3+ siteI9-111. The close proximity of these sites to the alumina support would sterically hinder access of the aromatic ring. If this combination of multi-layered MoS2 crystallites along with promotion by Group VIIi metals is critical in achieving high activity HDN catalysts then additives that merely promote MoS2 stacking should not make effective catalysts. This was found to be the case when phosphorus was combined with molybdenum in catalyst Mo/P(O.OO). The HRTEM results showed a multi-layered MoS2 structure that was similar to that seen with the Ni/Mo(0.37) catalyst. However, this system had a molybdenum dispersion when sulfided that was only one-third that of Ni/Mo(0.37). This Mo/P(O.OO) catalyst was also found to be very inactive for HDN[42]. While similar structures are formed with these two systems the role of phosphorus is clearly not identical to that of nickel. If the role of phosphorus i s not electronic as in the case of Group VIII metals then its role in hydrotreating catalysts is probably structural. Phosphorus is known to interact more strongly with the alumina support than it does with either cobalt or nickel. The phosphate group can react with the surface o f the alumina and take up a portion of the available surface area. This would result in a smaller surface area on which the Moo3 deposition could occur, leading to "taller" agglomerates of MoO3. This should in turn lead to multi-layered MoS2 crystallites with
37
s h o r t e r dimensions i n the l a t e r a l d i r e c t i o n t h a t are caused by t h e d i s r u p t i o n i n the surface chemistry and geometry by t h e phosphorus. Some of t h e phosphorus can s t i l l be associated w i t h t h e molybdenum oxide species even a f t e r c a l c i n a t i o n . This phosphorus could a l s o promote c r y s t a l growth i n t h e v e r t i c a l d i r e c t i o n by i n t e r f e r i n g w i t h t h e s u l f i d i n g process. This can be seen i n t h e XPS r e s u l t s by comparing t h e l o s s i n Mo d i s p e r s i o n when phosphorus i s added t o the molybdenum o n l y c a t a l y s t , Mo(O.00).
I n t h e oxide system t h e r e i s o n l y a 15% l o s s w h i l e f o r t h e
s u l f i d e case a 62% l o s s i s observed. While t h i s Mo/P(O.OO)
i s n o t o f i n t e r e s t from a c a t a l y t i c viewpoint i t
i s o f i n t e r e s t t o consider the p o s s i b i l i t y o f combining t h e phosphorus promotion o f MoS2 stacking w i t h Group
V I I I metals t o form even more a c t i v e
HDN c a t a l y s t s . The c o b a l t system w i l l be considered f i r s t . As p o i n t e d o u t
p r e v i o u s l y t h e a d d i t i o n o f phosphorus t o t h e Co/Mo c a t a l y s t causes a decrease i n t h e Mo d i s p e r s i o n and a n o t i c e a b l e increase i n t h e number o f b i - l a y e r and t r i - l a y e r MoS2 stacks. These f i n d i n g s suggest t h a t t h e Co/Mo/P(0.37) Co/Mo(0.37)
c a t a l y s t should be more a c t i v e f o r d e n i t r i f i c a t i o n than t h e c a t a l y s t i f t h e r e e x i s t s a l i n k between s t a c k i n g o f MoS2 and
HDN. This f i n d i n g has been confirmed by c a t a l y s t t e s t i n g where t h e
phosphorus c o n t a i n i n g system was 30% more a c t i v e on a v o l u m e t r i c b a s i s when compared t o t h e non-phosphorus c a t a l y s t [ 431. The n i c k e l c a t a l y s t s form a s i m i l a r p a t t e r n . Although MoS2 stacks a r e formed i n n i c k e l promoted c a t a l y s t s which do n o t c o n t a i n phosphorus t h e a d d i t i o n o f phosphorus appears t o increase t h e number o f stacks. A p i c t o r i a l r e p r e s e n t a t i o n o f a phosphorus promoted Ni-Mo c a t a l y s t i s shown i n F i g u r e 7. However, simply i n c r e a s i n g t h e number o f MoS2 stacks w i t h a combination o f N i and P does n o t guarantee t h a t a more a c t i v e system w i l l be obtained. The Ni/Mo/P(O.lO)
c a t a l y s t , f o r example, has many m u l t i p l e
MoS2 stacks b u t has a low surface area and poor Mo dispersion. This r e s u l t s i n an HDN a c t i v i t y t h a t i s 20% l e s s than t h a t f o r Ni/Mo(0.37)[431. An increase i n the n i c k e l promoter l e v e l t o 3 . 0 % ~increased t h e surface area o f t h e c a t a l y s t , Mo d i s p e r s i o n o f t h e c a t a l y s t , and t h e c a t a l y t i c a c t i v i t y . This c a t a l y s t was 50% more a c t i v e f o r HDN than t h e e q u i v a l e n t loaded c a t a l y s t prepared w i t h o u t phosphorus; however, t h e HDS a c t i v i t i e s were s i m i l a r [ 4 3 ] . F u r t h e r i n c r e a s i n g t h e Ni/Mo r a t i o as i n c a t a l y s t Ni/Mo/P(0.50)
r e s u l t e d i n lower molybdenum d i s p e r s i o n b u t t h e c a t a l y t i c
a c t i v i t i e s were e q u i v a l e n t on our screening t e s t t o t h e Ni/Mo/P(0.37) catalyst.
38
Phosphate
Fig. 7. Illustration of sulfided Ni/Mo catalyst supported on alumina and promoted with phosphorus. The nickel is shown as octahedrally coordinated between MoS2 layers and the phosphate occupies a portion of the alumina surface. All sulfurs are not shoen ont the nickel ion. This series of phosphorus promoted Ni-Mo catalysts points out that the correct combination of metal ratios and phosphorus levels are needed for optimum activity on alumina. These quantities will depend on the total metal loading of the catalyst along with the properties of the alumina support. While the data presented in this study were for alumina supported catalysts this does not imply that active catalysts cannot be prepared on other supports or that the addition of phosphorus is required for all supports. What is significant is that for the HDN reaction stacks of MoS2 promoted with Group V I I I metals is important. CONCLUSION The stacking of MoS2 layers in supported Ni/Mo and Co/Mo hydrotreating catalysts prepared by single-step impregnations has been shown to be an important feature of these catalysts. The Ni/Mo based catalysts, high in HDN activity, show stacks of MoS2 ranging u p to 10 layers. This can be explained by the NiZ+ promoter occupying the sites between adjacent MoS2 layers. Phosphorus, a known promoter of the HDN process for alumina based catalysts, has been seen to aid in the formation of stacks presumably by occupying part of the available surface area. The Co/Mo catalysts, primarily used for HDS, are quite different in nature. The MoS2 stacks are not formed at all for the Co/Mo(0.37) case. The MoS2 in these Co/Mo catalysts is spread out over the alumina surface in MoS2 monolayers.
39
One p o s s i b l e e x p l a n a t i o n f o r the d e v i a t i o n o f t h e Co promoted c a t a l y s t s from t h e N i promoted c a t a l y s t s i s t h a t under h y d r o t r e a t i n g c o n d i t i o n s t h e Co p r e f e r s t o be t e t r a h e d r a l l y coordinated r a t h e r than o c t a h e d r a l l y coordinated. Thus, the c o b a l t would n o t occupy t h e octahedral s i t e s between adjacent MoS2 l a y e r s b u t r a t h e r r e s i d e i n t h e same plane as the molybdenum atoms. The i n a b i l i t y o f t h e Co promoted c a t a l y s t s t o form stacks l i m i t s t h e f o r m a t i o n o f Mo3+ s i t e s b e l i e v e d needed f o r aromatics hydrogenation and HDN. ACKNOWLEDGEMENTS We wish t o thank M r . Dick Young f o r t h e p r e p a r a t i o n o f t h e c a t a l y s t s used i n t h i s study.
REFERENCES
1 J.M.J.G.
Lipsch and G.C.A. Schuit, J. Catal., 15 (1969) 163, 174,and 179. 2 J. Sonnemans, 1973 Ph.D. Thesis, TH Twente, The Netherlands. 3 J. Sonnemans and P. Mars, J. Catal., 31 (1973) 209. 4 J. Sonnemans, G.H. van den Berg, and P. Mars, J. Catal., 31 (1973) 220. 5 J. Sonnemans and P. Mars, J. Catal., 34 (1974) 215. 6 J. Sonnemans, W.J. Neyens, and P. Mars, J. Catal., 34 (1974) 230. 7 G.C.A. Schuit and B.C. Gates, Amer. I n s t . Chem. Eng. J., 19 (1973) 417. 8 M. LoJacono, A. Cimino, and G.C.A. Schuit, Gazz. Chim. I t a l . , 103 (197 1281. 9 R.J.H. Voorhoeve and J.C.M. S t u i v e r , J. Catal., 23 (1971) 228, 236, and 243. 10 A.L. Farragher and P. Cossee, i n J.W. Hightower(Ed.) C a t a l y s i s , Proc. 5 t h I n t . Cong. Catalysis, North Holland, Amsterdam, 1973, p.1301. 11 A.L. Farragher, Symposium on t h e Role o f S o l i d S t a t e Chemistry i n Catalysis, ACS Meeting, New Orleans, March 20-25, 1977. 12 B. Delmon, 3rd I n t . Conf. on t h e Chem. and Uses o f Molybdenum, (1979) p.73, and references c i t e d w i t h i n . 13 D.S. Thakur and B. Delmon, J. Catal., 9 1 (1985) 308. 14 D.S. Thakur, P. Grange, and B. Delmon, J. Catal. 9 1 (1985) 318. 15 H. Topsoe and B.J. Clausen, Catal. Rev.-Sci. Eng., 26 (1984) 395 and references c i t e d w i t h i n . 16 H. Topsoe, B.J. Clausen, N-Y Topsoe, and E. Pedersen, Ind. Eng. Chem. Fundam., 25 (1986) 25. 17 H. Topsoe and B.J. Clausen, Appl. Cat., 25 (1986) 273. 18 M.J. Ledoux, G. Maire, S. Hantzer, and 0. Michaux, i n M.J. P h i l l i p s and M. Ternan(Eds.), Proceedings o f t h e 9 t h I n t e r n a t i o n a l Congress on Catalysis, Vol I , The Chemical I n s t i t u t e o f Canada, Ontario,(1988) p.74. 19 G.C. Stevens and T. Edmonds, i n B. Delmon, P. Grange, P. Jacobs, and G. Poncelet(Eds.), Preparation o f C a t a l y s t s 11, E l s e i v e r , Amsterdam, 1979, p.507. 20 M. Breysse, B.A. Bennett, D. Chadwick, and M. V r i a n t , B u l l . SOC. Chim. Belg. 90 (1981) 1271. 21 J.C. Duchet. E.M. van0ers. V.H.J. deBeer. and R. Prins, J. Catal., 80 (1983) 386. 22 J.R. Katzer and R. Sivasubramanian, Catal. Rev.-Sci. Eng., 20 (1979) 155.
40
23 24 25 26 27 28
29
30 31 32 33 34 35 36 37
38 39 40 41 42
43
J.D. Colgan and N. Chomitz, U.S. Patent 3287280 (American Cyanamid). C.T. Adams, U.S. Patent 3629146 (Shell Oil Co.). R.N. Fleck, U.S. Patent 2547380 (Union Oil Co.). R.J. Mikovsky and A.J. Silvestri U.S. Patent 4128505 (Mobil Oil Company. ) I. Alstrup, I. Chorkendorff, R. Candia, B.S. Clausen, and H. Topsoe, J. Catal.. 77 11982) 397. M.M. Ramirez de Agudelo and A. Morales, in M.J. Phillips and M.Ternan(Eds.), Proceedings of the 9th International Congress on Catalysis, Vol. I, The Chemical Institute of Canada, Ontario, 1988, p.42. P.J. Argevine, J.C. Vartuli, and W.N. Delgass, in G.C. Bond, P.B. Wells, and F.C. Tompkins(Eds.),Proc. 6th Int. Cong. Catal., Imperial College, London, July 12-16, 1976, Burlington House, England, 1976,~. 64. S.C. Fung, J. Catal., 58 (1979) 454. M. Houalla and B. Oelmon, Surf. and Interface Anal., 3 (1981) 103. F.P.J.M. Kerkof and J.A. Moulijn, J. Phys. Chem., 83 (1979) 1612. M.P. Seah and W.A. Dench, NPL Rept. Chem 82 (Apri1,1978). L.F. Allard, J.S. Brinen, F.P. Oaly, and A.J. Garratt-Reed, Ultramicroscopy, 22 (1987) 135. 0. Sorensen, B.S. Clausen, R. Candia, and H. Topsoe. Appl. Catal., 13 (1985) 363. See K.S. Chung and F.E. Massoth, J. Catal., 64, 320(1980). N.P. Martinez, P.C.H. Mitchell, and P. Chiplunker. in P.C.H. Mitchell and A. Seaman(Eds.), 2nd Int. Conf. on the Chem. and Uses of Molybdenum, New College, Oxford, August 30-September 3, 1976, Climax Molybdenum Company Limited, London, England, 1976, p.164. S. Geller, Acta. Cryst., 15 (1962) 1195. F. Jellinek, in G. Nickless(Ed.), Inorganic Sulfur Chemistry, Elsevier, Amsterdam, 1968, p.719. E. Furimsky, Catal. Rev.-Sci. Eng., 22 (1980) 371. J.E. Huheey, Inorganic Chemistry, Harper and Row, New York, 1972, p.307 Catalyst activity testing results were obtained on a standard catalytically cracked heavy gas oil feedstock. The test conditions were 60 bar H2 pressure, 2.0 LHSV, and reactor temperature of 343°C. The results show that the Mo/P(O.OO) catalyst is 70% less active on a volumetric basis for HDN than Ni/Mo(0.37). Testing was performed using the same feed and conditions as was used in I411.
.
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Chevrel Phase HDS Catalysts:
41
Structural and Compositional Relationships to
Catalytic Activity
G. L . Schrader and M. E. Ekman Department of Chemical Engineering and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011
ABSTRACT The catalytic activities of "reduced" molybdenum sulfides, known as Chevrel phases, have been evaluated for hydrodesulfurization of thiophene and benzothiophene and hydrogenation of 1-butene. These materials have been found to have hydrodesulfurization activities comparable to or greater than model unpromoted and cobalt-promoted MoS2 catalysts; in contrast, Chevrel phases exhibit low activities for 1-butene hydrogenation. In this paper, a general 'discussionof the relationship between the solid state chemistry of Chevrel phases and their catalytic activity is presented. Structural properties appear to be an important factor: large cation Chevrel phases are the most active and stable materials. It is also likely that the most active phases resist surface oxidation vhich may occur if the ternary metal components undergo surface migration. "Reduced" molybdenum oxidation states are associated with the active sites, in direct analogy with conventional catalysts. INTRODUCTION Industrial hydrodesulfurization (HDS) catalysts are typically formed from oxides of Mo (or W) and Co (or Ni) supported on alumina. During use, the catalysts become sulfided. The historical origins of presently-used HDS catalysts dates from work conducted in pre-WWII Germany on the hydrogenation of coal and coal-derived liquids (refs. 1-2). Over the past forty years much research has been directed toward elucidating catalyst structure and composition and the nature of the active sites. Most of this work emphasizes the relationship of the active component in industrial catalysts to MoS2-based structures (refs. 3-6). However, characterization of these catalysts remains a challenging aspect of much current research. Several years ago we began to report research on a new class of HDS catalysts--"reduced" molybdenum sulfides referred to as Chevrel phase catalysts (refs. 7-10).
Considerable evidence has been offered that "reduced" Mo
oxidation states are associated with the active sites on even conventional HDS catalysts (refs. 11-12).
Chevrel phases have been shown to have activities
comparable to or exceeding those of conventional MoS2 or Co-Mo-S materials for thiophene and benzothiophene HDS.
In addition, the Chevrel phases apparently
42
favor desulfurization rather than hydrogenation (HYD), making them rather selective catalysts. Over twenty Chevrel phases have now been examined (refs. 13-14) resulting in the recent discovery of additional catalytically active compounds. It has also been possible to clarify some aspects of HDS reaction pathways and mechanisms using these catalysts (refs. 10,15-16). In this paper we present some of the relationships between catalytic activity and the structural and compositional properties of Chevrel phases. (ref. 17) reported in 1971 the initial synthesis and Chevrel g characterization of Mo chalcogenides referred to as Chevrel phases. The general formula for these compounds is MxMo6Z8 where M can be over forty different elements, x ranges from 1 to 4 , and Z is usually S, Se or Te. Much interest developed in these compounds because of the superconducting properties of some of the chalcogenides. Literature reviews have been provided by Yvon (ref. 1 8 ) , Chevrel and Sergent (ref. 19), and Chevrel (ref. 20). The basis for the structure of sulfide Chevrel phases is the Mo6S8 fundamental cubic unit (Figure 1). The sulfur atoms form a slightly distorted cube built around a molybdenum octahedron which is elongated along the ternary axis. The Mo-Mo bond distances are quite short--ranging from 2.65 to 2.80 A--compared to 2.72 A for metallic Mo. The Mo-Mo intracluster bond distance can be influenced by the addition of ternary metals: if the number of valence electrons is increased by increasing the concentration of the ternary component or by using ternary elements with a higher valence, the Mo-Mo bond distance decreases. This has led to the description of the unique structural character of Chevrel phases as consisting of "little bits of metal". The conductivity behavior (poor conductors becoming superconductors at temperatures as high as 15 K for PbMo6S8) has also been discussed in these terms (ref. 20). The Mo6S8 structural units may be stacked to form structures with rhombohedra1 or triclinic geometries. The Mo6S8 units are interconnected by
e.
e.
OX 0 Mo
Figure 1.
The Mo6S8 structural unit aligned along the ternary axis (ref. 18).
43
A.
The structures of the Chevrel phases tend to be highly stable because each unit is bonded to six other units through
short, covalent Mo-S bonds of 2.4-2.6
these linkages. The Mo6 clusters interact through Mo-Mo intercluster bonds of 3.1-3.4 A. The Chevrel phases can be grouped according to the ternary metal components which influence specific structural properties. The valence state and size of the ternary metal are particularly important. The ternary metals are located in "infinite channels" existing along the rhombohedra1 axes (Figure 2); thermal motion of the ternary atoms is highly anisotropic with large motion perpendicular to the ternary axis but with very little motion in the parallel direction.
Physically this is interpreted as giving rise to a delocalization of
the ternary atoms. However, the extent of the delocalization is primarily dependent on the size of the metal atom (Figure 3 ) .
On this basis, Chevrel
phases are classified as small cation, intermediate cation, and large cation compounds (Table 1). The Chevrel phases also demonstrate compositional ranges depending on the size of the cation. Small cation compound compositions (for ternary components such as Cu, Fe, Ni, Co) can be varied continuously within specific limits
lo%
)
and
to
avoid
diffusional
l i m i t a t i o n s and t h e i n f l u e n c e o f r e a c t i o n p r o d u c t s . Measurements were p e r f o r m e d after
18 h on-stream,
i.e.,
when a
pseudo-steady
s t a t e was reached.
The
c o m p o s i t i o n o f t h e gaseous phase was determined by o n - l i n e chromatography w i t h flame i o n i z a t i o n detection. TABLE 2 Reactions c o n d i t i o n s .
Reactant p r e s s u r e Total pressure
Pyr id i n e
Piperidine
hydrogenation
hydrogenolysi s
2 266x10 Pa 2oX1o5 Pa
Hydrogen s u l p h i d e p r e s s u r e
0
Reaction temperature
523 K
266x10' 20x10 330x10
Pa
5 2
548 K
Pa Pa
70
RESULTS Catalysts characterization
A TEM micrograph o b t a i n e d f o r a molybdenum s u l p h i d e on alumina sample i s g i v e n i n F i g u r e 1.
F i g . 1. TEM micrograph o f a MoS2/A1203 sample.
71
The molybdenum sulphide phase appears as small
c r y s t a l l i t e s having a
t y p i c a l l a m e l l a r s t r u c t u r e and c o n s t i t u t e d by one o r several l a y e r s separated by about 0.6 nm, representative
i n good accord w i t h t h e MoS2 s t r u c t u r e . T h i s micrograph i s
of
all
the
micrographs
obtained
in
our
studies.
These
micrographs allowed us t o measure, f o r a l l t h e samples s t u d i e d , t h e number of l a y e r s , N, and t h e l e n g t h o f t h e c r y s t a l l i t e s , L. D i s t r i b u t i o n s o f
N
and L were
then obtained on t h e basis o f a l a r g e sampling f o r each c a t a l y s t s t u d i e d ( s e v e r a l hundred c r y s t a l l i t e s f o r each sample) and average values, were c a l c u l a t e d f o r t h e d i f f e r e n t c a t a l y s t s .
-
N and
c,
Using hypotheses and equations
described p r e v i o u s l y (12) we then c a l c u l a t e d t h e c o n t r i b u t i o n s o f t h e basal ( S B ) and edge ( S E ) planes t o t h e s p e c i f i c s u r f a c e area ( S ) o f MoS2.
Table 3 g i v e s performed a f t e r
-
N,
L,
SB,
SE and S values obtained from TEM s t u d i e s
t h e determination o f p y r i d i n e hydrogenation and p i p e r i d i n e
hydrogenolysis r e a c t i o n r a t e s f o r a 2 wt.-% wt.-%
molybdenum on z i r c o n i a and an 11
molybdenum on alumina sample, compared w i t h those o b t a i n e d f o r t h e same
c a t a l y s t s before r e a c t i o n t e s t i n g .
TABLE 3 Morphological parameters
(1i n
2 nm; SB, SE and S i n m /g MoS2) o f some c a t a l y s t s
b e f o r e and a f t e r r e a c t i o n t e s t i n g . Catalyst
2 w t . - % Mo on Zr02 : b e f o r e t e s t
-
-
N
L
sB
'E
1.6
3.3
196
207
403
after piperidine t e s t
1.7
4.1
183
171
354
after pyridine t e s t
1.6
3.9
193
178
371
2.0
3.8
155
187
342
1.6
3.2
192
215
407
11 wt.-% Mo on A1203 : b e f o r e t e s t after pyridine t e s t
Table 3 i n d i c a t e s t h a t t h e d i f f e r e n c e s between t h e values o b t a i n e d b e f o r e and a f t e r r e a c t i o n t e s t i n g never exceed ca. 20%. Moreover, support,
iand L
depending on t h e
seem t o vary i n an o p p o s i t e way and we can t h e r e f o r e consider
t h a t t h e observed v a r i a t i o n s a r e w i t h i n t h e experimental e r r o r s ,
and t h a t no
s i g n i f i c a n t m o d i f i c a t i o n o f t h e c a t a l y s t s occurs d u r i n g t h e r e a c t i o n t e s t . The morphological c h a r a c t e r i s t i c s b e f o r e c a t a l y t i c t e s t i n g r e p o r t e d p r e v i o u s l y (12) are t h e r e f o r e a p p l i c a b l e . Table 4 g i v e s
i, L,
SB,
SE and S values obtained from TEM s t u d i e s o f t h e
11 w t . - % molybdenum on alumina sample sulphided a t d i f f e r e n t temperatures. No
72 s i g n i f i c a n t d i f f e r e n c e s appear between samples s u l p h i d e d a t 673 and 873 K,
but
when t h e c a t a l y s t i s s u l p h i d e d a t a h i g h e r temperature (1073 K ) , s i n t e r i n g o f t h e c r y s t a l l i t e s occurs along t h e a a x i s , g i v i n g r i s e t o an i n c r e a s e i n t h e i r length without modification
o f t h e average v a l u e o f t h e number o f l a y e r s , and
r e s u l t i n g i n a decrease i n SE and i n an unchanged v a l u e o f SB.
TABLE 4 Morphological parameters
(c in
nm; S8, SE and S i n m2/g MoS2) o f t h e 11 w t . - %
sample versus s u l p h i d a t i o n temperature ( i n K ) .
Mo/A1203
-
L
sB
sE
S
2.0
3.8 4.4 6.9
155 164 151
187 162 105
342 326 256
N
S u l p h i d a t i o n temperature
673 873 1073
1.9 2.1
Determination o f c a t a l y t i c p r o p e r t i e s Figures 2 and 3 show t h e v a r i a t i o n s o f t h e s p e c i f i c r a t e s o f p y r i d i n e hydrogenation
(rHN)
c a t a l y s t , given i n
and
piperidine
and
mol s-'
Mo l o a d i n g f o r t h e d i f f e r e n t supports. both r e a c t i o n s ,
but zirconia
s l i g h t l y active f o r piperidine
is
h y d r o g e n o l y s i s (rHDN) ( p e r gram o f g-'),
respectively,
as a f u n c t i o n o f
Pure alumina and s i l i c a a r e i n a c t i v e i n
inactive f o r
pyridine
hydrogenolysis (0.7
hydrogenation and mol
s-'
g-').
The
values given i n Figure 3 f o r z i r c o n i a - s u p p o r t e d c a t a l y s t s a r e t h e experimental values c o r r e c t e d by s u b t r a c t i n g t h e support a c t i v i t y . These r a t e s a r e found t o i n c r e a s e almost l i n e a r l y w i t h Mo l o a d i n g up t o a maximum o r a plateau.
However, t h e s l o p e o f t h e ascending p a r t o f t h e curve,
t h e amount o f Mo corresponding t o t h e maximum o r t o t h e b e g i n n i n g o f t h e p l a t e a u and t h e r e a c t i o n r a t e corresponding t o t h i s maximum o r t h i s p l a t e a u , vary w i t h t h e support and t h e r e a c t i o n considered. Table 5 shows t h e i n f l u e n c e o f t h e s u l p h i d a t i o n temperature on t h e r a t e s o f t h e p y r i d i n e hydrogenation ( rHN) and p i p e r i d i n e hydrogenolysis (rHDN) on t h e
11 w t . - %
molybdenum on alumina c a t a l y s t .
rHNincreases and rHDNremains
constant when t h e s u l p h i d a t i o n temperature increases from 673 t o 873 K . When t h i s temperature increases from 873 t o 1073 K b o t h r e a c t i o n r a t e s decrease considerably.
73
. . . . . . . .
I
0
W t Yo MO
10
*
Fig. 2. Variations of the pyridine hydrogenation rate as a function o f Mo loading for the different supports.
t
r HDN 110-7 moI s-lg-’)
’
A1203
1
0
.
.
.
.
1
10
.
.
.
.
1
*
W t ’lo MO
Fig. 3. Variations of the piperidine hydrogenolysis rate a s a function o f Mo loading for the different supports.
74
TABLE 5 V a r i a t i o n s o f t h e p y r i d i n e hydrogenation and p i p e r i d i n e hydrogenolysis r a t e s (in
lom8
and
lom7
mol
s-l
g-l)
of
the
11 wt.-%
Mo/A1203
catalyst with
s u l p h i d a t i o n temperature ( i n K ) . S u l p h i d a t i o n temperature
r~~
r~~~
673
4.2
3.2
a73
5.0
3.0
1073
3.1
1.3
DISCUSSION I t i s g e n e r a l l y accepted t h a t t h e c a t a l y t i c p r o p e r t i e s o f s o l i d s can be
discussed
in
terms
of
geometric
or
electronic effects.
Some c o n c l u s i o n s
r e l e v a n t t o these two e f f e c t s were o b t a i n e d i n a s t u d y d e a l i n g w i t h support e f f e c t s i n CO hydrogenation over t h e same MoS2 supported c a t a l y s t s ( 1 2 ) . The main f e a t u r e s a r e t h e f o l l o w i n g : f o r a given temperature o f s u l p h i d a t i o n , t h e d i s p e r s i o n decreases when t h e Mo l o a d i n g on t h e support i s increased, b u t t h e r a t i o o f t h e areas o f basal planes t o edge planes remains almost c o n s t a n t ; moreover, t h i s r a t i o ( b u t n o t t h e d i s p e r s i o n ) was found t o be independent o f t h e support (SB/SE = 0.8 i 0 . 1 ) ; an homothetical way,
t h i s suggests t h a t MoS2 c r y s t a l l i t e s grow i n
independently o f t h e n a t u r e o f t h e support;
XPS and
k i n e t i c data suggest t h a t t h e e l e c t r o n i c p r o p e r t i e s o f Mo a r e l i t t l e i n f l u e n c e d by t h e n a t u r e o f t h e support. MoS
2
d i s p e r s i o n e f f e c t s cannot f u l l y account f o r t h e v a r i a t i o n s i n CO
hydrogenation r a t e on changing t h e support and t h e above r e s u l t s r u l e o u t either
geometric
effects
linked with
changes
in
the
exposed
of
an Ma-support
planes
or
support-induced e l e c t r o n i c e f f e c t s . Support e f f e c t s were interaction:
i n the
interpreted
less active
i n terms
solids
(Mo
on alumina
or
chemical
silica)
some
Mo-0-support l i n k a g e s remaining a f t e r s u l p h i d a t i o n have an i n h i b i t i n g e f f e c t on the c a t a l y t i c properties;
i n contrast,
a c t i v e c a t a l y s t s have supports l i k e
c e r i a o r z i r c o n i a which a r e themselves a b l e t o undergo s u l p h i d a t i o n ,
thus
l e a d i n g o n l y t o Mo-S-support 1 inkages. Mo-0-support 1 inkages c o u l d i n h i b i t t h e a c t i v i t y e i t h e r by c r e a t i n g i n a c t i v e s u r f a c e Mo atoms ( 7 ) o r by changing t h e p o s i t i o n o f t h e MoS2 s t a c k i n g s w i t h r e s p e c t t o t h e support (16), t h u s v a r y i n g the density o f the active sites. We s h a l l now examin t h e e x t e n t t o which geometric o r e l e c t r o n i c e f f e c t s o r v a r i a t i o n s i n t h e d e n s i t y o f a c t i v e s i t e s due t o t h e presence o f Mo-0-support
75
1 inkages can account f o r t h e observed v a r i a t i o n s i n p y r i d i n e hydrogenation and p i p e r i d i n e hydrodenitrogenation r a t e s on changing t h e s u l p h i d a t i o n temperature o r t h e support. V a r i a t i o n s o f t h e r e a c t i o n r a t e s according t o s u l p h i d a t i o n temperature
5
Table
shows
that
the
pyridine
hydrodenitrogenation r a t e s on t h e 11 wt.-%
hydrogenation
way as a f u n c t i o n o f t h e s u l p h i d a t i o n temperature, proposed p r e v i o u s l y
(17),
that
and
piperidine
Mo/A1203 sample v a r y i n a d i f f e r e n t TS.
T h i s suggests,
as
t h e a c t i v e s i t e s i n t h e two r e a c t i o n s a r e
probably d i f f e r e n t . I n c r e a s i n g TS probably leads t o a decrease i n t h e number o f Mo-0-A1 linkages
which a r e s t i l l present a f t e r s u l p h i d a t i o n under moderate c o n d i t i o n s
( 1 8 ) . The hydrogenation a c t i v i t y f i r s t increases when TS increases from 673 t o 873 K (Table 5 ) ; t a k i n g i n t o account t h e corresponding l i m i t e d changes o f t h e morphological parameters o f MoS2 c r y s t a l l i t e s (Table 4),
t h e observed i n c r e a s e
i n hydrogenation a c t i v i t y cannot r e s u l t from changes i n d i s p e r s i o n , b u t c o u l d be a t t r i b u t e d t o t h e decrease i n Mo-0-A1 linkages, i n good accord w i t h what was proposed f o r CO hydrogenation. When TS i s increased t o 1073 K, t h e hydrogenation a c t i v i t y decreases, i n good agreement w i t h t h e observed decrease i n SE and t h e probably
limited
i s n e a r l y complete a t 873 K.
Another
increase o f
Mo s u l p h i d a t i o n which
conclusion i s t h a t t h e a c t i v e s i t e s f o r hydrogenation a r e probably l o c a t e d i n t h e edge planes, as SB appears t o be almost independent o f TS. Concerning t h e p i p e r i d i n e hydrodenitrogenation r e a c t i o n , Tables 4 and 5 show t h a t t h e a c t i v i t y f o l l o w s t h e changes i n SE, which confirms t h a t t h i s r e a c t i o n takes p l a c e on edge planes o f MoS2 and suggests t h a t i t i s n o t , o r l i t t l e , a f f e c t e d by t h e presence o f Mo-0-A1 l i n k a g e s . V a r i a t i o n s o f t h e r e a c t i o n r a t e s according t o t h e n a t u r e o f t h e support Figures
2
and
3
also
suggest
that
the
hydrogenation
and
hydrodenitrogenation s i t e s a r e d i f f e r e n t ; t h e a c t i v i t i e s o f t h e two r e a c t i o n s vary i n a d i f f e r e n t way according t o t h e metal l o a d i n g and t h e n a t u r e o f t h e support. The i n f o r m a t i o n obtained p r e v i o u s l y (12) and r e c a l l e d a t t h e b e g i n n i n g o f t h e d i s c u s s i o n a l l o w s us t o conclude t h a t t h e observed e f f e c t s o f t h e support on r e a c t i o n r a t e s cannot be i n t e r p r e t e d i n terms o f changes o f t h e exposed planes or
i n terms
of
electronic
effects.
We s h a l l
now
examine
if
the
support-induced v a r i a t i o n s can be i n t e r p r e t e d u s i n g t h e conclusions drawn from experiments w i t h v a r i a t i o n o f t h e s u l p h i d a t i o n temperature. Figure 3 shows t h a t t h e t h r e e supports g i v e s i m i l a r h y d r o d e n i t r o g e n a t i o n curves,
in
good agreement
with the
above-proposed absence
of
effect
of
76
Mo-0-support l i n k a g e s ( p r o b a b l y p r e s e n t on alumina and s i l i c a - b a s e d c a t a l y s t s and n o t on zirconia-based
catalysts,
as s u l p h i d i n g o f Zr02 i s e a s i e r than
s u l p h i d i n g o f A1203 o r Si02) on t h e h y d r o d e n i t r o g e n a t i o n r e a c t i o n . These curves are similar
t o those r e p o r t e d p r e v i o u s l y (12) w i t h maxima f o r t h e t h r e e
supports which f o l l o w t h e same sequence ( z i r c o n i a , s i l i c a , alumina) f o r t h e Mo l o a d i n g . Zirconia-supported c a t a l y s t s appear t o be t h e l e a s t a c t i v e systems i n t h e hydrodenitrogenation r e a c t i o n . Comparison o f Figures 2 and 3 shows t h a t t h e s o l i d s supported on z i r c o n i a behave v e r y d i f f e r e n t l y i n t h e two r e a c t i o n s , b e i n g much more e f f e c t i v e i n t h e hydrogenation r e a c t i o n . T h i s o b s e r v a t i o n c o u l d be r e l a t e d t o t h e absence o f i n h i b i t i n g Mo-0-support
interaction f o r zirconia,
leading t o t h i s
peculiar
behaviour i n c a t a l y t i c p r o p e r t i e s . However, according t o conclusions drawn f r o m experiments w i t h
sulphidation
temperature
variations,
it
might
have
been
expected t h a t t h e Zr02 supported samples would be t h e most a c t i v e f o r p y r d i n e hydrogenation, which i s n o t v e r i f i e d . T h i s f i r s t d i s c u s s i o n o f t h e support e f f e c t i s based o n l y on a d r e c t comparison o f experimental data. More d e t a i l e d i n t e r p r e t a t i o n s would need a comparison o f c a t a l y t i c data expressed per u n i t area o f t h e supported MoS2 a c t i v e phase and e s p e c i a l l y a b e t t e r knowledge o f t h e r o l e and p r o b a b l y
the
p o s i t i o n o f t h e Mo-0-support bonds; work i s now i n progress i n t h i s area.
CONCLUSION The
variations
in
the
catalytic
and
morphological
properties
A1203-supported MoS2 on changing t h e s u l p h i d a t i o n temperature suggest
of that
hydrogenation and hydrodenitrogenation s i t e s , i f b o t h l o c a t e d on edge planes o f MoS2,
are d i f f e r e n t :
Mo-0
-support
linkages
remaining a f t e r
sulphidation
i n h i b i t t h e former r e a c t i o n b u t n o t t h e l a t t e r . On changing t h e support, v a r i a t i o n s i n t h e c a t a l y t i c p r o p e r t i e s c o n f i r m t h a t t h e a c t i v e s i t e s i n t h e two r e a c t i o n s a r e d i f f e r e n t . cannot be explained by geometric e f f e c t s ( i . e .
These v a r i a t i o n s
changes i n t h e d i s p e r s i o n o r i n
t h e n a t u r e o f t h e exposed molybdenum planes) o r by e l e c t r o n i c e f f e c t s ( i . e . , changes i n t h e n a t u r e o f t h e a c t i v e s i t e s ) ; t h e y a r e n o t f u l l y e x p l a i n e d by t h e presence o f Mo-0-support 1inkages.
REFERENCES 1 V.H.J. de Beer, M.J.M. van d e r A a l s t , C.J. M a c h i e l s and G.C.A. S c h u i t , J . C a t a l . , 43 (1976) 78. 2 V.H.J. de Beer and G.C.A. S c h u i t , i n B. Delmon, P.A. Jacobs and G. P o n c e l e t ( 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: Proceedings, E l s e v i e r , Amsterdam, 1976, p.343. 3 H. Topsfie, B.S. Clausen, N. B u r r i e s c i , R . Candia and S. Morup, i n B. Delmon, P.A. Jacobs and G.Poncelet ( 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: Proceedings, E l s e v i e r , Amsterdam, 1976, p.479. 4 J.C. Duchet, E.M. van Oers, V.H.J. de Beer and R. P r i n s , J . C a t a l . , 80 (1983) 386. 5 G. M u r a l i d h a r , F.E. Massoth and J. Shabtai, J. C a t a l . , 85 (1984) 44. 6 F.E. Massoth, G. M u r a l i d h a r and J. Shabtai, J. C a t a l . , 85 (1984) 53. 7 J.P.R. V i s s e r s , B. S c h e f f e r , V.H.J. de Beer, J.A. M o u l i j n and R. P r i n s , J. C a t a l . , 105 (1987) 277. 8 T.F. Hayden, J.A. Dumesic, R.D. Sherwood and R.T.K. Baker, J. C a t a l . , 105 (1987) 299. 9 H. Shimada, T. Sato, Y. Yoshimura, J. H i r a i s h i and A. N i s h i j i m a , J . C a t a l . , 110 (1988) 275. 10 A . N i s h i j i m a , H. Shimada, T. Sato, Y. Yoshimura and J. H i r a i s h i , Polyhedron, 5 (1986) 243. 11 0. T o g a r i , T. Ono and M. Nakamura, J. Japan P e t r o l . I n s t . , 6 (1979) 336. 12 C. Mauchausse, H. Mozzanega, P. T u r l i e r and J.A. Dalmon, i n M.J. P h i l l i p s and M. Ternan ( E d i t o r s ) , Proc. 9 t h I n t . Cong. C a t a l . , Calgary, 1988, V o l . 2, p. 775. 13 J. Sonnemans, W.J. Neyens and P. Mars, J. C a t a l , 34 (1974) 230. 14 J.L P o r t e f a i x , M.L. V r i n a t , C. Gachet and M. C a t t e n o t , 8 t h F r e n c h - P o l i s h Symposium, P o i t i e r s , 1981. 15 J. Sonnemans, F. Goudriaan and P. Mars, Proc. 5 t h I n t . Cong. C a t a l . , Palm Beach, 1972, p. 1085. 16 T.F. Hayden and J.A. Dumesic, J . C a t a l . , 103 (1987) 366. 17 M. Breysse and coworkers, B u l l . SOC. Chim. Belg., 96 (1987) 829. 18 B.S. Clausen, H. Topsde, R. Candia, J. V i l l a d s e n , B. L e n g e l e r , J. A l s - N i e l s e n and F . C h r i s t e n s e n , J. Phys. Chem., 85 (1981) 3868.
.
This Page Intentionally Left Blank
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
79
THE EFFECT OF PHOSPHATE ON THE HYDRODENITROGENATION ACTIVITY AND SELECTIVITY O F ALUMINA-SUPPORTED SULFIDED Mo, Ni AND Ni-Mo CATALYSTS
S. EIJSBOUTS, L. VAN GRUIJTHUIJSEN, J. VOLMER, V.H.J. DE BEER and R. PRINSI Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands
ABSTRACT AlzOeupported Mo, Ni and Ni-Mo catalysts were pre ared via pore volume impregnation of the support with aqueous solutions of H 8 0 4 , (NH4r$110& and Ni(NO&. The catalysts were sulfided in situ and tested in the hydrodenitrogenation (HDN) of quinoline (643 K, 3.0 MPa) and the hydrodesulfurisation (HDS) of thiophene (673 K, 0.1 MPa) and were further characterized by X-ray photoelectron spectroscopy (XPS). The Ni/A1203 and Mo/A1203 catalysts had very low quinoline conversions to hydrocarbons which changed somewhat in the presence of phosphate. For the Ni-Mo catalysts the addition of phosphate resulted in an increased quinoline conversion to hydrocarbons and an increased selectivity for unsaturated N-free hydrocarbons. The effect of phosphate on the HDN activity and selectivity did not correlate with its effect on the dispersion of Ni or Mo as determined by XPS. Moreover the HDS activity was not influenced by the presence of phosphate. This indicates that the addition of phosphate does not lead to an increase in the formation of the active " N i - M d " metal sulfide sites but rather to the formation of a new type of HDN site which is associated with phosphate.
INTRODUCTION In the last decennium the importance of hydrodenitrogenation (HDN) in industrial hydrotreating has grown due to increased refining of heavy feed (crudes, vacuum residues), containing high percentages of S, N, 0 and metals (Ni, V). Hydrotreatment of such materials is mostly carried out using AlzOHupported Ni(Co)-Mo, often containing phosphate. Phosphate simplifies the preparation of the catalysts [1,2] and extends their life time through an improvement of the mechanical and thermal properties [3] as well as through a decrease in fouling [l]. Phosphate has also a positive effect on the hydrodesulfurisation (HDS) [l-2, 4-51 and HDN [1,4,6] activity of the catalysts. This positive effect of phosphate on the catalytic activity has been explained by the improvement of Mo [2,5] and Ni [5-6] dispersion as well as by the formation of MoSz stacks in Co-Mo and especially Ni-Mo catalysts, resulting in an increased number of the active "Co-Mo-S" or "Ni-Mo-S" sites in Present Address: Technisch Chemisches Laboratorium, ETH, 8092 Ziirich, Switzerland
80
these catalysts [7]. Phosphate interacts strongly with A1203 [8] and can form Alp04 [6,9] which hampers the formation of metal-aluminates and aluminium-moly bdate [8] and changes the acidity of the catalyst [I]. Changed support acidity results in decreased formation of coke [l],and affects the cracking and isomerisation [8] activity. If present in high concentrations, phosphate w a s reported to act as a poison in the Al2Oeupported catalysts (HDS) [4]. The influence of phosphate on the HDS performance of sulfided carbon-upported catalysts has already been studied for a number of different catalysts (Co, Mo, Co-Mo, Fe, Fe-Mo) [lo-111. The addition of phosphate to these systems resulted in a strong catalyst poisoning (HDS), indicating that the positive contribution of phosphate t o the catalyst performance is specific for the AlzOeupported catalysts. In the present study the effect of phosphate on the activity and selectivity of sulfided A l 2 O ~ u p p o r t e dMo, Ni and Ni-Mo catalysts for the HDN of quinoline has been studied. The HDN results have been compared with those of low pressure HDS experiments and X-ray photoelectron spectroscopy (XPS) measurements on oxidic catalysts in order to obtain a more complete picture of the phosphate effect in these catalysts. EXPERIMENTAL The y A 1 2 0 3 support [Ketjen 001-1.5E: A1203 > 97.3 wt%, surface area 280 mzgl, pore volume 0.67 cmxgl, particle diameter 0.2-0.5 mm] was sequentially impregnated with aqueous solutions of o-HsPO,, (NH4)6Mo7024.4H20 and Ni( NO&.6H20 (all Merck, p.a.). After each impregnation step the catalysts were dried in air at 383 K. All catalysts were calcined in air at 823 K after the last impregnation step. In the text the following notation will be used: Ni(x)Mo(y)P(z)/A1203, where x, y and z are wt% of metal and phosphorus. The elements are ordered according to the sequence of impregnation, starting from the support. The quinoline-HDN experiments have been carried out in the gas phase in a high pressure micro flow reactor with on-line GC analysis (121 using 0.5 g of in situ sulfided catalyst (643 K, 1.5 MPa). The reaction was carried out at 643 K and 3.0 MPa using a feed consisting of 12 ,ul.min-1 of liquid [23.8 mo1% quinoline, 3.8 mo1% dimethyldisulfide and 72.4 mol% decane] evaporated in 950 std cmsmin-1 H2. Besides the reaction products that belong to the main reaction pathways of quinoline-HDN [13-151 (Fig. 1) small amounts (total less than 5 % of Q mol equivalents) of byproducts from cracking and isomerisation reactions were found in the reaction product mixture. Based on the steady state data the catalyst call be characterized by (for abbreviations see legend of the Fig. 1) Q-conversion to hydrocarbons (46 of Q mol equivalents converted t o hydrocarbons (PCH+PBZ+PCHE) = Nhc], by the product distribution within the group of hydrocarbons and double ring N-compounds (DHQ+THQ5+Q+THQl), by the Q-conversion to OPA (% of Q mol equivalents converted to OPA = Nopa) and by Q-cracking and isomerisation (% of Q mol equivalents converted to byproducts = Nby).
81
WZ
14
If
Figure 1. Quinoline HDN reaction network (according to combined findings of references [13-151). Abbreviations : Q = quinoline, T H Q l = 1,2,3,4-tetrahydroquinoline, THQ5 = 5,6,7,~tetrahydroquinoline,DHQ = decahydroquinoline, PCHA = propylcyclohexylamine, PCHE = propylcyclohexene, PBZ = propylbenzene, PCH = propylcyclohexane. The thiophene-HDS experiments have been carried out a t 0.1 MPa and 673 K in a microflow reactor with on line GC analysis using 0.2 g of in situ sulfided catalyst [lo-121. The XPS measurements were carried out on oxidic catalysts using the same procedure and settings as described previously (12,161. The following elements have been scanned: Ni 2p1,2 and 2p3,2, Mo 3d3,2 and 3dS,Z, P 2p and 2s, A1 2p and 0 1s. Peaks of C Is, In 3d and N 1s have been used as internal standards for binding energy calibration. The error in the determination of the intensity ratios is 15 $4. RESULTS Quinoline HDN Pure A1203, P(5.2)/Ala03 and Ni(3.3)/A1203 had a negligible quinoline conversion to hydrocarbons (Nhc) (Table l ) , Ni(3.2)P(4.2)/A1203 was only slightly better. Both, Mo(7.O)/A1203 and Mo(6.8)P(4.2)/A1203 had low quinoline conversions t o hydrocarbons (Table 1, Fig. 2). The total amount of hydrogenated N-compounds was comparable t o that of the AlzO-upported Ni catalysts, but the fraction of THQ1 was lower, there were more products with hydrogenated benzene ring (DHQ, THQ5). The Q-conversion to compounds with opened N-containing ring (N,,,+Nh,) of the Mo catalysts was much higher than that of the Ni-catalysts. The Mo(2.1)P(4)/A1203 catalyst had a comparable quinoline conversion to hydrocarbons (Nhc) and OPA (Nopa) but much higher Q4racking and isomerisation (Nby) than the phosphate-free Mo-catalyst.
82
TABLE 1 Quinoline HDN (643 K, 3.0 MPa). Catalyst
Product Compositionb Nby Nhc Nopa N n
a
0.0 0.2 0.0 0.5 5.0 4.6 23.3 42.1 12.4 22.4
A1203
P(5.2) A1203 %[ 3.3j/A1203 Ni 3.2 P(4.2)/&03 M 7.0 /A1203 M$6.8]P(4.2) A1203 Ni 3.4 M 7.7 /A1203 Ni 3.1 M 7.1 P(4.4)/A1203 Nil 1.1 Ni 1.2/M~7.9/~A1203 M 6.7 (4.3)/A1&3
0.0 0.3 0.5 0.5 4.9 3.6 4.0 4.4 4.7 4.6
99.8 97.7 99.5 96.7 90.1 89.0 72.2 51.9 82.9 69.4
0.1 1.8 0.0 2.3 0.0 2.8 0.5 1.6 0.0 3.7
For catalyst notation see section Experimental. % of Q mol equivalents converted to hydrocarbons (PCH+PBZ+PCHE; Nhc), to OPA (No a), to byproducts (Nby) and present as double ring N-compounds (DHQ, THQ5, Q, TH#1; Nn). For details see section Experimental. a
b
60 70
d
Hey.
PCWHC
F e r n
Fa-EiHC
wWN
MQm
MQlM
QM
COMPONENT
Mo7.O/A1203
Mo6.8P4.2/ A1203
Figure 2. Quinoline HDN conversion (3.0 MPa, 643 K) and product distribution in the roup of hydrocarbons and N-compounds for the Mo(7.O)/A1203 and Mo(6.8)P(4.2 / I 1 2 0 3 catalysts. HC 46 = quinoline conversion to hydrocarbons = % (PCH+PBZ+PCHE] in the total product; PCH/HC, PBZ/HC, PCHE/HC = $4 in the group of hydrocarbons; DHQ/N,
THQ5/N, THQl/N, Q/N = % in the group of N-compounds.
83
70 60 50
40
s 30 20
10 0
HC Y.
PCWHC
Ni3.4Mo7.71 A1203
WUHC
PCHERK:
UlWN
MQSM
MQlCI
WN
COMPONENT Ni3.1Mo7.1 P4.41A1203
Figure 3. Quinoline HDN conversion (3.0 MPa, 643 K) and product distribution in the group hydrocarbons and N-compounds for the Ni(3.4)Mo(7.7)/A1203 and Ni(3.1)Mo(7.1)P(4.4)/A1203 catalysts. For details see Figure 2.
of
70
7
50
40
3 30
20 10
COMPONENT
Ni 1.2Mo7.91 A1203
Ni 1.1Mo6.7 P43A1203
Figure 4. Quinoline HDN conversion (3.0 MPa, 643 K) and product distribution in the group hydrocarbons and N-compounds for the Ni( 1.2)Mo(7.9)/A1203 and Ni( 1.1)Mo(S.7)P(4.3)/Al~O3catalysts. For details see Figure 2.
of
84
Addition of phosphate influenced the quinoline conversion t o hydrocarbons (Nhc) and the selectivity for unsaturated hydrocarbons of the Ni(3.4)Mo(7.7)/A1203 catalyst to a great extent (Table 1, Fig. 3). Although the surface area and pore volume decreased from 237 mzgl and 0.53 cm3gl to 151 mzgl and 0.38 crnsgl the activity increased strongly. Phosphate had no effect on the deactivation pattern. The Ni(3.1)Mo(7.1)P(4.4)/A1203 catalyst had a lower selectivity for PCH and a higher selectivity for PBZ. The N-compounds distribution was more or less equal to the equilibrium composition [13] for both catalysts. Also the activity of the Ni(l.l)M0(6.7)P(4.3)/Al203 catalysts was higher than that for the phosphatefree catalyst (Table 1, Fig. 4). Although the absolute increase in quinoline conversion to hydrocarbons (Nhc) in this couple of catalysts was lower than for the previous couple of Ni-Mo/AlzOs catalysts, the relative increase was similar. Just like for the Ni(3.4)Mo(7.1)P(4.4)/A1203 catalyst, also for the Ni(l.l)Mo(6.7)P(4.3)/Al?03 the selectivity for PCH decreased and that for PBZ increased in the presence of phosphate, while the product distribution of N-compounds did not change significantly. Interestingly, equilibrium w a s not established, the percentages of compounds with hydrogenated benzene ring (DHQ, THQ5) being lower than the equilibrium values [13]. Phosphate, however strongly increasing the quinoline conversion to hydrocarbons, left the N-product distribution almost unaffected. Thiophene HDS The results of the thiophene-HDS experiments at 673 K are listed in Table 2. The A1203 support and P(5.2)/&03 catalyst had no thiophene conversion. The thiophene-HDS
TABLE 2 Thiuphene HDS (673 K, 0.1 MPa). Catalyst
a
0.1
0.1 0.6 0.6 6.6 5.6 2.6 2. a
3.6 3.6 5.6 3.3 4.0 2.6
For catalyst notation see section Experimental. = first order reaction rate constants for thiophene conversion to hydrocarbons; = first order reaction rate constant for the consecutive butene hydrogenation. a
b khds
khydr
85
activity and butene-hydrogenation activity of Ni(3.3)/A1203 and Mo(7.0)/AI203 did not chauge with phosphate addition. For the Ni(l.l)Mo(6.7)P(4.3)/A1203, Ni(3.1)Mo(7.1)P(4.4)/Al203 catalysts there were small changes in HDS-activity and moderate changes in butene hydrogenation activity compared t o the phosphate-free catalysts, which resulted in a decreased khydr/khds ratio. If the changes of catalyst performance due to the phosphate addition were only quantitative (better dispersion, more "Ni-Mo-S" sites) the khydr/khds ratio should have been the same for the phosphate-free and phosphate-containing catalysts and the changes of the HDS activity should have paralleled those of the HDN activities, i.e. there should have been a promoting effect for the Ni-Mo.
XPS The results of the XPS measurements are listed in Table 3. The Ni/AI and Mo/Al XPS intensity ratios of the Ni(3.3)/A1203, Mo(7.0)/A1203 and Ni(l.2)Mo(7.9)/A1203 catalysts increased strongly in the presence of phosphate. This was not the case for the Ni( 3.1)Md 7.1)P( 4.3)/A1203 catalyst which had comparable Ni/AI and Mo/AI XPS intensity ratios as the phosphate-free catalyst. The measured Ni/Al and Mo/AI ratios were in all cases lower than the theoretical values calculated for monolayer coverage. The binding energies of the Ni 2p and Mo 3d peaks did not change significantly with phosphate addition. However, this does not exclude the possibility of the formation of Ni-P or Mo-P compounds since the differences between the binding energies of metal oxides and metal phosphates arc quite small. Phosphate influences thus the distribution of the metals on the A1203 support to
TABLE 3 XPS on Oxidic Catalysts. Catalyst
a
XPS Intensity Ratiosb Ni/AI
Mo/AI
P/Al
117 155
-
-
115 100 37 56
110 163 109 112 103 146
-
For notation see section Experimental Intensity ratios are based on the following peaks: Ni 2p3,2+ M3,2, A] 3, p 2P.
26
23
15
24
a
b
Ni 2pj,2,
Mo 3 d ~ , ~ + Mo
86
some extent but there is no simple correlation between the Ni and Mo dispersion and the IIDN (HDS) activity. DISCUSSION The addition of phosphate led to a significant increase of the quinoline conversion to hydrocarbons of Ni-Mo/AlzO.j catalysts. Simultaneously the selectivity for unsaturated PBZ increased and that for fully hydrogenated PCH decreased. Phosphate increased also the cracking and isomerisation capacity of these catalysts. The HDS conversion remained almost unaffected, whereas the hydrogenation of the hydrocarbons decreased somewhat. These effects of phosphate can be explained by a physical and/or chemical modification of the metal sulfide phase by phosphate, or by the formation of a new phosphate-containing active phase. The addition of phosphate leads to the formation of AlP04 on the A l 2 O e u p p o r t which decreases the surface area and pore volume of the support [6,9]. Phosphate is as AlPO4 strongly bound to the AlzOeurface and thereby changes also the concentration of the acid sites and their acid strength, an important parameter with respect to the cracking and isomerisation activity [8], as well as t o the coke formation [l]. But this acidity effect might be independent from the effect of phosphate on the HDS and HDN activities of the catalysts. The loss of surface area and pore volume found for the Ni(3.1)Mo(7.1)P(4.4)/A1203 catalyst caused by the AIP04 formation must be overcompensated by positive (physical and/or chemical) modifications of the active phase, since this loss could have a negative effect on the metal distribution and consequently on the catalytic performance. Also for this reason it can not be expected that XPS measurements can give a complete explanation of the phosphate promoting effect. For, a number of effects might work in opposite directions. The formation of A P O , and the concurrent decrease in the surface area and pore volume should lead to increased metal/Al XPS intensity ratios. The formation of "Ni-Mo-S" stacks (predicted by Kemp et al. [7]) would however lower the metal/Al intensity ratios. Besides, Mo and Ni species different from the "Ni-Mo-S" phase might be present in the catalyst, especially at higher loadings. The HDN and HDS experiments have shown that the changes of the catalytic performance are not merely due to an increase in the number of the same type of "Ni-Mo-S" sites in the phmphate-containing Ni-Mo/AlzOs catalysts. Firstly, the phosphatefree catalysts show a parallel increase of the HDN and HDS activity when Mo/A1203 catalysts are promoted by Ni, demonstrating that the "Ni-Mo-S" phase is both, HDS and HDN active. In the phosphatefree catalysts [Ni(3.4)Mo(7.7)/A1203 vs. Mo(7.O)/A1203], the HDN promoting effect of Ni on Mo is two times lower than in the phosphat e-cont aining catalysts [Ni(3.1)Mo( 7.1)P( 4.4)/Al20 3 vs. Mo( 6.8)P( 4.2)/ A 12031. However, this higher promoting effect of Ni on the HDN activity of Ni-Mo/AlzOs catalysts in the presence of phosphate has no parallel in the HDS reaction. Besides, in the absence of
87
Ni there is no increase of HDN and HDS activity for Mo/Ala03 due t o phosphate addition. If the number of edge sites of MoS2 would have increased [7l upon phosphate addition, then this increase itself (in the absence of Ni-promoter) apparenly did not result in a significant improvement of HDN and HDS activity. This is rather surprising since these MoS2 edge sites present in the Mo/A1203 catalysts are believed to be the catalytically active sites for both reactions. Secondly, not only the quinoline conversion t o hydrocarbons but also the selectivity for unsaturated hydrocarbons has changed in HDN as well as HDS. If the increase in the number of the active sites were the only effect of phosphate in this type of catalysts the thiophene conversion should have increased parallel to the quinoline conversion to hydrocarbons and the selectivity should not have changed in any of these reactions. The opposite trends found in our experiments strongly suggest that the active phase formed in the presence of phosphate must be qualitatively different (have different activity and selectivity) from the "Ni-Mo-S" phase present in the phosphatefree catalysts. Thirdly, the independence of the N-product distribution on the presence of phosphate and the differences in the N-product product distribution between the two sets of Ni-Mo catalysts exclude that the phosphate effect could only be due to the presence of higher amounts of the "Ni-Mo-S" sites due to the increased formation of MoS2 edges. The Ni( l.l)Mo(S.7)P(4.3)/Al203 catalyst, which has a rather low Ni/Mo ratio, probably contains already an excess of MoS2 edges and this means that the availability of MoS2 sites is not a limiting factor for the "Ni-Mo-S" formation. A higher number of active distribution of this "Ni-Mo-S" sites should also change the N-product Ni( l.l)Mo(6.7)P(4.3)/A1203 catalyst to make it closer to the Ni(3.4)Mo(7.7)/A1203 catalyst (higher number of " N i - M d " sites) which had a N-product distribution more or less equal to the equilibrium ratio. However, the quinoline conversion to hydrocarbons increased arid that means that also the formation of hydrogenated N-containing intermediates must have increased. Since their percentage found in the reaction mixture remained unchanged in the presence of phosphate, apparently, also their conversion must have been enhanced. Both might take place on the same site, possibly without the desorption of the intermediates. It can thus be concluded that the P-effect is not be due t o the higher number of "Ni-Mo-S" sites. The active phase formed in the presence of phosphate must be chemically different from the active phase in the phosphate-free catalysts. One possibility is that phosphate modifies the "Ni-Mo-S" metal sulfide phase. A phosphate-associated metal sulfide phase might, for instance, have a lower interaction with the A l 2 O ~ u p p o r t ,which makes it better sulfidable. In this context it should be noted that the S-content of metal sulfide catalysts is known to play an important role with respect to their hydrogenation properties. In HDS the existence of two different types of "Co-Mo-S" [17-181 and "Ni-Mo-S" phase (19-201 with different S-zontent and HDS activities have been reported for phosphate-free catalysts. The formation of different metal sulfide phases in the Ni-Mo/AlzO3 catalyst might be due to the stacking of MoS2 layers [7]. But this does not
88
necessarily have to result in an increase in the number of the active ("Ni-Mo-S") sites. It is also possible that the monolayer type sulfide phase contains sites which are not available for the HDN reaction (for steric reasons) but are active in HDS. These sites might be converted into sites also active for HDN when the texture of the metal sulfide phase changes, for instance, as result of MoS2 layer stacking. However, the textural changes must have affected also the chemical properties of the active sites (at least the %ontent). To agree with the experimental results, these new active sites should have a high HDN activity, about the same HDS activity as the original metal sulfide phase and a lower hydrogenation of hydrocarbons in HDN and HDS. They must be sites with a different activity and selectivity, i.e. chemically different sites. The changes of the catalytic performance can be explained if these new active sites were directly associated with phosphate or even consist of a metal phosphate. AlP04, BPO4 and also other metal phosphates (e.g. of Ni, Fe, Cr) are known as catalysts for reactions such as dehydration, isomerisation, alkylation, cyclisation, disproportionation etc. BP04 h a s even be reported to be a hydrorefining catalyst. The catalytic properties of metal phosphates are dependent on the concentration and strength of their acid sites, i.e. on the type of metal cation, P/cation ratio and preparation procedure. Neither the formation of phosphates of Ni or Mo besides A P 0 4 in phosphate-containing Al2Oeupported catalysts, nor their catalytic activity in the hydrorefining reactions can be excluded. HDN and HDS experiments on carbon-supported catalysts have given evidence that metal-P compounds might be able to catalyze these reactions [21]. It thus seems reasonable to assume that the new active sites associated with phosphate could be AlPO4, other metal phosphate (e.g. Ni-phosphate) or a modified metal sulfide (such as a metal phosphide-sulfide). These active sites could be able to catalyze all the reaction steps as well as just interact with the hydrogenated N-containing intermediates and hydrocarbons formed on the sulfide sites ("Ni-Mo-S") and act as a ring-opening, NHelimination or dehydrogenation catalyst. The final catalyst could then even he bifunctional. For a bifunctional catalyst, the same effect of phosphate should be obtained by a combination of a phosphate on A1203 with a AlzOeupported metal sulfide which can produce enough intermediates liable for further reactions on the phosphate-associated sitcs. At the same time the transfer of these intermediates from the metal sulfide to the phosphate associated active sites must be unimpeded. For all models, the final activity would he dependent on the metal loading as well as on the phosphate-loading. The independence of the N-product distribution for the Ni(l.l)Mo(6.7)P(4.3)/A120~ catalyst on the P-content contradicts, however, the bifunctional model. If the new active sites were not able to form their own intermediates, the percentage of N-containing hydrogenated intermediates in the reaction mixture should have decreased. Both the formation of new active sites associated with phosphate with a different activity and selectivity compared to the metal sulfide sites, as well as the formation of modified metal sulfide sites in the presence of phosphate can
[a],
89
better explain our experimental results. These two models have in common that the new active sites would be able to catalyze all the reaction steps, i.e. the formation of the hydrogenated N-containing intermediates as well as the N-removal. If the N-removal is fast compared to the desorption, this would not necessarily affect the N-product distribution. Such active sites could have lower affinity to the S-compounds (weaker adsorption than N-compounds) and would not affect the HDS that much. The composition of these new active sites and the dependence of the promoting effect of phosphate on the method of preparation [24] as well as the role of phosphate in carbon-eupported catalysts [21] will be the subject of further study. CONCLUSIONS Phosphate is an efficient HDN promoter for Ni-Mo/AlzOs catalysts. Parallel to the increase of quinoline conversion to hydrocarbons the selectivity for unsaturated hydrocarbons (propylbenzene) increases. The presence of phosphate also increases quinoline-cracking and isomerisation. The HDS activity does not change but, just like in the HDN, the selectivity for unsaturated hydrocarbons increases. These phosphate effects can be explained by assuming that, in addition to the metal sulfide sites ("Ni-Md?') a new type of active sites associated with phosphate is formed. The new active sites might be a phosphate modified metal sulfide sites, but it is also possible that it is Alp04 or Ni phosphate. ACKNOWLEDGEMENTS These investigations were supported by the Netherlands' Foundation for Chemical Research (SON) with financial aid from the Netherlands' Technology Foundation (STW). REFERENCES 1. 2.
3. 4.
5. 6. 7. 8. 9. 10.
11.
C.W. Fitz and H.F. Rase, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 40. D. Chadwick, D.W. Aitchison, R. Badilla-Ohlbaum and L. Josefsson, Stud. Surf. Sci. Catal., 16 (1982) 323. P.D. Hopkins and B.L. Meyers, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 421. RE. Tischer, N.K. Narain, G.J. Stiegel and D.L. Cillo, Ind. Eng. Chem. Res. 26 (1987) 422. P. Atanasova, T. Halachev, J. Uchytil and M. Kraus, Appl. Catal., 38 1988) 235. M.M. h m i r e z de Agudelo and A. Morales, Proc. 9th Int. Congress atal. Calgary, 1988, M.J. Philips and M.Ternan, Editors, The Chemical Institute of Canada, Ottawa 1988, Vol. I, p. 42. R A . Kemp, R.C. Ryan, and J.A. Smegal, Proc. 9th Int. Congress Catal., Calgary, 1988, M.J. Philips and M.Ternan. Editors. The Chemical Institute of Canada. Ottawa 1988, Val: I, p. 128. K. Gishti, A. Iannibello, S. Marengo, G. Morelli and P. Titarelli, Appl. Catal., 12 11984) 381. A. Morales, M.M. Ramirez de Agudelo and F. Hernandez, Appl. Catal. 41 (1988) 261. S.M.A.M. Bouwens, J.P.R. Vissers, V.H.J. de Beer and R Prins, to be published in J. Catal.. S.M.A.M. Bouwens, V.H.J. de Beer, R Prins, W.L.T.M. Ramselaar, E. Gerkema and A.M. van der Kraan, in preparation.
d
90
12. 13.
14. 15.
16. 17. 18.
19. 20. 21.
21. 23. 2.1.
S. Eijshoilts, I,. van Gruijthiiijsen, J. Volmer, V.H.J. de
Beer and R. Prins, in preparation. .J.F. Cocchetto and C.N. Satterfield, Ind. Eng. Chern. Proc. Des. Dev., 20 C.N. Satterfield and J.F. Cocchetto, Ind. En , Chern. Proc. Des. Dev., 20 H. Schulz, M. Schon and N.M.Rahrnan, Stu!, Surf. Sci. Catal., 27 (1986) J.P.R. Vissers, B. Scheffer, V.H.J. de Beer, J,A. Moulijn and R Prins, J. Catal., 105 (1987) 277. C. Wivel, B.C. Clausen, R. Candia, S. Morup and H. Topsoe, J. Catal., 87 (1984) 497. S.M.A.M. Bouwens, J.A.R. van Veen, D.C. Koningsberger, V.H.J. de Beer and R. Prins, in preparation. N.Y. Topsoe and H. Topsoe, J. Catal. 84 (1983) 3%. S.M.A.M. Bouwens, L.T. van der Klip, D.C. Koningsberger, V.H. J. de Beer and R Prins, in preparation. S. EijsLouts, J. Volrner, L. van Gruijthuijsn, V.H.J. de Beer and R. Prins, in preparation. J.B. Moffat, Catal. Rev. Sci. En 18 1978) 199. M. Zdrazil and M. Kraus. Stud. &rf. 6ci. Catal.. 27 (1986) 257 S. Eijsbouts, J. Volrner, id. van GruijthuijLn, E.M. ;an Oers, V.H.J. d e Beer and R Prins, in preparation.
91
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
INFLUENCE OF PREPARATION ON THE MORPHOLOGY AND MICROSTRUCTURE OF COBALT-MOLY BDENUM %PHI DES D l a z , F. P e d r a z a . H. and S. F u e n t e s
G.
R o j a s , J . C r u z , M. A v a l o s .
L.
Cota
INSTITUTO DE FI S I C A , UNAM. Apdo. P o s t a l 20-364. 01000 M B x i c o ,
D. F
ABSTRACT E l e c t r o n m i c r o s c o p y , X-ray d i f f r a c t i o n a n d Auger s p e c t r o s c o p y s t u d i e s w e r e made on u n s u p p o r t e d Co-Mo s u l p h i d e s i n order t o elucidate the influence of preparation on the structural The e f f e c t of v a r y i n g t h e c o b a l t p r o p e r t i e s of t h e s e s o l i d s . l o a d i n g i n t w o series of c a t a l y s t s p r e p a r e d by d i f f e r e n t methods was studied. The p r e c i p i t a t i o n C H S P ) a n d t h e i m p r e g n a t i o n C I T D > methods w e r e u s e d . X-ray and e l e c t r o n d i f f r a c t i o n p a t t e r n s showed t h a t t h e c r y s t a l l i n i t y of C O P S i n mixed s u l p h i d e s d e p e n d s s t r o n g l y on t h e method of p r e p a r a t i o n . Scanning micrographs r e v e a l e d d i f f er e n c e s i n m o r phol ogy f o r b o t h methods, p r eci p i t a t e d samples show irregular highly porous particles, whereas i mpregnated s a m p l e s show w e 1 1 d e f i ned s h a p e s for M o S z p a r ti c l es and o v e r them s o m e a g g r e g a t e s a s a f u n c t i o n of t h e p r e p a r a t i o n method. The p r e s e n t r e s u l t s show t h a t t h e f i n a l f o r m of c o b a l t i n mixed s u l p h i d e s depends s t r o n g l y on t h e way of p r e p a r a t i o n . The i m p r e g n a t i o n p r o c e d u r e a l l o w e d a good d i s t r i b u t i o n of c o b a l t o n t h e MoS2 s u r f a c e m a i n l y by f o r m i n g a p o o r l y c r y s t a l l i n e f o r m of CosSa and a t o m i c a l l y d i s p e r s e d c o b a l t .
I NTRODUCTI ON A knowledge of
t h e s t r u c t u r e of
promoted
MoSz i s i m p o r t a n t
when c o n s i d e r i n g t h e i n d u s t r i a l a p p l i c a t i o n s of t h e s e c a t a l y s t s I n s p i t e of
their
industrial
importance,
t h e fundamental
basis
for
t h e i r catalytic a c t i v i t y i s n o t w e l l understood. The s t r u c t u r e of and many p h y s i c a l property Cll. habits
noble m e t a l
aqd c h e m i c a l
Because of
where
the
basal
s u l p h i d e s is h i g h l y a n i s o t r o p i c properties
t h i s anisotropy.
plane
i s dominant
p i c t u r e of MoS2 s u g g e s t s t h a t e d g e p l a n e s . is incomplete.
with basal
M o S z crystallizes
C2>.
in
The s t r u c t u r a l
where t h e c o o r d i n a t i o n
must h a v e a n i n c r e a s e d c h e m i c a l r e a c t i v i t y compared
planes.
temperatures
are d e r i v e d f r o m t h i s
The a n i s o t r o p y of
a r e used
in
MoS2 i s i n c r e a s e d
its preparation.
c r y s t a l l i n e " i s u s e d t o name t h i s k i n d of
and
nm across.
when l o w
term
s o l i d C31.
t h e s t r u c t u r e of t h e s e s a m p l e s i s h i g h l y f o l d e d . and h u n d r e d s or t h o u s a n d s of
the
"poorly
Typically
5-6 l a y e r s t h i c k
High r e s o l u t i o n e l e c t r o n
92
microscopy
of
pictures
these
samples
suggest,
as
Delaney
C43
d e s c r i b e d , a " h i g h l y d i s o r d e r e d s t r u c t u r e which is formed by b a s a l s l a b s forming a d i s o r d e r e d
a r r a y of
c a l c u l a t i o n s of t h e s c a t t e r e d X-ray structures
poorly
C53
which
the
u s i n g computer
i n t e n s i t i e s constructed m o d e l concluded
that
p a t t e r n s c a n n o t be e x p l a i n e d o n t h e b a s i s of
ideal
of
experimental
in
crystallites
Liang et a l .
promoter c o u l d b e e n t r a p p e d " .
crystalline
m i c r o c r y s t a l l i t e models.
Only
They
MoSz.
by i n t r o d u c i n g d e f e c t s
into the
model, s u c h as r o t a t i o n . s h i f t i n g a n d f o l d i n g of p l a n e s , c o u l d t h e e x p e r i m e n t a l d i f f r a c t i o n p a t t e r n b e matched. R e g a r d i n g t h e c h e m i c a l r e a c t i v i t y , i t h a s been shown
that the
b a s a l p l a n e is r e l a t i v e l y i n e r t u n l e s s d e f e c t s a r e i n d u c e d o n t h e Thus Somorjai a n d c o - w o r k e r s
s u r f a c e (63.
C7.83 showed t h a t oxygen
a n d t h i o p h e n e are o n l y weakly p h y s i s o r b e d o n t h e MoS2 basal p l a n e . the
On
other
hand,
edge
planes
provided
significant
chemical
and c a t a l y t i c a c t i v i t y C l l 3 .
r e a c t i v i t y CQ.103
I n o r d e r t o c o r r e l a t e c a t a l y t i c a c t i v i t y w i t h t h e s u r f a c e area of MoSz,
Nz
adsorption
been
correlation
between
catalytic
On t h e o t h e r h a n d ,
C12a).
used.
Nevertheless.
with
HDS
activity.
a c t i v i t y and
seems t h a t
it
catalysts prepared by t h e s a m e procedure,
o n l y for
well
has
is a
linear
s u r f a c e area
found
oxygen c h e m i s o r p t i o n correlates f a i r l y although
controversy
some
about
the
e x p e r i m e n t a l c o n d i t i o n s r e m a i n s C12b.13.14). T h e r e i s good a g r e e m e n t i n many s t u d i e s t h a t c o b a l t or n i c k e l as
are placed
promoters
on
edge
Topsoe et a l p u s i n g I R s t u d i e s of
planes
MoS.
of
example,
For
NO a d s o r p t i o n a n d a n a l y t i c a l
e l e c t r o n microscopy. showed t h a t c o b a l t or n i c k e l i n c a t a l y t i c a l l y a c t i v e s a m p l e s w e r e p l a c e d a t t h e e d g e s of M o S Cl53. On t h e o t h e r
hand,
a new c o n c e p t of p r o m o t i o n by c o b a l t
has
been s u g g e s t e d t o b e d u e t o t h e f o r m a t i o n of a new c o b a l t s u l p h i d e p h a s e , v e r y a c t i v e f o r h y d r o d e s u l phur i z a t i on C 163. I n o r d e r t o g a i n m o r e i n s i g h t i n t o t h e n a t u r e of phases
present
s t r u c t u r e of
in
mixed
Co-Mo s u l p h i d e s .
we
have
t h e surface analysed
the
promoted molybdenum d i s u l p h i d e when c o p r e c i p i t a t i o n
and i m p r e g n a t i o n methods are i n v o l v e d i n i t s p r e p a r a t i o n . EXPERIMENTAL Two
series
prepared 0.9.
of
unsupported
w i t h atomic r a t i o s ,
cobalt-molybdenum r=Co/Co+Mo
of
0.1.
catalysts 0.3,
were
0.5. 0 . 7 ,
The p u r e s u l p h i d e s MoSz a n d CoeSe w e r e a l s o p r e p a r e d .
A
93
ser i es w a s p r e p a r e d by c o p r eci p i t a t i o n of ammoni um heptamol y b d a t e and c o b a l t n i t r a t e a c c o r d i n g t o t h e H S P method d e s c r i b e d by Candia et a1 C 1 7 ) . Another series w a s p r e p a r e d by i m p r e g n a t i o n of on a p r e c u r s o r reported
cobalt n i t r a t e
M o S z Cammonium t e t r a t h i o m o l y b d a t e .
of
recently
In
C183
the
latter
method
we
as w e
ATW,
utilized
the
r e a c t i o n between t h e ATM a n d t h e c o b a l t n i t r a t e t o d e p o s i t c o b a l t Decomposition of
i o n s on t h e s u r f ace.
ammoni um t h i o m 0 1 y b d a t e
at
l o w t e m p e r a t u r e s i s known t o p r o d u c e p o o r l y c r y s t a l l i n e M o S z C l ) .
f o r 4 h a t 673 K
The p r e c u r s o r s w e r e s u l p h i d e d under 15%H2S/H2 prior
to
being
characterized.
The
were
samples
under
stored
n i t r ogen a f t e r s u l phi d a t i on. E l e c t r o n microscopy w a s performed i n a JEOL 1OOCX S T E M u n i t . Samples w e r e t r a n s f e r e d under N2 t o n - h e p t a n e
and u l t r a s o n i c a l l y
d i s p e r s e d . F i n a l l y , t h e y w e r e mounted on copper g r i d s c o a t e d w i t h col 1o d i on and c a r b o n .
The
X-ray
diffraction
were
spectra
recorded
a
with
Siemens
i n s t r u m e n t u s i n g a m o l ybdenum c a t h o d e . Auger s p e c t r a w e r e obtai ned w i t h a P e r k i n E l m e r Model
cases
these
preparation.
samples For
PHI
were
Auger
550 s c a n n i n g Auger
to
exposed
anal y s i s
the
air
microscope.
In
during
specimen
were
degassed
samples
overni ght .
RESULTS S c a n n i n q E l e c t r o n Microscopy C S E M ) The morphology of
SEM.
A
typical
presented i n Fig.
samples w a s c h a r a c t e r i z e d
all
image la;
samples
of
particles
obtained with
h i g h l y porous t e x t u r e are observed.
an
by
by means of
precipitation
irregular
profile
is and
This micrograph corresponds
t o t h e c o m p o s i t i o n r = O . 3. n e v e r t h e l e s s . n o s u b s t a n t i a l d i f f e r e n c e s
i n s h a p e w e r e o b s e r v e d on c h a n g i n g t h e cobalt c o m p o s i t i o n . F i g u r e s lb-ld
show a s e q u e n c e of
0.5, r e s p e c t i v e l y ,
In
the
first
pseudomorphous defined observed.
shapes
MoS2 and mixed s u l f i d e s w i t h
r = 0 . 3 and
f o r s a m p l e s o b t a i n e d by d e c o m p o s i t i o n o f molybdenum
case,
with such
the
precursor
as
needles,
disulphide crystallites hexagons
and
particles of
ATM;
ATM.
are well
platelets
are
When c o b a l t is i m p r e g n a t e d , small a g g r e g a t e s a p p e a r on
t h e s u r f a c e of
M o S CFig.
aggregates progressively
lc). cover
As
the
shown p r e v i o u s l y C183. MoSz
surface
as
the
these cobalt
94
F i g . 1 . Scanning e l e c t r o n micrographs of H S P sample and Cb-d> I T D samples of r = O . 3 and r = 0 . 5 .
95
concentration
increases
CFig.
From
Id).
these
results
it
is
c l e a r l y observed f o r impregnated catalysts t h a t c o b a l t remains on t h e MoSz s u r f a c e f o r m i n g a g g l o m e r a t e s , a s i n t h e case of s u p p o r t e d Nevertheless,
metals.
i t is important
as w i l l
to note that
be
shown by Auger s p e c t r o s c o p y . c o b a l t is n o t o n l y p r e s e n t i n s u c h a a form.
X - R a v D i f f r a c t i o n CXRD> As shown by SEM.the s t r o n g l y d e p e n d s on t h e
morphology
of
method
preparation.
of
unsupported
sulphides However
to
e s t a b l i s h t h e i r s t r u c t u r e , i t is n e c e s s a r y t o p e r f o r m XRD a n a l y s i s on
The u s e of
t h e s e samples.
XRD h a s
been
t o give
suggested
l i t t l e i n f o r m a t i o n on t h e a c t i v e p h a s e i n s u l p h i d e s , i n p a r t i c u l a r
b e c a u s e w e l l c r y s t a l l i n e p h a s e s a r e almost i n a c t i v e i n HDS. been shown t h a t p o o r l y c r y s t a l l i n e s a m p l e s a r e b e t t e r than w e l l
crystallized
samples because t h i s
e n h a n c e s t h e f o r m a t i o n of
catalysts
"dispersion"
d e f e c t s a n d e x p o s u r e of
I t has effect
edge p l a n e s i n
MoSz C 193. Liang
et
a1
have
CS1
analysed
c r y s t a l l i n e MoSz u s i n g X-ray s t r u c t u r e is folded,
the
structure
of
poorly
d i f f r a c t i o n and concluded t h a t t h i s
rotated
and s h i f t e d between b a s a l
planes.
Also, t h e y s u g g e s t e d t h a t i n f o r m a t i o n a b o u t e d g e p l a n e s must b e I n t h e case of
o b s e r v e d i n t h e d i f f r a c t i o n r e g i o n C l l O 1 t o ClOO3.
mixed s u l p h i d e s t h e d i s p e r s i o n of c o b a l t i n M o s t c a n be f o l l o w e d t h r o u g h t h e d i f f r a c t i o n l i n e s of COPS. The s p e c t r a of c a t a l y s t s p r e p a r e d by b o t h methods are shown i n Figure
2.
From
these
spectra,
it
possible
was
d i f f e r e n c e s i n i n t e n s i t y and t h e s h i f t of l i n e s of C O O S f o r t h e e n t i r e
to
t h e main
composition range.
determine
diffraction
F i g u r e 2a shows
t h e s p e c t r a o b t a i n e d f o r HSP s a m p l e s b e f o r e s u l p h i d a t i o n .
No well
d e f i n e d s t r u c t u r e i s o b s e r v e d i n t h i s case; t h e COO21 l i n e of MoSz o n l y is s u g g e s t e d and n o d i f f r a c t i o n l i n e s of After
sulphidation.
the
spectra
of
all
these
CooSe are marked.
catalysts
change
s u b s t a n t i a l l y a n d i n t h i s case C o o 9 l i n e s a p p e a r i n t h e s p e c t r a . The i n t e n s i t y of t h e main l i n e s of d i f f r a c t i o n d e c r e a s e b e c a u s e of the lower
3 the
c o b a l t c o n t e n t i n mixed s u l p h i d e s CFig. 2b>.
h e i g h t or
i n t e n s i t y of
C440> and C3111
a g a i n s t t h e atomic composition. same
behaviour
suggesting
that
lines
I n Figure is
plotted
Both d i f f r a c t i o n l i n e s f o l l o w t h e the
orientation
e s s e n t i a1 1y t h e s a m e for a11 composi ti o n s .
of
COOS
is
96
HSP
R=0.9
I
52'
I
' 40'
I
' 18'
1
' 1'6. '
I
I
4" 20
Fi g . 2a. X-ray di ff r actogr a m f r o m H S P s a m p l e s b e f o r e s u l phi dati on C r =Co/Co+Mo>.
97
HSP I
I
I
RQ.9
I I
9 I
8
r I
I
R0.7
I
I I
Y I
>
I
-cnI-
I I I
z
W
5
I I
I
R0.5
I I
I
3 I I
I
pI
R.0.3
I I
I
Rz0.1
k I
I
1, 1 1
52"
40'
I
1
28'
16
4"
20
F i g . 2b. X-ray d i f f r a c t o g r a m s from H S P c a t a l y s t s a f t e r s u l p h i d a t i o n
C r =Co/Co+Mo) .
98
HSP
ITD
X-311
Y
0-440
0
Q
> + 5 z w k
1
z -
I
,I
0.3
0.l
0.7
0.5
0.9
ATOMIC COMPOSITION F i g . 3. I n t e n s i t y o f atomic composition.
C4403
From t h e s e r e s u l t s .
it
and is
C3113
lines
suggested
cogs8
of
that
CosSe
by
versus
the
coprecipitation
w i t h ammoni um s u l p h i d e , d i s p e r s e d cobal t a n d molybdenum s u l p h i d e s
are
formed,
but
this
well
relatively
dispersed
s i n t e r s t o COPSE b y s u l p h i d a t i o n a t 673 K. o r i e n t a t i o n of that
c r y s t a l l i t e s i n mixed
i n pure COPS,
i n t e n s i t i e s of amount
of
structure.
fact
is
excluded
I t is w e l l
from
rapidly that
the
s u l p h i d e s r e m a i n s close t o
s u g g e s t e d b y t h e c o n s t a n c y of
the diffraction lines,
cobalt
form
The
the
may i n d i c a t e t h a t a
binary
relative a great
cobalt-molybdenum
known t h a t s e g r e g a t i o n of
cobalt occurs
easi 1y on cobal t - m o l ybdenum s u l p h i d e s C 203.
For CFigure
samples p r e p a r e d by i m p r e g n a t i o n and b e f o r e s u l p h i d a t i o n the
4a3,
structures
of
precursors. decreases
spectra
Nevertheless. in
of and
CNH432MoSI
mixed
mixed
sulphides
CoCNGd2.6HzO.
t h e i n t e n s i t y of
samples.
The
d i f f r a c t i on 1i n e s c h a n g e o n c o b a l t
relative
agree
with
the
which
are
the
these lines strongly i n t e n s i t i e s of
i mpregnati on,
ATM
p r i n c i p a l 1y
in
t h e zone of s m a l l a n g l e s . s u g g e s t i n g t h a t c o b a l t is n o t J u s t l a i d on t h e s u r f a c e , b u t r e p l a c e s s o m e c a t i o n s i n t h e s t r u c t u r e .
After
99
s u l p h i d a t i o n C F i g u r e 4b3, and 0 . 7 a r e of
t h e CosSe p e a k s f o r s a m p l e s w i t h r = 0 . 9
v e r y similar i n t e n s i t y t o t h o s e for
H S P samples;
t h e s e peaks a l m o s t d i s a p p e a r . This is observed
however for r S O . 5 ,
m o r e c l e a r l y i n F i g u r e 3 f o r a l l samples with r=0.5 prepared
by
I TD. On t h e o t h e r hand,
MoS2 p e a k s C0023, ClOO3 a n d C1103 d e c r e a s e
g r a d u a l l y with i n c r e a s i n g c o b a l t c o nten t.
These r e s u l t s s u g g e s t
t h a t a d i s p e r s e d form of p o o r l y c r y s t a l l i n e c o b a l t i s o b t a i n e d o n t h e s u r f a c e of M o S z f o r i m p r e g n a t e d c a t a l y s t s . T r a n s m i s s i o n E l e c t r o n M i c r o S c O D Y CTEm I n o r d e r t o o b t a i n m o r e i n f o r m a t i o n on t h e m i c r o s t r u c t u r e of mi xed
s u l phi d e s , T E M
field,
dark
field
w a s carried o u t .
characterization and
performed on t h e m o s t
electron
diffraction
Br i g h t were
techniques
i n t e r e s t i n g s a m p l e s Cr=0.3 a n d 0 . 5 3 .
In
s o m e i n s t a n c e s molybdenum d i s u l p h i d e w a s a l s o i n c l u d e d as a means
of
comparison.
Bright
f i e l d CBF3 a n d dark
f i e l d CDF)
i m a g e s of
c o p r e c i p i t a t e d c a t a l y s t w i t h r z 0 . 3 a r e shown i n F i g u r e s Sa a n d 5b. The b r i g h t f i e l d
small
image shows a n e n s e m b l e of
forming i r r e g u l a r p a r t i c l e s .
agglomerates
The d a r k f i e l d image of
t h i s sample
shows c r y s t a l l i n e p a r t i c l e s i n t h e s i z e r a n g e 10-50 nm.
These
c r y s t a l l i n e p a r t i c l e s w e r e assumed t o be C O P S , i n a c c o r d a n c e w i t h x-ray d i f f r a c t i o n r e s u l t s . A b r i g h t f i e l d image of
shown i n
F i g u r e Sc.
t h e impregnated catalyst with r=0.3 is
Agglomerates
s u p p o r t e d on a s u b s t r a t e s e v e r a l good
agreement
results
p a t t e r n CEDP> of
diffraction shown
with
in
Figure
characteristic
of
The
6a.
an
MoSz
in
t h e s i z e range
t i m e s longer
obtained
by
monocrystal
in
nm
are o b s e r v e d .
in
SEM.
MoSz o b t a i n e d by hexagonal
30-50
An
precipitation
array the
electron
of
spots
z o n e axis
is
is
Cool>,
s u g g e s t i n g t h a t t h e b a s a l p l a n e is a c t u a l l y e x p o s e d . The EDP of r = O . 3 shown
samples. large
t h e c o b a l t - m o l ybdenum
in
Figure 6b
The p r e s e n c e
particles
are
of
is
characteristic
spots
formed
sampl e w i t h
copreci pi t a t e d of
polycrystalline
also suggests t h a t
with
preferential
relatively
orientations.
D i f f r a c t i o n s p o t s of MoSz and m a i n l y COD% w e r e i d e n t i f i e d i n t h i s pattern. The molybdenum d i s u l p h i d e from ATM g i v e s t h e r i n g p a t t e r n shown i n F i g u r e 6c.
T h i s p a t t e r n of p o o r l y c r y s t a l l i n e MoSz i s t y p i c a l
of m i c r o c r y s t a l l i n e s a m p l e s w i t h small c r y s t a l sizes b u t a l a r g e
100
ITD
R-0.7 A
R=0.5
-! . F i g . 4a. X-ray C r =Co/Mo+Co3.
40
'
- . 20
16
4
2s
d i f f r a c t o g r a m s of ITD s a m p l e s b e f o r e s u l p h i d a t i o n
101
ITD I
IL
I
I
I
I
1
i
I I
I
I I I
I I
I
I
4
I
I
1
-r
1
I
R.0.7
I
I
I
I
I I
I
I
/I.. I k I
I_
I
I
I
I
44
I
I
I
I
I
t
I
I
I
I
I I
I I
I
1
I I
R=OI
T
28'
I
I
I
"r.
I
40"
1
I
I
4
52"
I
^\.v
+ I
M.3
I
I
I
R0.5
I
I
I
1
I
I
I I
I
16"
I
4"
28 cogs8 _ _ . ~
-Moa
Fig. 4b. X-ray d i f f r a c t o g r a m s of ITD c a t a l y s t s a f t e r s u l p h i d a t i o n C r =Co/Co+MoI
102
F i g . 5 . E l e c t r o n micrographs of ITD and HSP c a t a l y s t s C r = O . S S . Ca3 Bright f i e l d image of an HSP c a t a l y s t ; Cb3 dark f i e l d image of t h e s a m e sample; Cc) b r i g h t f i e l d image of an ITD c a t a l y s t .
103
Fig. 6. E l e c t r o n d i f f r a c t i o n p a t t e r n s CEDP> of t h e samples prepared by both methods. Ca) EDP of M o S z o b t a i n e d by p r e c i p i t a t i o n . Cb3 EDP of Co-Mo C r = O . 3> c o p r e c i p i t a t e d sample;Cc> EDP of M o S prepared from ATM; Cdl Co-Mo c a t a l y s t C r = 0 . 3 > prepared by i mpregnati on C I TD) .
104 number
of
crystallites.
Figures
6d
and
correspond
6e
to
a
c a t a l y s t w i t h r = 0 . 3 . I n t h e f o r m e r i n s t a n c e t h e EDP w a s t a k e n o n a region
free
p a t t e r n of
from
agglomerates
and
poorly c r y s t a l l i n e M o S .
p i c t u r e i s c h a r a c t e r i s t i c of
is
typically
I n latter
a
diffraction
i n s t a n c e case t h e
r e g i o n with agglomerates and t h e
a
s p o t s o b s e r v e d c a m e from CooSa a n d MoS2 showing t h a t a g g l o m e r a t e s a r e formed by small c r y s t a l l i t e s of t h i s p h a s e C C O Q S ~ .
F i g . 6e. E l e c t r o n d i f f r a c t i o n p a t t e r n of a r e g i o n w i t h a g g l o m e r a t e s f o r a Co-Mo catalyst w i t h r = 0 . 3 p r e p a r e d by i m p r e g n a t i o n . S c a n n i n g Auqer
biiCrOSCODY
CSAbD
I n order t o e s t a b l i s h t h e s u r f a c e composition. w a s performed on t w o s a m p l e s w i t h r = O . B .
Auger
analysis
The atomic p e r c e n t a g e s of
t h e d i f f e r e n t elements found i n t h e o r i g i n a l
s u r f a c e and
after
s p u t t e r i n g w i t h A r i o n s are shown i n T a b l e 1 . The a n a l y s i s shown i n column CaI area C P A E S i n a p o r o u s z o n e of
corresponds
t h e catalyst.
to a restricted
The Co/Mo
ratio i n
t h i s i n s t a n c e i s h i g h e r t h a n t h e t h e o r e t i c a l v a l u e Co/Mo=l.
column
Cb) shows a n a v e r a g e a n a l y s i s CSAES3 o f a r e l a t i v e l y w i d e z o n e of the
same
sample.
The
cobalt
is
concentration
also
higher
c o n f i r m i n g t h e s u r f a c e e n r i c h m e n t by t h e p r o m o t e r . Analysis surface cobalt
with
Cc3
Ar
was
performed
ions
c o n c e n t r a t i on
for
30
decreases
after min.
s p u t t e r i n g of Under
n o t i ceably
these to
the
original
conditions.
1o w e r
val ues.
the We
c o n c l u d e t h a t s e g r e g a t i o n is o c c u r r i n g f o r c o p r e c i p i t a t e d s a m p l e s .
105
a n average a n a l y s i s o n
For t h e s a m p l e o b t a i n e d by i m p r e g n a t i o n .
a f l a t zone of MoSz f r e e f r o m a g g l o m e r a t e s i s shown i n t h e column
r a t i o i s close t o t h e t h e o r e t i c a l
Cd>. The Co/Mo
v a l u e and
the
s a m e h o l d s f o r a m o r e c o n f i n e d a n a l y s i s of t h e s a m e z o n e C e > . Column Cf> shows t h e a n a l y s i s c a r r i e d o u t o n t h e e d g e of particle.
atomic
The C o : M o : S
correlated
with
t h e mixed
ratio
phase
is
which
1:l:S.S.
t o be
proposed
a n MOSS could
located
on
be the
e d g e s of t h e MoSz s t r u c t u r e . Finally,
column
Cg>
agglomerates observed cobalt
preparation
By
Auger
into
involves
a
of
analysis
the
t h e MoSz p a r t i c l e s .
times
eight
is
taking
CITD3
the
on t h e s u r f a c e of
concentration
concentration.
shows
higher
account
that
this
surface
reaction
The
the
Mo
method
of
than
between
the
p r e c u r s o r s of Co and Mo. t h i s r e s u l t i s n o t s u r p r i s i n g . W e assumed c a t i o n s on t h e s u r f a c e of
t h a t Coz+ r e p l a c e s NHI+
a t o m i c d i s p e r s i o n of t h e promoter. the
excess
of
cobalt
as
are
catalysts"
active
coprecipitated catalysts C 1 8 > .
area
of
10-20
deposited
on
the
mz/g,
the
giving an
surface
The i n t e r e s t i n g f a c t i s t h a t
c o b a l t agglomerates. "model
be
will
ATM.
Once t h e s u r f a c e is c o v e r e d ,
as
or
more
forming
i n t h i s way, active
than
Although t h e y h a v e a small s u r f a c e catalytic
suggesting t h a t c o b a l t is w e l l
activity
d i s p e r s e d through
is
very
high,
t h e s u r f a c e of
MoSz . TABLE 1 S u r f a c e c o m p o s i t i o n by Auger e l e c t r o n s p e c t r o s c o p y C a t . YJ
E l ement Ca> 20
co Mo
ITD
HSP
7
S
47
0 C
20 6
Cb)
Cc>
21 8 51
19
65
13 7
4 5
7
c
Cd>
Ce)
Cf>
9 8 41
18 23 50
12 12 65
51 6
28
5
6
20
14
4
5
23
-
CONCLUSIONS
I n t h i s study all the
bul k
t h e characterization techniques applied t o
and s u r f ace show si g n i f i c a n t d i f f e r e n c e s d e p e n d i n g on
t h e method of p r e p a r a t i o n . observation segregation
that and
CoeSe
I n t h e c o p r e c i p i t a t i o n CHSP) method t h e forms
sintering
are
large occur
Also. t h e r e l a t i v e i n t e n s i t i e s of
particles during
suggests
thermal
that
processes.
t h e d i f f r a c t i o n l i n e s or s p o t s
106 suggest
that
most
of
the
cobalt
present
in
these
catalysts
is
i n v o l v e d as COP%.
o,-,t h e o t h e r hand, t h e i m p r e g n a t i o n method CITD3 g i v e s c o b a l t e x c l u s i v e l y o n t h e s u r f a c e of molybdenum d i s u l p h i d e . i n t w o f o r m s , a g g r e g a t e s a n d atomi c a l l y d i s p e r s e d c o b a l t .
The 1a t t e r
coul d
be
r e l a t e d t o t h e mixed a c t i v e p h a s e .
The c a t a l y t i c a c t i v i t y o f t h e s e s a m p l e s w a s f o u n d t o be s i m i l a r t o or
better
than
that
of
coprecipitated
samples
C181. I n
this
i n s t a n c e t h e s y n e r g i s t i c e f f e c t c a n o n l y b e e x p l a i n e d by a s u r f a c e model where a good d i s p e r s i o n of c o b a l t on t h e s u r f a c e o f
MoSz is
e n v i s i o n e d t o e x p l a i n t h e improved c a t a l y t i c p r o p e r ti es. ACKNOWLEDGEMENTS The a u t h o r s are g r a t e f u l t o Drs. J.M. Dominguez a n d P. Bosch f o r making t h e X-ray d i f f r a c t o m e t e r a v a i l a b l e a n d t o Mr. A. G o m e z f o r t e c h n i c a l a s s i s t a n c e w i t h t h e sample p r e p a r a t i o n . REFERENCES 1 F. R. Gamble, F. J . Disalvo. R. A. Klemm.and T. H. Geballe, S c i e n c e , 168, Cl9701, 568. 2 R . R . C h i a n e l l i . i n I n t . R e v . i n Phys. Chem., B u t t e r w o r t h s 1982 p. 2. C h i a n e l l i , E.B. Prestridge, T.E. Pecoraro and J . P . 3 R.R. Deneuf v i 11e , S c i e n c e , 203, C 19791 11 05. 4 F. Delaney. Applied C a t a l y s i s 16 C19851 135. 5 K . S. L i a n g , R. R . C h i a n e l l i , F. 2. C h i e n , and S.C . Moss, J. Non C r y s t . S o l i d s 79 C19861 251. 6 P. Ratnasamy, S . S i v a s a n k e r , C a t a . Rev. Ski. Eng. 22 C19801 401. 7 M. Salmeron. G . A . S o m o r j a i , A. Wold, R . R . C h i a n e l l i . a n d K . S . L i a n g , Chem. Phys. L e t t 90 C1982> 105. 8 M. H. F a r i a s . A. J . Gellman. G. A. S o m o r j a i ,R. R. C h i a n e l l i , a n d K . S . L i a n g . S u r f . Sci. 140 C19841 181. 9 K. Tanaka. and T. Okuhara. J . C a t a l . 78 C19821 155. 10 K. Suzuki M. Soma. T. O n i s h i , a n d K. Tamaru. J. E l e c t r o n S p e c t r o s c . R e l a t . Phenom. 24 C19811 28. 11 H. Topsoe. 9. S . C l a u s e n , R . C a n d i a , C. Wive1 and S. Morup, B u l l . Soc. Chim. Belg 90 C19813 1189. 12 a> M. B r e y s s e . R. F r e t t y . M. L a c r o i x . a n d M. V r i n a t , R e a c t . K i n e t . C a t a l . L e t t . 26 C19841 97. b1 S. J . T a u s t e r . T. A. P e c o r a r o , R. R . C h i a n e l l i , J . C a t a l . 63 C19803 515. 13 T.A. B o d r e r o , a n d C . H . Bartholomew. J . C a t a l . 84 C19831 145. 14 J . Valyon. and W . K . H a l l . J. C a t a l . 84 C1983> 216. 15 M. Topsoe, N. Y . Topsoe, 0. S o r e n s e n , R. C a n d i a . 9. S . C l a u s e n . K . Kallesoe. E. P e d e r s e n a n d R. Nevald. S o l i d State C h e m i s t r y i n C a t a l y s i s , p. 235 C19851. 16 M. J. Ledoux, 0. Michaux. G. A g o s t i n i , a n d P. P a n i s s o d . J . C a t a 1 . 93. CIS813 189. 17 R . C a n d i a . 9. J . C l a u s e n , a n d H. Topsoe. B u l l . Soc. Chim. B e l g 90 C19811 1225. 18 S. F u e n t e s . G. D l a z . F. P e d r a z a , H. R o j a s , N. R o s a s . J . C a t a l . 113 C19883 535. 19 R . R . C h i a n e l l i and M. Daage. F a l l Aiche M e e t i n g , Washington. D . C . nov. 1988. 20 R . W . P h i l l i p s a n d A . A . C o t e . J. C a t a l . 4 1 C19761 168.
.
M.L. Occelli and R.G. Anthony (Editors), Advances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
EFFECT OF 2,6-DIETHYLANILINE
107
AND HYDROGEN SULPHIDE ON HYDRODENITROGENATION
OF QUINOLINE OVER A SULPHIDED NiO-Mo03/A1 203 CATALYST
C.MOREAU,L.BEKAKRA,A.MESSALHI,J.L.OLIVE
and P.GENESTE
L a b o r a t o i r e de Chimie Organique Physique e t C i n 6 t i q u e Chimique Appl iquees, U.A.-C.N.R.S.
418, Ecole N a t i o n a l e S u p e r i e u r e de Chimie de M o n t p e l l i e r , 8 r u e
Ecole Normale
-
34075 M o n t p e l l i e r Cedex,France.
ABSTRACT The h y d r o d e n i t r o g e n a t i o n o f q u i n o l i n e was performed o v e r an i n d u s t r i a l s u l p h i d e d NiMo/Al 0 c a t a l y s t a t 340°C and 70 b a r H i n t h e presence o f H2S and 2 , 6 - d i e t h y l a n ? l ? n e under batch r e a c t o r c o n d i t i o n $ . The a d d i t i o n o f H S c o n f i r m s t h e p r e v i o u s f i n d i n g s c o n c e r n i n g t h e p r o d u c t d i s t r i b u t i o n f r o m 2 1,2,3,4-tetrahydroquinoline: H S i n c r e a s e s t h e p e r centage o f C - N bond cleavage and decreases t h a t o f r k g h y d r o g e n a t i o n . The e f f e c t o f 2 , 6 - d i e t h y l a n i l i n e on h y d r o d e n i t r o g e n a t i o n o f q u i n o l i n e has been shown t o be s i m i l a r t o t h a t o f H S. I n o r d e r t o account f o r t h i s s i m i l a r beha v i o u r , i t i s proposed t h a t H2S d o u l d i n c r e a s e t h e h y d r o g e n o l y s i s r a t e , whereas 2 , 6 - d i e t h y l a n i l i n e would, i n t u r n , decrease t h e h y d r o g e n a t i o n r a t e , t h u s leading t o a n e a r l y constant r e s u l t i n g e f f e c t .
INTRODUCTION I t i s w e l l known t h a t t h e presence o f H2S o r H2S p r e c u r s o r s i n c r e a s e s
s i g n i f i c a n t l y t h e r a t e o f h y d r o d e n i t r o g e n a t i o n (HDN) s u l p h i d e d NiMo/A1203 c a t a l y s t ( r e f . 1 ) .
o f q u i n o l i n e over a
T h i s e f f e c t has been observed under
f l o w - r e a c t o r c o n d i t i o n s i n b o t h t h e l i q u i d and vapour phase ( r e f . 2 ) . a l s o r e c e n t l y r e p o r t e d by S a t t e r f i e l d ( r e f . 3 ) as H2S,
I t was
t h a t H20 a c t s i n t h e same way
b u t t h e enhancing e f f e c t o f w a t e r a l o n e i s much l e s s t h a n t h a t
e x h i b i t e d by H2S alone. We
report
2,6-diethylaniline
here
the
effect
of
a
nitrogen-containing
molecule,
- on t h e mechanism o f t h e HDN o f q u i n o l i n e o v e r a (DEA),
commercial s u l p h i d e d Ni0-Mo03/A1203 c a t a l y s t a t 340°C and 70 b a r H2.
EXPERIMENTAL The c a t a l y s t used was P r o c a t a l y s e HR 346, which had t h e c o m p o s i t i o n 3% N i O , 14% Moo3 and 83% A1203. I t was s u l p h i d e d a t atmospheric p r e s s u r e u s i n g a
f l u i d i z e d - b e d t e c h n i q u e w i t h a gas m i x t u r e o f 15% H2S and 85% H2 by volume. The c a t a l y s t ( 5 g;
p a r t i c l e s i z e 0.100-0.125
mm) was heated i n a f l o w o f
H2/H2S (gas f l o w - r a t e 120 ml/min) f r o m 20 t o 400°C (8"C/min) and h e l d a t
108 400°C f o r 4 h, t h e n c o o l e d and f i n a l l y swept w i t h n i t r o g e n f o r 30 min. Experiments
were
carried
out
( A u t o c l a v e Engineers Magne-Drive),
in
a
0.3-litre
stirred
autoclave
o p e r a t i n g i n t h e b a t c h mode and equipped
w i t h a system f o r sampling o f l i q u i d d u r i n g t h e course o f t h e r e a c t i o n without stopping t h e a g i t a t i o n . The procedure was t y p i c a l l y as f o l l o w s . 2,6-diethylaniline
i n decane
or
dodecane
A m i x t u r e o f q u i n o l i n e and
(80 m l )
was
poured
into
the
autoclave. The s u l p h i d e d c a t a l y s t (0.8 g ) was r a p i d l y added t o t h i s s o l u t i o n under n i t r o g e n t o a v o i d c o n t a c t w i t h a i r . nitrogen,
After
i t had been purged w i t h
t h e t e m p e r a t u r e was i n c r e a s e d under n i t r o g e n u n t i l
reached
it
340°C. N i t r o g e n was t h e n removed and hydrogen was i n t r o d u c e d a t t h e r e q u i r e d pressure ( 7 0 b a r ) . Zero t i m e was t a k e n t o be when t h e a g i t a t i o n began. Analyses were performed on a G i r d e l 30 gas chromatograph equipped w i t h a f l a m e i o n i z a t i o n d e t e c t o r u s i n g hydrogen as c a r r i e r gas.
The w a l l - c o a t e d
o p e n - t u b u l a r f u s e d - s i l i c a c a p i l l a r y columns used were Chrompack C P - S i l (OV-1) o r CP-Si1 19 CB (OV-171,
10 m x 0.22 mm i . d .
5 CB
Products were i d e n t i f i e d
by comparison w i t h a u t h e n t i c samples and GC-MS a n a l y s i s . The r a t e c o n s t a n t s were deduced f r o m t h e e x p e r i m e n t a l p l o t s by c u r v e f i t t i n g and s i m u l a t i o n u s i n g an HP 9820 computer w i t h an HP 9826 A t r a c i n g t a b l e , assuming a l l t h e r e a c t i o n s t o be f i r s t o r d e r i n t h e o r g a n i c r e a c t a n t .
RESULTS AND DISCUSSION The k i n e t i c r e a c t i o n network f o r h y d r o d e n i t r o g e n a t i o n o f q u i n o l i n e a t 340°C and 70 b a r H2 under b a t c h r e a c t o r c o n d i t i o n s i s g i v e n i n F i g u r e 1 and does n o t d i f f e r f r o m t h o s e under f l o w r e a c t o r c o n d i t i o n s r e p o r t e d p r e v i o u s l y ( r e f s . 1-3).
Hydroprocessing o f quinoline alone
-
I n t h e absence o f any a d d i t i v e , t h e h y d r o d e n i t r o g e n a t i o n o f q u i n o l i n e
(2)occurs
according
to
the
following
1,2,3,4-tetrahydroquinoline (1,2,3,4-THQ) p r o p y l c y c l ohexane ( PCH)
.
sequence
: quinoline
(g)--+-
+ d e c a h y d r o q u i n o l i n e (5)
Hydroprocessing o f quinoline i n the presence o f H2Z I n t h e presence o f CS2 i n t h e feed,
a c t i n g as H2S p r e c u r s o r ,
Sat-
t e r f i e l d ( r e f . 1 ) has shown t h a t t h e amount o f d e c a h y d r o q u i n o l i n e decreases w h i l e t h e amount o f o - p r o p y l a n i l i n e i n c r e a s e s markedly,
as i l l u s t r a t e d i n
F i g u r e 1. These experiments were c a r r i e d o u t i n t h e l i q u i d phase and i n a t r i c k l e - b e d r e a c t o r . S i m i l a r o b s e r v a t i o n s have a l s o been r e p o r t e d by P e r o t u s i n g m e t h y l d i s u l p h i d e i n s t e a d o f CS2 as H2S g e n e r a t o r ( r e f . 4 ) .
109
Q
5,678-T H Q
1,2,3,4 - T HQ
OFA
DHQ
PC H
F i g . 1 K i n e t i c r e a c t i o n network f o r HDN o f q u i n o l i n e a l o n e o v e r a s u l p h i d e d NiMo/A1203 c a t a l y s t a t 340°C and 70 b a r H2 under b a t c h r e a c t o r c o n d i t i o n s . By o p e r a t i n g i n a b a t c h r e a c t o r a t s i m i l a r temperature and hydrogen
pressure, we have shown t h a t t h e a d d i t i o n o f gaseous H2S t o t h e i n i t i a l feed leads
to
similar
conclusions
concerning
the
product
distribution
1,2,3,4-tetrahydroquinoline as shown i n F i g u r e 3.
0
1
2
3
4
5
6
0
.5
F i g . 2 Product d i s t r i b u t i o n
F i g . 3 Amount o f OPA
(mole % ) v s w t % CS2 i n f e e d
(mole % ) vs pH2S ( b a r )
(data from r e f .1)
1.5
from
110 F a i r l y good p a r a l l e l i s m i s a l s o observed f o r t h e r a t e c o n s t a n t s f o r h y d r o g e n o l y s i s o f t h e C-N bond ( F i g . aromatic r i n g ( F i g .
1,
kl)
and f o r h y d r o g e n a t i o n o f t h e
1, k 2 ) o f 1,2,3,4-tetrahydroquinoline,
as r e p o r t e d i n
Tables 1 and 2. These o b s e r v a t i o n s a r e v a l i d whatever t h e s o u r c e o f H2S and type
of
reactor.
I t can be
noted is
1,2,3,4-tetrahydroquinoline
that
the
nearly
hydrogenolysislhydrogenation s e l e c t i v i t y (kl/k2)
rate
of
constant,
disappearance although
of
the
i n c r e a s e s on i n c r e a s i n g t h e
c o n c e n t r a t i o n i n CS2 ( T a b l e 1 ) o r H2S p r e s s u r e ( T a b l e 2 ) .
TABLE 1 : Rate c o n s t a n t s ( x 104 m o l / g o f c a t .
h ) f o r t h e disappearance o f
1,2,3,4-tetrahydroquinoline i n t h e presence o f CS2 a t 350°C, 6.9 MPa H2, i n a t r i c k l e - b e d r e a c t o r ( d a t a f r o m Ref .1)
%
cs2
kl
0 0.59 1.47 5.89
k2
28.5 33 30 22
kl
k2
kl/k2
2.5
26 25 20 14
0.1 0.3 0.5 0.6
a
10 8
TABLE 2 : Rate c o n s t a n t s ( x
lo4
% OPA 9 24 33 36
min-l/g o f cat.)
f o r t h e disappearance o f
1,2,3,4-tetrahydroquinoline a t 340"C, 70 b a r H2, i n a b a t c h r e a c t o r
P
H2S
kl k 2
0 0.5 1 1.5
75 77 77 132
kl
k2
llk2
0 18 25 53
75 59 52 79
0 0.3 0.5 0.7
% OPA 0 23 32 40
Hydroprocessing o f q u i n o l i n e i n t h e presence o f 2 , C - d i e t h y l a n i l i n e The r a t e c o n s t a n t s f o r t h e disappearance o f 1,2,3,4-tetrahydroquinoline i n t h e presence concentration
o f 2,6-diethylaniline in
2,6-diethylaniline
quinoline
(0.12
a r e r e p o r t e d i n Table 3 f o r a g i v e n M)
and
various
concentrations
in
and i n Table 4 f o r a t o t a l c o n c e n t r a t i o n i n N compounds
(quinoline + 2,6-diethylaniline
=
0.12 M ) .
111 4 . -1 TABLE 3 : Rate c o n s t a n t s ( x 10 min / g o f c a t . )
f o r t h e disappearance o f
1,2,3,4-tetrahydroquinoline i n t h e presence o f 2 , 6 - d i e t h y l a n i l i n e
a t 340°C,
70 bar H2, i n a b a t c h r e a c t o r ( [ Q ] = 0.12 M I .
[2,6-DEA],M ~
~~~
kl + k 2 ~~~
kl
k2
0 15 23 26
62 53 40 25
-
%OPA
kl'k2
~~
0 0.06 0.12 0.24
62 68 63 51
0 0.3 0.5 1
TABLE 4 : Rate c o n s t a n t s ( x 104 m i n - l / g o f c a t . )
0 21 36 49
f o r t h e disappearance o f
1,2,3,4-tetrahydroquinoline i n t h e presence o f 2 , 6 - d i e t h y l a n i l i n e
at
340°C,
70 bar H2, i n a b a t c h r e a c t o r ([Q]+[DEA] = 0.12 M)
[Ql,M
kl + k 2
[2,6-DEA],M
0.12 0.08 0.06 0.04
0.00 0.04 0.06 0.08
62 83 108 92
kl
k2
0 20 40 47
62 63 68 45
kl/k2 0 0.3 0.6 1
-
%OPA 0 24 37 51
The t a b l e s i n d i c a t e t h a t t h e amount o f o - p r o p y l a n i l i n e (DPA) - increases on
of
addition
various
amounts
of
2,6-diethylaniline
concentration i n quinoline o r f o r a given t o t a l i s i l l u s t r a t e d i n F i g u r e s 4 and 5, r e s p e c t i v e l y
1
0
-06
.I2
F i g . 4. Amount o f OPA (mole % ) vs [ 2,6 DEA];
[QI
= constant
.24
+ MP/o
0
.
for
a
given
N content i n t h e feed. This
-04 .06 -08
F i g . 5. Amount o f OPA (mole % ) v s [2,6 D E A ] Total N content = constant
112
It
should
also
be
s e l e c t i v i t y , g i v e n as kl/k2,
noted
that
the
hydrogenolysislhydrogenation i n c r e a s e s w i t h i n c r e a s i n g [ 2,6-DEA]/[Q] r a t i o ,
as i l l u s t r a t e d i n F i g . 6.
/ LQ7
DEA]
'
F i g . 6 S e l e c t i v i t y ( k , / k 2 ) f o r t h e disappearance o f 1,2,3,4-tetrahydroquinol i n e v s c o n c e n t r a t i o n r a t i o [ 2,6-DEA]/[Q]. The most i m p o r t a n t q u e s t i o n which t h e n a r i s e s i s t o account f o r t h e similar
behaviour
of
H2S
and
2,6-diethylaniline
on
the
hydrogenolysislhydrogenation r a t i o (kl/k2) f o r the disappearance of 1,2,3,4-tetrahydroquinoline. Indeed, t h e s e t w o a d d i t i v e s a r e known t o d i f f e r c o n s i d e r a b l y i n t h e i r acid-base p r o p e r t i e s . A p o s s i b l e e x p l a n a t i o n c o n s i s t s of
a
"push-pull"
effect,
i n which
H2S would
increase
the
number
of
h y d r o g e n o l y s i s s i t e s and b a s i c m o l e c u l e s would, i n t u r n , decrease t h e number of
hydrogenation s i t e s ,
the
resulting effect
on
t h e disappearance
of
1,2,3,4-tetrahydroquinoline b e i n g n e a r l y c o n s t a n t w i t h b o t h a d d i t i v e s , as i s observed e x p e r i m e n t a l l y . Table
5,
in
which
T h i s k i n d o f compensating e f f e c t i s i l l u s t r a t e d i n we
1,2,3,4-tetrahydroquinoline
report
the
in
the
disappearance presence
rate
of
constants
both
H2S
of and
2 , 6 - d i e t h y l a n i 1i n e . Although
a
slight
decrease
in
the
selectivity
(kl/k2)
and
the
percentage o f o - p r o p y l a n i l i n e i s observed f o r s i m u l t a n e o u s l y added 2,6-DEA and H2S compared w i t h i n d i v i d u a l l y added 2,6-DEA
and H2S,
we cannot draw
unambiguous c o n c l u s i o n s c o n c e r n i n g t h i s compensating e f f e c t compared w i t h t h e a d d i t i v i t y e f f e c t of H2S and H20 r e p o r t e d by S a t t e r f i e l d ( r e f . 5 ) .
113
4 . -1 TABLE 5 : Rate c o n s t a n t s ( x 10 min / g o f c a t . )
f o r t h e disappearance o f
1,2,3,4-tetrahydroquinoline i n t h e presence o f 2 , 6 - d i e t h y l a n i l i n e
and H2S a t
340°C, 70 b a r H2, i n a b a t c h r e a c t o r .
Additive
kl k2 +
2,6-DEA
108 77 110
H2S 2,6-DEA + H2S
kl
k2
kl/k2
40 25 33
68 52 77
0.59 0.48 0.43
I n i t i a l c o n c e n t r a t i o n s : [ Q ] = 0.06 M; [2,6-DEA]
= 0.06
%OPA 37 32 30
M;
pH2S = 1 b a r .
S a t t e r f i e l d and co-workers ( r e f s . 1 - 2 ) found t h a t t h e e f f e c t o f CS2 on t h e c o n v e r s i o n o f q u i n o l i n e was r e v e r s i b l e . Although an experiment t o t e s t such an o b s e r v a t i o n can be done o n l y i n a f l o w system,
a similar reversible
e f f e c t c o u l d be expected w i t h 2 , 6 - d i e t h y l a n i l i n e . The i n f l u e n c e o f H2S on t h e equi 1ib r i u m between h y d r o g e n o l y s i s and hydrogenation s i t e s and t h e n a t u r e o f t h e s e c a t a l y t i c s i t e s have a l r e a d y been considered (refs.1,6,7).
I n o r d e r t o e x p l a i n t h e e f f e c t o f water, S a t t e r f i e l d
assumed t h a t a d s o r p t i o n s o f H2S and H20 each i n c r e a s e t h e c a t a l y s t a c i d i t y and, consequently, C - N bond cleavage. W i t h t h e new r e s u l t s o b t a i n e d on t h e effect o f 2,6-diethylaniline,
an a l t e r n a t i v e e x p l a n a t i o n can be proposed. The
e q u i l i b r i u m between h y d r o g e n a t i o n and h y d r o g e n o l y s i s s i t e s can be r e g a r d e d as an acid-base e q u i l i b r i u m which would be s h i f t e d t o h y d r o g e n o l y s i s s i t e s by a d d i t i o n o f a c i d i c H2S and t o h y d r o g e n a t i o n s i t e s by a d d i t i o n 2,6-diethylani l i n e .
Water
would
be
expected
to
act
as
an
of
basic
amphoteric
substance. In
other
respects,
considered i n m i x t u r e s .
competitive We have
shown
adsorption
effects
must
also
be
i n t h e p r e c e d i n g paper t h a t t h e
hydrodeni t r o g e n a t i o n o f a1 k y l ani 1 i n e s is s t r o n g l y i n h i b i t e d by h e a v i e r
N-
c o n t a i n i n g molecules, whereas HDN o f t h e l a t t e r N - c o n t a i n i n g m o l e c u l e s a r e moderately i n h i b i t e d by a l k y l a n i l i n e s . The a c c e s s i b i l i t y o f m o l e c u l e s t o t h e c a t a l y t i c s i t e s i s an i m p o r t a n t parameter t o t a k e i n t o account, i n d e p e n d e n t l y of t h e number and t h e n a t u r e o f t h e s e c a t a l y t i c s i t e s . ACKNOWLEDGMENTS T h i s work was performed i n t h e framework o f t h e European C o n t r a c t "CCE-GERTH-CNRS: nouveaux c a t a l y s e u r s pour l ' h y d r o d k a z o t a t i o n de coupes 1ourdes"
.
114
R E F E R E N C E S 1 - Yang, S.H., and S a t t e r f i e l d , C.N., Ind. Eng. Chem. Process Des. Dev., 23( 1984120. 2 - S a t t e r f i e l d , C.N., and Yang, S.H., Ind. Eng. Chem. Process Des. Dev., 23( 1984) 11. and Morris, C.N., Ind. Eng. Chem. Process Des. Dev., 3 - S a t t e r f i e l d , C.N., 25( 19861942. 4 - Brunet, S., and Perot, G., React. K i n e t . Catal. L e t t . , 29,(1985)15. 5 - S a t t e r f i e l d , C.N., M o r r i s Smith, C., and I n g a l i s , M., Ind. Eng. Chem. Process Des. Dev., 24(1985)1000. 6 - Kwart, H., Katzer, J., and Horgan, J., J. Phys. Chem., 86,(1982)2641. 7 - Yang, S.H., and S a t t e r f i e l d , C . N . , J. Catal., 81(1983) 168 ; 81 (19831335.
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SEARCH
FOR
SIMPLE
MODEL
COMPOUNDS
TO
SIMULATE
ME
115
INHIBITION
OF
HYDRODENITROGENATION REACTIONS BY ASPHALTENES
C.MOREAU,L.BEKAKRA,R.DURANO,N.ZMIMITA
and P.GENESTE
L a b o r a t o i r e de Chimie Organique Physique e t C i n k t i q u e Chimique Appl qukes, U.A.-C.N.R.S.
418, Ecole N a t i o n a l e S u p k r i e u r e de Chimie de M o n t p e l l i e r
8 rue
Ecole Normale, 34075 M o n t p e l l i e r Cedex,France ABSTRACT The h y d r o d e n i t r o g e n a t i o n o f 2 , 6 - d i e t h y l a n i l i n e was performed o v e r an i n d u s t r i a l s u l p h i d e d NiMo/Al 0 c a t a l y s t a t 340°C and 70 b a r H i n the presence o f h e a v i e r N-conta?n?ng compounds such as q u i n o l i n e , G c r i dine, c a r b a z o l e and p h e n a n t h r i d i n e . These compounds have been f o u n d t o i n h i b i t t h e h y d r o d e n i t r o g e n a t i o n o f 2 , 6 - d i e t h y l a n i l i n e by f a c t o r s o f 5-25. The i n h i b i t i n g e f f e c t has been shown t o r e s u l t f r o m t h e presence o f a r o m a t i c o r s a t u r a t e d p o l y c y c l i c systems.
INTRODUCTION I t was r e c e n t l y shown ( r e f . 1 )
f o r t h e h y d r o d e n i t r o g e n a t i o n (HON) o f
d i s t i l l a t e s r e s u l t i n g from t h e conversion o f
heavy f e e d s t o c k s
that
the
c o n v e r s i o n o f b a s i c compounds, a l k y l a n i l i n e s i n p a r t i c u l a r , i s d i f f i c u l t i n t h e presence o f o t h e r compounds p r e s e n t i n t h e feed.
These compounds would
i n h i b i t t h e h y d r o g e n a t i o n o f t h e a r o m a t i c r i n g and,
as a consequence, t h e
c o n v e r s i o n o f a1 k y l ani 1 ines. Our c o n t i n u i n g i n t e r e s t i n t h e s t u d y o f t h e r e a c t i v i t y o f o r g a n i c model compounds i n h y d r o t r e a t i n g over s u l p h i d e d c a t a l y s t s ( r e f . 2 ) l e d us t o d e v e l o p a s i m p l e model capable, on a l a b o r a t o r y s c a l e , o f s i m u l a t i n g t h e i n h i b i t i o n o f HDN r e a c t i o n s b y asphaltenes. Although t h e i r exact s t r u c t u r e s a r e n o t w e l l d e f i n e d , a s p h a l t e n e s a r e g e n e r a l l y h i g h l y condensed ( l o w H / C r a t i o ) p o l y c y c l i c compounds c o n t a i n i n g heteroatoms, S, N and 0 (Fig.11, type structures ( r e f . 3 ) .
and a r e f r e q u e n t l y a s s o c i a t e d w i t h p o r p h y r i n
116
CH3
t
CH3
CH3 Fig. 1 Hypothetical asphaltene s t r u c t u r e . Whatever t h e proposed h y p o t h e t i c a l s t r u c t u r e f o r asphaltenes, framework
is
always
present,
i.e.,
the
condensed
a common
polyaromatic
system
c o n t a i n i n g heteroatoms, p a r t i c u l a r l y N atoms. T h i s framework i s expected t o be t h e most r e s i s t a n t t o h y d r o t r e a t i n g under c l a s s i c a l h i g h - t e m p e r a t u r e
and
high-hydrogen-pressure o p e r a t i n g c o n d i t i o n s . Recent r e s u l t s c o r r o b o r a t e t h i s hypothesis (ref.4):
t h e n i t r o g e n c o n t e n t i n asphaltenes
a f t e r severe h y d r o t r e a t i n g c o n d i t i o n s , (NiMo, N i W or C O W ) .
phenanthrjdine.
These The
models
are
problem
is
could simulate the i n h i b i t i n g e f f e c t o f quinoline, therefore
acridine, posed
competition.
@
a c t i v e phase
We t h e r e f o r e c o n s i d e r e d t h a t h e a v i e r N-heteroaromatics,
used as models i n o t h e r r e s p e c t s , asphaltenes.
remains unchanged
whatever t h e c a t a l y s t
N
@-N
in
carbazole terms
of
and
HDN/HDN
09 00
H Q u ino1 ine
Acri dine
Carbazole
Phenanthridine
117 EXPERIMENTAL The c a t a l y s t used was P r o c a t a l y s e HR 346, which had t h e c o m p o s i t i o n 3% N i O , 14% Moo3 and 83% A1203. I t was s u l p h i d e d a t atmospheric p r e s s u r e u s i n g a
f l u i d i z e d - b e d t e c h n i q u e w i t h a gas m i x t u r e o f 15% H2S and 85% HZ by volume. The c a t a l y s t ( 5 g; H2/H2S
p a r t i c l e s i z e 0.100-0.125
mm) was heated i n a f l o w o f
(gas f l o w - r a t e 120 m l / m i n ) f r o m 20 t o 400°C (8"C/min)
and h e l d a t
400°C f o r 4 h, t h e n c o o l e d and f i n a l l y swept w i t h n i t r o g e n f o r 30 min. Experiments
were
carried
out
in
a
0.3-litre
stirred
autoclave
( A u t o c l a v e Engineers Magne-Drive), o p e r a t i n g i n t h e b a t c h mode and equipped w i t h a system f o r sampling o f l i q u i d d u r i n g t h e c o u r s e o f t h e r e a c t i o n without stopping t h e a g i t a t i o n . The procedure was t y p i c a l l y 2,6-diethylaniline
as f o l l o w s .
An
equimolar
mixture o f
(0.06 M) and i n h i b i t o r (0.06 M ) i n decane o r dodecane ( 8 0
m l ) was poured i n t o t h e a u t o c l a v e . The s u l p h i d e d c a t a l y s t ( 0 . 8 g ) was r a p i d l y added t o t h i s s o l u t i o n under n i t r o g e n t o a v o i d c o n t a c t w i t h a i r . A f t e r i t had been purged w i t h n i t r o g e n , t h e t e m p e r a t u r e was i n c r e a s e d under n i t r o g e n u n t i l i t reached 340°C. N i t r o g e n was t h e n removed and hydrogen was i n t r o d u c e d a t
t h e r e q u i r e d p r e s s u r e ( 7 0 b a r ) . Zero t i m e was t a k e n t o be when t h e a g i t a t i o n began. Analyses were performed on a G i r d e l 30 gas chromatograph equipped w i t h a f l a m e i o n i z a t i o n d e t e c t o r u s i n g hydrogen as c a r r i e r gas.
The w a l l - c o a t e d
open t u b u l a r f u s e d s i l i c a c a p i l l a r y columns used were Chrompack C P S i l 5 CB (OV-1) o r C P S i l 19 CB (OV-171, 10 m x 0 . 2 2 mm i . d .
Products were i d e n t i f i e d
by comparison w i t h a u t h e n t i c samples and GC-MS a n a l y s i s . The r a t e c o n s t a n t s were deduced f r o m t h e e x p e r i m e n t a l p l o t s by c u r v e f i t t i n g and s i m u l a t i o n u s i n g an HP 9820 computer w i t h an HP 9826 A t r a c i n g t a b l e , assuming a l l t h e r e a c t i o n s t o be f i r s t o r d e r i n t h e o r g a n i c r e a c t a n t .
RESULTS AND DISCUSSION I n t h e absence o f
H2Z,
q u i n o l i n e , a c r i d i n e and c a r b a z o l e were f o u n d t o
i n h i b i t t h e hydrodenitrogenation o f 2,6-diethylaniline
b y a f a c t o r o f 6,
whereas p h e n a n t h r i d i n e was found t o lower t h e r a t e o f h y d r o d e n i t r o g e n a t i o n o f 2,6-diethylaniline
by
a
factor
disappearance o f 2 , 6 - d i e t h y l a n i l i n e
of
25.
The
rate
constants
for
the
i n t h e absence and i n t h e presence o f
h e a v i e r N-compounds a r e r e p o r t e d i n Table 1.
118 TABLE
1.
Disappearance
2,6-diethylaniline
rate
constants
(in
min-l.g.cat.-’)
for
HDN
of
i n t h e absence and presence o f h e a v i e r N - h e t e r o a r o m a t i c s
and a r o m a t i c s a t 340°C and 70 b a r H2 o v e r s u l p h i d e d NiMo/A1203 c a t a l y s t .
lo4
k x
Inhibitor
Inhibiting factor
None
100
Q u ino1 ine
18
6
Carbazol e
17
6
A c r i d i ne
18
6
P h e n a n t h r i d i ne
4
Ant hr ac ene
44
Phenanthrene
45
25 2 2
The i n h i b i t i o n o f t h e h y d r o d e n i t r o g e n a t i o n o f 2 , 6 - d i e t h y l a n i l i n e by c a r b a z o l e and p h e n a n t h r i d i n e i s i l l u s t r a t e d i n F i g u r e s 2 and 3, r e s p e c t i v e l y .
MI% I I
t
120
0
240
3602n.O
t-
120
240
Zn.
360
Fig. 2 : P l o t o f concentrations
Fig. 3 : P l o t o f concentrations
vs t i m e f o r simultaneous r e a c -
vs t i m e f o r s i m u l t a n e o u s r e a c -
t i on o f 2,6-di e t h y l a n i 1ine
t i o n o f 2,6-diethylaniline (
(
I
and c a r b a z o l e ( 0
2,6-diethylaniline
alone
1
and
(01.
and p h e n a n t h r i d i n e ( 0
1.
I
1
119 From Table 1,
i t can be seen t h a t t h e
inhibiting effect
i s more
pronounced f o r N - c o n t a i n i n g molecules ( a c r i d i n e and p h e n a n t h r i d i n e ) t h a n f o r t h e i r p a r e n t hydrocarbons (anthracene and phenanthrene). These r e s u l t s can be
Ant h r acene
Phenanthrene
compared w i t h t h e h y d r o d e s u l p h u r i z a t i o n o f t h i o p h e n e i n t h e pyridine.
Pyridine i s well
r e c e n t l y shown t h a t benzene,
known t o i n h i b i t thiophene HDS,
presence o f b u t we
have
t h e p a r e n t a r o m a t i c hydrocarbon o f p y r i d i n e ,
does n o t i n h i b i t t h e h y d r o d e s u l p h u r i z a t i o n o f t h i o p h e n e ( r e f . 5 ) .
This could
mean t h a t h e t e r o a t o m - c o n t a i n i n g compounds a r e adsorbed on t h e same t y p e o f catalytic
site
and
that
inhibiting
effects
result
from
competitive
a d s o r p t i o n s on t h i s s i t e , i n agreement w i t h p r e v i o u s assumptions ( r e f . 1 ) . I t can a l s o be seen f r o m Table 1 t h a t a l a r g e r i n h i b i t i n g e f f e c t o c c u r s
i n t h e presence o f p h e n a n t h r i d i n e . T h i s was n o t c o m p l e t e l y unexpected as t h e h y d r o d e n i t r o g e n a t i o n o f p h e n a n t h r i d i n e was e x t r e m e l y d i f f i c u l t . Only 20% o f d e n i t r o g e n a t e d compounds a r e p r e s e n t a f t e r 24 h o f h y d r o t r e a t m e n t . The l a s t p o i n t t o be n o t e d f r o m t h e s e simultaneous HDN/HDN r e a c t i o n s i s t h e s l i g h t i n h i b i t i n g e f f e c t ( b y a f a c t o r o f about 2) o f 2 , 6 - d i e t h y l a n i l i n e on t h e HDN o f q u i n o l i n e , a c r i d i n e , c a r b a z o l e and p h e n a n t h r i d i n e ( r e f . 5 ) . I n t h e presence o f H2S ( 1 b a r a t room t e m p e r a t u r e ) , analysis
of
the
inhibiting effect
of
quinoline
and
a more d e t a i l e d
phenanthridine
was
i n v e s t i g a t e d i n o r d e r t o understand t h e o r i g i n o f t h i s i n h i b i t i n g e f f e c t . C o n c e n t r a t i o n vs t i m e p l o t s f o r t h e HDN o f 2 , 6 - d i e t h y l a n i l i n e
i n t h e presence
o f q u i n o l i n e ( o r i t s i n t e r m e d i a t e s ) a r e g i v e n i n F i g u r e 4. The
inhibiting
disappearance
of
effect
is
observed
up
1,2,3,4-tetrahydroquinoline
to
the
nearly
(t=240-300
complete
min).
This
corresponds t o t h e c o n v e r s i o n o f t e t r a h y d r o q u i n o l i n e i n t o 2 - p r o p y l a n i l i n e .
We
have shown t h a t t h e r e was no i m p o r t a n t i n h i b i t i n g e f f e c t f o r s i m u l t a n e o u s HDN/HDN r e a c t i o n s between s u b s t i t u t e d a n i l i n e s ( 5 ) . As a consequence, t h e HDN
rate o f 2,6-diethylaniline i n t h e presence o f 2 - p r o p y l a n i l i n e i s t h a t n o r m a l l y expected ( 30 x l o 4 m i n - l . g . c a t . -1 a t 340°C, 70 b a r He, 1 b a r H 2 S ) . It can t h e r e f o r e be concluded t h a t
aromatic
N-compounds
(quinoline)
and
p a r t i a l l y s a t u r a t e d compounds (1,2,3,4-tetrahydroquinoline) a r e r e s p o n s i b l e f o r t h e i n h i b i t i o n o f t h e hydrodenitrogenation o f 2,6-diethylaniline.
120
F i g . 4 : P l o t o f c o n c e n t r a t i o n s v s t i m e f o r simultaneous 2 , 6 - d i e t h y l a n i l i n e and q u i n o l i n e i n t h e presence o f H2S.
reaction
of
The i n h i b i t i n g e f f e c t i s p a r t i c u l a r l y r e i n f o r c e d i n t h e presence o f p h e n a n t h r i d i n e and i t s two most i m p o r t a n t i n t e r m e d i a t e s , ( 1,2,3,4,5,6,7,8-octahydrophenanthri d i n e )
and
fully
p a r t i a l l y saturated
saturated
(perhydrophenanthridine), as i l l u s t r a t e d i n F i g u r e 5. 2 , 6 - D i e t h y l a n i l i n e
does
n o t r e a c t a t a l l i n t h e presence o f t h e N - p o l y c y c l i c compounds.
f M% , Et
I
43n
3/
n
I
I
ttrnin.
F i g . 5 : P l o t o f c o n c e n t r a t i o n s vs t i m e f o r simultaneous 2 , 6 - d i e t h y l a n i l i n e and p h e n a n t h r i d i n e i n t h e presence o f H2S.
reaction
of
121
CONCLUSION
The i n h i b i t i o n o f t h e h y d r o d e n i t r o g e n a t i o n o f a l k y l a n i l i n e s by h e a v i e r N - c o n t a i n i n g molecules r e s u l t s f r o m c o m p e t i t i v e a d s o r p t i o n on t h e same t y p e o f c a t a l y t i c s i t e as a l r e a d y assumed i n t h e l i t e r a t u r e . shown t h a t t h e presence o f p o l y c y c l i c molecules,
Moreover we have
aromatic o r p a r t i a l l y o r
t o t a l l y saturated, i s mainly responsible f o r t h i s i n h i b i t i n g e f f e c t . According t o t h e s e o b s e r v a t i o n s ,
p h e n a n t h r i d i n e and/or
i n t e r m e d i a t e s a r e s u i t a b l e models t o s i m u l a t e ,
i t s reaction
on a l a b o r a t o r y s c a l e ,
the
i n h i b i t i o n o f h y d r o d e n i t r o g e n a t i o n o f a l k y l a n i l i n e s by asphaltenes.
ACKNOWLEDGMENTS
This
work
was
"CCE-GERTH-CNRS:
1 ourdes"
performed
in
the
framework
nouveaux c a t a l y s e u r s pour
of
the
European
1 ' h y d r o d e s a z o t a t i o n de
Contract coupes
.
R E F E R E N C E S
1
-
Toulhoat H., and Kessas R.,Rev.Fr.I.F.P.,41(1986)511.
2 - Moreau C.,
and Geneste P.,
3 - Mc C u l l o c h D.C.,
i n B.E.
Catalysis,Vol.7,submitted f o r p u b l i c a t i o n . Leach ( E d i t o r ) , A p p l i e d I n d u s t r i a l C a t a l y s i s ,
Academic Press ,New York. 11 ( 1983169.
4 - Marseu R.,Martino G.,
and P l u m a i l J.C.,Proceedings
Congress on Catalysis,Calgary,(l988)144.
5
-
Zmimita N.,
Doctorat Thesis,Montpellier
(1987).
IXth
International
This Page Intentionally Left Blank
M.L. Occelli and R.G. Anthony (Editors), Aduances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
123
THE VERSATILE ROLE OF NICKEL I N Ni-MoS2/A1203 HYDROTREATING CATALYSTS AS SHOWN BY THE USE OF PROBE MOLECULES
J.P.
BONNELLE~, A.
WAMBEKE~, A.
KHERBECHE',
R.
HUBAUT',
L.
JALOWIECKI~,
s.
KASZTELAN112 and J. GRIMBLOT' 'Laboratoire de Catalyse Heterogene e t Homoggne, U.A. C.N.R.S. 402, U n i v e r s i t e des Sciences e t Techniques de L i l l e Flandres-Artois, F-59655 Villeneuve d'Ascq Cedex (France) n
LPresent adress : I n s t i t u t Francais du Petrole, Malmaison Cedex (France)
B.P.
311,
F-92506 R u e i l -
ABSTRACT Probe molecules have been used t o t e s t MoS2/AI203 and nickel-promoted MoS2/A1203 c a t a l y s t s w i t h c o n t r o l led S/metal r a t i o obtained by prereduction o f the samples a t d i f f e r e n t temperatures under hydrogen. Tests under m i Id condit i o n s , namely isoprene hydrogenation a t atmospheric pressure and low temperat u r e , o r t e s t s a t conventional high pressures and temperatures such as toluene hydrogenation and p y r i d i n e hydrodenitrogenation, have been used t o i n v e s t i g a t e the r o l e o f n i c k e l i n these c a t a l y s t s . The v e r s a t i l i t y o f n i c k e l is shown through a poisoning e f f e c t o f isoprene hydrogenation, a l a r g e promoting e f f e c t o f toluene hydrogenation and a small promoting e f f e c t o f p y r i d i n e hydrodenitrogenation. I n a d d i t i o n , t h e higher promoting e f f e c t observed f o r toluene hydroge n a t i o n disappears a f t e r reduction o f t h e c a t a l y s t . This e f f e c t i s due t o t h e d e s t a b i l i z a t i o n o f t h e n i c k e l species i n a decoration p o s i t i o n a t t h e edges o f the MoS2 slabs as shown by X-ray photoelectron spectroscopy. INTRODUCTION
Molybdenum -or tungsten- based hydrotreating c a t a l y s t s a f t e r s u l p h i d i n g can be described as small MoS2(WS2) p l a t e l e t s w e l l dispersed over t h e alumina support surface, as shown by h i g h - r e s o l u t i o n e l e c t r o n microscopy ( r e f s .
1-3).
These c a t a l y s t s have p r o p e r t i e s t h a t are g r e a t l y improved f o r many r e a c t i o n s involved i n the hydroprocessing o f o i l f r a c t i o n s when c o b a l t o r n i c k e l i s added as a promoter w i t h an optimum content such t h a t t h e atomic r a t i o o f N i t o (NitMo) = 0.3 i s s a t i s f i e d . I n general, a small p a r t o f t h i s promoter may remain i n association w i t h t h e alumina suDport, forminq a surface spinel phase. However, i t has been demonstrated t h a t the Dranoting e f f e c t r e s u l t s from t h e i n t e r a c t i o n o f cobalt o r n i c k e l
124
with the edge plane of the MoS2(WS2) platelets (refs. 4-5!, forming the socalled Co-Mo(W)-S or Ni-Mo(W)-S phases (refs. 4,6) where the promot.er is in a decoration position. The optimum promoter content can then be explained as corresponding to the saturation of the edge sites of very small MoS2(WS2) slabs (refs. 2.7). The promoting effect on the catalytic activity i s more or less important depending on the reaction considered. For instance, toluene hydrogenation (HYD) activity is known to be enhanced by a large factor of up to 20, whereas pyridine hydrodenitrogenation (HDN) is only mildly enhanced by a factor of up to 2 . In addition, the promoting effect. has been found to be dependent on the presence of H2S, as for example in quinoline HDN (refs. 8,s). The understanding of these differences and a definite explanation of the promotinq effect remain elusive. On the one hand structural effects such as the stability of the promoter in a decoration position have to be considered and on the other hand the effective catalytic role of the promoter remains to be elucidated. One means of investigating such questions is through the use of probe molecules. In previous work in this laboratory it has been shown that the sulphur unsaturation of the edge planes of the MoS2 slabs of supported or bulk catalysts could be monitored by hydrogen reduction at various temperatures. With no H2S in the feed, the S/Mo ratio of the active phase can be considered to be constant and the effect of the surface structure on the catalytic properties can be investigated. The results of such treatment were large variations in diene hydrogenation and isomerization activities, which have been proposed to be the consequence of the generation of different site structures on the (7010) edge plane of the MoS2 slabs (refs. 10-11). Such a possibility of monitoring the number and distribution of sites was considered particularly interesting for extension to high-pressure reactions and for investigating the role of Ni in promoted catalysts. Here we report results of a comparative study of conventional sulphided MoS2/A1203 (Mo) and Ni-MoS2/A1203 (NiMo) catalysts under particular conditions where the S/metal ratio was fixed by a prior reduction pretreatment test and where the tests were performed with a feed free of sulphur. Catalytic tests under very different conditions were performed, such as diene HYD under mild conditions and toluene HYD and pyridine HDN at high pressure and temperature. In addition, preliminary characterizations of Ni species by X-ray photoelectron spectroscopy (XPS) are reported. EXPERIMENTAL Two catalysts were studied, namely a 14 wt% Mo03/A1203 and a 3 wt% Ni0-14 wt% Mo03/A1203 prepared according to the usual procedures. For the isoprene HYD experiments the catalysts were sulphided with a H2/H2S (90/10 ~01%)gas mixture
125
a t 623 K for two hours. After sulphidation the catalysts were reduced with purified hydrogen a t different temperatures from 300 t o 1073 K f o r 12 hours. Isoprene (2-methyl-l,3-butadiene) HYD was performed i n an all-glass system a t atmospheric pressure and a t 323 K w i t h a 2.8 1.h-' flow-rate and H2/HC = 37 af t e r each reduction pretreatment. I n separate experiments, the S/metal ratios of the reduced catalysts were determined by measuring the amount of hydrogen sulphide removed by iodimetry . Further detai Is of these experiments have already been reported (re f. 1 0 ) . The HYO of toluene and HDN of pyridine were performed i n a high-pressure catalytic flow microreactor. The catalysts were sulphided a t 623 K and atmospheric pressure w i t h 33 vol%dimethyl-disulphide i n n-heptane. Again the catalyst was reduced by hydrogen a t different temperatures and a t atmospheric pressure prior t o being tested. The reactions were performed with sulphur free feed a t 5 MPa, 623 K, H2/HC = 50 and LSVH = 1.8 for tolune HYD and 3 MPa, 573 K, H2/HC = 75 and LSVH = 2 for pyridine HDN. The products were analysed by on-line gas chromatography w i t h a flame ionization detector and Carbowax-glass and SE-30 stainless-steel packed columns. Activities were calculated by considering the number of molecules converted per unit mass of catalyst and time, except for isoprene HYO, where the calculations are referred t o a single molybdenum atom (turnover-li ke definition). XPS measurements were performed on an AEI ES-2006 spectrometer equipped with a glove-box, a l l o w i n g transfer of the sample w i t h o u t exposure t o a i r . Binding energies were determined taking the A1 2p peak of the support as a reference ( B E = 74.8 eV). RESULTS
Reduction of the catalysts The effect of the reduction pretreatment of the fully sulphided catalysts is t o remove sulphur i n the form of hydrogen sulphide. The number of vacancies created i n the MoS2 active phase can be determined by measuring the amount of hydrogen sulphide evolved. Then, from the quantitative analysis of some chosen samples, the S/metal variation versus the reduction temperature can be determined, as already reported for Mo catalysts ( r e f . 1 0 ) . I n Figure 1 the results obtained for both Mo and NiMo catalysts can be compared in terms of both H2S removed and S/metal ratio. I t can be seen t h a t large variations of the S/metal ratio are obtained. Both curves have similar shapes b u t the S he t a l ratio i s higher for the NiMo catalyst because of the presence of Ni, as already reported ( r e f . 1 1 ) . Such curves have previously been separated i n t o three domains of temperature of reduction (TR), TR = 473 K, 473 K
> 1 and v/D q > > 1, because of the high enough flowP P rate of the carrier gas:
CP(Z',PO)
=
c(Z',po) (K;
k2k-lK/K )sinh qLeff
Dq(Po+k-,p) (P0+k2+k_l)
[Dpqpsinh qpLeff (
coth qpLeff
DPqP where q2 = po/D, and K '
GP
~
"Ap
Dpqp 2 2
.
PO
Po+k-lp
(33)
is given by the relation
being the overall mass transfer coefficient of the product in GP the gas. Now, sinh qLeff, sinhqpLeff and coth qpLeff/Dpqp are approxi5 mated [51 by qLeff, qpLeff and (1/p0 + l/Peff.p)/Lefff respectively, giving K
Cp(l'rPo) z
(TI 2k'k'
k' KB 2/Sp)C(l',po)2 -1 Ip
where v , k;, kll have the same meaning as before (cf. eqn.101, referring,to the product, are defined as while B, k' and kl 1P 1P follows:
Finally, substitution in eqn.35 of the right-hand side of eqn.9 for C ( Z', po) gives
222 TI2mk;kllki
Cp(2',P0)
KB2
=
The function c ( Z ' , to) = f(to) is found by taking the P inverse Laplace transformation with respect to p, of eqn.39, which depends on the product's parameters kip (for adsorption) and kl 1P (for desorption), as well as on those of the reactant k;, kil and ki. The first bracket on the right-hand side of 39 is the same with the denominator of eqn.9 of the reactant, so the roots of this polynomial are given by
The roots of the second bracket of eqn.39 are
where X and Y have the same meaning as in eqns.16 and 17. P P Substituting the four roots above in 39, and inversing the transformation, one obtains
where N3 =
n2kikllki KB2
n2mkikllk; KBPeff OK P
=
N2
Q1 = -Y[X + Y -(X + Y )BI[X P P
(43)
KP
+ Y -(Xp - Yp)B1/4
Q2 = Y[X - Y -(X + Y )B][X-Y -(X P P P
-
Q3 = -Y B[X + Y -(X + Y )B][X-Y-(X P P P P
Yp)B1/4
+ YP )B]/4
223
CALCULATION OF RATE CONSTANTS AND OTHER COEFFICIENTS FROM EXPERIMENTAL DATA For the kinetics of a given reaction on a certain amount of catalyst, at one temperature T1, four experiments are basically required: (1) An injection of a small amount (cf. Experimental) of reactant into the sampling cell is made without the presence of catalyst. Then, reversals of the flow direction of the carrier gas are performed for a constant short time interval, noting the time to when each reversal is made, as measured from the moment of the injection. The height h (in arbitrary units, say cm) of the sample peaks resulting from the flow reversals is measured as shown in Fig.2, and the diffusion band is constructed by plotting lnh versus to. An example for the band of such an experiment is given by Curve 1 in Fig.3. (2) The same experiment without catalyst as in ( 1 ) is repeated with the pure product (cf. Curve 2 in Fig.3). (3) After placing a known weight of catalyst at the bottom of vessel L2 of the same cell, conditioning the catalytic bed, etc. (cf. Experimental), an experiment like (11 is conducted with the reactant, each flow reversal being repeated after the recording of all sample peaks for reactant and product(s) due to the preceding reversal. A separate diffusion band is constructed for each substance, i.e. for each kind of sample peaks (cf. Curves 3 and 4 in Fig.3). (4) When the height of the sample peaks in the previous experiment has been decayed to a negligibly low detector signal, pure product is injeted and the experiment described in (2) is repeated in the presence of catalyst (cf. Curve 5 in Fig.3). The slope of the last linear part after the maximum of the diffusion bands resulting from the experiments ( 1 ) and ( 2 ) gives r2P = -Peff and r p = - PeffSp, respectively, at the temperature 2 P TI, according to eqn.6. The value of Leff for the cell is calculated from its volumes VG and Vh, without any kinetic experiment, by simply solving the quadratic equation (cf. eqn.1):
224
%
10
a-
'0, t 9 -
n
i8 1,
8
E
-?
4 - 7 E:
M
6
0
200
100
t,/min Fig.3. Diffusion bands of 1-butene and butane obtained at 403 K with a sampling cell of VG = 6.42 cm3, VG = 13.533 cm3 and L1 = 78 cm. Curve 1 : 1 cm3 of 1-butene injected without catalyst; curve 2 : 1 cm3 of butane injected without catalyst; curve 3 : 1 cm3 of 1-butene injected in the presence of 461 mg of 60% Ni/Al203 catalyst; curve 4 : butane obtained from the reaction of the injected 1-butene on the same catalyst as in curve 3; curve 5 : 1 cm3 of butane injected in the presence of the same catalyst as in curve 3 ; The carrier gas was pure H2 with a volume flow rate of 0.25 cm3s-1
.
(1.29
+ n 2Vi/VG)A2
t (4.29
+
n2V'/C G G) h
+
1 = 0
225
The smaller root r2 is used to calculate Leff by means of eqn.8.. From the distorted diffusion band of the reactant obtained in experiment (3), the two exponential coefficients (X + Y)peff/2 and ( X - Y)peff/2, and the two respective pre-exponential factors N2(1 + Z/Y)/2 and N2(1 - Z/Y)/2 are computed. This is done either by using a suitable computer program (non-linear regression analysis), or, if the last part after the maximum is linear, by finding the slope of this, say -(X -Y)peff/2 and the intercept In" 2 (1 - Z/Y)], and then reploting the initial data before the maximum as ln{h -N2(1- Z/Y)exp[-(X-Y)Peffto/21} versus to to find -(X+Y)peff/2 from the slope of the new straight line obtained, and ln[N2(1+Z/Y)I from its intercept. Having found the values of the exponential coefficients (XtY)peff/2 and (X-Y)peff/2, and the respective pre-exponential factors N2(1+Z/Y) and N2(1-Z/Y), it needs only simple arithmetic to calculate X , Y and Z, and from them the rate constants kl, k2 and k-l €or the reactant. For instance, addition of the two exponential coefficients and then division of their sum by Peff (found from experiment I) gives the value of X. Subtraction of the same coefficients and then division by Peff yields Y. Finally, from the ratio p of the two pre-exponential factors, one finds p = -
1 -Z/Y 1 + Z/Y
and from this
z=-1-P
(49)
l+P
The fact that arbitrary units are used for the height h of the sample peaks, from which a diffusion band is constructed (cf. p.13) does not influence the value of Z , since it is calculated from the ratio p of two intercepts pertaining to the same substance and to the same experiment, so that any unknown proportionality factors cancel out. The values of X, Y and Z are now used in conjunction with eqns.13, 14 and 15. According to this relations k;
= (X+Z
ki =
-2)/2n2
x2 - Y2 - 2 (X - Y) 2(X + Y) -4
(50)
(51)
226
and from these dimensionless rate constants, kl, k2 and k-l in s-1 are found by multiplication with Peff (cf. eqn.190). An alternative way 5[ 5 ] , without using the values of the preexponential factors, is to conduct two experiments at the same temperature with two different lengths Leff. Coming now to the calculation of the other physicochemical parameters, the distribution constant K and the overall mass transfer coefficients in the gas and in the solid phase KG and Ks, respectively, for the reactant are found using the relations: kl =
KGAs/aGLeff
k-l = KsAs/Vs K = KG/Ks An analogous procedure is used to determine klp, k-lp, KGp, K and K for the product from the results of experiment (4). SP P From the exponential coefficients (X +Yp)Peffep/2 and P p'( - 'p)Peff.p /2 of the product, using eqns.16 and 17, one finds from the product II of these coefficients
and from their sum
C
After that, K Ksp and K are easily calculated using eqns.37 GP' P and 38, and also the relation K = K /Ksp, all these being equiP GP valent to eqns.53, 54 and 55, for the product. Finally, a crucial confirmation for the parameters determined is to use their values to calculate the right-hand side of eqn.42, since X, Y, Xp, Peff' Peff.p are all known. The coefficient N3 is calculated using eqn.43 and the value N2 found from the two pre-exponential factors in experiment ( 3 ) . The simple addition of these two factors gives 2N2. The calculated diffusion band can then be compared with the actual experimental one obtained from the product sample peaks in experiment ( 3 ) . The factor 2 in eqn.3 must always be kept in mind.
227
TWO LIMITING CASES OF THE EQUATIONS DERIVED
If the distribution coefficient K or K has a high value, meaP ning a small value of the desorption rate constant k-l or kIP compared to the respective adsorption rate constants kl and k IP the concentration of the reactant and/or the product reaching the junction x = I' (cf. Fig.1) may be very low and the sample peaks recorded may have negligible height. If this happens only with the product, no parameter pertaining to this substance can be determined, but the rate constants kl, k-l, k2, the distribution constant K and the mass transfer coefficients KG and Ks for the reactant are normally measured, as already described, without being influenced. Experiments (2) and (4) are not needed in this case. An example belonging to this category is offered by the action of sulfur dioxide gas on marble, when the product calcium sulfate does not desorb from the solid. If the reactant does not desorb, but the product does, eqn.11 cannot be applied, but eqn.42 can, and using the values of X and P Y determined from experiment ( 4 1 , the coefficients (X+Y)peff/2 PI and (X-Y)peff/2 can be calculated using a suitable computer program. Ther, omitting kll from eqns.13 and 14, one obtains X = 1
+ nLk; + ki
(58
Y = 1
iIT
2k ' 1
(59
ki
meaning that the coefficient (X+Y)peff/2 equals(l+n2ki)peff, while (X-Y)peff/2 is equal to kiPeff, from which kl and k2 are easily found. In the limiting case described above all experiments (1)-(4) are necessary. An example of this case is the dehydration of a higher alcohol over an alumina catalyst yielding alkenes. REFERENCES 1 N.A.Katsanos and G.Karaiskakis, Adv.Chromatogr., 24(1984) 125-180. 2 N.A.Katsanos and G.Karaiskakis, Analyst, 112 (1987) 809-813. 3 N.A.Katsanos, Flow Perturbation Gas Chromatography, M.Dekker, New York, 1988. 4 N.A.Katsanos, J.Chromatogr., 446 (1988) 39-53. 5 J.Kapolos, N.A.Katsanos and A.Niotis, Chromatographia, submitted for publication. 6 N.A.Katsanos, P.Agathonos and A.Niotis, J.Phys.Chem., 92 (1988) 1645-1650.
This Page Intentionally Left Blank
M.1,. Occelli and ILG. Anthony (Editors),Aduances in Hydrotreating Catalysts 0 1989 Elsevier Science Puhlishers B.V., Amsterdam - Printed in The Netherlands
229
A MINIATURE ON-LINE CLOSED-CYCLE REACTOR FOR X-RAY PHOTOELECTRON SPECTROSCOPY STUDIES OF HYDRODESULPHURIZATION REACTIONS
~ P . A .SPEVACK, ~ L . L .COATSWORTH,
' N . s . MCINTYRE,
l
~
SCHMIDT . AND
2 ~ . BROWN ~ .
Surface Science Western, Room 6 , Natural Sciences Centre, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Energy Research Laboratory, CANMET, Energy, Mines and Resources Canada, Ottawa, Ontario, Canada K1A OG1
ABSTRACT An on-line reactor for the study of hydrodesulphurization reactions (HDS) on supported Co-Mo catalysts within which reduction, sulphidation and thiophene reaction stages can be carried out at temperatures up to 500°C and pressures to lo6 Pa has been developed. Gases are circulated through t h e reactor by a sealed, magnetically driven pump and the gas composition is sampled by an on-line gas chromatograph. A retractable transfer rod places pelletized or thin-film specimens, free of any sample holder or manipulator, inside the reactor. After reaction, samples are transferred in vacuo to a high resolution X-ray photoelectron spectrometer contiguous to the reactor. Thus, reactivity of a particular catalyst may be correlated with its surface composition. A detailed description of the apparatus and some preliminary experimental results are discussed. INTRODUCTION Surface science techniques have found wide application in the area of catalysis over the last decade.
They have become a standard tool for probing
single crystal and model supported catalysts in efforts to gain insight into the workings of process catalysts.
Mini catalyst reactors have been an
integral part of these studies. These expose the catalyst sample to gases and liquids experienced under process conditions and then allow the catalyst as well as the reaction products to be analyzed. Most of these reactors operate either in a flow or batch-type mode.
Flow reactors operate at low pressures
and therefore may be used within an ultra high vacuum environment.
Reactors
of this type permit truly in situ studies, where the sample may be examined during the course of the reaction (1).
The sample, often a single crystal
catalyst, is held inside a UHV chamber equipped with a range of analytical techniques
(LEED,
AES,
XPS)
capabilities, and ion guns. products are monitored by
a
as
well
as
gas
dosers,
heating/cooling
Reaction gases dose the sample and the reaction quadrupole mass spectrometer. A major limitation
on these studies is the pressure
regime in which the reactions can take place
230 normally < 10-l Pa. Many important catalytic processes take place on highly dispersed, large 5
surface area supported catalysts at pressures of 10
Pa or greater.
Thus,
questions have been raised as to the applicability of low pressure studies on idealized surfaces of single crystals to catalysis of supported catalysts at high reactant pressures. development of high specimens.
Efforts to answer these questions spurred the
pressure
reactors used
to
study
low
surface area
Used as stirred batch reactors, they permit reaction of gases and 5
liquids under high pressures
(Z
10
Pa) and temperatures with a small surface
area catalyst. Rapid transfer of the samples from the reactor to an analysis chamber equipped with a variety of surface sensitive techniques is required. Moreover, gas chromatographic analysis of the reactants and reaction products is desirable to correlate the surface composition
with catalytic activity.
High pressure reactions can extend the knowledge gained from fundamental studies of low surface area catalysts to improved development of commercial catalysts. DESIGN CRITERIA Reactor design should allow reacted specimens to be transferred from the reactor without
contamination or
oxidation altering
the
surface.
The
microreactor should thus be directly coupled to the UHV chamber to prevent any oxidation of air-sensitive samples. Inert-gas glove boxes have also been used
(2, 3 ) . but these are extremely clumsy and there is difficulty controlling the environment. The use of high pressure chambers externally mounted to the UHV analysis chamber, but separated by means of gate valves, alleviates the problem of sample transfer, but introduces new problems.
The designs mentioned by Kahn
( 4 ) , Brown (5) and Goodman ( 6 ) suffered from a relatively large reactor to
sample ratio which would promote side reactions with the reactor cavity walls. Additionally, the first two systems did not permit recirculation of reaction gases and liquids, although both were used as static Ichikawa’s design (7) also had this limitation. be
difficult with
these systems because
(batch)
reactors.
Accurate gas sampling would
of concentration gradients and
diffusion problems. More effort has been concentrated on high pressure cells mounted within UHV chambers.
Internally mounted cells have limitations in addition to those
mentioned for the external reactors. Most of the internally mounted cells are sealed with a piston or hand driven manipulator that squeezes the two halves of the microreactor together between an O-ring or metal gasket.
high pressure
microreactors have an upper
pressure limit
Some of these
of = lo5 Pa. This
231 limitation results from the use of gold ( 4 ) or viton O-rings or indium
gaskets (10).
(a),
copper ( 9 ) ,
These seals are all UHV compatible, but they were
not designed for high pressure work.
Additionally, these seals must be
replaced periodically after several uses, which may entail opening the UHV chamber to air.
Some authors have extended the usage of the gaskets by
various procedures including dulling the knife edge used for the seal (10,
ll), annealing and gold plating the copper gasket (9) or by remelting the seal (indium metal) (10) to regenerate the sealing surface. The
minireactor
systems that operate at pressures greater than lo5 Pa
are especially vulnerable.
The designs mentioned by Bracconi ( 1 2 ) ,
Blakely
(ll), Cabrera (9) and Rucker (13) claim to handle pressures of 3 , 100, 100 and 120 (x
l o 5 ) Pa, respectively. The first three designs use O-ring or metal
gasket seals which may be suspect to failure at high pressures. The possible failure of these seals is aggravated by the use of corrosive gases such as H,S and
thiophene which are used
research.
in hydrodesulphurization ( HD S)
Partial seal failure under such conditions could cause contamination of the UHV chamber and its components or possible internal damage during a complete seal failure. The majority of internal reactors use electrical feedthroughs for heating and
for
temperature
feedthroughs are not
measurement. designed
As
for high
Cabrera
(9)
pressures
or
points
out,
these
temperatures, and
I n general, most of the high
certainly not for corrosive environments.
pressure microreactors reported in the literature may be unreliable for continuous use because of possible seal failures. In an attempt to address many of these problems, a miniature on-line, closed-cycle reactor was designed and built to permit rapid and complete analysis of both the catalyst sample and the reactants/reaction products of the HDS reaction. The design was set out to meet the following requirements: Reactions of specially designed catalyst specimens should be able to be carried out at temperatures up to 600°C and at pressures up to lo6 Pa. The reactor cavity should be of small volume to minimize possible surface reactions with the reactor walls and to enhance detectability of products by maximizing the ratio of sample surface area to reactor volume. The sample must sit freely within
the reactor.
No
thermocouple,
electrical feedthrough, sample holder, manipulator or other device may be attached to the sample. devices
entering
a
UHV
This prevents possible contamination of any environment.
undesirable side reactions with
Additionally, this
foreign devices
avoids
(sample holders or
manipulators) from taking place within the catalytic reactor. Provision must be made to circulate gases across the catalyst specimen to
232 model a closed-loop batch reactor.
Injection of liquid samples into the
circulating gases should be allowed.
(5)
Gas
sampling
should
be
readily
accomplished
by
an
on-line gas
chromatograph capable of qualitative and quantitative analysis of the reactants and products. This paper describes the apparatus which was constructed to achieve these ambitious goals.
We disclose preliminary results obtained using this novel
system. EXPERIMENTAL A schematic diagram of the reactor, pumping system and gas chromatograph
is shown in Figure 1. The main features of the design are discussed below.
Reactor The reactor cavity is a stainless steel cylindrical tube of = 26 cm3 volume which is closed at one end by a welded plug providing for gas inlet and outlet
connections.
flange of a high pressure
The other end of the tube is welded to the end stainless steel ball valve.
As an added safety
feature, the flange was drilled to permit water cooling of the hall valve seats.
A removable
welded
plug.
shelf upon which the sample sits is attached to the
A well drilled
into the side of the tube near the s h e l f
permits a thermocouple sensor to be positioned close to the sample for accurate
temperature
monitoring.
Heating of the
reactor is
accomplished
by cartridge heaters located within two half copper blocks bolted to the exterior of
the tube.
Rapid cooling of
temperature is facilitated by a fan. by
a
microprocessor
k1"C.
temperature
by closing two
the reactor (V, and V, introduction
furnace to
ambient
accurate to within
isolated from the flow and
bellows valves
in Figure 1).
chamber by the
tube
controller
The reactor furnace assembly may be
circulation system
series.
based
the
Both heating and cooling are controlled
located on
the back of
The reactor is isolated from the UHV
ball valve and a manual UHV gate valve i n
The ball valve has a pressure rating far in excess of the ten
atmospheres required for these studies.
A pumping port is located midway
between the two valves to allow evacuation of the reaction chamber by a mechanical roughing pump. This ensures that traces of reaction gases will n o t contaminate the UHV chamber when the reactor is opened to the introduction chamber o f the XPS. Flow and Circulation Svstem The plumbing for the flow and circulation
system is compactly mounted on
233
Figure 1. Schematic diagram of microreactor and gas circulation system. an aluminum panel. stainless
The panel contains five stainless steel bellows valves,
steel needle
valve,
two
stainless steel
pressure
a
gauges, a
precision stainless steel flowmeter, and a stainless steel dome-loading regulator.
A l l plumbing is 0 . 1 2 5 "
steel tubing.
diameter chromatographic grade stainless
Circulating gases do not come in contact with any material
other than stainless steel with one exception. A special corrosion resistant lubricant (14) manufactured for sour gas applications i s used to lubricate the viton seals of the ball valves.
The circulation system and the gas lines
leading to the reactor are heated to prevent condensation of liquid thiophene used in our HDS experiments.
Gas lines leading to the gas chromatograph are
wrapped with insulating tape. The total volume of all plumbing (excluding the reactor) is = 20 cm3. Located directly behind the plumbing panel is a positive displacement stainless steel welded bellows pump encased within a reinforced stainless steel can.
The pump
is driven by a magnetically coupled motor.
The
reinforced can enables the exterior of the pump to be pressurized equally with the inside of the pump.
This pressure balancing enables the pump to operate
at pressures above one atmosphere. means of a dome-loading regulator. pressure inside regulator
the pump
is connected
Pressure equalization is accomplished by One port of the regulator "senses" the
(circulation lines).
to a
supply of
inert gas
The second port of the (argon) which is used to
234 pressurize the can.
The third port of the regulator is connected to the can
surrounding the pump.
As the internal pressure of the pump is increased
during high pressure reactions, the regulator automatically opens to deliver argon to the can.
The regulator automatically stops delivering argon to the
can as soon as the two pressures are equallized.
A bleed valve facilitates
depressurizing the can after reaction is complete. A 1 ml volume liquid reservoir containing thiophene is connected to a gas
chromatograph liquid sampling valve through a 3-port teflon valve. A 1 ml gas tight syringe is used to prime the liquid sampling valve for injection. This ensures that the 1 p1 sample loop is completely filled before the thiophene is injected into the circulating lines.
Gas
ChromatoaraDh Configured directly into the circulation line is a Hewlett Packard 5890 A
gas
chromatograph
controlled by
a HP 3393A integrator.
The chromatograph
is equipped with both a thermal conductivity detector and a flame ionization detector.
A 24'
x 1/8" stainless steel packed column is used.
phase is 20% BMEA on a support of Chromosorb P , AW 6 0 / 8 0 mesh. the hydrocarbon
The liquid
Calibration of
products was accomplished with standard gas mixtures and was
found to be linear over the concentration range of interest (10 - 1000 ppm). A 1 cm3 internally controlled gas sampling valve performs automated sample injections. Transfer Rod Samples in the form of 14 mm circular disks x 0.5 mm thick are picked up from slotted PHI-type flat sample holders by a manipulator.
The manipulator
consists of spring loaded tweezers mounted on a linear-motion feedthrough rod.
The rod is automated by a DC powered stepping motor and drive rod
assembly. Alignment of the transfer rod with the sample shelf in the reactor is accomplished by using a duplicate transparent plexiglass reactor.
Sample Introduction and Transfer Scheme Specimen disks resting on sample holders are introduced into the UHV introduction chamber of the spectrometer via an air lock system.
Once the
chamber is pumped down to UHV conditions, the automated transfer rod is brought in to pick up the sample disk free of its holder and place the disk on the sample shelf within the reactor.
Once the transfer rod is withdrawn, the
ball valve and gate valve may be closed and treatment of begin.
After
treatment of the
the sample may
sample is completed, the reactor is cooled to
235 ambient temperatures and thoroughly purged with hydrogen or argon gas.
Gas
chromatographic injections have been used to verify that the reactor and lines are free from corrosive gases or reaction vapours prior to evacuation. Figure
2 shows a cross-sectional view of the reactor and the gripper mechanism. After purging, the reactor is sealed from the circulation system by closing valves V,
and V,
(see Figure 1).
Opening the ball valve permits the reactor
to be evacuated by the mechanical pump.
The gate valve which separates the
evacuated reactor from the UHV introduction chamber of the XPS may now be opened.
Once the sample disk is placed back on the sample holder, a transfer
fork which travels perpendicular to the transfer rod picks up the sample holder and places it inside the analytical chamber for surface analysis. Transfer of a sample from the reactor into the analytical chamber (of base pressure
z
2.7 x
Pa) is accomplished in less than 5 minutes.
A top view of the reactor attached to the introduction chamber and the configuration
of the transfer
rod and transfer
fork is illustrated in
Figure 3 .
GAS IN AMPLE DISK
cu FURNACEBLOCK HALF
-BALL VALVE END FLANGE
-PUSH
THERMOCOUPLE WELL
0
I
2cm
u
MECHANISM
t
GAS
our
Figure 2. Microreactor cross-sectionalview.
ROD
236
FORK
f L ANALYSIS CHAMBER
]))-TURBO
PUMP
-TRANSFER ROD (FOR REACTOR)
Figure 3. Configuration of the reactor on the XPS introduction chainher illustrating the geometries of the transfer rod, transfer fork and analytical chamber. The spectrometer in use is an SSL SSX-100 analysis
instrument equipped
small spot XPS surface
with a monochromatized
A1 Ka X-ray source.
XPS peak positions are referenced to the C Is hydrocarbon contamination at 284.9 eV.
Further details of the instrument are presented elsewhere (15).
The reactor and circulation system are shown in Figure 4 , while the reactor and spectrometer air-lock introduction system are shown in Figure 5 . RESULTS AND DISCUSSION
lo5 Pa the data shown here was obtained at 1 x l o 5 Pa
Although the reactor has been fully tested at pressures up to 7 x and temperatures up to 555"C, and 350°C.
Catalyst performance was monitored with time, using a small portion of commercially composition
available
Co/Mo/-y-alumina
of this catalyst is 15 wt.
%
catalyst
pellet
Mo oxide and 3 wt.
7-alumina substrate. The surface area is 208 m2 g-'.
at 350°C.
% Co
The
oxide o n a
The pellet was secured
to an aluminum sample disk and placed inside the reactor.
reduced in flowing H, and sulphided in 2% H,S/H,
(MB226).
a
at 50 ml min"
The sample was for 30 minutes
The reactor and lines were purged with flowing H, under the same
conditions to remove excess H,S pressurized to 1 x
lo5
from the lines.
The reactor was then
Pa with hydrogen and 4 p1 of thiophene was introduced
into the lines (= 1% total volume). The gases were allowed to circulate over the catalyst and periodic injections were taken
with the gas chromatograph over a total reaction time
of six hours. The results are
presented in Figure 6 which plots the relative
237
Figure 4 . Photograph of the reactor, gas chromatograph, circulation system and associated plumbing.
Figure 5. Photograph illustrating the reactor (foreground) and the XPS sample introduction port (middle of photograph).
238
I 0'
A -
n-BUTANE -BUTANE
10'
.BUTENE I0'
-'ITEN€
\
JHIOPHEN E
I
1
3
2
4
5
6
TIME (hrs.1
Figure 6. HDS Activity of a Process Catalyst MB226. clarity.
1-butene was omitted for The products of
chromatographic signal intensity vs reaction time. reaction
of thiophene
the
with the MB226 catalyst include isobutane, n-butane,
t-2-butene,c-2-butene and H,S/l-butene (unresolved).
The data shows that all
of the thiophene was consumed in the reaction within four hours. Because of the external heating arrangement chosen in our design, we have introduced
a high level of catalytic activity from the reactor walls which
interferes with the determination of catalytic activity from a thin-film specimen alone.
To illustrate this point, the catalytic results for a thin-
film molybdenum on graphite catalyst will be discussed.
Molybdenum metal was
sputter deposited on a graphite disk to a thickness of = 1.4 nm. specimen was then air calcined at 200°C for two hours.
The
Details of the
preparation of thin-film catalysts are published elsewhere (15).
The sample
underwent the same treatments of sulphidation and thiophene reaction as the MB226 catalyst. A blank
was run immediately
before and
after the
sample catalyst
under identical conditions. The chromatographic results obtained from sample injections taken after three hours of reaction time are presented in Table 1. The fractional yield for each hydrocarbon (HC) produced in the reaction is determined as:
239 HCx ( % )
C(HC)x
=
CC(HC)x where x
+
x 100
‘(thiophene)
-butane, 1-butene, c-2-butene and t-2-butene. C(HC), denotes the ppm concentration obtained integrated results.
=
from
the
The amount of thiophene converted during the reaction is given as:
TABLE 1 HDS product composition following 3 hours of reaction processing with and without a thin-film Mo catalyst present PRODUCT BLANK t t n -butane 1-butene/H,S t-2-butene c-2-butene THIOPHENE
%
CONVERSION
FRACTIONAL YIELD ( % ) Mo ON GRAPHITE t t
1.0+0.1 3.6k0.6 1.IfO. 3 0.8f0.2
1.0 5.4fO.4 1.IfO. 2 0.8f0.2
6.4f0.9
8.2f0.1
t Residual H,S in the circulation lines has been subtracted from the data.
+
Fractional yields obtained for the blank are the average of 2 blanks run for each sample. Fractional yields for the catalyst are the average of 2 runs. Uncertainties were calculated as the standard deviation between the two sample runs and among the four blank runs. The results show an overall thiophene conversion of = 1.8% after the
blank is subtracted from the sample run. results.
Two points to note from these
First, the reproducibility of the results is very good despite the
low absolute reaction yields.
Second, the need to run frequent blanks is
essential to quantification. The ability of the reactor to maintain sample integrity between treatment and surface analysis without
alteration of
oxidation is shown in the next example.
the surface species through An alumina disk was sputter
deposited with molybdenum metal to a thickness of = 2 nm.
The sample was
calcined, reduced, sulphided, and reacted with thiophene for three hours under
240
Element
Atomic 70
01,
Mo3d
9.42 21.48
S2P
34.94
.
.
A'2p
6.38
ClS
24.56
n
N
A
Binding Energy
1000.0
0.0
(eV)
Figure 7. Broadscan and semiquantitative analysis of a thin-film molybdenum on alumina catalyst after reaction with thiophene for three hours at 3 5 0 ° C . the same conditions as those previously mentioned. from the reactor and analyzed by XPS.
The specimen was removed
The broadscan and semiquantitative
surface analysis of this sample is shown in Figure 7. interest may be noted.
Several features of
The oxygen to aluminum atomic ratio is 1.47, which is
close to the expected stoichiometric value for A1,0,. confirmed using both Ols/A12p and 02s/A12p ratios.
This ratio has been
Atomic compositions were
determined by a mathematical routine which uses sensitivity factors derived from Scofield (16).
The absence o f any excess oxygen suggests that only the
aluminum is oxidized while all molybdenum exists in sulphided form. The S/Mo ratio o f 1.63, is less than the ratio expected for MoS,.
This
sub-stoichiometric MoS, species has been observed by others (17) and may he attributed to the temperature used to purge the reactor and specimen prior to analysis.
Anionic vacancies are formed on the molybdenum sulphide covering
the alumina.
The complete sulphidation of the molybdenum phase and well
behaved stoichiometry of the alumina are both the result of a sample transfer system in which no contaminants are admitted. The analysis
flexibility of the reactor to permit
sequential
of the HDS catalyst is illustrated in the next
treatment and
example.
A
graphite
241
disk deposited with = 4 nm of molybdenum was air calcined, reduced, sulphided, and reacted with thiophene for 3 hours. after each treatment stage.
Figure 8 shows XPS Mo 3d spectra
The spectra show important changes in the
concentration of the two most prominent species.
The MoS, peak intensity has
increased dramatically between sulphidation and reaction treatments while the lower binding energy component has correspondingly decreased in intensity. The
identity of
this lower binding energy component has not yet been
unambiguously identified and requires further scrutinization.
However, the
ability to readily examine the catalyst after each stage of treatment was instrumental in revealing these subtle compositional changes.
I',
A
I
237.0
BINDING ENERGY (ev)
222
Figure 8 . XPS narrow scan analysis on the Mo 3d region for a molybdenum on graphite thin-film catalyst. a) Spectrum taken after sample underwent reduction and sulphidation steps. b) Spectrum taken after sample underwent reduction, sulphidation, and reaction with thiophene for three hours. Note increase in the peak due to MoS,. SUMMARY AND APPLICATIONS
The preceding examples illustrate the success of the reactor to fulfill the extensive requirements set out in its design. One of the few shortcomings of the system is manifested in the high background level of catalytic activity compared to the activity of the thin film specimens.
Efforts are currently
underway to coat the inside of the reactor with a thin gold film in order to reduce the catalytic activity of the furnace at high temperatures.
242 The system described is more robust and inherently more reliable than most catalytic reactors described in the literature.
Two separate specimens
may be reacted and analyzed simultaneously because the spectrometer is not dedicated exclusively to the catalyst reactor during its operation.
The
sample is not fixed to any manipulator or welded to any thermocouple device, permitting rapid sample changes in and out of the reactor.
Although the
reactor was designed for operation in a circulation mode at an upper pressure limit of
lo6
Pa, the reactor may potentially be operated at a substantially
higher pressure if used in the static mode. vacuum to certain
Transfer of the specimen under
instruments equipped with a PHI-style sample introduction
system is also possible because of a specially designed prototype vacuum transfer device.
This device may help increase the flexibility of analysis
for the researcher. ACKNOWLEDGEMENTS The authors acknowledge the contributions of Susan Choi and Tom Moy to the successful setup of the gas chromatograph.
The assistance of Bernie
Flinn during design and setup of the automated transfer rod is gratefully appreciated.
This work has been supported by the Department of Energy, Mines
and Resources (CANMET) under contract # 2 4 S T - 2 3 4 4 0 - 6 - 9 1 1 6 . REFERENCES
1) D. Barkowski, R. Haul, and U. Kretschmer, Surf. Sci. 107, L 3 2 9 , 1 9 8 1 . 2 ) P. Dufresne, J. Grimblot, and J.P. Bonnelle, Bull, SOC. Chim., 89, 1 9 8 0 . 3 ) J.S. Jepsen and H.F. Rase, Ind. Chem. Prod. Res. Dev. 20, 4 6 7 , 1 9 8 1 . 4 ) D.R. Kahn, E.E. Petersen, and G.A. Somorjai, J. Catalysis 3 4 , 2 9 4 , 1 9 7 4 . 5 ) J.R. Brown and M. Ternan, Ind. Eng. Chem. Prod. Res. Dev. 2 3 ( 4 ) , 557, 1984. 6 ) D.W. Goodman, R.D. Kelley, T.E. Madey, and J.T. Yates Jr., J. Catalysis 63, 226, 1980. 7 ) S . Ichikawa and M.S. Wilson, Rev. Sci. Instrum. 58 ( 2 ) , 3 1 7 , 1 9 8 7 . 8 ) J.J. Weimer and F.A. Putnam, Rev. Sci. Instrum. 55 ( 2 ) , 2 3 8 , 1 9 8 4 . 9 ) A.L. Cabrera, N.D. Spencer, E. Kozak, P.W. Davies, and G.A. Somorjai, Rev. Sci. Instrum. 5 3 ( 1 2 ) . 1 8 8 8 , 1 9 8 2 . 1 0 ) R.W. Judd, H.J. Allen, P. Hollins, and J . Pritchard, Spectrochimica Acta 43A ( 1 2 ) , 1 6 0 7 , 1 9 8 7 . 11) D.W. Blakely, E.I. Kozak, B.A. Sexton, and G.A. Somorjai, J . Vac. Sci. Technol. 13 ( 5 ) , 1091, 1 9 7 6 . 1 2 ) P. Bracconi, E. Porschke, K.H. Klatt, and R. U s s e r , J. Vac. Sci. Technol. A5 ( 2 ) , 2 3 4 , 1 9 8 7 .
13) T.G. Rucker, K. Franck, D. Colomb, M.A. Logan, and G.A. Somorjai, Rev. Sci. Instrum. 58 ( 1 2 ) , 2 2 9 2 , 1 9 8 7 . 1 4 ) KRYTOX 240AC manufactured by Du Pont 15) N.S. McIntyre, T.C. Chan, P.A. Spevack, and J.R. Brown, in preparation for this Proceedings. 1 6 ) J.H. Scofield, J. Electron Spectrosc. and Related Phenomenon 8 , 1 2 9 , 1976. 1 7 ) A. Wambeke,
L. Jalowiecki, S . Kasztelan, J. Grimblot, and J.P. Bonnelle, J. Catalysis 109, 3 2 0 , 1 9 8 8 .
M.L. Occelli and R.G. Anthony (Editors ), Advances i n Hydrotreating Catalysts 1989 Elsevier Science Publishers R.V., Amsterdam - Printed in T h e Netherlands
243
(0
CATALYTIC PROPERTIES I N HYDROTREATING REACTIONS OF RUTHENIUM SULPHIDES ON Y ZEOLITES : INFLUENCE OF THE SUPPORT ACIDITY
M. BREYSSE, M. CATTENOT, T. DECAMP, M. LACROIX, J.L. PORTEFAIX and
S. GOBOLOS*,
M. VRINAT I n s t i t u t de Recherches s u r l a Catalyse, CNRS, conventionne i l ' U n i v e r s i t 6 Claude Bernard LYON I, 2, Avenue A l b e r t E i n s t e i n , 69626 V i l l e u r b a n n e CBdex FRANCE *On l e a v e f r o m t h e C e n t r a l Research I n s t i t u t e f o r Chemistry o f t h e Hungarian Academy o f Sciences, 1025 Budapest, P u s z t a s z e r i u t 56-67, HUNGARY
ABSTRACT C a t a l y s t s c o n t a i n i n g ca.2% (w/w) o f r u t h e n i u m were prepared b y u s i n g [Ru(NH ) ] C l as a p r e c u r s o r compound and HY, NaY and KY z e o l i t e s as s u p p o r t s . Ru/KY 3 a l a l y I t m o d i f i e d by Na S v i a i m p r e g n a t i o n was a l s o i n v e s t i g a t e d . P r i o r t o t h e c a t a l y t i c t e s t s , t h e c a ? a l y s t s were s u l p h i d e d w i t h a H -H S m i x t u r e . The h y d r o d e s u l p h u r i z a t i o n (HDS) o f thiophene a t atmospheri$ $ r e s s u r e , the conversion o f biphenyl, t h e h y d r o g e n a t i o n (HN) o f p y r i d i n e and t h e h y d r o d e n i t r o g e n a t i o n (HDN) o f p i p e r i d i n e under medium-high p r e s s u r e were c a r r i e d o u t under dynamic c o n d i t i o n s . The f a s t d e a c t i v a t i o n and, t h u s , t h e l o w a c t i v i t y o f Ru/HY and Ru/NaY c a t a l y s t s i n t h e HDS o f t h i o p h e n e a r e a t t r i b u t e d t o coke f o r m a t i o n on t h e B r o n s t e d a c i d s i t e s o f t h e s u p p o r t . The s t a b i l i t y and t h e a c t i v i t y o f t h e c a t a l y s t s i n t h i s r e a c t i o n can be improved b y d e c r e a s i n g t h e s t r e n g t h o f t h e Bronsted a c i d s i t e s . I n t h e c o n v e r s i o n o f b i p h e n y l , t h e a c t i v i t y towards t h e f o r m a t i o n o f c r a c k i n g p r o d u c t s i n c r e a s e s w i t h t h e a c i d i t y o f t h e supports. The a c t i v i t y o f t h e c a t a l y s t s i n t h e HN o f p y r i d i n e and i n t h e HDN o f p i p e r i d i n e i s l e s s a f f e c t e d by t h e a c i d s t r e n g t h of t h e s u p p o r t . O n l y a s l i g h t decrease on t h e a c t i v i t i e s i s observed f o r t h e most a c i d i c s u p p o r t . INTRODUCTION The h y d r o t r e a t i n g c a t a l y s t s u s u a l l y employed a r e Mo o r W s u l p h i d e s , promoted by Co o r N i and s u p p o r t e d on alumina o r s i l i c a - a l u m i n a .
These c a t a l y s t s have
been w i d e l y s t u d i e d and i m p o r t a n t p r o g r e s s has been made i n t h e u n d e r s t a n d i n g o f t h e fundamental n a t u r e o f t h e c a t a l y s t systems and improvements i n t h e i r e f f i c i e n c y f o r t h e d i f f e r e n t r e a c t i o n s i n v o l v e d i n t h e h y d r o t r e a t i n g processes have
been
introduced.
synthetically concentrations
derived of
Nevertheless, feeds
nitrogen
presents compounds,
application new
to
problems,
which
have
heavy
residues
particularly
not
at
and high
been s a t i s f a c t o r i l y
s o l v e d . The design o f new c a t a l y s t s r e q u i r e s v e r y a c t i v e phases s u p p o r t e d on c a r r i e r s w i t h l a r g e s u r f a c e areas and t h e a b i l i t y t o produce h i g h d i s p e r s i o n s o f metals o r sulphides. I t has been found t h a t r u t h e n i u m s u l p h i d e ,
e i t h e r unsupported o r s u p p o r t e d
by carbon, i s one o f t h e most e f f e c t i v e systems f o r p e r f o r m i n g d i b e n z o t h i o p h e n e
244
hydrodesulphurization
(HDS)
and
biphenyl
hydrogenation
(HN)
(1-4).
More
r e c e n t l y , Harvey and Matheson ( 5 ) have shown t h a t ruthenium s u l p h i d e supported on Y z e o l i t e i s more a c t i v e i n t h e h y d r o d e n i t r o g e n a t i o n (HDN) o f q u i n o l i n e than conventional NiMo/A1203.
The
high a c t i v i t y
of
this
last
c a t a l y s t can
be
a s c r i b e d t o t h e p r o p e r t i e s o f t h e a c t i v e phase, b u t some p a r t i c u l a r p r o p e r t i e s o f t h e support, such as a c i d i t y , c o u l d a l s o be i m p o r t a n t parameters. The r o l e o f t h e a c i d i t y i n h y d r o t r e a t i n g r e a c t i o n s i s s t i l l a m a t t e r o f c o n t r o v e r s y and can v a r y w i t h t h e n a t u r e o f t h e r e a c t i o n s :
HDS, HDN or HN.
As t h e a c i d i t y o f
z e o l i t e s can be v a r i e d over a wide range, t h e q u e s t i o n o f t h e i n f l u e n c e o f t h i s parameter has
been examined
for
ruthenium s u l p h i d e
supported
on v a r i o u s
Y - z e o l i t e s . C a t a l y t i c a c t i v i t i e s were measured i n t e s t r e a c t i o n s c h a r a c t e r i s t i c o f hydrotreatment:
HDS o f
thiophene,
biphenyl
conversion,
p y r i d i n e HN and
p i p e r i d i n e HDN.
EXPERIMENTAL Catalyst preparation
A NaY z e o l i t e , t y p e LZ-Y52, s u p p l i e d by Union Carbide and HY and KY z e o l i t e s prepared from t h e s t a r t i n g NaY z e o l i t e were used as c a t a l y s t supports. The HY z e o l i t e was prepared from NaY by two successive i o n exchanges i n an aqueous s o l u t i o n o f NH4C1 (1 M ) a t room temperature f o r 24 h. The KY z e o l i t e was obtained from NaY by two successive exchanges i n an aqueous s o l u t i o n o f KN03
(1 M ) a t 333 K f o r 24 h. D e t a i l s o f t h e p r e p a r a t i o n o f potassium-exchanged Y z e o l i t e were described by Oukaci e t a l . ( 6 ) . A f t e r t h e exchange steps, b o t h HY and K Y z e o l i t e s were washed w i t h water t o remove NaCl and NaN03, r e s p e c t i v e l y , formed i n t h e exchange r e a c t i o n s . The samples were then d r i e d i n a i r a t 373 K f o r 24 h. Chemical a n a l y s i s o f K-exchanged z e o l i t e s showed t h a t exchange o f sodium by potassium was almost complete. Ruthenium
was
introduced
by
ion
exchange
according
to
the
following
procedure: 20 g o f z e o l i t e support was t r e a t e d w i t h 1 1 o f an aqueous s o l u t i o n containing
2
g
of
[ R u ( N H ~ ) ~ ] C (~s ~ upplied
by
Johnson-Matthey)
at
room
temperature f o r 24 h. On i o n exchange t h e f o l l o w i n g r e a c t i o n takes p l a c e ( 7 ) : m Ru(NH3)?
+ 3 mC1- + n Na+Y-[Ru(NH3)&,
3+ Na(n-3m) +
S i m i l a r r e a c t i o n s would occur w i t h o t h e r
Y + 3 m NaCl
zeolites.
A f t e r exchange,
the
c a t a l y s t s were washed t h r e e times w i t h water, then d r i e d a t 353 K under vacuum f o r 6 h. Chemical a n a l y s i s gave t h e amounts o f ruthenium i n t r o d u c e d by t h i s method ( t a k i n g i n t o account t h e w e i g h t l o s s o f 1273 K ) : Ru/NaY 2.6,
t h e support a f t e r d r y i n g a t
Ru/HY 2.3 and Ru/KY 2.3% (w/w).
245
One sample, r e f e r r e d as RuSNa/KY, was p r e p a r e d by i m p r e g n a t i o n of Ru/KY w i t h aqueous Na2S s o l u t i o n s a t a p p r o p r i a t e c o n c e n t r a t i o n s t o o b t a i n v a r i o u s Na2S/Ru ratios.
After
impregnation,
the
catalysts
were
dried a t
room t e m p e r a t u r e
overnight. P r i o r t o c a t a l y t i c t e s t s , t h e samples were s u l p h i d e d a t atmospheric p r e s s u r e i n a f l o w o f H2-H2S
a t 673 K f o r 4 h and c o o l e d t o room t e m p e r a t u r e under t h e
same atmosphere. C a t a l y t i c a c t i v i t y measurements H y d r o d e s u l p h u r i z a t i o n o f thiophene,
h y d r o g e n a t i o n o f b i p h e n y l and p y r i d i n e
and h y d r o d e n i t r o g e n a t i o n o f p i p e r i d i n e were performed
i n separate s e t s o f
experiments. A l l r e a c t i o n s were c a r r i e d o u t i n c o n t i n u o u s f l o w m i c r o r e a c t o r s under medium-high p r e s s u r e c o n d i t i o n s ( e x c e p t t h e HDS r e a c t i o n , p e r f o r m e d a t atmospheric
pressure).
Reaction
conditions
are
given
hydrocarbons were i n t r o d u c e d by a s a t u r a t o r - c o n d e n s e r .
in
Table
1.
Pure
For the hydrogenation
and h y d r o d e n i t r o g e n a t i o n t e s t s , H2S was added t o t h e f e e d i n o r d e r t o m a i n t a i n t h e s u l p h i d a t i o n s t a t e o f t h e c a t a l y s t s o r t o i n c r e a s e t h e a c t i v i t y f o r t h e HDN reaction.
TABLE 1 Reaction conditions Reaction
H2 p r e s s u r e
l o 5 Pa Thiophene HDS Biphenyl HN Pyridine HN Piperidine
H2S p r e s s u r e
10'
Hydrocarbon p r e s s u r e
Pa
Temperature
K
Pa
10'
-
25
623
29
21
8
550
30
665
266
573
30
665
266
573
1
HDN
I n a t y p i c a l run,
the f r e s h l y sulphided c a t a l y s t i s t r a n s f e r r e d i n t o t h e
r e a c t o r under an i n e r t gas t o m i n i m i z e i t s c o n t a c t w i t h a i r .
The r e a c t o r i s
t h e n connected t o t h e h i g h - p r e s s u r e equipment and t h e sample i s f l u s h e d under (HDS o f t h i o p h e n e ) o r H2-H2S f o r a few minutes, b e f o r e h e a t i n g t o t h e H2 r e a c t i o n temperature. A f t e r r e a c h i n g temperature and p r e s s u r e e q u i l i b r i u m , t h e r e a c t a n t i s i n t r o d u c e d i n t o t h e gas f l o w . T h i s s t e p d e f i n e s t h e i n i t i a l t i m e o f r e a c t i o n and t h e b e g i n n i n g o f t h e a n a l y s i s stage. automatic
sampling
valve
which
sends
all
the
The l a t t e r c o n s i s t s o f an products
chromatograph equipped w i t h a f l a m e i o n i z a t i o n d e t e c t o r .
through
a
gas
246
The s p e c i f i c r a t e i s c a l c u l a t e d u s i n g t h e f o l l o w i n g e q u a t i o n : T
A s = Q
-
(mol s-l g - l )
m
where Q
hydrocarbon f l o w - r a t e (mol s-'),
T = conversion and m
weight o f
c a t a l y s t . The t o t a l conversion, T, was always lower than 15%. Considerable d e a c t i v a t i o n occurs d u r i n g t h e f i r s t few hours on-stream f o r t h e thiophene HDS. I n o r d e r t o compare t h e d i f f e r e n t c a t a l y s t s , a l o g a r i t h m i c p l o t o f t h e conversion versus t i m e on-stream was u t i l i z e d . Above 200 min, l i n e a r r e l a t i o n s h i p i s observed and values o f t h e parameter")
characterize the
deactivation
example o f such a r e p r e s e n t a t i o n i s shown i n
0
1
slope n
properties o f
a
("deactivation
the catalysts.
An
F i g . 1.
2
3
lg t (min) F i g . 1. Logarithmic dependence o f conversion versus t i m e on-stream i n HDS o f t h i ophene ,Ru/NaY; 0,Ru/KY.
For t h e high-pressure r e a c t i o n s , t h e d e a c t i v a t i o n was much lower and almost similar f o r a l l the catalysts.
In a l l instances, t h e r e a c t i o n r a t e s were determined a f t e r 16 h on-stream.
247 RESULTS AND DISCUSSION Hydrodesulphurization o f thiophene The d e a c t i v a t i o n p r o p e r t i e s and consequently t h e s t e a d y - s t a t e a c t i v i t i e s o f t h e samples a r e c l e a r l y r e l a t e d t o t h e n a t u r e of t h e support (Table 2).
TABLE 2 C a t a l y t i c p r o p e r t i e s o f Ru/Y c a t a l y s t s n = d e a c t i v a t i o n parameter; r = r e a c t i o n r a t e i n lom8 mole s -1 g -1; c r a c k i n g s e l e c t i v i t y , d e f i n e d as
1/2 (benzene + cyclohexane)
I(products formed i n biphenyl conversion)
Thiophene n
SUDDOrt
NaY
1.2 0.17 1 0.03
KY HY
SNa-KY
Biphenyl
r
r
12 71 8 232
30 2
Pyri d i n e
S
90 40 180 100 2.5 50
P i p e r i d i ne
r
r
190 165 138 170
74 85 45 65
Ru/NaY and Ru/HY d e a c t i v a t e much f a s t e r than Ru/KY and, t h e r e f o r e ,
their
a c t i v i t i e s a f t e r 16 h on-stream a r e v e r y poor (8 f o r HY and 12 f o r Nay). I t can be assumed t h a t t h i s f a s t d e a c t i v a t i o n i s due t o coke formation. Although t h e mechanism o f
coke f o r m a t i o n i n HDS
i s not not f u l l y
established,
it i s
g e n e r a l l y accepted t h a t o l e f i n s a r e i n t e r m e d i a t e species. Butenes and butadiene produced d u r i n g t h e hydrogenolysis o f t h e thiophene c o u l d r e a c t w i t h a Bronsted s i t e on t h e c a t a l y s t surface, g i v i n g carbonium ions. These i o n s can condense t o form l a r g e r o l e f i n chains or, by a D i e l s - A l d e r mechanism, can produce aromatic compounds w i t h h i g h molecular weight (8). The presence o f a c i d i c s i t e s i s e v i d e n t f o r Ru/HY. For t h e o t h e r c a t a l y s t s , such as Ru/NaY and Ru/KY, Bronsted a c i d s i t e s c o u l d form on p a r t i a l r e d u c t i o n o f t h e s t a r t i n g ruthenium complex and f o r m a t i o n o f R u ( I 1 ) s u l p h i d o species on t h e z e o l i t e surface. Such a generation o f Bronsted a c i d i t y by r e d u c t i o n o f ruthenium complexes has a l r e a d y been observed i n z e o l i t e s (7). On t h e o t h e r hand,
i t i s accepted t h a t t h e s t r e n g t h o f
increasing
cation
radius
(9). The
Bronsted a c i d s decreases w i t h
results
obtained
on
Ru/KY
zeolite
( d e a c t i v a t i o n parameter much lower than f o r Ru/NaY and a c t i v i t i e s s i x times
248
h i g h e r ) c o n f i r m i n d i r e c t l y t h a t t h e s t r e n g t h o f Bronsted a c i d s i t e s s t r o n g l y i n f l u e n c e s t h e d e a c t i v a t i o n p r o p e r t i e s o f z e o l i t e supported Ru c a t a l y s t s . Another
possibility for
decreasing t h e
deactivation rate
of
the
Ru/Y
c a t a l y s t would be t o n e u t r a l i z e i t s Bronsted a c i d i t y by t h e i n t r o d u c t i o n o f basic additives.
I n t h i s study an aqueous s o l u t i o n o f Na2S
was used t o
i n t r o d u c e sodium c a t i o n s t o n e u t r a l i z e t h e z e o l i t e support. I n f a c t , f o r values o f t h e Na2S/Ru r a t i o between 1 and 2, t h e d e a c t i v a t i o n d u r i n g t h e f i r s t hour i s l e s s i m p o r t a n t and t h e steady s t a t e a c t i v i t y i s m u l t i p l i e d by a f a c t o r o f 3 (Table 2 ) . Conversion o f biphenyl The r e a c t i o n r a t e s o f t h e d i f f e r e n t Ru/Y z e o l i t e i n c r e a s e by t h r e e o r d e r s o f
CB>Ni-T3MPP = Ni-PH. The concept o f membrane bioreactors f o r o i l processing i s introduced. INTRODUCTION B i o p r o c e s s i n g i s p o i s e d t o become a m a j o r g r o w t h i n d u s t r y as we move towards t h e
21st century
(1,2).
The p r e l i m i n a r y e x p l o r a t o r y r e s e a r c h
d e s c r i b e d h e r e i s an overview o f Unocal's program i n v e s t i g a t i n g methods f o r biochemically
treating
model
oils
n i t r o g e n , and o i l - r e l a t e d m e t a l s .
to
remove
the
contaminants
sulfur,
The c a t a l y t i c c o n v e r s i o n o f hydrocarbon
molecules have t r a d i t i o n a l l y been conducted a t e l e v a t e d t e m p e r a t u r e s and pressures. I n contrast,
These processes a r e c a p i t a l i n t e n s i v e and expensive t o o p e r a t e . s i m i l a r reactions are
c a r r i e d out by b i o c a t a l y s i s
(whole
c e l l s and enzymes i s o l a t e d f r o m whole c e l l s ) w i t h process c o n d i t i o n s b e i n g l e s s severe t h a n t h o s e employed i n heterogeneous c a t a l y s i s .
Whole c e l l
growth and metabolism i s
atmospheric
n o r m a l l y o p t i m i z e d a t 3O"C-4O0C,
274
p r e s s u r e and w i t h i n a 6-8 pH range.
M i c r o b i a l processing o f f e r s m i l d
p r o c e s s i n g c o n d i t i o n s , a l o w e r c a p i t a l investment, and p o t e n t i a l l y a h i g h e r s e l e c t i v i t y f o r contaminant removal. We
have
studied
biological
oil
processing
by
microorganisms which c a t a l y z e t h e removal o f s u l f u r ,
the
isolation of
and n i t r o g e n f r o m
model heavy o i l s . Our i n i t i a l e x p e r i m e n t s were p e r f o r m e d i n s t i r r e d b a t c h r e a c t o r s where t h e o i l , aqueous phase and c u l t u r e a r e i n i n t i m a t e c o n t a c t . O i l d r o p l e t s a r e surrounded by t h e c o n t i n u o u s aqueous phase w h i c h c o n t a i n s
t h e microbes, n u t r i e n t s and oxygen ( F i g u r e 1).
I n a process a p p l i c a t i o n ,
i t i s d e s i r a b l e t o p h y s i c a l l y s e p a r a t e t h e o i l phase and t h e aqueous g r o w t h
medium c o n t a i n i n g t h e microorganisms;
therefore,
we sought t o d e v e l o p
a l t e r n a t e processes t o e x p l o i t t h e c a t a l y t i c a c t i v i t y o f o u r i s o l a t e s . The following
work
discusses
exploratory
microbial
activity
and
reactor
concepts f o r t h e u p g r a d i n g o f model heavy o i l s .
F i g . 1.
Diagram showing t h e f o u r phases o f a b i o r e a c t o r f o r m i c r o b i a l
upgrading o f o i l .
275
EXPERIMENTAL Microbial Culture The ex per i m e n t a l s t r a t e g y i n s e l e c t i v e enrichment c u l t u r e i s t o s t a r t w i t h a mixed c u l t u r e o f microorganisms which may have been exposed t o t h e contaminant one e v e n t u a l l y wishes m e t a b o lized.
A c u l t u r e i s grown i n a
d e f i n e d gro w t h medium c o n t a i n i n g a c a r e f u l l y s e l e c t e d c o n c e n t r a t i o n o f t h e contaminant. The c u l t u r e i s s u c c e s s i v e l y t r a n s f e r r e d t o f r e s h media a t i n t e r v a l s o f a few days. The c o n c e n t r a t i o n o f t h e contaminant may a l s o be inc re as ed i n successive growth media, so as t o s e l e c t i v e l y i s o l a t e o n l y these b a c t e r i a which w i l l s u r v i v e i n t h e presence o f t h e cont aminant . Process c o n d i t i o n s O i l -contaminated
soil
samples
obtained
f rom o i l
well
heads were
i n o c u l a t e d i n t o 250 m l b a f f l e d f l a s k s c o n t a i n i n g a 50:50 m i x t u r e o f a basal medium and m i n e r a l o i l supplemented w i t h 5% DBT. The c u l t u r e f l a s k s were inc ubat e d a t 30'C on a shaker (200 RPM) under a e r o b i c c o n d i t i o n s and were s u b c u l t u r e d ev e r y 72 hours t o f r e s h medium. The same procedure was used t o isolate
c arb a z o l e - (CB) ,
p h t h a l o c y a n i n e- (PH) ,
and
nickel -tetra(3-
me t h y lpheny l) p o r p h y r i n ( N i - T3 M P P ) - u t i l i z i n g microorganisms.
A l l experi -
mental c o n d i t i o n s and media c o m p o s i t i o n a r e g i v e n i n T able 1. between organisms
and s t r e s s
f a c t o r s of
t h e environment
Compet it ion selected f o r
microorganisms t h a t c o u l d grow w e l l i n t h e presence o f t h e s p e c i f i c contaminant i n the mineral o i l . Reactor d e s i q n L a t e r ex p e r i m e n t a t i o n was conducted i n more s o p h i s t i c a t e d r e a c t o r systems, such as membrane and h o l l o w f i b e r r e a c t o r s . The membrane b i o r e a c t o r i s a two compartment, h o r i z o n t a l d i a l y s i s c e l l system ( B e l l c o Glass,
I n c . Vineland,
NJ) d i v i d e d by a 5 micron p o r e s i z e membrane made
f rom polytetrafluoroethylene. t o permeate t h e pores.
The membrane i s hydrophobic, p e r m i t t i n g o i l One s i d e c o n t a i n ed t h e aqueous phase (100 m l )
inoculated w i t h previously
i s o l a t e d c a r b a z o l e - u t i l i z i n g microorganisms.
T h is compartment i s a e r a t e d .
The o t h e r s i d e o f t h e c e l l c o n t a i n s a m i n e r a l
o i l phase (100 m l ) c o n t a i n i n g 0.03 w t % c a r b a z o l e as t h e s o l e n i t r o g e n source f o r m i c r o b i a l growth.
Both compartments a r e s t i r r e d .
Growth i s
mo nit o re d a t 660 nm by c i r c u l a t i n g t h e aqueous phase i n t o a f l o w - t h r o u g h c u v e t t e equipped w i t h a U V / v i s i b l e spectrophotometer. t r a t i o n o f t h e o i l i s monitored over time.
The n i t r o g e n concen-
276
TABLE 1: EXPERIMENTAL CONDITIONS
TYPICAL BASAL MEDIUM ( g / l o f d i s t i l l e d H20)
CONCENTRATION
Beef E x t r a c t ( O i f c o ) Na HP04.12H20 KH* po4 Mg212.H20
4.08 9.5 1.4 0.2
(May a l s o i n c l u d e g l u c o s e o r ammonium s u l f a t e where appropriate) TYPICAL MODEL OIL 0.3 WT% (g/g) DIBENZOTHIOPHENE I N MINERAL O I L 0.03 WT% (g/g) CARBAZOLE I N MINERAL OIL 0.05 WT% (g/g) PHTHALOCYANINE I N MINERAL OIL
TYPICAL REACTORS SHAKE FLASK CSTR - BATCH, CONTINUOUS MEMBRANE REACTORS TYPICAL PROCESS CONDITIONS TEMPERATURE 32'C A I R RATE 1.5 L/MIN. 6.8-7.4 pH RANGE 0IL:WATER RATIO 50/50 BOTH PURE AND M I X E D CULTURES USED
The c a r b a z o l e - u t i l i z i n g c u l t u r e was a l s o used i n t h e h o l l o w f i b e r membrane r e a c t o r . c a p i 11 a r y
system
T h i s c u l t u r e was t r a n s f e r r e d t o t h e V i t a f i b e r a r t i f i c i a l manufactured
by
Amicon
(Danvers,
Massachuset t s).
Carbazole i n m i n e r a l o i l (50 m l ) i s pumped f r o m a heat ed r e s e r v o i r (35°C) t h ro ugh a f i l t e r t o a h o l l o w f i b e r u n i t . A h i g h biomass c o n c e n t r a t i o n immobiliz ed i n t h e s h e l l s i d e o f t h e f i b e r removes t h e n i t r o g e n i n t h e o i l as t h e o i l f l o w s ( 5 ml/min) t h r o u g h t h e lumen o f t h e f i b e r s .
A f t e r passing
t hro ugh t h e h o l l o w f i b e r u n i t , t h e o i l f l o w s t o a U V / v i s i b l e s p e c t r o p h o t o meter, where t h e decrease i n absorbance o f c a r b a z o l e i s m o n i t o r e d a t 250 nm.
The o i l i s t h e n r e t u r n e d back t o t h e r e s e r v o i r t o be r e c y c l e d t h r o u g h
the hollow f i b e r bioreactor.
277
System a n a l y s i s t h e absorbance i s d i r e c t l y p r o p o r t i o n a l t o t h e
For v a l u e s below 0.7,
b a c t e r i a l c e l l c o n c e n t r a t i o n (3) t h u s a l l o w i n g c a l c u l a t i o n o f c e l l growth rates.
Total
s u l f u r concentration
measured by X - r a y f l u o r e s c e n c e .
i n t h e o i l and aqueous phases were
CB, PH, and "I-PH
d e g r a d a t i o n was moni-
t o r e d by a decrease i n absorbance and 250 nm, 653 nm, and 665 nm, r e s p e c tively.
The t o t a l t r a c e n i t r o g e n c o n c e n t r a t i o n i n t h e o i l was measured by
chemiluminescence (UTM 561). RESULTS AND D I S C U S S I O N M i c r o b i a l d e s u l f u r i z a t i o n (MDSJ Our i n i t i a l program focused on t h e m i c r o b i a l d e s u l f u r i z a t i o n o f a model o i l .
The r a t i o n a l e o f o u r r e s e a r c h i s t h a t DBT i s c o n s i d e r e d t h e
major s u l f u r compound p r e s e n t i n o i l .
Rather t h a n a t t e m p t t o i n i t i a l l y
work w i t h a r e a l o i l , we f a b r i c a t e d a model s u l f u r - c o n t a i n i n g o i l m i x t u r e by d i s s o l v i n g DBT i n a c l e a r w h i t e m i n e r a l o i l .
The m i n e r a l o i l
is a
m i x t u r e o f hydrocarbons c o n s i s t i n g p r i m a r i l y o f naphthenes, p a r a f f i n s and isoparaffins;
i n i t i a l l y , i t i s s u l f u r , n i t r o g e n and m e t a l s f r e e .
There-
f o r e , DBT d i s s o l v e d i n m i n e r a l o i l p e r m i t t e d t h e d i r e c t assessment o f t h e efficiency o f a microbial culture f o r i t s desulfurization a c t i v i t y .
A p a r t i a l r e v i e w o f l i t e r a t u r e shows t h a t t h e m i c r o b i a l removal o f s u l f u r compounds f r o m o i l has been a t t e m p t e d s e v e r a l t i m e s d u r i n g t h e p a s t f o r t y years (Table 2).
Recent r e p o r t s have shown t h a t t h i o p h e n e d e r i v a -
t i v e s can be degraded by microorganisms.
Yamada e t .
al.
(7)
isolated
Pseudomonas a b i konensis and Pseudomonas j i a n i i which a r e capable of o x i d i z i n g DBT i n t o w a t e r - s o l u b l e
o r g a n i c compounds i n v o l v i n g s u l f u r .
s t u d i e s by Yamada e t . a l . (8) i d e n t i f i e d t h e s e m e t a b o l i c p r o d u c t s .
Further Cripps
( 9 ) i s o l a t e d an organism t h a t was capable o f u s i n g t h i o p h e n e - 2 - c a r b o x y l a t e
as t h e s o l e source o f carbon. source, degrade
Sagardia e t .
a1
benzothiophene.
.
When y e a s t e x t r a c t was p r o v i d e d as a carbon
(10) r e p o r t e d t h a t Pseudomonas a e r u q i n o s a can Also,
Hou
and
Laskin
(11)
reported
the
co-metabolism o f DBT by Pseudomonas aeruqinosa when grown on n - p a r a f f i n s a t u r a t e d w i t h DBT ( w i t h t h e accumulation o f 4-2(3-hydroxy)-thianaphthenyl2 - h y d r o x y - 3 - b u t a n o i c a c i d ) . K a r g i and Robinson (13) showed t h e a b i l i t y of S u l f o l o b u s a c i d o c a l d a r i u s t o o x i d i z e s u l f u r p r e s e n t i n DBT t o s u l f a t e . A more r e c e n t r e p o r t concerned w i t h t h e i s o l a t i o n and c h a r a c t e r i z a t i o n o f microorganisms t h a t o x i d i z e t h e model s u l f u r compound, DBT, was w r i t t e n by F i n n e r t y e t . a l . (14).
278
TABLE 2 L i t e r a t u r e on M i c r o b i a l O i l D e s u l f u r i z a t i o n
YEAR
AUTHOR
AFFILIATION
1950 Method o f D e s u l f u r i z i n g Crude O i l
R. S t r a w i n s k i (4)
Texaco Development Corp., New York,NY
C. Z o b e l l ( 5 )
Texaco Development Corp., New York,NY
1961 B a c t e r i o l o g i c a l D e s u l f u r i z a t i o n o f Petroleum
I . Kirshenbaum (6)
Esso Research and E n g i n e e r i n g Co., Linden, NJ
1968 M i c r o b i a l Conversion o f P e t r o - s u l f u r Compounds, P a r t I. I s o l a t i o n and Identification o f Dibenzothiopheneu t i l i z i n g Bacteria
K. Yamada ( 7 )
University o f Tokyo, C e n t r a l Research I n s t i t u t e o f E l e c t r i c Power I n d u s t r y , Abiko, Chiba P r e f e c t u r e
1972 The M i c r o b i a1 Metabol ism o f Thiophen-2-Carboxylate
R. C r i p p s ( 9 )
S h e l l Research L t d . S i tt ingbourne, U . K .
F. Sagardia (10)
University o f P u e r t o R i c o and the Office o f Pet roleum F uels A f f a i r s , San Juan, Puerto Rico
C . Hou A. L a s k i n (11)
Exxon Research and E n g i n e e r i n g Co., Linden, NJ
A. Laborde D. Gibson (12)
U n i v e r s i t y o f Texas A u s t i n , Texas
F. K a r g i J. Robinson (13)
Lehigh U n i v e r s i t y Bet h l ehem, Pennsylvania
W . R. F i n n e r t y K. Shockley H. Attaway (14)
University o f Georgia, Athens G eorgia
1953
1975
Process o f Removing S u l f u r f ro m Petroleum Hydrocarbons and Apparatus
D egra da t i o n o f Benzot h i ophene and Re1a t e d Compounds by a S o i l Pseudomonas i n O i l Aqueous Environment
1976 M i c r o b i a l Conversion o f Dibenzothiophene 1977
Metabol ism o f D i benzot hiophen e by a B e i j e r i n c k i a Species
1983 M i c r o b i a l O x i d a t i o n o f D i benzothiophene by t h e T h e r m o p h i l i c Organism Sul f o l o b u s a c i d o c a l d a r i u s 1983
Microbial Desulfurization and D e n i t r o g e n a t i o n o f Hydrocarbons
279
Two m i c r o b i a l species were i s o l a t e d t o d e s u l f u r i z e DBT i n m i n e r a l o i l t h ro ugh enrichment c u l t u r e
techniques.
Characterization
studies
have
i d e n t i f i e d t h e organisms as Pseudomonas aeruqinosa and A c i n e t o b a c t e r sp.. Using a m i x t u r e o f microorganisms, t h e d e s u l f u r i z a t i o n r a t e o f t h e o i l a t p r a c t i c a l l y ambient process c o n d i t i o n s was determined t o be 0.0019 s u l f u r / g oil.hr. Microbial denitroqenation
A
IMDN) o f c a r b a z o l e i n o i l
model n i t r o g e n - c o n t a i n i n g o i l was f o r m u l a t e d w i t h c a r b a z o l e (CB)
representing
t h e p r i n c i p a l n i t r o g e n contaminant p r e s e n t .
N i t r o g e n i s an
element e s s e n t i a l f o r p r o t e i n s y n t h e s i s , and hence c e l l growth.
I n our
experiments c a r b a z o l e i s s u p p l i e d as t h e o n l y n i t r o g e n source f o r growth. Thus, i f gro w t h i s observed, c a r b a z o l e ’ s n i t r o g e n must be i n c o r p o r a t e d i n t o t h e biomass. F i v e p ure c u l t u r e s were i s o l a t e d f r o m o i l - s o a k e d s o i l
samples and
tested f o r t h e microbial denitrogenation o f carbazole dissolved i n mineral oil.
Each pu r e c u l t u r e reduced t h e n i t r o g e n c o n c e n t r a t i o n o f t h e o i l
phase, alt h oug h n o t n e c e s s a r i l y i n a l i n e a r f a s h i o n .
The comparison o f
d e n i t r o g e n a t i o n d a t a f o r t h e p u r e c u l t u r e s i n d i c a t e t h a t t h e mixed c u l t u r e and i s o l a t e 3B p e r f o r m t h e b e s t ( F i g u r e 2 ) .
The maximum s p e c i f i c growt h
r a t e f o r each c u l t u r e does n o t show as d r amat ic a v a r i a t i o n w i t h c u l t u r e t y p e as does t h e d e n i t r o g e n a t i o n r a t e .
Culture denitrogenation r a t e
is
ranked as f o l l o w s : mixed c u l t u r e > 38 > 3A > 4 > 2 where 3B, 3A, 4 and 2 a r e i n d i v i d u a l s p e cies found i n t h e mixed c u l t u r e . I t i s t o be i n t u i t i v e l y expected t h a t t h e mixed c u l t u r e which c o n t a i n s a l l
i s o l a t e s (2, 4, 3A, 38) s h o u l d e x h i b i t a d e n i t r o g e n a t i o n r a t e t h a t exceeds each o f t h e i n d i v i d u a l s p e c i e s (as l o n g as no i n d i v i d u a l species produces intermediates t h a t are i n h i b i t o r y t o t h e others).
We a l s o n o t e t h a t if t h e
mixed c u l t u r e i s excluded, t h e r e i s an apparent i n v e r s e r e l a t i o n s h i p between d e n i t r o g e n a t i o n a b i l i t y and maximum s p e c i f i c growt h r a t e . T hat i s , slower d i v i d i n g c e l l s m e t a b o l i z e c a r b a z o l e n i t r o g e n a t a h i g h e r r a t e t han r a p i d l y gro w ing c e l l s . M i c r o b i a l d e n i t r o q e n a t i o n (MDN) o f DOrDhYrin t y p e comounds i n o i l
A l a r g e f r a c t i o n o f t h e n i t r o g e n i n a heavy r e s i d i s i n t h e asphaltenes.
One o f
t h e main b u i l d i n g
blocks
(or
monomers) asphal tene s t r u c t u r e i s t h e c y c l i c t e t r a p y r r o l i c nucleus (15). We
of
the
seek microbes which c o u l d c l e a v e open t h e p o r p h y r i n s t r u c t u r e , i n c o r p o r a t e
280
t h e n i t r o g e n i n t o c e l l biomass, and s e q u e s t e r t h e m e t a l .
To i n i t i a t e t h i s
r e s e a r c h , t h e model s u b s t r a t e s s e l e c t e d were p h t h a l o c y a n i n e (PH), benz-tetraza-porphyrin)
nickel
phthalocyanine
(Ni-PH)
and
nickel
tetra-
(111)
t e t r a (3-methyl p h e n y l ) p o r p h y r i n (Ni-T3MPP) ( F i g u r e 3 ) .
CULTURE Fig. 2.
D e n i t r o g e n a t i o n and s p e c i f i c g r o w t h r a t e s as a f u n c t i o n o f c u l t u r e
type.
1
2
c 3 2 HI8 N8 (F. WT. = 514.55)
Fig. 3.
S t r u c t u r e o f (1) p h t h a l o c y a n i n e (Ph;
and ( 2 ) n i c k e l p h t h a l o c y a n i n e ( N i - P h ) .
c32 Hr6 NI, Ni (F. WT. = 571.24)
tetrabenztetrazaporphyrin)
281
( a ) P h t h a l o c y a n i n e D e g r a d a t i o n (PH) The decrease i n absorbance concentration,
(653nm),
which
c u l t u r e system.
to
The i n i t i a l d e n i t r o g e n a t i o n r a t e s o f PH a r e 0.42
The PH d e g r a d a t i o n b e h a v i o r appeared
N/hour.
i s proportional
PH
was used t o m o n i t o r t h e d e g r a d a t i o n o f PH by t h e mixed ppm
t o be t h e same i r r e s p e c t i v e
o f whether t h e aqueous phase i s s p i k e d w i t h an a l t e r n a t e carbon source, o r
a n i t r o g e n source o r b o t h . ( b ) T o t a l N i t r o q e n Removal Versus P h t h a l o c y a n i n e N i t r o q e n Removal We measured n i t r o g e n c o n c e n t r a t i o n s by two independent t e c h n i q u e s . The t r a c e n i t r o g e n procedure g i v e s t h e t o t a l n i t r o g e n c o n c e n t r a t i o n i n t h e o i l i r r e s p e c t i v e o f i t s m o l e c u l a r form; whereas, UV s p e c t r o p h o t o m e t r y g i v e s t h e n i t r o g e n c o n c e n t r a t i o n which i s e x c l u s i v e l y bound up i n PH o r N i - P H . I f no i n t e r m e d i a t e s a r e produced d u r i n g t h e PH d e g r a d a t i o n ,
r e a d i n g s h o u l d agree.
t h e n t h e two
That i s , when PH i s e n z y m a t i c a l l y cleaved, a l l t h e
PH's nitrogen i s cell-associated.
W i t h PH as t h e s o l e n i t r o g e n and carbon
source i n t h e system, a r e d u c t i o n i n t o t a l n i t r o g e n c o n c e n t r a t i o n and PH c o n c e n t r a t i o n occurs by I - d a y ( F i g u r e 4 ) .
Although t o t a l n i t r o g e n
and PH
n i t r o g e n decrease, t h e y do n o t agree, s u g g e s t i n g t h e f o r m a t i o n o f m e t a b o l i c i n t e r m e d i a t e s , p r o b a b l y o x i d i z e d PH.
W i t h PH as t h e s o l e n i t r o g e n source,
t h e PH n i t r o g e n c o n c e n t r a t i o n decreases w i t h o u t a f f e c t i n g any change i n t h e total
nitrogen
concentration,
suggesting
the
formation
of
metabolic
intermediates.
YI
(I
0
E
z
405I
I
1
I
I
I
I
I
1
2
3
4
5
6
7
8
TIME (DAYS)
Fig. 4 .
T o t a l n i t r o g e n and n i t r o g e n i n p h t h a l o c y a n i n e v e r s u s t i m e .
i s s o l e n i t r o g e n and carbon source
-
F5-Ph i s s o l e n i t r o g e n source.
F3-Ph
282
( c ) N i c k e l P h t h a l o c v a n i n e D e q r a d a t i o n (Ni-PH) The N i - P H absorbance peak a t 665 nm was used t o m o n i t o r d e g r a d a t i o n . W it h i n c r e a s i n g time, t h e absorbance decreased, and t h e s p e c t r a f o r t h e o i l gave no i n d i c a t i o n o f f o r m a t i o n o f o p t i c a l l y a c t i v e i n t e r m e d i a t e s . We c a l c u l a t e d t h e d e n i t r o g e n a t i o n r a t e o f Ni-PH t o be 0.031 ppm N/hr. I f the assumption i s made t h a t t h e biomass c o n c e n t r a t i o n s a t any g i v e n t i m e i n t h e PH and Ni-PH d e g r a d a t i o n experiments a r e e q ual, t h e r a t e o f PH d e n i t r o g e n a -
t i o n by t h e mixed c u l t u r e i s a p p r o x i m a t e l y 13 t i m e s f a s t e r t h a n Ni-PH. (d ) N i c k e l T e t r a (3-Methyl Phenvl1 P o rDhvrin Deqradat ion (Ni-T3MPP) The mixed c u l t u r e was grown w i t h m i n e r a l o i l c o n t a i n i n g Ni-T3MPP as t h e s o l e source o f n i t r o g e n .
M i c r o b i a l growt h o c c u r r e d w i t h r a t e o f
d e n i t r o g e n a t i o n c a l c u l a t e d t o be 0.036ppm N/hr. Novel Reactor C o n f i q u r a t i o n s f o r MDN The u l t i m a t e g o a l i s t o d e v i s e n o v el b i o r e a c t o r c o n f i g u r a t i o n s f o r e x p l o i t i n g t h e b i o c a t a l y t i c a c t i v i t y o f microorganisms f o r c e r t a i n s p e c i f i c a p p l i c a t i o n s . Such r e a c t o r s w i l l be more s o p h i s t i c a t e d t h a n t r a d i t i o n a l heterogeneous c a t a l y t i c r e a c t o r s because o f t h e c o n d i t i o n s under which microorganisms grow (need f o r c o n t r o l o f n u t r i e n t composit ion, pH, temperature,
a b i l i t y t o remove t o x i c m e t a b o l i t e s ,
c o n t r o l a r e a few c o n s t r a i n t s ) .
and oxygen p a r t i a l pressure
So f a r we have r e p o r t e d on t h e use o f
b a c t e r i a f o r o i l d e s u l f u r i z a t i o n and d e n i t r o g e n a t i o n i n which a l l e x p e r i ments have been performed i n shake f l a s k s w i t h a l l f o u r phases ( c e l l s , o i l , aqueous growth medium, and a i r ) i n i n t i m a t e c o n t a c t . At t empt s t o p h y s i c a l l y s epara t e t h e o i l phase f r o m t h e aqueous phase c o n t a i n i n g t h e b i o c a t a l y s t s a r e now r e p o r t e d : membrane and h o l l o w f i b e r r e a c t o r c o n f i g u r a t i o n s have been used.
R e s u l t s a r e d i s c u s s e d below.
(a ) Membrane Reactor The o b j e c t i v e o f u s i n g membrane r e a c t o r experiment s was t o assess any p o s s i b l e b e n e f i t g a i n e d by p h y s i c a l l y s e p a r a t i n g t h e aqueous
and o i l
phases. A p l o t o f t h e absorbance o f t h e mixed c u l t u r e s ( o r o p t i c a l d e n s i t y ) o f t h e aqueous phase i n t h e membrane r e a c t o r w i t h CB as t h e s o l e n i t r o g e n source ( I ) and c o n t r o l ( 0 ) i s shown i n F i g u r e 5. experiment was preformed i n a b a f f l e d shake f l a s k .
The c o n t r o l
Bot h experiment s used
t h e same d e n i t r o g e n a t i o n c u l t u r e p r e v i o u s l y discussed.
The a b i l i t y o f t h e
membrane r e a c t o r t o a c h i e v e h i g h e r biomass d e n s i t i e s i s encouraging ( F i g u r e 5 ) . The f i n a l biomass d e n s i t y i n t h e membrane r e a c t o r i s h i g h e r t h a n t h a t
283
- 0.50 - 4.00
t 0
4 b DIALYSIS I
-0- CONTROL0
5
45
40
20
30
25
35
I
40
TIME (HOURS)
F i g . 5.
M i c r o b i a l d e n i t r o g e n a t i o n d i a l y s i s u n i t vs shake f l a s k .
achieved i n shake f l a s k s .
The f i n a l biomass c o n c e n t r a t i o n achieved i n t h e
membrane r e a c t o r i s 2 . 5 t i m e s t h a t o b t a i n e d i n shake f l a s k s .
However, t h e
observed maximum s p e c i f i c growth r a t e i n t h e membrane r e a c t o r i s o n l y 12 perc e nt o f t h a t o b t a i n e d i n shake f l a s k s . viewed as an e f f e c t i v e n e s s suggest t h a t : 1.
factor.
The 12 p e r c e n t f i g u r e can be
I n combinat ion,
this
I t i s b e n e f i c i a l t o p h y s i c a l l y separat e t h e microbe
f ro m t h e o i l phase t o achieve h i g h biomass d e n s i t i e s (and hence u l t i m a t e l y h i g h r e a c t i o n r a t e s ) .
Such a
r e s u l t i n d i c a t e s t h a t a t o x i c m e t a b o l i t e may be being produced which i n h i b i t s c e l l growth. 2.
The membrane decreases t h e c o n t a c t e f f e c t i v e n e s s between aqueous/oi l / a n d biomass phases. Simultaneous three-phase c o n t a c t i s necessary and t h e membrane i n t r o d u c e s an u n d e s i r a b l e mass t r a n s f e r r e s i s t a n c e .
information
284
The d e n i t r o g e n a t i o n r a t e achieved i n t h e membrane r e a c t o r was 3 . 2 t ime s t h e r a t e o b t a i n e d i n shake f l a s k s . The d e n i t r o g e n a t i o n r a t e i n t h e membrane r e a c t o r was 0.895 ppm N/hr whereas t h e d e n i t r o g e n a t i o n r a t e i n t h e shake f l a s k s was 0.279 ppmN/hr. T h i s d a t a i s encouraging and w a r r a n t s f u r t h e r st udy o f t h e membrane b i o r e a c t o r concept. (b) Whole C e l l I m m o b i l i z a t i o n i n a H ollow F i b e r React or f o r MDN o f c a r b a z o l e i n M i n e r a l O i l One o f t h e most s i g n i f i c a n t f a c t o r s l i m i t i n g m i c r o b i a l denitrogenation i s t h e q u a n t i t y o f b i o c a t a l y s t a v a i l a b l e t o perform the metabolism o f n i t r o g e n .
Since microbial c e l l s are d i f f i c u l t t o c u l t u r e
e f f e c t i v e l y a t h i g h d e n s i t y i n c o n v e n t i o n al chemostats, t h e i m m o b i l i z a t i o n o f a mixed c u l t u r e w i t h i n t h e h o l l o w f i b e r
r e a c t o r s has been s t u d i e d t o p r o v i d e an a l t e r n a t e method f o r achieving high c e l l d e n s i t i e s . A who1 e c e l l b i o r e a c t o r was devel oped employing h o l 1ow f i b e r membranes
f o r c e l l immobilization. or
surface
p o l y s u l fone
and
within
hollow-fiber
The mixed c u l t u r e system was grown on t h e e x t e r i the
macroporous
membranes.
matrix
Mineral
oil
of
asymmetric-wal l e d
containing
carbazole
( n i t r o g e n r i c h model f e e d s t o c k ) i s c i r c u l a t e d t h r o u g h t h e h o l l o w f i b e r r e a c t o r u n i t where t h e o i l d i f f u s e s a c r o s s t h e h o l l o w f i b e r membrane from t h e tube side t o the s h e l l side.
The c a r b a z o l e i s t h e sole n i t r o g e n source
a v a i l a b l e t o t h e microorganisms f o r growth. T h i s c e l l i m m o b i l i z a t i o n method when c o n t r a s t e d t o t h e more simp1 i s t i c b a t c h r e a c t o r , achieves h i g h e r c a r b a z o l e d e n i t r o g e n a t i o n r a t e s . The h o l l o w f i b e r MDN r a t e s a r e about 20 t i m e s l a r g e r t han r a t e s achieved i n a shake f l a s k s . The MDN r a t e f o r h o l l o w f i b e r process was 4 . 5 x l o 6 mole 6 N/g DCW.hr, w h i l e t h e MDN r a t e f o r b a t c h process was 0.24 x 10 mole N/g DCW.hr.
Scanning e l e c t r o n microscopy on t h e h o l l o w f i b e r s , p o s t - r u n , show
t h a t b a c t e r i a l r o d s p e n e t r a t e i n t o t h e f i b e r ( F i g u r e 6),
whereas, y e a s t
c e l l s remain i m m o b i l i z e d on t h e e x t e r i o r s u r f a c e o f t h e f i b e r ( F i g u r e 7 ) . Microbes a r e excluded f r o m t h e h o l l o w f i b e r w a l l as a r e s u l t o f t h e i r physical size.
285
Fig. 6. Penetration of hollow fiber by bacterial rods ( X 5000)
Fig. 7. Immobilization o f yeast cells on the exterior surface o f t h e hollow fiber ( X 5000).
286
CONCLUSIONS The h i g h a c t i v i t y and s e l e c t i v i t y o f b i o c a t a l y s t s i s one o f t h e i r m a j o r a s s et s .
I t s h o u l d be n o t e d t h a t t h e u n i t s o f r e a c t i o n r a t e we used
a r e e s s e n t i a l l y ppm/hr
( o r g moles / g o i l . h r ) .
n o t account f o r t h e mass o f b i o c a t a l y s t
T h i s d i m e n s i o n a l i t y does
present.
We d i d n o t measure
c e l l u l a r c o n c e n t r a t i o n s i n many o f t h e s e experiment s. A comparison o f t h e r a t e s i s o n l y v a l i d i f t h e c e l l u l a r d e n s i t i e s o f each c u l t u r e were appro x ima t e ly e q u a l .
We b e l i e v e t h i s assumption i s v a l i d .
The d e s u l f u r i z a t i o n o f DBT e x h i b i t s t h e h i g h e s t r a t e a t 55.3 x mg/l o i l . h r . of
the
N i t r o g e n removal r a t e s a r e l o w e r .
four
model
compounds
tested
r a nked
The d e n i t r o g e n a t i o n r a t e in
the
following
order
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 d e n i t r o g e n a t i o n
PH>CB>Ni-T3MPP = Ni-PH.
r a t e o f t h e n i c k e l p o r p h y r i n (Ni-T3MPP) and t h e n i c k e l a z a p o r p h y r i n (Ni-PH)
3 s m a l l e r t han
are almost i d e n t i c a l .
These r a t e s a r e about a f a c t o r o f 10
t h e d e s u l f u r i z a t i o n r a t e o f DBT. M i c r o b i a l r e a c t i o n r a t e s f o r d e s u l f u r i z a t i o n and d e n i t r o g e n a t i o n a r e compared w i t h s i m i l a r r a t e s achieved c a t a l y t i c a l l y a t h i g h t emperat ure (T able 3 ) .
TABLE 3 MICROBIAL AND CATALYTIC RELATIVE REACTION RATES M i c r o b i a l r e a c t i o n r a t e s o f a r b i t r a r i l y a s signed u n i t a c t i v i t y . RELATIVE REACTION RATES MICROBIAL (on wet c e l l b a s i s )
SOLID CATALYTIC REAL OILS
SULFUR
1
3.2 2.2
N I TROGEN
1
81
MODEL OILS 16
806 1778
The c a t a l y t i c r e a c t i o n r a t e s f o r r e a l r e s i d s and model compounds were used f o r comparison.
(i)
The f o l l o w i n g c o n c l u s i o n s can be drawn.
M i c r o b i a l d e s u l f u r i z a t i o n (MDS) r a t e s exceed m i c r o b i a l d e n i t r o g e n a t i o n (MDN) catalytic
r a t e s by a f a c t o r
desulfurization
rates
d e n i t r o g e n a t i o n by a f a c t o r o f ca. 20.
exceed
o f ca.
sol i d
900.
Solid
catalytic
287
HDS r a t e s o f r e a l o i l s exceed MDS r a t e s o f model o i l s by a f a c t o r
o f ca. 3. We s h o u l d r e c a l l t h a t t h e model o i l c o n t a i n s o n l y DBT which r e p r e s e n t s one o f t h e more r e f r a c t o r y s u l f u r - c o n t a i n i n g s pec i e s found i n a r e s i d . HDS r a t e o f model o i l s exceed MDS r a t e s o f model o i l s by a f a c t o r o f ca. 20 HDN r a t e s o f r e a l o i l s exceed MDN r a t e s o f model o i l s by a f a c t o r
o f ca. 100. HDN r a t e s o f model o i l s exceed MDN r a t e s o f model o i l s ba a
f a c t o r ca. 1000.
Our m i c r o b i a l d e s u l f u r i z a t i o n and d e n i t r o g e n a t i o n r a t e s a t 90°F a r e lo w er t han HDS r a t e s achieved o v e r s o l i d c a t a l y s t s a t 750°F. We b e l i e v e t h a t by enrichment c u l t u r e techniques we w i l l c o n t i n u e t o i s o l a t e microbes which w i l l
e x h i b i t higher a c t i v i t y .
We a l s o b e l i e v e t h a t biochemical
pro c es s ing i n t h e p e t r o l e u m i n d u s t r y demands work i n whole c e l l membrane r e a c t o r s o p e r a t i n g a t h i g h biomass d e n s i t y . ACKNOWLEDGEMENTS We w is h t o thank Unocal f o r s u p p o rt . Permission t o p u b l i s h t h i s account i s g r a t e f u l l y acknowledged. We a l s o would l i k e t o t hank D r . F. K a r g i and D r . W. R. F i n n e r t y f o r r e v i e w i n g t h i s manuscript . REFERENCES 1. 2.
I. S t i e f e l , Chem. Eng. Prog., Oct. 1987, 21-34. E. B ju rs t ro m , Chem. Eng. Feb. 18, 1985, 126-158. E.
3. P. Gerhardt, E d i t o r , Manual o f Methods f o r General B a c t e r i o l o g y , American S o c i e t y o f M i c r o b i o l o g y , Washington D.C. 1981, 193. 4. R. S t r a w i n s k i , U.S. P a t e n t No. 2,521,761 (1950). 5. C . Z o b e l l , U.S. P a t e n t No. 2,641,564 (1953). 6. I. Kirshenbaum, U.S. P a t e n t No. 2,975,103 (1961). 7. K. Yamada, Y. Monda, K. Kodama, S. Nakatani, and T. Akasaki, A g r i c . B i o l . Chem., 2 (1968) 840-845. 8. K. Yamada, K. Kodama, S. Nakatani, K. Umehara, K. shimizu, and Y . Minoda, A g r i c . B i o l . Chem., 34 (1970) 1320-1324. 9. R. Cripps , Biochem. J., 134 (1973) 353-366. 10. F . Sagardia, J. Rigau, A. M a r t i n e z - L a t t o z , F. Fuentes, C. Lopez and W. F l o r e s , A p p l i e d M i c r o b i o l o g y , 29 (1975) 722-725. 11. C . Hou and A. Laksin, Dev. I n d . M i c r o b i o l . , 11 (1976) 351. 12. A. Laborde and D . Gibson, A p p l i e d and Environmental M i c r o b i o l . , 3 (1977) 783-790.
288
13. F. Kargi and J. Robinson, Biotech. and Bioeng., 26 (1983) 687- 690. 14. W . Finnerty, K. Schockley and H. Attaway, M i c r o b i a l D e s u l f u r i z a t i o n and Deni trogenation o f Hydrocarbons, M i c r o b i a l Enhanced O i l Recovery, Pemwell Publishing Co., OK (1983). 15. A . H. Jackson, i n "The Porphyrins, Vol. 1, S t r u c t u r e and Synthesis: Part A . , " D . Dolphin, Ed., Academic, New York,
1978, 374-380.
289
AUTHOR I N D E X 181
Aboul -Ghei t, A. K. Avalos, M.
91
Bauer, S.H.
Beer, V.H.J. de
165
Gobolos, S.
243
79, 165 123
Breysse, M.
243
Brown, J.R.
187, 229
C a t t e n o t , M.
67, 243
Chan, T.C.
G r u i j t h u i j s e n , L. van
Chiu, N-S.
1 229
91
J a l o w i e c k i , L. Kapolos, J.
Dalmon, J.A.
21
Kherbeche, A.
van d e r
L a c r o i x , M.
243
79, 165
243
Denley, D.R.
147
21
Mauchausse, C.
67
Maxwell, I .E.
263
Messalhi, A. Moreau, C.
91
187, 229
107 107, 115
115 O l i v e , J.L.
E i j s b o u t s , S. Ekman, M.E. Esener, A.A.
165
147
M c I n t y r e , N.S.
147
Durand, R .
123
Kraan, A.M.
Li, Y-Xi
De Beer, V.H.J.
Diaz, G.
211
67
Davis, B.H.
T.
123
Katsanos, N.A.
165
1
Daage, M.
123 211
Lee, W-H.
107
79 41 263
Patras, L.E.
273
Pedraza, F.
91
P o r t e f a i x , J.L. Fuentes, S .
79
123
Kemp, R.A.
147
Coa t s w o r t h , L. L. C r a j e , M.W.J. Cruz, J . 91
Hubaut, R.
K a s z t e l a n , S.
187
C h i a n e l l i , R.R.
Decamp,
123
107, 115
B o n n e l l e , J.P.
Cota, L.
107, 115
G r i m b l o t , J.
147
Bekakra, L.
Geneste, P. Gerkema, E.
91
P r i n s , R.
79
67, 243
290
Rahimi, P.M. Rojas, H.
251 91
Ryan, R.C.
Van G r u i j t h u i j s e n , L.
21
Schmidt, I .
Van d e r Kraan, A.M. Volmer, J.
79
V r i n a t , M.
243
229
Schrader, G.L.
41
Wambeke, A.
Sekhar, M.V.C.
251
Webster, I . A .
Simpson, H.D. Smegal, J.A. Spevack, P.A. Spinnler, G.E. Summan, H.D.
123 273
133 21
187, 229 21 181
Zmimita, N.
115
165 79
29 1
SUBJECT INDEX Acid strength d i s t r i b u t i o n
Bulk s u l f i d e s , c h a r a c t e r i z a t i o n
123
91 Langmui r-Hinshelwood k i n e t i c s
C a t a l y s t design
Mass t r a n s f e r c o e f f i c i e n t s
133
Catalyst preparation
91
C a t a l y t i c reactors
211, 229
Chevrel phase c a t a l y s t s Closed-cycle r e a c t o r Cobal t / c a r b o n
263
1
Catalyst characterization
273
Microbial desulfurization
273
Mossbauer s t u d y
41
Molybdenum
229
165
229
Molybdenum d i s u l f i d e
165
Cobal t-molybdenum/al umi na
1, 133,
211
M i c r o b i a l deni t r o g e n a t i o n
Molybdenum s u l f i d e
21 41, 67
187 Cobal t-molybdenum/carbon
165
structure
91
1, 107,
133
91
Composi t i o n - a c t i v i ty r e 1 a t i onshi p 41 Coprocessing
147
N i c k e l -molybdenum/al umi na
Cobalt-molybdenum s u l f i d e s , preparation
251
Near edge X-ray a b s o r p t i o n f i n e
Cobal t-molybdenum s u l f i d e s , microstructure
Naphtha
N i c k e l -molybdenum-phosphorus/ alumina
79
251
C r y s t a l l o g r a p h i c concepts
133
Phosphate e f f e c t
79
P i p e r i d i n e hydrodeni t r o g e n a t i o n Desorption r a t e constants
211
D i f f e r e n t i a l scanning c a l o r i m e t r y 181
P l a t i n u m - t i n / a l umina
Promoter Extended X-ray a b s o r p t i o n f i n e structure
147
147
P1a t inum- ti n / s i1 ica Product properties
147 263
21
Promoting and p o i s o n i n g e f f e c t o f nickel
123
P y r i d i n e hydrodeni t r o g e n a t i o n Heavy o i l s Hydrocracking
Pyridine hydrogenation
273
67
263
Hydrodeni t r i f i c a t i o n
21
Hydrodenitrogenation
67, 79, 107,
115, 123, 133, 181, 251
67
Q u i n o l i n e hydrodenitrogenation 79, 107
123
292
Reactant p r e s s u r e e f f e c t
187
Sulfided catalysts Sulfides
Reversed-flow gas chromatography 211
133
79
Sulfidic catalysts
Rutheni urn s u l f i de/Y z e o l i t e
243
Sulfur/molybdenum s t o i c h i o r n e t r y Support a c i d i t y
Simultaneous r e a c t i o n s Stacked beds Stacking
107, 115
107, 115
Support e f f e c t
243 67
263
21
Toluene h y d r o g e n a t i o n
Structure-activity relationship
123
T r a n s i t i o n metal s u l f i d e s
I
41 Structure analysis
147
S t r u c t u r e - f u n c t i o n r e 1 a t i onshi p Sulfidation
X-ray p h o t o e l e c t r o n s p e c t r o s c o p y
1
187, 229
165
Sul f i d a t i o n temperature e f f e c t
67
Zeolites
263
123
293
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 I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417, 1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet yolume 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 t o 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 t o Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon 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, Bechyrie, September 29October 3, 1980 edited by M. LazniEka Volume 10 Adsorption at 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 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-1 6, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Prallaud, 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. JirO and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. A n 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
294 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 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-1 3, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirb, 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 International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Saga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-omwindermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S.Holloway Volume 27 Catalytic Hydrogenation edited by L. Cerveng Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 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 Preparationof 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 32 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 3 4 Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
295 Volume 3 5 Keynotes in Energy-Related Catalysis edited by S.Kaliaguine Volume 3 6 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. Chaney, R.F. Howe and S.Yurchak Volume 3 7 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13- 17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 3 8 Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 3 9 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 4 0 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Pbrot Volume 4 2 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pael Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 4 4 Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 4-8, 1988 edited by H.G. Karge and J. Weitkamp Volume 47 Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-1 6, 1988 edited by C. Morterra, A. Zecchina and G. Costa Volume 4 9 Zeolites: Facts, Figures, Future. Proceedings of the &th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Volume 5 0 Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony
