Studies in Surface Science and Catalysis 50 HYDROTREATING CATALYSTS Preparation, Characterization and Performance
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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
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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
V
CONTENTS Foreword
...................................................................
IX
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 .
R. P r i n s
Volmer, V.H.J.
de Beer and
...............................................................
79
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
H. Rojas, J. Cruz, M. Avalos, L. Cota and
.............................................................
Effect o f 2,6-diethylaniline
91
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
H.D. Simpson
...........................................................
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
...........................................
273
Author Index
..............................................................
289
S u b j e c t Index
.............................................................
291
L.E. Patras and I . A . Webster
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
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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.
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18
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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
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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
<
< 4.0 for CuxMo6S8).
Conversely, the concentration ranges for large Pb, or Sn is very small or nonexistent. For the light rare cations such as Ho, earths a composition of REl.0M06SE is found, but for the heavy rare earths i t is closer to RE1.2M06SE. Lead Chevrel phases cannot be prepared at PbMo6S8; rather, the most pure phases are obtained for PbMo6.2SE. For both Pb and Sn (1.6
x
Chevrel phases, a second ternary metal, such as a rare earth, may be 3
5
t
t
cu
Figure 2.
Chevrel phase structure projected on the hexagonal plane (1120), illustrating the arrangement of the ternary metal atoms in a) PbMo S8 (large cation compound), and b) CuxMo6S8 (small cation compoun8) (ref. 18).
44
cu
0.0
0 2
0 4
0 0
0 0
10
'2.
14
Delocallzatlon of Ternary Component (A)
Figure 3 .
The delocalization of the cation M as a function of the rhombohedra1 angle.
incorporated to produce a series of structurally related compounds with a nominal formula RExMl-xM~6S8. In some cases the solid solutions are complete. However, limitations in compositional ranges reflect some restrictions on the extent of rare earth insertion. In some compounds smaller cations are inserted into channel positions at low concentration while substitution for the large cation occurs at higher concentration. The oxidation state of Mo in the metal-rich Chevrel phases is low relative to MoS2. Based on simple calculations of formal oxidation states, the Mo6S8 binary compound has a valence of +2 2 / 3 . Introduction of the ternary metal decreases the Mo oxidation state by the transfer of electrons from the ternary component cations to the Mo octahedron. For example the formal oxidation state of Mo in C U ~ . ~ M Ocan ~ Sbe ~ calculated as t2. The Chevrel phases possess a broad range of possible compositions, structures, and oxidation states. The ability to control these properties make them attractive catalysts for studying the relationships between catalysis and solid state chemistry. EXPERIMENTAL PROCEDURE Catalyst Synthesis Several differences have been reported in the literature regarding the formulations required to produce pure, single phase Chevrel materials. For example, SnMo6S8 has been prepared with stoichiometry of SnxMo6-zS8
45
TABLE 1 Sulfur Chevrel phases MxMo6Saa Ternary components reported in the literature Li, Na, Mg, K, Ca, Sc, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Pd, Ag, Cd, In, Sn, Ba, La, Pb, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb Lu,
'J, NP
Examples of small cation compounds
CuxMo6S8 CoxMo6S8 N i.Mo6 S8
compositional ranges 1.6 < x < 4 1.32 < x < 2 1.32 < x < 2
Examples of intermediate cation compounds AgMo6S8 InMo6S8 Examples of large cation compounds HoMo6S8 PbMo6S8 SnMo6S8 aData from (ref. 19). ( 0 . 9 < x < 1.1; 0.2 < z < l ) , and PbMo6S8 has been reported with stoichiometry PbxMo6S8-y (0.85 < x < 1.05; 0.8 < y < 1.2) (ref. 21). "Pure" rare earth compounds have been prepared with compositions of RE1.0M06S8 and RE1.2M06S8 (ref. 22). Studies of single crystals have shown that the ideal stoichiometries
MleOMo6S8 (large cation compounds) and MxMo6S8 (small cation compounds) exist (refs. 23-24). Polycrystalline samples with compositions deviating from these "ideal" values may possibly contain some unidentified impurities. In this work, homogeneous pure polycrystalline samples were obtained for the rare earth materials at compositions of RE1.2M06S8. Representative small cation materials were prepared as MxMo6S8 (involving compositional ranges for some compounds such as Co where 1.5 < x 5 1.9) (ref. 9). Lead and tin compounds can be prepared at compositions of M 1.0M6.2S8, but alternate stoichiometries were also prepared. A series of lead-lutetium Chevrel phases were synthesized: L u ~ . ~ ~ P ~ S and ~ L u ~ . ~ ~ P (0.2 (0~< Mx ~ < 0.2) ~ ~ <- x~5 M1)~ (ref. ~ S 1~4). Details of these procedures are provided below.
46
Lead, tin, cobalt, and holmium Chevrel phase catalysts were synthesized from mixtures of MozSo6S3, molybdenum metal (reduced at lOOO'C in hydrogen for 18 h), and the appropriate metal sulfide of the ternary components (preparation method I). The powders were ground together thoroughly, pressed into pellets, and placed in fused-silica tubes which were evacuated to less than Torr. The synthesis tubes then were backfilled with argon to a pressure that would produce 1 atm at the reaction temperature. The tubes were sealed and heated to temperatures between 1000 and 1200°C for 24 to 48 h. PbMo6.2S8 and SnMo6.2S8 samples were reground in air, pressed into pellets, evacuated in fused-silica tubes, and reheated at temperatures between 1100 and 1200'C for 12 h. Dysprosium, lanthanum, lutetium, lead, and lead-lutetium Chevrel phase catalysts were prepared from mixtures of reduced molybdenum metal powdered sulfur, and sulfides of the ternary metals (preparation method 11). The materials were processed as described previously and then were heated in a muffle furnace from 450 to 750°C for a period of 48-72 h. The samples were transferred immediately to a high temperature box furnace at 1225°C for 24 h and quenched in air. After regrinding in air, the materials were pressed into pellets and placed in fused silica tubes which were evacuated and heated for 48 h at 1225'C. All synthesis tubes were opened in a nitrogen dry box where the pellets were lightly crushed. A 40-100 mesh portion was separated for use in the activity measurements; a small portion was reserved for X-ray photoelectron spectroscopy (XPS) analysis. All subsequent manipulations of these materials were performed in the dry box. Model unpromoted and cobalt-promoted MoS2 catalysts were prepared for comparisons of catalytic activity. Ammonium thiomolybdate was thermally decomposed at lOOO'C in a flow of helium resulting in.a catalyst referred to as 1000°C MoS2 (ref. 25). A cobalt-promoted MoS2 catalyst was synthesized with a cobalt to molybdenum ratio of 1:4 according to the homogeneous precipitation technique of Candia g &. (ref. 26). This material, referred to as C O ~ . ~ ~ 1--S, M O was pretreated at 450°C in a 2% H2S/H2 mixture for 4 h. Catalyst Characterization Catalysts were characterized both prior to and after 10 h of continuous H2-thiophene reaction. The bulk purity of the catalysts was examined with X-ray powder diffraction and laser Raman spectroscopy. X-ray powder diffraction patterns were obtained with a Siemans D500 diffractometer using CuKa radiation. Laser Raman spectra were obtained with a Spex 1403 monochromator utilizing the 514.5 nm line of a Spectra Physics argon ion laser operating at 200 mW (measured at the source). All spectra were collected using backscattering geometry from spinning catalyst pellets. Fifty scans were accumulated with a scanning speed
47
of 2 cm-l/s at 5 cm-l resolution. X-ray photoelectron spectra were obtained with an AEI 200B spectrometer using AIKa radiation to examine the surface composition and surface oxidation state of the molybdenum in the Chevrel phases. All spectra were referenced to a carbon 1s binding energy of 284.6 eV. Spectra of the unused catalysts were obtained from a freshly ground sample; spectra of the used catalysts were obtained from the reactor charge with no further grinding. The surface areas of the catalysts were determined by the BET method using a Micromeritics 2100E AccuSorb instrument. Krypton was used as the adsorbing gas at liquid nitrogen temperatures. Activity Measurements Thiophene HDS activities were measured at 400°C and atmospheric pressure using both pulse and continuous flow reactor techniques as described previously (refs. 7-9). Thiophene was fed with a Sage 341 syringe pump, and all gases were metered through Tylan RC-260 mass flow controllers. The catalyst loadings in the 1/4" stainless-steel reactor were adjusted to give approximately 3% conversion of thiophene after 20 min of continuous reaction (ranging from ~ -1.7327 S g for C O ~ . ~ M O ~ S ~The ) . reactor was filled 0.0074 g for C O ~ . ~ ~ - M Oto with fresh catalyst and heated from room temperature to 400°C in a stream of helium at 19 ml/min (STP). After about 1 h of helium purge at 4OO0C, between ten and twenty-five 0.25-ml pulses of 2 mol X thiophene in hydrogen were injected into the reactor at 30 min intervals. The flow was then replaced with a continuous flow of 2 mol X thiophene in hydrogen at 22 ml/min (STP). After 10 h of continuous thiophene reaction, the reactor was purged and cooled in a stream of helium. Benzothiophene HDS activities were performed utilizing the thiophene reactor apparatus with some slight modifications (ref. 15). Benzothiophene is a solid at room temperature (m.p. 29-32"C), and it was necessary to heat a small chamber surrounding the syringe pump to 40°C. The heated benzothiophene was pumped into a saturator (maintained at 230°C) where it was vaporized and mixed with hydrogen. The 1/4" stainless-steel reactor was loaded with between 0.3120 and 0.5404 g of Chevrel phase (PbMoge2S8) or 0.1640 g of model catalyst ( C O ~ . ~ ~ - M O ~ - S )The . reactor was heated from room temperature to the reaction temperature in a flow of helium at 19 ml/min (STP). After about 1 h helium purge at the reaction temperature, the flow was switched to a continuous flow of 2 mol % benzothiophene in hydrogen at 20 ml/min (STP). After 12 h of continuous reaction, the reactor was purged and cooled using flowing helium. Reaction temperatures ranging from 250 to 500°C were used. Activity measurements for HYD of 1-butene to ;-butane were also performed as described previously (refs. 7-9). The reactor was loaded with the same amount
48
of fresh catalyst as for the thiophene HDS activities. The reactor was heated from room temperature to 4OOOC while using a stream of helium at 19 ml/min (STP).
After about 1 h at 400°C ("fresh catalyst"), two 0.1-ml pulses of 2 mol
X 1-butene in hydrogen were injected into the reactor at 15 min intervals.
Twenty-five 0.1-ml pulses of 2 mol X thiophene were then introduced to the reactor, and the 1-butene pulses were repeated. The catalyst next underwent 2 h of continuous thiophene reaction (2 mol X thiophene in hydrogen at 22 ml/min). The reactor was purged with helium, and the 1-butene pulses were repeated. Product separation and analysis was performed with an Antek 310 gas chromatograph equipped with a flame ionization detector. Peak areas were measured by a Hewlett-Packard 33908 integrator. A 12 ft E-octane/Porasil C column was used for the thiophene HDS and 1-butene HYD studies. Identical retention times were found for e - 2 - b u t e n e and 1,3-butadiene requiring these materials to be combined in the data analysis. An 11 ft 3% SP-2100 on 100-120 mesh Supelcoport column was utilized for the benzothiophene HDS experiments. EXPERIMENTAL RESULTS Catalyst Characterization The purity of the bulk Chevrel phase structures was determined primarily by X-ray powder diffraction. For all Chevrel phases studied, there were no apparent changes in the X-ray pattern after thiophene reaction times of up t o
10 h:
there was no loss of crystallinity and no formation of other phases.
Laser Raman spectroscopy is a sensitive technique for the detection of both crystalline and poorly-crystalline MoS2 (bands at 383 and 409 cm-1) (ref. 27). A slight amount of MoS2 impurity was detected for the cobalt Chevrel phases (refs. 7-9).
After reaction, the amount of MoS2 in these materials increased.
The presence of a small amount of MoSZ was also detected in SnMo6S8; this amount remained approximately constant after 10 h thiophene reaction (ref. 13).
No
MoS2 was detected in any other fresh or used Chevrel phase catalysts. XPS binding energies of typical Chevrel phase materials are summarized in Table 2. Table 3 summarizes the binding energies for the compounds prepared at other stoichiometries. The Mo 3d5/2 binding energies for the fresh catalysts listed in Table 2 are grouped around 227.7 eV, ranging from 227.3 eV ( C O ~ . ~ M O ~to S ~228.1 ) eV (SnMo6.2S8);
this data clearly demonstrates the low
molybdenum oxidation state present in these materials. For comparison, the 3d5,2 binding energy for MoS2 (Mo4 + ) is 228.9 eV, and that for Moo3 (Mo6t ) is about 232.5 eV (ref. 28). These tables also summarize the changes in the Mo 3d spectra which occur after 10 h of thiophene reaction. For the large cation compounds, there are no significant shifts in the band positions. However, for the representative small cation compounds,
and Co1.,Mo6S8,
the Mo 3dSl2 bands shift from 227.8
49 TABLE 2
XPS binding energies and intensity ratios Catalyst
Mo
M
5
__
M~/MO
sbino
3d3/2
3d5/2
__
A
230.7
__ C __
--C
230.8
--C --
161.6
B
227.5 227.5
161.5
--
0.29 --d
A
231.3 231.3
228.0
_-C
228.0
__
__ C __
162.3 162.6
__ C __
__
2P
Large cation compounds E01.2Ma6S8
b‘1.2H06S8
B
--d
A
231.1
227.9
B
230.9
227.7
851.4e1 851.0
834.ge2 834.1
162.2 162.2
--
--d --
A
231.1 230.8
227.7 227.6
207.0f1 206.5
197.Zf2
B
196.8
161.8 161.8
0.17g 0.91
0.33 0.31
PbMo6.2S8 (prep. I)
A
230.9 230.8
227.5 227.5
142.3h1 142.5
137.gh2 137.5
161.6 161.8
0.561
0.27 0.28
PbHo6.2S8 (prep. 11)
A
231.1 231.0
227.7 227.9
143.4h1
138.gh2 138.3
162.3 162.4
0.501
143.0
SnHo6,ZS8
A
231.4
228.1
231.2
221,9
494.5el 494.5
486.Oe2 485.9
161.7 161.8
0.48’
B
A
231.2
227.8
796.Zk1
780.4k2
B
231.8
228.8
796.2
780.4
161.9 161.9
A B
230.6 231.9
227.3 228.7
795. lkl 796.6
77922 780.8
161.8 162.2
Lal.ZHo6S8
LU1.2H06S8
B
B
- _d
0.56
0.50
0.33 0.42
0.45
0.27 0.27
--d __
-_d __
0.291
0.24 0.31
Small cation compounds
NOTE:
0.23
A , fresh catalyst; B, after 10 h continuous E -thiophene reaction.
2
aRav area ratio of ternary component M electrons to Ho 3d electrons. bRav area ratio o f S 2p electrons t o no 3d electrons. ‘Ternary metal spectrum too diffuse to detect dInforrnation not available.
‘H 3d3/2 (el) and H 3d5/2 (e2). fH 4d3/2 ( f l ) and H 4d5/2 (f2). gRav area ratio of t4 4d to Ho 3d. h H 4f5/2 ( h l ) and H 4f7/2 (h2). area ratio of H Lf to Mo 3d. 1Rav area ratio o f H 3d to Ho 3d. kH 2p1/z (il) and M 2p3/2 (i2). ’Raw area ratio of M 2p3/2 to Mo 3d.
to 228.8 eV and from 227.3 to 228.7 eV, respectively. These small cation materials clearly show some oxidation of the surface molybdenum species. The
50
molybdenum 3dSl2 binding energy of fresh Pb0.92M06S8 is the highest of any of the Chevrel phases examined (228.5 eV), the binding energy drops to 227.8 eV after 10 h of thiophene reaction. This value is in good agreement with the other Chevrel phases. L u ~ . ~ P ~ ~ .also ~ ~ showed M o ~ Sa ~slight reduction in the Mo 3d5/2 binding energy after reaction (228.1 eV to 227.6 eV). The other lead-lutetium compounds remain stable with respect to molybdenum reduction after reaction. With the exception of L U ~ . ~ M O ~Cola7M06S8, S~, Pb0.92M06S8, and L U ~ , ~ P ~ M ~ ~ S ~ the binding energies of the ternary metal components do not change appreciably. Similarly, the sulfur 2p binding energy remains approximately constant (near 162.0 eV) for all catalysts. Tables 2 and 3 also show ratios of raw peak areas of a core electron orbital of the ternary metals compared to the molybdenum 3d electrons, and the ratios of the uncorrected peak areas for the sulfur 2p electrons compared to the molybdenum 3d electrons. These ratios are not corrected for instrumental or atomic sensitivity factors and are not intended to quantitatively reflect the compositions at the surface. Rather, they are provided to demonstrate changes which occur after thiophene reaction. For the lead and tin Chevrel phases, the ratios M/Mo and S/Mo remain approximately the same after reaction. For the small cation materials, the ratio M/Mo is smaller after thiophene reaction indicating the loss of the ternary component from the catalyst surface.
TABLE 3 XPS binding energies and intensity ratios: other stoichiometries Catalyst
Pb0.92M06S8
A
B LuC.8Ph0.33M06S8
A
B Lu0.4Pb0.67M06S8
A
B A
B NOTE:
A,
Pb
Mo
S
Lu
Pba/Mo Lub/Mo Sc/Ho
3d3/2
3d5/2
4f5/2
4f7/2
4d3/2
4d5/2
2p
231.8 231.0
228.5 227.8
143.1 142.5
138.3 137.6
--d
--
_-_
162.0 161.9
0.45 0.48
--
0.28 0.29
230.9 230.8
227.6 227.6
142.9 142.6
138.0 137.9
207.2 207.0
197.4 197.4
162.0 162.2
0.23 0.20
0.11 0.19
0.32 0.32
231.4 231.0
228.1 227.6
142.5 142.6
137.6 137.7
207.3 207.0
197.6 197.2
161.9 161.8
0.31 0.30
0.06
0.18
0.33 0.37
231.5 230.9
228.1 227.8
143.3 142.8
138.6 138.0
--e
--_
162.3 162.3
0.48 0.48
--e
0.33
--
0.42
--
fresh catalyst; B, after 10 h continuous K-thiophene reaction
aRav area ratio O E Pb 4f electrons to no 3d electrons. bRav area ratio o f Lu 4d electrons to Ho 3d electrons. ‘Rav area ratio o f S 2p electrons to Mo 3d electrons. dNot applicable. e ~ uconcentration too l o v to evaluate.
--d
51
Conversely, for the lutetium-containing Chevrel phases, there is an enrichment of the ternary metal at the surface after reaction (an increase in the M/Mo ratios). As shown in Table 4 , the surface areas of the Chevrel phases prepared by method I generally remained within 10% of the their initial values (fresh catalysts) after 10 h of thiophene HDS. However, the surface areas of the compounds prepared by method I1 increased significantly under thiophene reaction conditions. A comparison of PbMo6.2S8 samples prepared by the different methods indicates that preparation method I1 leads to higher surface area materials. Activity Measurements The continuous-flow thiophene HDS reaction results for the Chevrel phase catalysts and for the model MoS2-based materials are summarized in Table 4. The thiophene conversion rates have been normalized on the basis of the surface area of the catalysts. The initial surface areas were used for materials that exhibited no appreciable surface area changes
(
For catalysts displaying
an increase in surface area, the initial surface areas were used to normalize the HDS activities after 20 min of thiophene reaction, while the activities after 10 h of reaction were normalized using the final surface areas. A l l Chevrel phase materials exhibit thiophene HDS activities comparable to--or greater than--the model MoS2-based catalysts. The activities of the Chevrel phases can be grouped according to their structural classification (ref. 9). The large cation compounds are the most active, the intermediate cation compounds are less active, and the small cation compounds are the least active. The large cation Chevrel phases also show less deactivation over the 10 h period than do the model MoS2 catalysts. For example, the ratio of the activity after 10 h of thiophene reaction to that after 20 min is 0.91 for SnMo6.2Sa, 0.89 for D Y ~ . ~ M O ~and S ~0.88 , for H O ~ . ~ M O ~compared S~, to 0.40 for C O ~ . ~ ~ - M O ~and -S, 0.35 for 1000°C MoS2. The deactivation of small cation compounds is approximately equal to or greater than the model MoS2-based materials. For example, the ratio of the activity after 10 h to that after 20 min is 0.25 for
and 0.21 for C O ~ . ~ M O ~ S ~ . PbMo6.2Sa (method I) and Pb0.92M06S8 have similar thiophene HDS activities (Figure 4 ) (ref. 1 3 ) . However, changes in the stoichiometry of the tin Chevrel phases have a more dramatic effect on the catalytic properties (ref. 13). As
depicted in Figure 5, after 10 h of thiophene reaction, SnMo6Sa is 3 times more active than SnMo6.2Sa, which in turn is 6 times more active than Sn1.2M06Sa. Variations in the stoichiometry of the lead-lutetium Chevrel phases also have a pronounced effect on the catalytic activity (Figure 6 ) .
Luo.lPbMo6Sa is the
most active catalyst of this series, with activity generally decreasing with increasing lutetium concentration. Figure 7 shows the continuous thiophene
TABLE 4 Thiophene hydrodesulfurization (EDS) activities (4OOOC) Catalyst
Preparation method
Surface
Reaction
Thiophene
area
time
Conversion
C4 product distribution ( X )
BDS rate
(mol/s.m2)x108 !-butane
1-butene
(X)
(m2/9)
trans-
g-
2-butene 2-butene
Large cation compounds "1. Zno6'8
Dy1.2n06s8
Lal. 2n06s8
.l'L
2H06S8
I
I1
I1
I1
LU0.8Pb0.33H06S8 ' I
Lu0.4Pb0.67H06s8
Luo.lPbHo6S8
I '
I1
0.579
20 min
2.48
10 h
2.20
12.65 11.23
0.9
32.2
41.2
25.7
0.4
40.5
34.6
24.5
0.785
20 min
2.87
8.51
1.0
34.3
40.1
24.6
0.984
10 h
2.56
7.57
0.7
38.8
36.0
24.5
0.766
20 min
1.25
1.99
__ a
18.2
10 h
0.95
1.18
--a
46.6 49.4
35.2
0.990
33.9
16.7
0.693
20 min
2.06
1.80
3.6
24.4
10 h
3.48
1.93
3.0
37.3 28.6
34.7
1.093
40.0
28.4
0.689
20 min
1.64
1.55
2.3
44.1
32.4
21.2
1.033
10 h
3.17
2.00
0.7
34.7
38.8
25.8
0.563
20 min
1.59
4.43
1.3
48.4
31.9
18.4
0.644
10 h
1.36
3.30
_--a
47.9
34.3
17.8
0.649
20 min
2.60
8.43
0.8
53.5
26.7
19.0
0.952
10 h
2.84
6.27
0.6
55.0
25.5
18.9
Catalyst
Preparation method
Surface area
Reaction time
PbHo6.2%
Pb0.92H06S8
SnHo6.2’8
SnUo6S8
Snl. 2n06s8
I
I1
I
I
I
I
EDS rate (mol/s.m2)x108
C4 product distribution (%) :-butane 1-butene f ~ a n ~ - &-
(X)
(m2W
PbHo6.2S8
Thiophene Conversion
2-butene
2-butene
20 min
1.92
10.03
1.0
54.4
26.0
18.5
10 h
1.28
6.68
1.0
62.0
23.8
13.2
1.318
20 min
1.59
4.53
20.2
14.3
10 h
1.16
2.61
__ __ a
65.5
1.664
65.6
21.8
12.6
1.23
20 min
2.38
9.73
__ a
52.5
27.2
20.3
10 h
2.11
8.62
0.9
56.5
24.5
18.1
20 min
1.90
3.57
0.6
60.7
22.6
16.1
10 h
1.72
3.24
0.5
63.1
21.3
15.1
0.357
20 min
1.83
9.64
0.6
55.3
27.0
17.1
0.304
10 h
1.62
10.03
0.5
59.4
25.9
14.2
0.314
20 min
1.24
1.61
65.0
22.9
12.1
10 h
0.41
0.53
64.7
26.0
9.3
46.4 42.2
34.2
19.0
45.6
12.2
0.400
0.388
-_a --a
Small Cation Compounds “1 .5n06s8
I
0.150
20 min 10 h
2.06 0.54
3.16 0.82
0.4
__ a
(Continued on page 5 4 )
53
54
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55
Pb0.02M06SE PbMog,2Sg
0
0
1
2
3
1
5
8
7
8
D
10
Hours of Thiophene Reaction
Figure 4 .
HDS activities of lead Chevrel phases of various stoichiometries.
n
SnMo6Sg SnMo6.2SE
Snl.2M06S8
0
1
2
3
1
5
8
7
8
0
10
Hours of Thlophene Reactlon
Figure 5.
HDS activities of tin Chevrel phases of various stoichiometries.
56
0
1
2
3
4
5
6
7
8
10
0
Hours of Thlophene Reectlon
Figure 6.
HDS activities of lead-lutetium Chevrel phases.
\
2 0 mln rencllon
0
10 h i r e a c l l o n
2 hr raactlon
1.5
1.8
1.7
1.8
1.0
Compositlon of Co,MogSg
Figure 7.
HDS activities of cobalt Chevrel phases.
51
reaction results for the CoxMo6S8 series with 1.5 5 x 5 1.9. After 20 nlin of reaction, the activity increases in roughly a linear manner with increasing cobalt concentration. The activity differences decreased with reaction time and were approximately equal after 10 h of thiophene reaction (ref. 8). The C4 hydrocarbon product distributions resulting from thiophene HDS vary markedly for the Chevrel phase catalysts which have been examined. For example, the ratio of the 2-butenes to 1-butene after 10 h thiophene reaction was 2.4 for S ~ for , L U ~ . ~ M O ~1.5 S ~for , H O ~ . ~ M O ~0.58 S ~ ,for SnMoge2S8, 1.5 for C O ~ . ~ M O ~1.1 1000°C MoS2, and 1.7 for C O ~ . ~ ~ - M O ~ -These S . values differ from the thermodynamic equilibrium value at 4OO0C for which the ratio of 2-butenes to 1-butene is 2.8 (ref. 29). Catalyst activity measurements using benzothiophene are summarized for PbMogS2S8 (method 11) and C O ~ . ~ ~ - M Oin ~ -Table S 5. The benzothiophene the rate of production of ethylbenzene conversion activities were calculated as and were normalized on the basis of the catalyst surface area. HDS rates were determined at each reaction temperature after 12 h of continuous benzothiophene reaction, Comparisons between the catalytic activity of PbMogm2S8 (method 11) and C O ~ . ~ ~ - M Ocannot ~ - S be readily made from the data. However, i t should be noted that the lead Chevrel phase has a lower activity at 4OOOC for benzothiophene HDS (1.45 x mol/s-m2) than for thiophene HDS (2.61 x 10-8 2
mol/s-m ). Table 6 summarizes the results for the 1-butene HYD measurements for the Chevrel phase catalysts and for the model MoS2-based materials. The activities were calculated as the rate of production of !-butane normalized to the surface area of the fresh catalysts. HYD activities are reported for three different times: (A) fresh catalyst, (B) after 25 H2-thiophene pulses, and (C) after 2 h
TABLE 5
Benzothiophene hydrodesulfurization (EDS) activities (12 h reaction) Catalyst
Surface area (m2/g)
PbHo6.2S8 (prep. 11)
1. 66ha
C O ~ . ~ ~ - H O ~ 10.83 -S
Reaction Temperature
Benzothiophene Conversion
("C)
(X)
325 400 450 500
0.7 3.2 6.0 14.0
0.34 1.45
250 300
8.1 26.9
350
85.6
0.91 3.05 9.70
aSurface area after 10 h continuous E 2-thiophene reaction.
BDS rate (mol/s.m2)xd
2.12
3.95
58
continuous-flow thiophene reaction. All Chevrel phase catalysts have HYD activities much lower than the unpromoted and cobalt-promoted MoS2 catalysts (from about 7 to 30 times lower for the fresh catalysts to between 2 and 10 times lower after 2 h of thiophene reaction).
The ratio of HDS to HYD
activities after 2 h of continuous thiophene reaction is 39.5 for H O ~ . ~ M O ~ S ~ , 12.8 for Cole7M06S8, 7.6 for C O ~ . ~ ~ - M O ~and - S ,2.3 for 1000°C MoS2. The catalysts also show large variations in their ability to isomerize the 1-butene pulses to a mixture of 1-butene, =-2-butene, and a-2-butene. For example, after 2 h of continuous thiophene reaction, 46.1% of the 1-butene is not isomerized by Cole7M06S8, compared to 29.1% for D Y ~ . ~ M O ~ At S~. thermodynamic equilibrium (4OO0C), 26.5% 1-butene should be observed (ref. 29). LuOslPbMo6S 8 demonstrated no ability to convert 1-butene reaction conditions.
to
n-butane under the
TABLE 6 1-butene hydrogenation ( E D ) activities (400OC) Catalyst
8YD rate (mol/s.m2)x109
g-butane
C4 product distribution (%) 1-butene =-2-butene e-2-butene
Large Cation Compounds A
3.0
B
2.0
C
3.0
A
B C
0.06 0.05 0.06
90.5 93.3
0.00 0.69 1.03
0.00
84.8
0.02
75.1 29.1
A
0.53
1.06 0.79
0.02 0.04 0.03
69.3
B C A
0.23
0.03
B C
0.38
A
0.03
31.2
4.6 3.2 38.6 7.3 12.2 39.8
4.8 3.4
30.1
7.9 12.6 31.0
22.6
53.8
16.0 26.5 24.5
91.7 92.8
3.5
4.7
0.05
3.1
0.91
0.12
62.3
20.4
4.0 17.1
0.28 0.09 0.83
0.03 0.01 0.09
89.2 88.6 59.6
5.1
B C
A
0.22
B
0.66
0.01 0.03
C
0.88
0.04
50.7
14.6 21.6
5.5
5.7 5.8
21.5
18.8
12.6 19.9
12.6
61.6 54.3
24.1
74.8
18.5 21.5
59
Catalyst
Lug. 1PbMo6S8
KYC rate (mol/s~m2)x109
!-butane
A
0.00
0.00
B C
0.00
0.00
0.00
C4 product distribution (%) 1-butene g-2-butene e-2-butene
0.00
93.7 86.9 88.3
3.4 6.9 5.3
2.9 6.2 6.4
90.5 93.3 31.2
4.6 3.2 38.6
4.8 3.4 30.1
PbMo6.2S8
A
2.0
(prep. I)
6
2.0
C
2.0
0.03 0.03 0.03
A
0.24 0.24
0.01 0.01
89.1 59.5
5.6 21.8
5.3 18.6
1.2
0.05
46.2
28.6
25.2
3.0 7.0 2.0
0.08
0.19 0.06
69.3 39.3 41.4
16.2 33.9 32.1
14.4 26.6 26.4
1.0 1.0 1.0
0.05 0.04 0.06
47.o 39.3 46.6
27.9 32.5 27.8
25.0 28.1 25.5
1.0 2.0 1.0
0.06 0.09 0.06
47.4 40.9 46.1
26.4 29.5 29.7
26.1 29.5 24.1
26.0 11.0
0.76
B C
7.1
32.7 39.2 45.6
38.0 33.9 30.0
28.5 26.5 24.2
A
23.0 24.0 7.5
23.6 23.9 44.0
42.9 42.6
31.4 31.4
30.0
25.3
26.5
43.5
30.0
PbHo6. 8'2 (prep. 11)
Pb0.92n06S8
B
A
B C SnHo6.2S8
A
B C Small cation compounds c01.7M06s8
A
B C
Model MoS2 compounds C O ~ . ~ ~ - M ~ ~ -AS
IOOOOC nos2
B C
Calculated butene equilibrium at 400°Ca
0.33 0.21 2.05 2.08 0.66
-
NOTE: A. fresh catalyst; B, after 25 E2-thiophene pulses; C, after 2 h continuous E thiophene. 2aSee (ref. 2 9 ) .
DISCUSSION OF RESULTS Thiophene HDS activities of the Chevrel phase catalysts are comparable to--or greater than--those of the model unpromoted and cobalt-promoted MoS 2 -based catalysts. Comparisons between all catalysts are regarded as being approximate only, even though activities have been "normalized" to the surface areas. Correlations between total surface area and HDS activity have been shown to be
60
inadequate for some catalysts (ref. 3 0 ) . More definite comparisons can perhaps be made within the isostructural groups of Chevrel phases because of the similarity in surface areas. The activities of the Chevrel phase catalysts for thiophene HDS can be grouped according to their structural classification as reported previously (ref. 9). Large cation compounds are generally more active than intermediate cation compounds. Small cation compounds are generally the least active Chevrel phase catalysts. Also the cobalt (and nickel (ref. 9 ) ) Chevrel phase catalysts are among the least active materials, even though cobalt and nickel are widely used as promoters in conventional HDS catalysts. The most active thiophene H D S catalysts incorporate unusual "promoters", such as holmium, dysprosium, lead, and tin. Figure 8 illustrates the rate of thiophene H D S for several rare earth "promoted" Chevrel phase catalysts, as well as data for the model MoS2-based materials. After 10 h of thiophene reaction, the rare earth containing materials exhibit thiophene H D S activities comparable to or much greater than the model MoS2-based catalysts. The bulk structure of the Chevrel phase catalysts has been demonstrated t o be stable using X-ray diffraction and laser Raman spectroscopy analysis. No loss of crystallinity or formation of other phases was detected with X-ray diffraction. X-ray diffraction detected crystalline MoS2 in SnMo6S8, but none was detected in other fresh or used Chevrel phase catalysts. With the exception of the CoxMo6S8 materials, no poorly-crystalline MoSZ was detected by laser
0
1
2
3
4
6
8
7
8
D
10
Hours of Thlophene Reaction
Figure 8 .
HDS activities of rare earth Chevrel phases.
61
Raman spectroscopy. Synthesis of the Chevrel phase catalysts permits reduced molybdenum states to be examined directly. For the large cation compounds, the XPS data indicated that molybdenum underwent no apparent oxidation under reaction conditions. Pb0.92M06S8 and L u ~ . ~ P ~ ~ .underwent ~ ~ M o slight ~ S ~ reduction following thiophene reaction. For the CoxMo6S8 materials, surface oxidation of molybdenum after 10 h of thiophene HDS was accompanied by the formation of MoS2 (as detected by laser Raman spectroscopy).
From studies of other small cation materials i n
addition to those reported here, i t has been observed that the large cation materials generally are more stable with respect to surface oxidation of molybdenum than the small cation materials (ref. 9). For the CoxMo6S8 series, oxidation of the surface molybdenum was accompanied by a loss of the ternary component from the surface (Table 2 ) . L U ~ . ~ M O ~ S ~ (Table 2) and the lead-lutetium series materials (Table 3) demonstrate a surface enrichment by lutetium after thiophene reaction. However, lead and tin Chevrel phases exhibit no change in the concentration of the ternary component at the surface. The movement of the ternary metal is related to the "delocalization" of ternary atoms from their crystallographic positions in the Chevrel phase structures. The degree of delocalization is small for the large cation compounds and large for the small cation materials (ref. 18). Thus, under HDS conditions, the ternary component of the small cation materials can "retreat" into the bulk structure (ref. 9 ) . The low mobility of the ternary component of the large cation materials inhibits possible movement under HDS conditions. Figure 9 illustrates the relationship between the amount of ternary metal delocalization and thiophene HDS after 10 h thiophene reaction: the more immobile the ternary component is in the Chevrel phase structure, the greater the long-term catalytic activity. This relationship may provide a method for predicting the activities of other Chevrel phase materials. This relationship is not valid, however, for Lal.ZMo6S8.
The low HDS activity of Lal.ZMo6S8 is
unexpected and not completely understood. These observations may be explained by considering the stabilization effects of the ternary metals in the Chevrel phase structures. For example, the binary compound Mo6S8 cannot be prepared directly from the elements but rather must be formed by leaching the ternary component from a small cation compound. Ternary Chevrel phases are stable at high temperatures (with melting points of about 1700°C) (ref. 23). Mo6S8 decomposes at about 400°C (ref. 31) and forms large amounts of MoSz after thiophene reaction at temperatures as low as 300°C (ref. 9). Ternary metals involved in little delocalization from their positions in the Chevrel phase structures may be capable of stabilizing the catalytically active sites.
62
0.0
0.2
0.4
0.8
0.8
1.0
1.2.
1.4
Delocallzatlon of Ternary Component (A)
Figure 9.
HDS activities (10 h) versus delocalization of ternary component (adapted from (ref. 13)).
The investigation of various stoichiometries for lead, tin, and lead-lutetium Chevrel phases displayed some interesting features. A small change in the stoichiometry of the lead Chevrel phase changed the activity little (ref. 13). As shown in Figure 4 , similar continuous-flow thiophene HDS activities were found for Pb0.92M06S8 (Pb/Mo = 0.153) and PbMo6.2S8 (Pb/Mo = 0.161). However, as illustrated in Figure 5, changes in the nominal composition of the tin Chevrel phases had a more dramatic effect on the catalytic activity (ref. 13). SnMo6S 8 (Sn/Mo = 0.167), (contaminated with MoS2) had a higher activity than SnMo6.2S8 (Sn/Mo = 0.161). Both of these materials were significantly more active than Snl.ZMo6S8 (Sn/Mo = 0 . 2 0 0 ) . The lead-lutetium Chevrel phase catalysts also !emonstrate large differences in thiophene HDS activity with variations in composition (Figure 6 ) . It is thought that these differences in catalytic activity due to variations in stoichiometry are the result o f changes in bulk and surface properties, such as migration of the ternary components. Changes in structural properties of the Chevrel phases may also be important. Figure 10 illustrates the rate of thiophene HDS for the lead-lutetium materials as a function of the Chevrel phase unit cell volume. Decreasing the lutetium concentration (increasing the lead concentration) has the effect of increasing the unit cell volume. For this series of compounds, the highest thiophene HDS activity is observed for the material with the largest unit cell volume.
63
x,
Figure 10. HDS activities (10 h) versus volume of unit cell for lead-lutetium Chevrel phases.
A comparison of the effects of the preparation method on catalyst stability
and activity is also of interest. Lead Chevrel phases were prepared by preparation method I and method I1 to allow a direct comparison. For Chevrel phases prepared by only one method, comparisons are only indirect. The preparation method used for each catalyst is listed in Table 4. The most readily apparent difference surface area. P ~ M O (method ~ ~ ~ I) S has ~ 2 m /g) than PbMo6.2S8 (method 11) (1.318 reaction).
between the lead Chevrel phases is their significantly lower surface area (0.400 m 2/g fresh and 1.664 m 2/g after 10 h
The surface area of all Chevrel phases prepared by method I1
increases substantially after 10 h of thiophene HDS.
The surface areas of all
Chevrel phases prepared by method I (with the exception of SnMo6S8) change less than 10% after 10 h of thiophene reaction. The reasons for these effects are unexplained at this time. For example, PbMo6.2Sg (method I) and PbMo6.2S8 (method 11) have essentially identical Mo 3d XPS binding energies (Table 2 ) indicating the same low oxidation state of molybdenum is present in each sample. Furthermore, the Mo 3d binding energies do not shift after 10 h of continuous thiophene reaction for either compound. However, the lead Chevrel phases prepared by different methods do differ in PbMo6.2S8 (method I) is approximately 2.5 times more active for thiophene HDS than PbMo6.2S8 (method 11). The reason for
their activities for thiophene HDS.
this difference in activity is not clear. The degree to which Chevrel phase
64
catalytic activity is affected by the method of preparation warrants a more detailed investigation. The benzothiophene studies indicate that at 4OO0C, P ~ M O (method ~ ~ ~ 11) S ~has a lower activity for benzothiopbene HDS than for thiophene HDS (Table 5). This is not an unexpected result since benzothiophene is, in general, more difficult to desulfurize than thiophene (ref. 32).
Even so, i t has been suggested that
benzothiophene is actually a better model for the study of HDS processes, since i t and its derivatives are usually the most predominant type of thiophenic
compounds found in crude oils and coal liquids (ref. 33). Although possessing high thiophene HDS activities, Chevrel phase compounds exhibit 1-butene HYD activities which are much lower than the model unpromoted and cobalt-promoted MoS catalysts. This result has been reported previously 2 for several other Chevrel phases (refs. 7-9,14). The 1-butene HYD activity experiments also provide a measure of the isomerization activity of the Chevrel phase catalysts. The ability of the Chevrel phases to isomerize 1-butene is usually reflected in the butene distributions which result from thiophene HDS. For example, H O ~ . ~ M Oproduces ~ S ~ 32% 1-butene after 2 h of thiophene reaction, while 31% of the 1-butene feed is not isomerized during the 1-butene HYD activity measurements. Similarly Lal.ZMo6S8 produces 46% 1-butene from thiophene and 54% 1-butene from the 1-butene-hydrogen feed. Kolboe and Amberg (ref. 3 4 ) showed that for HDS of thiophene over MoS2 catalysts, the relative concentrations of the butenes departs from equilibrium. If 1-butene were the initial product of thiophene HDS, then 1-butene should be more readily observed for low conversions. Catalysts with little isomerization activity could also produce larger amounts of 1-butene. The selectivity of the Chevrel phases is also of interest regarding cracking products. Chevrel phases produce no detectable concentrations of cracking products from 1-butene (refs. 7-8). This is in sharp contrast to the model unpromoted and promoted MoS2-based catalysts. CONCLUSIONS Chevrel phases possess a rich solid state chemistry which permits the relationship between catalysis and structure, composition, and oxidation state to be examined. The vast majority of Chevrel phases which have been examined do have catalytic activity for thiophene HDS. The broad range of ("promoter") metals which may be used is remarkable. However, there are substantial differences in relative activity of the Chevrel phases. An important factor affecting catalytic activity appears to be structurally dependent. This is evidenced by the large cation Chevrel phases which permit little cation movement. This appears to also result in extended catalyst stability. Loss of ternary components from the surface of the catalysts leads to surface oxidation,
65
resulting in a decrease in the "metallic" nature of the catalysts and/or destabilization to form MoS2. A unique and apparently advantageous aspect of these Chevrel phases is the "reduced" oxidation state of molybdenum. The ability to systematically vary this oxidation state is an important property of Chevrel phase catalysts and has been the subject of other work (refs. 14,16). ACKNOWLEDGMENT This work was conducted through the Ames Laboratory which is operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research is supported by the Office of Basic Energy Sciences, Chemical Sciences Division. XPS spectra were obtained by J. W. Anderegg of the Ames Laboratory. REFERENCES 1. 2.
3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15.
16. 17. 18. 19. 20. 21.
22. 23. 24
I
0 . Weisser and S. Landa, Sulphide Catalysts, Their Properties and Applications, Pergamon, Oxford, 1973. B. C. Gates, J. R. Katzer and G. C. A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979. B. Delmon, in H. F. Barry and P. C. H. Mitchell (Eds.), Proceedings of the Climax Third International Conference on the Chemistry and Uses of Molybdenum, Climax Molybdenum Co., Ann Arbor, 1979, p. 73. P. Ratnasamy and S. Sivasanker, Catal. Rev.-Sci. Eng., 22 (1980) 401. H. Topsee and 8. S. Clausen, Catal. Rev.-Sci. Eng., 26 (1984) 395. H. Topsee, B. S. Clausen, N. Y. Topsee and E. Pedersen, Ind. Eng. Chem. Fund., 25 (1986) 25. K. F. McCarty and G. L. Schrader, in E. Ertl (Ed.), Proceedings of the 8th International Congress on Catalysis, Vol. IV, Berlin, Dechema, 1984, p. 427. K. F. McCarty and G . L. Schrader, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 519. K. R. McCarty, J. W. Anderegg and G. L. Schrader, J. Catal., 93 (1985) 375. K. F. McCarty and G. L. Schrader, J. Catal., 103 (1987) 261. A . J. A. Konings, A. Valster, V. H. J. deBeer and R. Prins, J. Catal., 76 (1982) 466. J. Valyon and W. K. Hall, J. Catal., 84 (1983) 216. K. F. McCarty, Ph.D. Dissertation, Iowa State University, Ames, Iowa, 1985. M. E. Ekman and G. L. Schrader, J. Catal., in press. S. C. Huckett, R. J. Angelici, M. E. Ekman and G. L. Schrader, J. Catal. 113 (1988) 36. M. E. Ekman, unpublished results. R. Chevrel, M. Sergent and J. Prigent, J. Solid State Chem., 3 (1971) 515. K. Yvon, in E. Kaldis (Ed.), Current Topics in Materials Science, Vol. 3, North Holland, Amsterdam, 1979, p. 53. R. Chevrel and M. Sergent, in 0 . Fischer and M. B. Maple (Eds.), Topics in Current Physics, Vol. 34, Springer, New York, 1982, p. 25. R. Chevrel, M. Hirrien and M. Sergent, Polyhedron, 5 (1986) 87. J. M. Tarascon, F. J. DiSalvo, D. W. Murphy, G. H. Hull, E. A. Rietman, and J. V. Waszczak, J. Solid State Chem., 54 (1984) 204. 0. Fischer, A. Treyvaud, R. Chevrel and M. Sergent, Solid State Comm., 17 (1975) 721. R. Flukiger and R. Baillif, in 0 . Fischer and M. B. Maple (Eds.), Topics in Current Physics, Vol. 34, Springer, New Yprk, 1982, p. 113. J. W. Lynn, G. Shirane, W. Thomlinson, R. Shelton and D. E. Moncton, Phys. Rev. B., 24 (1981) 3817.
66
25. J. C. Wildervanck and F. Z. Jellinek, Anorg. Chem., 328 (1964) 309. 26. R. Candia, B. S. Clausen and H. Topsae, Bull. SOC. Chim. Belg., 90 (1981) 1225. 27. T. J. Wieting and J. L. Verble, Phys. Rev. B., 3 (1971) 4286. 28. C. P. Li and D. M. Hercules, J. Phys. Chem., 88 (1984) 456. 29. S. W. Benson and A. W. Bose, J. Amer. Chem. SOC., 85 (1963) 1385. 30. S. J. Tauster, T. A. Pecoraro and R. R. Chianelli, J. Catal., 6 3 (1980) 515. 31. K. Y. Cheung and B. C. H. Steele, Solid State Ionics, 1 (1980) 3 3 7 . 32. D. R. Kilanowski and B. C. Gates, J. Catal., 62 (1980) 70. 33. G. H. Singhal, R. L. Espino and J. E. Sobel, J. Catal., 35 (1974) 353. 34. S. Kolboe and C. H. Amberg, Can. J. Chem., 44 (1966) 2623.
61
M.L. Occelli and R.G. Anthony (Editors), Advances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
INFLUENCE OF THE SUPPORT AND THE SULPHIDATION TEMPERATURE ON THE CATALYTIC PROPERTIES OF MOLYBDENUM SULPHIDE I N PYRIDINE HYDROGENATION AND PIPERIDINE HYDRODENITROGENATION
J.L. PORTEFAIX, M. CATTENOT, J.A.
DALMON and C. MAUCHAUSSE
I n s t i t u t de Recherches s u r l a C a t a l y s e , C.N.R.S., 69626 V i l l e u r b a n n e CBdex, France
-
2 avenue A . E i n s t e i n ,
ABSTRACT The c a t a l y t i c p r o p e r t i e s 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 h y d r o g e n o l y s i s o f molybdenum s u l p h i d e s u p p o r t e d on alumina, s i l i c a o r z i r c o n i a were determined. S i g n i f i c a n t d i f f e r e n c e s on changing t h e s u p p o r t o r t h e s u l p h i d a t i o n temperature were observed. To e x p l a i n t h e s e d i f f e r e n c e s , s e v e r a l i n t e r p r e t a t i o n s a r e examined: geometric e f f e c t s ( i .e., changes i n t h e d i s p e r s i o n o r i n t h e molybdenum exposed p l a n e s ) , 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 ) and v a r i a t i o n s i n t h e d e n s i t y o f t h e active sites. INTRODUCTION Owing t o t h e prominent r o l e t h a t t h e y p l a y i n h y d r o t r e a t i n g processes, s u p p o r t e d p u r e o r promoted molybdenum s u l p h i d e c a t a l y s t s
have been w i d e l y
s t u d i e d . The i n f l u e n c e o f t h e s u p p o r t on t h e i r c a t a l y t i c performance has o f t e n been examined,
b u t such s t u d i e s have d e a l t m a i n l y w i t h h y d r o d e s u l p h u r i z a t i o n
(HDS) and o n l y t o a s m a l l e x t e n t w i t h h y d r o d e n i t r o g e n a t i o n (HDN). Concerning h y d r o d e s u l p h u r i z a t i o n , t h e r e are, however, s t i 11 d i s c r e p a n c i e s i n t h e c o n c l u s i o n s drawn.
Thus de Beer and co-workers
(1,2) suggest t h a t t h e
n a t u r e o f t h e s u p p o r t i s n o t t h e most i m p o r t a n t f a c t o r g o v e r n i n g HDS a c t i v i t y , whereas o t h e r w o r k e r s ( 3 - 9 ) showed t h a t i t has a s i g n i f i c a n t i n f l u e n c e on t h e f i n a l s t r u c t u r e and t h e performance o f t h e c a t a l y s t ;
t h u s Duchet e t a l .
(4)
f o u n d t h a 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 a c t i v i t y p e r mole o f Mo decreases i n t h e o r d e r Mo/C opposite
>
Mo/Si02
trend,
>
Mo/A1203
Mo/A1203 whereas M u r a l i d h a r
>
Mo/Si02.
The
e t al.
observed v a r i a t i o n s
(5)
found
the
i n catalytic
p r o p e r t i e s on changing t h e s u p p o r t a r e most o f t e n e x p l a i n e d by t a k i n g i n t o c o n s i d e r a t i o n t h e i n t e r a c t i o n between t h e c a r r i e r and t h e Moo3 phase which e x i s t s before t h e sulphidation step; i n t e r a c t i o n govern t h e d i s p e r s i o n , p r o p e r t i e s o f t h e s u l p h i d e d phase.
t h e n a t u r e and t h e
strength o f
this
t h e s t u c t u r e and t h e i n t r i n s i c c a t a l y t i c
68
Concerning supports,
hydrodenitrogenation,
Ti02
and
A1203
are
equivalent
b u t a h i g h e r HDN a c t i v i t y i s o b t a i n e d w i t h Ti02-A1203
as
supported
c a t a l y s t s compared w i t h A1203 supported c a t a l y s t s ( 1 0 ) . A h i g h e r HDN a c t i v i t y i s a l s o obtained by adding Si02, B203, T i 0 2 o r Zr02 t o A1203 ( 1 1 ) ; t h i s l a s t r e s u l t i s t e n t a t i v e l y e x p l a i n e d i n terms o f a c i d i c p r o p e r t i e s o f t h e c a t a l y s t s . The aim o f t h i s work was t o o b t a i n i n f o r m a t i o n on t h e e f f e c t o f t h e support
in
HDN
processes.
We
compared
the
catalytic
properties
hydrodenitrogenation o f molybdenum s u l p h i d e supported on alumina, zirconia, physical effect
for
s i l i c a or
t h e c a t a l y t i c p r o p e r t i e s o f which f o r CO hydrogenation and some characteristics
can
be
had been p r e v i o u s l y determined
modified
during
the
sulphidation
(12).
step
of
The support
the
catalysts
p r e p a r a t i o n ; 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 was consequently examined f o r a Mo/A1203 sample. P y r i d i n e was chosen as t h e model molecule t o determine HDN performances. This
reaction
has been
extensively
used
for
the
evaluation o f
catalyst
hydrodenitrogenation p r o p e r t i e s . The r e a c t i o n scheme appears t o be complex and composed o f numerous consecutive and/or para1 1e l s t e p s (13) ; however, one can consider
that
there
are
two
main
consecutive
steps,
viz.,
pyridine
-
the actual
hydrogenation, g i v i n g p i p e r i d i n e , and p i p e r i d i n e hydrogenolysis hydrodenitrogenation s t e p are
hydrocarbons
and
-
g i v i n g r i s e t o C-N bond cleavage and whose p r o d u c t s
ammonia.
We
determined
separately
the
catalytic
performances o f our c a t a l y s t s i n b o t h steps. Moreover, p y r i d i n e hydrogenation i s v e r y l i t t l e i n f l u e n c e d by hydrogen s u l p h i d e wheras p i p e r i d i n e h y d r o g e n o l y s i s i s g r e a t l y enhanced by t h e presence o f t h i s compound ( 1 4 ) ;
consequently we
added H2S t o t h e r e a c t i o n m i x t u r e o n l y f o r t h i s l a t t e r r e a c t i o n . I n addition t o reaction tests,
several
transmission e l e c t r o n microscopy (TEM),
i.e.,
c a t a l y s t s were s t u d i e d u s i n g some samples
after
reaction
t e s t i n g and samples prepared by s u l p h i d a t i o n a t d i f f e r e n t temperatures. EXPERIMENTAL Catalysts preparation Supported molybdenum c a t a l y s t s were o b t a i n e d u s i n g t h e c l a s s i c a l i n c i p i e n t wetness impregnation method w i t h ammonium heptamolybdate s o l u t i o n s . For each metal-support
pair,
different
metal
loadings
were
prepared
and
the
corresponding s o l i d s a r e l i s t e d i n Table 1. A f t e r impregnation, t h e s o l i d s were d r i e d , then c a l c i n e d a t 773 K f o r 1 5 h. S u l p h i d a t i o n was f i n a l l y achieved u s i n g a 4 1 h-'
f l o w o f a 15% H2S-HZ
a t 673 K f o r 4 h ( h e a t i n g r a t e 8 K m i n - l ) .
In
order t o o b t a i n i n f o r m a t i o n on 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, t h e 11.3 wt.-%
Mo on alumina sample was a l s o s u l p h i d e d a t 873 and 1073 K.
69 TABLE 1 Supported Mo c a t a l y s t s prepared. Support
Mo l o a d i n g s (wt.-%)
A1203
-2 -1 (GFS Rhone Poulenc, 230 m g )
0.7
2.1
5.3
Si02
(S813 Rhone Poulenc, 190 m-'g-')
4.0
7.6
11.6
Zr02
( p r e p a r e d u s i n g flame r e a c t o r (Pi), -2 -1 0.7 75m 9 1
2.0
4.2
11.3
14.5
7.0
17.2
Catalyst characterization C l a s s i c a l 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 p i c s t u d i e s were performed u s i n g a JEOL 100 CX microscope. Determination o f c a t a l y t i c p r o p e r t i e s 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 h y d r o g e n o l y s i s r e a c t i o n r a t e s i n t h e presence o f t h e c a t a l y s t s were determined u s i n g a dynamic m i c r o r e a c t o r w o r k i n g i n t h e gaseous phase and under medium h i g h p r e s s u r e . The e x p e r i m e n t a l p r e s s u r e s and temperatures a r e g i v e n i n Table 2. reaction
rate
is
not
affected
by
the
Under such t e s t i n g c o n d i t i o n s , equilibrium
between
pyridine
the and
p i p e r i d i n e ( 1 5 ) and we d i d n o t observe p y r i d i n e as a r e a c t i o n p r o d u c t o f p i p e r i d i n e c o n v e r s i o n . The r e a c t a n t s f l o w s and c a t a l y s t w e i g h t s were choosen i n order
to
operate a t
low conversion
(
>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.
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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 |
126
t o the removal of the three different types of sulphur t h a t can be f o u n d i n MoS2 (refs. 7,lO). For reduction temperatures lower t h a n 473 K , large amounts of a weakly bound sulphur, assumed t o be the terminal sulphur ions present o n l y on the (1070) edge plane, are removed. Then, a t medium reduction temperatures the bridged sulphur ions present only on the (7010) edge plane are removed, whereas the basal plane sulphur ions need a high temperature of reduction t o be stripped o f f .
400 600 800
T (KI
Effect o f the temperature of reduction on the amount o f hydrogen sulphide removed ( l e f t ) or S/metal r a t i o (right) of ( a ) Mo and ( b ) NiMo on alumina catalysts.
f i g . 1.
D iene
hydrogenation The dependance of the t o t a l isoprene HYD activities a t 323 K on the temperature o f reduction of the Mo and NiMo catalysts i s reported i n Figure 2 . Two
127
similar "volcano" curves are obtained, w i t h no detectable HYD activity for a temperature of reduction lower t h a n 473 K or higher t h a n 1073 K . This range of reduction temperature corresponds t o the remval of the bridged sulphur ions i n the (7010) edge plane. Note t h a t the alumina support has been found t o be inactive for hydrogenation ( 1 1 ) . Thus i t has previously concluded t h a t the diene HYO sites were located exclusively i n the (7010) edge plane of the MoS2 slabs ( r e f . 1 0 ) . I t is worth recalling t h a t isomerization has also been found t o be sensitive t o the sulphur unsaturation of the (7010) edge plane. which seems t o be the only active surface of MoS2 (refs. 12,131. Interestingly, Figure 2 shows t h a t this i s not modified by the presence of Ni. The maximum activity of both the Mo and NiMo catalysts occurs a t the same reduction temperature, b u t surprisingly the activity for the NiMo catalyst is lower t h a n t h a t for the Mo catalyst. The product distributions, however, are different w i t h mobe t o t a l l y hydrogenated products obtained from the Mo catalyst t h a n w i t h NiMo, the latter g i v i n g more monohydrogenated products. To1uene hydrogenat ion
In high-pressure reactions, the starting temperature for reduction pretreatment was the reaction temperature. Figure 3 shows the toluene HYD activity of the Ni-Mo catalyst versus the temperature of reduction. The activity of the Mo catalyst is always low under our working conditions and decreases slighty when the prereduction temperature increases.
-
7
I
c
3, 2
I
400
600
800
(K)
F i g . 2 (Left). Isoprene (2-methyl-I ,3-butadiene) hydrogenation a c t i v i t y a t 323 K versus the temperature of prereduction by hydrogen of Mo and NiMo on alumina
catalysts.
F i g . 3 ( R i g h t ) . Toluene hydrogenation activity a t 623 K versus the temperature of prereduction of Mo and NiMo on alumina catalysts.
128
The effect of the addition of nickel on the Mo catalyst i s clearly important, as expected. An increase i n activity by a factor of 6 is found. However, af ter reduction a t T R > 623 K the activity decreases rapidly and tends t o reach a plateau
.
Pyridine hydrodenitrogenation In Figure 4 , the pyridine HDN activity reaches a small maximum after reduction a t 623 K and then decreases similarly for b o t h the Mo and N i b catalysts. The difference in activity between these catalysts i s now small, corresponding t o a small promoting effect, as is well known. Differences are observed, however, i n the product distribution (pentane + piperidine) calculated as S = lOO.pentane/(pentane t piperidine). The Mo catalyst produces more pentane with a 100% selectivity i n pentane for TR = 573-623 K. Apparently, hydrogenolysis seems t o be favoured by the Mo catalyst, b u t in fact b o t h the Mo and NiMo catalysts give the same production rate i n pentane, whereas hydrogenation o f pyridine i s favoured by the NiMo catalyst, leading t o the observed difference.
0) ~
500
700
800 T (K)
B.E
573
860
773
850
860
1 K)
850
Fig. 4 ( l e f t ) . HDN of pyridine on Mo and NiMo on alumina catalysts versus the temperature of prereduction : ( a ) activity ; ( b ) selectivity into pentane. F i g . 5 ( R i g h t ) . Evolution of the pyridine HDN of the NiMo on alumina catalysts versus the temperature of prereduction and Ni 2p3/2 XPS spectra (B.E. in eV) o f the sample prereduced a t 573 and 773 K and tested. The dashed curve ( b ) shows the reversibility on exposure of the catalyst reduced a t 773 K t o a sulphided feed.
129
Of particular importance is t o note in Figure 5 (dashed curbe b ) the t o t a l
reversibility of the HDN activity, as exposure t o a feed containing dimethyldisulphide after reduction a t 773 K restores the original activity of the catalyst. XPS measurements
To investigate the state of the Ni species i n the promoted catalyst, XPS analyses were performed on two samples obtained after reduction and testing i n pyridine HDN and f i n a l l y transferred t o the spectrometer w i t h o u t exposure t o a i r . The Ni 2p3,2 peak assignment is based on an XPS study on bulk and supported Ni-Mo catalysts ( r e f . 1 4 ) . Clearly, there i s a shift o f about 0.6 eV between the nickel species present i n b u l k nickel sulphides ( B E = 853.5 f 0.1 eV) and nickel on interaction w i t h MoS2 t o form the So-called "NiMoS" phase. The sample reduced a t 773 K give the Ni 2p3,2 spectrum reported i n Figure 5c, which i s characteristic of Ni being mainly i n a decoration position (hereafter abbreviated t o N i - D ) ( B E = 853.9 eV) w i t h traces of Ni oxide ( B E = 856 eV), whereas the Ni species observed after reduction a t 773 K i s characteristic o f a nickel bulk sulphide species ( B E = 853.5 eV) w i t h a slight increase in the Ni oxide peak. I n b o t h instances no metallic nickel can be detected ( B E = 852.8 eV). Hence i t is clear t h a t the changes i n the a c t i v i t y observed after reduction are associated w i t h a change in the nature of the dominant nickel species in the temperature of reduction range 573 t o 773 K. DISCUSSION
The presence of nickel (or cobalt) ions decorating the edge of the MoS2 slab is recognized as being the origin of the promoting effect (ref s. 4 - 6 ) . Hence a f i r s t aspect of the versatility of nickel i n these catalysts i s i t s location, such as i t s presence i n the alumina surface sites, in bulk sulphide particles or i n decoration positions. In the last instance the situation m i g h t be more complex because two types o f edge planes are distinguished on the MoS2 slabs, which can be expected t o accommodate two different types of Ni ions ( r e f . 7 ) . Other aspects of the versatility of Ni are the type of sites t h a t Ni creates and their reactivity. The results obtained on the unpromoted catalyst wi 11 be considered f i r s t . The volcano curve observed f o r the isoprene hydrogenation activity has been previously interpreted as evidence for the necessity t o have an optimum concentration of sulphur species on the active surface Ci.e. the (7010) edge plane1 t o o b t a i n the maximum activity. This corresponds t o the presence of a maximum number of active sites possessing a suitable structure. By s i t e structure is meant the adsorbed species, ligands and vacancies on an ensemble of metal ions
130
(refs. 12-13). I n other words, different s i t e structures are equivalent t o different environments of the adsorbed reactant. I n diene hydrogenation, the s i t e structure consists of a t least one Mo ion, originally three coordinatively unsaturated (cus) and one remaining sulphur. These cus are necessary t o adsorb the molecule and the hydrogen species. This has been discussed i n detail elsewhere ( r e f . 10) and w i l l not be considered further here. The variation of the activity of the Mo catalyst for pyridine HDN f i r s t indicates t h a t the experimental approach used, i .e., monitoring the sulphur content of the active surface through reduction pretreatment, i s extendable t o high-pressure reactions. Of course, d u r i n g the diene HYD t e s t , the temperature and hydrogen pressure are small enough not t o influence the S/Mo stoichiometry obtained after the prereduction step. This i s perhaps not always true for the reactions conducted a t h i g h H2 pressure, b u t the catalysts have been prereduced during a sufficiently long period t h a t the further S evolution during the t e s t should be small. This reaction also depends on the generation of a particular s i t e structure on the (7010) edge plane, because the variation i n activity occurs i n the same reduction temperature range as observed for isoprene HYD. However, the maximum activity occurs a t a lower temperature of reduction, resulting i n an active surface w i t h a higher sulphur ion concentration, which suggests t h a t the HDN s i t e structure i s different t h a n the diene HYD s i t e structure i n that i t will contain more sulphur species. This i s i n accordance w i t h the observation t h a t the presence of hydrogen sulphide promotes the hydrogenolysis step of the HDN reaction ( r e f . 8 ) . Toluene hydrogenation needs a less lacunar s i t e , as the variation of the activity versus the reduction temperature shows a maximum a t higher S/Mo stoichiometry t h a n for dienes. On Ni-promoted catalysts, two effects are superimposed i n the reported experiments : the promoting or poisoning effect and the change i n the sulphur/metal r a t i o , i .e., of the active surface structure. Hence, the sharp decrease i n the toluene hydrogenation activity of the N i b catalyst on reduction clearly suggests t h a t the N i - D i s destabilized, i n particular for temperatures of reduction higher t h a n 573 K. This i s confirmed by the XPS spectra i n Figure 5 , showing the change i n the N i species. However, this destabilized nickel i s not i n metallic form b u t mainly i n a sulphide form w i t h some oxide. For isoprene hydrogenation, activity starts after reduction a t 473 K. In the temperature of reduction range 473-573 K no differences between the Mo and NiMo catalysts can be observed. The MoS2 being correctly decorated by Ni, this observation implies t h a t there i s no promoting effect and t h a t the same type of sites are generated by the promoted surface. T h a t no promoting effect occurs may be the result of the atmospheric pressure used, as i t i s known that the promoting effect i s pressure dependent. T h a t the same s i t e structure is
131
generated would imply t h a t Ni has replaced Mo i n i t s normal lattice position and can be the active metal i o n . For reduction temperatures higher t h a n 573 K, the N i - D should be destabilized as suggested above. Therefore, the a c t i v i t y for isoprene HYD can now be attribu- ted t o Mo sites. However, the poisoning effect observed may be the result of a decrease i n the number of sites h a v i n g the appropriate structure because of a blocking effect of some sulphur ions through remaining Ni-S-Mo bonds. This would result i n selectivity changes as observed. I n other words, i t can be proposed t h a t on reduction of the NiMo catalyst, sulphur ions are removed which destabilized the N i - D w i t h o u t breaking a l l the Ni-S-Mo bonds w i t h the slab and eventually creating some bonds with the support. A t higher temperatures these latter will be broken and the Ni species allowed t o segregate i n t o sulphide particules. The reversibi l i t y clearly observed i n Figure 5 is also i n favour of a Ni species i n an intermediate position rather t h a n being segregated into sulphide particules where redispersion on sulphidat i o n would be more difficult. I n toluene hydrogenation, the S/Mo ratio a t the maximum of activity i s about 1.9. This ratio, essentialy corresponds t o the presence of 2 CUS a t the Mo i o n . This number is convenient for an on-side adsorption w i t h the plane of the ring parallel t o the surface and the formation of a n-complex surface intermediate. This so-called "horizontal adsorption" i s analogous t o t h a t proposed ( r e f . 15) for HDS of thiopene. I n the presence of Ni, the hydrogenation activity i s enhanced. As nickel atoms may also have two vacancies, we cannot reject the hypothesis t h a t these atoms are the adsorption sites themselves. Another possibility, or maybe a parallel mechanism, is t o consider an electron transfer from nickel acting as a promoter t o Mo, which becomes electron-rich. On this basis, back-bonding between the filled Mo d orbitals and the empty a n t i b o n d i n g x* orbitals of toluene destroys the aromaticity, as suggested by Harris and Chianelli for transition metal sulphides ( r e f . 1 6 ) . When nickel i s destabilized from the platelet and bound w i t h the support a t high reduction temperatures, these possibilities are no longer v a l i d . I n pyridine HDN, no major differences are found between the Mo and NiMo catalysts on reduction, i n contrast t o toluene HYD. The reduction has a similar effect on b o t h catalysts and the removal of sulphur is the dominant effect. The product distribution indicates t h a t on reduction the hydrogenolysis function i s not perturbed by the presence of Ni, whereas the hydrogenation is promoted. I t appears, therefore, t h a t the promoter acts on the hydrogenation b u t not on the hydrogenolysis f u n c t i o n , a l t h o u g h we are i n the region where the promoter should be destabilized. I t seems, therefore, t h a t Ni i n an intermediate position
132
is st 11 able to exert a promotional effect probably smaller than that of Ni in a ful decoration position. The origin of the specific effect of Ni on the HYD funct on remains unclear but it may be related to hydrogen activation. CONCLUSION The comparison of Mo and NiMo catalysts for model reactions performed at atmospheric or high pressures on prereduced samples reveals the complexity of the role of nickel in the promoted catalysts. Promoting effects, more or less important, and also apparent poisoning effects are found. These observations can result from both structural and reactivity effects. It has been shown that reduction leads t o destabilization of the nickel in a decoration position and modification of the surface S/Mo ratio. The former has a strong effect on the toluene hydrogenation owing to the high sensitivity of the hydrogenation functionality to promotion, whereas the latter has a strong effect on isoprene hydrogenation and pyridine HDN, for which only a small promoting effect is found. The different sensitivities of these reactions to variation of the S/Mo ratio suggest that they require different site structures. REFERENCES I J.V. Sanders, Chem. Scr., 14 (1979) 141. 2 S. Kasztelan, H. Toulhoat, J. Grimblot and J.P. Bonnelle, Bull. SOC. Chim. Belg., 93 (1984) 807. 3 R. Candia, 0. Sorensen, J. Villadsen. N.Y. Topsde, B.S. Clausen and H. Topsde, Bull. SOC. Chim. Belg., 93 (1984) 763. 4 H. Topsde, R. Candia, N.Y. Topsde and B.S. Clausen, Bull. SOC. Chim. Belg., 93 (1984) 783. 5 R.R. Chianelli, A.F. Ruppert, S.K. Behal, B.H. Kear, A. Wold and R. Kershaw, J. Catal., 92 (1985) 56. 6 M. Vrinat, M. Lacroix, M. Breysse and R. Frety, Bull. SOC. Chim. Belg., 93 (1984) 697. 7 S. Kasztelan, H. Toulhoat, J. Grimblot and J.P. Bonnelle, Appl. Catal., 13 (1984) 127. 8 C.N. Satterfield and S. Gultekin, Ind. Eng. Chem. Process Res. Dev., 20 (1981) 62. 9 G. Perot, S. Brunet, N. Hamze in M.J. Phillips and M. Ternan (Eds.), Proc. 9th Intern. Congress. Catalysis, Calgary (19881, The Chemical Institute of Canada, Vol. 1. 1988, p. 19. 10 A. Wambeke, L. Jalowiecki, S. Kasztelan, J. Grimblot and J.P. Bonnelle, J. Catalysis, 109 (1988) 320. 1 1 A. Wambeke, Thesis, Lille (1987). 12 S. Kasztelan. L. Jalowiecki, A. Wambeke. J. Grimblot and J.P. Bonnelle, Bull. SOC. Chim. Belg., 96 (1987) 1003. 13 S. Kasztelan, A. Warnbeke, L. Jalowiecki, J. Grimblot and J.P. Bonnelle, in preparation. 14 S. Houssenbaye, S. Kasztelan, H. Toulhoat. J.P. Bonnelle and J. Grimblot, submitted for publication in J. Phys. Chem.. 15 H. Kwart, G.C.A. Schuit and B.C. Gates, J. Catalysis 61 (1980) 128. 16 5. Harris and R.R. Chianelli, J. Catalysis 86 (1984) 400.
M.L. Occelli and R.G. Anthony (Editors), Advances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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A HISTORY OF THE DEVELOPMENT OF HIGH-METALS HYDROTREATING CATALYSTS. THE USE OF CRYSTALLOGRAPHIC CONCEPTS I N CATALYST DESIGN
H. D. SIMPSON Unocal C orp ora t i o n , CA 92621 USA
Science
& Technology D i v i s i o n ,
P.O.
Box 76,
Brea,
ABSTRACT The use o f c r y s t a l l o g r a p h i c d a t a i n f o r m u l a t i n g models u s e f u l i n t h e improvement of Mo03/A1 0 c a t a l y s t s promoted by Co and N i i s i l l u s t r a t e d . The a c t i v a t e d forms o f ?disting c a t a l y s t s were modeled w i t h CoMo S s t a r t i n g i n 1974. T h i s l e d t o t h e p r e d i c t i o n t h a t MOO l o a d i n g s h i g h e r t I ? a i t h o s e i n use a t t h a t t i m e should be p o s s i b l e w i t h c o % s t r u c t i v e r e s u l t s . The model was found t o be v a l i d i n t e s t s o f c a t a l y s t s w i t h v a r i o u s m e t a l s l o a d i n g s f o r i n i t i a l r e s i d d e s u l f u r i z a t i o n a c t i v i t y , and i n more extended t e s t i n g f o r t h e d e s u l f u r i z a t i o n o f l i g h t d i e s e l . A s e p arat e program s t a r t e d l a t e r l e d t o t h e development and e v e n t u a l c o m m e r c i a l i z a t i o n i n 1982 o f what a r e now among t h e w o r l d ’ s most a c t i v e d e n i t r o g e n a t i o n c a t a l y s t s . A r e v i e w o f t h e c u r r e n t s t a t u s o f fundamental r e s e a r c h i n t h e area o f Mo-based h y d r o t r e a t i n g c a t a l y s t s r e v e a l s 1 i n g e r i n g u n c e r t a i n t y o v e r t h e e x a c t i d e n t i t y o f t h e a c t i v e s i t e s . The most f a v o r e d p i c t u r e a t p r e s e n t i s a monolayer d i s t r i b u t i o n o f m i c r o c r y s t a l l i n e MoS2 o v e r t h e s u r f a c e o f t h e A1 0 support, w i t h t h e promoter atoms ( N i o r Co) i n s u r f a c e p o s i t i o n s i n a boid?ng c o n f i g u r a t i o n s i m i l a r t o t h a t o f Co i n CoMo S Futuristic p r e d i c t i o n s w i t h models based on (a) NiM S4, and (b) I&$i n monolayer I f (a) i s co e c t , t h e r e i s d i s p e r s i o n on A1 0 s u r f a c e s poses a d i l e m s t i l l s u b s t a n t i a ? j n c e n t i v e f o r improvement o f even t h e b e s t h y d r o t r e a t i n g c a t a l y s t s . I f ( b ) i s c o r r e c t , we have p r o b a b l y reached t h e l i m i t , a t l e a s t as f a r as t h e b e n e f i t o f m e t a l s l o a d i n g i s concerned. B e t t e r r e s o l u t i o n o f t h e e x ac t n a t u r e o f t h e s u r f a c e s p e c i e s i n t hese c a t a l y s t s would i n d i c a t e whether we s houl d c o n t i n u e t r y i n g t o improve t h e p r e s e n t systems, o r develop new ones.
a.
INTRODUCTION The s c ienc e o f c r y s t a l l o g r a p h y i n t h e modern sense u s u a l l y i n v o l v e s t h e use o f x - r a y s t o s t u d y t h e g e o m e t r i c arrangement o f t h e atoms i n s i n g l e crystals o f the materials o f interest.
T h i s i s made p o s s i b l e by t h e f a c t
t h a t r e g u l a r arrangements o f c h e m i c a l l y bonded atoms behave 1 i k e r e f r a c t i o n g r a t i n g s f o r t h e x - r a y s used (wavelength = 1-28). The i n t e n s i t i e s o f t h e d i f f r a c t e d x - r a y beams from t h e myriads o f m o l e c u l a r p l a n e s i n h e r e n t i n such r e g u l a r arrangements o f atoms c o n t a i n i n f o r m a t i o n about t h e c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e r e n c e caused i n t h e r a d i a t i o n by t h e atoms i n t h e planes. T h i s i n f o r m a t i o n can be m a t h e m a t i c a l l y transformed t o g i v e t h e i n d i v i d u a l atomic p o s i t i o n s i n t h e c r y s t a l .
When such i n f o r m a t i o n can be found f o r
134
substances t h a t are ( o r are believed t o be) r e l a t e d t o the actual a c t i v e s i t e s i n c a t a l y s t s , many c o n s t r u c t i v e inferences can be obtained. This paper shows how such inferences u l t i m a t e l y l e d t o t h e development o f a whole f a m i l y o f improved h y d r o t r e a t i n g c a t a l y s t s .
PROGRAM I N I T I A T I O N AND I N I T I A L POSTULATES I n t h e e a r l y 1970's, t h e major i n t e r e s t s i n h y d r o t r e a t i n g a t Unocal took the form o f an attempt t o develop an improved r e s i d d e s u l f u r i z a t i o n (HDS) c a t a l y s t . A t t h a t time, some twenty years o f i n d u s t r i a l s e r v i c e had been accumulated w i t h a succession o f c a t a l y s t s based on c o b a l t w i t h molybdenum on alumina, b u t increasing competition and operating s e v e r i t i e s d i c t a t e d another improvement. Although t h i s f a m i l y o f c a t a l y s t s had been i n commercial use f o r so long, e s s e n t i a l l y no knowledge had emerged about t h e nature o f the a c t i v e s i t e s . Two main f a c t s were known: (1) t h e a c t i v e surface consisted o f some form o f molybdenum and c o b a l t s u l f i d e s , and ( 2 ) the composition o f t h e best f i n i s h e d c a t a l y s t s was about 12 w t % Moo3 and 4 w t % COO on A1203. This composition corresponds t o a Mo/Co mole r a t i o o f about 2/1. Armed w i t h t h i s information, a l i t e r a t u r e search was made f o r mixed s u l f i d e s o f Mo and Co, w i t h the hope t h a t a substance w i t h t h e appropriate It was.
The c r y s t a l s t r u c t u r e It i s monoclinic, w i t h f o u r per u n i t c e l l . The s t r u c t u r e i s arranged so t h a t two o f the are roughly on t o p o f two others i n t h e b - a x i s d i r e c t i o n over
Mo/Co r a t i o would be found.
the substance CoMo2S4 ( r e f . 1 ) .
was found f o r formula u n i t s formula u n i t s a distance o f
I f i t i s assumed t h a t CoMo2S4 i s the a c t i v e component i n about 3.38. i t would almost c e r t a i n l y be most e f f e c t i v e i f c a t a l y s t s o f t h i s type, dispersed over the
support
surface
as a monolayer because t h i s would
represent t h e highest s t a t e o f dispersion.
This would suggest t h a t h a l f o f
the u n i t c e l l along the b - a x i s o f CoMo2S4 would c o n s t i t u t e an appropriate model.
A p r o j e c t i o n associated w i t h t h i s model i s shown i n F i g . 1.
t h i s figure,
i t i s seen t h a t ,
From
i n a d d i t i o n t o being bonded t o surrounding
s u l f u r s , the Mo atoms are a l s o bonded t o each o t h e r i n p a i r s roughly i n t h e c direction.
Thus, t h e r e might be some l o c a l i z e d m e t a l l i c character i n t h e
model compound.
1 i s given by and t h e r e i s Avagadro's number o f Mo atoms i n one gram atomic weight. Combining t h i s i n f o r m a t i o n w i t h the conversion f a c t o r o f 1010"A per meter, one can estimate t h a t an amount o f CoMo2S4 equivalent t o 0.28 g o f Moo3 and 0.07 g o f COO can The area o f one ( o f the two)
(a)(c)sin(p).
u n i t s shown i n F i g .
There are f o u r Mo atoms i n t h e p r o j e c t i o n ,
135
t h e o r e t i c a l l y be d i s p e r s e d as a monolayer on one gram o f A1203 h a v i n g a s u r f a c e area o f 200 m2 T h i s t r a n s l a t e s t o weight percentages o f 21 and 5
.
r e s p e c t i v e l y f o r t h e e q u i v a l e n t amounts o f Moo3 and COO.
F i g . 1. P o j e c t i o n o f CoMo S along t h e b a x i s . Space group: C2/m; a = 13.091k; b = 3.2271; c 4.89781; p = 118.910.
2
The i n f e r e n c e o f t h e above a n a l y s i s was t h a t i f one c o u l d p r o p e r l y o f Moo3 and COO on A1203 support s, a c t i v i t i e s p r o p o r t i o n a t e l y h i g h e r t h a n t h e st andard c a t a l y s t s i n use a t t h a t t i m e m i g h t be o b t a i n e d . An e x p e r i m e n t a l program was s t a r t e d t o e x p l o r e t h i s p o s s i b i 1 it y disperse
the
theoretical
amounts
.
I N I T I A L AUTOCLAVE EXPERIMENTS A lt h ough i t was r e c o g n i z e d a t t h e o u t s e t t h a t t h e i m p l i e d a c t i v i t y i n c r e a s e mig ht n o t be r e a l i z e d f o r r e s i d p r o c e s s i n g (because o f t h e h i g h d i f f u s i o n a l r e s i s t a n c e s a s s o c i a t e d w i t h t h e heavy n a t u r e o f t h i s f e e d s t o c k ) , an aut o c lav e f o r c o n d u c t i n g i n i t i a l a c t i v i t y t e s t s was a t t h e t i m e t h e o n l y means f o r e v a l u a t i n g t h e h y p o t h e s i s . A l so, t h e l i f e o f r e s i d c a t a l y s t s t ends t o be l i m i t e d by t h e d e a c t i v a t i o n o f t h e i r a c t i v e s i t e s by gradual b u t c ont in uous d e p o s i t i o n o n t o t h e c a t a l y s t s u r f a c e o f vanadium and n i c k e l ( p l u s some i r o n ) f ro m t h e feed. M e t a l s d e p o s i t i o n would n o t be expect ed t o s i g n i f i c a n t l y e f f e c t i n i n i t ia1 a c t i v i t y e v a l u a t i o n s , however. Compositional and s u r f a c e area i n f o r m a t i o n a r e summarized i n T able 1 f o r an i n i t i a l s e r i e s o f c a t a l y s t s prepared t o t e s t t h e t h e o r y . To o b t a i n t h e a c t i v i t i e s shown i n Table 1, t h i r t y grams o f each c a t a l y s t was f i r s t p r e s u l f i d e d i n a t u b u l a r q u a r t z r e a c t o r , t h e n unloaded i n t o m i n e r a l o i l t o preclude
air
exposure
before testing.
The
activity
test
itself
was
conducted i n an a u t o c l a v e equipped w i t h a s t i r r e r and a sampling valve,
136
u s i n g 165 g o f r e s i d w i t h p r o p e r t i e s shown i n T able 2 and a hydrogen pre s s ure o f 1000 p s i g . Samples were t a k e n a t one-hour i n t e r v a l s s t a r t i n g a t one hour i n t o t h e r u n , and s u b m i t t e d f o r s u l f u r a n a l y s i s .
A reaction rate
c o n s t a n t was c a l c u l a t e d f r o m t h e analyses assuming second o r d e r k i n e t i c s , and
related
to
the
r a t e constant
for
a
previously tested
commercial
r e f e r e n c e . Such e x p r e s s i o n s , e.g., r a t i o s o f r a t e constants t o t h e r a t e constant o f a reference c a t a l y s t , a r e c a l l e d R e l a t i v e A c t i v i t i e s . T able 1 P r o p e r t i e s and r e s u l t s f o r c a t a l y s t s screened i n i n i t i a l a c t i v i t y a u t o c l a v e test
C at a l v s t
m3 -
AB- 1 AB-2 AB-3 AB-4 AB-5 AB-6 AB-7 AB-8
22 22.2 18.5 24.5 24.5 22.3 20 20
Rel. W t . A c t i v i t y R e f erence1
1%o f 5 4.1 3.4 7.8 7.8 4.9 6 6
S u r f t c e Area (m / g) 200 188 194 285 263 236 359 350
166 130 145 123 129 130 101 115
Compositions w i t h o u t d e c i m a l s a r e nominal values.
Table 2 S e l e c t e d P r o p e r t i e s o f Resid Feedstock Used i n A u t o c l a v e T e s t s Gravity, 'API Sulfur, wt%
16.7 3.79
ASTM D i s t i l l a t i o n , D-1160, " F IBP/5 10/20 30/40 50/60 EP/Rec,
480/597 662/740 806/870 942/1024 vol%
1090/69.4
Conradson Carbon Residue, w t % Nitrogen, w t % ( N i t V ) wppm
8.5 0.218 46
Pour P o i n t , " F Asphal tenes, w t % Ash, d r y , w t % Oxygen, w t %
+45 5.6 0.012 0.45
137 A p l o t o f t h e r e l a t i v e a c t i v i t i e s v e r sus Mooj l o a d i n g f o r t h e c a t a l y s t s i n Table 1 appears i n F i g . 2. Both t h e a c t i v i t i e s and t h e Moo3 l o a d i n g have been n ormaliz e d by t h e s u r f a c e areas i n o r d e r t o e l i m i n a t e t h e e f f e c t s o f d i f f e r i n g c a t a l y s t d e n s i t i e s and s u r f a c e areas. The p l o t shows a v e r y n i c e e f f e c t o f me t a l s l o a d i n g d e n s i t y on t h e r e l a t i v e i n t r i n s i c a c t i v i t y . The presence o f CoMo04 i n t h e one sample shown i n F i g . 2 i n d i c a t e s reduced m e t a l s d i s p e r s i o n and e x p l a i n s t h e lowered a c t i v i t y r e l a t i v e t o t h e sample i n which no CoMo04 was found. COMOO4 CRYSTALLITES
1.0
a
k a
0.9
-
k3 0.7 -
Y
0
a
0.8
0.6
-
1
0.5
-
a
0.4 -
k I0 Y
0.3
-
0 0
a
I
0
I
1
I
I
0.05
0.10
0.15
0.20
% Mo03ISURFACE AREA
F i g . 2. S p e c i f i c i n i t i a l r e s i d f i n i n g a c t i v i t y vs. s p e c i f i c Mo% f o r selected c a t a l y s t s .
loading
DIESEL HYDROTREATING
Four c a t a l y s t s c o n t a i n i n g Moo3 l o a d i n g s f rom about 12 t o 22 w t . % ( w i t h a c o n s t a n t Mo03/Co0 w e i g h t r a t i o o f 4 : l ) were t e s t e d f o r t h e d e s u l f u r i z a t i o n of
light
diesel,
where
p o t e n t i a l s a r e much reduced r e l a t i v e t o r e s i d u a l o i l s .
and d e a c t i v a t i o n The p r o p e r t i e s o f a
typical
The c a t a l y s t s
l i g h t diesel
activated i n a
the
diffusional
feed a r e shown
bench
scale
flow
resistances
i n Table 3.
unit,
and t e s t e d
at
were
700 p s i g w i t h
temperatures r a n g i n g from 675°F t o 725'F over about 100 hours. T y p i c a l s u l f u r c onv ersi o n l e v e l s were 96-99%. R e a ct ion r a t e s were det ermined u s i n g 1.5 order kinetics,
and expressed r e l a t i v e t o a commercially a v a i l a b l e
c a t a l y s t used as a r e f e r e n c e .
138
Table 3 L i g h t D i e s e l Feed P r o p e r t i e s
G r a v i t y , 'API Sulfur, wt% T o t a l N i t r o g e n , ppm B a s i c N i t r o g e n , ppm
34.7 0.65 395 188
B o i l i n g Range, ' F (D-86)
IBP/5 10/20 30/40 50/60 70/80 90/95 Total Total Total Total
362/420 444/475 496/511 526/541 557/573 607/630 S at u r a t e s , w t % Monoaromatics, w t % D iaro m a t i c s , w t % Triaromatics, wt%
C/H r a t i o
76.1 13.5 5.3 0.5 86.0/13.2
FIA Analysis: Aromatics, % Olefins, % Saturates T o t a l S u l f u r Compounds, w t % B e nz ot h i ophenes D i benz o t h i ophenes A1 k y l Sul f i d e s Aromatic S u l f i d e s T hi ophenes
39.3 0 60.7 3.8
1.1 0 2.1 0.1 0.4
A p l o t o f r e l a t i v e a c t i v i t y v e r s u s wt.% Moo3 i s shown i n F i g . 3. t h i s case,
In
t h e t r e n d of a c t i v i t y v e r s u s m e t a l s l o a d i n g i s l i n e a r w i t h o u t
n o r m a l i z a t i o n ( t h e same s u p p o r t was used i n p r e p a r i n g a l l o f t h e samples). T h i s work was done i n 1974-1975.
Soon a f t e r , c a t a l y s t s w i t h s t i l l h i g h e r
a c t i v i t y , by v i r t u e o f even h i g h e r m e t a l s l o a d i n g s , were developed w i t h more advanced p r e p a r a t i v e t e c h n i q u e s .
139 iao
-
120
-
100
-
>
-
E
80-
t
0 Y
60-
5
-
C
8 40-
20 -
OENITROGENATION OF GAS O I L S A l l o f t h e f o r e g o i n g d i s c u s s i o n has been about h y d r o d e s u l f u r i z a t i o n
(HDS).
I n t h a t process, t h e emphasis is on t h e removal o f s u l f u r f rom t h e
hydrocarbon f e e d s t o c k .
I n a c t u a l i t y , b o t h s u l f u r and n i t r o g e n a r e always
removed i n any h y d r o t r e a t i n g process, because a1 1 hydrocarbon f eedst ocks s u b j e c t e d t o t h i s process c o n t a i n b o t h elements. What m a t t e r s i s t h e i r r e l a t i v e proportions.
I f t h e f e e d s t o c k has a h i g h s u l f u r c o n t e n t and n o t
I f the f e eds t o c k c o n t a i n s a l a r g e amount o f n i t r o g e n , t h e process and c a t a l y s t a r e s l a n t e d towards h y d r o d e n i t r o g e n n a t i o n (HDN). H y dro deni t r o g e n a t i o n c a t a l y s t s a r e t y p c i a l l y t e s t e d a t pressures o f about 1400 p s i g . The p r o p e r t i e s o f a t y p i c a l gas o i l used i n d e n i t r o g e n a t i o n t e s t i n g a r e shown i n Table 4. The temperature d u r i n g t h e t e s t is h e l d a t a v a l u e t h a t w i l l produce about 95% removal o f t h e n i t r o g e n a f t e r about 2-3 days. Experience g a i n e d between about 1950 and 1978 i n d i c a t e d t h a t somewhat d i f f e r e n t c a t a l y s t makeups a r e needed f o r HDN t han f o r HDS. Whereas CoO/Mo03 c a t a l y s t s seem t o be b e s t s u i t e d f o r HDS, NiO/Mo03 c a t a l y s t s appear t o be b e t t e r f o r HDN. The u s e f u l n e s s o f NiO/Mo03 c a t a l y s t s f o r HDN i s f u r t h e r enhanced by t h e i n c l u s i o n o f phosphorus. much n i t r o g e n , t h e process and c a t a l y s t a r e s l a n t e d towards HDS.
140
Ta ble 4 P r o p e r t i e s o f t y p i c a l g a s o i l f e e d used i n HDN s t u d i e s Gravity,
'API
24.6
B o i 1 ing Range (D-86) IBP/5 10/20 30/40 50/60 70/80 90/95 EP/Rec %
250/460 512/545 577/600 626/664 691/725 767/804 833/99.3
Sulfur, wt%
1.35
T o t a l N i t r o g e n , wppm B a s ic N i t r o g e n , wppm
iaio 738
H/C r a t i o
1 1 .a/85.
T o t a l Aromatics, v o l % Monoaromatics, v o l % Dia ro m a t i c s , v o l % Triaromatics, vol% T e t raar o m a t i c s , v o l % Pentaaromat i c s , v o l %
36.7 19.6 11.7 4.1 0.6 0.7
Total O l e f i n s , vol%
o
5.2
T o t a l S at u r a t e s , v o l %
44.3
I n t h e s p r i n g and summer o f 1978, a s e r i e s o f experiment s was begun w i t h t h e o b j e c t i v e o f o b t a i n i n g i n c r e a s e d a c t i v i t y f o r HDN t h r o u g h h i g h e r m e t a l s l o a d i n g s . The new t e c h n o l o g y t h a t r e s u l t e d l e d t o c a t a l y s t s w i t h 20-30% e x t r a a c t i v i t y r e l a t i v e t o commercial st andards. Subsequent d i s c u s s i o n s w i t h a commercial c a t a l y s t manuf act urer l e d t o a c o o p e r a t i v e e f f o r t w h i c h produced a p r e l i m i n a r y p i l o t sample i n September o f t h a t y ear.
T h i s sample had an a c t i v i t y o f about 140% r e l a t i v e t o commercial
standards, and about 20% b e t t e r t h a n t h e l a b o r a t o r y samples. some r e s e a r c h o f t h e i r own, f u r t h e r improve Unocal's
By v i r t u e o f
t h e c a t a l y s t manuf act urer had been a b l e t o
h i g h metals c a t a l y s t
invention.
In all,
about
t h r e e more y e a r s were r e q u i r e d b e f o r e a l l o f t h e scaleup and economic problems r e l a t i n g t o t h e manufacture o f t h e c a t a l y s t were r e s o l v e d . t h a t time, marke t i n g and s a l e s i n f o r m a t i o n were d i s s e m i n a t e d a l s o .
During
Early i n 1982 t h i s c a t a l y s t was f i n a l l y c o m m e r c i a lized and i t i s now an i n d u s t r y s t a ndard f o r gas o i l d e n i t r o g e n a t i o n .
141
A summary o f t h e o v e r a l l p r o g r e s s i s shown i n F i g . 4. The r e f e r e n c e i n t h i s case i s t h e o r i g i n a l v e r s i o n , used i n t h e m i d d l e 1950’s. It consisted of COO and Moo3 c o p e l l e t e d w i t h A1203. I n r a p i d succession, a s w i t c h f rom COO t o N i O - - and from p e l l e t i n g t o e x t r u s i o n t echniques f o r f o r m i n g t h e catalyst particles phosphorus was
--
was accomplished i n t h e mid-1960’s.
incorporated,
resulting
I n e a r l y 1970,
i n a dramatic a c t i v i t y
increase
because o f t h e i n c r e a s e d a c i d i t y from t h e phosphorus and t h e improvement i n t h e q u a l i t y o f t h e i m p r e g n a t i n g s o l u t i o n which t h e phosphorus a l s o p r o v i d e d .
1000 W
0
900
E
800
I
u
700
4
600
0
500
4 Z
400
300
a
t W
n
200
100 1950
1960
1970
1980
YEAR
F i g . 4. H i s t o r i c a l development o f HDN c a t a l y s t s : (I) c o p e l l e t e d Co/Mo; (11) c o p e l l e t e d Ni/Mo; (111) e x t r u d e d Ni/Mo; ( I V ) NiPMo on e x t r u d e d p a r t i c l e s ; ( V ) NiPMo on shaped e x t r u d e d p a r t i c l e s ; ( V I ) h i g h e r NiPMo l o a d i n g on shaped ex t ruded p a r t i c l e s . I n 1975, a f u r t h e r a c t i v i t y i n c r e a s e due t o lower d i f f u s i o n r e s i s t a n c e s was made by changing smaller.
t h e shape o f t h e c a t a l y s t
p a r t i c l e s and making them
The l a s t - - and h i g h e s t - - a c t i v i t y b a r i n F i g . 4 i s t h e r e s u l t o f
t h e r e c e n t h i g h m e t a l s technology. CURRENT STATUS D es pit e t h e s u c c e s s f u l use o f M o 4 -based h y d r o t r e a t i n g c a t a l y s t s f o r ov er 30 y ears , o u r knowledge about t h e n a t u r e o f t h e i r a c t i v e s i t e s i s s t i l l inc omp let e .
The Co/Mo/A1203 system has been s t u d i e d f a r more e x t e n s i v e l y
t h an t h e Ni/P/Mo/A1203
system, p r o b a b l y because i t was i n use f i r s t , and i t
i s s impler, hav i n g one l e s s c o n s t i t u e n t , summarized i n a n o t e by Massoth ( r e f . 2 ) .
The s i t u a t i o n as o f about 1976 was That work i n d i c a t e d a s t a t e of
142
general agreement t h a t the alumina support serves t o disperse the Moo3, probably as a monolayer,
i n the f i n i s h e d (but n o t a c t i v a t e d ) c a t a l y s t s .
This view s t i l l remains i n t a c t .
I n the meantime, however, various workers,
on t h e basis o f various data ranging a l l t h e way from surface spectroscopy t o a c t i v i t y t e s t s , tended t o d i v i d e themselves i n t o camps f a v o r i n g one o f t h r e e basic models c o n t r i v e d t o describe the surface s t r u c t u r e o f the promoted ( w i t h Co) and a c t i v a t e d ( s u l f i d e d ) working c a t a l y s t s . The f i r s t camp favored a model where t h e Mo i s distended over t h e surface o f the A1203 as MoS2.
I n t h i s model, t h e r o l e o f t h e Co i s t o coordinate a t t h e edges o f
very small MoS2 c r y s t a l l i t e s , thus a r r e s t i n g c r y s t a l growth and preserving the c o r r e c t chemical form o f Mo i n a very high s t a t e o f dispersion. This was known as the " i n t e r c a l a t i o n " model. The second camp favored the " S y n e r g i s t i c contact" model, i n which the small MoS2 c r y s t a l l i t e s are kept separated n o t by coordinated Co atoms, b u t by interspersed c r y s t a l 1 i t e s o f Cogs8. Some promotional a c t i o n was believed t o r e s u l t from s y n e r g i s t i c physical contact between t h e MoS; and Cogs8, beyond t h e e f f e c t o f t h e Cogss in
keeping
"monolayer,"
the
MoS2
highly
(
spersed.
The
third
camp
favored
the
model i n which both t h e Mo and Co are contained i n a s u l f i d e
monolayer which may n o t have a bu k counterpart, and which i s distended over
o f t h e support. schematically i n F i g . 5.
the
surface
All
of
these
models
are
illustrated
in
Co/Mo/A1203
II. SYNERGISTIC CONTACT
111. MONOLAYER
\ \ \ \ \ \ \A1203 SURFAC'E\, \ \ \ \' \ \ Fig. 5. Previous catalysts.
models
representing
active
surface
143
The d i v i s i o n o f t h o u g h t i m p l i e d by t h e preceding summary has g i v e n way i n t h e l a s t few y e a r s t o t h e "CoMoS" s t r u c t u r e advanced b y Topsoe, Clausen and co-workers ( r e f s . 3,4). Using r a d i o a c t i v e 57C0 c o n t a i n e d i n t h e c a t a l y s t s as sources, t h e Topsoe workers were i n i t i a l l y a b l e t o det ermine t h e Mossbauer s p e c t r a o f t h e s u r f a c e species. Augmenting t h i s i n f o r m a t i o n w i t h r e s u l t s f r o m EXAFS and I R experiments, and comparing t h e r e s u l t s w i t h r e s u l t s o b t a i n e d on b u l k s u l f i d e s and v a r i o u s r e l a t e d i n o r g a n i c c l u s t e r compounds,
they
concluded
that
the
Mo
is
very
MoSp-like,
with
the
c r y s t a l l i t e s b e i n g e x t r e m e l y s m a l l , and p r o b a b l y d i s p e r s e d as a d i s o r d e r e d monolayer on t h e s u p p o r t s u r f a c e . The Co atoms, on t h e o t h e r hand, appear t o be much ( b u t n o t e x a c t l y ) l i k e t h o s e i n CoMo2S4. I t i s b e l i e v e d t h a t t h e Co atoms occupy p o s i t i o n s a t t h e s u r f a c e of t h e MoS2-like phase, t o f orm a d i s p e r s e d composite t h a t has no known b u l k c o u n t e r p a r t . An e x c e l l e n t r e v i e w
5). An i d e a l i z e d c r y s t a l l o g r a p h i c i n t e r p r e t a t i o n o f t h e f a v o r e d model i s shown i n F i g . 7. The MoS2 s t r u c t u r e t o which i t i s r e l a t e d i s shown i n F i g . 6. The v i r t u a l CO p o s i t i o n s i n F i g . 7 may be p a r t i a l l y o r t o t a l l y occupied. From t h i s b r i e f overview, i t i s e v i d e n t t h a t t h e a c t i v e m a t e r i a l on t h e s u r f a c e o f Co/Mo/A1203 c a t a l y s t s s t i l l d e f i e s complete p h y s i c a l d e s c r i p t i o n , d e s p i t e f o r m i d a b l e e f f o r t s u t i l i z i n g t h e most modern a n a l y t i c a l methods. N ev ert heles s , t h e p r e s e n t work shows how a simple c r y s t a l l o g r a p h i c model n o t as o f about
1987 i s g i v e n by B a r t and V l a i c
(ref.
C
1 L
b
F i g . 6.
P r o j e c t i o n o f MoS a l o n g Space group c8/mmc; ; c = 12.2958. Since a = 3.160 the a t h e space group i s hexagonal, b = a.
.
axii[
F i g . 7. I d e a l i z e d r e p r e s e n t a t i o n o f p r e f e r r e d model. Space group and c e l l dimensions a r e t h e same as i n F i g . 6.
144
o n l y provided t h e i n c e n t i v e f o r pursuing increased a c t i v i t y through higher m e t a l s l o a d i n g s , b u t a l s o l e d t o good e s t i m a t e s o f t h e upper l i m i t o f ( p r o p e r l y d i s p e r s e d ) m e t a l s l o a d i n g s i n e x p e riment al c a t a l y s t s .
This provides
an e x c e l l e n t i l l u s t r a t i o n o f t h e power o f i n f e r e n c e and analogy i n r e s e a r c h - industrial i n particular. about Ni/P/Mo/A1203
Following t h i s s p i r i t ,
some f u r t h e r s p e c u l a t i o n
HDN c a t a l y s t s m i g h t be i n t e r e s t i n g .
FUTURE I n s earc h in g t h r o u g h
the l i t e r a t u r e f o r
structures
that
m i g h t be
re a s onably c l o s e t o t h e a c t i v e m a t e r i a l i n Ni/Mo/A1203 c a t a l y s t s , one f i n d s t h e compound NiMo3S4 ( r e f . 6 ) . I n t h i s substance, t h e Ni:Mo w e i g h t r a t i o i s about 1:6, j u s t about what i s observed i n t h e b e s t i n d u s t r i a l HDN c a t a l y s t s . No phosphorus i s i n t h e s t r u c t u r e , which i s tantamount t o assuming t h a t phosphorus has no r o l e i n t h e a c t i v e s i t e p e r se. T h i s does n o t p r e c l u d e i t s us ef u lnes s as an a c i d i c c o n s t i t u t e n t and i n making t h e i m p r e g n a t i n g s o l u t i o n f o r t h e c a t a l y s t , however. A view down one o f t h e axes o f t h e u n i t c e l l o f NiMojS4 i s shown i n F i g . 8. A lt h oug h a c t u a l l y rhombohedral, t h e s t r u c t u r e i s n e a r l y c u b i c . I t c o n s i s t s o f o c t a h e d r a l Mo c l u s t e r s c o o r d i n a t e d t o s u l f u r atoms which a r e i n t u r n bonded t o N i atoms s c a t t e r e d d i f f u s e l y among t h e r e g i o n s c o n s t i t u t i n g t h e corners o f t h e c e l l .
I n p r i n c i p l e , t h i s r e p r e s e n t s an a p p e a l i n g p i c t u r e o f
t h e d i s p o s i t i o n o f t h e p r o m o t e r atoms ( N i o r Co) i n Moog based c a t a l y s t s . I t i m p l i e s t h a t t h e promoter atoms m i g h t n o t occupy d i s c r e e t p o s i t i o n s , b u t
i n s t e a d "hop around" among v a r i o u s s y m m e t r i c a l l y e q u i v a l e n t p o s i t i o n s i n t h e catalyst "Mo3S4"
s urf ac e ,
e x e r t i n g j u s t enough bonding f o r c e
to
stabilize the
e n t i t i e s b u t n o t enough t o o t h e r w i s e m o d i f y t h e s t r u c t u r e .
By
v i r t u e o f b e i n g t h i s mobile, t h e N i atoms c o u l d a l s o be u n u s u a l l y a v a i l a b l e for c hemis o rb i n g the reactant m o l e cules. The octahedral Mo m i c r o c r y s t a l l i t e s c o u l d a l s o have h i g h c h e m isorbing c a p a c i t y because o f t h e h i g h p o t e n t i a l t h e y would have f o r c o o r d i n a t i v e u n s a t u r a t i o n . They m i g h t a l s o have some m e t a l l i c c h a r a c t e r , which would be expect ed t o be u s e f u l i n d i s s o c i a t i v e hydrogen c h e m i s o r p t i o n . From t h e u n i t c e l l s i z e and s t o i c h i o m e t r i c c o n s i d e r a t i o n s ,
i t can be
e s t i m a t e d t h a t a monolayer o f NiM4S4 would be e q u i v a l e n t t o even h i g h e r l e v e l s o f Moo3 and NiO t h a n a r e used i n t o d a y ' s b e s t c a t a l y s t s . Thus, even h i g h e r a c t i v i t i e s m i g h t be p o s s i b l e t h r o u g h s t i l l h i g h e r m e t a l s l o a d i n g s i f ways can be found t o approach p e r f e c t monolayers o f s upport s u r f a c e s .
NiM3Sq
on c a t a l y s t
145
F i g . 8. P r o j e c t i o n o f NiMo S4 a l o n g t h e c a x i s . Space group R3; a = 6.4628; a = 9 4 . 6 8 " . Since t h e space group i s rhombohedral, a = b = c . F o r completeness,
i t s h o u l d be s t a t e d t h a t a d i s p e r s e d monolayer o f
MoS2 (a p r o j e c t i o n o f which i s shown i n F i g . 6 ( r e f . 7 ) ) , w i t h t h e a p p r o p r i a t e amount o f N i d i s t r i b u t e d w i t h i n it, p r o v i d e s t h e same m e t a l s loading t h a t e x i s t s i n today's best c a t a l y s t s . I f t h e view t h a t t h e c a t a l y s t s urf ac e c o n s i s t s e s s e n t i a l l y of MoS2 i s c o r r e c t , t h e r e f o r e , we have pro bably reached t h e p i n n a c l e w i t h p r e s e n t systems and must hunt f o r c o m p l e t e l y new systems r a t h e r t h a n t r y i n g t o improve o l d ones. I f something l i k e NiMo3S4 should be a b e t t e r model f o r t h e a c t u a l s u r f a c e , t h e r e i s s t i l l hope f o r s u b s t a n t i a l improvement o f e x i s t i n g systems. T h i s c h o i c e can be r e s o l v e d i f a more d e f i n i t e c h a r a c t e r i z a t i o n o f t h e a c t u a l s u r f a c e species can be achieved. W i t h i n t h e l a s t y e a r , s e v e r a l r e f e r e n c e s have appeared which c a l l a l l o f t h e p r e v i o u s models i n t o q u e s t i o n . Recent papers by Van Veen, e t a l . ( r e f . 8 ) and Van d e r Kraan, e t a l . ( r e f s . 9 , 10) p r e s e n t d a t a s u p p o r t i n g t h e p o l t u l a t e by V i s s e r s e t a l . ( r e f . 11) t h a t most o f t h e a c t i v i t y i n s u l f i d e d Co/Mo c a t a l y s t s i s due t o c o b a l t s i t e s . I n t h i s scenario, t h e MoS2 m a i n l y f u n c t i o n s as a s u p p o r t f o r t h e d i s p e r s e d c o h b a l t . The s p e c i f i c a c t i v i t y f o r th iophene h y d r o d e s u l f u r i z a t i o n i s r e p o r t e d t o be h i g h e r when t h e a c t i v e m a t e r i a l i s supported on carbon t h a n when i t i s support ed on alumina o r s i l i c a ( r e f . 8 ) . I n r e l a t e d work, V i s s e r s, e t a l . a l s o r e p o r t increased thiophene h y d r o d e s u l f u r i z a t i o n a c t i v i t y w i t h Co/Mo c a t a l y s t s prepared on carbon covered alumina ( r e f . 1 2 ) .
146
Some o f t h e o t h e r papers i n t h i s symposium suggest o r support t h e i d e a that,
i n t h e best hydrodenitrogenation c a t a l y s t s ,
the active constituents
p r o j e c t f rom t h e s u p p o r t s u r f a c e i n columnar form, r a t h e r t h a n o c c u r r i n g as r a f t s . (See, f o r example, t h e works by R. C. Ryan, J. L. P o r t e f a i x , S.
It i s b e l i e v e d t h a t
E i j s h o u t s and t h e i r r e s p e c t i v e c o - w o r k e r s. )
this
columnar arrangement m i g h t p r e s e n t more metal s s u r f a c e area t o t h e reactants, h e l p i n g t o o f f s e t t h e c o n s t r a i n t s associated w i t h t h e r a t e l i m i t i n g hy dro g e n a t i o n s t e p . It w i l l be i n t e r e s t i n g t o see i f more a c t i v e h y d r o t r e a t i n g c a t a l y s t s
can be o b t a i n e d by p u r s u i n g t h e new models.
A p p l i c a t i o n t o t h e new models
of i d e a s l i k e t h e ones i l l u s t r a t e d i n t h e p r e s e n t work c o u l d be o f v a l u e i n o p t i m i z i n g them. ACKNOWLEDGMENT The i n i t i a l e x p e r i m e n t a l work on t h e development o f Unocal's activity
HDN c a t a l y s t s was
done
by D r .
R.
L. Richardson.
high
T echnical
a s s i s t a n c e was p r o v i d e d by Ms. P a u l i n e Borgens and Messrs. Jerome K a l i n o w s k i and Rudy Gonzales. REFERENCES
K. Anzenhofer and J. J. DeBoer, A c t a C r y s t . 825, 1419 (1969). F . E. Massoth, J. C a t a l . 50, 190 (1977). I . A l s t r u p , I . C h o r k e n d o r f f , R. Candia, B. Clausen and H. Topsoe, J. C a t a l . , 77(2), 397 (1982). 4 H. Topsoe, B. S . Clausen, N. Topsoe, E. Pedersen, W . Niemann, A. M u l l e r , H. Bogge, and B. L e n g e l e r , J. Chem. Soc., Faraday Trans. 1, 83, 2157 (1987). 5 J. C. J. B a r t and G. V l a i c , Adv. C a t a l . 35. 1 (1987). 6 J. G u i l l e v i c , 0. Bars and 0. Grandjean, J . . S o l i d S t a t e Chem. 7, 158119731. 7 See,' for'example, Ralph W . G . Wyckoff, " C r y s t a l S t r u c t u r e s , " Second Ed., I n t e r s c i e n c e , New York, 1963, Vol. 1, pp 280-282. 8 J. A. Rob van Veen, E. Gerkema, A. M. van d e r Kraan, and A. Knoester, J. Chem. SOC., Chem. Commun. (1987) 1684. 9 A. M. van d e r Kraan, M. W . J. C r a j e , E. Gerkema, W . L. T. M. Ramselaar, A p p l i e d C a t a l y s i s , 39 (1988) L7. 10 A. M. van d e r Kraan, M. W . J . C r a j e , E. Gerkema, W . L. T. M. Ramselaar and V. H. J. de Beer, t h i s symposium. 11 J. P. R. V i s s e r s , V . H. J. de Beer and R. P r i n s , J. Chem. SOC., Faraday Trans. 1, 83 (1987) 2145. 12 J. P. R. V i s s e r s , F . P. M. Mercx, S. M. A. M. Bouwens, V. H. J. de Beer and R. P r i n s , J. C a t a l . 114 (1988) 291. 1 2 3
147
M.L. Occelli and R.G. Anthony (Editors), Aduances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
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 Si02
A1203 Supports: NEXAFS
and EXAFS D i a g n o s t i c s
N-S. 1
Chiu',
W-H.
Lee',
Y-Xi Li',
S. H. Bauer
1 and B. H. D a v i s 2
Baker L a b o r a t o r y o f Chemistry, C o r n e l l U n i v e r s i t y , I t h a c a , NY 14853-1301
'Kentucky
Energy C a b i n e t L a b o r a t o r y , P. 0. Box 13015, L e x i n g t o n , KY 40512
ABSTRACT and Sn KX-ray f l u o r e s c e n c e and a b s o r p t i o n s p e c t r a , a t t h e P t LI edges, were r e c o r d e d f o r d r i e d , c a l c i n e d and reduced (773K a t 1 atm H2) p r e p a r a t i o n s o f Pt/Sn loaded c a t a l y s t s . The samples were p r e p a r e d b y t h e acetone c o m p l e x a t i o n procedure. W i t h P t m a i n t a i n e d a t 1%, t h e Sn c o n t e n t was v a r i e d f r o m 0.39% t o 3.40%. The o b j e c t i v e o f these s t u d i e s was t o d e t e r m i n e t h e dependence on t h e n a t u r e o f t h e s u p p o r t , o f t h e c o m p o s i t i o n s and s t r u c t u r e s o f t h e c o o r d i n a t i o n s h e l l s about t h e m e t a l l i c c o n s t i t u e n t s . I n general, whereas t h e c o n f i g u r a t i o n s about t h e t i n atoms v a r i e d somewhat, t h e s e were not o v e r l y s e n s i t i v e t o the substrate. I n contrast, the surroundings about t h e p l a t i n u m atoms were c l e a r l y d i f f e r e n t f o r A1203 ~s Si02. INTRODUCTION
The p r a c t i c a l importance o f Pt/Sn b i m e t a l 1 i c c a t a l y s t s , s u p p o r t e d on a l u m i n a o r s i l i c a , can be assessed f r o m t h e l a r g e number o f p u b l i c a t i o n s t h a t appeared r e c e n t l y ( r e f s . 1-12).
To f u l l y c h a r a c t e r i z e t h e s e m a t e r i a l s con-
s i d e r a b l e e f f o r t was expended i n d e t e r m i n i n g t h e c o m p o s i t i o n s o f t h e i n n e r c o o r d i n a t i o n s h e l l s about t h e m e t a l l i c c o n s t i t u e n t s .
The r e p o r t e d d i v e r s i t y
o f models i n d i c a t e t h a t t h e c r i t i c a l m o l e c u l a r parameters a r e h i g h l y s e n s i -
t i v e t o t h e p r e p a r a t i v e procedures and t o t h e n a t u r e o f t h e support.
To ob-
t a i n some measure o f t h e dependence o f s t r u c t u r e on sample h i s t o r y , we i n v e s t i g a t e d f o u r groups o f Pt/Sn c a t a l y s t s , w h e r e i n t h e p l a t i n u m c o n t e n t was k e p t a t a b o u t 1%, w h i l e t h e t i n l o a d i n g was v a r i e d f r o m 0.4 t o 3.4x. atom-centered,
We d e r i v e d
one-dimensional r a d i a l d i s t r i b u t i o n f u n c t i o n s (RDFs a r e b u l k
averages o f t h e a t o m i c c o o r d i n a t i o n around t h e p l a t i n u m and t i n atoms) f r o m near and extended X-ray f l u o r e s c e n c e (and a b s o r p t i o n ) s p e c t r a a t t h e P t LII and Sn K-edges.
Here we r e p o r t on a comparative s t u d y o f t h e s e b i m e t a l l i c
2 2 (250 m / g ) , prepared by t h e acetone-complexation procedure.
c a t a l y s t when supported on h i g h a r e a s i l i c a (700 m / g ) ~s h i g h a r e a a l u m i n a
EXPERIMENTAL F o r each p r e p a r a t i o n , 20 grams o f t h e s u p p o r t were w e t t e d w i t h 20 m l o f
148
acetone. Then w i t h c o n s t a n t s t i r r i n g , 20 m l o f an acet one s o l u t i o n c o n t a i n i n g t h e d e s i r e d amounts o f H2PtC16 and SnC12*2H20 were s l o w l y added. The impregnated m a t e r i a l was d r i e d a t room temperature, and t h e n heated ( i n a i r ) f o r 6 hours a t 393 K. hours.
F i n a l l y , t h e samples were c a l c i n e d a t -770 K f o r f o u r
W it h p o r t i o n s o f t h e d r i e d m a t e r i a l and o f t h e c a l c i n e d p r e p a r a t i o n s
re s erv e d f o r NEXAFS and EXAFS s p e c t r a l scans, t h e remaining p r e p a r a t i o n s were reduced i n f l o w i n g hydrogen a t a t m o s p h e r i c p r e s s u r e f o r a t o t a l o f about 5 hours a t 2770 K; c o o l e d i n H2 and t h e n swept w i t h h i g h p u r i t y argon.
All
reduced samples were k e p t s e a l e d i n t h e i r r e s p e c t i v e r e a c t o r s u n t i l opened i n an arg on f i l l e d g l o v e box f o r m o u n t i n g i n sample holders. T e s t s were made t o a s c e r t a i n t h e s e n s i t i v i t y o f t h e s e p r e p a r a t i o n s t o o x i d a t i o n by exposure t o t h e ambient atmosphere (see d e t a i l s below).
T able I i s a l i s t i n g o f t h e
m e t a l l i c c o n t e n t s o f t h e s i x p r e p a r a t i o n s c o nsidered i n t h i s r e p o r t . TABLE I The c a t a l y s t c o m p o s i t i o n s Content, wt.% Catalyst
Support
A
Pt
Sn
c1
1.0
0.44
>1.2
1.0
1.47
>1.2
1.0
3.40
>1.2
1.0
0.39
>1.2
1.0
0.51
>1.2
1.0
0.78
>1.2
A1 umi na
B ~ 2 5 0m Z/ g C D
Silica E
-700 m * / g F
X-ray s p e c t r a were r e c o r d e d a t t h e CHESS F a c i l i t y i n t h e f l u o r e s c e n c e mode, b o t h a t t h e P t and Sn edges.
Reference m a t e r i a l s were: P t and Sn as
me t a l f o i l s ; fi2PtC16; PtC12*2H20; P t 0 2 [ r u n a t t h e L I I I edge a t 11.59 kev]. A ls o, SnC14*5H20; SnC12*2H20; SnO and Sn02 [ r u n a t t h e K-edge a t 29.19 keV]. Scans were made o f a l l o y s w i t h nominal c o m posit ion PtSn and Pt3Sn. A l l t h e s p e c t r a were reduced u s i n g computer codes developed a t C o r n e l l , which p r o v i d e f o r s uc c es s iv e r e f i n e m e n t o f t h e ROFs [background c o r r e c t i o n ; t e r m i n a t i o n c o r r e c t i o n ; phase s h i f t ( r e f s . 13-15)].
Wh ile " s p l i t " s i d e bands appeared i n
t h e m a j o r RDF peak o f m e t a l l i c p l a t i n u m , f o r o t h e r s c a t t e r i n g p a i r s t h e y were o f low amp lit u de , n o t s i g n i f i c a n t l y l a r g e r t h a n t h e noise. Many v a r i a n t s i n t h e d a t a r e d u c t i o n procedures were t e s t e d ; u l t i m a t e l y a l l c o r r e l a t i o n s o f r a d i a l d i s t r i b u t i o n curves were made on t h e b a s i s o f s t r i c t l y comparable p r o cedures.
One t o t h r e e scans were r e c o r d e d f o r each sample.
I n i t i a l l y the
149
e n t i r e spectrum ( t o k 16 A - l ) was reduced even when t h e r e was much n o i s e a t h i g h k, i n o r d e r t o a c h i e v e t h e b e s t r e s o l u t i o n i n t h e RDF curves. Then, t h e s p e c t r a were t e r m i n a t e d a t k p 11 A-' t o m i n i m i z e t h e e f f e c t o f n o i s e and thu s we d e r i v e d q u a n t i t a t i v e peak areas f o r comparisons o f t h e v a r i o u s prepar a t i o n s . I t i s w o r t h n o t i n g t h a t t h e a r e a under an RDF peak i s n o t 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 amount o f t h e element p r e s e n t i n t h e sample. Given t h e d e r i v e d are a under a peak a t Rij (phase s h i f t e d ) , f o r an atom p a i r ( i j ) , where 1 d es ignat e s t h e c e n t r a l atom and I any one o f t h e surrounding atom typ e s i n t h a t c o o r d i n a t i o n s h e l l , then a f t e r m u l t i p l y i n g t h e area by R2 , t h e ij
r e s u l t i s r o u g h l y p r o p o r t i o n a l t o Z.cZ. f o r t h a t c o o r d i n a t i o n s h e l l . HowJi 1 ever, e x perie nc e i n d i c a t e s t h a t s t r u c t u r a l d i s t o r t i o n s reduce t h e e f f e c t i v e c o n t r i b u t i o n s o f t h e c o o r d i n a t i n g atoms w i t h i n s e l e c t e d s h e l l s . I n t h e f o l l o w i n g t h e r e s u l t s o f o u r a n a l yses a r e present ed i n sequence: NEXAFS f o r P t and Sn; EXAFS f o r P t and Sn, f o r t h e r e f e r e n c e compounds and c a t a l y s t s on h i g h a r e a alumina and s i l i c a . Our general c o n c l u s i o n s a r e t hen compared w i t h those d e r i v e d f r o m o t h e r d i a g n o s t i c s . NEXAFS
The near-edge p r o f i l e s a t t h e P t LII
and Sn K-edges show no d i s t i n c t i v e
f e a t u r e s f o r any o f t h e r e f e r e n c e substances o r t h e v a r i o u s c a t a l y s t preparations.
However, t h e steep i n c r e a s e s i n a b s o r p t i o n appear a t d i f f e r e n t l o c a -
t i o n s ; a l l edge p o s i t i o n s were determined f r o m t h e energy d e r i v a t i v e curves (ap/aE), and were measured r e l a t i v e t o those o f t h e corresponding met al f o i l s , assigned zero. G e n e r a l l y , p o s i t i v e d i splacement s c o r r e l a t e w i t h t h e o x i d a t i o n number.
U n f o r t u n a t e l y , t h e Eo v a l u es l i s t e d i n T able I 1 do n o t
p r o v i d e c l e a r - c u t assignments. a t t h e P t LIII-edge
I n p a r t i c u l a r , t h e Eo o f sample F(ca1cined)
i s anomolous [ c o n t r a s t w i t h B(calcined)] .
m a t e r i a l gave an unexpected RDF curve, as w e l l .
However, t h i s
F or e i t h e r support , when t h e
c a t a l y s t s were exposed t o a i r f o r about 2 hours, t h e r e was a measurable s h i f t i n t h e edge p o s i t i o n ( t o w a r d h i g h e r o x i d a t i o n ) a t t h e P t LIII-edge, as a 2-3 eV inc re a s e i n Eo a t t h e Sn K-edge.
as w e l l
The dependence o f edge p o s i t i o n
on o x i d a t i o n s t a t e o f t i n i s i l l u s t r a t e d i n F ig. 1.
W it h M y l a r windows on
t h e sample i n p l a c e , 20 m i n u t e s exposure t o t h e atmosphere showed no s i g n i f i ca nt changes i n e i t h e r NEXAFS o r EXAFS spectra. The r a p i d in c r e a s e s i n a b s o r p t i o n a t t a i n c h a r a c t e r i s t i c amplitudes. T h e i r v alues may be compared when t h e p a t t e r n s a r e normalized t o u n i t l e v e l , e s t a b l i s h e d by e x t r a p o l a t i n g t h e EXAFS background t o Eo. The corresponding [ P t LIII] ( r e f s . 16-18), and t o a b s o r p t i o n s have been assigned t o 2P3,2 +
a
1s
[Sn K] t r a n s i t i o n s . For a n analogous s e r i e s o f compounds, f o r a g i v e n c e n t r a l element, t h e r e s o l v e d peak h e i g h t i s a measure o f i t s i o n i c i t y , i.e.
+
f o r P t t h e number o f
3 electrons
p a r t i a l l y removed by chemical-bond
150
TABLE I 1 Near-edge features ( a t Sn K ) Materi a1
P t-meta 1 Pt/Sn a l l o y PtO, H,PtC1 PtC1,.2H,O
Sample B (dry) B (calcined) A (reduced) B (reduced) C (reduced) F F 0 E F
1.0*
3.45 4.21 8.63 8.60 4.00
3.0 1.0* 1.0* 1.0*
4.02 10.64 4.32 4.01 3.35
(0) 0-+1.5 3.0 3.0
E"
Sn-me t a 1 Sn-Pt a1 1oy SnC1,*2H20 SnO SnC1,*5H,O SnO,
(dry) (calcined) (reduced) (reduced) (reduced)
(0)
-1-+t2 A -0.6+0 0.5 4 5
B B A B C
(dry) (calcined) (reduced) (reduced) (reduced)
3 4. 0.
F F D E F
(dry) (calcined) (reduced) (reduced) (reduced)
2.0 3-4A -1+o,
:* I1
ii
A
:Displacement from metal *Under resolved Lorentzian .denotes s i m i l a r values Adenotes s i m i l a r values denotes s i m i l a r values formation ( r e f . 19). Horsley ( r e f . 20) showed t h a t t h e areas o f the threshold resonance l i n e s can be estimated by deconvoluting the absorption edge i n t o a Lorentzian component and an underlying "step", which represents the onset o f absorption due t o a continuum o f states. The t o t a l measured cross s e c t i o n can be l e a s t squares f i t t e d w i t h f o u r parameters (a l...a4): E-a4
1
o(E)
=
al 1+[ ( E-ap) /a3I2
t
{ ;; tan-' [T] t 0.53
The area under the Lorentzian measures the t r a n s i t i o n p r o b a b i l i t y t o the
a
o r b i t a l , and thus o f i t s n e t vacancy; the fwhm o f the Lorentzian i s i n v e r s e l y p r o p o r t i o n a l t o the l i f e t i m e o f the state. The values deduced f r o m our data are sumnarized i n Table 11; they a r e i n reasonable agreement w i t h H o r s l e y ' s areas f o r t h e reference compounds. Typical resolved near-edge pro-
vacant
f i l e s a r e i l l u s t r a t e d i n Figs. 2a,b,c,d.
The tabulated values i n d i c a t e t h a t
the higher s t a t e s o f o x i d a t i o n have l a r g e r
4
band vacancies, presumably due
t o enhanced i o n i c i t y o f the platinum centers.
Increased Sn loading on e i t h e r I n contrast, a l l o y i n g P t w i t h Sn,
support, decreases the
4
leads t o an increase.
Overall, alumina favors a higher
band vacancy.
4 band vacancy than
151
-50.0
-30.0
-10.0
10.0
30.0
50.0
70.0
Energy (ev) F ig. 1. High r e s o l u t i o n p r o f i l e s a t t h e Sn K-edges f o r c a t a l y s t preparat i o n s , supported on SiO,. The a b s c i s a ( i n eV) i s t h e d e p a r t u r e f rom Eo o f t h e me t a l. calcined;-dried;-reduced. (a) Sn (0.78%):f rom g l o v e ( b ) Sn (0.78%) t changes due t o a i r e x p o s u r e : - - - d i r e c t l y box;exposed 20 m i n u t e s t o a i r , w i t h m y l a r windows i n p l a c e ; e x p o s e d t o a i r f o r 60 minutes, w i t h m y l a r windows;--addit i o n a l 2 hours, m y l a r windows removed.
-
--
-
-
(4 Calcined f
(d)
(b)
4 -
-45.0
Reduced E
PtSn alloy
-25.0
-5.0 ErmOT
15.0
(4
35.0
-25.0
15.0
-5.0
-w
35.0
55.0
(4
Fig. 2 . P r o f i l e s a t 2 t LII edge, resolved i n t o overlapping Lorentzian and tan-1 f u n c t i o n s . The l a r g e dots snow how well the recorded data (-) a r e reproduced by the sums of the ( . . . ) functions.
153 does s i l i c a , wh ic h may be i n t e r p r e t e d as due t o h i g h e r l e v e l s o f bonding between t h e P t and A1203. These r e s u l t s a r e i n general agreement w i t h a r e p o r t by L y t l e e t a l . ( r e f . 21).
A c t i v i t y t e s t s f o r s i m i l a r preparations
may l e a d t o a c o r r e l a t i o n between r a t e s and
band vacancies.
The near-edge p r o f i l e s a t t h e Sn K-edge a l s o a r e f e a t u r e l e s s .
The peak
h e i g h t s o f SnC12*2H20 and SnO a r e 1.09 and 1.28, r e s p e c t i v e l y , r e l a t i v e t o t h e background. F o r SnC14*5H20 and Sn02 t h e peak h e i g h t s a r e 1.17 and 1.46, r e s p e c t i v e l y . On h i g h a r e a alumina, upon r e d u c t i o n o f t h e v a r i o u s preparat i o n s t h e edge l o c a t i o n s r e v e r t c l o s e t o zero, w i t h peak h e i g h t s o f 1.16, c o n s i s t e n t w i t h m i x t u r e s o f SnO and t h e reduced metal.
Exposure t o a i r f o r
about two hours b r i n g s t h e edge back t o a h i g h e r v a l u e b u t somewhat lower tha n t h a t o f Sn02. On s i l i c a ( r e c o r d e d f l u o r e s c e n c e s p e c t r a ) , we found: upon c a l c i n i n g t hese rose t o t h e d r i e d samples had peak h e i g h t s 1.08-1.10; -1.24. D
+
1.13;
Reduction decreased t h e i r values, depending on Sn l o a d i n g [sample E
+
1.11; F
+
1.091.
EXAFS The peaks i n t h e r a d i a l d i s t r i b u t i o n f u n c t i o n s f o r t h e r e f e r e n c e compounds, a f t e r background and t e r m i n a t i o n e r r o r c o r r e c t i o n s , p r o v i d e t h e b a s i s f o r assignments o f t h e c a t a l y s t RDFs.
The m a j o r peak i n Pt(meta1) appears a t
2.62 A (add phase s h i f t c o r r e c t i o n ; 0.15
A).
The (Pt-0) d i s t a n c e i n Pt02
( u n f o r t u n a t e l y i t s s t r u c t u r e i s n o t w e l l d e f i n e d ) i s 1.66
A w i t h 0.38 A phase
A. I n PtC12, t h e m a j o r peak A (add phase s h i f t c o r r e c t i o n ; 0.42 A ) , w i t h a small peak I n H2PtC16, (Pt-C1) = 1.93 A (add phase s h i f t a t 3.1 A assigned t o ( P t - P t ) . c o r r e c t i o n ; 0.39 A). As expected i t has a l a r g e r area t han i n PtC12. There s h i f t c o r r e c t i o n , w h i l e ( P t - P t ) appears a t 3.28 i s ( P t - C l ) = 1.94
a r e no o t h e r peaks above t h e n o i s e l e v e l i n t h i s RDF f u n c t i o n . I n c a l c i n e d Pt/A1203, prepared b y adding an H2PtC16 s o l u t i o n t o t h e support , t h e (Pt-0) peak i s prominent, w i t h a s h o u l d e r t h a t can be assigned t o a small f r a c t i o n o f (Pt-Cl).
F o r r e f e r e n c e , RDFs f o r Pt02, H2PtC16 and P t S n ( a l l o y ) , d e r i v e d
fro m EXAFS a t P t LIII-edge, and o f SnO, SnC14*5H20 and t h e a l l o y (Pt / Sn), d e r i v e d f rom EXAFS a t t h e Sn K-edge, a r e reproduced i n F ig. 3. A (Pt-Sn) ato m-pair s e p a r a t i o n o f 52.5 A i n t h e a l l o y ( uncorrect ed) i s c l e a r l y i n d i t h i s peak cannot be r e ca t e d i n b o t h RDFs. However, a t t h e P t LIII-edge so lv ed f ro m a c l o s e l y o v e r l a p p i n g ( P t - P t ) peak ( w i t h i n 0.02 A ) , as p r e s e n t i n m e t a l l i c p l a t i n u m and i n t h e a l l o y . Examination o f t h e RDFs d e r i v e d f r o m two s e t s o f scans f o r t h e s i x prepa r a t i o n s shows b o t h s i m i l a r i t i e s i n l o c a l s t r u c t u r e s and s t r i k i n g d i f f e r ences, which may be a s c r i b e d t o c o n t r o l by t h e supports.
I n the following
r e f e r e n c e w i l l be made t o " s t i c k " diagrams ( F i g u r e s 4,5),
presented t o i l l u -
s t r a t e t h e d i s t r i b u t i o n o f a t o m - p a i r d i s t a n c e s , p l o t t e d as d e r i v e d , w i t h o u t
or
Fig. 3. R a d i a l D i s t r i b u t i o n F u n c t i o n s d e r i v e d f o r t h e c a l i b r a t i n g rnateria,1 s: a,b,c a t t h e P t LIII-edge, and d,e,f a t t h e Sn K-edge.
155
1 .o
4.0
3.0
2.0
I
Sn-Sn
Sn-0
+ Sn-0
5.0
I
a
P:,
u 5 5
D I
5,
u)
b
C v)
I
6
C
I
4
v) C
I
Ji
v)
d
& i
1.o
1
2.0
4.0
3.0
5.0
Angstroms (phase shift uncorrected)
F i g . 4. Peak p o s i t i o n s ( n o t c o r r e c t e d f o r phase s h i f t ) a r e i n d i c a t e d by v e r t i c a l l i n e s , w i t h approximate peak h e i g h t s , d e r i v e d f r o m EXAFS s p e c t r a a t t h e Sn K-edge, f o r s e v e r a l r e f e r e n c e compounds and t h e s i x c a t a l y s t s l i s t e d i n Table ( I ) . ( a ) SnO, ( I ) and SnO ( ! ) : ( b ) SnC1,*5H2O and SnC1,.2H2O ( c ) Sn and F ( m e t a l ) : ( d ) Pt/Sn a l l o y ; (a) Dry p r e p a r a t i o n s : B and and F (y) Reduced samples: A ( B ) Calcined preparations: B Reduced samples: C and Reduced samples: B ( 1 ) and E (:): (E) D ); ( 6 ) F (!I.
(i
(1)
(1) (I);
(I)
(i);
(i);
(I)
(I)
156
1.o
1 .o
2.0
2.0
3.0
4.0
3.0
4.0
5.0
5.0
Angstroms (phase shift u n c o r r e c t e d )
F i g . 5. Peak p o s i t i o n s ( n o t c o r r e c t e d f o r phase s h i f t ) a r e i n d i c a t e d by v e r t i c a l l i n e s , w i t h approximate peak h e i g h t s d e r i v e d f r o m EXAFS s p e c t r a a t t h e P t LII -edge, f o r r e f e r e n c e compounds and t h e s i x c a t a l y s t s l i s t e d i n Table I . [ a ) Pt/Sn a l l o y ; ( b ) P t m e t a l ; ( c ) PtO,; ( d ) PtC1,.2H20 and and F (I ); (13) C a l c i n e d p r e p a r a H,PtCl, (! ) ; (a) Dry p r e p a r a t i o n s : B and F ( y ) Reduced samples: A and D (:); ( 6 ) Reduced tions: B I and F ( f ) . samples: B and E (E) Reduced samples: C
(I) (1)
(i);
(t);
(I)
(I)
(1)
(I)
157
c o r r e c t i n g f o r phase s h i f t s , and t o Tables 111 and I V wherein p r o p e r l y s c a l e d d i s t a n c e s and t h e i r r e l a t i v e i n t e n s i t i e s (peak areas) a r e sumnarized.
The
assignment o f peaks t o s p e c i f i c a t o m - p a i r s c a t t e r i n g i s l e s s ambiguous when t h e i r RDFs a r e compared d i r e c t l y w i t h those o f model compounds t h a t a r e p r e sumed t o i n c o r p o r a t e s i m i l a r c o n f i g u r a t i o n s i n t h e c o o r d i n a t i o n s h e l l s about t h e c e n t r a l element, whereas t h e i n t r o d u c t i o n o f phase s h i f t s i m p l i e s specif i c i d e n t i f i c a t i o n o f t h e unknown w i t h t h e known s t r u c t u r e s , which may n o t be s t r i c t l y isomorphous. The Sn K-edge s p e c t r a i n d i c a t e t h a t t h e arrangement o f atoms about t h e t i n s pec ies were e s s e n t i a l l y t h e same f o r t h e h i g h area alumina and h i g h area s i l i c a p r e p a r a t i o n s . From Fig. 4 i s i s e v i d e n t t h a t i n b o t h s e t s Sn-0 atomp a i r s dominate even a f t e r r e d u c t i o n , b u t t h e i r c o n t r i b u t i o n decreases somewhat w i t h Sn load i n g . SnC14.5H20
These (Sn-0) p a i r s a r e more l i k e t hose i n Sn02 and i n
t h an i n SnO o r i n SnC12*2H20.
I n b o t h s e t s t h e Sn-C1 peaks a r e
p r e s e n t b u t t h e i r c o n t r i b u t i o n s c l e a r l y decrease i n t h e sequence: d r y , c a l cined, reduced. I n t e r e s t i n g l y , t h e peak p o s i t i o n s d r i f t f r o m t h a t i n SnC14*5H20 t o SnC12*2H20 i n t h a t sequence. The c a l c i n e d samples a l s o show l o n g d i s t a n c e peaks t h a t may be assigned t o (Sn-Sn) and (Sn-0) s c a t t e r i n g , as p r e s e n t i n SnO; Sn02. e f f e c t o f t h e s up p o r t appears i n t h e r e g i o n 2.5-2.75 l o n g e r d i s t a n c e (Sn-Sn),
A.
A differential
Alumina f a v o r s a
s i m i l a r t o t h a t p r e s e n t i n Sn f o i l ; s i l i c a f a v o r s
t h e s h o r t e r d i s t a n c e assigned t o (Sn-Pt) s c a t t e r i n g i n (Pt/Sn).
A surprising
A peak appears Thus, t h e assignment o f t h e 2.5 A peak
obs e rv a t io n, i n t h e dr-y and c a l c i n e d samples (6;F) t h e 2.5 b o t h f o r alumina and s i l i c a supports.
t o a Pt-Sn i s somewhat q u e s t i o n a b l e (see below). I n sumnary, we f i n d t h a t t h e (Sn-0) peaks a r e much s t r o n g e r i n t h e s i l i c a support ed c a t a l y s t s tha n t h os e s uppor t e d on alumina; t h e (Sn-C1) peak i s weaker i n t h e s i l i c a tha n i n t h e alumina s u p p o r t ; a l s o , (Sn-Sn) i s l o n g e r C3.5-3.6 A compared t o 3.3 A] f o r t h e s i l i c a t h a n f o r t h e alumina, b e i n g much c l o s e r t o t h a t f ound f o r (Sn-Sn) i n Sn02. Reference t o Fi g . 5 shows t h a t t h e two s upport s l e a d t o p r o f o u n d l y d i f f e r e n t c o n f i g u r a t i o n s o f atoms about t h e p lat inum. Here t h e RDFs o f t h e r e f e r e n c e compounds do n o t p r o v i d e as c l e a n - c u t assignments as i n F ig. 4. The somewhat m y s t e r i o u s 2.5 A peak appears i n t h e dr-y samples f o r b o t h supp o r t s (B;F), and f o r t h e reduced sample C. T h i s peak has been assigned t o (Pt-Sn), as p r e s e n t i n t h e a l l o y ( r e f . 10).
However, i t s appearance i n t h e
o x i d i z e d s t a t e s o f these p r e p a r a t i o n s suggests t h a t a more p l a u s i b l e assumpt i o n i s t h e presence o f a solid s o l u t i o n o f t h e oxide, which i s i n c o m p l e t e l y decomposed by c a l c i n i n g and r e d u c t i o n . F o r p r e p a r a t i o n s on h i g h a r e a alumina, t h e f i r s t peak i n t h e d r y materi a l i s c l e a r l y due t o ( P t - C l ) ;
i n t h e c a l c i n e d stage t h e f i r s t peak i s due t o
TABLE I11 Reduced RDFs, d e r i v e d from EXAFS a t Sn K-edge [Atom-pair separations were c o r r e c t e d f o r phase s h i f t : areas m u l t i p l i e d by R f .] 1J
Position Reference Compounds SnCl,*5H,O (extended d a t a ) SnC1,.2tl,O (extended d a t a ) SnO, SnO Sn (metal) Pt,Sn ( a l l o y ) PtSn ( a l l o y )
2.10~
Area 118
2.16
40
2.05 2.18
276 114
Position
~ _ _ _ _ _
~~
~~~~~
Area
2.39~
248
2.59
101
Position
3.33A
Area
Position
Area
45
4.50A
63
3.19
277
3.71 3.48
665 373
2.82 2.md 2.73
220 1186
3.96d 3.61
90 1551
78 30
2.69*' 2.69*' 2.69*' 2.69*' 2.68*'
81 81 66 41 20
300
2.81e
112
2.44
53
2. 7ge
89
2.44bb 2.44b 2.45
89 37 59
2.96:
88
C a t a l y s t s on Alumina
Sn 1.47% (Dry) Sn 1.47% (Calcined) Sn 0.44% (Reduced) Sn 1.47% (Reduced) Sn 3.40% (Reduced)
1.95ia 1.96a 1.98a 1.98a 2.04a
130 200 224 133 100
b 2.4Ib 2.42
2. 46b 2.48
140 70
3. 4gC 3.34
43 50
3.0Id
102
3.73'
413
C a t a l y s t s on s i l i c a Sn 0.78% (Dry)
2.37b
Sn 0.78% (Calcined)
1.97a
291
Sn 0.39% (Reduced) Sn 0.51% (Reduced) sn 0.78% (Reduced)
1. 97a 1.97' 1. 93a
96 86 81
3. 02d 2.92
-
166 62 84
a(SnO) phase s h i f t o f Sncl,*SH,O. b(Sn-C1) phase s h i f t o f SnCl,*SH,O. c(Sn-Sn) phase s h i f t o f SnO, (-0.22A). d(Sn-Sn) phase s h i f t o f Sn f o i l (-0.12A). e ( p t - f n ) phase s h i f t o f PtSn a l l o y . "Perhaps the (Pt-Sn) s h i f t r a t h e r than (Sn-Sn) from SnO,, should have been used (add =0.08 t o t a b l e values),
[Atom-pair
TABLE I V Reduced RDFs, d e r i v e d from EXAFS a t P t L I I I - e d g e separations were c o r r e c t e d f o r phase s h i f t ; areas m u l t i p l i e d by R:
.I
1.I
Position Reference Compounds PtO, PtC1,*2H,O H,PtCl Pt,Sn (a1 l o y ) PtSn ( a l l o y )
2.05A
Area
Position
Area
2.36A 2.32 2.83 2.73c
658 1070 510 529
400
Position
Area
3.50A 3.32
300 20 1
3.97
90
3-96'
176
Position
4.79A (4.72'
2.77
P t (metal)
88 1
Area
94 267 284
C a t a l y s t s on Alumina Sn Sn Sn Sn Sn
1.47% 1.47% 0.44% 1.47% 3.40%
(Dry)
(Calcined) (Reduced) (Reduced) (Reduced)
2.26b
2.00; 2.26b 2.20b 2.24
270 498 168 137 118
2.58' 2.78' 2.98' 3.03' 2.57c
150 216 105 28 54
2.77c
700
3.87' 3.88'
3.4ZC
67
C a t a l y s t s on s i l i c a 923*
Sn 0.78% (Dry) Sn 0.783
(Calcined)
Sn 0.39% (Reduced)
2.55b'd
81
2.74'
180
Sn 0.51% (Reduced) Sn 0.78% (Reduced)
2.47b'd 2.47b'd
123 165
2.71' 2.77c
229 236
243
{
4.77'
267
5.40'
374
4. 73c 5.14'
a(Pt-0) phase s h i f t . b(Pt-C1) phase s h i f t . C(Pt-Pt) phase s h i f t from t h e 1 s t s h e l l o f t h e metal. :This i s an o v e r l a p o f Pt-C1 and s a t e l l i t e peak o f 1 s t s h e l l P t - P t (metal environment). Area i n c l u d e s t h e peak a t 2.28 A.
99 134
160
(Pt -0 );
i n t h e reduced samples a s u b s t a n t i a l peak remains t h a t i s b e s t i n t e r -
p r e t e d as a s u p e r p o s i t i o n a t t h e s e two t y p e s o f atom-pairs, w i t h a m p l i t u d e s t h a t decrease somewhat w i t h Sn l o a d i n g . There i s no (Pt-0) i n t h e s i l i c a su pport ed p r e p a r a t i o n s b u t t h e r e a r e RDF peaks t h a t may be assigned t o ( P t C 1 ) ( e x c e p t f o r t h e reduced s t a t e o f D). We c a l l a t t e n t i o n t o t h e presence o f a s a t e l l i t e peak on t h e l o w R s i d e o f ( P t - P t ) i n t h e f o i l . The d i s p l a c e -
ment o f t h e s a t e l l i t e f r o m t h e m a j o r ( P t - P t )
peak a t 2.6 A ( u n c o r r e c t e d ) and
i t s a m p l i t u d e i s somewhat v a r i a b l e and we presumed t h a t i t o v e r l a p s and t h e r e f o r e d i s t u r b s t h e ( P t - C 1 ) peak i n t h e s i l i c a p r e p a r a t i o n s . M e t a l l i c plat+num ( P t - P t s c a t t e r i n g ) i s c l e a r l y p r e s e n t a t a l l stages, b o t h c a l c i n e d and reduced f o r t h e s i l i c a s u p p o r t e d m a t e r i a l . T h i s i s n o t so e v i d e n t f o r We c o n clude t h a t t h e s u b s t r a t e s i l i c a t h e alumin a s u pp o r t e d samples (B;C). f a v o r s r e d u c t i o n t o t h e m e t a l l i c s t a t e even a t t h e c a l c i n e d stage.
To con-
t r a s t t h e d i f f e r i n g r o l e s o f t h e s u p p o r t s r e f e r t o F ig. 6. DISCUSSION
The areas l i s t e d i n Ta b l e I 1 1 and I V a r e measures o f t h e c o o r d i n a t i o n numbers f o r t h e i n d i c a t e d s h e l l s . Comparison o f t h e magnitudes d e r i v e d f rom Sn K-edge d a t a f o r b o t h t y p e s o f s u p p o r t s shows a c l e a r t r e n d w i t h i n c r e a s i n g Sn l o a d i n g ; i t decreases t h e e f f e c t i v e c o o r d i n a t i o n o f oxygens and c h l o r i n e atoms about t h e Sn atoms; t h i s i s more e v i d e n t f o r oxygen t h a n f o r c h l o r i n e . The e f f e c t i v e c o o r d i n a t i o n number ranges f r o m t h a t f ound i n Sn02 t o t h a t i n SnO. We c o n s i d e r t h i s t o be more o f a measure o f d i s t o r t i o n o f t h e f i r s t c o o r d i n a t i o n s h e l l r a t h e r t h a n an a c t u a l r e d u c t i o n i n t h e number o f atoms i n t h e imn ediat e v i c i n i t y o f t i n .
The c o o r d i n a t i o n around t h e p l a t i n u m (T able
I V ) f o l l o w s a p a t t e r n s i m i l a r t o t h a t o f t h e alumina support ed p r e p a r a t i o n s , b u t an i n v e r s e t r e n d ( w i t h t i n l o a d i n g ) when s i l i c a i s t h e support . T h i s i s consistent w i t h our observation t h a t platinum i n t e r a c t s l e s s w i t h Si02 s u r f a c e s t h an w i t h A1203.
Indeed, a s i m i l a r o b s e r v a t i o n was made b y Miura,
et a1 ( r e f .
22). I t i s i n t e r e s t i n g t o n o t e t h a t f o r t h e 0.78% Sn sample on s i l i c a i n t h e c a l c i n e d s t a t e , t h e a r e a under t h e ( P t - P t ) peak approaches t h a t o f m e t a l l i c p l a t i n u m ; i n t h e d r i e d sample i t i s somewhat lower whereas i n t h e c orres ponding reduced c a t a l y s t i t i s about o n e - f i f t h as l a r g e . c a t i v e o f a h i g h degree o f d i s p e r s i o n o f t h e plat inum.
This i s i n d i -
A s i m i l a r conclusion
a p p l i e s t o t h e a l u m i n a s u p p o r t e d c a t a l y s t where t h i s e f f e c t i n c r e a s e s modera t e l y w i t h platinum loading.
Nandi,
&
a. ( r e f .
23) a l s o r e p o r t e d t h a t
t h e r e appeared t o be 1 it t l e c o n t a c t a r e a between t h e p l a t i n u m c r y s t a l 1 it e s and t h e s i l i c a support.
From wide a n g l e d i f f r a c t i o n p a t t e r n s t h e y f ound t h a t t h e ( P t - P t ) d i s t a n c e s i n t h e i r p r e p a r a t i o n s d i f f e r e d l i t t l e f r o m t hose i n
b u l k plat in um. However, t h e y a l s o f o u n d t h a t w i t h met al l o a d i n g s o f 0.43% a p p r o x i m a t e l y 63% o f t h e p l a t i n u m was on t h e s u r f a c e , and t h a t exposure t o
161
.. ... ...
.. ... ... :E:
.. .... ... ..
.. ..... .
J .
.. .. .. ' ... ....... ..". .... *
.
8 0.0
*
*.
1 .o
2.0
.
.
.. .. .. .. .. .:v :. a . : . : ....cp-.. '. ... 3. : ...--*.: .,...' ...... *
3.0
4.0
5.0
6.0
R in Angstroms (phase shift uncorrected)
F i g . 6. Superposed RDFs d e r i v e d f r o m EXAFS a t t h e P t LIII-edge. F or p r e p a r a t i o n s on A1203 t h e peaks were d e s i g n a t e d n u m e r i c a l l y ; peaks w i t h Greek l e t t e r s a r e on Si02. Peaks 1,2,3 a r e a t o m - p a i r d i s t a n c e s i n c a l c i n e d 6 (1.47% Sn). They were assigned t o (Pt-0), ( P t - P t ) and ( P t - P t ) , r e s p e c t i v e l y , as i n Pt02. Peaks &,E,Q,~,$ a r e atom-pair d i s t a n c e s i n c a l c i n e d F (0.78% Sn); 6 was assigned t o P t - C l ) , p e r t u r b e d by a s a t e l l i t e f r o m t h e s t r o n g adj a c e n t ( P t - P t ) a t 2.62 (E); O,e,$ a r e as i n P t metal. Peaks 4,5 a r e atomp a i r d i s t a n c e s i n reduced C, (3.4% Sn). They were assigned, r e s p e c t i v e l y , t o The small t w i n peaks a t 3.1 and superposed ( P t - 0 ) + (Pt-C1) and t o (Pt-Sn). 3.6 A c o u l d be due t o r e s i d u a l PtC12*2H20. For comparison, peaks a,B,y a r e a t o m - p a i r d i s t a n c e s i n reduced F (0.78% Sn). T h i s RDF was c a l c u l a t e d w i t h an extended d a t a s e t and has h i g h e r r e s o l u t i o n . Peak a was a s s i g n e d t o ( P t - C l ) ; 0 t o (Pt-Sn) and y t o ( P t - P t ) .
8,
162
a i r g e n e r a t e d p a r t i c l e s c o n s i s t i n g o f c o r e s o f t h e m e t a l surrounded b y a Pt304 phase. Meitzner,
g.
( r e f . 10) r e p o r t e d on t h e i r s t u d i e s o f Pt/Sn c a t a l y s t s
and compared t h e e f f e c t s o f t h e Support ( a l u m i n a surrounding the m e t a l l i c constituents.
s i l i c a ) on t h e s t r u c t u r e s
Even though t h e i r d a t a r e d u c t i o n p r o -
cedures d i f f e r c o n s i d e r a b l y f r o m o u r s t h e r e i s g e n e r a l agreement as w e l l as several d i s t i n c t i v e differences.
We agree t h a t t h e p l a t i n u m appears t o be
more h i g h l y d i s p e r s e d on a l u m i n a when t i n i s p r e s e n t .
With respect t o the
s t a t e o f o x i d a t i o n we agree t h a t t h e t i n i s p r e s e n t as a m i x t u r e o f Sn',
Snf2
However, we found t h a t f o r e i t h e r s u p p o r t a m a j o r p o r t i o n o f t h e
and
m e t a l l i c atoms a r e c o o r d i n a t e d e i t h e r t o oxygen o r t o c h l o r i n e .
We do n o t
have h a r d e v i d e n c e f o r b i m e t a l l i c e n t i t i e s i n o u r p r e p a r a t i o n s .
Perhaps t h e
d i f f e r e n c e i s due t o o u r l o w e r r e d u c t i o n t e m p e r a t u r e (773 i n t e r e s t i n g t o n o t e t h a t Kuznetsov, o f Snt4,
a. ( r e f .
82510.
It i s
24) r e p o r t e d s e v e r a l t y p e s
3 t o 4 s t a t e s o f Sn+2 and Pt/Sn a l l o y s i n samples o f Pt-Sn/A1203
reduced w i t h hydrogen a t 823K, based on MBssbauer s p e c t r a . ACKNOWLEDGMENT The EXAFS s p e c t r a were r e c o r d e d a t t h e C o r n e l l H i g h Energy S y n c h r o t r o n Source, s u p p o r t e d by NSF g r a n t DMR-78/267.
Thanks a r e due t o t h e donors o f
t h e P e t r o l e u m Research Fund, a d m i n i s t e r e d by ACS, f o r p a r t i a l s u p p o r t o f t h i s research. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
14. 15. 16. 17.
18.
B.D. McNicol, J. C a t a l . 46 (1977) 438 (1977). R. Bacaud, P. B u s s i e r e , and F. F i g u e r a s , J. C a t a l . 69 (1981) 399. V.I. Kuznetsov, E.N. Yurchenlca, A.S. B e l y i , E.V. Z a l o l o k i n a , M.A. Smolikov, and V.K. D u p l y a k i n , React. K i n e t . C a t a l . L e t t . 21 (1982) 419. L i Yong-Xi and S h i a Yuan-Fu, "Proc. I n t . Conf. on t h e A p p l i c a t i o n s o f t h e MBssbauer E f f e c t , " ( J a i p u r ) , 438 (1982). R. Burch, P l a t i n u m Metal Rev. 22 (1978) 57. D.R. S h o r t , S.M. K h a l i d , J.R. K a t z e r , and M.J. K e l l e y , J. C a t a l . 72 (1981) 288. S.R. A d k i n s and B.H. Davis, J. C a t a l . 89 (1984) 371. B.A. Sexton, A.E. Hughes and K. Foger, J. C a t a l . 88 (1984) 466. L i Yong-Xi and S h i a Yuan-Fu, H y p e r f i n e I n t e r a c t i o n s 28 (1986) 875; 879. G. M e i t z n e r , G.H. Via, F.W. L y t l e , S.C. Fung, and J.H. S i n f e l t , J. Phys. Chem. 92 (1988) 2925. R. S r i n i v a s a n , R.J. DeAngelis and B.H. D a v i s , J. C a t a l . 106 (1987) 449. L i Yong-Xi, J.M. S t e n c e l and B.H. Davis, React. K i n . C a t a l . L e t t . submitted f o r p u b l i c a t i o n . N.-S. Chiu, S.H. Bauer, and M.F.L. Johnson, J. Mol. S t r u c t . 125 (1984) 33. N.-S. Chiu, S.H. Bauer, and M.F.L. Johnson, J. C a t a l . 89 (1984) 226. N.-S. Chiu, S.H. Bauer, and M.F.L. Johnson, J. C a t a l . 98 (1986) 32. M. Brown, R.E. P e i e r l s and E.A. S t e r n , Phys. Rev. 615 (1977) 738. D.R. S h o r t , A.N. Mansour, J.W. Cook, Jr., D.E. Sayers and J.R. K a t z e r , J. C a t a l . 82 (1983) 299. T.K. Sham, J. Chem. Phys. 84 (1986) 7054.
163
L y t l e , P.S.P.
Wei, R.B.
19.
F.W.
Greegor, G.H.
20.
J.A.
H o r s l e y , J. Chem. Phys. 76 (1982) 1451.
21.
F.W.
L y t l e , e t al.,
22.
H. M i u r a , S.S.
V i a and J.H.
S i n f e l d , J.
Chem. Phys. 70 (1979) 4849. C a t a l y s t C h a r a c t e r i z a t i o n Science, Chapter 14
(1985), ACS P u b l i c a t i o n . Feng, R. Saymeh and R.D.
Gonzalez, C a t a l y s t
C h a r a c t e r i z a t i o n Science, (ACS, 1985), Chapter 25. 23.
R.K.
24.
V.I.
Nandi,
Burwell,
F. M o l i n a r o , C. Tang, J.B. Cohen, J.B.
B u t t and R.L.
Jr., J. C a t a l y s i s , 78 (1982) 289. 3 J. C a t a l y s i s , 99 (1986) 159.
Kuznetsov,
a.,
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
M6SSBAUER
165
STUDY OF THE SULFIDATION OF HYDRODESULFURIZATION CATALYSTS: SO-CALLED
"CO-Mo-S" PHASE OBSERVED I N CARBON-SUPPORTED CO AND CO-MO SULFIDE CATALYSTS
CRAJEl, E. GERKEMA'.
M.W.J.
V.H.J.
DE BEER
2
VAN DER KRAAN'
and A.M.
'Interfacultair R e a c t o r I n s t i t u u t , D e l f t U n i v e r s i t y o f Technology, Mekelweg 15. 2629 J B D e l f t (The N e t h e r l a n d s ) 2Laboratory f o r I n o r g a n i c Chemistry and C a t a l y s i s , Eindhoven U n i v e r s i t y o f Technology, P.O. Box 513, 5600 MB Eindhoven (The N e t h e r l a n d s )
ABSTRACT I n - s i t u MBssbauer Emission Spectroscopy (MES) h a s been u s e d t o s t u d y t h e t y p e of phases p r e s e n t i n s u l f i d e d a c t i v a t e d carbon-supported Co and Co-Mo h y d r o d e s u l f u r i z a t i o n (HDS) c a t a l y s t s . Most o f t h e r e p o r t e d MES s t u d i e s are p e r formed on Co-Mo/A120 c a t a l y s t s . Co s p e c i e s i n t h e s o - c a l l e d 'To-Mo-S" p h a s e , s o f a r o n l y observed s u l f i d e d c a t a l y s t s c o n t a i n i n g Co and Mo. s h o u l d govern t h e HDS a c t i v i t y . The p r e s e n t o b s e r v a t i o n s show t h a t t h e same Co s p e c i e s i n s u l f i d e d Co/C and Co-Mo/C c a t a l y s t s , with t h e same quadrupole s p l i t t i n g (QS) as "Co-Mo-S" can b e formed. Similar r e s u l t s are o b t a i n e d f o r s u l f i d e d Fe/C and Fe-Mo/C c a t a l y s t s . Furthermore, i t t u r n s o u t t h a t t h e QS-value of t h e " a c t i v e phase" i n t h e s u l f i d e d Co/C and Co-Mo/C c a t a l y s t s depends on t h e s u l f i d i n g t e m p e r a t u r e and Co c o n t e n t . Hence, i t seems u n l i k e l y t h a t t h e r e w i l l b e o n l y one well d e f i n e d a c t i v e s u l f i d e phase which governs t h e HDS a c t i v i t y .
i2
INTRODUCTION In plied from
the
o i l - p r o c e s s i n g i n d u s t r y h y d r o t r e a t i n g is a large scale o p e r a t i o n ap-
to
remove h e t e r o atoms such as s u l f u r , n i t r o g e n and metal c o n t a m i n a t i o n s
o r g a n i c molecules p r e s e n t i n c r u d e o i l f r a c t i o n s . The i n d u s t r i a l l y a p p l i e d
hydrotreating
for
catalysts
contain
molybdenum
or
tungsten
o r n i c k e l , s u p p o r t e d on a h i g h - s u r f a c e - a r e a
cobalt
efficient
removal
of
the
s u l f i d e promoted by
alumina. The i n c r e a s i n g need
h e t e r o atoms is a c o n t i n u o u s d r i v e f o r f u r t h e r
development of h y d r o t r e a t i n g c a t a l y s t s . It
shown
is
results
in
an
that
t h e u s e o f carbon i n s t e a d o f alumina as carrier material
improvement
of
c a t a l y t i c a c t i v i t y (refs. 1-6). Vissers e t a l .
( r e f . 7 ) showed t h a t t h i s i s due t o t h e f a c t t h a t t h e c a r b o n s u p p o r t o n l y s e r v e s to
disperse
the
active
sulfide
phase
without
disturbing
its
catalytic
p r o p e r t i e s . More r e c e n t l y Vissers e t a l . ( r e f . 8) e x p l a i n e d t h e h i g h e r c a t a l y t i c activity of the that
the
o f Mo/C compared t o Mo/A1 0 t o be due t o d i f f e r e n c e s i n t h e s t r u c t u r e 2 3 s u l f i d e phases p r e s e n t and i n t h e i n t e r a c t i o n between t h e s e p h a s e s and
r e s p e c t i v e s u p p o r t s . R e c e n t l y i t h a s been shown by van Veen e t a l . ( r e f . on
similar
9)
A1 0 S i 0 2 and C a n a c t i v e Co-Mo s u l f i d e phase can b e p r e p a r e d with 2 3' d e g r e e s o f d i s p e r s i o n , and t h a t t h e s p e c i f i c a c t i v i t y o f t h i s phase f o r
166 the
hydrodesulfurization
ported
on
Scheffer
than when i t i s supported on alumina o r s i l i c a . Furthermore,
carbon
10) concluded from Temperature Programmed S u l f i d i n g experiments
(ref.
Mo c a t a l y s t s a r e more d i f f i c u l t t o s u l f i d e than carbon-
alumina-supported
that
of thiophene is h i g h e r when t h i s phase i s sup-
(HDS)
supported ones because of t h e s t r o n g i n t e r a c t i o n with t h e support of t h e former. sulfide for
o f t h e chemical s ta t e o f t h e Co o r N i promoter i o n i n t h e promoted
role
The
is much debated. D i f f e r e n t e x p l a n a t i o n s have been suggested
catalysts,
t h e observed c a t a l y t i c synergy i n Co-Mo c a t a l y s t s and a v a r i e t y of chemical
structures
has
demonstrated
been
proposed
Wive1 e t
by
for
al.
the
promoter
16) t h a t
(ref.
ions
( r e f s . 11-15). I t was
t h e HDS a c t i v i t y of alumina-
supported c a t a l y s t s was almost completely governed by t h e presence o f a d i s t i n c t so-called
"Co-Mo-S"
formed by i n c o r p o r a t i o n of Co atoms a t t h e edges of
phase
MoS - l i k e s t r u c t u r e s ( r e f s . 5,17,18). 2 Although t h e n a t u r e of t h e a c t i v e sites p r e s e n t i n t h i s phase was observed t o be d i f f e r e n t from t h e sites p r e s e n t i n unpromoted MoS2 ( r e f s . been
established
influences high
the
whether catalytic
thiophene
HDS
of Mo s u l f i d e sites. On t h e b a s i s o f t h e
properties
activity
, Duchet
catalysts
16.19). i t has not
Co atoms a r e t h e a c t i v e sites o r whether t h e Co
the
measured
et al. ( r e f .
f o r carbon-supported Co and N i s u l f i d e
3 ) and de Beer e t a l . ( r e f . 20) mentioned t h e
p o s s i b i l i t y of Co o r N i s u l f i d e a c t i n g as c a t a l y s t s i n s t e a d of promoters f o r t h e MoS2
phase. Recently Vissers e t a l . ( r e f . 21) d e r i v e d an a c t i v i t y f o r optimally
dispersed high
as
pure
c o b a l t s u l f i d e supported on a c t i v a t e d carbon what was n e a r l y a s of a Co-Mo/C c a t a l y s t . Based on t h i s f i n d i n g they concluded t h a t
that
a c t i v i t y o f s u l f i d e d Co-Mo/C c a t a l y s t s i s most l i k e l y completely due t o t h e
the
activity
of
catalysts
the
is
sites
cobalt
mainly
to
and
function
suggested
t h a t t h e r o l e of MoS2 i n t h e s e
as a support f o r o p t i m a l l y d i s p e r s e d c o b a l t
ions.
Via t h e a p p l i c a t i o n of a stepwise s u l f i d a t i o n procedure, we have been a b l e t o observe quadrupole s p l i t t i n g s i n t h e MBssbauer Emission s p e c t r a of both s u l f i d e d Co/C
and
catalysts
Co-Mo/C
catalysts
ascribed
we
Furthermore,
to
(refs.
the
22.23)
"Co-Mo-S"
phase
similar by
to
Tops$e
those i n Co-Mo/Al 0 2 3 al. (ref. 15).
et
have found i n d i c a t i o n s t h a t i n s u l f i d e d Fe and Fe-Mo c a t a l y s t s
supported
on
either
MOssbauer
parameters
alumina that
o r a c t i v a t e d carbon, Fe s p e c i e s are p r e s e n t with
can
be
ascribed
t o t h e s o - c a l l e d "Fe-Mo-S" phase
( r e f s . 24-27). I n t h e p r e s e n t paper w e attempt t o e l u c i d a t e f u r t h e r some a s p e c t s concerning catalysts. obtained
the
structure
of s u l f i d e d carbon-supported Co, Co-Mo. Fe and Fe-Mo
The s t r u c t u r a l information on t h e Co-containing c a t a l y t i c systems i s from
MBssbauer Emission Spectrocopy (MES). while M8ssbauer Absorption
Spectroscopy (MAS) i s used f o r t h e Fe-containing c a t a l y s t s .
167 EXPERIMENTAL The face
carbon c a r r i e r used was a Norit (RX3-Extra) a c t i v a t e d carbon with a s u r 2 a r e a of 1190 m /g and a pore volume of 1.0 cm3/g. The Fe/C and Co/C
were prepared by pore volume impregnation of t h e carrier with aqueous
catalysts solutions
of
and
3 3
respectively. Amersham)
Co(N0 ) .6H20 (Merck "for analysis") 3 2 Co/C c a t a l y s t s a s o l u t i o n of 5 7 ~ on i t r a t e (ex
Fe(N0 ) .9H20 In
was
case
of
the
added t o t h e primary s o l u t i o n . The Co-Mo/C and Fe-Mo/C c a t a l y s t s
were prepared by a two s t e p pore volume impregnation procedure. The Mo-phase was first
introduced
by impregnation with an aqueous s o l u t i o n of (NH4) 6M07024.4H20
(Merck, min 99.9%) followed by an i n t e r m e d i a t e drying i n s t a t i c a i r a t
16 h
383 K f o r
and Co o r Fe were introduced as described above. A f t e r Co o r Fe introduc-
t i o n t h e c a t a l y s t samples were d r i e d i n ambient a i r a t 293 K f o r 16 h . Precursor atomic
catalyst
absorption
compositions
(without 57C0) were determined by means of
using a Perkin-Elmer 300 AAS spectrometer. The
spectroscopy
c a t a l y s t s are denoted as Co(x)/C. Fe(x)/C, Co(x)-Mo(y)/C o r Fe(x)-Mo(y)/C with x y r e p r e s e n t i n g t h e w t % Co o r Fe. and Mo r e s p e c t i v e l y . P r i o r t o t h e s u l f i d a -
and
t h e samples were subjected t o an a d d i t i o n a l d r y i n g treatment i n a H -flow 2 r a t e 50 cm 3/min) and kept under an H2 atmosphere. During t h i s treatment
tion, (flow
353 and 393 K f o r 24 h a t each temperature.
the sample i s kept a t 313,
of t h e c a t a l y s t s was c a r r i e d o u t i n a Mdssbauer i n - s i t u r e a c t o r ,
Sulfidation which well
has
described elsewhere ( r e f . 2 8 ) . However, t h e h e a t i n g s e c t i o n as
been
as
the
outer
used i n t h i s study are made of s t a i n l e s s s t e e l ,
container
while
t h e Mylar windows are replaced by beryllium windows, vacuum brazed on t h e
outer
container.
S u l f i d i n g was c a r r i e d o u t i n a 10 mol % H2S i n H2 gas mixture
a flow r a t e of 60 c m 3/min. During t h e s u l f i d i n g procedure t h e following tem-
at
perature
program
was
sequentially
applied:
linear
increase
t o the desired
maximum s u l f i d a t i o n temperature f o r 1 h , holding a t t h i s temperature f o r 1 h and cooling i n t h e H2S/H The
MAS
spectrometer were
flow t o 293 K .
2
experiments with
carried
spectrometer
performed
in-situ
a
using
constant
velocity
a 5 7 ~ oi n Rh source a t room temperature. The MES experiments
out
at
room
temperature using a c o n s t a n t a c c e l e r a t i o n
triangular
mode
with
in-situ
a
in
were
a moving
single-line
absorber of
K Fe(CN) . 3 H 0 enriched i n 57Fe. The spectrometer was placed i n a v e r t i c a l posi4 6 2 t i o n , s o t h a t t h e c a t a l y s t s which had been prepared as e x t r u d a t e s , could be
measured a s such. Isomer s h i f t s are reported r e l a t i v e t o a source of 57C0 i n Rh, while
positive
source.
The
nitroprusside fitted lines, tion
by
velocities
velocity
correspond
to
the
absorber
moving away from t h e
s c a l e was c a l i b r a t e d by t h e Mdssbauer spectrum o f sodium
(SNP) obtained with t h e 57C0 i n Rh source. Mdssbauer s p e c t r a were
computer
with c a l c u l a t e d subspectra c o n s i s t i n g of Lorentzian-shaped by varying t h e Mdssbauer parameters i n a non-linear, i t e r a t i v e minimiza-
routine.
In
the
case
of
quadrupole
doublets
t h e l i n e widths and t h e
168 a b s o r p t i o n areas o f t h e c o n s t i t u e n t peaks were c o n s t r a i n e d t o b e e q u a l . RESULTS Preparational aspects From
our
extensive
carbon-supported aspects to
be
It
(refs.
24.25)
s p e c t r o s c o p i c s t u d y of a c t i v a t e d it
followed t h a t p r e p a r a t i o n a l
The observed d i f f e r e n c e s i n t h e c a t a l y t i c a c t i v i t y were demonstrated
to
due
v a r i a t i o n s i n t h e d i s p e r s i o n of t h e a c t i v e s u l f i d e p h a s e , w h i l e depends on 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 p r e c u r s o r material.
dispersion found
is
catalysts
absorption
an i m p o r t a n t r o l e i n t h e a c t i v i t y of t h e s e s u p p o r t e d i r o n s u l f i d e
play
catalysts. this
Fe
MOssbauer
t h a t t h e t h e r m a l s t a b i l i t y o f t h e c a t a l y s t p a r t i c l e s was improved
when h y g r o s c o p i c n i t r a t e a n i o n s are c o m p l e t e l y removed from t h e primary p r e p a r e d catalyst
precursor.
This
done by s u b j e c t i n g t h e samples t o an a d d i t i o n a l
was
t r e a t m e n t i n a H -flow ( f l o w rate 50 cm3.min-l) d u r i n g which t h e sample was k e p t 2 393 K f o r 24 h a t e a c h t e m p e r a t u r e . T h e r e f o r e , a l l o u r a c t i v a t e d
a t 313, 353 and
carbon-supported
Fe,
Co and Co-Mo c a t a l y s t p r e c u r s o r s are s u b j e c t e d t o
Fe-Mo,
t h i s additional H -treatment. 2
In
the
preparation
process
of hydrotreating c a t a l y s t s , sulfidation of the
o x i d i c c a t a l y s t p r e c u r s o r i s a c r u c i a l s t e p , because i t r e s u l t s i n t h e f o r m a t i o n of
the
actual
proceeds
and
phase. So, i t i s i m p o r t a n t t o know how t h e s u l f i d a t i o n
sulfide whether
the
final
r e s u l t ( t y p e o f s u l f i d e p h a s e formed and i t s
d i s p e r s i o n ) depends on t h e s u l f i d i n g p r o c e d u r e a p p l i e d . W e in the
have
a
s t u d i e d t h e s u l f i d a t i o n o f t h e a c t i v a t e d carbon-supported c a t a l y s t s
s t e p w i s e manner. F i r s t t h e MOssbauer spectrum was r e c o r d e d a t 293 K w h i l e sample
sample
was
i n s t a t i c H S/H a t a t m o s p h e r i c p r e s s u r e . Next, t h e same 2 2 s u b j e c t e d t o v a r i o u s s u c c e s s i v e s u l f i d a t i o n t r e a t m e n t s as d e s c r i b e d
was
kept
i n t h e Experimental s e c t i o n . The importance o f t h e a p p l i e d s u l f i d a t i o n procedure i s c l e a r l y demonstrated i n o u r Mtissbauer s t u d y o f a c t i v a t e d carbon-supported Fe-
Mo c a t a l y s t s ( r e f . 2 6 ) . When d u r i n g t h e 1 h s u l f i d i n g p r o c e d u r e i n a H S/H -flow 2 2 t h e c a t a l y s t p r e c u r s o r i s h e a t e d from room t e m p e r a t u r e up t o 673 K i n s t e a d of
a
K,
623
large
amount
of
Fel-xS was formed a t t h e e x p e n s e of Fe-Mo-S and a
d e c r e a s e i n t h e a c t i v i t y f o r t h i o p h e n e HDS was measured. Co-Mo/C and Fe-Mo/C c a t a l y s t s In
Fig.
1 t h e room t e m p e r a t u r e MOssbauer s p e c t r a of t h e Co(O.O8)-Mo(6.84)/C
and Fe(1.8)-Mo(9.5)/C c a t a l y s t s a f t e r t h e s u c c e s s i v e H2S/H - t r e a t m e n t s are 2 p r e s e n t e d t o g e t h e r w i t h t h o s e o f t h e c a t a l y s t p r e c u r s o r s which were s u b j e c t e d t o the
drying
quadrupole
treatment doublets.
ascribed
to
electron
capture).
in
high-spin
a
H2-flow. These l a s t s p e c t r a are a n a l y s e d w i t h two
case of t h e Co-Mo/C c a t a l y s t p r e c u r s o r one d o u b l e t is
In Fe
2+
and
one d o u b l e t b e l o n g s t o a Fe3+-species ( a f t e r
A s t h e spectrum o f t h e Fe-Mo/C c a t a l y s t p r e c u r s o r s c o n s i s t s
169 of
rather
doublets. the
broad
absorption
However,
Fe-Mo/C
lines,
this
spectrum is a l s o analysed u s i n g two
t h e MUssbauer parameters of t h e s e d o u b l e t s i n d i c a t e t h a t i n
precursor
the
i r o n is p r e s e n t as i r o n ( I I 1 ) o x i d e . I t appears t h a t
t h e s e doublets have i d e n t i c a l isomer s h i f t s ( I S ) b u t d i f f e r e n t quadrupole s p l i t -
(QS) (1s1=0.65 m m / s , QSl=0.55 m m / s ; IS2=0.66 m m / s , QS2=0.g2 r n m / s ) .
tings
on t h e r e s u l t s obtained with unsupported s m a l l a-Fe 0
2 3
a
composition is explained by a bulk- and surface-oxide c o n t r i b u t i o n .
spectral
The
Based
p a r t i c l e s ( r e f . 2 9 ) . such
spectral
surface-oxide
component
with
contribution.
Co(O.O8)-Mo(6.84)/C
and
the
largest
QS-value
can
be a s s i g n e d t o t h e
The r e s u l t s of computer a n a l y s e s of t h e s p e c t r a of
Fe(1.8)-Mo(9.5)/C
c a t a l y s t s are given i n t h e Tables 1
and 2 , r e s p e c t i v e l y .
is
As
shown i n Fig. 1. t h e s p e c t r a of b o t h c a t a l y s t p r e c u r s o r s changed as a
r e s u l t of exposure t o t h e H S/H2 gas mixture a t room temperature. I n t h e c a s e of 2
2.96
1.61
2.90
1.50
2.99
1.78
2-96
1.73
2.71
2.67
1.41
1.67
1.15
1.60
1.*7
3.3b
3.60
3.28
2.7?
L.20
2.07
4.02
Doppler velocity
(mm.s-'~
Fig. 1. I n - s i t u Mdssbauer emission s p e c t r a of s u l f i d e d C0(0.08)-Mo(6.84)/C and a b s o r p t i o n s p e c t r a of s u l f i d e d Fe(1.8)-Mo(9.5)/C c a t a l y s t s a t 293 K a f t e r v a r i o u s successive s u l f i d a t i o n treatments i n a H2S/H2 gas mixture.
170
Co-Mo/C sample
the
decreased,
while
the
in
spectral
contribution
of
the
high-spin Fe
t h e spectrum of t h e Fe-Mo/C sample a high-spin Fe
2+
-phase
2+
-phase
appeared. At
increasing
high-spin
sulfidation
Fe2+-phase
spectra
of
doublet
was
the
shows
the
s p e c t r a l contribution of the
decreased and f i n a l l y disappeared. I n comparison with t h e
c a t a l y s t p r e c u r s o r s roughly speaking one a d d i t i o n a l quadrupole
p r e s e n t . Although t h e computer f i t s of t h e s p e c t r a of t h e s u l f i d e d
were
Co-Mo/C c a t a l y s t s only
temperatures,
the
improved by u s i n g two doublets i n s t e a d of one, Table 1
numerical
r e s u l t s f o r t h e a n a l y s e s with one doublet as i n t h e
c a s e of t h e Co(O.O4)-Mo(6.84)/C and Co(2.25)-Mo(6.84)/Cc a t a l y s t s ( r e f . 23).
TABLE 1 M6ssbauer
parameters obtained a t 293 K of Co(O.O8)-Mo(6.84)/C,co(O.o8)/c and Co(O.38)/C c a t a l y s t s a f t e r v a r i o u s s u c c e s s i v e t r e a t m e n t s i n H2S/H2 g a s mixture. Experimental u n c e r t a i n t i e s : IS: 0.03 mm/s, QS: 0.05 mm/s, A: 5%. The QS-value of cogs8 i s constrained to 0.26 mm/s.
Treatment
Fe3+*
Fe2+*
" a c t i v e phases"
c09s8
CO (0.08)-Mo (6.84)/C
[393 K, H2] 0.25 0.75 24 [293 K,H+l [373 K.H2Sl [473 K,H$I [573 K.H$I [673 K.H$I
0.86 2.02 76 0.95 2.16 56 0.88 2.05 22
0.24 0.23 0.23 0.22 0.21
1.28 44 1.28 78 1.07 100 1.08 100 1.24 100 ~~
co (0.08) /c
[393 K, H2] 0.32 0.78 66 [293 K.H2S] 0.26 0.72 21 [373 K,H2Sl [473 K,HzSI [573 K,H2Sl C673 K,H2Sl
0.87 1.90 34 1.03 2.03 79 1.05 2.05 70
0.27 0.22 0.26 0.25
1.12 30 1.31 100 0.76 67
0.20 0.20 0.18 0.23 0.24
1.32 1.30 1.25 0.73
0.76 68
0.24 0.26 33 0.24 0.26 32
C0(0.38)/C
[393 K, H21 0.33 0.83 42 [293 K,HzSI [373 K,H2Sl [473 K,H2SI
[573 K.H$I [673 K.H2Sl
*
0.94 2.14 58 0.98 1.92 15
0.96 1.96
9
85 91 92 52
0.69 63
0.23 0.26 8 0.22 0.26 48 0.22 0.26 37
57Fe atoms produced by t h e deca o f 57Co. However, i n MES experiments t h e observed valence and s p i n s t a t e s of 57Fe may be d i f f e r e n t from those of t h e p a r e n t 57Co atoms.
171 It
turned
out
t h a t t h e s p e c t r a of t h e s u l f i d e d Fe-Mo/C c a t a l y s t s had t o be
f i t with t h r e e d i f f e r e n t s p e c t r a l components ( r e f . 3 0 ) . The Massbauer parameters of
one
0.57
of
these
mm/s,
s p e c t r a l components a r e c l o s e t o t h o s e o f p y r i t e (FeS2; IS =
QS = 0.62 m m / s ) .
However i t has been deduced from i n - s i t u MUssbauer F e ( x ) / C c a t a l y s t s a t 4.2 K ( r e f . 25) t h a t t h i s com-
experiments
with
ponent
t o be a s c r i b e d t o an "Fe-sulfide" having t h e Fel-xS-type
has
sulfided
structure.
Such an assignment i s i n agreement with t h e r e s u l t s of our s t u d y on t h e s u l f i d a tion
of
unsupported
sulfidation
up
transformed quadrupole Table
2
to
into
50 nm a-Fe 0 p a r t i c l e s ( r e f . 31). W e found t h a t a f t e r 2 3 573 K o r h i g h e r temperatures t h e i n i t i a l l y formed FeS2 is
Fel-xS.
In
order
t o show t h e temperature dependence of t h e
s p l i t t i n g s of t h e two o t h e r s p e c t r a l components, w e have included i n
the
numerical r e s u l t s o f i n - s i t u Massbauer experiments a t 293, 77 and
4 . 2 K of t h e Fe(1.8)-Mo(9.5)/C c a t a l y s t s u l f i d e d a t 623 K d u r i n g
4 h.
TABLE 2 Mdssbauer parameters obtained a t 293 K o f Fe(1.8)-Mo(9.5)/C and F e ( l . 8 ) / C catalysts after various successive t r e a t m e n t s i n H2S/H2 gas mixture. Experimental u n c e r t a i n t i e s : IS: 0.03 m m / s , QS: 0.05 m m / s , A: 5%. ( a ) , (b) and ( c ) a r e r e p r e s e n t i n g t h e d a t a measured a t 293, 77 and 4 . 2 K respect i v e l y of t h e Fe(1.8)-Mo(9.5)/C c a t a l y s t a f t e r a treatment i n H2S/H2 gas mixture a t 623 K during 4 h.
Treatment
Fe2+ IS
QS
(mm/s)
A (%)
"Fe-sulf i d e "
" a c t i v e phases"
*
*
IS
QS
(mm/s)
A
(%I
IS
QS
(mm/s)
A
(%I
IS
QS
(mm/s)
A (%)
Fe( 1.8)-Mo (9.5) / C [293 K.H2S] [473 K,H2S] [573 K,H2SI [673 K.H2Sl (a) (b)
1.46 2.20 26 1.28 2.38 13
(C)
0.55 0.59 0.58 0.58 0.57 0.67 0.68
0.61 0.66 0.64 0.62 0.49 0.65 0.64
0.57 0.57 0.58 0.65
0.61 0.61 0.57 0.44
7 29 42 42
53 34
34
0.60 0.60 0.59 0.57 0.59 0.67 0.68
0.97 1.04 1.04 1.07 0.98 1.12 1.09
50 31 41 39
0.63 0.64 0.65 0.67
1.06 0.98 0.95 0.91
32 27 26
26
44 34
0.60 0.61 0.61 0.55 0.59 0.67 0.68
1.35 1.38 1.47 1.54 1.47 1.60
1.60
17 27
17 19 21 22 32
Fe( 1 . 8 ) / C [293 K,H2S] [473 K,H2S] [573 K.H2Sl 1673 K.H2Sl
*
1.38 2.34
26
1.39 2.23
8
42
65 72
67
33
The parameters of t h e s p e c t r a l "Fe-sulfide" component are r a t h e r l i k e t h o s e of p y r i t e (FeS2). However, i n - s i t u Mdssbauer measurements a t 4.2 K of s u l f i d e d Fe(x)/C c a t a l y s t s showed t h a t t h i s component should be a t t r i b u t e d t o a w e l l d i s persed magnetic "Fe-sulfide" phase ( r e f . 2 5 ) . From i n - s i t u measurements a t 4.2 K of s u l f i d e d Fe(x)-Mo(9.5)/C c a t a l y s t s i t a l s o followed t h a t t h e s p e c t r a l components denoted as " a c t i v e phases" belonged t o well d i s p e r s e d magnetic p h a s e ( s ) , s i m i l a r t o t h e "Fe-sulfide" phase ( r e f . 3 0 ) .
172
3.15
3.09 2.73
* 2.67 L
m
1.27
I
1
3.20
T
-
1 . 1
I
1
W
293 K
Fig. 2 . I n - s i t u Mdssbauer emission s p e c t r a of s u l f i d e d Co(O.O4)-Mo(6.84)/C, Co(O.O8)-Mo(6.84)/C and Co(2.25)-Mo(6.84)/C c a t a l y s t s a t 293 K a f t e r t h e f i n a l s u l f i d a t i o n treatment i n a H2S/H2 gas mixture a t 673 K.
In
Fig. 2 t h e room temperature s p e c t r a of t h e Co(O.O4)-Mo(6.84)/C, c o ( 0 . 0 8 ) -
M0(6.84)/C
and
Co(2.25)-Mo(6.84)/C
s u l f i d a t i o n treatment a t
M0(6.84)/C
catalyst
now
catalysts
are
presented
a f t e r the f i n a l
673 K . I t is c l e a r from t h i s F i g u r e t h a t t h e Co(2.25)shows
a much smaller quadrupole s p l i t t i n g ( Q S = 0.87
m m / s ) t h a n t h e o t h e r two c a t a l y s t s (QS = 1 . 3 0 mm/s). Co/C and Fe/C c a t a l y s t s
3 shows t h e room temperature Mbssbauer s p e c t r a o f t h e C o ( O . O 8 ) / C and
Fig.
Co(O.h)/C c a t a l y s t s
after
the
s u c c e s s i v e H2S/H2-treatments
together with the
spectra
of t h e c a t a l y s t p r e c u r s o r s which were s u b j e c t e d t o d r y i n g t r e a t m e n t s i n
flowing
hydrogen.
Also f o r t h e s e c a t a l y s t s w e have analysed t h e spectra of t h e
p r e c u r s o r s with two quadrupole d o u b l e t s corresponding t o t h e p r e s e n c e of high2+ s p i n Fe and Fe3+-species. The r e s u l t s of computer a n a l y s e s of t h e s p e c t r a a r e a l s o given i n Table 1.
A s soon as c a t a l y s t p r e c u r s o r s t r e a t e d i n t o hydrogen are exposed t o t h e s u l f i d i n g H S/H gas mixture a t room temperature, t h e s p e c t r a are changed (see Fig. 2 2 3 ) . I n t h e case of c 0 ( 0 . 0 8 ) / c a high-spin Fe 2+ s p e c t r a l component i s predominant, Sulfiding After single those
at
is
which higher
s t i l l p r e s e n t a f t e r t h e s u l f i d a t i o n t r e a t m e n t a t 373 K . temperatures
causes
t h e spectrum t o change d r a s t i c a l l y .
t h e s u l f i d a t i o n treatment a t 473 K t h e spectrum c o n s i s t s e x c l u s i v e l y of a quadrupole of
the
doublet,
doublet
with Mbssbauer parameters ( I S and Q S ) i d e n t i c a l t o
observed f o r Co(O.O8)-Mo(6.84)/C s u l f i d e d a t 673 K ( s e e
173 1 ) . T h i s quadrupole d o u b l e t h a s p a r a m e t e r s similar t o t h o s e which Topsae
Table et
al.
(ref.
15) have a s c r i b e d t o t h e "Co-Mo-S" p h a s e . However, i t t u r n e d o u t
t h a t t h i s p a r t i c u l a r Co s p e c i e s i s n o t s t a b l e a t s u l f i d i n g t e m p e r a t u r e s o f
573 K
h i g h e r (see F i g . 3). The observed s u l f i d a t i o n b e h a v i o u r i s q u i t e s i m i l a r as
and
r e p o r t e d b e f o r e f o r a Co(O.O4)/C c a t a l y s t ( r e f . 23). 2+ For t h e c0(0.38)/c c a t a l y s t t h e h i g h - s p i n Fe s p e c t r a l component is h a r d l y observable of
a
after
s u l f i d a t i o n a t room t e m p e r a t u r e . The spectrum c o n s i s t s mainly
newly formed quadrupole d o u b l e t , which i s s t i l l p r e s e n t a f t e r s u l f i d a t i o n
473 K . The MUssbauer parameters o f t h e observed d o u b l e t are a l s o i n t h e range given by Topsae e t a l . ( r e f . 18) f o r t h e "Co-Mo-S" p h a s e . A f t e r s u l f i d a t i o n t r e a t m e n t s a t 573 K t h e s p e c t r a of b o t h Co/C c a t a l y s t s at
drastically free
changed.
These s p e c t r a are a n a l y s e d by one quadrupole d o u b l e t with
parameters and one d o u b l e t w i t h a quadrupole s p l i t t i n g c o n s t r a i n e d t o 0.26
0.79
0.72 5.29
5 2" 4.09
4.04
2.91
2.85 5.09
5.05 3.26
3.21 2.73
5.82
2.60 *.GI
5.28
5.18 I.0:
2.80
2.01
l.Pb
3.*7
3.37
Fig. 3. I n - s i t u M b s b a u e r e m i s s i o n s p e c t r a o f s u l f i d e d c0(0.08)/c and c0(0.38)/c c a t a l y s t s a t 293 K a f t e r v a r i o u s s u c c e s s i v e s u l f i d a t i o n t r e a t m e n t s i n a H2S/H2 gas m i x t u r e .
174 mm/s.
The
latter
spectrum of Co S
4
Fig. after
9 8'
splitting
can
be
considered
as
a good s i m u l a t i o n o f t h e
The deduced numerical v a l u e s are summarized i n T a b l e 1.
shows t h e room temperature Mossbauer s p e c t r a o f t h e F e ( 1 . 8 ) / C c a t a l y s t
the
successive
s u l f i d a t i o n t r e a t m e n t s t o g e t h e r w i t h t h e spectrum o f t h e
c a t a l y s t p r e c u r s o r . Although t h e a n a l y s e s o f t h e s e complex s p e c t r a are d i s c u s s e d in
detail
Table
elsewhere
(ref.
2 5 ) . t h e deduced numerical r e s u l t s are i n c l u d e d i n
2 . From t h e d a t a i n c l u d e d i n Table 2 , i t can b e concluded t h a t one o f t h e components
spectral
o f t h e F e ( l . 8 ) / C c a t a l y s t corresponds r a t h e r w e l l w i t h one
o f t h e components o f t h e spectrum of t h e Fe(1.8)-Mo(9.5)/C c a t a l y s t . For catalyst
reasons o f comparison t h e room temperature s p e c t r a o f t h e Fe(0.86)/A1 0 2 3 a f t e r t h e s u c c e s s i v e s u l f i d a t i o n t r e a t m e n t s a r e also shown i n F i g . 4 .
An "Fe(I1)-Aluminate" i s formed t o an a p p r e c i a b l e amount a t i n c r e a s i n g s u l f i d i n g temperature.
A more d e t a i l e d comparison of carbon- and alumina-supported Fe and
Fe-Mo c a t a l y s t s w i l l b e p r e s e n t e d elsewhere ( r e f . 3 2 ) .
1.53
8.51
B.*L
1.31
0.31
2.17
8.30
2.81
2.03
1.51
t.6.
l.8b
2.81
1.37
1.88
1.12
2.28
1.41
2.06
1.27
Doppler
velocity
(rnm.5.')
Fig. 4 . I n - s i t u M8ssbauer a b s o r p t i o n s p e c t r a o f s u l f i d e d F e ( l . 8 ) / C and Fe(0.86)/A120 c a t a l y s t s a t 293 K a f t e r v a r i o u s s u c c e s s i v e s u l f i d a t i o n treatments i n a HJ/H2 gas mixture.
175 DISCUSSION
i s already s t a t e d i n t h e i n t r o d u c t i o n , t h e r o l e and t h e chemical s t a t e of
As the
or
Co
N i promoter i o n s i n t h e promoted s u l f i d e c a t a l y s t s , is much debated
( r e f s . 17.33). It i s l a r g e l y due t o MOssbauer Emission Spectroscopy s t u d i e s t h a t
a
quantitative
(refs.
p i c t u r e of t h e s t r u c t u r e of c o b a l t i n HDS c a t a l y s t s has emerged
5,15,17.18).
Mo/A1203
Most
catalysts
of
the
reported
MES
s t u d i e s a r e performed on Co-
and only a few d e a l with Co/C and Co-Mo/C ( r e f s .
18.35.36).
Co-Mo/Al 0 c a t a l y s t s Topsae e t a l . ( r e f s . 2 3 5 , 1 5 , 1 7 , 1 8 ) and Wivel e t a l . ( r e f . 16) observed a quadrupole doublet (QS = 1.0 Tn
the
MES
spectra
of
sulfided
37) which could not be a s c r i b e d t o any known c o b a l t s u l f i d e o r cobalt-molybdenum s u l f i d e compound. Furthermore, i t was r e p o r t e d ( r e f . 37) t h a t 1 . 3 mm/s)
(ref.
quadrupole doublet was only observed i n s u l f i d i c c a t a l y s t s c o n t a i n i n g both
this
Mo and hence t h i s s p e c t r a l component was a s c r i b e d t o a Co phase denoted
Co
and
as
"Co-Mo-S".
in
t h e MES s p e c t r a governed almost completely t h e HDS a c t i v i t y of t h e c a t a l y s t .
Wivel e t a l . ( r e f . 16) reported t h a t t h e amount of t h i s Co-phase
t h e discovery of t h e s o - c a l l e d "Co-Mo-S" phase i n t h e MES s p e c t r a o f s u l -
Since
Co-Mo/Al 0 c a t a l y s t s , Topsae e t a l . ( r e f s . 5.17,18,37) demonstrated t h a t 2 3 t h e "Co-Mo-S" phase t h e Co i o n s are l o c a t e d a t edges of MoS2 c r y s t a l l i t e s .
fided in
Furthermore,
i s reported t h a t t h i s s t r u c t u r a l model of t h e a c t i v e HDS phase
it
n o t r e s t r i c t e d t o Co-Mo c a t a l y s t s but a l s o a p p l i e s f o r t h e Fe o r N i promoted
is
Mo o r W based c a t a l y s t s ( r e f . 37). it
However, quadrupole
follows from t h e numerical r e s u l t s i n Table 1 t h a t an i d e n t i c a l
doublet
can
be
observed
i n t h e MES s p e c t r a recorded f o r s u l f i d e d
C O ( O . O ~ ) / Cc ,0 ( 0 . 3 8 ) / c and Co(O.O8)-Mo(6.84)/C c a t a l y s t s , and t h a t t h i s doublet has
similar
parameters
"Co-Mo-S"
phase.
These
recently
reported
for
t o those which Topsde e t a l . ( r e f . 15) a s c r i b e d t o t h e observations
are
in
agreement
with those which are
Co(O.O4)/C and Co(O.O4)-Mo(6.84)/C c a t a l y s t s by van der
a l . ( r e f . 2 2 ) . These r e s u l t s i n d i c a t e t h a t t h e d e f i n i t i o n of t h e "Co-
Kraan
et
Mo-S"
phase
formation
i t s MOssbauer parameters is n o t unequivocally e s t a b l i s h e d . The
by
a
of
Co
s p e c i e s with s i m i l a r parameters as t h e s o - c a l l e d "Co-Mo-S"
phase does not n e c e s s a r i l y r e q u i r e t h e presence of MoS2. It
can
be
seen
in
Fig.
1 t h a t t h e s u l f i d a t i o n of t h e Fe(1.8)-Mo(9.5)/C
c a t a l y s t proceeds s i m i l a r l y t o t h e Co(O.O8)-Mo(6.84)/C c a t a l y s t . It i s even more clear the
when already
However (see
published
results
o f t h e Co(2.25)-Mo(6.84)/C c a t a l y s t ( r e f . 2 3 ) .
t h e spectrum of t h e s u l f i d e d Fe-Mo/C c a t a l y s t c o n t a i n s t h r e e components
Table
sulfide"
2).
of
together rupole
s u l f i d a t i o n of t h e Fe(1.8)-Mo(9.5)/C c a t a l y s t is compared with
the
form doublet
catalysts.
One
the
of t h e s e s p e c t r a l components has been a s c r i b e d t o an "Fe-
Fel-xS-type
roughly of
the
s t r u c t u r e . The remaining two s p e c t r a l components
speaking so-called
a doublet which is q u i t e similar t o t h e quad"Co-Mo-S"
phase i n t h e spectrum of Co-Mo/C
However, as such a quadrupole s p l i t t i n g i s a l s o observed i n s u l f i d e d
176
c a t a l y s t s without Mo, t h e s p e c t r a l components are n o t denoted by "Co-Mo-S"
Co/C
o r "Fe-Mo-S" i n Tables 1 and 2. r e s p e c t i v e l y , but as " a c t i v e phases". As
mentioned
before,
although t h e computer f i t s of t h e s p e c t r a of t h e s u l -
f i d e d Co-Mo/C c a t a l y s t s were improved by u s i n g two d o u b l e t s i n s t e a d o f one, only the
numerical
However,
results
t h e analyses with one d o u b l e t are given i n Table 1.
of
case of t h e Fe-Mo/C c a t a l y s t s t h e improvement o f t h e computer f i t s
in
was too l a r g e t o i g n o r e . The and
s i m i l a r i t y between t h e " a c t i v e phases" i n t h e s u l f i d e d Fe(1.8)-Mo(9.5)/C
Co-Mo/C
"active
et
Tops$e
is
catalysts al.
(ref.
is
information
supported
(see Table
phases"
2)
by
the
temperature dependence of t h e s e
i s comparable with t h e one r e p o r t e d by
which
37) f o r t h e so-called "Co-Mo-S" phase. Because a d d i t i o n a l
lacking
sofar,
we
suggest
t h a t t h e d i v i s i o n of t h e s p e c t r a l
i n t o two components i s r a t h e r due t o a d i s t r i b u t i o n i n quadrupole s p l i -
doublet
t t i n g s than t o two s p e c i f i c Co- or Fe-phases.
a
From
comparison
these
the
25)
the
and
results
spectral
components
denoted
magnetic
phase(s),
MoS2
in
Table 2 of t h e F e ( 1 . 8 ) -
so-called
in
(ref.
as
30)
I t turned out t h a t t h e
phases" i n Table 2 belonged t o w e l l -
"active
similar
catalysts.
to
the
"Fe-sulfide" phase. Hence, the
t h a t t h e formation of a Co s p e c i e s with similar MBssbauer parameters
conclusion the
results
of i n - s i t u measurements a t 4 . 2 K of s u l f i d e d Fe(x)/C
Fe(x)-Mo(9.5)/C
dispersed as
numerical
q u i t e a s i m i l a r s p e c t r a l component i s p r e s e n t . T h i s s i m i l a r i t y
catalysts
corroborates (ref.
of
c a t a l y s t with t h o s e of t h e F e ( l . 8 ) / C c a t a l y s t i t follows t h a t a l s o i n
Mo(9.5)/C
"Co-Mo-S" phase does not n e c e s s a r i l y r e q u i r e t h e presence of
carbon-supported
Co c a t a l y s t s , can be extended t o carbon-supported Fe
containing cat al y s t s . Although
of
sulfided
Co/C and Co-Mo/C c a t a l y s t s t h e same Co s p e c i e s can be
follows from comparison of t h e s p e c t r a i n F i g s . 1 and
it
formed, case
in
Co/C
temperature. A f t e r t h e s u l f i d i n g a t
sulfidation
3 t h a t i n the
c a t a l y s t s t h i s p a r t i c u l a r Co-species is not s t a b l e a t i n c r e a s i n g
673 K , t h e temperature a t which
thiophene a c t i v i t y tests are o f t e n performed, d i f f e r e n c e s between t h e s p e c t r a of Co/C
and
at
formed presence with
Co-Mo/C a
sulfiding
temperature
of
9 8 573 K o r h i g h e r . However, b e s i d e s the
i n t h e MES s p e c t r a of t h e Co/C c a t a l y s t s a s p e c t r a l component
of Co S
98
approximately
nificantly
a r e observed. Only i n t h e Co/C c a t a l y s t s i s Co S
catalysts
smaller
QS = 0.75 mm/s is measured. T h i s QS-value i s n o t only s i g than
the
QS-value observed
after
sulfidation
a t lower
temperatures b u t a l s o s m a l l e r than t h e lowest QS-value r e p o r t e d by Tops6e e t a l . (ref. ting
18) for "Co-Mo-S". A s i s r e p o r t e d b e f o r e ( r e f . 23) t h e quadrupole in
the
decreased
spectrum
split-
of t h e Co(2.25)-Mo(6.84)/C c a t a l y s t i s a l s o d r a s t i c a l l y
(from QS = 1.20 m m / s t o QS = 0.85 mm/s) by i n c r e a s i n g t h e s u l f i d i n g
373 K
temperature
from
increasing
sulfidation
till
473 K . Such a decrease i n quadrupole s p l i t t i n g with
temperature
so
far
is
only
observed
in
the
c a t a l y s t s with a r e l a t i v e l y high c o b a l t content (see Fig.
Co-Mo/C tion
remains
still
change
in
"active
the
phase".
obtained
by
Mo(7.7)/C
whether
structure In
the
of
3 ) . The ques-
decrease i n quadrupole s p l i t t i n g i s due t o a
the
"active
phase", o r due t o s i n t e r i n g of t h e
r e s p e c t i t i s important t o n o t e t h e r e s u l t s r e c e n t l y
this
e t a l . ( r e f . 38) who c h a r a c t e r i s e d C o ( b . l ) / C and C o ( l . 5 ) -
Bouwens
by means of EXAFS and XANES. They found t h a t as a r e s u l t of
catalyst
s u l f i d a t i o n s i m i l a r Co-phases a r e formed i n t h e s e c a t a l y s t s i . e . i n both cases a high
sulfur
of
coordination
t h e Co atoms was observed. These f i n d i n g s are i n
l i n e with t h e conclusions drawn by Vissers e t a l . ( r e f . 21) t h a t t h e a c t i v i t y of sulfided
MoS2
Co-Mo/C
should
species
allowing
i s most l i k e l y completely due t o t h e c o b a l t sites.
catalysts
be regarded as a support for t h e c a t a l y t i c a l l y a c t i v e c o b a l t
then
a higher
dispersion
of c o b a l t than i n t h e case of a carbon
support. The
high-spin Fe2+-phase i n t h e alumina-supported c a t a l y s t s and t h e p o s s i b l y
accompanying high-spin Fe3+-aliovalent s p e c i e s ( r e f . on-supported
4).
(see Fig.
ones
Therefore
the
39) are absent i n t h e carbMES s p e c t r a of t h e s u l f i d e d
carbon-supported c a t a l y s t s are less complex. CONCLUDING REMARKS
Only serve
by in
a stepwise s u l f i d a t i o n procedure has i t been p o s s i b l e t o ob-
using the
MES
spectra
of
both
sulfided
Co/C
and
Co-Mo/C c a t a l y s t s a
s p l i t t i n g with s i m i l a r parameters as those a s c r i b e d t o t h e "Co-Mo-S"
quadrupole
phase by Tops#e e t a l . ( r e f . 1 5 ) . This implies t h a t i n t h e s u l f i d e d Co/C and CoMo/C
c a t a l y s t s t h e same Co s p e c i e s i s formed, or t h a t t h i s quadrupole s p l i t t i n g
does n o t belong t o an unique Co s p e c i e s . Similar sulfided
results Fe/C
are
and
obtained
Fe-Mo/C
using Massbauer absorption spectroscopy with
c a t a l y s t s . Hence, study of t h e s e c a t a l y s t s i s very
h e l p f u l i n determining experimental d e t a i l s . The QS-value of t h e a c t i v e Co "phase" formed i n t h e s u l f i d e d Co/C and Co-Mo/C catalysts
depends
on t h e s u l f i d i n g temperature and Co c o n t e n t . Hence, i t seems
there
w i l l be only one w e l l defined a c t i v e phase (with s p e c i f i c
unlikely
that
spectral
characteristics)
which governs t h e HDS a c t i v i t y . However, a change of
QS-value of t h e a c t i v e Co "phase" due t o s i n t e r i n g of t h i s "phase" can not
the
be excluded. Although t h e r e s t i l l remain open q u e s t i o n s , t h e p r e s e n t r e s u l t s i n d i c a t e t h a t characterization means role
of
of
of
carbon-supported Co s u l f i d e and Co-Mo s u l f i d e c a t a l y s t s by
t h e 57Co-MES technique is a very promising approach i n s t u d i e s of t h e Co
i n commercial Co-Mo s u l f i d e c a t a l y s t s . Co i o n s do not d i f f u s e i n t o
t h e carbon-support as is t h e case with t h e alumina support.
178
ACKNOWLEDGEMENT The
information
in
this
paper
is
partly
d e r i v e d from a c o n t r a c t (EN3V-
0009/NL) concluded w i t h t h e European Economic Community. REFERENCES 1 J . L . Schmitt and G . A . C a s t e l l i o n , U . S . P a t e n t 4 . 032, 435 (1977). 2 D.G. Gavin and M.A. J o n e s , E.P. 0024109 (1981). 3 J . C . Duchet, E.M. van Oers. V . H . J . d e Beer and R . P r i n s . J . C a t a l . , 80 (1983 386. 4 C . K . Groot, V . H . J . de Beer, R. P r i n s , M . S t o l a r s k i and W.S. Niezwiedz, Ind Eng. Chem. Prod. Res. Dev., 25 (1986) 522. 5 H. Tops$e and B.S. Clausen, Appl. C a t a l . . 25 (1986) 273. 6 B.M. Reddy and V.S. Subrahmanyam, Appl. C a t a l . . 27 (1986) 1. 7 J.P.R. Vissers, C . K . Groot, E.M. van Oers. V . H . J . de Beer and R . P r i n s , B u l l . SOC. Chim. Belg.. 93 (1984) 813. 8 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 . Moulijn and R . P r i n s , J . C a t a l . , 105 (1987) 277. 9 J . A . R . van Veen, E. Gerkema. A.M. van d e r Kraan and A. Knoester, J . Chem. S o c . , Chem. Commun., (1987) p. 1684. 10 B. S c h e f f e r , T h e s i s , U n i v e r s i t y of Amsterdam, Amsterdam (1988). 11 B . C . Gates, J . R . Katzer and G.C.A. S c h u i t , i n Chemistry o f C a t a l y t i c P r o c e s s e s , McGraw-Hill, New York. 1st e d . , 1979, p. 411. 12 B . Delmon, i n H.F. Barry and P.C.H. M i t c h e l l ( E d i t o r s ) , Proc. 3 r d I n t . Conference on t h e Chemistry and Uses o f Molybdenum, Ann Arbor, 1979, Climax Molybdenum Co.. Ann Harbor, 1979, p . 73. 13 R . J . H . Voorhoeve and J . C . M . S t u i v e r . J . C a t a l . , 23 (1971) 243. 14 A.L. Farragher and P. Cossee, i n J.W. Hightower ( E d i t o r ) , P r o c . 5 t h I n t e r n . Congr. on C a t a l y s i s , Palm Beach, 1972, North Holland, Amsterdam, 1973, p .
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M.L. Occeili and R.G. Anthony (Editors), Adoances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
181
A NEW APPROACH FOR STUDYING THE ACID STRENGTH DISTRIBUTION I N HYDROTREATING CATALYSTS BY DIFFERENTIAL SCANNING CALORIMETRY A.K.
ABOUL-GHEIT and A.M.
SUMMAN
Chemistry Department, F a c u l t y of A p p l i e d Science & E n g i n e e r i n g , Umm A l - Q u r a University,
P.O. Box 3711, Makkah Al-Mukarramah (Saudi A r a b i a )
ABSTRACT Three h y d r o t r e a t i n g c a t a l y s t s i n t h e o x i d e form, Co-Mo-alumina, Ni-Mo-alumina and Ni-W-alumina, have been s u b j e c t e d t o t r i e t h y l a m i n e (TEA) a d s o r p t i o n u n t i l s a t u r a t i o n f o l l o w e d by i t s d e s o r p t i o n i n a d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) u n i t v i a a programmed i n c r e a s e i n temperature. A l l c a t a l y s t s show t h r e e DSC e f f e c t s , i n t h e temperature ranges 50-220, 220-420 and 420-600°C, i n d i c a t i n g t h r e e groups o f a c i d s i t e s , d e s c r i b e d h e r e as weak, i n t e r m e d i a t e - s t r e n g t h and strong, respectively. For the cobalt-molybdate c a t a l y s t , t h e strong s i t e e f f e c t appears t o have a l o w e r s t r e n g t h t h a n f o r t h e two o t h e r c a t a l y s t s . The number o f s t r o n g a c i d s i t e s can be arranged i n decreasing o r d e r as f o l l o w s : Ni-W-alumina > Co-Mo-alumina 7 Ni-Mo-alumina. The number o f i n t e r m e d i a t e s t r e n g t h a c i d s i t e s can be arranged i n t h e o r d e r Ni-Mo-alumina 7 Co-Mo-alumina > Ni-W-alumina, whereas t h e s t r e n g t h o f t h i s t y p e i s i n t h e o r d e r Ni-W-alumina 7 Co-Mo-alumina 7 Ni-Mo-alumina. The s t r e n q t h and number o f t h e s t r o n g e s t a c i d s i t e s has been found t o c o r r e l a t e w e l l w i t h t h e e f f i c i e n c y o f these c a t a l y t s t o hydrodenitrogenate a n i l i n e . INTRODUCTION Aboul-Gheit e t a l . ( r e f . 1) developed t h e use o f t h e r m o a n a l y t i c a l t e c h n ques f o r d e t e r m i n i n g t h e a c i d s i t e s t r e n g t h d i s t r i b u t i o n (ASSD) i n s o l i d c a t a l y s t s by a p p l y i n g d i f f e r e n t i a l thermal a n a l y s i s (DTA) t o desorb and d e t e c t p r e s o r b e d p y r i d i n e from t h e a c i d s i t e s o f m e t a l - c o n t a i n i n g mordenite c a t a l y s t s . They used d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) t o e v a l u a t e q u a n t i t a t i v e l y ASSD b y d e t e r m i n i n g t h e d e s o r p t i o n e n t h a l p y , AH, o f p r e s o r b e d TEA ( r e f . 2). T h i s DSC t e c h n i q u e has been developed t o e x c l u d e DSC e f f e c t s due t o t h e c a t a l y t i c m a t e r i a l i t s e l f , so t h a t t h e e f f e c t s due o n l y t o t h e d e s o r p t i o n o f t h e base from t h e a c i d s i t e groups appear i n t h e DSC thermogram ( r e f . 3 ) . T h i s t e c h n i q u e has been termed t h e " n u l l i f y i n g " technique. The ASSD i n v a r i o u s c a t a l y s t s and c a t a l y t i c m a t e r i a l s has been s u c c e s s f u l l y e v a l u a t e d u s i n g t h e n u l l i f y i n g technique ( r e f s . 4 - 6 ) . T h i s t e c h n i q u e has a l s o been used t o f o l l o w t h e h e a t t r e a t m e n t o f ammonium-exchanged z e o l i t e s , i . e . ,
deammoniation and d e h y d r a t i o n
( r e f . 7 ) . Moreover, DSC has been used f o r t h e p r e c i s e d e t e r m i n a t i o n o f w a t e r i n z e o l i t e s i n c o m b i n a t i o n w i t h thermogravimetry ( r e f . 8 ) .
182
I n t h i s work, the A S S B i n t h r e e i m p o r t a n t h y d r o t r e a t i n g c a t a l y s t s were
evaluated u s i n g the n u l l i f y i n q DSC technique. C o r r e l a t i o n o f ASSDs i n these t h r e e c a t a l y s t s w i t h t h e h y d r o d e n i t r o g e n a t i o n of a n i l i n e i s r e p o r t e d . EXPERIMENTAL Preparation o f the catalysts The Co-Mo-alumina c a t a l y s t was prepared by impregnating gamma-alumina w i t h c o b a l t n i t r a t e and ammonium molybdate s o l u t i o n s such t h a t t h e f i n i s h e d c a t a l y s t contained 2% COO and 8% Moo3. The Ni-Mo-alumina c a t a l y s t was prepared by i m p r e g n a t i n g the alumina w i t h n i c k e l n i t r a t e and ammonium molybdate s o l u t i o n s such t h a t the f i n i s h e d cs'alyst
contained 2% N i O and 8% Moo3. The Ni-W-alumina
c a t a l y s t was prepared by impregnating t h e alumina w i t h n i c k e l n i t r a t e and ammonium t u n g s t a t e s o l u t i o n s such t h a t the f i n i s h e d c a t a l y s t contained 2% N i O and 8% W03. The c a t a l y s t s were then d r i e d o v e r n i g h t a t l l O ° C and c a l c i n e d a t 53OoC f o r 4 h. EvalGation o f ASSD i n t h e c a t a l y s t s by DSC The c a l c i n e d c a t a l y s t s were soaked i n excess o f TEA o v e r n i g h t , then decanted and d r i e d a t 8OoC f o r 1 h b e f o r e measurement b y DSC. I n the sample c e l l t h e aluminium c r u c i b l e contained ca. 10 mg o f a c c u r a t e l y weighed TEA-presorbing c a t a l y s t , whereas t h e r e f e r e n c e c e l l c o n t a i n e d an aluminium c r u c i b l e c o n t a i n i n g e x a c t l y the same w e i g h t
o f t h e TEA-free c a t a l y s t . D e t a i l s of t h e n u l l i f y i n g DSC
technique are g i v e n i n r e f . 3. A M e t t l e r DSC-30 u n i t (TA-3000 system) was used under t h e f o l l o w i n g c o n d i t i o n s : s t a r t temperature, 5OoC; r a t e , 20 K/nin; f i n a l temperature, 60OoC; p l o t , 10 cm; f u l l - s c a l e range, 25 mW; weight, 10 mg; and w i t h o u t u s i n g purge gas. A n i l i n e HDN The r e a c t i o n was c a r r i e d o u t i n a b a t c h a u t o c l a v e a t 4OO0C and 100 b a r pressure and w i t h
r e a c t i o n p e r i o d s from 1 t o 6 h. D e t a i l s o f t h e apparatus
and c o n d i t i o n s a r e g i v e n elsewhere ( r e f . 9 ) . N e g l i g i b l e amounts o f cyclohexane and cyclohexylamine were d i s t i n g u i s h e d i n t h e products, p a r t i c u l a r l y w i t h t h e Ni-W-a1 umina c a t a l y s t . RESULTS AND DISCUSSION The DSC method used i n t h i s work t o e v a l u a t e ASSD i n t h e c a t a l y s t s appears t o have the same features as t h e temperature-programmed d e s o r p t i o n (TPD) method, as i n b o t h methods d e s o r p t i o n o f a base from t h e s t r o n g e s t a c i d s i t e s takes
183
p l a c e a t t h e h i g h e s t temperatures. However, t h e DSC method, a f t e r m o d i f i c a t i o n t o t h e n u l l i f y i n g method, shows advantages o v e r t h e TPD method i n some r e s p e c t s . DSC uses v e r y small samples compared w i t h t h e TPD method, w h i c h excludes
d i f f u s i o n l i m i t a t i o n t h a t r e s u l t s i n o v e r l a p p i n g o f t h e d e s o r p t i o n peaks. Hence, most i n v e s t i g a t o r s who use TPD o f ammonia o b t a i n o n l y one peak i n t h e i r TPD s p e c t r a , whereas we o b t a i n e d t h r e e w e l l r e s o l v e d peaks i n ammonia d e s o r p t i o n i n t h e DSC system ( r e f s . 4 and 6 ) . The TPD method depends on mass d e t e c t i o n (mass s p e c t r o m e t r y o r GC)
, so
probe molecules t h a t a r e t h e r m a l l y decomposed
w i t h i n t h e temperature range used cannot be employed; however, i n t h e DSC method, t h e chemisorbed fragment o f t h e probe molecule w i l l n o t s i g n i f i c a n t l y a f f e c t t h e DSC system a f t e r d e t a c h i n g a hydrocarbon fragment f r o m i t , i . e . , t h e A H v a l u e c o r r e s p o n d i n g t o a c e r t a i n group o f a c i d s i t e s does n o t d i f f e r markedly from t h a t o f a whole probe molecule. The TPD method does n o t d i f f e r e n t i a t e between p h y s i c a l l y and c h e m i c a l l y adsorbed phases o f a base, whereas t h e DSC method d i f f e r e n t i a t e s them as endothermic and e x o t h e r m i c peaks, r e s p e c t i v e l y .
Moreover, t h e DSC d e t e c t i o n system i s much more s e n s i t i v e t h a n TPD d e t e c t i o n . The DSC u n i t i s m e r e l y an i n e x p e n s i v e p a r t o f a t h e r m o a n a l y t i c a l system, whereas t h e TPD method r e q u i r e s a separate r e a c t o r i n a d d i t i o n t o an expensive i d e a l detector-mass spectrometer.
100
200
300
400
500
600
Temper at u r ePC F i g . 1. DSC thermograms f o r TEA d e s o r p t i o n f r o m t h e a c i d s i t e s o f t h e h y d r ot r e a t ing ca t a 1y s t s
.
184
F i g . 1 shows t h e DSC d e s o r p t i o n thermograms o f TEA from t h e a c i d s i t e s o f t h e t h r e e h y d r o t r e a t i n g c a t a l y s t s s t u d i e d . The thermograms r e v e a l t h r e e groups of a c i d s i t e s ( t h r e e DSC e f f e c t s ) d i f f e r i n g i n t h e i r s t r e n g t h ( t h e y appear i n t h r e e temperature r e g i o n s ) i n a l l c a t a l y s t s . These a c i d s i t e groups a r e described as weak,
i n t e r m e d i a t e - s t r e n g t h and s t r o n g , r e s p e c t i v e l y . A l l o f t h e
peaks appearing i n t h e thermograms a r e exothermic ( F i g . l ) , i n d i c a t i n g chemis o r p t i o n o n l y and t h a t t h e heat t r e a t m e n t o f t h e c a t a l y s t s p r i o r t o t h e DSC measurements is s a t is f a c t o r y
.
The a c i d s t r e n g t h of t h e c a t a l y s t s can be g e n e r a l l y c o r r e l a t e d according t o t h e e v o l u t i o n o f t h e most s t r o n g l y adsorbed f r a c t i o n o f t h e probe molecule. Hence, t h e highest-temperature DSC peak i s a h e l p f u l parameter f o r comparing t h e o v e r a l l a c i d i t y s t r e n g t h s o f t h e c a t a l y s t s . Accordingly,
i t may b e e v i d e n t
from F i g . 1 t h a t t h e Ni-W-alumina c a t a l y s t possesses t h e s t r o n g e s t a c i d i t y (peak maximum a t 562OC)
, and
Co-Mo-alumina
possesses t h e weakest a c i d i t y
(peak maximum a t 533OC). Although t h e Ni-Mo-alumina c a t a l y s t has i t s peak maximum a t approximately t h e same temperature
as t h e Ni-W-alumina
high-
temperature peak, t h e p a r t o f t h e peak t h a t has t o appear beyond 6OO0C f o r t h e Ni-W-alumina c a t a l y s t should be l a r g e r than t h a t f o r t h e Ni-Mo-alumina c a t a l y s t ( t h e maximum temperature a v a i l a b l e w i t h t h e DSC u n i t used i s 600OC). Table 1 shows t h a t t h e AH values f o r t h i s high-temperature peak a r e 212, 160 and 100 J/g f o r t h e Ni-W-, Co-Mo- and Ni-Mo-containing c a t a l y s t s , r e s p e c t i v e l y . TABLE 1 Peak temperatures and DSC e n t h a l p i e s f o r t h e e f f e c t s o b t a i n e d f o r TEA d e s o r p t i o n from t h e a c i d s i t e s o f the h y d r o t r e a t i n g c a t a l y s t s Peak
Co-Mo-alumina c a t a l y s t
Ni-Mo-alumina c a t a l y s t
Ni-W-alumina c a t a l y s t
Peak temp.
AH
Peak temp.
AH
Peak temp.
AH
(OC)
(J/g)
(OC)
(J/g)
(OC)
(J/g)
122
55
148
48
137
39
No.
1
2
332
132
325
153
340
128
3'
533
147
56 1
55
562
81
3*'
533
160
561
100
562
2 12
'Uncorrected **Values
values o f AH.
c o r r e c t e d f o r peak completion.
As s t r o n g a c i d s i t e s are e s s e n t i a l f o r C-C bond f i s s i o n , i t i s l o g i c a l t o
c o r r e l a t e a n i l i n e HDN on the c a t a l y s t s s t u d i e d w i t h t h e i r s t r o n g a c i d s i t e s .
185 F i g . 2 shows t h a t t h e HDN a c t i v i t y o f t h e c a t a l y s t s can be arranged i n t h e o r d e r Ni-W-alumina > Ni-Mo-alumina > Co-Mo-alumina, which i s t o a g r e a t e x t e n t c o m p a t i b l e w i t h t h e above-mentioned DSC f i n d i n g s . I t i s a l s o e v i d e n t t h a t t h e g r e a t e r s t r e n g t h o f s t r o n g a c i d s i t e s i n t h e Ni-Mo-alumina c a t a l y s t overcompensates t h e 1a r g e r number o f t h e r e l a t i v e l y waeker s t r o n g a c i d s i t e s i n t h e Co-Mo-alumina c a t a l y s t (peak maximum a t 56loC and AH o f 100 J / g v s .
533OC and 160 J/g, r e s p e c t i v e l y ) . It i s known t h a t N i - c o n t a i n i n g c a t a l y s t s possess h i g h e r h y d r o g e n a t i o n
e f f i c i e n c i e s t h a n Co-containing c a t a l y s t s ( r e f . 101. However, h y d r o g e n a t i o n i s a p r i m a r y s t e p f o r HDN ( r e f s . 11-13) of n i t r o g e n - c o n t a i n i n g r i n g molecules, b u t t h e a c i d i t y o f t h e c a t a l y t i c s u p p o r t p l a y s an i m p o r t a n t r o l e i n enhancing t h e o v e r a l l r e a c t i o n ( r e f . 1 4 ) . As a l l t h e c a t a l y s t s s t u d i e d were p r e p a r e d so as t o c o n t a i n i d e n t i c a l s u p p o r t s , d i f f e r e n c e s i n t h e ASSDs i n these
c a t a l y s t s should be due m a i n l y t o t h e metal o x i d e combinations. N e v e r t h e l e s s , HDN o f a n i l i n e does n o t appear t o be p r i m a r i l y dependent on a h y d r o g e n a t i o n
s t e p as w i t h o t h e r compounds possessing h e t e r o c y c l i c n i t r o g e n - c o n t a i n i n g r i n g s such as p y r i d i n e o r q u i n o l i n e . Hence, a n i l i n e HDN depends c h i e f l y on t h e a c i d strength o f the catalysts.
I
0
1
I
I
1
2
3
c
1
I
5
6
REACTION PERIOD, h F i g . 2. A n i l i n e HDN e f f i c i e n c y 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 s .
186
The i n t e r m e d i a t e - s t r e n g t h a c i d s i t e s (DSC peak No. 2 i n T a b l e 1) does n o t appear t o c o r r e l a t e w e l l w i t h a n i l i n e HDN. F i g . 1 and T a b l e 1 show t h a t t h e s e a c i d s i t e s can be arranged a c c o r d i n g t o t h e i r s t r e n g t h i n t h e o r d e r Ni-Walumina 7 Co-Mo-alumina 7 Ni-Mo-alumina.Although
t h e Ni-Mo-alumina c a t a l y s t
has t h e weakest i n t e r m e d i a t e - s t r e n g t h a c i d s i t e s , i t i s more a c t i v e t h a n t h e Co-Mo-alumina
c a t a l y s t . Also, t h e number o f t h e s e a c i d s i t e s ( T a b l e 1 ) does
n o t c o r r e l a t e w i t h t h e HDN a c t i v i t y . The Ni-Mo-alumina c a t a l y s t has t h e l a r g e s t number o f t h i s t y p e o f a c i d s i t e s , whereas Ni-W-alumina,
which i s
t h e most a c t i v e , possesses t h e s m a l l e s t number. Weak a c i d s i t e s which a r e a t t r i b u t e d t o hydrogen bonding do n o t seem t o have a r o l e i n t h e HDN a c t i v i t y f o r a n i l i n e . ACKNOIdLEDGEMENT P r o f e s s o r A.K.
Aboul-Gheit thanks t h e E g y p t i a n Petroleum Research I n s t i t u t e ,
Nasr City, C a i r o , Egypt, f o r
l e a v e o f absence.
REFERENCES
1 A.K. Aboul-Gheit, M.A. A l - H a j j a j i , M.F. Menoufy and S.M. Abdel-Hamid, Anal L e t t . . 19 (19861 529-536. 2 A.K. Aboul-Gheit and M.A. A l - H a j j a j i , Anal. L e t t . , 20 (1987) 553-559. 3 A.K. Aboul-Gheit, M.A. A l - H a j j a j i and A.M. Suman, Thermochim. Acta, 118 (1987) 9-16. 4 A.K. Aboul-Gheit, Thermochim. Acta, 132 (1988) 257-264. 5 A.K. Aboul-Gheit, J . C a t a l . , 113 (1988) 490-496. 6 A.K. Aboul-Gheit and A.M. Sunnnan, Thermochim. Acta, i n p r e s s . 7 A.K. Aboul-Gheit, Thermochim. Acta, 129 (1988) 301. 8 A.K. Aboul-Gheit, M.A. A l - H a j j a j i , A.M. Sunnnan and S.M. Abdel-Hamid, Thermochim. Acta, 126 (1988) 397. 9 A.K. Aboul-Gheit and I . K . Abdou, J . I n s t . P e t r o l . (London), 58 (1972) 305. 10 A.K. Aboul-Gheit, Appl. C a t a l . , 7 (1985) 39. 11 A.K. Aboul-Gheit and I . K . Abdou, J . I n s t . P e t r o l . (London), 59 (1973) 188. 12 A.K. Aboul-Gheit, Can. J . Chem., 53 (1975) 2575. 13 A.K. Aboul-Gheit, Rev. I n s t . Mex. P e t r o l . , 11 ( 3 ) (1979) 72. 1 4 A.K. Aboul-Gheit, P r e p r i n t s , Am. Chem. SOC., D i v . P e t r o l . Chem., 32 ( 2 ) (1987) 278.
M.L. Occelli and R.G. Anthony (Editors),Advances in Hydrotreating Catalysts Amsterdam - Printed in The Netherlands 0 1989 Elsevier Science Publishers B.V.,
187
SUPPORTED Go-Mo THIN FILM SULPHIDE CATALYSTS FOR HYDRODESULPHURIZATION. 1. XPS STUDIES OF THE EFFECTS OF REACTANT PRESSURE
'N.s. MCINTYRE, 'T.c. CHAN, 'P.A. SPEVACK AND Z ~ . BROWN ~ . Surface Science Western, Room 6 , Natural Sciences Centre, The University o f Western Ontario, London, Ontario, Canada N6A 587 Energy Research Laboratory, CANMET, Energy, Mines and Resources Canada, Ottawa, Ontario, Canada K1A OG1
ABSTRACT Supported cobalt-molybdenum catalyst surfaces have been prepared in thin film form for use in the study of hydrodesulphurization (HDS) mechanisms. The films consist of overlayed layers of cobalt and molybdenum (-5 nm thick) on an alumina substrate which are calcined, reduced and sulphided before being reacted at 230-350°C with a circulating thiophene hydrogen gas mixture in a reactor directly attached to an X-ray Photoelectron Spectrometer (XPS). The sulphided surfaces of certain of the thin film compositions were found to exhibit detectable HDS activity even though their surface areas were very low. Increased pressure of the C,H,S/H, gas resulted in increased activity. High resolution XPS studies revealed the presence of at least two other molybdenumsulphur species in addition to MoS,. These are ascribed to cationic and anionic sites on the Co-Mo-S surface phase presumed to have formed. HDS activity is found to correlate well with the measured concentration of these sites. Increased pressure results in an increase i n these sites, as well as an increase in the sulphidation of the alumina support.
INTRODUCTION The surface structure of supported cobalt-molybdenum sulphide catalysts h a s been the subject of extensive spectroscopic investigation.
Of particular
interest has been the promoting role of cobalt in creating active exchange sites on the molybdenum sulphide crystal which can greatly enhance the reduction of organic sulphides. has
been advanced largely
The concept of a discrete Co-Mo-S structure
through
the experimental work of Topsoe and co-
workers ( 1 - 2 ) who have used Mossbauer spectroscopy to identify a separate cobalt-containing phase within the supported bulk catalyst structure. Surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS or ESCA) have also been used extensively to probe the relationship between reactivity and structure ( 3 - 1 1 ) .
However, less structural information
has been forthcoming, due partly to the spectral resolution limits of the technique and to the mixture of surface phases normally encountered on a process catalyst surface.
188 This paper describes one approach to understanding the structural effects in the supported Co-Mo-S catalyst system. as
The supported phase(s) are exposed
a relatively homogeneous thin film whose well-characterized surface can be
exposed
to
hydrotreating
reactants, and
subsequently
analyzed
by
high
spectral resolution XPS. The development of catalyst structures in thin film form can be very advantageous
for fundamental
catalysis
studies.
Surface
spectroscopic
techniques such as XPS and Auger Electron Spectroscopy are somewhat limited in their capacities to fully analyze structures on pelletized process catalysts. Such surfaces are often highly insulating and the resultant peak shape distortion identify
during electron spectroscopic studies makes
subtle changes
in
lineshape which
could
it
suggest
difficult
to
a
in
change
electronic or molecular structure. There are other potential advantages to the use of supported thin films as model catalysts.
Precursor oxide composition may be more readily changed
and the relative surface composition of two or more active components may be As well, the orientation of
controlled more reliably when in thin film form.
each active phase with respect to one another, to the support phase and to the outer surface can be monitored and controlled. It
is, however, crucial
that the films produced
exhibit measurable
catalytic activity or no correlation can be made between surface structures observed and catalytic properties. surface
area
to
the
Since the films studied present minimal
reactants, the
structures must
have
high
surface
reactivity and in addition, very sensitive methods for monitoring the product gas are required. This present work describes the study of two different compositions of alumina supported cobalt-molybdenum thin film structures which are reacted under hydrodesulphurization (HDS) conditions with thiophene. measurable
quantities
hydrocarbons.
of
thiophene
to
be
converted
The films cause to
four-membered
The conversion efficiency is found to increase with reaction
pressure and concomitant structural changes in the molybdenum sulphur bonding can be identified. EXPERIMENTAL Preparation of Thin Film Oxide Precursors Thin films were prepared by deposition and calcination on an aluminum oxide support which had been thermally grown on a circular aluminum metal disk.
These disks
commercial by
(0.5 mm
grade Alcan
heating in air at
thick, 14 nun diameter) of "bright rolled",
aluminum, were sonicated
200°C for 30 minutes.
in methanol
The resultant
and oxidized
surface
oxide is
189 4-5
nm. in thickness and appears to consist of an amorphous mixture of
aluminum oxides and hydroxides, based on XPS and AES studies. Two series (A and C) of cobalt-molybdenum (CM) thin film formulations were prepared for this work.
Cobalt and molybdenum metals were deposited
sequentially on the alumina substrates using an Ion Tech sputter/deposition system equipped with a saddle field ion source. It was operated at 8 keV. and 5 mA.
using an argon ion beam.
Molybdenum foil (purity:
9 9 . 9 7 % , Alfa
products) and cobalt foil (purity: 9 9 . 9 9 6 4 % . Alfa products) were used as target materials.
The targets were each sputter cleaned in the chamber prior
to use. The sample disks rest on rotating holders providing even deposition. The deposited layers were then calcined by heating to 300°C in air (series C)
or to 500°C in 10-1 Pa oxygen (series A)
oxides.
obtain a variety of precursor
to
AES depth profiles obtained with a Physical Electronics ( P H I ) 600
Scanning
Auger
Microprobe
were
used
to
characterize
resultant
layer
structures. In Figures l(a) and (b) the near surface compositions of a series A thin film structure is shown before and after calcining. in a net outward migration of the cobalt component,
so
Calcining results
that both molybdenum
and cobalt concentrations were contained within the outer 3 nm.
A somewhat
thicker CM thin film formulation (series C) is shown in Figures 2 ( a ) and ( h ) , before
and after calcination.
In this
series, the
support phase was
deliberately distanced from the near surface phase where greatest overlays of cobalt and molybdenum occurred.
In some series C specimens, the near surface
cobalt concentration was predominant, such as the example in Figure 2 ; in other cases, the cobalt and molybdenum concentrations were comparable.
The
chemical and structural information of these precursor oxides, as revealed by
XPS, is discussed below. HDS Reactions of the Thin Films
The thin film oxide formulations were exposed to typical hydrotreating conditions in a closed-cycle mini reactor.
This reactor, described in detail
in a companion paper (12) is capable of operation at temperatures up to 5 0 0 ° C and in a pressure region from 0.1 circulating
through
the
reactor
can
1 MPa. be
chromatograph pneumatic injection system.
Reactant and product gases
sampled
"on
line"
by
a
gas
This system allows reactions at
elevated pressures to be monitored on a continuous fashion, if desired. The Hewlett-Packard 5890 A gas chromatograph, controlled by a HP 3393A integrator, is equipped with a 2 4 ' x 1/8" S S packed column. The liquid phase is 20% BMEA on a support of Chromosorb P , AW 6 0 / 8 0
mesh which allows separation of
hydrogen, hydrogen sulphide, thiophene reactants and the products cis- and t r a n s - 2-butene, and
butane. Detection of H,S and 1-butene was
prevented by
190
SPUTTER TIME (rnin)
Figure 1. Depth profiles obtained by Auger Electron Spectroscopy ( A E S ) o f a CM series A film before (a) and after (b) calcination. The Auger results were obtained on a 100 x 100 p m area using a 3 KeV argon ion beam rastered over a 2x2 mm area. The sputter rate for A1,0, under our conditions is 4 nm/rnin. Auger intensities, corrected for quantitative yield are plotted f o r Mo, Co, Al, and oxygen.
191
100 90
-
After Calcination
30P 3 I \
20
SPUTTER TIME (rnin.)
Figure 2 . AES depth p r o f i l e s o f a CM s e r i e s C f i l m before ( a ) and a f t e r (b) calcination. (a) Series A , calcined a t t o r r , 500'C and ( b ) s e r i e s C , c a l c i n e d a t 1 atm, 300°C.
192 peak overlap.
After quantitative calibration of this column, the thiophene
conversion efficiency ( E ) is determined from the expression: E(%)
=
C(cis-2-butene) + C(trans-2-butene) + C(butane) x 100 C(cis-2-butene) + C(trans-2-butene) + C(butane) + C(thiophene)
where C represents the area % of each constituent. The mini-reactor can be directly attached to the transfer chamber of a XPS system, and this allows a specimen to be transferred from the reactor to t h e XPS analysis system without exposing it to air.
In fact, the maximum pressure
t o which the sample was exposed during transfer is 5 x
Pa.
A study using a typical precursor oxide thin film specimen involved tlie following sequence:
(1) Placement of the specimen on a special shelf in the mini reactor using
R
remote manipulator. (2) Reduction of the specimen surface using hydrogen flowing at 50 ml min-' for 1/2 hour with the reactor at reaction temperature. ( 3 ) Sulphidation using 2% H,S/H,
flowing at 50 ml min-' for 1/2 hour with t h e
reactor a t reaction temperature. (4) Purging of H,S using 1 atm. H,
flowing at 50 ml min-' for 1/2 hour with
the reactor at reaction temperature.
HDS reaction using H, with 1%
thiophene were carried out at a variety of pressures and temperatures. During reaction the equilibrium composition could be sampled at frequent intervals without significant perturbation of the gas composition.
(5) At the end of the reaction time (typically 6 hours) the reaction chamber was evacuated and the gate valve opened into the transfer chamber of the XPS system.
X-ray Photoelectron Spectroscopy X-ray
photoelectron
spectra
were
Laboratories SSX-100 spectrometer. calibrated
to give an
Au(4f7/2)
taken
with
The energy scale
binding
energy
a
of
Surface
Science
the instrument was
position of
8 3 . 9 eV.
The
193 energy dispersion was set to give a difference of 857.1 eV between the Cu(2p3/2)
line
and
the
Cu(3p3/2)
line.
High
spectral resolution was
frequently used to analyze the fine structure of the elemental core lines detected on the thin film surfaces. Under the highest resolution conditions a ‘0
(3d5/2)
half-width of 0.55 eV was obtained
crystal of MoS,.
for a spectrum of basal plane
The lateral homogeneity of the thin film CM catalyst surface
was first investigated using micro-area XPS measurements at 1000 pm and 300 pin spot
sizes.
The
cobalt, molybdenum and
aluminum
surface compositions
determined at different positions of a single specimen did not vary by more than 10% relative to each other. Extensive
use was
made of XPS quantitative
measurements to assist in
the determination of the structure of reactive CM thin film surfaces. particular,
the
integrated
intensities of
Co(3p)
A1(2s),
In
S(2p) Mo(3d)
and O ( 1 s ) lines were used and corrected for photoelectron cross-section with the Scofield parameters (13) and for differences in electron mean free path correction with an E-0.7 correction.
Such corrections have been used in the
past in our laboratory to give O / A ~ ratios of 1.5 for A1,0, and S/Mo of 2.0 for MoS, (14). XPS lineshapes were resolved with a least-squares fitting program using
linewidths constrained to values previously observed
for the particular
species being resolved.
RESULTS The surfaces of the precursor oxide thin films were analyzed by XPS and their Co(2p) and Mo(3d) spectra are shown in Figures 3 and 4 respectively. On the series A surfaces, the Co(2p)
spectral shape and position (Figure 3(a))
suggest a cobalt inolybdate structure with binding energy (B.E.) of 780.3 eV on the basis (15).
of previous XPS studies of cobalt-molybdenum oxide surfaces
By contrast, the cobalt on the series C surface (Figure 3(b))
can be
clearly identified as Co,O, with a binding energy (B.E.) of 779.3 eV (15).
In
194
c02p
a 10-3torr O2,5O0'C
-='
CoMo04\
2 z 3
.:-.
-........... . . .
-*
*.'
2 -
.L
u
- .-:iatm.u2.300C; ~
I.............. .......... -.-.. ..
.i..
.. ..
............. ..- . :. ... ..........='.. -... - \ ...... . ... ..- . ....... ..... *.
"
.....
'oC, .
metal
Cn.0.
........*..."..... ......__..
: 6
-....._ ,..-_.._ 772.0
807.0
BINDING ENERGY (eV)
Figure 3. Co(2p) photoelectron spectra o f two series of precursor oxides used in the formation of thin film catalyst surfaces.
195
,...........:. 240
238
236
234
232
230
228
BINDING ENERGY (eV)
Figure 4 . Mo(3d) photoelectron spectra of two series of precursor oxides. (a) series A and (b) series C.
196 Figure 4(a),
the molybdenum (3d) on the series A surface is identified in
several forms:
CoMoO,, Mo'~,
molybdenum oxide (MOO,)
(15).
MOO,
and a
lower stoichiometric form of
In series C (see Figure 4(b))
the molybdenum
appears to be mainly bonded as a Mot'. Each precursor oxide thin film was
reduced, sulphided, and reacted at
two different pressures and temperatures of 350"C, 275°C and 230°C.
At 275
350°C the pressures used were 0.1 and 0.3 MPa while at 230"C, 0.1 and
and
0.7 MPa were used. Since the total amount of thiophene remained constant in each experiment the pressure increase in each case caused a common dilution of the thiophene present.
The percentage of thiophene converted in each run had to he
corrected for the amount converted on the iron oxide walls of the stainless steel
reactor
equalled
itself.
the rates
This "blank"
observed for
conversion
the thin film
rate in
some cases
catalysts surface and this,
at present, limits the sensitivity of the technique.
However in a number of
repeated experiments significant thiophene conversion could be attributed to the
thin film
catalyst itself.
This
was manifested not o n l y by the
increase in the total conversion rate, but also
in the change in the
composition
Conversion
of the
gaseous
products
produced.
on the
CM
thin film catalyst surface results in increased production of butanes compared to the gas compositions produced on the reactor wall surfaces.
Series A
specimens showed thiophene conversion rates of 0.6% and 1.0% at conditions of 230"C, 0.7 MPa
and
350"C, 0.3 MPa, respectively.
In most cases, the
percentage yields are determined after 1, 3 and 6 hours of reaction. reaction rates are higher than their equivalent low pressure counterparts.
(0.1 MPa)
The most significant conversion rates for Series A and C
occurred at 275°C and 0.3 MPa. Series A
The
Conversion rates rose from
between 3 and 6 hours
0.9 to 2.2%
for
reaction time, while Series C showed
increases of 0.1 to 1.1% over 6 hours.
Conversion was barely detectable a t
the lower pressure runs (0.1 MPa, 275"C), for both series. A later paper will
197 describe temperature and composition effects on activity. Molybdenum, cobalt and sulphur core line photoelectron spectra for series following reactions at 350°C at 0.1 and 0 . 3 MPa are compared
A formulations
in Figures 5 , 6 and 7. Peak centroid binding energies obtained for these and all
summarized in Table 1.
other species in the study are
the fitted
Mo(3d)
spectrum
I n Figure 5(a)
is seen to consist of two doublets.
Both of
these are believed to result from Mo-S bonding; essentially no peaks due to molybdenum
oxides
(MOO, or MOO,) can be
identified. Of the two molybdenum
species detected, the predominant one with a binding energy (B.E.) o f 229.0 2 0.1 eV can be identified with a MoS,
type structure on the basis of this
present work and previous studies ( 6 ) of CM catalysts. is also detected at 228.4 ? 0.1 eV. after hydrotreating at 0.3 MPa.
A second Mo(3d) peak
This peak is seen to be more prominent Other minor peaks above 229.0 eV also
contribute to the lineshape. A l l of these species above or below 229.0 eV can be thought of as representing sub- or super-stoichiometric phases caused by defects in the MoS, structure which would induce a change in partial charge on the molybdenum atom.
These may also be thought of as anionic or cationic
vacancies at a molybdenum site. Figures 6 (a) and (b) show Co(2p) spectra for the same reacted series A CM thin films.
A narrow nearly symmetric peak characteristic of a cobalt-
sulphide structure ( 6 ) is seen with no evidence of any oxide peaks present. The only
distinguishing
the high binding
feature is the satellite
energy side of the main peak.
structure about 4 eV on
The spectrum in Figure 6(a)
(0.1 MPa) is in excellent accord with that identified by Topsoe (1) and coworkers with Co,S,. possibly
be
stoichiometry.
The intensity of the satellite structure in 6(b) could
identified
with
a
cobalt
sulphide
of
slightly
It is worth emphasizing that while the Mo(3d)
different
lines change
shape quite dramatically with the pressure increase, no such change occurs with the Co(2p) lineshapes.
This militates against any explanations of the
Mo(3d) lineshape changes as being due to shifts in the Fermi level.
198
1 Y. thiophene/H2
P Z 3
s
v)
IZ 3
0 V
236
234
232
230
228
226
224
222
BINDING ENERGY (ev)
Figure 5 . Mo(3d) photoelectron spectra for Series A CM thin films following reaction in 2% H,S/H, at 350°C for 6 hours. (a) Pressure o f 1 atm. (0.1 MPa) (b) Pressure o f 3 atm. (0.3 MPa).
199
1x thiophene/ H,
COCJSE
a) 350'c ,1 atm
t
v)
I-z
$
I..
.. ........... .......-.:- ...
1
I
~
.-. .
.......
.............../...-- .-
. . r
a
.
.-.*.*..
f.
b) 35Oc.3atm
..... ....... .... ........*.......:...-=-
......................r..*........----* ..
807.0
BINDING ENERGY (EV)
-............ 772.0
Figure 6. Co(2p) photoelectron spectra for Series A CM thin films following reaction in 2% H,S/H, at 350°C for 6 hours. (a) Pressure of 1 atm. (0.1 MPa) (b) Pressure of 3 atm. (0.3 MPa).
200
2 Z 3 8
v,
53 8
166
165
164
I63
162
161
160
BINDING ENERGY (eV)
Figure 7.
S(2p) spectra for Series A CM thin films following reaction in 2% (a) Pressure of 1 atm. (0.1 MPa) (b) Pressure
H,S/H2 at 350°C for 6 hours. of 3 arm. (0.3 MPa).
201
TABLE 1
XPS binding energies measured in this study ( 2 0 . 1 eV)” SPECIES
Mo3d5/2
S2P
MOO,. MOO, Mo’ MOO, MoS, MoS, MOS,’,
228.5b 229.7b 231.6b 232.6b 228.4 229.0 229.5
__
coo
COQS8 CoMoO,
Co
_-_ __
161.5 161.9 163.5
--
__
--
__
777.8b 778.0 780.3b 779.3
162.3
231.9b
c03 0 4
a
2p3/2
Reference level Au 4€7/2 = 8 3 . 9 eV Further supporting information is provided in Reference (15)
The S(2p)
spectra shown in Figures 7 (a) and (b) reveal essentially
overlayed S(2p) doublets believed to be associated with
Cogs8
and MoS,. The
S(2p3/2) line centroid positions at 161.9 ? 0.1 eV for MoS, and 162.3 ? 0.1 eV for Cogs8 were determined from work on pure phases in this laboratory and they agree well with other published data (1).
A minor lower binding energy
doublet at 161.5 eV is tentatively identified as molybdenum sulfide substoichiometric phase in Figure 7(a).
At higher pressure this component
clearly increases in intensity.
This and the more electropositive species at
163.5 eV are identified with
sulphide vacancies which are respectively
anionic or cationic. A pressure effect on reactivity was most clearly observed at 275°C and concomitant changes in the Mo(3d) Figures 8(a)
and S(2p) spectra were also noted.
and (b) the changes in
Mo(3d)
In
lineshapes are seen to be
parallel to those in Figure 5 , however sub- and super-stoichiometric species at 228.4 eV and 229.5 eV are much more prominent than at 350°C.
equivalent S(2p) spectra
in Figures 9(a) and (b), the
additional
In the sulphide
202
....
./
.d
i
234
232
230
228
226
224
222
BINDING ENERGY (eV)
Figure 8 . Mo(3d) s p e c t r a f o r s e r i e s A CM t h i n f i l m s t r e a t e d a s t h o s e shown in Figures 5 - 7 b u t r e a c t e d a t 275°C. ( a ) P r e s s u r e o f 1 atm. ( 0 . 1 MPa) ( b ) P r e s s u r e o f 3 atm. ( 0 . 3 MPa).
203
s2p
1 % thiophene/Hq
a)
166
./ 7, ,
1 atrn. 275%
165
164
163
162
161
160
BINDING ENERGY (eV)
Figure 9 . S ( 2 p ) s p e c t r a f o r S e r i e s A CM t h i n f i l m s t r e a t e d a s shown i n Figures 5 - 7 but reacted a t 2 7 5 ° C . ( a ) Pressure of 1 a t m . (0.1 MPa) (b) Pressure o f 3 atrn. ( 0 . 3 MPa).
204
species at 161.5 eV is also more prominent than at higher temperature and it increases with pressure.
Another sulphur species, at 163.5 eV is again
identified with cationic vacancies.
HDS reactions at 2 3 0 ° C also showed some effects of pressure on reactivity and XPS line structure.
The major peak shown in Figure 10 assigned to sub-
stoichiometric MoS, may, in fact, also contain some oxide contribution, in contrast to the Mo(3d) spectra taken for high temperature reactions where the absence of M o - 0 bonding can be assured. Quantitative analysis of cobalt, molybdenum, aluminum, oxygen and sulphur species also help to establish surface structures through the measurement of stoichiometry.
I n Table 2 a quantitative assessment of the composition of
the outermost 1-2 nm is shown. composition ratios
with depth is
I n using such measurements a homogeneous
necessary to develop any confidence that the
observed are associated with chemical stoichiometry. Some indication
17- t hiophene/H2 7atm. 230.c
m k-
z
3
8
.....”........’
. .i .
, ,,,
Figure 10. Mo(3d) spectrum for series A thin film following reaction in 2 % H,S/H, at 2 3 0 ” 7 atm. (0.7 MPa) for 6 hours.
205
TABLE 2
Quantitative
surface
analyses
of
cobalt
molybdenum
as
a
f u n c t i o n of a t r e a t m e n t -
%
COMPOSITION (ATOMIC) SERIES A
Co Mo A1 C 0 S Ca 1c i n e d Reacted a t Reacted a t Reacted a t Reacted a t Reacted a t Reacted a t
350"C, 350°C, 275"C, 275"C, 230"C, 230"C,
14.1 10.4 12.5 13.0 12.1 16.9 14.6
0 . 1 MPa 0 . 3 MPa 0 . 1 MPa 0 . 3 MPa 0 . 1 MPa 0 . 7 MPa
11.1 9.0 9.4 8.4 7.5 9.1 8.6
8.2 12.2 10.2 9.8 13.3 3.4 4.5
26.2 23.6 22.9 25.0 21.0 27.8
40.5 15.7 9.6 15.9 16.2 10.7 11.5
31.8
-29.1 35.3 27.9 29.9 32.1 29.3
SERIES C
Co Mo A1 C 0 s Calcined Reacted a t Reacted a t Reacted a t Reacted a t Reacted a t Reacted a t
350"C, 350"C, 275"C, 275"C, 230°C, 230"C,
29.2 28.9 30.8 30.4 29.8
0 . 1 MPa 0 . 3 MPa 0 . 1 MPa 0 . 3 MPa 0 . 1 MPa 0 . 7 MPa
5.0 7.5 4.9 2.0 2.7
N.D. N.D. N.D. N.D. N.D.
18.3 20.6 22.7 24.5 23.5
the
Auger
47.5
1.9 1.6 1.7 2.5
-41.1 40.0 41.3 41.2
-
of
stoichiometry
precursor
oxides
c a n be which
obtained show
from
that,
before
HDS
depth
reaction
profiles the
of
the
cobalt
and
molybdenum a r e r e a s o n a b l y w e l l d i s t r i b u t e d t o g e t h e r and t h a t t h e y e s s e n t i a l l y c o v e r t h e aluminum oxide phase. A q u a n t i t a t i v e measure of t h e approximate S/Co r a t i o c a n b e deduced froin
the
series
C
film
sulphided
at
0.2
MPa
and 3 5 0 ° C .
In
this
case,
only
molybdenum and c o b a l t s u l p h i d e s a r e d e t e c t e d i n t h e p h o t o e l e c t r o n s p e c t r a w i t h t h e c o b a l t b e i n g t h e predominant p h a s e .
The molybdenum s u l p h i d e a p p e a r s t o
be e s s e n t i a l l y MoS,
as i n S e r i e s A ( s e e F i g u r e 5 ) .
detected.
r a t i o c a l c u l a t e d ( a f t e r c o r r e c t i n g f o r s u l p h u r bonded t o
The S / C o
Very l i t t l e oxygen i s
molybdenum) i s 0 . 9 , and t h i s i s i n good agreement w i t h t h e r a t i o e x p e c t e d f o r Cogs,.
For t h e e q u i v a l e n t Series A s u l p h i d e d s u r f a c e , a g a i n a v e r y low oxygen
c o n c e n t r a t i o n i s d e t e c t e d and a l l oxygen i s assumed t o be
bonded t o aluminum,
206
However, since the O/Al ratio is only 1.3, a small amount of aluminum may have sulphided.
After taking account of this sulphur and that bonded to
cobalt as Cogs,, the remaining sulphur gives a S/Mo ratio of 1.9. At 0.3 MPa and 350°C the O/Al
ratio has decreased to 0.95, showing that increased
sulphidation of the aluminum oxide occurs at higher HDS pressure. conducted
with
pure
A1,0,
films
under
the
same
Experiments
conditions
show
no
sulphidation; the sulphidation only appears to occur when there is a contact between sulphided cobalt and aluminum oxide
Assuming use of stoichiometric
quantities of sulphur to form appropriate amounts of A12S3 and Co,Ss, the amount of sulphur remaining gives a S/Mo rat
o
of 1.8. The overall S/Mo ratio
thus supports the observation of sub-stoichiometric species in the Mo(3d) and S(2p)
spectra.
This type of compositional "balance" has been used on other
thicker molybdenum-cobalt films to
demonstrate a
reduction in net S/Mo
stoichiometry under HDS conditions. At 275°C and 0.1 MPa the series C spectra show that the S/Co ratio has changed to 1 . 2 .
This information allows the S/Mo ratio in the equivalent
series A film to be deduced at 1.5; this is considerably lower than the ratio of 1.9 at 350°C. At 275°C and 0.3 MPa, after correcting for sulphur bonded to the same stoichiometry to cobalt and to the non-oxidized aluminum (A12S3), a S/Mo ratio of 1.55 is obtained.
This suggests, in the case of the 275°C
reactions, there is a clear correlation between a lower S/Mo ratio and the observation of significant sub-stoichiometric S(2p) and Mo(3d) peaks compared to 350°C. An increase in pressure at 275"C, however does not appear to cause a decrease in S/Mo ratio even though there is an increase in the observed substoichiometric Mo(3d) and S(2p) peaks. At 230°C it is more difficult to arrive at quantitative rationalization because of the absence of baseline data for a nearly pure cobalt phase (Series C).
As well, more oxygen is present in the sulphided films that can be
accounted for simply as aluminum oxide.
207 DISCUSSION The studies described here represent one of the first known attempts to model, in
thin
film form, interactive supported phases
molybdenum sulphides used in HDS reactions.
of
cobalt
and
Several aspects are noteworthy:
catalytic HDS activity has been confirmed on a film of very low surface area, reactant pressure affects the HDS activity of the thin film, and some of these are supported by structural alterations observed in the photoelectron core line spectra. The detection their
use as
of HDS
models
The observed levels of rate of area.
activity in such films is of course pivotal to
to correlate composition and structure to activity. activity
are quite low, partly because of the
low contact time, and low surface
circulation and the resultant
In an earlier
publication, we reported
thin films maintained
in a static
hydrogen/thiophene mixture
(16).
high
the activity of similar CM
pressure vessel
The reduction
in contact with a
of thiophene
to four-
membered hydrocarbons was, in such a case, much higher because of the increased contact time. The relatively high specific activity of these films can be related, in part, to the high level to which they are able to be sulphided.
It appears
that conventional supported catalysts have not been able to be sulphided at these low temperatures as completely as these thin films, and the excess oxygen present may result in reduced activity.
Further, mixing of cobalt and
molybdenum precursor oxide phases and the product sulphide phases is expected to be more complete because of the system of deposition. Increase in reactant pressure results in an increase in HDS activity, which
may be correlated
the Mo(3d) attributed
to the observation of some additional features i n
and S(2p) lineshapes. Some of
the additional peaks resolved are
to the presence of a sub-stoichiometric form of molybdenum
sulphide ( M O S ~ _ ~ ) .A
smaller concentration of molybdenum
detected as super-stoichiometric species
sulphide
is
( M O S ~ + ~ ) .In fact, these sub- and
208 super-stoichiometric species may be
thought of as anionic and cationic
vacancies associated with a S-Mo-S platelet structure, perhaps promoted by a substituted cobalt atom (1).
The mechanism for the hydrodesulphurization
reaction is generally thought to require an anion vacancy where the sulphurcontaining molecules (thiophene in this case) can be adsorbed (17).
While ESR
evidence for such vacancies have been reported, (18) no previous XPS study has detected these unambiguously. Indeed it is interesting that the concentration of such vacancies actually predominates over MoS,
at lower temperatures.
While such structures may have been part of the spectra obtained by other researchers it would have been difficult to identify them spectroscopically. In this work a higher resolution photoelectron spectrometer is employed, and the spectra are essentially free of the distortions wrought by differential charging on the catalyst surface during XPS analysis. XPS
intensity studies of the cobalt-rich C series allows the Cogs,
stoichiometry to be
confirmed for samples reacted at
however, more sulphur is associated with the cobalt phase less with the molybdenum sulphide phase.
At
350°C. (S/Co
=
275°C
1.2) and
Such temperature effects will be
discussed further in a later paper. An increase in pressure of the reactants over the surface also results in changes in the XPS intensity ratios. At 350°C, a change in reactant pressure from 0.1
-
0 . 3 MPa results in a decrease in the S/Mo ratio and an increase i n
the sub-stoichiometric MoS,.,
species.
affect the S/Mo ratio at 2 7 5 " C , obtained at 350°C.
Although increase in pressure did not
the ratios are significantly lower than those
This decrease in S/Mo
increase in the sub-stoichiometricMoS,.,
ratio also corresponds to an
species observed in the XP spectra.
Based on observed O/Al ratio, the increased pressure clearly causes some sulphide, previously bonded as MoS, to react with the alumina substrate thus creating more anionic vacancies on the molybdenum sulphide. pressure on the cobalt sulphide
The effect of
phase and its acknowledged ole in promoting
Mo-S activity is not evident from these
studies. No change in
the Co(2p)
209 spectra or in S/Co ratio is noted with pressure change at either 275°C or 350°C.
In cases o f high vacancy concentration on the molybdenum sulphide
surface phase, it might be thought that if this were promoted by cobalt, a change in Co(2p)
lineshape would result, consistent with the formation o f
CoMoSz ( 1 ) . The concentrations o f cobalt in the series A films used in this present study are rather high (Co/Mo
=
> 1.5) and the presence of the very
similar Cogs, phase may simply mask the presence of an interactive cobalt phase. Further thin films
work to
study the
may be useful
effect
in illuminating
the molybdenum sulphide structure.
of varying
Co/Mo ratios
in the
the role of cobalt in promoting
Additional studies on the effects of
temperature and catalyst aging are also underway.
CONCLUSIONS
1. An HDS-active catalyst in thin film form has been produced by deposition o f 5 nm thick layers of cobalt and molybdenum on an alumina substrate and
subsequent reduction and sulphidation. 2. An increase in H,/thiophene pressure during reaction resulted in a clear increase in HDS activity of the CM thin film. 3.
Increased HDS activity of a CM thin film could be correlated with changes in the photoelectron spectra of the Mo(3d)
and S(2p)
lines which suggest
an increased concentration of anionic sites on the molybdenum suphide.
ACKNOWLEDGEMENTS The authors acknowledge with thanks, the efforts of Ms. D. Johnston and Dr. L.L. Coatsworth in preliminary experiments on this system. The technical assistance of Mr. G. Good is also acknowledged. This work has been supported by the Department o f Energy Mines and Resources (CANMET) under contract #24ST23440-6-9116.
210
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211
ADSORPTION, REACTION AND DESORPTION RATE CONSTANTS INHETEROGENEOUS CATALYSIS, MEASURED SIEIULTANEOUSLY BY GAS CHROMATOGRAPHY
N.A.KATSANOS and J.KAPOLOS Physical Chemistry Laboratory, University of Patras, 26 110 Patras (Greece)
ABSTRACT Reversed-flow gas chromatography can be used to test solid catalysts with respect to their adsorptive and reactive properties, thus facilit