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Al2O3 Hydrotreating Catalysts as Shown by the Use of Probe Molecules
M.L. Occelli and R.G. Anthony (Editors), Aduances in Hydrotreating Catalysts 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
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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
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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
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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 She 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
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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
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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.
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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 af ter 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.
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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
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(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.
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
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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
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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 She 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
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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
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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.
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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 af ter 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.
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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
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(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
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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
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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.