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Support effects on hydrotreating catalysts
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Catufysis Today, 10 (1991) 489-505 Elsevier Science Publishers B.V., Amsterdam
Support effects on hydrotreating catalysts M. Breysse, J.L. Portefaix and M. Vrinat Institut de Becherches Villeurbanne Cddex
sur la Catalyse,
2, Avenue Albert Einstein,
69626
Table of contents
1. 2. 3.
4. 5. 6. 7. 8. 9.
Introduction Carbon Oxides 3.1. silica 3.2. Titania and zirconia Binary oxides Silica - Alumina 2kolites Mixtures of zeolites and other catalysts Clays, pillared clays and natural minerals Conclusions
1. INTRODUCTION
Over the years considerable effort has been devoted to the study of the properties of alumina supported, sulphided cobalt-molybdenum, nickel-molybdenum and nickel-tungsten catalysts. This has included attempts to define the active catalyst species, the role of promoters, the influence of preparation and activation conditions etc. Surprisingly, until recent years, much less attention has been paid to the role of the support. Nevertheless, it was recognized, in the very first studies related to CoMo or NiM0/Al203 catalysts, that alumina is not an inert carrier. The promoter ions, Co and Ni in particular, can react with the support and can occupy octahedral or tetrahedral sites in the external layers or even form COAl2O4 (NiAl204) depending on the conditions of preparation. The origin of the exclusive use of alumina can be ascribed to its oustanding textural and mechanical properties and its relatively low cost. One important factor is also the ability to regenerate catalytic activities after intensive use under hydrotreating conditions. Due to the necessity to develop hydrotreating catalysts with enhanced properties, other supports have been studied : carbon, silica, reolite, titania, 0920-5861/91/$05.95
0 199 1 Blsevier Science Publishers B.V. All rights reserved.
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sepiolite, etc. In several cases, it was claimed that higher activities were obtained than those of alumina,supported catalysts. This may be due to a number of reasons, such as the morphology of the sulphide phase, the existence of chemical bonds .with the carriers, the presence of different acidic properties, etc. Moreover, changing the nature of the support does not have a similar influence on the catalytic properties for the variety of reactions -hydrodesulphurixation (I-IDS), hydrodenitrogenation (HDN), hydrogenation (I-IYD) or hydrocracking (IX)- which take place over sulphide catalysts during the hydrotreating processes. The objective of this review is to examine these various sulphide-support interactions and to discuss the interpretations which have been proposed. For these reasons, results reported only in patents have not been examined.Literature data will be ordered according to the nature of the support : carbon, simple oxides (TiO2, ZrO2), binary oxides, silica-alumina, xeolites and clays. Results are given by comparison with the conventional alumina support which, means that the nature of the interaction of the sulphide phase with this particular support will also be examined. The influence of additives to alumina such as phosphorus or fluorine, which is another way to modify the active phase-carrier interaction, does not lie within the scope of this article.
2. CARBON Active Co-MO or Ni-Mo catalysts were prepared using carbon as support by Stevens and Edmonds [i], Duchet et al. [2], Bridgwater et al. [3], Breysse et al. [4], Tops+ et al. [5], Hoffmann et al. [6]. Generally, the catalytic activities reported in these studies are greater than those of typical alumina-supported catalysts. This high activity of the carbon-supported, mixed catalysts was explained by Tops+ et al [7] in relation to the CoMoS model. Using Mijssbauer emission spectroscopy, Topsoe et al. showed that the cobalt atoms are situated at MoS2 crystallite edges in a so-called “CoMoS” structure and that this structure almost completely governs the HDS activity [8]. These results were obtained for catalysts supported on either alumina or carbon [4-Q, or even unsupported [9,10]. Thus the support is not necessary for the creation of the mixed phases, but nevertheless it may play an important role by enabling dispersed phases to be produced, which is particularly the case for carbon. Later on, Candia et al. [ll] reported that two types of CoMoS are observed in alumina supported catalysts : type I at the normal sulphiding temperature and type II at high sulphiding temperature (type II being more active than type .I in I-IDS). The difference between type I and II was supposed to be the existence, in the first case, of remaining MO-O-Al linkages, while type II CoMoS is fully sulphided, which means that the interaction between active phase and support is only of the Van der Waals type. Topqte’s Massbauer results suggested that type II CoMoS is the one which most resembles the carbon-supported CoMoS. According to these authors, the catalysts have different activities essentially because they contain slightly different active phases. In order to check the validity of this
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interpretation, van Veen et al. prepared fully sulphided CoMoS [12] on carbon, alumina and silica using nitrilotriacetic acid to complex with both Co and MO, so that the precursor support interaction should be the same for all supports. They concluded that, even when the active phase (COWS II) is invariant and its dispersion is invariant, the support influences its specific activity, the effect of carbon being positive in comparison to alumina and silica. Nevertheless, since the samples were not characterixed by XPS or electron microscopy, it is difficult to know whether the morphology of the active phase, i.e. the crystallite dimensions and (or) orientations, was similar. The higher catalytic activity for MO/C! compared to M0/Al203 was also explained in terms of differences in the structure of the sulphide phase present and in the interaction between this phase and the respective supports [13]. A single slab monolayer strongly interacting with the support was assumed to be present on alumina, and small, three-dimensional particles essentially free of interaction on carbon. The net effect of the strong interaction with alumina is a nearly optimal dispersion of the MO sulphide phase and also an electronic transfer through the MoO-Al linkages. This electronic effect would explain the lower activity of alumina supported catalyst. Carbon supports may also be used for preparing highly active Co or Ni catalysts [2,14]. In fact, it was shown that Co and Ni are even more active than (Co) or equally active (Ni) as the corresponding M&based catalysts (see Fig. 1) when they are supported on carbon and much less active when supported on alumina. By comparing the EXAFS and XANEs spectra of Co/C with those of pure Co9S8 and Co+ it was shown by Bouwens et al. [15] that Co/C has a larger fraction of octahedral cobalt than Co9S8. Similarly, Tops+ et al. [7] and van der Kraan et al [ 161 showed by MZissbauer spectroscopy that for low Co concentration (C 1 %), the Co phases present in the carbon supported catalysts are different from Co9S8 (Fig. 2). why this phase is formed on carbon instead of Co9S8 as expected from thermodynamic considerations and why this phase is more active are still unanswered questions. According to Prins and coworkers, the great advantage of a material as inert as carbon is the possibility that all transition metal compounds present in the precursor state will be quantitatively converted into their active sulphide form. This is the reason why the same authors used this support for their thorough studies of the properties of transition metal sulphides [17] in HDS and HDN [18, 191. A similar approach was also used by Ledoux et al. [20] with the same objective of comparing the catalytic properties of various sulphides. An important drawback in the utilization of carbon materials for hydrotreating reactions is their extensive microporosity. For catalytic reactions involving large molecules the micropores are of little use, since part of the transition metals will be deposited in these pores and in effect will be wasted. Most mesoporous carbons, on the other hand, have poor crushing strengths, low bulk density or too low a surface area. To circumvent these problems, two different approaches were utilized by Vissers et al. [21, 221i
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Figure 1. Catalytic properties in thiophene hydrodesulphurisation of Co/C, Ni/C, MO/C and W/C (percentage conversion after 2 h run time, catalyst composition expressed as weight percentage, support : HCl treated Mekog carbon). Reprinted with permission from Ref. 2. Copyright 1983 - Academic Press. --_
I ___.__..
Figure 2. In situ MBssbauer emission spectra of two carbon supported catalysts @pm Co/C and 1 96 Co/C and Co9S8). Reprinted with permission from Ref. 7. Copyright 1984 - Societ& Chimiques Belges.
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The first one was the application of carbon black composite material. This was done by mixing the carbon particles with a poly-furfuryl alcohol binder and subsequently carbonizing the binder material at elevated temperatures. The composites demonstrated promising textural properties for use as supports for sulphide catalysts. Nevertheless, owing to the inert character of the composite surface, sintering of the MO phase took place during sulphidation and it was thus necessary to subject. the composites to oxidation treatments to increase their heterogeneity and thus their affinity towards the deposited MO phase. The second way to modify the sulphide support interaction was the covering of the Al203 surface with a thin layer of carbon prior to impregnation of the transition metals [22]. With this method the favorable carbon surface properties were combined with the optimum textural and mechanical properties of the Al203 support. A threefold increase in activity compared to Co/Al203 was obtained in HDS demonstrating the effective shielding by the carbon layer which reduces or eliminates the strong metal-alumina interaction.
3. OXIDES As mentioned above, the support which currently is most used as a support for industrial hydrotreating catalysts is the gamma cubic type alumina. Nevertheless, other oxides have been studied, primarily the most common and less expensive ones, such as silica and silica-aluminas. Later on, these studies were broadened to other oxides. The results concerning simple oxides will be given first and those concerning binary oxides in Parts .3 and 4 of the present review. 3.1. silica Generally, it is stated in the literature that silica leads to catalysts having a lower activity than alumina [23] [24]. In academic studies, silica was often used because of its inert character and consequently its smaller interaction with the sulphided phase which permits a better characterization. For example, see the thorough studies of Yermakov et al. dealing with the preparation, the physicochemical properties and the catalytic properties of NiMo and NiW/SiO2 [25]. Nevertheless, there is still some controversy about the existence of chemical interaction between molybdenum and the silica support, and a few studies are still carried out on this subject [26]. 3.2. Titanin and zirconla A few years ago, the first attempts at the use of unconventional oxides (TiO2, ZrO2 etc.) were carried out in various reactions, such as the treatment of heavy oils, the hydrocracking of bituminous coal [27, 281, the hydrotreatment of solvent refined coal [29] and solvent refined lignin [30]. In these preliminary papers significant support effects were .found on the hydrogenation and hydrocracking activities of
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molybdenum sulphide catalysts. The titania support and, to a lesser extent, the xirconia support appeared promising for upgrading the coal-derived liquids. In order to explain these effects, detailed characterizations of MoOg/TiO2 catalysts by Raman and Fourier transform infrared spectroscopy were reported by Ng and Gulari [31], Nishijima et al. [32], Wachs and Hardcastle [33] and Quincy et al. [34]. According to these authors, one of the most important differences in comparison to alumina, is the fact that as long as the loading is less than one monolayer, no molybdenum trioxide is formed on titania and that molybdate anions bind to the titania surface strongly and uniformly. Even after reduction, remaining M&J-Ti bonds contribute to the stabilization of the MO species on the titania surface without any segregation or sintering. Raman spectroscopic data [3 1, 341 suggest that different molybdenum species are present on oxidic Mo/Ti02 catalysts. Tetrahedral molybdate species formed at low MO loadings and surface polyoxomolybdate species with octahedral coordination formed at higher MO content This was rationalized on the basis of the surface characteristics of Ti02 : molybdenum will preferentially react first with the terminal hydroxyl group associated with one Ti4+ forming MO(T) species and this species will continue to react with bridging OH groups coordinated to two Ti4+ S&S. The molybdate spreading was also related to the difference in the distribution of surface OH groups over the support [31, 351. If Ti02 and Al203 present roughly comparable surface concentrations of anionic OH, on the titama surface Ti4+ are only tetrahedrally coordinated and the hydroxyl groups are uniformly distributed providing a homogenous surface for the adsorption of molybdate anions. On the other hand, on Al2O3, A13+ ions are octahedrally and tetrahedrally coordinated and hydroxyl groups are ordered in parallel rows [36]. Such preferential arrangement in rows requires an arrangement of the molybdate anions during the impregnation process which leads to molybdenum trioxide even at submonolayer loadings. These observations are in agreement with XPS data reported by Nag [371 and Caceres et al. [35] on the oxidic precursors. On TiO2 the Mo3d doublet displays an excellent resolution and a low FWHM (full width at half maximum) value suggesting that MO species are more homogeneously distributed and more similarly chemically coordinated than on Al203 where Mo6+ species with different chemical characteristics are present. The results reported above could be considered as slightly in contradiction with those of Wachs and Hardcastle [33] who observed exclusively tetrahedrally coordinated surface species for both MolTi and M0/Al203 samples. Nevertheless, it should be noted that this last study concerned only low amount of molybdenum (<5 I). On the other hand, using an equilibration adsorption method to introduce molybdenum on Ti02, Rim et al. [38] showed that at pH 3.98 the molybdenum oxide monolayer is composed of distorded octahedra. But in this last study, the sulphided state of the catalysts was not examined. This was the case in the work of Ng and Gulari [31] and Quincy et al [3 11 who proposed that tetrahedral molybdenum species are the precursors of the most active entities for HDS. Assuming that the surface structure of the sulphided catalyst could be related to that of the oxidic catalyst, interesting observations could be deduced from temperature
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programmed reduction studies, as was done by Shimada et al. [39], Leyrer et al. [40] and Caceres et al.[35]. The reducibility of molybdate depends on the support and MoEi is more easily reduced than M0/Al203 ; this mdt was confirmed by high temperature RSR techniques. Kohno et al. [41] using high pressure, Nevertheless, the interpretation of this effect is still a matter of controversy since some authors conclude that the interaction between MO and Ti02 is weak while some others maintain that it is strong. If there is a general agreement that sulphidation seems to be easier on Ti02 than on Al2O3, it is also clear that the differences observed in the structures of the catalyst are much smaller in the sulphided form than in the oxidic state. This fact could be iV*lstrated by XPS results since no variations of Mo3d binding energies over sulphioed MoiTi02 and M0/Al203 were observed by Ramirez et al. [42] and only small variations within the limit of the sensitivity of the technique were noticed by Shimada et al. [39] and Okamoto et al. [43]. In this last work some unsulphided Mo6+ species still remained after sulphidation. The absence of a significant shift in the MO binding energies, either for Ti02 or Al203 supported catalysts, with respect to unsupported MoS2, does not support the existence of an electronic effect as the main cause of the great differences in activity. Therefore, attention has been recently paid to the morphology of MoS2-like species in order to discuss these differences in terms of a geometrical effect. Thus, using thermal desorption spectroscopy of NO on sulphided catalyst, Okamoto et al. [43] demonstrated that there are at least two distinct NO adsorption sites assigned to triply and doubly coordinatively unsaturated MO sites (CUS). The proportion of these different sites depends on the support and MO loading in relation to variations in the morphology of the sulphided particles : MoS2 slabs produced on M0/Al203 catalysts expose a higher proportion of the (1010) edge planes than those on Momi catalysts. The evidence of a geometrical effect was also reported by Pratt et al. [44]. Using HREM examinations they found that the dominant morphology of MoS2 surface species on A4203 (or Si02) is that of flakes of MoS2 containing one to six layers and located verttcally to the surface of the support. By contrast, when molybdenum is supported on Tie;?, the Mo!+ consists of “raft-like” structures sitting flat on the support. This “skinning” phenomenon is not clear and differences in specific activities observed at low molybdenum loading are not really explained in relation to these observations. By contrast, the electron microscopic observations of Ramirez et al. [42] show clearly that for Mo/Ti02 samples, in comparison to MO/Al2O3, the length distribution of MoS2 crystal&s as well as the number of stacked layers shift toward the smaller particle range (see Fig. 3). The differences of the catalytic activities between both supported phases was explained in terms of different activities between the smaller and larger particles, this difference of activity being related to the MoS2 crystallite orientations. It was suggested that the role of Ti02 is to promote the formation of highly active edge-up MoS2 particles due to an interaction between the MoS2 edge planes and some planes of Ti02. Moreover, the synergetic (or promoting) effect of cobalt was found to be lower on Tie;! than on Al203 (3.2 and
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8, respectively) in agreement with previous results of Muralidhar et al. [24]. However, this promoting effect increased with increasing molybdenum loading for catalysts with the same ratio of Co to MO (Fig. 4). This was ascribed to the greater difficulty with which the mixed CoMoS species are formed on the edge-up particles that exist preferentially at low MOcontents.
b
20
NUMBER
OF
FRINGES
(N)
LENGTH
40
60
OF
80
100
CRYSTALLITE
120
Cil
Figure 3. Distribution.of the number of fringes (a) and of the length of cystallites (b) for TiO2 and Al203 molybdenum sulphide supported catalysts. (Data adopted from Ref. 41.
1.4
2.8
at Ma/sq.
5.6
nm.
Figure 4. (a) Synergetic effect of cobalt for various supported molybdenum catalysts; (b) Variation of the synergy with metal loading for CoMoITiO2 catalysts at constant Co/(Co+Mo)ratio. Data adopted from Ref. 41.
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While the support effect in hydrotreating catalysts has mainly been studied on Ti02, less attention has been devoted to other oxides such as ZrO2, CeO2, La203 and MgO. For ZrO2, in particular, the results are scarce and often contradictory. Nag [371 reported that MO oxidic species on Zr02 are more homogenously distributed and more similarly chemically coordinated than on Al2O3. According to Zaki et al. [45], the MO monolayer cannot be formed and MOO3 aggregates. Nevertheless, good hydrogenation properties of MolZrO2 catalysts have been found in the last few years. Large differences in CO hydrogenation, observed by Mauchausse et al. [46] for MolZrO2 and Mo/CeO2, compared to M0/Al203 were interpreted by variations in the density of the active sites depending on residual MoO-Al support bonds, in the most active solids, the support being slightly sulphided. These promising hydrogenation properties have also been observed for pyridine hydrogenation and piperidine hydrodenitrogenation [471. Using infrared spectroscopy to study the chemisorption of the probe CO molecules, Houssenbay et al. [48] also explain ‘the high activity of Mo/ZrO2 by an increase in the number of active sites. Detailed characterization of Mo/ZrO;! catalysts are given in the present issue [49]. An important drawback in the utilization of Zr02 as a support for hydrotreating catalysts arises from its textural and structural instability during high temperature treatments. A lot of studies (particularly for ceramic applications) have therefore been devoted to the stabilization of this oxide. .In order to obtain high surface area zirconia carriers, low temperature preparation methods are generally preferred. Among those methods, the precipitation of zirconia gel followed by impregnation with Y, Al, Ni, or molten salt preparation lead to interesting textural properties [50, 51, 521. Another approach to avoiding the instability of ZrO2 or the drawback of the low surface area of Ti02 is the study of mixed oxides. 4. BINARY OXIDES As discussed above, the most promising results were reported for TiO2 and ZrO2. Therefore, attempts to utilii mixed oxides as supports for hydrotreating catalysts have been primarily concerned with Ti02 and ZrO2. For MO catalysts supported on TiO2-Al203 and TiO2-Zr02, high hydrocracking activities of diphenylmethane were observed by Nishijima et al. [32] and a cooperative or synergetic effect of both components supports was evidenced. The maximum activity appeared for atomic ratios Ti/(Ti+Al) = 0.5 and Ti/(Ti+Zr) = 0.8. These catalysts also displayed higher HDN properties than simple oxides for the upgrading of coal derived liquids. These inherent HDS and HDN capabilities of titania and zirconia supports have been also applied in ternary compounds and a CoMo/TiO2-ZrO2-V205 catalyst was found to be twice as active as a commercial CoMo for HDS of heavy diesel and vacuum gas oil [533. It should be noted that the properties of such mixed oxides
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depend to a great extent on the preparation procedure. Moreover, CoMo/Ti0 2Zr02, CoMo/Ti02-Ce02 and CoMo/Ti02-Mn02 catalysts present very high initial HDS activities [54]. The promising properties of these mixed oxides is illustrated by the large number of very recent and interesting papers dealing with this subject ; see for example Refs. 55 and 56.
5. SILICA - ALUMINA The catalytic properties of molybdenum sulphide based catalysts supported on Si02-A1203 depend on the amount of Si02 and on the reaction, i.e. HDS, HYD, hydrocracking or RON. Muralidhar et al. [24] found that HOS and HYD activities decrease with increase in Si02 content from 10 to 75 %, while the activity for the hydrocracking of 2,4,4-trimethyl-l-pentene showed an opposite trend. These high hydrocracking activities have to be related to the properties of the protonic acid supports and not to the properties of the sulphide phase. For the HDN reactions, a Hofmann-type elimination and a nucleophilic displacement mechanism have been proposed. Such mechanisms are related to the acid-base properties of the catalysts. Consequently, it is to be expected that the support acidity will influence these reactions. In the hydrotreatment of heavy feedstocks, Toulhoat and Kessas [57] observed an increase of the HDN activity when 15 % Si0 2-AI203 was utilized as a support for NiMo catalyst in comparison with the properties of the same active phase on A1203. But a decrease was found at higher concentrations of silica, i.e. 25 %. The HDS activity decreased in both instances. On the other hand, in industrial processes, the high acidic properties of these catalysts tend to accelerate the formation of coke by increasing the rate of polymerization and alkylation reactions, which involve the olefins produced as intermediates by the various reactions of hydrogenolysis [23].
6. ZEOLITES Despite the extensive use of zeolites in catalysis, there has been relatively little work carried out on their applications to hydrotreating processes. However, zeolites have many features, such as high surface area, shape selectivity, the ability to produce high dispersion of metals, and a range of acid-base properties, which could be utilized in this type of reaction. The first attempts to use zeolites as a carrier for hydrotreating catalysts were reported in the "open" literature around 1970 but it is only in the last few years that publications have reported promising results concerning the utilization of these catalysts for hydrodenitrogenation reactions. In these first publications, the activities reported are generally lower than those of alumina supported catalysts. For example,
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Vrinat et al. [58] studied the properties of cobalt molybdenum in HY and .NaY xeolites. Cobalt was introduced by ion exchange and molybdenum by molybdenum carbonyl adsorption. For dibenxothiophene conversion these catalysts were less active than conventional cobalt molybdenum on alumina catalysts. One point should to be emphasised at this stage of our discussion, viz the diffusion problem. If the sulphide phase is well dispersed inside the xeolite framework, one would expect diffusion limitation with large molecules. This may be an explanation for the results obtained by Vrinat et al. [58]. Cid et al. [59, 60, 611 compared the properties of cobalt molybdenum catalysts on alumina, silica and NaY zeolite. All catalysts contained 12 wt.-% MoO3 and 5 % Co0 and were prepared by successive impregnations, first with ammonium heptamolybdate and then with cobalt nitrate [59]. The authors concluded that MO and Co are moderately dispersed into the cavities of the NaY xeolite, but located separately, without the formation of a clear &MO interaction phase. This lack of interaction between cobalt and molybdenum was supposed to be responsible for the relatively low hydrodesulphuration activity of the CoMo/NaY catalyst. Later on the same authors showed that better catalysts can be obtained by incorporating Co by ion exchange prior to molybdenum deposition, rather than by simultaneous impregnation of both Co and MO [60]. The appearance of a NiMoS phase was also claimed by Davidova et al. [62, 631. According to these authors, the samples based on Y and ZSM-5 xeolites display the maximal activity. This strong influence of the method of preparation on the catalytic results was also mentioned by Fornes et al. [64] for NiMo on ultrastable HY xeolites. Leglise et al. [65] used infrared spectroscopy to conlirm the existence of an interaction between the two components of NiMo sulphide catalysts loaded into Y xeolite, whether or not the latter was dealuminated. Nevertheless, the NiMoS phase was more active for benzene hydrogenation on the alumina carrier than in the zeolites which can be ascribed to the lower accessibility of the active phase to the hydrocarbon reagents. Moreover, side reactions such as alkylation, isomerixation and cracking, appeared on the xeolite catalyst due to its acidic properties. Okamoto et al, [66,67l used NaY and KY xeolites to prepare highly dispersed molybdenum sulphide from Mo(C0)6 or by an impregnation method. Catalysts prepared from Mo(C0)6 were found to be more active than those obtained by the more conventional method. It was suggested that the reolite basicity controls the strength of interaction between the xeolite oxygens and the subcarbonyl species and thus the final dispersion of sulphided molybdenum. Ruthenium sulphide supported on Y zeolite has good properties for hydrodenitrogenation reactions, which were first reported by Harvey and Matheson [68, 69, 703. An activity similar to (or slightly better than) that of conventional NiM0/Al203 was obtained for hydrodenitrogenation of quinoline with metal loadings much lower than those used commonly in commercial hydrodenitrogenation catalysts. The most active catalyst was obtained by ion exchange of lRu(NI-13)d3+ into NH4Y followed by sulphidation. Unexpectedly, the high activity of these catalysts for HDN was not reflected in a correspondingly high activity for HDS. The
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nature of the active ruthenium sulphur species was not established. From XRD examinations and TEM imaging of the sulphided catalysts, the authors concluded that the ruthenium sulphide was dispersed as aggregates composed of only a few molecules at most. XPS observations indicated a significant migration to the external Surface.
A systematic study of the influence of the support acidity was performed by G6biiliis et al. l?‘l] for ruthenium sulphide on HY, NaY and KY in a variety of reactions, i.e. the hydrodesulphurixation of thiophene, the conversion (hydrogenation and cracking) of biphenyl, the hydrogenation of pyridine, and the hydrodenitrogenation of piperidine. The rapid deactivation and thus the low activity of Ru/HY in the HDS of thiophene was attributed to coke formation on the Br6nsted acid sites of the support. The stability and the activity of the catalysts in this reaction can be improved by decreasing the acidity, which was particularly noticeable for R&Y. In the conversion of biphenyl, the activity towards the formation of cracking products increased with the acidity of the supports. The activity of the catalysts for the conversion of nitrogen containing molecules is less affected by the acid strength of the supports. Only a slight decrease of the activities is observed for the most acidic support. In conclusion, the study of hydrotreating catalysts deposited in xeolites appears to be promising but even more difficult than the studies relative to other carriers due to the peculiar structural properties of xeolites. The main problem is the determination of the position of the sulphided species, outside or inside the xeolite framework. As for carbon supported sulphides, highly dispersed species may present particular properties.
7. MIXTURES OF ZEOLITES AND OTHER CATALYSTS Several studies deal with hydrotreating catalysts composed of xeolite and alumina or silica-alumina [72, 731. The purpose of these studies was to combine the hydrogenation function due to CoMo, NiMo or NiW sulphide to a hydrogenolysis function brought about by the support and its acid centres. The further development of macroporosity by boron addition improved the HDN properties in upgrading vacuum gas-oil [73]. Another improvement of the HDN activity was obtained using a physical mixture of a conventional NiMo/Al203 and Ru/Y xeolite [68, 70, 741. The origin of this synergetic effect is still not completely understood. Moreover, this effect was not observed with any other transition metal sulphides supported on Y xeolite [75].
8. CLAYS, PILLARED CLAYS AND NATURAL MINERALS In the search for natural minerals as carriers for low cost hydrotreating catalysts, the first attempt was reported by Ianibello et al. using natural bauxite [76].
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The catalytic properties of the MO, CoMo and NiMo bauxite based catalysts resemble those of alumina catalysts in the metal removal reaction but lower activities were observed f6r the HDS and HDN reactions. Using natural clays themselves as catalysts, Sakata and Hamrin l77l compared the catalytic activities in HDS and HDN reactions of kaolinite, montmorillonite and illite API reference clays. The clay activity for HDS was correlated with the impurity iron content and compared to a commercial COMO/Al2O3 catalyst. Kaolinite clays gave good conversion for n-butylamine HDN. According to Mochida et al. [29], a NiMo supported on sepiolite is better than a commercial NiMo/Al203 for the conversion of the heavier fractions of solvent refined coal. This result was explained in relation to the pore structure of the support. Unfortunately this catalyst was less active for the removal of heteroatoms. It should be mentioned that acid pretreatment of these natural clays plays an important role in the preparation of the catalysts and good activities for thiophene HDS have been found over a N&IO/acid treated sepiolite catalyst by Corma et al.
ml. Nervertheless, among the various natural clays which were tested as catalyst support, the most interesting results were obtained over bentonites. Schultz et al. [79] found higher hydrocracking and HDS activities for MoS2 supported on acid treated Brazilian bentonites than for Al2O3, the bentonite itself contributing to the hydrocracking activity. Inasmuch as smectite clays have a long history of use as cracking catalysts, an important property of such materials is their ability to swell by intercalation of organic cations, organometallic complexes or mineral cations. High surface area bentonite pillared with inorganic pillaring agents was reported by several authors (see Occelli and Rennard, Ref. 80). Such pillared clays could find application as hydrotreating catalysts for the conversion of high molecular weight hydrocarbons. Used as support for a NiMo active phase, in composite catalysts, several bentonites intercalated with Al, Si or Zr cations displayed excellent hydrotreating activities WI. Along the same lines, a NiW catalyst supported on a cross-linked hydroxy titanium bentonite with a Ti(SO)4 solution shows higher pyridine HDN activity than an industrial NiW/Al203 catalyst [81]. Such an activity could be explained by the high surface area of the carrier and by the presence of titanium, since TiO2 has been recognized as an excellent support for the hydrotreating reaction.
9. CONCLUSIONS The objective of this review was to give an overview on the various support interactions encountered for hydrotreating catalysts. At the present time, this field of research is very active and every day sees the proposal of a new type of support for hydroprocessing catalysts [82, 831. Recent physicochemical characterisation techniques allow to progress in our knowledge of the sulphide support interaction
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and to distinguish between electronic and geometrical effects (size, stacking, orientations). Nevertheless, even if a lot of points still remain to be clarified, the results reported in the literature allow us to look to the future with optimism. A new generation of supports adapted to the various constraints of the hydrotreating processes will probably appear within the next few years. One should not underestimate the amount of work which this objective requires, since the development of new supports implies the optimization of the interaction with the active phase, of the sulphiding process and above all of the textural and mechanical properties.
Acknowledgments This work was carried out in the framework of the contract “New catalysts for hydrodenitrogenation of heavy cuts” of the “Non nuclear energy” R & D programme of the Commission of the European Communities (EN 3C - 0040 F). It received support from ELF, IFP and TOTAL and from the CNRS-PIRSEM.
10. REFERENCES 1
2 3 4 .5 6 7 8 9 10 11 12
G.C. Stevens and T. Edmonds, in “Preparation of Catalysts II”, (B. Delmon, P. Grange, P. Jacobs and G. Poncelet, Eds.), Elsevier, Amsterdam, 1979, p.507. J.C. Duchet, E.M. Van Oers, V.H.J. de Beer and R. Prins, J. Catal., (1983) 386. A.J.Bridgewater, R. Burch and P.C.H. Mitchell, Appl. Catal., 4 (1982) 267. M. Breysse, B.A. Bennett, D. Chadwick and M. Vrinat, Bull. Sot. Chim. Belg., 90 (1981) 1271. H. Tops+, B.S. Clausen, N. Tops+ and E. Pedersen, Ind. Eng. Chem. Fundam., 25 (1986) 25. C. Hoffman, M. Breysse, C. Leclerq and R. Frety, in “Actas 10” Simposio Ibero-American0 de Catalisis (Merida)“, (U.L.A., Eds), vol. 1 (1986) p.380. H. Tops@ R. Candia, N. Tops&? and B.S. Clausen, Bull. Sot. Chim. Belg., 93 (1984) 783. C. Wivel, R. Candia, B.S. Clausen, S. M@up and H. Tops+, J. Catal,., 68 (1981) 453. R. Candia, B.S. Clausen and H. Tops& J. Catal., 77 (1982) 564. M. Breysse, R. Frety, B. BenaIchouba and P. Buss&e, Radiochem. Radioanal. Lett., 59 (1983) 265. R. Candia, 0. Sorensen, J. Villadsen, N. Tops& B.S. Clausen and H. Tops& Bull. Sot. Chim. Belg., 93 (1984) 763. J.A. R. van Veen, E. Gerkema, A.M; van der Kraan and A. Knoester, J. Chem. Sot., Chem. Commun., (1987) 1684.
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D.S. Kim, Y. Kurusu, I.E. Wachs, F.D. Hardcastle and K. Segawa, J. Catal.; 120 (1989) 325. H. Shimada, T. Sato, Y. Yoshimura, J. Hiraishi and H. Nishijima, J. Catal., 110 (1988) 275. J. Leyrer, B. Vielhaber, M.I. Zaki, Z. Shuxian, J. Weitkamp and H. Kn&inger, Materials Chemistry and Physics, 13 (1985) 301. T. Kohno, T. Yokono, Y. Sanada, K. Yamashita, H. Hattori and K. Makino, Appl. Catal., 22 (1986) 201. J. Ramirez, S. Fuentes, G. Diaz, M. Vrinat, M. Breysse and M. Lacroix, Appl. Catal., 52 (1989) 211. Y. Okamoto, A. Maezawa and T. Imanaka, J. Catal., 120 (1990) 29. K.C. Pratt, J.V. Sanders and V. Cristov, J. Catal., 124 (1990) 416. M.I. Zaki, B. Vielhaber and H. Kn6zinger, J. Phys. Chem., 90 (1986) 3176. C. Mauchausd, H. Mozzanega, P. Turlier and J.A. Dalmon, in “Proc. 9th, Inter. Congr. Catal. (Calgary)“, (M.J. Phillips and M. Teman, Eds), vol. 2 (1988) p.775. J.L. Portefaix, M. Cat¬, J.A. Dalmon and C. Mauchausse, in “Advances in Hydrotreating Catalysts”, (M.L. Ocelli and R.G. Anthony, Eds), Elsevier Science Publishers, (1989) 243. S. Houssenbay, E. Payen, J. Grimblot, F. Mauge, J.C. Duchet, J.C. Lavalley and S. Kasztelan, Catalysis Today, this issue. F. Maugb, J.C. Duchet, J.C. Lavalley, S. Houssenbay and E. Payen, Catalysis Today, this issue. J.C. Duchet, M.J. Tilliette and D. Comet, Catalysis Today, this issue. D. Hamon, M. Vrinat, M. Breysse, B. Durand, M. Jebrouni, M. Roubin and P. Magnoux, Catalysis Today, this issue. P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burggraaf and J.R.H. Ross, Appl. Catal., 57 (1990) 127. I. Wang and R.C. Chang, J. Catal., 117 (1989) 266. F.P. Daly, J. Catal., 116 (1989) 600. W. Zhaobin, X. Qui, G. Xiexian, E.L. Sham, P. Grange and B. Delmon, Appl. Catal., 63 (1990) 305. F.J. Gil Lliambas, L. Bouyssieres and A. Lopez-Agudo, Appl. Catal., 65 (1990) 45. H. Toulhoat and R. Kessas, Revue de 1’Institut Fran&s du P&role, 41 (1986) 511. M. Vrinat, C.G. Gachet and L. de Mourgues, in “Catalysis by zeolites”, (B. Imelik et al., eds.), Elsevier, Amsterdam, (1980) 213. R. Cid, J. Villasenor, F. Orellana, J.L.G. Fierro and A. Lopez-Agudo, Appl. Catal., 18 (1985) 357. R. Cid, F. Orellana and A. Lopez-Agudo, Appl. Catal. 32 (1987) 327. A. Lopez-Agudo, R. Cid, F. Orellana and J.L.G. Fierro, Polyhedron, 5 (1986) 187.
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N. Davidova, P. Kovacheva and D. Shopov, in “Studies in Surface Science and Catalysis”, (B. Drzaj, S. Hocevar and S. Pejovnik, eds.), Elsevier, Amsterdam, 24 (1985) 659. N. Davidova, P. Kovacheva and D. Shopov, Zeolites, 6 (1986) 304. V. Fornes, M.I. Vazquez and A. Corma, Zeolites, 6 (1986) 304. J. Leglise, A. Jeanin, J.C. Lavalley and D. Comet, J. Catal., 114 (1988) 388. Y. Okamoto, A. Maezawa, H. Kane and T. Imanaka, in “Proc. 9th Inter. Congr. Catal., (Calgary)“, (M.J.Phillips and M. Ternan, Eds.), vol. 1 (1988) p.11. Y. Okamoto, A. Maezawa, H. Kane and T. Imanaka, J. Mol. Catal., 52 (1989) 337. T.G. Harvey and T.W. Matheson, J. Chem. Sot., Chem. Commun., (1985) 188. S.W. Oliver, T.D. Smith, J. Pilbrow, T. Harvey, T.W. Matheson and K.C. Pratt, Inorg. Chim. Acta, 117 (1986) L9-LlO. T.G. Harvey and T.W. Matheson, J. Catal., 101 (1986) 253. S. G(ibiil&, M. Breysse, M. Cattenot, T. Decamp, M. Lacroix, J.L. Portefaix and M. Vrinat, in” Advances in hydrotreating catalysts”, (M.L. Occelli and R.G. Anthony, Eds), Elsevier Science Publishers, (1989) 243. R.S. Mann, I.S. Sambi and K.C. Khulbr, Ind. Eng., Chem. Res. 27 (1988) 1788. M.L. Occelli and T.P. Debies, J. Catal., 97 (1986) 357. T.W. Matheson, K.C. Pratt and T.G. Harvey, International patent BOlJ 29/12, COlG 65/14, 27 March 1986. T.G. Harvey and K.C. Pratt, Appl. Catal., 47 (1989) 335. A. Iannibello, S. Marengo and A. Girelli, Appl. Catal., 3 (1982) 261. Y. Sakata and C.E. Hamrin Jr., Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 250. A. Corma, J.L.G. Fierro, J. Frontel, F. Melo, A. Mifsud and E. Sanz, in “Proc. 11” Simposio Iberoamericano de Catalisis, (Guanjuato)” (F. Cossio, 0. Bermudes, G. de1 Angel and R. Gomez, Eds), vol.2 (1988) p.537. G.W. Schultz, H. Shimada, Y. Yoshimura, T. Sato and A. Nishijima, Bull. Chem. Sot. Jpn., 58 (1985) 1077. M.L. Occelli and R.J. Rennard, Catalysis Today, 2 (1988) 309. S. Guida, Y. Fusan, Z. Huihua, and L. Zhonghui, Stud. Surf. Sci. Catal., 31 (1987)649. T. Sato, S. Kanzari, Y. Yoshimura, H. Shimada, N. Matubayashi, Y. Shibazaki and A. Nishijima, Chem. Letters, (1988) 315. N. Kimbara, A. Nishijima, T. Sato, Y. Yoshimura, H. Schimada, N. Matsubayashi and A. Yukimura, Cat. Letters, 6 (1990) 409.
Catufysis Today, 10 (1991) 489-505 Elsevier Science Publishers B.V., Amsterdam
Support effects on hydrotreating catalysts M. Breysse, J.L. Portefaix and M. Vrinat Institut de Becherches Villeurbanne Cddex
sur la Catalyse,
2, Avenue Albert Einstein,
69626
Table of contents
1. 2. 3.
4. 5. 6. 7. 8. 9.
Introduction Carbon Oxides 3.1. silica 3.2. Titania and zirconia Binary oxides Silica - Alumina 2kolites Mixtures of zeolites and other catalysts Clays, pillared clays and natural minerals Conclusions
1. INTRODUCTION
Over the years considerable effort has been devoted to the study of the properties of alumina supported, sulphided cobalt-molybdenum, nickel-molybdenum and nickel-tungsten catalysts. This has included attempts to define the active catalyst species, the role of promoters, the influence of preparation and activation conditions etc. Surprisingly, until recent years, much less attention has been paid to the role of the support. Nevertheless, it was recognized, in the very first studies related to CoMo or NiM0/Al203 catalysts, that alumina is not an inert carrier. The promoter ions, Co and Ni in particular, can react with the support and can occupy octahedral or tetrahedral sites in the external layers or even form COAl2O4 (NiAl204) depending on the conditions of preparation. The origin of the exclusive use of alumina can be ascribed to its oustanding textural and mechanical properties and its relatively low cost. One important factor is also the ability to regenerate catalytic activities after intensive use under hydrotreating conditions. Due to the necessity to develop hydrotreating catalysts with enhanced properties, other supports have been studied : carbon, silica, reolite, titania, 0920-5861/91/$05.95
0 199 1 Blsevier Science Publishers B.V. All rights reserved.
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sepiolite, etc. In several cases, it was claimed that higher activities were obtained than those of alumina,supported catalysts. This may be due to a number of reasons, such as the morphology of the sulphide phase, the existence of chemical bonds .with the carriers, the presence of different acidic properties, etc. Moreover, changing the nature of the support does not have a similar influence on the catalytic properties for the variety of reactions -hydrodesulphurixation (I-IDS), hydrodenitrogenation (HDN), hydrogenation (I-IYD) or hydrocracking (IX)- which take place over sulphide catalysts during the hydrotreating processes. The objective of this review is to examine these various sulphide-support interactions and to discuss the interpretations which have been proposed. For these reasons, results reported only in patents have not been examined.Literature data will be ordered according to the nature of the support : carbon, simple oxides (TiO2, ZrO2), binary oxides, silica-alumina, xeolites and clays. Results are given by comparison with the conventional alumina support which, means that the nature of the interaction of the sulphide phase with this particular support will also be examined. The influence of additives to alumina such as phosphorus or fluorine, which is another way to modify the active phase-carrier interaction, does not lie within the scope of this article.
2. CARBON Active Co-MO or Ni-Mo catalysts were prepared using carbon as support by Stevens and Edmonds [i], Duchet et al. [2], Bridgwater et al. [3], Breysse et al. [4], Tops+ et al. [5], Hoffmann et al. [6]. Generally, the catalytic activities reported in these studies are greater than those of typical alumina-supported catalysts. This high activity of the carbon-supported, mixed catalysts was explained by Tops+ et al [7] in relation to the CoMoS model. Using Mijssbauer emission spectroscopy, Topsoe et al. showed that the cobalt atoms are situated at MoS2 crystallite edges in a so-called “CoMoS” structure and that this structure almost completely governs the HDS activity [8]. These results were obtained for catalysts supported on either alumina or carbon [4-Q, or even unsupported [9,10]. Thus the support is not necessary for the creation of the mixed phases, but nevertheless it may play an important role by enabling dispersed phases to be produced, which is particularly the case for carbon. Later on, Candia et al. [ll] reported that two types of CoMoS are observed in alumina supported catalysts : type I at the normal sulphiding temperature and type II at high sulphiding temperature (type II being more active than type .I in I-IDS). The difference between type I and II was supposed to be the existence, in the first case, of remaining MO-O-Al linkages, while type II CoMoS is fully sulphided, which means that the interaction between active phase and support is only of the Van der Waals type. Topqte’s Massbauer results suggested that type II CoMoS is the one which most resembles the carbon-supported CoMoS. According to these authors, the catalysts have different activities essentially because they contain slightly different active phases. In order to check the validity of this
491
interpretation, van Veen et al. prepared fully sulphided CoMoS [12] on carbon, alumina and silica using nitrilotriacetic acid to complex with both Co and MO, so that the precursor support interaction should be the same for all supports. They concluded that, even when the active phase (COWS II) is invariant and its dispersion is invariant, the support influences its specific activity, the effect of carbon being positive in comparison to alumina and silica. Nevertheless, since the samples were not characterixed by XPS or electron microscopy, it is difficult to know whether the morphology of the active phase, i.e. the crystallite dimensions and (or) orientations, was similar. The higher catalytic activity for MO/C! compared to M0/Al203 was also explained in terms of differences in the structure of the sulphide phase present and in the interaction between this phase and the respective supports [13]. A single slab monolayer strongly interacting with the support was assumed to be present on alumina, and small, three-dimensional particles essentially free of interaction on carbon. The net effect of the strong interaction with alumina is a nearly optimal dispersion of the MO sulphide phase and also an electronic transfer through the MoO-Al linkages. This electronic effect would explain the lower activity of alumina supported catalyst. Carbon supports may also be used for preparing highly active Co or Ni catalysts [2,14]. In fact, it was shown that Co and Ni are even more active than (Co) or equally active (Ni) as the corresponding M&based catalysts (see Fig. 1) when they are supported on carbon and much less active when supported on alumina. By comparing the EXAFS and XANEs spectra of Co/C with those of pure Co9S8 and Co+ it was shown by Bouwens et al. [15] that Co/C has a larger fraction of octahedral cobalt than Co9S8. Similarly, Tops+ et al. [7] and van der Kraan et al [ 161 showed by MZissbauer spectroscopy that for low Co concentration (C 1 %), the Co phases present in the carbon supported catalysts are different from Co9S8 (Fig. 2). why this phase is formed on carbon instead of Co9S8 as expected from thermodynamic considerations and why this phase is more active are still unanswered questions. According to Prins and coworkers, the great advantage of a material as inert as carbon is the possibility that all transition metal compounds present in the precursor state will be quantitatively converted into their active sulphide form. This is the reason why the same authors used this support for their thorough studies of the properties of transition metal sulphides [17] in HDS and HDN [18, 191. A similar approach was also used by Ledoux et al. [20] with the same objective of comparing the catalytic properties of various sulphides. An important drawback in the utilization of carbon materials for hydrotreating reactions is their extensive microporosity. For catalytic reactions involving large molecules the micropores are of little use, since part of the transition metals will be deposited in these pores and in effect will be wasted. Most mesoporous carbons, on the other hand, have poor crushing strengths, low bulk density or too low a surface area. To circumvent these problems, two different approaches were utilized by Vissers et al. [21, 221i
492
Figure 1. Catalytic properties in thiophene hydrodesulphurisation of Co/C, Ni/C, MO/C and W/C (percentage conversion after 2 h run time, catalyst composition expressed as weight percentage, support : HCl treated Mekog carbon). Reprinted with permission from Ref. 2. Copyright 1983 - Academic Press. --_
I ___.__..
Figure 2. In situ MBssbauer emission spectra of two carbon supported catalysts @pm Co/C and 1 96 Co/C and Co9S8). Reprinted with permission from Ref. 7. Copyright 1984 - Societ& Chimiques Belges.
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The first one was the application of carbon black composite material. This was done by mixing the carbon particles with a poly-furfuryl alcohol binder and subsequently carbonizing the binder material at elevated temperatures. The composites demonstrated promising textural properties for use as supports for sulphide catalysts. Nevertheless, owing to the inert character of the composite surface, sintering of the MO phase took place during sulphidation and it was thus necessary to subject. the composites to oxidation treatments to increase their heterogeneity and thus their affinity towards the deposited MO phase. The second way to modify the sulphide support interaction was the covering of the Al203 surface with a thin layer of carbon prior to impregnation of the transition metals [22]. With this method the favorable carbon surface properties were combined with the optimum textural and mechanical properties of the Al203 support. A threefold increase in activity compared to Co/Al203 was obtained in HDS demonstrating the effective shielding by the carbon layer which reduces or eliminates the strong metal-alumina interaction.
3. OXIDES As mentioned above, the support which currently is most used as a support for industrial hydrotreating catalysts is the gamma cubic type alumina. Nevertheless, other oxides have been studied, primarily the most common and less expensive ones, such as silica and silica-aluminas. Later on, these studies were broadened to other oxides. The results concerning simple oxides will be given first and those concerning binary oxides in Parts .3 and 4 of the present review. 3.1. silica Generally, it is stated in the literature that silica leads to catalysts having a lower activity than alumina [23] [24]. In academic studies, silica was often used because of its inert character and consequently its smaller interaction with the sulphided phase which permits a better characterization. For example, see the thorough studies of Yermakov et al. dealing with the preparation, the physicochemical properties and the catalytic properties of NiMo and NiW/SiO2 [25]. Nevertheless, there is still some controversy about the existence of chemical interaction between molybdenum and the silica support, and a few studies are still carried out on this subject [26]. 3.2. Titanin and zirconla A few years ago, the first attempts at the use of unconventional oxides (TiO2, ZrO2 etc.) were carried out in various reactions, such as the treatment of heavy oils, the hydrocracking of bituminous coal [27, 281, the hydrotreatment of solvent refined coal [29] and solvent refined lignin [30]. In these preliminary papers significant support effects were .found on the hydrogenation and hydrocracking activities of
494
molybdenum sulphide catalysts. The titania support and, to a lesser extent, the xirconia support appeared promising for upgrading the coal-derived liquids. In order to explain these effects, detailed characterizations of MoOg/TiO2 catalysts by Raman and Fourier transform infrared spectroscopy were reported by Ng and Gulari [31], Nishijima et al. [32], Wachs and Hardcastle [33] and Quincy et al. [34]. According to these authors, one of the most important differences in comparison to alumina, is the fact that as long as the loading is less than one monolayer, no molybdenum trioxide is formed on titania and that molybdate anions bind to the titania surface strongly and uniformly. Even after reduction, remaining M&J-Ti bonds contribute to the stabilization of the MO species on the titania surface without any segregation or sintering. Raman spectroscopic data [3 1, 341 suggest that different molybdenum species are present on oxidic Mo/Ti02 catalysts. Tetrahedral molybdate species formed at low MO loadings and surface polyoxomolybdate species with octahedral coordination formed at higher MO content This was rationalized on the basis of the surface characteristics of Ti02 : molybdenum will preferentially react first with the terminal hydroxyl group associated with one Ti4+ forming MO(T) species and this species will continue to react with bridging OH groups coordinated to two Ti4+ S&S. The molybdate spreading was also related to the difference in the distribution of surface OH groups over the support [31, 351. If Ti02 and Al203 present roughly comparable surface concentrations of anionic OH, on the titama surface Ti4+ are only tetrahedrally coordinated and the hydroxyl groups are uniformly distributed providing a homogenous surface for the adsorption of molybdate anions. On the other hand, on Al2O3, A13+ ions are octahedrally and tetrahedrally coordinated and hydroxyl groups are ordered in parallel rows [36]. Such preferential arrangement in rows requires an arrangement of the molybdate anions during the impregnation process which leads to molybdenum trioxide even at submonolayer loadings. These observations are in agreement with XPS data reported by Nag [371 and Caceres et al. [35] on the oxidic precursors. On TiO2 the Mo3d doublet displays an excellent resolution and a low FWHM (full width at half maximum) value suggesting that MO species are more homogeneously distributed and more similarly chemically coordinated than on Al203 where Mo6+ species with different chemical characteristics are present. The results reported above could be considered as slightly in contradiction with those of Wachs and Hardcastle [33] who observed exclusively tetrahedrally coordinated surface species for both MolTi and M0/Al203 samples. Nevertheless, it should be noted that this last study concerned only low amount of molybdenum (<5 I). On the other hand, using an equilibration adsorption method to introduce molybdenum on Ti02, Rim et al. [38] showed that at pH 3.98 the molybdenum oxide monolayer is composed of distorded octahedra. But in this last study, the sulphided state of the catalysts was not examined. This was the case in the work of Ng and Gulari [31] and Quincy et al [3 11 who proposed that tetrahedral molybdenum species are the precursors of the most active entities for HDS. Assuming that the surface structure of the sulphided catalyst could be related to that of the oxidic catalyst, interesting observations could be deduced from temperature
495
programmed reduction studies, as was done by Shimada et al. [39], Leyrer et al. [40] and Caceres et al.[35]. The reducibility of molybdate depends on the support and MoEi is more easily reduced than M0/Al203 ; this mdt was confirmed by high temperature RSR techniques. Kohno et al. [41] using high pressure, Nevertheless, the interpretation of this effect is still a matter of controversy since some authors conclude that the interaction between MO and Ti02 is weak while some others maintain that it is strong. If there is a general agreement that sulphidation seems to be easier on Ti02 than on Al2O3, it is also clear that the differences observed in the structures of the catalyst are much smaller in the sulphided form than in the oxidic state. This fact could be iV*lstrated by XPS results since no variations of Mo3d binding energies over sulphioed MoiTi02 and M0/Al203 were observed by Ramirez et al. [42] and only small variations within the limit of the sensitivity of the technique were noticed by Shimada et al. [39] and Okamoto et al. [43]. In this last work some unsulphided Mo6+ species still remained after sulphidation. The absence of a significant shift in the MO binding energies, either for Ti02 or Al203 supported catalysts, with respect to unsupported MoS2, does not support the existence of an electronic effect as the main cause of the great differences in activity. Therefore, attention has been recently paid to the morphology of MoS2-like species in order to discuss these differences in terms of a geometrical effect. Thus, using thermal desorption spectroscopy of NO on sulphided catalyst, Okamoto et al. [43] demonstrated that there are at least two distinct NO adsorption sites assigned to triply and doubly coordinatively unsaturated MO sites (CUS). The proportion of these different sites depends on the support and MO loading in relation to variations in the morphology of the sulphided particles : MoS2 slabs produced on M0/Al203 catalysts expose a higher proportion of the (1010) edge planes than those on Momi catalysts. The evidence of a geometrical effect was also reported by Pratt et al. [44]. Using HREM examinations they found that the dominant morphology of MoS2 surface species on A4203 (or Si02) is that of flakes of MoS2 containing one to six layers and located verttcally to the surface of the support. By contrast, when molybdenum is supported on Tie;?, the Mo!+ consists of “raft-like” structures sitting flat on the support. This “skinning” phenomenon is not clear and differences in specific activities observed at low molybdenum loading are not really explained in relation to these observations. By contrast, the electron microscopic observations of Ramirez et al. [42] show clearly that for Mo/Ti02 samples, in comparison to MO/Al2O3, the length distribution of MoS2 crystal&s as well as the number of stacked layers shift toward the smaller particle range (see Fig. 3). The differences of the catalytic activities between both supported phases was explained in terms of different activities between the smaller and larger particles, this difference of activity being related to the MoS2 crystallite orientations. It was suggested that the role of Ti02 is to promote the formation of highly active edge-up MoS2 particles due to an interaction between the MoS2 edge planes and some planes of Ti02. Moreover, the synergetic (or promoting) effect of cobalt was found to be lower on Tie;! than on Al203 (3.2 and
496
8, respectively) in agreement with previous results of Muralidhar et al. [24]. However, this promoting effect increased with increasing molybdenum loading for catalysts with the same ratio of Co to MO (Fig. 4). This was ascribed to the greater difficulty with which the mixed CoMoS species are formed on the edge-up particles that exist preferentially at low MOcontents.
b
20
NUMBER
OF
FRINGES
(N)
LENGTH
40
60
OF
80
100
CRYSTALLITE
120
Cil
Figure 3. Distribution.of the number of fringes (a) and of the length of cystallites (b) for TiO2 and Al203 molybdenum sulphide supported catalysts. (Data adopted from Ref. 41.
1.4
2.8
at Ma/sq.
5.6
nm.
Figure 4. (a) Synergetic effect of cobalt for various supported molybdenum catalysts; (b) Variation of the synergy with metal loading for CoMoITiO2 catalysts at constant Co/(Co+Mo)ratio. Data adopted from Ref. 41.
497
While the support effect in hydrotreating catalysts has mainly been studied on Ti02, less attention has been devoted to other oxides such as ZrO2, CeO2, La203 and MgO. For ZrO2, in particular, the results are scarce and often contradictory. Nag [371 reported that MO oxidic species on Zr02 are more homogenously distributed and more similarly chemically coordinated than on Al2O3. According to Zaki et al. [45], the MO monolayer cannot be formed and MOO3 aggregates. Nevertheless, good hydrogenation properties of MolZrO2 catalysts have been found in the last few years. Large differences in CO hydrogenation, observed by Mauchausse et al. [46] for MolZrO2 and Mo/CeO2, compared to M0/Al203 were interpreted by variations in the density of the active sites depending on residual MoO-Al support bonds, in the most active solids, the support being slightly sulphided. These promising hydrogenation properties have also been observed for pyridine hydrogenation and piperidine hydrodenitrogenation [471. Using infrared spectroscopy to study the chemisorption of the probe CO molecules, Houssenbay et al. [48] also explain ‘the high activity of Mo/ZrO2 by an increase in the number of active sites. Detailed characterization of Mo/ZrO;! catalysts are given in the present issue [49]. An important drawback in the utilization of Zr02 as a support for hydrotreating catalysts arises from its textural and structural instability during high temperature treatments. A lot of studies (particularly for ceramic applications) have therefore been devoted to the stabilization of this oxide. .In order to obtain high surface area zirconia carriers, low temperature preparation methods are generally preferred. Among those methods, the precipitation of zirconia gel followed by impregnation with Y, Al, Ni, or molten salt preparation lead to interesting textural properties [50, 51, 521. Another approach to avoiding the instability of ZrO2 or the drawback of the low surface area of Ti02 is the study of mixed oxides. 4. BINARY OXIDES As discussed above, the most promising results were reported for TiO2 and ZrO2. Therefore, attempts to utilii mixed oxides as supports for hydrotreating catalysts have been primarily concerned with Ti02 and ZrO2. For MO catalysts supported on TiO2-Al203 and TiO2-Zr02, high hydrocracking activities of diphenylmethane were observed by Nishijima et al. [32] and a cooperative or synergetic effect of both components supports was evidenced. The maximum activity appeared for atomic ratios Ti/(Ti+Al) = 0.5 and Ti/(Ti+Zr) = 0.8. These catalysts also displayed higher HDN properties than simple oxides for the upgrading of coal derived liquids. These inherent HDS and HDN capabilities of titania and zirconia supports have been also applied in ternary compounds and a CoMo/TiO2-ZrO2-V205 catalyst was found to be twice as active as a commercial CoMo for HDS of heavy diesel and vacuum gas oil [533. It should be noted that the properties of such mixed oxides
498
depend to a great extent on the preparation procedure. Moreover, CoMo/Ti0 2Zr02, CoMo/Ti02-Ce02 and CoMo/Ti02-Mn02 catalysts present very high initial HDS activities [54]. The promising properties of these mixed oxides is illustrated by the large number of very recent and interesting papers dealing with this subject ; see for example Refs. 55 and 56.
5. SILICA - ALUMINA The catalytic properties of molybdenum sulphide based catalysts supported on Si02-A1203 depend on the amount of Si02 and on the reaction, i.e. HDS, HYD, hydrocracking or RON. Muralidhar et al. [24] found that HOS and HYD activities decrease with increase in Si02 content from 10 to 75 %, while the activity for the hydrocracking of 2,4,4-trimethyl-l-pentene showed an opposite trend. These high hydrocracking activities have to be related to the properties of the protonic acid supports and not to the properties of the sulphide phase. For the HDN reactions, a Hofmann-type elimination and a nucleophilic displacement mechanism have been proposed. Such mechanisms are related to the acid-base properties of the catalysts. Consequently, it is to be expected that the support acidity will influence these reactions. In the hydrotreatment of heavy feedstocks, Toulhoat and Kessas [57] observed an increase of the HDN activity when 15 % Si0 2-AI203 was utilized as a support for NiMo catalyst in comparison with the properties of the same active phase on A1203. But a decrease was found at higher concentrations of silica, i.e. 25 %. The HDS activity decreased in both instances. On the other hand, in industrial processes, the high acidic properties of these catalysts tend to accelerate the formation of coke by increasing the rate of polymerization and alkylation reactions, which involve the olefins produced as intermediates by the various reactions of hydrogenolysis [23].
6. ZEOLITES Despite the extensive use of zeolites in catalysis, there has been relatively little work carried out on their applications to hydrotreating processes. However, zeolites have many features, such as high surface area, shape selectivity, the ability to produce high dispersion of metals, and a range of acid-base properties, which could be utilized in this type of reaction. The first attempts to use zeolites as a carrier for hydrotreating catalysts were reported in the "open" literature around 1970 but it is only in the last few years that publications have reported promising results concerning the utilization of these catalysts for hydrodenitrogenation reactions. In these first publications, the activities reported are generally lower than those of alumina supported catalysts. For example,
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Vrinat et al. [58] studied the properties of cobalt molybdenum in HY and .NaY xeolites. Cobalt was introduced by ion exchange and molybdenum by molybdenum carbonyl adsorption. For dibenxothiophene conversion these catalysts were less active than conventional cobalt molybdenum on alumina catalysts. One point should to be emphasised at this stage of our discussion, viz the diffusion problem. If the sulphide phase is well dispersed inside the xeolite framework, one would expect diffusion limitation with large molecules. This may be an explanation for the results obtained by Vrinat et al. [58]. Cid et al. [59, 60, 611 compared the properties of cobalt molybdenum catalysts on alumina, silica and NaY zeolite. All catalysts contained 12 wt.-% MoO3 and 5 % Co0 and were prepared by successive impregnations, first with ammonium heptamolybdate and then with cobalt nitrate [59]. The authors concluded that MO and Co are moderately dispersed into the cavities of the NaY xeolite, but located separately, without the formation of a clear &MO interaction phase. This lack of interaction between cobalt and molybdenum was supposed to be responsible for the relatively low hydrodesulphuration activity of the CoMo/NaY catalyst. Later on the same authors showed that better catalysts can be obtained by incorporating Co by ion exchange prior to molybdenum deposition, rather than by simultaneous impregnation of both Co and MO [60]. The appearance of a NiMoS phase was also claimed by Davidova et al. [62, 631. According to these authors, the samples based on Y and ZSM-5 xeolites display the maximal activity. This strong influence of the method of preparation on the catalytic results was also mentioned by Fornes et al. [64] for NiMo on ultrastable HY xeolites. Leglise et al. [65] used infrared spectroscopy to conlirm the existence of an interaction between the two components of NiMo sulphide catalysts loaded into Y xeolite, whether or not the latter was dealuminated. Nevertheless, the NiMoS phase was more active for benzene hydrogenation on the alumina carrier than in the zeolites which can be ascribed to the lower accessibility of the active phase to the hydrocarbon reagents. Moreover, side reactions such as alkylation, isomerixation and cracking, appeared on the xeolite catalyst due to its acidic properties. Okamoto et al, [66,67l used NaY and KY xeolites to prepare highly dispersed molybdenum sulphide from Mo(C0)6 or by an impregnation method. Catalysts prepared from Mo(C0)6 were found to be more active than those obtained by the more conventional method. It was suggested that the reolite basicity controls the strength of interaction between the xeolite oxygens and the subcarbonyl species and thus the final dispersion of sulphided molybdenum. Ruthenium sulphide supported on Y zeolite has good properties for hydrodenitrogenation reactions, which were first reported by Harvey and Matheson [68, 69, 703. An activity similar to (or slightly better than) that of conventional NiM0/Al203 was obtained for hydrodenitrogenation of quinoline with metal loadings much lower than those used commonly in commercial hydrodenitrogenation catalysts. The most active catalyst was obtained by ion exchange of lRu(NI-13)d3+ into NH4Y followed by sulphidation. Unexpectedly, the high activity of these catalysts for HDN was not reflected in a correspondingly high activity for HDS. The
500
nature of the active ruthenium sulphur species was not established. From XRD examinations and TEM imaging of the sulphided catalysts, the authors concluded that the ruthenium sulphide was dispersed as aggregates composed of only a few molecules at most. XPS observations indicated a significant migration to the external Surface.
A systematic study of the influence of the support acidity was performed by G6biiliis et al. l?‘l] for ruthenium sulphide on HY, NaY and KY in a variety of reactions, i.e. the hydrodesulphurixation of thiophene, the conversion (hydrogenation and cracking) of biphenyl, the hydrogenation of pyridine, and the hydrodenitrogenation of piperidine. The rapid deactivation and thus the low activity of Ru/HY in the HDS of thiophene was attributed to coke formation on the Br6nsted acid sites of the support. The stability and the activity of the catalysts in this reaction can be improved by decreasing the acidity, which was particularly noticeable for R&Y. In the conversion of biphenyl, the activity towards the formation of cracking products increased with the acidity of the supports. The activity of the catalysts for the conversion of nitrogen containing molecules is less affected by the acid strength of the supports. Only a slight decrease of the activities is observed for the most acidic support. In conclusion, the study of hydrotreating catalysts deposited in xeolites appears to be promising but even more difficult than the studies relative to other carriers due to the peculiar structural properties of xeolites. The main problem is the determination of the position of the sulphided species, outside or inside the xeolite framework. As for carbon supported sulphides, highly dispersed species may present particular properties.
7. MIXTURES OF ZEOLITES AND OTHER CATALYSTS Several studies deal with hydrotreating catalysts composed of xeolite and alumina or silica-alumina [72, 731. The purpose of these studies was to combine the hydrogenation function due to CoMo, NiMo or NiW sulphide to a hydrogenolysis function brought about by the support and its acid centres. The further development of macroporosity by boron addition improved the HDN properties in upgrading vacuum gas-oil [73]. Another improvement of the HDN activity was obtained using a physical mixture of a conventional NiMo/Al203 and Ru/Y xeolite [68, 70, 741. The origin of this synergetic effect is still not completely understood. Moreover, this effect was not observed with any other transition metal sulphides supported on Y xeolite [75].
8. CLAYS, PILLARED CLAYS AND NATURAL MINERALS In the search for natural minerals as carriers for low cost hydrotreating catalysts, the first attempt was reported by Ianibello et al. using natural bauxite [76].
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The catalytic properties of the MO, CoMo and NiMo bauxite based catalysts resemble those of alumina catalysts in the metal removal reaction but lower activities were observed f6r the HDS and HDN reactions. Using natural clays themselves as catalysts, Sakata and Hamrin l77l compared the catalytic activities in HDS and HDN reactions of kaolinite, montmorillonite and illite API reference clays. The clay activity for HDS was correlated with the impurity iron content and compared to a commercial COMO/Al2O3 catalyst. Kaolinite clays gave good conversion for n-butylamine HDN. According to Mochida et al. [29], a NiMo supported on sepiolite is better than a commercial NiMo/Al203 for the conversion of the heavier fractions of solvent refined coal. This result was explained in relation to the pore structure of the support. Unfortunately this catalyst was less active for the removal of heteroatoms. It should be mentioned that acid pretreatment of these natural clays plays an important role in the preparation of the catalysts and good activities for thiophene HDS have been found over a N&IO/acid treated sepiolite catalyst by Corma et al.
ml. Nervertheless, among the various natural clays which were tested as catalyst support, the most interesting results were obtained over bentonites. Schultz et al. [79] found higher hydrocracking and HDS activities for MoS2 supported on acid treated Brazilian bentonites than for Al2O3, the bentonite itself contributing to the hydrocracking activity. Inasmuch as smectite clays have a long history of use as cracking catalysts, an important property of such materials is their ability to swell by intercalation of organic cations, organometallic complexes or mineral cations. High surface area bentonite pillared with inorganic pillaring agents was reported by several authors (see Occelli and Rennard, Ref. 80). Such pillared clays could find application as hydrotreating catalysts for the conversion of high molecular weight hydrocarbons. Used as support for a NiMo active phase, in composite catalysts, several bentonites intercalated with Al, Si or Zr cations displayed excellent hydrotreating activities WI. Along the same lines, a NiW catalyst supported on a cross-linked hydroxy titanium bentonite with a Ti(SO)4 solution shows higher pyridine HDN activity than an industrial NiW/Al203 catalyst [81]. Such an activity could be explained by the high surface area of the carrier and by the presence of titanium, since TiO2 has been recognized as an excellent support for the hydrotreating reaction.
9. CONCLUSIONS The objective of this review was to give an overview on the various support interactions encountered for hydrotreating catalysts. At the present time, this field of research is very active and every day sees the proposal of a new type of support for hydroprocessing catalysts [82, 831. Recent physicochemical characterisation techniques allow to progress in our knowledge of the sulphide support interaction
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and to distinguish between electronic and geometrical effects (size, stacking, orientations). Nevertheless, even if a lot of points still remain to be clarified, the results reported in the literature allow us to look to the future with optimism. A new generation of supports adapted to the various constraints of the hydrotreating processes will probably appear within the next few years. One should not underestimate the amount of work which this objective requires, since the development of new supports implies the optimization of the interaction with the active phase, of the sulphiding process and above all of the textural and mechanical properties.
Acknowledgments This work was carried out in the framework of the contract “New catalysts for hydrodenitrogenation of heavy cuts” of the “Non nuclear energy” R & D programme of the Commission of the European Communities (EN 3C - 0040 F). It received support from ELF, IFP and TOTAL and from the CNRS-PIRSEM.
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