TiO2–Al2O3 catalysts in gasoil and thiophene HDS and pyridine HDN: effect of the TiO2–Al2O3 composition

Applied Catalysis A: General 180 (1999) 53±61

Catalytic activities of Co(Ni)Mo/TiO2±Al2O3 catalysts in gasoil and thiophene HDS and pyridine HDN: effect of the TiO2±Al2O3 composition M.P. Borquea, A. LoÂpez-Agudoa,*, E. OlguõÂnb, M. Vrinatb, L. CedenÄoc, J. RamõÂrezc a

Instituto de CataÂlisis y PetroleoquõÂmica, CSIC, Cantoblanco, 28049 Madrid, Spain Institut de Recherches sur la Catalyse, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France c Unicat, Facultad de QuõÂmica UNAM, Cd. Universitaria, 04510 MeÂxico DF, Mexico

b

Received 25 May 1998; received in revised form 15 June 1998; accepted 15 September 1998

Abstract The effect of the TiO2±Al2O3 mixed oxide support composition on the hydrodesulfurization (HDS) of gasoil and the simultaneous HDS and hydrodenitrogenation (HDN) of gasoil‡pyridine was studied over two series of CoMo and NiMo catalysts. The intrinsic activities for gasoil HDS and pyridine HDN were signi®cantly increased by increasing the amount of TiO2 into the support, and particularly over rich- and pure-TiO2-based catalysts. It is suggested that the increase in activity be due to an improvement in reducing and sul®ding of molybdena over TiO2. The inhibiting effect of pyridine on gasoil HDS was found to be similar for all the catalysts, i.e., was independent of the support composition. The ranking of the catalysts for the gasoil HDS test differed from that obtained for the thiophene test at different hydrogen pressures. In the case of gasoil HDS, the activity increases with TiO2 content and large differences are observed between the catalysts supported on pure Al2O3 and pure TiO2. In contrast, in the case of the thiophene test, the pure Al2O3-based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also, in the thiophene test the difference in intrinsic activity between the pure Al2O3based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also in the thiophene test, the difference in intrinsic activity between the pure Al2O3- and pure TiO2-based catalysts is relatively small and dependent on the H2 pressure used. Such differences in activity trend among the gasoil and the thiophene tests are due to a different sensitivity of the catalysts (by different support or promoter) to the experimental conditions used. The results of the effect of the H2 partial pressure on the thiophene HDS, and on the effect of H2S concentration on gasoil HDS demonstrate the importance of these parameters, in addition to the nature of the reactant, to perform an adequate catalyst ranking. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Hydrodenitrogenation; Gasoil; Pyridine; CoMo/Al2O3 catalysts; NiMo/Al2O3 catalysts; Titania; Alumina

*Corresponding author. 0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00377-9

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M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

1. Introduction

2. Experimental

Due to the high intrinsic hydrodesulfurization (HDS) activity exhibited by TiO2-supported molybdena catalysts [1±5], in recent years the preparation and characterization of mixed oxides containing titania has attracted considerable attention as support for hydrotreating catalysts [6±13]. The TiO2±Al2O3 combination seems to be a promising support for hydrotreating catalysts since it allows to overcome the disadvantages of low surface titania, the low thermal stability of active anatase, and also the relatively poor sul®dation of the alumina-supported catalysts [7,14]. Thus, with the aim of obtaining highly dispersed TiO2 on Al2O3, different preparation methods have been used in the past [5±22]. The coprecipitation of titanium and aluminum isopropoxides is one of the methods often used for this purpose. This method leads to stable TiO2±Al2O3 mixed oxides with high surface areas even for TiO2-rich compositions [7]. It has also been suggested that the increased Lewis acidity of the supports favors molybdenum dispersion, less interaction with the support, and better sul®dation of the Mo phase, and consequently, higher intrinsic HDS activity of the supported Mo catalysts. In previous studies on thiophene HDS over Mo [4,5,7], W [13], CoMo [4,11] and NiMo [8,9,12] supported on Al2O3±TiO2 mixed oxides, it has been shown that the supported formulation has a strong in¯uence on the catalyst performance. Recently, it has been put in evidence that the activity trend is also to some extent dependent on the type of model molecule used in the study [23]. In the literature there is some discrepancy on the question if tests performed with model molecules (thiophene or DBT) represent adequately the HDS process occurring with a commercial gasoil feed [24± 26] which contains different sulfur and nitrogen compounds in a complex mixture of hydrocarbons. In view of this, it is the purpose of this work to analyze the performance of CoMo and NiMo catalysts supported on TiO2±Al2O3 mixed oxides under conditions similar to those used in industry, i.e., high pressure and a real gasoil feedstock. Additionally, experiments on the increased presence of nitrogen compounds (pyridine added) and H2S in the reaction stream were performed in order to analyze the effect of these variables on the performance of the different catalyst formulation.

2.1. Catalyst preparation The same CoMo/ and NiMo/TiO2±Al2O3 catalysts studied previously [23] were used in the present work. They were prepared by pore-volume impregnation of a series of TiO2±Al2O3 supports (covering a full range of composition of molar ratio TiO2/(TiO2‡Al2O3) from 0.0 to 1.0), with aqueous solutions of ammonium heptamolybdate and cobalt or nickel nitrate. The concentration of the impregnation solutions was appropriate to obtain samples with the same metal loading per square nanometer of support surface, i.e., 2.8 Mo atoms/nm2, and an atomic promoter to molybdenum ratio, rˆCo (or Ni)/Co (or Ni)‡Mo), of 0.3. Successive impregnation (®rst Mo and second Co or Ni) was used to prepare all the catalysts. After each impregnation, the samples were dried at 383 K for 12 h and then calcined at 673 K for 4 h. The supports were denoted by Ti(x)±Al, where x is the molar ratio TiO2/(TiO2‡Al2O3). 2.2. Catalytic activity measurements The activities of the catalysts were measured in three independent tests: HDS of gasoil in absence of pyridine, simultaneous HDS of gasoil and HDN of pyridine, and HDS of thiophene at different total pressures. The two ®rst tests were carried out consecutively (®rstly with gasoil and then with gasoil enriched with pyridine) on the same catalyst charge and at the same operating conditions. The thiophene HDS test was carried out on a fresh catalyst charge and at different operating conditions than with gasoil. All experiments were performed in a high-pressure ¯ow packed-bed microreactor. The standard experiments for gasoil HDS, and for the simultaneous gasoil HDS and pyridine HDN tests were carried out on a 0.2 g of catalyst (particle size between 0.15 and 0.25 mm) diluted with twice the volume of similar size inert SiC particles. Prior to the reaction, the catalysts was sul®ded in situ with a mixture of 7 vol% CS2 in gasoil (boiling range 468±667 K, density 0.877 g/cm3, sulfur 1.85 wt%, nitrogen 700 ppm) at a ¯ow rate of 13 cm3/h, 20 bar and 623 K for 4 h. After sul®dation of the catalyst, the reactor was pressurized to 30 bar with H2 and the

M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

gasoil was fed to the reactor . The standard reaction conditions were: temperature 623 K; total pressure 30 bar; liquid hourly space velocity (LHSV) 65 hÿ1; and H2 (gas)/gasoil (liquid) ratio of 400. After a stabilization time of 2 h on-stream, three successive samples were collected at regular intervals of 30 min and then analyzed for total sulfur and nitrogen contents. At the end of the gasoil HDS test, the gasoil feed was substituted by another one enriched with pyridine (8000 ppm of N), and the experiment to measure simultaneously gasoil HDS and pyridine HDN was started. The S and the N content of the feedstock and products were determined by ¯uorescence and chemiluminescence, respectively, with an Antek analyzer. The HDS and the HDN volumetric catalytic activities expressed by pseudo second-order rate constants for HDS (kHDS) and ®rst-order rate constant for pyridine HDN (kHDN), respectively, were calculated as described elsewhere [27]. For the thiophene HDS test, a catalyst charge of 0.2 g diluted with SiC particles in a 1:5 vol. ratio was used. Prior to the reaction, the catalysts were sul®ded in situ with a mixture of 10% H2S in H2 under atmospheric pressure at 673 K for 4 h. The reaction experiments were carried out with a feed consisting of 15% thiophene in cyclohexane (18 ml/h) and an H2 ¯ow of 120 STP ml/min at 623 K. The total pressure was varied in the increasing order of 3, 10, 20, and 30 bar. After 2.5 h on-stream, liquid samples were collected periodically for 30 min and anlayzed by gas chromatography using a column packed with n-octane Porasil C. The speci®c activity, As (mol/s g/

55

cat), was calculated after 3.5 h time on-stream at the pseudo stationary state according to the following equation: As ˆ F0 x=m; where F0 is the molar ¯ow rate of thiophene, x is the conversion of thiophene and m is the weight of the catalysts. 3. Results 3.1. HDS activity for gasoil in the absence of pyridine Table 1 lists the apparent reaction rate constants (per g catalyst) for the HDS of gasoil in the absence of pyridine at 623 K. Both the CoMo and NiMo catalysts followed similar HDS activity trends per gram of the catalyst with the support composition. However, the HDS activity of the catalyst, expressed per unit surface area of support, changes with the support composition as shown in Fig. 1. These results show, in general, that in the case of gasoil HDS, the activity is progressively increased with the TiO2 content in the support for both CoMo and NiMo catalysts, but they differed in activity level. In the mixed oxide support region, the NiMo catalysts present lower activity than the corresponding CoMo catalysts. This result is contrary to that observed previously for T and DBT model compounds [23]. However, on pure-Al2O3, the difference in gasoil HDS activity between CoMo and NiMo catalysts was

Table 1 Activities of CoMo/TiO2±Al2O3 and NiMo/TiO2±Al2O3 catalysts for gasoil HDS, and simultaneous gasoil HDS and pyridine HDN at 623 K Catalyst

Gasoil HDS 3

Simultaneous gasoil HDS and pyridine HDN

kHDS(G) (cm /g cat. h)

kHDS(G‡P) (cm3/g cat. h)

kHDN (cm3/g cat. h)

CoMo/Ti(0)±Al CoMo/Ti(0.2)±Al CoMo/Ti(0.5)±Al CoMo/Ti(0.95)±Al CoMo/Ti(1)±Al

7.8 12.2 28.2 64.7 36.6

8.1 10.6 25.4 62.1 38.3

34.9 33.0 47.3 50.4 40.7

NiMo/Ti(0)±Al NiMo/Ti(0.2)±Al NiMo/Ti(0.5)±Al NiMo/Ti(0.95)±Al NiMo/Ti(1)±Al

10.3 7.7 18.5 19.7 10.9

8.7 5.5 17.2 18.3 8.7

46.4 6.6 21.2 41.2 25.9

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M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

Fig. 1. Effect of the support composition on the HDS activity, expressed per unit surface, of gas oil (ÐÐÐ) and gasoil‡pyridine (


) at 623 K, over CoMo/ and NiMo/TiO2±Al2O3 catalysts.

very small, while on pure TiO2 was very large, about three times greater for CoMo than for NiMo. 3.2. Simultaneous HDS of gasoil and HDN of pyridine The activities per gram of catalysts, for gasoil HDS in the presence of pyridine at 623 K, are summarized in Table 1. The variations of the HDS activity per unit surface area of support with support composition are compared with those obtained in absence of pyridine in Fig. 1. These results clearly show that the catalyst HDS activity trend is virtually equal in absence than in presence of pyridine. However, the catalytic activity values were slightly lower in the presence of pyridine re¯ecting the known inhibition effect of pyridine on HDS. This effect has been explained by the strong interaction of basic nitrogen compounds with the catalyst surface [28±30]. In each catalyst series, no signi®cant differences in the inhibition factor, kHDS(G)/kHDS(G‡P), among catalysts were observed; the values ¯uctuated, without a de®ned trend, within the range 1.20.2 for the NiMo catalysts and 1.10.1 for the CoMo catalysts. How-

ever, as the original gasoil contains 700 ppm of N, the possibility that some inhibition due to such N-compounds could already occur in the experiments with gasoil in absence of added pyridine cannot be excluded. Nevertheless, if this would occur, the degree of inhibition due to the N-compounds in the original gasoil would be probably very similar for all the catalysts because the difference in HDS activity between the gasoil and gasoil‡pyridine tests is practically maintained along the catalyst series. In contrast, Wei et al. [31] found, for NiMo catalysts supported on titania-covered alumina, that the degree of in¯uence of pyridine on the thiophene HDS activity is different, especially in the primary stage of the reaction. The differences between our results and those of Wei et al. are likely due to differences in the support preparation and/or the reaction conditions used in the two studies. In the study of Wei et al. the titania-covered alumina support was prepared by impregnation of Al2O3 with a TiCl4. Additional studies will be necessary to clarify if the degree of inhibition is signi®cantly in¯uenced by the support composition. The pyridine HDN activities of the CoMo and NiMo catalysts are also summarized in Table 1. For this reaction, unlike for gasoil HDS, the changes in HDN activity with support composition are more pronounced for the NiMo than for the CoMo catalysts. The variation of the HDN activity per square meter of support with the TiO2/(TiO2‡Al2O3) molar ratio is shown in Fig. 2. It is seen that the general trend of the HDN activity curve is similar to that found for HDS activity (Fig. 1). The activity changes relatively little up to TiO2 content of about 50% and then increases steeply in such a way that for the catalyst supported on pure TiO2, the HDN activity values are about ®ve (NiMo) or nine (CoMo) times higher than on pure alumina-supported catalysts. Furthermore, the HDN activity values of the mixed oxides and pure TiO2supported CoMo catalysts were higher than for their NiMo counterparts. 3.3. Thiophene HDS activity at different H2 pressures To inquire if the differences in catalytic behavior between the high-pressure gasoil test and those observed previously for thiophene at atmospheric pressure [23] were a consequence of the different

M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

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Fig. 4. Effect of the support composition in NiMo/Ti(x)±Al catalyst on the intrinsic activity (Ai) for thiophene HDS at 623 K and different H2 pressures. Symbols as in Fig. 3. Fig. 2. Effect of the support composition on the HDN activity, expressed per unit surface, for simultaneous HDS and HDN of gasoil‡pyridine at 623 K of CoMo/ and NiMo/TiO2±Al2O3 catalysts.

operating H2 pressures, some experiments were performed with thiophene at different H2 pressures. For the NiMo/TiO2±Al2O3 catalysts, Fig. 3 shows the variations of the activity per gram of catalyst in

Fig. 3. Effect of the support composition in NiMo/Ti(x)±Al catalyst on the specific activity (As) for thiophene HDS at 623 K and different H2 pressures: (&) 3; (~) 10; (*) 20; (!) 30 bar.

thiophene HDS as a function of the TiO2/ (TiO2‡Al2O3) ratio at different H2 pressures. In contrast with the result for gasoil HDS, in the thiophene reaction the pure Al2O3-supported catalyst presents higher activity at all the pressures than the rest of the catalyst series. According to this ®gure, the difference in activity per gram of catalyst between the pure Al2O3 and pure TiO2-based catalysts was higher with increasing total pressure. However, if the activity is expressed per Mo atom, Fig. 4 shows that at low pressure the activity of the pure Al2O3-based catalyst is slightly lower than that of the pure TiO2-based catalyst, while at high pressure the result is the contrary. Therefore, the H2 pressure changes the activity ranking of the Al2O3- and pure TiO2-based catalysts. At all the pressures studied, a minimum in activity for the TiO2/(TiO2‡Al2O3) ratioˆ0.5 was observed. As expected, the thiophene activity curve at 3 bar (Fig. 4) has some similarity to that for the T test at atmospheric pressure [23]. An interesting observation is that the activity trend for the T reaction at 30 bar was quite different from that observed with gasoil (Fig. 1), which indicates that thiophene is not a good model molecule to rank the NiMo/TiO2±Al2O3 catalytic behavior with real feeds. The observed differences in activity trend between these tests are not exclusively due to the nature or reactivity of the feeds, but possibly also due to the operating conditions, particularly to the total H2 pres-

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M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

sure and H2S/H2 ratio which may affect differently the catalysts. Such effects has been recently observed in the HDS of T and DBT over CoMo catalysts supported on Al2O3 and on Ti(50)±Al(50) carriers [11]. In the present high-pressure T and gasoil tests the H2S/H2 ratios were different, although in both cases the total H2 pressure was equal, because their conversion levels were different, and therefore, different concentrations of H2S were present in each of the reactors. Thus, the possible inhibition effect of H2S in the gasoil test was examined in order to clarify the differences in activity ranking. These results will be given below. For the CoMo catalysts, the effect of the H2 pressure on thiophene activity was only examined for the pure Al2O3-based catalyst. For such a catalyst the variation of thiophene activity with H2 pressure was similar to that obtained for the NiMo counterpart, but the differences in activity between high and low pressures were smaller for the CoMo than for the NiMo catalyst. 3.4. Effect of H2S (CS2 as precursor) addition to the gasoil test In order to determine whether the support composition affects to the inhibition effect of H2S on the HDS of gasoil, the activity of NiMo supported on pure Al2O3, pure TiO2 and Ti(0.2)±Al carriers was measured at 623 K, 30 bar total pressure, and different H2S partial pressures. To modify concentration of H2S in the reaction, CS2 was added to the gasoil as a H2S precursor. In these experiments the feed consisted of gasoil, CS2 and a small amount of cyclohexane. The amount of the latter was varied in order to maintain the same LHSV in all experiments as the amount of CS2 was varied. To keep a low level of conversion, these experiments were performed at LHSV equal to 90 hÿ1. Fig. 5 shows the effect of H2S on the intrinsic activity for gasoil HDS. The addition of a small amount of H2S led to a slight increase in activity, which could be due to an insuf®cient sul®dation of the catalyst during the sul®dation treatment. With further H2S additions, the inhibiting effect of H2S was evidenced for the three catalysts, this being less pronounced for the pure- and rich-Al2O3 supported catalysts. It should be noted that this inhibiting effect was completely reversible.

Fig. 5. Effect of H2S added to the feedstock on the gasoil HDS activity at 623 K over NiMo/Ti(1)±Al, NiMo/Ti(0) and NiMo/ Ti(x)±Al catalysts.

4. Discussion From the catalytic activity results of gasoil HDS at high pressure (Fig. 1) and from those of the atmospheric and high-pressure T tests (Fig. 4), it is evident that the composition of the TiO2±Al2O3 supports in¯uences the catalytic properties of the CoMo and NiMo catalysts. This in¯uence was previously observed in atmospheric T and high pressure DBT tests [23]. It was common for the three activity tests (gasoil, DBT and atmospheric T tests) that the catalyst supported on pure TiO2 exhibited the highest intrinsic activity; as also observed in other studies [2±7]. However, signi®cant differences in activity trends were observed among the three activity tests when the results in the support region from pure alumina up to about 50% Al2O3 content were compared. In such a region the activity for gasoil HDS increased slightly with increasing TiO2 content, while for the T and DBT tests, a minimum in activity appeared in the TiO2/ (TiO2‡Al2O3) ratio of 0.5. This decrease in the activity of the alumina-rich catalysts was related to a loss of Co in the amorphous Al2O3 [23]. In principle, the absence of such a minimum for the gasoil tests seems to be due to a relative higher increase in HDS intrinsic activity of the TiO2-containing catalysts when the reaction test was conducted at high pressure. This is suggested by the fact that, for the atmospheric T test,

M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

the difference in HDS intrinsic activity between the pure Al2O3 and TiO2 based catalysts was a factor of about 2 or less, while for the gasoil test the difference was about 30 times for the CoMo and about 8 times for the NiMo catalysts. Since the different activity tests were carried out with the same catalysts, we must think that the type of feed and/or the reaction conditions have also an effect on the number or on the intrinsic activity of the HDS active sites of each catalyst. This effect may in turn be related to the extent of reduction and sul®dation of the Mo phase which is generally higher for the TiO2-supported catalysts than for those supported on alumina [2,3,5,8,12,14,32]. Then, under the high-pressure reaction conditions used for the gasoil test, the easier reduction and sul®dation of the TiO2-supported catalysts, as compared to the Al2O3-supported ones, may lead to a large number of catalytically active coordinatively unsaturated Mo sites in the former catalysts. For the T test at high pressure carried out here, the same behavior would be expected. However, the results of thiophene HDS at different H2 pressures (Fig. 4), showed that the pure Al2O3-based catalyst was more sensitive to the pressure than the pure TiO2based catalyst. This suggests that the observed difference in activity trends among the above three mentioned catalytic tests might not be only due to the effect of the H2 pressure used in each of the tests. Additionally, we must also consider that the level of conversion, and consequently, the H2S partial pressure in the reactor was different for the three catalytic tests. Thus, different degrees of inhibition of the reactant adsorption by H2S can be expected in each case. Moreover, whatever the model sulfur-compound used, the inhibiting effect of H2S can be different for each catalyst composition. In this respect, Fig. 5 shows that in the case of gasoil HDS, the pure TiO2 based catalyst is more susceptible to the inhibiting effect of H2S than the pure Al2O3 and rich-Al2O3 based catalysts. Similar results were observed in the HDS of DBT over the same catalysts [33]. In this study, it was also found that the magnitude of the inhibiting effect of H2S on HDS is strongly dependent on the support composition and the type of promoter (Ni or Co). Here we observed (Fig. 1), in contrast with previous reports for HDS of thiophene at atmospheric pressure and DBT at high pressure [23], that for gasoil HDS the CoMo catalysts were somewhat more active than the NiMo counter-

59

parts. This distinct behavior of the gasoil test can be accounted for by the differences in conversion and H2S produced among the three catalytic tests, and by the fact that the NiMo catalysts were more sensitive to the inhibiting effect of H2S than their CoMo counterparts [23,34,35]. Thus, the differences in the ranking of the catalysts for the HDS of gasoil, DBT, and atmospheric T tests, seem to be due to the differences in the feedstock reactants and in the reaction conditions, particularly the H2 and the H2S partial pressures. It is interesting to note that, in contrast to the above different sensitivity of the catalysts to the inhibiting effect of H2S on the HDS activity, the inhibiting effect of pyridine in HDS of gasoil was almost invariable with the support composition and the type of promoter (Ni or Co). The shape of the HDS activity curves versus the support composition (Fig. 1) was practically the same with and without addition of pyridine. The difference in the behavior of the catalysts in the presence of H2S and pyridine could be due to the nature of their inhibition. The presence of pyridine suppresses the HDS reaction because it is adsorbed competitively with the reactants and H2S on the same active sites. In contrast, H2S is also able to change the number of anion vacancy active sites of the catalyst and the distribution of other possible active sites such as H‡, Hÿ and SHÿ [34]. Concerning the pyridine HDN, it is clear from Fig. 2 that the composition of the catalyst supports exerts also a strong in¯uence on the HDN activity. This was notably increased for catalysts with TiO2/ (TiO2‡Al2O3) ratios>0.2. This increase in HDN activity arises essentially from an increase in the number of sulfur vacancy sites. Such an increase is brought about by the improving sul®ding±desul®ding process of TiO2-containing based catalysts, since the hydrogenation sites necessary for HDN, which are unsaturated Mo ions associated with sulfur anion vacancies [3,36], are strongly dependent on catalyst sul®dation. Considering that HDS and HDN are often claimed to occur on different active sites, the fact that HDN and HDS activities followed a similar general trend with support composition suggests, at ®rst sight, that the support composition did not change very much the relative proportion of the active sites for HDS and HDN. To corroborate this, the kHDN/kHDS ratios of all CoMo and NiMo catalysts were calculated and compared. It was found that for all catalysts, except for the

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M.P. Borque et al. / Applied Catalysis A: General 180 (1999) 53±61

pure Al2O3-supported CoMo and NiMo ones, the kHDN/kHDS ratios do not vary signi®cantly with the support composition since they ¯uctuate somewhat around the value of 2, without a clear trend. The slightly higher kHDN/kHDS ratio observed for the pure Al2O3-based catalysts would mean that such catalysts are slightly more selective to HDN than those supported on the mixed oxides and pure TiO2, at least under the present conditions. However, this apparent higher selectivity to HDN of the pure Al2O3 based catalyst cannot attribute to a relative higher hydrogenation function of this catalyst, since it was previously found in the DBT test that the formation of hydrogenated products was lower in the pure aluminasupported catalyst than over TiO2-containing supports [23]. An explanation of these results based on the consideration that the pure alumina-based catalyst would have a higher acidity than the TiO2 containing catalysts seems quite improbable. It was shown that the number of weak and strong acid sites in the mixed oxide supports are nearly the same as in the pure alumina, and that the number of medium acid sites increases linearly with TiO2 content [23]. A more likely explanation of the observed differences in catalyst selectivity could be a more favorable distribution of the acid sites in the vicinity of the hydrogenation sites for the pure alumina supported catalysts, as has been suggested for NiMoP/Al2O3 catalysts [37]. Alternatively, it might be considered the recent proposal of Kasztelan [38] that the HYD, HDS and HDN reactions occur in the form of elimination, addition and substitution reactions on a surface composed of coordinatively unsaturated Mo4‡ ions, sul®de ions, H and SH species. According to this, our results would suggest that the change in the composition of the support change essentially the number of sites where all the reactions take place. The observed differences in the kHDN/kHDS ratio between the Al2O3supported catalysts and those over TiO2-containing supports could then be due to the differences in the rate-determining step of the reactions. This explanation is consistent with the results of the kinetic study of the HDS of DBT over CoMo and NiMo catalysts supported on Al2O3, TiO2 and ZrO2 [33]. This study reports that the rate determining step of the DBT HDS reaction, which can occur by an addition or an E2 elimination mechanisms, is dependent on the nature of the support and the reaction conditions [33].

5. Conclusions The results from the above con®rm that the composition of the TiO2±Al2O3 mixed oxide supports has a strong in¯uence on the activity of CoMo and NiMo catalysts for the HDS and HDN of a commercial feedstock (gasoil), performed under experimental conditions close to those used in industry. In this case, catalysts supported on titanium-rich and pure titania carriers exhibit higher activity for HDS and HDN reactions than those supported on pure alumina. The catalysts ranking and the relative activity differences among the catalysts are strongly dependent on the type of feed (model molecule or gasoil). For the catalysts supported on the pure oxides, larger differences in HDS catalytic activity were found in the gasoil tests than in the thiophene tests. For catalysts supported on Al2O3±TiO2 mixed oxides, the HDS activity trend with support composition was completely different between the thiophene and gasoil tests. An adequate comparison ranking of catalyst formulations based in different supports should consider a study in a wide range of operating conditions, and particular care should be taken in comparing different catalyst formulations under similar H2/H2S ratios in the reaction stream. Acknowledgements Financial support by the Commission of the European Community (Contract no. C1I*CT92-0041), the DGICyT (Project CE93-0012), and the Scienti®c Cooperation Program CSIC (Spain)-CONACYT (MeÂxico), is gratefully acknowledged. References [1] G. Muralidhar, F.E. Massoth, J. Shabtai, J. Catal. 85 (1984) 44. [2] K.Y.S. Ng, E. Gulari, J. Catal. 95 (1985) 33. [3] J. Okamoto, A. Maezawa, T. Imanaka, J. Catal. 120 (1989) 29. [4] J. Ramirez, M. Vrinat, M. Breysse, M. Lacroix, Appl. Catal. 52 (1989) 211. [5] Z.B. Wei, Q. Xiu, X.X. Guo, P. Grange, B. Delmon, Appl. Catal. 75 (1991) 179. [6] A. Nishijima, H. Shimada, T. Sato, Y. Yoshimura, J. Haraishi, Polyhedron 5 (1986) 243.

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