Al2O3 sulfide catalysts for HDS

Applied Catalysis A: General 237 (2002) 161–169

Radioisotopic study of CoMo/Al2 O3 sulfide catalysts for HDS Part II. Temperature effects V.M. Kogan∗ ND Zelinsky Institute of Organic Chemistry, RAS 47, Leninsky Prospect, 117913 Moscow, Russia Received 8 March 2002; received in revised form 27 May 2002; accepted 28 May 2002

Abstract Using 35 S, reaction temperature effects within the range of 300–400 ◦ C on the number and reactivity of active sites of sulfidized Mo/A2 O3 catalysts (non-) promoted by Co in thiophene hydrodesulfurization (HDS) have been studied. On the basis of the results of the radioisotopic tests of the catalysts, changes in the number and mobility of the surface SH groups, as well as relationships between them and the coordinatively unsaturated sites (CUS) have been evaluated. Estimated dependencies have been explained in the terms of the “forcing out” mechanism. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Active sites; Catalysis; CoMo/Al2 O3 ; 3 H; 35 S; Radioisotopic methods; Reaction mechanisms; SH groups; Temperature effects; Thiophene; Vacancies

1. Introduction Investigation into the role of hydrogen in thiophene hydrodesulfurization (HDS) over sulfide catalysts is a relevant way to understand the mechanism of the reaction as a whole. A number of publications on this problem [1–10] deal with applications of isotopes to study the mechanism of thiophene HDS. Most researchers attempt to find a relationship between H–D exchange data and molecule adsorption on the catalyst surface and the catalytic HDS activity [1,3–5] or to determine the source of hydrogen that participates in thiophene C–S bond breaking: whether it is hydrogen from the thiophene ring or from gas phase [2]. However, the data obtained only in the course of such investigations and ∗ Tel.: +7-95-135-8910; fax: +7-95-135-5328. E-mail addresses: [email protected], [email protected] (V.M. Kogan).

not compared with the results of the studies of sulfide sulfur behavior do not allow the authors to come to final conclusions. In our previous studies [11–16] we compared the results of the investigations of the thiophene HDS mechanism over CoMo catalysts labeled by 35 S [11,12,14], by 3 H [13,14] and over non-labeled catalyst under 3 H hydrogen flow [15,16]. As reported in [11–19], in the course of thiophene HDS over a catalyst sulfided by 35 S, radioactivity was detected only in H2 S. During the reaction, H2 S was formed but there was no radioactivity in the thiophene, i.e. there was no exchange between sulfide sulfur and thiophene. Experiments in which the sulfided catalyst was treated in a flow of hydrogen or inert gas at 300–400 ◦ C for 2 h (reaction conditions) showed that no detectable amounts of H2 S or any sulfurorganic compounds were formed in the absence of thiophene. Thus, during HDS, catalyst sulfur was replaced by sulfur from thiophene or other sulfurorganic compounds. These data are evidence of the very important role

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played by catalyst sulfide sulfur in the HDS reaction. The amount of mobile sulfur, in turn, depends on the catalyst composition and the sulfidizing technique. The analysis of the curves of specific radioactivity of H2 S formed in the course of thiophene HDS gave us information about the amount and distribution of sulfide sulfur on the catalyst. The data showed that the latter is not uniform in its ability to participate in H2 S formation. Some of it is immobile, i.e. cannot participate, and some is able to transfer to hydrogen sulfide during the HDS reaction and can, thus, be considered as mobile. After mobile sulfur is removed as H2 S, the active site is formed. Mobile sulfur is not uniform. It can consist of more or less mobile parts (“rapid” and “slow” sulfur). We determined correlations between catalyst activity in HDS and the amount of mobile sulfur and mobile sulfur reactivity (mobility), or, what is the same, the productivity of active site Pi . Pi can be calculated as the ratio of the number of converted thiophene molecules to the number of the SH groups of the sites of a given type. In [17], we suggested an approach to evaluate the relative number of SH groups and coordinatively unsaturated sites (CUS). We assumed that CUS include places of thiophene adsorption and conversion—“functioning vacancies” (V)—and places that do not participate in the reaction under given conditions—“empty sites” (ES). In [11,14,16–19] it was shown that in promoted catalysts there are two types of SH groups different in their reactivity (mobility)—“rapid” and “slow” types. The vacancies related to these groups should also differ according to the SH group mobility. We indicated them as Vr and Vs . In [17] it was shown that together with the SH group, vacancies play very important role in the thiophene HDS reaction. Moreover, the existence of the vacancies belonging to the active sites of the given type is a critical factor that determines catalyst activity for hydrotreating of a given type of crude. The data obtained with the help of tritium [13,14] proved that under HDS conditions, H2 S is formed as the result of the replacement of a catalyst SH group by sulfur from a sulfurorganic compound. In other words, the detection of radioactive H2 S in the course of HDS on a sulfide catalyst labeled by 35 S is not caused by the isotopic exchange of H2 S with the catalyst SH groups but by forcing out SH groups in the form of H2 S into the gas phase.

Eventually, the “forcing out” mechanism of thiophene HDS over sulfide catalyst was suggested [15,16]. According to the mechanism, after adsorption of a thiophene molecule on an anion vacancy, one of the SH groups interacts with a H2 molecule and leaves catalyst surface as H2 S. As a result, a new vacancy is formed. H–H bond cleavage in the course of H2 interaction with a SH group is the limiting step of the reaction. However, some questions related to vacancy formation, interaction of the SH group with molecular hydrogen and a forcing our process remain to be answered. In particular, it is of crucial importance to evaluate the ratio of the number of SH groups to the number of the vacancies and to trace possible changes of this ratio depending on the reaction conditions. In this study, we used this approach to trace the effect of the reaction temperature on the relative number of SH groups, vacancies and empty sites, as well as on SH group mobility for non-promoted Mo and promoted CoMo sulfide catalysts and to compare the results with the data obtained in the course of our earlier investigations of the thiophene HDS mechanism. 2. Experimental Three Mo/Al2 O3 catalysts with Mo content 4, 8, and 12%, and two CoMo/Al2 O3 catalysts with 12% Mo, and 2 and 4% Co were used. Catalyst preparation, pre-treatment and sulfidation by elemental sulfur labeled by 35 S, as well as the details of radioisotopic tests and mathematical treatment of the data obtained were described in [16–19]. 3. Results 3.1. Unpromoted Mo/Al2 O3 catalysts The data of radioisotope tests of unpromoted catalysts that contain 4, 8 and 12% Mo are summarized in Table 1. The tests were carried out at 300–400 ◦ C. The same table gives the ratios of surface SH groups to CUS and of functioning vacancies V and empty sites ES calculated relatively to 1000 SH groups. With an increase in the temperature of the reaction, both thiophene conversion and the surface SH groups capable of participating in HDS increase. In the case of the samples with 4 and 8% Mo, the amount of mobile sulfur

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Table 1 Radioisotope testing results of Mo/Al2 O3 catalysts in the course of thiophene HDS at different reaction temperatures T (◦ C)

Υ (%)

Smob (wt.%)

Smob :Stotal (%)

P × 102

SH:CUS

V:ES:SH

4% Mo/Al2 O3 300 320 340 360 380 400

3.4 4.6 6.2 8.5 11.6 15.8

0.54 0.57 0.62 0.63 0.63 0.65

32.52 34.33 37.09 37.98 37.98 38.88

2.5 3.3 4.1 5.5 7.5 9.9

1.02 1.15 1.37 1.45 1.45 1.53

25:951:1000 33:839:1000 41:691:1000 55:637:1000 75:617:1000 99:553:1000

8% Mo/Al2 O3 300 320 340 360 380 400

5.7 7.7 10.5 14.3 19.5 26.6

1.07 1.11 1.16 1.18 1.22 1.23

31.54 32.85 34.09 35.42 36.02 36.44

2.2 2.8 3.7 4.9 6.5 8.8

1.0 1.1 1.2 1.3 1.3 1.4

22:979:1000 28:893:1000 37:815:1000 48:734:1000 65:687:1000 88:645:1000

12% Mo/Al2 O3 300 320 340 360 380 400

7.9 11.0 14.8 19.7 26.8 36.5

1.22 1.32 1.38 1.44 1.62 1.68

23.9 26.1 27.0 28.2 31.8 33.0

2.7 3.4 4.3 5.5 6.1 8.9

0.61 0.70 0.76 0.82 1.03 1.11

26:1602:1000 34:1385:1000 43:1271:1000 56:1169:1000 67:904:1000 88:823:1000

Reaction conditions: catalyst loading 100 mg, H2 flow = 20 ml/min, thiophene pulse volume—1 ␮l. Υ —thiophene HDS conversion (mol%).

increases slowly and when the reaction temperature is over 360 ◦ C, it comes to the plateau (Fig. 1). For 12% Mo catalyst, a linear dependence of the increase in the mobile sulfur on the reaction temperature is observed. The same regularities can be seen for the curves denoting a change in the share of mobile sulfur in the

total amount of sulfide sulfur (Fig. 2) and for SH:CUS (Fig. 3). It should be noted that changes in Smob :Stotal and SH:CUS depending on the reaction temperature are described by the same shape of the curves. Since mobile sulfur is actually of surface SH group that participate in the formation of H2 S and for a given catalyst

Fig. 1. Dependencies of Smob on the reaction temperature for Mo/Al2 O3 catalysts with different Mo loading.

Fig. 2. Dependencies of Smob :Stotal ratio on the reaction temperature for Mo/Al2 O3 catalysts with different Mo loading.

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Fig. 3. Dependencies of SH:CUS ratio on the reaction temperature for Mo/Al2 O3 catalysts with different Mo loading.

the total amount of sulfide sulfur is constant, it is evident that the number of CUS in unpromoted Mo catalysts does not depend on the reaction temperature. The number of only those SH groups that are able to participate in H2 S formation grows together with the reaction temperature. And that is a reason why the SH:CUS ratio grows. However, though the total number of CUS (V + ES) does not depend on the temperature, their ratio (V:ES) changes significantly. This is clearly illustrated by the data in the last column of Table 1 (V:ES:SH). With the rise of the reaction temperature, the number of functioning vacancies grows and that of the empty sites drops down. The productivities of the active sites of the catalysts with different Mo loading are similar and are characterized by the same temperature dependencies (Fig. 4). In [17], it was shown that with an increase of Mo in unpromoted Mo/Al2 O3 catalysts the share of mobile sulfur in the total amount of sulfide sulfur of these catalysts somewhat decreased. The decrease was related to the growth of the size of the MoS2 clusters. It was also shown that an increase in Mo content does not noticeably change the productivity of the active sites. The coincidence of the temperature dependencies of the active site productivities for the catalysts with different Mo loading means that the nature of the active sites of these catalysts is the same and does not depend on the size of the MoS2 cluster. These catalysts differ only in the ratio V:ES:SH. Comparing the temperature changes of the P (Fig. 4) and V:1000 SH (Fig. 5), one

Fig. 4. Dependencies of the active site productivity P on the reaction temperature for Mo/Al2 O3 catalysts with different Mo loading.

can see bi-unique correspondence between these values. It means that the number of V is determined by the value of active site productivity for non-promoted Mo catalysts. The fact that the amount of mobile sulfur slowly increases when the reaction temperature rises and that productivities of active sites P and the number of functioning vacancies V increase according to an exponential dependence shows that the catalytic activity of Mo catalysts depends on the number of the functioning vacancies which, in its turn, is determined by the activation of Mo–SH bond in the whole range of the temperatures under study.

Fig. 5. Dependencies of the V:1000 SH ratio on the reaction temperature for Mo/Al2 O3 catalysts with different Mo loading.

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Table 2 Radioisotope testing results of CoMo/Al2 O3 catalysts in the course of thiophene HDS at different reaction temperatures T (◦ C)

Υ (%)

Smob (wt.%)

Smob :Stotal (%)

Srapid (wt.%)

Sslow (wt.%)

Prapid × 102

Pslow × 102

SH:CUS

Ratio Vr

Vs

ES

SH

12% Mo, 300 320 340 360 380

2% Co/Al2 O3 20.4 1.65 33.1 1.75 37.8 1.78 48.7 1.96 62.3 2.31

28.91 30.56 31.11 34.32 40.51

0.51 0.65 0.60 0.84 1.11

1.14 1.10 1.18 1.12 1.21

11.5 15.8 18.7 18.8 19.0

2.1 2.9 3.5 3.5 3.5

0.84 0.93 0.96 1.17 1.76

36 59 63 81 91

14 18 23 20 18

1147 1002 956 751 459

1000 1000 1000 1000 1000

12% Mo, 300 320 340 360 380

4% Co/Al2 O3 34.5 2.17 44.7 2.31 58.6 2.32 75.8 2.45 96.1 2.79

32.77 34.90 35.09 36.97 42.14

0.65 0.70 0.78 0.89 1.19

1.52 1.61 1.54 1.56 1.59

12.1 15.3 19.7 24.8 25.0

4.0 4.6 5.6 5.6 5.7

1.14 1.31 1.33 1.51 2.18

36 46 66 90 107

28 32 37 36 33

810 682 648 537 319

1000 1000 1000 1000 1000

Reaction conditions: catalyst loading 100 mg, H2 flow = 20 ml/min, thiophene pulse volume—1 ␮l. Υ —thiophene HDS conversion (mol%).

3.2. Promoted CoMo/Al2 O3 The experimental results of the radioisotopic testing of promoted CoMo/Al2 O3 catalysts at various temperatures and the data of the estimated calculation of the ratios of SH groups to CUS, to functioning vacancies and to “empty sites” are given in Table 2. For the promoted Mo catalysts under study with different Co loadings and different reaction temperatures there exists a linear correlation between thiophene conversion and the amount of catalyst mobile sulfur (Fig. 6). It means that the catalyst activity to a first approximation is determined by the amount of mobile sulfur. Earlier we reported dependencies like that [12,14,18,19]. A similar dependence was found by Tétényi and co-workers [20]. However, this linear dependence is not universal and is true only for the cases when the nature of the active sites of the catalysts under study is similar and their productivities do not differ significantly [19,21]. A more detailed analysis of the data of Table 2 shows that the catalytic activity is determined not only by the amount of mobile sulfur. The temperature of the reaction differently affects the ratio of SH groups to CUS in unpromoted Mo catalysts and promoted CoMo catalysts (Fig. 7). In the first case, with an increase in the temperature, the SH:CUS ratio slowly and smoothly increases and in the second case, for CoMo catalysts at 320–350 ◦ C, the plateau is observed. When the temperature is further increased,

the latter ratio rapidly increases. This character of the changes of the SH:CUS ratio is determined by the corresponding temperature dependencies of “rapid” sulfur. This conclusion is the result of the comparison of Figs. 7 and 8. The amount of “slow” sulfur does not depend on the temperature but the temperature dependence of “rapid” sulfur actually coincides with the curves of the SH:CUS for the corresponding catalysts. Thus, the observed changes of the density of SH groups on the active phase surface of the CoMo catalyst are determined by changes in the number

Fig. 6. Dependence of thiophene HDS conversion on the amount of mobile sulfur Smob for CoMo/Al2 O3 catalysts containing 12% Mo and different amounts of Co.

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Fig. 7. Dependencies of SH:CUS ratio on the reaction temperature for CoMo/Al2 O3 catalysts containing 12% Mo and different amounts of Co.

of “raid” SH groups. According to the data obtained [16,18,19], these groups seem to be related to the Mo active sites while “slow” SH groups belong to the Co sites. The S-shape of the curves for “rapid” SH groups (Fig. 8) and the linear dependencies of the relative number of Vr vacancies (Fig. 9) on the reaction temperature show that for promoted CoMo catalysts at 320–350 ◦ C, some Vr vacancies appear while the number of related “rapid” SH groups is the same. Within

Fig. 8. Dependencies of the amounts of the Smob , Srapid and Sslow sulfur on the reaction temperature for CoMo/Al2 O3 catalysts containing 12% Mo and different amounts of Co.

Fig. 9. Dependencies of V:1000 SH ratio for V, Vr , Vs on the reaction temperature for CoMo/Al2 O3 catalysts contained 12% Mo and different amounts of Co (explanations are given in the text).

the same temperature range, the productivities of the “rapid” sites increase (Fig. 10), i.e. the catalytic activity is conditioned by Me–SH bond activation, the same as in non-promoted Mo catalysts. At higher temperature, the growth of the productivities of the “rapid” sites slows down and then the plateau area can be observed while the relative number of Vr continues to increase.

Fig. 10. Dependencies of active site productivities for P, Pr , and Ps on the reaction temperature for CoMo/Al2 O3 catalysts contained 12% Mo and different amounts of Co (explanations are given in the text).

V.M. Kogan / Applied Catalysis A: General 237 (2002) 161–169

4. Discussion The comparison of the temperature dependencies of active site productivities of promoted (Fig. 10) and non-promoted (Fig. 4) catalysts shows that in the initial temperature range—below 330 ◦ C for low promoted (2% Co) and below 350 ◦ C for higher promoted (4% Co) Mo catalysts—the shapes of the corresponding curves for “rapid” sites retrace those of analogous curves for non-promoted Mo catalysts. Besides, up to 340 ◦ C the productivities of the “rapid” sites of the catalysts with different Co loading practically coincide (Table 2). In other words, the observed growth of the catalytic activity of CoMo catalysts with a temperature rise up to 340–350 ◦ C is determined by Me–SH bond activation on the “rapid” sites. At temperature above 340 ◦ C, the curves of the productivities of “rapid” sites of the promoted catalysts diverge and achieve the plateau at different temperatures. The growth of the catalytic activity of the promoted catalysts at temperature > 340 ◦ C is determined by the growing number of functioning vacancies Vr (Fig. 9) and extra “rapid” SH groups involved into the reaction. However, in contrast to non-promoted Mo catalysts, this process is not accompanied by an increase in active site productivities, i.e. the number of H2 S molecules formed with the participation of one of the “rapid” SH groups remains approximately constant. There must be a factor that limits the formation of H2 S. It may be hydrogen adsorbed on the catalyst. Three types of hydrogen takes part in the HDS over sulfidized (Co)Mo/Al2 O3 catalysts: irreversibly adsorbed hydrogen from SH groups, reversibly and dissociatively adsorbed hydrogen and molecular hydrogen. Our experiments with 3 H-labeled sulfidized CoMo/Al2 O3 catalysts [13,14] showed that irreversibly adsorbed hydrogen participates in the HDS reaction but not in hydrogenation or in hydrogen exchange reactions. Comparing the results of the series of experiments of thiophene HDS over catalysts labeled by 35 S and 3 H, we found that both surface sulfur and irreversibly adsorbed hydrogen transfer to H2 S from one and the same SH group. It means that in the course of thiophene HDS catalyst, SH groups transfer to H2 S as a whole and that hydrogen from SH groups does not interact with a neighboring SH group. This hydrogen does not participate in isotopic exchange, or in the hydrogenation reaction or in C–S

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Table 3 Molar radioactivities (MR) of the products of thiophene HDS on CoMo/Al2 O3 in a hydrogen 3 H flow (MR of H2 flow = 5.9 GBq/mol, 360 ◦ C, 100 mg catalyst loading)

C4 H4 S C4 H8 C4 H10 H2 S

MR (GBq/mol)

RMR (GBq/mol)

9.33 19.53 25.25 0.04

4.67 4.88 5.05 0.04

RMR—reduced molar radioactivity (from [16]).

bond breaking. It was assumed that there must be an alternative source of a hydrogen atom needed for H2 S formation. In [15,16], we reported the data of our studies of thiophene conversion in an atmosphere of H2 labeled with tritium. In those experiments, radioactivity was found in all the reaction products. Table 3 (from [16]) shows that the molar radioactivity (MR) values of thiophene and hydrocarbons are proportional to the number of hydrogen atoms in these molecules. The reduced molar radioactivity (RMR) is a value corresponding to 1 mole of hydrogen in each compound. RMR of thiophene and hydrocarbons are slightly lower than that of the hydrogen of the gas phase. Thus, under the given conditions, a fast isotopic exchange takes place between the gas phase and thiophene hydrogen. Radioactivity was also discovered in H2 S. One would expect similar RMR values for H2 S and C4 -hydrocarbons. However, the RMR of H2 S was about 120 times smaller than in the other products. Here we observe a kinetic isotopic effect (KIE): low 3 H S radioactivity is caused by the changing rate 2 of dissociation of molecules with different isotope contents—H2 and T2 (or HT). One should stress that in fact the value of the KIE is a half of the observed experimental value. The observed ratio of the RMR of H2 S to that of the hydrocarbons is not due to a true KIE. This is so because unlike thiophene and C4 -hydrocarbon hydrogen, of which all atoms participate in isotopic exchange, only half of the total amount of gas phase hydrogen present in an H2 S molecule participates in H2 S formation. Hence, the introduction of heavy hydrogen into H2 S, observed in the former experiments, is in no way conditioned by hydrogen isotopic exchange in the SH groups. Thus, the ratio of the H2 S to H2 molar radioactivity calculated for a gram atom of hydrogen

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from the gas phase, can considered as the actual KIE value. In other words, if these C4 -hydrocarbon values are half of the RMR values presented in Table 3, then the H2 S RMR would remain constant. It then follows that the value of the actual KIE is about 60. Thus, we conclude that H2 S formation is related to the reaction of catalyst SH groups with molecular hydrogen. The KIE observed during the formation of H2 S proves that hydrogen activation is the limiting step in H2 S formation. This conclusion is supported by the results of Rodriguez et al. [10]. A KIE is not observed during the hydrogenation reactions because they proceed with the participation of preliminary dissociatively adsorbed hydrogen. According to, that suggested in [15,16] mechanism of thiophene HDS (Scheme 1), a thiophene molecule is adsorbed on the catalyst surface characterized by a definite ratio between the number of vacancies and SH groups, corresponding to its balanced state. C–S bond breaking takes place at the expense of dissociatively adsorbed hydrogen that does not belong to any

SH group. The break-up of a thiophene molecule results in the formation of a new SH group, and the ratio between the numbers of vacancies and SH groups changes in favor of the latter. Hence, the catalyst falls into a so-called “metastable” state which it can leave by forcing out any of the SH groups located on the catalyst surface as a result of its interaction with molecular hydrogen. H–H bond cleavage in the course of H2 interaction with an SH group is the limiting step of the reaction. The observed behavior of the “rapid” sites of the promoted CoMo catalysts versus temperature can be explained in terms of the suggested mechanism. At lower temperature, molecular hydrogen is adsorbed on the catalyst surface to excess and H2 S formation and, hence, thiophene HDS are determined by the mobility of SH groups (the activation of the Me–SH bonds) and by the number of functioning vacancies. When the reaction temperature rises, the amount of adsorbed molecular hydrogen decreases and its deficiency limits the formation of H2 S.

Scheme 1.

V.M. Kogan / Applied Catalysis A: General 237 (2002) 161–169

Then the problem is why the dependence of molecular hydrogen limits the functioning of “rapid” sites of promoted CoMo catalysts and does not affect the active sites of unpromoted Mo catalysts at the same reaction temperature. The reply could be found in the comparison of the productivities of these sites at the same reaction temperature. Indeed, the productivities of the “rapid” sites of CoMo catalysts (Table 2) are three to four times higher than those of the active sites of Mo catalysts (Table 1) and, thus, the demand of the “rapid” sites of the CoMo catalyst in molecular hydrogen is higher than that of the sites of Mo catalyst. In the case of the active sites of low productivity, the deficiency of adsorbed molecular hydrogen at temperature below 380 ◦ C is not observed.

5. Conclusions Dependencies of catalytic activity in thiophene HDS, the amount of mobile sulfur, the productivities of the active sites on the reaction temperature have been determined for Mo/Al2 O3 and CoMo/Al2 O3 catalysts of various compositions. Changes of the ratio between the CUS and the SH groups versus temperature have been evaluated. The data obtained shows that the catalytic activity of non-promoted Mo catalysts is determined by the number of the active sites and their productivities. Both values grow with the reaction temperature. Mo content does not affect the productivity or, what is the same, the activation of the surface SH groups within the whole range of the temperatures. Similar dependencies have been observed for the promoted CoMo catalysts at reaction temperatures up to 340–350 ◦ C. A further rise of the temperature causes higher catalytic activity due to the growing number of functioning vacancies and extra surface SH groups. At the same time, the growth of active site productivities of CoMo catalysts is limited. This phenomenon has been explained in terms of the “forcing out” mechanism of thiophene HDS, according to which the molecular hydrogen plays a key role in the reaction. It has been supposed that at the reaction temperature above 350 ◦ C,

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