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Investigation of the Influence of H2S on Hydrodesulfurization of Dibenzothiophene over a Bulk MoS2 Catalyst Hamdy Farag* Chemistry Department, Faculty of Science, Mansoura University 35516, Egypt
Kinya Sakanishi, Masatou Kouzu, Akimitsu Matsumura, Yoshikazu Sugimoto, and Ikuo Saito Institute for Energy Utilization, AIST, Tsukuba West, Ibaraki 305-8569, Japan
Bulk molybdenum sulfide catalyst was prepared by thermal decomposition of ammonium heptamolybdate at 800 °C in a flow of a 10% (v/v) H2S/H2 gas mixture. Investigation of the catalytic activity of the grinding catalyst for hydrodesulfurization (HDS) of dibenzothiophene (DBT) was carried out in a batch magnetically stirring microautoclave reactor at 340 °C and 3 MPa H2. The influence of a wide range of H2S concentration on the catalytic activity was studied. A significant promoting effect on HDS of DBT by H2S was observed. The experimental results showed the great enhanced contribution from the hydrogenation route with H2S in HDS of DBT. However, severe inhibition by H2S against the direct desulfurization route was observed. Kinetic analysis leads to the individual reaction rate constant in the reaction scheme performed. Introduction Hydrodesulfurization (HDS) is a crucial hydrotreating reaction that urgently needed to meet the nowadays regulations on the sulfur content of gas oil for the protection of the global environment. Currently, however, sulfur-free fuel (ultradeep HDS) is getting more interest for its applicability to the fuel cell process. The utility of producing an active catalyst or process while using the present industrial conditions has motivated researchers to investigate alternative catalyst preparations, oxidative HDS, adsorption, and/or process design.1-7 The activity of the catalyst, normally NiMo or CoMo/ Al2O3 for HDS, is highly affected by the other species in the real feed.1,8-14 However, there is no fundamental difference in responding to the reaction inhibition over various types of these catalysts. Still little is known about the mechanism of HDS reactions for the real feed because it is a complex process where many partners are competing on the same active sites. Nitrogen compounds, H2S, and aromatic compounds are components that show a retardation effect against HDS.8,9,13-20 In addition, the retardation extent is significantly increased for the most refractory sulfur species, i.e., dibenzothiophene (DBT) compounds. While numerous studies for supported CoMo and/or NiMo catalysts have been reported,1,8,9 relatively few studies15 have provided detailed information on the bulk MoS2, NiSx, and/or CoSx catalysts for HDS reactions. A fundamental question is whether the bulk catalysts respond to the inhibitors in the HDS reaction, H2S for example, in a way parallel with what was found for the supported and promoted Mo catalyst. It is this point that we want to investigate and put some clarification in this paper. H2S was announced to significantly reduce the catalytic activity of HDS for the most refractory sulfur * To whom all correspondence should be addressed. Present address: Institute for Energy Utilization, AIST, Tsukuba West, Ibaraki 305-8569, Japan.
compounds such as DBTs. In the bed reactor the major part of the catalyst bed operates in an environment of high concentration of H2S. This would be expected to put a more restricted load for the HDS of the refractory sulfur species. In a previous publication,13 it has been found that, even at the early stages of the HDS reaction of DBT over the CoMo catalyst supported on alumina or carbon, the inhibition by the self-produced H2S was significant. However, the extent of inhibition by H2S was found to be dependent on the catalyst supports applied. Thus, studies on the nature of inhibition may provide some fruitful information on the catalytic active sites, which are considered to be the breakthrough for developing much more active catalysts. Experimental Section Materials. Ammonium heptamolybdate, decane, DBT, and copper powder were commercially provided and used with no further purifications (purity > 99%). H2S (10%, v/v) balanced with H2 was used for catalyst sulfidation and H2S studies. Helium was used to flush out the catalyst after being sulfided. Catalyst Preparation. A detailed description of the catalyst preliminary preparation was stated elsewhere.21 Briefly, the ammonium heptamolybdate precursor was applied for temperature annealing at 800 °C with a rate of 10 °C/min while flowing the gas mixture of (10%, v/v) H2S/H2, 60 SCCM, from the beginning of the sample set. After the predetermined sulfidation time has passed, the sample was flushed with He for 20 min before cooling and the gas was kept flowing until arriving at the ambient temperature. Thereafter, the catalyst was ground for 24 h using the media-agitating mill. Zirconia beads (1 mm diameter) were used as grinding media, and the rotation speed of the mill was 3000 rpm. This leads to a particle size of ca. 4 nm as confirmed from the X-ray diffraction pattern. MoS2 after grinding was coded as MS-AMG. Finally, the catalyst was kept in a desiccator for further use.
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Figure 1. Yield-time profile of HDS of DBT at 340 °C and 3 MPa H2 over a MS-AMG catalyst with Cu addition (no H2S).
Activity Measurements. A solution of 1 wt % DBT in decane was prepared as a feedstock for the investigation run. A magnetically stirring sampler attached microautoclave reactor was used for the activity test. The reactor allows withdrawal of a small portion (∼0.1 mL) of the reaction fluid to be examined. A typical reaction run was performed as follows: The reactor was charged with 0.10 g of fresh catalyst powder and 10 g of a 1 wt % DBT solution, and in some certain runs (as stated in the text) 0.4 g of copper powder was added. This copper powder addition helps a semiquantitative trap of the continuously self-produced H2S from the HDS reaction zone. The HDS of DBT was investigated at 340 °C under 3 MPa H2 and a stirrer speed of 1000 rpm. This condition was fixed for all runs. The external feeding of H2S was accomplished from the cylinder of a 10% (v/v) H2S/H2 gas mixture. A variety of inserted H2S concentrations to the reaction zone were investigated. Sampling of ∼0.1 mL with time intervals was drawn after being heated to 340 °C. The capillary lines of the reactor were flushed back by means of a high-pressure H2 gas. Analysis. Gas chromatography equipped with a flame ionization detector (GC-FID; Agilent HP6890) and a capillary methylsiloxane column (0.32 mm × 50 m) was used for the reaction quantitative analysis. The qualitative analysis was fulfilled with the aid of GCMS (HP5970) along with the relevant standard compounds. Kinetic Analysis. A computer simulation model that considers the HDS of DBT as a consecutive reaction and puts limits on obtaining the individual reaction rate constants was applied.14 Results Reaction Pathways. HDS of DBT in the microautoclave reactor over a MS-AMG catalyst at 340 °C and 3 MPa H2 was performed at various time intervals. The products resulting from the reaction have been confirmed by GC-MS and GC-FID to be phenylcyclohexane (PC), biphenyl (BP), and partially hydrogenated DBT (H4-DBT) as a main yield. Trace amounts of bicyclohexane, methylnaphthalene, and some cracked species were also found. The trace product species is proposed to come mainly through the hydrogenation and hydrocracking of PC. It could also be possible that other species such as alkylbenzene or alkylcyclohexane were produced, but the interference of their retention time with that of solvent makes identifying them difficult. Yield-time profiles for the effect of H2S on the HDS of DBT are depicted in Figures 1 and 2. Figure 1 represents the run where copper powder, the scrubber of H2S,
Figure 2. Yield-time profile of HDS of DBT at 340 °C and 3 MPa H2 over a MS-AMG catalyst without Cu addition (with selfproduced H2S kept in the feed).
Figure 3. Reaction scheme of HDS of DBT over a MS-AMG catalyst at 340 °C and 3 MPa H2.
was included in the reaction feed. This product distribution leads to the well-known reaction scheme of HDS of DBT in which two reaction routes are involved, the hydrogenation through the H4-DBT intermediate and the direct C-S bond scission resulting in BP as illustrated in Figure 3. This represents only the main reaction scheme because other side reactions cannot be excluded. Obviously, a yield increase of PC and the hydrogenated DBT with H2S is noted as shown in a comparison of Figures 1 and 2. It preliminarily indicates the enforcement of the HDS reaction through the hydrogenation pathway. While apparent increase in the yield of PC was found, the yield of BP was considerably suppressed by the presence of H2S. The effect of H2S is clear even at the early beginning of the reaction, i.e., a conversion level below 10%. This may indicate its involvement in the HDS mechanism of DBT as soon as it is formed. A significant change in the half-life time of the reaction can easily be recognized. Kinetic Analysis. A kinetic analysis of pseudo-firstorder plots that shows the effect of self-produced H2S during the reaction on the catalytic activity of HDS of DBT is shown in Figure 4. From DBT conversion, the reaction rate constants were calculated by the following equation:
kt ) -(tw)-1 Ln(1 - x) where t is the reaction time, w is the catalyst weight, and x is the DBT conversion.
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Figure 4. Pseudo-first-order plot of HDS of DBT at 340 °C and 3 MPa H2 over MS-AMG (effect of H2S).
Figure 5. HDS selectivity curves of HDS of DBT over MS-AMG with self-produced H2S kept in the feed (340 °C and 3 MPa H2).
Figure 7. Effect of H2S on the product yield of PC for HDS of DBT over MS-AMG at 340 °C and 3 MPa H2.
Figure 8. Effect of the H2S concentration on the hydrogenation reaction route of DBT over MS-AMG at 340 °C and 3 MPa H2.
supported CoMo and/or NiMo catalysts because the increase in the partial pressure of H2S resulted in a decrease in their HDS activity. H2S and Selectivity Behavior
Figure 6. Activity change for HDS of DBT against the concentration of H2S over MS-AMG at 340 °C and 3 MPa H2; kt, ×10-4 s-1‚g of catalyst-1.
It is surprising that a significant enhancement in the catalytic activity by H2S is found. An extended investigation of the external addition of H2S up to 0.40 g in the feed (ratio of H2S to the sulfur in the feed is more than 20 times) on the catalytic activity of HDS was carried out. The reaction scheme was further considered for kinetic analysis simulation in order to obtain the individual rate constant in the above-mentioned reaction scheme. Figure 5 is an example that demonstrates the close matching of the experimental and theoretical estimated points. Figure 6 represents the correlation of the overall rate constants with a variety of H2S concentrations. Keeping the self-produced H2S from the reaction did promote the catalytic activity of HDS of DBT nearly 4 times its value if it was removed in situ by copper addition. Furthermore, the further externally added H2S almost did not change the activity even at a relatively high concentration that exceeds several times the prospected industry level. This is actually in contrast to what was found for
Promotion of the Hydrogenation Route. Selectivity of HDS of DBT versus H2S concentration is one that shows interesting results. It gives good insight into the reaction mechanism and the role of promotion as well. The previous kinetic analysis enables the measurements of the contribution of each reaction route on the HDS scheme in a quantitative way. The effect of the H2S concentration on the yield from the hydrogenation route (sum products of PC and bicyclohexane) is depicted in Figure 7. The corresponding estimated rate constant for hydrogenation, kHS1 in the reaction scheme, is presented in Figure 8. As the concentration of H2S increased, the activity sharply increased and thereafter remained almost fixed regardless of further addition of H2S up to a high level. It seems that H2S works on some certain active sites, and as soon as these sites become saturated with H2S, no more promotion could occur. It is surprising that even at a high level of H2S the HDS of DBT did not suffer any inhibition in contrary to the observed findings for the supported CoMo, NiMo, CoW, and/or NiW catalysts.7-9,13 These catalysts showed a serious inhibition effect even for the very small amount of H2S. Suppression of the Direct Desulfurization Route. Figure 9 shows the correlation between H2S and the yield of produced BP. The rate constant of the direct desulfurization route, kD0, against the amount of H2S is depicted in Figure 10. Conversely to what was found for the hydrogenation route, the direct desulfurization suffered from severe inhibition with the increase of H2S.
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Figure 9. Effect of H2S on the yield of BP for HDS of DBT over MS-AMG at 340 °C and 3 MPa H2.
Figure 10. Effect of the H2S concentration on the activity toward the direct desulfurization route of DBT over MS-AMG at 340 °C and 3 MPa H2.
Figure 11. Effect of the H2S concentration on the selectivity of HDS of DBT over MS-AMG at 340 °C and 3 MPa H2.
Then, it reached a constant level with a further increase in the amount of H2S inserted into the zone reaction. The curve looks similar to the curve for the hydrogenation route except its direction; the first is a reverse correlation, and the second is a direct correlation with early stages of added H2S. However, the promotion in the hydrogenation route is much more pronounced than the depression in the direct desulfurization pathway. Also, the hump in this curve occurs after a level of conversion that exceeds 99%. This indicates the starting hydrogenation reaction of BP to yield PC as shown in Figure 7. The selectivity, S, expressed as the ratio of kD0/kHS1 versus the added amount of H2S, is presented in Figure 11. The selectivity decreases with added H2S until about 0.04 g and then turns to show a constancy effect toward a further feeding level of H2S. Discussion Inhibition of HDS by H2S. H2S is produced as a byproduct from the HDS reactions, and it is believed to
certainly poison the catalyst. There is no fundamental difference in the impact of supported NiMo and/or CoMo catalysts on the inhibition effect by H2S.16-20 Supports probably play a role in the extent of such inhibition; nevertheless, the trend generally remains the same. Many studies show that removal of the generated H2S during HDS of gas oil over the commercial CoMo/Al2O3 catalysts either by the two-stage reaction process or through the addition of adsorbent has a pronounced beneficial effect on the catalytic activity.6,14 Although the results obtained in different laboratories are not always consistent upon comparison of the depression degree of HDS, it is clear that H2S seriously reduces the catalytic activity of HDS. A similar inhibition trend by H2S has been reported for the HDS of model sulfur compounds such as DBT and the most refractory 4,6dimethyldibenzothiophene.9 In addition, the inhibition effect seems to be independent of the reaction temperature. In the course of model compound studies, the direct desulfurization route in HDS is observed to suffer more inhibition than the hydrogenation route. The hydrogenation route remains unaffected or even gets a little enhanced by H2S. It is necessary to mention that at some certain level of H2S the HDS reaction rate reaches a state of constancy in which the catalyst activity remains stable. Different responses of the two HDS routes to H2S suggest that the hydrogenation of the ring in the polyaromatic sulfur compounds may take place over reaction sites other than the one used for the hydrogenolysis of the C-S bond. Promotion of HDS by H2S. Most studies have been performed with CoMo and/or NiMo catalysts; only a very few studies were done with the unsupported MoS2 catalysts. Studies encountering the bulk MoS2 catalysts should be concerned with the original precursors of obtaining these species. In a recent study, it was shown that the starting molybdenum precursors play an important role in the chemistry, morphology, and structure of the ultimate species as well.21 The present data show unique cases in which H2S greatly enhanced the HDS activity of DBT. Yet, this great promotion in catalytic activity for the HDS due to the presence of H2S never has been seen in the literature to the best of our information. The interesting result in the present study comes from the finding that the majority of activity promotion is due to the enlarged contribution from the hydrogenation route, whereas the contribution from the direct desulfurization was strongly inhibited. It is also to be noted that the generation yield of the H4-DBT intermediate is increased considerably with added H2S. From the kinetic rate constant, the reactivity of these species is by far much more than that of the parent DBT. The contribution from hydrogenation to the overall activity represents over than 85%. In the literature, the BP is the major product of HDS of DBT at nearly the same reaction conditions applied regardless of the H2S environment, in contrast to what was found in the present study. From the product distribution curve (Figures 1 and 2), it can be noted that the significant participation of the hydrogenation route eventually produces PC. This may lead to the survival of the point of view that the hydrogenation of such refractory sulfur compounds is a valuable alternative pathway for HDS instead of focusing only on the catalyst enhancement via the direct desulfurization route because this is geometrically limited by the steric hin-
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drance effect, especially for the most refractory polyaromatic sulfur compound. Thus, there is expectedly still room for improving the catalytic activity by providing the opportunity of directing the catalytic HDS reaction to the hydrogenation pathway. The stable catalytic activity after insertion of a relatively high concentration of H2S in the feed for HDS of DBT could lead to some informative structure of the active sites. The activity of the catalyst is nearly 3 times more active than the commercially promoted CoMo/ Al2O3 catalysts tested at close experimental conditions.14 The increase of the activity in the first stages of adding H2S is suggested to be due to the modification of the catalyst active site and probably conversion of some of the direct desulfurization active sites to be used for hydrogenation. The beginning constancy in the activity of the plateau as seen in the the curve of Figure 6 could be taken as an indication of the number of active sites. The plateau showing the stability of the catalytic activity with increasing H2S in the feed is suggested to be evidence of reaching a saturation level of the surface active sites that could be modified by H2S. Studies on the characterization of such catalysts are in progress. Conclusions H2S has been known for a long time as an inhibitor for HDS of the polyaromatic thiophenes. However, the present results show a quite surprising promotion effect of H2S in HDS of DBT over a bulk MoS2 catalyst in which the hydrogenation route was much more enhanced while the direct desulfurization was inhibited. Thus, the promotion and inhibition of HDS by H2S suggests its dual character. Formation of new sites either by replacement of the direct desulfurization sites or by creation of a new one is proposed. However, the efficiency of these sites is highly correlated to its geometrical feasibility to accommodate and accept the substrate. The results show the effective possibility of proceeding with the HDS reaction of DBT via the hydrogenation route by precise control of the catalyst synthesis, particle size, and reaction conditions. Acknowledgment This work has been entrusted by the New Energy and Industrial Technology Development organization under a subsidy of the Ministry of Economy, Trade and Industry. Literature Cited (1) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating catalysis; Springer-Verlag: Berlin, 1996. (2) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Oxidation of dibenzothiophene in an organic biphasis system and its application to oxidative desulfurization of light oil. Energy Fuels 2001, 15, 1535. (3) Aida, T.; Yamamoto, D.; Iwata, M.; Sakata, K. Development of oxidative desulfurization process for diesel fuel. Reviews on Heteroatom Chemistry; MYUKK: Tokyo, 2000; Vol. 22, p 241.
(4) Nishioka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Isolation of Sulfur Heterocycles from Petroleum and Coal-Derived Materials by Ligand Exchange Chromatography. Fuel 1986, 65, 270. (5) Sakanishi, K.; Ando, M.; Abe, S.; Mochida, I. Extensive Desulfurization of Diesel Fuel through Catalytic Two-Stage Hydrotreatment. Sekiyu Gakkaishi 1991, 34, 553. (6) Sakanishi, K.; Ando, M.; Mochida, I. Extensive Desulfurization of Diesel Fuel through Catalytic Two-Stage Hydrotreatment (Part 2). Sekiyu Gakkaishi 1992, 35, 403. (7) Ma, X.; Sakanishi, K.; Mochida, I. Three-Stage Deep Hydrodesulfurization of Diesel Fuel with CoMo and NiMo Catalysts at Relatively Low Pressure. Fuel 1994, 73, 1667. (8) Leglise, J.; van Gestel, J. N. M.; Finot, L.; Duchet, J. C.; Dubois, J. L. Kinetics of sulfur model molecules competing with H2S as a tool for evaluating the HDS activities of commercial CoMo/Al2O3 catalysts. Catal. Today 1998, 45, 347. (9) Kabe, T.; Aoyama, Y.; Wang, D.; Ishihara, A.; Qian, W.; Hosoya, M.; Zhang, Q. Effect of H2S on HDS of DBT and 4,6-DMDBT on alumina supported NiMo and NiW catalysts. Appl. Catal. A 2001, 209, 237. (10) Sie, S. T. Reaction order and role of hydrogen sulfide in deep HDS of gas oils: consequences for industrial reactor configuration. Fuel Process. Technol. 1999, 61, 149. (11) Vrinat, M.; Orozco, E. O. kinetics of dibenzothiophene hydrodesulfurization over MoS2 supported catalysts: modelization of the H2S partial pressure effect. Appl. Catal. A 1998, 170, 195. (12) Laredo, G. C.; De los Reyes, J. A.; Cano, J. L.; Castillo, J. J. Inhibiting effect of nitrogen compounds on the hydrodesulfurization of dibenzothiophene. Appl. Catal. A 2001, 207 (1-2), 103. (13) Farag, H.; Mochida, I.; Sakanishi, K. H2S and aromatic effects on HDS of dibenzothiophene over CoMo/C catalyst. Chemistry of diesel fuel; Taylor & Francis: New York, 2000; pp 123137. (14) Farag, H.; Mochida, I.; Sakanishi, K. Fundamental comparison studies on hydrodesulfurization of dibenzothiophenes over CoMo based carbon and alumina catalysts. Appl. Catal. A 2000, 194, 147. (15) Hermann, N.; Brorson, M.; Topsoe, H. Activities of unsupported second transition series metal sulfides for hydrodesulfurization of sterically hindered 4,6-dimethyldibenzothiophene and of unsubstituted dibenzothiophene. Catal. Lett. 2000, 65, 169. (16) Schulz, H.; Bohringer, W.; Ousmanov, F.; Waller, P. Refractory sulfur compounds in gas oils. Fuel Process. Technol. 1999, 61, 5. (17) Topsoe, H.; Knudsen, K. G.; Byskov, L. S.; Norskov, J. K.; Clausen, B. S. Advances in deep desulfurization. Stud. Surf. Sci. Catal. 1999, 121, 13. (18) Vrinat, M.; Gachet, C. G.; Cavalletto, G.; De Mourgues, L. Influence of Hydrogen Sulfide on the Hydrodesulfurization of Dibenzothiophene on a Sulfided Cobalt-Molybdenum/Alumina Catalyst. Appl. Catal. 1982, 3, 57. (19) Farag, H.; Sakanishi, K.; Mochida, I.; Whitehurst, D. D. Kinetic analysis and inhibition by naphthalene and H2S in hydrodesulfurization of 4,6-dimethyldibenzothiophene over CoMo based carbon catalysts. Energy Fuels 1999, 13, 449. (20) Kasahara, S.; Shimizu, T.; Yamada, M. Inhibiting effects of H2S on HDS of CoMo, NiMo and Mo/Al2O3. Catal. Today 1997, 35 (1-2), 59. (21) Farag, H. Effect of sulfidation temperature on the bulk structures of various molybdenum precursors. Energy Fuels 2002, 16, 944.
Received for review May 31, 2002 Revised manuscript received September 5, 2002 Accepted September 6, 2002 IE020404V
