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Ind. Eng. Chem. Res. 2004, 43, 2324-2329
Kinetics of Hydrodesulfurization of Dibenzothiophene Catalyzed by Sulfided Co-Mo/MCM-41 Yao Wang, Zhongchao Sun, Anjie Wang,* Lifeng Ruan, Mohong Lu, Jing Ren, Xiang Li, Chu Li, Yongkang Hu, and Pingjing Yao State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, People’s Republic of China
The kinetics of dibenzothiophene (DBT) hydrodesulfurization catalyzed by MCM-41-supported Co-Mo sulfides was investigated at temperatures of 280-340 °C and 5.0 MPa. Both the Langmuir-Hinshelwood (L-H) model and pseudo-first-order model were used to fit the experimental data in an integral operation. It was found that the L-H rate expression was not applicable in the study whereas the pseudo-first-order model fitted very well with the experimental data. Moreover, it is indicated that the L-H expression becomes first-order in DBT concentration if the adsorption of DBT is negligible and DBT concentration is low, as in a typical commercial hydrotreating process. It is shown that the rate constant of the hydrogenolysis route was influenced more greatly by the atomic ratio of Co to Mo than that of the hydrogenation route, implying that hydrogenolysis and hydrogenation might take place on separate active sites and the introduction of Co to the Mo matrix might enhance mainly the hydrogenolysis route. 1. Introduction A renewed interest in hydrodesulfurization (HDS) has come about due to increasingly stringent environmental legislation and the shortage of quality feedstocks in a refinery. To meet the specifications, many studies have been conducted on this subject in recent years to develop deep HDS catalysts. Moreover, it is of great interest to obtain better knowledge of the chemical reactions occurring in the hydrotreating process to develop highperformance catalysts. Among thiophenic compounds in petroleum fractions, dibenzothiophene (DBT) and its derivatives are the least reactive sulfur-containing constituents, and are, therefore, the key components in determining hydrotreating process kinetics.1 Steiner et al. studied the kinetics of DBT and alkyl-substituted DBTs in a blended LGO. And they found that HDS of DBTs could be represented as a first-order reaction. According to a correlation between the overall HDS conversion and the conversions of the individual components, they concluded that the behavior of DBT closely resembles the overall behavior of DBTs in the oil fraction at certain conditions. They therefore proposed that DBT could be used as a model compound for kinetic studies of an entire fraction.2 We have developed siliceous MCM-41-supported Mobased HDS catalysts which exhibited very high activity in the transformation of DBT.3,4 The goal of this research was to determine the kinetics of DBT HDS catalyzed by Co-Mo/MCM-41 using a high-pressure flow microreactor. 2. Experimental Section 2.1. Materials. Both ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24‚4H2O] and cobalt nitrate hexahydrate [Co(NO3)2‚6H2O] were of commercial GR grade. DBT was synthesized according to the method * To whom correspondence should be addressed. E-mail:
[email protected].
in the literature.5 Decalin was a product of Shanghai Chemical Reagents Co. and was used as a solvent without further purification. The preparation procedure of MCM-41 has been described in detail elsewhere.6 The structural parameters of the prepared MCM-41 were determined from its N2 adsorption and desorption isotherms. Its specific surface area is 875 m2‚g-1, and its BJH pore size is 27.4 Å. Co-Mo/MCM-41 catalysts were prepared in the following way: MCM-41 was impregnated with an aqueous solution of (NH4)6Mo7O24‚4H2O and Co(NO3)2‚6H2O for 12 h at room temperature, followed by the evaporation of solvent, drying at 120 °C for 5 h, and calcination in air at 540 °C for 5 h. A 20 wt % MoO3 loading level was chosen for preparing this series of catalysts. The content of cobalt was determined by varying the atomic ratio of Co to Mo in the range of 0.25-1.0. The catalysts were denoted as Co-Mo(x). The value in parentheses represents the atomic ratio of Co to Mo. 2.2. Reaction System and Procedure for HDS of DBT. The reaction system, on which the kinetics of DBT HDS was investigated, is illustrated in Figure 1. The HDS reaction was conducted in a trickle-bed reactor 8.0 mm in internal diameter. Between the furnace and the tubular reactor, a stainless steel tube was placed to improve the axial temperature distribution. Catalysts were pelleted and then crashed and screened to 20-35 mesh. A 0.1 g sample of the catalyst particles was charged into the reactor. The catalyst bed was located in the range of constant temperature along the axial dimension. The catalysts were presulfided before HDS reaction by a mixture of H2S/H2 (10 vol % H2S) at atmospheric pressure and 400 °C for 3 h. After the presulfidation, the reactor was cooled to the HDS reaction temperature in H2S/H2 and then pressurized to 5.0 MPa by H2. During the cooling period, a solution of 1.8 wt % DBT in decalin was fed into the reactor to wet the catalyst bed by a high-pressure metering pump. HDS reactions were carried out at temperatures of 280-340 °C, a total
10.1021/ie030856n CCC: $27.50 © 2004 American Chemical Society Published on Web 04/16/2004
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2325
Figure 2. DBT conversion as a function of the ratio of hydrogen to oil.
Figure 1. Schematic diagram of the HDS reaction system: 1, gas cylinder; 2, pressure regulator; 3, three-way valve; 4, pressure regulator; 5, gauge; 6, tubular reactor; 7, stainless steel; 8, electric furnace; 9, temperature indicator; 10, temperature-controlling unit; 11, high-pressure metering pump; 12, feed reservoir; 13, stop valve; 14, needle valve; 15, gas-liquid separator; 16, liquid sampling.
pressure of 5.0 MPa, and WHSV ) 30-90 h-1. Kinetic data were obtained by varying the space velocity of the liquid feed and monitoring the DBT conversion at steady-state operation. Sampling of liquid products was started 3 h after the reaction conditions had been achieved. At each condition, 6-8 samples were collected at an interval of 20 min. The products were analyzed by an Agilent-6890+ gas chromatograph equipped with a flame ionization detector using an HP-5 column. 2.3. Acquisition of Kinetic Data. Prior to the kinetic study, a set of experiments was performed to check the absence of intraparticle and interphase mass transfer limitations. It is indicated that a kinetic regime was established in all cases. Moreover, the life of the catalyst was investigated, and no deactivation was observed for all the catalysts for a period of over 60 h. Since DBT conversion through the catalyst bed was generally higher than 10%, the reactor should not be regarded as a differential one and the HDS rate could not be directly acquired from the change of DBT concentration across the catalyst bed. Consequently, an integral flow reactor model, i.e., plug-flow reactor, was adapted to acquire the reaction rate. 3. Results and Discussion 3.1. Effect of the Ratio of H2 to Oil. During HDS of DBT, H2 exists in large excess, as in the case of a typical commercial HDS reactor. Therefore, the effect of H2 on the rate constant is negligible in a typical DBT HDS reaction if the space velocity of H2 is high enough. To determine the minimum space velocity in the kinetic study, the effect of the ratio of H2 to oil on DBT conversion was investigated. Figure 2 shows the variation of DBT conversion with the ratio of H2 to oil during HDS of DBT catalyzed by Co-Mo(0.75) at 320 °C. It is indicated that DBT conversion was kept constant at a ratio of H2 to oil over
Figure 3. Variation of DBT conversion during HDS catalyzed by various Co-Mo sulfides: (9) Co-Mo(0.25), (b) Co-Mo(0.50), (2) Co-Mo(0.75), (1) Co-Mo(1.00).
600. It is, therefore, suggested that the kinetic experiments must be performed with a ratio of H2 to oil larger than 600 to eliminate the influence of H2 on DBT conversion. 3.2. DBT Conversion in HDS. Figure 3 illustrates the variations of DBT conversion with temperature during the kinetic study of HDS catalyzed by various MCM-41-supported Co-Mo sulfides at a WHSV of 30 h-1. It is indicated that DBT conversion increased steadily with reaction temperature. Moreover, Co-Mo(0.75) exhibited the highest HDS activity among the MCM-41-supported Co-Mo catalysts. 3.3. Kinetics of DBT HDS. In the kinetic studies of HDS reactions of thiophenic compounds, either the Langmuir-Hinshelwood (L-H) model or the pseudofirst-order model was used in the literature.7 The L-H rate equation gives both rate and adsorption equilibrium constants, which might be helpful to elucidate the reaction mechanism. Therefore, the L-H expression of rate equation has been widely used in studying the kinetics of HDS reactions. It should be noted that an average HDS rate was often used when the data were correlated in terms of the L-H kinetic model.1,8 This treatment assumed that the temperature, pressure, and concentrations are constant through the catalyst bed. In this case, the obtained kinetic parameters might be misleading or meaningless if the conversion is higher than 10%. Since
2326 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Figure 4. -[ln(1 - x)]/x versus W/(FDBT,0x) for DBT HDS: (9) 280 °C, (b) 300 °C, (2) 320 °C, (1) 340 °C.
Co-Mo/MCM-41 catalysts gave very high DBT conversion even at high liquid hourly space velocity, the HDS reaction in the reactor has to be treated as an integral operation. It is generally accepted that there exist two types of adsorption sites on the surface of the HDS catalyst, one site on which DBT and its products competitively adsorb and the other site on which H2 adsorbs.9 Accordingly, the following L-H equation was used in studying the kinetics of the DBT HDS reaction:
rHDS )
kHDSKDBTKH2CDBTPH2 (1 + KDBTCDBT + KH2SPH2S)(1 + KH2PH2)
(1)
where rHDS is the rate of HDS, kHDS is the rate constant of HDS, KDBT, KH2S, and KH2 are the adsorption equilibrium constants of DBT, hydrogen sulfide, and hydrogen on the surface; CDBT is the molar concentration of DBT in the feed, and PH2S and PH2 are the partial pressures of H2S and H2. During HDS of DBT, PH2 was kept constant, and the H2S partial pressure was negligible because of the low DBT concentration and high gas space velocity. Therefore, eq 1 can be simplified as follows:
rHDS )
k′HDSKDBTCDBT 1 + KDBTCDBT
where k′HDS is a composite of kHDS, PH2, and KH2. This model gives the following integrated rate expression for a plug-flow reactor:
-
(
)
ln(1 - x) W ) k′HDSKDBTCDBT,0 - KDBTCDBT,0 x FDBT,0x (2)
where W is the charge weight of the catalyst, FDBT,0 is the molar flow rate of DBT, CDBT,0 is the molar concentration of DBT in the feed, and x represents the DBT conversion. If -[ln(1 - x)]/x is plotted against W/FDBT,0x, a line should be obtained. From the slope and intercept of the line, k′HDS and KDBT will be determined. Nevertheless, when -[ln(1 - x)]/x was plotted against W/FDBT,0x for DBT HDS catalyzed by Co-Mo/MCM-41 catalysts at various temperatures (shown in Figure 4), the expected linear relationship was not observed. This suggests that the L-H model is not valid for the kinetics of DBT HDS. Singhal et al. studied the kinetics of DBT HDS at high DBT concentration in the liquid feed.9 They fitted the experimental data to the L-H kinetic model in a plugflow reactor, and found that a good agreement of the data with the model was obtained. Nevertheless, they deduced that the rate expression of the L-H model will become first-order in DBT concentration when the DBT concentration is at a low level. The pseudo-first-order kinetic model was frequently used to correlate data in HDS of DBT and its derivatives. The integrated rate equation of the pseudo-firstorder model for a plug-flow reactor is as follows:
-[ln(1 - x)] ) k′′HDS
W FDBT,0
(3)
where k′′HDS is the global rate constant (mmol‚g-1‚h-1),
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2327
Figure 5. -[ln(1 - x)] versus W/FDBT,0 for DBT HDS: (9) 280 °C, (b) 300 °C, (2) 320 °C, (1) 340 °C.
FDBT,0 is the initial DBT molar flow rate at the inlet of the reactor (mmol‚h-1), and W is the charge weight of the catalyst (g). -[ln(1 - x)] is plotted against W/FDBT,0 for HDS reaction catalyzed by each Co-Mo catalyst at different temperatures, as shown in Figure 5. Contrary to the L-H model, the pseudo-first-order model is well fitted to the experimental data. Comparing eq 2 with eq 3, one will find that eq 2 will give the same expression as eq 3 when KDBTCDBT,0 is negligible. In other words, in the HDS reaction at a low DBT concentration as in a typical commercial HDS process, the competitive adsorption of DBT on the surface of the catalyst is negligible, which is in agreement with the literature results.11,12 Hence, the rate expression becomes first-order with respect to DBT concentration, as reported by many researchers.2,12-16 The global rate constant of the first-order kinetic model is a composite of the intrinsic rate constant, H2 partial pressure, and adsorption constant of DBT. From the slope of each fitted line, the global rate constant was determined for each MCM-41-supported Co-Mo catalyst, as shown in Table 1. It is generally accepted that HDS of DBT involves two parallel routes: (1) hydrogenolysis of the C-S bonds to give biphenyl (HYG) and (2) hydrogenation of one of the benzenoid rings followed by rapid hydrogenolysis of the C-S bonds to give cyclohexylbenzene (HYD). It is reported that the rate of biphenyl (BP) hydrogenation to form cyclohexane (CHB) was typically 2 orders of magnitude slower than that of DBT hydrogenolysis.17,18 Therefore, the transformation of BP to CHB is negligible in studying the kinetics of DBT HDS.
Table 1. Pseudo-First-Order Rate Constant of DBT HDS Catalyzed by MCM-41-Supported Co-Mo Sulfides at Various Temperatures k′′HDS, mmol‚[(g of catalyst)‚h]-1 temp, °C
Co-Mo (0.25)
Co-Mo (0.5)
Co-Mo (0.75)
Co-Mo (1.0)
280 300 320 340
0.18 0.30 0.65 1.40
0.43 0.79 1.71 3.63
0.84 1.39 2.51 4.74
0.27 0.48 1.20 2.12
In other words, the rate constant of the HYG route can be calculated directly from the formation of BP. Provided that both HDS routes are also pseudo-first-order reactions, the selectivity of BP is expressed as follows:
SBP )
kHYGCDBT kHYG ) (4) kHYGCDBT + kHYDCDBT kHYG + kHYD
where SBP is the selectivity of BP and kHYG and kHYD represent the rate constants of the HYG and HYD routes, respectively. If the transformation of BP to CHB is neglected, then
kHDS ) kHYG + kHYD
(5)
According to eqs 4 and 5, kHYG and kHYD can be calculated from data in Table 1. The calculation results are listed in Tables 2 and 3. From Tables 2 and 3, it can be seen that both kHYG and kHYD reached the maxima at a Co/Mo atomic ratio of 0.75, and that the increment of the HYG rate constant was much higher than that of the HYD rate constant. It is therefore proposed that HYG and HYD might take
2328 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 2. Pseudo-First-Order Hydrogenolysis Rate Constants for DBT HDS over Co-Mo/MCM-41 at Various Temperatures kHYG, mmol‚[(g of catalyst)‚h]-1 temp, °C
Co-Mo (0.25)
Co-Mo (0.5)
Co-Mo (0.75)
Co-Mo (1.0)
280 300 320 340
0.12 0.20 0.45 1.05
0.31 0.56 1.25 2.61
0.66 1.11 1.93 3.57
0.17 0.34 0.86 1.52
Table 3. Pseudo-First-Order Hydrogenation Rate Constants for DBT HDS over Co-Mo/MCM-41 at Various Temperatures kHYD, mmol‚[(g of catalyst)‚h]-1 temp, °C
Co-Mo (0.25)
Co-Mo (0.5)
Co-Mo (0.75)
Co-Mo (1.0)
280 300 320 340
0.06 0.10 0.20 0.35
0.12 0.28 0.45 1.02
0.18 0.28 0.58 1.17
0.10 0.14 0.34 0.60
Figure 6. Arrhenius plot for DBT HDS catalyzed by the supported Co-Mo sulfides: (9) Co-Mo(0.25), (b) Co-Mo(0.50), (2) Co-Mo(0.75), (1) Co-Mo(1.00). Table 4. Apparent Activation Energies and Preexponential Factors of DBT HDS Catalyzed by MCM-41-Supported Co-Mo Sulfides
Ea, kJ‚mol-1 k0 × 10-8, mmol‚g-1‚h-1
Co-Mo (0.25)
Co-Mo (0.50)
Co-Mo (0.75)
Co-Mo (1.00)
111.5 43.8
101.4 15.0
81.4 0.4
99.6 6.5
place on separate active sites during HDS of DBT and that the enhancement of the HYG route is more pronounced with the introduction of Co to the Mo matrix. The Arrhenius plot for kHDS is shown in Figure 6 for each catalyst. From the intercept and slope of each line, the preexponential factor (k0) and apparent activation energy (Ea) can be calculated for each catalyst, as shown in Table 4. From Tables 1 and 4, it is indicated that a low activation energy led to high HDS activity. Conclusions Comparison of the L-H model with the pseudo-firstorder model revealed that the more complicated rate expression of the L-H model is simplified to an expression of a first-order reaction if the concentration of DBT is relatively low and the competitive adsorption of DBT is negligible. Since the concentrations of DBT and its
derivatives are low in the feedstocks in a typical deep HDS process and the adsorption constants for these compounds are small on the supported Mo-based catalysts, it is suggested that the pseudo-first-order model is applicable in the study of HDS of DBT. The rate constants of HYG and HYD indicated that HYG and HYD might take place on different active sites and Co species mainly promoted the HYG route. It is observed that a low activation energy led to high HDS activity. Acknowledgment We are grateful to the NSFC (Grants 20003002 and 20333030) and CNPC Innovation Foundation for financial support of the research. Literature Cited (1) Broderick, D. H.; Gates, B. C. Hydrogenolysis and Hydrogenation of Dibenzothiophene Catalyzed by Sulfided Co-Mo/γAl2O3: The Reaction Kinetics. AIChE J. 1981, 27 (4), 663. (2) Steiner, P.; Blekkan, E. A. Catalytic Hydrodesulfurization of a Light Gas Oil over a NiMo Catalyst: Kinetics of Selected Sulfur Components. Fuel Process. Technol. 2002, 79, 1. (3) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihara, A.; Qian, W. Hydrodesulfurization of Dibenzothiophene over Siliceous MCM41-Supported Catalysts: I. Sufided Co-Mo Catalysts. J. Catal. 2001, 199, 19. (4) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihara, A.; Qian, W., Yao, P. Hydrodesulfurization of Dibenzothiophene over Siliceous MCM-41-Supported Catalysts: II. Sulfided Ni-Mo catalysts, J. Catal. 2002, 210, 319. (5) Qian, W.; Ishihara, A.; Ogawa, S.; Kabe, T. Study of Hydrodesulfurization by the Use of 35S-Labeled Dibenzothiophene 1. Hydrodesulfurization Mechanism on Sulfided Mo/Al2O3. J. Phys. Chem. 1994, 98, 907. (6) Wang, A.; Kabe, T. Fine-tuning of pore size of MCM-41 by adjusting the initial pH of the synthesis mixture. Chem. Commun. 1999, 2067. (7) Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991, 30, 2021. (8) Orozco, E. O.; Vrinat, M. Kinetics of Dibenzothiophene Hydrodesulfurization over MoS2 Supported Catalysts: Modelization of the H2S Partial Pressure Effect. Appl. Catal., A 1998, 170, 195. (9) Singhal, G. H.; Espino, R. L.; Sobel, J. E.; Huff, G. A. Hydrodesulfurization of Sulfur Heterocyclic Compounds: Kinetics of Dibenzothiophene. J. Catal. 1981, 67, 457. (10) Kabe, T.; Ishihara, A.; Zhang, Q. Deep Desulfurization of Light Oil. Part 2: Hydrodesulfurization of Dibenzothiophene, 4-Methyldibenzothiophene and 4,6-Dimethyldibenzothiophene. Appl. Catal. 1993, 97, L1-L9. (11) Macaud, M.; Milenkovic, A.; Schulz, E.; Lemaire, M.; Vrinat, M. Hydrodesulfurization of Alkyldibenzothiophenes: Evidence of Highly Unreactive Aromatic Sulfur Compounds. J. Catal. 2000, 193, 255. (12) Laredo, G. C.; Altamirano, E.; De los Reyes, J. A. Inhibition Effects of Nitrogen Compounds on The Hydrodesulfurization of Dibenzothiophene: Part 2. Appl. Catal., A 2003, 243, 207. (13) Farag, F.; Mochida, I.; Sakanishi, K. Fundamental Comparison Studies on Hydrodesulfurization of Dibenzothiophenes over CoMo-Based Carbon and Alumina Catalysts. Appl. Catal., A 2000, 194/195, 147. (14) Farag, F.; Whitehurst, D. D.; Sakanishi, K.; Mochida, I. Carbon versus Alumina as A Support for Co-Mo Catalysts Reactivity towards HDS of Dibenzothiophenes and Diesel Fuel. Catal. Today 1999, 50, 9. (15) Andari, M. K.; Abu-Seedo, F.; Stanislaus, A.; Qabazard, H. M. Kinetics of Individual Sulfur Compounds in Deep Hydrodesulfurization of Kuwait Diesel Oil. Fuel 1996, 75 (14), 1664. (16) Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; de Beer, V. H. J.; Gates, E. C.; Kward, H. Hydrodesulfurization of Methyl-Substituted Dibenzothiophenes Catalyzed by Sufided CoMo/γ-Al2O3. J. Catal. 1980, 61, 523.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2329 (17) Sapre A. V.; Gates B. C. Hydrogenation of Aromatic Hydrocarbons Catalyzed by Sulfided CoO-Mo/γ-Al2O3: Reactivity, Reaction Networks, and Kinetics. Prepr., Div. Fuel Chem., Am Chem. Soc. 1980, 21 (1), 66. (18) Da Costa, P.; Potvin, C.; Manoli, J.-M.; Lemberton, J.-L.; G. Pe´rot, G.; Dje´ga-Mariadassou, New Catalysts for Deep Hydrotreatment of Diesel Fuel: Kinetics of 4,6-Dimethyldibenzothio-
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Received for review December 5, 2003 Revised manuscript received February 18, 2004 Accepted March 1, 2004 IE030856N
