Chen050427.qxd The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 1. Introduction Research interest in catalytic hydrodesulfurization (HDS) of petroleum feedstock has been increasing recent- ly. This is partly due to the necessity of processing feed- stocks that have large amounts of sulfur-containing com- pounds and also as a result of more stringent environmen- tal regulations with respect to emission of sulfur-contain- ing gases to the atmosphere. Sulfur removal has econom- ic benefits of prolonging catalyst life and decreasing cor- rosion of the process equipment downstream. Perhaps one of the most important factors that spurred interest in HDS is a recent review of the allowable sulfur content of fuels, flue gases and other petroleum products, in the U.S. ______________________________________ *Corresponding Author's e-mail: baba@squ.edu.om The Environmental Protection Agency (USEPA) guide- lines, for instance, restrict the amount of sulfur allowed in diesel fuels to 1.3 x 10-2 g/dL3 by the year 2010 (USA-EPA, 2001). This requires reduction of about 97% from the current level. Achieving this limit will require a major improvement or redesign of the catalysts employed for HDS. Therefore, it is necessary to gain further understanding of the chemical reactions involved. This could be achieved by studies of kinetics and networks of the reac- tions. Dibenzothiophene (DBT), as a typical sulfur-con- taining compound in the petroleum feedstock, has been studied extensively. A recent study has shown that at cer- tain conditions, the DBT kinetic closely resembles that of an overall behavior of DBTs in a light oil fraction (Steiner, et al. 2002). Aspects of the HDS of DBT kinetics have Kinetics of Hydrodesulfurization of Dibenzothiophene on Sulfided Commercial Co-Mo/γ-Al2O3 Catalyst Y.S. Al-Zeghayer1 and B.Y. Jibril*2 1Chemical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia 2Petroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33, Al-Khoud, PC 123, Muscat, Oman Received 27 April 2005; accepted 17 September 2005 Abstract: Kinetics of hydrodesulfurization of dibenzothiophene (DBT) has been studied on a commercial CoMo/γ-Al2O3 catalyst at 633 - 683 K and 10 atm. A low DBT concentration typically obtained in hydrodesulfurization operations was used. Pseudo-first-order model was found to fit the experimental data for the consumption of DBT. The activation energy for the conversion of DBT was found to be 51.7 kcal/mol. Biphenyl (BP) and cyclohexylbenzene (CHB) were obtained as dominant products. For the reaction network, both parallel and parallel-sequential routes were explored. The latter was found to give a better description of the BP and CHB distributions. The ratio of BP to CHB depended on the reaction temperature. The values of activation energies of DBT hydrogenolysis to BP (EBP), DBT hydrogenation to CHB (ECHB1) and hydrogena- tion of BP to CHB (ECHB2) were found to be in a decreasing order of ECHB2 > EBP > ECHB1. The result suggests the pres- ence of different catalytic sites leading to the two products on the catalysts. Keywords: Hydrodesulfurization, Dibenzothiophene, CoMo/Al2O3, Biphenyl, Cyclohexylbenzene, Kinetics IOÉŸ á«æ«Lh~«¡dG âjȵdG ádGPG äÓYÉØJ á«côMdibenzothiophene…QÉŒ »àjÈc …õØM πeÉY ≈∏Y ÒgõdG .¢S.…1πjÈL .….Ü h2@ á°UÓÿG IOÉŸ á«æ«LhQ~«¡dG âjȵdG ádGPEG äÓYÉØJ áÑ«côJ :dibenzothiopheneÚH Ée ájQGôM äÉLQO ~æY …QÉŒ »àjÈc …õØM πeÉY ≈∏Y É«∏ª©e â°SQhO633 ¤G683á≤∏£e áLQO ~æY h10 IOÉŸ áØ«©°V äGõ«côJ .…ƒL §¨°VDBTá«∏ª©ŸG èFÉàædG §Ñæà°ùj ¿G øµÁ áHPɵdG ¤h’G áLQ~dG øe »°VÉjQ êPƒ‰ ¿G âÑKG .âe~îà°SG á«æ«LhQ~«¡dG âjȵdG ádGPCG á«∏ªY øe É¡«∏Y π°UÉ◊G IOɢ˘ŸDBT IOÉe π˘jƒ˘ë˘à˘d á˘£˘°ûæŸG á˘bɢ£˘dG .ᢵ˘∏˘¡˘à˘°ùŸGDBT…hɢ°ùJ ɢ¡˘fG ~˘˘Lhkcal/mol51.7,,~L ɢª˘g ɢ¡˘«˘∏˘Y ∫ƒ˘°üÙG ᢫˘˘°ù«˘˘Fô˘˘dG äɢ˘é˘˘à˘˘æŸG ¿G(CHB) Biphenyl (BP) and cyclohexylbenzene»£©J á©HÉààŸG ájRGƒàŸG äÓYÉØàdG ¿G ~Lhh .âHôL á©HÉààŸG ájRGƒàŸGh …RGƒàŸG äÓYÉØàdG ɪgh äÓYÉØàdG áµÑ°T øe ÚæKG ¿Gh . ÚJOÉe ™jRƒàd π° aG ∞°UhCHBBP and IOÉe áÑ°ùf ¿Gh .BPIOÉe ¤GCHB IOÉe πjƒëàd ᣰûæŸG ábÉ£dG º«b ¿G .IQGô◊G áLQO ≈∏Y ~ªà©JDBT IOÉe ¤G) BPBP(E ¤G IOÉŸG ¢ùØf πjƒ–hCHB)CHB1(E IOÉe πjƒ–hBP ¤G)CHB2CHB (E ‹RÉæJ πµ°T ‘ É¡ª«b I~LhCHB1< EBP< ECHB2E.èFÉàædG ¿Gh õØ◊G πª©dG ≈∏Y ÚJOÉe êÉàf’ äOÉb iõØ◊G πeÉ©dG ≈∏Y áØ∏àfl ™bGƒe OƒLh ìÎ≤J.i áá««MMÉÉààØØŸŸGG ääGGOOôôØØŸŸGG ÚLhQ~«¡dG ᣰSGƒH âjȵdG ádGREG äÓYÉØJáÑ«côJ :,3O2 CoMo/A1,Cyclohexylbenzen, Biphenyl,.…QÉŒ »àjÈc …õØM πeÉY 39 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 been reported. The reaction was shown to follow a pseu- do-first-order kinetics and both parallel and consecutive reaction networks were identified (Girgis, et al. 1991; Farag, et al. 1997; Wang, et al. 2004; Pille, et al. 1994; Broderick, et al. 1981). When the reaction of DBT was tested on CoMo/MCM-41, a pseudo-first-order kinetic model was found to represent the experimental data better than a Langmiur-Hinshelwood model. This is a typical observation for low DBT concentrations. It has also been shown that the relative significance of hydrogenolysis and hydrogenation routes depended on Co/Mo ratios (Wang, et al. 2004). Furthermore, the two routes could also be substantially affected by the presence of Naphthalene or hydrogen sulfide for the HDS of DBT or 4,6- dimethyldibenzothiophene (Farag, et al. 1999). Sulfided promoted CoMo/Al2O3 and other transition metal catalysts have been widely researched and shown to be effective for the reaction (Topsoe, et al. 1984; Prins, et al. 1989; Yang, et al. 2002; Venezia, et al. 2002; Damyanova, et al. 2003; Papadopoulou, et al. 2003; Al- Zeghayer, et al. 2005). Further understanding of the nature of interactions between the reactants and catalysts and effects of reaction conditions thereupon is important. Therefore, the objective of this work was to describe the kinetics of DBT hydrodesulfurization on a sulfided com- mercial CoMo/γ-Al2O3 catalyst. Effects of variation of reaction temperature and feed flow on products were explored. 2. Experimental 2.1 Materials The materials used for the catalyst testing were Dibenzothiophene (DBT), Tetralin, Decalin and Dimethyldisulfide (CH3)2S2 (DMDS). All chemicals were of analytical grade quality supplied by Aldrich Chemical Company. A commercial catalyst was obtained from a supplier. Its composition and physical properties as given by the manufacturer are shown in Table 1. The catalyst was sulfided to a catalytically active form using a solution of dimethyldisulfide (2 wt% sulfur equivalent) in decalin solvent at 623 K until H2S breakthrough was observed. 2.2 Catalyst Testing The catalysts evaluation was carried out by passing solutions containing dibenzothiophene at a concentration of one or two weight percent in the hydrogen donor sol- vent (tetralin) through a fixed bed containing 1 g of the catalyst particles. The catalyst particle sizes and reaction conditions were chosen to minimize mass transfer limita- tions, based on separate preliminary experiments. The use of a hydrogen donor solvent ensured an excess supply of hydrogen readily available for the reaction without a need for adding gas phase hydrogen. The reaction was conduct- ed at temperature range of 633 - 683 K, pressure of 10 MPa, and flowrates required to give measurable conver- sions were between 0.5 and 2 g/min. Details on catalyst testing and GC analysis were reported earlier (Al- Zeghayer, 2005). 3. Results and Discussion The major products identified were biphenyl (BP) and cyclohexylbenzene (CHB). Negligible amounts of bicy- clohexyl were detected at high conversions. The conver- sion and products distribution over the catalyst are shown in Fig. 1. The figure shows variation in DBT conversion and BP and CHB selectivities with inverse of weight hourly space velocity (WHSV) from 0 - 250 g catalyst h/g feed. The effect of hydrogen concentration on the reaction was neglected as it was considered to be in excess. The DBT degree of conversion increased rapidly from 0 to 92% in the inverse WHSV range. At the initial conversion, BP appeared to be an exclusive product indicating that it is a primary product. At the highest conversion, its selec- tivity decreased to 80% with a corresponding increase in the selectivity to CHB from 0 to 20%. At certain conditions, the kinetics of HDS of Components wt% dry basis Molybdenum oxide (MoO 3) 15.40 Cobalt oxide (CoO 3.20 Sodium oxide (Na 2O) 0.03 Iron (Fe) 0.03 Sulphate (SO 4) 0.30 Silicon dioxide (SiO 2) 0.10 Physical properties Poured bulk density, lb/ft 3 33.0 Compacted bulk density, lb/ft 3 36.0 Crush strength, lb/m m 3.4 Surface area, m 2/gm 310.0 Pore volume, cm 3/gm 0.8 Table 1. Chemical and physical properties of the commercial catalyst C on ve rs io n (% ) an d Se le ct iv it y (% ) (1/WHSV)*1000 (g cat. h/g feed) Figure 1. Variation of DBT conversion and products distribution with 1/WHSV 40 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 Dibenzothiophene (DBT) was shown to represent that of an aggregate of DBTs in a petroleum feedstock (Steiner, 2002). In an earlier study of kinetics of HDS of DBT in liquid feed, the data were well fitted to a Langmuir- Hinshelwood (L-H) model in a plug-flow reactor (Singhal, et al. 1981). However, at a lower DBT concen- tration, as expected, a first order relation between the rate of consumption of DBT and concentration was obtained. It has been shown that for an analysis of the reaction net- work, even simple power law could give satisfactory results (Levenspiel, 1972). Therefore, for the catalyst under study, the kinetic data for the reaction were evaluat- ed assuming integral plug-flow reactor. A logarithmic plot of mole of DBT reacted and the inverse of WHSV showed a linear relation (Fig. 2). This indicates a pseudo- first order kinetic similar to earlier reports (Wang, et al. 2004; Farag et al. 1999; Singhal, et al. 1982). However, at a low temperature, DBT adsorption may be important despite the low concentration that was employed. The deviation of the data from the pseudo-first order for a low feed flow or high contact time may be due to adsorption of DBT. At a low temperature, such adsorption and species diffusion may be important. Therefore, the data may deviate from a first-order rate expression. The distribution of the products and reaction network were explored by changing the reaction temperature and feed flow rate (Fig. 3). Both show a similar trend of change in CHB/BP ratio with DBT conversions. But for all conversion levels, BP was the dominant product; con- trary to an earlier work on a similar catalyst (Singhal, et al. 1981; Broderick, 1980). Many workers have proposed a reaction network similar to Scheme I in Fig. 4. The products observed were biphenyl and cyclohexylbenzene. In addition, the presence of tetrahydrodibenzothiophene and hexahydrodibenzothiophene were also reported (Broderick, 1980). Other reports suggested their presence as undetected, highly reactive intermediates (Rollmann, 1977). Biphenyl was observed to react to give mainly cyclohexylbenzene (Singhal, et al. 1982; Broderick, 1980). It was concluded that the rate of reaction of biphenyl was slow in comparison with the disappearance of dibenzothiophene. Hydrogenation of cyclohexylben- zene to give bicyclohexyl was too slow to be measurable under typical hydrodesulfurization conditions (Broderick, 1980). The obvious path for further reaction of biphenyl is a series of hydrogenations, giving first cyclohexylbenzene and then bicyclohexyl. By comparing the relation between product ratios and conversions at constant temperature and also with increasing temperature, we consider that both parallel and sequential paths may be involved in the reaction network as suggested in Scheme I. Perhaps par- tially hydrogenated dibenzothiophene intermediates were formed. If the rate of hydrodesulfurization of the interme- diates (path 3') were very fast, the selectivity profiles would follow both parallel and sequential (complex) path. If the rate were slower, that would have hindered the appearance of cyclohexylbenzene in the early stages of the reaction; but contributed to its appearance by sequen- tial reactions. The reaction network in Scheme I was used to further lo g( m ol e of D B T re ac te d/ g fe ed ) (1/WHSV)*1000 g cat.h/g feed Figure 2. Logarithmic variation of mole of DBT converted versus 1/WHSV Effect of temperature Effect of flow rate P ro du ct r at io ( C H B /B P ) DBT conversion (%) Figure 3. Variation of CHB/BP ratio with DBT conversion 0.28 0.24 0.20 0.16 0 20 40 60 80 100 0.00 0.01 0.02 0.03 0.04 0 -1 -2 -3 Figure 4. Scheme I 41 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 study the kinetics of the reaction. The overall first-order rate constants and the activation energies were determined from the experimental data. To measure these parameters, it was assumed that at low conversion, the reaction of biphenyl could be neglected compared to its rate of appearance and that cyclohexylbenzene formed directly from dibenzothiophene (Sapre, et al. 1980). Arrhenius plot for the steady state consumption of DBT is shown in Fig. 5. The activation energy for the disappearance of DBT over the catalyst was calculated to be 51.7 kcal/mole as shown in Table 2. The same table compares the activa- tion energies with those obtained by others with the exception of the (5.3 kcal/mol) value obtained by Bartsch and Tanielian (1974); the energies of activation are in good agreement. The low values obtained by Bartasch and Tanielian indicate a diffusion-controlled reaction. Based on the relation between the rate constants for consumption of DBT and that of production of BP and CHB, the activation energies (El and E2) have been evalu- ated from the Arrhenius plots. The relation between the product ratio and the conversions obtained assuming the parallel-sequential path was fitted to the experimental data to determine the pseudo first-order rate constant for the reaction of biphenyl to cyclohexylbenzene (k3). The acti- vation energy (E3) was also calculated from the Arrhenius relation. The energies have the same order of magnitude as shown in the Table 3. The activation energy for the hydrogenolysis of dibenzothiophene to biphenyl is 50.3 kcal/mole. This is insignificantly greater than the activa- tion energy for hydrogenation of dibenzothiophene to cyclohexylbenzene by about 6.15 kcal/mole. This is con- trary to an earlier report that showed the rate of BP trans- formation to CHB to be two orders of magnitude slower than that of DBT hydrogenolysis, when the reaction was tested on alumina supported molybdenum carbide (Bartsh, et al. 1974). It was reported that at DBT conversions degrees high- er than 40%, the selectivity to CHB decreased with an increasing temperature (Rollmann, 1977). In the low con- version range, the results are in agreement with those of (Broderick, 1980). He used the ratio of rate equations to correlate the differential concentration data for the hydro- genation and hydrogenolysis reaction of dibenzothio- phene. However, this observation was limited because the study took account of the parallel reaction scheme without further reaction of biphenyl due to the low conversions (less than 15%). The results obtained by Singhal et al. favored the parallel and sequential reaction, which became increasingly important at high conversions (Singhal, et al. 1981). Accordingly, the change in selectiv- ity with temperature was said to reflect the differences in the activation energies. In this work, the results covered the full range of conversions. It shows that the activation energy for hydrogenolysis reaction of dibenzothiophene is greater than that for hydrogenation of dibenzothiophene to cyclohexylbenzene. The values of the activation energies for hydrogenolysis (E1) and hydrogenation (E2) of diben- zothiophene and the activation energy for hydrogenation reaction of biphenyl to cyclohexylbenzene (E3) are in a decreasing order of E3 > El > E2. Thus, the product distri- bution is partly determined by the temperature employed for the reaction. 4. Conclusions The pseudo-first-order kinetic model was found to fit the experimental data for the DBT consumption in 5.0 4.0 3.0 2.0 1.0 0.0 lo g e K ( g fe ed /g c at .h ) 1.45 1.50 1.55 1.60 1/T x 103 , K-1 Figure 5. Arrhenius plot for HDS of DBT on the catalyst Temp., K CHB/BP g feed / g catalyst h K, Kcal/mol k k1 k2 k3 E E1 E2 E3 633 653 683 0.000 0.145 0.152 4.03 18.04 83.73 4.03 15.76 72.69 0.00 2.28 11.04 0.00 0.53 5.00 53.1 50.3 44.1 63.8 k=k1+k2 and CCHB/CBP = k2/k1 Table 3. The Pseudo-first-order rate constants and activation energies for reactions in Scheme I Temperature range, K Ea, kcal/mol.K Solvent 633-683 558-623 573-723 not stated 548-598 53.1 39.1 36.0 5.3 28.0-32.8 Tetralin [this work] Tetralin [16] Aromatics [19] not stated [21] n-hexane [18] Table 2. Comparison of apparent activation energies for DBT consumption 42 The Journal of Engineering Research Vol. 3, No. 1 (2006) 38-42 hydrodesulfurization on a commercial CoMo/γ-Al2O3 cat- alyst. Biphenyl and cyclohexylbenzene were the major products observed. Their distributions suggest a reaction network that involves both parallel and sequential routes. The differences in activation energies of the respective reaction leading to biphenyl and cyclohexylbenzene indi- cate the importance of reaction temperature in determin- ing the products distribution. Further, there appeared to be separate sites responsible for production of biphenyl and cyclohexylbenzene. References Al-Zeghayer, Y.S., Sunderland, Y.S., Al-Masry, Y.W., Al- Mubaddel, F., Ibrahim, A.A., Bhartiya, B.K. and Jibril, B.Y., 2005, “Activity of CoMo/γ-Al2O3 as a Catalyst in Hydro-desulfurization: Effects of Co/Mo Ratio and Drying Condition," Appl. Catal. A, Vol. 282, pp. 1-10. Bartsh, R. and Tanielian, C., 1974, "Hydrodesulfurization: I. Hydrogenolysis of Benzothiophene and Dibenzothiophene over CoO-MoO3-Al2O3 Catalyst," J. Catal, Vol. 35, p. 353. Broderick, D.H., 1980, Ph.D Dissertation, University of Delaware, Newark, Delaware. Broderick, D.H. and Gates, B.C., 1981, "Hydro-desulfu- rization and Hydrogenation of Dibenzothiophene Catalyzed by Sulfided Co-Mo/γ-Al2O3: The Reaction Kinetics," AIChEJ, Vol. 27, p. 663. Da Costa, P., Potvin, C., Manoli, J.M., Lemberton, J.L., Perot, G. and Djega-Mariadassou, G., 2002, "New Catalysts for Deep Hydrotretment of Diesel Fuel: Kinetics of 4,6-Dimethyldibenzothiophene Hydro- desulfurization Over Alumina-Supported Molybdenum Carbide," J. Mol. Catal. A, Vol. 184 p. 323. Damyanova, S., Petrov, L. and Grange, P., 2003, "XPS Characterization of Zirconium-promoted CoMo Hydrodesulfurization Catalysts," Appl. Catal. A, Vol. 239, p. 241. Farag, H., Sakanishi, K., Mochida, I. and Whitehurst, D.D., 1999, "Kinetic Analyses and Inhibition by Naphthalene and H2S in Hydrodesulfurization of 4,6- dimethyldibenzothiophene (4,6-DMDBT) Over CoMo-based Carbon Catalyst," Energy and Fuel, Vol. 13, p. 449. Farag, H., Whitehurst, D.D., Sakanishi, K. and Mochida, I., 1997, "Catalysis in Fuel Processing and Environmental Protection," Proc. 214th National Meeting of ACS Sep 7-11, Las Vegas, USA, Am. Chem. Soc, Vol. 42 p. 569. Federal Register 65 (2000) 35430. Girgis, M.J. and Gates, B.C., 1991, "Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing," Ind. Eng. Chem. Res, Vol. 30, p. 2021. Levenspiel, O., 1972, Chemical Reaction Engineering, 2nd edn, John Wiley & Sons, New York. Papadopoulou, C., Vakros, J., Matralis, H.K., Kordulis, C. and Lycourghiotis, A., 2003, “On the Relationship between the Preparation Method and the Physicochemical and Catalytic Properties of the CoMo/γ-Al2O3 Hydrodesulfurization Catalysts," J. Coll. Interf. Sci, Vol. 261, pp. 146. Pille, R.C., Yu, C. and Froment, G.F., 1994, "Kinetic Study of the Hydrogen Sulfide Effect in the Conversion of Thiophene on Supported Co-Mo Catalysts," J. Mol. Catal, Vol. 94, p. 369. Prins, R., de Beer, V.H.J. and Somorjai, G.A., 1989, "Structure and Function of the Catalyst and the Promoter in Co-Mo Hydrodesulfurization Catalysts," Catal. Rev. Sci. Eng, Vol. 31 p. 1. Rollmann, L.D., 1977, "Catalytic Hydrogenation of Model Nitrogen, Sulfur, and Oxygen Compounds," J. Catal, Vol. 46, p. 243. Sapre, A.V. and Gates, B.C., 1980, "Prepr., Div. Fuel Chem., Hydrogenation of Aromatic Hydrocarbons Catalyzed by Sulfided CoO-Mo/γ-Al2O3: Reactivity, Reaction Network, and Kinetics," Am. Chem. Soc, Vol. 21, p. 66. Singhal, G.H., Espino, R.L., Sobel, J.E. and Huff, G.A., 1981, Hydrodesulfurization of Sulfur Heterocyclic Compounds: Kinetics of Dibenzothiophene," J. Catal, Vol. 67, p. 457. Steiner, P. and Blekkan, E.A., 2002, “Catalytic Hydro- desulfurization of a Light Oil Over a NiMo Catalyst: Kinetics of Selected Sulfur Components,” Fuel Process. Tech, Vol. 79 p. 1. Topsoe, H. and Clausen, B.S., 1984, "Importance of Co- Mo-S Type Structures in Hydrodesulfurization," Catal. Rev. Sci. Eng, Vol. 26, p. 395. Venezia, A.M., La Parola, V., Deganello, G., Cauzzi, D., Leonardi, G. and Predieri, G., 2002, "Influence of the Preparation Method on the Thiophene HDS Activity of Silica Supported CoMo Catalysts,” Appl. Catal. A, Vol. 229, p. 261. Wang, Y., Sun, Z., Wang, A., Ruan, A.L., Lifeng, L., Ren, J., Li, X., Li, C., Hu, Y. and Yao, P., 2004, "Kinetics of Hydrodesulfurization of Dibenzothiophene Catalyzed by Sulfided Co-Mo/MCM-41," Ind. Eng. Chem. Res, Vol. 43, p. 2324. Yang, P., Yan, F. and Liao, K., 2002, "A Study on the Effect of Preparation Parameters on the Catalytic Performance and Active Components of a new type Hydrodesulfurization Catalyst," Petr. Sci. Tech, Vol. 20, p. 763.