Hydrodesulfurization of 4,6-Dimethyldibenzothiophene Using Sol-Gel

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Chapter 7

Hydrodesulfurization of 4,6-Dimethyldibenzothiophene Using Sol-Gel Alumina-Supported Cobalt−Molybdenum Catalysts with High Metal Loading A. Ishihara,* N. Sato, S. Hayashi, T. Itoh, T. Hashimoto, and H. Nasu Mie University, 1577 Kurima Machiyacho, Tsu-City 514-8507, Japan *E-mail: [email protected]

In the present study, HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) using sol-gel CoMo/Al2O3 catalysts with high metal loading were investigated. Aluminiumtri-sec-butoxide, ammonium heptamolybdate and cobalt nitrate hexahydrate were used in the preparation of catalysts. The molar ratio of Co:Mo:Al was 1:1:2 or 1:1:15. Surface area (SA), pore volume (PV) and pore diameter (PD) for CoMo/Al2O3 (1:1:15) were 437m2/g, 1.02cm3/g and 5.5nm, respectively. Those for CoMo/Al2O3 (1:1:2) were 213m2/g, 0.82cm3/g and 14nm, respectively. In HDS of 4,6-DMDBT using the conventional catalysts, the conversion decreased in the order CoMo/Al2O3 (1:1:2) > Co-Mo/ref.Al2O3 (prepared by conventional impregnation) > CoMo/Al2O3 (1:1:15). CoMo/Al2O3 (1:1:2) also showed the highest methylcyclohexyltoluene (MCHT) selectivity. Although CoMo/Al2O3 (1:1:2) had large metal contents, the higher SA, PV and PD were maintained, which resulted in the higher activity and selectivity. Further, it was thought that hydrogenation of an aromatic ring of 4,6-DMDBT, which is needed for HDS of 4,6-DMDBT, was promoted by the formation of the larger amount of active sites for hydrogenation of the aromatic ring which was caused by the larger amounts of active metals loaded into the catalyst.

© 2012 American Chemical Society In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Introduction Sulfur concentrations of gasoline and gas oil have been regulated to less than 10 ppm. Such regulation may become severer in the near future because the regulation for NOx becomes severer. Further, if hydrogen for a fuel cell is prepared from naphtha, sulfur concentration in naphtha must be a ppb order. Thus the development of preparation methods for novel hydrodesulfurization (HDS) catalysts still has been required (1, 2). In recent years, much attention has been focused on NEBULA catalysts for hydrotreating (3, 4). However, the detail information for preparation has not been clarified. On the other hand, a sol-gel method enables us to make porous materials with fine structure and polycrystals with uniform composition and with more than one component at lower temperature (5). Active components Mo and Co in HDS catalysts were usually supported on Al2O3 by an impregnation method. In the case of the sol-gel method, since these active species can be added to an intermediate gel at the preparation of a support, very active catalysts with highly dispersed metal and with high metal loading may be obtained. In the present study, HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) using sol-gel CoMo/Al2O3 catalysts with high metal loading were investigated.

Experimental Aluminium-sec-tributoxide (Al(O-sec-C4H9)3, ASB) was dissolved in anhydrous ethanol. Then, aqueous solutions of ammonium heptamolybdate ((NH4)6Mo7O24⋅4H2O) and cobalt nitrate hexahydrate (Co(NO3)2⋅6H2O) were added to the ASB solution. Figure 1 shows the flow chart for the preparation of CoMo/Al2O3 with high metal loading by the sol-gel method. The molar ratio of Co:Mo:Al was 1:1:2 or 1:1:15. The obtained gel was dried and calcined at 500°C for 3h. The contents of MoO3 and CoO for CoMo/Al2O3 (1:1:2) were 45 and 23wt%, respectively. Those for CoMo/Al2O3 (1:1:15) were 15 and 8wt%, respectively. For comparison, the reference alumina-supported conventional catalysts were also prepared using the impregnation methods. Ref. Al2O3 was supplied from a petroleum company in Japan. CoMo/ref.Al2O3 represents the catalyst where Mo and Co components were supported in impregnation simultaneously. Mo-Co/ref.Al2O3 represents the catalyst where Co component was supported first and Mo component was second in impregnation. Co-Mo/ref.Al2O3 represents the catalyst where Mo component was supported first and Co component was second in impregnation on ref.Al2O3. Each catalyst has 20wt% of MoO3 and Co/Mo ratio 0.4. The measurement of XRD and N2 adsorption for the prepared catalysts was performed. HDS of decalin solution of 4,6-DMDBT (0.01wt%) was performed using a fixed-bed reactor after presulfiding by H2S/H2 at 400°C. Reaction conditions were as follows: WHSV 28/h, 50 kg/cm2, H2 18 l/h. Products dimethylbiphenyl (DMBP) and methylcyclohexyltoluene (MCHT) were determined by GC-FID. 88 In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. Flow chart for preparation of CoMo/Al2O3 by the sol-gel method (Co:Mo:Al=1:1:2 or 1:1:15).

Results and Discussion N2 adsorption measurement was performed and the results are shown in Table 1. Surface area (SA), pore volume (PV) and pore diameter (PD) for CoMo/Al2O3 (1:1:15) were 437m2/g, 1.02cm3/g and 5.5nm, respectively. Those for CoMo/Al2O3 (1:1:2) were 213m2/g, 0.82cm3/g and 14nm, respectively. SA and PV for CoMo/Al2O3 (1:1:2) were smaller than that for CoMo/Al2O3 (1:1:15) but PD was larger. Figure 2 shows the BJH pore size distribution. CoMo/Al2O3 (1:1:2) with high metal content showed very broad distribution while that for CoMo/Al2O3 (1:1:15) was very sharp and relatively narrow. There may be a slight cohesion in particles of CoMo/Al2O3 (1:1:2) because its content of transition metal species was much larger than that of CoMo/Al2O3 (1:1:15). However, CoMo/Al2O3 (1:1:2) still maintained enough large surface area and pore volume to catalyze hydrodesulfurization and its larger pore would have an advantage for the diffusion of 4, 6-DMDBT. According to XRD measurement in Figure 3, catalysts prepared by the conventional impregnation showed the peaks of gamma-alumina. Further CoMo/ref.Al2O3 revealed peaks of MoO3. It is thought that since active species could not be supported uniformly for CoMo/ref.Al2O3 the HDS activity decreased in comparison with that for Co-Mo/ref.Al2O3. In HDS of 4,6-DMDBT using the conventional catalysts, the conversion decreased in the order Co-Mo/ref.Al2O3 >CoMo/ref.Al2O3 >Mo-Co/ref.Al2O3 which is consistent with the order of surface area (Table 1 and Figure 4). When the performance of CoMo/Al2O3 (1:1:2) and CoMo/Al2O3 (1:1:15) was compared with that of Co-Mo/ref.Al2O3, CoMo/Al2O3 (1:1:2) showed the highest activity and MCHT selectivity (Figure 5). As shown in Figure 5, this catalyst exhibited the higher conversion than a typical commercial deep desulfurization catalyst supplied from a catalyst maker in Japan. Although CoMo/Al2O3 (1:1:2) had the larger metal contents, the higher SA, PV and PD were maintained, which resulted in the higher activity and selectivity. Further, it was thought that hydrogenation of 89 In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


an aromatic ring of 4,6-DMDBT, which is needed for HDS of 4,6-DMDBT, was promoted by the formation of the larger amount of active sites for hydrogenation of the aromatic ring which was caused by the larger amounts of active metals loaded into the catalyst. CoMo/Al2O3 (1:1:15) catalyst had higher surface area of 437m2g-1, but showed the lower activity. It is likely that the surface area is too high to be maintained under the hydrodesulfurization condition. Although chemisorption or N2 adsorption experiments for catalysts after the reaction have not been performed at the present time, there might be a collapse of the structure which decreased the metal dispersion and surface area for CoMo/Al2O3 (1:1:15).

Table 1. BET surface area, BET pore volume and BJH pore diameter for each catalyst

Figure 2. BJH pore size distribution of CoMo/Al2O3 (1:1:2) and CoMo/Al2O3 (1:1:15).

90 In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 3. XRD patterns Commercial catalyst, CoMo/Al2O3(1:1:2), CoMo/Al2O3 (1:1:15) (left) and ref.Al2O3 supported catalysts (right).

Figure 4. Effect of reaction temperature on conversion and selectivity for MCHT in HDS of 0.01 wt.% 4,6-DMDBT using catalysts prepared by the impregnation method.

Figure 5. Effect of reaction temperature on conversion and selectivity for MCHT in HDS of 0.01 wt.% 4,6-DMDBT using catalysts prepared by the sol-gel method . 91 In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Conclusions


In impregnation catalysts, Co-Mo/Al2O3 catalyst which had the highest surface area showed the highest activity, while Mo-Co catalyst which had the least surface area showed the lowest activity. When CoMo/Al2O3 catalysts were prepared by the sol-gel method, CoMo/Al2O3 (1:1:2) catalyst, which contained larger amounts of active species, showed the highest activity. CoMo/Al2O3 (1:1:15) catalyst had higher surface area of 437m2g-1, but showed the lower activity.

References 1.

2. 3. 4. 5.

Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation; Kodansha Scientific; Wiley-VCH: Tokyo; Weinheim, Germany, 1999. Ishihara, A. J. Jpn. Petrol. Inst. 2008, 51 (2), 73–82. Gochi, Y.; Ornelas, C.; Paraguay, F.; Fuentes, S.; Alvarez, L.; Rico, J. L.; Alonso-Nu´n˜ez, G. Catal. Today 2005, 107–108 –536. Eijsbouts, S.; Mayo, S. W.; Fujita, K. Appl. Catal., A: Gen. 2007, 322, 58–66. Lebihan, L.; Mauchaussé, C.; Duhamel, L.; Grimblot, J. J. Sol-Gel Sci. Technol. 1994, 2, 837.

92 In Nanocatalysis for Fuels and Chemicals; Dalai, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Hydrodesulfurization of 4,6-Dimethyldibenzothiophene Using Sol-Gel

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