1.3 Hydrodesulfurization (HDS) Process
1.3.3 Support for HDS catalyst
In HDS catalysts, the active components (mixed sulfides of Mo and Co or Ni as Co–
Mo–S or Ni–Mo–S phases) are supported on a carrier. The carrier or support material
usually provides high surface area and also provides mechanical strength to the catalyst. Al2O3 is the most widely used support material for HDS catalysts due to highly stability, content of acidic and basic sites, high surface area and porosity. New supports for HDS catalysts, viz. TiO2, ZrO2, MgO, carbon, SiO2, zeolites etc., are being explored for better catalytic activity, higher surface area and other suitable properties as catalyst support for given application (Rayo et al., 2009; Rana et al., 2005; Al–Dolama et al., 2006; Zhao et al., 2008). Okamoto and coworkers (2008, 2010) have studied the effect of nature of support (Al2O3, TiO2, ZrO2 and SiO2) using supported Mo–sulfide and of supported Co–Mo sulfide model catalysts on catalytic activity in two cases, viz. (1) HDS of thiophene, and (2) hydrogenation of butadiene.
Co–Mo catalysts were prepared by chemical vapor deposition of Co(CO)3NO on supported Mo–sulfide. Authors concluded that the Co species deposited in this way were located at the edges of the MoS2 particles. For HDS of thiophene, most active catalyst was SiO2–supported MoS2, while least active catalyst was Al2O3–supported MoS2. On the other hand, for hydro–genation of butadiene, most active catalyst was ZrO2–supported MoS2, while SiO2– supported MoS2 catalyst was the least active catalyst.
A few studies have also treated the matter of acidic and basic nature of support, and its effect on the nature of interaction with the active metal. TiO2 mixed oxide received relatively higher attention because it shows higher activity. Detailed explanation of characterization of hydrotreating catalysts supported on TiO2–Al2O3 binary oxides was given by Ramirez et al. (1993). The catalytic behavior over the TiO2–Al2O3 supported Mo catalysts, in particular for the 4,6–dimethyl–DBT, was much higher than that obtained over Al2O3 supported Mo catalyst. Muralidhar et al. (1984) have studied separately the HDS of thiophene and the hydrocracking of 2,4,4–
trimethylpentene over various supported CoMo catalysts. They compared the catalysts CoMo supported on different types of alumina to CoMo supported on SiO2–Al2O3, SiO2–MgO and TiO2. They concluded that the CoMo–Al2O3 catalysts were most active for HDS. However, the Al2O3–supported catalysts were less active in hydrocracking, which is an indication that they were less acidic than the other catalysts. The authors found that increasing the SiO2 content in SiO2–Al2O3 support decreased both the HDS and hydrogenation activities but increased the hydrocracking activity. Wang et al. (2015) have studied the Co–Mo–S catalysts supported on tri–
modal porous Al2O3 for the hydrodesulfurization of fluid catalytic cracking gasoline.
To improve the performance of CoMoS catalysts applied to the hydrodesulfurization (HDS) of fluid catalytic cracking (FCC) gasoline, a series of CoMoS/Al2O3 catalysts were prepared with alumina of different pore structures, and their HDS performance was evaluated with real FCC gasoline. This study indicated that CoMoS/Al2O3
catalysts prepared with micro or meso–porous alumina possessed high HDS activity but low selectivity, whereas macro–porous alumina enhanced the selectivity of CoMoS catalysts. Flego et al. (2001) aimed at improving the activity of CoMo catalysts in view of deep desulfurization of fuels. The authors prepared CoMo catalysts with various supports containing metal (viz. Zr, Si, and B) oxides having different acid–base properties and measured their activity in the HDS of thiophene.
The authors also estimated the hydrogenation activity of the catalysts from the butane selectivity in the HDS products. Both HDS and hydrogenation activities increased with increasing acid site density. The authors interpreted the relative decrease in hydrogenation activity of the samples containing boron oxide by supposing that the olefins were too strongly adsorbed on the acid sites to migrate to the sulfide phase.
Klimova et al. (1998) used Mg–Al mixed oxides prepared by the sol–gel method as
supports for Mo and NiMo catalysts. The results indicated that even with the incorporation of small amounts of magnesia in alumina, the hydrogenation function of the un–promoted Mo–catalyst was substantially reduced, while the thiophene conversion was much less affected. The gain in HDS/hydrogenation selectivity resulting from the addition of magnesia was attributed to the fact that the size of MoS2 crystallites increased with increasing magnesia content, as compared to their size on pure alumina. This interpretation implies that hydrogenation takes place at corner sites, the addition of magnesia leads to an increased proportion of edge versus corner sites. However, with Ni promoted catalysts with Ni/ (Ni + Mo) ratio of 0.3, there was continuous and substantial drop in HDS activity with increasing magnesia loading, which paralleled the decrease in hydrogenation activity. Kagami et al. (2005) also studied the catalytic activities of NiMo/Al2O3 catalysts on various model compounds like thiophene, tetrahydrothiophene, dibenzothiophene and 4,6–
dimethyldibenzothiophene.
The use of zeolites and amorphous silica–alumina as supports for HDS active phases (e.g. Mo, CoMo, NiMo etc.) have been the subject of many studies. Zeolites are highly acidic and contain Bronsted acidity that can improve the HDS activity for refractory sulfur compounds such as DBT, 4,6–DMDBT. The Bronsted acidity of the support can promote isomerization of the alkyl groups in the alkyl DBTs and suppress their steric hindrance (Perot, 2003). The main problem in using zeolites as supports for HDS catalysts is the difficulty in the impregnation and dispersion of the active phase (e.g. MoS2, Co–Mo–S or Ni–Mo–S) on the support surface. Another type of HDS catalyst, nano–porous alumino–silicate based materials such as MCM–41, Al–
MCM–41, SBA–15 have received considerable attention as supports for Co and Ni promoted MoS2 catalysts (Corma et al., 1995, 1996; Song et al., 1999; Klimova et al.,
1998; Gutierrez et al., 2008; Ren et al., 2008). Carbon–supported catalysts have also been found to show high HDS activity with reduced deactivation by coke deposition due to a lower acidity of the carbon support (Kouzu et al., 2004). Rapid sintering of the active phase under reaction conditions and its unsuitability for regeneration (after deactivation) are major disadvantages in using this support.