Introduction
Recently, with the increasingly strict environmental regulations, various novel supports for hydrogenation catalysts are studied to remove the sulfur in the oil product.[1, 2] For designing good-performance hydrotreating catalysts, it is significant to understand the HDS reaction mechanisms and kinetics of sulfur-containing molecules. According to the structure model proposed by Topsøe, the catalytic active sites of Co-Mo-S are located on the edge of MoS2 slabs.[3] It was reported that MoS2 slabs principally including two types of edges: the sulfur-terminated edge (S-edge) and Mo-terminated edge (M-edge). [4] Prins reported that DDS reaction may be likely to occur on the metal edges of the Co-MoS2crystallites, and the HYD reaction occurs on the brim sites close to the edges of the MoS2 plane and Co sites located at the metal edge.[5] Moses et al. report that the HYD and DDS routes could occur on the Mo-edge and S-edge, but the breakage of C-S bond is easier to take place on S-edge sites, which is also considered as the rate limiting step in HDS reaction.[6] Oliviero et al reveal that S-edge is intrinsically more active than Mo-edge due to its activation of adsorbing thiophene reactant for subsequent C-S bond breakage in HDS reaction. The adsorption of H on S-edge is stronger than M-edge.[7] Topsoe et al propose that the S-H groups with high hydrogenolysis activity are mainly associated with S-edge and B acidic sites.[8] Han et al disclose that B acid sites not only can promote the formation of coordinatively unsaturated sites (CUS) sites, but also increase the acidity of -SH sites and further improve the hydrogenolysis activity.[9] Meanwhile, the fully sulfide Mo-edge sites can promote the HYD route.[10]
Besides understanding the effect of different active sites, the supports of hydrotreating catalysts with excellent physico-chemical properties for promoting the dispersion of active metals and reactant diffusion should also be developed. Noticeably, mesoporous materials have been wildly applied in the fields of catalysis, adsorption and separation due to their high pore size, pore volume, surface area and structural order degree.[11, 12] Among these materials, SBA-16 silica with highly ordered degree possesses large cage-like mesopores arranged in cubic body-centered Im3m symmetry, which will lead to lower mass transfer resistance and further facilitate the catalytic reactions.[13] However, SBA-16 pure silica presents low acidity and poor dispersion of active metals, which is adverse to HDS reaction. Therefore, it is necessary to incorporate other atoms into the SBA-16 material for increasing the acidity of catalysts and dispersion of active metals. Typically, Al modification into pure silica materials can strongly improve the amount of acidity.[14] Meanwhile, the incorporation of Ti or Zr atoms into pure silica materials can improve the properties of active metals.[15-18] Mere incorporation of single atoms into SBA-16 pure silica shows advantages and disadvantages. Therefore, it is necessary to incorporate dual metals into SBA-16 materials for obtaining high-efficiency catalysts.
In this study, serial Al-Ti-SBA-16 composites with different acidities and physico-chemical properties were successfully prepared. Before introducing Al species, the Ti-SBA-16 precursors were prepared by a two-step method through pre-hydrolysis of TEOS before the addition of tetrabutyl titanate for protecting the original structure of SBA-16 silica. Various characterization methods were carried out on Al-Ti-SBA-16 composites and corresponding NiMo/Al-Ti-SBA-16 catalysts to investigate the influence of Al and Ti species on their properties. Moreover, the HDS performances of NiMo/SBA-16 and NiMo/Al-Ti-SBA-16 catalysts were also evaluated to correlate the relationship between the HDS activity and selectivity and properties of support and catalysts. An appropriate composition of Al and Ti species in the NiMo/Al-Ti-SBA-16 catalysts, which will allow the highest HDS efficiency, were obtained. The relationship between S-edge and Mo-edge sites and HDS performance and selectivity was also proposed. Finally, the kinetic and thermodynamic analyses were applied to investigate the influence of S-edge and Mo-edge on the intrinsic HDS reactivity for various catalysts.