The HDS rate constants kHDS, kHYD, kDS, kDDS, kDDS/kHYD and TOF values are provided in Table 7. The values of kHYD, kDS and kDDS kinetic parameters could reflect the reactivity of each reaction steps. kDDS/kHYD may be applied to investigate the HDS selectivity. It shows that the kHYD and kDDS present an increasing tendency with the Ti and Al contents in the NiMo catalysts, demonstrating that incorporation of Ti and Al into SBA-16 silica can enhance the selectivity of HYD route and DDS route respectively. Noticeably, the kDS value presents an increasing tendency with the amount of B acidity for various catalysts. The NiMo/AT-7.5 exhibits the highest kDS value of 2080 10-4 mol g-1 h-1may be due to its relative high amount of acidity and the highest amount of S-edge sites.
The kHDS and TOF values could be applied to investigate the intrinsic reactivity of DBT HDS reaction. The kHDSvalues for serial catalysts present the same tendency with their HDS efficiencies from Fig. 1, confirming that the enhancement of acidity and properties of active phases could improve the DBT HDS performance. NiMo/SBA-16 catalyst shows the lowest kHDS and TOF values of 6.2×10-4 mol g-1h-1 and 5.1 h-1, and NiMo/AT-7.5 catalyst presents the highest kHDS and TOF values of 16.3×10-4 mol g-1h-1 and 9.0 h-1 respectively. It is wildly accepted that larger amount of vacant sites for active metals will allow a better HDS performance.[59] However, Vogelaar proposed that the fully sulfide catalyst with the lowest amount of CUS sites presents the best HDS performance for thiophene reactant.[54] Combining the CO-IR results and HDS activity, our results show that the HDS efficiencies are mostly linked with the total concentrations of Mo-edge and S-edge, but the NiMo/AT-7.5 exhibit the highest HDS performance even though its total concentration of Mo-edge and S-edge is not the highest. This phenomenon should be correlated with the special properties of -SH groups in S-edge. The acidity of S-edge sites can be enhanced through easily accepting the H+ from B acid site, which can significantly enhance the DDS route of HDS reaction. Meanwhile, the -SH groups can also act as the H2 source during the HDS reaction.[8] For NiMo/AT-7.5 catalyst, the S-edge concentration promoted by Al modification is the highest, and the concentration of fully sulfide Mo-edge sites promoted by Ti modification is relatively high. Therefore, the highest kHDS and TOF values for NiMo/AT-7.5 catalyst can be ascribed to the synergistic effect of the enhancing acidity and distribution and dispersion of active metals. Above all, it can conclude that incorporating appropriate contents of Al and Ti atoms into SBA-16 silica can maximize HDS performance for the corresponding NiMo/Al-Ti-SBA-16 catalyst. The principle is shown in Scheme 2.