Fig. 3 (A) the product distributions of DBT HDS reaction; (B) the HYD/DDS ratios for various catalysts; (C) the correlation between S-edge/Mo-edge ratio and HYD/DDS ratio.
DBT HDS reaction includes HYD and DDS routes, whose corresponding products are tetrahydrodibenzothiophene (THDBT), biphenyl (BP) and cyclohexylbenzene (CHB). The distributions of DBT HDS reaction routes for serial NiMo catalysts are displayed in Fig. 3A. It can be seen that DDS route should the primary path in DBT HDS reaction due to the HYD and DDS ratios are lower than 1.0. From Fig. 3B, the HYD and DDS ratios follows the order of NiMo/AT-10 < NiMo/AT-7.5 < NiMo/AT-5 < NiMo/SBA-16 < NiMo/AT-2.5 < NiMo/AT-0. The distribution of HDS route can be related with the acidity of catalyst. Compared with NiMo/SBA-16 catalyst, the selectivity of DDS routes for NiMo catalysts containing Al species are enhanced, which should be due to the fact that B acid site can promote the formation of coordinatively unsaturated sites (CUS) by accepting an electron from MoS2 active phase.[9] NiMo/AT-10 catalyst presents the lowest HYD and DDS ratio of 0.25. In addition, the HYD and DDS ratio for NiMo/AT-0 catalyst is the highest of 0.45, verifying that the incorporation of Ti atoms into SBA-16 structure can improve the selectivity of HYD route. The improvement of HYD selectivity after Ti modification should be assigned to the special structure and electronic properties of active phases formed on surface of catalyst.[57] Meanwhile, CO-IR results can also disclose the correlation between HDS route and types of active metals.[6] It is recognized that vacant (CUS) sites are responsible for S removal through DDS route, whereas the HYD pathway should be enhanced by a fully sulfide MoS2phase.[58] Caused the Mo-edge and S-edge sites play different roles in HDS reaction route, the improvement of DDS route must be at the expanse of HYD route. Therefore, the concentration ratio of S-edge and Mo-edge may be applied to correlate with the HDS selectivity. The relationship between S-edge/Mo-edge ratio and HYD/DDS ratio is shown in Fig. 3C. It shows that the HYD/DDS ratio decreases with the increasing S-edge/Mo-edge ratio. Therefore, the HYD/DDS ratios can be well linked with the S-edge/Mo-edge ratios.
Table 7 the pseudo-first-order constants (10-4 mol g-1 h-1) and TOF (h-1) of DBT HDS reaction routes for serial NiMo catalysts, which are obtained at the HDS conversions lower than 30%.