Hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene over NiMo supported on yolk-shell silica catalysts with adjustable shell thickness and yolk size

Mesoporous yolk-shell silica spheres with different shell thicknesses and yolk sizes (YxSy) were synthesized by incubating mesostructured silica nanospheres with water. Al-modified YxSy-supported NiMo catalysts were prepared and applied to hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The shell thickness and yolk size have a large effect on the HDS activities. Among the as-made catalysts, NiMo/Al-Y30S13 catalyst with relatively low shell thickness (13 nm), yolk size (30 nm) and proper structural stability exhibit the highest activities for HDS of DBT and 4,6-DMDBT at the weight time of 1.39-13.76 g min mol-1, of which its DBT conversion at 1.39 g min mol-1 (50.4%) is 1.5 times as that of the reference NiMo/Al2O3 (33.6%), four times as that of NiMo/Al-Y83S28 (12.3%) and what's more, the HDS conversion of Al-Y30S13 could reach 99.5% at 13.76 g min mol-1 (about 2.5 ppm S remained). Its 4,6-DMDBT conversion at 1.39 g min mol-1 (27.1%) is almost 2 times as that over NiMo/Al2O3 (13.8%), and 2.7 times as that over NiMo/Al-Y83S28 (9.9%). Additionally, the 95.7% HDS conversion (about 16.6 ppm S remained) of Al-Y30S13 could be obtained at 13.76 g min m ol-1. The good HDS performance of the NiMo/Al-Y30S13 catalyst could be derived from the synergistic effect of moderate shell thickness and relatively small yolk size, proper structural stability, appropriate acidity, moderate metal-support interaction (MSI), suitable dispersion and desirable stacking morphology of the Ni and Mo species. DBT HDS over the NiMo/Al-Y30S13 catalyst shows the lowest direct desulfurization (DDS)/hydrodesulfurization (HYD) ratio (2.70), demonstrating that the increase of HYD proportion could improve the ability of ultra-deep desulfurization of catalysts. 4,6-DMDBT HDS over the NiMo/Al-Y30S13 catalyst shows the highest selectivity of isomerization (ISO) route (69%), illustrating that the ISO route is the dominant pathway. Furthermore, the mechanisms of DBT and 4,6-DMDBT HDS are proposed over NiMo/Al-YxSy materials.