Hydrodesulfurization Of Dibenzothiophene And Its Hydrogenated Intermediates Over Bulk Metal Phosphide

Transition metal phosphides possess excellent physical and chemical properties. They have attracted much more attention as a new family of hydrotreating catalysts due to their high activity and stability. In the present study, the HDS performances of the bulk Ni2P, MoP, and WP were investigated using dibenzothiophene (DBT) and its hydrogenated intermediates,1,2,3,4,4a,9b-hexahydro-dibenzothiophene (HH-DBT) and1,2,3,4-tetrahydro-dibenzothiophene (TH-DBT), as the model sulfur-containing compounds both in the presence and absence of piperidine. Combined with the characterization results of XRD, N2physisorption, CO chemisorption, XRF, and XPS, some insights into the structure-activity relationships and the in-depth understanding of the HDS reaction mechanisms and kinetics of sulfur-containing molecules over this new family of HDS catalysts were obtained.In the HDS of DBT over Ni2P, the selectivity to BP, the only product of the direct desulfurization (DDS) pathway of DBT HDS, was more than80%. Assuming pseudo-first-order kinetics for the HDS of DBT, the DDS pathway rate constant KDDS was4.9times higher than that of the hydrogenation pathway (kHYD), indicating that DBT mainly desulfurizes by the DDS pathway over bulk Ni2P. Both HYD and DDS pathways were inhibited by piperidine. After the addition of piperidine, the ratio KDDS/KHYD increased to14, suggesting that the HYD pathway was more inhibited than the DDS pathway. CHEB-1, CHEB-2, and CHEB-3were all observed in the HDS products. Separate hydrogenation showed that only CHEB-2isomerized to CHEB-1and CHEB-3, but that CHEB-1did not isomerize. The desulfurization of HH-DBT occurred mainly by a β-elimination mechanism in which the hydrogen atom attached to carbon atom C(4) was involved. TH-DBT underwent desulfurization mainly through hydrogenolysis. A minor amount of BP was formed by disproportionation of CHEB.Both HYD and DDS pathways played an important role in the HDS of DBT over MoP. HDS activity of MoP increased with reaction time during HDS reactions, mainly due to an increase of DDS activity. The XPS results showed that, the surface S/Mo ratio (0.35) of the spent MoP was almost eight times higher than the bulk value (0.045), indicating sulfur was mainly incorporated on the surface of MoP during the HDS reaction, probably leading to the formation of new active sites, which possessed higher direct desulfurization (DDS) activity than the fresh MoP. The sulfur coordinated on surface of the spent MoP that constitutes the new active sites originates from DBT but not from H2S. DMDS pretreatment was an effective method for the surface sulfidation of MoP, leading to increase in the HDS performance and DDS activity of MoP. Piperidine strongly inhibited the HDS of DBT and its hydrogenated intermediates over MoP. It not only competed with these sulfur-containing molecules for the adsorption on the active sites but also slowed down the sulfidation of MoP surface.HDS activity of WP was slightly lower than that of MoP, but higher than that of Ni2P. The inhibition effect of piperidine on WP was similar to that on Ni2P and MoP. HYD pathways played an important role in the HDS of DBT over WP. WP possessed high hydrogenation/dehydrogenation activity becasuse certain amount of DBT were detected in the HDS of TH-DBT and HH-DBT, suggesting that WP behaved like noble metal catalysts.