| The metal-rich transition-metal phosphides have gained increasing attention due to their high activity and stability in hydrotreating process. In the present study, the performances of bulk Mi2P and WP in the hydrodesulfurization (HDS) of model sulfur-containing compounds were investigated. Bulk Ni2P and WP were prepared by coprecipitation and in situ temperature-programmed reduction in hydrogen. Dibenzothiophene (DBT),4,6-dimethyldibenzothiophene (4,6-DMDBT) and their hydrogenated intermediates, including 1,2,3,4-tetrahydro-dibenzothiophene (TH-DBT), l,2,3,4,4a,9b-hexahydro-dibenzothiophene (HH-DBT),1,2,3,4-tetrahydro-4,6-dimethyldibenzothiophene (TH-4,6-DMDBT), and 1,2,3,4,4a,9b-hexahydro-4,6-dimethyldibenzothiophene (HH-4,6-DMDBT), were synthesized, and used as the model sulfur-containing compounds in HDS. Based on the HDS results, reaction mechanisms and kinetics, some insights into the structure-performance relationship were discussed. We also compared the HDS performance of the Ni2P and WP catalysts.The HDS of 4,6-DMDBT over bulk N2iP occurred predominantly through the hydrogenation (HYD) pathway, and the HYD and direct desulfurization (DDS) pathways were about equally inhibited by piperidine. Piperidine inhibited the desulfurization of 4,6-DMDBT and TH-4,6-DMDBT similarly, but did not affect the desulfurization of HH-4,6-DMDBT. These differences in inhibition were ascribed to different C-S bond cleavage mechanisms of these molecules.4,6-DMDBT and TH-4,6-DMDBT underwent desulfurization mainly through hydrogenolysis. While the desulfrization of HH-4,6-DMDBT over bulk Ni2P occurred mainly by a β-elimination mechanism.Both DBT and 4,6-DMDBT reacted mainly through the HYD pathway over bulk WP. Piperidine inhibited the HDS of 4,6-DMDBT much more strongly than that of DBT. The desulfurization rates were in the order HH-DBT> TH-DBT> DBT, and TH-4,6-DMDBT> HH-4,6-DMDBT> 4,6-DMDBT, which indicated that the desulfrization of 4,6-DMDBT, TH-4,6-DMDBT, HH-4,6-DMDBT, DBT, and TH-DBT over bulk WP occurred mainly by hydrogenolysis mechanism, while HH-DBT underwent desulfurization mainly through (3-elimination mechanism. The formation of cyclopentylphenylmethane and cyclopentylcyclohexylmethane in the HDS of DBT, TH-DBT and HH-DBT over bulk WP could be due to the heterolytic cleavage of the cycloalkyl C-S bond in HH-DBT or dodecahy dro-dibenzothiophene.The methyl groups in 4,6-DMDBT, TH-4,6-DMDBT, and HH-4,6-DMDBT have several effects according to the HDS results. The DDS pathway of DBT was faster than that of 4,6-DMDBT, while the HYD pathway of DBT was slower than that of 4,6-DMDBT. The dehydrogenation of TH-4,6-DMDBT and HH-4,6-DMDBT were faster than that of TH-DBT and HH-DBT, respectively. These means that the methyl groups not only suppressed the direct desulfurization of 4,6-DMDBT by steric hindrance, but also accelerate the hydrogenation/dehydrogenation of 4,6-DMDBT and its hydrogenated intermediates. In addition to this, the methyl groups affected the C-S bond cleavage mechanism in the HDS of HH-DBT and HH-4,6-DMDBT over bulk WP.Bulk Ni2P and WP showed similar metallic character. Substantial dehydrogenation of TH-4,6-DMDBT to 4,6-DMDBT,3,6-DMDBT and 2,6-DMDBT occurred over bulk Ni2P and WP. The skeletal isomerization of 1-heptene was negligible over bulk M2P and WP in the absence of H2, demonstrating that bulk M2P and WP had very low acidity. Thus, the formation of these DMDBT isomers, as well as the hydrocracking of 1-methyl-4-(3-methylcyclohexyl)-benzene over bulk WP, are not due to the low acidity of bulk M2P and WP but probably to their metallic nature.The active-site-based rate constants for the HDS of DBT,4,6-DMDBT and their hydrogenated intermediates over bulk M2P and WP were calculated. WP possessed a higher hydrogenation/dehydrogenation activity than bulk Ni2P, and thus the higher activity for the HDS of 4,6-DMDBT indicated taht WP is a promising catalyst for deep HDS. Nevertheless, WP was more sensitive to piperidine than M2P. |
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