Mechanism And Kinetics Of The Desulfurization Of Dibenzothiophene And Its Critical Hydrodesulfurization Intermediates Over Transition-metal Phosphides

At present,the environmental regulations are increasingly strict,while the trend of heavy and low-quality crude oil feed stocks is progressively prominent,making deep desulfurization of the petroleum fractions economically and efficiently a long-term and difficult task for clean fuel production.It is urgent to develop efficient hydrodesulfurization(HDS)catalysts.Transition-metal phosphides have attracted more and more attention due to their excellent activity and stability.Dibenzothiophene and its alkylated derivatives(DBTs)are the most refractory sulfur-containing compounds.Hence,an in-depth understanding of the HDS mechanisms and the dynamics of these molecules,and the structure-performance relationship of HDS catalyst are prerequisite for the development of efficient HDS catalysts and catalytic processes.This paper focused on the HDS mechanism of DBTs and the structure-performance relationship over transition-metal phosphides,which are the core scientific question of the deep HDS of petroleum fractions.CoP and Co2P were prepared by H2 temperature-programmed reduction of the mixtures of Co(OH)2 and H3PO3 with stoichiometric P/Co ratios.HDS reactions of DBT and its hydrogenated intermediates 1,2,3,4-tetrahydro-dibenzothiophene(TH-DBT)and 1,2,3,4,4a,9b-hexahydro-dibenzothiophene(HH-DBT)over CoP and C02P as well as the effect of piperidine were all studied.CoP was much more active and more nitrogentolerant than Co2P.Over the two cobalt phosphide catalysts,the direct desulfurization(DDS)and the hydrogenation(HYD)pathway of DBT HDS were almost equally important.The cleavage of the aryl C-S bonds in TH-DBT and HH-DBT can occur prior to the cleavage of the vinyl C-S bond in TH-DBT and the cycloalkyl C-S bond in HH-DBT.The desulfurization of DBT occurred through hydrogenolysis,the desulfurization of HH-DBT occurred mainly through ?-elimination,but the two catalysts behaved differently in the desulfurization of THDBT.Piperidine strongly inhibited the hydrogenation/dehydrogenation reactions and the direct desulfurization(DDS)of DBT,but its inhibition on the desulfurization of TH-DBT and HHDBT were less pronounced.The HDS reactions over CoP and Co2P followed a pseudo-firstorder kinetics and a pseudo-zero-order kinetics,respectively.For an in-depth understanding of the cleavage mechanisms and dynamics of the cycloalkyl C-S bond over transition-metal phosphides as well as the influence of the nitrogen-containing compounds on the cleavage of the cycloalkyl C-S bonds,2-phenylcyclohexanethiol(2-PCHT)was designed and synthesized in this paper,and its reactions both under H2 and Ar over Ni2P,MoP,WP,CoP,and Co2P were studied.We found that the hydrodesulfurization of 2-PCHT proceeded through three parallel pathways under H2:?-elimination,hydrogenolysis,and dehydrogenation.The dehydrogenation pathway consisted of two parallel reactions:the dehydrogenation-desulfurization reaction with BP as the product and the dehydrogenation-ring closure reaction with TH-DBT as the product.Under Ar,the parallel pathways were ?elimination,hydrogenolysis and/or homolytic C-S bond cleavage,and dehydrogenation.2PCHT can directly produce HH-DBT by a ring closure pathway over CoP.Under H2,?elimination dominated the hydrodesulfurization of 2-PCHT over Ni2P,CoP,and Co2P,while hydrogenolysis was as fast as ?-elimination over MoP and WP.Under Ar,?-elimination and dehydrogenation pathways were about equal over Ni2P,whereas ?-elimination was the major pathway over CoP,Co2P,MoP,and WP.The inhibiting effects of piperidine depended on the reaction and catalyst,which can be an indication of the complexity of the cleavage of the cycloalkyl C-S bond in 2-PCHT and the active sites in the surfaces of the transition-metal phosphide catalysts.The HDS of 2-PCHT over the phosphide catalysts can be fitted by a pseudo-first-order rate model,but the desulfurization of 2-PCHT under Ar cannot be described by the power-law rate expression.Phosphosulfide phases can be formed by the direct reaction of the phosphide catalysts with the sulfur-containing compounds,even in the absence of H2.