Theoretical Investigation Of Methanthiol Hydrodesulfurization Mechanism

A large number of sulfur compounds are produced on the process of oil refining andconsuming, which not only are very detrimental to the refining equipment, but also causeenvironmental pollutions. The strict laws have been laid down to reduce the sulfur content.The exploration of more active desulfurization catalyst and the desulfurization mechanismhas become the urgent task of the researchers. In this thesis, with the use of density functionaltheory, we focus on the hydrodesulfurization mechanism for methanthiol on the surface of thetraditional MoS2and CoMoS cluster catalyst, as well as the decomposition desulfurizationmechanism on Pd (111) surface, and describes the C S bond scission mechanism, theinfluence of the additives Co on MoS2by the comparative analysis, as well as the differencesbetween precious metals and the traditional catalyst, provides a theoretical guidance for thedesign of desulfurization catalyst to a certain extent.Firstly, this thesis verifies that the dissociation reaction of hydrogen molecules in theMoS2edge is in the way of heterolytic cleavage, and studies reaction mechanism of the CUSformation process via hydrogenation reaction. For this procedure, analysis shows that there isonly a small amount of CUSs in the short-term homeostasis, and that the increase in the H2and the reduction of H2S condition can promote the formation of CUS. The methanthioladsorbed on bridge site of CUS is most stable adsorption configuration. The S H bondcleavage of methanthiol has minimum energy barrier, but methane formation directly by S Hand C S bonds initial scission is a way of least resistance. This thesis also exploratoryresearches desulfurization reaction under hydrogen atmosphere and the study find that Hatoms adsorbed on the edge inhibit the adsorption capacity of methanthiol, but promote thedesulfurization reaction of methanthiol. Secondly, this thesis studies the hydrodesulfurization reaction of methanthiol on theCoMoS cluster to analyse influence of the additives Co on MoS2catalyst. On hydrogenationreaction, Co atom is not effective activation reaction site, enhances the activity of adjacentMo reaction site and promotes cleavage of H2. The most stable adsorption configuration ofmethanthiol is parallelly adsorbed on Co-top site of CUS on CoMoS cluster and the optimalreaction path is the the S H initial scission and then resulting methane through the H atomtransfer reaction. The study shows that the additive Co is beneficial to methanethioladsorption, decomposition and methane formation. Electron density analysis shows that theCo atom has a strong electrophilic property, thereby changes the electron population andimproves the electrophilic capacity of the surrounding atoms. Mo S bond presents ionicproperty but Co S bond presents covalent property becouse of Co changing chemical bond.Frontier orbital theory found that Co atoms change dispersion and stretch direction of orbitalsresult in the different adsorption configuration and adsorption energy.Finally, the decomposition desulfurization mechanism of methanthiol on Pd (111)surface was researched. Methanthiol is stablily adsorbed on top site on Pd (111) via S atomand the optimal reaction path is CH3SHâ†'CH3S(brg)â†'CH3S(fcc)â†'CH2Sâ†'CH2+S. During the decomposition reaction of methanthiol, the initial S H bond cleavage hasminimum energy barrier, but C H bond scission of intermediates is relatively difficult. Thereare two adsorption configurations of CH3S, which is an important intermediate, and bothconfigurations are easy to obtain and mutual conversion. The d-PDOS verifies energybarrier variation rule of initial S H bond, C S bond and C H bond, which be grouped intorelational formula by BEP method, respectively. Pd surface poisons by S atoms, which arevery easy to obtain and stable absorption, but is difficult to deposit by carbon.