Theoretical Study On Phenol Hydrogenation Catalysis On Zeolite-encapsulated Platinum Metal Cluster

With the over-exploitation and utilization of energy such as oil and coal,and the non-recycling of fossil resources such as oil,energy sources are becoming less and less,and it is urgent to find alternative energy sources.Biomass energy is a new type of energy that is processed from biomass resources such as agriculture,forestry and plants as raw materials.Heterogeneous catalysis can effectively convert renewable biomass resources into transportation fuels and value-added chemicals.Among them,catalytic conversion of phenolic compounds in lignin is widely used to produce various industrial raw materials such as cyclohexanol.Therefore,the development of high-performance and highly stable catalysts is vital for large-scale practical applications.In this thesis,density functional theory（DFT）calculations were used to systematically study the bifunctional catalytic phenol hydrogenation over zeolite encapsulated metal platinum（Pt）clusters.The thesis work including the following two major parts:（1）The stability of a seris of encapsulated Pt_n（n=1～6）clusters in the framework of pure silica zeolite（Silicalite-1）and protonated aluminum-containing zeolite（H-MFI）was studied.Calculation results show that as the size of the Pt_ncluster increases,the interaction between the Pt_n cluster and the zeolite frameworks become stronger.The Pt_n clusters encapsulated near the Br（?）nsted acid site（BAS）site of H-MFI forms more stable Pt_nH_x（x=1,2）clusters.In the presence of water,the stability of zeolite encapsulated Pt₆ clusters will be enhanced.In addition,as the water concentration in the pores of zeolite increases,the water dissociation on the Pt₆@Silicalite-1 becomes easier,while it is more difficult on the Pt₆H₂@H-MFI.At the same time,it is found that the proton（H⁺）at the BAS site in H-MFI zeolite can be transferred to the encapsulated Pt₆ cluster via the dynamic H⁺（H₂O）_n cluster,thereby forming a more stable Pt₆H₂@H-MFI configuration.（2）The adsorption,dissociation and hydrogenation of phenol in the pore of H-MFI zeolite and H-MFI encapsulated Pt_n clusters were studied.Calculation results show that the mixedσ-πinteraction of phenol through the aromatic ring on the Pt₆ cluster is beneficial to its adsorption in the zeolite.However,the dissociation of phenol into a phenoxy group is found to be thermodynamically and kinetically unfavorable.In the Pt₆@H-MFI system,the ortho C positon on the aromatic ring of phenol is the most likely postion for initial hydrogenation.The key intermediate for further hydrogenation is theα-C positon of hydrogenated phenol on the Pt₆ clusters.The activation barrier for phenol hydrogenation is significantly lower than that for the hydroxyl deprotonation of phenol.Therefore,we believe that the main route of phenol hydrogenation is direct hydrogenation to cyclohexanol.