Theoretical Study On The Sn-Beta Zeolite In Catalytic Conversion Of Biomass

Sn-Beta zeolite is an important catalyst in the process of biomass transformation,and plays an important role in improving the utilization of biomass energy.The active sites of Sn-Beta zeolite will change under experimental conditions.The modification and catalytic mechanism of Sn-Beta zeolite were studied theoretically,which could provide theoretical guidance for the directional regulation of catalytic activity and selectivity of target products of Sn-Beta zeolite.Therefore,the coordination of Sn(?)and active sites of Sn-Beta zeolite as well as the catalytic mechanism of aldose-ketose interconversion catalyzed by Sn-Beta zeolite under real conditions were theoretically studied by methods of DFT and cluster model.The main results are as follows:(1)The active sites of Sn-Beta zeolite under real conditions were investigated by using 38T cluster model and ONIOM method.It was thermodynamically positive for Sn-Beta zeolite to hydrate and hydrolyze in aqueous solution,which transformed to the final Open six-coordinated Sn(?)site with double acidity real model.The corresponding results of ammonia and pyridine adsorption energy showed that the hydrolyzed T5 Open active sites of Sn-Beta zeolite exhibited strong Lewis and Br(?)nsted acidity.In addition,119Sn NMR with bi-acid Open six-coordination Sn(IV)active sites was calculated.The results show that the proposed model is reasonable and reliable.(2)Two paths for the isomerization of a-GLU catalyzed by Sn-Beta zeolite were studied by using the 8T monohydrated Open model with double acidity and density functional method.In path 1,the Lewis-Br(?)nsted acidic active sites of Sn-Beta zeolite acted as a synergistic reaction,while the synergistic reaction in path 2 transformed to the Lewis acidity and Br(?)nsted alkali.The results showed that path 1 was a more favorable route for kinetics.The isomerization of ?-GLU catalyzed by Sn-Beta zeolite was the result of synergistic catalysis between B acid site and metal active site(Lewis acid)on the zeolite.The synergistic mechanism is that in the 12HS rate-determining step,the acidic SiOH proton of Brnsted transfers to activate the carbonyl group of glucose,and then promotes the transfer of hydrogen anion of C2 to carbonyl group,thus effectively reducing the activation energy of the reaction.Additionally,the activation energy of path 1 catalyzed by three-hydrated Sn-Beta zeolite was the lowest 61.5 kJ/mol.(3)Three paths for the isomerization of glyceraldehyde to dihydroxyacetone catalyzed by Sn-Beta zeolite were studied by using the 8T monohydrated Open model with double acidity and density functional method.The best path was the same as the isomerization of a-GLU catalyzed by Sn-Beta zeolite.It was the path 1,where the Lewis-Br(?)nsted acidic active sites of Sn-Beta zeolite acted as a synergistic reaction.Besides,the energy barrier(52.2 kJ/mol)for isomerization of glyceraldehyde catalyzed by monohydrated Sn-Beta zeolite was the lowest.