Theoretical Calculations For The Stereoselectivity Of Epoxidation For Cis/trans Hex-2-enes In TS-1/H₂O₂ System

Titanium silicalite?TS-1?is a kind of selective oxidation catalyst.In H₂O₂ solution,TS-1 zeolite has excellent catalytic oxidation performance for many little organic compounds.In this work,density functional theory calculations are performed to study the structures of the Ti-peroxide species,and investigate the geometrical configuration,stability and atomic charges of the Ti-peroxide species at six different T sites,providing microscopic evidence for catalytic oxidation structure with exist of hydrogen peroxide in TS-1/H₂O₂ catalytic system.On this basis,we also study for the catalytic oxidation reaction mechanism for cis/trans hex-2-enes on the Ti-peroxide active center at T10 site.The main results have obtained as follows:1.Based on 40T cluster model including seven layers atoms,we investigate the structure and stability of Ti-peroxide species at T2,T4,T8,T10,T11 and T12 sites.We have examined four Ti-peroxide species,including Ti-?2?OOH?and Ti-?1?OOH?complexes with 5-coordination,and Ti-?2?OOH?H2O and Ti-?1?OOH?H2O complexes with 6-coordination.The geometric optimization and the formation energies of these species have been performed.The results indicated that the stability order at T10 is as Ti-?1?OOH?>Ti-?2?OOH?H2O,while on the other T sites are as Ti-?2?OOH?H2O>Ti-?1?OOH?.Coordination with one H2O molecule lead to more stable six coordination Ti-peroxide of Ti-?2?OOH?H2O and Ti-?1?OOH?H2O complexes.2.The NBO calculation analysis showed that the electropositivity of the active oxygen Oa decreases in the following order:Ti-?2?OOH?>Ti-?1?OOH?.After adsorption of water molecule,the electropositivity of O?in Ti-?1?OOH?is reduced,while in Ti-?2?OOH?it is improved.Overall,the adsorption of water molecule has little influence on the electropositivity of O?.For the Ti-peroxides at T4,T8,T10 and T11 sites,the electropositivity of O? are relatively higher than that at T2 and T12 sites,implying a higher activity.This is just in aggremment with the stability order of the Ti location.3.The NBO orbital analysis showed that the O?-O? bond is ? bond,and Ti-O? and Ti-O?bonds are ? bonds in Ti-?2?OOH?and Ti-?2?OOH?H2O.There exist intramolecular hydrogen bond in Ti-?1?OOH?and Ti-?1?OOH?H2O structures.4.We also studied the epoxidation reaction mechanisms of cis/trans-hex-2-enes over the Ti-peroxide active centers Ti-?1?OOH?at T10 site.The calculated activivation energies of epoxidation of cis and trans isomers are 95 and 101 kJ/mol,respectively,showing higher cis-selectivity,which is in good aggreement with the experimental results.