Theoretical Calculations On The Structure And Catalytic Activity Of Novel Titanium Species In Ti-YNU-1 Zeolite

Ti-YNU-1 zeolite with expanded 12-MR interlayer channels has higher catalytic activity and product selectivity than Ti-Beta,TS-1,3D Ti-MWW and Ti-MOR zeolites in the reactions of olefinic epoxidation.In this paper,density functional theory（DFT）was applied to investigate the structure of framework Ti species,the stability of the active centers and solvent effect,and to evaluate its catalytic activity and reaction mechanisms for 1-hexene and cyclohexene epoxidations,which is of great significance to reveal the catalytic properties of titanosilicates microscopically.The main results are as follows:1.The IR vibrational spectra of the defective Ti species and the formation energies of Ti-hydroperoxo active centers in the interlayer of Ti-YNU-1 zeolite were calculated.The results indicate that the characteristic peak of 930 cm^-1 in Ti-YNU-1 zeolite may be ascribed to the[Ti6OH（OSi）₃]species.The stability order of the Ti-hydroperoxo active centers at the T6 site is Ti-η²（OOH）-H₂O>Ti-η¹（OOH）>Ti-η²（OOH）-CH₃CN>Ti-η²（OOH）.2.The structures of adsorption complexes of the Ti-hydroperoxo active centers with H₂O,CH₃OH,CH₃CN and CH₃COCH₃ at the T6 site in Ti-YNU-1 zeolite were studied,and the adsorption energies and NBO charges were calculated.The results show that the adsorption of H₂O and CH₃OH on the Ti center is stronger than that of CH₃COCH₃ and CH₃CN.On the Ti6-η²（OOH）active center,The solvent molecules prefer to adsorb on H_βthrough hydrogen bonding.The NBO analysis shows that the adsorption of solvent molecules on the Ti center and H_βis not a simple electrostatic interaction,but coordination adsorption and hydrogen bonding interactions.3.The mechanisms of the 1-hexene epoxidation over different active centers in Ti-YNU-1 and Ti-MWW zeolites were studied.In Ti-YNU-1 zeolite,the catalytic activity of the active centers decrease as Ti6-η²（OOH）>Ti6-η²（OOH）-H₂O>Ti6-η²（OOH）-CH₃CN.The structure of Ti6-η¹（OOH）active center converts to that of Ti6-η²（OOH）-H₂O during the epoxidation of 1-hexene.In Ti-MWW zeolite,a stable transition state of 1-hexene epoxidation on the Ti1-η¹（OOH）active center was obtained,but its activation energy barrier is higher.The products of all epoxidation reactions were adsorbed on the Ti center.The desorption processes are endothermic with much higher desorption energies than the corresponding activation energies.Therefore,the desorption of the products are suggested as the rate-determing step.The adsorption of solvent molecules on the Ti center can affect the catalytic activity.Adsorption of CH₃CN results in the electronegativity of the atomic charges of O_αto be decreased,and also facilitates the desorption of the products.The results show that the adsorption of solvent on H_βis not conducive to the epoxidation reaction.4.The mechanism of epoxidation of cyclohexene at T6 and T3 sites in Ti-YNU-1/H₂O₂system was studied and compared with the epoxidation of 1-hexene.The results show that the order of catalytic activity was Ti6-η²（OOH）-H₂O>Ti6-η²（OOH）-CH₃CN>Ti3-η²（OOH）,and the activation energy of cyclohexene epoxidation was lower than that of 1-hexene epoxidation.However,T3 site is located inside the 10-membered ring sinusoidal channels that cyclohexene is difficult to transfer onto the active center.Therefore,with the increase of Ti content,the number of Ti3-η²（OOH）active centers increased,and the conversion of cyclohexene was lower than that of 1-hexene due to the diffusion control of the channel.