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

Ti-BEA zeolite possesses a three-dimensional network of intersecting 12-membered ring channels,the diameter of the channel is 7.3?×6.0?which is beneficial to the catalytic conversion for macromolecule reactants.In this paper,density functional theory?DFT?were applied to investigate the structures of the Ti-peroxo intermediates and the solvent effect,and to evaluate the catalytic activity and reaction mechanisms for styrene epoxidation on different active centers.By using ONIOM method together with different functionals,the confinement effect for styrene epoxidation in Ti-BEA zeolite was studied.The main results are as follows:1.The Ti/Si substitution energies at nine different T sites and the IR vibrational spectra of the Ti species were calculated.The results show that the Ti atom prefers to locate at the T1 and T2 sites in Ti-BEA zeolite.The characteristic vibration frequency of[Ti?OSi?₄]species was observed around 960 cm^-1,which in good agreement with the characteristic peak in experimental IR for framework Ti?IV?species.2.The stability of the Ti-peroxo intermediates Ti-BEA/H₂O₂ system were examined by theoretical calculations.The stability order of the Ti-peroxo intermediates at the T2 site is as Ti-?¹?OOH?-H₂O>Ti-?²?OOH?-H₂O>Ti-?²?OOH?-CH₃CN>Ti-?¹?OOH?>Ti-?²?OOH?.3.The reaction mechanisms of the styrene epoxidation over different active centers in Ti-BEA were studied.The activation energy barriers for styrene epoxidation on different active centers increase along with Ti-?²?OOH?<Ti-?²?OOH?-H₂O<Ti-?²?OOH?-CH₃CN.The produced epoxide is adsorbed on the Ti center.The desorption process is an endothermic process with the desorption energy higher than the corresponding activation energy,implying the rate-determining step for styrene epoxidation reaction.The adsorption of one solvent molecule on the Ti center can influence the catalytic activity.The acetonitrile solvent as ligand can improve the positivity of O_?and also favors for olefin approach as well the desorption of the epoxide.4.By using B3LYP,B3LYP-D3,B97D and M06-L functional together with ONIOM model of Ti-BEA zeolite,the confinement effect for styrene epoxidation was studied.The activation energy barriers calculated by B3LYP-D3 and B97D which include the dispersion effect and M06-L which include the long-range effect are lower than the results calculated by B3LYP functional.The dispersion of the confinement effect in the channel not only favor the stability of adsorption state of the substrate molecules,but also approve the stability of the transition state.