Theoretical Calculation On The Structure And Electronic Spectra Of Titanium–peroxo Catalytic Active Center In Ti-MWW/H₂O₂ Catalyst

Ti-MWW/H₂O₂ catalytic system have unique solvent effect and stereo selectivily in the selective oxidation reactions of organic compounds.There are structure-effect relationship between the microscopic structures and catalytic perfofrmance.Because the active center in this catalytic system is Ti-peroxo intermediate,it is difficult to determine its real construction.In this work,density functional theory calculations are performed to study the structure and stability of different Ti-peroxo species located in different channels of Ti-MWW zeolite.Natural bond orbital analysis was used to explore electronic prpoperties,and Time-dependent DFT method was used to calculate UV-vis spectra,in order to determine the possible active center,providing the fundament for further study of the catalytic oxidation reaction mechanism.The ontained results are as follows:1)DFT method was applied to study the strucuture and stability of the active species of Ti-η¹（O（H）OH）,Ti-η¹（OOH）and Ti-η²（OOH）at T1 and T3 sites in Ti-MWW zeolite.The results obtained by B3LYP and M06L exchange-corelation functional were compared,which indicates that M06L functional is better for the researched systems.The structure of Ti-peroxo species is dependent on the location and microscopic structures of framework Ti species.On Ti1OH/Si6OH model,only the five-coordinated Ti-η¹（OOH）and Ti-η²（OOH）species can be formed.On Ti1OH/Si7OH model,the formation energies of five-coordinated Ti-peroxo species have the order of Ti-η¹（O（H）OH）<Ti-η¹（OOH）<Ti-η²（OOH）,which easily adsorb water to produce six-coordinated Ti-peroxo species.By contrast,all Ti-peroxo species on Ti₃OH/Si3OH model have lower stability.On Ti1OH/Si7OH model,Ti-η²（OOH）H₂O is more stable than Ti-η¹（OOH）,implying that the inner molecular hydrogen boding is not the key factor for the stability of Ti-peroxo species.2)NBO analysis revealed that the positivity of O_αin Ti-η²（OOH）on Ti₃OH/Si3OH model is most high.The next is that of O_αin Ti-η²（OOH）on Ti1OH/Si7OH and Ti1OH/Si6OH models.Therefore,it was suggested that Ti-η²（OOH）has the highest activity.3)TD-DFT calculations indicated that all kind of mono-and bidenated Ti-peroxo species showed excitation wavelength in the region of 230-310 nm.Ti-η²（OOH）shows lower excitation wavelength than that of Ti-η¹（OOH）.Adsorption with water displays minor influence on the excitation wavelength.