Theoretical Calculations On The Distribution Of Framework Titanium And Vibrational Spectra Of TS-1 Zeolite

Titanium silicalite?TS-1?is a kind of selective oxidation catalyst.In H₂O₂?30%aqueous?solution,TS-1 zeolite has excellent selective oxidation performance for a variety of organic compounds.There is a direct structure-activity relationship between Ti?IV?location,microscopic structure,properties of the electronic structure and catalytic activity in TS-1.Relevation of the microscopic structure and spectral properties of the framework Ti?IV?have instruction for optimal design and spectroscopic characterization of catalyst.In this work,we perform density functional theory calculations to investigate the location of Ti?IV?at the 12crystallographically different tetrahedtral sites?T site?,the geometric structure and IR vibrational spectra for TS-1 zeolite,providing microscopic information for the experimental IR spectrum and ascription of 960 cm^-1 band.The main results are obtained as follows:1.The[Ti?OSi?₄]species of Ti?IV?at 12 different T sites were optimized and the frequencies were calculated using about 40T cluster model including seven layers atoms.Based on Ti/Si substitution energies and structure parameters,the preference for Ti?IV?substituting the Si?IV?are at the T10,T4,T11 and T8 sites.The larger the average Ti-O bond,the lower substitution energy.The locatin of Ti?IV?in the zeolite framework is not random.2.Frequency analysis indicates that there exist three stretching vibration modes associated with the framework Ti?IV?in TiO₄ unit,the vibration frequency are at 900-980cm^-1region.Wherein,the 960 cm^-1 vibration peak was found on the[Ti?OSi?₄]species at T10,T4,T11 and T8 sites,while at other T sites the vibrational frequencies of Ti?IV?species appear at higher wavenumber.At the same time theoretical calculations show that the 960 cm^-1vibration peak depends on the microscopic structure and stability of framework Ti?IV?.There are no vibrational frequencies at 900-1000 cm^-1 region in pure silica zeolites,which is consistent with the experimental IR spectral data.So it is suggested that the experimental infrared spectra of 960 cm^-1 vibration peak attributable to[Ti?OSi?₄]species of T10,T4,T11and T8 sites.3.The thermodynamic stability of[Ti?OSi?₃OH]species are studied.The formation energies at the T10,T4,T9,T1 and T12 sites are lower.T10 and T4 sites are not only in favor of perfective[Ti?OSi?₄]species existing,but also in favor of defective[Ti?OSi?₃OH]structures existing;while T9,T1 and T12 only benefit defective[Ti?OSi?₃OH]structure existing.4.According to the results of frequencies,the characteristic vibration frequency of[Ti?OSi?₃OH]species at the T10,T4,T8 and T11 sites move 15-20 cm^-11 to the high wavenumber,appears around 985 cm^-1,so do the[Ti?OSi?₃OH]species at the T2,T12 sites,while the characteristic vibration frequency of[Ti?OSi?₃OH]species of T9 and T1 sites appears at 970 cm^-1.