Theoretical Calculations On The Structure And Vibrational Spectra Of New Framework Ti Species

Titanosilicate zeolites are environmentally friendly catalysts for selective oxidation.There is a direct structure-activity relationship between the microscopic structures of framework Ti species and catalytic performance.Calclulated vibrational spectra of may provide reference for experimental characterization of catalysts.In this work,density functional theory calculation method based on 35Tcluster models is applied to study the geometric structures,energies,and vibrational spectra of different Ti species.The main results are as follows:1)The structures of adsorption complexes of[Ti?OSi?₄]and[Ti?OSi?₃OH]species at T1and T3 sites with H₂O and NH₃ were studied,the adsorption energies and vibrational frequencies were calculated.It was indicated that Ti?OSi?₃OH]species have larger adsorption energies with H₂O and NH₃ than[Ti?OSi?₄]species.For Ti location,Ti?IV?at T1 site shows higher Lewis acidity than Ti?IV?at T3 site.For ligand molecules,ammonia cooperated with the central Ti?IV?to a greater extent than water does.The vibrational frequencies of absorption complexes can be divided to two regions,namely the Ti-specific vibration region and the hydroxyl group vibration region.After adsorption with H₂O and NH₃,the Ti-specific band of[Ti?OSi?₄]species shifted from 960 to 970cm^-1,and the Ti-specific bands of[Ti?OSi?₃OH]species at the and shifted separately from 990 cm^-1?T1 site?and 970cm^-1?T3site?to 980 cm^-1.The??O–H?and??N–H?vibration modes showed slight blue shift,reflecting the Lewis acidity of the Ti species.The H-bonded TiOH group displayed obvious red shift by500–1100 cm^-1,appeared in the 2700–3200cm^-1 region.2)The structures and vibrational spectra of[Ti?OSi?₃OH]species formed by defect interlayer.If the T6 site is defect,a hydroxyl nest was formed,or a SiOH at T6 site was formed,the stable[Ti?OSi?₃OH]species can be produced on the adjacent T1 site.Both[Ti?OSi?₃OH]species have a pair of vibration peaks around 930 and 1030 cm^-1.Further analysis of the frequencies with isotop exchanged models revealed that for ¹⁸O exchange,the Ti specific vibrational frequencies shifted red by about 30 cm^-1,while for ²⁹Si,⁴⁹Ti,and ²H exchanges,the vibrational frequencies showed no change.The vibration are ascribed to the motion of O in Ti-O-Si bonds.