Density functional theory investigations into the structure and spectroscopic properties of the Ti4+ species in Ti-MWW zeolite

The location and structure of the framework Ti4+ species in Ti-MWW zeolites were studied by density functional theory (DFT) based on the cluster models mimicking the Ti-MWW zeolite with a Si/Ti ratio of 35. The geometry, spectroscopic properties and energies of substitution for the Ti4+ species at different T sites were investigated. The results indicated that the Ti4+ species in form of Ti(OSi)4 prefers to locate at the T1 and T3 sites, positioning on the edge of 12-membered ring cavity and the intersection of the 10-membered ring intralayer channels in MWW, respectively. In their calculated infrared (IR) spectra, the vibrational mode at 960 cm(-1) band is actually a collective vibration of the antisymmetric stretching of Ti(-O-Si)(4) in a deformed tetrahedral geometry. The totally symmetric Ti(OSi)(4) tetrahedron at the Ti4 site is absent from the 960 cm(-1) band because the vibrational mode is infrared inactive. Hydrolysis of a Ti-O-Si bridge was thermodynamically favorable and resulted in a tripodal defect Ti species with an inversed Ti-OH group and a remaining Si-OH in the adjacent T site. The calculated IR spectra of the tripodal Ti species have a blue shift in the characteristic IR band to 990-1000 cm(-1). The UV-vis spectra of the Ti4+ species were calculated by the time-dependent DFT method. The calculated IR and UV-vis spectra are in very good agreement with the experimental measurements, and provide microscopic features for the identification of the structure of framework titanium in Ti-MWW zeolites. (C) 2014 Elsevier Inc. All rights reserved.