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

• Structure and spectroscopic features of Ti-MWW are calculated by DFT and TD-DFT.
• Ti4+ prefers to locate at T1 and T3 sites in form of Ti(OSi)4.
• The 960 cm−1 band is attributed to the out-of-phase stretching of Ti(OSi)4.
• The excitation at 210 nm is contributed by electronic transition of p(O) → π∗(Ti–O).

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.

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