Density Functional Theory Investigations Into The Structure And Spectroscopic Properties Of Framework Ti⁴⁺ Species In Ti-MWW Zeolite

Ti-MWW zeolites exhibit excellent catalytic activities in the oxidation of various organiccompounds with aqueous hydrogen peroxide as an oxidant under mild conditions. Ti-MWWzeolites possess a unique pore structure comprised of two independent10-membered-ring(MR) channels and12-MR supercages systems, displaying highly catalytic activities inoxidation reactions for both of small and larger molecules. Theoretical study of thespectroscopic properties may provide valuable theoretical references for attribution of theexperimentally determined absorption on960cm-1band in FT-IR spectra.In this work, the location and structure of framework Ti4+species in Ti-MWW zeolitewere studied by density functional theory method (DFT) based on the cluster models. Thegeometry, energies, and spectroscopic properties of different Ti4+species were investigated.Based on the calculated substitution energies, Ti4+prefers to locate at T1positioned on theedge of12-member ring cavity in MWW. The vibration at960cm1band is attributed to theout-of-phase stretching of central Ti(OSi)4unit, which is actually a collective vibration of theantisymmetric stretching of four Ti–O–Si arranged in deformed tetrahedral symmetry. Thetotally symmetric Ti(OSi)4tetrahedron at Ti4site is absent from960cm1band due to thevibration mode being infrared inactive. The defect Ti species containing inversed Ti–OHgroup and remained Si–OH in adjacent T site is thermodynamically favorable under wateradsorption. Formation of Ti–OH resulted in a blue shift of the characteristic IR band to990–1000cm1. The excitation state calculated by time-dependent density functional theoryfor the Ti4+species revealed that the low-lying electronic transitions are mainly contributed bythe electron transfers from the occupied p orbitals of four bridging oxygens in Ti–O–Si to theunoccupied π*(Ti–O) orbitals, and the transition with the highest oscillator strength occurredat210nm. The Ti–OH species led to some transitions extended to220–230nm regions. Thecalculated results are in very good agreement with the experimental measurements and allowthe identification of the framework structure of titanium sites in Ti-containing zeolites.The adsorption of NH3probe on Ti-MWW were studied by DFT calculation. Thevibrational properties of the adsorption complex were investigated. It was found that NH3prefer to approach the central Ti(IV) cation at the T1site. After adsorption of NH3the Ti-Obecame longer. Compared to Ti-OH,the central Ti(IV) cation showed stronger Lewis acidity. Introducing a F to Ti(OSi)4produced the Si-Fgroup, resulting in stroger Lewis acidity of Ti. Ifthe Si-OH adjacent to the Ti-OH defect sitewas repaced by Si-F, the acidity of both Ti andTi-OH decreased.Adsorption of NH3leads to blue shift of the vibrational frequency ofTi-O-Si.