Vibrational structure of titanium silicate catalysts. A spectroscopic and theoretical study

A thorough analysis of the vibrational features of the titanium silicalite- 1 (TS-1) catalyst is presented, based on quantitative IR measurements, Rama n and resonant Raman experiments, quantitative XANES, and quantum chemical calculations on cluster and periodic models. The linear correlation of the intensity of the IR and Raman bands located at 960 and 1125 cm(-1) and the XANES peak at 4967 eV with the amount of tetrahedral Ti are quantitatively demonstrated. Raman and resonant Raman spectra of silicalite and TS-1 with variable Ti content are presented, showing main features at 960 and 1125 cm -1 associated with titanium insertion into the zeolite framework. The enhan cement of the intensity of the 1125 cm-1 feature and the invariance of the 960 cm(-1) feature in UV-Raman experiments, are discussed in terms of reson ant Raman selection rules. Quantum chemical calculations on cluster models Si[OSi(OH)(3)](4) and Ti[OSi(OH)(3)](4) at the B3LYP/6-31G(d) level of theo ry provide the basis for the assignment of the main vibrational contributio ns and for the understanding of Raman enhancement. The resonance-enhanced 1 125 cm-1 mode is unambiguously associated with a totally symmetric vibratio n of the TiO4 tetrahedron, achieved through in-phase antisymmetric stretchi ng of the four connected Ti-O-Si bridges. This vibration can also be descri bed as a totally symmetric stretching of the four Si-O bonds pointing towar d Ti. The resonance enhancement of this feature is explained in terms of th e electronic structure of the Ti-containing moiety. Asymmetric stretching m odes of TO4 units show distinct behavior when (i) T is occupied by Si as in perfect silicalite, (ii) T is occupied by Ti as in TS-1, or (iii) the oxyg en atom belongs to an OH group, such as in terminal tetrahedra of cluster m odels and in real defective zeolites. Asymmetric SiO4 and TiO4 stretching m odes appear above and below 1000 cm(-1), respectively, when they are achiev ed through antisymmetric stretching of the T-O-Si bridges, and around 800 c m(-1) (in both SiO4 and TiO4) when they involve symmetric stretching of the T-O-Si units. In purely siliceous models, the transparency gap between the main peaks at 800 and 1100 cm(-1) contains only vibrational features assoc iated with terminal Si-OH groups, while in Ti-containing models it contains also the above-mentioned asymmetric TiO4 modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of si licalite and TS-1 free of CH groups using the QMPOT embedding method correc tly reproduce the transparency gap of silicalite and the appearance of asym metric TiO4 vibrations at 960 cm(-1) in TS-1. Finally, we demonstrate, for the first time, that the distortion of the tetrahedral symmetry around Ti c aused by water adsorption quenches the UV-Raman enhancement of the 1125 cm( -1) band.