Effect of Ti insertion in the silicalite framework on the vibrational modes of the structure: an ab initio, and vibrational study

We report a thorough theoretical study based on both “conventional” cluster and embedded cluster models on the effect induced on the vibrational modes of the MFI framework by the isomorphous insertion of a Ti atom. On an industrial ground, this insertion has generated one of the most important catalyst of the last two decades: titanium silicalite (TS-1). To allow a direct comparison of TS-1 with the parent Ti-free structure (silicalite), quantum chemical calculations on both cluster models Ti[OSi(OH)3]4 and Si[OSi(OH)3]4 have been performed. In both cases we have employed the B3LYP/6-31G+(d,p) level of theory in order to provide the basis for the assignment of the main vibrational contributions. On an experimental ground, dehydrated TS-1 exhibits a IR spectrum characterized by a well defined band located at 960 cm-1 and a Raman spectrum showing two components at 960 and 1125 cm-1, being the latter enhanced in case of resonant Raman effect achieved using an UV laser source. In this work, the enhancement 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 resonant Raman selection rules. The resonance-enhanced 1125 cm-1 mode is unambiguously associated with a totally symmetric vibration of the TiO4 tetrahedron, achieved through in-phase antisymmetric stretching of the four connected Ti-O-Si bridges. This vibration can also be described as an in-phase stretching of the four Si-O bonds pointing towards Ti. The resonance enhancement of this feature is explained in terms of the electronic structure of the Ti-containing moiety. Asymmetric stretching modes of TO4 units show distinct behavior when T is occupied by Si or Ti, or when the oxygen atom belongs to OH groups (such as in terminal tetrahedra of cluster models and in real defective zeolites). Asymmetric SiO4 and TiO4 stretching modes appear above and below 1000 cm-1 respectively, when they are achieved through antisymmetric stretching of the T-O-Si bridges, and around 800 cm-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 associated with terminal Si-OH groups, while in Ti- containing models it contain also the above mentioned asymmetric TiO4 modes, which in turn are strongly coupled with Si-OH stretching modes. Calculations on periodic models of silicalite and TS-1 free of OH groups using the QMPOT embedding method, correctly reproduce the transparency gap of silicalite and the appearance of asymmetric TiO4 vibrations at 960 cm-1 in TS-1.