Resonance Raman effects in TS-1: the structure of Ti(iv) species and reactivity towards H2O, NH3 and H2O2: an in situ study

Abstract

The isomorphous insertion of 1–2 wt% of Ti into the MFI framework leads to a Titanium silicalite-1 (TS-1) material, which is an active and highly selective catalyst in a remarkable number of low-temperature oxidation reactions with aqueous H₂O₂ as oxidant. Such Ti(IV) species exhibit a local T_d-like symmetry, forming [TiO₄] units, and induces to the hosting MFI matrix two Ti-specific vibrational modes at 960 and 1125 cm⁻¹. We report a Raman study on the perturbation caused by interaction with H₂O, NH₃ and H₂O/H₂O₂ on the vibrational modes of the [TiO₄] unit embedded in the MFI framework. The selective use of different excitation laser sources in the near-IR (1064 nm; 9398 cm⁻¹), visible (442 nm; 22 625 cm⁻¹), near-UV (325 nm; 30 770 cm⁻¹) and far-UV (244 nm; 40 985 cm⁻¹) allowed us to progressively enter into the oxygen to titanium charge transfer transition and thus to switch on the resonance effects on the 1125 cm⁻¹ mode, which is the only Ti-specific mode exhibiting the same symmetry of the charge transfer transition. Interaction with both water and ammonia causes the formation of [Ti(H₂O)₂O₄] or [Ti(NH₃)₂O₄] complexes which destroy the T_d-like symmetry and thus the Raman enhancement of the 1125 cm⁻¹ mode. Upon dosing a H₂O/H₂O₂ to TS-1, the powders turn yellow as a consequence of the appearance a new charge transfer transition around 385 nm (26 000 cm⁻¹). In order to single out the vibrational mode of the active peroxo complex formed on Ti, we have performed Raman experiments using a visible laser source (442 nm; 22 625 cm⁻¹). In these conditions we have observed the strong enhancement of a mode at 618 cm⁻¹, which has been attributed to the symmetric breathing mode of the Ti(O)₂ ring.