On the structure of the active site of Ti-silicalite in reactions with hydrogen peroxide: A vibrational and computational study

Ti-Silicalite (TS-1) is an excellent catalyst for the selective oxidation of alkenes to epoxides, of NH3 to hydroxylammine and of the ammoximation of cyclohexanone, using H2O2 as oxidant. This paper reports spectroscopic (IR/Raman) and computational (HF, cluster models) studies on the complexes formed upon interaction of the Ti catalytic centre with water and hydrogen peroxide. Evidence is presented of the formation of unstable hydroperoxidic species upon interaction with neutral aqueous H2O2 solutions, which can be converted into a more stable ionic complex in a basic environment. The vibrational spectra of these complexes have been recorded in appropriately designed low-temperature experiments on solution-soaked powders bearing some resemblance with the catalytic reactor environment. The assignment of the spectra was confirmed by ab initio calculations and by parallel experiments on the structurally similar Ti-free silicalite. (C) 1998 Academic Press