Hydroxylation of aromatics with hydrogen peroxide over titanosilicates with MOR and MFI structures: Effect of Ti peroxo species on the diffusion and hydroxylation activity

The shape selectivity in the liquid-phase hydroxylation of aromatic hydrocarbons with hydrogen peroxide has been studied on two kinds of titanosilicate, the large-pore Ti-M with MOR structure and medium-pore TS-1 with MFI structure, and the liquid-phase diffusion of aromatics into both catalysts was compared in the presence of H₂O and H₂O₂ to clarify the origin of the shape selectivity. The hydroxylation rate on TS-1 decreased monotonically with increasing molecular size in the order benzene > toluene ≫ ethylbenzene > cumene, while Ti-M showed the maximum rate for toluene hydroxylation. The apparent diffusivity of aromatics was lowered roughly by 1 order by adding H₂O₂ to H₂O solvent for both titanosilicates but was not affected for their Ti-free derivatives, mordenite and silicalite-1. The reduction in diffusion rate in the presence of H₂O₂ was more pronounced for TS-1 and for Ti-rich samples. It is concluded that a bulky Ti peroxo species (Ti-OOH) formed by the interaction of Ti site with H₂O₂ mainly causes a transition-state shape selectivity in the hydroxylation of bulky aromatics in titanosilicate/H₂O₂ systems.