Vanadium-Modified TS-1 Catalysts with Enhanced Lewis Acid and Charge Density for Efficient Hydroxylation of Benzene to Phenol

Direct hydroxylation of benzene to phenol is an environmentally friendly strategy to produce phenol compared with the conventional process. In this work, vanadium-modified titanium silicate-1 (TS-1) with enhanced Lewis acid and charge density were prepared as catalysts for efficient hydroxylation of benzene to phenol in the presence of hydrogen peroxide as an oxidant under mild conditions. The hierarchical TS-1 zeolites with a high framework Ti content were synthesized in the presence of NH4HCO3, and then vanadium doping was used to change the acidity and charge density. It was found that NH4HCO3 favored the titanium substitution of framework sites and nanocrystal stacking of TS-1. Vanadium doping increased the amount of Lewis acid sites and the charge density associated with the framework Ti. The enhancement in acid and electron characteristics boosted the intrinsic catalytic reactivity of Ti-OOH species, which further improved the performances toward benzene hydroxylation. The presented strategy and mechanistic results clarified the relationship between the structure and acidity in TS-1 zeolite. It was also verified that hydroxyl radicals (•OH) rather than superoxide radicals (•O2–) were the reactive agents responsible for the consecutive selective oxidation of benzene.