Photocatalytic selective oxidation of phenol to produce dihydroxybenzenes in a TiO₂/UV system: Hydroxyl radical versus hole

Photocatalytic oxidation plays an important role in organic synthesis. This research systematically studied the effect of reactive oxidation species (ROSs) such as surface-bounded hydroxyl radicals (·OH_b), free hydroxyl radicals (·OH_f), and hole (h⁺) on the photocatalytic selective oxidation of phenol in the TiO₂/UV system and its corresponding important chemical intermediates, dihydroxybenzenes. Experimental results showed that (1) the oxidation of phenol by hydroxyl radicals, whether in free (·OH_f) or bounded states (·OH_b), led mainly to the ortho- (catechol) and para-oriented hydroxy derivatives (hydroquinone), whereas the meta-oriented hydroxy derivative (resorcinol) became the main product when hole was used as the active species; (2) in naked TiO₂ suspensions, where ·OH_b was the main ROSs, the selectivity of the catechol detected in the solution was much lower than that on the surface of the fluorinated TiO₂ system (TiO₂-F), where ·OH_f served as the main ROSs. This finding can be attributed to the very strong catechol adsorption on the surface of naked TiO₂. However, this adsorption was severely inhibited in the presence of fluorine because of of the competitive adsorption between catechol and fluoride ions over TiO₂ photocatalyst; and (3) the hole oxidation of phenol provided a slower oxidation rate but a higher yield of dihydroxybenzenes compared with hydroxyl radical-mediated oxidation. Copyright © 2016 Elsevier B.V.