Photocatalytic selective oxidation of phenol to produce dihydroxybenzenes in a TiO 2 /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 (OHb), free hydroxyl radicals (OHf), 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 (OHf) or bounded states (OHb), led mainly to the ortho- (catechol) and para-oriented hydroxy derivatives (hydroquinone), whereas the meta-oriented hydroxy derivative (resorcinol) became the main product when holes were used as the active species; (2) in naked TiO2 suspensions, where OHb 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 OHf 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 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.