Selectivity of hydroxyl radical in the partial oxidation of aromatic compounds in heterogeneous photocatalysis

The photocatalytic oxidation of different benzene derivatives has been investigated in order to understand how the substituent group affects the selectivity to hydroxylated compounds. Experimental runs were performed by using TiO2 (Merck) aqueous suspensions at natural pH irradiated by near-UV light. The organic molecules used as substrate contained an electron withdrawing group (EWG) (nitrobenzene, cyanobenzene, benzoic acid, 1-phenylethanone), an electron donor one (EDG) (phenol, phenylamine, N-phenylacetamide) or both an EWG and an EDG (4-chlorophenol). The results clearly indicated that the primary photocatalytic oxidation of aromatic compounds containing an EDG gives rise mainly to ortho and para mono-hydroxy derivatives while in the presence of an EWG all the mono-hydroxy derivatives are obtained. This finding can open a new green route for the synthesis of hydroxylated aromatic compounds. Moreover, in the presence of both an ED and an EW group, as in the case of 4-chlorophenol and hydroxy-cyanobenzenes, the attack of the hydroxyl radical takes place only in the positions activated by -OH. A competing reaction pathway to total oxidation was also observed from the starting of the irradiation; this pathway was more important for compounds containing an EWG. This evidence can be explained by considering the strong interaction of these molecules with the TiO2 surface. In fact, adsorption in the dark, measured for all the compounds, resulted to be significant only for molecules having strongly EWG's.