Photocatalytic transformation of aromatic compounds in aqueous zinc oxide suspensions: effect of substrate concentration on the distribution of products

The photocatalytic transformations of 4-methoxybenzyl alcohol and 4-hydroxybenzyl alcohol were studied as a function of the substrate concentration in the presence or absence of a hydroxyl radical scavenger (isopropanol). In the absence of isopropanol, hydroxylated derivatives and aldehydic products are formed. The formation of aldehydic products is clearly favoured at high substrate concentrations. In the presence of isopropanol, the formation of hydroxylated derivatives is suppressed, whereas the formation of aldehydic products is only partially reduced. These results show that hydroxyl radicals and positive holes are both involved in the transformation of these compounds. Direct oxidation by positive holes, yielding the aldehydic products, is the main process at high substrate concentrations. The photocatalytic transformation of 4-hydroxyphenethyl alcohol yields many products. In the presence of isopropanol, only one product is observed. Its formation rate is clearly enhanced in the presence of the hydroxyl radical scavenger. (C) 1997 Elsevier Science S.A.