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-hy droxybenzyl alcohol were studied as a function of the substrate concen tration in the presence or absence of a hydroxyl radical scavenger (is opropanol). In the absence of isopropanol, hydroxylated derivatives an d aldehydic products are formed. The formation of aldehydic products i s clearly favoured at high substrate concentrations. In the presence o f isopropanol, the formation of hydroxylated derivatives is suppressed , whereas the formation of aldehydic products is only partially reduce d. These results show that hydroxyl radicals and positive holes are bo th involved in the transformation of these compounds. Direct oxidation by positive holes, yielding the aldehydic products, is the main proce ss at high substrate concentrations. The photocatalytic transformation of 4-hydroxyphenethyl alcohol yields many products. In the presence o f isopropanol, only one product is observed. Its formation rate is cle arly enhanced in the presence of the hydroxyl radical scavenger. (C) 1 997 Elsevier Science S.A.