Pathways of phenol and benzene photooxidation using TiO2 supported on a zeolite

The use of photocatalysts supported on adsorbents is gaining substantial attention. Photooxidation of phenol and benzene initiated by light absorption by TiO2 supported on ZSM-5 zeolite is reported to learn in what ways loading TiO2 on an adsorbent high silica zeolite modifies the reactions pathways. At equal TiO2 loading, rates of phenol loss were similar to reactions on P25. In acid media, the major primary products were the result of expected o-, p-hydroxylation. Catechol and hydroquinone were formed and consumed at comparable rate so that a near steady state persisted for about 400 min. In contrast, secondary reactions were faster on P25 and catechol and hydroquinone concentrations decline systematically from 60 min onward, Beyond trihydroxy species, ring opening became important. The open chain oxygenated species built up little and appear to undergo further oxidation rapidly. The zeolite supported photocatalyst had a maximum efficiency at pH = 3.4 and lost reactivity at higher pH. In basic media, hydroquinone becomes the dominant primary product. Catechol yields decline, but phloroglucinol, undetected in acid, is found. There is evidence of some polymerization with TiO2 on the adsorbent zeolite support in base that interferes with mineralization. Reactions of benzene may not be simply limited to initial formation of phenol. The are some differences in the pathway for benzene that suggest other initial steps. It is reasonable to infer that there are differences between phenol and benzene adsorption on the catalyst and that benzene can react with species other than an adsorbed OH radical in an alternate the initial step. (C) 2002 Elsevier Science B.V. All rights reserved.