Pathways of phenol and benzene photooxidation using TiO 2 supported on a zeolite

Abstract

The use of photocatalysts supported on adsorbents is gaining substantial attention. Photooxidation of phenol and benzene initiated by light absorption by TiO 2 supported on ZSM-5 zeolite is reported to learn in what ways loading TiO 2 on an adsorbent high silica zeolite modifies the reactions pathways. At equal TiO 2 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 TiO 2 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.