Active Sites in Fe-exchanged Zeolites and Catalysis of Benzene Hydroxylation to Phenol

Abstract

Fe-exchanged zeolites show great promise for the direct benzene hydroxylation to phenol, one of the most important industrial chemicals. In this review, the recent achievements on the active sites and benzene hydroxylation have been summarized and discussed. Firstly, the catalytic results that are assumed in absence of Fe ions are given; secondly, the indispensible role of Fe ions has been revealed. Four types of Fe-involved active sites reported so far have been elaborated, and each type may correpond to a variety of active-site structures. Then the catalytic mechanisms of the mono- and binuclear-Fe sites are sucessively exploited, which are found to be quite similar. The comprising Fe3+ ions are catalytically unfavorable owing to the formation of specailly stable phenolate intermediates that result in deactivation to the catalysts. The Fe2+ ions, whether in the mononuclear or binuclear sites, are potentially active for the catalytic reaction. Finally, a brief concluding remark is presented listing some unresolved problems that may seriously impede this catalytic reaction on an industrial scale. Keywords: Active site, Activation barrier, Benzene hydroxylation, Reaction mechanism, Zeolite framework.