Investigation of zeolite catalysts for the direct partial oxidation of benzene to phenol

Abstract

The direct oxidation of benzene to phenol using nitrous oxide as an oxidant has been investigated on a range of zeolite catalysts. It has been found that ZSM-5 type zeolites are excellent catalysts for this reaction, giving up to 99% selectivity at high conversions. Zeolites having the EU-1 type structure had low activities and selectivity, and amorphous acidic silica-alumina catalysts were inactive. The acidity of the various catalysts has been assessed using FT-IR of adsorbed pyridine coupled with thermal desorption, and by temperature-programmed desorption of adsorbed ammonia. It has been found that all the types of catalysts have similar acidities even though they have very different catalytic activities. Elimination of Brønsted acid sites has demonstrated that such sites are a necessary but not sufficient requirement for high activity and selectivity in the benzene-to-phenol reaction. The possible role of iron ions has been investigated and it has been concluded that these are not involved in the catalytic reaction on these particular materials. Possible reaction mechanisms have been considered and the importance of the detailed structure of the zeolite has been examined.