Investigations on the selective oxidation of benzonitrile using nitrous oxide catalyzed by modified ZSM-5 zeolites

Abstract

The direct oxidation of benzonitrile to hydroxybenzonitriles using nitrous oxide as oxidant has been studied on ZSM-5 type zeolites as catalyst. The influence of catalyst properties as well as of reaction conditions on the conversion and on the selectivity of the reaction has been investigated systematically. It has been found that catalysts prepared without addition of iron gives up to 73% selectivity to hydroxybenzonitriles at a benzonitrile conversion of 10%. The three isomers of hydroxybenzonitrile have been found with an o: m: p ratio of 1:6:3. At higher reaction temperatures a higher benzonitrile conversion of up to 22% has been obtained, and the decomposition of nitrous oxide is increased. An excess of nitrous oxide is beneficial for higher conversions. However, with increasing conversion the selectivity to hydroxybenzonitriles is decreased. The role of aluminum and iron in the ZSM-5 zeolite has been investigated. The initial activity of the catalyst is increased by decreasing the Si/Al ratio. The lower the amount of Al, generating acid sites, the higher the selectivity to hydroxybenzonitriles. The catalyst activity is increased by addition of iron, but the higher activity results in a strong loss of selectivity caused by stronger total oxidation and a more rapid deactivation of the catalyst. Moreover, it seems possible that both components, aluminum and iron, are responsible for the catalytic activity of the zeolite. A reaction network for selective benzonitrile oxidation has been developed which includes the oxidation to hydroxybenzonitrile as well as competitive and consecutive reaction steps.