Formation of phenazine from azobenzene over H-ZSM5: Reaction control by a molecular constrained environment

Abstract

Azobenzene undergoes a unique reaction in the channels of H-ZSM5 zeolite at 593–673 K to give phenazine which is mainly retained within the zeolite. The yield of phenazine is increased by the addition to the reaction carrier gas of oxygen (1% vol.) which probably functions as a hydrogen acceptor. No reaction occurs when low acidity silicalite is used. On H-USY zeolite the yield of phenazine is low and the main product is a black, insoluble material which is probably polymeric. Benzo(c)cinnoline which is a known dehydrocyclization product from azobenzene under strongly acidic reaction conditions is never more than a trace product using H-ZSM5 or H-USY. A reaction scheme is proposed involving homolysis of the N ~ N bond in protonated trans-azobenzene, forming two radicals which, in a H-ZSM5 channel, are constrained in their mutual orientation so as to lead to the formation of phenazine.