Oxidation of benzene to phenol by nitrous oxide - Activity of iron in zeolite matrices of various composition

Catalytic properties of Fe introduced in the pentasil type zeolite matrices of various composition (B-ZSM-5, Al-ZSM-5, Ga-ZSM-5, Ti-ZSM-5) have been studied. All the matrices were shown to be inert in the title reaction. However, upon iron introduction they may serve as a basis for the development of very active and selective catalysts. The amount of iron needed for this purpose depends dramatically on the matrix composition: Al-ZSM-5 and Ga-ZSM-5 zeolites exhibit high activity even in the presence of 0.01-0.03 wt.% Fe, while B-ZSM-5 and Ti-ZSM-5 zeolites need a 10-100 times more Fe. The distinctions in zeolite activity relate to the fact that the formation of Fe-containing alpha-sites, catalyzing the reaction, strongly depends on the Bronsted acidity and proceeds with different efficiency in various matrices. But being formed, a-sites in their catalytic performance have only slight dependence on the matrix composition. Activity of B-ZSM-5, Al-ZSM-5, and Ti-ZSM-5 zeolites is described by a common dependence on the concentration of alpha-sites, revealing their identity in these matrices. In Ga-ZSM-5 matrix, a-sites exhibit a two to three times higher activity. The role of Fe admixtures in the literature disagreement on the origin of zeolite activity is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.