Preparation and catalytic properties of ionic sodium clusters in zeolites

Abstract

Exposure of high-alumina zeolites to sodium metal vapour produces coloured centres1–5 which have been associated with the formation of ionic sodium clusters, located in the zeolite cages. To form these centres, excess alkali metal is required, which contaminates the reaction vessel and makes the system unsuitable as a heterogeneous catalyst. The pioneering work of Pines6 in the area of basic catalysis on sodium metal deposited on alumina implies that ionic clusters in zeolites should exhibit interesting catalytic properties, but there have been no reports of such activity because the procedure yields a material which is difficult to reproduce. Here we present a new method for the preparation of such clusters in zeolites, consisting of the impregnation of a dehydrated zeolite with alcoholic solutions of sodium azide, followed by the controlled decomposition of the azide. As demonstrated by electron-spin resonance (ESR) spectroscopy, ionic sodium clusters are formed in the zeolite pores. These clusters exhibit catalytic properties in both isomerization and hydrogenation reactions of alkenes and alkynes. We believe that this represents the first realistic method for the preparation of alkali metal clusters in zeolites as well as the first catalytic characterization of such systems.