Catalytic and ESR studies of ethylene dimerization on palladium-exchanged NaX and CaX zeolites

Abstract

Oxygen-pretreated, palladium-exchanged Na-X and Ca-X zeolites are catalytically active for ethylene dimerization in both static and flow reactors. The catalytic activity of the zeolites for this reaction is shown to be due to palladium species and is greatly dependent on the type of major cocation (Na/sup +/ or Ca/sup 2 +/) in the zeolites. The cocations influence the location of the active palladium species. In NaPd-X zeolites, palladium cations occupy sites (SI or SI') which are inaccessible to ethylene, while in CaPd-X zeolites palladium cations occupy sites (SII' or SII) which are relatively accessible to ethylene. As a result, the reaction occurs after a long induction period in NaPd-X zeolites due to migration of palladium species toward more accessible locations. This induction period decreases with an increase of reaction temperature. Paramagnetic species, giving ESR signals at g/sub parallel/ = 2.53 and g/sub parallel/ = 2.33-2.34, both with g/sub perpindicular/ at 2.10, are assigned to Pd/sup +/ coordinated to ethylene and are detected prior to butene formation. Consequently, monovalent palladium cations are considered are considered to be catalytically active sites for ethylene dimerization.