Cation motion in dehydrated, partially ca2+-exchanged zeolites

Abstract

Ionic conduction in dehydrated partially Ca2+-exchanged synthetic faujasites is due to Na+ ions on sites II. The mechanism is a vacancy diffusion. Activation energies lie in the range 38–59 kJ mol–1 and are mainly determined by the positive charge density in the hexagonal prisms and in the cubo-octahedra. Deep bed heating prior to the conduction experiments leads to an increase of the activation energy for migration of Na+ ions in X zeolites but not in Y zeolites. Two relaxations are observed. The low frequency relaxation is a Maxwell–Wagner effect. The high frequency relaxation is ascribed to the movement of Ca2+ ions on sites I and I′ between neighbouring sites. The activation energies for this relaxation range between 84 and 106 kJ mol–1. The electrical permittivities, uncorrected for pellet heterogeneity, are in the range 2.32–2.84.