Mechanisms Responsible for Dielectric Properties of Various Faujasites and Linde Type A Zeolites in the Microwave Frequency Range

Abstract

Microwave energy is used in zeolite sorption process intensification and more specifically in the desorption step. To optimize a microwave process, the knowledge of dielectric properties of the used materials is required. The complex permittivities of several zeolites (NaA, NaK-LSX, NaX, NaY, and DAYs) were measured and interpreted using phenomenological models. Permittivities are linked to the various properties of the zeolites such as structures (LTA and FAU), number of exchangeable cations, silanols density, and hydration level. Three phenomena have been proven to occur: two relaxation processes and one ionic conductivity contribution. Rotational polarization of water molecules adsorbed is revealed, as well as interfacial polarization of charges in intercrystalline void and orientational polarization of water molecules adsorbed on hydroxyl sites. Water loading strongly affects the charge carriers mobility and improves the conductivity phenomenon observed for low and intermediate silica zeolites. Cation location is also of main importance in the involved mechanisms.