A dielectric study of the synthetic linde type- A zeolite-I. Experimental methods and the linde 5- A-water system

Abstract

A system, designed particularly for the study of the dielectric properties of zeolites on the adsorption of vapours, is described. It enables compacted disc samples to be dehydrated at 450°C and 10 -5Torr., and then measured, in the frequency range 5 Hz–148 kHz, with any equilibrium pressure, up to 760 Torr., of most gaseous adsorbates. Measurements of the permittivity and dielectric loss factor have been made for the Linde Molecular Sieve zeolite type 5- A in the dehydrated state and with adsorbed water at all water contents up to saturation. By the use of conventional dielectric measuring techniques the water saturated material has been measured in the frequency range 5 Hz–8.5 GHz. A well defined region of dielectric absorption, occurring in the low frequency (< 148 kHz) range, has been ascribed to movement, within the zeolite cavities, of the interstitial cations. The behaviour of this absorption with variation of water content and temperature has been investigated. A further low frequency absorption, attributed to the adsorbed water, has been observed and its behaviour with water content and temperature studied. The results suggest that the total dipole polarisation due to the water relaxes in two stages: one is of a low frequency ice-like character, the other, relatively much smaller dispersion, gives absorption(s) throughout the MHz region. The permittivity increment shows a sequence of linear increases with water content which have been interpreted as indicating coverage of the first and second most active adsorption sites in the zeolite cavities. Possible mechanisms of these processes are discussed.