The diffusion and sorption of water in zeolites—II. Intrinsic and self-diffusion

Abstract

The solution of three-dimensional diffusion equations is discussed with respect to the applicability of a √ t relationship to describe diffusion. The kinetics of the rates of H 2O sorption and of D 2O-H 2O exchange have been followed in the zeolites chabazite, gmelinite and heulandite in the temperature range 30–80°C, and intrinsic diffusion coefficients and self-diffusion coefficients of water in the crystal evaluated from the respective sets of measurements. Self-diffusion coefficients in all three zeolites have values intermediate between the self-diffusion coefficients of liquid water, at the same temperature, and ice near its melting point. Small changes in total water content near saturation produce a slight decrease in the self-diffusion coefficients in chabazite and gmelinite and a more marked decrease in heulandite. Using equilibrium sorption data it is shown that the equation D = D∗ (dln a d lnc) is a reasonably satisfactory description of the relationship between the diffusion coefficients and this provides an indication that both intrinsic and self-diffusion processes involve molecules as diffusing units.