Heats of adsorption on mixed-cation LiNa-LSX: Estimating SIII site occupancy by Li

Li-LSX (low silica type-X) zeolite is the sorbent of choice for air separation by adsorption today. Li-LSX is obtained by ion exchange of Na-LSX. The occupancy of Site III (SIII, which is the open site in the supercage) by Li⁺ is crucially important for N₂ adsorption. In this work, mixed-cation LiNa-LSX zeolites were prepared and N₂ adsorption isotherms were measured. The isosteric heats of adsorption of N₂ were obtained from the temperature dependence. The heats of adsorption remained at approximately 17.7 kJ/mol (i.e., that for pure Na-LSX) at Li⁺ exchange below approximately 70%, and increased upon further increase in Li⁺ exchange, to 22.6 kJ/mol (i.e., that for pure Li-LSX). The increase was not linear (i.e., concave shaped), however, which indicated that one N₂ molecule could likely adsorb on two adjacent cation sites. The relationship between heat of adsorption and Li ion occupancy on site SIII was obtained. It is suggested that this relationship may be used to estimate the occupancy by Li⁺ on SIII of faujasite type of zeolites.