Cation Dependence of Gas Adsorption in Clinoptilolite and RHO Zeolites Using Monte Carlo Simulation: A Linear Model

Zeolites have tremendous potential as adsorbents for the selective separation and storage of gases in many industrial processes. The extent and selectivity of gas adsorption are dependent on a number of zeolite properties, such as pore diameter, accessible volume, and silicon-to-aluminum ratio. In addition, the existence of aluminum in the framework requires the presence of cations to maintain charge neutrality, and the identity of these extra-framework cations will also have a significant effect on adsorption. In this work, the continuous fractional component Monte Carlo method was used to examine the effect of the size and the charge of these cations on the adsorption of a variety of gases (CH4, N2, and CO2) in two representative zeolite frameworks, clinoptilolite (heulandite) (CLI) and RHO at 298.15 K, using a range of pressures from 0.1 to 60 bar for a variety of mono- and divalent cations (Na+, K+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+). Adsorption calculations in the single-cation systems revealed unique trends in the size and charge for each gas. To examine the combined effect of multiple cation types, adsorption was calculated at 0.5 and 30 bar in several two-cation systems, in which the effects of the size and charge were isolated and examined. The resulting adsorption varied linearly with the mole fraction of each two-cation mixture. A linear model is proposed to describe the loading of gases in CLI and RHO zeolites containing complex cation mixtures. Loading predicted by this model showed excellent agreement with direct simulation of gas adsorption in randomized seven-cation systems, suggesting that the adsorption effect of each cation is additive among the zeolite frameworks tested.