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.