CO2 adsorption in Y and X zeolites modified by alkali metal cation exchange

Ion exchange was performed on NaY and NaX zeolites with alkali metal cations Li+, K+, Rb+, and Cs+ and studied by adsorption of CO2. This is the first study to examine adsorption equilibrium isotherms and capacities of CO2 on the alkali metal series for both Y and X zeolites under mild conditions. CO2 capacity increased as Cs < Rb ≈ K < Li ≈ Na for Y zeolites. For X zeolites, the capacity for CO2 increased in the order Cs < Rb < K < Na < Li (the order of decreasing ionic radii). For both zeolites the larger cation forms (Cs, Rb, K) exhibited strongly nonlinear concave downward isotherms, which is indicative of strong interactions between CO2 and the zeolite. This is consistent with an increased basicity of the framework compared to the smaller cation forms, given that CO2 is a weakly acidic gas. This is also reflected by the Henry’s law slopes obtained from the Toth isotherm equation. Our measurements show that, in general, CO2 capacities are greatest for the Li forms, in which the ion–quadrupole interaction is dominant. Adsorption equilibrium measurements of CO2 on each ion-exchanged material reveal behaviors and trends based on cation size and acid–base surface properties that can have an important impact on tuning adsorptive properties of zeolites by ion exchange.