Effects of the ion-exchange sequence on the CO2 uptake and CO2–over–N2 selectivity of zeolite NaKA

Abstract

The CO₂ and N₂ adsorption on small-pore zeolite |Na_12-xK_x|-A was hypothesized to be affected by the ion exchange sequence used for the zeolite preparation. Zeolites were prepared by ion exchange of a commercially available zeolite |Na₁₂|-A (4A) and a zeolite |K₁₂|-A (3A) composition prepared from zeolite 4A. The CO₂ and N₂ adsorption properties were studied experimentally, and the binary CO₂-over-N₂ selectivity was estimated from single-component adsorption data using the apparent Henry's law coefficients. It was observed that the level of CO₂ adsorption was reduced by increasing the K content for both series of zeolite NaKA. Zeolite |Na_12-xK_x|-A-from-4A had the highest CO₂ adsorption capacity (at 1 atm and 273 K) for a given K content. At low K content, zeolite |Na_12-xK_x|-A-from-3A had the highest CO₂-over-N₂ selectivity. At an intermediate K content, the zeolites prepared from 4A had the highest selectivity. These differences show that non-equilibrium processes during the ion exchange are important for the CO₂ and N₂ adsorption properties of the derived zeolites. As of now, we refrain from speculating whether they relate to the detailed positioning of K⁺ and Na⁺ cations in the local structure of the zeolite or to mass-transport-related concentration gradients of the cations in the structure. Irrespectively, it was observed that the ion-exchange sequence affects the CO₂ and N₂ adsorption properties of the zeolites, which could be of general importance when it comes to the tuning of the properties of cation-rich zeolites.