Alkali Metal Cations Influence the CO₂ Adsorption Capacity of Nanosized Chabazite: Modeling vs Experiment

Abstract

The CO2 adsorption behavior of nanosized chabazite (CHA) containing different compositions of extra-framework alkali metal cations (Na+, K+, Cs+) with a constant Si/Al ratio of 2.2 was investigated experimentally (adsorption isotherms and in situ Fourier transform infrared (FTIR) spectroscopy) and theoretically by Grand Canonical Monte Carlo (GCMC) modeling. The CO2 adsorption isotherms were in good agreement with the GCMC modeling in the pressure range of 0–60 kPa for all samples except the Cs+-containing nanosized CHA. The GCMC modeling did not consider the kinetics of adsorption; thus, the CO2 capacity of the Cs-CHA sample was overestimated; the partial blockage of the pores and inaccessibility of CO2 to the free voids of the CHA structure was experimentally observed by recording adsorption isotherms and in situ FTIR study. The FTIR results revealed that CO2 adsorbed mainly physically on all samples with the sorption capacity decreasing in the following order: Na-CHA ≃ K-CHA > Cs-CHA. The Na-CHA demonstrated significant micropore volume compared to the other samples as measured by N2 physisorption due to the inability of Na+ to effectively occupy the eight-membered rings (8MRs). This study implies that the different alkali metal cation forms of the nanosized CHA zeolite are of practical interest for flue gas purification, where a high CO2/N2 selectivity is required.