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

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The CO 2 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 CO 2 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 CO 2 capacity of the Cs-CHA sample was overestimated; the partial blockage of the pores and inaccessibility of CO 2 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 CO 2 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 N 2 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 CO 2 /N 2 selectivity is required.