Effect of framework Si/Al ratio on the mechanism of CO2 adsorption on the small-pore zeolite gismondine

Here, we compare the CO2 adsorption properties of the Na+ form of a series of gismondine (framework type GIS) zeolites with Si/Al ratios of 1.5–4.7 in order to understand the effect of framework Al (thus extraframework Na+) content on the actual adsorption mechanism on this type of small-pore zeolites. Negligible CO2 uptake (≤0.7 mmol g−1 at 25 °C and 1.0 bar) was observed for Na-GIS-1.5 and Na-GIS-2.2, because the number of extraframework cations near their 8-ring windows are large enough to sterically hinder CO2 adsorption. Unlike that on Na-GIS-4.7 with an uptake of 3.0 mmol g−1, on the other hand, CO2 adsorption on Na-GIS-2.5, Na-GIS-2.8, and Na-GIS-3.0 was found to show one clear step but at different pressures, probably due to small differences in the number of gating cations with high site occupancy located near the distorted 8-ring windows. While the adsorption behavior of both Na-GIS-2.8 and Na-GIS-3.0 appears to begin by the relocation of some gating cations with high site occupancy without notable structural breathing, i.e., the cooperative cation gating-breathing mechanism, with the former effect being more dominant, the behavior of Na-GIS-2.5 and Na-GIS-4.7 can be understood based on the cation gating and molecular sieving effects, respectively. The overall results of this study demonstrate that the framework Si/Al ratio of GIS-type zeolites is key to governing their CO2 adsorption mechanism.