Effect of framework Si/Al ratio on the adsorption mechanism of CO2 on small-pore zeolites: II. Merlinoite

Potassium merlinoite with a framework Si/Al ratio of 2.9 (K-MER-2.9) is synthesized using K-MER with a higher Si/Al ratio of 3.8 (K-MER-3.8) as seed crystals. The CO2 adsorption properties of K-MER-2.9 are investigated at 25-45 ?C and 0-1.0 bar and compared with four other K-MER zeolites with different Si/Al ratios (1.7-4.2) in order to investigate the effect of framework Si/Al ratio (or extraframework cation content) on the CO2 adsorption mechanism on MER-type zeolites. While the CO(2 )uptake (0.8 mmol g-1) at 25 ?C and 1.0 bar of K-MER-1.7 is negligible, due to a large number of gating cations occupying the 8-membered ring windows, the opposite holds for the uptakes (3.0-3.8 mmol g(-1)) of K-MER-2.3, K-MER-2.9, K-MER-3.8, and K-MER-4.2. In addition, while the former two zeolites show a clear one step adsorption isotherm but at different CO2 partial pressures, the latter two ones exhibit steep CO2 adsorption from a low pressure. Structural analysis reveals that the gating cations with high site occupancy in dehydrated K-MER-2.9 relocate upon CO2 adsorption, which is accompanied by the distinct framework relaxation, i.e., the cooperative cation gating-breathing mechanism like the case of K-MER2.3, but with the latter effect being more dominant. In contrast, the CO2 adsorption behavior of K-MER-3.8 and K-MER-4.2 can be explained by the breathing and molecular sieving effects, respectively. These findings extend and confirm our recent report that CO2 adsorption mechanism on small-pore zeolites can differ notably according to the framework Al content, as well as the framework topology.