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.

Automated summary

The study investigates the CO2 adsorption properties of potassium merlinoite zeolites with different Si/Al ratios and reveals the significant impact of framework Si/Al ratio on the CO2 adsorption mechanism.