Li+/ZSM-25 Zeolite as a CO2 Capture Adsorbent with High Selectivity and Improved Adsorption Kinetics, Showing CO2-Induced Framework Expansion

The structure of ZSM-25, a RHO family zeolite, was resolved recently. Recent reports focused on Na-ZSM-25 as a promising CO, adsorbent with high CO, working capacity and exceptional CO2/CH4 ideal selectivity for pressure swing adsorption. However, these reports discuss single-gas adsorption performance of Na-ZSM-25 and are unable to provide the CO2/CH4 separation behavior in practical scenarios where the adsorption is performed using mixed gases. Furthermore, Na-ZSM-2S suffers from slow adsorption kinetics that limits its industrial applications. In this study, Li+/ZSM-25 zeolites (LZZs) are developed to enhance the CO, adsorption kinetics without compromising selectivity. The adsorption performance was examined using both single-(isotherm) and binary-(breakthrough) gas measurements. The results indicate that the CO, adsorption rate of LZZ is 9.84 times that of Na-ZSM-25 with a simultaneous increase of the CO, adsorption capacity by 6.1% at 303 K and 9.5 bar. The very high CO2/CH4 adsorptive selectivity is successfully inherited from Na-ZSM-25 zeolite in both single- and binary-gas adsorption. Furthermore, in situ XRD reveals CO2-induced framework expansion, which explains the origin of the high working capacity and type-II-like isotherms of the ZSM-25-based zeolites. This study implies promising application of LZZ in natural gas purification and biogas upgrading.