Adsorption of CO2 on high silica MFI and DDR zeolites: Structural defects and differences between adsorbent samples

Abstract Equilibrium isotherms have been measured for adsorption of CO2 on several different samples of high silica zeolites (silicalite, the aluminum free form of ZSM-5 and DDR, the aluminum free form of ZSM-58), prepared by different synthesis methods. The isotherms for the various samples show significant differences but detailed analysis reveals that the normalized adsorption equilibrium constants are substantially the same for all samples. Comparison with theoretical isotherms, calculated by molecular simulation, shows that, for both silicalite and DDR, the measured isotherms for the lowest capacity samples lie very close to the theoretical isotherms for the ideal structures, which conform closely to the simple Langmuir model. The heats of adsorption are almost independent of loading (as required by the Langmuir model) The substantially higher capacities of some of the DDR samples can be explained by an increase in the accessible micropore volume arising from structural defects. For silicalite the capacity sequence follows the hydroxyl content but this correlation appears to be indirect since a higher hydroxyl content implies a more defective structure.