(113a) Adsorption of CO2 on High Silica Zeolites: Structural Defects and Differences between Adsorbent Samples

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 and therefore containing different levels of framework hydroxyls. 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. The differences in the isotherms appear to be due to differences in the saturation capacities. Comparison with theoretical isotherms, derived by molecular simulation, shows that, for both silicalite and DDR, the measured isotherms for the lowest capacity samples are very close to the theoretical values. The theoretical isotherms conform almost exactly to the simple Langmuir model and the heats of adsorption are essentially independent of loading. For the silicalite samples the capacity increases somewhat with hydroxyl content but there is no stoichiometric relationship. The increase in capacity appears to be associated with some mesoporosity arising from structural defects. Only one of the DDR samples showed an adsorptive capacity substantially greater than that of the ideal structure. In contrast to silicalite the increased capacity of DDR appears to be associated with higher energy sites, suggesting access to the smaller cages in some DDR crystals.

Acknowledgement: This study was carried out as part of a GOALI project. The financial support provided by ExxonMobil Corporation and the National Science Foundation is gratefully acknowledged. We are grateful to Wieslaw Roth for zeolite samples and Clarence Chase for the assistance in carrying out the XRD and NMR measurements.