Anomalous Henry's law behavior of nitrogen and carbon dioxide adsorption on alkali-exchanged chabazite zeolites

This work presents adsorption equilibria of N2 and CO2 on a fully exchanged potassium chabazite (KCHA), sodium chabazite (NaCHA) and lithium chabazite (LiCHA) zeolites. Isotherms were measured at 273, 303 and 333 K over pressure ranges from 0.1 to 103 kPa for N2 and from 0.001 to 103 kPa for CO2, using a volumetric apparatus. In all cases, CO2 adsorption capacities were remarkably higher than those for N2 with unusual non-linear isotherm trends at pressures lower than 1.0 kPa. Accordingly, extrapolation to zero pressure for the determination of Henry's constants is problematic and could be erroneous. The maximum loadings of N2 and CO2 were obtained on LiCHA, and very low loadings of N2 on KCHA (attributed to pore blockage) suggest KCHA is a promising adsorbent for N2/CO2 mixture separation. The adsorbed phase density of CO2 on LiCHA was found to be fairly consistent with the theoretical value derived from van der Waals equation, while densities on NaCHA and KCHA deviated significantly from van der Waals’ values. This discrepancy was attributed to the underestimation of pore volumes in NaCHA and KCHA obtained from N2 isotherms at 77 K. Henry's law constants were extracted from virial plots for the measured isotherms. The virial plots for CO2 isotherms on KCHA and NaCHA exhibit “negative-slope” profiles at 273 K and pressures lower than about 1.0 kPa. The temperature dependence of the Henry's law constants obtained from the virial plots indicates that at these conditions we do not have a true equilibrium condition. Rather, we suggest that kinetic hindrance dominated CO2 adsorption at the low coverage region partially due to pore blockage.