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

Abstract

This work presents adsorption equilibria of N 2 and CO 2 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 N 2 and from 0.001 to 103 kPa for CO 2, using a volumetric apparatus. In all cases, CO 2 adsorption capacities were remarkably higher than those for N 2 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 N 2 and CO 2 were obtained on LiCHA, and very low loadings of N 2 on KCHA (attributed to pore blockage) suggest KCHA is a promising adsorbent for N 2/CO 2 mixture separation. The adsorbed phase density of CO 2 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 N 2 isotherms at 77 K. Henry's law constants were extracted from virial plots for the measured isotherms. The virial plots for CO 2 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 CO 2 adsorption at the low coverage region partially due to pore blockage.