SYNTHETIC HETEROGENEITY IN X-ZEOLITE FOR GAS-ADSORPTION

Energetic heterogeneity for gas adsorption was synthetically created by physically admixing NaX and BaX crystals and by binary ion exchange of X zeolite framework with Na+ and Ba2+ ions. Isotherms for adsorption of pure N2 and CO at two temperatures were measured on the single and mixed ion crystals in order to characterize their degree of heterogeneity. It was found that both NaX and BaX crystals acted like homogeneous substrates for adsorption of N2. The adsorption of CO on the NaX crystals also exhibited homogeneous behavior, but its adsorption of BaX was significantly heterogeneous. The adsorption of both gases on the simulated heterogeneous adsorbents could be described by simple weighted additivity of the adsorption contributions of each site, indicating that there was insignificant influence between the adsorption force fields of Na+ and Ba2+ ions even when they were in close proximity to each other within the unit cell of X zeolite framework. It was shown that a substrate can be energetically heterogeneous for a gas even when the isosteric heat of adsorption does not vary with surface coverage, which is the common evidence of the existence of adsorbent heterogeneity. The existence of heterogeneity can also be masked when the adsorbate is weakly adsorbed. Consequently, blind curve fitting of adsorption isotherms with a homogeneous adsorption model can be very misleading, especially when extrapolating isotherm data and predicting multicomponent adsorption equilibrium. © 1991.