Low temperature adsorption study of CO2 in ZIF-8

We present the results of a thermodynamic and kinetics of adsorption study of CO2 sorbed in ZIF-8. Adsorption isotherms were measured at ten temperatures between 133 K and 237 K. Consistent with what has been reported for this system at higher temperatures, there is only one step in the sorption isotherms up to reaching the saturated vapor pressure. We obtained the isosteric heat of adsorption as a function of loading from the isotherm data. The isosteric heat displays a non-monotonic dependence on loading: there is a maximum in the isosteric heat at high sorbent loadings. For all loadings below the maximum, the isosteric heat is an increasing function of loading. We also measured the time required to reach equilibrium after a dose of gas was added to the sorbent to establish the sorption kinetics for this system. While the general trend in the equilibration times is that equilibration times are shorter for increasing loading, we found a small local maximum in a region in the neighborhood of where the isosteric heat has its peak value. The thermodynamic and kinetic results obtained for CO2 in ZIF-8 are compared with those obtained in prior studies of 02 and Xe in ZIF-8. (C) 2017 Elsevier Inc. All rights reserved.