O2 Adsorption on ZIF-8: Temperature Dependence of the Gate-Opening Transition

We present the results of an adsorption isotherm study of O2 on the metal–organic framework ZIF-8. This material undergoes a structural transition ("gate-opening") as a function of increasing pressure and sorbent loading, which manifests itself in the isotherm data as a quasi-vertical substep. We used this feature to explore the temperature dependence of the structural transition; we have found that the transition occurs below the saturated vapor pressure only for temperatures below 95.39 K. The adsorption isotherm data measured at various temperatures were used also to determine the isosteric heat of adsorption of O2 on this sorbent for different sorbent loading values. We have studied the adsorption kinetics for this system, that is, how the equilibration times for adsorption change as a function of sorbent loading. The sorbent loading dependence of the equilibration time is nonmonotonic; this characteristic appears to be related to the structural transition in the sorbent.