Adsorption of Gases on Zeolitic Imidazolate Frameworks: Modeling with Equations of State for Confined Fluids and Pore Size Distribution Estimation

Certain gas separations, as that of propene from propane, are difficult and costly via methods such as distillation, conventionally used in oil and gas industries. Membrane-based separations using metal organic frameworks (MOFs) and, in particular, zeolitic imidazolate frameworks (ZIFs) are potential candidates for less expensive and more energy-efficient processes. Two effects define the performance of a given separation: diffusion and adsorption equilibrium. This works focuses on the latter: the adsorption equilibrium of gases on ZIFs has been correlated and predicted using an equation of state for confined fluids, which extends the Peng–Robinson model, widely used for process design in the oil and gas industries. Excellent agreement between experimental and calculated adsorption isotherms is obtained. In addition, we fitted pore size distributions (PSDs), under the initial assumption that these PSDs might present multiple peaks. In all cases, the fitted PSDs presented a single, narrow peak, which is consistent with the well-defined pore structure of the investigated ZIFs.