Evaluation of Energy Heterogeneity in Metal Organic Frameworks: Absence of Henry's Region in MIL-53 and MIL-68 Materials?

We present in dos study a detailed thermodynamic analysis of N-2 adsorption at 77.4 K and low pressures in a series of metal-organic framework materials displaying different structures, pore textures, and framework flexibility. The energy heterogeneity of these materials has been quantified using a thermodynamic isotherm model developed in previous studies through two characteristic parameters. This formulation also allows the generation of an equation differing from the Dubinin-Astakhov isotherm that allows a proper description or gas adsorption at low pressures. Unlike zeolites or rigid coordination polymers, highly flexible MIL-53(AI) and MIL-68(In) materials are characterized by N-2 adsorption isotherms lacking of a characteristic Henry's region (linear trend) at low relative pressures. We argue about an unexpected promotion of energy heterogeneity on these materials due to sorbate-induced elastic deformation ascribed to compression tensions to explain this observation.