Phase Transformations in MOFs Induced by Adsorbate Exchange

Deformation of nanoporous materials induced by gas adsorption is a ubiquitous phenomenon that plays an important role in adsorption separations, gas and energy storage, nanosensors, actuators, secondary gas recovery, and carbon dioxide sequestration in coal and shale reservoirs. One of the most prominent examples is the breathing phase transformation in metal-organic frameworks (MOF) associated with significant volume variations upon adsorption and desorption of guest molecules. Here, we present a theoretical framework for the quantitative description of the breathing transitions upon adsorption of binary mixtures, drawing on the practically important example of the displacement of methane by carbon dioxide in the MIL-53 MOF. The proposed approach, which is based on the concept of adsorption stress, reveals the mechanisms of framework deformation and breathing phase transformation between the large pore (LP) and narrow pore (NP) conformations. We show that when pure CH₄ adsorption proceeds entirely in the LP phase, even a small addition of CO₂ makes the LP phase unstable and triggers conversion to the NP phase, and the reverse NP-LP transformation occurs upon further displacement of CH₄ by CO₂. The theoretical predictions of adsorption and strain isotherms are confirmed by an agreement with the literature experimental studies performed on MIL-53(Al) at different CH₄-CO₂ mixture pressures and temperatures. The proposed general approach is applicable to other flexible nanoporous structures and gas mixtures.