日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Flexible metal–organic frameworks (MOFs) can undergo structural transitions induced by gas adsorption, called gate adsorption. Because this transition occurs abruptly at a specific pressure, the adsorption isotherm exhibits a step-like increase in uptake. Across this pressure, small pressure changes result in large variations in amount adsorbed, making this phenomenon promising for energy-efficient separation technologies as an alternative to conventional distillation. However, gate adsorption is typically accompanied by adsorption–desorption hysteresis, in which the transition pressures differ between the pressurization and depressurization processes, and the fundamental understanding of this behavior remains limited. In particular, the adsorption–desorption behavior strongly depends on the particle size of the sample. Specifically, many MOFs exhibit larger hysteresis as the particle size decreases, but the origin of this dependence has not been clarified. In this review, we focus on ELM-11, a layered MOF, and first present the particle-size-dependent differences in adsorption–desorption behavior. We then model the solid-phase transition occurring across the entire particle and provide a thermodynamic interpretation of the observed phenomena.