The Behavior of Flexible MIL-53(Al) upon CH4 and CO2 Adsorption

TL;DR

This study combines thermodynamic modeling and gas adsorption experiments to analyze the phase behavior of flexible MIL-53(Al) during CH4 and CO2 adsorption, revealing general breathing phenomena and a transferable stress model.

Contribution

It introduces a generic temperature-loading phase diagram and demonstrates the transferability of the stress model from xenon to methane adsorption in MIL-53(Al).

Findings

Breathing effect occurs across a limited temperature range.

The stress model applies to methane, not just xenon.

Phase mixtures exist near transition pressures without inhomogeneity.

Abstract

The use of the osmotic thermodynamic model, combined with a series of methane and carbon dioxide gas adsorption experiments at various temperatures, has allowed shedding some new light on the fascinating phase behavior of flexible MIL-53(Al) metal-organic frameworks. A generic temperature-loading phase diagram has been derived; it is shown that the breathing effect in MIL-53 is a very general phenomenon, which should be observed in a limited temperature range regardless of the guest molecule. In addition, the previously proposed stress model for the structural transitions of MIL-53 is shown to be transferable from xenon to methane adsorption. The stress model also provides a theoretical framework for understanding the existence of lp/np phase mixtures at pressures close to the breathing transition pressure, without having to invoke an inhomogeneous distribution of the adsorbate in the porous sample.