Charting the Complete Thermodynamic Landscape of Gas Adsorption for a Responsive Metal-Organic Framework

New nanoporous materials are able to revolutionize adsorption and separation processes. In particular, materials with adaptive cavities have high selectivity and may display previously undiscovered phenomena, such as negative gas adsorption (NGA), in which gas is released from the framework upon an increase in pressure. Although the thermodynamic driving force behind this and many other counterintuitive adsorption phenomena have been thoroughly investigated in recent years, several experimental observations remain difficult to explain. This necessitates a comprehensive analysis of gas adsorption akin to the conformational free energy landscapes used to understand the function of proteins. For the first time, we constructed the complete thermodynamic landscape of methane adsorption on DUT-49, a system that demonstrates NGA. Traversing this complex landscape correctly reproduces the experimentally observed structural transitions, the temperature dependence of the NGA phenomenon and the observed hysteresis between adsorption and desorption. The complete thermodynamic description presented here provides unparalleled insight into the process of adsorption and provides a framework to understand other adsorbents that challenge our preconceptions.