The Role of Temperature and Adsorbate on Negative Gas Adsorption in the Mesoporous Metal-Organic Framework DUT-49

Unusual adsorption phenomena, such as breathing and negative gas adsorption (NGA), are rare and challenge our understanding of the thermodynamics of adsorption in deformable porous solids. In particular, NGA appears to break the rules of thermodynamics by exhibiting a spontaneous release of gas accompanying an increase in pressure. This apparent anomaly is in fact due to long-lived metastable states, and a fundamental understanding of this process is required for the discovery of new materials with this exotic property. Interestingly, NGA was initially observed upon adsorption of methane in the metal-organic framework DUT-49 at relatively low temperature, close to the respective standard boiling point of the adsorptive, and no NGA was observed in the same host/guest system at higher temperatures. In this contribution, we present an extensive investigation of adsorption of a range of different gases at various temperatures in DUT-49, a material which features an NGA transition. Experiments at temperatures ranging from 21 to 308 K, were used to identify, for each guest, a critical temperature range in which NGA occurs. The experimental results were complemented by molecular simulations that rationalize the absence of NGA at elevated temperatures and the non-monotonic behavior observed upon temperature decrease. Furthermore, this in-depth analysis highlights the crucial thermodynamic and kinetic conditions for NGA that is unique to each guest and potentially other solids with similar effects. We expect this exploration to provide detailed guidelines for experimentally discovering NGA and related “rule breaking” phenomena in novel and already reported materials, and furthermore, specify conditions required for the practical application of this effect, for example as pressure amplifying materials.