Negative gas adsorption of flexible metal-organic frameworks

Counterintuitive phenomena have led to tremendous improvement of modern technologies and often result in a paradigm shift in sciences. The results of this work rationalize the serendipitous discovery of negative gas adsorption (NGA) as a novel, counterintuitive adsorption phenomenon with the potential to change common thinking about adsorption in porous solids. Upon adsorption of methane at 111 K in a mesoporous metal-organic framework DUT-49 (Dresden University of Technology No. 49) a negative adsorption gradient with a corresponding drop in uptake, ΔnNGA, of 8.62 mmol g-1 was observed presenting the initial discovery of NGA. By in depth analysis of the underlying structural transitions, the mechanism of NGA could be rendered as an interplay between adsorption-induced stress and cooperative structural contraction in the mesoporous solid DUT-49 (Dresden University of Technology No. 49). In situ diffraction experiments in combination with in silico adsorption analysis illuminate the underlying pore-filling mechanism supporting the role of small mesoporosity in the adsorption mechanism of NGA. Upon structural contraction and corresponding reduction in porosity adsorbate is forced out of the pores and released into the gas phase resulting in an artificial self-inflicted pressure amplification, unprecedented in any porous solid. Extensive studies on materials properties and adsorption behavior of DUT-49 define crystal size, framework defects, nature of the adsorptive, and adsorption temperature as key physical parameter that influence NGA. Rational variation of these factors leads to a vast increase in the amount of gas released upon NGA from the original observations. An empirical correlation between the critical temperature of the fluid and the adsorption temperature at which NGA is found to reach its maximum is established, allowing to predict adsorption conditions at which NGA is expected to occur. Computationally aided investigations of the thermodynamics and kinetics of the transition allow to further describe the metastability of the transition and define conditions under which NGA can be expected for solids other than DUT-49.