Volume shrinkage of a metal-organic framework host induced by the dispersive attraction of guest gas molecules

Using a density functional theory calculation including van der Waals (vdW) corrections, we report that H-2 adsorption in a cubic-crystalline microporous metal-organic framework (MOF-5) leads to volume shrinkage, which is in contrast to the intuition that gas adsorption in a confined system (e. g., pores in a material) increases the internal pressure and then leads to volumetric expansion. This extraordinary phenomenon is closely related to the vdW interactions between MOF and H-2 along with the H-2-H-2 interaction, rather than the Madelung-type electrostatic interaction. At low temperatures, H-2 molecules adsorbed in the MOF-5 form highly symmetrical interlinked nanocages that change from a cube-like shape to a sphere-like shape with H-2 loading, helping to exert centrosymmetric forces and hydrostatic (volumetric) stresses from the collection of dispersive interactions. The generated internal negative stress is sufficient to overcome the stiffness of the MOF-5 which is a soft material with a low bulk modulus (15.54 GPa).