Unveiling non-monotonic deformation of flexible MOFs during gas adsorption: From contraction and softening to expansion and hardening

Flexibility of metal-organic frameworks (MOFs) plays an important role in their applications, particularly in adsorption separations, energy and gas storage, and drug delivery. As an important practical example, we study adsorption of CH₄, and CO₂ on isoreticular MOF-1 (IRMOF-1) crystal at different temperatures using an original computational scheme of iterative grand canonical Monte Carlo (GCMC) and isothermal-isobaric ensemble molecular dynamics (NPT-MD) simulations. Our findings reveal that thermal fluctuations and flexibility of the host framework affect adsorption of guest molecules, which in turn exert a significant adsorption stress, up to 0.1 GPa, on the framework causing its deformation that occurs in a counterintuitive manner. Contrary to the expected gradual swelling during adsorption, we observe non-monotonic deformation, characterized by sharp contraction during the pore filling, followed by partial expansion. During the pore-filling process, guest molecules engender softening of the host structure to a nearly 100% increase in compressibility. However, upon the pore filling and further densification of the adsorbed phase, the structure hardens and compressibility decreases. These findings are supported by quantitative agreement with adsorption experiments on IRMOF-1 and are expected to be applicable to various degrees, to other MOFs and nanoporous materials.