Thermal Fluctuations and Framework Flexibility of IRMOF-1 upon CH4 and CO2 Adsorption

Flexibility of metal-organic frameworks (MOFs) plays an important role in their applications for adsorption separations, energy and gas storage, and drug delivery. Here, we demonstrate 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. We find that in contrast with expected gradual swelling during adsorption, framework deformation is non-monotonic with sharp contraction during the pore filling followed by partial expansion. As an important example, we study adsorption of CH4, and CO2 on iso-reticular 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. The effects or non-monotonic framework flexibility are confirmed by quantitative agreement with adsorption experiments and are expected to be characteristic to different degrees to other MOFs.