Dynamic structural and microstructural responses of a metal–organic framework type material to carbon dioxide under dual gas flow and supercritical conditions

The structural and microstructural responses of a model metal–organic framework material, Ni(3-methyl-4,4′-bipyridine)[Ni(CN)₄] (Ni-BpyMe or PICNIC-21), to CO₂ adsorption and desorption are reported for in situ small-angle X-ray scattering and X-ray diffraction measurements under different gas pressure conditions for two technologically important cases. These conditions are single or dual gas flow (CO₂ with N₂, CH₄ or H₂ at sub-critical CO₂ partial pressures and ambient temperatures) and supercritical CO₂ (with static pressures and temperatures adjusted to explore the gas, liquid and supercritical fluid regimes on the CO₂ phase diagram). The experimental results are compared with density functional theory calculations that seek to predict where CO₂ and other gas molecules are accommodated within the sorbent structure as a function of gas pressure conditions, and hence the degree of swelling and contraction in the associated structure spacings and void spaces. These predictions illustrate the insights that can be gained concerning how such sorbents can be designed or modified to optimize the desired gas sorption properties relevant to enhanced gas recovery or to addressing carbon dioxide reduction through carbon mitigation, or even direct air capture of CO₂.