Correlation between structure and dynamics of CO2 confined in Mg-MOF-74 and the role of inter-crystalline space: A molecular dynamics simulation study

Mg-MOF-74 is a metal-organic framework (MOF) with a high capacity for CO2 adsorption. Given the importance of CO2 confinement in Mg-MOF-74 for capture and storage applications, it is important to understand CO2 behavior in Mg-MOF-74. While most molecular simulation studies use ideal single crystal models of nano-porous substrates, the existence of inter-crystalline space is shown to have profound effects on the adsorption, structure and dynamics of the adsorbed fluid. Here we report a molecular dynamics simulation study at 300 K, of CO2 confined in Mg-MOF-74 with inter-crystalline spacing of different widths. Six strong sites of CO2 adsorption are found at the periphery of the Mg-MOF-74 pores in addition to a relatively weak adsorption at the pore center. On insertion of inter-crystalline spacing, additional sites of strong adsorption are seen close to the pore opening, which delocalize as the inter-crystalline space is widened while adsorption in the pore center grows. This redistribution has important implications for the dynamics of CO2. With wider inter-crystalline spacing, anom-alous loading dependence of translational diffusivity is observed. In general, inter-crystalline spacing suppresses the rotational motion of CO2. Translational motion, however, exhibits an enhancement on the introduction of inter-crystalline space but gets suppressed when this space is widened.