Correlation between structure and dynamics of CO2 confined in Mg-MOF-74 and the role of inter-crystalline space: A molecular dynamics simulation study
Abstract
Mg-MOF-74 is a metal-organic framework (MOF) that exhibits 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 the structural and dynamical behavior of CO2 in Mg-MOF-74 pores. While most molecular simulation studies use ideal single crystal models of nano-porous substrates, the existence of inter-crystalline space has been shown to have profound effects on the sorption, structure and dynamics of the adsorbed fluid. To address these issues, we report a molecular dynamics simulation study at 300 K, of CO2 confined in several models of Mg-MOF-74 with systematically inserted inter-crystalline spacing of different widths. Both structural and dynamical behavior of CO2 is studied in 5 models of Mg-MOF-74, each at 4 different loadings. Six strong sites of CO2 adsorption are found at the periphery of the pores of Mg-MOF-74 in addition to a relatively weak adsorption at the center of the pore. On insertion of inter-crystalline spacing, additional six sites of strong adsorption are seen in the inter-crystalline space close to the pore opening. These additional sites delocalize as the inter-crystalline space is widened and the population of guest molecules adsorbed at the pore center grows at the expense of peripheral population. This redistribution of guest molecules has important implications for their dynamics. While in the model without inter-crystalline space, translation motion is found to be slower at higher loadings, as wider inter-crystalline space is introduced, anomalous loading dependence of translational diffusivity is observed. In general, inserting inter-crystalline spacing is found to enhance both translational as well as rotational motion of the guest molecules. The results reported here provide valuable insight to carbon capture and storage.
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