  Mg-MOF-74 is a metal-organic framework (MOF) that exhibits a high capacity
for CO$_2$ adsorption. Given the importance of CO$_2$ confinement in Mg-MOF-74
for capture and storage applications, it is important to understand the
structural and dynamical behavior of CO$_2$ 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 CO$_2$ confined in several models of Mg-MOF-74 with systematically
inserted inter-crystalline spacing of different widths. Both structural and
dynamical behavior of CO$_2$ is studied in 5 models of Mg-MOF-74, each at 4
different loadings. Six strong sites of CO$_2$ 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.
