Coulombic effect on permeation of CO2 in metal-organic framework membranes

Comparing the aperture size of a membrane to the kinetic diameters of gases to be separated has long been regarded as the rule of thumb for identifying potential membrane materials. This method based upon molecular sieving could however fail for CO2 separations using metal-organic frameworks (MOFs). While the challenge may be attributed to the breathing effect or linker rotation for some highly flexible MOFs, the adsorbent-adsorbate Coulombic interaction can also significantly influence the diffusion of CO2 in MOFs. This work is aimed to investigate how the Coulombic interaction can alter the free energy profile of CO2 in a MOF channel. MOFs with one-dimensional (1D) channels (98 structures) in the orthorhombic system are probed in this in silico study. Coulombic interactions are identified to cause a notable offset between the transport bottleneck (the spot where the free energy reaches maximum) and the topological bottleneck (the spot where the pore limiting diameter is located). As a result, the transport bottleneck of CO2 and that of H-2 or CH4 are found to be at different locations for a decent portion of studied MOF structures. This makes the pore limiting diameter of a MOF fail to serve as an effective indicator for molecular sieving. This study also investigates the Coulombic effect on the permeability of CO2 in MOFs, and suggests that a highly CO2 permeable and selective MOF would have strong CO2 adsorption sites that create an energetically homogeneous channel for transport. Finally, we also discuss the Coulombic effect on the diffusion of CO2 in MOFs when their flexibility is considered.