Investigating the potential of MgMOF-74 membranes for CO₂ capture

Abstract

MgMOF-74 is a metal-organic framework (MOF) with exposed metal cation sites that has one-dimensional 1.1 nm sized hexagonal-shaped channels. On the basis of information available in the published literature, it appears that MgMOF-74 has significant advantages over other MOFs, with respect to its uptake capacity for CO₂. The primary objective of the present communication is to investigate the performance of MgMOF-74 membranes in separating CO₂/H₂, CO₂/N₂, CO₂/CH₄, and CH₄/H₂ mixtures, that are important in carbon capture. To achieve this objective all the parameters required for modeling MgMOF-74 membrane permeation were obtained using molecular simulations. Specifically, Configurational-Bias Monte Carlo (CBMC) simulations were used to determine pure component adsorption isotherms, and isosteric heats of adsorption. Molecular dynamics (MD) simulations were performed to determine the self-diffusivities, D_i,self, and the Maxwell-Stefan (M-S) diffusivities, Ð_i, of guest molecules. The MD simulations show that the zero-loading diffusivity Ð_i(0) is consistently lower, by up to a factor of 10, than the values of the Knudsen diffusivity, D_i,Kn. The ratio Ð_i(0)/D_i,Kn is found to correlate with the isosteric heat of adsorption, which in turn is a reflection of the binding energy for adsorption at the pore walls. The stronger the binding energy, the lower is the ratio Ð_i(0)/D_i,Kn. Using the Maxwell-Stefan formulation for binary mixture permeation, along with data inputs from CBMC and MD simulations, the permeation selectivities for CO₂/H₂, CO₂/N₂, CO₂/CH₄, and CH₄/H₂ mixtures were determined for a range of upstream pressures. The model calculations show that increased upstream pressures lead to significant enhancement in permeation selectivities; this enhancement is directly traceable to diffusional correlations within the 1D channels. Such correlations have the effect of slowing-down the more mobile partner species in the mixtures. MgMOF-74 membrane permeation selectivities for CO₂/H₂, and CO₂/N₂ mixtures are higher than those reported in the published literature with zeolite membranes.