Investigating the potential of MgMOF-74 membranes for CO 2 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 CO 2. The primary objective of the present communication is to investigate the performance of MgMOF-74 membranes in separating CO 2/H 2, CO 2/N 2, CO 2/CH 4, and CH 4/H 2 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 CO 2/H 2, CO 2/N 2, CO 2/CH 4, and CH 4/H 2 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 CO 2/H 2, and CO 2/N 2 mixtures are higher than those reported in the published literature with zeolite membranes.