Computational assessment of MOF membranes for CH4/H2 separations

Abstract

MOFs have received significant attention as gas separation membranes due to their wide range of pore sizes, permanent porosities and high surface areas. Thousands of MOFs have been reported to date. However, membrane performance of only a small number of MOFs has been experimentally reported since fabrication of thin-film MOF membranes is challenging. In this study, we used atomically-detailed simulations to assess membrane-based CH4/H-2 separation performances of 172 different MOF structures. Adsorption selectivity, diffusion selectivity, membrane selectivity and gas permeability of MOFs were calculated using atomically-detailed simulations to identify the most promising membrane materials. Our results show that a significant number of MOF membranes exhibits high CH4 selectivity over H-2 and a small number of MOF membranes exhibits mediocre H-2 selectivity over CH4. Gas permeabilities and selectivities of MOF membranes were compared with traditional membranes such as polymers and zeolites. Several MOFs were identified to exceed the upper bound established for polymeric membranes and many MOF membranes showed higher gas permeabilities and selectivities than zeolites LTA, ITQ-29 and MFI. We also carried out flexible molecular dynamics simulations to examine the effect of MOF's flexibility on the predicted membrane performance. Considering flexibility of the framework made a negligible effect on the gas permeability and selectivity of the material having large pores whereas more pronounced changes were seen in gas permeabilities of the material having narrow pores. The results of this computational study will be helpful to guide the experiments to the most promising MOF membranes for CH4/H-2 separations.