Dual-Force Zone Nonequilibrium Molecular Dynamics Simulations on Nanoporous Metal–Organic Framework Membranes for Separation of H2/CH4 Mixtures

Recently, metal–organic frameworks (MOFs) have emerged as powerful nanoporous materials for selective separation of complex gas mixtures because of possible modifications of their structural characteristics. Herein, to investigate the effect of MOF functionalization on their gas separation performance at nanoscale, we have performed separation of H2/CH4 mixtures through functionalized and unfunctionalized IRMOF-1 membranes. A molecular-level understanding is developed by employing a dual-force zone nonequilibrium molecular dynamics (DFZ-NEMD) simulation method. The DFZ-NEMD method was validated by performing H2/CH4 separation through IRMOF-1 and comparing these results with experimental and other computational results reported in the literature. The linkers in different directions with respect to the gas flow were functionalized with acetamide, urea, and methyl groups to explore the effect of structural and chemical features of functional groups on gas separation performance. These results show that both the size and chemical nature of a functional group can play an important role in determining the size of the MOF nanopores and thus the selectivity and permeability of an MOF membrane. Moreover, the carbon atoms on the six-membered ring of linkers of IRMOF-1 were systematically functionalized with urea to understand the effect of the position of functional groups on the gas separation performance and mechanism. We show that by changing only this position, one could generate MOF pore entrances with a diverse density of functional groups, generating MOF membranes with different selectivities and permeabilities. Our gas separation results show that the change in this density of functional groups at the pore entrance can further result in different diffusion mechanisms, including Knudsen-type diffusion, adsorption–diffusion, and molecular sieving. Overall, molecular-level insights from the results of this research can pave a path for the synthesis of functionalized MOFs to control the separation of different gas molecules and hydrocarbons from their mixtures.