Interfacial Engineering of MOF-Based Mixed Matrix Membrane through Atomistic Simulations

We engineer the interfacial characteristics of a ZIF-8-6FDA-durene polyimide (PI) polymer mixed matrix membrane in silico by using an ionic liquid (IL) at the polymer-ZIF interface for improved CO2/CH4 pure-compo-nent perm-selectivity as determined by equilibrium molecular dynamics simulations. Incorporation of ZIF-8 into PI results in the formation of a low-density polymer region near the surface having thickness around 7-9 angstrom and larger pores and in which CO2 diffuses an order of magnitude faster, while CH4 diffuses 2 orders of magnitude faster than in the bulk polymer. This leads to a 3 -fold increase in CO2 permeability in the ZIF-8-6FDA-durene composite compared to that in the neat polymer membrane, albeit with reduced CO2/CH4 perm-selectivity due to the presence of larger voids. Interestingly, inclusion of ZIF-8 in the PI polymer leads to improved plasticization resistance against CO2 due to reduction in the degrees of freedom over which the polymer can swell in the presence of the solid surface. To promote compatibility between 6FDA-durene polymer and ZIF-8, an ionic liquid (BMIM-BF4) having favorable interactions with the polymer as well as ZIF-8 is identified. The absence of a leaky interface following incorporation of IL -modified ZIF-8 crystals in PI results in a lower gas diffusion coefficient for both gases than that in the composite system with unmodified crystals, with the diffusivity of CH4 reducing significantly more than that of CO2, without significantly affecting the solubility. Thus, a pronounced increase in CO2/CH4 perm-selectivity with a small decrease in CO2 permeability is observed in the composite membrane having IL at the polymer-ZIF-8 interface, leading to performance well above the Robeson upper bound.