Influence of chain length on structural properties of carbon molecular sieving membranes and their effects on CO[formula omitted], CH[formula omitted] and N[formula omitted] adsorption: A molecular simulation study

Abstract

Carbon molecular sieving membranes (CMSM) derived from 6FDA/BPDA-DAM polymer precursors (6F-CMSM) show excellent adsorption and selectivity towards CO2. Using molecular dynamics (MD) simulations, we investigate how the length of individual carbon chains determines the morphologies emerging from the same monomers. Smaller chains produce densely packed structures, whereas longer chains arrange in layers, creating larger voids comprising both micropores and ultra-micropores. The pure gas adsorption isotherms of CO2, CH4, and N2 inside various 6F-CMSM morphologies, obtained from MD, provide theoretical insights into most feasible structures formed in experiments, thereby providing an estimate of the corresponding 6F-CMSM density. Using grand canonical Monte Carlo simulations, we observe that high isosteric heats of adsorption for CO2 leads to enhanced adsorption capacity and high sorption selectivity from binary mixtures of these gases. Studying the asphericity of carbon chains, we report the existence of an optimum chain length of highest curvature, leading to low density, high porosity, and high CO2 adsorption capacities that compare well with available experimental data, without compromising sorption selectivity of 6F-CMSM. This work provides a theoretical basis of how slight modifications of the physical arrangements of chemical components lead to significant changes in porosity and gas adsorption capacities of molecular sieving membranes.