Molecular Dynamics Investigation into the High Permeability and High Selectivity of Nano-Porous Polyimide Membranes for the “Green” Separation of Natural Gas

Abstract

Molecular modeling techniques were used to investigate the permeability and selectivity of various natural gas components through nano-porous polyimide membranes for the environmentally-friendly "green" separation of natural gas. The polyimide membranes showed the ability of creating nano-scale channels within the polymeric matrix during the molecular mobility of the polymeric chains through which specific gas molecules can penetrate the membrane surface. The four natural gas components investigated in this study were methane, ethane, propane and butane, all hydrocarbon materials of similar basic chemical structures. The self-diffusion coefficients of the gas molecules were thus used to express the permeability of the various gases through the membranes since the solubility of the gas molecules in the polymeric substances were assumed to be constant. Methane showed a noticeably high self-diffusion coefficient calculated from the application of Einstein relation to the generated molecular dynamics trajectories. All other gases had similar values for the self-diffusion coefficients, which indicate the ability of the methane molecules to penetrate the polymeric membrane in a much larger speed due to a possible matching between the methane molecular size and the size of the interconnected nano-channels within the membranes. The results also showed that the polymer molecules had lower self-diffusion coefficients than the gas ones due to the large size of the polymeric segments. Other structural parameters such as the radial distribution function in direct relationship to the local packing of the polymeric segments and penetrant molecules are also illustrated.