Temperature dependence of gas transport and sorption in carbon molecular sieve membranes derived from four 6FDA based polyimides: Entropic selectivity evaluation

Abstract

Temperature dependences of gas transport and sorption properties on four previously isothermally studied 6FDA based carbon molecular sieve (CMS) membranes: 6FDA/DETDA, 6FDA:BPDA(1:1)/DETDA, 6FDA/DETDA:DABA(3:2) and 6FDA/1,5-ND:ODA(1:1), were examined over the temperature range from 35 °C to 50 °C. The permeability, sorption and diffusivity of these materials are reported, and activation energies of permeation and diffusion as well as heats of sorption for gases CO2, CH4, O2, and N2 of these materials are compared. Permselectivity of these materials at 35 °C was probed for three gas pairs: CO2/CH4, O2/N2, and N2/CH4, and results show that diffusion selectivity is the dominant factor in providing permselectivity. Diffusion selectivity can be factored further into an energetic selectivity and an entropic selectivity. The temperature dependence of both energetic and entropic selectivity and their contributions to overall diffusion selectivity are discussed in detail in this study. All four CMS membranes showed larger than unity entropic selectivity discriminations. 6FDA/DETDA derived CMS membrane, with the most compact structure, showed the largest entropic selectivity for CO2/CH4, O2/N2, and N2/CH4 as about 7.40, 4.24, and 2.17 respectively. Analysis of the selectivity factors provides insights into the effects of polymer precursor backbone structures on key transport and sorption properties in CMS materials.