The significance of entropic selectivity in carbon molecular sieve membranes derived from 6FDA/DETDA:DABA(3:2) polyimide

This study reports and analyzes gas transport and sorption properties between 35 and 50 °C for polymeric and carbon molecular sieve (CMS) membranes derived from an in-house synthesized polyimide referred to as 6FDA/DETDA:DABA(3:2). The analysis of activation energies of permeation and diffusion and heats of sorption for CO2, CH4, O2, and N2 in these membrane materials provides insights regarding the permeability and selectivity changes at increasing pyrolysis temperature between 550 °C and 800 °C. The diffusion selectivity is factored further into “energetic” and “entropic” selectivity contributions to show that higher permselectivity of CMS membranes compared to polymeric membrane arises from a larger than unity entropic selectivity. CMS membranes pyrolyzed at 800 °C are also shown to have significantly elevated entropic selectivity compared with membranes pyrolyzed at 550 °C. Analysis of the selectivity factors provides fundamental insights into the importance of entropic factors as tools to tailor membrane performance.