Evidence of Entropic Selectivity of Xylene Isomers in Carbon Membrane

The purification of benzene derivatives, particularly xylene isomers, is one of the most important organic mixture separations practiced in industry. The separation of xylene isomers is especially challenging due to the similarity of their physical properties, such as kinetic diameter, molecular weight and boiling point. Carbon molecular sieve (CMS) membranes are promising materials capable of performing such challenging solvent separations. Indeed, CMS materials possess excellent thermal and chemical stability, even in aggressive solvent environments. Xylene isomer transport and sorption properties in polymer of intrinsic microporosity-1 (PIM-1) derived CMS materials were examined over a temperature ranging from 35 to 55 ^oC. The diffusivity, sorption coefficient and permeability under different operating conditions are reported. Diffusion and permeation activation energies as well as apparent heats of sorption for the p-xylene and o-xylene in the CMS are compared. The results showed that diffusive selectivity was the dominant factor in contributing to the permselectivity. Moreover, the contributions of “energetic” and “entropic” selectivity to the diffusive selectivity are studied in detail and reveal that the entropic factors dominate the xylene selection mechanism. Overall, this study shows that the rigid PIM-1 derived carbon membrane provide a strong molecular sieving effect that distinguishes between p-xylene and o-xylene based on molecular shape.