Zeolite-like performance for xylene isomer purification using polymer-derived carbon membranes

Abstract

Significance Xylenes are essential feedstocks for manufacturing packaging materials, versatile chemicals, industrial solvents, etc. The purification of xylene isomers is one of the most important yet energy-intensive organic mixture separations in the chemical industry. We achieved the separation of xylene isomers using carbon molecular sieve (CMS) membranes derived from a spirobifluorene-based polymer of intrinsic microporosity (PIM-SBF), which could potentially reduce the energy consumption, carbon emissions, and equipment footprint. CMS membranes are solvent- and temperature-resistant materials that can withstand high transmembrane pressures when fabricated into the form of hollow fibers. The new CMS membrane produced here shows competitive performance with state-of-the-art zeolites under high xylene loadings, and its development has provided fundamental insight and guidance into the manipulation of CMS pore structure. Polymers of intrinsic microporosity (PIMs) have been used as precursors for the fabrication of porous carbon molecular sieve (CMS) membranes. PIM-1, a prototypical PIM material, uses a fused-ring structure to increase chain rigidity between spirobisindane repeat units. These two factors inhibit effective chain packing, thus resulting in high free volume within the membrane. However, a decrease of pore size and porosity was observed after pyrolytic conversion of PIM-1 to CMS membranes, attributed to the destruction of the spirocenter, which results in the “flattening” of the polymer backbone and graphite-like stacking of carbonaceous strands. Here, a spirobifluorene-based polymer of intrinsic microporosity (PIM-SBF) was synthesized and used to fabricate CMS membranes that showed significant increases in p-xylene permeability (approximately four times), with little loss in p-xylene/o-xylene selectivity (13.4 versus 14.7) for equimolar xylene vapor separations when compared to PIM-1–derived CMS membranes. This work suggests that it is feasible to fabricate such highly microporous CMS membranes with performances that exceed current state-of-the-art zeolites at high xylene loadings.