Carbon Molecular Sieve Membranes with a Rationally Designed Polymer Precursor for Improved Propane/Propylene Separation

Membrane separation of propane and propylene provides a more energy-efficient route than traditional separation methods, such as distillation and absorption. Carbon molecular sieve (CMS) membranes with excellent chemical and physical stability have great potential in this separation process. The chemical structure of the polymer precursor greatly influences the separation performance of the CMS membrane. In this study, a benzimidazole-based polyimide is designed to manufacture CMS membranes for enhanced propane and propylene separation. The heterocyclic benzimidazole ring provides a flat chain structure to construct ultramicropores. Moreover, metal ions are homogeneously embedded in the polyimide precursor via a strong coordination interaction with the benzimidazole moiety. The microporous structure of the CMS membrane is further tailored owing to the steric hindrance of metal ions, which makes it difficult for larger C3H8 to penetrate. The pore structure changes in pristine CMS and metal ion modified CMS (M-CMS) are analyzed by wide-angle X-ray diffraction and gas adsorption measurement. The gas permeation data reveals that the M-CMS membrane demonstrates an improved C3H6/C3H8 selectivity (49.3) much higher than pristine CMS (18.6) and its comprehensive separation performance surpasses most of the reported CMS membranes for C3H6/C3H8 separation. This study demonstrates an effective strategy for preparing CMS membranes with tailored micropores for efficient propane/propylene separation.