Exploratory development of dual-layer carbon–zeolite nanocomposite hollow fiber membranes with high performance for oxygen enrichment and natural gas separation

A type of novel precursor, namely dual-layer polyethersulfone (PES)-zeolite beta/BTDA-TDI/MDI co-polyimide (P84) composite hollow fibers, was applied to fabricate the dual-layer carbon–zeolite nanocomposite hollow fiber membranes through pyrolysis in this work. After pyrolysis at 800 °C, these newly developed nanocomposite hollow fibers exhibit a significantly enhanced O2/N2 and CO2/CH4 selectivity of 11.3 and 152, respectively, in the pure gas measurement, and meanwhile, they also show a comparable CO2/CH4 selectivity of 140 in the mixed gas measurement. TGA curves indicate that the presence of zeolite beta may assist the formation of carbon–zeolite nanocomposite structure in the outer layer and both outer layer and inner layer inside the dual-layer hollow fibers may experience a delayed decomposition pattern during pyrolysis compared with their corresponding single-layer membranes. After pyrolysis at 800 °C, X-ray diffraction (XRD) patterns suggest that the monolayer graphite sheets have been formed in the outer-layer carbon matrix with a d-space of 3.73 Å, scanning electron microscope (SEM) images reveal a uniform and compact dispersion of zeolite particles in the outer-layer carbon matrix, and both XRD patterns and energy dispersion of X-ray (EDX) data demonstrate the integrity of zeolite beta by an analysis of the crystalline structure and elemental composition. These above-mentioned characterizations support a conjecture from various aspects that a combining effect of carbon molecular sieve (CMS) and zeolite materials results in a remarkable improvement of dual-layer hollow fiber membranes in the performance of oxygen enrichment and natural gas separation through the molecular sieving mechanism.