Ultrahighly permeable carbon molecular sieving membranes enabled by blocking the precursor polyimide molecules with 6FAP moieties

Background

Selecting appropriate polymer precursors is essential for developing high-performance carbon molecular sieve membranes (CMSMs) for gas separation, as precursor structure significantly impacts the resulting membrane's thermal stability, microstructure, and gas transport properties.

Methods

This study reports the synthesis of a novel polyimide for CMSMs fabrication, which features a main chain backbone of tetrahydrocyclobuta (1,2-c:3,4-c') difuran-1,3,4,6-tetraone-1,4-bis (4-amino-2-trifluoromethylphenoxy) benzene (CBDA-6FAPB), with 2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP) diamine as a block monomer via ternary co-polymerization. The thermal stability, surface functional groups, microstructure and morphology of the membranes were characterized by thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The effects of varying 6FAP proportions, and the permeation conditions on the thermal stability of polyimide, the microstructure and gas separation performance of CMSMs were systematically investigated.

Significant findings

Results show that increasing the proportion of 6FAP enhances the selectivity of CMSMs, with the gas permeability initially rising then decreasing. Exceptionally ultrahigh permeabilities are achieved, corresponding to 7690.0 Barrer (H₂), 2578.6 Barrer (CO₂) and 1466.5 Barrer (O₂), along with the selectivities of 31.0 (H₂/N₂), 10.4 (CO₂/N₂) and 5.9 (O₂/N₂), for CMSMs made by the proportion of 5% of 6FAP segments. In summary, the as-prepared CMSMs are excellent and promising with attractively commercial prospect.