Synthesis and gas separation performance of intrinsically microporous polyimides derived from sterically hindered binaphthalenetetracarboxylic dianhydride

Abstract

A novel dianhydride, 3,3′-di-tert-butyl-2,2′-dimethoxy-[1,1′-binaphthalene]-6,6′,7,7′-tetracarboxylic dianhydride (TNTDA), was prepared via a multi-step process. A model compound made using TNTDA and aniline has a dihedral angle of 75° between the two naphthalimide rings, evidenced by single crystal X-ray diffraction data. High temperature polycondensation of TNTDA with four different diamines afforded four intrinsically microporous polyimides (PIM-PIs). The fractional free volumes (FFV), surface areas, and average inter-chain distances of the polyimides were 0.181–0.214, 379–564 m² g⁻¹, and 0.635–0.682 nm, respectively. Furthermore, the rigid, bulky, and contorted structures of the PIM-PIs led to the formation of roughly 0.6 nm ultramicropores and roughly 1.2 nm larger micropores. The ultramicropores can improve the gas size-sieving capability of the polyimides, while the larger micropores can facilitate high permeability. Consequently, the TNTDA-based PIM-PIs showed an improved gas separation performance compared to polymers made using the benchmark monomer, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride. In particular, polymers from TNTDA and 2,6-diaminotriptycene (DAT) exhibited the best performance, with CO₂/CH₄ and O₂/N₂ separation metrics approaching or surpassing the 2008 Robeson upper bounds, because of the presence of additional ultramicropores related to the internal free volume of the triptycene moiety.