Highly permeable polyimides incorporating Tröger's base (TB) units for gas separation membranes

Two novel polyimides incorporating Tröger's Base (TB) units, Bio-PITB-1 and Bio-PITB-2, were facilely synthesized from lignin-based imide-containing diamines via TB polymerization. The polymers exhibited excellent thermal stability, finely tuned microporous structures with high BET surface areas (~569 m2 g−1), high fractional free volumes (~0.224), and appropriate inter-chain distances (~0.68 and ~0.38 nm). Mechanically tough Bio-PITB membranes were successfully achieved due to their high molecular weights (~1.24 × 105 g mol−1) and good solubility in organic solvents. Bio-PITBs presented superior gas transport properties with improved gas permeability compared to the previously reported PI-TBs as well as Bio-TBPIs prepared from the same lignin-based dianhydrides but different synthetic routes. As a result, Bio-PITBs located near the 2008 upper bounds in H2/CH4, H2/N2, and CO2/CH4 gas pairs with H2 and CO2 permeabilities over 1000 Barrer. Furthermore, the effects of membrane drying protocols and physical aging on gas transport behaviors of Bio-PITBs were investigated.