Tröger's Base (TB)-containing polyimide membranes derived from bio-based dianhydrides for gas separations

Ultramicroporous polyimides obtained from bio-based anhydrides were synthesized for gas separation membranes. Two novel dianhydrides, 5,5'-(mesitylmethylene)bis(4-methylphthalic anhydride) (MMDA) and 5,5'-(9H-fluorene-9,9-diyl)bis(4-methylphthalic anhydride) (FDDA), were successfully synthesized from guaiacol, a lignin-based bio-material. Due to the fixed center of o-methyltriphenylmethane and o-methyldiphenylfluorene units, the bio-based dianhydrides had high energy barriers to rotation, resulting in highly rigid polymer chains. After polymerization with a Tröger's Base (TB)-containing diamine using a one-step imidization, two kinds of bio-based polyimides incorporating TB units (Bio-TBPI-1 and Bio-TBPI-2) were prepared with reasonable molecular weights and outstanding thermal properties. Bio-TBPIs demonstrated a microporous structure determined by high BET surface areas of ~500 m3 g−1, high fractional free volumes (FFVs) of ~0.2, and d-spacings of ~0.7 nm. In addition, Bio-TBPIs exhibited narrow cavity size distributions around 0.5 nm, and molecular sieving characteristics due to the ultramicroporosity. Bio-TBPI membranes displayed excellent gas separation performance, which was located near the upper bounds of important gas pairs such as H2/CH4, H2/N2, O2/N2, and CO2/CH4. The gas separation performance of Bio-TBPIs was better than that of corresponding TB-based polyimides containing commercial dianhydrides. The effect of physical aging on gas transport behaviors of Bio-TBPIs was monitored for 200 days, resulting in loss of permeability and enhanced selectivities.