Triptycene dimethyl-bridgehead dianhydride-based intrinsically microporous hydroxyl-functionalized polyimide for natural gas upgrading

The synthesis and gas permeation properties of a high-performance hydroxyl-functionalized PIM-polyimide (TDA1-APAF) prepared from a novel 9,10-dimethyl-2,3,6,7-triptycene tetracarboxylic dianhydride (TDA1) and a commercially available 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) diamine monomer are reported. The microporous polymer had a BET surface area based on nitrogen adsorption of 260 m2 g−1. A freshly prepared sample exhibited excellent gas permeation properties: (i) CO2 permeability of 40 Barrer coupled with a CO2/CH4 selectivity of 55 and (ii) H2 permeability of 94 Barrer with a H2/CH4 selectivity of 129. Physical aging over 250 days resulted in significantly enhanced CO2/CH4 and H2/CH4 selectivities of 75 and 183, respectively with only ~25% loss in CO2 and H2 permeability. Aged TDA1-APAF exhibited 5-fold higher pure-gas CO2 permeability (30 Barrer) and two-fold higher CO2/CH4 permselectivity over conventional dense cellulose triacetate membranes at 2 bar. In addition, TDA1-APAF polyimide had a N2/CH4 selectivity of 2.3, thereby making it potentially possible to bring natural gas with low, but unacceptable nitrogen content to pipeline specification. Gas mixture permeation experiments with a 1:1 CO2/CH4 feed mixture demonstrated higher mixed- than pure-gas selectivity and plasticization resistance up to 30 bar. These results suggest that intrinsically microporous hydroxyl-functionalized triptycene-based polyimides are promising candidate membrane materials for removal of CO2 from natural gas and hydrogen purification in petrochemical refinery applications.