Gas Separation Using Cross-Linkable 6FDA-Based Co-Polyimides Grafted With Cyclodextrins

Natural gas purification and olefin/paraffin separation are important processes in petrochemical industries and petroleum refining. CO₂/CH₄ separation is very important for natural gas sweetening and CO₂ captures and olefins are used for the production of esters, alcohol, polymers, and other synthesis intermediates. Using a cross-linkable co-polyimide (6FDA-Durene/DABA (9/1)), we have developed new flexible and high- performance gas separation membranes. Both membrane permeability and plasticization resistance can enhance simultaneously by grafting various sizes of cyclodextrin to the polyimide matrix and then decomposing them at elevated temperatures. The gas permeability of thermally treated pristine polyimide (referred as the original PI) and CD grafted co-polyimide (referred as PI-g-CDs for 200 and 300 °C and PPM-CDs for 350, 400, and 425 °C) has been determined using O₂, N₂, CO₂, CH₄, C₃H₆, and C₃H₈ at 35 °C. The permeability of all gases increases with an increase in thermal treatment temperature from 200 to 425 °C. However, permeability increases more for those grafted with bigger size CD. Interestingly, the permeability ratios of PPM-α-CD to PPM-γ-CD and PPM-β-CD to PPM-γ-CD at 400 and 425 °C are around 0.6 and 0.8, respectively. These numbers are almost the same as the cavity diameter ratios of α-CD to γ-CD and β-CD to γ-CD. Clearly, the bigger CD creates the bigger micro-pores. Permselectivity decreases first with an increase in thermal treatment temperature up to 350 °C and then increases. Permselectivity of thermally treated CD grafted co-polyimide membranes is also slightly higher than that of the original PI due to higher degrees of cross-linking in CD grafted co-polyimide membranes.  In addition, for co-polyimide membranes grafted by CDs, the higher thermal treatment temperature results in membranes with the better plasticization resistance to CO₂ and the better separation performance for 50:50 CO₂/CH₄ mixed gases. The best result for pure gas tests is achieved for PPM-γ-CD-425 that has a CO₂ permeability of 4211 Barrers with a CO₂/CH₄ ideal selectivity of 22.44 and a C₃H₆ permeability of 521 Barrers with a C₃H₆/C₃H₈ ideal selectivity of 18.09. It can also resist against CO₂ plasticization until 30 atm. The CO₂ permeability drops slightly to 3976 Barrers with almost the same CO₂/CH₄ selectivity of 22.84 in mixed gas tests.