Natural gas purification and olefin/paraffin separation using cross-linkable 6FDA-Durene/DABA co-polyimides grafted with α, β, and γ-cyclodextrin

Using a cross-linkable co-polyimide (6FDA-Durene/DABA (9/1)), we have developed new flexible and high-performance gas separation membranes that can enhance both membrane permeability and plasticization resistance 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 partially pyrolyzed membranes (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. Permeability of the original PI thermally treated at 425 °C is about 4–6 times higher than that treated at 200 °C. The permeability increase jumps to 8–10 times for PPM-α-CD and 15–17 times for PPM-γ-CD due to CD decomposition at high temperatures and bigger CD creating bigger micro-pores. 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. This membrane 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.