Natural Gas Purification and Olefin/Paraffin Separation Using Thermal Cross-Linkable Co-Polyimide/ZIF-8 Mixed Matrix Membranes

Cross-linkable mixed matrix membranes represent an attractive technology that promises both outstanding separation properties and swelling resistance for the natural gas purification and olefin/paraffin separation. Using three 6FDA-based polyimides (6FDA-Durene, 6FDA-Durene/DABA (9/1), 6FDA-Durene/DABA (7/3)) and nano-size zeolitic imidazolate framework-8 (ZIF-8), we have fabricated mixed matrix membranes (MMMs) with uniform morphology comprising ZIF-8 as high as 40 wt% loading by directly mixing as-synthesized ZIF-8 suspension into the polymer solution. Permeability of all gases increases rapidly with an increase in ZIF-8 loading. However, the addition of ZIF-8 nano-particles into the polymer matrix increases the ideal CO₂/CH₄ selectivity of only 6.87%, while the ideal C₃H₆/C₃H₈ selectivity improves 134% from 11.68 to 27.38 for the MMM made of 6FDA-Durene/DABA (9/1) and 40 wt% ZIF-8. Experimental data show that the plasticization resistance and gas pair selectivity of MMMs are strongly dependent on the amount of cross-linkable moiety and annealing temperature. MMMs made of 6FDA-Durene do not show considerable improvements on resistance against CO₂-induced plasticization after annealing at 200 – 400 °C, while MMMs synthesized from cross-linkable co-polyimides (6FDA-Durene/DABA (9/1) and 6FDA-Durene/DABA (7/3)) show significant enhancements in CO₂/CH₄ and C₃H₆/C₃H₈ selectivity as well as plasticization suppression characteristics up to a CO₂ pressure of 30 atm after annealing at 400 ºC due to the cross-linking reaction of the carboxyl acid (COOH) in the DABA moiety. The MMM made of 6FDA-Durene/DABA (9/1) and 40 wt% ZIF-8 possess a notable ideal C₃H₆/C₃H₈ selectivity of 27.38 and a remarkable C₃H₆ permeability of 47.3 Barrer. After thermally annealed at 400 ºC, the MMM made of 6FDA-Durene/DABA (9/1) and 20 wt% ZIF-8 shows a CO₂/CH₄ selectivity of 19.61 and an impressive CO₂ permeability 728 Barrer in mixed gas tests. The newly developed MMMs may have great potential for industrial nature gas purification and C₃H₆/C₃H₈ separation.