Fabrication of high-flux defect-free hollow fiber membranes derived from a phenolphthalein-based copolyimide for gas separation

In the realm of gas purifications, hollow fiber membranes have gained significant prominence due to their unique advantages. However, achieving defect-free membranes poses a considerable challenge. In this study, we addressed this challenge with a novel copolymer, 6FDA-DAM:DAP(2:1), comprising 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,4.6-Trimethy-m-phenylenediamine (DAM), and a phenolphthalein-derived diamine of 3,3′-diaminophenolphthalein (DAP). We found that defect-free hollow fiber membranes could be prepared when 20 wt% or more ethanol was added in the polymer dope solution, but this led to the formation of oval hollow fibers that would be flattened under high pressure. We solved this limitation by lowering ethanol content to 15 wt%, adding 2 wt% LiNO3, and increasing the air–gap distance. These measures reduced the difference in phase inversion rates between the surface region and the bulk phase of the nascent hollow fiber. The CO2 permeance of this defect-free hollow fiber membrane reached 282 GPU with a CO2/CH4 selectivity of 50.2. Highly permeable defective hollow fiber membranes were also developed using a polymer dope without LiNO3 and a low THF content of 5 wt%. After coated by silicone rubber, CO2 permeance of the membrane reached 554 GPU with a CO2/CH4 selectivity of 45.0. In mixed gas tests, the two membranes exhibited O2 permeances of approximately 43.3 and 75.3 GPU, with O2/N2 selectivities of 5.5 and 5.4, respectively. Their CO2 permeances were 281 and 465 GPU, with CO2/CH4 selectivities of 45 and 41, CO2/N2 selectivities of 35.7 and 33.3, respectively. These separation performances were among the best of state-of-art polymeric hollow fiber membranes and demonstrated great potential for gas separation applications such as natural gas sweetening, flue gas treatment, and air separation.