Fluorene-functionalized intrinsic microporous polyimide thermal reforming membrane [PIM-PI(TR) membrane] for biogas purification: High CO₂ permeability and CO₂/CH₄ selectivity exceeding the Robeson

The continuous increase in greenhouse gas emissions is having a significant impact on the global climate system. To address this issue, efforts worldwide are promoting the development of clean energy source, with methane-rich biogas being a primary focus. However, the dilution of biogas with CO₂ significantly reduces its calorific value. Therefore, efficient methane purification and resource utilization are urgently required. This study specifically developed a high-efficiency gas separation membrane suitable for large-scale industrial applications. By precisely separating CO₂ from biogas, it provides crucial technological support for enhancing biogas energy quality and promoting its industrialization. Specifically, using a conventional polyimide membrane (6FDA-APAF) as the template, a series of intrinsically microporous polyimide membranes (PIM-PI membranes) were innovatively synthesized by incorporating fluorene-containing microporous units—Namely, 9,9′-bis(4-aminophenyl)fluorene (FDA) or 9,9′-bis(4-amino-3-fluorophenyl)fluorene (FFDA). Subsequent thermal rearrangement (TR) treatment produced PIM-PI(TR) membranes. Both the introduction of fluorene groups and TR treatment increased the interchain spacing of the membranes, with the thermally rearranged PIM-PI(TR) membranes exhibiting larger d-spacings and broader micropore peaks. The 6FDA-FFDA (PIM-PI membrane) achieved a CO₂ permeability coefficient of 151.87 barrer, which is 9.9 times higher than that of the nonporous conventional polyimide membrane (6FDA-APAF, 15.4 barrer). The 6FDA-APAF/FDA (8,2) TR400 [PIM-PI(TR) membrane], subjected to thermal rearrangement, further increased its CO₂ permeability coefficient to 813.80 barrer, representing a 19-fold improvement over the untreated membrane. Simultaneously, this membrane exhibited a CO₂/CH₄ permeability selectivity factor as high as 39.03, surpassing the 2019 Robeson upper bound. Furthermore, both macro- and micro-characterizations reveal structurally uniform and intact membranes. With superior fabrication quality that lays a solid foundation for industrial applications. These results highlight the promising potential of TR-treated PIM-PI (TR) membranes containing fluorene-based pore-forming units for gas separation. This study also offers essential technical support for the industrial application of polyimide membranes in biogas purification