6FDA-polyimide thin-film composite hollow fiber membranes for hydrofluorocarbons and CO2 separations

Herein, high molecular weight fluorinated aromatic polyimides were assessed for the first time to recover difluoromethane (R-32) from its blends with other hydrofluorocarbons and hydrofluoroolefins (R-134a: 1,1,1,2-tetrafluoroethane, R-125: pentafluoroethane, and R-1234yf: 2,3,3,3-tetrafluoropropene). First, a screening was performed with thick flat membranes made of the 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and three different amines: 2,2′-bis(4-aminophenyl)hexafluoropropane (6FpDA), 2,4,6-trimethyl-m-phenylenediamine (TMPD), and 2,3,5,6-tetramethyl-1,3-phenyldiamine (durene). As a result, 6FDA-TMPD was selected for the fabrication of defect-free hollow fiber-thin film composite membranes (HF-TFCM) due to its superior performance for the separation of R-32. These HF-TFCMs showed excellent separation performance to reclaim high-purity R-32 (permeate concentration >99 vol%) from the commercial binary blends R-410A and R-454B (a mixture of R-32 and R-1234yf). In addition, we report for the first time the membrane recovery of R-32 from the ternary blend R-407C (R-32/R-134a/R-125 38.2:43.8:18 vol%). Eventually, the separation of CO2 from synthetic gas mixtures of CO2/CH4 (50:50 vol%) and CO2/N2 (15:85 vol%) was performed for benchmarking, showing that the prepared HF-TFCM kept the separation performance of the 6FDA-TMPD thick membranes.