Novel hexa-fluorinated intrinsically porous polyimide membranes for the desalination of high saline water by air-gap membrane distillation

Hexa-fluorinated porous membranes were prepared from 4,4′-(hexafluoroisopropylidene) diphtalic anhydride (6FDA)- 2,3,5,6- tetramethyl-m-phenylenediamine (TMPD) (6FDA-TMPD), which prepared by a polycondensation reaction between the dianhydride 6FDA and the diamine TMPD at 200 °C, by nonsolvent induced phase inversion process for the desalination of highly saline water (70,000 mg L⁻¹) via membrane distillation (MD) technology. Here, for the first time, we demonstrate the significance of the intrinsic porosity of 6FDA-TMPD polyimide membranes, that not only result in an excellent performance in MD without the addition of porogen chemicals but also outperform other membranes including commercially available membranes. The membrane characteristics were optimized by varying the coagulation bath, dope solution concentration, and thickness. Notably, the developed membranes possess high porosity which can be attributed to the high BET surface area (550 m² g⁻¹) of 6FDA-TMPD. Membranes developed from an ethanol-based coagulation bath resulted in spongy-like morphology and demonstrated excellent desalination performance with a high flux (∼ 18.20 kg m⁻² h⁻¹) and stable salt rejection (> 99.96 %). The reported fabrication approach can be used to delineate the key design parameters for the fabrication of highly efficient intrinsically porous polyimide-based membranes which may pave the way for the practical application of MD for the desalination of highly saline water. Graphical abstract.