Reverse osmosis separation of particular organic solvent mixtures by a perfluorodioxole copolymer membrane

Reverse osmosis (RO) separation of organic solvent mixtures where solvent molecular weights are < 100 Da is challenging especially for powerful solvents that swell most uncrosslinked polymers. To avoid polymer swelling by solvents, a particular perfluoropolymer, perfluoro-2,2-dimethyl-1,3-dioxole copolymerized with tetrafluoroethylene, (PDD-TFE), designated CMS-7, was studied. This amorphous glassy extremely hydrophobic copolymer has a very high free volume (FV) fraction. The maximum dimension of FV regions is less than similar to 0.65 nm allowing only single solvent molecule permeation. Further, interactions between polarity, dimensions and shapes of solvent molecules with those of polymer FV elements can lead to extraordinarily selective permeation. RO separation of the following binary systems through a thin 1.67 mu m film of this polymer supported on an e-PTFE support was studied over 1000-3500 kPa feed pressure: N-Methyl-2-pyrrolidone (NMP)-toluene; dimethylformamide (DMF)-toluene; dimethylsulfoxide (DMSO)-toluene; NMP-methanol; n-butanol-ethanol. Pure toluene appeared as permeate for three polar aprotic-aromatic systems from highly toluene-rich feeds; the membrane rejected polar molecules having dimensions similar to those of toluene. High osmotic pressure of the feed mixtures employed vis-a-vis feed pressures used reduced toluene permeation flux and the range of separable feed mixtures. Pure methanol permeate was obtained from particular polar aprotic-polar protic NMP-methanol mixtures due to significantly smaller methanol dimensions. Pure ethanol permeate was also similarly obtained from a particular mixture with n-butanol, a case of polar protic1-polar protic2 system where ethanol molecules were smaller. Such novel and clean-cut pressure-driven separations may be due to various combinations of the dimensions of the solvent molecules and the polymer FV regions, high polymer hydrophobicity, polarity of the aprotic/protic solvents and very low swelling of the polymer. Solvent sorption studies of dense polymers provide an insight into swelling of the polymer by the solvent and potential solvent-specific interactions. Sorption studies using 25 mu m polymer samples for polar aprotic solvents, NMP, DMF, DMSO, yielded very low levels of sorption, 0.4-0.9 wt%; those for toluene and methanol were already known to be only 1.34 and 1.2 wt% respectively. Permeation behaviors of other solvent mixtures, toluene-n-heptane, NMP-tetrahydrofuran, methanol-water, ethanol-water as well as individual phases of the immiscible mixture of NMP and the nonpolar solvent n-heptane were also studied. To improve understanding, permeation of pure water in reverse osmosis mode was also investigated.