Two-dimensional molybdenum disulfide based membranes for ionic liquids separation

Abstract

Ionic liquids (ILs) are solvents with negligible volatility and corrosiveness that are non-flammable and thermally stable. These environmentally friendly properties allow ILs to be widely used in various applications. However, the high cost of ILs has been the major constraint that limits their widespread use in industry. In this paper, we report the application of a two-dimensional nanofiltration membrane, MoS2, to recover ILs from their aqueous solution since ILs are often diluted with water in many applications. MoS2 membranes from the common octahedral coordinated (1T) phase possess a laminar structure that allows water molecules to permeate through the interlayer nanochannels between nanosheets. Often the interlayer spacing is smaller than many ILs, enabling separation of water from ILs including 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and 1-hexyl-3-methylimidazolium chloride (HmimCl). In this paper we first demonstrated that the rejection rate of BmimBF4 and HmimCl from IL/water mixtures reached 40.8% and 86.2% respectively. More importantly, we found that IL rejection rate can be further increased by 25% for BmimBF4, 8% for HmimCl under higher operating pressures, indicating that the applied transmembrane pressure reduces the interlayer spacing of MoS2 membranes. This hypothesis was proved by our XRD characterization. Finally, the effects of hydrophilicity and charge of dopamine-modified 2D MoS2 membranes on the IL rejection were also investigated, revealing that increased surface hydrophilicity and more positively charged membrane surfaces enabled further increase in IL rejection.