A study of fluid properties and microfiltration characteristics of room temperature ionic liquids [C 10-min][NTf 2] and N 8881[NTf 2] and their polar solvent mixtures

Abstract

Room temperature ionic liquids (RTILs) are non-volatile and exhibit distinctive fluid properties with high viscosity typically 2–3 orders of magnitude higher than water, rendering their separation, recovery and reuse from some reaction mixtures a difficult and highly challenging task for conventional separation processes. This work investigates the potential application of microfiltration process in ionic liquid separation based on experimental studies of fluid properties and filtration characteristics of two ionic liquids C 10-min[NTf 2] and N 8881[NTf 2]. It was found that direct filtration of the pure ionic liquids through Nuclepore microfiltration membranes yielded very low permeation rate, a direct consequence of exceedingly high viscosity. The ionic liquid viscosity can be drastically reduced by mixing with a small volumetric proportion of simple diluting agents of methanol or ethanol, and the mixed solution permeated through the microfiltration membranes with ease. Methanol and ethanol were easily separated by additional post processing of evaporation, taking advantage of the non-volatile properties of the ionic liquids. The decrease of viscosity with increasing methanol to ionic liquids ratio follows an exponential fashion, and the viscosity–temperature relationship was found following Arrhenius equation for the pure ionic liquids with simple asymmetric cations without functional groups. The mixing ratio of the simple diluting agents to ionic liquids had significant impact on permeation rate, but effective filtration can be carried out at 20% (v/v) diluting agents to ionic liquid ratio. The difference in molecular structure of the asymmetric ionic species was further analysed for their effect on the microfiltration permeation rate.