Supported ionic liquid membranes for water and volatile organic compounds separation: Sorption and permeation properties

Supported ionic liquid membranes (SILMs) were prepared by immobilizing the room temperature ionic liquids (RTILs) ([C4C1iml[BF4] and [CnC1im][PF6] (n=4 and 6)) in a porous Matrimid membrane. The Matrimid membrane was prepared by water vapor-induced phase inversion, and its porous structure was characterized by SEM. After immersion of the porous Matrimid membrane in RTILs, the prepared SILMs were composed of 60% of RTIL phase. From the bubble point test, the SILMs displayed an excellent retention of their RTIL phase, and thus showed a good stability in the face of increased pressure on one of their faces. The feasibility of the prepared SILMs for the separation of water and volatile organic compounds (VOCs) such as ethanol and cyclohexane by vapor permeation was investigated. For this purpose, a preliminary study of water and VOC vapor sorption in RTILs was necessary to determine the affinity between each solute and RTILs. Only [C4C1im][BF4] showed both the best sorption capacities and the good sorption selectivities. The vapor permeation measurements were therefore carried out only through the SILMs impregnated with [C4C1im][BF4]. The SILM permeability was found to increase in the same sorption affinity order as for [C4C1im][BF4], i.e. cyclohexane < ethanol < water (in molar unit); moreover, the best ideal selectivities of the SILM based on [C4C1im][BF4] were related to the separation of cyclohexane with αwater/cyclohexane = 371 and αethanol/cyclohexane=217. Moreover, the SILMs retained most of their [C4C1im][BF4] phase regardless of the vapor used, and thus showed satisfactory stability as well as durability under the permeation conditions used.