Highly Efficient and Recyclable Carbon-Nanofiber-Based Aerogels for Ionic Liquid–Water Separation and Ionic Liquid Dehydration in Flow-Through Conditions
Li Zhi Guan, María C. Gutiérrez, María J. Roldán?Ruiz, Ricardo Jiménez, M. Luisa Ferrer, Francisco del Monte
Ionic liquids (ILs) are being widely used in many diverse areas of social interest, including catalysis, electrochemistry, etc. However, issues related to hygroscopicity of many ILs and the toxic and/or nonbiodegradable features of some of them limit their practical use. Developing materials capable of IL recovery from aqueous media and dehydration, thus allowing their recycling and subsequent reutilization, in a single and efficient process still poses a major challenge. Herein, electrically conductive aerogels composed of carbon nanofibers (CNFs) with remarkable superhydrophobic features are prepared. CNF-based 3D aerogels are prepared through a cryogenic process, so called ice-segregation-induced self-assembly (ISISA) consisting of the unidirectional immersion of an aqueous chitosan (CHI) solution also containing CNFs in suspension into a liquid nitrogen bath, and subsequent freeze-drying. The CNF-based 3D aerogels prove effective for absorption of ILs from aqueous biphasic systems and recovery with quite low water contents just through a single process of filtration. Moreover, the electrical conductivity of CNF?based 3D aerogels is particularly interesting to treat highly viscous ILs because the Joule effect allows not only shortening of the absorption process but also enhancement of the flux rate when operating in flow-through conditions.
a–f) Scanning electron microscopy (SEM) images of the CNF-based 3D aerogel resulting from unidirectional freezing and freeze-drying before (a–d) and after (e,f) HDI treatment. Longitudinal views in (a,b), and cross-sectional views in (c–f). Scale bars are 1 mm in (a, c, and e), 50 µm in (b) and 100 µm in (d,f). The red arrow in (a) indicates the direction of freezing.