Nonaqueous Lyotropic Ionic Liquid Crystals: Preparation, Characterization and Application in Extraction

NonAqueous Lyotropic ionic Liquid Crystals: Preparation, Characterization and Application in Extraction

Xianxian Liu, Qiwei Yang, Zongbi Bao, Baogen Su, Zhiguo Zhang, Qilong Ren, Yiwen Yang, and Huabin Xing*

Key Laboratory of Biomass Chemical Engineering of Ministry of Education,°°College of Chemical and Biological Engineering, Zhejiang University,°°Hangzhou 310027, P. R. China

Abstract

Ionic liquids (ILs) as unique materials have been used in various extraction processes and drawn much attention in the past decade because of their reduced environmental impact and enhanced separation selectivity. As we all know, ILs with their structural tunable property can be task-specifically designed and take more advantages than common solvents in the separation of H-bond donor solutes (such as tocopherols, oleic acid, naphthenic acid and phenol). However, the key drawback for IL-mediated extraction processes is the low extraction capacity for organic molecules due to the intrinsically strong polarity of ILs resulting from their charged structures, which seriously limits their application. In order to overcome this long-standing problem and expand the application of ILs, we designed a class of new non-aqueous lyotropic liquid crystals (LLCs) based on anionic surfactant carboxylate ILs (ASC-ILs) and developed an efficient IL-based LLC extraction process for some typical H-bond donor solutes, the distribution coefficients of these solutes, selectivity of targets solutes to typical impurities and recycle of ILs were performed through batch extraction experiments. The IL-based LLCs have self-assembled anisotropic nanostructures constituted by an ordered hydrogen-bond receptor interface and a nonpolar domain, which not only overcome the strong polarity of IL, but also provide outstanding cooperative effects between the hydrogen-bond and the van der Waals interactions. Particularly, comparisons of the extraction capacity between IL-based LLCs and other common extractants were also investigated. In addition, POM, infrared (IR) spectroscopy and X-ray diffraction (XRD) measurements were employed to investigate the interactions of ASC-IL-based LLCs and the microstructure of extraction phase. As expected, the distribution coefficients of IL-based LLCs to ¦Ä-tocopherol reached unprecedented values of 50-60 at very high feed concentrations of more than 100 mg/ml, which were 800-1000 times greater than those of common ILs, traditional organic and polymeric extractants. Similarly, excellent extraction capacities for other H-bond donor molecules, such as oleic acid, naphthenic acid and phenol were also found with IL-based LLCs as extractants. The IL-based LLCs also demonstrate excellent separation selectivity for structure-related compounds and even homologues because of their great molecular recognition ability, which provides a new platform for the production of high-purity drugs and clean energies. In the cases of separation of ¦Ä-tocopherol to methyl linoleate, oleic acid to triolein and naphthenic acid to octane, surprisingly high selectivity were obtained, indicating the molecular recognition of the LLC extractant to these H-bond donor solutes is very accurate and exclusive. What's more, the IL-based LLC extraction process has the merit of easy recovery of solutes and simple regeneration of ILs through switching off the self-organized nanostructures of LLCs. Prospectively, IL-based LLCs, as new soft materials, successfully combine the unique properties of ILs and LCs open a new avenue for the development of high-performance extraction methods. And, we anticipate that this study will expand the application of ILs and LLCs to a wide variety of fields.

 

Figure 1. a) Distribution coefficient of d-tocopherol D_¦Ä-toc in extractant¨Chexane biphasic systems at different feed concentrations C_f. The molar fraction of IL in the extractant was 0.1; b) Effect of the molar ratio of IL on D_¦Ä-toc at a C_f of 100 mgmL_1; c) D_¦Ä-toc values for different IL-based LLCs as extractants at C_f=100 mgmL_1. LLC1: [Ch][Lau]-acetonitrile (10:90, molar), LLC2: [Ch][Myr]-DMSO (10:90, molar), LLC3: [Ch][Pal]-DMSO (10:90, molar), LLC4: [N₂₂₂₂][Lau]-DMSO (50:50, molar), and LLC5: [Emim][Lau]-DMSO (40:60, molar).

References

1.      Liu X. X.; Yang Q. W.; Bao Z. B.; Su B. G.; Zhang Z. G.; Ren Q. L.; Yang Y. W.; Xing H. B. Chem. Eur. J. DOI: 10.1002/chem.201500306.

2.      Yang Q. W.; Xu D.; Zhang J. Z.; Zhu Y. M.; Zhang Z. G.; Qian C.; Ren Q. L.;Xing H. B. ACS Sustainable Chem. Eng. 2015, 3, 309-316.

Acknowledgements

The research was supported by the National Natural Science Foundation of China (21222601, 21476192, and 21436010), the Zhejiang Provincial Natural Science Foundation of China (LR13B060001) and the Program for New Century Excellent Talents in University of China (NCET-13-0524).