RSC Advances REVIEW View Article Online View Journal | View Issue Recovery and purification of ionic liquids from solutions: a review Cite this: RSC Adv.,2018,8,32832 Jingjing Zhou,ab Hong Sui,abc Zhidan Jia,ab Ziqi Yang,ab Lin He *abc and Xingang Liabc With low melting point, extremely low vapor pressure and non-flammability, ionic liquids have been attracting much attention from academic and industrial fields. Great efforts have been made to facilitate their applications in catalytic processes, extraction, desulfurization, gas separation, hydrogenation, electronic manufacturing, etc. To reduce the cost and environmental effects, different technologies have been proposed to recover the ionic liquids from different solutions after their application. This review is mainly focused on the recent advances of the recovery and purification of ionic liquids from solutions. Several methods for recovery of ionic liquids including distillation, extraction, adsorption, membrane separation, aqueous two-phase extraction, crystallization and external force field separation, are Received 29th July 2018 introduced and discussed systematically. Some industrial applications of ionic liquid recovery and Accepted 13th September 2018 purification methods are selected for discussion. Additionally, considerations on the combined design of Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DOI: 10.1039/c8ra06384b different methods and process optimization have also been touched on to provide potential insights for rsc.li/rsc-advances future development of ionic liquid recovery and purification. 1. Introduction Ionic liquids (ILs) are a class of molten salts with melting points below 100 C. Generally, this kind of chemical is composed of organic cations and organic/inorganic anions. This article is licensed under a aSchool of Chemical Engineering and Technology, Tianjin University, 300072 Tianjin, The cations are mainly, but not limited to alkylated imidazole, China. E-mail: [email protected]; Tel: +86-022-27404701 pyrrole or pyridine derivatives, quaternized alkyl amines and b National Engineering Research Center of Distillation Technology, 300072 Tianjin, alkyl phosphines (Table 1), etc. Some representative anions of China Open Access Article. Published on 21 September 2018. Downloaded 10/5/2021 11:22:11 AM. ionic liquids are halides, alkyl sulfates, uorinated hydrocar- cCollaborative Innovation Center of Chemical Science and Engineering, 300072 1–3 Tianjin, China bons, carboxylic acids and amino acids, as shown in Table 2. Jingjing Zhou was born in Hebei, Professor Hong Sui received her China. She received her B.Sc. B.Sc. degree from Hebei Univer- degree in chemical engineering sity of Technology and Ph.D. from Dalian University of Tech- degree in Chemical Engineering nology in 2016. And she majored from Tianjin University. She in the process simulation of worked as a postdoctoral fellow waste heat recovery system of in National Pei Yang Distillation coke oven gas. Aer that, she Technology Engineering Limited joined Prof. Xingang Li's group Company and then joined the as a master student in School of faculty of Tianjin University as Chemical Engineering and a professor. She has published Technology, Tianjin University. more than 100 scientic papers At present she is focusing on the and about 40 invention patents recovery of ionic liquids aer applications of ionic liquids in oil– granted as the rst inventor or co-inventors. Her main research solid separation, interfacial characterization, etc. areas are related to the surface science in complex uid, oil–solid separation, solvent extraction, adsorption, functional chemicals synthesis and applications and also the advanced methodologies for remediating contaminated sites. 32832 | RSC Adv.,2018,8, 32832–32864 This journal is © The Royal Society of Chemistry 2018 View Article Online Review RSC Advances In addition to the two most common IL types, i.e.,proticand non-aqueous environment through industrial waste stream.21,22 aprotic ILs,4 some other ILs subclasses have also been re- Although there are multiple ILs created from GRAS (Generally ported, including polymeric ILs,5 double salt ILs,6 dicationic Recognized As Safe) list ions,23,24 the leaking of some ILs with ILs,7 deep eutectic solvent,8 chiral ILs,9 solvate ILs,10 etc. Some certain toxicity may cause potential environmental impact.25 On ionic liquids mentioned in the review are listed in Table 3. the other hand, a major concern associated with adopting ILs in During the past decades, ILs have attracted much attention large scale industrial processes is their relatively high cost in from researchers and engineers due to their unique charac- comparison with conventional solvents. The expense can be teristics such as negligible vapour pressure, non- offset to some extent if ILs are recycled.26 Therefore, efficient ammability, high thermal stability and a wide electro- separation and recovery of ILs from different solutions are chemical (conductivity) window. In addition, the physical essential for their ecological and economic applications. and chemical properties (e.g., solubility, polarity, viscosity During the past decades, great attempts have been made and acidity) of ILs are also found to be able to be designed by by researchers for the recovery and recycling of ILs, including combination and modication of their cations and anions. In distillation,27 extraction,28 adsorption,29 membrane separa- this way, the ILs have been applied in certain processes, such tion,30 aqueous two-phase extraction,31 crystallization32 and as catalytic reactions,11,12 separations,13,14 desulfurization,15,16 force eld separation,33 etc., shown in Fig. 1. Among these hydrogenation,17,18 and electrochemistry.19,20 methods, distillation and extraction are two of the most However, ILs are, at least, partially miscible with water or commonly used ways. Distillation, especially vacuum distil- hydrocarbons, which will inevitably end up in the aqueous or lation, is usually employed as the nal step for separation of Zhidan Jia was born in Hebei, Lin He was born in Jiangxi, China. She received her B.Sc. China. He received his B.Sc. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. degree from Hebei University of degree from Wuhan University of Technology in 2016. And she Science & Technology in 2009. focused on the preparation of Aer that, he joined Prof. Xin- crystal whiskers with MgSO4. gang Li's group as a Ph.D. Aer that, she studied in Tianjin candidate. Then, he worked as University as a master student a postdoctoral fellow in Collab- and joined Prof. Xingang Li's orative Innovation Center of group. She is majoring in Chemical Science and Engi- – This article is licensed under a behaviors of ionic liquids at oil neering. In 2017, he joined the water interface and applications faculty of Tianjin University as of ionic liquid in oil industry. an assistant professor. His research focuses on the interfacial sciences of unconventional oil Open Access Article. Published on 21 September 2018. Downloaded 10/5/2021 11:22:11 AM. processing, gas adsorption and desorption, interfacially active materials synthesis and applications, soil remediation. He pub- lished over 35 papers and obtained over 10 granted patents. Ziqi Yang received her B.Sc. Xingang Li received his Ph.D. degree in Chemistry from Nan- from Tianjin University in 1992. kai University in 2015. During He is now a full-professor in her pursuing the bachelor School of Chemical Engineering degree, she focused on the & Technology, in Tianjin synthesis of functionalized gra- University. He is also the head of phene for glycoproteins enrich- National Engineering Research ment. Aer that, she joined Prof. Centre of Distillation Tech- Xingang Li's group, majoring in nology, China. His research the research on the synthesis of focuses mainly on the separation CO2-responsive materials and elds, including distillation, their applications in oil–solid extraction, soil remediation, separation. She received unconventional oil exploitation, her M.Sc. degree in Chemical Engineering from Tianjin University waste gas treatment, chemical synthesis, chemical processing of in 2018. Now, she is a Ph.D candidate in chemical engineering. coal, etc. He obtained the National Progress Awards in Science and Technology (2007), National Invention Award (2013) and other scientic awards. He has published more than 300 articles including original papers, reviews and chapters of books. This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8, 32832–32864 | 32833 View Article Online RSC Advances Review Table 1 Cations of ionic liquids Acronym Name Structure Reference [Hmim]+ 1-Methylimidazolium 43 [Mmim]+ 1,3-Dimethylimidazolium 30 + [C2mim] 1-Ethyl-3-methylimidazolium 44 [Amim]+ 1-Allyl-3-methylimidazolium 45 + [C4mim] 1-Butyl-3-methylimidazolium 46 + [C6mim] 1-Hexyl-3-methylimidazolium 47 + [C8mim] 1-Octyl-3-methylimidazolium 48 + Creative Commons Attribution-NonCommercial 3.0 Unported Licence. [C16mim] 1-Hexadecyl-3-methylimidazolium 49 [PDmim]+ 1-Propyl-2,3-dimethylimidazolium 50 + [(CH2)nCOOHmim] 1-Alkylcarboxylic acid-3-methylimidazolium 51 + This article is licensed under a [C4(mim)2] 1,1-Bis(3-dimethylimidazolium-1-yl)butane 52 + [C4-py] 1-Butylpyridinium 53 Open Access Article. Published on 21 September 2018. Downloaded 10/5/2021 11:22:11 AM. + [C6-py] 1-Hexylpyridinium 54 + [C8-py] 1-Octylpyridinium 55 + [C4-3-mpy] 1-Butyl-3-methylpyridinium 56 + [C4-4-mpy] 1-Butyl-4-methylpyridinium 47 [Bmpyr]+ 1-Butyl-1-methylpyrrolidinium