molecules Review Industrial Applications of Ionic Liquids Adam J. Greer 1,* , Johan Jacquemin 2,3,* and Christopher Hardacre 1,* 1 Department of Chemical Engineering and Analytical Science, the University of Manchester, the Mill, Sackville Street, Manchester M13 9PL, UK 2 Laboratoire PCM2E, Université de Tours, Parc de Grandmont, 37200 Tours, France 3 Materials Science and Nano-Engineering, Mohammed VI Polytechnic University, Lot 660-Hay Moulay Rachid, Ben Guerir 43150, Morocco * Correspondence: [email protected] (A.J.G.); [email protected] (J.J.); [email protected] (C.H.); Tel.: +44-(0)-161-306-2672 (A.J.G.) Academic Editor: György Keglevich Received: 21 October 2020; Accepted: 3 November 2020; Published: 9 November 2020 Abstract: Since their conception, ionic liquids (ILs) have been investigated for an extensive range of applications including in solvent chemistry, catalysis, and electrochemistry. This is due to their designation as designer solvents, whereby the physiochemical properties of an IL can be tuned for specific applications. This has led to significant research activity both by academia and industry from the 1990s, accelerating research in many fields and leading to the filing of numerous patents. However, while ILs have received great interest in the patent literature, only a limited number of processes are known to have been commercialised. This review aims to provide a perspective on the successful commercialisation of IL-based processes, to date, and the advantages and disadvantages associated with the use of ILs in industry. Keywords: ionic liquids; industrial applications; commercial processes; designer solvents; synthesis 1. Introduction Research in the field of ionic liquids (ILs) has been steadily increasing over the last two decades, since their initial discovery in 1914 by Paul Walden [1]. ILs are liquid salts comprised entirely of cations and anions and are frequently described as being liquid below an arbitrary value, such as 100 ◦C, or at room temperature (RT). It should be noted that this temperature constraint is not required for a substance to be considered an IL [2]. ILs are differentiated from other salts, such as sodium chloride, which only become molten at extremely high temperatures (>800 ◦C) due to their tightly packed structure and strong ionic interactions. The lower melting point of an IL is attributed to the bulky and unsymmetrical structure of the constituent ions, which can be paired together in different combinations to tailor their thermophysical properties [3]. Some examples of common cations and anions are shown in Figure1, together with their preferred abbreviations [4]. ILs were branded as “solvents of the future” in 2003, due to the wide range of applications in which they could be used, particularly as replacements for traditional solvents in industrial processes which were often toxic, flammable and highly volatile [5]. Their low vapour pressure led to an assumption that ILs could also be considered as green solvents, due to low levels of atmospheric pollution, however, it is now well known that the manufacturing, use, and disposal of the solvent itself, must also be considered [6–10]. Furthermore, IL toxicity has been identified as an emerging problem as the use of ILs increases, particularly towards aquatic organisms, and studies have shown that this can be affected by changes in IL structure, such as the chosen cation/anion, alkyl chain length, and functionalised alkyl groups [11–13]. Some ILs containing fluorine anions, such as [PF6]− or [BF4]− are hydrolytically unstable and liberate HF, leading to an increase in popularity of the costly, but water stable, [NTf2]− and Molecules 2020, 25, 5207; doi:10.3390/molecules25215207 www.mdpi.com/journal/molecules Molecules 2020, 25, 5207 2 of 31 Molecules 2020, 25, x FOR PEER REVIEW 2 of 31 tris(perfluoroalkyl)trifluorophosphate ([FAP]−) anions [14,15]. However, it is important to remember − − thatwater in stable, many cases,[NTf2] the and solvent tris(perfluoroalkyl)trifluorophosphate that the IL is replacing can be much ([FAP] more) anions harmful [14,15] (to. bothHowever, the user it andis important the environment), to remember such that as in carcinogenic many cases, chromium the solvent (IV) that salts the IL used is replacing in chromium can be electroplating much more processes,harmful (to or both HF used the user in alkylation and the reactions;environment), yet nonetheless, such as carcinogenic detailed toxicity chromium testing (IV) of salts proposed used in ILs ischromium still essential. electroplating processes, or HF used in alkylation reactions; yet nonetheless, detailed toxicity testing of proposed ILs is still essential. Cations Anions - - Cl F Cl - F F R Br F F N N Al P N halide R - - - B F F R Cl Cl N Cl /Br /F Cl F F F F 1-alkyl-3 tetrachloroaluminate hexafluorophosphate methylimidazolium 1-alkyl-1-methylpiperidinium - - + + tetrafluoroborate [Cnmim] [Cnmpip] [AlCl4] - [PF6] [BF4] O 1-alkyl-1-methylpyrrolidinium O O + [Cnmpyr] CF3 S O R R O O CF3 O P N trifluoromethanesulfonate acetate trifluoroacetate - - - - R R R R [OTf] or [CF3SO3] [AcO] [tfa] R R tetraalkylphosphonium tetraalkylammonium O O O O + + N N [Pn,n,n,n] [Nn,n,n,n] S S S S F F CF3 CF3 O O O O bis(flurosulfonyl)imide bis(trifluromethanesulfonyl)imide - - - - [fsi] or [N(SO2F)2] [NTf2] or [N(SO2CF3)2] Figure 1. Examples of common cations and anions found in ionic liquids, and their preferred abbreviations. Figure 1. Examples of common cations and anions found in ionic liquids, and their preferred Manyabbreviations. ILs have a high thermal stability, and a low flammability owing to their negligible vapour pressure at ambient temperature. These properties can increase the safety of high temperature solvent applications,Many ILs such have as a inhigh batteries, thermal or stabil evenity, allow and theira low use flammability in space [ 16owing]. It shouldto their benegligible noted, however, vapour thatpressure many at ILs ambient can still temperature. be distilled, These but often properties extreme can conditions increase the are safety required of high [17]. temperature Additionally, solvent as ILs consistapplications, exclusively such ofas ions,in batteries, they also or have even higher allow thermaltheir use and in space electrical [16] conductivities. It should be noted in comparison, however, to commonthat many laboratory ILs can still solvents, be distilled, as well but as often larger extreme electrochemical conditions windows. are required This [17] has. Additionally, meant that they as haveILs consist found usesexclusively as electrolytes of ions, and they heat also transfer have fluidshigher and thermal can e ffandectively electrical disperse conductivities heat produced in duringcomparison a reaction. to common laboratory solvents, as well as larger electrochemical windows. This has meantThe that idea they that have ILs couldfound replace uses as conventionalelectrolytes and solvents heat transfer created afluids lot of and interest can effectively in the academic disperse and industrialheat produced communities. during a reaction. This substitution is enforced by the EU REACH (Registration, Evaluation, AuthorisationThe idea that and ILs Restriction could replace of Chemicals) conventional regulations solvents that created are designeda lot of interest to improve in the safetyacademic and protectand industrial the environment, communities. and their This restricted substitution substances is enforced list, which by the phases EU outREACH hazardous (Registration, chemicals andEvaluation, provides Authorisation further opportunities and Restriction for safer, of IL-basedChemicals) processes regulations [18]. that However, are designed prior toto beingimprove able tosafety substitute and protect any organic the environment, solvent used and in their a given restricted process, substances the compatibility list, which of phases a selected out hazardous IL must be chemicals and provides further opportunities for safer, IL-based processes [18]. However, prior to checked in respect to all the operation units of a process. A comparison of the main properties of organic being able to substitute any organic solvent used in a given process, the compatibility of a selected IL solvents vs. those of ILs originally reported by Plechkova and Seddon in 2008 has been updated herein, must be checked in respect to all the operation units of a process. A comparison of the main properties as reported in Table1, to highlight the main di fferences between these two classes of materials [19]. of organic solvents vs. those of ILs originally reported by Plechkova and Seddon in 2008 has been The tuneable nature of ILs allows for the synthesis of a solvent with specific properties tailored to a updated herein, as reported in Table 1, to highlight the main differences between these two classes particular application, which is less the case for organic solvents. In light of such a comparison, while of materials [19]. The tuneable nature of ILs allows for the synthesis of a solvent with specific ILs have uses as alternative solvents, many other applications exist as reported in Figure2[20,21]. properties tailored to a particular application, which is less the case for organic solvents. In light of such a comparison, while ILs have uses as alternative solvents, many other applications exist as reported in Figure 2 [20,21]. Molecules 2020, 25, 5207 3 of 31 Table 1. Comparison of organic solvents with ionic liquids (ILs), updated from work by Plechkova and Seddon