Author: Dr Natalia V. Plechkova, QUILL Research Centre, School of Chemistry and Chemical Engineering, Queen’s University Belfast Earlier this year, the website www.topbritishinnovations.org invited the public to choose, from a list of 87 topics, what they thought was the most important British innovation of the last 100 years. The winner was the universal machine described by Alan Turing in the 1930s, which formed the basis of modern computers. It would be at this point practically impossible, and even cruel, to imagine your life without a computer. In the same survey, from a list of 12 options, the question was asked: ‘What recent innovation would help define the future?’ Perhaps surprisingly, ionic liquids were chosen by a remarkable margin (33% of votes). This was surprising to many, but it was not to the scientists who work on this innovative class of compounds. With health and safety stated as the number one business objective of many industrial companies, it is no wonder that hazardous solvents, heated to dangerous temperatures, are now the bad guys. Fortunately, there is a new superhero in town, who flexes his muscles at temperatures less than 100ºC, dissolves materials others fear to dissolve and who will not evaporate or spontaneously combust at the first sign of trouble. The cape wearer is none other than ionic liquids. Like the universal machines in their time, a lot is unknown and undiscovered regarding ionic liquids (as we will see later in this article). Hopefully in the future, our lives will be unimaginable without them, too. Most ionic liquids, consisting solely of ions, are molten salts, which combine the advantages of classical inorganic salts and molecular solvents. Inorganic salts have a similar size of cation and anion, so their crystal structure is highly organised, which makes their melting points high. [login type="readmore"] In contrast, ionic liquids have different sizes of anions and cations, which results in low melting points, in some cases less than 100°C, and they may even be liquid at room temperature (see Fig. 1). Compared to molecular solvents, most ionic liquids have negligible vapour pressure, so they do not evaporate noticeably at room temperature. This is one of the reasons why some ionic liquids earned the name of ‘green’ solvents. This also means that many do not burn, even when directly heated with a blow torch. Conveniently, they are now supplied (on large and small scale) by various companies, such as CYTEC, IoLiTec, BASF, Merck and Koei. SPECTRUM OF APPLICATIONS So, what could these advantages bring in terms of engineering applications? From extracting cellulose and other useful chemicals from biomass to plants and flowers, the spectrum of applications is limited only by imagination. Processes can be carried out in ionic liquids at room temperature, where many ionic liquids remain liquid, resulting in good mass-transfer. Hazards are greatly reduced, as many ionic liquids are inflammable and do not evaporate under ambient conditions. [caption id="attachment_3560" align="alignright" width="409"] Fig 1: Handling an ionic liquid in the laboratory[/caption] The use of ionic liquids will give the reaction an ionic environment, so most processes could proceed in a different way compared to a molecular solvent, resulting in the generation of new and different products, or cheaper and safer routes to known products (Earle et al, 2004). Kerogen dissolution (or lack of it) has been a thorn in the paw of the petroleum industry, which now may be painlessly extracted using ionic liquids (Patell et al, 2003). Useful compounds for pharmaceutical, nutritional and cosmetic applications can be extracted from biomass that would otherwise be burned or wasted. Ionic liquids can dissolve wood, wool, cork, keratin, cellulose, sulfur, algae et cetera and then (using anti-solvents) lead to the isolation of the target product. Exciting developments in many fields of chemistry stem from ionic liquid applications such as catalysis, electrochemical analysis, organic synthesis, protein purification, extraction/separation and protein crystallisation and detection (Kokorin, 2011). Hydrated ionic liquids allow acid sample storage, important in biotechnology and forensic sciences, and provide long-term storage for DNA (Nishimura et al, 2005; Wang et al, 2007; de Zoysa et al, 2009; Vijayaraghavan et al, 2010). They have even been used for carbon dioxide remediation, a process becoming known as CCS – carbon capture and storage (Atkins et al, 2011). Ionic liquids have been investigated for potential applications from as early as the 1940s, where they found use for the electrodeposition of aluminium, but it was not until the end of the 1990s that research centres and industries started concentrating their efforts on ionic liquid studies. Ionic liquids could be made liquid crystalline, hydrogen-bonded, Lewis/Brønsted acidic or basic, and hydrophobic or hydrophilic, with various functional groups. Ionic liquid are prepared in such a way to suit the process for which they are required, which is why they are often referred to as ‘designer solvents’. The possibilities are endless, as practically any heterocyclic cation and any anion can be embedded in the structure of an ionic liquid. There are also binary and ternary mixtures of ionic liquids, which are interesting from the engineering point of view as they can reduce the price of the overall mixture, but keep the physicochemical properties suitable for a specific process. There are well over one million possible simple ionic liquids, and over one trillion (10¹⁸) possible ternary mixtures. Prof Kenneth R. Seddon (chair of Inorganic Chemistry at Queen's and director of QUILL) and I have reviewed the industrial applications of ionic liquids in detail elsewhere (Plechkova and Seddon, 2008), but here I now summarise some of the highlights. INDUSTRIAL SUCCESS Currently, one of the most known and successful examples of an industrial success using ionic liquid technology is the BASIL^TM (biphasic acid scavenging utilising ionic liquids) process, first introduced by BASF to the site in Ludwigshafen in Germany in 2002, in order to produce the generic photoinitiator precursors, alkoxyphenylphosphines. Many chemical processes produce acids as by-products – most commonly hydrochloric acid (HCl), which normally has to be scavenged. Tertiary amines are used as scavengers, creating an ammonium salt which can be removed via an aqueous extraction phase. With reaction mixtures sensitive to water, it is not straightforward, as a slurry of ammonium salts is formed which means highly viscous solutions and limited heat transfer. In addition, the ammonium salt has to be separated by filtration.  However, if instead of triethylamine, 1‑methylimidazole is used as an acid scavenger, liquid 1-methylimidazolium chloride (melting point = 75 °C) is formed, while the reaction mixture is at ca. 80°C. After the reaction, there are two clear liquid phases that can easily be worked up by a simple phase separation. The upper phase is the product phase. The lower phase is the pure ionic liquid, which can react with sodium hydroxide to reform the 1‑methylimidazole, which is the initial reagent, so it is a continuous process. This new technology increased the space-time yield of the process from 8 to 690,000 kg m^-3 h^-1 per year – an 80,000-fold increase. Gas manufacturer Linde has designed the so-called 'ionic compressor’ for compressing gases at constant temperature and high pressure, using an ionic liquid, which has a very low compressibility. Linde pointed out that “in contrast to a conventional piston compressor, with some 500 moving parts, we now need only eight”. The system can sustain a constant gas pressure of 250 bar while delivering 500 cubic meters of natural gas per hour. The potential applications are natural gas filling stations in association with dihydrogen. HIGH PERFORMANCE LUBRICANTS Ionic liquids were first reported as potential high-performance lubricants over ten years ago, and have drawn considerable attention in the area of tribology since then, because of both their excellent lubrication and anti-wear properties in comparison with the lubrication oils which are generally used. The advances in this field are covered elsewhere (Zhou et al, 2009). [caption id="attachment_3584" align="alignright" width="481"] Fig 2: Ionic liquids effectively do not evaporate under normal conditions[/caption] Ionic liquids effectively do not evaporate under normal conditions (Fig 2). This quality leads to applications such as using ionic liquids as the electrolyte in metal-air batteries in place of water, as they evaporate more slowly leading to improved battery life.  The electrochemical properties of ionic liquids allow for more power from a smaller battery, as they have  electrochemical windows of up to six volts (versus 1.23 V for water), meaning energy densities of 900-1600 W h kg^-1 are possible (Hamilton, 2009). Ionic liquids can be utilised as dispersing agents in paints to enhance finish, appearance and drying properties (Armand et al, 2009) and for nanoparticles at IoLiTec. They are also finding potential applications as heat transfer and storage mediums in solar thermal energy systems. Solar installations focus the sun’s energy to a common receiver, which may reach up to 600 °C. This heat is transformed into electrical energy through another process, but when the sun dies down, the heat is rapidly lost. Conventionally, since the 1980s, nitrate salts have been used as the medium of choice to store the heat, but they solidify around 200°C and above, and thus require heating to prevent solidification. Ionic liquids, however, have a more useful liquid-phase temperature range and would be ideally suited to this application (Wu et al, 2001). Ionic liquids also have potential for the recycling of synthetic goods, plastics and metals. They are able to separate similar compounds from each other, for example polymers in plastic waste streams, at lower temperatures than current approaches (Armand et al, 2009) and could help avoid incinerating plastics or dumping them in landfill. The diversity of ionic liquid types and structures mean that their toxicity is not yet widely studied and well understood, but ionic liquids ranging from harmless cholinium acetate (cholinium chloride is used as chicken feed, while acetic acid is vinegar for our chips) to highly toxic cyanide-based systems have been prepared. However, we can (and have) designed ionic liquids to be non-toxic and biodegradable – a green paradigm. WORLD'S BEST CHEMISTS In 2011, Thomson Reuter named the best 100 chemists in the world, based on the number of citations divided by the number of papers over the last 10 years (Thomson Reuters, 2011).  The top two scientists from the UK were Prof Kenneth R. Seddon and Dr John D. Holbrey, both of whom work at the Queen’s University Ionic Liquid Laboratories (QUILL) Research Centre (based in Belfast, founded in 1999), which specialises in ionic liquids. A decade before QUILL was born, Prof William James (Jim) Swindall CBE founded the Queen’s University Environmental Science and Technology Research (QUESTOR) Centre in 1989. The QUESTOR Centre was the first industry/university co-operative research centre in Europe and it remained such until QUILL was co-founded by Profs Swindall and Seddon in 1999. Prof Swindall served as the sole director of QUESTOR until 2002, when he decided to focus all his efforts on the rapidly growing QUILL Centre. The QUILL Centre aims to promote collaborative industry-university research on ionic liquids and is an industrial-academic consortium, with about 16 members from industry who pay an annual membership fee. A recent QUILL industrial success, for which Seddon, Holbrey and their team of researchers are responsible, is an environmental mitigation technology for the reactive capture of mercury from natural gas streams that went live in October 2011. [caption id="attachment_3564" align="alignright" width="781"] Fig 3: Mercury Removal Unit at Petronas, which utilises ionic liquid technology[/caption] It was delivered from first fundamental screening to laboratory scale and subsequently to multi-ton commercial deployment in less than four years (circa three-to-four years faster than normal in the energy sector) (Abai et al, 2012). The industrial partner was Petronas, with a team of engineers led by Prof Martin P. Atkins. The mercury removal unit is shown in Fig 3. It has the potential to save $3-10 per plant per annum, in addition to licensing opportunities to a market running to over 300 plants worldwide. The success of the project can be laid at the feet not only of ionic liquid technology, but at the synergy between the skills and focus of the academic and industrial teams, and expert project management. Many examples of industrial applications have been shown in this article, but since only a very limited number of ionic liquids have been studied, the current applications of ionic liquids are far away from exhaustive. The best is yet to come, which is why increasing numbers of scientists in academic, industrial and government laboratories around the world are now devoting their energies to working in the field of ionic liquids. This trend is strikingly illustrated in Fig 4. [caption id="attachment_3572" align="aligncenter" width="941"] Fig 4: Publication growth trends for ionic liquids (black), Buckminsterfullerene, C60 (blue), superconductivity (green), and supercritical fluids (red) between 1980 and 2011 (Deetlefs M, Fanselow M, unpublished data)[/caption] Technologies that will accommodate our futures lives, often unnoticed in the background, will utilise the unique properties of ionic liquids. When we start our car and drive to the Slieve League Cliffs in Donegal for the weekend, would many be aware that the hydrogen powering the engine comes from an innovative compressor in the filling station, which uses a type of ionic liquid piston? Would many even care that the hydrogen engine is not driving the wheels directly, but used as generator to charge batteries containing ionic liquids as electrolyte, which then power electric motors in the wheels? Few would know that when they return home from that trip on a very cloudy day and take a hot shower powered by solar energy, that ionic liquids are being used as a heat transfer and storage system, storing energy from the previous sunny day. And when they wash their dirty clothes from the trip, how many would realise that they have used an ionic liquid as part of the cleaning process? Like all great technologies, such as the universal machine, ionic liquids’ greatest strength is that they do go unnoticed, but power the gears and cogs which keep the world turning. Dr Natalia V. Plechkova attained her BSc and MSc in Chemical Engineering at the Russian Mendeleev University of Chemical Technology, Moscow, and her PhD in QUILL in Chemistry, under the guidance of Prof Kenneth R. Seddon. Since then she has been a research fellow and project manager in the QUILL (Queen’s University Ionic Liquid Laboratories) Research Centre, focusing on various aspects of ionic liquids, including their synthesis, characterisation, properties and applications.  References Abai M, Atkins MP, Cheun KY, Holbrey J, Nockemann P, Seddon K, Srinivasan G, Zou Y. World Patent, WO2012/046057, 2012. Armand M, Endres F, MacFarlane DR, Ohno H, Scrosati B. ‘Ionic-liquid materials for the electrochemical challenges of the future.’ Nat Mater 2009;8:621-629. Atkins M, Kuah YC, Estager J, Ng S, Oliferenko A, Plechkova N, Puga A, Seddon K, Wassell D, Removal of Carbon Dioxide from a Gas Stream by Using Aqueous Ionic Liquid, World Patent, WO2011/114168, 2011. de Zoysa RSS, Jayawardhana DA, Zhao Q, Wang D,  Armstrong DW, Guan X. Slowing DNA Translocation through Nanopores Using a Solution Containing Organic Salts. J Phys Chem B 2009;113:13332-13336. Earle MJ, Katdare SP, Seddon KR. ‘Paradigm confirmed: the first use of ionic liquids to dramatically influence the outcome of chemical reactions.’ Org Lett 2004;6:707-710. http://archive.sciencewatch.com/dr/sci/misc/Top100Chemists2000-10/, 'Essential Science Indicators^SM from Thomson Reuters', January 1, 2000-October 31, 2010. http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=home.showFile&rep=file&fil=Zemships_Flyer_EN.pdf, ‘Climate protection to the fore: the first ever fuel cell driven passenger ship’. http://www2.basf.us/corporate/051004_ionic.htm, “BASF to present BASIL ionic liquid process at technology transfer forum” (Press release). BASF. 2004-05-10. Kokorin A. Ionic liquids: Applications and Perspectives, Rijeka, Croatia, 2011. Nishimura N, Nomura Y, Nakamura N, Ohno H. ‘DNA strands robed with ionic liquid moiety.’ Biomaterials 2005;26:5558-5563. Patell Y, Seddon KR, Dutta L, Fleet A. ‘The dissolution of kerogen in ionic liquids’, in Green Industrial Applications of Ionic Liquids, ed. Rogers RD, Seddon KR, Volkov S, NATO Science Series, II: Mathematics, Physics and Chemistry, Kluwer, Dordrecht, 2003, Vol. 92, pp. 499-510. Plechkova NV, Seddon KR. ‘Applications of ionic liquids in chemical industry.’ Chem Soc Rev 2008;37:123-150. Stark A, Seddon KR. ‘Ionic Liquids’ in Kirk-Othmer Encyclopaedia of Chemical Technology, ed. Seidel A. John Wiley & Sons, Inc., Hoboken, New Jersey, 2007, Vol. 26, pp. 836–920. http://www.technologyreview.com/news/416150/betting-on-a-metal-air-battery-breakthrough/, Hamilton T, ‘Betting on a Metal-Air Battery Breakthrough’, November 2009. Vijayaraghavan R, Izgorodin A, Ganesh V, Surianarayanan M, MacFarlane, DR. ‘Long term structural and chemical stability of DNA in hydrated ionic liquids.’ Angew Chem Int Edit 2010;49:1631-1633. Wang JH, Cheng DH, Chen XW, Du Z, Fang ZL. ‘Direct extraction of double-stranded DNA into ionic liquid 1‑butyl-3-methylimidazolium hexafluorophosphate and its quantification.’ Anal Chem 2007;79:620-625. Wu B, Reddy RG, Rogers RD. ‘Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems.’ Proceedings of Solar Forum 2001: Solar Energy: The Power to Choose, April 21-25, 2001, Washington, DC ASME, Washington, DC, 2001;445–451. Zhou F, Liang Y, Liu W. ‘Ionic liquid lubricants: designed chemistry for engineering applications.’ Chem Soc Rev 2009;38:2590-2599. Further reading Brennecke JF, Maginn EJ. Ionic liquids: innovative fluids for chemical processing. AIChE J 2001;47(11):2384-2389. Davis Jr JH. Task-Specific Ionic Liquids. Chem Lett 2004;9(33):1072-1077. Freemantle M. An Introduction to Ionic Liquids. 2009. Royal Society of Chemistry. ISBN 978-1-84755-161-0. Parvulescu VI, Hardacre C. Catalysis in ionic liquids. Chem. Rev. 2007;107:2615-2665. Niedermeyer H., Hallett JP, Villar-Garcia IJ, Hunt PA,Welton T. Mixtures of ionic liquids. Chem Soc Rev 2012;41:7780-7802. Ohno H. (ed.) Electrochemical aspects of ionic liquids. 2011, John Wiley & Sons. Plechkova NV, Seddon KR. (eds.), Ionic Liquids UnCOILed: Critical Expert Overviews. 2013, Wiley. van Rantwijk F, Sheldon RA, ‘Biocatalysis in ionic liquids.’ Chem. Rev. 2007;107: 2757-2785. Wasserscheid P, Welton T. (eds.) Ionic liquids in synthesis. 2008, Wiley. Welton T. ‘Room-Temperature Ionic Liquids.’ Chem Rev 1999;99: 2071-2084. Torrecilla J (ed.). The Role of Ionic Liquids in the Chemical Industry (Biochemistry Research Trends: Chemistry Research and Applications), 2012, Nova Science Publishers Inc.