Info: What are ionic liquids?

Just as ionic inorganic compounds (for instance sodium chloride), ionic liquids consist entirely of positive and negatively charged ions. While the melting point of ionic inorganic compounds is typically several hundreds degrees centigrade, the melting point of ionic liquids is by definition below 100 °C. Several types of ionic liquids are liquid at room temperature (room temperature ionic liquids or RTILs).

The cation of an ionic liquid is often a large organic cation, such as an imidazolium, a pyridinium or a quaternary ammonium ion. As anions, Cl-, Br- , BF4-, PF6- or CF3SO3- are popular choices. An example of a well-known ionic liquid is 1-ethyl-3-methylimidazolium hexafluorophosphate, which is abbreviated to [C2mim][PF6]. Because the properties of ionic liquids (miscibility with water and other solvents, dissolving ability, polarity, viscosity, density) can be tuned by an appropriate choice of the anion and the cation, ionic liquids are often considered as designer solvents. Presently, there exists worldwide an intense research activity in the field of ionic liquids, because these solvents have several interesting properties. The vapour pressure of an ionic liquid is extremely low, so that ionic liquids are non-volatile and do not evaporate. Ionic liquids can be used as environmentally friendly substitutes for volatile organic compounds (VOCs).
After reaction, the reaction products can be separated from the ionic liquid by distillation. Ionic liquids are fluid over a broad temperature range, from the melting point to the onset of thermal decomposition. Because ionic liquids are non-flammable and non-explosive, they are much safer to work with in the lab than the conventional organic solvents. Due to their ionic nature, ionic liquids conduct electricity. Ionic liquids have a high electrochemical stability, which means that they are very resistant to oxidation and reduction. By dissolving metal salts in ionic liquids, reactive metals can be deposited and purified by electrolysis. Ionic liquids are polar solvents, and their polarity is comparable with the polarity of the lower alcohols. In contrast to other polar organic solvents, ionic liquids are non-coordinating and non-solvating.
Ionic liquids are good solvents for many organic and inorganic compounds, but at the same time they are not very corrosive. For instance, ionic liquids are one of the very few solvents in which cellulose is soluble. By choosing ionic liquids as the solvent for organic reactions, it is possible to obtain a higher reaction rate, higher yields and a better selectivity than when conventional organic solvents are used. For a long time, ionic liquids were an academic curiosity. Research was mainly focused on haloaluminate ionic liquids as ion-conductive electrolytes for batteries and as a medium for electrodeposition of reactive metals.

Typical first-generation ionic liquids are mixtures of aluminium chloride and N-butylpyridinium chloride in different molar ratios. Although these first-generation ionic liquids performed well, they were cumbersome to handle due to their moisture- and oxygen-sensitivity. Therefore, one has to work with these solvents in a glove-box. A major breakthrough was the development of the second-generation of ionic liquids like the 1-alkyl-3-methylimidazolium hexafluorophosphates, which are moisture and air-stable. Because these ionic liquids can be handled on the lab bench like an conventional organic solvent, and because they have at the same time the advantageous properties of molten salts like a low vapour pressure and remarkable solubilising properties, this second generation of ionic liquids attracted the attention of the wide scientific community. Researchers began to use the ionic liquids for applications other than battery electrolytes and electrolyte baths for electrodeposition of metals, for instance as solvents for organic synthesis and especially as solvents for catalytic reactions. Of special interest is the fact that enzymes retain their activity in ionic liquids, so that these solvents can be used in biocatalysis and in biochemical technology.

The interest of the scientific community in the field is reflected by the exponential increase of the number of research papers since 2000. In 2006 alone, more than 2000 papers on ionic liquids and related topics will be have been published. These papers are about the development of new ionic liquids, the physicochemical properties of ionic liquids, the use of ionic liquids as reaction solvents and in other applications, and about the toxicity and biocompatibility of ionic liquids. Although a vast number of different ionic liquids and ionic liquids mixtures are possible, the majority of studies (> 95%) are devoted to one single class of ionic liquids, being the 1-alkyl-3-methylimidazolium ionic liquids. This popularity can be partially explained by the fact that this was the first class of moisture- and air-stable ionic liquids, that they are widely commercially available and that they have interesting properties. However, it is unfortunate that the potential of other ionic liquids remains largely unexplored.

Figure 1. Examples of cations commonly used in ionic liquids: (1) 1-alkyl-3-methylimidazolium; (2) 1-alkylpyridinium; (3) quaternary ammonium; (4) 1,1’-dialkylpyrrolidinium; (5) 1,1’-dialkylmorpholinium; (6) phosphonium.

Figure 2. Commonly used imidazolium ionic liquids.

^ Top




Applications of ionic liquids

Due to their structural variety, the actual properties of ionic liquids are very much dependent on their composition. As a consequence, ionic liquids offer a perspective for a wide spectrum of future applications. Applications include:

• Energy conversion: electrolytes in batteries and dye-sensitized solar cells, membrane additive for fuel cells;
• Heat storage: large-heat-capacity medium for storing latent heat, heat carriers for solar installations;
• Chemistry: solvents for reactions in organic synthesis, catalytic reactions, biphasic reactions, medium for microwave reactions, polymerization reactions;
• Biotechnology: solvents for biocatalysis and purification of proteins, biomass conversion, biocides, dissolution and regeneration of cellulose;
• Chemical engineering: solvents for extraction and separation, azeotrope breakers in extractive distillation, heat transfer liquids, lubricants, medium voor gas absorption;
• Metallurgy: electrolyte baths for electrodeposition of reactive metals, electropolishing, electroplating, solvents for metal extraction processes;
• Analytics: stationary phases for gas chromatography; matrices for mass spectrometry
• Surfactants: cationic surfactants;
• Pharmaceutics: disinfectants, antibacterials, personal health care products, drug delivery;
• Nuclear industry: processing of burnt nuclear fuel rods;
• Other applications: wood preservatives, metal cleaning products, embalming liquid, liquid crystals, functional composite materials, plasticiser, secondary dispersant in pigment pastes, antistatics, liquid mirrors.....

Figure 3. Dissolution of a copper(II) oxide layer on a copper plate by the [Hbet][Tf2N] ionic liquid.

Industrial processes based on ionic liquids

BASF has developed industrial processes utilizing ionic liquids. These processes are called Basil™ processes. The abbreviation BASIL stands for “Biphasic Acid Scavenging utilizing Ionic Liquids.” BASF makes use of this technology for the synthesis of alkoxyphenylphosphines, These compounds are important raw materials for the production of BASF’s Lucirin© photoinitiators that are used to cure coatings and printing inks by exposure to UV radiation. HCl gas is formed during the synthesis of alkoxyphenylphosphines, and this gas has to be scavanged to avoid product decompostion. In the past, scavenging was done with with a tertiary amine. This resulted in a thick, non-stirrable slurry, which was difficult to filter off from the product. If 1-methylimidazole is used as an acid scavenger, an ionic liquid is formed: 1-methylimidazolium chloride, which has a melting point of 75 °C. After the reaction two clear liquid phases occur that can easily be separated. The upper phase is the pure product, the lower phase the pure ionic liquid.
Eastman Chemicals is using a phosphonium ionic liquid in a process for isomerization of epoxybutene to 2,5-dihydrofuran.
Several industrial processes take advantage of the low vapor pressure of ionic liquids.
Air Products has developed an ionic-liquid-based technology for delivering hazardous gases for ion implantations in the electronics industry. The hazardous gases are stored in gas-cylinders filled with an ionic liquid. The [Rmim][Cu2Cl3] ionic liquids can absorb toxic gases like phosphine (PH3) and arsine (AsH3) by complex formation. [Rmim][BF4] ionic liquids can be used to absorb boron trifluoride (BF3). By application of vacuum to the gas-storage cylinders the gases can be released in the actual implantation process and no loss of ionic liquid is observed. Due to the superior heat transfer capability of ionic liquids, gases are easier to load and deliver compared to solid zeolite or carbon-based adsorption systems.
Linde Gas GmbH uses ionic liquids as liquid piston in pumps for the compression of gases (natural gas and hydrogen). A liquid piston has advantages over a solid metal piston. A solid piston has the disadvantage that they wear and become leaky with time. The ionic liquid seals the cylinder space, in which the liquid piston moves, completely and there is no wear. This technology, developed in partnership with proionics, saves a substantial proportion of the energy required to compress gases.