Development of ionic liquids that exhibit useful and unique properties has created huge untapped potential for commercial applications to increase operating efficiencies of many chemical production operations. These include highly selective, high-yielding and previously unavailable intermediate synthesis, and energy efficient azeotropic distillations, i.e., as entrainers in azeotropic or extractive distillations or in extraction processes. One application might be in separation of aromatics from aliphatics, or separation of phenols from reaction mixtures. Additional applications exist in the textile, paper, plastics, nuclear waste treatment, electrolytes for batteries and capacitors, and the oil and gas industries. A quick search of patent literature indicates many new applications to come.
Ionic liquids, a new class of purely ionic, salt-like materials, are liquid below 100oC. This is a functional definition. Commonly, they have melting points below room temperature, with some even below 0oC. These new materials are liquid over a range of 300 oC to 400oC, from the melting point (m.p.) to the decomposition temperature of the ionic liquid.
Compared to table salt, NaCl, with a melting point of 801oC, 1-ethyl-3-methyl-imidazolium ethylsulfate, a typical ionic liquid, has a melting point of -20oC. Figure 1 shows NaCl on the left and an ionic liquid on the right. Figure 1 shows how the irregular shapes of cations and anions in the ionic liquid pack poorly in the crystal structure compared to the regular pattern in NaCl. This leads to low melting points in ionic liquids. In addition, the amorphous nature of ionic liquids spreads out the charges of the cations and anions. This contributes to lowering the melting point. In some cases, especially if long aliphatic side chains are involved, glass transition states can be observed instead of a melting point.
The strong ionic (Coulomb-) interaction within ionic liquids results in negligible vapor pressures, unless decomposition occurs. Below the decomposition temperature, the material is non-flammable and highly stable thermally, mechanically and electrochemically. Furthermore, it imparts very appealing solvent properties and immiscibility with water or organic solvents that results in biphasic systems.
Ionic liquids are frequently reported as being non-flammable. This is only true up to the temperatures at which decomposition takes place. Some ionic liquids already start to decompose at 120°C, others are stable up to nearly 400°C. Upon decomposition neutral and volatile molecules are formed, which, of course, can burn. This explains why ionic liquids indeed have flashpoints even if they are usually much higher than 100°C. However, ionic liquids can easily be disposed by incineration, which is usually done at temperatures of several hundred degrees Celsius. At these very high temperatures even the toughest organic ionic liquids will break down.
The cation has a strong impact on the ionic liquids properties and will often define its thermo-stability and electrochemical stability. Furthermore, choice of the anion controls the chemistry and functionality of the ionic liquid in general. For example, the choice of a hexaflurophosphate anion with 1-butyl-3-methylimidazolium (BMIM) cation will lead to an ionic liquid that is insoluble in water.
However, if the anion is chloride, the ionic liquid is water soluble. By changing the anion, the ionic liquid can be acidic, neutral, or basic.
There are about 1018 theoretical combinations of cations and anions that could potentially form ionic liquids. A realistic number is much lower. Approximately 1,000 ionic liquids are described in the literature today; perhaps 300 are available commercially. Figure 2 shows typical structures that combine organic cations with inorganic or organic anions.
Creating combinations of organic cations and anions enables chemists to design, fine-tune and tailor ionic liquids to possess the physical and chemical properties that they need for their specific application.
Applications can be characterized by use of ionic liquids either as process chemicals or as performance chemicals. In 1948, the 1-butyl-pyridinium chloride/AlCl3 class of electrically conducting materials drew attention for their use as electrolytes for the electro-deposition of aluminum. Other important applications in this context include their use as electrolytes for batteries, in fuel cells as well as in super capacitors. Today, a large choice of ionic liquids is commercially available, providing a broad range of properties as a result of selection of cations, and especially anions.
Over the past five years, other interesting applications have been suggested. Research is active in batteries, where electrical conductivity and nonflammability are important; and in thermal fluids and lubricants because of thermal stability and low vapor pressure. It is reasonable to expect that potential users will bring many more applications forward, with some of them commercialized in the next few years.
A broad range of commercial processes and applications now are being investigated. As a result, a number of large-scale applications could be realized in 2006 and 2007.
Application as acid scavenger
At this time, the most important application of ionic liquids involves their use as reaction media for chemical processes. Many processes produce byproduct acid, most commonly hydrochloric acid. When the reaction product needs to be protected from decomposition or other side reactions, this acid byproduct must be scavenged. Usually, tertiary amines, such as triethylamine, are added to the reaction mixture, generating an ammonium salt; the salt is an unwanted byproduct. In many cases, this ammonium salt can be removed by using an aqueous extraction phase. However, for reaction mixtures that are sensitive to water, the situation is more complicated. In these cases, generation of ammonium salts leads to the formation of a viscous slurry during the reaction. The slurry causes poor mixing, which leads to poor heat transfer, i.e., hot spots, and difficulty controlling the reaction.