Ionic liquids-useful reaction green solvents for the future

Abstract

Ionic liquids (IL) represent fascinating new class of solvents with unusual physical and chemical properties; low melting salts (up to 100⁰C). The main driving force for research in this area is the need to find replacement for environmentally damaging solvents in a wide range of chemical processes. To date, most chemical reactions have been carried out in molecular solvents. For the past twenty years, most of our understanding of our chemistry has been based upon the behavior of molecules in the solution phase in molecular solvents. Recently a new class of solvents has emerged called as Ionic liquids. An ionic liquid is an organic salt in which the ions are poorly coordinated, which results in these solvents being liquid below 100°C, or even at room temperature (room temperature ionic liquids, RTIL's). At least one ion has a delocalized charge and one component is organic, which prevents the formation of a stable crystal lattice. Ionic liquids are composed entirely of ions. For example, molten sodium chloride is an ionic liquid; in contrast, a solution of sodium chloride in water (a molecular solvent) is an ionic solution. The term “ionic liquids” has replaced the older phrase “molten salts” (or “melts”), which suggests that they are high-temperature, corrosive, viscous media (like molten minerals). The reality is that ionic liquids can be liquid at temperatures as low as –96°C. Furthermore, room-temperature ionic liquids are frequently colourless, fluid, and easy to handle. In the patent and academic literature, the term “ionic liquids” now refers to liquids composed entirely of ions that are fluid around or below 100°C¹. Properties, such as melting point, viscosity, and solubility of starting materials and other solvents, are determined by the substituents on the organic component and by the counter ion. Many ionic liquids have even been developed for specific synthetic problems. For this reason, ionic liquids have been termed "designer solvents”. This means that their properties can be adjusted to suit the requirements of a particular process. Properties such as melting point, viscosity, density, and hydrophobicity can be varied by simple changes to the structure of the ions. For example, the melting points of 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates are a function of the length of the 1-alkyl group, and form liquid crystalline phases for alkyl chain lengths over 12 carbon atoms. Another important property that changes with structure is the miscibility of water in these ionic liquids. For example, 1-alkyl-3-methylimidazolium tetrafluoroborate salts are miscible with water at 25 °C where the alkyl chain length is less than 6, but at or above 6 carbon atoms, they form a separate phase when mixed with water. This behaviour can be of substantial benefit when carrying out solvent extractions or product separations, as the relative solubility’s of the ionic and extraction phase can be adjusted to make the separation as easy as possible. In addition, ionic liquids have practically no vapour pressure which facilitates product separation by distillation. There are also indications that switching from a normal organic solvent to an ionic liquid can lead to novel and unusual chemical reactivity. This opens up a wide field for future investigations into this new class of solvents in catalytic applications. Research into ionic liquids is booming. The first industrial process involving ionic liquids was announced in March 2003, and the potential of ionic liquids for new chemical technologies is beginning to be recognized. One of the primary driving forces behind research into ionic liquids is the perceived benefit of substituting traditional industrial solvents, most of which are volatile organic compounds (VOCs), with non-volatile ionic liquids. Replacement of conventional solvents by ionic liquids would prevent the emission of VOCs, a major source of environmental pollution.  Ionic liquids are not intrinsically “green”—some are extremely toxic—but they can be designed to be environmentally benign, with large potential benefits for sustainable chemistry. There are four principal strategies to avoid using conventional organic solvents: No solvent (heterogeneous catalysis), water, supercritical fluids, and ionic liquids. The solventless option is the best established, and is central to the petrochemical industry, the least polluting chemical sector. The use of water can also be advantageous, but many organic compounds are difficult to dissolve in water, and disposing of contaminated aqueous streams is expensive². Supercritical fluids, which have both gas- and liquid-like properties, are highly versatile solvents for chemical synthesis. This technology was recently commercialized by Thomas Swan & Co., Ltd., in a chemical plant designed for multipurpose synthesis. Together with ionic liquids, these alternative solvent strategies (sometimes referred to as alternative reaction media or green solvents) provide a range of options to industrialists looking to minimize the environmental impact of their chemical processes. What are the advantages of using a room-temperature ionic liquid in an industrially relevant catalytic process? As noted above, ionic liquids have no detectable vapour pressure, and therefore contribute no VOCs to the atmosphere. But this is not the only reason for using ionic liquids. Another is that at least a million binary ionic liquids, and 10¹⁸ ternary ionic liquids, are potentially possible³. (For comparison, about 600 molecular solvents are in use today.) The next decade should see ionic liquids being used in many applications where conventional organic solvents are used today. Furthermore, ionic liquids will enable new applications that are not possible with conventional solvents. In the future, solvents will be designed to control chemistry, rather than the chemistry being dictated by the more limited range of molecular solvents currently used⁴. As discussed above, Ionic liquids are salts consisting of cations such as imidazolium, pyridinium, quarternary ammonium and quarternaryphosphonium, and anions such as halogen, triflate, trifluoroborate and hexafluorophosphate, which exists in the liquid state at relatively low temperatures. Their characteristic features include almost no vapour pressure, non-inflammability, non-combustibility, high thermal stability, relatively low viscosity, wide temperature ranges for being liquids and ionic liquid conductivity.

KEY WORDS: ionic liquids, green solvents, designer solvents, low melting salts.