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. The burgeoning interest in the field was obvious at the recent American Chemical Society (ACS) meeting in New York, where ionic liquids were the focus of 10 sessions (1).
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 colorless, 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°C.
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 nonvolatile 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 (2).
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 expensive. Supercritical fluids, which have both gas-and liquid-like properties, are highly versatile solvents for chemical synthesis (3). This technology was recently commercialized by Thomas Swan & Co., Ltd., in a chemical plant designed for multipurpose synthesis. Together with ionic liquids (4–6), 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 vapor 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 possible (7). (For comparison, about 600 molecular solvents are in use today.)
This diversity enables the solvent to be designed and tuned (2) to optimize yield, selectivity, substrate solubility, product separation, and even enantioselectivity. Ionic liquids can be highly conducting (8), dissolve enzymes (9), form versatile biphasic systems for separations (10), can form both polymers and gels for device applications (8), are media for a wide range of organic and inorganic reactions (4–6), and are the basis for at least one industrial process, called the BASIL process (see the figure) (11).
The BASIL process was developed and is operated by BASF. At the meeting, Matthias Maase (BASF) revealed that use of the BASIL process increases the productivity of their alkoxyphenylphosphine formation process by a factor of 80,000 compared with the conventional process. Other companies are also pursuing the use of ionic liquids. Bernd Weyershausen (Degussa) presented an ionic liquid-based process for the synthesis of organosilicon compounds. Use of an ionic liquid solvent enabled the catalyst to be easily recycled and reused without further treatment after separation from the product at the end of the reaction. Christian Mehnert (ExxonMobil) described biphasic hydroformylation with rhodium catalysts in ionic liquids.
Because research into ionic liquids is at an early stage, many of their properties remain to be elucidated. Nonetheless, ionic liquids have already provided access to new chemical processes. Recent papers describe their potential application as embalming fluids (12), in ion drives for space travel (13), for desulfurization of fuels (14), and as lubricants (15).
Ionic liquids have already found many laboratory applications in synthesis, catalysis, batteries, and fuel cells (4–6, 8, 16), and numerous new combinations of ionic liquid solvent properties are available or predicted. 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 used.
Science 31 October 2003:
Vol. 302 no. 5646 pp. 792-793