Ionic liquids

In the last few years a new research area has emerged called “ionic liquids.” These are solvents composed of chemical compounds carrying ionic charges. In the past they have been known as “molten salts.” That many substances melt is common knowledge to everyone. Indeed, most substances will either melt or decompose if heated to a high enough temperature. This is also true of many organic compounds. The melting point for organic compounds is related to their structure. Both the size, shape of the molecule, and the type of functional groups contained can effect the melting point. In the case of ionic organic compounds, e.g. alkylammonium, pyridinium, imidazolium, etc. compounds, the melting point in a series of similar compounds seems to depend largely on the size of the respective cation and anion. In some systems, the size of at least one of the components of the ionic pair can be increased enough to lower the melting point below room temperature. While ionic liquids, as molten salts, have been known for centuries, and even organic ionic liquids have been known for the last hundred years, their use as novel solvent systems has only been investigated since the early 1980's. Two of the better studied ionic liquid cations are 1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium.

The development of organic room temperature ionic liquids (RTIL’s) has precipitated intense academic and industrial research activity. A variety of new chemical processes have been and are being developed which incorporate ionic liquids both as solvents and catalysts. The advantages of ionic liquids are many:

1. They are good solvents for a large variety of organic, inorganic, and polymeric materials. However, they do not dissolve glass, polyethylene, or Teflon. High solubility usually implies small reactor volumes in the final process.

2. Robust. Large working temperature range (-40 to +200 ⁰C), and (for selected ionic liquids) resistance to water and air oxidation.

3. Non-volatility. Ionic liquids have essentially zero vapor pressure. This eliminates the volatile organic carbon compounds (VOC’s) associated with non-ionic, or molecular solvents, e.g. acetone and methanol. VOC’s are tightly regulated by environmental agencies and a significant source of air pollution.

4. This zero volatility allows for high temperature reactions without the requirement of a pressure vessel to contain the vapors.

5. Designability. Some ionic liquids are water soluble, others are not. Some dissolve typical organic solvents, others do not. It has been estimated there are 10⁸ different possible ionic liquids. Thus, a large range of conditions and functionality are available when designing an ionic liquid-based process.

6. They can exhibit acidic or basic properties, and have the potential to catalyze certain reactions in certain systems.

7. They are relatively inexpensive and typically easy to prepare.

Ionic liquids have been shown to be excellent reaction mediums for Diels-Alder reactions with a variety of compounds. Their advantages for product separation have been utilized in hydrogenation reactions, where the reaction took place in the ionic liquid and the product was extracted in a molecular solvent. Ionic liquids have also been used to couple vinyl compounds to aromatics via the Heck reaction:

There are still many areas of chemistry where the potential for ionic liquid solvents is largely unexplored. Lignocellulosics are one of these unexplored areas. Here ionic liquids can serve as solvents, reaction catalysts, and components of novel separation schemes. We are currently exploring this research area with a grant from the USDA National Research Initiative, Competitive Grants Program.