Ionic liquids are a fascinating area of ongoing chemical research. All molecules are comprised of atoms (the elements from the Periodic Table) held together by chemical bonds. These bonds consist of a pair of electrons, and they can either be evenly shared between atoms (a covalent bond) or unequally distributed between the two atoms that are bonded together, which is called an ionic bond. A simple comparison can be made with two items from the kitchen: salt and table sugar. Table sugar molecules are held together with covalent bonds, and as such the material has a relatively low melting point; you can melt sugar on your stovetop. Table salt, however, is comprised of solely ionic bonds, and has a melting point that is much higher. We normally don’t think of liquids being ionic, as the melting points of the corresponding solids are so high that we don’t run into these liquids every day.
The exceptions to this guideline are the class of compounds known as ionic liquids. These compounds are liquid at room temperature, yet contain ionic bonds in their structure. They are exciting because for all practical purposes, they never boil. This makes them attractive for chemical syntheses as there are no vapor byproducts or hazardous emissions that need to be collected and recycled. The disadvantage to ionic liquids, of course, is that eventually (through repeated heating / cooling cycles) they become contaminated, and need to be disposed of in an environmentally conscious way. However, because they are not readily incinerated (which is the standard method of disposal for most covalent organics), ionic liquids pose a particular problem when it comes to the end of the lifetime: how can chemists get rid of them?
New research from a Chinese university indicates that it may be something as simple as sound waves. By adding a small amount of hydrogen peroxide and acetic acid to a water solution of the ionic liquid and then blasting the flask with ultrasonic high frequency sound waves, the scientists report (in the journal Dalton Transactions) that after three days of gentle heating, the molecular fragments that comprise the ionic liquid are oxidized and break down. This conversion is over 99% complete after 72 hours, and doesn’t require any sophisticated catalysts or special handling. This process was followed by kinetic studies (which measure the concentrations of chemicals as a function of time) combined with gas chromatography-mass spectrometry, which is a very powerful analytical technique capable of identifying fleeting intermediates and complex structures.
This approach of using ultrasonics to degrade ionic liquids is a key breakthrough. Ionic liquids will never make any ground in the area of “green”, environmentally friendly chemistry if there is no easy method of disposing of the ionic liquids at the end of the process. By using such benign reagents as vinegar, dilute peroxide, and sound waves, the potentially toxic ionic liquids are broken down into simple and nontoxic byproducts that can be easily discarded. With the rapid increase of ionic liquid research, it is only a matter of time before they begin to become incorporated into industrial syntheses, and this method of disposal is one of the most promising methods of treating the liquids at the end of their lifetime.
“Ultrasonic chemical oxidative degradations of 1,3-dialkylimidazolium ionic liquids and their mechanistic elucidations”.
Xuehui Li, Jinggan Zhao, Qianhe Li, Lefu Wang and Shik Chi Tsang.
Dalton Transactions, a publication of the Royal Society of Chemistry.