Ionic liquids – safer and more environmentally friendly solvents

Organic solvents have long contributed to the smooth running of chemical production thanks to their diverse structures and good solvency capabilities. However, organic solvents also have a serious disadvantage, that is, it is too easy to turn into a gas and evaporate into the air. A typical example is that when sterilizing the surface of the skin with alcohol, we feel a slight cooling sensation because the rapid evaporation of the alcohol carries away the heat.

The volatile nature of organic solvents poses two serious problems: firstly, solvent molecules escaping into the air can cause a range of environmental and health problems. For example, a number of organic substances can react with ozone and nitrogen oxides in the air when exposed to light, forming the infamous photochemical smog. If the volatilization of organic solvents occurs in relatively closed and poorly ventilated indoor environments, the solvent molecules can easily enter the human body, thus causing damage to health.

Secondly, most organic solvents are flammable. The vapors that are constantly being emitted further exacerbate the fire hazard and inevitably keep users on their toes. The emergence of ionic liquids has given people a new hope. Do ionic liquids not evaporate? Not really. This is because in the ionic liquid, although the attraction between positive and negative charges is not enough to maintain the anion and cation in a fixed position, but still has enough strength to prevent them from moving in a wider range, to make them into a gas can be said to be “difficult”. While liquid molecular compounds boil and vaporize when temperatures continue to rise, ionic liquids remain liquid, evaporation into the air is negligible, and ionic liquids do not usually burn.

Therefore, if ionic liquids are used to replace conventional organic solvents, the safety factor in chemical production can be greatly improved. The advantages of using ionic liquids as solvents go beyond safety. In ionic liquids, we can change the chemical structure of anions and cations separately, so ionic liquids are more versatile than conventional organic solvents. For example, many chemical processes require the use of catalysts, which are traditionally dissolved in a solvent, but it is now possible to simplify the reaction process by adjusting the chemical structure so that the ionic liquid acts as both solvent and catalyst.

In conventional organic solvents, the alkylation of olefins and aromatics cannot be carried out, while in ionic liquids, catalyzed by Sc(OTf)3, the reaction can be carried out smoothly at room temperature, with yields as high as 96%, and the ionic liquid catalysts can be reused.

Cellulose hydrolysis has given researchers quite a headache. Cellulose forms dense crystals due to strong intermolecular hydrogen bonding, which makes them insoluble in most solvents. Without the ability to form a solution, cellulose hydrolysis is difficult to perform efficiently. Fortunately, in recent years researchers have discovered that many ionic liquids can dissolve a percentage of cellulose by breaking the hydrogen bonds between cellulose molecules, and some can even swallow the entire lignocellulosic biomass whole. Once a solution is formed, we have the potential to further process cellulose to make better use of the treasure trove nature has given us.