Green Solvent – Ionic Liquid

Ionic liquid refers to an ionic compound that has a melting point below room temperature and is therefore in liquid form at room temperature. However, in actual practice, we often include ionic compounds whose melting points are higher than room temperature but lower than 100 °C, because it does not require a very high temperature to melt them. It should be noted here that aqueous solutions of ionic compounds (such as salt water) cannot be regarded as a member of ionic liquids, because although they are also liquids at room temperature, they contain a large number of molecular compounds. In ionic liquids, the force between anions and cations is Coulomb force, the size of which is related to the number of charges and radius of anions and cations. The larger the ionic radius, the smaller the force between them, and the lower the melting point of this ionic compound. The anions and cations of some ionic compounds are large in size and loose in structure, resulting in a low force between them, so that the melting point is close to room temperature.

As early as 1914, German chemist Paul Walden discovered that the melting point of ionic compounds such as ethylamine nitrate was only 12 °C, which is recognized as the earliest discovered ionic liquid. But for a long time, scientists have only regarded ionic liquids as some “alternative” compounds and have not conducted too much in-depth research on them. This situation did not change until the end of the last century and the beginning of this century, when research on ionic liquids began to grow explosively.

The reason why ionic liquids suddenly attracted attention after decades of silence is that scientists have discovered that these flowing salts can help us create a greener and more environmentally friendly world.

In the following examples, let’s feel what difference ionic liquids can make.

Ionic liquids – safer and more environmentally friendly solvents.

Organic solvents have long made great contributions to the smooth progress of chemical production with their diverse structures and good solubility. However, organic solvents also have a serious disadvantage, which is that they are too easy to turn into gases and evaporate into the air.

A typical example is that when we use alcohol to disinfect the surface of the skin, we will feel a little cool, because the rapid evaporation of alcohol takes away heat.

The volatile nature of organic solvents brings two serious problems:

First, after the solvent molecules escape into the air, they may cause a series of environmental and health problems. For example, many organic substances can react with ozone and nitrogen oxides in the air under light to form the notorious photochemical smog. If the volatilization of organic solvents occurs in a relatively closed and poorly ventilated indoor environment, the solvent molecules can easily enter the human body, causing damage to health.

Secondly, most organic solvents are flammable. The continuously volatilized steam further exacerbates the fire hazard, which inevitably makes users nervous. The emergence of ionic liquids has given people new hope. Will ionic liquids not volatilize? Really not. This is because in ionic liquids, although the attraction between positive and negative charges is not enough to keep anions and cations in a fixed position, it still has enough strength to prevent them from moving in a larger range. It is “difficult to turn them into gas”. When the temperature continues to rise, liquid molecular compounds boil and vaporize, while ionic liquids can continue to remain in liquid form, and the volatilization into the air is almost negligible, and ionic liquids usually do not burn. Therefore, if ionic liquids are used to replace traditional organic solvents, the safety factor in chemical production can be greatly improved.

The advantages of using ionic liquids as solvents are more than just safety. In ionic liquids, we can change the chemical structure of anions and cations separately, so ionic liquids are more diverse than traditional organic solvents. It is estimated that the number of compounds that can theoretically form ionic liquids can be as high as an astonishing 1018! Although far fewer have been put into practical use – there are only about 300 ionic liquids available for purchase, this still provides the chemical industry with a wealth of choices. For example, many chemical productions require the use of catalysts. In traditional processes, we usually dissolve catalysts in solvents, but now we can adjust the chemical structure to allow ionic liquids to play both the role of solvent and catalyst, thereby simplifying the reaction process.

The first company to apply ionic liquids in chemical production and taste the sweetness was probably the German chemical giant BASF. Among BASF’s many products, one is an organophosphorus compound, which produces an acidic byproduct of hydrogen chloride during its synthesis, which needs to be neutralized with an alkali, and then the salt generated by the reaction is removed. The traditional production process usually uses triethylamine to neutralize hydrogen chloride, and the resulting triethylamine hydrochloride is a solid that is insoluble in organic matter and is quite difficult to separate. In 2002, BASF adopted a new process called BASIL, using 1-methylimidazole, which also has alkalinity, instead of triethylamine. The resulting 1-methylimidazole hydrochloride is an ionic liquid, just like oil and water are immiscible, forming two layers of liquid with a clear interface, making it easier to separate from the reaction product. Although this small change is underestimated, the benefits it brings are quite considerable: not only has the reaction yield increased from 50% to 98%, but the production efficiency and reactor volume production capacity have increased to nearly 90,000 times the original! In addition, breakthroughs have been made in the application of ionic liquids in other fields. The Institute of Process Engineering of the Chinese Academy of Sciences has made certain achievements in the industrialization of ionic liquids. For example, it has successfully developed a green set of new technologies for the efficient conversion of CO2 to prepare various carbonate new materials using ionic liquids. A 150,000-ton industrial unit has been built and is in full production. It has successfully developed the world’s first ionic liquid method for melamine tail gas separation industrial demonstration unit, and has built a billion-square-meter melamine tail gas ammonia-carbon separation industrial demonstration unit. In traditional organic solvents, the alkylation reaction of olefins and aromatics cannot be carried out. In ionic liquids, under the catalysis of Sc(OTf)3, the reaction can proceed smoothly at room temperature with a yield of up to 96%, and the ionic liquid catalyst can be reused. Cellulose hydrolysis is a headache for researchers. Due to the strong hydrogen bonds between molecules, cellulose forms dense crystals, which makes them insoluble in most solvents. If a solution cannot be formed, cellulose hydrolysis is difficult to carry out efficiently. Fortunately, in recent years, researchers have found that many ionic liquids can dissolve a certain proportion of cellulose by destroying the hydrogen bonds between cellulose molecules, and some ionic liquids can even swallow the entire lignocellulosic biomass. Once the solution is formed, it is possible to further process the cellulose to make better use of the treasure trove given to us by nature.

The root cause of the safety hazards of lithium-ion batteries is the flammable organic solvents in the electrolyte. Can we replace it with ionic liquids that do not burn? A study from Canada in 2010 showed that if 40% of ionic liquid was added to the electrolyte of traditional lithium-ion batteries, the performance of the battery was not significantly affected, but the flammability of the electrolyte was significantly reduced, and it would not burn even in the face of bright flames. With such an electrolyte, the safety hazards of lithium-ion batteries may become history.

However, although ionic liquids have shown unique advantages in many fields, a considerable number of applications related to them are still in the laboratory research stage. Although the reasons behind this are varied, there are several common “roadblocks” that are worth noting.

First, compared with traditional solvents, ionic liquids are still a bit expensive. It is estimated that the current price of ionic liquids is around US$20 per kilogram. If ionic liquids are to be used to replace traditional organic solvents, the price should be reduced to at least US$2.5 per kilogram to make it profitable for producers who are used to careful calculations.

Secondly, although many ionic liquids are liquids at room temperature, they have high viscosity and their fluidity is much worse than that of traditional organic solvents, which may bring many inconveniences to the production and use process.

Third, most of the ionic liquids developed in the early stage are easy to absorb moisture, which is also a troublesome thing in many occasions. For example, although many ionic liquids are good solvents for cellulose, once a small amount of water is mixed into them, their ability to dissolve cellulose will drop sharply; in addition, some ionic liquids, such as the hexafluorophosphate mentioned above, will decompose once they come into contact with water, releasing highly toxic hydrofluoric acid, which is a serious threat to the personal safety of users. If the use of ionic liquids must be carried out under extremely dry conditions, their own advantages will be greatly reduced. In addition, how to recycle and reuse ionic liquids after use, and how to reduce the toxicity of certain ionic liquids are all issues worthy of attention.

There is no doubt that there are still many difficulties to overcome in the future development of ionic liquids. However, decades of practice have shown us that these mobile salts are indeed promising. Hainan Sincere Industries Co., Ltd. has been promoting the development of ionic liquids for more than 20 years and firmly believes that the future will bring us a greener and more environmentally friendly life.