Ionic liquids [ILs] – New emerging technology

Ionic liquids offer new possibilities for solvent engineering for biocatalytic reactions. They may just simply replace organic solvents. Ionic liquids (ILs) have been widely applied for CO2 separation owing to their distinctive properties, such as nonvolatility, tunability of structures, and good affinity with CO2. IL-based membranes exhibit hybrid properties of both ILs and membranes, which is a prospective method to achieve efficient CO2 separation from other light gases. ILs are fast emerging as a new technology with huge potential in multidimensional applications. The graph below shows the number of publications about ionic liquids published in journals. Though the data are a bit old nonetheless show interest in ILs. This reflects on the growing academic and industrial interest in Ionic Liquid technology. Today about 1000 Ionic Liquids are described in the literature, and approximately300 are commercially available.

Currently, the amine is used for CO2 separation. Because it is highly corrosive it adds a huge burden on the capital costs of equipment. 

Current results indicated the technical viability of ILs-based CO2 capture processes compared to available technologies, but further analysis (i.e cost estimations, environmental impacts, and life cycle assessment) are required to ensure the sustainability of a new process.

CO2 capture and sequestration are huge challenges today for the growth H2 business

The total estimated value of the market for hydrogen raw materials is $ 115 billion, and it is expected that it will only grow, reaching $ 155 billion by 2022. Demand for hydrogen has grown more than threefold since 1975. But it almost entirely supplied from fossil fuels, with 6% of global natural gas and 2% of global coal generating around 830 million tons of carbon dioxide per year. Only a small fraction of global hydrogen is produced today from non-carbon emitting water electrolysis. Until non-carbon emitting new technologies are developed and scaled up sequestration of CO2 play a very important role if the H2 market has to grow

What is Ionic liquid? 

 In contrast to ordinary liquids like water or petrol which are made of electrically neutral molecules with no electricity in the molecules, an ionic liquid is made of ions. These substances are variously called liquid electrolytes. They have melting points lower than 100 °C. ILs have many advantages over traditional organic solvents including negligible vapor pressure at room temperature because of strong ionic bonds as well as high thermal stability, making them nearly ideal solvents in extraction techniques. The physical and chemical properties including thermal stability, viscosity, and solubility in water and other organic solvents can be varied by simply changing the combination of cations and anions.

Technical

What is liquid?

A liquid is a nearly incompressible fluid that retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, and plasma), and is the only state with a definite volume but no fixed shape. A liquid is made up of tiny vibrating particles of matter, such as atoms, held together by intermolecular bonds. Most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena. Water is, by far, the most common liquid on Earth.

What is the difference between a molecule and an ion?

A molecule is when a bunch of non-charged atoms bond together to form the smallest and most basic unit of an element or a compound. If they are the same type of atoms, they form an element and if they are different types of atoms, they form a compound. Molecules usually hold the chemical properties of the elements or compounds they make up. An ion is basically an atom or molecule that has lost or gained one or more electrons, causing it to have an unequal number of protons in the nucleus and electrons in the orbitals so that it now has a positive (if it loses electrons) or negative (if it gains electrons) charge.

Application of ionic liquids

CO2 separation and sequestration

by ionic liquidsHow does it work?

 Principle: Amines are the most prevalent absorbent in post-combustion carbon capture technology today. In particular, mono-ethanolamine (MEA) has been used in industrial scales in post-combustion carbon capture, as well as in other CO2 separations, such as "sweetening" of natural gas. However, amines are corrosive, degrade over time, and require large industrial facilities. Ionic liquids, on the other hand, have low vapor pressures due to the strong ionic (Coulomb-) attractive forces within these substances. Vapor pressure remains low through the substance's thermal decomposition point (typically >300 °C). In principle, this low vapor pressure simplifies their use and makes them "green" alternatives. Additionally, it reduces the risk of contamination of the CO2 gas stream and of leakage into the environment. The solubility of CO2 in ionic liquids is governed primarily by the anion, less so by the cation. The hexafluorophosphate (PF6–) and tetrafluoroborate (BF4–) anions have been shown to be especially amenable to CO2 capture.

Process

A typical amine gas treating process flow diagram.

Ionic liquids for use in CO2 capture by absorption could follow a similar process.

Properties of molecules vs ions:

It is obvious that some of the unique properties of ions have made ionic liquids so special. 

Molecule’s properties

In simple words, a molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. Molecules are held together by either covalent bonding or ionic bonding.

Covalent bonding

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are termed shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding

Ionic bonding

Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions and is the primary interaction occurring in ionic compounds. The ions are atoms that have lost one or more electrons (termed cations) and atoms that have gained one or more electrons (termed anions) This transfer of electrons is termed electrovalence in contrast to covalence.

The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids.

Ion’s properties

An ion is a particle, atom, or molecule with a net electrical charge.

Properties of ions: That makes ions unique

Ions in their gas-like state are highly reactive and will rapidly interact with ions of opposite charge to give neutral molecules or ionic salts. Ions are also produced in the liquid or solid state when salts interact with solvents (for example, water) to produce solvated ions, which are more stable, for reasons involving a combination of energy and entropy changes as the ions move away from each other to interact with the liquid. Electrons, due to their smaller mass and thus larger space-filling properties as matter waves, determine the size of atoms and molecules that possess any electrons at all. Thus, anions (negatively charged ions) are larger than the parent molecule or atom, as the excess electron(s) repel each other and add to the physical size of the ion because its size is determined by its electron cloud. Cations are smaller than the corresponding parent atom or molecule due to the smaller size of the electron cloud. One particular cation (that of hydrogen) contains no electrons and thus consists of a single proton - much smaller than the parent hydrogen atom.

Ionic liquid emerging technologies

Unique properties of ionic liquids

[1] are liquid at unusually low temperatures.

[2] the melting point below room temperature, some of them even have melting points below 0 °C

[3] liquid over a wide range of temperature range (300–400 °C)

[4] negligible vapor pressure due to the strong ionic (Coulomb-) interaction within these substances

[5] non-flammable

[6] high thermally, mechanically as well as an electrochemically stable product

[7] flexible, properties can be tailored according to needs. The choice of the cation has a strong impact on the properties of the Ionic Liquid and will often define the stability.

What makes ionic liquids unique solvent? One property that is ionic bonds

Everything of ionic liquids hinges on their ionic bonds

Ionic bonds form stronger than van der Waals attractive forces gives it low vapor pressure, melting points below room temperature; some of them even have melting points below 0 degc and stay as a liquid over a wide temperature range (300–400 degc). Most importantly flexibility to insert anions and cations to tailor the 

During the CO2 capture process, the anion and cation play a crucial role in the dissolution of CO2. Furthermore, intermolecular forces, such as hydrogen bonds, van der Waals bonds, and electrostatic attraction, contributes to the solubility of CO2 in ionic liquids. This makes ionic liquids promising candidates for CO2 capture because the solubility of CO2 can be modeled accurately by the regular solubility theory (RST), which reduces operational costs in developing a more sophisticated model to monitor the capture process.