Green solvents are the future for rice straw conversion!
Rice straws are worldwide by-products in the production of rice. Often falsely depicted as waste, they can be converted into high-value products such as biofuels. To achieve this, conventionally high-energy, non-sustainable pre-treatment of the rice straw biomass is necessary to separate the compounds it is made out of (cellulose and lignin). Ionic liquids and deep eutectic solvents as green solvents are the future of this process as they can be utilized for this process while being much more climate-friendly. But what exactly are these liquids and what are they made of? Read the paragraphs below to find out more about them!
Ionic liquids are defined as compounds completely composed of ions with low melting point (below 100 °C) 1 or most simply they are salts that are liquid at, or close to, room temperature.2
In common salts, such as sodium chloride (NaCl), the cations (Na+) and the anions (Cl-) are tightly packed together because the ions are small and symmetrical (Figure 1). In ionic liquids however, both cations and anions are larger and asymmetrical and cannot come close enough together. This is the main reason why they do not establish a crystalline structure and are fluid at room temperature.2
Figure 1 a) Crystal structure of sodium chloride showing the ions tightly packed together; b) Structure of the Ionic Liquid 1-butyl-3-methylimidazolium hexafluorophosphate. (Hydrogen = white, Carbon = grey, Nitrogen = blue, Phosphor = pink, Fluorine = yellow).
Ionic liquids have different chemical and physical properties than “normal” liquids. When most liquids are heated, their molecules absorb energy and evaporate into the atmosphere but this is not the case with ionic liquids. They cannot evaporate and ‘pollute’ the atmosphere also they cannot burn. This is an excellent property and makes them ‘green solvents’. They are the safest option for industrial plants where large volumes of solvents are required.2
Because they are composed of ions, ionic liquids are also very good electrical conductors.
The advantages and characteristics of ionic liquids can be summarised below:3, 4
- They are liquid in a wide range of temperatures
- They are highly polar
- They are non-volatile
- They have very high electrical conductivity
- They are stable and resistant to heat up to 300 °C
The most relevant applications of ionic liquids are:3, 4
- Green solvents as alternative to volatile (organic) solvents
- Use in catalytic reactions (as two-phase catalysts or as stabilizers for other catalysts)
- Super Solvents: they are very good at dissolving a wide range of compounds at low temperatures, even compounds that were traditionally considered insoluble2
- Electrochemistry: ionic liquids are excellent electrolytes in rechargeable lithium batteries and are widely used in electrochemical devices such as fuel cells, photovoltaic cells, power storage
- Liquid-liquid extraction: it is a separation method widely used in industry for its energy efficiency. In liquid-liquid extractions two non-miscible liquids are used to promote the separation of compounds between the two non-miscible phases. Ionic liquids are a good alternative to organic (and volatile) solvents. They are mainly used for the extraction of valuable metal ions like gold or for the removal of toxic ions like mercury or cadmium, from drinking water.
Deep Eutectic Liquids
Deep eutectic liquids are obtained from the combination of two compounds in a specific ratio that results in a mixture (the deep eutectic liquid) with a much lower melting temperature than each of the starting compounds.5 They are obtained from the combination of a hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA). HBDs are compounds that contain a strong electronegative (electron attracting) atom like Nitrogen, Oxygen and Fluorine that is covalently bound to a hydrogen atom. An HBA is a compound with an electronegative atom with a lone pair of electrons that participate in the hydrogen bond. This is depicted in Figure 2a below.
A classic example of a deep eutectic liquid is the mixture of choline chloride and urea in a 1:2 ratio. Such a mixture has a melting point of 12 °C whereas the starting compounds choline chloride and urea have melting points of 302 °C and 133 °C respectively.5
Figure 2 a) Schematic representation of a Hydrogen Bond between a Hydrogen Bonding Donor (HBD) and a Hydrogen Bonding Acceptor (HBA) shown as dashed lines; b) structure of the deep eutectic liquid choline chloride and urea in a 1:2 ratio. (Hydrogen = white, Carbon = grey, Oxygen = red, Nitrogen = blue, Chlorine = green)
Applications of Deep Eutectic Solvents (DESs):6, 7
- Green solvents
- Metal processing thanks to the high solubility of metals and metals salts in DESs. Under this umbrella, the following applications can be found:
Metal electrodeposition: Suitable DES could achieve the deposition of new alloys and semiconductors as well as new coating methods for the deposition of metals resistant to corrosion like Titanium, Aluminum and Tungsten. The plating of technologically important systems such as Nickel, Cobalt and Chromium is hampered by the use of aqueous precursors which are carcinogenic. DESs could offer a solution and avoid the use of such precursors. The metal deposition processes of five metals aided by DESs have been studied in detail: Chrome, Aluminum, Copper, Nickel and Zinc plating.Metal electropolishing: This is the controlled dissolution of a metal surface to reduce surface roughness, increase optical reflectivity and hence increase resistance to corrosion and lubricity in engines and decrease wear. DESs allowed the application to be extended to the electropolishing of Aluminum, Titatium, Nickel/Cobalt alloys and super alloys for the first time.
Metal extraction: Metals and a variety of metal oxides are soluble in DESs. At an industrial scale the processing and reprocessing of metals produces a large volume of aqueous waste which requires treatment for the acid and basic byproducts before disposal. This is chemically and energy demanding. DESs could offer a more environmentally friendly option.
- Separation processes: DESs are used in liquid-liquid extractions at industrial scale and are showing promise for the recovery of compounds like radioactive ions and transition metal ions from aqueous solutions. Their application in the desulfurization of fuels is also growing in relevance.
- Gas capture: DESs have been used to prepare recyclable solid sorbents for the capture of CO2.
Battery technologies: DESs are being considered as electrolytes in lithium-ion batteries and redox flow batteries.
- Biocatalysis which is the chemical modification of a chemical compound performed by a microbe or enzyme: The most commonly used solvents are aqueous solvents. DESs can be used as reaction media for the biocatalysis or as cosolvents to aid the dissolution of non-polar compounds into the aqueous solution.
- Biomass processing.
- Pharmaceutical and medical research: DESs are drawing increasing attention as vehicles for drug delivery.
IONIC LIQUIDS VS DEEP EUTECTIC LIQUIDS: DIFFERENCES AND SIMILARITIES
The main difference between Deep eutectic liquids and Ionic liquids is the nature of their starting materials: ILs are a combination of organic cations (positive charge) with organic or inorganic anions (negative charge) while DESs are a combination of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs). In both cases the number of potential combinations is unlimited.8 The differences and similarities are nicely summarised in the figure below (Figure 3).6, 8
Figure 3 Differences and similarities of ILs and DESs adapted from Płotka-Wasylka et al. and Hansen et al..6, 8
Did we catch your interest?
If you want to find out more about deep eutectic solvent, check out our short article “TRUNGS ARTICLE” which explains its utilization in more detail. To find out more about the products the rice straws can be converted into, have a look at the following articles: “Bioethanol: a CO2 neutral energy source”, “Malic acid is part of your daily life”.
1. Lei, Z.; Chen, B.; Koo, Y.-M.; MacFarlane, D. R., Introduction: Ionic Liquids. Chemical Reviews 2017, 117 (10), 6633-6635.
2. Deetlefs, M.; Seddon, K. Ionic Liquids - The discovery most likely to shape the 21st century.
3. Kianfar, E., Ionic Liquids: Properties, Application, and Synthesis. 2020.
4. Mutelet, F.; Butet, V.; Jaubert, J.-N., Application of inverse gas chromatography and regular solution theory for characterization of ionic liquids. Industrial & engineering chemistry research 2005, 44 (11), 4120-4127.
5. Scelsi, E.; Angelini, A.; Pastore, C., Deep eutectic solvents for the valorisation of lignocellulosic biomasses towards Fine Chemicals. Biomass 2021, 1 (1), 29-59.
6. Hansen, B. B.; Spittle, S.; Chen, B.; Poe, D.; Zhang, Y.; Klein, J. M.; Horton, A.; Adhikari, L.; Zelovich, T.; Doherty, B. W., Deep eutectic solvents: A review of fundamentals and applications. Chemical reviews 2020, 121 (3), 1232-1285.
7. Smith, E. L.; Abbott, A. P.; Ryder, K. S., Deep eutectic solvents (DESs) and their applications. Chemical reviews 2014, 114 (21), 11060-11082.
8. Płotka-Wasylka, J.; De la Guardia, M.; Andruch, V.; Vilková, M., Deep eutectic solvents vs ionic liquids: Similarities and differences. Microchemical Journal 2020, 159, 105539.