Green solvents[modifier | modifier le code]

A solvent is a substance, liquid at its temperature of use, which has the property to dissolve, dilute or extract other substances without modifying them chemically and without itself being modified. The traditional organic solvents (acetone, NMP, toluene, etc.), although very effective, raise today many problems as they are associated with adverse effects, both on the health and safety of workers (burns, cancer, eczema, brain disorder (encephalopathy), fetotoxicity (via the placenta), and inflammations of several peripheral nerves at the same time, as well as on the environment and health of the general population (smog precursors, water, air and ozone layer pollution). Solvents represent a major part of the chemical use in various domains (paints, coating, synthesis, …) and in consequence define a lot of the environmental performance of the chemical industry.

In 2015, the UN defined a new sustainability focused development plan based on 17 sustainable development goals. It recognizes the need of "green chemistry" and "green solvent" for a more sustainable chemistry in the future. At the same time, more and more companies want to go green, to ensure that their activities and products are part of a sustainable development process. It is in this context that so-called green, ecological, biodegradable, and sustainable solvents have emerged, which were developed as a more environmentally friendly solvents, or biosolvents, derived from the processing of agricultural crops, an alternative to petrochemical solvents.

Classification[modifier | modifier le code]

Whereas regular solvents are easily classified in two categories being polar and nonpolar, it is much harder to do so for green solvents as their chemical structure and source can differ a lot. In fact, green solvents are established from trials and errors in search for substitutes of existing hazardous solvents.

Here is a non-exhaustive list of substances that research have found advantages in favor of being qualified as green solvents, based on their production method, or the raw materials from which they are produced:

Water[modifier | modifier le code]

Water is not an organic solvent because it contains no carbon atoms, and it is the first green solvent that comes to mind when thinking of solvent-solute mixtures. Water is a polar protic solvent thanks to its chemical structure, non-toxic and renewable. For instance in living matter, ions and proteins are all dissolved in water. It is the cheapest and most abundant solvent for a large range of reactions and processes in industrial chemistry. There are cases where traditional organic solvents can be replaced by aqueous preparations. Water-based coatings have largely replaced petroleum-based paints for the construction industry. However, traditional solvent-based anti corrosion paints remain the most used today.

Supercritical fluid[modifier | modifier le code]

Some substances that occur in the gas phase at ambient temperature and pressure can act as solvents if heated to temperatures and pressures above their critical value. This is an area where the gas and liquid states exist in a single phase with properties intermediate between liquid and gas, including the mobility of gas and the dissolving power of liquid: a supercritical fluid.

- Supercritical water (SCW) is obtained at a temperature of 374.2°C and a pressure of 22.05 MPa. It behaves as a dense gas with a dissolving power equivalent to that of organic solvents of low polarity. However, the solubility of inorganic salts in SCW is radically reduced. SCW is used as a reaction medium, especially in oxidation processes for the destruction of toxic substances such as those found in industrial aqueous effluents. The use of supercritical water has two main technical challenges, namely corrosion and salt deposition.

- Supercritical CO₂ Carbon dioxide (CO₂) is the most commonly used supercritical fluid because of the relatively easy to reach conditions. Temperatures above 31°C and pressures above 7.38 MPa are sufficient. It then behaves as a good apolar solvent.

Solvents derived from carbohydrates[modifier | modifier le code]

- Ethanol is the second most used solvent after water. Thus, it is found used in toiletries, cosmetics, some cleaners and coatings. Studies show that simple alcohols such as methanol and ethanol are to be preferred environmentally, unlike formaldehyde or dioxane. The results of this case study indicate that methanol-water or ethanol-water mixtures are environmentally friendly compared to pure alcohol or propanol-water mixtures.

- Ethyl lactate, made from lactic acid obtained from corn starch, is notably used as a mixture with other solvents in some paint strippers and cleaners. Ethyl lactate has replaced solvents such as toluene, acetone, and xylene, resulting in a much safer workplace.

- Bioethanol is made industrially by fermentation of sugars and starch, one of the oldest known chemical processes, but also from cellulose.

- Biobutanol (butyl alcohol, various isomers) is also produced by fermentation of sugars. Isobutanol and ter-butanol are used as solvents in paints.

- Tetrahydrofurfuryl alcohol (THFA) is obtained from hemicellulose.

Solvents derived from lipids[modifier | modifier le code]

Lipids (triglycerides) themselves can be used as solvents. But they are mostly hydrolyzed to fatty acids and glycerol (glycerin). Fatty acids can be esterified with an alcohol to give fatty acid esters, e.g., FAMEs (fatty acid methyl ester) if the esterification is performed with methanol. Usually derived from natural gas or petroleum, the methanol used to produce FAMEs can also be obtained by other routes, including thermochemical gasification of biomass and household hazardous waste. Glycerol from lipid hydrolysis can be used as a solvent in synthetic chemistry as some of its derivatives.

Deep eutectic solvents (DES)[modifier | modifier le code]

By combining certain substances in given proportions, it is possible to obtain a mixture whose melting point is lower than that of the constituents: this is called an eutectic mixture. Many solid substances mixed in this way become liquids that can be used as solvents, especially when the melting point depression is very large, hence the term deep eutectic solvent (DES). One of the most commonly used substances to obtain DES is the quaternary ammonium salt called choline chloride (trimethyl hydroxyethylammonium chloride). Smith, Abbott, and Ryder report that a mixture of urea (melting point: 133°C) and choline chloride (melting point: 302 °C) in a 2:1 molar ratio has a melting point of 12°C.^[1]

Natural deep eutectic solvents (NADES) are also a research area, (Liu et al. 2018), the latter being easy to produce from only two low-cost and well-known ecotoxicity components, a hydrogen-bond acceptor and a hydrogen-bond donor).^[2]

Solvents obtained by extraction[modifier | modifier le code]

Solvents in a diverse class of natural substances called terpenes are obtained by extraction from certain parts of plants. All terpenes are structurally presented as multiples of isoprene with the gross formula (C₅H₈)_n. D-limonene, a monoterpene, is one of the best known solvents in this class, as is turpentine. D-limonene is extracted from citrus peels while turpentine is obtained from pine trees (sap, stump) and as a by-product of the Kraft paper-making process (Sell, 2006). Turpentine is a mixture of terpenes whose composition varies according to its origin and production method. In Canada and the United States, a range of mass concentrations of 40 to 65% α-pinene and 20 to 35% β-pinene, but also 2 to 20% d-limonene are found. α-pinene can replace n-hexane for the extraction of vegetable oil and as a substitute solvent for extracting molecules such as carotenoids used especially as food additives. Turpentine, formerly used as a solvent in organic coatings, is now largely replaced by petroleum hydrocarbons. Nowadays, it is mainly used as a source of its constituents, including α-pinene and β-pinene.

Ionic liquids[modifier | modifier le code]

Ionic liquids are molten organic salts that are generally fluid at room temperature. The most popular cations, variously substituted, include imidazolium, pyridinium, ammonium and phosphonium. Anions include halides, tetrafluoroborate, hexafluorophosphate, and nitrate. Bubalo et al. (2015) argue that ionic liquids are non-flammable, chemically, electrochemically and thermally stable, with negligible volatility. Indeed, they are named green solvents, as their low volatility allows them to limit VOC emissions compared to conventional solvents. Optimistically, ionic liquids from renewable and biodegradable materials have recently emerged. But their ecotoxicity and poor degradability had been recognized in the past because the resources typically used for their production are non-renewable, as is the case for imidazole and halogenated alkanes, derived from petroleum. Plus, their availability is up for debate because their production cost is high.^[2]

Switchable solvents[modifier | modifier le code]

Bubbling CO₂ into water or an organic solvent results in changes to certain properties of the liquid such as its polarity, ionic strength, and hydrophilicity. As an example, this allows an organic solvent to form a homogeneous mixture with otherwise immiscible water. Furthermore, the process is reversible. Jessop et al (2012) developed this technology which has the potential to be used in synthetic chemistry, extraction and separation of various substances. The degree of how green this technology is, is measured by the energy and material savings it provides; thus, one of the advantages of switchable solvents is the reuse of solvent and water.

Solvents from waste materials[modifier | modifier le code]

1st generation biorefineries exploit food-based substances such as starch and vegetable oils. For example, corn grain is used to make ethanol. Second-generation biorefineries use residues or wastes generated by various industries as feedstock for the manufacture of their solvents.

2-Methyltetrahydrofuran, derived from lignocellulosic waste, would have the potential to replace tetrahydrofuran, toluene, DCM, and diethyl ether in some applications. Levulinic acid esters from the same source would have the potential to replace DCM in paint cleaners and strippers.

Used cooking oils can be used to produce FAMEs (Byrne, F. et al., 2017). Glycerol, obtained as a byproduct of the synthesis of these, can in turn be used to produce various solvents such as 2,2-dimethyl-1,3-dioxolane-4-methanol, usable as a solvent in the formulation of inks and cleaners.

Fusel oil, a mixture of amyl alcohol isomers, is a byproduct of ethanol production from sugars as seen previously. Green solvents derived from fusel oil such as isoamyl acetate or isoamyl methyl carbonate could be obtained . When used to manufacture nail polishes, VOC emissions report a minimum reduction of 68% compared to the emissions by using traditional solvents.

Petrochemical solvents with certain green characteristics[modifier | modifier le code]

Notably due to the high price of new sustainable solvents, Clark et al. (2017) list twenty-five solvents that are currently considered acceptable to replace hazardous solvents even if they are petrochemical derived.

These include propylene carbonate and DBEs. Propylene carbonate and DBEs have been the subject of monographs on solvent substitution. Propylene carbonate and two DBEs are considered green in the manufacturer GlaxoSmithKline's GSK Solvent Sustainability Guide used in the pharmaceutical industry. Propylene carbonate can also be produced from renewable resources. On the other hand, the DBEs that have appeared on the market in recent years are more and more obtained by valorizing by-products of the synthesis of polyamides, derived from petroleum. Other petrochemical solvents are variously referred to as green solvents, such as certain halogenated hydrocarbons like PCBTF. This one has been used since the early 1990s in paints to replace smog-forming solvents.

Siloxanes are compounds containing silicon, oxygen, carbon and hydrogen atoms. They are known in industry, especially in the form of polymers (silicones, R-SiO-R'), for their thermal stability and elastic and non-stick properties. The early 1990s saw the emergence of low molecular weight siloxanes (methylsiloxanes), which can be used as solvents in precision cleaning, replacing stratospheric ozone-depleting solvents.

A final category of petrochemical solvents that qualify as green involves polymeric solvents. The International Union of Pure and Applied Chemistry (IUPAC), defines the term polymer solvent as follows: a polymer that acts as a solvent for low-molecular weight compounds. In industrial chemistry, poly(ethylene glycols) (PEGs, H(OCH2CH2)nOH) are one of the most widely used polymeric solvent families. PEGs, with molecular weights below 600 Da, are viscous liquids at room temperature while heavier PEGs are waxy solids.

Soluble in water and readily biodegradable, liquid PEGs have the advantage of negligible volatility (< 0.01 mmHg or < 1.3 Pa at 20°C). PEGs are synthesized from ethylene glycol and ethylene oxide, both of which are petrochemicals derived molecules. Ethylene glycol from renewable sources (cellulose) is commercially available.

Physical Properties[modifier | modifier le code]


Classification	Solvent	Structure & Chemical formula	Boiling point (°C)	Density	Viscosity

Uses[modifier | modifier le code]

Conforming to their various chemical nature, green solvents come to use for applications in a lot of domains. Not only are they preferred for their low impact on the environment, but they have been found to have other pros such as more efficient physicochemical properties or reaction yield than when using traditional solvents. However, the results obtained are for the most part observations from experiments on particular green solvents, so they cannot yet be generalized.

Organic synthesis[modifier | modifier le code]

Green solvents’ efficiency has mainly been proven in extractions and separations in comparison to traditional solvents.

- Supercritical CO₂ is largely used in the food industry as an extraction solvent. Among other processes like flavoring agents, fragrances, essential oils, or lipids extraction from plants, sc-CO₂ is a green substitute to dichloromethane in coffee decaffeination, avoiding the use of a hazardous solvent and additional synthesis steps. Sc-CO₂ can also apply to polymerization reactions, specifically in PTFE (polytetrafluoroethylene) formation to manipulate monomers safely and avoid explosive reactions of peroxide with dioxygen. Although the original process involves water, a green solvent itself, sc-CO₂ allows less waste materials, thus reducing the E factor^[3], indicator of how “green” a reaction is.

$Efactor={\frac {\text{Mass of all waste materials}}{\text{Mass of desired product}}}$

- Observations report that the higher DES’ hydrophobicity, the higher the extraction efficiency of neonicotinoids from aqueous solutions, although the exact trend has not been established yet (Osch et al., 2015).

Industrial chemistry[modifier | modifier le code]

- Ethyle lactate has an excellent cleaning power, resulting in various use to clean metal surfaces, to remove greases, oils, adhesives and solid fuels. That explains their role in aqueous preparations used for industrial degreasing, but also in coatings, adhesives and inks.

- THFA mixtures with other green solvents are studied for their cleaning properties. As an example, the mixture of THFA with FAME and ethyl lactate has been proposed as a paint stripper (Bergemann et al., 2000).

- FAMEs, through studies, have been used as a reactive diluent in coatings for continuous metal strip coating (e.g., the interior coating of food cans), reducing the amount of volatile solvent in this type of coating, and so doing, lowering their toxicity at work and for the environment (Johansson and Johansson, 2007).

- Ionic liquids particularly have applications in electrodeposition.

Toxicity[modifier | modifier le code]

Legislation[modifier | modifier le code]

References[modifier | modifier le code]

↑ (en-US) « Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) > Publications and Tools > IRSST's Production > IRSST Publication », sur Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST)
↑ ^{a et b} Neil Winterton, « The green solvent: a critical perspective », Clean Technologies and Environmental Policy, vol. 23, n^o 9,‎ 30 septembre 2021, p. 2499–2522 (ISSN 1618-954X et 1618-9558, DOI 10.1007/s10098-021-02188-8, lire en ligne)
↑ (en) Roger A. Sheldon, « Atom utilisation, E factors and the catalytic solution », Comptes Rendus de l'Académie des Sciences - Series IIC - Chemistry, vol. 3, n^o 7,‎ 1^er octobre 2000, p. 541–551 (ISSN 1387-1609, DOI 10.1016/S1387-1609(00)01174-9, lire en ligne)

Bibliography[modifier | modifier le code]

Capello, Christian, et al. « What Is a Green Solvent? A Comprehensive Framework for the Environmental Assessment of Solvents ». Green Chemistry, vol. 9, n^o 9, août 2007, p. 927‑34, https://doi.org/10.1039/B617536H.
Green Solvents: Foundations, Health, Safety, Environment and Substitution, https://www.irsst.qc.ca/en/publications-tools/publication/i/101074/n/green-solvents

[:0-1] (en-US) « Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) > Publications and Tools > IRSST's Production > IRSST Publication », sur Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST)

[:1-2] {a et b} Neil Winterton, « The green solvent: a critical perspective », Clean Technologies and Environmental Policy, vol. 23, n^o 9,‎ 30 septembre 2021, p. 2499–2522 (ISSN 1618-954X et 1618-9558, DOI 10.1007/s10098-021-02188-8, lire en ligne)

[3] (en) Roger A. Sheldon, « Atom utilisation, E factors and the catalytic solution », Comptes Rendus de l'Académie des Sciences - Series IIC - Chemistry, vol. 3, n^o 7,‎ 1^er octobre 2000, p. 541–551 (ISSN 1387-1609, DOI 10.1016/S1387-1609(00)01174-9, lire en ligne)

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