Green solvents are environmentally friendly chemical solvents that are used as a part of green chemistry. They came to prominence in 2015, when the UN defined a new sustainability-focused development plan based on 17 sustainable development goals, recognizing the need for green chemistry and green solvents for a more sustainable future. Green solvents are developed as more environmentally friendly solvents, derived from the processing of agricultural crops or otherwise sustainable methods as alternatives to petrochemical solvents. Some of the expected characteristics of green solvents include ease of recycling, ease of biodegradation, and low toxicity.
Examples
Water Although not an organic solvent, water is an attractive solvent because it its non-toxic and renewable. It is a useful solvent in many industrial processes. Traditional organic solvents can sometimes be replaced by aqueous preparations. Water-based coatings have largely replaced standard petroleum-based paints for the construction industry; however, solvent-based anti-corrosion paints remain among the most used today.
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 carbon dioxide Supercritical carbon dioxide (CO2) is the most commonly used supercritical fluid because of its relatively easy to use. Temperatures above 31 °C and pressures above 7.38 MPa are sufficient to obtain supercriticality, at which point it behaves as a good nonpolar solvent.
Alcohols and esters Ethanol is used in toiletries, cosmetics, some cleaners and coatings. Bioethanol, made industrially by fermentation of sugars, starch, and cellulose is widely available. Biobutanol (butyl alcohol, various isomers) is also produced by fermentation of sugars.
Tetrahydrofurfuryl alcohol (THFA) is a specialty solvent that may be obtained from hemicellulose.
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 in some applications.
Lipid-derived solvents
Lipids include a broad range of nonpolar substances of biological origin, of which triglycerides (TGs) such as those found in refined vegetable oil are the cheapest and most abundant. TGs themselves can be used as solvents, but 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 esters) 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 gasification of biomass and household hazardous waste. Glycerol from lipid hydrolysis can be used as a solvent in synthetic chemistry, as can some of its derivatives.
Deep eutectic solvents Deep eutectic solvents (DES) have low melting points, can be cheap, safe and useful in industries. One example is octylammonium bromide/decanoic acid (molar ratio of [1:2]) has a lower density compared to water of 0.8889 g.cm−3, up to 1.4851 g.cm−3 for choline chloride/trifluoroacetamine [1:2]. Their miscibility is also composition-dependent.
A mixture whose melting point is lower than that of the constituents is called an eutectic mixture. Many such mixtures 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 ammonium salt choline 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.
Natural deep eutectic solvents (NADES) are also a research area relevant to green chemistry, being easy to produce from two low-cost and well-known ecotoxicity components, a hydrogen-bond acceptor, and a hydrogen-bond donor.
Terpenes
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, 20 to 35% β-pinene, and 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 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
Ionic liquids are molten organic salts that are generally fluid at room temperature. Frequently used cationic liquids, include imidazolium, pyridinium, ammonium and phosphonium. Anionic liquids include halides, tetrafluoroborate, hexafluorophosphate, and nitrate. Bubalo et al. (2015) argue that ionic liquids are non-flammable, and chemically, electrochemically and thermally stable. These properties allow for ionic liquids to be used as green solvents, as their low volatility limits VOC emissions compared to conventional solvents. The ecotoxicity and poor degradability of ionic liquids has 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). Ionic liquids produced from renewable and biodegradable materials have recently emerged, but their availability is low because of high production costs.
Switchable solvents 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. This allows an organic solvent to form a homogeneous mixture with the otherwise immiscible water. This process is reversible, and was developed by Jessop et al. (2012) for potential uses in synthetic chemistry, extraction and separation of various substances. The degree of how green switchable solvents are is measured by the energy and material savings it provides; thus, one of the advantages of switchable solvents is the potential reuse of solvent and water in post-process applications.
The properties of switchable solvents are caused by the strength of their conjugate acid's pKa and octanol-water partition coefficient ratio Kow. They must have a pKa above 9.5 to be protonated by carbonated water and also a log(Kow) between 1.2 and 2.5 to be switchable, otherwise they will be hydrophilic or hydrophobic. These properties depend on the volumetric ratio of the compound compared to water. For example, N,N,N-Tributylpentanamidine is a switchable solvent, and for a volumetric ratio of compound to water of 2:1, it has a log(Kow)= 5.99, which is higher than 2.5.
Solvents from waste materials
First-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. 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, an isomeric mixture of amyl alcohol, is a byproduct of ethanol production from sugars. Green solvents derived from fusel oil such as isoamyl acetate or isoamyl methyl carbonate could be obtained. When these green solvents are used to manufacture nail polishes, VOC emissions report a minimum reduction of 68% compared to the emissions caused by using traditional solvents.
Petrochemical solvents with green characteristics Due to the high price of new sustainable solvents, in 2017, Clark et al. listed twenty-five solvents that are currently considered acceptable to replace hazardous solvents, even if they are derived from petrochemicals.
These include propylene carbonate and dibasic esters (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, which is used in the pharmaceutical industry. Propylene carbonate can be produced from renewable resources, but DBEs that have appeared on the market in recent years are obtained as by-products of the synthesis of polyamides, derived from petroleum. Other petrochemical solvents are variously referred to as green solvents, such as halogenated hydrocarbons like parachlorobenzotrifluoride, which has been used since the early 1990s in paints to replace smog-forming solvents.
Siloxanes are compounds known in industry 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 defines the term "polymer solvent" as "a polymer that acts as a solvent for low-molecular weight compounds". In industrial chemistry, polyethylene 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 petrochemical-derived molecules, though ethylene glycol from renewable sources (cellulose) is commercially available.
Physical properties The physical properties of solvents are important in identifying the solvent used according to the reaction conditions. In particular, their dissolution properties make it possible to assess the use of a particular solvent for a chemical reaction, such as an extraction or a washing. Evaporation is also important to consider, as it can be indicative of the potential volatile organic compound (VOC) emissions.
The following table shows selected properties of green solvents in each category: +ClassificationSolventMolar mass