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In pursuit of the perfect green solvent: what qualities would typify the perfect green solvent?

Solvents are a chemist's best friend. Thanks to their heat capacities, solvents can control temperature, they provide mass transport, and they facilitate separations of one chemical from another. Would we be ungrateful if we took a second look at solvents with a more critical eye?

Waste generated by cleaning, extractions, and chemical manufacturing is primarily waste solvent. Apart from the economic cost associated with waste solvent disposal, there are environmental consequences of solvent use including smog formation, global warming, stratospheric ozone depletion, and ground-level ozone production. Health hazards associated with solvents include flammability, carcinogenicity, toxicity, mutagenicity, and a host of other ills. How can we reap the benefits that solvents have to offer, without also suffering from the problems they create?

Green solvents are those that are less damaging to health and environment than those that have been used traditionally. What characteristics should a green solvent have? Perhaps it is easier to describe the characteristics it should not have. Green solvents should not be flammable, toxic to any life form, carcinogenic, or able to contribute to smog formation, ozone depletion, or eutrophication of natural waters. Green solvents should not require a large amount of energy for their production (from renewable raw materials) or for the separation of the solvent from solutes or products. Finally, green solvents should not be expensive, or they will rarely be used. Besides avoiding all of these negatives, a green solvent must have the right physical properties to perform well in the intended application. Unfortunately, as you may have guessed, there is currently no perfect green solvent that can meet all of these requirements. However, some solvents are clearly "greener" than others. This article is a summary of the types of green solvents, especially in the context of solvents as reaction media.

The greenest solvent is sometimes no solvent at all. Reactions and separations in neat liquids without added solvent are obviously desirable because of the greatly reduced volume of waste. (1) However, even this has potential disadvantages in many cases, including excessive viscosity, poor mass transfer, and poor temperature control. The energy required to stir or grind the solventless mixture can have greater environmental impact than a solvent would have.

Volatile green solvents

Green or otherwise--most solvents used in industry or by consumers are volatile. The attraction is obvious. Volatile solvents can be easily removed after use. Volatility, however, comes at an environmental price. Volatile solvents, with a few exceptions, are flammable, are readily lost by vapour emissions to the atmosphere, contribute to smog formation, and enter the human body by inhalation. Green volatile solvents must have features that minimize these adverse properties or minimize the damage done after the solvent vapour enters the atmosphere or the human body.

Water

Nature's solvent has great appeal as a green solvent. Water is obviously nonflammable, nontoxic, and quite inexpensive. Research by C.J. Li, MCIC, and colleagues at McGill University have demonstrated that water can even be used as a solvent for Grignard and other formerly water-incompatible reactions. (2) For some reactions, there is no need for the reagents to be soluble in water. Barry Sharpless at Scripps has introduced "on-water" reactions in which organic reagents floating on top of water react more quickly due to the presence of water. (3) Water's primary disadvantage is the large amount of energy or time that often must be expended in order to sufficiently separate the product from the water before the product can be used and the water discarded.

Supercritical water (sc[H.sub.2]O)

Supercritical water differs from liquid water in being able to readily dissolve both organic compounds and gases--a fortuitous circumstance that makes it appropriate as a medium for reactions between the two. However, its primary disadvantage is its very high temperature (greater than the critical temperature of 374[degrees]C). As a result, sc[H.sub.2]O is primarily investigated as a medium for the complete destruction of toxic wastes and chemical weapons. Steven Rogak at The University of British Colmnbia is investigating flow fouling and heat transfer in super-critical water oxidation (SCWO) processes using Canada's largest SCWO pilot plant.

Supercritical carbon dioxide (scC[O.sub.2])

The critical temperature of C[O.sub.2] is a mild 31[degrees]C, so that scC[O.sub.2] can be used as a solvent for synthetic chemistry (4) (e.g. DuPont's new green process for the production of fluoropolylners) or extraction of natural products (5) (e.g. caffeine from coffee beans). Ill addition to being nontoxic and nonflammable, scC[O.sub.2] has excellent mass transport properties that make it ideal for applications that might be mass transport-limited in conventional solvents. For example, Garry Rempel, FCIC, at the University of Waterloo has developed a process for the chemical modification of artificial rubber by homogeneous catalysts dissolved in scC[O.sub.2]. The prime disadvantages of scC[O.sub.2] are the energy required for corn pressing the gas and the capital costs of the high-pressure equipment.

C[O.sub.2]-expanded liquids

At pressures above 20 bar, gaseous C[O.sub.2] readily dissolves in organic liquids. This causes a volumetric expansion of the liquid phase accompanied by changes in the physical properties of the liquid including increased acidity and lowered viscosity, melting point, and polarity. Volumetric expansion of very viscous liquids such as liquid polymers, ionic liquids, and crude oil is typically smaller, but the viscosity lowering is dramatic. The largest application of expanded liquids in Canada is enhanced oil recovery, where compressed C[O.sub.2] is often piped into an underground oil reservoir so that it will dissolve in and lower the viscosity of the oil and facilitate its transportation to the production well. As reaction or extraction solvents, expanded liquids have the mass transport benefits of scC[O.sub.2] at only a fraction of the pressure.

Biomass-derived solvents

Ethanol has widespread use as a solvent and is considered green because of its biomass origins and its biodegradability, but its flammability is an obvious disadvantage. Terpenes such as D-limonene (from orange peel) or [alpha]- and [beta]-pinene (from pine) are used, either pure or mixed with water, as industrial cleaning solvents. They are biodegradable and have a pleasant smell. But pinene is still flammable and both limonene and pinene have rather high boiling points that limit the rate at which they can be removed or evaporated from products.

Switchable solvents

Some unusual liquids developed recently at Queen's University have the ability to reversibly switch from one kind of solvent to another whenever some form of trigger is applied. (7) Such switchable solvents would be useful in industry for any process in which one process stage (e.g. a reaction or extraction) requires one kind of solvent and the subsequent process stage (e.g. another reaction or a separation) requires a very different kind of solvent. The normal approach in such a situation is to remove the first solvent after the first stage is complete and replace it with a second solvent. The resulting waste in solvent, time, and energy is unfavourable both environmentally and economically. A switchable solvent could be used for both stages, but it is too early in the research to predict whether solvents of this kind will be economically viable.

Nonvolatile green solvents

In some applications, nonvolatile solvents are acceptable, leading to significant "green" advantages including nonflammability, no solvent emissions to the atmosphere, no contribution to smog formation, and lack of human exposure through solvent inhalation. However, if nonvolatile solvents are to be used as reaction media, then a mechanism by which the product can be removed from the solvent must be identified. One could distill the product (if it is volatile), extract it with a volatile green solvent, or cause the product to precipitate by expansion of the solvent with C[O.sub.2]. Academic researchers are increasingly looking at nonvolatile solvents and considering how they can best be used to benefit both industry and environment.

Ionic liquids (ILs)

Ionic liquids (salts melting below 100[degrees]C) are highly polar and are included in discussions of green solvents because they are nonvolatile and nonflammable. Of course, there are exceptions. A few are volatile--especially dimethylammonium dimethylcarbamate. Some are less polar--especially the tetraalkyl-phosphonium ionic liquids made by Cytec Canada. ILs are being investigated and considered by industry for a wide range of applications including as reaction solvents, lubricants, electrolytes, and sensors. (8) Jason Clyburne and Rob Singer at St. Mary's University in Halifax are key researchers in IL chemistry in Canada. They have independently been investigating the use of ILs as solvents for reactions involving reactive organometallic reagents. Unfortunately, ILs do have disadvantages. Some of them are more toxic than others. Their preparation from raw materials can be energetically and economically expensive, and separation of ILs from solutes (without using traditional solvents) can be problematic.

Liquid polymers

Liquid fractions of polyethers (such as poly(ethylene glycol) or PEG) and siloxane polymers are particularly inexpensive nonvolatile green solvents. (6) They are nonflammable, generally nontoxic to both humans and aquatic life, biodegradable (less so for the siloxanes), and available in a wide range of polarities. The principal disadvantages are concerns about long-term stability of the polyethers and, as always for nonvolatile solvents, concerns about product/solvent separation.

Other nonvolatile solvents

Soybean oil, canola oil, and their methyl esters, (9) like ethanol, have biomass origins and biodegradability. Unlike ethanol, they are neither flammable or toxic. The oils are already used as solvents for pigments and the esters are being considered as industrial degreasing solvents. DuPont has been a proponent of "dibasic esters" (i.e., mixtures of methyl esters of adipic, glutaric, and succinic acids) as green solvents due to their low toxicity, carcinogenicity, and volatility. The esters, generated from unwanted by-products of nylon manufacture, are a green alternative to methylene chloride for paint stripping. (10)

Conclusion

There is no perfect green solvent, but there are a wide variety of solvents that are green in one or more aspects. Careful selection of green solvents is required to simultaneously optimize solvent performance and minimize environmental impact. Current research in green solvents highlights their advantages as reaction media, but the lessons learned can be applied to many other applications of solvents.

End notes

(1.) K. Tanaka, Solvent-free Organic Synthesis (Weinheim: VCH-Wiley, 2003).

(2.) C.J. Li, Chem. Rev. 2005, 105, (8), pp. 3095-3166.

(3.) S. Narayan; Muldoon, J.; Finn, M. G.; Fokin, V. V.; Kolb, H. C.; Sharpless, K. B., Angew. Chem. Int. Ed. 2005, 44, (21), pp. 3275-3279.

(4.) P.G. Jessop; Leitner, W. (Eds.), Chemical Synthesis Using Supercritical Huids. (Weinheim: VCH/Wiley, 1999).

(5.) M. McHugh; Krukonis, V. Supercritical Fluid Extraction. 2nd ed.; (Boston: Butterworth-Heinemann, 1994).

(6.) D.J. Heldebrant; Witt, H.; Walsh, S.; Ellis, T.; Rauscher, J.; Jessop, P. G., Green Chem. 2006, 8, pp. 807-815.

(7.) P.G. Jessop; Heldebrant, D. J.; Xiaowang, L.; Eckert, C. A.; Liotta, C. L., Nature 2005, 436, (25 August), 1102.

(8.) P. Wasserscheid; Welton, T. (Eds.), Ionic Liquids in Synthesis; (Weinheim: VCH-Wiley, 2002).

(9.) J. Hu; Du, Z.; Tang, Z.; Min, E., Ind. Eng. Chem. Res. 2004, 43, (24), pp. 7928-7931.

(10.) N. E. Kob, In Clean Solvents; M. A. Abraham, L. M., Ed. (Washington: ACS, 2002), pp. 238-253.

Philip G. Jessop, MCIC, is a Canada Research Chair in green chemistry and an associate professor in the chemistry department at Queen's University. His research interests include green solvents (expanded, polymeric, or switchable), switchable surfactants, and the chemistry of C[O.sub.2] and [H.sub.2].
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Title Annotation:liquid polymers, ionic liquids, Nonvolatile green solvents
Author:Jessop, Philip G.
Publication:Canadian Chemical News
Geographic Code:1CANA
Date:Feb 1, 2007
Words:1922
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