Green Chemistry and Biosurfactant Research - Biosurfactants: Research Trends and Applications - page 6

Green solvents : Solvents are auxiliary materials used in chemical synthesis. They are
not an integral part of the compounds undergoing reaction, yet they play an important
role in chemical production and synthesis. By far, the largest amount of “auxiliary
waste” in most chemical productions is associated with solvent usage. In a classical
chemical process, solvents are used extensively for dissolving reactants, extracting
and washing products, separating mixtures, cleaning reaction apparatuses, and dis-
persing products for practical applications (Li and Trost, 2008). The development of
green chemistry redefines the role of a solvent: an ideal solvent facilitates the mass
transfer but does not dissolve ! In addition, a desirable green solvent should be natu-
ral, nontoxic, cheap, and readily available. More desirably, it should have additional
benefits of aiding the reaction, separation, or catalyst recycling (Li and Trost, 2008).
Water : The only natural solvent on earth is water. Life requires the construction of
chemical bonds in an aqueous environment. It is obvious that water is the most inex-
pensive and environmentally benign solvent. Since it was reported that Diels-Alder
reactions could be greatly accelerated by using water as a solvent instead of organic
solvents, there has been considerable attention dedicated to the development of organic
reactions in water (Li and Trost, 2008). In many cases, because of hydrophobic effects,
using water as a solvent not only accelerates reaction rates but also enhances reaction
selectivity, even when the reactants are sparingly soluble or insoluble in the medium.
Furthermore, the low solubility of oxygen gas in water, an important property in the
early development of life in an anaerobic environment, can facilitate air-sensitive tran-
sition metal catalysis in open air (Li, 2002). The use of water as a solvent also implies
the elimination of tedious protection-deprotection processes for certain acidic hydro-
gen-containing functional groups, which contributes to the overall synthetic efficiency.
Supercritical CO 2 : Breakthroughs in the use of supercritical fluids such as carbon
dioxide have met with success in the research laboratory as well as commercially.
Supercritical fluids offer a number of benefits, such as the potential to combine reac-
tion and separation processes and the ability to tune the solvent through variations in
temperature and pressure. In the supercritical fluids area, CO 2 has received the most
attention because its critical temperature and pressure ( T c 31.1°C, P c 7.7 MPa) are
more accessible than those of other solvents (water, e.g., has T c 374°C and P c 221 bar).
CO 2 offers numerous advantages as a benign solvent: it is nontoxic, nonflammable,
and inexpensive and can be separated from the product by simple depressurization.
Applications of supercritical CO 2 are found in the dry cleaning industry, where CO 2
replaces per chloroethylene (PCE) as a solvent (Micell Technologies and Hughes
Environmental Systems) in semiconductor manufacturing, where the low surface
tension of supercritical CO 2 avoids the damage caused by water in conventional pro-
cessing (Gleason and Ober, 2001) and in chemical processing.
Ionic liquids : Ionic liquids, a relatively new area of solvent investigation, are
attractive because of their negligible vapor pressure and their use in polar sys-
tems to generate new chemistries. A plethora of ionic liquids can be produced
by varying the cations and anions, permitting the synthesis of ionic liquids tai-
lored for specific applications (Anastas and Kirchoff, 2002). The 2001 Kenneth
G. Hancock Memorial Student Award in Green Chemistry was presented to