Carbon dioxide is often promoted as a sustainable solvent, as CO 2 is non-flammable, exhibits a relatively low toxicity and is naturally abundant. However, injudicious use of carbon dioxide in a process or product can reduce rather than enhance overall sustainability. This review specifically examines the use of CO 2 to create greener processes and products, with a focus on research and commercialization efforts performed since 1995. The literature reveals that use of CO 2 has permeated almost all facets of the chemical industry and that careful application of CO 2 technology can result in products (and processes) that are cleaner, less expensive and of higher quality.

Supercritical and near-critical CO2 in green chemical synthesis and processing

Fluids near their critical point have dissolving power comparable to that of liquids, are much more compressible than dilute gases, and have transport properties intermediate between gas-and liquid-like. This unusual combination of physical properties can be advantageously exploited in environmentally benign separation and reaction processes, as well as for new kinds of materials processing.

Supercritical fluids as solvents for chemical and materials processing

This review summarizes recent applications of ionic liquids (ILs) as ‘green’ solvents in extractions of a variety of substances, including metal ions, organic and bio-molecules, organosulfur from fuels, and gases. ILs could also be used along with another ‘green’ technology, supercritical fluid extraction (SFE), for a more effective separation of products from ILs. In addition to their environmentally-benign feature, ILs have other favorable properties over organic solvents used for extraction, such as adjustable hydrophobicity, polarity and selectivity. Copyright © 2005 Society of Chemical Industry

Use of ionic liquids as ‘green’ solvents for extractions

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(97)01013-5.pdf

Why are enzymes less active in organic solvents than in water

Chemical and biochemical transformations in ionic liquids

Supercritical fluids are materials above their critical point that represent a unique class of nonaqueous media for biocatalysis and bioseparation. The inherent gas-like low viscosities and high diffusi vities of supercritical fluids increase the rates of mass transfer of substrates to enzyme. Conversely, the liquid-like densities of supercritical fluids result in higher solubilizing power than those observed for gases. Unlike the behavior of gases and liquids, the physical properties of a supercritical fluid can be adjusted over a wide range by a relatively small change in pressure or temperature. In a supercritical fluid, the careful regulation of the density enables reactant and product solubility to be controlled, thus simplifying downstream separations. The extraction power of supercritical carbon dioxide has been used extensively in both the chemical and food industries.The use of supercritical fluids as a dispersent for biocatalysis was first described in 1985, and there is now a growing tr...

Enzyme Activity in Supercritical Fluids

Biocatalytic transesterification of methylmethacrylate is possible in many different solvents. The reaction rate is readily controlled by variation in solvent physical properties. The reaction proceeds better in hydrophobic solvents, and activity can be restored in hydrophilic solvents by the addition of water. We have now demonstrated that supercritical carbon dioxide is not a good solvent for the reaction between 2-ethlhexanol and methylmethacrylate. It appearance that the supercritical carbon dioxide may either alter the pH of the microaqueous environment associated with the protein or reversibly form covalent complexes with free amine groups on the surface of the enzyme. Although supercritical carbon dioxide is a poor solvent for acrylate transesterification, many other supercritical fluids (ethane, ethylene, sulfur hexafluoride, and fluoroform) are better than most conventional solvents. In supercritical ethane it is possible to control the activity of the enzyme by changing pressure, and the enzyme appears to follow Michaelis-Menten Kinetics. We find that sulfur hexafluoride, the first anhydrous inorganic solvent in which biocatalytic activity has been reported, is a better solvent than any conventional or supercritical organic fluid tested.

Biocatalytic synthesis of acrylates in organic solvents and supercritical fluids: I. optimization of enzyme environment.

We have previously demonstrated that the activity of the lipase (Candida cylindracea) catalyzed transesterification reaction between methylmethacrylate and 2-ethylhexanol in supercritical carbon dioxide is comparatively low. In this article, we have investigated the same reaction in supercritical carbon dioxide with a special emphasis on determining the extent of any interaction between the enzyme and carbon dioxide. Transesterification reaction rates in hexane and supercritical carbon dioxide are compared at different temperatures. In supercritical carbon dioxide, temperature was found to have no significant effect on reaction rate in the range of 40 degrees to 55 degrees C. Above 55 degrees C, however, the reaction rate increased significantly as a function of temperature. It appears that carbon dioxide forms reversible complexes with the free amine groups on the surface of the enzyme. Direct evidence of modification was obtained using mass spectroscopy to detect the extent of modification of a pure protein. The kinetics of the reaction have been studied in hexane, and they obey a ping-pong bi-bi mechanism with inhibition by 2-ethylhexanol. The effect of bubbling carbon dioxide and/or fluoroform on the reaction rate in hexane at different temperatures suggests that the enzyme undergoes shear inactivation in hexane. (c) 1995 John Wiley & Sons, Inc.

Biocatalytic synthesis of acrylates in organic solvents and supercritical fluids: III. Does carbon dioxide covalently modify enzymes?

We describe, for the first time, the ability of a polyoxyethylene sorbitan trioleate-isopropanol microemulsion in hexane to solubilize pure proteins. The dependences of cytochrome c extraction and buffer solubilization by the reverse micellar system on ionic strength of the aqueous phase, detergent concentration, and cosurfactant concentration result in increased extraction. In addition, subtilisin (a serine protease) is shown to be active in this microemulsion. Further the activity of the enzyme can be regulated by the water content of the micelles, enabling control of enzyme activity by "solvent engineering."

Sanjay V. Kamat

Papers

Enzyme Activity in Supercritical Fluids

Biocatalytic synthesis of acrylates in organic solvents and supercritical fluids: I. optimization of enzyme environment.

Biocatalytic synthesis of acrylates in organic solvents and supercritical fluids: III. Does carbon dioxide covalently modify enzymes?

Protein extraction and activity in reverse micelles of a nonionic detergent

Control of enzyme enantioselectivity with pressure changes in supercritical fluoroform