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Benefits of Supercritical Fluids
Supercritical fluids are materials in a state above their critical temperature and critical pressure. They show unique and tunable physicochemical properties (both gaseous and liquid) and have found numerous applications in extraction, particle formation, cleaning, dyeing, etc. They are also promising alternatives for the conventional solvents used in organic synthesis as solubility, mass transfer, solvent strength, reaction kinetic can be easily manipulated by relatively small changes in operating pressure and temperature. Especially supercritical carbon dioxide and water are attractive because these reaction media are non toxic, non flammable, inexpensive and environmentally benign.
- Relatively low operating pressure and temperature
- Easy recuperable by reducing pressure
- Higher solubility for organic substrates
Depending on the pressure and/or temperature the dielectric constant varies from 2 to 30. This is similar to the range from a nonpolar solvent such as hexane (1.8) to a polar solvent such as methanol (32.6)
- Acid catalytical properties
Under supercritical conditions, an increase of about 30 times the proton concentration that occurs under room temperature and atmospheric pressure can be obtained. Hence, the use of an extra acid catalyst can be avoided for specific applications.
Although the many advantages, the use of supercritical fluids remains technically demanding and a basic knowledge is necessary for the save and efficient use of these promising reaction media on laboratory scale. The technique can also be used on industrial scale: a unique multi-purpose processing plant using supercritical fluid (SCF) as solvent has recently begun production (Thomas Swan & Co). The plant replaces conventional solvents with inert SCFs for reactions like hydrogeneration, Friedel-Crafts alkylations and acylations, hydroformylations and etherification. The result is cleaner, faster, more selective processes with no volatile organic solvents to be recovered and little waste.
The web site "Supercritical Fluids in Organic Synthesis" is an excellent starting point for both a researcher new to the field as an expert who is looking for a intuitively organized web site about this subject.
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