Supercritical fluids (scf) are promising alternatives to hazardous organic solvents. For the past decade, intensive studies have been carried out to develop a variety of environmentally-acceptable and highly efficient transition-metal-catalyzed reactions including hydrogenation, carbonylation, carbon-carbon bond formations, and oxidation in scf, and it has been found that the use of scf can offer great oppotunities to improve the rate of the reactions and to control the selectivity because of high miscibility with gaseous reactants and tunable physicochemical properties as well as the absence of a liquid/gas-phase boundary. Among those scf, CO₂ is most often used because of its non-toxicity, inexpensiveness, and nonflammability. Besides, the high volatility of CO₂ allows it to be completely and easily removed from the reaction product, resulting in overall solventless reactions. Thus, scf have been recognized as not only merely alternatives to conventional organic solvents but also very attractive media for organic synthesis. Multiphase reaction systems in which scf are employed in combination with other media (H₂O, ionic liquids, etc.) are also attractive and useful reaction media.

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A new method for rapid expansion from supercritical solution with a nonsolvent (RESS-N) to obtain polymer microparticles containing proteins, medicine and inorganic microparticles is presented in the following. On this method, a suspension of microparticles such as protein in carbon dioxide (CO₂) containing a cosolvent and dissolved polymer is sprayed through a nozzle into atmosphere. Polymer solubility, such as that of poly (ethylene glycol) and poly (methyl methacrylate), in CO₂ was found to increase significantly through use of low molecular weight alcohols as cosolvents. Subsequent to expansion, the particles do not agglomerate, since pure cosolvent functions as nonsolvent for the polymer. Polymer coating thickness on about core materials, mean particle diameter and particle size distribution can be controlled according to polymer feed composition.

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Supercritical methanol, which has 512.6 K of critical temperature and 8.09 MPa of critical pressure, is a promising solvent for advanced chemical reaction processes. As this solvent can change the solvent properties much more widely and successively than liquids solvents and control the solvation structure easily by manipulating the temperature and pressure, we can realize the high reaction rate and the selective conversion to a desirable product at the same time.
In this review, we show the dependence of the microscopic solvent properties of sub-and supercritical methanol on temperature and pressure and explain the relationship between the properties and reactivity. Then we describe the highly selective and non-catalytic methylation of aromatic ring, N-methylation. O-methylation and production of acetal from ketone using supercritical methanol

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Considerable attention has been directed to enzymatic processes in supercritical fluids (SCFs) for theoretical and industrial applications. SCFs have unique physicochemical properties that are readily altered through changes in pressure and temperature, especially in the near-critical region and such changes may lead to different reactivities in enzymatic processes. SCFs as reaction media may result in better mass-transfer properties compared to liquids. Increased flow rate was found to promote reaction speed and water-content, choice of cosolvent and SCFs were found to have strong effect on enzymatic reaction rates with consequent effect on solvation state of substrate, product or even enzyme. In the near-critical region not only reaction rate, but stereoisomerism were noted to be improved. In the near-critical region, local density and composition of solution underwent considerable change, and occasionally solute-solvent clustering was found to affect reactivity. The effects of this formation on reactions were thus examined at the molecular level and those of the conformation of active enzymes on reactivity were elucidated. SCFs in the near-critical region may be considered to activate the enzymes by altering their characteristic structures and, thus a stereoselective mechanism would appear operative.

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Supercritical carbon dioxide have been attracting much attention as an environmentally-friendly alternative solvent that can replace organic solvents because of its low critical temperature and pressure, non-toxicity, inflammability, and physical properties similar to hexane. In this review article, recent research on the synthesis of polymeric materials with supercritical carbon dioxide as a solvent was described.

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