ABSTRACT
The physicochemical properties of scCO2 offer a widespread set of alternatives for novel processing protocols and strategies. Strategies of the use of scCO2 for material-processing purposes could be classified into four main groups, as depicted in Fig. 9.1.
9.1 Strategy I: Use of scCO2 as a SolventExtraction of natural compounds, particle formation processes (e.g., rapid expansion of a supercritical solution, RESS), polymer impregnation, cleaning of impurities, solvent removal, and the reaction medium (e.g., in Diels-Alder, silanization, hydrogenation, hydroformilation, oxidation, polymerization, and enzimatic reactions) are among the most promising applications exploiting the solvent power of supercritical carbon dioxide (scCO2) [1-6]. CO2can provide not only environmental advantages but also chemical advantages when applied strategically [7]: (i) CO2 cannot be oxidized; in essence, CO2 is the result of complete oxidation of organic compounds and it is, therefore, particularly useful as a solvent in oxidation reactions, (ii) CO2 is benign and hence cross-contamination during extraction is not significative. (iii) CO2 is an aprotic solvent that can be employed without penalty in cases where labile protons could interfere with the reaction. (iv) CO2 is generally immune to free radicals and does not support chain transfer to solvent during free-radically-initiated polymerization; thus, it is an ideal solvent for use in such polymerizations, despite the fact that it is typically a poor solvent for high-molecular-weight polymers. (v) CO2 is miscible with gases in all proportions above 304 K. (vi) CO2 exhibits solvent properties with a variety of low-molecular-weight organic liquids, as well as with many common fluorous (perfluorinated) solvents. (vii) CO2 exhibits a liquid viscosity only 1/10 that of water and, hence, the Reynolds number for flowing CO2 is approximately 10 times that for conventional fluids at comparable fluid velocity, improving convective heat transfer. Obviously, CO2 also exhibits some inherent disadvantages as a solvent:
