Supercritical, Near-critical and Expanded Solvents in Chemical Reactions

Supercritical, Near-critical and Expanded Solvents in Chemical Reactions 

As already mentioned, supercritical solvents have been used as chemical reaction solvents for a long time. However, their study in the context of clean processes or Green Chemistry is relatively recent It was during the 1990s that research on carbon dioxide as a reaction medium developed. The main goal was the reduction or complete ehmination of volatile organic solvents used in laboratory and industrial processes. 

When more complex molecules are involved, the changes can be more pronounced. The phase behavior itself can even become more complicated, as the formation of a third phase, involving Hquid-liquid-vapor equilibrium, is very common at pressures and temperatures not very far from the critical values of the solvent 

This focus on phase behavior brought about the understanding that some of the reactions that had been successfully accomplished in supercritical carbon dioxide were actually taking place in the liquid phase (or at the interface) of a heterogeneous liquid-vapor system. Similarly, reactions carried out in liquid, near-critical propane had shown that this solvent displayed the same advantages as those of truly supercritical ones. The fact is that the densities of liquid mixtures close to the mixture s critical line are sufficiently lower than those of a classical Hquid to exhibit the same type of property values (lower viscosity, higher diffusivity, higher so- 

Although the relatively poor solvent power of carbon dioxide may be used to advantage for selective separations, as explained above, it is a key technical issue that is limiting its widespread use. It can, in fact, spoil the economics of otherwise highly attractive processes, namely integrated reaction-separation. The recent trend is to tackle this problem of solubility by using biphasic mixtures of COj and liquid compounds, where the liquid phase retains some of the advantages of supercritical fluids, but with enhanced solvent power. 

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