Supercritical fluid reactant addition

Figure 2. Supercritical fluid reactant addition system.

Investigations of organic reactions in supercritical solvents are subject to several constraints, one attributable to supercritical fluid properties and others imposed for interpretive and experimental simplicity. Because supercritical fluid properties are affected by changes in temperature, a reaction should be selected which does not require heat for initiation and is not highly exothermic. Additionally, for experimental simplicity and clarity of interpretation, a clean, well-understood reaction should be chosen and one should expect an experimentally observable response to changes in pressure. Finally, a unimolecular reaction which produces a single product obviates the complication of controlling the concentrations of two reactants and simplifies product analysis. The photoisomerization of trans-stilbene meets these requirements. 

Supercritical fluid extraction (SEE) turns out to be very effective in the isolation of all three surfactant classes from solid matrices. While supercritical CO2 alone did not affect significant recovery of surfactants, the addition either of modifiers or of reactants resulted in nearly quantitative recoveries. Thus, LAS and secondary alkane sulphonates (SAS) are extracted from sewage sludges in the form of tetrabufylammonium ion pairs. Lee et al. extracted NP from sewage... 

The use of supercritical CO2 necessitates additional unit operations to allow the conditioning of the fluid so that it is at the correct conditions when the reactants are added. The reactants and solvents are mixed and then taken to the reactor. 

Whilst it is obviously valuable to measure the solubility of reagents in the SCF, it is important to be aware that the solubility in a multicomponent system can be very different from that in the fluid alone. It is also important to note that the addition of reagents and catalysts can have a profound effect on the critical parameters of the mixture. Indeed, at high concentrations of reactants, the mole fraction of C02 is necessarily lower and it may not be possible to achieve a supercritical phase at the temperature of interest. Increases in pressure (i.e. further additions of C02) could yield a single liquid phase (which would have a much lower compressibility than scC02). For example, the Diels-Alder reaction (see Chapter 7) between 2-methyl-1,3-butadiene and maleic anydride has been carried out a pressure of 74.5 bar and a temperature of 50 °C, assuming that this would be under supercritical conditions as it would if it were pure C02. However, the critical parameters calculated for this system are a pressure of 77.4bar and a temperature of 123.2 °C, far in excess of those used [41]. 

---