Gas-expanded liquids are tunable media for reaction and separation. We report that gas-expanded liquids under CO2 pressure are unique media for amine formation and separation. In the heterogeneous hydrogenation of benzonitrile and phenylacetonitrile with NiCl2/NaBH4 in CO2-expanded ethanol, the primary amines are protected by CO2 so that the yield of the primary amines is greatly increased and the production of the secondary amines is effectively suppressed. In the homogeneous hydrogenation of benzonitrile and phenylacetonitrile with RhH(P-i-Pr3)3 and benzophenone imine with Rh(1,5-C8H12){P(C6H5)3}2]PF6 in CO2-expanded tetrahydrofuran, the primary amines are separated in situ in the form of solid carbamic acids and/or ammonium carbamates with increased yield while the catalyst remains in the solution. These results demonstrate the potential for using modest pressures of CO2 to facilitate reactions as well as to separate products.

Supercritical fluid chromatography is a powerful technique for analysis, and it also provides a useful tool for the measurement of thermodynamic properties. For any application, the method involves equilibration between the mobile supercritical fluid phase and the polymeric stationary phase. The mobile-phase properties are often modified substantially with the addition of polar or protic cosolvents, but these adducts also cause changes in the stationary phases. In situ FTIR spectroscopy and the swelling of poly(dimethylsiloxane) (PDMS) are combined to quantify these changes due to partitioning of the cosolvent into and the swelling of the polymeric stationary phase as a function of fluid pressure. These results are used to evaluate the magnitude of the corrections for the changes in retention time of analytes, which affects both analytical methods and thermophysical property measurement.

The first example of a phase-transfer-catalyzed reaction carried out in a supercritical fluid (SCF) is reported. The kinetics of the nucleophilic displacement of benzyl chloride with a bromide ion in SCF solvent CO2 in the presence of acetone cosolvent is presented. In addition, the solubility of tetraheptylammonium bromide (THAB) in SCF CO2 in the presence of acetone cosolvent is reported. The reaction rate was measured at 50 and 75 °C at pressures to 200 bar; the reaction was found to follow first-order, reversible kinetics for the reactions catalyzed by THAB and zero-order kinetics for reactions catalyzed by 18-crown-6.

An alternative means of epoxidation is reported that uses environmentally benign supercritical carbon dioxide as both a solvent and reactant in combination with aqueous H2O2, which is made possible through the in situ formation of peroxycarbonic acid. Experiments were conducted at 40 °C and 120 bar in which cyclohexene was epoxidized to 1,2-cyclohexene oxide and 1,2-cyclohexanediol in this aqueous−organic biphasic system. Through the addition of NaHCO3 and the hydrophilic cosolvent dimethylformamide, the conversion increased from 0.4 mol % (without additives) to 12.6 mol % (with 0.1 mol % NaHCO3 and 13 mol % dimethylformamide). The results suggest that the reaction occurs within the aqueous phase, which led to investigations using the water-soluble olefin 3-cyclohexen-1-carboxylate sodium salt as a means of verifying the reaction location. Epoxidation of 3-cyclohexen-1-carboxylate sodium salt went to completion in less than 20 h at 40 °C and 120 bar with an epoxide yield of 89 mol % and diol yield of 11 mol %.

The hetero-Diels-Alder reaction between anthracene and excess 4-phenyl-1,2,4-triazoline-3,5-dione has been investigated in supercritical CO 2 at 40°C and pressures between 75 and 216 bar. Bimolecular reaction rate constants have been measured via fluorescence spectroscopy by following the decrease in anthracene concentration with reaction time. The reaction rate is elevated in the vicinity of the critical pressure. This difference is consistent with local composition enhancement and can be modeled with the Peng-Robinson equation of state.

We report the selective removal of a desired reaction intermediate with a supercritical fluid in a new synthesis route to poly(ethylene terephthalate), avoiding ethylene glycol. We have successfully catalyzed the esterification of terephthalic acid with ethylene oxide in a supercritical fluid with a series of quaternary ammonium salts to form mono(2-hydroxyethyl terephthalate). This desired monoester was removed from the involatile bed of terephthalic acid and catalyst by continuous extraction with supercritical fluid before subsequent reaction to the diester could take place.

The first example of a phase-transfer-catalyzed alkylation reaction under supercritical fluid conditions is reported. The reaction is that of phenylacetonitrile and ethyl bromide in the presence of tetrabutylammonium bromide and potassium carbonate in supercritical ethane at 45, 60, and 75 °C and 138 bar. Results show that the reaction will go to completion in less than 24 h in the presence of the catalyst but that only a few percent conversion is achieved without it during the same period of time. The effects of catalyst concentration, temperature, and cosolvents are investigated. Catalyst solubility estimates and kinetic analyses suggest that the reaction takes place on the surface of the potassium carbonate particles. When the same reaction is attempted in supercritical carbon dioxide, both carboxylation and alkylation are observed. Cycloalkylation reactions between phenylacetonitrile and dibromoalkanes are also discussed.