The mutual solubility of carbon dioxide and alcohols over a wide range of temperature and pressure provides a useful and tunable medium for reactions and separations. For many years, researchers have used alcohols as cosolvents in supercritical CO2, and recently CO2-swollen alcohols have been used for antisolvent crystallization and as mobile phases for chromatography. However, little consideration has been given to chemical interaction between the alcohols and CO2. We have confirmed that such an interaction does exist and can create an acidic environment. By isolating reaction products we have demonstrated that alcohol−CO2 complexes react similarly to carboxylic acids with diazodiphenylmethane, a compound typically used to evaluate acid strengths. Our evidence indicates that the behavior of CO2−alcohol systems is comparable to that of CO2−water systems, where carbonic acid is formed.

We demonstrate the use of room-temperature ionic liquids as a catalytic, environmentally benign solvent for the cyanide displacement on benzyl chloride, replacing phasetransfer catalyzed biphasic systems and thus eliminating the need for a volatile organic solvent and hazardous catalyst disposal.

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.

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Compressed CO 2 and other gases can serve as media for catalysis in a number of different ways. While use of such gases as supercritical fluids (SCFs) under single-phase conditions has been studied intensively over the past decade, the use of inert gases under split phase conditions has received less attention. Having a condensed phase such as water, an ionic liquid, or even a solid below a SCF allows one to perform reactions combined with simultaneous or subsequent separation of product from catalyst or excess reagent. Performing a reaction in a condensed phase below a subcritical gas allows one to modify the reaction behaviour by adjusting the choice of inert gas or pressure of the gas. Examples of these possibilities are described, with an emphasis on applications to homogeneous hydrogenation and phase-transfer catalysis. (2,3). Much of this research has focused on the use of single-phase conditions (particularly for homogeneous catalysis), because it is thereby easier to establish that the reaction is taking place in the supercritical phase. However, for practical purposes it may be preferable to perform the reaction under splitphase (i.e. biphasic) conditions. This paper summarizes several studies of reactions performed in this manner. There has been a great deal of attention paid recently to the use of supercritical fluids as substitute media for chemical reactions (1), including catalytic reactionsLiquid/SCF biphasic systems, where the liquid is essentially insoluble in the SCF, have many advantages over single-phase SCF media, including the ability to separate the catalyst from the product and the ability to use catalysts