CO2-Catalyzed Acetal Formation in CO2-Expanded Methanol and Ethylene Glycol

Gas-expanded liquids are tunable media for reactions and separations. We report for the first time that CO 2 -expanded alcohols are catalytic media for cyclohexanone acetal formation. This process offers the opportunity to replace some environmentally harmful acid catalysts, as the CO 2 -based acid catalysts are self-neutralizing and result in no solid waste. We propose that the alkylcarbonic acids formed in situ from CO 2 and alcohols are the acid catalysts and that the catalytic capacity can be tuned with CO 2 pressure and temperature. The kinetics of cyclohexanone acetal formation in CO 2 -expanded methanol was investigated under various CO 2 pressures over a temperature range of 25−50 °C without the addition of acid. Pseudo-first- and second-order rate constants were evaluated. These rate constants go through a maximum and the superficial activation energies go through a minimum between 10 and 40 bar of CO 2 .