We have used solid acids (both Bronsted and Lewis Acids) to catalyse continuous reactions in scCO2
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- Alkylation of aromatics, where the presence of CO2 appears to improve the selectively of alkylation (e.g. “Continuous Reactions in Supercritical Fluids; a Cleaner, More Selective Synthesis of Thymol in Supercritical CO2” (R. Amandi, J.R. Hyde, T. Lotz, S. K. Ross and M. Poliakoff) Green Chem. (2005) 7, 288-93. “The Synthesis of o-Cyclohexylphenol in Supercritical Carbon Dioxide: Towards a Continuous Two-Step Reaction” R. Amandi, K. Scovell, P. Licence, T. J. Lotz and M. Poliakoff) Green Chem. (2007) 9, 797-801.
- Etherification of alcohol, where the only by-product is water “The Continuous Acid Catalysed Dehydration of Alcohols in Supercritical Fluids: A New Approach to the Cleaner Synthesis of Acetals, Ketals and Ethers with High Selectivity” (W. K. Gray, F. R. Smail, M. G. Hitzler, S. K. Ross and M. Poliakoff) J. Am. Chem. Soc. (1999) 121, 10711-18. “The Automation of Continuous Reactions in Supercritical CO2: The Acid-Catalysed Etherification of Short Chain Alcohols.” (B. Walsh, J. R. Hyde, P. Licence, M. Poliakoff) Green Chem., (2005) 7, 456–63.
- Etherification of diols, where manipulation of the phase behaviour of the reaction mixture can switch selectivity from mono-to bis-methylation. “The Selective Mono-Protection of 1,n-Terminal Diols in Supercritical Carbon Dioxide, a Striking Example of Solvent Tunable Desymmetrization.” (P. Licence, W. K. Gray, M. Sokolova and M. Poliakoff) J. Am. Chem. Soc. (2005), 127, 293-298.
- The aldol reaction was found to proceed over a variety of heterogeneous acidic and basic catalysts and with increased selectivity compared to using neat reactants. "The continuous self aldol condensation of propionaldehyde in supercritical carbon dioxide: a highly selective catalytic route to 2-methylpentenal" (J. G. Stevens, R. A. Bourne and M. Poliakoff) Green Chem. (2009), 11, 409 - 416
We always welcome e- mails to martyn.poliakoff@nottingham.ac.uk from those interested in this area or who would like reprints of papers.