Selective catalytic conversion of pulegone in supercritical carbon dioxide towards natural compounds: Carvone, thymol or menthone

• Diversity of products from pulegone was obtained with a high selectivity.
• New conversion of pulegone to thymol.
• Noble metal catalyst drives selectivity of hydrogenation.
• Time of reaction is reduced when supercritical CO2 is applied.
• Pulegone as a useful plant source and toxic substance is converted into useful products.

Conversion of pulegone in CO2 can suit as an alternative method to synthesis of menthone, carvone or thymol. The use of high-density carbon dioxide in the catalytic conversion of pulegone, for the first time, was investigated over 0.5 wt% Pd, or Rh, or Ru supported on alumina via hydrogenation allowing to eliminate use of organic solvent. It was possible to achieve various key natural flavour products with high selectivity and conversion. Reaction catalysed by all the catalysts was faster in a single supercritical phase (fluid reagents) than in a biphasic system (liquid + fluid reactants). The reaction catalysed by Pd produced fully hydrogenated CC isomers of menthone with a selectivity above 98% in a short time of 15 min. The Rh catalyst was found to be more active and selective towards thymol's formation (99.8%). The Rh catalysed reaction exhibited >98% selectivity of thymol with the conversion of >98% after a 75 min reaction. A 25 min-reaction with the Ru catalyst converted pulegone in 99% to carvone with a 99.4% selectivity. High conversion and diverse and high selectivity accompanied by beneficial reduction in reaction time depending on the catalyst used are achieved in CO2 applied compared to those reported for conventional organic solvents used.

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