An easy and environmentally friendly route to a supported palladium catalyst for highly selective partial hydrogenations has been achieved by European researchers.

Frédéric Goettmann, Maria-Magdalena Titirici and colleagues from the Max-Planck Institute of Colloids and Interfaces in Germany and the Institut de Chimie Séparative de Marcoule in France made the new catalyst, based on palladium particles supported on hydrophilic carbon, in an aqueous one-pot reaction by a process called hydrothermal carbonisation.

The catalyst exhibits high selectivity towards batch partial hydrogenation of phenolic compounds. The team tested it to form cyclohexanone, one of the main intermediates in the preparation of compounds used to manufacture nylon-6, nylon-6,6 and polyamide resins.

Cyclohexanone is normally prepared from phenol with a supported palladium catalyst in the gas phase, but this is not an easy task in batch reactors, and functional phenolic compounds are difficult to hydrogenate selectively. Only a few studies report successful reactions in a liquid phase, as the selectivity of hydrogenation is poor, leading to difficult separation steps.

But with Goettmann's new catalyst, water can be used as a solvent, and in one of their reactions, cyclohexanone was obtained in 95% yield with a conversion rate close to 99%.

Goettmann plans to compare the new catalyst with palladium on charcoal catalysts used not only in other hydrogenation reactions, but also in C-C bond forming reactions such as Heck and Sonogashira couplings, functionalisation of allylic derivatives and dehydrogenation reactions. 'Our catalyst opens the possibility to test all these reactions directly in water and, possibly, to observe alternative selectivities,' he said.

The catalyst, however, is slower than palladium on charcoal, and Goettmann wonders if this would be too much of a drawback for potential applications.

Elinor Richards