Citral and citronellal are two compounds that are commonly used as a flavouring and in the perfumery industry, both having a sweet lemon scent. Now, scientists from Germany have developed an ionic liquid catalyst that can be used in the production of citronellal from citral that has high activity and excellent selectivity.

The hydrogenation of citral is usually performed with a palladium-carbon catalyst. Unfortunately, this reaction does not selectively stop at citronellal, often hydrogenating further to make other products.

 

The ionic liquid coating improves the hydrogenation catalysts performance

 

Peter Claus and colleagues from the Technical University of Darmstadt have been looking for a catalyst that would produce citronellal selectively. 'This is very important to the chemical industry, where consecutive hydrogenations must very often be prevented,' says Claus. 

Ionic liquids are salts in liquid form, and are frequently used as green solvents and stabilisers in chemical reactions. Claus found that by impregnating an already known Pd/SiO₂ catalyst with an ionic liquid (called butylmethylimidazolium dicyanamide) an almost 100% yield of citronellal is obtained, and very little further reaction.

What's more, by using the ionic liquid as a coating on the catalyst, rather than a bulk solvent, the production cost decreased, and there was no need to separate out the product at the end of the reaction, making the process one step shorter. 

Douglas MacFarlane from the ionic liquids group at Monash University, Australia, says this development opens up new areas for catalyst applications, 'This is a fascinating piece of work that shows how effective an ionic liquid layer can be in controlling the selectivity of a conventional solid phase catalyst.'

Rebecca Brodie