UK scientists have come up with a method that for the first time enables medium-chain aldehydes to be synthesised in a continuous-flow process using a homogeneous catalyst.

Homogeneous catalysts have always been difficult to use in industrial-scale continuous-flow synthesis, because they have to be both soluble in and separable from the reaction mixture. David Cole-Hamilton's team from the University of St Andrews, UK, has now overcome this obstacle by using the product of the reaction to dissolve the catalyst, and then extract it as it is being formed with supercritical CO₂.

The hydroformylation of alkenes is an important industrial process, and involves adding hydrogen and carbon monoxide to an alkene to generate an aldehyde. Supercritical carbon dioxide (scCO₂) has been much used as a solvent for these reactions because of its green credentials. However, the catalysts used tend not to dissolve in the scCO₂, requiring the addition of other liquids as co-solvents. Unfortunately, because the products tend to be soluble in these liquids too, an extraction step is needed. Cole-Hamilton and colleagues have now found the products of the hydroformylation of medium-length alkenes can dissolve the catalysts themselves, dispensing completely with the need for co-solvents.

Cole-Hamilton's system works by mixing the alkene (octene) and aldehyde (nonanal) together, then forming the rhodium imidazolium catalyst while pressurising the system with CO, H₂ and scCO₂. Once the reaction gets going, 'scCO2 can be used to transport everything continuously into and out of the reactor,' said Cole-Hamilton.

Philip Jessop, from Queen's University in Kingston, Canada, said the new method 'is a brilliant twist' on the concept of adding a liquid to the mixture to dissolve the catalyst. Future work, said Cole-Hamilton, 'will aim to increase the reaction rate and to extend the methodology to a wide variety of other reactions.'

David Barden