The largest transition metal-catalysed process in the chemical industry could get an efficiency boost in the future, thanks to the isolation of a key catalytic intermediate.

UK chemists have investigated a new series of catalysts in an effort to up the efficiency of one of the largets scale catalytic processes in the chemical industry - the conversion of carbon monoxide (CO) and methanol into acetic acid. Matthew Clarke and colleagues at St Andrews University, UK, in collaboration with BP scientists (Hull, UK) have examined a series of diphosphine ligands in the rhodium-catalysed carbonylation of methanol, alongside investigating the reactivity of the catalytic intermediates. 'This is one of the simplest organic reactions there is, but because of the scale, it must be done so incredibly efficiently and relatively small improvements can make a large change to the total energy used in the process' said Clarke.

The industrial process used currently, BP's Cativa process, is already efficient compared to previous technology, but does require the starting CO to be purified, and the almost pure acetic acid to be distilled - both of which are energy-expensive processes. Now BP is funding a project to look for the next generation of catalyst to get around either, or both, of these drawbacks.

Cheaper, lower grade CO includes large amounts of hydrogen, so to use it you need a catalyst that reacts with the CO but not the hydrogen, said Clarke. 'We identified a ligand, dppx [bis(diphenylphosphino)-o-xylene], that was very selective,' he said. 'We isolated the catalytic intermediate, and showed it doesn't react with hydrogen, where the less selective catalysts do, so this is likely to be the origin of the selectivity.

'We're already making new catalysts based on this work, trying to gain a rational understanding of the selectivity,' Clarke added. 'We haven't cracked it yet, but I have an inkling there is a ligand out there that is active, selective, stable and commercially viable.'

Andreas Danopoulos, who studies catalysis at the University of Southampton, UK, said the work was a good approach to understanding the catalyst. 'But I'm not sure that rational design is the best way to solve this problem - I think a combinatorial approach [to screen a large number of possible ligands] would be better,' he said.

James Mitchell Crow