I capture the catalyst

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

UK chemists have made a new series of catalysts in an effort to up the efficiency of one of the most widely used, and energy-intensive, 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, have examined a series of diphosphine ligands in the rhodium-catalysed carbonylation of methanol. 'This is one of the simplest organic reactions there is, but because of the scale, it must be done so incredibly efficiently,' said Clarke.

The industrial process used currently, BP's Cativa process, requires the starting CO to be purified, and the final product 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 [tetrakis(diphenylphosphino)-p-xylene], that was very selective,' he said. 'We isolated the catalytic intermediate, and showed it doesn't react with hydrogen, where the others do, so this is likely to be the origin of the selectivity.

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

Link to journal article

Evaluation of C₄ diphosphine ligands in rhodium catalysed methanol carbonylation under a syngas atmosphere: synthesis, structure, stability and reactivity of rhodium(I) carbonyl and rhodium(III) acetyl intermediates
Gareth Lamb, Matthew Clarke, Alexandra M. Z. Slawin, Bruce Williams and Lesley Key, Dalton Trans., 2007
DOI: 10.1039/b712974b

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