C1 to acetyls: catalysis and process

Carbonylation of methanol and methyl acetate to acetic acid and acetic anhydride are the only current examples of C₁ processes displacing petrochemical routes for the manufacture of commodity chemicals. Modem carbonylation processes are based upon an iodide promoted, homogeneous rhodium catalyst operating under mild reaction conditions (150°C–200°C, 25–45 bar). This highly selective catalytic cycle is described, and contrasted with other catalytic metals. The embodiment of Rh/I catalysis in commercial BP Chemicals methanol and methyl acetate carbonylation processes is also described, illustrating the process simplicity arising from high selectivity.

The now extensive literature on attempts to support the rhodium catalyst in iodide promoted systems is reviewed. In the liquid phase, catalytic behaviour similar to that for the homogeneous system is claimed, although proper comparative data and conclusive evidence for catalyst immobilisation is generally lacking. A number of supported rhodium systems are reported to be active under vapour phase conditions, and activated carbon is also found to promote carbonylation activity of certain base metals, notably nickel. Issues affecting commercial viability include catalyst lifetime, gaseous by-product formation and efficiency of utilisation of precious metal components.

Literature on carbonylation in the absence of iodide promoter, and on direct routes to acetyls from synthesis gas or methanol only is also reviewed. All require step changes in catalytic performance in order to approach commercial viability.