(583az) Mechanistic Studies of Ethylene Carbonylation Over Molybdenum Hexacarbonyls

The carbonylation of alkenes is potentially an important route to alkanoic acid intermediates. Iodide-promoted molybdenum hexacarbonyl has been reported to be active for carbonylation of ethylene and carbon monoxide into propionic anhydride or propionic acid.[1] As compared with the nickel tetracarbonyl-based process commercialized by BASF, the molybdenum hexacarbonyl catalysts operate at relatively lower temperature and pressure. However, using iodide can lead to chronic corrosion of the reactors. Further understanding of the catalyst mechanism may lead to improved catalyst designs or altered promoter requirements. Currently, mechanistic studies are scarce. It has been demonstrated that zero valent Mo species are oxidized to a Mo(II) species by ethyl iodide,[2] but the step in which ethylene enters the catalytic cycle is not fully understood.

Here, we discuss the effect of various process parameters on ethylene carbonylation, including the partial pressure of CO, ethylene and hydrogen, T, solvent, water, iodide source and the catalyst loading. A key finding is that the apparent conversion of ethyl iodide decreases with increasing reaction time, suggesting the initiation of the catalytic cycle with ethyl iodide followed by the incorporation of ethylene while the solution iodide recovers and reaches a steady state concentration. A detailed mechanism will be presented, based on the quantitative product analysis by GC and FTIR, results from isotopic labeling, and FTIR analysis of metal carbonyl catalyst species.

[1] J.R. Zoeller, E.M. Blakely, R.M. Moncier, T.J. Dickson, Molybdenum catalyzed  carbonylation of ethylene to propionic acid and anhydride, Catalysis Today, 36 (1997) 227-241.

[2] J.R. Zoeller, N.L. Buchanan, T.J. Dickson, K.K. Ramming, Molybdenum catalyzed ethylene carbonylation. II. Spectroscopic investigation of the reactions and equilibria of molybdenum hexacarbonyl and molybdenum halocarbonyls under reaction conditions, Catalysis Today, 49 (1999) 431-440.