The Gas-Phase Autoxidation of Ketene

Peter J Roden, Moray S Stark and David J Waddington

Department of Chemistry, University of York, York, YO1 5DD, United Kingdom

14th International Symposium on Gas Kinetics, Leeds University, September, 1996

Ketene and the ketyl radical have been reported as important intermediates in the oxidation of hydrocarbons and related compounds (1), and specifically in the oxidation of methane (2), acetylene (3), and acetone (4). However, there have only been two detailed studies of the autoxidation of ketene itself in the gas phase, by Barnard and Kirschner (5) and by Michaud and Ouellet (6,7). Their findings, obtained over a temperature range of 603-748K, are broadly similar, the major products being carbon monoxide, carbon dioxide and formaldehyde. These products were also observed in a later study on the reaction of ketene with hydroxyl radicals (8).

These experimental results cannot easily be rationalised using the mechanisms proposed by earlier workers (5-7). Indeed some of the reactions they proposed now appear unnecessarily complex. We have therefore constructed a reaction mechanism to model their results (eg. figure 1). Addition of hydroxyl and hydroperoxyl radicals to the beta-carbon atom of ketene account for the majority of the observed products.

OH + CH2CO = CO + HOCH2 (+ O2) = CH2O + HO2
HO2 + CH2CO = OH + CH2O + CO

However the rate and the autocatalytic character of the reaction appear to be governed by addition of radicals to the alpha-carbon atom. In the simulations, radical chain branching is by the decomposition of methylhydroperoxide (observed as a product at 603 K), with the methyl radicals formed predominantly by addition of hydroxyl (9) and hydroperoxyl (10) radicals to the alpha-carbon, followed by an internal hydrogen transfer reaction.

OH + CH2CO = CH2C(O)OH = CH3 + CO2
HO2 + CH2CO = CH2C(O)O2H = CH3C(O)O2 = CH3 + O2 + CO

It is difficult to simulate the rapid, autocatalytic overall rate of reaction for ketene autoxidation without having an unusually fast rate for the addition of hydroperoxyl to ketene. This is consistent with recent work (11) on the epoxidation of acrolein by peracetyl radicals, which demonstrated that having a carbonyl group in the vicinity of a C=C double bond greatly increases the rate of addition of peroxyl radicals, when compared with unsubstituted alkenes.


References

1. Salooja, K. C., Combust. Flame, 10: 11-21 (1966).
2. Dagaut, P., Boettner, J., Cathonnet, M., Combust. Sci. and Technol., 77: 127-148 (1991).
3. Miller, J. A., Mitchell, R. E., Smooke, M. D., Kee, R. J. 19th Symp. (Int.) on Comb., The Combustion Institute, Pittsburg,1982, p181-196.
4. Barnard, J. A., Honeyman, T. W., Proc. Roy. Soc. London A, 279: 236-247 (1964).
5. Barnard, J. A., Kirschner, E., Combust. Flame, 11: 496-500 (1967).
6. Michaud, P., Ouellet, C., Combust. Flame, 12: 395-398 (1968).
7. Michaud, P., Ouellet, C., Canad. J. Chem., 49: 297-302, (1971).
8. Faubel, C., Wagner, H. G., Hack, W., Ber. Bunsenges. Phys. Chem., 81: 689-692 (1977).
9. Brown, A. C., Canosa-Mas, C. E., Parr, A. D., and Wayne, R. P., Chem. Phys. Lett., 161: 491-496 (1989).
10. Tidwell, T. T., Ketenes. J. Wiley & Sons, New York, 1995, p636-639.
11. Roden, P. J., Stark, M. S. And Waddington, D. J., The Epoxidation of Acrolein by Peracetyl Radicals, 14th Int. Symp. on Gas Kinetics, Leeds, 1996.


(Image: Comparison of experimental and simulated results)
Figure 1. Autoxidation of ketene. Conditions, 603 K, 80 mbar, [CH2CO]/[O2]initial = 1/2. The points are experimental results of Michaud and Ouellet (7) and the lines are simulations.



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