The Gas Phase Oxidation of Acetaldehyde Reaction Mechanism and Kinetics

Abstract

The mechanism of the low temperature oxidation of gaseous acetaldehyde was investigated in the temperature range of 1 50-400?°C. The minor, intermediate and major products were identified and measured quantitatively by sampling directly into the ionization chamber of an MS10-C2 mass spectrometer from the reactor. The formation of H2O, CO, CO2, HCOOH, H2, HCHO, CH3COOH and CH3OH as the major products, the presence of H2O2, (CH3)2C0, (CH3CO)2O2, (CH3CO)2 and CH4 as the minor products, the production of the unstable product CH3OOH and the existence of peracetic acid as a degenerate branching intermediate were confirmed. The exprimental results led to a proposed degenrate branched chain mechanism for the gas-phase oxidation of acetaldehyde. This paper attempts to explain the kinetics in the region of lower slow combustion (at 150?°C), cool flames, and upper slow combustion (at 400?°C) with an initial total pressure between 4 to 12 cm., Hg. The major products detected were (in order of concentrations): H2O > CO > CO2 > H2 > CH3COOH > HCHO > HCOOH The reaction orders obtained were 0.38 with respect to oxygen pressure and 1.85 with respect to acetadehyde pressure for maximum rate. The reaction rate was found to be independent of the total pressure. In temperature range of 1 50-250?°C, the reaction was accompanied by a pressure decrease, while in 275-400?°C, a pressure increase was observed. The low activation energies of 53.50 KJ mole-1 (in 150-250?°C region) and 57 KJ mole-1. (in 275-400?°C region) indicate that the reaction is a degenerate branching chain type for which the net-branching factor is correlated with the acetaldehyde pressure and the activation energy as f = K [PCH3CHO]0.8exp[-E/RT]. The kinetics data obtained by the mass spectrometer were analyzed by computer programing and the results were presented by the appropriate kinetics plots.