Low Temperature Gas-phase Oxidation of Aldehydes: Part 1.—The Reaction Below 120°C

Abstract

The gas-phase oxidation of acetaldehyde, propionaldehyde and the butyraldehydes, in an unpacked and a packed vessel between 45 and 120°C, has been studied using a static system with a very small dead volume. Contrary to earlier reports, below 70–80°C with very high aldehyde/oxygen ratios the final pressure decreases were not greater than, but agreed very closely with, the overall stoichiometry RCHO + O2→RCO3H. The variation of pressure with time agreed with that deduced from the usual autocatalytic chain mechanism 2 and confirmed, in particular, that the initiation process is first order with respect to both aldehyde and oxygen, that the addition of oxygen to CH3CO radicals is second order and that the branching process is first order with respect to both peracid and aldehyde. The variation of the second order rate coefficients with temperature and surface/volume ratio indicated that both initiation and branching are heterogeneous. However, the addition process is homogeneous, the rate coefficient being about 1.2 × 107 dm3 mol–1 s–1 between 62.5 and 83.5°C. Analyses of the minor products (CO, CO2, H2O, CH4, C2H6, CH2O, CH3OH, CH3COCH3, CH3COCOCH3) of the acetaldehyde oxidation and of the peracetic acid/acetaldehyde interaction after the oxygen was used up has given information on the chemical nature of the elementary steps occurring in initiation and branching.