Products and Pathways of Aldehydes Oxidation in the Negative Temperature Coefficient Region

Aldehydes are major intermediates in oxidation and pyrolysis of hydrocarbons and particularly biofuels. While the high temperature oxidation chemistry of C-3-C-5 aldehydes have been studied in the literature, a comprehensive low temperature kinetics remains unaddressed. In this work, acetaldehyde, propanal, and 2-propenal (acrolein) oxidation was investigated at low-temperature combustion condition (500-700 K). The isomer-specific product concentrations as well as the time-resolved profiles were studied using Sandia's multiplexed photoionization mass spectroscopy (MPIMS) with synchrotron radiation from the advanced light source (ALS). The laser-pulsed photolysis generates chlorine atoms which react with aldehydes to form the parent radicals. In the presence of excess oxygen, these radicals react with O-2 and form RO2 radicals. The temperature-dependent product yields are determined for 500K to 700K and the competition between the channels contributing to the formation of each product is discussed. In acetaldehyde oxidation, the formation of the main products is associated with HO2 elimination channel from QOOH or direct H atom elimination from the parent radicals. In propanal oxidation, the most intensive signal peak was associated with acetaldehyde (m/z = 44) which was formed through the reaction of alpha'-R with O-2. The alpha'-RO2 intermediate decomposes to acetaldehyde+OH+CO via Waddington mechanism and formation of five-member ring transition state. In 2-propenal oxidation, the unsaturated radical produced from alpha-R reacts with O-2 to form the primary products.