Combustion of Aldehydes in the Negative Temperature Coefficient Region: Products and Pathways

Aldehydes are major intermediates in oxidation and pyrolysis of hydrocarbons and particularly biofuels. While the high temperature oxidation chemistry of C₃-C₅ 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 products 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 O₂ and form RO₂ radicals. The temperature dependent products yields are determined for 500 K to 700 K and the competition between the channels contributing to the formation of each product is discussed. In acetaldehyde oxidation, the formation of the main products are 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 α′-R with O₂. α′-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 α-R reacts with O₂ to form the primary products.