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

摘要

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 (500-700K). 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 RO_2 radicals. The temperature-dependent product 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 is associated with HO_2 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 O_2-The α'-RO_2 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 O_2 to form the primary products.