The C2H4O isomers in the oxidation of ethylene

Abstract

A combined rotational spectroscopy, thermochemistry, and kinetic modeling study explores the mechanism of acetaldehyde (ethanal), vinyl alcohol (ethenol), and ethylene oxide (oxirane) formation in the oxidation of ethylene (ethene). Multiplexed quantitative detection of oxidation intermediates and products exiting a SiO₂/SiC microreactor at 1700 K is demonstrated with the BrightSpec W-band chirped-pulse Fourier transform millimeter-wave spectrometer. The broadband rotational spectrum contains transitions of formaldehyde (CH₂O), methoxy (CH₃O), methanol (CH₃OH), ketene (CH₂CO), acetaldehyde (CH₃CHO), syn-vinyl alcohol (syn-CH₂CHOH), anti-vinyl alcohol (anti-CH₂CHOH), oxirane (c-CH₂OCH₂), propyne (CH₃CCH), and syn-propanal (syn-CH₃CH₂CHO). We focus on the three C₂H₄O species and deduce their concentration ratio [CH₃CHO]:[CH₂CHOH]:[c-CH₂OCH₂] = 1:0.7(2):0.06(2) by comparing the observed line intensities to those simulated with the PGOPHER software. Detailed thermochemistry of the C₂H₄O isomers and conformers is provided by Active Thermochemical Tables. The observed excess concentrations of vinyl alcohol and oxirane relative to the more stable acetaldehyde compared to the equilibrium ratio at 1700 K (1:0.087:0.000024), point to direct chemical pathways to these higher energy isomers. The mechanism for the formation of the three C₂H₄O isomers is analyzed using a 0-D homogenous reactor kinetics simulation for ethylene oxidation. The ratios of the C₂H₄O isomers concentrations predicted by the kinetic model are compared to the experimental values.