Ethanol oxidation: Kinetics of the α-hydroxyethyl radical + O 2 reaction

Document Type

Article

Publication Date

8-6-2009

Abstract

Bioethanol is currently a significant gasoline additive and the major blend component of flex-fuel formulations. Ethanol is a high-octane fuel component, and vehicles designed to take advantage of higher octane fuel blends could operate at higher compression ratios than traditional gasoline engines, leading to improved performance and tank-to-wheel efficiency. There are significant uncertainties, however, regarding the mechanism for ethanol autoignition, especially at lower temperatures such as in the negative temperature coefficient (NTC) regime. We have studied an important chemical process in the autoignition and oxidation of ethanol, reaction of the α-hydroxyethyl radical with O2(3P), using first principles computational chemistry, variational transition state theory, and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation simulations. The α-hydroxyethyl + O2 association reaction is found to produce an activated a-hydroxy-ethylperoxy adduct with ca. 37 kcal mol-1 of excess vibrational energy. This activated adduct predominantly proceeds to acetaldehyde + HO2, with smaller quantities of the enol vinyl alcohol (ethenol), particularly at higher temperatures. The reaction to acetaldehyde + HO2 proceeds with such a low barrier that collision stabilization of C2O3H 5 isomers is unimportant, even for high-pressure/low-temperature conditions. The short lifetimes of these radicals precludes the chain-branching addition of a second O2 molecule, responsible for NTC behavior in alkane autoignition. This result helps to explain why ignition delays for ethanol are longer than those for ethane, despite ethanol having a weaker C-C bond energy. Given its relative instability, it is also unlikely that the a-hydroxy-ethylperoxy radical acts as a major acetaldehyde sink in the atmosphere, as has been suggested. © 2009 American Chemical Society.

Identifier

68149147496 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp903210a

ISSN

10895639

PubMed ID

19594149

First Page

8923

Last Page

8933

Issue

31

Volume

113

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