Role of (H2O) n (n = 1-2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products.

Li Xu,Junyao Li,Narcisse T Tsona,Shanshan Tang,Lin Du

doi:10.1021/acsomega.9b00145

Abstract

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH3CH2OH + •OH reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH3CH2OH, the bared reaction proceeds through three elementary paths, producing CH2CH2OH, CH3CH2O, and CH3CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH3CHOH is favored over the temperature range of 216.7–425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH3CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH2CH2OH, CH3CH2O, and CH3CHOH are computed to be 5.2 × 10–13, 8.6 × 10–14, and 9.0 × 10–11 cm3 molecule–1 s–1, respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3–5 and 6–8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH3CH2OH + •OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it.

Highlights

Environmental and energetic crises have led to the increasing use of ethanol (CH3CH2OH) as one of the environment friendly and renewable biofuels and especially as a fuel additive or extender.[1,2] The enhanced use of ethanol is likely to result in its increased release into the atmosphere as unburnt fuel, fugitive emissions, and discharge during its production
The different reactant intermediates and reaction paths in which the hydroxyl radical abstracts one hydrogen atom from the methyl site, hydroxyl site, and methylene site on ethanol are marked with letters a, b, and c, respectively
Three elementary reaction paths have been found depending on the ethanol hydrogen [−OH, −CH2− (α-hydrogen), −CH3 (β-hydrogen)] abstracted by the hydroxyl radical

Summary

Introduction

Environmental and energetic crises have led to the increasing use of ethanol (CH3CH2OH) as one of the environment friendly and renewable biofuels and especially as a fuel additive or extender.[1,2] The enhanced use of ethanol is likely to result in its increased release into the atmosphere as unburnt fuel, fugitive emissions, and discharge during its production. Ethanol is acknowledged as a biogenic volatile organic compound released from living plants with an emission rate of 26 (range, 10−38) Tg year−1, which contributes significantly to a total global ethanol source of 42 (range, 25− 56) Tg year−1.1,3 In the atmosphere, most of ethanol is in its ground state and about three-quarters of ethanol are removed by the reaction with hydroxyl radicals in the gaseous and aqueous phases, occurring on a time scale of 4 days, and the remainder is lost through dry deposition and wet scavenging.[1,4,5] In the oxidation reaction, a hydroxyl radical can attack and abstract one hydrogen atom of the ground-state ethanol from three different sites (methyl, hydroxyl, and methylene), leading to the formation of different C2H5O isomers. These C2H5O isomers subsequently undergo atmospheric oxidation with molecular oxygen to generate important secondary pollutants and radicals. Formaldehyde is primarily formed from the β-hydroxyalkyl radical produced from eq 1a through the following processes[4,6−8]

Methods

Results

Conclusion