Kinetics of the reactions of the hydroxyl radical with CH3OH and C2H5OH between 235 and 360 K

doi:10.1016/S1010-6030(03)00073-X

Journal of Photochemistry and Photobiology A: Chemistry

Volume 157, Issues 2–3, 5 May 2003, Pages 237-245

https://doi.org/10.1016/S1010-6030(03)00073-X Get rights and content

Abstract

The kinetics of the reaction of the hydroxyl radical (OH) with methanol (1, rate coefficient k₁) and ethanol (2, rate coefficient k₂), has been studied as a function of temperature (T=235–360 K and T=227–360 K, respectively) by using laser photolysis of a suitable OH-precursor to generate OH and laser-induced fluorescence to detect it. The rate coefficients for reactions (1) and (2) are given by the following expressions (in cm³ molecule ⁻¹ s⁻¹): k₁(T)=(3.6±0.8)×10⁻¹² exp(−(415±70)/T) and k₂(T)=(4.3±0.7)×10⁻¹² exp(−(85±35)/T), respectively, where the uncertainties include the precision of the fit to the Arrhenius expression and estimated systematic errors (±2σ). Our results are consistent with H-atom abstraction taking place primarily at the methyl site of methanol and at the methylene site for ethanol. The atmospheric implications of these reactions are also discussed.

Introduction

Methanol and ethanol appear to be ubiquitous in the atmosphere. They are released due to the usage as fuels (especially, ethanol), additives to fuels, and in paints, as well as from biomass burning and from damaged plants. Methanol and ethanol are removed from the atmosphere via their reaction with hydroxyl radicals, OH. The reaction of OH with alcohols is also a crucial step in their combustion. $OH + CH_{3} OH → products$ $OH + C_{2} H_{5} OH → products$ Reaction (1) can proceed by two exothermic channels; the H-atom abstraction from the OH-group or the H-atom abstraction from the methyl group [1], [2]. $OH + CH_{3} OH → H_{2} O + CH_{3} O, Δ H_{298}^{∘} =−62.4 kJ mol^{−1}$ $OH + CH_{3} OH → H_{2} O + CH_{2} OH Δ H_{298}^{∘} =−99.6 kJ mol^{−1}$ The possible exothermic pathways in Reaction (2) are: $OH + C_{2} H_{5} OH → H_{2} O + C_{2} H_{5} O, Δ H_{298}^{∘} =−56.3 kJ mol^{−1}$ $OH + C_{2} H_{5} OH → H_{2} O + CH_{3} CHOH, Δ H_{298}^{∘} =−110 kJ mol^{−1}$ $OH + C_{2} H_{5} OH → H_{2} O + CH_{2} CH_{2} OH Δ H_{298}^{∘} =−63 kJ mol^{−1}$ The enthalpies of these reactions, ΔH₂₉₈^∘, are taken from Meier et al. [3].

Previously, k₁ has been measured at 298 K by using absolute techniques [4], [5], [6] and relative methods [7], [8], [9], [10]. The rate coefficients k_1a and k_1b have been also calculated using ab initio methods [11]. The kinetic studies of k₂ are less numerous than for k₁, but they have also been measured using both absolute [5], [6] and relative methods [7], [10], [12]. Branching ratios in reactions (1) and (2) have been determined at room temperature by several authors [3], [4], [11], [13], [15], [16]. According to these studies, it is assumed that H-atom abstraction from the aliphatic chain (Reactions (1b) and (2b)) is the dominant channel at atmospheric temperatures.

Even though the temperature dependence of k₁ and k₂ have been studied at T>298 K, they are not well characterized at tropospheric temperatures [3], [11], [13], [14], [15], [16], [17], [18]. For example, k₁ has been measured only at 260 K by Greenhill and O’Grady [19] and at 240 K by Wallington and Kurylo [20], and k₂ has been measured at 255 and 273 K by Greenhill and O’Grady [19] and at 240 K by Wallington and Kurylo [20]. This lack of data at temperatures relevant to the atmosphere has led to large uncertainties in the recommended values of k₁ and k₂.

In this work, we report the temperature dependence of k₁(235–360 K) and k₂(227–360 K) at temperatures relevant for the atmosphere.

Section snippets

Experiments

The apparatus and procedures employed in the current kinetic study have been previously described in detail [21]. The experiments involve excimer laser photolysis of mixtures of an OH precursor, reactant, and He followed by laser-induced fluorescence detection of the OH radical. A jacketed Pyrex reaction cell with an internal volume of ca. 200 cm³ was used in all experiments. The temperature inside the cell was held constant (±1 K) by circulating through a jacket heated ethylene glycol above room

Results and discussion

All experiments were carried out under pseudo-first order conditions in OH ([ROH]>1000[OH]₀; [ROH] is the methanol or ethanol concentration). Under these conditions, the OH temporal profiles followed the pseudo-first order rate law given by: $ln [OH]_{t} = ln [OH]_{0} −k_{i} ′t$ where, $k_{i} ′=k_{i} [ROH]+k_{0} ′$ k_i′ is the measured pseudo-first order decay rate coefficient and k_i is the second-order rate coefficient for the reaction of OH with methanol (i=1) or ethanol (i=2). k₀′ is the pseudo-first order rate coefficient

Acknowledgements

This work was funded in part by the NASA Upper Atmospheric Research Program. E. Jiménez acknowledges a fellowship from the University of Castilla-La Mancha (Spain).

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$k_{1} (C H_{3} OH + OH) = 3.25 \times 10^{- 13} {(\frac{T}{300 K})}^{2.55} \exp (297.8 \frac{K}{T}) 210 - 1344 K$
$k_{2} (C D_{3} OH + OH) = 4.69 \times 10^{- 13} {(\frac{T}{300 K})}^{2.24} \exp (- 59.8 \frac{K}{T}) 293 - 1287 K$
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$k_{1 a}^{H} (CH 2 OH channel) = 2.55 \times 10^{- 11} \exp (- 2287.1 K / T)$
$k_{1 b} (CH 3 O channel) = 4.30 \times 10^{- 11} \exp (- 3463.2 K / T)$

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¹: Permanent address: Departamento de Quı́mica Fı́sica, Facultad de Ciencias Quı́micas, Universidad de Castilla-La Mancha, Camilo José Cela 10, 13071 Ciudad Real, Spain.

²: Current address: Ernest Orlando Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA.

³: Also associated with the Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO 80309, USA.

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Kinetics of the reactions of the hydroxyl radical with CH3OH and C2H5OH between 235 and 360 K

Abstract

Introduction

Section snippets

Experiments

Results and discussion

Acknowledgements

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chemical kinetic data base for combustion chemistry. Part 2. Methanol

J. Phys. Chem. Ref. Data

Kinetic studies of the reactions of F and OH with CH3OH

J. Phys. Chem.

Rates of OH radical reactions. 4. Reactions with methanol, ethanol, 1-propanol, and 2-propanol at 296 K

J. Phys. Chem.

Kinetic rate constants for the reaction of OH with methanol, ethanol, and tetrahydrofuran at 298 K

J. Phys. Chem.

The gas-phase reaction of hydrazine and ozone: a non-photolyitic source of OH radicals for measurements of relative OH radical rate constants

Int. J. Chem. Kinet.

Quantitative erfassung der photochemischen transformationsprozesse in der troposphere

Chemiker Zeitung.

Kinetics of the reactions of the hydroxyl radical with CH₃OH and C₂H₅OH between 235 and 360 K

Kinetic studies of the reactions of F and OH with CH₃OH