Shock Tube Measurements of the Rate Constant for the Reaction

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Shock Tube Measurements of the Rate Constant for the Reaction Ethanol + OH Ivo Stranic,*,† Genny A. Pang,‡ Ronald K. Hanson,† David M. Golden,† and Craig T. Bowman† †

Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States Institute for Biological and Medical Imaging (IBMI), Helmholtz Center Munich, Ingoldstädter Landstraße 1, 85764 Neuherberg, Germany

‡

ABSTRACT: The overall rate constant for the reaction ethanol + OH → products was determined experimentally from 900 to 1270 K behind reflected shock waves. Ethan18ol was utilized for these measurements in order to avoid the recycling of OH radicals following H-atom abstraction at the β-site of ethanol. Similar experiments were also performed with unlabeled ethan16ol in order to infer the rate constant that excludes reactivity at the β-site. The two data sets were used to directly infer the branching ratio for the reaction at the β-site. Experimental data in the current study and in previous low-temperature studies for the overall rate constant are best fit by the expression koverall = 5.07 × 105 T[K]2.31 exp(608/ T[K]) cm3 mol−1 s−1, valid from 300 to 1300 K. Measurements indicate that the branching ratio of the β-site is between 20 and 25% at the conditions studied. Pseudo-first-order reaction conditions were generated using tert-butylhydroperoxide (TBHP) as a fast source of 16OH with ethanol in excess. 16OH mole fraction time-histories were measured using narrow-line width laser absorption near 307 nm. Measurements were performed at the linecenter of the R22(5.5) transition in the A−X(0,0) band of 16OH that does not overlap with any absorption features of 18OH, thus producing a measurement of the 16OH mole fraction that is insensitive to the presence of 18OH.

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INTRODUCTION The harmful emissions and diminishing availability of fossil fuels highlight the need for renewable and environmentally friendly alternatives. Biofuels, primarily ethanol, currently account for approximately 3% of overall road-transport fuel use globally.1 The share of biofuels in road-transport fuel is expected to increase to 27% worldwide by 2050, with ethanol accounting for approximately 40% of the total biofuel quantity.2 Because of the increasing demand for ethanol, there is significant interest in developing accurate combustion models for this fuel. Rate constants for reactions of ethanol with OH radicals are critical for accurately modeling ethanol combustion. In this study, the overall high-temperature rate constant for the reaction ethanol + OH → products was measured using isotopic labeling of 18O in the alcohol group of ethanol. Isotopic labeling was utilized in order to eliminate the interference of OH-producing reaction pathways that typically perturb rate constant measurements for reactions of alcohols with OH radicals at high temperatures. Experiments using unlabeled ethanol were also performed in order to determine the rate constant for the title reaction that excludes reactivity at the βsite (non-β). By combining measurements of the overall and nonβ reaction rate constants, the branching ratio of the title reaction at the β-site (BRβ) was inferred.

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k1o, respectively. The reaction pathways for these reactions as well as structural formulas of relevant species are illustrated in Figure 1. C2H5OH + OH → CH3CHOH + H 2O

→ CH 2CH 2OH + H 2O

(Reaction 1b)

→ CH3CH 2O + H 2O

(Reaction 1o)

Reaction of ethanol with OH radicals at the β-site (Reaction 1b) produces CH2CH2OH radicals that rapidly decompose at temperatures above 500 K via Reaction 2 to form ethylene and OH, thus resulting in zero net OH consumption.3,4 CH 2CH 2OH → C2H4 + OH

(Reaction 2)

Therefore, high-temperature rate constant measurements for the reaction ethanol + OH performed by monitoring the rate of removal of OH radicals typically exclude the contribution from the β-site. A branching ratio for the reaction at the β-site is defined as BRβ =

REACTION PATHWAYS

k1b k1a + k1b + k1o

Received: November 4, 2013 Revised: January 8, 2014 Published: January 9, 2014

The reaction ethanol + OH proceeds via three possible reaction sites, α, β, and o, defined by reaction rate constants k1a, k1b, and © 2014 American Chemical Society

(Reaction 1a)

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shock tube into a 29.9 m multipass cell was used to verify that that the ethanol concentration inside the shock tube was equal to the manometric calculations. 16 OH species time histories were measured using direct absorption of light in the A−X(0,0) band near 307 nm. Measurements of 16OH in ethano16ol and ethan18ol were performed at the R11(5.5) and R22(5.5) transitions, respectively. The R11(5.5) transition was preferred in the unlabeled experiments because it has a strong absorption coefficient that has been studied in greatest detail.26,27 The R22(5.5) transition was necessary in labeled experiments in order to avoid spectral overlap between 16OH and 18OH at the target wavelength, thus resulting in a 16OH concentration measurement that is independent of the presence of 18OH. The target wavelengths were accessed by frequency-doubling the visible output of a narrow-line width ring dye laser. Visible light near 614 nm was produced by pumping Rhodamine 6G dye in a Spectra Physics 380A laser cavity using a Coherent Verdi 5 W continuous wave laser at 532 nm. A temperature-tuned AD*A nonlinear crystal was used for intracavity frequency-doubling. Further details on the 16OH detection system as well as the 16 OH spectrum can be found elsewhere.26,27

Figure 1. Dominant reaction pathways related to ethanol + OH reactions.

The rate constant for the reaction of ethanol with OH radicals has been studied extensively in previous work. Measurements were performed near atmospheric temperatures by Wallington and Kurylo,5 Jiménez et al.,6 Greenhill and O’Grady,7 Dillon et al.,8 and Orkin et al.,9 at intermediate temperatures by Carr et al.,10 Hess and Tully,3 and Meier et al.,11 and at combustion temperatures by Sivaramakrishnan et al.12 and Bott and Cohen.13 Theoretical studies have also been performed by Xu and Lin,14 Zheng and Truhlar,15 and Galano et al.,16 and chemical kinetic mechanisms for ethanol combustion have been developed by Marinov,17 Leplat et al.,18 Natarajan and Baskharan,19 Norton and Dryer,20 and Dunphy and Simmie.21 Notably, experiments by Hess and Tully3 at 295 and 599 K utilized isotopically labeled H218O as the 18OH precursor, thus overcoming the recycling of OH described previously. A comparison of their measurements using unlabeled and labeled OH radicals clearly indicate that the rate of removal of OH in unlabeled experiments begins to lose sensitivity to H-abstraction at the β-site near 500 K, with a complete loss of sensitivity above 650 K. Therefore, high-temperature measurements performed using unlabeled ethanol by Carr et al.,10 Sivaramakrishnan et al.,12 and Bott and Cohen13 do not account for reactivity at the β-site.

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KINETIC MODELING The rate constants for the title reaction were determined by fitting the simulated 16OH time histories from the kinetic model to the experimental data using the ethanol + OH reaction rate constants as free parameters that affect the pseudo-first-order decay rate of 16OH. A detailed explanation of pseudo-first-order kinetics for similar measurements of the rate constant for the reaction tert-butanol + OH can be found elsewhere.28 As discussed in the introduction and demonstrated in Figure 2, the concentration of 16OH in the presence of excess

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EXPERIMENTAL METHODS The rate constants for the reaction ethanol + OH → products was inferred by fitting the measured pseudo-first-order decay rate of 16OH following the shock heating of ethanol/TBHP/ water/argon mixtures using kinetic simulations (see Kinetic Modeling section for details). Experiments performed with ethan18ol and ethan16ol were used to infer the overall and non-β rate constants for the title reaction, respectively. TBHP (tert-butyl hydroperoxide) was used as a fast source of 16OH, and an ethanol/TBHP ratio of at least 13 ensured that the ethanol concentration remains approximately constant throughout the measurement time, resulting in pseudo-first-order 16OH decay. Experiments were performed behind reflected shock waves in the Stanford Kinetics Shock Tube with a 14.13 cm inner diameter, the further details of which are provided elsewhere.22−24 The initial temperature and pressure in the reflected shock region are known to within ±0.3% and 0.6%, respectively.25,26 Ethan16ol (anhydrous >99.5%), ethan18ol (99% purity, 98.5% enrichment), and TBHP (70% wt in H2O solution) were obtained from Sigma Aldrich. The water/ TBHP ratio in the reacting mixtures was approximately 4:1, though the presence of water does not affect measurements or simulations of 16OH time histories. Mixtures were prepared manometrically inside a stainless steel mixing tank, and direct laser absorption at 3.39 μm through gases sampled from the

Figure 2. Sensitivity analysis of 16OH in a labeled experiment: T = 1032 K, P = 1.08 atm, 349 ppm ethan18ol, 28 ppm TBHP, 80 ppm H2O, and diluted in argon. Sensitivity of 16OH concentration to reaction i is defined as Si(t) = {∂[16OH](t)/∂ki}/{[16OH](t)/ki}.

ethan18ol exhibits sensitivity to all three ethanol + OH reaction sites. However, as shown in Figure 3, the concentration of 16 OH in the presence of excess ethan16ol is not sensitive to reaction at the β-site due the fast decomposition of the 823

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the decomposition rate of the CH3CHOH is relatively slow, its concentration may reach levels similar to those of CH3 radicals, and thus, its potential to consume a non-negligible amount of 16 OH radicals must be considered. Critically, an accurate kinetic model must contain reasonable rate constant estimates for both the decomposition of CH3CHOH, which primarily affects its absolute concentration, as well as for the reaction CH3CHOH + OH, which affects the rate of removal of 16OH radicals. It is concluded that the Leplat et al.18 mechanism does not contain accurate rate constants for either reaction. The decomposition reaction for the CH3CHOH radical was described by a bimolecular rate constant expression that was estimated in previous work19 on ethanol ignition in shock tubes. A comparison with more recent work described in the following paragraph indicates that this rate constant estimate is too low at the conditions in this study. The reaction CH3CHOH + OH was described in the Leplat et al.18 mechanism using a temperature-independent rate constant of 5 × 1012 cm3 mol−1 s−1, which is also significantly too low because it is 30% slower than the measured rate constant in this work for the reaction ethanol + OH. It is expected that the rate constant for this reaction would be comparable to that of other hydrocarbon radical + OH reactions, whose rate constants are typically on the order of 2 × 1013 cm3 mol−1 s−1. In the current mechanism, the rate constant for the decomposition of the CH3CHOH radical was taken from recent theoretical calculations by Dames.4 That work utilizes the RRKM/Master Equation approach with electronic energies, molecular geometries, and force constant from computations by Senosian et al.30 The rate constant calculations by Dames4 are preferred to similar calculations by Xu et al.,31 though the latter are slower by approximately a factor of 4 at the conditions of the current experiments. The rate constants for the reaction channels CH3CHOH + OH → CH2CHOH + H2O and CH3CHOH + OH → CH3CHO + H2O were assumed to be equal to the rate constant for the reaction C2H5 + OH → C2H4 + H2O estimated by Tsang and Hampson.32 The effect of the uncertainties in the above rate constants on the inferred rate constant for the reaction ethanol + OH is discussed in the uncertainty analysis. Notably, data reduction preformed using the Leplat et al.18 and Marinov17 mechanisms with updated rate constants for the reactions discussed above resulted in nearly identical inferred values of the title reaction rate constant. It is further noted that based on similar arguments outlined in previous work28 on rate constant measurements for the reaction tert-butanol + OH, the kinetic isotope effect is negligible when inferring the rate constant for the title reaction. Furthermore, conversion of 18OH to 16OH through the reaction 18OH + H216O ↔ 16OH + H218O is also negligible.28

Figure 3. Sensitivity analysis of 16OH in an unlabeled experiment. Sensitivity due to reaction at the β-site is not plotted in the figure because it is negligible: T = 1029 K, P = 1.03 atm, 354 ppm ethan16ol, 14 ppm TBHP, 40 ppm H2O, and diluted in argon. Sensitivity defined in Figure 2.

CH2CH2OH radical via Reaction 2. Therefore, experimental data in the labeled and unlabeled experiments may be used to infer the overall and non-β rate constants for the title reaction, respectively. It is noted that the relative branching of the reaction ethanol + OH at the α- and o-sites, with rate constants k1a and k1o, respectively, remains an undetermined free parameter in the kinetic simulations. However, brute force sensitivity analysis indicates that variations in the ratio k1o/k1a from 0 to 1, which is a reasonable range based on theoretical calculations,14,15 do not perturb the measurements of the overall or non-β rate constants for the title reaction by more than 4%. Experiments were modeled using the CHEMKIN-PRO29 kinetics solver designed by Reaction Design. Simulations were performed assuming a constant volume and constant internal energy model, using a modified version of the ethanol mechanism proposed by Leplat et al. 18 The primary modifications to the mechanism were the addition of reactions necessary for modeling TBHP decomposition, as well updates to the rate constants for reactions of OH radicals with CH3 radicals, which are the principal source for secondary OH removal. Rate constants for both of these reaction sets were taken from work by Pang et al,24 and they have been verified against measured 16OH time histories during neat TBHP pyrolysis from both the current and past studies.24,28 The kinetic mechanism was also updated to include duplicate reactions for ethan18ol and its labeled fragments that are assumed to have equivalent reaction rate constants as their unlabeled counterparts. An examination of the literature revealed that the decomposition time scale of the CH3CHOH radical at experimental temperatures near 900 K is similar to the ∼100 μs time scale of 16OH decay. This slow decomposition rate is a consequence of the geometry of CH3CHOH radical, which only contains β-scission pathways that require the rupture of C−H bonds. Similar radicals with different structures such as CH2CH2OH or CH3CH2O decompose much more rapidly at similar conditions because β-scission pathways are available that rupture the weaker C−C or C−O bonds, respectively.4,30 Since

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RESULTS AND DISCUSSION Measurements were acquired between 910 and 1274 K near 1.1 atm for a variety of mixture compositions. The non-β rate constant was measured in 31 unlabeled experiments, and the concentrations of ethan16ol and TBHP were varied from 205 to 885 ppm and 10−38 ppm, respectively. Rate constant measurements show excellent repeatability for a variety of mixture compositions, which suggests that secondary reactions are accurately described in the kinetic mechanism. Because of the high cost of ethan18ol, the overall rate constant was measured in only 15 labeled experiments, and the concentrations of ethano18ol and TBHP were varied from 285 to 349 ppm and 16−23 ppm, respectively. All overall and non-β 824

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Table 1. Summary of the Measurements of the Overall Rate Constant for the Title Reaction; Mixtures Are Balanced in Argon mixture

T (K) P (atm)

349 ppm ethan18ol 108 ppm TBHP + H2O 1032 1113 1078 1191 353 ppm ethan18ol 93 ppm TBHP + H2O 1232 1230 1263 995 963 954 285 ppm ethan18ol 80 ppm TBHP + H2O 935 926 917 1110 1141

1.08 1.04 1.06 1.04 1.03 0.98 1.00 1.13 1.17 1.19 1.20 1.23 1.16 1.10 1.08

koverall × 10−12 (cm3 mol−1 s−1) 7.90 9.08 8.56 10.35 11.00 10.90 11.40 7.75 7.54 7.40 7.27 7.03 7.28 9.20 9.73

Figure 4. Representative 16OH time histories for ethan16ol/TBHP/ argon mixtures (knon‑β in units of cm3 mol−1 s−1). Postreflected shock conditions: T = 1023 K, P = 1.03 atm. Discrepancy in the rise of 16OH is caused by the limited time resolution of the diagnostic (∼5 μs).

Table 2. Summary of the Measurements of the Non-β Rate Constant for the Title Reaction; Mixtures Are Balanced in Argon mixture

T (K) P (atm)

354 ppm ethan16ol 64 ppm TBHP + H2O 1032 979 1023 1075 1147 205 ppm ethan16ol 60 ppm TBHP + H2O 1137 914 951 988 386 ppm ethan16ol 133 ppm TBHP + H2O 1090 1274 1196 1207 1247 1148 1111 229 ppm ethan16ol 65 ppm TBHP + H2O 986 959 939 921 932 885 ppm ethan16ol 170 ppm TBHP + H2O 1065 1024 207 ppm ethan16ol 59 ppm TBHP + H2O 930 913 800 ppm ethan16ol 59 ppm TBHP + H2O 1020 930 1123 1213 910 922

1.05 1.08 1.03 0.99 0.94 0.98 1.05 1.05 1.07 0.85 1.03 0.99 0.98 1.01 1.02 1.03 1.09 1.10 1.13 1.14 1.13 1.12 1.16 1.22 1.18 1.20 1.26 1.16 1.08 1.29 1.27

knon‑β × 10−12 (cm3 mol−1 s−1) 6.23 5.62 6.04 6.44 7.18 7.10 5.65 5.78 5.95 6.41 8.45 7.76 7.80 8.15 7.20 6.83 5.90 5.84 5.74 5.60 5.74 6.40 6.04 5.83 5.62 5.90 5.50 6.70 7.60 5.49 5.60

Figure 5. Comparison of the measured overall and non-β rate constants for the title reaction with previous theoretical and experimental work at high temperatures. Curves by Zheng and Truhlar15 represent calculations using the M08-SO/6-31+G(d,p) method. Curve labeled “Fit” was generated based on all experimental data shown in Figure 7.

and TBHP precursor) and Carr et al.10 (flash photolysis/laserinduced fluorescence). The single measurement from the study by Bott and Cohen33 (absorption and TBHP precursor) is 35% lower and lies outside of the uncertainty bounds of the current data. A comparison of the overall and non-β rate constant measurements, which differ by approximately 20%, can be used to infer the temperature-dependent branching ratio of reaction at the β-site (BRβ), which is shown in Figure 6. Since labeled and unlabeled experiments were not carried out at identical temperatures, it is not possible to calculate point values for BRβ. Instead, for the purposes of determining BRβ, separate best fits to

rate constant measurements are tabulated in Tables 1 and 2, respectively. Measured 16OH time histories exhibit a high signalto-noise ratio, and simulations indicate excellent sensitivity to the title reaction rate constants, as shown in Figure 4. As shown in Figure 5, measurements of the non-β rate constant for the title reaction agree within the uncertainty limits with previous studies by Sivaramakrishnan et al.12 (absorption

Figure 6. Comparison of the measured branching ratio BRβ with previous theoretical work. 825

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However, Xu and Lin14 predict a branching ratio for the β-site that is below the current measurement at temperatures above 1100 K and is essentially at the limit of our experimental uncertainty. The discrepancy between the theoretical calculations by Sivaramakhrishnan et al.12 and the experimental data for the branching ratio of the β-site could be explained by the fact that their calculations for abstraction at the OH- and β-sites were performed at a lower level of theory compared to calculations for abstraction at the α-site, which is the primary abstraction channel. Nonetheless, fair agreement between the three studies is encouraging given that critical parameters such as molecular geometries and potential energies were calculated using different theoretical methods, the details of which can be found in the respective publications. It should be noted that Zheng and Truhlar15 compute rate constants for the various pathways of the title reaction using several density functionals that produce results that differ by up to a factor of 4 at room temperatures. However, all of their calculations predict a branching ratio for the β-site that is between 0.15 and 0.3 from 900 to 1200 K. In this study, the results using the M08-SO/6-31+G(d,p) method are presented because they show superior agreement with the experimental data. It may also be noted that calculations by Galano et al.,16 which were performed using conventional TST, significantly underpredict the measured data at intermediate temperatures where the calculations were performed. Uncertainties in this study were calculated using the same procedures outlined in previous work.28 An example uncertainty analysis for a labeled experiment is shown in Figure 8. Though most

the current data valid from 900 to 1270 K were generated, yielding 8.14 × 10−6 T[K]5.39 exp(4162/T[K]) for the overall rate constant and 1.57 × 10−8 T[K]6.11 exp(5194/T[K]) for the non-β rate constant (units of cm3 mol−1 s−1). These expressions are then used to compute a curve for BRβ using the following expression: BRβ = (Fitoverall − Fitnon‐β)/Fitoverall

The result is plotted in Figure 6. A comparison of the measurement for the overall rate constant with previous measurements at lower temperatures is shown in Figure 7. Generally, there is good agreement among

Figure 7. Comparison of the measured overall rate constant for the title reaction with previous theoretical and experimental work. Data from past studies are excluded if they were performed at conditions that are not sensitive to reactivity at the β-site. Data are best fit by the expression: koverall = 5.07 × 105 T2.31 exp(608/T) cm3 mol−1 s−1.

the nine experimental data sets that are presented, and the data are well fit using the following expression valid from 250 to 1300 K: koverall = 5.07 × 105T[K]2.31 exp(608/T[K]) cm 3 mol−1 s−1

It is observed that measurements of both the overall and non-β rate constants for the title reaction exhibit a slight reduction in temperature dependence from 900 to 1000 K. This is evident in Figures 5 and 7, where the slopes of the experimental data from the current study between 900 and 1000 K (Figure 5) are lower than those of the fit to the aggregated experimental data across the full temperature range (Figure 7). The authors believe that the apparent reduction in the temperature dependence in the current study may be caused by inaccuracies of the CH3CHOH radical chemistry in the kinetic mechanism, which are discussed in detail in the uncertainty analysis later in this section. Adjustments of the rate constant for relevant CH3CHOH radical chemistry reactions within their uncertainties can reduce the measured title reaction rate constant by up to 7% at 900 K. Nonetheless, the authors do not believe that rate constants for these reactions should be adjusted based only on the measurements of the overall and non-β rate constant for the ethanol + OH reaction in this study. Theoretical calculations show good agreement with the experimental data across a variety of temperatures, as shown in Figures 5−7. Quantum chemistry calculations by Xu and Lin,14 Zheng and Truhlar,15 and Sivaramakhrishnan et al.,12 which were performed using variational transition state theory (VTST), muti-structural VTST, and variable reaction coordinate TST, respectively, are in excellent agreement with the experimental data for the overall reaction rate constant.

Figure 8. Magnitude of the uncertainty in the measured overall rate constant for the title reaction associated with each factor considered in the analysis. Random uncertainty factors are indicated by *; the rest are systematic. Uncertainties are ± , unless specified otherwise: 205 ppm ethan18ol, 12 ppm TBHP, 35 ppm H2O, and diluted in argon. T = 914 K and P = 1.09 atm.

of the uncertainty factors depicted in Figure 8 are straightforward, the uncertainty due to CH3CHOH radical chemistry must be discussed in further detail. The uncertainties in the decomposition rate of CH3CHOH and the rate of reaction CH3CHOH + OH are considered to be a factor of 3 and 4, respectively. Uncertainties in these reactions rate constants manifest themselves as uncertainties in the measurement of the rate constant for the title reaction depending on the extent to which the CH3CHOH radical acts as a sink for 16OH radicals. An extreme scenario for secondary 16OH removal by CH3CHOH occurs if the decomposition rate of CH3CHOH is low, thus resulting in higher concentrations of this radical, and if the rate constant for the reaction CH3CHOH + OH is high, thus allowing rapid removal of 16OH. If this worst case 826

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and Dr. Enoch Dames for valuable insights and advice provided throughout the study.

scenario is assumed in the kinetic mechanism, a lower overall and non-β rate constant for the title reaction is required in order for the kinetic simulations to fit the experimental data. Similarly, the converse scenario occurs if the decomposition rate of CH3CHOH is high, thus resulting in lower concentrations of the radical, and if the rate constant of the reaction CH3CHOH + OH is low, thus reducing the secondary removal of 16OH. In this case, a higher rate constant for the title reaction will be required in order for the kinetic simulations to match the experimental data. It is noted that reactions of 16OH with other stable secondary species such as ethylene or acetaldehyde do not significantly contribute to the uncertainties in this study because their rate constants are known relatively accurately, and they are slower by approximately a factor of 5 compared to the rate constant for the reaction CH3CHOH + OH. The uncertainties in the measurements of the overall and non-β rate constants in the current study are approximately +13% at 1250 K, ± 17% at 1000 K, and +21%, −26% at 900 K. The increase in the uncertainties at low temperatures is primarily caused by the uncertainties in the rate constants for TBHP decomposition and CH3CHOH radical chemistry. Since the decomposition rates of TBHP and CH3CHOH exhibit large temperature dependence, their decomposition rates at low temperatures are relatively slow, and the sensitivity of the simulations to their rate constants is significant. At high temperatures, these molecules decompose at time scales much greater than those of 16 OH removal, thus rendering the uncertainties in their rate constants unimportant. The uncertainties in the overall and non-β rate constants were mathematically propagated into an overall uncertainty in the inferred value of BRβ. Systematic errors in the overall and non-β rate constants largely cancel out when calculating BRβ, thus resulting in a value of BRβ that exhibits primarily random errors. As indicated by the low scatter in the measurements, random errors are relatively small in the current study.

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(1) Biofuels. http://www.iea.org/topics/biofuels/. (2) Eisentraut, A.; Brown, A.; Fulton, L. Technology Roadmap: Biofuels for Transport; OECD Publishing: Paris, France, 2011. (3) Hess, W. P.; Tully, F. P. Catalytic Conversion of Alcohols to Alkenes by OH. Chem. Phys. Lett. 1988, 152, 183−189. (4) Dames, E. E. Master Equation Modeling of the Unimolecular Decomposition of A-Hydroxyethyl(CH 3 CHOH) and Ethoxy(CH3CH2O) Radicals. Int. J. Chem. Kinet. 2013, to be published. (5) Wallington, T. J.; Kurylo, M. J. The Gas Phase Reactions of Hydroxyl Radicals with a Series of Aliphatic Alcohols Over the Temperature Range 240−440 K. Int. J. Chem. Kinet. 1987, 19, 1015− 1023. (6) Jiménez, E.; Gilles, M. K.; Ravishankara, A. R. Kinetics of the Reactions of the Hydroxyl Radical with CH3OH and C2H5OH between 235 and 360 K. J. Photochem. Photobiol., A 2003, 157, 237− 245. (7) Greenhill, P.; O’Grady, B. V. The Rate Constant of the Reaction of Hydroxyl Radicals with Methanol, Ethanol, and (D3)Methanol. Aust. J. Chem. 1986, 39, 1775−1787. (8) Dillon, T. J.; Hölscher, D.; Sivakumaran, V.; Horowitz, A.; Crowley, J. N. Kinetics of the Reactions of HO with Methanol (210− 351 K) and with Ethanol (216−368 K). Phys. Chem. Chem. Phys. 2005, 7, 349−55. (9) Orkin, V. L.; Khamaganov, V. G.; Martynova, L. E.; Kurylo, M. J. High-Accuracy Measurements of OH Reaction Rate Constants and IR and UV Absorption Spectra: Ethanol and Partially Fluorinated Ethyl Alcohols. J. Phys. Chem. A 2011, 115, 8656−68. (10) Carr, S. A.; Blitz, M. A.; Seakins, P. W. Site-Specific Rate Coefficients for Reaction of OH with Ethanol from 298 to 900 K. J. Phys. Chem. A 2011, 115, 3335−45. (11) Meier, U.; Grotheer, H.-H.; Riekert, G.; Just, T. Reactions In a Non-Uniform Flow Tube Temperature Profile: Effect On The Rate Coefficient for the Reaction C2H5OH + OH. Chem. Phys. Lett. 1987, 133, 162−164. (12) Sivaramakrishnan, R.; Su, M.-C.; Michael, J. V.; Klippenstein, S. J.; Harding, L. B.; Ruscic, B. Rate Constants for the Thermal Decomposition of Ethanol and Its Bimolecular Reactions with OH and D: Reflected Shock Tube and Theoretical Studies. J. Phys. Chem. A 2010, 114, 9425−39. (13) Bott, J. F.; Cohen, N. A Shock Tube Study of the Reactions of the Hydroxyl Radical with Several Combustion Species. Int. J. Chem. Kinet. 1991, 23, 1075−1094. (14) Xu, S.; Lin, M. C. Theoretical Study on the Kinetics for OH Reactions with CH3OH and C2H5OH. Proc. Combust. Inst. 2007, 31, 159−166. (15) Zheng, J.; Truhlar, D. G. Multi-Path Variational Transition State Theory for Chemical Reaction Rates of Complex Polyatomic Species: Ethanol + OH Reactions. Faraday Discuss. 2012, 157, 59. (16) Galano, A.; Alvarez-Idaboy, J. R.; Bravo-Perez, G.; Ruiz-Santoyo, M. E. Gas Phase Reactions of C1−C4 Alcohols with the OH Radical: A Quantum Mechanical Approach. Phys. Chem. Chem. Phys. 2002, 4, 4648−4662. (17) Marinov, N. M. Kinetic Model for High Temperature Ethanol Oxidation. Int. J. Chem. Kinet. 1998, 31, 183−220. (18) Leplat, N.; Dagaut, P.; Togbé, C.; Vandooren, J. Numerical and Experimental Study of Ethanol Combustion and Oxidation in Laminar Premixed Flames and in Jet-Stirred Reactor. Combust. Flame 2011, 158, 705−725. (19) Natarajan, K.; Bhaskaran, K. A. An Experimental and Analytical Investigation of High Temperature Ignition of Ethanol. Shock Tubes Shock Waves 1981, 834−842. (20) Norton, T. S.; Dryer, F. L. The Flow Reactor Oxidation of C1− C4 Alcohols and MTBE. Proc. Combust. Inst. 1991, 23, 179−185. (21) Dunphy, M. P.; Simmie, J. M. High-Temperature Oxidation of Ethanol. J. Chem. Soc., Faraday Trans. 1991, 87, 1691−1696.

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CONCLUSIONS The overall and non-β rate constants for the reaction ethanol + OH → products were measured behind reflected shock waves in a shock tube. The two data sets were also used to directly calculate the branching ratio for the reactivity at the β-site. Isotopic labeling of 18O in ethan18ol was used as a critical tool for overcoming the recycling of OH radicals that typically occurs when measuring the overall rate constants for reactions of alcohols with OH radicals at high temperatures. By spectrally distinguishing the recycled 18OH radicals from the consumed 16OH radicals, the decay rates of 16OH in the labeled experiments were sensitive to reactivity of OH at all three reaction sites of ethanol. Measurements in the current study show excellent agreement with previous experiments, though the current work exhibits lower scatter and overall uncertainties. Data also show excellent agreement with recent theoretical calculations that were performed using different computational methods.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(I.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, with Dr. Wade Sisk as contract monitor. The authors would like to thank Dr. David Davidson 827

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dx.doi.org/10.1021/jp410853f | J. Phys. Chem. A 2014, 118, 822−828