Acetone, 2-Butanone, 3-Pentanone, and 2-Pentanone - American

Article pubs.acs.org/JPCA

High-Temperature Measurements of the Reactions of OH with a Series of Ketones: Acetone, 2-Butanone, 3-Pentanone, and 2-Pentanone King-Yiu Lam,* David F. Davidson, and Ronald K. Hanson Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States ABSTRACT: The overall rate constants for the reactions of hydroxyl radicals (OH) with a series of ketones, namely, acetone (CH3COCH3), 2-butanone (C 2 H 5 COCH 3 ), 3-pentanone (C 2 H 5 COC 2 H 5 ), and 2-pentanone (C3H7COCH3), were studied behind reflected shock waves over the temperature range of 870−1360 K at pressures of 1−2 atm. OH radicals were produced by rapid thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP) and were monitored by the narrow line width ring dye laser absorption of the well-characterized R1(5) line in the OH A−X (0, 0) band near 306.69 nm. The overall rate constants were inferred by comparing the measured OH time histories with the simulated profiles from the detailed mechanisms of Pichon et al. (2009) and Serinyel et al. (2010). These measured values can be expressed in Arrhenius form as kCH3COCH3+OH = 3.30 × 1013 exp(−2437/T) cm3 mol−1 s−1, kC2H5COCH3+OH = 6.35 × 1013 exp(−2270/T) cm3 mol−1 s−1, kC2H5COC2H5+OH = 9.29 × 1013 exp(−2361/T) cm3 mol−1 s−1, and kC3H7COCH3+OH = 7.06 × 1013 exp(−2020/T) cm3 mol−1 s−1. The measured rate constant for the acetone + OH reaction from the current study is consistent with three previous experimental studies from Bott and Cohen (1991), Vasudevan et al. (2005), and Srinivasan et al. (2007), within ±20%. Here, we also present the first direct high-temperature rate constant measurements of 2-butanone + OH, 3-pentanone + OH, and 2-pentanone + OH reactions. The measured values for the 2-butanone + OH reaction are in close accord with the theoretical calculation from Zhou et al. (2011), and the measured values for the 3-pentanone + OH reaction are in excellent agreement with the estimates (by analogy with the H-atom abstraction rate constants from alkanes) from Serinyel et al. Finally, the structure−activity relationship from Kwok and Atkinson (1995) was used to estimate these four rate constants, and the estimated values from this group-additivity model show good agreement with the measurements (within ∼25%) at the present experimental conditions.

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INTRODUCTION Ketones are listed as a class of volatile organic compounds (VOCs) and are massively produced and used as solvents or polymer precursors in industries. As one of the common pollutants, substantial amounts of ketones are emitted into the atmosphere from a variety of natural and anthropogenic sources. The reactions with OH radicals are the primary removal pathways for ketones in the atmosphere, which may result in the formation of ozone and other components of the photochemical smog in urban areas.1 In combustion environments (T > 1000 K), ketones are also formed as intermediate products during the oxidation of hydrocarbon and oxygenated fuels. Additionally, due to their attractive photophysical properties, they are commonly used as fuel tracers for quantitative planar laser-induced fluorescence measurements in order to monitor temperature fields and species concentrations in internal combustion engine studies.2−5 Despite their practical importance, very few experimental and theoretical studies are available that study high-temperature ketone oxidation. In particular, the H-atom abstraction by OH radicals for ketones, which is one of the major fuel consumption pathways during oxidation, is not very well-understood at high temperatures. Hence, an accurate knowledge of H-atom © 2012 American Chemical Society

abstraction reactions is needed in the development of successful detailed mechanisms suitable for high-temperature applications. Due to their significant roles in atmospheric chemistry, the rate constants for the reactions of OH radicals with a series of ketones, including acetone, 2-butanone, 3-pentanone, and 2pentanone, have been extensively studied by many researchers1,6−16 over the temperature range of 240−400 K. However, the kinetic data on ketones + OH at combustion-relevant conditions are generally scarce. There were a few experimental studies for the acetone + OH reaction rate constant over 500− 1300 K. Yamada et al.17 utilized two different OH precursors (HONO and N2O/H2O) and measured the rate constants for OH + CH3COCH3 and CD3COCD3 in a reactor over 298− 832 K using the pulsed laser photolysis/pulsed laser-induced fluorescence technique. Bott and Cohen18 pioneered the use of tert-butyl hydroperoxide (TBHP) as a OH precursor and monitored the OH decay in a shock tube using the UV lamp absorption method at 309 nm in order to study the rate constant for the acetone + OH reaction near 1200 K and 1 atm. Received: April 21, 2012 Revised: May 16, 2012 Published: May 18, 2012 5549

dx.doi.org/10.1021/jp303853h | J. Phys. Chem. A 2012, 116, 5549−5559

The Journal of Physical Chemistry A

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Similarly, Vasudevan et al.19 and Srinivasan et al.20 both measured the acetone + OH rate constant using shock tubes and UV absorption methods over the combustion-relevant temperature range of 980−1300 K. These measurements are in good agreement with each other. In contrast to acetone, there was only one experimental study available for larger ketone + OH kinetic data. Tranter and Walker21 added small amounts of ketones (acetone, 2-butanone, and 3-pentanone) individually to slowly reacting mixtures of H2 + O2 at 753 K and measured the consumption of ketones and H2 with the use of gas chromatography. This method allowed them to study the relative rate constants for the reactions of H and OH with ketones at 753 K. Furthermore, Zhou et al.22 recently performed a theoretical study on the mechanism and kinetics of the reactions of OH with three methyl ketones, acetone, 2butanone, and isopropyl methyl ketone. They employed the computationally less expensive G3 and G3MP2BH&H methods to calculate the energy barriers and utilized the Variflex code including Eckart tunneling corrections to compute the total rate constants over 500−2000 K. In addition, all possible abstraction channels have been accounted for in their calculation. However, except for acetone, their theoretical calculations have not been validated against any high-temperature experimental data. The overall rate constants for the reactions of OH with four ketones, namely, acetone (CH 3 COCH 3 ), 2-butanone (C2H5COCH3), 3-pentanone (C2H5COC2H5), and 2-pentanone (C3H7COCH3), were determined behind reflected shock waves over the temperature range of 870−1360 K at pressures of 1−2 atm. CH3COCH3 + OH → products

(1)

C2H5COCH3 + OH → products

(2)

C2H5COC2H5 + OH → products

(3)

C3H 7COCH3 + OH → products

(4)

manometrically in a 40 L stainless steel tank heated uniformly to 50 °C and mixed with a magnetically driven stirring vane. A double-dilution process was employed to allow for more accurate pressure measurements in the manometrical preparation of a highly dilute mixture. A highly concentrated mixture was first prepared and mixed for at least 2 h to ensure homogeneity and consistency, and the mixture was then further diluted with argon and mixed for an additional 2 h prior to the experiments. The gas utilized in this study was argon (research grade) 99.999%, which was supplied by Praxair and used without further purification. The liquid chemicals were 70% TBHP in water, CHROMASOLV grade acetone (≥99.9%), CHROMASOLV grade 2-butanone (≥99.7%), ReagentPlus grade 3-pentanone (≥99%), and ReagentPlus grade 2pentanone (≥99%) from Sigma-Aldrich and were purified using a freeze−pump−thaw procedure to remove dissolved volatiles and air prior to mixture preparation. The mixture composition was confirmed by sampling a portion of the mixture (from near the endwall) in an external multipass absorption cell with a path length of 29.9 m and monitoring the fuel concentration in the cell with a Jodon helium−neon laser at 3.39 μm. The details of the laser diagnostic setup are discussed elsewhere.23 Beer’s law was used to convert the measured absorption data into the fuel mole fraction. The absorption cross sections of ketones for Beer’s law were directly obtained from the PNNL database,24 and the measured fuel concentrations were consistent with the values expected from the manometrical preparation within ±5%. The OH radical concentration was measured using the frequency-doubled output of a narrow line width ring dye laser near 306.69 nm. The laser wavelength was tuned to the peak of the well-characterized R1(5) absorption line in the OH A−X (0, 0) band. Visible light near 613.4 nm was generated by pumping rhodamine 6G dye in a Spectra Physics 380A laser cavity with the 5 W, cw output of a Coherent Verdi laser at 532 nm. The visible light was then intracavity frequency-doubled using an angle-tuned LBO nonlinear crystal to generate ∼1 mW of light near 306.69 nm. Using a common-mode-rejection detection scheme, a minimum absorbance of 0.1% could be detected, which resulted in a minimum detection sensitivity of ∼0.2 ppm at 1400 K and 1.5 atm. Further details of the OH laser diagnostic setup are discussed elsewhere.19,25 The OH species concentration can be calculated from Beer’s law, I/Io = exp(−kOHXOHPL), where I and Io are the transmitted and incident laser intensities, kOH is the OH absorption coefficient, XOH is the OH mole fraction, P is the total pressure, and L is the path length (15.24 cm). The overall estimated uncertainty in the measured OH mole fraction (XOH) is approximately ±3%, mainly due to the uncertainty in temperature (±0.7%). Measurements were also conducted with the laser tuned off from the absorption line to verify that there was no significant interference absorption or emission. All data were recorded at 1 MHz using a high-resolution (14 bit) data acquisition system.

These measurements include the first direct high-temperature measurements of the overall rate constants for reactions 2−4. These high-temperature kinetic data, along with the earlier work,1,6−21 are compared with the theoretical calculation (from Zhou et al.22) and estimates using the group-additivity model.

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EXPERIMENTAL SETUP Experiments were performed in a stainless steel, high-purity, low-pressure shock tube at Stanford. The shock tube is comprised of a 3.7 m driver section and a 10 m driven section, with an inner diameter of 15.24 cm. Reflected shock temperatures and pressures were determined from the incident shock speed at the endwall using standard normal shock relations, with uncertainties of approximately ±0.7 and ±1%, respectively, mainly due to the uncertainty in the measured shock velocity (±0.2%). The endwall incident shock speed was measured using a series of five piezoelectric pressure transducers over the last 1.5 m of the shock tube and linearly extrapolated to the endwall. The OH laser diagnostic, along with a Kistler piezoelectric pressure transducer for pressure measurements, was located at a test section 2 cm from the driven section endwall. Between experiments, the shock tube and mixing assembly were routinely turbomolecular pumped down to ∼6 μTorr to ensure purity of the test mixtures. Test mixtures were prepared

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KINETIC MEASUREMENTS A total of 58 reflected shock wave experiments were performed to determine the overall rate constants for the reactions of OH with four ketones (acetone, 2-butanone, 3-pentanone, and 2pentanone) at near-pseudo-first-order conditions. Experiments were carried out over the temperature range of 870−1360 K at pressures of 1−2 atm using different initial fuel concentrations: acetone (304 ppm), 2-butanone (152 ppm, 161 ppm and 206 ppm), 3-pentanone (151 ppm and 211 ppm), and 2-pentanone 5550



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known in this study, and the overall rate constant for reaction 4 cannot be inferred accurately at T > 1300 K. A thorough theoretical or experimental study for 2-pentanone decomposition pathways is required. Nevertheless, at T < 1300 K, the consumption of 2-pentanone in the present study is predominantly controlled by the H-atom abstraction reactions by OH radicals, and the overall rate constant for reaction 4 was determined over a narrower temperature range of 900−1300 K. In addition, all simulations were performed using the CHEMKIN PRO package under the standard constant internal energy and volume assumption. Acetone + OH Kinetics. An OH radical sensitivity analysis for the mixture of 304 ppm acetone with 28 ppm TBHP (and 73 ppm H2O) in Ar at 1148 K and 1.95 atm is provided in Figure 1. The OH sensitivity is calculated as SOH = (∂XOH/∂ki)

(161 ppm). These ketones were prepared with 53−101 ppm TBHP/water and diluted in argon. To properly simulate the consumption of OH radicals by ketones, the Pichon et al. mechanism of NUI Galway26 was chosen as the base mechanism for acetone, and the Serinyel et al. mechanism of NUI Galway27,28 was utilized as the base mechanism for 2-butanone, 3-pentanone, and 2-pentanone. Additionally, a TBHP chemistry set was included in these base mechanisms. TBHP (or (CH3)3−CO−OH) is an excellent OH radical precursor for the present study because it rapidly decomposes to form an OH radical and a tert-butoxy radical, (CH3)3CO, at temperatures greater than 1000 K.29 The tert-butoxy radical further falls apart to form acetone and a methyl radical. Concurrently, TBHP can also react with the OH radical to form other products, and the TBHP chemistry set is described as follows (CH3)3 −CO−OH → (CH3)3 CO + OH

(5)

(CH3)3 CO → CH3COCH3 + CH3

(6)

(CH3)3 −CO−OH + OH → H 2O + O2 + tert ‐C4 H 9 (7)

(CH3)3 −CO−OH + OH → H 2O + HO2 + iso‐C4 H8 (8)

Thermodynamic parameters for TBHP and tert-butoxy radical were taken from the thermodynamic database from Goos et al.,30 and the thermodynamic parameters for OH were updated with the values from Herbon et al.25 The rate constants for reactions 5, 7, and 8 were obtained from Pang et al.,31 and the rate constant for reaction 6 was taken from Choo and Benson.32 In addition, the initial decomposition pathways for acetone, 2-butanone, and 3-pentanone are described as follows: CH3COCH3 → CH3CO + CH3

Figure 1. OH sensitivity plot for the rate constant measurement of acetone + OH at 1148 K and 1.95 atm.

(9)

C2H5COCH3 → C2H5 + CH3CO

(10a)

C2H5COCH3 → CH3 + C2H5CO

(10b)

C2H5COCH3 → CH3 + CH3COCH 2

(10c)

C2H5COC2H5 → C2H5 + C2H5CO

(11a)

C2H5COC2H5 → CH3 + C2H5COCH 2

(11b)

× (ki/XOH), where XOH is the local OH mole fraction and ki is the rate constant for reaction i. The analysis reveals that the reaction of OH with acetone (reaction 1) is the dominant reaction over the time frame of the experiment, with some minor interference from the secondary reactions.

For 2-butanone and 3-pentanone (and 2-pentanone), the initial decomposition pathways consist of multiple channels. Hightemperature decomposition pathways for 2-butanone and 3pentanone were first investigated by Serinyel et al.27,28 Recently, Lam et al.33,34 have performed experimental studies during high-temperature acetone, 2-butanone, and 3-pentanone pyrolyses. In their studies, they measured the rate constant for reaction 9 and the overall values for reactions 10 and 11 at pressures near 1.6 atm. At T > 1300 K, the consumption of ketones in the present study is mainly controlled by the H-atom abstraction reactions by OH radicals and the ketone decomposition pathways. Hence, reactions 9−11 are pertinent to the determinations of the overall rate constants for reactions 1−3 at higher temperatures, and the rate constants for reactions 9−11 were updated with the values from Lam et al.33,34 In addition, a review of the literature shows that there is currently no experimental or theoretical study for high-temperature 2pentanone pyrolysis. Thus, 2-pentanone decomposition pathways, along with the corresponding rate constants, are not

CH3 + OH → CH 2(s) + H 2O

(12)

CH3OH ( + M) → CH3 + OH ( + M)

(13)

CH3 + CH3 ( + M) → C2H6 ( +M)

(14)

The rate constant for reaction 12 was updated with the value of 1.65 × 1013 cm3 mol−1 s−1 adopted by Pang et al.31 In particular, this value is in good agreement with the calculated values from Jasper et al.35 and the measurements from Vasudevan et al.36 and Srinivasan et al.20 (within ±35%). The rate constant for reaction 13 was updated with the measured values from Srinivasan et al.20 at ∼0.3−1.1 atm, and their values agree well with the calculated values from Jasper et al.35 and the measured values from Vasudevan et al.36 at 1.3 atm. In addition, the rate constant for reaction 14 was updated with the measured values from Oehlschlaeger et al.37 (in the reverse direction), and the measurements from Oehlschlaeger et al. are in excellent agreement with another experimental study from Kiefer et al.38 Figure 2 shows a sample measured OH concentration time history for the mixture of 304 ppm acetone in Ar at 1148 K and 1.95 atm, and the measured peak OH mole fraction is ∼28 ppm. Due to wall adsorption and condensation of TBHP, the 5551



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at 1148 K. The primary contributions to the uncertainties in the rate constants are (a) temperatures (±1%), (b) the mixture composition (±5%), (c) OH absorption coefficient (±3%), (d) wavemeter reading in the UV (±0.005 cm−1), (e) fitting the data to computed profiles (±5%), (f) locating time zero (±0.5 μs), (g) the rate constant for CH3 + OH → CH2(s) + H2O (uncert. factor = 2), (h) the rate constant for CH3OH (+M) → CH3 + OH (+M) (uncert. factor = 2), and (i) the rate constant for CH3 + CH3 (+M) → C2H6 (+M) (±20%). As shown in Figure 3, the individual error sources were introduced

Figure 2. Sample acetone + OH rate constant measurement using the mixture of 304 ppm acetone with ∼28 ppm TBHP (and 73 ppm water) in Ar at 1148 K and 1.95 atm. Simulation from the Pichon et al. mechanism for the best-fit rate constant, along with perturbations of ±50%, is also shown.

initial TBHP mole fraction was assumed to be the same as the measured peak OH mole fraction, which was formed immediately after the decomposition of TBHP behind the reflected shock wave at T > 1000 K. Note that a 70%, by weight, solution of TBHP in water in the liquid phase corresponds, initially, to 69% water and 31% TBHP in the vapor phase, based on Raoult’s law.39 Therefore, a 101 ppm TBHP/water mixture should have at most 31.3 ppm TBHP. In the present study, the mixtures of 101 ppm TBHP/water consist of ∼28−30 ppm TBHP, based on the measured peak OH yields. In addition, the test mixtures were chosen such that the ratio of the initial acetone concentration to the initial TBHP concentration is ∼10, thereby achieving near-pseudofirst-order conditions. For the conditions described in Figure 2, a best-fit overall rate constant for reaction 1 of 3.83 × 1012 cm3 mol−1 s−1 was obtained between the experimental data and the simulation. Simulations for the perturbations of ±50% in the inferred rate constant are also illustrated in Figure 2. In addition, Table 1 summarizes the rate constant measurements of reaction 1 at 872−1355 K and 1.69−2.12 atm. A detailed error analysis was performed to estimate the uncertainty limits of the measured rate constant for reaction 1

Figure 3. Uncertainty analysis for the rate constant of CH3COCH3 + OH → products at 1148 K and 1.95 atm.

separately, and their effects on the rate constant for reaction 1 were determined. These uncertainties were combined in a root-sum-squared method to give an overall uncertainty estimate of ±28% at 1148 K. Figure 4 shows the Arrhenius plot for the present rate constant measurements of reaction 1 at T = 872−1355 K, along

Table 1. CH3COCH3 + OH → Products: Rate Constant Data T5 [K]

P5 [atm]

k1 [cm3 mol−1 s−1]

101 ppm TBHP (and water), 304 ppm CH3COCH3, Ar 934 2.01 2.52 × 1012 1008 2.09 2.72 × 1012 1011 1.85 2.83 × 1012 1111 1.98 3.47 × 1012 1148 1.95 3.83 × 1012 1221 1.86 4.50 × 1012 1247 1.82 4.96 × 1012 1280 1.78 4.90 × 1012 1307 1.69 5.29 × 1012 1355 1.75 5.63 × 1012 92 ppm TBHP (and water), 304 ppm CH3COCH3, Ar 872 1.96 2.14 × 1012 996 2.12 2.79 × 1012

Figure 4. Arrhenius plot for acetone + OH (k1) at temperatures above 833 K.

with the previous measurements of Vasudevan et al.19 from the same laboratory. The current measurements agree well with the previous values (within ±5%). These measured values can then be expressed in Arrhenius form as k1 = 3.30 × 1013 exp(−2437/ T) cm3 mol−1 s−1 over 872−1355 K. Bott and Cohen18 also utilized TBHP as the OH precursor and employed both the 5552



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conditions depicted in Figure 5, with some minor interference from the secondary reactions (reactions 5, 12, and 14). As shown here, as the temperature decreases, the reaction for TBHP decomposition becomes more important at the early times. Figure 6 shows an example of the overall rate constant measurement (k2 = k2a + k2b + k2c) for reaction 2 at 1039 K and

shock tube and UV lamp absorption method at 309 nm to monitor the OH decay and study reaction 1 near 1200 K and 1 atm. The current measurements are consistent with Bott and Cohen’s measured value within 20%. In addition, Srinivasan et al.20 used a similar method to investigate the rate constant for reaction 1 and provided the rate constant of 4.40 × 1012 cm3 mol−1 s−1 over 1178−1299 K. Their value is in close accord with our previous and current measurements. Figure 4 also shows the rate constants for reaction 1 adopted by two different detailed mechanisms, Pichon et al.26 and Herbinet et al.40 The values of k 1 from the Pichon et al. mechanism are approximately 24 and 43% faster than the current measurements at 1000 and 1250 K, respectively; the values employed from the Herbinet et al. mechanism are in excellent agreement with the current measured values (within ±11%). Additionally, a theoretical calculation from Zhou et al.,22 which modeled all possible abstraction channels, was performed using the computationally less expensive G3 and G3MP2BH&H methods to calculate the energy barriers and using the Variflex code including Eckart tunneling corrections to compute the total rate constants for the reactions of OH with ketones (acetone, 2-butanone, and isopropyl methyl ketone) over 500−2000 K. As shown in Figure 4, the computed values from Zhou et al. are consistently lower than all hightemperature experimental data by ∼55%. 2-Butanone + OH Kinetics. The OH sensitivity analysis was also carried out for the rate constant determination of 2butanone + OH → products (reaction 2) using the mixture of 152 ppm 2-butanone with 14 ppm TBHP (and 41 ppm water) in Ar at 1039 K and 1.41 atm, as shown in Figure 5. Note that

Figure 6. Sample 2-butanone + OH rate constant measurement using the mixture of 152 ppm 2-butanone with ∼14 ppm TBHP (and 41 ppm water) in Ar at 1039 K and 1.41 atm. Simulation from the Serinyel et al. mechanism for the best-fit rate constant, along with perturbations of ±50%, is also shown.

1.41 atm. The mixture is 152 ppm 2-butanone in Ar, with the measured peak OH yield of ∼14 ppm. Thus, we infer that the initial TBHP mole fraction is 14 ppm. The model predictions from the Serinyel et al. mechanism with the best-fit overall rate constant of k2 = 6.82 × 1012 cm3 mol−1 s−1 and the variations of ±50% in the inferred rate constant are also shown in Figure 6. Due to the near-pseudo-first-order conditions, the measured overall rate constant should be insensitive to the branching ratios of the individual channels. The effect of the branching ratios on the rate constant determination was also investigated at 1039 K by interchanging the branching ratios of channels 2a and 2b while maintaining the total value, and a negligible change in the inferred rate constant was found. In addition, a detailed error analysis (similar to the analysis for reaction 1) was performed for the rate constant measurement of reaction 2 at 1039 K and 1.41 atm, and the overall uncertainty was estimated to be ±22%. Table 2 summarizes the rate constant measurements of reaction 2 at 879−1364 K and 1.21−1.63 atm. Three different mixture compositions were employed to confirm that the inferred rate constants are independent of any secondary chemistry effects. Figure 7 shows the Arrhenius plot for the overall rate constant measurements of reaction 2 at T > 833 K, along with the estimated values adopted in the Serinyel et al. mechanism and the theoretical values from Zhou et al. The measured values can be expressed in Arrhenius form as k2 = 6.35 × 1013 exp(−2270/T) cm3 mol−1 s−1 over 879−1364 K. The values used in the Serinyel et al. mechanism are ∼40% lower than the measurements. Interestingly, the theoretical values from Zhou et al. are in excellent agreement with the measurements within 10%. Note that the measurements and the theoretical calculations both exhibit some slight non-Arrhenius curvature at the present test conditions.

Figure 5. OH sensitivity plot for the rate constant measurement of 2butanone + OH at 1039 K and 1.41 atm.

reaction 2 consists of three different abstraction channels, as described in the Serinyel et al. mechanism27,28 C2H5COCH3 + OH → CH 2CH 2COCH3 + H 2O

(2a)

C2H5COCH3 + OH → CH3CHCOCH3 + H 2O

(2b)

C2H5COCH3 + OH → C2H5COCH 2 + H 2O

(2c)

At 1100 K, channel 2b is the dominant pathway with a branching ratio of 0.53 due to the weaker C−H bond energy at the secondary site, and channel 2a is the next most important pathway with a branching ratio of 0.41. However, channel 2c is nearly insignificant with a branching ratio of 0.06. More importantly, reaction 2 is the most sensitive reaction at the 5553



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Table 2. C2H5COCH3 + OH → Products: Rate Constant Data T5 [K]

P5 [atm]

pentanone are the dominant pathways for the consumption of OH at 1188 K and 1.94 atm. In particular, reaction 3 consists of two channels, in which the OH radical can abstract the H-atom from 3-pentanone at the primary or secondary site.

k2 [cm3 mol−1 s−1]

55 ppm TBHP (and water), 152 ppm C2H5COCH3, Ar 962 1.51 6.10 × 1012 999 1.45 6.42 × 1012 1039 1.41 6.82 × 1012 1119 1.27 8.25 × 1012 1174 1.24 9.01 × 1012 1247 1.26 1.05 × 1013 53 ppm TBHP (and water), 161 ppm C2H5COCH3, Ar 879 1.57 5.06 × 1012 955 1.63 5.93 × 1012 1110 1.39 8.00 × 1012 1282 1.26 1.09 × 1013 1297 1.23 1.14 × 1013 65 ppm TBHP (and water), 206 ppm C2H5COCH3, Ar 905 1.55 5.30 × 1012 1088 1.42 7.51 × 1012 1104 1.34 7.80 × 1012 1320 1.21 1.17 × 1013 1364 1.25 1.24 × 1013

C2H5COC2H5 + OH → CH 2CH 2COC2H5 + H 2O (3a)

C2H5COC2H5 + OH → CH3CHCOC2H5 + H 2O

(3b)

On the basis of the Serinyel et al. mechanism, the branching ratios of channels 3a and 3b are 0.42 and 0.58, respectively, at 1188 K. In addition, there is some minor interference from the following reactions at later times: CH3 + OH → CH 2(s) + H 2O

(12)

C2H4 + H ( +M) → C2H5 ( +M)

(15)

CH3COCH3 + OH → CH3COCH 2 + H 2O

(1)

In the current analysis, the rate constant for reaction 1 was updated with our Arrhenius expression, with an uncertainty of approximately ±28%. In addition, the rate constant for reaction 15 adopted by the Serinyel et al. mechanism was used, and we assumed that its uncertainty is approximately a factor of 2. Figure 9 shows a representative OH time history trace at 1188 K and 1.94 atm using the mixture of 213 ppm 3-

Figure 7. Arrhenius plot for 2-butanone + OH (k2) at temperatures above 833 K. Figure 9. Sample 3-pentanone + OH rate constant measurement using the mixture of 213 ppm 3-pentanone with ∼17 ppm TBHP (and 59 ppm water) in Ar at 1188 K and 1.94 atm. Simulation from the Serinyel et al. mechanism for the best-fit rate constant, along with perturbations of ±50%, is also shown.

3-Pentanone + OH Kinetics. As illustrated in Figure 8, the OH sensitivity reveals that the reactions of OH with 3-

pentanone with 17 ppm TBHP (and 59 ppm H2O) in Ar. The model predictions from the Serinyel et al. mechanism with the best-fit rate constant of 1.23 × 1013 cm3 mol−1 s−1 and the variations of ±50% in k3 are also shown in Figure 9. Note that the overall rate constant for reaction 3 is insensitive to the branching ratios of its individual channels due to the nearpseudo-first-order conditions. A detailed error analysis was then conducted for k3 at 1188 K and 1.94 atm, and the overall uncertainty was estimated to be ±20%. Table 3 summarizes the overall rate constant determinations of reaction 3 at T = 878−1353 K and P = 1.21−2.20 atm. Note that two different mixture compositions were used to confirm that the inferred rate constants are free of any secondary chemistry effects, and the values determined from these two mixtures are consistent with each other. Figure 10 shows the Arrhenius plot for our measured values, along with the

Figure 8. OH sensitivity plot for the rate constant measurement of 3pentanone + OH at 1188 K and 1.94 atm. 5554



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Table 3. C2H5COC2H5 + OH → Products: Rate Constant Data T5 [K] 76 1140 1188 1248 1296 1310 76 936 1025 1068 75 878 955 981 1093 1339 53 1091 1157 1192 1258 1301 1353

P5 [atm] ppm TBHP (and water), 1.61 1.94 1.76 1.79 1.74 ppm TBHP (and water), 2.02 2.13 1.94 ppm TBHP (and water), 1.98 2.20 2.13 1.91 1.71 ppm TBHP (and water), 1.46 1.34 1.29 1.27 1.21 1.23

C3H 7COCH3 + OH → C2H4 + CH3COCH 2 + H 2O (4a)

C3H 7COCH3 + OH → C3H6 + CH3CO + H 2O

k3 [cm3 mol−1 s−1] 213 ppm C2H5COC2H5, Ar 1.12 × 1013 1.23 × 1013 1.33 × 1013 1.57 × 1013 1.61 × 1013 211 ppm C2H5COC2H5, Ar 7.38 × 1012 8.90 × 1012 9.54 × 1012 211 ppm C2H5COC2H5, Ar 6.61 × 1012 7.86 × 1012 8.09 × 1012 1.06 × 1013 1.67 × 1013 151 ppm C2H5COC2H5, Ar 1.03 × 1013 1.16 × 1013 1.26 × 1013 1.43 × 1013 1.57 × 1013 1.69 × 1013

(4b)

C3H 7COCH3 + OH → C2H5CHCO + CH3 + H 2O (4c)

C3H 7COCH3 + OH → n‐C3H 7 + CH 2CO + H 2O (4d)

Channel 4a describes the H-atom abstraction from 2-pentanone at the γ site to form a CH2CH2CH2COCH3 radical and a H2O molecule. Through β-scission, the fuel radical decomposes very rapidly to form a C2H4 molecule and a CH3COCH2 radical. Due to the rapid decomposition of the fuel radical, we assumed that the products from the fuel radical are formed immediately after the H-atom abstraction. Similarly, channels 4b and 4c describe the H-atom abstraction from 2-pentanone at the β and α sites, respectively. Due to its similar structure to methyl butanoate, the rate constants for channels 4a−4c were approximated to be the same as the rate constants for methyl butanoate (MB) + OH reactions at the α, β, and γ sites, and these values for MB + OH reactions were obtained from the Dooley et al. mechanism.41 Among these three channels, channel 4b (the H-atom abstraction at the β position) should be the fastest route for the removal of OH, which was also suggested in previous experimental studies.1,7 In addition, the rate constant for channel 4d was assumed to be the same as that of channel 2c (C2H5COCH3 + OH → C2H5COCH2 + H2O). The resulting branching ratios of channels 4a−4d at 1186 K are 0.23, 0.38, 0.37, and 0.02. These four channels were then incorporated into the Serinyel et al. mechanism.27,28 As expected, the estimated branching ratios of channels 4a−4d have no discernible effect on the determinations of the overall rate constant at near-pseudo-first-order conditions. In the present analysis, we also included the pathways for the reactions of H with 2-pentanone in the Serinyel et al. mechanism, which can be described as follows:

Figure 10. Arrhenius plot for 3-pentanone + OH (k3) at temperatures above 833 K.

C3H 7COCH3 + H → C2H4 + CH3COCH 2 + H 2

(16a)

C3H 7COCH3 + H → C3H6 + CH3CO + H 2

(16b)

C3H 7COCH3 + H → C2H5CHCO + CH3 + H 2

(16c)

C3H 7COCH3 + H → n‐C3H 7 + CH 2CO + H 2

(16d)

In a similar way, the rate constants for channels 16a−16c were assumed to be the same as the rate constants for the reactions of H with methyl butanoate at the α, β, and γ sites, and these values were also adopted from the Dooley et al. mechanism.41 Additionally, the rate constant for channel 16d was assumed to be the same as that of 2-butanone (C2H5COCH3 + H → C2H5COCH2 + H2). Interestingly, the addition of reactions 16a−16d has negligible influence on the overall rate constant determinations of reaction 4. Figure 11 shows a representative OH time history trace at 1186 K and 1.30 atm using the mixture of 161 ppm 2pentanone with 15 ppm TBHP (and 45 ppm H2O) in Ar. The simulations from the Serinyel et al. mechanism with the best-fit rate constant of k4 = 1.24 × 1013 cm3 mol−1 s−1 and the variations of ±50% in k4 were also illustrated. A detailed error analysis was then conducted to estimate the overall uncertainty in k4 at 1186 K, and the uncertainty was found to be ±24%. Table 4 summarizes the overall rate constant measurements of reaction 4 at 902−1302 K and 1.23−1.59 atm, and Figure 12

estimated values from the Serinyel et al. mechanism, at temperatures above 833 K. The measured values can be expressed in Arrhenius form as k3 = 9.29 × 1013 exp(−2361/T) cm3 mol−1 s−1 over 878−1353 K. Interestingly, the values for reaction 3 from the Serinyel et al. mechanism were estimated by analogy with the H-atom abstraction rate constants from alkanes,28 and their values are in close accord with the present measurements within ±5%. As is evident in Figure 10, the measurements and the estimated values from Serinyel et al. both experience slight non-Arrhenius curvature at the current experimental conditions. 2-Pentanone + OH Kinetics. As mentioned previously, a comprehensive mechanism for high-temperature 2-pentanone kinetics is not available in the literature. In the present work, we have assumed the pathways for the reactions of OH with 2pentanone to be similar to those of methyl butanoate.41 5555



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values obtained for the 3-pentanone + OH reaction at our experimental conditions.

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COMPARISON WITH LOW-TEMPERATURE DATA Figure 13 presents the current data along with some earlier measurements of reaction 1 at temperatures greater than 250 K.

Figure 11. Sample 2-pentanone + OH rate constant measurement using the mixture of 161 ppm 2-pentanone with ∼15 ppm TBHP (and 45 ppm water) in Ar at 1186 K and 1.30 atm. Simulation from the Serinyel et al. mechanism for the best-fit rate constant, along with perturbations of ±50%, is also shown.

Table 4. C3H7COCH3 + OH → Products: Rate Constant Data T5 [K]

P5 [atm]

Figure 13. Arrhenius plot for acetone + OH → products (k1) at all temperatures.

k4 [cm3 mol−1 s−1]

60 ppm TBHP (and water), 161 ppm C3H7COCH3, Ar 902 1.59 7.58 × 1012 1104 1.37 1.14 × 1013 1186 1.30 1.24 × 1013 1216 1.23 1.31 × 1013 1302 1.26 1.51 × 1013 63 ppm TBHP (and water), 161 ppm C3H7COCH3, Ar 955 1.50 8.45 × 1012 1009 1.46 9.38 × 1012 1042 1.43 1.04 × 1013 1093 1.37 1.12 × 1013 1125 1.30 1.17 × 1013 1264 1.25 1.48 × 1013

In the study from Wollenhaupt et al.,6 the rate constant for reaction 1 was measured over 202−395 K at 20−100 Torr of Ar or N2 bath gas using the pulsed laser photolysis technique to generate OH radicals from the sequential two-photon dissociation of NO2 in the presence of H2 at 439 nm or from the photolysis of HONO at 351 nm. They monitored the OH radicals using either resonance fluorescence or a laser-induced fluorescence detection scheme, and they also concluded that their measurements are independent of pressure. Similarly, Le Calve et al.1 and Gierczak et al.9 studied k1 over 199−383 K by generating OH via pulsed laser photolysis and detecting it via laser-induced fluorescence, while Wallington and Kurylo7 investigated k1 over 240−440 K using the flash photolysis/ resonance fluorescence measurement technique. Moreover, Yamada et al.17 examined k1 over a wide temperature range of 298−832 K using the pulsed laser photolysis/pulsed laserinduced fluorescence technique. They then performed a detailed analysis using variational transition state theory and suggested that the dominant products of reaction 1 are CH3COCH2 and H2O through direct abstraction at all temperatures (particularly above 450 K). Additionally, Tranter and Walker21 added small amounts of acetone to slowly reacting mixtures of H2 + O2 at 753 K and monitored the consumption of acetone and H2 with the use of gas chromatography. This method allowed them to infer the relative rate constant for reaction 1 at 753 K. It is pertinent to note that these low-temperature measurements are in excellent agreement with each other. As is evident in Figure 13, the rate constants employed in the comprehensive mechanisms of Pichon et al.26 and Herbinet et al.40 are able to predict the lowtemperature data reasonably well over 298−832 K. In particular, the rate constant from Herbinet et al. provides much better agreement with the existing high-temperature data. In addition to the values from the detailed mechanisms, the theoretical calculation from Zhou et al.22 agrees well with earlier low-temperature measurements (at T < 500 K), but the

Figure 12. Arrhenius plot for 2-pentanone + OH (k4) at temperatures above 900 K.

also presents the Arrhenius plot for these measured values. These measured values are expressed in Arrhenius form as k4 = 7.06 × 1013 exp(−2020/T) cm3 mol−1 s−1 over 902−1302 K. It should be noted that the overall rate constant measurements for the 2-pentanone + OH reaction are quite similar to the 5556



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calculated values are at least 40% lower than the measurements at T > 500 K. Figure 14 shows the Arrhenius plot for the overall rate constant measurements of reaction 2 at temperatures greater

Figure 15. Arrhenius plot for 3-pentanone + OH → products (k3) at all temperatures.

at 753 K (using the same approach as the one that they employed for reactions 1 and 2). Atkinson et al.16 also measured the relative rate constant for reaction 3 at 299 K using methylnitrite (CH3ONO) photolysis in air as a source of OH radicals. They monitored the organic reactants using gas chromatography with flame ionization detection. In their work, they took advantage of their previous knowledge of the rate constant for the cyclohexane + OH reaction and inferred the rate constant for reaction 3 from the ratio of k3/kcyclohexane+OH at 299 K. Moreover, Wallington and Kurylo7 determined the absolute rate constant for reaction 3 over 240−440 K using the flash photolysis/resonance fluorescence measurement technique, and they suggested that k3 did not exhibit any temperature dependence at their test conditions. Furthermore, the rate constant from the Serinyel et al. mechanism is able to predict the existing data rather accurately over 440−1353 K. Similarly, Figure 16 illustrates the current high-temperature data and previous low-temperature measurements7,11,16 of

Figure 14. Arrhenius plot for 2-butanone + OH → products (k2) at all temperatures.

than 250 K. Kinetic measurements of reaction 2 were performed at room temperature by different researchers using both relative12−15 and absolute1,7,10,11 methods. In general, these room-temperature measurements are in close accord with each other, except for the value obtained from Atkinson et al.12 Concurrently, the rate constant for reaction 2 was examined as a function of temperature (213−598 K) by Wallington and Kurylo7 using the flash photolysis/resonance fluorescence technique and by Le Calve et al.,1 Carr et al.,10 and Jimenez et al.11 using the pulsed laser photolysis/laser-induced fluorescence technique. Their measurements are in excellent agreement with each other, and no pressure dependence can be found at their experimental conditions. On the basis of these low-temperature data, k2 exhibits only slight positive temperature dependence over 250−400 K. In addition to the rate constant determination for the acetone + OH reaction, Tranter and Walker21 measured the relative rate constant for reaction 2 at 753 K. Figure 14 also presents the estimated values of k2 from Serinyel et al.27,28 and the theoretical values from Zhou et al.22 As described previously, the calculated values from Zhou et al. are consistent with the present high-temperature data (at T > 879 K) within 10%. However, the calculated values are faster than the earlier low-temperature data by a factor of 2 at 500 K and by a factor of 6 at 250 K. Consequently, the theoretical study predicts a pronounced negative temperature dependence of k2 over 250−500 K, and this effect does not appear in the existing data. On the other hand, the estimated values from Serinyel et al. are ∼40% lower than the current hightemperature data and are in good agreement with the lowtemperature data over 345−600 K. In addition, the overall rate constant from Serinyel et al. does not exhibit any negative temperature dependence over 250−500 K. Figure 15 also shows the current high-temperature data (at T > 878 K) and three previous low-temperature measurements (at T < 800 K) for the reaction of OH with 3-pentanone, along with the rate constant from the Serinyel et al. mechanism.27,28 As compared to acetone and 2-butanone, fewer experimental and theoretical studies are available in the literature. Tranter and Walker21 measured the relative rate constant for reaction 3

Figure 16. Arrhenius plot for 2-pentanone + OH → products (k4) at all temperatures.

reaction 4 over 250−1302 K. More importantly, the rate constant provided by Jimenez et al.11 exhibits a pronounced negative temperature dependence over 248−388 K, and this trend does not appear in the kinetic measurements of reactions 1−3. 5557



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s−1, kC2H5COCH3+OH = 6.35 × 1013 exp(−2270/T) cm3 mol−1 s−1, kC2H5COC2H5+OH = 9.29 × 1013 exp(−2361/T) cm3 mol−1 s−1, and kC3H7COCH3+OH = 7.06 × 1013 exp(−2020/T) cm3 mol−1 s −1 . Detailed error analyses, which account for both experimental and secondary chemistry contributions, yielded the uncertainty estimates of ±28% at 1148 K for k1, ±22% at 1039 K for k2, ±20% at 1188 K for k3, and ±24% at 1186 K for k4. In addition, the SAR developed by Atkinson and his coworkers42−44 was used to estimate the rate constants for reactions 1−4, and the estimated values are in good agreement with the present high-temperature data (within ∼25%).

COMPARISON WITH ESTIMATION METHOD The measured overall rate constants for reactions 1−4 over 250−1360 K can be compared with the estimated values using the structure−activity relationship (SAR) developed by Atkinson and his co-workers.42−44 Their method of calculating the rate constants for the reactions of OH with organic compounds is based on the estimation of primary (−CH3), secondary (−CH2−), and tertiary (−CH

Acetone, 2-Butanone, 3-Pentanone, and 2-Pentanone - American

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