ARTICLE pubs.acs.org/JPCA
Reactions of OH with Butene Isomers: Measurements of the Overall Rates and a Theoretical Study Subith S. Vasu,*,†,‡ Lam K. Huynh,§,||,^ David F. Davidson,† Ronald K. Hanson,† and David M. Golden† †
Mechanical Engineering Department, Stanford University, Stanford, California 94305-3032, United States Combustion Research Facility, MS 9055, Sandia National Laboratories, Livermore, California 94551-0969, United States § School of Biotechnology, International University VNUHCM, Vietnam Institute for Computational Science and Technology at Ho Chi Minh City, Vietnam ^ Chemical Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States
)
‡
ABSTRACT: Reactions of hydroxyl (OH) radicals with 1-butene (k1), trans-2-butene (k2), and cis-2-butene (k3) were studied behind reflected shock waves over the temperature range 8801341 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH3)3COOH, and monitored by narrow-line width ring dye laser absorption of the well-characterized R1(5) line of the OH AX (0, 0) band near 306.7 nm. OH time histories were modeled using a comprehensive C5 oxidation mechanism, and rate constants for the reaction of OH with butene isomers were extracted by matching modeled and measured OH concentration time histories. We present the first high-temperature measurement of OH þ cis-2-butene and extend the temperature range of the only previous high-temperature study for both 1-butene and trans-2butene. With the potential energy surface calculated using CCSD(T)/6-311þþG(d,p)//QCISD/6-31G(d), the rate constants and branching fractions for the H-abstraction channels of the reaction of OH with 1-butene were calculated in the temperature range 3001500 K. Corrections for variational and tunneling effects as well as hindered-rotation treatments were included. The calculations are in good agreement with current and previous experimental data and with a recent theoretical study.
’ INTRODUCTION Alkenes are not only important as fuels13 but also as intermediates in the oxidation of most hydrocarbon fuels,47 and they may be formed in large quantities in practical combustors.8 Alkenes also contribute to soot deposits through products formed via hydrogen abstraction.9,10 Furthermore, ignition delay times and species time histories for hydrocarbon fuels can be strongly sensitive to alkene concentration levels and oxidation rates.4,11 Hence, the oxidation kinetics of alkenes is important to the hierarchical development of the kinetic mechanisms of real fuels.1214 OH reactions with alkenes represent one of their major oxidation routes in both combustion and atmospheric chemistry.4,1517 Butene is the smallest alkene with isomers, and each isomer can react with OH to form a different set of products OH þ 1-C4 H8 ¼ various products
ð1Þ
OH þ trans-2-C4 H8 ¼ various products
ð2Þ
OH þ cis-2-C4 H8 ¼ various products
ð3Þ
r 2011 American Chemical Society
Various products from reactions 13 are formed via addition, abstraction, and other bimolecular processes, and therefore, the overall rate constant k = ktotal(T) =kabs þ kaddition þ k(other_processes). At low temperatures (typically below ∼500 K) hydrogen abstraction is very slow; at higher temperatures (T > 1000 K) this process dominates. Experiments in the intermediate temperature range contain the convolved effects of addition, back-dissociation, abstraction, and isomerization processes (relevant detailed discussion regarding OH þ alkene reactions can be found in refs 10, 18, and 19). A review of the literature suggests that kinetic data on OH þ alkenes at combustion-relevant conditions are scarce.10,1922 Recently, we measured the rates of OH reactions with ethylene,21 propene,21 and 1,3-butadiene19 (the simplest conjugate alkene). High-level ab initio theoretical studies of OH reactions with these alkenes were found to agree reasonably well with these experiments.10,18,19,21,22 Received: December 27, 2010 Revised: February 16, 2011 Published: March 09, 2011 2549
dx.doi.org/10.1021/jp112294h | J. Phys. Chem. A 2011, 115, 2549–2556
The Journal of Physical Chemistry A There are only a few studies of OH reactions with butene isomers at low temperatures, most notably by Atkinson and Pitts,23 who used a flash photolysis-resonance fluorescence technique to measure k13 between 297 and 425 K. Above this temperature, there exist just two experimental studies: Tully15 measured k1abs using a laser photolysis/laser-induced fluorescence technique in the range 650833 K, and using a laser pyrolysis/laser fluorescence method Smith16 measured k1 and k2 near 1225 and 1275 K. Huynh et al.24 used reaction-class transition state theory to study vinylic hydrogen abstraction from alkenes by OH. Recently, Sun and Law25 investigated H-abstraction reactions of butene isomers by the OH radical at the CCSD(T)/6-311þþG(d,p)// BHandHLYP/6-311G(d,p) level and CCSD(T)/6-311þþG(d, p)//BHandHLYP/cc-pVTZ level quantum chemistry. To the best of our knowledge, other than the ones mentioned here, there are no other experimental or theoretical studies of OH þ butene isomers above 500 K. In this paper we present direct, high-temperature measurements of the reaction between three butene isomers (1-butene, trans-2-butene, and cis-2-butene) and OH radicals in the range 8801341 K. We also report ab initio calculations of the rate constant and branching fractions of the abstraction channels of k1 on the CCSD(T)/6-311þþG(d,p)//QCISD/6-31G(d) potential energy surfaces using transition state theory (TST) together with corrections for variational, tunneling effects, and hinderedrotation approximation.
’ EXPERIMENTAL SETUP All experiments were performed behind reflected shock waves in a stainless steel, helium-driven, unheated shock tube with an inner diameter of 14.13 cm. Mixtures were prepared manometrically in an unheated stainless steel mixing chamber equipped with a magnetic stirrer assembly and mixed for 6 h to ensure homogeneity and consistency between experiments. Mixtures were made using argon (99.999%, Praxair), 70% tert-butyl hydroperoxide (TBHP) in water and butene isomers (1-butene, g99.5%; trans-2-butene, g99.5%; and cis-2-butene, g99.6%) supplied by Sigma-Aldrich. Details of the shock tube and mixing procedure can be found elsewhere,19,2629 and only a brief description is given here. In all cases the mixing assembly and shock tube driven section were held at room temperature. Preshock GC analysis for such TBHP mixtures showed that less than 0.3 ppm of TBHP decomposes in the mixing tank to form acetone.27,30,31 Modeling the reaction system with this decomposition taken into account showed no discernible effect on the rate constant measurements. Also, kinetic simulations also showed that water vapor in the initial reactant mixture had no significant effect on the rate constant measurements. The shock tube test section and mixing manifold were routinely evacuated below 1 μTorr using two turbo-molecular pumps (Varian V-250 and V-80, respectively) to ensure purity of the test mixture. The leak-plus-outgassing rates for the shock tube were less than 5 μTorr/min. Incident shock velocity was measured using five axially spaced PCB Piezotronics 113A26 piezoelectric transducers (PZT’s) and a 6-channel amplifier (PCB 483B08) connected to four Philips PM6666 interval timers with a resolution of 0.1 μs and linearly extrapolated to the end wall. Average incident shock wave attenuation rates were between 0.6% and 2% per meter. Ideal shock relations and the thermodynamic database from ref 32 were used to calculate the initial reflected shock temperature and pressure (T5 and P5).
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Uncertainty in the initial reflected-shock temperature and pressure (i.e., test conditions) is typically less than 0.7% and 1%, respectively, arising mostly from the 0.3% uncertainty in the end wall shock velocity measurements (see refs 26, 28, and 33). OH radicals were monitored using a narrow-line width ring dye laser tuned to the center of the R1(5) absorption line in the (0,0) band of the A2Σþ r X2Π transition at 306.7 nm. This particular absorption feature was chosen since it had the highest peak absorption and is well isolated from adjacent lines at the current experimental conditions. Absorption spectroscopy of the OH radical is well established at combustion temperatures and near-atmospheric pressures, and the current laser absorption method is fully described in refs 4, 2628, and 30. The minimum detectable absorption was less than 0.1%, leading to ppm-level detectivities for OH at microsecond time scales. OH mole fractions (XOH) were determined from the measured absorption using the BeerLambert relation: I/Io = exp{kνP5XOHL}, where P5 (atm) is the initial reflected shock pressure and L is the path length (14.13 cm). The well-characterized absorption constant, kν, was calculated incorporating the collision-broadening and collision-shift parameters previously measured.26,28 Several uncertainties contribute to the determination of XOH with uncertainties in temperature (T5) having the largest effect. All data were recorded at 2 MHz using a high-resolution data acquisition system. The overall estimated uncertainty in measured XOH is ∼3%. Measurements were also performed with the laser tuned off the OH absorption line and with the laser turned off to verify that there was no significant interfering absorption or emission.
’ KINETIC MEASUREMENTS AND ANALYSIS A total of 26 shock wave experiments were carried out to determine the overall rate constants k13 at near-pseudo-firstorder conditions. Measurements were conducted over temperatures from 880 to 1341 K and pressures from 1.92 to 2.49 atm for the following initial concentrations of butene isomers: 1-butene (272 ppm and 150 ppm), trans-2-butene (142 ppm), and cis-2butene (144 ppm). Nominal mixtures with 5664 ppm TBHP and water (based on partial pressures) dilute in argon were prepared. To model the OH mole fraction time history, the Galway mechanism12 was chosen as the base mechanism. At T ≈ 1000 K, TBHP decomposes almost instantaneously (half-life ≈ 0.1 μs) to form an OH radical and a tert-butoxy radical, (CH3)3CO, which immediately (half-life < 0.1 μs) falls apart to form acetone and a methyl radical (acetone and methyl radicals can also react with OH) ðCH3 Þ3 CO OH f OH þ ðCH3 Þ3 CO
ð4Þ
ðCH3 Þ3 CO f CH3 þ CH3 COCH3
ð5Þ
The above reactions were added to the base mechanism, and the rate constant measured by Vasu et al.21 was used for k4, while evaluation of Benson and O’Neal34 was used for k5. For all species, Galway mechanism12 thermodynamic data were used except for the following species: (CH3)3COOH, cis-2-butene, trans-2-butene, and (CH3)3CO, for which data were taken from Goos et al.32 Because of their importance as revealed by sensitivity analysis, values from recent studies were used for reactions 69 CH3 COCH3 þ OH f CH3 COCH2 þ H2 O 2550
ð6Þ
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Figure 1. Example OH þ 1-butene (k1) rate measurement. Initial reflected shock conditions: 1097 K, 2.12 atm, 150 ppm 1-butene, 11.8 ppm TBHP in argon. Model predictions using the best-fit predictions (k1 = 1.4 1013 cm3/mol/s) for the target rate constant and 50% variation from the measured rate are also shown.
C2 H6 ð þ MÞ f CH3 þ CH3 ð þ MÞ
ð7Þ
CH3 þ OH f 1 CH2 þ H2 O
ð8Þ
CH3 þ OHð þ MÞ f CH3 OHð þ MÞ
ð9Þ
Figure 2. OH sensitivity plot for conditions of Figure 1: 1097 K, 2.12 atm, 150 ppm 1-butene, 11.8 ppm TBHP in argon (note that reactions 6 and 8 overlap).
Table 1. Overall Rate Constants of OH þ Butene Isomers, k13, Data T (K)
P (atm)
rate constant (1013 cm3 mol1 s1)
272 ppm 1-butene, ∼12 ppm TBHP in Ar, k1
Rate constant values measured by Vasudevan et al.35 and Oehlschlaeger et al.36 were used for k6 and k7, respectively. Calculated values for k8 and k9 from Jasper et al.37 were used because the total rate of CH3 þ OH = products calculated by ref 37 agreed well with the measurements by Vasudevan et al.38 (uncertainty = (35%) and Bott and Cohen.39 All simulations were conducted assuming homogeneous adiabatic conditions behind reflected shock waves, with the common constant-internal-energy, constant-volume constraint (constant U, V) using CHEMKIN 4.1.1.40 An example OH absorption trace for 1-butene is shown in Figure 1 for the middle of the 1-butene temperature range. For the conditions of the experiment shown (1097 K, 2.12 atm, 150 ppm 1-butene 11.8 ppm TBHP in argon), the measured peak OH yield is 11.8 ppm. Because wall adsorption and condensation of TBHP is possible, the initial TBHP mole fraction was inferred directly from the OH data. In all experiments, the initial OH yield (and thus nominally the initial TBHP mole fraction) was ∼12 ppm. Using only an OH diagnostic, it is not possible to distinguish the various product channels of reactions 13 as only the total rate constant is determined from the OH decay and not the product formation. Fortunately, because of the pseudofirst-order behavior of the removal of OH at the low concentrations involved in the present experiments, the total rate is independent of the branching ratios (similar to our previous studies19,21,29,35,38), i.e., the choice of products has no discernible effect on our overall rate determination. An OH radical sensitivity analysis (for the conditions in Figure 1) is provided in Figure 2. Here sensitivity is defined as S = (∂XOH/∂ki)(ki/XOH), where XOH is the local OH mole fraction and ki is the rate constant of reaction i. Sensitivity
880 894
2.30 2.47
1.02 1.05
951
2.48
1.13
1000
2.30
1.24
150 ppm 1-butene, ∼11.6 ppm TBHP in Ar, k1 980
2.20
1.21
1057
2.19
1.32
1095
1.92
1.40
1097 1223
2.12 2.23
1.40 1.80
1250
2.17
1.88
1329
2.23
2.30
1337
2.12
2.41
1341
2.12
2.36
142 ppm trans-2-butene, ∼12.8 ppm TBHP in Ar, k2 1024
2.38
1.33
1028
2.49
1.36
1102 1135
2.35 2.20
1.50 1.60
1138
2.28
1.60
1185
2.13
1.80
1269
2.17
2.20
1337
2.14
2.55
1339
2.16
2.60
144 ppm cis-2-butene, ∼11.7 ppm TBHP in Ar, k3
2551
1169 1221
2.27 2.24
1.60 1.70
1258
2.22
1.80
1296
2.18
2.00
dx.doi.org/10.1021/jp112294h |J. Phys. Chem. A 2011, 115, 2549–2556
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Figure 3. Example OH þ trans-2-butene (k2) rate measurement. Initial reflected shock conditions: 1185 K, 2.13 atm, 142 ppm trans-2-butene, 14 ppm TBHP in argon. Model predictions using the best-fit predictions (k2 = 1.8 1013 cm3/mol/s) for the target rate constant and 50% variation from the measured rate are also shown.
Figure 4. Example OH þ cis-2-butene (k3) rate measurement. Initial reflected shock conditions: 1258 K, 2.22 atm, 144 ppm cis-2-butene, 12.2 ppm TBHP in argon. Model predictions using the best-fit predictions (k3 = 1.8 1013 cm3/mol/s) for the target rate constant and a factor of 2 variation from the measured rate are also shown.
analysis clearly shows that reaction between OH and 1-butene is the only dominant sensitive reaction over the entire time frame of the experiment. Because the initial concentration ratio of the reactants ([1-butene]0/[TBHP]0) ratio is very large (∼13 and 23 times, respectively, for the two 1-butene concentrations used in current experiments), the chemistry is essentially pseudo-firstorder, with only slight interference from reactions 69 (see Figure 2). At the lowest temperatures, there is minor influence from reaction 4 (TBHP decomposition) at early times ( 625 K) with previous measurements from Tully15 and Smith16 and with theoretical calculations from Sun and Law.25
obtained for both fuel concentrations. Table 1 summarizes the current measurements of the rate constant of the OH þ 1-butene reaction. OH þ trans-2-butene (k2) and OH þ cis-2-butene (k3) were reduced in the same manner as the OH þ 1-butene (k1) data, and these results are tabulated in Table 1. Example OH absorption traces at the middle of trans- and cis-2-butene temperature ranges are shown in Figures 3 and 4, respectively. Overall rate constants of 1.8 1013 cm3 mol1 s1 were obtained using the best-fit approach, for the conditions of the experiments shown in Figures 3 and 4, for k2 and k3, respectively. Model predictions for variations from the measured rates are also shown in Figures 3 and 4, highlighting the sensitivity to the target reactions. Sensitivity analyses (not shown here) for OH for the conditions in Figures 3 and 4 are similar to that presented in Figure 2: the analysis shows that OH concentration profiles are predominantly sensitive to the target reactions. Slight interference from reactions 69 (as in the case of 1-butene) is seen. A detailed error analysis was carried out to set uncertainty limits on the measured rate constants. The individual contributions of each error source were determined by perturbing each error source to the estimated positive and negative bounds of its 2-σ uncertainty and refitting the experimental traces by adjusting the rate constant of interest (for details see refs 29, 35, and 41). The resulting uncertainties in the rate constant for each individual error source were combined using the root-sum-squares (RSS) method, which assumes that the uncertainty contributions are uncorrelated and estimated to similar probabilities (e.g., 2-σ probability that the true value falls within the ( error limit). The main uncertainty categories and their effect on the target reaction rates of OH þ butene isomers (k13) for the conditions in Figures 1, 3, and 4 are shown in Table 2. The following uncertainty estimates are obtained: (7.7% at 1097 K and 2.12 atm for k1; (8.7% at 1185 K and 2.13 atm for k2; and (8.3% at 1258 K and 2.22 atm for k3. Figure 5 presents the current data for k13 along with earlier measurements from Tully15 and Smith16 and with theoretical calculations from Sun and Law.25 Current measurements show that OH þ trans-2-butene (k2) is 10% higher than OH þ 1-butene (k1) near 1250 K, while OH þ cis-2-butene (k3) is 5% lower. The current measurements have very low uncertainties (less than 10%) and lie slightly higher than the Smith16 data (the 2552
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Table 2. Uncertainty Analysis for OH þ Butene Isomers Reactionsa 2-σ uncertainty in error source
error source
uncertainty in
uncertainty
2-σ average
k1, k2, k3 from positive perturbation
in k1, k2, k3 from negative perturbation
uncertainty contribution in k1, k2, k3
experimental uncertainties temperature
(0.7%41
(þ0.1%, þ0.4%, þ0.4%)
(0.1%, 0.4%, 0.4%)
(0.1%, 0.4%, 0.4%)
fitting (signal/noise)
(7%
(þ7.2%, þ7%, þ7%)
(3.6%, 7%, 7%)
(5.4%, 7%, 7%)
OH absorption constant (kν)
(3%41
(þ3.6%, þ3%, þ3%)
(3.6%, 3%, 3%)
(3.6%, 3%, 3%)
mixture concentration
(0.43 ppm41
(0.3%, þ0.5%, þ0.5%)
(þ0.3%, 0.5%, 0.5%)
(0.3%, 0.5%, 0.5%)
wavemeter reading
(0.0.01 cm130
(4.3%, 4%, 2%)
(2.9%, 4%, 4%)
(3.6%, 4%, 3%)
reaction 6: CH3COCH3 þ OH f CH3COCH2
(30%30
(0.1%, 0.4%, 0.4%)
(þ0.1%, þ0.4%, þ0.4%)
(0.1%, 0.4%, 0.4%)
reaction 8: CH3 þ OH f 1CH2 þ H2O
(uncert. factor = 1.75)37
(2.1%, 1.5%, 1.5%)
(þ1.4%, þ0.7%, þ1%)
(1.8%, 1.1%, 1.3%)
reaction 9: CH3 þ OH(þM) f CH3OH(þM)
(uncert. factor = 1.75)37
(0.7%, 0.5%, 0.5%)
(þ0.4%, þ0.4%, þ0.4%)
(0.6%, 0.5%, 0.5%)
reaction 7: CH3 þ CH3(þM) f C2H6(þM)
(20%36
(
