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Products From the Oxidation of N-Butane From 298-735 K Using Either Cl Atom or Thermal Initiation: Formation of Acetone and Acetic Acid – Possible Roaming Reactions? Edward William Kaiser, and Timothy J. Wallington J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06608 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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The Journal of Physical Chemistry

Products From the Oxidation of n-Butane From 298-735 K Using Either Cl Atom or Thermal Initiation: Formation of Acetone and Acetic Acid – Possible Roaming Reactions? E. W. Kaisera* and T. J. Wallingtonb* a

Department of Natural Sciences, University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI 48128, bResearch and Advanced Engineering, Ford Motor Company, Dearborn, MI 48121-2053 ABSTRACT The oxidation of 2-butyl radicals (and to a lesser extent 1-butyl radicals) has been studied over the temperature range 298 K to 735 K. The reaction of Cl atoms (formed by 360 nm irradiation of Cl2) with n-butane generated the 2-butyl radicals in mixtures of n-C4H10, O2, and Cl2 at temperatures below 600 K. Above 600 K, 2-butyl radicals were produced by thermal combustion reactions in the absence of chlorine. The yields of the products were measured by gas chromatography using a flame ionization detector. Major products quantified include acetone, acetic acid, acetaldehyde, butanone, 2-butanol, butanal, 1- and 2- chlorobutane, 1-butene, trans-2-butene, and cis-2-butene. At 298 K, the major oxygenated products are those expected from bimolecular reactions of 2-butylperoxy radicals (butanone, 2-butanol, and acetaldehyde). As the temperature rises to 390 K, the butanone decreases while acetaldehyde increases because of the increased rate of 2-butoxy radical decomposition. Acetone and acetic acid first appear in significant yield near 400 K, and these species rise slowly at first then sharply, peaking near 525 K at yields of ~25 mole percent and ~20 mole percent, respectively. In the same temperature range (400 K to 525 K), butanone, acetaldehyde, and 2-butanol decrease rapidly. This suggests that acetone and acetic acid may be formed by previously unknown reaction channels of the 2-butylperoxy radical which are in competition with those that lead to butanone, acetaldehyde, and 2-butanol. Above 570 K the yields of acetone and acetic acid fall rapidly as the yields of the butenes rise. Experiments varying the Cl atom density, which in turn controls the entire radical pool density, were performed in the temperature range 410 K to 440 K. Decreasing the Cl atom density increased the yields of acetone and acetic acid while the yields of butanone, acetaldehyde, and 2-butanol decreased. This is consistent with the formation of acetone and acetic acid by unimolecular decomposition channels of the 2-butylperoxy radical, which are in competition with the bimolecular channels that form butanone, acetaldehyde, and 2-butanol. Such unimolecular decomposition channels would be unlikely to proceed through conventional transition states because those states would be very constrained. Therefore, the possibility that these decomposition channels proceed via roaming should be considered. In addition, we investigated and were unable to fit our data trends by a simplified ketohydroperoxide mechanism.

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1. INTRODUCTION Because it is the simplest hydrocarbon fuel exhibiting many of the characteristics of larger hydrocarbon fuel molecules during combustion, the study of the oxidation of n-butane is of considerable interest to combustion science. Two recent publications have examined the products formed from the oxidation of butyl radicals at 550-800 K, and these studies included extensive bibliographies of previous work, which will not be repeated here.1,2 Our experiments on the subject of 2-butyl and 1-butyl radical oxidation cover the temperature range 298-735 K primarily near atmospheric pressure, although very limited data are presented at reduced pressure. Concentrations of several major products detectable by a flame ionization detector (GC/FID) have been measured throughout the 298-735 K temperature range. A main focus of this study is to gain a better understanding of the formation of one major product in our experiments, acetone, which has been observed twice previously.1,3 In contrast, Eskola et al.2 make no mention of observing acetone in their extensive n-butane oxidation experiments over the 575-700 K temperature range. Although it is a major product in our experiments and in those of Herbinet et al,1 its source is not known as noted by Herbinet et al. Acetone seems to have no simple source, since it requires rearrangement and subsequent decomposition of a 2-butylperoxy radical or its derivative 2-butoxy radical through intermediates that are very strained. In addition, the formation of significant amounts of acetic acid is observed herein and also in the experiments of Herbinet et al.1 Again, this species is not mentioned in the experiments of Eskola et al.2 Formation of acetic acid also requires passage through a very strained intermediate with subsequent extensive rearrangement. We examine the formation of acetone and acetic acid and explore a possible reason why these species are not mentioned by Eskola et al. Possible formation of acetone by a simplified ketohydroperoxide mechanism was also investigated. Most of our experiments were carried out in the presence of Cl2, which is photolyzed to Cl atoms by UV radiation. The Cl atoms form butyl radicals by reaction with n-butane in the mixture permitting studies at low temperature. The butyl radicals subsequently react primarily with O2 in the initial mixture. Experiments at T > 600 K were performed in the absence of Cl2 by conventional combustion. During the Cl2 initiated experiments, the steady-state Cl density was varied by changing the UV flux into the reactor.

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This changed certain product yields, providing important clues concerning the formation mechanisms of acetone and acetic acid. 2. EXPERIMENTS The GC/FID analysis has been described in detail previously.4 The primary GC column was a 30m DB-1 with 5 µm coating. Experiments were performed at 298-590 K using initial mixtures of Cl2 (purity = 99.7%), n-butane (99.99%), and oxygen (99.999%) diluted by N2 (99.999% min). These mixtures were prepared by partial pressure using a vacuum manifold. Freeze/thaw degassing cycles were performed on the butane and Cl2 reactants. In addition, degassed CF2Cl2 (99%) was included in the reaction mixtures for internal calibration of the GC samples. This molecule does not react with Cl and is thermally stable at the maximum temperature and reaction time of these experiments.5 Chlorine atoms were generated by irradiation of the unreacted mixture with UV light peaking near 360 nm from a single Sylvania F6T5 BLB fluorescent lamp for temperatures below 500 K. After irradiation for a chosen time at ambient temperature, a portion of the contents was removed from the reactor into a 2.5 cm3 gas-tight syringe set to 1 cm3 (Hamilton) using the vacuum manifold. This sample was analyzed by injection into the injector port (373 K) of the gas chromatograph. The presence of an internal calibration species, CF2Cl2, permitted corrections to be made for uncertainty in the precise amount of sample injected into the GC using the syringe. At temperatures from 550-590 K, no irradiation was used since the thermal dissociation of Cl2 was faster than that caused by irradiation. Leaving Cl2 in the mixture above 600 K produced a reaction that was too fast to control. Therefore, above 600 K, the Cl2 was removed from the mixture, and the reaction proceeded as a conventional thermal oxidation of nbutane. Experiments at 298 K were carried out in a 500 cm3 spherical Pyrex flask equipped with a teflonsealed stopcock for connection to the vacuum system. The unreacted mixture placed in the 500 cm3 reactor was irradiated multiple times. A sample was withdrawn into the syringe and analyzed after each irradiation. All experiments in this reactor were carried out at a total pressure of 1000 Torr.



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Elevated temperature experiments were performed over the range 366-735 K using a ~40 cm3, cylindrical, Pyrex reactor (26 mm ID x ~7 cm length) with a thermocouple well along the axis and a Teflon-sealed, glass stopcock attached to a Pyrex capillary tube at the end opposite the thermocouple well. This reactor was placed inside a tube oven, whose lid remained open approximately 6 mm to allow radiation from the fluorescent lamp to enter. The calibration of the chromel-alumel thermocouple was checked in ice and boiling water. The temperature along the axis of the reactor was uniform to ±2 K from the mean. A portion of the unreacted mixture in a 500 cm3 Pyrex storage flask was expanded into the high-temperature reactor at a pressure of ~760 Torr. The mixture was then irradiated for a chosen time, and a sample of the contents was withdrawn into the gas-tight syringe using the vacuum manifold, after removing a small amount of gas to purge the low temperature dead volume. Only one irradiation was possible per sample placed into this reactor during the high temperature experiments because substantial pressure loss occurred during sampling from the low-volume reactor. GC/FID retention times and the FID response factors relative to n-butane (response=1.0) were obtained for acetone (0.463), butanone (0.645), acetic acid (0.25), and acetaldehyde (0.234) by injecting known concentrations of the pure species into the GC in the presence of the internal calibration species. The FID response factors for 1- and 2-chlorobutane, 1-butene, and c- and t-2-butene have been determined to be identical to that of n-butane within experimental error previously. This was verified for 1- and 2-chlorobutane by running an experiment in the absence of O2 at low butane consumption. In this experiment, the sum of the two chlorobutanes formed was equal to that of the butane consumed to within experimental error. The retention times of butanal and 1- and 2-butanol were determined directly from pure samples in these experiments, but the GC/FID response factors of these compounds were not measured. The FID response factors of 1- and 2-butanol and butanal were assumed equal to that measured for butanone (0.645 relative to butane defined as 1.0) as has been observed in a previous study of FID response factors.6

Some loss of acetic acid was observed during calibration tests at low

concentration and for longer than the standard 30 sec residence time in the syringe as might be expected for a polar, low-vapor-pressure species.


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Secondary consumption of the products by Cl will make the apparent yield of the products decrease with increasing consumption of n-butane. All of the products formed have rate constants for reaction with Cl which are less than the rate constant for n-butane reaction with Cl. The rate constants used to correct for secondary consumption of product species by reactions with Cl are: acetone (1.5 x 10-11 e-1200/RT cm3 molec-1 s-1);7 acetic acid (2.8 x 10-14 cm3 molec-1 s-1, T dependence unknown);8 2-Clbutane (7 x 10-11 cm3 molec-1 s-1 at 298 K)9 and 1-Cl-butane (1.1 x 10-10 cm3 molec-1 s-1 at 298 K);9 nbutane (kn-C4H10 = 2.2 x 10-10 cm3 molec-1 s-1 at 298 K);9 acetaldehyde (7.2 x 10-11 cm3 molec-1 s-1 independent of T);10 butanone (4 x 10-11 cm3 molec-1 s-1 independent of T);11 (n-butanal 1.4 x 10-10 cm3 molec-1 s-1 independent of T);12 and ethyl chloride (8.3 x 10-12 cm3 molec-1 s-1 at 298 K assumed temperature independent).13 Rate constants for reactions of Cl with butenes are not available at elevated temperature; we assumed values of 1.5 x 10-10 cm3 molec-1 s-1 independent of temperature. 3.0 RESULTS AND DISCUSSION 3.1 Experiments with Cl2 (298-587 K) Table 1 contains 32 data points, which are representative of the 50+ data point set that was obtained in experiments using Cl atom initiation to study the oxidation of 2- and 1-butyl radicals. This table gives: mole fractions of O2, n-C4H10, and Cl2 in the initial mixtures; the total pressure (balance N2) and temperature; reaction time (either the time of irradiation (tirr), or, for thermal Cl2 dissociation, the residence time in the reactor); and the fraction of the initial n-butane remaining (C/C0). Product yields are presented in units of mole percent, which is defined as Yield = 100 x (moles of product formed)/(moles of n-butane consumed). Each number/letter set of data represents experiments carried out with the same initial mixture but different reaction times. The data in Table 1 span the temperature range 298 to 587 K. Because the rate constants for reaction of Cl with acetone, acetic acid, and ethyl chloride are small relative to that of n-butane, no corrections for secondary consumption were required for these species. Corrections for the other species were calculated using the Acuchem chemical kinetics solver.14



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Table 1. Initial conditions and product yields for selected experiments studying the reaction Cl + C4H10 in N2/O2 diluent from 298-597 K. data Cl2 C4H10 P T Cb acetone butanone butanal Acetic Acid CH3CHO 1-Cl-C4H9 2-Cl-C4H9 Σ butenes C2H5Cl 2-butanold tirra O2 c c c set (min) (ppm) (ppm) (ppm) (Torr) (K) C0 (mole %) (mole %) (mole %) (mole %)c (mole %)c (mole %)c (mole %)c (mole %)c (mole %)c (mole %)c 4 1a 2 9.3x10 2205 993 1000 298 0.94 24.4 9.8

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