Products from the Oxidation of n-Butane from 298 to 735 K Using Either Cl Atom or Thermal Initiation: Formation of Acetone and Acetic Acid-Possible Roaming Reactions?

The oxidation of 2-butyl radicals (and to a lesser extent 1-butyl radicals) has been studied over the temperature range of 298-735 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 and then sharply, peaking near 525 K at yields of ∼25 and ∼20 mol %, respectively. In the same temperature range (400-525 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 of 410-440 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.

Rate Constants for the Reaction of Cl with a Series of C4 to C6 Ketones Using the Relative Rate Method

Rate constants for the reaction of Cl with eight ketones were measured relative to the rate constant of propane in approximately 900 Torr of N2 at ambient temperature. Experiments were carried out in a Pyrex reactor with GC analysis of the consumption of the ketones and propane. Chlorine atoms were generated by irradiation of Cl2 in the initial mixture using a black-light-blue fluorescent lamp. The rate constants determined in these experiments (10-11 cm3 molecule-1 s-1) are: butanone (3.8 +/- 0.3); 2-pentanone (11.6 +/- 1.0); 3-pentanone (8.3 +/- 0.7); 2-hexanone (19.4 +/- 1.9); 3-hexanone (15.3 +/- 1.1); cyclopentanone (10.4 +/- 0.9); 3-methyl-2-butanone (6.2 +/- 0.5); and 4-methyl-2-pentanone (12.8 +/- 1.0). The results for 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, 3-methyl-2-butanone, and 4-methyl-2-pentanone are significantly higher (by a factor of 3 for 2-hexanone) than reported in two previous absolute rate studies. The likely explanation for this discrepancy is discussed.

Kinetics, Products, and Stereochemistry of the Reaction of Chlorine Atoms with cis- and trans-2-Butene in 10−700 Torr of N2 or N2/O2 Diluent at 297 K

The reactions of Cl atoms with cis- and trans-2-butene have been studied using FTIR and GC analyses. The rate constant of the reaction was measured using the relative rate technique. Rate constants for the cis and trans isomers are indistinguishable over the pressure range 10-900 Torr of N2 or air and agree well with previous measurements at 760 Torr. Product yields for the reaction of cis-2-butene with Cl in N2 at 700 Torr are meso-2,3-dichlorobutane (47%), DL-2,3-dichlorobutane (18%), 3-chloro-1-butene (13%), cis-1-chloro-2-butene (13%), trans-1-chloro-2-butene (2%), and trans-2-butene (8%). The yields of these products depend on the total pressure. For trans-2-butene, the product yields are as follows: meso-2,3-dichlorobutane (48%), dl-2,3-dichlorobutane (17%), 3-chloro-1-butene (12%), cis-1-chloro-2-butene (2%), trans-1-chloro-2-butene (16%), and cis-2-butene (2%). The products are formed via addition, addition-elimination from a chemically activated adduct, and abstraction reactions. These reactions form (1) the stabilized 3-chloro-2-butyl radical, (2) the chemically activated 3-chloro-2-butyl radical, and (3) the methylallyl radical. These radicals subsequently react with Cl2 to form the products via a proposed chemical mechanism, which is discussed herein. This is the first detailed study of stereochemical effects on the products of a gas-phase Cl+olefin reaction. FTIR spectra (0.25 cm(-1) resolution) of meso- and DL-2,3-dichlorobutane are presented. The relative rate technique was used (at 900 Torr and 297 K) to measure: k(Cl + 3-chloro-1-butene) = (2.1 +/- 0.4) x 10(-10), k(Cl + 1-chloro-2-butene) = (2.2 +/- 0.4) x 10(-10), and k(Cl + 2,3-dichlorobutane) = (1.1 +/- 0.2) x 10(-11) cm3 molecule(-1) s(-1).

Automotive fuels and internal combustion engines: a chemical perspective

Commercial transportation fuels are complex mixtures containing hundreds or thousands of chemical components, whose composition has evolved considerably during the past 100 years. In conjunction with concurrent engine advancements, automotive fuel composition has been fine-tuned to balance efficiency and power demands while minimizing emissions. Pollutant emissions from internal combustion engines (ICE), which arise from non-ideal combustion, have been dramatically reduced in the past four decades. Emissions depend both on the engine operating parameters (e.g. engine temperature, speed, load, A/F ratio, and spark timing) and the fuel. These emissions result from complex processes involving interactions between the fuel and engine parameters. Vehicle emissions are comprised of volatile organic compounds (VOCs), CO, nitrogen oxides (NO(x)), and particulate matter (PM). VOCs and NO(x) form photochemical smog in urban atmospheres, and CO and PM may have adverse health impacts. Engine hardware and operating conditions, after-treatment catalysts, and fuel composition all affect the amount and composition of emissions leaving the vehicle tailpipe. While engine and after-treatment effects are generally larger than fuel effects, engine and after-treatment hardware can require specific fuel properties. Consequently, the best prospects for achieving the highest efficiency and lowest emissions lie with optimizing the entire fuel-engine-after-treatment system. This review provides a chemical perspective on the production, combustion, and environmental aspects of automotive fuels. We hope this review will be of interest to workers in the fields of chemical kinetics, fluid dynamics of reacting flows, atmospheric chemistry, automotive catalysts, fuel science, and governmental regulations.