Experimental and Kinetic Modeling Study of 2-Methyl-2-Butene: Allylic Hydrocarbon Kinetics

Abstract

Two experimental studies have been carried out on the oxidation of 2-methyl-2-butene, one measuring ignition delay times behind reflected shock waves in a stainless steel shock tube, and the other measuring fuel, intermediate, and product species mole fractions in a jet-stirred reactor (JSR) . The shock tube ignition experiments were carried out at three different pressures, approximately 1.7 atm, 11.2 atm, and 31 atm, and at each pressure, fuel-lean =0.5), stoichiometric (=1.0), and fuel-rich (=2.0) mixtures were examined, with each fuel/oxygen mixture diluted in 99% Ar, for initial post-shock temperatures between 1330 and 1730K. The JSR experiments were performed at nearly-atmospheric pressure (800 Torr), with stoichiometric fuel/oxygen mixtures with 0.01 mole fraction of 2M2B fuel, a residence time in the reactor of 1.5s, and mole fractions of 36 different chemical species were measured over a temperature range from 600 to 1150K. These JSR experiments represent the first such studies reporting detailed 2 species measurements for an unsaturated, branched hydrocarbon fuel larger than iso-butene. A detailed chemical kinetic reaction mechanism was developed to study the important reaction pathways in these experiments, with particular attention on the role played by allylic C-H bonds and allylic pentenyl radicals. The results show that, at high temperatures, this olefinic fuel reacts rapidly, similar to related alkane fuels, but the pronounced thermal stability of the allylic pentenyl species inhibits low temperature reactivity, so 2M2B does not produce "cool flames" or negative temperature coefficient behavior. The connections between olefin hydrocarbon fuels, resulting allylic fuel radicals, the resulting lack of low temperature reactivity, and the gasoline engine concept of Octane Sensitivity are discussed.

Understanding the reactivity of unsaturated alcohols: Experimental and kinetic modeling study of the pyrolysis and oxidation of 3-methyl-2-butenol and 3-methyl-3-butenol

“…Similar to some of the other resonantly stabilized radicals encountered in this work, recombination with hydroperoxy radicals is the major reaction channel. This is in agreement with the conclusions by Welz et al for 2-methyl-2-butene oxidation [69]. Hydrogen addition to 2-methyl-1,3-butadiene can also form non-resonantly stabilized radicals, i.e.…”

“…Characteristic for the oxidation of prenol is the occurrence of aldehyde peaks below 700 K. Such early formation of oxygenated products has not been observed during the oxidation of 2-methyl-2-butene [69], the hydrocarbon backbone of prenol.…”

“…Resonantly stabilized radicals, such as allyl radicals, have a relatively long life time compared to alkyl radicals. Recombination with hydroperoxy radicals therefore is an important consumption channel, as shown for allyl in propene oxidation [50] and 2-methyl-butenyl in 2-methyl-2-butene oxidation [69]. The resulting hydroperoxide has a weak O-O bond and scission results in a hydroxyl radical and an oxy radical.…”

“…Besides hydrocarbons and small oxygenated molecules, the reactor effluent includes acetone, 2-methyl-2-propenal and 3-buten-2-one. A recent kinetic modeling study regarding the oxidation of 2-methyl-2-butene, which is the hydrocarbon backbone of prenol, highlighted the importance of hydroperoxy recombination with allylic radicals [69]. Reaction path analysis showed that the latter reactions, and subsequent decomposition, can lead to 2-methyl-2-propenal and 3-buten-2-one.…”

“…Reaction path analysis showed that the latter reactions, and subsequent decomposition, can lead to 2-methyl-2-propenal and 3-buten-2-one. Hydroxyl radical addition on 2-methyl-2-butene and reaction with molecular oxygen, the Waddington mechanism, leads to acetaldehyde, acetone and hydroxyl radical [69].…”