Chlorine atom-initiated low-temperature oxidation of prenol and isoprenol: The effect of C C double bonds on the peroxy radical chemistry in alcohol oxidation

Abstract

The chlorine atom-initiated oxidation of two unsaturated primary C5 alcohols, prenol (3-methyl-2buten-1-ol, (CH 3) 2 CCHCH 2 OH) and isoprenol (3-methyl-3-buten-1-ol, CH 2 C(CH 3)CH 2 CH 2 OH), is studied at 550 K and low pressure (8 Torr). The time-and isomer-resolved formation of products is probed with multiplexed photoionization mass spectrometry (MPIMS) using tunable vacuum ultraviolet ionizing synchrotron radiation. The peroxy radical chemistry of the unsaturated alcohols appears much less rich than that of saturated C4 and C5 alcohols. The main products observed are the corresponding unsaturated aldehydesprenal (3-methyl-2-butenal) from prenol oxidation and isoprenal (3-methyl-3butenal) from isoprenol oxidation. No significant products arising from QOOH chemistry are observed. These results can be qualitatively explained by the formation of resonance stabilized allylic radicals via H-abstraction in the Cl + prenol and Cl + isoprenol initiation reactions. The loss of resonance stabilization upon O 2 addition causes the energies of the intermediate wells, saddle points, and products to increase relative to the energy of the initial radicals and O 2. These energetic shifts make most product channels observed in the peroxy radical chemistry of saturated alcohols inaccessible for these unsaturated alcohols. The experimental findings are underpinned by quantum-chemical calculations for stationary points on the potential energy surfaces for the reactions of the initial radicals with O 2. Under our conditions, the dominant channels in prenol and isoprenol oxidation are the chain-terminating HO 2forming channels arising from radicals, in which the unpaired electron and the OH group are on the same carbon atom, with stable prenal and isoprenal co-products, respectively. These findings suggest that the presence of C=C double bonds in alcohols will reduce low-temperature reactivity during autoignition.

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

“…C-H β-scission of the cyclic radical forms 3-methyl-furan, which was experimentally observed in this work and in the oxidation study by Welz et al [29].…”

“…The α-prenol radicals can react with molecular oxygen forming 3-methyl-2-butenal and hydroperoxy radical (reactions 49 and 50 in Table 1). In their study regarding the chlorine atom-initiated low-temperature oxidation of prenol, Welz et al highlighted that this is a low-energy reaction path [29]. At pyrolysis conditions, 3-methyl-2-butenal is formed by direct O-H β-scission of α-prenol radicals (reaction 31 in Table 1).…”

“…Recently, Welz et al investigated the oxidation of 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), see Figure 1, at 550 K and low pressure (8 Torr) [29]. In their experiments, chlorine atoms, which were generated from Cl2 by pulsed laser photolysis, abstract hydrogen from prenol and isoprenol.…”

Chemical kinetics of cyclic ethers in combustion

“…This unsaturated CE, the formation of which is well predicted by the model developed by [573], is formed through 3-methyl-2-butenal, a major product of prenol oxidation, by abstraction of a primary H-atom followed by cyclization. During the study of the chlorine atom-initiated low-temperature oxidation of prenol in a FR (550 K, 8 Torr) using SVUV-PIMS, Welz et al [582] recorded a strong signal at m/z 82 (C5H6O) and tentatively assigned it as 3-methylfuran. The slow rise of the 3-methylfuran signal suggests that it was formed as a secondary product.…”

The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study

“…Recently, Ninnemann et al 23 investigated the combustion behavior of these fuels by conducting ignition delay times, laminar burning velocities, and CO 2 time-history measurements. The effect of a double bond at low temperatures was studied by Welz et al 24 Particularly, the authors investigated an O 2 addition to prenol and isoprenol radicals theoretically and experimentally at 500 K and 8 torr. Allylic radicals were found to be the major products produced via hydrogen abstraction reactions initiated by Cl atoms, which underwent O 2 addition forming a/g-RO 2 radicals.…”