title: Low-temperature combustion chemistry of novel biofuels: resonance-stabilized QOOH in the oxidation of diethyl ketone.
abstract: The Cl˙ initiated oxidation reactions of diethyl ketone (DEK; 3-pentanone; (CH3CH2)2C=O), 2,2,4,4-d4-diethyl ketone (d4-DEK; (CH3CD2)2C=O) and 1,1,1,5,5,5-d6-diethyl ketone (d6-DEK; (CD3CH2)2C=O) are studied at 8 Torr and 550-650 K using Cl2 as a source for the pulsed-photolytic generation of Cl˙. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron radiation. Adding a large excess of O2 to the reacting flow allows determination of products resulting from oxidation of the initial primary (Rp) and secondary (Rs) radicals formed via the Cl˙ + DEK reaction. Because of resonance stabilization, the secondary DEK radical (3-oxopentan-2-yl) reaction with O2 has a shallow alkyl peroxy radical (RsO2) well and no energetically low-lying product channels. This leads to preferential back dissociation of RsO2 and a greater likelihood of consumption of Rs by competing radical-radical reactions. On the other hand, reaction of the primary DEK radical (3-oxopentan-1-yl) with O2 has several accessible bimolecular product channels. Vinyl ethyl ketone is observed from HO2-elimination from the DEK alkylperoxy radicals, and small-molecule products are identified from β-scission reactions and decomposition reactions of oxy radical secondary products. Although channels yielding OH + 3-, 4-, 5- and 6-membered ring cyclic ether products are possible in the oxidation of DEK, at the conditions of this study (8 Torr, 550-650 K) only the 5-membered ring, 2-methyltetrahydrofuran-3-one, is observed in significant quantities. Computation of relevant stationary points on the potential energy surfaces for the reactions of Rp and Rs with O2 indicates this cyclic ether is formed via a resonance-stabilized hydroperoxyalkyl radical (QOOH) intermediate, formed from isomerization of the RpO2 radical.
authors.affiliation: Combustion Research Facility, Sandia National Laboratories, MS 9055, Livermore, CA 94551 USA. cataatj@sandia.gov.
authors.forename: Adam M
authors.initials: AM
authors.lastname: Scheer
authors.forename: Oliver
authors.initials: O
authors.lastname: Welz
authors.forename: Judit
authors.initials: J
authors.lastname: Zádor
authors.forename: David L
authors.initials: DL
authors.lastname: Osborn
authors.forename: Craig A
authors.initials: CA
authors.lastname: Taatjes
mesh_terms: D056804:Biofuels / Q000032:analysis*; D004987:Ethers / Q000737:chemistry*; D006358:Hot Temperature; D008422:Materials Testing; D010084:Oxidation-Reduction; D010422:Pentanones / Q000737:chemistry*
chemical_list: D056804:Biofuels; D004987:Ethers; D010422:Pentanones; C027452:diethyl ketone