ISSN:
0538-8066
Schlagwort(e):
Chemistry
;
Physical Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
The thermal reaction of propene was examined around 800 K in the presence of less than 20% oxygen. At initial time, the production of H2, CH4, C2H4, C2H6, allene, C3H8, 1,3-butadiene, butenes, 3- and 4-methylcyclopentene, a mixture of 1,4- and 1,5-hexadienes, methylcyclopentane (or dimethylcyclobutane), 4-methylpent-1-ene, and hex-1-ene, was observed along with hydrogen peroxide, CO, and small quantities of ethanal and CO2. Oxygen increases the initial production of hydrogen and of most hydrocarbons and, particularly, that of C6 dienes and of cyclenes. However, the production of allene, methylcyclopentane (or dimethylcyclobutane), and 4-methylpent-1-ene is practically not affected. A kinetic study confirms the mechanism proposed for the thermal reaction of propene. Formation of allene, thus, involves a four-center-unimolecular dehydrogenation of propene, that of 4-methylpent-1-ene is explained by an ene bimolecular reaction while methylcyclopentane (or dimethylcyclobutane) probably arises from a bimolecular process involving a biradical intermediate. Other products arise from a conventional chain radical mechanism.A kinetic scheme is proposed in which chains are primarily initiated by the bimolecular step:C3H6+O2→HO2·+C3H5·which competes with the second-order initiation of propene pyrolysis. Since allene production is not affected by oxygen, it is concluded that allyl radicals are not dehydrogenated by oxygen; but they oxidize in a branching step involving allylperoxyl radicals; r. radicals other than methyl, and allyl are dehydrogenated according to the conventional process:r·+O2→unsaturated+HO2·and account for the production of a large excess of C6 diolefins, methylcyclopentenes, and hydrogen peroxide, when r. stands for C6H11, the allyl adduct. Hydrogen peroxide gives rise to a degenerate branching of chains. Based on the proposed scheme, a modeling of the reaction is shown to account fairly well for the concentration-time profiles. Rate constants of many steps are evaluated and discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 503-522, 1998
Zusätzliches Material:
7 Ill.
Materialart:
Digitale Medien
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