Propane oxidation activation energies

Above 300°C. the effective reaction of an alkyl radical with oxygen may be Reaction 3 rather than 2 because of the reversibility of Reaction 2. If it is assumed that Reaction 3 is important at about 450°C., its rate can be estimated from the competition between pyrolysis and oxidation of alkyl radicals. Falconer and Knox (21) observed that the ratio of (pro-pene)/(ethylene) from the oxidation of propane between 435° and 475°C. increased with oxygen concentration and decreased with temperature—the apparent activation energy difference for the two reactions forming the olefins being 27 =t 5 kcal. per mole. They interpreted this result in terms of a competition between Reactions 1 and 3. The observed ratio (propene)/(ethylene) was 3.5 at 435°C. and 10 mm. of Hg pressure. If log ki(propyl) = 13.2 — 30,000/2.30RT, the value for the n-propyl radical (34), then log k3 = 8.0. If the A factor is 109-3, we derive the Arrhenius equation... 

A criterion for the suitability of a spectroscopy cell for investigations of working catalysts can be formulated as follows the activity or selectivity data and activation energy values have to be in agreement with the catalytic performance data measured with a conventional fixed-bed reactor. Table 1 is a comparison of the conversion and selectivity values characterizing an alumina-supported molybdenum-vanadium oxide catalyst during propane ODH obtained with a conventional fixed-bed reactor and with a spectroscopic cell that fulfills this requirement (Banares and Khatib, 2004). Similar considerations have also been reported earlier for other methods, such as X-ray diffraction (Clausen et al., 1991). 

The mechanism of formation of the cracking products had not been resolved at the time of publication of Shtern s review [2]. Semenov [3], however, had pointed out that the direct decomposition of prop-l-yl and prop-2-yl radicals at 300 °C was most unlikely, due to the large activation energies involved (27—29 and 40 kcal. mole", respectively). He therefore suggested the following alkylperoxy radical isomerization and decomposition reactions to explain the formation of propene and ethylene in propane oxidation, viz. 

Competitive studies have been very useful in providing relative rates and activation energies. For example, reaction of Cp Rh PMe3) with a 1 1 mixture of benzene and propane showed a preference for oxidative addition of the benzene carbon-hydrogen... 

Oxidized fish oils, rich in n-3 polyunsaturated fatty acids, produced volatile compounds more readily than oxidized vegetable oils, rich in linoleic acid. Activation energy for the formation of propanal from fish oils was lower than for the formation of hexanal from vegetable oils. A mixture of aldehydes contributed to the characteristic odors and flavors of oxidized fish, described as rancid, painty, fishy and cod liver oil-like (Table 11.21). Oxidation of unsaturated fatty acids in fish was related to the formation of 2-pentenal, 2-hexenal, 4-heptenal, 2,4-heptadienal and 2,4,7-decatrienal. The fishy or trainy characteristic of fish oil was attributed to 2,4,7-decatrienal. Studies of volatiles from boiled trout after storage showed significant increases in potent volatiles by aroma extraction dilution analysis (Table 11.22). Volatiles with the highest odor impact included l,5-octadien-3-one, 2,6-nonadienal, 3-hexenal, and 3,6-nonadienal. 3,6-Nonadienal and 3-hexenal were considered to contribute most to the fatty, fishy flavor in stored boiled fish. 

Supported vanadium oxides are also active in the activation of propane and critically depend upon the vanadium dispersion and the surface acidity, which appears to be controlled by its interaction with the support. The activation energy for C-H bond activation is thought to increase in the following order " ... 

Hydrocarbon conversions can. In general, be represented fairly well by first-order reaction kinetics, and the conversion levels for runs made with a constant hydrocarbon flow rate as a general rule Increased significantly as the temperature Increased. Figures 1, 2, and 3 show typical results for ethane, propane, ethylene, and propylene. Based on first-order kinetics, the activation energies for ethane, propane, ethylene, and propylene were determined In the various reactors tested. In the Vycor reactor, these activation energies were approximately 51, 57, 56, and 66 k cal/g mole respectively. They were much lower In metal reactors especially after the reactor was oxidized. In a relatively new and unoxidized Incoloy reactor, the activation energies were 15, 47, 27, and 26 k cal/g mole respectively. 

Table 1.6. Oxidation of propane over Pd, Pt and Rh catalysts. Kinetic orders with respect to O2 (m) and to C3H8 (n) and activation energies. From Ref. 9.

That this is a promotion is evidenced from the absence of conversion in oxygen-free mix-fures of alkanes wifh NO and by a sharp reduction in the effective activation energy upon NO addition, calculated from fhe femperafure dependence of fhe conversion of alkanes displayed in Fig. 9.2 (Table 9.1). Nofe that, for the oxidation of methane, it becomes even lower than that for the oxidation of ethane and propane without NO additives. In addition, imder conditions of these experiments (Paik = 20, P02 = 10 kPa, Pno = 2 kPa, Ptot = 101 kPa, and fr = 6 s), the alkane conversion per NO molecule was 5.0 for CH4 (at 650 °C), 4.1 for C2H6 (at 550 °C), and 4.2 for CsHg (at 500 °C) [184]. 

The CBS-QB3 potential energy surface accounts for the various experimentally observed products, including hydroperoxyl radical, propene, HO, propanal, and oxirane (c-CsHgO). The activation barrier for simultaneous 1,4-H transfer and HO2 expulsion, obtained via calculations, compares well to the experimentally observed barrier (26.0kcal/mol) of DeSain et al. This work provides some ramifications for larger alkylperoxy radicals multiple conformers of long alkylperoxy radicals are likely to play a role in the overall oxidation chemistry and dictate consideration for correct treatment of thermochemistry at lower temperatures T< 500 K), unimolecular reactions dictate peroxy radical chemistry. 

In the same vein, with propane, the primary C-H bond reacted twice as fast as the secondary despite its high bond dissociation energy, meaning antiradical reactivity. Whereas radical H-abstraction occurs from the weakest C-H bond of hydrocarbons (selectivity tertiary C-H > secondary C-H > primary C-H), this new activation mode proceeding with the opposite selectivity using C-H oxidative addition by... 

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