Hydroperoxyalkyl Radicals Research Articles

Comparison of oxidation and autoignition of the two primary reference fuels by rapid compression

New experimental data on autoignition delays and product distributions during two-stage autoignitions for the two primary reference fuels n -heptane and iso -octane (2,2,4-trimethylpentane) have been obtained by rapid compression in the low and intermediate range of temperature for enginelike conditions of stoichiometry and dilution. The lower reactivity of iso -octane has been compensated by a four times increase in pressure. A good correlation between our data and that published is obtained when the compressed charge density of the core gas is considered. Both fuels show many common features in this temperature range: a marked negative temperature coefficient region that shifts to higher temperatures as the pressure is increased and a similarity in the nature of the intermediate species. However, the importance of the cool flame zone is greater for n -heptane, and the negative temperature coefficient region extends toward higher temperatures. The evolution of the main intermediate products formed during the two-stage autoignition is presented and discussed according to a common generic mechanism that takes into account the various isomerizations of alkylperoxy radicals and scissions of the hydroperoxyalkyl radicals. Cyclic ethers are important intermediates. For both hydrocarbons, tetrahydrofurans are the major O heterocycles formed in cool flames, especially in the case of iso -octane. The observed high selectivity in the lower alkenes demonstrates the importance of β carbon-carbon scission of the hydroperoxyalkyl radicals that leads to terminal alkenes in the case of n -heptane and to methylpropene and substituted pentenes in the case of iso -octane. These channels have to be taken into account in the improvement of detailed mechanisms for good predictions of pollutants.

Reaction of hydroperoxy t-butyl radicals and t-butyl peroxyt-butyl radicals in relation to chain propagation in combustion

Hydroperoxyl t -butyl radicals (I) or t -butyl peroxy t -butyl radicals (II) were generated in an oxygen-free system. This allowed an unequivocal measure of the ratio of olefin to epoxide formation to be made. For hydroperoxy alkyl radicals, the two paths are competitive over the temperature range 413–473 K with olefin production becoming more dominant as the temperature is raised. Some controversy surrounds the primary mechanism for the oxidation of alkyl radicals at temperatures up to 900 K. Strong arguments have been put forward for both exclusive olefin and exclusive epoxide formation. By comparing our results with this work, we conclude that the primary path is indeed exclusive olefin formation. The controversy may be reconciled by proposing that epoxide formation arises as the result of secondary reactions involving the addition of HO 2 or RO 2 to the primary olefin product. Pressure-dependent processes complicate the issue. However, it is difficult to see how any collisional stabilisation process would lead to increased epoxide formation in view of the present evidence available. This work shows that the addition of RO 2 to olefins results in the almost exclusive production of epoxide, at least at lower temperatures. For t -butyl peroxy t -butyl radicals over the temperature range 363–403 K, the path involving the production of epoxide is ≥98%. Further evidence is required on the reaction of hydroperoxy and alkylperoxy alkyl radicals.

The reaction of hydroperoxy-propyl radicals with molecular oxygen

The addition of hydroperoxy-alkyl radicals to molecular oxygen is an important process leading tochain branching at conditions related to autoignition and engine knock, as well as in the low-temperature oxidation of paraffins. Rate constants and product channels for the reaction of hydroperoxy-propyl radicals with O2 are estimated using thermodynamic properties, bimolecular quantum Kassel analysis, and transition state theory. Thermochemistry of the relevant molecules and radicals is estimated using group additivity and bond dissociation groups for radicals. Results show that the rates of the hydroperoxy-propyl radical addition to O2 are near their high-pressure limits for pressures of 1 atm and above. The main products at 0.1–15 atm are stabilization, reverse reaction to hydroperoxy-propyl + O2, and alkyl-carbonyl-hydroperoxide + OH. Dissociation rates and product channels of the stabilized adducts are estimated using unimolecular quantum Kassel analysis because stabilization is the most important hydroperoxy-propyl radical + O2 product channel. At low temperatures, below about 700 K, the stabilized peroxy adducts react primarily to alkyl-carbonyl-hydroperoxide + OH, products that lead to chain branching. At higher temperatures, above about 700K, the stabilized peroxy adducts react primarily to hydroperoxy-propyl radical + O2, initial reactants, which inhibits the overall oxidation. The switch in decomposition product channels, from low to high temperature, correlates well with the experimentally observed negative temperature coefficient behavior for propane oxidation. Rate expressions for reaction of each of the three hydroperoxy-alkyl isomers with O2 and for dissociation of the stabilized adducts are calculated for a series of pressures over a temperature range of 300–2100 K. Accurate rate expressions for alkylperoxy and hydroperoxy-alkyl reactions are essential to predict the negative temperature coefficient behavior for alkanes.

ChemInform Abstract: Formation, Isomerization, and Cyclization Reactions of Hydroperoxyalkyl Radicals in Hexadecane Autoxidation at 160‐190 . degree.C

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Formation, isomerization, and cyclization reactions of hydroperoxyalkyl radicals in hexadecane autoxidation at 160-190.degree.C

Kinetic and mechanistic investigations of the effects of oxygen pressure on liquid-phase autoxidation reactions in hydrocarbons at elevated temperatures were carried out with hexadecane at 160-190 o C and at oxygen pressures from 4 to 120 kPa using a stirred-flow reactor

ChemInform Abstract: Arrhenius Parameters for the Addition of HO2 Radicals to (E)‐But‐2‐ene Over the Range 400‐520 °C.

Studies have been made of the addition of HO2 radicals to (E)-but-2-ene in the temperature range 400–520°C, by use of the co-oxidation of (E)-but-2-ene and propene in the presence of tetramethylbutane as a source of HO2 radicals. From measurements of the relative yields of 2,3-dimethyloxirane and methyloxirane and the known Arrhenius parameters for reaction (6), values of A7= 108.61 ± 0.30 dm3 mol–1 s–1 and E7= 50.0 ± 4 kJ mol–1 have been obtained. HO2+ C3H6→ C3H6O + OH (6), HO2+(E)-C4H8-2 → C4H8O + OH (7)The values are compared with data for HO2 addition to other alkenes. The excellent correlation between the activation energy for addition and the ionisation energy of the alkene is used to provide a kinetic data base for HO2 addition to alkenes over the range 600–1000 K, where the reactions are of major importance.From studies of the relative yields of 2,3-dimethyloxirane and (Z)-but-2-ene from (E)-but-2-ene + H2+ O2 mixtures, further evidence is presented to show that the decomposition of hydroperoxyalkyl radicals, such as CH3CH(OOH)CHCH3, into alkene + HO2 is at most a minor process compared with formation of an oxirane + OH radical.

Arrhenius parameters for the addition of HO2radicals to (E)-but-2-ene over the range 400–520°C

Studies have been made of the addition of HO2 radicals to (E)-but-2-ene in the temperature range 400–520°C, by use of the co-oxidation of (E)-but-2-ene and propene in the presence of tetramethylbutane as a source of HO2 radicals. From measurements of the relative yields of 2,3-dimethyloxirane and methyloxirane and the known Arrhenius parameters for reaction (6), values of A7= 108.61 ± 0.30 dm3 mol–1 s–1 and E7= 50.0 ± 4 kJ mol–1 have been obtained. HO2+ C3H6→ C3H6O + OH (6), HO2+(E)-C4H8-2 → C4H8O + OH (7)The values are compared with data for HO2 addition to other alkenes. The excellent correlation between the activation energy for addition and the ionisation energy of the alkene is used to provide a kinetic data base for HO2 addition to alkenes over the range 600–1000 K, where the reactions are of major importance.From studies of the relative yields of 2,3-dimethyloxirane and (Z)-but-2-ene from (E)-but-2-ene + H2+ O2 mixtures, further evidence is presented to show that the decomposition of hydroperoxyalkyl radicals, such as CH3CH(OOH)CHCH3, into alkene + HO2 is at most a minor process compared with formation of an oxirane + OH radical.

Mechanism of ketone formation in the thermooxidation and radiolytic oxidation of low density polyethylene

During the initial stage of the thermooxidation of unstabilized LDPE at 95°C the rate of initiation has been found to be first order in hydroperoxide concentration whereas the initiation of radiation-induced oxidation at 25°C was independent of the hydroperoxide concentration. In both cases, the ketone accumulation is much more important than that of hydroperoxide. According to a kinetic analysis of thermooxidation, this result can be attributed to a chain reaction in which secondary peroxy radicals abstract tertiary hydrogen atoms from secondary hydroperoxides to form hydroperoxy alkyl radicals R 1Ċ(OOH)R 2 which decompose by β-scission to give ketones and hydroxyl radicals. The overall result of these reactions is the production of ketones and water in equal amounts and the regeneration of hydroperoxide. Kinetic analysis of the radiation-induced oxidation leads to the conclusion that hydroperoxy radicals HO . 2 also participate in the attack on the tertiary hydrogen of the secondary hydroperoxides.

The role of radical-radical reactions in hydrocarbon oxidation

The yields of products obtained by Euker and Leinroth in the oxidation ofn-butane in a flow system at 325°C have been interpreted on the assumption that the oxygenated products, except butene oxides, are formed from butoxy radicals produced by the mutual reaction of alkyperoxy radicals. On this basis, by taking reasonable velocity constants, it has been possible to use the observed value ofd[CH3CHO]/dt to calculate the concentration of C4H9O2 radicals, and hence a value ofd[C4H8]/dt consistent with the experimental value. The same treatment, applied to results obtained by Berry, Cullis, and Trimm, explains why their value ofd[CH3CHO]/dt is lower by a factor of 2000, whereasd[C4H8]/dt is only lower by a factor of 40. By taking reasonable values for the velocity constants involved, it is shown that a consistent and quantitative interpretation is possible. The uncertainties in the calculation preclude the establishment of a unique mechanism and further work, in which both rates and yields of products are measured, is necessary to confirm the importance of radical-radical reactions. An activation energy of 30 kcal mole−1 has been obtained for the reactions-C4H9O2 = dimethyl oxiran + OH; this probably corresponds to the activation energy for the isomerisation of the peroxy radical into the hydroperoxyalkyl radical. A similar treatment has been used to explain the formation of formaldehyde in the oxidation of ethane at 320°–340°C.

Studies of the combustion of branched-chain hydrocarbons

The cool-flame oxidation of both 3-ethylpentane and 3-methylpentane has been studied in the temperature range 250°–410°C. The results show that, with these two branched-chain hydrocarbons, the principal chain-propagating cycle involves alkylperoxy radical isomerization, followed by decomposition of the resulting hydroperoxyalkyl radicals; evidence is presented for the first time that ethyl group shifts can occur during the breakdown of the latter radicals. The “alkene-hydroperoxy radical addition” route appears to play only a minor role, even at 403°C. It is also shown that the higher the octane number of a hydrocarbon fuel the more likely it is to lead to β -scission products. Since many of these products are lower molecular weight alkenes, this result is very relevant to the problem of pollution from vehicle exhausts, since alkenes may react photochemically to form smog.

The low-temperature combustion of n-pentane

The reaction of n -pentane with oxygen has been investigated in the temperature range 257°–280°C at initial pressures which cover both the slow-oxidation and cool-flame regions. The oxidation has been followed by the direct analysis of reaction mixtures, together with the changes in pressure and temperature. The analytical results show that acetone is the major primary organic product of the slow reaction, and that it accounts for about 60% of the pentane consumed in the early stages of the reaction. In contrast, cool-flame formation is accompanied by the production of relatively large amounts of olefins, while the importance of acetone as a product is markedly decreased in comparison with the slow-oxidation region. These results are discussed in terms of a reaction mechanism that includes isomerization reactions of the alkylperoxy radical, and it is concluded that acetone must be formed through the formation of a hydroperoxyalkylperoxy radical rather than a decomposition reaction of a hydroperoxyalkyl radical. It is shown qualitatively that the relative importance of the various products obtained is in accord with the suggested mechanism for the over-all reaction. Simultaneous records of the pressure and temperature changes during the appearance of a cool flame show that the temperature rise is preceded by an abrupt, almost isothermal pressure rise. With n -pentane as fuel, therefore, it is suggested that branching reactions play an important role in cool-flame formation, and that the cool-flame limit is not governed solely by thermal factors. * Present address: Department of Chemical Engineering, University of Salford.

The Cool Flames of Hydrocarbons

AbstractThis review describes the properties of the cool‐flame and two‐stage ignition processes that characterize the gaseous oxidation of hydrocarbons and discusses the chemical reactions that are responsible for these phenomena. Cool flames result from a chainthermal acceleration of reaction rate. It is probable that the free‐radical chain involved is propagated by the reaction of an alkyl radical with oxygen to give an alkyperoxy radical which isomerizes to a hydroperoxyalkyl radical. The decomposition of this radical produces a hydroxyl radical, which attacks the hydrocarbon rapidly and unselectively to regenerate an alkyl radical. Branching probably results from the pyrolysis of mono‐ and dihydroperoxides, from the oxidation of aldehydes, and from radical‐molecule reactions. This reaction scheme explains the existence of a low‐temperature ignition peninsula and the relation of the extent and shape of this peninsula to the molecular structure of the hydrocarbon. The chemical relevance of cool flames to abnormal combustion phenomena, such as knock, in gasoline engines is discussed.