Thermochemical kinetic studies of organic peroxides relevant to the combustion of hydrocarbons
In the combustion of fuels and related organic compounds the initial step consists of a free radical forming process occurring either homogeneously or heterogeneously, such as RH + O2 → R + HO_2 (1) The radical R, reacts with oxygen to produce an alkyl or other peroxy radical: R + O_2 ↔ RO2 (2) One of the controversies involved in the mechanism for the oxidation of hydrocarbons is the route for the unimolecular decomposition of the hydroperoxy alkyl radical (R-HOOH). This would be produced as a result of the isomerisation of the alkyl peroxy radical (RO2). There are three possible unimolecular paths for R-HOOH together with the addition of oxygen to form hydroperoxy alkyl peroxy radical. This study is concerned with the generation of an alkyl peroxy alkyl radical and its decomposition to both epoxide and olefin formation and at lower temperatures predominantly follows the thermochemically more favourable route. No direct information is available about the rate constants of the two decomposition routes of alkyl peroxyalkyl/hydroperoxy alkyl radicals. There are different ways to find out the rate constants for the decomposition of alkyl peroxy alkyl/hydroperoxy alkyl radical to olefin and oxirane. One such way was a study of the gas phase, hydrogen chloride catalysed decomposition of di-t-butyl peroxide. A surrogate hydroperoxy alkyl radical was generated via this study and the most favourable route for the decomposition of dtBP-H is confirmed. Again, on thermochemical grounds, the formation of isobutene oxide predominates over the formation of isobutene. The modelling of this study assisted considerably in choosing the reaction steps for a probable mechanism and in the assessment of rate parameters for the individual steps. A bonafide hydroperoxy alkyl radical was generated via the study of the sensitized decomposition of t-butyl hydroperoxide in an uncoated, coated reaction vessel and also in the presence of oxygen. The Arrhenius parameters for the ratio of the rate of formation of isobutene to isobutene oxide was observed experimentally, and are in good agreement with the estimated values in the coated reaction vessel but in uncoated and in the presence of oxygen, this ratio is nearly doubled which suggests that isobutene is formed heterogeneously and surface played an important role. In order to observe the effect of surface: volume ratio on product formation, this system was studied in four different coated reaction vessels and it was concluded that the surface effect was negligible on a coated spherical reaction vessel. The bond dissociation energy DHo(RO-OH) in alkyl hydroperoxides, is important because the value of the rate constant is critical to cool flames production. The pyrolysis of t-butyl hydroperoxide was carried out, in a bath of isobutane in order to isolate the tBuO-OH bond breaking step. Acetone formation constituted a direct measure of the rate of decomposition of t-butyl hydroperoxide. The O-O bond dissociation energy was found experimentally, which is in good agreement with other group workers values.