Butane photo-oxidation

Butane photo-oxidation and auxiliary mechanism assessment Chemistry of butane photo-oxidation... 

To allow a full evaluation, however, it is necessary to consider not only the experimental data for the butane-NOx system, but also the NOx photo-oxidation runs for intermediate products, MEK, CH3CHO and HCHO. The aim of the mechanism refinements for butane degradation, therefore, was to provide an adequate, self-consistent description of all these systems, whilst simultaneously identifying any required modifications to the auxiliary mechanism parameters. Mechanism testing and refinement was therefore carried out by an iterative procedure, considering butane photo-oxidation and each of the sub-systems in turn. The adapted version of MCM v3, resulting firom identified modifications, is denoted MCM v3a throughout this article. 

Because there is no primary radical source during butane photo-oxidation, this parent system was the most sensitive to changes in the chamber radical source parameter. In conjunction with the optimizations described above, it was found that the best fit to the butane-NOx photo-oxidation data was obtained by multiplying the Carter chamber radical source values used in the SAPRC evaluation (Carter, 2000) by a factor of 1.35 for the ITC and ETC chambers, and by a factor of 1.2 for the DTC, CTC and XTC chambers. 

In this regard, it is well to remember the role which the wall plays on the nature of the products obtained from gas phase oxidation. There is certainly common agreement that walls and wall reactions are important in this respect. For example, Hay et al. (11) have shown the importance of the walls in determining the nature and composition of the oxygenated products from 2-butane + 02 at 270°C. Cohens study on the photo-oxidation of acetone also illustrates this point (10). He found that if acetone is photolyzed by itself in a quartz vessel, the normal products—methane, ethane, carbon monoxide, and methyl ethyl ketone— are produced. 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

As with the chamber characterisation studies of Carter (Carter, 2000), the series of butane-NOx photo-oxidation experiments was used for initial assessment of the auxiliary mechanism parameters. This system is believed to provide a good test for the chamber wall effects because the degradation chemistry of butane is quite well characterized (Carter and Lurmann, 1991, Carter et al, 1995a), and because of the large set of experiments for which data are available. 

With these changes, the adapted MCM v3a butane mechanism was found to give generally good fits to D(03-N0) data in the complete series of butane-NOx photo-oxidation experiments. The scatter in the results was indicative of run-to-run variability, with most of the data being fit by the mechanism to within + - 30%, with no consistent biases. 

The MCM v3 butane degradation chemistry has been evaluated using chamber data on the photo-oxidation of butane, and on photo-oxidation of its degradation products, methylethyl ketone (MEK), acetaldehyde (CH3CHO) and formaldehyde (HCHO), in conjunction with an initial evaluation of the chamber-dependent auxiliary mechanisms for the series of relevant chambers. 

The MCM v3 mechanism for butane was found to provide an acceptable reaction framework for describing the NOx-photo-oxidation experiments on the above systems, although a number of parameter modifications and refinements were identified and introduced which resulted in an improved performance. These generally relate to the magnitude of sources of free radicals from carbonyl photolysis processes, which are currently under review in MCM development activities. Specifically, recommendations are made to update the photolysis parameters for HCHO, MEK, either in line with data reported since the MCM mechanism development protocol (Jenkin et ai, 1997), or on the basis of optimization in the current study. 

Table 1 Reaction products in the photo-oxidation of butane. 54 ppm (= 24.07 pmol) n-butane 2.5 + 60 ppm (= 26.60 pmol) H2O2. We identified additionally ethanol, propanal, methylhydroperoxide, formic acid and acetic acid.

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

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