Alkanes kinetic parameters

Table XV summarizes kinetic parameters for hydrogenolysis reactions of alkanes and cycloalkanes over film catalysts and over supported catalysts for which it can be reasonably assumed that reaction is confined to the metallic phase. These kinetic parameters refer to the overall reaction, i.e., to the rate of disappearance of the parent molecule. It will be evident from Table XV that the catalytic activity of a given metal with a given...

The structural, spectroscopic and electrochemical properties of oxoruthenate (IV) complexes have been summarised, and a representative compilation of kinetic parameters for their oxidation reactions with alcohols, alkanes and alkenes presented [20],... 

Extensive studies have been described of the reactions of linear alkanes with H2 over EUROPT-l [6, 7, 12, 15, 16]. Table 2 summarizes the kinetic parameters reported for ethane, propane, and -butanc. Skeletal isomerization is of course possible with this last molc-... 

Table 2. Kinetic parameters for the hydrogenolysis of linear alkanes on EUROPT-1 TOF and selectivities at 573 K [6, 15].

Hydrogenolysis of alkanes and related reactions on these catalysts have been studied by a number of groups [18, 19, 21-23], Kinetic parameters for ethane, propane, and n-butane on both catalysts are summarized in Table 3. With propane, selectivities to ethane on... 

Argyle, M.D., Chen, K.D., Iglesia, E. and Bell, A.T. (2005) In situ UV-visible spectroscopic measurements of kinetic parameters and active sites for catalytic oxidation of alkanes on vanadium oxides. Journal of Physical Chemistry B, 109 (6), 2414-20. 

The correlation of k or Egct with bond strengths are the most commonly used for estimation purposes but correlations with other properties of the molecules are possible. For example, it is possible to correlate the rate constant for OH + alkane reactions with the ionization potential of the alkane [10] and, for other types of reaction, it has been suggested that both electron affinities and ionization potentials of the species involved may correlate with the kinetics parameters [51]. However, because of the great sensitivity of the rate constant to small errors in Eact, correlations aimed at predicting Eact are very rarely used. 

Undoubtedly, improved numerical simulations of the variation in the low-temperature reactivity amongst alkanes and their isomers will emerge. Perhaps, more importantly there will be reduced numerical models, obtained by the methods discussed in Chapter 4, which encapsulate the appropriate kinetic dependences in a fundamental way. The difficulty with a number of current approaches to reduced kinetic modelling [217] is that, although some of the elements of (74) may be identified in the reduced schemes, the implementation of the schemes relies too heavily on empirically derived kinetic parameters. 

The above formulas allow us to rationalize the experimental data concerning the thermal dissociation of alkanes and the corresponding free-radical recombinations. Interesting predictions of various thermodynamic and kinetic parameters of these reactions can also be made. 

The most common initiation or homolysis reaction is the breaking of a covalent C-C bond with the formation of two radicals. This initiation process is highly sensitive to the stability of the formed radicals. Its activation energy is equal to the bond dissociation enthalpy because the reverse, radical-radical recombination reaction is so exothermic that it does not require activation energy. C-C bonds are usually weaker than the C-H bonds. Thus, the initial formation of H radicals can be ignored. The total radical concentration in the reacting system is controlled both by these radical initiation reactions and by the termination or radical recombination reactions. In accordance with Benson (1960), the rate constant expressions of these unimolecular decompositions are calculated from the reverse reaction, the recombination of two radical species to form the stable parent compound, and microscopic reversibility (Curran et al., 1998). The reference kinetic parameters for the unimolecular decomposition reactions of K-alkanes for each single fission of a C-C bond between secondary... 

Cyc/o-alkanes are typically cyc/o-pentanes and cyc/o-hexanes with a certain degree of methylation and a single more or less long alkyl side chain. The C-C cleavage in the side chain follows the same rules and applies the same reference kinetic parameters as the initiation reactions of normal and branched alkanes. 

H-abstraction reactions of cyc/o-alkanes follow the same rules and apply the same reference kinetic parameters as the analogous reactions of normal and branched alkanes. For example, Fig. 6 shows the main cyclo-hexyl radical pyrolysis pathways. For simplicity s sake, most of the dehydrogenation reactions are not reported. 

The complete set of the reference kinetic parameters for cyc/o-alkane and alkene pyrolysis is reported in Table VII. 

Reference Kinetic Parameters for the Pyrolysis of Cyclo-Alkane and Alkenes (Units are kcal, kmol, l, s)... 

Below we will focus mainly on the problems of compiling micro-chemical (micro-kinetic) schemes of alkane oxidation and populating it with kinetic parameters. Of course, we had to pay certain attention to some closely connected questions but it was impossible to analyze all of them in detail in the framework of one paper. 

An alternative approach implies that all kinetic parameters used in the model are determined independently from the modeling itself. The probability of serious errors is still high in this case too, although for many parameters important for alkane oxidation and combustion such errors are evaluated (see, for instance, Tsang and Hampson, 1986 Warnatz, 1984). Despite that, it seems that in this case we are dealing with the lesser of two evils. Moreover,—and this is of principal importance—every value included into the scheme can be independently corrected as soon as newer and more reliable data appear. This does not require any modification of any other kinetic parameter in the model. 

Anthracenes linked by a much longer chain also give internal photodimers, and this is demonstrated by the formation (with reasonably high quantum yield) of photoproducts from bis(9-anthryl)polyoxa-alkanes (187) with linking chain lengths of up to 19 atoms. Kinetic parameters for this reaction have also been reported. An elaboration of this system to the cyclophane (188), which has one... 

These reactions consist in the abstraction of an H atom of an alkane R H molecule by a free radical R to give a molecule RH and a free radical R . The kinetic parameters of a... 

The inclusion of only one carbon double bond in a longer carbon chain does not decrease the oxidation stability significantly, as the example of 1-octadecene demonstrates (Table 3-17). The onset point has a temperature of 185 °C, the maximum temperature of the first peak is 215 °C. The kinetic parameters of the first oxidation peak are nearly equivalent to those of the n-alkanes. For the second peak the data are higher and for the last peak lower compared to the n-alkanes. 

Allara DL, Shaw RA (1980) A compilation of kinetic parameters for thermal decomposition of n-alkanes molecules. J Phys Chem Ref Data 9 523-559... 

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