The kinetics and mechanisms of hydrocarbon thermal cracking

Among the elementary steps involved in the thermal cracking reactions, the most important ones are  

The next section will review how to use ab initio calculations to study the detailed mechanisms of HC thermal cracking. By calculating the reactants and products as well as the relevant transition states, the full reaction coordinate of the key elementary reactions involved in the overall thermal cracking can be drawn. This includes the bonding pictures of all the species involved in the reaction, as well as the key kinetic parameters such as BDEs and activation energies from first principles. The comparison between the calculated results and experimental data will be discussed. 

Several simple paraffin molecules, such as butane, pentane, and octane, are used to represent the parent HCs (Xiao et al. 1997). It is assumed that the dynamics of the cracking reactions are controlled by short-range covalent forces. Therefore, the results based on these small paraffin molecules can be extended to much larger linear systems. 

In the following sections, we will review the results of ab initio calculations on the three basic steps of HC thermal cracking initiation cracking, H-transfer reaction, and radical decomposition. The results of their corresponding reverse reactions are discussed. The calculated structures and energies are compared with experimental data. 

Ideally, one would apply these high-accuracy methods as a routine for thermochemistry and kinetics studies. However, these methods can be very expensive for even a moderate system. For example, a G2(MP2) calculation on a octane molecule exceeds 100 cpu hours on a CRAY-YMP supercomputer. Petersson (1998) did extensive benchmark study and his conclusion is that most of the CBS and G2 methods are not practical for systems that have more than a half dozen non-hydrogen atoms. Table 2 shows the summary of error measurements for the G2 test set of 125 reactions. Most of these reactions involve very small gas molecules. 

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In the previous section, the kinetics and mechanisms of hydrocarbon thermal cracking were examined using ab initio quantum mechanical calculations. In this section we will review how transition state theory (TST) can be applied to calculate the carbon isotopic fractionation associated with natural gas generation. 

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