Elementary reactions versus overall hydrocarbon cracking

Elementary reactions versus overall hydrocarbon cracking 

Once we have determined the detail mechanisms of HC thermal cracking, it is important to link the atomistic, elemental reactions to the overall petroleum and natural gas generation. One of the common questions is how to compare the calculated activation energies with the measured ones. From atomic theory, an activation energy is the energy difference between the reactant and transition state of an elementary reaction. It is directly linked to the nature of the chemical bond in a molecular system. From a phenomenological approach, activation energy is derived from the classical Arrhenius equation  

Assuming the pre-exponential factor. A, is constant, the activation energy can be defined as 

To compare the ab initio results of the elemental reactions with the overall reactions of HC cracking, one needs use the following equation (Savage and Klein 1989)  

The calculated BDE of reaction (22) is 68 kcal/mol at the B3LYP/6-31G level, which is considerably lower than the BDE of a typical C-C bond. When the carbon atom at the P position is replaced by heteroatoms such as sulfur or oxygen, which is quite common in kerogen structures (Siskin et al. 1995), the linkages become even weaker. For example, consider the following reactions, 

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Elementary reactions versus overall hydrocarbon cracking... 

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