Cracking classic reaction mechanism

The classical HCK mechanism on bifunctional catalysts separates the metallic action from that of the acid by assigning the metallic function to the creation of an olefin from paraffin and the isomerization and cracking of the olefins to the acid function. Both reactions are occurring through carbenium ions [102],... 

In the cases of mesoporous silica, AMS and quartz chip, the 0.5 C3/C4 ratio being close to unity means that two reactions proceed with almost equal probability to each other. This is in accordance with the classical radical mechanism of alkane cracking supposing that the energy required to form tertiary radical is not so different from that required for secondary radical and that both radicals are cracked by P-scission mechanism shown below [13]. Thus, the results shown in Fig. 4 strongly suggest that isohexane is cracked via the radical mechanism on the mesoporous silica catalysts, or, in other words, MCM-41, both with and without aluminum impurity, and FSM-16 exhibit radical type catalytic function. 

As shown, the ratio was very high on zeolite catalysts, while that on mesoporous silica was as low as those on AMS and quartz chip. The high ratio on zeolites can not be explained by classical mechanism of acid-catalyzed cracking supposing higher stability of tertiary carbenium ion and its cracking by P-scission, because this supposition predicts that the reaction (2) proceeds in preference to the reaction (1). Rather, a-scission of carbocation [12] may rationalize the higher C3/C4 ratio on zeolite catalysts. 

The diffusion, adsorption and chemical steps for the dehydrogenation and cracking reactions of light alkanes catalyzed by zeolites were studied using a combined classical mechanics (MM, MD and MC) / quantum mechanics approach. 

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 ... 

In heterogeneous catalysis, nth-order kinetics may be the result of adsorption on a nonideal catalyst surface. In homogeneous systems, nth-order kinetics may represent the overall rate of the underlying elementary reactions, e.g., the classical Rice-Herzfeld mechanism for thermal cracking of hydrocarbonsFor simplicity, n is assumed to be constant for all species. This is not a strong assumption for many petroleum processes. 

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