Kinetic Analysis of Monomolecular Cracking of Normal Alkanes (C4–C6) over Brønsted Acid Site of Zeolitic Type Catalyst with Energetic Evaluation of Transition States Using Quantum-Chemical Modeling

The work aims to determine the kinetic parameters of reactions for production of light olefins via catalytic cracking reactions of C4–C6 n-alkanes based on the energy characteristics of the transition state using quantum chemical calculations. Cracking reactions of C4–C6 n-alkanes proceed via protolytic mechanism on the Brønsted acid sites of zeolite-containing catalysts. For kinetic studies in this work, the thermochemical parameters of the intermediate stages, including hydrocarbon adsorption and transition state were determined, then the activation energies and rate constants were determined over the temperature range of catalytic cracking process from 773–903 K (500–630°C). The results showed that DFT method in combination with B3LYP and ωB97X-D functionals, and 3-21G basis demonstrated quite high accuracy in determining thermochemical parameters, including enthalpy, entropy and Gibbs free energy at both energetic levels of adsorption and transition state. Then, modeling continued by calculations of activation energies and rate constants of reactions. Obtained kinetic parameters made it possible to determine the reactivity of hydrocarbons with different chain length. It was obtained that the rate constants of butane cracking reactions with the formation of ethylene are 54–90 times higher than the formation of propylene. The rate constants of pentane cracking reactions with the formation of butylene are on average 5 times higher than the formation of propylene. The rate constants for hexane cracking reactions with the formation of butylene are 2.9–3.7 times higher compared to the formation of propylene.