Analysis of Ethylbenzene and Methylbutene Dehydrogenation Processes in the Context of Kinetic Modeling and Electronic Theory of Heterogeneous Catalysis

Under the homogeneous approximation, kinetic models of the heterogeneous catalytic dehydrogenation of ethylbenzene and methylbutenes over iron oxide catalysts of different phase compositions were constructed and parametrized using experimental data. It was shown that (1) the target (forward) reaction in the reversible ethylbenzene dehydrogenation determines the overall rate and ethylbenzene conversion, while both the forward reaction and the cracking of ethylbenzene to benzene and ethylene are key for selectivity; (2) the target (forward) reactions in the reversible methylbutene dehydrogenations and the cracking of 2-methylbut-1-ene are key for the overall rate, methylbutene conversion, and selectivity; (3) the kinetic parameters obtained (pre-exponential factors and activation energies in the temperature-dependent Arrhenius expressions for the rate constants) for the reactions that are key for conversion and selectivity are consistent with values typical of monomolecular reactions; (4) the activity ranking of the iron oxide catalysts of different phase compositions in ethylbenzene dehydrogenation shows a negative correlation with the work function of the metals; (5) the rate-limiting step of the catalytic cycle of the target reaction in ethylbenzene dehydrogenation is the electron transfer from the iron oxide catalyst together with a hydrogen molecule, because an increase in the rate of this step accelerates the overall process, which is consistent with the electronic theory of heterogeneous catalysis; and (6) in the dehydrogenation of methylbutenes, the iron oxide catalyst heat-treated at 1073 K gives higher conversion and selectivity than the catalyst heat-treated at 923 K, because the former contains predominantly more densely packed potassium polyferrite, whereas the latter contains less densely packed potassium monoferrite. The denser packing reduces the work function and, according to the electronic theory, accelerates the desorption of hydrogen molecules from the catalyst surface, thereby increasing the process rate.