Mechanistic Investigations and Kinetic Modeling of Hydrogen Transfer Reactions in Olefin Cracking Processes over H-ZSM-5

Reaction kinetic investigations with a commercial H-ZSM-5 catalyst were performed to model the hydrogen transfer reactions for the olefin cracking process in a temperature range of 420–490 °C. The model presented in this work is an extension of our previously published olefin interconversion model. (F. Warnecke et al., Ind. Eng. Chem. Res. 2020, 59 [28] 12696–12709, https://pubs.acs.org/doi/10.1021/acs.iecr.0c01747.) The lumped reaction kinetic model in the present work describes the byproduct formation from olefins with a high degree of detail. To analyze the byproduct formation with high resolution, a custom-made method using an online-GC was developed. An additional offline GC-MS was used to identify the complex product composition of the hydrocarbon species. Based on these investigations, the product spectrum was divided into 21 lumped component groups, which consist, in total, of 124 individual components. Olefins, paraffins, cyclo-olefins, aromatics, and naphthenes are considered in the kinetic model. The kinetic model is based on the hydrogen transfer mechanism, which considers bimolecular hydrogen transfer between a carbenium ion and a second hydrocarbon molecule. A custom-made approach for the reaction rates which is using acceleration factors is applied. This approach imitates the bimolecular driving forces successfully and additionally reduces model complexity. To the best of our knowledge, this is the first kinetic model with a meaningful hydrogen balance as it considers cyclo-olefins as hydrogen poor components in addition to the commonly considered group of aromatics. Our experimental and modeling results demonstrate that this extension is of high relevance for the hydrogen transfer, and in particular for the hydrogen balance at low contact times. The concepts allows for an accurate prediction of the components and gives a realistic impression about the olefin as well as for the byproduct formation.