On the transition to gasoline-to-olefins chemistry in the cracking reactions of 1-octene over H-ZSM-5 catalysts

The cracking reactions of 1-octene over H-ZSM-5 zeolite are studied via micro-reactor and off-line spectroscopic techniques for up to 72 h on stream and a temperature range of 473-673 K. 1-Octene is found to react via a twocycle hydrocarbon pool mechanism, with strong similarities to that reported for methanol-to-hydrocarbons chemistry. This dual-cycle mechanism requires temperatures of 673 K or higher to function with full efficiency, with lower temperatures deactivating portions of the cyclic mechanism, leading to premature deactivation of the catalyst through over-production of coke species. Inelastic neutron scattering is used to study the coke composition, identifying two distinct deactivation mechanisms depending on reaction temperature. The catalyst is also found to slowly progress from an aromatic-heavy to an olefin-heavy product regime even at full efficiency due to progressive blockage of active sites by amorphous carbon-rich coke. Artificial aging of the zeolite, through steam treatment, is found to shift the catalyst lifetime so that it commences at a later stage in this process, resulting in increased light olefin production. The reduced aromatic production also means that deactivation of the catalyst occurs more slowly in steamed catalysts than in fresh ones, after an equivalent time-onstream. Collectively, these observations connect with the application of ZSM-5 catalysts to facilitate gasoline-toolefins chemistry in fluidised catalytic cracking unit operations.

Automated summary

The cracking reactions of 1-octene over H-ZSM-5 zeolite were investigated, and it was found that it follows a dual-cycle hydrocarbon pool mechanism similar to methanol-to-hydrocarbons chemistry. Higher temperatures are required for full efficiency, while lower temperatures lead to deactivation and coke deposition. Inelastic neutron scattering was used to study coke composition, revealing two distinct deactivation mechanisms depending on reaction temperature. Steam treatment of the zeolite delayed catalyst activation and resulted in increased light olefin production, with slower deactivation compared to fresh zeolite.