Fe-ZSM-5 outperforms Al-ZSM-5 in paraffin cracking by increasing the olefinicity of C3-C4 products

Abstract

Iron-modified Al-ZSM-5 increases selectivity to propene, a key petrochemical resulting from fluid catalytic cracking (FCC). However, the type and role of active iron species remain unclear, hindering efforts to streamline the design of selective FCC additives. Here, we investigated Al-free Fe-ZSM-5 catalysts containing iron species in the form of framework Fe3+, extra-framework Fe3+, oxidic clusters, and oxide micro aggregates in n-octane cracking (FCC model) to assess their effect on catalytic cracking. DR-UV–Vis spectroscopy, 57Fe Mössbauer Spectroscopy, FTIR studies of pyridine adsorption, and n-octane cracking tests at 500 °C revealed that framework-associated coordinatively unsaturated Fe3+ species, which induce strong Lewis acidity, are responsible for paraffin cracking initiation, whereas bulk iron oxides on the zeolite surface are inactive. In comparison with Al-ZSM-5, Fe-ZSM-5 increases the olefinicity of the valuable C3-C4 fractions (selectivity to propene and butenes) and promotes aromatization reactions due to the lower relative strength of Fe-induced Brønsted acid sites and dehydrogenation properties. As shown by our 57Fe Mössbauer study (performed at −269 °C) of the catalyst in calcined, spent, and regenerated states, Fe-ZSM-5 deactivation is associated with the loss of tetrahedrally coordinated Fe3+ species. Therefore, tuning Fe-ZSM-5 C3-C4 selective FCC additives requires stabilizing framework Brønsted and framework-associated Lewis acid sites while decreasing the concentration of iron oxide species. Ultimately, these findings may enable us to meet the demand for propene derived from FCC cracking, which is expected to grow in the foreseeable future.