Catalytic cracking of n-heptane over Fe modified HZSM-5 nanosheet to produce light olefins

Aiming at the acid modification and stabilization of ordered mesopores in HZSM-5 nanosheet (NS) zeolite for catalytic cracking, two kinds of Fe-modified NS zeolite catalysts were prepared by incipient wetness impregnation (Fe/NS) and an in-situ hydrothermal synthesis method (Fe@NS, N-[3-(trimethoxysilyl)propyl]ethylenediamine complexed Fe as precursors). Fe modification conveyed a lower n-heptane cracking conversion by the Fe/NS catalyst because of the decreased total acid content compared with the parent NS catalyst. However, the decreased Bronsted to Lewis acid ratios by Fe modification restricted the deep hydrogen transfer of olefins, leading to an enhanced ethylene and propylene yields in the product. Furthermore, for Fe@NS catalyst, the highly dispersed FeOx between the nanosheet layers stabilized the mesopore arrangement, and the facilitated diffusion by highly connected channels further alleviated olefin consuming. At reaction temperature of 550 degrees C with a weight hourly space velocity of 8.64 h-1, a 41.8 wt% yield of light olefins (ethylene of 12.2 wt% and propylene of 29.6 wt%) by the Fe@NS catalyst was realized, which was 10.4% and 21.7% higher than that of the Fe/NS and parent NS catalysts, respectively. Moreover, Fe@NS also showed a good catalytic stability with a deactivation rate of only 4.37% after 18 h reaction.

Automated summary

Fe modification of NS zeolite catalysts resulted in lower n-heptane cracking conversion but enhanced ethylene and propylene yields due to decreased acid content and restricted deep hydrogen transfer. The Fe@NS catalyst showed improved stability and higher light olefin yields compared to Fe/NS and parent NS catalysts.