Hydrothermally stable acid-modified ZSM-22 zeolite for selective propylene production via steam-assisted catalytic cracking of n-hexane

To enhance the textural properties and the hydrothermal stability of porous catalysts, ZSM-22 zeolite samples were treated with different concentrations of HF aqueous solutions (0.7, 1.0 and 1.5 wt%). ZSM-22 samples that were treated with 0.7 and 1.0 wt% HF aqueous solutions showed a selective removal of aluminum atoms from TON framework. On the other hand, more concentrated HF (1.5 wt %) caused the leaching of silicon atom from the zeolites framework. The HF treated ZSM-22 samples showed smaller crystals size, better hydrothermal stability as reflected by intact textural properties as well as their mesoporosity. The modified samples also showed enhanced acidity with lower Bronsted acidity than the parent ZSM-22. ZSM-22 samples modified with low HF concentrations (0.7 and 1.0 wt%) showed better initial conversions in steam catalytic cracking (SCC) of hexane, which can be attributed to the enhancement in the acidity. However, deactivation rate was not enhanced after the HF treatment. The HF modified ZSM-22 samples showed more BTX selectivity which can be attributed to their larger external surface areas. The HF-treated samples lost more acidity, especially Bronsted acidity, after SCC reaction than the parents ZSM-22 zeolite.