Hierarchical ZSM-5@SiO2 Catalysts: A Novel Approach to Optimizing Olefin Yield from Heavy Atmospheric Gas Oil

In this study, we introduce a hierarchical core–shell ZSM-5@SiO2 zeolite catalyst to explore its effects on the catalytic cracking of heavy atmospheric gas oil into olefins and how the water/oil ratio influences the process. The modified core–shell zeolite and ZSM-5 catalysts were characterized, employing a suite of techniques, including BET, SEM, HR-TEM, XRD, and NH3-TPD, both before and after a steaming treatment. The parent ZSM-5 zeolite, encapsulated by a 24 nm thin amorphous-SiO2 layer, undergoes significant physical changes upon steaming, which induces the formation of additional mesopores and decreases the catalyst acidity. In the absence of steam cofeeding, the catalytic cracking favored bimolecular reactions, yielding limited olefin selectivity. However, introducing steam into the process significantly enhanced the performance, increasing olefin selectivity dramatically from 23.8 to 69%. Furthermore, the propylene/ethylene ratio was favorably stirred from 0.7 to 1.4 with an increasing water/oil ratio, highlighting the critical role of steam in shifting the reaction toward monomolecular pathways and promoting the production of lighter olefins. This study emphasizes the synergistic effects of mesoporosity enable large-molecule diffusion, moderate acidity, and steam cofeeding in optimizing the catalytic cracking process for higher olefin yield.