The Role of External Acidity of Hierarchical ZSM-5 Zeolites in n-Heptane Catalytic Cracking

The hierarchical ZSM-5 zeolite, which contains mesopores in addition to intrinsic micropores, is expected to enhance the bulky molecular diffusion and accessibility of acid sites. These sites, distributed in the internal surface (micropore) or external environment (including external surface and mesopore walls), play different roles in chemical reactions. To explore the effect of internal/external acidity on the reaction, in this study, a series of hierarchical ZSM-5 zeolites with similar pore structures but different external acid properties were synthesized. Tartaric acid was employed to poison the acidic sites in the external environment selectively and extract Al from the framework. The catalytic cracking of n-heptane (the main compound of naphtha) was chosen as a model reaction. The results show that elimination of external active sites tends to reduce the coke formation. The more external acid sites are selectively eliminated, the better catalytic performance can be achieved because of suppressing the bimolecular reactions of ethylene and propylene.