Insight into reaction path and mechanism of catalytic cracking of n-hexane in HZSM-5 zeolites

The n-hexane was used as a model compound to study the catalytic cracking behavior of light hydrocarbon in HZSM-5 zeolites, and the law of product selectivity of real acid-catalyzed reaction was investigated by analyzing the product distributions. The results showed that no pyrolysis reaction was found at 300 ℃. Only the acid catalytic reaction took place by the mechanism of carbocation, whose activity was positively correlated to the amount of Brønsted (B) acid sites. The selectivity of ethane, ethylene and propane was negatively correlated, while that of propylene was positively correlated with the Si/Al ratios and catalyst to oil ratios, suggesting that low acid density might be more favorable for monomolecular cracking reactions. It was worth nothing that the total selectivity of C₄ products was much higher than that of C₂ products. Combined with the quantum chemistry calculation results, it could be confirmed that the super-stability of \rmC_2\rmH_5^ + carbenium ion from the monomolecular cracking of n-hexane made it difficult to produce ethylene and ethane through hydrogen transfer reaction. It’s easier to form a C₈ carbenium ion (\rmC_8\rmH_19^ +) with another n-hexane molecule, and then to generate more C₄ products. These results revealed the nature of the low selectivity of ethylene in light hydrocarbon catalytic cracking products. It could be concluded that the product selectivity of catalytic cracking of light hydrocarbons could be modulated by controlling reaction paths depending on the catalyst acid properties and the catalyst to oil ratios. This work will provide important theoretical support for the catalyst design and process development of naphtha catalytic cracking.