Study on the deactivation mechanism of HZSM-5 in the process of catalytic cracking of n-hexane

The coke deactivation over HZSM-5 zeolites with different Si/Al ratios in the n-hexane cracking reaction was studied in 200 h of reaction. In the products, alkanes, C₂⁼, C₃⁼, C₄⁼ and aromatic compounds such as benzene, toluene, and xylene (BTX) were formed. The selectivity variations of C₃⁼ and C₄⁼ were inverse proportional to that of BTX, indicating that C₃⁼ and C₄⁼ intermediates were the precursors of BTX compounds. The internal coke was preferentially formed inside zeolite micropores in the initial reaction period (0–70 h), which significantly diminished the micropore volume, covered the strong acid sites, and strongly inhibited catalytic activity of the catalysts with lower Si/Al ratio. The external coke was principally formed after 70 h of reaction and had a less influence on the catalytic activity. The internal coke (density 1.03 g·cm⁻³ and a H/C ratio of 1.16) could be effectively removed by H₂ treatment at 850 °C; while the external coke (density 1.46 g·cm⁻³ and a H/C ratio of 0.28) with graphite-like structure resisted H₂ treatment. For the zeolites with low Si/Al molar ratio (25 and 50), the rapid coking stage (0–70 h) occurred at high n-hexane conversion followed by a slow deactivation stage at low conversion (70–200 h); for zeolites with high Si/Al molar ratio (85 and 130), a very slow deactivation stage took place in the reaction period (0–70 h), followed by a faster deactivation stage (70–200 h). This work demonstrated that zeolite deactivation behavior was correlated not only with the strong acid sites and microporosity of catalyst, but also affected by the location, amount, structure, and type of coke. The mechanisms for internal and external coke formation and H₂ decoking strategy for the deactivated catalyst regeneration were discussed and proposed.