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

Abstract

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.