Insights into the coke deposited on HZSM-5, Hβ and HY zeolites during the cracking of polyethylene

The effect of the zeolite structure (HZSM-5, Hβ and HY) on coke deposition during the cracking of high-density polyethylene has been studied by combining the results of multiple spectroscopic and analytical techniques: FTIR, Raman, UV–vis, 13C NMR and coke extraction, followed by GC-MS and 1H NMR analysis. In addition, by combining FTIR and temperature programmed oxidation (TPO) analysis we obtained information on the coke: properties, burn-off, and changes in composition during catalyst regeneration. Samples of the spent catalysts were obtained in a state-of-the-art pilot plant (conical spouted bed reactor) after the continuous treatment of 900 g (1 g min−1, 15 h) of high-density polyethylene at 500 °C, using 30 g of catalyst. The results show that as the pore diameter of the zeolite is increased, bimolecular reactions (hydrogen transfer and oligomerizations), condensations and cyclizations are enhanced, yielding more aromatic coke. Furthermore, the pore topology of the HZSM-5 zeolite improves the flow of coke precursors (also favored by the high flow rate of N2) to the outside of the catalyst; viz. HZSM-5 catalyst preserves its activity for longer.

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► We studied the principles of coke deposition on zeolites in the pyrolysis of HDPE.
► Two stages of coke development: (1) aromatic and (2) aliphatic growth.
► The number of aromatic rings in the coke follows the trend HY > Hβ > HZSM-5.
► The high flow of N2 for the pyrolysis helps by prolonging the life of the catalyst.