The effect of the zeolite structure (HZSM-5, H beta 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, C-13 NMR and coke extraction, followed by GC-MS and H-1 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 degrees 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 N-2) to the outside of the catalyst: viz. HZSM-5 catalyst preserves its activity for longer. (C) 2011 Elsevier B.V. All rights reserved.