Catalytic cracking of a polyolefin mixture consisting of polypropylene and both low- and high-density polyethylene has been studied at 400 °C over a variety of acid solids as catalysts. The highest activities were obtained over HMCM-41, n-HZSM-5 zeolite, with nanometer crystal size, and HBeta zeolite. The high surface area and large pores present in HMCM-41 are responsible for the high conversions obtained with this catalyst. Likewise, in the case of n-HZSM-5, the presence of a high external surface area enhances its cracking activity, because the zeolite external acid sites are not sterically hindered for the conversion of the bulky polyolefin molecules. Significant differences are observed in the product distribution: n-HZSM-5 shows the highest selectivity toward C1−C4 gaseous hydrocarbons (50 wt %), HBeta leads mainly to liquid hydrocarbons in the range C5−C12 (60 wt %), whereas HMCM-41 yields both C5−C12 (54 wt %) and C13−C 30 (32 wt %) fractions. A certain loss of activity of these catalysts has been observed after one cycle of regeneration. For HMCM-41, this phenomenon is caused by both dealumination and particle aggregation that take place during the regeneration treatment.
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Serrano et al. (2000) studied this question.