The Effect of Impurities in Post-Consumer Plastic Waste on the Cracking of Polyolefins with Zeolite-based Catalysts

Impurities present in post-consumer plastic waste (PCPW) are a major limiting factor for the catalytic conversion of polyolefin waste into useful chemicals. While the detrimental effect of common impurities present in crude oil, like sulfur and nitrogen as well as metals and their complexes, have been studied extensively, the same cannot be said yet for impurities in PCPW. In this work, we systematically study the effects of a variety of these impurities during catalytic cracking of polyolefins using ultrastable zeolite Y (US-Y) as catalyst. We have prepared model contaminated polymer materials with controlled concentration of individual impurities and tested their impact on the catalyst activity by repeated thermogravimetric analysis with progressively more contaminated catalyst material. Our findings indicate that the most critical impurities are typical zeolite poisons, e.g., alkali metal and earth-alkali metal ions. Na, K, Ca, Mg and Cu led to noticeable zeolite deactivation at 0.14 wt% loading on pristine polymer. Inert metal oxides, such as SiO2 and TiO2 only cause deactivation at high loadings (~1 wt%) by blocking access to the zeolite surface. By contrast, cracking of S and Cl contaminated polymer did not lead to deactivation. In addition, examination of the effect of real, mixed PCPW revealed that zeolite US-Y still retains considerable activity even after processing 60 times its weight in PCPW. Metal and metal oxide impurities largely remain on the zeolite material, highlighting the potential of catalytic cracking yielding cleaner pyrolysis oil compared with thermal pyrolysis.