Novel Catalyst for Single Step Conversion of Plastic Waste with 85% Olefin Yield

The interest in the chemical recycling of plastic waste via thermochemical processes such as pyrolysis is on the rise since it bears the potential to promote a carbo-circular economy, limiting further sourcing of fossil resources. Within this study, the pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals for the production of polyolefins, such as light olefins and monoaromatics, in a circular fashion at high selectivity. As a catalyst for upgrading the pyrolysis vapors, HZSM-5 with different properties, affected by different Si/Al ratios, incorporation of mesopores (via desilication), stabilization with phosphorus, and steam-treatment, were characterized using ²⁷Al, ²⁹Si, and ³¹P NMR, NH₃-TPD, Pyridine FTIR, XRD, and physisorption. Different feeds, (virgin and post-consumer), catalyst/feed ratios, and reaction temperatures using a micropyrolysis reactor coupled to two-dimensional gas chromatography.

After a steam-treatment at 800 °C, a phosphorus-stabilized microporous HZSM-5 and its desilicated version showed high selectivity for production of valuable base chemicals when upgrading pyrolysis vapors at 600 °C, with ~85% C₂-C₄ olefins and ~5% aromatics produced from virgin PE. Increasing the reaction temperature from 600 to 700 °C, increased the aromatics yield to ~10 wt. % while producing similarly high overall yields of C₂-C₄ olefins, with increased yields of ethylene and 1,3-butadiene at the expense of propylene and C₄ olefins. The coke yield for these severe cracking conditions was 1 wt. %. Processing post-consumer mixed polyolefin resulted in 5-10 wt. % lower olefins yield than using virgin PE but produced ~5 wt. % more aromatics due to its contamination with PS and PET. While parent HZSM-5 rapidly deactivated for repeated runs owing to its high coking propensity, phosphorus-modified and steamtreated HZSM-5 showed almost no deactivation during 130 runs with stable conversion towards C₅₊ aliphatics and high C₂-C₄ olefins selectivity with a 70% reduction in coke deposition.