Unprecedently high olefin yields were obtained with a phosphorous-modified conventional and mesoporous HZSM-5 zeolite. They were demonstrated to be suitable catalysts for producing valuable base chemicals from different virgin and post-consumer waste polyolefins. After steam treatment at 800 °C, the acidity of the catalysts was reduced to ~0.10-0.15 mmol NH3/g, resulting in a lower activity. However, these catalysts can be considered stable under reaction and regeneration conditions which are not expected to exceed 700 °C in the process of cracking plastics pyrolysis vapors. Varying the catalyst/feed ratio between 10 and 80 and the catalyst temperature between 500 and 700 °C affected the proportion of different monomer products. This provides great flexibility for plant operators to steer the product slate towards the highest market demand and maximize C4 olefins, ethylene, propylene, or aromatics production, all while keeping carbon losses to CH4 and coke low, below 1.5% and 0.5 %, respectively. The yield structure obtained from a synthetic polyolefins mixture of LLDPE, LDPE, HDPE and PP matched the relative contributions of the single polymers, and an extremely high monomer yield of 84 wt. %, i.e., 79 wt. % C2-C4 olefins and 5 wt. % aromatics was obtained. When using post-consumer MPO feed, the monomer yield was slightly lower with 74 wt. % C2-C4 olefins and 9 wt. % aromatics, and this is attributed to the higher aromatics content in the feed from PS and the presence of some PET. While parent HZSM-5 rapidly deactivated and showed a high coking propensity (27 µg of coke /m2 catalyst), HZSM-5 that was modified with 2 wt. % P and steam treated at 800 °C showed almost no deactivation during 130 runs with stable conversion of C5+ aliphatics and high C2-C4 olefins selectivity with a much lower coke deposition of 8 µg/m2. Finally, the studied catalytic process constitutes a sustainable alternative process for plastic waste valorization since it leads to about an order of magnitude (0.3 versus 3.1 kg CO2-eq./kgplastics) lower environmental burden than the current practice of incineration.

Selective conversion of mixed polyolefins to valuable base chemicals

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ACS Fall 2022

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