Production of monocyclic aromatics and light olefins through ex-situ catalytic pyrolysis of low-density polyethylene over Ga/P/ZSM-5 catalyst

Abstract

Catalytic pyrolysis is considered as a potential route to convert petroleum-based plastic waste to high-valued chemicals such as aromatics and olefins, towards a circular economy. In this study, ex-situ catalytic pyrolysis of low-density polyethylene (LDPE) was carried out in a two-stage fixed bed, whereby the primary products containing paraffin and gas from LDPE pyrolysis in the upper stage entered the lower catalyst stage for catalytic pyrolysis to produce monocyclic aromatics and light olefins. The effects of catalyst types (HZSM-5, Ga/ZSM-5 and Ga/P/ZSM-5) and temperature (450–600 °C) in the lower stage on catalytic pyrolysis were mainly investigated. The results indicated that Ga/ZSM-5 and Ga/P/ZSM-5 catalysts were beneficial to the production of olefins and aromatics due to the increase of their dehydrogenation ability compared with HZSM-5. Furthermore, the induction of P into Ga/ZSM-5 was beneficial to producing more monocyclic aromatics and reducing carbon deposition through adjusting the acidity of Ga/ZSM-5. The selectivity of monocyclic aromatics in oil reached as high as 90.7% and that of light olefins in gas was 25.6% during ex-situ catalytic pyrolysis over Ga/P/ZSM-5 at 550 °C. The results from ex-situ catalytic pyrolysis were compared with those of the in-situ catalytic pyrolysis, suggesting that ex-situ catalytic pyrolysis was a better method to process plastic waste in terms of monocyclic aromatics production. Finally, a catalytic pyrolysis mechanism of LDPE was deduced in accordance with product distribution from catalytic pyrolysis.