Upgrading of polyethylene to hydrocarbon fuels over the Fe-modified Pt/Al2O3 catalysts at a mild condition without external H2

Abstract

Upcycling of waste plastic is an emerging topic in environmental chemical and process engineering. In this work, we investigated the pyrolysis of polyethylene (PE) over Fe-modified Pt/Al2O3 catalysts in the absence of external H2 to produce hydrocarbon fuels at a mild condition (330℃). The critical interest is to achieve the mild conversion of PE by using the bimetallic catalyst to generate in-situ H instead of external H2 for hydrogenolysis. Results show that a slight impregnation of Fe on Pt/Al2O3 (Fe/Pt = 0.25) can improve the oil yield and light oil selectivity as well as promote the formation of aromatics and alkenes in oil after PE pyrolysis in a batch reactor. Multiple active sites including Pt, Fe and Lewis acid site of Al2O3 work systematically to drive the tandem catalysis without H2. The Pt plays the major role in PE pyrolysis via hydrogenolysis assisted by in-situ generated H, but shows the mutual inhibition with the acid site of Al2O3 that is contributive to PE cracking via a traditional carbocation chemistry. Fe modification on Pt/Al2O3 plays multiple roles in catalysis: promotes cyclization reaction, benefits the hydrogenolysis activity of Pt by providing active H during aromatic formation, and decreases catalyst acidity to reduce gas production. However, an over-impregnation of Fe (Fe/Pt = 1) partially covers the Pt sites, resulting in the decrease of fuel oil production but an increase in residue yield due to the deactivation of Pt sites. The post-reaction catalyst can be almost completely regenerated after combustion and reduction.