Catalytic Pyrolysis of Waste Polyethylene into Aromatics by H3PO4-Activated Carbon

H3PO4-activated carbon was used as the catalyst in the pyrolysis of waste polyethylene (PE) to study its catalytic effect on the enrichment of the aromatics. The effect of the mass fraction ratio of phosphorus to wood chips (defined as the P/WC ratio) during impregnation and the residence time of the catalytic reaction on the product yield and the oil components were investigated, and the catalytic mechanism was discussed. The increase of the P/WC ratio from 10% to 50% and the residence time from 1 to 5 s would increase the oil yield from 23.2% to 41.8% and 40.8%, respectively. Alkanes, alkenes, and aromatics were the main components in the oils. The content of aromatics increased along with the residence time and could be up to 30.0% when the P/WC ratio was 40%. Monocyclic aromatics, mainly alkyl and alkenyl benzenes with carbon atom numbers between 7 and 10, were the main aromatic compositions, the highest content of which was up to 23.8% when the P/WC ratio was 40% and the residence time was 3 s. A high P/WC ratio would accelerate the conversion of isomerized products to aromatized ones. The hydrogen transfer reactions catalyzed by Bronsted acid sites such as P-OH in C-O-PO3, C-2-PO2, and C-PO3 and the direct dehydrogenation catalyzed by dehydrogenation active sites like P=0 were the main aromatization processes.