Sustainable production of aromatics via catalytic pyrolysis of polyolefins towards the carbon cycle for plastics

Catalytic pyrolysis has been emerging as a promising chemical recycling technology to convert plastic waste into olefins and aromatics that can be used as alternatives to petroleum-based chemicals and promote the carbon cycle for plastics. However, the above procedure typically suffers from poor selectivity of the target product. Herein, catalytic pyrolysis of polyolefin plastics was carried out in a two-stage fixed bed reactor for highly selective production of monocyclic aromatic hydrocarbons (MAHs), over Zn-P/HZSM-5 catalysts. After loading 3.0 wt% Zn and 0.5 wt% P on HZSM-5 catalyst, catalytic pyrolysis of low-density polyethylene (LDPE) at 500 C achieved the highest liquid yield of 55.9 wt% and exhibited a remarkably high selectivity up to 87.44 % towards MAHs, especially in which the BTEX (benzene, toluene, ethylbenzene, xylene) content reached 75.16 %. The introduced Zn species decreased Bronsted acid sites while increased Lewis acid sites of HZSM-5, which in turn improved the aromatization activity. Simultaneously, the addition of P decreased the acidity strength, inhibiting the formation of coke and polycyclic aromatics hydrocarbons. Furthermore, catalytic co-pyrolysis of LDPE, high-density polyethylene (HDPE) and polypropylene (PP) were investigated to reveal the synergistic effect in terms of BTEX selectivity. Intriguingly, a high liquid yield of 55.7 wt% can be obtained at a feeding ratio of HDPE:LDPE: PP = 1:1:1, and an enhanced selectivity of MAHs up to 87.66 % was achieved, with the highest BTEX content of 79.89 %. Finally, carbon emission assessments of catalytic plastic pyrolysis indicated that it presented great potential to reduce carbon emission and facilitate carbon cycle.