Co-production of light olefins and aromatics with high PX selectivity from catalytic pyrolysis of polypropylene over shaped ZSM-5 microspheres

Abstract

White pollution has intensified due to plastics’ inherent degradation resistance. Catalytic pyrolysis can convert waste plastics into valuable chemicals like light olefins and aromatics while reducing pollution. However, insufficient selectivity for target products remains a challenge. In this study, the shaped ZSM-5 (Z5) microspheres from industry was used and modified via 3.0 wt% impregnation of P, Ga, and Zn to enhance the yield of light olefins and aromatics in ex-situ catalytic pyrolysis of polypropylene (PP). Incorporating P, Ga, or Zn modulated catalyst acidity, promoting the dehydrogenation /aromatization reactions and significantly increasing liquid, light olefin and aromatic yields. The Zn-Z5 microspheres gave the highest light olefin yield (36.7 wt%), and decreased alkane yield by 24.6 wt%, sharply increasing olefin-alkane ratio from 0.52 to 1.56, compared with Z5. Concurrently, the yields of toluene, ethylbenzene, xylene and C_9 + aromatics in the liquid products were increased, with the yield of aromatics rising from 22.6 wt% (Z5) to 38.4 wt% (Zn-P/Z5). Notably, the p-xylene (PX) selectivity in xylenes significantly improved, with the selectivity of Zn-P/Z5 attaining 75.3 %. These results show that employing P-, Ga-, and Zn-modified Z5 microspheres for the catalytic pyrolysis of plastics facilitates the high-value recycling of waste plastics, achieving co-production of light olefins and aromatics with high PX selectivity.