Role of Bifunctional Metal-Promoted Zeolitic Catalyst in Microwave-Assisted Pyrolysis of Polyethylene to Monocyclic Aromatics

Abstract

In the context of promoting sustainability and conserving resources, the exponential growth of discarded plastic materials worldwide presents a unique opportunity for chemical recycling. However, efficiently catalyzing plastic pyrolysis faces challenges due to its energy-intensive, endothermic nature, constraints in catalyst spatial distribution, size, and molecule diffusivity. To address these limitations, we proposed a rational design of a microwave-assisted catalytic process, integrating pyrolysis and catalysis in a single unit to enhance mass and heat transfer. This design involves promoting zeolite HZSM-5 with zinc and nickel species, which were subsequently coated onto silicon carbide foam as a microwave absorber. The metallic nature of zinc and nickel was investigated within the HZSM-5 zeolite, analyzing their distributions, synergistic effects, and roles in polyethylene decomposition and aromatization. Focusing on low-density polyethylene decomposition as a representative of major waste plastics, we aimed to improve pyrolytic liquid yield and monocyclic aromatics under moderate operating conditions of 360 °C. The Zn-promoted HZSM-5 catalyst exhibited superior aromatization activity while yielding a total liquid product of 43.6 wt %, predominantly comprising 93.5% aromatics with 63.7% benzene, toluene, ethylbenzene, and xylene (BTEX) selectivity. Thorough characterization analyses revealed isolated Zn2+ and [ZnOH]+ species as active sites for C–H bond scission and dehydrogenation reactions facilitating aromatization. The process demonstrated sensitivity to the zinc loading, as forming macrocrystalline ZnO clusters in a catalyst loaded with 6 wt % zinc led to a decrease in aromatics content to 81.7% in the produced liquid. Introducing Zn species promoted the aromatization mechanism, including a hydrocarbon pool of C1–C3, followed by oligomerizations and cyclization over zeolitic Brønsted acid sites. In conclusion, the integration of electrification into this process offers a promising avenue toward sustainable exploitation of plastic waste for valuable aromatics production.