Role of Bifunctional Metal-Promoted Zeolitic Catalyst in Microwave-Assisted Pyrolysis of Polyethylene to Monocyclic Aromatics
Fatemeh Vatankhah, Adrián Carrillo García, Jamal Chaouki
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 Zn 2+ 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 C 1 –C 3, 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.