Catalytic conversion of polyolefins to aromatics by ultra-thin b-axis oriented ZSM-5 zeolite

The development of plastics has significantly heightened the demand for these versatile materials across various industries, primarily due to their lightweight composition, durability, and cost-effectiveness. Nonetheless, despite their extensive utilization, only approximately 9% of global plastic waste is successfully recycled. The limited capacity for recycling has resulted in a concerning accumulation of plastic waste in landfills and the environment, leading to serious ecological issues such as soil and water pollution, as well as increased greenhouse gas emissions. As a result, there is an urgent need for effective recycling strategies and technologies to address the escalating global plastic crisis. Chemical recycling technologies offer a promising solution by transforming waste plastics into small molecules that can serve as feedstock for industrial production. Aromatics, which are significant platform molecules and fuel additives, are extensively used in the synthesis of materials, construction, and pharmaceuticals, garnering considerable attention in this area. Currently, the primary method for producing aromatic hydrocarbons is naphtha catalytic reforming, which faces significant supply and demand discrepancies. Therefore, selectively converting waste polyolefin plastics into aromatic hydrocarbons offers a dual advantage: it helps address the supply imbalance of aromatic hydrocarbons while also providing an effective solution to reduce plastic pollution. Zeolite catalysts, known for their unique pore structures, adjustable acidity, and excellent thermal stability, have demonstrated considerable promise in the selective catalytic conversion of polyolefins into high-value products. However, traditional zeolite structures often impose diffusion constraints due to their micropores, which negatively impact catalytic performance. Herein, we synthesized a series of ZSM-5 flaky catalysts with varying b-axis lengths of 30 nm, 90 nm, and 300 nm using seed solution and direct hydrothermal methods. The impact of b-axis thickness on the aromatization of high-density polyethylene (HDPE) was investigated. The b-ZSM-5 catalyst with a b-axis length of 30 nm demonstrated exceptional cracking performance for HDPE, achieving a product yield of approximately 80% at 400℃ within one hour. This significantly outperformed the b-ZSM-5-90 nm and b-ZSM-5-300 nm catalysts, which had similar acidity levels. The catalytic efficiency of the ZSM-5 flake catalyst in HDPE conversion was notably affected by the concentration of external acid centers and diffusion factors, as confirmed by 2,6-DTBPy-IR and intelligent gravimetric analyzer (IGA) technologies. A nearly linear correlation was found between polyolefin conversion and external acid concentration, highlighting the importance of external acid sites in enhancing conversion efficiency. Additionally, thermogravimetric (TG) curves and UV-Vis spectra indicated that the used catalysts exhibited relatively low external coke formation, primarily due to the adsorption of methylated aromatics. As the b-axis thickness of the catalysts decreased, the amount of coke generated also reduced. This suggests that a shorter b-axis length improves the diffusion of aromatic molecules within the zeolite channels, minimizing the accumulation of aromatics on the catalyst’s external surface and subsequently lowering coke formation. Furthermore, the b-ZSM-5-30 nm catalyst was utilized to depolymerize real polyolefin plastics, including HDPE sheets, low-density polyethylene (LDPE) films, and polypropylene (PP) sheets. The b-ZSM-5-30 nm catalyst exhibited excellent catalytic performance, achieving high product yields (~95 wt%) and aromatic yields (~34 wt%) in the depolymerization of all real polyolefin plastics, confirming its versatility for practical applications. Overall, this study highlights the critical role of b-axis thickness in optimizing catalytic performance for polyolefin conversion and reveals how the synergistic interaction between external acidity and diffusion markedly enhances both reaction activity and catalyst stability. These findings provide essential insights for improving the efficiency of waste plastic recycling.