Sustainable Chemical Recycling of PET: Catalyst Development and Waste Valorization

Abstract

Plastics production is growing significantly each year, but due to inadequate management of post-use plastics, the volume of wasted plastic waste is also increasing. The persistent nature of plastics leads to their accumulation, creating severe environmental effects. Among other plastics, polyethylene terephthalate (PET) is often used for food packaging, making its lifespan short and subject to severe purity regulations. To solve this problem of waste PET, glycolysis, one of the chemical recycling methods, is attracting attention as the fastest and most feasible commercialization method. This dissertation presents a chemical recycling method for waste PET using glycolysis and develops the research with commercialization in mind for real-world application. Among various catalysts, an eco-friendly and cost-effective Fe3O4 catalyst was developed to establish a PET glycolysis system. The study successfully synthesized recycled PET from actual PET products and highlighted the necessity of a metal impurity removal process. Furthermore, industrial catalysts were developed, addressing the requirements for enhanced recovery. Lastly, the research demonstrates waste valorization by recycling waste PET using industrial and biomass waste, presenting these as viable sustainable resources. Chapter 1 provides a comprehensive overview of plastic waste management challenges, focusing on innovative recycling strategies and catalyst development. It explores regulatory measures, PET recycling methods, and introduces a novel "waste tackle waste" approach that utilizes industrial and biomass waste as catalysts for chemical recycling. The chapter sets the stage for the dissertation's research on sustainable solutions to plastic waste management at an industrial scale. Chapter 2 establishes a lab-scale PET glycolysis system and determines the optimal Fe3O4 catalyst. To advance the reaction, the study optimized reaction conditions and developed both qualitative and quantitative reaction systems for efficient performance evaluation. Among five Fe3O4 catalysts synthesized via different methods, Fe3O4-CP (co-precipitation) exhibited the highest performance owing to its large surface area. The optimal reaction conditions were determined as 195 °C, 2 h, with 1 wt% catalyst and a 5.5 EG-to-PET weight ratio. Chapter 3 transitions to a pilot-scale glycolysis reaction using actual waste PET products. Metal impurities were removed using Amberlite IRC-120 resin, and their effect was confirmed by synthesizing recycled PET (r-PET). Removing metal impurities enhanced crystallinity and melting point, aligning the r-PET more closely with commercial PET. This step was designated as essential for practical waste treatment processes. Furthermore, bis(2-hydroxyethyl) terephthalate obtained from glycolysis was used to synthesize a polyol by reacting with CO2, showcasing its potential to produce eco-friendly, high-performance polyurethane foam. Chapter 4 addresses the limitations of powder-type catalysts observed in pilot-scale reactions by developing industrial millimeter-sized bead-type catalysts. The Fe3O4-coated Al2O3 bead catalyst demonstrated performance comparable to powder-type Fe3O4, while offering superior recovery and reusability. This innovation bridges academia and industry, simplifying the process and highlighting its potential for commercialization. Chapter 5 explores chemical recycling processes for colored waste PET using catalysts and activated carbon derived from other environmental wastes, suggesting a pathway where waste tackles waste. Industrial waste, iron rust, was used to synthesize powder- and bead-type iron oxide catalysts, which exhibited high PET glycolysis performance. Biomass waste, kraft lignin, was employed to produce adsorbents that outperformed commercial activated carbon in decolorization. These findings demonstrate the potential of iron rust and kraft lignin for various applications, confirming their feasibility for colored PET chemical recycling and paving the way for broader field utilization. Through these studies, this dissertation proposes sustainable chemical recycling for PET while offering pathways for waste valorization, contributing significantly to eco-friendly practices and industrial applicability.