Converting waste PET plastics into automobile fuels and antifreeze components

Polyethylene terephthalate (PET), a widely used polyester plastic, poses a significant environmental challenge due to its recalcitrance to degradation. Millions of tons of PET waste enter the oceans annually, harming marine life. While depolymerization and reuse are common solutions, existing methods often suffer from harsh reaction conditions, low yields, and purification difficulties. Chemical depolymerization methods like hydrolysis, glycolysis, ammonolysis, and pyrolysis aim to preserve the chemical composition of plastics, converting them into stable monomer molecules; however, these methods face limitations in terms of harsh reaction conditions, low product yields, and purification difficulties. Recent research has explored alternative approaches, including using Ru/Nb2O5 catalysts for PET conversion to arenes, but these methods can be costly and result in low selectivity for desired products like p-xylene (PX). This study addresses the need for a cost-effective and efficient method for PET conversion, especially relevant in island contexts with limited waste management infrastructure. The researchers developed a hydrogen-free, one-pot method using a low-cost Cu-based catalyst to directly convert PET waste into valuable products, namely gasoline fuels (PX) and antifreeze components (EG), using methanol as both solvent and hydrogen source. The study investigated the catalyst's structure, reaction mechanism, and scalability, aiming to provide a sustainable solution for plastic waste management in regions where other options may be limited.

Existing methods for PET recycling, such as hydrolysis, glycolysis, ammonolysis, and pyrolysis, are often hindered by issues like harsh reaction conditions, low product yields, and difficulties in purification. Researchers have investigated the use of various catalysts, including Ru/Nb2O5 and Co/TiO2, for converting PET into arenes, but these approaches have limitations. Ruthenium-based catalysts, while effective, are expensive. Other studies have achieved high monomer yields but with low selectivity for p-xylene (PX), a valuable gasoline component. Furthermore, some approaches rely on external hydrogen sources, adding complexity and cost. The use of alcohols as both solvents and hydrogen sources has been explored in recent literature. For instance, the conversion of PET into aromatics using Ru/Nb2O5 in water with the ethylene glycol fragments from the PET structure providing a hydrogen source has been investigated. However, these methods still have their shortcomings, particularly in achieving high selectivity to PX. Other studies have looked at electrocatalytic and photocatalytic methods for PET upcycling, but these are still nascent technologies with potential limitations. The chemical processing of other plastics like polyethylene (PE) has also been extensively studied, with methods involving hydrogenolysis and aromatization showing promise for converting waste PE into higher-value products. However, a method specifically tailored to the challenges of island environments, where waste management options are limited, remained lacking. This research aims to fill this gap.

The researchers developed a hydrogen-free, one-pot method for converting PET waste into p-xylene (PX) and ethylene glycol (EG). The process uses methanol as both a solvent for PET methanolysis and a source of hydrogen for DMT hydrodeoxygenation. The core of the method is a novel Cu-based catalyst, Cu/SiO2 (HT), synthesized through a hydrothermal method. The influence of various factors on catalyst performance was systematically investigated. Different catalysts (Co/SiO2, Ni/SiO2, Fe/SiO2, Cu/TiO2, Cu/CeO2, Cu/ZrO2) were tested, alongside different synthetic methods for Cu/SiO2 (hydrothermal (HT), impregnation (IM), deposition-precipitation with urea (DPU), deposition-precipitation with ammonia (DPA)). The addition of alkali metals (Na, Li, K, Rb, Cs) as chlorides during hydrothermal synthesis was also explored. The optimal catalyst, CuNa/SiO2 (HT), was characterized using various techniques including XRD, N2 adsorption-desorption, XPS, XAES, H2-TPR, FTIR, TGA, and TEM. The reaction mechanism was studied using kinetic and in-situ FTIR spectroscopy. The impact of the Na+/Cu2+ molar ratio on catalyst formation and activity was investigated. Catalytic tests were performed using different alcohols (methanol, ethanol, isopropanol) as solvents and hydrogen donors, as well as a different polymer (PBT). The efficiency of the developed method was assessed using green chemistry metrics (energy economy (ξ coefficient), environmental factor (E), and environmental energy impact (Φ)). Finally, a preliminary on-site test of the method was conducted using PET waste collected from beach sediments in Phuket Island, Thailand, to demonstrate its practical applicability.

The study demonstrated the successful conversion of PET into PX and EG using a CuNa/SiO2 catalyst in a one-pot, hydrogen-free process. The use of methanol as both solvent and hydrogen donor proved highly efficient. The catalyst’s high activity was attributed to a high Cu+/Cu2+ ratio, resulting from the dense and granular copper silicate precursor formed by the addition of NaCl during hydrothermal synthesis. Different catalyst supports (TiO2, CeO2, ZrO2) showed significantly lower activity, while the hydrothermal synthesis method proved superior to impregnation, deposition-precipitation with urea, and deposition-precipitation with ammonia methods. The addition of NaCl during the hydrothermal synthesis played a crucial role in shaping the catalyst's structure and activity. A Na+/Cu2+ molar ratio of 5:1 was found to be optimal, yielding 100% PX. Kinetic studies revealed a four-step reaction pathway for DMT hydrodeoxygenation to PX, with the hydrogenation of methyl 4-methylbenzoate (C) as the rate-determining step. In-situ FTIR spectroscopy confirmed the proposed reaction pathway. The method was also shown to effectively convert PBT. Green chemistry metrics revealed that the developed process is highly efficient compared to previous methods, exhibiting a high ξ coefficient and low E factor and Φ. Finally, a successful on-site demonstration of the process was conducted using various PET waste samples collected from Phuket Island, achieving 100% PX yield, showcasing the method's potential for real-world application.

The findings of this study address the urgent need for sustainable solutions to PET plastic waste. The developed one-pot, hydrogen-free method for PET conversion offers significant advantages over existing techniques. The use of a low-cost, readily available Cu-based catalyst is economically advantageous. The integration of in-situ hydrogen production simplifies the process and reduces its environmental footprint. The high efficiency of the process and its successful demonstration on real-world waste samples from Phuket Island highlight its potential for large-scale implementation, particularly in regions with limited waste management resources. The high Cu+/Cu2+ ratio was identified as a key factor in the catalyst's high activity, offering insights for future catalyst design. The detailed reaction pathway elucidation provides a deeper understanding of the process’s kinetics and mechanism, allowing for further optimization. The work's success also underscores the potential for other similar waste-to-energy technologies, encouraging further research into sustainable chemical recycling processes.

This research successfully developed a highly efficient and cost-effective method for converting waste PET and PBT plastics into valuable fuels and antifreeze components. The key innovation lies in the use of a novel CuNa/SiO2 catalyst and methanol as both solvent and hydrogen source, eliminating the need for external hydrogen. The process's high efficiency, demonstrated through both laboratory experiments and an on-site trial, positions it as a strong candidate for addressing the global challenge of plastic pollution, particularly in resource-limited environments. Future research could explore further catalyst optimization, examining other metal dopants and exploring alternative hydrogen donor molecules, potentially expanding the applicability of this methodology to other types of plastic waste. Scaling up the process for industrial applications is another crucial next step.

While the study demonstrated high efficiency in laboratory and pilot-scale settings, the long-term stability and durability of the CuNa/SiO2 catalyst under continuous operation need to be further investigated. The cyclic reaction tests showed some degree of catalyst deactivation after multiple uses, suggesting that continuous regeneration or replacement strategies may be necessary for industrial applications. The study focused primarily on PET and PBT, and further research should evaluate the method's effectiveness on other types of polyester plastics and complex plastic mixtures. The economic viability of the process on a truly industrial scale requires a more comprehensive cost-benefit analysis. The scalability of the process needs further research to optimize the process design and parameters for large-scale deployment. While the on-site test in Phuket Island was encouraging, more extensive field trials are required to confirm its applicability in diverse environmental conditions.