Heterogeneous catalysts for the degradation and upcycling of plastic waste
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Date
2025
Authors
Breen, Rachel
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University College Cork
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Abstract
This thesis investigates the development and optimisation of heterogeneous catalytic processes to convert waste polymers into high-value product streams, providing a sustainable alternative to current global plastic management strategies. An emphasis is placed on catalyst design and characterisation, optimisation of reaction parameters such as catalyst and reagent loadings, reaction time and temperature and product analysis. The aim for Chapter 3 was to design and synthesise various heterogeneous catalysts for the glycolysis of polyethylene terephthalate (PET) to its monomer bis(2-hydroxyethyl)terephthalate (BHET). A porous SiO2 support was implemented and functionalised with different amine ligands which were suitable for iron ion immobilisation. The nature of the organic ligand influenced the steric and electronic effects which influenced the catalytic behaviour. Nanoparticles were synthesised via calcination and chemical reduction from the ion immobilised catalysts which were found to possess superior catalytic activity. The nanoparticle catalyst synthesised by chemical reduction gave the highest mass loss PET and isolated yield BHET. X-ray photoelectron spectroscopy (XPS) analysis revealed the preservation of the organic ligand for the chemically reduced nanoparticle catalyst which was responsible for the enhanced activity relative to other catalysts owing to dual catalytic activity occurring from the iron nanoparticle and the amine ligand.
Some activity from the ligand-only catalysts in Chapter 3 influenced the work presented in Chapter 4 which focuses on the development of metal-free heterogeneous catalysts for the depolymerisation of PET and PLA. Three guanidine catalysts were immobilised on porous SiO2 with a range of nitrogen environments and were evaluated under conventional thermal and microwave assisted heating methods to find the best method for converting these plastics to their monomers with high yields. The use of microwave-assisted heating dramatically shortened reaction times. The guanidine ligand with the most nitrogen environments was the best performing, indicative that the availability of nitrogen environments was crucial for both PET and PLA depolymerisation.
The motivation for Chapter 5 arose from a review of the literature, which highlighted that polyolefins, such as polyethylene, have significantly higher global production rates than polyesters like PET. Additionally, literature article outputs on the chemical recycling of polyolefins significantly lacked when
compared with output of polyesters, likely owing to the challenges associated with chemical inertness and stability of polyolefins, making them resistant to chemical degradation. Chapter 5 describes the method development for the Fenton oxidation of low-density polyethylene (LDPE). The product
mixture was screened using high performance liquid chromatography (HPLC) and the main products identified were short chain mono and di-carboxylic acids. Two catalyst families were developed: metal oxide composite catalysts and carbon-based catalysts, which were used to optimise the Fenton
oxidation process. The superior activity of a Fe2O3-CeO2 composite in Chapter 5 motivated the development of Chapter 6, while the use of carbon-based catalysts formulated the motivation behind Chapter 7.
Chapter 6 describes the synthesis of a Fe-CeO2 solid solution catalyst synthesised in 3 Fe:Ce ratios. Each of the solid solution catalysts outperformed bare Fe2O3 and CeO2 which was attributed to a synergistic and stabilisation effect of the Fe-CeO2 composite. The presence of solid solution formation was confirmed using Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM), with CeO2 lattice saturation and formation of Fe2O3 aggregates evident for the catalyst with the highest Fe loading. The catalyst with a 1:1 molar ratio resulted in the highest mass loss LDPE and carboxylic acid yield highlighting the importance of tuning metal molar ratio in the catalyst. The catalysts were also effective at pH7 and exhibited excellent recyclability owing to the stability of the solid solution structure. An appendix to chapter 6 outlines the results of a collaborative project carried out with the School of Physics in Trinity College Dublin on the synthesis and characterisation of quasi 2D Fe-CeO2 nanoflakes obtained from cryogenic exfoliation.
Chapter 7 examines the application of unmodified biochar as a sustainable, carbon-based catalyst for the Fenton-type oxidation of low-density polyethylene (LDPE) into low-molecular-weight carboxylic acids. The influence of biochar source and pyrolysis temperature on material properties was
characterised using multiple analytical techniques, and catalytic performance was evaluated in relation to these physicochemical properties. As part of collaborative work with the School of Engineering in University College Cork, anaerobic digestion was employed to convert the product mixtures of
carboxylic acids to methane as a potential strategy to mitigate complex downstream processing for short-chain carboxylic acids. An appendix to Chapter 7 outlines the development of a sustainable materials workshop developed for undergraduate students on the characterisation of biochar. The
workshop is centred around the complexity of biochar’s structure and promotes collaborative efforts between undergraduate and postgraduate students. This thesis will be presented in the format of Thesis by Publication.
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Keywords
Circular economy , Heterogeneous catalysis , Sustainable materials , Chemical recycling , Depolymerisation , Upcycling , Microwave chemistry
Citation
Breen, R. 2025. Heterogeneous catalysts for the degradation and upcycling of plastic waste. PhD Thesis, University College Cork.