Thesis

Recycling of polyethylene terephthalate (PET) through ionic liquid-catalysed glycolysis : mechanistic insights and sustainable strategies

Downloadable Content

Download PDF
Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T17570
Person Identifier (Local)
  • 202050069
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Since their discovery in the early 20th century and subsequent industrial scale production beginning in the 1950s, plastics have become near-ubiquitous across modern society. Their extensive application in packaging and manufacturing has seen incredible growth, largely attributed to their durability, which ironically has also led to widespread environmental damage. Polyethylene terephthalate (PET), is a thermoplastic used for primarily food packaging and textiles. While its recycling rate is actually greater than alternative plastics, the volume in which it is produced means much more work is needed to be done to improve the end-of-life management. Catalysed PET glycolysis has been identified as a promising process for the chemical recycling of PET waste, to its monomer bis(2-hydroxyethyl) terephthalate (BHET). In recent years, ionic liquids (ILs) have garnered interest as catalysts for PET glycolysis, due to their non-flammability and tuneability. However, while some metal based ILs have shown exceptional activity and reusability, their high toxicity and cost of synthesis are a major hurdle to their scalability. Alternatively, choline-based ILs have been proposed as non-metal, non-toxic and cheaply-produced alternatives to their metal-containing counterparts. Despite the sustainability of these catalysts, issues such as low activity and unclear recyclability still remain. This thesis aims to bridge the gap in performance between choline- and metal-based ILs, while retaining the green credentials and low-cost of the former. Density functional theory (DFT) calculations were employed to screen two non-metal choline-based ILs, [Ch][For] and [Ch][OAc], and two imidazolium-based ILs, [bmim]Cl and [bmim]2[CoCl4]. Subsequently, the ILs identified as being the most promising, [bmim]2[CoCl4] and [Ch][OAc], were further evaluated using experimental techniques. Experimentally-derived BHET yields and DFT-calculated energy barriers revealed the metal IL’s superior catalytic performance, however BHET yields were low compared to literature standards. It was proposed that this was a result of moisture contamination hindering the degradation process, as well as potential redepolymerisation of the monomer. The role of moisture was later explored via DFT and experiment, highlighting its hindering effects when in contact with the hygroscopic ILs within the reaction. Further DFT analyses elucidated the catalytic mechanism of both, indicating that the metal IL’s enhanced activity was influenced by the unique coordination ability of the anion’s cobalt centre, while the organic IL was limited to non-covalent interactions. Results revealed that the mechanistic role of the cation, in both cases, was limited, challenging the conventional reaction mechanism commonly described in literature. The results indicate the insufficiency of “off-the-shelf” catalytic mechanisms, emphasising the need for bespoke analysis of individual catalytic systems. A number of approaches to enhance the effectiveness and sustainability of the process were explored. Microwave (MW) heating was found to rapidly heat cobalt containing reaction samples, while organic IL-containing systems responded poorly. The results indicated that [Ch][OAc] is unsuitable for future studies of MW-assisted PET glycolysis, unless combined with some form of MW-absorbing “cladding” materials. Finally, a waste biomass-derived, KOH-activated biochar (BC) catalyst support was introduced as a potential method of enhancing the IL performance, especially in the organic case. The [Ch][OAc]-catalysed process observed a remarkable 24% increase in BHET yield, which was attributed to the catalyst’s efficient dispersion across the BC surface, as well as a potential synergistic catalytic role played by the support. Raman spectroscopy of separated BC indicated the potential reusability of the [Ch][OAc]-BC catalyst. These findings demonstrate the potential for use of a green, cheaply synthesised BC-supported [Ch][OAc] IL catalyst for PET glycolysis.
Advisor / supervisor
  • Zhang, Xiaolei, 1986-
Resource Type
DOI

Relations

Items

Thumbnail Title Date Uploaded Visibility Actions
file details PDF of thesis T17570 2026-07-30 Public Download