The study of catalytic removal of acetaldehyde from reprocessed pet: from catalyst design to kinetics

Hawkins, Ben 2023. The study of catalytic removal of acetaldehyde from reprocessed pet: from catalyst design to kinetics. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

PET plastics are prevalent in everyday life due to their properties that make them suitable for a wide range of uses. Strong, durable, and light, they can withstand large forces without breaking, will not noticeably degrade or decompose, and they add very little weight as a packing product. They are malleable so can be shaped for purpose, and transparent for visibility of stored materials. However, as they do not decompose, they can rapidly accumulate within the environment. This results in significant health and pollution risks. Currently there are few suitable alternatives, so research is focussed on reusing, recycling, and reprocessing already made PET products. Avient is one of the leading research bodies on PET plastics and has been working on methods to increase the number of reprocessing cycles possible for PET materials. Reprocessing PET requires exposure to high temperatures and pressures to melt down and remould into products with a renewed life cycle. This can cause the polymer chains to degrade, resulting in lower quality plastics and the production of by-products, each with their own complexities. Degradation increases with each repeated cycle. The focus of this thesis is on acetaldehyde, the main product of PET degradation. Carcinogenic at concentrations > 200 ppm, it can also be detected by human taste and with concentrations as low as 60 ppb. Current methods for acetaldehyde scavenging involve using 500 ppm of anthranilamide to react with acetaldehyde in a 1:1 ratio. The efficiency of this is 70 % and is the benchmark for any new prospective technologies. This study investigates the potential of using a heterogeneous catalyst to act as an acetaldehyde scavenger as it forms during PET reprocessing. While breakdown of acetaldehyde and other VOCs (Volatile Organic Compounds) is already well-documented, this research is done entirely within anaerobic environments. Therefore, this research is leading for VOC removal where total oxidation is not viable. Key properties that determine a catalysts success were identified. First, catalysts must be redox active to transfer oxygen to and from reactants. Second, they must also have either a strong Lewis-acid or Lewis-base environment to activate acetaldehyde. Next, two potential mechanisms were identified for acetaldehyde conversion in anaerobic environments. The first is a bimolecular mechanism, where carbon dioxide gas rapidly evolved in the initial stages. This was followed by either acetone when a metal oxide catalyst was employed, or hydrocarbons in the presence of a zeolite. The second mechanism was unimolecular and identified to be decarbonylation of acetaldehyde to form a 1:1 ratio of methane and carbon monoxide as the only products. Only catalysts containing nanoparticles of palladium(IV) (or similar) supported on an already active metal-oxide catalyst via impregnation methods could perform mechanism two. TGA analysis revealed coking to be the deactivation method of both types of catalysts. Analysis of an exhausted sample showed that all carbon lost through the gas-phase reactor was deposited onto the surface of the catalyst. Initial mechanistic studies and thermal studies were also later carried out and revealed that turnover frequency and lifetime of metal-oxide catalysts increased with higher temperatures. Intermediates therefore either react with subsequent acetaldehyde molecules to form acetone in a carboxylate coupling mechanism or undergoes coking on the surface. Initially iron(III) oxide was also identified as a potential scavenger for acetaldehyde, however further studies revealed that these compounds were also releasing oxygen to form iron(II) oxide, acting more as an oxidising agent than a catalyst. Later studies showed that as a co-metal ion, iron(III) can improve the lifetime of a metal-oxide catalyst by releasing oxygen to prevent coking from occurring on the catalyst surface. A positive correlation was found between efficiency of metal-oxide catalysts when tested in a gas-phase model reactor and when removing acetaldehyde from PET. Further, ceria based mixed-metal catalysts showed no significant effects on either optical properties or the tensile strength properties of the PET plastics up to a loading of 1000 ppm. Finally, this project showcases the flexibility of heterogeneous catalysis. The most active metal-oxide catalysts showed comparable removal of acetaldehyde with the current industrial methods, with an optimised ceria-zirconia-iron(III) oxide catalyst outperforming anthranilamide as an acetaldehyde scavenger within certain PET resins.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	11 November 2024
Last Modified:	02 Dec 2024 16:33
URI:	https://orca.cardiff.ac.uk/id/eprint/173822

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