2024 AIChE Annual Meeting

Separations in Polyethylene Terephthalate (PET) Depolymerization: Utilizing Carbon Dioxide for Terephthalic Acid Production

Polyethylene Terephthalate (PET) is one of the most commercially available polyesters; primarily used in bottles for drinking water, shampoo bottles, packaging, and textiles. Despite its versatility, PET is not biodegradable and is often landfilled after being physically recycled a few times. Methods to chemically depolymerize PET have been proposed such as glycolysis, solvolysis, alcoholysis, and hydrolysis.

Alkaline hydrolysis achieves about 100% conversion by reacting PET with NaOH to produce disodium terephthalate (Na2TPA) under optimized conditions. However, to precipitate terephthalic acid (TPA), the original PET precursor, large volumes of high-concentration acids are required.

Here, carbon dioxide (CO2) will be used as a sustainable alternative to strong acids to precipitate TPA from a Na2TPA solution. This approach can potentially help solve two major environmental issues: the accumulation of greenhouse gases in the atmosphere and the sustainable recycling of PET.

CO2 is introduced into an aqueous Na2TPA solution a high-pressure membrane filtration cell. A TPA suspension is formed and filtered at constant pressure. The filtrate is then analyzed using 1H NMR to determine the residual Na2TPA composition and solid TPA is weighed gravimetrically. Depolymerization co-products like ethylene glycol and excess reactants such as sodium hydroxide are possible contaminants, therefore, separations with varying concentrations of ethylene glycol and NaOH have been performed to test TPA yield. The ethylene glycol results produce a Na2TPA removal of 82% at 5 weight-percent ethylene glycol. Also, there is little deviation from the removals obtained from strictly aqueous Na2TPA for tests up to 17 weight-percent ethylene glycol. The sodium hydroxide exhibits 74% removal with only the small presence of 0.001 mols NaOH.

Further, a depolymerization reaction derived from previous literature is performed in a microwave reactor with 0.2 M sodium hydroxide (NaOH) and 10-weight percent PET to produce disodium terephthalate. Latest results include testing the monomer yield of alkali decomposition produced Na2TPA in the precipitation reaction. The precipitation reaction produced a terephthalic acid yield of 72.3%, similar to the 75.1% TPA yield that was achieved in the absence of NaOH, and ethylene glycol with commercial disodium terephthalate. The overall terephthalic acid yield was 61%.

Future work includes optimizing the experimental procedure and repolymerizing TPA into biodegradable polymers like polybutylene adipate terephthalate (PBAT).