(322c) Electrochemically Mediated Alkaline Hydrolysis and Methanolysis of Poly(ethylene terephthalate) (PET)

Decades of uncontrolled plastic production and disposal have resulted in large discharges of plastics into the environment and runaway ecological effects. The current plastic industry is unsustainable and hazardous, as it rapidly consumes finite resources and emits greenhouse gasses. Recycling plastic is an approach to mitigating these environmental effects; however, the most commonly used recycling processes employ mechanical methods of shredding and melting plastic waste, which degrades the polymers’ properties and upholds an unsustainable, linear plastic economy. Chemical recycling offers a pathway to circularizing the plastic industry by utilizing reactive recycling processes to recover high-quality monomer feedstocks for repolymerization. Two such chemical recycling methods, alkaline hydrolysis and methanolysis, have been demonstrated to effectively depolymerize common plastics such as poly(ethylene terephthalate) (PET). However, both methods require environmentally intensive chemical feedstocks, as well as high energy inputs for extreme operating conditions (e.g., temperature and pressure), preventing their full-scale deployment. Our approach aims to reinvent chemical recycling by integrating it with electrochemical processes to reduce both costs and environmental impacts. We investigate electrochemically mediated alkaline hydrolysis and methanolysis to achieve the following advantages over conventional methods: access to more extreme reactivity from applying an electrochemical driving force, process intensification, integration with renewable energy, and application of more moderate operating conditions.

Methods:

Electrochemically mediated recycling converts plastic waste back into its monomer units by generating reactants electrochemically and depolymerizing plastic in situ. In a two-chamber cell separated by a cation-exchange membrane, both alkaline (pH>12) and acidic (pH < 2) environments are achieved via hydrogen evolution reaction (HER) in the cathode chamber and oxygen evolution reaction (OER) in the anode chamber, respectively. Conducting HER in a mixed solvent system of both methanol and water produces methoxide and hydroxide ions for use in depolymerization. The supporting electrolyte for both chambers is 0.1M NaClO4, with a Pt coil as the cathode, an Ir/Ta mixed metal oxide mesh as the anode, and Ag/AgCl as a reference electrode. We targeted poly(ethylene terephthalate) (PET) for this approach because it is commercially available and a significant contributor to single-use plastic waste. Electrolysis was conducted at a constant current (20mA) for 5 hours and evaluated by three metrics: total PET conversion measured by mass difference in solids; the molecular weight distribution of the remaining oligomers measured by gel permeation chromatography (GPC); and the yield of monomer products, terephthalic acid (TPA), monomethyl terephthalate (MMT), and dimethyl terephthalate (DMT) measured by high-performance liquid chromatography (HPLC). We investigated the effects of several experimental conditions on the performance, including water/methanol ratio in the catholyte, catholyte temperature, and anolyte composition. While high PET conversion is one important goal, industrial feasibility of this recycling approach requires the selective production of either TPA or DMT for eventual repolymerization into high-quality plastic products.

Results and implications:

Our investigation of experimental conditions revealed several key insights for designing electrochemically mediated recycling. First, various performance regimes exist as a function of the catholyte water/methanol ratio, indicating that methanol plays a key role in both depolymerizing PET and improving mass transfer of reactants to PET. A minimum threshold of methanol content (≥30 mol% methanol) is needed to disrupt surface tension and allow for effective mixing of PET in the catholyte. In the 30-80 mol% methanol regime, there is a positive correlation between methanol content and total PET conversion. Interestingly, TPA selectivity also increases in this regime despite the decreasing water content, with water being the feedstock for the reactant that produces TPA. For the 100 mol% methanol case, selectivity shifts towards MMT and DMT, while total conversion decreases. TPA production still occurs in this case due to presence of a small amount of water that was transferred from the anode chamber throughout the 5-hour electrolysis period. Second, despite not being the direct reaction environment , the anolyte can also be engineered to improve overall performance. Using a buffer (Na3PO4) as the supporting electrolyte in the anode chamber can improve depolymerization by preventing the transfer of protons to the cathode chamber that would have otherwise neutralized methoxide and hydroxide ions. We observed >2x increase of PET conversion in the catholyte when a buffered anolyte was used. Additionally, when all considered experimental conditions were optimized, we achieved >60% PET conversion. The knowledge gained from studying electrochemically mediated depolymerization of PET will be applied to other common and novel plastic designs to expand the scope of this work and contribute to the movement towards a circular plastic economy.