Electro-Oxidation of Diols in Polymer Recycling: Facilitating Separation and Refining Products
Authors: Haß Jan , Gausmann Marcel , Jupke Andreas 
Published: February 2024

Increasing environmental awareness has highlighted the importance of plastic recycling and rightfully so, considering plastic pollution and an ever-increasing consumption of plastic products. Currently, numerous recycling strategies are being investigated with varying approaches depending on the type of polymer [1]. One attractive process is the chemical recycling of polyesters via alkaline hydrolysis which produces the dicarboxylic acid salts and diols of the respective polyester. The dicarboxylic acids can be separated by crystallization, while the diol separation remains a challenge. A process concept is proposed that utilizes two electrochemical separation technologies to process the polyester hydrolysis solution. A selective electro-oxidation of diols is proposed to mediate and simplify the challenging diol separation. The dicarboxylic acids can be crystalized using an electrochemical pH shift crystallization, which can simultaneously provide the base required for the alkaline hydrolysis of polyesters.

The electrochemical pH shift crystallization utilizes water electrolysis in a membrane-separated parallel plate electrolyzer. The pH of the anode and cathode chambers is shifted towards acidic and basic pH values, respectively [2]. The acidification of the anolyte shifts the dissociation equilibrium of the respective dicarboxylic acid salt to the less water-soluble, fully protonated dicarboxylic acid. Crystallization is carried out within the electrolyzer, while the alkaline catholyte solution may be applied in the polyester hydrolysis.

To circumvent the challenging removal of diols from aqueous solution [3], a selective electro-oxidation of said diols is proposed. The oxidation towards hydroxycarboxylic acids or dicarboxylic acids facilitates simplified separation processes such as pH shift extraction or crystallization, respectively. Rather than a closed-loop recycling towards both monomers, the proposed process suggests an upgrading of the diols to value-added molecules and crystallization of the dicarboxylic acid, which facilitates and simplifies separation.

The study investigates the selective electro-oxidation of diols from three different polyesters: polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene succinate (PBS). A three-electrode setup utilizing plate electrodes as working electrodes is used in a custom cyclic voltammetry cell. Electro-oxidation is carried out in 1M KOH as electrolyte with diol concentrations ranging from 0.2 to 1M. Following the electrocatalyst activity studies, the selectivity of the electro-oxidation utilizing the most promising catalyst is studied in membrane-separated parallel plate electrolyzer experiments. A custom Luggin-Haber capillary setup monitors and controls the oxidation potential during the experiment. This presentation features the preliminary results of plate electrode catalysts’ electro-oxidation of three different diols in alkaline media. Selectivity studies for the oxidation of the three diols (ethylene glycol, 1,3-propane diol, and 1,4-butane diol) in the membrane-separated parallel plate electrolyzer are presented.

[1] Hamad, K., Kaseem, M., & Deri, F. (2013). Recycling of waste from polymer materials: An overview of the recent works. Polymer Degradation and Stability, 98(12), 2801–2812. DOI: 10.1016/j.polymdegradstab.2013.09.025

[2] Kocks, C., Wall, D., & Jupke, A. (2022). Evaluation of a Prototype for Electrochemical pH-Shift Crystallization of Succinic Acid. Materials (Basel, Switzerland), 15(23), 8412. DOI: 10.3390/ma15238412

[3] Garcia-Chavez, L. Y., Hermans, A. J., Schuur, B., & Haan, A. B. de (2012). COSMO-RS assisted solvent screening for liquid–liquid extraction of mono ethylene glycol from aqueous streams. Separation and Purification Technology, 97, 2–10. DOI: 10.1016/j.seppur.2011.11.041