Sodium ethoxide as an environmentally benign and cost-effective catalyst for chemical depolymerization of post-consumer PET waste

Abstract

Polyethylene terephthalate (PET) waste is mounting up in the environment due to its poor biodegradability and low recycling rate. Glycolysis is a promising chemical recycling technique to convert PET into its monomer bis(2-hydroxyethyl)terephthalate (BHET). Here, we present our work on the glycolytic depolymerization of post-consumer PET waste using sodium ethoxide (EtONa) as a low-cost catalyst. In order to optimize the reaction in terms of PET conversion and BHET yield, response surface methodology (RSM) based on the Box–Behnken design was applied for the reaction temperature (160–190 °C), the molar ratio of PET : EtONa (50–150), the molar ratio of ethylene glycol to PET (EG : PET) (3–7), the reaction time (2–6 h) and the PET particle size (0.25–1 mm). Based on the experimental results, regression models as a function of significant process parameters were obtained and evaluated by analysis of variance (ANOVA) to predict the depolymerization performance of EtONa. By further optimization and expanding the parameter space beyond the initial upper limits, high PET conversion (98%) and an isolated yield of BHET (76%) were achieved. Under similar conditions, its depolymerization performance was compared to other widely studied catalysts, such as zinc acetate (PET conversion 97%, BHET yield 75%) and cobalt acetate (PET conversion 93%, BHET yield 70%). BHET precipitation without water is also demonstrated and it was found that EG and catalyst recycling is possible at least for 5 recycled runs with persistent conversion. Hence, EtONa is a very promising low-cost catalyst for PET depolymerization which has potential feasibility for a large-scale process.