Our work aims to address these challenges through microwave-assisted hydrolysis. Through hydrolysis, PET can be chemically recycled into its monomers, terephthalic acid (TPA) and ethylene glycol, which can then be used to synthesize virgin-quality PET. We propose microwave heating as an energy-efficient and selective alternative to conventional conduction/convection heating as it enables direct and volumetric heating of the reaction medium. Furthermore, microwave-assisted PET depolymerization can become a desirable recycling strategy by introducing active, easily separable, reusable, and inexpensive catalysts.
First, we compared monomer yields from microwave heating with those from conventional heating for uncatalyzed PET hydrolysis. Performing microwave-assisted experiments at different reaction severities allowed comparison with conventionally heated processes reported in the literature. Next, we studied HZSM-5 zeolite catalysts with different Si/Al ratios (23, 50, and 80) for the hydrolysis of PET in a microwave reactor. We demonstrated that HZSM-5 catalysts with higher Si/Al ratios (50 and 80) are active for PET hydrolysis. With these catalysts, TPA yields increased from 14% to 39% at 230 °C for 30 minutes and from 64% to 80% at 245 °C for 30 minutes, compared to the uncatalyzed experiments. Moreover, due to microwave-absorbing properties of zeolites, heating ramp times were reduced by 50% compared to the uncatalyzed runs. Finally, we optimized catalyst acidity, catalyst-to-feedstock ratio, reaction time, and temperature to identify the best conditions for the hydrothermal recycling of PET using zeolite catalysts. These findings support microwave-assisted, catalyst-driven hydrolysis is an energy-efficient alternative to conventional PET chemical recycling.