Mechanoenzymatic Depolymerization of Highly Crystalline Polyethylene Naphthalate under Moist-Solid Conditions

Plastics have dramatically improved our quality of life. Their lightweight, chemical resistance, and barrier properties are some of the advantages that make them superior to other materials, delivering health and energy-saving benefits among others. However, plastic recycling technologies are urgently needed to address the overwhelming accumulation of plastics in the environment. Here, we report a strategy that combines enzymatic catalysis and mechanical mixing, otherwise known as mechanoenzymology, with moist-solid reaction conditions to enable the clean hydrolysis of the recalcitrant plastic polyethylene naphthalate (PEN) to its building block 2,6-naphthalenedicarboxylic acid (2,6-NDA). Using a commercial variant of Humicola insolens cutinase (HiC, Novozym 51032) added in batches allowed us to achieve 56% yield of 2,6-NDA from high crystallinity (Xc = 41%) PEN. Remarkably, comparable reactions under standard aqueous conditions afforded 30 times less product. The high yield obtained together with the lack of increase in plastic crystallinity (Xc) over the course of the reaction suggest that under moist-solid reaction conditions, both the amorphous and crystalline regions of the plastic are depolymerized. Preliminary mechanistic and kinetic studies are also presented to pave the way for future optimization.