Mechanochemical Recycling of Polyethylene Terephthalate with Monohydrate Barium-Hydroxide

Mechanochemical depolymerization has emerged as a green and energy-efficient approach for plastic recycling due to its solvent-free operation and mild reaction conditions. Although NaOH exhibits high effectiveness, its corrosiveness and environmental risks necessitate more sustainable alternatives. Herein, barium hydroxide monohydrate (Ba(OH)2·H2O) was identified to be highly efficient for solvent-free mechanochemical depolymerization of polyethylene terephthalate (PET) via ball milling, achieving 88.2% yield of terephthalic acid (TPA) without external heating. The optimized conditions were 4 h of ball milling, 550 rpm of ball milling speed at room temperature and atmospheric pressure. The presence of crystalline water in Ba(OH)2·H2O facilitated the release of active hydroxide ions under mechanical force, resulting in superior reactivity compared to its anhydrous counterpart. Mechanistic studies revealed that mechanical force simultaneously disrupts PET crystallinity and activates the reagent, facilitating ester bond cleavage. The broad applicability of this method was demonstrated with other polyester-type substrates, including PBT, PC, and PLA, affording the corresponding monomers in high yields, with 73.1% TPA from PBT, 81.5% lactic acid from PLA, and 87.9% bisphenol A from PC. It also exhibited high selectivity in degrading PET from blends with PE, PP, or PS, leaving these polymers intact for separate recycling. This work offered both a practical reaction system and mechanistic insights for the sustainable valorization of complex plastic wastes.