Upcycling polymethyl methacrylate to methacrylic acid†

Abstract

Waste polymethyl methacrylate (PMMA) has become a more prominent contributor to global plastic waste in the aftermath of the COVID-19 pandemic. Recycling PMMA relies either on mechanical recycling or thermal depolymerization. Mechanical properties deteriorate after several mechanical recycling cycles. Depolymerization technologies operate in an inert atmosphere and require costly monomer purification downstream. Therefore, neither chemical nor mechanical recycling of PMMA is economically viable. Here, we demonstrate a sustainable recycling method through catalytic hydrolysis to upcycle PMMA while reaching higher product purity. PMMA reacts over zeolites and produces methacrylic acid instead of methyl methacrylate offering technical, economical, and market benefits. Direct hydrolysis of PMMA over an H-type zeolite with an SiO₂/Al₂O₃ ratio of 80 produced methacrylic acid with a yield of 56% and a selectivity of 58%. Coke formed within the framework of large-pore zeolites, causing reversible deactivation of medium–strong acid sites and Brønsted acid sites. The catalytic decarboxylation of methacrylic acid primarily produces acetone and CO, and six-membered glutaric anhydride forms in solid residues.