Electrophilic Substitution in Poly(ethylene terephthalate) Waste for Upcycling to Functional Metal-Organic Frameworks

Abstract

In contrast to polymer-to-polymer recycling of waste polyethylene terephthalate (wPET), chemical upcycling into added-value molecules beyond terephthalic acid is a promising strategy. Among possible transformations, electrophilic substitution offers a direct path, yet has remained unrealized due to the electron-deficient nature of the aromatic ring. Here, we demonstrate that substitution and depolymerisation can be merged in a single step, enabling the direct electrophilic functionalisation of wPET – a transformation previously considered inaccessible. We report a one-step method for electrophilic functionalisation of various wPET sources, such as post-consumer food boxes, bottles, industrial blends, and textile residues – into 2-nitroterephthalic acid (TA-NO₂) and 2-iodoterephthalic acid (TA-I). Mechanistic studies reveal that PET hydrolysis precedes nitration under the investigated conditions, alongside partial electrophilic functionalisation of the polymer surface. Upcycling optimisation was guided by the environmental energy impact factor, providing a quantitative metric to balance yield, resource efficiency, and sustainability. The developed protocols afford TA-NO₂ and TA-I in >70% yields and >95% purity across various PET sources. The synthesised TA-NO₂ and TA-I were subsequently applied as linkers in the synthesis of MOFs UiO-66-NO₂ and UiO-66-I, which exhibited crystallinity, porosity, and thermal stability comparable to MOFs obtained from commercial linkers. Overall, this work establishes electrophilic substitution-assisted upcycling for wPET valorisation that broadens the chemical space of PET-derived building blocks and delivers clear added value through enhanced functionality and reduced cost.