Polyoxometalate-Based Zinc–Organic Network as a Dual-Site Acid Catalyst for the Conversion of Polyethylene Terephthalate Plastic to Terephthalic Acid

Chemical depolymerization and recycling of poly(ethylene terephthalate) (PET) constitute a sustainable, resource-efficient, and environmentally beneficial approach, which requires the development of efficient heterogeneous catalysts. Herein, polyoxometalate(POM)-based zinc-organic networks were synthesized as dual-site acid catalysts for the alcoholysis of PET into the value-added terephthalic acid (TPA) product, with formulas of [Zn₂(μ₂-Cl)(H₂O)₂(DTAB)₃][PW₁₂O₄₀]·4H₂O (1), [Zn₂(DTAB)₄][SiW₁₂O₄₀]·4H₂O (2), and [Zn₂(H₂O)₄(DTAB)_2.5][HBW₁₂O₄₀]·8H₂O (3) (DTAB = 1,4-di(4H-1,2,4-triazol-4-yl)benzene). Structural analysis showed that compounds 1 and 2 were composed of 2D Zn-ligand networks embedded POM clusters in an "egg-in-a-box" manner and that compound 3 consisted of a 3D POM-based host-guest framework constructed by {Zn₂(H₂O)₄(N-N)₃} units and [HBW₁₂O₄₀]^4- clusters. Three compounds incorporate the Zn²⁺ Lewis acid centers and heteropolytungstate clusters, having the acid strength order compound 3 > compound 1 > compound 2. When employed as dual-site acid catalysts, the three compounds exhibited efficient catalytic performance for PET alcoholysis into TPA with >92% conversion rate and >94% selectivity of the TPA product, along with excellent recyclability and structural stability. This work offers a novel perspective for the development of POM-based heterogeneous catalysts in the upcycling of plastic waste.