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

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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 [Zn 2 (μ 2 -Cl)(H 2 O) 2 (DTAB) 3 ][PW 12 O 40 ]·4H 2 O ( 1 ), [Zn 2 (DTAB) 4 ][SiW 12 O 40 ]·4H 2 O ( 2 ), and [Zn 2 (H 2 O) 4 (DTAB) 2.5 ][HBW 12 O 40 ]·8H 2 O ( 3 ) (DTAB = 1,4-di(4 H -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 {Zn 2 (H 2 O) 4 (N-N) 3 } units and [HBW 12 O 40 ] 4– clusters. Three compounds incorporate the Zn 2+ 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.