Synthesis and DFT studies of a new Zn(ii) coordination polymer containing a paddle-wheel hetero-ligand system as a catalyst for the degradation of microplastics

Abstract

We report the synthesis of a new zinc(II) coordination polymer ([Zn(MIM)₂(TPA)₂]) containing a hetero-ligand system of terephthalic acid (TPA) and 1-methylimidazole (MIM), designed for catalytic applications. Characterization of the compound was carried out using spectroscopic techniques, single crystal X-ray diffraction analysis (SCXRD), powder X-ray diffraction (PXRD), elemental analysis and transmission electron microscopy (TEM). SCXRD revealed a four-coordinate ZnN₂O₂ polymeric paddle-wheel-like structure with the Zn(II) centre linked by two (µ-N)₂ coordinated MIM and two (µ-O)₂ coordinated TPA. The [Zn(MIM)₂(TPA)₂] is seen to crystallize in the triclinic crystal system with space group P. We further investigated the degradation of high-density polyethylene (HDPE) microplastics (MPs) using [Zn(MIM)₂(TPA)₂] as a catalyst with H₂O₂ as an oxidant. Notably, the Fenton-like degradation reactions were observed to achieve degradation of the HDPE MPs into mainly oxidized fragments of C₁₅–C₃₇ organics at neutral pH, suggesting a beneficial pathway for environmental applications. DFT studies, specifically molecular electrostatic potential (MESP) analysis, revealed a positive electrostatic potential for the Zn metal center and a negative electrostatic potential for the peroxide oxygen, which influences the stability of the compound. Furthermore, the calculated solvation energy of −11.49 kcal mol⁻¹ confirmed the stability of the Zn–H₂O₂ system in water.