Synthesis, computational analysis and application of the hetero-ligand system Co(II) coordination polymer as catalyst for the degradation of microplastics

Abstract

The cobalt(II) coordination polymer containing 1,2,4,5-benzenetetracarboxylic acid (HBTC) and 1-methylimidazole (MIM), formulated as [Co(MIM) 2 (HBTC) 2 (H 2 O) 2 ]·(H 2 O) (CoCP), was synthesized. Characterization of the compound was achieved by elemental analysis, single-crystal X-ray diffraction analysis (SCXRD), powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The molecular, kinetic, thermodynamic and binding properties of the catalyst were computationally investigated using DFT/B3LYP/6–31(d), Monte Carlo (MC) simulation and pair distribution function (PDF) analysis. SCXRD analysis revealed an octahedral structure having the Co(II) ion coordinated with two water molecules, two imidazole and two benzene tetracarboxylic acid ligands in a CoN 2 O 4 manner. It crystallizes in the monoclinic crystal system with space group C 2/ c . The catalytic property of the compound for the degradation of microplastics was investigated using high-density polyethylene (HDPE) microplastics (MPs) with H 2 O 2 as oxidant. The Fenton-like reactions for the degradation process were observed to achieve degradation of the HDPE MPs into C 19 – C 44 organics with selectivity of 51.60% to oxidized fragments at neutral pH. From the computational analysis, CoCP presented a narrow band gap of 2.14 eV (Fermi level = −3.65 eV) and strong electrophilicity (11.92 eV). The carboxylate oxygen, imidazole nitrogen and cobalt nodes were identified as the binding and catalytic sites. Degradation of HDPE was confirmed to be nonspontaneous (ΔG = 2351 – 3186 au) and endothermic (ΔH = 2351 – 3185 au) as revealed by the computed thermodynamic analysis. An interplay of physical and chemical interactions between the CoCP-HDPE pair was observed from the MC and PDF analysis.