Structure-activity correlation in 2D Co(II) coordination polymers for efficient photocatalytic tetracycline degradation and water remediation

Two new cobalt(II) coordination polymers (CPs), [Co(Hbtc)(bmib)]ₙ ( 1 ) and [Co(sc)(bmib)]ₙ ( 2 ) [bmib = 4,4′-bis(2-methylimidazol-1-yl)biphenyl, Hbtc = benzene-1,3,5-tricarboxylate, and sc = suberate], were synthesized under solvothermal conditions and comprehensively characterized by single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric analysis (TGA), and UV–Vis diffuse reflectance spectroscopy. Single-crystal X-ray diffraction analysis revealed that both CPs possess two-dimensional coordination frameworks based on distorted octahedral Co(II) centers. In CP ( 1 ), μ₃-Hbtc²⁻ and μ₂-bmib ligands cooperatively construct a corrugated layered network, while in CP ( 2 ), μ₂-suberate and μ₂-bmib linkers generate an extended two-dimensional architecture. Optical studies showed that CPs ( 1 ) and ( 2 ) own narrow band-gap energies of 2.28 and 2.44 eV, respectively, showing their ability to absorb visible light and generate photoinduced charge carriers. The photocatalytic activities of CPs ( 1 ) and ( 2 ) were evaluated toward the degradation of several antibiotics under visible-light irradiation. Among them, CP ( 1 ) exhibited superior performance for tetracycline (TCY) degradation, achieving a maximum degradation efficiency of 83.52% under the optimized conditions of 3 mg catalyst loading, 40 ppm TCY concentration, and pH 8, while in CP ( 2 ) showed significantly lower activity, affording a degradation efficiency of only 24.47% within 10 min. The degradation process followed pseudo-first-order kinetics, and radical trapping experiments found superoxide radicals (•O₂⁻) as the dominant reactive species. Furthermore, CP ( 1 ) displayed excellent recyclability, structural stability, and resistance to coexisting ions while keeping high activity in real water samples. The enhanced photocatalytic performance of CP ( 1 ) is attributed to its lower band-gap energy and favorable framework architecture, which help visible-light absorption and efficient charge separation. These findings highlight a clear structure–activity relationship between framework structure and photocatalytic behavior and prove the potential of CP ( 1 ) as an efficient and robust photocatalyst for antibiotic-contaminated wastewater remediation.