Robust Two-Dimensional Porphyrinic Copper Metal-Organic Framework for Rapid and Efficient Cr(VI) Removal from Water

Hexavalent chromium (Cr-(VI)) is a highly toxic and mobile water contaminant, and its efficient removal under environmentally relevant conditions remains a significant challenge. Herein, we report the synthesis of a two-dimensional porphyrinic copper metal-organic framework (2D-CuTpyP_MOF) containing mixed-valence Cu-(I)/Cu-(II) sites and evaluate its performance for rapid Cr-(VI) adsorption from aqueous solution. The framework was synthesized through the coordination-driven assembly of 5,10,15,20-tetra-(4-pyridyl)-porphyrin (TpyP) with Cu-(II) acetate and characterized using FT-IR, powder X-ray diffraction, SEM-EDS, X-ray photoelectron spectroscopy (XPS), and N₂ sorption analysis. Comparison of the experimental and simulated p-XRD patterns confirmed the successful formation of the targeted crystalline framework, while XPS revealed the coexistence of Cu-(I) and Cu-(II) species. Despite its relatively low BET surface area (12 m² g^-1), 2D-CuTpyP_MOF exhibited a maximum Cr-(VI) adsorption capacity of 96.51 mg g^-1 and achieved 95.5% removal within 10 min at pH 7.0. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic model and was found to be spontaneous and endothermic. Structural characterization after adsorption demonstrated excellent framework stability, whereas the combination of adsorption performance and physicochemical characterization suggests that Cr-(VI) uptake is governed predominantly by chemically active Cu-porphyrinic sites rather than by extensive internal porosity. These findings demonstrate that layered porphyrinic MOFs with mixed-valence metal sites are promising adsorbents for rapid oxyanion removal and provide new insights into the design of low-porosity, chemically active frameworks for water purification.