Green synthesis of HKUST-1 at room temperature for superior dark Fenton-like degradation of indigo carmine: joint DFT mechanistic insights.

Abstract

Developing green and facile synthesis strategies for copper-based metal-organic frameworks (Cu-MOFs) is critical for sustainable wastewater treatment. Herein, we report a KI-assisted, solvent-free, room-temperature synthesis of HKUST-1 (RT-HKUST-1) as an environmentally friendly approach that avoids organic solvents, elevated temperatures, and harsh conditions. The prepared RT-HKUST-1, exhibits a high specific surface area (BET: 1509 m2 g-1) and decreasing particle size confirmed by SEM, XRD, BET, FT-IR, TGA and XPS analysis. Its catalytic performance was evaluated for Indigo Carmine (IC) degradation via a dark Fenton-like degradation process using H2O2 under neutral pH. Complete IC removal was achieved within 8 min using a low catalyst dosage (10 mg L-1) and 100 ppm H2O2. Kinetic and thermodynamic analysis indicate a first-order exothermic reaction. Radical scavenging experiments suggest that hole-like oxidizing species dominate the degradation pathway. RT-HKUST-1 also shows good stability and reusability over multiple cycles. Density functional theory (DFT) calculations explain the role of KI. At pristine Cu2+-Cu2+ paddlewheels, homolytic O-O cleavage of H2O2 is strongly uphill (ΔG = +157 kJ mol-1), so these sites cannot generate ˙OH. At the mixed-valence Cu2+/Cu+ sites formed by KI-assisted synthesis, Cu+ reduces bound H2O2 to Cu2+-OH- and ˙OH in a single concerted step with a free-energy barrier of only 62 kJ mol-1. The same Cu+ centers can also transfer an electron directly to adsorbed IC. The superior performance of RT-HKUST-1 is therefore attributed to its mixed-valence Cu2+/Cu+ sites, which both produce ˙OH and activate the dye via electron transfer. Hence, this study demonstrates that KI-assisted RT-HKUST-1 is an efficient, reusable, and environmentally friendly catalyst for rapid dye degradation under mild, dark Fenton-like conditions.

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