The researchers synthesized HKUST-1 at room temperature from copper acetate and trimesic acid in an ethanol-water mixture, then opened its clogged pores using two different activation strategies. One sample, labeled HT, was heated at 100 degrees Celsius under vacuum for 24 hours. The other, labeled DCM, was repeatedly soaked in dichloromethane and dried over seven cycles. Because solvent molecules trapped inside the framework normally block the coordinatively unsaturated copper sites that drive catalysis, the choice of activation method matters enormously, and the team wanted a direct, like-for-like comparison that the literature rarely provides.

Characterization revealed that the two routes leave distinct fingerprints. Powder X-ray diffraction showed that heat treatment preserved the crystalline HKUST-1 framework, while solvent exchange reduced crystallinity and shifted peaks, a sign that capillary forces during evaporation had partially collapsed the structure. Nitrogen sorption measurements confirmed the consequence: the HT sample retained a surface area of 360 square meters per gram, whereas the DCM sample fell to 96 square meters per gram. X-ray photoelectron spectroscopy showed both samples contain a near-equal mix of Cu+ and Cu2+ states, a mixed valency that facilitates charge transfer, along with oxygen vacancies that are more prominent in the heat-treated material.

Photocatalytic tests used 10 milligrams per liter of methylene blue and a modest catalyst dose of 0.25 grams per liter under 100 milliwatts per square centimeter of simulated sunlight. The results were strikingly pH-dependent. In acidic solution, HT removed only about 53 percent of the dye in 180 minutes and DCM about 47 percent, because few hydroxide ions are available to form the reactive hydroxyl radicals that do the chemical heavy lifting. At neutral pH the efficiencies climbed to 93 percent for HT and 74 percent for DCM. Under basic conditions, degradation accelerated dramatically, with HT destroying 99.04 percent of the dye in just 60 minutes and DCM reaching 98.72 percent.

To identify the active species, the team added radical scavengers. Isopropyl alcohol, which mops up hydroxyl radicals, slashed degradation to roughly 23 to 27 percent, while EDTA, which traps holes, actually improved performance. Combined with cyclic voltammetry measurements of the band edges, this confirmed that hydroxyl radicals generated by valence-band holes oxidizing water are the dominant degrading agents, since the conduction band sits too positive to reduce oxygen to superoxide. Electrochemical impedance spectroscopy added another piece of evidence: the HT electrode showed lower charge-transfer resistance than DCM, meaning photogenerated electrons and holes separate and migrate more efficiently, suppressing the recombination that cripples many photocatalysts.

The most dramatic result came from destroying the framework altogether. Annealing HKUST-1 at 400 degrees Celsius converted it into a copper metal oxide, labeled CMO, composed of roughly spherical nanoparticles averaging 19 nanometers with rough, porous surfaces. This transformation collapsed the band gap from about 3.7 electronvolts in the parent MOF to just 1.41 electronvolts, allowing the oxide to absorb light across nearly the entire ultraviolet-visible spectrum. The payoff was speed: CMO degraded 98.80 percent of methylene blue within only 30 minutes at pH 12, with a pseudo-first-order rate constant of 143.52 x 10^-3 per minute, far outpacing the parent framework, and it still achieved 68.73 percent degradation in acidic conditions where the MOFs struggled.

Recyclability tests over six cycles showed efficiency losses of roughly 12 to 13 percent for all three catalysts, an acceptable decline attributable to gradual loss of active sites. Notably, post-cycling X-ray diffraction revealed that the HT and DCM frameworks partially hydrolyze under alkaline conditions, forming new copper-trimesate phases, whereas the CMO pattern remained essentially identical to the fresh catalyst. The monoclinic CuO phase resists both hydrolysis and photocorrosion, suggesting the derived oxide is the more durable option for repeated wastewater treatment duty.

Perhaps the most practically relevant part of the study concerns mixed pollutants, which most research ignores. When the cationic methylene blue and the anionic methyl orange were degraded together, the outcome depended sharply on pH. In acidic solution the catalyst surface becomes positively charged, attracting the anionic methyl orange, which was degraded 89.70 percent, while methylene blue removal dropped to 31.67 percent, inhibited by electrostatic repulsion and competitive adsorption. At neutral pH the split was 57.11 percent for methylene blue and 96.85 percent for methyl orange, and at pH 10 both dyes exceeded 97 percent removal within 180 minutes, turning the green mixed solution completely transparent.

Kinetic analysis of the binary system exposed another subtlety: the classical pseudo-first-order model that fits single-dye degradation broke down, with pseudo-second-order kinetics providing better fits in most cases. This deviation reflects the competing adsorption and reaction of two dyes on the same surface, a warning that multicomponent effluents cannot be predicted from single-pollutant studies alone. For engineers designing real treatment systems, the message is that pH can be used as a dial to select which dye is destroyed first, or to eliminate both simultaneously.

Taken together, the study delivers a comprehensive map linking activation strategy, physicochemical properties, and photocatalytic performance for copper-based materials. It shows that a simple, chemical-free activation choice such as vacuum heating can outperform solvent exchange by preserving porosity, and that converting a MOF into its oxide trades surface area for a dramatically narrower band gap, faster kinetics, and superior stability. As industries seek sustainable alternatives to Fenton chemistry, which works only in narrow acidic windows and generates metal sludge, these sunlight-driven copper catalysts, active across pH 2 to 12 and reusable over multiple cycles, offer a compelling blueprint for the rational design of next-generation water purification materials.

Subject of Research: Photocatalytic degradation of organic dyes using activated HKUST-1 metal-organic framework and its derived copper oxide

Article Title: Photocatalytic degradation of single and binary dyes using activated HKUST-1 and its derived copper oxide

Article References: Sonia, Singh, V., Yarramaneni, S., & Singh, V. (2026). Photocatalytic degradation of single and binary dyes using activated HKUST-1 and its derived copper oxide. Discover Chemistry, 3(1), Article 522. https://doi.org/10.1007/s44371-026-00977-y

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00977-y

Keywords: HKUST-1, metal-organic framework, photocatalysis, copper oxide, methylene blue, methyl orange, wastewater treatment, band gap, hydroxyl radicals, dye degradation, pH-dependent selectivity, MOF-derived catalysts