Choosing the metal at the heart of a metal-organic framework may matter as much as the framework itself. In a study published in Discover Polymers, researchers Lebogang Manamela and Nolwazi Nombona built two composite adsorbent films that were deliberately identical in every respect except one: the metal node of the embedded MOF. One film carried a copper terephthalate framework, Cu-BDC; the other carried its nickel analogue, Ni-BDC. Both were embedded in the same cellulose acetate matrix reinforced with cellulose nanocrystals, fabricated under the same conditions, and tested against the same dye. The result was a unusually clean demonstration of how a single chemical decision cascades through crystal structure, pore architecture and, ultimately, water-cleaning performance.
The motivation came from a persistent blind spot in the adsorption literature. Most studies of cellulose-MOF composites evaluate one material at a time, so differences in MOF chemistry, cellulose support and fabrication method are tangled together. When two adsorbents are compared, it is rarely clear whether performance gaps reflect the metal node, the support or the preparation route. By holding the polymer composition, MOF loading, organic linker and processing conditions constant, the new work isolated metal node identity as the only intentional variable, allowing its influence on crystallinity, pore structure, surface chemistry and adsorption behavior to be assessed directly.
Characterization revealed that the two frameworks were structurally quite different despite sharing the same terephthalate linker. X-ray diffraction showed Ni-BDC as the triclinic phase Ni3(BDC)2(OH)2·4H2O with broad, low-intensity peaks, a signature of defect-rich, poorly ordered frameworks. Cu-BDC, by contrast, matched the monoclinic structure with sharper, more intense reflections indicating substantially higher crystallinity and long-range order. Scanning electron microscopy reinforced the contrast: Ni-BDC formed layered flake-like structures aggregated into flower-like clusters, while Cu-BDC consisted of well-defined spherical particles roughly 500 to 900 nanometers across, built from densely packed nanoscale blocks. After immobilization in the polymer matrix, these differences persisted, with the copper-based film retaining more discernible crystalline reflections than its nickel counterpart.
Crucially, the properties that often confound such comparisons were matched. Fourier transform infrared spectroscopy showed comparable carboxylate coordination environments in both MOFs, and both composite films displayed the characteristic cellulose acetate bands, with a slight shift of the carbonyl band near 1737 cm-1 suggesting hydrogen bonding between the matrix, the nanocrystals and the MOF surfaces. X-ray photoelectron spectroscopy found essentially equivalent surface chemical compositions dominated by carbon and oxygen, with no distinct metal signals, indicating the MOF particles sit embedded within the polymer rather than exposed at the film surface. Thermogravimetric analysis estimated MOF loadings of approximately 12.2 percent for the Cu-BDC/CNC film and 12.8 percent for the Ni-BDC/CNC film. Nitrogen sorption measurements gave nearly identical BET surface areas of 19.99 and 19.98 square meters per gram respectively.
Where the films diverged was in how that surface was organized. The Cu-BDC/CNC film had an average pore diameter of 1.75 nanometers and a pore volume of 0.264 cubic centimeters per gram, versus 1.20 nanometers and 0.22 cubic centimeters per gram for the Ni-BDC/CNC film. Microscopically, the copper-based film showed a highly porous morphology with interconnected pores from roughly 200 nanometers to 1 micrometer distributed uniformly across the surface, while the nickel-based film was flatter with irregularly distributed pores and non-uniform channels. The authors argue this difference in pore architecture, not surface area or chemistry, governs how easily methylene blue molecules can diffuse to binding sites buried inside the film.
Adsorption testing bore this out. Both films performed best at pH 11, where deprotonation of surface hydroxyl and carboxylate groups above the point of zero charge, measured at 7.0 for the copper film and 6.1 for the nickel film, generates negatively charged surfaces that attract the cationic dye. Under those optimum conditions, the Cu-BDC/CNC film reached an equilibrium adsorption capacity of 3.60 ± 0.075 milligrams per gram with 60 ± 1.25 percent methylene blue removal, while the Ni-BDC/CNC film achieved 2.78 ± 0.08 milligrams per gram and 46 ± 1.34 percent. At the lowest initial concentration of 1 milligram per liter, however, removal efficiencies converged at roughly 84 and 85 percent, showing both films work well when adsorption sites outnumber dye molecules. Equilibrium data fit the Langmuir isotherm best, yielding maximum capacities of 4.36 and 3.13 milligrams per gram for the copper and nickel films respectively, with separation factors confirming favorable adsorption.
Kinetic analysis added texture to the mechanism. Both films showed rapid uptake in the first hour, a slower approach between the second and fifth hours, and equilibrium after about five hours, with 80 to 90 percent of the equilibrium capacity reached within the first two to three hours. The pseudo-second-order model fit best, predicting equilibrium capacities of 4.34 and 3.7 milligrams per gram against experimental values of 3.60 and 2.78 milligrams per gram. Weber-Morris, Bangham and Boyd analyses indicated that intraparticle diffusion contributes but is not the sole rate-limiting step, with boundary-layer and film diffusion also involved. Activation energies of 42.3 kilojoules per mole for the copper film and 50 kilojoules per mole for the nickel film point to activated surface interactions rather than purely diffusion-controlled physisorption, with the copper composite facing lower energetic resistance.
Practical durability also favored the copper version, though not overwhelmingly. After five methanol-regenerated adsorption-desorption cycles, the Cu-BDC/CNC film retained about 73 percent of its initial capacity, falling from 3.6 to 2.64 milligrams per gram, while the Ni-BDC/CNC film retained 67 ± 0.54 percent, at 1.85 ± 0.015 milligrams per gram. X-ray diffraction of the cycled films showed their main crystalline features largely preserved, with minor changes in peak position and intensity possibly reflecting local structural rearrangement or residual dye. In competitive tests pairing cationic methylene blue with anionic methyl orange, both films showed pH-responsive selectivity, favoring the anionic dye under acidic conditions and the cationic dye under alkaline conditions, consistent with the surface-charge analysis.
The most instructive surprise came from real textile industry effluent. There the ranking flipped: the Ni-BDC/CNC film produced an approximately 62 percent reduction in integrated UV-Vis spectral absorbance, against roughly 33 percent for the Cu-BDC/CNC film. The authors attribute this to the complex multicomponent matrix, in which diverse dyes, salts and organic species compete for finite sites, and suggest the less ordered nickel framework offers a more heterogeneous adsorption environment able to engage a broader range of contaminants, whereas the copper film’s sites favor the larger cationic methylene blue molecule. The lesson is a caution against extrapolating single-solute laboratory results to real wastewater: metal node selection governs not only capacity and kinetics but selectivity, and practical adsorbent design should weigh structural stability and performance in complex mixtures alongside maximum capacity. The study also acknowledges that its reported capacities sit below those of highly porous powders, aerogels and nanofibrous cellulose-MOF materials, a trade-off accepted in exchange for the mechanical integrity, easy recovery and reusability of a standalone film.
Subject of Research: How metal node identity in cellulose-supported metal-organic framework composite films governs methylene blue adsorption performance
Article Title: Influence of copper and nickel nodes on the adsorption behavior of metal organic framework cellulose nanocrystal and acetate composite films
Article References: Manamela, L., & Nombona, N. (2026). Influence of copper and nickel nodes on the adsorption behavior of metal organic framework cellulose nanocrystal and acetate composite films. Discover Polymers, 3(1), Article 28. https://doi.org/10.1007/s44347-026-00062-8
Image Credits: AI Generated
DOI: 10.1007/s44347-026-00062-8
Keywords: metal-organic frameworks, cellulose acetate, cellulose nanocrystals, methylene blue, adsorption, copper, nickel, textile wastewater, Langmuir isotherm, pore architecture, water treatment, Influence
News Source: Bethany Barker. (October 28, 2026). Copper or nickel: metal choice steers dye capture in cellulose-MOF films. Scienmag.
