Biodiesel, composed of fatty acid methyl esters, has long been promoted as a cleaner alternative to petroleum diesel because it is non-toxic, biodegradable, and burns with a higher cetane number and lower viscosity than conventional fuel. It is typically produced by reacting free fatty acids or triglycerides with methanol or ethanol, a process that depends critically on the catalyst employed. Industrial production has traditionally relied on homogeneous alkali catalysts, which achieve high conversions under mild conditions but bring serious drawbacks: they are difficult to separate from the product, generate hazardous wastewater during washing, and saponify when the feedstock contains high levels of free fatty acids. Liquid acid catalysts can handle low-grade, acidic feedstocks, but they are corrosive, non-reusable, and equally hard to recover.

Heterogeneous acid catalysts, which remain solid during the reaction and can be filtered out afterwards, promise to solve these problems. Among the most promising acid candidates are heteropolyacids such as phosphotungstic acid, or HPW, a Keggin-type compound prized for its strong Brønsted acidity and stability. Pure HPW, however, dissolves readily in polar solvents like methanol, which makes recovery nearly impossible and prevents continuous operation. Its low specific surface area also means that only a fraction of its acid sites is accessible to reactant molecules. The logical remedy is to immobilize the acid on a porous solid support, and metal-organic frameworks, with their enormous surface areas, tunable pores, and chemical versatility, have emerged as particularly attractive hosts.

The Anshun team, led by Qiuyun Zhang and Yutao Zhang, chose UiO-67, a zirconium-based framework built from zirconium oxide clusters connected by 4,4′-biphenyldicarboxylic acid linkers. Their key insight was that the amount of organic linker used during synthesis could be used as a tuning knob. By varying the linker quantity from 2 to 6 millimoles in a one-pot hydrothermal synthesis, the researchers produced three composite catalysts, designated HPW@UiO-67-1, -2, and -3, each with different porosity, morphology, and acidity. The synthesis itself was straightforward: zirconyl chloride, the linker, benzoic acid as a modulator, and HPW were dissolved in dimethylformamide and heated at 130 degrees Celsius for eight hours in a sealed autoclave.

Extensive characterization confirmed that the composites formed successfully and revealed why the lowest linker concentration produced the best catalyst. Fourier-transform infrared spectroscopy detected the characteristic W-O and P-O vibrations of the Keggin unit alongside the Zr-O bands of the framework, while X-ray diffraction showed that the UiO-67 crystalline structure was preserved, with no separate HPW peaks, indicating that the acid was well dispersed within the matrix. Nitrogen physisorption measurements were especially revealing: the UiO-67-1 support displayed a type IV isotherm typical of mesoporous materials and a remarkable BET surface area of 2219.2 square meters per gram, whereas samples made with more linker showed micropore-dominated type I isotherms. The researchers attribute this difference to competitive coordination between the biphenyldicarboxylic acid linker and benzoic acid modulator for the zirconium clusters, which generates structural defects and opens up mesopores.

After HPW loading, the surface area of the composites dropped sharply, as expected when guest molecules occupy the framework pores. Crucially, however, HPW@UiO-67-1 retained a mesopore surface area of 465.8 square meters per gram, far exceeding the 17.7 and 16.8 square meters per gram measured for the two composites synthesized with higher linker amounts. Scanning electron microscopy showed that the low-linker composite featured irregular particles with numerous small particles decorating larger ones, a morphology that exposes more active sites, while the higher-linker samples suffered from hard agglomeration. Elemental mapping confirmed that carbon, oxygen, zirconium, phosphorus, and tungsten were uniformly distributed throughout the composite, and thermogravimetric analysis demonstrated that the material remains stable well beyond the temperatures required for esterification.

Acidity measurements tied the structure directly to performance. Ammonia temperature-programmed desorption showed that HPW@UiO-67-1 possessed the highest total acid capacity of the series at 0.95 millimoles per gram, compared with 0.58 for the bare framework and 0.45 to 0.65 for the other composites. Pyridine-adsorbed infrared spectroscopy revealed that embedding HPW doubled the Brønsted-to-Lewis acid site ratio, from 0.117 to 0.257 at 150 degrees Celsius, and introduced a distinct Brønsted acid band at 1540 wavenumbers. X-ray photoelectron spectroscopy added further evidence of strong electronic interaction: the zirconium 3d peaks shifted to lower binding energies after HPW incorporation, indicating partial electron transfer from the framework to the acid units, while tungsten 4f peaks confirmed the presence of intact Keggin species.

Catalytic testing bore out the design logic. Without any catalyst, oleic acid esterification reached only 31.8 percent conversion, and the bare UiO-67 supports performed poorly due to insufficient active sites. Pure HPW achieved 98.7 percent conversion in one hour, but only by acting as a homogeneous, unrecoverable catalyst. Among the solid composites, HPW@UiO-67-1 consistently outperformed its siblings, and the team optimized the reaction conditions through single-factor experiments. The best results, 93.9 percent conversion of oleic acid to methyl oleate, were obtained at 140 degrees Celsius over five hours with a methanol-to-acid molar ratio of 20:1 and 0.1 grams of catalyst. Beyond these optima, conversion declined slightly, a consequence of the reverse esterification reaction between the ester product and water, mass-transfer limitations at higher catalyst loadings, and methanol saturation of the active sites. The catalyst also converted lauric, myristic, palmitic, and stearic acids with conversions above 80 percent, demonstrating broad applicability across fatty acid feedstocks.

Perhaps most importantly for real-world use, the catalyst proved durable. Recovered by simple centrifugation after each run and reused without any pretreatment, HPW@UiO-67-1 retained more than 80 percent of its activity over four consecutive cycles. Post-reaction analysis showed that the framework structure survived largely intact, with the modest activity decline attributed to residual organic contaminants on the surface and minor catalyst loss during recovery rather than structural collapse. A hot filtration test, in which the catalyst was removed mid-reaction and the filtrate allowed to continue reacting, confirmed that the catalysis was genuinely heterogeneous: conversion barely increased in the absence of the solid, ruling out significant leaching of soluble acid species. Compared with previously reported solid acid catalysts, the composite achieved comparable or superior conversions under milder conditions of temperature, time, and catalyst loading.

The study illustrates a broader principle in modern catalysis research: that performance is not simply a matter of combining active ingredients, but of engineering the architecture at the nanoscale. By adjusting a single synthesis parameter, the organic linker concentration, the researchers controlled crystallinity, pore structure, particle morphology, and acid density simultaneously, and the resulting mesopore-rich, acid-dense composite delivered the best of both worlds, the strong Brønsted acidity of heteropolyacids and the recyclability of a robust porous framework. As the search for sustainable alternatives to fossil fuels intensifies, such defect-engineered metal-organic framework composites could help make biodiesel production from low-grade, high-acid feedstocks both economically viable and environmentally sound, turning a laboratory curiosity into a practical tool for clean energy.

Subject of Research: UiO-67-supported heteropolyacid catalysts for biodiesel production via oleic acid esterification

Article Title: Construction of UiO-67 supported heteropolyacid with enhanced catalytic activity for biodiesel production via esterification

Article References: Zhang, Q., Wu, H., Chen, L., Chen, L., Fan, L., Deng, T., Ma, J., & Zhang, Y. (2026). Construction of UiO-67 supported heteropolyacid with enhanced catalytic activity for biodiesel production via esterification. Journal of Saudi Chemical Society, 30(2), Article 19. https://doi.org/10.1007/s44442-026-00068-8

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00068-8

Keywords: biodiesel, metal-organic frameworks, UiO-67, heteropolyacid, phosphotungstic acid, esterification, oleic acid, heterogeneous catalysis, mesoporous materials, solid acid catalyst, renewable energy, catalyst reusability

The material itself is a study in green chemistry. The researchers cleaned and dried sweet flag plants, ground the biomass through a 100-mesh sieve, and pyrolyzed it in a limited-oxygen environment inside a muffle furnace, ramping the temperature to 600 degrees Celsius at 5 degrees per minute and holding it there for 90 minutes. To introduce the dopants, they soaked the powdered biomass in solutions of manganese chloride and urea, sonicated the mixture, stirred it for 24 hours, and then repeated the calcination step. By varying the relative amounts of biochar precursor, urea, and manganese salt, the team produced a family of catalysts with different doping ratios, ultimately identifying a 1:1:1 ratio of biochar to nitrogen to manganese as the optimum. The approach recycles waste biomass, sequesters carbon, and avoids the energy-hungry activation methods, such as ultraviolet lamps, heating, or electrochemical cells, that many persulfate systems demand.

Characterization revealed why the co-doped material works so well. Scanning electron microscopy showed that the original biochar has a rough, sheet-like surface, and that manganese and nitrogen modification roughens it further, decorating the layered carbon with stacked spherical Mn-N nanoparticles that multiply the available active sites. Energy-dispersive X-ray spectroscopy confirmed the presence of carbon, oxygen, manganese, and nitrogen, with manganese accounting for a remarkable 35.35 percent of the sample mass. X-ray diffraction identified manganese oxides in multiple oxidation states, including MnO2, MnO, and Mn3O4, while Fourier-transform infrared spectroscopy detected characteristic Mn-O bonds and C-NH-C linkages, confirming that both dopants were chemically incorporated rather than merely deposited on the surface.

Not every physical change was an improvement. Brunauer-Emmett-Teller measurements showed that the specific surface area of the biochar collapsed from 117.534 square meters per gram in the pristine material to just 7.232 square meters per gram after doping, a consequence of nitrogen species and manganese particles partially filling the micropores and mesopores. Nitrogen adsorption-desorption isotherms, showing combined type I and type IV behavior with H4 hysteresis loops, indicated that both micropores smaller than 2 nanometers and mesopores between 2 and 50 nanometers coexist in the material. The trade-off proved worthwhile: although adsorption performance declined, the doping introduced a wealth of catalytically active sites and structural defects, including pyridine nitrogen, graphitic nitrogen, and manganese oxide species, that more than compensated by dramatically accelerating persulfate activation.

X-ray photoelectron spectroscopy provided the most detailed picture of the catalytic machinery. The nitrogen 1s spectra revealed pyridine N, pyrrole N, and graphite N at binding energies of 398.10, 399.29, and 400.20 electronvolts, with pyridine nitrogen dominating at 52.58 percent before reaction. Manganese appeared as Mn(II), Mn(III), and Mn(IV), and the valence distribution shifted as the reaction proceeded: the fractions of Mn(II) and Mn(III) fell from 44.31 and 30.75 percent to 39.75 and 23.72 percent, while Mn(IV) rose to 36.52 percent. This valence climb is the fingerprint of electron transfer, in which the lower-valence manganese ions donate electrons to persulfate, cleaving its O-O bond and generating sulfate radicals as the metal centers oxidize toward higher states.

Performance testing under optimized conditions, 2 millimolar peroxydisulfate, 1.0 gram per liter of catalyst, and 20 milligrams per liter of tetracycline at pH 7, delivered a removal rate of 93.76 percent in 120 minutes, with a pseudo-first-order rate constant of 0.0199 per minute. The comparisons were telling. Peroxydisulfate alone did essentially nothing, plain biochar adsorbed 48.52 percent of the antibiotic, and singly doped catalysts paired with persulfate managed 68.00 percent for manganese-only and 60.65 percent for nitrogen-only materials. The co-doped system outperformed a comparable delta-MnO2/biochar persulfate system reported previously, which achieved 85.5 percent removal, and it did so through simple catalyst addition rather than the energy-intensive three-dimensional electrochemical activation used in some rival designs.

The team systematically probed the operational window. Increasing the peroxydisulfate concentration from 0.5 to 2 millimolar raised removal from 72.82 to 93.76 percent, but further additions plateaued because excess oxidant self-quenches the very sulfate radicals it generates. Raising the catalyst dose from 0.3 to 1.0 grams per liter improved degradation, though pushing to 2.0 grams per liter brought no further gain, likely because superfluous doped species compete with pollutants for radicals. Degradation efficiency exceeded 93 percent at low initial tetracycline concentrations and declined gradually as pollutant loading rose. Encouragingly, the system proved remarkably pH-tolerant, holding removal near 93 percent across the range from pH 3 to 9, with only strongly alkaline conditions at pH 11 dropping performance to 74.89 percent, a flexibility that matters for real wastewater streams of variable composition.

Electron paramagnetic resonance spectroscopy and radical quenching experiments untangled the chemistry driving the degradation. The EPR spectra captured three reactive species: sulfate radicals, hydroxyl radicals, and singlet oxygen, the latter a non-radical oxidant generated at structural defects and carbonyl groups on the nitrogen-doped carbon. Kinetic calculations from quenching tests using methanol, tert-butanol, L-histidine, and p-benzoquinone assigned contribution rates of 73.7 percent for superoxide radicals, 51.5 percent for singlet oxygen, 39.2 percent for hydroxyl radicals, and 13.4 percent for sulfate radicals, revealing that the non-radical and superoxide pathways, long underappreciated in persulfate chemistry, actually dominate. Liquid chromatography-mass spectrometry identified twelve intermediates and allowed the researchers to propose three degradation routes involving deamination, demethylation, dehydroxylation, ring cleavage, and ring opening of the tetracycline skeleton.

Durability and practicality rounded out the assessment. Across three consecutive reuse cycles the catalyst degraded 93.8, 86.0, and 80.3 percent of the antibiotic, retaining more than 80 percent of its initial activity, with the gradual decline attributed to leaching of manganese active centers, a known challenge for metal-carbon composites. In a proof-of-concept flow experiment, catalyst-loaded polyurethane sponges packed into a reaction column fed by a peristaltic pump removed 83.1 percent of tetracycline initially and stabilized around 69 percent after adsorption saturation. Computational toxicity prediction with the TEST software suggested that acute toxicity, measured as LC50 values against fathead minnow and the water flea Daphnia magna, generally decreased as tetracycline was converted into smaller intermediates, although the authors caution that these are model predictions rather than bioassays and that some fragments retain significant toxicity.

The study, published open access on 29 September 2026 and supported by China’s National Key Research and Development Program, arrives amid growing alarm over antibiotic residues in the environment and the resistance crisis they accelerate. Its significance lies less in any single number than in the demonstration that a cheap, plant-derived carbon, doped with two abundant elements and paired with a common oxidant, can rival far more elaborate treatment trains without external energy input. The authors are candid that the flow-through results and cycling stability represent early-stage proof of concept rather than an engineered solution, and that manganese leaching and residual toxic intermediates must be addressed. Even so, the work charts a credible path from laboratory beaker toward continuous water treatment, and it adds sweet flag, a wetland plant once prized in traditional medicine, to the growing roster of biomass feedstocks being reborn as environmental catalysts.

Subject of Research: Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water

Article Title: Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water

Article References: Cui, H., Li, L., Jia, Y., Xu, N., Wang, Z., Liu, Z., Zhang, J., Zhang, C., Gong, W., Feng, W., Shan, Y., & Xue, H. (2026). Mn/N co-doped biochar-activated persulfate degradation of tetracycline in water. Journal of Saudi Chemical Society, 30(5), Article 74. https://doi.org/10.1007/s44442-026-00113-6

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00113-6

Keywords: biochar, persulfate activation, tetracycline, advanced oxidation processes, manganese doping, nitrogen doping, Acorus calamus, antibiotic pollution, water treatment, singlet oxygen, sulfate radicals, catalyst reusability