Perovskite Nanoparticles Deliver Near-Perfect Triazole Yields in Water

Chemists have turned a well-known electroceramic perovskite into a workhorse for click chemistry. In a study published in Discover Catalysis, Zahra Ghaffari, Mojtaba Amini and Farzad Seidi report that calcium copper titanate (CaCu3Ti4O12, or CCTO) nanoparticles, prepared by a straightforward sol-gel route, drive the azide–alkyne cycloaddition to 1,2,3-triazoles with a 99% yield in water at 70 °C within four hours. The catalyst works heterogeneously, can be recovered and reused for six consecutive reaction cycles, and showed no detectable loss of activity through leached copper in a hot filtration test.

The reaction at the heart of the study is the copper-catalyzed version of Huisgen’s 1,3-dipolar cycloaddition, the flagship transformation of click chemistry that pairs an organic azide with a terminal alkyne to give a single 1,4-disubstituted 1,2,3-triazole isomer. Copper(I) systems, famously developed by Sharpless and Meldal, deliver this regioselectivity, but free Cu(I) species are chemically fragile: they oxidize readily to Cu(II) or disproportionate into Cu(0) and Cu(II), which complicates their practical use. Anchoring the catalytic copper inside a robust perovskite lattice is one way to sidestep that instability, and it is precisely the strategy the authors pursued.

To make the catalyst, the team dissolved calcium nitrate tetrahydrate and copper nitrate trihydrate in ethanol to form solution A, while titanium butoxide in ethanol formed solution B. Citric acid was added to promote uniform gel formation, and after combining the two solutions with ethylene glycol, sky-blue gels appeared. These were dried at 120 °C for 12 hours and then calcined at 800 °C for 4 hours, yielding the CCTO powder. The sol-gel route was chosen, the authors note, because it delivers high purity within a short timeframe compared with other synthesis processes tested for this material.

Characterization confirmed that the synthesis had done what it was designed to do. X-ray diffraction patterns matched the standard CCTO reference pattern (JCPDS file No. 75–2188), with sharp, narrow peaks indicating good crystallization and no extra peaks to suggest impurities. Energy-dispersive X-ray spectroscopy recorded 5.10% copper in the sample, and atomic absorption spectrometry measured a copper concentration of 1.142 mg per liter; in a 0.004 g portion of catalyst, the AAS analysis found approximately 4.568 µg of copper. SEM mapping showed the elements calcium, copper, titanium and oxygen distributed uniformly across the catalyst matrix, with the weaker copper signal consistent with its relatively low fraction in the CaCu3Ti4O12 stoichiometry.

Electron microscopy put numbers on the particle scale. Transmission electron microscopy revealed evenly distributed particles with an estimated average size of 16.94 nm and a remarkably narrow distribution, with a standard deviation of only 0.20 nm. Scanning electron microscopy showed irregular polycrystals ranging from about 200 nm to several micrometers. Nitrogen adsorption–desorption measurements classified the material as mesoporous: the Type IV isotherm, with an H3- or H4-type hysteresis loop, pointed to slit-shaped or complex pore structures, and the greatest pore volume fell in the 2 to 6 nm pore-width range, giving the material the high surface area that benefits catalytic activity.

With the material in hand, the authors optimized the three-component reaction of phenylacetylene, benzyl chloride and sodium azide, in which the organic azide forms in situ. Without catalyst, no product formed. Increasing the catalyst amount from 0.001 g to 0.004 g raised the yield from 64% to 99%. Screening water, ethanol, methanol, propanol, toluene and n-hexane identified water as the best solvent, and raising the temperature from room temperature to 70 °C lifted the yield from 82% to 99%, with no further benefit beyond that point. Four hours proved sufficient for completion. The final conditions—0.004 g of CCTO, water, 70 °C, 4 hours—delivered the 1,2,3-triazole in 99% yield.

The substrate scope revealed predictable structure–reactivity trends. Replacing benzyl chloride with benzyl bromide improved yields, exemplified by 72% for 5-chloro-2-hydroxybenzaldehyde versus 97% for its bromo analogue, which the authors attribute to the stronger carbon–chlorine bond being harder to break during the cycloaddition. Ortho substitution on the benzyl halide reduced efficiency through partial steric hindrance, as seen in the 72% yield for 2-nitrobenzyl chloride. Substrates such as propargyl alcohol also gave lower yields than phenylacetylene, ascribed to interactions of the alcohol with water.

Two tests addressed the question that decides whether a catalyst is genuinely heterogeneous. In the hot filtration experiment, the solid catalyst was removed by filtration at 70 °C after 50% conversion, and the filtrate was allowed to continue reacting under identical conditions: no additional product formed, indicating that catalysis was not mediated by leached copper species. The reaction efficiency in the filtrate was calculated at 10%, supporting the heterogeneous character of the catalyst and the feasibility of recovery. In practice, the catalyst was separated by centrifugation, washed and dried, and reused for up to six cycles before its performance declined. FT-IR spectra of the fresh and six-times-reused catalyst were essentially identical, with the perovskite lattice bands near 580–600 cm−1 (Ti–O and Cu–O vibrations) and at 495–436 cm−1 (Ti–O–Ti) intact, suggesting no major phase transformation or degradation during the reaction cycles. The authors suggest the slight yield decline over cycles may reflect surface fouling or minor morphological changes rather than significant copper leaching.

Against other reported copper-based catalytic systems, the authors highlight three practical advantages: shorter reaction times, a smaller required catalyst amount, and no need for added ligands or additives. Unlike catalysts based on metal–organic frameworks, polymers or ionic liquids, the CCTO system operates in pure water without organic solvents or magnetic supports, and it uses less copper than most alternatives. The perovskite structure, the authors argue, provides thermal and mechanical stability while immobilizing the copper, addressing the leaching and deactivation that plague conventional Cu-based catalysts.

The proposed mechanism follows the Fokin–Finn pathway: copper coordinates to the acetylenic carbon, lowering electron density at the sp-hybridized carbon and promoting deprotonation; the resulting copper acetylide coordinates the benzyl azide, a pentagonal ring forms, copper exits the cycle, and the proton returns to yield the 1,2,3-triazole. The study remains a laboratory-scale optimization on model substrates, and the reported yields and recyclability describe exactly those tested conditions. Even so, the combination of a simple sol-gel preparation, aqueous operation, near-quantitative yield and demonstrated six-cycle reuse makes CCTO nanoparticles a noteworthy addition to the growing toolbox of recyclable copper catalysts for sustainable triazole synthesis.

Subject of Research: Synthesis and catalytic testing of calcium copper titanate nanoparticles for azide–alkyne cycloaddition reactions

Article Title: Synthesis and characterization of CaCu3Ti4O12 nanoparticles for highly efficient azide–alkyne cycloaddition reactions

Article References: Ghaffari, Z., Amini, M., & Seidi, F. (2026). Synthesis and characterization of CaCu3Ti4O12 nanoparticles for highly efficient azide–alkyne cycloaddition reactions. Discover Catalysis, 3(1), Article 4. https://doi.org/10.1007/s44344-025-00028-1

Image Credits: AI Generated

DOI: 10.1007/s44344-025-00028-1

Keywords: calcium copper titanate, CCTO nanoparticles, click chemistry, azide–alkyne cycloaddition, 1,2,3-triazoles, heterogeneous catalysis, perovskite, green chemistry, sol-gel synthesis, water as solvent, catalyst recycling, copper catalysis