High-Resolution Imaging Validates 3D Network Formation in Novel Catalytic Polyoxometalate Aerogels

September 24, 2025 – Researchers from the Universities of Mainz, Ulm, and Jena (Germany) have achieved the first conversion of a polyoxometalate-based organogel into a catalytically active aerogel. The TRIS-functionalized Anderson polyoxometalate forms a gel within just 10 seconds when combined with ZnCl₂ in acetonitrile. Electron microscopy reveals the aerogel's layered structure of thin fibrous sheets, while elemental mapping confirms a uniform distribution of all components. The new material shows promising catalytic activity for selective alcohol oxidation reactions, opening pathways to functional POM-based porous catalysts for energy and separation technologies.

Supramolecular gels represent a promising class of functional soft materials with unique structure and function. [1,2,3] Metallogels are particularly attractive because classical coordination chemistry between metal cations and suitable organic ligands can be exploited to design and tune gel properties and reactivities. [4,5] These linkages are further stabilized by non-covalent interactions such as hydrogen bonding, electrostatic forces, or hydrophobic interactions. [6,7] This versatility has enabled applications ranging from catalysis and sensing to optoelectronics and magnetism. [8,9,10,11] Metallogels can also be converted into technologically important aerogels [12] through freeze-drying, [13] an approach that removes the solvent from the gel structure while preserving its three-dimensional architecture. Aerogels are remarkable materials featuring high specific surface area, low density, and hierarchical pore structures. [14] These exceptional properties have driven their development in areas such as water treatment. [15] The use of diverse materials and their modification in aerogel construction has broadened applications from photocatalysis to the food industry. [16,17,18]

Molecular metal oxide anions, known as polyoxometalates (POMs), [19] have recently emerged as a new class of low molecular weight gelators for designing metallogels. POMs are versatile molecules with tunable structures and reactivities. In 2020, Centellas et al. developed dumbbell-type dimers linked via Co(II) or Co(III), constructed from Keggin and Dawson POMs functionalized with terpyridine ligands. The resulting metallogels exhibited intriguing solvent exchange properties, opening pathways to hydrogel formation. [20]

One prototype gel-forming POM is the organofunctionalized Anderson-Evans polyoxometalate [21] [XMo₆O₁₈{RC(CH₂O)₃}₂]^3- (where X = Mn, Fe, Co, and R = -NH₂, -CH₃, -CH₂OH), which features two organic ligands arranged linearly on opposite sides of the cluster, tethered by tris-alkoxide groups (Fig. 1a). More recently, Casimiro et al. (2024) designed supramolecular metallogels by linking terpyridine-functionalized Anderson anions with Zn(II) or Co(II) cations. These hybrid gels exhibited intriguing properties including luminescence, birefringence, and spin-crossover behavior. [22] Until now, no conversion of pure POM-based metallogels into aerogels had been demonstrated.

In recent years, Anderson-Evans POMs have been extensively studied as catalysts for various oxidation reactions, demonstrating their potential to oxidize organic compounds such as thioanisoles and furan-based chemicals. [23,24] Building on these studies, the present work demonstrates the use of a POM-based aerogel for the oxidation of primary alcohols, paving the way for POM-based porous catalysts in organic transformations. The researchers developed a POM-based organogel that forms within 10 seconds by reacting ZnCl₂ with TRIS-functionalized Anderson POM in acetonitrile solution. The resulting organogel can readily be converted into an aerogel through freeze-drying. The study provides comprehensive characterization of the aerogel and demonstrates its suitability for selective alcohol oxidation reactions using benzyl alcohol, furfuryl alcohol, and octanol as model substrates. Additionally, the team presents initial insights into the scope of gel formation, focusing on the roles of the POM, ZnCl₂, and the solvent.

The POM-based organogel was prepared by combining acetonitrile solutions of ZnCl₂ and the TRIS-functionalized Anderson anion (ⁿBu₄N)₃[MnMo₆O₁₈{(OCH₂)₃CNH₂}₂]. [25] The vial inversion test [26] confirmed the stability and viscoelasticity of the gel (Fig 1b). Subsequently, the organogel was converted into aerogel (1) by freeze-drying (Fig. 1c).

To gain deeper structural insights into the gelation process, the researchers performed ¹H NMR spectroscopic titrations, adding 0 to 2 molar equivalents of ZnCl₂ to the MnMo₆ solution in deuterated acetonitrile (Figure 2). Notably, the methylene protons of the TRIS-NH₂ experience a significant paramagnetic shift due to the Mn(III) center of the Anderson POM. [25] Upon adding increasing amounts of ZnCl₂, the team observed broadening and intensity decrease of all ¹H NMR signals. The reduced molecular mobility resulting from gelation partially reintroduces anisotropic chemical shift and dipole-dipole couplings, affecting the spectral lines. Previous reports have shown similar line broadening of characteristic peaks when Keggin-type POMs triggered hydrogel formation with gamma-cyclodextrin. [27]

Thermogravimetric analysis (TGA) in air atmosphere revealed the thermal stability of compound 1. A significant weight loss of 27.6% (calculated: 28.3%) occurs between 200 and 385 °C, attributed to the loss of three ⁿBu₄N⁺ cations. A second weight loss of 18.4% between 410 and 555 °C corresponds to the simultaneous decomposition of the TRIS moieties [28] and volatilization of ZnCl₂. [29,30] X-ray photoelectron spectroscopy revealed a spin-orbit coupled doublet of Mn 2p at binding energies of 640.9 and 652.7 eV, assigned to Mn 2p_3/2 and Mn 2p_1/2 of Mn(III), respectively (Figure 3b). [31] The Cl 2p spectrum (Fig. 3c) shows binding energies at 200.2 eV (Cl 2p_1/2) and 198.5 eV (Cl 2p_3/2), characteristic of Cl-Zn coordination. [32] The Zn 2p XP spectrum revealed two peaks at 1045.3 and 1022.4 eV, assigned to the Zn 2p_1/2 and Zn 2p_3/2 signals of Zn(II) species (Figure 3d). [33]

Scanning electron microscopy (SEM) revealed the morphology of compound 1 (Fig. 4a,b), showing a layered structure composed of stacked and agglomerated sheets. For further insights into the morphological and structural properties, the scientists employed transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and scanning-transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS). The images reveal that compound 1 consists of thin, elongated fibrous sheets that appear stacked (Fig 4c,d). HRTEM analysis (Fig. 4e,f) confirms the existence of both long fibrous and layered structures, likely resulting from the supramolecular linkage of Anderson anions.

High-angle annular dark-field scanning TEM (HAADF-STEM) imaging and corresponding EDS elemental mapping verified the uniform distribution of C, N, O, Cl, Mn, Zn, and Mo throughout compound 1 (Fig. 5a). Local selected area electron diffraction (SAED) analysis (Fig. 5b) indicates crystalline characteristics, with a calculated d-spacing of 4.83 nm^-1 that agrees well with the d-spacing value of 4.40 nm^-1 obtained from powder X-ray diffraction (pXRD) (Fig. 5c).

To assess the elemental distribution of Mn, Zn, Cl, and Mo in the bulk material, the researchers employed 2D micro X-ray fluorescence spectroscopy (2D-microXRF; resolution approximately 25 micrometers) on a small piece of compound 1 (Fig. 6). Homogeneous distribution of all four elements is evident from similar intensities across the bulk material, particularly within the selected (1 x 1) mm² area indicated by the green square (Fig. 6a). This becomes more apparent when examining the intensity histograms generated for this area (Fig. 6b), which display a normal distribution of intensities without extreme values.

POMs have been extensively investigated as catalysts for various organic oxidation reactions. [34,35,36] Anderson-type POMs in particular have proven highly efficient for alcohol oxidation. [37,38,39] Following the reaction, the catalyst was separated via centrifugation, and product analysis was performed using quantitative ¹H NMR spectroscopy. The oxidation data for benzyl alcohol demonstrate that compound 1 achieves significantly higher benzaldehyde yields compared to the blank sample (Fig. 7a). Similarly, substantially higher furfural and octanal yields were observed with compound 1 as catalyst (Fig. 7b) over time. These results indicate that compound 1 exhibits higher activity for benzyl alcohol oxidation than for furfuryl alcohol or octanol oxidation, possibly related to the more complex molecular structures of furfuryl alcohol and octanol.

The study demonstrates a facile approach to synthesize compound 1 based on TRIS-functionalized Anderson polyoxometalate anions as low molecular weight gelators when crosslinked with ZnCl₂ in acetonitrile. The resulting gel showed promising initial alcohol oxidation reactivity for benzyl alcohol and the bio-based furfuryl alcohol. The findings provide initial insights into the individual roles of POM anion, metal salt, and solvent, offering guidance for the future development of this new class of POM-based porous catalysts.

Resource:
Sachdeva, G., Maloul, S., Zolg, J., Müller, R., Mondeshki, M., Ebrahimi, E., Abid, D., Chala, S. A., Neumann, C., Turchanin, A., Biskupek, J., Kaiser, U., Leopold, K., Streb, C. (2025).
Polyoxometalate Aerogels Formed by Organofunctionalized Anderson Polyoxometalates as Low Molecular Weight Gelators.
Adv. Mater. Interfaces 2025, e00597.
https://doi.org/10.1002/admi.202500597