Novel tetra-propyl/butylammonium encapsulated Keggin-type polyoxotungstates: synthesis, structural characterization, and catalytic capability in oxidative cleavage of unsaturated fatty acids

Abstract

Four tungsten oxide (TO)-based catalysts, (1) TO (without any organic moiety), (2) TO-TMA, (3) TO-TPA, and (4) TO-TBA (TMA: tetramethylammonium, TPA: tetrapropylammonium, and TBA: tetrabutylammonium cations) were straightforwardly prepared via a mild hydrothermal approach, based on oxidative dissolution of tungsten bare powder, followed by a facile encapsulation of the tetraalkylammonium cations (NR₄⁺). The significant impact of the NR₄⁺ cations on crystallization of the amorphous and metastable peroxotungstic species, formed during the synthesis, has been discussed through different characterization techniques including XRD, FTIR, TGA and DTG, TEM, EDS, N₂ adsorption/desorption isotherms, zeta potential measurement, and elemental analysis. TO-TPA and TO-TBA were composed of two novel hybrid inorganic–organic polyoxotungstates, [(CH₃CH₂CH₂)₄N]_2.75[H_5.25W₁₂O₄₀]·7.42H₂O and [(CH₃CH₂CH₂CH₂)₄N]_3.31[H_4.69W₁₂O₄₀]·1.08H₂O, respectively. The capability of the synthesized catalysts in oxidation of unsaturated fatty acids (UFAs) has been examined in the reaction of oleic acid, the most abundant UFA, with H₂O₂. This is the first report on employing heterogeneous Keggin clusters of tungsten oxide in the oxidative cleavage of UFAs. The catalysts exhibited excellent efficiencies, particularly TO-TPA and TO-TBA, owing to the strong and organo-protected acidity of their Keggin structures, which resulted in full conversion of oleic acid and superior yields of the desired products. Despite their pseudo-homogeneous behavior in the reaction, the NR₄⁺-encapsulated catalysts demonstrated efficient recovery and steady reusability without any significant decay in their activities up to at least 4 cycles.