Synthesis, structural characterization and properties of a series of heteropolyoxomolybdates: [AlMo6(OH)6O18]3−, [GeMo12O40]4−, [GeMo6O22(Hmal)3]7−, [SiMo12O40]4−, and [TeMo6O24]6−

Abstract

Eight heteropolyoxomolybdates were synthesized and characterized, comprising three Anderson-type compounds [Co(ox)(NH₃)₄]₃[AlMo₆(OH)₆O₁₈]·7H₂O (1), [Co(ox)(NH₃)₄]₆[TeMo₆O₂₄]·16H₂O (2), and Na₂[Co(ox)(en)₂]₄[TeMo₆O₂₄]·16H₂O (3), four Keggin-type compounds [Co(ox)(NH₃)₄]₄[GeMo₁₂O₄₀]·6H₂O (4), [Co(ox)(NH₃)₄]₄[SiMo₁₂O₄₀]·6H₂O (5), [Co(ox)(en)₂]₄[GeMo₁₂O₄₀]·12H₂O (6), and [Co(ox)(en)₂]₄[SiMo₁₂O₄₀]·12H₂O (7), as well as one coordination polymer Na₄[Co(ox)(NH₃)₄]₃[GeMo₆O₂₂(Hmal)₃]·7.75H₂O (8) featuring an extensive three-dimensional coordination framework. Additionally, two reaction intermediates, [{Co(ox)(NH₃)₄}₂Na₂(H₂O)₆{H₄Mo₈O₂₈}]·4H₂O (1a) and [Co(ox)(NH₃)₄]₂[Al(mal)₂(H₂O)₂]·Hmal·2H₂O (1b), were also isolated. Compounds [Co(NH₃)₆][Al(mal)₂(H₂O)₂](NO₃)₂·H₂O (9) and [Co(en)₃]₂[Mo₈O₂₆(H₂O)₂]Cl₂·6H₂O (10) were the only products obtained from reactions involving Al³⁺ as the heteroatom and complex cations [Co(NH₃)₆]³⁺ or [Co(en)₃]³⁺. All compounds were prepared by hydrothermal synthesis and the mechanochemical method followed by vapour-assisted ageing. The reactions between sodium molybdate and [Co(ox)(NH₃)₄]NO₃·H₂O in the presence of AlCl₃ or GeO₂ and malonic acid were monitored in situ by Raman spectroscopy. The initial transformation involves a remarkably fast reaction between sodium molybdate and malonic acid, generating new species that subsequently reacted with other mixture components. The photocatalytic activities of compounds 1, 2, 4 and 5 in the degradation of the safranin T dye under visible light irradiation, both with and without hydrogen peroxide, were also examined. Compounds 4 and 5 exhibit the highest activities. To investigate the mechanisms driving the crystallization process, we monitored the transformation kinetics of amorphous intermediates into the final crystalline products within an aqueous slurry at room temperature. We employed ex situ powder X-ray diffraction (PXRD), a technique allowing real-time observation of structural changes as they occur. The resulting PXRD data, which generated complex, multi-dimensional datasets (data tensors), were analyzed using the 2^nd-order tensor decomposition method, principal component analysis (PCA).