Three-Dimensional Silver-Containing Polyoxotungstate Frameworks for Photocatalytic Aerobic Oxidation of Benzyl Alcohol

Abstract

Photocatalytic organic transformation derived by functionalized polyoxometalate (POM)-based metal-organic frameworks provides a feasible route for fine chemical synthesis. Herein, three kinds of photoactive three-dimensional silver-containing polyoxotungstate frameworks are synthesized with the formulas [Ag₃L₂(OH)][Na(H₂O)_0.5][PW₁₂O₄₀]·H₂O (<b>1</b>), [Ag₄L₃][SiW₁₂O₄₀] (<b>2</b>), and [Ag(H₂O)][Ag₄L₃][BW₁₂O₄₀]·9H₂O (<b>3</b>) (L = 1,4-di(4H-1,2,4-triazol-4-yl)benzene). In compounds <b>1</b>-<b>3</b>, the cationic Ag-triazole clusters with diverse nuclei serve as nodes to assemble with rigid bridging ligands (L) and polyoxoanions to extend into stable three-dimensional frameworks, in which Keggin-type anions act as guests or pendants. When using them as heterogeneous photocatalysts, compounds <b>1</b>-<b>3</b> show high catalytic activity and selectivity for the photocatalytic aerobic oxidation of benzyl alcohol to benzoic acid under 10 W 365 nm light irradiation. Among them, compound <b>1</b> exhibits the highest performance with ca. 99% benzyl alcohol conversion and 99% selectivity of benzoic acid in 9 h. Compounds <b>2</b> and <b>3</b> show ca. 79 and 88% conversions of benzyl alcohol, respectively, which are higher than those of the individual Keggin-type precursors. Moreover, mechanism investigation suggests that the synergistic cooperation occurring between cationic Ag-triazole clusters and Keggin-type polyoxoanions modulates the energy band structures of compounds <b>1</b>-<b>3</b>, resulting in the efficient separation of photogenerated carriers and accelerating the aerobic oxidation of benzyl alcohol. This work provides some important guidance for the design and development of efficient POM-based photocatalysts for practical organic transformation.