Polyoxometalate-Based Metal–Organic Frameworks with Unique High-Nuclearity Water Clusters

The maximum exposure of polyoxometalates (POMs) is of great significance to enhance the catalytic performance of HKUST-1 with incorporated Keggin-type POMs. Herein, two phosphovanadomolybdates were encapsulated into the HKUST-1 via a hydrothermal method to obtain two polyoxometalate-based metal–organic frameworks, formulated as [Cu₁₂(BTC)₈(H₂O)₁₂]­[H₄PMo₁₁VO₄₀]@(H₂O)₃₀ (1) and [Cu₁₂(BTC)₈(H₂O)₁₂]­[H₅PMo₁₀V₂O₄₀]@(H₂O)₄₉ (2). Single-crystal X-ray diffraction analysis indicates that two compounds contain unique high-nuclearity water clusters without organic counter cations. The octahedral-shaped water cluster (H₂O)₃₀ was constructed from square-pyramid-shaped (H₂O)₅ for compound 1, while the huge cage-shaped water cluster (H₂O)₄₉ of compound 2 consisted of crown-like (H₂O)₈ and one water molecule, which substitute the organic counter cations involved in the structural construction. More importantly, after removing the water clusters via simple heat treatment, the active sites of the two compounds were fully exposed, leading to good catalytic activities for both benzene hydroxylation reaction and oxidative desulfurization. Furthermore, the catalytic test confirmed that compound 2 may be a bifunctional heterogeneous catalyst with great promise for both benzene hydroxylation and oxidative desulfurization.