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.