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.

多钼氧酸盐（POMs）的最大暴露度对于提升包含Keggin型POMs的HKUST-1的催化性能具有重要意义。本研究中，通过水热法将两种磷钒钼酸盐封装于HKUST-1中，以获得两种基于多钼氧酸盐的金属-有机框架材料，分别以[Cu12(BTC)8(H2O)12]­[H4PMo11VO40]@(H2O)30（1）和[Cu12(BTC)8(H2O)12]­[H5PMo10V2O40]@(H2O)49（2）的化学式表示。单晶X射线衍射分析表明，这两种化合物均含有独特的、高核性的水簇结构，且不含有机反离子。对于化合物1，八面体形的水簇(H2O)30由正方形金字塔形(H2O)5构成；而化合物2中，巨大的笼状水簇(H2O)49由冠状(H2O)8和一个水分子构成，替代了参与结构构建的有机反离子。更为重要的是，通过简单的热处理去除水簇后，两种化合物的活性位点得以完全暴露，从而在苯酚化反应和氧化脱硫反应中展现出良好的催化活性。此外，催化测试证实，化合物2可能是一种具有双功能性的异相催化剂，在苯酚化反应和氧化脱硫反应中均具有巨大的应用潜力。