Abstract
This review describes the synthesis and characterization of inorganic materials containing polyoxometalates encapsulated in oxide matrices. Examples illustrating key aspects in terms of synthesis and applications are presented according to the nature of the final hybrid material: those based on non-structured silicas, on mesostructured silicas, on macrostructured silicas and on other oxides. In each part, key points of the synthetic protocols are highlighted and structural features and properties of the resultant hybrid nanocomposites are discussed.
Highlights
Polyoxometalates (POMs) are a class of molecularly defined inorganic metal-oxide clusters formed from early transition metals (V, Nb, Ta, Mo, W) in their highest oxidation states [1]
The preparation of POM-oxide materials via direct synthesis leads to materials with larger pore sizes without pore blockage and more stable and uniform distribution of the active phase in the solid which in turn can enhance the performance of the resulting nanocomposites
Electronic transfers occurred resulting in a reduction of the metal salt and the formation of metal nanoparticles which were very small in size and well dispersed on the surface of the polyoxometalate/organic cation/silica composite material
Summary
Polyoxometalates (POMs) are a class of molecularly defined inorganic metal-oxide clusters formed from early transition metals (V, Nb, Ta, Mo, W) in their highest oxidation states [1]. The conventional post-synthesis impregnation method is undoubtedly the simplest route to access functionalized materials, but it suffers from several drawbacks including difficulty in achieving high POMs loading without significant decrease in surface area and ordering, loss of the initial high dispersity of supported POM units via leaching, and loss of homogeneity due to minor changes in the structure All of these lead to reduced activity or stability of the immobilized POMs. On the contrary, the preparation of POM-oxide materials via direct synthesis leads to materials with larger pore sizes without pore blockage and more stable and uniform distribution of the active phase in the solid which in turn can enhance the performance of the resulting nanocomposites. We conclude with an example where the POM is integrated with both organic and inorganic matrices in the sophisticated synthesis of metal nanoparticules, mirroring the broader current trend in catalyst design toward nanometric or molecular scale structural control of active sites and their surroundings
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