Polyoxometalates Catalysts for Sustainable Oxidations and Energy Applications

Polyoxometalates Catalysts for Sustainable Oxidations and Energy Applications 

Mauro CARRARO, Giulia FIORANI, Andrea SARTOREL and Marcella BONCHIO  

Polyoxometalates (POMs) are oxo-clusters of early transition metals in their highest oxidation state, namely Mo(VI), W(VI), V(V), Nb(V) and Ta(V). A simple classification is based on their chemical composition, with two types of general formula  

Polyoxometalates are generally formed only by transition metals with dimensions (cationic radius) compatible with an octahedral coordination. In addition, empty low energy d-orbitals on the metal ion are required to form at least one terminal M=0 double bond. The aggregation of octahedral units MOe occurs upon condensation between two central metal ions through p,-oxo bridged bonds, while the presence of less basic, terminal oxygen atoms prevents the linear polymerization (as in common metal oxides silicates, germanates, tellurates) and promotes the formation of discrete molecular units.  

These discrete, soluble multimetal oxides are characterized by a formidable structural variety that can be achieved by tuning reaction parameters (such as concentrations and/or stoichiometric ratio of the reagents, temperature and pH), thus yielding nanosized complexes with a different shape, charge density and surface reactivity. When considering isostructural POMs, in particular, such properties can be controlled at the molecular level by changing their constituent elements (other transition metals or the heteroatom X). 

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