Oxidation Catalysis by POMs

Some relevant examples of the use of POMs in different oxidation processes will be presented in the next section, including a brief discussion of possible heteroge-nization/recovery strategies of the POM-based catalysts. 

Dioxygen is the most attractive terminal oxidant, since it is cheap and abundant in the atmosphere. Moreover, it presents the highest active oxygen content with no harmful by-products. Several POMs have thus been used as catalysts to activate dioxygen by using different approaches.  

Regioselective cleavage of 2-hydroxycyclohexanone by HPA-2 has given adipic acid or its dimethyl ester as the major products (yields 80-90%) at 65 C in aqueous acetic acid (in 3.5 h) or methanol (in 7 h), respectively. 

HPA-3 has been used with benzoyl derivatives at room temperature with completion of dioxygen uptake within 5 h, and methyl benzoate and/or benzoic acid have been formed with 90-100% selectivity.  

Pyridine-modified HPA-1 ha also been applied for the liquid-phase oxidation of benzene to phenol with 2.0 MPa of dioxygen at 1 lO C, in acetic acid. When using ascorbic as reducing reagent, phenol was obtained with 9% yield in 10 h. A 13% yield was observed in the presence of -cyclodextrin as phase transfer agent, under analogue conditions.  

---