The heteropolyacids "H 3þn [PMo 12en V n O 40 ]$aq" (denoted as HPA-n; n ¼ 2, 3, 8) catalyse the oxidation of aldehydes to carboxylic acids in the presence of dioxygen with very good yields. The effect on the catalytic activity of various parameters such as the precursors, solvent, temperature or catalyst/substrate ratio was examined. The process is particularly selective for linear and aromatic aldehydes. The oxidation of adipaldehyde with dioxygen in mild conditions, in the presence of HPA-2 as a catalyst, leads to the formation of adipic acid together with a significant amount of other byproducts. Thus, several modifications of the catalytic systems have been carried out to improve their selectivity. The effect of cocatalysts was investigated and, among the species tested, complex Ni(acac) 2 was found to be the most efficient yielding 60% of adipic acid.

A commercial montmorillonite clay catalyst, K-10 montmorillonite, was tested for catalytic oxidation of aldehydes in the presence of molecular oxygen under mild conditions. K-10 montmorillonite catalysed the oxidation of aldehydes with good activity and excellent selectivity toward the formation of the corresponding acids. The effects of the amount of catalyst, temperature and solvent on the catalytic activity were investigated. Remarkably, this catalyst was reusable without any appreciable loss in activity and selectivity.

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The oxidation of aldehydes with molecular oxygen was investigated in the presence of MoO2(acac)2(Bis(acetylacetonato)dioxomolybdenum(VI)), and under mild condition (temperature and pressure). In the oxidation reaction system, the effect of the catalyst has been evaluated. In addition, the factor of solvents was deeply discussed. The process is particularly selective for linear and aromatic aldehydes. Finally, the use of environmentally-friendly oxidation system, including mild conditions and inexpensive oxidant, can make this process very attractive in the fine chemical synthesis.