Keggin heteropolyacids (i.e. H3PW12O40, H3PMo12O40 and H4SiW12O40) were converted to potassium lacunary salts and afterwards their vacancy were filled by metal cations (i.e. Cu2+, Co2+, Fe3+, Ni2+ or Al3+). These solid catalysts were assessed on oxidation of benzaldehyde to benzoic acid by hydrogen peroxide. Remarkably, among heteropoly salts investigated, the K6SiW11CoO39 was the most active catalyst. High conversion (ca. 90%) and benzoic acid selectivity (ca. 90%) were achieved. The activity of solid catalyst remained unaltered after successive cycles of reuse. Effect of catalyst nature, temperature, reactants concentration on conversion and selectivity were assessed. [GRAPHICS] .