Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol Using (NH4)xAyPMo12O40 (A: Sb, Sn, Bi) Mixed Heteropolysalts and Hydrogen Peroxide in Free Solvent

Abstract

Clean synthesis of adipic acid (AA) from oxidation of cyclohexanone, cyclohexanol or mixture cyclohexanol/cyclohexanone, was carried out at 90 °C, in the presence of hydrogen peroxide (30%) in free solvent, using Keggin-type polyoxometalates, (NH4)xAyPMo12O40 (An+=Sb3+, Bi3+ or Sn2+), as catalysts. HPLC analysis of reaction mixture showed the formation of adipic, succinic and glutaric acids and unidentified products. The salts were found to be effective for AA synthesis. Whatever the composition of the catalyst, the alcohol oxidation favors the formation of the unidentified products, unlike the ketone oxidation which favors that of the adipic acid. (NH4)0.5Sn1.25PMo12O40 led to the highest AA yield (56%) from cyclohexanone oxidation, after 20 h of reaction. In addition, 31P NMR analysis showed that it has conserved the Keggin structure contrary to others catalysts and that it can be used at least 3 times with reaction times of 20 h, without regeneration. From different catalytic tests and 31P NMR data, reaction pathways have been proposed. The active species could be peroxo-polyoxometalates.