The primary stages of polyoxomolybdate catalyzed cyclohexanone oxidation by hydrogen peroxide as investigated by in situ NMR.

The adipic acid (AA) production was carried out in two stages: oxidation of cyclohexanone (-one) by Keggin-type polyoxometalate (POM), followed by oxidation of this latter by hydrogen peroxide. The process lasts 20 h and the temperature is maintained at 90 °C. AA is then recovered by cold crystallization (4 °C). The POMs have as formula HMPMo 12 O 40 (M:Co, Ni, Mn, Cu or Zn). The materials were characterized by FT-IR and UV-Vis spectroscopies and by thermogravimetric analysis. The purity of adipic acid was confirmed by FT-IR and 13 C and 1 H NMR analysis. The effects of POM composition, catalyst/-one molar ratio and the cyclohexanol addition to-one on adipic acid yield were examined. The whole catalysts were found to be effective toward cyclohexanone oxidation and the highest yield (53%) was obtained with HZnPMo 12 O 40 system for a catalyst/-one molar ratio of 1.89 × 10 −3. The alcohol addition to-one has a negative effect on adipic acid formation.

The oxidation of cyclohexanone and/or cyclohexanol to adipic acid (AA) was performed at 90 °C with a reaction time of 20 h, in the presence of H2O2 as oxidant and transition metal substituted ammonia polyoxometalates of formula, (NH4)xHyMzPMo12O40 (M: Fe, Co, or Ni, and x = 2.5 or 2.28) as catalysts. The catalytic results showed that the AA yield is sensitive to the transition metal nature and to the reaction conditions (sample weight and substrate amount). The (NH4)2.29H0.39Co0.16PMo12O40 was found to be the better catalytic system toward AA synthesis from cyclohexanone oxidation, with 40% of AA yield