Abstract
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 HMPMo12O40 (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 13C and 1H 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 HZnPMo12O40 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.
Highlights
Adipic acid (AA) is a raw material for the production of polyamides, nylon 6-6, polyesters and polyurethanes and food additives [7, 17]
In recent work, it has been shown that the proton presence in the POM allows the activation of cyclohexanone by keto-enolic tautomerization, before its oxidation into more oxygenated compounds [2]. In this context and to understand the influence of the metal nature on adipic acid formation, we tested a series of substituted POMs by introducing both single metal atom with an oxidation state of 2 and single proton as counter-ion of the [PMo12O40]3− Keggin anion
The UV–visible spectra of different solids (Fig. 2) show a large charge transfer band oxygen–molybdenum(VI) in 200–480 nm range, consisting of two components located at 200–300 and 300–450 nm [11, 27]. The latter are associated with the different types of oxygen atoms of the Keggin anion, inter-anion charge transfer transitions and counter-ion nature
Summary
Adipic acid (AA) is a raw material for the production of polyamides, nylon 6-6, polyesters and polyurethanes and food additives [7, 17]. Keywords Keggin-type heteropoly salts · Oxidation · Hydrogen peroxide · Cyclohexanone · Adipic acid
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