Evidence for the presence of alternative mechanisms in the oxidation of cyclohexanone to adipic acid with oxygen, catalysed by Keggin polyoxometalates
Graphical abstract
The oxidation of cyclohexanone with air, conducted with polyoxometalates catalysts and water–acetic acid co-solvents, occurs with either a catalytic-redox or an autoxidation mechanism, in function of the conditions used.
Introduction
Adipic acid (AA) is one of the most used chemicals in the world. The global AA capacity is around 3 million metric tons per year; currently, the growth of AA production is close to 3% per year; nylon 6,6 accounts for the greater part of the total AA consumption [1], [2], [3], [4].
The principal industrial process is done in two steps. The first step is the oxidation of cyclohexane with air to form a mixture of cyclohexanol and cyclohexanone—the so-called KA oil; the yield of this step is between 85% and 90%. In a variant of this first step, cyclohexanol is obtained by the hydrogenation of phenol. The second step is the oxidation of KA oil with an excess of 50–60% HNO3 and a Cu/V catalyst. The molar yield obtained at total KA oil conversion is as high as 95%; the by-products are glutaric acid (GA) and succinic (SA) acid. The co-products of oxidation with nitric acid are nitrogen oxides; NO and NO2 can be reoxidized and recycled, whereas N2O is a pollutant gas that has to be abated before the emission of tail-gases into the atmosphere.
This synthetic approach by two-step oxidation was developed during the 1940s but several technological improvements have been adopted with positive results on energy consumption, the final quality of the product, and the safety and environmental impact of the process. In any case, the study of an alternative synthetic pathway in which air is the oxidising agent is a challenge for modern chemistry, with potential industrial applications due to the savings resulting from the elimination of both the nitric acid production and the recovery plant.
Various catalysts were proposed in the literature for the aerobic oxidative cleavage of cyclic ketones, vicinal diols and α-ketols under mild conditions [5], [6], [7], [8], [9], [10], [11], [12], including Keggin-type polyoxometalates [13], V oxo and dioxo salts and complexes [14], and heterogenized VO catalysts [15]. Table 1 shows some literature results for cyclohexanone oxidation to AA. Keggin-type polyoxometalates (POMs) offer a unique structural versatility that makes it possible to tune the composition in function of the requirements of the reaction [16], [17]. For instance, POMs are amongst the very few systems that catalyse the oxidation of organic substrates with air at a moderate reaction temperature through a redox-type mechanism [18], [19], [20], [21], [22], [23], [24]. To this regard, experimental evidence was reported for O-transfer from the oxidized POM to anthracene (to produce anthraquinone) and xanthene (to produce xanthone), whereby the reduced lacunary POM is then reoxidized by O2 [21], [22], in the same way as it occurs in the Mars-van Krevelen mechanism with mixed oxides catalysts in gas-phase oxidations.
In a previous paper, we reported on the catalytic behaviour of Keggin-type P/Mo/V POMs in the aerobic oxidation of cyclohexanone to AA conducted in water solvent; it was found that reactivity performance was greatly affected by the POM composition, e.g. the number of V atoms incorporated in the Keggin anion and the nature of the cation [20]. However, the cyclohexanone conversion and AA yield achieved were relatively low. In this paper, we have analysed the reactivity of the same catalysts under conditions that are more favourable to achieving higher cyclohexanone conversion, and investigated the reaction mechanism in the presence of acetic acid solvent.
Section snippets
Experimental
Catalytic experiments were carried out in a semi-continuous stirred autoclave reactor (100 mL volume) made out of glass, at an operating pressure of between 1 and 10 atm (Buchi Miniclave). The reactor was equipped with a vapours condenser (cooling fluid temperature: −10 °C), to avoid major loss of cyclohexanone during reaction. Oxygen was fed continuously to the reactor, with a standard flow rate of 300 N mL/min. The liquid phase was loaded batchwise; typical reaction conditions, unless otherwise
Experimental evidence of the presence of two different mechanisms
The use of water as the solvent for cyclohexanone oxidation has positive implications from the environmental point of view, due to both lower corrosion problems and avoiding of toxic solvents, although at the end of the process additional costs have to be borne in order to degradate organic contaminants in water before disposal. A solvent is necessary, however, because the reaction is strongly exothermal, and a ballast component contributes to the removal of the heat generated. However, the
Conclusions
The reactivity of homogeneous Keggin-type P/Mo/V polyoxometalates as catalysts for the liquid-phase oxidation of cyclohexanone with oxygen was investigated under various conditions. The use of acetic acid as a co-solvent led to a strongly enhanced cyclohexanone conversion with respect to the reaction carried out with water-only solvent. The conditions used for catalytic experiments also affected the reaction mechanism: either with a water-only solvent, or in the presence of acetic acid/water
Acknowledgements
The ISA (Institute of Advances Studies), The University of Bologna, is acknowledged for their financial support to K.R.
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