Gold catalyzed oxidation of aldehydes in liquid phase

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Abstract

Gold on carbon oxidizes aldehydes to carboxylic acids in water solution under mild conditions without loss of activity on recycling, as does not occur for platinum on carbon. Also scarcely soluble aldehydes can be oxidized but the reaction rate is slower. Experiments carried out in an organic solvent such as CCl4 show a speed up of the reaction, in this solvent being recyclable both Au/C and Pt/C. Another explored possibility was represented by carrying out the reaction in the absence of solvent. However, for solid aldehydes like p- and o-OH-PhCHO the water solvent represent the unique alternative.

The reaction was carried out in water at 363 K under 300 kPa of O2. The reaction rate depends on the nature of R. Alternatively oxidation was carried out in CCl4 or without solvent (in air). Gold on carbon catalyst can be recycled without loss of activity.

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Introduction

The oxidation of aldehydes with O2 as the oxidant is actually performed in the liquid phase using homogeneous catalytic systems based on salts of copper, iron, cobalt, manganese, etc. [1]. However, the use of heterogeneous systems would be advantageous as metal recovery is easier and more efficient than in homogeneous systems and, furthermore, far less damaging from an environmental point of view.

From the industrial point of view, gas phase oxidation is more attractive as the absence of solvent facilitates product collection, but limitations due to high boiling points or thermal instability sometimes preclude this choice.

Liquid phase catalytic oxidation can normally be carried out under milder conditions than the gas phase and, by employing O2 as the oxidant, the choice of solvent determines the active mechanism, organic solvents favoring the radical pathway [2], whereas water privileges the ionic pathway [3]. Moreover, water is a safe and environmentally compatible solvent, and is favored from the industrial point of view. However, in liquid aqueous phase oxidation the classical heterogeneous catalysts based on platinum-group metals suffer deactivation when O2 is the oxidant, thus catalyst lifetime is compromised [3], [4].

In order to meet both environmental (liquid aqueous phase and heterogeneous system) and industrial requirements (safety and high productivity), we turned our attention to gold based catalytic systems, gold having been reported as being much more resistant to poisoning in liquid phase oxidation where O2 is the oxidant than platinum metal based catalysts [5].

This paper deals with the use of gold catalysts in the oxidation of aldehydes in the liquid phase, extending promising results from the oxidation of d-glucose to gluconic acid [6]. We also compared catalyst activity and catalyst lifetime in different solvent systems (pure water, organic solvent and mixture of both), evaluating the differences in reactions using platinum catalyst as a function of reagent and solvent employed.

Section snippets

Materials

Gold of 99.9999 purity in sponge from Fluka and activated carbon from Carbosorb (MK; SA=900–1100 m2/g; PV=1.5 ml/g; pH 9–10) were used. NaBH4 of purity >96% from Fluka and polyvinylalcohol (PVA) (M=10,000 Da) from Aldrich were used. Gaseous oxygen from SIAD was 99.99% pure. Aldehydes of the maximum purity grade (>98%) were from Fluka and used without any further purification.

Pt/C (5%) commercial catalyst was from Engelhard (Escat 21, water content 54%).

Gold sol preparation

The procedure has been reported elsewhere [7]

Results and discussion

We recently reported that gold on carbon was able to oxidize d-glucose (an emiacetal) to gluconic acid, even in the absence of a base that neutralized the acid formed [6]. Contrarily to the reaction carried out in the presence of a base, we noted a slow deactivation process of the catalyst that we ascribed to metal leaching and sintering of gold particles. Comparing gold to PGM-based catalysts, however, we evidenced a better resistance of gold to poisoning at low pH.

In this research, we have

Conclusion

Gold on carbon showed good activity in oxidizing aldehydes in water solution and, contrary to Pt/C, there was no deactivation of the catalyst on recycling. In water solvent, almost all the tested aldehydes are oxidized, the reaction rate depending on the nature of substrate. Scarcely soluble reagents show a slower reaction rate. Using organic solvents we highlighted the worst behavior of CH3CN whereas CCl4 produced an enhancement of the reaction rate in most cases. An alternative for liquid

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