Oxidation of tensidic alcohols to their corresponding carboxylic acids via Au- and AuPt-catalysts

Reducing the use of environmentally hazardous chemicals in industrial processes plays a key role in the science of catalysis. Particularly the liquid-phase oxidation of tensidic alcohols to the corresponding carboxylic acids holds a great potential for improvement towards a “greener” process. Currently ether carboxylic acids are either synthesized via Williamson’s ether-synthesis using chloroacetic acid derivatives. Incomplete conversion and consequential excess use of chloroacetic acid lead to impurities (e.g. educt residues and by-products) which accelerate the deterioration of the acid and impair its solubility. Or they can be produced by oxidising the corresponding alcohol with dioxygen using supported Pt- or Pd- catalysts. However, difficulties such as metal leaching and incomplete conversion may occur during this process. Our research group is first to develop and employ mono- and bimetallic Au-catalysts for the liquid-phase oxidation of fatty alcohol ethoxylates and related model compounds, i.e. alkyl ethoxylates and ethoxylates, to their corresponding carboxylic acid.
Several preparation methods, catalyst supports and Au-Pt-ratios (for bimetallic catalysts) were screened. For all model compounds monometallic Au-catalysts featured a selectivity of 100 % to the carboxylic acid. Maintaining total selectivity, the activity could be increased significantly by using a bimetallic Au-Pt-catalysts with a gold:platinum ratio of 90:10. Thus, for comparable reaction conditions our catalyst was ten times as active as a Pt-catalyst.
The experiments were carried out at elevated pressures (5-30 bar) in thermostatted stainless steel autoclaves at constant pH (9 -11). Variation of reaction parameters and kinetic studies revealed a dependency of the activity on temperature (80 – 130 °C), oxygen pressure, pH-value and educt concentration (5 – 80 %) – all of which do not affect the selectivity.
The reaction mechanism should be identical for all model compounds. However, during the oxidation of certain fatty alcohol ethoxylates with monometallic Au-catalysts intermediate metal leaching occurs. This phenomenon does not arise with the other two model compounds. Preliminary results suggest that metal leaching can be reduced considerably by employing bimetallic catalysts with a proper Au-Pt-ratio.