(14i) Electrochemical Anthraquinone Process Enabled By Phase Transfer Catalysis
Existing methods for the direct electrochemical production of hydrogen peroxide rely on precious metal catalysis, using palladium or platinum, as well as toxic alloying metals such as mercury. Furthermore, the production is limited by both solubility of oxygen in water, and the possibility of a buildup of hydrogen peroxide short-circuiting the device. Furthermore, a general existing problem with direct electrolysis is that H2O2 is generated in some electrolyte which must be removed, as well as the instability of peroxides in such catholytes.
We have developed a method for the electrolytic reduction of quinones to form hydroquinones in aqueous solutions; in effect an electrochemical analog of the anthraquinone process. Where precious metal oxygen evolution catalyst is avoided by the use of a bipolar membrane, allowing quinone reduction at acidic pH and oxygen evolution at basic pH using an iron-nickel (FeNi) water oxidation catalyst. The peroxide forming step is decoupled from the initial reduction by phase transfer catalysis, using a tetrabutylammonium salt and a water immiscible phase to shuttle the quinone between an electrolyte medium and an oxygenated salt-free water flow in which the product can be formed with high efficiency. A device utilizing this method was fabricated and the continuous production of hydrogen peroxide was demonstrated with a Faradaic efficiency of >90% at concentrations of > 2000 ppm at neutral pH.