(14i) Electrochemical Anthraquinone Process Enabled By Phase Transfer Catalysis

Currently, hydrogen peroxide is produced via the anthraquinone process, using hydrogen as terminal reductant for two-electron reduction of oxygen to H₂O₂. This is in order to ensure selectivity for two-electron reduction as well as to avoid mixing hydrogen and oxygen within the explosive concentration limits. However, this process is difficult to downscale because of the reliance on the use of hydrogen generally produced on-site by steam reforming. Electrochemical delivery of two protons and two electrons can be envisioned as an alternative, however there is currently no commercial production of hydrogen peroxide using electricity and very few small scale demonstrations.

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.