Lin, C.; Chung, T.; Huang, S. (National Cheng Kung University) U.S. Patent Application 2025/0382711, 18-Dec-2025
Even though tetrabutylammonium phosphate is not being used as a phase transfer catalyst in this patent application, we find the following aspects of its use to be interesting. The chemistry involves the electrochemical reduction of 2,5-furandicarboxylic acid at a bismuth or lead cathode in acidic aqueous media, resulting primarily in 2-hydroxyadipic acid, with adipic acid formed under more strongly reducing conditions. This overall reaction is a furan ring opening with hydrogenation in a single electrochemical step, using water as the hydrogen source and avoids molecular hydrogen, high temperature, or precious-metal catalysts, under mild conditions.
Tetrabutylammonium phosphate plays a critical role in this system and appears to suggest some structure–activity relationships. It is speculated that the symmetric tetrabutylammonium cation, with four equivalent C4 alkyl groups, is particularly effective at organizing the electrochemical double layer at the cathode surface. Compared with non-symmetric quaternary ammonium ions such as tributylmethylammonium, the fully symmetric cation packs more uniformly and reproducibly at the metal–electrolyte interface. This improved interfacial ordering appears to promote productive adsorption and activation of 2,5-furandicarboxylic acid while suppressing competing pathways such as unselective hydrogen evolution.
The choice of phosphate as the counterion also appears to be an important choice. Halide-containing quaternary ammonium salts, especially bromides and chlorides, are known in electrochemistry to engage in specific anion adsorption, perturb surface electronic structure, and in some cases accelerate corrosion or surface reconstruction of metal electrodes. In contrast, phosphate and dihydrogen phosphate are comparatively non-aggressive anions under acidic cathodic conditions. Their use minimizes halide-induced surface poisoning and may contribute to improved electrode stability, particularly for bismuth and lead surfaces that are sensitive to halogen chemistry.
The other quat salts shown in the graph are tetrapentylammonium bromide and tetraethylammonium perchlorate. It is not clear why these quat salts were chosen. Tetrapentylammonium bromide appears to give similar results to tetrabutylammonium phosphate (see graph), but we might be concerned about electrode corrosion.
While I have been focusing on structure-activity relationships for quat salts in PTC systems for 49 years, I am always curious to see structure-activity relationships for quat salts in non-PTC systems. When the use of several quat salts in electrochemical systems gives different results for different quat salts, I like to understand if there is any learning value or insight that might affect the way I choose quat salts for synthetic applications.
