Anodic Generation of Lewis Acid for the Electrochemical Synthesis of Methyl Formate from CO2

Abstract

Electro-oxidation of neutral substrates to produce cationic species, such as protons or metal cations to serve as Bronsted acids or Lewis acids, can potentially be coupled with the CO2 reduction reaction at the cathode. In particular, coupling the electrochemical CO2 reduction reaction with the anodic generation of a Lewis acid can be a new approach to synthesizing new products from CO(2 )while avoiding a high proton environment. In this study, we explored the coupling of the anodic Lewis acid generation by the dissolution of a titanium anode and the cathodic production of formate from CO2 in a methanol electrolyte. As a result, we observed methyl formate as a CO2 conversion product with the highest faradaic efficiency of 69% under ambient conditions. Interestingly, methyl formate was observed only when NaBF4 was used in an undivided electrochemical cell, whereas only formic acid/formate was produced when sodium salts of other anion counterparts were utilized. We have shown that the anodic generation of Ti4+ cations surrounded by less coordinating anions led to fluoride abstraction from BF4- anions. This resulted in BF3, the active Lewis acid in the esterification reaction of formate and methanol, producing methyl formate.