Ionic Liquids as Potential Supporting Electrolytes for the Anodic Oxidation of 4-methylanisole

Abstract

The subject of this article is the mechanism and kinetics of 4-methylanisole methoxylation in two ionic liquid (IL) environments, namely triethylsulfonium bis(trifluoromethylsulfonyl)imide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, dissolved in methanol. The investigations focused on the electrochemical behavior of 4-methylanisole, 4-methoxy-benzyl-methyl-ether and 4-methoxybenzaldehyde dimethyl acetal as the substrate and intermediates of the reaction studied. Voltammetry on a glassy carbon stationary electrode and a rotating disk electrode was used to accomplish this task. The results showed that both ILs behave as an indifferent supporting electrolyte in methanol solutions. The methoxylation kinetics of the individual compounds was described in terms of Tafel kinetics. A generalized form of the Tafel equation was utilized to determine the rate-determining step of the methoxylation process. A positive finding is that ILs stabilize intermediates and hinder an oligomerization reaction. Batch experiments revealed that the selectivity of methoxylation in the presence of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide improves significantly compared to NaClO4 as the supporting electrolyte. The selectivity can be further improved by the addition of KF to the electrolyte solution. Selectivity to 4-methoxybenzaldehyde dimethyl acetal, obtained using this electrolyte composition, almost attains the value observed for pure KF solution in methanol, while significantly reducing cell voltage and thus process energy consumption.