Alain Fankhauser
Electrochemical acetoxylation has been widely investigated about 40 years ago. However, there are only a limited number of studies in non-aqueous electrolytes, mainly because of difficulties related with the extremely low electrical conductivity of organic media. This work focuses on electrochemistry in organic media and deals with the problem of low conductivity by using ultramicroelectrodes which allow neglecting the ohmic drop. Precise electrochemical measurements were thus accessible and electrochemical acetoxylation of aromatic compounds in a glacial acetic acid media has been investigated using three electrode materials: platinum (electrocatalytic), graphite (industrial electrodes) and boron-doped diamond (BDD) (non-electrocatalytic). In the first part of this work, a glacial acetic acid medium has been characterized on the three electrode materials. Strong similarities in the behavior of the electrodes in glacial acetic acid and in aqueous electrolytes have been outlined. Furthermore an oxidation mechanism of acetate depending on the electrode material has been proposed. Methane and methyl acetate are the main side-products formed during this oxidation but only in negligible quantities. The proposed mechanisms for the formation of these side-products are based on a model similar to that for the oxidation of organic substrates in aqueous solutions on BDD. In the second part of this work, the electrochemical behavior of p-methylanisole (p-MA) in acetonitrile has been investigated. It has been shown that two oxidation peaks appear on the three electrode materials. The main anodic reaction of p-MA is polymerization and both peaks are related to this reaction. The first peak is attributed to the oxidation of p-MA to the corresponding radical cation, whereas the second one is supposed to correspond to the oxidation of a cationic intermediate. Finally it has been shown that the electrode deactivation due to the deposition of polymeric material on the electrode surface occurs at different potentials on the three investigated electrode materials. A mechanism involving two polymerization routes has been proposed. Acetoxylation of substituted aromatic substrates has been investigated using different substrates: p-MA, toluene and nitrobenzene. It has been shown that mainly three types of reactions occur: acetoxylation, methylation and polymerization. Acetoxylation is favored on platinum, while on BDD electrodes polymerization is privileged. Concerning methylation, it has been shown to occur only if the oxidation potential of the substrate is close to that of the solvent. A general reaction scheme depending on the electrode material has been proposed. In the fourth part of this work, acetoxylation of p-MA and toluene using graphite electrodes has been performed in a lab-scale monopolar electrolyzer and in a bench-scale bipolar cell. It has been found that due to the cell design, acetoxylation of p-MA can be conducted at high current efficiencies and at high selectivities even for a conversion as high as 90%. Finally, first milestones have been set for the continuation of this work. A kinetic model based on the previously proposed mechanism of acetate oxidation, acetoxylation and polymerization depending on the electrocatalytic activity of the electrode has been simulated and compared to the obtained results in the first parts of this work. Furthermore, the importance of the supporting electrolyte has been highlighted. Moreover, it has been shown that dimensionally stable anodes (DSA) can be used in glacial acetic acid media. Finally, a bipolar reactor with a sub-mm interelectrode gap and internal recirculation has been constructed.
EPFL2007
