Electro-Oxidation Of Alcohols To Aldehydes In Supercritical Carbon Dioxide With Ionic Liquids

Aromatic aldehydes are versatile intermediates applied broadly in industries of pharmaceuticals, plastic additives, perfumes, and so on. The oxidation of aromatic alcohol to its corresponding aldehyde is an important transformation. As a clean process, electro-organic synthesis could obtain high selectivity by controlling electrolysis voltage. Ionic liquids (ILs) are promising alternatives to aqueous solvents because of stable physical and chemical properties, high ionic conductivity and wide electrochemical window. Moreover, supercritical CO2(scCO2) possess low viscosity and high diffusion rate which could increase mass transfer rate.Electro-oxidation of benzyl alcohol to benzaldehyde was conducted in homogeneous supercritical CO2/[Bmim][PF6]/MeCN solutions, where ionic liquid [Bmim][PF6] was used as the supporting electrolyte and acetonitrile as co-solvent to enhance the solubility of [Bmim][PF6] in supercritical CO2. The results of cyclic voltammetry experiments indicate that electro-oxidation of benzyl alcohol in the scCO2/[Bmim][PF6]/MeCN is an irreversible reaction and the electro-oxidation of benzyl alcohol is a diffusion-controlled process. High selctivity and yield of benzaldehyde can be obtained by tuning electrolysis potential, quantity of passed electricity, and pressure of the reaction system. Our results have demonstrated that both the selectivity and the yield of benzyldehyde can reach more than99.8%at an electrolysis potental of2.7V, system pressures of16-17MPa and a temperature of318.15K, which may offer a new research way to improve the selectivity of aromatic aldehyde from aromatic alcohol.On the basis of study of electro-oxidation of benzyl alcohol to benzaldehyde, we futher researched electro-oxidation of other alcohols, such as cinnamic alcohol, phenylpropyl alcohol, hexyl alcohol, cyclohexanol and1-octanol in scCO2/[Bmim][PF6]/MeCN. The results demonstrate that electro-oxidation of cinnamic alcohol and phenylpropyl alcohol could be carried out and both are irreversible reactions. However, high selevtivity and yield could not be obtained simultaneously. The electro-oxidation of hexyl alcohol, cyclohexanol and1-octanol are hard to happen.