Electro-Oxidation Of Benzyl Alcohol In Supercritical CO₂ That Dissolves Ionic Liquids

The electrochemical reaction in supercritical CO2 that dissolves ionic liquids can exert the advantages of electrochemistry, ionic liquid and supercritical fluids, such as the controllable electrochemical reaction, wide electrochemical windows of ionic liquids, the low viscosity, high diffusibility and no surface tension of supercritical fluids. Aromatic aldehydes are versatile intermediates for the production of pharmaceuticals, plastic additives, and perfumes, so the oxidation of an aromatic alcohol into its corresponding aldehyde is an important transformation. However, a major drawback of such oxidation reactions is their lack of selectivity owing to the easy over-oxidation of aldehydes into carboxylic acids since aldehydes are more readily amenable to oxidation than aromatic alcohols. In this thesis, the electro-oxidation reaction of benzyl alcohol was selected as a model reaction to investigate the potential advantages related to electro-organic synthesis in the homogeneous supercritical CO2. When supercritical CO2 is employed as solvent in chemical reactions that are conducted at high pressures, it is very important to understand the phase behavior for the significant influence on reaction rate, selectivity, mass transfer properties, etc. So the high-pressure phase equilibria for the binary system CO2+ benzyl alcohol was studied firstly.The contents of our work are shown as follows:(1) In this work, the phase equilibria for the system CO2+benzyl alcohol were measured at 298.15,306.35 and 313.15 K, under pressures from 1.03 to 16.15 MPa. Three-phase equilibria exist at temperatures lower than 307.45 K, which were observed at temperatures from 279.75 to 307.45 K. The phase behavior of CO2+ benzyl alcohol mixtures probably belongs to Type-Ⅲclass according to the classification of van Konynenburg and Scott.(2) With acetonitrile as cosolvent, electro-oxidation of benzyl alcohol in supercritical CO2 that dissolves ionic liquid has been studied in high-pressure cell with three electrodes. The experimental results show that the benzyl alcohol can be oxidated into benzaldehyde without any over-oxidation in constant voltage electrolysis with 2.7 V, both the yield and selectivity of benzaldehyde can reach 100%, and the corresponding faradic efficiency is 88%. When the benzyl alcohol is oxidated into benzaldehyde on working electrode, the CO2 is deoxygenated into oxalic acid on auxiliary electrode.