Emulsion electrosynthesis in the presence of power ultrasound Biphasic Kolbe coupling processes at platinum and boron-doped diamond electrodes

The electrochemical oxidation of aliphatic carboxylic acids, hexanoic, hept anoic, and lauric acid, under biphasic conditions is studied as a model sys tem for ultrasound enhanced Kolbe electrosynthesis processes. Power ultraso und is used to generate an in situ emulsified medium and to remove reaction products continuously from the electrode surface. A clean and highly effic ient process at platinum electrodes with formation of 'one electron' produc ts only occurs in marked contrast to processes in monophasic media. For hex anoic acid the Kolbe dimer product R-R is formed in up to 75% yield with 45 % current efficiency at 0.18 A cm(-2) current density and in the presence o f 190 W cm(-2) ultrasound. The mechanism is explained in terms of a dynamic ally modified electrode surface, at which hydrophobic products are immediat ely 'trapped' via partitioning into a non-polar organic phase and transport ed away into the emulsion system. Kolbe electrosynthesis is undertaken both at platinum electrodes and at free-standing polycrystalline boron-doped di amond electrodes, in order to minimize the surface erosion effect induced b y power ultrasound. The type and yield of products obtained from the biphas ic Kolbe electrolysis process at diamond electrodes are essentially identic al to those observed at platinum and based on this observation, the presenc e of a biphasic reaction layer at the electrode surface is postulated to go vern the process. (C) 2001 Elsevier Science B.V. All rights reserved.