Paired electrooxidation Part III: Production ofn-butyric acid fromn-butanol

Abstract

It is well known that, except for methanol and benzyl alcohol, alcohol oxidation is not easy in the absence of a redox mediator since direct oxidation of alcohols at platinum and carbon anodes is limited because of the requirement of high oxidation potentials. In earlier work, nickel in aqueous potassium hydroxide was used as the anode to cause the oxidation of primary alcohol to form carboxylic acid in current yields in excess of 90% [1]. Nickel oxide hydroxide NiO(OH), is continuously produced by the electrooxidation of nickel hydroxide. The application of NiO(OH), prepared by chemical methods, to alcohol oxidation has been well studied [2-4]. Recently, a nickel hydroxide electrode was used for alcohol oxidation through indirect electrochemical oxidation [1, 5-10]. Many papers have reported that the oxidation of alcohol has both high current efficiency and yield using a nickel or nickel hydroxide plated electrode as anode. However, when oxidation takes place in such a system, the current at the cathode does not contribute to the electrolysis process. Therefore, paired electrolysis is examined in this study with a view to increasing the total current efficiency. In paired electrolysis, hydrogen peroxide is produced at the cathode to oxidize the reactant, here n-butanol. Thus, oxidation occurs at both anode and cathode. Several papers have reported the idea of paired electrolysis [11, 12]. In these papers, oxidation did not occur by direct electrooxidation at the cathode but required indirect oxidation through redox mediators, such as, Mn3+/ 3+ 2+ 2 2 Mn 2+, V5+/V 4+, Ce /Ce ,8208-/SO4-. These redox mediators are expensive and require separation from the electrolyte. Some authors have carried out cathodic reduction of oxygen to produce hydrogen peroxide [13-16]. Some papers have reported using H202 combined with metallic redox mediators for cathodic oxidation [17, 18]. But paired electrolysis by simultaneous oxidation through peroxide compounds generated both at anode and cathode has not been reported. In this study, the combination of anodic and direct cathodic oxidation without redox mediators was carried out and paired electrolysis was examined at different controlled current values in divided and undivided cells. Though n-butanol cannot be easily oxidized, very high current efficiency was obtained. The reactions in the paired electrochemical oxidation of n-butanol to n-butyric acid may be expressed as follows. Anodic reaction [8-10]: