Kolbe electrolysis of acetic acid in a polymer electrolyte membrane reactor

A polymer electrolyte membrane (PEM) reactor is described for use in Kolbe electrolysis: the anodic oxidation of an. alkyl carboxylic acid with subsequent decarboxylation and coupling to yield a dimer, 2RCOOH --> R-R + 2CO(2) + 2e(-) + 2H(-). Platinized Nafion(R) 117 is the PEM and functions simultaneously as the electrolyte and separator. Results demonstrating the feasibility of Kolbe electrolysis in a PEM reactor are presented for the oxidation of gaseous acetic acid (in a nitrogen diluent) to ethane and carbon dioxide, with hydrogen evolution at the counter electrode. The investigation includes the following effects on current density, current efficiency, and product selectivity: acetic acid partial pressure (P-total approximate to 1 atm), cell voltage and temperature, phase of the catholyte (liquid water or humidified nitrogen), and the procedure used to prepare the membrane-electrode assembly Current densities from 0.06 to 0.4 A/cm(2) with Kolbe current efficiencies of 10 to 90% were obtained for cell voltages ranging from 4 to 10 V. The best results were obtained using PEMs plantinized by a nonequilibrium impregnation-reduction method; a 75% current efficiency at 0.3 A/cm(2) with a cell voltage of 6 V were measured at the following reaction conditions: 42 degrees C reactor, 58 mm Hg acetic acid (50 degrees C acetic acid dew point), and 42 degrees C liquid water to the cathode. These initial results are encouraging for Kolbe electrolysis in a PEM cell; additional work, however, is needed to determine if the PEM strategy may be employed using a liquid-phase reactant. In addition, optimal reaction conditions and downstream mass-transfer separation requirements remain to be determined, both of which are reactant specific.