Kolbe Electrolysis of Acetic Acid in a Polymer Electrolyte Membrane Reactor

Abstract

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, . Platinized Nafion® 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 (Ptotal ≈ 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/cm2 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 platinized by a nonequilibrium impregnation‐reduction method; a 75% current efficiency at 0.3 A/cm2 with a cell voltage of 6 V were measured at the following reaction conditions: 42°C reactor, 58 mm Hg acetic acid (50°C acetic acid dew point), and 42°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.