Electrochemical Hydrogenation of Soybean Oil Using Transition Metal Carbide Based Electrocatalysts
Adam Lausche, Saemin Choi, Kanako Okada, and Levi Thompson. Chemical Engineering, University of Michigan, 3074 H.H. Dow, 2300 Hayward, Ann Arbor, MI 48105
More than 8,300,000 metric tons of partially hydrogenated vegetable oil are produced in the United States annually. Most of this oil is processed using a high temperature, high pressure thermochemical process with hydrogen gas and a solid nickel catalyst. Partially hydrogenated vegetable oils produced using these conventional methods typically contain significant amounts of trans-fatty acids (TFA). These TFAs have been linked to coronary artery disease. Recently an electrochemical approach has been reported for the partial hydrogenation of soybean oil. This process was carried out at low temperatures using a Pd black cathode electrocatalyst and resulted in a significant reduction in the formation of TFAs. Research described in this paper investigated the use non-noble metal catalysts for the electrochemical hydrogenation of soybean oil. Early transition metal carbides have demonstrated good activities and selectivities for this reaction. The experiments were carried out using a reactor that resembled a proton exchange membrane fuel cell. A Pt-black anode was used with Nafion 117 to produce the membrane electrode assemblies (~5 cm2), along with an early transition metal carbide or Pd-black based cathode. The activities and selectivities were evaluated at 60-80 °C, with a current density of 0.16 A/cm2 and oil flow rate of 100 mL/min. The oils were characterized using gas chromatography (according to American Oil Chemists' Society standards of fatty acid methyl esters) and iodine value measurements. The electrocatalysts were characterized using x-ray powder diffraction, BET surface area analysis, and x-ray photoelectron spectroscopy. The product chemistries and current efficiencies varied significantly with the type of carbide electrocatalyst. The NbC electrocatalyst was most active among the carbides and was nearly as active as the Pd black electrocatalyst. As shown in Figure 1, the NbC cathode exhibited better selectivities than the Pd-black cathode, producing lower concentrations of stearic acid (C18:0). Both materials produced a lower TFA content (10-15% at an iodine value of 110) when compared to a commercial reactor with Raney nickel catalyst (up to 40%). The reaction products were consistent with a Horiuti-Polanyi mechanism for the hydrogenation of triacylglycerols, which involves the hydrogenation of unsaturated fats (e.g. linolenic, linoleic, and oleic acids) and isomerization to produce geometric and positional isomers (e.g. elaidic acid). These and other results will be presented in this paper.