(254e) Electrocatalytic Hydrogenation of Triglycerides Using Transition Metal Carbide Based Catalysts

Introduction More than 8,300,000 metric tons of partially hydrogenated vegetable oil are produced in the United States annually [1]. Most of this oil is produced using a high temperature, high pressure thermochemical process with hydrogen and a solid nickel catalyst. The resulting partially hydrogenated oils 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 triglycerides [2]. This process has been carried out at low temperatures using noble metal cathode electrocatalysts and resulted in very low TFA contents. Research described in this paper investigated the use of non-noble metal catalysts for the electrochemical hydrogenation of vegetable oil. In particular, the properties of high surface area, early transition metal carbide based electrocatalysts were evaluated. Materials and Methods The electrochemical hydrogenation experiments were performed using a reactor that resembled a proton exchange membrane fuel cell. An early transition metal carbide or Pd-black based cathode and Pt-black anode were used with Nafion 117 to produce the membrane electrode assemblies (~5 cm2). The carbide catalysts were prepared by temperature programmed carburization of the metal oxides (e.g. WO3, NbO3) in 15% CH4/H2. We also evaluated the performance of several carbide supported metal catalysts. These catalysts were prepared using the incipient wetness method with aqueous metal salt solutions (e.g. H2PtCl6?6H2O, (NH3)4Pd(NO3)2).The resulting materials were reduced in H2 at 450 °C for four hours then passivated for five hours using a mixture of 1% O2/He. The activities and selectivities were evaluated at 60-80 °C, 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 [3]. The electrocatalysts were characterized using x-ray powder diffraction, BET surface area analysis, x-ray photoelectron spectroscopy and inductively coupled plasma (ICP) spectroscopy. Results and Discussion The early transition metal carbides demonstrated good activities and selectivities for the electrochemical hydrogenation of triglycerides in soybean oil. Surface areas for the carbides ranged from 10 to 100 m2/g. The product chemistries and current efficiencies varied significantly with the type of carbide. The NbC electrocatalyst was most active and was nearly as active as the Pd black electrocatalyst. The NbC cathode also 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) than the commercial process that uses Raney nickel catalysts (up to 40%). Carbide supported metal catalysts have been shown for certain reactions to exhibit activities and selectivities that exceed those of the individual materials [4]. The early transition metal carbide supported catalysts also exhibited good selectivities during triglyceride hydrogenation with respect to saturated and trans fat formation. As shown in Figure 1, a Pd/WC electrocatalyst produced less saturated fat than Pd black. All catalysts tested produced approximately the same amount of trans fat. The WC support did not exhibit significant hydrogenation activities, however, its presence appeared to reduce the selectivity of the supported Pd towards stearic acid (C18:0) while maximizing the selectivity to oleic acid, the desired product. This trend was also observed for the 6% Ru/WC and 12% Pt/Mo2C catalysts. Significance Early transition metal carbides have been reported to possess catalytic properties that resemble those of noble metals. In this paper we demonstrate their performance for an important industrial food processing reaction. The carbides are less expensive than the noble metals, and their low selectivities towards TFA production make them attractive for consideration as commercial catalysts for the electrochemical hydrogenation of triglycerides. References [1] Margarine and Cooking Oil Processing in the US ? Industry Report, IBISWorld, 31122, 41 (2008). [2] Pintauro, P.N., Gil, M.P., Warner, K., List, G., Neff, W. Ind. Eng. Chem. Res. 44, 6188 (2005). [3] ?Official methods and recommended practices of the AOCS?. American Oil Chemists' Society, 5th Edition, Champagne, Illinois (1998). [4] Setthapun, W., Bej, S.K., Thompson, L.T. Top. Catal. 49, 73-80 (2008).