Electro-Catalytic Reforming of Ethylene Glycol

Abstract

Biofuels continue to receive national attention motivated by concerns about finite energy supplies, geopolitical issues surrounding petroleum imports, and the environmental impact of fossil fuels. Biomass is an abundant resource, but is not easily converted to a usable transportation fuel. The aqueous phase reforming (APR) process [1] shows promise for converting compounds derived from biomass into hydrogen and alkanes at moderate temperatures (~200 <super>O</super>C) in the aqueous phase. However, hydrogen selectivity in APR is a major hurdle, especially as carbon chain length of the feed molecules increase [2]. An alternative and complementary process for reforming carbohydrates is electro-catalytic reforming (ECR) which involves electrooxidation of carbohydrates and reduction of generated protons to produce hydrogen at the cathode. In this work we use ethylene glycol as a prototypical carbohydrate, as it is the simplest sugar-like molecule. The primary reactions in this system are equations 1 and 2 for the anode and cathode respectively. C₂2H₆O₂ + 2H₂O → 2CO₂ + 10H<super>+</super> + 10e<super>-</super> (1) 2H<super>+</super> + 2e<super>-</super> → H₂ (2) Protons formed at the anode reduce and form gaseous hydrogen at the cathode. The formation of hydrogen in a separate compartment from the reactants, simplifies hydrogen removal and purification, and improves overall hydrogen efficiency by preventing deleterious homogeneous reactions. Successful electro-catalytic reforming rests largely on effective electrooxidation, which typically is hampered by formation of poisons at the anode [3]. We developed a reactor that utilized a proton exchange membrane fuel cell architecture and operated up to 140 <super>O</super>C and 3.04 MPa with a liquid phase reactant. The reaction was studied by cyclic voltammetry and step potential measurements. We measured reaction products by gas chromatography (GC) and high pressure liquid chromatography (HPLC). We corroborated findings that saw a decease in electrooxdation potential of ethylene glycol with increasing temperature [4]. We focused on increasing the operating temperature of the ECR reactor to improve its catalyst poison tolerance and to approach operating conditions of APR. However, to increase temperature a high temperature electrolyte needed to be developed. We studied cross-linked sulfonated poly ether ether ketone membranes to test their viability to increase the temperature range of ECR. Using a Nafion electrolyte, and GC and HPLC data, we were able to close the mass balance on the ECR reaction. We measured five major products of the reforming reaction: carbon dioxide, hydrogen, glycolic acid, glycolaldehyde, and oxalic acid. For operating conditions of 137 <super>O</super>C and 0.7 V we calculated that ECR uses 36 % less platinum than APR at 265 <super>O</super>C, to reform an equivalent amount of ethylene glycol.