Reaction Pathways of the Electrochemical Hydrogenation and Hydrogenolysis of Furanic Species

Abstract

Electrochemical conversion of lignocellulosic biomass is gaining interest as a renewable method to produce value-added chemicals and fuels. Furfural (FF) and 5-(hydroxymethyl)furfural (HMF) are two biomass derived platform molecules of particular interest for their variety of possible products and abundance. Electricity may be used to upgrade FF to furfuryl alcohol (FA), a chemical intermediate, and 2-methylfuran (MF), a biofuel, via electrochemical hydrogenation and hydrogenolysis (ECH) reactions, respectively. Similarly, HMF may be upgraded to the hydrogenation product, 2,5-dihydroxymethylfuran (DHMF), a chemical intermediate, and the hydrogenolysis product, 2,5-dimethylfuran (DMF), a biofuel. While existing studies have shown the effects of various reaction parameters, such as electrode, electrolyte, applied potential, and temperature on reaction efficiency and product selectivity, the fundamental mechanism of the ECH reactions of furanic species is still not fully understood. This work seeks to uncover insights into the reaction pathway through bulk electrolysis ECH of furanic species.The ECH reactions were carried out via potentiostatic bulk electrolysis in an H-type cell with Cu flag working electrode in acidic electrolyte (0.5M H2SO4). The applied potential was varied from near ECH onset potential to more negative potentials in order to study the reaction in potential regimes where the competing hydrogen evolution reaction (HER) was expected to be insignificant, moderate, and highly competitive compared to ECH. The reaction was sampled at various times throughout the electrolysis; the concentration of HMF products was determined by HPLC and the concentration of FF products was determined by GCMS. We have shown that FF likely forms FA and MF in separate, parallel pathways. This is evidenced by the FF product distribution with reaction time: during 90 minutes of reaction time at -500mV versus RHE, the concentrations of both FA and MF steadily increased, indicating that FA is likely not an intermediate in a series reaction to MF. Further, when FA is used as the starting molecule for ECH, it is not converted to MF.