Upgrading biomass derived furan aldehydes by coupled electrochemical conversion over silver-based electrocatalysts

A coupled electrolysis system with silver-based electrocatalyts was developed to achieve efficiently electro-oxidation of furfural and reduction of 5-hydroxymethylfurfural (HMF) to obtain furoic acid (2-FA) and 2,5-furandimethanol (BHMF), respectively. Ag2O was loaded on nickel foam (NF) by soaking in AgNO3 solution followed by calcination resulting in preparation of Ag2O@NF anode. Metallic Ag was loaded on copper foam (CuF) by electrodeposition obtaining Ag@CuF cathode. The prepared silver-based electrodes were then used in a flow electrolysis cell to achieve co-production of 2-FA and BHMF. At the optimal voltage of 1.7 V, 100 % furfural conversion with 98.9 % 2-FA yield and 100 % HMF conversion with 98.7 % BHMF yield were obtained on anode and cathode, respectively. The coupled flow electrolysis system achieved 150.6–188.7 % Faradaic efficiency (FE) depending on the reaction time and much higher added value based on consumption of per unit of electric energy, compared with the uncoupled system. Possible mechanisms for furfural oxidation and HMF reduction were proposed. It was found that the oxidation of furfural on Ag2O catalysts proceeded by two-electron and single-electron pathways, with the former being the major pathway, while in the later pathway hydrogen gas was simultaneously formed on the anode. The formation of BHMF might be achieved by electrocatalytic hydrogenation (ECH) mechanism or direct electro-reduction mechanism, while ECH mechanism seems to be more preferable. Hydrogen evolution reaction (HER) was the most important competing reaction on the cathode, leading to relatively low (20–80 %) FE for BHMF, which was greatly dependent on the voltage applied. However, the finding of this work may provide new inspiration on electricity-driven upgrading of biomass derived aldehydes to produce high value-added products.