Zinc-Catalyzed Electrocatalytic Hydrogenation of Furfural at Neutral pH

Electrocatalytic hydrogenation (ECH) of biomass-derived platform chemicals is a sustainable way to produce value added fuels and chemicals as compared to chemo-catalytic hydrogenation pathway [1,2]. In this work, ECH of furfural was carried out to produce furfuryl alcohol (FAL) and 2-methylfuran (MF) which have applications in pharmaceutical, polymer and fuel industries [1-3]. In this work, Zn metal was used as a novel catalyst for the ECH of furfural to study its yield and Faradaic efficiency (FE) for the desired products. The activity of Zn metal was compared to the well-known catalysts such as Cu and Ni. The effect of electrolyte pH was also studied, and it was found to significantly affect FE and product profile. Potentiostatic electrolysis in acidic and basic pH electrolytes suffered from poor product yield and FE, either due to dominant hydrogen evolution reaction (HER) or degradative electrodimerization and polymerization reactions. Electrolysis at neutral pH (pH 6 to 8) exhibited increased yields and FE as compared to acidic and basic pH. We correlate this to optimum proton concentration in neutral electrolytes that would restrict HER in addition to providing a pH atmosphere that would minimize side reactions. At neutral pH, the reaction was more selective towards FAL formation than MF. The best activity of Zn catalyst was obtained with 0.5 M sodium bicarbonate (NaHCO 3 ) electrolyte (pH = 8.4) at -0.7 V/RHE with 77.5 % FE for FAL. The zinc catalyst displayed a total FE of 94 % for the ECH of furfural. At the same reaction conditions, the conversion, yield and FE for ECH with Zn was remarkably higher as compared to Cu and Ni metal electrodes. Zinc oxide precipitates were observed on the wire during electrolysis and were confirmed by EDS. The increased activity of Zn suggests a mechanistic role of adhered ZnO precipitates, possibly enhancing H adsorption and transfer to furfural. References: [1] Z. Li, S. Kelkar, C.H. Lam, K. Luczek, J.E. Jackson, D.J. Miller, C.M. Saffron, Aqueous electrocatalytic hydrogenation of furfural using a sacrificial anode, Electrochim. Acta. 64 (2012) 87–93. doi:10.1016/j.electacta.2011.12.105. [2] X.H. Chadderdon, D.J. Chadderdon, J.E. Matthiesen, Y. Qiu, J.M. Carraher, J.P. Tessonnier, W. Li, Mechanisms of Furfural Reduction on Metal Electrodes: Distinguishing Pathways for Selective Hydrogenation of Bioderived Oxygenates, J. Am. Chem. Soc. 139 (2017) 14120–14128. doi:10.1021/jacs.7b06331. [3] S. Jung, E.J. Biddinger, Electrocatalytic Hydrogenation and Hydrogenolysis of Furfural and the Impact of Homogeneous Side Reactions of Furanic Compounds in Acidic Electrolytes, ACS Sustain. Chem. Eng. 4 (2016) 6500–6508. doi:10.1021/acssuschemeng.6b01314. Figure 1