Reduced metal nanocatalysts for selective electrochemical hydrogenation of biomass-derived 5-(hydroxymethyl)furfural to 2,5-bis(hydroxymethyl)furan in ambient conditions

Abstract

Selective electrochemical hydrogenation (ECH) of biomass-derived unsaturated organic molecules has enormous potential for sustainable chemical production. However, an efficient catalyst is essential to perform an ECH reaction consisting of superior product selectivity and a higher conversion rate. Here, we examined the ECH performance of reduced metal nanostructures, i.e., reduced Ag (rAg) and reduced copper (rCu) prepared via electrochemical or thermal oxidation and electrochemical reduction process, respectively. Surface morphological analysis suggests the formation of nanocoral and entangled nanowire structure formation for rAg and rCu catalysts. rCu exhibits a slight enhancement in ECH reaction performance in comparison to the pristine Cu. However, the rAg exhibits more than two times higher ECH performance without compromising the selectivity for 5-(HydroxyMethyl) Furfural (HMF) to 2,5-bis(HydroxyMethyl)-Furan (BHMF) formation in comparison to the Ag film. Moreover, a similar ECH current density was recorded at a reduced working potential of 220 mV for rAg. This high performance of rAg is attributed to the formation of new catalytically active sites during the Ag oxidation and reduction processes. This study demonstrates that rAg can potentially be used for the ECH process with minimum energy consumption and a higher production rate.

Keywords: 2,5-bis(hydroxymethyl)furan (BHMF); 5-(hydroxymethyl)furfural (HMF); biomass; electrocatalysts; electrochemical hydrogenation; nanocoral Ag.

FIGURE 1

Schematic of ECH and HER: The protons can either produce hydrogen or it can be used for ECH reaction for HMF to BHMF formation (the simplest possible reaction pathway).

FIGURE 2

Physical analysis of catalysts: SEM image of (A) Ag after 12 h oxidation in 0.1 M KCl solution at 0.3 V versus RHE (in the inset pristine Ag) (B) Ag after reduction in 0.1 M KHCO₃ solution at 0.3 V versus RHE, (C) XRD analysis of pristine Ag oxidized Ag [AgCl (111), AgCl (220) and AgCl (222)] and reduced Ag, (D) Cu after thermal oxidation at 400°C for 1 h (in the inset pristine Cu), (E) Cu after electrochemical reduction in 0.1 M KHCO₃ solution at 0.3 V versus RHE. The scale bar is 5 µm for (A, B) and 2.5 µm for (D–F) XRD analysis of pristine Cu, oxidized Cu [ Cu₂O (110) Cu₂O (111) Cu₂O (200) Cu₂O (220) and Cu₂O (311)] and reduced Cu.

FIGURE 3

Electrochemical performance of the catalysts: LSV collected in buffer solution and buffer solution with 20 mM HMF solution for (A) Ag, rAg, (B) cu and rCu catalysts. (C) Comparison of current densities obtained at 0.6 V versus RHE for all catalysts in buffer and buffer with 20 mM HMF solution. (D) Tafel slope for Ag, rAg, Cu and rCu catalysts obtained from LSV collected in 20 mM HMF buffer solution. (E) CA experiments for Ag, rAg, Cu and rCu at 0.56 V versus RHE performed in 20 mM HMF buffer solution and (F) corresponding 1H NMR spectra for product identification.

FIGURE 4

Comparative ECH performance of the rAg and rCu catalysts: (A) chronoamperometry (CA) results collected in buffer solution with 20 mM HMF solution at different potentials for rAg. (B) Corresponding NMR spectra for the collected electrolyte after 30 min CA experiments. (C) BHMF NMR spectra peak (B3) to HMF NMR spectra peak (A1) ratio. (C) CA results collected in buffer solution with 20 mM HMF solution at different potentials for rCu. (D) Corresponding NMR spectra for the collected electrolyte after 30 min CA experiments. (E) BHMF NMR spectra peak (B3) to HMF NMR spectra peak (A1) ratio. The results confirm higher ECH activity of rAg catalysts.