Electrocatalytic valorization of 5-hydroxymethylfurfural coupled with hydrogen production using tetraruthenium-containing polyoxometalate-based composites

Abstract

Electrocatalytic biomass valorization represents a promising route to produce sustainable and carbon-neutral products, especially when coupled with hydrogen production. Here we report a Ru₄/PEI-rGO electrocatalyst, prepared by electrostatic integration of a tetraruthenium-containing polyoxometalate, Rb₈K₂[Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]·25H₂O (Ru₄), with polyethyleneimine (PEI) modified rGO, which can efficiently electrocatalyze biomass-derived 5-hydroxymethylfurfural (HMF) to value-added furanic products simultaneously coupled with H₂ production in near neutral media. Under minimally optimized conditions, the conversion of HMF and the yield of H₂ reached 50.8% and 55.4 μmol after 6 hour electrocatalysis at 0.94 V vs. Ag/AgCl, respectively. 2,5-Diformylfuran (DFF) was detected as the dominant oxidation product with a selectivity of 66.2%. The faradaic efficiencies (FEs) of the anode and cathode reached 97.0 ± 3% and 100.2 ± 2%, respectively. In the presence of the 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) additive, the conversion of HMF reached 99.7%, and the distribution of the oxidation product has been greatly altered, wherein furanic carboxyl products are dominant. Various electrochemical characterization studies and spectrogram analyses, especially Fourier Transformed Alternating Current (FTAC) voltammetry, were employed to disclose that Ru^V centers were catalytically active species, and the three-electron oxidized Ru₄ species are responsible for the excellent electrocatalytic activity. Long-term electrocatalysis demonstrated that the present Ru₄/PEI-rGO composite possessed good stability for catalysis.