Furfural electrovalorisation using single-atom molecular catalysts

The electrochemical conversion of biomass to high-value platform chemicals as a potential means to reduce our reliance on crude oil offers environmental advantages, in comparison to thermal catalysis as it takes place at room temperature and pressure. In this study, we investigate the electrochemical reduction of a biomass-derived chemical, furfural, using Cu and Co single-atom catalysts supported on carbon electrodes via non-covalent adsorption. Under mild basic electrolyte (pH 10) we have selectively produced hydrofuroin, a valuable and promising precursor to sustainable drop-in jet fuels. Density functional theory (DFT) calculations were carried out to unveil that the weak-binding molecular catalysts could give rise to a facile reaction path towards the electrochemical production of hydrofuroin. The calculated adsorption energies of furfural and hydrogen on different types of catalysts aid in mapping the experimental selectivity for furfural electroreduction, offering a general catalyst design blueprint for this reaction. Based on theoretical calculations, we tested Co and Cu phthalocyanines (Pc) adsorbed onto multi-walled carbon nanotubes (MWCNTs) to show that these single-atom molecular catalysts can display up to 65.3% faradaic efficiency for hydrofuroin production with low amounts of hydrogen evolution in pH 10 at -0.50 V vs. RHE. The stability of CoPc and CuPc towards furfural reduction is probed by time-of-flight secondary ion mass spectrometry (ToF-SIMS), where CoPc is found to be more stable with only 16% reduction in normalised counts, compared to 67% for CuPc. Mechanistically, we suggest that the rate-determining step for hydrofuroin formation on single-atom molecular catalysts should be the first proton-coupled electron transfer prior to radical coupling, which could possibly happen in solution. Our work provides new insights into utilizing single-atom catalysts for the biomass electrovalorisation to value-added fuels and chemicals.