Selective hydrogenation of vanillin to vanillyl alcohol over Pd, Pt, and Au catalysts supported on an advanced nitrogen-containing carbon material produced from food waste

Abstract

Food waste worldwide has been threatening the environment and resulting in high carbon emissions. Although anaerobic fermentation is a potential solution transforming food waste into valuable chemicals, a large amount of the fermentation residue remains as solid waste. For closing the circular economy loop for food waste valorization, we proposed a thermo-chemical activation of a fermentation residue produced from a mixture of food waste and sludge to prepare a nitrogen-enriched, advanced carbon material. This could be used as a support to synthesize Pd/C, Pt/C and Au/C catalysts for the aqueous-phase hydrogenation of vanillin to vanillyl alcohol at mild reaction conditions. The produced alcohol increased sixfold the commercial value of the original aldehyde. The catalytic performance (based on the vanillin conversion and vanillyl alcohol yield) followed the order of Pd/C > Pt/C > Au/C. The Pd/C catalyst showed an excellent catalytic activity (>99% vanillin conversion and > 99% vanillyl alcohol selectivity) at optimized reaction conditions (i.e., 30 °C and 0.7 MPa H2 for 90 min, with 2 mmol vanillin/10 mg catalyst), along with high reusability and stability (up to four consecutive runs). These catalytic features outperformed those of a commercial Pd/C catalyst owing to: (i) high reduction degree and stabilization of the Pd particles on the carbon support, which accommodated a higher proportion of pyridinic than pyrrolic nitrogen, and (ii) rapid adsorption of the aldehyde group on the catalyst combined with rapid desorption of the newly formed hydroxymethyl group. This exceptional catalytic behavior was corroborated by efficient hydrogenation of other lignin-derived aromatic aldehydes, including p-hydroxybenzaldehyde and syringaldehyde, to their respective alcohols. Our results can bring a novel use for food waste in catalysis and represent a sustainable and efficient conversion of biomass into value-added chemicals and advanced materials.