Oxidation of biomass derived 5-hydroxymethylfurfural using heterogeneous and electrochemical catalysis

The concept of biorefineries involves multiple catalytic processes to convert biomass feedstock into valuable chemical products. While conventional catalytic biomass-related processes are thermally activated, the catalytic activation of biomass conversion reactions using an electrical interfacial potential (electrocatalysis) is new and poorly explored to date. Here, we report a comparative study of biomass catalysis using thermal heterogeneous and electrocatalysis in liquid-phase. First, the oxidation of the biomass-derived model molecule 5-hydroxymethyl-2-furfural (HMF) was studied using aqueous-phase heterogeneous catalysis under mild temperature (50 °C) and pressure (10 bar O2) conditions. Oxidation reactions were carried out in a semi-batch reactor to test catalytic activity of Pt/C catalyst and compared with Au/TiO2, Ru/C, Rh/C and Pd/C. The important reaction parameters such as influence of pH, effect of pressure and type of catalytic metal surface were explored. HMF degradation at higher pH in the absence of metal catalyst was discussed using in situ NMR study. At lower pH (≤7), the alcoholic group of HMF oxidizes faster than aldehyde on Pt surface, whereas at higher pH (≤13), oxidation of alcoholic group appears as the rate-limiting step. At given reaction conditions, Au shows better catalytic activity than Pt, Pd, Ru and Rh at pH 13. The heterogeneously catalyzed oxidation of HMF was then compared to the corresponding electrochemical oxidation catalysis. Electrochemical catalysis offers an added advantage by providing the electrode potential and the faradaic current as two additional external control parameters. These are helpful to tune the thermodynamic driving force, activation energy and thus the reaction rate and selectivity of complex reaction processes. The electrochemical activation of water at anodic electrode potentials results in the in situ generation of reactive oxygenated surface species from the aqueous solvent and thus eliminates the use of molecular oxygen. The electrocatalytic oxidation of HMF was found very selective for the formation of 2,5-furandicarbaldehyde.