To alloy or not to alloy? The unexpected power of Pd-Au catalyst physical mixtures in efficient HMF oxidation to FDCA

Bimetallic palladium (Pd) and gold (Au) systems are active for promoting the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a key building block for producing polyethylene furanoate, a bio-based polymer to substitute polyethylene terephthalate. Here, an FDCA yield of ~99% was achieved over a physical mixture of 1.5 wt.% Au/C and 1.5 wt.% Pd/C (Pd:Au molar ratio of 5:1) under mild conditions (90 °C, 1 bar O2), outperforming bimetallic core-shell Au@Pd/C (~90% FDCA yield) or alloyed AuPd/C (~73% FDCA yield) systems. To gain insights into the synergy between the two monometallic catalysts, a series of kinetic studies were conducted employing either HMF or its intermediates as the substrates in catalytic oxidation systems over either Pd/C or Au/C. The results show distinct selectivity preference of the two catalysts: Pd/C favours the 2,5-diformylfuran pathway (DFF), whilst Au/C follows the 5-hydroxymethyl-2-furancarboxylic acid (HFCA) pathway, as well as the presence of base-induced Cannizzaro disproportionation (CD) reactions. The advantage of the physical mixture system is largely attributed to synergy between the two metals, which promotes the DFF pathway (over the HFCA route) and suppresses CD reactions, facilitating a more rapid progression of the overall oxidation cascade process. Catalyst recycling studies reveal deactivation of the physical mixture system (FDCA yield dropped to 62% after 3 cycles), with detailed comparative characterization of the fresh and used catalysts identifying operando Pd leaching and subsequent deposition onto Au/C, forming a core (Au)-shell (Pd) structure, as the origin of the diminished activity. Our findings challenge the conventional view regarding the alloy superiority in the selective oxidation of HMF, showing that the systems based on simple physical mixtures of monometallic catalysts could be a more effective and practical strategy for progressing FDCA production via selective HMF oxidation.