Mechanistic Insights into the Aerobic Oxidation of 2,5-Bis(hydroxymethyl)furfural to 2,5-Furandicarboxylic Acid on Pd Catalysts

2,5-Furandicarboxylic acid (FDCA) is one of the top selected value-added chemicals, which can be obtained by the aerobic oxidation of 2,5-bis(hydroxymethyl)furfural (BHMF) over a Pd-based catalyst. However, the elucidation of the reaction mechanism was hindered by its rapid kinetics. Herein, employing the density functional theory (DFT) calculations, we delve into the detailed reaction pathways of the BHMF oxidation into FDCA over Pd(111) and PdH x (111) identifying the rate-determining steps. The results demonstrated that 2,5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA) are the important intermediates, while the oxidation of FFCA is the rate-limiting step with the energy barrier of 0.68 and 0.51 eV on Pd(111) and PdH1/4(111), respectively. By comparison of the d-band center of Pd(111) and PdH x (111) surfaces and the overall energy barrier of this reaction over these two surfaces, it makes clear that the occupation of H atoms in the Pd bulk changes the surface electronic structures and enhances the binding energy of BHMF with the PdH x surface, which consequently speeds up the conversion of BHMF into FDCA. Water plays a crucial role in facilitating the activation of O2 via the H-transfer by constructing a hydrogen-bonding chain with the O2 and OH*. The activation of molecular oxygen experiences enhancement through the synergy of OH* and H2O, resulting in the production of actomic oxygen (O*). Both O* and OH* actively participate in the BHMF oxidation, where O* improved the activation toward initial critical reaction pathways on Pd(111) while OH* exhibited its pronounced impact on the latter two key processes on both Pd(111) and PdH1/4(111). This study will contribute to well understanding the oxidation mechanism of BHMF into FDCA over Pd-based catalysts and establish a theoretical framework for the potential development of an effective catalyst.