A mechanistic switch in H₂ activation reverses selectivity in HMF hydrogenation over SACs

The selective hydrogenation of 5-hydroxymethylfurfural (HMF) to high-value chemicals is a crucial yet challenging reaction in biomass conversion. Herein, we systematically investigated the catalytic mechanisms of g-C₃N₄-supported Pd and Rh single-atom catalysts (SACs) for HMF hydrogenation, with a focus on the role of H₂ activation modes in reaction pathways and selectivity. For Pd@C₃N₄, in the first hydrogenation of HMF, the Pd site activates HMF and H₂ independently, preferentially hydrogenating the CO bond to yield BHMF via the Pd-mediated pathway. In the second hydrogenation of HMF, following BHMF formation, the catalytic system relies on the g-C₃N₄ support to heterolytically cleave H₂via the Pd/N-cooperative pathway. With the help of the by-product H₂O for H-transfer, the hydrogenation of the CC bond of BHMF toward BHMTHF is more favorable than the hydrogenation of the CH₂OH group. In contrast, Rh@C₃N₄ enables the homolytic cleavage of H₂ at the Rh center without the assistance of the g-C₃N₄ support and consistently favors the hydrogenation of the CH₂OH group to produce DMF. The divergence in selectivity between Pd@C₃N₄ and Rh@C₃N₄ originates from the distinct H₂ activation modes, particularly the accessibility of intermediate 16-the key species where H₂ undergoes homolytic cleavage at the metal center. The strong binding of the species in 16 makes the first H attack at the CC bond energetically unfavorable, which makes the second H attack the rate-determining step with a much higher barrier than that for the CH₂OH group's hydrogenation. This work reveals that the H₂ activation mode acts as a decisive mechanistic switch to reverse reaction selectivity. The proposed principle offers theoretical guidance for the rational design of high-efficiency single-atom catalysts toward selective HMF hydrogenation.