Regulating the oxidation state of Pd to enhance the selective hydrogenation for 5-hydroxymethylfurfural

The highly selective hydrogenation of 5-hydroxymethylfurfural to 2,5-dihydroxymethylfuran is an important reaction in the field of biomass hydrogenation, because it is a bridge between biomass resources and chemical industry. Here, we precisely constructed carbon nitride supported Pd-based catalysts by a simple impregnation-reduction method. By changing the reduction temperature, catalysts with different oxidation state could be precisely constructed. Moreover, the important correlation between the ratio of Pd⁰/Pd²⁺ and catalytic activity is revealed during the selective hydrogenation of HMF. The Pd/g–C₃N₄–300 catalyst with a Pd⁰/Pd²⁺ ratio of 3/2 showed the highest catalytic activity, which could get 96.9% 5-hydroxymethylfurfural conversion and 90.3% 2,5-dihydroxymethylfuran selectivity. Further density functional theory calculation revealed that the synergistic effect between Pd⁰ and Pd²⁺ in Pd/g–C₃N₄–300 system could boost the adsorption of the substrate and the dissociation of hydrogen. In this work, we highlight the important correlation between metal oxidation state and catalytic activity, which provides valuable insights for the rational design of precious metal catalysts for hydrogenation reactions.