Microenvironment engineering of nitrogen-doped hollow carbon spheres encapsulated with Pd catalysts for highly selective hydrodeoxygenation of biomass-derived vanillin in water

Development of efficient and stable metal catalysts for the selective aqueous phase hydrodeoxygenation (HDO) of biomass-derived oxygenates to value-added biofuels is highly desired. An innovative surface microenvironment modulation strategy was used to construct the nitrogen-doped hollow carbon sphere encapsulated with Pd (Pd@NHCS-X, X: 600–800) nanoreactors for catalytic HDO of biomass-derived vanillin in water. The specific surface microenvironments of Pd@NHCS catalysts including the electronic property of active Pd centers and the surface wettability and porous structure of NHCS supports could be well-controlled by the calcination temperature of catalysts. Intrinsic kinetic evaluations demonstrated that the Pd@NHCS-600 catalyst presented a high turnover frequency of 337.77 h^–1 and a low apparent activation energy of 18.63 kJ/mol. The excellent catalytic HDO performance was attributed to the unique surface microenvironment of Pd@NHCS catalyst based on structure-performance relationship analysis and DFT calculations. It revealed that pyridinic N species dominated the electronic property regulation of Pd sites through electronic metal-support interaction (EMSI) and produced numerous electron-rich active Pd centers, which not only intensified the dissociation and activation of H₂ molecules, but also substantially improved the activation capability of vanillin via the enhanced adsorption of –C=O group. The fine hydrophilicity and abundant porous structure promoted the uniform dispersion of catalyst and ensured the effective access of reactants to catalytic active centers in water. Additionally, the Pd@NHCS-600 catalyst exhibited excellent catalytic stability and broad substrate applicability for the selective aqueous phase HDO of various biomass-derived carbonyl compounds. The proposed surface microenvironment modulation strategy will provide a new consideration for the rational design of high- performance nitrogen-doped carbon-supported metal catalysts for catalytic biomass transformation.