Earth-abundant Co nanoparticles encapsulated in N-doped hollow carbon sphere for highly selective hydrodeoxygenation of biomass-derived vanillin

Catalytic hydrodeoxygenation (HDO) of lignin-derived bio-oils to value-added biofuels is of great importance to efficient utilization of renewable biomass. Precisely tailoring the surface microenvironment of supported metal nanocatalysts plays a significant role in regulating the interaction between reactants and catalytic active sites and facilitates the acquiring of high-performance HDO catalysts for lignin upgrading. Herein, we report a step-economic synthesis of N-doped hollow carbon sphere encapsulated with Co nanoreactors (Co@NHCS-X, X: 700-1000) via the template-directed in-situ crystallization of Co-based ZIF-67 (zeolite imidazolate framework) and pyrolysis process. The optimized Co@NHCS-900 nanocatalyst presented a 100% conversion and 96% selectivity towards valuable 2-methoxy-4-methylphenol (MMP) biofuel through HDO of bio-oils-based vanillin. Such excellent catalytic performance was attributed to the specific structure properties and surface microenvi-ronment of Co@NHCS nanocatalysts. It was demonstrated that the electronic interaction between embedded Co nanoparticles and surface nitrogen species of NHCS nanoreactors constructed desired microenvironment and generated abundant surface Co active species, which could effectively enhance activation capabilities of re-actants and intermediate, and achieve highly selective HDO of vanillin to MMP via promoted adsorptions of C--O/C-O bonds as confirmed by intrinsic kinetic evaluations and density functional theory (DFT) calculations. Moreover, a two-step of hydrogenation/hydrogenolysis tandem reaction pathway of vanillin HDO to MMP and the corresponding catalytic reaction mechanism were proposed. Besides, the hierarchical porous structure of NHCS facilitated the dispersion and stabilization of encapsulated Co nanoparticles and endowed Co@NHCS-900 nanocatalyst with good stability in recycling tests. This work provided new insights into rational design of efficient and stable metal nanocatalysts for biomass conversion.