Solvent-Mediated Synthesis of Hierarchical MOFs and Derived Urchin-Like Pd@SC/HfO₂ with High Catalytic Activity and Stability

Carbon materials with hierarchical morphologies, pores, and compositions have attracted extraordinary attention due to their unique structural advantages and widespread applications. However, their controllable synthesis remains a grand challenge. Herein, a solvent-mediated strategy was demonstrated for the preparation of an urchin-like superstructure via modulating the hydrothermal condition (acetic acid/water ratio) of metal–organic frameworks (MOFs). The direct pyrolysis of a hierarchical NUS-6 precursor produced a well-defined carbon-based composite consisting of sulfur-doped carbon (SC) and HfO2 with an urchin-like morphology and micro-/mesoporosity, while the doped S sites and oxygen vacancies of HfO2 can help to anchor and activate Pd nanoparticles (NPs) through the strong host–guest interaction, which was further confirmed by the calculated results of the binding energy and differential charge density through density functional theory (DFT). The synthesized Pd@SC/HfO2 composite exhibited extremely high catalytic activity and stability toward the water-phase hydrodeoxygenation of vanillin (conversion >99%, selectivity >99%), as well as good universality for the hydrogenation of a series of unsaturated hydrocarbons in an aqueous system. Remarkably, the catalytic activity and structural stability of Pd@SC/HfO2 were largely maintained even after successive 10 cycles.