Monolithic wood-based carbon supported Pd nanoparticles with tunable exposure for boosting semi-hydrogenation of alkynols†

The selective hydrogenation of alkynols to enols catalyzed by carbon-supported Pd catalysts is a highly attractive reaction for the production of fine chemicals. However, the limited dispersion and stability of Pd species on the carbon support often hindered their catalytic activity. In this study, we developed a “carbon-layer anchored” method to synthesize Pd@NMC monolithic catalysts with tunable Pd exposure by the coating of nitrogen-containing polymer on the wood framework, followed by Pd loading, and carbonization procedures. The exposure of Pd nanoparticles can be controlled through a partial oxidation strategy. As a result, the semi-embedded Pd catalyst (Pd@NMC-250) exhibited superior activity and selectivity (91%) in the semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The reaction rate can reach 5110 mol mol_Pd⁻¹ h⁻¹, which is 4.5-fold higher than that of the commercial Lindlar catalyst under identical conditions (308 K, 5 bar H₂). Moreover, the monolithic catalyst presented good stability for enduring five cycles. The nitrogen species in the support and the carbon layer deposited on Pd nanoparticles during pyrolysis synergistically promote the Pd anchoring. After mild oxidation, the carbon layer is partially removed to form a semi-embedded Pd@C nanostructure, with the surface exposed and the bottom embedded on the carbon support, which enhances the stability. Furthermore, the monolithic wood-based carbon catalysts are easy to separate from the solution and thus have the potential for operating a variety of liquid-phase catalytic reactions beyond hydrogenation.