Surface palladium nanoparticles in ionic liquids modified with phosphorus ligands for enhanced catalytic semi-hydrogenation

Abstract

Thermodynamic stability of nanoparticles necessitates the use of stabilizing agents to provide steric and electronic protection. Nevertheless, their activity and selectivity remain suboptimal under moderate reaction conditions. In this study, we present high-performance Pd nanoparticles with a distinctive Pd-phosphate surface that is akin to a “quasi nano-frustrated Lewis pair” architecture, where electron donation from ionophilic phosphine species enhances the electron density of the Pd NPs. Solid state NMR and XPS analyses disclose the strong coordination of phosphine species on Pd NPs. DFT calculations reveals the geometry and conformations of the coordinated phosphine, where one of the phenyl rings is nearly parallel to the facets of the nanoparticle, such interaction occurs through the six carbon atoms of the π system. We investigate the structure-activity relationships (SARs) exhibited by these NPs in the efficient semi-hydrogenation of phenylacetylene (TOF = 3.85 s⁻¹), 2-cyclohexen-1-one (TOF = 0.8 s⁻¹), and 1,3-cyclohexadiene (TOF = 12.82 s⁻¹) at 40°C and 2-4 bar H₂ in BMIm.NTF2 ionic liquid. The higher activity and selectivity are related to; (i) the formation of ionic liquid cages/membrane around NPs akin to catalytic active membranes that tune the diffusion affinity of reactants, reactive intermediates, and products to catalytic active sites, and (ii) the hindrance provided by the Pd-P bonds.