Silica nanospheres-encapsulated polymer ligands-bound Pd nanoparticles as highly efficient and selective catalysts for semi-hydrogenations of alkynes

Abstract

In this work, we report that poly (acrylic acid) (PAA) and poly (ethylenimine) (PEI) capped Pd nanoparticles (NPs) encaged in hollow silica nanospheres (PAA/PEI-Pd@HSNs) function as highly efficient and selective catalysts for hydrogenations of a series of alkynes. We used the coordination of Pd⁴⁺ ions with PEI and PAA to build the micelles system in an ethanol-water system, which was employed as the templates for silica deposition and following NaBH₄ reduction to give PAA/PEI-Pd@HSNs. The materials feature polymer-coordinated small Pd NPs inside silica nanospheres with thin shells and large cavities. The PAA/PEI-Pd@HSNs exhibit extremely high catalytic efficiency and reusability with enhanced alkene selectivity of ∼90 % at near complete conversions for hydrogenation of a series of alkynes, and maintain high selectivity even with a significantly extended reaction time. The enhanced catalytic performance of PAA/PEI-Pd@HSNs is ascribed to their thin silica shells/large cavities to improve the catalytic activity, PEI/PAA ligands that inhibit the deep hydrogenation of alkenes to alkanes on Pd NPs, and the protection of silica shells for inner ligands and Pd NPs to improve the reusability.