The semihydrogenation of alkynes to alkenes is one of the most important and fundamental reactions for the synthesis of bulk and fine chemicals. Pd supported on poly(ethyleneimine) (Pd/PEI) and those supported on silica have recently been studied as catalysts, however, they still suffer from leaching of Pd NPs and PEI during catalytic cycles, resulting in a low selectivity for alkenes and decreased reusability of catalyst. In this study, we report hollow silica spheres encapsulating Pd nanoparticles (NPs) together with PEI inside the hollow cavities (Pd+PEI@HSS) act as stable catalyst for the semihydrogenation of alkynes. Pd+PEI@HSS was synthesized by a self-assembly approach using poly(ethyleneimine) (PEI) as a template. TEM images showed that Pd+PEI@HSS is composed of dispersed hollow spheres with an average particle size of ca. 160 nm containing Pd NPs and PEI in their void spaces (see Figure). The silica shell thickness was determined to be ca. 50 nm, and the average diameter of Pd NPs was determined to be ca. 9 nm. N2 adsorption measurements verified the existence of randomly-arranged micro-pores (ca. 1 nm) in the silica shell. In the semihydrogenation of diphenylacetylene, Pd+PEI@HSS afforded the corresponding alkene with more than 95% alkene selectivity (cis/trans = 95:5), which was far superior to those of Pd@HSS (without PEI) and unsupported Pd/PEI, demonstrating that the encapsulation of PEI inside the hollow silica drastically improves the alkene selectivity. This hybrid catalyst was also applicable to terminal alkynes, phenylacetylene, and was reusable at least 5 catalytic cycles without any significant loss of activity and selectivity because of the presence of protective silica shell.

Pd nanoparticles and aminopolymers encapsulated in hollow silica spheres as stable heterogeneous catalysts for semihydrogenation of alkynes

256th National Meeting (2018)

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