Design and Synthesis of Yolk–Shell Nanostructured Silica Encapsulating Metal Nanoparticles and Aminopolymers for Selective Hydrogenation Reactions

Yolk–shell nanostructure consisting of catalytically active core materials encapsulated by hollow silicate materials has been regarded as a promising platform for the design of heterogeneous catalysts because of the enclosed void space useful for encapsulation and compartmentation of guest molecules and the outer silica shell acting as a physical barrier to protect them from the surrounding environment. In this chapter, the design and development of a new type of yolk–shell nanostructured silica composites encapsulating metal nanoparticles (NPs) and aminopolymer, poly(ethyleneimine) (PEI), inside the hollow silicas are described. The synthesis of yolk–shell structured silica-Pd NPs-PEI nanocomposites by a self-assembly approach using PEI as a template is described. Such a yolk–shell nanostructured catalyst shows high selectivity and reusability in the semihydrogenation of both internal and terminal alkynes to produce the corresponding alkenes, owing to the poisoning effect of PEI on Pd NP surface, as well as the ability of silica shell to prevent leaching/aggregation of the encapsulated components. Furthermore, a yolk–shell nanostructured catalyst encapsulating PdAg NPs together with PEI shows an excellent catalytic activity under moderate reaction conditions and reusability in the CO2 hydrogenation to produce formic acid, owing to the CO2 capturing ability of PEI and the protective effect of the silica shell. The synergistic interaction mechanisms of metal NPs and PEI within the limited nanospace of hollow silicas are also addressed. It is proposed that the design and synthesis of such nanocomposites with yolk–shell structures are beneficial (i) for creating a unique catalytic field with close proximity of active metal sites and PEI as a macroligand, thereby promoting the catalytic performances in terms of selectivity and activity, and (ii) for increasing the catalyst durability and reusability by inhibiting the leaching/aggregation of the encapsulated components under severe reaction conditions.