Palladium Nanoparticles Captured in Microporous Polymers: A Tailor-Made Catalyst for Heterogeneous Carbon Cross-Coupling Reactions

摘要

A new strategy based on polymerization-induced phase separation (PIPS) techniques was proposed for fabricating palladium nanoparticles (PdNPs) captured in a microporous network polymer. Pd(OAc)_2 was premixed with a monomer having a poly(amidoamine)-based dendrimer ligand, and subsequently this was thermally polymerized with an excess amount of ethylene glycol dimethacrylate under PIPS conditions. In this system, the formation of PdNPs occurred concurrently with the polymer synthesis in a one-pot process, even with no additional reducing reagent. The resultant microporous polymer was found to have a mesoporosity; the nitrogen sorption analysis gave a specific-surface area of 511 m~2 g~(-1), an average pore diameter of 9.9 nm, and a total pore volume of 1.01 mL g~(-1). The TEM images of the polymer revealed that the created PdNPs were very small with a diameter of mainly ca. 2.0 nm; the high-resolution images were lattice-resolvable, showing the crystalline nature of the PdNPs (Pd(111) facets). Catalytic performances of the PdNP-containing microporous polymers were investigated for a heterogeneous Suzuki-Miyaura reaction of 4'-bromoacetophenone and phenylboronic acid in water. In the presence of 10~(-2) molar equiv of the polymer, the reaction efficiently proceeded at 80 ℃ and gave the desired product, 4-acetylbiphenyl, in >90% yield after 2 h. On the basis of the ICP-AES analysis, the Pd content released into the solution phase was estimated to be only 0.27% of the initial charge. Thereby, this polymer was successfully recovered by simple filtration and reused with only a minimal loss of activity (yield >90% even at the eighth run). When the catalytic reaction was examined with a low amount of the polymer catalyst, the turnover number (TON) reached 8.5 × 10~4 while maintaining a good yield. Finally, the dendrimer template effect of the polymer catalyst was discussed by referring to the catalytic performances of a control polymer prepared with nonintegrated ligand monomers.