Water compatible Pd nanoparticlecatalysts supported on microporous polymers: their controllable microstructure and extremely low Pd-leaching behaviour

Abstract

This paper reports on fabrication of water-compatible palladium nanoparticle (PdNP) catalysts supported on microporous polymers, which show high activity and extremely low Pd leaching behaviour in the usage for carbon cross-coupling reactions. The supported PdNP catalysts were fabricated by utilizing the polymerization-induced phase separation (PIPS) method. An outstanding point of the present study is to use a small molecule ligand monomer (D-m) based on N,N-dimethyl ethylenediamine, instead of the PAMAM dendrimer-based monomer used previously having integrated ligand sites. When D-m that had been complexed with Pd²⁺ was copolymerized with excess 1,6-hexanediol dimethacrylate (HX) in the PIPS setup, a highly active catalytic polymer (D-HX) having extremely low Pd leaching characteristics was successfully obtained. The TEM images of D-HX revealed that the created PdNPs were small with a diameter of mainly ca. 5.0 nm. Catalytic performances of D-HX were investigated for a heterogeneous Suzuki–Miyaura reaction in water. In the presence of 10⁻² molar equiv. of the polymer, the reaction efficiently proceeded at 80 °C and gave the desired product in ca. 90% yield after 2 h. D-HX could be easily recovered by simple filtration and reused with only a minimal loss of activity. Intriguingly, the hot filtration test indicated that nearly no Pd species was thermally released from D-HX, which is in large contrast with the results of other polymers tested herein leading to a lot of Pd leaching. Through this study, it was demonstrated that the microscopic structure of the polymer supports could be well-controlled by changing the structure of monomers and it was closely related to the Pd leaching behaviour of the present heterogeneous catalysis systems.