M. Lim, H. Kim, Y. Jang, H. Rhee
Keywords: heterogeneous catalyst, Pd nanoparticle, green chemistry, Suzuki coupling, polymer-inorganic hybrid support
Summary:The development of heterogeneous metal catalysts supported on a solid support has attracted attention. This catalyst system has the advantages of ease of catalyst separation and reusability, and has higher reactivity due to the larger active surface with nano-sized metal. Previously, we developed two types of solid supports which are aminopropyl-functionalized reverse phase silica gel and thermoresponsive poly(N-isopropylacrylamide-co-4-vinylpyridine) (pNIPAM-4VP). These solid supports were applied to the syntheses of Pd, Au, Cu immobilized heterogeneous catalysts and various organic reactions such as hydrogenations, Suzuki-Miyaura couplings, Heck-Mizoroki couplings, Sonogashira couplings, Tsuji-Trost reactions, and A-3 coupling reactions, azide–alkyne cycloaddition reactions that were performed in water. In this study, polymer-inorganic hybrid supports by the combination of polymer and silica were synthesized and applied to organic reactions in water. The two polymers were grafted onto aminopropyl silica gel by the reversible addition−fragmentation chain-transfer (RAFT) polymerization. Poly(4-vinylpyridine), which is expected to hold the metal more tightly, was grafted onto silica gel and followed by Pd metal immobilization (Catalyst 1). Poly(N-isopropylacrylamide) (PNIPAM), which have hydrophilicity and hydrophobicity depending on temperature, was also grafted onto silica gel and followed by Pd metal immobilization (Catalyst 2). Various characterization techniques such as XPS(X-ray Photoelectron Spectroscopy), ICP(Inductively Coupled Plasma), SEM(Scanning Electron Microscopy), EDXA(Energy Dispersive X-ray Analysis), TEM(Transmission Electron Microscope) are used to verify the efficiency of the catalysts. Both catalysts have nano-sized Pd metal based on TEM image. The reactivity of these catalysts was tested on Suzuki-Miyaura coupling reactions in water. Results showed that PNIPAM grafted catalyst was more efficient than poly(4-vinylpyridine) grafted catalyst. Additionally, the recovered catalyst was successively subjected to the next runs of the coupling reaction under the same reaction conditions without considerable loss on its activity. Inductively Coupled Plasma (ICP) showed negligible leaching of the Pd metal into the solution. The study shall be seen in the light of the large interest in the development of a catalyst that can combine the benefits of heterogeneous catalysis and green chemistry. We believe the developed Pd catalyst capable enough to equally catalyze other organic reactions and transformations.