Abstract
An insight into the nano- and micro-structural morphology of a polymer supported Pd catalyst employed in different catalytic reactions under green conditions is reported. The pre-catalyst was obtained by copolymerization of the metal-containing monomer Pd(AAEMA)2 [AAEMA− = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with ethyl methacrylate as co-monomer, and ethylene glycol dimethacrylate as cross-linker. This material was used in water for the Suzuki-Miyaura cross-coupling of aryl bromides, and for the reduction of nitroarenes and quinolines using NaBH4 or H2, as reductants. TEM analyses showed that in all cases the pristine Pd(II) species were reduced in situ to Pd(0), which formed metal nanoparticles (NPs, the real active species). The dependence of their average size (2–10 nm) and morphology on different parameters (temperature, reducing agent, presence of a phase transfer agent) is discussed. TEM and micro-IR analyses showed that the polymeric support retained its porosity and stability for several catalytic cycles in all reactions and Pd NPs did not aggregate after reuse. The metal nanoparticle distribution throughout the polymer matrix after several recycles provided precious information about the catalytic mechanism, which was truly heterogeneous in the hydrogenation reactions and of the so-called “release and catch” type in the Suzuki coupling.
Highlights
The value of nanoparticles (NPs) is one of the discoveries that has recently been developed to an extent that scientists could have never imagined a century ago, in the field of catalysis [1,2,3,4].The main problem arising with the use of metal nanoparticles as catalysts is their tendency to form agglomerates, which decreases their activity
A morphological study on Pd NPs supported onto a polymer matrix (Pd-pol) was carried out by Transmission electron microscopy (TEM), Energy Dispersive Spectroscopy (EDS) and micro-IR techniques
These analyses were performed on such material before, during and after its exposure to prolonged catalytic runs under green conditions in water solvent in the following organic reactions: the Suzuki-Miyaura coupling between aryl bromides and phenylboronic acid and the hydrogenation of nitroarenes and quinolines, using dihydrogen or NaBH4, as reducing agent
Summary
The value of nanoparticles (NPs) is one of the discoveries that has recently been developed to an extent that scientists could have never imagined a century ago, in the field of catalysis [1,2,3,4]. An alternative way to prepare highly dispersed polymer stabilized nanoparticles consists in the copolymerization of metal-containing monomers with suitable co-monomers [26], followed by reduction of the supported metal. In order to obtain a material with a uniform distribution of the catalytically active sites, the pre-catalyst was not synthetized by classical immobilization of palladium centres onto a prefabricated support, but it was synthetised in a rather unconventional way, by co-polymerization of the metal-containing monomer Pd(AAEMA)2 [AAEMA− = deprotonated form of 2-(acetoacetoxy)ethyl methacrylate] with a suitable co-monomer (ethyl methacrylate) and cross-linker (ethylene glycol dimethacrylate) (Pd-pol pre-catalyst, Scheme 1) [28,29].
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