Polymers as NP Stabilizers

Polymers provide metal NP stabilization not only because of the steric bulk of their framework, but also by weak binding to the NP surface by the heteroatom, playing the role of ligands. Poly(N-vinyl-2-pyrrolidone) (PVP) is the most commonly used polymer for NP stabilization and catalysis, because it fulfils both steric and ligand requirements [5f. For instance Pt-, Pd- and RhNPs stabilized by PVP, are synthesized by refluxing ethanolic reduction of the corresponding metal halide and immobilized in an ionic liquid, l-n-butyl-3-methylimidazolium hexafluorophos-phate ([BMI][PF 5]), and are very efficient olefin and benzene hydrogenation catalysts at 40 °C that can be recycled without loss of activity (see Chart 1.1 for the two major polymer formulas used for NP catalysis) [12k]. 

Chart 1.1 Two major polymer families used as metal NP supports for catalysis 

A very important concept pioneered in the 1970s is that of catalysis using two different metals such as Au and Pd in the same NP [15]. This idea has been beautifully developed by Toshima s group who used PVP to stabUize core-shell bimetallic Au-PdNPs, i.e. for instance NPs in which the core is Au whereas Pd atoms are located on the shell (Fig. 1.6) [16]. Subsequent to co-reduction, this structure is controlled by the order of reduction potentials of both ions and the coordination abilities of both atoms to PVP. The location of Au in the core and Pd on the shell was demonstrated by EXAFS, and it was shown that such heterobimetallic 

Au-cored PdNPs are more active in catalysis than simple PVP-stabilized PdNPs. Thus, the Au core enhances the catalytic properties of PdNPs at the PdNP surface [5f,g]. Conversely, design strategies can lead to the opposite core-shell structure (Pd core, Au shell), and specific catalytic properties were obtained for methylacrylate hydrogenation [16]. 

Cyclohexene hydrogenation was catalyzed with PdNPs stabilized by highly branched amphiphilic polyglycerol (75% esterified with palmitoyl chloride) and this system was submitted to a continuously operating membrane reactor for recovery and recycling of the PdNP catalyst [16c]. 

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