PEGStabilized Metal Nanoparticles

Metal nanoparticles, and in particular Pd-nanoparticles (also called Pd-colloids), are stabilized by the presence of, among others, ammonium

salts, surfactants, PEGs, ' polysiloxane, and organic thiol mono layers.39 The highly polar ethylene glycol was shown earlier to be a catalyst-philic phase for SAPC. Analogously, polyethylene glycols are an attractive polar and high-boiling class of compounds that can be considered catalyst-philic in a number of instances, and that can be coupled with a second immiscible phase.

An L_L_S system based on PEG, is, for example, the one described by Leitner and co-workers, where the ability of PEGs to stabilize dispersed Pd-nanoparticles was coupled with the use of scCO2. The model reaction that was investigated in this system was the oxidation of alcohols with oxygen in the presence of Pd-clusters of structure [Pd561-phen60(OAc)180] (phen = 1,10-phenanthroline) (Figure 6.14).40

The performance of the highly dispersed Pd-clusters—in the oxidation of the alcohols in scCO2—when embedded in a PEG-1000 matrix was particularly good and showed high selectivity (>99%), with no overoxidation observed. PEG-1000 is a solid at room temperature, but became a liquid under the investigated conditions, wherein the catalyst was well dispersed. A 2-h induction period was observed, but once the reaction was terminated, the products could be extracted quantitatively with CO2 (80°C, 14.5 MPa), and the catalyst-philic PEG phase remained in the reactor. Negligible amounts of PEG were extracted, and a very low contamination of the product by Pd was observed (<2.3 ppm). The PEG_Pd catalytic ensemble could be used repeatedly without loss of activity, and without the induction period of the first run. Control experiments showed that the activity and selectivity of the catalytic system were higher than those with conventional heterogeneous catalysts.

PEG-1000

Figure 6.14 Selective oxidation using PEG-stabilized Pd-nanoparticles.

PEG-1000

Figure 6.14 Selective oxidation using PEG-stabilized Pd-nanoparticles.

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