245
electron microscopy, and the corresponding images clearly showed two different
types of nanoparticle organization.
Thus, for the Rh nanoparticles stabilized by the polymer containing cyclodextrins, a mono and well-dispersed size distribution was clearly obtained, whereas
those prepared from the cyclodextrin-free polycarboxylate (control polymer) was
organized into non-ordered superstructures, in which the nanoparticles were
entrapped in string-like assemblies with a bigger average particle size. The catalytic
properties were evaluated in the hydrogenation of various alkene and arene derivatives. For instance, in the case of 1-tetradecene, the turnover frequency was equal to
2000 h
−1
and 420 h
−1
for the hydrogenation reaction catalyzed by Rh nanoparticles
stabilized by polyCOONa-β-cyclodextrin and polyCOONa, respectively. These
results were correlated with both the average particle size and the dispersion of the
rhodium nanoparticles stabilized by the above stabilizers. In order to have a better
insight into the beneficial effect of the structure of the polymer, an additional experiment has been performed using a physical mixture, i.e., adding the same amount of
β-cyclodextrin in the polycarboxylate solution. The turnover frequency was very
close to the value of the polycarboxylate alone, but the colloidal suspension after the
catalytic test was unstable. All these experiments undoubtedly supported the view
that the β-cyclodextrin covalently linked to the polymer chain induced a significant
effect in terms of activity and reusability. The recyclability was studied by reusing
the aqueous catalytic layer during five successive hydrogenation runs of 1- tetradecene
with no loss of stability and activity. The rhodium leaching in the organic phase of
each catalytic test was very low (<0.2 ppm), while the transmission electron microscopy experiments confirmed the robustness of the colloidal suspensions, with no
change in terms of particle size and morphology.
A deeper study on the stability and the catalytic activity of the rhodium nanoparticles was performed by modifying the pH value of the solution before the reduction
step and by using different ratios of grafted cyclodextrin (Noël et al. 2014).
According to transmission electron microscopy experiments, homogeneous dispersions of the metal nanoparticles were observed for the lowest initial pH value (5.1),
and, on the contrary, unstable colloids were observed for highest pH value (7.7). It
seemed that there was no link between the mean particle size and the amount of
grafted cyclodextrin onto the polymer backbone. The catalytic activities were evaluated in the hydrogenation of methyl linoleate under 10 bar of hydrogen at
30 °C. Several Rh nanoparticles called controls were also synthesized (Rh nanoparticles stabilized by a β-cyclodextrin and polycarboxylate mixture with a ratio corresponding to the grafted polycarboxylates). According to the catalytic results, the
grafted polycarboxylates gave higher activities, and, in terms of recovery, the colloids with the physical mixture showed stable emulsions which led to nonrecoverable catalytic systems.
Herbois et al. (2015) reported another work which consisted into the encapsulation of fine water-dispersible Ru nanoparticles (with size of ~1.8 nm) into 3D
β-cyclodextrin-based polymer (poly(CTR-β-cyclodextrin) derived from controlled
polycondensation of β-cyclodextrin with citric acid). The synthetic procedure
included reduction of Ru precursor, ruthenium nitrosyl nitrate, by NaBH 4 in the
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications
electron microscopy, and the corresponding images clearly showed two different
types of nanoparticle organization.
Thus, for the Rh nanoparticles stabilized by the polymer containing cyclodextrins, a mono and well-dispersed size distribution was clearly obtained, whereas
those prepared from the cyclodextrin-free polycarboxylate (control polymer) was
organized into non-ordered superstructures, in which the nanoparticles were
entrapped in string-like assemblies with a bigger average particle size. The catalytic
properties were evaluated in the hydrogenation of various alkene and arene derivatives. For instance, in the case of 1-tetradecene, the turnover frequency was equal to
2000 h
−1
and 420 h
−1
for the hydrogenation reaction catalyzed by Rh nanoparticles
stabilized by polyCOONa-β-cyclodextrin and polyCOONa, respectively. These
results were correlated with both the average particle size and the dispersion of the
rhodium nanoparticles stabilized by the above stabilizers. In order to have a better
insight into the beneficial effect of the structure of the polymer, an additional experiment has been performed using a physical mixture, i.e., adding the same amount of
β-cyclodextrin in the polycarboxylate solution. The turnover frequency was very
close to the value of the polycarboxylate alone, but the colloidal suspension after the
catalytic test was unstable. All these experiments undoubtedly supported the view
that the β-cyclodextrin covalently linked to the polymer chain induced a significant
effect in terms of activity and reusability. The recyclability was studied by reusing
the aqueous catalytic layer during five successive hydrogenation runs of 1- tetradecene
with no loss of stability and activity. The rhodium leaching in the organic phase of
each catalytic test was very low (<0.2 ppm), while the transmission electron microscopy experiments confirmed the robustness of the colloidal suspensions, with no
change in terms of particle size and morphology.
A deeper study on the stability and the catalytic activity of the rhodium nanoparticles was performed by modifying the pH value of the solution before the reduction
step and by using different ratios of grafted cyclodextrin (Noël et al. 2014).
According to transmission electron microscopy experiments, homogeneous dispersions of the metal nanoparticles were observed for the lowest initial pH value (5.1),
and, on the contrary, unstable colloids were observed for highest pH value (7.7). It
seemed that there was no link between the mean particle size and the amount of
grafted cyclodextrin onto the polymer backbone. The catalytic activities were evaluated in the hydrogenation of methyl linoleate under 10 bar of hydrogen at
30 °C. Several Rh nanoparticles called controls were also synthesized (Rh nanoparticles stabilized by a β-cyclodextrin and polycarboxylate mixture with a ratio corresponding to the grafted polycarboxylates). According to the catalytic results, the
grafted polycarboxylates gave higher activities, and, in terms of recovery, the colloids with the physical mixture showed stable emulsions which led to nonrecoverable catalytic systems.
Herbois et al. (2015) reported another work which consisted into the encapsulation of fine water-dispersible Ru nanoparticles (with size of ~1.8 nm) into 3D
β-cyclodextrin-based polymer (poly(CTR-β-cyclodextrin) derived from controlled
polycondensation of β-cyclodextrin with citric acid). The synthetic procedure
included reduction of Ru precursor, ruthenium nitrosyl nitrate, by NaBH 4 in the
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications