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J. A. Delgado and C. Godard
[103] NPs and their evaluation in the semi-hydrogenation of alkynols. Interestingly,
catalysts prepared in the presence of PEG were ca. 4 times more active compared to
those prepared in absence of the polymer.
If amines have been the classical additives for the moderation of the reactivity of
palladium catalysts, the incorporation of amine groups in the structure of polymer
stabilizer could play the double role of stabilizing the NPs and enhancing the
alkene selectivity. Following this approach, Long et al. reported the preparation
of PdNPs supported on a mesoporous silica material functionalized with branched
poly(ethyleneimine) polymers (PEI) composite and its application in the selective
hydrogenation of alkynes. Interestingly, the over-hydrogenation rate was significantly reduced by increasing the support porosity and the molecular weight and
branching structure of the polymer [104]. More recently, Yamashita et al. reported
the preparation of a yolk-shell nanostructured composite composed of Pd NPs stabilized by PEI, confined in hollow silica spheres and their application in the semihydrogenation of alkynes [105]. Using a one-pot method, the yolk-shell nanostructured Pd-PEI-silica composite (Pd + PEI@HSS) was fabricated as a Pd core of
ca. 5–9 nm in diameter and a porous silica shell of ca. 30–50 nm thickness. This
composite material provided a high alkene selectivity in the semi-hydrogenation
of phenylacetylene to styrene due to the strong poisoning effect of amine groups
of the PEI. Using a novel approach, Studer et al. reported a facile light-mediated
preparation of small polymer-coated Pd NPs and their application as catalysts for
alkyne semi-hydrogenation [106, 107]. The photoactive polymers acted as reagents
for the photochemical reduction of Pd ions and as stabilizers for the Pd NPs generated
in situ. These materials revealed efficient hydrogenation catalysts with high activity
and Z-selectivity in the semi-hydrogenation of alkynes.
Very recently, Xinwu et al. reported the preparation of sulphur-containing
polymer-supported palladium nanoparticles (Pd/SPMB) using cross-linked
poly(N,N-methylene bis(acrylamide)) (SPMB) as platform and stabilizer of the NPs
[76]. This material exhibited excellent activity, stability, and selectivity in the semihydrogenation of both internal and terminal alkynes. Notably, the selectivity enhancement towards alkene was attributed to the coverage of sulphide and thiolate on the
Pd surface with the appropriate degree of poisoning.
Micelles are another family of widely applied stabilizers of M-NPs. Depending
on the solvent´s polarity, the structure of the amphiphilic compound and the metallic
precursor, it is possible to tune the hydrophobic or hydrophilic environment of the
miscelle´s core that embed the M-NPs. For instance, the semi-hydrogenation of 2butyne-1,4-diol catalysed by PdNPs stabilized in the micelle core of poly(ethyleneoxide)-block-poly-2-vinylpiridine (PEO-b-P2VP) was reported by Kiwi-Minsker
et al. [108]. The structural characteristics of the amphiphilic co-polymer permitted
the stabilization of PdNPs by the pyridine units inside the core of the micelle, while
the PEO chains constituting the shell were accountable of its dispersion in aqueous
media (Fig. 10.9). The high selectivity was ascribed to a modification of the Pd surface
with the 2-vinylpyridine units in the micelle´s core. Using a simple ultra-filtration
procedure, the authors demonstrated the recyclability of the micelles.
J. A. Delgado and C. Godard
[103] NPs and their evaluation in the semi-hydrogenation of alkynols. Interestingly,
catalysts prepared in the presence of PEG were ca. 4 times more active compared to
those prepared in absence of the polymer.
If amines have been the classical additives for the moderation of the reactivity of
palladium catalysts, the incorporation of amine groups in the structure of polymer
stabilizer could play the double role of stabilizing the NPs and enhancing the
alkene selectivity. Following this approach, Long et al. reported the preparation
of PdNPs supported on a mesoporous silica material functionalized with branched
poly(ethyleneimine) polymers (PEI) composite and its application in the selective
hydrogenation of alkynes. Interestingly, the over-hydrogenation rate was significantly reduced by increasing the support porosity and the molecular weight and
branching structure of the polymer [104]. More recently, Yamashita et al. reported
the preparation of a yolk-shell nanostructured composite composed of Pd NPs stabilized by PEI, confined in hollow silica spheres and their application in the semihydrogenation of alkynes [105]. Using a one-pot method, the yolk-shell nanostructured Pd-PEI-silica composite (Pd + PEI@HSS) was fabricated as a Pd core of
ca. 5–9 nm in diameter and a porous silica shell of ca. 30–50 nm thickness. This
composite material provided a high alkene selectivity in the semi-hydrogenation
of phenylacetylene to styrene due to the strong poisoning effect of amine groups
of the PEI. Using a novel approach, Studer et al. reported a facile light-mediated
preparation of small polymer-coated Pd NPs and their application as catalysts for
alkyne semi-hydrogenation [106, 107]. The photoactive polymers acted as reagents
for the photochemical reduction of Pd ions and as stabilizers for the Pd NPs generated
in situ. These materials revealed efficient hydrogenation catalysts with high activity
and Z-selectivity in the semi-hydrogenation of alkynes.
Very recently, Xinwu et al. reported the preparation of sulphur-containing
polymer-supported palladium nanoparticles (Pd/SPMB) using cross-linked
poly(N,N-methylene bis(acrylamide)) (SPMB) as platform and stabilizer of the NPs
[76]. This material exhibited excellent activity, stability, and selectivity in the semihydrogenation of both internal and terminal alkynes. Notably, the selectivity enhancement towards alkene was attributed to the coverage of sulphide and thiolate on the
Pd surface with the appropriate degree of poisoning.
Micelles are another family of widely applied stabilizers of M-NPs. Depending
on the solvent´s polarity, the structure of the amphiphilic compound and the metallic
precursor, it is possible to tune the hydrophobic or hydrophilic environment of the
miscelle´s core that embed the M-NPs. For instance, the semi-hydrogenation of 2butyne-1,4-diol catalysed by PdNPs stabilized in the micelle core of poly(ethyleneoxide)-block-poly-2-vinylpiridine (PEO-b-P2VP) was reported by Kiwi-Minsker
et al. [108]. The structural characteristics of the amphiphilic co-polymer permitted
the stabilization of PdNPs by the pyridine units inside the core of the micelle, while
the PEO chains constituting the shell were accountable of its dispersion in aqueous
media (Fig. 10.9). The high selectivity was ascribed to a modification of the Pd surface
with the 2-vinylpyridine units in the micelle´s core. Using a simple ultra-filtration
procedure, the authors demonstrated the recyclability of the micelles.