10 Progress in the Selective Semi-hydrogenation of Alkynes …
323
surfactants and is the responsible of the dispersion of the NPs in aqueous media
[100]. In a subsequent study, the same authors reported the application of PdNPs using
chiral tetraalkyl ammonium phosphate stabilizers for the semi-hydrogenation of 3hexyn-1-ol [127]. No relevant differences in catalytic performance where observed
in comparison with HHDMA.
To understand the origin of the high alkene selectivity displayed by the Pd Nanoselect catalyst, Pérez-Ramírez et al. investigated the selectivity patterns and accessibility constraints by means of DFT calculations [128]. According to the computed
structure, the hydrogen phosphate groups of the HHDMA appear firmly adsorbed on
the Pd atoms. Such a configuration resulted in the isolation of Pd ensembles (which
can be considered a geometric effect of the stabilizer), and a reduction of the hydride
coverage (electronic effect, Fig. 10.12b). Additionally, the authors compared the
catalytic activity of terminal and internal alkynes, and proposed that the adsorbed
HHDMA and its distribution on the Pd surface create accessibility constrains which
condition the catalytic activity (Fig. 10.12b). These observations reflect the essential role of the stabilizer in the catalytic performance of M-NPs [128]. In a subsequent report, the same authors discussed the effect of the amount of stabilizer on the
performance of catalysts analogous to Pd Naloselect prepared with distinct HHDMA
contents (0.3–16.8 wt%) [129]. Higher activities were obtained at higher HHDMA
contents during the semi-hydrogenation of 1-hexyne, and with inputs from DFT, the
authors proposed that the configuration of the adsorbed HHDMA on the Pd NPs
highly depended on the stabilizer concentration: when the HHDMA concentration is
low, the surfactant lies flat on the metal surface thus blocking the accessibility of the
substrate to the active sites, while at higher surfactant concentrations, its configuration
is the one described previously (with cavities open to receive reagents).
More recently, Scarso et al. reported a method for the preparation of Pd nanoparticles in aqueous medium stabilized by anionic sulfonated surfactants [130]. The
nanoparticles were prepared from Pd(OAc) 2 solutions in the presence of anionic
surfactant and using hydrogen gas as the reducing agent. The aqueous PdNP suspensions were tested in the dechlorination of aromatic substrates, hydrogenation and
dihydroxylation of carbonyl groups and semi-hydrogenation of alkynes. In all cases,
the micellar medium was crucial for stabilizing the metal nanoparticles, dissolving
substrates, steering product selectivity and enabling the recycling.
Regarding the semi-hydrogenation of acetylene catalysed by colloidal NPs or
supported catalysts prepared through colloidal approaches, scarce reports are identified in the literature. Ji et al. reported the preparation of water-soluble Pd NPs stabilized by three polymers, hydroxyethyl cellulose (HEC), polyquaternium-10 (quaternized hydroxyethyl cellulose) and sodium carboxymethyl cellulose (CMC) and
its application in the acetylene semi-hydrogenation [131]. From the tested polymers, the NPs stabilized by CMC evidenced the highest performance of the series
(50% selectivity at full conversion). Kiwi-Minsker et al. investigated the effect of
the stabilizer on the performance of carbon nanofibres supported PdNPs during the
semi-hydrogenation of acetylene [39]. In this study, polyvinylalcohol (PVA), sodium
di-2-ethylhexylsulfosuccinate (AOT) and polyvinyl pyrrolidone (PVP) were evaluated as stabilizers. Differences in activity were justified by geometric and electronic
323
surfactants and is the responsible of the dispersion of the NPs in aqueous media
[100]. In a subsequent study, the same authors reported the application of PdNPs using
chiral tetraalkyl ammonium phosphate stabilizers for the semi-hydrogenation of 3hexyn-1-ol [127]. No relevant differences in catalytic performance where observed
in comparison with HHDMA.
To understand the origin of the high alkene selectivity displayed by the Pd Nanoselect catalyst, Pérez-Ramírez et al. investigated the selectivity patterns and accessibility constraints by means of DFT calculations [128]. According to the computed
structure, the hydrogen phosphate groups of the HHDMA appear firmly adsorbed on
the Pd atoms. Such a configuration resulted in the isolation of Pd ensembles (which
can be considered a geometric effect of the stabilizer), and a reduction of the hydride
coverage (electronic effect, Fig. 10.12b). Additionally, the authors compared the
catalytic activity of terminal and internal alkynes, and proposed that the adsorbed
HHDMA and its distribution on the Pd surface create accessibility constrains which
condition the catalytic activity (Fig. 10.12b). These observations reflect the essential role of the stabilizer in the catalytic performance of M-NPs [128]. In a subsequent report, the same authors discussed the effect of the amount of stabilizer on the
performance of catalysts analogous to Pd Naloselect prepared with distinct HHDMA
contents (0.3–16.8 wt%) [129]. Higher activities were obtained at higher HHDMA
contents during the semi-hydrogenation of 1-hexyne, and with inputs from DFT, the
authors proposed that the configuration of the adsorbed HHDMA on the Pd NPs
highly depended on the stabilizer concentration: when the HHDMA concentration is
low, the surfactant lies flat on the metal surface thus blocking the accessibility of the
substrate to the active sites, while at higher surfactant concentrations, its configuration
is the one described previously (with cavities open to receive reagents).
More recently, Scarso et al. reported a method for the preparation of Pd nanoparticles in aqueous medium stabilized by anionic sulfonated surfactants [130]. The
nanoparticles were prepared from Pd(OAc) 2 solutions in the presence of anionic
surfactant and using hydrogen gas as the reducing agent. The aqueous PdNP suspensions were tested in the dechlorination of aromatic substrates, hydrogenation and
dihydroxylation of carbonyl groups and semi-hydrogenation of alkynes. In all cases,
the micellar medium was crucial for stabilizing the metal nanoparticles, dissolving
substrates, steering product selectivity and enabling the recycling.
Regarding the semi-hydrogenation of acetylene catalysed by colloidal NPs or
supported catalysts prepared through colloidal approaches, scarce reports are identified in the literature. Ji et al. reported the preparation of water-soluble Pd NPs stabilized by three polymers, hydroxyethyl cellulose (HEC), polyquaternium-10 (quaternized hydroxyethyl cellulose) and sodium carboxymethyl cellulose (CMC) and
its application in the acetylene semi-hydrogenation [131]. From the tested polymers, the NPs stabilized by CMC evidenced the highest performance of the series
(50% selectivity at full conversion). Kiwi-Minsker et al. investigated the effect of
the stabilizer on the performance of carbon nanofibres supported PdNPs during the
semi-hydrogenation of acetylene [39]. In this study, polyvinylalcohol (PVA), sodium
di-2-ethylhexylsulfosuccinate (AOT) and polyvinyl pyrrolidone (PVP) were evaluated as stabilizers. Differences in activity were justified by geometric and electronic