10 Progress in the Selective Semi-hydrogenation of Alkynes …
325
10.5.2 Structure of the Substrate
The structure of the substrate has proved to also define to some extent the alkene
selectivity. Geometric hindrance at the triple bond (internal vs. terminal) or electronic
properties (determined by the presence of donating or withdrawing groups) could
condition the adsorption strength of the substrate to the metal surface, thus influencing its propensity to be or not selectively hydrogenated. Furthermore, the interplay
between the substrate structure and the stabilizer properties (e.g. steric hindrance,
site isolation, etc.) determine the substrate-metal interaction, thus increasing the
complexity of the phenomena and preventing the identification of general patterns.
For instance, during the semi-hydrogenation of terminal alkynes, the hydrogenation of the triple bond is commonly slower than that of the alkene product (for Pt, Rh,
Ni, Pd) [136, 137]. Moreover, the hydrogenation of terminal alkenes proceeds faster
than that of internal ones due to accessibility constraints to the metal surface [91,
138]. Conversely, in a recent report, the semi-hydrogenation of 4-octyne catalysed by
a Pd-based catalyst proceeded faster than that of 1-octyne (full hydrogenation after 10
and 30 min, respectively, at 3 bar H 2 , 30 °C) [62]. Comparison of the reactivity with
an iso-structural internal alkyne (dimethyl acetylenedicarboxylate) suggested that the
presence of electron-withdrawing groups might result in an important decrease in
the hydrogenation rate (47% conversion after 3 h at 3 bar H 2 , 30 °C). This argument
could also explain the larger reaction rate observed for 4-octyne, which contains
a more electron rich triple bond than that of 1-octyne. A similar behaviour was
also reported by Corma et al., who evaluated the semi-hydrogenation of a series of
phenylacetylenes substituted in para positions catalysed by Ni 2 P NPs [3, 4]. The
lowest activity and selectivity was observed for the substrate provided with the most
electron-withdrawing NO 2 group (X = 3%, and S C=C = 43%). Another common
reactivity pattern is the observation of retarded hydrogenation rate when hydroxyl
groups are present in the alkyne structure [79]. For instance, based on experimental
evidence, Kelsen et al. proposed that the hydrogenation rate of Fe NPs is decreased by
the presence of hydroxyl groups in the alkynol substrate, a phenomena that favoured
the alkene selectivity [85].
10.5.3 Effect of the Use of Additives
The use of additives has been one of the traditional strategies to improve the alkene
selectivity in the semi-hydrogenation of alkynes. As a general view, a successful
additive should advantageously compete with the alkene product for active sites and
thus prevent the over-hydrogenation reaction. Its efficiency is normally a function of
their concentration in the reaction media and their adsorption properties at the NPs
surface versus those of reagents and products. The use of nitrogenated compounds
was early employed for this purpose. For instance, Brown et al. studied the effect of
a series of amine additives in the semi-hydrogenation of alkynes catalysed by P 2 Ni
325
10.5.2 Structure of the Substrate
The structure of the substrate has proved to also define to some extent the alkene
selectivity. Geometric hindrance at the triple bond (internal vs. terminal) or electronic
properties (determined by the presence of donating or withdrawing groups) could
condition the adsorption strength of the substrate to the metal surface, thus influencing its propensity to be or not selectively hydrogenated. Furthermore, the interplay
between the substrate structure and the stabilizer properties (e.g. steric hindrance,
site isolation, etc.) determine the substrate-metal interaction, thus increasing the
complexity of the phenomena and preventing the identification of general patterns.
For instance, during the semi-hydrogenation of terminal alkynes, the hydrogenation of the triple bond is commonly slower than that of the alkene product (for Pt, Rh,
Ni, Pd) [136, 137]. Moreover, the hydrogenation of terminal alkenes proceeds faster
than that of internal ones due to accessibility constraints to the metal surface [91,
138]. Conversely, in a recent report, the semi-hydrogenation of 4-octyne catalysed by
a Pd-based catalyst proceeded faster than that of 1-octyne (full hydrogenation after 10
and 30 min, respectively, at 3 bar H 2 , 30 °C) [62]. Comparison of the reactivity with
an iso-structural internal alkyne (dimethyl acetylenedicarboxylate) suggested that the
presence of electron-withdrawing groups might result in an important decrease in
the hydrogenation rate (47% conversion after 3 h at 3 bar H 2 , 30 °C). This argument
could also explain the larger reaction rate observed for 4-octyne, which contains
a more electron rich triple bond than that of 1-octyne. A similar behaviour was
also reported by Corma et al., who evaluated the semi-hydrogenation of a series of
phenylacetylenes substituted in para positions catalysed by Ni 2 P NPs [3, 4]. The
lowest activity and selectivity was observed for the substrate provided with the most
electron-withdrawing NO 2 group (X = 3%, and S C=C = 43%). Another common
reactivity pattern is the observation of retarded hydrogenation rate when hydroxyl
groups are present in the alkyne structure [79]. For instance, based on experimental
evidence, Kelsen et al. proposed that the hydrogenation rate of Fe NPs is decreased by
the presence of hydroxyl groups in the alkynol substrate, a phenomena that favoured
the alkene selectivity [85].
10.5.3 Effect of the Use of Additives
The use of additives has been one of the traditional strategies to improve the alkene
selectivity in the semi-hydrogenation of alkynes. As a general view, a successful
additive should advantageously compete with the alkene product for active sites and
thus prevent the over-hydrogenation reaction. Its efficiency is normally a function of
their concentration in the reaction media and their adsorption properties at the NPs
surface versus those of reagents and products. The use of nitrogenated compounds
was early employed for this purpose. For instance, Brown et al. studied the effect of
a series of amine additives in the semi-hydrogenation of alkynes catalysed by P 2 Ni