10 Progress in the Selective Semi-hydrogenation of Alkynes …
331
acetylene) within the glass. Moreover, low activity towards oligomerization was
detected for this material thus suggesting high resistance against deactivation via
this pathway. This report highlights that encapsulation of the active phase could be
a promising strategy for the selective transformations of small molecules such as
acetylene. Extending this concept, Qin et al. proposed the utilization of a porous
TiO 2 /Pt/TiO 2 sandwich catalyst for the semi-hydrogenation of alkynes [166]. This
catalyst displayed enhanced alkene selectivity when compared with the uncovered
Pd catalyst, indicating once more that encapsulation is an efficient strategy for the
moderation of the reactivity of highly reactive (normally unselective) metal phases.
Similarly, Beller et al. reported the deposition of nitrogen-doped graphitic layers onto
the surface of CoNPs immobilized on silica (Co/phen@SiO 2 -800) and the evaluation
of the resulting catalyst in the semi-hydrogenation of alkynes [167]. Good to excellent
alkene selectivities were observed for both, internal and terminal alkynes.
More recently, Gong et al. reported the activation and spill-over of hydrogen
on sub-1 nm palladium nanoclusters confined within sodalite zeolite for the semihydrogenation of acetylene [168]. According to the authors, the small pore of the
sodalite is crucial as it only allows H 2 diffusion into the channels to reach the
encapsulated Pd nanoclusters and thus avoids over-hydrogenation to form ethane.
In an attempt to rationalize the effects of carbon supports in the semihydrogenation of acetylene catalysed by single atoms and small metal clusters,
Richards et al. reported the deposition of Pd on carbon nanotubes or carbon nanofibres [169]. Stabilization of the most finely dispersed palladium (at the atom scale)
was observed on carbon nanofibres with a stack structure in comparison with other
carbon supports. This catalyst demonstrated the highest selectivity in the semihydrogenation of acetylene to ethylene. In addition, the doping of carbon nanofibres
by nitrogen atoms resulted in the relative increase of the ethylene selectivity but with
a concomitant decrease of the activity.
10.5.6 External Parameters
In the semi-hydrogenation of alkynes, the selection of the adequate reaction conditions (P and T) play a fundamental role in the reaction kinetics, at the time of defining
the desired conversion level which is indeed directly linked with the alkene selectivity. Generation of excessive subsurface hydrides under relatively high hydrogen
pressures with concomitant over-hydrogenation consequences have been frequently
documented for Pd catalysts [133]. Conversely, catalysts based on Ag, Au and CeO 2 ,
with intrinsic lower hydrogen activation activity, normally require higher pressures
[158, 162].
Frequently underestimated, reaction parameters such as the mixing rate (relevant
for liquid phase) might importantly impact the reaction output, and a careful consideration should be taken at the beginning of experimentation. For instance, Bruehwiler
et al. investigated the reaction kinetics and their interplay with the mass transfer
for the semi-hydrogenation of 2-methyl-3-butyn-2-ol catalysed by Pd/CaCO 3 [170].
331
acetylene) within the glass. Moreover, low activity towards oligomerization was
detected for this material thus suggesting high resistance against deactivation via
this pathway. This report highlights that encapsulation of the active phase could be
a promising strategy for the selective transformations of small molecules such as
acetylene. Extending this concept, Qin et al. proposed the utilization of a porous
TiO 2 /Pt/TiO 2 sandwich catalyst for the semi-hydrogenation of alkynes [166]. This
catalyst displayed enhanced alkene selectivity when compared with the uncovered
Pd catalyst, indicating once more that encapsulation is an efficient strategy for the
moderation of the reactivity of highly reactive (normally unselective) metal phases.
Similarly, Beller et al. reported the deposition of nitrogen-doped graphitic layers onto
the surface of CoNPs immobilized on silica (Co/phen@SiO 2 -800) and the evaluation
of the resulting catalyst in the semi-hydrogenation of alkynes [167]. Good to excellent
alkene selectivities were observed for both, internal and terminal alkynes.
More recently, Gong et al. reported the activation and spill-over of hydrogen
on sub-1 nm palladium nanoclusters confined within sodalite zeolite for the semihydrogenation of acetylene [168]. According to the authors, the small pore of the
sodalite is crucial as it only allows H 2 diffusion into the channels to reach the
encapsulated Pd nanoclusters and thus avoids over-hydrogenation to form ethane.
In an attempt to rationalize the effects of carbon supports in the semihydrogenation of acetylene catalysed by single atoms and small metal clusters,
Richards et al. reported the deposition of Pd on carbon nanotubes or carbon nanofibres [169]. Stabilization of the most finely dispersed palladium (at the atom scale)
was observed on carbon nanofibres with a stack structure in comparison with other
carbon supports. This catalyst demonstrated the highest selectivity in the semihydrogenation of acetylene to ethylene. In addition, the doping of carbon nanofibres
by nitrogen atoms resulted in the relative increase of the ethylene selectivity but with
a concomitant decrease of the activity.
10.5.6 External Parameters
In the semi-hydrogenation of alkynes, the selection of the adequate reaction conditions (P and T) play a fundamental role in the reaction kinetics, at the time of defining
the desired conversion level which is indeed directly linked with the alkene selectivity. Generation of excessive subsurface hydrides under relatively high hydrogen
pressures with concomitant over-hydrogenation consequences have been frequently
documented for Pd catalysts [133]. Conversely, catalysts based on Ag, Au and CeO 2 ,
with intrinsic lower hydrogen activation activity, normally require higher pressures
[158, 162].
Frequently underestimated, reaction parameters such as the mixing rate (relevant
for liquid phase) might importantly impact the reaction output, and a careful consideration should be taken at the beginning of experimentation. For instance, Bruehwiler
et al. investigated the reaction kinetics and their interplay with the mass transfer
for the semi-hydrogenation of 2-methyl-3-butyn-2-ol catalysed by Pd/CaCO 3 [170].