304
J. A. Delgado and C. Godard
over-hydrogenation or oligomerization of the alkyne substrate [5, 6]. When referred
to catalysts employed in the semi-hydrogenation of alkynes, a wide variety of transition metals has been employed, covering a wide range of particle sizes, most of them
classified as either metal nanoparticles (M-NPs), metal nanoclusters (M-Clusters)
and single-atom catalysts (SACs) (Scheme 10.1) [7].
Metals that easily activate the hydrogen molecule (e.g. Pt, Ni) normally display a
high over-hydrogenation activity and suffer of poor control of the alkene selectivity.
Conversely, metals with low hydrogen activation activity (e.g. Au, Ag) frequently
display good alkene selectivities but require harsh reaction conditions in terms of both
temperature and pressure [8]. Palladium appears as the metal that apparently meets
the right balance towards hydrogen activation and outstanding activity under mild
conditions, but its control of the selectivity when approaching full alkyne conversion
is extremely poor [9].
In terms of catalyst preparation, solid supports such as oxides (SiO 2 , Al 2 O 3 , TiO 2 ),
CaCO 3 and carbon-based materials (carbon nanofibres or nanotubes, activated C,
graphite) have been classically employed for the dispersion of the metallic palladium
phase [10]. Although impregnation methodologies have dominated the preparation
of heterogeneous catalysts, control of the NPs size or their distribution onto the
support are common issues. In contrast, the use of colloidal methods provides access
to well-defined M-NPs with accurate control of their size, shape and structure at the
nanoscale [11]. Even though sophisticated synthetic methods prevent the scaling-up
for productive applications, the possibility to prepare series of materials with welldefined properties has provided the scientific community with model catalysts that are
ideal for fundamental studies and have contributed enormously to the understanding
of the phenomena involved in the semi-hydrogenation of alkynes [12]. In addition,
one of the recent trends in research on heterogeneous catalysis consists in the use of
small metal clusters (M-clusters) and single-atom catalysts (SACs) [13]. Their special
alkyne
R
R
H 2
R
R
H
H
alkene
+
M-NPs: Pd, Ni, Cu, Ag, Au, Fe, Ru, Pt..
M-Clusters: Pd 2 , Cu 11 , Ni 6
SACs: Pd, Ni, Ru
M-NPs
R
alkane
M-Clusters
SACs
Scheme 10.1 Catalysts employed in the semi-hydrogenation of alkynes
J. A. Delgado and C. Godard
over-hydrogenation or oligomerization of the alkyne substrate [5, 6]. When referred
to catalysts employed in the semi-hydrogenation of alkynes, a wide variety of transition metals has been employed, covering a wide range of particle sizes, most of them
classified as either metal nanoparticles (M-NPs), metal nanoclusters (M-Clusters)
and single-atom catalysts (SACs) (Scheme 10.1) [7].
Metals that easily activate the hydrogen molecule (e.g. Pt, Ni) normally display a
high over-hydrogenation activity and suffer of poor control of the alkene selectivity.
Conversely, metals with low hydrogen activation activity (e.g. Au, Ag) frequently
display good alkene selectivities but require harsh reaction conditions in terms of both
temperature and pressure [8]. Palladium appears as the metal that apparently meets
the right balance towards hydrogen activation and outstanding activity under mild
conditions, but its control of the selectivity when approaching full alkyne conversion
is extremely poor [9].
In terms of catalyst preparation, solid supports such as oxides (SiO 2 , Al 2 O 3 , TiO 2 ),
CaCO 3 and carbon-based materials (carbon nanofibres or nanotubes, activated C,
graphite) have been classically employed for the dispersion of the metallic palladium
phase [10]. Although impregnation methodologies have dominated the preparation
of heterogeneous catalysts, control of the NPs size or their distribution onto the
support are common issues. In contrast, the use of colloidal methods provides access
to well-defined M-NPs with accurate control of their size, shape and structure at the
nanoscale [11]. Even though sophisticated synthetic methods prevent the scaling-up
for productive applications, the possibility to prepare series of materials with welldefined properties has provided the scientific community with model catalysts that are
ideal for fundamental studies and have contributed enormously to the understanding
of the phenomena involved in the semi-hydrogenation of alkynes [12]. In addition,
one of the recent trends in research on heterogeneous catalysis consists in the use of
small metal clusters (M-clusters) and single-atom catalysts (SACs) [13]. Their special
alkyne
R
R
H 2
R
R
H
H
alkene
+
M-NPs: Pd, Ni, Cu, Ag, Au, Fe, Ru, Pt..
M-Clusters: Pd 2 , Cu 11 , Ni 6
SACs: Pd, Ni, Ru
M-NPs
R
alkane
M-Clusters
SACs
Scheme 10.1 Catalysts employed in the semi-hydrogenation of alkynes