8 Earth-Abundant d-Block Metal Nanocatalysis for Coupling …
251
renewable solvents, and combinations thereof …) and solvent-free chemistry are
useful and efficient approaches for many applications, needing investment and effort
to adapt the current processes to the new reaction conditions [69]. In the frame of this
chapter, we are interested in polyols, largely used in the industrial sector (highlighting
the production of polymers such as polyurethanes, polyvinyl alcohol …), because
they present lower environmental impact than low-weight and volatile organic compounds, and they are particularly attractive for the synthesis and stabilization of
nano-sized metal clusters.
Focusing on this type of nanomaterials, well-defined metal-based nanoparticles
(objects showing dimensions in the range 1–100 nm) have known a huge expansion
since the 1980s due to their distinctive properties, both physical and chemical, in
comparison to molecular and bulk materials. Actually, this is a consequence of their
electronic and structural features, making possible a vast number of applications
[111, 90, 133], in particular in catalysis [1, 56, 89, 111, 112]. Even though metal
nanoparticles (MNPs) have been largely applied in classical heterogeneous catalysis, with crucial participation in industrial processes mainly those related to oil area
[137], nanocatalysis, which concerns MNPs dispersed in a solvent, has only been
developed since the end of the last century. This exponential growth has benefited of
the recent advances in characterization techniques (including operando approaches)
which has permitted the design of catalysts at nanometric scale by means of controlling morphology and surface state of nanoobjects [5, 10, 63, 28]. Noticeably,
reproducible synthetic methodologies are crucial for their further applications [107,
122]. The chemical strategies (commonly named bottom-up syntheses) often include
solvents, from conventional organic compounds to alternative ones (showing a lower
environmental impact), such as water [21, 110], ionic liquids [105], scCO 2 , [32,
100, 134], and polyols [23, 40, 43]. It is important to mention that solvents may be
involved in different aspects, for example, as reducing agents, stabilizers, or medium
for trapping, nanocatalysts preserving their morphology during the catalytic transformation and thus facilitating their recycling. This is the case when polyols are
present. In 1989, the polyol methodology was for the first time reported by Fiévet
and coworkers where metal salts were reduced in ethylene glycol, obtaining welldefined MNPs [44, 45]. In this method, the polyol acts as solvent, reducing agent, and
stabilizer when higher polyols are involved (e.g., polyphenols or polysaccharides),
which prevents the agglomeration of MNPs in solution [36, 43].
These privileged physicochemical properties favor the preparation of tailor-made
first-row transition metal nanocatalysts and derived nanocomposites in a controlled
manner. Rational catalyst design is essential for the preparation of well-defined clusters and nanoparticles with optimal properties in terms of redox control, cooperative
effects, and increased surface areas, all of them key factors for catalysis. Beyond the
dual catalytic behavior related to both surface and reservoir of molecular species,
nanocatalysis brings novel reaction manifolds due to the unique structural properties of MNPs, particularly their differential electronic structure as compared to bulk
metals (e.g., facilitating single electron transfer processes via the Fermi level for
electrons [42]).
251
renewable solvents, and combinations thereof …) and solvent-free chemistry are
useful and efficient approaches for many applications, needing investment and effort
to adapt the current processes to the new reaction conditions [69]. In the frame of this
chapter, we are interested in polyols, largely used in the industrial sector (highlighting
the production of polymers such as polyurethanes, polyvinyl alcohol …), because
they present lower environmental impact than low-weight and volatile organic compounds, and they are particularly attractive for the synthesis and stabilization of
nano-sized metal clusters.
Focusing on this type of nanomaterials, well-defined metal-based nanoparticles
(objects showing dimensions in the range 1–100 nm) have known a huge expansion
since the 1980s due to their distinctive properties, both physical and chemical, in
comparison to molecular and bulk materials. Actually, this is a consequence of their
electronic and structural features, making possible a vast number of applications
[111, 90, 133], in particular in catalysis [1, 56, 89, 111, 112]. Even though metal
nanoparticles (MNPs) have been largely applied in classical heterogeneous catalysis, with crucial participation in industrial processes mainly those related to oil area
[137], nanocatalysis, which concerns MNPs dispersed in a solvent, has only been
developed since the end of the last century. This exponential growth has benefited of
the recent advances in characterization techniques (including operando approaches)
which has permitted the design of catalysts at nanometric scale by means of controlling morphology and surface state of nanoobjects [5, 10, 63, 28]. Noticeably,
reproducible synthetic methodologies are crucial for their further applications [107,
122]. The chemical strategies (commonly named bottom-up syntheses) often include
solvents, from conventional organic compounds to alternative ones (showing a lower
environmental impact), such as water [21, 110], ionic liquids [105], scCO 2 , [32,
100, 134], and polyols [23, 40, 43]. It is important to mention that solvents may be
involved in different aspects, for example, as reducing agents, stabilizers, or medium
for trapping, nanocatalysts preserving their morphology during the catalytic transformation and thus facilitating their recycling. This is the case when polyols are
present. In 1989, the polyol methodology was for the first time reported by Fiévet
and coworkers where metal salts were reduced in ethylene glycol, obtaining welldefined MNPs [44, 45]. In this method, the polyol acts as solvent, reducing agent, and
stabilizer when higher polyols are involved (e.g., polyphenols or polysaccharides),
which prevents the agglomeration of MNPs in solution [36, 43].
These privileged physicochemical properties favor the preparation of tailor-made
first-row transition metal nanocatalysts and derived nanocomposites in a controlled
manner. Rational catalyst design is essential for the preparation of well-defined clusters and nanoparticles with optimal properties in terms of redox control, cooperative
effects, and increased surface areas, all of them key factors for catalysis. Beyond the
dual catalytic behavior related to both surface and reservoir of molecular species,
nanocatalysis brings novel reaction manifolds due to the unique structural properties of MNPs, particularly their differential electronic structure as compared to bulk
metals (e.g., facilitating single electron transfer processes via the Fermi level for
electrons [42]).