catalysts
Review
Merging Metallic Catalysts and Sonication:
A Periodic Table Overview
Claudia E. Domini 1 , Mónica B. Álvarez 1 , Gustavo F. Silbestri 1, *, Giancarlo Cravotto 2
and Pedro Cintas 3, *
1
2
3
*
Instituto de Química del Sur (INQUISUR), Departamento de Química,
Universidad Nacional del Sur (UNS)-Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET), Avenida Alem 1253, Bahía Blanca 8000, Argentina;
[email protected] (C.E.D.); [email protected] (M.B.Á.)
Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via P. Giuria 9, 10125 Turin, Italy;
[email protected]
Departamento de Química Orgánica e Inorgánica, Facultad de Ciencias-Universidad de Extremadura,
and Instituto del Agua, Cambio Climático y Sostenibilidad (IACYS)-Unidad de Química Verde y Desarrollo
Sostenible, Avda. de Elvas s/n, 06006 Badajoz, Spain
Correspondence: [email protected] (G.F.S.); [email protected] (P.C.);
Tel.: +54-291-459-5101 (G.F.S.); Tel.: +34-924-289-300 (P.C.)
Academic Editor: Keith Hohn
Received: 8 March 2017; Accepted: 14 April 2017; Published: 19 April 2017
Abstract: This account summarizes and discusses recent examples in which the combination of
ultrasonic waves and metal-based reagents, including metal nanoparticles, has proven to be a useful
choice in synthetic planning. Not only does sonication often enhance the activity of the metal
catalyst/reagent, but it also greatly enhances the synthetic transformation that can be conducted
under milder conditions relative to conventional protocols. For the sake of clarity, we have adopted
a structure according to the periodic-table elements or families, distinguishing between bulk metal
reagents and nanoparticles, as well as the supported variations, thus illustrating the characteristics
of the method under consideration in target synthesis. The coverage focuses essentially on the
last decade, although the discussion also strikes a comparative balance between the more recent
advancements and past literature.
Keywords: metal catalysts; nanoparticles; organic synthesis; sonication; supported metal
reagent/catalyst; ultrasound
1. Introduction
The interaction of ultrasound with metals dates back to early sonochemistry, from the 1950s
onwards, when some scientists discovered, rather by serendipity, that organometallic reagents and
their subsequent coupling with organic compounds could be greatly improved by sonication in
ultrasonic baths. The method clearly rivaled traditional procedures employed for metal reagents, as the
ultrasonic protocol worked well without inert conditions and undried solvents. These pluses were
initially associated to cleaning effects that removed the passivating coat of the otherwise unreactive
zero-valent metals. There has been considerable progress since, and the reader is referred to past
and recent major reviews that document in detail the mechanisms involved in activation of metal
surfaces, notably by creating reactive sites by atomic dislocation and corrosion effects, as well as
facile metalations and further reactions with an organic compound [1–5]. Of major significance in
this context is the use of sonication in Barbier and Barbier-like reactions, where tedious and complex
operations required to activate bulk metals are usually converted into simpler one-pot reactions [6,7].
Catalysts 2017, 7, 121; doi:10.3390/catal7040121
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Catalysts 2017, 7, 121
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The activation also applies to metal nanoparticles (NPs) [8–10], although their stability and reactivity
may be substantially different from those of bulk metals, even though finely divided, and often
require polymeric additives and/or deposition on inert supports, thus increasing the lifetimes of NPs
in solution.
Electron microscopy and X-ray diffraction reveal morphological surface changes induced by
sonication on the crystal structure of metals such as Al, Cu, Zn, or Ag, which exhibit different
hardness and natural roughness. As expected, cavitation plays a pivotal role, especially through
mechanical effects generated by the collapse of microbubbles and the action of shock waves on a given
surface. Both damage and deformation translate into structural reorganization of crystallites leading
to small dimensional variations, weak grain cohesion, and atomic defects [11]. Chemical effects,
especially surface oxidation induced by reactive species formed by substrate or solvent pyrolysis and
released after bubble implosion, also contribute to structural changes of metal surfaces [12].
It is now well established that effects caused by cavitation can globally be rationalized by
assuming a spherical model of bubbles, as microreactors, where three sonochemical reaction zones
are identified [13]. The core or inner environment provides high local temperatures and pressures
for volatile solutes capable of entering into the bubble and undergoing such intense conditions
when it collapses. Given the extremely fast collapse and concomitant cooling rates (exceeding 1010 K
per second), the structural reorganization and/or crystallization is highly hindered, thereby resulting in
amorphous products. The interfacial region, between the core and the surrounding liquid, provides less
intense conditions, yet high enough—ca. 1900 K—to bring about a chemical reaction. The interfacial
region appears to be the prevalent zone where non-volatile substrates or those unable to enter into
the bubble may react. Given the fast kinetics of collapse, the growth of the nuclei will also be
considerable restricted and this may result in nanostructured or nanocrystalline materials depending
on the temperature. The third zone comprising the bulk liquid at ambient conditions can also be the
site for sonochemical reactions to occur, notably after releasing of reactive species generated by bubble
collapse. However, the formation rate will also be lower [14].
The interplay between ultrasonic irradiation and metal activation can also be regarded within
the green chemistry domain, even if some metal particles are inherently toxic materials and organic
solvents cannot be completely avoided. However, these drawbacks are more than compensated for
by the milder, faster, and more efficient processes run under sonication. Aqueous environments are
often tolerated and new mechanistic pathways may afford functionalized derivatives not attained
by thermal activation, thus switching conventional routes. In addition, it is worth pointing out that
ultrasound along with other sustainable and enabling technologies provide sufficient activation to
perform chemical reactions under solvent-free (i.e., neat reagents) and catalyst-free conditions [15].
This perspective is aimed at providing a good coverage of recent developments in the use
of metal derivatives and metal NPs, generated and manipulated under ultrasound, hoping to
be illustrative enough of the breadth and strength of the technique. The goals are focused on
the capture of fundamental details that make ultrasound advantageous in a particular catalytic
application. The preparation of micro- and nanomaterials with potential catalytic activity has grown
almost exponentially. Since our analysis cannot be overlong, thus aiding readability and outlook
of the applications presented, the article is largely restricted to synthetic strategies and/or specific
applications, i.e., the sonochemical preparation of metal nanostructures or metal NPs without such
detailed explorations cannot be treated, apart from a few exceptions in terms of discussion or
comparative purposes. Accordingly, the authors hope to marry the properties of concision and
representative cases, yet being informative and insightful.
2. Transition Metals and Supported Transition Metal Reagents
The use of transition metals still represents the main domain of applications in synthesis and
catalysis aided by ultrasound. Metal powders or metal alloys as well as other non-zero valent
precursors, cheap metal oxides or metal halides for instance, are usually employed as precursors and
Catalysts 2017, 7, 121
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subjected to cavitation-induced treatments to generate active reagents and/or improve the synthetic
transformation still further. To decrease metal loadings and improve the stability of metal particles,
doping strategies on appropriate organic and inorganic supports are performed. Unfortunately,
the nature of the resulting composite or soluble species is not always specified. The discussion
Catalysts 2017, 7, 121
3 of 29
hereafter
identifies
or
Catalysts
2017, 7, 121the material as stated in the original sources, while applications involving
3 of NPs
29
nanostructured
will be
later. is not always specified. The discussion hereafter
nature of themorphologies
resulting composite
or treated
soluble species
nature
of
the
resulting
composite
or
soluble
species
is
not always
specified.
The
discussion
hereafter
Late-transition
metals,
copper
in
particular,
a while
wide
range
of
synthetic
applications
identifies the material as stated in the originalprovide
sources,
applications
involving
NPs or as
identifies
the
material
as
stated
in
the
original
sources,
while
applications
involving
NPs
morphologies
will be and
treated
later.
Cu(I)nanostructured
species are involved
in efficient
expeditious
couplings. Click reactions, generallyorbased
nanostructured
morphologies
will
be
treated
later.
Late-transition[3+2]-cycloadditions
metals, copper in particular,
provide
a wide
of synthetic
applications
on copper-catalyzed
between
azides
and range
alkynes
(CuAAC)
have metasmany
metals,incopper
inand
particular,
provide
a wide Click
rangereactions,
of synthetic
applications
as
Cu(I)Late-transition
species
are involved
efficient
expeditious
couplings.
generally
based on
applications
in materials
science,
as well
as
macromolecular
and biomedicinal
chemistries
[16–18].
Cu(I)
species
are
involved
in
efficient
and
expeditious
couplings.
Click
reactions,
generally
based
on
copper-catalyzed
[3+2]-cycloadditions
between azides
and alkynes
have1,5-disubstituted
met many
Relative
to the conventional
thermal cycloaddition,
which usually
yields(CuAAC)
both 1,4- and
copper-catalyzed
[3+2]-cycloadditions
azides and and
alkynes
(CuAAC)
have met[16–18].
many
applications in materials
science, as wellbetween
as macromolecular
biomedicinal
chemistries
1,2,3-triazoles,
the
Cu-catalyzed
version
is faster
and highly regioselective
leading
to 1,4-regioisomers.
applications
in
materials
science,
as
well
as
macromolecular
and
biomedicinal
chemistries
[16–18].
Relative to the conventional thermal cycloaddition, which usually yields both 1,4- and 1,5The orthogonal
of this coupling
in living organisms
make ityields
ideal both
for specific
labeling
of
Relative
to character
the
conventional
cycloaddition,
usually
1,4- leading
and
1,5disubstituted
1,2,3-triazoles,
thethermal
Cu-catalyzed
version iswhich
faster and
highly regioselective
to
biomolecules.
The
active
salt to
theofcorresponding
copper
acetylides
as intermediates
is
disubstituted
1,2,3-triazoles,
thegenerate
Cu-catalyzed
version
is fasterinand
highly
regioselective
to
1,4-regioisomers.
The orthogonal
character
this coupling
living
organisms
make
itleading
ideal for
The
orthogonal
character
of
this
coupling
in
living
organisms
make
it
ideal
for
Cu(I),1,4-regioisomers.
although
the
latter
can
be
formed
in
situ
from
a
Cu(II)
salt
using
a
reducing
agent,
especially
specific labeling of biomolecules. The active salt to generate the corresponding copper acetylides as
specific
labeling
of biomolecules.
Thelatter
active
salt
generate
the corresponding
copper
acetylides
as
sodium
ascorbate.
intermediates
is Cu(I),
although the
can
betoformed
in situ
from a Cu(II) salt
using
a reducing
intermediates
is
Cu(I),
although
the
latter
can
be
formed
in
situ
from
a
Cu(II)
salt
using
a
reducing
agent,of
especially
sodium
ascorbate. protocols was reported by Sreedhar and Surendra Reddy
One
the first
sonochemical
agent,
especially
sodium
ascorbate.protocols
One
of thea first
sonochemical
wasinreported
by Sreedhar
Surendra Reddy
in 2007 [19].
in 2007
through
three-component
reaction
aqueous
solution and
at ambient
temperature
One of
the
first sonochemical
protocols
was reported
by Sreedhar
and Surendra
Reddy
in 2007
through
a
three-component
reaction
in
aqueous
solution
at
ambient
temperature
[19].
The
The cycloaddition was performed in a common ultrasonic bath and hence, its reproducibility
in
through
a three-component
reaction
in ultrasonic
aqueous solution
at
ambient
temperature in[19].
The
cycloaddition
was
performed
in
a
common
bath
and
hence,
its
reproducibility
terms
ofmass
terms of acoustic power should be treated with caution. However, sonication clearly enhanced
cycloaddition
performed
in a with
common
ultrasonic
bathsonication
and hence,clearly
its reproducibility
in terms
of
acoustic powerwas
should
be treated
caution.
However,
enhanced mass
transfer
transfer
and yielded
triazoles
(3) in less
than
30 min
(Scheme
1). Theclearly
best results
were
obtained
using
acoustic
power
should
be
treated
with
caution.
However,
sonication
enhanced
mass
transfer
and yielded triazoles (3) in less than 30 min (Scheme 1). The best results were obtained using Cu(I)
Cu(I)and
iodide
(92%
yield),
while
other
Cu(I)
salts
as
well
as
the
use
of
Cu(II)
or
Cu(0)
species
gave
rise
yielded
triazoles
(3)
in
less
than
30
min
(Scheme
1).
The
best
results
were
obtained
using
Cu(I)
iodide (92% yield), while other Cu(I) salts as well as the use of Cu(II) or Cu(0) species gave rise to
to lower
yields
and
poor
selectivity.
iodide
(92%
yield),
while
other
Cu(I)
salts
as
well
as
the
use
of
Cu(II)
or
Cu(0)
species
gave
rise
to
lower yields and poor selectivity.
lower yields and poor selectivity.
Scheme 1. Ultrasound-promoted three-component Cu-catalyzed alkyne-azide cycloaddition
Scheme 1. Ultrasound-promoted three-component Cu-catalyzed alkyne-azide cycloaddition (CuAAC)
Scheme
Ultrasound-promoted three-component Cu-catalyzed alkyne-azide cycloaddition
(CuAAC) 1.
in water.
in water.
(CuAAC) in water.
A further study by Cravotto and associates compared the efficiency of soluble and
A further
study
by Cravotto
and asassociates
compared
theparameters
efficiency
ofand
soluble
and
heterogeneous
catalysts,
well
the
influence
ofefficiency
other
like
solvent
and
A
further
studycopper
by Cravotto
andasassociates
compared
the
of soluble
heterogeneous
heterogeneous
copper
catalysts,
as
well
as
the
influence
of
other
parameters
like
solvent
and
temperature.
Moreover,
activation
was
carried
out
by
either
microwaves
(MW)
or
simultaneous
copper catalysts, as well as the influence of other parameters like solvent and temperature. Moreover,
temperature.
Moreover, activationirradiation
was carried
outAn
byanalysis
either microwaves
or simultaneous
microwave-ultrasound
[20].
(Table 1) of(MW)
data
gathered
for the
activation
was carried out(MW/US)
by either microwaves
(MW)
or simultaneous
microwave-ultrasound
microwave-ultrasound
(MW/US)
irradiation
[20].
An
analysis
(Table 1) of data
gathered
for the
cycloaddition
shown
in
Scheme
2
indicates
the
pluses
of
heterogeneous
catalysis
using
an
(MW/US) irradiation [20]. An analysis (Table 1) of data gathered for the cycloaddition shown
in
cycloaddition
shown in SchemeCu(I)
2 indicates
the pluses
of Cu(I)
heterogeneous
catalysis
using
an
inexpensive charcoal-supported
reagent prepared
from
and sonication,
as well
as the
Scheme
2 indicates
the pluses of heterogeneous
using
inexpensive
charcoal-supported
inexpensive
charcoal-supported
Cu(I) reagentcatalysis
prepared
froman
Cu(I)
and sonication,
as well as the Cu(I)
benefits of using
the MW/US combination.
reagent
prepared
from
Cu(I)
and
sonication,
as
well
as
the
benefits
of
using
the
MW/US
combination.
benefits of using the MW/US combination.
Scheme 2. CuAAC reaction activated by microwaves (MW) or simultaneous microwave-ultrasound
Scheme
2.irradiations.
CuAAC
reaction
activatedby
bymicrowaves
microwaves (MW)
microwave-ultrasound
Scheme
2. CuAAC
reaction
activated
(MW)ororsimultaneous
simultaneous
microwave-ultrasound
(MW/US)
(MW/US)
irradiations.
(MW/US) irradiations.
An improved sonochemical protocol also developed by the Cravotto group employs
An improved
sonochemical
protocol
developed smoothly
by the triggered
Cravotto the
group
commercially
available
Cu turnings as
catalyst also
[21]. Ultrasound
redoxemploys
process
commercially
available
CuCu(II)
turnings
as catalyst
[21]. Ultrasound
smoothly
triggered
the
redox
process
between metallic
Cu and
oxide
on the metal
surface leading
to Cu(I)
species.
This
protocol
is
between
metallic
and Cu(II)than
oxideother
on the
metal surface
leading
to Cu(I)reactions
species. This
protocol
is
much simpler
andCu
inexpensive
catalysts
employed
for CuAAC
and works
well
much simpler and inexpensive than other catalysts employed for CuAAC reactions and works well
Catalysts 2017, 7, 121
4 of 29
An improved sonochemical protocol also developed by the Cravotto group employs commercially
available Cu turnings as catalyst [21]. Ultrasound smoothly triggered the redox process between
metallic Cu and Cu(II) oxide on the metal surface leading to Cu(I) species. This protocol is much
Catalysts 2017,
121
4 of 29
simpler
and 7,inexpensive
than other catalysts employed for CuAAC reactions and works well in water,
where the latter itself serves as ligand and metallic wastes are easily removed by filtration. As depicted
in water,
latter itself
serves
as ligand
andand
metallic
wastes areMW/US
easily removed
by filtration.
in
Figure where
1, boththe
sonication
alone
(with
a Ti horn)
simultaneous
(pyrex horn)
can be
As
depicted
in
Figure
1,
both
sonication
alone
(with
a
Ti
horn)
and
simultaneous
MW/US
(pyrex
successfully employed [21,22].
horn) can be successfully employed [21,22].
Table 1. Cu-catalyzed alkyne-azide cycloaddition (CuAAC) reactions activated under different
Table 1.conditions.
Cu-catalyzed alkyne-azide cycloaddition (CuAAC) reactions activated under different
thermal
thermal conditions.
Catalyst
Catalyst(10
(10mol
mol%)
%)
(a)
Cu(II)/C,
Cu(II)/C,LL-ascorbic
-ascorbicacid
acid(a)
Cu(II)/C,
Cu(II)/C, LL-ascorbic
-ascorbic acid
acid
(b)
recycled
recycledCu(II)/C
Cu(II)/C (b)
Cu(II)/C
Cu(II)/C
Cu(II)/C
Cu(II)/C
Cu(I)/C
Cu(I)/C
Cu(I)/C
Cu(I)/C
Cu(I)/C
Cu(I)/C
(a)
(a)
C = charcoal.
◦ C)
Conditions(at(at8585°C)
Reaction
Time Yield
Yield
Conditions
Reaction
Time
(%)(%)
bath
oiloil
bath
2 h2 h
79 79
MW
min
MW
1010
min
83 83
MW
min
MW
3030
min
84 84
bath
oiloil
bath
3 h3 h
30 30
MW
45 min
81
MW
45 min
81
oil bath
2h
78
oil bath
2h
78
MW
10 min
89
MW
1010
min
89 93
MW/US
min
MW/US
10 min
93
(b)
Catalyst recycled from the above reaction.
C = charcoal. (b) Catalyst recycled from the above reaction.
Figure 1.
1. Click
and the
the
Figure
Click cycloadditions
cycloadditions with
with metallic
metallic copper
copper activated
activated by
by ultrasound
ultrasound alone
alone (left)
(left) and
combined
action
of
US
and
MW
irradiations
in
a
professional
MW
reactor
(right).
Set-ups
from
combined action of US and MW irradiations in a professional MW reactor (right). Set-ups from
Cravotto’s group
group laboratory
laboratory at
at the
the University
University of
of Turin.
Turin.
Cravotto’s
An advantageous application of the above procedure deals with the structural modification of
An advantageous application of the above procedure deals with the structural modification of
cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well
cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well
as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion
as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion
compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming
compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming
purifications using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at
purifications using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at
high temperature affords cyclodextrin-triazole derivatives (9 and 10, Scheme 3) as white powders,
high temperature affords cyclodextrin-triazole derivatives (9 and 10, Scheme 3) as white powders,
with only trace amounts of the metal [22].
with only trace amounts of the metal [22].
cyclodextrins, which constitute versatile ligands and scaffolds in supramolecular chemistry, as well
as stationary phases in chiral chromatography. Cyclodextrins form stable sandwich-type inclusion
compounds with copper ions, giving rise to a green-grayish coloration that require time-consuming
purifications using chelate agents [23]. Metallic copper as catalyst in dimethylformamide (DMF) at
high temperature
Catalysts
2017, 7, 121 affords cyclodextrin-triazole derivatives (9 and 10, Scheme 3) as white powders,
5 of 29
with only trace amounts of the metal [22].
Catalysts 2017, 7, 121
Catalysts 2017, 7, 121
5 of 29
5 of 29
Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates.
Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates.
Scheme 3. CuAAC reactions in the presence of Cu(0) with β-cyclodextrin derivatives as substrates.
Further extension
extension of the same conceptual basis to imidazolium-functionalized
alkynes led to
Further
imidazolium-functionalized alkynes
Further extension of the same conceptual basis to imidazolium-functionalized alkynes led to
positively charged triazole-bridged
triazole-bridged cyclodextrins
cyclodextrins (Scheme
(Scheme 4),
4), which
which exhibit
exhibit hybrid
hybrid properties
properties of
positively charged triazole-bridged cyclodextrins (Scheme 4), which exhibit hybrid properties of
cyclodextrins as encapsulating hosts and ionic liquids [24]. This transformation can be conducted in
cyclodextrins as encapsulating hosts and ionic liquids [24]. This transformation can be conducted in
irradiation.
an aqueous environment and is more efficient
efficient under
under the
the simultaneous
simultaneous action
action of
of US/MW
US/MW irradiation.
an aqueous environment and is more efficient under the simultaneous action of US/MW irradiation.
Scheme 4. CuAAC
CuAAC leading
leading to
to hybrid
hybrid cyclodextrin-triazole-based ionic liquids.
Scheme 4. CuAAC leading to hybrid cyclodextrin-triazole-based ionic liquids.
Jacob
etal.
al. developedporous
porous
glasses
a novel
catalyst
support
for CuAAC
reactions.
The
Jacob et
glasses
as aas
novel
catalyst
support
for CuAAC
reactions.
The catalyst
Jacob
et al.developed
developed porous
glasses
as
a novel
catalyst
support
for CuAAC
reactions.
The
catalyst
could
easily
be
generated
by
sonication
in
5–10
min
from
Cu(II)
acetate
in
aqueous
medium
could
easily
be
generated
by
sonication
in
5–10
min
from
Cu(II)
acetate
in
aqueous
medium
using
catalyst could easily be generated by sonication in 5–10 min from Cu(II) acetate in aqueous medium
using
an ultrasonic
calcined
catalyst
was
then
employedininthe
the click cycloaddition
cycloaddition under
an ultrasonic
bath. bath.
TheThe
calcined
catalyst
was
then
employed
under
using
an ultrasonic
bath.
The
calcined
catalyst
was
then
employed in theclick
click cycloaddition under
MW-assisted
heating
(Scheme
5)
[25].
This
resulted
in
excellent
regioselectivity
and yields
within
MW-assisted
heating
(Scheme
5)
[25].
This
resulted
in
excellent
regioselectivity
and
yields
within
short
MW-assisted heating (Scheme 5) [25]. This resulted in excellent regioselectivity and yields within
short
reaction
times.
The experimental
conditions
wereusing
optimized
using theofcycloaddition
of
reaction
times.
The
experimental
conditions
were
optimized
the
cycloaddition
phenylacetylene
short reaction times. The experimental conditions were optimized using the cycloaddition of
phenylacetylene
(5) and
azide (12).
Recyclability
studies
thebecatalyst
(5) and benzyl azide
(12).benzyl
Recyclability
studies
also showed
thatalso
the showed
catalyst that
could
reused could
up to
phenylacetylene
(5) and
benzyl
azide (12).
Recyclability
studies
also
showed
that
the catalyst
could
be
reused
up
to
four
times
with
comparable
efficiency.
four
times
with
comparable
efficiency.
be reused up to four times with comparable efficiency.
Scheme 5. Click-reaction using a porous glass support.
Scheme
Scheme 5.
5. Click-reaction
Click-reaction using
using aa porous
porous glass
glass support.
support.
With the advent and development of flow processes, which not only avoid hazards on
With the advent and development of flow processes, which not only avoid hazards on
manipulating
substances,
but also miniaturization
and which
intensification,
interesting
With the toxic
advent
and development
of flow processes,
not onlyan
avoid
hazardsclick
on
manipulating toxic substances, but also miniaturization and intensification, an interesting click
reaction
in continuous
flow hasbut
also
been
reported [26].and
The
reactor in question
consistsclick
of a reaction
coppermanipulating
toxic substances,
also
miniaturization
intensification,
an interesting
reaction in continuous flow has also been reported [26]. The reactor in question consists of a coppermade
cassette that
a piezoelectric
transducer.
1,4-Disubstituted
triazolesofwere
obtained in
in continuous
flowintegrates
has also been
reported [26].
The reactor
in question consists
a copper-made
made cassette that integrates a piezoelectric transducer. 1,4-Disubstituted triazoles were obtained in
yields ranging from 15% to 55% and within 50–150 °C depending on the starting substrates in only
yields ranging from 15% to 55% and within 50–150 °C depending on the starting substrates in only
5–10 min.
5–10 min.
Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by
Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by
sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the
sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the
addition of metal acetylides to in situ generated imines leading to propargylamines (17), a protocol
Catalysts 2017, 7, 121
6 of 29
cassette that integrates a piezoelectric transducer. 1,4-Disubstituted triazoles were obtained in yields
ranging from 15% to 55% and within 50–150 ◦ C depending on the starting substrates in only 5–10 min.
Couplings other than alkyne-azide cycloadditions mediated by Cu species and activated by
sonication have also proven to be useful in synthesis. Thus, ultrasonic irradiation was applied to the
addition of metal acetylides to in situ generated imines leading to propargylamines (17), a protocol
described more than one decade ago (Scheme 6). Even though the process was conducted in a common
ultrasonic bath, good yields were obtained at room temperature using CuI in aqueous solution [27].
More recently, Cargnelutti et al. described a straightforward C-S coupling of aryl halides (18)
with thiols (19) in glycerol, catalyzed by CuI/bis(2-pyridyl)diselenoethers and using ultrasound
(as
alternative
energy source) to accelerate the process (Scheme 7) [28]. The efficiency of such66 of
C-S
Catalysts
2017,
29
Catalysts
2017, 7,
7, 121
121
of
29
couplings in glycerol is analogous to other cross-coupling reactions in toxic organic solvents, which also
also
employ
expensive
catalysts
on transition
metals.
The method
allowed
theofuse
of a
employ
moremore
expensive
catalysts
basedbased
on transition
metals.
The method
allowed
the use
a wide
wide
of functionalized
reaction
partners,
the desired
products
in yields.
good yields.
rangerange
of functionalized
reaction
partners,
givinggiving
the desired
products
in good
Scheme
Scheme 6.
6. Cu(I)-mediated
Cu(I)-mediated
synthesis of
of propargylamines.
propargylamines.
Cu(I)-mediated synthesis
Scheme 7. Synthesis
Synthesis of aryl
aryl sulfides using
using bis(2-pyridyl)diseleno-ethers.
Scheme
Scheme 7.
7. Synthesis of
of aryl sulfides
sulfides using bis(2-pyridyl)diseleno-ethers.
bis(2-pyridyl)diseleno-ethers.
Cheaper and
andabundant
abundanttransition
transition
metals
invariably
represent
valuable
resources
for metalmetals
invariably
represent
valuable
resources
for metal-assisted
assisted
transformations.
Pirola
et
al.
prepared
supported
iron
catalysts
for
Fischer-Tropsch
(FT)
transformations. Pirola et al. prepared supported iron catalysts for Fischer-Tropsch (FT) reactions
reactions
using different
both conventional
and non-conventional
[29]. Metal
deposition
on
using different
methods,methods,
both conventional
and non-conventional
[29]. Metal
deposition
on silica
silica
was conducted
either
ultrasound
(US)
microwave
(MW),thereby
therebyincreasing
increasing the
the catalyst
was conducted
withwith
either
ultrasound
(US)
or or
microwave
(MW),
activity. Data obtained for FT reactions showed that the catalysts generated under sonication were
the most efficient,
efficient, especially under an inert
inert (Ar)
(Ar) atmosphere.
atmosphere. MW-prepared
MW-prepared catalysts
catalysts afforded
afforded results
comparable to those obtained with conventional protocols.
Copper
22Cr
22O255O
), 5sometimes
referred
to as
catalyst,
and
Copper chromite
chromite (with
(withformula
formulaCu
Cu
), sometimes
referred
tothe
as Adkins
the Adkins
catalyst,
2 Cr
TiO
copper
chromite
catalysts
have have
been been
prepared
by means
of different
methods,
and
and22-supported
TiO2 -supported
copper
chromite
catalysts
prepared
by means
of different
methods,
their
catalytic
performance
assessed
in
the
liquid-phase
hydrogenation
of
furfural
giving
rise
and their catalytic performance assessed in the liquid-phase hydrogenation of furfural giving rise to
furfuryl alcohol [30]. TiO22-supported
-supported catalysts
catalysts obtained
obtained under
under ultrasonication
ultrasonication exhibited
exhibited the highest
activity. Notably,
temperature
and
studies
conducted
at
Notably, the
thecatalytic
catalyticactivity
activitywas
wasdependent
dependentononthe
the
temperature
and
studies
conducted
◦
140,
170,
andand
200200
°C showed
the the
bestbest
results
at the
temperature.
The catalyst’s
efficiency
was
at 140,
170,
C showed
results
at lowest
the lowest
temperature.
The catalyst’s
efficiency
ascribed
to thetoamount
of reducible
Cu(II)Cu(II)
ions present
in the in
catalyst
and the
metallic
surface.
Cu(0)
was ascribed
the amount
of reducible
ions present
the catalyst
and
the metallic
surface.
was generated by reduction of the catalyst with H22. The activity was completely lost when carbon
deposited on the surface or decreased the amount of metal species.
Arena et al. also conducted a systematic research on the effects of Zn/Cu ratios on the structure
and adsorption of Cu–ZnO/ZrO22 catalysts (Zn/Cu ratios = 0–3; ZrO22 = 42–44 wt %), synthesized via
the reverse co-precipitation under ultrasound energy. Cu/ZrO22 and Cu–ZnO/ZrO22 systems ensured
Catalysts 2017, 7, 121
7 of 29
Cu(0) was generated by reduction of the catalyst with H2 . The activity was completely lost when
carbon deposited on the surface or decreased the amount of metal species.
Arena et al. also conducted a systematic research on the effects of Zn/Cu ratios on the structure
and adsorption of Cu–ZnO/ZrO2 catalysts (Zn/Cu ratios = 0–3; ZrO2 = 42–44 wt %), synthesized via
the reverse co-precipitation under ultrasound energy. Cu/ZrO2 and Cu–ZnO/ZrO2 systems ensured
a superior activity in the hydrogenation reaction of CO2 to methanol with respect to conventional
catalysts [31].
3. Main Group Metal Derivatives
Unlike zero-valent transition metals, some exhibiting high hardness and being reluctant to
undergo a direct metalation process [1,5], some main group elements, especially Group 13 and
14
metals,
Catalysts
2017,often
7, 121 show characteristics typical of alkaline or alkaline-earth elements and can7 also
of 29
Catalysts 2017, 7, 121
7 of 29
tolerate hydrophilic media and sensitive functional groups. Both metalation and couplings can
conducted
the action
of ultrasound
waves. Itwaves.
suffices,
for instance,
to mention
that
be smoothlyunder
conducted
under the
action of ultrasound
It suffices,
for instance,
to mention
conducted under the action of ultrasound waves. It suffices, for instance, to mention that
organostannane
compounds
(alkyl,
alkenyl,
and and
arylaryl
derivatives),
which
are are
typically
prepared
by
that organostannane
compounds
(alkyl,
alkenyl,
derivatives),
which
typically
prepared
organostannane compounds (alkyl, alkenyl, and aryl derivatives), which are typically prepared by
transmetalation
of the
Grignard
or organolithium
reagents
with bis(tributyltin)
oxide,
by transmetalation
of corresponding
the corresponding
Grignard
or organolithium
reagents
with bis(tributyltin)
transmetalation of the corresponding Grignard or organolithium reagents with bis(tributyltin) oxide,
can
however
be synthesized
via avia
direct
Barbier-like
reaction
under
oxide,
can however
be synthesized
a direct
Barbier-like
reaction
undersonication
sonication[32,33].
[32,33].A
A similar
similar
can however be synthesized via a direct Barbier-like reaction under sonication [32,33]. A similar
strategy employing
employinggermanium
germaniumhalides,
halides,Mg
Mgturnings,
turnings,
and
organohalides
under
sonication
enables
and
organohalides
under
sonication
enables
the
strategy employing germanium halides, Mg turnings, and organohalides under sonication enables
the preparation
of corresponding
the corresponding
organogermanium
compounds
preparation
of the
organogermanium
compounds
[34].[34].
the preparation of the corresponding organogermanium compounds [34].
Despite their
areare
versatile
reagents
to accomplish
a variety
of metaltheirtoxicity,
toxicity,organostannates
organostannates
versatile
reagents
to accomplish
a variety
of
Despite their toxicity, organostannates are versatile reagents to accomplish a variety of metalmediated
couplings,
the Stille
reaction
in particular.
In 2009
Lamandé-Langle
et et
al.al.
described
a
metal-mediated
couplings,
the Stille
reaction
in particular.
In 2009
Lamandé-Langle
described
mediated couplings, the Stille reaction in particular. In 2009 Lamandé-Langle et al. described a
general
procedure
that
allows
the
efficient
inin
following
a
a general
procedure
that
allows
the
efficientsynthesis
synthesisofofvarious
variousorganotin
organotincompounds
compounds
following
general procedure that allows the efficient synthesis of various organotin compounds in following a
Barbier-like
strategy
a Barbier-like
strategyasaswell
well(Scheme
(Scheme8)8)[35].
[35].The
Theprocess
processoffers
offersaa number
number of
of advantages
advantages such
such as
Barbier-like strategy as well (Scheme 8) [35]. The process offers a number of advantages such as
simplicity and
a
and good
good yields,
yields, and
andreagent
reagentquality
quality(both
(bothsolvents
solventsand
andtin
tinhalides)
halides)does
doesnot
notconstitute
constitute
simplicity and good yields, and reagent quality (both solvents and tin halides) does not constitute a
critical
issue.
Moreover,
careful
purification
before
use
and
the
presence
of
an
additive
are
not
a critical issue. Moreover, careful purification before use and the presence of an additive are
critical issue. Moreover, careful purification before use and the presence of an additive are not
required. Numerous organotin compounds could easily be synthesized in a simplified and improved
required. Numerous organotin compounds could easily be synthesized in a simplified and improved
one-step ultrasound-assisted Barbier reaction via cross-coupling of a variety
variety of
of organic
organic bromides
bromides (25)
(25)
one-step ultrasound-assisted Barbier reaction via cross-coupling of a variety
of
organic
bromides
(25)
stannyl
Bu
2SnCl
2, BuSnCl
3, SnCl
4) promoted
by magnesium.
The diwith
stannyl chlorides
chlorides(24:
(24:Bu
Bu33SnCl,
Bu
by magnesium.
Theand
di3 SnCl,
2 SnCl
2 , BuSnCl
3 , SnCl
4 ) promoted
with stannyl
chlorides
(24:
Bu
SnCl,
Bu
2SnCl2, BuSnCl3, SnCl4) promoted by magnesium. The di- and
tri-functional
derivatives
(26)(26)
were
further
used
in in
a aStille
cross-coupling
reaction,
with
different
and
tri-functional
derivatives
were
further
used
Stille
cross-coupling
reaction,
with
tri-functional derivatives (26) were further used in a Stille cross-coupling reaction, with different
iodovinyl acids (or esters) to determine how many groups were
were transferred.
iodovinyl acids (or esters) to determine how many groups were transferred.
transferred.
Scheme
8. Sonochemical
organotin compounds.
Scheme
Sonochemical synthesis
synthesis of
of
Scheme 8.
8. Sonochemical
synthesis
of organotin
organotin compounds.
compounds.
In 2012, Domini et al. reported an improved Stille synthesis of unsymmetrical biaryls (29) when
In 2012, Domini et al. reported an improved Stille synthesis of unsymmetrical biaryls (29) when
conducting the coupling under sonication (Scheme 9) [36]. Yields ranged from 57% to 97% in 30 min,
conducting the coupling under sonication (Scheme 9) [36]. Yields ranged from 57% to 97% in 30 min,
whereas the silent transformation took ca. 20 h to afford similar results.
whereas the
the silent
silent transformation
transformation took
took ca.
ca. 20
20 hh to
to afford
afford similar
similar results.
results.
whereas
Scheme 9. Synthesis of unsymmetrical biaryls.
Scheme 9.
9. Synthesis
unsymmetrical biaryls.
biaryls.
Scheme
Synthesis of
of unsymmetrical
Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides (31)
Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides (31)
in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10) [37]. The
in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10) [37]. The
protocol also works for aryl vinyl ketones. Yields were moderate to good (42%–84%) and,
protocol also works for aryl vinyl ketones. Yields were moderate to good (42%–84%) and,
remarkably, the transformation can be run under neat conditions, without requiring an organic
remarkably, the transformation can be run under neat conditions, without requiring an organic
solvent. Sonication dramatically reduced the reaction times compared to conventional conditions
solvent. Sonication dramatically reduced the reaction times compared to conventional conditions
Catalysts 2017, 7, 121
8 of 29
Arylstannanes (30) have been recently employed as useful partners with alkanoyl chlorides
(31) in an indium-mediated regioselective synthesis of alkyl aryl ketones (32) (Scheme 10) [37].
The protocol also works for aryl vinyl ketones. Yields were moderate to good (42–84%) and, remarkably,
the transformation can be run under neat conditions, without requiring an organic solvent. Sonication
dramatically reduced the reaction times compared to conventional conditions (from 3–32 h to 10–70 min
depending on the starting materials). Indium appears to be a radical initiator and the authors suggest
Catalysts 2017, 7, 121
8 of 29
a catalytic
Catalysts
2017,cycle
7, 121involving a single-electron transfer (SET) mechanism.
8 of 29
Scheme 10. Indium-mediated regioselective synthesis of alkyl aryl ketones.
Scheme
Scheme 10. Indium-mediated
Indium-mediated regioselective
regioselective synthesis of alkyl aryl ketones.
Certainly indium shows an ionization potential comparable to that of electropositive metals, and
Certainly
indium shows
potential comparable
to thatthat
of electropositive
metals,
and
Certainly
showsan
anionization
ionization
of
electropositive
metals,
enables
a broadindium
compatibility
with
numerouspotential
functionalcomparable
groups and,tounlike
alkaline
or alkaline-earth
enables
a
broad
compatibility
with
numerous
functional
groups
and,
unlike
alkaline
or
alkaline-earth
and
enableshas
a broad
compatibility
numerous
functional
and, unlike
alkaline or
alkaline-earth
elements,
the advantage
of with
performing
reactions
in groups
an aqueous
environment
[38,39].
In past
elements,
has
the
advantage
of
performing
reactions
in
an
aqueous
environment
[38,39].
elements,
hasinvolving
the advantage
performing
reactions
an aqueous
[38,39]. In
In past
past
development
Barbierofallylation
reactions,
the in
allylindation
of environment
1H-indole-3-carboxaldehyde
development
involving
Barbier
allylation
reactions,
the
allylindation
of
1H-indole-3-carboxaldehyde
development
involving
Barbier
allylation
reactions,
the
allylindation
of
1H-indole-3-carboxaldehyde
with azoles (pyrazole and imidazole compounds) was conducted in aqueous media under sonication.
with
azoles
and
compounds)
was
in aqueous
under
sonication.
with
azoles (pyrazole
(pyrazole
and imidazole
imidazole
compounds)
was conducted
conducted
aqueousmedia
media
underleading
sonication.
Mechanistically,
the one-step
protocol
involves dehydration
andinnucleophilic
addition
to a
Mechanistically,
the
one-step
protocol
involves
dehydration
and
nucleophilic
addition
leading
to a
Mechanistically,
the one-step
protocol
involves11).
dehydration
and nucleophilic
leadingthe
to a overall
variety
variety of indole
derivatives
(Scheme
Sonication
significantlyaddition
accelerates
variety
indole (Scheme
derivatives
(Scheme significantly
11). Sonication
significantly
accelerates
the without
overall
of
indole of
derivatives
11). Sonication
accelerates
the overall
transformation
transformation
without affecting
reaction yields
[40]. The
work inspired
a multi-component
one-pot
transformation
without
affecting
reaction
yields
[40].
The
work
inspired
a
multi-component
one-pot
affecting
reaction yields
[40]. The workA,
inspired
a multi-component
one-pot with
domino
synthesis of
domino synthesis
of convolutamydine
a natural
marine natural product
antinociceptive
domino
synthesisA,ofa convolutamydine
A, a naturalwith
marine natural product
withasantinociceptive
convolutamydine
natural marine
natural
effects [41],
well
as further
effects [41], as well
as further
extension
to theproduct
preparationantinociceptive
of other functionalized
indolyl
derivatives
effects
[41],
as
well
as furtherofextension
to the preparation
of
other functionalized
indolyl derivatives
extension
to
the
preparation
other
functionalized
indolyl
derivatives
[42].
[42].
[42].
Scheme
promoted by
by sonication.
sonication.
Scheme 11.
11. Indole
Indole syntheses
syntheses involving
involving Barbier
Barbier allylindation
allylindation promoted
Scheme 11. Indole syntheses involving Barbier allylindation promoted by sonication.
Soengas and associates have also paid attention to the effect of sonication on some indiumSoengas and
andassociates
associateshave
have
attention
the of
effect
of sonication
some
Soengas
alsoalso
paidpaid
attention
to thetoeffect
sonication
on someon
indiumpromoted reactions, which can be conducted under milder conditions thus preserving high levels of
indium-promoted
can be under
conducted
under
milderthus
conditions
thus
preserving
promoted
reactions,reactions,
which canwhich
be conducted
milder
conditions
preserving
high
levels of
diastereoselection. The group applied the strategy to, for instance, the synthesis of β-lactam
high levels of diastereoselection.
The group
appliedto,
thefor
strategy
to, for
diastereoselection.
The group applied
the strategy
instance,
the instance,
synthesisthe
of synthesis
β-lactam
carbohydrates [43], β-amino acid and spirodiketopiperazines from sugar lactones [44], or allylation
of β-lactam carbohydrates
acid and spirodiketopiperazines
from sugar
[44],
carbohydrates
[43], β-amino[43],
acidβ-amino
and spirodiketopiperazines
from sugar lactones
[44], lactones
or allylation
and Reformatsky reactions on oxime ethers [45]. An interesting variation of the classical Reformatskyor allylation
and Reformatsky
reactions
on [45].
oxime
[45]. An
interesting
variation
the classical
and
Reformatsky
reactions on oxime
ethers
Anethers
interesting
variation
of the
classicalof
ReformatskyClaisen rearrangement has been described by Ishihara et al. The reaction of substituted allyl αReformatsky-Claisen
rearrangement
has beenby
described
al. The of
reaction
of substituted
Claisen
rearrangement
has been described
Ishiharaby
et Ishihara
al. The et
reaction
substituted
allyl αbromoacetates with indium and indium(III) chloride under ultrasound leads to functionalized
allyl α-bromoacetates
with indium
and indium(III)
chloride
under
ultrasoundleads
leadstoto functionalized
functionalized
bromoacetates
with indium
and indium(III)
chloride
under
ultrasound
building blocks bearing quaternary centers [46]. The transformation is run in the presence of a
centers
[46].[46].
The The
transformation
is runis
in run
the presence
of a silylating
building blocks
blocksbearing
bearingquaternary
quaternary
centers
transformation
in the presence
of a
silylating reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers
silylating reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers
promising avenues in chiral transfer, as exemplified in the asymmetric reactions shown in Scheme 12
promising avenues in chiral transfer, as exemplified in the asymmetric reactions shown in Scheme 12
starting from diastereomerically different products. It should be noted that the preparation of
starting from diastereomerically different products. It should be noted that the preparation of
univalent indium salts can be successfully accomplished from indium metal employing sonication as
univalent indium salts can be successfully accomplished from indium metal employing sonication as
well [47].
well [47].
Catalysts 2017, 7, 121
9 of 29
reagent, and involves presumably the in-situ formation of In(I) species. The protocol offers promising
avenues in chiral transfer, as exemplified in the asymmetric reactions shown in Scheme 12 starting
from diastereomerically different products. It should be noted that the preparation of univalent indium
salts
can2017,
be successfully
accomplished from indium metal employing sonication as well [47]. 9 of 29
Catalysts
7, 121
Catalysts 2017, 7, 121
9 of 29
Scheme
12. Sonochemical
Reformatsky-Claisen
rearrangement on
on chiral substrates.
substrates.
Scheme 12.
12.
Sonochemical Reformatsky-Claisen
Reformatsky-Claisen rearrangement
rearrangement
Scheme
Sonochemical
on chiral
chiral substrates.
Like indium, lighter metallic gallium has attracted considerable interest in recent years. It is
Like indium, lighter metallic gallium has attracted considerable interest in recent years.
years. It is
fairly stable under air and moisture and exhibits a low first ionization potential. Moreover, due to its
stable under
underair
airand
andmoisture
moistureand
andexhibits
exhibitsa alow
lowfirst
first
ionization
potential.
Moreover,
to
fairly stable
ionization
potential.
Moreover,
duedue
to its
low melting point (ca. 29 °C), it◦ can easily be handled in the liquid phase [48]. Ga(0) has been exploited
its low
melting
point
(ca.°C),
29it can
C), it
can easily
be handled
in thephase
liquid
phase
[48].
has been
low
melting
point
(ca. 29
easily
be handled
in the liquid
[48].
Ga(0)
hasGa(0)
been exploited
in a few stoichiometric Barbier reactions [49,50]. The ionic forms show distinctive properties; while
exploited
in a few stoichiometric
Barbier reactions
[49,50].
The
ionicshow
formsdistinctive
show distinctive
properties;
in
a few stoichiometric
Barbier reactions
[49,50]. The
ionic
forms
properties;
while
Ga(III) is a strong Lewis acid and has been employed in catalytic reactions, the chemistry of Ga(I) is
while
Ga(III)
is
a
strong
Lewis
acid
and
has
been
employed
in
catalytic
reactions,
the
chemistry
Ga(III) is a strong Lewis acid and has been employed in catalytic reactions, the chemistry of Ga(I) of
is
largely underestimated as the low oxidation state tends to undergo disproportionation to Ga(III) and
Ga(I) isunderestimated
largely underestimated
the low oxidation
tends todisproportionation
undergo disproportionation
to
largely
as the lowasoxidation
state tendsstate
to undergo
to Ga(III) and
Ga(0). In order to use Ga(0) in catalytic fashion, it should be converted into unstable Ga(I) species. In
Ga(III)In
and
Ga(0).
In order
Ga(0) fashion,
in catalytic
fashion,
should beinto
converted
into
unstable
Ga(I)
Ga(0).
order
to use
Ga(0)toinuse
catalytic
it should
beitconverted
unstable
Ga(I)
species.
In
a recent study, Qin and Schneider described the first catalytic use of elemental gallium through in
In a recent
study,
Qin anddescribed
Schneiderthe
described
the first
use ofgallium
elemental
gallium
aspecies.
recent study,
Qin and
Schneider
first catalytic
usecatalytic
of elemental
through
in
situ oxidation by Ag(I) and ultrasonic activation [51]. Ga(I)-catalyzed carbon-carbon bond formations
through
in situby
oxidation
byultrasonic
Ag(I) andactivation
ultrasonic[51].
activation
[51]. Ga(I)-catalyzed
carbon-carbon
bond
situ
oxidation
Ag(I) and
Ga(I)-catalyzed
carbon-carbon
bond formations
involving allyl or allenyl boronic esters and acetals, ketals or aminals, proceeded with essentially
formationsallyl
involving
allylboronic
or allenyl
boronic
esters and
acetals,
ketals or
aminals, proceeded
with
involving
or allenyl
esters
and acetals,
ketals
or aminals,
proceeded
with essentially
complete chemo- and regioselectivity in good overall yields (Scheme 13). The reaction time was
essentiallychemocomplete
chemoand regioselectivity
good overall
yields (Scheme
13).reaction
The reaction
complete
and
regioselectivity
in good in
overall
yields (Scheme
13). The
time time
was
significantly decreased by switching from conventional heating and stirring (40 °C in ◦dioxane, 24 h)
was
significantly
decreased
by switching
conventional
heating
and stirring
C in dioxane,
significantly
decreased
by switching
from from
conventional
heating
and stirring
(40 °C(40
in dioxane,
24 h)
to sonication (40–45 °C, 8◦ h). The Ga(0)/Ag(I) ratio and the virtual Ga(I) loading were decreased to
24 sonication
h) to sonication
(40–45
C, 8The
h). The
Ga(0)/Ag(I)
the virtual
Ga(I)
loading
were
decreased
to
(40–45
°C, 8 h).
Ga(0)/Ag(I)
ratioratio
and and
the virtual
Ga(I)
loading
were
decreased
to
2:1 and 5 mol %, respectively, under such conditions without loss of activity. An external ligand [18],
to 2:1
and
5 mol
respectively,
undersuch
suchconditions
conditionswithout
withoutloss
lossofofactivity.
activity.An
Anexternal
externalligand
ligand [18],
[18],
2:1
and
5 mol
%,%,
respectively,
under
crown-6, employed to stabilize Ga(I) catalyst, proved to be critical for full conversion. Remarkably,
catalyst, proved
proved to be critical for full conversion.
conversion. Remarkably,
Remarkably,
crown-6, employed to stabilize Ga(I) catalyst,
this reaction could be achieved on a gram scale at low catalyst loading (0.1 mol %).
this reaction could be achieved on a gram scale at low
low catalyst
catalyst loading
loading (0.1
(0.1 mol
mol %).
%).
Scheme 13. Ga(I)-catalysis under sonication for C-C bond formation.
Scheme
Scheme 13.
13. Ga(I)-catalysis
Ga(I)-catalysis under
under sonication
sonication for
for C-C
C-C bond
bond formation.
formation.
Finely dispersed gallium microspheres with concomitant encapsulation of organic dyes (which
Finely dispersed gallium microspheres with concomitant encapsulation of organic dyes (which
include
Congo
red, crystal
violet,microspheres
phenanthroline,
and concomitant
rhodamine 6G)
within them could
be achieved
Finely
dispersed
gallium
with
encapsulation
of organic
dyes
include Congo red, crystal violet, phenanthroline, and rhodamine 6G) within them could be achieved
by means
of ultrasonication
in water
forphenanthroline,
a few minutes at
55rhodamine
°C at a temperature
which
Gabe
is
(which
include
Congo red, crystal
violet,
and
6G) withinat
them
could
by means of ultrasonication in water for a few minutes at 55 °C at◦ a temperature at which Ga is
molten. This
application,
reported byinGedanken
[52,53],
though disconnected
achieved
by means
of ultrasonication
water for aand
fewco-workers
minutes at 55
C at even
a temperature
at which Ga
molten. This application, reported by Gedanken and co-workers [52,53], even though disconnected
from
synthesis,
provides potentiality
in Gedanken
removing organic
pollutants
by such
microspheres
aided by
is
molten.
This application,
reported by
and co-workers
[52,53],
even
though disconnected
from synthesis, provides potentiality in removing organic pollutants by such microspheres aided by
the mild
and relatively
inexpensive
use
sonication.
Scanning
electron
(SEM) (Figure
2)
from
synthesis,
provides
potentiality
in of
removing
organic
pollutants
bymicroscopy
such microspheres
aided by
the mild and relatively inexpensive use of sonication. Scanning electron microscopy (SEM) (Figure 2)
and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules
and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules
takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption
takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption
onto the metal surface or interlattice inclusions were ruled out.
onto the metal surface or interlattice inclusions were ruled out.
Catalysts 2017, 7, 121
10 of 29
the mild and relatively inexpensive use of sonication. Scanning electron microscopy (SEM) (Figure 2)
and transmission electron microscopy (TEM) images indicated that entrapment of organic molecules
takes place in the voids of hollow Ga particles, whereas alternative mechanisms based on adsorption
onto
the2017,
metal
surface or interlattice inclusions were ruled out.
Catalysts
7, 121
10 of 29
Figure 2.
2. SEM
formed
after
sonication
in
Figure
SEM (scanning
(scanningelectron
electronmicroscopy)
microscopy)images
imagesofofgallium
galliumparticles
particles
formed
after
sonication
(A)(A)
distilled
water
1,10-phenanthroline. Reproduced
Reproduced with
with
in
distilled
waterand
and(B)
(B)aqueous
aqueoussolution
solution(9.1
(9.1 mM)
mM) of
of 1,10-phenanthroline.
permission
from
Ref.
[53].
Copyright
2014
the
Royal
Society
of
Chemistry.
permission from Ref. [53]. Copyright 2014 the Royal Society of Chemistry.
4. Nanostructured
Nanostructured Materials
Materials from
from Transition
Transition Elements
Elements
4.
As mentioned
mentioned above,
above, the
the application
application of
of ultrasound
ultrasound to
to the
the preparation
preparation of
of micromicro- and
and nanosized
nanosized
As
materials
from
inorganic
and
organic
materials
represents
a
fertile
field
of
modern
research
materials from inorganic and organic materials represents a fertile field of modern research [8,10,54],
[8,10,54],
and cavitational
cavitational implosion
implosion not
not only
only makes
makes their
their synthesis
synthesis easier
easier but
but also
also induces
induces profound
profound
and
morphological changes
changes that
that affect
affect the
the reactivity
reactivity against
against other
othermolecules.
molecules. Although
Although zero-valent
zero-valent
morphological
elements can be employed, sonochemical procedures often harness the facile formation of metal ion
elements can be employed, sonochemical procedures often harness the facile formation of metal
species in solution to induce surface modifications and formation of supported catalysts with high
ion species in solution to induce surface modifications and formation of supported catalysts with high
activity. The following examples include cases involving early, middle, and late transition metals, as
activity. The following examples include cases involving early, middle, and late transition metals,
well as a few cases of so-called inner transition metals comprising the lanthanide f-block.
as well as a few cases of so-called inner transition metals comprising the lanthanide f -block.
4.1. First-Row
First-Row Transition
Transition Metal
Metal NPs
NPs
4.1.
Rahmani et
et al.
al. synthesized,
synthesized, under
under the
the action
actionofofultrasonic
ultrasonicirradiation,
irradiation,Cr/clinoptilolite-ZrO
Cr/clinoptilolite-ZrO22
Rahmani
nanocatalyst
to
investigate
the
CO
2
-enhanced
dehydrogenation
of
ethane
[55].
Sonication
was carried
carried
nanocatalyst to investigate the CO2 -enhanced dehydrogenation of ethane [55]. Sonication was
out by
byapplying
applyingaapulse
pulseratio
ratio(on:off)
(on:off)
0.3:0.1
at input
an input
power
W60for
60 min.
The samples
out
of of
0.3:0.1
s ats an
power
of 90ofW90for
min.
The samples
were
were
thoroughly
characterized
by
means
of
various
physicochemical
techniques,
and
the
authors
thoroughly characterized by means of various physicochemical techniques, and the authors determined
determined
that
sonication
generated
metal
oxide-NPs
with sizes
of about
4–8 nm,
that
sonication
generated
metal
oxide-NPs
with
sizes of about
4–8 nm,
promoting
the promoting
distributionthe
of
◦
distribution
of
metal
particles
and
strengthening
the
chromium-support
interaction.
The
best
metal particles and strengthening the chromium-support interaction. The best performance at 700 C
performance
at 700 °C
forthe
5 h Cr/CLT-Z25(U)
was attained with
the Cr/CLT-Z25(U)
nanocatalyst
giving 38% ethylene
for
5 h was attained
with
nanocatalyst
giving 38%
ethylene yield.
yield.Iron-based nanomaterials are especially popular in terms of facile preparation from inexpensive
Iron-based
are especially
popular in
terms
of facile
preparation
from
inexpensive
precursors.
Both nanomaterials
synthetic and degradation
applications
are
frequently
reported
for such
NPs,
which can
precursors.
Both
synthetic
and
degradation
applications
are
frequently
reported
for
such
NPs, which
be reused and/or recycled, thus contributing to sustainable methodologies. Thus, Dai et al. obtained
an
can be reused
and/or
recycled,precipitation
thus contributing
to sustainable
Thus, Dai et[56].
al.
ozonation
catalyst
via chemical
and further
calcination,methodologies.
assisted by ultrasonication
obtained
an
ozonation
catalyst
via
chemical
precipitation
and
further
calcination,
assisted
by
The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3 O4 magnetic core, a silica
ultrasonication
[56].
The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3O4
mid-layer,
and CeO
2 outer layer (47: Fe3 O4 @SiO2 @CeO2 ) for the degradation of aspirin (acetylsalicylic
magnetic
silica could
mid-layer,
CeO2the
outer
layer
(47: Fe
3O4@SiO2@CeO2) for the degradation of
acid,
ASA,core,
46) awhich
also and
enhance
total
organic
carbon
removal (Scheme 14). The ASA
aspirin
(acetylsalicylic
acid,
ASA,
46)
which
could
also
enhance
total 81.0%,
organicwhile
carbon
removal
removal with Fe3 O4 @SiO2 @CeO2 as catalyst conducted for 60 minthe
reached
67.3%
with
(Scheme 14). The ASA removal with Fe3O4@SiO2@CeO2 as catalyst conducted for 60 min reached
81.0%, while 67.3% with Fe3O4@SiO2, 66.1% with Fe3O4, and only 64.1% with O3 alone. This type of
catalyst had magnetic recyclability and low metal leaching.
precursors. Both synthetic and degradation applications are frequently reported for such NPs, which
can be reused and/or recycled, thus contributing to sustainable methodologies. Thus, Dai et al.
obtained an ozonation catalyst via chemical precipitation and further calcination, assisted by
ultrasonication [56]. The authors prepared magnetic core/shell CeO2 nanoparticles with a Fe3O4
magnetic
core, a silica mid-layer, and CeO2 outer layer (47: Fe3O4@SiO2@CeO2) for the degradation
of
Catalysts 2017, 7, 121
11 of 29
aspirin (acetylsalicylic acid, ASA, 46) which could also enhance the total organic carbon removal
(Scheme 14). The ASA removal with Fe3O4@SiO2@CeO2 as catalyst conducted for 60 min reached
Fe3 O4 @SiO
66.1% with
with Fe
Fe33O
O44@SiO
, and2, only
alone.
type
of catalyst
magnetic
81.0%,
while
66.1%64.1%
with with
Fe3O4O
, 3and
onlyThis
64.1%
with
O3 alone.had
This
type of
2 , 67.3%
recyclability
and
low
metal
leaching.
catalyst had magnetic recyclability and low metal leaching.
Catalysts 2017, 7, 121
11 of 29
Scheme 14. Degradation of a pharmaceutical compound using an iron-based ozonation catalyst in the
Scheme 14. Degradation of a pharmaceutical compound using an iron-based ozonation catalyst in the
presence of ultrasound.
presence of ultrasound.
Gobouri et
have also
also shown
shown that
that simultaneous
simultaneous irradiation
irradiation of
O33-NPs
-NPs (iron
(iron oxide
Gobouri
et al.
al. have
of Fe
Fe22O
oxide
nanoparticles)
with
light
and
sonication
can
enhance
their
catalytic
effects
in
degradation
processes
[57].
nanoparticles) with light and sonication can enhance their catalytic effects in degradation processes
The
catalytic
activity
was
evaluated
in
the
degradation
of
Eosin
Y
and
Rhodamine
B
under
certain
[57]. The catalytic activity was evaluated in the degradation of Eosin Y and Rhodamine B under
experimental
conditions
(pH, amount
catalyst,
organic
The catalytic
certain
experimental
conditions
(pH, of
amount
of and
catalyst,
andadditives).
organic additives).
Theefficiency
catalytic
increased
significantly,
from
53%
to
72.5%
and
5.59%
to
20.79%,
respectively,
when
the
degradation
efficiency increased significantly, from 53% to 72.5% and 5.59% to 20.79%, respectively, when the
was
conducted
under
simultaneous
irradiation. irradiation.
degradation
was
conducted
under simultaneous
In line
line with
In
with the
the use
use of
of advanced
advanced oxidation
oxidation processes
processes (AOPs)
(AOPs) as
as degradative
degradative strategies,
strategies, aa novel
novel
method
of
decomposing
ibuprofen
(IBP),
a
widely
used
anti-inflammatory
drug,
was
developed by
by
method of decomposing ibuprofen (IBP), a widely used anti-inflammatory drug, was developed
Ziylan
and
Ince
[58].
Catalytic
ozonation
using
high-frequency
ultrasound
and
Fe-based
species
was
Ziylan and Ince [58]. Catalytic ozonation using high-frequency ultrasound and Fe-based species was
−1
applied. The
The optimum
optimum concentration
concentration of
of IBP,
IBP, O
flow rate,
applied.
O33 flow
rate, and
and US
US power
power were
were 50
50 µM,
μM, 12
12mg
mgmin
min−1,,
−
1
and 0.23
respectively. The
Themost
mostcritical
criticalparameter
parameterwas
was found
found to
to be
be pH,
pH, as
as the
the latter
latter
and
0.23 W
W mL
mL−1, , respectively.
determined the
the mass
masstransfer
transferand
anddecomposition
decompositionofofO3Oas
well
as the
diffusion
of solutes
3 as
determined
well
as the
diffusion
of solutes
fromfrom
the
the
bulk
liquid
to
the
interfaces
(both
gas–liquid
and
solid–liquid).
The
maximum
degree
of
oxidation
bulk liquid to the interfaces (both gas–liquid and solid–liquid). The maximum degree of oxidation
could be
be obtained
obtained with
with zero-valent
zero-valent iron
iron (ZVI)
(ZVI) nanoparticles
at pH
pH 3.0
The synergy
synergy of
of
could
nanoparticles at
3.0 (100%,
(100%, 58%).
58%). The
sonication and
and ZVI
ZVI at
at acid
acid pH
pH was
was ascribed
ascribed to
to aa series
series of
of factors
factors such
such as
as the
the existence
existence of
of surface
surface areas
areas
sonication
enriched
with
large
reaction
and
nucleation
sites,
the
role
of
active
Fe,
reactive
oxygen
species
which
enriched with large reaction and nucleation sites, the role of active Fe, reactive oxygen species which
promote Fenton-like
andand
finally
the contribution
of hydrodynamic
shear forces
to continuous
promote
Fenton-likereactions,
reactions,
finally
the contribution
of hydrodynamic
shear
forces to
cleaning
of
the
catalytic
surface.
The
oxidation
and
mineralization
of
IBP
was
efficiently
accelerated
in
continuous cleaning of the catalytic surface. The oxidation and mineralization of IBP was efficiently
1-h
ozonation.
accelerated in 1-h ozonation.
The use
has
also
been
applied
to
The
use of
of transition
transition metal
metalnanocomposites
nanocompositesasascatalysts
catalystsunder
undersonication
sonication
has
also
been
applied
the
synthesis
of
monoand
disubstituted
dihydroquinazolinones
(52)
in
good
yields
[59].
The
reaction
to the synthesis of mono- and disubstituted dihydroquinazolinones (52) in good yields [59]. The
depicted depicted
in Schemein15Scheme
involves15
a three-component
reaction of isatoic
anhydride
(49), anhydride
primary amines
reaction
involves a three-component
reaction
of isatoic
(49),
or, alternatively
ammonium
acetate
(50), with
aryl(50),
aldehydes
catalyzed
some transition
primary
amines or,
alternatively
ammonium
acetate
with aryl(51)
aldehydes
(51)by
catalyzed
by some
metal multi-walled
carbon nanotubes
(MWCNTs).
The activities
of these species
found
to
transition
metal multi-walled
carbon nanotubes
(MWCNTs).
The activities
of thesewere
species
were
decrease
in the order:
Co–MWCNTs
> Pt–MWCNTs
> Cu–MWCNTs
> Ag–MWCNTs.
The protocol
found
to decrease
in the
order: Co–MWCNTs
> Pt–MWCNTs
> Cu–MWCNTs
> Ag–MWCNTs.
The
showed
advantages
as
mild
reaction
conditions,
easy
work-up,
green
character,
energy
efficiency
and
protocol showed advantages as mild reaction conditions, easy work-up, green character, energy
reusable
catalysts.
efficiency and reusable catalysts.
Scheme
15.One-pot
One-pot
multicomponent
condensation
catalyzed
bymulti-walled
metal multi-walled
carbon
Scheme 15.
multicomponent
condensation
catalyzed
by metal
carbon nanotubes
nanotubes
(MWCNTs)
under
ultrasound
irradiation.
(MWCNTs) under ultrasound irradiation.
A related multicomponent strategy was reported by Safari and Zarnegar leading to an improved
preparation of dihydropyrimidin-one/thione derivatives (56) starting from aromatic aldehydes (53),
urea/thiourea (54), and β-dicarbonyl compounds (55), using Fe3O4–MWCNT as nanocatalyst under
ultrasound irradiation [60] (Scheme 16). This Biginelli transformation tolerates numerous functional
groups in the reaction partners and proceeds in short times and good yields, due to the high catalytic
Catalysts 2017, 7, 121
12 of 29
A related multicomponent strategy was reported by Safari and Zarnegar leading to an improved
preparation of dihydropyrimidin-one/thione derivatives (56) starting from aromatic aldehydes (53),
urea/thiourea (54), and β-dicarbonyl compounds (55), using Fe3 O4 –MWCNT as nanocatalyst under
ultrasound irradiation [60] (Scheme 16). This Biginelli transformation tolerates numerous functional
groups in the reaction partners and proceeds in short times and good yields, due to the high catalytic
activity of the NPs. The catalyst was reused without significant losses in performance and was,
in addition,
easily
Catalysts 2017,
7, 121recovered from the reaction mixture by means of an external magnet).
12 of 29
Scheme
Multicomponent
synthesis
of dihydropyrimidin-ones/thiones
using
3O4-MWCNT
Scheme
16. 16.
Multicomponent
synthesis
of dihydropyrimidin-ones/thiones
using
Fe3 OFe
4 –MWCNT
under
ultrasonication.
under ultrasonication.
Nickel NPs, in the form of montmorillonite-supported nickel nanoparticles (Ni-MMT) are good
Nickel NPs, in the form of montmorillonite-supported nickel nanoparticles (Ni-MMT) are good
catalysts that can be easily generated through an ultrasound-assisted cation exchange impregnation
catalysts that can be easily generated through an ultrasound-assisted cation exchange impregnation
method within ca. 30 min [61]. Montmorillonites, which are phyllosilicates with a lamellar structure,
method within ca. 30 min [61]. Montmorillonites, which are phyllosilicates with a lamellar structure,
are ideal candidates for developing a low-cost, anti-coke, and anti-sintering support, thus preventing
are ideal candidates for developing a low-cost, anti-coke, and anti-sintering support, thus preventing
the agglomeration of NiNPs. The catalysts were evaluated in the production of H2 by the glycerol
the agglomeration of NiNPs. The catalysts were evaluated in the production of H2 by the glycerol
steam reforming reaction (GSR) with low CO concentration. The effect of Ni contents and calcination
steam reforming reaction (GSR) with low CO concentration. The effect of Ni contents and calcination
temperatures indicated that a mesoporous Ni-MMT catalyst with 20.9% Ni calcinated at 700 °C
temperatures indicated that a mesoporous Ni-MMT catalyst with 20.9% Ni calcinated at 700 ◦ C showed
showed the highest activity and stability for GSR conducted at 600 °C.
the highest activity and stability for GSR conducted at 600 ◦ C.
A nanostructured Co3O4 catalyst enables the selective oxidation of vanillyl alcohol to vanillin at
A nanostructured Co3 O4 catalyst enables the selective oxidation of vanillyl alcohol to vanillin at
75
°C
and ambient pressure with H2O2 as the sole oxidant and water as solvent in conjuction with
75 ◦ C and ambient pressure with H2 O2 as the sole oxidant and water−1 as solvent in conjuction with
ultrasound (at 19.95 kHz and volume acoustic power = 0.25 W mL
determined by calorimety) [62].
ultrasound (at 19.95 kHz and volume acoustic power = 0.25 W mL−1 determined by calorimety) [62].
Although both yield and selectivity were modest, such figures were higher under ultrasound in only
Although both yield and selectivity were modest, such figures were higher under ultrasound in
15 min (compared to 1 h under silent conditions). Moreover, a commercial Co3O4 nanopowder (with
only 15 min (compared to 12 h−1under silent conditions). Moreover, a commercial Co3 O4 nanopowder
a surface area do 32 m g
) was also less efficient under sonication. The results were ascribed to
(with a surface area do 32 m2 g−1 ) was also less efficient under sonication. The results were ascribed
enhanced physical (mass transfer) and chemical (production of OH radicals by sonicating H2O2)
to enhanced physical (mass transfer) and chemical (production of OH radicals by sonicating H2 O2 )
effects promoted by ultrasound on the catalyst surface.
effects promoted by ultrasound on the catalyst surface.
Noble and coinage transition metal NPs have been extensively used in recent years in view of
Noble and coinage transition metal NPs have been extensively used in recent years in view of
the numerous organic transformations accomplished with such metals. The use of NPs often provide
the numerous organic transformations accomplished with such metals. The use of NPs often provide
more efficient and environmentally benign approaches (by reducing metal loadings and wastes) than
more efficient and environmentally benign approaches (by reducing metal loadings and wastes)
the corresponding conventional catalysts employed under homogeneous or heterogeneous
than the corresponding conventional catalysts employed under homogeneous or heterogeneous
conditions. As a representative example, CuAAC reactions affording triazoles with high
conditions. As a representative example, CuAAC reactions affording triazoles with high regioselectivity
regioselectivity in good yields, have been performed with stable, water-soluble Cu-NPs synthesized
in good yields, have been performed with stable, water-soluble Cu-NPs synthesized by means of
by means of a double-hydrophilic block copolymer as template under 10-min sonication. Such Cua double-hydrophilic block copolymer as template under 10-min sonication. Such Cu-NPs could
NPs could further be reused three times without any substantial loss of catalytic efficiency [63].
further be reused three times without any substantial loss of catalytic efficiency [63].
Surface modification of micro-sized TiO2 using high-intensity ultrasound was reported by
Surface modification of micro-sized TiO2 using high-intensity ultrasound was reported by
Stucchi and co-workers in order to overcome the lack of photocatalyst activity of the semiconductor
Stucchi and co-workers in order to overcome the lack of photocatalyst activity of the semiconductor
under visible light [64]. The authors studied the photodegradation of acetone and acetaldehyde
under visible light [64]. The authors studied the photodegradation of acetone and acetaldehyde
(hydrophilic and polar indoor pollutants) in the gas phase, using an LED lamp. The US-assisted
(hydrophilic and polar indoor pollutants) in the gas phase, using an LED lamp. The US-assisted
synthesis of metallic copper and copper oxides species at the TiO2 surface employed CuCl2 as
synthesis of metallic copper and copper oxides species at the TiO2 surface employed CuCl2 as
precursor (Cu amounts ranged from 1 to 75 wt %), showing diverse morphologies in accordance with
copper content. Ultrasonic parameters (20-kHz frequency, 200 W, and a horn of 13-mm diameter)
were important for the preparation of nanoparticles with a good distribution on the TiO2 support.
The optimal ultrasonic operating conditions for the copper deposition (50 W cm−2, 2.5 h, 62 °C)
accelerated the diffusion of solute in the reaction medium, generating available active sites on the
TiO2 surface, thereby increasing the visible light absorption and improving the photocatalytic
Catalysts 2017, 7, 121
13 of 29
precursor (Cu amounts ranged from 1 to 75 wt %), showing diverse morphologies in accordance
with copper content. Ultrasonic parameters (20-kHz frequency, 200 W, and a horn of 13-mm diameter)
were important for the preparation of nanoparticles with a good distribution on the TiO2 support.
The optimal ultrasonic operating conditions for the copper deposition (50 W cm−2 , 2.5 h, 62 ◦ C)
accelerated the diffusion of solute in the reaction medium, generating available active sites on the TiO2
surface, thereby increasing the visible light absorption and improving the photocatalytic activity.
4.2. Pd and Pt NPs
Versatile enough among metal catalysts, palladium has proven its efficiency and broad scope in
promoting the quintessential point of organic synthesis, i.e., carbon-carbon bond-forming reactions.
Pd-NPs
serve
precatalysts, which are produced in situ from Pd salts, usually Pd(II) precursors,
Catalysts
2017,as
7, 121
13 of 29
Catalysts 2017, 7, 121
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then reduced to Pd(0), forming Pd-NPs [65].
Pd-NPs serve
asstudied
precatalysts,
which are produced
in situ from
Pd salts, usually Pd(II)
precursors,
Zhang
al. as
an aqueous
reaction
halogeno-arenes
with
olefins (58)
Pd-NPs et
serve
precatalysts,
which are Heck
produced
in situof
from
Pd salts, usually(57)
Pd(II)
precursors,
then
reduced to
Pd(0), forming
Pd-NPs [65].
under
ultrasonic
irradiation
at
ambient
temperature
affording
the
corresponding
products
(59) in
then reduced to Pd(0), forming Pd-NPs [65].
Zhang
et
al.
studied
an
aqueous
Heck
reaction
of
halogeno-arenes
(57)
with
olefins
(58)
under
moderateZhang
to high
yields
(Scheme
17) [66].
in-situ were
also olefins
a recyclable
catalyst.
et al.
studied
an aqueous
HeckPd-NPs
reaction prepared
of halogeno-arenes
(57) with
(58) under
ultrasonic irradiation at ambient temperature affording the corresponding products (59) in moderate
Furthermore,
Heck reaction
under
such benign
conditions
offered high
regioselection
of para-over
ultrasonicthe
irradiation
at ambient
temperature
affording
the corresponding
products
(59) in moderate
to high yields (Scheme 17) [66]. Pd-NPs prepared in-situ were also a recyclable catalyst. Furthermore,
to high yields (Scheme
17) [66].
Pd-NPs
prepared
in-situ
also a recyclable groups.
catalyst. Furthermore,
ortho-substitution
in phenyl
iodides
(57),
notably
withwere
electron-donating
Ultrasound was
the Heck reaction under such benign conditions offered high regioselection of para-over orthothe
Heck
reaction
under
such
benign
conditions
offered
high
regioselection
of para-over
orthoessential
in
the
formation
of
Pd-NPs;
without
it,
slow
generation
and
no
aggregation
of the
substitution in phenyl iodides (57), notably with electron-donating groups. Ultrasound was essential
substitution
in
phenyl
iodides
(57),
notably
with
electron-donating
groups.
Ultrasound
was
essential
nanomaterial
took
place.
The
insertion
of
palladium
into
the
C-I
bond
benefits
from
the
combined
in the formation of Pd-NPs; without it, slow generation and no aggregation of the nanomaterial took
in the formation of Pd-NPs; without it, slow generation and no aggregation of the nanomaterial took
actionplace.
between
the surface
energy ofinto
Pd-NPs
and
thebenefits
energyfrom
derived
from ultrasonic
cavitation.
The insertion
of palladium
the C-I
bond
the combined
action between
the
place. The insertion of palladium into the C-I bond benefits from the combined action between the
surface
energy
of Pd-NPs
andso-called
the energySonogashira
derived from reaction,
ultrasonic cavitation.
A
related
coupling,
the
which enables the preparation of
surface energy of Pd-NPs and the energy derived from ultrasonic cavitation.
A related
coupling,
the
so-called
Sonogashira
reaction,
which enables
the preparation of
disubstituted
acetylenes
(62;the
Scheme
18), Sonogashira
has also been
conducted
nanoparticulated
A related
coupling,
so-called
reaction,
whichwith
enables
the preparationcatalysts
of
disubstituted
acetylenes
(62;
Scheme
18),
has
also
been
conducted
with
nanoparticulated
catalysts
and ultrasonic
activation
Gholap
and
[67].conducted
The protocol
could be performed
in short
disubstituted
acetylenesby(62;
Scheme
18),associates
has also been
with nanoparticulated
catalysts
and ultrasonic
activation
by Gholap and associates
[67].
The protocol
could be performed
insonication.
short
reaction
withactivation
high chemoselectivity
and good[67].
yields
room temperature,
underin
and times
ultrasonic
by Gholap and associates
The at
protocol
could be performed
short
reaction times with high chemoselectivity and good yields at room temperature, under sonication.
reaction times
withwere
high obtained
chemoselectivity
and good
yields at room
temperature,
sonication.
Comparative
analyses
from reactions
conducted
in acetone
and in under
an ionic
liquid, in the
Comparative analyses were obtained from reactions conducted in acetone and in an ionic liquid, in
Comparative
analyses
were and
obtained
from reactions
in acetone
and in anirradiation
ionic liquid,not
in only
absence
of a phosphine
ligand
co-catalysts.
It wasconducted
determined
that ultrasonic
the absence of a phosphine ligand and co-catalysts. It was determined that ultrasonic irradiation not
the absence
of a phosphine
ligand
and
co-catalysts.
It was
determined
that ultrasonic
irradiation
not
generated
the
Pd(0)-NPs
as
the
active
catalyst,
but
also
enhanced
the
catalytic
activity
of
this
species
in
only generated the Pd(0)-NPs as the active catalyst, but also enhanced the catalytic activity of this
only
generated
the
Pd(0)-NPs
as
the
active
catalyst,
but
also
enhanced
the
catalytic
activity
of
this
the cross-coupling
reaction.
species in the cross-coupling
reaction.
species in the cross-coupling reaction.
Scheme 17. Sonochemical Heck reaction with PdNPs in aqueous medium.
Scheme
17.17.
Sonochemical
withPdNPs
PdNPs
aqueous
medium.
Scheme
SonochemicalHeck
Heck reaction
reaction with
inin
aqueous
medium.
Scheme 18. Sonochemical Sonogashira coupling with in situ PdNPs.
Scheme
SonochemicalSonogashira
Sonogashira coupling
in in
situ
PdNPs.
Scheme
18.18.
Sonochemical
couplingwith
with
situ
PdNPs.
Palladium NPs merged with different supports have also found a variety of applications. The
Palladium NPs merged with different supports have also found a variety of applications. The
electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4/C) surfaces
electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4/C) surfaces
have been investigated recently [68]. Both Pd-NPs and deposition onto the support were obtained by
have been investigated recently [68]. Both Pd-NPs and deposition onto the support were obtained by
sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural,
sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural,
morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction
morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction
(XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on the
(XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on the
surface of LiFePO4/C were tested using various concentrations of Pd(II). It is worth pointing out that
Catalysts 2017, 7, 121
14 of 29
Palladium NPs merged with different supports have also found a variety of applications.
The electrochemical performance of Pd-NPs deposited onto carbon-coated LiFePO4 (LiFePO4 /C)
surfaces have been investigated recently [68]. Both Pd-NPs and deposition onto the support were
obtained by sonochemical procedure and stirring (at a frequency of 200 kHz and 8.4 W). The structural,
morphological and electrochemical performances were evaluated by means of TEM, X-ray diffraction
(XRD), and charge–discharge measurements. The size of NPs (homogenous size distributions) on
the surface of LiFePO4 /C were tested using various concentrations of Pd(II). It is worth pointing
out that the technique can be used to ameliorate the electrochemical performance of LiFePO4 for
battery applications.
Heterometallic NPs can also be prepared by similar strategies of stabilization and deposition,
which are enhanced by ultrasonic irradiation, such as the sonochemical reduction of Pd and Au salts
to the zero-valent oxidation state followed by immobilization of the resulting nanoparticles on TiO2
surfaces [69]. The sonolytic oxidation of paracetamol in aqueous solution catalyzed by Pd–TiO2 and
Pd/Au–TiO2 was especially effective due to their considerably smaller size (than that of Au–TiO2 ).
Catalysts 2017, 7, 121
14 of 29
Furthermore, the activity of the Pd–TiO2 nanocomposite was improved under ultrasonic and UV
zero-valent
irradiationtoatthe254
nm. oxidation state followed by immobilization of the resulting nanoparticles on TiO2
surfaces [69]. The sonolytic oxidation of paracetamol in aqueous solution catalyzed by Pd–TiO2 and
Kim et al. prepared Pd-Sn NPs under ultrasonic irradiation at 20 kHz with an input power
Pd/Au–TiO
especially effective due to their considerably smaller size (than that of Au–TiO2).
2 for 22 was
of 42 W/cm
hthe[70].
The
electrocatalytic
activitywasfor
oxygenunder
reduction
in
Furthermore,
activity
of the
Pd–TiO2 nanocomposite
improved
ultrasonicwas
and evaluated
UV
0.5 M KOH
and was
found
irradiation
at 254
nm. to be greater than those of Pt or Pd NPs in alkaline media. Pd was
al. metallic
prepared Pd-Sn
under
ultrasonic
at 20 kHz
withofan3–5
input
power
of 42
present chieflyKim
in et
the
stateNPs
in the
Pd-Sn
NPsirradiation
(with average
size
nm).
The
factors that
W/cm2 for 2 h [70]. The electrocatalytic activity for oxygen reduction was evaluated in
affected the particle size and dispersion of Pd-Sn NPs and, therefore controlled the electroactivity
0.5 M KOH and was found to be greater than those of Pt or Pd NPs in alkaline media. Pd was present
for oxygenchiefly
reduction
were state
the molar
ratioNPs
of (with
Pd toaverage
Sn ions
initial
concentrations,
in the metallic
in the Pd-Sn
sizein
of solution,
3–5 nm). Thetheir
factors
that affected
the volume
ethanol
increasing
radicals
duringthe
sonolysis),
as well
as the amount
theofparticle
size(thereby
and dispersion
of Pd-Snhydrogen
NPs and, therefore
controlled
electroactivity
for oxygen
reduction
were
the
molar
ratio
of
Pd
to
Sn
ions
in
solution,
their
initial
concentrations,
the
volume
of citric acid (acting as a stabilizer and controlling the surface properties). In order toofassess the
ethanol (thereby increasing hydrogen radicals during sonolysis), as well as the amount of citric acid
electrocatalytic
action, indium-doped tin oxide (ITO) electrodes were modified with Pd or Pd-Sn
(acting as a stabilizer and controlling the surface properties). In order to assess the electrocatalytic
NPs, which
involve
the initial
oxidation
of thewere
electrode
make
OH-terminated,
followed by
action, indium-doped
tin oxide
(ITO) electrodes
modifiedtowith
Pd oritPd-Sn
NPs, which involve
◦
aminosilanization
3-aminopropyl-triethoxysilane
(APS) and
subsequent
polymerization
the initial with
oxidation
of the electrode to make it OH-terminated,
followed
by aminosilanization
with at 100 C.
3-aminopropyl-triethoxysilane
(APS) and
subsequent
polymerization
100 °C. The
resulting
APSThe resulting
APS-modified ITO electrode
was
immersed
in a citrate at
solution
of Pd-Sn
NPs
(Scheme 19).
modified ITO electrode was immersed in a citrate solution of Pd-Sn NPs (Scheme 19).
Scheme
19. In-doped
tin oxide
(ITO)
electrodes modified
modified with
Pd–Sn
NPs,NPs,
obtained
by sonochemical
Scheme 19.
In-doped
tin oxide
(ITO)
electrodes
with
Pd-Sn
obtained
by sonochemical
Reproduced with permission from Ref. [70]. Copyright 2009 Elsevier Ltd.
reduction.reduction.
Reproduced
with permission from Ref. [70]. Copyright 2009 Elsevier Ltd.
A simple process to synthesize C60-Pd polymer spherical nanoparticles (20–200 nm in diameter)
was
reported
Brancewicz
et al.C[71].
The polymer was obtained from a benzene solution containing
A simple processbyto
synthesize
60 -Pd polymer spherical nanoparticles (20–200 nm in diameter)
precursors such as the well-known complex of zero-valent palladium (Pd2(dba)3·CHCl3) and fullerene
was reported by Brancewicz et al. [71]. The polymer was obtained from a benzene solution containing
(C60), and showed a tendency to precipitate in large cubic particles (20–80 μm) which are composed
precursorsofsuch
the well-known
complex
zero-valent
palladium
(Pd2 (dba)
·CHCl
3 ) and fullerene
smallasspherical
nanoparticles
(20–200 of
nm).
The procedure
takes advantage
of 3the
ultrasound
(C60 ), and energy
showed
a tendency
to precipitate
in large
cubic particles
(20–80
µm) which
composed of
effect
to disintegrate
the cubic particles
in spherical
nanoparticles.
Moreover,
the size are
of such
C60-Pd nanoparticles
NPs can be fine-tuned
by the
concentration
of the parent
time of
small spherical
(20–200
nm).
The procedure
takessolution,
advantage
ofpolymerization,
the ultrasound energy
temperature, and stirring conditions. A solution containing a 3:1 ratio of [Pd] to [C60] led to the most
homogeneous size of NPs. Dispersed C60-Pd NPs are stable in many organic protic and aprotic
solvents. The thin solid films formed from chemically synthesized nanoparticles exhibited reversible
electrochemical properties, e.g., high efficiency of electro-reduction and good capacitance.
Highly homogeneous, spherical porous Pd nanostructures (SPPNs) with rough surfaces were
obtained through a facile and rapid ultrasound-promoted reduction [72]. Sonication plays the dual
role of speeding up the spontaneous assembly of Pd-NPs to avoid oriented attachment and
Catalysts 2017, 7, 121
15 of 29
effect to disintegrate the cubic particles in spherical nanoparticles. Moreover, the size of such C60 -Pd
NPs can be fine-tuned by the concentration of the parent solution, time of polymerization, temperature,
and stirring conditions. A solution containing a 3:1 ratio of [Pd] to [C60 ] led to the most homogeneous
size of NPs. Dispersed C60 -Pd NPs are stable in many organic protic and aprotic solvents. The thin
solid films formed from chemically synthesized nanoparticles exhibited reversible electrochemical
properties, e.g., high efficiency of electro-reduction and good capacitance.
Highly homogeneous, spherical porous Pd nanostructures (SPPNs) with rough surfaces were
obtained through a facile and rapid ultrasound-promoted reduction [72]. Sonication plays the
dual role of speeding up the spontaneous assembly of Pd-NPs to avoid oriented attachment and
maintaining a homogeneous dispersion of such nanostructures. The synthesis involves the sonication
of an aqueous solution of K2 PdCl4 plus ascorbic acid for 7 min at 40 ◦ C without any additives or
templates. Isolated structures with a narrow size distribution and diameters ranging from 40 to 100 nm
(controllable through the concentration of the Pd salt) were obtained. Typical products are formed
by loosely packed grains of 2–3 nm with a diameter of 52 nm. The electro-oxidation of formic acid
Catalysts 2017, 7, 121
15 of 29
was employed to assess the catalytic performance of the SPPNs. Long-term stability and recyclability
an aqueous solution
of K2PdCl
4 pluscatalyst
ascorbic acid
for 7 min
at 40 °C without
any additives or
makes thisofnanomaterial
a suitable
anode
for formic
acid-based
fuel cells.
templates.
structuresassisted
with a narrow
size distribution
diameters ranging
from
40 to 100
Likewise,
the Isolated
ultrasonically
synthesis
of Pd-Cuand
nanocatalysts
with
various
Pd loadings
nm (controllable through the concentration of the Pd salt) were obtained. Typical products are
and their activities toward CO oxidation has also been reported [73]. The results of catalytic tests
formed by loosely packed grains of 2–3 nm with a diameter of 52 nm. The electro-oxidation of formic
indicated acid
thatwas
NPsemployed
containing
a high
of Pd showed
a better
to assess
the amount
catalytic performance
of the
SPPNs. catalytic
Long-termperformance
stability and at lower
recyclability
makes this nanomaterial
a suitablePd(1.5%)-Cu(20%)/Al
anode catalyst for formic acid-based
fuel
cells.
temperatures.
A nanomaterial
of composition
O
exhibited
the best activity
2 3
Likewise,
the
ultrasonically
assisted
synthesis
of
Pd-Cu
nanocatalysts
with
various
Pd
loadings
after 95 min of ultrasonic irradiation exposure.
and their activities toward CO oxidation has also been reported [73]. The results of catalytic tests
Colmenares
et al. developed an outstanding photodeposition method combined with ultrasonic
indicated that NPs containing a high amount of Pd showed a better catalytic performance at lower
irradiation,
i.e.,
sonophotodeposition
method
[74,75], for preparing
bimetallic
temperatures. A nanomaterial of (SPD)
composition
Pd(1.5%)-Cu(20%)/Al
2O3 exhibited
the best Pd-Au
activity alloy NPs
95 min of
ultrasonic irradiation
exposure.
supportedafter
on titania
(Pd-Au/TiO
The preparation was accomplished under mild conditions
2 P90) [76].
Colmenaresand
et al.pressure),
developed anin
outstanding
photodeposition
method
combinedstrong
with ultrasonic
(ambient temperature
short times,
and without
requiring
reducing agents.
irradiation, i.e., sonophotodeposition (SPD) method [74,75], for preparing bimetallic Pd-Au alloy NPs
The bimetallic
system
exhibited
remarkable
catalytic
activity
(83%),
selectivity
(70%),
and stability in
supported on titania (Pd-Au/TiO2 P90) [76]. The preparation was accomplished under mild
the oxidation
of methanol
yield methyl
formate. inThe
enhanced
catalytic
rationalized
conditions
(ambient to
temperature
and pressure),
short
times, and
without activity
requiringwas
strong
reducing
agents.effects
The bimetallic
system exhibited
remarkable
catalytic
activity
(83%),
selectivity of Pd-Au
in terms of
synergistic
by combining
Au- and
Pd-NPs,
and the
strong
interaction
(70%), and stability in the oxidation of methanol to yield methyl formate. The enhanced catalytic
with titania.
activity was rationalized in terms of synergistic effects by combining Au- and Pd-NPs, and the
Pd NPs
(with sizes between 10 and 17 nm) have also been prepared by a sustainable method that
strong interaction of Pd-Au with titania.
involves the use
a (with
bioreductant,
namely
Perilla frutescens
leaf, method
under sonication
Pd of
NPs
sizes between
10 andan
17aqueous
nm) have extract
also beenofprepared
by a sustainable
◦
that
involves
the
use
of
a
bioreductant,
namely
an
aqueous
extract
of
Perilla
frutescens
leaf,
under
(ultrasonic bath for 2 h at 60 C) without any additional surfactant or capping agent [77].
It is thought
sonication
(ultrasonic
bath
for
2
h
at
60
°C)
without
any
additional
surfactant
or
capping
agent
[77]. The NPs
that polyphenols and flavonoids present in the bio-organic extract reduced Pd(II) to Pd(0).
It is thought that polyphenols and flavonoids present in the bio-organic extract reduced Pd(II) to
were thenPd(0).
applied
to a high-yielding (81%–95%), three-component synthesis of aryl-substituted
The NPs were then applied to a high-yielding (81%–95%), three-component synthesis of arylpyrazolylphosphonates
from the corresponding
aldehydes,
triethylphosphite
in
substituted pyrazolylphosphonates
from thepyrazolones,
corresponding aryl
pyrazolones,
aryl and
aldehydes,
and
triethylphosphite
in ethanol (Scheme 20).
ethanol (Scheme
20).
Scheme
20. Ultrasonic
generation
PdNPs using
using bioreduction
with with
P. frutescens
and subsequent
Scheme 20.
Ultrasonic
generation
of of
PdNPs
bioreduction
P. frutescens
and subsequent
multicomponent synthesis of pyrazolone derivatives. Reproduced with permission from Ref. [77].
multicomponent synthesis of pyrazolone derivatives. Reproduced with permission from Ref. [77].
Copyright 2015 the Royal Society of Chemistry.
Copyright 2015 the Royal Society of Chemistry.
Together with Pd, the heavier congener Pt can be obtained as nanostructured material, stabilized
by different ligands and additives under sonication. Pt-NPs have met a myriad of catalytic functions
in basic and applied research, some largely related to photocatalysis and electrochemistry. Thus, from
2006 onwards, Colmenares et al. have been working with some titania-based catalysts doped with
different transition metals, obtained by the sol-gel method using titanium tetraisopropoxide as
precursor [78]. Magnetic stirring (MS) or ultrasound (US) were used to produce the aging of the gel.
Sonication enabled the synthesis of pure anatase nanoparticles with increased surface areas, which
Catalysts 2017, 7, 121
16 of 29
Together with Pd, the heavier congener Pt can be obtained as nanostructured material,
stabilized by different ligands and additives under sonication. Pt-NPs have met a myriad of catalytic
functions in basic and applied research, some largely related to photocatalysis and electrochemistry.
Thus, from 2006 onwards, Colmenares et al. have been working with some titania-based catalysts
doped with different transition metals, obtained by the sol-gel method using titanium tetraisopropoxide
as precursor [78]. Magnetic stirring (MS) or ultrasound (US) were used to produce the aging of
the gel. Sonication enabled the synthesis of pure anatase nanoparticles with increased surface
areas, which influence the photocatalytic performance. Further pursuits included not only titanium
tetra-isopropoxide, but also titanium tetrachloride as titanium precursor, employing MS, US,
microwave (MW), and reflux methodologies for the sol-gel aging procedure [79]. The synthetic
protocol was optimized and applied to the generation, by coprecipitation, of a Pt-doped TiO2 system
that was evaluated against the selective photoxidation of propan-2-ol in the gas phase.
The same research group also reported the complete degradation of 3-chloropyridine under the
action of photocatalysts based on titania. This reaction was practically completed after 480 min of
irradiation [80]. The synthesis of TiO2 -based catalysts was performed using titanium tetra-isopropoxide
and acetyl acetonates derived from different metals. The catalysts were obtained by aging the gel
under US, MW and heating at reflux. The best solid in terms of material properties—100% anatase
NPs, high crystallinity, and high surface area—was obtained under ultrasonic irradiation. Otherwise,
the modification of TiO2 with metals (Fe, Pt, Pd) was unfavorable for photocatalysis. The team also used
titanium isopropoxide or titanium tetrachloride as precursors of TiO2 -based catalysts, for the selective
photo-conversion of 2-butenol to 2-butenal in the liquid phase, as well as gas-phase photo-oxidation of
2-propanol to acetone [81].
Vinodgopal et al.
employed an ultrasound dual frequency arrangement at 20 kHz
(ultrasound horn-type transducer) and 211 kHz (hig- frequency transducer) operating in tandem
to obtain bi- and single layered graphene with Pt-NPs dispersed on that 2D-material [82]. These hybrid
composites showed good electrocatalytic activity in the oxidation of methanol. The dual frequency
arrangement enabled the reduction of the graphene oxide sheets while ensuring a high level of
exfoliation. The latter issue is critical as optimal properties of graphene largely depend on its thickness;
ultrasound being a common energy source to achieve successfully liquid-phase exfoliation [83,84].
The enantioselective hydrogenation of 1-phenyl-1,2-propanedione (63) was performed by different
Pt-modified catalysts: Pt/Al2 O3 , Pt/SiO2 , Pt/SF (silica fiber), and Pt/C catalysts, also decorated with
cinchonidine in a pressurized reactor subjected to ultrasonic irradiation (Scheme 21) [85]. The initial
rate along with regio- and enantioselection trends were interrogated for different pretreatments,
solvents, and US powers. Sonication improved significantly both enantioselection and activity of the
Pt/SF catalyst. On using 5 wt % Pt/SF, pronounced enhancements of the initial rate, enantio- and
regioselectivity could be observed under sonication relative to silent experiments. In solvents of high
vapor pressure (e.g., methyl acetate) ultrasound had negligible effects on both reaction rate and product
selection, while solvents exhibiting low vapor pressures (mesitylene in particular) resulted in the
highest ultrasound-induced enhancements. From a chemical viewpoint, this asymmetric reduction is
a complex process giving rise to four isomeric hydroxyketones (65 and 66), which can undergo further
hydrogenation to the corresponding diols (64 as diastereomeric mixture). The authors only assessed the
first hydrogenation step, measuring discrete enantiomeric excesses (47% ee at 50% conversion and up
to 60% ee at 98% conversion). The effects caused by ultrasound were dependent on the catalyst and its
pretreatment. The enhancement of ee under sonication was only noticeable with Pt/SF (34% ee versus
17% ee under silent conditions at 50% conversion). This was attributed to a substantial modification of
metal particle size distribution (as viewed by SEM) under acoustic irradiation.
mixture). The authors only assessed the first hydrogenation step, measuring discrete enantiomeric
excesses (47% ee at 50% conversion and up to 60% ee at 98% conversion). The effects caused by
ultrasound were dependent on the catalyst and its pretreatment. The enhancement of ee under
sonication was only noticeable with Pt/SF (34% ee versus 17% ee under silent conditions at 50%
conversion).
Catalysts 2017, 7,This
121 was attributed to a substantial modification of metal particle size distribution
17 of(as
29
viewed by SEM) under acoustic irradiation.
Scheme
21.21.
Stereoselective
hydrogenation
ofof
1-phenyl-1,2-propanedione
Scheme
Stereoselective
hydrogenation
1-phenyl-1,2-propanedioneimproved
improvedbybysonication.
sonication.
Highly dispersed Pt-NPs could also be incorporated into CeO2 nanopowders through a one-step
Highly dispersed Pt-NPs could also be incorporated into CeO2 nanopowders through a one-step
ultrasound-promoted reduction [86]. The efficiency of the resulting
Pt/CeO2 catalysts was tested in
ultrasound-promoted reduction [86]. The efficiency of the resulting Pt/CeO2 catalysts was tested in the
the combustion reaction of ethyl acetate. Total conversion could be achieved
at low temperature. It
combustion reaction of ethyl acetate. Total conversion could be achieved at low temperature. It should
be noted that the sonochemical preparation of the catalyst led to Pt-NPs with diameters of 2–4 nm and
favored the homogeneous intercalation into CeO2 nanopowders.
Selvaraj et al. reported the preparation of a new supporting material formed by polythiophene
(PTh) and multiwalled carbon nanotubes (MWCNTs) (i.e., PTh-CNTs), which could be obtained
through in situ polymerization of thiophene on carbon nanotubes using FeCl3 as oxidant under
ultrasound [87]. Such polythiophene/CNT composites were further doped with Pt- and Pt-Ru NPs by
reduction of the corresponding metal salts with formaldehyde as reducing agent at pH 11. The resulting
Pt/PTh-CNT and Pt-Ru/PTh-CNT materials were tested in the electrocatalytic oxidation of ethylene
glycol, finding that the bimetallic catalyst, Pt-Ru/PTh-CNT showed a prolonged stability and better
storage characteristics than Pt/PTh-CNT.
Nanoparticles made of Pt3 Co with Pt-enriched shells on a carbonaceous support have been
generated by a one-step ultrasonic polyol procedure from Pt(acac)2 (acac = acetyl acetonate) and
Co(acac)2 as metal precursors [88]. Sonication promoted the transformation of Co(acac)2 and delayed
the conversion of Pt(acac)2 into nanoparticles; an observation consistent with the different vapor
pressures of Pt(II) and Co(II) acetyl acetonates. This sonochemical variation afforded Pt-NPs that
were superior electrocatalysts for oxygen reduction than other commercially available catalysts like
Pt/C. Likewise, Jiang et al. investigated the coverage of Pt atoms on PtCo-NPs and their catalytic
effect on oxygen reduction [89]. PtCo-NPs having different coverages of Pt atoms were synthesized
according to various methods, which included microemulsion synthesis and ultrasound-assisted
microemulsion, the latter being especially efficient and yielding the most active catalyst (ascribed to
an increase of active surface coatings of Pt atoms on NPs). The rate of oxygen reduction catalyzed
by such ultrasound-generated PtCo-NPs was approximately four times higher than that of the PtCo
catalyst obtained through a conventional method, and they could catalyze the oxygen reduction to
water without detection of H2 O2 as intermediate product.
Nagao et al. evaluated the effect of sonication during the impregnation of bimetallic Pt-Ru-NPs on
a carbonaceous support (ultrasonically dispersed) by the reduction of Pt and Ru ions with NaBH4 [90].
The catalysts prepared under sonication were single-nanometer-sized particles, while Pt-Ru particles
with significant aggregation were instead detected in the conventional, non-irradiated sample.
In addition, the former catalyst was much more efficient for methanol oxidation than Pt-Ru-NPs
generated under silent conditions. The enhanced effect of ultrasound on the catalytic activity was
also established at concentrations of Pt, Ru, and carbonaceous support as high as 3.0 mM, 1.2 mM,
and 1065 mg L−1 , respectively. In other words, ultrasonically-activated NPs remained unaffected in
catalytic activity as the concentrations of metal ions and carbon increased.
In a related study, series of Ag-, Pt-, and Pt/Ag-NPs supported on carbon substrates as
electrocatalysts were generated under ultrasonic irradiation without consecutive thermal treatments [91].
Bimetallic-NPs were sphere-shaped agglomerates with a higher particle size than the corresponding
single-metal species, which were less than 10 nm in diameter. The Pt-Ag/C system showed significant
Catalysts 2017, 7, 121
18 of 29
enhancement of electrochemical activity (oxygen reduction), 1.5 times higher, with respect to a Pt/C
catalyst from commercial suppliers.
4.3. Ag and Au NPs
The preparations of silver and gold NPs have been widely documented in recent years and this
trend will continue for years to come in view of their high versatility. Sonication has obviously added
to the repertoire of methods employed in synthesis. Ag-NPs, currently obtained by different green
routes [92], exhibit a remarkable antibacterial activity, whose mechanism of action has been investigated
in detail as well [93]. Thus, Manjamadha and Muthukumar synthesized Ag-NPs in only 10 min using
an aqueous extract of Lantana camara L. as bioreductant and capping reagent under ultrasound [94].
Both TEM (Figure 3) and Fourier transform infrared spectroscopy (FTIR) studies confirmed that the
weed plant played a key role in the bioreduction and stabilization of NPs by electrostatic interaction
of OH groups
present in the polyphenols and amide linkages of the protein. Finally, the antioxidant
Catalysts
2017, 7, 121
18 of 29
activity exerted by such Ag-NPs was checked using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) test.
Moreover, the synthesized
hadthe
remarkable
antibacterial
activity
versusantibacterial
both Gram positive
picrylhydrazyl
(DPPH) test.Ag-NPs
Moreover,
synthesized
Ag-NPs had
remarkable
activity
and Gram
specimens.
versus
bothnegative
Gram positive
and Gram negative specimens.
Figure
TEM
(transmission
electron
microscopy)
imagesimages
of Ag-NPs
generated
from Lantana
Figure 3.3. (a) (a)
TEM
(transmission
electron
microscopy)
of Ag-NPs
generated
from
camara
L.
extract
under
ultrasound.
The
box
shows
the
resulting
particle
size
distribution;
(b) SAED
Lantana camara L. extract under ultrasound. The box shows the resulting particle size distribution;
(selected
electron
analysis) pattern.
with permission
from Ref. from
[94].
(b) SAEDarea
(selected
areadiffraction
electron diffraction
analysis)Reproduced
pattern. Reproduced
with permission
Copyright
2016
Springer.
Ref. [94]. Copyright 2016 Springer.
Highly ordered TiO2 nanotube arrays loaded with Ag-NPs were synthesized at low AgNO3
Highly ordered TiO2 nanotube arrays loaded with Ag-NPs were synthesized at low AgNO3
concentrations using a common
UV lamp through ultrasound-aided photochemistry [95]. It was
concentrations using a common UV lamp through ultrasound-aided photochemistry [95]. It was
shown that the concentration of Ag-NPs deposited on TiO2 nanotubes was sufficient to considerably
shown that the concentration of Ag-NPs deposited on TiO nanotubes was sufficient to considerably
improve the photoelectrical and photocatalytic activities of2 the TiO2 nanotube array. Compared with
improve the photoelectrical and photocatalytic activities of the TiO2 nanotube array. Compared with
pure TiO2, Ag-doped TiO2 nanotubes generated from 0.006 M AgNO
3 through the hybrid US-assisted
pure TiO2 , Ag-doped TiO2 nanotubes generated from 0.006 M AgNO3 through the hybrid US-assisted
photochemical methods had higher photocatalytic degradation rate (3.7 times) and higher
photochemical methods had higher photocatalytic degradation rate (3.7 times) and higher photocurrent
photocurrent (1.2 times). The enhanced photocatalytic behavior of the Ag-TiO2 system was assessed
(1.2 times). The enhanced photocatalytic behavior of the Ag-TiO2 system was assessed through the
through the elimination of methylene blue (MB) dye in aqueous solution
(at an initial concentration
elimination
of methylene blue (MB) dye in aqueous solution (at an initial concentration of 8 mg L−1 ).
−1
of 8 mg L ). A 200-W high pressure Hg lamp with an emission wavelength of 365 nm was employed
A 200-W high pressure Hg lamp with an emission wavelength of 365 nm was employed as UV-light
as UV-light source, while the changes in MB concentration were obtained by measuring the
source, while the changes in MB concentration were obtained by measuring the absorbance intensity
absorbance intensity at 660 nm. Other authors have also synthesized Ag/TiO2 composite NPs in
at 660 nm. Other authors have also synthesized Ag/TiO2 composite NPs in ethylene glycol in basic
ethylene glycol in basic medium (NaOH) through a sonochemical
process [96].
medium (NaOH) through a sonochemical process [96].
A new and stable nanostructured catalyst with high activity towards oxygen electro-reduction
A new and stable nanostructured catalyst with high activity towards oxygen electro-reduction
in acid electrolytes was introduced by Pech-Pech and co-workers [97,98]. The authors reported the
in acid electrolytes was introduced by Pech-Pech and co-workers [97,98]. The authors reported the
preparation of Ag-rich NPs within a PtPd-enriched area ([email protected]). The catalyst was
preparation of Ag-rich NPs within a PtPd-enriched area ([email protected]). The catalyst was synthesized
synthesized in a one-pot protocol by applying intense ultrasound, reducing Ag, and then Pt and Pd
salts with NaBH4 in the presence of trisodium citrate, and finally adding XC-72 (a multi-purpose
conductive carbon black). Structural characterization of the resulting catalysts revealed that the
amount of Pt and Pd in the shell had a pronounced effect on electro-reduction. The optimal activity
was obtained for a catalyst of composition [email protected]/C, which was a useful alternative to Pt/C.
Nanostructured silver vanadates with nano-rod shape (diameter of 50–200 nm and length of 0.5–
Catalysts 2017, 7, 121
19 of 29
in a one-pot protocol by applying intense ultrasound, reducing Ag, and then Pt and Pd salts with
NaBH4 in the presence of trisodium citrate, and finally adding XC-72 (a multi-purpose conductive
carbon black). Structural characterization of the resulting catalysts revealed that the amount of Pt and
Pd in the shell had a pronounced effect on electro-reduction. The optimal activity was obtained for
a catalyst of composition [email protected]/C, which was a useful alternative to Pt/C.
Nanostructured silver vanadates with nano-rod shape (diameter of 50–200 nm and length
of 0.5–2.0 µm) could be obtained by means of hydrothermal treatment assisted by US and MW
techniques [99]. Silver vanadates showed a strong absorbance in the UV–vis zone at 470–500 nm and
had much higher photocatalytic efficiency in the decomposition of MB under visible light exposure
compared to common TiO2 NPs. Notably, such Ag vanadates also exhibited antibacterial activity,
thus adding extra value to potential applications in environmental remediation.
An easy and efficient technique for the synthesis of Ag-NP colloids with cherry- or raspberry-type
morphologies was reported by Lei et al. [100]. The approach involves the self-assembly of a copolymer
to micelles, which served as a compartment for the generation of NPs with the assistance of
ultrasound. The above-mentioned morphologies depended on the amount of AgNO3 loading,
the nature of the reducing agent and external conditions. Sonication favored the diffusion of AgNO3
and polyethyleneimine (PEI) affording nucleation sites that were well distributed through the micellar
cores and led to cherry-like Ag-NP colloids. Moreover, PEI is a nontoxic reducing agent with a slow
reaction rate capable of controlling both nucleation and growth processes. Likewise, it serves as
protective agent for the resulting Ag-NPs.
Saha et al. described an ultrasound-assisted green procedure for the synthesis of carbohydrate
polymer-inspired Ag-NPs using tyrosine and starch, both being naturally-occurring substances [101].
Starch-stabilized Ag-NPs were prepared in water using a small amount of tyrosine as a non-harmful
reducing agent. The cytotoxicity of the resulting Ag-NPs on the peritoneal macrophages of Wistar rat
was evaluated and confirmed a higher selectivity to filaricidal and larvicidal activity (Figure 4) than
that of commercially available ones.
The sonogeneration and catalytic effect of Au-NPs is well documented in recent literature.
Kuo et al. synthesized Au-NPs coated on TiO2 (ranging from 2 to 80 nm in diameter) by varying the
pH values from 3 to 7, based on sonoelectrochemical reductions for 3 h [102]. Acetaldehyde solutions
in ethanol were almost completely degraded (ca. 95%) based on Au-NPs having the smallest mean
diameter of 2 nm. The catalytic activity for acetaldehyde decomposition was markedly reduced in the
presence of Au-NPs of greater diameters. Moreover, the optimal sizes of Au-NPs on TiO2 to produce
the strongest surface-enhanced Raman scattering (SERS) effects were ca. 60 nm, under a 785-nm
irradiation for probe molecules of Rhodamine 6G and polypyrrole.
In line with the above-mentioned applications, a 2011 review by Wen et al. detailed the use of
supported noble metal NPs (Au/TiO2 , Au/ZrO2 , Ag/AgCl) as efficient catalysts for selective chemical
syntheses and degradation of environmental pollutants [103]. Thus, the NPs could effectively catalyze
solar-energy conversion into chemical energy (i.e., oxidation of alcohols, thiols, and benzene into
carbonyls, disulfides, and phenol, respectively, or the reduction of nitro-aromatic compounds to the
corresponding azo derivatives). Under ultrasound irradiation, the supported NPs could also catalyze
the generation of hydrogen from water. Furthermore, a variety of contaminants (bearing different
functional groups: aldehyde, alcohol, carboxylic acid, phenolic compounds, and organic dyes) were
significantly decomposed, and completely degraded under the action of ultrasound.
Two catalysts containing Au/TiO2 /C were produced by means of US and MW irradiations by
George et al. [104]. The deposition of Au colloids onto powdered TiO2 /C, was performed by means of
a solvated metal atom impregnation (SMAI) method, which provided highly-dispersed Au particles
without requiring high-temperature calcination and reduction steps, which often lead to particle
sintering. The catalytic performance of 1 wt % Au-TiO2 /C-based catalysts, both sonochemically and
microwave irradiated was evaluated for the oxidation of CO in the range of 0–300 ◦ C and compared
to that of a commercial catalyst containing 1 wt % Au-TiO2 (Degussa-P25). Figure 5 shows the
Catalysts 2017, 7, 121
20 of 29
different patterns of the above-mentioned catalysts viewed by transmission electron microscopy (TEM).
An increase in CO conversion was observed for the ultrasonically-derived catalyst at low temperature.
Furthermore, the reactivity observed for this oxidation followed the order Au/TiO2 /C (US) > Au/TiO2
(P25)
> Au/TiO2 /C (MW).
Catalysts 2017, 7, 121
20 of 29
Figure
Figure4.4.Ethidium
Ethidiumbromide
bromide(EB)/acridine
(EB)/acridine orange (AO) stained
stained oocyte,
oocyte, microfilaria
microfilaria of
of S.
S. cervi
cerviand
and
larvae
quinquefasciatus.
(a) Control,
(b) parasites
treatedtreated
with 8.85
µg/mL
Ag-NP10),
(c) parasites
larvaeofofC.C.
quinquefasciatus.
(a) Control,
(b) parasites
with
8.85 of
μg/mL
of Ag-NP10),
(c)
treated
with
28.3 µg/mL
Ag-NP10,
(d) control,
larvae
21.95
µg/mL
of Ag-NP10,
parasites
treated
with 28.3ofμg/mL
of Ag-NP10,
(d) (e)
control,
(e)treated
larvae with
treated
with
21.95 μg/mL
of Agand
(f) and
larvae
with 117.1
of Ag-NP10.
Reproduced
with permission
from Ref.
[101].
NP10,
(f)treated
larvae treated
withµg/mL
117.1 μg/mL
of Ag-NP10.
Reproduced
with permission
from
Ref.
Copyright
2015 Elsevier
Ltd. Ltd.
[101]. Copyright
2015 Elsevier
Sonochemically prepared
prepared Au,
could
be be
successfully
immobilized
ontoonto
TiO2TiO
after
Sonochemically
Au, Pt
Ptand
andPd
PdNPs
NPs
could
successfully
immobilized
2
prolonged
sonication
[105].
Sonochemically-generated
catalysts
showed
higher
activities
than
those
after prolonged sonication [105]. Sonochemically-generated catalysts showed higher activities
prepared
conventional
protocols protocols
as evaluated
in the production
of hydrogen
from ethanolthan
those by
prepared
by conventional
as evaluated
in the production
of hydrogen
from
containing
aqueous
solutions.
The
reactivity
was
markedly
dependent
on
the
type
of
metal
NPs
and
ethanol-containing aqueous solutions. The reactivity was markedly dependent on the type of metal NPs
their
dimensions.
Small
Pt-NPs
did
effectively
restrict
side
reactions
arising
from
recombination
of
and their dimensions. Small Pt-NPs did effectively restrict side reactions arising from recombination
electrons
and
holes
and
released
of
electrons
and
holes
and
releasedhydrogen
hydrogenatata ahigher
higherrate.
rate.
Core-shell
nanostructures
consisting
of
Au-NPs
within
shells
of Ag
NPs-filled
polymer
(AuCore-shell nanostructures consisting of Au-NPs within
shells
of Ag
NPs-filled
polymer
core/Ag-PVP-shell)
were
synthesized
by
Kan
and
co-workers
in
a
two-step
sonication-assisted
(Au-core/Ag-PVP-shell) were synthesized by Kan and co-workers in a two-step sonication-assisted
reduction procedure
procedure [106].
[106]. Ag-NPs
Ag-NPs acted
acted as
as reductive
reductive agent
agent for
for Au(III)
Au(III) ions
ions in
in the
the presence
presence of
of
reduction
poly(vinylpyrrolidone)
(PVP).
After
sonication,
solidification
and
spherulite
growth
of
the
polymer
poly(vinylpyrrolidone) (PVP). After sonication, solidification and spherulite growth of the polymer
chainwas
wasinduced
induced
the as-formed
Au-NPS,
such
that
tiny Ag-NPs
were embedded
in the
chain
on on
the as-formed
Au-NPS,
such that
tiny
Ag-NPs
were embedded
in the crystalline
crystalline
matrix
of
polymer
spherulites.
The
formation
of
core-shell
NPs
was
dependent
on
the
matrix of polymer spherulites. The formation of core-shell NPs was dependent on the redox potentials
redox
potentials
of
ions
in
solution,
the
coordinating
ability
of
such
ionic
species
to
stabilize
PVP,
as
of ions in solution, the coordinating ability of such ionic species to stabilize PVP, as well as the
well as theand
crystalline
and interlinkage
degrees
of suchduring
a polymer
duringirradiation.
ultrasonic irradiation.
An
crystalline
interlinkage
degrees of such
a polymer
ultrasonic
An ultrasonic
ultrasonic
cleaner,
working
at
40
kHz
with
an
output
power
of
360
W,
was
sufficient
to
produce
the
cleaner, working at 40 kHz with an output power of 360 W, was sufficient to produce the Au/Ag
Au/Ag composite
in theofabsence
oxygen.
The role
stabilizing
role of
polymers
andinsurfactants
in
composite
in the absence
oxygen.ofThe
stabilizing
of polymers
and
surfactants
nanoparticle
nanoparticle
hashighlighted
likewise been
highlighted
Park et al.,
who synthesized
colloidal,
production
hasproduction
likewise been
by Park
et al., whoby
synthesized
colloidal,
uniformly dispersed
uniformly
dispersed
hybrid
nanocomposites
constituted
by
polypyrrole
(PPy)
and
Au-NPs
(or Pthybrid nanocomposites constituted by polypyrrole (PPy) and Au-NPs (or Pt-NPs) [107]. The procedure,
NPs)
[107].
The
procedure,
assisted
by
ultrasonic
irradiation,
involved
reduction
of
Au(III)
ions
and
assisted by ultrasonic irradiation, involved reduction of Au(III) ions and pyrrole monomer oxidation
pyrrole
monomer
oxidation
in an sodium
aqueousdodecyl
solutionsulfate
containing
in
an aqueous
solution
containing
(SDS).sodium dodecyl sulfate (SDS).
Catalysts 2017, 7, 121
Catalysts 2017, 7, 121
21 of 29
21 of 29
Figure
TEMimages
imagesofof(a)
(a)1%
1%Au-TiO
Au-TiO22 (anatase)/C
(anatase)/C (under
Figure
5. 5.TEM
(under US),
US),where
whereblack
blackdots
dotsare
areAu-NPs;
Au-NPs;(b)
2 (anatase)/C
(under MW), and (c) 1% Au/TiO2-P-25, the last support coming from
(b)1%
1%Au/TiO
Au/TiO
2 (anatase)/C (under MW), and (c) 1% Au/TiO2 -P-25, the last support coming from
Degussa.
Reproduced
with
permission
from
Ref.
[104].
Copyright
2007
Elsevier
B.V.
Degussa.
Reproduced
with
permission
from
Ref.
[104].
Copyright
2007
Elsevier
B.V.
The construction of a highly efficient anode catalyst for oxygen evolution reaction (OER) was
The construction of a highly efficient anode catalyst for oxygen evolution reaction (OER) was
reported in 2015 and consisted of a combination of multiwalled carbon nanotubes, metallic Au, and
reported in 2015 and consisted of a combination of multiwalled carbon nanotubes, metallic Au,
transition metal-oxide (i.e., CNTs-Au@Co3O4) generated under high-intensity ultrasound [108]. The
and transition metal-oxide (i.e., CNTs-Au@Co3 O4 ) generated under high-intensity ultrasound [108].
tubular architecture provides enough active centers for OER and enables fast mass and charge
The tubular architecture provides enough active centers for OER and enables fast mass and charge
transports. The Co3O4 layer protects Au-NPs against further detachment, and reciprocally Au
transports. The Co3 O4 layer protects Au-NPs against further detachment, and reciprocally Au
facilitated the production of Co(IV) species for OER due to its greater electronegativity.
facilitated the production of Co(IV) species for OER due to its greater electronegativity.
It should be finally pointed out that Au-NPs can be efficiently generated by means of highIt should be finally pointed out that Au-NPs can be efficiently generated by means of high-intensity
intensity focused ultrasound (HIFU), working at 463 kHz [109]. Formation of NPs proceeds through
focused ultrasound (HIFU), working at 463 kHz [109]. Formation of NPs proceeds through the
the reduction of Au(III) ions to Au(0) by radicals generated under ultrasonic cavitation. TEM images
reduction of Au(III) ions to Au(0) by radicals generated under ultrasonic cavitation. TEM images
revealed that NPs exhibit irregular shapes at low power (30 W), mainly icosahedral at 50 W, and
revealed that NPs exhibit irregular shapes at low power (30 W), mainly icosahedral at 50 W, and finally
finally nanorods at 70 W. The size decreased with a narrower size distribution as the acoustic power
nanorods at 70 W. The size decreased with a narrower size distribution as the acoustic power increased.
increased.
4.4. Lanthanide NPs
4.4. Lanthanide NPs
Vilela and co-workers prepared highly crystalline lanthanide composite materials [Ln(Hpmd)
highly
lanthanide
materials
3+ ions,crystalline
(H2 O)],Vilela
where and
Ln3+ co-workers
can be Eu3+ , prepared
Gd3+ , or Tb
and the organic
linker composite
is 1,4-phenylenebis
[Ln(Hpmd)(H
2O)], where Ln3+ can be Eu3+, Gd3+, or Tb3+ ions, and the organic linker is 1,4(methylene)diphosphonic acid (H4 pmd) as building block [110]. The authors used three different
phenylenebis(methylene)diphosphonic
4pmd) as building block [110]. The authors used three
strategies
based on heating procedures ofacid
the (H
reaction
containers, including classical hydrothermal
different
strategies
based
on
heating
procedures
of
reaction
including
classical
◦
synthesis (180 C for 3 days), MW-assisted heating (50-Wthe
power
at 40 ◦containers,
C for 5 s), and
US-promoted
hydrothermal
synthesis
(180
°C
for
3
days),
MW-assisted
heating
(50-W
power
at
40
°C
for
5
s), and
3+
synthesis which allowed, for highly diluted mixtures (Ln :H4 pmd:H2 O ratio = 1:1:7200), the generation
3+:H4pmd:H2O
US-promoted
synthesis
which
allowed,
for
highly
diluted
mixtures
(Ln
of isolated nano-crystals at room temperature within 5 min of acoustic irradiation. The Eu-containing
ratio = 1:1:7200),
the generation
of isolated nano-crystals
at room
temperature
5 min ofdiverse
acoustic
◦ C showing
compound
was evaluated
in the methanolysis
of styrene oxide
conducted
at 55within
irradiation.
The
Eu-containing
compound
was
evaluated
in
the
methanolysis
of
styrene
oxide
catalytic activity and selectivity. Using the ultrasonic procedure it was feasible to significantly reduce
conducted
at 55 °C showing
diverse
activity and and
selectivity.
Using
ultrasonic
◦ C tocatalytic
both
the temperature
(from 180
room temperature)
reaction
timesthe
(from
3 daysprocedure
to 5 s),
it
was
feasible
to
significantly
reduce
both
the
temperature
(from
180
°C
to
room
temperature)
and
without affecting phase purity and crystallinity.
reaction times (from 3 days to 5 s), without affecting phase purity and crystallinity.
Catalysts
2017, 7,
1217, 121
Catalysts
2017,
22 of 29
22 of 29
In a recent and very detailed study on rare earth-doped nanopowders, ZrO2:Dy3+ NPs were
3+
In
a Catalysts
recent
and
earth-doped
obtained
by
sonication
(ultrasonicstudy
horn on
at rare
20 kHz,
300 W at nanopowders,
333 K) in the ZrO
presence
2017,
7, 121 very detailed
22 of 229:Dy of NPs
(CTAB)
as surfactant
[111]. 300
The resulting
NPs
were cetyltrimethylammonium
obtained by sonicationbromide
(ultrasonic
horn
at 20 kHz,
W at 333
K)exhibited
in the different
presence of
3+ NPs were
In
a
recent
and
very
detailed
study
on
rare
earth-doped
nanopowders,
ZrO
2:Dy
shapes
and
sizes
depending
on
the
irradiation
times
and
the
quantity
of
CTAB
as
revealed
by SEM
cetyltrimethylammonium bromide (CTAB) as surfactant [111]. The resulting NPs exhibited
different
obtained
by sonication
(ultrasonic
horn
at 20after
kHz,
300 W at
333 K) in
the
presence
of
images.
Flower-like
morphologies
were
obtained
prolonged
irradiation
(up
to
6
h)
and
increased
shapes and
sizes
depending
on
the
irradiation
times
and
the
quantity
of
CTAB
as
revealed
by SEM
cetyltrimethylammonium bromide (CTAB) as surfactant [111]. The resulting NPs exhibited different
with
frequency (from
20 to 24 kHz).
This
suggests
a complex
evolution
of self-assembly
and
growth
images.
Flower-like
morphologies
were
obtained
after
prolonged
irradiation
(up
to
6
h)
and
increased
shapes and sizes depending on the irradiation times and the quantity of CTAB as revealed by SEM
of NPs, dictated to a significant extent by the surrounding surfactant (Figure 6). The nanopowders
images.(from
Flower-like
morphologies
were obtained
after
prolonged evolution
irradiation (up
6 h) and increased
with frequency
20 to
24 kHz). This
suggests
a complex
oftoself-assembly
and growth
had excellent
luminescent
were utilized
to reveal
latent
on various
with frequency
(from 20properties
to 24 kHz). that
This suggests
a complex
evolution
of fingerprints
self-assembly(LFPs)
and growth
of NPs,
dictated
to
a
significant
extent
by
the
surrounding
surfactant
(Figure
6).
The
nanopowders
surfaces
(Figure
7). to
Inaaddition,
photocatalytic
behavior
of ZrO(Figure
2:Dy3+ NPs
was
evaluated in the
of NPs,
dictated
significantthe
extent
by the surrounding
surfactant
6). The
nanopowders
had excellent
luminescent
properties
that
were
utilized
to
reveal
latent
fingerprints
(LFPs) on various
degradation
of
methylene
blue
dye.
had excellent luminescent properties that were utilized to reveal latent fingerprints (LFPs) on various
3+ NPs was evaluated in the
surfaces (Figure
In addition,
the the
photocatalytic
behavior
surfaces 7).
(Figure
7). In addition,
photocatalytic behavior
of of
ZrOZrO
2:Dy3+
NPs was
evaluated in the
2 :Dy
degradation
of methylene
degradation
of methylene
blueblue
dye.dye.
3+ -NPs
6. Schematic
diagramdepicting
depicting thethe
formation
of ZrO2of
:Dy ZrO
-NPs :Dy
(3 mol
%) with(3and
without
Figure 6. Figure
Schematic
diagram
formation
mol
%) with and
2
Figure
6. Schematic diagrambromide
depicting
the formation
ZrO2:Dyof3+-NPs
(3 mol %)
with and without
cetyltrimethylammonium
(CTAB)
and SEMof images
the resulting
morphologies.
without
cetyltrimethylammonium
bromide
(CTAB)
and
SEM
images
of
the
resulting
morphologies.
cetyltrimethylammonium
bromide
(CTAB)
and SEM
of the
resulting
morphologies.
Reproduced with permission
from Ref.
[111]. Copyright
2017images
the American
Chemical
Society.
Reproduced
withwith
permission
from
Ref.
[111].
theAmerican
American
Chemical
Society.
Reproduced
permission
from
Ref.
[111].Copyright
Copyright 2017
2017 the
Chemical
Society.
3+
Figure 7. Sequential illustration highlighting the development of latent fingerprint (LFP) detection
using ZrO2:Dy3+ (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the
substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white
7. Sequential
illustration
highlightingthe
the development
development ofoflatent
(LFP)
detection
FigureFigure
7. Sequential
illustration
highlighting
latentfingerprint
fingerprint
(LFP)
detection
using
ZrO
2:Dy3+ (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the
3+
using ZrO2 :Dy (3 mol %): (i) impression of fingerprints on different substrates; (ii) applied onto the
substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white
substrate to stain the fingerprint; (iii) UV light is then used to irradiate the fingerprint to emit white
light, and (iv) revealing the fingerprint with high sensitivity and contrast. Reproduced with permission
from Ref. [111]. Copyright 2017 the American Chemical Society.
Catalysts 2017, 7, 121
23 of 29
light,
(iv) revealing the fingerprint with high sensitivity and contrast. Reproduced with
Catalysts
2017,and
7, 121
23 of 29
permission from Ref. [111]. Copyright 2017 the American Chemical Society.
4.5. Main-Group
Main-Group NPs
NPs
4.5.
Aluminum NPs
NPs have
have been
Aluminum
been isolated
isolated and
and stabilized
stabilized by
by reaction,
reaction, under
under ultrasonication,
ultrasonication, of
of SiCl
SiCl44
and
LiAlH
in
the
presence
of
poly(vinylpyrrolidone)
(PVP)
at
ambient
temperature
[112].
and LiAlH44 in the presence of poly(vinylpyrrolidone) (PVP) at ambient temperature [112]. The
The sonochemical
procedure
(1 h under
nitrogen
stream)
was used
to produce
spherical
Al with
NPs
sonochemical
procedure
(1 h under
nitrogen
stream)
was used
to produce
spherical
Al NPs
with
reasonably
narrow
distribution
(2–15
nm),
integrated
random
positionsofofthe
thePVP
PVPmatrix.
matrix.
reasonably
narrow
size size
distribution
(2–15
nm),
integrated
at at
random
positions
This transformation
also releases
and volatile
volatile hydrosilanes
hydrosilanes as
as side
side products.
Thermal analysis
analysis of
of
This
transformation also
releases H
H2 and
products. Thermal
◦
such composites
composites unveiled
C by
by gradual
such
unveiled their
their stability
stability up
up to
to 422
422 °C
gradual oxidation
oxidation in
in air.
air.
In
another
interesting
application,
Kumar
et
al.
described
the
production
of enantio-selective
enantio-selective
In another interesting application, Kumar et al. described the production of
gallium particles by synthesizing them in aqueous solutions of D- or L-tryptophan, thereby inducing
some imprinted chirality in their structure [113]. Chiral imprinting could be achieved by ultrasonic
◦ C) of molten Ga covered by an aqueous solution of D- or L-tryptophan,
irradiation (for 3 min at 55 °C)
of molten Ga covered by an aqueous solution of D- or L-tryptophan,
The resulting
resulting
affording a grey suspension of particles, which were centrifuged
centrifuged at 6000 rpm for 10 min. The
micro- or
or nano-particles
nano-particles of
ofGa
Gaencapsulating
encapsulatingone
oneofofthe
theenantiomers.
enantiomers.After
After
leaching
products are microleaching
of
of
the
enantiomer
in
water,
molecular
scaffolds
are
left
on
the
surface
of
the
Ga
particles,
which
then
the enantiomer in water, molecular scaffolds are left on the surface of the Ga particles,
enabled the
of the
enantiomer
from the racemate
DL-tryptophan).
Unfortunately,
enabled
theentrapment
entrapment
ofspecific
the specific
enantiomer
from the(i.e.,
racemate
(i.e., DL-tryptophan).
the
enantiomeric
excesses
were
poor
(from
6%
to
12%
ee)
as
determined
by
polarimetry,
circular
dichroism
Unfortunately, the enantiomeric excesses were poor (from 6% to 12% ee) as determined
by
and
chiral
HPLC.
However,
the
method
appears
to
be
promising
enough
en
route
to
chiral
imprinting
polarimetry, circular dichroism and chiral HPLC. However, the method appears to be promising
mediated
metal
metal
ion NPs generated
sonication.
enough
enby
route
to or
chiral
imprinting
mediated under
by metal
or metal ion NPs generated under sonication.
More recently
Gedanken
and
his
group,
who
have
significantly
advanced
the subject
of Ga-based
recently Gedanken and his group, who have significantly
advanced
the subject
of Gamaterials
sonofabrication,
reported
a one-step
procedure
that
yields
based
materials
sonofabrication,
reported
a one-step
procedure
that
yieldsGa-doped
Ga-dopedcarbon-dots
carbon-dots on
polyethylene glycol
glycol (PEG 400) and molten gallium [114]. The resulting
resulting nanomaterial,
nanomaterial,
Ga-NPs from polyethylene
Ga@C-dots@Ga-NPs,
was
deposited
on
a
glass
substrate
and
further
evaluated
for
neural
growth.
Ga@C-dots@Ga-NPs, was deposited on a glass substrate and further evaluated for neural growth.
showed a 97% increase in the number of branches
Cells grown on the aforementioned substrate showed
resulting. ItItisisnoteworthy
noteworthythat
that
the
sonochemical
protocol
(conducted
with
US probe
20 kHz,
the sonochemical protocol
(conducted
with
a USa probe
at 20at
kHz,
70%
◦
70% amplitude,
forh)2.5enabled
h) enabled
facile
fabrication
coating.
latter
be attributed
amplitude,
70 °C70forC2.5
bothboth
facile
fabrication
andand
coating.
TheThe
latter
cancan
be attributed
to
to ultrasonic
cleaning
that
prevents
metaloxidation
oxidationby
byatmospheric
atmosphericoxygen
oxygenand
andavoids
avoidsthe
the concomitant
concomitant
ultrasonic
cleaning
that
prevents
metal
passivation on the C-dot surface.
Safari et al. reported
reported aa convenient
convenient synthesis
synthesis of
of 1,2,4,51,2,4,5In a more-oriented synthetic application, Safari
tetrasubstituted imidazoles
imidazoles (70)
(70) through a four-component one-step condensation
condensation of a substituted
tetrasubstituted
aromatic aldehyde (67), benzyl (68), phenyl amine (69), and ammonium acetate with nanocrystalline
in ethanol
ethanol under
under ultrasound
ultrasound (Scheme
(Scheme 22)
22) [115]. The
The protocol
protocol show
magnesium aluminate (MgAl2O44)) in
the typical
advantages
of
sonochemically
accelerated
reactions
in
terms
of
high
yields,
mild
conditions,
typical advantages of sonochemically accelerated reactions in terms of high yields,
mild
short reaction
times,
and easy
work-up.
conditions,
short
reaction
times,
and easy work-up.
Scheme 22. Ultrasound-promoted synthesis of 1,2,4,5-tetrasubstituted imidazoles mediated by Mg/Al
Scheme 22. Ultrasound-promoted synthesis of 1,2,4,5-tetrasubstituted imidazoles mediated by
NPs.
Mg/Al NPs.
5. Conclusions
5. Conclusions
In this review we have provided some aspects and applications, mainly gathered in the last
In this review we have provided some aspects and applications, mainly gathered in the last
decade, of metals and metal ions, either massive or micro/nano-structured form, under the action of
decade, of metals and metal ions, either massive or micro/nano-structured form, under the action of
an ultrasonic field. The latter constitutes a well-known and established method that rivals and often
an ultrasonic field. The latter constitutes a well-known and established method that rivals and often
surpasses the effects of conventional thermal protocols. Along with evident pluses of acceleration,
surpasses the effects of conventional thermal protocols. Along with evident pluses of acceleration,
Catalysts 2017, 7, 121
24 of 29
selectivity, and good overall yields, sonication represents a low-cost strategy, affordable by most
laboratories, which can be fine-tuned by simply adjusting empirical parameters such as frequency,
input power, or solvents of choice as sound propagation and cavitational implosion are largely
influenced by the fluid’s properties. In addition, ultrasonic devices can be combined with other
irradiation and activation methods, and scaling up can also be easily accomplished. Clearly, one should
look forward to developing and improving organic and organometallic reactions in the near future by
placing metal derivatives in the vicinity of ultrasound beams.
Acknowledgments: The authors deeply acknowledge Universidad Nacional del Sur (UNS) and Consejo Nacional
de Investigaciones Científicas y Técnicas (CONICET), the Junta de Extremadura-FEDER (Grant GR15022), and the
University of Turin (Ricerca locale 2015) for financial support.
Author Contributions: All authors contributed equally to the redaction of this review. All authors have given
approval to the final version of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest. The founding institutions had no role in the
design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, as well
as in the decision to publish the results.
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