Acknowledgement
First of all, I would like to acknowledge the jury members Prof. Alain Roucoux and
Dr. Marc Taillefer for accepting as reviewers of my PhD dissertation, Prof. Philippe Serp
and Prof. Hanh Nguyen for accepting as examiners of my PhD defense. It was my honor to
discuss with them and to receive their valuable evaluations for my works. My eyes widen
and my horizon is broadened…
I truly thank Dr. Didier Bourissou to accept me as a PhD student in Laboratoire
Hétérochimie Fondamentale et Appliquée. I also appreciate Prof. Hanh Nguyen and Prof.
Blanca Martin-Vaca as bridges to connect me and my supervisor, for the days I was still in
Vietnam. The experiences in LHFA were really fantastic and I will keep everything in my
mind, forever…
From the bottom of my heart, the biggest acknowledgement dedicates to Prof.
Montserrat Gómez for accepting as supervisor of my PhD study. Her endless support,
guidance and patience have pointed out to me where I should keep moving on. With
strictly mother-like attitude, she is an energetic and responsible supervisor, inspiring and
motivating me from the first days… It is a long story!
Dr. Isabelle Favier, she is the biggest “gift” to me, from Montserrat and Gods. I
have never seen an enthusiastic and understanding person like her. She trained me in the
lab as my co-supervisor and treated me out-of-the work as my big-sister. She always gives
me a reason why I should love my chemistry, over different optimistic sightseeing,
particularly in desperate times. Especially, she is also my translator in French.
I am grateful to Dr. Daniel Pla, who usually shares with me about variety of topics,
including chemistry, food, culture, plans of life after defense and postdoc… Thanks a lot
for these sharing moments and for his valuable friendship.
My special thanks dedicate to Mr. Christian Pradel for TEM analyses and for his
endless help in the last 3 years. I will never forget the “big” man in LHFA!
Many thanks to Ms. Alix Sournia-Saquet (from LCC) and Mr. Alain Moreau (from
LCC) for electrochemistry experiments and for their valuable scientific discussions; Ms.

Sonia Mallet-Ladeira (from ICT) for X-ray diffraction analyses; Ms. Caroline Toppan
(from ICT) for nuclear magnetic resonance experiments. They are so kind!
I would like to express my gratitude to all LHFA members, especially Ms. Maryse
Beziat and Ms. Sérah Noel for their help in administrative works; Mr. Olivier Volpato, Mr.
Olivier Thillaye du Boullay, Mr. Julien Babinot and Mr. Romaric Lenk for their supports
in the lab. Without their helps, my works would be much more difficult.
Writing for SYMAC team-mates, my greatest friends…
Antonio Reina Tapia, a warmly boy from Mexico… We had a lot of funny and
memorable moments. I cannot describe how amazing you are! Keep in touch!
Garima Garg, my Lazy Gaga from India… She doesn’t know how sweet she is! I
“love” you, Garima!
Lorena Soria Marina, my same-age friend from Spain… We are still young and we
will take a beer soon!
Many thanks to Marta Rodríguez-Rodríguez (a little girl from Spain), Yingying Gu
(an elegant lady from China), Stéphanie Foltran (a gorgeous lady from France), Julian A.
W. Sklorz (a gentleman from Germany), Marie-Lou Toro (a cute girl from France), Jésica
Ortiz (a beautiful girl from Mexico), Alejandro Serrano (a young man from Mexico) and
Tiago Gomes Duarte (my same-age friend from Portugal) for their coming and making my
life more colorful. I will bring everything from France to Vietnam and to anywhere I will
come!
Making a PhD in Toulouse is not my choice. It is my destiny!
Loving all…


Index
Abbreviations and Acronyms
General Introduction and Objectives
Chapter 1: Mono- and Bi- Metallic Nanoparticles in Catalysis
1.1 Metal nanoparticles........................................................................................................ 11
1.1.1 Synthesis and stabilization of metal nanoparticles ................................................. 12

1.1.2 Metal nanoparticles involving first-row transition metals in “wet” catalysis ......... 16
1.1.2.1 Water ................................................................................................................ 16
1.1.2.2 Alcohols ........................................................................................................... 22
1.1.2.3 Ionic liquids ...................................................................................................... 29
1.2 Bimetallic nanoparticles ................................................................................................ 34
1.2.1 Synthesis of bimetallic nanoparticles ..................................................................... 35
1.2.2 Factors influencing the synthesis of bimetallic nanoparticles ................................ 37
1.2.3 Bimetallic nanoparticles applied in catalysis .......................................................... 39
1.2.3.1 Water ................................................................................................................ 39
1.2.3.2 Alcohols ........................................................................................................... 45
1.2.3.3 Ionic liquids ...................................................................................................... 47
1.3 References ……………………………………………………………………………..52
Chapter 2: Zero-valent Copper Nanoparticles in Glycerol: Synthesis,
Characterization and Catalytic Applications
2.1 Introduction ................................................................................................................... 65
2.2 Synthesis and characterization of Cu(0)NPs in glycerol ............................................... 66
2.2.1 Synthesis of Cu(0)NPs in glycerol .......................................................................... 66
2.2.1.1 Effect of reaction temperature .......................................................................... 67
2.2.1.2 Effect of poly(vinylpyrrolidone) as stabilizer .................................................. 69
2.2.1.3 Effect of reducing agent ................................................................................... 71
2.2.1.4 Microwave-assisted synthesis of CuNPs in glycerol ....................................... 74
2.2.2 Characterization of CuA ......................................................................................... 76
2.3 Catalytic applications .................................................................................................... 85
2.3.1 C-N bond formation processes ............................................................................... 85
2.3.2 Synthesis of propargyl amines .............................................................................. 105
2.3.2.1 Cross-dehydrogenative coupling .................................................................... 106

Page | 3



2.3.2.2 One-pot three-component aldehyde-amine-alkyne (A3) coupling ................. 115
2.3.2.3 One-pot three-component ketone-amine-alkyne (KA2) coupling .................. 125
2.3.2.4 A3 coupling/cycloisomerization tandem processes ........................................ 127
2.4 Conclusions ................................................................................................................. 136
2.5 Experimental section ................................................................................................... 137
2.6 References ……………………………………………………………………………165
Chapter 3: Bimetallic Palladium-Copper Nanoparticles in Glycerol: Synthesis,
Characterization and Impact in Catalysis
3.1 Introduction…………………………………………………………………………..179
3.2 Synthesis and characterization of PdCuNPs in glycerol ............................................. 180
3.2.1 Synthesis of PdCuNPs by co-reduction of mixed metal precursors ..................... 180
3.2.1.1 Nature of metal precursors ............................................................................. 180
3.2.1.2 Effect of solvent ............................................................................................. 183
3.2.1.3 Effect of poly(vinylpyrrolidone) as stabilizer ................................................ 184
3.2.1.4 Effect of reaction temperature ........................................................................ 185
3.2.1.5 Effect of reaction time .................................................................................... 186
3.2.1.6 Effect of Pd/Cu ratio ...................................................................................... 189
3.2.2 Characterization of PdCuNPs ............................................................................... 190
3.2.3 Synthesis and characterization of PdCuNPs by sequential reduction processes .. 210
3.3 Catalytic applications: reactivity-structure relationship study .................................... 216
3.3.1 Effect of the second metal on activity and selectivity .......................................... 216
3.3.1.1 Pd-catalyzed selective hydrogenation of alkynes........................................... 216
3.3.1.2 Cu-catalyzed azide-alkyne cycloaddition (Cu-AAC) .................................... 222
3.3.2 Multi-task catalyst for one-pot AAC-Coupling processes .................................... 224
3.4 Conclusions ................................................................................................................. 233
3.5 Experimental section ................................................................................................... 235
3.6 References ................................................................................................................... 245
Chapter 4: Rh(I)-catalyzed Hydroaminomethylation in Glycerol
4.1 Introduction ................................................................................................................. 255
4.2 Results and Discussion ................................................................................................ 264

4.3 Conclusions and Perspectives ...................................................................................... 267
4.4 References ................................................................................................................... 268
Conclusions and Perspectives
Résumé de Thèse

Page | 4


Abbreviations and Acronyms
A3 Aldehyde, amine and alkyne

ICP-AES Inductively coupled plasma

AAC Azide-alkyne cycloaddition

atomic emission spectroscopy

ATR Attenuated total reflectance

J Coupling constant

BF-TEM Bright field transmission

ILs Ionic liquids

electron microscopy

IR Infrared spectroscopy

BMNPs Bimetallic nanoparticles


KA2 Ketone, amine and alkyne

Bn Benzyl

Mes Mesityl

CDC Cross-dehydrogenative coupling

MNPs Metal nanoparticles

cod Cyclo-1,5-octadiene

MS Mass spectroscopy

Conv. Conversion

MW Microwave

CV cyclic voltammetry

NMR Nuclear magnetic resonance

Cy Cyclohexyl

OAc Acetate

dba Dibenzylideneacetone

ppm Part per million


EDX Energy-dispersive X-ray

PEG Polyethylene glycol

spectroscopy

Ph Phenyl

EELS Electron energy loss spectroscopy

PVA Polyvinyl alcohol

equiv. Equivalents

PVP Poly(vinylpyrrolidone)

EXAFS Extended X-ray absorption fine

PXRD Powder X-ray diffraction

structure

RT Room temperature

fcc Face-centerd cubic

Sel. Selectivity

FFT Fast fourier transform


SEM Scanning electron microscopy

FT-IR Fourier transform infrared

SPR Surface plasmon resonance

spectroscopy

STEM Scanning transmission electron

GC Gas chromatography

microscopy

h Hours

t Time

HAADF High-angle annular dark-field

T Temperature

HAM Hydroaminomethylation

TEM Transmission electron microscopy

HR-TEM High-resolution transmission

TMEDA N,N,N’,N’-


electron microscopy

tetramethylethylenediamine

Hz Hertz

TOF Turnover frequency

-1-


TON Turnover number
TPPTS Tris(3-sulphophenyl)phosphine
trisodium salt
UV-vis Ultraviolet–visible spectroscopy
XANES X-ray absorption near edge
structure
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
XRF X-ray fluorescence

-2-


General Introduction and Objectives
Catalysis plays an important role in chemistry, where homogeneous and
heterogeneous catalysis are considered as the two main domains. Homogeneous catalysts
(molecular complexes often dissolved in solution) show high activity and selectivity, but
relative low stability and hard recycling. In contrast, heterogeneous catalysts (materials

often grafted onto a solid support) are highly stable and show an easy recycling (long life),
however, harsh reaction conditions and mass transport problems represent important
concerns [1,2].
From the late-90s of the last century, along with the development of nanoscience
and nanotechnology, nanocatalysis (based on the design of well-defined metal nanostructures) has offered a revolutionary change by combining the advantages of both
classical catalytic systems, leading to high efficiency and selectivity [3-5]. Distinctive
reactivity exhibited by metal nanoparticles (MNPs) can be tuned by size, shape and
composition of these MNPs, through the control of synthetic conditions, solvent and
stabilizer… In order to avoid the agglomeration of MNPs and facilitate the recovery of the
nanocatalysts, MNPs can be supported on solids or in liquid phases. Numerous studies
focused on MNPs supported on classical solids, such as carbon-based materials [6-13],
silica-based materials [10,11], metal oxides [10-13] and polymers [11,13]. In comparison
with solid-supported MNPs, the synthesis of MNPs in liquid supports, such as polyols or
ionic liquids, permits to obtain small MNPs with narrow size distribution, even at high
metal concentration [14-17]. Besides, immobilization of the catalytic ionic liquid phase
containing MNPs on suitable solid supports has been developed, permitting an easy
catalytic recycling and a reduction of ionic liquid amount [18-23].
Among the most relevant solvents for catalytic purposes, glycerol exhibits some
advantages, such as low cost, low toxicity, non-flammability, biodegradability, high
boiling point, negligible vapor pressure, high solubility for both organic and inorganic
compounds, and low miscibility with other organic solvents [15,24,25]. In this work, we
are interested in glycerol thanks to its ability for the immobilization of MNPs, avoiding
their agglomeration and then facilitating the recycling of the catalytic phase. In our team,
the synthesis of MNPs (PdNPs, NiNPs and Cu2O NPs) in glycerol, in the presence of

-3-


stabilizers (phosphines, polymers, cinchona-based alkaloids…), has been studied [26-31].
These catalytic systems showed excellent reactivity in a diversity of organic

transformations,

such

as

CC

and

Cheteroatom

bond

formation

processes,

hydrogenations and carbonylative cyclisations… On this basis, the objectives of this Thesis
are:
 The use of glycerol for the synthesis of mono- and bi- metallic nanoparticles:


Synthesis of zero-valent copper nanoparticles in neat glycerol and their
applications in catalysis, in particular in C-heteroatom bond formation
processes and one-pot tandem multi-step processes.



Synthesis of palladium-copper bimetallic nanoparticles in neat glycerol

and study of the structure-reactivity relationship, as well as their catalytic
applications as multi-task catalysts.

 The use of glycerol as solvent for Rh-catalyzed hydroaminomethylation reaction
to synthesize amines from alkenes, carbon monoxide and hydrogen.
The manuscript of this Thesis is organized in 4 chapters as follows:
In Chapter 1, a literature survey of monometallic nanoparticles, including
synthetic methodologies and role of stabilizing agents/supports, are presented. In
particular, the synthesis of first-row transition metal nanoparticles (mainly Cu, Ni and Co)
immobilized in unusual solvents (water, alcohols and ionic liquids), and their catalytic
applications are described. On the other hand, the general structures, synthetic routes and
factors influencing in the synthesis of bimetallic nanoparticles, are presented, followed by
the synthesis of bimetallic nanoparticles and their reactivity in water, alcohols and ionic
liquids, highlighting the most representative examples reported in the literature.
In Chapter 2, the synthesis, characterization and catalytic applications of zerovalent copper nanoparticles (CuNPs) immobilized in glycerol and stabilized by
poly(vinylpyrrolidone) (PVP) are discussed. The influence of reaction parameters (such as
nature of copper precursors, Cu/PVP ratio, Cu/H2 ratio, temperature, reaction time) on the
formation of CuNPs is evaluated. The as-prepared CuNPs were fully characterized by
(HR)TEM, EDX, UV-vis, IR, XRD, XPS, cyclic voltammetry and elemental analysis, both
in glycerol solution and solid state. CuNPs dispersed in glycerol proved to be a robust and
versatile catalyst for a diversity of C-N bond formation reactions: synthesis of di- (via

-4-


cross-dehydrogenative coupling), tri- (via aldehyde-amine-alkyne A3 coupling) and tetrasubstituted propargyl amines (via ketone-amine-alkyne KA2 coupling); different types of
heterocycles were also obtained, in particular indolizines, benzofurans and quinolines, by
tandem A3-cycloisomerization processes, using ortho-functionalized benzaldehydes as
substrates. The recycling of the catalytic phase was studied, getting metal-free organic
compounds.

In Chapter 3, bimetallic palladium-copper nanoparticles (PdCuNPs) dispersed in
glycerol, synthesized by both co-reduction and sequential reduction methods, are
described. The optimization of the different parameters involved in the PdCuNPs synthesis
is detailed (nature of metal precursors, solvent, metal/PVP ratio, temperature and reaction
time). The as-prepared PdCuNPs were fully characterized by different techniques, such as
XRD, FT-IR, XPS, cyclic voltammetry, elemental analysis, HR(TEM), HAADF-STEM,
EDX mapping profile and EDX line-scanning profile, both in glycerol solution and solid
state. In terms of catalytic applications, the synergetic effect between both metals on
activity and selectivity was evaluated through the selective hydrogenation of alkynes
towards alkenes and azide-alkyne cycloaddition reaction. In addition, PdCuNPs in
glycerol, acting as a multi-task catalytic system, were applied in one-pot processes,
involving Cu-catalyzed azide-alkyne cycloaddition (CuAAC) followed by Pd-catalyzed CC cross coupling (Sonogashira, Suzuki and Heck) reactions.
In Chapter 4, a preliminary study corresponding to the synthesis of amines from
alkenes by Rh-catalyzed tandem hydroaminomethylation reaction, using glycerol as
solvent instead of common organic solvents, is presented. The results obtained points to an
efficient catalyst, competitive in relation to the best Rh-based systems described in the
literature.
The main conclusions and perspectives are included in the last part of this
manuscript.

-5-


References
[1]

Catalyst Separation, Recovery and Recycling, (Eds.: D. Cole-Hamilton, R. Tooze),
Springer, Dordrecht, 2006.

[2]


J. M. Thomas, ChemCatChem 2010, 2, 127–132.

[3]

Nanomaterials in Catalysis, (Eds.: P. Serp, K. Philippot), Wiley-VCH, Weinheim,
2013.

[4]

Nanoparticles and Catalysis, (Ed.: D. Astruc), Wiley-VCH, Weinheim, 2008.

[5]

Nanoparticles from Theory to Application, (Ed.: G. Schmid), Wiley-VCH,
Weinheim, 2004.

[6]

M. R. Axet, R. Basca, B. F. Machado, P. Serp, Adsorption on and Reactivity of
Carbon Nanotubes and Graphene, in: Handbook of Carbon Nano Materials, (Eds.:
F. D’Souza, K. M. Kadish), World Scientific, Singapore, 2014, 5, pp 39–183.

[7]

P. Serp, Carbon Nanotubes and Nanofibers in Catalysis, in: Carbon Materials for
Catalysis, (Eds.: P. Serp, J. L. Figueiredo), Wiley-VCH, Weinheim, 2008, pp 309–
372.

[8]


P. Serp, E. Castillejos, ChemCatChem 2010, 2, 41–47.

[9]

P. Serp, M. Corrias, P. Kalck, Appl. Catal., A. 2003, 253, 337–358.

[10]

Y. Wang, Z. Xiao, L. Wu, Curr. Org. Chem. 2013, 17, 1325–1333.

[11]

M. B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X. Huang, R. Silva, X. Zou,
R. Zboril, R. S. Varma, Chem. Rev. 2016, 116, 3722–3811.

[12]

D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed. 2005, 44, 7852–7872.

[13]

R. J. White, R. Luque, V. L. Budarin, J. H. Clark, D. J. Macquarrie, Chem. Soc.
Rev. 2009, 38, 481–494.

[14]

H. Dong, Y.-C. Chen, C. Feldmann, Green Chem. 2015, 17, 4107–4132.

[15]


F. Chahdoura, I. Favier, M. Gómez, Chem. Eur. J. 2014, 20, 10884–10893.

[16]

I. Favier, D. Madec, M. Gómez, Metallic Nanoparticles in Ionic Liquids –
Applications in Catalysis, in: Nanomaterials in Catalysis, (Eds.: P. Serp, K.
Philippot), Wiley-VCH, Weinheim, 2013, pp 203–249.

[17]

J. Dupont, J. D. Scholten, Chem. Soc. Rev. 2010, 39, 1780–1804.

-6-


[18]

C. P. Mehnert, Chem. Eur. J. 2005, 11, 50–56.

[19]

J. Scholz, M. Haumann, Supported Ionic Liquid Thin Film Technology, in
Nanomaterials in Catalysis, (Eds.: P. Serp, K. Philippot), Wiley-VCH, Weinheim,
2013, pp 251–280.

[20]

L. Rodríguez-Pérez, C. Pradel, P. Serp, M. Gómez, E. Teuma, ChemCatChem
2011, 3, 749–754.


[21]

J. Huang, T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu, Y. Chang, G. Zhao, Angew.
Chem. Int. Ed. 2004, 43, 1397–1399.

[22]

M. Ruta, G. Laurenczy, P. J. Dyson, L. Kiwi-Minsker, J. Phys. Chem. C 2008, 112,
17814–17819.

[23]

P. Migowski, K. L. Luska, W. Leitner, Nanoparticles on Supported Ionic Liquid
Phases – Opportunities for Application in Catalysis, in: Nanocatalysis in Ionic
Liquids, (Ed.: M. H. G. Prechtl), Wiley-VCH, Weinheim, 2017, pp 249–273.

[24]

Y. Gu, F. Jérơme, Green Chem. 2010, 12, 1127–1138.

[25]

A. E. Díaz-Álvarez, J. Francos, B. Lastra-Barreira, P. Crochet, V. Cadierno, Chem.
Commun. 2011, 47, 6208–6227.

[26]

F. Chahdoura, C. Pradel, M. Gómez, Adv. Synth. Catal. 2013, 355, 3648–3660.


[27]

F. Chahdoura, S. Mallet-Ladeira, M. Gómez, Org. Chem. Front. 2015, 2, 312–318.

[28]

F. Chahdoura, I. Favier, C. Pradel, S. Mallet-Ladeira, M. Gómez, Catal. Commun.
2015, 63, 47–51.

[29]

F. Chahdoura, C. Pradel, M. Gómez, ChemCatChem 2014, 6, 2929–2936.

[30]

A. Reina, C. Pradel, E. Martin, E. Teuma, M. Gómez, RSC Adv. 2016, 6, 93205–
93216.

[31]

A. Reina, A. Serrano-Maldonado, E. Teuma, E. Martin, M. Gómez, Catal.
Commun. 2018, 104, 22–27.

-7-


-8-


Chapter 1


Chapter 1

Mono- and Bi- Metallic Nanoparticles in Catalysis

-9-


Chapter 1

Index
1.1 Metal nanoparticles................................................................................................... - 11 1.1.1 Synthesis and stabilization of metal nanoparticles ............................................ - 12 1.1.2 Metal nanoparticles involving first-row transition metals in “wet” catalysis .... - 16 1.1.2.1 Water ........................................................................................................... - 16 1.1.2.2 Alcohols ...................................................................................................... - 22 1.1.2.3 Ionic liquids ................................................................................................. - 29 1.2 Bimetallic nanoparticles ........................................................................................... - 34 1.2.1 Synthesis of bimetallic nanoparticles ................................................................ - 35 1.2.2 Factors influencing the synthesis of bimetallic nanoparticles ........................... - 37 1.2.3 Bimetallic nanoparticles applied in catalysis ..................................................... - 39 1.2.3.1 Water ........................................................................................................... - 39 1.2.3.2 Alcohols ...................................................................................................... - 45 1.2.3.3 Ionic liquids ................................................................................................. - 47 1.3 References ................................................................................................................ - 52 -

- 10 -


Chapter 1
1.1 Metal nanoparticles
In general, metal nanoparticles (MNPs) exhibit a size in the range of 1-100 nm [14]. MNPs reveal high surface-to-volume ratios and their unique optical, electronic and
magnetic properties which are absolutely different compared to bulk materials [5,6]. For
instance, the melting point of bulk gold is 1064 oC, but slowly falls down for materials
showing nanoparticles of mean size ca. 20 nm (1000 oC) and dramatically drops for those
exhibiting smaller sizes (ca. 3 nm, 350 oC). This fact seems to be related to higher
number of surface atoms in the nanoparticles, in relation to bulk metal; these surface atoms
are less coordinated than inner atoms, leading to a easy mobility at high temperature, and
then reducing the melting point [7,8]. Besides, the difference in color between bulk gold
(yellow) and gold nanoparticles (red) could be explained as a consequence of the localized
plasmon resonance observed at nanosize below 50 nm [7,8].
One of the earliest contributions in preparation of MNPs is related to gold
nanoparticles synthesized by Faraday in 1857, by reduction of Na[AuCl 4] with white

phosphorus in carbon disulphide [9]. One century later, this gold-based material was
structurally identified by transmission electron microscopy, revealing the presence of
nanoparticles (mean diameter of 6 ± 2 nm) [10].
Over the past few decades, nanoscience and nanotechnology have been attractive in
diverse fields, such as chemistry, physics, material science, life science and medicine
[4,11]. Among them, one of the most developed areas is catalysis. From the late-90s of the
last century, nanocatalysis has emerged as a domain at the interface between homogeneous
and heterogeneous catalysis. Nanocatalysts can combine the advantages of the classical
catalysts, leading to a more efficient reactivity and novel pathways and easy
recovery/recycling (supported MNPs) (Figure 1.1) [12-15]. Indeed, the distinctive
catalytic activity exhibited by MNPs is influenced by the energy of the surface atoms, size,
morphology, different surface sites showing different coordination numbers (corners,
edges, faces, steps), capped stabilizers and supports, among the main factors related to the
surface properties [4,11,16,17].

- 11 -


Chapter 1

Figure 1.1 Main properties of MNPs compared to homogeneous and heterogeneous catalysts.

1.1.1 Synthesis and stabilization of metal nanoparticles
Metal nanoparticles could be prepared following two main approaches, i.e. by topdown and bottom-up methodologies (Figure 1.2) [8,15,18-20].

Figure 1.2 Schematic illustration of preparative methods of MNPs [15].

Top-down approaches are physical methods involving thermal and mechanical
subdivision of bulk metals giving nano-size metal particles; their stabilization can be
reached by addition of protecting agents. Some of the most commonest synthetic


- 12 -


Chapter 1
techniques are: spray drying (a heated gas stream dries an atomized fluid permitting to
obtain nano-powders, but broad size distributions) [21], laser ablation (MNPs are removed
from a solid by irradiation with a laser beam, then condensed in solution to obtain MNPs;
however, this technique is quite expensive) [22], and sputtering (MNPs are bombarded by
energetic ions and then condensed on a thin film). Among these latter techniques,
sputtering leads to a better control of size and morphology in a reproducible way than the
others, but it still remains less accessible [23,24].
On the contrary, the most appropriate methodologies for the preparation of MNPs
for catalytic purposes are those coming from bottom-up approaches, due to the better
control on size, morphology and composition [1,3,19,20,25]. The main four techniques are
following highlighted:
 Chemical reduction of transition metal salts, carried out by using reducing
agents such as H2, CO, hydrides (NaBH4, LiAlH4), hydrazine, alcohols, etc. [2628].
 Decomposition of organometallic complexes, carried out by reduction of
coordinated ligands and/or metal under H2, CO pressure, etc. [1,29-32].
 Thermal, photochemical or sonochemical decompostition of metal precursors,
carried out at high temperatures, light or ultrasound irradiation triggering the
reduction [33-37].
 Electrochemical reduction, preparation of MNPs on the cathode surface of the
electrode, where anode acts as a source of metal [38-42].
From a mechanistic point of view, the formation of MNPs starts by the reduction of
a metal precursor towards zero-valent metal atoms; and then the formation of nuclei by
collision of metal atoms or autocatalytical pathway. Then, the growth of the nuclei gives
instable metal nanoparticles which could be controlled by chemical factors (mainly by
adding a stabilizing agent) to prevent agglomeration towards bulk metal as described by

Turkevich (Figure 1.3) [43]. Therefore, the size and shape of MNPs can be controlled by
reaction conditions, such as nature and concentration of the metallic precursor, nature of
stabilizer, reducing agent, temperature, solvent, reaction time, etc.

- 13 -


Chapter 1

Figure 1.3 Mechanism of formation of MNPs following a chemical reduction approach [42,43].

At nanoscale, the excess of surface free energy, compared to the lattice energy,
induces thermodynamically unstable MNPs, in consequence, they tend to aggregate
towards bulk metal by van der Waals forces [8]. Therefore, the use of stabilizing agents
(such as polymers, dendrimers, ligands, surfactants, etc.) is necessary to prevent the
agglomeration and to control the size and/or dispersion of MNPs [20]. The stabilization of
MNPs can be classified into three main categories: electrostatic, steric and electrosteric
stabilization (Figure 1.4) [44-47].

Figure 1.4 Schematic representation of electrostatic, steric and electrosteric stabilization of MNPs
[44-47].

 Electrostatic stabilization

(Derjaguin-Landau-Verwey-Overbeek stabilization,

DLVO) [48-50]. Anions are adsorbed on the electrophilic surface of MNPs to form
a layer around the nanoparticles and then generate a Coulombic repulsion between
neighboring particles, which is against attractive van der Waals forces between
particles [44-51]. Halides and carboxylates are commonly used to afford this type

of stabilization [45-47,52].

- 14 -


Chapter 1
 Steric stabilization. Metal nanoparticles are coated by polymers providing a sterical
barrier and thus preventing particle aggregation [44-47]. The most common
polymeric stabilizer is poly(vinylpyrrolidone) (PVP), due to its bulky structure and
weak binding to metal surface [46,47,53]. Besides, many other polymers have been
applied to stabilize MNPs, such as poly(2,5-dimethylphenylene oxide) (PPO),
polyacrylonitrile, polyacrylic acid, polyurea, poly(N,N-dialkylcarbodiimide), etc.
[13,46,47]. Furthermore, dendrimers exhibit a well-defined polydispersity acting as
macromolecular box, permitting the confinement of MNPs, and thus, the control of
size and morphology of MNPs [39,40]. On the other hand, ligands such as thiols,
phosphines and amines can stabilize MNPs by strong dative -interactions and/or
-back donation from metal to the Lewis base [46,47,54-56].
 Electrosteric stabilization. The stabilization of MNPs is induced by both
electrostatic and steric effects; the most used stabilizers are tetraammoniumalkyl
halides, polyoxoanions ([CH2CH(CO2)]nn-, P2W15Nb3O629-, SiW9Nb3O407-,
C6H5O73-, etc.) [45].
On the other hand, MNPs can be deposited on solid supports or dispersed on liquid
phases, permitting to stabilize MNPs as well as favoring an easy recycling when MNPs are
applied as catalysts.
 Solid supports: there are numerous studies focusing on the preparation of
supported-MNPs

using

carbon-based


materials

(activated carbon,

carbon

nanotubes, graphenes, graphite, etc.) [27,46,57-62], silicas (SiO2, zeolite,
mesoporous silica as SBA, MCM, etc.) [27,62] or metal oxides (Al2O3, CeO2, ZnO,
MgO, TiO2, etc.) [27,46,57,62].
 Liquid supports: solvents as ionic liquids have been proved to be efficient liquid
supports to immobilize MNPs (colloidal solution), through the electrostatic
stabilization [45-47,63-67]. In addition, polyols have been also used to synthesize
MNPs with uniform size and shape as well as low agglomeration [26]. Recently,
glycerol has become a convenient medium to synthesize MNPs, mainly thanks to
its supramolecular arrangement [68-72].
Nowadays, the synthesis of MNPs in liquids which can act also as stabilizers (i.e.
polyols, ionic liquids), has become more and more attractive. In comparison with solid

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Chapter 1
supports, the synthesis of MNPs in these types of solvents permits to obtain small MNPs
with “monodisperse” size distribution, even at high metal concentration. In the case of
colloidal MNPs (MNPs dispersed in a liquid phase), the access of reagents to catalytic
active sites is in general easier, because mass transfer concerns are minimized [15].
Furthermore, these catalytic systems can be easily recycled, like for solid-supported MNPs,
thanks to the immobilization of MNPs in these like liquid supports [26,45-47,65,68].
Compared to noble metals, first-row transition metals have been less studied in the

synthesis of MNPs in such media (polyols, ionic liquids) [26,45-47,65,68].
Besides, in comparison with common organic solvents, water is considered as a
green solvent and an interesting alternative for economic reasons. Water has some
advantages, such as owning highly polar character (tuning original activity and selectivity
in catalytic reactions) and its low miscibility with most organic solvents (recovery and
recycling through a water-organic biphasic approach). However, it shows some concerns
due to the low solubility of organic compounds and the instability of some metal-based
catalysts [25].
In the next part of this chapter, we discuss the synthesis of colloidal first-row
transition metal nanoparticles as well as their catalytic applications in water, polyols and
ionic liquids.
1.1.2 Metal nanoparticles involving first-row transition metals in “wet” catalysis
Among the first-row transition metals, copper, nickel and cobalt are the most
frequently used to synthesize metal-based nanoparticles. In this part, some examples of the
synthesis of Cu-, Ni- and Co-based nanoparticles in three kinds of solvents (water, alcohols
and ionic liquids) and their catalytic applications (if concerned) are presented. Metal
nanoparticles supported on solids will not be considered.
1.1.2.1 Water
Water is a sustainable solvent due to its non-toxicity, availability, friendly
environmentally impact, ability to solubilize some reagents (metal salts, reducing agents,
stabilizers). However, zero-valent MNPs can be easily oxidized towards metal oxides in
this medium [25].

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Chapter 1
Copper nanoparticles (CuNPs). Many works use reducing agents such as NaBH4,
N2H4, glucose, ascorbic acid, etc. to synthesize CuNPs in water by chemical reduction.
Veerappan and co-workers reported the synthesis of CuNPs starting from CuCl2, using

hydrazine as reducing agent and pectin as stabilizer at room temperature [73]. The
resulting CuNPs were analyzed by UV-vis, PXRD and (HR)-TEM, proving the formation
of spherical nanoparticles (ca. 5 nm) constituted by zero-valent copper with face-centered
cubic (fcc) crystalline structure (Figure 1.5). These as-prepared CuNPs were applied in the
catalytic reduction of nitrobenzenes in aqueous solution by NaBH4. Substrates were
efficiently converted into anilines in less than 5 minutes (67 mol% CuNPs), whereas the
reaction did not work in the absence of CuNPs. Furthermore, these CuNPs were also
applied in C-N cross-couplings of amines with bromobenzene in dimethylsulfoxide,
affording good yields (69-85%) [73].

Figure 1.5 (A) Schematic preparation of pectin-stabilized CuNPs from CuCl2; (B) PXRD pattern
of CuNPs; (C) HR-TEM micrograph of CuNPs, showing one isolated nanoparticle (inset).
Reproduced from reference [73] with permission of the Royal Society of Chemistry and Copyright
Clearance Center.

Duan and co-workers reported the synthesis of CuNPs in water from CuSO4, using
hydrazine as reducing agent and dodecyl benzene sulfonic acid sodium (DBS) as
stabilizing agent at 100 oC [74]. The CuNPs were characterized by PXRD, indicating the
presence of Cu(0) with fcc structure. TEM analyses showed uniform and well-dispersed
nanoparticles (dmean = 100 nm). The obtained CuNPs catalyzed the reduction of

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Chapter 1
nitrobenzenes using THF:H2O (1:2) as solvent to afford the corresponding anilines in
moderate-to-high isolated yields (66-95%). The reduction of nitrobenzene in H2O at 50 oC
gave aniline in 66% yield after 4 h, whereas using THF:H2O (1:2) permitted to obtain 98%
yield of aniline after 2 h. The catalyst was recycled three times (Scheme 1.1). PXRD
analysis of the catalyst after the 1st run showed the presence of copper oxides, probably

responsible of the activity decrease [74].

THF:H2O (1:2)
o
50 C, 2 h

isolated yields

Ar = 4-Me-C6H4, 3-Me-C6H4,
4-Cl-C6H4, 2-Cl-C6H4,
4-HO-C6H4, 4-OMe-C6H4,
4-CHO-C6H4, 4-C2H5COO-C6H4, etc.

Scheme 1.1 Reduction reactions of nitrobenzenes catalyzed by CuNPs [74].

Following the same strategy, Shen and co-workers reported the synthesis of CuNPs
from CuSO4.5H2O in water by T-shaped microfluidic chip at room temperature, using
NaBH4 as reducing agent in the presence of poly(vinylpyrrolidone) (PVP) as stabilizer
(Figure 1.6) [75]. The obtained CuNPs were characterized by TEM, EDX and UV-vis,
showing uniform size distribution (mean diameter of 8.95 nm) without any evidence of
copper oxides.

Figure 1.6 Schematic representation of the synthesis of CuNPs by T-shaped microfluidic chip at
room temperature. The length, width and height of the channel are 10 nm, 200 m and 30 m,
respectively. Reproduced from reference [75] with permission of the Royal Society of Chemistry
and Copyright Clearance Center.

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Chapter 1
Copper nanoparticles were also prepared in water by an electrochemical method.
Theivasanthi and co-workers reported the electrochemical synthesis of CuNPs using
CuSO4.5H2O as metal precursor [76]. Surface-cleaned copper electrodes were used (15 V,
6 A). CuNPs were obtained on the cathode surface and characterized by PXRD, FT-IR and
TEM, showing the formation of Cu(0) nanoparticles (mean size of 24 nm). Similarly,
Hashemipour et al. also synthesized CuNPs from CuSO4 in water with electrical power of
3 V and 7 A for 20 min. As a result, the spongy layers of CuNPs were deposited on the
plating electrode, exhibiting a mean particle size of 10 nm [77].
353
353 K
K

2 M hydrazine

3 M hydrazine

5 M hydrazine

298 K

Figure 1.7 SEM images of CoNPs synthesized at 298 K and 353 K using different hydrazine
concentrations [78]. Open access article distributed under the Creative Commons Attribution
License.

Cobalt nanoparticles (CoNPs). Compared to CuNPs, the synthesis of CoNPs (and
NiNPs) in water has been less developed because of their easy oxidation. Salman and coworkers studied the synthesis CoNPs from CoSO4.7H2O, using hydrazine as a reducing
agent. In this work, citric acid acts as capping agent to protect and stabilize Co(0)NPs. In

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Chapter 1
the absence of citric acid, Co(OH)2 was detected; the addition of citric acid allowed the
formation of hexagonal close-packed (Co) nanoparticles. Besides, the effect of hydrazine
concentration and temperature on morphology of CoNPs was evaluated as described in
Figure 1.7 [78]. No catalytic applications were discussed in this paper.
Mukherjee et al. reported CoNPs as reusable catalysts for the reduction of 4nitrophenol [79]. In this work, CoNPs were synthesized from CoSO4 in water, using
NaBH4 as reducing agent and tetrabutyl ammonium bromide (TBAB) as surfactant, at
room temperature for 1 h. The TBAB-stabilized CoNPs were characterized by TEM, FTIR and XPS proving the formation of Co(0) particles at nanosize scale (the average size in
the range 9095 nm). Godard and co-workers synthesized colloidal cobalt nanocatalysts
applied in Fischer-Tropsch process [80]. A series of CoNPs were also prepared in water by
chemical reduction of CoCl2 in the presence of polymers as stabilizers and using NaBH4 as
reducing agent, giving spherical nanoparticles showing mean diameters of ca. 2.6 nm
(Figure 1.8). Boron doping on surface of CoNPs and Co/B ratio were detected by XPS and
ICP analyses, respectively. The presence of boron on metal surface has been frequently
observed in the synthesis of MNPs using NaBH4 as reducing agent [81,82]. The isolated
CoNPs (by centrifugation) were re-dispersed in water and applied in Fischer-Tropsch
synthesis, under 31.5 bar total pressure (H2:CO:Ar = 2:1:0.15) at 180 oC for 12 h. Except
Co4, the selectivity of CO2, CH4, C2–C4 and C5–C12 was ranged between 23–43, 18–47,
16–40 and 8–24 wt%, respectively (Figure 1.9). The nature and structure of the stabilizers
influenced the reduction degree of cobalt and the B-doping of CoNPs, thus affecting the
reactivity in the aqueous phase Fischer-Tropsch synthesis [80].

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