(Bi) metallic nanoparticles in glycerol synthesis, characterization and catalytic applications

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 appli

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

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…

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

2.3.2.3 One-pot three-component ketone-amine-alkyne (KA ) 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

A 3 Aldehyde, amine and alkyne

AAC Azide-alkyne cycloaddition

ATR Attenuated total reflectance

BF-TEM Bright field transmission

FFT Fast fourier transform

FT-IR Fourier transform infrared

ICP-AES Inductively coupled plasma

atomic emission spectroscopy

J Coupling constant ILs Ionic liquids

IR Infrared spectroscopy

KA 2 Ketone, amine and alkyne

Mes Mesityl MNPs Metal nanoparticles

MS Mass spectroscopy

MW Microwave NMR Nuclear magnetic resonance OAc Acetate

ppm Part per million PEG Polyethylene glycol

Ph Phenyl PVA Polyvinyl alcohol PVP Poly(vinylpyrrolidone) PXRD Powder X-ray diffraction

RT Room temperature Sel Selectivity

SEM Scanning electron microscopy SPR Surface plasmon resonance STEM Scanning transmission electron

microscopy

t Time

T Temperature TEM Transmission electron microscopy

TMEDA

N,N,N’,N’-tetramethylethylenediamine

TOF Turnover frequency

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

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 nano-structures) 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

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

zero-valent 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

substituted propargyl amines (via ketone-amine-alkyne KA 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 C-

C 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

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Chapter 1

Mono- and Bi- Metallic Nanoparticles in Catalysis

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

-1.1 Metal nanoparticles

In general, metal nanoparticles (MNPs) exhibit a size in the range of 1-100 nm

[1-4] 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[AuCl4] 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]

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

top-down 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

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

[26-28]

 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

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]

 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

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]

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

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]

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

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]

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 workers 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

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

4-nitrophenol [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, FT-

IR 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]

Figure 1.8 TEM micrographs and size histograms of CoNPs stabilized by polymers Reprinted

with permission from reference [80] Copyright (2017) MDPI

Figure 1.9 Product selectivity in the aqueous phase Fischer-Trosch synthesis catalyzed by CoNPs

Reprinted with permission from reference [80] Copyright (2017) MDPI

Nickel nanoparticles (NiNPs) Zhao et al reported the synthesis of NiNPs, in

water by reduction of NiCl2.6H2O using NaBH4 as reductant [83] During the reaction,

NaBH4 reduced Ni2+ and water towards Ni0 and H2 Normally, NiNPs can be easily oxidized by O2 dissolved in H2O In this case, the in situ generated H2 favored the reduction to Ni(0), excluding its oxidation PXRD analysis evidenced the presence of fcc Ni(0) structure in the absence of crystalline oxidized phases TEM micrographs showed NiNPs with a size range of 10 to 30 nm and irregular shapes due to the lack of stabilizers

Chen et al reported the synthesis of NiNPs from NiCl2, using hydrazine as reducing agent in an aqueous solution of cationic surfactants cetyltrimethylamonium bromide and tetradodecylammonium bromide (CTAB/TC12AB) [84] The pure Ni

crystalline of fcc structure was confirmed by PXRD analysis The mean size of NiNPs (ca

1036 nm) decreased when hydrazine concentration increased or NiCl2 concentration decreased On the other hand, Musaev and co-workers reported the formation of NiNPs by

laser ablation in water [85] Laser ablation was performed on a 1 cm  1 cm  8 mm Ni block The target was immersed in water and irradiated by a nitrogen laser operating at 337

nm, with pulse duration of 10 ns and a repetition rate of 5 Hz The obtained NiNPs were characterized by TEM, EDX and PXRD exhibiting the presence of nickel oxides in the as-prepared materials

1.1.2.2 Alcohols

In contrast to water, alcohols can act as reducing agents, in particular following the

polyol methodology [26] In this part, the synthesis of MNPs in ethanol, benzyl alcohol,

and polyalcohols (ethylene glycol, glycerol, etc.) and their applications in catalysis (if

concerned) are discussed

Ethanol Bakr and co-workers reported the synthesis of complex branched

nanocages Cu(OH)2 by reduction of CuCl2 in ethanol , using NaBH4 as reducing agent and

PVP as stabilizer [86] According to bright-field TEM (BF-TEM) micrographs, the size of

nanocages’ core was from 19 to 240 nm and the length of the branches was from 85 to 232

nm The structure and size of nanocages was tuned by temperature and the ratio of

metal/surfactant used (Figure 1.10) Furthermore, polycrystalline CuO branched

nanocages were formed from Cu(OH)2 by dehydration at 200 oC for 24 h Besides, reduction of Cu(OH)2 by hydrazine permitted to obtain Cu2O nanoframes In this work,

only CuO branched nanocages were tested in the C-S cross-coupling between iodobenzene

and 1-octanethiol in DMSO, giving 91% conversion in 2 h (Figure 1.11)

Figure 1.10 BF-TEM micrographs of the nanostructures at different PVP/Cu ratios at 25 oC and 65

o

C Reprinted with permission from reference [86] Copyright (2014) American Chemical Society

Figure 1.11 Catalytic study of C-S cross coupling between iodobenzene and 1-octanethiol over time, catalyzed by CuO branched nanocages Reprinted with permission from reference [86]

Copyright (2014) American Chemical Society

Xiao’s group reported a simple method to prepare CoNPs with homogeneous

spherical shape and narrow size distribution [87] This synthetic technique is relied on

chemical reduction of CoCl2 in ethanol using NaBH4 as reducing agent, giving spherical CoNPs with mean diameter of 4.7 nm PXRD pattern of the as-prepared CoNPs showed only a very broad peak corresponding to the (111) diffraction line of (fcc) cobalt All diffraction lines of fcc Co(0) structure were observed after annealing at 600 oC for 1 h under argon flow Interestingly, Co-B alloy pattern which is usually observed in the syntheses of MNPs using borohydride salts, was not detected in this case It seems that

chemical reductions in nonaqueous solutions permit to avoid possible formation of metal oxides and borides

Benzyl alcohol Benzyl alcohol acted as both solvent and reducing agent to obtain

Cu-based nanoparticles under microwave-assisted conditions [88,89] Shivashankar and

co-workers reported the synthesis of copper nanostrutures by reduction of Cu(acac)2 in benzyl alcohol under microwave irradiation (800 W) for 3 min under reflux conditions at room temperature PXRD and XPS analyses showed the formation of Cu and Cu2O NPs The BF-TEM micrograph revealed small and spherical nanoparticles (dmean = 7.7 ± 3.8 nm)

[88] On the other hand, Prasad et al reported the synthesis of CuO and Cu2O NPs from Cu(OAc)2 in benzyl alcohol under microwave conditions (700 W) From the same precursor, the formation of CuO or Cu2O NPs was controlled by the pH of the solution as the only variable parameter The morphology, phase purity and optical properties of the obtained nanoparticles were characterized by TEM, SEM, XRD and optical spectroscopy

[89]

Ethylene glycol It represents the most frequently used alcohol to synthesize MNPs

Kawasaki et al described the synthesis of monodispersed spherical CuNPs (ca 3.5 nm) in

ethylene glycol (high metal concentration, up to 0.6 M copper salt) from Cu(OAc)2, using hydrazine as reducing agent and 1-amino-2-propanol as stabilizer, at room temperature

[90] Luo’s group also developed the synthesis of CuNPs by chemical reduction of CuCl2

in ethylene glycol, using L-ascorbic acid as reducing agent in the absence of any other

capping agent [91] In this work, highly dispersive CuNPs with a mean particle size of less

than 6 nm were characterized by (HR)TEM, UV-vis and EDX The influence of the order

of reagent addition on the size of CuNPs was evaluated as described in Figure 1.12

Similarly, the synthesis of Cu(0)NPs was carried out by reduction of CuSO4.5H2O in ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP) or sodium dodecyl

sulphate (SDS) as stabilizers [92]; vanadium(II) sulfate was used as reductant The

Cu(0)NPs were obtained with a mean size ca 50 nm in the absence of oxidized copper

Figure 1.12 TEM micrographs and size distribution of CuNPs with different ways of reagent

addition (A) L-ascorbic acid solution was drop-wised added into CuCl2 solution heating at 100 oC;

(B) CuCl2 solution was drop-wised added into L-ascorbic acid solution heating at 100 oC; (C) the mixture solution of CuCl 2 and L-ascorbic acid was drop-wised added into the hot ethylene glycol

solvent; (D) solution of CuCl 2 and solution of L-ascorbic acid were mixed together at 100 oC

Reproduced from reference [91] with permission of John Wiley and Sons and Copyright Clearance

Center

Diethylene glycol (DEG) was also used as solvent for the synthesis of

monodisperse CuNPs by the polyol methodology [26] In fact, poly(vinylpyrrolidone)

(PVP) acting as capping agent and sodium phosphinate monohydrate (NaH2PO2.H2O) as

reducing agent allowed the formation of particles with a mean diameter of 45 ± 8 nm [93]

The particle size and its distribution were tuned by optimization of reaction parameters such as concentration of reducing agent, temperature and precursor injection rate The XPS analyses exhibited the presence of amorphous CuO and chemisorbed PVP surrounded on the surface of copper

Polyol process has been also applied to synthesize cobalt nanoparticles (CoNPs) Jeyadevan and co-workers prepared CoNPs by reduction of Co(OAc)2, CoCl2 or Co(OH)2

in ethylene glycol, in the presence of either sodium metal or sodium hydroxide at high temperature (>190 oC) as described in Scheme 1.2 [94]

Scheme 1.2 Synthesis of CoNPs using different cobalt precursors in ethylene glycol Reproduced from reference [94] with permission of the Royal Society of Chemistry and Copyright Clearance

Center

Glycerol (propane-1,2,3-triol) is produced in high amounts as co-product in the

production of biodiesel [95] Since beginning of the 21st century, glycerol has been considered as a sustainable solvent for organic transformations and in particular for the

synthesis of MNPs [96] 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 [26,97,98] Especially, the hydroxyl groups trigger a complex supramolecular network [69-72], permitting to trap and disperse MNPs, avoiding their agglomeration Up

to date, mainly second- and third-row transition metal nanoparticles were synthesized in

glycerol [68] Regarding first-row transition metals, to the best of our knowledge, only

works involving Cu, Ni or Mn have been reported, leading to mixtures of zero-valent metal

and metal oxide nanoparticles [99,100,102,103]

Nisaratanaporn and co-workers reported the preparation of ultrafine Cu powders from Cu(NO3)2.3H2O in a solution of NaOH in glycerol (with different molar ratios of NaOH:Cu(NO3)2.3H2O in the range of 0:1 to 5:1), at 120–160 oC The size, shape and composition of Cu powders were controlled by the concentration of NaOH and the reaction

time PXRD and SEM analyses pointed out the formation of Cu(0) and Cu2O particles with

mean particle sizes in the range of 60–400 nm [99] Recently, Khan’s group prepared

CuNPs in glycerol by reduction of CuCl2 using hydrazine as reducing agent [100a] The

as-prepared materials were characterized by TEM (showing an average particle size of 5 nm) and XANES (X-ray Absorption Near Edge Structure), evidencing the formation of mixtures of Cu(0) and CuO NPs However, no further catalytic applications were studied Bartůněk and co-workers reported the synthesis of copper nanoparticles stabilized in a polyvinyl alcohol-glycerol matrix CuNPs were characterized by UV-vis, showing a small shoulder band around 580 nm (but not clearly) PXRD analysis showed the presence of crystalline Cu(0) (fcc structure) with the mean diameter of particles of 37 nm However,

these results could not exclude the presence of copper oxides in the samples [100b]

In our group, we published the preparation of Cu2O NPs (dmean = 4.7 ± 1.5 nm) immobilized in glycerol by reduction reaction of Cu(OAc)2 under H2 (3 bar) at 100 oC,

using PVP as stabilizer (Figure 1.13) [101] The as-prepared Cu2O NPs were fully characterized by (HR)TEM, XRD, FT-IR and EDX, evidencing the exclusively formation

of Cu2O The preformed Cu2O NPs were applied in C-heteroatom couplings and alkyne cycloadditions (CuAAC), giving the expected products in high isolated yields (86-97%) Furthermore, the catalytic phase was recycled up to 10 times without loss of the

azide-catalytic reactivity (Scheme 1.3)

Figure 1.13 TEM micrograph and the corresponding size histogram for Cu2 O NPs dispersed in

glycerol after centrifugation Reproduced from reference [101] with permission of John Wiley and

Sons and Copyright Clearance Center

Scheme 1.3 a) C-heteroatom couplings and b) AAC catalyzed by Cu2O NPs in glycerol [101]

Concerning nickel nanoparticles, the group of Shen published the synthesis of Ni and Ni/NiO core-shell nanoparticles with particle sizes in the range of 12 to 30 nm, by the

polyol approach [102] Ni(OAc)2.4H2O or Ni(NO3)2.6H2O was dissolved in glycerol at 80

o

C for 30 min; the mixture was then aged at 80 oC for 1 h, after addition of Na2CO3

aqueous solution The obtained solids were dried at 100 oC and then calcined at 400 oC for

4 h under nitrogen atmosphere, permitting to form pure Ni(0)NPs; or at 400–600 oC under air, obtaining Ni/NiO core-shell nanoparticles with different structures

On the other hand, the synthesis of Mn(OH)2 (ca 20 nm) and MnCO3 (ca 60 nm)

nanocrystals was developed by Li’s group following a hydrothermal reduction, using glycerol and glycol as both solvents, reducing agents (KMnO4 was used as metal

precursor) and stabilizing agents [103]

1.1.2.3 Ionic liquids

Ionic liquids (ILs) exhibit distinctive physico-chemical properties (low vapor pressure, wide temperature range for the liquid phase, low coordinative properties, thermal and chemical stability, broad electrochemical window) [63-67,104]; these features permit

to control the assembling, size and shape of metal nano-systems However, there are still

main drawbacks, as high cost, high viscosity, toxicity, etc [104]

Figure 1.14 Some frequently used cations and anions for ionic liquids [104]

The synthesis of MNPs in ionic liquids started in the beginning of the 2000s Dupont’s group firstly reported the synthesis of iridium nanoparticles by reduction of [IrCl(cod)]2 (cod = 1,5-cyclooctadiene) in 1-n-butyl-3-methylimidazolium

hexafluorophosphate ([C1C4Im][PF6]), under H2 (4 atm) at 75 oC for 10 min [63] This

catalytic system was applied in hydrogenations of different olefins, giving high conversions (56-100%)

ILs favor the electrostatic stabilization of NPs, playing the role in stabilization, size

and solubility of the NPs [13,45-47,63-66] However, ILs are non-innocent; at high

temperature, palladium N-heterocyclic carbene complexes can be generated from

deprotonation of the imidazolium salt, then bounding to the surface of NPs or forming mononuclear mono- or bis-carbene complexes to Pd atoms leached from PdNPs surface

[13,46] Over the past decade, the numbers of publications related to the synthesis of

second- and third-row transition metal nanoparticles in different ionic liquids, as well as their catalytic applications have been enormously increased Nevertheless, the synthesis of first-row transition metal nanoparticles in ILs has been less developed In this part, we discuss some works focused on the synthesis of Cu-, Ni- and Co-based nanoparticles as well as their catalytic applications in ionic liquids (if concerned)

Copper nanoparticles Prechtl’s group reported the synthesis of Cu2O NPs in ionic

liquids [105,106] In this work, n-Bu4POAc ionic liquid acted as both reducing agent and solvent for reduction of CuCO3 to form Cu2O NPs at 120 oC The as-prepared Cu2O NPs showed a mean diameter in the range of 5.58.0 nm These nanoparticles were applied in amination reactions of aryl iodides, giving the expected products in high conversions (up to

99%), without the use of stabilizers or additives (such as bases) (Scheme 1.4) [105]

Besides, protodecarboxylation of 2-nitrobenzoic acid and other derivatives were carried out in ILs using this catalyst under smooth conditions; the IL catalytic phase was recycled

ten times without loss of activity [106]

Scheme 1.4 Amination of aryl iodides catalyzed by Cu2O NPs in n-Bu4 POAc Figures indicate conversions (determined by 1H NMR using hexamethyldisilane as internal standard) [105]

Strong reducing agents are commonly used to fully convert Cu2+ into Cu0 In fact, colloidal CuNPs were prepared using hydrazine as reducing agent of Cu(OAc)2 in imidazolium-based ILs and PVP or PVA as stabilizing agents TEM micrographs evidenced that PVP is a better stabilizer than PVA Spherical particles were obtained in 1-butyl-3-methylimidazolium tetrafluoroborate ([C1C4Im][BF4]) and cubic particles in 1-butyl-3-methylimidazolium hexafluorophosphate ([C1C4Im][PF6]) In terms of catalytic

applications, polydispersed spherical CuNPs of size between 80 and 130 nm generated in [C1C4Im][BF4],were used for CuAAC at room temperature, obtaining the corresponding

triazoles in high yields under mild conditions (Scheme 1.5) [107]

Scheme 1.5 Azide-alkyne cycloaddition catalyzed by CuNPs in [C1 C 4 Im][BF 4] [107]

Nickel nanoparticles Dupont and co-workers reported the formation of spherical

NiNPs based on the decomposition of [Ni(cod)2] in different imidazolium-based ILs under

H2 (4 bar) at 75 oC The obtained NiNPs were constituted by a Ni(0) core with a shell of

NiO, showing similar mean diameters in different ILs (Figure 1.15) [64]

Figure 1.15 1-Alkyl-3-methylimidazolium N-bis(trifluoromethanesulfonyl) amide ILs used in the

synthesis of NiNPs and the corresponding particle sizes [64]

The preparation of amino-functionalized ionic liquids, IL-NH2, for the synthesis of

NiNPs was reported by Hou and co-workers as described in Scheme 1.6 [108] In this

work, IL-NH2 acted as stabilizer agent and hydrazine as reducing agent The as-prepared NiNPs were characterized by UV-vis, exhibiting the presence of a weak interaction of the functionalized IL with Ni2+ and Ni0 complexes PXRD analysis confirmed the fcc crystalline structure of Ni(0) TEM micrographs showed the formation of small NiNPs with a mean diameter of 6-7 nm, tending to aggregate in order to form larger nanoparticles

(35 nm) These nanocatalysts were applied in the selective hydrogenation of

4-phenyl-3-buten-2-one in aqueous medium (Scheme 1.7)

Scheme 1.6 Synthetic route of functionalized IL-NH2 and the synthesis of NiNPs [108]

Scheme 1.7 Hydrogenation of 4-phenyl-3-buten-2-one in water [108]

Following a similar strategy, Jiang’s group also synthesized NiNPs from the reduction of Ni(OAc)4.4H2O with NaBH4 or hydrazine in the presence of 1-butyl-2,3-

dimethylimidazolium (S)-2-pyrrolidinecarboxylic acid salt as stabilizing agent [109] The

obtained Ni(0)NPs exhibited a fcc structure (by PXRD), showing a mean particle size of 5.1 nm Besides, UV-vis, TGA and XPS were conducted to study the interaction between NiNPs and IL The IL immobilized NiNPs were applied as highly efficient catalyst in

chemoselective hydrogenation of quinoline and nitroaromatic compounds (Scheme 1.8)

Scheme 1.8 Hydrogenation of quinoline catalyzed by NiNPs [109]

Conv = 92%

Selectivity (A:B) = 93:7

Cobalt nanoparticles The group of Dupont carried out the synthesis of CoNPs by

in situ decomposition of [Co2(CO)8] dispersed in 1-n-decyl-3-methylimidazolium

N-bis(trifluoromethanesulfonyl)imidate ([DMI][NTf2]) at 150 oC [110] These CoNPs

exhibited a bimodal size distribution with a mean size of 79 ± 17 nm for large particles (cubic shape) and 11 ± 3 nm for small ones (spherical nanoparticles) Only CoNPs with cubic shape (dmean = 53 ± 22 nm) were obtained in 1-n-decyl-3-methylimidazolium

trifluoro-tris-(pentafluoroethane) phosphate ([DMI][FAP]) [110] Following the same

approach, CoNPs were also prepared from [Co2(CO)8] at 150 oC for 20-40 min, in different ILs such as 1-n-butyl-3-methylimidazolium-N-bis(trifluoromethanesulfonyl)imidate

([C1C4Im][NTf2]), 1-n-decyl-3-methylimidazolium tetrafluoroborate ([DMI][BF4]) and

1-n-tetradecyl-3-methylimidazolium-N-bis(trifluoromethanesulfonyl)imidate

([TDMI][NTf2]) The resulting spherical CoNPs showed a mean size in the range of 7.7 nm The immobilized zero-valent CoNPs in [C1C4Im][NTf2] were efficient catalysts for

4.5-the Fischer-Tropsch process (Scheme 1.9) [111]

Scheme 1.9 Fischer-Tropsch synthesis catalyzed by CoNPs immobilized in [C1 C 4 Im][NTf 2] [111]

To sum up, the synthesis of first-row transition metal nanoparticles (mainly Cu-, Ni- and Co-based nanoparticles) in different non-conventional solvents (water, alcohols and ionic liquids) has been discussed The use of water as solvent for the synthesis of MNPs usually requires strong reducing agents and/or stabilizing agents For these MNPs, often metal(0)-core and metal oxide-shell is observed On the other hand, in some cases,

polyols (glycerol, ethylene glycol, etc.) and ionic liquids can play the role of solvent,

reducing agent and also stabilizer Moreover, immobilization of MNPs in polyols or ionic liquids permits to avoid the oxidation of zero-valent MNPs and to easily recycle the catalytic systems In particular, we are interested in copper nanoparticles due to their versatile applications in catalysis To the best of our knowledge, up to date, well-defined Cu(0)NPs dispersed in glycerol have not ever been reported in literature In this Thesis, we have focused our efforts on the synthesis of Cu(0)NPs in glycerol and their catalytic applications

1.2 Bimetallic nanoparticles

In early 1980s, the concept of “bimetallic clusters” was introduced by John H Sinfelt, mainly focused on the well-dispersed bimetallic entities supported on alumina or

silica [112] In comparison to monometallic nanoparticles, bimetallic nanoparticles

(BMNPs) have been less developed, in particular for catalytic purposes However, in the past decade, the use of BMNPs as catalysts in organic transformations has significantly

increased [4,113-121] BMNPs differ from their corresponding monometallic structures in

terms of morphology, composition and structure, inducing distinctive physical and

chemical properties [122] The synergy between both metal partners, modulated by the

different BMNP structural arrangements, introduces a wide diversity in reactivity, mainly

due to the electronic tuning of metals [116,123]

Bimetallic nanoparticles (BMNPs) consist of two metal components, performing as

alloy, core-shell structure or cluster-in-cluster as described in Figure 1.16 [119] The

addition of a guest metal modifies the electronic properties of the active sites of the host metal by electron transfer between both partners (electronic effect) Furthermore, the coordination of the guest metal to the surface of the host metal gives new geometries of active sites (geometric effect) In some cases, the atoms of the two metals play a unique role, as adsorption for different reactants or intermediates (synergetic or bi-functional

effect) [116,117]

Figure 1.16 Structures of BMNPs: a) mixed alloys; b) order alloys; c) subclusters with two

interfaces; d) subclusters with three interfaces; e) subclusters with a small number of A-B bonds; f) core-shell nanoparticles; g) multishell core-shell nanoparticles; h) multiple small core material coated by single shell material; i) movable core within hollow shell material Reproduced from

reference [119] with permission of Elsevier and Copyright Clearance Center

1.2.1 Synthesis of bimetallic nanoparticles

Bimetallic nanoparticles can be prepared in gas-phase, in solution, or by deposition

on solid supports according to various bottom-up methodologies, such as chemical reduction, thermal decomposition of precursors, electrochemical synthesis, radiolysis

synthesis and sonochemical synthesis [124]

 Chemical reduction This methodology appears to be the most appropriate way to

control the synthetic parameters and thus the final structure of BMNPs The metal precursors, in an appropriate solvent containing the stabilizer, are commonly reduced by NaBH4, N2H4 or H2, etc [4,15,119,124] BMNPs can be prepared

following 2 main approaches, as follows:

 Co-reduction of mixed metal precursors Two different metal precursors

are reduced simultaneously Depending on the reduction rate of constituted metal ions and chemical nature of metals (reduction potential, structure), this method permits to prepare different types of BMNPs Using reducing agents such as NaBH4 or N2H4, the reduction process is fast, and leads to independent nucleation processes for both metals without formation of

structured BMNPs [124-126] To overcome this problem, the choice of

appropriate metal precursor, reducing agent, stabilizer and solvent permits

to gain better control in the reduction step of the synthesis of BMNPs [127]

Besides, the reduction process is more controllable if using milder reducing agents, such as borane-tert-butylamine complex (TBAB), CO, superhydrides (LiBEt3H), oleylamine, ethylene glycol, L-ascorbic acid, etc

[128-133]

 Sequential reduction of metal precursors Following this strategy, the

second metal is deposited on the surface of pre-formed monometallic nanoparticles This method is quite suitable to prepare core-shell structured

BMNPs [15,119,124]