XRD X-RAY DIFFRACTION

LIST OF ABBREVIATIONS AND ACRONYMS

INTRODUCTION

Materials containing functional voids can be easily found in nature. The presence of pores in solids has sparked the curiosity of scientists who tried to reproduce, at first, and then optimize their characteristics. Indeed, porosity translates into a large specific surface area available in a small sample volume and accessible for interaction with guest species, thus being exploitable for applications.

Among the porous materials produced in laboratory and industry, an important class is represented by metal-organic frameworks (MOFs) which show crystalline structures consisting of metallic clusters or ions bound by organic ligands. The hybrid nature combined with the chemical tunability gives rise, through targeted synthetic methods, to frameworks with varied pore shapes and sizes and record surface areas. MOFs' extreme versatility has made them suitable for a large range of applications in several different domains. Their success has been possible through the synthesis of chemically and thermally stable frameworks, allowing their use in reactive environments and at relatively high temperature, up to several hundreds of degrees Celsius. However, the large extent of voids in the MOFs structure makes these systems prone to mechanical instabilities when submitted to an external stress. Knowing the key factors that most influence the mechanical properties of these materials is of fundamental importance and can be revealed submitting MOFs to high pressure, in the GPa range, by using diamond anvil cells (DACs). Furthermore, mechanical stability of MOFs is of paramount importance for industrial applications, where practical processing involving compression are commonly performed.

MOFs, as other porous systems, can be classified by pore size in microporous (pore diameter d < 2 nm), mesoporous (2 nm < d < 50 nm) and macroporous (d > 50 nm). Much attention has been devoted to mesoporous MOFs since their large structural voids allow new physical and chemical functionalities. Mesoporous MOFs are of particular interest for the encapsulation of guests as large molecules or nanoparticles at ambient conditions, opening the way for new composites combining the advantages of both parent materials. In particular, metallic nanoparticles have been successfully embedded in several MOFs boosting their performances for hydrogen storage, catalysis and gas sensing, among others.

Two prototypical mesoporous frameworks are the chromium-based MIL-101(Cr) and MIL-100(Cr) (Matériaux de l'Institut Lavoisier) synthesized for the first time by G. Férey and collaborators.

MIL-101(Cr), with its large pores, has ideal textural properties, thermal and chemical stability for nanoparticles confinement. Various metals have been inserted in its pores with different metal loadings and nanoparticle sizes. MIL-100(Cr) presents the same metallic cluster as MIL-101(Cr) but a different linker which give rise to a similar structure with smaller pores and porous volume.

The aim of this Ph.D. thesis is to assess the intrinsic mechanical stability of two mesoporous MOFs, MIL-101(Cr) and MIL-100(Cr), and understand how the presence of confined guests of different chemical nature within their pores could affect the high-pressure behavior of the host framework. Since high-pressure studies with DACs on mesoporous MOFs were lacking in the literature, we first had to establish the right experimental conditions and methodology needed to reach a coherent set of results. We then studied the high-pressure response of the pristine frameworks surrounded by different kinds of pressure transmitting media (typically used in DACs to ensure a uniform hydrostatic pressure on the sample), particularly focusing on how the MOFs mechanical stability is affected when the media can penetrate within the pores. This thesis is structured as follows:

• Chapter one presents a brief overview of porous materials, focusing on MOFs structures, properties and application. Particular attention is placed on the description of MIL-101(Cr) and MIL-100(Cr) structures and on high-pressure studies on microporous MOFs.

• Chapter two is dedicated to the description of the sample preparation and of the experimental techniques employed, from laboratory post-synthetic characterization to high-pressure measurements at synchrotron facilities. Principles of the diamond anvil cell used for the highpressure generation and elements of synchrotron radiation production, X-ray diffraction and infrared spectroscopy are given.

• Chapter three reports on the results obtained on the high-pressure behavior of pristine MIL-101(Cr) with different pressure transmitting media and using X-ray diffraction and infrared spectroscopy at synchrotron facilities.

• Chapter four focuses on the mechanical stability of Pd loaded MIL-101(Cr) composites highlighting how the host framework high-pressure response is affected by the presence of Pd nanoparticles confined within its pores with different metal loadings, i.e. 20 and 35 wt.%.

• Chapter five is centered on MIL-100(Cr) behavior under pressure and the results are compared to those obtained for MIL-101(Cr).

• Conclusions summarizes the main findings of this thesis and future work is proposed.

METAL-ORGANIC FRAMEWORKS

In the last two decades, Metal-Organic Frameworks (MOFs) have drawn the attention of the scientific community for their outstanding properties and their large range of applications. MOFs are assembled by connecting metallic clusters with organic linkers through coordination bonds to create crystalline networks with permanent porosity. 1 The number of synthesized compounds has exponentially grown, and more than 100000 MOF-like frameworks have been deposited in the Cambridge Structural Database up to now. 2 The aim of this chapter is giving an overview on the MOFs properties and applications with a special focus on the frameworks investigated in this thesis, i.e. MIL-101(Cr) and MIL-100(Cr).

Introduction to porous materials

The term "porous materials" identifies any solid containing void space, i.e. pores. The material size is typically larger than the voids and, according to the International Union of Pure and Applied Chemistry (IUPAC), the cavities or channels should be deeper than wider. 3 Several quantities are defined to characterize porous solids. Among the most important ones we find the internal surface, which indicates the accessible internal surface per unit of mass of the material, the porosity consisting in the volume fraction of the pores, and the total volume being the volume of the solid phase summed to the volume of the pores. For nanoporous materials, i.e. materials with pores in the nanometer range, the IUPAC proposes the following classification according to the pore size 3 :

• microporous solids: pore diameter below 2 nm;

• mesoporous solids: pore diameter between 2 and 50 nm;

• macroporous solids: pore diameter above 50 nm.

Cavities are classified by their availability to external fluids in four different pores geometries, as shown in Figure 1.1. Pores not connected to the surface are named closed pores (C). If a single connection to the surface is present than it is a blind pore (B) while through pores (T) present apertures on both sides of the solid. The volume that communicates between pores is called interconnected pore (I). Several different families of porous materials have been characterized and classified. Among them there are silicates which may have an amorphous nature, such as amorphous mesoporous silicas, or crystalline structure such as zeolites, inorganic microporous aluminosilicates with interconnected pores. 4 Activated carbons mainly consists of amorphous carbon in powder or granular form. 4 Clay minerals naturally occur in soil and sediments and should also be mentioned due to their wide range of pore size. 5,[START_REF] Diamond | Pore Size Distributions in Clays[END_REF] In the 1990s, a new class of porous coordination polymers, periodic arrays constituted of metal entities linked by ligands through coordination bonds [START_REF] Batten | Coordination Polymers, Metal-Organic Frameworks and the Need for Terminology Guidelines[END_REF] , emerged as really promising due to its hybrid character and crystalline structure: Metal-Organic Frameworks (MOFs). Combining the properties of organic and inorganic components, MOFs rapidly became among the most studied porous materials. [START_REF] Li | Metal-Organic Frameworks for Separations[END_REF] The aforementioned porous solid families are summarized on their average pore sizes in

.2 Examples of micro-, meso-, and macroporous materials, showing pore size domains. 4 In the following sections we will focus only on MOFs, introducing the principle of reticular chemistry, MOFs structures, along with their properties and applications.

MOFs structure, chemical composition and applications 1.2.1 Structure, components and topology

MOFs are porous materials composed of metallic centers, constituting the nodes of the crystal lattice, self-assembled by organic molecules. Their hybrid nature and the self-assembling process between the organic and inorganic components taking place in the synthesis [START_REF] Stock | Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites[END_REF] allow creating MOFs with a large variety of structures, pore sizes and topologies (see Figure 1.3). The organic ligands are typically bi-, tri-, or tetradentate components such as carboxylates, amines or phosphonates. Unlike zeolites whose inorganic structure withstands only few metals, MOFs can be synthesized from a wide range of metal species. Generally, inorganic clusters are formed by transition metal cations (Zn 2+ , Cu 2+ , Cr 3+ ...). Depending on the metal used and its degree of oxidation, the coordination number can vary from two to seven leading to the formation of metal complexes with very different geometries, i.e. linear, tetrahedral, octahedral or pyramidal with a square base.

Lanthanides are also used because they offer a higher coordination than transition metals which allows the formation of complexes where the lanthanide cation is surrounded by seven to ten donor atoms. [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF] The metallic clusters are usually called secondary building units (SBUs) and represent the starting point to synthesize MOFs using the reticular synthesis approach introduced for the first time by O.

Yaghi and collaborators. [START_REF] Yaghi | Reticular Synthesis and the Design of New Materials[END_REF] MOFs reticular synthesis. Using this method, the SBUs preserve their integrity during the construction process and their intrinsic geometric properties facilitate the network design. Moreover, it is possible to synthesize isoreticular MOFs, i.e. sharing identical topology, starting from the same SBU but changing the organic linker, as in the so-called IRMOF series. [START_REF] Eddaoudi | Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage[END_REF] Examples of some of the possible combinations of SBUs with different organic linkers are given in Figure 1. 5. This shows that it is possible to obtain from the same inorganic cluster and organic ligands of different nature, materials with very different crystal structures (pore size and geometry), which then are translated into different adsorption properties as in the case of MOF-5 and MOF-177. [START_REF] Rowsell | Hydrogen Sorption in Functionalized Metal-Organic Frameworks[END_REF][START_REF] Chae | A Route to High Surface Area, Porosity and Inclusion of Large Molecules in Crystals[END_REF] Due to the ever growing number of new frameworks synthesized, chemists started to describe the complex structures of MOFs in term of periodic networks. This strategy allows to schematize the structures with nets consisting of nodes, the metallic counterpart, and linkers, the organic component. [START_REF] Li | Topological Analysis of Metal-Organic Frameworks with Polytopic Linkers and/or Multiple Building Units and the Minimal Transitivity Principle[END_REF][START_REF] Kim | Topology Analysis of Metal-Organic Frameworks Based on Metal-Organic Polyhedra as Secondary or Tertiary Building Units[END_REF] The net is also known as topology and a network structure's topology is invariant to bending, stretching, and squeezing, but not to bond breaking. [START_REF] Öhrström | Let's Talk about MOFs-Topology and Terminology of Metal-Organic Frameworks and Why We Need Them[END_REF] The topology mainly depends on the connectivity and the symmetry of the metallic clusters and organic nodes. [START_REF] O'keeffe | Deconstructing the Crystal Structures of Metal-Organic Frameworks and Related Materials into Their Underlying Nets[END_REF] MOFs topologies are identified with three letter codes defined in the Reticular Chemistry Structure Resource, RCSR, which also lists all the most common topologies. [START_REF] Öhrström | Let's Talk about MOFs-Topology and Terminology of Metal-Organic Frameworks and Why We Need Them[END_REF] [START_REF] Öhrström | Let's Talk about MOFs-Topology and Terminology of Metal-Organic Frameworks and Why We Need Them[END_REF] New MOFs structures are usually named after the Institute or the University where they were first synthesized. Among the hundreds of MOFs, a few of them have become particularly widespread in the field, because of the ease of their synthesis, or commercial availability, stability, or applicability. This is the case of Cu-BTC, also known as HKUST-1 (Hong Kong University of Science and Technology), [START_REF] Chui | A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n[END_REF] UiO-66 named after the University of Oslo, [START_REF] Cavka | A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability[END_REF] NU-1000 from Northwestern University, [START_REF] Mondloch | Vapor-Phase Metalation by Atomic Layer Deposition in a Metal-Organic Framework[END_REF] and ZIF-8 (Zeolitic Imidazolate Framework). [START_REF] Park | Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks[END_REF] Another important series of MOFs are the frameworks synthesized by Férey's group and named MIL from the name of the institute (Matériaux de l'Institut Lavoisier) and a numbers identifying the specific structure. Among the others, two MOFs of the MIL family are particularly noteworthy for their mesoporous nature: MIL-100 [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF] and MIL-101 [START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] . In general, these frameworks can be synthesized using several metals such as Cr, Al and Fe. Cr-based MIL-100 and MIL-101 are the main systems studied in this thesis and their structures will be later described in Section 1.7.

Applications

Due to the very interesting properties and their tunability, MOFs are ideal candidates for a large variety of applications from gas adsorption [START_REF] Li | Porous Metal-Organic Frameworks for Gas Storage and Separation: Status and Challenges[END_REF] to drug delivery. [START_REF] Wang | Metal-Organic Frameworks for Stimuli-Responsive Drug Delivery[END_REF] The most relevant applications for this Ph.D. work are briefly described in the following sections.

Gas storage

The need of new energy sources to overcome the dependence to fossil fuels demands alternative routes for energy storage. In the last years, gases such has hydrogen (H2) [START_REF] Sankir | Hydrogen Storage Technologies[END_REF][START_REF] Hirscher | Materials for Hydrogen-Based Energy Storage -Past, Recent Progress and Future Outlook[END_REF] and methane (CH4) [START_REF] Celzard | Preparing a Suitable Material Designed for Methane Storage: A Comprehensive Report[END_REF] have been proposed as substitutive fuels. These gases are conventionally stored by liquefaction or pressurized vessels but both methods have limitations for safety and energy penalty associated with the compression or the liquefaction processes. [START_REF] Mokhatab | Handbook of Natural Gas Transmission and Processing -Principles and Practices[END_REF] An alternative gas storage technique is based on the gas adsorption in highly porous materials, which could provide higher volumetric and specific capacities at lower pressures and higher temperatures. Moreover, adsorption, and in particular physisorption which is based on Van der Waals interactions, does not involve chemical reactions between the gas and the material thus overcoming poor reversibility observed in other solid-state storage methods. Among all the porous materials studied for storage (i.e. carbons [START_REF] Zlotea | A Round Robin Characterisation of the Hydrogen Sorption Properties of a Carbon Based Material[END_REF] , zeolites and silica [START_REF] Menon | Porous Adsorbents for Vehicular Natural Gas Storage: A Review[END_REF] ), MOFs are one of the most promising owing to their ultrahigh porosity and chemical tunability. [START_REF] Van Der Voort | Introduction to Porous Materials[END_REF][START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF] The H2 uptake of several MOFs has been evaluated both experimentally [START_REF] Hirscher | Materials for Hydrogen-Based Energy Storage -Past, Recent Progress and Future Outlook[END_REF] and computationally [START_REF] Goldsmith | Theoretical Limits of Hydrogen Storage in Metal-Organic Frameworks: Opportunities and Trade-Offs[END_REF][START_REF] Ahmed | Predicting Hydrogen Storage in MOFs via Machine Learning[END_REF] and correlations were identified between the MOFs textural properties and the higher adsorption capacities. First, the H2 uptake is influenced by the micropore volume since H2 molecules condense at low-pressure in the micropores. Therefore, the optimal MOF material should present a high internal surface area but must have narrow pores which size closely matches the diameter of H2 to increase the van der Waals potential fields from the opposing pore walls. [START_REF] Van Der Voort | Introduction to Porous Materials[END_REF][START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF] In general, usable capacities are linearly related with pore volume, specific surface area and void fraction while inversely correlated to single crystal density and volumetric surface area. [START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF] Similar relationships between textural properties and storage capacity have been found for CH4, bearing in mind that the adsorption occurs at higher temperature than for H2 due to the specific thermodynamic properties of bulk methane. [START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF][START_REF] Lee | Can Metal-Organic Frameworks Attain New DOE Targets for On-Board Methane Storage by Increasing Methane Heat of Adsorption?[END_REF] This relationship between the MOFs structural parameters and their adsorption performance shows that new efforts are needed to tune the pore size and minimize the MOFs density. One of the solutions proposed to improve the volumetric capacity is the post-synthetic densification of MOFs powders into monoliths, with continuous and homogeneous morphology, or pellets which present distinct morphological regions due to the presence of different materials. [START_REF] Connolly | Shaping the Future of Fuel: Monolithic Metal-Organic Frameworks for High-Density Gas Storage[END_REF][START_REF] Samantaray | Volumetrics of Hydrogen Storage by Physical Adsorption[END_REF] This can be achieved through various processes, such as mechanical/hydraulic pressing, extrusion, granulation and some densified MOFs have shown improved volumetric adsorption capacities and separation performances. [START_REF] Samantaray | Volumetrics of Hydrogen Storage by Physical Adsorption[END_REF][START_REF] Blanita | Hydrogen Cryo-Adsorption by Hexagonal Prism Monoliths of MIL-101[END_REF][START_REF] Nandasiri | Separation, and Catalytic Properties of Densified Metal-Organic Frameworks[END_REF] Alternative strategies to increase the gas storage capacity are also based on the presence of exposed metal sites in the structure and framework flexibility. [START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF] The first can strengthen the attraction between the gas, especially CH4, and the framework while the second can result in hysteretic behavior which in turn enhances the usable capacity of the material, as demonstrate in two flexible MOFs for CH4 adsorption. 45

CO2 capture and separation

The fossil fuels combustion produces large amounts of CO2 which is then released into the atmosphere causing global warming. [START_REF] Sumida | Carbon Dioxide Capture in Metal-Organic Frameworks[END_REF] MOFs are considered good candidates for CO2 adsorption and separation in both post-combustion and pre-combustion capture. [START_REF] Sumida | Carbon Dioxide Capture in Metal-Organic Frameworks[END_REF][START_REF] Li | Recent Advances in Gas Storage and Separation Using Metal-Organic Frameworks[END_REF] Post-combustion capture involves CO2 separation over mainly N2-rich flue gas stream at low-pressure (partial pressure of CO2 around 0.15 bar). [START_REF] Glover | Gas Adsorption in Metal-Organic Frameworks -Fundamentals and Applications[END_REF] In the pre-combustion stage, the CO2 is instead captured from CO2/H2 stream at high pressure, typically 5-40 bar. [START_REF] Li | Recent Advances in Gas Storage and Separation Using Metal-Organic Frameworks[END_REF] Therefore, MOFs selectivity for CO2 over N2 or water vapor is of paramount importance for these applications, along with having high gravimetric and/or volumetric uptake of CO2. [START_REF] Van Der Voort | Introduction to Porous Materials[END_REF] Selective capture of CO2 over other molecules can be obtained through two different mechanisms. In size-based selectivity, the pore diameter may allow molecules only up to a given size to penetrates the framework. Molecular size is usually expressed using the kinetic diameter, which is defined as the size of a molecule in a gas and is related to the probability that the molecule will collide with others. [START_REF] Mehio | Quantum Mechanical Basis for Kinetic Diameters of Small Gaseous Molecules[END_REF] Since N2 presents a kinetic diameter (3.64 Å) larger than CO2 (3.3 Å), CO2/N2 separation based on molecule sizes has been achieved in microporous MOFs such as HKUST-1 and MOF-11(Mg). [START_REF] Li | Microporous Metal-Organic Frameworks for Gas Separation[END_REF] However, almost all the materials with high adsorption capacities for CO2 exhibit pore windows larger than the molecules size thus leading to the study of adsorptive selectivity based on the different interactions that occurs between the specific molecule and the framework. The different polarizability and quadrupole moment between CO2 and other molecules as N2 or H2 results in a higher affinity of the surface of the material for CO2. [START_REF] Van Der Voort | Introduction to Porous Materials[END_REF] As schematized in

Catalysis

MOFs have demonstrated potentiality for a vast set of catalytic reactions such as oxidation-, reduction-, acid-and base catalysis. [START_REF] Van Der Voort | Introduction to Porous Materials[END_REF] MOFs catalysts can be categorized in three classes depending on the type of active center: (1) those with active site on the framework itself, (2) those with encapsulated active species, and (3) those with active guest sites attached through post-synthetic modification. [START_REF] Ranocchiari | Catalysis by Metal-Organic Frameworks: Fundamentals and Opportunities[END_REF][START_REF] Huang | Multifunctional Metal-Organic Framework Catalysts: Synergistic Catalysis and Tandem Reactions[END_REF] Regarding the first class, the catalytic activity can be associated with the metallic cluster or the functional groups of the organic component. Activating the MOFs upon heating in vacuum, CUS sites are exposed and can act as Lewis-acid sites as has been proved, for example, for a copper-based MOF, namely Cu(5-methylisophthalate), active for air oxidation of CO and CO2. [START_REF] Zou | Probing the Lewis Acid Sites and CO Catalytic Oxidation Activity of the Porous Metal-Organic Polymer [Cu(5-Methylisophthalate)[END_REF] As for the linker active site, MOFs with basic nitrogen-containing ligands as building blocks have been proved being catalytic active. [START_REF] Zhu | Metal-Organic Frameworks for Heterogeneous Basic Catalysis[END_REF] However, the employment of pristine MOFs as catalysts is limited by their mechanical, thermal, and chemical stability and by the minimal presence of active sites. In order to satisfy realistic catalytic applications, the other two aforementioned classes are currently preferred and largely studied. [START_REF] Chen | Metal-Organic Framework Composites for Catalysis[END_REF] In the second category, the most widespread species incapsulated in MOFs are metallic nanoparticles (NPs) which act as active centers for catalytic reactions while the main MOF function is to provide a stable environment. [START_REF] Chen | Metal-Organic Framework Composites for Catalysis[END_REF] In particular, Pd NPs stabilized in porous frameworks are among the most used in hydrogenation reactions for their high activity. [START_REF] Chen | Boosting Catalysis of Pd Nanoparticles in MOFs by Pore Wall Engineering: The Roles of Electron Transfer and Adsorption Energy[END_REF]

Functionalization of MOFs

The reticular chemistry approach allows designing and synthesizing frameworks with specific topologies/properties and with very different structures. However, additional functionalities can be introduced to optimize MOFs for targeted applications. Chemical functionalization of MOFs can be mainly divided in three different categories: pre-synthetic, in situ and post-synthetic modification (PSM). Using pre-synthetic and in situ functionalization, functionalized linkers or other entities are incorporated into the framework during the synthesis. On the contrary, PSM does not deeply alter the basic structure, porosity and crystallinity. In this way, a wide range of isoreticular MOFs presenting tailored functionalities not accessible by direct synthesis can be prepared. PSM can be grouped according to the type of interaction arising between the added entity and the pristine framework. For example, the incorporation of guest molecules involves weak interactions, while the replacement of the ligand or metal exchange are classified as strong PSM. Figure 1.8 summarizes all the different categories of functionalization and some examples for each class are also given. [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF] A large number of PSM has been reported to date and providing a detailed description of all of them is beyond the scope of this section. Two functionalization processes more related to this thesis work will be described in the following. In particular, the next subsections will focus on weak interactions post-synthetic functionalization and more specifically the possibility to encapsulate guest entities, such as metallic nanoparticles and polymers, within the framework.

Nanoparticles incorporation

The high porosity and the crystalline structure of MOFs allow the incorporation of metal and metal oxide nanoparticles within the pores giving rise to new composite materials with enhanced properties. [START_REF] Falcaro | Application of Metal and Metal Oxide Nanoparticles@MOFs[END_REF][START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF] Ultrasmall well-dispersed nanoparticles (NPs), usually thermodynamically unstable due to the high surface energy, can be realized inside MOFs. In the past years, metallic NPs have been successfully embedded in several MOFs (Metal@MOF) boosting their performances for hydrogen storage [START_REF] Zlotea | Pd Nanoparticles Embedded into a Metal-Organic Framework: Synthesis, Structural Characteristics, and Hydrogen Sorption Properties[END_REF][START_REF] Malouche | Interactions of Hydrogen with Pd@MOF Composites[END_REF] , catalysis [START_REF] Yang | Metal-Organic Frameworks Meet Metal Nanoparticles: Synergistic Effect for Enhanced Catalysis[END_REF] and gas sensing 62 , among others. [START_REF] Falcaro | Application of Metal and Metal Oxide Nanoparticles@MOFs[END_REF] A broad range of synthetic methods, taking into account the specific characteristic of the chosen MOF and metal, have been developed to optimize the preparation of Metal@MOF composites.

In situ modification during the MOF synthesis can be used but PSM involving the loading of precursor molecules with successive activation under thermal or chemical conditions is usually preferred. [START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF] The idea behind the latter synthetic methods is schematically presented on the left side of Figure 1.9 and the three most common strategies are chemical vapor infiltration, solution impregnation and incipient wetness impregnation. Chemical vapor infiltration is based on loading the MOFs with volatile precursors in the gas phase by a sublimation process. Although this method allows high metal loadings in a single step, composites usually show NPs agglomeration as reported, for example, for Mg@SNU-90 presenting metal NPs average size of 60 nm. [START_REF] Lim | Magnesium Nanocrystals Embedded in a Metal-Organic Framework: Hybrid Hydrogen Storage with Synergistic Effect on Physi-and Chemisorption[END_REF] In the solution impregnation approach, the porous material is immersed in an aqueous solution containing a metal salt precursor. When the amount of impregnation solution corresponds to the MOF pore volume the method is called incipient wetness. Differently from the chemical vapor method, the precursor needs to be reduced in order to form NPs by introducing external chemical agents. [START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF][START_REF] Juan-Alcañiz | Metal-Organic Frameworks as Scaffolds for the Encapsulation of Active Species: State of the Art and Future Perspectives[END_REF] To prevent NPs aggregation and encapsulate the NPs inside the pores, a variation of the solution impregnation method involving two solvents has been conceived. This synthetic strategy, named "double solvent method", consists in soaking the MOF in an excess hydrophobic solvent and a hydrophilic one containing the precursor in a volume equal to, or slightly less, than the available pore volume. The double solvent method was successfully used the first time to synthesize Pt@MIL-101 composites with well-dispersed ultra-small NPs. [START_REF] Aijaz | Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of Metal-Organic Framework: A Double Solvents Approach[END_REF] Metal@MOFs are classified with respect of the nanoparticle's location and size in three main groups. The schematic representations of these families, named A, B and C, are reported on the right part of Figure 1.9. Composites belonging to class A have NPs with large size distribution exceeding the pore diameter and located on the external surface of the MOF crystal. Class B describes particles with large size distribution exceeding the size of the pores but inserted within the framework. In this case, the porosity of the host framework is obviously degraded by nanoparticles insertion. The last case is represented by class C in which the framework totally retains its crystallinity, and the average NPs size matches the pore size. However, this ideal result is particularly challenging to achieve and a combination of case B and C is usually obtained with the probability of forming particles with large size distribution, as in case A, with increasing the metal loading amount. [START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF] It is worth noting that structural characterization of NPs@MOFs materials by X-ray diffraction (XRD) is challenging due to the small NPs size. [START_REF] Wiktor | Transmission Electron Microscopy on Metal-Organic Frameworksa Review[END_REF] Transmission Electron Microscopy (TEM) is a powerful technique to reveal the morphology and the size distribution of the embedded NPs although the high degree of instability of MOFs towards the high energy electrons used in TEM can be a major obstacle. [START_REF] Wiktor | Transmission Electron Microscopy on Metal-Organic Frameworksa Review[END_REF] NPs confinement is usually confirmed by indirect observations, such as a NPs size distribution lower than the pore diameter and the decrease of the internal surface area after the insertion. [START_REF] Malouche | Hydrogen Absorption in 1 Nm Pd Clusters Confined in MIL-101(Cr)[END_REF][START_REF] Chen | Location Determination of Metal Nanoparticles Relative to a Metal-Organic Framework[END_REF] Very rarely, direct observations by electron tomography have been reported, as for Au@ZIF (ZIF-8 and ZIF-90) reported by Esken et al. [START_REF] Esken | Stabilization and Encapsulation of Cavity-Size Matching Gold Clusters inside Functionalized Zeolite Imidazolate Frameworks[END_REF] the Au NP nanoparticle distribution as well as the edges of the imaged crystal. [START_REF] Esken | Stabilization and Encapsulation of Cavity-Size Matching Gold Clusters inside Functionalized Zeolite Imidazolate Frameworks[END_REF] However, important artifacts are introduced by this technique (MOF damage by the electron beam and NPs agglomeration) which hinder a widespread application of electron tomography to directly demonstrate the effective confinement of NPs in the MOF structure. [START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF] Despite the large number of studies published about NPs@MOFs composites, this field still has enormous potential for emerging MOFs applications, such in plasmonic, electro-and thermochromic areas. [START_REF] Falcaro | Application of Metal and Metal Oxide Nanoparticles@MOFs[END_REF] Moreover, the mechanical stability of NPs@MOF composites has never been investigated before and little is known about the capacity of these materials to sustain the densification processes necessary to spread their application. The possible different mechanical response of the host framework to an external stimulus, such as pressure, after the NPs insertion could also provide a further evidence to verify if the NPs are effectively confined as depicted in case C.

Hybridization of MOFs and polymers

The combination of MOFs with other functional materials is also a successful strategy to form novel composites with advanced functions hard to obtain from individual components. [START_REF] Kitao | Hybridization of MOFs and Polymers[END_REF] This strategy is usually named "hybridization" since two or more materials with different chemical nature are mixed. [START_REF] Rejab | Historical Development of Hybrid Materials[END_REF] Thanks to their unique properties, polymers are currently employed in many different applications and modification of MOFs with polymers is an emerging field rapidly growing. [START_REF] Schmidt | Metal-Organic Frameworks in Polymer Science: Polymerization Catalysis, Polymerization Environment, and Hybrid Materials[END_REF] Synthetic approaches to obtain hybrid polymer/MOFs materials are numerous and varied. A schematic illustration of different methods is shown in Figure 1.11. Among post-synthetic modifications, the polymerization of monomers introduced in the pores is a simple and convenient technique (see (a) in Figure 1.11). Indeed, small molecules such as monomers are easy to spread into the framework and allowing the synthesis of structure-controlled polymers. [START_REF] Kitao | Hybridization of MOFs and Polymers[END_REF] Polymer chains already formed can also be inserted using other approaches, including melt processing and solution-mediated incorporation. [START_REF] Park | Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo-Polymerization Reaction[END_REF] Pre-synthetic or in situ modification involving the growth of MOFs in the presence of polymers are also widely performed. [START_REF] Park | Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo-Polymerization Reaction[END_REF] Finally, the organic counterpart of MOFs is a good candidate to be polymerized giving birth to polymer chains within the framework, as schematized in panel (b) of

The stability of MOFs

The MOFs success in application is mainly due to their porous nature and high chemical versatility. However, in order to fully benefit of their properties and make them exploitable for applications, MOFs must be stable, namely the structure must resist to degradation once exposed to operating conditions. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF] Three main class of stabilities are usually considered: chemical, thermal and mechanical stability. Chemical stability is one of the most basic requirements that a material should satisfy to resist in aqueous media or harsh chemical conditions, potentially encountered in industrial processes as well. For practical applications high-temperatures are also usually necessary thus claiming for high thermal stability. Finally, mechanical stability is of paramount importance for post-synthetic processing through mechanical compression. [START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF] In the last years, great attention has been paid to synthesize frameworks chemically and thermally stable and different experimental approaches have been used to assess these stabilities. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF][START_REF] Ding | Improving MOF Stability: Approaches and Applications[END_REF] On the contrary, mechanical stability has been determined only for few microporous MOFs and the systematic investigations of many new synthesized frameworks is currently lacking. [START_REF] Ding | Improving MOF Stability: Approaches and Applications[END_REF] This Ph.D. work deals with this void and aims to evaluate the stability at high pressure of prototypical mesoporous MOFs for the first time.

In the next sections, the three categories of stabilities and their importance in MOFs will be presented and summarized. Moreover, strategies allowing the synthesis of robust MOFs will be also briefly discussed.

Chemical stability

Chemical stability is the ability of a given material to withstand chemical treatment without any significant change in its structure. The most used method to evaluate if a framework is chemically stable consists in comparing the XRD patterns before and after soaking the material in an acidic, basic or neutral solution. An additional test is the estimation of the MOF surface area after the treatment by measuring the nitrogen adsorption isotherms. Indeed, loss of internal surface area concomitantly with a partial modification in the XRD pattern is usually indicative of partial pore collapse and amorphization. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF] Using these methods allowed assessing the pH-dependent ranges in which MOFs are chemically stable, as summarized in Figure 1.12 for some MOFs. 76 The principal factor determining the chemical stability of MOFs under many operating conditions is the metal-ligand bond strength which mainly depends on the charge density of the metal cation. [START_REF] Yuan | Stable Metal-Organic Frameworks: Design, Synthesis, and Applications[END_REF] The interaction between the organic and the inorganic moieties can be seen as a Lewis acidbase interaction where the carboxylate-based linkers are considered as hard bases and the metal ions charges and ionic radii determine the acidity. In particular, strong coordination bonds originate from high charges and small ionic radii of the metal. [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF] For this reason, high-valence metal based MOFs are among the most chemically stable including the Al 3+ , Fe 3+ , and Cr 3+ based MIL series synthesized for the first time by Férey and coworkers. [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF][START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] Particular attention should be paid to hydrolytic stability, i.e. structural integrity in water.

Since water vapor is a major component of industrial streams, the effect of water on MOFs has been carefully evaluated in the past. [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF][START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF][START_REF] Burtch | Water Stability and Adsorption in Metal-Organic Frameworks[END_REF][START_REF] Leus | Systematic Study of the Chemical and Hydrothermal Stability of Selected "Stable" Metal Organic Frameworks[END_REF][START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF] Two main mechanisms have been identified as the origin of the MOFs degradation in the presence of water: hydrolysis and linker replacement. [START_REF] Low | Virtual High Throughput Screening Confirmed Experimentally: Porous Coordination Polymer Hydration[END_REF] By comparing all the framework studied, the highest stability in water is provided by the presence of anionic, nitrogencontaining linkers as in the case of zeolitic imidazolate frameworks (ZIFs). [START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF] Several strategies have been proposed to improve MOFs stability and in particular to strengthen the metal-ligand interaction. Among the others, using highly complexing ligands or highly charged cations is one of the most successful method but also the employment during the synthesis of modulators, i.e. an organic molecules bearing the same binding group as the linker but having a lower topicity and different acid dissociation constant [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF] , such as monocarboxylic acids is very efficient. 75,82

Thermal stability

The thermal stability of a material can be defined as its ability to retain its chemical properties and physical structure upon heating to a relatively high temperature. Thermogravimetric analysis (TGA) and differential-scanning-calorimetry are the main experimental techniques used to record mass loss or a thermal effect (exothermic or endothermic) during increasing the temperature which indicates the decomposition or major structural changes. For MOFs, TGA curves usually show distinct steps of mass loss at different temperatures, as reported for example in Figure 1.13 for MIL-100(Cr). [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF] A first decrease is observed below 100-150 °C and is assigned to the evacuation of guest molecules, such as solvent and/or water. The first mass loss is typically followed by a plateau where the MOF is stable until the decomposition temperature (Td) where a second mass loss event is recorded corresponding to the framework degradation. However, some processes happening upon heating, such as phase transitions, melting or pore collapse, can dramatically affect the physical and chemical properties of the material without causing any mass loss. For this reason, complementary technique should be used to evaluate the thermal stability of MOFs. For example, XRD patterns can be measured during or after the thermal treatment to assess if the structure is retained or not. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Healy | The Thermal Stability of Metal-Organic Frameworks[END_REF] Figure 1.13 Thermogravimetric analysis of MIL-100(Cr) adapted from ref [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF] .

As schematized in Figure 1.14, experimental Td values for some of the most known MOFs families ranges from around 200 °C for Hybrid Ultra Microporous materials (HUMs) up to 600 °C for UL-MOFs (University of Limerick MOFs). In MOFs, thermal degradation is usually the result of the linker combustion subsequently to the node-linker bond breakage due to, among others, node-cluster dehydration or linker dehydrogenation or graphitization. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF] Other phenomena that could take place upon heating but preserve the chemical composition are amorphization and melting. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF] Interestingly, various frameworks of the ZIF family undergo melting transition without decomposing and this exotic thermal behavior emerges from the combination of low node stability with high ligand stability. [START_REF] Healy | The Thermal Stability of Metal-Organic Frameworks[END_REF][START_REF] Frentzel-Beyme | Meltable Mixed-Linker Zeolitic Imidazolate Frameworks and Their Microporous Glasses: From Melting Point Engineering to Selective Hydrocarbon Sorption[END_REF] Figure 1.14 Observed thermal decomposition temperature values for a range of MOF families. [START_REF] Healy | The Thermal Stability of Metal-Organic Frameworks[END_REF] The thermal stability of a given frameworks depends on multiple factors and finding trends and correlation between MOFs properties and their Td is not straightforward. However, some rules of thumbs were proposed in the past years to improve the stability upon heating of MOFs. Regarding the inorganic counterpart, metal ions at their most stable oxidation state and inorganic chains or sheets instead of metal nodes should be preferred. Moreover, short aromatic linkers or phosphonate and sulfonate-based result to ensure high stability. [START_REF] Garcia | Metal-Organic Frameworks: Applications in Separations and Catalysis[END_REF][START_REF] Healy | The Thermal Stability of Metal-Organic Frameworks[END_REF]

Mechanical stability

Systems are mechanically stable if they can withstand external forces. In the peculiar case of porous materials, also the ability of the framework to retain porosity after the removal of the solvent residual guest molecules during the activation process should be taken into account. [START_REF] Yaghi | Introduction to Reticular Chemistry -Metal-Organic Frameworks and Covalent Organic Frameworks[END_REF] Indeed, some mesoporous MOFs collapse after the activation presenting internal surface area and porosity dramatically lower than that expected from simulation of single-crystal data. This is due to significant capillary forces creating high strain when the framework is activated. [START_REF] Mondloch | Activation of Metal-Organic Framework Materials[END_REF] Using low surface tension solvents including CH2Cl2, n-hexane and liquid CO2 is a successful strategy to activate MOFs without losing the porosity. [START_REF] Yuan | Stable Metal-Organic Frameworks: Design, Synthesis, and Applications[END_REF] Although this is one of the main issue for porous materials stability, it is also important to evaluate the ability of the activated sample to retain its structure when external forces are applied. As already mentioned, MOFs need to be densified in the form of monoliths and/or pellets for targeted applications, and determining whether the structure or textural properties are preserved during the compression or not is crucial for their large-scale commercialization.

The mechanical properties of a given framework can be assessed by evaluating its response to compression, shear stress or tension. [START_REF] Burtch | Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications[END_REF] In Figure 1.15, common mechanical stresses and the related properties are shown. The stiffness under unidirectional loading is measured by the Young's modulus, E, while the bulk modulus, K, evaluates the isotropic volumetric elasticity under hydrostatic pressure.

The shear modulus, G, also called rigidity modulus, measures the rigidity of a material when subjected to opposing shear forces and the Poisson's ratio is defined as the ratio between the lateral and axial strain upon axial loading. [START_REF] Burtch | Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications[END_REF] Excluding the bulk modulus, the aforementioned properties are anisotropic.

Moreover, it is possible to establish the maximum load that the system can hold before changing its phase or structure. The extreme case of pressure-induced structural modification induced is the framework amorphization, usually accompanied by partial or total pore collapse, thus marking the upper limit of the mechanical stability. 87 The most common technique to assess the amorphization pressure of MOFs is isotropic compression, which is also the most widespread method to evaluate the bulk modulus of materials. In particular, performing XRD while compressing the sample allows tracking the evolution of the crystalline structure, as done in this Ph.D thesis (see Chapters 3, 4 and 5), and identify when the MOF undergoes amorphization. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] Computational methods have also been fruitfully used to calculate the elasticity parameters and the pressure range where the MOFs structure is mechanically stable. [START_REF] Moghadam | Structure-Mechanical Stability Relations of Metal-Organic Frameworks via Machine Learning[END_REF] The behavior at high-pressure of some of the most famous microporous MOFs has been studied and unusual effect were observed on compression along with pressure-induced amorphization.

Due to the importance of these previous studies for the present work, a detailed overview about MOFs at high-pressure (GPa scale) will be given in the dedicated section later in this chapter together with a brief overview of high-pressure studies on other porous materials.

High pressure studies on porous materials

The compression of materials having cavities in their structure is particularly fascinating from the fundamental point of view. Indeed, the presence of pores yields to a wide range of physical responses to pressure, some of them unusual and unexpected. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF][START_REF] Redfern | Mechanical Properties of Metal-Organic Frameworks[END_REF][START_REF] Tan | Mechanical Properties of Hybrid Inorganic-Organic Framework Materials: Establishing Fundamental Structure-Property Relationships[END_REF] Among all the porous material families, zeolites have been deeply investigated at high pressure in the past years and comparing the results on different structures, some general conclusion on their behavior under pressure have been drawn. [START_REF] Gatta | Zeolites at High Pressure: A Review[END_REF] First, this class of open frameworks silicates present a wide range of compressibility with bulk modulus values between 15 and 70 GPa. It has been shown that compressibility and microporosity are not strictly correlated, with a higher degree of porosity not necessarily resulting in higher compressibility. In fact, some microporous zeolites present higher bulk moduli than nonporous, rockforming minerals. [START_REF] Gatta | Zeolites at High Pressure: A Review[END_REF][START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF] In zeolites, being the silicon atoms tetrahedrally coordinated with each other through shared oxygen atoms, the structural compression is mainly governed by the tilting of rigid tetrahedra, as for example reported for the 1D siliceous zeolites TON, schematically represented in Figure 1.16. [START_REF] Gatta | Zeolites at High Pressure: A Review[END_REF][START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON[END_REF] The last remarkable aspect on zeolites is that the pressure-induced deformation mechanisms is generally determined by the framework's topology more than the metallic ions position. 92 Although zeolites present many common points with MOFs, the insights into the highpressure response of zeolites may not be directly extended to MOFs due to the specific chemical nature of these two materials families. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF][START_REF] Mckellar | Structural Studies of Metal-Organic Frameworks under High Pressure[END_REF] Moreover; it is worth remembering that zeolites are mainly microporous systems (see Figure 1.2) while an ever growing number of mesoporous MOFs are synthesized.

Regarding the high-pressure response of materials with larger pores, a few studies have been performed mainly on mesoporous silicas. These class of materials present a honeycomb-like structure made by amorphous silica, with cylindrical pores in the mesoporous range, lacking of ordered crystalline structure at the atomic scale. [START_REF] Bruce | Porous Materials[END_REF] For this reason, the mechanical properties have been assessed

exploiting small angle X-ray diffraction or small angle X-ray scattering. [START_REF] Wu | High-Pressure Stability in Ordered Mesoporous Silicas: Rigidity and Elasticity through Nanometer Scale Arches[END_REF][START_REF] Kizzire | Studies of the Mechanical and Extreme Hydrothermal Properties of Periodic Mesoporous Silica and Aluminosilica Materials[END_REF][START_REF] Mayanovic | Mechanical and Hydrothermal Stability of Mesoporous Materials at Extreme Conditions[END_REF] The bulk moduli have thus been estimated from the low-pressure linear response region performing a linear fit on the relative volume change [START_REF] Wu | High-Pressure Stability in Ordered Mesoporous Silicas: Rigidity and Elasticity through Nanometer Scale Arches[END_REF][START_REF] Kizzire | Studies of the Mechanical and Extreme Hydrothermal Properties of Periodic Mesoporous Silica and Aluminosilica Materials[END_REF] These studies demonstrate that mesoscopic order in mesoporous silicas is retained even up to 12 GPa in specific cases [START_REF] Wu | High-Pressure Stability in Ordered Mesoporous Silicas: Rigidity and Elasticity through Nanometer Scale Arches[END_REF] showing a remarkable mechanical stability although dependent on the pore shape and size. [START_REF] Mayanovic | Mechanical and Hydrothermal Stability of Mesoporous Materials at Extreme Conditions[END_REF] However, these results are not straightforwardly extendable to other mesoporous crystalline systems such as MOFs. For this reason, a detailed investigation on different MOFs under compression is needed in order to identify common behaviors and draw some generalizations on this other class of materials.

Since a detailed review of previous studies on inorganic or amorphous porous materials is beyond the scope of this thesis, in the next section, the state of the art of high-pressure studies on MOFs, especially microporous, will be reported as well as the importance of assessing their mechanical properties for practical applications.

Microporous MOFs: state of the art

Besides the fundamental importance of understanding the behavior of porous systems such as MOFs under compression, evaluating how these materials behave when compressed is important for industrial applications as well. Indeed, common synthetic techniques provide MOFs as powders with low density, technically unsuitable for applications. Increasing the material's density and consequently its mechanical strength, has thus become an important research direction. [START_REF] Connolly | Shaping the Future of Fuel: Monolithic Metal-Organic Frameworks for High-Density Gas Storage[END_REF] The densification of MOFs into the form of pellets or monoliths has been achieved through various processes, such as mechanical/hydraulic pressing, extrusion, granulation and some densified MOFs have shown improved volumetric adsorption capacities and separation performances. [START_REF] Samantaray | Volumetrics of Hydrogen Storage by Physical Adsorption[END_REF][100][101][102][103] MOFs monoliths are also easier to handle and present high mass transfer rates, 104,105 which is particularly beneficial for gas sorption applications and catalytic reactions. The high-pressure behavior of MOFs is commonly unveiled by XRD experiments but also other techniques, such as neutron diffraction or vibrational spectroscopies can provide important insights. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] From the experimental point of view, studying the high-pressure behavior of MOFs needs particular care. Indeed, MOFs are soft materials and samples should be properly degassed in order to exclude a possible effect of adsorbed molecules on the mechanical response of the framework to compression. Unfortunately, in the literature, it is rarely specified whether the degassing procedure has been performed or not before high-pressure experiments, often leading to incoherent results. Due to their complex structure, computational simulations have been also carried out in many cases to establish relations between the structure and the mechanical properties. [START_REF] Moghadam | Structure-Mechanical Stability Relations of Metal-Organic Frameworks via Machine Learning[END_REF][START_REF] Redfern | Mechanical Properties of Metal-Organic Frameworks[END_REF] However, density functional theory (DFT) calculations for systems presenting unit cells with thousands of atoms, such as MOFs, are highly computational and time demanding. Therefore, experimental studies often turn out to be the only investigation possible, especially for mesoporous MOFs.

Before this thesis work, only the mechanical properties of a limited range of microporous MOFs have been assessed. In the following, a brief overview about the high-pressure response of these systems will be given. First, the compression mechanisms employed, both isotropic and anisotropic, will be summarized. Later, different types of phase transitions occurring on compression will be discussed, including displacive, reconstructive transitions, and amorphization. Finally, guest-inclusion phenomena will be presented focusing on the role of the pressure transmitting media used during the various experiments.

Isotropic and anisotropic compression on MOFs

When a material is subjected to an isotropic compression, as already mentioned in section 1.4.3, its resistance to pressure is quantified by the bulk modulus K0 defined as

𝐾 0 = -𝑉 ( 𝜕𝑃 𝜕𝑉 ) 𝑇 (1.1)
where P is the pressure, V is the volume and T the temperature of the material. This quantity can be extracted from the pressure dependence of the volume through fitting the data using an equation of state (EoS). The bulk modulus is inversely related to the compressibility by the relation 106

𝛽 = - 1 𝑉 ( 𝜕𝑉 𝜕𝑃 ) 𝑇 . (1.2)
Several EoS have been derived for solid materials, but no EoS has been specially derived for porous materials up to date. Therefore, the volume-pressure relation of MOFs is usually described employing an isothermal Birch-Murnaghan EoS, originally formulated for minerals but successfully used in previous studies on porous systems. This EoS is derived by the Taylor expansion of the Helmholtz free energy, in the approximation of infinitesimal strain. 106 Considering the expansion at the second order, the Birch-Murnaghan equation presents the following formulation 107,108 :

𝑃(𝑉) = 3𝐾 0 2 [( 𝑉 0 𝑉 ) 7 3 -( 𝑉 0 𝑉 ) 5 3
]

(1.3)
while taking into account the third order term of the Helmholtz free energy expansion, the EoS becomes

𝑃(𝑉) = 3𝐾 0 2 [( 𝑉 0 𝑉 ) 7 3 -( 𝑉 0 𝑉 ) 5 3 ] {1 + 3 4 (𝐾 𝑝 -4) [( 𝑉 0 𝑉 ) 2 3 -1]} (1.4)
with V0 the volume at ambient pressure and 𝐾 𝑝 the derivative of the bulk modulus

𝐾 𝑝 = ( 𝜕𝐾 0 𝜕𝑃 ) 𝑇 . (1.5)
In absence of guest molecules, experimental studies on MOFs at high-pressure reveal relatively low values for K0 ranging from 4 to around 30 GPa. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF]109 A complete and systematic highpressure study of two Zr-MOFs family, i.e. UiO-type (fcu topology) and NU-900 series (scu topology), has been performed by the group of O. Farha. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF] As schematized in Figure 1.18, they found that the pressure response mainly depends on the distortion of the metal-linker-metal entities in this specific MOF class. In particular, sample with "predistorted" ligands present a lower bulk modulus than the undistorted analogues. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF] Another factor that influences the compressibility and hence the mechanical stability of porous frameworks is the functionalization of the linker. This has been demonstrated for MIL-53 by P. Yot and collaborators. 110 They demonstrated that the bulk modulus changes in samples with Cl, CH3 or NO2 functionalized linkers. 110 The enhancement of the bulk modulus by post-synthetic modification of the linker has also been recently reported for another MOF, namely a 4,8-connected Zr-based framework, NU-901. 111 with distortions in their structures at ambient pressure. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF] MOFs compressibility is also affected by the presence of defect sites, i.e. locations where a node or a linker is missing from the ideal structure. This has been proved both computationally and experimentally for UiO-66. Coudert and collaborators demonstrated that although defects increase the porosity and internal surface area of this MOF, as shown in panel (a) of Figure 1.19, missing linkers or nodes lower the bulk modulus. 112 The experimental evidence corroborating the influence of defects on the compressibility of UiO-66 was provided by Dissegna et al. 113 Three defective samples were prepared and the pressure dependence of the volume was probed confining the sample together with silicone oil into a sealed plastic capillary, then immersed in a water-filled cell which provides finer pressure control at pressures lower than 0.2 GPa. The slope of the unit cell volume as function of pressure, quantified by the bulk modulus (see Equation 1.1), clearly depends on the defects concentration, with lower bulk modulus for more defected samples (see Figure 1.19(b)). 113 As mentioned above, computational studies are of paramount importance to assess relations between the mechanical properties and MOFs structures. Moghadam and collaborators explored the mechanical stability of more than 3000 frameworks with 42 different topologies. [START_REF] Moghadam | Structure-Mechanical Stability Relations of Metal-Organic Frameworks via Machine Learning[END_REF] Their results demonstrate, at least theoretically, that the bulk modulus is inversely correlated with the pore diameter, and that this correlation is affected by the framework topology. This relation is reported in Figure 1.20

where different colors identify different topologies. In other cases, the compressed system can display different geometrical changes on different directions resulting in anisotropic compression. This behavior is mainly observe in non-cubic MOFs, especially ones with wine-rack topology which exhibit negative linear compressibility and even rarer negative area compressibility, i.e. expansion of two directions under pressure. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] In particular, MIL-53 has been the subject of several studies for its flexible behavior and a transition between a large and narrow form can be induced by mechanical pressure but also by other external stimuli such as removal/introduction of guest molecules, as shown in Figure 1.20 for the case of water. 118,119 In the gas adsorption case, this phenomenon has been defined as "breathing effect". 120

Displacive and reconstructive phase transitions

Usually, the changes in bonds length and angles on compression cause the lowering of the crystal symmetry through a phase transition. In the so-called displacive phase transitions, the change of the symmetry is only caused by small collective displacements of individual atoms characterized by a phonon mode with a frequency that falls to zero at the phase transition. 121 When, instead, the chemical bonds are destroyed and reformed a reconstructive phase transition takes place. A peculiar phase transition observed in selected porous frameworks is the pressure-induced transition between a large and narrow form. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF]122,123 In particular, MIL-53 has been the subject of several studies for its flexible behavior and it has been shown that this transformation can be induced by mechanical pressure but also by other external stimuli such as removal/introduction of guest molecules, as shown in Figure 1.22 for the case of water. 118,119 In the gas adsorption case, this phenomenon has been defined as "breathing effect". 120

Pressure-induced amorphization

The extreme case of symmetry lowering is the transition to an amorphous form, i.e. form where the molecules or atoms are not arranged in periodic arrays and long-range order is lacking. As already discussed in section 1.4.3, the presence of large voids in the structure makes MOFs prone to amorphization when an external pressure is applied. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF][START_REF] Redfern | Mechanical Properties of Metal-Organic Frameworks[END_REF][START_REF] Mckellar | Structural Studies of Metal-Organic Frameworks under High Pressure[END_REF] The amorphization is usually irreversible, although isolated cases of recovered crystallinity on pressure release have been published. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] As one of the prototypical zeolitic imidazolate frameworks, ZIF-8 was one of the first MOFs studied at high-pressure and its amorphization has been well characterized in several studies. [125][126][127] Chapman et al reported evidences of the ZIF-8 pressure induced amorphization exploiting powder XRD. 128

Guest inclusion

Usually, in high-pressure experiments the sample needs to be surrounded with a suitable pressure transmitting medium (PTM) in order to ensure a homogenous pressure distribution thus providing hydrostatic conditions. Previous high-pressure studies revealed the medium can have a strong impact on the MOF response under pressure, since small molecules can penetrate inside the pores. [START_REF] Tan | Mechanical Properties of Hybrid Inorganic-Organic Framework Materials: Establishing Fundamental Structure-Property Relationships[END_REF] PTM are thus differentiated in penetrating and non-penetrating ones. The use of largemolecules or polymeric fluids, such as silicone oil or Fluorinert-70 should, in principle, prevent the insertion inside the pores, at least in microporous frameworks. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] The previous sections described highpressure phenomena of "empty-pore" frameworks, without taking into account the eventual presence of guest molecules from the PTM or, e.g., water absorbed from the atmosphere and not removed with an appropriate degassing procedure.

A clear example of PTM penetration within the pores of a microporous MOFs has been reported by Chapman and collaborators on HKUST-1 (Cu-btc) 125 where its compressibility was studied with Fluorinert-70, iso-propanol and a mixture of methanol and ethanol (MEW) as PTM. In the case of iso-propanol and MEW, two regimes of near-linear compressibility are observed in the low-pressure and high-pressure ranges (see Figure 1.25) with a corresponding bulk modulus of 118 GPa and 30 GPa, respectively. This phenomenon, explained as the PTM being inserted by pressure in the low-pressure regime, is now commonly indicated in the literature as "hyperfilling". After that the MOF is hyperfilled, the compressibility slope changes meaning that the MOF and PTM are now compressed together. On the contrary, when Fluorinert-70 is used, the framework volume is monotonically reduced and the amorphization is observed around 2 GPa, i.e. at a lower pressure than when hyperfilling occurs. 125 The penetration of fluid PTM within the pores at high pressure was also reported in MIL-68(In) exploiting in situ IR spectroscopy. 129 The authors conclude that Nujol, a mineral oil formed by alkanes mixture and used as PTM, is included into the framework since the pressure-induced modification observed on the IR spectrum are reversible at odd with what observed without any PTM.

The study at high-pressure of guest-included species in MOFs can be particularly meaningful to localize the adsorption sites and it could provide an insight on the adsorption properties of the porous structures. [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF] A clear example is given by a high-pressure single crystal diffraction and computational study performed by Hobday et al on ZIF-8 with several penetrating gases, i.e. O2, N2, Ar and CH4.

With Ar, six different absorption sites fully occupied and ordered were located at 0.75 GPa (see Figure 1.26) while with O2, N2 the same sites present positional disorder. 130 It is worth noting that the linkers are of different topocity: H2BDC is ditopic, i.e. has two sites at which another compound can form a complex, and H3BTC is tritopic. [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF][START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] In both frameworks, the ST are formed by joining trinuclear Cr3OL3(-COO)6 (where L is the solvent) SBUs with trigonal tritopic BTC or linear ditopic BDC linkers, respectively. The difference lies in the position of the linker in the ST:

in MIL-101(Cr) the linkers represent the ST edges while in MIL-100(Cr) the tritopic BTC linker is located on the ST faces. 11 The Cr octahedra present in both systems have a negative default charge per trimer which is compensated by an anion such as F -, Cl -or a hydroxyl group, depending on the synthetic method. The other two Cr atoms are linked to terminal water molecules which can be removed by degassing the sample under dynamic vacuum at a temperature above 423 K leading to the formation of two coordinative unsaturated sites (CUS). 139,140 The presence of CUS translates in high adsorption capacities, in particular for CO2 and CH4 uptakes, as revealed by adsorption experiments. 140 MIL-101(Cr) and MIL-100(Cr) show excellent hydrothermal and chemical stability in acid and basic environments. [START_REF] Ding | Improving MOF Stability: Approaches and Applications[END_REF][START_REF] Leus | Systematic Study of the Chemical and Hydrothermal Stability of Selected "Stable" Metal Organic Frameworks[END_REF] Moreover, the large pores result in really high internal surface area:

around 3500 cm 3 g -1 for MIL-101(Cr) and 2000 cm 3 g -1 for MIL-100(Cr). [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF][START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] The combination of their chemical and textural properties makes MIL-100(Cr) and MIL-101(Cr) ideal mesoporous MOFs for many applications. 140-143

MATERIALS AND METHODS

In this chapter, an overview of the experimental methods employed in the present work is given. The first section is dedicated to synthetic procedures and sample characterization. Basic theoretical principles for the experimental techniques here employed and a brief description of the experimental setups are given. Particular attention will be paid to diamond anvil cells, which have been exploited to generate high pressure in all the measurements presented. The Infrared and X-ray diffraction synchrotron beamlines where the experiment have been carried out will be also briefly described.

Materials preparations and characterization

The pristine samples used in this thesis work were prepared by the group of Dr. Gabriela

Synthesis

MIL-101(Cr)

MIL-101(Cr), with chemical formula (Cr3O)Clx(OH)1-x(H2O)2[O2C-C6H4-CO2]3(H2O)n, is prepared following a green synthetic path. The reaction mixture, obtained from terephthalic acid (98 wt.%) and CrCl3•6H2O (96 wt.%) and water, in ratio 1:1:500, is mixed with trifluoroethanol (TFE) pot and is placed in a stainless-steel autoclave. The prepared compound is then heated in an electric oven at a temperature of 463 K for 16 hours, with a heating ramp of 6 hours. After cooling, the resulting solid (green) is separated by filtration and dried at room temperature. For activation, i.e. removal of the residual solvent molecules within the pores, the as-synthesized MIL-101(Cr) is heated in ethanol under reflux for 72 hours. The activated sample is then filtered and dried at 423 K. It is worth noting that activated samples can easily absorb water molecules from the atmosphere which can be removed by degassing the powder by heating under vacuum. In particular, samples used in this work have been degassed before any experiment at 493 K for 18 hours under secondary vacuum (below 10 -5 mbar) and successively handled avoiding air exposure.

MIL-100(Cr

Nanoparticles confinement

The synthetic method used to confine palladium nanoparticles (NPs) within the MIL-101(Cr) pores is the Double Solvent Method (DSM), which is considered the most effective method to avoid NPs aggregation on the external surface of the framework. [START_REF] Aijaz | Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of Metal-Organic Framework: A Double Solvents Approach[END_REF] As already described in section 1.3.1, this method involves the combination of a hydrophilic solvent (water) and a hydrophobic solvent (pentane).

The first contains a volume of the metal precursor equal or slightly less than the pore volume of the adsorbent while the latter facilitates the impregnation process minimizing the deposition of the metal precursor on the outer surface of the absorbent.

First, the pristine sample is degassed as described in the previous paragraph. This procedure is necessary to prevent the presence of adsorbed water molecules. An amount of 150 mg of degassed sample is suspended in 30 mL of pentane, sonicated for 20 minutes, and then agitated via magnetic stirring for 30 minutes. After, 0.2 mL of an aqueous solution containing H2PdCl4 (1.56x10 -3 mol/mL of Pd for 20 wt.% loading) is added. The solution volume is calculated considering the total pore volume of MIL-101(Cr), i.e. 1.72 cm 3 /g. The mixture is then stirred at 750 rpm for 4 hours at room temperature in a beaker covered by parafilm. After this time interval, the magnetic stirring is lowered to 250 rpm and the parafilm is removed to have a complete evaporation of the solvents. The recovered solid is dried at 343 K in an autoclave overnight. Finally, the powder is reduced under Ar/H2 flow (0.5 L/min) at 573 K for 15 minutes. 144 This procedure limits the incorporation of palladium in the framework to 20 wt.% since, for a higher concentration of the impregnation solution, PdCl2 starts to precipitate. However, composites containing 20 wt.% of Pd present a high percentage of pore volume still available. To overcome this limitation, loadings up to 35 wt.% have been achieved performing a second impregnation on the powder loaded at 20 wt.%, properly degassed, before proceeding with the reduction. It is important to stress that the synthesis of all doped samples has been performed using the same batch of pristine MIL-101(Cr). The composites obtained were named x-Pd@MIL-101 where x = 20, 35 wt.% indicates the percentage of metal over the total sample mass.

Characterization techniques

The physicochemical properties of all samples, pristine and loaded with metal nanoparticles, are evaluated at ambient conditions exploiting laboratory powder X-ray diffraction, transmission electron microscopy and N2 physisorption. In the following sections, a brief description of the experimental techniques and the corresponding apparatus is given.

Powder X-Ray Diffraction

Crystalline materials present ordered structures formed by atoms arranged in a threedimensional periodic array. The structure geometry and symmetry can be described defining a primitive unit cell as the smallest repeating unit of the crystal lattice which is characterized by six parameters: three edges (a, b and c) and three angles (α, β and γ). 145 Depending on the unit cell, several set of atomic parallel planes characterizing the crystalline structure can be identified. Let's define an interplanar distance d that in a solid is typically of the order of an angstrom (10 -8 cm). From this derives that an electromagnetic wave undergoes diffraction effects caused by the microscopic structure of a solid if it has an energy of order

ℏ𝜔 = ℎ𝑐 𝜆 → ℎ𝑐 10 -8 𝑐𝑚 ≈ 12.3 × 10 3 𝑒𝑉 (2.1)
which corresponds to X-rays. Therefore, it is possible to reveal the locations of the atoms in a crystal structure studying the distribution of X rays scattered by the periodic array. The simplest structural information that can be obtained arises from the Bragg law, formulated by W.H. and W.L. Bragg in 1913, which links the angle of incidence 𝜃 of the wave, its wavelength 𝜆 and the interplanar distance 𝑑 through the relation:

𝑛𝜆 = 2𝑑 𝑠𝑖𝑛 𝜃. (2.2)
this equation implies that the path difference between the incident and the scattered beam 2𝑑 𝑠𝑖𝑛 𝜃 must be an integer of wavelengths. A geometrical representation of the Bragg law is shown in Figure It is important to stress that diffraction can occur only if the wavelength is of the same order of magnitude as the distance between scattering centers. Since 𝑠𝑖𝑛 𝜃 cannot excess the unity, the follow condition, directly derived from the Bragg law (equation 2.2), needs to be satisfied:

𝑛𝜆 2𝑑 = 𝑠𝑖𝑛 𝜃 < 1. (2.3)
Knowing that the smallest value for n is 1, the required condition for having diffraction at any observable angle 2𝜃 is

𝜆 < 2𝑑. (2.4)
In order to satisfy the Bragg law, 𝜆 or 𝜃 should change during the experiment. If the sample consists of a very fine powder, which is the case for the samples measured in this work, it is possible to use a monochromatic beam since each particle corresponds to a tiny crystal randomly oriented. In this way, the diffraction pattern produced coincides to the one that would have been obtained by combining the diffraction patterns for all possible orientations of a single crystal. 146 The same case of randomly oriented small crystals can be treated from the reciprocal lattice point of view. Performing the Fourier transform of the set of lattices, it is possible to obtain randomly oriented identical reciprocal lattices with a common origin. The reciprocal lattice vectors d*hkl are the differences between the incident k0 and the corresponding scattered wavevectors k1, aligned along the surface of the cone, as shown in Figure 2.2. The apex of the cone coincides with the center of a sphere with radius equal to 1/λ, named Ewald's sphere, the cone axis is parallel to k0 and the solid cone angle is 4θ. In the hypothesis that the small crystals number approaches infinity, the density of scattered wavevectors k1 becomes constant on the surface of the cone thus resulting in a constant diffracted intensity around the cone circumference. The intersection between the cone and a plane perpendicular to the incident beam, where a planar area detector is usually placed (see Figure 2.2), gives rise to rings which intensities and diameters depends on the specific reciprocal lattice vector. 147

Transmission Electron Microscopy

Fundamental particles, such as electrons, possess also a wave character and their wavelength, as proposed by Louis De Broglie, is given by

𝜆 = ℎ 𝑝 = ℎ 𝑚𝑣 (2.5)
where h is the Planck's constant and p, m and v represent the electrons' momentum, mass and speed.

Electrons can be accelerated by a voltage V and their velocity, related to V taking into account relativistic effects, gives the following expression for the electronic wavelength

𝜆 = ℎ √ 2𝑚𝑒𝑉 (1 + 𝑒𝑉 2𝑚𝑐 2 ) (2.6)
where e is the electron charge, m its mass and c the light speed. 149 Considering the order of magnitude of the quantities involved (e≈10 -19 C, m≈10 -31 kg, c≈10 8 m/s), this relation shows that the electrons' wavelength is orders of magnitude shorter than visible light. Therefore, an electron beam, once focused, can be exploited for Transmission Electron Microscopy (TEM) to image materials at the nanometers scale, overcoming the diffraction limit of optical microscopes. 150 The main components of the TEM are shown schematically, along with the corresponding ray diagram, in Figure 2.4. The electron beam is emitted by an electron gun at the top of the microscope and is focused through electromagnetic lenses into a fine, coherent beam. This beam, which travels under vacuum, strikes a very thin sample (thickness around 100 nm) and the electrons can be either transmitted or diffracted depending on the structure, thickness, and transparency to electrons of the specimen. After being focalized again, the transmitted portion of the beam hits a phosphor screen, or a charge-coupled device (CCD) camera and the image is produced. In bright field mode, the areas where few electrons are transmitted appear darker in the image while the others are lighter. In this work, TEM imaging has been performed on a 200 kV FEG TEM (FEI Tecnai F20 equipped with a Gatan Energy Imaging Filter, resolution 0.24 nm). Powder samples are dispersed in ethanol before being deposited on a sample holder grid which allows the observation of the MOF grains. Usually, the grains thickness (between 100 and 500 nm) is too high to be crossed by the electron beam. For this reason, images need to be collected at the grain's edges. It is possible to characterize the average size of nanoparticles confined within MOF pores, by measuring the diameter of the nanoparticles observed in several TEM images. A histogram of particles size is then produced to calculate the average size of the distribution.

Performing Transmission Electron Microscopy on MOFs is challenging due to their instability and possible damage under the high energy electrons used. [START_REF] Wiktor | Transmission Electron Microscopy on Metal-Organic Frameworksa Review[END_REF]151 For this reason, minimum electron dose and minimum flux have been used on these porous samples.

Specific surface area and total pore volume

Gas adsorption is the most popular technique to characterize the textural properties of porous materials, e.g. pore size distribution, pore geometry and connectivity. 152 Adsorption is defined as the adhesion of molecules, atoms or ions to a surfaces and it can be classified depending on the strength of the interactions between the adsorbate and the adsorbent. Two main processes are distinguished: chemisorption, which involves the formation of chemical bonding, and physisorption when weaker forces are present. Thanks to its characteristics, physisorption is more suitable for surface area and pore volume determinations, being a non-destructive reversible process unlike chemical adsorption.

Moreover, it may lead to surface coverage by more than one layer of adsorbate. 153 This phenomenon is described through adsorption and desorption isotherms which show the relation, at constant temperature, between the amount adsorbed and the equilibrium relative pressure P/P0 where P is the equilibrium pressure and P0 the saturation vapour pressure at the adsorption temperature. 152 Depending on the pore size, the adsorbentadsorbate interaction and the surface type, the IUPAC has defined eight possible types of isotherms that can be observed in porous materials (Figure 2.5). Type I a and b are typical of microporous materials and represent monolayer physisorption process. In this case, the limiting uptake is being governed by the accessible micropore volume rather than by the internal surface area. In nonporous or microporous systems, unrestricted monolayermultilayer adsorption can occur which give rise to Type II sorption isotherms. The inflection point B indicates the moment when multilayer adsorption begins to occur. Type IV a and b describe multimolecular physisorption and it is emblematic for mesopores. Pore condensation causes the hysteresis loop in the desorption and the plateau of the isotherm indicates complete pore filling. Type III and Type V are uncommon convex isotherms which can be observed if weak attractive interactions between the adsorbent and the adsorbate arise. The last type is representative of layer-by-layer adsorption on a highly uniform nonporous surface. 152,153 In this work, the specific surface area and the total pore volume of samples have been determined from nitrogen adsorption/desorption isotherms measured at liquid nitrogen temperature (77 K) using an automated volumetric apparatus (Autosorb IQ Quantachrome) which mainly consists of a secondary vacuum pump, a sample holder immersed in the cryogenic liquid, a reference volume and a gas system (see Figure 2.6 for a schematic setup). Measurements were performed by Dr. Gabriela Blanita's group in the NIRDIMT laboratory (Romania). Before measuring the adsorption isotherm of samples, other physiosorbed molecules need to be removed from the internal surface through the degassing procedure previously described. After that, the dead volume of the sample holder is determined using He at the measuring temperature. Then, successive doses of the adsorbate gas are admitted into the gas manifold and sample holder. When the sample starts to adsorb, the gas pressure falls until the adsorbate and the adsorbent are in equilibrium.

The N2 adsorption isotherm is thus constructed point by point until the maximum relative pressure is reached. The reverse procedure stands for the desorption when gas is removed successively from the gas phase.

Starting from the measured N2 isotherm, the specific surface area is calculated using the S. Brunauer, P.H. Emmett and E. Teller (BET) equation, while the total pore volume is obtained from the isotherm value at maximum relative pressure P/P0 = 0.95. The BET theory established in 1938

states that 1 𝑣 ( 𝑃 0 𝑃 -1) = 1 𝑣 𝑚 𝐶 + 𝐶 -1 𝑣 𝑚 𝐶 • 𝑃 𝑃 0 (2.7)
where 𝑣 is the volume adsorbed at relative pressure P/P0 and 𝑣 𝑚 is the volume of the adsorbate forming a complete monolayer. The BET constant C depends to the adsorption energy of the first layer E1, to the liquefaction energy of the adsorbate EL, to the temperature T of the adsorption and to the gas constant R according to the following relation:

𝐶 = 𝑒 ( 𝐸 1 -𝐸 𝐿 𝑅𝑇 ) (2.8)
The BET theory is based on several hypotheses: adsorption occurs only on well-defined sites of the solid phase surface, several adsorbate layers can cover the surface of the adsorbent (multilayer adsorption), from the second layer the adsorption energy 𝐸 2 corresponds to the liquefaction energy 𝐸 𝐿 , gas molecules physically adsorb on a solid in layers infinitely and finally the Langmuir theory can be applied to each adsorbate within a monolayer (Langmuir's basic assumptions). Usually, in the 𝑃 𝑃 0 range 0.05-0.35, the equation 2.7 can be linearized using the function 𝑦 = 𝑞 + 𝑚𝑥 with

𝑞 = 1 𝑣 𝑚 𝐶 𝑎𝑛𝑑 𝑚 = 𝐶 -1 𝑣 𝑚 𝐶 (2.9)
and the monolayer volume is given by

𝑣 𝑚 = 1 𝑞 + 𝑚 . (2.

10)

The value of the specific surface area 𝑆 is then deduced by the following relation

𝑆 = 𝑣 𝑚 𝑀 𝑣 𝑊 𝜎 𝑎𝑑 𝑁 𝐴 (2.11)
with 𝑁 𝐴 Avogadro's number, 𝑊 weight of the sample, 𝑀 𝑣 the molar volume of the adsorbate gas and σ ad the cross sectional area of the adsorbate (16.2 Å for N2).

Inductively Coupled Plasma -Optical Emission Spectrometry

Inductively coupled plasma optical emission spectrometry (ICP-OES) is an analytical technique that can quantitatively measure chemical elements at ultra-low-concentration (ppt = parts per trillion = ng/l). The underlying physical principle of ICP-OES relies on measuring the specific energy of the photons emitted when previously excited electrons return to the ground state. This analytic technique can be used to check the percentage of metal content present in composites containing metallic nanoparticles. In this work, powder samples have been digested in HCl-HNO3 water solution following standard protocols and processed using a Varian Vista-PRO CCD Simultaneous ICP-OES instrument.

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) is a technique which allows measuring the mass of a substance while changing the temperature at a constant rate in a controlled atmosphere. The mass variation provides information about physical phenomena occurring to the sample: a mass loss is linked to processes such as dehydration, decomposition, desorption, and reduction while a mass gain refers to adsorption, oxidation or carbonation, among the others. In particular, TGA can be used to evaluate the thermal stability of a material. The experimental apparatus consists of a high precision thermobalance and a furnace that can apply controlled temperatures. A thermocouple is placed in the holder to measure the temperature on the sample. Data obtained from TGA analysis are represented as a plot of the mass weight, absolute or percentage, as function of the temperature. A derivative weight loss curve can also be obtained to highlight where the weight loss is most apparent. In this work, TGA data were collected with the DSC SDT Q600 Thermogravimetric Analyzer apparatus in the NIRDIMT laboratory (Romania) in collaboration with Dr. Gabriela Blanita.

High-Pressure technique: the diamond anvil cell

In this section, description of the diamond anvil cell, the device used to perform high-pressure experiments, is given. Special emphasis is placed on the hydrostatic limits of pressure transmitting media relevant for IR spectroscopy, determined in this work.

Principle

The Diamond Anvil Cell (DAC) is the most popular device to generate high static pressures because of its extraordinary versatility and compatibility with many experimental techniques. Indeed, the mechanical and optical properties of diamonds make this device suitable for studies involving radiation ranging from X-rays to infrared, from a few GPa up to hundreds of GPa and combined with low/high temperatures.

As shown in Figure 2.8, the working principle of the DAC is based on the use of two diamonds as opposed anvils. A perpendicular force is applied to the external diamond faces and the ratio ( 10

Diamonds

Diamond is an allotropic form of carbon, consisting of tetrahedrally covalently bonded carbon atoms. This carbon structure results in the hardest known material thus giving the best candidate for high-pressure experiments. 157 Moreover, since diamonds are transparent in a wide range of the electromagnetic spectrum, they result well-suitable for studies involving interactions between light and matter, such as IR spectroscopy and XRD. Diamonds can be classified according to the presence of nitrogen impurities which affect their optical properties. Type Ia diamonds contain around 0.05-0.2% of nitrogen and present a characteristic IR absorption band between 1100 and 1300 cm -1 , as shown in Figure 2.9. 157 On the contrary, type IIa gems, which are almost impurity free, only have the intrinsic two-phonon absorption of diamond centered at about 2000 cm -1 and 2500 cm -1 (see Figure 2.9). 158 has a 2-4 mm diameter, and the diamond thickness is of 1-3 mm depending on the design. Two main anvil designs are commonly used: standard cut and conical / Bohler-Almax cut, both shown in Figure 2.10. For the former, the table of conventional anvils is mounted on flat tungsten carbide plates with a central conical opening. On the contrary, conical anvils are fit into tungsten carbide seats which provide lateral support to the diamonds. The main advantages of this design consist of larger apertures and thinner anvils, important features for both IR spectroscopy and X-ray diffraction. 159 Alignment and centering of the two anvils is of paramount importance to produce and maintain static highpressure conditions. Indeed, the tungsten carbide backing plates where diamonds seat have fine adjustments to set the anvils parallelism and centering. These operations are performed by direct observation of the diamonds through the optical axis under a microscope. 157 The parallelism is verified looking at the Newton interference fringes generated when illuminating with a white light source.

Fringes arise in the region where the culets are separated by a distance multiple of λ/2 and disappear when they are perfectly parallel. This technique ensures parallelism with an error of the order of 10 -3 rad. 158 In this work, synthetic IIa type diamonds with culet ranging from 500 µm to 1 mm were mounted into the diamond anvil cell.

Gasket

The gasket is an essential component of the DAC and has two main critical functions: its hole provides the high-pressure chamber where the sample is isolated, it gives lateral support for the culets of the anvils and avoid the direct contact between the culet significantly reducing the diamond failure risk. 160 Gaskets are metallic foils with a typical initial thickness of 100-250 μm. The metal is chosen depending on the experimental conditions. Usually, stainless steel, copper-beryllium and rhenium are widely employed with the latter more suitable for ultrahigh-pressure experiments for its higher yield strength. Moreover, the gasket material needs to be inert to the sample. As showed in Figure 2.11, the metal sheet is placed between the anvils and pre-indented to a typical value of 30-50 μm. The sample cavity is then produced by drilling a cylindrical hole by means of an electro-erosion machine equipped with tungsten wires having diameters ranging from 100 to 300 μm. An experimental rule of thumbs states that the hole should have a diameter of one third to one half of the culet size. Moreover, it needs to be well centered to guarantee stability to the sample area when the load is applied. Indeed, the pressure distribution of the diamond culets is strongly inhomogeneous with a pronounced peak at the center and decreases going to the edges of the gasket. 157

The membrane DAC mechanism

Among the diamond anvil cells available at the SMIS beamline, there are two membrane DACs manufactured by BETSA (France) and an in-house designed cell. The computer-aided design (CAD) representation of the BETSA DAC is showed in Figure 2.12. The in-house manufactured cell presents a similar design with the peculiarity that it can be sealed using four screws. This characteristic is of paramount importance for this work, as it will be explained in the paragraph 2.2.7 about the DAC loading procedure. These DACs are equipped with IIa diamonds with 1 mm, 600 and 500 μm culets diameters. The anvils are placed on two tungsten carbide backing plates and one of them is mounted on a tungsten carbide hemisphere which allows tilting and alignment of the diamond anvil one over the other. Thanks to the Boehler-Almax design, the optical aperture results to be of 70°. 159 When the cell is closed, its body is screwed in an outer case and a metallic membrane is placed between the two.

Force is applied inflating the membrane with an inert gas, usually helium or nitrogen. On increasing the gas pressure into the membrane, the upper anvil is pushed against the bottom one thus generating high pressure on the sample. Four metallic rods keep the alignment of the anvils. It is worth noting that gas flow to the membrane can be remotely controlled making membrane DAC particularly useful in the case of spectroscopy measurements at high/low temperature since the experimental chamber is not accessible when the measure is running and during XRD experiments which are performed in the experimental hutch of synchrotron beamlines. 161 In this work, the automatic pressure controller PACE5000 was used to finely control the gas flow into the membrane.

𝑷[𝑮𝑷𝒂] = 𝟏. 𝟖𝟕(±𝟎. 𝟎𝟏) × 𝟏𝟎 𝟑 ( ∆𝝀 𝝀 𝟎 ) [𝟏 + 𝟓. 𝟔𝟑(±𝟎. 𝟎𝟑) ( ∆𝝀 𝝀 𝟎 )] (2.12)
where 𝜆 0 is the wavelength of the R1 line at ambient condition. 166 However in the 0-20 GPa range, the shift of the fluorescence line R1 may be approximated as linear and the following relation

𝑷 = 𝜶𝜟𝝀 (2.13)
with 𝛼= 2.74 GPa/nm can be used. 163 The error on the pressure values thus determined is ± 0.1 GPa under hydrostatic conditions. 158 It is finally worth to notice that the spectral shape of the fluorescence and namely the dip between the R1 and R2 lines can be used as a first rough marker of the hydrostaticity of the environment. 157 An alternative method is to derive the pressure value from a known equation of state of a volumetric calibrant, such as NaCl or Au, loaded in the high-pressure chamber together with the sample. This method can be easily applied when carrying out an X-ray diffraction experiment and provides a more accurate pressure measurement (error of ± 0.005 GPa) than the ruby fluorescence. 167 In this work, the ruby-luminescence method has been used to measure the pressure in IR spectroscopy experiments while the equation of state of NaCl has been the chosen technique during XRD measurements. NaCl was preferred to other gauges, such as Au, for its larger compressibility at low pressure, i.e. below 1 GPa, which allows a more precise pressure measurement in this range.

Pressure transmitting media

The study of physical and chemical properties of materials under pressure should be performed in ideal hydrostatic conditions. However, the DAC design (see Figure 2.8) induces an uniaxial stress on the sample and the lack of hydrostaticity as well as the presence of pressure gradients across the sample can change the sample's behaviour at high pressure. For this reason, a pressure transmitting medium (PTM) surrounding the sample is commonly used. Gases and fluids (e.g. helium, neon, argon, or ethanol-methanol mixtures) provide the best hydrostatic conditions in different pressure ranges. 168,169 However, hydrostaticity is not always the only parameter to consider. Other requirements must be met depending on the experiment, such as the PTM compatibility with the desired pressure and temperature ranges, i.e. without phase transitions or transformations, and its chemical reactivity to avoid undesired interactions and modifications of the sample. 170 Moreover, the PTM must be appropriate for the experimental technique. A suitable PTM for high pressure IR spectroscopy, which is one of the main techniques used in this work, must be transparent in the investigated frequency window to not interfere with or mask the sample signal. Besides gaseous media, solid PTM exist that are transparent either in the THz/far-IR range (10-500 cm -1 ) or in the mid-/near-IR (500-8000 cm -1 ) range. However, solids cannot be totally hydrostatic but are usually considered quasi-hydrostatic. It is also worth to stress that when IR transmission measurements are performed, as in this thesis work, the sample thickness must be adjusted according to the desired absorption; namely, it cannot be too thin to absorb sufficiently nor too thick to avoid signal saturation. For powder samples such as MOFs, filling a part of the gasket hole with solid PTM allows for a better control of sample thickness. By controlling the PTM thickness, the desired sample thickness can be obtained. Owing to these reasons and their easy loading into the DAC, powders of solid PTM such as alkali halides are widely used in high pressure IR spectroscopy. 171 However, the degree of quasi-hydrostaticity that they can ensure has never been evaluated before and needs to be addressed before to use them as PTM for MOFs. The presence of strong pressure gradients could impact the high-pressure behavior of the samples being investigated, resulting in non-reproducible results. This prompted us to study the hydrostatic limit of four solid PTM, KBr, CsI, NaCl and polyethylene, of potential interest for this work below 20 GPa. 171 The method used to assess the hydrostaticity is based on the measurement of the fluorescence emissions from multiple rubies loaded inside a DAC together with the chosen PTM and has been introduced by Klotz et al. 169 The ruby-fluorescence peaks, R1 and R2, shift toward lower energies with increasing pressure, whereas their widths, Γ, broaden when pressure gradients are present. 172 Therefore, the R1 frequency obtained from several rubies can be used to calculate the pressure standard deviation:

𝝈 = √ 𝟏 𝑵 ∑(𝑷 𝒊 -𝑷 ̅ ) 𝟐 𝑵 𝒊=𝟏 (2.14)
where 𝑃 ̅ is the average pressure and Pi the pressure obtained from the i th ruby probe. Thus, σ can be used as an estimation of the pressure inhomogeneity in the sample chamber. 169 Indeed, if the medium is completely hydrostatic and the measured pressure is identical for all the rubies placed in the DAC, σ results to be zero. Table 2.1 summarizes the most used PTM described in current literature with their hydrostatic limits, pressure standard deviations at two selected pressure values, and transmission windows limited to the far-and mid-IR ranges (20-4000 cm -1 ). The hydrostatic limits of the studied PTM are relatively low (1-2 GPa) compared with those of fluid media. However, a comparison between the pressure standard deviations in the 10-20 GPa range shows that alkali halides and polyethylene perform better than widely used PTM such as methanol-ethanol mixtures and silicone oil; namely, the first exhibits lower average pressure gradients than the latter. In particular, the σ value of silicone oila widely used PTM for high-pressure studiesat 10 GPa is comparable with those of salts and polyethylene.

However, at higher pressures, its pressure standard deviation is much higher, with σ reaching 2.3 GPa at 20 GPa, which is approximately 2-5 times higher than the σ values of the investigated solid PTM at equal pressure. Similarly, the 4:1 methanol-ethanol mixture is completely hydrostatic in the 0-10 GPa range. At 10.5 GPa, the standard deviation rapidly increases above the hydrostatic limit. In the 10-20

GPa pressure range, all four investigated PTM exhibit lower standard deviations than 4:1 methanolethanol. Daltons corresponding to an average size between 2.4 and 3 nm 174 while Nujol molecules size is 5-7 Å. 129 The solid PTM are finely milled, dried and stored at 120 °C in an oven before loading in the DAC.

DAC loading procedure

Usually, the DAC can be handled in atmosphere and the sample is loaded using an optical microscope. However, as mentioned above, MOFs can easily adsorb water molecules present in the atmosphere thus demanding a meticulous DAC preparation. Indeed, the presence of adsorbed water molecules hinders the measurement of a spectral region of the IR spectrum of the sample and could affect the structural response of the framework under compression. For these reasons, the previously described degassing procedure is performed before any experiment. The as-activated samples are loaded into the DAC in a glovebox equipped with a microscope under N2 or Ar gas atmosphere (O2 and H2O levels < 1 ppm). The DAC is sealed to a pressure value around 0.1 GPa before removing it from the glovebox. Particular attention is paid to avoid any compression of samples during the loading procedure, i.e. while opening and closing the DAC repeatedly. We implemented different protocols for the DAC loading to ensure the best quality of the collected data depending on the experimental technique. For infrared spectroscopy measurements, the ratio between PTM and sample thickness is carefully checked in order to prevent the saturation of the most intense IR peaks in the MOF. Together with the sample, a small ruby chip is placed for the pressure determination. In the case of X-ray diffraction experiments, the sample is loaded together with the chosen PTM and few grains of NaCl used to measure the pressure. In some cases, NaCl acts both as PTM and pressure gauge.

Synchrotron-based techniques

A brief introduction on the production of synchrotron radiation is presented, stressing the advantages of the synchrotron source compared to a classical thermal source, in the infrared case.

Afterwards, basic theoretical principles of infrared spectroscopy are given. Finally, the beamlines where the experiments of this work have been performed will be briefly described, with a particular focus on the SMIS beamline of the synchrotron SOLEIL.

Synchrotron radiation 2.3.1.1 Emission mechanism

Synchrotron radiation is an electromagnetic emission produced when charged particles travelling at ultra-relativistic energies are deviated from their trajectory by an external magnetic field.

When an electron with charge q = -e and mass 𝑚 𝑒 travels through a magnetic field 𝐵 ⃗ , the Lorentz force

𝐹 = 𝑞𝑣 × 𝐵 ⃗ (2.15)
acts on it, bending its trajectory in a circle with radius ρ, and centripetal acceleration 𝑣 2 /𝜌 in a direction perpendicular to its velocity 𝑣 and to the applied magnetic field 𝐵 ⃗ . The centripetal force acting on the electron, and thus the acceleration, is then related to the Lorentz force by the equation:

𝑚 𝑒 𝑣 2 𝜌 = 𝑒𝑣𝐵 . (2.16)
The equation 2. 16 shows as the velocity of the particle depends on the intensity of the magnetic field 𝐵 ⃗ and on the radius ρ. The accelerated particle emits the electromagnetic radiation whose total radiated power P is given by the Larmor's equation formulated at first for a non-relativistic particle, i.e. 𝛽 = 𝑣 𝑐 ≪ 1. P depends on the charge q, the acceleration 𝑎 = 𝑣 2

𝜌

and the speed of light 𝑐:

𝑃 ∝ 𝑞 2 𝑎 2 𝑐 3 .
(2.17)

Introducing the relativistic contribution in Larmor's equation, it is possible to write the power radiated per unit solid angle as follows:

𝑑𝑃 𝑑𝛺 = 𝑞 2 𝑎 2 4𝜋𝑐 3 𝑠𝑖𝑛 2 𝜃 (1 -𝛽 cos 𝜃) 3 [1 - 𝑠𝑖𝑛 2 𝜃𝑐𝑜𝑠 2 𝜑 𝛾 2 (1 -𝛽 cos 𝜃) 2 ]
(2.18)

where 𝜃 and 𝜑 are the angles when spherical coordinates are adopted, and 𝛾 = 1 √1-𝛽 2 the Lorentz factor. 175 In the classical (non-relativistic) condition with 𝛽 ≪ 1, the radiation power is isotropically distributed and is proportional to 𝑠𝑖𝑛 2 𝜃, as shown in with 𝐸 energy of the electron. 175,176 Thanks to its emission mechanism and the peaked shape of the radiation power typical of accelerated electrons, synchrotron radiation exhibits interesting properties such as:

• broad spectrum, from X-rays to microwaves;

• high flux which corresponds to high intensity of the photon beam;

• high brightness defined as the photon flux per unit area and per unit solid angle at the source. In the linear accelerator, electrons are extracted from a cathodic tungsten filament to which a voltage of -90 kV is applied. In the accelerator cavities, an electromagnetic field increases the particles kinetic energy reaching 110 MeV. A transfer line allows the electron beam to pass through the booster, the second accelerator, which is a polygon composed of a succession of magnetic elements and straight sections. Once the maximum energy of 2.75 GeV is attained, the beam can be injected into the storage ring whose circumference measures 354 m. The ring is composed of several magnetic elements essential on one hand to accelerate and control the trajectory of the electron beam and on the other hand to produce and extract the synchrotron radiation. Dipole-bending magnets force the electrons to follow the circular path and are alternated with focusing and de-focusing magnets (quadrupoles and sextupoles). The energy loss of the electron beam inside the storage ring is compensated by the presence of radiofrequency cavities which transfer electromagnetic energy to the electrons. Bending magnets and insertion devices, undulators or wigglers, generates the synchrotron. The latters consist of an array of small magnets with alternating polarity which force electrons to oscillate thus producing very intense radiation. The wavelength can be tuned by changing the gap between the rows of magnets.

The radiation produced is then extracted from the ring and directed through optical systems to the beamlines. Synchrotron radiation produced at SOLEIL covers a wide energy range extending from the infrared to the X-rays, i.e. from 0.001 eV to 80 KeV. 177

Characteristics and advantages

Synchrotron radiation can be characterized by two quantities: flux and brightness. The flux is defined as the number of photons per second with 0.1% bandwidth collected in the beam plane and integrated over the opening angle, while the brightness represents the photon flux per unit area and per unit solid angle at the source. 178 High incident flux, high brightness due to small cross section of the electron beam and high degree of collimation of the radiation represents the biggest advantages of synchrotron radiation.

As explained in section 2.2.1, the space available for the sample in a DAC is limited. For XRD experiments, the low flux of conventional sources calls for long exposure times and the large beam divergence involves contributions from the gasket in the diffraction patterns. Using extremely brilliant synchrotron radiation source, acquisition times are dramatically cut and the possibility to collimate intense X-ray beams to a few micrometers reduces the problem of beam shadowing by the gasket. 179 Moreover, samples studied in this thesis work contain light atoms, such as H, C and O, which implies low X-ray scattering since the scattering amplitude is proportional to Z 4 where Z is the atomic number. In this case, the high flux provided by the synchrotron facilitates the measurements.

The small dimensions of the sample make challenging high-pressure infrared experiments as well, especially in the far infrared range. The spot size at the sample is diffraction limited, therefore it depends on the measured spectral range and becomes larger at lower energies. For this reason, high brightness is necessary to perform infrared high-pressure experiments.

Infrared spectroscopy

Theoretical principles

A molecule is a set of two or more atoms chemically bonded which can be modeled, in first approximation, as harmonic oscillators. A simple diatomic molecule can thus be described as two atomic masses m1 and m2 connected by a spring of constant K. In the classical limit, the force exerted on the atoms is given by the Hooke's law:

𝐹 = -𝐾(𝑟 -𝑟 𝑒𝑞 ) (2.20)
where 𝑟 𝑒𝑞 is the equilibrium bond distance. The resulting vibrational energy is

𝐸 𝑣𝑖𝑏 = 1 2 𝐾(𝑟 -𝑟 𝑒𝑞 ) 2 (2.21)
while the vibration frequency

𝜈 = 1 2𝜋 √𝐾 ( 1 𝑚 1 + 1 𝑚 2 ). ( 2

.22)

Moving to quantum mechanics, the harmonic vibration of a diatomic molecule is treated by solving the corresponding time-dependent Schrodinger equation and the energy levels are quantized as follows:

𝐸 𝑛 = (𝑛 + 1 2 ) ℎ𝜈 (2.23)
with 𝜈 frequency of the vibration and 𝑛 the quantum number identifying the energy level.

Electromagnetic radiation can be characterized by its energy 𝐸(eV), wavelength λ (nm), frequency ν (Hz) or wavenumber 𝜈̅ (cm -1 ), defined as the number of waves per unit of distance

𝐸 = ℎ𝜈 = ℎ𝑐 𝜆 = ℎ𝑐𝜈̅ . ( 2

.24)

As stated above, when radiation interacts with matter, the photon energy may be absorbed by the molecules causing a transition to an excited state. In particular, the characteristic energy of vibrational transitions corresponds to the infrared range, as shown in Figure 2.18. Infrared refers to that part of the electromagnetic spectrum between the visible and microwave (energy range 1.7 eV -1.24 meV) and it is conventionally divided into three regions: the near-infrared (14000 -4000 cm -1 ), the mid-infrared (4000 -400 cm -1 ) and the far-infrared (400 -10 cm -1 ). 180

( 𝜕𝜇 𝜕𝑄 ) 0 ≠ 0 (2.26)
where Q is the vibrational amplitude. Finally, it is important to stress that the measured IR band intensity is proportional to the square of the change in the dipole moment:

𝐼 𝐼𝑅 ∝ ( 𝜕𝜇 𝜕𝑄 ) 2 (2.27)

The infrared spectrum

Generally speaking, when an incident radiation with intensity I0 interacts with a sample, it can be reflected 𝐼 𝑅 , absorbed 𝐼 𝐴 or transmitted 𝐼 𝑇 :

𝐼 0 = 𝐼 𝑅 + 𝐼 𝐴 + 𝐼 𝑇 .

(2.28)

where we neglected scattering phenomena for sake of simplicity. From the experimental point of view, any of the above intensity components can be used to measure the infrared spectrum of the sample which is defined as the sample-dependent change induced on the intensity distribution of infrared radiation emitted by a source in the infrared range.

The relationship between the intensities of the incident and transmitted IR radiation and the sample thickness is classically approximated by the Lambert-Beer law:

𝐴 = 𝜀𝑐𝑙 (2.29)
where 𝐴 represents the absorbance, 𝜀 is the molar extinction coefficient (absorptivity), 𝑐 the concentration and 𝑙 pathlength of the sample. 182 If the light is totally absorbed by the sample and thus 𝐼 𝑅 + 𝐼 𝐴 = 0, the transmittance 𝑇 is given by the ratio between the intensities of the incident (𝐼 0 ) and transmitted (𝐼 𝑇 = 𝐼) light after passing through the sample:

𝑇 = 𝐼 𝐼 0 . ( 2

.30)

Hence, the absorbance defined in Equation 2.29, in a non-reflecting sample, is also equal to

𝐴 = 𝑙𝑜𝑔 𝐼 0 𝐼 = 𝑙𝑜𝑔 1 𝑇 (2.31)
where 𝐼 0 and 𝐼 are respectively the reference and the sample spectra.

Graphically, the spectrum is a plot of the intensity (absorbance or transmittance) versus a property of light, usually the wavenumbers 𝜈̅ (cm -1 ) and in the case of absorbance, vibration transitions appears as peaks in the IR spectrum.

Fourier Transformed Infrared Spectroscopy (FTIR)

Dispersive spectrometers are the first instruments developed to record IR spectra. In these instruments, the polychromatic source is monochromatized by a prism or a grating and the separated frequencies are then measured independently. 180 On the contrary, in Fourier transform (FT) instruments all frequencies are measured simultaneously since the source is modulated into an interferogram that contains a broad frequency region. The three main advantages of the FT spectrometer over a dispersive one are:

i. the Fellgett or multiplex advantage which, thanks to the simultaneous collection of information from all wavelengths, provides rapid collection times and high signal-to-noise ratio (this quantity increases as the square root of the number of collected scan);

ii. the Jacquinot or high throughput advantage ensures that more light reaches the sample and hence the detector since any grating is present in FT-IR spectrometers;

iii. the Connes or wavelength accuracy advantage involving the use of a laser to precisely measure the position of the moving mirror and calibrate the wavelength scale. 182 One of the most widespread interferometer used in FT-IR spectrometers is the Michelson type where, as schematically illustrated in i.e. the measured spectrum. However, in the not ideal case the translating mirror moves over a finite distance, thus meaning that the integral must be replaced by a sum extended between finite values thus limiting the spectral resolution. Greater optical path differences provide a higher number of data point for a fixed spectral range and hence a better resolution. The latter is then linked to the path difference and defined by ∆𝜈 = 1/𝛿. 182 To properly resolve all the peaks, the wavenumber resolution of an FT-IR spectrometer must be set at a value (in cm -1 ) smaller than the Full Width at Half Maximum (FWHM) of the bands to be observed. 183 In this work, the spectrometer resolution has been set at 2 cm -1 .

Detectors

Several detectors can be used with a FTIR spectrometer, depending on the energy range of interest. Among the most used, we find the Mercury Cadmium Telluride (HgCdTe -MCT) detectors and the bolometer, both used in the IR measurements performed in this thesis.

MCT are liquid nitrogen cooled photodetectors sensitive to photons with energy usually between 600 -10000 cm -1 . The incident radiation induces electronic transitions in the semiconductor compound. Photons with energy greater than the semiconductor bandgap excite electrons into the conduction band, thereby increasing the conductivity of the material and changes in conductivity are directly related to the amount of incident photons.

For frequencies below 600 cm -1 , a thermal detector consisting of boron-doped silicon thermistor, called bolometer, is generally used. The absorber of heat capacity is thermally connected to a body at constant temperature, called heat reservoir, by a resistance. Any thermal radiation that strikes the detector causes a temperature change which in turn causes a resistance change. This is then amplified and measured as a voltage difference, as a regulated current is applied to the detector element. In order to reveal only the temperature change caused by the incoming IR radiation to detect, the bolometer is kept under vacuum and cooled down at liquid helium temperature (4.2 K).

SMIS beamline

All the high-pressure infrared measurement reported in this Ph.D. thesis have been performed at the SMIS (Spectroscopy and Microscopy in the Infrared using Synchrotron) beamline of SOLEIL synchrotron. SMIS is dedicated to IR spectromicroscopy and provides a very bright IR source in the 200 -8000 cm -1 range exploiting synchrotron radiation emitted from a dedicate bending magnet inside the storage ring. In particular, SMIS collects both edge radiation, emitted at the entrance or exit of the bending magnet, and dipolar radiation, emitted during the circular trajectory in the magnet, as shown on the left part of Figure 2.22. These two sources have different characteristics as shown in the calculated spatial distribution of photons at 10 μm plotted on the right of Figure 2.22: edge sources present an almost cylindrical symmetry, whereas the dipolar radiation is constant in the orbit plane. 184,185 The fringes are due to interference between the two edge sources. We will now focus only on SMIS-2 where this work was carried out. Here, a home-built horizontal microscope equipped with large working distance Schwarzschild objectives allows highpressure measurements from the near to the far infrared domain. The light from the synchrotron or the globar source is sent to a Thermo-Fisher iS50 FTIR interferometer coupled with the microscope. As noticeable from the comparisons between the flux of synchrotron radiation measured on the SMIS beamline and a blackbody source shown in Figure 2.24, synchrotron infrared radiation presents higher flux at large wavelengths in the THz range, whereas the flux is lower in the mid-IR. However, the brightness of synchrotron emission at SMIS is between two to four orders of magnitude higher than that of a globar, which is essential to measure small micrometric samples as in IR microscopy and high-pressure experiments. 184

X-Ray Diffraction

As discussed in section 2.1.3.1, XRD is the main experimental technique to investigate the crystal structure of materials. Depending on the quality of the data and on their analysis method, this technique can provide information about the crystal lattice symmetry, the atomic positions and electronic density maps. Therefore, structural phase transition induced by thermodynamic parameters as pressure and temperature can be detected by XRD. For high pressure studies involving the DAC, the quality and completeness of XRD data obtained is typically lower than those obtained at ambient conditions. This is due to the Compton scattering caused by the diamond anvils and to an angular range limited by the numerical aperture of the DAC. Moreover, the sample volume contained in a diamond anvil cell is limited thus causing extremely weak signal. The latter limitation can be partly overcome by exploiting a high photon flux such that provided by synchrotron sources. The X-ray diffraction measurements at high-pressure reported in this Ph.D. thesis have been performed on PSICHE and CRISTAL beamlines at SOLEIL synchrotron and on XRD1 beamline at ELETTRA (Trieste, Italy). In the next sections the beamline layouts will be briefly described.

PSICHE beamline

PSICHE (Pressure, Structure and Imaging by Contrast at High Energy) beamline is dedicated to diffraction and tomography experiments at extreme conditions from 15 to 80 keV. 188 X-rays are delivered through a wiggler insertion device which provides the highest energy at SOLEIL synchrotron and gives a continuous energy spectrum with a peak at 25 keV.

For high-pressure XRD measurement using diamond anvil cells, the beam is focused by Kirkpatrick-Baez mirrors on a 35 µm 2 spot and the diffracted photons are detected using a DECTRIS CdTe 4M detector. In this work, all data have been collected at 25 keV which correspond to a wavelength of 0.48975 Å. Before starting the measurement, the appropriate diffraction geometry, wavelength, and sample to detector distance (around 619 mm), is calibrated taking a diffraction pattern of a standard material, such as CeO2 or LaB6. The cell, mounted on a x -y -z motorized stage, is aligned and centered by looking at the transmission profile collected by a photodiode. Since the absorption inside the gasket hole is lower than on the gasket, this procedure allows to center the sample chamber on the beam path. The cell, once aligned and centered, is rotated by ±10° around the axis perpendicular to the x-ray beam during the exposure time to improve the powder average. Measuring the diffraction pattern of the standard (LaB6) allowed obtaining the calibrated wavelengths which resulted to be 0.5790 Å and 0.5802 Å.

CRISTAL beamline

CRISTAL (Crystallography -Cristallographie in

XRD1 beamline

XRD1 is an X-ray diffraction beamline at ELETTRA synchrotron which collects synchrotron radiation ranging from 4 to 21.5 KeV produced by a wiggler. The beam is then collimated by a cylindrical mirror before being send to the monochomator which selects the specific energy. The beam size at sample can be shaped by using precision slits and the detector used is a Dectris Pilatus 2M.

Thanks to the beamline configuration, it is possible to measure samples at ambient condition in capillaries or perform high-pressure experiments using a DAC. The data reported in this work have been collected at about 17 KeV corresponding to a wavelength of 0.7 Å while the sample-to-detector distance was about 247 mm. The calibration of t was performed collecting the XRD pattern of the LaB6 loaded in the DAC.

Data analysis

In this section general procedures used to perform data analysis of infrared and X-ray diffraction data are described.

IR peaks, distinctive of the measured sample, can be described by a peak function characterized by three parameters: location (frequency), height (intensity) and line width. Specific infrared modes have been analyzed with a standard least-square fitting procedure with Gaussian or Lorentzian functions described in equation 2.33 and 2.34:

𝑦 = 𝑦 0 + 𝐴 𝑤√𝜋/2 𝑒 -2 (𝑥-𝑥 𝑐 ) 2 𝑤 2
(2.33)

𝑦 = 𝑦 0 + 2𝐴 𝜋 𝑤 4(𝑥 -𝑥 𝑐 ) 2 + 𝑤 2 (2.34)
where 𝑦 0 is the offset, 𝑥 𝑐 , 𝐴 and 𝑤 the peak center, width and area respectively with corresponding errors as well.

Representative spectral deconvolution obtained with the OriginPro software is shown in Water vapor and CO2 absorptions due to the atmosphere are subtracted from the MIR spectra through the atmospheric correction tool available on the Omnic software provided by Thermo-Fisher which is also used to collect the spectra. This procedure is not available in the FIR range, therefore spectra are smoothed below 400 cm -1 using the Savitzky-Golay method at the fifth polynomial order to remove the residual water vapor lines. An example of the data treatment is reported in Figure 2.29 for the MIL-100(Cr) spectrum collected with CsI as PTM at 0.1 GPa.

When spectra covering both the FIR and MIR range will be shown in the next chapter, it is important to note that a normalization procedure has been applied. MIR and FIR spectra are divided / multiplied by an arbitrary factor to normalize the height of a peak observed in both spectral ranges, i.e.

the region between 600 and 700 cm -1 where the bolometer and the MCT detectors response overlap. In general, lattice parameters can be obtained from 1D patterns performing a diffraction profile fitting technique. One of the most common procedures is the Le Bail's method (or 'pattern matching') which models the diffraction pattern without using a full structural model, i.e. the atomic positions, but only the unit cell parameters. First, it is necessary to enter the initial lattice parameters and the space group of the compound measured and chose a function to model the line profile which can be adjusted though six different parameters. A polynomial function is used to model the continuous background. Unit cell parameters are refined in a standard fitting procedure where the residuals between the observed and model pattern are minimized and the uncertainty range on the obtained values is also calculated. 191 With this method, only the position and shape of the lines are considered neglecting the calculated intensities. Since the process is iterative, numerous cycles are needed to converge to the best parameters. All data analysis have been performed using the Fullprof Suite software. 192 It is important to notice that the compounds studied in this work present really complex structures and the unit cell consists of more than 10000 atoms. [START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] Therefore, a full structural determination based on the Rietveld analysis is complex, if not impossible, and beyond the scope of this work.

MIL-101(CR) AT HIGH PRESSURE

In this chapter the mechanical stability of the mesoporous MIL-101(Cr) will be discussed.

First, a brief overview about the sample at ambient condition is given. The effect of pressure on this system will be then revealed through high-pressure X-ray diffraction and infrared spectroscopy. In particular, the role of the pressure transmitting media will be elucidated.

Characterization of MIL-101(Cr) at ambient conditions

Preliminary characterization at ambient conditions of the activated MIL-101(Cr) is reported in this paragraph. Structural, microstructural and textural properties were determined by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, nitrogen porosimetry and infrared spectroscopy.

Crystalline structure

At ambient conditions, MIL-101(Cr) crystallizes in a cubic structure with space group Fd3 ̅ m and an unit cell containing more than 10000 atoms, as reported by Férey and collaborators. [START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] The measured XRD pattern of the activated sample is shown in Figure 3.1 and it is well compatible with previous reports. The sample was then degassed following the protocol described in section 2.1.1 and placed in a capillary inside a glovebox under a 1 bar of Ar atmosphere to measure the diffraction pattern at the XRD1 beamline of the ELETTRA synchrotron. This pattern is shown in Figure 3.2, together with the Le Bail refinement obtained with the cubic structure, Fd3 ̅ m space group. The unit cell parameter and volume are 88.31±0.03 Å and 689400±100 Å 3 , respectively, consistent with values of 88.87 Å and 701860 Å 3 previously reported. [START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] It is important to remind that even if the capillary was prepared in the glovebox and a the sample was properly degassed, it was necessary to expose the sample to air for less than one minute to seal the capillary outside the glovebox. Therefore, although we cannot exclude the adsorption of minimal quantities of water, its content inside the MOF has been minimized by this procedure.

Microstructure

The microstructural characterization of the activated sample exposed to air was carried out by TEM. As shown in the TEM image in

Thermal stability

Thermogravimetric analysis was performed under dry air on the activated MIL-101(Cr) in order to evaluate its thermal stability and decomposition temperature. The TGA curve plotted in Figure 3.4 presents a first mass loss upon heating up to around 100 °C which is explained as the evacuation of adsorbed water molecules within the pores and corresponds to about the 15% of the total weight. This initial loss is then followed by a slightly tilted plateau where sample mass is roughly constant up to 350°C. At this temperature, a second more pronounced mass loss is detected, marking the decomposition temperature of MIL-101(Cr) and thus the limit of its thermal stability.

Textural properties

The textural properties of the activated MIL-101(Cr) were estimated by nitrogen adsorption/desorption isotherms at 77 K, as shown in Figure 3.5. The isotherm of the dehydrated sample is of type IV and has the typical features expected for mesoporous materials (see Figure 2.5).

The large uptake at very low P/P0 indicates the presence of micropores which are located in the superthetrahedra (see

Vibrational spectrum

Infrared spectroscopy measurements at ambient conditions were performed on the degassed sample in order to identify the main molecular groups of MIL-101(Cr) and the related infrared absorption bands. Moreover, IR is very sensitive to adsorbed species from the atmosphere, in particular H2O, allowing to check the amount of water adsorbed before performing the high-pressure experiments.

The spectrum of the degassed sample isolated in the DAC is shown in Figure 3. UiO-66(Zr), 196 MIL-100(Cr) 139 and MIL-100(Al) 197 allows an identification of the main spectral features. All modes observed are listed in Table 3.1 together with their tentative assignments when possible and a brief description of their appearance in the spectrum. Starting from the FIR range, the peak at 389 cm -1 (labeled ν1) has not been reported before and it is assigned a posteriori to a collective mode, based on its response to pressure (see Section

3.3.2)

. The peak at 589 cm -1 (ν2) is attributed to a CrO vibration. The bands between 700 and 1200 cm - 1 are assigned to the bending of benzene rings, and the most intense bands between 1300 and 1700 cm - 1 to the stretching modes of the carboxylate groups. The broad band in the 2700-3600 cm -1 region originates from OH stretching vibrations of residual water molecules inside the pores and from the MOF hydroxyl groups perturbed by the hydrogen bonding with water. 193,196 On the high-frequency side of this band two peaks emerge, a weak one at 3590 cm -1 (ν28) and another one more intense at 3669 cm -1 (ν29). Based on previous works on other MOFs 193 , in particular on MIL-100(Al) 197 , the first mode can be assigned to the structural Cr-OH stretching while the second originates from water molecules adsorbed on a free CUS site or coordinated with the hydroxyl group. This is confirmed by IR in situ measurements on MIL-101(Cr) performed while pumping and heating the sample by Dr.

Annalisa Paolone at Sapienza University of Rome and presented in Appendix 1. shows that this procedure causes a weight decrease of 30% in the sample. As a triatomic molecule, water in the gas phase presents three infrared active vibrations: symmetric and antisymmetric OH stretching observed at 3657 cm -1 and 3756 cm -1 , respectively and deformation modes at 1595 cm -1 (bending). 193 The presence of adsorbed water molecules in the framework results in the IR spectra as a broad intense band in the 2500-3600 cm -1 frequency region appearing already a few minutes after air exposure, as shown in Figure 3.9. The appearance of this strong signal obstacles the measurement the OH peaks ν28 and ν29 described in the previous paragraph.

Effect of water adsorption in MIL

Moreover, other subtle changes are observed in the MIL-101(Cr) spectrum due to the presence of adsorbed water around 650 cm -1 , 1300 cm -1 and 1700 cm -1 . This characterization then allows discarding DAC loading where the sample spectrum presents these spectral features, thus performing high-pressure measurements only on samples where the IR spectrum is compatible with a minimal adsorbed water content.

X-Ray Diffraction measurements

In this section, high-pressure synchrotron XRD measurements on MIL-101(Cr) with several PTM will be presented and discussed. First, the high-pressure response of the framework with a solid medium, namely NaCl, will be shown. Then, the behavior of the same sample compressed when a fluid is used as medium will be illustrated.

Compression with a solid medium: NaCl

Selected high-pressure XRD patterns of MIL-101(Cr) collected at the CRISTAL beamline (λ=0.5802 Å) with NaCl as PTM are shown in Figure 3.10 in the low 2θ region while the full 2θ range is shown in Figure 3.11. The intensity of the Bragg peaks gradually decreases under pressure up to about 0.17 GPa, where the XRD pattern of MIL-101(Cr) is no longer detectable. This indicates a complete transition to an amorphous phase which is not reversible since the Bragg reflections are not detected in the pattern collected after the pressure release.

In order to track the structural evolution of the sample under pressure, the unit cell parameters were determined at each pressure step through a Le Bail refinement. The cubic Fd3 ̅ m structure was used assuming that this is maintained up to the crystal-to-amorphous transition. The lattice parameter a at the lowest pressure point, i.e. 0.01 GPa, results to be 87.88±0.02 Å. Figure 3.12 shows the pressure dependence of the lattice parameter a up to 0.1 GPa: a initially decreases up to 0.05 GPa and then a plateau is observed. Above 0.07 GPa the refinement is no longer possible due to the complete disappearance of the Bragg reflections.

Compression with fluid media: silicone oil and Nujol

The high-pressure response of MIL-101(Cr) was also evaluated using two different fluid PTM: a high-viscosity PDMS silicone oil and the mineral oil Nujol. As shown in Figure 3. Increasing pressure, Bragg reflections shift toward higher angles up to about 7 GPa, reflecting the lattice compression. Above this pressure, surprisingly, peaks shift towards lower angles while simultaneously becoming broader and less intense. This effect is also observed in the high angle region reported in Figure 3.15 and Figure 3.16. Note that the first reflection observed at around 0.9° is considerably less intense than in the case of the solid PTM (see Figure 3.10). Since the sample consists of a very fine powder, this effect is unlikely due to preferred orientations but point out an effect of the PTM on the framework, as it will be discussed in the following. The enhanced framework stability under pressure observed with fluid PTM, compared to the solid PTM case, strongly suggests the penetration of the fluid into the MIL-101(Cr) pores, [START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF][START_REF] Redfern | Mechanical Properties of Metal-Organic Frameworks[END_REF][START_REF] Mckellar | Structural Studies of Metal-Organic Frameworks under High Pressure[END_REF]129 which is not possible with the solid NaCl. This would also be responsible of the reduced intensity of the Bragg peak around 0.9°. Indeed, a similar effect has been previously observed using CO2 as PTM in Silicalite 199 or in neutron diffraction studies at low temperature performed on zeolites with adsorbed gas molecules. 200 This is probably due to the changes in the electronic density and thus in the structure factor when guests are encapsulated into the pores. [199][200][201] Since the penetration under pressure of small molecules was already proved for other microporous MOFs, the inclusion of Nujol, made of longchain alkanes, was expected since the average molecular size (5-7 Å) is smaller than MIL-101(Cr) pore windows (12-16 Å). 129,202 Therefore, in this study, Nujol is used as benchmark for penetrating medium and observing that using silicone oil as PTM results in a similar but unexpected behavior, suggests that even this fluid can enter into the mesopores despite its high viscosity made of long PDMS chains.

Le Bail refinement was also performed on these data and the lattice parameter and volumes obtained for the lowest pressure points for all the experimental runs are compared in Table 3.2 to those obtained at ambient pressure in a capillary. This comparison shows that when fluid PTM are used, the cubic lattice parameter a results to be larger than the lattice parameter obtained with the pristine MIL-101(Cr) in the capillary and in the DAC with NaCl as PTM. In particular, a increases of 1.07 Å with silicone oil and of 0.82 Å with Nujol compared to the run with NaCl. This indicates that the insertion of fluid PTM already starts when the DAC is initially closed, at a pressure lower than 0.1 GPa, resulting in a slightly expanded lattice. The relative change in the lattice parameter a under pressure is shown in Figure 3.17 showing a threefold increase compression above 0.2 GPa. This behavior is reminiscent of that observed for the Cu-btc framework under pressure, with a fluid PTM of small molecules, such as methanolethanol mixture and isopropyl alcohol (see Section 1.5.4). 125 In analogy with this study, MIL-101(Cr) is less compressible in the low-pressure regime since the PTM progressively penetrates into the cavities until they are hyperfilled with silicone oil at a about 0.2 GPa. After this pressure value, the framework and PTM are compressed together and the whole pressure response can be considered as originating from the composite material. This also explains why MIL-101(Cr) with Nujol does not show the two-regime behavior. The framework is already hyperfilled when the DAC is loaded at pressures lower than 0.1 GPa, owing to the smaller size of alkanes in Nujol. The pressure dependence of the lattice parameter a thus confirms the insertion of Nujol and silicone oil inside the pores of MIL-101(Cr) at high-pressure since the framework is less compressible compared to the "empty-pore" case with NaCl (see Figure 3.17). This behavior agrees with both the slight lattice expansion observed upon the DAC closing and the pressure-induced amorphization shifted to higher pressure when fluids are used. It is important to stress that the obtained bulk moduli reflect the response of the composite material MIL-101(Cr) + PTM, thus it is not characteristic of pure MIL-101(Cr). The bulk modulus of the material obtained is one order of magnitude higher than the one resulting from the "empty-pore" framework pointing out that filling the pores with fluids enhances the mechanical stability of the host system.

Let's now shift our focus to even higher pressures in Figure 3.22, where a is shown as a function of pressure up to 15 GPa, for the two fluid PTM. Initially, the lattice parameter decreases in a similar way for PTM, up to about 7 GPa. At higher pressure, the lattice parameter unexpectedly starts to increase with pressure. This effect is more pronounced for silicone oil than Nujol, indicating a possible link with the molecular size of the PTM. At these pressures, the MIL-101(Cr) structure is quite disordered, as reflected by the XRD patterns shown in Figure 3.14-16, with low-intensity and high-angle Bragg reflections (above about 3°) not detectable anymore, while the main peaks are broad. This points to the nucleation of an amorphous phase coexisting with a crystalline one (which gives the diffraction signal). These two phases and the PTM represent a closed system, whose total volume has to decrease with increasing pressure according to thermodynamics. Therefore, the observed lattice expansion of the crystalline phase is allowed if a strong volume reduction of another component present inside the DAC sample chamber takes place at the same time. The main hypothesis is that the amorphous phase has a higher density because its pores are collapsed, as it has been already observed in other MOFs. 203,204 This creates the space necessary for the expansion of the crystalline phase. The migration of the guest molecules from the collapsed pores of the amorphous into the crystal could also contribute to the expansion, ex-plaining the larger increase of the parameter a observed with silicone oil, as shown in Figure 3.22. This phenomenon, although complex, has been recently proposed to explain a similar pressure-induced expansion in microporous systems. 205,206

Infrared spectroscopy measurements

High-pressure synchrotron-based IR measurements were performed in the 200-4000 cm -1 range in order to further confirm the observed structural transformations, the PTM inclusion and to obtain local and chemical information. In the case of solid PTM, CsI was selected instead of the NaCl The spectral differences induced by the different PTM at low pressure will be now discussed.

Later, the evolution under pressure of MIL-101(Cr) spectra will be shown, discussed and compared with those obtained by XRD.

Infrared spectra with different media before compression

The IR spectra of MIL-101(Cr) loaded inside the DAC at 0.1 GPa are shown in Figure 3.24 in the 200 -3800 cm -1 spectral range, for the three PTM: CsI, Nujol and silicone oil. The comparison between the spectrum collected with CsI and the two with Nujol and silicone oil points out some spectral modifications induced by the fluid PTM. Besides a new peak appearing with Nujol around 1300 cm -1 , the main differences are found between 300 and 650 cm -1 (Figure 3.25(a)) and around 3600 cm -1 , in the OH stretching region (Figure 3.25(b)).

The collective mode at 389 cm -1 (ν1), the CrO mode at 589 cm -1 (ν2) and the ν28-ν29 OH modes all appear broader and shifted to lower frequencies by the presence of fluid PTM. To quantify this shift, the ratio between the frequencies of the most intense peaks with CsI and Nujol is calculated and plotted in Figure 3.26, labeled as in Table 3.1. when CsI is used as PTM are listed in the table and numbered as in Table 3.1.

It is apparent that only the aforementioned peaks ν1 ν2, ν28, and ν29 have a relative shift larger than -0.6% as highlighted by the threshold line in

Compression with a solid medium: CsI

High-pressure IR spectra of MIL-101(Cr) measured with CsI in the two relevant spectral ranges where the ν1 ν2, ν28, and ν29 modes appear are shown in Figure 3.27, whereas the whole frequency range is shown in Figure 3.28.

The collective mode around 389 cm -1 and the intensity of the two OH modes ν28 and ν29 at 3590 and 3669 cm -1 become weaker as pressure increases without an apparent frequency shift, until they completely disappear at about 0.4 GPa. The CrO peak at 589 cm -1 shows a clear softening and broadening up to 0.4 GPa, while little effect is observed at higher pressures. The pressure-induced spectral changes are not reversible after the pressure release. This behavior is consistent with the irreversible crystalline-to-amorphous transition observed in the XRD measurements (see Figure 3.9), and it confirms that the peak at 389 cm -1 is a collective vibrational mode since it disappears when the long-range order is lost. Regarding the OH stretching region between 3500 and 3800 cm -1 shown in Figure 3.27(b), the intensity of the ν28 and ν29 peaks also decreases as pressure increases without a clear shift and around 0.4 GPa the peaks fade into the broad band below 3700 cm -1 . This can be ascribed to the increased interaction of the terminal OH bonds with the surrounding environment. Similarly to ν1 and ν2 modes, the changes induced by the pressure to the spectrum are not reversible. At first sight, any dramatic change is observed between 650 and 1800 cm - 1 besides the weakening of the overall spectrum intensity which is probably linked to the amorphization. It is possible to notice that the amorphization pressure determined by IR data is slightly different from that identified by XRD, 0.4 GPa instead of 0.09 GPa. However, a more detailed analysis on selected vibrational modes, will provide further insights, as discussed in section 3.3.4 in comparison with the results obtained on the same modes with the fluid PTM.

Compression with fluid media: silicone oil and Nujol

With fluid PTM, a common behavior is observed in both the FIR and OH spectral regions (Figure 3.29 (a)-(b) for silicone oil, (c)-(d) for Nujol). The whole MIR range is showed for Nujol in Figure 3.30 whereas it is not reported with silicone oil due to the strong peaks it presents in this region (see Figure 3.23 and 3.24) The collective mode ν1 around 389 cm -1 is not detectable with silicone oil due to the strong PTM absorption at this frequency, but it can be measured with Nujol. In this case the peak hardens and broadens under pressure but it is present over the whole pressure range. The CrO vibration (ν2 ~ 590 cm -1 ), accessible with both Nujol and silicone oil, also hardens and broadens increasing pressure. In the OH stretching region both ν28 and ν29 show a remarkable and unexpected pressure-induced softening, along with a line-shape broadening. Therefore, with fluid PTM, both FIR and OH modes have an opposite behavior than with CsI.

The presence of the collective mode up to 15.3 GPa with Nujol and the lack of new peaks appearing, confirms XRD data and demonstrate the enhanced framework stability at high-pressure with a fluid PTM. Moreover, IR spectroscopy shows that the chemical bonds are locally preserved. In particular, the OH modes are detected in the whole pressure range explored, although shifted for the guest-host interactions arising between the inserted fluid and the framework. This behavior suggests that in the CsI case, ν28 and ν29 peaks probably disappear for the high degree of structural distortion at the local level which causes the approaching of the hydroxyl groups to the neighboring atoms. Peak broadening witnesses the pressure-induced disorder although a complete amorphization is not reached in the IR experiments with fluid media. In fact, releasing the pressure, the ν1, where detected, and ν2 modes shift back to the starting position and their widths narrow resembling the original peak shapes.

A similar behavior is observed in all the other MIR detectable modes shown in Figure 3.30 and the OH stretching modes reported in Figure 3.29(b-d) although the spectrum still appear slightly broader compared to the ambient condition spectrum due to the hysteresis of the pressure. This is in well agreement with XRD results, which showed that MIL-101(Cr) cubic structure is still present, although stressed, between 11 and 13.2 GPa with silicone oil and at 15.3 GPa with Nujol.

Comparison of the IR results for the three PTM

In order to stress the different behavior of MIL-101(Cr) under pressure when a solid or fluid PTM is employed, a Lorentzian least-square peak fitting procedure has been performed on the most significant IR peaks, providing peak frequencies and full widths at half maximum (FWHMs). In particular, attention is focused on the following peaks: the collective mode ν1 and the CrO vibration ν2 below 600 cm -1 , the C-H benzene ring mode ν9 at about 880 cm -1 , and the ν28 and ν29 OH stretching modes around 3600 cm -1 .

The pressure dependence of the ν1 and ν2 frequencies are compared in Figure 3.31(a)-(b) in the low-pressure range (0-1.1 GPa). In the presence of CsI, the collective and CrO modes soften while for the two fluid PTM a clear hardening is observed. Moreover, the two peaks broaden when MIL-101(Cr) is compressed with a solid PTM, as shown in Figure 3.32(a) and (b) while with a fluid, the FWHMs of ν1 and ν2 are nearly constant in the pressure range considered, i.e. 0-1.1 GPa. From XRD data, we have shown that pristine MIL-101(Cr) undergoes amorphization below 0.1 GPa (see section 3.2.1). The bond instability proved by the anomalous pressure dependence of ν2 frequencies is linked to the transition to the amorphous phase. Indeed, when the long-range order is still present and the crystalline structure is stable, as XRD data show in the fluid PTM case (see section 3.2.2), the ν1 and ν2 peaks frequencies harden, usual behavior observed as a result of bond-stiffening in crystalline system under pressure. 157 Moreover, the collapse of the structure leads to atoms approaching causing local interactions and anharmonic effects as supported by the observed peaks broadening. The fitting procedure of experimental data also provides the pressure evolution of the peak heights, a quantity that can be used to track the suppression of the collective mode. In particular, the heights ratio between the CrO mode (ν2) and the collective mode (ν1) is shown in Figure 3.33 for CsI and Nujol as PTM. This quantity is about 1.5 at 0.1 GPa, i.e. in the crystalline phase in both cases.

However, when increasing pressure using Nujol, the ratio stays constant over the whole pressure range shown, i.e. 0-1.1 GPa. On the contrary in presence of a solid PTM, it immediately starts to increase with rising pressure until the ν1 mode disappears. Therefore, the departure from the threshold value of although less drastic than ν2 since the changes in the peak frequencies are below 2 cm -1 . Regarding the FWHMs, peak broadening is observed with both PTM but with CsI the effect is more pronounced.

This proves that also the organic linker is sensitive to the guest inclusion and the behavior of this mode under pressure provides an additional evidence that after the amorphization the short-range order is lost although the chemical bonds of the framework organic components are not broken and still detected at 0.6 GPa (see Figure 3.28). The results of the lineshape analysis for two other modes assigned to vibration of the organic counterpart, i.e. ν5 assigned to the C-C bending 193 and ν10 originating from the C-H deformations, are reported in Appendix 2 and shows similar trends but on a smaller scale (changes below 1 cm -1 ). Error bars are within the symbol size.

Regarding the OH region, the pressure effect on the frequency of the ν28 and ν29 peaks with the three PTM is shown in Figure 3.35(a) and (b), respectively. Note that when silicone oil is used, these two peaks merge in a unique spectral feature that is identified with ν29 for its initial frequency and stronger intensity. When CsI is used as PTM, the ν28 and ν29 vibrations stay constant before being totally suppressed by the pressure and above 0.4 GPa merge in the broad band observed below 3600 cm -1 . With the fluid PTM, both modes soften in the pressure range considered although a slightly different trend is observed for Nujol and silicone oil in the ν29 peak. The continuous and apparent softening of the two OH modes with a fluid PTM indicates that the PTM-MOF interaction are effectively and continuously tuned by pressure, when PTM and Cr-OH /Cr-H2O groups get closer under pressure. Even if the overall behavior under pressure of MIL-101(Cr) observed with XRD is similar using Nujol and silicone oil, the IR spectrum differs for the two fluid PTM especially in the OH frequency region. This highlights that at local level the different nature of the inserted fluids leads to different interactions between the included molecules and the host framework. Indeed, the structural hydroxyl groups point inside the pores and result to be important probes for the phenomena arising from the guest molecules and the host frameworks making ν28 and ν29 behavior under pressure particularly meaningful to study guest-host interactions.

Finally, it is worth noting that the CrO and the ligand vibrational peaks, shown in Figure 3.27(a) and 3.29(a)-(c), are still detected at the highest pressures reached regardless the PTM used.

Therefore, the MIL-101(Cr) building units, i.e. the chromium cluster and terephthalic acid, are stressed by the pressure treatment but still preserved even when the MIL-101(Cr) becomes completely amorphous.

Conclusions

In this chapter, we discussed the effect of high-pressure on the mesoporous MIL-101(Cr) using different (fluid and solid) pressure transmitting media. 209 The high-pressure behavior of emptypore MIL-101(Cr) can be studied when a solid PTM is used. Indeed, with NaCl a crystal-to-amorphous transitions is completed at 0.1 GPa, where the whole XRD pattern disappears. This was confirmed by the suppression of the IR-active collective and OH stretching modes at a similar pressure and stressed by the pressure dependence of the central peak frequency of several modes of both inorganic and organic components. Moreover, we found that the ratio between the heights of the peaks assigned to the chromium oxide stretching and the collective mode represent a reliable spectroscopic marker of the amorphous phase formation. From now on, this quantity could be used to determine whether sample preparations and applications involving a compression transform MIL-101(Cr) into an amorphous phase without resorting to XRD measurements. It is important to stress that the amorphous MIL-101(Cr) phase here mentioned in this work could be metastable, i.e. not the true thermodynamically stable phase at room temperature and high-pressure. To clarify this point, it would be important to perform a high-pressure and high-temperature study to check the stability of this phase in the PT phase diagram. It is important to notice that no sharp and intense diffraction peak of the fcc Pd structure can be identified, despite the high metal loadings. Only a board and weak contribution is noticed close to the expected Pd (111) main diffraction peak. The lack of sharp peaks and the broad shape of this feature due to the inserted NPs can be explained by the very short coherence length of ultra-small NPs. 210 This also demonstrates the absence of undesired large NPs agglomerates outside the powder grains.

Chemical composition

The Pd concentration of the composites was checked by inductively coupled plasma optical emission spectroscopy (ICP-OES) and the results (Table 4.2) are well in agreement with the nominal expected values. As already done for pristine MIL-101(Cr) (paragraph 3.1.5) the intensity of the broad band centered at 3100 cm -1 , ascribed to residual water, has been used to check whether the sample adsorbed water or not during the storage or the manipulation.

X-Ray Diffraction measurements

Hereinafter, the effect of pressure on the crystal structure of 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) will be discussed and the results will be compared to the previous ones obtained on the pristine MIL-101(Cr). Since the role of the PTM was elucidated in Chapter 3 and the fluid penetration into the pores was proved for MIL-101(Cr), the discussion will be particularly focused on the high-pressure response of x-Pd@MIL-101(Cr) composites when NaCl is used as PTM, since it does not penetrate the framework. XRD measurements at high-pressure of Pd loaded samples with silicone oil as medium will also briefly presented.

Compression with a solid medium: NaCl

The high-pressure XRD data were collected on the CRISTAL beamline at a wavelength Note that 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) have a background increasing at low 2θ, at variance with the original MOF (Figure 3.10). This is attributed to the strong small-angle scattering caused by the presence of ultra-small Pd NPs. 210,212 When increasing pressure, Bragg reflections weakens becoming no longer detectable above a sample-dependent pressure value and they are not restored after the pressure release, thus indicating an irreversible pressure-induced amorphization for both composites (see Figure 4.6(a-b)). The loss of the XRD patterns occurs around 0.25 GPa and 0.32 GPa for 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr), respectively. Although this is the clearest effect, peak shift, corresponding to the lattice compression, is also observed and will be tracked though data analysis later in this chapter. The evolution of these XRD patterns is qualitatively analogous to the one observed for the pristine MIL-101(Cr) and discussed in section 3.2.1 (see Figure 3.10(b)). However, the patterns of MIL-101(Cr) vanished at 0.17 GPa, i.e. at lower pressure as compared to x-Pd@MIL-101(Cr) composites. For the three samples, the unit cell volume shows a qualitatively similar behavior under pressure, with an initial decrease then followed by a plateau. At higher pressures, the refinement is not possible due to the complete disappearance of Bragg reflections. From The bulk modulus clearly increases with the metal loading, reaching a value about five-times larger for the highest Pd content, i.e. 35-Pd@MIL-101(Cr). The pressure value where the volume reduction is over and the volume start to be constant under pressure is identified as the amorphization onset. In fact, as discussed in Chapter 3, the end of the volume compression on the crystalline phase, which occurs when its Bragg reflections start to disappear, implies the concomitant nucleation of the amorphous phase. 209 The amorphization onset clearly shifts to higher pressure with increasing Pd The transition to the amorphous phase is then completed at slightly higher pressure, when the XRD patterns are not present anymore, as previously discussed. Finally, once the crystalline order of the porous host is lost, any Bragg reflection is detected for bulk Pd which means that NPs do no agglomerate and are still well dispersed in the MOF's amorphous phase (see Figures 4.7 and 4.8).

Compression with a fluid medium: silicone oil

High-pressure XRD measurements of x-Pd@MIL-101(Cr) composites have been performed also with a fluid PTM, namely silicone oil. In this paragraph, the behavior on compression of these composites will be discussed in the low-pressure range (0-1 GPa) and afterwards the high-pressure response will be reported up to 14 GPa for only one composite, 20-Pd@MIL-101(Cr).

High pressure range

The high-pressure response of 20-Pd@MIL-101(Cr) with silicone oil was also evaluated up to 14 GPa. Selected XRD patterns collected at the XRD1 beamline of ELETTRA synchrotron (λ=0.7 Å) are shown in Figure 4.17 The evolution under pressure of the crystalline structure of 20-Pd@MIL-101(Cr) shows a common behavior with the one observed for pristine MIL-101(Cr). On compression, Bragg peaks shift toward higher angles up to about 9 GPa meaning that the lattice is shrinking. Thereafter, peaks shift towards lower angles while simultaneously becoming broader and less intense. After pressure release, the XRD pattern at 1.2 GPa resembles the broad profile measured at the highest pressure but shifted to lower angles, meaning that the original crystallinity is not re-established, and the system is mostly amorphous even if some long-range order is still present.

The pressure dependence of the unit cell volume, as obtained through a Le Bail refinement, is compared to that of MIL-101(Cr) in In both cases, with and without Pd NPs, the unit cell volume decreases under pressure to further increase again after a certain pressure value. However, in the case of 20-Pd@MIL-101(Cr) the minimum volume value is reached at higher pressure, i.e. around 9 GPa, compared to the pristine framework whose volume decrease ends around 7 GPa. Although, as stated before, the high-pressure behavior is deeply affected by the PTM incorporation, this result suggests that the presence of the NPs within the pores delays the amorphization onset.

TEM images collected on the recovered samples are reported in Appendix 3. Note that crystallites cannot be seen anymore, and well-defined pore channels are lacking. This observation is compatible with the agglomeration of the crystallites into a larger amorphous material inside the DAC.

It is also interesting that Pd NPs are still well detectable and dispersed in this amorphous phase.

Infrared spectroscopy measurements

XRD measurements revealed that the high-pressure response of x-Pd@MIL-101(Cr) composites when a fluid PTM is employed is deeply affected by the pore filling with the PTM, as also noted for pristine MIL-101(Cr). Therefore, for high-pressure synchrotron-based IR spectroscopy, we will focus mainly on the solid PTM case, namely CsI. IR measurements performed on the 35-Pd@MIL-101(Cr) with fluid PTM will be briefly discussed later. The spectrum collected when the pressure was released is shown in green. In analogy with the behavior of the pristine framework (see section 3.3.3), the CrO mode ν2, detectable with both PTM, blueshifts in the pressure range explored, i.e. 0.1-1 GPa. The same effect is observed on the collective ν1 mode, accessible when Nujol is used and reported in panel (a). At variance with CsI case, the ν1 peak is still detected above 0.4 GPa. This confirms that the amorphization is shifted to higher pressure by the PTM penetration within the pores of the composite material and this is consistent with the XRD results reported in section 4.2.2. After the pressure release, the peak positions shift back to the values initially observed and the original peak shapes are also recovered, thus demonstrating that the pressure-induced deformations on the 35-Pd@MIL-101(Cr) composite are reversible.

The pressure dependence of the peak frequency for the ν1 and ν2 modes 35-Pd@MIL-101(Cr), as obtained from the fitting procedure, is compared in

Nanoparticles effect: conclusions

In this chapter, we have studied the effect of pressure on the mesoporous MIL-101(Cr) functionalized with Pd nanoparticles up to 35 wt.% exploiting two complementary synchrotron-based techniques, X-ray diffraction and infrared spectroscopy. Such high metal loadings are achieved through a double solvent synthetic method and confirmed by laboratory techniques. In the case of a solid PTM, XRD and IR data consistently show that embedding Pd nanoparticles inside the pores of the framework results in an increased stability of the framework itself. This phenomenon is likely due to steric effects of the Pd nanoparticles inside the pores since the stability range and the bulk moduli of the framework increase proportionally to the Pd loading, i.e. increasing the pore filling with NPs.

In chapter 3, we shown that the insertion of polymers into the pores of the pristine MIL-101(Cr) enhanced the framework stability by one order of magnitude but the control on the pore filling was lacking. 213 The present strategy, with its fine control of the metal loading inside the MOF, allows tuning, as demonstrated, the mechanical properties of the framework, indicating that the pore volume is gradually filled by Pd NPs increasing the metallic loading. The pressure amorphization value shifts from about 0.1 GPa, for the pristine MIL-101(Cr), to around 0.3 GPa for the highest metal loading, i.e.

Microstructure

Textural properties

Nitrogen porosimetry was performed at 77 K on the as-synthesized sample to estimate its textural properties. The obtained isotherm, shown in Figure 5.4, is between type I and IV (see section 2.1.3.3) and a slight secondary uptake is observed, indicating the presence of both micro-and mesopores. The BET surface area and the specific pore volume determined from experimental data are 1679±10 m 2 g -1 and 0.92±0.01 cm 3 g -1 respectively. In Figure 5.5, the pore size distribution calculated thought DFT is reported and it reveals the presence of different pore sizes ranging from 6 to 24 Å.

Thermal analysis

Thermal stability of MIL-100(Cr) was determined by thermogravimetric analysis (TGA). As shown in Figure 5.6, three steps of the weight loss were observed as function of temperature: the first (~10% of initial weight) between 25 and 100°C and the second, an additional 15% loss of the initial proceed to an assignment of the observed peaks, besides those below 500 cm -1 which are here reported for the first time and assigned to collective modes similarly to the MIL-101(Cr) spectrum. The bands in far-IR region, i.e. below 500 cm -1 , correspond to collective modes. The peaks at 527 cm -1 (ν4) and 664 cm -1 (ν5) are attributed to CrO vibrations of the metallic clusters. The mid-IR range (700-3800 cm - 1 ) is dominated by the organic linker internal vibrations. In particular, the region between 700 and 1200 cm -1 shows bending modes of the benzene rings and the peak at 1028 cm -1 is assigned to a ring deformation. The most intense bands between 1300 and 1700 cm -1 originates from the stretching vibrations of the carboxylate groups while the peak at 3088 cm -1 from the CH stretching of the aromatic rings. The latter emerges from the broad band due to the OH stretching vibrations of residual water molecules inside the framework's pores. At higher frequencies, three main peaks are detected: the first one at 3585 cm -1 (ν26) is attributed to the OH stretching mode of the structural hydroxyl group bound to the chromium atoms while the other two features at 3617 and 3675 cm -1 (ν27 and ν28) to the OH stretching mode of coordinated water molecules. It is worth noting that the IR profile between 3500 and 4000 cm -1 strongly depends on the hydroxylation degree of the sample, as reported in a previous infrared study performed by Vimont and collaborators. 139 In particular, the intensity, position and shape of ν27 and ν28 are affected by the amount and species of coordinated water. In Table 5.1 the main peaks observed are listed along with their tentative assignment.

X-ray diffraction measurements

The effect of pressure on the MIL-100(Cr) crystalline structure was evaluated using both NaCl and silicone oil as PTM. The first experimental run was performed at the CRISTAL beamline (λ=0.5790 Å) while XRD patterns with silicone oil were collected on the PSICHE beamline (λ=0.48593 Å) of SOLEIL synchrotron. Selected diffractograms collected increasing pressure are reported in Figure 5.8. In the NaCl case (panel (a)), the intensity of the Bragg peaks gradually decreases on compression up to about 0.6 GPa. This indicates a complete transition to the amorphous phase which is not reversible after the pressure release. On the contrary, when silicone oil is used as medium (panel (b)), the main XRD peaks are still detectable up to around 18 GPa pointing out that the framework is still partially crystalline at this pressure value. The pattern collected at the minimum pressure obtained after the pressure release, i.e. around 3.8 GPa, resembles those measured at the highest pressure meaning that the original order is not re-established. However, a complete transition to an amorphous phase is not observed with a fluid PTM in the pressure range explored. Bearing this in mind, we performed a fit to the unit cell volume data with a second and thirdorder Birch-Murnaghan equation of state up to 0.4 GPa, as shown in Figure 5.10. In the case of a second-order EoS fit (dashed red line), the bulk modulus results to be K0 = 9.0±0.4 GPa. Although the fit converged, the curve obtained does not perfectly match the experimental points. For this reason, we also performed the fit using the third-order equation (dotted red line) which gave a bulk modulus K0 = 12.0±0.4 GPa and its first derivative Kp = -8.6±1.4 GPa. A negative value for Kp indicates that the framework becomes more compressible on increasing pressure and it is an unusual effect for ordinary solid systems. However, a similar response has been previously reported for ZIF-8. 128 The authors proposed that this could originates from the bending of the organic linker and may be a widespread phenomenon for porous materials since the volume shrinkage at low pressure could not be sufficient to cause unfavorable close contact among atoms. 128

Infrared spectra with different media before compression

As already discussed in the previous chapters, CsI was used instead of NaCl as solid PTM due to its larger transmission window in the IR while the silicone oil strong absorption masks the sample signal especially in the MIR range (see Figure 3.23 for silicone oil spectrum). Figure 5.12 shows the comparison between MIL-100(Cr) spectra collected with the two PTM at 0.1 GPa. It can be noticed that the most intense peaks are saturated with CsI and the accessible range with silicone oil is limited. For these reasons, the discussion will focus only on the far-IR and OH regions, shown in

IR spectra at high-pressure

High-pressure IR spectra of MIL-100(Cr) in the FIR and OH regions are reported in Figure The trend changes observed for ν2 and ν4 frequencies under pressure can be also observed for the peak widths at similar pressure values, as can be seen in Figure 5.16(c)-(d). Indeed, the ν2 and ν4

FWHMs show two slope changes at about 0.4 and 0.8 GPa. The frequency blueshift observed for the ν4 mode can be related with the transition to the amorphous phase during which the increased interaction with the adjacent atoms weakens CrO bonds. Usually, an abrupt linewidth increase under pressure can be related to the occurrence of a phase transition which modifies the network of the interactions, such as in the case of the amorphization. [START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON[END_REF] The IR data confirms that the pressure induced transition to the amorphous phase starts around 0.4 GPa and it is completed around 0.8 GPa, in agreement with the results obtained by XRD (see section 5.2).

The spectral analysis also provides the peak heights as function of pressure and it is possible to calculate the h(ν4)/h(ν1) ratio, as done for the other samples studied in this work. Note that MIL-100(Cr) has two collective modes that could be used to calculate this ratio. However, only ν1 can be considered since it is detectable when silicone oil is used. The peak ratios, calculated for the solid and fluid PTM cases and compared in Figure 5.17, have a value around 5 at 0.1 GPa. This value is mantained with silicone oil in the pressure range here explored. With a solid PTM instead, the ratio starts to increase above 0.7 GPa, meaning that the intensity of the collective mode decreases compared to the intensity of the ν4 peak. The abrupt change in the trend occurs when the transition to the amorphous phase is completed and it is not observed in the fluid medium cases. For this reason, ν1 can be assigned to a collective mode since its pressure response is directly linked to the amorphization and differs from that observed for the ν4 vibration. This effect has been already reported for pristine MIL-101(Cr), as discussed in section 3. To have complementary information on the high-pressure behavior of ν2, FIR spectra of MIL-100(Cr), reported in Appendix 4, were also collected using Nujol as PTM. Indeed, this fluid allows to access all the vibrational modes of the framework below 700 cm -1 (see Figure 3.23 for Nujol IR spectrum). In this case, the ratio between the heights of ν4 and ν2 can be calculated and compared to the same quantity obtained when CsI is used as PTM, as shown in Figure A.9. However, the h(ν4)/h(ν2) ratio gradually increases with Nujol and does not show clear differences between the solid and fluid thus hindering the use of this quantity as a marker for the transition to the amorphous phase. This also suggests that ν2 does not have a full collective character, directly probing the long-range order, as the ν1 mode. Further investigation and computational studies are needed to provide a detailed assignment of the vibrational modes.

The lineshape fitting procedure of experimental data was also carried out on the OH frequency range in the case of the solid PTM. Particular attention is put on the ν26 peak assigned to the structural hydroxyl group linked to the chromium atoms of the metallic cluster. 139 Finally, it is important to stress that the small discrepancy in the pressure values for the end of the amorphization, i.e. 0.6 GPa in XRD patterns and 0.8 GPa in IR data, probably originates from the higher sensitivity of IR to the local environment and the observed delay suggests that short-range order is still present after the amorphization, between 0.6 and 0.8 GPa, when it is completely lost.

Comparison with MIL-101(Cr)

MIL-101(Cr) and MIL-100(Cr) have similar cubic structures with the same topology consisting of microporous supertetrahedral units linked by rigid carboxylate ligands: terephthalate and trimesate, respectively. The presence of a different linker results into different pore sizes and windows which in turn is reflected on the internal surface areas and total pore volumes: MIL-101(Cr) displays larger pores and higher total pore volume compared to MIL-100(Cr). To summarize, the structural, textural and mechanical parameters for the samples here studied are reported and compared in Table 5.2. 3549±10 1679±10 Total pore volume (cm 3 g -1 )

1.72±0.01 0.92±0.01 Bulk modulus 2 nd BM (GPa) 1.1±0.9 9.0±0.4 Amorphization pressure (GPa) 0.1 0.6

The comparison of the results obtained during this thesis work on the mechanical properties of these two frameworks shows that MIL-101(Cr) and MIL-100(Cr) behave qualitatively similarly when an external pressure is applied. When a solid medium as NaCl is employed, an irreversible amorphization of the empty structure occurs for both systems while fluid media such as silicone oil and Nujol can penetrate into the pores enhancing the structural stability of one order of magnitude.

However, a closer inspection reveals quantitative differences especially in the case of a solid PTM:

MIL-100(Cr) bulk modulus is considerably higher, almost 9 times in the case of the second-order equation of state fit, and the amorphization occurs at higher pressure, i.e. it is completed around 0.6

GPa for MIL-100(Cr) instead of 0.1 GPa for MIL-101(Cr). This is particularly noticeable when the normalized unit cell volumes of the two frameworks are compared under pressure, as in Figure 5.19. This evidence clearly points out that MIL-100(Cr) is more resistant to compression than MIL-101(Cr). In order to give a tentative explanation to this result, we need to recall the differences between the two frameworks.

As stated in Table 5.2, MIL-100(Cr) presents lower porosity compared to MIL-101(Cr).

Indeed, the total pore volume is 0.92 cm 3 g -1 for the first sample and 1.72 cm 3 g -1 for the second one.

The two frameworks have identical mtn topology with 6-connecting SBUs and two kinds of mesoporous cages [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF][START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] , but different linkers: BDC and BTC linkers for MIL-101(Cr) and MIL-100(Cr), respectively. [START_REF] Kim | Topology Analysis of Metal-Organic Frameworks Based on Metal-Organic Polyhedra as Secondary or Tertiary Building Units[END_REF] At odd than BDC, BTC is a polytopic linker meaning that it connects more than two SBUs, as in the MIL-100(Cr) structure (see Figures 1.27 and 1.28). [START_REF] Li | Topological Analysis of Metal-Organic Frameworks with Polytopic Linkers and/or Multiple Building Units and the Minimal Transitivity Principle[END_REF] The schematic representations linkers. [START_REF] Kim | Topology Analysis of Metal-Organic Frameworks Based on Metal-Organic Polyhedra as Secondary or Tertiary Building Units[END_REF] Redfern and coworkers proposed that in the hypothesis that the primary source of lattice rigidity is due to the steric repulsion of the framework components, the presence of larger void space could host greater distortions and in turn lead to amorphization. [START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF] However, previous experimental and theoretical studies proved that also the length and shape of the linker affect the framework stability. [START_REF] Burtch | Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications[END_REF][START_REF] Redfern | Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series[END_REF]218,219 The influence on the mechanical stability of the ligand connectivity has never been explored before and the comparison between MIL-101(Cr) and MIL-100(Cr) could be a first study in this direction. Our results suggest that the polytopic nature of the BTC linker and the higher interconnection between the metal SBUs in MIL-100(Cr) are key factors to achieve higher mechanical stability in porous frameworks. However, totally decoupling the ligand connectivity and the textural properties, in particular the total pore volume, is not possible. Therefore, the higher mechanical stability of MIL-100(Cr) compared to MIL-101(Cr) likely originates from the organic linker substitution in the structure which is also the root cause for the different textural properties and degree of porosity.

CONCLUSIONS

In this Ph.D. thesis, we investigated the mechanical stability of two prototypical mesoporous Metal-Organic Frameworks (MOFs), i.e. MIL-101(Cr) and MIL-100(Cr), and of two composites based on MIL-101(Cr) with embedded Pd nanoparticles, revealing the pressure-induced modification of their structural and vibrational properties. These frameworks present similar cubic crystalline structure with quasi-spherical pores made up from trimeric chromium oxide units combined with 1,4-benzenedicarboxylate and 1,3,5-benzenetricarboxylate as linkers, respectively.

Since high-pressure studies on mesoporous MOFs were missing in the literature before this work, we first focused on finding the experimental conditions needed to obtain reliable and reproducible results. We evaluated the suitability of several PTM assessing the hydrostaticity of selected alkali halides and we proved that NaCl and CsI provide acceptable hydrostatic conditions in the lowpressure range. Moreover, being solid, there are no doubts they are non-penetrating media even for mesoporous systems, ensuring the absence of any guest related effect in high-pressure response of the MOF. Thereafter, we selected two fluids PTM: a silicone oil made by large PDMS polymers chains, a priori considered as non-penetrating for microporous frameworks, and Nujol whose small molecules (around 5 -7 Å) make it instead representative for penetrating media. We paid also particular attention to the sample preparation and to the diamond anvil cell loading procedure.

Indeed, we adopted a thorough preparation with samples properly degassed and avoiding air exposure at all the stages, at variance with previous results on other MOFs where different unclear preparation routes led to scattered results. Exploiting two complementary techniques, such as X-ray diffraction and infrared spectroscopy, provided a clear and coherent picture about the intrinsic mechanical stability of all the systems studied and the eventual guest role. In the following, we summarize the main results trying to further discuss them and to draw some general conclusions.

Using a solid PTM, MIL-101(Cr) undergoes a crystal-to-amorphous transition below 0.1 GPa, as proved by the suppression of the Bragg reflection peaks in the XRD pattern, the disappearance of representative modes, such as the OH bands, and the discontinuity in the pressure dependence of peak widths, heights and frequencies in the IR spectrum. In particular, the pressureinduced disappearance of a peak in the far-IR spectrum never reported before allowed us to assign such peak to a collective mode. On the contrary, XRD and IR data proved the insertion of PDMS polymers, unexpected due to their large size, as confirmed by using Nujol as a control. This results in a new composite material showing enhanced framework mechanical stability of one order of magnitude, up to around 7 GPa. Therefore, we demonstrate that high-pressure allows the incorporation of PDMS silicone oil inside the large pores of MIL-101(Cr), proving that hybridization of MOFs with polymers can be achieved by industrially accessible pressure treatments, in addition with widely reported chemical synthetic methods. This could be of interest for very large-molecules segregation, separation and other technological applications made possible by mesoporous MOFs.

Thereafter, we successfully embedded well dispersed Pd nanoparticles within the pores of MIL-101(Cr), following a double solvent impregnation method with two different metal loadings: 20 and 35 wt.%. In particular, performing two consecutive impregnations allowed the preparation of Pd nanoparticles with average size of 1.6 nm for very high metal loading (35 wt.%), one of the highest reported so far for such small nanoparticles. This method provides fine control on the pore filling which was not possible with the PTM inclusion. We then studied the high-pressure response of these composites named x-Pd@MIL-101(Cr) (x=20, 35 wt.%) from both the structural and vibrational point of view, with particular focus on the solid PTM case.

The control of the metal content allowed us to demonstrate that Pd nanoparticles tune the mechanical stability of MIL-101(Cr), with a bulk modulus and a crystalline-amorphous transition pressure increasing with the Pd loading. Further confirmation of the mechanical stability enhancement is provided by the analysis of the FIR spectra, where the evolution of peak frequencies and intensities under pressure presents strong signatures at the crystal-to-amorphous transition. This enhancement is attributed to the nanoparticles steric hindrance whereas the presence of host-guest chemical interactions is ruled out by infrared spectroscopy. Pd@MIL-101(Cr) composites thus result to be better candidates, compared to pristine MIL-101(Cr), for post-synthetic modification involving compression. The enhanced stability of Pd@MIL-101(Cr) composites against the amorphization also represents new evidence for the effective presence of Pd nanoparticles inside the pores which is often deduced exclusively from the compatibility of the average NPs size with the available pore diameters.

Finally, we studied the mechanical stability of the pristine MIL-100(Cr) which, in the case of solid PTM, undergoes amorphization at 0.6 GPa, higher than the pressure value of 0.1 GPa observed for MIL-101(Cr), showing that MIL-100(Cr) presents higher mechanical stability than MIL-101(Cr). Accordingly, MIL-100(Cr) exhibits a considerably higher bulk modulus, about ten times greater than the one obtained for MIL-101(Cr). These two MOFs have same topology but the presence of a linker with different connectivity, i.e. two for MIL-101(Cr) and three for MIL-100(Cr), culminates in different porosity and pore size. Although the linker connectivity and the porosity are inevitably correlate, our results suggests that the presence in the structure of a polytopic linker provides higher mechanical stability to MIL-100(Cr). Further studies should aim to study MOFs with different linker connectivity but similar pore volume, in order to disentangle these two parameters and explore how the first affects the mechanical stability.

In perspective, further studies on the adsorption properties at high-pressure of these frameworks, both the pristine MOFs and the Pd@MIL-101(Cr) composites, are planned. Indeed, the scientific community is actively engaged in the discovery of new functional materials for the capture of polluting gases (CO2) or the storage of clean energy (H2, CH4) and MOFs are among the most promising materials for these applications. However, it has not been investigated yet if and how the adsorbed gas molecules affect the mechanical stability of such complex systems. Indeed, guest molecules adsorbed on the internal surface of the pores could affect the mechanical properties of MOFs through chemical interactions or their steric hindrance. In order to investigate these aspects, in the final stages of this Ph.D. thesis, we performed preliminary experiments on the structural stability of pristine MIL-101(Cr) exposed to liquid CO2 cryogenically loaded in the DAC exploiting a pressurized vessel surrounded by liquid nitrogen. In this experimental condition, liquid CO2 plays the double role of PTM and guest being both around and inside the sample within the pores. In Figure 1, the XRD patterns of MIL-101(Cr) increasing pressure with liquid CO2 are shown up to 4 GPa while Figure 2 shows the patterns collected on decompression. Panels (a) of Figures 1 and2 show that Bragg peaks of MIL-101(Cr) are detected up to 4 GPa and the crystallinity of the framework is retained under pressure. This is due to the penetration of CO2 molecules within the pores that, in analogy with the fluid PTM cases, enhances the framework mechanical stability. Note that at the lowest measured pressure, i.e. 0.4 GPa, we observe a broad contribution between 8° and 12° which is assigned to the diffracting signal of liquid CO2, 220 both surrounding and penetrated into the pores. Increasing the pressure, new Bragg peaks at 0.7 GPa appear (marked with the * symbols in panel (b) of Figure 1). Both peaks and transition pressure are typical of the well-known liquid-to-solid phase transition of bulk CO2. 221 However, at the same pressure and above, the contribution between 8° and 12° of liquid CO2 is still detected. The alleged contradiction can be easily solved by the following explanation: CO2 outside the MOF, i.e. that acting as a PTM, becomes solid while the CO2 confined into the MIL-101(Cr) pores persists in a liquid state (eventually glassy at higher pressures). This is confirmed by the pattern collected after the pressure release, shown in green in Figure 2. Indeed, releasing the pressure, the DAC opens causing the leaking of the CO2 and the consequent disappearance of the broad feature between 8° and 12° (see Figure 2 (b)).

Simultaneously, the intensity of the framework diffraction signal increases revealing that MIL-101(Cr) is still crystalline. In order to have an insight on the guest-host interactions between the adsorbed CO2 and the porous framework, we also performed preliminary IR spectroscopy experiments on MIL-101(Cr) loaded with liquid CO2 and the data analysis is ongoing. This configuration, where liquid CO2 totally fills the available pore volume of the framework, represents the limit case for the study of adsorbed gas molecules. Further structural and spectroscopic investigations involving the exposure of the MOF to gas CO2 are needed to clarify how the adsorption mechanism affects the mechanical stability of MIL-101(Cr). Moreover, exploring the pressure response of other MOFs with CO2, for example MIL-100(Cr), or changing gas to probe different active sites could also provide information on the adsorption properties of such materials. In particular, high-pressure experiments on pristine MIL-101(Cr) and Pd@MIL-101(Cr) composites exposed to hydrogen may be carried out. Indeed, the formation at high-pressure of hydrides with high hydrogen-to-metal ratio has been proved for bulk materials and could arise from the interaction with the nanoparticles confined within the pores of MIL-101(Cr).

In conclusions, our results proves that the mechanical stability of mesoporous frameworks is linked to their total pore volume and can be enhanced by introducing guest molecules of different chemical nature, such as polymers or metallic nanoparticles. Such complex composites with new physical and chemical properties result to be not only more functional but also more stable.

  INTRODUCTION ............................................................................................................................. 1 METAL-ORGANIC FRAMEWORKS ...................................................................................... 1.1 INTRODUCTION TO POROUS MATERIALS .......................................................................................... 1.2 MOFS STRUCTURE, CHEMICAL COMPOSITION AND APPLICATIONS .................................................. 1.2.1 Structure, components and topology .......................................................................................... 1.2.2 Applications ................................................................................................................................ 1.3 FUNCTIONALIZATION OF MOFS .................................................................................................... 1.3.1 Nanoparticles incorporation .................................................................................................... 1.3.2 Hybridization of MOFs and polymers ...................................................................................... 1.4 THE STABILITY OF MOFS .............................................................................................................. 1.4.1 Chemical stability ..................................................................................................................... 1.4.2 Thermal stability ....................................................................................................................... 1.4.3 Mechanical stability .................................................................................................................1.5 HIGH PRESSURE STUDIES ON POROUS MATERIALS ......................................................................... 1.5.1 Microporous MOFs: state of the art ......................................................................................... 1.5.1.1 Isotropic and anisotropic compression on MOFs ...................................................................................................................... 1.5.1.2 Displacive and reconstructive phase transitions ....................................................................................................................... 1.5.1.3 Pressure-induced amorphization .............................................................................................................................................. 1.5.1.4 Guest inclusion .......................................................................................................................................................................... 1.5.1.5 MOFs for mechanical energy storage ........................................................................................................................................

  Finally, we have explored how the content of Pd nanoparticles inside the pores tunes the mechanical stability of MIL-101(Cr). The composites are named x-Pd@MIL-101(Cr) where x indicates the weight percentage of Pd (x=20, 35 wt.%). Pristine samples have been obtained through the collaboration with the group of Dr. Gabriela Blanita (National Institute for Research and Development of Isotopic and Molecular Technologies, Romania) while nanoparticles functionalized samples have been synthesized during this Ph.D. at the Institute de Chimie et de Matériaux Paris-Est. We have studied the effect of pressure on the frameworks exploiting two complementary techniques on four different synchrotron beamlines: X-ray diffraction (Psiché and Cristal beamlines of SOLEIL Synchrotron, Gif-sur-Yvette, France and XRD1 beamline of ELETTRA synchrotron, Trieste, Italy) and Infrared spectroscopy (SMIS beamline, SOLEIL, other partner of the present work). The first technique provides information about the pressure-induced structural modifications while the latter probes the vibrational dynamics of the chemical bonds and reveals the existence of guest-host interactions. The high brightness provided by synchrotron radiation allowed us to investigate samples loaded into the micrometric chamber of the DAC. Additional infrared characterizations were performed in collaboration with Dr. Annalisa Paolone (Consiglio Nazionale delle Ricerche, Sapienza University, Italy).

3

 3

Figure 1 . 1

 11 Figure 1.1 Cross-section of a porous solid. Adapted from ref 3 .

Figure 1

 1 Figure 1.2.

Figure 1 . 3

 13 Figure 1.3 Schematic representation of MOF assembly, their structural and chemical versatility.10

Figure 1 .

 1 4 shows typical Zn and Cu-based SBUs commonly employed in

Figure 1 . 4

 14 Figure 1.4 Examples of SBUs.In blue are represented the metal polyhedra, in black the carbon atoms, in red the oxygen atoms, in green the nitrogen atoms, and in purple the chlorine atoms.[START_REF] Kaskel | The Chemistry of Metal-Organic Frameworks-Synthesis, Characterization, and Applications[END_REF]

Figure 1 . 5

 15 Figure 1.5 Structures of different MOFs with their respective SBUs and ligands: MOF-5, IRMOF-3, MOF-177, IRMOF-62, MOF-199, MOF-74.17

Figure 1 .

 1 6 shows the schematic representations of three different topologies with four-connected branching points or vertices.

Figure 1 . 6

 16 Figure 1.6 (a) The dia-net based on the diamond structure; (b) The chiral qtz-net based on the quartz structure; (c) The 2D sql-net.[START_REF] Öhrström | Let's Talk about MOFs-Topology and Terminology of Metal-Organic Frameworks and Why We Need Them[END_REF]

Figure 1 . 7 ,

 17 this effect can be enhanced by the presence of coordinately unsaturated metal sites CUS (panel (a)) and pre-or postsynthetic functionalization of the framework, such as integration of aromatic amine (panel (b)).[START_REF] Trickett | The Chemistry of Metal-Organic Frameworks for CO2 Capture, Regeneration and Conversion[END_REF]

Figure 1 . 7

 17 Figure 1.7 Structural design features for selective CO2 capture in MOFs. (a) CUS in the secondary building unit in Mg-MOF-74. (b) Covalently linked polar functionalities (aromatic amine) in CAU-1. 50

Figure 1 . 8

 18 Figure 1.8 Categories of chemical functionalization of MOFs. 11

Figure 1 . 9

 19 Figure 1.9 Schematic representation of most used synthesis methods to embed NPs in MOFs and the three classifications for the resulting composites.[START_REF] Rösler | Metal-Organic Frameworks as Hosts for Nanoparticles[END_REF]

  Tomographic reconstruction of these composites is shown in Figure 1.10, black arrows indicate embedded Au NPs while white arrows highlight the NPs positioned on the surface of the MOF.

Figure 1 .

 1 Figure 1.10 TEM Au@ZIF (a) Bright-field TEM images of Au@ZIF-8. (b) Slice through the tomographically reconstructed volume of the crystal. (c) Image of the 3D rendered volume showing

Figure 1 .

 1 Figure 1.11.[START_REF] Park | Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo-Polymerization Reaction[END_REF]

Figure 1 . 11

 111 Figure 1.11 Sketch of different methods to form polymers/MOFs materials. (a) Polymerization within nanochannels of MOFs. (b) Polymerization of ligands. (c) Introduction of polymer chains into MOFs nanochannels. (d) Polymer containing ligand monomers. (e and f) Post-synthetic polymertreated ligands. (g) Mixed matrix membrane based on MOFs.[START_REF] Kitao | Hybridization of MOFs and Polymers[END_REF]

Figure 1 . 12

 112 Figure1.12 The chemical (acid-base) stability of some representative metal-organic frameworks based on literature data. The bar length indicates the pH range that the metal-organic frameworks (MOFs) can tolerate.[START_REF] Howarth | Thermal and Mechanical Stabilities of Metal-Organic Frameworks[END_REF]

Figure 1 . 15

 115 Figure 1.15 Depictions of selected mechanical properties in solids.[START_REF] Burtch | Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications[END_REF]

Figure 1 . 16

 116 Figure 1.16 Polyhedral representation of the structure of TON at ambient pressure (left) and the at around 1.5 GPa (right). Silicone and oxygen atoms are represented in blue and red, respectively.[START_REF] Thibaud | High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON[END_REF]

Figure 1 .

 1 Figure1.17 schematizes the phenomena observed in MOFs at high-pressure. Besides amorphization and phase transitions, MOFs display other intriguing mechanical phenomena under compression. Among them, negative linear compressibility (NLC) involves the expansion of the framework along one or more direction under pressure with the concomitant reduction along the remaining directions.[START_REF] Mckellar | Structural Studies of Metal-Organic Frameworks under High Pressure[END_REF]

Figure 1 . 17

 117 Figure1.17 Schematic representations of the most common high-pressure response of MOFs.[START_REF] Collings | Metal-Organic Frameworks under Pressure[END_REF]

Figure 1 .

 1 Figure 1.18 (a) MOFs with undistorted structures are more resistant to compression than (b) those

Figure 1 .

 1 Figure 1.19 (a) Computationally calculated directional Young's modulus and accessible porosity (in blue) of defective UiO-66 112 and (b) experimental pressure dependence of volume for pristine UiO-66 and three defective samples. 113

Figure 1 . 20

 120 Figure 1.20 Bulk modulus as function of the largest cavity diameter. Each point represents a different MOF and the color indicate the topology.[START_REF] Moghadam | Structure-Mechanical Stability Relations of Metal-Organic Frameworks via Machine Learning[END_REF]

  For a closed thermodynamic systems, volume should decrease on compression thus implying the occurrence of positive linear compressibility in directions perpendicular to the NLC axis, as shown in Figure 1.21. 114,115 NLC is usually observed in MOFs with wine-rack topology, such as [NH4][Zn(HCOO)3] 115 , MIL-53 116 , ZnTAu(CN)2U2 114 and ZAG-4 117 .

Figure 1 .

 1 Figure 1.21 NLC in [NH4][Zn(HCOO)3]: schematic representation of the structure at ambient condition and on compression and pressure dependence of a axis and c axis. 115

Figure 1 . 22

 122 Figure 1.22 Breathing effect in MIL-53. 118

Figure 1 .

 1 Figure 1.23 (a) Ambient pressure and (b) high-pressure phases of [tmenH2][Er(HCOO)4]2. Blue arrows indicate the bond rearrangement.124

Figure 1 . 24

 124 Figure 1.24 Irreversible amorphization of ZIF-8. 128

Figure 1 . 25

 125 Figure 1.25 Relative volume change as function of pressure of HKUST-1 with several media. 125

Figure 1 . 26 1 . 5 . 1 . 5

 1261515 Figure 1.26 Ar adsorption sites in ZIF-8-HP at 1.20 GPa (298 K) crystal structure. Zn, N and C atoms are depicted in light grey, light blue and grey respectively. Framework atoms are drawn as capped sticks while Ar atoms are colored according to the six symmetry-independent sites. 130

Figure 1 . 27 135 1. 6

 1271356 Figure 1.27 Absorbed mechanical energy as a function of maximum applied pressure for individual UiO-66 nanocrystals. Insets: (a) side view TEM of a single crystal of UIO-66 before compression; (b) same crystal after compression to 2000 mN, and (c) top view, SEM of a compressed nanocrystal. 135

Figure 1 . 28

 128 Figure 1.28 Representation of the BTC linker (left), the chromium oxide trimeric building block (center) and the BDC linker (right).

Figure 1 . 29 Figure 1 . 30

 129130 Figure 1.29 Supertetrahedra of MIL-101(Cr) (right) and MIL-100(Cr) (left). Cr, C and O atoms are depicted in blue, gray and red respectively.

Figure 1 . 31

 131 Figure 1.31 The mesoporous cages of MIL-101(Cr): (a) pore windows and (b) pore diameters. 137

Figure 1 .

 1 Figure 1.32 MIL-100(Cr) pores windows (top) and sizes (bottom). Adapted from ref 138 .

Blanita(

  National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania) while their preparation, characterization, and functionalization with the Pd nanoparticles were performed at the Institut de Chimie et des Matériaux Paris-Est (Thiais, France).

)

  MIL-100(Cr) (chemical formula Cr3F(H2O)3O[C6H3-(CO2)3]2(H2O)n with n~28) is synthesized by a modified hydrothermal synthesis developed by Llewellyn et al.140 Trimesic acid(8.4 g, 40 mmol), HF 49% (1.44 ml, 0.8 g, 40 mmoli HF) and H2O (192 ml) are added to Cr2O3 (4 g, 4 mmoli). The obtained mixture is stirred for few minutes at room temperature and then introduced in a Teflon-lined stainless-steel autoclave for 96 hours at 493 K. The resulting green solid is separated by filtration, washed with water, and dried at 393 K in an autoclave. The as-synthesized MIL-100(Cr) is purified by Soxhlet extraction with ethanol.

2. 1 ,

 1 where two beams are specularly reflected by adjacent planes. It can be seen that 2𝑑 𝑠𝑖𝑛 𝜃 corresponds to the path difference between the two beams and therefore, when the Bragg law is satisfied, the rays interfere constructively, and sharp diffraction peaks (also called Bragg peaks) are observed. Across other directions of space, the scattered beams are out of phase and destructive interference occurs. The angle between the diffracted beam and the transmitted beam is 2𝜃 which is known as the diffraction angle and in angle-dispersive configuration, diffraction data consists of a record of photon intensity versus 2𝜃.146

Figure 2 . 1 A

 21 Figure 2.1 A Bragg reflection from a family of lattice planes spaced by a distance d. Incident and reflected rays are shown for the two neighboring planes.

Figure 2 . 2

 22 Figure 2.2 Powder diffraction cone produced by randomly oriented microcrystals when irradiated by an incident beam. The detector is perpendicular to both the direction of the incident beam and cone axis. 147

Figure 2 . 3

 23 Figure 2.3 Schematic illustration of a diffractometer working in the Bragg-Brentano geometry. 148

Figure 2 . 4

 24 Figure 2.4 Schematic ray diagram of a conventional TEM microscope.

Figure 2 . 5

 25 Figure 2.5 IUPAC classification of physisorption isotherms. 154

Figure 2 . 6

 26 Figure 2.6 Simplified static volumetric apparatus. 153

  The instrument consists in two main parts: a plasma source and an optical spectrometer. Ions are generated by the high-temperature plasma and emit radiation at several wavelengths depending on the components of the sample. The light emitted is then directed into a monochromator to separate the wavelengths. Finally, the light intensity is measured by a photodetector such as a charge coupled device (CCD). A schematic representation of an ICP-OES setup is showed in Figure2.7.

Figure 2 . 7

 27 Figure 2.7 Schematic cross section of an ICP-OES. 155

2 -10 3 )

 23 between the external and the internal smaller face, called culet, allows the generation of pressures up to the Mbar range. The two culets are separated by a pre-indented metallic gasket which is drilled at the center, to create an experimental chamber where the sample is placed and surrounded by a pressure transmitting medium, which ensures hydrostatic conditions. Pressure within the cell is measured through a pressure gauge, i.e. the fluorescence emission of microscopic ruby sphere or the equation of state of a well-known solid. The maximum accessible pressure depends on several factors such as the diamond culet diameter, the hole size and the thickness and material of the gasket. Usually, culets of 400-600 µm allow to reach 20-40 GPa while a better control of the low-pressure range (below 5 GPa) can be achieved using a larger culet diameter, i.e. 1 mm.

Figure 2 . 8

 28 Figure 2.8 Schematic representation of a diamond anvil cell and the pressure chamber. 156

Figure 2 . 9

 29 Figure 2.9 Mid-Infrared absorbance spectra comparison of type Ia and IIa diamonds.157

Figure 2 . 10

 210 Figure 2.10 Schematic view of conventional (a) and conical (b) high-pressure anvils. 157

Figure 2 . 11

 211 Figure 2.11 Procedure for gasket preparation: (a) the metal foil is placed between the anvils and (b)deformed applying pressure; (c) the as prepared gasket is pierced in the center realizing the highpressure chamber (yellow area) and placed again between the diamonds the same orientation as it was indented.157

Figure 2 . 12

 212 Figure 2.12 Schematic representation of BETSA DAC. 162

Figure 2 .

 2 Figure 2.13 R1 and R2 fluorescence lines of ruby (Al2O3 with Cr +3 impurities) at ambient pressure and 10 GPa.

  The standard deviations of the four investigated PTM as function of the pressure are shown in Figure2.14. All PTM are not perfectly hydrostatic, i.e. quasi-hydrostatic, but they exhibit σ values below 0.2 GPa up to 5 GPa thus making them suitable for high-pressure experiments in this pressure range.

Figure 2 . 14

 214 Figure 2.14 Comparison between the pressure dependence of the standard deviations for the four samples: NaCl (green), KBr (blue), CsI (red), and polyethylene (black).171

b σ at 18 GPa*

 18 The considered frequency range is limited to 20-4000 cm -1 , which is suitable for mid-and far-IR experiments.Several PTM are used in the high-pressure measurements performed during this work: two fluids, polydimethylsiloxane (PDMS) silicone oil (viscosity 30000 cSt at 25°C, CAS Number 63148-62-9) and the mineral oil Nujol (viscosity 100 cSt at 25°C, CAS nr. 8012-95-1), and solid salts, namely NaCl and CsI. PDMS polymer chains forming the silicone oil present a molecular weight around 100k

Figure 2 . 2 𝜃 2 , 1 𝛾 2

 22212 15(a). If the electron travels at speed close to the light speed and 𝛽 ≈ 1, the factor (1 -𝛽 cos 𝜃)5 vanishes for small theta values, thus giving a maximum on the theta in the angular distribution of the emitted radiation (panel (b) in Figure2.15). At this point, setting 𝜑=0 in equation 2.18 and expanding the cosine function for small angles cos 𝜃 ≈ 1 -1 gives that the radiation power inversely depends on the term ( emitted in a cone (panel (b) in Figure2.15) within an angle of

Figure 2 . 15 2 . 3 . 1 . 2

 2152312 Figure 2.15 Qualitative radiation patterns related to charged particles moving in a circular orbit for (a) slow particles (β = v/c << 1) and (b) in the relativistic β ≈ 1 case. 176

Figure 2 . 16

 216 Figure 2.16 Schematic representation of a third-generation synchrotron.

Figure 2 . 17

 217 Figure 2.17 Molecular energies. Typical gap energies of electronic, vibrational and rotational states.

Figure 2 . 18

 218 Figure 2.18 Schematic representation of the changes in the dipole moment when the molecule is irradiated by IR radiation. 180

Figure 2 . 19

 219 Figure 2.19 Possible vibrational modes of molecules.

Figure 2 .

 2 20, the photon beam entering in the interferometer is split into two perpendicular beams by a beam splitter. The transmitted light strikes a stationary mirror while the reflected light strikes a movable mirror. After the reflection on the mirrors, the two light-beams recombine at the beam splitter. The recombined beam is then directed through the sample and then to the detector. As the movable mirrors scan different positions as a function of time, a path difference (δ) is created between 0 and a maximum value, giving rise to a series of constructive and destructive interferences.

Figure 2 . 20

 220 Figure 2.20 Schematic representation of a Michelson interferometer.

Figure 2 . 21

 221 Figure 2.21 Interferograms of (a) monochromatic source, (b) dichromatic source, and (c) polychromatic source.[START_REF] Van Der Voort | Introduction to Porous Materials[END_REF]

Figure 2 . 22

 222 Figure 2.22 Sketch of the two sources, edge and dipolar, collected at the SMIS beamline with the calculated spatial distribution at 10 μm. 185

Figure 2 . 23

 223 Figure 2.23 The layout of the SMIS (Spectroscopy and Microscopy in the Infrared using Synchrotron) beamline at SOLEIL.

Figure 2 . 24

 224 Figure 2.24 Photon flux, on the left, and brightness, right, measured at the SMIS beamline for the synchrotron radiation and a conventional blackbody source.

Figure 2 . 25 186 Figure 2 . 26

 225186226 Figure 2.25 Scheme of the horizontal microscope at the SMIS beamline when a MCT detector is used. 186

Figure 2 . 27

 227 Figure 2.27 Scheme of a Schwarzschild reflective objective where the light path is indicated by solid lines. 187

  French) is an undulator-based XRD beamline, dedicated to the study of single crystals and powders in the 4 to 30 keV range. For high-pressure experiments, the diamond anvil cell is mounted on the phi stage of the 6circle diffractometer. The DAC positioning and centering onto the beam spot (~40x40 µm 2 ) is similar to the procedure described for PSICHE. The diffraction patterns are revealed by a Rayonix SX165 detector and the distance between the sample and the detector is about 182 mm. XRD patterns shown in the next chapters have been collected in two different beamtimes at around 21.4 keV.

Figure 2 . 28 .

 228 Figure 2.28. The evolution of these quantities as function of pressure provides valuable insight about the sample behavior under compression. 189

Figure 2 . 28

 228 Figure 2.28 Example of the fitting procedure on the MIL-101(Cr) spectrum in the FIR range. Empty dots represent the experimental data, green solid lines the main spectral components while the red solid line is the cumulative fitting curve.

Figure 2 . 29

 229 Figure 2.29 Example of data treatment to remove the water absorption in the FIR spectra.

Figure 2 .

 2 Figure 2.30 Top: typical diffraction image showing powder diffraction rings of MIL-101 within the diamond anvil cell before and after the masking procedure. Bottom: integrated one dimensional pattern.

Figure 3 . 1

 31 Figure 3.1 Laboratory X-ray diffraction pattern of MIL-101(Cr).

Figure 3 . 2

 32 Figure 3.2 Measured and simulated X-ray diffraction pattern of MIL-101(Cr) inside a capillary at the XRD1 beamline of the ELETTRA synchrotron. The simulated pattern has been obtained through a Le Bail refinement.

Figure 3 . 3 ,

 33 Figure 3.3).

Figure 3 . 3

 33 Figure 3.3 TEM image of MIL-101(Cr) crystallites.

Figure 3 . 4

 34 Figure 3.4 TGA of MIL-101(Cr) under dry air at a 10°C/min heating rate

Figure 1 .

 1 28) while the two secondary adsorption steps around 0.1 and 0.2 P/P0 highlight the presence of two kinds of mesopores (see Figure1.29). The BET surface area and the specific pore volume determined from experimental data are of 3549±10 m 2 g -1 of 1.72±0.1 cm 3 g -1 respectively. From the adsorption data, the pore size distribution of the sample, shown in Figure3.6, can be also obtained through DFT. The resulting pores sizes are about 27 and 32 Å, well in agreement with previous results.[START_REF] Férey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF]

Figure 3 . 5

 35 Figure 3.5 N2 adsorption and desorption curves at 77 K for MIL-101(Cr).

Figure 3 . 6

 36 Figure 3.6 Pose size distribution of MIL-101(Cr).

  7. The spectral range between 1800 and 2400 cm -1 is not accessible due to the strong diamond absorption, but MIL-101(Cr) does not have any fundamental IR active vibration in this region.193 Spectral regions where the signal saturates are masked and the inset in Figure3.7 shows the spectrum recorded for a thinner sample in the 1300-1700 cm -1 frequency region. Indeed, since MIL-101(Cr) presents vibrational peaks with very different intensities, two spectra were collected with different sample thicknesses to appreciate both weak and strong peaks.

Figure 3 . 7

 37 Figure 3.7 Infrared spectra of MIL-101(Cr) isolated inside the DAC. The inset shows the 1300-1700 cm -1 frequency region.

  Figure 3.8 for MIL-100(Al). As previously said in Chapter 2, these molecules can be removed by degassing the sample at high temperature under vacuum. TGA measurement reported in section 3.1.3

Figure 3 . 8

 38 Figure 3.8 Interaction of water molecules with Al 3+ coordinative unsaturated sites in MIL-100(Al).197

Figure 3 . 9 .

 39 Figure 3.9 . Infrared spectra of MIL-101 in controlled atmosphere (black) and after about 5 minutes of air exposure (red).

Figure 3 . 10

 310 Figure 3.10 Selected powder XRD patterns of MIL-101 under pressure with NaCl. Patterns are vertically stacked for sake of clarity with increasing pressure values in GPa. Patterns collected after the pressure release, at the lowest possible pressure, are shown in green.

Figure 3 .

 3 Figure 3.11 Selected XRD patterns (λ=0.5802 Å) of MIL-101 on compression with NaCl up to 7.5° collected at the CRISTAL beamline. Pattern collected when the pressure was released is shown in green.

Figure 3 . 12

 312 Figure 3.12 Pressure dependence of the lattice parameter a of MIL-101(Cr) with NaCl as PTM.

Figure 3 . 13

 313 Figure 3.13 Unit cell volume of MIL-101(Cr) in the 0-0.05 GPa pressure range. Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points.

  14(a)-(b) in the low 2θ region, XRD patterns with fluid PTM present a common behavior as a function of pressure.

Figure 3 . 14

 314 Figure 3.14 Selected powder XRD patterns (λ=0.48975 Å) of MIL-101(Cr) under pressure with (a) silicone oil and (b) Nujol. Patterns are vertically stacked for sake of clarity with increasing pressure values in GPa. Patterns collected after the pressure release, at the lowest possible pressure, are shown in red.

Figure 3 .

 3 Figure 3.15 Selected XRD patterns (λ=0.48975 Å) of MIL-101(Cr) on compression with silicone oil as PTM up to 6°. The pattern collected after the pressure release is shown in red.

Figure 3 .

 3 Figure 3.16 Selected XRD patterns (λ=0.48975 Å) of MIL-101(Cr) on compression with Nujol as PTM up to 6°. The pattern collected after the pressure release is shown in red.

up to 0. 8 GPa

 8 , for the three PTM. When MIL-101(Cr) is loaded with Nujol, a decreases almost linearly, reaching a -1.5% relative change at 0.7 GPa. With silicone oil, interestingly, the lattice parameter relative change exhibits two different regions of almost linear compressibility, below and above 0.2 GPa, as shown in detail in Figure3.18. First, it decreases with a slope of -0.01 GPa -1 and then of -0.03 GPa -1 , thus

Figure 3 . 17

 317 Figure 3.17 Relative change in the unit cell parameter a of MIL-101(Cr) under pressure up to 0.8 GPa. NaCl (green symbol), silicone oil (purple) and Nujol (orange) are the PTM employed.

Figure 3 . 18

 318 Figure 3.18 Linear fit of the relative change in the lattice parameter a when silicone oil is used as PTM.

Figure 3 . 19

 319 Figure 3.19 Unit cell volume of MIL-101(Cr) in the 0-0.25 GPa pressure range when silicone oil is used as PTM. Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points.

Figure 3 . 20

 320 Figure 3.20 Unit cell volume of MIL-101(Cr) in the 0.25-2.0 GPa pressure range when silicone oil is used as PTM. Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points. Error bars are within the symbol size.

Figure 3 . 21

 321 Figure 3.21 Unit cell volume of MIL-101(Cr) in the 0-2.0 GPa pressure range when Nujol is used as PTM. Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points. Error bars are within the symbol size.

Figure 3 . 22

 322 Figure 3.22 Pressure dependence of the lattice parameter a. Silicone oil (purple symbols) and Nujol (orange) are the PTM employed. Error bars are within the symbol size.

  used in XRD measurements since it has the widest transmission window among the alkali halides ranging from 180 to 20000 cm -1 , thus allowing to measure also the FIR range. 207 On the contrary, silicone oil presents strong active modes in the IR range here explored which prevents the study of the whole MIL-101(Cr) spectrum. The attenuated total reflection (ATR) IR spectra of the two fluid PTM, i.e. silicone oil and Nujol, are reported in Figure 3.23.

Figure 3 . 23

 323 Figure 3.23 Infrared spectra of silicone oil (purple) and Nujol (orange).

Figure 3 .

 3 Figure 3.24 IR spectra of MIL-101(Cr) loaded in the DAC with CsI (green), Nujol (orange) and silicone oil (purple) at 0.1 GPa. Spectral regions where signal saturates due to silicone oil or Nujol absorptions are masked. Spectra are vertically stacked for clarity.

Figure 3 .

 3 Figure 3.25 (a) FIR region where the collective mode and the CrO vibration appear. Grey dashed lines represent the peak central frequency with CsI and highlight the frequency shift with fluid PTM. (b) Spectral region of the OH stretching vibrations. Spectra are vertically stacked for clarity.

Figure 3 . 26

 326 Figure 3.26 Frequency shift of the main IR peaks of MIL-101(Cr) with Nujol relative to the MIL-101(Cr) with CsI. The grey dashed line indicates the threshold at 0.994. Peak frequencies measured

Figure 3 . 26 .Figure 3 . 8 )

 32638 Figure 3.8). The fluid probably forces the water molecules closer to each other or to the structural hydroxyl groups thus increasing the hydrogen bond strength. This, in turn, also affects the CrO vibrational frequency. These results create a coherent vibrational picture consistent with the PTM insertion within the MOF pores and indicate that the four modes are ideal vibrational probes of the host-guest interaction.

Figure 3 .

 3 Figure 3.27 Selected FIR and OH stretching regions of MIL-101(Cr) spectra measured under pressure with CsI. Spectra collected after the pressure release, at the lowest possible pressure, are shown in red. Spectra are vertically stacked for clarity with increasing pressure values.

Figure 3 .

 3 Figure 3.28 MIR spectra of MIL-101(Cr) increasing the pressure with CsI used as PTM. The spectrum collected when the pressure was released is shown in red.

Figure 3 .

 3 Figure 3.29 Selected FIR and OH stretching regions of MIL-101(Cr) spectra measured under pressure with silicone oil (a)-(b) and Nujol (c)-(d) used as PTM. In panel (a), the absorption around 400 cm -1 of silicone oil is masked. Spectra collected after the pressure release, at the lowest possible pressure, are shown in red. Spectra are vertically stacked for clarity with increasing pressure values.

Figure 3 .

 3 Figure 3.30 MIR spectra of MIL-101(Cr) increasing the pressure with Nujol used as PTM. The spectrum collected when the pressure was released is shown in red.

Figure 3 . 31

 331 Figure 3.31 Pressure dependence of the collective ν1 (a) and the CrO ν2 peak frequency (b) modes of MIL-101(Cr) up to 1.1 GPa with different PTM: CsI (green symbol), silicone oil (purple) and Nujol (orange). When the silicone oil is used the collective mode is not accessible. Error bars are within the symbol size.

Figure 3 . 32

 332 Figure 3.32 Pressure dependence of the width of the MIL-101(Cr) collective ν1 (a) and the CrO ν2 (b) peaks up to 1.1 GPa with different PTM: CsI (green symbol), silicone oil (purple) and Nujol (orange). When silicone oil is used the collective mode ν1 is not accessible. Error bars are within the symbol size.

1. 5 ,

 5 indicative of the crystalline phase, marks the amorphization onset. The delayed disappearance of the collective ν1 mode compared to the amorphization pressure value observed in XRD data indicates that short-range order is still present after the amorphization, at least in a narrow pressure range, and it is then completely lost for further compression. This is in agreement with a recent pair distribution function study on MIL-100(Fe) showing that short-range correlations are retained even in the amorphous phase obtained by ball milling treatment.208

Figure 3 . 33

 333 Figure 3.33 Pressure dependence of the ratio between peak heights of the two modes for each pressure value

Figure 3 . 34

 334 Figure 3.34 Pressure dependence of the peak frequency (a) and width (b) of MIL-101(Cr) benzene ring deformation ν5 mode up to 0.6 GPa with different PTM: CsI (green symbol) and Nujol (orange).

Figure 3 . 35

 335 Figure 3.35 Pressure dependence of the OH stretching ν28 (a) and the ν29 peak frequency (b) modes of MIL-101(Cr) up to 1.1 GPa with different PTM: CsI (green symbol), silicone oil (purple) and Nujol (orange). When the silicone oil is used the structural OH mode is not observed. Error bars are within the symbol size.

  Using a fluid PTM, as Nujol or high-viscosity PDMS silicone oil, the framework stability increases up to 7 GPa, due to PTM penetration inside the pores. Above this pressure, a peculiar pressure-induced expansion of the crystalline phase is observed which is explained by the concomitant nucleation of a denser amorphous phase. The size of this expansion is dependent on the fluid PTM.The amorphization process is finished around 15 GPa with silicone oil, whereas with Nujol it is still not completed at about 14 GPa. The insertion of the fluid PTM is further confirmed by IR spectroscopy, where the OH stretching vibrations of the MOF hydroxyl groups and their pressure dependence are strongly modified by the presence of the guest PTM inside the pores. In particular, high-pressure allows the incorporation of high-viscosity PDMS silicone oil inside the mesopores of MIL-101(Cr).4 PD@MIL-101(CR) AT HIGH PRESSUREIn this chapter the high-pressure response of MIL-101(Cr) with embedded Pd nanoparticles is given. The effect of the presence of Pd nanoparticles on the mechanical properties of the framework is investigated exploiting synchrotron X-ray diffraction and infrared spectroscopy. A comparison between the pristine framework behaviour under compression and the metal loaded samples is given.4.1 Characterization of x-Pd@MIL-101(Cr) (x=20, 35 wt.%) at ambient conditionsCharacterization at standard conditions of temperature and pressure of the as-synthesized x-Pd@MIL-101(Cr) (x= 20 and 35) composites is reported in this paragraph. Structural, microstructural and textural properties were determined by X-ray diffraction, transmission electron microscopy, nitrogen porosimetry and infrared spectroscopy.

4. 1 . 1

 11 Crystalline structurePd nanoparticles (NPs) are formed inside the MIL-101(Cr) pores following the impregnation and reduction procedures described in section 2.1.2. In order to check that the MIL-101(Cr) crystalline structure is preserved we performed laboratory XRD measurements on the composites x-Pd@MIL-101(Cr) (x=20, 35 wt.%) at ambient conditions which were compared to those of the pristine sample.This comparison, shown inFigure 4.1, reveals that XRD patterns of Pd doped and pristine samples agree well, clearly indicating that the NPs insertion does not alter the framework structure. All diffraction peaks from the porous solids host are visible for the Pd loaded composites proving the stability of the porous framework in our NPs synthetic conditions (reduction under H2/Ar flow at 573 K).

Figure 4 . 1

 41 Figure 4.1 XRD patterns of pristine MIL-101(Cr) (green) and x-Pd@MIL-101(Cr) composites (orange and blue for x=20 and 35 wt.% Pd, respectively).

4. 1 . 2 Microstructure

 12 The presence of well-dispersed ultra-small Pd NPs is confirmed by Transmission Electron Microscopy. Typical TEM images of the two composites 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) are shown in Figure 4.2. The Pd NPs are homogeneously distributed within the composite, for both metal loadings.

Figure 4 . 2

 42 Figure 4.2 Typical TEM images of 20-Pd@MIL-101(Cr) (left) and 35-Pd@MIL-101(Cr) (right).

Figure 4 . 4

 44 Figure 4.4 N2 adsorption (filled points) and desorption (empty) curves of MIL-101(Cr) (green), 20-Pd@MIL-101(Cr) (orange) and 35-Pd@MIL-101(Cr) (blue).

Table 4 . 1

 41 Textural properties of MIL-101(Cr) and x-Pd@MIL-101(Cr), as determined by N2 porosimetry: BET surface area (SBET) and total porous volume. Both values are reported per mass of composite.

4. 1 . 5

 15 Vibrational spectraInfrared spectra of samples isolated inside the diamond anvil cell were collected in the IR range from 100 to 5000 cm -1 . The comparison between the spectrum of 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) and pristine MIL-101(Cr) is shown in Figure4.5. The three spectra are basically similar with no major differences observed. This points out the absence of strong chemical interactions between the embedded NPs and the framework in the composite materials. Therefore, all the observed peaks of x-Pd@MIL-101(Cr) are assigned to the vibrational modes of pristine MIL-101(Cr) already mentioned in paragraph 3.1.5. This also confirms that the MIL-101(Cr) local molecular structure is preserved after the impregnation procedure, in agreement with the preservation of crystalline structure revealed from the XRD measurements reported in Figure4.1. As for the pristine MIL-101(Cr), no vibrational modes were detected neither below 400 cm -1 nor above 4000 cm -1 .

Figure 4 . 5

 45 Figure 4.5 Infrared spectra of pristine MIL-101(Cr) (green), 20-Pd@MIL-101(Cr) (orange) and 35-Pd@MIL-101(Cr) (blue) isolated inside the DAC.

  λ=0.5802 Å. Selected patterns at high-pressure for 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) are shown in Figure 4.6 in the low 2θ range, where the main diffraction peaks are visible. In panel (a), the range below about 1.5° is not available for patterns collected at 0.874 GPa and at 0.123 GPa (pressure released) because of signal saturation due to an increased exposure time of the detector. The full 2θ range is shown in Figure 4.7 and 4.8 for the two composites.

Figure 4 . 6

 46 Figure 4.6 Selected powder XRD patterns (λ = 0.5802 Å) of (a) 20-Pd@MIL-101(Cr) and (b) 35-Pd@MIL-101(Cr) under pressure. Patterns are vertically stacked for sake of clarity. Patterns collected after the pressure release are shown in green.

Figure 4 . 7 .

 47 Figure 4.7 . Selected powder XRD patterns (λ = 0.5802 Å) of 20-Pd@MIL-101(Cr) under pressure. The patterns collected after the pressure release is shown in green. Black dashed lines indicate the position of fcc Pd Bragg peaks (not observed) while the grey area highlights the NaCl (220) Bragg reflection.

Figure 4 . 8

 48 Figure 4.8 Selected powder XRD patterns (λ = 0.5802 Å) of 35-Pd@MIL-101(Cr) under pressure. The patterns collected after the pressure release is shown in green. Black dashed lines indicate the position of fcc Pd Bragg peaks (not observed) while the grey area highlights the NaCl (220) Bragg reflection.

Figure 4 .

 4 10, it is noticeable that the slope, which is related to the compressibility of the material (see Section 1.5.1), is affected by the amount of Pd NPs. To quantify this difference, the volume data were fitted with a second-order Birch-Murnaghan EoS to obtain the bulk modulus, as shown inFigures 4.11

  and 4.12. The comparison between the bulk modulus of pristine MIL-101(Cr) (see paragraph 3.2.1) and the results here obtained are summarized in Figure 4.13(a) and listed in Table4.3.

Figure 4 . 9

 49 Figure 4.9 Pressure dependence of the lattice volume of 20-Pd@MIL-101(Cr) (orange) and 35-Pd@MIL-101(Cr) (blue).

Figure 4 . 10

 410 Figure 4.10 Unit cell volume as a function of the pressure for pristine MIL-101(Cr) (green), 20-Pd@MIL-101(Cr) (orange) and 35-Pd@MIL-101(Cr) (blue) up to 0.3 GPa. Volume values are normalized to the lowest pressure values for each sample.

Figure 4 . 11

 411 Figure 4.11 Unit cell volume of 20-Pd@MIL-101(Cr). Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points.

Figure 4 . 12

 412 Figure 4.12 Unit cell volume of 35-Pd@MIL-101(Cr). Dashed line represents the second-order Birch-Murnaghan equation-of-state best fit to the experimental points.

Figure 4 .

 4 Figure 4.13 (a) Relationship between the bulk modulus and the Pd loading. (b) Sketch of Pd content dependence of the amorphization pressure values. The grey part indicates the pressure range where the transition to the amorphous phase occurs.

content from 0 .

 0 05 to 0.09 and 0.21 GPa for MIL-101(Cr), 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr), respectively, as highlighted by the arrows in Figure 4.10 and schematized in Figure 4.13(b).

4. 2 . 2 . 1

 221 Low pressure range XRD data of 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) upon the increase of pressure with silicone oil as PTM were collected on the CRISTAL beamline (λ=0.5802 Å) and are shown in

Figure 4

 4 Figure 4.14.

Figure 4 . 14

 414 Figure 4.14 Selected powder XRD patterns (λ = 0.5802 Å) of (a) 20-Pd@MIL-101(Cr) and (b) 35-Pd@MIL-101(Cr) under pressure with silicone oil as PTM. Patterns are vertically stacked for sake of clarity.

Figure 4 . 15

 415 Figure 4.15 Pressure dependence of the volume for MIL-101(Cr) (purple) 20-Pd@MIL-101(Cr) (pink) and 35-Pd@MIL-101(Cr) (violet). Silicone oil is the PTM employed. Volume values are normalized to the lowest pressure values for each sample. Error bars are within the symbol size.

Figure 4 . 16

 416 Figure 4.16 Pressure dependence of the unit cell volume of 35-Pd@MIL-101(Cr). Silicone oil (purple symbols) and NaCl (blue) are the PTM employed. Error bars are within the symbol size.

  . The first reflections, i.e. Bragg peaks below 1.8°, are not accessible due to the beamline specifications.

Figure 4 . 17

 417 Figure 4.17 Selected powder XRD patterns (λ=0.7 Å) of 20-Pd@MIL-101(Cr) under pressure with silicone oil. Patterns are vertically stacked for sake of clarity. Pattern collected after the pressure release is shown in red.

Figure 4

 4

Figure 4 . 18

 418 Figure 4.18 Pressure dependence of the unit cell volume of MIL-101(Cr) (purple symbols) and 20-Pd@MIL-101(Cr) (dark cyan) with silicone oil as PTM. Error bars are within the symbol size.

Pd@MIL-101(Cr) at high pressure 123 4 . 3 . 1

 431 Compression with a solid medium: NaClParticular attention has been paid to the far-IR region where the collective mode (ν1) at 389 cm -1 and the CrO stretching (ν2) around 589 cm -1 are present, as discussed in the previous chapter.209 Selected high-pressure FIR and MIR spectra for the 20-Pd@MIL-101(Cr) and35-Pd@MIL-101(Cr) are shown in Figure 4.19-21. The IR spectra of the x-Pd@MIL-101(Cr) closely resembles those of pristine MIL-101(Cr) (see Figures 3.7and 4.5) thus showing the lack of chemical interactions between the guest NPs and the host framework under pressureIn both samples, the intensity of the ν1 peak decreases under pressure finally becoming hardly detectable above 0.6 GPa. On the contrary, the ν2 peak broadens being still present over the whole pressure range. These spectral modifications are not reversible after the pressure release, in agreement with XRD results.

Figure 4 . 19 .

 419 Figure 4.19 . Selected FIR spectra of 20-Pd@MIL-101(Cr) (a) and 35-Pd@MIL-101(Cr) (b) measured under pressure. Spectra collected after the pressure release are shown in green. Spectra are vertically stacked for sake of clarity.

Figure 4 .

 4 Figure 4.20 Selected MIR spectra of 20-Pd@MIL-101(Cr) increasing the pressure with CsI as PTM.

Figure 4 .

 4 Figure 4.21 Selected MIR spectra of 35-Pd@MIL-101(Cr) increasing the pressure with CsI as PTM.The spectrum collected when the pressure was released is shown in green.

Figure 4 . 23 4 . 3 . 2

 423432 Figure 4.23 Peak frequency of the ν1 (a) and ν2 (b) modes as a function of pressure. Arrows highlight the pressure values where the amorphization is completed, as obtained from the XRD analysis.

Figure 4 .

 4 Figure 4.24 Selected FIR spectra of 35-PdMIL-101(Cr) measured under pressure with Nujol (a) and silicone oil (b) used as PTM. In panel (b), the absorption around 400 cm -1 of silicone oil is masked. Spectra collected after the pressure release, at the lowest possible pressure, are shown in blue. Spectra are vertically stacked for clarity with increasing pressure values.

Figure 4 .

 4 25 for different PTM. When fluid PTM are used both peaks continuously redshift up to 1 GPa, whereas with CsI, a softening is observed at the crystal-to-amorphous transition. This comparison strongly corroborate that the mode softening is linked to the amorphization.

Figure 4 . 25

 425 Figure 4.25 Peak frequency of the ν1 (a) and ν2 (b) modes as a function of pressure for 35-Pd@MIL-101(Cr) composite with CsI, Nujol and silicone oil as PTM.

5 . 1 5 . 1 . 1

 51511 35 wt.% Pd. The bulk modulus is also increased by the NPs presence, reaching nearly a fivefold increase for the 35-Pd@MIL-101(Cr) composite. The fact that the insertion inside MIL-101(Cr) of completely different chemicals, as a polymeric fluid and metallic NPs, increases the host's stability range confirms that the observed enhancement is mainly driven by a purely steric filling mechanism, where an ultimate "hyperfilling" of the pores is reached with the fluid. This also suggests that other host-guest interactions have little effect on the structural stability of MIL-101(Cr) under pressure.The lack of such chemical interactions is also demonstrated by the infrared spectra of the Pd loaded materials, which do not show new spectral features or modification when compared to the pristine MIL-101(Cr). In the IR spectra, we identified a spectroscopic quantity tracking the amorphization process, i.e. the ratio between the heights of the peaks assigned to the chromium oxide stretching and the collective mode, which can be exploited from now on as a vibrational marker of the framework stability. The pressure dependence of the vibrational frequencies of the same IR modes suggests structural or electronic local effects on the chromium oxide clusters taking place during the amorphization process. Both XRD and IR results show that increasing the Pd loading tunes the mechanical properties of the host framework enhancing its structural stability. This clearly points out that Pd nanoparticles are effectively inserted inside the MIL-101(Cr) pores, thus showing that the application of high mechanical pressure could be exploited as a new tool to demonstrate the successful insertion of well-dispersed nanometric objects inside MOFs' pores, at least for high metal loadings.5 MIL-100(CR) AT HIGH PRESSUREThe behaviour at high pressure of the mesoporous MIL-100(Cr) will be presented and discussed in this chapter. Structural, microstructural and textural properties of MIL-100(Cr) at ambient conditions were determined with laboratory XRD, TGA, TEM and N2 porosimetry measurements. The behavior at high-pressure was then studied by synchrotron XRD and IR spectroscopy. By analogy with MIL-101(Cr), two different pressure transmitting media were used, namely solid alkali halides and fluid silicone oil. Finally, the mechanical properties of MIL-101(Cr) and MIL-100(Cr) will be compared and the role of the organic linker on the mechanical stability will be highlighted. Characterization of MIL-100(Cr) at ambient conditions Crystalline structure MIL-100(Cr) structure is built up from trimers of chromium octahedra as metallic clusters and benzene-1,3,5-tricarboxylate (BTC) as organic linker. Similarly to MIL-101(Cr), the chromium clusters are assembled in supertetrahedral building units forming a 3D network with cubic symmetry (space group Fd3 ̅ m).[START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF] The crystalline structure of the activated MIL-100(Cr) was verified by both laboratory and synchrotron X-ray diffraction at ambient conditions as shown in Figures 5.1and 5.2, respectively.

Figure 5 . 1

 51 Figure 5.1 Laboratory XRD pattern of MIL-100(Cr).

Figure 5 . 2

 52 Figure 5.2 Measured and simulated X-ray diffraction patterns for MIL-100(Cr) inside a capillary at the synchrotron. The simulated pattern has been obtained through a Le Bail refinement.

Figure 5 .

 5 Figure 5.3 shows typical TEM images of MIL-100(Cr) grains. Grains size results to be between 100 and 200 nm and the image of a single grain reveals the crystalline nature of the sample.

Figure 5 . 3

 53 Figure 5.3 Typical TEM images of MIL-100(Cr) with different magnification.

Figure 5 . 4

 54 Figure 5.4 N2 adsorption and desorption curves at 77K for MIL-100(Cr).

Figure 5 . 5

 55 Figure 5.5 Pose size distribution of MIL-100(Cr).

Figure 5 . 8 Figure 5 . 9

 5859 Figure 5.8 Selected powder XRD patterns of MIL-100(Cr) under pressure with (a) NaCl (λ=0.5790 Å) and (b) silicone oil (λ=0.48593 Å). Patterns are vertically stacked for sake of clarity. Patterns collected after the pressure release are shown in blue.

Figure 5 . 9

 59 Figure 5.9 Pressure dependence of MIL-100(Cr) lattice parameter a up to 0.65 GPa. NaCl (pale blue symbols) and silicone oil (light green) are the PTM employed. If not displayed, error bars are within the symbol size.

Figure 5 . 10

 510 Figure 5.10 Unit cell volume of MIL-100(Cr) in the 0-0.5 GPa pressure range. Dashed and dotted lines represent the second and third-order Birch-Murnaghan equation-of-state fit of the experimental points, respectively.

Figure 5 . 13 .

 513 Figure 5.13. In the far-IR region below 700 cm -1 -panel (b) -silicone oil induces a slight blueshift of about 2 cm -1 of the CrO stretching mode ν4 at 529 cm -1 compared to the peak position with CsI. On the contrary, in the OH region -panel (b) -the fluid medium on the MIL-101(Cr) peaks is moreremarkable: one broad peak is detected at 3642 cm -1 , whereas three peaks at 3585 (ν26), 3617 and 3675 (ν28) with a solid PTM. This proves that some silicone oil is already inserted inside the MIL-100(Cr) pores and interacts with the hydroxyl group and the residual water molecules.

Figure 5 .

 5 Figure 5.12 IR spectra of MIL-100(Cr) loaded in the DAC with CsI (pale blue) and silicone oil (green) at 0.1 GPa. Spectral regions where signal saturates due to silicone oil absorptions are masked. Spectra are vertically stacked for clarity.

Figure 5 .

 5 Figure 5.13 (a) FIR region where the collective mode and the CrO vibration appear. (b) Spectral region of the OH stretching vibrations. Spectra are collected at 0.1 GPa and vertically stacked for clarity.

5 .

 5 Figure 5.13, indicates a preservation of the pressure-induced modifications, which agrees with the irreversible transition to the amorphous phase observed around 0.6 GPa in the XRD experiments.Focusing now on the silicone oil case, spectra shown in Figure5.13(c)-(d) present a quite different evolution under pressure: the CrO ν4 mode blueshifts without a noticeable intensity variation until 6 GPa, followed by a broadening. The OH mode redshifts until it merges with the broad residual water band detected between 2700 and 3500 cm -1 . After the pressure release, all the peaks shift back toward the initial frequencies and the original spectral lineshapes are partially recovered. The observed effects on the high-pressure IR spectra of MIL-100(Cr) with silicone oil are consistent with the XRD data discussed in the previous section which points out the structural endurance of the framework up to 18 GPa (see Figure5.8(b)).

Figure 5 .

 5 Figure 5.14 Selected FIR and OH stretching regions of MIL-100(Cr) spectra measured under pressure with CsI(a)-(b) and silicone oil (c)-(d) used as PTM. In panel (c), the absorption around 400 cm -1 of silicone oil is masked. Spectra collected after the pressure release, at the lowest possible pressure, are shown in blue. Spectra are vertically stacked for clarity with increasing pressure values.

and 5 .Figure 5 .

 55 Figure 5.16(a) and (c)) without showing any relevant anomaly, consistently with the absence of phase transitions. On the contrary, a different behavior is observed with a solid PTM (blue symbols). The ν1 peak central frequency is more or less constant up to 0.4 GPa. Above this value, it starts to harden until 0.8 GPa (see Figure 5.15(a)). The second collective mode ν2 and the CrO mode ν4 instead harden up to

Figure 5 . 15

 515 Figure 5.15 Pressure dependence of the collective ν1 (a), ν2 (b) and the CrO ν4 (c) peak frequency modes of MIL-100 up to 1.1 GPa with different PTM: CsI (pale blue symbol) and silicone oil (green). When silicone oil is used the collective mode ν2 is not accessible. Error bars are within the symbol size.

Figure 5 . 16

 516 Figure 5.16 Pressure dependence of the width of the MIL-100 collective ν1 (a), ν2 (b) and the CrO ν4 (c) peaks up to 1.1 GPa with different PTM: CsI (pale blue symbol) and silicone oil (green). When silicone oil is used the collective mode ν2 is not accessible. Error bars are within the symbol size.

  3.3 and shown in Figure 3.32.

Figure 5 . 17

 517 Figure 5.17 Pressure dependence of the ratio between peak heights of the two modes for each pressure value with silicone oil (green symbols) and CsI (blue).

Figure 5 .

 5 20 shows the trends under pressure of the central frequency (panel (a)) and the linewidth (panel (b)). Both quantities result to be constant until 0.4 GPa, above this pressure value the peak softens while the FWHM increases. Above 0.8, concomitantly with the amorphization, the OH peaks are not detected anymore. As already suggested for MIL-68(In), 129 the softening of the OH modes may be due to the formation and strengthening of hydrogen bonds between the hydroxyl groups and the neighboring C-H groups of the organic linker during the framework distortions caused by the amorphization process. This could also explain the softening observed on the CrO mode ν4 (see Figure5.15(c)).

Figure 5 . 18

 518 Figure 5.18 Pressure dependence of the central frequency (a) and width (b) of the ν26 with CsI as PTM.

Figure 5 . 19

 519 Figure 5.19 Unit cell volume as a function of the pressure for MIL-101(Cr) (green) and MIL-100(Cr) (red) up to 0.7 GPa when NaCl is employed as medium. Volume values are normalized to the lowest pressure values for each sample.

Figure 5 . 20

 520 Figure 5.20 Relative unit cell volume as a function of the pressure for MIL-101(Cr) (green points) and MIL-100(Cr) (red) up to 19 GPa. Volume values are normalized to the lowest pressure values for each sample. Error bars are within the symbol size.

of the 3 -

 3 Figure 5.21.

Figure 5 . 21

 521 Figure 5.21 The 3-D network of (a) MIL-101(Cr) and (b) MIL-100(Cr) with the BDC and BTC

Figure 1

 1 Figure 1 Selected XRD patterns of MIL-101(Cr) on increasing pressure with liquid CO2 as PTM in the low-angles (a) and high-angles (b) regions. Stars indicate the peaks assigned to solid CO2 after the phase transition.

Figure 2

 2 Figure 2 Selected XRD patterns of MIL-101(Cr) on decreasing pressure with liquid CO2 as PTM in the low-angles (a) and high-angles (b) regions.

Figure A. 3

 3 Figure A.3 IR spectra in the OH stretching region of MIL-101(Cr) while increasing pressure from 0.1 mbar to 1 bar (ambient pressure).

  Figure 1.24 shows that Bragg reflections broadening on compression until they are not Song and coworkers 127 did not reveal any pressure-induced chemical transformation of the SBUs up to 39 GPa although the original spectrum is not recovered due to the irreversible amorphization. It is worth noting that this is one of the first high-pressure study on MOFs involving

	detectable anymore above 0.34 GPa in both hydrostatic and non-hydrostatic conditions, i.e. with and
	without a pressure transmitting medium (see Section 2.2.6). Infrared spectroscopy (IR) measurements
	performed by

vibrational spectroscopy which proved the power of in situ IR spectroscopy to provide new insights on the chemical bonding and the local structure.

127

Table 2 . 1

 21 List of the most used PTM and their hydrostatic limits, standard deviations at 10 and 20 GPa, and transmission windows.

	PTM	Hydrostatic	σ at 10 GPa	σ at 20 GPa	IR transmission
		limit (GPa)	(GPa)	(GPa)	window (cm -1 )*
	Helium 169	23	0	0.05	20-4000
	Neon 169	15	0	0.15	20-4000
	Argon 169	1.4	0.1	0.2	20-4000
	Nitrogen 169	10	0	0.4	20-4000
	1:1 iso-n-pentane 169	7.4	0.4	0.9 a	600-1200
					1700-2700
					3200-4000
	5:1 iso-n-pentane 169	7.0	0.4	1.0 a	600-1200
					1700-2700
					3200-4000
	4:1 methanol-ethanol 169	10.5	0	2.0	1800-2600
	Silicone oil 169	3.0	0.3	2.3	1400-3000
	1:1 Fluorinert FC-84,	2.0	0.45	0.9 b	1600-4000
	FC-87 169
	Daphne 7474 169,173	3.7	0.32	-	200-600
					1500-3000
	16:3:1 methanol-ethanol-	10.5	0	2.2	1800-2600
	water 169
	NaCl 171	1.0	0.18	0.5	625-4000
	KBr 171	2.0	0.20	1.3	400-4000
	CsI 171	2.0	0.25	0.5	180-4000
	Polyethylene 171	2.0	0.34	0.7	20-700
					900-1400
					1600-2700
	a σ at 15 GPa

Table 3 . 1

 31 Vibrational bands observed for MIL-101(Cr), their appearance and proposed assignment.The * symbol marks the modes assigned in this work.

	Label	Frequency (cm -1 )	Appearance	Assignment 193
	ν1		medium, sharp	Collective*
	ν2		strong, sharp	Cr-O
	ν3		weak, sharp
	ν4		medium, broad
	ν5		weak, sharp	C-C bending
	ν6		strong, sharp	Benzene ring bending
	ν7		weak, sharp
	ν8		weak, sharp
	ν9		weak, sharp	C-H deformation 198
	ν10		medium, sharp	Benzene ring deformation
	ν11		weak, broad
	ν12		weak, broad
	ν13		weak, sharp	C-H deformation 198
	ν14		weak, broad
	ν15		weak, broad
	ν16		weak, broad
	ν17		weak, broad
	ν18		weak, broad
	ν19		strong, sharp	COO -symmetric stretching
	ν20		strong, shoulder	COO -symmetric stretching
	ν21		strong, sharp	C=C stretching
	ν22		strong, shoulder	COO -asymmetric stretching
	ν23		strong, sharp	COO -asymmetric stretching
	ν24		weak, broad
	ν25		weak, broad	C=O
	ν26		weak, broad	C-H stretching
	ν27		weak, broad	C-H stretching
	ν28		weak, sharp	Cr-OH stretching
	ν29		weak, sharp	H2O or H2O/H2O stretching

Table 3 . 2

 32 Unit cell parameters of MIL-101(Cr) obtained through the Le Bail refinement for sample inside the capillary and inside the DAC with different PTM at the lowest pressure.

	PTM	P (GPa)	a (Å)	V/10 2 (Å 3 )
	None (Capillary)	0	88.31±0.03	6893.8±1.0
	NaCl	0.01	87.88±0.02	6786.9±2.4
	Silicone Oil	0.05	88.95±0.03	7037.5±7.6
	Nujol	0.05	88.70±0.03	6977.6±6.2

Table 3 . 3 .

 33 Summary of the values of the bulk moduli for different PTM as obtained by a second order Birch-Murnaghan equation-of-state.

	PTM	Pressure range (GPa)	Bulk modulus (GPa)
	NaCl	0 ̶ 0.05	1.1±0.9
	Silicone oil	0 ̶ 0.25	31±2
	Silicone oil	0.25 ̶ 2.0	9.4±0.4
	Nujol	0 ̶ 2.0	13.4±0.2

Table 4 . 2

 42 Pd concentration obtained by ICP-EOS.

	Sample	Pd content (wt..%)
	20-Pd@MIL-101(Cr)	19.9±0.5
	35-Pd@MIL-101(Cr)	35.0±0.9

Table 4 . 3

 43 Volumes at ambient pressure and bulk moduli of pristine MIL-101(Cr), 20-Pd@MIL-101(Cr) and 35-Pd@MIL-101(Cr) obtained from a second-order Birch-Murnaghan equation of state.

	Sample	V0 (Å 3 ) /10 3	Bulk Modulus (GPa)
	MIL-101(Cr)	680.3±6.1	1.1±0.9
	20-Pd@MIL-101(Cr)	686.4±2.4	2.4±0.3
	35-Pd@MIL-101(Cr)	695.8±0.7	4.9±0.3

Table 5 . 1

 51 Vibrational bands of MIL-100(Cr) and proposed assignment. The * symbol marks the modes assigned in this work.

	Label	Frequency (cm -1 )	Appearance	Assignment 139,193
	ν1		weak, sharp	Collective*
	ν2		medium, sharp	Collective*
	ν3		weak, sharp
	ν4		strong, sharp	Cr-O
	ν5		medium, broad	Cr-O
	ν6		strong, sharp	C-C bending
	ν7		strong, sharp	Benzene ring bending
	ν8		weak, sharp
	ν9		weak, broad
	ν10		weak, sharp
	ν11		medium, sharp
	ν12		medium, broad
	ν13		medium, sharp
	ν14		medium, broad	Benzene ring deformation
	ν15		weak, broad
	ν16		medium, broad
	ν17		weak, broad
	ν18		weak, broad
	ν19		strong, sharp	COO -symmetric stretching
	ν20		strong, sharp	COO -symmetric stretching
	ν21		strong, shoulder	COO -asymmetric stretching
	ν22		strong, shoulder	C=C stretching
	ν23		strong, sharp	COO -asymmetric stretching
	ν24		weak, broad	C=O
	ν25		weak, broad	C-H stretching
	ν26		weak, sharp	Cr-OH stretching
	ν27		weak, broad	H2O stretching
	ν28		weak, sharp	H2O/H2O stretching

Table 5 . 2

 52 Characteristic parameters for MIL-101(Cr) and MIL-100(Cr). The bulk modulus obtained through the second-order Birch-Murnaghan (BM) EoS is taken into account for both frameworks.

		MIL-101(Cr)	MIL-100(Cr)
	Pore diameters (Å)	29 and 34	25 and 29
	Pore windows (Å)	11.7 and 16	5 and 8.6
	BET surface (m 2 g -1 )

NPs size distributions are given in Figure 4.3 for the two composites and were obtained through the statistical analysis of NPs sizes measured from ten to fifteen TEM images, counting at least 300 NPs. The average sizes result to be 1.1 nm with a standard deviation of σ = 0.3 nm for 20-Pd@MIL-101(Cr) and 1.6 nm (σ = 0.5 nm) for 35-Pd@MIL-101(Cr). Since these values are smaller than the pore sizes of MIL-101(Cr) (diameters of 29 and 34 Å), this is usually considered as an indirect proof of the effective insertion of the Pd NPs within the framework pores. 144,211 It is worth noting that, to the best of our knowledge, this is the first time that such high metal loading (35 wt.%) are achieved for the insertion of Pd nanoparticles with average size of 1.6 nm.

Textural properties

The N2 adsorption/desorption isotherms at 77 K for the two composites are shown in Figure 4.4 and compared to the pristine MIL-101(Cr) curve already reported in Figure 3.5. The three isotherms are typical of mesoporous materials as from the IUPAC classification discussed in section 2.1.3.3.

Overall, the isotherms of both composites show a lower N2 adsorption when compared to the pristine porous material. Both the specific surface area and the total pore volume, listed in Table 4.1, decrease by metal doping. This effect is due to the sample weight increase by Pd doping and to the presence of NPs which might hinder the access of N2 into the mesopores. [START_REF] Chen | Boosting Catalysis of Pd Nanoparticles in MOFs by Pore Wall Engineering: The Roles of Electron Transfer and Adsorption Energy[END_REF] that of the CrO mode when the amorphization process is completed, as tracked by the increase of the h(ν2)/h(ν1) ratio. This effect is clearly influenced by the presence of Pd NPs, in perfect agreement with the shift of the amorphization onset obtained from XRD.

From the peak fitting it is also possible to extract the peak frequencies which are represented in Figure 4.23(a) and (b) for both modes. The ν1 and ν2 frequencies are constant (or slightly redshift) in the pristine sample while a distinct behavior appears in the NPs doped samples, with a blue-shift followed by a red-shift as a function of pressure. The pressure value where this trend changes is affected by the metal loading, being 0.25 GPa for 20-Pd@MIL-101(Cr) and 0.4 for 35-Pd@MIL-101(Cr). These values, once again, correlates well with the amorphization onset. Indeed, when the system is still crystalline with the Bragg reflections still detected (see Figure 4.6), both modes show a pressure-induced hardening reflecting the usual reduction of interatomic distances. 157 On the contrary, the transition to the amorphous phase, i.e. the loss of long range order, is accompanied by the softening of these modes which suggest short-range instabilities. The delayed disappearance of the collective mode ν1 compared to the amorphization pressure value observed in XRD data indicates that shortrange order is still present after the amorphization, at least in a narrow pressure range, and it is then completely lost on further compression, as already discussed for the pristine MIL-101(Cr) in Section weight occurring between 100 and 300 °C, are attributed to the degassing of guest water molecules; the third one, between 300 and 400°C, is due to the decomposition of the framework.

Vibrational spectrum

The infrared spectrum of degassed MIL-100(Cr) isolated inside the DAC (without air exposure) was measured in the IR range from 100 to 5000 cm -1 and it is shown in Figure 5.7. Based on previous works about MIL-100(Cr) 139,214 , MIL-100(Al) 197 , chromium based MOFs with benzenetricarboxylic acid as linker, such as MIL-96(Cr) 215 and HKUST-1 216,217 , it is possible to When silicone oil is used as medium, the lattice parameter slightly increases up to 0.2 GPa, as shown in Figure 5.9(a), and then starts to decrease with pressure. This small volume expansion of the unit cell can be explained by a pressure-promoted hyperfilling of the MIL-100(Cr) pores by the PDMS, as already observed on MIL-101(Cr) (see section 3.2.2) at a lower extent. Indeed, MIL-100(Cr) presents two pore cages with diameters of about 24 Å and 29 Å, respectively, which are smaller than MIL-101(Cr) ones (see Figure 1.30). As stated in section 2.2.6, the average size of PDMS polymer chains is larger than the MOF cavities, hindering their penetration within the pores at ambient pressure.

Infrared spectroscopy measurements

The high-pressure behavior of MIL-100(Cr) with a solid and fluid PTM was also studied exploiting infrared spectroscopy in the 100-5000 cm -1 frequency range. The spectral differences observed with the two PTM at the lowest possible pressure will be first reported. Later, the evolution of MIL-100(Cr) spectra under pressure will be shown and discussed.