Thomas Vosegaard

Nielsen

Dedicated to my parents Nancy and Jaime who with all their effort, wisdom and love, made me the person I am today Dedicado a mis padres Nancy y Jaime, quienes con su esfuerzo, sabiduria y amor, me han hecho la persona que soy hoy Development of high-field (DNP)-NMR methods to detect quadrupolar nuclei on surfaces i

the materials. Furthermore, they can be used to acquire heteronuclear correlation spectra of protons and quadrupolar nuclei at fast MAS (62.5 kHz).

For the observation of homonuclear proximities between quadrupolar nuclei, we have compared the performances of symmetry-based homonuclear dipolar recoupling schemes for nuclei with spin value I = 3/2, such as 11 B, and 5/2, such as 27 Al. For both isotopes, the double-quantum (DQ) -single-quantum (SQ) sequences are recommended, which yields good efficiency for both the cross-and auto-correlation peaks. For 11 B isotope, the chemical shift differences are small or moderate, and therefore, bracketed recoupling schemes are recommended. For 27 Al nuclei, the chemical shift differences are larger and un-bracketed recoupling schemes are recommended.

We also investigated the surface of boron nitride and oxide supported on dendritic fibrous nanosilica, which are catalyst for the oxidative dehydrogenation of propane.

Through a combination of 1D and 2D multinuclear ( 1 H, 29 Si and 11 B) solid-state NMR experiments, it was demonstrated that BN phase undergoes a transformation to B2O3 (oxidation) during the reaction, which led to a larger fraction of hydrogen-bonded BOH groups at the surface of oxidized DFNS/BN with respect to as-prepared DFNS/B2O3. This structural difference could explain the higher selectivity towards propene of oxidized DFNS/BN.

Résumé

Les surfaces sont des régions-clefs des matériaux. Les propriétés de surface jouent un rôle important pour de nombreuses applications, telles que les nanosciences, l'électrochimie, l'électronique, la conversion et le stockage d'énergie, ou encore la médecine. Par conséquent, la compréhension des processus de surface et la caractérisation de leurs propriétés chimiques sont d'une grande importance dans le développement de nouveaux matériaux. Cependant, la caractérisation structurale des espèces de surface au niveau atomique est souvent difficile en raison de leur absence d'ordre à longue distance et de leur faible abondance par rapport aux autres sites. La spectroscopie RMN à l'état solide fournit des informations uniques sur la structure atomique locale. Cependant, la faible sensibilité intrinsèque de la RMN représente une limitation importante pour la caractérisation de la surface, surtout lorsque les matériaux étudiés contiennent des noyaux quadripolaires.

Les noyaux quadripolaires de spin demi-entier, tels que 11 B, 17 O et 27 Al, constituent environ les deux tiers des noyaux stables magnétiquement actifs et sont présents dans de nombreux solides organiques, inorganiques et biologiques. Cependant, l'observation par RMN de ces noyaux sur des surfaces solides reste souvent difficile en raison de la taille importante de la matrice de densité ainsi que de l'interaction quadripolaire, qui élargit les raies et complique la dynamique de spin. Le signal RMN de surface peut être détecté sélectivement par transfert cohérent de la polarisation des protons vers les noyaux de surface. Dans le cas des noyaux quadripolaires demientiers, la technique de polarisation croisée en rotation à l'angle magique (crosspolarization under magic-angle spinning, CPMAS, en anglais) manque de robustesse et il est préférable d'utiliser des méthodes n'utilisant pas de verrouillage de l'aimantation (spin lock en anglais) des noyaux quadripolaires. Deux types de séquences de ce type ont été proposées: la séquence D-RINEPT et la séquence PRESTO.

Dans ce travail, nous avons introduit de nouveaux recouplages dipolaires hétéronucléaires robustes basés sur la symétrie construits à partir d'impulsions d'inversion composites, qui peuvent être incorporées dans les séquences D-RINEPT et PRESTO pour transférer la polarisation des protons aux noyaux quadripolaires à des fréquences MAS supérieures à 20 kHz. Ces transferts peuvent être combinés avec la polarisation dynamique nucléaire (dynamic nuclear polarization, DNP, en anglais) pour augmenter les signaux RMN des noyaux quadripolaires, en particulier près de la surface des matériaux. De plus, ils peuvent être utilisés pour enregistrer des spectres de corrélation hétéronucléaire de protons et de noyaux quadripolaires à des fréquences MAS élevées (62,5 kHz).

Pour l'observation des proximités homonucléaires entre noyaux quadripolaires, nous avons comparé les performances de recouplages dipolaires homonucléaires basés sur la symétrie pour les noyaux de spin I = 3/2 (tels que 11 B) et 5/2 (tels que 27 Al). Pour les deux isotopes, les séquences de corrélation double-quantum (DQ) simple-quantum (SQ) sont recommandées car elles conduisent à une bonne efficacité, à la fois pour les pics de corrélation et d'auto-corrélation, fournissant l'ensemble des proximités spatiales, même pour les résonances ayant des fréquences de résonance proches ou identiques.

Nous avons également étudié au cours de cette thèse la structure à l'échelle atomique de nitrure et d'oxyde de bore supportés sur des nanoparticules de silice fibreuses dendritiques, qui sont des catalyseurs pour la déshydrogénation oxydative du propane. Grâce des expériences de RMN à l'état solide multinucléaires 1D et 2D ( 1 H,

List of Figures

. 27 Al 1D spectra of -alumina at 18.8 T with R = 20 (a) and 62.5 (b) kHz acquired using 1 H → 27 Al transfers with RINEPT-CWc sequence and SR4 1 2 (tt), SR4 1 2 (270090180) and R12 3 5 (270090180) recoupling, or PRESTO scheme and (a) R22 2 7 (1800) or R18 2 5 (1800), or (b) R16 7 6 (270090180) and R14 6 5 (270090180) recoupling. The  delays and 1/1,max RF-fields were fixed to their optimum values given in Tables 7 and9 27 AlO6 on-resonance signal of -alumina at R = 20 kHz as function of 1 or 1,max for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table 7. ..................

. 27 AlO6 signal of -alumina at R = 20 kHz as function of offset for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 7. ...............

. 27 Al 1D spectra of AlPO4-14 at 18.8 T with R = 20 (a) and 62.5 (b) kHz acquired using 1 H → 27 Al transfers with RINEPT-CWc sequence and SR4 1 2 (tt), SR4 1 2 (270090180) and R12 3 5 (270090180) recoupling, or PRESTO sequence and (a) R22 2 7 (1800) and R18 2 5 (1800), or (b) R16 7 6 (270090180) and R14 6 5 (270090180) recoupling. The  delays and 1/1,max RF-fields were fixed to their optimal values given in Tables 8 and10. The resonance at ca. 11 ppm in (a) is due to an impurity. ..............

. 1 H MAS spectrum of AlPO4-14 acquired at B0 = 18.8 T and R = 20 kHz by averaging 16 transients separated by a recycle interval of 1 s, using the DEPTH pulse sequence for probe background suppression, with ν1  208 kHz. 72

. 27 AlO4 signal of AlPO4-14 at R = 20 kHz as function of 1 or 1,max of the recoupling for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table 8. ..................

. 27 AlO4 signal of AlPO4-14 at R = 20 kHz as function of offset of the recoupling for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 8 27 AlO6 on-resonance signal of -alumina at R = 62.5 kHz as function of 1 or 1,max for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table 9.

. 27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of 1 or 1,max of the recoupling for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table 10.

. 27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of offset of the recoupling for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 10 27 Al HETCOR 2D spectra of AlPO4-14 recorded with RINEPT-CWc-SR4 1 2 (270090180), SR4 1 2 (tt), SR4 1 2 (180090180) and PRESTO-R16 7 6 (270090180) transfers. All 2D spectra were acquired using NUS 25% in 72 min. .......................... , and (e) BR2 2 1 . In (a-c,e), white and grey colors mean phases of 90° and 270° respectively, and in (d) yellow and green colors mean phases of 45° and 315° respectively. In each case, the number is the flip-angle for the CT ] and BR2 2 1 , sequences versus (a) 1 ranging from 1 to 8 kHz and (b) offset ranging from +10 to -10 kHz. The spectra obtained with SR2 2 1 scheme are not shown because it is much less efficient than these three schemes (see Fig. recoupling for DQ-SQ sequence at B0 = 18.8 T with a MAS frequency of 20 kHz. An expansion of the cross-peaks is also shown. All 2D spectra were acquired using uniform sampling. The 2D DQ-SQ and SQ-SQ spectra result from averaging 64 and 48 transients, respectively, using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 2h30 min and 1h50 min, respectively. The signal was normalized with respect to the number of transients. We employed optimal rf field strength equal to 1 = 3.7 kHz for BR2 2 1 , 4.1 kHz for [BR2 2 1 ] and [SR2 2 1 ] and 1 = 3.9 kHz for SR2 2 1 . ...........................

Figure 46.

2D 27 Al SQ-SQ D-HOMCOR spectrum of AlPO4-14 using (SR2 2 1 ) 4 recoupling at B0 = 18.8 T with a MAS frequency of 20 kHz acquired with NUS retaining 33% of the t1 points. Cross-and diagonal peaks are indicated with orange and blue ovals, respectively. The extra framework impurity at -2 kHz is indicated with a star. We used 𝜏 𝑚𝑖𝑥 = 27 Al DQ-SQ D-HOMCOR spectrum of AlPO4-14 using BR2 2 1 recoupling acquired with NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz. Cross-and auto-peaks are indicated with orange and blue circles, respectively. The 2D SQ-SQ spectrum results from averaging 64 transients using a relaxation delay RD = 1 s for each of 330 t1-increments, leading to an experimental time Texp = 2h. The recoupling used the following parameters: 1 = 2.9 kHz, offset = 4 kHz (19. 10,[14][15][16]46,[51][52][53][54][55][56][57][58][59][60][61][62]77 It should be noted that the 1 values presented here result from a linear combination of 2 and 1(non-sheared). They do not directly correspond to iso (the isotropic chemical shift). 35,78

List of Tables

General introduction

Surfaces are a key region of materials. Their physical and chemical properties define the way materials interact with the environment and determine their reactivity.

Understanding the role of surfaces is of increasing relevance in many fields as diverse as heterogeneous catalysis, corrosion inhibition, nanoscience, electronics, energy conversion and storage, as well as health and medicine. [1][2][3][4] With the emerging new technologies, there is a need for understanding of fundamental processes and controlling the assembly of atoms and molecules of highly complex surfaces.

However, the structural characterization of solid surfaces is frequently challenging because of the significantly small proportions of atoms at the very surface, and the difficulty of detecting them and distinguishing them from those located in the bulk region of the material. Solid-state NMR spectroscopy has emerged as a powerful characterization tool for solid surfaces. It provides unique information on the local atomic-level of surface sites, thus probing short-range ordering and dynamics.

Nevertheless, its lack of sensitivity poses a major limitation to surface characterization, especially when the investigated materials contain quadrupolar nuclei.

Quadrupolar nuclei with a half-integer spin value (I = 3/2, 5/2, 7/2 and 9/2) represent two-thirds of the NMR actives isotopes. 5,6 Nuclei, such as 11 B, 17 O and 27 Al, are present in organic, inorganic and biological solids. Nevertheless, the NMR observation of these nuclei near surfaces remains often more challenging than those of spin-1/2 nuclei. This difficulty comes notably from the large anisotropic quadrupolar interaction, which broadens the resonance, decreases their intensity and complexifies the spin dynamics. Moreover, some of the relevant quadrupolar isotopes have low gyromagnetic ratios (γ), or low natural abundance, which represent additional challenges for the NMR detection.

Dynamic Nuclear Polarization (DNP), which consists in the microwave-driven transfer of polarization from unpaired electrons to surrounding nuclei, has enabled the selective enhancement of NMR signals of surface species, notably for spin-1/2 nuclei.

This approach has been called dynamic nuclear polarization surface-enhanced spectroscopy (DNP-SENS) and generally relies on indirect DNP via protons, since 1 H-1 H spin diffusion efficiently transports the polarization within the sample. The DNPenhanced 1 H polarisation is usually transferred to the detected nuclei using crosspolarization under magic-angle spinning (CPMAS) sequence. 7 In the case of inorganic This thesis aims at developing methods for detecting quadrupolar nuclei near surfaces, based on symmetry-based recoupling schemes, for reintroducing hetero-and homo-nuclear dipolar couplings. These new heteronuclear dipolar recoupling techniques will be combined with PRESTO and D-RINEPT techniques to transfer DNP-enhanced 1 H polarization to half-integer quadrupolar isotopes. We will also compare different approaches to probe proximities between identical quadrupolar nuclei. Therefore, this dissertation makes an important contribution to the observation of quadrupolar nuclei in materials, notably near surfaces. The overall structure of the study takes the form of four chapters organized in the following way.

Chapter 1 lays out the importance of surfaces and presents the theoretical background of NMR of quadrupolar nuclei and methods used for this study.

Chapter 2 introduces new recoupling schemes based on single and composite π pulses for PRESTO and D-RINEPT sequences. Their performances were evaluated for 1 H→ 27 Al magnetization transfers at moderate (20 kHz) and high (62.5 kHz) MAS frequencies, using samples of γ-Al2O3 and AlPO4-14, which exhibit small and moderate 1 H-1 H dipolar interactions, respectively. These techniques are expected to be useful for transferring the DNP-enhanced magnetization of 1

Applications to materials science

Surfaces and interfaces play a significant role in defining the boundary region of a material and controlling the interactions with its environment. 1 Both the nature and strength of these interactions are defined by interfacial properties. 2 Many critical processes, such as corrosion, adhesion, adsorption, friction and catalysis, take place at the boundaries of a material 3,4 and can affect the chemical composition and/or atomic and electronic structure of the surfaces. 1 Understanding the role of surfaces and interfaces is relevant to fields as diverse as catalysis, surface physics, corrosion, nanoscience, tribology, electronics, geochemistry, electrochemistry, energy conversion and storage, chemical and petroleum processing, as well as health and medicine. 1,[5][6][7][8][9] Furthermore, modern surface science has become increasingly important in the field of nanomaterials, 7,10 where surfaces are critical in determining the properties, stability and functionalities of materials such as quantum dots, 11,12 battery materials, 13 nanoparticles, [14][15][16] nanocrystals 17 and nanowires. 18 Generally, the chemical composition of the surfaces differs from that of the bulk.

Surface atoms have fewer neighbours and therefore cannot satisfy their bonding requirements, resulting in higher chemical reactivity. 5,[19][20][21] This property makes surface chemistry unique, and a favoured medium for many chemical and biological reactions to occur. 1 With the advent of new technologies and applications, there is a need for improved understanding and controlling the assembly of atoms and molecules at well-defined surfaces in complex environments. 5,7 Therefore, fundamental and applied studies of surface and interface processes and characterization of the electronic, magnetic and chemical properties are of central importance in developing new materials to meet today's challenges.

Techniques to characterize surfaces

One of many grand challenges of surface chemistry is the difficulty of experimentally determining surface structure. This stems from the significantly small proportion of atoms at the very surface, and the difficulty of detecting them and distinguishing them from those located in the bulk region of the material. 1,22 In recent years, many surface science techniques have been developed and adapted to study interfacial phenomena at the molecular level. 1 Adsorption-desorption methods 5 and microscopies, can provide information about the topology of the surface and its surface area. The microscopies encompass scanning probe microscopies, such as atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), 5,22,23 and electron microscopies, such as transmission electron microscope (TEM) and the scanning electron microscope (SEM). 24,25 Information on the chemical composition and atomic-level structures of the surfaces can be obtained using surface-sensitive spectroscopies, including X-ray photoelectron and Auger electron spectroscopies, as well as secondary ion mass spectrometry (SIMS). Furthermore, compared to techniques mentioned in the previous paragraph, they can be employed in practical working environments (operando or insitu). 14,26,27 They are based on stimulating the surface of the sample with incoming (incident) particles (photons, electrons and ions) and monitoring the signals produced by the ejected particles (photons, electrons and ions) that are only emitted by the surface, as a result of said stimulation. 6,[28][29][30] The analysis can reveal key information about the elemental composition as well as the atomic-level structure of the surface of the studied material. 6,14 The main characteristics and type of information that can be extracted from most commonly used surface methods are summarized in Table 1.

Table 1. General characteristics of common surface analysis methods

In X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), photons are used as a probe and electrons are detected. 31,32 high sensitivity. [33][34][35] In ion scattering spectroscopy (ISS), also known as low-energy ion scattering (LEIS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), ions are used as a probe and the detected particles. 36,37 These different surface characterization methods are summarized in Figure 1. 1.

Each technique has a specific sampling profile and its own unique capabilities. Most of these methods have a nanometric resolution in at least one dimension and require that the analysis be performed in an ultra-high vacuum apparatus. 1,6 Generally, no single technique provides enough information to solve a complex real-world problem.

However, different surface analysis techniques can provide complementary and comprehensive information. 8 The methods presented here are under continuous development. Advances in technology and instrumentation, such as more efficient vacuum systems made possible characterization of the structure and reactivity of a wide range of surfaces.

NMR of surfaces

Recently, solid-state NMR spectroscopy has emerged as a powerful characterization tool for solid surfaces. 38 It allows determining the atomic-level structure of surface species, thus probing short-range ordering and dynamics. 7,39 This makes solid-state NMR an ideal tool for establishing structure-property relationships and the development of advanced materials including catalysts, [39][40][41][42] thin-films, [43][44][45] and nanoparticles [46][47][48][49] .

Compared to diffraction techniques, which detect long-range ordering and require some structural regularity, 38,50 solid-state NMR is sensitive to local interactions and geometries 51 of real-world materials on the micro-and nano-scale and thus, is wellsuited for disordered or non-crystalline surfaces. 7,52,53 However, the low concentration of the surface sites and the inherent low sensitivity of NMR poses limit to a broader application of this technique. 51,54,55 Recent advances in polarization techniques that selectively enhance surface NMR signals 56 and selective labelling, 57,58 have potentiated its application and opened new insights in surface chemistry.

Isotropic chemical shifts are the most employed information obtained from solidstate NMR. 51 They allow structural assignments of various surface chemical species of the nucleus under study, as the chemical shifts are directly related to the chemical environment that arises from the interaction of the probed nucleus and its electronic cloud. 38,59 Figure 2 summarizes the internal interactions in solid-state NMR and the structural information that can be obtained from the NMR spectra. However, unlike in solution-state NMR, the effect of anisotropic interactions, such as dipolar coupling and chemical shift anisotropy (CSA), can broaden the NMR signals and prevents the measurement of the isotropic chemical shifts. 60 By spinning the sample at the magic angle (54.74°) with respect to the direction of the magnetic field, it is possible to average these anisotropic interactions, and to measure reliably the isotropic chemical shifts, iso. 61,62 One-dimensional (1D) solid-state magic angle spinning NMR (MAS NMR) experiments have been used to reveal detailed structural information on the surface of a wide-range of materials. 7,38,63 1 H, 13 C, 29 Si and 31 P nuclei are the most commonly detected spin-1/2 isotopes to solve questions regarding local structures and chemical processes on surfaces. 39,52 Details of these nuclei and other common NMR-active nuclei found on solid surfaces and their nuclear magnetic properties are listed in Table 2.

Table 2. Summary of the nuclear magnetic properties of some nuclei commonly found in solid surfaces. 64 The development of MAS and cross-polarization (CP) 66,67 has enabled the acquisition of 1D spectra under high-resolution. The CPMAS technique, which transfers the polarization of protons to another isotope through dipolar couplings, allows the detection of insensitive nuclei like 13 C and 15 N. 67 Under Hartman-Hann conditions, 68 the CPMAS sequence is commonly employed to obtain selectively NMR spectra of surface nuclei, such as 29 Si or 31 P, in materials, such as oxides, in which protons are only located on the surface. 69 A wide range of surface species has been characterized using CPMAS spectroscopy. [70][71][72] For example, 1D 1 H- 29 Si CPMAS and 1 H-31 P CPMAS experiments have been applied to probe the surface of siliconnanocrystals, 54 Si-nanopowders 73 and functionalized silica nanoparticles, 74 providing key information to resolve the chemical structure of the surface.

In addition to 1D experiments, multidimensional experiments are often needed to elucidate structural information and confirm assignments made from one-dimensional experiments. One of the most commonly employed 2D methods is heteronuclear correlation spectroscopy (HETCOR) using CP transfer (CP-HETCOR). 75 For instance, CP-HETCOR NMR is used to determine the proximity between protons and heteronuclei (e.g. 13 C, 29 Si, 31 P) by establishing correlations between neighbouring atoms. 76 Advances in proton homo-nuclear decoupling schemes 54 have enabled the detection of high-resolution 2D-HETCOR experiments. For instance, 1 H- 29 Si HETCOR NMR spectroscopy allowed selectively filtering the NMR signal of 1 H close to 29 Si surface sites in hybrid organic-inorganic interfaces in surfactant templated silicas that have been prepared under acidic and basic conditions, showing the clear difference in the number of Si-OH groups between the acidic and basic systems. The acidic sample exhibits a silanol rich surface with a longer distance between the Si surface sites and the polar head groups, whereas the basic sample exhibits a small amount of Si-OH groups and a short distance between the Si-O-surface groups and the surfactant polar head group. 77 Remarkable gains in sensitivity have been achieved by recording HETCOR experiments through the detection of 1 H rather than moderate to low-γ nuclei such as 13 C, as 1 H has a larger gyromagnetic ratio (γ) and natural abundance of 99.985%. This indirect detection of heteronuclei can be achieved using double CP transfer. The gain in sensitivity provided by indirect detection allows the acquisition of through-space 2D HETCOR spectra of species bound to a surface at natural abundance with a reduced acquisition time. 78 As an illustration, 2D 1 H-13 C HETCOR NMR and 1 H- 15 N HETCOR NMR spectra of allyl groups covalently bound to the surfaces of mesoporous silica nanoparticles reveal the spatial proximity between allyl groups and the silica surface. 79 More recently, Swanson and collaborators demonstrated the application of 1 H- 29 Si DQ-SQ MAS NMR also have been used to probe through-space proximities between all silicon species in the surface of functionalized mesoporous silica nanoparticles. [START_REF] Kobayashi | Spatial Distribution of Organic Functional Groups Supported on Mesoporous Silica Nanoparticles: A Study by Conventional and DNP-Enhanced 29 Si Solid-State NMR[END_REF][START_REF] Zhao | Atomic-Scale Structure of Mesoporous Silica-Encapsulated Pt and PtSn Nanoparticles Revealed by Dynamic Nuclear Polarization-Enhanced 29 Si MAS NMR Spectroscopy[END_REF] The acquisition of a train of echoes using Carr-Purcell Meiboom-Gill (CPMG) sequence [START_REF] Hung | Application of the Carr-Purcell Meiboom-Gill Pulse Sequence for the Acquisition of Solid-State NMR Spectra of Spin-1/2 Nuclei[END_REF] can further enhance the NMR signal of these 29 Si DQ-SQ experiments.

Importance of quadrupolar nuclei near surfaces

Many inorganic materials contain quadrupolar nuclei with spin I ≥ 1, which represent about 75% of NMR-active isotopes. [START_REF] Ashbrook | Structural Information from Quadrupolar Nuclei in Solid State NMR[END_REF] . Nuclei, such as 27 Al (I = 5/2), 17 O (I = 5/2), 6 Li (I = 1), 7 Li (I = 3/2), 11 B (I = 3/2), 14 N (I = 1), 23 Na (I = 3/2), 25 Mg (I = 5/2),

Cl (I = 3/2), and 39 K (I = 3/2), are present in catalysts, inorganic oxides, polymers, clays, glasses, energy materials, pharmaceutical and biomaterials. 65,[START_REF] Stone-Weiss | An Insight into the Corrosion of Alkali Aluminoborosilicate Glasses in Acidic Environments[END_REF][START_REF] Xu | Molecular-Level Environments of Intercalated Chloroaluminate Anions in Rechargeable Aluminum-Graphite Batteries Revealed by Solid-State NMR Spectroscopy[END_REF][91] Compared to spin-1/2 nuclei, an additional difficulty for the detection of quadrupolar nuclei near surfaces is that their NMR signals are broadened by the quadrupolar interaction, which results from the interaction between the electric quadrupolar moment of the quadrupolar nuclei and the electric field gradient. This interaction is generally stronger than the dipolar and CSA interactions and gives rise to broadening often over several megahertz, which cannot be removed by MAS. [START_REF] Ashbrook | Structural Information from Quadrupolar Nuclei in Solid State NMR[END_REF][START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF] Second, some of the commonly studied nuclei have low gyromagnetic ratios (γ), or high quadrupole moments and or low natural abundances, which represent additional challenges for the NMR detection. [START_REF] Walkley | Solid-State Nuclear Magnetic Resonance Spectroscopy of Cements[END_REF][START_REF] Smith | Low-γ Nuclei in Solid Samples[END_REF] Furthermore, the lower coordination number of surface atoms results in higher electric field gradient and hence, larger quadrupolar interaction.

However, structural information can be obtained by evaluating the quadrupolar interaction, which depends on the quadrupolar coupling constant (CQ) and the asymmetry parameter of the electric field gradient (ηQ). [START_REF] Ashbrook | Structural Information from Quadrupolar Nuclei in Solid State NMR[END_REF] Both parameters are sensitive to the coordination environment, and combined with isotropic chemical shifts iso, they can help determining the local structure of surface sites. 60 The NMR spectra of half-integer spin quadrupolar nuclei (e.g. 27 Al, 11 B, 17 O) are often dominated by the central transition (CT) between energy levels mI = +1/2 and -1/2 since this transition is not broadened by the first-order quadrupolar interaction but only by the second-order quadrupolar interaction. [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Nevertheless, this secondorder quadrupolar interaction is not completely averaged out by MAS. 61 Conversely, high magnetic fields reduce the broadening due to the second-order quadrupolar interaction and thus, improve the resolution of the NMR spectra of quadrupolar nuclei. [START_REF] Lin | High Magnetic Field Solid-State NMR Analyses by Combining MAS, MQ-MAS, Homo-Nuclear and Hetero-Nuclear Correlation Experiments[END_REF][START_REF] Gan | Seeking Higher Resolution and Sensitivity for NMR of Quadrupolar Nuclei at Ultrahigh Magnetic Fields[END_REF] Furthermore, multiple-quantum MAS (MQMAS) experiment is a 2D NMR method, which allows the acquisition of high-resolution NMR spectra of quadrupolar nuclei by removing broadening due to second-order quadrupolar interaction. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF] This technique correlates different transitions within the spin system and yields an isotropic spectrum from the projection onto the indirect dimension. Other approaches have been proposed to record high-resolution spectra of quadrupolar nuclei. Some of these techniques involve sample rotation around two different angles: simultaneously for double rotation (DOR), and sequentially for dynamic-angle spinning (DAS). [START_REF] Ashbrook | Recent Advances in Solid-State Nuclear Magnetic Resonance Spectroscopy[END_REF] The MQMAS, DOR and DAS experiments will be presented in more details in Section 1.4.

An example of quadrupolar nucleus detected near surface is 27 as isotropic chemical shifts as well as CQ and ɳQ quadrupolar parameters. [START_REF] Ashbrook | Structural Information from Quadrupolar Nuclei in Solid State NMR[END_REF] In particular, the coordination of the aluminium can be deduced from its isotropic chemical shift.

For example, Xu et al. used 27 Al MAS and MQMAS NMR experiments combined with high magnetic fields to study a series of transition aluminium oxides where the γ-, δ-, or θ-Al2O3 dominates. 101 Specifically, MQMAS was used to resolve and obtain the quadrupolar parameters of the different aluminium sites, thus, allowing to distinguish the aluminium environments of each of the -, and -phases. It was found that -Al2O3 contains three distinct octahedral sites and two aluminium tetrahedral sites. These results allowed to determine also that the -Al2O3 consists of local structures similar to those found in θ-Al2O3 as well as δ-Al2O3 phases but with less ordering, which before could not be observed at low field-NMR. More recently, Kaushik et al. reported the use of 1D and 2D NMR experiments (including 27 Al MAS, 27 Al{ 1 H} and 29 Si{ 27 Al} D-HMQC and 27 Al DQ/SQ) to get detailed insight into the structure of the surface species on alumina oxide/silica thin-films prepared by atomic layer deposition (ALD). 102 The results showed that in films with 3 wt% of Al, a submonolayer is formed and possibly contains aluminium species ( [n] Al where n = 4, 5 or 6) which have been substituted by Si in the second coordination sphere such as [4] Al(3Si), [4] Al(4Si) and [5] Al(2Si) on the silica surface, with most of these sites attached to OH groups, whereas films with 9 or 15 wt% of Al exhibit characteristics of an amorphous alumina phase with a high concentration of [5] Al species and OH groups.

In addition, the results gave further evidence that the most likely species at the interface between silica and alumina are [4] Al(2Si), [4] Al(3Si) and [5] Al(2Si).

Another isotope of interest for the characterization of surfaces is 17 for all the materials, while D-HMQC helped to probe the proximities between 17 O and 1 H sites and gain further information on the spatial configuration of the surface species.

These results show that 17 O NMR can provide unprecedented information on the interactions between the metallic complexes and their support in supported catalysts. 17 O NMR spectroscopy has been also applied to study CeO2 nanoparticles. 49,106,107 They made use of hyperpolarization techniques, in particular dynamic nuclear polarisation (DNP), to selectively enhance the NMR signals of the surface species on isotopically-labelled samples. This technique so-called DNP-SENS 108 will be discussed in more detail in section 1.5.4.

QUADRUPOLAR INTERACTION

Quadrupolar nuclei, as mentioned above, have spin I larger than 1/2. These nuclei have a non-spherical distribution of charge in the nucleus that gives rise to an electric quadrupolar moment (eQ), independent of its environment. [START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF][START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF] The quadrupolar interaction then, results when the quadrupolar moment interacts with the surrounding electric field gradient (EFG). The EFG is generated by the electronic charge distribution and is defined by three components, 𝑉 𝑋𝑋 , 𝑉 𝑌𝑌 and 𝑉 𝑍𝑍 , in its principal axis system (PAS) (Figure 3). 109 The quadrupolar interaction is anisotropic i.e., depends upon orientation and is usually described in terms of two parameters: the quadrupolar coupling constant, 𝐶 𝑄 = 𝑒𝑄𝑉 𝑍𝑍 /ℎ (with h = 6.62610 -34 J.Hz -1 being the Planck constant and 𝐶 𝑄 in units of hertz) which quantifies its magnitude, and the asymmetry parameter, 𝜂 𝑄 = (𝑉 𝑋𝑋 -𝑉 𝑌𝑌 )/𝑉 𝑍𝑍 , which describes the shape of the interaction and takes values between 0 ≤ 𝜂 𝑄 ≤ 1. 110,111 The 𝐶 𝑄 values are typically in the order of a few megahertz to tens of megahertz for quadrupolar spins in sites of non-cubic symmetry. [START_REF] Ashbrook | Structural Information from Quadrupolar Nuclei in Solid State NMR[END_REF] However, in most practical cases the quadrupolar interaction is much smaller than the Zeeman interaction, and thus, its effect can be considered as a perturbation to the Zeeman energy levels. [START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF]110 The quadrupolar interaction can be divided into two contributions: the first-order, 𝐻 𝑄 (1) , and second-order, 𝐻 𝑄 (2) . 109,112 Figure 4 shows the Zeeman energy levels for nuclei with I = 3/2 and the perturbation caused by the quadrupolar interaction when considering 𝐻 𝑄 (1) and 𝐻 𝑄 (2) .

The first-order quadrupolar interaction generates small shifts of the energy levels and splits the NMR signal into 2I allowed transitions between the 2I + 1 Zeeman levels. 65,[START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF]109 For half-integer quadrupolar nuclei, they are referred as the central transition (CT) occurring between energy levels 𝑚 𝐼 = +1/2 and -1/2, and the satellite transitions (STs) between energy levels mI and mI+1. The resonance frequency of STs depends on both first-and second-order quadrupolar interactions. As the strength of the first-order quadrupolar interaction generally exceeds the MAS frequency, this interaction cannot be completely removed by MAS and the STs exhibits several spinning sidebands. 111 Conversely, the central transition (CT) is unaffected by the quadrupolar interaction to the first-order 113 and hence, is in many cases the only transition observed experimentally. 92 The effect of 𝐻 𝑄 (2) is to shift further all the lines, including the CT, 109 as observed in Figure 4. This interaction has a more complex angular dependence than the firstorder quadrupolar interaction, therefore the 𝐻 𝑄 (2) effect on the NMR spectrum is only partially averaged by MAS. 111 Nevertheless, the second-order quadrupolar broadening is proportional to 𝐶 𝑄 2 /B0 and hence the use of high magnetic fields improves the spectral resolution. 110

EFFECTS OF RF FIELD FOR QUADRUPOLAR NUCLEI

Radiofrequency (rf) field irradiation allows the manipulation of the magnetization and coherences in NMR. 114 In contrast to their spin I = 1/2 counterparts, the rf interaction with quadrupolar nuclei in a high magnetic field is much more complicated, and renders NMR experiments of these nuclei challenging. [114][115][116] The response of the spins to the rf irradiation depends on the relative magnitudes of three parameters: (i) the rf amplitude (ω 1 ) of the pulse, (ii) the first-order quadrupolar splitting (ω Q ), and

(iii) the irradiation offset (∆ω 0 ), which is the difference between the resonance frequency and the carrier frequency of the rf pulse. 115,117,118 Usually, the first-order quadrupolar interaction is much larger than the achievable nutation frequencies (i.e.

rotation of the magnetization under the influence of the rf field), and therefore cannot be neglected during the excitation of the spin system. 110,118 The rf pulses on quadrupolar nuclei can be classified as selective or nonselective, 119 as illustrated in Figure 5 for a spin I = 3/2. Consequently, the behaviour of quadrupolar spins under rf irradiation, can be summarized in three regimes. 117 In the limiting case where the rf pulse amplitude is greater than the strength of the first-order quadrupolar interaction, ω 1 ≫ ω Q , this corresponds to a non-selective or hard pulse irradiation where all NMR transitions (central and satellite transitions) are excited simultaneously (Case A). 118 Under this regime, the nutation frequency of the CT is given by ω 𝑛𝑢𝑡 = ω 1 . 117,118 These non-selective RF pulses are used primarily for the acquisition of one-dimensional (1D) NMR spectra. In addition, for acquiring quantitative 1D NMR spectra, the quadrupolar nuclei are excited by a single nonselective pulse that produces small flip angle rotations of the CT magnetization. 110 ) ω 𝑟𝑓 , where ω 𝑟𝑓 is the rf nutation frequency for an isotropic liquid or a solid with 𝐶 𝑄 = 0. 110,119,120 Most NMR studies 121 of half-integer quadrupolar nuclei involve selective excitation of the CT transition. 119 These CT-selective pulses are mainly used to tilt the magnetization related to the CT away from the B0 axis by an arbitrary angle. 110 For example, 90° and 180° CT-selective pulses are employed in numerous pulse sequences that are described in section 1.4.

Furthermore, selective pulses can be used to excite the satellite transitions STs (Case C) by using a nutation frequency ω 𝑛𝑢𝑡 = ω 1 √3. However this particular case only involves single crystal samples, since selective excitation of STs cannot be achieved simultaneously for all crystallites in a powder sample. 115 In between these two regimes, there is a fourth regime, where ω Q ≈ ω 1 . The spin dynamics of quadrupolar spins become complex and the degree of excitation highly depends on the strength of the quadrupolar interaction. 110,117 Nevertheless, the magnetization can be also converted into multiple quantum coherences. 117,122 In particular, triple quantum coherences can be excited with a hard pulse, thus allowing experiments as MQMAS. 121 This experiment is explained in more detail in section 1.4.1.

HIGH-RESOLUTION NMR SPECTRA OF QUADRUPOLAR NUCLEI

The power of solid-state NMR to probe the structure of materials relies strongly on the availability of recording high-resolution spectra. 123 As mentioned earlier, the second-order quadrupolar interaction poses a limit to obtaining high resolution spectra of quadrupolar spins, since it cannot be completely removed by MAS due to its complex angular dependence. 119 There are several sophisticated methods to remove the broadening caused by this interaction. 120 One experimental approach to overcome this is to find an angle  such that 𝑃 2 (cos 𝜃) = 𝑃 4 (cos 𝜃) = 0 (where P2 and P4

denote the second-and fourth-order Legendre polynomials) is satisfied. 117 Two techniques, DOR (Double Rotation) 124 and DAS (Dynamic Angle Spinning) 125 , were initially developed to remove the second-order broadening of the CT. They involved space averaging of the second-order quadrupolar interaction through mechanical rotation of the sample holder. 117 In DOR, the sample is placed within an inner rotor, which is spun inside an outer rotor, and the assembly is spun simultaneously around two different axes (30.56° and 54.74°) relative to B0. 126 The DOR technique enables the elimination of the first-and second-order terms of the quadrupolar interaction in a 1D experiment. 127 On the other hand, DAS is a 2D experiment that works by rapidly switching the rotor between two axes. 128,129 Therefore, it requires specialized instrumentation for the rapid switching and stabilization of the rotor during the experiment. 130,131 Since this is a 2D experiment, experimental times are typically longer than those for DOR. 116,131 Even if the development of DOR and DAS allowed an elegant solution to the high resolution problem, their use is limited nowadays since they both require dedicated NMR probes. 110 The advent of 2D experiments such as MQMAS 121,132 and later satellite transition MAS (STMAS) 133,134 , have revolutionized the observation of half-integer quadrupolar nuclei. 116 They use conventional MAS hardware and provide high-resolution NMR spectra by cancelling out the broadening due to both first-and second order quadrupolar interactions. 110,117

MQMAS

The MQMAS approach introduced by Frydman and Harwood, 121 is the most popular method for studying quadrupolar nuclei. [START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF]117,135 Unlike the DAS and DOR techniques, the MQMAS experiment requires rotation of the sample around a single angle (i.e. at the magic-angle, which also eliminates dipole and CSA interactions) and therefore, can be performed using a conventional MAS probe. [START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF]111 The elimination of the quadrupolar second-order interaction is achieved in a two-dimensional experiment by correlating multiple quantum coherences, typically triple-quantum coherences (between energy levels 𝑚 𝐼 = +3/2 ↔ 𝑚 𝐼 = -3/2, indicated as 3Q in Figure 4), with the CT coherence. [START_REF] Ashbrook | New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei[END_REF][START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]136 The simplest MQMAS experiment involves two-pulses and served as foundation for the subsequent developments in MQMAS. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]129 The first RF pulse excites all possible (non-observable) multi-quantum (MQ) coherences during the 𝑡 1 evolution.

The second RF pulse converts these MQ coherences into observable -1Q-CT coherence during 𝑡 2 . [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]135 The 2D spectra provides an isotropic high-resolution spectrum along the indirect dimension (F1), correlated with its corresponding MAS spectrum in the detection dimension (F2). 117,118 This is achieved by selecting only coherences between energy levels with opposite magnetic quantum numbers 𝑝/2 and -𝑝/2 with 𝑝 = 2𝑚 with 𝑚 = -𝐼, -𝐼 + 1, . . . , 𝐼, that corresponds to 𝑝Q transitions which are not affected by the first-order quadrupolar broadening (only second-order broadening is present for these transitions with opposite magnetic quantum numbers). [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]117 For spins I > 3/2, higher order coherences can be excited, thus, 𝑝 can take values as 3, 5, etc, corresponding to 3Q and 5Q coherence pathways, respectively. 129 Nevertheless, 3QMAS experiment remains as the most used to the date owing to its higher sensitivity. 116 Most of the MQMAS spectra are presented after a shearing transformation, which is implemented to produce an isotropic spectrum in the indirect dimension that is free from the second-order quadrupolar broadening. The projection of the spectrum onto the F1 dimension reveals one narrow NMR peak for each individual crystallographic site. Therefore, the 3QMAS experiment allows the determination of the number of crystallographic sites, and their NMR parameters as the isotropic chemical shifts and the quadrupolar parameters, which are not possible to obtain with the conventional 1D MAS spectra, in which signals of different sites often overlap. 118 The most commonly used MQMAS sequences are composed of three-pulses: zfilter and split-𝑡 1 . The pulse sequence and coherence pathways of these two schemes are shown in Figure 6. The z-filter approach proposed by Amoureux et al 137 is robust and easy to optimize. It involves two hard pulses (excitation and conversion) followed by a short z-filter delay to store the magnetization along the z-axis as 0Q, after which the magnetization is transferred into observable 1Q coherences by a soft 𝜋/2

(selective) pulse that only excites the CT. The purpose of the last pulse is to symmetrize the echo and antiecho coherence pathways (Figure 6a). As it acquires both signals with the same intensity, the z-filter method produces a 2D pure-absorption lineshape. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF] Experimentally, the rf field strength of the hard-pulses must be set to the highest power level that is compatible with the rf power specification of the probehead.

Usually, a z-filter delay of 10-20 s and an rf-field ν 1 = 10-20 kHz for the last softpulse, are employed. 117

The split-𝑡 1 approach was introduced as an alternative to the z-filter method. 138 In this method, 𝑡 1 is split into MQ and SQ evolution periods for time fractions of 𝑡 1 /(1 + 𝑅) and 𝑅𝑡 1 /(1 + 𝑅), respectively (Figure 6b), where 𝑅 is a factor that takes values for 3Q excitation of +7/9 and 12/19 for I = 3/2 and 5/2, respectively. 121 This factor avoids the need for the shearing transformation after the acquisition. As a result, the second order anisotropic broadening is refocused at the end of the 𝑡 1 period, and the echo will appear at the same 𝑡 2 time for all 𝑡 1 periods. 117 The split-𝑡 1 experiment gives a higher signal-to-noise ratio (S/N) than the z-filter approach. Its sensitivity is usually √2 higher than the z-filter experiment because a full echo is acquired. 110 This experiment does not require a shearing transformation.

One of the most relevant benefits of MQMAS, is that it provides a direct separation of the contributions from the chemical shift and quadrupolar coupling. After shearing transformation (for the z-filter MQMAS) and scaling of the MQMAS spectrum; information about the isotropic chemical and quadrupolar-induced shifts (CS and QIS) are obtained by analyzing the 2D spectrum. Indeed, the analysis and interpretation of the MQMAS spectra is facilitated by introducing three axes: CS, QIS and anisotropic (A) axes. In the obtained spectrum, the resonances with zero quadrupolar coupling are sharp and appear along the CS axis. On the other hand, resonances with significant quadrupole coupling experience a shift of the centre of gravity away from the CS axis in the direction of the QIS axis, due to the quadrupole second-order shift. The A axis, defined by the orientation of anisotropic ridges in the MQMAS spectrum, appears parallel to the direct F2 axis. The slopes of the CS and QIS axes are determined based on the general convention for scaling the isotropic dimension after shearing. 139 The slopes of CS and QIS axes proposed by Amoureux et al. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF] are 1 and -10/17, respectively.

Although the MQMAS is a widely used and robust method, its intrinsically poor sensitivity is a limiting factor for many practical applications. This is due to the inefficient excitation of MQ coherences and the subsequent conversion of these MQ coherences to SQ, since MQ transitions are prohibited for the quantum mechanics rules. 123,140 In addition, the efficiency of MQMAS is highly dependent on the 𝐶 Q value of the crystallographic sites and the rf field. Several methods have been proposed to improve the generation and/or conversion of MQ coherences. 135,141 In particular, the conversion process remained as the central target. 140 Figure 7 summarizes some of the excitation/conversion schemes that can be used for this purpose. transfers. However, this last method is mainly efficient for I = 3/2. A representation of these types of pulses can be also observed in Figure 7. Further methodological developments involving MQMAS include its combination with polarization transfer between the quadrupolar isotope and neighbouring spins I = 1/2. 110 It is possible to combine CP with MQMAS for either spectral editing of the MQMAS spectra or to acquire heteronuclear correlation 2D spectra endowed with high resolution. The MQMAS scheme can be inserted before or after the CP transfer. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF] However, polarization transfers with CP are difficult to optimize due to the complex spin dynamics of quadrupolar nuclei during spin lock. The heteronuclear correlation with MQ-HETCOR provides high-resolution in the indirect F1 dimension and has been utilized in many applications. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]150 For example, 3Q-HETCOR experiment with SPAM conversion was used to correlate 27 Al-31 P nuclei on AlPO4-14, showing its utility in the structural study of these microporous materials. 151 Alternatively the CP transfer can be replaced by REDOR scheme for spectral editing as well as dipolarmediated refocused INEPT to acquire high-resolution HETCOR 2D spectra. [START_REF] Amoureux | NMR: Experimental Strategies and Applications[END_REF]152 In particular, REDOR-MQMAS allows for measurement of the dipolar interactions and internuclear distances between pairs of spin-1/2 and quadrupolar nuclei with highresolution for the latter isotope. 153

STMAS

The STMAS experiment is an alternative method to MQMAS to obtain high resolution spectra of half integer quadrupolar nuclei. 133 This 2D technique correlates the 1Q-ST and 1Q-CT coherences, which increases the sensitivity significantly. This correlation generates isotropic spectra, in a similar way than MQMAS, but with higher efficiency since the coherence transfer occurs between 1Q coherences. 134 However, the STs are affected by the first-and second-order quadrupolar interactions, making the experiment difficult to carry out. To overcome this, STMAS experiment requires extremely precise magic angle adjustment (with better than 0.002° accuracy), stable rotation rate and that the applied pulses are timed accurately. These experimental settings have greatly increased the difficulty of its implementation and thus, limited its use. 134 The most frequently used sequences of the STMAS experiment are composed of three pulses: two hard pulses (the first one for the creation of inner STs and the second for transferring the coherences to the CT) and one soft pulse. The correlation of the dephasing occurring on the STs in 𝑡 1 and the CT during 𝑡 2 , is achieved because the refocusing of the second-order quadrupole frequencies are equal for two symmetrical STs, therefore, producing isotropic peaks. 110,154 The appearance of the resulting STMAS spectrum is very similar to that of a MQMAS spectrum. 155 However, due to the difficult experimental setting mentioned above, this method does not belong to the routine NMR experiments.

Another difficulty of the STMAS approach is the presence of large diagonal peaks in the 2D spectra, which arise from unwanted autocorrelation signals of the CT evolution during 𝑡 1 and 𝑡 2 periods, and in a minor extent from the STs, which cannot be cancelled out by phase cycling. 156,157 The DQF-STMAS z-filter experiment introduced by Kwak and Gan addresses this problem. 158 In this pulse sequence (Figure 6c), a CT selective 𝜋 pulse is introduced at the end of 𝑡 1 delay and right before the 1Q conversion in order to produce and select the 2Q coherences. This DQF allows filtering out the unwanted CT-CT and ST-CT transitions are filtered out with a suitable phase cycling scheme. This double-quantum excitation is performed with almost no sensitivity loss. 157 Furthermore, many of the techniques designed to improve the sensitivity of MQMAS can also be employed for STMAS. For example, SPAM has been successfully applied with DQF-STMAS by Sasaki et al. 147 More recently, with the advent of LP, a new type of STMAS has been introduced (LP-STMAS), which, like LP-MQMAS, requires less rf-field to be implemented. 159

METHODS TO ENHANCE THE SENSITIVITY FOR QUADRUPOLAR NUCLEI 1.5.1 High-fields

High magnetic fields are valuable for solid-state NMR of quadrupolar nuclei. They improve the spectral resolution by reducing the second-order quadrupolar broadening ( 𝐵 0 -1 ), that remains under MAS. 113,160 Another advantage of high fields is to improve the sensitivity of quadrupolar nuclei which is proportional to 𝐵 0 11/4 . 161 Since the chemical shifts scale linearly with 𝐵 0 , high magnetic fields provide a direct means to determine chemical shifts experimentally, and detect and evaluate the CSA with accuracy. [START_REF] Gan | Seeking Higher Resolution and Sensitivity for NMR of Quadrupolar Nuclei at Ultrahigh Magnetic Fields[END_REF]162 The availability of high field NMR spectrometers (𝐵 0 > 18.8 T) is particularly useful for studying quadrupolar nuclei with lineshapes that are dominated by significant second-order quadrupole broadening. [START_REF] Gan | Seeking Higher Resolution and Sensitivity for NMR of Quadrupolar Nuclei at Ultrahigh Magnetic Fields[END_REF] More recently, the advent of ultrahigh magnetic field of up to 35.2 T ( 1 H Larmor frequency of 1.5 GHz) with dramatically improved sensitivity and resolution, 160 opened a new stage for the study of quadrupolar nuclei and make possible the observation of difficult low- and/or lowabundance nuclei including [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mg, 17 O and 67 Zn. 161,163,164

Population transfer

Another way to improve the sensitivity of quadrupolar nuclei involves population transfer methods. They are used to enhance the CT of a quadrupolar nucleus by perturbing the populations of the outer satellite transitions. 165 Generally, there are two mechanisms to describe this process: ST saturation and ST inversion. The ST saturation can be accomplished by equalizing the population of energy levels related to STs. The enhancements of the CT (I + 1/2) that can be achieved are 2, 3, 4 and 5 for nuclei with spins of 3/2, 5/2, 7/2 and 9/2 respectively. 166 On the other hand, the magnetization of the ST can be also transferred to the CT by a series of ST inversions.

Considering that all the outer STs are inverted before that the inner ones, this can lead to enhancements (2I) equal to 3, 5, 7 and 9 for spins 3/2, 5/2, 7/2 and 9/2 respectively. 166 Different populations transfer methods for the saturation and inversion of spin populations have been suggested. They involve DFS (double frequency sweeps), RAPT (rotor-assisted population transfer) and HS (adiabatic inversion with hyperbolic secant pulses). 166-168

DFS

DFS allows for signal enhancement by adiabatically inverting the STs so that the population of the outer spin levels are transferred to the CT energy levels before they are selectively excited. 142,169 The inversion of population between two energy levels can be accomplished by sweeping a continuous rf-field through the entire spectrum while keeping the amplitude of the pulse constant. 168 These pulses irradiate simultaneously the high-and low-frequency sides of the CT using waveform generators. 168 DFS schemes with a large sweep range gives the best signal enhancement for NMR spectra of stationary powdered samples. 169

RAPT

RAPT is a method to enhance the CT sensitivity by a selective saturation of the STs. It consists of pulse train with constant amplitude and alternating phase. 170 Unlike DFS that uses a frequency sweep, RAPT uses only a constant frequency offset. The advantage of RAPT over other methods is that can be applied to polycrystalline samples during MAS, thus allowing to obtain simultaneously CT enhancement for all crystalline orientations. 171

HS

Shaped adiabatic pulses are also used to invert the populations associated with the STs.

For the hyperbolic secant pulse, the adiabatic pulses are created with both amplitude and phase modulation. 172,173 The pulse is smoothly turned on and off as the amplitude is modulated by a hyperbolic secant function, whereas the phase is modulated to induce a sweep with a hyperbolic tangent profile. Under MAS conditions, HS yields an enhancement of the CT that exceeds that obtained by DFS or RAPT. 166,168

QCPMG

The quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) method, [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] adapted from the CPMG sequence, is commonly used in solid-state NMR to enhance the sensitivity of half-integer quadrupolar nuclei. 174 The sensitivity enhancement is achieved by applying a train of CT selective  pulses that refocus the signal during acquisition, thus, allowing the spin-echo to be recorded several times in the full timescale of the transverse relaxation/dephasing. 175 After a Fourier transformation of the echo train, a spectrum is obtained consisting of narrow peaks, often called spikelets, whose manifold reproduces that of the conventional powder spectrum. 174 The origin of the sensitivity improvement from QCPMG compared to a conventional echo experiment (Figure 8a), comes from the acquisition of multiple echoes in every FID. 166 The QCPMG pulse sequence, presented in Figure 8b, is composed of two parts: a /2 pulse that is used for excitation and N successive  pulses, separated by 2τ, for refocusing. A train of echoes is collected in the time domain, which after a Fourier transformation results in a spectrum with narrow spikelets. The envelope of spikelets contains information on the inhomogeneous interactions, such as CSA, quadrupolar coupling, heteronuclear dipolar coupling, whereas each individual spikelet contains information regarding the homogeneous interactions, such as the homonuclear dipolar coupling and dynamics. 166 The resolution of the QCPMG spectrum is determined by the spikelet's spacing in the frequency domain (2τ) -1 and their width, which is related to T2'. 176 The QCPMG approach is effective if the spin-spin relaxation time (i.e. the decay of the echo envelope T2') is longer than the magnetization decay caused by local interactions. 176 Therefore, the maximum possible enhancement will depend on the value of T2' of the sample, which determines how many echoes can be recorded in each scan. The gain in S/N achieved with QCPMG, compared to a regular echo experiment, is determined by 𝐺 𝑆/𝑁 = 2√𝑇 2 ′ 𝑇 off (2𝜏) -1 , where T2' is the effective spinspin relaxation time and Toff is the time for the FID of the individual echo to become comparable with the noise level of a standard echo experiment. 167 A modified version of the QCPMG sequence was developed by O'Dell and Schurko. 177 This sequence is similar to the standard QCPMG scheme but employs frequency sweep WURST (wideband uniform rate smooth truncated) pulses instead of rectangular pulses (Figure 8c). The frequency sweep of the WURST pulses is achieved by simultaneous modulation of the amplitude and phase of the pulses. The WURST-QCPMG sequence is useful for the acquisition of wideline solid-state NMR spectra due to the high excitation bandwidths which affords a large reduction in the number of sub-spectra that must be acquired. 178

Dynamic Nuclear Polarization (DNP)

As shown in previous sections, solid-state NMR is a powerful tool for studying the surface of inorganic materials. It provides a unique insight into the atomic-level structure and dynamics of surface species, which is essential for establishing structureactivity relationships. 179 However, the inherent low sensitivity of solid-state NMR often prevents its application for the characterization of inorganic solids and the observation of their surfaces. NMR sensitivity is, thus, one of the main limitations for further developments in surface chemistry. 180 Such low sensitivity arises from the small magnetic moments of nuclear spins that yields small Boltzmann polarization, which is used to saturate the electron spins. To achieve this, DNP takes advantage of the large magnetic moments of electrons (γ e ) and thus, a theoretical signal enhancement of ~ 660 (γ e /γ H ) is predicted for protons. 181 To perform a DNP experiment, an exogenous radical polarising agent (PA) is dispersed into the system of interest. This PA provides the unpaired electrons that are the source of the spin polarization. The choice of the PA highly depends on the magnetic field used for the experiment and the nucleus that will be polarized, 1 H or a low-γ nucleus. 183 DNP experiments are typically performed at cryogenic sample temperatures of ca.

100 K and high-power gyrotron microwave sources (>10 W) are required to successfully saturate the electron paramagnetic resonance (EPR) transitions of the PA. 181 The low temperatures increase electron and nuclear relaxation times, allowing for a higher accumulation of nuclear spin polarization and facilitate a more complete saturation of the electron spins. 182 In addition, a Boltzmann related signal enhancement is also obtained due to the cryogenic temperatures employed. The sensitivity of the DNP experiment is usually expressed in terms of DNP enhancement, ɛon/off, and is determined by measuring the gain in signal intensity with and without microwave irradiation. 182

Instrumentation

The unique experimental conditions of DNP (cryogenic temperatures, high-power, high-frequency w irradiation) require the implementation of specialized hardware. 184 In recent years, DNP has made important advances, and although the technique was initially developed for low magnetic fields, the scope has been extended to instruments operating at fields of 14.1 and 21.1 T. 184 The DNP NMR system consists of several components and it can be considered as an extension of the solid-state NMR spectrometer. A schematic drawing of typical DNP MAS spectrometer is presented in The gyrotron is the instrument that generates high-power microwaves that are required for the polarization transfer from unpaired electrons to NMR-active nuclei. It consists of a gyrotron tube that is sealed inside a strong magnetic field and operates by transferring energy from an electron beam to an electric wave. 184,185 Gyrotrons can provide reliable continuous w irradiation frequencies of up to 1 THz and deliver continuous w power in the order of 10 W and above. 56 Such high-power conditions are necessary for MAS DNP as the size of the sample (in the order of a few millimetres) is comparable to the wavelength of the w, therefore, excluding the use of DNP NMR probes with w resonant structures. 181 The w transmission line, also called waveguide, consists of a series of interconnected aluminium tubes. Its purpose is to transfer the w beam delivered by the gyrotron to the sample situated inside the NMR probe. At the end of the waveguide, the w are diffracted and reflected by the RF coil, which have a pitch that is comparable to the wavelength of the w. Due to the reflection of the w, the distribution of the w field within the sample is non-uniform, and hence, resulting in an inhomogeneous distribution of the DNP enhancement, with higher gains in the centre of the rotor. Furthermore, transparent sapphire rotors are preferred for DNP MAS experiments because of their higher thermal conductivity and their w transparency. 181,185 In addition to the typical capabilities of regular solid-state NMR probes and w transmission, DNP MAS probes are designed to operate at low temperature (LT) and provide stable MAS in this condition. 56,184 Besides cold bearing and drive gas, an additional cooling gas is used. This gas passes through a heat exchanger and is transported to the MAS stator via thermally isolated transfer lines. The LT MAS DNP probe is place within an NMR magnet. The NMR magnets used for MAS DNP are similar to conventional wide-bore solid-state magnets. However, they are equipped with an additional sweep coil, that is used to adjust the static magnetic field in order to fulfill the DNP matching conditions for maximal DNP enhancements. 181

Direct and Indirect DNP

In general terms, DNP experiments can be carried out in two ways: indirect and direct. In an indirect DNP experiment, the 1 H nuclei that are nearby the PA are first polarized, and the 1 H spin diffusion mechanism distributes the magnetization through the sample. 182 Consequently, the enhanced polarization can be transferred to less receptive nuclei by using solid-state NMR methods like CP, or more recently introduced PRESTO and R-INEPT methods. 186 In a direct DNP method, the PA polarize directly the observed (low-) nuclei, and then, followed by homonuclear spin diffusion, the polarization is distributed to more remote nuclei (Figure 10). In these experiments, the NMR signal is measured by direct excitation experiment. 187 Indirect DNP is more commonly used that direct DNP. This is mainly due to the long build-up curves observed in direct DNP experiments because spin diffusion mechanism between low- nuclei is not as efficient as that of 1 H. 182 However, direct and indirect experiments have been proven to be complementary in surface science. Indirect DNP has shown to be selective to the first atomic layer whereas direct DNP probes the layers of the sub-surface. 179 180 This effect led to the development of 'dynamic nuclear polarization surface-enhanced nuclear spectroscopy' (DNP SENS). 180 The high sensitivity gains provided by DNP enable to selectively enhance the NMR signals from surfaces through a combination of incipient wetness impregnation and cross polarization methods (Figure 10). 188 DNP SENS has been shown to increase the signal intensity of conventional solid-state NMR spectra by one to three orders of magnitude at high-field and has been successfully applied to probe surfaces of a variety of materials. 55,189,190 DNP SENS has notably been applied to enhance the sensitivity of half-integer quadrupolar nuclei, such as 27 Al 191 and 17 O 106,192 . Very recently, DNP SENS approach has been successfully applied to enhance the NMR signal of low- nuclei, such as 47,49 Ti, [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mo and 67 Zn at the surfaces of MoO3/TiO2 and Al-doped ZnO nanoparticles. 193

Sample formulation

Sample formulation is an essential step to ensure the success of a DNP SENS experiment. Incipient wetness impregnation (IWI) is often used to coat the surface of materials with PA solution. 194 This method requires the addition of stable radicals as source of unpaired electrons, usually nitroxide biradicals, such as AMUPol and TEKPol, which are dissolved in aqueous media (e.g. glycerol-d8/D2O/H2O 60/30/10 v:v:v, also called DNP-juice) or in organic solvents (e.g. 1,1,2,2-tetrachloroethane shorten as TCE), respectively. Impregnation process brings the PA molecules into close proximity with the NMR-active nuclei on the surface, thus, allowing them to be polarized faster by DNP than the bulk. 108,182 As DNP experiments are usually done at cryogenic temperatures, the polarizing matrix freezes to form a glassy solution. This frozen solution plays an important role in providing a rigid protonated matrix that diffuses hyperpolarization through the sample, which is advantageous for indirect DNP. 182 However, the frozen solvent dilutes the investigated material thus, reducing the effective amount of sample in the rotor. To circumvent this issue, DNP SENS experiments are often performed with a minimized amount of the frozen solvent. 108 Furthermore, in some cases, the radical solution can also modify the structure and dynamics of the surface.

The concentration of PA also influences the DNP sensitivity. For higher concentrations of PA, each unpaired electron polarizes fewer nuclei, and the build-up of the DNP-enhanced nuclear polarization occurs more rapidly, which tends to increase the sensitivity. The difference in polarization between the unpaired electrons can be generally enhanced by increasing the w power. 181 High-energy and high-frequency w generated by gyrotrons are used to saturate one of the EPR transitions, and consequently drive the polarization transfer from electron to the 1 H through the threespin "flip-flop-flip" process. 56,195,196 However, high-energy w can cause sample heating and solvent melting, hence reducing the DNP enhancement. 56 The optimal w power is thus optimized depending on the sample. In addition, cryogenic temperatures (c.a. 100 K) are required to slow down the electron spin relaxation rates and ensure efficient DNP polarization transfer. 108,183

Polarizing agents

Since CE requires two unpaired electrons dipolar coupled, most of high field DNP experiments have been carried out using exogeneous nitroxide-based biradicals. 188 The covalent link between the two nitroxide radicals in these molecules, results in an intramolecular dipolar coupling. 181,197 Generally, PAs with broad and anisotropic EPR spectra favour the CE mechanism since sufficient inter-or intramolecular electronelectron (e-e) dipolar coupling are present. 56 Nitroxide biradicals, such as TOTAPOL, AMUPol and TEKPol, have been proven to be efficient CE polarizing agents. 181,197 The efficiency of the PAs depends on various parameters as the solubility, concentration and electron spin properties at operational temperatures. 56 Generally, PAs are evaluated based on several factors, such as the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, its g-factor anisotropy, and the nuclear Larmor frequency. 108,181 The g anisotropy is the major cause of broadening at high magnetic fields. The structures of bi-nitroxides have been improved over the recent years by adjusting the length and rigidity of the organic tether, in order to optimize the e-e coupling and the relative orientation of their g-tensors. Other efforts have been made to change the functional groups to modulate the electron relaxation times and improve the solubility. 198 This has led to efficient polarizing agents as those presented in Figure 11. However, the performance of these radicals decreases significantly when increasing B0. This is attributed to the fact that the efficiency of the CE scales with the inverse of the magnetic field strength. 199 Very recently, HyTEK2 radical has demonstrated high DNP efficiency and outstanding performance at both 18.8 and 21.1 T, compared to its widely used counterparts AMUPol and TEKPol. 198,200 The development and design of new PAs for efficient DNP transfer under MAS and high magnetic field are still active areas of research. 56,201

OUTLINE OF THE THESIS

In chapter 1, we reviewed theory and methods which are helpful to understand the presented methodological achievements presented in Chapter 2 and 3 of this thesis. Amoureux contributed to the analysis of the results, the experimental design and the writing of the paper. All authors contributed equally to the editing of the article.

INTRODUCTION

Half-integer quadrupolar nuclei (I = 3/2, 5/2, 7/2 or 9/2) represent over 60% of the NMR-active isotopes. 2 They are in proximity with protons in a wide range of materials including nanoparticles, microporous and hybrid materials, heterogeneous catalysts and biomaterials. [3][4][5][6] In solid-state NMR, the proximities between these isotopes can be probed by transferring polarization from 1 H to half-integer quadrupolar nuclei via heteronuclear dipolar couplings under MAS conditions. [7][8][9][10][11] In recent years, the polarization transfer from protons, combined with dynamic nuclear polarization (DNP), have improved the sensitivity for the detection of quadrupolar nuclei. The sensitivity gain achieved with this approach has enable the detection of quadrupolar nuclei, such as 17 O, 27 Al, 67 Zn, 47,49 Ti, [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mo and 43 Ca, near the surface of materials. 12- 20 The polarization transfer from 1 H to half integer quadrupolar isotopes was first achieved by cross polarization (CP) under MAS conditions. 21 However, CPMAS lacks robustness and efficiency due to the spin-locking of the quadrupolar magnetization. 22- 24 One of the main challenges of cross polarization involving quadrupolar nuclei is that it requires a careful adjustment of the applied RF-fields in order to fulfill one of the with 𝑝 = 0, 1, 2 and 3. 23,25 In addition, the quadrupolar magnetization cannot be spinlocked efficiently for some crystallite orientations, which results in line-shape distortions. [26][27][28] More recently, other schemes, such as PRESTO (phase shifted recoupling effects a smooth transfer of order) 13,29,30 (Fig. 13a,c) and D-RINEPT (through-space refocused INEPT) 20,31 , have facilitated the coherent transfer of polarization to half-integer quadrupolar isotopes and circumvent the limitations mentioned above. In these two techniques:

i) No spin-lock is applied to the half-integer quadrupolar nucleus, which limits the polarization losses and improves the robustness to the CQ value;

ii) Only two or three CT selective pulses are applied on the quadrupolar nucleus, thus providing transfers that are robust to offsets on this channel;

iii) Since they do not have to fulfill any RF-field matching condition, the transfers are robust to RF-field inhomogeneity.

Consequently, for these schemes the dipolar interactions between the proton and quadrupolar isotopes are reintroduced by symmetry-based RN 𝑛 𝑣 recoupling schemes that are applied on the 1 H channel. 32,33 The PRESTO sequence uses γ-encoded recoupling schemes such as R18 2 5 , which does not require the synchronization of the delays between successive recoupling periods with the MAS. 34 However, PRESTO sequence, like CPMAS, suffers from dipolar truncation, which prevents long-range transfers, and is sensitive to the 1 H CSA. 29 In the case of D-RINEPT, non-γ-encoded recoupling schemes, such as SR4 1 2 , 33 are used. Therefore, the recoupling periods must be rotor-synchronized. In addition, D-RINEPT is not affected by dipolar truncation, hence allowing for transfers over long distances. Furthermore, this sequence benefits from higher robustness to 1 H CSA and RF-field inhomogeneities but, is more sensitive to 1 H-1 H dipolar interactions at low MAS frequencies owing to the low RF-field requirement of SR4 1 2 , ν1 = 2𝜈 R . 19,20,31 More recently, Nagashima et al. introduced a new variant of the D-RINEPT sequence which uses a novel variant of SR4 1 2 recoupling and is more robust to the 1 H-1 H dipolar losses (Fig. 13b,c). 19,20 In this new sequence:

(i) the SR4 1 2 dipolar recoupling is built from tanh/tan (tt) adiabatic inversion pulses;

(ii) continuous-wave (CW) irradiations are applied during the windows, in order to decouple 1 H-1 H interactions;

(iii) composite π/2 and π pulses are applied on the 1 H channel to improve the robustness to RF inhomogeneities of the sequence and hence, its transfer efficiency.

This sequence, denoted as D-RINEPT-CWc-SR4 1 2 (tt) in the following, has proven to be more efficient than PRESTO and CPMAS at 𝜈 R = 12.5 kHz, and has been combined with DNP to successfully detect the NMR signals of quadrupolar nuclei, especially those with small dipolar couplings with protons and/or small gyromagnetic ratios, such as 47,49 Ti, 17 O, [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mo and 67 Zn. In the case of quadrupolar nuclei with large coupling with protons, D-RINEPT-CWc-SR4 1 2 (tt) using only two pulses on the quadrupolar channel has shown to be more efficient than PRESTO. 19 As mentioned above, polarization transfer methods like D-RINEPT can be applied for indirect DNP experiments where low spinning speeds are used (e.g. 𝜈 R = 12.5 kHz). However, many experiments in solid-state NMR require the polarization transfer from 1 H to half-integer quadrupolar isotopes at 𝜈 R > 12.5 kHz. One reason for this, is that MAS frequencies greater than 20 kHz are required to avoid the overlapping of the bands of the CT and the spinning sidebands of STs, in particular for 27 Al experiments recorded at 18.8 T. Second, through-space heteronuclear correlation (D-HETCOR) 2D

spectra between 1 H and the quadrupolar nucleus can be acquired at 𝜈 R ≥ 60 kHz with high resolution along the 1 H dimension, since high MAS rates can average out the 1 H- Globally, no systematic study of the R𝑁 𝑛 𝑣 recouplings built from composite π pulses have been carried out.

This dissertation seeks to investigate the use of D-RINEPT-CWc pulse sequence with an adiabatic recoupling at the MAS frequencies of 𝜈 R = 20 and 62.5 kHz.

Through numerical simulations of spin dynamics and NMR experiments performed on γ-alumina and isopropylamine-templated microporous AlPO4-14 (shorten AlPO4-14 in the following), it was shown that the RF-field requirement of this sequence increases with the 1 H-1 H dipolar interactions present in the sample of study. In practice, this RF requirement is not compatible with most of the MAS probes specifications at 𝜈 R ≥ 20 kHz, even for moderate 1 H-1 H interactions. As an alternative, this study provides new variants of the PRESTO and D-RINEPT-CWc with recoupling schemes built from single rectangular or composite π pulses, that were selected by average Hamiltonian (AH) theory. Consequently, these new schemes were tested on γ-alumina and AlPO4-14, which exhibit small and moderate 1 H-1 H dipolar interactions, respectively, and identified the most efficient and robust 1 H- 27 Al PRESTO and D-RINEPT transfers at B0= 18.8 T with 𝜈 R = 20 and 62.5 kHz. Finally, this study demonstrated how these magnetization transfers can be used to acquire 2D 1 H- 27 Al HETCOR spectra at 𝜈 R = 62.5 kHz using a non-uniform sampling (NUS) 37 scheme.

THEORETICAL BACKGROUND

Symmetry-based dipolar recouplings

Symmetry based elements are widely used in solid-state NMR under MAS conditions to achieve selective recoupling of the heteronuclear dipolar interactions. 38 The design of recoupling pulse sequences is based on the symmetry-based recoupling framework established by Levitt et al. 38 Two main families of pulse sequences are identified: The C-and R-type. In the C-type sequences, the basic element is a 2π element, which is a RF cycle, and the pulse sequence is built by repeating this cycle with incremented phases. These cycles return the irradiated spins to their initial states in the absence of other interactions. 32,39 In the R-type sequences, the basic element is a π rotation element also known as R. Recoupling of heteronuclear dipolar interaction by a C𝑁 𝑛  or R𝑁 𝑛  symmetry-based sequence requires to choose the symmetry numbers N, n and , which are integers, in order to recouple selectively the heteronuclear spin interactions, while decoupling other interactions, such as the effect of 1 H-1 H spin interactions, to first order in the AH theory. 40 The transformation properties of a given spin interaction with respect to the rotation of the samples and the rotation of the nuclear spin polarizations by RF pulses are described by the space and spin ranks, denoted l and λ, respectively. 38 The space rank l has 2l+1 components m that takes values between l and -l, whereas the spin rank λ has 2λ+1 components μ ranging from -λ to λ. Therefore, different interaction Hamiltonians take different values of l and λ, which allows for the design of rotor synchronized pulse sequences in order to select the desired interaction and remove the unwanted ones. 41 For example, the CSA and heteronuclear dipolar coupling interactions both have (l, λ) = (2, 1), whereas the homonuclear dipolar coupling has (l, λ) = (2, 2). 32 The recoupling sequences considered in this work correspond to the R𝑁 𝑛  class, which have more selective selection rules than C𝑁 𝑛  class. The R𝑁 n  scheme is obtained by repeating the pair RϕR'-ϕ N/2 times, with ϕ = πν/N radians an overall phase shift.

R is an inversion pulse with a duration of nTR/N, where TR = 1/νR is the rotor period, and R'-ϕ is an inversion pulse derived from R by changing the sign of all phases. R and R' are identical when they are amplitude-modulated, i.e. all phase shifts are multiple of π. The rf-field requirement of R𝑁 𝑛  is equal to:

𝜈 1 = 𝑁 𝑛 𝜉 𝑡𝑜𝑡 2𝜋 𝜈 𝑅 , (1)
where

𝜉 𝑡𝑜𝑡 = ∑ 𝜉 𝑖 𝑃 𝑖=1
is the sum of the flip angles of the P individual pulses of the R element. The element R must rotate the resonant spins through the angle π about the x-axis of the rotating frame, in the absence of other spin interactions. 42 Consequently, R can be a π pulse with phase ϕ = 0, a composite pulse, or a pulse with shaped amplitude and modulated frequency. 32 In general, R𝑁 n  schemes can be classified as non γ-encoded and γ-encoded. The definition of γ-encoding is derived from the fact that the amplitude of the recoupled AH does not depend on the γ Euler angle. 34,40,43 The non-γ-encoded R𝑁 𝑛  sequences are compatible with the use of supercycle, in order to improve the robustness to unwanted terms, such as 1 H-1 H dipolar interactions or 1 H CSA. For example, SR4 1 2 scheme, which uses a three-step multiple-quantum supercycle, is widely used for heteronuclear recoupling with protons in D-HMQC 44 and D-RINEPT 31 experiments.

Conversely, γ-encoded R𝑁 𝑛  sequences can provide a maximum transfer efficiency 25% higher than their non-γ-encoded counterparts. 45 Furthermore, they do not require the synchronization with MAS of the delay between successive recoupling periods.

Scaling factor κ

The recoupling of heteronuclear dipolar interactions in R𝑁 𝑛  scheme is scaled by a factor κ, which theoretically depends on the chosen symmetry numbers N, n and  as well as the basic inversion element. 38 In order to accelerate the coherence transfer, it is desirable to use symmetry schemes providing large scaling factors. 40,42 In particular, this is important when carrying out experiments that involve the reintroduction of weak couplings bij, that carry valuable long-range structural information and therefore, require the application of the recoupling sequence over long-time intervals. 42

Adiabatic pulses

Adiabatic RF pulses play an important role in spin inversion due to their robustness to RF-field inhomogeneity and resonance offsets. 46 They utilize the adiabatic principle wherein the magnetization vector M is manipulated by a slow passage (adiabatic) of the effective magnetic field Beff through resonance. The advantage of this adiabatic passage (or sweep) is that spins having different resonant frequencies are inverted or manipulated at different times. 47 To achieve broadband inversion, this sweep rate must satisfy the adiabatic condition which is expressed by: 48,49 |dθ/dt| << ωeff (2) where ωeff = γ |Beff| is the effective field expressed in angular frequency units, given by the resultant of the applied radiofrequency field ω1 = γ |B1| and the resonance offset Δω = γ |ΔB|, and θ is the angle made by Beff with the +x axis. This is often rearranged into an adiabaticity factor Q defined as:

Q = ωeff /|dθ/dt| (3)
which should be sufficiently larger than unity. Thus, the adiabatic condition can be met by ensuring that the sweep speed is sufficiently slow or that the magnitude of Beff is sufficiently large. 50 Adiabatic pulses usually consist of a frequency-and amplitude modulated pulse, which creates Beff that sweeps adiabatically from an initial orientation parallel to the applied magnetic field B0 to finish antiparallel to it (adiabatic full passage AFP). 50,51 These type of pulses can be implemented by means of tanh/tan pulses (denoted tt hereafter), which are relatively short, an advantage for solid-state NMR, and employs a hyperbolic tangent (tanh) modulation of the RF-field amplitude and a tangent (tan)

sweep of the frequency. 52 During the (tt) pulse, the instantaneous rf-amplitude is equal to:

𝜔 1 (𝑡) = 𝜔 1,max { tanh [ 8ξ𝑡 𝑇 R ] 0 ≤ 𝑡 < 𝑇 R /8 tanh [2ξ (1 - 4𝑡 𝑇 R )] 𝑇 R /8 ≤ 𝑡 < 𝑇 R /4 (4)
where ω1,max is the peak amplitude of the rf-field, t refers to the time since the start of the pulse, which lasts TR/4 when incorporated into the SR4 1 2 recoupling scheme. The parameter ξ determines the rise and fall times of the pulse. Hence, in the frequencymodulated (FM) frame, 47 the phase is

𝜙 𝐼 (𝑡) = Δν 0,max 2tan() ln {cos [θ (1 -8 𝑡 𝑇 R )]} . ( 5
)
where Δ0,max is the peak amplitude of the carrier frequency modulation and  determines the frequency sweep rate in the center of the pulse. Here, ξ = 10 and θ = 87° = atan (20) were employed. 20,53,54 However, one drawback of adiabatic pulses is that they require high RF power compared with rectangular pulses, 55 which is usually not compatible with the RF power specifications of most MAS probes.

PRESTO-III sequence

In the PRESTO sequence (Fig. 13a,c), symmetry-based γ-encoded R𝑁 𝑛  schemes applied to the 1 H channel reintroduce the |𝑚| = 2 space components and the singlequantum (SQ) terms of the hetero-nuclear dipolar couplings between the protons and the quadrupolar nuclei, as well as the 1 H CSA, while they suppress the contributions of 1 H isotropic chemical shifts, the hetero-nuclear J-couplings with protons, and the 1 H-1 H dipolar couplings to the first-order AH. 30 As mentioned above, the heteronuclear dipolar interaction is characterized by a space rank l = 2 and a spin rank  = 1.

A γ-encoded |𝑚| = 2 SQ hetero-nuclear dipolar recoupling must selectively reintroduce the two components {l, m, , μ} = {2, 2, 1, μ} and {2, -2, 1, -μ} of the hetero-nuclear dipolar coupling and 1 H CSA with μ = 1, while all other components must be suppressed.

During these recoupling schemes, the contribution of the dipolar coupling between I = 1 H and S nuclei to the first-order Hamiltonian is equal to 30 :

𝐻 ̅ 𝐷,𝐼𝑆 (1) = 𝜔 𝐷,𝐼𝑆 𝑆 𝑧 [𝐼 + exp(𝑖2𝜑) + 𝐼 -exp(-𝑖2𝜑)], (6)
where I ± = Ix  iIy are the shift operators, and the magnitude and phase of the recoupled I-S dipolar coupling are given by

𝜔 𝐷,𝐼𝑆 = -𝜅 √3 2
𝑏 𝐼𝑆 sin 2 (𝛽 𝑃𝑅 𝐷,𝐼𝑆 )

and 𝜑 = 𝛾 𝑃𝑅 𝐷,𝐼𝑆 -𝜔 𝑅 𝑡 0 , (

respectively, where bIS is the dipolar coupling constant in rad/s, and κ is the scaling factor of the recoupled hetero-nuclear dipolar interaction, which depends on the R𝑁 𝑛  symmetry and the R element. The Euler angles {0, 𝛽 𝑃𝑅 𝐷,𝐼𝑆 , 𝛾 𝑃𝑅 𝐷,𝐼𝑆 } relate the I-S vector to the MAS rotor frame, and t 0 refers to the starting time of the recoupling. The norm of 𝐻 ̅ 𝐷,𝐼𝑆 (1) does not depend on the 𝛾 𝑃𝑅 𝐷,𝐼𝑆 angle, since these recoupling schemes are encoded. 34,43 The Hamiltonian of Eq. 6 does not commute among different spin pairs, and hence, the PRESTO sequence is affected by dipolar truncation, i.e., the transfer to distant nuclei is attenuated by the stronger couplings with nearby spins 56 . As mentioned above, the SQ hetero-nuclear dipolar recoupling schemes also reintroduce the 1 H CSA with the same scaling factor κ. However, this term does not commute with the recoupled 1 H-S dipolar interactions. Therefore, in the case of large 1 H CSA, for instance at high magnetic fields, this interaction can interfere with the 1 H-S dipolar couplings, especially with the small ones. These interferences can be limited by the use of the PRESTO-III variant, depicted in Fig. 13a,c 30 , in which three CT-selective pulses are applied to the S channel. Indeed, the CT-selective π-pulses partly refocus the 1 H CSA, which limits these interferences.

Recoupling schemes for PRESTO

PRESTO using schemes, such as R18 1 7 and R18 2 5 , built from single rectangular pulses has been used for hetero-nuclear dipolar recoupling at moderate MAS frequencies (νR  10 kHz) 32 . Other schemes, such as R12 27 Al nuclei at νR = 62.5 kHz. 31 In this work, R𝑁 𝑛  schemes built from single rectangular and composite π-pulses to achieve γ-encoded |𝑚| = 2 hetero-nuclear SQ dipolar recoupling at νR = 20 or 62.5 kHz, were selected based on the AH and spin dynamics simulations. Three types of composite π pulses were tested: i) (270090180), which is designed to be offset compensated and amplitude modulated, thus improving the robustness to RF inhomogeneity. This composite pulse has been used as a basic element in several R𝑁 𝑛  sequences; 31,38,39,57 ii) (90024090900), which compensates both RF inhomogeneity and offset; 58,59 and iii) (90-45904590-45), which was introduced by Madhu et al., exhibits homo-nuclear decoupling properties. 60 .

Adiabatic pulses cannot be employed for SQ hetero-nuclear dipolar recoupling since they yield vanishing scaling factors for the rotational components with μ ≠ 0 54 . The corresponding selected R𝑁 𝑛  sequences are listed in Tables 3 and4 for νR = 20 and 62.5 kHz, respectively. These R𝑁 𝑛  symmetries eliminate the contribution of 1 H- 1 H dipolar interactions to the first-order Hamiltonian, but not their contribution to the second-order. The magnitude of the cross-terms between 1 H-1 H interactions in the second-order Hamiltonian was calculated with the Euclidean norm ‖𝜅 {1,2} 𝐷𝐷 1 ×𝐷𝐷 2 ‖ 2 , for the selected symmetries (Annex 1). 64 Likewise, the magnitude of the cross terms between 1 H CSA or offset were also calculated. with the Euclidean norms

‖𝜅 {1,2} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖ 2 and ‖𝜅 {1,2} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖ 2
, respectively. The calculated values of these three parameters are presented in the last three columns of the Tables 3 and4.

For νR = 20 kHz, according to the AH, the R𝑁 𝑛  sequence with the highest robustness to 1 H-1 H dipolar interactions is R22 2 7 (180 0 ). However, this recoupling is slightly less robust to 1 H offset than R18 2 5 (180 0 ), which has already been reported. For this MAS frequency, the R𝑁 𝑛  schemes using the chosen composite pulses either required RFfields greater than 120 kHz, e.g. ν1 = 130 and 173 kHz for the R26 3 7 schemes built from (90-45904590-45) and (270090180) pulses, or did not suppress efficiently the second-order cross-terms between 1 H-1 H interactions because of small RF-field (ν1 ≤ 62.5 kHz). For νR = 62.5 kHz, the R𝑁 𝑛  sequences using composite π-pulses recouple the 1 H-S dipolar interaction with a higher scaling factor than those built from single π-pulses.

According to AH, the (90024090900) basic element leads to the highest robustness to kHz. 31,57 The scaling factors κ of the 1 H-S dipolar interaction of the R𝑁 𝑛  schemes built from single π-pulses are small with 45 ≤ ϕ ≤ 135°, and hence the schemes given in Table 4 were also selected with an extended ϕ range of 20-160°. These recoupling schemes are less robust to offset than the R𝑁 𝑛  schemes built from (270090180) element.

D-RINEPT sequence

In the D-RINEPT sequence, the 1 H-S dipolar interactions are reintroduced under MAS by applying a non-γ-encoded two-spin order dipolar recoupling to the 1 H channel. These schemes reintroduce the m= 2 space components and the zeroquantum (0Q) terms of the 1 H-S dipolar interaction and 1 H CSA, i.e., the rotational components {l, m, , μ} = {2, 2, 1, 0}, while they suppress the contributions of 1 H isotropic chemical shifts, the hetero-nuclear J-couplings with protons, and the 1 H-1 H dipolar couplings to the first-order AH. 33,61 The contribution of the 1 H-S dipolar coupling to this Hamiltonian is equal to: 31,33,65 𝐻 ̅ 𝐷,𝐼𝑆 (1) = 2𝜔 𝐷,𝐼𝑆 𝐼 𝑧 𝑆 𝑧 (9) where 𝜔 𝐷,𝐼𝑆 = 𝜅𝑏 𝐼𝑆 sin 2 (𝛽 𝑃𝑅 𝐷,𝐼𝑆 )cos(2𝜑).

The norm of 𝐻 ̅ 𝐷,𝐼𝑆 (1) depends on the  phase, given by Eq. 8, and hence on the 𝛾 𝑃𝑅 𝐷,𝐼𝑆 angle. Therefore, these two-spin order dipolar recoupling schemes are non--encoded.

The Hamiltonian of Eq. 9 commutes among different spin pairs and hence, these recoupling schemes are not affected by dipolar truncation. Similarly, the recoupled 1 H CSA contribution to the first-order Hamiltonian is proportional to Iz and hence, also commutes with the recoupled 1 H-S dipolar interactions and does not interfere with the hetero-nuclear dipolar recoupling.

The new D-RINEPT-CWc sequence introduced by Nagashima et al., 19,20 is displayed in Fig. 13b,c. The 1 H-S dipolar couplings are reintroduced by applying the R𝑁 𝑛  schemes, such as SR4 1 2 , during the defocusing and refocusing delays , which are identical in this article, even if distinct delays can improve the transfer efficiency. 19 As the two-spin order recoupling schemes are non-γ-encoded, they must be rotorsynchronized. A delay of TR is used between two successive R𝑁 𝑛  blocks. In the D-RINEPT-CWc sequence, a CW irradiation is applied during these delays in order to limit the losses due to 1 H-1 H dipolar interactions. 19 The nutation during this CW irradiation is eliminated by employing CW irradiations with opposite phases.

Furthermore, the robustness to 1 H rf-field inhomogeneity is improved by replacing the first π and second π/2 pulses by composite (90018090900) and (9090900) pulses, respectively, the CW irradiation being applied between the individual pulses. 58,66 .

Recoupling schemes for D-RINEPT

Different R𝑁 𝑛  sequences have been proposed to achieve non-γ-encoded |𝑚| = 2 two-spin order dipolar recoupling, including: i) symmetries R(4𝑛) 𝑛 2𝑛-1 = R12 3 5 , R16 4 7 , R20 5 9 , R24 6 11 , R28 7 13 and R32 8 15 for n = 3, 4, 5, 6, 7 and 8 using single π-pulses as basic element, which have been employed to measure 1 H- 17 O dipolar couplings at νR = 50 kHz, 61 ii) SR4 1 2 recoupling built from a single π-pulse, which corresponds to the

[R4 1 2 R4 1 -2 ] 0 [R4 1 2 R4 1 -2 ] 120 [R4 1 2 R4 1 -2
] 240 sequence and has been employed in the RINEPT scheme, 19,31 iii) R12 3 5 and SR4 1 2 schemes using a (90-45904590-45) composite π-pulse as a basic element, which have been incorporated into D-HMQC at νR = 36 kHz, 36 and iv) SR4 1 2 schemes built from a (tt) adiabatic pulse, which have been used in the RINEPT sequence. 19,20 A schematic representation of this selection is presented in Fig. 17.

The selected R𝑁 𝑛  sequences are listed in Table 6, along with the parameters of the SR4 1 2 schemes built from the different basic elements R for the sake comparison. The scaling factor, κ, of the recoupled 1 H-S dipolar interaction was calculated using the 'C and R symmetries' Mathematica package. 39,[62][63][64] . In addition, the amplitudes of the cross-terms between 1 H- , respectively. They are presented in the last three columns of the Table 6. 6.

According to the AH, the (90024090900) composite -pulse yields the highest robustness to 1 H-1 H dipolar interactions. However, the rf-field requirement of the R𝑁 𝑛  sequences built from this composite pulse, 1 = 1.16NR/n, is not compatible at R = 62.5 kHz with most 1.3 mm MAS probes (e.g. 1 = 291 kHz for SR4 1 2 ). Furthermore, the highest robustness to 1 H CSA and offset is achieved using the (270090180) composite -pulse. The SR4 1 2 schemes benefit from the highest robustness to 1 H CSA, because of the three-step multiple-quantum super-cycle. 33,64 Contrary to the R𝑁 𝑛  with |𝑚| = 2 SQ hetero-nuclear dipolar recouplings, the rf-field of the R𝑁 𝑛  with |𝑚| = 2 two-spin order schemes is always higher than 2R since these symmetries with 2n

> N, such as R12 9 5 , have smaller κ scaling factors for the basic elements employed here. In the case of the adiabatic R𝑁 𝑛  (tt) sequences, the determination of the scaling factors of the first-and second-order terms of the effective Hamiltonian is more complex since they depend on the 1,max, Δ0,max, ξ and θ parameters. 54 For example, the scaling factor of the R12 3 5 and SR4 1 2 schemes is κ = 0.31 for 1,max/Δ0,max = 0.685, ξ = 10 and θ = 87°, and this value monotonously decreases for increasing 1,max/Δ0,max ratios.

1 H→ 27 Al PRESTO AND D-RINEPT NMR EXPERIMENTS

Samples and experimental conditions

Experiments were performed on isotopically unmodified -alumina purchased from Merck, and isopropylamine-templated AlPO4-14, which was prepared as described by Antonijevic et al. 67 All 1 H → 27 Al RINEPT-CWc and PRESTO-III NMR experiments were recorded at B0 = 18.8 T on a Bruker BioSpin Avance NEO spectrometer equipped with a 1.3 mm MAS probe spinning at R = 20 (to test the R𝑁 𝑛  schemes with large rf-field requirement) or 62.5 kHz. The tested recoupling schemes are listed in Tables 7 and8 for experiments recorded at R = 20 kHz and Tables 9 and10 for R = 62.5 kHz. The RF-field of the /2 and  pulses on the 1 H channel, which do not belong to the recoupling scheme, was equal to 208 kHz, that of the continuous wave irradiation to 147 kHz, and that of 27 Al CT-selective π/2 and π pulses to 10 kHz. The defocusing and refocusing delays  are given in Tables 7 to 10. The pulses on the 1 H channel were applied on-resonance, whereas those on 27 Al channel were applied: (iii) in the middle of the AlO4 and AlO6 peaks for the 1D spectra shown in Figs.

These differences in offset explain some changes in the relative efficiencies of the recoupling between the figures. These 1D spectra resulted from averaging 64 transients with a relaxation delay of 1 s. The 27 Al isotropic chemical shifts were referenced at 0 ppm to 1 mol.L ─1 [Al(H2O)6] 3+ solution.

2D 1 H→ 27 Al D-HETCOR spectra of AlPO4-14 using RINEPT-CWc-SR4 1 2 were acquired with (1800), (270090180) and tt pulses as well as PRESTO-R16 7 6 (270090180).

These 2D spectra were acquired using a non-uniform sampling (NUS) with an exponentially biased sampling retaining 25% of the points with respect to uniform sampling. The 2D spectra resulted from 8 transients for each of the 500 t1 increments with a recycle delay of 1 s, i.e. an acquisition time of 72 min.

The decay of the transverse proton magnetization of AlPO4-14 was also measured during a spin echo sequence (Fig. 18), in which the refocusing -pulse was identical to that used in the defocusing part of the RINEPT-CWc sequence (Fig. 13b). This decay was measured at R = 20 and 62.5 kHz either with no recoupling or by applying a SR4 1 2 recoupling built from (1800), (270090180) and tt pulses during the delays of the spin echo sequence. The RF-fields applied during the two recoupling blocks were equal to their optimal values given in Tables 6 and8.

Figure 18. Spin echo sequence used to measure the decay of the transverse 1 H magnetization.

Performances of PRESTO and D-RINEPT at R = 20 kHz γ-alumina

The 1D spectra of γ-alumina acquired using 1 H → 27 Al RINEPT and PRESTO sequences, shown in Fig. 19, exhibit two resonances at 70 and 10 ppm, assigned to tetra-(AlO4) and hexa-coordinated (AlO6) resonances, respectively. 68 . The signal of penta-coordinated (AlO5) sites, which are mainly located in the first surface layer, is barely detected because of the lack of sensitivity of conventional solid-state NMR spectroscopy. 69 The most intense peak, AlO6, was used to compare the transfer efficiencies of RINEPT and PRESTO sequences with different recoupling schemes.

Table 7 lists the measured performances of 1 H → 27 Al RINEPT-CWc and PRESTO transfers using various recoupling for γ-alumina at R = 20 kHz. The PRESTO sequences using R22 2 7 (1800) and R18 2 5 (1800) recoupling were compared with the RINEPT-CWc scheme using SR4 1 2 and R12 3 5 with: single (1800), composite (270090180) and (90-45904590-45) or (tt) adiabatic pulses. A low transfer efficiency was obtained for RINEPT-CWc-SR4 1 2 (90024090900) because of its low scaling factor, κ = 0.131, and hence its performances are not reported in Table 7. The recoupling schemes based on the symmetry SC2 1 0 , corresponding to the

[C2 1 0 ] 0 [C2 1 0 ] 120 [C2 1 0 ] 240 sequence
with a basic element C = (9045901359045902259031590225), or C6 3 0 built from C' = (9030901209030902409033090240) were also tested. These basic elements, which derive from (90-45904590-45), have recently been proposed. 36 As seen in Table 7 and Fig. 19a, the sequences yielding the highest transfer efficiencies are by decreasing order RINEPT-CWc with SR4 1 2 (tt) or R12 The  delays and 1/1,max RF-fields were fixed to their optimum values given in Tables 7 and9.

The PRESTO sequences using R22 2 7 (1800) and R18 2 5 (1800) recoupling also result in good transfer efficiencies, but lower than RINEPT-CWc-SR4 1 2 (tt). However, they use RF-fields of ν1/νR = 5.5 and 4.5, which are compatible with the specifications of 3.2 mm MAS probes. The higher transfer efficiency of R22 2 7 (1800) with respect to R18 2 5 (1800) stems from its weaker second-order cross-terms between 1 H-1 H interactions as shown in Table 3.

The efficiency of the RINEPT-CWc-SR4 1 2 (270090180) sequence, with ν1 = 4νR, is comparable to that of PRESTO-R18 2 5 (1800), but with a higher robustness to offset and RF inhomogeneity. We can notice that amplitude modulated recoupling schemes, for which the phase shifts are equal to 180°, such as SR4 1 2 (270090180) and SR4 1 2 (1800), exhibit a high robustness to RF-field maladjustments (Fig. 20). 39 The use of (270090180) composite pulses with SR4 1 2 symmetry, instead of single π pulses, improves its transfer efficiency as well as its robustness to offset and RF-field inhomogeneity.

In summary, for R = 20 kHz in γ-alumina, the RINEPT-CWc-SR4 1 2 (270090180) sequence achieves efficient and robust transfers of magnetization from protons to 27 Al nuclei using a moderate RF-field of ν1 = 4νR. For 1 H spectra with a width smaller than 20 kHz and MAS probes with a good RF-homogeneity, PRESTO-R22 2 7 (1800) can result in slightly higher transfer efficiencies. 7. Vertical bars correspond to the error on AlO6 intensity, determined by the inverse of the S/N of the optimum offset value.

AlPO4-14

Fig. 22a shows the 1 H → 27 Al RINEPT and PRESTO 1D spectra of AlPO4-14 recorded with R = 20 kHz. They exhibit three 27 Al resonances at 43, 21 and -2 ppm assigned to AlO4, AlO5 and AlO6 sites, respectively. 70 The AlO5 and AlO6 sites are directly bonded to OH groups. The 1 H MAS spectrum is shown in Fig. 23. According to the literature, the 27 AlO4 signal subsumes the resonances of two AlO4 sites with quadrupolar coupling constants CQ = 1.7 and 4.1 MHz, whereas those of AlO5 and AlO6 sites are equal to 5.6 and 2.6 MHz, respectively. 67,71 The 1 H-1 H dipolar couplings within the isopropylamine template molecule are larger than in γ-alumina. We used the most intense peak, AlO4, to compare the 1 H → 27 Al transfer efficiencies of RINEPT-CWc and PRESTO sequences with different recoupling schemes, and the results are given in Table 8. The six sequences yielding the highest transfer efficiencies are the same as for γ-alumina and their relative efficiencies are comparable for the AlO4 peak of AlPO4-14 and the AlO6 signal of γ-alumina. The  delays and 1/1,max RF-fields were fixed to their optimal values given in Tables 8 and10. The resonance at ca. 11 ppm in (a) is due to an impurity.

Nevertheless, the RF requirement of the SR4 1 2 (tt) and R12 3 5 (tt) schemes is higher for AlPO4-14 than for γ-alumina because of the larger 1 H-1 H dipolar couplings. This fact is in agreement with the numerical simulations (not shown) which demonstrate that higher 1 H-1 H dipolar couplings require higher RF-field so that the (tt) pulses remain adiabatic. 1 This RF requirement prevents the use of these adiabatic recoupling schemes at R = 20 kHz with most 3.2 mm MAS probes. The RF requirement of the other sequences, and their robustness to offset and rf-field inhomogeneity are similar for both samples (Table 8 and Figs. 24 and25).

In the case of AlPO4-14, PRESTO sequence yields a higher efficiency than RINEPT for AlO5 and AlO6, contrary to the AlO4 resonance, since (i) these Al sites are directly bonded to OH groups and (ii) R22 2 7 (1800) and R18 2 5 (1800) schemes are subject to dipolar truncation (section 2.3.4), which prevents to transfer the 1 H magnetization of these OH groups to 27 AlO4 nuclei.

Hence, at R = 20 kHz, for both AlPO4-14 and γ-alumina, the RINEPT-CWc-SR4 1 2 (270090180) and PRESTO-R22 2 7 (1800) sequences are the best choices to transfer the 1 H magnetization to 27 Al nuclei. 8. Vertical bars correspond to the error on AlO4 intensity, determined by the inverse of the S/N of the optimum offset value.

Performances of PRESTO and D-RINEPT at R = 62.5 kHz

Similar comparisons of the performances of the various RINEPT-CWc and PRESTO sequences were performed for γ-alumina and AlPO4-14 at R = 62.5 kHz.

γ-alumina

The corresponding data for γ-alumina are given in Table 9. The sequences yielding the highest transfer efficiencies are by decreasing order: RINEPT-CWc with SR4 1 2 (tt)

or R12 3 5 (tt) > RINEPT-CWc-SR4 1 2 (270090180)  PRESTO-R16 7 6 (270090180) > PRESTO-R14 6 5 (270090180) > RINEPT-CWc-R12 3 5 (270090180).
The nominal RF requirements of the RINEPT sequences using adiabatic or (270090180) composite π-pulses correspond to ν1max  5νR (313 kHz) or 4νR (250 kHz), which exceed the specifications of our 1.3 mm MAS probe, and the sequences were tested only up to ν1max = 208 kHz (Fig. 26). This suboptimal RF-field may limit the transfer efficiencies of these sequences.

The PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) sequences yield transfer efficiencies comparable to those of RINEPT-CWc-SR4 1 2 (270090180), but with a significantly lower RF-field, 1  137 kHz  2.3νR. Furthermore, the robustness to offset of these PRESTO sequences is comparable to that of RINEPT-CWc-SR4 1 2 (270090180) (Fig. 27). PRESTO-R22 4 3 (1800) and -R16 3 2 (1800) sequences with a small phase shift of 2ϕ ≤ 52° are less efficient because they are sensitive to RF inhomogeneity. Table 9. Comparison of the performances of 1 H→ 27 Al RINEPT-CWc and PRESTO transfers using various recouplings for the AlO6 signal of γ-alumina at R = 62.5 kHz.

a AlO6 signal normalized to that with 1 H → 27 Al RINEPT-CWc-SR4 1 2 (tt). b The absolute error on AlO6 signal intensity is  0.08. c FWHM of the robustness to rf-field was not measured for RINEPT-SR4 1 2 (tt) and -R12 3 5 (tt) (Fig. 26).

AlPO4-14

In the case of AlPO4-14, the relative transfer efficiencies for 27 AlO4 species follow a similar order as for γ-alumina, except that the transfer efficiencies of PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) are significantly lower than that of RINEPT-CWc-SR4 1 2 (270090180) (Table 10). This decreased efficiency of the PRESTO schemes for AlO4 stems notably from the dipolar truncation, which prevents the transfer of magnetization from the OH groups bonded to AlO5 and AlO6 sites to AlO4, since these 27 AlO4 nuclei are significantly more distant to protons as shown in Table 11.

Furthermore, the amplitude-modulated SR4 1 2 (270090180) recoupling benefits from a higher robustness to RF-field inhomogeneity than the PRESTO schemes (Fig. 28).

Conversely, the robustness to offset of these three sequences are comparable (Fig. 29), In summary, at R = 62.5 kHz, for both γ-alumina and isopropylamine-templated AlPO4-14, PRESTO-R16 7 6 (270090180) and RINEPT-CWc-SR4 1 2 (270090180) are the best methods to transfer the polarization of protons to quadrupolar nuclei. However, the first sequence requires a much lower rf-field than the second one. (tt) and -R12 3 5 (tt) (Fig. 28). 27 Al HETCOR EXPERIMENTS Fig. 30 demonstrates the possibility to acquire 2D 1 H- 27 Al D-HETCOR spectra using RINEPT-CWc-SR4 1 2 (2700,90180) transfer at R = 62.5 kHz. This spectrum was recorded on AlPO4-14 using a NUS scheme retaining 25% of the t1 points, which would be acquired using the conventional uniform sampling. In this spectrum, the CH proton only correlates with the AlO4 site since it is too distant from AlO5 and AlO6 sites as shown in Table 11. The other two 1

MEASUREMENT OF THE DECAY OF THE TRANSVERSE 1 H MAGNETIZATION DURING RECOUPLING

The decay of the 1 H transverse magnetization was measured during a spin-echo experiment (Fig. 18), in which the refocusing -pulse was the composite one employed in the defocusing part of the RINEPT-CW sequence (Fig. 13b). These experiments were performed on AlPO4-14 since the 1 H- At R = 20 kHz, the T2' constants are significantly shorter under SR4 1 2 (1800) and SR4 1 2 (270090180) than without recoupling. This faster decay can stem from the reintroduction of 1 H-1 H dipolar interactions in the second-and higher-order terms of the AH by the recoupling as well as the effect of pulse transients 74 . Conversely, the T2' constants under SR4 1 2 (tt) are much longer than without recoupling, showing that the adiabatic pulses using large RF-field efficiently decouple the 1 H-1 H dipolar interactions, whereas the continuous variation of the phase and amplitude during these pulses minimizes the transients.

At R = 62.5 kHz, the T2' constants without recoupling are lengthened with respect to those at R = 20 kHz since faster MAS better averages the 1 H-1 H dipolar interactions. 75 . Conversely, the T2' constants under SR4 1 2 (270090180) recoupling are shorter at R = 62.5 than at 20 kHz. This counter-intuitive reduction may stem from the shorter pulse lengths at R = 62.5 kHz, which results in a larger number of transients. For the same reason, the T2' constants under SR4 1 2 (1800) are only slightly longer at high MAS frequency. The T2' constants under SR4 1 2 (tt) recoupling are much shorter at R = 62.5 than at 20 kHz because the adiabaticity criterion is not fulfilled at R = 62.5 kHz and hence, the elimination of 1 H-1 H dipolar interactions is less effective (Fig. 28).

CONCLUSIONS

In with RF-requirements of ca. 4νR and 2.3νR, respectively, result in the most robust and efficient transfers. At both MAS frequencies, the RINEPT and PRESTO techniques complement each other since the latter is dipolar truncated, whereas the former is not.

As a result, the RINEPT sequences must be chosen to observe simultaneously protonated and unprotonated sites, whereas the PRESTO schemes can be employed for the selective observation of quadrupolar nuclei in proximity to protons. These techniques are expected to be useful for transferring the DNP-enhanced magnetization of protons to quadrupolar nuclei in indirect MAS DNP experiments at R ≥ 20 kHz, notably used at high magnetic fields. 20,76,77 It was demonstrated that they can be used to correlate the NMR signals of protons and quadrupolar nuclei at high MAS frequencies.

INTRODUCTION

Dipolar homo-nuclear correlation (D-HOMCOR) 2D spectra are important tools for solid-state nuclear magnetic resonance (ssNMR) spectroscopy. They allow for the analysis of through-space proximities between different nuclei belonging to the same isotopes. This D-HOMCOR information is mostly acquired through singlequantum  single-quantum (SQ-SQ) or double-quantum  single-quantum (DQ-SQ) 2D spectra. To enhance the resolution, the experiments must be performed under magic-angle spinning (MAS) of the sample. Therefore, they require rotorsynchronized recoupling schemes to reintroduce the dipolar interactions, which are otherwise canceled by the sample rotation. A large armory of these two types of MAS experiments has been largely developed for spin-1/2 nuclei, 1 and presently these methods can be considered as routine tools. However, the development of these D-HOMCOR sequences is still ongoing for quadrupolar nuclei with a spin value I > ½.

The quadrupolar nuclei with a half-integer spin value (I = 3/2, 5/2, 7/2 and 9/2) represent two-thirds of the NMR actives isotopes. Several types of HOMCOR methods have been proposed for quadrupolar nuclei. They were initially based either on quadrupolar-driven 2,3 or proton-driven [4][5][6] dipolar recoupling mechanisms.

However, the first methods are not very efficient, and the second require protonated samples. In the case of half-integer quadrupolar nuclei, several new homonuclear recoupling sequences have more recently been proposed. [7][8][9][10][11][12][13][14][15] They are based on the R𝑁 𝑛  rotor-synchronized MAS schemes developed by Levitt and co-workers, 16,17 and they have largely been developed by Edén et al, who have recently published several research and review articles on this topic. [8][9][10][11][12][13] However, to the best of our knowledge, there is no global comparison of the advantages and limitations of the two types of sequences (SQ-SQ and DQ-SQ) for half-integer quadrupolar nuclei. Moreover, the previously published D-HOMCOR analyses have mostly been performed either on spin-3/2 or 5/2 nuclei, and rarely on the two categories of nuclei simultaneously.

Starting from the conclusions provided in previous publications, [8][9][10][11][12][13] this work has sought to determine the best schemes for the aforementioned methods and provide a quantitative comparison of their efficiencies. In this work, the analysis has been simultaneously performed on nuclei with spin 3/2 ( 11 B) or 5/2 ( 27 Al). It is important to note that the theoretical explanations used in this manuscript have been developed

for spin-1/2 nuclei, for the sake of ease of spin dynamics of these nuclei with respect to quadrupolar nuclei. This means (i) that they are only based on symmetry properties, 17 (ii) that the nuclei are not subject to the quadrupolar interaction, and (iii)

hence that the rf-field is much larger than all other microscopic interactions.

However, contrary to spin-1/2 nuclei, for half-integer quadrupolar nuclei the spins are often subject to large quadrupolar interactions, and hence the rf-field must be limited to a few kHz to control the trajectory of the magnetization, which limits the excitation bandwidth of the recoupling sequences.

PULSE SEQUENCES

In Fig. 32, the D-HOMCOR sequences applied to half-integer quadrupolar nuclei are presented. The purpose of these sequences is to correlate the signal of identical nuclei, which are coupled by dipolar interaction, and simultaneously cancel the signal created by the coherences that are developing within the same nucleus. To achieve this, perfect control of the transfers of the magnetizations is required, which practically means using rf-fields that are much larger than all microscopic interactions. This is feasible for spin-1/2 nuclei, because dipolar and CSA interactions are often limited to a few tens of kHz, whereas MAS probes can deliver rf-fields up to ca. 100 kHz. However, these rf-fields cannot control the magnetization of the satellite transitions (ST, m  m-1 with m  1/2) of half-integer quadrupolar nuclei since these ST transitions are subject to the first-order quadrupolar interactions, which often extend over several MHz. In this case, the only way to control the magnetization transfers is to apply weak rf-fields of only a few kHz to manipulate selectively the magnetization of the Central-Transition (CT) between the 1/2 energy levels. This is possible because these levels are only subject to the much smaller second-order quadrupolar interaction. However, as a result, these weak CT-selective pulses lead to poor robustness to offsets. In this 'fictitious spin-1/2 approximation', the effective rffield acting on these CT levels is the rf field strength multiplied by the factor

(𝐼 + 1 2 ⁄ ).
All the HOMCOR sequences presented derive from the HORROR condition, 7,[18][19][20][21][22] which means that the nutation frequency of the pulses is given by

𝜈 1 = 𝜈 𝑅 (2𝐼 + 1) ⁄ , (1)
where 𝜈 𝑅 = 1 Τ 𝑅 ⁄ is the MAS spinning frequency. All following sequences are derived from the R𝑁 𝑛  rotor synchronized recoupling schemes introduced by Levitt for spin-1/2 nuclei. 16,17 However, it is important to remind that these schemes follow symmetry considerations, but that they do not account for the quadrupolar interaction. In the following, 𝜏 mix will be referred to the recoupling time either in the center of the SQ-SQ sequence or in the excitation and reconversion periods of the DQ-SQ scheme (Fig. 32).

The SQ-SQ sequence shown in Fig. 32a is identical to that applied to spin-1/2 nuclei. After a /2-pulse, the CT coherence evolves during t1 on the 1Q levels. Then, a second /2-pulse transfers these magnetizations along z, where a zero-quantum (ZQ) mixing sequence is applied. A three or four steps super-cycling is used, which means that all phases of the ZQ mixing sequence are incremented every 120° or 90°. In the following, these super-cyclings will be labelled with the index 𝑀 = 3 or 4, respectively.

Then a third /2-pulse is applied to transfer the magnetization onto the -1Q level for CT observation during t2 period.

The DQ-SQ sequences are composed of two recoupling periods of identical length, 𝜏 mix , one for excitation and one for reconversion (Fig. 32b). They transfer the magnetization from 0Q to 2Q and from 2Q to 0Q, respectively. The recoupling periods are composed of symmetry-based schemes, bracketed or not with /2-pulses.

They are denoted as [R𝑁 𝑛

 ] or R𝑁 𝑛  , respectively. The DQ-SQ sequence shown in Fig. 32b differs from that applied to spin-1/2 nuclei by the blue -pulse in the middle. By using a phase-cycling with at least 8 phases, the 2Q → ∓2Q inversion pathways are selected. As a result, all single nucleus contributions are canceled and hence, only remain those between different nuclei, which contribute to the DQ-SQ HOMCOR signal. The 𝜏 -180 -𝜏 sorting part of the sequence does not introduce any dephasing for the detected signal. Each R𝑁 𝑛  individual block lasts 𝑛𝛵 R and hence, 𝜏 mix is a multiple number of rotor periods. Presently, all DQ recoupling schemes are not encoded, which means that the starts of the excitation and reconversion blocks must be separated by an integer number of rotor periods. As a result, the delay between the end of the 1 st and the beginning of the 2 nd R𝑁 𝑛  recoupling parts must also be rotorsynchronized (Fig. 32b: 𝑀𝛵 R ). The DQ evolution time is also often rotorsynchronized, t1 = jTR, (i) to fold back all sidebands along F1 onto the center-band, to simplify the spectrum and enhance the S/N ratio, and (ii) to always keep rotorsynchronized the starts of excitation and reconversion parts. The spinning speed may not be sufficient for the frequency spread along F1, which is the double of that along F2. In that case, the position of the -pulse (shown in blue in Fig. 32b) should be changed to decrease the required indirect spectra-width by redistributing the evolution time before and after this pulse. 14,23 To limit the losses, the  delay should be as small as possible, and thus equal to (TR -180 -90)/2 or (TR -180}/2, when respectively using or not bracketing /2-pulses before and after the R𝑁 𝑛  blocks. The two recoupling 𝜏 mix blocks use the same R𝑁 𝑛  scheme, except with a 90° phase shift.

SYMMETRY-BASED MATHEMATICAL DESCRIPTION

It is important to remind here that the R𝑁 𝑛  schemes follow symmetry considerations, but that they do not account for the quadrupole interactions, which are often much larger than the rf-field.

SQ-SQ

Following the two most recent reviews, 12,13 the five best recoupling schemes for SQ-SQ experiments have been analyzed in this work. These schemes are described in The first four sequences, called SRN n 1 , use a phase inversion super-cycle, which results from the combination of two inverse recoupling schemes, R𝑁 𝑛 1 R𝑁 𝑛 -1 . This supercycling is often used with spin-1/2 nuclei to make the sequence more robust to CSA.

The last sequence, BR2 2 1 , can be written as (R2 2 1) n (R2 2 -1 ) n . It uses a different supercycling, introduced with the R 3 recoupling, 7 which corresponds to a global phase reversal at the midpoint of the 𝜏 𝑚𝑖𝑥 interval. All these sequences were initially developed for DQ-SQ experiments. (e) BR2 2 1 . In (a-c,e), white and grey colors mean phases of 90° and 270° respectively, and in (d) yellow and green colors mean phases of 45° and 315° respectively. In each case, the number is the flip-angle for the CT-selective pulse.

It must be noted that to fulfill the HORROR condition (Eq. 1), the SR2 4 1 and SR2 8 1 schemes replace the 180° square pulses of SR2 2 1 with composite -pulses, such as (900270180) pulses for SR2 4 1 (Fig. 33b). In the following, Σ and  will be referred to the sum and difference of chemical shifts for a pair of nuclei j and k.

The SRN n 1 recoupling schemes shown in Fig. 33a-d are periodic, and hence they can be described with an average Hamiltonian:

This super-cycling concatenates M blocks of SRN n 1 schemes with the phases of all the pulses of a block shifted by 360°𝑀 ⁄ with respect to the previous block shown in Fig. 33. However, it must be noted that if on one hand it increases the robustness of the 𝑆 schemes, on the other hand it multiplies by 𝑀 their cycle times. Therefore, to limit the global cycle time, only the 𝑀 = 3 or 4 super-cyclings are used.

In the following, the conclusions obtained by Edén and co-workers for SQ-SQ HOMCOR analyses concerning half-integer quadrupolar nuclei are summarized. 12,13 These conclusions have been obtained with extensive simulations and experiments at 14.1 T and with 𝜈 𝑅 ranging from 10 to 30 kHz.

-First conclusion: BR2 2 1 is not recommended as it requires a high-power 1 H decoupling during the long 𝜏 𝑚𝑖𝑥 time, because it also recouples the hetero-dipolar interactions.

-Second conclusion: an enhanced robustness to offset and rf-inhomogeneity is observed with (SR2 2 1 )M < (SR2 4 1 )M < (SR2 8 1 )M for increasing chemical shift difference, .

In the case of a small-moderate separation, Δ ≤ 3 -4 kHz, (SR2 2 1 ) 𝑀 works better than (SR2 4 1 ) 𝑀 , (SR2 8 1 ) 𝑀 and (SR4 4 1 ) 𝑀 . As example, this was the case for 11 B in a NCBS glass (Fig. 6g in ref. 13 ).

In the case of a large separation, Δ > 3 -4 kHz, from the simulations, (SR2 8 1 ) 𝑀 is better than (SR2 2 1 ) 𝑀 and (SR2 4 1 ) 𝑀 . However, this was not confirmed experimentally, and hence the authors recommended (𝑆𝑅2 4 1 ) 𝑀 . This was the case for 27 Al in aluminophosphate AlPO-CJ19, where Δ 𝑖𝑠𝑜,𝑚𝑎𝑥 ≈ 9.6 kHz and 27 Al- 27 Al dipolar interactions are small (conclusion in ref. 12 ).

DQ-SQ

The same SRN n 1 recoupling schemes shown in Fig. 33a-d can be applied to DQ-SQ sequences. Their Hamiltonian given in Eq. 2 are mixtures of DQ and ZQ operators.

Pure DQ recoupling can be obtained with sandwiching these schemes with two 90° pulses, and the sequences, then denoted [SRN n 1 ] double the scaling factor and eliminate the ZQ terms:

𝐻 [𝑆] 𝑗𝑘 = 𝐾. 𝐹 𝑗𝑘 . { 𝐼 + 𝑗 𝐼 + 𝑘 + 𝐼 - 𝑗 𝐼 - 𝑘 } (4)
The theoretical maximal efficiencies of SRN n 1 and [SRN n 1 ] sequences are identical.

However, practically, [SRN n 1 ] is often slightly more efficient due to (i) increased losses in SRN n 1 related to lower scaling factor, and (ii) interferences of ZQ and DQ terms, which do not commute for distinct spin-pairs.

For the BR2 2 1 sequence, no basic periodic unit can be defined, and therefore the conventional average Hamiltonian cannot be applied. However, a Magnus expansion can be applied to the Hamiltonians of the R2 2 1 schemes, and after the n th point of BR2 2 1 , at 4𝑛Τ 𝑅 , it gives in an offset-toggling frame: 15

𝐻 𝐵𝑅2 2 1 𝑗𝑘 = 0.5𝐾. 𝐹 𝑗𝑘 . {(1 -0.5(SΔ -SΣ))(𝐼 + 𝑗 𝐼 - 𝑘 + 𝐼 + 𝑘 𝐼 - 𝑗 ) -2(SΔ + SΣ)𝐼 𝑗z 𝐼 𝑘z + 2(CΣ + CΔ)2𝐼 𝑗y 𝐼 𝑘z + 2(CΣ -CΔ)𝐼 𝑗z 𝐼 𝑘y + (1 + 0.5 (SΔ -SΣ))(𝐼 + 𝑗 𝐼 + 𝑘 + 𝐼 - 𝑗 𝐼 - 𝑘 )} (5)
where

𝑆Ψ = sin(8𝑛Ψ𝛵 R ) (8𝑛Ψ𝛵 R ) ⁄ and 𝐶Ψ = {1 -cos(8𝑛Ψ𝛵 R ) (8𝑛Ψ𝛵 R ) ⁄ },
with Ψ = Δ or Σ, and these terms tend to 0 when 8𝑛Ψ𝛵 R ≫ 1. In Eq. 5, the 1 st and 2 nd terms are ZQ, the 3 rd and 4 th are SQ, and the 5 th is the DQ contribution. These five terms depend on the length of the BR2 2 1 recoupling, 4𝑛𝛵 R , and on the sum (Σ), and difference (Δ) of chemical shifts of the two coupled spins. In the case of small dipolar couplings and hence large n values, if simultaneously 𝑛𝛵 R ≫ 1/(8Δ) and 1/(8Σ), Eq. 5 simplifies and only remains

𝐻 𝐵𝑅2 2 1 𝑗𝑘 = 0.5𝐾. 𝐹 𝑗𝑘 . {(𝐼 + 𝑗 𝐼 - 𝑘 + 𝐼 + 𝑘 𝐼 - 𝑗 ) + (𝐼 + 𝑗 𝐼 + 𝑘 + 𝐼 - 𝑗 𝐼 - 𝑘 )} (6)
With (Δ, Σ) = (≠ 0, 0), (0, ≠ 0) or (Δ, ±Δ), it may also lead to the same DQ contribution multiplied by 0.5, 1.5 or 1, when 𝑛𝛵 R ≫ 1/(8Δ) or 1/(8Σ). This shows that there is no constant DQ-SQ dipolar scaling factor for the BR2 2 1 recoupling, even in the simple case of spin-1/2 nuclei. Edén and co-workers have shown with simulations, that depending on the quadrupolar interaction and the dipolar value, the scaling factor is 'intermediate in between the bracketed or not bracketed sequences'. 11 In the case of [BR2 2 1 ], the Hamiltonian of Eq. 5 simplifies to Eq. 4, which leads to an increase of the scaling factor.

MATERIALS AND METHODS

Samples

Crystalline Li2O•2B2O3 and isopropylamine templated AlPO4-14 (AlPO4-14 hereafter) were used for 11 B-11 B and 27 Al- 27 Al HOMCOR experiments, respectively.

Crystalline Li2B4O7 was synthetized using the procedure detailed below whereas AlPO4-14 was prepared as described by Antonijevic et al. 26

Synthesis of lithium diborate

Li2O•2B2O3 was prepared by mixing lithium carbonate (Li2CO3) and boron oxide (B2O3) in appropriate proportions. The mixture was melted in a platinum crucible in a furnace at 1000 °C for 5 h before quenching it on an iron plate at room temperature.

The glass sample was crystallized by heating it at 540 °C during 24 h and allowing it to cold down slowly. The crystallization temperature was verified by Differential Thermal Analysis (DTA).

Phase identification

After synthesis, powder XRD measurements were performed to determine the phase purity of the crystal sample. The patterns were recorded at room temperature using a Bruker D8 Advance X-ray diffractometer working in the in the Bragg-Brentano geometry with Cu-Kα (λ= 1.5406 Å) radiation. The diffractometer was operated over the angular range of 2θ = 10-60° with a scan step width of 0.02° and a scan rate of 0.5 s/step.

The XRD-patterns were analyzed using the DIFFRAC.EVA V4.0 software (Bruker). The profile-matching refinement of the crystal structures were carried out using the single crystal structural model for each compound imported in the JANA2006 crystallographic system. 27 The background was fitted using Chebyshev polynomial functions, and the peak shapes were described by a Pseudo-Voigt function.

The XRD pattern of the crystallized sample is shown in Fig. 34. It is in good agreement with that of reported Li2B4O7 crystal (JCPDS card 79-0963). The main high intensity reflections are assigned to the Li2B4O7 phase. However, some peaks with smaller intensities cannot be assigned to Li2B4O7, which indicates that the sample contains an impurity. In order to identify this impurity, profile-matching refinement was done using the JANA2006 27 software. α-LiBO2 (JCPDS card 51-0517) was identified as a secondary phase and can be attributed as the impurity of the crystallized sample (Fig. 36).

NMR experiments

All experiments 11 B-11 B and 27 Al- 27 Al through-space homonuclear correlation (D-HOMCOR) experiments have been recorded at two magnetic fields: 9.4 and 18.8 T, with 3.2 mm rotors spinning at R = 20 kHz. They provided the same information, and hence the results obtained at 18.8 T are mainly presented in this dissertation because the resonances are four times more resolved than at 9.4 T, and hence much easier to explain. 11 B chemical shifts were referenced using a solid sample of NaBH4 (iso = -42.05 ppm), whereas 27 Al chemical shifts were referenced at 0 ppm to 1 mol L -1

[Al(H2O)6] 3+ solution.

Experiments at 9.4 T

Experiments at B0 = 9.4 T (0 = 400 MHz for 1 H) were recorded with a wide-bore magnet equipped with a Bruker AVANCE NEO console, using a 3.2 mm HXY magic angle spinning (MAS) probe operated in double-resonance mode.

Two-dimensional (2D) 11 B MQMAS spectrum was obtained using the z-filtered sequence 28 at R = 16 kHz. Excitation and conversion pulses lasted p = 6 and 1.8 μs with ν1 ≈ 122 kHz and the central transition (CT) selective pulse lasted 10 μs with ν1 ≈ 12.5 kHz. The spectra are the result of averaging 12 transients (using a cogwheel phase cycling COG12 for coherence pathway selection) separated by a recycle delay of 10 s for each of the 300 t1 increments of 62.5 μs. The quadrature detection method of States et al. 29 was used to achieve sign discrimination in the indirect dimension.

After acquisition, a 2D Fourier transformation followed by a shearing transformation was performed to obtain a spectrum where the contour ridges were parallel to F2, allowing the isotropic spectrum to be obtained directly from a projection onto F1. 30

Experiments at 18.8 T

The experiments at B0 = 18.8 T were performed on a Bruker AVANCE NEO spectrometer equipped with a 3.2 mm double resonance 1 H/X probe.

Quantitative 11 B NMR spectra were acquired at R = 20 kHz by using single-pulse NMR experiments with a pulse length of 1 μs and an RF field strength of 120 kHz, corresponding to a short tip angle. 31 Quantitative 11 B NMR spectra were acquired by averaging 4 transients, separated by a recycle interval of 20 s.

11 B and 27 Al SQ-SQ and DQ-SQ D-HOMCOR 2D spectra were acquired by using the pulse sequences displayed in Fig. 32a andb, respectively. The tested recoupling schemes for SQ-SQ experiments are SR2 2 1 , SR2 4 1 and SR2 8 1 (Fig. 33a-c) using the supercycle 𝑀 = 4. For DQ-SQ sequences, the tested recouplings are SR2 2 1 and BR2 2 1 (Fig. 33a ande, respectively), and their bracketed counterparts, [SR2 2 1 ] and [BR2 2 1 ],

respectively. As said before, HOMCOR methods of half-integer quadrupolar nuclei are very sensitive to offsets. Therefore, the carrier frequency, which is then a very important parameter, will be indicated with an arrow in all spectra. The recycle delays were 1 s for all experiments. The spinning speed was R = 20 kHz for each experiment.

Additional experimental details are given in the figure captions.

RESULTS AND DISCUSSION

11 B-11 B D-HOMCOR experiments on Li2B4O7

11 B is a spin-3/2 isotope with a natural abundance of 80.1 % and a Larmor frequency of 0 = 256.8 MHz at B0 = 18.8 T. Boron species are either three-or four-coordinated, and they are named B III and B IV , respectively. These nuclei are subject to quadrupole interactions with CQ values ranging between 2.4-2.9 MHz for B III and 0.4 -0.8 MHz for B IV . [32][33][34][35] The difference of chemical shifts between the B III and B IV sites is often limited to Δ ≤ 5 kHz at B0 = 18.8 T. Nevertheless, these two types of species are resolved on the 1D spectra at this field, which simplifies the interpretation of the NMR spectra.

Borate glasses are important systems with various potential applications in optical, optoelectronic and biomedical devices. [36][37][38][39] Crystalline Li2B4O7 (Li2O•2B2O3) was chosen as a test sample. There are two boron species in this compound (JCPDS card 79-0963), one B 1 III and one B IV sites, with a chemical separation of Δ ≈ 4.2 kHz at B0 = 18.8 T.

The sample was obtained after a 24 h recrystallization process at 540 °C, from the glass obtained after melting during 5 h at 1000 °C of Li2B4O7.10H2O. The crystalline sample contains 90% of Li2B4O7, 40,41 with 10% of another compound with only one B III species (denoted B 2 III hereafter) (Fig. 38). A detailed analysis of the X-ray diffraction pattern has shown that the second species is α-LiBO2 (Fig. 36). [42][43][44] The DMfit deconvolution of the 1D spectrum (Fig. 37) gives proportions of 45, 10 and 45 % for B 1 III , B 2 III and B IV species, with cs (ppm), CQ (MHz), Q = (18.4, 2.63, 0.18), (16.5, 2.36, 0.60) and (2.1, 0.48, 0.51), respectively (Table 13). It must be noted that the two B III resonances are narrow and well separated in the MQMAS spectrum (Fig. 38), which means that the two different compounds form distinct crystallized phases.

Hence, no spatial proximity between B 1 III and B 2 III species is expected. Fitting was carried out using int2QUAD model in DMFit. 45 The relative amounts of Li2B4O7 and α-LiBO2 phases are 90 and 10 %.

Table 13. Data obtained from the deconvolution of the 11 B 1D spectrum of the Li2B4O7

(using the int2QUAD model in DMFit. 45 ) presented in Fig. 37. In the following, the two B 1 III and B 2 III resonances are considered as a single one, and the sample will be called Li2B4O7, neglecting the 10% impurity. Hereafter, B 3 and B 4 will denote the auto-correlation peaks of (B 1 III + B 2 III ) and B IV species, and B 34 and B 43 the cross-peaks observed along F2 at (B 1 III + B 2 III ) and B IV frequencies, respectively.

B

SQ-SQ

As previously demonstrated, 𝑀 = 3 and 4 provide similar robustness to offset and rf-inhomogeneity, but 𝑀 = 4 often performs slightly better'. 12 So, following these conclusions, the three best (SR2 n 1 ) SQ-SQ recoupling schemes were tested on Li2B4O7 using this super-cycling: (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 . 12,13 It must be noted that at 𝜈 𝑅 = 20 kHz the mixing step of these (SR2 n 1 ) 4 sequences is long: Δ𝜏 𝑚𝑖𝑥 = 8𝑛𝛵 𝑅 = 0.8, 1.6 and 3.2 ms with n= 2, 4 and 8, respectively.

2D SQ-SQ D-HOMCOR spectra exhibit large diagonal signals. However, these signals are produced by magnetization that is not transferred during the sequence or by transfers between nuclei with very close or identical resonance frequencies.

Therefore, they do not provide information on the spatial proximities. In the case of the sample tested here, the B III and B IV resonances are well resolved at B0 = 18.8 T, and there is no spatial proximity between the B 1 III and B 2 III species. Nevertheless, the diagonal peaks can arise from magnetization, which is not transferred, as well as transfers of magnetization between nearby 11 B nuclei in the same environment.

Consequently, it is not easy to analyse the SQ-SQ spin dynamics with only 1D (t1 = 0) experiments, due to the superposition of useful (B 34 and B 43 cross-peaks) and useless (B 3 and B 4 auto-peaks) signals onto the F2 axis. In Fig. 39, a series of 2D spectra acquired with three different τ mix values are presented.

Two points can be noted by comparing the spectra displayed in Fig. 39: (i) the B 34 cross-peak is negative for νoffset = 3 kHz in the case of (SR2 8 1 ) 4 , which indicates a higher sensitivity to offset of this sequence and (ii) the B 43 cross peak is less intense than the B 34 peak owing to the faster decay of B III SQ coherences with respect to that of B IV SQ coherences. In particular, B 43 cross peak does not appear in the case of In Fig. 40, we show a series of 2D spectra acquired with three different offsets, offset, as indicated on the Figures. For each recoupling scheme, the τ mix value was chosen for optimum B 34 cross-peak value. As observed in Fig. 39, whatever the offset value, the B 43 cross-peak either does not appear in Fig. 40 with (SR2 8 1 ) 4 or is hardly visible with (SR2 4 1 ) 4 . This is not the case with (SR2 2 1 ) 4 . For this recoupling scheme, the maximum B 34 signal with the least line-shape distortion is observed with offset = 3 kHz. especially with (SR2 8 1 ) 4 recoupling, large variations of the amplitude for this crosspeak are observed. This amplitude can even become negative for (SR2 8 1 ) 4 . This is not the case with (SR2 2 1 ) 4 , which is much more robust than the two other recoupling schemes. Fig. 41 is focused onto the behavior of the informative B 34 cross-peak.

(SR2 4
However, it is very important to keep in mind that the observation of the informative cross-peaks is often hampered by the large amplitude of the useless diagonal signals,

as shown in Fig. 42.

SQ-SQ vs DQ-SQ

In a last stage for the spin-3/2 analyses, in Fig. 45 is presented the comparison of the rows corresponding to the two species with the four most efficient sequences: SQ-SQ with (SR2 2 1 ) 4 , and DQ-SQ with [BR2 2 1 ], [SR2 2 1 ] and BR2 2 1 . In order to make an easy comparison of the 2D spectra, the DQ-SQ ones were sheared by a factor of two in order to be displayed as SQ-SQ 2D spectra with the same scales on the two axes (not shown). ] and BR2 2 1 recoupling for DQ-SQ sequence at B0 = 18.8 T with a MAS frequency of 20 kHz. An expansion of the cross-peaks is also shown. All 2D spectra were acquired using uniform sampling. The 2D DQ-SQ and SQ-SQ spectra result from averaging 64 and 48 transients, respectively, using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 2h30 min and 1h50 min, respectively. The signal was normalized with respect to the number of transients. We employed optimal rf field strength equal to 1 = 3.7 kHz for BR2 2 1 , 4.1 kHz for [BR2 2 1 ] and [SR2 2 1 ] and 1 = 3.9 kHz for (SR2 2 1 ) 4 .

The bracketed [SR2 2 1 ] and [BR2 2 1 ] sequences are the two most efficient DQ-SQ methods for the B 34 cross-peak and B 4 auto-peak (Fig. 45), whereas for the B 43 and B 3 peaks they give the same sensitivity as BR2 2 1 . The SQ-SQ method with (SR2 2 1 ) 4 is twice more efficient that DQ-SQ methods for the B 34 cross-peak, but the least efficient for the B 43 cross-peak owing to the faster decay of B III SQ coherences. Moreover, it provides no through-space information through the B 3 and B 4 auto-peaks, which are mostly related to magnetizations that have not been transferred during the sequence.

As a conclusion for spin-3/2 nuclei with small or moderate chemical shift differences, such as 11 B, the DQ-SQ sequence with [BR2 2 1 ] and [SR2 2 1 ] bracketed recouplings seem to be the better choices. Indeed, they deliver a good efficiency simultaneously for the cross-and auto-peaks, which both then provide an analysis of the spatial proximities. Conversely, the SQ-SQ sequences lead to cross-peaks with similar intensities as the DQ-SQ ones, but they do not allow analyzing the spatial proximities with resonances that have close or identical frequencies.

3.6.2 27 Al- 27 Al experiments on AlPO4-14

In 27 Al NMR, the chemical shift range is much larger than for 11 B. Indeed, the difference between the isotropic chemical shifts of Al IV and Al VI nuclei reaches Δ ≈ 10 kHz at B0 = 18.8 T, which is approximately three-fold larger than that between those of B III and B IV nuclei. In this case of large  values, Edén et. al have recommended the (SR2 2 1 ) 4 and especially the (SR2 4 1 ) 4 SQ-SQ sequence. This was the case for 27 Al in CJ19, where Δ iso,max ≈ 9.6 kHz and 27 Al- 27 Al dipolar interactions are small. 13 Therefore, the SQ-SQ sequences using (SR2 2 1 ) 4 and (SR2 4 1 ) 4 recoupling and the DQ-SQ schemes incorporating [BR2 2 1 ] and BR2 2 1 recoupling will be compared in the following.

AlPO4-14 presents four aluminum species: two tetrahedral Al IV (Al 14). 46 Moreover, the tested sample contained an extra framework impurity at -2 kHz indicated with a star. Along with Al-O-P-O-Al connectivities, which are common for all aluminophosphate molecular sieves, there also exist some Al-O-Al connectivities in AlPO4-14 due to edge sharing between AlO6 octahedra and vertex sharing between AlO5 and AlO6 polyhedra. As a result, the 27 Al- 27 Al inter-nuclear distances are equal to 2.9, 3.6 and at Hz, respectively. All other distances are much larger and hence the dipolar interactions are much smaller than 100 Hz. To simplify the analysis, the 2D 27 Al HOMCOR spectra are represented as those of 11 B presented above. As example, Al 4 and Al 56 denote to the Al IV -Al IV auto-peak and the Al V -Al VI cross-peak, respectively. In addition, the two It is important to note that, due to the large differences of the dipolar couplings, the relative intensities of the peaks are largely changing depending whether the experiment is optimized for the shortest (Al IV -Al IV , Al VI -Al VI , Al V -Al VI and Al VI -Al V ) distances, as here, or longest (all other peaks) ones. It must also be noted that, owing to the weak rf-field used, the intensities of the peaks are strongly related to the offset value, as can be observed in Fig. 49.

CONCLUSIONS

In this work, it has been analyzed and compared the efficiencies of HOMCOR experiments for half-integer quadrupolar nuclei, namely 11 B and 27 Al. For both isotopes, the DQ-SQ sequences are recommended, which deliver a good efficiency simultaneously for the cross-and auto-correlation peaks, hence providing a global analysis of the spatial proximities, even for resonances that have close or identical frequencies.

In the first case ( 11 B, I = 3/2), the chemical shift differences are small or moderate, and therefore, the [SR2 2 1 ] or [BR2 2 1 ] bracketed recoupling schemes are recommended.

In the second case ( 27 Al, I = 5/2), the chemical shift differences are larger, and can reach ca. 10 kHz at 18.8 T. In this case, the BR2 2 1 un-bracketed recoupling scheme is recommended.

In any case, the acquisitions are time-consuming to obtain a reasonable signal-tonoise ratios. It must be noted that the experimental time can be largely decreased by using an initial CT enhancement obtained by the transfer of population from the STs to the CT. 14 The related gain increases with the spin value. The experimental time can also be decreased by a non-uniform sampling of the indirect dimension. 47 This chapter focuses primary on the solid-state NMR study of the DFNS/BN and DFNS/B2O3 catalysts along with insights into their structures.

INTRODUCTION

The catalytic oxidative dehydrogenation (ODH) of alkanes to produce alkenes is an important process in the modern chemical industry. [2][3][4][5] An important product of this reaction is propene, which is a very important building block for a large number of chemical products, such as the widely used poly(propylene) polymer. It is also used to produce a large quantity of other useful chemicals, such as acrolein, acrylonitrile, cumene, propylene oxide and butyraldehyde. 2,6 However, even after decades of research, propene selectivity remains too low due to over-oxidation of propene producing CO2 emission. 4 Consequently, there is an urgent need for a heterogeneous catalytic process which can convert propane to propene at low temperatures and with good selectivity. The discovery of boron-based catalysts, such as boron nitride (h-BN) and boron nitride nanotubes (BNNT), with highly favourable catalytic properties, provided an alternative route for the production of olefins from alkanes, which is more sustainable than the conventional protocol. 7,8 For these boron-based catalysts, it was hypothesized that the oxidised edges were the catalytic active sites. 6 Additional studies on various boron-containing materials with oxidised surfaces supported that assumption. [9][10][11][12] In particular, 11 B solid-state NMR and in operando infrared spectroscopies showed the oxidation of h-BN surface under ODH conditions. 10,12 DFT calculations also proved that the formation of a disordered boron oxide phase at the surface of h-BN is feasible. 13 More recently it has been shown that boron oxide supported on mesoporous silica or silica nanoparticles can catalyze the ODH of propane. 14,15 Conversely, it was shown that zeolites containing isolated BO3 units incorporated into the zeolite framework shows no catalytic activity for the ODH of propane to propene and the catalysis of ODH reaction requires the presence of aggregated boron sites. 16 In this work, new boron nitride supported on dendritic fibrous nanosilica (DFNS/BN) catalysts are introduced for the ODH of propane. DFNS as support provides highly accessible large surface area owing to its unique fibrous morphology, for which the internal surface is more accessible than the porous structures found in, for example, in MCM-41 and SBA-15 type materials. [17][18][19] demonstrated to be a powerful probe of the local structure of materials. [20][21][22][23][24] Indeed, the successful use of solid-state NMR to better understand catalytic sites and catalytic mechanisms has been proven. [25][26][27] The present work has sought to determine which the active phase of DFNS/BN catalysts is and how the BN and B2O3 phases are anchored in the SiO2 support in these materials.

STATE OF THE ART

Catalysts for ODH of light alkanes

The dehydrogenation reaction of alkanes to produce alkenes is an important process in the modern chemical industry. 2,4 The products from this reaction such as ethylene (named ethene according to IUPAC nomenclature) and propylene (also named propene) are the building blocks for various chemicals including polymers and fine chemicals. 4,5 Therefore, the amount of ethene or propene production is an indicator of the scale of a chemical industry. 4 Currently, 10% of the global propene production is obtained via propane dehydrogenation (PDH), which is considered economically and environmentally viable. 3 This reaction can be categorized in two routes depending on whether the oxidant is used or not, which are called oxidative or direct dehydrogenation, respectively. 4 The direct dehydrogenation (DH) is a thermodynamically limited reaction with endothermic properties. The conventional catalysts that are used for DH include Pt-and Cr-based oxides. However, the coke formation during the process deactivates the catalysts. 5 Conversely, for the oxidative dehydrogenation (ODH) (Fig. 52), which is an exothermic reaction, the introduction of an oxidant (e.g. O2) raises the thermodynamics limit and prevents coke formation.

Nevertheless, the main drawback of ODH is its low selectivity of the desired olefin. In the past few decades, considerable efforts have been made to develop efficient catalytic systems for the ODH reaction of light alkanes, which mostly used transition metal oxides (e.g., V2O5, MoO3, and NiO) and alkaline-earth metal oxychlorides. 3,4 These metal-based catalysts displayed high reactivity towards the oxidative conversion of alkanes, but suffered from oxidation to COx and thus leading to CO2 emission. 3 The recent discovery of metal-free boron-based catalysts, such as h-BN and BNNTs, with high selectivity and activity to olefins and with reduced formation of CO2, has revolutionized the catalytic oxidative dehydrogenation and paved a new route to the selective cleavage of C-H bonds. 3,6,8,11 Subsequently, a series of boron-based catalysts have been prepared with similar performance to h-BN which has been the referent for this reaction. 8,9,15 However, there is a need for further understanding of the catalytic mechanisms and nature of the active sites.

Catalytic activity of DFNS/BN and DFNS/B2O3

The catalytic properties of DFNS/BN and DFNS/B2O3 were examined for the oxidative dehydrogenation (ODH) of propane. They were studied using a fixed bed micro-activity flow reactor under atmospheric pressure (1 bar) at 490 °C. The catalytic activity of these samples was evaluated in terms of conversion and productivity. These measurements were performed by our collaborators in India.

It was found that during 19 h, DFNS/BN is moderately stable at the studied temperature (490 °C) and that the conversion percentage decreases by ~18%. This catalyst then exhibits a selectivity of around 59.4% for propene and around 16.5% for ethene (Fig. 55a). In terms of productivity, significant amounts of propene (38360 µmol.gcat -1 .h -1 ) and ethene (10657 µmol.gcat -1 .h -1 ) were produced. However, it was observed that the productivity towards propene decreased with the reaction time which

indicates the formation of coke on the catalyst surface (Fig. 55b). It is required a separate study to understand this phenomenon, but it is not part of the scope of this manuscript.

On the other hand, it was observed that DFNS/B2O3 was also stable for 19 h, with a selectivity around 17.9% for propene and 6.8% for ethene, and more than 70% for COx with x = 1 or 2, with a conversion rate of 24.8% (Fig. 55c). The high selectivity towards the CO when using DFNS/B2O3 needs further investigation. However, this is not part of the scope of this manuscript. In terms of productivity, this catalyst showed a good performance towards propene (19520 µmol gcat -1 h -1 ) and ethene (7441 µmol.gcat -1 .h -1 ), which are almost half of the productivities obtained with DFNS/BN (Fig. 55d). This fact indicates that B2O3 is catalytically active for propane dehydrogenation, although to a lesser extent than BN supported on DFNS. propane into propene with high selectivity, but the productivity is lower than that obtained with DFNS/BN and DFNS/B2O3 or the other tested catalysts. It was found that DFNS/BN exhibits a better performance than h-BN in terms of propane conversion and propene productivity, while BNNT showed better propene productivity compared to DFNS/BN (Fig. 56). DFNS/B2O3 also displayed better conversion and selectivity than h-BN, but in general, exhibits lower productivity towards propene compared to B2O3 supported on SBA-15 (BOS-10) and BNNT. The better performance of both DFNS/BN and DFNS/B2O3 for ODH of propane can be attributed to the highly accessible active sites as a result of the fibrous morphology and high surface area that DFNS provides as a support. Besides, these DFNS/BN and DFNS/B2O3 supported catalysts showed better performance than vanadium (V) supported on silica, which is one of the best-known silica-supported catalysts for this reaction (Table 15). 2 This result shows the high performance of the introduced catalysts and its potential application for the ODH of light alkanes.

SOLID-STATE NMR EXPERIMENTS

For all solid-state NMR experiments, rotors were packed inside an argonatmosphere glovebox to prevent contact with moisture. 11 B chemical shifts were referenced using a solid sample of NaBH4 (iso = -42.05 ppm), whereas 1 H and 29 Si chemical shifts were referenced using neat tetramethylsilane (TMS) (iso = 0 ppm).

Samples

A set of three samples were used for the solid-state NMR analyses: i) boron nitride supported on dendritic fibrous nanosilica, which was prepared using a solution-phase deposition process (as-prepared DFNS/BN hereafter); ii) boron oxide supported on dendritic fibrous nanosilica, which was prepared using solid-phase deposition techniques (as-prepared DFNS/B2O3 hereafter), and iii) DFNS/BN after catalysis.

These samples were prepared by our collaborators in India. or 586 s (pure h-BN).

Experiments at 9.4 T

Two-dimensional (2D) 11 B MQMAS spectra were obtained using the z-filtered sequence 33 at R = 20 kHz, with a 3. 34 was used to achieve sign discrimination in the indirect dimension. After acquisition, a 2D

Fourier transformation followed by a shearing transformation was performed to obtain a spectrum where the contour ridges were parallel to F2, allowing the isotropic spectrum to be obtained directly from a projection onto F1. 35 11 B-{ 1 H} D-HMQC experiments with 11 B detection were acquired at R = 20 kHz with a 3.2 mm triple-resonance HX probe. 36,37 The 1 H-11 B dipolar couplings were reintroduced by applying SR4 1 2 scheme on the 1 H channel. 38 The total duration of the two SR4 1 2 recoupling blocks was τD = 0.75 ms. The 29 Si dipolar couplings were reintroduced the SFAM1 dipolar recoupling scheme on the 29 Si channel. 40 This experiment was calibrated using a sample of 29 Si-enriched borosilicate glass. 41 The total duration of the two SFAM1 recoupling blocks was τD = 2 ms. 29 Si π/2 pulses were applied with ν1 ≈ 59 kHz and 11 B CT selective pulses were applied with ν1 ≈ 17 kHz.

The peak values of the frequency and amplitude sweeps of SFAM1 on 29 1D 11 B NMR data for the three aforementioned samples is presented in Figure 57d, e and f. The spectrum of as-prepared DFNS/BN (see Figure 57d) shows three broad signals with centres of gravity at ca. 0, 15 and 26 ppm. The signal at ~26 ppm is assigned to tri-coordinated (B III ) boron species of type BN3, BN2O and BNO2 since these sites have δiso values of 30, 27-28 and 20-24 ppm, respectively and quadrupolar coupling constant CQ = 2.7-2.9 MHz. 10,15,55,56 The BN2O and BNO2 sites are located on the edges of the BN sheets. The signal at ~15 ppm is assigned to tri-coordinated BO3 environments. 14,15,52,54,[57][58][59][60][61] The signal at ~0 ppm is assigned to tetra-coordinated (B IV ) BO4 environments. DFNS/B2O3 display a small peak at ~0 ppm that can be assigned to BO4 environments.

For sake of comparison, Fig. 58 compares the 11 B spectra of as-prepared DFNS/BN and DFNS/B2O3 with that of the h-BN. The spectrum of h-BN displays a broad resonance with centre of gravity at ~28 ppm, which is assigned to boron species of type BN3 with a quadrupolar coupling of CQ = 2.9 MHz, which is in agreement with previous investigations. 10,56 This feature of h-BN assisted in the identification of boron sites of as-prepared DFNS/BN. To confirm the above assignment and probe 1 H-11 B proximities, the 2D 11 B-{ 1 H} through-space heteronuclear multiple-quantum coherence (D-HMQC) spectra of DFNS/BN (as-prepared and after catalysis) and as-prepared DFNS/B2O3 were recorded (Fig. 59). The D-HMQC spectrum of as-prepared DFNS/BN (Fig. 59a) is dominated by a cross-peak between hydroxyl groups on the edge of BN sheets at 6.7 ppm and 11 BO3 signal at 15 ppm. This observation is consistent with the formation of a disordered boron oxide phase at the surface of h-BN. 10,13 The BO3 signal also correlates with the 1 H signal at 4 ppm assigned to BO2OH sites, which substantiates the presence of these sites in this sample. The lower intensity of the cross-peak at 4 ppm with respect to that at 6.7 ppm is consistent with the relative intensities of these signals in the 1D The D-HMQC spectrum of DFNS/BN after catalysis (Fig. 59b) exhibits a broad correlation between the 11 B signal at 14 ppm ascribed to BO3 sites and a 1 H signal centered at 5 ppm. This 1 H isotropic chemical shift corresponds to that of BO2OH sites on the surface of oxidized h-BN phase. 15 The D-HMQC spectrum of as-prepared DFNS/B2O3 (Fig. 59c) is dominated by a correlation between BO3 units and 1 H signal ranging from 2.3 to 4 ppm, which is assigned to BO2OH sites with second neighbours, B or Si, as well as silanol groups near BO3 moieties. [14][15][16][51][52][53] The comparison of Fig. 59b and c confirms the higher chemical shifts of BO2OH protons in DFNS/BN after catalysis than as-prepared DFNS/B2O3. This shift could stem from a higher fraction of hydrogen-bonded BO2OH sites. A cross-peak between the 1 H signal at 1.7 ppm of isolated silanol and BO3 signal is also visible. Hence, the 11 B-{ 1 H} D-HMQC spectrum confirms the close proximity between the silica surface and the boron atoms.

Probing 29 Si environments and 29 Si-11 B proximities

1D 1 H→ 29 Si CP MAS experiments were also performed on DFNS/BN, DFNS/BN after catalysis and as-prepared DFNS/B2O3 (Fig. 61). Spectral fitting was carried out using DMFit 63 (with a Gaussian/Lorentzian model) in order to identify the individual species present. Detailed information on these fits is presented in Table 16 and Fig. 62.

The spectra of all three samples show the presence of Si(SiO)n(OH)4-n sites (denoted Q n ) with n = 2, 3 and 4, characteristic of silica-type materials. 27,64,65 The spectrum of as-prepared DFNS/BN also displays a broad shoulder in the region attributable to SiNnO4-n sites with n = 1, 2 or 3 (Fig. 61a and Fig. 62a), since the SiN3O, SiN2O2 and

SiNO3 sites resonate at ca. -63, -75 and -90 ppm, respectively. [66][67][68][69][70][71] This signal indicates the presence of Si-N bonds in this material. The spectrum of DFNS/BN after catalysis (Fig. 61b and Fig. 62b) reveals that primarily Q 4 species remain in this sample, indicating that most of the Si-OH surface groups have been consumed or transformed, likely as a result of the catalytic reaction process. 72 This reduced amount of surface hydroxyls also leads to a lower efficiency of the CP transfer and hence, lower signal-to-noise ratio for this sample, despite the collection of a larger number of transients. 73,74 The shoulder centred at ca. -73 ppm (attributed to SiNnO4-n sites) is also no longer visible after catalysis, in agreement with the conversion of BN into B2O3 inferred from PXRD and XPS (not shown) and 11 B NMR. The spectrum of as-prepared DFNS/B2O3 can be also fitted with three components (Q 2 , Q 3 and Q 4 ) as shown in Table 16 and Fig. 62c, but with some differences in the integrated areas with respect to DFNS/BN after catalysis. 63 In (c), the small peak at ca. -20 ppm in the 1 H→ 29 Si CP MAS spectrum of as-prepared DFNS/B2O3 is probably due to residual organic species, so was not included in the fitting process. To further probe the interactions between the silica surface and the supported BN and B2O3 phases, the 2D 11 B-{ 29 Si} D-HMQC spectra were acquired. The corresponding spectrum of as-prepared DFNS/BN shown in Fig. 63a is dominated by a cross-peak between the Q 4 site and the BO3 signal at ~12 ppm assigned to B(OSi)x(OB)3-x environments with x = 1, 2 or 3. 14,16,53,57 These B(OSi(Q 4 ))x(OB)3-x sites mainly corresponds to the subsurface region of the material. The spectrum also exhibit cross-peaks between the Q 4 and Q 3 site and the BO3 signal at ~14 ppm as well as between the Q 3 sites and the BO3 signal at ~16 ppm, which are assigned to surface sites B(OSi(Q 3,4 ))x(OB)2-xOH with x = 1 or 2 and BOSi(Q 3 )(OH)2, respectively. 16,53,57 Cross-peaks between the BO4 signal at ~0 ppm and the Q 3 sites as well as the BO4 signal at ~-2 ppm and the Q 4 sites were observed. The cross-peak of the B IV signal at ~0 ppm can be ascribed to the surface B(OSi(Q 3 ))x(OB)3-xOH site with x = 1, 2 or 3, whereas the cross-peak of the B IV signal at ~-2 ppm can be ascribed to the subsurface B(OSi(Q 4 ))x(OB)4-x sites with x = 1, 2, 3 or 4. 13,54 These cross-peaks between BO3 and BO4 signals and those of Q 4 and Q 3 sites demonstrate the presence of B-O-Si bonds in the as-prepared DFNS/BN. Furthermore, the BN2OH signal at 26 ppm correlates with that of SiN2O2 site at -73 ppm. A weak cross-peak between the BNO2 sites at 23.5 ppm and the SiNO3 signal at -93 ppm is also observed. 46,67,69,71 These cross-peaks suggest the presence of B-N-Si bonds in the sample. The 2D 11 B-{ 29 Si} D-HMQC spectrum of DFNS/B2O3 (Fig. 63b) is dominated by cross-peaks between the BO2OH signal at ~14 ppm and the Q 4 and Q 3 sites, which suggests the anchoring of B2O3 phase on silica surface via B-O-Si bonds. 14

Insights into the boron species

To better resolve the 11 In order to obtain detailed insight into the distinct functional groups that were present in the boron surface layers, spectral fitting of the lineshapes in the F1 dimension of the acquired MQMAS spectra was performed using DMFit. 63 Since the F1 dimension of an MQMAS spectrum presents resonances that are free from the effects of quadrupolar broadening, 23,63 it was possible for a Gaussian/Lorentzian model to be employed. The simulated spectra are shown in Figs. 65 and66. Table 17 lists the shifts, δ1, along the indirect dimension of the sheared MQMAS spectra of the simulated peaks for the three investigated samples. Due to the significant overlap of sites in these MQMAS spectra, fitting of the total lineshapes to obtain iso and CQ/PQ values with a sufficient degree of accuracy was not possible.

In the 2D MQMAS spectrum of as-prepared DFNS/BN (Fig. 64a and 65), three peaks at δ1 = 30.5, 28.6 and 27.2 ppm can be resolved in the most deshielded signals.

The peak at 30.5 ppm is assigned to bulk BN3 sites, whereas the signals at 28.6 and 27.2 ppm are likely produced by BN2OH and BN2O sites, respectively, located at the edge of BN sheets, since the 11 B chemical shift increases for decreasing number of bridging oxygen atoms. 46,61,77 The BNO2 signal may not be detected because of the lack of sensitivity of MQMAS experiments. The BO3 signal can be simulated by three peaks at δ1 = 18.4, 17.2 and 16.3 ppm, assigned to BO2OH(ring), BO2OH(non-ring) and BO3(ring), respectively since for the same number of bridging oxygens, 11 B nuclei residing in boroxol rings exhibit more deshielded signals as compared to those in the chain. 14,61,77 BO3(non-ring) sites resonating at δ1 ≈ 14 ppm (Fig. 64a) can also contribute to the signal at δ1 =16.3 ppm. Furthermore, according to the 2D 11 B-{ 29 Si} D-HMQC spectrum (Fig. 63a), a fraction of the BO3 moieties are anchored to the silica surface. In particular, the peak at δ1 = 18.4 ppm subsumes the signal of BOSi(Q 3 )(OH)2 and B(OSi(Q 3,4 ))x(OB)2-xOH surface sites, whereas the signal of B(OSi(Q 4 ))x(OB)4-x sites also overlaps with those of BO2OH(non-ring) and BO3(ring) environments. The BO4 signal can be deconvoluted as the sum of three peaks at δ1 = 0.1, -1 and -1.9 ppm, which may be ascribed to B(OSi(Q 3 ))x(OB)3-xOH, B(OB)4 and B(OSi(Q 4 ))x(OB)4-x sites, respectively (Fig. 65c), based on the 2D 11 B-{ 1 H} and 11 B-{ 29 Si} D-HMQC spectra (Fig. 59a and63a). 17 The BO4 peak is folded, and thus, not shown. Table 17. Possible boron species present in DFNS supported catalysts obtained from fitting the F1 dimensions of MQMAS spectra of DFNS/BN before and after catalysis, and DFNS/B2O3 before catalysis. 10,[14][15][16]46,[51][52][53][54][55][56][57][58][59][60][61][62]77 It should be noted that the 1 values presented here result from a linear combination of 2 and 1(non-sheared). They do not directly correspond to iso (the isotropic chemical shift). 35,78 a Value not reported because peak was folded. 78,79 As seen in Fig. 66a, the BO3 signal in the projection of the 2D MQMAS spectrum of DFNS/BN after catalysis can be simulated as the sum of five components resonating at δ1 = 21.1, 19.1, 17.1, 15.7 and 14.1 ppm, assigned to BO(OH)2, BO2OH(ring), BO2OH(non-ring), BO3(ring) and BO3(non-ring), respectively. 14,61,77 The projection also displays a folded BO4 signal, which can be simulated as a single resonance (not shown). For DFNS/B2O3, the BO3 signal in the projection of the 2D MQMAS spectrum can be simulated as the sum of five components at δ1 = 20.5, 19, 17.2, 15.8 and 14.4 ppm (Fig. 66b). These shifts are similar to those used to simulate the BO3 signal DFNS/BN after catalysis. The BO4 signal of DFNS/ B2O3 catalysis can also be simulated as a single resonance (not shown with the larger amount of hydrogen bonded hydroxyl groups attached to B for this material (the resonance of BO3(ring), which is bonded to hydroxyl group, is represented in red in Fig. 66). 15,16 This structural difference could explain the difference in productivity between DFNS/BN after catalysis and DFNS/B2O3. Table 18 summarizes the main plausible local environments, which have been identified in the investigated samples on the basis of solid-state NMR data. 27 Al- 27 Al HOMCOR experiments were performed at R = 20 kHz on two samples, Li2B4O7 and AlPO4-14, respectively. For both isotopes, the DQ-SQ sequences are recommended, which yields a good efficiency for both the cross-and auto-correlation peaks, hence providing a global analysis of the spatial proximities, even for resonances that have close or identical frequencies. In the case of 11 B, the chemical shift differences are small or moderate, and therefore, the 1 ] bracketed recoupling schemes are recommended. Conversely, for 27 Al, the chemical shift differences are larger, and can reach ca. 10 kHz at 18.8 T. In this case, the BR2 2 1 un-bracketed recoupling scheme is recommended. The insights gained from this study may be of assistance to the observation of homonuclear correlations between half-integer quadrupolar nuclei at high magnetic fields. Before We have presented in Chapter 1 the available methods for acquiring high-resolution NMR spectra of quadrupolar nuclei, notably the MQMAS and STMAS sequences. In general, MQMAS scheme produces high-resolution spectra and is more robust and easier to set up than STMAS experiment, but its low efficiency leads to low sensitivity.

[SR2 2 1 ] or [BR2 2
In order to circumvent this issue, we explored how the MQMAS scheme can be combined with MAS-DNP. The results shown below correspond to unpublished material, which is in development, and for this reason were not included in the central work of this thesis.

The D-RINEPT sequence (introduced in this thesis) use on the S channel three CTselective pulses: two -pulses to refocus the 1 H CSA and one /2-pulse in the middle for the coherence transfer. At the end of the sequences, the detected S signal is that of the single-quantum (SQ) of the CT between energy levels mI = -1/2 and +1/2. These SQ-CT coherences are broadened by the 2 nd -order quadrupolar interaction (HQ2) of S isotopes. This broadening can be refocused by converting the SQ-CT coherences into triple-quantum (3Q) coherences using FAM-N scheme (see Fig. 67). These 3Q coherences are converted back into SQ-CT coherences by another FAM-N. The signal can be detected using QCPMG scheme in order to enhance the NMR signal. nuclei with high resolution for this material. The isotropic projections of these 2D spectra are presented in Fig. 69. It is observed that the BN3 signal is detected in the spectrum acquired with direct excitation using the split-t1 MQMAS pulse sequence, but it is absent in the D-RINEPT-MQMAS-QCPMG spectra (Fig. 68). This finding suggests that BN3 sites are remote from protons by several angstroms so that D-RINEPT cannot efficiently transfer the DNP-enhanced polarization of protons to the BN3 sites. Likewise, BN2OH groups are not observed in the RINEPT-MQMAS spectrum. This result can be attributed to the fact that they are not present at the surface or because their signal is too low to be detected. Therefore, these findings suggest that the BN3 and BN2OH sites may not be found on the outermost surface of the material.

For the 11

DNP-ENHANCED SOLID-STATE NMR OF QUADRUPOLAR NUCLEI IN MoO3/TiO2 NANOPARTICLES

MoO3 supported on TiO2 are important heterogeneous catalysts used for the selective oxidation of hydrocarbons and alcohols, as well as a precursor of hydrodesulfurization catalysts (HDS). [1][2][3][4] The catalytic activity of MoO3/TiO2 is notably related to its Brønsted acidity, resulting from the presence of OH groups on their surface. However, the local environment of these OH groups, and in general, of their surface sites remains elusive.

Recently, Nagashima et al. 5 were able to record the 17 O, 47,49 Ti and [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mo NMR spectra of the surface of these catalysts, thanks to the nuclear dynamic polarization Nevertheless, the MoOTi and Mo2OTi local environments could not be detected. The DNP-enhanced 1 H→ 47,49 Ti D-RINEPT spectrum indicated that the amorphous phase is located near the surface of the particles (see Fig. 72). Nevertheless, this spectrum lacks resolution since the signals of anatase, rutile and amorphous phases as well as surface sites overlap. Furthermore, the DNP-enhanced 1 H→ [START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF] Mo D-RINEPT spectrum shows the presence of tetra-or penta-coordinated Mo sites (not shown).

Nevertheless, this spectrum again lacks resolution and the exact coordination number and location of these MoO4 or MoO5 sites was not determined. Table 21. Proportion of Mo(V) species found in MoO3/TiO2 catalysts before and after impregnation with TEKPol solutions.

DNP at high field

In order to improve the resolution of 17 O NMR spectra of MoO3/TiO2, we also performed DNP-enhanced NMR experiments on this sample at 18.8 T since high magnetic field reduce the second-order quadrupole broadening. We recorded the 17 O NMR spectrum at 18.8 T of MoO3/TiO2(P25) impregnated with 110 mM TEKPol solution in TCE using DFS-QCPMG sequence and enhanced by direct DNP (see Fig.

A.2. SIMPSON input file for D-HOMCOR-DQ-SQ-𝐁𝐑𝟐

Figure 1 .

 1 Figure 1. Schematic representation of the surface analysis methods presented in Table 1. ........................................................................................................

Figure 2 .

 2 Figure 2. Summary of the most important interactions in solid-state NMR and the available information. ............................................................................

Figure 3 .

 3 Figure 3. Schematic representation of the nuclear quadrupolar interaction which arises from the coupling between the electric quadrupolar moment eQ and the electric field gradient (EFG) E. (left). The EFG is mathematically described by a 3x3 matrix and is defined by its three tensor components 𝑉 𝑋𝑋 , 𝑉 𝑌𝑌 and 𝑉 𝑍𝑍 , with 𝑉 𝑍𝑍 ≥ 𝑉 𝑌𝑌 ≥ 𝑉 𝑋𝑋 by convention (middle). The orientations of the PAS for the EFG tensor with respect to the external magnetic field, B0 (right). ................................

Figure 4 .

 4 Figure 4. Schematic energy level diagram for a spin I = 3/2 nucleus showing the effect of the Zeeman, first-order and second-order quadrupolar interactions. The first-order quadrupolar interaction splits the NMR spectrum into 2I -1 lines with resonance frequencies affected by the quadrupolar splitting parameter 𝜔 𝑄 , while the central transition CT remains unaffected at the Larmor frequency 𝜔 0 . The second-order quadrupolar interaction affects all the lines. ................................................

Figure 5 .

 5 Figure 5. Schematic frequency domain representation of rf irradiation of quadrupolar nuclei with spin I = 3/2. ...........................................................

Figure 6 .

 6 Figure 6. Pulse sequences and coherence pathways for (a) the z-filter and (b) the split-𝑡1 shifted-echo 3QMAS experiments (the coherence pathway is given for the z-filter approach on a spin I = 3/2, depicted at the top, with echo and antiecho pathway signals represented by solid and dashed lines, respectively) and (c) DQF-STMAS z-filter experiment.....................

Figure 7 .

 7 Figure 7. Different types of schemes that can be used for the excitation and conversion for the MQMAS experiment: hard pulses (HP), rotationinduced adiabatic coherence transfer (RIACT), soft-pulses added mixing (SPAM), double frequency sweeps (DFS), fast-amplitude modulation pulse trains (FAM-N), long pulses (LP) and cosine long pulses (Cos-LP). ...........................................................................................

Figure 8 .

 8 Figure 8. Pulse sequences of (a) Quadrupolar echo, (b) QCPMG and (c) WURST-QCPMG. .......................................................................................

Figure 9 .

 9 Figure 9. High-field DNP-NMR system with gyrotron microwave source (gyrotron tube indicated in purple), microwave transmission line (yellow) and low-temperature NMR probe (light blue). ..............................

Figure 10 .

 10 Figure 10. Schematic representation of indirect and direct DNP experiments. After microwave irradiation, polarization from electrons can be transferred directly to nuclei of the investigated materials (direct DNP) or indirectly, by transferring the polarization first to 1 H and then to the detected nuclei by CP or other methods (indirect DNP). ............................

Figure 11 .

 11 Figure 11. Structures of some of the organic radicals used as PA for high-field MAS DNP. ...................................................................................................

Figure 12 .

 12 Figure 12. Schematic representation of two R𝑁 𝑛  schemes employed in this work: R22 2 7 built from rectangular 1800 pulses and R16 7 6 built from composite 270090180 pulses. .........................................................................

Figure 13 . 1 H

 131 Figure 13. 1 H → 27 Al (a,c) PRESTO-R𝑁 𝑛  and (b,c) D-RINEPT-CWc-R𝑁 𝑛  pulse sequences. Those applied to 1 H and 27 Al channels are shown in (a,b) and (c), respectively. The narrow and broad black bars represent /2 and -pulses, respectively. The acquisition of the FIDs (indicated with the vertical dashed line) starts after (a) the end of the R𝑁 𝑛  block in the case of PRESTO or (b) on top of the echo shifted with R/2 with respect to the end of the last recoupling block in the case of RINEPT. .......

Figure 14 .

 14 Figure 14. Type of pulses employed in R𝑁 𝑛  sequences for PRESTO-III. ................

Figure 15 .

 15 Figure 15. Schematic representation of the selection of R𝑁 𝑛  schemes for PRESTO based on the symmetry numbers N, n and ν, the RF requirement and the ϕ angle. The selected R𝑁 𝑛  schemes are presented in Tables 3-5. ................................................................................................

Figure 16 .

 16 Figure 16. Type of pulses employed in R𝑁 𝑛  sequences for D-RINEPT-CWc. ........

Figure 17 .

 17 Figure 17. Schematic representation of the selection of R𝑁 𝑛  schemes for D-RINEPT-CWc based on the symmetry numbers N, n and ν, and the ϕ angle. The selected R𝑁 𝑛  schemes are presented in Table 6. ........................

Figure 18 .

 18 Figure 18. Spin echo sequence used to measure the decay of the transverse 1 H magnetization. ..............................................................................................

  Figure19.27 Al 1D spectra of -alumina at 18.8 T with R = 20 (a) and62.5 (b) kHz acquired using 1 H →27 Al transfers with RINEPT-CWc sequence and SR4 1 2 (tt), SR4 1 2 (270090180) and R12 35 (270090180) recoupling, or PRESTO scheme and (a) R22 27 (1800) or R18 25 (1800), or (b) R16 76 (270090180) and R14 65 (270090180) recoupling. The  delays and 1/1,max RF-fields were fixed to their optimum values given in Tables7 and 9. .........................................................................................................

Figure 20 .

 20 Figure20.27 AlO6 on-resonance signal of -alumina at R = 20 kHz as function of 1 or 1,max for PRESTO-R22 27 (1800) and -R18 25 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table7. ..................

  Figure 23. 1 H MAS spectrum of AlPO4-14 acquired at B0 = 18.8 T and R = 20 kHz by averaging 16 transients separated by a recycle interval of 1 s, using the DEPTH pulse sequence for probe background suppression, with ν1  208 kHz. 72 .....................................................................................

  ..........................................................................................

Figure 27 .

 27 Figure 27. 27 AlO6 signal of -alumina at R = 62.5 kHz as function of offset for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 9. ........................................................................................................

  Figure28.27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of 1 or 1,max of the recoupling for PRESTO-R16 76 (270090180) and -R14 65 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 35 (270090180). For each curve,  was fixed to its optimum value given inTable 10.........................................................................................

  Figure 29. 27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of offset of the recoupling for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 10. ..............................................................

Figure 30 .

 30 Figure 30. 1 H-27 Al D-HETCOR 2D spectrum of AlPO4-14, along with its skyline projections, at B0 = 18.8 T and νR = 62.5 kHz acquired in only 72 min with only ca. 2.5 L of active volume with NUS 25% using RINEPT-CWc-SR4 1 2 (270090180) transfer. ....................................................

Figure 31 .

 31 Figure 31. Skyline projections along a) F2 and b) F1 of 1 H-27 Al HETCOR 2D spectra of AlPO4-14 recorded with RINEPT-CWc-SR4 1 2 (270090180), SR4 1 2 (tt), SR4 1 2 (180090180) and PRESTO-R16 7 6 (270090180) transfers. All 2D spectra were acquired using NUS 25% in 72 min. ..........................

Figure 32 .

 32 Figure 32. Pulse sequences used for (a) SQ-SQ and (b) DQ-SQ HOMCOR experiments of half-integer spin quadrupolar nuclei. All pulses are CTselective with 𝜈 1 = 𝜈 𝑅 (2𝐼 + 1). The black squares are /2-pulses, whereas the blue one in (b) is a -pulse. In (b) a [𝑅𝑁 𝑛  ] bracketed recoupling is shown with five /2-pulses, whereas an un-bracketed version should only require the last reading /2-pulse. The R𝑁 𝑛  recoupling parts are represented with grey zones. .....................................

Figure 33 .

 33 Figure 33. Minimum cycle times with (a) SR2 2 1 , (b) SR2 4 1 , (c) SR2 8 1 , (d) SR4 4 1 , and (e) BR2 2 1 . In (a-c,e), white and grey colors mean phases of 90° and 270° respectively, and in (d) yellow and green colors mean phases of 45° and 315° respectively. In each case, the number is the flip-angle for the CT-selective pulse. ...............................................................................

  Figure 33. Minimum cycle times with (a) SR2 2 1 , (b) SR2 4 1 , (c) SR2 8 1 , (d) SR4 4 1 , and (e) BR2 2 1 . In (a-c,e), white and grey colors mean phases of 90° and 270° respectively, and in (d) yellow and green colors mean phases of 45° and 315° respectively. In each case, the number is the flip-angle for the CT-selective pulse. ...............................................................................

Figure 34 .

 34 Figure 34. Comparison of powder XRD patterns of crystallized lithium diborate Li2O•2B2O3 and the glass sample initially prepared. Vertical lines represent the positions of diffraction lines of B4Li2O7...............................

Figure 35 .

 35 Figure 35. Crystalline structure of Li2B4O7 based on the cif file. The diborate unit is indicated with the atoms of boron tri-and four-coordinated, B3 and B4, respectively. ..................................................................................

Figure 36 .

 36 Figure 36. Comparison and difference between the powder XRD pattern of crystallized lithium diborate Li2O•2B2O3 and reference XRD patterns of α-LiBO2 and B4Li2O7. ............................................................................

Figure 37 .

 37 Figure 37. Lineshape deconvolution obtained from the 11 B 1D spectrum of the Li2B4O7 crystallized sample recorded at 18.8 T and R = 20 kHz. Fitting was carried out using int2QUAD model in DMFit. 45 The relative amounts of Li2B4O7 and α-LiBO2 phases are 90 and 10 %. .......................

Figure 38 .

 38 Figure 38. (a) 2D 11 B z-filtered MQMAS spectrum of Li2B4O7 recorded at B0 = 9.4 T and R = 16 kHz. Panel (b) is an expansion of panel (a). The oblique lines are the Chemical-Shift axes. .................................................

Figure 39 .

 39 Figure 39. Series of 2D 11 B SQ-SQ D-HOMCOR spectra of Li2B4O7 acquired at B0 = 18.8 T with a MAS frequency of 20 kHz and a non-uniform sampling (NUS) retaining 50% of the t1 points, with the carrier frequency optimized (Fig. 40) at offset = 3 kHz (11.7 ppm), shown with an arrow between the B III and B IV resonances. The spectra acquired with recoupling schemes, (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 , are shown in the first, second and third column, respectively. The negative cross-peaks are shown in green. The length of 𝜏 𝑚𝑖𝑥 = 3.2, 6.4 or 9.6 ms, is indicated in red on each panel. The top projections are those related to 𝜏 𝑚𝑖𝑥 = 3.2 ms. The 2D spectra result from averaging 48 transients using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 48-55 min. ..........................................................

Figure 40 .

 40 Figure 40. 2D 11 B SQ-SQ D-HOMCOR spectra of Li2B4O7 acquired at B0 = 18.8 T with a MAS frequency of 20 kHz and a non-uniform sampling (NUS) retaining 50% of the t1 points with offset = 0, 3, 6 kHz, as indicated in black and with arrows. The spectra acquired with recoupling schemes (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 are shown in the first, second or third column, respectively. The negative signals are shown in green. The

Figure 41 .

 41 Figure 41. Slices of the 2D spectra corresponding to the B 34 (B III -B IV ) crosspeak of Li2B4O7 observed with the 11 B SQ-SQ D-HOMCOR sequence and (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 recoupling schemes, shown in the first, second or third column, respectively. (a,c,e) offset = 3 kHz and 𝜏 𝑚𝑖𝑥 = 1.6, 3.2, 6.4 and 9.6 ms; (b,d,f) offset = 0, 1.5, 3, 4.5, 6 kHz and 𝜏 𝑚𝑖𝑥 =(b) 3.2, (d) 1.6 and (f) 6.4 ms. These slices are extracted from the 2D spectra shown in Figs. 39 and 40. ........................................................

Figure 42 .

 42 Figure 42. 1D slices of the 11 B SQ-SQ D-HOMCOR spectra shown in Fig.8 of Li2B4O7 with (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 schemes, with offset = 3 kHz (11.7 ppm) and 𝜏 𝑚𝑖𝑥 = 3.2 ms. These slices show that the B 34 (B III -B IV ) and B 43 (B IV -B III ) cross-peaks are hardly observable with (SR2 4 1 ) 4 , and especially (SR2 8 1 ) 4 . .............................................................................

Figure 43 .

 43 Figure 43. Experimental 1D 11 B DQ-filtered signal of Li2B4O7 observed with a 1D (t1 = 0) version of the [SR2 2 1 ], [BR2 2 1 ] and BR2 2 1 , sequences versus (a) 1 ranging from 1 to 8 kHz and (b) offset ranging from +10 to -10 kHz. The spectra obtained with SR2 2 1 scheme are not shown because it is much less efficient than these three schemes (see Fig. 44). The optimum values are indicated with *. ........................................................

  Figure 43. Experimental 1D 11 B DQ-filtered signal of Li2B4O7 observed with a 1D (t1 = 0) version of the [SR2 2 1 ], [BR2 2 1 ] and BR2 2 1 , sequences versus (a) 1 ranging from 1 to 8 kHz and (b) offset ranging from +10 to -10 kHz. The spectra obtained with SR2 2 1 scheme are not shown because it is much less efficient than these three schemes (see Fig. 44). The optimum values are indicated with *. ........................................................

Figure 44 .

 44 Figure 44. Comparison of the (a) F2 or (b) F1 projections of 2D DQ-SQ 2D spectra of Li2B4O7 acquired at B0 = 18.8 T with a MAS frequency of 20 kHz with NUS retaining 33% of the t1 points and recorded with [SR2 2 1 ], [BR2 2 1 ], SR2 2 1 and BR2 2 1 sequences. The 2D spectra result from averaging 64 transients using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 45-50 min. We employed optimal rf field strength equal to 1 = 3.7 and 4.1 kHz for BR2 2 1 / SR2 2 1 and [BR2 2 1 ] / [SR2 2 1 ], respectively. .......................................

Figure 45 .

 45 Figure 45. Comparison of F2 slices containing (a) B 4 or (b) B 3 diagonal peaks of 2D 11 B the SQ-SQ and DQ-SQ D-HOMCOR spectra of Li2B4O7 using (SR2 2 1 ) 4 recoupling for SQ-SQ scheme and [BR2 2 1 ], [SR2 2 1 ] and BR2 2 1

3. 2

 2 ms, 1 = 2.4 kHz and offset = 4 kHz (19.2 ppm) for the recoupling. The 2D SQ-SQ spectrum results from averaging 48 transients using a relaxation delay RD = 1 s for each of 330 t1-increments, leading to an experimental time Texp = 1h 40 min. ..........................................................

Figure 47 .

 47 Figure 47. Comparison of the cross-peak intensities of the SQ-SQ spectra of AlPO4-14 at B0 = 18.8 T with a MAS frequency of 20 kHz acquired with NUS retaining 33% of the t1 point, obtained from 1D slices taken from the 2D spectra recorded with (SR2 2 1 ) 4 (𝜏 𝑚𝑖𝑥 = 4.8 ms, offset = 2.82 kHz (13.5 ppm)) and (SR2 4 1 ) 4 (𝜏 𝑚𝑖𝑥 = 3.2 ms, offset = 4 kHz (19.2 ppm)) recoupling schemes. The auto-peaks, which bring no information, have been truncated at 5 % of their maximum values. We employed rf field strength equal to 1 = 2.4 kHz. The other experimental parameters are identical to those of Fig. 46. .......................................................................

Figure 48 .

 48 Figure48. 2D27 Al DQ-SQ D-HOMCOR spectrum of AlPO4-14 using BR2 2 1 recoupling acquired with NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz. Cross-and auto-peaks are indicated with orange and blue circles, respectively. The 2D SQ-SQ spectrum results from averaging 64 transients using a relaxation delay RD = 1 s for each of 330 t1-increments, leading to an experimental time Texp = 2h. The recoupling used the following parameters: 1 = 2.9 kHz, offset = 4 kHz (19.2 ppm) and mix = 0.8 ms. ..............................................

  Figure48. 2D27 Al DQ-SQ D-HOMCOR spectrum of AlPO4-14 using BR2 2 1 recoupling acquired with NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz. Cross-and auto-peaks are indicated with orange and blue circles, respectively. The 2D SQ-SQ spectrum results from averaging 64 transients using a relaxation delay RD = 1 s for each of 330 t1-increments, leading to an experimental time Texp = 2h. The recoupling used the following parameters: 1 = 2.9 kHz, offset = 4 kHz (19.2 ppm) and mix = 0.8 ms. ..............................................

Figure 49 .

 49 Figure 49. F2 slices of three 2D 27 Al DQ-SQ D-HOMCOR spectra of AlPO4-14 acquired using BR2 2 1 recoupling and NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz for offset = 3, 4 or 5 kHz. The other experimental parameters are identical to those of Fig. 48. ...............................................................................................................

Figure 50 .

 50 Figure 50. Robustness with respect to (a) rf-field and (b) offset of 1D 27 Al DQfiltered D-HOMCOR spectra of AlPO4-14, recorded with BR2 2 1 (blue, mix = 0.8 ms) or [BR2 2 1 ] (red, 𝜏 𝑚𝑖𝑥 = 0.4 ms) recoupling at B0 = 18.8 T with a MAS frequency of 20 kHz. The 1D NMR spectra result from averaging 64 transients with RD = 1 s. In panel (a), offset = 4 kHz, whereas in panel (b), 1 = 2.9 and 3 kHz for BR2 2 1 and [BR2 2 1 ], respectively. ...............................................................................................

Figure 51 .

 51 Figure 51. Comparison of three F2 slices of 2D 27 Al DQ-SQ and SQ-SQ D-HOMCOR spectra of AlPO4-14 acquired using BR2 2 1 and (SR2 2 1 ) 4 recoupling with 𝜏 𝑚𝑖𝑥 = 0.8 and 3.2 ms, respectively, and NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz. The offset of the recoupling schemes was equal to offset = 4 kHz (19.2 ppm). The acquisition parameters of DQ-SQ and SQ-SQ experiments are identical to those of Figs. 46 and 48, respectively. The signal of the SQ-SQ spectrum has been multiplied by 1.33 to compensate for the difference in the number of transients. The auto-correlation peaks of SQ-SQ spectrum bring no information, and hence, these signals are truncated to the percentage of their maximal intensity indicated on each panel. ..........................................................................................................

Figure 52 .

 52 Figure 52. Schematic representation of the ODH of propane to propene. ..............

Figure 53 .

 53 Figure 53. Fibrous morphology of DFNS/BN revealed by (a) SEM and (b) TEM. The BN loading and textural properties of this material is described below. .........................................................................................................

Figure 54 .

 54 Figure 54. Fibrous morphology of DFNS/B2O3 revealed by (a) SEM and (b) TEM. The BN loading and textural properties of this material is described below. ........................................................................................

Figure 55 .

 55 Figure 55. (a, c) Conversion and selectivity for propane dehydrogenation at 490 °C and (b, d) productivity for the same reaction and temperature using (a, b) DFNS/BN and (c, d) DFNS/B2O3. ...................................................

Figure 56 .

 56 Figure 56. Comparison of DFNS/BN and DFNS/B2O3 with a selection of previously reported unsupported and supported catalysts for propane dehydrogenation. 6,14 ...................................................................................

Figure 57 .

 57 Figure 57. 1D (a, b, c) 1 H DEPTH and (d, e, f) 11 B quantitative (short tip angle) MAS NMR spectra of (a, d) as-prepared DFNS/BN, (b, e) DFNS/BN after catalysis and (c, f) as-prepared DFNS/B2O3. Experiments were recorded at B0 = 18.8 T with R = 20 kHz. ................................................

Figure 58 .

 58 Figure 58. 11 B quantitative (i.e., short tip angle) 1D NMR spectra of pure BN, as-prepared DFNS/BN and DFNS/B2O3. Experiments were recorded at B0 = 18.8 T with R = 20 kHz. ...................................................................

Figure 59 .

 59 Figure 59. 2D 11 B-{ 1 H} D-HMQC (a) as-prepared DFNS/BN (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3 along with their skyline projections in both dimensions. The spectra were acquired at B0 = 18.8 T and R = 20 kHz. .....................................................................................

Figure 60 .

 60 Figure 60. Comparison of 1D 11 B quantitative (short tip angle) spectrum and the summation projection in the F2 dimension of the 11 B-{ 1 H} D-HMQC spectrum of as-prepared DFNS/BN. The spectra were normalized to the intensity at 12 ppm. ....................................................................................

Figure 61 .

 61 Figure 61. 1 H→ 29 Si CP MAS NMR spectra of (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3. Experiments were recorded at B0 = 9.4 T with R = 10 kHz. ..........................................

Figure 62 .

 62 Figure 62. Deconvolutions of the lineshapes obtained from 1 H→ 29 Si CP MAS experiments on (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3. Fitting was carried out using the Gaussian/Lorentzian model in DMFit. 63 In (c), the small peak at ca. -20 ppm in the 1 H→ 29 Si CP MAS spectrum of as-prepared DFNS/B2O3 is probably due to residual organic species, so was not included in the fitting process. ............................................................................................

Figure 63 .

 63 Figure 63. 2D 11 B-{ 29 Si} D-HMQC NMR spectra of (a) as-prepared DFNS/BN and (b) as-prepared DFNS/B2O3 along with their skyline projections in both dimensions. The spectra were recorded at B0 = 18.8 T and R = 14.286 kHz. ................................................................................................

Figure 64 .

 64 Figure 64. 2D 11 B MQMAS spectra of (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3. The projections in F1 indirect dimension of MQMAS spectra along with their deconvolution

Figure 65 .

 65 Figure 65. (a-c) Deconvolutions of the lineshapes obtained from total projections of the F1 dimension of the 11 B MQMAS spectrum of asprepared DFNS/BN. Fitting was carried out using the Gaussian/Lorentzian model in DMFit. 63 ....................................................

Figure 66 .

 66 Figure 66. Deconvolutions of the lineshapes obtained from total projections of the F1 dimension of the 11 B MQMAS spectrum of (a) DFNS/BN after catalysis and (b) as-prepared DFNS/B2O3. Fitting was carried out using the Gaussian/Lorentzian model in DMFit. 17 The BO4 peak is folded, and thus, not shown. ...................................................................................

Figure 67 . 1 H

 671 Figure 67. 1 H → S D-RINEPT-MQMAS-QCPMG. (a) Pulse sequence, with the red selective -pulse, just after the 2 nd FAM-N pulse, omitted in the case of S = 3/2. (b,c) Coherence transfer pathways for quadrupolar spin in the case of S  5/2 (b) or S = 3/2 (c). R = 7/9, 19/12, 101/45, 91/36 for S = 3/2, 5/2, 7/2, 9/2, respectively. .............................................................

Figure 68 .

 68 Figure 68. 2D 11 B high-resolution MQMAS spectra of DFNS/BN acquired at νR=10 kHz and 9.4 T with direct excitation (split-t1 MQMAS pulse sequence -in black) and D-RINEPT-MQMAS-QCPMG sequence with recoupling time dip = 1440 µs (in blue) and 320 µs (in red). ....................

Figure 69 .

 69 Figure 69. Isotropic projection of the 2D 11 B high-resolution MQMAS spectra of DFNS/BN presented in Fig. 68. .............................................................

Figure 70 .

 70 Figure 70. DNP-enhanced 1 H→ 13 C CPMAS spectra at B0 = 9.4 T and 105 K with νR=10 kHz of MoO3/TiO2(P25) impregnated with TEKPol solution in TCE. 5 ........................................................................................

Figure 71 .

 71 Figure 71. DNP-enhanced 1 H→ 17 O RINEPT-SR2 2 1 (tt)-QCPMG spectra of isotopically unmodified MoO3/TiO2 impregnated with 110 mM TEKPol solution in TCE with τ = (a) 1.9 and (b) 0.1 ms at B0 = 9.4 T and νR = 10 kHz. The spectra are the FT of the sum of QCPMG echoes. 5 ....................................................................................................................

Figure 72 .

 72 Figure 72. 47,49 Ti QCPMG NMR spectra of isotopically unmodified MoO3/TiO2 impregnated with 110 mM TEKPol solution in TCE at B0 = 9.4 T with νR = 10 kHz enhanced by (a) indirect DNP using 1 H→ 47,49 Ti RINEPT-SR2 2 1 (tt) transfer with τ = 2.9 ms and (b) DFS scheme. The spectra are the FT of the sum of QCPMG echoes. 5 .....................................................

Figure 73 .

 73 Figure 73. Raman spectrum of MoO3/TiO2(P25). The signals corresponding to the MoO3 particles are observed at 815 and 992 cm -1 , while the signal from the polyoxomolybdates phase appears at 980 cm -1 . ........................

Figure 74 .

 74 Figure 74. Raman spectrum of MoO3/TiO2(rutile ) with 2, 7 and 10 %wt content of MoO3. .....................................................................................................

Figure 75 .

 75 Figure 75. Mo 3d XPS spectrum of MoO3/TiO2(P25) indicating the presence of Mo(V) and Mo(VI) species. .......................................................................

Figure 76 .

 76 Figure 76. DNP-enhanced 17 O DFS-QCPMG NMR spectrum of MoO3/TiO2(P25) impregnated with 110 mM TEKPol solution in TCE acquired at B0 = 18.8 T and νR = 10 kHz. ................................................... 194

  schemes for single-quantum (SQ)-SQ and double-quantum (DQ)-SQ through-space homonuclear correlation (D-HOMCOR) sequences. Their performances were evaluated for 11 B-11 B and27 Al-27 Al D-HOMCOR at R = 20 kHz using samples of Li2B4O7 and AlPO4-14, respectively. The most efficient recoupling schemes for the two HOMCOR variant were identified. Furthermore, we compared the efficiencies of these HOMCOR variants using these recoupling schemes.

Figure 1 .

 1 Figure 1. Schematic representation of the surface analysis methods presented in Table1.

Figure 2 .

 2 Figure 2. Summary of the most important interactions in solid-state NMR and the available information.

Figure 3 .

 3 Figure 3. Schematic representation of the nuclear quadrupolar interaction which arises from the coupling between the electric quadrupolar moment eQ and the electric field gradient (EFG) E. (left). The EFG is mathematically described by a 3x3 matrix and is defined by its three tensor components 𝑉 𝑋𝑋 , 𝑉 𝑌𝑌 and 𝑉 𝑍𝑍 , with |𝑉 𝑍𝑍 | ≥ |𝑉 𝑌𝑌 | ≥ |𝑉 𝑋𝑋 | by convention (middle). The orientations of the PAS for the EFG tensor with respect to the external magnetic field, B0 (right).

Figure 4 .

 4 Figure 4. Schematic energy level diagram for a spin I = 3/2 nucleus showing the effect of the Zeeman, first-order and second-order quadrupolar interactions. The first-order quadrupolar interaction splits the NMR spectrum into 2I -1 lines with resonance frequencies affected by the quadrupolar splitting parameter ω Q , while the central transition CT remains unaffected at the Larmor frequency ω 0 . The second-order quadrupolar interaction affects all the lines.

Figure 5 .

 5 Figure 5. Schematic frequency domain representation of rf irradiation of quadrupolar nuclei with spin I = 3/2.

Figure 6 .

 6 Figure 6. Pulse sequences and coherence pathways for (a) the z-filter and (b) the split-𝑡 1 shifted-echo 3QMAS experiments (the coherence pathway is given for the z-filter approach on a spin I = 3/2, depicted at the top, with echo and antiecho pathway signals represented by solid and dashed lines, respectively) and (c) DQF-STMAS z-filter experiment.

  scheme developed by Colaux et al., has been proposed for the efficient 1Q-CT  3Q transfers.136

Figure 7 .

 7 Figure 7. Different types of schemes that can be used for the excitation and conversion for the MQMAS experiment: hard pulses (HP), rotation-induced adiabatic coherence transfer (RIACT), soft-pulses added mixing (SPAM), double frequency sweeps (DFS), fast-amplitude modulation pulse trains (FAM-N), long pulses (LP) and cosine long pulses (Cos-LP).

Figure 8 .

 8 Figure 8. Pulse sequences of (a) Quadrupolar echo, (b) QCPMG and (c) WURST-QCPMG.

  and slow longitudinal relaxation (T1), which ultimately results in weak NMR signals.181,182 Dynamic Nuclear Polarization (DNP) has emerged in the recent years as a technique to enhance the sensitivity of solid-state NMR experiments by 1-3 orders of magnitude. 182 DNP enhances the NMR signal by transferring the polarization from unpaired electrons to nearby NMR-active nuclei via microwave (w) irradiation,

Figure 9 .

 9 Figure 9. The principal components of the DNP NMR system are: gyrotron, transmission line and the NMR magnet.

Figure 9 .

 9 Figure 9. High-field DNP-NMR system with gyrotron microwave source (gyrotron tube indicated in purple), microwave transmission line (yellow) and low-temperature NMR probe (light blue).

Figure 10 .

 10 Figure 10. Schematic representation of indirect and direct DNP experiments. After microwave irradiation, polarization from electrons can be transferred directly to nuclei of the investigated materials (direct DNP) or indirectly, by transferring the polarization first to 1 H and then to the detected nuclei by CP or other methods (indirect DNP).

Figure 11 .

 11 Figure 11. Structures of some of the organic radicals used as PA for high-field MAS DNP.

Chapter 2 Chapter 2 :

 22 introduces new robust symmetry-based heteronuclear dipolar recoupling built from composite inversion pulses for PRESTO and D-RINEPT methods to transfer the polarization of protons to quadrupolar nuclei at MAS frequencies of 20 and 62.5 kHz. Chapter 3 compare various symmetry-based homonuclear dipolar recouplings for DQ-SQ and SQ-SQ HOMCOR experiments between quadrupolar nuclei with spin I = 3/2 and 5/2 at 20 kHz. Chapter 4 presents new insights into the surface of boron nitride and oxide supported on dendritic fibrous nanosilica, DFNS/BN and DFNS/B2O3, through a combination of 1D and 2D multinuclear ( 1 H, 29 Si and 11 B) solid-state NMR experiments. The conclusions and perspectives finalize this document. The appendices further include SIMPSON input files (Appendix A), relevant pulse programs (Appendix B) and an AU program used for the shearing process for a 2D spectrum acquired with NUS (Appendix C). . Improved NMR transfer of magnetization from 1 H to half-integer quadrupolar nuclei at 20 and 62.5 kHz MAS 2.1 STATEMENT OF CONTRIBUTION Chapter 2 of this thesis includes the outcome of a publication in Magnetic Resonance, 1 for which I am the first author and that has the following co-authors: Andrew G. M. Rankin, Julien Trébosc, Frédérique Pourpoint, Yu Tsutsumi, Hiroki Nagashima, Olivier Lafon and Jean-Paul Amoureux. In all cases, only my primary contribution towards this publication is included in this manuscript. I carried out the NMR experiments on -alumina and AlPO4-14, analyzed the experimental data and assisted with the editing of the paper. I also performed some preliminary numerical simulations of spin dynamics using Simpson software in order to compare the performances of the different composite pulses. Nevertheless, these simulations are not included in the manuscript since they are performed under conditions different from those used in experiments. A. Rankin and J. Trébosc provided assistance in experimentation. Y. Tsutsumi contributed to the spin dynamics simulations and performed the NMR experiments on L-Histidine•HCl. H. Nagashima and F. Pourpoint contributed to the analysis and interpretation of the results. O. Lafon contributed to the experimental design, conducted the numerical evaluations of the average Hamiltonian theory for the recoupling schemes and assisted with the writing of the paper. J-P.

  Fig.12displays two of the recoupling schemes employed in this work: symmetry-based scheme R22 27 built from 1800 pulses and R16 76 scheme built from composite 270090180 inversion pulses, where the phases of the individual rectangular pulses forming the composite pulse are indicated as subscripts.The R22 27 scheme consists of a repetition of two-pulse element 18057.27180-57.27 and the required RF-field strength corresponds to a nutation frequency of 5.5 times the MAS frequency. This R16 7 6 scheme consists of a repetition of four-pulse element 27067.590247.5270-67.590115.7 and its RF-field strength then corresponds to a nutation frequency of 2.28 times the MAS frequency.

Figure 12 .

 12 Figure 12. Schematic representation of two R𝑁 n  schemes employed in this work: R22 2 7 built from rectangular 1800 pulses and R16 7 6 built from composite 270090180 pulses.

Figure 13 . 1 H

 131 Figure 13. 1 H → 27 Al (a,c) PRESTO-R𝑁 𝑛  and (b,c) D-RINEPT-CWc-R𝑁 𝑛  pulse sequences. Those applied to 1 H and 27 Al channels are shown in (a,b) and (c), respectively. The narrow and broad black bars represent /2 and -pulses, respectively. The acquisition of the FIDs (indicated with the vertical dashed line) starts after (a) the end of the R𝑁 𝑛  block in the case of PRESTO or (b) on top of the echo shifted with R/2 with respect to the end of the last recoupling block in the case of RINEPT.

Figure 14 .

 14 Figure 14. Type of pulses employed in R𝑁 𝑛  sequences for PRESTO-III.

Figure 15 .

 15 Figure 15. Schematic representation of the selection of R𝑁 𝑛  schemes for PRESTO based on the symmetry numbers N, n and ν, the RF requirement and the ϕ angle. The selected R𝑁 𝑛  schemes are presented in Tables 3-5.

Figure 16 .

 16 Figure 16. Type of pulses employed in R𝑁 𝑛  sequences for D-RINEPT-CWc.

Figure 17 .

 17 Figure 17. Schematic representation of the selection of R𝑁 𝑛  schemes for D-RINEPT-CWc based on the symmetry numbers N, n and ν, and the ϕ angle. The selected R𝑁 𝑛 

  with AlO6 signal of -alumina in Figs. 20 and 26, Tables 7 and 9, as well as in Figs. 21 and 27 when the offset is null, (ii) on-resonance with AlO4 signal of AlPO4-14 in Figs. 24 and 28, Tables 8 and 10 as well as in Figs. 25 and 29 when the offset is null, and

  ). Figs.20 and 21 display the signal intensity of these sequences as function of the RF-field amplitude and offset, respectively. The highest transfer efficiencies are obtained with the RINEPT-CWc sequence incorporating a (tt) adiabatic pulse. This recoupling also leads to the highest robustness to offset and RF inhomogeneity, and SR4 1 2 (tt) and R12 3 5 (tt) yield identical transfer efficiency and robustness. Hence, the three-step multiple-quantum super-cycle of the SR4 1 2 symmetry does not improve the robustness in the case of a (tt) basic element. However, these recoupling schemes require maximum RF-fields of 1,max  8νR = 160 kHz, which may exceed the RF power specifications of most 3.2 mm MAS probes.

Figure 19 .

 19 Figure 19. 27 Al 1D spectra of -alumina at 18.8 T with R = 20 (a) and 62.5 (b) kHz acquired using 1 H → 27 Al transfers with RINEPT-CWc sequence and SR4 1 2 (tt), SR4 1 2 (270090180) and R12 3 5 (270090180) recoupling, or PRESTO scheme and (a) R22 2 7 (1800) or R18 2 5 (1800), or (b) R16 7 6 (270090180) and R14 6 5 (270090180) recoupling.

Figure 20 .

 20 Figure 20. 27 AlO6 on-resonance signal of -alumina at R = 20 kHz as function of 1 or 1,max for PRESTO-R22 2 7(1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table7. Vertical bars correspond to the error on AlO6 intensity, determined by the inverse of the S/N of the optimum 1.

Figure 21 .

 21 Figure 21. 27 AlO6 signal of -alumina at R = 20 kHz as function of offset for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 7. Vertical bars correspond to the error on AlO6 intensity, determined by the inverse of the S/N of the optimum offset value.

Figure 22 .

 22 Figure 22. 27 Al 1D spectra of AlPO4-14 at 18.8 T with R = 20 (a) and 62.5 (b) kHz acquired using 1 H → 27 Al transfers with RINEPT-CWc sequence and SR4 1 2 (tt), SR4 1 2 (270090180) and R12 3 5 (270090180) recoupling, or PRESTO sequence and (a) R22 2 7 (1800) and R18 2 5 (1800), or (b) R16 7 6 (270090180) and R14 6 5 (270090180) recoupling.

Figure 23 . 1 H

 231 Figure 23. 1 H MAS spectrum of AlPO4-14 acquired at B0 = 18.8 T and R = 20 kHz by averaging 16 transients separated by a recycle interval of 1 s, using the DEPTH pulse sequence for probe background suppression, with ν1  208 kHz.72

Figure 24 .

 24 Figure 24. 27 AlO4 signal of AlPO4-14 at R = 20 kHz as function of 1 or 1,max of the recoupling for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table 8. Vertical bars correspond to the error on AlO4 intensity, determined by the inverse of the S/N of the optimum 1.

Figure 25 .

 25 Figure 25. 27 AlO4 signal of AlPO4-14 at R = 20 kHz as function of offset of the recoupling for PRESTO-R22 2 7 (1800) and -R18 2 5 (1800) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 8. Vertical bars correspond to the error on AlO4 intensity, determined by the inverse of the S/N of the optimum offset value.

Figure 26 .

 26 Figure 26. 27 AlO6 on-resonance signal of -alumina at R = 62.5 kHz as function of 1 or 1,max for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2(270090180) and -R12 3 5 (270090180). For each curve,  was fixed to its optimum value given in Table9. Vertical bars correspond to the error on AlO6 intensity, determined by the inverse of the S/N of the optimum 1.

Figure 27 .

 27 Figure 27. 27 AlO6 signal of -alumina at R = 62.5 kHz as function of offset for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 9. Vertical bars correspond to the error on AlO6 intensity, determined by the inverse of the S/N of the optimum offset value.

Figure 28 .

 28 Figure 28. 27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of 1 or 1,max of the recoupling for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve, was fixed to its optimum value given in Table10. Vertical bars correspond to the error on AlO4 intensity, determined by the inverse of the S/N of the optimum 1.

Figure 29 .

 29 Figure 29. 27 AlO4 signal of AlPO4-14 at R = 62.5 kHz as function of offset of the recoupling for PRESTO-R16 7 6 (270090180) and -R14 6 5 (270090180) as well as RINEPT-CWc-SR4 1 2 (tt), -SR4 1 2 (270090180) and -R12 3 5 (270090180). For each curve,  and 1 or 1,max were fixed to their optimum values given in Table 10. Vertical bars correspond to the error on AlO4 intensity, determined by the inverse of the S/N of the optimum offset value.

  H signals correlate with the three Al environments. Similar 2D spectra (not shown) were acquired using RINEPT-CWc transfer based on SR4 1 2 (1800) and SR4 1 2 (tt) recoupling as well as PRESTO-R16 7 6 (270090180). Their skyline projections are shown in Fig. 31. The projections along F2 are in agreement with the results presented in Fig. 22b.

Figure 30 .

 30 Figure 30. 1 H-27 Al D-HETCOR 2D spectrum of AlPO4-14, along with its skyline projections, at B0 = 18.8 T and νR = 62.5 kHz acquired in only 72 min with only ca. 2.5 L of active volume with NUS 25% using RINEPT-CWc-SR4 1 2 (270090180) transfer.

Figure 31 .

 31 Figure 31. Skyline projections along a) F2 and b) F1 of 1 H-27 Al HETCOR 2D spectra of AlPO4-14 recorded with RINEPT-CWc-SR4 1 2 (270090180), SR4 1 2 (tt), SR4 1 2 (180090180) and PRESTO-R16 7 6 (270090180) transfers. All 2D spectra were acquired using NUS 25% in 72 min.

  this work, novel symmetry-based hetero-nuclear dipolar recoupling schemes were introduced, which can be incorporated into the RINEPT and PRESTO sequences to transfer the magnetization from 1 H to half-integer quadrupolar nuclei at R = 20 or 62.5 kHz. These new recouplings have been compared to the existing ones. It was shown that the RINEPT-CWc-SR4 1 2 (tt) sequence with adiabatic pulses, which produces efficient and robust transfers at R  10-15 kHz, 20 requires rf-fields incompatible with the specifications of most MAS probes for R ≥ 20 kHz. Conversely, the introduced RINEPT-CWc-SR4 1 2 (270090180) and PRESTO-R22 2 7 (1800) techniques with rf-fields of ca. 4R and 5.5R, respectively, are the methods of choice at R = 20 kHz to transfer the magnetization from protons to quadrupolar nuclei. At R = 62.5 kHz, the RINEPT-CWc-SR4 1 2 (270090180) and PRESTO-R16 7 6 (270090180) sequences

Chapter 3 :

 3 Comparison of double and single quantum dipolar NMR correlations of quadrupolar nuclei3.1 STATEMENT OF CONTRIBUTIONChapter 3 of this thesis incorporates unpublished material for a publication for which I will be the first author, and which will be co-authored with Julien Trébosc, Nghia Tuan Duong, Frédérique Pourpoint, Olivier Lafon and Jean-Paul Amoureux. In all cases, only my primary contribution towards this publication is included in this manuscript. I carried out the NMR experiments on Li2B4O7 and AlPO4-14, analyzed the experimental data and assisted with the editing of the paper. I also performed some preliminary numerical simulations of spin dynamics using Simpson software in order to compare the performances of the different recoupling schemes. Nevertheless, these simulations are not included in the manuscript since they are under progress. J. Trébosc provided assistance in experimentation. Nghia Tuan Duong assisted with the spin dynamics simulations. F. Pourpoint contributed to the analysis and interpretation of the results. O. Lafon and J-P. Amoureux contributed to the experimental design, analysis and interpretation of the results and the writing of the paper. All authors contributed equally to the editing of the article.

Figure 32 .

 32 Figure 32. Pulse sequences used for (a) SQ-SQ and (b) DQ-SQ HOMCOR experiments of half-integer spin quadrupolar nuclei. All pulses are CT-selective with 𝜈 1 = 𝜈 𝑅 (2𝐼 + 1) ⁄ . The black squares are /2-pulses, whereas the blue one in (b) is a -pulse. In (b) a [𝑅𝑁 𝑛  ] bracketed recoupling is shown with five /2-pulses, whereas an un-bracketed version should only require the last reading /2-pulse. The R𝑁 𝑛 

Fig. 33 :

 33 Fig. 33: S = (a) SR2 2 1 , (b) SR2 4 1 , (c) SR2 8 1 , (d) SR4 4 1 and (e) the block-cycled BR2 2 1 .

Figure 33 .

 33 Figure 33. Minimum cycle times with (a) SR2 2 1 , (b) SR2 4 1 , (c) SR2 8 1 , (d) SR4 4 1 , and

Figure 34 .

 34 Figure 34. Comparison of powder XRD patterns of crystallized lithium diborate Li2O•2B2O3 and the glass sample initially prepared. Vertical lines represent the positions of diffraction lines of B4Li2O7.

Figure 35 .

 35 Figure 35. Crystalline structure of Li2B4O7 based on the cif file. The diborate unit is indicated with the atoms of boron tri-and four-coordinated, B3 and B4, respectively.

Figure 36 .

 36 Figure 36. Comparison and difference between the powder XRD pattern of crystallized lithium diborate Li2O•2B2O3 and reference XRD patterns of α-LiBO2 and B4Li2O7.

Figure 37 .

 37 Figure 37. Lineshape deconvolution obtained from the 11 B 1D spectrum of the Li2B4O7 crystallized sample recorded at 18.8 T and R = 20 kHz. Fitting was carried out using int2QUAD model in DMFit.45 The relative amounts of Li2B4O7 and α-LiBO2 phases are 90 and 10 %.

Figure 38 .

 38 Figure 38. (a) 2D 11 B z-filtered MQMAS spectrum of Li2B4O7 recorded at B0 = 9.4 T and R = 16 kHz. Panel (b) is an expansion of panel (a). The oblique lines are the Chemical-Shift axes.

1 ) 4

 14 and (SR2 8 1 ) 4 . Conversely, with (SR2 2 1 ) 4 recoupling the two cross-peaks are visible and positive.

Figure 39 .

 39 Figure 39. Series of 2D 11 B SQ-SQ D-HOMCOR spectra of Li2B4O7 acquired at B0 = 18.8 T with a MAS frequency of 20 kHz and a non-uniform sampling (NUS) retaining 50% of the t1 points, with the carrier frequency optimized (Fig. 40) at offset = 3 kHz (11.7 ppm), shown with an arrow between the B III and B IV resonances. The spectra acquired with recoupling schemes, (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 , are shown in the first, second and third column, respectively. The negative cross-peaks are shown in green. The length of 𝜏 mix = 3.2, 6.4 or 9.6 ms, is indicated in red on each panel. The top projections are those related to 𝜏 mix = 3.2 ms. The 2D spectra result from averaging 48 transients using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 48-55 min.

Figure 40 .

 40 Figure 40. 2D 11 B SQ-SQ D-HOMCOR spectra of Li2B4O7 acquired at B0 = 18.8 T with a MAS frequency of 20 kHz and a non-uniform sampling (NUS) retaining 50% of the t1 points with offset = 0, 3, 6 kHz, as indicated in black and with arrows. The spectra acquired with recoupling schemes (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 are shown in the first, second or third column, respectively. The negative signals are shown in green. The length of τ mix = 1.6, 3.2 or 6.4 ms is indicated in red on each sub-figure. The top projections are those related to the spectra on the 1 st line. The other experimental parameters are the same as in Fig. 39.

Figure 41 . 1 ) 4 , 4

 41144 Figure 41. Slices of the 2D spectra corresponding to the B 34 (B III -B IV ) cross-peak of Li2B4O7 observed with the 11 B SQ-SQ D-HOMCOR sequence and (SR2 2 1 ) 4 , (SR2 4 1 ) 4 and (SR2 8 1 ) 4 recoupling schemes, shown in the first, second or third column, respectively. (a,c,e) offset = 3 kHz and τ mix = 1.6, 3.2, 6.4 and 9.6 ms; (b,d,f) offset = 0, 1.5, 3, 4.5, 6 kHz and τ mix =(b) 3.2, (d) 1.6 and (f) 6.4 ms. These slices are extracted from the 2D spectra shown in Figs. 39 and 40.

Figure 45 .

 45 Figure 45. Comparison of F2 slices containing (a) B 4 or (b) B 3 diagonal peaks of 2D 11 B the SQ-SQ and DQ-SQ D-HOMCOR spectra of Li2B4O7 using (SR2 2 1 ) 4 recoupling for SQ-SQ scheme and [BR2 2 1 ], [SR2 2 1 ] and BR2 2 1 recoupling for DQ-SQ sequence at

IV 1 :

 1 iso = 42.7 ppm, CQ = 1.72 MHz, Q = 0.57; Al IV 2: iso = 43.5 ppm, CQ = 3.90 MHz, Q = 0.83), one pentavalent Al V (iso = 27.1 ppm, CQ = 5.61 MHz, Q = 0.93) and one octahedral Al VI (iso = -1.3 ppm, CQ = 2.55 MHz, Q = 0.67) (Table

least 4 . 3 Å

 43 for Al VI -Al VI , Al V -Al VI and Al IV j-Al IV k, (j, k = 1,2), which correspond to dipolar coupling constants |bVI-VI|/(2π) = 333, |bV-VI|/(2π) = 174, and |bIVj-IVk|/(2π)  102

Al 1 IV

 1 and Al 2 IV overlapping resonances are not distinguished in the following.

Figure 47 . 3 . 6 . 2 . 2 Figure 48 .

 47362248 Figure 47. Comparison of the cross-peak intensities of the SQ-SQ spectra of AlPO4-14 at B0 = 18.8 T with a MAS frequency of 20 kHz acquired with NUS retaining 33% of the t1 point, obtained from 1D slices taken from the 2D spectra recorded with (SR2 2 1 ) 4 (τ mix = 4.8 ms, offset = 2.82 kHz (13.5 ppm)) and (SR2 4 1 ) 4 (τ mix = 3.2 ms, offset = 4 kHz (19.2 ppm)) recoupling schemes. The auto-peaks, which bring no information, have been truncated at 5 % of their maximum values. We employed rf field strength equal to 1 = 2.4 kHz. The other experimental parameters are identical to those of Fig. 46.

Figure 49 .

 49 Figure 49. F2 slices of three 2D 27 Al DQ-SQ D-HOMCOR spectra of AlPO4-14 acquired using BR2 2 1 recoupling and NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz for offset = 3, 4 or 5 kHz. The other experimental parameters are identical to those of Fig. 48.

Figure 50 .

 50 Figure 50. Robustness with respect to (a) rf-field and (b) offset of 1D 27 Al DQ-filtered D-HOMCOR spectra of AlPO4-14, recorded with BR2 2 1 (blue, mix = 0.8 ms) or [BR2 2 1 ] (red, τ mix = 0.4 ms) recoupling at B0 = 18.8 T with a MAS frequency of 20 kHz. The 1D NMR spectra result from averaging 64 transients with RD = 1 s. In panel (a), offset = 4 kHz, whereas in panel (b), 1 = 2.9 and 3 kHz for BR2 2 1 and [BR2 2 1 ],respectively.

Figure 51 .

 51 Figure 51. Comparison of three F2 slices of 2D 27 Al DQ-SQ and SQ-SQ D-HOMCOR spectra of AlPO4-14 acquired using BR2 2 1 and (SR2 4 1 ) 4 recoupling with τ mix = 0.8 and 3.2 ms, respectively, and NUS retaining 33% of the t1 points at B0 = 18.8 T with a MAS frequency of 20 kHz. The offset of the recoupling schemes was equal to offset = 4 kHz (19.2 ppm). The acquisition parameters of DQ-SQ and SQ-SQ experiments are identical to those of Figs. 46 and 48, respectively. The signal of the SQ-SQ spectrum has been multiplied by 1.33 to compensate for the difference in the number of transients. The auto-correlation peaks of SQ-SQ spectrum bring no information, and hence, these signals are truncated to the percentage of their maximal intensity indicated on each panel.

Chapter 4 :

 4 Boron nitride and oxide coated dendritic fibrous nanosilica for oxidative dehydrogenation: Insights into the catalytic sites from solid-state NMR 4.1 STATEMENT OF CONTRIBUTION Chapter 4 of this thesis is based on a publication in ACS Sustainable Chemistry and Engineering, 1 for which I am a co-author together with: Andrew G. M. Rankin, Julien Trébosc and Olivier Lafon from the University of Lille, and our collaborators from the Tata Institute of Fundamental Research and the National Chemical Laboratory in India: Rajesh Belgamwar, Ayan Maity, Amit Kumar Mishra, Chathakudath P. Vinod and Vivek Polshettiwar. The co-authors from the University of Lille were in charge of carrying out the solid-state NMR characterization and strongly contributed to the writing of the article. My contribution to this article was to participate to the NMR experimentation, analysis of results and helping with the writing and editing of the paper. The co-authors from India, were in charge of the synthesis of DFNS/BN and DFNS/B2O3 materials and their characterization by SEM, TEM, XRD and XPS, as well as performing the catalytic activity tests, and assisting with the writing of the article. All the authors contributed equally to the editing of the article.

4

 4

Figure 52 .

 52 Figure 52. Schematic representation of the ODH of propane to propene.

Figure 53 .

 53 Figure 53. Fibrous morphology of DFNS/BN revealed by (a) SEM and (b) TEM. The BN loading and textural properties of this material is described below.

Figure 54 .

 54 Figure 54. Fibrous morphology of DFNS/B2O3 revealed by (a) SEM and (b) TEM.The BN loading and textural properties of this material is described below.

Figure 55 .

 55 Figure 55. (a, c) Conversion and selectivity for propane dehydrogenation at 490 °C and (b, d) productivity for the same reaction and temperature using (a, b) DFNS/BN and (c, d) DFNS/B2O3.

Figure 56 .

 56 Figure 56. Comparison of DFNS/BN and DFNS/B2O3 with a selection of previously reported unsupported and supported catalysts for propane dehydrogenation.6,14

  2 mm HX probe. Excitation and conversion pulses lasted p = 6.5 and 2 μs with ν1 ≈ 100 kHz and the central transition (CT) selective pulse lasted 12.5 μs with ν1 ≈ 20 kHz. SPINAL 1 H decoupling 28 (∼100 kHz) was applied during acquisition. Spectra are the result of averaging 240 (as-prepared DFNS/BN), 1032 (as-prepared DFNS/B2O3) or 1704 (DFNS/BN after catalysis) transients separated by a recycle delay of 1 (as-prepared DFNS/B2O3 and DFNS/BN after catalysis) or 3.5 s (as-prepared DFNS/BN) for each of the 256 (as-prepared DFNS/BN) or 32 (as-prepared DFNS/B2O3 and DFNS/BN after catalysis) t1 increments of 25 (as-prepared DFNS/BN) or 50 μs (as-prepared DFNS/B2O3 and DFNS/BN after catalysis). The quadrature detection method of States et al.

  Si channel were (𝜈 ref max , 𝜈 1 max ) = (70, 57) kHz. The double frequency sweep (DFS) method 42 was used in order to enhance the polarisation of the central transition (CT) by the manipulation of the populations of the satellite transitions (ST). The DFS pulse lasted 2 ms and used ν1 ≈ 17 kHz. During the DFS pulse, the frequencies of the rf spikelets were linearly swept in a symmetric manner from 1 to 85.8 kHz with respect to the CT. Spectra are the result of averaging 448 (DFNS/BN) or 1024 (DFNS/B2O3) transients separated by a recycle interval of 4 s for each of the 32 (DFNS/BN) or 42 (DFNS/B2O3) t1 increments of 35 μs. The States-TPPI quadrature detection method 39 was used to achieve sign discrimination in the indirect dimension. Data processing was carried out using Bruker TopSpin (version 4.0.8) and ssNake (version 1.1).43

Figure 57 .

 57 Figure 57. 1D (a, b, c) 1 H DEPTH and (d, e, f) 11 B quantitative (short tip angle) MAS NMR spectra of (a, d) as-prepared DFNS/BN, (b, e) DFNS/BN after catalysis and (c, f) as-prepared DFNS/B2O3. Experiments were recorded at B0 = 18.8 T with R = 20 kHz.

Figure 58 .

 58 Figure 58. 11 B quantitative (i.e., short tip angle) 1D NMR spectra of pure BN, asprepared DFNS/BN and DFNS/B2O3. Experiments were recorded at B0 = 18.8 T with R = 20 kHz.

Figure 59 .

 59 Figure 59. 2D 11 B-{ 1 H} D-HMQC (a) as-prepared DFNS/BN (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3 along with their skyline projections in both dimensions. The spectra were acquired at B0 = 18.8 T and R = 20 kHz.

Figure 60 .

 60 Figure 60. Comparison of 1D 11 B quantitative (short tip angle) spectrum and the summation projection in the F2 dimension of the 11 B-{ 1 H} D-HMQC spectrum of asprepared DFNS/BN. The spectra were normalized to the intensity at 12 ppm.

Figure 61 .

 61 Figure 61. 1 H→ 29 Si CP MAS NMR spectra of (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3. Experiments were recorded at B0 = 9.4 T with R = 10 kHz.

Figure 62 .

 62 Figure 62. Deconvolutions of the lineshapes obtained from 1 H→ 29 Si CP MAS experiments on (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) asprepared DFNS/B2O3. Fitting was carried out using the Gaussian/Lorentzian model in DMFit.63 In (c), the small peak at ca. -20 ppm in the 1 H→29 Si CP MAS spectrum of as-prepared DFNS/B2O3 is probably due to residual organic species, so was not included in the fitting process.

Figure 63 .

 63 Figure 63. 2D 11 B-{ 29 Si} D-HMQC NMR spectra of (a) as-prepared DFNS/BN and (b) as-prepared DFNS/B2O3 along with their skyline projections in both dimensions. The spectra were recorded at B0 = 18.8 T and R = 14.286 kHz.

  B signals, 2D11 B MQMAS spectra of as-prepared DFNS/BN and DFNS/B2O3 and DFNS/BN after catalysis (Fig.64) were also acquired. These spectra correlate the anisotropic MAS spectra of the 11 B central transitions shown along the direct δ2 dimension to their isotropic shift detected along the indirect δ1 dimension. The projection along the vertical isotropic direction indicates that each of the three11 B sites has a wide distribution of δiso values, which stems from the distribution of local environments, as the second neighbour of boron atoms can be B, Si or H.44,49

Figure 64 .

 64 Figure 64. 2D 11 B MQMAS spectra of (a) as-prepared DFNS/BN, (b) DFNS/BN after catalysis and (c) as-prepared DFNS/B2O3. The projections in F1 indirect dimension of MQMAS spectra along with their deconvolution are shown in Figs. 65 and 66. The MQMAS spectra were acquired at B0 = 18.8 T and R = 20 kHz.

Figure 65 .

 65 Figure 65.(a-c) Deconvolutions of the lineshapes obtained from total projections of the F1 dimension of the 11 B MQMAS spectrum of as-prepared DFNS/BN. Fitting was carried out using the Gaussian/Lorentzian model in DMFit.63

Figure 66 .

 66 Figure 66. Deconvolutions of the lineshapes obtained from total projections of the F1 dimension of the 11 B MQMAS spectrum of (a) DFNS/BN after catalysis and (b) asprepared DFNS/B2O3. Fitting was carried out using the Gaussian/Lorentzian model in DMFit.17 The BO4 peak is folded, and thus, not shown.

6 CONCLUSIONS 2 ( 11 B

 6211 DFNS/BN Peak at δ1 = 30.5 ppm in 11 B MQMAS as-prepared DFNS/BN Cross-peak {δ( 1 H), δ( 11 B)} = {6.7 ppm, 26 ppm} in D-HMQC as-prepared DFNS/BN Cross-peak {δ( 29 Si), δ( 11 B)} = {-73 ppm, 26 ppm} in D-HMQC as-prepared DFNS/BN Cross-peak {δ( 29 Si), δ( 11 B)} = {-93 ppm, 23.5 ppm} in D-HMQC with R = B or Si(Q 4 ) as-prepared DFNS/BN as-prepared DFNS/B2O3 Cross-peak {δ( 29 Si), δ( 11 B)} = {-110 ppm, 12 ppm} in D-HMQC with R = B or Si(Q 4 ) and A = H or Si as-prepared DFNS/BN as-prepared DFNS/B2O3 Cross-peak {δ( 29 Si), δ( 11 B)} = {-101 and -110 ppm, 14 ppm} in D-HMQC as-prepared DFNS/BN as-prepared DFNS/B2O3 Cross-peak {δ( 29 Si), δ( 11 B)} = {-101 ppm, 16 ppm} in D-HMQC with A = B or Si(Q 4 ) as-prepared DFNS/BN Cross-peak {δ( 29 Si), δ( 11 B)} = {-110 ppm, -2 ppm} in D-HMQC with A = B or Si(Q 3 ) as-prepared DFNS/BN Cross-peak {δ( 29 Si), δ( 11 B)} = {-101 ppm, 0 ppm} in D-HMQC with A = B or Si as-prepared DFNS/BN DFNS/BN after catalysis as-prepared DFNS/B2O3 Peak at δ1 ≈ 16 (BO3) and 19 (BO2OH) ppm in 11 B MQMAS with A = B or Si as-prepared DFNS/BN DFNS/BN after catalysis Cross-peak {δ( 1 H), δ( 11 B)} = {4 ppm, 15 ppm} in D-HMQC Peak at δ1 = 17.1 ppm in 11 B MQMAS with A = B or Si as-prepared DFNS/B2O3 Cross-peak {δ( 1 H), δ( 11 B)} = {2.7 ppm, 15 ppm} in D-HMQC 4.Through a combination of analytical techniques, notably 1D and 2D multinuclear ( 1 H, 29 Si, 11 B) solid-state NMR experiments, it has been shown that the supported BN phase undergoes a transformation to B2O3 (oxidation) during the propane dehydrogenation process, whilst avoiding deactivation (up to 19 h). This has been attributed to the stabilising effect of DFNS through the covalent linkage of silica and B2O3 via Si-O-B bonding, allowing B2O3 to remain on the DFNS surface and act as a catalyst. The presence of these linkages was confirmed by solid-state NMR spectroscopy. Furthermore, 2D 1 H-11 B heteronuclear correlation and 11 B high resolution spectra acquired at B0 = 18.8 T indicate a larger fraction of hydrogen-bonded BOH groups at the surface of oxidised DFNS/BN with respect to as-prepared DFNS/B2O3. This structural difference could explain the higher selectivity towards propene of oxidised DFNS/BN. Several symmetry-based homonuclear recoupling schemes used for SQ-SQ and DQ-SQ HOMCOR sequences were analysed and compared. Their performances were evaluated simultaneously for half-integer quadrupolar nuclei with spin value I = 3/) or 5/2 ( 27 Al). Notably, 11 B-11 B and

  this study, the efficiency of the SQ-SQ and DQ-SQ experiments were analyzed separately. Therefore, this work appears to be the first comprehensive study that compares the performances of SQ-SQ and DQ-SQ simultaneously. However, further simulations of the spin dynamics of the spins I = 3/2 and 5/2 are needed to understand the results obtained here and to explain the differences between the two spins. One of the possible extensions to this work would be to decrease the experimental time by using an initial CT enhancement obtained by the transfer of population from the STs to the CT.The aim of the last part of this thesis was to investigate the surface of catalysts for the oxidative dehydrogenation of propane, notably BN and B2O3 supported on dendritic fibrous nanosilica (DFNS). A combination of 1D and 2D multinuclear ( 1 H,29 Si and 11 B) solid-state NMR methods provided new insights into the structure of both materials. These findings suggest that the supported BN phase undergoes a transformation to B2O3 (oxidation) during the propane dehydrogenation process, whilst avoiding deactivation. The results of this study support the idea that DFNS offers a stabilizing effect through the covalent linkage of silica and B2O3 via Si-O-B bonding, allowing B2O3 to remain on the DFNS surface and act as a catalyst. The NMR methods used for this research confirmed the presence of these linkages. These findings have significant implications for the understanding of how the oxidized boron phase is anchored to the support and how this affects the stability of the catalyst. The second major finding was the larger fraction of hydrogen-bonded BOH groups at the surface of oxidized DFNS/BN with respect to as-prepared DFNS/B2O3, evidenced by 2D 1 H-11 B HETCOR and 11 B MQMAS spectra. This structural difference could explain the higher selectivity towards propene of oxidized DFNS/BN. This study has raised important questions about the nature of the hydroxyl groups and should serve as a guide for the synthesis of more efficient catalysts for the oxidative dehydrogenation of propane.A combination of above developed solid-state NMR techniques could contribute to a better characterization of homo-and hetero-nuclear through-space proximities in various materials, notably near their surface. Therefore, this improved characterization of surfaces of materials will facilitate the rational improvement of their properties.Perspectives DNP-ENHANCED MQMAS EXPERIMENTS USING D-RINEPTTRANSFERThis thesis has focused on the development of methods to improve the detection of quadrupolar nuclei near surfaces. Chapter 2 was dedicated to the study of the transfer of polarization from 1 H to quadrupolar nuclei, such as27 Al, based on the PRESTO and D-RINEPT-CWc sequences. In particular, this variant of D-RINEPT, D-RINEPT-CWc-SR4 1 2 (tt), has proven to be efficient for the indirect DNP of quadrupolar nuclei of low natural abundance, such as 17 O, or low gyromagnetic ratio, such as 47.49 Ti, in DNP experiments at 9.4 T. In addition, it has been demonstrated that this new technique is more sensitive than both direct and indirect DNP experiments using PRESTO. Nevertheless, the CT in these D-RINEPT spectra are broadened by secondorder quadrupolar interactions, which can mask differences in isotropic chemical shifts.

Figure 67 . 1 H

 671 Figure 67. 1 H → S D-RINEPT-MQMAS-QCPMG. (a) Pulse sequence, with the red selective -pulse, just after the 2 nd FAM-N pulse, omitted in the case of S = 3/2. (b,c) Coherence transfer pathways for quadrupolar spin in the case of S  5/2 (b) or S = 3/2 (c). R = 7/9, 19/12, 101/45, 91/36 for S = 3/2, 5/2, 7/2, 9/2, respectively.

  B high-resolution MQMAS spectra acquired D-RINEPT-MQMAS-QCPMG with short recoupling time (dip = 320 µs), it is possible to observe the signals of groups BO2OH and B(OSi(Q 3 ))x(OB)3-xOH, indicating that these sites are located on the surface of this material and are responsible of the catalytic activity of DFNS/BN. Conversely, for long recoupling time (dip = 1440 µs), the signals of protonated BO3 sites decay, whereas only the signal of BO3 sites not bonded to OH groups are detected in the 2D 11 B D-RINEPT-MQMAS spectra. These sites must be located in the subsurface of the material. Therefore, these results are of great importance and support those presented in Chapter 4. These results demonstrate the efficiency and usefulness of the newly developed sequence and open a new way for the observation of surface and subsurface species in various materials with high resolution.

Figure 68 .

 68 Figure 68. 2D 11 B high-resolution MQMAS spectra of DFNS/BN acquired at νR=10 kHz and 9.4 T with direct excitation (split-t1 MQMAS pulse sequence -in black) and

Figure 69 .

 69 Figure 69. Isotropic projection of the 2D 11 B high-resolution MQMAS spectra of DFNS/BN presented in Fig. 68.

  Fig. 70). Surprisingly, no TEKPol decomposition was observed for TiO2 (P25/TiO-15) alone or when MoO3 was supported on SiO2 or Al2O3 nanoparticles.

Figure 70 .

 70 Figure 70. DNP-enhanced 1 H→ 13 C CPMAS spectra at B0 = 9.4 T and 105 K with νR=10 kHz of MoO3/TiO2(P25) impregnated with TEKPol solution in TCE.5

Figure 71 .

 71 Figure 71. DNP-enhanced 1 H→ 17 O RINEPT-SR4 1 2 (tt)-QCPMG spectra of isotopically unmodified MoO3/TiO2 impregnated with 110 mM TEKPol solution in TCE with τ = (a) 1.9 and (b) 0.1 ms at B0 = 9.4 T and νR = 10 kHz. The spectra are the FT of the sum of QCPMG echoes. 5

Figure 72 .

 72 Figure 72.47,49 Ti QCPMG NMR spectra of isotopically unmodified MoO3/TiO2 impregnated with 110 mM TEKPol solution in TCE at B0 = 9.4 T with νR = 10 kHz enhanced by (a) indirect DNP using 1 H→47,49 Ti RINEPT-SR4 1 2 (tt) transfer with τ = 2.9 ms and (b) DFS scheme. The spectra are the FT of the sum of QCPMG echoes.5

Figure 73 .

 73 Figure 73. Raman spectrum of MoO3/TiO2(P25). The signals corresponding to the MoO3 particles are observed at 815 and 992 cm -1 , while the signal from the polyoxomolybdates phase appears at 980 cm -1 .

Figure 74 .

 74 Figure 74. Raman spectrum of MoO3/TiO2(rutile) with 2, 7 and 10 %wt content of MoO3.

76 ).

 76 This spectrum exhibits signals of OMo4, OMo5 and HOMo sites. Nevertheless, no signal was detected for 1 H→ 17 O D-RINEPT-CWc-SR4 1 2 (tt) experiment owing to the lower DNP enhancement (on/off = 9) and longer build-up time (RD = 2.9 s) at 18.8 T, instead of on/off = 76 and RD = 1 s at 9.4 T. We also impregnated MoO3/TiO2(P25) nanoparticles with solution of AsympolPOK 6 nitroxide biradical in [ 2 H8]glycerol/ 2 H2O/H2O mixture with volume fractions 60/30/10 v/v/v. This radical solution leads to slightly higher DNP enhancement of 1 H signal (on/off = 11) and shorter build-up times (RD = 1.5 s) than those measured for TEKPol solution in TCE. Nevertheless, 1 H→ 17 O D-RINEPT-CWc-SR4 1 2 (tt) spectrum of this sample only exhibits the 17 O signal of water. In the future, we plan to test other radicals, such as HyTEK2 7 in TCE, in order to improve the sensitivity.

Figure 76 .

 76 Figure 76. DNP-enhanced 17 O DFS-QCPMG NMR spectrum of MoO3/TiO2(P25) impregnated with 110 mM TEKPol solution in TCE acquired at B0 = 18.8 T and νR = 10 kHz.

  (eta)*$par(eta)/3.0)*($par(CQ1)/$Larmor)*($par(CQ1)/$Larmor)*($Larmor/ 1.0e6)] set QIS2 [expr -25000*(1+$par(eta)*$par(eta)/3.0)*($par(CQ2)/$Larmor)*($par(CQ2)/$Larmor)*($Larmor/ 1.0e6)] set f [fsimpson [list [list shift_1_iso [expr ($QIS1+$QIS2)/2-(-3000)]] [list shift_2_iso [expr ($QIS1+$QIS2)/2-(-3000)]]]] set c 0 for {set i 1} {$i <= $par(np)} {incr i} { incr c set Sr [findex $f $c -re] set Si [findex $f $c -im] puts $FileRe "[expr 1.0e3*$i/$par(sw)] [expr $Sr]" puts $FileIm "[expr 1.0e3

;

  100m mc #0 to 2 F1PH(calph(ph4, +45) & calph(ph23, +45) & calph(ph24,+45) & calph(ph5, +45) & calph(ph6, +45), caldel(d0,+in0)) exit ;NSmin = 64 ;Delta p = +2 or -2 for first [BR2] block ph4=Delta p = +2 or -2 for central pi pulse ph6=(8) {0 0

  . ....................................................................................................

	_Toc93174639Figure 26.

Table 1 .

 1 General characteristics of common surface analysis methods ......................

Table 2 .

 2 Summary of the nuclear magnetic properties of some nuclei commonly found in solid surfaces.64,65 ..........................................................................

Table 3 .

 3 Selected R𝑁 𝑛  = 2 SQ hetero-nuclear dipolar recoupling for νR = 20 kHz. ..............................................................................................................

Table 4 .

 4 Selected R𝑁 𝑛  = 2 SQ hetero-nuclear dipolar recoupling with 45° ≤ ϕ ≤ 135° for νR = 62.5 kHz. ................................................................................

Table 5 .

 5 Selected R𝑁 𝑛  = 2 SQ hetero-nuclear dipolar recoupling built from single π pulses with 20° ≤ ϕ ≤ 160° and κ ≥ 0.15 for νR = 62.5 kHz. ..........

Table 6 .

 6 Selected R𝑁 𝑛

 = 2 two-spin order hetero-nuclear dipolar recoupling. .........

Table 7 .

 7 Comparison of the performances of 1 H →

27

Al RINEPT-CWc and PRESTO transfers using various recouplings for AlO6 signal of γalumina at R = 20 kHz ................................................................................

Table 8 .

 8 Comparison of the performances of 1 H →

27

Al RINEPT-CWc and PRESTO transfers with AlPO4-14 at R = 20 kHz. .....................................

Table 9 .

 9 Comparison of the performances of 1 H→

27

Al RINEPT-CWc and PRESTO transfers using various recouplings for the AlO6 signal of γalumina at R = 62.5 kHz. ............................................................................

Table 10 .

 10 Comparison of the performances of 1 H →

27

Al RINEPT-CWc and PRESTO transfers using various recouplings for AlPO4-14 at R = 62.5 kHz. ..............................................................................................................

Table 11 .

 11 Distances

between the different H atoms and their closest Al neighbours in the structure of isopropylamine templated AlPO4-14 determined from X-ray diffraction.

73

The H and Al atoms are numbered according to the cif file. ...............................................................................

Table 12 .

 12

1 H T2' values of AlPO4-14 without recoupling or with SR412 recoupling built from (1800), (270090180) or tt inversion element. The estimated error bars are equal to 7 %. ..........................................................

Table 13 .

 13 Data obtained from the deconvolution of the 11 B 1D spectrum of the Li2B4O7 (using the int2QUAD model in DMFit.45 ) presented in Fig.37.

....................................................................................................................

Table 14 .

 14 Aluminum species present in AlPO4-14 reported by Sasaki et al.46 .......

Table 15 .

 15 Comparison of conventional catalysts with DFNS/BN and DFNS/B2O3 for propane dehydrogenation. 6,14 ...........................................

Table 16 .

 16 Data obtained from the deconvolutions of the 1 H→29 Si CP MAS NMR spectra (using the Gaussian/Lorentzian model in DMFit63

Table 17 .

 17 Possible boron species present in DFNS supported catalysts obtained from fitting the F1 dimensions of MQMAS spectra of DFNS/BN before and after catalysis, and DFNS/B2O3 before catalysis.

Table 18 .

 18 ......................................................... Main plausible local environments in as prepared DFNS/BN, DFNS/BN after catalysis and as-prepared DFNS/B2O3 based on solidstate NMR data. ..........................................................................................

Table 19 .

 19 Synthesized MoO3/TiO2(rutile) catalysts with various content of MoO3. .........................................................................................................

Table 20 .

 20 Specific area BET and pore volume for the various TiO2 supports. .......

Table 21 .

 21 Proportion of Mo(V) species found in MoO3/TiO2 catalysts before and after impregnation with TEKPol solutions. ...............................................

  particles, for which the bulk region contains no or a limited amount of protons, this CPMAS transfer allows the selection of the surface nuclei. Nevertheless, in the case of half-integer quadrupolar nuclei, the CPMAS technique lacks robustness and it is preferable to use methods for which no spin-lock is applied to the quadrupolar nucleus.8 Two types of sequences, without any spin-lock, can be used in this case:

through-space refocused INEPT (D-RINEPT) and PRESTO, which use symmetrybased heteronuclear dipolar recouplings solely applied to the 1 H channel. More recently a new variant of the D-RINEPT scheme was introduced, which uses adiabatic recoupling and continuous wave irradiation in order to improve the decoupling of 1 H-1 H interactions. These transfers were combined with DNP to detect the NMR signals of quadrupolar isotopes with low natural abundance ( 17 O) and low gyromagnetic ratio

( 47,49

Ti,

67

Zn,

[START_REF] Larsen | Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors[END_REF]

Mo), notably near the surface of MoO3/TiO2 and Al-doped ZnO nanoparticles.

Technique Probe/Detected Lateral resolution Information depth Depth resolution Type of information X-ray photoelectron spectroscopy (XPS)

						Surface
						elemental
		Photons/Electrons ~ 2-10 m	2-10 nm	~ 2 nm	and chemical composition.
						Depth
						profiling
						Elemental
						composition
	Auger electron spectroscopy (AES)	Electrons/Electrons ~ 10 nm	2 nm	~ 2 nm	of whole material or individual large particles.
						Depth
						profiling
						Detection of
						fingerprint
	Surface					signals of
	enhanced Raman spectroscopy	Photons/Photons	~ 2 m	>10 nm	Single molecule	molecules and for the investigation
	(SERS)					of surface
						reactions
						intermediates
						Elemental
	Low-energy					composition
	ion scattering	Ions/Ions	~ 5 m	∼10 nm	∼0.2 nm	at the outer
	(LEIS)					atomic layer
						of a sample
	Conversely in Auger Electron Spectroscopy (AES), electrons are used
	both a probe and a detected particle, whereas in Raman spectroscopy, the probe and
	the detected particles are photons. Recent advances in Raman spectroscopy led to
	surface enhanced Raman spectroscopy (SERS), a powerful detection technique with

Time-of- flight secondary ion mass spectrometry (ToF-SIMS)

					Surface
					elemental
	Ions/Ions	~60 nm	1 nm	<1 nm	composition
					and isotope
					distributions

) a ν 0 at 18.8 T (MHz) b Q (fm 2 ) c

  ,65 Q is the nuclear quadrupolar moment, eQ, divided by the elementary charge e = 1.60210-19 C.

	Nucleus NA (%1 Spin H 1/2 99.99	800.00	-
				H	1	0.01	122.81	0.286
	13 C	1/2	1.07	201.16	-
	6 Li	1	7.59	117.75	-0.0801
	7 Li	3/2	92.41	310.96	-4.010
	11 B	3/2	80.4	256.71	4.059
	14 N	1	99.63	57.81	2.044
	15 N	1/2	0.37	81.09	-
	17 O	5/2	0.04	108.45	-2.558
	17 F	1/2	100.00	752.75	-
	23	Na	3/2	100.00	211.61	10.400
	25	Mg	5/2	10.00	48.97	19.940
	27 Al	5/2	100.00	208.46	14.660
	29 Si	1/2	4.68	158.94	-
	31 P	1/2	100.00	323.85	-
	35 Cl	3/2	75.76	78.39	-8.165
	39 K	3/2	93.26	37.33	5.850
	47 Ti	5/2	7.44	45.11	30.200
	49 Ti	7/2	5.41	45.12	24.700
	67	Zn	5/2	4.10	50.05	15.000
	95	Mo	5/2	15.70	52.14	-2.200

a Natural abundance. b Larmor frequency at 18.8 T. c

  56,181 Nevertheless, high concentrations of PA can induce paramagnetic quenching of surface NMR signals and hence, decreases the absolute NMR sensitivity. In DNP SENS experiments, low concentrations of PA are preferred, especially for long polarization transfers. 181 Nevertheless, PAs can be physisorbed onto surfaces, thus, the concentration of the PA must be optimized in function of the specific surface area of the samples. DNP can occur in solid-state NMR through several mechanisms. The three most important mechanisms for MAS DNP transfers at 𝐵 0  5 T are the solid effect (SE),

	Overhauser effect (OE) and cross effect (CE). 160 The SE and OE are the main
	mechanisms involved for DNP MAS using radicals with narrow EPR lines, for
	example trityl-and BDPA-type radicals. Conversely, polarization transfers with
	radicals with broad EPR line widths, which are the most commonly used radicals (e.g.
	nitroxide-based radicals), can be explained by the CE. 108,195
	The CE mechanism has been proven to be the most efficient one at high magnetic

189

Typically, concentrations of 16 mM for TEKPol in TCE and 10 mM for AMUPol in DNP juice, have been found to be optimal for the impregnation of high-surface area porous materials.

189

Cross effect mechanism fields. It involves two unpaired electrons (𝑆 𝑎 and 𝑆 𝑏 ), which are coupled by J-exchange or dipolar interactions, and one nucleus, typically 1 H. The polarization transfer is achieved if the EPR frequency difference between the two electrons matches the Larmor frequency of the polarized nucleus, typically protons: |𝜔 𝑒𝑎 -𝜔 𝑒𝑏 | = 𝜔 1𝐻 .

108

  The PRESTO and D-RINEPT recoupling schemes can be built from different composite pulses, which have been designed to improve their performances. Giovine et al. have demonstrated that the efficiency of PRESTO at 𝜈 R ≥ 60 kHz can be improved by using the R16 76 recoupling built from composite π pulses, such as 270090180.31 This composite pulse, which is used as a basic inversion element, was proposed by Madhu et al. to improve the robustness to RF-field and offset.35 Other

	recoupling schemes, such as SR4 1 2 and R12 3 5 built from 90+4590-4590+45 composite

1

H interactions.

pulses, have been proposed by Perras et al. recently to decrease the effect of 1 H-1 H interactions,

36

but up to now, have not been incorporated into the D-RINEPT transfers.

  CSA at fast MAS frequencies, νR  60-70 kHz.57 More recently, Giovine et al., used PRESTO with R16 3 2 recoupling built from a single rectangular pulse to transfer the 1 H polarization to

	5 4 , R14 6 5 , R16 7 6 , R14 8 5 , R18 8 7 ,
	R16 9 6 , R20 9 8 and R18 10 7 using composite (270090180) pulses, were applied for the
	measurement of 1 H

Table 3 .

 3 Selected R𝑁 𝑛

 |𝑚| = 2 SQ hetero-nuclear dipolar recoupling for νR = 20 kHz.

  , which have already been employed for the measurement of 1 H CSA and the transfer of 1 H polarization to half-integer quadrupolar nuclei at νR ≥ 60

	R14 6 5 and R16 7 6
	R	R𝑁 𝑛 	ϕ° 1/R	κ	κ/‖𝜅 {𝟏,𝟐} 𝐷𝐷 1 ×𝐷𝐷 2 ‖	2	κ/‖𝜅 {𝟏,𝟐} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖ 2	κ/‖𝜅 {𝟏,𝟐} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖ 2
		R22 2 7	57	5.5 0.178	162		7.12	17.58
	1800	R28 3 5	51 4.67 0.176	156		5.08	18.29
		R18 2 5	50	4.5 0.175	140		7.20	18.49

1

H-1 H interferences. Even if the amplitude of the cross-terms is inversely proportional to the MAS frequency (Table

5

), the amplitude of these terms is lower at νR = 20 than 62.5 kHz. The (270090180) element is less robust to 1 H-1 H interferences, but benefits from a high robustness to offset. The selected R𝑁 𝑛  symmetries for this element include

Table 4 .

 4 Selected R𝑁 𝑛  |𝑚| = 2 SQ hetero-nuclear dipolar recoupling with 45° ≤ ϕ ≤ 135° for νR = 62.5 kHz.

Table 5 .

 5 Selected R𝑁 𝑛  |𝑚| = 2 SQ hetero-nuclear dipolar recoupling built from single π pulses with 20° ≤ ϕ ≤ 160° and κ ≥ 0.15 for νR = 62.5 kHz.

	R		R𝑁 𝑛 	ϕ°	1/R	κ	κ/ ‖𝜅 {𝟏,𝟐} 𝐷𝐷 1 ×𝐷𝐷 2 ‖ 2	κ/ ‖𝜅 {𝟏,𝟐} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖	2	κ/ ‖𝜅 {𝟏,𝟐} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖	2
			R10 4 3	54	2.92 0.227	39.63		2.82	12.63
	90024090900	5 R14 6 R12 5 4	64.3 60	2.72 0.232 2.80 0.230	36.33 36.08		1.87 2.25	12.39 12.47
			R12 7 8	120	2.00 0.227	35.96		1.61	7.72
			R16 7 6	67.5	2.28 0.150	17.96		1.85	3.5010 10
	270090180	10 R16 7 R14 6 5	112.5 2.28 0.150 64.3 2.33 0.150	17.96 15.90		1.85 2.33	3.5010 10 3.5810 10
			R14 6 9	115.7 2.33 0.150	15.90		2.15	3.5810 10
			R10 4 3	54	1.88 0.186	16.70		2.97	15.07
	90-45904590-4	5 R18 7 R14 6 5	50 64.3	1.93 0.189 1.75 0.177	15.73 15.55		1.98 2.09	25.49 5.49
			R12 5 4	60	1.80 0.181	15.17		2.47	8.11
			R14 6 5	64.3	1.16 0.085	5.35		2.26	1.34
	1800	9 R14 6 R16 7 6	115.7 1.16 0.085 67.5 1.14 0.082	5.35 4.90		2.26 1.98	1.34 1.09
			R16 7 10	112.5 1.14 0.082	4.90		1.98	1.09
	R	R𝑁 𝑛 	ϕ /° 1/R	κ	κ/‖𝜅 {𝟏,𝟐} 𝐷𝐷 1 ×𝐷𝐷 2 ‖	2	κ/‖𝜅 {𝟏,𝟐} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖ 2	κ/‖𝜅 {𝟏,𝟐} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖ 2
		R28 5 4 25.7 2.75 0.163	24.42			3.34	26.42
	1800	R22 4 3 24.5 2.75 0.162	22.84			4.10	27.24
		R16 3 2 22.5 2.67 0.161	16.26			5.21	28.89

  1 H interactions, 1 H CSA and offset were calculated with the

	Euclidean norms ‖𝜅 {1,2} 𝐷𝐷 1 ×𝐷𝐷 2 ‖ 2	, ‖𝜅 {1,2} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖	2	and ‖𝜅 {1,2} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖	2

Table 6 .

 6 Selected R𝑁 𝑛  |𝑚| = 2 two-spin order hetero-nuclear dipolar recoupling.

	R	R𝑁 𝑛 	ϕ°	1/R	κ	κ/‖𝜅 {𝟏,𝟐} 𝐷𝐷 1 ×𝐷𝐷 2 ‖ 2	κ/‖𝜅 {𝟏,𝟐} 𝐶𝑆𝐴×𝐶𝑆𝐴 ‖	2	κ/‖𝜅 {𝟏,𝟐} 𝛿𝑖𝑠𝑜×𝛿𝑖𝑠𝑜 ‖	2
		R16	101	4.66 0.131	63.17	16.48		9.31
		R20 5 11	99	4.66 0.131	60.68	16.59		14.45
		R12	105	4.66 0.131	51.25	16.11		9.70
		R16	79	4.66 0.131	45.52	15.76		13.60
	90024090900	R28 7 10	64	4.66 0.131	44.55	14.06		11.98
		R20	81	4.66 0.131	44.30	15.95		14.46
		R12	75	4.66 0.131	43.91	15.40		12.83
		SR4	90	4.66 0.131	42.37	22.65		10.48
		R28 7 11	71	3	0.191	39.81	10.05		6.10
		R20	72	3	0.191	39.74	10.26		5.49
		R8 2 3	67.5	3	0.191	39.43	9.42		7.88
		R8 2	67.5	3	0.191	39.43	9.42		7.88
	90-45904590-4	R24 6 10	75	3	0.191	39.32	10.66		4.22
		R28 7 10	64.3	3	0.191	38.82	8.65		10.13
		R12	75	3	0.191	38.33	10.66		4.22
		SR4	90	3	0.191	19.95	19.48		1.33
		R24 6 11	82.5	4	0.212	33.12	25.46		8.6710 10
		R20	81	4	0.212	31.85	25.19		8.6710 10
		R20 5 11	99	4	0.212	31.85	25.19		8.6710 10
		R16	78.8	4	0.212	28.56	24.69		8.6710 10
	270090180	R16	101.2	4	0.212	28.56	24.69		8.6710 10
		R12	75	4	0.212	20.84	23.58		8.6710 10
		R12	105	4	0.212	20.84	23.58		8.6710 10
		SR4	90	4	0.212	35.21	149.93		8.6710 10
		R16	78.8	2	0.25	19.65	10.52		2.78
		R16	115.7	2	0.25	19.65	10.52		2.78
	1800	R12	75	2	0.25	18.9	9.89		3.74
		R12	105	2	0.25	18.9	9.89		3.74
		SR4	90	2	0.25	13.2	22.98		1.56

Table 7 .

 7 Comparison of the performances of 1 H →27 Al RINEPT-CWc and PRESTO transfers using various recouplings for AlO6 signal of γ-alumina at R = 20 kHz

	PRESTO /RINEPT	Recoupling	 /s	1/1,max /kHz	AlO6 a,b Δν0 c /kHz	Δν0/ν1	Δν1 d /kHz	Δν1/ ν1
	RINEPT	2 (tt) SR4 1 R12 3 5 (tt)	400 400	160 160	1.00 1.00	110 110	0.68 0.68	> 100 e > 100 e	> 0.62 > 0.62
	PRESTO R22 2 7 (1800)	400	110	0.73	30	0.27	39	0.35
	RINEPT SR4 1 2 (270090180)	400	80	0.63	50	0.63	44	0.55
	PRESTO R18 2 5 (1800)	400	90	0.61	28	0.31	27	0.30
		R12 3 5 (270090180)	400	80	0.50	40	0.50	35	0.44
		SR4 1 2 (90-45904590-45) 400	63	0.42	14	0.22	14	0.22
		SR4 1 2 (1800)	400	45	0.40	17	0.38	24	0.53
	RINEPT	R12 3 5 (1800)	400	45	0.35	10	0.22	15	0.33
		R12 3 5 (90-45904590-45) 400	66	0.35	11	0.17	18	0.27
		SC2 1 0 (C)	400	63	0.31	14	0.22	45	0.71
		C6 3 0 (C')	400	66	0.28	10	0.15	40	0.60

a AlO6 signal normalized to that with 1 H→

27

Al RINEPT-CWc-SR4 1 2 (tt). b The relative error bars were determined from the S/N. For the AlO6 signal intensity, they are equal to  0.03. c FWHM of the robustness to offset. d FWHM of the robustness to RF-field. e Only a lower bound of RF-field could be determined due to probe RF specifications (Fig.

20

).

Table 8 .

 8 Comparison of the performances of 1 H →

	PRESTO /RINEPT	Recoupling	 /s	1/1,max /kHz	AlO6	Intensity a AlO5	AlO4	Δν0 /kHz	Δν0/ν1	Δν1 /kHz	Δν1/ν1
					b	c	d
	RINEPT	2 (tt) SR4 1 R12 3 5 (tt)	800 800	208 208	1.00 0.99	1.00 0.99	1.00 0.98	120 120	0.58 0.58	-e -e	-e -e
	PRESTO R22 2 7 (1800)	600	114	1.54	1.07	0.67	26	0.23	38	0.33
	RINEPT SR4 1 2 (270090180)	800	77	0.72	0.65	0.67	45	0.58	48	0.62
	PRESTO R18 2 5 (1800)	600	94	1.45	1.03	0.62	25	0.27	26	0.28
		R12 3 5 (270090180)	800	77	0.58	0.50	0.48	46	0.60	36	0.47
		SR4 1 2 (1800)	600	43	0.64	0.45	0.36	14	0.33	23	0.53
		SR4 1 2 (90-45904590-45) 800	61	0.56	0.43	0.25	16	0.26	20	0.32
	RINEPT	SC2 1 0 (C)	800	68	0.54	0.41	0.24	18	0.26	52	0.73
		R12 3 5 (90-45904590-45) 600	61	0.43	0.30	0.21	8	0.13	18	0.29
		R12 3 5 (1800)	600	45	0.34	0.28	0.21	8	0.18	18	0.40
		C6 3 0 (C')	600	68	0.52	0.36	0.21	10	0.15	42	0.61

27

Al RINEPT-CWc and PRESTO transfers with AlPO4-14 at R = 20 kHz.

a Intensities of AlO6, AlO5 and AlO4 resonances normalized to their intensities with 1 H →

27

Al RINEPT-CWc-SR4 1 2 (tt).

b

The absolute error on AlO6 signal intensity is  0.02.

c

The absolute error on AlO5 signal intensity is  0.03.

d

The absolute error on AlO4 signal intensity is  0.01. e FWHM of the robustness to RF-field was not measured for RINEPT-SR4 1 2 (tt) and -R12 3 5 (tt) (Fig.

24

).

Table 10 .

 10 Comparison of the performances of 1 H →27 Al RINEPT-CWc and PRESTO transfers using various recouplings for AlPO4-14 at R = 62.5 kHz.

	PRESTO /RINEPT	Recoupling	 /s	1/1,max /kHz	AlO6 b	Intensity a AlO5 c	AlO4 d	Δν0 /kHz	Δν0/ν1	Δν1 /kHz	Δν1/ν1
		SR4 1 2 (tt)	480	208	1.00	1.00	1.00	48	0.23	-e	-e
		R12 3 5 (tt)	480	208	1.07	1.00	1.06	44	0.21	-e	-e
	RINEPT	SR4 1 2 (270090180)	480	208	1.05	0.95	0.97	85	0.41	90	0.43
		R12 3 5 (270090180)	480	208	0.91	0.84	0.91	80	0.38	68	0.33
		R16 7 6 (270090180)	672	146	1.71	1.21	0.76	80	0.55	50	0.34
	PRESTO	R14 6 5 (270090180)	576	146	1.72	1.27	0.76	86	0.59	45	0.31
		SR4 1 2 (1800)	480	129	0.84	0.79	0.75	48	0.37	64	0.49
	RINEPT	R12 3 5 (1800)	480	136	0.72	0.67	0.74	18	0.13	54	0.40
		R22 4 3 (1800)	512	157	1.47	1.18	0.69	60	0.38	20	0.33
	PRESTO	R16 3 2 (1800)	480	147	1.17	0.83	0.52	64	0.44	20	0.31
		R12 3 5 (90-45904590-45) 256	190	0.48	0.27	0.14	32	0.17	75	0.39
		C6 3 0 (C')	256	193	0.47	0.28	0.14	28	0.15	78	0.40
	RINEPT	SR4 1 2 (90-45904590-45) 256	196	0.48	0.14	0.14	36	0.18	77	0.39
		SC2 1 0 (C)	256	188	0.53	0.25	0.14	44	0,23	80	0.43
	a Intensities of AlO6, AlO5 and AlO4 resonances normalized to their intensities with 1 H→ 27 Al RINEPT-
	CWc-SR4 1 2 (tt). b The absolute error on AlO6 signal intensity is  0.04. c The absolute error on AlO5
	signal intensity is  0.06. d The absolute error on AlO4 signal intensity is  0.02. e FWHM of the
	robustness to rf-field was not measured for RINEPT-SR4 1 2

Table 11 .

 11 Distances between the different H atoms and their closest Al neighbours in the structure of isopropylamine templated AlPO4-14 determined from X-ray diffraction.73 The H and Al atoms are numbered according to the cif file.

	H	Al	rHAl /Å
		Al4O6	2.496
	H1 (OH)	Al4O6 Al1O5	2.499 2.503
		Al2O4	4.299
	H2 (NH3)	Al4O6 Al2O4	3.069 3.779
	H3 (NH3)	Al3O4 Al4O6	3.778 3.960
	H4 (NH3)	Al2O4 Al1O5	3.479 3.801
	H5 (CH)	Al2O4 Al1O5	3.737 4.850
	H6 (CH3)1	Al1O5 Al3O4	3.655 4.594
	H7 (CH3)1	Al3O4 Al1O5	4.082 4.320
	H8 (CH3)1	Al2O4 Al3O4	3.772 4.651
	H9 (CH3)2	Al4O6 Al3O4	3.888 4.124
	H10 (CH3)2	Al4O6 Al3O4	3.509 4.502
	H11 (CH3)2	Al4O6 Al3O4	3.970 4.048

Table 12 . 1

 121

	1 H dipolar interactions are larger in this

  8,9,24

  With 𝐹 𝑗𝑘 . = 𝑏 𝑗𝑘 sin(2𝛽 𝑗𝑘 ) cos(𝛾 𝑗𝑘 ) where 𝑏 𝑗𝑘 is the dipolar coupling constant and (𝛽 𝑗𝑘 , 𝛾 𝑗𝑘 ) are the Euler angles relating the 𝑗𝑘 inter-nuclear vector in the MAS rotor-

	fixed frame. 𝐾 = 3 (8√2) ⁄	is the scaling factor of the homonuclear dipolar interaction.
	All these sequences results in a Hamiltonian containing both DQ (𝐼 + 𝑗 𝐼 +	𝑘 +
	𝐼 -𝑗 𝐼 -	𝑘 ) and ZQ (𝐼 -𝑗 𝐼 +	𝑘 -4𝐼 𝑗𝑧 𝐼 𝑘𝑧 ) operators, which can interfere in multi-spin
	systems. The sequence in Fig. 32a requires a ZQ recoupling, which can be obtained by
	filtering the DQ terms with a 𝑀 ≥ 3 super-cycling, and the sequences are then noted
	(𝑆) 𝑀≥3 . The super-cycled Hamiltonian is thus simplified with respect to Eq. 2:
		𝐻 𝑆(𝑀≥𝟑) 𝑗𝑘	= 0.5𝐾. 𝐹 𝑗𝑘 . { 𝐼 + 𝑗 𝐼 -	𝑘 + 𝐼 -𝑗 𝐼 +	𝑘 -4𝐼 𝑗𝑧 𝐼 𝑘𝑧 }
					8,25
	𝐻 𝑆 𝑗𝑘 = 0.5𝐾. 𝐹 𝑗𝑘 . {𝐼 + 𝑗 𝐼 +	𝑘 + 𝐼 -𝑗 𝐼 -	𝑘 + 𝐼 + 𝑗 𝐼 -	𝑘 + 𝐼 -𝑗 𝐼 +	𝑘 -4𝐼 𝑗𝑧 𝐼 𝑘𝑧 }	(2)

Table 14 .

 14 Aluminum species present in AlPO4-14 reported by Sasaki et al.46

	Al site	iso (ppm)	CQ (MHz),	Q
	Al IV 1	42.7	1.72	0.57
	Al IV 2	43.5	3.90	0.83
	Al V	27.1	5.61	0.93
	Al VI	-1.3	2.55	0.67
	3.6.2.1 SQ-SQ
	The (SR2 2 1 ) 4 and (SR2 4 1 ) 4 schemes were tested with the SQ-SQ sequences on
	AlPO4-14. The optimized results were very similar. In Fig. 46 the best 2D spectrum
	recorded with the second scheme is shown. All cross-peaks are observed, even the
	Al IV -Al VI ones with a large chemical shift difference of 10 kHz and a small dipolar
	coupling of bIV-VI = 52 Hz related to the long inter-nuclear distance of 5.4 Å.

  3,4

Table 15 .

 15 Comparison of conventional catalysts with DFNS/BN and DFNS/B2O3 for BN is and how the BN and B2O3 phases are anchored on the SiO2 support. The aim of this study is to correlate the atomic structures to the catalytic performance.

		propane dehydrogenation. 6,14
	Name of catalyst	Temperature (°C)	WHSV -1 (kgcat mol C3H8 -1 )	Conversion (%)	Productivity h -1 ) (golefin gcat -1	Reference
	DFNS/BN	490	1.47	25.5	1.9	This work
	DFNS/B2O3	490	1.47	24.7	1.0	This work
	BOS-10	450	0.18	14.8	1.2	78
	BNNT	490	2-4	16.5	~4.1	5
	h-BN	490	15-40	14.3	~0.4	5
	V-SiO2	490	5-15	8.4	~0.5	5
	As seen in Figs. 55 and 56, DFNS/BN displays higher catalytic activity than h-BN,
	while DFNS/B2O3 also shows catalytic activity for ODH of propane, although with
	lower selectivity and productivity than DFNS/BN. This dissertation seeks to
	investigate the structures of both DFNS/BN and DFNS/B2O3 by solid-state NMR
	through a combination of 1D and 2D experiments, in order to understand what the

  The samples were spun at a MAS rate, R = 10 kHz. The initial π/2 pulse lasted 2 μs and was followed by a CP contact time equal to 8 ms. During the CP transfer, the RF nutation frequency on the29 Si channel was constant and equal to 50 kHz, while the 1 H nutation frequency was linearly ramped from 36 to 40 kHz. The

	4.4.3 Experiments at 18.8 T
	All experiments at B0 = 18.8 T (0 = 800 MHz for 1 H) were performed with a
	narrow-bore magnet equipped with a Bruker AVANCE NEO console. One-
	dimensional (1D) 1 H and 11 B experiments were acquired at R = 20 kHz, using 3.2 mm
	double-resonance 11 B-31 P (featuring an MgO stator block to prevent 11 B background
	signal) and 3.2 mm double-resonance HX probes for 11 B and 1 H experiments,
	respectively. 1 H NMR spectra were acquired by averaging 16 transients separated by
	a recycle interval of 3 s, using the DEPTH pulse sequence for probe background
	suppression, 31 with ν1 ≈ 93 kHz. Quantitative 11 B NMR spectra were acquired by using
	single-pulse NMR experiments with a pulse length of 0.5 μs and an RF field strength
	of 71 kHz, corresponding to a short tip angle. 32 The quantitative 11 B NMR spectra were
	acquired by averaging 2 (pure hexagonal (h) BN), 4096 (as-prepared DFNS/BN and
	DFNS/B2O3) or 16 (DFNS/BN after catalysis) transients, separated by a recycle
	interval of 1 (as-prepared DFNS/BN and DFNS/B2O3), 0.8 (DFNS/BN after catalysis)
	1 H→ 29 Si CP MAS NMR experiments were recorded at B0 = 9.4 T (0 = 400 MHz
	for 1 H) with a wide-bore magnet equipped with a Bruker AVANCE II console, using a
	3.2 mm triple-resonance HXY magic angle spinning (MAS) probe operated in double
	resonance mode. 1 H→ 29 Si CP MAS spectra were recorded using recycle intervals of 1.95 s (as-prepared
	DFNS/BN), 17 s (as-prepared DFNS/B2O3) or 27.29 s (DFNS/BN after catalysis) and
	were the result of averaging 4096 (as-prepared DFNS/BN and DFNS/B2O3) or 7200
	transients (DFNS/BN after catalysis). SPINAL 1 H decoupling 28 (∼125 kHz) was
	applied during acquisition. The Hartmann-Hahn condition 29,30 was calibrated using a
	powdered sample of octakis(trimethylsiloxy)silsesquioxane (Q8M8).

  52,54,57,58,62 The 11 B spectrum being quantitative, the integrated intensities of the different signals indicates that BN sheets contains 37% of boron atoms, the remaining ones forming boron oxide phases as the result of the oxidation of

	BN sheets. The spectra of DFNS/BN after catalysis and as-prepared DFNS/B2O3 (Fig.
	57e and f) appear to be relatively similar. Both display lineshape components with
	centres of gravity at ~14 ppm attributed to BO3 sites. This line is slightly broader for
	the DFNS/BN sample after catalysis. The signal at 26 ppm of BN3 and BN2O sites has

a very low intensity for the DFNS/BN sample after catalysis, which further confirms the oxidation of BN. As expected, this signal is not observed in the

11

B spectrum of as-prepared DFNS/B2O3. Spectra of DFNS/BN after catalysis and as-prepared

  1 H spectrum. The 11 B signals at ~0 ppm of BO4 sites also exhibit weaker correlations with 1 H signals at 4 and 6.7 ppm, which indicates the close proximity between BO4 environments and hydroxyl groups on the edge of BN sheets as well as BO2OH sites. The 11 B signal at 26 ppm exhibits further reduced correlations with 1 H signals at 6.7 ppm, which could stem from BN2(OH) sites at the edge of BN sheets, as well as at 4 ppm, which indicates the close proximity between BO2OH sites and BN sheets.

Table 16 .

 16 Data obtained from the deconvolutions of the 1 H→29 Si CP MAS NMR spectra (using the Gaussian/Lorentzian model in DMFit63 ). Parameters include: peak assignment;27,[64][65][66][67][68][69]71,72,75,76 chemical shift, ; full width at half maximum, FWHM; absolute integrated area (%).

	Sample	Assignment	 (ppm)	FWHM (ppm) Area (%)
		SiN2O2	-73.2	19.1	6.2
	As-prepared	Q 2	-91.1	14.6	14.6
	DFNS/BN	Q 3	-101.0	12.1	34.2
		Q 4	-109.5	11.8	45.0
	DFNS/BN after catalysis	Q 2 Q 3 Q 4	-98.3 -103.7 -110.9	5.1 7.6 8.1	9.3 24.4 66.3
	As-prepared DFNS/B2O3	Q 2 Q 3 Q 4	-93.1 -101.0 -110.4	6.5 8.2 9.9	3.6 47.1 49.3

  ). A key difference between DFNS/BN after catalysis and DFNS/B2O3 is the larger relative intensities of BO(OH)2 and BO2OH(ring) sites as compared to BO3(ring) for the first sample, which is consistent

	Sample	Species	1 (ppm)
		BN3	30.5
		BN2OH	28.6
		BN2O	27.2
	As-prepared DFNS/BN	BO2OH(ring) BO2OH(non-ring) BO3(ring)	18.4 17.2 16.3
		B(OSi(Q 3 ))3OH	0.1
		B(OB)4	-1.0
		B(OSi(Q 4 ))4	-1.9
		BO(OH)2	20.5
		BO2OH(ring)	19.0
	As-prepared	BO2OH(non-ring)	17.2
	DFNS/B2O3	BO3(ring)	15.8
		BO3(non-ring)	14.4
		BO4	a
		BO(OH)2	21.1
		BO2OH(ring)	19.1
	DFNS/BN after	BO2OH(non-ring)	17.1
	catalysis	BO3(ring)	15.7
		BO3(non-ring)	14.1
		BO4	a

Table 18 .

 18 Main plausible local environments in as prepared DFNS/BN, DFNS/BN after catalysis and as-prepared DFNS/B2O3 based on solid-state NMR data.

Si et 11 B), il a été montré que la phase BN se transforme en B2O3 (oxydation) au cours de la réaction, conduisant à une plus grande fraction de groupement BOH impliqués dans des liaisons hydrogène à la surface des matériaux du DFNS/BN oxydé que des nanoparticules DFNS/B2O3. Cette différence structurelle pourrait expliquer la plus grande sélectivité envers le propène du DFNS/BN oxydé.

Acknowledgements

work with him. I would also like to thank Frédérique Pourpoint, for co-supervising my thesis, for her time and advice. I also would like to acknowledge the funding received towards my PhD from the project funded by the European Soft Matter Infrastructure EUSMI (H2020 INFRAIA 2016). Thanks also to Laurent Delevoye, director of the RMN2I team, for his support, Institute in India, for his collaboration with the DFNS/BN project. From Japan, thanks to Yu Tsutsumi, from Bruker Japan for his collaboration with the D-RINEPT simulations, and Nghia T. Duong from RIKEN Institute for his help with the D-HOMCOR project. Special thanks to Hiroki Nagashima from the AIST Institute in Japan

and τ mix = 3.2 ms. These slices show that the B 34 (B III -B IV ) and B 43 (B IV -B III ) crosspeaks are hardly observable with (SR2 4 1 ) 4 , and especially (SR2 8 1 ) 4 .

DQ-SQ

The SRN n 1 schemes shown in Fig. 33a-d mostly differ in their behavior with respect to rf-inhomogeneity and offset due to the use of a small rf-field to fulfill the HORROR condition. From experiments and simulations, M. Edén and co-workers have made several conclusions concerning DQ-SQ experiments with SRN n 1 recoupling applied to half-integer quadrupolar nuclei. 11 The first one is that 'an enhanced compensation for offsets can be observed following the order SR2 2 1 < SR2 4 1 ≈ SR4 4 1 . 8,9 The second point is that SR2 2 1 is the most robust scheme to rf inhomogeneity, whereas SR2 4 1 is the most robust to offset, but exhibits a low efficiency. The third point is that bracketed recoupling schemes are 'favored, although their more rapid DQ excitation dynamics is sometimes a disadvantage'. The fourth conclusion is that 'the strong sensitivity of SR2 2 1 to the coupling of offset and rf-inhomogeneity restricts its application to  smaller than a few kHz, yet encompassing many 11 B and 23 Na applications, and that SR2 4 1 is recommended for larger Δ values, which must remain lower than 10 kHz'.

Concerning the BR2 2 1 scheme, Edén and co-workers concluded that 'provided τ mix is sufficient, its robustness to offsets and rf-inhomogeneity is the best'. From numerical simulations, the dipolar scaling factor of BR2 2 1 was found intermediate between bracketed or not bracketed sequences in Fig. 33a-d. 11 However, the bracketed [BR2 2 1 ] recoupling scheme has never been tested up to now.

Following these conclusions, the 2D 11 A great advantage of DQ-SQ over SQ-SQ is that most 2D experiments can be easily optimized in a 1D way, with t1 = 0, because all peaks are related to through-space proximities and are thus meaningful. It must be first noted that the minimum mixing step of these four DQ-SQ sequences, Δ𝜏 𝑚𝑖𝑥 = 200 s at R = 20 kHz, is much shorter than with the previous (SR2 n 1 ) 4

recoupling employed in SQ-SQ D-HOMCOR schemes. Furthermore, a ratio of two was found between the optimum recoupling times for the bracketed and un-bracketed versions: τ mix,opt = 200 and 400 s, for [SR2 (not shown). In Fig. 43, the robustness of the three most efficient sequences versus rffield and offset is presented.

In Fig. 43a it is observed that the experimental optimum rf-fields are slightly smaller than the theoretical value of 1 = 5 kHz (Eq. 1). Moreover, the two bracketed schemes give similar rf-robustness, higher than their un-bracketed version. This is the opposite in Fig. 43b: the two bracketed versions give similar robustness to offset, but the highest one is achieved for BR2 The 2D spectra result from averaging 64 transients using a relaxation delay RD = 1 s for each of 120 t1-increments, leading to an experimental time Texp = 45-50 min. We employed optimal rf field strength equal to 1 = 3.7 and 4.1 kHz for BR2 2 1 / SR2 2 1 and In Fig. 50, the sensitivity and the robustness to rf-field and offset of the BR2 = 4800 Hz). This efficiency only reaches a few percent in agreement with previous simulations. 15 The S/N ratio of these results is smaller than the one previously published on the same sample with the same spectrometer. 14 However, in this previous study an initial hyper-secant CT enhancing was used and the experiments were recorded using a 4 mm rotor instead of a 3.2 mm one, like in this dissertation. On one hand, using a 4 mm rotor allows increasing the S/N due to the larger sample amount, but on the other hand, it limits the spinning speed to ca. 15 kHz. For 27 Al DQ-SQ experiments performed at B0 = 18.8 T, a MAS frequency of 15 kHz requires a rescaling of the evolution time t1 using the -pulse displayed in blue in Fig. 32b. 14 Moreover, it has been shown that the offset robustness increases with the MAS frequency, since the rf-field of the recoupling is proportional to the MAS frequency (compare Figs. 3c and d of ref 14 ). A better offset robustness is indeed observed at R = 20 kHz (Fig. 40b), compared to 13.89 kHz (Fig. 3b of ref. 15 ). In Fig. 51, it is observed that the cross-peak intensities for DQ-SQ D-HOMCOR scheme using BR2 2 1 are always equal to or larger than those observed with SQ-SQ D-HOMCOR using (SR2 4 1 ) 4 recoupling.

Therefore, as a conclusion, for 27 Al (I = 5/2) nuclei, the DQ-SQ D-HOMCOR variant using BR2 2 1 sequence is recommended since it yields cross-peaks of higher CHAPTER 4

4.5 RESULTS AND DISCUSSION

Probing 1 H and 11 B environments and their proximities

The 1D 1 H and 11 B spectra on DFNS/BN (as-prepared and after catalysis) and asprepared DFNS/B2O3 are shown in Fig. 57. The 1 H NMR spectrum of as-prepared DFNS/BN is dominated by a peak at 6.8 ppm and also exhibits a broad resonance with a maximal intensity at 4.3 ppm extending down to 0 ppm (Fig. 57a). The peak at 6.8

ppm is assigned to hydroxyl groups on the edge of BN sheets. [44][45][46] This signal can also subsume contributions from the NB2H sites of the armchair edges of BN sheets as well as BO3OH moieties since the reported isotropic chemical shifts, δiso, of these sites are ca. 6 and 8 ppm, respectively. 15,46 The broad signal centred at 4.3 ppm subsumes the contributions of adsorbed water, [47][48][49][50] BO2OH sites having δiso values ranging from 5 to 2.7 ppm depending on the formation of hydrogen bonds and the nature of the second neighbour, B or Si, [14][15][16]51,52 the NB2H sites of the zigzag edges of BN sheets resonating at ca. 3 ppm, 46 and silanol groups with δiso values ranging from 3 to 1.6 ppm depending on the formation of hydrogen bonds and the nature of the second neighbour, B or Si. 14- 16,47,48,51,52 The 1 H NMR spectrum of DFNS/BN after catalysis shown in Fig. 57b displays a broad peak centred at 4.4 ppm, which subsumes the contribution of adsorbed water, BO2OH sites and silanol groups. The peak at 0.1 ppm arises from coke formation or the surface residual organic species. The marked reduction in the intensity of the resonance at 6.5 ppm indicates the conversion of BN into B2O3. This fact is in agreement with the results obtained with XPS and powder X-ray diffraction (PXRD)

(not shown) and further confirms the conversion of BN into B2O3. The 1 H spectrum of as-prepared DFNS/B2O3 shown in Fig. 57c exhibits peaks at 2.9 and 1.7 ppm, which are ascribed to BO2OH and isolated silanol sites, respectively. Silanol groups close to B atoms with δiso values ranging from 2.2 to 2.7 ppm can contribute to the 1 H signal. 16,[52][53][54] The peak at 0 ppm is due to mobile residual organic species. The higher chemical shift of BO2OH signal for DFNS/BN after catalysis than for as-prepared DFNS/B2O3 could result from the larger amount of hydrogen-bonded BO2OH sites. 15

General conclusions

This PhD work focused on the development of novel solid-state NMR methods to detect half-integer quadrupolar nuclei near surfaces of materials. These methods are notably based on hetero-and homo-nuclear dipolar recoupling schemes. The main achievements and new insights can be summarized as follows.

Novel symmetry-based heteronuclear recoupling schemes built from single and composite π pulses have been introduced for PRESTO and D-RINEPT sequences.

Their performances have been tested for the 1 H→ 27 Al transfers at R = 20 and 62.5 kHz in two samples, notably γ-Al2O3 and AlPO4-14, which exhibit low and moderate 1 H-1 H dipolar interactions. As a result, 1 H magnetization can be efficiently transferred to 27 Al nuclei using D-RINEPT with 𝑆R4 1 2 (270090180) and PRESTO with R22 2 7 (1800) at R = 20 kHz. These new recouplings have been compared to D-RINEPT-SR4 1 2 with adiabatic pulses, which still produce efficient and robust transfers, but requires large rf fields that may not be compatible with the specifications of most MAS probes.

Therefore, the introduced recouplings will prove useful in indirect DNP experiments at R = 20 kHz and high-magnetic fields to enhance the signal of quadrupolar nuclei via protons. On the other hand, transfers at high MAS are also required for many NMR experiments, including indirect DNP as well as HETCOR experiments between protons and half-integer quadrupolar nuclei. D-RINEPT with SR4 1 2 (270090180) and PRESTO with R16 7 6 (270090180) produce robust and efficient transfers at R = 62.5 kHz.

These recouplings were demonstrated to be useful to acquire 2D 1 H- 27 Al HETCOR spectra, allowing for the correlation of the NMR signals of protons and quadrupolar nuclei. At both MAS frequencies, the RINEPT and PRESTO techniques complement each other since the latter is dipolar truncated, whereas the former is not.

Consequently, the RINEPT sequences must be chosen to observe simultaneously protonated and unprotonated sites, whereas the PRESTO schemes can be employed for the selective observation of quadrupolar nuclei in proximity to protons. Further research might explore the use of these techniques using the composite (270090180) pulse to detect other challenging quadrupolar nuclei with low receptivity such as 17 O (small N.A), and/or 47,49 Ti, 67 Zn and 33 S (low gyromagnetic ratio γ).

Synthesis of catalysts

We synthesized MoO3/TiO2 with TiO2(rutile) support and various content of MoO3 (see Table 19). Other MoO3/TiO2 samples with TiO2 anatase and P25 and various content of MoO3, will be synthesized as well. The surface area and porous volume of the TiO2 supports were determined using N2 adsorption (see Table 20). The MoO3/TiO2(rutile) catalysts (Table 20) were prepared by impregnation of TiO2(rutile) with an aqueous solution containing a known quantity of (NH4)6Mo7O24•4H2O (HMA). The samples were dried at 80°C for 48 h, followed by calcination in air at 450°C for 3 h.

Raman

Raman spectroscopy allows easily distinguishing the MoO3 particles and polyoxomolybdates in these materials since they give distinct Raman resonances (see ;d6 probe dead time (should be D6=DE) ;d7 RF offset delay ;d5 Delay after last recoupling for Tr/2 ;d8 Delay after last recoupling for Tr/4 ;pl1 p1 and p2 power level ;pl12 Heteronuclear dipolar decoupling ;pl19 Presat pulse ;pl2 not used ;pl20 Presat pulse ;pl21 p3 and p4 power level ;pl22 initial spin lock ;pl33 CW23 decoupling ;pl43 CW45 decoupling ;pl44 CW67 decoupling ;pl11 dipolar recoupling power (sr4/sfam) ;spnam5 dipolar recoupling shape pulse ;sp5 power for recoupling shape ;p16 : requested recoupling time ;p17 : actual recoupling time ;l11 sr4/sfam repetition ;cnst30: Tanh/tan offset ;cnst31: spinning speed in Hz ;cnst3: Tanh/tan shape pulse step (ns) ;p1 90 degree pulse for X ;p2 180 degree pulse for X ;p3 90 degree pulse for 1H ;p4 180 degree pulse for 1H ;p6 pulse of the recoupling sequence ;p19 presat pulse for 1H ;p20 presat pulse for X ;p22 initial spin lock for Tr/2 ;p23 initial spin lock for Tr/4 PRESAT2(f2) d1 rpp16 rpp17 rpp14 rpp15 ; not necessary to use different phases and reset but... PRESAT1(f1) (10u pl21):f2 (10u pl1 ph2):f1 (p3 ph1):f2 void apod(char *curdat) ; void fetchparmod(char *curdat,int *wdw, int *phmod, int *bcmod, int *ftmod, int *memod) ; void zeroparmod(char *curdat) ; void storeparmod(char *curdat,int wdw, int phmod, int bcmod, int ftmod, int memod) ; void ft2(char *curdat); void ift2(char *curdat); void xfb(char *curdat) ; int parmode; int mq, nspin, wdw2, bcmod, phmod, stsr2, stsi1, stsi2, fnmode, mc2; int option1=0, option2=0; double ratio, final, ph0, ph1, ph2; float Noise, spin, offset1, offset2, off2, off1, sr2, sr1; char nucl [80], yes [20],

pulprog [80], ti [80], ShearOpt[PATH_MAX];

// select current dataset // ====================== GETCURDATA;

(void) strcpy (yes,"yes");

// is-it a 2D spectrum and taken in States, States-TPPI or EA? // ===================== FETCHPARS("PPARMOD",&parmode); if ( parmode != 1 ) { STOPMSG("Program is only suitable for 2D data!");} FETCHPAR1S("FnMODE",&fnmode); if ( fnmode == 0 ) { FETCHPAR("MC2",&mc2); if ( mc2 != 3 && mc2 != 4 && mc2 != 5 ) STOPMSG("mc2 must be States, States-TPPI, or Echo-Antiecho!"); } else if ( fnmode != 4 && fnmode != 5 && fnmode != 6) STOPMSG("FnMode must be in States, States-TPPI, or Echo-Antiecho!"); // if auto selected, don't aks for ABS2 and additional F1 shift // ============================================================ // automatic baseline correction for MQMAS and STMAS ? // =================================================== FETCHPARS("USERP1",yes); GETSTRING("Apply ABS2 ? : ",yes); // additional F1 shift ? // ===================== f1=0.; FETCHPAR1S("NOISF1",&Noise); FETCHPARS("TI",ti); if (!strcmp(ti,"shearing done")) f1=Noise; if (mq != 1) GETFLOAT(" F1 shift in ppm ? : ",f1); // init some variables // =================== FETCHPAR("WDW",&wdw2) STOREPAR("WDW",0) ratio=2.; final=1.; GETDOUBLE(" Enter initial slope ratio :",ratio); GETDOUBLE(" Enter finalslope ratio :",final);

// ignore all strip parameters // =============================================== FETCHPAR("STSI",&stsi2); FETCHPAR1("STSI",&stsi1); STOREPAR("STSR",0); STOREPAR("STSI",0); STOREPAR1("STSR",0); STOREPAR1("STSI",0); if (stsi2 != 0 || stsi1 != 0) Proc_err(0,"Strip-FT not yet implemented,\nparameters disabled"); // calculate F2 transform // ====================== SETCURDATA XPY("apod2D") XF2 // AUERR=CPR_exec( "xf2 raw",WAIT_TERM); if ( yes[0]=='y') {ABS2 XHT2} int PH_mod2; FETCHPAR("PH_mod",&PH_mod2) STOREPAR("PH_mod",0) calculateShear(curdat, ratio, f1, &ph0, &ph1, &ph2) ; newdoshear(curdat, ph0, ph1, ph2); //Apply apodisation // ift2(curdat); //apod(curdat); // ft2(curdat); // shift time domain back to t=0 with final R ratio newdoshear(curdat,-ph0,-ph1/ratio*final,0); // direct shearing //newdoshear(curdat,/*ph0=*/0,ph1/*/ratio*(ratio-final)*/,0); // store TI to recall shearing and shifting // ======================================== STOREPARS("TI","shearing done"); STOREPAR1S("NOISF1",f1); STOREPARS("USERP1",yes); } void fetchparmod(char *curdat,int *wdw, int *phmod, int *bcmod, int *ftmod, int *memod) { FETCHPAR("WDW",wdw); FETCHPAR("BC_mod",bcmod); FETCHPAR("PH_mod",phmod); FETCHPAR("FT_mod",ftmod); FETCHPAR("ME_mod",memod); } void zeroparmod(char *curdat) { STOREPAR("WDW",0); STOREPAR("BC_mod",0); STOREPAR("PH_mod",0); STOREPAR("FT_mod",0); STOREPAR("ME_mod",0); } void storeparmod(char *curdat,int wdw, int phmod, int bcmod, int ftmod, int memod) { STOREPAR("WDW",wdw); STOREPAR("BC_mod",bcmod); STOREPAR("PH_mod",phmod); STOREPAR("FT_mod",ftmod); STOREPAR("ME_mod",memod); } void apod(char *curdat) { int wdw, phmod, bcmod, ftmod, memod; float lb=-0.; fetchparmod(curdat, &wdw, &phmod, &bcmod, &ftmod, &memod) ; zeroparmod(curdat); STOREPAR("WDW",2); STOREPAR("GB",0.5); FETCHPAR("LB",&lb); if (lb>0.0) {lb=-lb; STOREPAR("LB",lb);} XTRFP2; storeparmod(curdat, wdw, phmod, bcmod, ftmod, memod) ; } void ft2(char *curdat) { int wdw, phmod, bcmod, ftmod, memod; fetchparmod(curdat, &wdw, &phmod, &bcmod, &ftmod, &memod); zeroparmod(curdat); STOREPAR("FT_mod",6); XTRFP2 storeparmod(curdat, wdw, phmod, bcmod, ftmod, memod) ; } void ift2(char *curdat) { int wdw, phmod, bcmod, ftmod, memod; fetchparmod(curdat, &wdw, &phmod, &bcmod, &ftmod, &memod); zeroparmod(curdat); STOREPAR("FT_mod",2); XTRFP2 storeparmod(curdat, wdw, phmod, bcmod, ftmod, memod) ; } void xfb(char *curdat) { int wdw, phmod, bcmod, ftmod, memod; fetchparmod(curdat, &wdw, &phmod, &bcmod, &ftmod, &memod); zeroparmod(curdat); CPR_exec("xfb nc_proc 0",WAIT_TERM); storeparmod(curdat, wdw, phmod, bcmod, ftmod, memod) ; }