Study of Xenon Mobility in the Two Forms of MIL-53(Al) Using Solid-State NMR Spectroscopy

The Al-based metal–organic framework (MOF) MIL-53­(Al) exhibits a structural transition between a large-pore (<i>lp</i>) form and a narrow-pore (<i>np</i>) one. Such change is induced by temperature, external pressure, or the adsorption of guest molecules. ¹²⁹Xe solid-state NMR experiments under static and magic-angle spinning (MAS) conditions have been used to study the <i>lp</i>–<i>np</i> transition in MIL-53­(Al) initially loaded with xenon gas under a pressure of 5 × 10⁴ Pa (at room temperature). The conversion of the <i>lp</i> form into the <i>np</i> one when the temperature decreases from 327 to 237 K and the reopening of the pores below 230 K are then observed. Furthermore, ¹H → ¹²⁹Xe cross-polarization under MAS (CPMAS) experiments demonstrate the possibility to observe the <i>np</i> phase at <i>T</i> ≤ 230 K, while the <i>lp</i> one is unseen because the xenon residence time is too short for successful cross-polarization transfer. Moreover, even for the <i>np</i> phase at 199 K, the xenon atoms still exhibit significant motion on time scale faster than a few milliseconds. We prove the exchange of Xe atoms between the <i>lp</i> and <i>np</i> forms at room temperature with the two-dimensional (2D) ¹²⁹Xe EXchange SpectroscopY (EXSY) NMR method. Using ¹²⁹Xe selective inversion recovery (SIR) experiments, the rate for this exchange has been measured at 43 ± 6 s^–1

Giant Modulation of Refractive Index from Correlated Disorder

Correlated disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical and mechanical properties of materials. However, the possibility of obtaining novel optical and opto-electronic properties from such correlated disorder remains an open question. Here, we show unambiguous evidence of correlated disorder in the form of anisotropic, sub-angstrom-scale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS3, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS6 chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. 47/49Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between correlated disorder and the optical response in BaTiS3, this study opens a pathway for designing optical materials with high refractive index and functionalities such as a large optical anisotropy and nonlinearity.Comment: 24 pages, 3 figure

Use of advanced solid-state NMR methods for the characterization of Metal-Organic Frameworks : study of structural transformations and steam effects

Les Metal-Organic Frameworks (MOF) sont des matériaux hybrides poreux cristallisés dont l’importance grandissante est liée à leurs propriétés intéressantes. Malgré l’engouement pour ces solides, leur déploiement dans des procédés industriels reste faible et s’explique par leurs plus faibles stabilités thermiques et chimiques. Plusieurs méthodes peuvent être utilisées pour étudier les modifications structurales pouvant intervenir comme la diffraction des rayons X, la mesure de porosité ou la Résonance Magnétique Nucléaire (RMN). Dans cette thèse, différentes techniques avancées de RMN ont été utilisées pour mettre en évidence ces transformations (MQMAS, D HMQC et SFAM-RESPDOR). Ces expériences ont notamment montré un changement de coordinence du scandium dans le MIL-100(Sc) (validé par des calculs ab initio de paramètres RMN) ainsi que des variations de distance internucléaire (MIL-100(Sc) et MIL 53(Al)). La localisation d’espèce piégée (acide téréphtalique) a également été réalisé dans le MIL-53(Al). La respiration de ce matériau a également été suivi par RMN 129Xe et la dynamique de ce gaz a été suivi en fonction de la température (1H→129Xe CPMAS, EXSY et inversion sélective). L’évaluation de la stabilité des MOF à la vapeur d’eau a également été initiée pendant ces travaux avec le HKUST-1. Ses produits de décomposition formés par hydrolyse ont été identifiés par RMN et sa stabilité sous vapeur à 200 °C ouvre de nouvelles perspectives pour de ce type de matériau. Enfin, un travail de méthodologie en RMN des solides a été effectué sur les séquences PRESTO III et D-R-INEPT. Cette dernière semble être prometteuse notamment pour une application en DNP RMN.Metal-Organic Frameworks (MOF) are hybrids porous crystalline materials of high interest due to their interesting properties. However, their use in industrial processes is rather low when compared to other porous materials and can be explained by the lower thermal and chemical stabilities of MOF. Such structural modifications can be probed by different technics such as powder X ray diffraction, porosity measurement or Nuclear Magnetic Resonance. In this thesis, advanced solid-state NMR experiments were used (MQMAS, D HMQC and SFAM RESPDOR) to characterized these transformations. These experiments were used to highlight scandium coordination changes e.g. in MIL-100(Sc) (supported by ab-initio calculation of NMR parameters) or to measure variations of internuclear distances (MIL-100(Sc) and MIL 53(Al)). Trapped terephthalic acid were localized by these measurements in MIL 53(Al). Breathing effect of MIL 53(Al) and the dynamic of trapped xenon gas were investigated by 129Xe NMR as a function of temperature (1H→129Xe CPMAS, EXSY and selective inversion recovery). The evaluation of MOF stability when exposed to steam was initiated in this thesis with HKUST 1 Decomposition products formed by hydrolysis were identified by NMR and the stability of HKUST 1 treated under steam flow at 200 °C opens news outlooks for these materials. Besides these studies of structural transformations within MOF, the development of two new NMR experiments was done on: PRESTO-III and D R INEPT sequences. The D-R-INEPT sequence appears to be the most promising scheme especially for DNP-NMR application

Solid state NMR to study the metal-organic frameworks

National audienc

Solid-state NMR helps to elucidate the structure of the Sc-based Metal-Organic Frameworks

International audienc

Magnetization transfer from protons to quadrupolar nuclei in solid-state NMR using PRESTO or dipolar-mediated refocused INEPT methods

International audienceIn solid-state NMR spectroscopy, the through-space transfer of magnetization from protons to quadrupolar nuclei is employed to probe proximities between those isotopes. Furthermore, such transfer, in conjunction with Dynamic Nuclear Polarization (DNP), can enhance the NMR sensitivity of quadrupolar nuclei, as it allows the transfer of DNP-enhanced 1H polarization to surrounding nuclei. We compare here the performances of two approaches to achieve such transfer: PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order), which is currently the method of choice to achieve the magnetization transfer from protons to quadrupolar nuclei and which has been shown to supersede Cross-Polarization under Magic-Angle Spinning (MAS) for quadrupolar nuclei and D-RINEPT (Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer) using symmetry-based 〖SR4〗_1^2 recoupling, which has already been employed to transfer the magnetization in the reverse way from half-integer quadrupolar spin to protons. We also test the PRESTO sequence with 〖R16〗_7^6 recoupling using 270090180 composite π-pulses as inversion elements. This recoupling scheme, which has previously been proposed to reintroduce 1H Chemical Shift Anisotropy (CSA) at high MAS frequencies with high robustness to rf-field inhomogeneity, has not so far been employed to reintroduce dipolar couplings with protons. These various techniques to transfer the magnetization from protons to quadrupolar nuclei are analyzed using (i) an average Hamiltonian theory, (ii) numerical simulations of spin dynamics, and (iii) experimental 1H 27Al and 1H 17O transfers in as-synthesized AlPO4-14 and 17O-labelled fumed silica, respectively. The experiments and simulations are done at two magnetic fields (9.4 and 18.8 T) and several spinning speeds (15, 18-24 and 60 kHz). This analysis indicates that owing to its -encoded character, PRESTO yields the highest transfer efficiency at low magnetic fields and MAS frequencies, whereas owing to its higher robustness to rf-field inhomogeneity and chemical shifts, D-RINEPT is more sensitive at high fields and MAS frequencies, notably for protons exhibiting large offset or CSA, such as those involved in hydrogen bonds

Capture de radionucléides dans les solides poreux de type Metal-Organique Framework

International audienc

Capture of radionuclides in Metal-Organique Framework (MOF)

International audienc

Local characterization of Metal-Organic Frameworks by Solid-State NMR: applications in depollution strategies

International audienc