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

Implication of Water Molecules at the Silica – Ibuprofen Interface in Silica-Based Drug Delivery Systems Obtained through Incipient Wetness Impregnation

International audienceThe dynamical behavior of ibuprofen or benzoic acid (model molecule of ibuprofen) encapsulated in mesoporous silica is very specific as a very high mobility is evidenced at ambient temperature due to the existence of confinement effect. In this contribution, we demonstrate through variable temperature 1H MAS NMR experiments that this specific dynamical behavior is also related to a fast chemical exchange that takes place between protons of the COOH group of the organic molecule and protons from water molecules at the surface of the silica for materials obtained through incipient wetness impregnation. This phenomenon implies a weak interaction between the guest molecule and the silica surface that is related to the fast release profile of encapsulated ibuprofen observed in vitro

Probing the mobility of ibuprofen confined in MCM-41 materials using MAS-PFG NMR and hyperpolarised-Xe-129 NMR spectroscopy

International audienceThe continuous-flow hyperpolarised (HP)-Xe-129 NMR and magic angle spinning-pulsed field gradient (MAS-PFG) NMR techniques have been used for the first time to study the distribution and the dynamics of ibuprofen encapsulated in MCM-41 with two different pore diameters

Exploring the Complex Porosity of Transition Aluminas by ¹²⁹Xe NMR Spectroscopy

Crystalline mesoporous γ-, δ-, and θ-alumina samples with complex porosity and various surface areas (70 to 330 m²/g) were characterized by ¹²⁹Xe NMR spectroscopy and nitrogen adsorption. Experimental conditions have been optimized to measure ¹²⁹Xe NMR chemical shifts independent of the nature of the surface and solely dependent on the pore size. Xenon adsorption constants have been determined from xenon adsorption isotherms and from the fit of the chemical shift versus volume to surface ratio (V/S) curves with a simple exchange model. The discrepancy observed between those values has been attributed to the rugosity of the alumina pore surface. A direct correlation between the observed chemical shift and the alumina pore diameter was obtained for the whole series of alumina samples

Xenon Capture on Silver-Loaded Zeolites: Characterization of Very Strong Adsorption Sites

INGENIERIE+SAG:YSC:DFAThe number and strength of adsorption sites for Xe in silver-modified zeolites are estimated from isotherm measurements at various temperatures over a broad range of pressure (from 1 ppm to atmospheric pressure). Fully and partially exchanged silver zeolites were synthesized starting from Na-ZSM-5(25), Na-ZSM-5(40), Na-Beta, NaX, and NaY. We have discovered that silver-modified zeolites may present one or two distinct adsorption sites depending on the nature of the material and silver loadings. The strongest adsorption sites are characterized by isosteric heat of adsorption in the order of -40 to -50 kJ.mol(-1). For Pentasil-type zeolites, we observe a linear 2:1 correlation between the total amount of silver and the number of strong sites. The highest concentration of strong sites is found for fully silver exchanged ZSM-5 (5.7 X 10(-4) mol/g), which presents the largest silver content for Pentasil-type zeolite. The equilibrium constant of Ag-ZSM-5 at low pressure is about 50 times larger than that of AgX. Qualitative correlations were established between Xe adsorption isotherms and Xe NMR signals. We show that Xe NMR could be used as a quantitative method for the characterization of the strength and of the number of strong Xe adsorption sites on silver-exchanged zeolites. The numbers of strong adsorption sites responsible for the Xe adsorption at 10-1000 ppm can be determined by the length of the plateau observed at low Xe uptake. In practice, our findings give guidelines for the discovery and optimization of silver-loaded zeolites for the capture of Xe at ppm levels. It appears that the amount of silver is a key parameter. Silver-modified ZSM-5 shows adsorption capacities 2-3 orders of magnitude larger than currently applied adsorbents for atmospheric Xe capture

Xenon Capture on Silver-Loaded Zeolites: Characterization of Very Strong Adsorption Sites

The number and strength of adsorption sites for Xe in silver-modified zeolites are estimated from isotherm measurements at various temperatures over a broad range of pressure (from 1 ppm to atmospheric pressure). Fully and partially exchanged silver zeolites were synthesized starting from Na-ZSM-5(25), Na-ZSM-5(40), Na-Beta, NaX, and NaY. We have discovered that silver-modified zeolites may present one or two distinct adsorption sites depending on the nature of the material and silver loadings. The strongest adsorption sites are characterized by isosteric heat of adsorption in the order of −40 to −50 kJ·mol^–1. For Pentasil-type zeolites, we observe a linear 2:1 correlation between the total amount of silver and the number of strong sites. The highest concentration of strong sites is found for fully silver exchanged ZSM-5 (5.7 × 10^–4 mol/g), which presents the largest silver content for Pentasil-type zeolite. The equilibrium constant of Ag-ZSM-5 at low pressure is about 50 times larger than that of AgX. Qualitative correlations were established between Xe adsorption isotherms and Xe NMR signals. We show that Xe NMR could be used as a quantitative method for the characterization of the strength and of the number of strong Xe adsorption sites on silver-exchanged zeolites. The numbers of strong adsorption sites responsible for the Xe adsorption at 10–1000 ppm can be determined by the length of the plateau observed at low Xe uptake. In practice, our findings give guidelines for the discovery and optimization of silver-loaded zeolites for the capture of Xe at ppm levels. It appears that the amount of silver is a key parameter. Silver-modified ZSM-5 shows adsorption capacities 2–3 orders of magnitude larger than currently applied adsorbents for atmospheric Xe capture