9 research outputs found
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. <sup>129</sup>Xe 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 × 10<sup>4</sup> 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, <sup>1</sup>H → <sup>129</sup>Xe 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) <sup>129</sup>Xe EXchange SpectroscopY (EXSY) NMR method. Using <sup>129</sup>Xe selective inversion recovery (SIR) experiments, the rate for this
exchange has been measured at 43 ± 6 s<sup>–1</sup>
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 <sup>129</sup>Xe NMR Spectroscopy
Crystalline
mesoporous γ-, δ-, and θ-alumina
samples with complex porosity and various surface areas (70 to 330
m<sup>2</sup>/g) were characterized by <sup>129</sup>Xe NMR spectroscopy
and nitrogen adsorption. Experimental conditions have been optimized
to measure <sup>129</sup>Xe 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<sup>–1</sup>. 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<sup>–4</sup> 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