A. Gédéon

Nuclear Magnetic Resonance of Physisorbed 129Xe Used as a Probe to Investigate Porous Solids

Abstract Xenon is an inert gas with a large electronic environment that makes it sensitive to any interaction, even physical. In the case of 129Xe isotope (spin 1/2), the resulting electronic perturbation is directly transmitted to the nucleus and, therefore, affects the nuclear magnetic resonance chemical shift. In this review, we report exhaustively up to 1996 the many applications of this technique in both fundamental and applied research in the fields of microporous and mesoporous solids.

129Xe NMR overview of xenon physisorbed in porous solids

Xenon is an inert gas whose large polarizability makes it very sensitive to electronic perturbation which is directly transmitted to the nucleus and therefore markedly affects the chemical shift. This overview, which is not exhaustive, presents some characteristic examples of the use of 129Xe NMR to study micro and mesoporous solids. Copyright

Mechanical properties of mesoporous silicas and alumina–silicas MCM-41 and SBA-15 studied by N2 adsorption and 129Xe NMR

Abstract Purely siliceous and Al-containing MCM-41 and SBA-15 materials have been prepared with pore diameters of 2.4–3.0 nm for MCM-41 and 6.5–7.5 nm for SBA-15. The effect of compression on the mesopore structure has been studied by N 2 adsorption–desorption, X-ray diffraction and 129 Xe NMR experiments. The pore size distributions obtained from N 2 adsorption data show that the mesopore structure is partially destroyed and that small size pores are formed at the expense of the original ones (in the loose powder) when the powder is compressed at high pressure (up to 520 MPa). The 129 Xe NMR spectra are interpreted in terms of an exchange between adsorbed Xe (in the channels) and gaseous Xe atoms (in the interparticle spaces), which depends on the particle size and the compaction of the powder. For both MCM-41 and SBA-15 materials, the Al-containing samples are more fragile than the purely siliceous materials.

Direct incorporation of A1 in SBA mesoporous materials: characterization, stability and catalytic activity

Abstract Aluminum-incorporated SBA mesoporous materials have been obtained by direct synthesis; the resulting materials retain the hexagonal order and physical properties of purely siliceous SBA-15 and present higher catalytic activity in cumene cracking reaction than A1MCM-41 solids. The stability of these mesoporous molecular sieves after various treatments (calcination, vapor treatment and treatment in solution at different pH) is also studied using XRD, 27 A1 MAS NMR and N 2 adsorption/desorption techniques. All results show that SBA after treatments has much higher stability than MCM-41 owing to its large wall thickness. The incorporation of aluminum into purely siliceous SBA-15 improves its stability. The stability of the state of aluminum coordination as well as the mesoporous structure of A1SBA is also higher than A1MCM-41, especially in the different pH solutions.

Surface acidity diagnosis and catalytic activity of AlSBA materials obtained by direct synthesis

Abstract The objective of this work is to give a characterization of aluminosilicate AlSBA materials in particular with respect to acidity and the nature of acid sites. The number of acidic centers and the strength of these sites are determined by ammonia adsorption and temperature-programmed desorption (TPD). The Bronsted acidity of AlSBA is also assessed by measuring the relative intensity of the laser-induced fluorescence (LIF) quinolinium ion band peaking at 390 nm when quinoline is adsorbed on the surface of the material. When Lewis acid sites are present, a complex formed between these sites and quinoline is observed. The AlSBA samples showed a high and durable activity in the cumene cracking.

A surfactant-assisted preparation of well dispersed rhodium nanoparticles within the mesopores of AlSBA-15: characterization and use in catalysis

Well dispersed and efficient Rh(0) hydrogenation catalysts were obtained by the reduction of Rh(III)-exchanged mesoporous aluminosilicates by sodium borohydride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride.

Characterization of Cu+ ions in CuHZSM-5 zeolites and CuSAPO-34 molecular sieve Cu+ laser-induced luminescence and 129Xe-NMR diagnosis

Abstract We report on an attempt to characterize by laser-induced phosphorescence spectroscopy Cu+ sites at low concentration in CuHZSM-5 and CuSAPO-34 samples which have been heated to 623 K in vacuum. Evidence is given that atleast two different sites for the copper ion location are available in the ZSM-5 structure. The laser-induced luminescence diagnosis of Cu+ ions allows a determination of the relative concentration of these sites in the samples. It provides evidence that the concentration of sites with square pyramidal symmetry varies linearly with the total number of Cu+ ions per unit cell. The signal intensity of Cu+ is highly dependent on the site environment and especially on the Al position. 129Xe-NMR confirms the presence of Cu+ and that there is no Cu2+ interacting with Xe in the zeolite pores.

A 129-Xe NMR study of the Co2+-Xe interactions in partiallyexchanged CoNaY zeolites: influence of the hydration level

Sodium Y zeolites partially exchanged with Co 2+ have been studied by 129-XeNMR spectroscopy. Large chemical shifts due to paramagnetic effects of Co 2+ ions are observed when the water concentration is less than 6 H 2 O/Co 2+ . The 0 term (σ value for zero xenon concentration) derived from a second-order polynomial expansion is characteristic of the Co 2+ -Xe interactions. This term increases exponentially when [H 2 O] decreases.

Spectroscopic evidence of SBA-15 mesoporous solids functionalization

Abstract In order to enhance the acid properties of mesoporous materials, the functionalization of the SBA-15 silica surface with arene-sulfonic acid groups has been adopted via a one-step synthesis procedure. The structures of the sulfonic acid surface-functionalized mesoporous silica were studied and identified by using nitrogen adsorption, X-ray diffraction (XRD), FT-IR, TEM, CP/MAS 29 Si and 13 C NMR, and hyperpolarized 129 Xe NMR techniques. The number of acid groups was determined by ammonia adsorption and catalytic properties were also studied in liquid phase. The structure of the obtained materials showed hexagonal mesoscopic order, possessed large surface areas with a heterogeneous distribution of sulfonic functions within the pore of the mesoporous solid. The anchoring of acid groups has been proved by 29 Si, 13 C CP-MAS-NMR measurements and using the recent technique of hyperpolarized 129 Xe NMR. The number of acid groups is directly proportional to the functionalization ratio. Moreover, these acidified solids display interesting catalytic properties for the esterification of acetic acid with ethanol.

Aluminum incorporation and interfacial structures in AISBA-15 mesoporous solids: double resonance and optically pumped hyperpolarized 129 Xe NMR studies

Aluminum-incorporated SBA-15 mesoporous materials have been obtained by direct synthesis. The surfactant—aluminosilicate interaction during synthesis was studied by double resonance NMR and confronted with the structural properties of the materials obtained after calcinations. Continuous-flow laser-polarized 129Xe NMR spectroscopy was applied for the first time to explore the porosity of the AlSBA-15 mesoporous molecular sieves. TRAPDOR experiments firmly established a strong interaction between segments of the PEO block of the surfactant with the silica-alumina framework. 1H Dipolar Dephasing revealed that the amount of segments rigidified by this interaction increased with the maturation time. The increased rigidity of the surfactant is to be linked with the increased mesoscopic ordering during maturation, resulting in the higher mesoporous surface obtained after calcinations. The invariability of the TRAPDOR effect proved that the strength of the interaction, that is the degree of interpenetration of the organic/inorganic phases remained the same irrespective of maturation time. Togethe with the dramatic decrease of the microporous volume with maturation time, this established that the origin of the microporosity of AlSBA-15 is to be found in the incomplete hydrolysis of the TEOS precursor itself rather than in the incomplete PEO— aluminosilicate phase separation.