B. F. Chmelka

NMR-STUDY OF XENON DYNAMICS AND ENERGETICS IN NA-A ZEOLITE

Abstract For xenon atoms adsorbed in Na—A zeolite, electronic interactions cause shifts in NMR frequencies, resulting in a spectrum with discrete peaks from xenon atoms in cages with different xenon occupancies. Using two-dimensional exchange NMR, it is possible to determine the microscopic rates of intercage motion and to relate them to the adsorption and activation energies of the xenon atoms. The dependence of the adsorption energies on xenon cage occupancy reflects the importance of the intracage interactions and is directly related to the cage occupancy distribution. Variable temperature measurements yield an activation energy of about 60 kJ/mol for the transfer of a xenon from one cage to another.

Some developments in nuclear magnetic resonance of solids.

Nuclear magnetic resonance (NMR) spectroscopy continues to evolve as a primary technique in the study of solids. This review briefly describes some developments in modern NMR that demonstrate its exciting potential as an analytical tool in fields as diverse as physics, chemistry, biology, geology, and materials science. Topics covered include motional narrowing by sample reorientation, multiple-quantum and overtone spectroscopy, probing porous solids with guest atoms and molecules, two-dimensional NMR studies of chemical exchange and spin diffusion, experiments at extreme temperatures, NMR imaging of solid materials, and low-frequency and zero-field magnetic resonance. These developments permit increasingly complex structural and dynamical behavior to be probed at a molecular level and thus add to our understanding of macroscopic properties of materials.

Macroscopic or microscopic information of Y zeolite from 129Xe n.m.r. line splitting

Abstract The origin of 129Xe n.m.r. line splitting observed in the study of Y zeolite has been investigated by comparing the n.m.r. spectra obtained before and after mixing various Y zeolites containing different cations or metal clusters at room temperature and also by low-temperature n.m.r. study of the zeolite mixtures. The results show that two 129Xe n.m.r. lines separated by as much as 30 ppm apart can be completely coalesced at room temperature after mixing. The xenon exchange rate between ordinary Y zeolite crystallites of 1 μm size is usually so fast that the n.m.r. signal can only provide information averaged over many crystallites. Only at low temperatures, for very large chemical shift differences or for specially prepared Y zeolites with much larger crystal size, the xenon n.m.r. signal can be specific to the local environment inside the zeolite crystal.

Adsorption effects in aluminophosphate molecular sieves studied by27Al double-rotation NMR

Abstract27Al double rotation NMR (DOR) spectroscopy is used to investigate structural changes in the framework of several aluminophosphate molecular sieves upon adsorption of water. The shapes, widths, and positions of the spectral lines yield information on the aluminum environments, adsorption sites, and degree of structural disorder undergone upon water adsorption.

Framework Ordering in Aluminophosphate Molecular Sieves Studied by27Al Double Rotation NMR

Abstract Aluminum-27 Double Rotation NMR spectroscopy (DOR) has been used to investigate framework ordering in the aluminophosphate molecular sieves VPI-5, AIPO 4 -5, and AIPO 4 -8. Well-resolved peaks in the 27 Al DOR spectra of both hydrated and dehydrated VPI-5 allow isotropic shifts to be correlated with local framework structure. More distorted aluminum environments are reflected by broader lines in 27 Al DOR spectra of AlPO 4 -5 and AlPO 4 -8.

Calcination-Dependence of Platinum Cluster Formation in NAY Zeolite: A Xenon-129 NMR Study

Xenon-129 NMR spectroscopy of adsorbed xenon is used to monitor the location of metal clusters and cluster precursors as a function of calcination conditions for NaY zeolite-supported platinum catalysts. Our results indicate that for the 673 K reduction conditions imposed, the formation of highly dispersed platinum clusters within the Y-zeolite matrix is best achieved by employing a calcinatoin temperature close to 673 K. Incomplete decomposition of the ion-exchanged Pt(NH 3 ) 4 2+ complex during calcination at 473 K results in migration of nearly all platinum to the exterior surface of the zeolite crystallite during reduction. Calcination temperatures significantly above 673 K induce decomposition of the shielded precursor species and subsequent migration of the metal into the sodalite cavities. A substantial amount of the platinum confined within the sodalite cavities migrates back into the supercages during reduction at 673 K. The 129 Xe NMR data are corroborated by hydrogen chemisorption and transmission electron microscopy experiments.

Multinuclear Solid-State Nmr Study of a Sodalite Semiconductor Supralattice

23 Na double rotation NMR (DOR) and 81 Br magic angle spinning (MAS) are used to investigate the synthesis and assembly of sodium, silver-bromide clusters encapsulated in the porous framework of sodalite. The NMR spectra provide site-specific information on the nano-environments of the charge balancing, extraframework Na + cations, and the encapsulated Br − anions. The results indicate the possibility of a percolation threshold beyond certain Na + and Br − loadings.