Gian C. Gobbi

Detailed interpretation of the 29Si and 27Al high-field MAS n.m.r. spectra of zeolites offretite and omega

Abstract The 29Si MAS n.m.r. spectra of highly dealuminated zeolites are shown to exhibit separate Si(OAl) resonances due to crystallographically inequivalent lattice sites. The chemical shift dispersion due to crystallographic inequivalence is of the same order of magnitude as that from the presence of aluminium atoms in the first coordination sphere and can result in the overlapping of resonances. Thus, the interpretation of the 29Si MAS n.m.r. spectra of zeolites of low Si Al ratios can be ambiguous. The correct peak assignments are presented for the zeolites offretite and omega and the problems involved in obtaining quantitatively reliable Si Al ratios from the spectra discussed. In the case of offretite it is not possible to completely describe the distribution of silicon and aluminium over its two non-equivalent crystallographic sites, although the Si Al ratios for a number of samples are shown to agree with a random distribution. For zeolite omega it is possible to completely describe the non random Si distribution and to predict the aluminium distribution, which is in excellent agreement with that obtained independently from the high field (14.1T) 27Al MAS n.m.r. spectrum. All data indicate that Si and Al distributions in omega are not random in nature.

High-resolution solid-state nuclear magnetic resonance studies of dealuminated zeolite Y

The products of hydrothermal treatment of zeolite NH4-Na-Y and of treatment of zeolite Na-Y with silicon tetrachloride vapour under different conditions were examined by 29Si and 27Al high-resolution solid-state n.m.r. with magic-angle spinning (m.a.s.n.m.r.) at high magnetic fields. 29Si m.a.s.n.m.r. in combination with chemical analysis allowed the amount of framework and non-framework aluminium to be readily and quantitatively determined. However, not all non-framework aluminium is observed by 27Al m.a.s.n.m.r. in most samples. Hydrothermal dealumination yielded materials with Si/Al ratios of up to 6.92, while the zeolite dealuminated with Si/Cl4 had framework Si/Al ratios of up to 25 and contained little interstitial aluminium. M.a.s.n.m.r. is superior to X-ray diffraction as a method of determining zeolite composition.

Al,Si ordering on cordierite using ?magic angle spinning? NMR: I. Si29 spectra of synthetic cordierites

Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 − Al:Si=0.80:0.20, T2−Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.

Dealumination of mordenite using silicon tetrachloride vapour

Abstract Reaction of SiCI 4 vapour with synthetic large-port Na-mordenite at 700°C enhances the silicon content of the zeolite. The product is highly crystalline to X-rays and i.r.; the unit cell dimensions are reduced. The reaction has been studied using 29 Si and 27 Al MAS n.m.r. The former permits the Si, Al composition of the aluminosilicate framework to be determined; the latter unmistakably and quantitatively distinguishes between tetrahedral aluminium in the lattice and octahedrally coordinated aluminium cationic positions in the zeolitic channels.

Demonstration of contact induced ion exchange in zeolites.

In this paper, it is demonstrated that exchange of the cationic species Na/sup +/, Li/sup +/, K/sup +/, and Ca/sup 2 +/, can occur between different zeolite crystals based only on a simple physical contact between the crystallites. The structural changes accompanying the exchange have been monitored by powder XRD and /sup 29/Si MAS NMR/sup 3/ studies which, because they are sensitive to long- and short-range ordering effects, respectively, complement each other and give a complete description of the structures. Zeolite A has been chosen for this work as it has a well-defined structure and XRD pattern and gives a single sharp absorption in its /sup 29/Si MAS NMR spectrum. 4 references, 1 figure.

The characterisation of the nature of silica in biological systems

The silica in some biominerals has been investigated by electron microscopy and i.r. and n.m.r. spectroscopy and has been shown to be amorphous and to contain many Si–OH units.

Structural studies of sialon ceramics by high-resolution solid-state NMR

High-resolution solide-state 27Al NMR with magic-angle spinning (MASNMR) readily monitors the quantity and coordination (four- and six-fold) of aluminium in two ceramic materials of the SiAlON system. Sialon X-phase, of approximate composition Si3Al6O12N2, contains aluminium-centred octahedra and tetrahedra in the ratio ca. 1.9:1.0, while another sample containing a mixture of sialon polytypoids shows AlO6 octahedra and a large quantity of what is most probably nitrogen-coordinated tetrahedral aluminium. In addition, 29Si MASNMR detects two different kinds of silicon in the latter sample in a 2:1 ratio. These observations are interpreted satisfactorily in terms of the crystal structures of the compounds and provide further examples of the potential of MASNMR in the investigation of complex ceramic systems.

Characterization of local atomic environment and quantitative determination of changes in site occupancies during the formation of ordered synthetic cordierite by 29Si and 27Al magic-angle spinning n.m.r. spectroscopy

Eight distinct tetrahedral sites for silicon can be identified and assigned in freshly crystallized (hexagonal) synthetic cordierite, Mg2Al4Si5O18, four in the chain manifold and four in the six-membered aluminosilicate rings, but there are only two sites (one in the rings, the other in the chains) in the aged (orthorhombic) product; Si:Al ratios within each manifold can be determined from the 29Si magic-angle spinning n.m.r. spectra which also permit the course of Si, Al ordering to be charted as thermodynamic equilibrium is approached.

29Si magic angle spinning N.M.R. investigation of local ordering effects in zeolite ZSM-39

29 Si Magic angle spinning n.m.r. analysis of zeolite ZSM-39 has resolved the number of tetrahedral sites and the deviation from ideal symmetry.