Catalysts Based on Fiberglass Supports: I. Physicochemical Properties of Silica Fiberglass Supports

The physicochemical properties and structure of supports prepared by leaching soda–silica fiberglass materials were studied using a set of physicochemical techniques (BET; IR spectroscopy; transmission electron microscopy; and ²⁹Si, ²³Na, ²⁷Al, ¹³³Cs, and ¹²⁹Xe (of adsorbed molecules) NMR spectroscopy). A matrix that corresponded in chemical composition to SiO₂was formed at high degrees of leaching; however, it was considerably different from ordinary silica gels in properties. The structure and properties of this matrix are most adequately described by the model of a pseudolayer intercalation structure, which includes alternating layers of several silicon–oxygen tetrahedrons separated by narrow (<4 Å) cavities. Considerable amounts of OH groups (5000 mol/g) are contained in these cavities, and these OH groups are different from the surface hydroxyl groups of ordinary globular silica. Although the interlayer spaces are small, comparatively bulky cations can be intercalated into them.     

Distinguishing surface versus buried cation sites in aluminosilicate mesoporous materials

Mesoporous MCM-41 aluminosilicates were prepared through direct synthesis and surface grafting resulting in the incorporation of aluminum into the pore walls and onto the wall surface, respectively. 7Li and 23Na NMR studies of ion-exchanged Li and Na-Al-MCM-41 were able to distinguish between cations in the surface region and those buried deeper in the pore walls. Thus it was demonstrated that most of the cations in the grafted Al-MCM-41 locate in the surface region, whereas the cations in the synthesized Al-MCM-41 are distributed throughout the pore walls. The NMR spectra of dehydrated Li- and Na-MCM-41 resemble those of glassy materials, reflecting the amorphous nature of this class of mesoporous materials. 7Li NMR studies of dehydrated Li-Al-MCM-41 prepared from direct synthesis in the presence of oxygen showed that most of the Li+ cations are not accessible to O2, while the Li+ cations in Al-grafted Li-Al-MCM-41 are accessible, which also confirms their locations. This study provides valuable insights for the understanding of the structure and properties of aluminosilicate mesoporous materials.     

Probing the location and distribution of paramagnetic centers in alkali metal-loaded zeolites through (7)Li MAS NMR

The nature and surroundings of lithium cations in lithium-exchanged X and A zeolites following loading with the alkali metals Na, K, Rb, and Cs have been studied through (7)Li solid-state NMR spectroscopy. It is demonstrated that the lithium in these zeolites is stable with respect to reduction by the other alkali metals. Even though the lithium cations are not directly involved in chemical interactions with the excess electrons introduced in the doping process, the corresponding (7)Li NMR spectra are extremely sensitive to paramagnetic species that are located inside the zeolite cavities. This sensitivity makes (7)Li NMR a useful probe to study the formation, distribution, and transformation of such species.     

Experimental and computational characterization of the 17O quadrupole coupling and magnetic shielding tensors for p-nitrobenzaldehyde and formaldehyde

We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-(17)O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 +/- 0.2 MHz, etaQ = 0.45 +/- 0.10, delta11 = 1050 +/- 10, delta22 = 620 +/- 10, delta33 = -35 +/- 10, alpha = 90 +/- 10, beta = 90 +/- 2, gamma = 90 +/- 10 degrees. The principal component of the 17O CS tensor with the most shielding, delta33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, delta11, lies along the C=O bond. For the 17O QC tensor, the largest (chi(zz)) and smallest (chi(xx)) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: sigma11 = -1139 +/- 80, sigma22 = -533 +/- 80, sigma33 = 431 +/- 5, and sigma(iso) = -414 +/- 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), M?ller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.     

Catalysts Based on Fiberglass Supports: II. Physicochemical Properties of Alumina Borosilicate Fiberglass Supports

The properties of borosilicate fiberglass supports were studied using a set of physicochemical techniques (BET; transmission electron microscopy; IR spectroscopy; and ¹¹B, ²³Na, ²⁷Al, and ²⁹Si NMR spectroscopy). Unleached fiberglass was characterized by the presence of silicon primarily in the Q ²form (two silicon atoms in the second coordination sphere of silicon–oxygen tetrahedrons) and by a comparatively homogeneous distribution of concomitant heteroatoms (B, Al, and Na). Under the action of an acid, the extraction of nonsilica components and the rearrangement of a silicon–oxygen framework (the transition of Q ²to Q ³+ Q ⁴) took place simultaneously. This was accompanied by the development of the surface and by the formation of channels with a wide range of sizes: from channels 15–50 Å in diameter (the adsorption properties of these channels are similar to those of mesopores in ordinary silica supports) to microchannels, which are typical of pseudolayer intercalation structures described previously for leached soda–silica fiberglass.