Study of the Constitutive Superficial Water of Precipitated Amorphous Silicas using 1H NMR: Broad-Line at 4 K and HR MAS at 300 K

Spectroscopically Distinct Sites Present in Methyltrioxorhenium Grafted onto Silica−Alumina, and Their Abilities to Initiate Olefin Metathesis

Deposition of CH3ReO3 onto the surface of dehydrated, amorphous silica-alumina generates a highly active, supported catalyst for the metathesis of olefins. However, silica-alumina with a high (10 wt %) Re loading is no more active than silica-alumina with low (1 wt %) loading, while CH3ReO3 on silica is completely inactive. Catalysts prepared by grafting CH3ReO3 on silica-alumina contain two types of spectroscopically distinct sites. The more strongly bound sites are responsible for olefin metathesis activity and are formed preferentially at low Re loadings (< or =1 wt %). They are created by two Lewis acid/base interactions: (1) the coordination of an oxo ligand to an Al center of the support and (2) interaction of one of the adjacent bridging oxygens (AlOSi) with the Re center. At higher Re loadings (1-10 wt %), CH3ReO3 also interacts with surface silanols by H-bonding. This gives rise to highly mobile sites, most of which can be observed by 13C solid-state NMR even without magic-angle spinning. Their formation can be prevented by capping the surface hydroxyl groups with hexamethyldisilazane prior to grafting CH3ReO3, resulting in a metathesis catalyst that is more selective, more robust, and more efficient in terms of Re use.

Unusual properties of water at hydrophilic/hydrophobic interfaces

The behaviour of water at mosaic hydrophilic/hydrophobic surfaces of different silicas and in biosystems (biomacromolecules, yeast cells, wheat seeds, bone and muscular tissues) was studied in different dispersion media over wide temperature range using 1H NMR spectroscopy with layer-by-layer freezing-out of bulk water (close to 273 K) and interfacial water (180 < T < 273 K), thermally stimulated depolarization current (TSDC) (90 < T < 270 K), infrared (IR) spectroscopy, and quantum chemical methods. Bulk water and water bound to hydrophilic/hydrophobic interfaces can be assigned to different structural types. There are (i) weakly associated interfacial water (1H NMR chemical shift delta(H) = 1.1-1.7 ppm) that can be assigned to high-density water (HDW) with collapsed structure (CS), representing individual molecules in hydrophobic pockets, small clusters and interstitial water with strongly distorted hydrogen bonds or without them, and (ii) strongly associated interfacial water (delta(H) = 4-5 ppm) with larger clusters, nano- and microdomains, and continuous interfacial layer with both HDW and low-density water (LDW). The molecular mobility of weakly associated bound water is higher (because hydrogen bonds are distorted and weakened and their number is smaller than that for strongly associated water) than that of strongly associated bound water (with strong hydrogen bonds but nevertheless weaker than that in ice Ih) that results in the difference in the temperature dependences of the 1H NMR spectra at T < 273 K. These different waters are also appear in changes in the IR and TSDC spectra.

Solid-state NMR characterization of diastereoisomeric chiral stationary phases and their soluble models

Two diastereoisomeric chiral stationary phases (CSPs), devised for enantioselective HPLC, showing unexplained differences in their chromatographic performances, have been characterized, together with their soluble models, by means of 13C-cross polarization/magic angle spinning (CP/MAS), 1H-MAS and 1H-free induction decay (FID) analysis. The NMR investigation of the soluble models has not highlighted significant structural/conformational differences between the two diastereoisomers, but has constituted a useful support for the analysis of the more complex NMR data of the CSPs. The organic chiral selectors of the stationary phases show a poor internal mobility and no conformational differences between the two diastereoisomers have been observed. On the contrary, interesting differences between the two CSPs have been found involving the silanols on the silica surface and the dynamics of the linking chains between the organic selectors and the silica surface itself. An explanation of the chromatographic behaviour has been proposed in terms of different proximity of the organic moieties with respect to the inorganic surface in the two CSPs.

Theoretical and experimental investigation of the effect of proton transfer on the (27)al MAS NMR line shapes of zeolite-adsorbate complexes: an independent measure of solid Acid strength

Assessing the degree of proton transfer from a Brønsted acid site to one or more adsorbed bases is central to arguments regarding the strength of zeolites and other solid acids. In this regard certain solid-state NMR measurements have been fruitful; for example, some (13)C, (15)N, or (31)P resonances of adsorbed bases are sensitive to protonation, and the (1)H chemical shift of the Brønsted site itself reflects hydrogen bonding. We modeled theoretically the structures of adsorption complexes of several bases on zeolite HZSM-5, calculated the quadrupole coupling constants (Q(cc)) and asymmetry parameters (eta) for aluminum in these complexes and then in turn simulated the central transitions of their (27)Al MAS NMR spectra. The theoretical line width decreased monotonically with the degree of proton transfer, reflecting structural relaxation around aluminum as the proton was transferred to a base. We verified this experimentally for a series of adsorbed bases by way of single-pulse MAS and triple quantum MQMAS (27)Al NMR. The combined theoretical and experimental approach described here provides a strategy by which (27)Al data can be applied to resolve disputed interpretations of proton transfer based on other evidence.

Water Adsorption on Pyrogenic Silica Followed by 1H MAS NMR

On the surface of two commercial pyrogenic silicas (Degussa and Cabot), five resonances were identified on the basis of the chemical shift, homonuclear coupling (T2), and spin-lattice relaxation behavior (T1). In accordance with previous studies we observed three different types of silanol groups: (i) weakly coupled (long T2), water inaccessible, isolated "internal" silanols at 1.8 ppm; (ii) weakly coupled, external "free" silanols revealed upon dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between 3 and 7 ppm. The resonance of water, whose position between 2.6 and 4.6 ppm depended on water content, corresponded to two unresolved species of slightly different T1. By equating this resonance to the weighted average of two distinct populations of water, we were able to distinguish the first layer of strongly hydrogen bound water at 2.7 ppm from liquid-like water at 5 ppm. The first layer is complete for water relative humidity as low as 3.6% and corresponds to a surface coverage of 4.75 H2O/nm2. If we assumed a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. This ratio was the same for the two silicas regardless of surface area. Copyright 1997 Academic Press. Copyright 1997Academic Press