Robert Vincent

Acidity of partially and non-dealuminated HY zeolites measured by 4-K broad-line and 300-K high-resolution magic-angle spinning 1H nuclear magnetic resonance spectroscopy: Synergy between Brönsted and Lewis acid sites

Abstract Previous experimental results on the acidity of partially hydrated [non-dealuminated or dealuminated by (NH4)2SiF6] HY zeolite samples obtained using both 4-K broad-line and 300-K magic-angle spinning (MAS) 1H nuclear magnetic resonance (NMR) spectroscopy are discussed. An attempt is made to explain the synergy between Bronsted and Lewis acid sites.

Flexibility in zeolites:29Si NMR studies of ZSM-5 frame transitions

Zeolites are most often perceived as rigid solids. Recent evidence has demonstrated that the temperature or the adsorption of molecules whose dimensions approach the pore dimensions induce changes in the solid structure, i.e., a flexing of the solid lattice. For pentacil zeolites, as ZSM-5, a transition between monoclinic and orthorhombic forms of the crystalline structure is found with a change in temperature or the adsorption of ring containing molecules. We find that the temperature at which this transition occurs depends on the Si/Al ratio within ZSM-5 and the discrete, reversible transition can be measured by29Si NMR, confirming prior X-ray studies. A similar dependence is found for ZSM-11. Phosphorous modification of the ZSM-5 does not change the transition temperature; however, steam treating of the zeolite does. The implications of these measurements to the flexibility and to the potential for transport is discussed. An analogy between transport and flexibility in one-dimensional polymers and that in the three-dimensional, open, solid network of zeolites is suggested.

Wide-line 1H NMR : a new application for the study of Brönsted acidity strength of solids with HY zeolite as example

Abstract The fitting of 4 K 1 H wide-line NMR spectra of Bronsted-acidic solid samples loaded with known amounts of water allows the determination of the concentration of the following species: H 3 O + , H 2 O…HO; and of the remaining initial species: OH and H 2 O.

Brønsted acidity of amorphous silica–aluminas studied by 1H NMR

Abstract1H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H3O+ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.

Adsorption, desorption, and interparticle motion of hydrogen on silica‐supported ruthenium: A study by in situ nuclear magnetic resonance

1H NMR line shapes of hydrogen adsorbed on silica‐supported ruthenium at pressures of 10−6–10 Torr were studied by using selective excitation via DANTE sequences. A transition from inhomogeneous to homogeneous line broadening was observed at hydrogen coverage of ∼0.5. The spectra were simulated by using generalized Bloch equations that included N‐site exchange processes. The homogeneous line shape originates from increased hydrogen mobility, whereas proton–proton dipolar couplings are negligibly small. A rate parameter k obtained from this model quantifies the average mobility of hydrogen in the exchange process. This parameter increases by more than three orders of magnitude when the hydrogen coverage changes from 0.4 to 0.8. The simulations of line shapes obtained at variable temperatures showed that k exhibits Arrhenius behavior with an activation energy of 52 (±5) kJ/mol and preexponential factor k0=4×1010 s−1. It is implied that the motion of hydrogen must involve desorption, interparticle diffusion,...

Niobium oxide acidity studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

The quantitative study of the Brønsted acidity of niobic acid (Nb2O5·xH2O) using broad-line1H NMR at 4 K has been performed by interacting niobic acid, pretreated at 573 K under vacuum, with water molecules. The number of oxyprotonated species (H3O+ and H2O...HO species formed, unreacted acidic OH groups or excess H2O molecules) deduced from the simulations of the broad-line1H NMR spectra shows a continuous increase in the number of H3O+ species with adsorbed water molecules. This increase may be due to a classical dilution effect or to a synergistic interaction between Brønsted and Lewis acid sites. These results are compared with those of some HY zeolites with or without framework defects.

Interaction between CD3OH and a H-mordenite studied by 1H NMR, broad-line at 4 K and MAS at 300 K

The simulation of 4 K 1H broad-line NMR spectra of CD3OH interacting with the acidic OH groups of H-mordenite zeolite shows no CD3OH2+ ions but hydrogen-bonded complexes. For high CD3OH concentrations, clusters of hydrogen-bonded methanol molecules are formed, also hydrogen-bonded to Brønsted acid sites of the zeolite. The 300 K 1H MAS spectra prove that the desorbed H-mordenite sample contains also silanol groups in electrostatic interaction with the framework; these silanols do not interact with methanol under our experimental conditions.

The acidity of niobic acid studied by 1H broad-line NMR at 4 K and H MAS NMR at room temperature: comparison with other solid acids

Abstract The acidity of niobium pentoxide calcined at 573 K has been studied by interacting water molecules and the solid acid using 1 H broad-line NMR at 4 K and MAS NMR at room temperature. 1 H Broad-line NMR in ‘rigid lattice’ conditions allows the determination of the numbers of the oxygen-protonated species (H 3 O + and H 2 O ··· HO species, unreacted OH groups or excess H 2 O molecules) present when water molecules used as base react with a Bronsted acid. With niobic acid, the method showed a continuous increase in the hydronium ion concentration with the number of water molecules. The ionization coefficient, determined when the ratio of the number of water molecules to that of the acidic hydroxyl groups of niobic acid is one, is 0.2. This value is similar to the ionization coefficients found for some HY zeolites.

Application of NMR Methods to Catalysis

The aim of this Section is to give a brief historical account of NMR spectroscopy and to present the various sections of this work.

A NEW AREA OF APPLICATION OF BROAD-LINE PROTON NMR. STUDY OF THE STRENGTH OF BRONSTED ACID SITES ON THE SURFACE OF SOLIDS (HY ZEOLITE)

ConclusionsThe number of oxygen-protonated groups formed as a result of adsorption of water on the surface of Bronsted acid sites can be measured by broad-line proton NMR at 4 K. To obtain reliable results in calculating these spectra, it is necessary to consider the presence of hydrogen bonds between the molecules of water and acid OH groups. The number of H3O ions is a function of not only the number of molecules of water but also of H2O...HOb groups. The experimental data with a larger number of molecules of water confirm this dependence and the possibility of altering the geometry of the H2O...HOb, complex.Applied to HY zeolite with a number of adsorbed molecules of water less than or equal to the number of acid OH groups, it was found that: all water molecules form hydroxonium ions or hydrogen bonds with OH groups; the number of these ions increases with the amount of water.The results of a complete quantitative study will be published.