Patrice Batamack

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.

Interaction of the perfluorinated polymer Nafion-H with water studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

Broad-line1H NMR at 4 K shows that, when Nafion-H interacts with water molecules at low concentrations, the only species formed is the hydronium ion. The chemical shift of the unsolvated hydronium ion in Nafion-H with 1 H2O/SO3−, determined by1H MAS NMR at room temperature, is 10.4 ppm relative to external TMS.

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 acid strength of zeolites studied by 1H NMR: scaling, influence of defects

Abstract Many industrial chemical reactions, particularly in petrochemistry, require acidic catalysts (alone or associated with a hydrogenating phase capable of activating strong covalent bonds (C-C, C-H), However it is important that the acidity (density and strength of the acid sites) be adapted to the desired reaction; hence the need to know the strength of the acid sites of a catalyst before its use in a given reaction. The acid strength of an OH group depends not only on the ionic character of the bond but also, and often predominantly, on its polarizability. It must therefore be studied in the presence of a proton acceptor. We have shown that by studying the dipolar interactions by 1 H NMR at 4 K it is possible to characterize the different species in the equilibria solid-OH + H 2 O = solid-OH…OH 2 =solid-O − + H 3 O + and, consequently, to define a broad Bronsted acidity scale going practically from non-acidic solids to superacids. We apply here this technique to the study of various zeolites.

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.

Comparison of amorphous silica-alumina and highly dealuminated HY zeolite by1H high resolution MAS-NMR of solids

Abstract1H MAS-NMR spectroscopy reveals analogies between partly dehydrated samples of silica-alumina and strongly dealuminated HY zeolite. There is a fundamental difference between the dehydration processes of crystalline and amorphous samples. A scheme for the reactions occurring during dehydration and dehydroxylation is proposed.

Comparison of the acidity of dealuminated and non-dealuminated HY zeolite by two1H-NMR methods: wide line at 4 K and high resolution MAS at 300 K

Abstract1H-Wide-Line and MAS-NMR at 4 and 300 K, respectively, for HY zeolite samples, non-dealuminated and partially dealuminated with (NH4)2SiF6 reveal significant differences.

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.

Solid-state 1H NMR studies of the acidity of mordenites

The interaction of known numbers of water molecules with the acidic OH groups of H-form commercial mordenites, giving H3O+ and H2O⋯HO, is studied using 1H broad-line NMR at 4 K and MAS NMR at 300 K. The H3O+ concentration for one adsorbed water molecule per Bronsted-acid site is related to the Si/Al ratio. The differences between the H3O+ concentrations for the two samples at a higher water concentration is associated, semi-quantitatively, with synergy between Bronsted- and Lewis-acid sites.

1H broad-line and MAS NMR: application to the study of acid sites of desilicated zeolite ZSM-5

Broad-line1H NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts lower than or equal to that of the Brønsted acid sites. The dissociation coefficient of the acid OH groups, currently associated with the acid strength, was shown not to be affected by the selective removal of Si from the zeolite ZSM-5 framework which resulted in more Brønsted acid sites per unit surface area. On the other hand, by using MAS NMR, bridging Brønsted acid sites hydrogen-bonded to the zeolite framework were identified on the “anhydrous” surface of the desilicated ZSM-5. Moreover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.