D. Freude

Nuclear magnetic resonance studies on the acidity of zeolites and related catalysts

A review is given on the possibilities of the various nuclear magnetic resonance techniques which have been applied up till now to study quantitatively the acidity of zeolites and related catalysts. From the intensity of the wide line proton magnetic resonance signal of unloaded catalysts, the total concentration of protons can be determined. A loading of the samples with (deuterated) pyridine reduces drastically the mean residence time of the acidic protons at the oxygen atoms of the surface through formation and thermally activated motion of pyridinium ions. From the proton magnetic relaxation of the pyridine molecules and pyridinium ions a microdynamical model for the pyridine loaded H-Y zeolites could be derived. The rate constant for the decomposition of the pyridinium-ion-surface-comp)ex which can be determined in this way is used as a measure for the strength of acidity of the OH groups involved. Through magic angle spinning of thermally activated samples (contained in sealed glass ampoules to prevent adsorption of water) it became possible for the first time to determine quantitatively the concentration of non-acidic OH groups, of OH groups having different strength of acidity, and of residual ammonium ions. The results are related to the catalytic activity (cumene cracking) of the amorphous silica-aluminas, Hmordenites and zeolites H-Y studied in the present paper. In contrast to Br~nsted acidity, the current application of n.m.r, techniques to study Lewis acidity is more complicated because experimental difficulties, mainly due to fast exchange of adsorbed bases between different kinds of adsorption sites, arise.

NMR intensity measurements of half-integer quadrupole nuclei

Abstract The intensity of the NMR spectrum of a half-integer spin nucleus, subject only to the quadrupolar interaction, is calculated as a function of the length τ of the on-resonance excitation pulse and the quadrupole coupling paramters. For τ ⩽ π/ 4ω1 (I + 1/2) the intensity is proportional to τ and independent of the quadrupole coupling constant, whilst for larger τ the intensity passes through a maximum at a value which is dependent upon the parameters describing the quadrupole interaction. This is numerically calculated for I = 512. Values of the quadrupole coupling constant can be determined from measurements of the intensity as a function of τ.

1H MAS NMR studies on the acidity of zeolites

Proton magic-angle-spinning nuclear magnetic resonance (‘H MAS NMR) spectra contain quantitative information about Bransted acidity and structure defects in z&&es. The strength of acidity of bridging OH groups increases with the Si/Al ratio from 1.4 to 7 but remains constant above Si/Al ~10. Two signals of acidic hydroxyl protons observed in zeolites HY are correlated with the so-called high- and low-frequency band in infrared spectroscopy.

Magic-angle spinning nuclear magnetic resonance studies of water molecules adsorbed on Brønsted- and Lewis-acid sites in zeolites and amorphous silica–aluminas

On weakly rehydrated dealuminated zeolites and amorphous silica–aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements. A 1H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 1H NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.

Solid-state NMR studies of the geometry of brønsted acid sites in zeolitic catalysts

Abstract 1 H, 27 Al and 29 Si solid-state NMR was used to determine the geometry of the Bronsted acid site in zeolites. The mean distance between acidic protons in bridging hydroxyl groups and the nearest framework Al atom is 2.38±0.04 A in zeolite H–Y and 2.48±0.04 A in H-ZSM-5. The quadrupole frequency of framework Al is 1.02±0.05 and 3.1±0.8 MHz in dehydrated Na-Y and H-Y, respectively.

N.m.r. studies of aluminium in zeolites

27AI nuclear magnetic resonance (n.m.r.) techniques were used to determine the number of AI atoms in lattice positions and in extra-lattice positions of decationated zeolite Y. A sample activated under deep bed conditions at 400°C exhibits 50% of the total amount of aluminium in extra-lattice positions. One part of the extra-lattice aluminium in the rehydrated samples could be shown to exist in the form of mobile complexes. Further results are that (i) threefold oxygen coordinated lattice aluminium and (ii) OH nests could not be found in the course of the usual processes of dehydroxylation and dealumination. A comparison of the 27A1 n.m.r, shift as determined by magic angle spinning (MAS) of the samples for various cation-exchanged zeolites A, X and Y showed that the resonance line in zeolite A is shifted by 3.5+-1.0 ppm to higher field with respect to zeolite Y. The results are not inconsistent with Loewensteins rule applied to zeolite A, in contrast to interpretations of similar 29Si n.m.r, results published in the literature.

Magic-angle-spinning NMR studies of acid sites in zeolite H-ZSM-5

1H, 11C, 17Al, and 29Si magic-angle-spinning (MAS) NMR was used to elucidate the nature of the catalytic activity of zeolite H-ZSM-5. 1H MAS NMR of sealed samples after mild hydrothermal dealumination shows that the enhanced activity for n-hexane cracking is not due to an enhanced Bronsted acidity. The concentrations of the various OH groups and aluminous species suggest that the reason for the enhanced catalytic activity is the interaction of the n-hexane molecule with a bridging hydroxyl group and with extra-framework aluminium species. Loading the samples with HCOOH or HCl shows that those extra-framework aluminium species, which give rise to the enhanced activity, cannot be easily removed from their positions, and are therefore immobilized by the zeolitic framework.

Study of brønsted acidity of zeolites using high-resolution proton magnetic resonance with magic-angle spinning

Abstract By applying magic angle spinning to proton magnetic resonance of zeolites, two different lines were separated. Comparison of the corresponding resonance shifts with values of OH groups of other adsorbents and of organic compounds in solution showed that the signals must be due to hydroxyl groups of different acidity.

NMR studies concerning the dehydroxylation of zeolites HY

Abstract By applying the magic-angle spinning technique to proton magnetic resonance of zeolites HY two or three different lines could be separated. Measurement

NMR line shifts caused by the second-order quadrupolar interaction

Abstract The quadrupole shift for the central transition in magic-angle-spinning spectra of nuclei with non-integer spins in powder samples is smaller than the half-width of the line.