Influence of cation exchange on the Al-27-NMR spectra of zeolites

The influence of cation exchange on the Al-27-NMR spectra of NaA-zeolites has been studied by Al-27-MAS- and MQ-MAS-Solid State-NMR. From the Al-27-spectra a characterization of the different At sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the Al-27-NMR-spectra than exchange with larger cations like Ba2+. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites

NMR study of role of the cross-relaxation effect in the cortex and the nucleus rabbit lens fragments

We have experimentally studied the coupling between the longitudinal magnetization of macromolecular protons and water protons in the cortex and the nucleus rabbit lens fragments (cross-relaxation). The NMR lines of water protons in the cortex and the nucleus of rabbit lens are inhomogenously broadened and relative narrow. We have assumed that the NMR line of crystalline protons in the lens is homogeneously broadened, very broad (the halfwidth order of 25 kHz) with small intensity. To detect the effect of cross-relaxation we adopted concept of the magnetization transfer used in magnetic resonance imaging. We irradiated the sample 20 kHz off resonance of the water protons with a train of low power RF pulses forming a selective saturation pulse and then after variable delay excited with a high-power 90 degrees, detection pulse. Finally we have measured the sample NMR line intensity as a function of duration of selective pulse for several fixed delays between selective and detection pulses. We have found the significant difference between time constants of decays for the cortex and the nucleus. We attributed this difference to longer time of hydration water molecules in the cortex spend within proximity of the protein surface than in the nucleus. We have interpreted these finding as an effect of chemical protons exchange between macromolecular and water protons rather then cross-relaxation effects, and it is discussed in terms of the Levy walk mechanism. (C) 1999 Elsevier Science B.V. All rights reserved

Influence of Cation Exchange on the ²⁷Al-NMR Spectra of Zeolites

The influence of cation exchange on the ²⁷Al-NMR spectra of NaA-zeolites has been studied by ²⁷Al-MAS- and MQ-MAS-Solid State-NMR. From the ²⁷Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the ²⁷Al-NMR-spectra than exchange with larger cations like Ba²⁺. Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites

Microcrystallization of benzene-d(6) in mesoporous silica revealed by H-2 solid-state nuclear magnetic resonance

Benzene-d(6) confined in a mesoscopically organized, controlled, porous-glass mesocellular foam (MCF), with a pore diameter of 30 nm, is studied by low-temperature solid-state H-2 nuclear magnetic resonance (H-2 NMR) spectroscopy in the temperature range of 90-180 K. The resulting spectra are compared to bulk benzene-d(6) and benzene confined in a mesoporous silica (SBA-15). The comparison shows that the spectra in the MCF are the superposition of an amorphous surface phase and a crystalline inner bulk phase. For the inner crystalline phase, the activation energies coincide with that of bulk benzene. The pore volume and the filling factor indicate that approximately three molecular layers of benzene are present on the inner surfaces