Tracing compartment exchange by NMR diffusometry: Water in lithium-exchanged low-silica X zeolites.

The two-region model for analyzing signal attenuation in pulsed field gradient (PFG) NMR diffusion studies with molecules in compartmented media implies that, on their trajectory, molecules get from one region (one type of compartment) into the other one with a constant (i.e. a time-invariant) probability. This pattern has proved to serve as a good approach for considering guest diffusion in beds of nanoporous host materials, with the two regions ("compartments") identified as the intra- and intercrystalline pore spaces. It is obvious, however, that the requirements of the application of the two-region model are not strictly fulfilled given the correlation between the covered diffusion path lengths in the intracrystalline pore space and the probability of molecular "escape" from the individual crystallites. On considering water diffusion in lithium-exchanged low-silica X zeolite, we are now assuming a different position since this type of material is known to offer "traps" in the trajectories of the water molecules. Now, on attributing the water molecules in the traps and outside of the traps to these two types of regions, we perfectly comply with the requirements of the two-region model. We do, moreover, benefit from the option of high-resolution measurements owing to the combination of magic angle spinning (MAS) with PFG NMR. Data analysis via the two-region model under inclusion of the influence of nuclear magnetic relaxation yields satisfactory agreement between experimental evidence and theoretical estimates. Limitations in accuracy are shown to result from the fact that mass transfer outside of the traps is too complicated for being adequately reflected by simple Fick's laws with but one diffusivity.

17O NMR studies of low silicate zeolites.

Multiple-quantum magic-angle spinning and double-rotation NMR techniques were applied in the high field of 17.6 T to the study of oxygen-17-enriched zeolites A and LSX with the ratio Si/Al = 1. A monotonic correlation between the isotropic value of the chemical shift and the Si-O-Al bond angle alpha (taken from X-ray data) could be found. Hydration of the zeolites causes a downfield 17O NMR chemical shift of about 8 ppm with respect to the dehydrated zeolites. Ion exchange of the hydrated zeolites generates stronger chemical shift effects. The increase of the basicity of the oxygen framework of the zeolite LSX is reflected by a downfield shift of approx. 10 ppm going from the lithium to the cesium form, and the substitution of sodium by thallium in the zeolite A causes a shift of 34 ppm for the O3 signal. 17O DOR NMR spectra are superior to 17O 3QMAS NMR spectra, featuring a resolution increase by a factor of 2 and are about equal with respect to the sensitivity. The residual linewidths of the signals in the 17O DOR and 17O 5QMAS NMR spectra can be explained by a distribution of the Si-O-Al angles in the zeolites.

Laser-supported high-temperature MAS NMR for time-resolved in situ studies of reaction steps in heterogeneous catalysis.

The temperature of zeolite samples containing various adsorbed molecules was rapidly changed (within 15 s) from room temperature to 600 K by means of a laser beam. The location of the sealed glass ampoule in a boron nitride container decreases the temperature gradient in the sample and avoids laser-induced reactions. The technique facilitates time-dependent magic-angle-spinning (MAS) NMR spectroscopy of high-temperature reactions which take place within 60 s. The H-D exchange in the hydrogen form of zeolites loaded with fully deuterated molecules, the methanol-to-gasoline conversion and the catalytic ethylbenzene disproportionation in zeolites were monitored by 13C and 1H MAS NMR by means of a "stop and go" method.

207Pb NMR detection of spinning-induced temperature gradients in MAS rotors.

The position of the 207Pb MAS NMR resonance from Pb(NO3)2 at a given inlet temperature of the spinning air depends on the rotation frequency, the diameter and geometry of the rotor, the position of the sample in the rotor, and the bearing air pressure. Taking these effects into consideration, the use of Pb(NO3)2 as a chemical shift thermometer gives a temperature accuracy of (0.1 K for a very small sample and a MAS-induced temperature gradient of 4 K for a 4 mm sample spun at 10 kHz.

Solid-state nuclear magnetic resonance studies of acid sites in zeolites.

2H Magic-angle spinning nuclear magnetic resonance (MAS NMR), echo 27Al NMR, two-dimensional (2D) echo 1H MAS NMR and 1H, 27Al and 29Si MAS NMR have been applied to study the acid sites of dehydrated zeolites. The quadrupole coupling constants CQCC determined from 2H MAS NMR spectra of Si-OH-Al sites increase with the framework aluminium content of the zeolites from 208 Hz (H-ZSM-5) to 236 kHz (H-X and H-Y) due to the decreased acid strength of the bridging OH groups. The 27Al signal of the Si-OH-Al sites, which was considered "NMR-invisible" in the past, yields CQCC = 16 MHz for zeolite H-ZSM-5. The majority of non-framework aluminium species could also be observed in dealuminated and dehydrated zeolites H-ZSM-5 giving CQCC approximately 9 mHz. 2D echo 1H MAS NMR spectra yield values of 16-40 ms for the lower limits of the lifetime of hydroxyl species at room temperature. A lifetime of more than 40 ms was obtained from echo 2H MAS NMR spectra for protonated sites giving a signal at ca. 6.5 ppm in partially dealuminated and weakly rehydrated samples.