Toralf Mildner

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.

Temperature-switched MAS NMR: A new method for time-resolved in situ studies of reaction steps in heterogeneous catalysis

Abstract The temperature of zeolite samples containing adsorbed methanol molecules was switched from room temperature to 500–600 K within 30–40 s by means of a laser beam. The catalytic reaction was monitored by 13 C MAS NMR and 1 H MAS NMR, the latter in steps of one second. A two-dimensional Fourier-transformation of the difference between the time-dependent and the equilibrium state of the system gives a good representation of reaction steps. For the methanol conversion it was found that the primarily formed C 3 /C 4 alkanes are still involved in further reactions, whereas methane is an inert product under these conditions.

Surface methoxy groups in zeolites studied by multinuclear MAS n.m.r. spectroscopy

Solid-state n.m.r. spectroscopy of 1 H and 13 C nuclei has been applied in order to study chemisorbed methyl species on zeolites with different intermediate electronegativity of the framework. The secondary influence of the framework via the carbon atoms, which is expressed in the 1 H MAS n.m.r. spectra, gives a similar measure of the basicity of the zeolite as the 13 C MAS n.m.r. of methoxy groups. The basicity of the zeolite determines the reactivity of the surface methoxy groups.

Time-resolved 13C MAS NMR investigation of chemical reactions: the MTG process†

Sample heating with a laser beam facilitates time‐dependent magic angle spinning (MAS) NMR spectroscopy of high‐temperature reactions in situ. The methanol‐to‐gasoline conversion (MTG process) was monitored by 13C MAS NMR spectroscopy using the stop‐and‐go method in addition to the continuous heating of sealed samples. It was found that effective alkane formation is forced by interrupted laser heating. With a decrease of the dimethyl ether concentration a constant concentration of methanol was observed under both continuous and interrupted heating conditions. Copyright

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

The temperature of zeolite samples containing adsorbed methanol molecules was switched from room temperature to 500600 K within 30-40 s by means of a laser beam. The catalytic reaction was monitored by 13C MAS NMR and ‘H MAS NMR, the latter in steps of one second. A two-dimensional Fourier-transformation of the difference between the time-dependent and the equilibrium state of the system gives a good representation of reaction steps. For the methanol conversion it was found that the primarily formed Q/C4 alkanes are still involved in further reactions, whereas methane is an inert product under these conditions.

Temperature switched in situ 1H and 13C MAS NMR studies of the catalytic conversion of methanol on zeolite ZSM-5

We have observed the time dependence of the first steps of the methanol to hydrocarbons conversion (MTG process) in zeolite H-ZSM-5 at 230 °C using the MAS (magic-angle spinning) technique with a laser heating system. It was found that the concentration of protons in the species with stronger polarized O-H bonds increases during the reaction up to 60% at equilibrium. Moreover, the appearance of carbon monoxide during the conversion of methanol to hydrocarbons were elucidated.

2.1 NMR Studies Concerning the Acidity and Catalysis in Zeolites

Abstract Two NMR techniques, namely 2H MAS NMR and echo Fourier 27AI NMR have been applied for the first time to study Bronsted acidity of dehydrated zeolites. The 27AI signal of Si-OH-AI sites, which was NMR-invisible until now, yield a quadrupole coupling constant Cqcc=16 MHz for zeolite H-ZSM-5. The values of Cqcc determined from the 2H MAS NMR spectra of Si-OH-AI sites increase with the framework aluminium content of the zeolites from 208 kHz (H-ZSM-5) to 236 kHz (H-X and H-Y) due to the decreased acid strength of the bridging OH groups. The state of art of the two techniques, 2H MAS NMR and 1H MAS NMR, applied to study Bronsted acidity of dehydrated zeolites is compared. Concerning the methanol to hydrocarbons conversion (MTG process) in the zeolite H-ZSM-5 we observed the time dependence of the first step of the reaction at 200°C using a MAS (magic-angle spinning) probe with laser heating system and found that the concentration of protons in the species with stronger polarized O-H bonds increases during the reaction up to 60% at equilibrium.

Temperatur-geschaltete 2D MAS NMR. Eine neue Methode für zeitaufgelöste Untersuchungen katalytischer Reaktionen an Festkörperoberflächen

The temperature of zeolite samples containing adsorbed methanol molecules was switched from room temperature to 500 within 30 seconds by means of a laser beam. The catalytic reaction on the zeolite H-ZSM-5 was monitored by ,3C MAS NMR and MAS NMR, the latter in steps of one second. A two-dimensional Fourier-transform of the difference between the time dependent and the equilibrium state of the system gives a good representation of the elementary steps of catalytic reactions.