MAS NMR rotor reactor

Fig. 12. Schematic representation of an MAS NMR rotor reactor for MAS NMR investigations under CF reaction conditions. Reproduced with permission from 60. Copyright 1999 Kluwer Academic.

Unlike other spectroscopic methods requiring samples under vacuum or very low gas pressures, NMR spectroscopy of working catalysts is not limited by the so-called pressure gap. The flow techniques described in Section III.B are suitable for catalytic reaction experiments under atmospheric pressure. If necessary, a higher pressure inside the MAS NMR rotor reactor can be used. The gas pressure inside batch samples may be limited by the strength of the walls of the glass inserts or the type of the cap used to seal the MAS NMR rotor after the preparation of the reaction system. In both cases, at least atmospheric pressure can be reached inside the sample volume. 

As an example. Fig. 18 shows CP/MAS NMR spectra recorded during the investigation of surface ethoxy species (7S) formed on acidic zeolite HY ( si/ Ai = 2.7) by a SF protocol. Figure 18a shows the CP/MAS NMR spectrum recorded after a continuous injection of C-1-enriched ethanol, CHI CHzOH, into the MAS NMR rotor reactor containing calcined zeolite HY. The ethanol was injected at room temperature for 10 min. Subsequently, the loaded zeolite was purged with dry nitrogen (200 mL/min) at room temperature for 2h. 

Fig. 18. C CP/MAS NMR spectra of zeolite HY (/isi/mai = 2.7) recorded after a continuous injection of CHPCH2OH into the MAS NMR rotor reactor at room temperature and subsequent purging with dry nitrogen at room temperature (a) after a subsequent purging with dry nitrogen at 453 K (b) and after water had been injected into the MAS NMR rotor reactor at room temperature (c). The CP/ MAS NMR spectra of ethylated zeolite Y (Cl l/f l F-Y) were recorded after thermal treatments at 52 K (d) and 623 K (e). Asterisks denote spinning sidebands.

In a recent work (207), in situ ll and Al MAS NMR spectroscopy was used to investigate the Bronsted acid sites of zeolite HZSM-5 during the conversion of methanol under CF conditions. As a consequence of the application of MAS instead of the spin-echo technique, only framework Al atoms involved in quadrupolar interactions according to Cqcc < 5 MHz could be observed. Upon injection of methanol into the MAS NMR rotor reactor filled with calcined zeolite HZSM-5, a single Al MAS NMR signal arose at 54 ppm at reaction temperatures up to... 

Applying the equipment shown in Fig. 13 (Section III.B) the authors performed a simultaneous analysis of the reaction products leaving the MAS NMR rotor reactor by on-line gas chromatography and an NMR characterization of the compounds adsorbed on the catalyst under steady-state conditions. These investigations showed that the intensity of the signals at ca. 80 ppm correlates with the yields of MTBE determined by gas chromatography (60). An increase of the reaction temperature of the exothermic synthesis of MTBE from 333 to 353 K, led to a simultaneous... 

K during the methanol conversion on zeolite HY ( si/KAi = 2.7) under flow conditions (74). In these experiments, a flow of C-enriched methanol with a modified residence time of WIF — lOOgh/mol was continuously injected into the spinning MAS NMR rotor reactor. Simultaneously, the yields of DME, Tdme, were determined by on-line gas chromatography (Fig. 32, middle). 

The role of surface methoxy species during the conversion of methanol to DME was investigated by SF MAS NMR spectroscopy (f4). After the preparation of pure surface methoxy species by conversion of C-cnrichcd methanol on zeolite HY ( si/ Ai — 2.7) (Fig. 34a), a flow of methanol with a natural abundance of C-isotopes ( CII3OI1) was injected at 433 K for 10 min into the spinning MAS NMR rotor reactor. In the " C CP/MAS NMR spectrum shown in Fig. 34b, weak signals are evident at 60.5 and... 

To ensure that meaningful results were obtained by C CF MAS NMR spectroscopy, catalytic experiments were performed with a conventional fixed-bed reactor and, under the same conditions, a spinning MAS NMR rotor reactor. The reaction products from both reactors were analyzed by on-line gas chromatography (261). A comparison of the results showed a reasonably good agreement in particular, the methanol conversion takes place in the same temperature range, and the same qualitative behaviors of the conversion and yield curves were found for the two reactors (261). 

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