In-situ MAS NMR investigations

The most frequently applied technique for in situ MAS NMR investigations of heterogeneously catalyzed reactions under flow conditions is based on the injection... 

The first in situ MAS NMR investigation of the synthesis of MTBE on acidic zeolites was performed by Mildner et al. (228) under batch reaction conditions. In this investigation, the temperature-jump MAS NMR technique (stop-and-go experiment, see Section III.A) was applied to characterize the reaction dynamics under non-equilibrium conditions on a boron-modified pentasil zeolite ( si/... 

Fig. 26. Design of the MAS rotor (a) and the turbine (b) modified for in-situ MAS NMR investigations of solid-state catalysts (Reproduced by permission of The Royal Society of Chemistry, Cambridge).

The first in situ MAS NMR investigation of the synthesis of MTBE on acidic zeolites was performed by Mildner et al. (228) under batch reaction conditions. In this investigation, the temperature-jump MAS NMR technique (stop-and-go experiment, see Section III.A) was applied to characterize the reaction dynamics under non-equilibrium conditions on a boron-modified pentasil zeolite ( si/ Mg = 80). The catalyst was calcined in a glass insert, which was sealed after the loading with MTBE. H MAS NMR spectra were recorded during the heating period of 100 s. Then the laser power was switched off and the temperature of the samples fell back to room temperature within about 60s. During the stop period of 1 h, when the reaction state was frozen, a C MAS NMR spectrum was recorded. By repetition of the stop-and-go periods for several times, the complete reaction could be measured by both H and MAS NMR spectroscopy. 

Trapped by a suitable compound, a transient intermediate can be converted into a more stable species for unequivocal identification. Stepanov and Luzgin (82) investigated the reaction of acetonitrile with 1-octene or tert-butyl alcohol on acidic zeolite HZSM-5 ( 2si/ Ai = 49) at 296 K by in situ MAS NMR spectroscopy under batch reaction conditions. Upon coadsorption of acetonitrile and 1-octene, a C MAS NMR signal at 108 ppm was observed, indicative of TV-alkylnitrilium ions 2 in Scheme 3. As depicted in Scheme 3a, the formation of these cations was explained by trapping the chemically unstable alkylcarbenium ions (formed from the adsorbed... 

Isotopic labeling is a powerful tool being used to understand the nature of transition states as well as the mechanisms of surface-catalyzed reactions. By in situ MAS NMR spectroscopy, the scrambling of labels at room temperature was investigated upon adsorption of p C-1]- 1-octene on calcined zeolite HZSM-5 (wsi/... 

A new MAS NMR probe for in-situ investigations of hydrocarbon conversion on solid catalysts under continuous-flow conditions has been developed by Hunger et al. (88). This design allows the injection of gaseous educt compounds into the MAS rotor during NMR experiments. Figure 26 shows the in-situ MAS NMR probe built on the basis of a commercial Broker double-bearing 7 mm MAS probe. 

Despite the above cautions, some interesting mechanistic work has already been accomplished, such as a detailed characterization of framework-bound alkoxys by in situ MAS NMR. These species, a subject of much current research including theoretical investigations 193], have previously been proposed to be intermediates in several mechanisms for carbon-carbon bond formation. For example, Chang and coworkers suggested the mechanism in Scheme III as an... 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts 93,94). By in situ MAS NMR spectroscopy (95), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h 94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product 94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide 93). 

In the last decade a number of in situ NMR spectroscopic studies on chemical reactions catalyzed by zeolites have been reported. Surveys of the most important static in situ NMR [ 1 ] and in situ MAS NMR [4,235] investigations have been published. 

Hunger M, Seiler M, Buchholz A In situ MAS NMR spectroscopic investigation of the conversion of methanol to olefins on sdicoaluminophosphates SAPO-34 and SAPO-18 under continuous flow conditions, Catal Lett 74 61—68, 2001. 

Hunger M and Horvath T 1995 A new MAS NMR probe for in situ investigations of hydrooarbon oonversions on solid oatalysts under oontinuous-flow oonditions Chem. Commun. 1995 1423-4... 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

In more recently introduced equipment, the calcination and loading of the catalyst samples can be performed under shallow-bed conditions. For example, the equipment developed by Zhang et al. (51) (Fig. 9) allows a calcination of the powder in a horizontal tube inside a heater at temperatures of up to 1000 K. After loading of the catalyst with probe molecules or reactants, the powder is added to an MAS NMR rotor at the bottom of the equipment, sealed with a rotor cap from a plug rack, and transferred to the NMR spectrometer. As in the case of the former approaches, the samples prepared in the equipment of Zhang et al. 151) can be used for ex situ as well as in situ NMR investigations under batch reaction conditions. Furthermore, this equipment is suitable for ex situ investigations of solid-catalyzed reactions under flow conditions. In this case, the horizontal tube inside the heater is used as a fixed-bed reactor. 

Generally, several protocols are used for the characterization of sohd-catalyzed reactions under batch reaction conditions by NMR spectroscopy. In ex situ experiments, the conversion of reactants adsorbed on the catalyst is carried out in an external oven and stopped after a given reaction time by quenching, for example, in liquid nitrogen. Subsequently, the reaction products formed on the catalyst surface are investigated at room temperature by use of a standard MAS NMR probe. This protocol is repeated with a stepwise increment of the reaction time at the same temperature or with a stepwise increment of the reaction temperature for the same duration. In an in situ experiment, the catalytic conversion of the reactants is measured inside the NMR spectrometer by use of a high-temperature MAS NMR probe. 

The photocatalysis of TCE on the TiO2-coated optical microfibers in the presence of oxygen was further investigated by in situ C MAS NMR spectroscopy 191 ). The major products are phosgene and carbon dioxide, with small quantities of DCAA and trichloroacetaldehyde remaining on the catalyst at the end of the reaction. Solid-state NMR spectroscopy has also been applied successfully for the investigation of the photocatalytic degradation of dichloromethane (192,193). 

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