Ex situ NMR

Since it was proposed in the early 1980s [6, 7], spin-relaxation has been extensively used to determine the surface-to-volume ratio of porous materials [8-10]. Pore structure has been probed by the effect on the diffusion coefficient [11, 12] and the diffusion propagator [13,14], Self-diffusion coefficient measurements as a function of diffusion time provide surface-to-volume ratio information for the early times, and tortuosity for the long times. Recent techniques of two-dimensional NMR of relaxation and diffusion [15-21] have proven particularly interesting for several applications. The development of portable NMR sensors (e.g., NMR logging devices [22] and NMR-MOUSE [23]) and novel concepts for ex situ NMR [24, 25] demonstrate the potential to extend the NMR technology to a broad application of field material testing. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

The insertions of CO into Pd-Me and of various strained alkenes into Pd-COR supported by the dinitrogen ligand Ar-BIAN have been studied by Vrieze and Elsevier by means of ex situ NMR spectroscopy [22, 51], For example, the sequential insertions of CO and norbomadiene into Pd-alkyl and Pd-acyl bonds allowed the detection and isolation of several intermediates such as acyl(carbonyl) and P-chelate (Scheme 7.24). The Pd complexes isolated after alkene insertion were found to have a structure arising from cis addition of Pd-C(0)R to the exo face of the olefin. 

Figure 23 Ex-situ NMR and in-situ CO infrared data for CO adsorbed on Pt/carbon with different Pt particle sizes. (A) Correlation between the extrapolated IR frequency (see text)...

Mechanical rotation of a sample at 54.7° with respect to the static magnetic field, MAS, is well known. It has been shown that narrowing effects in the spectra can also be achieved for a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the Bq field (Bq-MAS). A static sample of solid hyperpolarised xenon at 3.4 mT was used for experimental demonstration. The method is especially useful in cases where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging. 

Perlo J, Casanova F, Bliimich B (2007) Ex situ NMR in highly homogeneous fields H spectroscopy. Science 315 1110-1112... 

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

Aurbach and co-workers performed a series of ex situ as well as in situ spectroscopic analyses on the surface of the working electrode upon which the cyclic voltammetry of electrolytes was carried out. On the basis of the functionalities detected in FT-IR, X-ray microanalysis, and nuclear magnetic resonance (NMR) studies, they were able to investigate the mechanisms involved in the reduction process of carbonate solvents and proposed that, upon reduction, these solvents mainly form lithium alkyl carbonates (RCOsLi), which are sensitive to various contaminants in the electrolyte system. For example, the presence of CO2 or trace moisture would cause the formation of Li2COs. This peculiar reduction product has been observed on all occasions when cyclic carbonates are present, and it seems to be independent of the nature of the working electrodes. A single electron mechanism has been shown for PC reduction in Scheme 1, while those of EC and linear carbonates are shown in Scheme 7. ... 

Frei and co-workers also extended this reaction to other zeolites showing that almost identical behavior was observed in BaY, BaX, and in the K+ and Ba " forms of zeolite L [45,46]. Xiang et al. [47] have also studied the photooxidations of a series of 1-alkenes in the more acidic BaZSM-5 [48] and Ba- 3. The extensive polymerization of propylene in these zeolites demonstrates the detrimental effect of Bronsted acid sites on the reaction selectivity. These workers also used ex situ nuclear magnetic resonance (NMR) allowing more detailed... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

The primary methods for analysis were usually gravimetric, thermal, and spectroscopic in nature but not necessarily correlated with in situ analysis (XPS, AFM, TEM, etc.) or ex situ analysis of surface-bound polymers by de-grafting (NMR, MW, polydispersity, etc.). Colloidal stability and homogeneity of the grafting process is a primary concern. A range of these systems were analogous to what has been done in solution and in bulk and should be thoroughly examined in terms of chemistry on flat substrate surfaces. Several examples follow. 

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. 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

Fig. 10 Ex situ Raman spectra near the Ag(2) mode of C60, measured as a function of reaction time. Characteristic lines for pristine material (1469 cm-1), dimers (1464 cm-1), and chains (1459 cm-1) have been fitted to the spectra to show the structural evolution with time. Reprinted with permission from P-A Persson, U Edlund, P Jacobsson, D Johnels, A Soldatov, and B Sundqvist, NMR and Raman characterization of pressure-polymerized C60 , Chem. Phys. Lett. vol. 258 (1996) 540-46 [59]. Copyright 1996 Elsevier Science BY...

As already mentioned at the end of Sec. 7.2, autoclaves in hydrothermal synthesis are black boxes , and all the attempts to explain the formation of the porous solids originated from ex situ experiments. The above hypothesis concerns reactions within the autoclave and requires for its analysis in situ measurements in hydrothermal conditions which were never performed so far. These experiments must give information both on the solution and on the solid. Moreover, in the systems chosen for such studies, the kinetics of formation must be adapted to the possibilities of observation of the chosen technique. Two of them are particularly useful for this purpose in situ diffraction studies using synchrotron radiation (SR) and in situ NMR (liquid and solid state). 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

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