129Xe-NMR results

XH and 13C NMR Measurements. In order to confirm these 129Xe NMR results obtained indirectly from the co-adsorbed xenon, we performed separate NMR experiments on the adsorbed benzene. Specifically, the and 13C NMR of benzene were measured with the same samples used in the 129Xe... 

Through the analysis of adsorption isotherms and 129Xe NMR results of the co-adsorbed xenon, we have shown that the dispersal of benzene molecules depends on not only the cation distribution but also the amount of benzene adsorbate within the supercage of zeolite adsorbents. We have also demonstrated for the first time that this well known indirect technique has the capability not only to probe the macroscopic distribution of adsorbate molecule in zeolite cavities but also to provide dynamic information about the adsorbate at the microscopic level. Conventional H and 13C NMR which directly detect the adsorbate species, although providing complimentary results, are relatively less sensitive. 

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

One of the resulting 129Xe NMR spectra is shown in Figure 5.3.8 (solid line 2) in comparison with the spectrum of the same initial mixture without combustion (dashed line 1). Referenced with 0 ppm is the gas phase peak at room temperature. 

The adsorbed-129Xe NMR spectrum allows us to choose between these interpretations. The chemical shifts extrapolated to N = 0, (0), are 59.6 and 58.0 0.5 ppm for NaY zeolite and NaY zeolite-V205 (.R = 0.05), respectively. Although the difference between these values is small, it may be associated with a difference in the Na+ concentration (19). However, according to the relationship between 6S and the pore free space (79), this result proves that the size of cavities where xenon is adsorbed remains unchanged. On the other hand, the variation of 6 with N (6 = f[N]) is rectilinear, with good correlation coefficients (0.9998 6) for NaY zeolite and the sample with R = 0.05 (Figure 4). 

This exchange model is supported by experiments on samples with more carbon black (Figure 12.19). In samples with more carbon black the Xe chemical shift shifts in the direction of pure carbon black, just as expected for simple exchange effects. More work is needed to clarify, for example, the decrease of the line width with decreasing temperature. The results show, however, that 129Xe NMR has the potential to characterise the interface between the polymer and filler particles in carbon black filled materials, for which it is known that they are difficult to characterise by other spectroscopic techniques. 

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

NMR spectrometry of Xenon-129 adsorbed in coked samples of the totally protonated H-ZSM-5 zeolite and the modified Na, H-ZSM-5 showed variations attributable to differences in coke distribution. 129Xe NMR spectrometry is extremely useful for probing microporous materials. Ito et al.(2) demonstrated, for example, that NMR spectrometry of adsorbed xenon in coke-fouled H-Y zeolite could probe the deposits after coking and the nature of the internal surfaces after decoking. The NMR results in this study are consistent with a distribution of coke restricted by size selectivity of the acidifying medium. 

A modified pentasil was prepared in which protonated sites reside externally and cationic sites reside internally. Coked samples of this zeolite were characterized by 129Xe NMR. Even at high coke levels, only slight blockage of channels was observed. In contrast the fully protonated H-ZSM-5 was shown to undergo coke deposition resulting in channel blockage. These observations can be rationalized by a model in which deposition of coke in the channels or at... 

Finally, 29xe is a very suitable and sensitive isotope for probing the pore architecture of zeolitic materials. The extended Xe electron cloud is easily deformable due to interactions between, e.g. the Xe atoms and the channel wall of a zeolitic ftamewoik, and deformation results in a large low-field shift of the Xe resonance. In addition, 129xe NMR can be used to study metal particles in zeolites, while reduction-oxidation reactions can be monitored (13). Table 2 summarizes the NMR properties of a number of nuclei which have been used in NMR investigations of zeolitic materials (11). 

These results confirm that 129xe NMR can be used to probe the location of paramagnetic cations and to measure the paramagnetic effect of these cations. We have shown that it is only when xenon can enter the first sphere of coordination of Co2+/ i.e. when [H20] < 6 / Cq2+ that chemical shift at zero xenon concentration drastically increases. The Oq values derived from a second-order polynomial... 

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