Zeolite simulated spectrum

The sodium-23 MASNMR spectra of the Y zeolite samples with these particular levels of cation exchange are shown in Figure 2, along with the spectrum of the Na-Y zeolite. Simulations of these spectra indicate that about 25, 10, and 20% of the integrated NMR intensity of the respective exchanged zeolites arises from residual Na cations in the supercages. The different Na NMR lines are best resolved in the case of the (NH, Na)-Y zeolites. 

Fig. 15. IINS spectra (INlBeF, ILL) of (a) HZSM-5 at low water loading (the spectrum of the dry zeolite has been subtracted) (b) simulated spectrum of water hydrogen-bonded to a bridging hydroxyl group, and (c) simulated spectrum of H30. Reproduced from Reference (76) with permission of the American Chemical Society.

Figure 7.9. A. MAS NMR spectum of dehydrated zeolite NaX at a magnetic field of 14.1 T, with the simulated spectrum and the five components used in the simulation. B. Na DOR spectrum acquired at a magnetic field of 9.4 T with an outer rotor speed of 1.5 kHz, with simulated spectrum and the components used in the simulation. C. Na 2D nutation spectrum acquired at a magnetic field of 9.4 T. The asterisk denotes spinning side bands. From Feuerstein et al. (1996), by...

Fig. 21. Cs MAS NMR spectrum of a dehydrated 72% cesium-exchanged zeolite Y (a), the simulated spectrum (b), the components (c), and spinning sidebands ( ) [187]...

Weitkamp et al. [36], who prepared a series of alkaline metal zeolites (K-Y,Rb-Y, Cs-Y) via solid-state ion exchange in order to study the effect of the basicity of such zeolites on their catalytic behavior, used Cs MAS NMR for the characterization of Cs-Y. Figure 15 shows (a) the spectrum of Cs-Y, (b) the simulated spectrum and (c) the individual components derived from a decomposition of spectrum (b). 

Figure 4.37 MAS NMR of a calcined Y zeolite. The top spectrum is the experimental Si MAS NMR spectrum and the middle spectrum is based on computer simulation...

Fig. 11. High-resolution 29Si MAS NMR spectra of synthetic zeolites Na-X and Na-Y at 79.80 MHz (58). Experimental spectra are given in the left-hand columns Si(nAl) signals are identified by the n above the peaks. Computer-simulated spectra based on Gaussian peak shapes and corresponding with each experimental spectrum are given in the right-hand columns. Individual deconvoluted peaks are drawn in dotted lines.

The distributions of the Xe atoms among the cages of the zeolite are provided by the relative intensities of the Xe1 Xe2, Xe3,. ..Xe8 peaks seen individually in the 129Xe NMR spectrum. Thus, the NMR experiment provides a direct measure of the distribution of Xe atoms among the cavities, e.g., what fraction of the zeolite cages have 5 exactly Xe atoms This is reproduced very well by the grand canonical simulations described above. 

In a combined simulation and experimental project we set out to assess the adsorption properties of a series of zeolites. In the present work the adsorption properties of n-butane and iso-butane on MFI are being studied. The experimental part consists in the validation of the molecular simulation model, by confirming its results. The experiments were performed in a constructed in-house manometric apparatus coupled with a NIR spectrometer (Perkin Elmer, FT-IR system, GX Spectrum). Figure 1 is a scheme of the experimental set-up. 

This has become routine analysis for zeolites with cubic symmetry, i.e. a single crystallographic site, such as X and Y. The problem becomes more complicated when the number of crystallographic sites increases for example, in offretites and erionites (2 sites), or mordeni-tes (4 sites), each of the sites has a five-line NMR spectrum, each spectrum being offset in relation to the other. This results in numerous cases of interference and the solution can only be extracted via simulation-optimisaiion programs which arc only resolved fora limited number of cases. 

Detailed calculations of the spinning sidebands in DAS spectra have been carried out using average Hamiltonian and irreducible tensor approaches (Sun et al. 1992). In DAS spectra the sideband intensities and their moments depend on the relative rotor phase between the two evolution periods. The sideband intensities additionally depend on the ratio of the time spent at each angle. The 2D O DAS spectrum of zeolite Sil-Y (Figure 3.22) shows three lines in the ratio 2 1 1 (Bull et al. 1998). Simulation of the anisotropic slices from the O 2D DAS spectrum for each peak allows extraction of xq and t] for each resonance. 

Figure 5.15. A. Al MQMAS spectrum of steam-treated faujasite zeolite H-Y obtained at 18.8T. B. Single-pulse 18.8T MAS NMR spectrum (top) simulated with two tetrahedral, one Al and one octahedral resonance (lower profiles) using intensities derived from the MQMAS spectrum (A). Asterisks denote spinning side bands. After Fyfe et al. (2000), by permission of the Royal...

The sodium-23 MASNMR spectrum of a Na-Y zeolite at 132 MHz (11.7 Tesla) is shown in Figure la. As discussed below, the features of this spectrum arise from the presence of at least two separate NMR lines. A simulation of this spectrum, with symmetric lines of mixed Gaussian/Lorentzian character, is also shown in Figure la, along with the component lines of the simulation. Such a spectrum is difficult to simulate uniquely because of the overlap of the lines and the errors introduced into the spectrum as a result of spectral phasing and baseline correction. 

To determine if the features of the spectrum of Figure la indicate the existence of two or more lines, as simulated, or arise from a single NMR line with a lineshape due to second order quadrupole interactions, sodium-23 MASNMR spectra of a Na-Y zeolite... 

Fig. 16. (a) Extra-framework cation sites in X- and Y-type zeolites, (b) Far- infrared spectrum of Na-Y with band assignments to cation sites according to [232]. (c) Experimental IR spectrum in comparison to simulated spectra calculated according to the shell model and occupancy of different cation sites, (d) Experimental spectrum in comparison to power spectra simulated by MD at occupancy of different cation sites (parts c and d from [79] with permission)... 

As can be seen, two bands at 102 and 73 cm" arise. However, a reliable interpretation of this spectrum is quite difficult. The key problem in interpreting far-infrared spectra of silicon-rich zeolites such as ZSM-5 is connected with the fact that, due to the low cation concentration, structural information about cation sites are so far rather scarce. Under the mentioned conditions it would certainly be a substantial progress if the vibrational assignment in the far-infrared region could be assisted by other suitable cation-sensitive techniques which provide additional information. One way, as chosen in Ref. [363], is to start from X-ray absorption spectroscopy (XAS) giving access to the local environment of the cations and their coordination spheres. For the dehydrated Ba-ZSM-5 sample a six-fold oxygen coordination at a distance of 2.75 A was obtained for Ba " ions by EXAFS analysis of the XAS spectrum. In a second step, positions in the unit cell of ZSM-5, which fulfill these criteria, were searched by computer simulation... 

A typical Ag spectrum of zeolite A is shown in Fig. 16A. Simulation with Gaussian lines gives a somewhat better fit of the experimental spectrum than simulation with a Lorentzian line shape. The g and A values are in both cases equal (Table 5). When the spectrum becomes complex, as in the case of I > /2 nuclei, second derivative spectra are helpful in the assignment and the determination of the nuclearity (Fig. 16B). 

The spectra can become quite complex when the nuclear quadrupole coupling is appreciable, as for nitric oxide (NO) introduced into Na-A zeolite discussed in Chapter 6. It is also known that NO tends to dimerize forming an 5 = 1 species under these conditions [39]. A triplet state complex is formed interacting with one or more Na nuclei in the zeolite [40]. The spectrum obtained after FT of the three-pulse ESEEM signal in Fig. 2.22 is difficult to analyze by visual inspection. Methods to obtain the hyperfine and quadrupole couplings by simulations are described in Chapter 3. 

Combined zero-field and g-tensor anisotropy The g-tensor anisotropy can be appreciable for transition metal ion complexes, but also for some triplet state molecules. The Q-band spectrum of an (NO)2 surface complex in zeolite LTA has been analyzed to have rhombic symmetry for the g-tensor and the zero-field splitting, see Fig. 6.5 in Chapter 6. Complications due to overlap with another spectrum (in this example an NO surface complex) are common in practical applications. Variations of the experimental conditions, e.g. the sample composition (amount of nitric oxide in this example) and measurements at different microwave frequencies can then give support for the assignment. Refinement of the visual assignment by simulations as discussed in Section 3.4.1.7 is also frequently employed. 

Fig. 335 Experimental and simulated 3-pulse (a) and 4-pulse (b) ESEEM spectra of (NO)2 dimer in Na-LTA zeolite. The 4-pulse spectrum was obtained with the pulse sequence shown in Eig. 3.33 by varying ti, while t2 was kept equal to ti. The simulated curves were obtained with Aj = 4.6 MHz, A = 8.2 MHz, Qj = -0.3 MHz, Q = 0.6 MHz attributed to Na in an 5 = 1 complex of the type (NO)2-Na. The figure is adapted from [D. BigUno et al. Chem. Phys. Lett. 349, 511 (2001)] with permission from Elsevier...

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