29Si MAS NMR spectra

Figure 2. 29Si MAS NMR spectra of the silica substrate treated with CH3Si(OC2H5)3 (1) and n-CgHi7Si(OC2H5)3 (2), respectively. 

Interesting results have been presented by Baleizao et al. for chiral vanadyl Schiff base complex [50].120 The 29Si MAS NMR spectra have confirmed covalent grafting of the vanadyl complex into the silicate skeleton. 

The ZSM-4 sample was prepared following the previously described procedures.[20] The elemental analysis showed that the Si/Al ratio was 3.0. 29Si MAS NMR spectra were recorded at 11.7 T MHz on a Varian InfinityPlus 500 spectrometer on a sample loaded in a 7.5 mm MAS rotor spinning at 4 kHz using a rc/2 rad pulse length and a recycle delay of 360 s. The 29Si chemical shifts are referenced with respect to an external solution of TMS (5Si = 0.0 ppm). 

Si MAS NMR spectra of Ti-Beta gels and powders were recorded on a 600 MHz (14.1 T) Varian NMR spectrometer. The spectra of samples a) to f) were obtained using a... 

Increased dealumination resulted in minor changes in the 29Si MAS-NMR spectra (Fig. lb), indicating that the zeolite structure remains intact. The appearance of sharper peak shoulders between -107 and -117 ppm is ascribed to a reduced contribution of Qm(nAl) Si(OSi)m n(OAr)n(OH)4 m sites (with 4 > m > n > 1). This, slightly shifted, contribution overlaps with the all-silica contribution and masks a clear distinction of the individual peaks in the parent material. 

Figure 1.29Si MAS-NMR spectra of a) H-MCM-22 treated at different alkaline (NaOH) concentrations for 45 min at 323 K and b) H-MCM-22 steamed at 773 K for various periods.

The steam treatment does however affect the Al-surroundings in the zeolite crystal. As seen in Fig. 2b, the intensity of both the tetragonally (at 0 ppm) and octahedrally (at 55 ppm) coordinated aluminum species decreases considerably after steam treatments for more than 4 h. Steam treatment for more than 8 h did not lead to a further decrease in the signal intensities. The decreases confirm that aluminum is extracted from the framework during the steaming process, as was also concluded from the 29Si MAS-NMR spectra (Fig. 1 b). This may lead to the formation of additional (micro) porosity, but the aluminum extraction could negatively affect the catalytic activity. 

Figure 13. 29Si-MAS NMR spectra of analcite (left) and zeolite Na-Y (right). Key top, deconvolved spectra middle, 79.5 MHz spectra (400 MHz 1H) and bottom, 17.9 MHz spectra (90 MHz 1H). (Reproduced with permission from Ref. 14. Copyright 1982, J. Magn. ResonJ...

Conclusions, some of them contrary to the above, were reached more recently by Zhuang et al. (145) from a combination of 31P and 1H MAS NMR spectroscopy of adsorbed trimethylphosphine. These authors found not only Lewis acid sites (vide infra), but also Brpnsted acid sites in TS-1 (145). They claimed that the 1H, 29Si MAS NMR spectra and the resonance related to Brpnsted acid sites in the 31P MAS NMR demonstrated clearly that the presence of Ti in the framework results in the formation of a new OH group, titanols, which is more acidic than the silanols of silicalite-1 (145) . The peak at 4.3 ppm in the 31P MAS NMR spectra was assigned to a ((CH3)3P-H)+ complex arising from the interaction of (CH3)3P with Brpnsted acid sites present on TS-1. The origin of this proton is not clear at present, especially because the MAS NMR spectra of the same TS-1 samples did not differ significantly from those of silicalite-1 (145) the latter, when free from impurities, is not known to be a Brpnsted acid. 

Figure 1.16 29Si MAS NMR spectra for NaY zeolites with three different (0, 1, and 3 wt%) Ru loading [121], The slight changes in relative intensities among the different peaks seen in these data are interpreted in terms of changes in Al coordination around the individual silicon atoms, as indicated by the diagram on the right [122], (Reproduced with permission from Elsevier and The American Chemical Society.)...

Si MAS NMR spectra of the uncalcined MCM-41 samples synthesized normally and with TPA+ and Na+ are shown in Fig 4. It was observed that the ratio of Q4/Q3 peaks was higher in samples synthesized with additional cations. The effect was most pronounced with TPA" as the additional cation. The higher Q4/Q3 ratio indicates that the silicate polymerization during the formation of the mesostructure was enhanced by the presence of the additional cations, Upon calcination, the free silanol groups are forced to condense to form Si-O-Si bond and 29Si MAS NMR of the samples showed predominantly Q4 peak. However, these... 

Figure 2.29Si MAS NMR spectra of (a) siliceous MCM-41, (b) C2H5-grafted MCM-41 and (c) C2H4-Si203 hybrid mesoporous materials.

Figure 3. 29Si-MAS.NMR spectra of Figure 4. Carbon content of trimethylsilylated...

The quantitative analysis of the deconvolved 29Si-MAS-NMR spectra permits to calculate the SiMe3/SiC>2 ratios which give a close value to that obtained by chemical analysis as it is shown in figure 4 indicating that there is no unreacted HMDS located within the pores of Ti-MCM-41. 

Figure 2 29Si MAS NMR spectra of silicalite-1 (a) and MCM-41 (b) based molecular sieves (as = as-synthesized, c = calcined)...

Figure 2a shows the 29Si MAS NMR spectra of calcined NbS-1, TaS-1 and ZSM-5 The spectral shapes of NbS-1 and TaS-1 are similar to those of aluminum-containing ZSM-5 with ns/nAi = 45, Three lines are observed for NbS-l(41) and TaS-l(74) whose chemical shifts are -103, -112 and -115 ppm. The line at -103 ppm is due to the presence of [SiO (OH)] units in defect sites within the silicalite-1 structure This assignment was confirmed by H - 29Si crosspolarization (CP) experiments in which the intensity of the signal at -103 ppm increased... 

In order to confirm this the trimethylsilylation of MCM-41 was carried out. The surface -OH groups is expected to be converted into trimethylsilyl groups. Actually it was observed in 29Si MAS NMR spectra that the shoulder peak for Q3 sites decreased remarkably and the peak for Q4 sites became almost symmetric by the... 

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.

Vaughan et al. (76) and Thomas et al. (77) measured 29Si MAS NMR spectra of the sodium forms of (Si,Ga)-sodalites and (Si,Ga)-faujasites. All preparations in ref. 76 contained some aluminum (attempts at preparing completely Al-free compounds were unsuccessful), but the amounts involved were so small (less than 5%) that the influence of Al on 29Si spectra was... 

In the light of what has been said in Sections III,B-III,D about the appearance and interpretation of 29Si MAS NMR spectra of zeolites, one might expect the spectrum of a highly siliceous zeolite to be uncomplicated displaying a single Si(4Si) signal, sometimes with a smaller Si(3Si) resonance,... 

A few cases have come to light in which crystallographic nonequivalence of tetrahedral sites for silicon has been reflected in the 29Si MAS NMR spectra of zeolites. The most complex and the best resolved example is the spectrum of silicalite which was discussed in Section III,F another is the spectrum of the well-ordered natural zeolite scolecite (57), which contains two signals corresponding to nonequivalent Si(3 Al) units (see Fig. 23). The correctness of... 

Fig. 24. 29Si MAS NMR spectra at 79.80 MHz (ahove) of zeolite Y, zeolite omega (synthetic mazzite), offretite, and mordenite, and their dealuminated forms (below).

Fig. 29. 29Si MAS NMR spectra (101) at 39.76 MHz of silicalite (Si/Al) > 4400) containing sorbed organic molecules. (A), parent material (B), containing ethanol (C), containing 1-propanol (D), containing n-decane (E), containing benzene.

Woessner and Trewella (132) observed indirect (electron-coupled) nuclear spin interactions in the 29Si MAS NMR spectra of low albite. Three signals were measured, corresponding to T2M, T20, and T,M silicon sites, respectively. The last two signals were split and the magnitude of the splitting was field independent. This is the first time such effects, which are not normally observed in true solids (3), have been found for the 29Si nucleus. 

The analysis of 29Si MAS NMR spectra of layer silicates with a wide range of tetrahedral compositions (Si/Al ratio 2.7-1.7) indicates (483, 486) that the distribution of Si and Al in these materials is indeed determined by (i) the local balance of charges, and (ii) the Loewenstein rule. In muscovite, phlogopite, and vermiculite, aluminum is randomly distributed in... 

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