29Si MAS NMR

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). 

Figure 2. 29Si MAS NMR spectrum of zeolite ZSM-4 recorded at 11.7 T with 4.0 kHz MAS and (inset) the previously proposed peak assignments.

Samples of Y faujasites were prepared by sodium exchange of a starting ultrastable Y zeolite (H form, denoted in the following as USY). Global Si/Al ratio is 16 according to X fluorescence measurements framework Si/Al is 21 as measured by 29Si MAS NMR. 

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... 

All preparations were structurally characterized by means of XRD (Siemens 5005). TEM imaging was performed with a Philips CM200 instrument. 27A1 and 29Si MAS NMR (Broker 500 MFlz and 360 MFlz respectively) was used to study the microporous phase and the kinetic of its formation. The relaxation delays were 0.2s and 200s respectively. Acidity was determined by the adsorption of carbon monoxide after activating the samples in vacuum (10 6 mbar) at 450°C for 1 h. The spectra were recorded on a Equinox 55 Broker spectrometer with a resolution of 2 cm 1 and normalized to 10 mg of sample. 

Recrystallization procedure applied to the amorphous aluminosilicates of different chemical composition resulted in the formation of the dispersed zeolitic domains of the FAU and BEA structure in porous matrices. The structural transformation into the composite material was proved with TEM, XRD and 27Al and 29Si MAS NMR spectroscopies. The IR data revealed that strong Bronsted acid centers were main active sites generated in the composite materials, irrespectively of the Al content. 

Figure 1 29Si MAS NMR spectrum of Si-CHA samples. (Full line) before (starting material) and (Dotted line) after 4 water intrusion-extrusion cycles. The spectral region between -96 and -108 ppm is expanded in the insert.

All unmodified and modified MCM-22 samples were subsequently analyzed by 27A1 and 29Si MAS-NMR using a Varian VXR 300S. 

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 3 a) 29Si MAS-NMR and b)27Al MAS-NMR of parent MCM-22 with various Si/Al-ratios. 

The 2VSi MAS-NMR of the MCM-22 zeolites with higher Si/Al ratio (Fig. 3a), shows a sharper distinction between the individual 29Si MAS-NMR peaks, ascribed to the various Si-positions in the framework. Similarly as for the steam-treated samples, this can be explained by a lower contribution of Qm(nAl) species (with 4 > m > n > 1) (see for instance Fig. lb and Fig. 3a). Furthermore, the 27A1 MAS-NMR shows comparable... 

The 27A1 and 29Si MAS-NMR show that due to the alkaline treatment, silicon is extracted from the framework and defect Si-OFl groups are formed. The aluminum is unaffected by the alkaline treatment. The steam treatment leads to the extraction of aluminum from the framework as evidenced by the reduced intensities in the 27A1 MAS-NMR, and the shift of the 29Si MAS-NMR towards those of higher Si/Al-ratios. 

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...

Figure 15, Dealumination of zeolite Na-Y using SiClk vapor studied by 29Si-MAS NMR at 79.8 MHz. Key top, parent Na-Y zeolite and bottom, after treatment with SiClk. (Reproduced from Ref. 30. Copyright 1982, American Chemical Society.)...

Figure 17. 29Si-MAS NMR spectrum at 79.8 MHz of zeolite ZK-4 showing five peaks at the chemical shifts indicated, corresponding to the silicon environments shown in the figure. The vertical arrow indicates the frequency of the single peak in the spectrum of zeolite-A,...

Fig. 2 Experimental analysis of the chemical shift anisotropy of high-silica ZSM-5 zeolite, (a) 29Si MAS NMR and (b) extracted CSA lineshapes from a two-dimensional CSA recoupling sequence dashed lines are simulated lineshapes. Adapted with permission from [79]. Copyright 2008 American Chemical Society...

Fig. 8 13C and 29Si MAS-NMR of two SiC polytypes. Left panel-, see text for assignments. Right panel-. Experimental and calculated ultra-slow MAS-NMR spectra at ultra-high field of 21.1 T. Modified and reprinted with permission from [133]. Copyright 2009 by the American Chemical Society... 

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.)...

Figure 4.32 29Si MAS NMR spectrum of a NaY zeolite (Si/Al = 2.6) and the assignment of the different signals.

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.

---