29Si MAS NMR spectroscopy

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. 

The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by 27A1 MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and 29Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. Re-inserted Al gives rise to Bronsted acidity. 

Fig. 46. Dealumination of zeolite Na-Y using SiCl4 vapor studied by 29Si MAS NMR spectroscopy at 79.80 MHz (206). (a) Parent material (Si/Al = 2.61) (b) after complete dealumination (corresponding to the 27A1 MAS NMR spectrum in Fig. 47d) (Si/Al = SS).

Two-dimensional DQ H MAS-NMR spectra were used to investigate the local structure of a surfactant-templated silicate thin film.498 Silicalite-1 powders (20-1000 nm. crystals) were characterised by 29Si MAS-NMR spectroscopy.499 13C and 29Si solid-state NMR data were used to characterise organic-inorganic mesoporous silica-based materials.500... 

A phyllosilicate-rich clay was studied by 29Si MAS-NMR spectroscopy.520 13C CP/MAS-NMR and 29Si MAS-NMR spectra were used to characterise silylated montmorillonites, and to estimate the extent of the silylation reaction.521 Ab initio calculations have been made of 29Si NMR chemical shifts for silicate complexes with carboxylates, amino acids and multicarboxylic acids.522 Structural changes on thermal treatment of kaolinite were followed using 29Si MAS-NMR spectra.523... 

NMR spectra, and chemical analysis by X-ray fluorescence, the five samples El to E5 are dioctahedral 2 1 layer silicates, mainly constituted of Si and Al as framework-forming elements, containing Al for Si substitutions in tetrahedral layers and fluorine for hydroxyl groups substitutions. 29Si MAS NMR spectroscopy gives rise to their (Si/AI)iv tetrahedral ratio with a good reliability (Table 10-5). The well-controlled and well-known hydrogels composition indicates that only dioctahedral layer silicates, with two Al and one vacancy per... 

I. Jaymes, A. Douy, D. Massiot and J. P. Coutures, Characterization of mono- and diphasic-mullite precursor powders prepared by aqueous routes. 27AI and 29Si MAS-NMR spectroscopy investigations, J. Mater. Sd., 31,4581-4589 (1996). 

The second approach consists of synthesizing first the complex MLra 1(L X) with the desired ratio (L )/(M) this complex bears the reactive fragment X which then reacts with the surface of the silica. This method is of limited interest, because the synthesis and isolation of these functionalized complexes is not straightforward. One of the successful examples concerns the synthesis of nickel carbonyl complexes anchored to the surface via two bonds in an attempt to increase the stability through a sort of chelate effect. Initial attempts to achieve this by the methods described in Equation(5) (initial functionalization of silica) and Equation(6) (initial functionalization of complex) failed, as demonstrated by 29Si and 31P CP MAS NMR spectroscopies.51... 

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. 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

TS-1 has been obtained by the hydrothermal crystallization of a gel obtained from TEOS and TEOT in the presence of tetrapropylammonium hydroxyde (TPA-OH). The structure of TS-1 has been demonstrated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), microprobe analysis, and 29Si magic-angle spinning (MAS) NMR spectroscopy. Furthermore, an absorption band in the IR spectrum at 960 cm-, present in TS-1 and absent from that of silicalite, was initially considered a fingerprint for the characterization of TS-1. However, later work (discussed below) has shown that this band is also present in many other silica compounds, and therefore its relation to framework Tilv is not straightforward. 

The interaction between the PFES and silica was investigated by NMR spectroscopy. JH and 29Si MAS NMR spectra of SiC>2 and PFES-Si02 were made under the same... 

Tetrahedral and octahedral aluminium sheets in montmorillonite were studied by 27A1 MQ/MAS-NMR spectroscopy.405 19F, 27A1 and 29Si MAS-NMR spectra were used to characterise montmorillonite with various contents of Mg(II) or Zn(II) substituted for aluminium in octahedral sheets.406 Solid-state NMR spectra can be used to characterise and quantify the nanodispersion of variously modified montmorillonite clays in polystyrene matrices.407 27A1 and 29Si MAS-NMR spectra were used to follow hydration processes for Portland cement.408,409... 

Si 3H CP/MAS-NMR was used to probe interactions of transition metal carbonyl clusters (Ru3H(CO)M, Os2FI(CO)ii, Co(CO)4 ) deposited in the mesoporous aluminosilicate material MCM-41.637 A 29Si MAS-NMR study has been made of rhodium-amine complexes on Si02 surfaces.638 31P CP/MAS-NMR spectroscopy was able to characterise Cu6(TePh)6(PPh2Et)5 clusters in the pores of MCM-41.639... 

Discussions of old and new techniques used to study the surface of silica, including diffuse reflectance Fourier transform (DRIFT) infrared spectroscopy, 29Si cross polarization magic-angle nuclear magnetic resonance (CP MAS NMR) spectroscopy, as pioneered by Maciel and Sindorf... 

Geminal silanols are silanediols groups located in Q2 silicon sites (1). Their existence was postulated by Peri and Hensley (49) but not confirmed experimentally until the advent of solid state 29Si CP MAS NMR spectroscopy. On the basis of this technique, Fyfe et al. (50) determined in the particular case of a commercial silica gel that the ratio of Q4/Q3/Q2 silicon sites was 8.8/5.7/1. 

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