27A1-NMR spectra

Figure 17. 27A1-NMR spectrum of hydrothermally dealuminated faujasite. (Reproduced with permission from ref. 62. Copyright 1987 The Royal Society of Chemistry.)...

All the investigated samples exhibit exclusively the signal centred around 55 ppm in the SP (single pulse) 27A1 MAS NMR spectrum. This signal corresponds to the A1 atoms with a tetrahedral coordination [15], The 3Q 27A1 MAS NMR method was employed to resolve the individual resonances corresponding to different tetrahedrally coordinated A1 atoms. 

Although it does not relate to polytype identification, we mention a 14N static and MAS-NMR study of WZ AIN, which revealed a fortuitously small Cq < 10 kHz [167]. For comparison, the 27A1 Cq in the same compound was measured to be 1.913 MHz, and had an unusual positive temperature coefficient that has been seen for other metal nuclei in WZ structures. The 14N MAS-NMR spectrum of BN yielded a Cq of 142 kHz corresponding to an axially-symmetric tensor in the hexagonal ring structure [167]. 

Figure 1-a shows the 27A1 MAS NMR spectrum of the AISBA sample. It exhibits three lines at 52 ppm, about 35 ppm and 0 ppm, corresponding to four-, penta- and hexa- coordinated aluminum species. This proves that a part of the aluminum source is incorporated in the framework of this sample, though there is still some non-framework aluminum. These non-framework aluminum can be eliminated by washing the solid in NH4CI solution (Figure 1-b). 

Equation (13) works well for materials with framework Si/Al ratios below ca. 10. For more siliceous zeolites, the 29Si MAS NMR spectrum is dominated by the Si (4 Al) signal and the estimation of composition becomes inaccurate. In these cases, the relative amounts of framework and nonframework aluminum can be estimated from 27A1 NMR spectra (Section HI,I). 

Samoson et al. (141) illustrate their arguments with the 27A1 MAS NMR spectrum of sillimanite. For the signal from six-coordinated Al in sillimanite, s = 9.1 kHz (148), which means that the signal cannot be narrowed by MAS however, for four-coordinated Al, s = 5.3 kHz (line shape which compares well with theoretical calculations is indeed found in the spectrum. 

Important structural information is provided by the 27A1 MAS NMR spectrum of silicalite, a porous solid isostructural with zeolite ZSM-S (see Section III,F). It has been argued (81) that silicalite has no aluminum and no cations in this structure and therefore no ion-exchange properties. In other words, it was claimed not to be a zeolite, and the original patent (80) describes it as a porous form of silica, and asserts that such aluminum as may be present is in the form of A1203 impurity. Because it is not possible to monitor the coordination of very small amounts of A1 (of the order of hundreds of ppm) by X-ray crystallographic methods, these claims could not be tested directly. Fyfe et al. (82) have demonstrated that ... 

Fig. 34. High-resolution 27A1 MAS NMR spectrum of silicalite at 104.22 MHz (155) 176,214 free induction decays were accumulated repetition time was 0.1 sec.

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).

Fig. 48. 27A1 MAS NMR spectrum at 130.32 MHz of zeolite omega dealuminated with silicon tetrachloride vapor (209).

Fig. 8 (a) A 29 i NMR spectrum of a monomeric Na silicate solution of the composition 1.0 mol% SiC>2, R = 0.1 and an 27A1 spectrum of a monomeric Na aluminate solution of the composition 1 M NaAlC>2 (b)-(d) 29si and 27A1 spectra of aluminosilicate solutions with increasing AI concentration. 29si spectral frequencies are referenced to Si(OH)4 and Al spectral frequencies are referenced to the octahedral Al + ion in an aqueous solution of AICI3. 

The intensity of the 27A1 MAS NMR spectrum of the realuminated sample R-3 (Figure 2) is greater than that of the dealuminated sample D-2. The F2 projection indicates that aluminium in the latter sample is in the tetrahedral coordination. The nutation spectra (Figure 3) clearly show that... 

Fig. 16. 27A1 NMR spectra of dehydrated and partially rehydrated VPl-5. a) MAS spectrum of dehydrated VPI-5. b) DOR spectrum of dehydrated VPI-5. c) DOR spectrum after 2 days of rehydration, d) DOR spectrum after 23 days of rehydration (24). Structure of VPI-5 (right) (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

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