Aluminosilicates, chemical shift ranges

Figure 4.33 Si MAS NMR chemical shift ranges for aluminosilicates. This figure is redrawn based on references [152-155], Here Q" is the tetrahedral silicon connected to n aluminum atoms n = 0 ) via oxygen bridges.

TABLE II 27AI NMR CHEMICAL SHIFT RANGES (PPM) FOR ALUMINOSILICATE SOLUTIONS. 

NMR spectroscopy is ideally suited for characterizing the silicate and aluminosilicate species present in the media from which zeolites are formed. The nuclei observable include 29si, 27 Al, and all of the alkali metal cations. The largest amount of information has come from 29gi spectra ri-31. This nucleus has a spin of 1/2, no quadrupole moment, and a chemical shift range of about 60 ppm. As a consequence, it is possible to identify silicon atoms in specific chemical structures. 27 Al, on the other hand, is a spin 3/2 nucleus and has a sizeable quadrupole moment. This results in broad lines and limits the amount of information that can be extracted from 27Al spectra. 

The silicate species discussed in the preceding section can react with aluminate anions, Al(OH)4 to produce aluminosilicate anions. Si NMR spectra of solid silicates and aluminosilicates indicate that the replacement of Si by A1 in the second coordination sphere of a give Si causes a low-field shift of about 5 ppm. Since each Si atom can have up to four metal atoms in its second coordination spere, fifteen possible Qn(mAl) structural units can be envisioned. The estimated chemical shift ranges for these units are given in Table 3. It is apparent from this table that the 29si spectrum of an aluminosilicate solution in which A1 and Si atoms were statistically distributed would be much more complex than that of an analogous solution containing only silicate species. 

Table III. Estimated 2 Si Chemical Shift Ranges of Q11 and Q mAl) Structural units of Silicate and Aluminosilicate Anions in Solutions (18)...

Chemical Shift Ranges for Si NMR of Aluminosilicates (3) Si-nAl Chemical Shift (ppm vs IMS)... 

The situation in aluminosilicate glasses is complicated by the additional effects of next-nearest-neighbour Al on the Si shifts. Although some Q" sites can be unambiguously assigned, others such as Q (OAl) and Q (3A1) occur in the same chemical shift range and cannot be differentiated on this basis. Systematic variations in the Si peak positions and widths of sodium aluminosilicate and calcium aluminosilicate glasses can... 

Fig. 6. chemical shifts of Si(nAl) imits in zeolite frameworks. The dotted lines for Si(4Al) designate the chemical shift range observed for 1 1 aluminosilicate sodalites with different cage... 

Fig. 13. Ranges of Si chemical shift for Si(nAl) building blocks in framework aluminosilicates.

The chemical shifts of the octahedral A1 are typically in the range 0-10 ppm, but no systematic change of the octahedral chemical shift with aluminosilicate composition was detected (Kinsey et al. 1985). 

Figure 5. Typical ranges of29Si chemical shifts ofQ4(mAl) units in aluminosilicates. (Reproduced with permission from reference 40. Copyright 1990.)...

Figure 5 Typical range of Si NMR chemical shift values of Q"(0AI) and Q (0-4AI) structural units in aluminosilicates. Typical range of Al NMR chemical shift values for aluminum atoms depending on its state of coordination.

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