Aluminosilicates, chemical shift

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.

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

Fig. 31. Plots of isotropic 29Si MAS NMR chemical shifts (in ppm from TMS) versus dTT (in A), the calculated total Si T nonbonded distance, for Si atoms in the five kinds of Si(nAl) tetrahedral environments. Aluminosilicates corresponding to the various points can be identified by consulting ref. US from which the figure is taken.

Alkali and alkaline-earth aluminosilicates are insoluble, and Muller et al. (138) resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/Al ratios and the pure TMA aluminate solution were studied. The molar ratio TMAOH Si Al varied from 3 0 2 to 9 6 2. Theoretically there are 15 distinct Q"(mSi) units with Q = Al (n 0-4 m 0-n). However, dimeric aluminate anions are found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of Al—O—Al linkages limits the number of possibilities to five... 

The work of Muller et al. (138), who measured 27A1 chemical shifts in TMA-aluminosilicate solutions, has been discussed in Section 111,1. Briefly, they identified four kinds of Al- centered units, i.e., Q°, Ql(l Si), Q2(2Si), and Q3(3Si), and suggested that no Al—O—Al linkages are present in aluminosilicate anions. 

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

Figure 6 Variation of the chemical shifts (ppm) of AI(OH)4 and AI(2Si) peaks with silica content for aluminosilicate solutions 1 e - 8e.

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)...

High resolution Si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. NMR has proven useful in identifying the connectivity of A1 to Si,... 

On the basis of the pioneering work of Lippmaa et aj (3) on 29 i MASNMR, and that of Engelhardt et a l (U) on 29 i NMR of silicate solutions, it became apparent that 29si chemical shifts are sensitive to the substituents present in the second coordination sphere. Correlations between chemical shift and structure were established, and these were soon utilized (5 7) for the structural studies of both soluble and insoluble silicates and aluminosilicates. A wide variety of zeolitic solids has by now been investigated (6-18) using 27/ i and several other... 

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

Engelhardt, G., and R. Radeglia (1984). A semiempirical quantum chemical rationalization of the correlation between Si-O-Si angles and Si NMR chemical shifts in silica polymorphs and framework aluminosilicates. Chem. Phys. Lett. 108, 271-74. 

Relationships between Si NMR Spectra and Structure/Bonding Eour-Coordinated Si-O-Compounds Tetrahedral Si Chemical Shifts in Silicates Si Chemical Shifts in Aluminosilicates Effects of Other Nearest Neighbours on the Si Shift... 

The aluminosilicates constitute an important class of inorganic compounds. The occurrence of aluminium atoms in the second coordination sphere of the silicon, to which they are bonded through oxygen atoms, produce systematic changes in the Si chemical shift in a similar way to the changes which are associated with differences in the... 

Thus, the secant relationship (4.20) with the angular definition modified for aluminosilicates (MacKenzie et al. 1985) has been used to shed light on the possible existence of a Si-Al spinel suggested to form when the clay mineral kaolinite is heated to 980°C. On the basis of the known structure of the closely-related 7-alumina spinel, the T-O-T angle of the postulated tetrahedral Si site in such a structure predicts a chemical shift of - 79 ppm for this site. Such a resonance would normally be masked by the broad resonance of the amorphous Si02 also present, but when this material was selectively removed by leaching with KOH solution, the predicted Si peak was detected (MacKenzie et al. 1996) (Figure 4.13). Measurements of the relative intensity of this... 

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... 

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