27Al chemical shifts

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

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. 

Nuclear magnetic resonance spectroscopy of the 27Al nucleus, both in solution and the solid state, has proved useful for the determination of the coordination numbers and, to some extent, the stereochemistry of Al. Four-, 5-, and 6-coordinate geometries can be differentiated on the basis of their chemical shifts. 

Fig. 9. Evolution with time of the NMR characteristics of AIPO4—02. (a) pH variation at 150°C from, 4N NMR (b) A1 atomic fraction in solution versus time at 150°C (c) in situ 27Al NMR spectra during the first 5 h of synthesis at 150°C (d) in situ l9F at 150"C (e) in situ 27A1 NMR spectra during nucleation and growth (f) chemical shift evolution of the prenucleation cluster peak vs time (with courtesy of C. In Gerardin and F. Taulelle).

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

Al MAS NMR spectra of an AIPO4-8 sample in hydrated form and after dehydration in vacuum at 293 K are shown in Fig. 11. In the dehydrated sample, the aluminium resonance is at 35 ppm, characteristic of tetrahedral coordination. A trace of aluminium exhibits a chemical shift of ca. 10 ppm, typical of aluminium monohydrate. It indicates that the dehydration of this particular sample is not... 

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