Molecular Sieves: Crystalline Systems

Improved sensitivity and quantification for 29Si NMR experiments on solids using UDEFT (Uniform Driven Equilibrium Fourier Transform)

“…Si NMR spectroscopy has been used for the characterization of a wide range of solids, including silicon alloys [1][2][3][4], silicon-containing organic compounds [5,6] and polymers [7], silicon nitride and carbide ceramics [8,9], crystalline and amorphous silicates, including minerals [10][11][12], zeolites [13,14], cements [15,16], silica-supported catalysts [17][18][19], acid heterogeneous catalysts made of amorphous aluminosilicates [20] and glasses [21][22][23]. 29 Si is a spin-1/2 isotope, but its NMR sensitivity is small due to its low natural abundance (4.7 %), its moderate gyromagnetic ratio (29Si  0.21H) and its very long longitudinal relaxation times, T1, which can reach tens of hours [24,25].…”

Recent developments in solid-state NMR spectroscopy of crystalline microporous materials

“…45 This has generally limited its application in comparison to MQMAS, although it has been applied to phosphate-based frameworks. 4,5,46 Although useful structural information can be obtained from high-resolution spectra, more detailed information can be obtained by exploiting the interactions between nuclei, through experiments that "recouple" a particular interaction or that transfer magnetisation between nuclear species. 29 Experiments that use the dipolar coupling enable internuclear distances or proximities to be probed, while those that use the scalar coupling provide information on covalent connectivity.…”

“…11 The nature of the vertices and links depends on the class of material in question, and the same underlying topology (i.e., network connectivity) can be constructed from many chemically-distinct species. Aluminosilicate zeolites are composed of SiO 4 classified by the International Zeolite Association (IZA), which currently recognises 218 framework types, although more are added every year.…”

“…Readers are referred to more comprehensive recent reviews for more detail. [4][5][6][7][8][9][10] The structures of microporous materials can generally be considered as nets, with cationic vertices and anionic links between the vertices, as shown schematically in Figure 1(a) for the sodalite (SOD) net. 11 The nature of the vertices and links depends on the class of material in question, and the same underlying topology (i.e., network connectivity) can be constructed from many chemically-distinct species.…”

“…16,18 The (cationic) SDA typically remains in the pores of the AlPO after synthesis and its charge is balanced by the presence of Al-bound F − or OH Gallophosphates (GaPOs) generally exhibit frameworks analogous to other zeolites, although some frameworks are unique to GaPOs. 11 Like AlPOs, GaPOs are composed of strictly alternating GaO 4 and PO 4 tetrahedra, giving an overall neutral GaPO 4 framework.…”