Solid-state Si-29 MAS NMR study of titanosilicates

The relationship among the Si-29 NMR chemical shift, the number of titanium polyhedra coordinating each silicon tetrahedron, and the oxygen formal charge is examined for titanosilicates with known crystal structures. A systematic downfield chemical shift is observed when increasing numbers of titanium polyhedra coordinate a given silicon tetrahedron, which is similar to the trend seen for aluminosilicates. The chemical shift increases linearly with increasing oxygen formal charge for both titanosilicates and aluminosilicates. Summation of the oxygen formal charge accounts for effects on the chemical shift from coordination geometry, nonbridging oxygen, and cation valence. Silicates with a high oxygen formal charge, or more than one nonbridging oxygen per silicon, deviate from the linear trend observed for aluminosilicates and titanosilicates. The chemical shift/structure correlations derived from titanosilicates with known crystal structures are used to determine the local bonding configurations for a new titanosilicate, CsTiSi2O6.5, and for an Engelhard formulation designated ETS-4.