Synthesis and characterization of a (Zn, Ti)-substituted layered silicate

Abstract

The incorporation of Ti^IV into the framework of zinc-substituted fluorohectorite is reported. Various spectroscopic techniques point to the substitution of Ti^IV into predominantly the octahedral rather than tetrahedral sublattice of the smectite framework. For clay particles prepared from a reactant composition with Ti/Ti + Si = 0.063 (ZTFH), energydispersive X-ray analysis of the particles observed by transmission electron microscopy indicates that Ti^IV accounts for ca. 6% of the Ti + Si. X-Ray absorption near-edge spectra indicate that the Ti^IV in these clay particles is predominantly in an octahedral coordination. This is supported by electron paramagnetic resonance (EPR) spectra of an oriented film sample in which Ti^IV is reduced to Ti^III by heating the sample to 450 °C in 30 Torr of CO. The observed single-ion Ti^III EPR spectrum shows dramatic dependence of the orientation of the clay film in the magnetic field. This orientation dependence is consistent with the principal g-tensor axis of the Ti^III site being perpendicular to the clay layers. The EPR parameters of this Ti^III species are consistent with Ti^III occupying an octahedral site. Three-pulse electron spin-echo modulation data taken at the Ti^III EPR line position shows modulation at the ⁷Li nuclear frequency and simulation indicates that Ti^III ions are at a distance of 0.30–0.35 nm from Li^I ions, a distance that is consistent with these two ions occupying adjacent sites in the octahedral sheet. The MAS NMR spectrum of the zinc/titanium-substituted fluorhectorite consists of an intense Q³ resonance at –96.4 ppm and a weak shoulder at ca.–92.5 ppm. These two resonances are interpreted as due to variations in the octahedral sheet composition. Therefore Ti^IV appears to have a strong preference for occupation of the octahedral sites in the zinc-substituted fluorohectorite framework.