Sandia octahedral molecular sieves (SOMS): Structural and property effects of charge-balancing the M^IV-substituted (M = Ti, Zr) niobate framework

Sandia octahedral molecular sieves (SOMS) is an isostructural, variable composition class of ion exchangers with the general formula Na₂Nb_2-x M^IV_xO_6-x (OH)_x·H₂O (M^IV = Ti, Zr; x = 0.04-0.40) where up to 20% of the framework Nb^V can be substituted with Ti^IV or Zr^IV. This class of molecular sieves is easily converted to perovskite through low-temperature heat treatment (500-600 °C). This report provides a detailed account of how the charge imbalance of this Nb^V-M^IV substitution is compensated. X-ray powder diffraction with Rietveld refinement, infrared spectroscopy, thermogravimetric analysis, ²³Na MAS NMR, and ¹H MAS NMR were used to determine how the framework anionic charge is cation-balanced over a range of framework compositions. All spectroscopic evidence indicated a proton addition for each M^IV substitution. Evidences for variable proton content included (1) increasing OH observed by ¹H MAS NMR with increasing M^IV substitution, (2) increased infrared band broadening indicating increased H-bonding with increasing M^IV substitution, (3) increased TGA weight loss (due to increased OH content) with increasing M^IV substitution, (4) no variance in population on the sodium sites (indicated by Rietveld refinement) with variable composition, and (5) no change in the ²³Na MAS NMR spectra with variable composition. Also observed by infrared spectroscopy and ²³Na MAS NMR was increased disorder on the Nb^V/M^IV framework sites with increasing M^IV substitution, evidenced by broadening of these spectral features. These spectroscopic studies, along with ion exchange experiments, also revealed the effect of the Nb^V/M^IV framework substitution on materials properties. Namely, the temperature of conversion to NaNb_1-xM^IV_xO₃ (M = Ti, Zr) perovskite increased with increasing Ti in the framework and decreased with increasing Zr in the framework. This suggested that Ti stabilizes the SOMS framework and Zr destabilizes the SOMS framework. Finally, comparing ion exchange properties of a SOMS material with minimal (2%) Ti to a SOMS material with maximum (20%) Ti revealed the divalent cation selectivity of these materials which was reported previously is a function of the M^IV substitution in the framework. A thorough investigation of this class of SOMS materials has revealed the importance of understanding the influence of heterovalent substitutions in microporous frameworks on material properties.