From Layered Double Hydroxides to Layered Double Hydroxide-Based Nanocomposites-A Solid-State NMR Study

摘要

The local structure of pristine and surfactant-modified aluminum layered double hydroxides (LDH) has been characterized by ~(27)Al and ~1H solid-state NMR. Values for the ~(27)Al quadrupole coupling constants and the isotropic chemical shifts obtained from ~(27)Al triple-quantum (3Q)MAS NMR have been applied to fit the one-dimensional ~(27)Al MAS spectra and to characterize the structural changes in the different stages of LDH modification by the regeneration method quantitatively. Six-coordinated (octahedral) aluminum is found in all LDHs studied, indicating that the LDH structure is composed of octahedra formed by six hydroxyl groups. Two octahedral Al sites with different local environments have been distinguished in the ~(27)Al 3QMAS NMR spectra. The calcined and subsequently surfactant-treated LDHs show additionally a considerable fraction of 4-fold coordinated (tetrahedral) aluminum. The quantitative analysis shows that ca. 29% of aluminum has been converted from octahedral to tetrahedral sites in bis(2-ethylhexyl)phosphate (BEHP)-modified LDH, while modification with sodium dodecyl benzenesulfonate (SDBS) leads to formation of only ca. 17% tetrahedral Al sites. The presence of tetrahedral aluminum is attributed to calcination products, which are not converted after rehydration. Differences in the relative contents of AlO4 in surfactant-modified LDH is explained by the presence of interlayer water molecules in SDBS-LDH that is confirmed by ~1H MAS NMR.