Abstract
The local structure of pristine and surfactant-modified aluminum layered double hydroxides (LDH) has been characterized by 27Al and 1H solid-state NMR. Values for the 27Al quadrupole coupling constants and the isotropic chemical shifts obtained from 27Al triple-quantum (3Q)MAS NMR have been applied to fit the one-dimensional 27Al 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 27Al 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)...
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