Structural characterisation of aluminium layered double hydroxides by Al-27 solid-state NMR

Al-27 solid-state NMR has been applied to study the local structure of pristine and chemically modified aluminium layered double hydroxides (LDH). The pristine LDH only shows six-fold coordinated, octahedral, aluminium, while the calcined and subsequently surfactant treated LDH sample shows a significant fraction of four-fold coordinated tetrahedral aluminium. The co-existence of two types of octahedral sites with different quadrupolar parameters is clearly observed in both samples. Quadrupolar coupling constants and isotropic chemical shifts have been measured from the Al-27 triple-quantum MAS NMR allowing to fit the Al-21 MAS spectra and quantify the different species in the samples. The quantitative analysis reveals that 30% of the aluminium is in four-fold coordination in the surfactant-modified LDH. We show that this chemical modification retains the two types of AlO6 sites with a decreased intensity of the site showing the lowest quadrupolar coupling constant. (C) 2009 Elsevier Inc. All rights reserved.