Interlayer structure and dynamics of Cl--LiAl2-layered double hydroxide:: 35Cl NMR observations and molecular dynamics modeling

Cl-35 NMR experiments and molecular dynamics modeling provide significant new insight into the structure and dynamics of the interlayer species in the layered double hydroxide (LDH) LiAl2(OH)(6)Cl.nH(2)O. The LiAl2 LDHs have wide ranging potential applications as catalysts, as filtration and exchange materials, and in drug and gene-therapy delivery systems, and many of these applications depend on the behavior of interlayer species. Room-temperature MAS and static Cl-35 NMR data indicate that the Cl- environments vary significantly with hydration state. The fully hydrated paste form has three types of Cl-: surface Cl- that yields a sharp solution-like peak at 0 ppm, interlayer Cl- that yields a resonance dominated by uniaxial chemical shift anisotropy (CSA), and interlayer Cl- that yields a structureless peak that is not fully narrowed by MAS. The relative abundance of the CSA dominated peak increases with decreasing hydration and is the only resonance observed for fully dried samples. For the dehydrated samples, this resonance corresponds to the one Cl- site in the crystal structure. Both types of interlayer Cl-35(-) sites are present for paste and partially hydrated samples, and comparison to the results of molecular dynamics modeling suggests that the CSA-dominated resonance is due to Cl- on two types of prism sites with time averaged uniaxial symmetry and that the structureless peak is due to Cl- on dynamically averaged sites stabilized by H-bonding from both OH groups and interlayer waters. Variable-temperature NMR experiments show a disordered, static interlayer at -90 degreesC. With increasing temperature, surface Cl- has observable dynamical motion at -60 degreesC, whereas appreciable motion in the interlayer does not begin until above -30 degreesC. At 70 degreesC, signal for the two types of interlayer sites is nearly fully averaged, indicating site exchange at frequencies of at least 103 Hz. This observation is consistent with relatively rapid site hopping observed in the MD simulations. The simulations also demonstrate the presence of hopping libration of the interlayer water molecules, as previously observed for other LDH compounds, provide important insight into the structure of the interlayer H-bonding network, and provide structural understanding of the smaller interlayer water content of the LiAl2 LDH phases compared to the Ca2Al and Mg3-xAlx phases.