  Layered double hydroxides (LDHs) serve a score of applications in catalysis,
drug delivery, and environmental remediation. Smarter crystallography,
combining X-ray diffraction and NMR spectroscopy revealed how interplay between
carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH.
Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH
10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal
treatment to avoid extensive recrystallisation. In ideal configuration, all Al
cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR
revealed two different Al local environments were present in all samples in a
ratio dependent on synthesis pH. NMR-crystallography, integrating NMR
spectroscopy and X-ray diffraction, succeeded to identify them as Al residing
in the highly ordered crystalline phase, next to Al in disordered material.
With increasing synthesis pH, crystallinity increased, and the side phase
fraction decreased. Using 1 H-13 C, 13 C-27 Al HETCOR NMR in combination with
27 Al MQMAS, 27 Al-DQ-SQ measurements and Rietveld refinement on
high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs
for CO3 2-over HCO3-was linked to strict Al and CO3 2-ordering in the
crystalline LDH. Even upon equilibration of the LDH in pure NaHCO3 solutions,
only CO3 2-was adsorbed by the LDH. This reveals the structure directing role
of bivalent cations such as CO3 2-during crystallization of [M 2+ 4M 3+ 2(OH)2]
2+ [A 2-]1.yH2O LDH phases.
