Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy
Paul J. Sideris,1,2
Ulla Gro Nielsen,1,2*
Zhehong Gan,3
Clare P. Grey1,2
The anion-exchange ability of layered double hydroxides (LDHs)
has been exploited to create materials for use in catalysis,
drug delivery, and environmental remediation. The specific cation
arrangements in the hydroxide layers of hydrotalcite-like LDHs,
of general formula Mg
2+1–xAl
3+xOH
2(Anion
n–x/n)·
yH
2O,
have, however, remained elusive, and their elucidation could
enhance the functional optimization of these materials. We applied
rapid (60 kilohertz) magic angle spinning (MAS) to obtain high-resolution
hydrogen-1 nuclear magnetic resonance (
1H NMR) spectra and characterize
the magnesium and aluminum distribution. These data, in combination
with
1H-
27Al double-resonance and
25Mg triple-quantum MAS NMR
data, show that the cations are fully ordered for magnesium:aluminum
ratios of 2:1 and that at lower aluminum content, a nonrandom
distribution of cations persists, with no Al
3+-Al
3+ close contacts.
The application of rapid MAS NMR methods to investigate proton
distributions in a wide range of materials is readily envisaged.
2 Center for Environmental Molecular Sciences, Stony Brook University, Stony Brook, N11794–3400, USA.
3 Center of Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA.
* Present address: Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark.
To whom correspondence should be addressed. E-mail: cgrey{at}notes.cc.sunysb.edu
