11-02
M.H.W. Verkuijlen, R. de
Gelder, P.J.M. van Bentum, A.P.M. Kentgens,
Oxidation products of NaAlH4 studied by Solid-State
NMR and X-ray diffraction,
J. Phys. Chem. C 114 (2011) 7002-7011
Abstract
The oxidation products of NaAlH(4) were
studied using solid-state nuclear magnetic resonance experiments combined with
X-ray diffraction (XRD) experiments. We studied the coordination of aluminum
with oxygen using single-pulse (27)Al experiments. (27)Al-{(1)H} rotational
echo double-resonance, (23)Na-{(27)Al}/(27)Al-{(23)Na} transfer of population
in double-resonance (TRAPDOR), and combined (27)Al-(23)Na
TRAPDOR-multiple-quantum magic-angle spinning (MQMAS) experiments were used to
get qualitative information about Al-H and Al-Na proximities. These NMR
experiments show that the intermediate oxidation product is an amorphous sodium
aluminum hydroxide with Al in a tetrahedral coordinated site. A (27)Al MQMAS
experiment shows distributions of chemical shift and quadrupolar
coupling parameters. Therefore, the material is probably highly disordered, and
multiple compounds maybe present. XRD measurements show that this phase indeed
lacks long-range order. The end product contains mainly octahedral coordinated
aluminum hydroxide. Crystalline aluminum hydroxides, consisting of gibbsite and
bayerite, and sodium carbonate hydrates, Na(2)CO(3)
center dot H(2)O and Na(3)(CO(3))(2) center dot 2H(2)O, are observed by XRD
measurements. Our results show that relatively harmless and nontoxic compounds
are formed. However, the materials should handled carefully because of the
strong basicity of compounds like sodium carbonate.