Oxidation Products of NaAlH4 Studied by Solid-State NMR and X-ray Diffraction

Abstract

The oxidation products of NaAlH4 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 27Al experiments. 27Al−{1H} rotational echo double-resonance, 23Na−{27Al}/27Al−{23Na} transfer of population in double-resonance (TRAPDOR), and combined 27Al−23Na 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 27Al MQMAS experiment shows distributions of chemical shift and quadrupolar coupling parameters. Therefore, the material is probably highly disordered, and multiple compounds may be 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, Na2CO3·H2O and Na3(CO3)2·2H2O, 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.