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Abstract

Ball milling of MgCl₂ and CaCl₂ with NaAlH₄ or LiAlH₄ can be used for the preparation of magnesium, calcium and lithium–magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure state. The alanates were characterized by X-ray diffractometry, solid-state ²⁷Al NMR and IR spectroscopy and thermovolumetric (TV) and differential scanning calorimetry (DSC) measurements. Mg(AlH₄)₂ dissociates thermally in one step to MgH₂, Al and hydrogen; at a higher temperature, MgH₂ and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH₄)₂ and of LiMg(AlH₄)₃ (in mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH₂ and of a MgH₂/LiH mixture, respectively, in addition to Al and H₂. Possible intermediates of these two steps are CaAlH₅ and LiMgAlH₆. Higher temperature dissociations include formation of MgH₂ (LiH) and Ca–Al alloys from CaH₂, CaH₂ and Al, respectively. Upon ball milling of MgCl₂ or CaCl₂ with NaAlH₄ or LiAlH₄ in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained. This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the thermodynamic stability of Mg(AlH₄)₂ (ΔH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ΔH values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol, respectively.