Dehydrogenation and rehydrogenation reactions in alanates

Alkali metal alanates undergo dehydrogenation in the 200-300 °C temperature range (Li, 201 °C Na, 265 °C K, 290 °C) to give aluminum metal and the corresponding alkali metal hydrides (see Eq. 14.1). 

The occurrence of these two distinct reactions during the dehydrogenation process has been verified in subsequent studies of the thermolysis of LiAlH4, NaAlH4, KAlH4by a variety of techniques including differential thermal X- 

Since the entropy of hydrogenation must be negative, this is a non-spontaneous process under all conditions. A second dehydrogenation reaction, seen in Eq. 

This reaction was found to be endothermic with a A// of 25kJ/mol H2. Einally, the dehydrogenation of LiH was observed in the 370 83 °C temperature range. The enthalpy of the dehydrogenation of LiH(s) has subsequently been determined to be 140kJ/mol.  

In contrast to the alkali metals, studies of the dehydrogenation of Mg(AlH4)2 by Fichtner et al showed that at 163 °C, the dehydrogenation of the hydride proceeds according to Eq. (14.6) without the involvement of [AlHs] as an intermediate phase. 

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