Synthesis, Crystal Structure Analysis and Decomposition of RbAlH4

Abstract

RbAlH 4 , a member of the complex metal aluminum hydride family, can be synthesized phase pure by different synthesis routes. Synthesis from the metals by a mechanochemical reaction requires the presence of a catalyst, but also emphasizes the reversibility of hydrogenation. The structure refinement of neutron diffraction data confirms that RbAlD 4 is isostructural to KAlD 4. The decomposition proceeds via two distinct processes at temperatures above 275 • C. However, the structures formed during decomposition seem to be different from the compounds formed during hydrogen release of early alkali metal aluminum hydrides.

Crystal structure evolution of complex metal aluminum hydrides upon hydrogen release

“…Melting processes together with the appearance of unsolved intermediate structures lead to the simple hydrides RbH and CsH [58] . TG-DSC measurements of RbAlH 4 showed that only 45% of the expected hydrogen release is achieved up to 650 K [38] .…”

“…In the orthorhombic structure of RbAlD 4 (space group Pnma ) the Rb + cation is coordinated by ten deuterium atoms, coordination [10] , within a coordination distance between 2.818(6) and 3.332(3) Å ( Fig. 4 d) [38] . The deuterium atoms belong to 7 different [AlD 4 ] − deuteride anions.…”

Alanates, a Comprehensive Review

“…If this material follows the group 1 tendency regarding dehydrogenation reactions, RbAlH 4 could reach a 2.5 wt.% of reversible hydrogen storage. Weidenthaler et al reported the synthesis of RbAlH 4 from the metals Al, Rb, and with TiCl 3 as an additive; milling was performed in a hydrogen atmosphere (200 bar) [176]. Adkis et al reported the synthesis of RbAlH 4 by the reaction between LiAlH 4 and metallic Rb [177].…”

“…This reaction yield was explained by the formation of a complex between the halide salts and the triethylaluminum, i.e., a Ziegler-type complex. RbAlH 4 , and the deuterated species were also produced by the metathesis reaction between NaAlH 4 , LiAlH 4 , or LiAlD 4 with RbCl or RbF promoted by ball milling [176]. RbAlH 4 or RbAlD 4 were further heated in an autoclave and then purified [176].…”

“…RbAlH 4 decomposes in two steps at 300 °C and 350 °C (peak temperatures in TG-DCS curves) [176]. However, no complete dehydrogenation and full reversibility have been demonstrated.…”