Promoted hydrogen release from ammonia borane by mechanically milling with magnesium hydride: a new destabilizing approach

Abstract

Ammonia borane (NH(3)BH(3), AB) is an intriguing molecular crystal with an extremely high hydrogen capacity and moderate thermal stability. In the present study, we show a simple but effective approach for destabilizing AB for promoted hydrogen release at moderate temperatures. It is found that mechanically milling with magnesium hydride (MgH(2)) can dramatically improve the dehydrogenation properties of AB, on both the kinetic and thermochemical aspects. For the mechanically milled AB/0.5MgH(2) material, over 8 wt% hydrogen can be released from AB within 4 h at around 100 degrees C without undesired volatile by-products. Moreover, the dehydrogenation reaction of the AB/0.5MgH(2) sample becomes significantly less exothermic than that of neat AB. In situ X-ray diffraction results demonstrate that the MgH(2) additive well maintains its phase stability during the ball-milling and the subsequent heating processes. Meanwhile, Raman spectroscopy and in situ(11)B NMR studies show that the MgH(2) additive exerts considerable influence on the chemical bonding state and decomposition process/products of AB. Combined phase/structure analyses results suggest that MgH(2) exerts effect via developing solid phase interaction with AB.

Ammonia borane–metal alanate composites: hydrogen desorption properties and decomposition processes

“…This phenomenon was also observed in other metal amidoboranes. 17,20,21,23,24 From this result, the formation of metal amidoborane phase was also suggested. …”

Mono- and Bimetalic Amidoboranes

“…Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/6/8/88/s1, Selected figures illustrating properties of: α-LiAB [18,19,26], NaAB [18,[22][23][24]26], KAB [25,26], RbAB [26], CsAB [26], Mg(AB) 2 [28,29], Ca(AB) 2 [19,31,32], Sr(AB) 2 [33], Zn(AB) 2 [34], Al(AB) 3 [35], Y(AB) 3 [38], LiNa(AB) 2 [24,39], LiAl(AB) 4 [40], NaAl(AB) 4 [41], NaMg(AB) 3 [42], Na 2 Mg(AB) 3 [44,45], K 2 Mg(AB) 3 [45].…”

“…Crystal structure was solved by Owarzany et al from X-ray powder data [26]. The lattice parameters are a = 9.1182 (8) A number of research groups tried to synthesize Mg(AB) 2 for several years but without success [19,27,28] (see S11 in Supplementary Materials). All of these approaches were unsuccessful because of the low reactivity of MgH 2 against ammonia borane.…”

“…Moderately strong dihydrogen bonds between oppositely charged hydrogen atoms in two different amidoborate anions are found at 1.96(1)-2.28(1) Å. DFT calculations of the unit cell vectors of NaAl(AB) 4 NaMg(AB) 3 was synthesized via a mechanochemical reaction of NaMgH 3 with ammonia borane in molar ratio 1:3 by Kang et al [42]. The milling must be followed by isothermal treatment at 45˝C overnight to complete the synthesis, according to Equation (28). During this thermal treatment 3.4 wt% of hydrogen is being released, which corresponds to 0.8 equivalents of hydrogen per formula unit thus suggesting an incomplete conversion of substrates (this was confirmed by X-ray powder diffraction [42]).…”

Mixed-metal (Li, Al) amidoborane: synthesis and enhanced hydrogen storage properties

“…21 The thermal stability of the amidoboranes can then be tuned towards favourable dehydrogenation properties. A noteworthy feature is that, due to the incorporation of other structurestabilizing ligands [NaNH 2 BH 3 ], the Mg(NH 2 BH 3 ) 2 moiety, which is structurally unstable according to a previous report, 22 is present in some of these mixed-metal amidoboranes, demonstrating the functions of dual-metal ions towards stabilization of unstable complex hydrides, as similarly observed in the synthesis of MZn(BH 4 ) 3 (M ¼ Li, Na), in which the coordination of alkali metals gives rise to the stabilization of Zn(BH 4 ) 2 under ambient conditions. 23 Furthermore, signicantly improved hydrogen desorption properties were realized for the mixed-metal amidoboranes in comparison with the monometallic amidoboranes, e.g., complete suppression of ammonia was observed for NaMg(NH 2 BH 3 ) 3 , 19 while there is $15 mol% ammonia release for NaNH 2 BH 3 .…”