Zn(BH 4) 2 •2NH 3 , a new ammine metal borohydride, has been synthesized via simply ball-milling a mixture of ZnCl 2 •2NH 3 /2LiBH 4. Structure analysis shows that the subsequent complex has a monoclinic structure with unit-cell parameters of a = 6.392(4) Å, b = 8.417(6) Å, c = 6.388(4) Å and β = 92.407(4) ° and space group P2 1 , in which Zn atoms coordinate with two BH 4 groups and two NH 3 groups. The interatomic distances reported herein show that Zn-H bonding in Zn(BH 4) 2 •2NH 3 is shorter than Ca-H bonds in Ca(BH 4) 2 •2NH 3 and Mg-H in Mg(BH 4) 2 •2NH 3. This reduced bond contact leads to an increase in the ionic character of H. This study is able to show a good correlation between the reduced M-H distance and enhanced dehydrogenation behavior of the hydride material. Dehydrogenation results showed that this novel compound is able to release 8.9 wt. % hydrogen below 115 °C within 10 min without concomitant release of undesirable gases such as ammonia and/or boranes, thereby demonstrating the potential of Zn(BH 4) 2 •2NH 3 to be used as a solid hydrogen storage material.
The recyclable dehydrogenation of ammonia borane (AB) is achievable within a graphene oxide (GO)-based hybrid nanostructure, in which a combined modification strategy of acid activation and nanoconfinement by GO allows AB to release more than 2 equiv of pure H(2) at temperatures below 100 °C. This process yields polyborazylene (PB) as a single product and, thus, promotes the chemical regeneration of AB via reaction of PB with hydrazine in liquid ammonia.
A new ammine dual-cation borohydride, LiMg(BH(4))(3)(NH(3))(2), has been successfully synthesized simply by ball-milling of Mg(BH(4))(2) and LiBH(4)·NH(3). Structure analysis of the synthesized LiMg(BH(4))(3)(NH(3))(2) revealed that it crystallized in the space group P6(3) (no. 173) with lattice parameters of a=b=8.0002(1) Å, c=8.4276(1) Å, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH(4) units. Strong N-H···H-B dihydrogen bonds exist between the NH(3) and BH(4) units, enabling LiMg(BH(4))(3)(NH(3))(2) to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH(4))(3)(NH(3))(2)/LiBH(4) composite is able to release over 8 wt% hydrogen below 200 °C, which is comparable to that released by Mg(BH(4))(3)(NH(3))(2). More importantly, it was found that release of the byproduct NH(3) in this system can be completely suppressed by adjusting the ratio of Mg(BH(4))(2) and LiBH(4)·NH(3). This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.
The crystal structure of a promising hydrogen storage material, calcium borohydride monoammoniate (Ca(BH 4 ) 2 $NH 3 ), is reported. Structural analysis revealed that this compound crystallizes in an orthorhombic structure (space group Pna2 1 ) with unit-cell parameters of a ¼ 8.4270A, b ¼ 12.0103 A, c ¼ 5.6922A and V ¼ 576.1121 A 3 , in which the Ca atom centrally resides in a slightly distorted octahedral environment furnished by five B atoms from BH 4 units and one N atom from the NH 3 unit. As Ca(BH 4 ) 2 $NH 3 tends to release ammonia rather than hydrogen when heated in argon, a novel aided-cation strategy via combining this compound with LiBH 4 was employed to advance its dehydrogenation. It shows that the interaction of the two potential hydrogen storage substances upon heating, based on a promoted recombination reaction of BH and NH groups, enables a significant mutual dehydrogenation improvement beyond them alone, resulting in more than 12 wt% high-pure H 2 (>99%) released below 250 C. The synergetic effect of associating the dihydrogen reaction with mutually aided-metal cations on optimizing the dehydrogenation of this kind of composites may serve as an alternative strategy for developing and expanding the future B-N-H systems with superior and tuneable dehydrogenation properties.
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