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Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System.

ABSTRACT: Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH?)?) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH and Mg(NH?)? lead to the fast hydrogen desorption. To preserve the potential hydrogen desorption properties, thermochemical and mechanochemical rehydrogenation processes are investigated. Although the only thermochemical process results in slowing the reaction rate due to the crystallization, the ball-milling can recover the original hydrogen desorption properties. Furthermore, the mechanochemical process at 150 °C is useful as the rehydrogenation technique to preserve the suitable crystalline size and mixing state of the reactants. As a result, it is demonstrated that the 8LiH and 3Mg(NH?)? system is recognized as the potential hydrogen storage material to desorb more than 5.5 mass% of H? at 150 °C.

SUBMITTER: Miyaoka H

PROVIDER: S-EPMC5455667 | biostudies-literature | 2015 Jun

REPOSITORIES: biostudies-literature

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Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System.

Miyaoka Hiroki H Wang Yongming Y Hino Satoshi S Isobe Shigehito S Tokoyoda Kazuhiko K Ichikawa Takayuki T Kojima Yoshitsugu Y

Materials (Basel, Switzerland) 20150629 7

Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH₂)₂) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH ...[more]

PMID: 28793414

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Project description:The dimeric ?-diketiminato magnesium hydride, [(BDI)MgH]2, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the ?,?-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C[double bond, length as m-dash]C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]2 and 1-hexene using density functional theory (DFT) indicated that the initial Mg-H/C[double bond, length as m-dash]C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (?H = -14.1 kcal mol-1) traverses a barrier of 18.9 kcal mol-1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol-1. This Mg-H/C[double bond, length as m-dash]C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH3, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg-H/C[double bond, length as m-dash]C insertion and classical Si-H/Mg-C ?-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol-1, is found to be rate determining.

Dataset Information

Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System.

Publications

Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System.

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