Regeneration of alkaline metal amidoboranes with high purity
Graphical abstract
A high-efficient and self-sufficient recyclable regeneration strategy for the thermal dehydrogenation products of alkaline metal amidoboranes has been established successfully, achieving a high purity of 98%.
Introduction
In the endeavor to decrease greenhouse gas emissions and fossil fuel dependence, hydrogen has been considered as one of the best alternative energy carriers because of its abundance, high energy density, and environmental friendliness. Highly efficient and convenient hydrogen storage materials/technology are still the main challenge for today's development of the hydrogen economy [1]. A reliable and economical hydrogen storage system has to satisfy several criteria, including high storage capacity, safety, low operation temperature, reversibility, and low cost [2], [3].
Recently, the ammonia–borane (H3NBH3, AB) complex has been identified as one of the leading candidates as a hydrogen reservoir owing to its high hydrogen content (19.6 wt %, mat.-based), high air-stability, and nontoxicity [4], [5], [6]. The practical application of AB and its derivatives, however, is still obstructed by crucial issues, especially, slow kinetics at low temperatures (below 90 °C) and excessive volatile impurities (borazine, ammonia, and diborane) during its pyrolysis dehydrogenation [7], [8], [9], [10], [11], [12], [13], [14]. To overcome the above drawbacks, significant efforts have been made to investigate improvement of the dehydrogenation of AB [15], [16], [17], [18], [19], [20], [21], [22]. As part of this research effort, modifying the structure of AB to enhance the kinetics and dynamics of H2 release has been extensively studied. For example, replacing one H atom on the N of AB with a metal cation generates a metal amidoborane (MAB), such as LiNH2BH3 (LiAB), KNH2BH3 (KAB), etc., which not only has fast kinetics and decreased enthalpy for H2 release but also suppresses the emission of gaseous impurities [23], [24], [25], [26], [27].
The viability of any storage system is critically dependent on efficient recyclability. Reports on the regeneration of metal amidoboranes are quite sparse, however. In our previous work, regenerable hydrogen storage of LiAB was first achieved through the route of chemical hydrogenation of its dehydrogenated products by treatment with hydrazine in liquid ammonia [28]. Nevertheless, only ∼60% of the LiAB was obtained, because during regeneration, some of LiAB react with liquid ammonia to form aminelithium amidoborane (LiNH2BH3NH3), which decomposes at the regeneration temperature, thus resulting in a severe degradation of regeneration purity. In addition, hydrazine is energetically demanding, dangerously unstable, and toxic, so it is very difficult to handle on an industrial scale [29]. Therefore, it is highly preferable to design new strategies for the regeneration of MAB that feature safety, practicability, and high purity. In this paper, we develop a chemical recycling for MAB (M = Li, K), in which some common reagents are used to accomplish the whole cycle, and high regeneration purity of MAB can be achieved at mild conditions.
Section snippets
Materials and preparation
The materials, ammonia borane (NH3BH3, AB, 97%), LiH (95%), KH (30 wt% in paraffin), LiAlH4-tetrahydrofuran (THF) solution (2 M L−1), NH4Cl (99%), anhydrous CH3OH (99.9%), B(OCH3)3 (99.5%) and anhydrous THF (99.9%), were purchased from Sigma–Aldrich and used in as-received form without further purification. KH was washed to remove the paraffin in THF prior to use. AB (200 mg) was dissolved in THF (20 mL) under magnetic stirring for 3 min. LiH (51.9 mg) and KH (260 mg) was added into AB solution
Results and discussions
The thermal dehydrogenation product that comes from loss of two equivalents of H2 molecular of MAB, MNBH, has an analogous structure to polyborazylene (NBH2, PB) [30], [31]. PB is the final pyrolysis residue of AB and other B–N-based compounds and can be regenerated through a three-step strategy, including 1) digestion of NBH2 residue by the formation of a series of B species, 2) reduction of these digestion products with a strong reducing agent, and 3) ammoniation of the reduction products to
Conclusion
In summary, we have established a highly efficient and self-sufficient regeneration recycling system for the thermal dehydrogenation products of alkaline metal amidoboranes (MAB, M = Li, K), in which a facile and safe chemical pathway was adopted to reproduce MAB with an unprecedentedly high purity of 98%. More importantly, our finding of successful scission of dehydrogenated polymeric MAB residues into small molecule B species that guarantees the facilitation of the following regeneration
Acknowledgments
This work was partially supported by the National Natural Science Foundation of China (21271046, 51471053), the PhD Programs Foundation of the Ministry of Education of China (20110071110009), the Science and Technology Commission of Shanghai Municipality (11JC1400700), and an Australian Research Council Discovery Project (DP140102858). The authors also would like to thank Dr. Tania Silver for critical reading of the manuscript.
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Ziwei Tang and Lijun Zhang contributed equally to this work.