Diammoniate Of Diborane Research Articles

Exploration of the Dehydrogenation Pathways of Ammonia Diborane and Diammoniate of Diborane by Molecular Dynamics Simulations Using Reactive Force Fields.

Ammonium aminodiboranate (AADB) and diammoniate of diborane (DADB) are two isomers of ammonia borane (AB), which have been intensively studied for hydrogen storage. Their high hydrogen contents give them the high potential to serve as hydrogen storage materials. To explore their dehydrogenation pathways, molecular dynamics (MD) simulations with a reactive force field (ReaxFF) were applied. Temperature ramping simulations of their thermolysis were carried out. For AADB, at low temperatures, its hydrogen release can be realized mainly via intermolecular dehydrogenations. As the temperature of the simulated system increases, the formations of B-N bonds begin to occur. In the case of DADB, we found that this molecule could release hydrogen at a lower temperature with the cleavage of the B-N bond. The compositional analysis of the simulated systems was also conducted to monitor the potential intermediates along their dehydrogenation pathways. Our current work provides a detailed picture of the initial dehydrogenation steps of AADB and DADB and highlights the difference in their respective dehydrogenation processes.

A New Perspective on Borane Chemistry: The Nucleophilicity of the B-H Bonding Pair Electrons.

A number of recently discovered nucleophilic boron compounds, such as boryl anions and borylenes, are breaking the rules regarding boron and boron-containing compounds and their reputation as Lewis acids/electrophiles. In a similar fashion, the B-H bonding pair electrons in boranes also show nucleophilicity which is ascribed to the lower electronegativity of boron relative to that of hydrogen. However, this nucleophilicity of the B-H bond has received far less attention. Explorations of the nucleophilicity of the B-H bonding pair electrons have led to the formation of B-H-B bonded units and B-H⋅⋅⋅H-Y dihydrogen bonds, based on which new chemistry has been uncovered, including the elucidation of the mechanism of formation of aminodiborane (ADB), the diammoniate of diborane (DADB), and lithium or sodium salts of octahydrotriborates (B3 H8 - ), as well as the development of more convenient and straightforward synthetic routes to these reagents. Moreover, the recognition of the nucleophilic properties of the B-H bonding pair electrons will also help to more deeply understand the different mechanisms operating in hydroboration reactions.

The Roles of Dihydrogen Bonds in Amine Borane Chemistry

A dihydrogen bond (DHB) is an electrostatic interaction between a protonic hydrogen and a hydridic hydrogen. Over the past two decades, researchers have made significant progress in the identification and characterization of DHBs and their properties. In comparison with conventional hydrogen bonds (HBs), which have been widely used in catalysis, molecular recognition, crystal engineering, and supramolecular synthesis, chemists have only applied DHBs in very limited ways. Considering that DHBs and conventional HBs have comparable strength, DHBs could be more widely applied in chemistry. Over the past several years, we have explored the impact of DHBs on amine borane chemistry and the syntheses and characterization of amine boranes and ammoniated metal borohydrides for hydrogen storage. Through systematic computational and experimental investigations, we found that DHBs play a dominant role in dictating the reaction pathways (and thus different products) of amine boranes where oppositely charged hydrogens coexist for DHB formation. Through careful experiments, we observed, for the first time, a long-postulated reaction intermediate, ammonia diborane (AaDB), whose behavior is essential to mechanistic understanding of the formation of the diammoniate of diborane (DADB) in the reaction of ammonia (NH3) with tetrahydrofuran borane (THF·BH3). The formation of DADB has puzzled the boron chemistry community for decades. Mechanistic insight enabled us to develop facile syntheses of aminodiborane (ADB), ammonia borane (AB), DADB, and an inorganic butane analog NH3BH2NH2BH3 (DDAB). Our examples, together with those in the literature, reinforce the fact that DHB formation and subsequent molecular hydrogen elimination are a viable approach for creating new covalent bonds and synthesizing new materials. We also review the strong effects of DHBs on the stability of conformers and the hydrogen desorption temperatures of boron-nitrogen compounds. We hope that this Account will encourage further applications of DHBs in molecular recognition, host-guest chemistry, crystal engineering, supramolecular chemistry, molecular self-assembly, chemical kinetics, and the syntheses of new advanced materials.

Metal-free, polyether-mediated H2-release from ammonia borane: roles of hydrogen bonding interactions in promoting dehydrogenation

Polyetheral additives were found to be efficient promoters to enhance the rate of H2-release from ammonia borane (AB) at various temperatures. In particular, tetraethylene glycol dimethyl ether (T4EGDE, 29 wt% relative to AB + T4EGDE) exhibited significantly improved activities for AB dehydrogenation, with the material-based hydrogen storage capacity of 10.3 wt% at 125 °C within 40 min. In situ FT-IR spectroscopy indicated the formation of B-(cyclodiborazanyl)amino-borohydride (BCDB), borazine, and μ-aminodiborane as gaseous byproducts. In addition, (11)B nuclear magnetic resonance (NMR) spectroscopy further revealed that diammoniate of diborane (DADB) was initially formed to give polyaminoborane as liquid and/or solid spent-fuel, consistent with previous reports. Density Functional Theory (DFT) calculations suggested that hydrogen bonding interactions between AB and a polyetheral promoter initially played an important role in increasing the reactivity of B-H bonds of AB by transferring electron density from oxygen atoms of the promoter into B-H bonds of AB. These partially activated, hydridic B-H bonds were proposed to help promote the formation of diammoniate of diborane (DADB), which is considered as a reactive intermediate, eventually enhancing the rate of H2-release from AB. In addition, our in situ solid state (11)B magic angle spinning (MAS) NMR measurements further confirmed that the rate of DADB formation from AB with a small quantity of T4EGDE was found to be much faster than that of pristine AB even at 50 °C. This metal-free method for H2-release from AB with an added, small quantity of polyethers would be helpful to develop feasible hydrogen storage systems for long-term fuel cell applications.

Large‐Scale and Facile Preparation of Pure Ammonia Borane through Displacement Reactions

Ammonia borane (AB) is the most widely studied hydride for hydrogen storage in addition to being a useful reducing agent. Attempts to synthesize pure AB through simple displacement reactions date back to the 1960s; but have been thwarted by the formation of the diammoniate of diborane (DADB), an ionic byproduct. Based on our recent characterization of the formation mechanism of DADB, we have developed a large-scale synthesis of pure AB by both increasing the basicity of the Lewis base of the borane carrier and using a nonpolar solvent to limit the formation of an intermediate, the ammonia diborane (AaDB). Conditions were optimized for the preparation of pure AB by two displacement reactions, either ammonia with dimethylsulfide borane or ammonia with dimethylaniline borane in toluene at room temperature. These procedures are also suitable for preparation of other amine boranes which had the same problem of forming ionic byproducts during displacement reactions.

A Convenient Synthesis and a NMR Study of the Diammoniate of Diborane

DADB synthesis: The diammoniate of diborane (DADB) was synthesized in a new metathesis reaction between the diammoniate of monochloroborane and potassium borohydride in liquid ammonia. (1)H and (11)B NMR spectra of DADB are reported. The stability in THF was examined by variable-temperature (11)B NMR spectroscopy.

Hypercrosslinked porous poly(styrene-co-divinylbenzene) resin: a promising nanostructure-incubator for hydrogen storage

A novel strategy of adopting the reversible swelling effect of a promising scaffold—hypercrosslinked porous poly(styrene-co-divinylbenzene) resin (PSDB) to nanoconfine hydrogen storage materials is reported, in which nanoconfined ammonia–borane (AB) is endowed with high loading ratio and significantly improved hydrogen storage capabilities. To verify the importance of swelling behavior displayed by this polymeric scaffold in dehydrogenation, an ammonia-dissolving route (PSDB–AB (NH3)) that does not involve swelling, was performed to enable direct comparison with the methanol-dissolving route (PSDB–AB (CH3OH)). Moreover, solid-state 11B NMR measurements were employed to illustrate the different reaction mechanisms in these two PSDB-confined AB samples, where decomposition involving both the diammoniate of diborane (DADB) and linear dimer are observed for PSDB–AB (NH3) but only the latter route is seen for PSDB–AB (CH3OH), deeply demonstrating the difference of dehydrogenation properties in these two samples. Our findings establish a prospective approach via utilizing this class of polymers for promoting the design and construction of advanced energy materials with high performances.

Concentration‐Dependent Dehydrogenation of Ammonia–Borane/Triglyme Mixtures (Eur. J. Inorg. Chem. 1/2012)

AbstractThe cover picture shows a graph of the hydrogen release rate from ammonia–borane dissolved in glyme solutions (left). On the right, the compounds formed during the decomposition of ammonia–borane under these conditions as detected by NMR spectroscopy are depicted. The increase of the hydrogen release rate during the course of the reaction as well as the spectroscopic analysis indicate the presence of a complex process governed by multiple pathways. Similarly to the hydrogen release from solid ammonia–borane and in contrast to the low concentration case, the analysis of the decomposition of ammonia–borane in highly concentrated glyme solutions demonstrates that the diammoniate of diborane (DADB) plays a vital role in the overall decomposition scheme. Details are discussed in the article by F. Mertens et al. on p. 49 ff. magnified image

CO2-Enhanced Thermolytic H2 Release from Ammonia Borane

This work presents the accelerated dehydrogenation of ammonia borane (AB) at 85 °C under carbon dioxide atmosphere. The presence of CO2 significantly enhances the thermolysis kinetics, and the hydrogen release becomes faster as the CO2 pressure increases from 1.70 to 3.67 bar. Around 4 bar of CO2, 1.33 equiv of hydrogen is released from AB in 1 h. The accelerated AB thermolysis is due to CO2 reacting with AB, or intermediates like diammoniate of diborane (DADB), or primary thermolysis products including linear dimers of aminoborane (LDAB), which accelerates AB thermolysis, and this reaction has been confirmed by ATR-FTIR, Raman, and 11B and 13C solid-state MAS NMR measurements.

Facile solid-phase synthesis of the diammoniate of diborane and its thermal decomposition behavior

The recent mechanistic finding of the hydrogen release pathways from ammonia borane (AB) has sparked new interest in the chemistry and properties of the diammoniate of diborane (DADB), an ionic isomer of AB. We herein report a facile one-step solid-phase synthesis route of DADB using inexpensive starting materials. Our study found that mechanically milling a 1 : 1 NaBH(4)/NH(4)F powder mixture causes the formation of crystalline DADB via a NH(4)BH(4) intermediate. The produced DADB can be readily separated from the sodium fluoride (NaF) by-product by a purification procedure using liquid ammonia at -78 °C. The thermal decomposition behavior of DADB was studied using synchronous thermal analyses, particularly in comparison with AB. It was found that the decomposition steps and products of DADB are similar to those of AB. But meanwhile, DADB exhibits a series of advantages over AB that merit its potential hydrogen storage application, such as lower dehydrogenation temperature, free of foaming and lack of an induction period in the thermal decomposition process. Our study further found that the volatile non-hydrogen products from DADB can be effectively suppressed by milling with MgH(2).

Solid state NMR study on the thermal decomposition pathway of sodium amidoborane NaNH2BH3

The thermal decomposition pathway of sodium amidoborane (NaAB; NaNH2BH3) has been investigated in detail by using solid state NMR spectroscopy. 23Na MAS/3QMAS NMR spectra suggested that NaH and an amorphous Na–N–B–H phase started to be formed as decomposition products even at 79 °C, although NaAB was prepared from NaH and NH3BH3 by ball milling at room temperature. Based on the quantitative analyses of the 23Na MAS spectra, we proposed a decomposition reaction to 200 °C to be NaNH2BH3 → Na0.5NBH0.5 + 0.5NaH + 2.0H2. The hypothetical phase Na0.5NBH0.5 is amorphous, where the basic molecular unit of the original NaAB is polymerized into a [–BN–]n network structure. It was also found that the diammoniate of diborane (DADB) and polyaminoborane (PAB) were not formed during the decomposition of NaAB, which are both key compounds on the pyrolysis of ammonia borane (AB).

Thermal Decomposition and Spectroscopic Studies of Preheated Ammonia Borane

This paper presents enhanced dehydrogenation of solid ammonia borane (AB, NH3BH3) at 85 °C after it was preheated at 80 °C for 2−4 h. Thermal treatments and subsequent hydrogen release experiments were carried out in a Hastelloy cell mounted on a high-pressure differential scanning calorimeter. With increased preheating durations, not only does the induction period for hydrogen release shorten but also the hydrogen release becomes fast. It is deduced that diammoniate of diborane (DADB, [(NH3)2BH2]+[BH4]−), oligomers or polymers, and AB form a solid solution, shifting the melting temperature of AB to a low value. Hydrogen evolution, as the result of the initial stage of oligomerization, is observed after maintaining pristine AB at 80 °C for 20 min. FTIR-ATR spectra of thermally treated AB indicated that only linear dimers of aminoborane (LDAB, NH3BH2NH2BH3) are detected for preheating periods shorter than 3 h, whereas other oligomers and polymers form for the longer thermal treatment at 80 °C.

Decomposition Pathway of Ammonia Borane on the Surface of Nano-BN

Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While chemical additives have been shown to lower the temperature for hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal boron nitride (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time and less exothermicity, and inert nano-BN may be easily removed during any regeneration of the spent AB. The samples were prepared by mechanically alloying AB with nano-BN. Raman spectroscopy indicates that the AB/nano-BN samples are physical mixtures of AB and h-BN. The release of hydrogen from AB/nano-BN mixtures as well as the decomposition products was characterized by 11B magic angle spinning (MAS) solid state NMR spectroscopy, TGA/DSC/MS with 15N-labeled AB, and solution 11B NMR spectroscopy. The 11B MAS solid state NMR spectrum shows that diammoniate of diborane (DADB) is...

The diammoniate of diborane: crystal structure and hydrogen release

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics.

Ammonia Borane Hydrogen Release in Ionic Liquids

The rate and extent of H(2)-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material, was found to be enhanced in ionic-liquid solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) (50:50-wt %) exhibited no induction period and released 1.0 H(2)-equiv in 67 min and 2.2 H(2)-equiv in 330 min at 85 degrees C, whereas comparable solid-state AB reactions at 85 degrees C had a 180 min induction period and required 360 min to release approximately 0.8 H(2)-equiv, with the release of only another approximately 0.1 H(2)-equiv at longer times. Significant rate enhancements for the ionic-liquid mixtures were obtained with only moderate increases in temperature, with, for example, a 50:50-wt % AB/bmimCl mixture releasing 1.0 H(2)-equiv in 5 min and 2.2 H(2)-equiv in only 20 min at 110 degrees C. Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H(2)-release rates compared to the solid-state, and produced a system that achieved 11.4 materials-weight percent H(2)-release. Solid-state and solution (11)B NMR studies of AB H(2)-release reactions in progress support a mechanistic pathway involving: (1) ionic-liquid promoted conversion of AB into its more reactive ionic diammoniate of diborane (DADB) form, (2) further intermolecular dehydrocoupling reactions between hydridic B-H hydrogens and protonic N-H hydrogens on DADB and/or AB to form neutral polyaminoborane polymers, and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene materials.

The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane

The thermal decomposition of ammonia borane (AB) in the absence and presence of chemical additives was investigated to develop an approach for reducing the induction period for hydrogen release in the solid state. Gas chromatography techniques were used to measure the yield of hydrogen as a function of time under isothermal conditions between 75 and 90 °C to set the baseline. Solid-state 11B-NMR spectroscopy of the products produced after 1 mol equiv of hydrogen had been desorbed from AB (i.e., polyaminoborane) showed a complex mixture of sp3 boron species. Raman microscopy was used to follow the transformation of crystalline AB to amorphous AB upon heating and the subsequent formation of the diammoniate of diborane (DADB). A gas buret was used to monitor the time-dependent release of hydrogen from AB in the presence of chemical additives. The combination of these approaches provides insight into the mechanism of hydrogen release from solid AB. The release of molecular hydrogen is described by a process i...

The thermal decomposition of ammonia borane: A potential hydrogen storage material

One equivalent of hydrogen gas is evolved from ammonia borane (NH3BH3) when it is heated above 70°C. The initial stages of this process have been examined using TG/DSC, optical microscopy, and high temperature X-ray diffraction. Two exothermic events have been observed, the first of which takes place without hydrogen evolution. During this stage, the sample loses its crystallinity and birefringence. The products are believed to be a more mobile form of NH3BH3 and the diammoniate of diborane ([NH3BH2NH3]+[BH4]−). These products subsequently react in the second exothermic stage to generate hydrogen.

In situ solid state 11B MAS-NMR studies of the thermal decomposition of ammonia borane: mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material

The mechanism of hydrogen release from solid state ammonia borane (AB) has been investigated via in situ solid state (11)B and (11)B{(1)H} MAS-NMR techniques in external fields of 7.1 T and 18.8 T at a decomposition temperature of 88 degrees C, well below the reported melting point. The decomposition of AB is well described by an induction, nucleation and growth mechanistic pathway. During the induction period, little hydrogen is released from AB; however, a new species identified as a mobile phase of AB is observed in the (11)B NMR spectra. Subsequent to induction, at reaction times when hydrogen is initially being released, three additional species are observed: the diammoniate of diborane (DADB), [(NH(3))(2)BH(2)](+)[BH(4)](-), and two BH(2)N(2) species believed to be the linear (NH(3)BH(2)NH(2)BH(3)) and cyclic dimer (NH(2)BH(2))(2) of aminoborane. At longer reaction times the sharper features are replaced by broad, structureless peaks of a complex polymeric aminoborane (PAB) containing both BH(2)N(2) and BHN(3) species. The following mechanistic model for the induction, nucleation and growth for AB decomposition leading to formation of hydrogen is proposed: (i) an induction period that yields a mobile phase of AB caused by disruption of the dihydrogen bonds; (ii) nucleation that yields reactive DADB from the mobile AB; and (iii) growth that includes a bimolecular reaction between DADB and AB to release the stored hydrogen.

Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids

Ionic liquids are shown to provide advantageous media for amineborane-based chemical hydrogen storage systems. Both the extent and rate of hydrogen release from ammonia borane dehydrogenation are significantly increased at 85, 90, and 95 degrees C when the reactions are carried out in 1-butyl-3-methylimidazolium chloride compared to analogous solid-state reactions. NMR studies in conjunction with DFT/GIAO chemical shift calculations indicate that both polyaminoborane and the diammoniate of diborane, [(NH3)2BH2+]BH4-, are initial products in the reactions.

Synthesis of cyclopentaborazane from diammoniate of diborane

Synthesis of cyclopentaborazane from diammoniate of diborane