Synergetic Effects of In Situ Formed CaH2 and LiBH4 on Hydrogen Storage Properties of the Li–Mg–N–H System

Hydrogen storage properties and mechanisms of the Ca(BH₄)₂‐doped Mg(NH₂)₂–2 LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH₄)₂ and LiH readily occurs to yield CaH₂ and LiBH₄ during ball milling. The Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt % of H₂ with an onset temperature of about 90 °C for dehydrogenation and 60 °C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt % of H₂ is rapidly desorbed from the Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite within 100 minutes at 140 °C, and rehydrogenation can be completed within 140 minutes at 105 °C and 100 bar H₂. In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH₂)₂–2 LiH–0.1 Ca(BH₄)₂ composite are decreased by about 16.5 % and 28.1 %, respectively, and thus are responsible for the lower operating temperature and the faster dehydrogenation/hydrogenation kinetics. The fact that the hydrogen storage performances of the Ca(BH₄)₂‐doped sample are superior to the individually CaH₂‐ or LiBH₄‐doped samples suggests that the in situ formed CaH₂ and LiBH₄ provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH₂)₂–2 LiH system.     

[Ca(BH<Subscript>4</Subscript>)<Subscript>2</Subscript>]<Subscript> <Emphasis Type="Italic">n</Emphasis> </Subscript> clusters as hydrogen storage material: A DFT study

Calcium borohydride is widely studied as a hydrogen storage material. However, investigations on calcium borohydride from a cluster perspective are seldom found. The geometric structures and binding energies of [Ca(BH₄)₂]_n (n = 1–4) clusters are determined using density function theory (DFT). For the most stable structures, vibration frequency, natural bond orbital (NBO) are calculated and discussed. The charge transfer from (BH₄) to Ca was observed. Meanwhile, we also study the LUMO–HOMO gap (E_g) and the B–H bond dissociation energies (BDEs). [Ca(BH₄)₂]₃ owns higher E_g, revealing that trimer is more stable than the other forms. Structures don’t change during optimization after hydrogen radical removal, showing that calcium borohydride could possibly be used as a reversible hydrogen storage material. [Ca(BH₄)₂]₄ has the smallest dissociation energy suggesting it releases hydrogen more easily than others.     

Crystal structures and thermodynamic investigations of NaSc(BH4)4 from first‐principles calculations

The double cation borohydride NaSc(BH₄)₄ has a total H₂ content of 12.67 wt.% and has been suggested as a potential candidate for hydrogen storage applications. This study reports first‐principles calculations of the structure and reaction thermodynamics of NaSc(BH₄)₄. The calculations indicate that NaSc(BH₄)₄ is decomposed into a mixture of ScB₂, NaBH₄, and Na₂(B₁₀H₁₀) with H₂ release of 9.3 wt.% at 118 K at a partial pressure of H₂ of 100 bar. Reactant compositions that can destabilize NaSc(BH₄)₄ were evaluated. This effort identified four destabilization reactions that are predicted to have reaction thermodynamics for hydrogen release within the temperature range of 78–109 K. Even though the reactions conclusively produce undesired compounds, such as refractory materials or kinetically stable B₁₂H₁₂‐containing species, the thermodynamic study suggests a direction for improving the thermodynamics of double cation borohydride‐based systems being actively considered for hydrogen storage applications. © 2012 Wiley Periodicals, Inc.     

Synthesis and Thermal Decomposition Behaviors of Magnesium Borohydride Ammoniates with Controllable Composition as Hydrogen Storage Materials

An ammonia‐redistribution strategy for synthesizing metal borohydride ammoniates with controllable coordination number of NH₃ was proposed, and a series of magnesium borohydride ammoniates were easily synthesized by a mechanochemical reaction between Mg(BH₄)₂ and its hexaammoniate. A strong dependence of the dehydrogenation temperature and purity of the released hydrogen upon heating on the coordination number of NH₃ was elaborated for Mg(BH₄)₂?x NH₃ owing to the change in the molar ratio of H^δ+ and H^δ?, the charge distribution on H^δ+ and H^δ?, and the strength of the coordinate bond N:→Mg²⁺. The monoammoniate of magnesium borohydride (Mg(BH₄)₂?NH₃) was obtained for the first time. It can release 6.5 % pure hydrogen within 50 minutes at 180 °C.     

Sorption and desorption properties of a CaH2/MgB2/CaF2 reactive hydride composite as potential hydrogen storage material

The hydrogenation behavior of 3CaH₂+4MgB₂+CaF₂ composite was studied by manometric measurements, powder X-ray diffraction, differential scanning calorimetry and attenuated total reflection infrared spectroscopy. The maximum observed quantity of hydrogen loaded in the composite was 7.0 wt%. X-ray diffraction showed the formation of Ca(BH₄)₂ and MgH₂ after hydrogenation. The activation energy for the dehydrogenation reaction was evaluated by DSC measurements and turns out to be 162±15 kJ mol⁻¹ H₂. This value decreases due to cycling to 116±5 kJ mol⁻¹ H₂ for the third dehydrogenation step. A decrease of ca. 25–50 °C in dehydrogenation temperature was observed with cycling. Due to its high capacity and reversibility, this composite is a promising candidate as a potential hydrogen storage material.     

Calcium–Amidoborane–Ammine Complexes: Thermal Decomposition of Model Systems

Hydrocarbon‐soluble model systems for the calcium–amidoborane–ammine complex Ca(NH₂BH₃)₂ ? (NH₃)₂ were prepared and structurally characterized. The following complexes were obtained by the reaction of RNH₂BH₃ (R=H, Me, iPr, DIPP; DIPP=2,6‐diisopropylphenyl) with Ca(DIPP‐nacnac)(NH₂) ? (NH₃)₂ (DIPP‐nacnac=DIPP? NC(Me)CHC(Me)N? DIPP): Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₂, Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₃, Ca(DIPP‐nacnac)[NH(Me)BH₃] ? (NH₃)₂, Ca(DIPP‐nacnac)[NH(iPr)BH₃] ? (NH₃)₂, and Ca(DIPP‐nacnac)[NH(DIPP)BH₃] ? NH₃. The crystal structure of Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₃ showed a NH₂BH₃^? unit that was fully embedded in a network of BH???HN interactions (range: 1.97(4)–2.39(4) Å) that were mainly found between NH₃ ligands and BH₃ groups. In addition, there were N? H???C interactions between NH₃ ligands and the central carbon atom in the ligand. Solutions of these calcium–amidoborane–ammine complexes in benzene were heated stepwise to 60 °C and thermally decomposed. The following main conclusions can be drawn: 1) Competing protonation of the DIPP‐nacnac anion by NH₃ was observed; 2) The NH₃ ligands were bound loosely to the Ca²⁺ ions and were partially eliminated upon heating. Crystal structures of [Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)]_∞, Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃) ? (THF), and [Ca(DIPP‐nacnac){NH(iPr)BH₃}]₂ were obtained. 3) Independent of the nature of the substituent R in NH(R)BH₃, the formation of H₂ was observed at around 50 °C. 4) In all cases, the complex [Ca(DIPP‐nacnac)(NH₂)]₂ was formed as a major product of thermal decomposition, and its dimeric nature was confirmed by single‐crystal analysis. We proposed that thermal decomposition of calcium–amidoborane–ammine complexes goes through an intermediate calcium–hydride–ammine complex which eliminates hydrogen and [Ca(DIPP‐nacnac)(NH₂)]₂. It is likely that the formation of metal amides is also an important reaction pathway for the decomposition of metal–amidoborane–ammine complexes in the solid state.     

Efficient Reduction of Aryl Azides and Aryl Nitro Compounds to Their Corresponding Amines with Sulfurated Calcium Borohydride CA(BH2S3)2. A New Modified Borohydride Agent

Ca(BH₂S₃)₂ is easily prepared by metathetical reaction between NaBH₂S₃ and CaCl₂ in THF. Ca(BH₂S₃)₂ is much more stable than NaBH₂S₃ therefore, is a more practical reagent. Sulfurated calcium borohydride can easily reduce aryl nitro and aryl azido functions to their amines in high yields in refluxing THF. Reduction of nitro groups is accompanied with regioselectivity. Very high chemoselectivity is also observed for the reduction of an azido functional group in the presence of nitro functionality.     

Self-catalyst growth of single-crystalline CaB6 nanostructures

Large-scale calcium hexaboride (CaB₆) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca) powders and boron trichloride (BCl₃) gas mixed with hydrogen and argon. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) were used to characterize the compositions, morphologies, and structures of the samples. Our results show that the nanowires are highly single crystals elongated preferentially in the [1 1 0] direction. The growth mechanism based on the self-catalyst process is simply discussed.     

Hydrogen Storage Properties of Ca(BH4)2–LiNH2 System

Ca(BH₄)₂ is one of the promising candidates for hydrogen storage materials because of its high gravimetric and volumetric hydrogen capacity. However, its high dehydrogenation temperature and limited reversibility has been a hurdle for its practical applications. In an effort to overcome these barriers and to adjust the thermal stability, we make a composite system Ca(BH₄)₂–LiNH₂. Interaction of Ca(BH₄)₂ and LiNH₂ leads to decreased dehydrogenation temperatures and increased hydrogen desorption capacity in comparison to pristine Ca(BH₄)₂. More than 7 wt % of hydrogen can be detached at a temperature as low as approximately 178 °C from the cobalt‐catalyzed Ca(BH₄)₂–4 LiNH₂ system.     

Kinetic Enhancement of Direct Hydrogenation of MgB2 to Mg(BH4)2 upon Mechanical Milling with THF,MgH2, and/or Mg

Modification of magnesium diboride, MgB₂, by mechanical milling with THF, MgH₂, and/or Mg results in a lowering of the conditions required for its direct, bulk hydrogenation to magnesium borohydride, Mg(BH₄)₂, by 300 bar and 100 °C. Following mechanical milling with MgH₂ or THF and Mg, MgB₂ can be hydrogenated to Mg(BH₄)₂ at 300 °C under 700 bar of H₂ while achieving ∼54–71 % conversion to the borohydride. The discovery of a means of dramatically lowering the conditions required for the hydrogenation of MgB₂ is an important step towards the development of a practical onboard hydrogen storage system based on hydrogen cycling between Mg(BH₄)₂ and MgB₂. We suggest that mechano-milling with THF, Mg, and/or MgH₂ may possibly introduce defects in the MgB₂ structure which enhance hydrogenation. The ability to activate the MgB₂ through the introduction of structural defects transcends its relevance to hydrogen storage, as a method of overcoming its chemical inertness provides the key to harnessing other interesting properties of this material.     

The structure and dynamics of FeH(DMPE)2(BH4), FeH(DEPE)2(BH4) and FeH(DPrPE)2(BH4): Iron complexes containing monodentate borohydride ligands

FeH(DMPE)₂(BH₄) [DMPE = 1,2-bis(dimethylphosphino)ethane] is a stable, diamagnetic complex which can be synthesized readily by borohydride reduction of FeH(DMPE)₂Cl or by treatment of Fe(DMPE)₂H₂ with borane. The complex contains an unsupported B? H? Fe hydrogen bridge. Analogous complexes with bulkier ligands, FeH(DEPE)₂(BH₄), [DEPE = 1,2-bis(diethylphosphino)ethane] and FeH(DPrPE)₂(BH₄) [DPrPE = 1,2-bis(di-n-propylphosphino)ethane], are less stable. In all complexes, in solution the borohydride ligand undergoes rapid internal motion, with all four boron-bound hydrogens interchanging environments. The barriers for BH₄ reorientation (measured by NMR spectroscopy) are in the sequence FeH(DMPE)₂(BH)₄ > FeH(DEPE)₂(BH)₄ > FeH(DPrPE)₂(BH₄).     

Metallboranate und Boranatometallate. 14. Chlorid-tetrahydridoborate des Calciums und Strontiums

Metal Tetrahydridoborates and Tetrahydridoborato Metalates. 14. Chloro Tetrahydridoborates of calcium and Strontium The action of hydrogen chloride on E(BH₄)₂ (E ? Ca, Se) in a 1:1 mole ratio in tetrahydrofuran yields the title compounds isolated as their tetrahydrofuran adducts. No intermediates were detected by ¹¹B n.m.r. spectroscopy in the reaction of methanol with Ca(BH₄)₂. The final product is Ca[B(OCH₃)₄]₂ · THF.     

Trimeric cluster of lithium amidoborane—the smallest unit for the modeling of hydrogen release mechanism

A detailed first‐principle DFT M06/6‐311++G(d.p) study of dehydrogenation mechanism of trimeric cluster of lithium amidoborane is presented. The first step of the reaction is association of two LiNH₂BH₃ molecules in the cluster. The dominant feature of the subsequent reaction pathway is activation of H atom of BH₃ group by three Li atoms with formation of unique Li₃H moiety. This Li₃H moiety is destroyed prior to dehydrogenation in favor of formation of a triangular Li₂H moiety, which interacts with protic H atom of NH₂ group. As a result of this interaction, Li₂H₂ moiety is produced. It features N^?? H⁺? H^? group suited near the middle plane between two Li⁺ in the transition state that leads to H₂ release. The transition states of association and hydrogen release steps are similar in energy. It is concluded that the trimer, (LiNH₂BH₃)₃, is the smallest cluster that captures the essence of the hydrogen release reaction. © 2016 Wiley Periodicals, Inc.     

A Composite of Complex and Chemical Hydrides Yields the First Al‐Based Amidoborane with Improved Hydrogen Storage Properties

The first Al‐based amidoborane Na[Al(NH₂BH₃)₄] was obtained through a mechanochemical treatment of the NaAlH₄–4 AB (AB=NH₃BH₃) composite releasing 4.5 wt % of pure hydrogen. The same amidoborane was also produced upon heating the composite at 70 °C. The crystal structure of Na[Al(NH₂BH₃)₄], elucidated from synchrotron X‐ray powder diffraction and confirmed by DFT calculations, contains the previously unknown tetrahedral ion [Al(NH₂BH₃)₄]^?, with every NH₂BH₃^? ligand coordinated to aluminum through nitrogen atoms. Combination of complex and chemical hydrides in the same compound was possible due to both the lower stability of the Al?H bonds compared to the B?H ones in borohydride, and due to the strong Lewis acidity of Al³⁺. According to the thermogravimetric analysis–differential scanning calorimetry–mass spectrometry (TGA–DSC–MS) studies, Na[Al(NH₂BH₃)₄] releases in two steps 9 wt % of pure hydrogen. As a result of this decomposition, which was also supported by volumetric studies, the formation of NaBH₄ and amorphous product(s) of the surmised composition AlN₄B₃H_(0–3.6) were observed. Furthermore, volumetric experiments have also shown that the final residue can reversibly absorb about 27 % of the released hydrogen at 250 °C and p(H₂)=150 bar. Hydrogen re‐absorption does not regenerate neither Na[Al(NH₂BH₃)₄] nor starting materials, NaAlH₄ and AB, but rather occurs within amorphous product(s). Detailed studies of the latter one(s) can open an avenue for a new family of reversible hydrogen storage materials. Finally, the NaAlH₄–4 AB composite might become a starting point towards a new series of aluminum‐based tetraamidoboranes with improved hydrogen storage properties such as hydrogen storage density, hydrogen purity, and reversibility.     

Trans-stereospecific polymerization of butadiene and random copolymerization with styrene using borohydrido neodymium/magnesium dialkyl catalysts

The combination of a neodymium borohydride, Nd(BH₄)₃(THF)₃ (1) or Cp^*Nd(BH₄)₂(THF)_x (2), with MgⁿBuEt (BEM), affords an efficient and highly selective (up to 96.7% 1,4-trans) catalyst for butadiene polymerization. In the presence of excesses of Mg co-catalyst, polymer chain transfer takes place between neodymium and magnesium, and significant amounts of 1,2-units are observed. When considered for butadiene-styrene statistical copolymerization, the catalytic system based on 2 showed a good ability to produce poly[(1,4-trans-butadiene)-co-styrene)], with strong impact of the Mg/Nd ratio on the yield and on the copolymer microstructure, including the percentage of inserted styrene (up to 16.9 mol%). Whatever the co-monomers concentration the polybutadiene backbone remained 1,4-trans. The precise microstructure of the polymers and copolymers was thoroughly analyzed by means of high resolution NMR spectroscopy (900 MHz) and MALDI-ToF spectrometry.     

Chloride substitution in sodium borohydride

The dissolution of sodium chloride and sodium borohydride into each other resulting in formation of solid solutions of composition Na(BH₄)_1−xCl_x is studied. The dissolution reaction is facilitated by two methods: ball milling or combination of ball milling and annealing at 300 °C for three days of NaBH₄-NaCl samples in molar ratios of 0.5:0.5 and 0.75:0.25. The degree of dissolution is studied by Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data. The results show that dissolution of 10 mol% NaCl into NaBH₄, forming Na(BH₄)_0.9Cl_0.1, takes place during ball milling. A higher degree of dissolution of NaCl in NaBH₄ is obtained by annealing resulting in solid solutions containing up to 57 mol% NaCl, i.e. Na(BH₄)_0.43Cl_0.57. In addition, annealing results in dissolution of 10-20 mol% NaBH₄ into NaCl. The mechanism of the dissolution during annealing and the decomposition pathway of the solid solutions are studied by in situ SR-PXD. Furthermore, the stability upon hydrogen release and uptake were studied by Sieverts measurements.     

Synthesis of amorphous manganese borohydride in the(NaBH_4–MnCl_2) system,its hydrogen generation properties and crystalline transformation during solvent extraction

The mixture of(2NaBH_4+ MnCl_2) was ball milled in a magneto-mill. No gas release was detected. The XRD patterns of the ball milled mixture exhibit only the Bragg diffraction peaks of the Na Cl-type salt which on the basis of the present X-ray diffraction results and the literature is likely to be a solid solution Na(Cl)x(BH4)(1-x), possessing a cubic Na Cl-type crystalline structure. No presence of any crystalline hydride was detected by powder X-ray diffraction which clearly shows that NaBH_4 in the initial mixture must have reacted with MnCl_2 forming a Na Cl-type by-product and another hydride that does not exhibit X-ray Bragg diffraction peaks. Mass spectrometry(MS) of gas released from the ball milled mixture during combined MS/thermogravimetric analysis(TGA)/differential scanning calorimetry(DSC) experiments, confirms mainly hydrogen(H2) with a small quantity of diborane gas, B_2H_6. The Fourier transform infra-red(FT-IR) spectrum of the ball milled(2NaBH_4+ MnCl_2) is quite similar to the FT-IR spectrum of crystalline manganese borohydride, c-Mn(BH_4)_2, synthesized by ball milling, which strongly suggests that the amorphous hydride mechano-chemically synthesized during ball milling could be an amorphous manganese borohydride. Remarkably, the process of solvent filtration and extraction at 42 °C, resulted in the transformation of mechano-chemically synthesized amorphous manganese borohydride to a nanostructured,crystalline, c-Mn(BH_4)_2hydride.     

Synthesis and some properties of calcium tetradecahydroundecaborate Ca(B<Subscript>11</Subscript>H<Subscript>14</Subscript>)<Subscript>2</Subscript> · 4Dg (Dg = diglyme)

The reaction of Ca(BH₄)₂ · 1.5Dg with decaborane-14 in diglyme at 85°C yields Ca(B₁₁H₁₄)₂ · 4Dg. The duration of the reaction is 20 h. The molar ratio of Ca(BH₄)₂ · 1.5Dg: B₁₀H₁₄ is 1: 3.5. The yield is 84.7%. The synthesized calcium tetradecahydroundecaborate Ca(B₁₁H₁₄)₂ · 4Dg is a stable compound soluble in water and a number of organic solvents. The study of the compound was carried out using elemental analysis and IR and NMR ¹¹B {¹H} spectroscopy. Calcium undecaborate can be used for synthesizing other salts with boron hydride nido-borates.     

Zur Darstellung und Charakterisierung von Calciumhydrogensulfaten

Preparation and Characterization of Calcium Hydrogen Sulfate CaSO₄ · H₂SO₄ was identified as calcium hydrogen sulfate whereas CaSO₄ · 3 H₂SO₄ is an adduct of CaSO₄ with H₂SO₄. Depending on the excessive amount of H₂SO₄ both compounds exist side by side up to a temperature of 343 K, whereas above this temperature only Ca(HSO₄)₂ is stable. The DTA curve of Ca(HSO₄)₂ shows two maxima at 488 K and 523 K, according to the separation of H₂O under formation of pyrosulfate and decomposition of this compound under elimination of SO₃. In comparison with other hydrogen sulfates Ca(HSO₄)₂ shows a considerable increased O? H distances. The d-values of Ca(HSO₄)₂ are calculated and represented.