The Onset of Dehydrogenation in Solid Ammonia Borane: An Ab Initio Metadynamics Study

The discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H₃BNH₃, AB) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use ab‐initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H₃NBH₂(μ‐H)BH₃) leads to the release of NH₄⁺, which in turn triggers an autocatalytic H₂ production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum‐mechanical simulations on a cluster.     

Beiträge zur Chemie des Phosphors. 239 [1]. Reaktion von Diphosphan(4) mit Diboran(6) und mit THF-Boran: Bildung von Diphosphan-boran,P2H4 · BH3, und Diphosphan-1,2-bis(boran), BH3 · P2H4 · BH3

Contributions to the Chemistry of Phosphorus. 239. On the Reaction of Diphosphane(4) with Diborane(6) and with THF-Borane: Formation of Diphosphane-borane, P₂H₄ · BH₃, and Diphosphane-1,2-bis(borane), BH₃ · P₂H₄ · BH₃ Diphosphane(4) always reacts with diborane(6) in the temperature range of ?118 to ?78°C, to furnish a mixture of diphosphane-borane, P₂H₄ · BH₃ ( 1 ), and diphosphane-1,2-bis(borane), BH₃ · P₂H₄ · BH₃ ( 2 ), in addition to small amounts of triphosphane-1,3-bis(borane), BH₃ · P₃H₅ · BH₃, and phosphane-borane, BH₃ · PH₃, irrespective of the molar ratios of the reactants employed. The formation of the 1 : 1 adduct P₂H₄ · B₂H₆ reported in the literature [4] could not be confirmed. The structures of compounds 1 and 2 were investigated by nuclear magnetic resonance spectroscopy which revealed the complete, homolytic cleavage of diborane(6). As a result of the bonding of one BH₃ group to diphosphane(4), the Lewis basicity of the other PH₂ group is markedly reduced. Similar mixtures of products are obtained when the borane adduct THF · BH₃ is employed in an analogous reaction. In the case of a 1 : 1 molar ratio of P₂H₄ : THF · BH₃ at ?78°C, the reaction furnishes compound 1 exclusively. This product can be isolated in the pure state and is found to be appreciably more stable than diphosphane(4).     

“Spontaneous” Ambient Temperature Dehydrocoupling of Aromatic Amine–Boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine–borane adducts ArNH₂ ? BH₃ ( 3 a – c ; Ar= a : Ph, b : p‐MeOC₆H₄, c : p‐CF₃C₆H₄) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H₂. From reactions of 3 a , b in concentrated solutions in THF at 22 °C over 24 h cyclotriborazanes (ArNH‐BH₂)₃ ( 7 a , b ) were isolated as THF adducts, 7 a , b? THF, or solvent‐free 7 a , which could not be obtained via heating of 3 a – c in the melt. The μ‐(anilino)diborane [H₂B(μ‐PhNH)(μ‐H)BH₂] ( 4 a ) was observed in the reaction of 3 a with BH₃?THF and was characterized in situ. The reaction of 3 a with PhNH₂ ( 2 a ) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)₂BH ( 5 a ), which has potential generality. This observation, together with further studies of reactions of 4 a , 5 a , and 7 a , b , provided insight into the mechanism of the catalyst‐free ambient temperature dehydrocoupling of 3 a – c in solution. For example, the reaction of 4 a with 5 a yields 6 a and 7 a . It was found that borazines (ArN‐BH)₃ ( 6 a – c ) are not simply formed via dehydrogenation of cyclotriborazanes 7 a – c in solution. The transformation of 7 a to 6 a is slowly induced by 5 a and proceeds via regeneration of 3 a . The adducts 3 a – c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline–borane derivative 3 c , the linear iminoborane oligomer (p‐CF₃C₆H₄)N[BH‐NH(p‐CF₃C₆H₄)]₂ ( 11 ) was obtained. The single‐crystal X‐ray structures of 3 a – c , 5 a , 7 a , 7 b? THF, and 11 are discussed.     

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.     

Metal‐Free Ammonia–Borane Dehydrogenation Catalyzed by a Bis(borane) Lewis Acid

The storage of energy in a safe and environmentally benign way is one of the main challenges of today’s society. Ammonia–borane (AB=NH₃BH₃) has been proposed as a possible candidate for the chemical storage of hydrogen. However, the efficient release of hydrogen is still an active field of research. Herein, we present a metal‐free bis(borane) Lewis acid catalyst that promotes the evolution of up to 2.5 equivalents of H₂ per AB molecule. The catalyst can be reused multiple times without loss of activity. The moderate temperature of 60 °C allows for controlling the supply of H₂ on demand simply by heating and cooling. Mechanistic studies give preliminary insights into the kinetics and mechanism of the catalytic reaction.     

CuCo/BNNSs纳米催化剂对固态储氢材料氨硼烷水解的催化性能

采用共还原法将CuCo双金属负载到通过聚乙烯吡咯烷酮(PVP)辅助离子插层法制备的少层氮化硼纳米片(BNNSs)上,获得了平均粒径为2.7 nm且高度分散的铜钴/氮化硼纳米片(CuCo/BNNSs)纳米催化剂。通过原子力显微镜(AFM)、X射线衍射(XRD)、傅里叶转换红外光谱(FTIR)、拉曼光谱(Raman)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)和高分辨率透射电镜(HRTEM)对载体及催化剂的结构及形貌进行表征,并研究了CuCo/BNNSs的催化性能。研究发现,由于Cu、Co、BNNSs和OH-之间高效的四重效应协同使得Cu0.5Co0.5/BNNSs纳米催化剂在室温、pH=14条件下对氨硼烷(AB,NH3BH3)水解释氢具有极高的催化活性。转化频率(TOF)值达到104.52 molH2·molmetal^-1·min^-1,且CuCo/BNNSs纳米催化剂具有良好的稳定性,6次循环利用后仍保持较高催化活性。     

Metal‐Borohydride‐Modified Zr(BH4)4⋅8 NH3: Low‐Temperature Dehydrogenation Yielding Highly Pure Hydrogen

Due to its high hydrogen density (14.8 wt %) and low dehydrogenation peak temperature (130 °C), Zr(BH₄)₄ ? 8 NH₃ is considered to be one of the most promising hydrogen‐storage materials. To further decrease its dehydrogenation temperature and suppress its ammonia release, a strategy of introducing LiBH₄ and Mg(BH₄)₂ was applied to this system. Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄ and Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂ composites showed main dehydrogenation peaks centered at 81 and 106 °C as well as high hydrogen purities of 99.3 and 99.8 mol % H₂, respectively. Isothermal measurements showed that 6.6 wt % (within 60 min) and 5.5 wt % (within 360 min) of hydrogen were released at 100 °C from Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄ and Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂, respectively. The lower dehydrogenation temperatures and improved hydrogen purities could be attributed to the formation of the diammoniate of diborane for Zr(BH₄)₄ ? 8 NH₃–4 LiBH₄, and the partial transfer of NH₃ groups from Zr(BH₄)₄ ? 8 NH₃ to Mg(BH₄)₂ for Zr(BH₄)₄ ? 8 NH₃–2 Mg(BH₄)₂, which result in balanced numbers of BH₄ and NH₃ groups and a more active H^δ+ ??? ^?δH interaction. These advanced dehydrogenation properties make these two composites promising candidates as hydrogen‐storage materials.     

Microwave spectrum of aminoborane,BH2NH2

The unstable species aminoborane, BH₂NH₂, has been identified as a reaction product of ammonia with diborane by microwave spectroscopy. The rotational constants determined are A = 138212 ± 4 MHz, B = 27487.83 ± 0.10 MHz and C = 22878.44 ± 0.11 MHz for ¹¹BH₂NH₂ and A = 138199 ± 6 MHz, B = 28420.36 ± 0.11 MHz and C = 23520.78 ± 0.12 MHz for ¹⁰BH₂NH₂. The dipole moment is 1.844 ± 0.015 D.     

Metallboranate und Boranato-metallate. V. Solvate des Cadmiumboranats und Boranatocadmate

The reaction of CdCl₂ or CdBr₂with LiBH₄, in ether yields no pure Cd(BH₄)₂, but Li₂Cd(BH₄)₄ was isolated as an oily etherate. Similarly, NaCd(BH₄)₃ was obtained from CdCl₂ and NaBH₄ in ether and tetrahydrofurane as solvents. LiCd(BH₄)₃ and NaCd(BH₄)₃ were also formed from the components in ether solution. In these solutions Cd migrates to the anode confirming their formulation as tetrahydroborato-cadmates. Cadmiumtetrahydroborate was formed in the reaction of cadmium methoxide with diborane in tetrahydrofurane (THF) and isolated as crystalline solvates. It reacts with pyridine to give Cd(BH₄)₂ · 3 NC₅H₅ and with NH₃ to yield Cd(NH₃)₆(BH₄)₂.     

Kinetic Analysis and Modeling of Thermal Decomposition of Ammonia Borane

Ammonia borane (AB; NH₃BH₃) is one of the most promising materials for hydrogen storage applications, mainly due to its high gravimetric hydrogen storage capacity of 19.6 wt%. In this paper, we present an exclusive kinetic analysis of AB thermolysis. Three methods are used for kinetic analysis of the thermal decomposition of AB, namely the Kissinger method, isoconversional model‐free fitting method, and solid‐state kinetics model–based method. Finally, a need to device a new model for thermal kinetics of AB was observed and hence a new kinetic model for AB thermolysis is proposed.     

石墨型氮化碳对氨硼烷放氢性能的影响

利用石墨型氮化碳(C₃N₄)和氨硼烷(NH₃BH₃,AB)球磨制备了AB-C₃N₄体系,发现C₃N₄的加入使AB放氢反应温度明显降低,但是副产物氨气浓度有所升高. 因此,利用LiBH₄改性的C₃N₄(LC₃N₄)同AB球磨合成出了AB-LC₃N₄体系,并采用X射线衍射、程序升温脱附-质谱联用、热重-差热分析及核磁共振等技术考察了该体系的脱氢性能. 结果表明,由于LC₃N₄的加入,AB的放氢反应温度明显降低,放氢反应速率加快,放氢诱导期缩短,同时抑制了副产物无机苯的生成. 另外,C₃N₄的化学修饰也降低了AB-LC₃N₄放氢过程中生成氨气的浓度. 动力学分析和核磁共振结果表明,AB-LC₃N₄分解过程依然遵循NH₃BH₂NH₃BH₄诱导的氨硼烷自分解机理.     

Bimetallic Au–Ni Nanoparticles Embedded in SiO2 Nanospheres: Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane

Gold–nickel nanoparticles (NPs) of 3–4 nm diameter embedded in silica nanospheres of around 15 nm have been prepared by using [Au(en)₂Cl₃] and [Ni(NH₃)₆Cl₂] as precursors in a NP‐5/cyclohexane reversed‐micelle system, and by in situ reduction in an aqueous solution of NaBH₄/NH₃BH₃. Compared with monometallic Au@SiO₂ and Ni@SiO₂, the as‐synthesized Au–Ni@SiO₂ catalyst shows higher catalytic activity and better durability in the hydrolysis of ammonia borane, generating a nearly stoichiometric amount of hydrogen. During the generation of H₂, the synergy effect between gold and nickel is apparent: The nickel species stabilizes the gold NPs and the existence of gold helps to improve the catalytic activity and durability of the nickel NPs.     

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.     

Thermal Dehydrogenation of Base‐Stabilized B2H5+ Complexes and Its Role in CH Borylation

Thermally induced dehydrogenation of the H‐bridged cation L₂B₂H₅⁺ (L=Lewis base) is proposed to be the key step in the intramolecular C? H borylation of tertiary amine boranes activated with catalytic amounts of strong “hydridophiles”. Loss of H₂ from L₂B₂H₅⁺ generates the highly reactive cation L₂B₂H₃⁺, which in its sp²‐sp³ diborane(4) form then undergoes either an intramolecular C? H insertion with B? B bond cleavage, or captures BH₃ to produce L₂B₃H₆⁺. The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH₂C₆F₅ complexes through disproportionation of L₂B₂H₅⁺ HB(C₆F₅)₃^?.     

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.     

Quantum mechanics investigation on initial decomposition of ammonia borane in glyme

The chemical kinetics of ammonia borane (AB) in glyme solution is studied using quantum mechanics (QM) based calculations along with experimental results available in the literature. The primary objective of this study is to propose a detailed reaction mechanism that explains the formation of species observed during AB decomposition for temperatures ranging from 323 to 368 K. The quantum mechanics investigation uses transition state theory to identify the relevant reaction pathways. Intrinsic reaction coordinate calculations use the identified transition‐state structure to link the reactants to the products. These calculations were performed using the Gaussian 09 program package, including the solvation model based on density (SMD) with acetonitrile as the solvent. Thermodynamic properties of species at equilibrium or at transition states were computed using the G4(MP2) compound method. Sensitivity analysis was performed using a species conservation model to identify reactions and species that play a critical role. This study confirms the previous experimental observation regarding the initiation of decomposition of AB in glyme. It also elucidates the role of DADB, ammonium borohydride salt ([BH₄]⁻[NH₄]⁺) and BH₂NH₂ in hydrogen release and intermediates formed during initial phase of AB decomposition. This work shows how QM calculations along with experimental results can contribute to our understanding of the complex chemical kinetics involved during AB dehydrogenation.     

核壳结构Cu@CoCr的合成和对硼氨配合物水解的催化性能

通过一步法原位合成了核壳结构的Cu@CoCr三元合金催化剂,该合成方法条件温和、步骤简便。25℃下,Cu0.4@Co0.5Cr0.1三元合金催化剂对于硼氨配合物水解反应的TOF值达到0.242 0 molH2.moLcat-1.s-1,接近Pt、Pd等贵金属的催化活性,反应的活化能仅为35 kJ.mol-1,5次重复使用后仍能保持初始活性的35%。与非核壳结构的CuCoCr合金相比,核壳结构的Cu@CoCr三元合金催化剂的催化性能及稳定性均有明显提高。此外材料自身具有磁性,可进行磁分离,适合于实际应用。     

Chemical and Phase Transformations in the Systems Hydrogen-Sorbing Intermetallic Compound-Diborane

The reactions of the intermetallic compounds CeFe₂, CeCo₂, and 3-ScFe₂ with B₂H₆ at 4.8×10³ Pa, 293-573 K, and various contact times were studied. CeFe₂ decomposes in the presence of diborane to give CeH_1.9-2.2, Ce₂Fe₁₇H_8-10, -Fe, CeCo₂ decomposes into the cerium trihydride CeH_2.7-2.8 and -Co, whereas ₃-ScFe₂ forms an ScFe₂H_0.4-0.5 hydride phase. Therewith, with CeFe₂ and CeCo₂, diborane decomposes into solid nonvolatile X-ray amorphous various-composition compounds (BH _mn , as well as H₂ and B₅H₉, while with ₃-ScFe₂, into H₂, B₄H₁₀, B₅H₉, B₅H₁₁, and (BH _{m) n} .     

Synthesis and Crystal Structure of Cesium Hexamminesodium Decahydro‐closo‐decaborate‐Ammonia(1/1), Cs[Na(NH3)6][B10H10]·NH3

Cs[Na(NH₃)₆][B₁₀H₁₀]·NH₃ was synthesised from cesium and disodium‐decahydro‐closo‐decaborate Na₂B₁₀H₁₀ in liquid ammonia, from which it crystallized in form of temperature sensitive colorless plates (triclinic, P1¯, a = 8.4787(7) Å, b = 13.272(1) Å, c = 17.139(2) Å, α = 88.564(1)°, β = 89.773(1)°, γ = 81.630(1)°, V = 1907.5(3) ų, Z = 4). The compound is the first example of an alkali metal boranate with two different types of cations. The decahydro‐closo‐decaborate dianions [B₁₀H₁₀]^2— and the cesium cations form a equation/tex2gif-stack-1.gif[Cs₂(B₁₀H₁₀)₂]^2— layer parallel to the ac plane. These layers are separated by N—H···N‐hydrogen bonded hexamminesodium cations.