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.     

Preparation,characterization and thermal behaviour of polymeric complex of cadmium hexamethylenetetramine nitrate

The preparation of cadmium nitrate complex with bridged hexamethylenetetramine-[{Cd(HMTA)(NO₃)₂(H₂O)₂}_n], CHNC, of polymeric nature has been reported here for the first time. It was characterized by X-ray crystallography, ¹H NMR, FT-IR and elemental analysis. The crystal structure is stabilized by hydrogen bonding between the oxygen atom of the nitrate group with the methyl hydrogen of HMTA and hydrogen of the coordinated water molecule via C–H⋯O and O–H⋯O interactions. The thermolysis of this complex was investigated by TG-DSC and ignition delay measurements. The model-free isoconversional and model-fitting kinetic approaches have been applied to isothermal TG data for kinetics investigation of thermal decomposition of the complex. At higher temperatures, the complex gets ignited to produce highly thermally stable residue most closely resembles CdO proved by XRD.     

Promoted H2 Generation from NH3BH3 Thermal Dehydrogenation Catalyzed by Metal–Organic Framework Based Catalysts

The application of ammonium borane (AB) as a hydrogen storage material is limited by the sluggish kinetics of H₂ release. Two catalysts based on metal–organic frameworks (MOFs) have been prepared either by applying MOF as precursors or by the in situ reduction method. In the release of H₂ from AB, the high H₂ content of the whole system, the remarkably lower reaction onset temperature, the significantly increased H₂ release rates at ≤90 °C, and the decreased reaction exothermicity have all been achieved with only 1.0 mol % MOF‐based catalyst. Moreover, the clear catalytic diversity of three catalysts has been observed and discussed. The in situ synthesized Ni⁰ sites and the MOF supports in the catalysts were proven to show significant and different effects to promote the catalytic activities. With MOF‐based catalysts, both the enhanced kinetics and the high H₂ capacity of the AB system present great advantages for future use.     

Spectroscopic and Thermal Studies on 2,4,6-trinitro Toluene (TNT)

The kinetics and mechanism of the initial stage of thermal decomposition of 2,4,6-trinitro toluene (TNT), a widely used high explosive, have been studied, together with its morphology and evolved gaseous products using thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy (IR) and hot-stage microscopy. The kinetics of the thermolysis has been followed by IR after suppressing volatilisation by matrixing and by isothermal TG without suppressing volatilisation to simulate actual user conditions. The best linearity was obtained for Avrami-Erofeev equation for n=1 in isothermal IR and also in isothermal TG. The activation energy was found to be 135 kJ mol⁻¹, with logA (in s⁻¹) 12.5 by IR. The effect of additives on the initial thermolysis of TNT has also been studied. Evolved gas analysis by IR showed that CO₂, NO₂, NO and H₂O are more dominant than N₂O, HCN and CO. The decomposition involves the initial rupture of the C-NO₂ bond, weakened by hydrogen bonding with the labile hydrogen atom of the adjacent CH₃ group, followed by the abstraction of the hydrogen atom of the methyl group by NO₂, generated in the initial step. This revised version was published online in August 2006 with corrections to the Cover Date.     

Power‐Law Kinetic Models for Synthesis of Ammonia Borane

In the past few years, there have been increasing numbers of studies for the production and dehydrogenation of ammonia borane (NH₃BH₃, AB), which has become a significant hydrogen storage material. However, kinetic model studies based on the synthesis of AB in the literature have not been encountered, though there are many kinetic modeling studies on dehydrogenation of AB (Akbayrak et al., Appl Catal B 2016, 198, 162–170; Choi et al., Phys Chem Chem Phys 2014, 16(17), 7959–7968; Esteruelas et al., Inorg Chem 2016, 55(14), 7176–7181; Park et al., Int J Hydrogen Energy 2015, 40(46), 16316–16322; Rakap, Appl Catal B 2015, 163, 129–134; Tonbul et al., Int J Hydrogen Energy 2016, 41(26), 11154–11162; Zhang et al., Int J Hydrogen Energy 2016, 41(39), 17208–17215). The paper describes the development of a kinetic model for synthesis of ammonia borane by using borohydride (NaBH₄) and ammonium salt (NH₄)₂SO₄. The synthesis of AB experiments was carried out at different temperature ranges between 25 and 50°C, different inlet molar ratios (NaBH₄/(NH₄)₂SO₄ = 1–4), and different molarities with respect to NaBH₄ (0.11–0.67 M NaBH₄). After the parametric experiments were conducted, empirical power law was evaluated for the synthesis reaction. The power‐law model represented the trends of the kinetics of the synthesis reaction and was reproduced as .     

Nanostructured Ni2P as a Robust Catalyst for the Hydrolytic Dehydrogenation of Ammonia–Borane

Ammonia–borane (AB) is a promising chemical hydrogen‐storage material. However, the development of real‐time, efficient, controllable, and safe methods for hydrogen release under mild conditions is a challenge in the large‐scale use of hydrogen as a long‐term solution for future energy security. A new class of low‐cost catalytic system is presented that uses nanostructured Ni₂P as catalyst, which exhibits excellent catalytic activity and high sustainability toward hydrolysis of ammonia–borane with the initial turnover frequency of 40.4 mol_(H2) mol_(Ni2P)^?1 min^?1 under air atmosphere and at ambient temperature. This value is higher than those reported for noble‐metal‐free catalysts, and the obtained Arrhenius activation energy (E_a=44.6 kJ mol^?1) for the hydrolysis reaction is comparable to Ru‐based bimetallic catalysts. A clearly mechanistic analysis of the hydrolytic reaction of AB based on experimental results and a density functional theory calculation is presented.     

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.     

The Non‐Isothermal Decomposition Kinetics of NaNTO·H2O and the Relationship between its Structure and Properties

NaNTO·H₂O was prepared by mixing 3‐nitro‐1,2,4‐triazol‐5‐one (NTO) aqueous solution and sodium hydroxide aqueous solution. Its thermal decomposition and kinetics were studied under non‐isothermal conditions by DSC and TG/DTG methods. The kinetic parameters were obtained from analysis of the DSC and TG/DTG curves by the Kissinger method, the Ozawa method, the differential method and the integral method. The most probable mechanism function for the thermal decomposition of the first stage was suggested by comparing the kinetic parameters. The critical temperature of thermal explosion (T_b) was 240.93 °C. The theoretical investigation on the structure unit of the title compound was carried out by DFT‐B3LYP/CEP‐31G methods; atomic net charges and the population analysis were discussed.     

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.     

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.     

Renewed Insight into the Promoting Mechanism of Magnesium Hydride on Ammonia Borane

Our previous study found that mechanically milling with magnesium hydride (MgH₂) could dramatically improve the dehydrogenation property of ammonia borane (AB). Meanwhile, it appears that the MgH₂ additive maintains its phase stability in the milling and subsequent heating process. In an effort to further the mechanistic understanding of the AB/MgH₂ system, we reinvestigated the property and structure evolution in the hydrogen release process of the AB/0.5MgH₂ sample. Property examination using volumetric method and synchronous thermal analyses showed that the AB/0.5 MgH₂ sample releases ～13.8 wt % hydrogen after being heated at 300 °C. This hydrogen amount is in excess of that available from AB, indicative of the participation of a faction of MgH₂ in the dehydrogenation process of AB. Structural and chemical state analyses using Fourier transformation infrared spectroscopy and solid‐state ¹¹B nuclear magnetic resonance techniques further showed that part of MgH₂ participates in the dehydrogenation process of AB from the first step, resulting in the formation of Mg? B? N? H intermediate species. The incorporation of Mg in AB is believed to be a crucial event leading to dehydrogenation property improvements, particularly for the release of the last equivalent of H₂ in AB at relatively moderate temperature. These findings have provided renewed insight into the promoting mechanism of MgH₂ on the hydrogen release from AB.     

Synthesis and thermal decomposition kinetics of the complexes [Sm(o‐NBA)3bipy]2·2H2O and [Sm(o‐BrBA)3bipy]2· 2H2O

Two new complexes [Sm(o‐NBA)₃bipy]₂·2H₂O ( 1 ) and [Sm(o‐BrBA)₃bipy]₂·2H₂O ( 2 ) (where o‐NBA is o‐nitrobenzoic acid, o‐BrBA is o‐bromobenzoic acid, and bipy is 2,2′‐bipyridine) were prepared and characterized by elemental analysis, IR, UV, and molar conductance, respectively. The thermal decomposition behaviors of the two complexes were investigated by means of TG–DTG and IR techniques. The thermal decomposition kinetics was studied by using advanced double equal‐double steps method, nonlinear integral isoconversional method, and nonlinear differential isoconversional method. The kinetic parameters of the second‐step process for the two complexes were obtained, respectively. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 607–616, 2008     

Solid-state thermolysis of ammonia borane and related materials for high-capacity hydrogen storage

Ammonia borane (NH(3)BH(3), AB) is a unique molecular crystal containing an intriguingly high density of hydrogen. In the past several years, AB has received extensive attention as a promising hydrogen storage medium. Several strategies have been successfully developed for promoting H(2) release and for suppressing the evolution of volatile by-products from the solid-state thermolysis of AB. Several potentially cost-effective and energy-efficient routes for regenerating AB from the spent fuels have been experimentally demonstrated. These remarkable technological advances offer a promising prospect of using AB-based materials as viable H(2) carriers for on-board application. In this perspective, the recent progresses in promoting H(2) release from the solid-state thermolysis of AB and in developing regeneration technologies are briefly reviewed.     

A Rapid Microwave‐Assisted Thermolysis Route to Highly Crystalline Carbon Nitrides for Efficient Hydrogen Generation

Highly crystalline graphitic carbon nitride (g‐C₃N₄) with decreased structural imperfections benefits from the suppression of electron–hole recombination, which enhances its hydrogen generation activity. However, producing such g‐C₃N₄ materials by conventional heating in an electric furnace has proven challenging. Herein, we report on the synthesis of high‐quality g‐C₃N₄ with reduced structural defects by judiciously combining the implementation of melamine–cyanuric acid (MCA) supramolecular aggregates and microwave‐assisted thermolysis. The g‐C₃N₄ material produced after optimizing the microwave reaction time can effectively generate H₂ under visible‐light irradiation. The highest H₂ evolution rate achieved was 40.5 μmol h⁻¹, which is two times higher than that of a g‐C₃N₄ sample prepared by thermal polycondensation of the same supramolecular aggregates in an electric furnace. The microwave‐assisted thermolysis strategy is simple, rapid, and robust, thereby providing a promising route for the synthesis of high‐efficiency g‐C₃N₄ photocatalysts.     

Evidence of a Kinetic Isotope Effect in Nanoaluminum and Water Combustion

The normally innocuous combination of aluminum and water becomes violently reactive on the nanoscale. Research in the field of the combustion of nanoparticulate aluminum has important implications in the design of molecular aluminum clusters, hydrogen storage systems, as well as energetic formulations which could use extraterrestrial water for space propulsion. However, the mechanism that controls the reaction speed is poorly understood. While current models for micron‐sized aluminum water combustion reactions place heavy emphasis on diffusional limitations, as reaction scales become commensurate with diffusion lengths (approaching the nanoscale) reaction rates have long been suspected to depend on chemical kinetics, but have never been definitely measured. The combustion analysis of nanoparticulate aluminum with H₂O or D₂O is presented. Different reaction rates resulting from the kinetic isotope effect are observed. The current study presents the first‐ever observed kinetic isotope effect in a metal combustion reaction and verifies that chemical reaction kinetics play a major role in determining the global burning rate.     

Theory and Practice: Bulk Synthesis of C3B and its H2‐ and Li‐Storage Capacity

Previous theoretical studies of C₃B have suggested that boron‐doped graphite is a promising H₂‐ and Li‐storage material, with large maximum capacities. These characteristics could lead to exciting applications as a lightweight H₂‐storage material for automotive engines and as an anode in a new generation of batteries. However, for these applications to be realized a synthetic route to bulk C₃B must be developed. Here we show the thermolysis of a single‐source precursor (1,3‐(BBr₂)₂C₆H₄) to produce graphitic C₃B, thus allowing the characteristics of this elusive material to be tested for the first time. C₃B was found to be compositionally uniform but turbostratically disordered. Contrary to theoretical expectations, the H₂‐ and Li‐storage capacities are lower than anticipated, results that can partially be explained by the disordered nature of the material. This work suggests that to model the properties of graphitic materials more realistically, the possibility of disorder must be considered.     

CuxCo1−xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High‐Efficiency Hydrolysis of Ammonia–Borane

Ammonia–borane (AB) is an excellent material for chemical storage of hydrogen. However, the practical utilization of AB for production of hydrogen is hindered by the need of expensive noble metal‐based catalysts. Here, we report Cu_xCo_1?xO nanoparticles (NPs) facilely deposited on graphene oxide (GO) as a low‐cost and high‐performance catalyst for the hydrolysis of AB. This hybrid catalyst exhibits an initial total turnover frequency (TOF) value of 70.0 (H₂) mol/(Cat‐metal) mol?min, which is the highest TOF ever reported for noble metal‐free catalysts, and a good stability keeping 94 % activity after 5 cycles. Synchrotron radiation‐based X‐ray absorption spectroscopy (XAS) investigations suggested that the high catalytic performance could be attributed to the interfacial interaction between Cu_xCo_1?xO NPs and GO. Moreover, the catalytic hydrolysis mechanism was studied by in situ XAS experiments for the first time, which reveal a significant water adsorption on the catalyst and clearly confirm the interaction between AB and the catalyst during hydrolysis.     

Kinetics of Transesterification of Methyl Acetate and Ethanol Catalyzed by Ionic Liquid

The kinetic behavior and chemical equilibria for the transesterification of methyl acetate with ethanol to ethyl acetate and methanol catalyzed by an ionic liquid 4‐(3‐methyl‐1‐imidazolio)‐1‐butanesulfonic acid hydrogen sulfate ([HSO₃‐bmim]HSO₄) were studied experimentally. The relationship of chemical equilibrium constants versus temperature was obtained from kinetic experimental data. The effects of temperature, initial molar ratio of ethanol to methyl acetate, and catalyst concentrations were investigated. Based on the reaction mechanism, an ideal homogeneous (IH) and two nonideal homogeneous models (NIH‐1, NIH‐2) were proposed to correlate the reaction kinetic data. The activity coefficient of the catalyst was considered in NIH‐2 but not in the NIH‐1 model. The nonideal homogeneous model NIH‐1 was the best model to describe the kinetics of the transesterification reaction, and the accuracy of the reaction kinetic model is not improved by considering the effects of catalyst on the activities of the reactants and products.     

Transition-metal bimaleates and their solid solutions: Synthesis,structural, and thermoanalytical study

Transition-metal hydrogen maleates of composition M(C₄H₃O₄)₂ · 4H₂O, where M = Mn, Fe, Co, and Ni, and their solid solutions were synthesized and characterized by X-ray crystallography, IR spectroscopy, mass spectrometry, and thermal analysis. X-ray crystallography and IR spectroscopy showed that both intramolecular and intermolecular H-bonds exist in these compounds. The generation of continuous substitutional solid solutions with cation substitution in these compounds was studied. The thermolysis mechanism was studied for both transition-metal hydrogen maleates and their solid solutions. The composite produced by thermolysis is stable up to 1200°C in flowing helium.     

Carbothermal synthesis of vanadium nitride: Kinetics and mechanism

Constant rate thermal analysis (CRTA) has been used for the first time to study the kinetics of the carbothermal reduction of V₂O₅ in nitrogen to obtain vanadium nitride. It is noteworthy to point out that CRTA method allows both a good control of pressure in the sample surroundings and the use of reaction rates low enough to keep temperatures gradients at a negligible level to avoid any heat or mass transfer phenomena. This method allows one to control the texture and the structure of many materials through kinetic control of the thermal treatment of the precursors. The precise control of the external parameters of the reaction shows that CRTA is an attractive method for kinetic studies and leads to more reliable kinetic data. It has been shown that the carbothermal synthesis of vanadium nitride is best described by a three‐dimensional diffusion kinetic model (the Jander equation) with an activation energy which falls in the range of 520–540 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 369–375, 2006