Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase.

The hydrogenation of benzaldehyde to benzyl alcohol on carbon-supported metals in water, enabled by an external potential, is markedly promoted by polarization of the functional groups. The presence of polar co-adsorbates, such as substituted phenols, enhances the hydrogenation rate of the aldehyde by two effects, that is, polarizing the carbonyl group and increasing the probability of forming a transition state for H addition. These two effects enable a hydrogenation route, in which phenol acts as a conduit for proton addition, with a higher rate than the direct proton transfer from hydronium ions. The fast hydrogenation enabled by the presence of phenol and applied potential overcompensates for the decrease in coverage of benzaldehyde caused by competitive adsorption. A higher acid strength of the co-adsorbate increases the intensity of interactions and the rates of selective carbonyl reduction.

Solid-state hydrogen rich boron-nitrogen compounds for energy storage.

Boron compounds have a rich history in energy storage applications, ranging from high energy fuels for advanced aircraft to hydrogen storage materials for fuel cell applications. In this review we cover some of the aspects of energy storage materials comprised of electron-poor boron materials combined with electron-rich nitrogen elements with the goal of moderate temperature release of hydrogen. The parent compounds of ammonium borohydride, ammonia borane, and diammoniate of diborane provide approaches for storing high gravimetric and volumetric densities of hydrogen. Here we provide a review with a historical perspective and current developments in the area of solid state B and N containing compounds. This review highlights developments in synthesis of ammonia borane and its derivatives over the last 80 years. Thermodynamics and kinetics of hydrogen release in the solid state are discussed. By changing either substituents on the boron and nitrogen atoms or the physical environment by embedding in mesoporous scaffolds, the thermodynamics can be modified to reduce the exothermicity of hydrogen release and minimize formation of volatile impurities. Several mechanistic studies are reviewed identifying the key distinctions between homopolar and heteropolar H2 release. Strategies for economical and efficient regeneration of the hydrogen storage materials via chemical transformation are critically reviewed. The limited efficiency of these chemical regeneration has limited some of the potential applications.

Kinetics study of solid ammonia borane hydrogen release--modeling and experimental validation for chemical hydrogen storage.

Ammonia borane (AB), NH3BH3, is a promising material for chemical hydrogen storage with 19.6 wt% gravimetric hydrogen capacity of which maximum 16.2 wt% hydrogen can be released via an exothermic thermal decomposition below 200 °C. We have investigated the kinetics of hydrogen release from AB and from an AB-methyl cellulose (AB/MC) composite at temperatures of 160-300 °C using both experiments and modeling. The hydrogen release rate at 300 °C is twice as fast as at 160 °C. The purpose of our study was to show safe hydrogen release without thermal runaway effects and to validate system model kinetics. AB/MC released hydrogen at ∼20 °C lower than neat AB and at a faster release rate in that temperature range. Based on the experimental results, the kinetics equations were revised to better represent the growth and nucleation process during decomposition of AB. We explored two different reactor concepts; auger and fixed bed. The current auger reactor concept turned out to not be appropriate, however, we demonstrated safe self-propagation of the hydrogen release reaction of solid AB/MC in a fixed bed reactor.

Methods to stabilize and destabilize ammonium borohydride.

Ammonium borohydride, NH(4)BH(4), has a high hydrogen content of ρ(m) = 24.5 wt% H(2) and releases 18 wt% H(2) below T = 160 °C. However, the half-life of bulk NH(4)BH(4) at ambient temperatures and pressures, ~6 h, is insufficient for practical applications. The decomposition of NH(4)BH(4) (ABH(2)) was studied at variable hydrogen and argon back pressures to investigate possible pressure mediated stabilization effects. The hydrogen release rate from solid ABH(2) at ambient temperatures is reduced by ~16% upon increasing the hydrogen back pressure from 5 to 54 bar. Similar results were obtained using argon pressure and the observed stabilization may be explained by a positive volume of activation, ca. 73 ± 17 cc mol(-1), in the transition state leading to hydrogen release. Nanoconfinement in mesoporous silica, MCM-41, was investigated as alternative means to stabilize NH(4)BH(4). However, other factors appear to significantly destabilize NH(4)BH(4) and it rapidly decomposes at ambient temperatures into [(NH(3))(2)BH(2)][BH(4)] (DADB) in accordance with the bulk reaction scheme. The hydrogen desorption kinetics from nanoconfined [(NH(3))(2)BH(2)][BH(4)] is moderately enhanced as evidenced by a reduction in the DSC decomposition peak temperature of ΔT = -13 °C as compared to the bulk material. Finally, we note a surprising result, storage of DADB at temperature <-30 °C transformed, reversibly, the [(NH(3))(2)BH(2)][BH(4)] into a new low temperature polymorph as revealed by both XRD and solid state MAS (11)B MAS NMR.

A thermodynamic and kinetic study of the heterolytic activation of hydrogen by frustrated borane-amine Lewis pairs.

Calorimetry is used to measure the reaction enthalpies of hydrogen (H(2)) activation by 2,6-lutidine (Lut), 2,2,6,6-tetramethylpiperidine (TMP), N-methyl-2,2,6,6-tetramethylpiperidine (MeTMP), and tri-tert-butylphosphine (TBP) with tris(pentafluorophenyl)borane (BCF). At 6.6 bar H(2) the conversion of the Lewis acid Lewis base pair to the corresponding ionic pair in bromobenzene at 294 K was quantitative in under 60 min. Integration of the heat release from the reaction of the Frustrated Lewis Pair (FLP) with H(2) as a function of time yields a relative rate of hydrogenation in addition to the enthalpy of hydrogenation. The half-lives of hydrogenation range from 230 s with TMP, ΔH(H2) = -31.5(0.2) kcal mol(-1), to 1400 s with Lut, ΔH(H2) = -23.4(0.4) kcal mol(-1). The (11)B nuclear magnetic resonance (NMR) spectrum of B(C(6)F(5))(3) in bromobenzene exhibits three distinct traits depending on the sterics of the Lewis base; (1) in the presence of pyridine, only the dative bond adduct pyridine-B(C(6)F(5))(3) is observed; (2) in the presence of TMP and MeTMP, only the free B(C(6)F(5))(3) is observed; and (3) in the presence of Lut, both the free B(C(6)F(5))(3) and the Lut-B(C(6)F(5))(3) adduct appear in equilibrium. A measure of the change in K(eq) of Lut + B(C(6)F(5))(3) ⇔ Lut-B(C(6)F(5))(3) as a function of temperature provides thermodynamic properties of the Lewis acid Lewis base adduct, ΔH = -17.9(1.0) kcal mol(-1) and a ΔS = -49.2(2.5) cal mol(-1) K, suggesting the Lut-B(C(6)F(5))(3) adduct is more stable in bromobenzene than in toluene.

3-Methyl-1,2-BN-cyclopentane: a promising H2 storage material?

We provide detailed characterization of properties for 3-methyl-1,2-BN-cyclopentane 1 that are relevant to H(2) storage applications such as viscosity, thermal stability, H(2) gas stream purity, and polarity. The viscosity of 1 at room temperature is 25 ± 5 cP, about one fourth the viscosity of olive oil. TGA/MS analysis indicates that liquid carrier 1 is thermally stable at 30 °C but decomposes slowly at 50 °C. RGA data suggest that the H(2) desorption from 1 is a clean process, producing relatively pure H(2) gas. Compound 1 is a polar zwitterionic-type liquid consistent with theoretical predictions and solvatochromic studies.

Immobilizing highly catalytically active Pt nanoparticles inside the pores of metal-organic framework: a double solvents approach.

Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal-organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a "double solvents" method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation.

Control of hydrogen release and uptake in amine borane molecular complexes: thermodynamics of ammonia borane, ammonium borohydride, and the diammoniate of diborane.

Molecular complexes of Lewis acid-base pairs can be used to activate molecular hydrogen for applications ranging from hydrogen storage for fuel cells to catalytic hydrogenation reactions. In this paper, we examine the factors that determine the thermodynamics of hydrogen activation of a Lewis acid-base pair using the pedagogical examples of ammonia borane (NH3BH3, AB) and ammonium borohydride ([NH4][BH4], ABH2). At ambient temperatures, ABH2 loses hydrogen to form the Lewis acid-base complex AB, suggesting that free energy drives the reaction to release hydrogen. However, direct measurement of the reaction enthalpy is not straightforward given the complex decomposition pathways leading to the formation of the diammoniate of diborane ([NH3BH2NH3][BH4], DADB). In this work, we compare two approaches for deriving the thermodynamic relationships among AB, DADB, and ABH2.

Reversible dehydrogenation of magnesium borohydride to magnesium triborane in the solid state under moderate conditions.

Thermal decomposition of magnesium borohydride, Mg(BH(4))(2), in the solid state was studied by a combination of PCT, TGA/MS and NMR spectroscopy. Dehydrogenation of Mg(BH(4))(2) at 200 °C en vacuo results in the highly selective formation of magnesium triborane, Mg(B(3)H(8))(2). This process is reversible at 250 °C under 120 atm H(2). Dehydrogenation at higher temperature, >300 °C under a constant argon flow of 1 atm, produces a complex mixture of polyborane species. A borohydride condensation mechanism involving metal hydride formation is proposed.

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.

Synthesis, structure and dehydrogenation of magnesium amidoborane monoammoniate.

Magnesium amidoborane monoammoniate (Mg(NH(2)BH(3))(2) x NH(3)) which crystallizes in a monoclinic structure (space group P2(1)/a) has been synthesized by reacting MgNH with NH(3)BH(3). Dihydrogen bonds are established between coordinated NH(3) and BH(3) of [NH(2)BH(3)](-) in the structure, promoting stoichiometric conversion of NH(3) to H(2).

Determination of structure and phase transition of light element nanocomposites in mesoporous silica: case study of NH3BH3 in MCM-41.

Nanocomposition of molecular crystal ammonia borane (AB) by embedding it in mesoporous silica leads to a remarkable enhancement of the hydrogen storage properties. To investigate the nature of a nanophase AB, we used atomic pair distribution function (PDF) analysis of synchrotron X-ray powder diffraction data to follow the structural evolution of AB embedded within MCM-41 at temperatures ranging from 80 to 300 K. We found that the nanophase AB residing within the mesoporous scaffold does not undergo the structural phase transition at 225 K that was observed in the neat molecular crystal. Rather, it stays in the tetragonal phase over a wide temperature range of 110 to 240 K and starts to lose structural correlation above 240 K. This finding strongly suggests that nanoconfinement of AB within mesoporous scaffolds stabilizes the high-temperature disordered tetragonal phase at a much lower temperature. PDF analyses of composite materials composed of excess AB (i.e., AB:MCM-41 > 1:1) indicates that the excess AB forms aggregates outside the mesoporous scaffold and that these aggregates have structural properties similar to neat AB, that is, the orthorhombic-to-tetragonal structural phase transition is observed at 225 K upon warming. These results may provide important insight into the mechanism behind the enhanced hydrogen storage properties of this system.

Experimental and computational studies on collective hydrogen dynamics in ammonia borane: incoherent inelastic neutron scattering.

Incoherent inelastic neutron scattering is used to probe the effects of dihydrogen bonding on the vibrational dynamics in the molecular crystal of ammonia borane. The thermal neutron energy loss spectra of (11)B enriched ammonia borane isotopomers ((11)BH(3)NH(3), (11)BD(3)NH(3), and (11)BH(3)ND(3)) are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics. A harmonic vibrational analysis on NH(3)BH(3) clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50-500 cm(-1)). Some assignment of modes to simple harmonic motion, e.g., NH(3) and BH(3) torsion in the molecular crystal is possible, and it is confirmed that the lowest modes are dominated by collective motion. We show that the vibrational dynamics as modeled with ab initio molecular dynamics provides a more complete description of anharmonic and collective dynamics in the low frequency region of the inelastic incoherent neutron scattering spectra when compared to the conventional harmonic approach.

Interaction of lithium hydride and ammonia borane in THF.

The two-step reaction between LiH and NH(3)BH(3) in THF leads to the production of more than 14 wt% of hydrogen at 40 degrees C.

High-capacity hydrogen storage in lithium and sodium amidoboranes.

The safe and efficient storage of hydrogen is widely recognized as one of the key technological challenges in the transition towards a hydrogen-based energy economy. Whereas hydrogen for transportation applications is currently stored using cryogenics or high pressure, there is substantial research and development activity in the use of novel condensed-phase hydride materials. However, the multiple-target criteria accepted as necessary for the successful implementation of such stores have not yet been met by any single material. Ammonia borane, NH3BH3, is one of a number of condensed-phase compounds that have received significant attention because of its reported release of approximately 12 wt% hydrogen at moderate temperatures (approximately 150 degrees C). However, the hydrogen purity suffers from the release of trace quantities of borazine. Here, we report that the related alkali-metal amidoboranes, LiNH2BH3 and NaNH2BH3, release approximately 10.9 wt% and approximately 7.5 wt% hydrogen, respectively, at significantly lower temperatures (approximately 90 degrees C) with no borazine emission. The low-temperature release of a large amount of hydrogen is significant and provides the potential to fulfil many of the principal criteria required for an on-board hydrogen store.