In Situ Multinuclear NMR Spectroscopic Studies of the Thermal Decomposition of Ammonia Borane in Solution

Abstract

The development of condensed-phase hydrogen-storage materials for fuel-cell-powered vehicles that meet the 2015 system target goals of the US Department of Energy of a volumetric density greater than 82 g H 2 per liter and a gravimetric density greater than 90 g H 2 kg À1 has attracted recent interest. [1,2] Ammonia borane, NH 3 BH 3 (AB), which has a volumetric density of 146 g H 2 per liter and a gravimetric density of 194 g H 2 per kilogram, shows promise as a material that could meet these long-term storage targets. [3] The details of the mechanisms for hydrogen release from AB are not completely understood; however, significant progress has been made in furthering our understanding of these mechanisms. Wolf and co-workers showed that the thermal decomposition of AB in the solid state occurs stepwise by a series of exothermic reactions to yield 12 wt % hydrogen. [4,5] The first step yields H 2 and polyaminoborane (PAB); subsequent decomposition of PAB yields another equivalent of H 2 and polyiminoborane (PIB). One important breakthrough for our understanding of the reaction mechanisms for H 2 release from AB was the recent in situ NMR spectroscopic study that showed formation of the diammoniate of diborane, [NH 3 BH 2 NH 3 ], to be an essential first step for hydrogen release from solidstate AB.[6] Once DADB is formed, hydrogen release from AB follows, whereby the release of the first equivalent of H 2 yields acyclic amineborane oligomers.DADB is also observed in the decomposition of AB in ionic liquids; [7] however, no one has reported the observation of DADB in organic solvents. Indeed, Mayer observed that the addition of DADB to a glyme solution results in decomposition and the evolution of hydrogen at À20 8C.[8]Wang and Geanangel followed the decomposition of AB at 80 8C in several solvents by ex situ NMR spectroscopy.[9] They did not report the observation of DADB or linear decomposition products in glyme, but they did observe the formation of cyclic (-BH 2 NH 2 -) n species. Unfortunately, the lower resolution did not permit them to discriminate between the n = 2, n = 3, and n = 4 cyclic products. These results suggest that the decomposition of AB in organic solvents occurs by a different reaction mechanism to that observed in the solid state or ionic liquids. It is important to understand the mechanism of hydrogen release, as this mechanism should provide critical insight into the stability of AB solutions that may be used as hydrogen-storage media. For example, Rassat et al. were able to predict the thermal stability of AB at 60 8C on the basis of the mechanism of hydrogen release in the solid state.[10] The mechanistic details of decomposition permit the evaluation of the long-term thermal stability of the storage compound, whether it is stored in the solid or solution phase.Herein, we revisit the initial thermolysis pathways of AB in the solution phase. In the process, we introduce a new highpressure, spinning NMR cell that enables the in situ observation of the decomposition of AB by 11 B NMR spect...

Dehydrogenation of ammonia-borane by cationic Pd(ii) and Ni(ii) complexes in a nitromethane medium: hydrogen release and spent fuel characterization

“…2, besides the small peak nearly at −310 ppm, most of the N containing species of sfAB can be associated with tetrahedrally coordinated N sites that are typically found in PAB (-NH 2 BH 2 -). 27 The corresponding 11 B signal, tetrahedrally coordinated B sites and especially those peaks at ∼−10 ppm, explains the remaining B-N moieties in the spent fuel. The −310 ppm peak might show the presence of B 2 NH trigonal sites in a hexagonal ring structure as shown there.…”

Quasi in situ Mössbauer and XAS studies on FeB nanoalloy for heterogeneous catalytic dehydrogenation of ammonia borane

“…2) reveal that little H 2 is released from the neat AB at 60 • C even holding the temperature for 40 h, which is probably due to extremely long induction period [29,30]. Under the same condition, the Fe-doped AB samples evolve hydrogen immediately upon reaching the temperature showing clear difference from the diammoniate of diborane (DADB in short) initiated sigmoidal kinetic behavior of neat AB reported in the literature [29][30][31]. By comparison, the FeCl 3 -doped AB sample exhibits superior performance than FeCl 2 , i.e.…”

Amineborane Based Chemical Hydrogen Storage - Final Report

“…The initial formation of DADB has also been proposed as a key step in thermally-induced AB H 2 -release reactions in the solid state and in organic solvents. 7,50 While a DADB pathway may contribute to the H 2 -release observed in the bmimCl/PS solutions, the observed rate enhancements and DSC properties of the Proton Sponge reactions suggest that there is also another mechanistic pathway for H 2 -release in these systems. Proton Sponge (Equation 4.6) is a strong base and because of its low nucleophilicity is frequently used as a deprotonating agent in organic and inorganic syntheses that can eliminate the undesirable side reactions often found with more coordinating bases.…”

“…The combined solid-state and solution 11 B NMR studies of AB/ionic-liquid and DADB/ionic-liquid H 2 -release reactions in progress support a AB dehydrogenation pathway in ionic liquids (Figure 3.17) involving: (1) ionic-liquid promoted conversion of AB into its more reactive ionic DADB form, (2) further intermolecular dehydrocoupling reactions between hydridic B-H hydrogens and protonic N-H hydrogens on DADB and/or AB to form polyaminoborane polymers and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene materials. The initial formation of DADB has also been proposed as a key step in thermally-induced AB H 2 -release reactions in the solid state 7,49 and in organic solvents, 50 but the highly polar medium provided by ionic liquids promotes DADB formation and appears to be the key activating feature of these ionic liquid reactions.…”