Thermal Decomposition of T‐Butylamine Borane Studied by in Situ Solid State NMR

Abstract

Amine boranes are a class of hydrogen storage materials which release significant yields of hydrogen gas upon heating to temperatures relevant for automotive applications. The fhermodynamic stability of this class of amine boranes is correlated with the extensive dihydrogen bonding between hydrides bound to boron and amine protons. Further, the hydrogen evolution follows a bimolecular pathway via the di-hydrogen bonding network. The thermal decomposition of t-butylamine borane (tBuAB), (CH 3 )3CNH 2 BH3, has been studied in order to understand the reaction pathway of hydrogen sorption and the impact of the t-butyl substitution dehydrogenation thermodynamics. Ή , U B , and l3 C solid state nuclear magnetic resonance (NMR) spectroscopy has revealed that heating initiates two separate reaction pathways: isomerization and hydrocarbon abstraction resulting in varying yields of isobutane and hydrogen. It is also possible that tBuAB dissociates about the N-B bond giving rise to borane stretching modes in the gas FTIR. Trapped t-butylamine (tBuA) which slowly diffuses from the tBuAB solid in 13 C NMR studies appears to be present; however, this spectral region is convoluted by other decomposition products. n B NMR indicates that the major reaction pathway results in hydrogen evolution with isobutane formation being present in smaller yields. The t-butyl substitution lowers the thermodynamic stability -compared to NH3BH3-but results in impure hydrogen gas stream and lowered capacity due to isobutene evolution. INTRODUCTIONDevelopment of suitable materials to store hydrogen for automotive use has received pointed attention over the past decade [1], Significant progress has been made with the discovery of novel chemical hydrides, complex metal hydrides, and adsorption substrates which continue to optimize both thermodynamics and kinetics of hydrogen sorption [2]. Chemical hydrides typically offer the largest theoretical gravimetric capacities. Autrey et al. [3] have recently shown that mechanical milling of alkali metal hydrides with ammonia borane can further lower the decomposition temperature. In all cases, however, many challenges remain in order to meet the current US DOE performance targets [4].Amine boranes are being considered for hydrogen storage materials since they contain significant quantities of hydrogen which potentially can be released at low temperatures (80-150 °C) via chemical reactions. Ammonia borane, NH3BH3, is one of the most promising in this class as it decomposes to release greater than two moles of pure hydrogen gas (14 wt %) below 160 °C [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], Although isoelectronic to ethane, NH3BH3 is a solid at room temperature due to the di-hydrogen bonding network formed between the amine protons and boron hydrides in the solid state lattice. Further, it has been shown that the hydrogen release mechanism involves transformation and isomerization to an ionic dimer where a hydride migrates from one boron to the adjacent boron in the dimer [20]. The greatest ch...