The effect of solvent properties on the hydrogen producing performance of catalytic liquid phase ammonia borane

In this study, the influence of solvent type (H2O, methanol, ethanol and ethylene glycol) and properties(acid, alkali) on catalytic liquid phase ammonia borane for hydrogen production are investigated and the influence mechanism are analyzed via theoretical analysis, DFT calculation and systematic experimental investigation. The results proved that both acid and alkali can improve the efficiency of hydrogen production by catalytic ammonia borane hydrolysis, the complete reaction time of AB hydrolysis in solvents pH = 7, pH = 3 and pH = 12 are 8.48 min, 4.3 min and 5.87 min, respectively. The reason of alkaline solution enhance the catalytic ammonia borane hydrolysis is not that OH- remove NH4+ attached to the catalyst surface, which realized by auxiliary activate H-O bond of H2O molecule(Rate-control of AB hydrolysis) under the action of the catalyst. 1 equivalent of ammonia borane reacts with acid solution can produce 3 equivalent of hydrogen, and it follows the analogous principle of 'the strong acid tend to convert into the weak acid', the reaction product is the weaker acid-H3BO3. The reaction of AB with CH3COOH follows the rule of stepwise incomplete reaction, the reaction energy barrier of the first step, second step and third step are 0.149 eV, 0.312 eV and 0.768 eV, respectively. The reaction product are the complex mixtures of CH3COOBH2NH3, (CH3COO)(2)BHNH3 and (CH3COO)(3)BNH3. 1 equivalent of ammonia borane reacts with CH3COOH only produce 1.22 equivalent of hydrogen due to the H of -BH3 in AB is not fully released. The hydrogen production rate and final hydrogen yield of catalytic AB reaction in alcohol solvent are lower than that in water solvent, and the reaction rates of hydrogen production in different alcohol solvents are also different, the hydrogen production rate and complete reaction time are CH3OH(9 min)> CH3CH2OH(9.5 min) > (CH2OH)(2)(24.5 min), which may be attributed to the different strength of the O-H bond of the hydroxyl group in the alcohol molecule.