Influence of Si/Al molar ratio of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

In this work, we investigated influence of Si/Al molar ratio of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane. The hollow spheres were prepared by polystyrene templates method. In this method, silica-alumina composite shell were coated on polystyrene particles via sol-gel reaction, and the PS template particles were removed by calcination. From the result of transmission electron microscopy, the obtained hollow spheres with Si/Al molar ratios of 12, 23, 49, 92, and 186 were almost similar morphologies. The activities of these hollow spheres for the hydrolytic dehydrogenation of ammonia borane were compared. The amount of hydrogen evolution of hollow spheres with a Si/Al molar ratio of 49 was the highest of all the hollow spheres. From the result of solid-state 27Al MAS NMR and NH3-TPD, the dispersion and amount of aluminum species play important roles in determining the amount of hydrogen evolution. Additionally, the amount of hydrogen evolution depends on the amount of Brønsted acid sites of the hollow spheres.