Influence of alcohol solvents on morphology of hollow silica–alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane

In this study, we reported effect of alcohol solvents on morphology of hollow silica–alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane. Silica–alumina composite shell was coated on polystyrene template particles via sol–gel reaction in various alcohol solvents. The hollow spheres were prepared in methanol, ethanol and 2-propanol for 36, 17 and 8 h, respectively. The results indicate that the order of sol–gel reaction rate was 2-propanol > ethanol > methanol. Amount of hydrogen evolution of the hollow spheres prepared in methanol and ethanol were higher than that of the hollow spheres prepared in 2-propanol. From the results of solid-state 27Al nuclear magnetic resonance spectra, we found that aluminum species of the hollow spheres prepared in methanol and ethanol were highly dispersed in their silica matrix. The results of temperature programed dispersion of ammonia showed that all the hollow spheres have weak and strong Bronsted acid sites. The amount of weak Bronsted acid sites in all the hollow spheres were of same level, while the amount of strong Bronsted acid sites increase with decrease of sol–gel reaction rate. These results indicate that the sol–gel reaction rate influences the dispersion of aluminum species, and highly dispersed aluminum species lead to increase in the amount of Bronsted acid sites.