(218b) Critical Loading Level for Low-Temperature, Simultaneous Release of Two Equivalents of Hydrogen From Ammonia Borane Nanocomposites

Ammonia borane (NH3BH3, AB) has a gravimetric density of 196 g H2/kg AB; however, it slowly releases about 7 wt % hydrogen at temperatures below 85 degree Celsius. Although the dehydrogenation kinetic can be accelerated by compositing AB with mesoporous silica, the hydrogen density decreases significantly. The reduced density of nanocomposites can be offset by initiating the second hydrogen release step. However, this only occurs at low AB loading levels. Here, we report the critical loading level (or the maximum loading level) for the low-temperature, simultaneous release of two equivalents of hydrogen from MCM-41-AB nanocomposites. At a loading level of 0.15 (mass ratio of AB to matrix), 2.1 equivalents of hydrogen releases in 1 hour at 89.5 degree Celsius. Also, we find that nanoconfined AB (AB resides inside mesoporous channels) is amorphous and aggregated AB (AB resides outside mesoporous channels) has a tetragonal structure. Additionally, the first dehydrogenation of aggregated AB peaks at a low temperature compared with pristine AB. The critical AB loading level for nanocomposites involving MCM-41 and AB is found between 0.15 and 0.21. The findings improve understanding of mesoporous matrix-AB nanocomposites and are critical for design of novel AB nanocomposites.