Hyperpolarized 129Xe NMR Investigation of Ammonia Borane in Mesoporous Silica

Hyperpolarized (HP) Xe-129 NMR was used to probe the porosity of mesoporous silica (MCM) infused with ammonia borane (AB). Variable-temperature HP Xe-129 NMR measurements have been systematically carried out on a series of MCM-41 materials with AB loading ranging from 33 to 75 wt % (1:2 to 3:1 AB:MCM). Three distinct types of pore environments are clearly evident: pristine mesopores, pores coated with AB inside the meso-channels, and interparticle spacing formed from AB aggregates outside the meso-channels. We found similarly uniform coating of AB on mesoporous silica channels with 1:2 and 1: 1 AB:MCM loading (ratio of weight percent). When the loading of AB to MCM is greater than 1: 1, AB starts to aggregate outside the meso-channels. Further increases in loading (>= 3: 1) result in the formation of partially blocked meso-channels as a result of excessive AB. The detailed information obtained from this study on how supported AB resides in nanoporous channels and how it evolves with the increase of AB loading is helpful for the rational design of novel materials with optimal hydrogen storage and release properties.