Pressure-Induced Insertion of Ammonia Borane in the Siliceous Zeolite, Silicalite-1F

A combination of Raman spectroscopy and X-ray diffraction was used to investigate the insertion of ammonia borane in the 5.5 angstrom diameter pores of the hydrophobic, all-silica zeolite, silicalite-1F in the pressure range up to 4.8 GPa. Insertion and nanoconfinement result in the appearance of new Raman modes, especially in the N-H stretching region and significant changes in the intensities and pressure dependencies of a large number of other modes. Orientational disorder of the -BH3 and -NH3 groups persists to higher pressures in nanoconfined as compared to the bulk ammonia borane. The structure of the recovered sample was determined by single crystal X-ray diffraction. Each pore in the unit cell was found to contain between 2 and 3 molecules of ammonia borane forming single-molecule chains due to the spatial constraints. In situ, high pressure, X-ray powder diffraction indicated that the compressibility of the ammonia borane-silicalite-1F composite is three times lower than that of empty silicalite-1F due to pore filling. These results show that silicalite-1F can be a suitable nanoscaffold for this important chemical hydrogen-storage material.