Methods to stabilize and destabilize ammonium borohydride.

Ammonium borohydride, NH(4)BH(4), has a high hydrogen content of ρ(m) = 24.5 wt% H(2) and releases 18 wt% H(2) below T = 160 °C. However, the half-life of bulk NH(4)BH(4) at ambient temperatures and pressures, ~6 h, is insufficient for practical applications. The decomposition of NH(4)BH(4) (ABH(2)) was studied at variable hydrogen and argon back pressures to investigate possible pressure mediated stabilization effects. The hydrogen release rate from solid ABH(2) at ambient temperatures is reduced by ~16% upon increasing the hydrogen back pressure from 5 to 54 bar. Similar results were obtained using argon pressure and the observed stabilization may be explained by a positive volume of activation, ca. 73 ± 17 cc mol(-1), in the transition state leading to hydrogen release. Nanoconfinement in mesoporous silica, MCM-41, was investigated as alternative means to stabilize NH(4)BH(4). However, other factors appear to significantly destabilize NH(4)BH(4) and it rapidly decomposes at ambient temperatures into [(NH(3))(2)BH(2)][BH(4)] (DADB) in accordance with the bulk reaction scheme. The hydrogen desorption kinetics from nanoconfined [(NH(3))(2)BH(2)][BH(4)] is moderately enhanced as evidenced by a reduction in the DSC decomposition peak temperature of ΔT = -13 °C as compared to the bulk material. Finally, we note a surprising result, storage of DADB at temperature <-30 °C transformed, reversibly, the [(NH(3))(2)BH(2)][BH(4)] into a new low temperature polymorph as revealed by both XRD and solid state MAS (11)B MAS NMR.