Tetrahydroborate sodalite nanocrystals: Low temperature synthesis and thermally controlled intra-cage reactions for hydrogen release of nano- and micro crystals

Tetrahydroborate sodalite nanoparticles were successfully synthesized under low temperature hydrothermal conditions (333 K) from high alkaline aluminosilicate gels and NaBH4 salt. The products were characterized by X-ray powder diffraction, scanning electron microscopy, FTIR spectroscopy and thermogravimetry in comparison with the microcrystalline sample Na8[AlSiO4]6(BH4)2· NaBH4-sodalite of 25 nm average crystal size agglomerated by X-ray amorphous hydrosodalite type phase to bigger particles up to ∼100 nm was observed after a reaction period of 12 h. The sodalite host framework protects the BH4- ions from hydrolysis under open conditions at room temperature. Hydrogen release is detected via nitrate tracer reduction in temperature dependent IR (TIR) experiments. A total conversion of the BH4 groups for the nanocrystalline sodalite sample is reached at 773 K whereas a larger amount of the BH4--units remained stable within the microcrystalline sample under the same conditions. This can be explained by the smaller crystal size and the high amount of inter grown hydrosodalite type phase in the BH4--sodalite nanocrystals compared to the microcrystals. The minor hydrosodalite type phase also present in the microcrystalline sample can be reloaded with water and the main hydrogen release reaction BH4- + 2H2O => 4H2 + BO2− can be continued.