Thermal behaviour of NaBH4-sodalites with alumosilicate framework: Influence of cage water content and the surrounding conditions

Sodalites Na8[AlSiO4]6(BH4)1.8(H3O2)0.2 and Na8[AlSiO4]6(BH4)(H3O2)0.5(CO3)0.25(H2O) were examined to characterize the influence of the mixed cage fillings and the surroundings on the thermal behaviour of enclathrated BH4− and the stability of the alumosilicate framework. The samples were investigated in a NaCl matrix during temperature dependent IR-measurements (TIR) and by thermal analysis in inert atmosphere (helium) and in synthetic air. The thermal products were further characterized by XRD and SEM/EDS.It could be shown that the extent of the BH4− decomposition strongly depends on the cage-fillings of the individual sodalite phase and on the environmental experimental conditions. At low to medium temperatures hydrolysis of BH4− by water molecules from decomposition of H3O2− and H2O/CO32− templates leads to B(OH)4−/BH3OH− and via subsequent dehydration to BO(OH)2− and finally BO2−. At elevated temperatures direct oxidation of BH4− to BO2− occurs inside the sodalite cages.Further heating results in sodalite framework destruction and finally formation of nepheline with some boron in its structure besides some unknown polymeric borates outside the alumosilicate phase. Framework stability of the sodalites was found to decrease in the direction → heating in NaCl → heating in He-atmosphere → heating in air.

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► Thermal behaviour of NaBH4 enclathrated sodalites with alumosilicate framework.
► Influence of mixed cage fillings (H2O, H3O2−, CO32−) on thermal behaviour of BH4−.
► Extend of intra cage reactions depend on templates and experimental surroundings.
► Formation of B(OH)4−/BH3OH−, BO(OH)2− and finally BO2− inside the cages.
► Formation of nepheline with boron in its structure after framework destruction.