Hydrothermal stability of NaBH4 enclathrated sodalites with aluminosilicate and gallosilicate framework

The time dependent stability of tetrahydroborate sodalites with aluminosilicate- and gallosilicate framework was studied in water at room temperature and under hydrothermal conditions at 353 K. The experiments were carried out in different time intervals (4–48 h) using an excess of water according a solid:liquid mass ratio of 1:150. The kinetics of decomposition was followed by registration of weight loss, change of the pH-value and by X-ray powder diffraction as well as FT-IR spectroscopy.Both samples undergo remarkable time dependent decomposition. The decomposition rate is higher for the gallosilicate sodalite even though its crystal size is larger compare to the aluminosilicate sodalite. Besides formation of some X-ray amorphous material in both cases precipitation of crystalline aluminosilicate or gallosilicate was found, always connected with impurities of carbonate anions.FT-IR clearly showed that the remaining parts of sodalite structure of both samples were always pure tetrahydroborate sodalite. No borate species other than BH4− were detected. Thus, hydrolysis of the moisture sensitive BH4− anions by an entrance of water into intact cages can be excluded under the described experimental conditions.