Experiments on the thermal activation of hydrogen release of NaBH4-sodalites characterized by IR- and MAS-NMR spectroscopy

Abstract

New results on the reactions of the (BH4)-anion enclathrated in the cages of sodalites are reported. Hydrothermally synthesized NaBH4-sodalites (ǀNa8(BH4)2ǀ[SiAlO4]6) always contain hydro-sodalite type cages (ǀNa3(H2O)4ǀ[SiAlO4]3). With increasing temperature dehydration occurs. Above 250 °C a limited reaction of the residual water is going on with (BH4)-cage fillings releasing hydrogen and the appearance of certain borate specifications enclosed in the sodalite cages. The effect of a reaction of oxygen with the (BH4)-anions in the sodalite-cages at temperatures above 400 °C is also shown. The degree of (BH4)-conversion using wet and dry N2 stream is further followed by IR and MAS NMR investigations. External supply of water largely enhances the degree of reaction, e.g. at 400 °C from 16% to 44% loss of (BH4)-absorption intensity. However, 11B MAS NMR shows 8% and 22% of a conversion of (BH4)-cage fillings into new borate species in dry and wet N2 stream, respectively. These lower values are explained by a loss of B-ions via formation and evaporation of BH3 from the sodalite. Further evaluation of the 11B MAS NMR spectra could resolve the formation of (B(OH)3)-, (BO(OH)2)-, (B(OH)4)- along with unreacted (BH4)-species in the cages. 1H-MAS NMR shows a −3.8 ppm signal related to cage isolated (OH)-, which suggests an initial reaction step via (H+ + BH4−) to (BH3 + H2). The formation of (B(OH)3), however, also indicated sufficient water for a reaction of BH3 releasing further hydrogen. The formation of (BH3) could be observed in temperature dependent IR investigations at temperatures above 400 °C.