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

• Published in: International journal of hydrogen energy 2022-10, Vol.47 (85), p.36175-36189
• Main Authors: Rüscher, Claus Henning, Schomborg, Lars, Bredow, Thomas
• Format: Article
• Language: English
• Subjects: Hydro-sodalite; Inter cage reactions; NaBH4-Sodalite
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/j.ijhydene.2022.05.302

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. •NaBH4-sodalites enclosing H2O-cage fillings in addition were synthesized for inter-cage reactions above 250 °C.•New insights on the hydrogen release mechanism are based on systematic IR and NMR spectroscopic investigations.•BH3 is obtained as intermediate, which reacts quickly to B(OH)3, stabilized in the sodalite cages.•The formation of cage isolated (OH)- indicates jumping H+ and Na+ through the cage windows for charge neutrality.•A channel structure only opens above 400 °C for oxygen uptake for further reaction.