Thermochemical Formation of Sodium Borohydride from Sodium Tetramethoxyborate

Abstract

Sodium borohydride (NaBH₄) can be used as a hydrogen export material, but thermochemical regeneration from NaBO₂ is considered too costly to be feasible mainly due to the cost associated with recycling the metal hydride reagents. This study explores an alternative regeneration route of NaBH₄ using NaB(OCH₃)₄ instead of the widely studied NaBO₂. In situ synchrotron X-ray diffraction (SXRD) was utilized to detect the formation of NaBH₄ from NaB(OCH₃)₄ or NaBO₂ using various metal hydride reducing agents (NaH, MgH₂, Mg₂FeH₆, LiAlH₄, NaAlH₄, CaNi₅H_x, and LaNi₅H_x). NaBH₄ formation was detected using NaB(OCH₃)₄ with NaH (265–320 °C), MgH₂ (310–440 °C), NaAlH₄ (>140 °C), and LiAlH₄ (>20 °C). In contrast, regeneration from NaBO₂ required higher temperatures, with NaBH₄ formation using MgH₂ (>430 °C), Mg₂FeH₆ (>460 °C), NaAlH₄ (>140 °C), and LiAlH₄ (>110 °C). Notably, the reaction between NaB(OCH₃)₄ and NaH yields NaOCH₃, whereas the reaction between NaB(OCH₃)₄ and MgH₂ yields Mg(OCH₃)₂, which may offer lower energy recycling of the metal hydrides. This study also underscores the critical role of hydridic hydrogen (H^–) in solid-state thermochemical reactions for NaBH₄ formation and presents an alternative regeneration route using NaB(OCH₃)₄ that could offer cost and energy savings.