Thermochemical Formation of Sodium Borohydride from Sodium Tetramethoxyborate

Abstract

Sodium borohydride (NaBH 4 ) can be used as a hydrogen export material, but thermochemical regeneration from NaBO 2 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 NaBH 4 using NaB-(OCH 3 ) 4 instead of the widely studied NaBO 2 . In situ synchrotron Xray diffraction (SXRD) was utilized to detect the formation of NaBH 4 from NaB(OCH 3 ) 4 or NaBO 2 using various metal hydride reducing agents (NaH, MgH 2 , Mg 2 FeH 6 , LiAlH 4 , NaAlH 4 , CaNi 5 H x , and LaNi 5 H x ). NaBH 4 formation was detected using NaB(OCH 3 ) 4 with NaH (265−320 °C), MgH 2 (310−440 °C), NaAlH 4 (>140 °C), and LiAlH 4 (>20 °C). In contrast, regeneration from NaBO 2 required higher temperatures, with NaBH 4 formation using MgH 2 (>430 °C), Mg 2 FeH 6 (>460 °C), NaAlH 4 (>140 °C), and LiAlH 4 (>110 °C). Notably, the reaction between NaB(OCH 3 ) 4 and NaH yields NaOCH 3 , whereas the reaction between NaB(OCH 3 ) 4 and MgH 2 yields Mg(OCH 3 ) 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 NaBH 4 formation and presents an alternative regeneration route using NaB(OCH 3 ) 4 that could offer cost and energy savings.