Thermochemical Formation\nof Sodium Borohydride from\nSodium Tetramethoxyborate

Sodium borohydride (NaBH<sub>4</sub>) can be used as\na hydrogen\nexport material, but thermochemical regeneration from NaBO<sub>2</sub> is considered too costly to be feasible mainly due to the cost associated\nwith recycling the metal hydride reagents. This study explores an\nalternative regeneration route of NaBH<sub>4</sub> using NaB(OCH<sub>3</sub>)<sub>4</sub> instead of the widely studied NaBO<sub>2</sub>. <i>In situ</i> synchrotron X-ray diffraction (SXRD) was\nutilized to detect the formation of NaBH<sub>4</sub> from NaB(OCH<sub>3</sub>)<sub>4</sub> or NaBO<sub>2</sub> using various metal hydride\nreducing agents (NaH, MgH<sub>2</sub>, Mg<sub>2</sub>FeH<sub>6</sub>, LiAlH<sub>4</sub>, NaAlH<sub>4</sub>, CaNi<sub>5</sub>H<sub><i>x</i></sub>, and LaNi<sub>5</sub>H<sub><i>x</i></sub>). NaBH<sub>4</sub> formation was detected using NaB(OCH<sub>3</sub>)<sub>4</sub> with NaH (265–320 °C), MgH<sub>2</sub> (310–440\n°C), NaAlH<sub>4</sub> (>140 °C), and LiAlH<sub>4</sub> (>20\n°C). In contrast, regeneration from NaBO<sub>2</sub> required\nhigher temperatures, with NaBH<sub>4</sub> formation using MgH<sub>2</sub> (>430 °C), Mg<sub>2</sub>FeH<sub>6</sub> (>460\n°C),\nNaAlH<sub>4</sub> (>140 °C), and LiAlH<sub>4</sub> (>110\n°C).\nNotably, the reaction between NaB(OCH<sub>3</sub>)<sub>4</sub> and\nNaH yields NaOCH<sub>3</sub>, whereas the reaction between NaB(OCH<sub>3</sub>)<sub>4</sub> and MgH<sub>2</sub> yields Mg(OCH<sub>3</sub>)<sub>2</sub>, which may offer lower energy recycling of the metal\nhydrides. This study also underscores the critical role of hydridic\nhydrogen (H<sup>–</sup>) in solid-state thermochemical reactions\nfor NaBH<sub>4</sub> formation and presents an alternative regeneration\nroute using NaB(OCH<sub>3</sub>)<sub>4</sub> that could offer cost\nand energy savings.