Efficient Synthesis of Sodium Borohydride: Balancing Reducing Agents with Intrinsic Hydrogen Source in Hydrated Borax

作者：Zhu, Yongyang; Ouyang, Liuzhang^*; Zhong, Hao; Liu, Jiangwen; Wang, Hui; Shao, Huaiyu; Huang, Zhenguo^*; Zhu, Min

来源：ACS Sustainable Chemistry & Engineering, 2020, 8(35): 13449-13458.

DOI：10.1021/acssuschemeng.0c04354

• Sodium borohydride
• Borax
• Hydrogen storage
• Hydrogen
• Chemical synthesis
• Metal hydrides

Sodium borohydride (NaBH4) has been identified as one of the most promising hydrogen storage materials; however, it is still challenging to produce NaBH4 with low cost and high efficiency, which are largely determined by the sources of boron and hydrogen and reducing agents used. Herein, we report an economical method to produce NaBH4 by ball milling hydrated borax (Na2B4O7 center dot 10H(2)O and/or Na2B4O7 center dot 5H(2)O) with different reducing agents such as MgH2, Mg, and NaH under ambient conditions. The direct use of natural hydrated borax avoids the dehydration process (at 600 degrees C) and consequently reduces cost and improves overall energy efficiency. A high yield of 93.1% can be achieved for a short ball mill duration (3.5 h) for Na2B4O7 center dot 5H(2)O-NaH-MgH2 system. In this system, H-2 is generated in situ which subsequently reacts with Mg forming MgH2. Low cost Mg is therefore employed to replace the majority of MgH2, leading to an attractive yield of 78.6%. To further reduce the cost of raw materials and improve the utilization of hydrogen source in the hydrated borax, Na2B4O7 center dot 10H(2)O is used to partially substitute for Na2B4O7 center dot 5H(2)O, leading to a complete replacement of MgH2. Compared with literature results, the optimized recipe features low cost and high efficiency since it utilizes hydrogen from the hydrated water in natural borax and avoids high temperatures. Our finding is expected to facilitate applications of NaBH4 for hydrogen storage.