Regeneration Of Sodium Borohydride Based On Mg-Al Reduction

The hydrolysis of sodium borohydride?NaBH₄?can release a large amount of hydrogen in a controllable way at room temperature,which is a practical method for hydrogen production on-site.However,the hydrolysis reaction is irreversible,and the resulting NaBH₄ regeneration technology has become a key problem to be solved urgently in the large-scale application of NaBH₄ hydrolysis.Based on the existing NaBH₄ regeneration technology,anhydrous NaBO₂is basically used as the raw material.The direct by-product of hydrolysis,NaBO₂·2H₂O or NaBO₂·4H₂O,needs to be dehydrated at high temperature at 350? first,which is high in energy consumption.At the same time,additional H containing raw material needs to be found as the renewable hydrogen source of NaBH₄,which further increases the regeneration cost.Therefore,in this thesis,sodium metaborate hydrate?NaBO₂·xH₂O?,a direct by-product of NaBH₄ hydrolysis,was used as a raw material and serves as a source of Na,B,and H.NaBH₄ was regenerated by ball milling at room temperature,and an efficient and economical NaBH₄ regeneration technology was discussed.Firstly,based on the low price of metal Al and having a higher oxidation state than Mg,it is more efficient as a NaBH₄ regeneration reducing agent.NaBH₄ regeneration was achieved by using the mixed powder of metal Al and Mg as reducing agent and the direct by-product of hydrolysis,NaBO₂·2H₂O,through high-energy ball-milling reaction.This process converts H⁺ in crystal water to H^- without external hydrogenation source,so as to achieve energy storage.Through the optimization and adjustment of process parameters such as the molar ratio of reducing agent and ball milling time,the yield of NaBH₄ regeneration was improved,which reached 26%.The effect of NaH additive on the yield of NaBH₄ regeneration was also discussed.It was observed that the addition of NaH could inhibit the Fe peeling caused by ball milling for a long time and significantly improve the yield of NaBH₄.The XPS test proved that Al was used as the reducing agent to participate in the NaBH₄ regeneration reaction in the ball milling process.Secondly,in order to improve the ball milling effect of the ductile reducing agent metal Mg and Al mixed powder,the brittle intermetallic compound Mg₁₇Al₁₂ was used as the reducing agent,and the NaBH₄ regeneration was achieved by high energy ball milling with NaBO₂·2H₂O or NaBO₂·4H₂O at room temperature.The titration method confirmed that the yield of NaBH₄ regeneration was 29%.At the same time,the effects of the addition of NaH,Na OH,Na₂O,and Na₂CO₃ and the amount of addition on the regeneration reaction were explored.It was shown that the addition of all additives can improve the regeneration effect of NaBH₄.Among them,the addition effect of 10 wt.%NaH was the most obvious,and the highest NaBH₄ regeneration yield could reach about 60%.The mechanism of NaBH₄ regeneration was discussed through XRD,FT-IR,NMR and MS.It was clarified that the regeneration of NaBH₄ is mainly the process of conversion of[B?OH?₄]^- group to[BH₄]^-group.In this process,the[OH]^- of the[B?OH?₄]^-group is gradually replaced by the H^-resulting from the reduction and eventually forms[BH₄]^-.Finally,in order to further take advantage of the low price and high valence of Al,Mg₂Al₃containing higher Al content than the intermetallic compound Mg₁₇Al₁₂ was used as the reducing agent,and NaH was added as an additive.NaBH₄ was synthesized by the reaction of high energy ball milling with NaBO₂·xH₂O?x=2,4?in the atmosphere of argon.By adjusting the reducing agent ratio and ball milling time,it was observed that the molar ratio of Mg₂Al₃ to NaBO₂·2H₂O was 2:1,and the highest yield was 46%after 20 hours of ball milling.At the same time,using XRD and FT-IR methods,it was determined that Mg₂Al₃ was difficult to convert[OH]^-to H^-by short-time ball milling,while in long-term ball milling,the yield of NaBH₄ increased with the increase of the amount of Mg₂Al₃ reducing agent.