Production and Regeneration of Sodium Borohydride Based on Hydrates

Author:Zhong Hao

Supervisor:ou yang liu zhang michael felderhoff


Degree Year:2019





Hydrogen generation via hydrolysis is one of the most promising solution for hydrogen storage and application.But its irreversibility should be solved for the large-scale application of hydrolysis.In this thesis,borax(Na2B4O5(OH)4·8H2O)and direct hydrolysis by-product(NaB(OH)4)are used for sodium borohydride(NaBH4)production and regeneration.Sustainable hydrogen source and ball milling are used to achieve a low cost and mild condition NaBH4 production and regeneration method.First,in the traditional industrial and high temperature and hydrogen pressure lab methods for NaBH4 production,borax should be dehydrated at a high temperature,which is energy consumed and high cost because an additional hydrogen source should be used in the NaBH4 production process.In this thesis,natural borax(Na2B4O5(OH)4·8H2O)is used to react with Mg/Mg2Si and Na2CO3 under argon via ball milling and achieve the production of NaBH4.In this process,the[OH]-group from the borax works a hydrogen source.The generated NaBH4 owns similar hydrolysis property as the commercial NaBH4.And the direct by-product NaB(OH)4 is found after the hydrolysis.Iodate method is used to obtain the NaBH4 production yield.In the reaction between Mg,Na2CO3 and Na2B4O5(OH)4·8H2O,the highest NaBH4 yield is 66%,while the highest NaBH4 yield of the reaction between Mg2Si,Na2CO3 and Na2B4O5(OH)4·8H2O is 78%.In the mechanism study,XRD,FTIR,NMR and MS are used for analysis and it is found that there are two steps in the NaBH4 generation reaction.First,Na2B4O5(OH)4·8H2O reacts with Mg/Mg2Si and Na2CO3 to generate NaB(OH)4.Then NaB(OH)4 continuously to react with Mg/Mg2Si to produce NaBH4.Generation of active Mg-O-Si-H intermedia might be the reason for the high NaBH4 yield when Mg2Si is used as reducing agent.Second,Mg2Si is used to react with the direct by-product NaB(OH)4/NaB(OH)4·2H2O under argon via ball milling,which achieves NaBH4 regeneration.The regenerated NaBH4 has an excellent hydrogen generation property via hydrolysis and achieves 6.3wt%systemetic hydrogen generation property(including the NaBH4,water and catalyst).The NaBH4 yield of achieved by the reaction between Mg2Si and NaB(OH)4 is 78%,while the NaBH4 yield of the reaction between Mg2Si and NaB(OH)4·2H2O is 74%.In the mechanism study,XRD,FTIR,NMR and MS are used for analysis and there are also two step in the reaction between Mg2Si and NaB(OH)4·2H2O.NaB(OH)4·2H2O is dehydrated in the reaction with Mg2Si to generate NaB(OH)4.The next step is the reaction between Mg2Si and NaB(OH)4.In this process,a Mg-O-Si-H intermedia forms and accelerates the transformation of[OH]-to H-.With the substitution of H-for[OH]-,NaBH4 is generated afterward.However,changing the shaker mill to planetary ball milling for the reaction between Mg2Si and NaB(OH)4 reduces the NaBH4 yield to 1.7%.Using hydrogen pressure can promote the NaBH4 yield in the reaction via planetary ball mill and achieve a NaBH4 yield of 46%.After adding hydrogen pressure in the milling process,it is found that there are three pathway in the regeneration reaction.The three pathway are,first,Mg2Si decomposes during ball milling under hydrogen pressure and the generated MgH2 reacts with NaB(OH)4 to regenerate NaBH4;second,the reaction between H2,Mg2Si and NaB(OH)4 to regenerate NaBH4;finally,the reaction between Mg2Si and NaB(OH)4.But using an alloy(Mg2Ni)having good hydrogenation property in the reaction with NaB(OH)4 under argon via ball milling can not achieve the regeneration.Because Ni can catalyze the hydrolysis reaction between the generated NaBH4 and NaB(OH)4,which also demonstrates the effect of Mg-O-Si-H intermedia for NaBH4 regeneration.Third,to prove the general applicability of using hydrate to regenerate NaBH4,Ca/CaSi2 is used to react with NaB(OH)4 under argon via ball milling and achieve the regeneration of NaBH4.Here,Ca is more abundant than Mg on the earth,so Ca is more possible than Mg to be used in the future with a long time.In the reaction between Ca and NaB(OH)4,the highest NaBH4 yield is 37%.It may result from the bad ability of Ca to transform[OH]-to H-.But the ability in CaSi2 is more powerful,which achieves a 57%NaBH4 yield.It is found that CaSi2 is a more efficient reducing agent to transform[OH]-to H-,thus,which achieves a much higher NaBH4 yield.