Production of sodium borohydride by hydrogenation of anhydrous borax at high temperature and pressure in the presence of the magnesium

Borohydrides have attracted great attention as hydrogen storage media, for hydrogen generation or as a fuel for the direct borohydride fuel cell (DBFC) due to their high hydrogen storage capacity. In order to increase borohydride applications for hydrogen generation, however, it is necessary to reduce the cost of the borohydride production. Therefore, more researches are needed to develop new borohydride synthesis technology to lower the cost effectively. Turkey, the land of boron minerals, has a considerable capacity of refined borates besides being the biggest borax ore producer of the world. As Turkey has 72 % of world boron reserves, it is important to use these deposits to produce borohydrides.The purpose of this study is to produce NaBH4 from suitable boron minerals. This study focused on two main parts. In the first part of the study CaH2, Na2CO3 and anhydrous boric acid (B2O3) were used as reactants to perform mechano-chemical reaction with a ball milling system at room temperature, and in the second part a novel reactor system was used for the production of sodium borohydride by hydrogenation of anhydrous borax (Na2B4O7) at high temperature and pressure in the presence of the magnesium was studied. Effects of the inlet temperature of hydrogen gas fed to the reactor, production of NaBH4 by stepwise heating of the reactor, different reaction temperatures, use of both stoichiometric and excess amounts of reactants, the effect of adding chemicals as additional sodium sources (Na2CO3, NaOH, Na2O, Na2O) and the effect of adding Al to produce catalytic improvement on sodium borohydride yields were investigated.Sodium borohydride was extracted from solid product by ammonia solution and then MgO and other solid products were separated by filtration. Afterwards, solid sodium borohydride was obtained by evaporating ammonia solution. Concentration of sodium borohydride in the product was determined with three different methods; iodometric, spectrophotometric and voltammetric. In this study, sodium borohydride yields were calculated using the concentrations determined from iodometric method which is also commonly cited in the literature. For the characterization of the products FTIR and XRD techniques were used.The highest yield was obtained as 93 % in the experiment performed at a reactor temperature of 550oC, reaction time of 4 hours, the hydrogen gas given to the reactor at 25 bar and 400oC and using a stoichiometric mixture of anhydrous borax with 200 % excess amount Mg.