Novel Approach for Synthesis of Magnesium Borohydride, Mg(BH4)2

Mg(BH4)2 is a complex hydride with one of the highest hydrogen contents (∼ 15%) known yet. Several synthesis routes have been reported for it, all based on the metathesis reaction of MgCl2 with NaBH4 performed in a ball-mill or in suitable solvents.In the present study a new approach for synthesis of Mg(BH4)2 will be presented in which the more reactive MgBr2 is used instead of MgCl2. For this purpose a mixture of MgBr2 and NaBH4 (molar ratio: 1: 2 and 1:2.15) was ball-milled for 6, 12 and 18 h, respectively. Mg(BH4)2 was extracted from the reaction product (Mg(BH4)2 + NaBr) by Soxhlet with diethylether over a day. The remaining residue after solvent evaporation was dried in vacuum at 150°C for 24 h and 5 h at 190 °C. The intermediate and final products of the reactions were analyzed using XRD, DTA/TG, Mass and Vibrational Spectroscopy. The XRD diagrams of the mixture after ball milling showed only the characteristic reflections of NaBr and the patterns obtained after solvent extraction was in all cases consistent with β- Mg(BH4)2. The additional weak MgBr2 reflections, decreased by increasing the ball milling time from 6 to 18 h. The DTA/TG coupled with MS revealed ∼11% mass loss when the product was heated up to 600 °C. The result of MS detected that the exhaust gas is exclusively H2.Compared to MgCl2, the use of MgBr2 has two advantages: the reaction time is considerably shorter and the excess of MgBr2 can act as additive lowering the onset temperature for hydrogen release from 290 °C - for pure Mg(BH4)2 - to ∼ 220 °C.