Development of Magnesium Borohydride Etherates as Hydrogen Storage Materials

Abstract

The studies presented in this dissertation are to aim at understanding the kinetic enhancement of Lewis base THF-adduct of Mg(BH4)2 and evaluating the potential for hydrogen storage under moderate reaction conditions. The physiochemical properties of independently synthesized Mg(BH4)2(THF)x (x = 0.25 - 3) are studied using in-situ XRD and NMR spectroscopy, and differential scanning calorimetry (DSC). We find that the complex mixture undergoes solid-solid phase transformation and endothermic melting at temperature <100 oC. Further heating to 180 - 200 oC results in release of H2 to form MgB10H10 as the major product. Using Pressure-Composition-Temperature (PCT), the reaction can be cycled between MgB10H10 and Mg(BH4)2 by maintaining the supervicous state of the material. Ca. 25% conversion through 3 cycles, dehydrogenation at 4 bar H2 back pressure and rehydrogenation at 90 bar H2, is observed. It is noted that crystallization of the dehydrogenated state at reaction times longer than 10 hours or cooling of the melt to room temperature curtails the reversibility. We also examine the role of magnesium tetrabutoxyborate, proposed to arise from the ring opening of THF by Mg(BH4)2 attack, in the reversible cycling between MgB10H10 and Mg(BH4)2. No reversibility of MgB10H10 is observed upon introducing independently synthesized Mg[B(OBu)4]2 (Bu = C4H9) in the hydrogenation attempts. Intrigued by the reversibility of THF-solvate Mg(BH4)2/ MgB10H10, similar approach is conducted for newly independently synthesized MgB12H12(THF)3. No melting of MgB12H12(THF)3 upon heating and no formation of BH4-, characterized by NMR analysis, is observed.