Synthesis of ammine dual-metal (V, Mg) borohydrides with enhanced dehydrogenation properties

The penta-ammine vanadium (III) borohydride, i.e. V(BH4)3·5NH3, was successfully synthesized via ball-milling of VCl3·5NH3 and LiBH4 in a molar ratio of 1:3. This compound was shown to release 11.5 wt% hydrogen with a H2-purity of 85 mol% by 350 °C. To improve the dehydrogenation purity of V(BH4)3·5NH3, Mg(BH4)2 with various molar ratios was mixed with V(BH4)3·5NH3 to synthesize expected ammine metal-mixed borohydrides, among which the formed VMg(BH4)5·5NH3 was indexed to be a monoclinic unit cell with lattice parameters of a = 19.611 Å, b = 14.468 Å, c = 6.261 Å, β = 93.678° and V = 1772.75 Å3. Dehydrogenation results revealed that the Mg(BH4)2 modified V(BH4)3·5NH3 system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH4)3·5NH3/2Mg(BH4)2 sample, 12.4 wt% pure hydrogen can be released upon heating to 300 °C. Further investigation on the dehydrogenation mechanism of the VMg(BH4)5·5NH3 system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at 75 °C) as complementary to the prime combination of BH···HN.

► V(BH4)3·5NH3 was synthesized via the reaction of VCl3·5NH3 with LiBH4.
► VMg(BH4)5·5NH3 was indexed to be a monoclinic unit cell.
► For V(BH4)3·5NH3/2Mg(BH4)2, 12.4 wt% pure H2 can be released.