Theoretical and experimental study on Mg(BH4) 2-Zn(BH4)2 mixed borohydrides

After a screening of possible systems prone to give an enthalpy of decomposition close to 30 kJ mol(H2)(-1), i.e. suitable for a dehydrogenation process close to room temperature and pressure, the Zn dissolution into Mg(BH4)(2) has been investigated. The total energy of pure compounds and solid solutions has been computed by DFT calculations using the CRYSTAL09 code. To generate the Mg(1-x)Znx(BH4)(2) solid solution, alpha-phase of Mg(BH4)(2) (space group P6(1)22) has been considered, with a replacement of Mg2+ with Zn2+ ions, without lowering the symmetry of the crystalline structure. On the basis of DFT results, the enthalpy of decomposition has been estimated, considering MgH2, Zn, alpha-B and H-2 as products, and a value of 30 kJ mol(H2)(-1) has been calculated for x = 0.2. In order to verify the results of calculations, mixtures of Mg(BH4)(2) and ZnCl2 with 1.0:0.7 ratio have been ball milled, both at room temperature and in cryo-conditions. Samples have been analyzed with a combination of experimental techniques (XRD, DSC, IR-ATR, TGA, TPD, PCI). The phase mixture obtained after the synthesis strongly depends on the milling conditions. For prolonged times, the formation of Zn and MgCl2 has been observed, suggesting the delivering of B-containing species during the milling. After heating, a hydrogen release, coupled with diborane delivering, has been observed for temperatures close to 100 degrees C, suggesting a significant decrease of the decomposition temperature with respect to pure Mg(BH4)(2). Theoretical and experimental results have been discussed on the basis of the possible reaction paths, as estimated from available thermodynamic databases.