Synthesis of amorphous manganese borohydride in the(NaBH_4–MnCl_2) system,its hydrogen generation properties and crystalline transformation during solvent extraction

The mixture of(2NaBH_4+ MnCl_2) was ball milled in a magneto-mill. No gas release was detected. The XRD patterns of the ball milled mixture exhibit only the Bragg diffraction peaks of the Na Cl-type salt which on the basis of the present X-ray diffraction results and the literature is likely to be a solid solution Na(Cl)x(BH4)(1-x), possessing a cubic Na Cl-type crystalline structure. No presence of any crystalline hydride was detected by powder X-ray diffraction which clearly shows that NaBH_4 in the initial mixture must have reacted with MnCl_2 forming a Na Cl-type by-product and another hydride that does not exhibit X-ray Bragg diffraction peaks. Mass spectrometry(MS) of gas released from the ball milled mixture during combined MS/thermogravimetric analysis(TGA)/differential scanning calorimetry(DSC) experiments, confirms mainly hydrogen(H2) with a small quantity of diborane gas, B_2H_6. The Fourier transform infra-red(FT-IR) spectrum of the ball milled(2NaBH_4+ MnCl_2) is quite similar to the FT-IR spectrum of crystalline manganese borohydride, c-Mn(BH_4)_2, synthesized by ball milling, which strongly suggests that the amorphous hydride mechano-chemically synthesized during ball milling could be an amorphous manganese borohydride. Remarkably, the process of solvent filtration and extraction at 42 °C, resulted in the transformation of mechano-chemically synthesized amorphous manganese borohydride to a nanostructured,crystalline, c-Mn(BH_4)_2hydride.