Synthesis of amorphous manganese borohydride in the (NaBH4–MnCl2) system, its hydrogen generation properties and crystalline transformation during solvent extraction

Abstract

The mixture of (2NaBH4 + MnCl2) 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 NaCl-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 NaCl-type crystalline structure. No presence of any crystalline hydride was detected by powder X-ray diffraction which clearly shows that NaBH4 in the initial mixture must have reacted with MnCl2 forming a NaCl-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, B2H6. The Fourier transform infra-red (FT-IR) spectrum of the ball milled (2NaBH4 + MnCl2) is quite similar to the FT-IR spectrum of crystalline manganese borohydride, c-Mn(BH4)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(BH4)2 hydride.