PNNL

Abstract

Alkyl magnesium compounds such as di-butyl magnesium can be effectively decomposed to produce MgH2 and Mg metal for applications such as hydrogen storage, single site catalysis, powder metallurgy or as the last step of comprehensive processes to produce Mg metal from magnesium salts. For these applications it is important to understand the decomposition reactions of di-butyl magnesium and to quantify its decomposition products. To avoid interference from air and moisture, an in-situ X-ray diffraction (XRD) coupled with mass spectroscopy system was developed in this work to investigate phase changes and released products in real time for the decomposition reactions. For the di-butyl magnesium (MgBu2n-n) with two n-butyl groups, transformation to MgH2 occurred at 250 °C and transformation to Mg metal occurred near 384 °C. The diffraction data suggest the possibility of an intermediate phase with one butyl group formed during the transformation to MgH2. Decomposition of MgH2 under vacuum is accelerated by eliminating H2 from the system as it is evolved. The sec-butyl n-butyl Magnesium (MgBu2s-n) was observed to decompose at as low as 50 °C indicating the increased lability of the sec-butyl group in the di-butyl magnesium isomer. Thermogravimetric analysis (TGA) for the MgBu2n-n and MgBu2s-n loaded in air-free conditions recorded conversion rates higher than 98% for the MgH2 to magnesium metal transition.