Ammonia borane thermolytic decomposition in the presence of metal (II) chlorides

In the present work, we studied the effect of metal chlorides, MCl₂, on the thermal decomposition of ammonia borane NH₃BH₃ (AB). Some metals from row n = 4 of the periodic table were chosen and used as MCl₂: namely, FeCl₂, CoCl₂, NiCl₂, CuCl₂, and ZnCl₂. In addition, three metals from column VIII of the periodic table were considered: NiCl₂, PdCl₂ and PtCl₂. The AB decomposition was followed by TGA and DSC; the decomposition gases analyzed by μGC/MSD coupling, and the solid by-products identified by XRD, IR and XPS. We observed that the presence of CuCl₂ in AB is beneficial, making the decomposition occur in much milder conditions than for pristine AB; for example, the dehydrogenation of CuCl₂-doped AB started at 25 °C, with the sample losing about 14 wt% at 85 °C. However, MCl₂ does not hinder the evolution of the undesired borazine; it only contributes to a decrease in its content compared to pristine AB. To rationalize the better performance of CuCl₂, we propose that Cu offers an optimal doping activity with intermediate binding energies for the intermediates: i.e. with H not too strongly bonded but optimally bonded to the N of AB. The germ Cu?NH₂–BH₂, then formed, acts as a Lewis acid through B and has an optimized reactivity towards a new AB molecule (head-to-tail dehydrocoupling). This is discussed herein.