Rapid release of 1.5 equivalents of hydrogen from CO₂-treated ammonia borane

This work addresses the accelerated dehydrogenation of ammonia borane (AB, NH₃BH₃) in two separate processes of CO₂ pre-treatment of AB and dehydrogenation of the treated AB. Decoupling these two processes can still keep the dehydrogenation activity of CO₂-treated AB and eliminate the purification step of H₂ from gas phase. When AB is exposed to 1.38 MPa of carbon dioxide (CO₂) at 70 °C, it shows the most favorable and controllable operating condition for the CO₂ pre-treatment. The pre-treatment enhances not only the rate but also the amount of hydrogen release at the dehydrogenation step; 1.5 mol H₂ per mol of AB rapidly desorbs at 85 °C in 1 h, corresponding to 10.1 wt.% of hydrogen with regard to pristine AB. Also, our observations show that the fast dehydrogenation resulted from the CO₂ pre-treatment is preserved for more than four days of storage. The degree of dehydrogenation is further confirmed by ATR-FTIR spectroscopic and elemental analyses of the solid product. The spectra display the N-H stretching mode involving π-bonded nitrogen (sp² N) at ca. 3434 cm^-1,while the atom ratio of H:B is found to be 2.84:1. Based on the hydrogen release measurements, spectroscopic observations and elemental analyses, we deduce that the predominant solid product of dehydrogenation of CO₂-treated AB at 85 °C is a polymer with an empirical formula of (NBH₃)_n. It corresponds to the solid product after 1.5 equivalent hydrogen release of AB.