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.

Graphical abstractAfter ammonia borane (AB) is exposed to 1.38 MPa of CO2 at 70 °C for 38 min, it rapidly desorbs 10.1 wt. % of hydrogen regarding pristine AB at 85 °C, which is equivalent to about 1.5 H2 mol(AB)−1. The rapid hydrogen desorption is due to promoters for AB dehydrogenation forming during the CO2-pretreatment, which are stable even after removing CO2 from the treated AB.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► CO2-pretreatment of AB at 70 C and 1.38 MPa CO2 evidenced 10.1 wt% H2 (1.5 H2) release. ► IR and elemental analyses confirm the solid dehydrogenation product as (NBH3)n. ► Decoupling pre-treatment and dehydrogenation steps eliminate additional separation between H2 and CO2. ► No further separation of catalysts and promoters needed for regenerating the spent AB.