Delayed ammonia release of ammonia borane hydrolysis by citric acid

Abstract

Ammonia borane (AB), with high hydrogen density and stability under ambient conditions, readily releases hydrogen via catalyzed hydrolysis. However, ammonia, a byproduct of hydrogen release, limits its utilization. Citric acid, a safe and cost-effective acid catalyst, for the acid catalyst of AB hydrolysis was investigated in this study to mitigate ammonia emissions while enhancing hydrogen release. Maximal hydrogen release (3 eq.H2) and ammonia emission suppression (<0.5 ppm) were achieved using a 2-mol/L AB aqueous solution adding into citric acid powder (molar ratio of AB: citric acid = 1:1). In contrast, the amount of hydrogen release at other concentrations and/or using the mixing method was below 2.7 mol/mol AB within several hours. Ammonia re-released even after hydrogen release in the case of high AB concentration (e.g. >4 mol/L AB aqueous solution with citric acid powder (molar ratio of AB: citric acid = 1:1)) in the solution because of formation of borate ester (1:2 borate–diol molar ratio) via mono-ammonium citrate (a borate-ester precursor), which was revealed by the solution NMR. These results reveal a cost-effective and simple on-way method for hydrogen release with suppressed impurities. Furthermore, understanding impurity generation mechanisms can contribute to utilizing AB-derived hydrogen.