Borates in hydrolysis of ammonia borane

•In hydrolysis of ammonia borane, the hydrolysate is aqueous B(OH)3.•The solid by-products are mainly ammonium borate hydrates and H3BO3.•With the ammonium borate hydrates, the atomic ratio N/B is <1.•When NH3 is extracted at 80 and 500 °C, H3B3O6 and B2O3 form respectively.•H3B3O6 and B2O3 could be recycled to ammonia borane.

Ammonia borane NH3BH3 (19.5 wt% H) is able to release hydrogen by hydrolysis in the presence of a catalyst in ambient conditions. This reaction has received considerable attention since 2006, with special focus on the catalytic material. In comparison, important aspects like the nature of the hydrolysis by-product(s) have been much less investigated while a good identification of the borate(s) is required for approaching recyclability. In this context, we present a work based on a systematic approach that aims at characterizing the hydrolysate, its stability in time, and the borate(s) recovered after drying. It is shown that the hydrolysate consists in aqueous B(OH)3 and that the solution (catalyst-free) is stable when stored 6 months under argon atmosphere at 30 °C. The extraction of the water from the hydrolysate was performed at different conditions (vacuum, or air; from −50 to 500 °C). It is observed that the higher the temperature, the lower the hydration degree of the borates. The total dehydration, with the formation of B2O3, can be obtained at heating at 500 °C. The main problem with the hydrolysate is the release of NH3 during the drying stage. A solution is to remove NH3 after hydrolysis and to dry the NH3-free hydrolysate. By this way, H3B3O6 forms. Hence, B2O3 and H3B3O6 could be recovered and recycled into ammonia borane. Besides the identification of the borates, the suitability of ammonia borane for hydrogen production by hydrolysis is discussed, especially in comparison with sodium borohydride NaBH4.