Regenerating spent ammonia borane

26 march 2011 - Recycling

Ammonia borane (AB) is a potential hydrogen carrier for the hydrogen economy but regenerating spent AB fuel presents a challenge. When using a catalyst based on a transition metal and an N-heterocyclic carbene more than two equivalents of hydrogen are extracted leaving a residue called polyborazylene, an odd mix of compounds with formal formula BNH. In 2009 Dixon et al. described AB regeneration from BNH based on ortho-benzenedithiol (converting B-N bonds to B-S bonds) / tributyltin hydride (reducing agent) (DOI) but the tin reagent proved too expensive for industrial application. In 2011 Dixon has made a switch to hydrazine (Sutton et al. DOI).
Direct reaction of BHN with hydrazine yields the hydrazine-borane complex which in itself can be used as a hydrogen fuel if it wasn't for the even more obscure waste-product obtained than PB without even a remote chance of regeneration. An ammonia replacement reaction then. At room temperature the hydrazine-borane complex is unimpressed by the presence of liquid ammonia but with heat applied the ammonia-borane is regenerated with 85% efficiency. The overall reaction taking place is: 4BNH + 5 N2H4 -> 4H3N.BH3 + 5N2. Efficiency (>92%) is greatly improved by reaction of PB with hydrazine in liquid ammonia: hydrazine is displaced in-situ from the hydrazine-borane complex and available again as a reducing agent.
The only new problem to solve according of Dixon is the limited global production capacity of hydrazine but integration of BNH regeneration and hydrazine production is envisioned.