Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration

Interest in sustainable non-hydrocarbon-based fuels for transportation has grown as the realization that the supply of fossil fuels is limited and the deleterious environmental effects of burning them has come into public focus. The use of hydrogen (H-2) has been proposed as an alternative, but its use in pure form is undesirable due to the high pressures or low temperatures required to store useful quantities. Approaches to ameliorate this issue, which stem from the low volumetric energy density of H-2, include the pursuit of sorbents capable of containing H-2 in greater density than liquid H-2 at reasonable temperatures and pressures, metal hydrides such as NaBH4, and chemical hydrides such as ammonia borane (AB, NH3BH3). AB contains 19.6 wt.-% H-2, which is well suited to practical applications, but issues with extracting the optimal quantities of H-2 at reasonable temperatures and at useful rates as well as recycling of the spent fuel back into AB with good efficiency and reasonable cost remain to be solved. These problems are inextricably intertwined, as the needs of the regeneration process dictate which catalysts are used (i.e. what kind of spent fuel is generated). This Microreview focuses on recent developments in transition metal complexes for the catalytic dehydrogenation of AB. Neither solvolysis of AB nor thermolysis of AB in the absence of a catalyst are covered in this review.