Chi-Wing Tsang

Catalytic Solvolysis of Ammonia Borane

An energy source with a low environmental impact remains a crucial goal for our society. While energy consumption is a broader concern, transportation is an area of keen interest. Hydrogen is an attractive alternative to petrochemical resources because its combustion produces only water as a by-product. Unfortunately, the physical properties of hydrogen, which complicate its safe, efficient, and economical storage, remain a significant barrier toward establishing hydrogen as a viable source of energy. Of the known hydrogen storage technologies (i.e. compression and liquefaction, metal hydrides, chemical hydrides , and carbon nanotube adsorption) chemical hydrides have the highest gravimetric storage capacity. Despite recent determinations by the Department of Energy (DOE) on the status of sodium borohydride, ammonia borane remains one of the most compelling candidates for hydrogen storage because of its higher hydrogen content (19.6 wt %) and stability. 6] Indeed, the aforementioned DOE report goes so far as to suggest that the decision to not use sodium borohydride should not impact continued research on ammonia borane (AB). Moreover, applications outside of transportation remain equally worthy of consideration, not only as a means to further the refinement of developing technologies, but also to encourage the development of critical aspects connected with the establishing of new energy sources, such as the supply and distribution channels. Several homogeneous catalysts have been shown to catalyze the release of one equivalent of hydrogen from ammonia borane at ambient temperature. For example, a very efficient homogeneous iridium catalyst for the dehydrogenation of ammonia borane was reported by Goldberg and coworkers, who demonstrated the fast release of hydrogen within 20 minutes at room temperature. Relevant pincertype catalysts have shown similar efficacies as demonstrated by the research groups of Fagnou and Schneider. Manners and co-workers have demonstrated that pincerbased catalysts can catalyze the linear polymerization of ammonia borane to form poly(aminoborane). Baker and coworkers described the acid-initiated dehydrogenation of ammonia borane as well as a homogeneous nickel-containing catalyst capable of effecting the dehydrogenation of ammonia borane wherein a 94% yield of hydrogen was observed in three hours at 60 8C. Despite these advances, dehydrogenation of ammonia borane remains limited both in terms of hydrogen yield and reaction rate. In contrast, the hydrolysis of ammonia borane in the presence of a heterogeneous catalyst can provide up to three equivalents of hydrogen per mole of ammonia borane at room temperature at satisfactory rates. Several reports have appeared (see for example Xu and Chandra, Manners and co-workers, Ramachandran and Gagare, and Jagirdar and co-workers), which detailed heterogeneous catalysts containing noble or basic metals and used for the hydrolysis of ammonia borane. Unfortunately, these systems require relatively high catalyst loadings and the catalysts have proven difficult to recover with no option for reuse. Recently, reusable monodisperse nickel nanoparticles have emerged as useful catalysts that display five cycles of catalytic activity. Nonetheless, the most practical issue—the systemic wt % of hydrogen—is rarely addressed for hydrolysis-based systems. For example, the system wt % of hydrogen for the hydrolysis of ammonia triborane (where the system weight is defined as NH3B3H7 + water + catalyst) is 6.1% when a base metal heterogeneous catalyst is used. The comparison of this value with the modified DOE target of 7.5% systemic gravimetric capacity for the year 2015 shows that the systemic wt % of hydrogen is among the most significant hurdles for the development of an efficient system for the generation of hydrogen by means of hydrolytic methods. That is, the requirement for the reaction media (i.e. organic solvent or water in the case of solvolytic or hydrolytic processes), which contributes greatly to the total weight of the system, significantly diminishes the hydrogen wt % of the system. Herein, we describe a system for the solvolysis of ammonia borane that constitutes significant progress toward addressing the issues described above. The simple and robust system displays rapid and quantitative evolution of hydrogen from ammonia borane and employs a homogeneous iridium catalyst with exceptionally low loadings and minimal use of solvent. [*] Dr. T. W. Graham, Dr. C.-W. Tsang, X. Chen, Dr. R. Guo, Dr. W. Jia, Dr. S.-M. Lu, Dr. C. Sui-Seng, C. B. Ewart, Dr. D. Amoroso, Dr. K. Abdur-Rashid Kanata Chemical Technologies Inc. 101 College Street, Office 230, MaRS Centre, South Tower, Toronto, ON, M5G 1L7 (Canada) Fax: (+ 1)416-981-7814 E-mail: damoroso@kctchem.com kamal@kctchem.com Homepage: http://www.kctchem.com