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Ammonia–Borane: the Hydrogen Source Par Excellence?

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Ammonia–Borane: the Hydrogen Source Par Excellence? View Article Online / Journal Homepage / Table of Contents for this issue PERSPECTIVE www.rsc.org/dalton | Dalton Transactions Ammonia–borane: the hydrogen source par excellence? Frances H. Stephens, Vincent Pons and R. Tom Baker* Received 1st March 2007, Accepted 3rd May 2007 First published as an Advance Article on the web 31st May 2007 DOI: 10.1039/b703053c Ammonia–borane, H3NBH3, is an intriguing molecule for chemical hydrogen storage applications. With both protic N–H and hydridic B–H bonds, three H atoms per main group element, and a low molecular weight, H3NBH3 has the potential to meet the stringent gravimetric and volumetric hydrogen storage capacity targets needed for transportation applications. Furthermore, devising an energy-efficient chemical process to regenerate H3NBH3 from dehydrogenated BNHx material is an important step towards realization of a sustainable transportation fuel. In this perspective we discuss current progress in catalysis research to control the rate and extent of hydrogen release and preliminary efforts at regeneration of H3NBH3. 1 Introduction dioxide produced from burning gasoline is captured on board the vehicle and shipped to a processing plant where it is reformed back The success of global economies has stemmed largely from to gasoline—a tall order indeed! This need for regeneration also 1 access to sources of inexpensive energy. In developed countries, hinders commercialization of borohydride hydrolysis as there are transportation of people and goods exhausts 65% of refined currently no energy efficient routes to convert the stable borate petroleum products, a trend that is likely to escalate significantly in product back to borohydride. In this perspective we explore the the near term as developing countries expand their transportation dehydrogenation chemistry of ammonia–borane, H3NBH3 (1), 2 infrastructure. The hydrogen economy has been touted as a po- (Chart 1), a remarkable molecule which contains both hydridic tential multisource energy solution for transportation that can also B–H and protic N–H bonds and a strong enough B–N bond that reduce emissions of nitrogen and sulfur oxides, carbon particulates under most conditions hydrogen loss is favored over dissociation and voluminous quantities of carbon dioxide that constitute the to ammonia and borane. Its combination of low molecular weight 3 present status quo. Indeed several nations have already turned (30.7 g mol−1) and high gravimetric hydrogen capacity (19.6 wt%) to hydrogen-powered bus fleets using internal combustion engines has attracted a flurry of recent investigations into hydrogen release or more efficient, low-temperature polymer electrolyte membrane from 1, highlights of which will be summarized here. We conclude (PEM) fuel cells to reduce the pollution burden in populous urban with a description of preliminary efforts to regenerate 1 from Downloaded by University of Washington on 06 March 2013 settings.4 As we look to the future and attempt to utilize hydrogen Published on 31 May 2007 http://pubs.rsc.org | doi:10.1039/B703053C dehydrogenated BNHx materials. for longer haul transportation, it becomes obvious that vehicular hydrogen storage needs drastic improvement to be practical.5 Compressed hydrogen, presumably stored on-board in lightweight 2 Preparation and physical properties of tanks,6 lacks the necessary volumetric capacity to drive 500 km ammonia–borane, aminoborane and iminoborane without refueling and would involve a major rework of our fuel distribution infrastructure. While significant progress is being Ammonia–borane is a colorless solid that is stable at room 7–9 temperature and soluble in relatively polar coordinating solvents made on complex metal hydrides such as Ti-doped NaAlH4 17–19 ◦ 19,20 and new porous sorbent materials10–12 for hydrogen storage, these (Table 1). The melting point of 1 is in the range 110–114 C. ◦ 17,21 materials tend to be restricted to material capacities of less than Two reports claim that ultra-pure material melts at ca. 125 C. 8 wt% hydrogen, excluding other system contributions such as fuel The two most important methods for laboratory-scale preparation tank, lines, etc.13 of 1 are salt metathesis and direct reaction. While a number 22 23 20 A fourth option for on-board hydrogen storage takes advantage of ammonium salts such as chloride, sulfate, and carbonate of hydrogen stored as E–H bonds where E is a light main group can be employed for salt metathesis, combination of ammonium element such as C, B, N, or O.14 In an early example, hydrolysis formate with sodium borohydride with sonication gives 1 in high 24 of sodium borohydride was controlled using a heterogeneous yield. This method also permits the facile isotopic labeling of 1 10/11 1/2 14/15 ruthenium catalyst in Millenium Cell’s Hydrogen on DemandTM ( B, H, N). Direct reaction of ammonia gas with diborane 22,23,25–27 · 17 · 28 technology that was demonstrated in the NatriumTM car.15,16 The (B2H6, 2), BH3 SMe2 or BH3 THF also affords 1 in promise (and possibly the downfall) of such chemical hydrogen workable yields. storage schemes lies in the need to regenerate the fuel, thus When 2 is treated with ammonia, cleavage of the B2H6 frame- approaching a sustainable solution to our transportation needs. work can occur symmetrically to generate 1 or unsymmetrically To fully appreciate this concept one can imagine that the carbon to generate an ionic isomer, [BH2(NH3)2][BH4](3) (Scheme 1). A large scale synthetic method for 3 has been reported,28 wherein 2 is passed into liquid ammonia at −78 ◦C. In contrast to the Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM, USA. E-mail: [email protected]; Fax: +1 (505) 667-9905; Tel: +1 (505) relative thermal stability of 1, glyme solutions of 3 have been 667-9274 reported to undergo hydrogen loss at temperatures as low as This journal is © The Royal Society of Chemistry 2007 Dalton Trans., 2007, 2613–2626 | 2613 View Article Online Fran Stephens was born in Fitchburg, Massachusetts, in 1977 and grew up in Circle Pines, Minnesota, USA. After she received her BS degree from the University of St. Thomas in 1999, she joined the group of Kit Cummins at MIT, where she studied the activation of white phosphorus by low-valent molybdenum and uranium complexes. She obtained her PhD in 2004 and subsequently joined the chemical hydrogen storage research team at Los Alamos National Laboratory (LANL) as a Director’s Postdoctoral Fellow with Tom Baker. Fran was promoted to LANL Technical Staff Member in 2006 in the Inorganic, Isotope, and Actinide Chemistry group. Her research interests include the application of main-group chemistry to energy security. Vincent Pons was born in Toulouse, France, in 1974. After obtaining his Diplomeˆ d’Etudes Approfondies at the Universite´ Paul Sabatier (Toulouse, France) in 1999, he joined the group of Professor D. Michael Heinekey at the University of Washington (Seattle, United States) in the fall of the same year. During his graduate studies, he studied the binding and activation of dihydrogen by transition metals. He earned his PhD in 2004 and, after a first postdoctoral stay at the University of Washington working with both Professors D. Michael Heinekey and Karen I. Goldberg, he moved to Los Alamos National Laboratory where he has been working since 2006 with Dr R. Tom Baker and Professor Larry G. Sneddon (University of Pennsylvania) as a joint postdoctoral research associate. His current research interests include the understanding of the mechanism of dehydrogenation of amine–borane adducts and the investigation of nanoparticles in ionic liquids as dehydrogenation catalysts. R. Tom Baker grew up in Tsawwassen, British Columbia, Canada. He received his BSc degree from UBC in 1975 and his PhD from UCLA in 1980 with M. Frederick Hawthorne. After a postdoctoral stint with Philip S. Skell at Penn State, he joined the staff at DuPont Central Research in Wilmington, DE, where he applied inorganic and organometallic chemistry and homogeneous catalysis to the nylon, fluoroproducts, and titanium dioxide businesses. He joined the Chemistry Division at Los Alamos National Laboratory in 1996 and is currently involved in multifunctional catalysis approaches to low-temperature hydrocarbon functionalization and chemical hydrogen storage. Downloaded by University of Washington on 06 March 2013 Published on 31 May 2007 http://pubs.rsc.org | doi:10.1039/B703053C Frances H. Stephens Vincent Pons R. Tom Baker Chart 1 Boron compounds referred to in text. 2614 | Dalton Trans., 2007, 2613–2626 This journal is © The Royal Society of Chemistry 2007 View Article Online Table 1 Solubility of ammonia–borane at room temperature21 Solvent Solubility/g (100 g solvent)−1 Ammonia 260 Water 33.6 Tetrahydrofuran 25 Diethyl ether 0.74 Alcohols: Ethanol 6.5 Isopropanol 4 Isobutanol 1 Scheme 1 Symmetric and unsymmetric cleavage of diborane by ammonia. Fig. 1 Low-temperature (orthorhombic) crystal structure of H NBH . − ◦ 19 3 3 20 C. Although compound 3 is commonly referred to as the View along crystallographic c axis. N, B, and H atoms are depicted in diammoniate of diborane, its molecular structure was in dispute blue, purple, and gray, respectively. The dihydrogen bonds are delineated 22,23,26,29,30 for several years. In the first report Schlesinger and Burg by dashed lines. The unit cell is demarcated by gray lines.54 31 proposed a structure of formula [NH4][NH2(BH3)2](3 ). This structure was later disproved by a detailed series of papers by Parry Aminoborane was prepared, isolated, and characterized at low 22,23,29,30,32 11 33 34 ◦ and Shore, subsequent BNMR and Raman studies, temperature (below −160 C) by subjecting borazine, [HNBH]3 35,36 61 and finally an X-ray crystal structure of [BH2(NH3)2][Cl]. The (6), to a radiofrequency discharge. Other preparative methods B–N bonds in the latter molecule are both 1.58(2) A˚ ,andthere and details of the spectroscopic properties of 4 have been are relatively short Cl–N distances (ca.
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