Abstract
As potential hydrogen storage mediums, ammonia borane and its derivatives have been paid an increasing attention owing to their higher hydrogen capacities and facile dehydrogenation properties under moderate conditions. In this chapter, we presented extensive studies on thermodynamic tailoring of dehydrogenation of metal amidoboranes, metal borohydride-ammonia borane complexes, and metal amidoborane ammoniates as well as their derivatives, with special focus on the syntheses, crystal structures, and dehydrogenation properties. Finally, future perspective was given toward the practical applications.
Keywords
- Hydrogen storage
- dehydrogenation
- metal amidoborane
- B-N-containing compounds
- thermodynamics
- kinetics
1. Introduction
Mainly driven by two global issues of energy crisis and environmental pollution, great attention to a relatively new concept of the hydrogen economy has been paid in the last decades. However, hydrogen storage is one of bottlenecks in hydrogen economy. Generally, the materials with high hydrogen content have received an increasing interest for chemical storage concepts. As a chemical hydride, ammonia borane (NH3BH3, AB) has attracted significant attentions owing to the extremely high hydrogen capacity up to gravimetric density of 19.6 wt% and volumetric density of 146 g H2 L-1 [1]. Unlike metal borohydrides, solid crystalline AB is not very sensitive to air at ambient condition. In AB molecules, there are both hydridic H atoms in BH3 groups and protic H atoms in NH3 groups. Consequently, the chemical potential of the interaction between protic Hδ+ and hydridic Hδ- to the formation of hydrogen has been known as one of the prime driving powers for hydrogen release [2,3]. However, the relatively high kinetic barriers are observed in its multistep decomposition, which heightens the temperatures for hydrogen release more than 100 °C [4,5]. Other disadvantages of AB dehydrogenation consist of the discharge of some detrimental by-products (borazine and ammonia) and severe sample expansion and foaming. Moreover, the intrinsic exothermicity of dehydrogenation reveals that reversible hydrogenation under moderate conditions is thermodynamically impracticable. In recent years, a number of techniques including nanoconfinement [6-8], using ionic liquid medium [9], and catalytic modifications [10-14] were successfully employed in extensive studies. More importantly, another strategy proposed to improve the properties of AB thermolysis is the formation of metal-substituted AB-compounds, i.e., metal amidoboranes (MABs), which have been proven to be a practical method for tuning the dehydrogenation thermodynamics of AB [15-20]. Because of the quite high hydrogen content and reasonable dehydrogenation properties, this new type of materials has been paid intensive attention and grown to become one of the likeliest candidates for storing hydrogen.
In this chapter, we briefly summarize the recent progress of nanoconfinement and nanocatalysis of AB, metal amidoboranes (including single- and mixed-metal amidoboranes), and their derivatives (including metal amidoborane ammoniates, metal borohydride-ammonia borane complexes, and other complexes related to AB or MAB), with a particular emphasis on the synthesis, crystal structure, and dehydrogenation.
2. Nanoconfinement of AB
The pioneering work on AB nanoconfined in porous materials was performed by Autrey and his coworkers [6]. They loaded AB into SBA-15 (a kind of mesoporous material with specific surface area of about 900 m2 g−1, pore size of 7.5 nm in diameter, and porous volume of 1.2 cm3 g−1) by impregnating SBA-15 in a saturated methanol solution of AB. The obtained sample was determined to contain 50 wt% AB (denoted as AB:SBA-15). Compared to pristine AB, the sample nanoconfined in SBA-15 has two significant differences, as shown in Figure 1. First, the dehydrogenation peaks shifted to lower temperatures. Second, the great suppression of borazine evolution was successfully achieved, which indicates that the dehydrogenation reaction is probably tuned. The change of reaction pathway was further verified by differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments. From isothermal DSC test, the dehydrogenation reaction enthalpy of AB:SBA-15 was found to significantly reduce to 1 ± 1 kJ (mol AB)−1, compared to pristine AB of 21 ± 1 kJ (mol AB)−1. Also, NMR tests showed that the post-dehydrogenated product was only polyaminoborane (PAB) for AB:SBA-15 upon heating to 85 °C, which is different from pristine AB.
Isothermal dehydrogenation showed that the releasing rate of hydrogen from AB:SBA-15 was much more rapid than that from pristine AB. As an example, it needed less than 10 min for AB:SBA-15 to reach 50% reaction conversion at 80 °C, while 290 min for pristine AB. Also, a much lower activation energy of 67 ± 5 kJ mol−1 was determined for dehydrogenation from AB:SBA-15. However, the total amount of released hydrogen was undoubtedly sacrificed after AB loading into SBA-15, which was not specifically mentioned in their work.
In another study [7], carbon cryogel with specific surface area of 300 m2 g−1, pore diameter in the range of 2-20 nm, and pore volume of 0.7 cm3 g−1 was selected as a nanoscaffold to incorporate AB. Sample with a loading of 24 wt% AB into carbon cryogel was obtained (denoted as C−AB nanocomposite) by wet impregnation using tetrahydrofuran (THF) as solvent. Nitrogen physisorption test revealed that there is a loss of ~30% in pore volume after loading AB. From Figure 2, it was found that carbon cryogel as a nanoscaffold exhibited an even more pronounced effect on dehydrogenation than SBA-15. The peak for hydrogen release shifted to a low temperature of ~90 °C, and no signal for borazine was observed throughout the experiment. However, the total amount of released hydrogen was significantly less than that from pristine AB (9 wt%
As a newly developed kind of porous material, metal-organic frameworks (MOFs) consist of polydentate organic ligands and metal ions, achieving 3D extended frameworks [21,22]. A unique feature of MOFs is its adjustable porosity, which enables it with special properties like the combination of active metal sites and nanoporosity. Taking advantage of its porosity, an MOF-confined AB system was successfully synthesized by an infusion method using a solvent of methanol [23]. Y(BTC)(H2O)⋅DMF (denoted as JUC-32-Y) was used as a nanoscaffold because of its high specific surface area, suitable pore size, and high thermal stability. Most importantly, it has many unsaturated metal sites after the removal of the terminal H2O upon heating at 300 °C under vacuum (Figure 3A). After loading AB in 1:1 molar ratio, the framework of JUC-32-Y retained its integrity, even after dehydrogenation of AB/JUC-32-Y. Temperature programmed desorption coupled with mass spectrometry (TPD-MS) results shown in Figure 3B demonstrated that the dehydrogenation of AB/JUC-32-Y started at a lower temperature of ~50 °C, with a peak at 84 °C for most rapid releasing rate, which has a reduction of 30 °C in comparison with pristine AB. Furthermore, no volatile products such as B2H6, NH3, and borazine were detected during the decomposition process. The suppression of ammonia was ascribed to the existence of unsaturated coordination of Y3+. Figure 3C indicates a notable improvement of the dehydrogenation kinetics of AB/JUC-32-Y. At 95 °C, 10.2 wt% H2 could be released in initial 10 min while 8.0 wt% H2 at a reduced temperature of 85 °C. Pristine AB displayed no emission of any hydrogen under same conditions. Note that AB/JUC-32-Y could release ~13 wt% H2 at 95 °C within 3 h, corresponding to 2 Eq of H2 in pristine AB.
Soon afterward, Gadipelli et al. [24] reported the confinement of AB in Mg-MOF-74 with 1D pores (denoted as AB/Mg-MOF-74), in which the dehydrogenation kinetics had a remarkable improvement under 100 °C. Furthermore, in the released hydrogen, the detrimental concomitants such as ammonia, borazine, and diborane were not detectable throughout the dehydrogenation process. More importantly, owing to the relatively high specific surface area of 1100 m2 g−1 and low atomic weight of metal cation, Mg-MOF-74 could contain about 26 wt% AB determined by theoretically structural optimization, while only about 8 wt% AB in JUC-32-Y [23]. Almost 14 wt% of pure H2 could be released at a temperature of 125 °C in one hour for the AB/Mg-MOF-74 sample, which exceeded 13 wt% of H2 (2 Eq) from pristine AB. Further systematic investigation showed that the dehydrogenation kinetics of AB/Mg-MOF-74 were very dependent on the loading amount of AB. Other MOFs such as Ni-MOF-74 and HKUST-1 were also tried in this work. However, the structures collapsed with AB loading, which enabled them to be unsuitable for serving as a nanoscaffold. In our related study [25], new tactics for loading AB in MIL-101 (AB/MIL-101) or Ni-modified MIL-101 (AB/Ni@MIL-101) were proposed through wet impregnation. As shown in Figure 4, the dehydrogenation of AB/MIL-101 proceeded without an induction period or any detectable borazine and diborane, but with some ammonia. For the case of AB/Ni@MIL-101, the dehydrogenation peak for the most rapid rate was reduced to a lower temperature of 75 °C. Furthermore, the detrimental by-products of borazine, diborane, and ammonia were not detected during the whole experiments. Theoretical calculations indicated the establishment of B−O and Cr−N bonds and the existence of strong interaction between MIL-101 and NH2BH2 species, which could facilitate the dehydrogenation.
3. Nanocatalysis of AB
Ni1-
A “co-precipitation” process was developed using a typical wet-chemical method for fabricating nanoalloy to catalyze AB dehydrogenation [27]. Typically, a mixture of 2.0 mol% of CoCl2 and 10 mL of THF was stirred by ultrasound at room temperature. Then AB was introduced into the resultant THF solution, which was immediately distilled under a reduced pressure at room temperature. Finally, the Co-doped sample was obtained after continuous evacuation to eliminate the rest of THF. Ni-doped sample could be prepared in the same way. The size of most Ni and Co particles was below 3 nm, which indicated the peculiarity of the “co-precipitation” method in synthesizing nanoscaled metal catalysts. TPD-MS curves in Figure 7A showed that the catalyst-doped AB started to release H2 at a very low temperature of 50 °C and exhibited broad dehydrogenation processes peaked at about 113 °C. Isothermal dehydrogenation test indicated that nearly 1 Eq of H2 was released from the catalyst-doped AB at 59 °C without any induction period (Figure 7B). However, after being held at this temperature for more than 24 h, there was no evolution of any H2 from pristine AB. Moreover, the detrimental by-product of borazine and sample forming were not observed during the catalytic dehydrogenation. Determined by the Kissinger model, the activation energy
Because the chlorides of cobalt and nickel as precursors have better performance in catalyzing the dehydrogenation of AB, FeCl3 and FeCl2 were tried in the following studies [28,29]. Different amounts of FeCl3 or FeCl2 were introduced in solid AB using a “co-precipitation” method. Generally, FeCl3 exhibited much better performance in catalyzing AB dehydrogenation than FeCl2. Because of the formation of FeB nanoalloy in particle size of 2-5 nm, AB doped with 5 mol% FeCl3 gave excellent performance with an onset temperature for hydrogen release at about 55 °C. Volumetric release tests (Figure 8) showed that FeCl3-doped AB evolved H2 immediately at 60 °C without an induction period observed. With the increase of temperature to 80 °C and 100 °C, it could release more than 1.2 and 1.5 Eq of H2, respectively. Also, sample foaming upon heating and the by-products (borazine and ammonia) were significantly suppressed. More importantly, crystalline linear polyaminoborane (PAB) was found upon dehydrogenation of FeCl3-doped AB, which was the first case on the preparation of crystalline PAB through solid-state reaction. Theoretical calculations showed that the growth of crystalline PAB may follow the dehydrogenation-chain growth mechanism. When the loading amount of FeCl3 was reduced to 2.0 mol%, the optimal performance of the formation of crystalline PAB was achieved.
As shown in Figure 9, a nanocomposite of AB/Li-CMK-3 was successfully synthesized by an impregnation method, i.e., AB loaded into a Li-doped CMK-3 (a class of carbon material with an ordered mesoporous structure) [30]. It could release more than 7 wt% H2 at 60 °C, which was ascribed to the synergy between catalytic effect and nanoconfinement in AB/Li-CMK-3 system. These effects resulted in a significant enhancement of the dehydrogenation kinetics and a great suppression of detrimental by-products. In reference [31], two novel catalysts based on MOFs (denoted as MOF1cat and MOF2cat) were developed, in which in situ-formed metal Ni and the retained frameworks greatly promoted their catalytic activity. Loading with only 1.0 mol% catalyst in AB would give rise to higher dehydrogenation capacity, more rapid hydrogen releasing rates, lower onset temperature, and much less reaction exothermicity. At 90 °C, about 8 wt% H2 could be released from catalyzed samples. The reaction enthalpies of AB dehydrogenation catalyzed by MOF1cat and MOF2cat were determined to be −4.3 and −7.9 kJ mol−1, respectively, much less than −21 kJ mol−1 for pristine AB. The apparent activation energies of 131 and 160 kJ mol−1 were also much lower than 184 kJ mol−1 for pristine AB.
Other catalysts consisting of noble metals were fabricated. For instance, the catalytic dehydrogenation properties of AB by carbon nanotubes functionalized with Pt nanoparticles (denoted as Pt@CNTs) were reported, in which Pt@CNTs was synthesized through an “ammonia-deliquescence” method [32]. From Figure 10, AB/Pt@CNTs had two peaks for hydrogen release at temperatures of about 108 °C and 150 °C. Furthermore, great suppression of borazine and no sample foaming and expansion were observed during the whole dehydrogenation. Of particular concern is achieving much less exothermicity for dehydrogenation. Meanwhile, AB/Pt@CNT had ameliorative dehydrogenation kinetics, i.e., releasing 1 Eq of H2 in 5 h at 70 °C, which was evidenced by the reduced activation energy of 106.2 kJ mol−1. It was also indicated that the combination of the nanoconfinement of AB into CNTs and the synergetically catalytic effects of Pt nanoparticles and CNTs were primarily responsible for enhancing the dehydrogenation properties. In addition, Pd, Pt, and Ni nanoparticles well deposited on MCM-48 (a silica-based material with mesoporous structure) were developed by a magnetron sputtering technique [33]. Then the resultant catalysts were introduced into AB by impregnation method using anhydrous diethyl ether as solvent and displayed obvious catalytic effects for hydrogen release from AB. For example, the onset temperature for dehydrogenation was about 93 °C for AB/MCM-48-Pd (weight ratio of 2:1), with a shift of 12 °C to reduced temperature compared with pristine AB. For all samples catalyzed by well-dispersed Pd, Pt, and Ni nanoparticles on MCM-48, borazine as a detrimental by-product was significantly suppressed, and there was no foaming and expansion during dehydrogenation process.
4. Metal amidoboranes, MABs
In 1938, sodium amidoborane (NaNH2BH3, NaAB, similarly hereinafter for other amidoboranes) was initially prepared by a reaction of borane etherate with sodium and ammonia [34]. For the case of lithium amidoborane (LiNH2BH3, LiAB), it was synthesized through deprotonation of AB in THF using
4.1. Lithium amidoborane (LiNH2BH3, LiAB) and sodium amidoborane (NaNH2BH3, NaAB)
As mentioned above, LiAB or NaAB could be prepared by carefully milling a mixture of AB and an equivalent LiH or NaH. LiAB crystallizes in an orthorhombic cell (space group
LiAB could also be synthesized by reaction of LiH and AB in THF after the evolution of 1 Eq of H2. Surprisingly, it then releases another 2 Eq of H2 in THF at a lowest temperature of 40 °C reported so far [48]. The dehydrogenation of LiAB in THF was determined to be zero-order based on the concentration of LiAB. However, the dehydrogenation mechanism is still unclear and needs further investigations. Through an electrospinning technique, LiAB nanoparticles were well dispersed in carbon nanofibers (denoted as LiAB@CNFs), which had onset and peak temperatures of about 40 °C and 80 °C, significantly lower than pristine LiAB. Upon heating to 100 °C, all hydrogen in LiAB of 10.6 wt% could be released in only 15 min [49]. For the case of NaAB, it could also be prepared through a wet-chemical method by reacting AB with NaH at -3 °C or with NaNH2 at 25 °C in THF [50]. After dehydrogenation of LiAB or NaAB, the solid residues were found to be poorly crystalline phases. A following study indicated that solid NaH was detected for NaAB [50]. However, a recent investigation on the post-dehydrogenated products of NaAB at 200 °C suggested that except NaH, an amorphous phase with the chemical composition of Na0.5NBH0.5 (or regarded as a mixture of NaNBH and
4.2. Lithium amidoborane-ammonia borane (LiNH2BH3⋅NH3BH3, LiAB⋅AB) and β -LiAB
Careful exploration by XRD technique during ball milling of AB and LiH (in 1:1 molar ratio) indicated a stepwise process in LiAB formation [53]. A new complex hydride of lithium amidoborane-ammonia borane (LiNH2BH3⋅NH3BH3, LiAB⋅AB) was formed as an intermediate [38,53]. Pure LiAB⋅AB could be prepared by ball milling a mixture of AB/LiAB or 2AB/LiH [54]. It crystallizes in a monoclinic cell (space group
4.3. Potassium amidoborane (KNH2BH3, KAB)
Through the reaction of AB with an equivalent KH, polycrystalline KAB was synthesized in THF, while single crystal was obtained in a mixed solvent of diglyme and hexane at room temperature [40]. Similar to LiAB and NaAB, KAB crystallizes in an orthorhombic cell (space group
4.4. Magnesium amidoborane (Mg(NH2BH3)2, MgAB)
MgAB is supposed to be a promising material for storing hydrogen because it has a hydrogen content of 11.8 wt%. However, it is unsuccessful to prepare crystalline MgAB either by ball milling method or
4.5. Calcium amidoborane (Ca(NH2BH3)2, CaAB)
Due to the strong coordination between solvent and metal cation, CaAB⋅2THF adduct rather than pure CaAB was first synthesized through reacting CaH2 with two equivalent AB in THF [39]. The specially octahedral coordination of Ca2+ with two NH2, two BH3, and two O resulted in an extended chain-like structure. After desiccation under a dynamic flow of argon, about 10% remaining THF was evolved from 70 °C to 105 °C, and hydrogen was released mainly from 120 to 245 °C. Another adduct of [(DIPP-nacnac)CaNH2BH3(THF)2] was also reported [56], in which Ca2+ is octahedrally coordinated to DIPP-nacnac [(2,6-iPr2C6H3)NC(Me)C(H)C(Me)N(2,6-iPr2C6H3)] bidentate ligand, [NH2BH3]-, THF. Using mechanical milling method from CaH2 and two equivalent AB, Wu et al. [38] synthesized a solvent-free CaAB, which crystallizes in a monoclinic cell (space group
4.6. Strontium amidoborane (Sr(NH2BH3)2, SrAB)
Through reacting SrH2 with AB in a 1:2 molar ratio, SrAB was successfully synthesized by moderate milling and the following heating treatment at 45 °C [41]. Similar to CaAB, SrAB also crystallizes in a monoclinic cell (space group
4.7. Yttrium amidoborane (Y(NH2BH3)3, YAB)
Ball milling YCl3 with three equivalent LiAB gave rise to the formation of YAB [42]. In fact, it was a homogenous mixture with a side product of LiCl, which is very difficult to separate. YAB has a monoclinic cell (space group
4.8. Na[Li(NH2BH3)2]
Na[Li(NH2BH3)2] was reported to be the first mixed-cation amidoborane, which was prepared by milling a mixture of NaH/LiH/2AB performed on a high-energy disk mill [58]. It crystallizes in a triclinic cell (space group
4.9. NaMg(NH2BH3)3
Almost at the same time, another dual-metal (alkali metal and alkaline-earth metal) amidoborane, namely, NaMg(NH2BH3)3, was reported [60]. It was prepared by mechanically milling 3AB/NaMgH3 powdery mixture and the following treatment at 45 °C according to the reaction equation: 3NH3BH3 + NaMgH3
4.10. Na2Mg(NH2BH3)4
Na2Mg(NH2BH3)4 was first synthesized by ball milling a mixture of NaH/MgH2/2AB [61]. It was identified to have a space group of
4.11. K2Mg(NH2BH3)4
K2Mg(NH2BH3)4 was synthesized by ball milling method according to the reaction equation: 2Mg(NH2BH3)⋅NH3 + 2KH
4.12. (Li,Al) amidoborane
Bimetallic (Li,Al) amidoborane was prepared by ball milling of 4AB/Li3AlH6 mixture [63]. It was also observed after heating a post-milled mixture of LiAlH4/AB at 170 °C [64]. The information of crystal structure was not reported in both references [63,64]. It could release about 10 wt% H2 at 200 °C, with a significant suppression of volatile by-products. The post-dehydrogenated products of (Li,Al) amidoborane could be regenerated through reduction by hydrazine in liquid ammonia. Around 3.5 wt% H2 was released upon heating to 400 °C, corresponding to a 35% regeneration yield [63].
5. Metal amidoborane ammoniates
Ammonia was a hydrogen-rich compound (17.7 wt%). As a strong Lewis base, it is used as a ligand to form amine complexes with various compounds for hydrogen storage [65-69]. It is noted that because of the difference in electronegativity, positively charged H in NH3 can interact with protic H in metal amidoboranes and borohydrides for facilitating hydrogen release.
5.1. Lithium amidoborane ammoniate (LiNH2BH3⋅NH3, LiAB⋅NH3)
A new hybrid material with the formula of LiNH2BH3NH3 was produced from the mixed AB and LiNH2 in a 1:1 molar ratio, which can be treated as a monoammoniate of LiAB. It rapidly released 11.9 wt% H2 under 250 °C, with the most rapid rate observed at 60 °C [70]. In fact, LiAB⋅NH3 was synthesized by reacting LiAB with an equivalent NH3 [71]. At room temperature, it is a sticky matter in an amorphous state. Upon cooling to −20 °C, LiAB⋅NH3 becomes a solid and crystallizes in an orthorhombic cell (space group
5.2. Calcium amidoborane diammoniate (Ca(NH2BH3)2⋅2NH3, CaAB⋅2NH3)
In 2009, Chua et al. [72] reported the synthesis, structure, and dehydrogenation of CaAB⋅2NH3. They provided two synthesizing methods, i.e., ball milling of Ca(NH2)2/2AB mixture and reacting CaAB with two equivalent NH3. It crystallizes in an orthorhombic cell (space group
5.3. Calcium amidoborane monoammoniate (Ca(NH2BH3)2⋅NH3, CaAB⋅NH3)
CaAB⋅NH3 was prepared by ball milling a mixture of CaNH/2AB [73]. Alternatively, ball milling a mixture of Ca(NH2)2/CaH2/4AB could also give rise to the formation of CaAB⋅NH3 with accompanying 2 Eq of H2 released. It crystallizes in a monoclinic cell (space group of
5.4. Magnesium amidoborane monoammoniate (Mg(NH2BH3)2⋅NH3, MgAB⋅NH3)
Ball milling a mixture of Mg(NH2)2/2AB could give rise to the formation of magnesium amidoborane diammoniate (Mg(NH2BH3)2⋅2NH3, MgAB⋅2NH3). Unlike solid CaAB⋅2NH3, MgAB⋅2NH3 is a liquid matter at ambient condition. Upon releasing 1 Eq of NH3, solid MgAB⋅NH3 was formed [74]. Alternatively, MgAB⋅NH3 could also be prepared by ball milling a mixture of MgNH/2AB [74]. In a monoclinic cell (space group of
6. Metal borohydride-ammonia borane complexes
6.1. 2LiBH4⋅NH3BH3, 2LiBH4⋅AB
In 2010, the first metal borohydride-ammonia borane complex, namely, 2LiBH4⋅AB, was reported [75]. It was synthesized by ball milling AB with two equivalent LiBH4 and had an orthorhombic structure (space group
6.2. LiBH4⋅NH3BH3, LiBH4⋅AB
LiBH4⋅AB was prepared by mechanically milling AB with an equivalent LiBH4 [77]. It crystallizes in a monoclinic cell (space group
6.3. Ca(BH4)2⋅NH3BH3, Ca(BH4)2⋅AB
Ca(BH4)2⋅AB was prepared by ball milling Ca(BH4)2 with an equivalent of AB, which had an orthorhombic cell (space group
6.4. Mg(BH4)2⋅2NH3BH3, Mg(BH4)2⋅2AB
Mg(BH4)2⋅2AB was prepared by ball milling Mg(BH4)2 with 2 equivalent AB [78,79]. It has an orthorhombic unit cell (space group
6.5. Al(BH4)3⋅NH3BH3, Al(BH4)3⋅AB
Al(BH4)3⋅AB was synthesized through a reaction between Al(BH4)3 and an equivalent AB in a closed reactor at room temperature for 3 days, which could be accelerated by ball milling [80]. The resulting white crystals of Al(BH4)3⋅AB revealed two polymorphs: the low-temperature
7. Other derivatives
7.1. Mg(BH4)2⋅2NH3⋅NH3BH3, Mg(BH4)2⋅2NH3⋅AB
Mg(BH4)2⋅2NH3⋅AB was prepared by ball milling Mg(BH4)2⋅2NH3 with an equivalent AB [78]. It had a tetragonal cell in space group of
For dehydrogenation improvement, Zr(BH4)4⋅8NH3 was ball milled with four equivalent AB. Although no new compound formed after ball milling and the subsequent heating treatment, it had an onset temperature at about 60 °C for dehydrogenation and could release 7.0 wt% H2 at 100 °C in 45 min [81].
7.2. LiCa(NH2)3(BH3)2
LiCa(NH2)3(BH3)2 was reported to be prepared by ball milling CaAB with an equivalent LiNH2 [82]. Primary analysis showed that it crystallized in a monoclinic cell with lattice parameters of
7.3. Sodium aminodiborane (NaNH2(BH3)2)
NaNH2(BH3)2 with an infrequent NH2(BH3)2− anion was first reported by Girolami and coworkers in 2010 [83]. It was synthesized by the reduction of AB with excess Na in THF at room temperature and the following reflux. However, the target compound failed to isolate from the resulting THF adduct. In a very recent paper [84], solvent-free NaNH2(BH3)2 was prepared by reacting NaNH2 and two equivalent AB in THF. In a closed reactor, NaNH2(BH3)2 could release 2 Eq of H2 (6.0 wt%) at 271 °C, forming solid products of NaBH4 and highly condensed polyborazylene.
7.4. LiNH2BH3⋅LiBH4, LiAB⋅LiBH4
LiAB⋅LiBH4 was synthesized by ball milling LiAB with an equivalent LiBH4 [85]. It was ascribed to an orthorhombic cell (space group
7.5. M(BH3NH2BH2NH2BH3), M = Li or Na
Preparation of M(BH3NH2BH2NH2BH3) (denoted as M(B3N2), M = Li or Na) was performed through a wet-chemical method in THF, according to the reaction: MH + 3NH3BH3
8. Conclusions
In this chapter, metal amidoboranes, metal borohydride-ammonia borane complexes, and metal amidoborane ammoniates as well as their derivatives were summarized with focus on their structures and dehydrogenation properties. In addition, nanoconfinement and nanocatalysis of AB for improving the dehydrogenation were also included. From scientific point of view, some new B−N-containing compounds and their crystal structures were reported, which enriches the chemistry of the scope of B and N elements. For the case of practical applications, the fast dehydrogenation rate at around 60 °C and the stability under 40 °C is one of the most important targets. Due to the difficulties in reversible hydrogenation of AB-related materials, off-board regeneration of spent fuel with high-energy efficiency is another issue to be solved. This would make it vitally important for altering the dehydrogenation thermodynamics from exothermicity to endothermicity. What is exciting is the fact that some cases of endothermic dehydrogenation were observed [60,62,80]. In this regard, it is indispensable for material design. That means that both a large number of experimental studies and the related theoretical calculations are urgently needed in the future. Moreover, catalytic modification should be considered for the dehydrogenation kinetics. In addition, how to eliminate the un-wanted by-products including ammonia, diborane, and borazine in the evolved hydrogen is also a critical issue, which needs special attention to be given because they are detrimental to proton exchange membrane (PEM) fuel cell, especially ammonia even with trace amount. As an example, there is a controversial debate of NaAB dehydrogenation for ammonia contamination [36,49,51]. Therefore, the mechanism for NH3 formation during dehydrogenation is another task of further study.
Acknowledgments
The related research in this chapter was financially supported by the National Natural Science Foundation of China (51361006, 51401059, 51461010, 51361005, and 51371060), the Guangxi Natural Science Foundation (2014GXNSFAA118043, 2014GXNSFAA118333, 2013GXNSFBA019034, and 2013GXNSFBA019239), and the Guangxi University Research Project (YB2014132). This work was partially sponsored by Guangxi Collaborative Innovation Center of Structure and Property for New Energy Materials.
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