Open access peer-reviewed chapter
By Hailiang Chu, Shujun Qiu, Lixian Sun and Fen Xu
Submitted: October 27th 2014Reviewed: June 12th 2015Published: November 25th 2015
DOI: 10.5772/61048
Downloaded: 1039
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.
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.
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.
TPD/MS tests of AB nanoconfined in SBA-15 (AB:SBA-15). For comparison, neat AB was also included. Reproduced from Ref. [6] with permission.
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% vs. 13 wt%). When confined in carbon cryogel, AB underwent the expected reactions for dehydrogenation. Unexpectedly, the enthalpy of dehydrogenation reaction for C−AB nanocomposite was determined to be an extremely high value of ~120 kJ mol−1 (compared to pristine AB of 21 ± 1 kJ mol−1 and AB:SBA-15 of 1 ± 1 kJ mol−1). However, it remains so far unclear what caused the tremendous increase in reaction enthalpy. The authors suggested that the improvements (for AB loaded into both carbon cryogel and SBA-15) were ascribed to the increase of defect sites and surface energy of AB, resulting from nanoconfinement and/or the possible catalysis by the terminal SiO−H groups in SBA-15 and carboxylic groups in carbon cryogel. For carbon cryogel used as a nanoscaffold, the smaller the pore diameter, the lower the temperature for the dehydrogenation. In-depth studies of pore size effect on the structure and dehydrogenation are needed in the forthcoming research.
(A) DTA, MS for (B) H2 and (C) borazine of AB nanoconfined in carbon cryogel (C-AB nanocomposite) at 1 °C/min. For comparison, neat AB was also included. Reproduced from Ref. [7] with permission.
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.
(A) 3D images of (a) JUC-32-Y and (b) AB/JUC-32-Y. Views of 1D chains of JUC-32-Y (c) before and (d) after removal of terminal H2O and (e) after interaction with AB. Y: bright blue; C: gray; O: pink; N: blue; and B: purple. (B) TPD-MS and (C) Isothermal dehydrogenation of AB/JUC-32-Y. For comparison, neat AB was also included. Reproduced from Ref. [23] with permission.
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.
TPD-MS of AB, AB/Ni@MIL-101, and AB/MIL-101. Reproduced from Ref. [25] with permission.
Ni1-xPtx (x = 0-0.12) hollow spheres were prepared with an assistance of a template called as poly(styrene-co-methacrylic acid) and then used as catalysts for generating hydrogen from AB [26]. The morphology and the composition of the obtained Ni0.88Pt0.12 sample were shown in Figure 5. There are both broken and intact hollow spheres appeared in the sample. TEM images revealed that the porous shells of hollow spheres with a thickness of 20-40 nm contained many smaller nanowhiskers and nanoparticles, which enabled the increase of specific surface area of hollow spheres. Then the hollow spheres with a chemical composition of Ni0.88Pt0.12 were introduced into a methanol solution of AB. The mixture was vacuumed to obtain a dry solid sample with homogeneous loading of about 1.8 wt% Pt. TPD tests in Figure 6 showed that the onset temperature for hydrogen release was reduced to about 56 °C and the peak shifted to about 100 °C.
(a) SEM image, (b, c, d) TEM images, and (e) HRTEM image and (f) EDX spectrum of Ni0.88Pt0.12 hollow spheres. Reproduced from Ref. [26] with permission.
TPD-MS of AB mixed with Ni0.88Pt0.12 hollow spheres (red line). For comparison, neat AB (black line) was also included. Reproduced from Ref. [26] with permission.
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 Ea was reduced to 117 and 123 kJ mol−1 for Co- and Ni-doped AB. For the case of Co-doped AB, from electron paramagnetic resonance (EPR) characterization, Co2+ was partially reduced in the preparation, which was regarded as the catalytically active species for AB dehydrogenation.
(A) TPD-MS of pristine and 2.0 mol% Co- or Ni-doped AB samples: (a) H2 signal and (b) borazine signal. (B) Isothermal hydrogen release tests at 59 °C on (a) pristine, (b) Co-doped, and (c) Ni-doped AB samples. Reproduced from Ref. [27] with permission.
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.
Volumetric release of 5.0 mol% FeCl3-doped AB samples (□). For comparison, pristine AB (▲) was also included as a reference. Reproduced from Ref. [28] with permission.
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.
Schematic model of AB/Li-CMK-3 nanocomposites (AB: dark gray spheres; Li: light gray spheres; CMK-3 substrate: gray rod). Reproduced from Ref. [30] with permission.
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.
TPD-MS of pristine AB and loaded AB/Pt@CNTs. Reproduced from Ref. [32] with permission.
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 n-butyllithium at 0 °C and then used as a reducing reagent [35]. However, the crystal structure of LiAB and NaAB was not reported until 2008 [36-38]. A solid-state synthetic method was developed for MABs through controlled mechanical ball milling AB with MH (M = Li or Na) in an equivalent molar amount. Calcium amidoborane (Ca(NH2BH3)2, CaAB) was prepared by a wet-chemical reaction between AB and CaH2 in THF [39]. As a matter of fact, CaAB⋅2THF rather than pure CaAB was obtained in final product due to the strong coordination of THF with Ca2+. Wu et al. [38] synthesized solvent-free CaAB by ball milling solid-state CaH2 and AB. More recently, some new MABs such as potassium amidoborane (KNH2BH3, KAB) [40], strontium amidoborane (Sr(NH2BH3)2, SrAB) [41], and yttrium amidoborane (Y(NH2BH3)3, YAB) [42] were successfully synthesized. In general, there are two synthetic approaches for the preparation according to the reported MABs, i.e., solid-state mechanical ball milling and wet-chemical synthesis. In solid-state ball milling method, the chemical interaction between protic H and hydridic H was regarded as one of main driving forces for hydrogen release [43-45]. It should be pointed out that the milling conditions are crucial during the synthesis of MABs because of their lower temperatures for dehydrogenation. In a wet-chemical reaction, the adducts rather than pure MABs are always obtained because the solvent is easily coordinated with amidoborane, especially alkaline-earth metal amidoboranes [39]. Because of relatively strong combination between solvent molecules and metal cations, special procedures are needed for removal of solvent, during which the decomposition of MABs should be avoided.
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 Pbca) with lattice parameters of a = 7.11274(6) Å, b = 13.94877(14) Å, and c = 5.15018(6) Å (Figure 11) [36]. Moreover, NaAB is an isostructural compound with LiAB and the lattice parameters are a = 7.46931(7) Å, b = 14.65483(16) Å, and c = 5.65280(8) Å [36]. In their crystal structures, the bond lengths of Na-N and Li-N are 2.35 and 1.98 Å. The substitution of one H atom in NH3 group by Li+ gave rise to the slight shortening of bond length for B-N (1.56 Å) compared to that in AB (1.58 Å) [46]. Moreover, the B-H distance is 1.079 Å in LiAB and 1.245 Å averagely in NaAB, which are different from that that in AB (1.11 Å) [47]. The establishment of an ionic Li-N or Na-N bond and the resulting reinforce of B-N bond in [NH2BH3]- group in LiAB or NaAB resulted in an improved dehydrogenation properties. LiAB and NaAB started to release H2 at 85 °C and had a peak at about 90 °C [36,37]. In addition, no borazine was detectable during the dehydrogenation. Furthermore, the heat for dehydrogenation from LiAB and NaAB was determined to be ~-3 and -5 kJ (mol H2-1), remarkably less exothermic than that of pristine AB (-21 kJ (mol H2-1)) [4]. At 91 °C, about 7.4 and 11 wt% H2 could be released from NaAB and LiAB, respectively (Figure 12).
Crystal structure of LiAB. B: orange spheres; N: green spheres; H: white spheres; and Li: red spheres. Reproduced from Ref. [36] with permission.
Isothermal dehydrogenation of LiAB, NaAB, and post-milled AB samples at about 91 °C. Reproduced from Ref. [36] with permission.
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 h-BN) was identified by NMR technique [51]. Unfortunately, the results from Fijałkowski et al. [52] showed that a substantial amount of NH3 was released during synthesis by ball milling and the following thermal decomposition of NaAB.
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 P21/c) with lattice parameters of a = 7.0536(9) Å, b = 14.8127(20) Å, c = 5.1315(7) Å, and β = 97.491(5)°. There is the intergrowth of AB and LiAB layers in crystal structure of LiAB⋅AB. As shown in Figure 13, each Li+ is bonded with one N atom from [NH2BH3]- with a Li-N bond in 2.05 Å and coordinated with hydrogen atoms from BH3 group in two LiAB and one AB. The dihydrogen bonding of NH⋅⋅⋅HB is 1.902 Å in AB layer. About 14.0 wt% H2 could be released at 228 °C from LiAB⋅AB after its melting with an onset temperature of 58 °C. Furthermore, another allotrope of β-LiAB was found during ball milling process. Interestingly, it transformed from initially formed α-LiAB and could transform to α-LiAB upon extended milling [53]. β-LiAB also crystallizes in an orthorhombic cell (space group Pbca) with lattice parameters of a = 15.15 Å, b = 7.726 Å, and c = 9.274 Å. However, its unit cell has a double volume of α-LiAB, in which two alternative LiAB layers were observed along the a axis. The coordination of Li+ was found to be distortedly tetrahedral in β-LiAB due to different distances for Li1-B and Li2-B (Figure 13b). The bond length of Li-N (1.93 and 2.04 Å for Li1-N and Li2-N) is similar to that in α-LiAB (1.98 [36] or 2.02 Å [53]). Both allotropes of LiAB displayed identical dehydrogenation behaviors, i.e., 10.8 wt% H2 could be released at 180 °C [53].
Coordination of Li+ in (a) α-LiAB, (b) β-LiAB, and (c) LiAB⋅AB. Li: purple spheres; B: brown spheres; N: blue spheres; and H: white spheres. Reproduced from Ref. [53] with permission.
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 Pbca) with lattice parameters of a = 9.4304(1) Å, b = 8.26112(1) Å, and c = 17.3403(2) Å. However, different from Li+ in LiAB and Na+ in NaAB, K+ is octahedrally coordinated by [NH2BH3]- groups with the formation of three K-N bonds (3.0207-3.1345 Å) and three K⋅⋅⋅BH3 coordinations (Figure 14A). The distance of closest NH⋅⋅⋅HB (2.265 Å) in KAB is smaller than those in LiAB (2.372 Å) and NaAB (2.717 Å). As shown in Figure 14B, an endothermic peak for melting was observed before an exothermic event for hydrogen release. It could release 1.5 Eq of H2 (6.5 wt%) at ~80 °C. With the temperature increase to 160 °C, another 0.5 Eq of H2 was released. No ammonia or borazine was detected throughout the dehydrogenation.
(A) Crystal structure of KAB. K: green spheres; N: blue spheres; B: cream spheres; and H: white spheres. (B) (a) DSC and (b) IGA data for KAB decomposition. Reproduced from Ref. [40] with permission.
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 via wet-chemical method until the beginning of 2013. MgAB was synthesized through aging treatment of post-milled 2AB/MgH2 or 2AB/Mg powder, in which the formation of MgAB was not a straightforward process but experienced a prerequisite step of phase transition [55]. However, the crystal structure was failed to solve. In 2010, its structure was successfully proposed through theoretical simulation [15]. MgAB has a monoclinic cell (space group C2) with lattice parameters of a = 8.5772 Å, b = 5.6048 Å, c = 5.6216 Å, and β = 85.8476°. Two Mg-N bonds with the length of 2.111 Å and two Mg⋅⋅⋅BH3 coordinations are around Mg2+ (Figure 15). As a stable compound at room temperature, MgAB could release ~10 wt% H2 at 300 °C, without volatile by-products detectable [55].
Simulated crystal structure of MgAB. Mg: green spheres; N: blue spheres; B: orange spheres; and H: white spheres. Reproduced from Ref. [15] with permission.
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 C2) with lattice parameters of a = 9.100(2) Å, b = 4.371(1) Å, c = 6.441(2) Å, and β = 93.19°. Ca2+ is octahedrally coordinated through two Ca-N bonds with the length of ~2.466 Å and four coordinations of Ca⋅⋅⋅BH3 with the distance in the range of 2.87-3.03 Å (Figure 16). Different from the CaAB adducts, there are two steps for hydrogen release with peaks at 100 and 140 °C. Upon heating to 250 °C, 2 Eq of H2 was released with the solid residues in amorphous state.
(Top) Crystal structure of CaAB. Ca: orange spheres; B: green spheres; N: blue spheres; and H: white spheres. (Bottom) Coordination environment of Ca2+. Reproduced from Ref. [38] with permission.
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 C2) with lattice parameters of a = 8.1660(4) Å, b = 5.0969(3) Å, c = 6.7258(4) Å, and β = 94.392(4)°. Sr2+ is octahedrally coordinated by [NH2BH3]- groups to form two Sr-N bonds with a length of 2.68 Å and four Sr⋅⋅⋅BH3 coordinations (Figure 17). Upon heating, SrAB started to decompose into H2 and Sr(NBH)2 at about 60 °C. In an isothermal dehydrogenation performed at a temperature of 80 °C, 4 Eq of H2 could be rapidly released in a few minutes. However, the evolved hydrogen was contaminated by undesirable by-products of NH3 and B2H6 evolved from the decomposition of the resulting Sr(NBH)2.
Crystal structure of SrAB. Sr: red spheres; N: yellow spheres; and B: blue spheres. Reproduced from Ref. [41] with permission.
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 C2/c) with lattice parameters of a = 13.18902(63) Å, b = 7.82233(38) Å, c = 14.874274(68) Å, and β = 92.42620(40)°. However, it was thermodynamically unstable and spontaneously decomposed in several days at room temperature. A large amount of H2 with NH3 impurity was released from freshly prepared sample upon heating to 250 °C. Motivated by the synthesis of YAB, the attempt to synthesize iron (III) amidoborane (Fe(NH2BH3)3, FeAB) through metathesis of FeCl3 and three equivalent LiAB in THF was unsuccessful [57]. Instead of FeAB formation, 1.5 Eq of H2 was released and a black solid containing LiCl was produced. Meanwhile, Fe3+ was reduced during dehydrogenation evidenced by Mössbauer and XAFS characterizations.
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
Crystal structure of Na[Li(NH2BH3)2] determined theoretically (A) and experimentally (B). Coordinations of Li+ and Na+ are displayed under each structure. Li: big red spheres; Na: big yellow spheres; N: small light blue spheres; B: small green spheres; and H: small pink spheres. Reproduced from Ref. [59] with permission.
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
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 I41/a and approximate lattice parameters of a = 9.415 Å and c = 12.413 Å. The crystal structure illustrated in Figure 19 shows that Mg2+ is coordinated with four [NH2BH3]− groups via Mg-N bonds (2.104 Å) to form [Mg(NH2BH3)4]2− tetrahedron. Na+ is octahedrally coordinated only with six BH3 groups with the distances of Na-B ranging from 2.900 to 3.634 Å. The distances of Na-H coordinations between Na+ and its close hydridic H in BH3 groups are in the range of 2.383-2.943 Å. As a consequence, Mg2+ and Na+ are linked through different coordinations with both ends of the bridging [NH2BH3]− groups and form an ordered structure in coordinations. Na2Mg(NH2BH3)4 had an onset temperature for dehydrogenation at ~65 °C and a total amount of 8.4 wt% H2 was released upon heating to 200 °C, with contamination by trace amount of borazine and ammonia. In another paper [62], a wet-chemical method was proposed to synthesize pure Na2Mg(NH2BH3)4. Typically, a solid mixture of Mg(NH2)2/2NaH/4AB was introduced into THF. After 0.7 Eq of gaseous products (H2 and NH3) was released, pure Na2Mg(NH2BH3)4 as the precipitate was obtained through filtration, which agrees well with the one synthesized by ball milling method [61].
Crystal structure of Na2Mg(NH2BH3)4. Na: pink spheres; Mg: yellow spheres; N: blue spheres; B: orange spheres; and H: white spheres. Reproduced from Ref. [61] with permission.
K2Mg(NH2BH3)4 was synthesized by ball milling method according to the reaction equation: 2Mg(NH2BH3)⋅NH3 + 2KH
Crystal structure of (a) K2Mg(NH2BH3)4, (b) Na2Mg(NH2BH3)4, (c) Coordination of Mg2+, and (d) Coordination of K+. Reproduced from Ref. [62] with permission.
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].
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.
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 Pbca), which has the lattice parameters of a = 9.711(4) Å, b = 8.7027(5) Å, and c = 7.1999(1) Å. However, the crystal structure was not reported in reference [71]. At room temperature, LiAB⋅NH3 decomposed into ammonia and LiAB under vacuum. However, at temperatures more than 40 °C, favorable dehydrogenation was observed. Volumetric release test showed that about 6.2 wt% H2 could be released at 70 °C under argon, while 11.2 wt% H2 at 60 °C under ammonia.
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 Pna21) with lattice parameters of a = 18.673(3) Å, b = 5.2283(8) Å, and c = 8.5748(12) Å. Figure 21 shows that each Ca2+ has octahedral coordination through two Ca⋅⋅⋅NH3 coordinations (2.517 and 2.521 Å), two Ca⋅⋅⋅BH3 coordinations (2.650 and 2.807 Å), and two Ca-N bonds (2.397 and 2.521 Å), which results in a bonding network of NH⋅⋅⋅HB dihydrogen due to NH3 coordinations. CaAB⋅2NH3 decomposed into CaAB and 2 Eq of NH3 under 100 °C in an open system. On the contrary, in a closed reactor, H2 rather than NH3 was released predominantly due to the existence of NH3 equilibrium pressure, with an onset temperature at ~70 °C. Almost 9 wt% H2 could be released upon heating to 300 °C. Compared to CaAB, the onset temperature has a 50 °C reduction and the dehydrogenation capacity at 300 °C has a 2 wt% increase.
Crystal structure of CaAB⋅2NH3. Ca: green spheres; N: blue spheres; B: pink spheres; and H: white spheres. Reproduced from Ref. [72] with permission.
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 P21/c) with lattice parameters of a = 10.5831(14) Å, b = 7.3689(11) Å, c = 10.2011(13) Å, and β = 120.796(6)° [73]. Unlike CaAB⋅2NH3, Ca2+ in CaAB⋅NH3 has a geometry of trigonal bipyramid (Figure 22), in which Ca2+ is surrounded by two NH2 from [NH2BH3]− groups through Ca-N bonds (2.400 and 2.465 Å), two BH3 from [NH2BH3]− groups through Ca⋅⋅⋅BH3 interactions (2.669-3.182 Å), and one NH3 through Ca⋅⋅⋅NH3 interaction (2.472 Å). The special structure resulted in the network of NH⋅⋅⋅HB dihydrogen bond. TG-DSC-MS tests revealed that NH3 was predominantly released under 100 °C with a weight loss of 12.6%, while 6.2 wt% H2 was solely released from 100 to 300 °C. Compared with the theoretical value for deammoniation of CaAB⋅NH3 (~14.5 wt%) and for dehydrogenation of CaAB (~8 wt%), some NH3 should participate in dehydrogenation under 100 °C, which is evidenced by the concurrent release of H2 from MS. However, in a closed reactor, hydrogen release in two steps was observed with an exothermic event detected in the first step. At 300 °C, 6 Eq of H2 (~10.2 wt%) could be released, in which the amount of by-product NH3 is less than 0.1 Eq.
A) Crystal structure of CaAB⋅NH3. (B) Coordination of Ca2+. Ca: green spheres; N: blue spheres; B: pink spheres; and H: white spheres. Reproduced from Ref. [73] with permission.
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 P21/a), MgAB⋅NH3 has the lattice parameters of a = 8.8815(6) Å, b = 8.9466(6) Å, c = 8.0701(5) Å, and β = 94.0744(48)°. As shown in Figure 23, each Mg2+ has a tetrahedral coordination, in which Mg2+ forms two Mg-N bonds (2.104 and 2.129 Å) with the adjacent [NH2BH3]− groups, a Mg⋅⋅⋅NH3 coordination (2.157 Å) with the adjacent NH3, and a Mg⋅⋅⋅BH3 coordination (2.126 Å) with the adjacent [NH2BH3]− group. In MgAB⋅NH3, a dihydrogen bonding network was established through the interactions of NH⋅⋅⋅HB between NH3 and BH3 groups. Different from CaAB⋅2NH3 and CaAB⋅NH3, the decomposition of MgAB⋅NH3 in an open system exhibited direct desorption of H2 instead of NH3, indicating easier interaction between Hδ+ in NH3 and Hδ- in BH3 to the formation of H2 or stronger Mg⋅⋅⋅NH3 coordination. Hydrogen release of MgAB⋅NH3 started at about 50 °C, with 5.7 Eq of H2 (11.4 wt%) released up to 300 °C. An amorphous solid residue with chemical composition of MgB2N3H was left. NH3 was undetectable during the whole dehydrogenation process, which evidenced the complete conversion of NH3.
(a) Crystal structure of MgAB⋅NH3, and (b) Coordination of Mg2+. Mg: green spheres; N: blue spheres; B: orange spheres; and H: white spheres. Reproduced from Ref. [74] with permission.
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 Pnma) with lattice parameters of a = 8.3118(8) Å, b = 12.428(1) Å, and c = 6.5944(7) Å. Figure 24 shows that there are alternating layers of borohydride and AB in 2LiBH4⋅AB. And Li+ and BH4− in the layer of borohydride remain their original configuration in pristine LiBH4. Li+ has a tetrahedral coordination with one AB and three BH4− groups. The distances between Li and H in BH4− are in the range of 2.023-2.246 Å, analogous to those between Li and its two close hydridic H in AB (2.078 and 2.321 Å). The BH⋅⋅⋅HN distances between BH4− and its nearby AB range from 2.248 to 2.254 Å, whereas the BH⋅⋅⋅HN distance between the adjacent AB is 2.439 Å. Such a weak BH⋅⋅⋅HN interaction in AB layers may be ascribed to the reinforced interactions of AB with the nearby H in BH4− and also with Li+. Consequently, the intercalation of AB molecules into the crystal structure of LiBH4 is basically stabilized by the interactions of AB with Li+ and BH4−. There were two steps for dehydrogenation of 2LiBH4⋅AB, i.e., the first step occurring at 105-135 °C and the second one at 360 °C, which are determined to be the dehydrogenation of pristine AB and LiBH4, respectively. In addition, the suppression of ammonia was achieved compared with pristine AB. It is worth noting that the dehydrogenated products of 2LiBH4⋅AB could reabsorb 2.4 wt% H2 at 400 °C under 82 bar H2 pressure. In contrast, MBH4-AB (M = Na, K) do not form new compounds during mechanochemical treatment [76].
Crystal structure of (a) 2LiBH4⋅AB and (b) Ca(BH4)2⋅AB. Coordination of (c) Li+ and (d) Ca2+. Li: pink spheres; Ca: orange spheres; B: green spheres; N: blue spheres; and H: white spheres. Reproduced from Ref. [75] with permission.
LiBH4⋅AB was prepared by mechanically milling AB with an equivalent LiBH4 [77]. It crystallizes in a monoclinic cell (space group P21) with lattice parameters of a = 14.3131 (11) Å, b= 4.3634 (5) Å, c= 15.3500(13) Å, and β= 90.325(11)°. In LiBH4⋅AB, there are LiBH4 columns running along the b axis (Figure 25), which is notably different from 2LiBH4⋅AB. LiBH4 columns are segregated by the adjacent AB molecules and thus lose the structural similarity to pristine one. In such an arrangement, Li+ has two different coordination environments, both of which have a trigonal-planar coordination. One is coordinated by two BH4− and AB groups and the other by three BH4− groups. The unique coordination of Li+ results in the remarkable variations of the interactions of Li+ with its adjacent anions. For example, the distances of Li−B are ranging from 2.22 to 2.81 Å. While the distances between Li and its two close hydridic H in AB are 1.78 and 2.07 Å, much smaller than those in 2LiBH4⋅AB (2.08 and 2.32 Å) [75], which indicates a stronger interaction between AB and Li+. The BH⋅⋅⋅HN distances between AB and the adjacent BH4− are in a range of 1.85−2.31 Å, and those between nearby AB in AB layers are 1.86−2.39 Å, which demonstrates the presence of strong dihydrogen bonding throughout LiBH4⋅AB. For thermolysis, LiBH4⋅AB first decomposed into 2LiBH4⋅AB and AB at about 54 °C and then exhibited a three-step dehydrogenation, with release of a total amount of ~15.7 wt% H2 upon heating to 450 °C. Interestingly, h-BN was found to form in the post-dehydrogenated product at a moderate temperature of 450 °C (compared with other methods for h-BN preparation), which resulted in the improved rehydrogenation and the following dehydrogenation properties of LiBH4⋅AB sample.
(a) Crystal structure of LiBH4⋅AB. (b) Li+ coordination. (c) Schematic arrangement of LiBH4 and AB components in the crystal structure. Li: yellow spheres; B: green spheres; N: blue spheres; and H: white spheres. Reproduced from Ref. [77] with permission.
Ca(BH4)2⋅AB was prepared by ball milling Ca(BH4)2 with an equivalent of AB, which had an orthorhombic cell (space group Aba2) with lattice parameters of a = 8.265(1) Å, b = 13.478(2) Å, and c = 8.136(1) Å [75]. Each Ca2+ has an octahedral coordination, surrounded by two AB molecules with Ca−B distances ranging from 2.899 to 2.923 Å and four BH4− groups (Figure 24). Unlike 2LiBH4⋅AB, the BH⋅⋅⋅HN distance between neighboring AB in AB layers of Ca(BH4)2⋅AB is 1.735 Å, which indicates more powerful dihydrogen bonding. Through dihydrogen bonding, AB also interacts with its adjacent BH4− with the BH⋅⋅⋅HN distances of 1.986−2.037 Å. Furthermore, the distances between Ca2+ and its two close hydridic H of AB are 2.441 and 2.504 Å, similar to those between Ca2+ and H in BH4− (2.412−2.477 Å). As a consequence, different from 2LiBH4⋅AB, AB layers and Ca(BH4)2 layers are interlaced primarily by dihydrogen bonding in Ca(BH4)2⋅AB. For the case of hydrogen release, it has similar dehydrogenation behaviors with 2LiBH4⋅AB, i.e., independent dehydrogenation of AB and Ca(BH4)2 was observed, although the dehydrogenation temperature was slightly reduced to some extent.
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 P212121) with lattice parameters of a = 14.4135(2) Å, b = 13.2084(2) Å, and c = 5.1118(1) Å. As shown in Figure 26, each Mg2+ has a tetrahedral coordination, surrounded by two BH4− groups and two AB groups with Mg-B distances in the range of 2.394−2.500 Å. The distances between Mg2+ and its close H in AB range from 2.062 to 2.106 Å, while the distances between Mg2+ and its close H in BH4− are in the range of 1.987-2.033 Å. Between AB and its nearby BH4− groups, there are BH⋅⋅⋅HN dihydrogen bonds with the distances in the range of 2.210-2.274 Å. Mg(BH4)2⋅2AB melted at 48 °C and started to release H2 at 75 °C [79]. It had a broad dehydrogenation peak at 126 °C with a releasing trace amount of NH3, B2H6, and borazine. The following dehydrogenation in the range of 200-400 °C was observed with two peaks at 297 °C and 340 °C. Volumetric release tests implied that the dehydrogenation behaviors of Mg(BH4)2⋅2AB look like a combination of pristine AB in 30-200 °C and Mg(BH4)2 in 200-400 °C.
Crystal structure of (a) Mg(BH4)2⋅2AB and (b) Mg(BH4)2⋅2NH3⋅AB. Coordination of Mg2+ in (c) Mg(BH4)2⋅2AB and (d) Mg(BH4)2⋅2NH3⋅AB. Mg: red spheres; B: pink spheres; N: blue spheres; and H: white spheres. Reproduced from Ref. [78] with permission.
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 α-phase and the high-temperature β-phase. Pure α-phase was found only in fresh samples, and it slowly transformed to β-phase at room temperature. Pure β-phase was obtained upon heating α-phase to about 62 °C with 1 °C min−1, and it could not transform back to α-phase even cooling to -173 °C. Both phases crystallize in monoclinic cells with space group of P21/c and Cc for α- and β-phase, respectively. α-phase has the lattice parameters of a = 7.8585(2) Å, b = 6.86473(14) Å, c = 15.7136(8) Å, and β = 96.429(4)°, while β-phase of a = 10.8196(8) Å, b = 7.2809(4) Å, c = 11.3260(9) Å, and β = 107.695(8)°. The crystal structures of Al(BH4)3⋅AB in α-phase and β-phase are shown in Figure 27. In both structures, Al3+ is coordinated with one AB and three BH4− to form a distorted tetrahedron of AlB4. The interactions of NH⋅⋅⋅HB between NH3 and BH4− groups were formed in Al(BH4)3⋅AB to sustain its 3D structure. The distances between Al and B in BH4− narrowly range from 2.21 to 2.23 Å, while the distance between Al and B in BH3 from AB is 2.31 Å. The distances between Al and H in BH4− are in the range of 1.65−1.81 Å, while the distances between Al and H in BH3 range from 1.86 to 1.96 Å. The thermal decomposition of Al(BH4)3⋅AB at 70 °C gave rise to the release of 2 Eq of H2 (5 wt%), with a unique intermediate found in [6H + 1H] coordination of Al3+. It is worth noting that the endothermic dehydrogenation of Al(BH4)3⋅AB (39 kJ mol−1) was achieved.
Crystal structure of Al(BH4)3⋅AB: (a) α-phase and (b) β-phase. Reproduced from Ref. [80] with permission.
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 P4bm and the lattice parameters of a = 9.4643(8) Å and c = 5.5229(8) Å (Figure 26). Each Mg2+ is tetrahedrally coordinated by two BH4− with Mg-B distance of 2.30 Å and two NH3 groups with Mg-N distance of 2.14 Å. NH3 groups interact with their close BH4− and AB groups through the formation of BH⋅⋅⋅HN dihydrogen bonds(1.821-2.368 Å). Different from Mg(BH4)2⋅2AB, Mg(BH4)2⋅2NH3⋅AB exhibited significant depression of by-products and tremendously improved H2 release. The onset dehydrogenation was reduced to 75 °C, and about 9.6 wt% H2 could be released by 170 °C. Upon heating to 400 °C, a total amount of 13.8 wt% H2 was desorbed.
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].
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 a = 7.3021 Å, b = 12.5513 Å, c = 5.0595 Å, and β = 120.98°. However, the detailed crystal structure was not given. The onset temperature for dehydrogenation was at about 50 °C and a total amount of 6.8 wt% H2 could be released exothermically upon heating to 300 °C, without the detectable borazine and diborane.
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.
LiAB⋅LiBH4 was synthesized by ball milling LiAB with an equivalent LiBH4 [85]. It was ascribed to an orthorhombic cell (space group Pbca) with lattice parameters of a = 9.2824(18) Å, b = 14.3092(28) Å, and c = 7.6194(12) Å. In LiAB⋅LiBH4 structure shown in Figure 28, alternative LiAB and LiBH4 layers are stacked along the b axis. A strong interaction is resulting from the formed ionic bonds by Li+ with [NH2BH3]− and BH4−. Li+ has two tetrahedral coordination environments. One is surrounded by one BH3 of [NH2BH3]− and three BH4−, while the other by one NH2 of [NH2BH3]−, one BH4−, and two BH3 of [NH2BH3]−. Furthermore, the BH⋅⋅⋅HN dihydrogen bondings range from 2.181 to 2.387 Å. However, the detailed dehydrogenation behavior of this compound was not given, although it had a similar process to 2LiAB/LiBH4 reported in detail.
(a) Crystal structure of LiAB⋅LiBH4 and (b, c) Coordination of Li+. Li: green spheres; N: blue spheres; B: cyan spheres; and H: pink spheres. Reproduced from Ref. [85] with permission.
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
Crystal structure of Li(B3N2) and Na(B3N2): Li: green spheres; Na: yellow spheres; B: red spheres; N: blue spheres; and H: grey spheres. Reproduced from Ref. [86] with permission.
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.
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.
© 2015 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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