A New Complex Borohydride LiAl(BH4)2Cl2
by
, ,
Oleksandr Dolotko
1,*
,
Takeshi Kobayashi
1,
Ihor Z. Hlova
1,
Shalabh Gupta
1 and
Vitalij K. Pecharsky
1,2 1
U.S. DOE Ames Laboratory, Iowa State University, Ames, IA 500011, USA
2
Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA
*
Author to whom correspondence should be addressed.
Inorganics 2021, 9(5), 35; https://doi.org/10.3390/inorganics9050035
Submission received: 5 April 2021
/
Revised: 28 April 2021
/
Accepted: 30 April 2021
/
Published: 4 May 2021
(This article belongs to the Special Issue Metal Hydrogen Systems: Fundamental Properties and Current Applications for Energy Storage and Conversion)
Abstract
A new mixed alkali metal–aluminum borohydride LiAl(BH4)2Cl2 has been prepared via mechanochemical synthesis from the 2LiBH4–AlCl3 mixture. Structural characterization, performed using a combination of X-ray powder diffraction and solid-state NMR methods, indicates that the LiAl(BH4)2Cl2 phase adopts a unique 3D framework and crystallizes in an orthorhombic structure with the space group C2221, a = 11.6709(6) Å, b = 8.4718(4) Å, c = 7.5114(3) Å. The material shows excellent dehydrogenation characteristics, where hydrogen evolution starts at Tons = 70 °C, releasing approximately 2 wt.% of nearly pure (99.8 vol.%) hydrogen and a very small amount (~0.2 vol.%) of diborane. When compared to halide-free mixed alkali metal–aluminum borohydrides, the presence of Al‒Cl bonding in the LiAl(BH4)2Cl2 structure likely prevents the formation of Al(BH4)3 upon decomposition, thus suppressing the formation of diborane.
1. Introduction
Complex borohydrides continue to attract significant attention as potential hydrogen storage materials due to their large gravimetric hydrogen contents, for example, 10.5 wt.% hydrogen in NaBH4, 14.8 wt.% in Mg(BH4)2, and 18 wt.% in LiBH4. Borohydrides are, however, thermodynamically stable and they release hydrogen in successive steps, some of which require rather high temperatures [1,2,3,4]. One of the most effective approaches to tailor the hydrogen desorption properties of borohydrides is through the synthesis of mixed-cation or/and mixed-anion borohydrides with weakened B–H bonds. Consequently, modifications of the structure and properties of metal borohydrides by anion substitutions when alkali or alkali earth halide salts form solid solutions with the corresponding borohydrides, such as LiBH4–LiX and Ca(BH4)2–CaX2 systems, where X = F, Cl, Br or I, were recently demonstrated [5,6,7,8,9]. Further, when mixed-cation borohydrides are formed, hydrogen desorption onset temperatures decrease with increasing Pauling electronegativity (χP) of the partner cation [10], as has been demonstrated by the syntheses and characterizations of LiK(BH4)2, LiSc(BH4)4, NaSc(BH4)4, LiZn2(BH4)5, and Li4Al3(BH4)13 [11,12,13,14,15,16]. These materials also show enhanced hydrogen desorption kinetics compared to pristine LiBH4. Functionality of these mixed complex borohydrides, however, remains impeded by either or both high desorption temperatures and emissions of gaseous products other than hydrogen, one of which is often diborane [17]. Further, hydrogen reversibility remains a major challenge because extreme pressures and high temperatures are required, and the hydrogen absorption rates are usually very low.
On the other hand, some of the mixed-cation mixed-anion borohydrides, such as NaY(BH4)2Cl2, release no measurable amounts of diborane during their decomposition, retaining boron in the solid, which is a prerequisite for reversible hydrogen storage [18]. Further, synthesized via mechanochemical reactions in the LiBH4–SiS2 system, mixed-cation mixed-anion Lix(SiS2)y(BH4)x, where x and y depend upon the ratio of the starting materials, demonstrate improved kinetics of hydrogen desorption when compared to pure LiBH4 [19]. Recently, we also reported suppression of diborane release in the LiBH4–AlCl3 system via simultaneous cation and anion substitutions in LiBH4 [20]. Here, the covalent character of the Al−H bonds and the presence of Al‒Cl bonds in the resultant borohydride materials are responsible for the purity of the released hydrogen. Samples reported at that time were, however, multiphase mixtures. In this work, we report the successful synthesis, crystal structure, and hydrogen desorption properties of a new mixed-cation mixed-anion complex borohydride LiAl(BH4)2Cl2.
2. Results and Discussions
2.1. Initial Phase Analysis
LiBH4 reacts with AlCl3 upon ball milling over a short period of time. After 3 h of milling, the reaction is completed. Depending on the ratio of the starting materials in the mixture, the formation of different phases was observed, as listed in Table 1. As earlier reported by Lindemann et al., when the system is LiBH4-rich, one of the phases in this system corresponds to the Li4Al3(BH4)13 compound [15]. The stoichiometry of this compound requires a molar LiBH4/AlCl3 ratio of 4.33:1 (or 13:3) to complete the following metathesis reaction:
13LiBH4 + 3AlCl3 → Li4Al3(BH4)13 + 9LiCl.
When the concentration of LiBH4 in a system is reduced below the ideal stoichiometric ratio of 4.33:1, minor shifts in Bragg reflections corresponding to this compound are observed, as illustrated in Figure 1. Bragg reflections are highlighted with pink rectangles. The reduction in the unit cell parameter of the cubic Li4Al3(BH4)13 compound can be explained by partial substitutions of BH4− anions with Cl− due to an excess of AlCl3 over the stoichiometry of reaction 1. These substitutions were also reported by Lindemann et al. when the reaction products of the 4:1 mixture (instead of the ideal 4.33:1 stoichiometry as in reaction 1) were characterized by Rietveld refinement [15]. A further increase in the AlCl3 concentration leads to progressive shifts of the Bragg reflections of the Li4Al3(BH4)13-xClx phase, which continues down to the 3.65(or slightly lower):1 stoichiometry. When the LiBH4/AlCl3 ratio is lowered to 3:1, the Li4Al3(BH4)13-xClx phase coexists with a new phase and LiCl (Figure 1 and Table 1).
When the molar LiBH4/AlCl3 ratio becomes 2:1, only a new phase that coexists with LiCl is observed with no detectable traces of Li4Al3(BH4)13-xClx. Considering the as-weighed stoichiometry, the underlying metathesis reaction can be described as the following:
2LiBH4 + AlCl3 → LiAl(BH4)2Cl2 + LiCl.
The stoichiometry of this new mixed-cation mixed-anion phase, that is, LiAl(BH4)2Cl2, is also confirmed by the Rietveld refinement of the XRD data described in the next section. In addition to 3:1 and 2:1 stoichiometries, mixtures with 1.3:1 and 1:1 molar component ratios after ball milling show the presence of the same phase as in the 2:1 reaction but with slightly shifted Bragg reflections. Similar to the Li4Al3(BH4)13-xClx compound, partial substitutions between the BH4− and Cl− anions in the structure of the new phase are responsible for these lattice parameter variations. When the LiBH4/AlCl3 ratio is reduced below 2:1 in the starting mixture, the new LiAl(BH4)2Cl2 phase coexists with LiAlCl4 in addition to LiCl. LiAlCl4 forms as a result of the reaction of excess AlCl3 with the newly formed LiCl via the following reaction:
LiCl + AlCl3 → LiAlCl4..
Hence, the mechanochemical reaction at the 1:1 stoichiometry can be described by the following overall scheme:
2LiBH4 + 2AlCl3 → LiAl(BH4)2Cl2 + LiAlCl4.
2.2. Crystal Structure Refinement
The LiAl(BH4)2Cl2 phase that forms when LiBH4 and AlCl3 are ball-milled in a 2:1 molar ratio is a bimetallic halide-containing borohydride, which crystallizes in a C2221 space group with the lattice parameters (a = 11.6698(5) Å, b = 8.4724(4) Å, c = 7.5116(3) Å). The atomic positions, bond distances, and bond angles determined from the Rietveld refinement are listed in Table 2 and Table 3. The best fit (Figure 2) was obtained for random (within two standard deviations) occupancy of the two available 4a sites with Al and Li atoms. The Rietveld refinement was performed with the application of additional constraints, where the overall occupancies of the 4a sites were fixed to 1 and the overall Al to Li ratio was 1:1. The refinements of the models where Al and Li are ordered in M1 and M2 positions lead to an increase in Rp and Rwp from 4.8 and 6.5% to ~5.8 and ~8.5%, respectively, in both of the ordered models.
The structure of the LiAl(BH4)2Cl2 compound consists of layers of M(BH4)2 groups, separated by 2Cl atoms along the a-direction (Figure 3), where the M atoms consist of Li and Al, randomly occupying the 4a positions (also see Table 1). The coordination environment of the Li and Al atoms consists of two BH4 units and two Cl atoms (i.e., M(BH4)2Cl2), with the interatomic M–Cl distances of 2.1613(1) Å and 2.2337(1) Å. The M1 atoms are bridged to the M2 via two chlorine atoms, while two M2 atoms are connected through BH4 units (Figure 3d). The chlorine atoms are coordinated to two M atoms (M1 and M2), while the coordination of each BH4 unit is made of two M atoms. The structure can also be described as a network of tetrahedral (M(BH4)2Cl2) complexes connected along the a-axis via alternating bridging of M–BH4–M and M–Cl–M units.
Considering the Bragg peak shifts discussed above, LiAl(BH4)2Cl2 should also contain mixed-anion sites, effectively making it LiAl(BH4)2±δCl2±δ, which is also common for the earlier known Li4Al3(BH4)13-xClx. The unusual feature of the LiAl(BH4)2Cl2 compound is the mixed occupancy of the same sites by Al and Li atoms. Despite the difference in their ionic radii, the structural model, which has identical (within experimental errors, see Table 2) concentrations of Al and Li atoms in both 4a positions, converges to the lowest residuals during the Rietveld refinement.
2.3. NMR Characterization
To identify the species formed in the ball-milled products of the 2LiBH4–1AlCl3 mixture, we used 11B and 27Al ssNMR. In both spectra, practically one predominant signal is observed, as follows: the 11B signal at −37 ppm assigned to the (BH4)− unit (Figure 4a), and the 27Al signal at 74 ppm attributed to Al(BH4)2Cl2 (Figure 4b) [20]. Most likely, the latter species are the same as those found in the final products from the 3:1 and 3.67:1 mixtures and as an intermediate in the formation of Li4Al3(BH4)13 from the 4.33:1 mixture. Indeed, the corresponding 27Al signal in the 2:1 mixture appears in the 27Al{1H} J–HMQC spectrum (Figure 4c), which relies on a through-bond correlation, indicating the covalent character of the Al‒H bonding [20]. More importantly, however, all boron and aluminum configure a single compound in the ball-milled product of the 2:1 mixture, further confirming the chemical makeup of the title material. The minor 27Al signal at 60 ppm was also identified to Al(BH4)2Cl2, whose coordination geometry is slightly different from that yielding the main 27Al signal at 74 ppm, suggesting the presence of a disordered phase as an impurity [21].
2.4. Dehydrogenation
The TPD characteristics of the as-milled products for the LiBH4–AlCl3 mixtures taken in different molar ratios of the starting materials are compared in Figure 5 and Table 4. The desorption of the reaction product obtained after ball-milling the mixture of LiBH4 and AlCl3 taken in the 4.33:1 ratio, that is, of the Li4Al3(BH4)13 compound, starts at ~80 °C, where 2.8 wt.% of hydrogen is released in 80 h (Figure 5). The RGA of the released gas shows the presence of 16 vol.% of diborane and 84 vol.% of hydrogen. The amount of released hydrogen (2.8 wt.%) is much less than its theoretical content in the Li4Al3(BH4)13 compound (7.6 wt.%). This is attributed to the decomposition of the Li4Al3(BH4)13 with the formation of LiBH4, which is stable at 385 °C, loss of materials by vaporization of intermediate products, and the formation of diborane [15].
The thermal decomposition of the ball-milled 3.67:1 mixture reveals the release of 3.5 wt.% of hydrogen with an onset temperature of 61 °C (Figure 5). This amount corresponds to 51% of all the available hydrogen in the mixture (6.8 wt.%), which is higher compared to the 4.33:1 stoichiometry. This increase is likely related to a lower amount of LiBH4 , which is stable at the highest temperature (385 °C) and forms upon the decomposition of Li4Al3(BH4)13-xClx. The amount of released diborane (3 vol.%) is much smaller when compared to Li4Al3(BH4)13 (16.0 vol.%).
Decreasing the LiBH4 content in the system with AlCl3 to a 3:1 molar ratio leads to further suppression of the diborane release when heating the ball-milled mixture to 385 °C (Table 4). The RGA of the released gas from the 3:1 ratio product shows the presence of only 0.3 vol.% diborane. The hydrogen desorption, which initiates at ~50 °C, has similar kinetics compared to the two mixtures with higher starting LiBH4 contents (Figure 5). In 80 h, the products of the mechanochemical reaction in the 3:1 system, which now include LiAl(BH4)2±δCl2±δ in addition to Li4Al3(BH4)13-xClx, release ~3.0 wt.% of hydrogen, which corresponds to ~50% of the hydrogen available in the 3:1 mixture (6.0 wt.%).
The TPD study of the ball-milled 2:1 mixture, which is nearly pure LiAl(BH4)2Cl2 together with inert LiCl, shows H2 desorption at a significant rate starting at 66 °C (Figure 5). The kinetics of the hydrogen desorption of this mixed-cation mixed-anion compound is much higher when compared to all other compositions. In 6 h, while the temperature in the system is ramped from ~20 to 385 °C, the mixture releases ~1.8 wt.% of hydrogen. Upon stabilization of the temperature and gas pressure in the system, the total H2 release reaches ~2.0 wt.%, which corresponds to ~44% of the hydrogen available in the 2:1 mixture (4.5 wt.%). Improvement in the purity of the released H2 is observed, with only 0.2 vol.% of B2H6 detected, the remainder being pure hydrogen (Table 4).
The XRD study of the solid products after decomposition of the 2:1 system shows the formation of a mixture of different products. Among the crystalline products are LiCl and Al. However, the presence of multiple unknown Bragg reflections and the possible formation of amorphous byproducts does not allow us to make definitive conclusions about a decomposition pathway. This study requires additional efforts, and the nature of the products after temperature-induced decomposition will be described when available.
The analysis of the TPD illustrated in Figure 5 shows two-step decompositions. The first step starts at a low temperature, is relatively fast, and ends around 100 °C regardless of the LiBH4/AlCl3 ratio in the starting mixture. The second step, which starts around 150–160 °C is observed for all mechanochemical reaction products in LiBH4-rich materials. This step is very slow, and it extends for a long period of time without reaching saturation after 80 h. A different behavior is clearly observed for LiAl(BH4)2Cl2. The region between the first and second steps is smeared out and is not as pronounced as in all other samples. The second step of decomposition almost reaches its saturation upon heating the sample to 385 °C. Only 0.2 wt.% of additional H2 is released when the sample is kept at 385 °C for an additional 14 h.
3. Materials and Methods
3.1. Sample Preparation
The starting materials, LiBH4 (>95 wt.% purity) and AlCl3 (99.99 wt.% purity), were used as purchased from MilliporeSigma, Inc. The precursors taken in desired molar ratios were ball-milled in a 50 mL hardened steel vial using 20 g of steel balls (two large balls weighing 8 g each and four small balls weighing 1 g each) in an SPEX 8000M mill for 3 h. All manipulations were conducted in a glovebox under inert argon atmosphere with oxygen and moisture levels below 1 ppm.
3.2. Powder X-Ray Diffraction
Powder X-ray diffraction (XRD) data for phase analysis of the reaction products and determination of the crystal structure were collected at room temperature on a PANalytical powder diffractometer using Cu Kα1 radiation with a 0.02° step, in the range of Bragg angles 10° ≤ 2θ ≤ 80°. The measurements were carried out using a sample holder covered by the polyimide (Kapton) film to protect the samples from the ambient air. Presence of the film resulted in an enhanced amorphous-like background in the XRD patterns between 13° and 20° of 2θ.
3.3. Hydrogen Desorption
The kinetics of hydrogen desorption were measured using PCTPro-2000—a fully automated Sieverts-type instrument, which enables experiments in the temperature range from 25 to 400 °C. While still in the glove box, pellets of materials to be examined were placed in a PCTPro-2000 autoclave. The closed autoclave was then connected to the PCT instrument and evacuated. Volume calibrations using high-purity helium gas were performed at room temperature before every hydrogen desorption kinetic measurement. The temperature-programmed desorption (TPD) was performed by heating the samples from ambient temperature to 385 °C with a heating rate of 1 °C/min. When the pressure caused by gas release stabilized, samples were cooled down to room temperature and evacuated. The nature and composition of gaseous products released during the heating was tracked by using the RGAPro-2500 residual gas analyzer, connected to the PCT autoclave. For more precise gas analysis, in order to avoid losing the less volatile B2H6 during mass spectroscopy, the length of the connection between the autoclave and the gas analyzer was the shortest possible.
3.4. Solid-State NMR
The 11B and 27Al solid-state nuclear magnetic resonance (ssNMR) experiments were performed on a Varian spectrometer operated at 14.1 T, equipped with a 3.2 mm triple-resonance magic-angle spinning (MAS) probe. The samples were packed in a MAS zirconia rotor in a glovebox under argon atmosphere and sealed with double O-ring caps to minimize contamination of oxygen and moisture, and were spun at 12.5 kHz. One-dimensional (1D) 11B and 27Al direct polarization (DP)MAS and 1D 27Al{1H} MAS-J–HMQC experiments were carried out. All spectra were acquired using SPINAL64 1H decoupling [22]. Detailed experimental conditions are given in the figure captions using the following symbols; νRF(X) is the magnitude of the radio frequency magnetic field applied to X spins, τmax is the optimum J evolution time, and τRD is the recycle delay. The 11B and 27Al shifts were referenced to the diethyl ether–boron trifluoride complex (BF3·OEt2) and 1.0 M aqueous solutions of Al(NO3)3 at 0 ppm, respectively.
3.5. Structural Characterization
The XRD data of the ball-milled 2LiBH4–AlCl3 mixture containing Bragg peaks of LiCl and an unknown phase were used for indexing and structure solution. A total of 11 lowest angle Bragg peaks with the highest intensities, presumably belonging to the unknown phase, were successfully indexed ab initio using TREOR [23] in a base-centered orthorhombic cell. Correctness of indexing was verified by Le Bail refinement using FullProf [24]. The initial structural model was obtained in the C2221 space group through the global optimization in direct space using FOX [25]. During the structure solution, the positions of two cations, one Cl− anion and centers of gravity of two (BH4)− groups were optimized. H-atoms were refined with 1.15(1) Å restrains on the B‒H distances and with 109.5(5)° restraints on the H‒B‒H angles in BH4 tetrahedra. The final structure refinement was performed by Rietveld analysis with soft restraints on interatomic bond lengths and bond angles using FullProf. The background was approximated using linear interpolations between data points selected from regions with no Bragg peaks present. The pseudo-Voigt function was used for the peak shape description. The scale factor, lattice parameters, fractional coordinates of atoms and their isotropic displacement parameters, zero shift, peak shape parameters, and half width (Caglioti) parameters were allowed to vary during the refinement.
4. Conclusions
In summary, we synthesized a new double-cation double-anion complex LiAl(BH4)2Cl2 by mechanochemical synthesis from the 2LiBH4–AlCl3 mixture. This phase shows a unique 3D framework and crystallizes in a new orthorhombic structure with the space group C2221 and lattice parameters a = 11.6709(6) Å, b = 8.4718(4) Å, c = 7.5114(3) Å. The replacement of about a half of the BH4− anions with Cl− in the coordination sphere of Al results in the coexistence of covalent Al−H and more ionic Al‒Cl bonds in the structure, which are not present in the earlier reported Li4Al3(BH4)13 and are present to a much lesser extent in Li4Al3(BH4)13-xClx.
We also studied the hydrogen desorption properties of the mechanochemically prepared phases starting with LiBH4–AlCl3 mixtures taken in different molar ratios, including the 2:1 product. Temperature-programmed desorption analysis shows that despite the high theoretical hydrogen content and the low desorption temperature, all LiBH4-rich mixtures with AlCl3 (with more than 3 moles of LiBH4 per 1 mole of AlCl3) release significant amounts of diborane. Decreasing the concentration of LiBH4 in the starting mixture suppresses diborane emission without affecting the onset temperature of hydrogen release. The 4.33:1 and 3.67:1 ratio mixtures are keeping the Li4Al3(BH4)13-like structures for the hydride phase and release nearly 16 and 3 vol.% of B2H6 respectively. The 3:1 ratio mixture contains halide derivatives of two structure types—the cubic Li4Al3(BH4)13 and the new orthorhombic LiAl(BH4)2Cl2. The 2:1 composition contains the nearly stoichiometric LiAl(BH4)2Cl2 compound, which releases almost pure hydrogen upon its decomposition. Most likely, the formation of Al‒H bonds in the (Al(BH4)2Cl2)− complex in the new compound weakens the B‒H bonds and decreases the dehydrogenation temperature, while the ionic Al‒Cl bonds prevents the formation of Al(BH4)3, which is a known intermediate leading to the formation of diborane.
Author Contributions
O.D. contributed to the design of experiments, performed materials synthesis, Rietveld refinements, hydrogen storage investigations, and writing the manuscript; I.Z.H. performed the material synthesis and hydrogen storage properties measurements, contributed to the manuscript writing; T.K. performed solid-state NMR characterization, contributed to interpretation of result and manuscript writing; S.G. contributed to design the experiments, interpretation of results and manuscript writing, V.K.P. guided the study, contributed to structural characterization, data interpretation, and the manuscript writing. All authors provided comments and edits during the preparation of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency & Renewable Energy under the Fuel Cell Technologies Office, award number DE-EE-0007047. The research was performed at the Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data that support plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
Acknowledgments
The authors thank Yaroslav Filinchuk from the Institute of Condensed Matter and Nanosciences, and the Université Catholique de Louvain, Belgium for help in the finding the model of the crystal structure of the new LiAl(BH4)2Cl2 compound using FOX.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
(a) Powder XRD patterns of samples with different LiBH4/AlCl3 ratios ball-milled for 3 h; (b) details between 12 and 33° of 2θ, where the Bragg reflections of the Li4Al3(BH4)13 phase are highlighted by the pink rectangles and those of the LiAlCl2(BH4)2 phase are highlighted by the blue rectangles. Locations of the Braggs peaks of the reaction products are shown as the vertical markers at the bottom of the plot; for Li4Al3(BH4)13 and LiAlCl2(BH4)2 the locations are for the ideal stoichiometries observed at 4.3:1 and 2:1 molar LiBH4:AlCl3 ratios, respectively.
Figure 1.
(a) Powder XRD patterns of samples with different LiBH4/AlCl3 ratios ball-milled for 3 h; (b) details between 12 and 33° of 2θ, where the Bragg reflections of the Li4Al3(BH4)13 phase are highlighted by the pink rectangles and those of the LiAlCl2(BH4)2 phase are highlighted by the blue rectangles. Locations of the Braggs peaks of the reaction products are shown as the vertical markers at the bottom of the plot; for Li4Al3(BH4)13 and LiAlCl2(BH4)2 the locations are for the ideal stoichiometries observed at 4.3:1 and 2:1 molar LiBH4:AlCl3 ratios, respectively.
Figure 2.
Rietveld-refined XRD pattern of the 2:1 (molar) mixture of LiBH4 and AlCl3 after 3 h of milling. Short vertical bars mark calculated positions of the Bragg peaks of LiAl(BH4)2Cl2 and LiCl.
Figure 2.
Rietveld-refined XRD pattern of the 2:1 (molar) mixture of LiBH4 and AlCl3 after 3 h of milling. Short vertical bars mark calculated positions of the Bragg peaks of LiAl(BH4)2Cl2 and LiCl.
Figure 3.
Unit cell of the LiAl(BH4)2Cl2 compound (a) in a–c (b) and a–b (c) directions. Representation of the coordination of atomic groups in unit cell(d) (M1 and M2 atoms represent cation, which consists of Li and Al atoms, partially occupying the 4a positions (Table 2)).
Figure 3.
Unit cell of the LiAl(BH4)2Cl2 compound (a) in a–c (b) and a–b (c) directions. Representation of the coordination of atomic groups in unit cell(d) (M1 and M2 atoms represent cation, which consists of Li and Al atoms, partially occupying the 4a positions (Table 2)).
Figure 4.
(a) 11B DPMAS, (b) 27Al DPMAS, and (c) 27Al{1H} J–HMQC spectra of the products of the 2LiBH4-1AlCl3 mixture ball-milled for 3 h. The DPMAS spectra (a, b) were acquired with a single pulse excitation, using flip angle of 10°, νRF(11B) = 50 kHz, νRF(27Al) = 50 kHz, νRF(1H) = 50 kHz for SPINAL-64 decoupling, and τRD = 0.5 s. The J–HMQC spectrum was obtained using νRF(1H) = 100 kHz for the FSLG decoupling and short pulses, and 50 kHz for SPINAL-64 decoupling during the acquisition, νRF(27Al) = 50 kHz and 20 khz for the initial π/2 pulse and the inversion π pulses, respectively, τmax = 2.8 ms, and τRD = 1.0 s.
Figure 4.
(a) 11B DPMAS, (b) 27Al DPMAS, and (c) 27Al{1H} J–HMQC spectra of the products of the 2LiBH4-1AlCl3 mixture ball-milled for 3 h. The DPMAS spectra (a, b) were acquired with a single pulse excitation, using flip angle of 10°, νRF(11B) = 50 kHz, νRF(27Al) = 50 kHz, νRF(1H) = 50 kHz for SPINAL-64 decoupling, and τRD = 0.5 s. The J–HMQC spectrum was obtained using νRF(1H) = 100 kHz for the FSLG decoupling and short pulses, and 50 kHz for SPINAL-64 decoupling during the acquisition, νRF(27Al) = 50 kHz and 20 khz for the initial π/2 pulse and the inversion π pulses, respectively, τmax = 2.8 ms, and τRD = 1.0 s.
Figure 5.
(a) TPD of ball-milled LiBH4–AlCl3 mixtures taken in different molar ratios of starting materials. The green line represents temperature profile. The desorption curves were obtained during heating from room temperature to 385 °C with a heating rate of 1 °C/min. (b) Zoomed plot representing the TPD during the first 7 h of measurements.
Figure 5.
(a) TPD of ball-milled LiBH4–AlCl3 mixtures taken in different molar ratios of starting materials. The green line represents temperature profile. The desorption curves were obtained during heating from room temperature to 385 °C with a heating rate of 1 °C/min. (b) Zoomed plot representing the TPD during the first 7 h of measurements.
Table 1.
Results of the phase analysis based on XRD data of the LiBH4–AlCl3 system obtained after 3 h of ball milling of the mixtures with different molar ratios of the starting materials. Lattice parameters of LiAlCl4 (Sp. Gr. P21/c; a = 7.050(1) Å, b = 6.499(1) Å, c = 13.076(2) Å, β = 93.1(1)°) and LiCl (Sp. Gr. Fm-3m, a = 5.146(1) Å) are constant. Lattice parameters of stoichiometric Li4Al3(BH4)13 and LiAlCl2(BH4)2 are for the 4.3:1 and 2:1 reactions, all other represent Li4Al3(BH4)13-xClx and LiAlCl2±y(BH4)2±y stoichiometries.
Table 1.
Results of the phase analysis based on XRD data of the LiBH4–AlCl3 system obtained after 3 h of ball milling of the mixtures with different molar ratios of the starting materials. Lattice parameters of LiAlCl4 (Sp. Gr. P21/c; a = 7.050(1) Å, b = 6.499(1) Å, c = 13.076(2) Å, β = 93.1(1)°) and LiCl (Sp. Gr. Fm-3m, a = 5.146(1) Å) are constant. Lattice parameters of stoichiometric Li4Al3(BH4)13 and LiAlCl2(BH4)2 are for the 4.3:1 and 2:1 reactions, all other represent Li4Al3(BH4)13-xClx and LiAlCl2±y(BH4)2±y stoichiometries.
| LiBH4:AlCl3 Molar Ratio | Li4Al3(BH4)13 | LiAlCl2(BH4)2 | LiAlCl4 | LiCl |
|---|---|---|---|---|
| 4.3:1 | a = 11.390(1) Å | not present | not present | present |
| 4:1 | a = 11.367(1) Å | not present | not present | present |
| 3.65:1 | a = 11.350(1) Å | not present | not present | present |
| 3:1 | a = 11.291(2) Å | a = 12.159(4) Å, b = 8.493(4) Å, c = 7.377(3) Å | not present | present |
| 2:1 | not present | a = 11.6698(5) Å, b = 8.4724(4) Å, c = 7.5116(3) Å | not present | present |
| 1.3:1 | not present | a = 11.577(1) Å, b = 8.436(1) Å, c = 7.553(1) Å | present | present |
| 1:1 | not present | a = 11.575(2) Å, b = 8.417(1) Å, c = 7.541(1) Å | present | not present |
Table 2.
Crystallographic data for LiAl(BH4)2Cl2 compound determined from Rietveld refinement of the structure model obtained using FOX: space group C2221 (#20), a = 11.6698(5) Å, b = 8.4724(4) Å, c = 7.5116(3) Å. Fit residuals (not corrected for background) Rp = 4.78%; Rwp = 6.49%.
Table 2.
Crystallographic data for LiAl(BH4)2Cl2 compound determined from Rietveld refinement of the structure model obtained using FOX: space group C2221 (#20), a = 11.6698(5) Å, b = 8.4724(4) Å, c = 7.5116(3) Å. Fit residuals (not corrected for background) Rp = 4.78%; Rwp = 6.49%.
| Atom | Site | x | y | z | Uiso, Å2 | Occupancy |
|---|---|---|---|---|---|---|
| M1 (Al1) | 4a | 0.633(2) | 0 | 0 | 0.081(2) | 0.45(2) |
| M1 (Li1) | 4a | 0.633(2) | 0 | 0 | 0.081(2) | 0.55(2) |
| M2 (Al2) | 4a | −0.119(2) | 0 | 0 | 0.081(2) | 0.55(2) |
| M2 (Li2) | 4a | −0.119(2) | 0 | 0 | 0.081(2) | 0.45(2) |
| Cl | 8c | 0.752(1) | 0.8070(2) | 0.0306(3) | 0.098(1) | 1 |
| B1 | 4b | 0 | 0.479(1) | ¼ | 0.052(3) | 1 |
| H11 | 8c | 0.063 | 0.401(1) | 0.327 | 0.052(3) | 1 |
| H12 | 8c | 0.050 | 0.558(1) | 0.152 | 0.052(3) | 1 |
| B2 | 4b | 0 | 0.983(1) | ¼ | 0.052(3) | 1 |
| H21 | 8c | 0.065 | 1.061(1) | 0.175 | 0.052(3) | 1 |
| H22 | 8c | 0.048 | 0.904(1) | 0.350 | 0.052(3) | 1 |
Table 3.
Bond distances (Å) and angles (°) in the LiAl(BH4)2Cl2 compound. (M1 and M2 atoms consist of Li or Al, partially occupied the 4a positions.). Since only the centers of gravity of two symmetrically independent (BH4)− groups were refined, the M–H bond lengths are tentative.
Table 3.
Bond distances (Å) and angles (°) in the LiAl(BH4)2Cl2 compound. (M1 and M2 atoms consist of Li or Al, partially occupied the 4a positions.). Since only the centers of gravity of two symmetrically independent (BH4)− groups were refined, the M–H bond lengths are tentative.
| M1–H11 | 2.7202(1) | 2x |
| M1–H11 | 2.7628(1) | 2x |
| M1–H12 | 1.5758(1) | 2x |
| M1–B1 | 2.4411(1) | 2x |
| M1–Cl | 2.1613(1) | 2x |
| M2–H21 | 2.5683(1) | 2x |
| M2–H21 | 2.5715(1) | 2x |
| M2–H22 | 1.6158(1) | 2x |
| M2–B2 | 2.3385(1) | 2x |
| M2–Cl | 2.2337(1) | 2x |
| M1–Cl–M2 | 82.515(4) | 2x |
| B1–H11 | 1.1503(1) | 2x |
| B1–H12 | 1.1508(1) | 2x |
| H–B1–H | 109.1(1)–109.8(1) | |
| B2–H21 | 1.1502(1) | 2x |
| B2–H22 | 1.1502(1) | 2x |
| H–B2–H | 109.4(1)–109.6(1) |
Table 4.
Hydrogen desorption characterization and RGA analysis of decomposition products of LiBH4–AlCl3 mixtures ball-milled for 3 h in different molar ratios.
Table 4.
Hydrogen desorption characterization and RGA analysis of decomposition products of LiBH4–AlCl3 mixtures ball-milled for 3 h in different molar ratios.
| LiBH4:AlCl3 Ratio | H2 Release (wt. %) 1 | H2 (vol. %) | B2H6 (vol. %) |
|---|---|---|---|
| 4.33:1 | 2.8 | 84.0 | 16.0 |
| 3.67:1 | 3.5 | 97.0 | 3.0 |
| 3:1 | 3.0 | 99.7 | 0.3 |
| 2:1 | 2.0 | 99.8 | 0.2 |
1 wt.% of hydrogen released after 80 h.
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Dolotko, O.; Kobayashi, T.; Hlova, I.Z.; Gupta, S.; Pecharsky, V.K. A New Complex Borohydride LiAl(BH4)2Cl2. Inorganics 2021, 9, 35. https://doi.org/10.3390/inorganics9050035
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Dolotko O, Kobayashi T, Hlova IZ, Gupta S, Pecharsky VK. A New Complex Borohydride LiAl(BH4)2Cl2. Inorganics. 2021; 9(5):35. https://doi.org/10.3390/inorganics9050035
Chicago/Turabian StyleDolotko, Oleksandr, Takeshi Kobayashi, Ihor Z. Hlova, Shalabh Gupta, and Vitalij K. Pecharsky. 2021. "A New Complex Borohydride LiAl(BH4)2Cl2" Inorganics 9, no. 5: 35. https://doi.org/10.3390/inorganics9050035
APA StyleDolotko, O., Kobayashi, T., Hlova, I. Z., Gupta, S., & Pecharsky, V. K. (2021). A New Complex Borohydride LiAl(BH4)2Cl2. Inorganics, 9(5), 35. https://doi.org/10.3390/inorganics9050035
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