Structural Characterization of the Products by Solid-State NMR

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Mechanochemistry of the LiBH-AlCl System: Structural Characterization of the Products by Solid-State NMR Takeshi Kobayashi, Oleksandr Dolotko, Shalabh Gupta, Vitalij K. Pecharsky, and Marek Pruski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10856 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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The Journal of Physical Chemistry

Mechanochemistry of the LiBH4-AlCl3 System: Structural Characterization of the Products by SolidState NMR T. Kobayashi*,1, O. Dolotko1, S. Gupta1, V.K. Pecharsky,*1, 2, M. Pruski,*1, 3 1 2

U.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA

Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA 3

Department of Chemistry, Iowa State University, Ames, IA 50011, USA

ABSTRACT: The double-cation metal borohydride, Li4Al3(BH4)13, mechanochemically produced from a 13:3 mixture of lithium borohydride (LiBH4) and aluminum chloride (AlCl3), has a low hydrogen desorption temperature; however, the material’s decomposition is accompanied by a large emission of toxic diborane (B2H6). We found that a decrease of the LiBH4:AlCl3 ratio in the starting mixture yields increased amounts of partially chlorinated products that also dehydrogenate at low temperature, but release negligibly small amounts of diborane. Extensive characterization by solid-state NMR spectroscopy (SSNMR) and powder X-ray diffraction (XRD) found that the 11:3 ratio product maintains the Li4Al3(BH4)13-like structure, with additional Cl- anions substituting for BH4- compared to the 13:3 mixture. Further decrease of relative LiBH4 concentration in the starting mixture to 9:3 results in a different product composed of tetrahedral [Al(BH4)4]- and [Al(BH4)2Cl2]- complexes, in which two hydrogen atoms of each borohydride group are bridged to aluminum sites. Additionally, SSNMR revealed the covalent character of the Al‒H bonds, which is not observed in Li4Al3(BH4)13. These 1 ACS Paragon Plus Environment

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findings suggest that the Al‒Cl bonding present in the chlorinated complexes prevents the formation of Al(BH4)3, which is a known intermediate leading to the formation of diborane during thermal dehydrogenation of the nearly chlorine-free Li4Al3(BH4)13.

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INTRODUCTION Among various classes of potential solid-state hydrogen storage media, metal borohydrides, M(BH4)n consisting of alkali or alkaline earth metal cations (Mn+) and borohydride anions (BH4-), have attracted considerable interest due to their high gravimetric density of hydrogen.1 Despite their high hydrogen content, many borohydrides release hydrogen at temperatures much higher than 100 °C. For example, LiBH4 contains ~18 wt.% of hydrogen but hydrogen desorption initiates at around 400 °C.2 To enhance the decomposition kinetics of metal borohydrides, doping by various additives, including transition metals,3-4 metal oxides,5-6 hydrides,7-8 amides,9-11 halides,12-14 or graphite15-16 has been widely explored. Nakamori et al. reported that the dehydrogenation temperature (Td) correlates with the Pauling electronegativity (χP) of metal cations in the borohydrides.17 Since the manipulation of Td in singlecation borohydrides is limited by the nature of the individual metal, they introduced a second metal cation to tune the dehydrogenation process. For example, the Td value of the products obtained from the ball-milled mixture of zirconium chloride and lithium borohydride (ZrCl4 + nLiBH4, n = 4-6) is related to the average value of χP for ZrLin-4, and thus can be altered by changing n.18 Subsequent studies achieved more precise and extensive tuning by the mechanochemical synthesis of double-cation borohydrides19-24 via metathesis or double-substitution reactions. Among the double-cation borohydrides, Li4Al3(BH4)13 is an attractive compound with a high hydrogen content (17.1 wt.%) and low Td (70-80°C). The compound can be synthesized at room temperature in 3 hours by ball milling, which effects a simple mechanochemical reaction between LiBH4 and aluminum chloride (AlCl3):25-26 13LiBH4 + 3AlCl3 → Li4Al3(BH4)13 + 9LiCl.

(1)

Upon heating to 70-80°C, Li4Al3(BH4)13 decomposes into LiBH4 and Al(BH4)3, whereas the latter decomposes into elemental aluminum releasing hydrogen and diborane according to eqs 2 and 3: Li4Al3(BH4)13 → 3Al(BH4)3 + 4LiBH4

(2)

3Al(BH4)3 → 3Al + 9H2 + 6B2H6.

(3) 3 ACS Paragon Plus Environment


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The formation of the volatile Al(BH4)3 (b.p. ~45°C) results in the loss of aluminum from the reaction mixture during thermal dehydrogenation, which precludes the potential reversibility of this system. In the present work, we found that a decrease of relative LiBH4 concentration in the starting LiBH4:AlCl3 mixture from the stoichiometric 13:3 (eq 1) to 11:3 and 9:3 suppresses diborane formation during thermal dehydrogenation, without affecting the Td. Solid-state (SS)NMR techniques combined with powder XRD analysis revealed a discontinuous change in the structure of the ball-milled products upon reduction of the LiBH4 content. In particular, the 9:3 mixture yields a partially chlorinated aluminum borohydride [Al(BH4)2Cl2]- complex in which, unlike in the Li4Al3(BH4)13 structure, the nonhydrogen atoms (Al, B, Cl) form a tetrahedral AlB2Cl2 skeleton with two hydrogen atoms of each borohydride group being covalently bridged between the aluminum and boron. We note that our study is focused solely on the chemical composition and structure of the ball-milled materials, whereas local motions were important to us only to the extent that they affected the SSNMR spectra. The reorientation of [BH4]- anions or the translational diffusion of Li+ cations in metal borohydrides can be studied by NMR via the temperature dependence of spin-lattice relaxation rates.27-28

EXPERIMENTAL Sample preparations. The starting materials LiBH4 (>95 wt.% purity) and AlCl3 (99.99 wt.% purity) were purchased from Sigma-Aldrich and used as such. Due to the sensitivity of the starting materials and the products to moist air, all manipulations were carried out in a glove box under a continuously purified and monitored argon atmosphere. For synthesis, mixtures with various molar ratios of starting components weighing ~1 g total were individually loaded in a 65 ml hardened-steel vial and ball-milled for 3 hours, as per the earlier report,25 in an 8000 M SPEX mill containing two 8 g and four 1 g balls. Each synthesis was repeated at least 3 times and the reproducibility of the products was confirmed by subsequent analyses.


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Hydrogen desorption. The kinetics of hydrogen desorption was measured using an automated Sievert’s-type pressure-composition-temperature (PCT) apparatus (PCTPro-2000, SETARAMHyEnergy LLC), which enables experiments in the temperature region from 25°C to 400°C and up to 200 bar of hydrogen pressure. While still in the glove box, the investigated samples were pressed into pellets and placed in the sealed autoclave, which was then connected to the instrument and evacuated. Volume calibration using high-purity helium gas was performed at room temperature before the kinetic measurements. The samples were heated from 25 °C to 390 °C with a heating rate of 1 °C/min under continuously monitored pressure and held at this temperature until the pressure in the system was stable, or up to 80 hours for reactions with slow kinetics. Qualitative analysis of the released gas was carried out using a residual gas analyzer (RGA, RGAPro-2500, SETARAM-HyEnergy LLC), connected to the autoclave. Following dehydrogenation, the samples were cooled to room temperature using forced air cooling, evacuated and removed in a glove box for further analyses. However, due to the highly amorphous nature of dehydrogenated materials, XRD did not yield any discernable patterns other than one attributable to LiCl. The SSNMR spectra, although detectable, were very broad and uninformative, and thus will not be further discussed. XRD. The products were characterized by powder XRD analysis at room temperature on a PANaytical powder diffractometer using Cu Kα1 radiation with a 0.02° 2θ step, with Bragg angles (2θ) ranging from 10° to 80°. During the measurements, a polyimide (Kapton) film was used to protect the sample from moisture and oxygen, which resulted in a weak amorphous-like background in the XRD patterns between 13° and 20°. Solid-state NMR. The

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B and

27

Al SSNMR experiments were performed at room temperature

(~295 K) on a Varian 600 MHz (14.1 T) NMR spectrometer, equipped with a 3.2-mm triple-resonance magic-angle spinning (MAS) probe, and an Agilent 400 MHz (9.4 T) spectrometer equipped with a 3.2mm double-resonance MAS probe. The samples were packed in MAS zirconia rotors in a glove box under argon atmosphere and sealed with double O-ring caps to avoid possible reaction with oxygen and 5 ACS Paragon Plus Environment


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moisture. The following one- and two-dimensional (1D and 2D) experiments were carried out: 1D 11B and

27

1D

27

Al direct polarization (DP)MAS, 2D

Al{1H} MAS-J-HMQC,33 and

27

27

Al triple-quantum (3Q)MAS,29-30

27

Al{11B} REDOR,31-32

Al{1H} J-modulated spin-echo experiment34. All spectra were

acquired using SPINAL64 1H decoupling.35 Detailed experimental conditions are given in the figure captions, where B0 denotes the strength of the external magnetic field, νR is the MAS rate, νRF(X) is the magnitude of the radio frequency magnetic field applied to X spins, τrec is the REDOR recoupling time, τmax is the optimum J evolution time, ∆t1 is the increment of t1 during the 2D experiment, τRD is the recycle delay, and NS is the number of scans. The

11

B and

27

Al shifts were referenced to the diethyl

ether-boron trifluoride complex (BF3·OEt2) at δ 11B = 0 ppm and 1.0 M aqueous solutions of Al(NO3)3 at δ 27Al = 0 ppm, using methyl proton signal of sodium-3-(trimethylsilyl)propanesulfonate (DSS) at δ 1

H = 0 ppm as a secondary external reference.36

RESULTS AND DISCUSSION Characterization of the ball-milled products of LiBH4-AlCl3 mixtures; PCT, RGA and XRD analyses. The results of thermal dehydrogenation of products obtained with LiBH4:AlCl3 ratios of 13:3, 11:3 and 9:3 are shown in Figure 1A and Table 1. The 13:3 ratio product, i.e. Li4Al3(BH4)13, started desorption at around ~80°C,25 and released 2.8 wt% of hydrogen below 390°C in 80 h. The amount of released hydrogen was much lower than the available hydrogen contained in the initial mixture (7.6 wt.%), which is attributed to the decomposition of some of the Li4Al3(BH4)13 to a more stable LiBH4 (eq 2) and the formation of diborane.25 Indeed, the RGA of the gaseous product showed release of 16 vol.% of B2H6 and 84 vol.% of H2. Note that our PCT-RGA apparatus is not set for real-time monitoring of the gaseous decomposition products and the hydrogen desorption curves were normalized based on the hydrogen content in the overall gaseous products. Thus, due to the release of B2H6, these curves may not represent the exact kinetics of hydrogen desorption. In the thermal treatment of the 11:3 ratio product, gas was released with a lower onset temperature of ~61°C. The concentration of diborane in the released gas was suppressed to 3 vol.%, whereas the 6 ACS Paragon Plus Environment

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release of hydrogen was higher (3.5 wt.%), which corresponds to 51 % of all hydrogen available in the mixture (6.8 wt.%). Finally, the 9:3 ratio product initiated hydrogen desorption at an even lower temperature (~50°C) and released 3.0 wt.% of hydrogen in 80 h. Similar to the 11:3 stoichiometry, the gaseous product contained ~50 % of the available hydrogen in the starting mixture (6.1 wt.%), with very small contamination of diborane of 0.3 vol.%. Table 1. Hydrogen desorption characterization and RGA analysis of decomposition products of LiBH4AlCl3 mixtures of different molar ratios, ball milled for 3 hours.

a)

LiBH4:AlCl3 ratio

H2 release (wt.%)a

H2 (vol.%)

B2H6 (vol.%)

13:3

2.8

84.0

16.0

11:3

3.5

97.0

3.0

9:3

3.0

99.7

0.3

wt.% of hydrogen released during 80 h of thermal treatment. The XRD pattern of the 13:3 ratio product displays Bragg peaks assigned to a mixture of

Li4Al3(BH4)13 (a = 11.390(1) Å) and LiCl (first two major peaks at 2θ = 30.0° and 34.8°)25. The XRD pattern of the 11:3 ratio product is similar, except that the set of Bragg peaks corresponding to the Li4Al3(BH4)13 phase is shifted towards slightly larger Bragg angles (Figure 1C), indicating a smaller size of the cubic unit cell (a = 11.351(1) Å). As will be shown later, the decreased cell size results from a partial substitution of the BH4- unit with Cl-, i.e. Li4Al3(BH4)13-xClx (vide infra). The 9:3 ratio product also yielded Bragg peaks characteristic of LiCl and a new set of Bragg reflections that could not be assigned to any known structure, and thus was extensively studied by SSNMR.



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Figure 1. (A) Hydrogen desorption curves versus time and versus temperature (inset) of the LiBH4:AlCl3 mixtures taken at 13:3, 11:3 and 9:3 molar ratios and ball-milled for 3 hours. The desorption curves were obtained during heating from room temperature to 390°C at a heating rate of 1°C/min. (B) The XRD powder patterns of the ball-milled products before the desorption. Asterisks (*) represent the Bragg reflections assigned to Li4Al3(BH4)13. The peaks representing LiCl at 2θ = 30.0° and 34.8° are truncated to reveal low-intensity details. (C) Expanded XRD powder patterns of the 13:3 and 11:3 ratio products illustrate the shift of the Bragg peaks and, therefore, the smaller-size of the unit cell in the latter. Characterization of the ball-milled products of LiBH4:AlCl3 mixtures; SSNMR. The discussion of SSNMR results is organized as follows. We first use the

11

B DPMAS,

27

Al DPMAS and

27

Al

3QMAS results, in concert with the DFT calculations of NMR parameters, to identify the 11B and 27Al sites present in the 3-hr ball-milled LiBH4:AlCl3 mixtures taken in the molar ratios of 13:3, 11:3 and 9:3. Since the structural analysis of the 9:3 sample proved to be challenging not only for XRD, but also for routine SSNMR, we carried out three additional experiments to fully characterize this sample. First, we demonstrated the existence of covalent Al‒H bonds by applying the 1D version of the 1H-27Al MASJ-HMQC scheme.33 Second, we quantified the number of BH4 units that coordinate with each of the Al 8 ACS Paragon Plus Environment

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cations by using the 27Al{11B} rotational double echo resonance (REDOR) experiment.31-32 Finally, we validated the multiplicity of such covalent bonds by J-modulated spin-echo measurements.34 11

B DPMAS, 27Al DPMAS, 27Al 3QMAS and DFT calculations. The 11B and 27Al DPMAS spectra of

ball-milled products are shown in Figures 2A and 2B, respectively. As in the case of XRD, the 13:3 and 11:3 ratio products yielded similar spectra, which differ significantly from those observed for the sample prepared using the lowest LiBH4 content. Based on the crystallographic structure of Li4Al3(BH4)13,25 we assigned the 11B signal at -36 ppm to the dominant BH4- unit bridging the Al and Li atoms and the minor signal at -44 ppm to the BH4- unit surrounded by four Li atoms. The signal at -42 ppm corresponds to unreacted LiBH4 (see Figure S1 in Supporting information). The 9:3 ratio product showed two signals at -36 and -37 ppm, which are also characteristic of the BH4- units. We attribute these signals to the Al(BH4)4-xClx species as discussed below.

Figure 2. 11B (A) and 27Al (B) DPMAS spectra of the products of LiBH4-AlCl3 mixture ball-milled for 3h. The spectra were acquired at B0 = 14.1 T with a single pulse excitation, using flip angles of 10°, νR = 12.5 kHz, νRF(11B) = 50 kHz, νRF(27Al) = 50 kHz, νRF(1H) = 50 kHz for SPINAL64 1H decoupling, and τRD = 0.5 s. The spectra were normalized to a constant height. The interpretation of

27

Al spectra proved to be more intricate, but provided structural insights

unobtainable with other methods. We first note that the 27Al DPMAS spectrum of the 13:3 ratio product 9 ACS Paragon Plus Environment


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is dominated by the MAS-averaged second-order quadrupolar powder pattern centered at around 38 ppm, which is assigned to Li4Al3(BH4)13. Based on the analysis of the corresponding 3QMAS spectrum shown in Figure 3, the isotropic chemical shift δcs,iso and the second-order quadrupolar effect parameter PQ were estimated to be 46 ppm and 5.8 MHz, respectively. The DFT calculations of the NMR parameters carried out for the Li4Al3(BH4)13 structure25 yielded very similar values (Table 2; see Supporting Information for the calculation details), thereby unambiguously confirming the assignment. Note that the same signal is also dominant in the 11:3 ratio product; however, it is also accompanied by a weaker signal (by a factor of ~5) centered at ~55 ppm. Again, by combining the results of

27

Al

3QMAS and DFT calculations, we assign this weaker resonance to an Al(BH4)3Cl unit in the Li4Al3(BH4)13-xClx structure. Indeed, the experimental line shape parameters obtained for this sample from the

27

Al 3QMAS spectrum in Figure 3, δcs,iso = 59 ppm and PQ = 4.2 MHz, agree well with the

DFT calculations for such a structure, δcs,iso = 61.8 ppm and PQ = 4.7 MHz (see Table 2). Evidently, the substitution of a single BH4- with Cl- in the 11:3 ratio product resulted in the smaller size of the unit cell observed by XRD (Figure 1C). We also calculated the NMR parameters for a similar Al site with two Cl atoms, Al(BH4)2Cl2, but in this case the results were inconsistent with the experimental values (see Table S2 in Supporting Information). We finally note that a small amount of partially chlorinated product(s) was already discernable in the 13:3 sample (Figure 2B), suggesting an incomplete metathesis of the mechanochemical reaction product. In fact, the signal attributed to Al(BH4)3Cl features prominently in the

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Al 3QMAS NMR spectrum of the intermediates from the 13:3 mixture obtained

after 15-min ball-milling (See Figure S2 in Supporting Information).


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Figure 3. 2D

27

Al 3QMAS spectra of the products of LiBH4-AlCl3 mixtures ball-milled for 3h. The

spectra were obtained at B0 = 9.4 T using νR = 16 kHz, νRF(27Al) = 200 kHz and 12.5 kHz for hard and soft pulses, and νRF(1H) = 64 kHz for SPINAL64 1H decoupling. The data were acquired in 128 rows with ∆t1 = 8.33 µs, NS = 96 per row, and τRD = 1 s. The spectra were sheared and scaled as described elsewhere.37

Table 2. The experimentally and computationally obtained quadruple coupling parameters of

27

Al

spectra for ball-milled samples of LiBH4-AlCl3 mixtures. LiBH4:AlCl3

Phase

Al sites

molar ratio 13:3

11:3

CQ

(ppm)

(MHz)

ηQ

PQ (MHz)

Li4Al3(BH4)13a

Al(BH4)4

46b

calc

Al(BH4)4

46.0

Li4Al3(BH4)13-xClx

Al(BH4)4

46b

5.7

Al(BH4)3Cl

59b

4.2

Al(BH4)4

46.6

5.6

0.10

5.6d

Al(BH4)3Cl

61.8

4.7

0.25

4.7d

New phase

Al(BH4)4

46b

5.6

Al1

Al(BH4)2Cl2

77b

2.4c

Al2

Al(BH4)2Cl2

65b

3.1c

Al3

Al(BH4)4

51b

2.1c

calc

9:3

δcs,iso

5.8c 5.7

0.01

5.7d

Minor Cl substitution is expected.25 b δcs,iso was obtained from the MQMAS spectra, as described in reference 34. c PQ was obtained from the analysis of the 2D 27Al 3QMAS spectra. d The second-order a

(

)

12

quadrupolar effect parameter PQ is given by PQ = CQ 1 + ηQ 3 , where CQ and ηQ are the quadrupole coupling constant and the asymmetry of the electric field gradient tensor, respectively. 2



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As in the case of XRD and 11B SSNMR, further reduction of the LiBH4 content in the LiBH4:AlCl3 starting mixture to 9:3 resulted in a substantial change of the product, the

27

27

Al DPMAS spectrum. In the reaction

Al signal centered at 55 ppm disappeared, the signal at 38 ppm significantly decreased,

and three new peaks appeared at 74, 60, and 48 ppm, which will be hereafter denoted as Al1, Al2, and Al3. Additional double-resonance SSNMR methods were used to refine our understanding of these sites, as described below. 1

H-27Al MAS-J-HMQC. In analogy to the liquid-state HMQC experiment, the 2D MAS-J-HMQC

sequence relies on the polarization transfer via scalar (J) coupling, and hence probes the heteronuclear correlations between pairs of directly bonded nuclei. Knowing that only the BH4 protons are present in the studied materials, we used a simplified, 1D version of the 1H{27Al} HMQC experiment, in which Jpolarized

27

Al magnetization was observed with the evolution time t1 set to 0. The sequence used the

frequency-switched Lee-Goldburg (FSLG) homonuclear decoupling scheme38 to remove the 1H-1H dipolar coupling during the polarization transfer.33,39 The MAS-J-HMQC spectra (Figure 4) unequivocally demonstrate that Al1, Al2, and Al3 sites in the 9:3 ratio product are covalently bonded to the hydrogen of BH4 units, or at least the Al‒H bonding has some degree of covalent character. The magnitudes of the JAlH-couplings could not be estimated from this experiment, because the maximum transfer was observed at τmax = 2.8 ms due to fast T2’ relaxation (note that we measured the JAlHcouplings using the J-modulated spin-echo experiment described below). It is interesting to note that the 27

Al signal from Li4Al3(BH4)13 did not appear in the MAS-J-HMQC experiment, which suggests that

the BH4 units are present as isolated anions in the Li4Al3(BH4)13 structure.


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Figure 4. 1D

27

Al{1H} MAS-J-HMQC spectra of the 13:3 and 9:3 ratio products. The spectra were

obtained at B0 = 14.1 T using νR = 12.5 kHz, νRF(1H) = 100 kHz during 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 and the inversion π pulses, respectively, τmax = 2.8 ms, τRD = 1 s, and NS = 2400. 27

Al{11B} REDOR. In the REDOR experiment, a series of rotor-synchronized π pulses are typically

applied at the resonance frequency of non-observed nuclei (spins I), to reintroduce the heteronuclear dipolar coupling with the observed nuclei (spins S), which is otherwise fully or partially removed by MAS. Such experiments result in the so-called REDOR dephasing curve, which is constructed as a normalized difference, ∆S/S0 = [S0(τrec)-S(τrec)]/S0(τrec), between two spin-echo decays S0(τrec) and S(τrec), measured as a function of the recoupling time τrec without and with the recoupling π pulses, respectively. In the case of an isolated pair of spins with I = S = 1/2, ∆S/S0 can be fitted to a simple analytical formula which yields the dipole-dipole coupling constant, and thereby the internuclear distance rIS. The interpretation of the REDOR dephasing curves becomes more complicated in the presence of multiple-spin systems, such as SIn, because for long recoupling times the dipolar dephasing curves are sensitive to their geometry.40 It turns out, however, that in such case the initial slope of the REDOR curve, measured within the limit 0 < ∆S/S0 < ~0.2, can be fitted to a simple analytical formula,30 2 ∆S 4 = τ rec M 2IS 2 S 0 3π

( )

(4)

with 13 ACS Paragon Plus Environment


M 2IS =

4  µ0  15  4π

n  2 2 ( ) γ γ h I I + 1 rI−k 6S ,  I S ∑  Ik

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(5)

where M 2IS is the van Vleck’s heteronuclear dipolar second moment calculated over all spins I surrounding spin S,41 µ0 is the vacuum permeability, τrec is given by the multiple of the rotor period (NTR), γx is the gyromagnetic ratio of spin x, and rI k S (k = 1,2,…n) represent the internuclear I-S distances. Accordingly, in SIn spin systems with a single (and known) internuclear distance rIS the order ‘n’ can be determined from the initial slope of the REDOR curve. Further complications can arise if one, or both, of the spins involved are half-integer quadrupolar. However, in relatively common situations when only the central transition is effectively excited by the π pulses, eq. (4) can be written as ∆S f (τ rec )2 M 2IS , = 2 S0 I (I + 1)π

(6)

where the scaling factor f accounts for incomplete saturation of the non-observed spins and other experimental imperfections.42-43 The 27Al{11B} REDOR curves for the Al1, Al2, and Al3 sites in the 9:3 ratio product are shown in Figure 5. In order to use eq. (6), we first determined the scaling factor f, by measuring the 27Al{11B} REDOR curve for the known structure of Li4Al3(BH4)13 (f = 0.20). Since we have established the presence of covalent bonds between hydrogen in BH4 units and all three aluminum sites, we could safely assume that the Al-B pairs are approximately equidistant and are fixed in the solid structure of the 9:3 ratio product (i.e., do not undergo any motions that would affect the second moment M 2IS ). Finally, by using the internuclear distance rAl-B = 2.1 Å (which is a value known for Al(BH4)344), the number of BH4 units n in the first coordination sphere of each Al site becomes the sole fitting parameter in eq (6). The resulting fits, carried out over the range 0 < ∆S/S0 < 0.2 (Figure 5) show that the numbers of coordinated BH4 units are 2, 2, and 4 for Al1, Al2, and Al3, respectively. With the observed 27Al NMR


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shifts being consistent with tetrahedral coordination, the Al1 and Al2 sites must be coordinated to two chlorine atoms (Al(BH4)2Cl2).

Figure 5. 27Al{11B} REDOR curves for the Al1-Al3 sites in the 9:3 products. The solid lines represent the analytical curve assuming two (Al1 and Al2) and four (Al3) equivalent boron neighbors with an Al‒B distance of 2.1 Å. The experiments were carried out at B0 = 14.1 T using νR = 12.5 kHz, νRF(1H) = 100 kHz for the π pulse during τrec and 50 kHz for SPINAL-64 decoupling during the acquisition, νRF(27Al) = 50 kHz and 20 kHz for the initial π/2 and the inversion π pulses, respectively, τRD = 0.5 s, and NS = 2400.

J-modulated spin-echo. As the final confirmation of the HMQC and REDOR results, we estimated the JAl-H values by measuring the evolution of J-modulated spin-echoes for Al1, Al2, and Al3.34 As in the MAS-J-HMQC experiment, we used 1H-1H FSLG decoupling to extend the spin-echo evolution, which is given by

S (τ ) = S0 cosm (2πJ IS s f τ )e −2τ T2′ ,

(7)

where τ is the echo delay, S0 is the signal intensity without J-coupling, m represents the number of attached protons, T2’ is the previously mentioned transverse relaxation time that is non refocusable by a π pulse, and sf is the scaling factor introduced by homonuclear decoupling (1/√3 for FSLG). The experimental conditions and the echo decays measured for Al1, Al2, and Al3 in the range 0 < τ < 5 ms are reported in the Supporting Information (Figure S3 in Supporting Information). The experimental data were subsequently fitted to eq (7) in order to determine the value of m which yields the JAl‒H couplings that are in agreement with those observed for Al(BH4)3.45-48 For Al2, we obtained m = 4. For 15 ACS Paragon Plus Environment


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the remaining sites, two values of m yielded fits of similar quality: m = 3 and 4 for Al1, and m = 7 and 8 for Al3. Given that in the molecular substructures present in the 9:3 product each BH4 anion bonds to Al via two hydrogens, the even values of m appear more likely. Taken together, our SSNMR results suggest that in each Al(BH4)xCl4-x complex, the nonhydrogen atoms form a tetrahedral AlBxCl4-x skeleton with two hydrogen atoms of each borohydride group being bridged between the aluminum and boron atoms, as depicted in Figure 6.21-22, 25, 49-51

Figure 6. Schematic diagrams of aluminum coordination described in this study: (A) Al(BH4)4 unit in Li4Al3(BH4)13 (13:3 product), (B) Al1 and Al2 sites in Al(BH4)2Cl2 (9:3 product), and (C) Al3 site in Al(BH4)4] (9:3 product). Al, B, Cl, and H atoms are shown in light blue, dark green, light green, and white, respectively. 4. Conclusion In summary, the combined use of SSNMR, XRD, volumetric analysis and RGA measurements provided new insights into a series of hydrogen-containing materials mechanochemically synthesized from LiBH4 and AlCl3. While the high total hydrogen content and the low hydrogen desorption temperature of Li4Al3(BH4)13 produced from the 13:3 ratio mixture are clearly advantageous, this compound releases a significant amount of diborane. We found that a decrease of LiBH4 content in the starting mixture leads to formation of complexes in which diborane emission can be suppressed without affecting the onset temperature of hydrogen release. The 11:3 ratio mixture yields a partially chlorinated product, Li4Al3(BH4)13-xClx, which preserves the Li4Al3(BH4)13 structure, albeit with a minor reduction of the phase volume due to partial replacement of the borohydride with a chlorine anion. The 9:3 ratio mixture no longer leads to the chlorinated derivative of Li4Al3(BH4)13; instead, a compound containing 16 ACS Paragon Plus Environment

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[Al(BH4)2Cl2]- complex was identified, in which the non-hydrogen atoms form a tetrahedral AlB2Cl2 skeleton with two hydrogen atoms of each borohydride group being bridged between the central aluminum and boron atoms. In addition, SSNMR experiments indicate the covalent character of the Al– H bonding, which was not observed in Li4Al3(BH4)13. These results suggest that the insertion of Cl alters the nature of the Al–BH4 bonding and consequently affects the dehydrogenation process. Even though the majority of past efforts have been devoted to improving the dehydrogenation process of metal borohydrides by substituting various metal cations and creating double-cation borohydrides, the results of this work clearly suggest an alternative, i.e. tuning the dehydrogenation process of metal borohydrides by manipulating the anion moieties instead. ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publication website at DOI. The SI contains: 1D 11B DPMAS spectrum of LiBH4 reference, 2D 27Al 3QMAS NMR spectrum of the 13:3 ratio product after 15 min of ball-milling, theoretical DFT calculation details, and evolution curves of the 27Al signal intensities in the J-modulated spin-echo experiment. Corresponding Authors *T.K.: [email protected]. *V.K.P.: [email protected] *M.P.: [email protected] Notes The authors declare no competing financial interests. Acknowledgement We would like to thank Dr. Viktor Balema and Dr. Ihor Hlova for fruitful discussions of the results. This work was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency & 17 ACS Paragon Plus Environment


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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.


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Structural Characterization of the Products by Solid-State NMR

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