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Article

Lewis Base Complexes of Magnesium Borohydride: Enhanced Kinetics and Product Selectivity upon Hydrogen Release

by
Marina Chong
1,
Tom Autrey
1,2,* and
Craig M. Jensen
2,*
1
Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99352, USA
2
Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822, USA
*
Authors to whom correspondence should be addressed.
Inorganics 2017, 5(4), 89; https://doi.org/10.3390/inorganics5040089
Submission received: 3 November 2017 / Revised: 27 November 2017 / Accepted: 28 November 2017 / Published: 6 December 2017
(This article belongs to the Special Issue Functional Materials Based on Metal Hydrides)
Browse Figures
Versions Notes

Abstract

:
Tetrahydofuran (THF) complexed to magnesium borohydride has been found to have a positive effect on both the reactivity and selectivity, enabling release of H2 at <200 °C and forms Mg(B10H10) with high selectivity.

Graphical Abstract

1. Introduction

Over the past decade, there has been a significant international effort involving chemists, materials scientists and physicists to discover and demonstrate a solid-state hydrogen storage material that would enable a fuel cell electric vehicle 5 min refueling time and a 500 km driving range. Only a few of the thousands of materials investigated have garnered as much interest as Mg(BH4)2 [1,2,3,4,5,6,7,8,9,10,11,12]. The high gravimetric density of H2, ca. 14.7 wt % H2 and thermodynamics for H2 release lie in the narrow window required for reversibility under moderate pressure and temperature. The dehydrogenation of the borohydride to MgB2 has a calculated ΔH0 of 38.6 kJ/(mol H2) and ΔS of 111.5 J/(K·mol H2), predicting a plateau pressure of 1 bar H2 of 73 °C [13]. These thermodynamic properties together with the borohydride’s high gravimetric hydrogen density, and demonstrated hydrogen cycling compatibility [1,9] suggest its application as a reversible hydrogen carrier for PEM fuel cell applications. Two critical challenges remaining are (i) the slow rates of hydrogen release and (ii) the thermodynamic stability of the major dehydrogenation product, magnesium dodecaborane, Mg(B12H12), occasionally referred to as the dead-end for reversibility.
At temperatures greater than 460 °C the borohydride releases ~14 wt % hydrogen, giving mixture of products, i.e., MgB2, MgH2, Mg and amorphous boron, depending on reaction conditions [6,10,11,14,15]. Hydrogenation of this product mixture at 400 bar H2 and 270 °C results in the uptake of 6.1 wt % hydrogen [5]. NMR studies concluded that MgB12H12, forming at temperatures greater than 250 °C, is a thermodynamic endpoint, preventing re-hydrogenation to Mg(BH4)2 [4]. On the other hand, the reversal of MgB2 to Mg(BH4)2 occurs, albeit, under extreme conditions of 950 bar H2 and 400 °C [9]. This demonstrated that reversibility can be achieved, however, under conditions that are impractical for commercial hydrogen storage applications. Similarly, the lithium, sodium, and potassium salts of B12H122− have been hydrogenated, in the presence of metal hydrides, to the corresponding borohydride under 1000 bar of H2 at 500 °C [16]. Whether the pathway for the hydrogenation of MgB2 to Mg(BH4)2 involves MgB12H12 remains an open question.
The use of additives to enhance kinetics of hydrogen release from Mg(BH4)2 has been the subject of several investigations [17,18,19,20]. An early study found that TiCl3 lowered the onset temperature of hydrogen release from 262 to 88 °C [17]. More recently, significant reductions in the onset temperature of hydrogen release have been observed upon the addition of NbCl5 and a Ti–Nb nanocomposite [18]; metal fluorides such as CaF2, ZnF2, TiF3, and NbF5 [18,19,20] and ScCl3 [19]. Hydrogen release is also induced by mechanically milling Mg(BH4)2 with TiO2 resulting in release of 2.4 wt % H2 at 271 °C while undergoing reversible dehydrogenation to Mg(B3H8)2 [20]. Alternatively, the thermal dehydrogenation of Mg(BH4)2 has been shown to be accelerated in eutectic mixtures with LiBH4 [21,22,23]. Another study claimed that the addition of LiH to Mg(BH4)2 induced hydrogen evolution at temperatures as low as 150 °C and enabled the cycling of 3.6 wt % H2 through 20 cycles at 180 °C [24].

2. Results

The high temperature and pressure required for reversibility led us to explore the decomposition pathways at lower temperatures. The decomposition of Mg(BH4)2 over a prolonged period (5 weeks) under 1 bar nitrogen at 200 °C yields Mg(B3H8)2 as the major product [1]. While formation of the B3H8− anion has been recognized from thermal condensation studies of BH4− in solution [25], this finding provided evidence that an analogous process may take place during solid state decomposition contrary to theoretical predictions. Furthermore, under 120 bar hydrogen pressure and 250 °C, the Mg(B3H8)2 intermediate was completely converts back to Mg(BH4)2 after 48 h. The subsequent hydrogenation of independently synthesized Mg(B3H8)2·THF (THF = tetrahydofuran), where attempts to remove the solvent were unsuccessful, then demonstrated that quantitative re-hydrogenation to Mg(BH4)2 could be achieved under 50 bar H2 and 5 h at 200 °C [26]. We concluded that the faster rate exhibited by the solvated sample resulted from a phase change induced by the coordination of the THF. Studies of borohydrides and boranes in the context of hydrogen storage, have typically focused on complete solvent removal. The presence of residual solvent is generally considered problematic and the various synthetic routes to Mg(BH4)2 often call for rigorous efforts to obtain a pure, solvent-free product. However, our findings suggested that the solvent coordination might have the beneficial effect of enhancing dehydrogenation kinetics. Only a handful of studies have explored the dehydrogenation of Mg(BH4)2 coordinated to a solvent, the majority of which have highlighted nitrogen donors [27,28,29,30].
Our observation of the kinetic enhancement of the hydrogenation of Mg(B3H8)2 to Mg(BH4)2 prompted us to further explore how solvent coordination affects hydrogen release temperatures. We have examined the effect of dimethyl sulfide (DMS), diethyl ether (Et2O), triethylamine (TEA), diglyme (Digly), dimethoxy ethane (DME) and THF, encompassing a range of Lewis basicity, on the decomposition of Mg(BH4)2. Alternative syntheses, complex polymorphism, predicted thermodynamic properties, and attempts to improve the hydrogen cycling capacity of Mg(BH4)2 have been widely explored and reviews of these activities were recently published [20,31]. However, the solid-state chemistry of the interconversion of the borane intermediates involved in these systems remains largely unexplored. Therefore, a unique aspect of this work has been the direct observation and characterization of the borane products and metastable reaction intermediates by MAS and solution phase 11B NMR studies.
The TEA, Et2O, Digly, DME and THF solvates of Mg(BH4)2, were prepared by adding an excess of solvent to Mg(BH4)2 at room temperature. Subsequently the solvent was removed en vacuo to obtain a crystalline solid. The stoichiometry of the solvates was determined from the relative integrated intensities of the signals observed in the 1H NMR spectra as summarized in Table 1. Where we could find crystal structure information for solvates of Mg(BH4)2, the stoichiometry of solvate to Mg cation determine by NMR in our work is slightly greater than reported for Mg(BH4)2·DME 1:1.5 and slightly lower for Mg(BH4)2·THF 1:3 [33].
Unsolvated Mg(BH4)2 and solvate powders were dehydrogenated via combinatorial screening equipment made by Unchained Labs® (Pleasanton, CA, USA), consisting of a 24 well plate design. Heating of the samples was conducted in a screening pressure reactor at 180 °C for 24 h under N2 flow. Product ratios determined by 11B NMR are shown in Table 2. Entry 1 shows the low reactivity of unsolvated Mg(BH4)2 at 180 °C with 93% BH4− remaining. This result is typical of the slow kinetics of dehydrogenation for borohydride complexes at temperatures below 300 °C. Dehydrogenation of the TEA complex favored formation of B3H8−, along with a trace amount of B10H102−. The ether additives showed higher levels of dehydrogenation at 180 °C. Another difference found with the ether complexes is the observation of B10H102− as the major product, suggesting either a competing dehydrogenation path or that the presence of these ether ligands encourages further reactivity of the B3H8− to form more deeply dehydrogenated products. Of the ether solvates, DME and THF provided the highest conversion of BH4− with B10H102− as the major product. Only small amounts of B12H122−, demonstrating that the decomposition was ~10× more selective for B10H102− than B12H122−, much higher than the ~1.5× selectivity exhibited by the Digly solvate. These findings motivated further exploration of the dehydrogenation reaction.

3. Discussion

A recent study asserted that closo-boranes are secondary products formed upon aqueous workup of the low temperature dehydrogenation reactions [34]. To determine if formation of B10H102− occurs directly in the solid state reaction, the 11B VT MAS NMR spectrum of dehydrogenated (1 atm N2 at 180 °C for 24 h) Mg(BH4)2·THF was obtained (Figure 1). At room temperature, the observed resonances were broad, typical of solid state spectra for quadrupolar nuclei. Heating the sample to 160 °C sharpened the BH4− peak and the resonances for B10H102− at −2 and −27 ppm could be resolved. The peaks at −2 and −27 ppm assigned to the basal and apical boron in B10H10 based on the 1:4 ratio integration ratio in the solid state spectrum at 160 °C. At 20 °C the peaks are barely perceptible from the baseline. The sample is subsequently dissolved in a mixture of THF/D2O for solution NMR analysis. A solution phase spectrum of the dehydrogenated complex was obtained for comparison of product distribution and line width after dissolution in THF/D2O (Figure 1c). The same high selectivity for B10H102− is observed in both solution (−2, −30 ppm) and solid-state NMR with respective yields of 19% and 18%.
In situ VT MAS 11B NMR studies of the Mg(BH4)2·THF complex provides additional insight. As seen in Figure S1, the room temperature spectrum contains resonances for both unsolvated and solvated Mg(BH4)2 at −41 and −44 ppm respectively. See Figure S2 for a reference spectrum of unsolvated Mg(BH4)2. The downfield shift of the THF solvated BH4− complex is comparable to the downfield shift reported for Mg(BH4)2·4NH3 [37]. Upon heating the two peaks collapse into a single narrow peak at 90 °C. We interpret the narrow line width (FWHM = 32 Hz) as being indicative of a fluid phase. This is consistent with the observation of the melting of the THF solvate between 80–100 °C in a melting point apparatus and similar to the m.p. of 90 °C reported for Mg(BH4)2·2NH3 [36].
A comparison of the IR spectra of the solvated and unsolvated Mg(BH4)2 complex is shown in Figure 2. The single prominent stretch observed in the B–H stretching region between 2300–2500 cm−1 [38] for Mg(BH4)2 is indicative of lack of directional bonding between the Mg cation and the tetrahedral environment of BH4−. The additional coordination of THF molecules results in the BH4− also bonding in a mono or bidentate mode to the Mg cation. This lowering of symmetry leads to a spectrum with a number of overlapping bands occur between 2000–2500 cm−1. The modified coordination may play a role in the dehydrogenation mechanism and energetics.
The melting of the THF adduct is also likely to be a contributing factor to the enhanced kinetics. However, the onset of dehydrogenation occurs at temperatures above the melting point of the THF complex. The THF may also reduce the activation energy of clustering to form more deeply dehydrogenated products by altering the coordination mode between Mg2+ and BH4− through donation of electron density or steric interactions. The high selectivity for MgB10H10 over MgB12H12 is surprising. Either THF influences the reaction pathway, i.e., lower the barrier for a branching point that pushes the reaction towards MgB10H10 formation or THF flips the thermodynamic stability of the closoboranes making MgB10H10 more stable than MgB12H12.

4. Materials and Methods

All sample preparation and storage was conducted either in a nitrogen glovebox or on a Schlenk line. Solvents were dried over molecular sieves and verified by NMR for purity before use.

4.1. Synthesis of Mg(BH4)2

Magnesium borohydride was synthesized following a method described by Zanella et al. Di-n-butylmagnesium (Sigma Aldrich, Milwaukee, WI, USA) was added dropwise to borane-dimethylsulfide complex (Sigma Aldrich) in toluene according to the reaction scheme:
3Mg(C4H9)2 + 8BH3·S(CH3)2 → 3Mg(BH4)2·2S(CH3)2 + 2B(C4H9)3·S(CH3)2
The mixture was allowed to stir at room temperature for a minimum of 3 h and subsequently filtered, washed with toluene, and dried en vacuo at room temperature for 6 h and then at 75 °C overnight. The product, a fine white powder, was found to consist of >95% α-Mg(BH4)2 by XRD analysis.

4.2. Synthesis of Solvent Adducts of Mg(BH4)2

The TEA, Et2O, Digly, and THF solvates of Mg(BH4)2, were typically prepared by adding an excess of solvent to Mg(BH4)2 at room temperature and stirring for 30 min. Excess solvent was then removed en vacuo either at room temperature or up to 45 °C for higher boiling point solvents, for as long as needed to obtain a crystalline solid. The DMS adduct was obtained during the synthesis of Mg(BH4)2 as described above prior to removal of the DMS by heating.

4.3. Characterization of Mg(BH4)2 Adducts and Decomposition Products by Solution NMR

Powders were typically dissolved in a 1:2 mixture of THF:deuterium oxide (D2O) and analyzed within 10 min on a Varian 300 MHz spectrometer with 11B chemical shifts referenced to BF3·Et2O (δ = 0 ppm) and 1H referenced to TMS (δ = 0 ppm). 11B was measured at 96.23 MHz and 1H was measured at 299.95 MHz. A relaxation delay of 15 s was used for all 11B analyses with a 90° pulse width of 6 μs. An external standard was added to the quartz NMR tubes to determine the solubility of the powder in the THF/D2O mixture. The standard consisted of an aqueous solution of sodium tetraphenylborate (NaBPh4) sealed in a glass capillary. Calculation of percent composition of decomposed products was based on peak areas.

4.4. Solid State NMR

Sample powders were packed into 4 mm zirconium oxide rotors and spun at 12 kHz on a Varian 500 MHz spectrometer (Varian, Palo Alto, CA, USA) equipped with a HX 4 mm probe.

4.5. In Situ NMR

The characterization of Mg(BH4)2·THF during heating to 200 °C was conducted by variable temperature (VT) solid state magic angle spin (MAS) NMR in a Varian 500 MHz spectrometer 5 mm HXY probe. 1H and 11B shifts were referenced to tetramethylsilane at 0 ppm and lithium borohydride at −41.6 ppm and measured at 499.87 and 160.37 MHz respectively. 1H and 11B spectra were obtained with a 2 s and 5 s relaxation delay and 90° pulse width of 6 μs. The sample powder was packed in a 5 mm zirconia rotor under 1 atm N2 with a Teflon spacer and then capped with a customized plastic bushing capable of withstanding pressures up to 200 bar. The details of the rotor design are given in detail elsewhere [32,33] and have been modified to accommodate 5 mm rotors. The rotors were spun at 5 kHz at room temperature and subsequently heated at a rate of about 6 °C/min and held at specific temperatures during the ramp at which 11B and 1H spectra were obtained. The duration of the analyses at the set temperatures was approximately 45 min.

5. Conclusions

In summary, characterization of the dehydrogenation products arising from Mg(BH4)2·THF complex by solution and solid-state NMR shows that the dehydrogenation mechanism is highly selective for B10H102− over B3H8− (theoretical H2 release 8.1 wt % vs. 2.5 wt % in the absence of solvates) and B12H122−, a kinetic dead end. The dehydrogenation of Mg(BH4)2 at temperatures below 200 °C and potential for cycling between Mg(BH4)2 and MgB10H10 have significant implications for hydrogen storage applications. Further studies into optimizing the reaction through modification of ligand to Mg ratios are currently underway.

Supplementary Materials

The following are available online at www.mdpi.com/2304-6740/5/4/89/s1, Figure S1: In situ VT MAS 11B NMR of Mg(BH4)2·THF at room temperature and 90 °C. Figure S2: 11B MAS spectra of unsolvated Mg(BH4)2 and solvated Mg(BH4)2·THF.

Acknowledgments

The authors gratefully acknowledge research support from the Hydrogen Materials—Advanced Research Consortium (HyMARC), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. We authors thank Junzhi Yang for preliminary experimental results on solvated magnesium borane complexes, Heather Job for assistance with the combinatorial decomposition experiments, Gary Edvenson for insightful discussion on borane cluster chemistry and David Hoyt and Sarah Burton from the Environmental Molecular Science Laboratory (EMSL) for assistance with the solid state NMR. EMSL is a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL a multi-program national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830.

Author Contributions

Marina Chong, Tom Autrey and Craig Jensen conceived and designed the experiments; Marina Chong performed the experiments; Marina Chong, Tom Autrey and Craig Jensen contributed to analyzing the data and writing the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 05 00089 g001 550
Figure 1. 11B NMR spectra of Mg(BH4)2·THF (tetrahydofuran) dehydrogenated (a) solution phase dissolved in 1:2 THF:D2O, (b) solid state collected at 160 °C and (c) solid state collected at 20 °C. Experimental set-up described in references [35,36].
Figure 1. 11B NMR spectra of Mg(BH4)2·THF (tetrahydofuran) dehydrogenated (a) solution phase dissolved in 1:2 THF:D2O, (b) solid state collected at 160 °C and (c) solid state collected at 20 °C. Experimental set-up described in references [35,36].
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Inorganics 05 00089 g002 550
Figure 2. Attenuated Total Reflectance-Infrared spectra of unsolvated Mg(BH4)2 blue spectra with simple B–H stretching region and Mg(BH4)2·THF red spectrum with complex B–H stretching frequency.
Figure 2. Attenuated Total Reflectance-Infrared spectra of unsolvated Mg(BH4)2 blue spectra with simple B–H stretching region and Mg(BH4)2·THF red spectrum with complex B–H stretching frequency.
Inorganics 05 00089 g002
Table
Table 1. Ligand ratios in synthesized solvates, determined by 1H NMR.
Table 1. Ligand ratios in synthesized solvates, determined by 1H NMR.
SolvateMg:Ligand Ratio
Mg(BH4)2·DMS §1:0.34
Mg(BH4)2·TEA1:1.8
Mg(BH4)2·Et2O1:0.36
Mg(BH4)2·Digly1:1.18
Mg(BH4)2·DME1:2.2
Mg(BH4)2·THF1:2.8
§ The dimethyl sulfide (DMS) solvate was obtained through the synthetic protocol as described by Zanella et al. [32]. The DMS is weakly bond to the magnesium cation and readily removed by heating.
Table
Table 2. Distribution of products of Mg(BH4)2 solvates determined from integration of 11B NMR peaks in mol % after dehydrogenation at 180 °C, 24 h, 1 atm N2. The balance of products consist of trace quantities of boric acid due to hydrolysis of unstable polyboranes.
Table 2. Distribution of products of Mg(BH4)2 solvates determined from integration of 11B NMR peaks in mol % after dehydrogenation at 180 °C, 24 h, 1 atm N2. The balance of products consist of trace quantities of boric acid due to hydrolysis of unstable polyboranes.
SampleB10H102−B3H8−B12H122−BH4−
Mg(BH4)2 3 93
Mg(BH4)2·TEA26 89
Mg(BH4)2·Et2O44 88
Mg(BH4)2·Digl52382
Mg(BH4)2·DME4614430
Mg(BH4)2·THF3112339

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Chong, M.; Autrey, T.; Jensen, C.M. Lewis Base Complexes of Magnesium Borohydride: Enhanced Kinetics and Product Selectivity upon Hydrogen Release. Inorganics 2017, 5, 89. https://doi.org/10.3390/inorganics5040089

AMA Style

Chong M, Autrey T, Jensen CM. Lewis Base Complexes of Magnesium Borohydride: Enhanced Kinetics and Product Selectivity upon Hydrogen Release. Inorganics. 2017; 5(4):89. https://doi.org/10.3390/inorganics5040089

Chicago/Turabian Style

Chong, Marina, Tom Autrey, and Craig M. Jensen. 2017. "Lewis Base Complexes of Magnesium Borohydride: Enhanced Kinetics and Product Selectivity upon Hydrogen Release" Inorganics 5, no. 4: 89. https://doi.org/10.3390/inorganics5040089

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