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Article

Release of Pure H2 from Na[BH3(CH3NH)BH2(CH3NH)BH3] by Introduction of Methyl Substituents

by
Ting Zhang
,
Timothy Steenhaut
,
Michel Devillers
and
Yaroslav Filinchuk
*
Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Place Louis Pasteur 1, 1348 Louvain-la-Neuve, Belgium
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(5), 202; https://doi.org/10.3390/inorganics11050202
Submission received: 10 April 2023 / Revised: 2 May 2023 / Accepted: 5 May 2023 / Published: 7 May 2023
(This article belongs to the Special Issue State-of-the-Art and Progress in Metal-Hydrogen Systems)
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Abstract

Over the last 10 years, hydrogen-rich compounds based on five-membered boron–nitrogen chain anions have attracted attention as potential hydrogen storage candidates. In this work, we synthesized Na[BH3(CH3NH)BH2(CH3NH)BH3] through a simple mechanochemical approach. The structure of this compound, obtained through synchrotron powder X-ray diffraction, is presented here for the first time. Its hydrogen release properties were studied by thermogravimetric analysis and mass spectrometry. It is shown here that Na[BH3(CH3NH)BH2(CH3NH)BH3], on the contrary of its parent counterpart, Na[BH3NH2BH2NH2BH3], is able to release up to 4.6 wt.% of pure hydrogen below 150 °C. These results demonstrate that the introduction of a methyl group on nitrogen atom may be a good strategy to efficiently suppress the release of commonly encountered undesired gaseous by-products during the thermal dehydrogenation of B-N-H compounds.

1. Introduction

In the field of chemical hydrogen storage [1,2,3], boron–nitrogen–hydrogen (B-N-H) compounds have emerged as promising candidates owing to the light weight of boron and nitrogen and to their ability of bearing multiple hydrogens. Additionally, B–H and N–H bonds tend to be hydridic and protic, respectively, resulting in normally facile hydrogen release [4,5,6,7,8,9,10]. A typical representative of B-N-H materials is ammonia borane (NH3BH3, or AB), which contains three hydridic and protic hydrogens on the N and B atoms, respectively. Ammonia borane has attracted consideration attention for hydrogen storage due to its high gravimetric storage density (up to 19.6 wt.%), high stability under ambient conditions, low toxicity, and high solubility in common solvents [11,12,13,14,15]. However, one of the drawbacks of NH3BH3 for hydrogen storage is the decomposition temperature. It starts releasing the first equivalent of hydrogen at about 120 °C, and a second hydrogen elimination step occurs at approximately 145 °C; the remaining amount of hydrogen is not released until more than 500 °C. Moreover, its decomposition is exothermic and thus irreversible, and it releases multiple volatile byproducts such as NH3, N3B3H6, and B2H6, making the chemical hydrogenation process more challenging. In addition, the thermal decomposition of AB is furthermore paired with severe foaming and volume expansion [14,16,17]. To overcome these disadvantages, several strategies have been employed, including nanoconfinement using nanoscaffolds, catalytic effects, ionic liquid assistance, the hydrolysis reaction, and chemical modification of NH3BH3 through replacing one of the H atoms in the –NH3 group of NH3BH3 by a metal, forming metal amidoboranes (MABs) [7,15,18,19,20,21,22,23,24,25]. Among these strategies, the formation of metal amidoboranes as a popular option show a number of advantages over neutral NH3BH3: (i) lower hydrogen release temperatures than that of pristine NH3BH3 [26]; (ii) generally the released hydrogen is not contaminated with undesirable borazine by-products [19,27,28]; (iii) the de-hydrogenation process is much less exothermic, about 3 to 5 kJ/mol [26,29], vs. 22.5 kJ/mol for NH3BH3 [17,30]. Furthermore, the introduction of metals increases the diversity of hydrogen storage candidates based on B-N-H compounds. Recently, five-membered chain anions having the general formula [BH3NH2BH2NH2BH3]−, also known under the abbreviation [B3N2]−, have emerged as a novel group of ammonia borane derivatives [31,32,33,34,35,36,37,38]. M[B3N2] compounds have a higher hydrogen content than MABs, and the Li and Na [B3N2]− derivatives are stable at room temperature, on the contrary of their respective MABs [33]. However, the interest in M[B3N2] is much more recent than for MAB, and therefore there are only few reports about their synthesis, structure, characterization, and hydrogen storage properties. In 2011, the salt of Verkade’s base (2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane, C18H39N4P, VB, chemical formula see Figure S1) with [B3N2] was synthesized, and its structure was characterized, with the aim of studying the activating effect of VB on the rate and extent of H2 release from NH3BH3 [31]. Two years later, the same authors reported the synthesis of the sodium salt and two substituted Na[BH3N(R)HBH2N(R)HBH3] salts (R = H, Me, and benzyl), to further study the growth of aminoborane oligomers through the de-hydrocoupling reactions of NH3BH3 [32]. Interestingly, since 2014, the salts of the [BH3NH2BH2NH2BH3]− anion with different cations have been synthesized with a focus on the study of their hydrogen storage properties (see Table S1). Generally, these kinds of complexes that are studied for their hydrogen storage properties can be classified into three types, based on their cations: ionic liquids, ammonium, and alkali metal salts. A total of four [B3N2]− ionic liquids have been described: [Bu4N][B3N2], [Et4N][B3N2], [C(N3H6)][B3N2], and [C(N3H5CH3)][B3N2] [35]. Among them, [Bu4N][B3N2] and [Et4N][B3N2] release pure hydrogen below 160 °C [35]. [NH4][B3N2] was reported this year, as the minor component of a 1:3 mixture with NH3BH2NH2BH2NH2BH3. Despite its impressive hydrogen content, this system releases H2 with substantial contamination by borazine and traces of ammonia and diborane [38]. Among alkali metal (Li–Cs) salts of [B3N2]−, only Li[B3N2] was shown to release pure hydrogen during thermal decomposition [33,34]. However, it is the sodium salt, Na[B3N2], that was studied the most in the literature until now, with five synthesis approaches (four wet chemical and one dry mechanochemical) reported between 2013 and 2021. In 2013, Sneddon and co-workers reported that Na[B3N2] could be obtained from NaN(SiMe3)2–3 NH3BH3 in fluorobenzene at 50 °C for 24 h [32]. Grochala et al. synthesized the same compound from NaH-3NH3BH3 in THF at room temperature for 24 h and obtained Li[B3N2] by a similar approach [33]. The same authors later used a metathesis method to obtain Na[B3N2] from VBH[B3N2] and M[Al{OC(CF3)3}4] (M = Na) in CH2Cl2 at room temperature for 1 h and were able to obtain the related K, Rb, and Cs salts by this method as well [34]. Although the metathesis is fast, the two precursors involved in this kind of reaction need to be synthesized first, adding a second step to the preparation of the salt. In 2021, Chen et al. reported a facile synthetic method to obtain Na[B3N2], based on the reaction of NaNH2BH3 with NiBr2 or CoCl2 as a catalyst [36]. Results showed that the reaction with 0.05 equiv. of NiBr2 in THF at 0 °C could produce the final Na[B3N2] after 10 h, with a yield of 60%. The main advantages of the dry mechanochemical synthesis are that it avoids the use of solvent and usually simplifies the drying process [39,40]. However, the reported procedures for the synthesis of Li[B3N2] and Na[B3N2] require two stages of milling at room temperature, followed by a removal of the by-products (NH3) upon heating [33,34]. Moreover, long time reaction times and/or complicated operating processes are usually needed for the synthesis of the alkali metal [B3N2] compounds.
With its 12.7 wt.% hydrogen content, Na[B3N2] has potential for hydrogen storage. However, the hydrogen released when heating this compound is contaminated by unwanted by-products, including NH3, B2H6, and larger fragments detected by mass spectrometry [33,34]. In one of our previous studies, we found that, compared to NH3BH3, the reaction of CH3NH2BH3 with NaAlH4 leads to a product with a completely different thermal behavior. This is likely due to the space hindrance and the electronic effect caused by the introduction of the methyl group [41]. Similar introduction of a methyl group on the N atoms of [BH3NH2BH2NH2BH3]− in Na[B3N2] would affect the geometry of the B-N-B-N-B skeleton and change the inter-anion dihydrogen bonds, potentially positively affecting the hydrogen properties of the compound. With this in mind, we synthesized Na[BH3(CH3NH)BH2(CH3NH)BH3] (abbreviated here as Na[B3(MeN)2]) through a new convenient mechanochemical synthesis method from easily accessible NaH and CH3NH2BH3. We also report that its structure, solved from synchrotron powder X-ray diffraction (PXRD), enables a better understanding of the structure–properties relationships. Its thermal dehydrogenation was also investigated, by thermogravimetric analysis (TGA) and mass spectrometry, revealing a release of pure hydrogen and thus confirming our hypothesis. The purity of hydrogen released from Na[BH3NH2BH2NH2BH3] was enhanced by the introduction of methyl groups on N atoms. This achievement represents the first successful suppression of the unwanted by-product release through the introduction of -CH3 groups on the nitrogen atoms of [B3N2]−. Furthermore, the structure of Na[BH3(CH3NH)BH2(CH3NH)BH3] was analyzed for the first time helping to understand the potential reasons behind the improved hydrogen purity. This study provides valuable insights into the relationship between the hydrogen release properties and the structure of B-N-H compounds. The introduction of methyl groups on the nitrogen atoms of Na[B3N2] to enhance hydrogen purity could potentially be extended to the other M[B3N2] or even other M-B-N-H compound. It could even be expanded to include the use of other small alkyl groups or small electron-donating groups instead of the methyl group.

2. Results and Discussion

The reaction between NaH and NH3BH3 in various molar ratios was reported to produce different hydrogen rich B-N compounds, i.e., NaNH2BH3 (1:1), NaBH3NH2BH3 (1:2), NaBH3NH2BH2NH2BH3 (1:3), NaBH3NH2BH2NH2BH2NH2BH3, and NaBH3NH2BH(NH2BH3)2 (1:4) [42]. All of those have potential for application in hydrogen storage due to their high H content (see Table S2). Despite the potential of this system, there is only one report of the reaction between the methyl-substituted CH3NH2BH3 and NaH, in a 1:3 molar ratio [32]. We thus investigated the reaction of NaH and CH3NH2BH3 by mechanochemistry (Figure 1A), to avoid an incorporation of or a reaction with solvents. Upon milling NaH-CH3NH2BH3 systems in different molar ratios, new peaks appeared on the PXRD pattern of all the tested ratios, along with some unreacted NaH for the 1:1 system (Figure 1B). The 11B NMR spectra of the resulting products furthermore showed the appearance of a new quadruplet signal located between −14.45 and −16.07 ppm, which likely belongs to the BH3 unit of Na[CH3NHBH3]. When the NaH:CH3NH2BH3 ratio was increased to 1:2, the PXRD pattern of the obtained product shows peaks corresponding to crystalline Na[BH3(CH3NH)BH2(CH3NH)BH3]. Although the 11B NMR spectrum of the product displays a triplet signal of BH2 (−2.24 ppm) and quadruplet signal of BH3 (−16.39 ppm), expected for the [BH3(CH3NH)BH2(CH3NH)BH3]− anion, other signals on the spectrum reveal the presence of unknown non-crystalline by-products (Figure 1C). Further increasing the ratio to 1:3 leads to the appearance of only the signals of BH2 and BH3 from the [BH3(CH3NH)BH2(CH3NH)BH3]− anion on the 11B NMR spectra. The phase purity of the compound obtained upon 27 h of ball milling the 1:3 mixture was confirmed by temperature programmed synchrotron powder X-ray diffraction (PXRD) measurements. Indeed, the complete set of peaks of the pattern disappeared at once at around 150 °C (Figure S2). Based on the 11B NMR spectrum and the temperature ramping synchrotron PXRD experiment, we deduce that relatively pure Na[B3(MeN)2] with five membered B-MeN-B-MeN-B chains was formed and that the reaction shown in Figure 1A was complete.
The obtained Na[B3(MeN)2] was further characterized by infrared (IR) spectroscopy (Figure 2). On the spectrum, it can be seen that the asymmetry of the N-H stretching band (3162 cm−1) disappeared, compared with the CH3NH2BH3 precursor. This is because one hydrogen on the nitrogen of CH3NH2BH3 is released, combined with a hydride atom from NaH to form H2. In addition, the N-H bending of Na[B3(MeN)2] (1457–1491 cm−1) is redshifted compared to CH3NH2BH3 (1596 cm−1). This can be attributed to the introduction of weak electron donating methyl group on N atom, influencing the electron density of N and further having an effect on the N-H band. The broad band located in the region of 2000–2500 cm−1 belongs to the B-H stretching band. There is no significant difference compared to the CH3NH2BH3 precursor. However, the signal of the B-N stretching is widened and split in Na[B3(MeN)2] (692–716 cm−1), due to the presence of two types of B-N bands in Na[B3(MeN)2], whereas CH3NH2BH3 exhibits only one B-N band.
All of the aforementioned differences between Na[B3(MeN)2] and CH3NH2BH3 align with our previous analysis based on XRD and 11B NMR, further confirming the proposed formula of the product as shown in Figure 1A.
The structure of Na[B3(MeN)2] was determined by direct space methods from synchrotron powder X-ray diffraction (PXRD) data, indexed in the monoclinic space group P21/n; the final Rietveld refinement profile is shown in Figure S3. The [BH3(CH3NH)BH2(CH3NH)BH3]− anion is a five-membered B-N chain with an alternance of B and N atoms connected in a similar way as in the reported [BH3NH2BH2NH2BH3]− [33]. Although the N atoms in Na[B3(MeN)2] have four different substituents and are therefore chiral, the crystal structure reveals that the anions are integrated in the solid as a meso compound, as both N atoms possess opposite chirality. This is in agreement with reported DFT calculations, which indicate that the meso isomer is the preferred stereoisomer for this anion [32]. Due to the introduction of the methyl substituents, the skeleton of the B-N-B-N-B chain shows a twisted geometry, which is in contrast with the linear geometry adopted by the [B3N2]− anion in the Li, Na, and K salts but is similar to the reported Rb and Cs [B3N2]− salts [34]. This type of geometry enables the formation of intramolecular dihydrogen bonds, of 2.04 and 2.12 Å (Figure 3A). With this type of geometry of the chain anion, an increase in the intramolecular interactions is expected, which should have a positive influence on the hydrogen release properties of Na[B3(MeN)2]. Interanion dihydrogen bonds are also present between H atoms of the NH and terminal BH3 groups, as well as between H atoms of the NH and the ones of the central BH2, as can be seen in Figure 3B and in Table S3. Unlike in the reported K[B3N2] and Rb[B3N2], the N-BH2 distances are not shorter than the N-BH3 ones (Table S4) [34,43]. Na+ cations have a distorted triangular bipyramidal coordination geometry with five B atoms from four distinct [BH3(CH3NH)BH2(CH3NH)BH3]− anions (Figure 3B,C). The coordination is performed through six hydridic H atoms of the BH3 (green balls in Figure 3C) groups from four different chain anions. Two other hydridic H atoms from the BH2 (red balls in Figure 3C) of one of above four chain anions complete the coordination around Na. This is different from the unsubstituted Na[BH3NH2BH2NH2BH3], where Na atoms are coordinated only to hydrogen atoms of the terminal [BH3] groups. This may be one reason of the release of large undesirable gaseous species during the thermal dehydrogenation of Na[B3N2] [33].
The thermal stability of Na[B3(MeN)2] was investigated by thermogravimetric analysis (TGA) under inert argon atmosphere, from room temperature to 150 °C. A single step decomposition event occurs at about 80 °C, accompanied by a weight oscillation due to the so-called “jet” effect [14,44] (Figure 4A). The solid decomposition products isolated upon heating at 150 °C were identified as being crystalline NaBH4, along with some unknown crystalline and possibly amorphous compounds, based on PXRD and IR analyses (Figure 4B,C). Those by-products are expected to contain B, C, and N atoms, based on 4.6 wt.% the experimental weight loss as compared to 26.2 wt.% B, 19.4 wt.% C, and 22.7 wt.% N in the sample before decomposition. It is interesting to note that thermally decomposing alkali metal salts of the unsubstituted anion (Li – Cs [B3N2]) also leads to the formation of BH4− compounds, similarly to the title compound. The observed mass loss during the thermal decomposition of Na[B3(MeN)2], of 4.6 wt.%, is in accordance with the possible release of pure H2, as the compound has a theoretical hydrogen content of 8.09 wt.% (excluding H atoms from the methyl groups). This is interesting, as the parent Na[B3N2] shows a larger mass loss (~20 wt.%) than its theoretical hydrogen content (12.7 wt.%) when heating below 200 °C, resulting in the single-step release of undesirable gaseous decomposition by-products like diborane and ammonia [34].
The purity of the gas released during the thermal de-hydrogenation of Na[B3(MeN)2] was analyzed by means of temperature-programmed mass spectrometry between 40 °C and 150 °C. Hydrogen was the only gas detected, and the experiment confirmed that NH3, B2H6, CH4, and CH3NH2 were not released during the decomposition (Figure 5). This confirms that the methyl-substituted Na[B3(MeN)2] indeed releases about 4.6 wt.% of pure hydrogen upon heating to 150 °C. This confirmed that the introduction of a methyl group on the nitrogen atoms efficiently suppresses the release of unwanted by-products during thermal hydrogen desorption.

3. Materials and Methods

All samples were obtained from commercially available NaH (95%), NaBH4 (97%), CH3NH2·HCl (98%), and anhydrous THF (≥99.9%) that were purchased from Sigma Aldrich Co., Ltd. (St. Louis, MI, USA). All operations were performed in gloveboxes with a high purity argon atmosphere.

3.1. Syntheses

Synthesis of CH3NH2BH3: CH3NH2BH3 was synthesized following a procedure adapted from the literature [45]. Initially, powdered NaBH4 (3.79 g, 0.1 mol), CH3NH2·HCl (13.50 g, 0.1 mol), and THF (300 mL) were added to a 500 mL three-neck round-bottom flask. The resulting mixture was then vigorously stirred at ambient temperature under an argon atmosphere for 48 h. Filtration was performed to remove the solid by-product (NaCl) from the reaction mixture, and the collected filtrate was subjected to evaporation under reduced pressure using a rotary evaporator. The resulting white solid of CH3NH2BH3 was then dried under vacuum overnight to eliminate any residual THF. The purity of the product was confirmed through characterization using 1H, 11B, and 13C NMR and PXRD, as depicted in Figures S4–S7.
Synthesis of Na[BH3(CH3NH)BH2(CH3NH)BH3]: Totals of 1 eq. of NaH (30.0 mg) and 3 eqs. of CH3NH2BH3 (168.4 mg) were placed into an 80 mL stainless steel vial with three 10 mm diameter stainless steel balls (ball-to-powder mass ratio of 60:1). The reactants were then milled in a planetary ball mill (Fritsch Pulverisette 7 Premium line), with a rotation speed of 500 rpm for 55 milling cycles of 30 min interrupted by 5 min cooling breaks. The product was obtained as a white powder.

3.2. Instrumental

Samples were carefully filled into 0.7 mm thin-walled glass capillaries (Hilgenberg GmbH, Malsfeld, Germany) within an argon-filled glovebox. To prevent contact with air, the capillaries were sealed with grease before being taken out of the glovebox. The sealed capillaries were then cut and promptly placed into wax on a goniometer head, ensuring that no air entered the capillary. Diffraction data were immediately collected using a MAR345 image-plate detector equipped with an Incoatec Mo (λ = 0.71073 Å) Microfocus (lµS 2.0) X-ray source operating at 50 kV and 1000 µA. The resulting two-dimensional images were azimuthally integrated using the Fit2D software, with LaB6 serving as a calibrant.
Synchrotron PXRD patterns were collected with a PILATUS@SNBL diffractometer (SNBL, ESRF, Grenoble, France) equipped with a Dectris PILATUS 2M single-photon counting pixel area detector (λ = 0.77509 Å). Powder patterns were obtained by using raw data processed by the SNBL Toolbox software using data for LaB6 standard. The synchrotron PXRD data for Na[BH3(CH3NH)BH2(CH3NH)BH3] were indexed in a monoclinic unit cell, and its structure was solved by global optimization using the FOX software [46]. The anions were modeled by conformationally free z-matrices with restrained bond distances and angles. Since the N-atom of methylamidoborane is chiral, all combinations of these chiral centres were examined. The final structure showed the best fit to the data but also satisfied crystal-chemical expectations, such as the formation of dihydrogen bonds (N-H∙∙∙H-B) and the coordination of Na+ to H atoms of BH3 and BH2 groups. Rietveld refinements were done in Fullprof [47], refining all non-hydrogen atoms of the anions individually using restraints from DFT-refined geometry. Hydrogen atoms were refined using the rigiding model, with Na as free atoms. The symmetry was confirmed with ADDSYM routine in the PLATON software. RB = 7.9%, Rp = 14.2, Rwp = 12.5, χ2 = 424 (mind that the counting statistics is very high).
Fourier transform infrared spectroscopy (FTIR): Attenuated total reflectance (ATR)-IR spectra were recorded using a Bruker Alpha spectrometer. The spectrometer was equipped with a Platinum ATR sample holder, which featured a diamond crystal for single bounce measurements. The entire experimental setup was located within an argon-filled glovebox to maintain an inert atmosphere during the measurements.
Thermogravimetric analysis (TGA): TGA measurements were conducted using a Netzsch STA 449 F3 TGA/DSC. The TGA/DSC was equipped with a stainless-steel oven and located within an argon-filled glovebox to ensure an inert atmosphere during the measurements. The samples were loaded into Al2O3 crucibles and subjected to a heating rate of 5 K/min under an argon flow of 100 mL/min.
Mass spectrometry: Mass spectrometry measurements were conducted using a Hiden Catlab reactor coupled with a Quantitative Gas Analyser (QGA) Hiden quadrupole mass spectrometer. Prior to the experiment, the samples were loaded into a quartz tube with two layers of quartz wool, all within the protective atmosphere of an argon-filled glovebox. The ends of the quartz tube were sealed with Parafilm before being removed from the glovebox. Subsequently, the quartz tube was placed in the sample holder outside the glovebox after quickly removing the Parafilm. The argon flow (40 mL/min) was immediately initiated to prevent any contact of the sample with air. The samples were then heated to 40 °C and held isothermally for approximately 2 h to stabilize the temperature. Heating was then performed at a rate of 5 °C/min until reaching 150 °C, followed by a 1 h isotherm. Gas evolution was monitored by recording the peak with the highest intensity for each gas, specifically the m/z values of 2, 15, 17, 18, 26, 28, and 30, corresponding to H2, CH4, NH3, H2O, B2H6, N2, and CH3NH2, respectively. The absence of H2O and N2 signals in the collected data confirmed the absence of leaks, ensuring that the sample remained under a protective argon atmosphere throughout the measurement.

4. Conclusions

We synthesized Na[BH3(CH3NH)BH2(CH3NH)BH3] (Na[B3(MeN)2], 130.5 g H2/kg, 126 g H2/L, Table S5), a methyl-substituted Na salt with five-membered B-N chain anions, by a novel mechanochemical approach from NaH and CH3NH2BH3. Its crystal structure was determined for the first time based on synchrotron PXRD, showing that the introduction of -CH3 groups on the N atoms leads to the introduction of the anion in a kinked geometry into the solid, unlike its unsubstituted parent counterpart (Na[B3N2]), that possesses straight B-N chains. Na[B3(MeN)2] releases up to 4.6 wt.% of pure hydrogen below to 150 °C, contrary to its unsubstituted analogue that releases undesirable gaseous by-products during heating. This indicates that the introduction of methyl (or other) substituents on the nitrogen atoms of similar compounds is a promising approach to suppress the release of unwanted volatile by-products during thermal hydrogen release.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11050202/s1, Figure S1: Chemical formula of VB; Table S1: H-contents, mass losses and by-products formed during thermal treatment of several M[B3N2] compounds; Table S2: H-content in NaNH2BH3, NaBH3NH2BH3, NaBH3NH2BH2NH2BH3, NaBH3NH2BH2NH2BH2NH2BH3, and NaBH3NH2BH(NH2BH3)2; Table S3. Inter-anion dihydrogen bond lengths and angles in Na[B3(MeN)2]; Figure S2: temperature ramping synchrotron PXRD patterns of Na[B3(MeN)2]; Figure S3: Rietveld refinement of the synchrotron PXRD pattern of Na[B3(MeN)2]; Table S4: B-N bond lengths in CH3NH2BH3, M[B3N2] (M = Li – Cs), and Na[B3(MeN)2]; Figures S4–S7: NMR and PXRD of CH3NH2BH3; Table S5: The mole mass, density, and gravimetric and volumetric hydrogen density of Na[B3(MeN)2].

Author Contributions

Conceptualization, software, T.Z. and Y.F.; methodology, validation, formal analysis, investigation, data curation, writing—original draft preparation, and visualization, T.Z.; writing—review and editing, T.S., M.D. and Y.F., supervision, M.D. and Y.F.; project administration, funding acquisition, Y.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the FNRS (CC J.0073.20, EQP U.N038.13, EQP U.N022.19) and the Communauté Française de Belgique (ARC 18/23-093). Ting Zhang was supported through the China Scholarship Council fellowship (201809370045).

Data Availability Statement

CCDC number: 2254456 contains supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Center.

Acknowledgments

We thank the ESRF for the beamtime allocation at the SNBL. We also thank François Devred for help with the mass spectrometry measurements.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 11 00202 g001
Figure 1. (A) Equation of the mechanochemical reaction; (B) PXRD patterns of NaH and CH3NH2BH3 ball-milled in different molar ratios, along with patterns of NaH and CH3NH2BH3 (λ = 0.71073 Å); (C) 11B NMR spectra of ball-milled NaH and CH3NH2BH3 mixtures in different molar ratios, along with the spectrum of CH3NH2BH3.
Figure 1. (A) Equation of the mechanochemical reaction; (B) PXRD patterns of NaH and CH3NH2BH3 ball-milled in different molar ratios, along with patterns of NaH and CH3NH2BH3 (λ = 0.71073 Å); (C) 11B NMR spectra of ball-milled NaH and CH3NH2BH3 mixtures in different molar ratios, along with the spectrum of CH3NH2BH3.
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Figure 2. IR spectra of Na[B3(MeN)2] and CH3NH2BH3.
Figure 2. IR spectra of Na[B3(MeN)2] and CH3NH2BH3.
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Figure 3. Ball and stick plot of the [BH3(CH3NH)BH2(CH3NH)BH3]− anion with indication of the intramolecular dihydrogen bond (A), crystal packing of Na coordination polyhedra with boron atoms (hydrogen atoms are omitted for clarity) in Na[BH3(CH3NH)BH2(CH3NH)BH3] projected along the c axis, indicating interanion dihydrogen bonds (B), and coordination of H atoms around the Na+ cation (central B was highlighted by red color) (C). Color code: N = blue, B = green, C = grey, H = white, and Na = pink. Dihydrogen bonds are displayed by red dotted lines.
Figure 3. Ball and stick plot of the [BH3(CH3NH)BH2(CH3NH)BH3]− anion with indication of the intramolecular dihydrogen bond (A), crystal packing of Na coordination polyhedra with boron atoms (hydrogen atoms are omitted for clarity) in Na[BH3(CH3NH)BH2(CH3NH)BH3] projected along the c axis, indicating interanion dihydrogen bonds (B), and coordination of H atoms around the Na+ cation (central B was highlighted by red color) (C). Color code: N = blue, B = green, C = grey, H = white, and Na = pink. Dihydrogen bonds are displayed by red dotted lines.
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Figure 4. TG analysis of Na[B3(MeN)2] (A); PXRD patterns (λ = 0.71073 Å) (B) and IR spectra (C) of the product upon heating at 150 °C, compared to the starting Na[B3(MeN)2] and NaBH4.
Figure 4. TG analysis of Na[B3(MeN)2] (A); PXRD patterns (λ = 0.71073 Å) (B) and IR spectra (C) of the product upon heating at 150 °C, compared to the starting Na[B3(MeN)2] and NaBH4.
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Figure 5. Mass spectrometry analysis of gases released during the thermal decomposition of Na[B3(MeN)2] under argon, between 40 °C and 150 °C.
Figure 5. Mass spectrometry analysis of gases released during the thermal decomposition of Na[B3(MeN)2] under argon, between 40 °C and 150 °C.
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MDPI and ACS Style

Zhang, T.; Steenhaut, T.; Devillers, M.; Filinchuk, Y. Release of Pure H2 from Na[BH3(CH3NH)BH2(CH3NH)BH3] by Introduction of Methyl Substituents. Inorganics 2023, 11, 202. https://doi.org/10.3390/inorganics11050202

AMA Style

Zhang T, Steenhaut T, Devillers M, Filinchuk Y. Release of Pure H2 from Na[BH3(CH3NH)BH2(CH3NH)BH3] by Introduction of Methyl Substituents. Inorganics. 2023; 11(5):202. https://doi.org/10.3390/inorganics11050202

Chicago/Turabian Style

Zhang, Ting, Timothy Steenhaut, Michel Devillers, and Yaroslav Filinchuk. 2023. "Release of Pure H2 from Na[BH3(CH3NH)BH2(CH3NH)BH3] by Introduction of Methyl Substituents" Inorganics 11, no. 5: 202. https://doi.org/10.3390/inorganics11050202

APA Style

Zhang, T., Steenhaut, T., Devillers, M., & Filinchuk, Y. (2023). Release of Pure H2 from Na[BH3(CH3NH)BH2(CH3NH)BH3] by Introduction of Methyl Substituents. Inorganics, 11(5), 202. https://doi.org/10.3390/inorganics11050202

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