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

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. R_B = 7.9%, R_p = 14.2, R_wp = 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.