Surface-Controlled Conversion of Ammonia Borane from Boron Nitride

Abstract

“One-pot regeneration”, which is simple regneneration method of ammonia borane (AB) using hydrazine and liquid ammonia, enables conversion of AB from hexagonal boron nitride (h-BN) after milling hydrogenation. Solution ¹¹B-NMR revealed the presence of AB after NH₃/N₂H₄ treatment of milled h-BN (BNH_x) although the yield of AB was less than 5%. The conversion mechanism was clarified as B-H bonds on the h-BN surface created by ball-milling under hydrogen pressure have an ability to form AB, which was confirmed by Thermogravimetry-Residual Gas Analysis (TG-RGA) and Infrared (IR) analysis. The reaction routes are also the same as regeneration route of polyborazylene because intermediates of AB such as (B(NH₂)₃ and hydrazine borane were found by solution ¹¹B-NMR after soaking BNH_x in liquid NH₃ and hydrazine, respectively. Because of the fact that all reactions proceed on the h-BN surface and no reaction proceeds when neat h-BN is treated, breaking of B₃N₃ ring structure and then creation of B-H bond is the key issue to increase conversion yield of AB.

1. Introduction

In the future, a major hurdle of realizing carbon constraining world is storage and transportation of energy. While there are many potential green sources of electrical energy generation (e.g., solar and wind), both the intermittency and remoteness of these sources will severely limit their practical application at large scale [1]. Storage of the electric energy produced by intermittent sources would allow a leveling of the supply that should enable the “green-grid”. There are many possible ways to store the energy, but chemical bonds represent perhaps the cheapest option. Hydrogen generated from electrolysis suffers from several drawbacks: it is a gas and therefore requires either large volumes or high pressures to store [2]. Similar issues arise in the application of hydrogen as a fuel in automotive applications. Although the commercial hydrogen fuel cell vehicle can store 70 MPa of hydrogen with about 5.5 mass% including tank, improvement of hydrogen storage technology with higher hydrogen density (U.S. DOE ultimate target: 6.5 mass% of system) is required [2]. The promising way to store hydrogen with high density is hydrogen storage material; therefore, many different hydrogen storage media have been considered to improve their gravimetric and volumetric density of usable hydrogen [3]. One of the recent high profile candidates is ammonia borane (AB; NH₃BH₃) because of its 19.6 mass% of hydrogen capacity and moderate initial dehydrogenation temperature (~110 °C) [4]. AB desorbs hydrogen over a wide range of temperatures in three idealized steps, as follows [5]:

NH₃BH₃ → “NH₂BH₂” + H₂, 6.5 mass% < 114 °C

(1)

“NHBH” + 2H₂, 13.1 mass% < 200 °C

(2)

BN + 3H₂, 19.6 mass% > 400 °C

(3)

In reality, the decomposition pathway of AB is more complicated and the “NH₂BH₂” and “NHBH” species are better represented as polymers. While, on the face of it, AB looks like an excellent material for hydrogen storage, there are several issues to be overcome before practical use could be a reality [6]. Issues with the kinetics of hydrogen release have been well-examined by many research groups using a wide range of approaches such as catalysts [7], Lewis and Brønsted acids [8], metal hydrides [9], proton sponge activation [10], ionic liquids [11], confinements [12], and metal-substitution as metal amidoboranes [13]. However, the major limitation of AB has always been the exothermic nature of hydrogen release although this nature has advantages from a technological point of view, in that no extra energy is required to obtain hydrogen from AB. This nature prohibits the direct rehydrogenation of the dehydrogenated material form back to AB by simple pressurizing hydrogen gas. Only chemical recycle route (regeneration) is possible to recharge hydrogen into dehydrogenated AB. The dehydrogenated material, or “spent fuel” as it is often called, is a complex mixture of polymeric {BNH_x}_n species (polyborazylene: PB), generally containing hydrogen. PB has linear or B₂N₃ ring structures with many degrees of polymerization, which depends on amount and condition of released hydrogen. The full release of hydrogen from AB, which is the ceramic boron nitride (h-BN), is also spent fuel, but h-BN was considered too robust to be ever converted back to AB due to quite strong bonds in B₃N₃ ring structure. Therefore, less than 2.5 equivalents of hydrogen released from AB would be favorable for regeneration, which means hydrogen in the {BNH_x}_n species would facilitate the chemical reformation of AB. The proto-typical spent fuel, which is ring-structured PB made from borazine (B₃N₃H₆) [14], has been successfully converted back to AB by using benzenethiol [15] as a digesting agent followed by tin hydrides [16] for hydrogen transfer. Moreover, we eventually established a quite simple one-pot AB regeneration technique from PB using hydrazine (Hz) in liquid ammonia (liq. NH₃) as the hydrogen transfer reagent [17]. This “one-pot regeneration” rapidly proceeds within 24 h via two reaction routes: hydrazine reacts with PB to form hydrazine borane (N₂H₄BH₃) and then AB forms by the ligand exchange from N₂H₄ to NH₃ [17]; HB(NH₂)₂ forms by the reaction between PB and ammonia in the liquid ammonia and then AB and B(NH₂)₃ form by the disproportionation of HB(NH₂)₂ [18]. The theoretical approach indicates that only B-H bond can react with NH₃ and N₂H₄ [17,18], which means that B-H bonding is mandatory for regeneration.

While some reports of AB regeneration [19,20,21] and lithium amidoborane (LiNH₂BH₃) [22] have been published, there is still a great challenge remaining which is full-regeneration of AB from hexagonal boron nitride (h-BN). Based upon simple thermodynamic argument, h-BN is too far downhill to be considered as a precursor for AB (endothermic reaction, ΔH = 215.4 kJ/mol) due to full-dehydrogenated state of AB [23,24]. Another fact, that h-BN has no B-H bonding, is also a big issue for regeneration. Therefore, direct hydrogenation of bulk h-BN to AB might be difficult to proceed. Graphite, which has similar structure to h-BN, is also difficult to hydrogenate directly at room temperature but ball-milling under hydrogen pressure, a strong hydrogenation tool, realizes hydrogenation of carbon at room temperature (RT) [25]. Even ball-milling hydrogenation method has been employed, no AB formed but only surface on h-BN was hydrogenated taking the form of “BNH_x”, which still has many B₃N₃ rings (hklm = 0002) supported by X-ray diffraction (XRD) [26]. The hydrogenation of h-BN during the ball milling process relies upon the formation of defects at the surface of h-BN, which is an apparently thermodynamic favorable process. Although BNH_x is smaller hydrogen coordination (x < 0.5) than PB (x = 1.0) and has a similar structure to that of PB (Figure 1), the infrared (IR) spectrum of BNH_x showed both B-H and N-H stretching peaks with a structure similar to polyaminoborane (PAB; {BH₂NH₂}_n). Therefore, BNH_x is different from PAB in terms of BH₂ species and number of B₃N₃ ring structure. Now activated BNH_x is no longer as thermodynamically stable as h-BN and direct chemical conversion to AB using N₂H₄/NH₃ is possible. Hence, the purpose in this study is to transform back to AB upon N₂H₄/NH₃ treatment from B-H bonds in milling hydrogenated h-BN. We also investigated the reaction mechanism of this transformation including the reaction route.

2. Materials and Methods

The starting material, h-BN nanopowder (<150 nm, 99%, Aldrich Co. Ltd.) was used as received or after dried at 200 °C for 8 h under vacuum condition. BNH_x and BN-NH_x (N-H bonding coated h-BN) were synthesized by using the ball milling method under 1 MPa of H₂ or 0.8 MPa of NH₃ at RT for 80 h using vibration milling (SEIWA GIKEN Co. Ltd., RM-10) as the previous report [26]. In a typical reaction, 100 mg of each sample (neat h-BN, BNH_x, and BN-NH_x) was placed into stainless steel vessel (about 50 mL) and cooled down to −78 °C using a dry ice/ethanol mixture. Then NH₃ was condensed into the container. After the addition of about 30 mL of liquid NH₃, 200 µL of anhydrous hydrazine (98%, Aldrich Co. Ltd.) was added. The reaction vessel was then sealed and heated at 40 °C for 1 week. These vessels were opened under atmospheric pressure containing condensed NH₃ by cooling in liquid nitrogen before opening to prevent blowing up sample in the container. After warming up and the evaporation of NH₃ under atmospheric pressure for 5 h, residual NH₃ and N₂H₄ were completely removed under dynamic vacuum maintained for 5 h. All procedures were carried out without exposing air.

¹¹B and ¹H solution nuclear magnetic resonance (NMR: Bruker, AVANCE 300, 96.29 MHz for ¹¹B and 300.13 MHz for ¹H) of BNH_x and N₂H₄/NH₃ treated samples were performed at room temperature using d8-THF as the solvent. Samples for NMR were prepared by filtering by THF (about 50 mL) then dynamic evacuation of filtered solution. External calibration of ¹¹B NMR was performed using BF₃-etherate standard (0.0 ppm). We also measured ¹¹B solution NMR (Bruker, AVANCE 400, 128.38 MHz) of BNH_x after soaking in each solvent (NH₃ and N₂H₄) at RT for 24 h in order to confirm intermediates.

Thermogravimetric analysis (TGA) of BNH_x after the N₂H₄/NH₃ treatment (whole sample including soluble and insoluble species) was performed by LABSYS evo (SETARAM). At the same time, gas analyses were performed by residual gas analyzer (RGA: INFICON, Transpector^® CPM) and infrared (IR: Thermo Nicolet, AVATAR 360 FT-IR) TGA-RGA measurement of BNH_x before the treatment and residual (after removal of AB) treated BNH_x was simultaneously performed by TG 8120 (Rigaku) coupled with M-QA200TS (Canon Anelva Corporation). Both measurements were performed with a scanning rate of 10 °C/min. Powder X-ray diffraction (XRD) measurements were operated by RINT-2100 (Rigaku, Cu Kα radiation) for BNH_x and D8 Discover (Bruker, Cu Kα radiation) for N₂H₄/NH₃ treated BNH_x. All procedures have been performed without exposing the samples to air by handling in the Ar-filled glovebox except the TGA-RGA-IR measurement of BNH_x after the treatment, where the exposure time was minimized (<10 sec).

3. Results and Discussion

The regeneration of AB is most simply followed by ¹¹B solution NMR. The reaction of neat h-BN with N₂H₄/NH₃ showed no evidence of the formation of AB or any other molecular species even after prolonged time (Figure S1a in supplementary materials). Extraction of BNH_x, hydrogenated h-BN by ball milling, with THF yielded no observable B-H species in solution (Figure 2a). In contrast, when BNH_x was treated with N₂H₄/NH₃ for 1 week, a soluble BH₃ containing species (−20.4 ppm, J_B,H = 94 Hz) is presented (Figure 2b). This resonance is identified as AB by comparison with commercial AB. In addition, mixture of this soluble species and the commercial AB showed single peak in ¹¹B NMR spectrum (Figure S1b). Although hydrazine borane (HzB) can be produced as an intermediate compound or by the reaction between AB and N₂H₄, in liquid NH₃ all of HzB can be converted to AB by metathesis (Figure 2b). The ¹H NMR spectrum of this product also shows the presence of resonances identified as triplet NH₃ species in AB at 4ppm (Figure S2).

Unfortunately, the yield of AB is not significant overall being less than 5% based upon the mass of treated BNH_x. It is important to reveal the mechanism of this reaction in order to improve AB yield using this method. It is likely that only the surface of h-BN is hydrogenated during the ball milling process and only this surface is susceptible to be attacked by NH₃ and N₂H₄. We have reported that BNH_x, hydrogenated h-BN by ball milling, is still dominated by the hexagonal N₃B₃ ring structure of h-BN, even after 80 h milling [26]. Indeed, our X-ray diffraction analysis (XRD) of BNH_x and the product after N₂H₄/NH₃ treatment have diffraction peaks corresponding to h-BN ring structure (hklm = 0002, Figure S3). BN-NH_x, which is milled h-BN under ammonia pressure for 80 h [22], has not been shown in our hands to generate AB by the Hz/NH₃ treatment (Figure S1c), which is the same as the failure of neat h-BN to show any reaction with N₂H₄/NH₃. This is part of the evidence that presence of the B-H bonds is critical to this reaction as confirmed. Moreover, the calculation results in the previous report support that B-H bonds are only reactive with NH₃ and N₂H₄ but not N-H bonds [17,18]. TG-RGA-IR analysis could support this speculation. RGA profile of BNH_x shows wide temperature range of hydrogen (and small amount of N₂ and NH₃) release (Figure 3a). On the other hand, residue (N₂H₄/NH₃ treated BNH_x after removal of AB) desorbed mainly NH₃ (and small amount of N₂ and H₂) below 500 °C (Figure 3b), which is similar to that of BN-NH_x denoted in Figure 3c. This phenomenon indicates that weak B-H bond in BNH_x was substituted from NH₃ and N₂H₄ during treatment. Increase of BNH_x mass after the treatment (100 to 170 mg) can be this evidence. Additional evidence has been found in IR results because the residue has weaker B-H (about 2500 cm⁻¹) and stronger N-H peaks (about 3500 cm⁻¹) than these BNH_x peaks (Figure S4). Overall untreated BNH_x loses about 2.5 mass% up to 900 °C whereas the residue loses 11.5 mass% mostly due to NH₃ desorption. This is also evidence of substitution from B-H to N-H bonds.

The reaction path is also an important issue to be clarified for improvement of this method. As mentioned, the intermediate of N₂H₄ route is N₂H₄BH₃, while the intermediates of NH₃ route are HB(NH₂)₂ and B(NH₂)₃ (decomposition product of HB(NH₂)₂). The simple way to confirm their presence is NMR measurement after soaking BNH_x in liquid NH₃ or N₂H₄. ¹H-decoupled ¹¹B solution NMR spectra (solvent: THF) after soaking in both N₂H₄ and liquid NH₃ had a weak peak at −20.2 ppm (Figure 4a) and a strong peak at 26.5 ppm with a small shoulder (Figure 4b), respectively. Although the signals in the coupled spectrum of N₂H₄-soaked sample were too weak to identify, this peak position is close to N₂H₄BH₃. The peak in the NMR spectrum in liquid NH₃-soaked sample locates almost the same as B(NH₂)₃. Other peak was not observed in both spectra. Thus, regeneration route of BNH_x could be the same as PB regeneration route as we reported.

When we consider these observations, a probable reaction mechanism for the formation of the AB from h-BN can be envisioned. Firstly, all of the reactions, from the ball milling through to the reactions with N₂H₄/NH₃, only occur at the BN surface. Previous calculations indicate that only the B-H bonds in (BNH_x)_n have an ability to form AB on reaction with N₂H₄ and NH₃, while the B₃N₃ ring structure is unaffected by them. Once B-H bonds created by milling are attacked by N₂H₄ and NH₃, N_mH_n bonds are formed resulting in release of AB and an N_mH_n coated surface. The sample mass may increase due to this substitution reaction. The coated N_mH_n surface could be similar to BN-NH_x. The B-H bonds reacted with NH₃ and N₂H₄ could be weak bond energy with N atom. Considering the main B-H bond in BNH_x is PAB ({BNH₂}_n), this weak B-H bond could be BH₂. A schematic depiction of this model is displayed in Figure 5 but the real reaction might be more complicated because the edge of BNH_x would have many kinds of structure. We need to identify “active” edge by the theoretical approach to clarify deeper reaction mechanism of this regeneration method. Further experimental work is also necessary to minimize or break the volume of N₃B₃ ring structure and therefore increase the amount of B-H bonds in the BNH_x by ball-milling (or other method).

4. Conclusions

In this study, the synthesis of AB from h-BN was successful using ball milling under hydrogen pressure and then soaked in N₂H₄-liquid NH₃ solution. The THF-soluble product of this reaction was shown by ¹¹B NMR to be AB. H₂H₄/NH₃ treated BNH_x which removed AB, had similar RGA profile to BN-NH_x below 500 °C, indicating substitution of N-H bonds for B-H bonds proceeded. The key aspect of this chemistry is the creation of the defects by breaking B₃N₃ ring at the surface of h-BN during the milling under H₂. This phenomenon enables the formation of B-H species that can be attacked by both H₂H₄ and NH₃ to form AB. These B-H bonds in BNH_x would be low bond energy with N atom likewise broken B₃N₃ ring structure, which can desorb hydrogen below 500 °C. The presence of intermediates (N₂H₄BH₃ and B(NH₂)₃) in NMR indicates that their reaction routes could be the same as those of PB regeneration. Although this method is not commercially useful immediately, unless there is improvement of its low production yield (about 5%), such surface chemistry could open up new possibilities of other thermodynamically tough chemical reactions. Of course, it may be possible to envision a hydrogen storage system using full hydrogen from AB (19.6 mass% H₂) if high AB yield of regeneration using our method is realized. If an effective method to break the strong bonding in B₃N₃ ring and then create B-H bonding is found, this “one-pot regeneration” probably gives high yield of AB.