Hydrazine Synthesis Essay

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  • 1. Introduction

    Access to energy has been one of the most important events in recent human history. The world entered into a new era with technological development related to widespread use of coal in the 19th century. With oil emerging in the 20th century, the world has entered into another era, characterized by faster technological progress, which has, unfortunately, negatively impacted the environment. Nowadays, and in light of past experience, a new era must begin. The 21st century could be that of the hydrogen century. Hydrogen is attractive owing to abundance via various sources, high mass energy density (120 MJ/kg) and oxidation into water. However, the transition CxHy (≤25 wt% H) → H2 (100 wt% H), i.e., the development of a near-future energy economy, is very challenging. Important technical/scientific issues touching production, storage and end-use have to be addressed [1,2,3].

    Storage of hydrogen is particularly critical and problematic, mainly because molecular hydrogen is a gas, even the lightest one. Accordingly, it has a low volumetric energy density (10.7 kJ·L−1 at 27 °C and 1 bar). Solutions have been investigated in order to make safe and efficient technologies emerge. First, the conventional storage methods (i.e., compressed gas up to 700 bars and cryogenic liquid at −253 °C) were considered while the efforts have been concentrated on storage system (i.e., tank, pipes, and so on) in terms of safety and performance. Then, alternative methods, involving materials, which are generally called hydrogen storage materials, emerged [4,5,6].

    Hydrogen storage materials enable a safer storage than the compressed and cryogenic technologies, and naturally carry 7–20 wt% H. Depending on their nature, there is distinction between physical storage (i.e., cryo-adsorption) and chemical storage. With the former, porous materials store molecular hydrogen in conditions (−196 °C and 10–120 bars of H2) that are milder than those for cryogenic liquid [4,5,6,7]. With respect to chemical hydrogen storage materials, atomic hydrogen is chemically bonded to a heteroatom and molecular hydrogen is released by solvolysis or thermolysis [4,5,6]. Borohydrides [8] and nitrogen-containing boranes (also called B–N–H compounds or boron- and nitrogen-based materials) [9] are typical examples.

    Hydrazine borane N2H4BH3 is one of the most recent boron- and nitrogen-based materials in the field of hydrogen storage. Though discovered and known since more than fifty years [10], the current energy context has been an opportunity to revive scientific interest on it, especially in view of the high gravimetric hydrogen density (15.4 wt% H). Hence, since 2009, hydrazine borane and new derivative compounds, the alkali hydrazinidoboranes MN2H3BH3, have positioned themselves as being potential candidates for chemical hydrogen storage, then focusing more and more attention. This is the core topic of the present review, which for the first time aims at specifically focusing on these materials, giving a timely and detailed overview about fundamentals, and tentatively discussing application prospects on the basis of the recent achievements.

    2. Brief Historical View of Hydrazine Borane

    Hydrazine borane was first reported by Goubeau and Ricker in 1961, in an original paper written in German [10]. The article provides experimental details about the synthesis as well as useful data about the molecular and crystal structures. Interestingly, it is mentioned the formation of a shock-sensitive solid residue upon the release of 2 equivalents of H2. Later, in 1967, Gunderloy stressed on the shock-sensitivity and flammability of hydrazine borane but no further detail can be found in the report [11]. Yet, one year later, the same author wrote in a patent that hydrazine borane “is highly stable at room temperature (25 °C) and is neither impact nor friction sensitive” [12]. With hindsight, the contradiction does not appear to be so critical, since the borane-hydrazine compounds were demonstrated to be potential solid-state monopropellants for rocket devices [12,13,14] and fast hydrogen generating systems [15,16].

    From an academic point of view, little research was carried out on hydrazine borane from 1961 to 2009. In 1971, the standard enthalpy was determined [17]. In 1997, hydrazine borane was used as precursor of porous boron nitride obtained by self-propagating high-temperature synthesis (Equation (1)) [18]. In 1999, the structure of hydrazine borane and that of its protonated analogue were calculated by the density functional theory method [19].

    N2H4BH3 → BN + 0.5N2 + 3.5H2

    In the 2000s, ammonia borane NH3BH3 was the only boron- and nitrogen-based material under intense research for chemical hydrogen storage [20]. One of the strategies was to destabilize it by chemical modification (synthesis of derivatives) [21]. This is in this context that hydrazine borane, which can be seen as a derivative of ammonia borane, emerged in 2009. The same year, Hamilton et al. [22] dedicated very few lines to pristine hydrazine borane in a review paper about the boron- and nitrogen-based materials and, on the basis of the aforementioned 1960s’ literature, suggested unsuitability for chemical hydrogen storage.

    3. Hydrazine Borane

    3.1. Synthesis

    The original synthesis procedure of hydrazine borane (Equation (2)) is based on the reaction of sodium borohydride NaBH4 with hydrazine sulfate (N2H4)2SO4 in dioxane at around 30 °C for 5–15 h [10]. It may be qualified as the classical procedure, reused by Hügle et al. in 2009 [23] and then, revisited and improved in terms of yield, purity and overall cost by Moury et al. in 2012 [24]. This is today the main procedure for the preparation of hydrazine borane at lab-scale.

    Hydrazine borane can also be synthesized by reaction of sodium borohydride with magnesium chloride MgCl2 either in hexahydrated form MgCl2·6H2O implying then the use of iced hydrazine N2H4 (Equation (3)) or in the form of a tetrahydrazinate MgCl2·4N2H4 (Equation (4)) with tetrahydrofuran C4H8O as solvent [12,25]. Instead of the chloride salt, a hydrazine salt N2H4·HX with X = Cl or CH3COO can be used (Equation (5)), the reaction taking place in tetrahydrofuran at temperatures between 50 and 100 °C [11,12,26]. The BH3 source can be changed also. Trimethylamine borane N(CH3)3BH3 can be reacted with hydrazine (Equation (6)) in benzene C6H6 at 50 °C for several hours [27].

    NaBH4 + 0.5(N2H4)2SO4 → N2H4BH3 + 0.5Na2SO4 + 0.5H2

    2NaBH4 + MgCl2·6H2O + 2N2H4 → 2N2H4BH3 + 2NaCl + 2H2 + Mg(OH)2 + 4H2O

    2NaBH4 + MgCl2·4N2H4 → 2N2H4BH3 + 2NaCl + Mg(N2H3)2 + 2H2

    NaBH4 + N2H4·HCl → N2H4BH3 + NaCl + H2

    N(CH3)3BH3 + N2H4 → N2H4BH3 + N(CH3)3

    In chemistry, synthesis generally makes two or more reactants react in order to get the targeted molecule. This was the classical strategy for the reactions mentioned above. Often, the first attempts fail. Sometimes, a surprising result stands out, like the formation of hydrazine borane while trying to get ammonia borane. Sutton et al. [28,29] were widely involved in finding an efficient chemical route to form ammonia borane from one of its solid residue, polyborazylene. Hydrazine was tentatively used as reducing agent of polyborazylene in tetrahydrofuran. After 12 h of reaction under stirring in room conditions, hydrazine borane was found to form. This is somehow an alternative route for synthesizing hydrazine borane. This would be also a way of regeneration, provided the thermolysis of hydrazine borane mostly leads to polyborazylene.

    The heat of formation of solid-state hydrazine borane was determined by pyrolysis in a bomb calorimeter under 29.6 bars of argon. Hydrazine borane decomposed into boron nitride BN (Equation (1)). The heat of formation of solid-state hydrazine borane was found to be 42.7 ± 0.4 kJ·mol−1 [17].

    3.2. Molecular and Structural Analyses

    The FTIR spectrum of hydrazine borane (Figure 1a) is typical of a boron- and nitrogen-based material, with numerous vibration bands, especially those ascribed to the N–H and B–H stretching regions (2600–3500 and 2100–2600 cm−1). Compared to the spectrum of ammonia borane, it is roughly comparable, but shows several additional bands of different intensity [10]. Particularly, there are two small bands at 1915 and 2015 cm−1 (B–H stretching region), suggesting strong interactions between H of BH3 and other elements. Another example is the band at 910 cm−1 in the BN–N asymmetric and N–N symmetric stretching region [24].

    The solution-state 11B NMR spectrum of hydrazine borane (Figure 1b) shows a signal at δ between −20 and −17.1 ppm, and the 11B{1H} spectrum a quartet (1:3:3:1) characteristic of the BH3 group (1JBH 94 ± 1 Hz) [23,24,30]. The presence of the N2H4 moiety can be verified in the 1H NMR spectrum (Figure 1c) via two singlets at δ 3.44 ppm (NH2–N) and δ 5.45 ppm (NH2–B). The BH3 group is also confirmed by a quartet (1:1.1:1.1:1) centered at δ 1.41 ppm due to the heteronuclear coupling between 11B and 1H and some small signals (three visible over the δ range 1.12–1.72 ppm and four overlapped) attributed to the heteronuclear coupling between 10B and 1H. The solid-state 11B NMR spectrum (Figure 1d) shows two signals (due to quadrupolar coupling) centered at about δ −24 ppm and whose sharpness indicates high crystallinity.

    Figure 1. Molecular identification of hydrazine borane N2H4BH3. (a) FTIR spectrum; (b) solution-state 11B and 11B{1H} NMR spectra; (c) solution-state 1H NMR spectrum; and (d) solid-state 11B NMR spectrum. Adapted from [24]—Reproduced by permission of the Physical Chemistry Chemical Physics (PCCP) Owner Societies.

    Figure 1. Molecular identification of hydrazine borane N2H4BH3. (a) FTIR spectrum; (b) solution-state 11B and 11B{1H} NMR spectra; (c) solution-state 1H NMR spectrum; and (d) solid-state 11B NMR spectrum. Adapted from [24]—Reproduced by permission of the Physical Chemistry Chemical Physics (PCCP) Owner Societies.

    Hydrazine borane is a white crystalline solid and a Lewis acid-base adduct. By XRD of a single crystal, Goubeau and Ricker reported an orthorhombic Pccn (56) space group [10]. More recently, the structure was solved using an orthorhombic Pbcn (60) space group with all of the B, N and H atoms belonging to the 8 d sites; Further, the cell parameters were refined [24,31,32]. As shown in Table 1, the cell parameters a, b and c are in good agreement. Neutron diffraction experiment permitted to obtain correct coordinates of the H atoms [31]. The N–B, N–N, N–H and B–H bonds as well as the N–H···H–B and N–H···N interactions were described. The nature of the N–B coordinative (or dative) bond (1.596 Å) exhibits an electrostatic feature mainly, but with substantial contribution of covalence with a large electron population of ~2.1 electrons and a small donation of ~0.05 electron from the Lewis base (N) to the Lewis acid (B). With respect to the N–H···H–B intermolecular weak interaction (2.01(1)–2.41(1) Å) [31], it allows a head-to-tail network of the hydrazine borane molecules (Figure 2), which rationalizes the solid state of the material [24]. The N–H···N intermolecular interactions (2.114 Å) occur with the head-to-tail network, according to planes parallel to the a-axis. Of note is a dipole moment of 4.18 D determined by Goubeau and Ricker for hydrazine borane [10].

    Table 1. Crystallographic data of hydrazine borane (HB) from various works (with ref. as reference and No. as number).

    FeatureHB in ref. [10]HB in ref. [31]HB in ref. [24]HB in ref. [32]
    Analyzed sampleSingle crystalSingle crystalSingle crystalPowder
    Crystal size (mm3)2.5 × 0.5 × 0.50.3 × 0.3 × 0.20.45 × 0.5 × 0.5
    Temperature (K)not given95173Room
    Crystal systemOrthorhombicOrthorhombicOrthorhombicOrthorhombic
    Space group (No.)Pccn (56)Pbcn (60)Pbcn (60)Pbcn (60)
    Z8888
    a (Å)13.0512.974(2)12.9788(5)13.1227(11)
    b (Å)5.125.070(1)5.0616(2)5.1000(5)
    c (Å)9.559.507(1)9.5087(4)9.5807(9)
    B–N bond (Å)1.5961.5871.592
    N–N bond (Å)1.4521.4521.458

    Figure 2. Head-to-tail network of the hydrazine borane molecules N2H4BH3 determined from XRD data [24

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