Beyond Hydrolysis: Scalable, On-Demand Dihydrogen Release from NaBH₄ Enables Circular and Sustainable Process Design

Abstract

Hydrogen storage in its elemental form poses significant safety and economic challenges. Metal hydrides, particularly sodium borohydride, offer a promising alternative because of their superior safety profiles and enhanced transportability. This study presents a scalable hydrogen release system based on sodium borohydride and commercially available alcohols and acids. The system enables rapid, controlled hydrogen generation, achieving quantitative yields. Quantum chemical calculations were performed to propose a mechanism for the alcoholysis of NaBH₄ with isopropyl alcohol (IPA) and acid present. It was demonstrated that the reaction proceeds via isopropoxy-substituted borane derivatives BH_(3−n)(O ⁱ Pr) _n (for n = 0, 1, 2, 3), which can form Lewis acid–base adducts with IPA. These Lewis acid–base adducts serve as reaction complexes for σ-bond metathesis, upon which an equivalent of hydrogen gas is released. Notably, the spent fuel can be regenerated to sodium borohydride using established chemical reactions, ensuring the system's sustainability and applicability for larger-scale hydrogen production.

Keywords

Hydrogen storage - NaBH₄ - Energy transition - Alcoholysis - Semi-alcoholysis

Significance

The study presents a novel hydrogen release system using NaBH₄ and isopropanol, enabling rapid, controlled hydrogen generation. This system addresses limitations in existing hydrogen technologies, by allowing easy storage, transport, and on-demand hydrogen release. The spent fuel is regenerated to NaBH₄ using established reactions, making it a sustainable and scalable solution for hydrogen economy applications. Mechanistic studies shed light on the physical organic interactions driving the alcoholysis of organo-boron compounds, thereby expanding the fundamentals of Lewis acid–base chemistry.

Introduction

The transition from fossil fuel-based energy systems to a hydrogen economy has been significantly impeded by insufficient storage capacity [1]. Hydrogen gas exhibits low volumetric density, and its liquefaction demands substantial energy input [2]. Moreover, safety considerations, transportation challenges, and boil-off losses hinder the widespread adoption of hydrogen as an energy storage medium [3] [4] [5]. To address these limitations, various chemical-based hydrogen storage methods have been proposed, including ammonia, liquid organic hydrogen carriers (LOHCs), and metal hydrides [6] [7] [8] [9].

Ammonia exhibits several advantages, including a relatively high volumetric energy density (~13.5 MJ/L) as opposed to liquefied hydrogen gas (~8 MJ/L), along with well-established large-scale commercial production processes [8]. Nevertheless, concerns related to safety, environmental impact, and toxicity significantly constrain its potential as a widely adopted hydrogen storage method [10], [11]. Conversely, most LOHCs are regarded as relatively safe and nontoxic. However, their development remains in its infancy due to challenges such as high dehydrogenation temperatures and severely limited large-scale synthesis [12], [13].

Metal hydrides are well-studied, offer superior safety profiles and enhanced transportability, and mitigate hydrogen loss during storage [14]. However, the liberation of dihydrogen from these compounds typically necessitates elevated temperatures and pressures [15], [16]. Additionally, the synthesis of these materials often requires critical raw materials, which presents potential resource constraints. These factors collectively impact the practical implementation and scalability of metal hydride-based hydrogen storage systems [17].

NaBH₄ has been a promising candidate among metal hydrides for hydrogen storage, exhibiting favorable characteristics that alleviate several of the limitations mentioned above, due to its high gravimetric and volumetric energy density (21.4 wt % H₂ and 137 kg/m², respectively, based on the reaction with acid and isopropanol), low toxicity, and superior safety [18], [19]. Despite the U.S. Department of Energy's decision in 2007 to discontinue research on NaBH₄ as a vehicular hydrogen energy carrier, recent advancements have demonstrated NaBH₄'s potential for portable and remote fuel cell applications [20] [21] [22] [23] [24].

Initial investigations into the utilization of NaBH₄ as a solid-state hydrogen storage vector primarily centered on its hydrolysis reaction, resulting in the liberation of dihydrogen gas and the formation of sodium metaborate as a byproduct. However, this reaction is characterized by notoriously slow kinetics and produces a variety of hydrated metaborate species, significantly reducing the effective gravimetric hydrogen storage capacity [25], [26], [27]. The incorporation of catalysts can enhance the reaction kinetics and provide greater control over byproduct formation of the hydrolysis of NaBH₄ ([Scheme 1]) [28], [29]. Nevertheless, this approach compromises the system's recyclability and circularity, presenting challenges for sustainable long-term use. Addition of acid also improves kinetics, but large-scale applications necessitate complex transportation and storage systems for the reagents, which presents logistical challenges ([Scheme 1]) [30].

Scheme 1 Different reaction routes of releasing hydrogen from NaBH₄. (1) Catalytic hydrolysis with sodium metaborate as a side product. (2) Acid-mediated hydrolysis with boric acid as a side product. (3) Alcoholysis with tetraalkoxy borate as a side product. (4) Acid-mediated alcoholysis with isopropanol.

An alternative and potentially more effective method for the liberation of dihydrogen from NaBH₄ is through alcoholysis reactions, particularly using methanol or other small alcohols ([Scheme 1]) [31]. These reactions are generally exothermic and have well-defined spent fuels. However, the difficulties in transport and storage remain. Moreover, in the absence of catalysts, these reactions still exhibit slow kinetics, and precise control over dihydrogen release becomes difficult to obtain [32].

As an important matter in this respect, the balance between the costs of the required time, energy input, and the created value of the produced hydrogen has to be economically feasible. In this context, a system that allows the premixing of alcohols and borohydrides, combined with a chemical trigger to initiate the dihydrogen release, would be an incredibly valuable tool.

The method herein described uses a cheap and commercially available acid to modulate the production of hydrogen gas out of a premixed borohydride alcohol solution, allowing its release on demand in one step with fast reaction kinetics. This system could give rise to a highly tunable and controllable industrial application allowing the direct feed of resulting spent fuel into existing industrial processes.

Results and Discussion

The study was initiated by mixing commercially available alcohols with NaBH₄. For a selection of alcohols, various steric and electronic properties were examined, along with considerations of economic feasibility. The ¹¹B, ¹H, and ¹³C NMR spectra indicated complete conversion of NaBH₄ to tetra-alkoxy borates upon mixing of NaBH₄ with methanol ([Figs. S1–S3]), ethanol ([Figs. S4–S6]), propanol ([Figs. S7–S9]), and butanol ([Figs. S10-S12]), respectively [33] [34] [35]. Meanwhile, the ¹¹B and ¹H NMR spectra of a mixture of isopropanol and NaBH₄, as well as tert-butanol and NaBH₄, were identical to those recorded initially after mixing both components with no line broadening and no solvation, even when heated to reflux for multiple days. Collectively, these experiments indicate that both isopropanol and tert-butanol can be used for a premixed formulation with NaBH₄ creating a suitable fuel for on-demand hydrogen release. However, due to the instability and poor reaction kinetics of tert-butanol, this manuscript will focus on the premixed isopropanol/NaBH₄ system.

Inspired by these initial results and the work of Paskevicus et al. [19], in which they synthesized tetraalkoxy borates from alcohols and NaBH₄, an investigation was conducted to determine if a premixed isopropanol/NaBH₄ suspension allows the clean formation of dihydrogen gas after addition of an acid concomitant with the selective formation of trialkyl borates as a recyclable spent fuel.

A reaction system consisting of a premixed suspension of NaBH₄ and isopropanol was subjected to the addition of various commercially available acids (SI.3). The progress of the reaction was monitored using ¹¹B and ¹H NMR spectroscopy. Among the acids tested, only H₂SO₄ displayed sharp, well-defined peaks, during and after the reaction. In contrast, all other acids examined resulted in extreme line broadening, likely due to the coordination of the respective anions, significantly hindering the ability to effectively monitor the reaction progress. Therefore, H₂SO₄ was deemed the most suitable acid for this system.

With the use of H₂SO₄ as an acid concomitant, the formation of Na₂SO₄ as an unwanted byproduct was acknowledged as a potential limitation for large-scale applications. Na₂SO₄ is already abundant as it is not widely used as a resource chemical and therefore generally regarded as a waste product in the chemical industry [36]. Consequently, alternative acids may warrant consideration for bigger-scale reactions.

Full conversion of NaBH₄, isopropanol, and acid to triisopropyl borate was observed at room temperature, monitored via ¹¹B and ¹H NMR recorded from the reaction mixture, and no distinguishable side products were detected in the recorded NMR spectra [37], [46], [47]. To confirm the high purity of the released dihydrogen, the evolved gas was passed through a D₂O-filled bubbler, in order to trap any released volatiles from the reaction mixture. Subsequent analysis of the D₂O by ¹H and ¹¹B NMR spectroscopy confirmed the absence of other volatile species in the developed gas.

Further evidence for the formation of dihydrogen gas from the system was gained by volumetric hydrogen gas measurement using a modified setup from Zheng et al [38]. Indeed, quantitative dihydrogen gas formation was observed for the reaction based on an isopropanol/NaBH₄ premix within the error margin of our setup ([Scheme 2]).

Scheme 2 Gas displacement measurement of the one-step release of dihydrogen gas from premixed NaBH₄ and IPA with a 20 min acid addition time. Relatively slow hydrogen gas release was still measured 2 h after acid addition was completed. The black line is the corresponding moving average of the measured displacement, whereas the yellow area indicates the error margin. 100% corresponds to the maximum expected yield.

A sub-stochiometric 0.25:1 ratio of H₂SO₄ to NaBH₄ facilitated the release of the theoretically predicted volume of gas and resulted exclusively in the formation of the desired product. In line with expectations, after subsequent addition of the remaining 0.25 equiv of H₂SO₄, full conversion was achieved and the residual amount of dihydrogen gas was released. It is the modularity of our approach, combined with the initial option of preparing a stable premix system, which distinguishes this approach from previously reported ones [39] [40] [41].

Gifted with the simple design of the premix isopropanol/NaBH₄ system, which demonstrated efficient and quantitative on-demand dihydrogen release, we proceeded to evaluate the scalability of our approach. A 20-g batch reaction was conducted under similar conditions in the previously described fashion. Upon the addition of sulfuric acid, a quantitative conversion to triisopropoxy borate was achieved, resulting in the release of 53 L of pure dihydrogen gas over a total reaction time of 40. The resulting azeotropic mixture was separated by distillation from the excess of alcohol and Na₂SO₄. Separation of this azeotropic mixture via distillation requires significant separation power or expensive membranes, which is not achievable on our lab scale. However, separation of azeotropic mixtures by pressure-swing distillation on industrial scale is well-established, therefore not hindering the intended application [42]. Hence, this premixed NaBH₄-based fuel system demonstrates potential for scalable, controlled dihydrogen generation with efficient separation of the spent fuel.

To elucidate the reactivity trends of the alcohol/NaBH₄ system, particularly the stability of the isopropanol/NaBH₄ premixed fuel, and to further comprehend the reaction kinetics of the acid-mediated hydrogen release in this premixed system, extensive mechanistic studies were conducted. Quantum chemical density functional theory (DFT) calculations were performed to propose a mechanism for the alcoholysis of NaBH₄ with IPA and acid present. All computational details are available in the Supporting Information. The proposed pathway (M06-2X/6-311++G**/GD3/CPCM (2-propanol) level of theory, (SI.1) describes NaBH₄ alcoholysis with isopropanol (3 equiv) and protonated isopropanol ( ⁱ PrOH₂ ⁺, 1 equiv, serving as a model compound for the isopropanol/H₂SO₄ mixture) in stoichiometric amounts ([Scheme 1b]). Alcoholysis ([Scheme 3]) is initiated with the formation of a thermodynamically stable complex between BH₄ ⁻ and ⁱ PrOH₂ ⁺ (R to RC1, −13.6 kcal/mol), followed by a barrierless proton transfer from the protonated alcohol to BH₄ ⁻ (RC1 to INT1, −11.4 kcal/mol), generating a BH₃-H₂ Lewis acid–base type adduct (SI.7.2). Subsequently, the BH₃-H₂ adduct can undergo a thermodynamically favorable exchange of H₂ with IPA (INT1 to INT2, −16.6 kcal/mol) via TS1 with a reaction barrier of +5.2 kcal/mol, releasing the first equivalent of hydrogen gas. The formed BH₃-IPA adduct, again characterized by Lewis acid–base type HOMO/LUMO interactions (SI.7.3), reacts via σ-bond metathesis (TS2, +27.9 kcal/mol) to mono-isopropoxy borane (INT3, BH₂O ⁱ Pr) releasing the second equivalent of H₂ gas. Thereafter, BH₂O ⁱ Pr (INT3) forms a Lewis acid–base adduct with IPA (RC3). The formation of this adduct was found to be slightly uphill (+3.5 kcal/mol), predicted due to steric shielding and increasing occupation of the isopropyl-substituted borane LUMO ([Fig. S27]). The resulting BH₂O ⁱ Pr-isopropanol complex can again undergo σ-bond metathesis to form BH(O ⁱ Pr)₂ (INT4) via TS3 (+29.3 kcal/mol). This two-step process of complex formation with subsequent σ-bond metathesis is repeated once more (INT4 to RC4, then RC4 to P via TS4). The third σ-bond metathesis (INT4 to P) has a barrier of +41.5 kcal/mol (TS4) and is rate-determining. This relatively high barrier for the release of the fourth equivalent of hydrogen gas can be overcome by heat evolution due to the extremely exergonic nature of the overall reaction (−118.9 kcal/mol) and aligns with experimental findings showing that active cooling of the reaction generates B-NMR visibility of the BH(O ⁱ Pr)₂ intermediate ([Fig. S17]). To conclude, the alcoholysis of NaBH₄ with IPA and acid present can proceed via isopropoxy-substituted borane derivatives BH_(3−n)(O ⁱ Pr) _n (for n = 0, 1, 2), which can form Lewis acid–base adducts with IPA. These Lewis acid–base adducts serve as reaction complexes for σ-bond metathesis, upon which an equivalent of hydrogen gas is released. After three consecutive cycles of σ-bond metathesis, the triisopropyl borate product (B(O ⁱ Pr)₃) was formed.

Scheme 3 Proposed mechanism for NaBH₄ alcoholysis with isopropyl alcohol and sulfuric acid in isopropyl alcohol as a solvent. Free energies (∆G_298K, in kcal/mol) calculated with DFT at the M062X/6-311++G**/disp-3/CPCM, 2-propanol (ultrafine grid) level of theory.

Despite the clean acid-initiated on-demand release of dihydrogen from premixed alcohol/NaBH₄, it is undeniable that an economically favorable hydrogen production on a larger scale requires the spent fuel to be recovered in a low-cost and accessible manner. Challenging aspects in this context are the separation of the excess alcohol from the respective trialkoxy borates and reconversion of borates to NaBH₄ to close the loop. Notably, there are numerous literature procedures available describing the recycling of a variety of trialkoxy borates to NaBH₄ [43], [44]. However, the recycling of triisopropyl borate to NaBH₄ has not been previously demonstrated. In order to close that gap, we conducted a small-scale reaction using the established Brown–Schlesinger conditions for the recycling of triisopropyl borate to NaBH₄.

Indeed, full conversion to NaBH₄ was achieved in 4 hours at the applied scale and work-up ([Scheme 4]), as confirmed by ¹H and ¹¹B NMR analyses with moderate isolated yield (48%) [Scheme 4] [45], [48]. The observed compatibility of triisopropyl borate with the Brown–Schlesinger reaction indicates that its integration into existing industrial processes would require only minor modifications to the current infrastructure.

Scheme 4 Recycle reaction of triisopropyl borate to sodium borohydride with sodium hydride.

Conclusions

In conclusion, the novel approach of a premixed system of the widely available hydrogen carrier sodium borohydride with isopropanol exhibits major advantages such as long-term storage, on-demand hydrogen release, and scalability. It was demonstrated that NaBH₄ alcoholysis with IPA and acid via BH_(3−n) (O ⁱ Pr) _n (n = 0, 1, 2, 3) species is thermodynamically feasible and highly exergonic. After proton transfer and H₂ elimination at the boroncenter by the protonated alcohol, the reaction proceeds via consecutive σ-bond metathesis with IPA, with Lewis acid–base type orbital interactions as a driving force for the formation of the reaction complexes. The spent fuel can be readily separated and recycled back into NaBH₄ in one step, using well-established chemical reactions.

Contributors’ Statement

J. C. Slootweg devised the project, the main conceptual ideas and proof outline. P. W. Wessels, T. Wesselingh and P. Germanacos carried out the experiments. C. J. Verhoef planned and carried out the simulations. G. B. De Jong aided in interpretation of results and design of experiments. F. Buß supervised the project and made contributions to the manuscript. V.J. Geiger supervised the project and processed the analytical data. All authors discussed the results and commented on the manuscript.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgment

We thank Ben Meijer for providing suggestions in the context of industrial applications.

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Publication History

Received: 25 November 2024

Accepted after revision: 23 December 2024

Accepted Manuscript online:
24 December 2024

Article published online:
07 February 2025