Kinetic Evaluation and Catalytic Efficiency of Sebacic Acid as a Novel Catalyst in Hydrogen Generation via NaBH₄ Alcoholysis Reactions

Abstract

This study explores the use of sebacic acid, a catalyst not previously examined in the literature, for hydrogen production from NaBH₄ through methanolysis and ethanolysis reactions. Solutions of sebacic acid with concentrations ranging from 0.1 M to 0.4 M were prepared and tested. At a concentration of 0.3 M, 90% of the hydrogen from a 0.33 M NaBH₄ solution was released within 3 s, and full release was achieved in 4 s. Hydrogen production rates reached 4500 mL/min for ethanolysis and 4845 mL/min for methanolysis, with methanolysis reactions proving faster. The activation energies for methanolysis and ethanolysis were calculated as 7.17 kJ/mol and 52.3 kJ/mol, respectively. These results demonstrate that sebacic acid enables rapid and efficient hydrogen production, offering a new approach that significantly advances current hydrogen production methods.

1. Introduction

Energy and its sources play a crucial role in the economic and social development of nations, with fossil fuels traditionally serving as the primary energy source. However, the depletion of these resources, coupled with growing environmental awareness, has driven the pursuit of clean, safe, cost-effective, and highly efficient energy alternatives [1]. In recent times, modern society has been in pursuit of a clean, renewable, and cost-effective energy source that can serve as a viable alternative to fossil fuels. Hydrogen production and storage technologies have been extensively researched in recent years [2]. Hydrogen, recognized as an environmentally benign energy carrier, has been identified as a promising solution to critical challenges such as the depletion of energy resources and the environmental pollution caused by the extensive use of fossil fuels [3]. Widely acknowledged as a clean energy source, hydrogen requires only oxygen for combustion, resulting in water as the sole by-product. The use of hydrogen fuel in engines offers the potential to significantly reduce greenhouse gas emissions, smog, and acid rain. Despite these benefits, the practical implementation of hydrogen as a fuel is hindered by significant challenges related to its storage and transportation [4]. The development of efficient hydrogen storage and transportation technologies highlights its potential to establish a robust and flexible energy infrastructure [5]. Hydrogen, the most abundant element in the universe, plays a crucial role in the transition to a sustainable energy future due to its versatility and clean energy properties. As an energy carrier, hydrogen can be efficiently transformed between chemical and electrical forms, making it essential in various sectors such as transportation, industry, and power generation [6]. This has driven ongoing research and innovation in hydrogen technologies to overcome current challenges and enhance its role in the global energy sector [7].

Currently, hydrogen is stored using two primary methods: physical and chemical storage. Physical storage is less favored due to safety concerns and the need for larger tank volumes. Conversely, chemical storage, particularly using chemical hydrides such as NaBH₄, KBH₄, LiH, and NaH, is preferred because of its higher hydrogen storage capacity (e.g., NaBH₄ at 10.8%), safe hydrolysis/alcoholysis processes, and production of pure hydrogen [8]. Sodium borohydride (NaBH₄) has attracted significant interest as a commonly used chemical hydride for producing pure hydrogen for fuel cells. Its advantages include a high theoretical hydrogen capacity (10.8 wt.%), excellent stability and storability, non-toxic nature, safe reaction conditions, and the generation of environmentally friendly by-products [9].

The alcoholysis of NaBH₄ using alcohols such as methanol or ethanol is a viable alternative, especially in environments where water freezing poses a problem. This method is advantageous because it involves fewer steps for recycling the spent borohydride and eliminates issues related to water freezing [10,11]. Methanol and ethanol have several advantages over water. First, the alcoholysis reactions of NaBH₄ or KBH₄ with these alcohols exhibit faster reaction kinetics. Second, the lower melting points of these alcohols result in improved catalytic performance at reduced temperatures. Third, the alcoholysis reaction produces no undesirable by-products. However, despite these benefits, the reaction with ethanol has been reported to be relatively slow. Additionally, ethanol’s greater weight compared to methanol reduces the concentration of gravimetric hydrogen in the system [12].

The methanolysis reaction of sodium borohydride occurs spontaneously, with studies indicating that the kinetic reaction constant for this spontaneous alcoholysis is higher than that for its spontaneous hydrolysis at ambient temperature without any catalyst [13]. The activation energy required for the NaBH₄ methanolysis reaction is lower than that for hydrolysis, and the by-product NaB(OCH₃)₄ can be recycled to regenerate methanol. Additionally, methanolysis can be conducted at much lower temperatures (with methanol’s melting point at −97.6 °C) compared to hydrolysis, which is limited by water’s freezing point below 0 °C. The presence of a catalyst plays a crucial role in controlling the hydrolysis/methanolysis of NaBH₄ [14]. The spontaneous nature of sodium borohydride’s methanolysis reaction has been corroborated by previous reports, which note that the kinetic reaction constant for this alcoholysis is higher than that for spontaneous hydrolysis at room temperature without a catalyst. The by-product NaB(OCH₃)₄ resulting from the alcoholysis reaction dissolves in methanol, preventing inhibition of catalyst activity by the by-products and potentially offering advantages over hydrolysis [15]. In recent years, the use of alcohol solvents, particularly methanol, instead of water in the NaBH₄ reaction has garnered significant attention. The NaBH₄ methanolysis reaction, as shown in Equation (2), exhibits higher hydrogen production rates at relatively lower temperatures compared to NaBH₄ hydrolysis reactions, along with better kinetic properties.

Unlike methanol, ethanol serves as another potential solvent for the ethanolysis reaction of NaBH₄. While the use of alcohols offers certain advantages, ethanolysis reactions are notably slower than methanolysis reactions. The slower kinetics of the ethanolysis process highlight the need for an efficient catalyst. Additionally, hydrogen generation from the solvolysis of NaBH₄ in ethylene glycol–water mixtures offers notable improvements in hydrogen generation kinetics and operational temperature compared to pure hydrolysis or glycolysis systems. However, the molecular weight of ethanol is higher than that of methanol, which results in a lower gravimetric hydrogen density in the ethanol system [12].

The commercialization of hydrogen production from NaBH₄ encounters significant obstacles, including limited hydrogen yield at room temperature and poor solubility of NaBH₄ and NaBO₂ by-products in highly alkaline solutions. To overcome these challenges, developing effective catalysts is crucial to improve the efficiency of NaBH₄ hydrolysis reactions. Methanol and ethanol offer several advantages over water in alcoholysis reactions. First, the reaction kinetics of NaBH₄ or KBH₄ with these alcohols are significantly faster. Second, their lower melting points enhance catalytic performance at lower temperatures. Finally, alcoholysis reactions do not produce undesirable by-products, thus improving the overall efficiency and cleanliness of the process.

Equations (1) and (2) illustrate the ethanolysis (ER), and methanolysis reactions (MR) of NaBH₄ in the presence of a catalyst [16].

NaBH₄ + 4CH₃CH₂OH → NaB(OCH₂CH₃)₄ + 4H₂

(1)

NaBH₄ + 4CH₃OH → NaB(OCH₃)₄ + 4H₂

(2)

Sebacic acid (C₁₀H₁₈O₄, CASRN 111-20-6), produced through the caustic fusion of castor oil, serves as a crucial organic intermediate in the synthesis of nylon, alkyd resins, plasticizers, lubricants, and more [17]. Understanding the solubility of sebacic acid in various solvents is essential for designing crystallization processes and other chemical handling methods [18]. However, information on its solubility is limited in the literature. At room temperature, sebacic acid appears as white crystals in flake form. It is slightly soluble in water but insoluble in benzene, petroleum ether, and carbon tetrachloride. In contrast, it is soluble in ethanol and ethyl ether. There are no known studies in the literature where sebacic acid has been used as a catalyst for hydrogen production. The NaBH₄ hydrogen production system initially used catalysts like oxalic, boric, formic, and citric acids. These acids significantly influenced the NaBH₄ reaction, leading to a search for safer alternatives. A variety of acids, including phosphoric, nitric, acetic, sulfuric, hydrochloric, and formic acids, are typically employed in the hydrolysis reactions of NaBH₄ [19]. The use of acid catalysts in NaBH₄ hydrolysis increases the hydrogen production rate by enhancing the proton concentration in the solution. This process can be executed either directly or indirectly, with the acidic solution generally added to solid NaBH₄ [20]. The primary benefits of acid hydrolysis include the production of very dry hydrogen gas and environmentally benign waste by-products. In acidic solutions, NaBH₄ can be readily converted into sodium salts and boron compounds. The addition of an acid catalyst to the NaBH₄ solution increases the hydrogen generation rate (HGR) due to the elevated proton concentration. The literature contains numerous studies on the hydrolysis, methanolysis, and ethanolysis of NaBH₄ [21].

While the literature has extensively explored hydrogen generation from NaBH₄ through hydrolysis, research on its reactions in methanol and ethanol remains limited. Arzac, G. M., and Fernández, A., demonstrated that acetic acid offers an optimal balance between hydrogen generation rates, conversion efficiency (which improves with increasing amounts of accelerator), and environmental friendliness. The highest experimental gravimetric hydrogen density (GHD) recorded was 2.1 wt.%. Although adding water to ethanol can enhance hydrogen production rates, it also decreases conversion efficiency. Moreover, using ethanol–methanol mixtures without any catalyst can further improve reaction rates. Among the acidic accelerators, hydrochloric acid has shown the best performance, followed by acetic and citric acids. Ascorbic and tartaric acids (with the latter being poorly soluble in ethanol) and cationic resins exhibited conversion rates in the range of 40–55%. Cationic resins performed similarly to tartaric acid. The hydrogen evolution curves for these acidic accelerators indicate a positive order (>0) in sodium borohydride [12]. Another study about organic acids as catalysts from Balbay, A., and Saka, C., on the effects of hydrochloric acid, acetic acid, and temperature on hydrogen generation from NaBH₄ revealed that semi-methanolysis reactions with these acids were completed in 4 and 5 s, respectively. The highest hydrogen production rates achieved in semi-methanolysis reactions were 4875 mL min⁻¹ with 1 M hydrochloric acid and 3960 mL min⁻¹ with acetic acid [22].

There is extensive literature on hydrogen production from NaBH₄ via the hydrolysis method. However, research on alcoholysis reactions is limited, and only a few organic acids have been used as catalysts in these studies. The purpose of this study is to investigate the use of sebacic acid, which has not been previously explored in the literature, as a catalyst for hydrogen production from NaBH₄. In this context, sebacic acid solutions in various concentrations were prepared and the alcoholysis reactions were conducted under different conditions. This study also investigated the kinetics of hydrogen production via the methanolysis and ethanolysis reactions of NaBH₄ with sebacic acid catalysts with a focus on how varying NaBH₄ concentrations influence the reaction rate.

2. Materials and Methods

Sebacic acid (99%) and methanol (99.8%) were acquired from Aldrich Chemistry (St. Louis, MO, USA), while ethanol (≥99.9%) was supplied by Isolab (Eschau, Germany). NaBH₄ (97%) was obtained from AFG Bioscience (Northbrook, IL, USA) for the experiments. The chemicals used in the reactions were of analytical grade.

The catalytic performance of sebacic acid was evaluated by determining the hydrogen yield from NaBH₄ ethanolysis (NaBH₄-ER) and methanolysis (NaBH₄-MR). The reaction method was applied as Balbay, A., and Saka [22] for the production of hydrogen via alcoholysis reactions. The reaction setup involved immersing a reaction vessel in a water bath with temperatures controlled between 30 and 50 °C. The reaction flask was placed in a thermostatically controlled water bath to maintain a stable reaction temperature, which could be adjusted by regulating the water bath’s temperature. One of the flask’s necks was sealed with a cork connected to a hose, allowing the produced hydrogen to flow through the hose into a wet gas meter. The volume of hydrogen generated was measured using this meter. Initially, the remaining two necks of the flask were left open but were sealed immediately after the catalysts were introduced into the solution. The generated hydrogen gas was collected and analyzed using an inverted burette connected through appropriate conduits. Subsequently, 10 mL of NaBH₄–water solutions with concentrations of 0.165 M and 0.33 M were introduced into a 50 mL reaction flask. Following this, 10 mL of sebacic acid solutions were added to the flask, with concentrations of 0.1, 0.2, 0.3, and 0.4 M for methanol, and 0.1, 0.2, and 0.3 M for ethanol, corresponding to NaBH₄-MR and NaBH₄-ER. The stoichiometric ratio of sebacic acid to NaBH₄ for the hydrogen generation (HG) reactions was maintained at 1:1. The volume of hydrogen gas produced over time was measured using a piston displacement apparatus filled with water. Hydrogen gas measurements were systematically captured with a high-speed camera. Schematic diagram of the process like Dandan Ke et al. [23] was created and studies were carried out as in the reference. The appearance of the process is shown in Figure 1.

3. Result and Discussion

Effect of Sebacic Acid for Hydrogen Production from Methanolysis and Ethanolysis Reactions

In this study, hydrogen production from NaBH₄ via alcoholysis (methanolysis and ethanolysis) reactions was conducted using sebacic acid as a catalyst. To this end, sebacic acid solutions were prepared in varying concentrations with both methanol and ethanol. Additionally, the impact of different NaBH₄ concentrations was also investigated. The experiments were primarily conducted at 30 °C, with additional tests performed at 40 and 50 °C to observe the effects of temperature variations. As shown in Figure 2, ethanolysis reactions with 0.1, 0.2, and 0.3 M sebacic acid concentrations were performed using 0.33 M NaBH₄. The results showed that at a 0.1 M sebacic acid concentration, half of the hydrogen was obtained within 3 s, after which the hydrogen production rate decreased. A total of 234 mL of hydrogen was collected in 30 s, but complete hydrogen production required a longer time. At a 0.2 M concentration, half of the hydrogen was similarly produced in 3 s, with 90% obtained in 30 s, and complete hydrogen production achieved after 60 s. At a 0.3 M concentration, 90% of the hydrogen in NaBH₄ was produced within 3 s, with complete hydrogen production achieved in 4 s. The hydrogen generation rate for the 0.3 M ethanolysis reaction was 4500 mL min⁻¹, indicating efficient and rapid hydrogen production.

Figure 3 presents the results of the methanolysis reactions. These experiments were carried out at 30 °C using 0.33 M NaBH₄ in conjunction with sebacic acid solutions at concentrations of 0.1 M, 0.2 M, 0.3 M, and 0.4 M. In the 0.1 M sebacic acid methanolysis reaction, half of the hydrogen was produced within 3 s, with 92% achieved by the 15th second, and complete hydrogen production was observed by the 30th second. In the 0.2 M experiment, 90% of the hydrogen was generated in 3 s, with full production completed at 6 s. For the 0.3 M reactions, 93% of the hydrogen yield was attained within 3 s, with total production achieved by the 4th second. Similarly, in the 0.4 M methanolysis reactions, 90% of the hydrogen was obtained within 4 s, and complete production was reached by the 5th second. The hydrogen production rates for these reactions were 610, 3200, 4845, and 3960 mL min⁻¹, respectively. Notably, the 0.3 M and 0.4 M methanolysis studies demonstrated high hydrogen yields within short durations, achieving significant hydrogen production in 3 s.

Figure 4 compares the methanolysis reactions at a 0.33 M NaBH₄ concentration across temperatures of 30, 40, and 50 °C, using sebacic acid concentrations of 0.2 M and 0.4 M. At a 0.2 M sebacic acid concentration, complete hydrogen production was achieved in 6 s at 30 °C, while at 50 °C, the complete production was reached in 4 s. When the sebacic acid concentration was increased to 0.4 M, over 90% of the hydrogen was produced within 4 s at all three temperatures. However, with increasing temperature, the total time required for complete hydrogen production extended. It is observed that increasing the temperature benefits hydrogen production time at 0.2 M sebacic acid concentration, while at 0.4 M, higher temperatures disadvantageously lengthen the duration needed for complete production. It can be inferred that at a 0.4 M sebacic acid concentration, hydrogen production occurs more rapidly at 30 °C. For the 0.2 M concentration at 50 °C, the hydrogen production rate was measured at 4950 mL min⁻¹ for the total duration of 4 s. If calculated for the 3 s mark, this rate increases to 6010 mL min⁻¹, noting that 93% of the hydrogen had been produced by that time.

Figure 5 shows the results of ethanolysis reactions with a 0.2 M sebacic acid concentration at three different temperatures. At this concentration, half of the hydrogen was produced within 3 s. By 30 s, 90% of the hydrogen was obtained, and complete hydrogen production was achieved at 60 s. When the temperature was increased to 40 °C, 92% of the hydrogen was produced within 4 s, and total production was completed by the 9th second. At 50 °C, the hydrogen production rate increased significantly, with 99% of the hydrogen generated within 4 s and complete production reached in 5 s. These findings indicate that increasing the temperature positively affects hydrogen production in ethanolysis reactions.

Figure 6 illustrates the effects of varying NaBH₄ concentrations on the hydrogen production rate. This study focused on 0.4 M sebacic acid methanolysis reactions conducted at 30 °C, comparing hydrogen production rates at NaBH₄ concentrations of 0.33 M and 0.165 M. The results indicate that the duration of hydrogen production remained consistent for both concentrations, with 97% of the hydrogen from NaBH₄ being produced by the 4th second and complete production achieved by the 5th second. This suggests that altering the amount of NaBH₄ does not significantly impact the time required for hydrogen production in methanolysis reactions.

Finally, a comparison was made between hydrogen production via ethanolysis and methanolysis reactions in the presence of a sebacic acid catalyst, as shown in Figure 7. For this study, reactions were conducted at 30 °C with 0.33 M NaBH₄ and 0.2 M sebacic acid. The results indicated that in ethanolysis reactions, half of the hydrogen was produced within 3 s, with 90% achieved by 30 s, and complete production reached at 60 s. In contrast, in the methanolysis reactions at the same conditions, complete hydrogen production was achieved within 6 s at 30 °C. These findings suggest that methanolysis reactions enable faster hydrogen production compared to ethanolysis reactions when using sebacic acid as a catalyst.

This study also investigated the kinetics of hydrogen production via the methanolysis and ethanolysis reactions of NaBH₄, with a focus on how varying NaBH₄ concentrations influence the reaction rate. Since NaBH₄ serves as the primary reactant in this system, the hydrogen generation rate is directly proportional to the concentration of NaBH₄. A higher NaBH₄ concentration implies a greater availability of hydrogen sources, indicated by an increased concentration of sodium and borohydride ions in the solution. To assess the impact of temperature on these reactions, kinetic studies were conducted at three different temperatures (30, 40, and 50 °C) using 0.2 M sebacic acid as a catalyst. The results show that an increase in temperature leads to a higher volume of hydrogen produced during the methanolysis and ethanolysis reactions of NaBH₄, facilitated by the sebacic acid catalyst. In kinetic studies, the rate constants (k) and activation energy (Ea) are calculated step-by-step. Initially, the change in NaBH₄ concentration over time at each temperature is analyzed to determine the rate constants. Subsequently, the Arrhenius equation is employed to calculate the activation energy. The concentration of NaBH₄ decreases over time from an initial concentration of 0.33 M. To accurately determine the rate constant, it is crucial to establish the reaction order (0th-, 1st-, or 2nd-order kinetics). The data clearly indicate a decrease in NaBH₄ concentration over time, allowing us to observe how the reactant concentration evolves.

Calculation was made assuming a first-order reaction to calculate the rate constant.

The rate law for a first-order reaction is given in Equation (3):

ln[NaBH₄] = ln[NaBH₄]₀ − kt

(3)

Logarithmic concentration–time graphs are created for 303 K, 313 K, and 323 K to determine the rate constants. These rate constants are then used to construct an Arrhenius plot, enabling the calculation of the activation energy.

The Arrhenius equation (Equation (4)) is expressed as follows:

k = Ae^−Ea/RT

(4)

where k is the rate constant, A is the frequency factor, Ea is the activation energy (J/mol), R is the gas constant (8.314 J/(mol K)), and T is the temperature (K). Taking the logarithm of this equation, it becomes linear. This linear equation is used to plot lnk against 1/T, and linear regression is applied to determine the value of Ea.

In this study, the rate constants for NaBH₄ methanolysis reactions were determined at three different temperatures (303 K, 313 K, and 323 K) and the activation energy was calculated via the Arrhenius equation. The results indicate that the reaction rate increases with temperature. Specifically, the rate constants were determined as k₃₀₃ = 0.7381 s⁻¹, k₃₁₃ = 0.7963 s⁻¹, and k₃₂₃ = 0.8863 s⁻¹ and the activation energy was calculated as Ea = 7.175 kJ/mol. In comparison, the activation energies obtained for methanolysis reactions using sebacic acid were lower than those reported in the literature for many other catalysts, such as the ATC-supported CoeCeeB catalyst for alcoholysis (29.51 kJ/mol) [24], the poly(4-vinyl pyridine)-based polymeric catalyst for NaBH₄ methanolysis (13.07 kJ/mol) [25], and the CoCl₂ solution catalyst for alcoholysis (25–29 kJ/mol) [15]. This activation energy value highlights the energy requirements and temperature dependence of methanolysis reactions. Such kinetic studies are essential for understanding reaction mechanisms and for the development of more efficient catalysts.

The plots in Figure 8 show the first-order kinetics for each temperature (30 °C, 40 °C, and 50 °C), with the calculated rate constants (k) for each temperature fitted to the data. The red dashed lines represent the linear fits used to determine the rate constants. In the ethanolysis reactions, the rate constants determined based on the calculations and graphs are as follows: k₃₀₃ = 0.0694 s⁻¹, k₃₁₃ = 0.7564 s⁻¹, and k₃₂₃ = 0.9585 s⁻¹. Using these rate constants, the Arrhenius plot was constructed and the slope was used to determine the activation energy (Ea). The activation energy obtained was approximately Ea = 52.3 kJ/mol. These results suggest that the reaction of NaBH₄ with ethanol is highly temperature-sensitive and requires overcoming a significant energy barrier. The kinetic and activation energy data are crucial for optimizing hydrogen production processes and enhancing efficiency.

4. Conclusions

The objective of this study was to explore the use of sebacic acid, an unexamined catalyst in the literature, for hydrogen production from NaBH₄. Various concentrations of sebacic acid solutions were prepared, and alcoholysis reactions were performed under different conditions. At a concentration of 0.3 M, 90% of the hydrogen contained in 0.33 M NaBH₄ was obtained within 3 s, with complete hydrogen extraction achieved within 4 s. The hydrogen generation rate for the 0.3 M ethanolysis reaction was 4500 mL min⁻¹, demonstrating efficient and rapid hydrogen production. For methanolysis reactions, the hydrogen production rates were 610, 3200, 4845, and 3960 mL min⁻¹ for 0.1, 0.2, 0.3, and 0.4 M concentrations, respectively. Notably, the 0.3 M and 0.4 M methanolysis studies yielded high hydrogen production within a short duration of 3 s. When comparing hydrogen production through ethanolysis and methanolysis reactions in the presence of sebacic acid, methanolysis was found to facilitate faster hydrogen production. In similar studies, the highest normalized hydrogen generation rate (HGR) achieved from the semi-methanolysis of KBH₄ using 1 M phosphoric acid as a catalyst was reported to be 5779 mL min⁻¹ g⁻¹ [26]. The hydrogen evolution from the alcoholysis reaction of sodium borohydride using H₃BO₃ as a catalyst was investigated. The completion times for the NaBH₄-methanolysis reaction with H₃BO₃ concentrations of 0.2, 0.4, 0.5, 1 M, and a saturated acid solution were approximately 50, 15, 10, 2, and 1 min, respectively [27]. The use of sebacic acid in NaBH₄-methanolysis and NaBH₄-ethanolysis reactions allowed for complete hydrogen conversion within 3–4 s, presenting a novel, rapid, and effective method that significantly contributes to the literature. The rate constants for NaBH₄ methanolysis reactions were determined at three different temperatures (303 K, 313 K, and 323 K) and the activation energy was calculated as Ea = 7.175 kJ/mol and the activation energies obtained for methanolysis reactions using sebacic acid are lower than those reported in the literature for many other catalysts reactions. In the ethanolysis reactions, the activation energy obtained was approximately Ea = 52.3 kJ/mol.