Investigation of Calcium Phosphate Catalysts in Sodium Borohydride Methanolysis for Improved Hydrogen Production

Abstract

In this study, calcium-rich resource minerals such as brushite, tricalcium phosphate (TCP), and hydroxyapatite were tested as catalysts for the methanolysis of alkaline solutions of NaBH₄ to generate hydrogen H₂. The synthesis of calcium phosphate compounds was characterized by means of X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The hydrogen generation rate with the TCP catalyst (15,214 mL min⁻¹ g⁻¹) was higher than with the hydroxyapatite catalyst (12,437 mL min⁻¹ g⁻¹) and brushite catalyst (6210 mL min⁻¹ g⁻¹) for the methanolysis of 250 mg NaBH₄ at 298 K using 25 mg of catalyst. The impact of TCP weight on hydrogen generation was studied. The methanolysis reaction led to a higher hydrogen volume generation over time with an increase in the weight of the TCP catalyst at a temperature of 308 K. The calculated activation energy for NaBH₄ hydrolysis with the TCP catalyst was 23.944 kJ mol⁻¹, suggesting the high catalytic activity of TCP. The values of enthalpy (ΔH) and entropy (ΔS) were calculated, and the results showed that ΔH was 21.28 kJ mol⁻¹ and ΔS was −93.096 J·mol⁻¹. ΔH was positive, meaning that the reaction was endothermic, and the negative ΔS meant a decrease in the disorder of the methanolysis reaction. The stability of the catalysis was tested in successive methanolysis tests. The catalyst’s efficiency decreased to 89% after four cycles.

1. Introduction

Energy is essential for humanity to power our daily activities, fuel economic development, and improve our quality of life. We rely on energy to heat our homes, operate vehicles, produce food, provide healthcare, and much more. Different energy sources have advantages and disadvantages, and some can have negative effects on the environment or human health [1]. Fossil fuels such as oil, coal, and natural gas are among the most widely used energy sources [2]. Air pollutants emitted from burning fossil fuels contribute to climate change and air pollution [3,4]. Additionally, using biomass from deforestation can lead to the loss of natural habitats and biodiversity. The combustion of certain types of biomass can release atmospheric pollutants, such as oxides of nitrogen like NO and NO₂, and oxides of sulfur like SO₂, contributing to air pollution [1]. Green energy is often seen as an important solution to environmental problems [5]. This type of energy has several benefits, including being renewable, having fewer negative effects on the environment, and helping to reduce greenhouse gas emissions [6]. Solar energy is abundant, renewable, and does not produce greenhouse gas emissions during its production [7]. Wind energy is generated by harnessing the power of the wind to turn wind turbines, which convert this kinetic energy into electricity [8]. It is renewable and does not produce air pollution. Hydroelectricity is produced by harnessing the kinetic energy of water flow. It is considered a renewable energy source [9,10]. Geothermal energy is produced by harnessing the natural temperature of the earth [11]. A recent energy vector was based on the generation of hydrogen, one of the cleanest energies available, which could contribute to solving energy and environmental problems. The creation, storage, and distribution of hydrogen-based energy remain a challenge, however [12]. Almost 95% of hydrogen comes from fossil fuel sources and wood [13]. This method is called natural gas reforming, and it involves heating natural gas (methane) to high temperatures, using an appropriate catalyst, to produce hydrogen and carbon monoxide (CO) [14]. After this, the CO can react with steam to produce additional hydrogen in a process called steam reforming. Other techniques to produce hydrogen include water or oxygen electrolysis [15,16] and the gasification method [17,18]. Electrolysis separates a water molecule into hydrogen and oxygen. Like fuel cells, this method converts electrical energy into chemical energy through the retention of electrons in stable chemical bonds. Gasification makes it possible to produce, via combustion, a mixture of CO and H₂ from coal or biomass. However, the transformation of hydrogen into energy is limited because of the difficulties of storing hydrogen [19]. When compared to numerous other hydrogen storage techniques, chemical hydrides provide significantly more benefits. This storage method is more efficient due to its safety and because hydrogen production occurs under ambient reaction conditions. Different metal hydrides or complex hydrides have been used as hydrogen sources, including LiH [20,21], MgH₂ [22,23], CaH₂ [24], AlH₃ [25,26], LiBH₄ [27,28], and NaBH₄ [29,30]. Specifically, sodium borohydride (NaBH₄) is promising for hydrogen generation due to its high gravimetric hydrogen storage capacity of approximately 10.8 wt.% [31]. Therefore, the generation of hydrogen can be carried out via the hydrolysis of NaBH₄ [32,33] in the presence of water (Equation (1)) or via the alcoholysis (Equation (2)) method of NaBH₄ [34,35] in the presence of alcohols.

NaBH₄ + 2 H₂O → NaBO₂ + 4 H₂

(1)

NaBH₄ + 4 CH₃OH → NaB(OCH₃)₄ + 4 H₂

(2)

For the alcoholysis method, alcohols such as ethanol [36] or methanol [37,38] were chosen to increase the rate of hydrogen generation [39] and limit the freezing problems associated with the use of water. In general, the generation of hydrogen with NaBH₄ via alcoholysis or hydrolysis is very limited, and the reaction yield is low. However, several studies have been carried out to speed up the reaction and generate more hydrogen using catalysts. Numerous substances have been investigated as catalysts for the generation of H₂, such as nickel Ni [40], Co [41], Ru [42], and Pt [43], as well as metal oxides such as CaO [44], MgO [45], Fe₂O₃ [46], and CuO [47]. While each type of catalyst offers specific advantages for hydrogen generation, there are notable drawbacks to consider. Noble metal catalysts, such as Pt and Ru, are highly efficient but have high costs, limited availability, and potential toxicity. Non-noble metal catalysts like Ni and Co are more affordable but often have lower catalytic activity, are susceptible to corrosion, and raise environmental concerns due to their toxicity. Metal oxides, including CaO, MgO, Fe₂O₃, and CuO, can provide stability and affordability but may suffer from lower catalytic activity, long-term stability issues, and sensitivity to reaction conditions. Given these challenges, ongoing research is crucial to developing more economical, sustainable, and effective catalysts for hydrogen generation applications.

Calcium phosphates were used as catalysts in this study due to their efficiency in activating the hydrolysis of NaBH₄, allowing a rapid and large hydrogen release. They offer a large specific surface area, giving more active sites for hydrolysis and improving hydrogen generation [48]. The porous structure of calcium phosphates allows easy diffusion of reactants and products, improving the efficiency of the reaction. Furthermore, calcium phosphates are stable in aqueous and alkaline media, which is beneficial for the hydrolysis conditions of NaBH₄. The non-toxic and biocompatible nature of calcium phosphates means they have a low environmental impact, and they can be recycled and reused in several hydrolysis cycles, reducing waste and production costs [49]. Other catalysts, such as noble metals, transition metals, and non-metallic catalysts, are expensive, rare, and may present toxicity problems.

This study investigated several hydrolysis parameters in order to achieve a high hydrogen generation rate, including the quantity of catalyst, the temperature of the reaction, and the quantity of the reducing agent. The reaction kinetics and the regeneration and reusability efficiency of the catalyst were also investigated.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. X-ray Diffraction Analysis

The structural analysis of the catalysts was carried out using the X-ray diffraction method with a Cu Kα monochromator of wavelength 1.54 Å (Figure 1). The hydroxyapatite XRD spectrum (Figure 1a) showed peaks in the 2θ angle regions, with intensities of 25.94°, 28.84°, and 32.02° corresponding to the crystal planes denoted by the Miller indices (0 0 2), (2 1 0), and (2 1 1). However, the crystallite structure of hydroxyapatite was hexagonal, with a space group of P 63/m, and the unit cell dimensions values were found to be a = 9.41 Å, c = 6.89 Å. The beta-tricalcium phosphate XRD spectrum (Figure 1b) showed peaks with intensity. β-TCP crystallized in the rhombohedral space group R3c, and the unit cell dimensions were as follows: a = b = 10.39 Å, and c = 37.22 Å. The results showed that brushite crystallized in the monoclinic space group I2/a. The unit cell dimensions were a = 5.82 Å, b = 15.22 Å, c = 6.27 Å, β = 116.41 (Figure 1c).

Table 1. The phase composition and lattice parameters of calcium phosphates using MATCH! Software, version 4, analysis.

Phase	Ca/P Molar Ratio	Crystal Structure	a (Å)	b (Å)	c (Å)	ß°	Unit Cell Volume (Å3)
Brushite CaHPO4·2H2O	1	Monoclinic	5.82	15.22	6.27	116.41	497.43
Tricalcium Phosphate Ca3(PO4)2	1.5	Trigonal (hexagonal axes)	10.39	-	37.22	-	3479.57
Hydroxyapatite Ca10(PO4)6(OH)2	1.62	Hexagonal	9.41	-	6.89	-	528.03

shows the results of the software analysis.

2.1.2. Surface Morphology Using SEM

The morphologies of brushite (Figure 2A), hydroxyapatite (Figure 2B), and TCP (Figure 2C) are shown in Figure 2. Brushite consists of platy fragments that form angles with the c-axis. Additionally, this image shows a rectangular shape of various sizes, measuring a few micrometers [50]. The SEM micrographs of hydroxyapatite show the particles in a rod shape. The direction of the rods is random, and their size varies between a few nanometers and 200 nm in length [51]. Figure 2 presents monophasic brushite, which has an equal molar ratio of Ca and P. As the Ca/P ratio increased to 1.5 and 1.6, the SEM images showed nanosized fibrous crystals of TCP and HA, respectively.

The SEM analysis verified the XRD data, indicating that monocline brushite, nano-fibrous trigonal TCP, and nano-fibrous hexagonal HA crystals were formed.

2.2. Determination of the Catalytic Activity of the Prepared Catalysts

NaBH₄ methanolysis using the catalysts brushite, hydroxyapatite, and TCP in methanol was investigated. The results of these reactions are depicted in Figure 3. The temporal evolution of the hydrogen volume revealed a pronounced acceleration in the hydrogen yield facilitated by the catalysts compared to free NaBH₄. The evolution of the quantity of hydrogen during the methanolysis of NaBH₄ with the catalysts was probably linked to the increase in the specific surface area and a high concentration of surface sites [52,53]. As a result, a significant amount of BH₄⁻ ions could be efficiently adsorbed and activated by the catalyst [54]. This caused a large amount of hydrogen to be produced very quickly [55,56].

The comparative analysis of catalysts involved the calculation of the hydrogenation generation rate (HGR), defined as the change in hydrogen volume (mL) over time (min) per gram of catalyst weight (g). From the curves depicted in Figure 3, the HGR values for the TCP, hydroxyapatite, and brushite catalysts were 15,214 mL min⁻¹ g⁻¹, 12,437 mL min⁻¹ g⁻¹, and 6210 mL min⁻¹ g⁻¹, respectively. The results indicated a maximum HGR value for the catalyst, TCP. TCP offered the highest hydrogen generation rate due to its porous structure and stability, which ensured efficient interactions with the reactants [52]. Hydroxyapatite, although stable and providing a good surface area, had a slightly lower density of active sites [53]. Brushite demonstrated a lower hydrogen generation rate because of its low surface area and non-porous structure [57].

Calcium phosphates can achieve a high rate of hydrogen generation with several catalysts.

Table 2. A comparison of some catalysts in terms of hydrogen generation.

Catalyst	Activ. Energy (kJ mol−1)	HGR (mL g−1 min−1)	Ref.
Ni	-	325	[58]
Ni50Co50	26	1600	[58]
Fe2O3@OMWCNTs	15.92	405	[59]
Cu1.7Ca0.3O/GO	-	9809	[60]
a-Fe2O3@N-C NSs	-	637	[46]
NiCoP/Ni	38.3	3986	[61]
CuCo2O4	22	1370	[62]
MWCNTeCOOH	20.1	8766	[63]
Brushite	-	6210	This work
Hydroxyapatite	-	12,437	This work
TCP	23.9	15,214	This work

shows the HGR values of some catalysts and compares the HGR values of calcium phosphate catalysts.

2.3. Impact of TCP Weight Variation on Rates of Hydrogen Generation

The impact of TCP weight as a catalyst on the rate of hydrogen production was investigated. The NaBH₄ and TCP methanolysis reactions showed a high hydrogen volume value for a weight of 60 mg of TCP at a temperature of 308 K. The hydrogen volume over time increased as a function of TCP catalyst weight (Figure 4). The catalyst offered the BH₄⁻ ions a surface to be adsorbed onto and made it easier for the BH₄⁻ ions to transfer their electrons to the catalyst’s surface. As a result, H⁻ ions were formed; therefore, the rate of hydrogen generation increased. This phenomenon was probably due to the evolution of surface area achieved by varying the weight of the TCP catalyst [64]. The small particle size of TCP can enhance the hydrogen production yield [65].

2.4. The Influence of Temperature on the Methanolysis Process of NaBH₄ Catalyzed by TCP

The reaction temperature significantly impacts the methanolysis of NaBH₄. The hydrogen generation rate (HGR) increased with increasing temperature (Figure 5). The HGR values were calculated as 15,214, 30,102, 42,768, 46,800, and 54,240 mL min⁻¹ g⁻¹ for 25, 35, 45, 55, and 65 °C, respectively. The variation in the hydrogen volume versus time at different temperatures was used to calculate the activation energy of the catalyst using the Arrhenius Equation (3):

$$\mathrm{K}=\mathrm{A}{\mathrm{e}}^{{\displaystyle \frac{\mathrm{E}\mathrm{a}}{\mathrm{R}\mathrm{T}}}}$$

(3)

where K is the hydrogen generation rate (mL min⁻¹ g⁻¹), A is the pre-exponential factor, Ea is the activation energy (kJ mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is the temperature (K). The activation energy was determined from the plot Ln (K) versus 1/T based on the slope of the line. A slope of −2.88 was found (Figure 6a). The TCP catalyst’s activation energy was 23.944 kJ mol⁻¹, which was a significantly lower activation energy compared to many other catalysts, suggesting that TCP has a high catalytic capacity for the methanolysis of NaBH₄ [65]. A lower activation energy implies a lower energy requirement for the chemical reaction, thus facilitating low-cost hydrogen production.

Chemical kinetics, describing changes in the reaction rate against temperature, were analyzed using the Eyring equation to determine the thermodynamic parameters (Equation (4)). Thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) were obtained using the Eyring plot method. The Eyring equation involves plotting Ln (K/T) versus (1/T) and deriving the slope of the line (−ΔH/R). The value of ΔH was determined to be 21.28 kJ mol⁻¹, and ΔS was found to be −93.096 Jmol⁻¹ K⁻¹ by intercepting the line (Ln(k_B/h) + ΔS/R) [66] (Figure 6b).

$$\mathrm{L}\mathrm{n}\left({\displaystyle \frac{\mathrm{K}}{\mathrm{T}}}\right)=-\left({\displaystyle \frac{\mathsf{\Delta }\mathrm{H}}{\mathrm{R}\mathrm{T}}}\right)+\mathrm{L}\mathrm{n}\left({\displaystyle \frac{{\mathrm{k}}_{\mathrm{B}}}{\mathrm{h}}}\right)+{\displaystyle \frac{\mathsf{\Delta }\mathrm{S}}{\mathrm{R}}}$$

(4)

• k_B is Boltzmann’s constant (1.381 × 10⁻²³ J/K);
• T is the absolute temperature in Kelvin (K);
• h is Planck’s constant (6.626 × 10⁻³⁴ Js);
• R is the gas constant (8.314 J K⁻¹ · mol⁻¹).

At the temperature of T = 298 K, the free enthalpy ΔG was calculated according to Equation (5), resulting in a ΔG value of 49.002 kJ mol⁻¹.

$$\mathsf{\Delta }\mathrm{G}=\mathsf{\Delta }\mathrm{H}-\mathrm{T}\mathsf{\Delta }\mathrm{S}$$

(5)

A change in enthalpy that is positive means that the reaction is endothermic, and a change in entropy that is negative means a decrease in the disorder of the methanolysis reaction. A reaction with a positive ΔG value is called an endergonic reaction.

2.5. HGR of the TCP Catalyst

In the reusability test of NaBH₄ methanolysis, the stability and yield of the catalyst over several reaction cycles were evaluated. Practically, a catalyst is more effective when it can be used more times without being replaced. The TCP catalyst was subjected to the methanolysis of NaBH₄ under the same constant conditions over five cycles. The results, as shown in Figure 7, were compared to the first used to assess any decline in activity after each cycle. The catalyst efficiency decreased to a value of 89% after four cycles. This slight decrease was probably due to tetramethoxyborate (B(OCH₃)₄⁻), a by-product of the NaBH₄ methanolysis reaction, which decreases catalytic activity by reducing the active sites of the catalyst.

3. Experiments

3.1. Materials and Methods

The chemicals used in this study were obtained from various suppliers. Diammonium hydrogen phosphate ((NH₄)₂HPO₄) was obtained from Techno Pharmchem in Delhi, India. Ca(NO₃)₂·4H₂O was purchased from LOBA Chemie. Sodium borohydride (NaBH₄) powder, ≥98.0%, was purchased from Sigma-Aldrich (St. Louis, MO, USA). All other reagents and solvents were obtained from commercial sources and were utilized without further purification. Qualitative mineralogical analysis and phase composition of the samples were performed using an XRD diffractometer 6000, Shimadzu (Kyoto, Japan). To analyze the samples, a cobalt tube was used, and scans were carried out in a 2-theta range of 10° to 60° with a scan rate of 2°/min. Crystal Impact—MATCH was employed to process the powder XRD diffraction data for crystal analysis and phase composition, specifically software version 3.15 (Bonn, Germany). An Inspect F50 instrument (FEI Company, Eindhoven, The Netherlands) was used to acquire a thorough understanding of the product morphology.

3.2. Synthesis of Calcium Phosphate Compounds

Three different calcium phosphate compounds, brushite, tricalcium phosphate, and hydroxyapatite, were synthesized using solutions of (NH₄)₂HPO₄ and Ca(NO₃)₂·4H₂O, respectively, as illustrated in Figure 8. The (NH₄)₂HPO₄ solution was combined with 100 mL of the Ca(NO₃)₂·4H₂O solution at a regulated flow rate of roughly 2 mL/min. Using a glass funnel with a stopcock, the mixture was mixed at a 450 rpm stirring speed until a certain Ca/P molar ratio of 1, 1.5, or 1.62 was reached, which took around an hour at room temperature (RT). For 60 min, the resultant mixture was mixed by stirring. To maintain the pH within the specified range, ammonia (~15 mol/L) or diluted HCl was added, as shown in Figure 8. Then, the resulting white precipitate was separated. To keep it from clumping, the filter cake was washed three times using ethanol and deionized water. Lastly, the sample was dried for seven days in an oven set at 40 °C.

3.3. Hydrolysis Experiment

Figure 9 presents a schematic of the methanolysis reaction setup employed to quantify both the hydrogen production rate and yield. This experiment involved maintaining a constant volume of methanol (10 mL) and a known quantity of NaBH₄ (0.25 g), along with a catalytic quantity of catalyst (0.025 g), all placed within a Pyrex glass reactor. This reactor had two openings, one for methanol addition and the other for hydrogen exhaust via a connected pipe. The hydrogen production began when the NaBH₄ and catalyst came into contact with methanol. The generated hydrogen was exhausted through the pipe, increasing the pressure in the water chamber. Another pipe is extended to an empty collecting chamber positioned on an electronic weighing balance. Throughout the reaction, the hydrogen gas produced in the reaction chamber passed through the pipe, causing a pressure increase inside the water chamber. This increase in pressure resulted in the displacement of an equal amount of water into the collection chamber placed on the electronic balance. The values were recorded and analyzed over time by the computer. Consequently, the liberated H₂ gas displaced an equal volume of water in the collecting chamber, indicating the amount of H₂ produced during the hydrolysis. Finally, the catalyst was washed with methanol three or four times and dried for 9 h at 80 to 90 °C in an oven. This catalyst was also utilized after regeneration in the hydrolysis reaction.

4. Conclusions

This study investigated the catalytic performance of various calcium phosphate catalysts in the methanolysis of sodium borohydride (NaBH₄) for hydrogen production, with tricalcium phosphate (TCP) emerging as the most effective catalyst. TCP demonstrated the highest hydrogen generation rate, reaching up to 15,214 mL min⁻¹ g⁻¹ at 25 °C. This study also revealed that increasing the weight of the TCP catalyst significantly enhanced hydrogen production, with the maximum volume observed at 60 mg of TCP at 308 K. The activation energy for TCP-catalyzed methanolysis was determined to be 23.944 kJ mol⁻¹, indicating high catalytic efficiency. Additionally, the enthalpy (ΔH) and entropy (ΔS) were calculated to be 21.28 kJ mol⁻¹ and −93.096 J mol⁻¹ K⁻¹, respectively, with a free energy (ΔG) of 49.002 kJ mol⁻¹ at 298 K. Although the reusability tests showed a slight efficiency decline to 89% after four cycles, likely due to the formation of tetramethoxyborate by-products, TCP’s overall performance in hydrogen generation is promising. This study highlighted the importance of catalyst weight and reaction temperature in optimizing hydrogen production, providing valuable insights for developing efficient catalytic processes.