Mechanochemical Synthesized CaO/ZnCo₂O₄ Nanocomposites for Biodiesel Production

Abstract

Biodiesel has been recognized as an environmentally friendly, renewable alternative to fossil fuels. In this work, CaO/ZnCo₂O₄ nanocomposites were successfully synthesized via simple mechanochemical reaction between ZnCo₂O₄ and CaO powders by varying the CaO loading from 5 to 20 wt.%. The synthesized materials were found to be highly efficient heterogeneous catalysts for transesterification of tributyrin with methanol to produce biodiesel. The nanocomposite, which contained 20 wt.% CaO and 80 wt.% ZnCo₂O₄ (CaO/ZnCo₂O₄-20), exhibited superior and stable transesterification activity (98% conversion) under optimized reaction conditions (1:12 TBT to methanol molar ratio, 5 wt.% catalyst and 180 min. reaction time). The experimental results revealed that the reaction mechanism on the CaO/ZnCo₂O₄ composite followed pseudo first-order kinetics. The physicochemical characteristics of the synthesized nanocomposites were measured using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier-transformed infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), N₂-physisorption, and CO₂- temperature-programmed desorption (CO₂-TPD) techniques. The results indicated the existence of coalescence between the CaO and ZnCo₂O₄ particles, Additionally, the CaO/ZnCo₂O₄-20 catalyst was found to possess the greater number of highly basic sites and high porosity, which are the key factors affecting catalytic performance in transesterification reactions.

1. Introduction

Most of the energy consumed globally originates from fossil fuels such as coal, petroleum, and natural gas. Indeed, fossil fuels currently account for around 86% of total energy usage [1,2]. Nonetheless, these sources are limited and may run out in the next century [3]. The combustion of fossil fuels emits air pollutants and toxic gases such as SO_x, NO_x, and CO_x, volatile organic compounds, and heavy metals which can cause dangerous diseases. In addition, CO₂ emissions are one of the most significant direct contributors to global warming [4]. As such, many researchers and decision-makers across the world have started to seek alternative and clean energy sources. Alternative energy sources should be cost-effective, technically feasible, simple to store and transport, environmentally friendly, and widely available to the public [5,6,7,8,9,10].

In the last two decades, biodiesel has emerged as a promising renewable alternative to petroleum diesel [11]. Biodiesel is a general term for several ester-based oxygenated fuels derived from renewable sources, such as vegetable oils and animal fats [12,13,14]. Biodiesel resembles petroleum diesel in terms of its physical, chemical, and combustion properties, but it is sulphur-free, making it a cleaner-burning fuel [15,16,17]. Biodiesel can be obtained from the transesterification of vegetable oils or animal fats with alcohols [18]. The choice of feedstocks for biodiesel production is a key factor in making it a viable alternative to petroleum-based diesel. Plenty of oils from plants and animal fat have been used as feedstocks for the synthesis of biodiesel. Tributyrin (TBT), also known as glyceryl tributyrate, with chemical formula C₁₅H₂₆O₆, is a short-chain aliphatic ester. TBT is naturally found in animal fats, vegetable oils, dairy products, etc., and is mainly used in the processing of margarine [19]. Synthetically, TBT can be produced by reacting glycerin and butyryl chloride with an organic base. In this research, TBT is used as a raw material to produce biodiesel. The presence of a catalyst in the transesterification process is essential to enhance the rate of reaction and the yield. For industrial biodiesel production, homogeneous catalysts are used; however this approach suffers from significant drawbacks, such as high production costs due to the need to separate of K⁺ and Na⁺ ions and glycerol [20]. The utilization of enzyme catalysts has significant advantages, but it is currently not feasible due to the relatively high cost and unstable behaviour of enzymes.

The utilization of an efficient heterogeneous catalyst could be advantageous because, contrary to homogeneous catalysts, no separation procedures are required. In most cases, the catalysts can be reused and recycled for an extended time, allowing for continuous operation on a large scale [21]. For biodiesel production, a variety of heterogeneous basic catalysts, such as alkaline earth metal oxides, alkali metals exchanged zeolites, and hydrotalcites, has been studied. Because of their high basicity, alkaline earth metal oxides offer high biodiesel yields [22]. Li et al. [23] utilized Li/NaY zeolite catalysts for biodiesel production and observed yields of 98.6%. The spent catalyst characterization results showed that the catalyst has excellent stability with a moderate regenerative capacity. Sulaiman et al. [24] studied the conversion of refined and used cooking oils into safer and low toxicity biodiesel via base-catalysed transesterification. Alumina supported magnesium, calcium, strontium, and barium oxide-based catalysts with iron as a dopant offered a maximum biodiesel yield of 84.0%.

Several studies have indicated that calcium oxide (CaO) is a promising catalyst for the transesterification for biodiesel production because of its low cost, ease of production from natural and waste raw materials, high catalytic activity and lack of significant negative environmental effects [25]. Sai Bharadwaj et al. [26] synthesized biodiesel from non-edible rubber seed oil using CaO prepared from eggshells. A conversion rate of 99.7% of oil to biodiesel was obtained with a 12:1 methanol to oil molar ratio and 4 wt.% catalyst in 3 h. However, the leaching of Ca²⁺ is a major drawback of the use of CaO as a catalyst; additionally, it is sensitive to free fatty acids (FFAs), as the leached Ca²⁺ species react with FFAs to form undesired soap products [27]. Several researchers have attempted to substitute CaO for another metal oxide to improve the stability of the process. Yu et al. [28] reported the production of biodiesel over CaO–CeO₂ catalysts by the transesterification of Pistacia chinensis oil, obtaining a yield of 91% at 110 °C.

In recent years, spinel oxides have emerged as an alternative to conventional heterogeneous supports due to their unique crystal structure and chemical and thermal stability [29]. These compounds are typically expressed with the general formula of AB₂O₄ [30]. The co-precipitation method is the most common method used for the preparation of spinel nanomaterials, but this method is laborious. Liu et al. [31] synthesized CaO/MgFe₂O₄ catalysts by co-precipitation and used it for to produce biodiesel from soybean oil. In traditional methods, the synthesis of catalysts generally involves multi-step processes, heating, and/or the addition of expensive and hazardous reagents. By contrast, the mechanochemical method can overcome these drawbacks by using simple grinding or milling under optimized conditions. In mechanochemical reactions, the reactants are broken apart by mechanical force, which leads to an amorphous mixture of all reagents and a higher surface for the reaction to take place on. The effect of the mechanical action can reduce the particle size and create active sites by generating new active surfaces which can connect to other particles, thereby increasing the chemical activity of the materials [32]. In the present study, we synthesized low-cost heterogeneous CaO/ZnCo₂O₄ nanocomposites by a mechanochemical synthesis method and used the materials as catalysts for the transesterification of tributyrin with methanol. The influence of different reaction conditions, such as the oil to methanol molar ratio, reaction time and temperature, on the performance of the catalysts was also studied. The synthesized materials were analyzed using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FT-IR), N₂-physisorption, X-ray photoelectron spectrometer (XPS) and CO₂-temperature-programmed desorption (CO₂-TPD) techniques. The correlation between the catalytic performance and the physicochemical characteristics of the catalysts was also studied.

2. Results and Discussion

2.1. XRD

XRD analysis was used for phase detection and to investigate the crystal structure of the synthesized ZnCo₂O₄, CaO and CaO/ZnCo₂O₄ composite catalysts; the obtained patterns are shown in Figure 1. The bare ZnCo₂O₄ sample exhibited reflections at 19°, 31.6°, 37.4° and 44.6°, corresponding to the (110), (220), (310), (400), (511) and (440) planes of cubic structure of the ZnCo₂O₄ spinel [JCPDS file#00-001-1149]. No other reflections were observed for any other crystal phase, indicating the high purity of the synthesized ZnCo₂O₄ material.

The XRD pattern of the synthesized bare CaO sample showed all reflections due to the presence of a cubic CaO structure [JCPDS file#01-074-1226]. The CaO/ZnCo₂O₄ composite samples featured the characteristic reflections of both ZnCo₂O₄ and CaO crystal phases. It is interesting to note that reflections due to the crystalline CaO phase were observed even composite with low CaO loading (5 wt.%); this was probably due to the high crystallinity of CaO. The intensity of the CaO reflections increased with an increase of CaO loading from 5 to 20 wt.%. The average crystallite sizes of both the ZnCo₂O₄ and CaO phases for all the synthesized materials were calculated using the Scherrer equation.

Table 1. The crystallite sizes of ZnCo₂O₄ and CaO phases for the synthesized samples.

Catalyst	Crystallite Size (nm)
	ZnCo2O4	CaO
CaO	-	38
ZnCo2O4	27	-
CaO/ZnCo2O4-5	16	30
CaO/ZnCo2O4-10	14	28
CaO/ZnCo2O4-15	12	25
CaO/ZnCo2O4-20	12	25

illustrates the calculated crystallite sizes of the ZnCo₂O₄ and CaO phases for the synthesized samples. It is clear that the crystallite size of CaO is large compared to that of ZnCo₂O₄; interestingly, the crystallite sizes of both CaO and ZnCo₂O₄ were slightly decreased in the composites, probably due to the mechanical grinding.

2.2. HRTEM Analysis

The HRTEM images and particle size distribution of the synthesized catalysts are shown in Figure 2. The images of bare ZnCo₂O₄, CaO and CaO/ZnCo₂O₄-20 samples consist of isolated and agglomerated spherical nanoparticles. The average particle size of CaO and ZnCo₂O₄ samples is around 31 nm and 28 nm, respectively. On other hand, the particle size of the CaO/ZnCo₂O₄-20 sample was around 21 nm, which is less than that of the parent compounds of the nanocomposites, as shown in the histograms (Figure 2), which is in accordance with the crystallite sizes determined from XRD results. The HRTEM images of the CaO and ZnCo₂O₄ samples shows the lattice fringes with a distance of 0.24 nm, corresponding to the (200) plane of the CaO phase, and 0.286 nm, which was attributed to the d-spacing of the (200) plane of the spinel structure of ZnCo₂O₄ [33,34]. The HRTEM image of the mechanochemically synthesized CaO/ZnCo₂O₄-20 composite sample showed the presence of lattice fringes for both ZnCo₂O₄ and CaO particles. The image of the composite sample clearly shows existence of interface (marked with red color eclipse) between the particles revealing coalescence between the CaO and ZnCo₂O₄ particles. This observation is indicating that the two crystalline phases grown together due to vigorous milling and thermal treatment procedures adapted during the synthesis of CaO/ZnCo₂O₄ composite.

2.3. FT-IR

FT-IR spectroscopy was employed to investigate the functional groups present in the synthesized samples. Figure 3 shows the FT-IR spectra for the synthesized catalysts obtained at room temperature. The bare ZnCo₂O₄ has a characteristic band in the region of 3440 to 3615 cm⁻¹, which could be attributed to H-O-H stretching vibrations, indicating the presence of water molecules. The intense IR bands observed at 1628 cm⁻¹ and at 1387 cm⁻¹ were assigned to CO₃⁻² groups [35]. The strong absorption bands at 666 and 566 cm⁻¹ were due to the intrinsic stretching vibrations of Zn-O at tetrahedral lattice sites and Co-O at octahedral sites, respectively. The observed results indicated the successful formation of cubic structured ZnCo₂O₄, which is in concurrence with the XRD results [36].

The spectrum of the bare CaO sample displayed an absorption band at 3647 cm⁻¹, corresponding to the bending vibration modes of the OH groups from the remaining hydroxide or from H₂O molecules on the external surface of the sample, likely present as a result of handling the specimen before recording the spectra [37]. The broad band around 1400–1500 cm⁻¹, as well as the weak band at 873 cm⁻¹, is characteristic of C-O bonds related to the carbonation of CaO [38]. The characteristic IR band at 520 cm⁻¹ for CaO signified the presence of a Ca-O functional group. The spectra of the CaO/ZnCo₂O₄ nanocomposite samples showed similar absorption bands to those of the bare CaO and bare ZnCo₂O₄ samples, indicating the presence of functional groups in both the CaO and ZnCo₂O₄ phases.

2.4. N₂ Gas Physisorption Measurements

Figure 4 illustrates the N₂ gas adsorption/desorption isotherms of the samples. All the isotherms could be identified as type IV, as per the IUPAC classification [39]. The hysteresis loops in the isotherms indicated the mesoporous nature of all catalysts. The presence of H3-type hysteresis loops in the isotherms indicated that the catalysts possessed slit pores which formed due to the aggregation of sheet like particles [40].

Table 2. Textural properties of different catalysts from N₂ gas physisorption.

Sample	SBET (m2/g)	Smicro (m2/g)	Smeso (m2/g)	Vtotal (cc/g)	Vmicro (cc/g)	Vmeso (cc/g)	Average Pore Size (nm)
CaO/ZnCo2O4-20	23.0	7.0	14	0.033	0.006	0.023	3.0
ZnCo2O4	32.0	12.0	16	0.041	0.007	0.027	3.0
CaO	6.0	0.0	5.8	0.01	0.00	0.01	4.0

displays the calculated textural properties, such as the BET surface area (S_BET), micropore surface area (S_micro), mesopore surface area (S_meso), total pore volume (V_total), micropore volume (V_micro) and mesopore volume (V_meso) for representative samples. The highly crystalline CaO sample exhibited a low BET-surface area (6.0 m²/g), while the ZnCo₂O₄ sample possessed a surface area of 32 m²/g, which is high for a spinel structured sample. Interestingly, the composite samples showed surface areas less than that of ZnCo₂O₄ but higher than that of CaO. For instance, CaO/ZnCo₂O₄-20 exhibited a surface area of 23 m²/g, possibly due to presence of CaO particles in the pore channels of ZnCo₂O₄. A pore size distribution analysis was carried out using NLDFT method. The pore size distribution patterns indicated that the samples had multimodal pores in the micro, meso and macro size ranges [41]. The pore volume and average pore size properties of the composite samples also showed a similar trend to that of the surface area.

2.5. XPS

The surface elemental composition and oxidation state of the elements presented in the synthesized catalysts were analyzed using XPS analysis. The deconvoluted XPS spectra of the samples are shown in Figure 5. For the sake of clarity, only 2p_3/2 components for the Zn 2p and Co 2p spectra are presented in the figure. The Zn 2p_3/2 component present in the synthesized ZnCo₂O₄ catalyst showed three deconvoluted Zn 2p_3/2 peaks at 1020 eV, 1021.2 eV and 1022.2 eV. The peaks observed at about 1020 eV and 1021.2 eV could be attributed to the Zn-O bonds in the ZnCo₂O₄, while the other peak at 1022.2 eV could be assigned to Zn(OH)₂ species [42]. Interestingly, the CaO-ZnCo₂O₄ composite sample only exhibited two peaks at 1021.1 eV and 1022 eV, corresponding to Zn-O and Zn(OH)₂ species, respectively.

The Co 2p_3/2 peak in the ZnCo₂O₄ and CaO/ZnCo₂O₄ composite samples could be fitted into two peaks. The fitted peaks at 780.0 eV and 781.6 eV could be assigned to Co³⁺, and Co²⁺ species. It has been reported that bulk cobalt oxides such as Co₃O₄ possess mixed oxidation states of Co²⁺ and Co³⁺ and are expected to show satellite features due to both Co²⁺ and Co³⁺ states. Both spectra showed a satellite peak at around 786 eV for the Co²⁺ species [43]. Moreover, the integral area corresponding to the Co³⁺ peak was much larger than that of the Co²⁺ peak, which revealed that the main oxidation state of Co in the three spinel samples was Co³⁺ [44]. These observations indicated that the surface of the synthesized samples had a composition containing Co³⁺ and Zn²⁺. The O 1s spectra of the two samples could be fitted into three peaks. The peak at 529.5 eV matched the oxygen in the crystal lattice that bonded with metal ions, the peak at 531.5 eV corresponded to the oxygen of the OH⁻ groups and the peak at 533.3 eV corresponded to the oxygen of the physically adsorbed H₂O molecules [45]. The Ca 2p peak of CaO/ZnCo₂O₄ composite sample showed the spin-orbit doublet for Ca 2p_3/2 and Ca 2p_1/2 components. A reasonably well-defined valley between the Ca 2p_3/2 and Ca 2p_1/2 existed in the spectra of the samples, indicating that the samples did not possess many physically adsorbed carbonate or hydroxyl groups due to calcination. The BE of the Ca 2p_3/2 and Ca 2p_1/2 components appeared at 347.4 eV and 351.1 eV and could be attributed to the Ca²⁺ oxidation state [46]. The XRD, TEM and XPS results clearly indicated the formation of a CaO/ZnCo₂O₄ composite by the mechanochemical synthesis method.

2.6. CO₂-TPD

The basicity of the synthesized catalysts was evaluated using CO₂-temperature-programmed desorption. Figure 6 illustrates the deconvoluted CO₂-TPD profiles of three representative catalysts. A broad desorption peak in the temperature range of 500–680 °C was observed for the bare CaO sample. On other hand, the bare ZnCo₂O₄ sample exhibited one small desorption peak centered at 620 °C and a major desorption peak at 720 °C. It is interesting that the CaO/ZnCo₂O₄-20 sample showed the presence of three desorption peaks at 750 °C, 800 °C and 908 °C. The CO₂ desorption temperature of all the samples were found to be higher than 400 °C, suggesting the high strength of the CO₂ adsorption sites, indicating that all the catalysts possessed strong basic sites [47]. CO₂ uptake was also quantified; the results indicated that the CaO/ZnCo₂O₄-20 catalyst uptake (1276 μmol/g,

Table 3. Data obtained from CO₂-TPD analysis of CaO, ZnCo₂O₄ and CaO/ZnCo₂O₄-20 samples.

Catalyst	Temp. (°C)	CO2 Uptake (μmol/g)	Total CO2 Uptake (μmol/g)
CaO/ZnCo2O4-20	750	251	1276
	800	539
	908	486
ZnCo2O4	620	152	677
	720	545
CaO	638	904	904

) was higher than those of both ZnCo₂O₄ (677 μmol/g) and CaO (904 μmol/g). The improved basicity of the CaO/ZnCo₂O₄-20 composite sample was due to the synergetic effect between ZnCo₂O₄ and CaO. Furthermore, it was observed that the CaO/ZnCo₂O₄-20 catalyst exhibited CO₂ desorption peaks at high temperatures compared to bare ZnCo₂O₄ and CaO, which indicates that the composite sample possessed more strong basic sites than its parent oxides. The observed characterization results indicated that the CaO/ZnCo₂O₄ composite samples could offer high catalytic activity for a base catalyzed transesterification process [47].

2.7. Catalytic Activity

The synthesized samples were used as catalysts for the transesterification of tributyrin with methanol to produce biodiesel (methyl esters). The transesterification of tributyrin with methanol produces methyl esters and glycerin via three different elementary reactions (Equations (1)–(3)). The reaction between triglycerides (TAG) and methanol (MeOH) yields methyl-esters and diglycerides (DAG) in the first step. The reaction between diglycerides and methanol in the next step produces methyl esters and monoglycerides (MAG). In the final step, the monoglycerides are converted to methyl-esters and glycerin (Gly).

TAG + MeOH → DAG + Methyl ester

(1)

DAG + MeOH → MAG+ Methyl ester

(2)

MAG + MeOH → Gly + Methyl ester

(3)

A ¹H NMR and ¹³C NMR analysis was performed to characterize the synthesized biodiesel (Figures S1 and S2). The ¹H NMR of the purified separated sample showed two triplet signals at δ 0.82 and 2.24 ppm due to CH₃ and CH₂ groups, respectively, a multiplet signal at 1.42–1.54 due to CH₂ groups, and a singlet signal at δ 3.71 ppm due to OCH₃ groups, which is in complete agreement with the structure of methyl butyrate.

To optimize the reaction conditions in order to achieve the highest catalytic activity, activity measurements over catalysts were carried by varying parameters such as the methanol/TBT molar ratio, reaction temperature and reaction time. Preliminarily, the influence of the reaction time on the conversion of TBT and yield of biodiesel over the synthesized catalysts was studied at 80 °C; the results are shown in Figure 7a. As the reaction time increased to 180 min, the TBT conversion levels increased gradually (not reaching 100% conversion) with the ZnCo₂O₄, CaO/ZnCo₂O₄-5, CaO/ZnCo₂O₄-10 and CaO/ZnCo₂O₄-15 samples. The conversion levels did not change significantly as the reaction time was increased 60 min, indicating that the equilibrium had been reached within 60 min. With a further increase of the reaction time, the conversion levels slightly increased and reached a maximum of 20% for CaO/ZnCo₂O₄-10 and 80% for CaO/ZnCo₂O₄-15. This was mainly due to the presence of a limited number of active sites on the surface of these catalysts. Moreover, transesterification progresses as a stepwise reaction of the triglycerides, resulting in the formation of intermediate diglycerides and monoglycerides, which are then converted into biodiesel and glycerin [48]. However, in case of bare CaO, and the CaO/ZnCo₂O₄-20 samples, the TBT conversion levels increased drastically within 30 min. and reached 100% in just 60 min. This was due to the presence of a greater number of active sites in these two catalysts, resulting high conversion levels in the case of CaO/ZnCo₂O₄-20 and the bare CaO catalyst. Although bulk CaO offered high catalytic activity, its recyclability is difficult; this difficulty was not observed in the case of the CaO/ZnCo₂O₄-20 composite.

The influence of the molar ratio of TBT and methanol on biodiesel yield is shown in Figure 7b. It is well known that the transesterification reaction is reversible, and a higher molar ratio is required to shift the reaction equilibrium to the right. Figure 7b shows that as the amount of methanol increased, the biodiesel production increased, with the maximum biodiesel production of 100% being achieved with a 1:12 oil-to-methanol molar ratio over the CaO/ZnCo₂O₄-20 catalyst. This was possibly due to the formation more methoxy species on the surface of the catalyst which shifted the reaction equilibrium to the right to achieve maximum biodiesel production [49]. The transesterification reaction was catalyzed by synthesized ZnCo₂O₄, CaO, and the CaO/ZnCo₂O₄ catalysts and was carried out in the temperature range of 40–90 °C to assess the effect of the reaction temperature on the conversion of TBT to biodiesel (see Figure 7c). The obtained results clearly indicate that CaO and CaO/ZnCo₂O₄-20 samples are efficient catalysts for the transesterification of TBT compared to other samples due to the presence of large numbers of basic sites, as shown in the CO₂-TPD analysis. The conversion of TBT was enhanced with an increase in the reaction temperature in all catalyst samples. The possible reason for this is that increasing the temperature lowered the viscosity of TBT, enhanced the collision rate of molecules and increased the effective collisions between the molecules, thereby leading to a higher conversion of TBT to biodiesel [50].

2.8. Selectivity

As shown in Equations (1)–(3), it was possible to form different glycerides along with methyl ester; therefore, the determination of selectivities for different products is important. It was observed that the selectivity to methyl butyrate was between 97% and 99% for the highly active CaO/ZnCo₂O₄-20 catalyst (Figure 8). There was no significant change in the selectivity to methyl butyrate when the reaction temperature was changed from 40 °C to 90 °C. This was possibly due to fact that the CaO/ZnCo₂O₄-20 catalyst possessed relatively large surface area, pore diameter and highly active basic sites for transesterification, which enhanced the diffusion of the reactant and product molecules.

2.9. Reusability

Experiments were carried out to test the reusability of the CaO/ZnCo₂O₄-20 and CaO catalysts for biodiesel production; the results are shown in Figure 9. After each reaction, the CaO/ZnCo₂O₄-20 and CaO catalysts were separated from the reaction mixture using centrifugation, washed with methanol and dried fully for reutilization. It was observed that the conversion of TBT decreased sharply over the bare CaO catalyst and achieved less than 50% conversion in the fifth cycle. The deactivation of the CaO catalyst may have been caused by three possible factors. The first is CaO leaching into the reaction mixture due to the reaction with methanol to form calcium methoxide; the second is CaO surface poisoning due to the adsorption of fatty acid and glycerol molecules; and third is structure collapse [49]. The CaO/ZnCo₂O₄-20 catalyst exhibited better catalytic performance compared with bare CaO, although the conversion of TBT dropped to only 10% after the fifth run probably due to a loss of catalyst mass during the reuse procedure. Therefore, the CaO/ZnCo₂O₄-20 catalyst was found to be economically suitable for industrial application from a practical point of view.

2.10. Kinetic Studies

The optimized reaction conditions were used to perform kinetic studies of the transesterification of tributyrin with methanol over the synthesized catalysts. The reaction requires a stoichiometric amount of 3 moles of methanol and 1 mole of tributyrate to form 3 moles of fatty acid methyl ester and 1 mole of glycerol [51]. The transesterification reaction is as follows:

TBT + 3 MeOH → Gly + 3 Methyl ester

(4)

The kinetic model used in this work is based on the following assumptions:

(1)

The reaction does not depend on the methanol concentration (due to it being an excess reagent), and as such, the reaction could be fitted well with pseudo-first-order rate law [51].

(2)

The production of intermediate species is negligible.

$$\mathrm{Rate} =-\mathrm{r} =\frac{- \mathrm{d} \left[\mathrm{TBT}\right]}{\mathrm{dt}}= \mathrm{k} \left[\mathrm{TBT}\right]{\left[\mathrm{MeOH}\right]}^3$$

(5)

Based on the assumption:

$$-\mathrm{r} =\frac{- \mathrm{d} \left[\mathrm{TBT}\right]}{\mathrm{dt}}={\mathrm{k}}_{\mathrm{TBT}}$$

(6)

After the integration of the Equation (6):

$$\ln \frac{\left[\mathrm{TBT}\right]\mathrm{o}}{\left[\mathrm{TBT}\right]}={\mathrm{k}}_{\mathrm{TBT}} .\mathrm{t}$$

(7)

where [TBT]_o is the initial concentration of tributyrate, [TBT] is the tributyrate concentration after time t and k_TBT is the pseudo first order rate constant.

A plot of ln $\frac{ \left[\mathrm{TBT}\right]\mathrm{o}}{\left[\mathrm{TBT}\right]}$ as a function of time is shown in Figure 10a.

Table 4. Reaction rate constant and activation energy over different catalysts at different reaction temperatures.

Catalyst	Reaction Rate Constant (k)						Ea kJ mol−1
	40 °C	50 °C	60 °C	70 °C	80 °C	90 °C
CaO	34 × 10−3	41 × 10−3	52 × 10−3	64 × 10−3	75 × 10−3	82 × 10−3	17
ZnCo2O4	0.07 × 10−3	0.1 × 10−3	0.2 × 10−3	0.4 × 10−3	0.5 × 10−3	0.7 × 10−3	48
CaO/ZnCo2O4-20	32 × 10−3	41 × 10−3	49 × 10−3	55 × 10−3	70 × 10−3	79 × 10−3	16
CaO/ZnCo2O4-15	2 × 10−3	7 × 10−3	9 × 10−3	11 × 10−3	14 × 10−3	18 × 10−3	22
CaO/ZnCo2O4-10	0.3 × 10−3	0.7 × 10−3	1 × 10−3	1.4 × 10−3	1.7 × 10−3	4 × 10−3	34
CaO/ZnCo2O4-5	0.1 × 10−3	0.3 × 10−3	0.4 × 10−3	0.6 × 10−3	1 × 10−3	1.6 × 10−3	36

shows the reaction rate constants at different reaction temperatures. The data presented in Table 4 demonstrate that the reaction rates constant ‘ka’ for the CaO/ZnCo₂O₄-20 and CaO catalysts were similar and higher than those of the other synthesized catalysts. The activation energy was calculated through the Arrhenius equation:

$$\ln k= \ln \mathrm{A} -\frac{E_a}{RT}$$

(8)

where ‘k’ is the reaction constant, ‘A’ is the frequency or preexponential factor, ‘E_a’ is the activation energy of the reaction, R is the gas constant and T is the absolute temperature [52]. The plot between ln k and 1/T (Figure 10b) was used to determine the activation energy. The experimental data obtained using CaO/ZnCo₂O₄-20 and CaO catalysts clearly demonstrate that the reaction followed pseudo first-order kinetics.

The catalytic transesterification activity and characterization results indicated that CaO nanoparticles are the major active phase for the transesterification reaction; however, their stability was poor during the reaction, and the presence of ZnCo₂O₄ nanoparticles as a catalyst carrier in the CaO/ZnCo₂O₄-20 composite offered superior transesterification catalytic activity with added stability. The HRTEM results clearly indicated the existence of coalescence between the CaO and ZnCo₂O₄-particles. In addition, the CaO/ZnCo₂O₄-20 nanocomposite catalyst had high surface area and pore diameters with a greater number of strong basic sites; thus, the CaO/ZnCo₂O₄-20 composite exhibited the highest catalytic performance.

3. Materials and Methods

3.1. Materials

Zinc nitrate hexahydrate and cobalt nitrate hexahydrate were obtained from Sigma-Aldrich, St. Louis, MO, USA. Calcium chloride, sodium hydroxide were obtained from BDH Limited, Poole, England. Pure urea was obtained from Loba Chemie Pvt. Ltd., Mumbai, India. Ethanol was obtained from Fisher Scientific, Waltham, MA, USA. All chemicals were used as received.

3.2. Synthesis of Nanosized Materials

3.2.1. Synthesis of ZnCo₂O₄ Spinel

The ZnCo₂O₄ spinel was prepared by a co-precipitation followed by hydrothermal method (Scheme 1). In a typical synthesis, one mmol of Zn(NO₃)₂·6H₂O was mixed with two mmol of Co(NO₃)₂·6H₂O, which was dissolved in 50 mL of deionized water under vigorous stirring. Then, 60 mmol of urea was dissolved in 100 mL of deionized water and added to the above solution to maintain the pH at around 11 under constant stirring. Stirring continued for 30 min to obtain a homogeneous precipitate. The obtained precipitate was placed in a Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 180 °C for 6 h. Then, the obtained cake was filtered and washed five times with deionized water and ethanol, before being dried in an oven at 60 °C for 6 h. The dried cake was calcined at 400 °C for 3 h in air at a heating rate of 3 °C/min.

3.2.2. Synthesis of Calcium Oxide

Initially, Ca(OH)₂ was synthesized by dropwise addition of 1 M of NaOH solution to 100 mL of 0.5 M CaCl₂ solution under constant stirring at 80 °C, during which the pH was maintained at 11. At the end of the precipitation process, the resultant precipitate was collected and washed five times with distilled water until a pH of 7 was reached. Then, the obtained cake was dried in an oven at 60 °C for 12 h and then calcinated at 650 °C for 2 h under a N₂ atmosphere at a heating rate of 5 °C/min to obtain CaO powder.

3.2.3. Synthesis of CaO/ZnCo₂O₄ Nanocomposites

The CaO/ZnCo₂O₄ nanocomposites were prepared via a mechanochemical method by varying the CaO weight percent (5–25%). For the synthesis of the sample with different compositions, a suitable amount of ZnCo₂O₄ was mixed with a desirable amount of CaO and the prepared mixed powder was ground for 40 min using an agate mortar. Then, it was calcined at 300 °C for 4 h.

3.3. Material Characterization

The crystallographic structures of all prepared solid samples were characterized by X-ray diffraction using a Bruker diffractometer (Brucker D8 Advance, Karlsruhe, Germany) equipped with Cu Kα (λ = 1.5405 Å) and a monochromator at 40 kV and 40 mA, in the 2θ range of 5°–80°. The Scherrer equation was used, i.e., 𝑑 (nm) = $\frac{0.9\times \lambda }{\mathrm{B} \times \mathrm{Cos}\left(\mathsf{\theta }\right)}$, where 𝑑 is the average crystallite size of the samples under investigation, λ is the wavelength of the radiation employed, 0.89 is the Scherrer constant, B is the full width at half maximum (FWHM) of diffraction peak and θ is the Bragg diffraction angle. The HRTEM images of the samples were obtained using a FEI Tecnai G2 F30 TEM microscope (Portland, OR, USA) in Bright Field imaging mode. The FT-IR spectra of the materials were obtained using a PerkinElmer FT-IR spectrometer (Waltham, MA, USA). The BET surface area, total pore volume and pore radius of different catalysts were determined from the results of N₂ physisorption performed at −196 °C using a Quantachrome Autosorb-iQ instrument (Boynton Beach, FL, USA) after degassing the samples at 200 °C for 5 h. The surface elemental compositions of the samples were analyzed by X-ray photoelectron spectroscopy utilizing a Thermo-Fisher scientific (Waltham, MA, USA) with monochromatic X-rays of Al Kα radiation at a size of 400 μm and a pressure of 10^–9 mbar. A full-pass energy spectrum of 200 eV and in a narrow 50 eV band was applied. The binding energy of the transverse carbon line (C 1s) was used for calibration, and the positions of the peaks were corrected relative to the C 1s signal position. The CO₂-TPD patterns of the catalysts were obtained using a Chembet-3000 (Quantachrome, Boynton Beach, FL, USA) instrument. The sample was heated at 120 °C under helium for 1 h to remove the physically adsorbed water. Then, it was cooled to 30 °C and exposed to CO₂ gas for 1 h. Next, the sample was purged by flowing helium gas for 30 min, and then the temperature was increased from 25 °C to 900 °C at a rate of 10 °C/min. The NMR spectra were recorded at 298 K on a Bruker Avance III 400 (9.4 T, 400.13 MHz for ¹H, 100.62 MHz for ¹³C) spectrometer (Bruker, Billerica, MA, USA) with a 5mm BBFO probe. Chemical shifts (δ in ppm) were relative to internal standard DMSO-d6 (δ 2.50) for ¹H NMR.

3.4. Transesterification of Tributyrin with Methanol

Transesterification was carried out in a 100 mL, three-necked round-bottomed flask with a reflux condenser and heated in a precisely controlled oil bath under N₂. In a typical run, 1 mmol of tributyrin (TBT) was added to 12 mmol of methanol. Then, a calculated amount of catalyst (5.0 wt.% relative to tributyrate weight) was added to the reaction mixture. The moment of adding the catalyst was regarded as the initial reaction time. The liquid samples were withdrawn periodically to determine the product distribution using a Shimadzu GC17A gas chromatograph integrated with a DB-1 capillary column and flame ionization detector. The following equations were used to determine the % conversion of tributyrin and % selectivity to the methyl butyrate [53]

$$\% \mathrm{Conversion} \mathrm{of} \mathrm{TBT} =\frac{{\left[\mathrm{TBT}\right]}_{\mathrm{t}=0}-{\left[\mathrm{TBT}\right]}_{\mathrm{t}}}{{\left[\mathrm{TBT}\right]}_{\mathrm{t}=0}}\times 100$$

(9)

$$\% \mathrm{Selectivity} \mathrm{of} \mathrm{product} =\frac{{\left[\mathrm{Methyl} \mathrm{butyrate}\right]}_{\mathrm{t}}}{3\times {\left[\mathrm{TBT}\right]}_{\mathrm{t}=0}}\times 100$$

(10)

4. Conclusions

CaO/ZnCo₂O₄ nanocomposite catalysts were successfully prepared via a simple mechanochemical method. They exhibited better catalytic performance in the production of biodiesel from tributyrate. A methyl ester yield of 98% was achieved over the CaO/ZnCo₂O₄-20 composite catalyst under the optimum reaction conditions. In addition, the nanocomposite catalyst could be reused up to five times with good activity. The excellent performance and stability of the CaO/ZnCo₂O₄-20 composite catalyst for the transesterification reaction were possibly due to the existence of coalescence between the CaO and ZnCo₂O₄-20 particles. Kinetic studies revealed that the reaction following the pseudo first-order kinetics and the number of highly basic sites in the composite catalysts were key factors affecting the catalytic performance in the transesterification reaction. These results indicate that ZnCo₂O₄ spinel can be utilized as a support for the active CaO nanoparticles to develop efficient and stable heterogeneous catalysts for biodiesel production.