Catalytic Performance of CO:Ni Mixed Oxide Based Catalyst for Green Fuel Synthesis from Biomass Derived Citrullus Colocynthis Oil

Piyush Shrivastava ^†, Jharna Gupta ^*, Rushil Bhatt ^†, Aditi S. Vetal ^†

1. Department of Chemical Engineering, The Maharaja Sayajirao University of Baroda, Pratapgunj, Vadodara, Gujarat 390002, India

† These authors contributed equally to this work.

* Correspondence: Jharna Gupta

Academic Editor: Lucília Graciosa de Sousa Ribeiro

Special Issue: Advanced Catalysis for Green Fuel Production from Biomass and Wastes

Received: September 18, 2024 | Accepted: December 18, 2024 | Published: January 08, 2025

Catalysis Research 2025, Volume 5, Issue 1, doi:10.21926/cr.2501002

Recommended citation: Shrivastava P, Gupta J, Bhatt R, Vetal AS. Catalytic Performance of CO:Ni Mixed Oxide Based Catalyst for Green Fuel Synthesis from Biomass Derived Citrullus Colocynthis Oil. Catalysis Research 2025; 5(1): 002; doi:10.21926/cr.2501002.

© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

As the demand for alternative fuels has increased in recent years, many researchers have attempted to find sustainable solutions, from commercial fuel to eco-friendly and renewable sources. A Better alternative to fossil fuels is biofuel, which can replace diesel with biodiesel as an eco-friendly fuel source [1]. Biodiesel has attracted significant attention owing to its significant advantages such as nontoxicibility, high lubricity with renewable sources, sulphur-free nature, high flash point, oxygenated nature, and best biodegradable fuel alternative, which results in the reduction of greenhouse gases and demands finite petroleum resources [2,3].

Both edible and non-edible seed oils have been identified as potential feedstock for biodiesel synthesis. While edible seed oils such as soybean, canola, palm, and sunflower are readily available, abundant, and contain high levels of oil [4,5], their use as biodiesel feedstock raises concerns about food security and affordability. This is because diverting these oils from production can increase prices and limit supplies. Non-edible oils are increasingly being explored as alternative feedstocks for biodiesel production because they offer several advantages, including competition for food resources and speculative deployment of marginal lands [6]. Some non-edible oils commonly used for biodiesel include Thumba oil, Jatropha Oil, Tung Oil, Jojoba Oil, and Pongamia Oil. Recently, researchers have focused on increasing the efficiency and scalability of biodiesel synthesis from non-edible oil feedstocks as a renewable energy source [7,8]. The utilization of non-edible oil in domestic use is nearly negligible because of its toxic components, which do not suit human health even when grown in forest regions and barren lands without much cultivation process, and is even its cost-effective than edible oil [9]. Citrullus colocynthis is increasingly being recognized for its potential as a biodiesel feedstock. Globally, Citrullus colocynthis oil production is relatively limited and often region-specific, with notable cultivation in countries such as India, parts of the Middle East, and North Africa. Its economic potential has gained attention for both industrial and pharmaceutical applications owing to the high quality of its fatty acids. Citrullus colocynthis plants are drought-resistant, with each fruit containing up to 1,000 seeds, making it an efficient source for oil extraction. Citrullus colocynthis seeds contain 22-53% oil, depending on the variety and growth conditions [10]. The oil yield per hectare can exceed 3,000 kg, making it comparable to or even higher than that of many traditional oilseed crops, such as soybean and rapeseed. The primary application of Citrullus colocynthis oil is biodiesel production. It is an eco-friendly alternative to fossil fuels, emitting fewer hydrocarbons and contributing to lower greenhouse gas emissions. This makes it an attractive option for sustainable energy initiatives [11,12].

Homogeneous catalysts (NaOH, KOH, CH₃ONa, CH₃OK and H₂SO₄) are commonly used industrial catalysts because of their fast reaction rates under moderate conditions [13]. Homogeneous acidic or basic catalysts produce higher catalytic activity in biodiesel yield under mild operating conditions [14]. However, some disadvantages, such as additional separation, purification steps, and soap formation in the reaction, require attention for synthesizing heterogeneous catalysts for biodiesel synthesis [15,16]. Biodiesel production often uses heterogeneous catalysts. These solid catalysts offer two key advantages: effortless recovery from the reaction system and the ability to be reused multiple times. Researchers have explored a wide range of solid catalysts for this purpose, including metal oxides, combinations of different oxides, metal oxides treated with sulfates, carbon materials with sulfonic acid groups, ion exchange resins, heteropolyacids, and specific materials such as calcium oxide, nickel oxide, magnesium oxide, zeolites and biowaste catalysts (Psidium guajava L. leaves catalyst and Parkia speciosa catalyst) [17,18,19,20,21]. Heterogeneous catalysts offer several advantages in biodiesel production, including convenient separation from reaction components, reduced soap formation, and potential for catalyst recycling. However, homogeneous catalysts may require higher reaction temperatures and longer reaction times [22].

Transesterification was studied by most of the research as the best biodiesel production method, with glycerol as a secondary product from its feedstock [23]. Transesterification yields high-purity biodiesel with lower viscosity, making it more compatible with diesel engines than pyrolysis, microemulsion, or blending. It operates under milder conditions than pyrolysis, reducing energy costs and avoiding the stability issues observed in microemulsions. Unlike blending oils, which are chemically altered to produce biodiesel with consistent performance and lower emissions, Transesterification also produces glycerol as a helpful byproduct, which adds economic value [24]. This process involves the reaction of the oil with an alcohol in the presence of catalysts, which may be either homogeneous or heterogeneous.

A literature survey found that cobalt oxide has good basicity characteristics, high catalytic activity in the reaction mixture, and a surface area of 121.1 m²/g. However, it is not widely used for biodiesel production through transesterification reactions [25]. Conversely, nickel oxide has gained significant interest because of its extensive surface area, cost-effectiveness, and potential porous structure [26]. Hence, mixed-metal cobalt and nickel oxide catalysts have emerged as effective heterogeneous catalysts for biodiesel production from Citrullus colocynthis oil. These catalysts offer enhanced alkalinity and specific surface area, increasing activity and stability during the transesterification reaction. Various mixed metal oxides, such as CaZnO, CaO-MgO, TiO₂-MgO, NaZrO₃ and CaMgO have been employed as catalysts in transesterification reactions [27,28,29]. However, no one has yet attempted to produce biodiesel using a catalyst based on varying Co and Ni ratios, specifically the Nickel cobaltite (NiCo₂O₄) catalyst. The advancement of heterogeneous catalysts based on cobalt and nickel shows promising results, as cobalt oxide demonstrates significant catalytic activity in the transesterification process and the potential for combined effects with nickel oxide to enhance durability and activity. The combination of nickel and cobalt within the spinel structure creates a synergistic effect, improving catalytic activity and stability. Nickel enhances hydrogenation activity, while cobalt contributes to oxidation-reduction properties, making the catalyst more effective for transesterification.

Therefore, this analytical research intends to synthesize the heterogeneous catalyst with different Co: Ni ratios and good catalytic activity for biodiesel synthesis from Citrullus colocynthis oil. The reaction parameters and reusability tests were also analyzed for biodiesel synthesis. Studies Fourier-Transform Infrared spectroscopy (FTIR) and gas chromatography (GC) were performed and interpreted to characterize the biodiesel. To characterize the biodiesel, studies using Fourier-Transform Infrared spectroscopy (FTIR) by FT-IR spectrometer (Perkin Elmer spectra two) in the range of 400-4500 cm^-1 and gas chromatography (GC) by gas chromatography (Model: Master GC Dani Instruments, Italy) with a capillary column of length 30 m, Inner diameter 0.32 mm and film thickness 0.25 µm were used for biodiesel analysis.

2. Materials and Methods

2.1 Materials and Chemicals

Citrullus colocynthis oil was purchased from a nearby market and research was performed using analytical-grade anhydrous methanol, cobalt oxide (Co₃O₄), and nickel oxide (NiO) from Merck Limited, Mumbai, India.

2.2 Characterization of Oil

Citrullus colocynthis oil was evaluated for quality parameters, as shown in Table 1, following the ASTM D6751 standard test method. These parameters included the density, acid value, saponification number, viscosity, and molecular weight. Kinematic Viscosity was measured using a Saybolt viscometer (temperatures range from 21°C to 99°C).

Table 1 Properties of Citrullus colocynthis oil used in the present study.

2.3 Catalyst Synthesis

A co-precipitation method was employed to prepare the catalysts, as shown in Figure 1. The co-precipitation method prepared three different Co:Ni ratios and supported on zirconium dioxide (ZrO₂). The precipitate was aged at 60°C for 1 h. Subsequently, it was filtered using Whatman filter paper, rinsed with distilled water, and placed in a hot-air drying unit operating at 100°C. The dried material was calcined at 500°C for 3 h and placed in a desiccator for further study, as shown in Figure 1 [30]. Catalysts with various Co₃O₄ and NiO ratios were prepared and designated as the Co:Ni ratios as shown in Table 2.

Figure 1 (a) Catalyst Preparation (b) Catalyst Filtration (c) Prepared Catalyst.

Table 2 Synthesized catalyst composition with varying Co:Ni ratios.

2.4 Characterisation of Catalyst

This investigation deployed various characterization techniques, including Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD), to gain insights into the catalysts' structure and behavior. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analysis were carried out using the FESEM FEI system (Nova Nano 450) and an X-Pert Pro powder analytical instrument, respectively.

2.5 Biodiesel Production by Transesterification Method

Using an H₂SO₄ acid catalyst, the esterification procedure was initially performed to lower the FFA content at 60-70°C. A two-necked flat-bottom flask with a magnetic stirrer, temperature sensors, and a reflux condenser was used in the experimental setup [31]. Before initiating the reaction, 100 milliliters of Citrullus colocynthis oil was heated to atmospheric pressure at 60°C. The methanol and oil reaction mixture was combined in a 6:1 molar ratio with a set amount (1 ml) of acid catalyst. A two-hour batch period was chosen to complete the esterification process. The esterification reaction mixture was moved to a colloquial step funnel for phase separation after the completion of the reaction. After phase separation, the upper phase containing the esterified ester was used for transesterification.

Finally, the transesterification reaction used different CO:Ni ratios as solid catalysts. The esterified oil mixture (100 ml) was first heated and poured into a similar flask that was already filled with a heterogeneous mixture of methanol to oil molar ratio (3:1-12:1) and catalyst (1.5-5.5 g) under vigorous stirring using a magnet bar. The transesterification reaction occurred between 0.5 and 3.5 hours, with a constant temperature of 65°C. Once the response was concluded, the product was transferred to a funnel to enable phase partitioning. The solid heterogeneous catalyst was then separated from the reaction mixture by applying centrifugal force for 20 minutes at 5,000 rpm. A colloquial step funnel was then used to separate the glycerol. FAME was then used for characterization. The experimental procedure is shown in Figure 2. Biodiesel yield was determined and calculated using Equation (1).

\[ Biodiesel\,Yield\,\mathrm{(\%)}\,=\,\frac{Vol.\,of\,\textit{citrullus Biodiesel}\,\mathrm{(ml)} }{Vol.\,of\,\textit{citrullus coloynthis oil}\,\mathrm{(ml)} } \tag{1} \]

Figure 2 Biodiesel production employing the transesterification method. 1. Methanol and solid catalyst mixture. 2. Three-necked flask. 3. Magnetic stirrer with a hot plate. 4. Temperature-sensor. 5. Coil condenser. 6. Esterified Citrullus colocynthis oil. 7. Transfer the product mixture into a separating funnel. 8. Separation of glycerol with catalyst from biodiesel. 9. Collection of biodiesel. 10. Centrifuge process.

Repeated transesterification reactions evaluated the catalyst's recyclability. After each reaction, the solid catalyst was separated via centrifugation, dried, and calcined before reuse in the next cycle. The following optimized reaction conditions were maintained throughout: 3.5 wt% catalyst, 65°C reaction temperature, 2.5 h of time, and a 9:1 methanol-to-oil molar ratio.

2.6 Biodiesel Characterisation

Gas chromatography with a 30 m long and 0.25 mm thick capillary column was used to assess the purity of the biodiesel. Chromatographic variables were set as follows: detection: 280°C, injector: 240°C, column: 200°-232°C. First, two milliliters of hexane were used as the solvent to dissolve a 100 mg mixture of pure methyl esters (C8-C24). This solution was fed into the GC capillary column in 0.5-1 ml increments, and the elution times of each component were recorded.

3. Result and Discussion

3.1 Catalyst Characterisation

3.1.1 XRD Analysis

All XRD results show the presence of little traces of cobalt oxide (Co₃O₄) phase after the reaction of Co₃O₄ and NiO in different ratios and very low indication of Cobalt oxide phase left in the sample at 2θ = 32.35°, 36.69°, 44.76°, 50.1°, 59.42° and 67.55° as compared with the JCPDS card number 00-073-1701. A new phase of nickel cobaltite (NiCO₂O₄) was also found in comparison with JCPDS card number 47-1049. However, Co₃O₄ and NiCO₂O₄ phases have the same spinel structure but different lattice parameters (Co₃O₄ (8.083 Å and NiCO₂O₄ 8.111 Å) [31]. Therefore, it isn't easy to differentiate these two phases. Still, deep analysis of the lattice parameter confirms the presence of NiCO₂O₄ phases in all the XRD graphs, mainly at 2θ = 31.23 and 38.30 as shown in Figure 3 [31]. Comparing all three graphs for different CO:Ni ratios, we found that the CO:Ni (33:67) ratio shows the major phase of NiCO₂O₄, as shown in Figure 3(b).s The other ratio i.e. CO:Ni (50:50) and CO:Ni (67:33) confirmed the presence of two phases i.e. NiCO₂O₄ and NiO as shown in Figure 3(a) and 3(c), respectively.

Figure 3 XRD analysis.

3.1.2. SEM Analysis

The surface topography of the sample with varying Co:Ni ratio are shown in Figure 4. The SEM images of all the prepared catalysts show irregular crystal structures with varied particle sizes. These different particle sizes are bound together by coalescence with different grain sizes and are distributed on the catalyst surface [31]. According to the literature, a smaller grain size of the catalyst results in a higher catalyst surface area. A higher surface area is responsible for catalytic activity. The observed surface morphology suggests a less porous structure because the porosity of the catalyst is directly influenced by its heat treatment. A smaller number of pores shows the higher treatment temperatures of the catalyst because the porous wall fractures at higher temperatures [32]. NiO peaks in each CO:Ni ratio catalyst occurred because of the thermal decomposition of NiCO₂O₄ during the calcination process above 400°C.

Figure 4 SEM image of three different catalyst based on variation of Co:Ni. (a) Co:Ni (50:50). (b) Co:Ni (67:33). (c) Co:Ni (33:67). (d) Reused catalyst after 5 runs.

3.2 Optimisation Study of Reaction Parameters

Figure 5(a) illustrates the effect of the methanol-to-oil molar ratio on biodiesel production. The biodiesel yield increased from 82% to 90%. It decreased from 90% to 85% when the methanol to oil ratio (mol/mol) was used from 6:1 to 12:1. The formation of a methoxy functional group on the catalyst surface steered the reaction towards the forward pathway. The biodiesel yield decreases when the methanol to oil ratio (mol/mol) increases to 12:1. Oil becomes more diluted owing to a higher amount of methanol, which results in a reversible reaction. Glycerol recovery becomes difficult and limits the biodiesel yield. Thus, the optimized value of methanol to oil ratio (mol/mol) was chosen to be 9:1 in this study [33].

Figure 5 Optimization study of reaction parameters.

As seen in Figure 5(b), the transesterification processes were conducted using varying quantities of catalyst, ranging from 1.5 to 4.5 g. The graph shows that when the catalyst amount increases from 1.5 to 4.5 g, biodiesel production also increases, and the biodiesel yield drops. Due to a high amount of catalyst, methanol and oil were not mixed properly, and separating the ester became difficult [34].

One of the most crucial elements in transesterification reaction is response time. Under ideal circumstances, the impact of the response time on biodiesel was investigated, as shown in Figure 5(c). The illustration shows that the biodiesel yield increased with increasing time and subsequently somewhat reduced after 2.5 h. This is because the saponification reaction reversibly produces soap, which hinders ester separation and lowers biodiesel production [35,36,37,38]. Thus, the ideal reaction time for the catalysts was 2.5 h. Several studies have documented biodiesel production using different heterogeneous catalysts, as outlined in Table 3. The reaction conditions obtained in this research were compared with similar research studies [39,40,41,42,43,44,45,46,47,48] from the literature (Table 3). Typically, the catalyst concentration is a little high for the transesterification reaction, possibly due to the neutralization of the acid used in the esterification step. Comparing biodiesel yield across different reaction conditions, it becomes evident that using a sufficient amount of catalyst (3.5 wt%) and methanol to oil molar ratio of 9:1 over 2.5 h resulted in the highest biodiesel yield and improved biodiesel properties.

Table 3 Investigation into the performance of other solid heterogeneous catalysts for biodiesel synthesis.

3.3 Reusebility Study

In biodiesel industries, the reusability test of the catalyst plays a vital role from an economic perspective. As depicted in Figure 6, Several cycles of the transesterification process were used to examine the Co:Ni (33:67) catalyst's reusability. The transesterification reaction was repeated in the flask with the same volume of esterified oil and methanol added. As seen in Figure 6, the catalytic activity in terms of biodiesel yield decreases from 90% to 53%. No more studies were carried out as the tiny particles of the catalyst were broken up by vigorous stirring, and the little particles were released with the glycerol supernatant.

Figure 6 Reusability study.

The SEM image of the catalyst after being reused for 5 cycles, as shown in Figure 4(d), reveals the development of clusters and a rough and irregular surface, which reduces the catalyst activity.

3.4 Biodiesel Characterisation

The properties of Citrullus colocynthis oil biodiesel were analyzed and compared with the ASTMD6751 Standard test method, as illustrated in Table 4.

Table 4 Analyzed properties of Citrullus colocynthis oil biodiesel.

3.4.1 GC Characterisation

Gas chromatography analysis shows the fatty acid methyl ester (FAME) distribution in Citrullus colocynthis oil-based biodiesel. This analysis represents specific ester constituents, including oleic acid methyl ester, stearic acid methyl ester, palmitic acid methyl ester, and linoleic acid methyl ester. Gas chromatography (GC) was subsequently employed to authenticate the purity of the synthesized biodiesel by its FAME profile with a standard reference [49]. Table 5 shows the different fatty acid compounds and their retention time in biodiesel products.

Table 5 Fatty acid compounds and their retention time in biodiesel product.

3.4.2 FTIR Analysis

The FTIR approach is advantageous over GC because it can evaluate entire samples, including the precipitated fraction, without additional processing. Figure 7 shows biodiesel's FTIR spectra obtained using Co:Ni (33:67) catalysts. According to published research, triglyceride and methyl ester have comparable chemical structures, which account for their similar spectra. The primary range in which the chemical structures of biodiesel and oil can differ is 1500-900 cm^-1. The peak at 1436 cm^-1 shows the presence of asymmetric stretching of -CH₃ in the biodiesel sample. Based on studies, the peaks in the 2800-3000 cm-1 regions and 1675-1725 cm^-1, respectively, represent the stretching vibrations of CH₃(-CO-O-CH₃) and the carbonyl group (-C=O). Additionally, the lack of a wide adsorption band in the 3300-2500 cm^-1 range suggests that the biodiesel is dry, confirming the sample’s dry phase (no moisture present) [49].

Figure 7 FTIR analysis of Biodiesel.

4. Conclusion

This study investigated the impact of varying Co: Ni ratios in nickel-based catalysts on biodiesel synthesis. Transesterification reaction tests and catalyst characterization techniques such as XRD and SEM were used to explain these effects. The transesterification reaction test indicates that the Co:Ni (33:67) catalyst exhibits the best catalytic activity for biodiesel synthesis among all the cobalt-nickel mixed oxide ratios. Compared to other catalysts primarily responsible for biodiesel synthesis and have proven to be excellent catalysts, the XRD results confirm that the NiO phase and NiCo₂O₄ spinel structure are significantly deformed. Nickel cobaltite (NiCo₂O₄) catalysts exhibit excellent properties for biodiesel production due to high surface area and porosity, synergistic Effect of Nickel and Cobalt, high thermal stability, enhanced electron transfer for faster and more efficient conversion of biodiesel, resistance to catalyst deactivation such as leaching and sintering. Characterized by an extensive surface area and superior catalytic behavior, the Co:Ni (33:67) spinel mixed metal oxide catalyst attained the maximum biodiesel yield (exceeding 90%) from non-food-grade Citrullus colocynthis oil, surpassing the performance of all other synthesized catalysts under specific reaction conditions, including a 2.5 h reaction time, a 9:1 methanol to oil molar ratio, a reaction temperature of 65°C, and a catalyst loading of 3.5% by weight. After five transesterification runs, the Co:Ni (33:67) catalyst's reusability test revealed that the biodiesel yields were higher than 50%.

Acknowledgments

The Maharaja Sayajirao University of Baroda, India provided support and funding for this work.

Author Contributions

Mr. Piyush Shrivastava, Mr. Rushil Bhatt and Miss Aditi S. Vetal have collected the data and written the manuscript. Dr. Jharna Gupta has given valuable suggestions for finalizing the manuscript.

Funding

The Maharaja Sayajirao University of Baroda, India provided support and funding for this work.

Competing Interests

The authors declare that they have no conflict of interest regarding this publication.