Biodiesel Synthesis from Calophyllum Inophyllum Utilizing Modified Zeolite and Bentonite Catalysts

At present, the reserves of fossil fuels like crude oil, coal, and natural gas are dwindling due to escalating usage and their non-renewable nature [1]. Consequently, there is a pressing need for the development of alternative and renewable fuel sources [2]. Indonesia, as a nation, boasts a diverse range of natural resources, encompassing both renewable and non-renewable ones.

The demand for crude oil as the world's primary fuel source is substantial. However, this demand is not accompanied by an awareness that the combustion of processed crude oil products such as gasoline, diesel, and others has negative impacts on the environment and tends to harm the Earth in the long term [3]. The adverse effects of excessive fossil fuel usage include climate change due to global warming, often referred to as global warming [4-6].

Nowadays, the ongoing and swift consumption of crude oil as a fuel is leading to its diminishing reserves. Therefore, there is a demand for alternative energy sources and fuels that can evolve and hold promise for the future [4]. Biodiesel is a renewable alternative fuel for diesel engines derived from renewable sources such as vegetable oil, animal fats, and algae [7]. It exhibits combustion properties remarkably similar to diesel fuel, hence holds potential for substituting petroleum diesel [8-11].

Calophyllum inophyllum, a thick brown-colored vegetable oil with a strong caramel-like scent [12], finds various uses among farmers in Kebumen and Cilacap, Central Java, Indonesia. It serves as a coating for roof tiles, aids in batik making, and is utilized in embalming [13]. In West Java, some fishermen rely on Calophyllum inophyllum as a fuel for their boats during maritime activities [14]. This oil is rich in unsaturated fatty acids, notably oleic acid, and contains unsaponifiable components like fatty alcohols, sterols, xanthones, coumarin derivatives, phytols, isophytols, isophtalates, capillary acids, pseudobrasilic acid (cyclohexadienone derivatives), and triterpenoid compounds, ranging from 0.5% to 2%, which possess medicinal properties [13].

Advantages of using Calophyllum inophyllum as a source of biofuel include its high oil content (up to 74%), its non-competition with food production and its availability is abundant in tropical countries like Indonesia [15]. Calophyllum inophyllum offers various benefits for biodiesel production and development: it naturally grows abundantly across Indonesia, easily regenerates, and bears fruit throughout the year, indicating its resilience to environmental conditions. It can be cultivated with ease either in single-crop fields or mixed forests and thrives in dry climates [16]. Nearly all parts of the Calophyllum inophyllum plant are valuable, yielding various economically significant products. Using Calophyllum inophyllum biofuel can also help reduce deforestation for firewood, given its high seed productivity compared to other plants [13].

Biodiesel derived from Calophyllum inophyllum also offers advantages, including its comparatively high oil yield (40-73%) when compared to other plants like jatropha and palm oil [17]. Some quality parameters already meet Indonesian standards, and Calophyllum inophyllum burns twice as long as kerosene. Additionally, it holds future potential as a blending component. Compared to fossil fuels, biodiesel has several benefits, such as renewability, higher energy efficiency, emissions reduction, and the possibility of using it as a complete diesel substitute with advanced processing technology [18]. It also offers better emission quality than diesel and can serve as a bio-kerosene alternative [19].

Recent developments in biodiesel production from Calophyllum inophyllum oil have advanced in several areas, including improved processing technology, the use of more efficient and eco-friendly catalysts, and better purification methods. Future progress is expected to focus on further reducing production costs through innovations and optimizations in the production process, as well as the introduction of new, more cost-effective and efficient catalysts. The importance of producing biodiesel from Calophyllum inophyllum oil lies in diversifying energy sources, fostering local economic growth, benefiting the environment, and providing a sustainable renewable energy option. Energy diversification is achieved by utilizing Calophyllum inophyllum oil as a feedstock for biodiesel, offering an alternative renewable energy source, decreasing reliance on fossil fuels, and strengthening energy security [15].

Local economic development is facilitated by the creation of economic opportunities, such as jobs for farmers and communities in tropical regions, through the planting and cultivation of nyamplung trees, which in turn boosts community income. Environmentally, biodiesel produced from Calophyllum inophyllum oil offers the advantage of reducing greenhouse gas emissions when compared to fossil fuels. Moreover, cultivating nyamplung trees can enhance soil quality and aid in carbon absorption. Nyamplung is a renewable and sustainable energy source, as it is a non-food plant that is easy to cultivate, grows rapidly, and requires minimal operational costs [17, 18].

The rationale for this study lies in the limited utilization of Calophyllum inophyllum oil, despite its considerable potential as a replacement for fossil fuels. Previous investigations commonly utilized acids and bases as catalysts, whereas there's limited research on employing modified zeolite and bentonite in Calophyllum inophyllum oil production. Despite their potential for higher yields compared to acids and bases, these modified catalysts are rarely explored in prior studies. An additional advantage of modified heterogeneous catalysts is the ease of separating the catalyst after the reaction, enabling its reuse and preventing any waste generation from the catalyst. This differs from the biodiesel production process using acid-base catalysts, which produces waste acids and bases during the separation of biodiesel products from the leftover catalyst. Hence, our research identifies a gap in the extant research in utilizing modified zeolite and bentonite as promising catalysts for achieving greater yields. In addition to these factors, the use of modified zeolite and bentonite as catalysts for producing biodiesel from Calophyllum inophyllum oil offers various innovative aspects that can enhance the efficiency and sustainability of the process. Here are some of the innovative aspects of this approach [15, 17-19]:

1. Use of Nature-Based Materials
   
   • Zeolite: Zeolite is a natural mineral with an excellent porous structure, allowing it to function as a catalyst with high absorption and selectivity capabilities. By modifying zeolite, we can enhance its catalytic activity in the transesterification of Calophyllum inophyllum oil.
   • Bentonite: Bentonite is a clay rich in montmorillonite, which can be modified to improve its catalytic properties. This modification often involves changes to the surface structure or the addition of active materials.
2. Catalyst Modification
   
   • Metal Addition: Modifying zeolite and bentonite with transition metals or other active metals can enhance their catalytic capabilities for the transesterification reaction. These metals can act as active centers for the chemical reaction.
   • Enhancing Acid/Base Properties: Chemical or physical modifications to zeolite and bentonite can improve the acid or base properties of the catalyst, which are crucial for speeding up the transesterification process.
3. Energy Efficiency
   
   • Low-Temperature Reaction Process: By using modified catalysts, the transesterification process of Calophyllum inophyllum oil can be carried out at lower temperatures compared to conventional catalysts, reducing energy consumption and operational costs.
   • Reusable Catalysts: Modified zeolite and bentonite generally exhibit good stability and can be reused across multiple reaction cycles without significant loss of catalytic activity.
4. Use of Renewable Resources
   
   • Calophyllum inophyllum oil: Calophyllum inophyllum oil is a renewable raw material, and utilizing modified zeolite and bentonite-based catalysts supports an environmentally friendly biodiesel production process.
5. Improvement in Biodiesel Quality
   
   • Purification and Product Quality: Modified catalysts can enhance the conversion of oil into biodiesel, resulting in higher quality biodiesel, including improved purity and physical characteristics that meet standards.
6. Waste Reduction
   
   • Minimized By-Products: The use of efficient catalysts can reduce the amount of by-products and waste, making the process cleaner and more environmentally friendly.
7. Availability and Cost
   
   • Affordable Catalysts: Zeolite and bentonite are relatively inexpensive materials that are abundantly available in Indonesia, so modifying them for use as catalysts in biodiesel production can reduce raw material and operational costs.
8. Adaptive Catalyst Design
   
   • Customizable Catalysts: Modifying zeolite and bentonite allows for adjustments to meet specific needs in the transesterification process, such as tailoring pore size or surface properties to enhance reactivity with Calophyllum inophyllum oil.

Overall, the innovation in using modified zeolite and bentonite as catalysts for producing biodiesel from Calophyllum inophyllum oil offers great potential to enhance process efficiency, reduce costs, and support sustainability in biodiesel production [17, 18]. The modification of zeolite and bentonite for catalytic applications in biodiesel production involves several unique and innovative aspects that can provide significant benefits for the advancement of biodiesel production technology. Here is a description of the uniqueness of these modifications, their application mechanisms, and innovations.

Uniqueness of Zeolite and Bentonite Modifications

1. Porous Structure and Catalytic Properties
   
   • Zeolite: Zeolite features a crystalline structure with regular pores and specific pore sizes, allowing it to act as a catalyst with high absorption capacity and good reaction selectivity. Modifying zeolite can enhance its acid or base properties, increase pore size, or add active metals to accelerate transesterification reactions.
   • Bentonite: Bentonite is a clay with a laminar structure providing a large surface area for chemical interactions. Modifications typically involve changes in chemical composition or the addition of active materials to improve catalytic capacity [12-14].
2. Chemical and Physical Modifications
   
   • Zeolite: Chemical modifications, such as ion exchange and the addition of transition or other active metals, can enhance catalytic properties. Zeolites modified with active metals (e.g., Cu, Ni) can improve catalytic activity in transesterification reactions.
   • Bentonite: Modifications may include altering the structure or surface properties by adding activating agents or using techniques like filtering or precipitation to change acid/base properties and enhance catalytic activity [13, 14, 17].

Mechanisms of Catalyst Modification with Hydrochloric Acid

Specific Activation of Zeolite and Bentonite Catalysts Using Hydrochloric Acid (HCl) The modification of zeolite and bentonite catalysts with hydrochloric acid aims to improve the catalytic properties of these materials through several key mechanisms:

1. Removal of Impurity Cations
   
   • Mechanism: HCl interacts with alkali or alkaline earth metal cations trapped in the structure of zeolite or bentonite. These cations include sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), and calcium (Ca²⁺).
   • Reaction: HCl ionizes and releases H⁺ ions, which replace the cations in the material's structure, resulting in the release of these cations as dissolved chlorides (e.g., NaCl, KCl).
   • Outcome: This process enhances the acidity of active sites in zeolite or bentonite and improves their catalytic activity.
2. Dealumination
   
   • Mechanism: Dealumination is the removal of aluminum (Al) from the crystalline framework of zeolite or bentonite. Under certain conditions, H⁺ ions from HCl can attack and extract Al³⁺ from the framework, which is then carried away as AlCl₃ or related compounds.
   • Reaction: The process can be described by the reaction: Al-O-Si + 3H⁺ → H-O-Si + Al³⁺, where the released Al³⁺ forms complexes with Cl⁻ ions from HCl.
   • Outcome: Removal of aluminum increases the number of Bronsted acid sites on the material's surface, enhancing acid properties and catalytic ability, particularly for reactions requiring high acidity.
3. Increased Surface Area
   
   • Mechanism: HCl can remove amorphous material or unstructured components covering the pore surfaces of zeolite or bentonite. This process opens up more access to active sites within the material’s pores.
   • Reaction: HCl dissolves metal oxides or other unstructured components that might obstruct the pores.
   • Outcome: The specific surface area of the material increases, enhancing its ability to act as a catalyst.
4. Modification of Crystal Structure
   
   • Mechanism: Under certain conditions, acid treatment can cause changes in the crystal structure of zeolite or bentonite, such as in pore size or distribution.
   • Reaction: This can occur through delamination or recrystallization due to the high acidity of HCl.
   • Outcome: These changes can produce materials with an optimal structure for specific catalytic applications, such as cracking or isomerization.
5. Removal of Organic Impurities
   
   • Mechanism: HCl can also help remove organic impurities present in zeolite or bentonite, which might affect catalytic efficiency.
   • Reaction: HCl reacts with organic compounds, especially if they are basic, forming salts that dissolve in the acid solution.
   • Outcome: Removal of these impurities improves catalyst activity by preventing blockage of active sites.

Based on the above description, modifying zeolite and bentonite with hydrochloric acid significantly enhances acidity, surface area, and catalytic activity, making them more effective for various catalytic applications [12, 15, 17-19]. Variations in HCl concentration during modification significantly affect the physical and chemical properties of the resulting catalysts. Factors influenced by HCl concentration include acidity, structural stability, surface area, adsorption capacity, and overall catalytic properties. Low concentrations of HCl moderately increase acidity without damaging the structure, while higher concentrations significantly enhance acidity and surface area but risk damaging the material’s structure. Therefore, optimizing HCl concentration is necessary to balance increased acidity with structural stability for specific catalytic applications. The optimal HCl concentration for modifying zeolite is typically between 0.1 M and 2 M, while for bentonite, it ranges from 0.5 M to 3 M. Adjustments should consider material type, modification goals, and treatment conditions (time and temperature) to balance catalytic activity enhancement with maintaining material structural stability. In this study, zeolite and bentonite were modified using a 0.5 M HCl solution, which represents the optimum concentration for modifying both zeolite and bentonite.

Modification Application Mechanisms

1. Modification Processes
   
   • Zeolite:
     • Ion Exchange: This process replaces sodium ions in zeolite with active metal ions such as K⁺, Ca²⁺, or other transition metals to enhance catalytic activity.
     • Metal Impregnation: Involves adding active metals to zeolite, which can improve catalytic capability for specific reactions.
   • Bentonite:
     • Addition of Catalytic Agents: Adding active materials such as metals or acids/bases to the surface of bentonite to improve catalytic activity.
     • Activation or Treatment: Methods like heating or using chemicals to alter the surface properties of bentonite, enhancing its catalytic ability [16-18].

2. Application in Biodiesel Production
   
   • Transesterification: Modified zeolite and bentonite are used in the transesterification process to convert Calophyllum inophyllum oil into biodiesel. These catalysts help accelerate the reaction between triglycerides and methanol or ethanol, producing methyl or ethyl esters (biodiesel) and glycerol.
   • Batch and Continuous Processes: Modified catalysts can be applied in both batch and continuous processes. They can be adapted for different reactor types, such as flow reactors or batch reactors [17-19].

Innovations for Technological Advancement

1. Energy Efficiency and Cost
   
   • Low-Temperature Processes: Modified catalysts enable reactions to occur at lower temperatures, reducing energy consumption and operational costs.
   • Recovery and Reusability: Modified catalysts generally have good durability and can be reused across multiple reaction cycles, reducing the need for new materials and lowering production costs [17-19].
2. Improved Biodiesel Quality
   
   • Purification: Modified catalysts enhance the conversion of oil to biodiesel, resulting in high-quality biodiesel that meets stringent quality standards [13, 14].
3. Waste Reduction and Environmental Impact
   
   • Minimized By-Products: Efficient catalysts reduce the amount of by-products and waste, making the process cleaner and more environmentally friendly.
4. Adaptation for Various Feedstocks
   
   • Flexibility: Modifications to zeolite and bentonite allow adaptation to various types of feedstocks, not just Calophyllum inophyllum oil but also other vegetable oils, expanding the technology's application and flexibility [14, 15].
5. Integrated Catalyst Design
   
   • Multi-Functional Catalysts: Development of catalysts with multifunctional capabilities, such as those that can perform multiple reactions simultaneously (e.g., transesterification and free fatty acid removal), improves process efficiency and reduces additional processing steps [18, 19].

Overall, modifying zeolite and bentonite for catalytic applications in biodiesel production offers significant advantages in efficiency, cost, and environmental impact. This innovative approach has the potential to drive technological progress in the biodiesel industry and enhance the sustainability of renewable energy production.

Life Cycle Assessment (LCA) Results for Zeolite and Bentonite Catalysts Modified with Acid

The Life Cycle Assessment (LCA) for zeolite and bentonite catalysts modified with acid involves a thorough evaluation of environmental impact, energy consumption, and emissions throughout the entire life cycle of these catalysts. Below are the analysis and strategies to optimize the process to reduce environmental impact:

1. Energy Consumption
   
   • Production Stage
     • Extraction and Processing of Raw Materials: The energy required to mine and process zeolite and bentonite includes activities such as drilling, grinding, and drying. These activities require significant energy consumption.
     • Acid Modification: The acid modification process, including heating and chemical use, requires additional energy. Chemical reactions and separation processes also contribute to energy consumption.
   • Usage Stage
     • Catalyst Activation: If catalysts need to be activated before use, for example through heating, this also adds to energy consumption. However, this stage is usually lower compared to production.
     • Process Efficiency: Efficient catalysts allow biodiesel transesterification to occur at lower temperatures, reducing energy requirements in biodiesel production.
   • Optimization Strategies
     • Energy Efficiency: Apply energy-efficient technologies in the modification and activation processes, such as using high-efficiency heaters or heat recovery systems.
     • Renewable Energy Use: Consider using renewable energy sources, such as solar or geothermal energy, in the production and modification processes [15-17].
2. Environmental Impact
   
   • Production Stage
     • Chemical Use: Acid modification involves chemicals that can potentially be hazardous. Proper handling and effective waste management methods are needed to minimize environmental impact.
     • Environmental Degradation: Mining and processing raw materials can cause environmental damage such as habitat disruption and soil or water pollution.
   • Usage Stage
     • Catalyst Efficiency: More efficient catalysts reduce the amount of raw materials needed and decrease by-product production, which positively impacts the environment.
     • Recycling and Reuse: Catalysts that can be recycled or reused reduce waste and the environmental impact of biodiesel production.
   • Optimization Strategies
     • Green Chemistry: Use less hazardous acids and chemicals in the modification process. Develop effective waste management solutions.
     • Sustainable Mining Practices: Implement environmentally friendly mining practices to reduce the environmental impact of raw material extraction [13-15, 17].
3. Emissions
   
   • Production Stage
     • Air Emissions: The acid modification process can produce emissions such as volatile organic compounds (VOCs) and hazardous gases. Effective emission control and ventilation systems are required.
     • Water Pollution: The use of acids and other chemicals can generate liquid waste that needs treatment to prevent pollution.
   • Usage Stage
     • Emissions During Use: Efficient catalysts produce fewer emissions during the biodiesel production process compared to conventional catalysts.
   • Optimization Strategies
     • Emission Control: Implement effective emission control technologies to capture and treat gases and pollutants from the production process.
     • Waste Treatment: Apply efficient waste treatment systems to manage liquid and solid waste from the modification process [13, 15-18].
4. Overall Process Optimization
   
   • Environmentally Conscious Design
     • Sustainable Catalyst Design: Design catalysts with a focus on efficiency, durability, and recyclability. Choose materials and processes that minimize environmental impact.
     • Recyclability: Ensure that catalysts can be recycled or reused efficiently to reduce waste.
   • Ongoing Evaluation and Improvement
     • Periodic Assessment: Conduct Life Cycle Assessments (LCA) regularly to identify areas for improvement in the catalyst life cycle.
     • Technological Innovation: Invest in research and development to create cleaner and more efficient modification and production processes.
   • Stakeholder Engagement
     • Collaboration: Work with stakeholders such as suppliers, customers, and regulators to promote environmentally friendly practices and sustainability in catalyst production [17-20].

By implementing these strategies, the environmental impact of producing and using zeolite and bentonite catalysts modified with acids can be minimized, contributing to a more sustainable and environmentally friendly biodiesel production process.

This study involves several phases: preparation of catalysts, extraction of Calophyllum inophyllum seeds using n-hexane solvent to obtain Calophyllum inophyllum and subsequent analysis of its characteristics, esterification stage to reduce the free fatty acid content in Calophyllum inophyllum, and transesterification stage to produce biodiesel. The yield of the resulting biodiesel is then calculated. The biodiesel with the highest product yield is further analyzed for its characteristics according to the American Society for Testing and Materials (ASTM) procedures and follows the requirements of the Indonesia National Standard (SNI) for biodiesel (SNI 04-7182-2006), including viscosity, density, cetane number, flash point, and methyl ester content. The types of modified catalysts (i.e., zeolite, bentonite, and a mixture of zeolite and bentonite) and the transesterification reaction temperature were chosen as factors in this study.

Below are the research steps:

1. Materials

The primary materials used in this study include Calophyllum inophyllum fruit obtained from the Ampel area of Surabaya and moisture of crushed seeds was removed under natural sunlight, natural zeolite and bentonite obtained from Pacitan, East Java, Indonesia. The reagents used include n-hexane, hydrochloric acid (purity 33%), and methanol Merck (purity 99.9%), sourced from PT. Kurniajaya, Surabaya.

2. Preparation of modified catalysts

The modified heterogeneous catalysts used were zeolite and bentonite. Prior to use, natural zeolite and bentonite was crushed using a JUNKE and KUNKEL micro hummer mill to obtain zeolite and bentonite with particle size of 170/240 mesh. Subsequently, the zeolite and bentonite was treated with 30% of hydrogen peroxide solution to remove the organic impurities. Excess of hydrogen peroxide solution was removed by gently heating in a water bath and the solution was separated from the zeolite and bentonite. The purified zeolite and bentonite was repeatedly washed and dispersed in distilled water. After the water have been separated from zeolite and bentonite, the purified zeolite and bentonite was then dried in an oven at 110℃ for 24 hours to remove moisture content from its structure. The dried zeolite and bentonite was finally crushed in a micro hummer mill to obtained powder zeolite with particle size of 170/240 mesh. Modification of zeolite and bentonite were prepared with HCl solution. A brief procedure of catalyst preparation is as follows: 100 g of natural zeolite powder was weighed and placed into a three-neck flask. Then, 400 mL of 0.5 M HCl solution was added until all the zeolite powder was submerged. The mixture was then heated to 100℃ and stirred for 24 hours. Subsequently, zeolite and HCl solution was separated by filtration using a vacuum filter system. The zeolite catalyst was then oven-dried at 110℃ for 24 hours and the resulting zeolite solid was calcined at 550℃ for 4 hours. The same procedure was carried out for bentonite.

3. Calophyllum inophyllum extraction

The extraction process of dried Callophyllum inophyllum seeds was conducted to obtain Calophyllum inophyllum oil using n-hexane solvent with a ratio of 1:4 (weight:volume) by the Soxhlet method. The extracted oil was separated from the solvent mixture by distillation at 110℃ for an hour to obtain Calophyllum inophyllum oil [31].

4. Analysis of free fatty acid content in Calophyllum inophyllum

A total of 10 g of Calophyllum inophyllum was weighed and mixed with 25 mL of neutral 96% alcohol. The mixture was heated in a water bath until boiling, then cooled to room temperature and 2 drops of phenolphthalein indicator were added. This solution was then titrated using 0.1 N NaOH solution until it turned pink, and then the content of free fatty acids in the Calophyllum inophyllum was calculated. This testing was also conducted on the oil resulting from the esterification process [31].

5. Esterification

The esterification process aims to decrease the free fatty acid content by reacting Calophyllum inophyllum with methanol at a molar ratio of 1:20 using a potent acid catalyst, specifically concentrated HCl. Calophyllum inophyllum is combined with 370 mL of methanol in a three-necked flask, and concentrated HCl, acting as the catalyst, is added at a quantity equivalent to 10% of the oil volume. The mixture undergoes a 2-hour reaction at 60℃. Upon completion of the reaction, the resulting methyl esters are separated from the mixture using a separating funnel, and the product is then analyzed to determine its free fatty acid level [37].

6. Transesterification

Transesterification of Calophyllum inophyllum oil into biodiesel was carried out in a 500 mL three-neck flask equipped with reflux condenser, temperature indicator, and controlled water bath heater. The transesterification reaction is carried out by adding modified catalysts, namely zeolite and bentonite. The molar ratio used between Calophyllum inophyllum oil and methanol is 1:6, with the catalyst amounting to 10% of the oil's weight. In this study, 100 mL of Calophyllum inophyllum oil resulting from esterification is placed in a three-necked flask and mixed with 24.3 mL of methanol and 10% of the weight of modified zeolite or bentonite catalyst. The mixture is then stirred using a magnetic stirrer, and the reaction takes place for 5 hours at various temperatures, namely 50, 60, 65, and 70℃. After the reaction is completed, the mixture is filtered using a Buchner funnel under vacuum condition to separate the catalyst from the biodiesel and glycerol mixture. The biodiesel and glycerol are then separated using a separating funnel. The resulting biodiesel is subsequently dried in a vacuum rotary evaporator to remove its water content and the excess methanol. Biodiesel product was then repeatedly washed using warm distilled water (60℃). The fatty acid methyl esther or FAME layer (upper layer) was separated from water layer (bottom layer) using a separating funnel. The yield and characteristics of the resulting biodiesel are then analysed and calculated.

7. Characterization of biodiesel and catalyst

The biodiesel product obtained is subjected to characterization testing under conditions of maximum biodiesel yield, following SNI and ASTM parameters, including density, viscosity, flash point, cetane number, and methyl ester content. Density testing is performed using a pycnometer according to ASTM D1298, viscosity testing with a Brookfield Viscometer spindle no.21 at 100 rpm according to ASTM D445-10, 2010, flash point with a Pensky Marten Closed Tester according to ASTM D93, cetane number with a Cetane Number Tester GD-R3035 following ASTM D613 and D6887, while the fatty acid methyl ester content is analyzed using Gas Chromatography (GC Shimadzu 2014). The GC was equipped with a DB-wax Capillary column (30 m × 0.25 mm i.d. × 0.1 m film thickness, Agilent JW Scientific) and flame ionization detector (FID). Helium was employed as carrier gas at 40 cm/s. The injector temperature was 250℃ at splitless condition. The FID was set at 300℃. The initial oven temperature was set at 50℃ with equilibration time of 3 min. After isothermal period, oven temperature was increased to 250℃ at heating rate of 10℃/min and held for 8 min. Peaks of methyl esters were identified by comparing them with the reference standard. The yield of biodiesel was determined by the following equation:

$\begin{gathered}\text { Yield }(\%)= \frac{(\text { weight of biodiesel } \times \% \text { FAME in sample }) \times 100 \%}{(\text { weight of Calophyllum inophyllum oil) })}\end{gathered}$         (1)

These FAME tests are conducted at the Pertamina laboratory in Jagir, Surabaya. The limitations of this research are the characterization of modified zeolite and bentonite catalysts includes X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) tests conducted at the Energy Laboratory, Sepuluh Nopember Institute of Technology, Surabaya. Characterization analysis using X-ray Photoelectron Spectroscopy (XPS) was not employed. XRD testing utilizes a Philips X-pert Powder Analytical diffractometer with an X-ray diffractometer operating at 30 kV and 15 mA, with 2θ measurements ranging from 5° to 90°. While FTIR employs an Invenio-Bruker Corp. spectrometer.

The study indicates that Calophyllum inophyllum oil can effectively be transformed into biodiesel through esterification and transesterification processes using modified zeolite and bentonite catalysts. The biodiesel yield achieved with these modified catalysts is 88.19%, matching that of biodiesel produced with acid and base catalysts, which is 88.27%. The advantage of using these modified catalysts is their easy separation and ability to be reused in the biodiesel production process, making them eco-friendly. In contrast, traditional acid-base catalysts are difficult to separate and cannot be reused, resulting in industrial waste that requires management.

The use of modified catalysts offers potential for advancing biodiesel science and industry by reducing waste and lowering separation costs in the future. The resulting biodiesel meets the specifications outlined by the Indonesian National Standard (SNI), confirming its viability as a renewable fuel option.

Future research on the biodiesel production process from Calophyllum inophyllum oil using modified zeolite and bentonite catalysts holds significant potential for innovation and improvement. Innovation focus can be directed towards catalyst optimization, broader applications, and developing solutions for technical and economic challenges, which will play a key role in making the process more efficient and sustainable.

Catalyst optimization can be achieved through the development of hybrid catalysts by combining zeolite and bentonite with other catalytic materials (such as metals or metal oxides) to enhance catalytic activity and selectivity. Modifying the structure and composition of zeolite and bentonite to improve accessibility and reactivity with Calophyllum inophyllum oil, and using nanotechnology to create nanoparticles from zeolite and bentonite to increase active surface area and catalyst efficiency are also potential areas for optimization.

Improvements in the transesterification process can be achieved through integrated process technologies that combine transesterification reactions with biodiesel separation and purification, as well as optimizing operating conditions to enhance conversion and reaction efficiency.

For sustainability and material availability, alternative materials that can replace or complement zeolite and bentonite can be used, and recycling catalysts and managing waste from the modification process can help reduce environmental impact.

Broader application improvements can be pursued by diversifying feedstocks, adapting modified catalysts for various types of biodiesel feedstocks, or using a mix of Calophyllum inophyllum oil with other non-food oil feedstocks. Developing zeolite and bentonite catalyst modifications for industrial scale, integrating with new sustainable process technologies, and automating and controlling processes to enhance consistency and biodiesel production efficiency are also important.

To address technical challenges, solutions can focus on high stability of modified catalysts and long service life for industrial scale use. For economic challenges, efforts can be directed towards reducing the production costs of catalyst modifications. With the right approach, it is possible to significantly enhance biodiesel production efficiency and reduce environmental impact.

Recommendations for further research include exploring biodiesel production from other non-edible oils and testing catalysts derived from other environmentally friendly natural materials to improve biodiesel yield, as well as conducting characterization of modified zeolite and bentonite catalysts using X-ray Photoelectron Spectroscopy (XPS). This advanced characterization technique is highly useful for understanding chemical surface changes in modified catalysts and can help in understanding how modifications affect oxidation states, composition, and surface reactivity. This is crucial for designing more efficient and durable catalysts. Through XPS analysis, researchers can monitor and optimize the catalyst modification process for better applications in industrial chemical reactions.

Another useful recommendation for future research is the design of a mini-reactor for biodiesel production from Calophyllum inophyllum oil. This involves designing a portable mini-reactor with a small but adequate capacity to meet local needs. It should include essential components such as a reaction tank, mixer, heater, filtration system, and separation tank. Safety and ease of use should be prioritized in the design, while mobility and durability are ensured through the use of high-quality materials and a compact design. With this design, it is expected that small-scale biodiesel production can be carried out efficiently and safely at various locations.