Abstract

Non-renewable fossil fuels like Rubber Seed Oil-Based Biodiesel are currently employed to meet global energy demands. However, concerns surrounding their future scarcity, volatile crude oil prices, and environmental impact have spurred widespread research into renewable energy alternatives. Biodiesel, which shares comparable properties to fossil diesel while offering biodegradability, non-toxicity, and environmental friendliness when used in diesel engines, has emerged as a promising supplement and potential substitute for traditional diesel fuel. Among various non-edible oil-bearing plants, rubber seeds have been recognized as a strong contender for biodiesel production due to their high oil content ranging from 35% to 45% by weight. This study focuses on recent advancements in the fuel qualities of rubber seed oil (RSO) biodiesel and evaluates its performance in diesel engines. The investigation also encompasses techniques for rubber seed oil extraction, biodiesel production, factors influencing the transesterification process of RSO, and the future prospects and challenges associated with rubber seed oil-based biodiesel. Numerous studies have demonstrated that rubber seed oil biodiesel, specifically in the form of Fatty Acid Methyl Ester (FAME), exhibits comparable viscosity, flash point, calorific value, and cetane number to conventional petroleum diesel. As a result, it can be directly utilized in internal combustion engines (ICEs) without the need for extensive modifications. Performance studies involving engines fueled by rubber seed oil biodiesel and its blends with petrodiesel indicate that all tested RSO biodiesel variants exhibit engine performance characteristics similar to those observed when using conventional petrodiesel.

Ikhazuagbe H. Ifijen¹*

*Corresponding authors: larylans4u@yahoo.com, ifijen.hilary@rrin.gov.ng

Keywords: Rubber seed oil, biodiesel, diesel engine, efficiency, environmental performance, sustainable fuel

Introduction

The utilization of plant oils in various industrial and food products has been a long-standing practice [1-3]. One of the applications of plant oils is in the production of biodiesel, which involves different types of oils. Biodiesel, according to ASTM, is a liquid fuel derived from vegetable oil and animal fat, specifically the fatty acid alkyl ester of long-chain fatty acids [4]. Common feedstocks for biodiesel production include oil seeds (vegetable oil), animal fats, algae, and low-quality materials like used cooking oil and greases [5]. Initially, biodiesel production relied heavily on edible oils such as soybean, palm, and canola. However, the excessive use of these edible oils led to a food-versus-fuel crisis [6]. The high cost of biodiesel remains a significant obstacle to its widespread commercialization, as the feedstock price influences around 80% or more of the overall biodiesel cost [5]. To address this, utilizing raw materials with high free fatty acid content or non-edible oils has been proposed as a practical approach to reduce biodiesel costs [7-8]. Non-edible oils, like rubber seed oil, have emerged as a promising feedstock due to their lower price compared to edible oils, which are in high demand [9] [10-13].

Various non-edible oil sources such as jatropha, Moringa oleifera, Pongamia pinnata, and camelina sativa have been employed for biodiesel production [14-15]. Rubber seed oil, with its oil content of 35-45% by weight, proves to be a strong contender among non-edible oil-bearing plants for biodiesel production. Its renewable nature, environmental friendliness, and ease of local production contribute to the viability of rubber seed oil as a substitute for diesel fuel [4, 16]. Moreover, rubber seed oils have low sulfur content, pose no storage issues, and exhibit excellent lubricating properties [4, 17]. Additionally, vegetable oil-producing trees absorb more carbon dioxide during photosynthesis than they release when burned, which aids in reducing atmospheric carbon dioxide levels [18]. Replacing diesel with renewable fuels derived from vegetable oils can lead to significant foreign exchange savings for many countries, including major oil exporters [19-21]. Such projects can help developing nations address their environmental concerns while boosting their economies. With their numerous advantages, vegetable oils have the potential to eventually replace petroleum-based fuels.

Rubber seed oil has been proven to be an effective biodiesel resource that meets the criteria set by ASTM and EN in terms of its physicochemical qualities and suitability for biodiesel synthesis [6, 15]. It has been found that diesel engines perform well with biodiesel derived from rubber seed oil and can even be partially fueled by it [22]. The use of biodiesel in diesel engines results in increased efficiency, reduced pollution emissions, less wear and tear on engine components, and neutral effects on lubricating oil. An intriguing aspect is that the use of biodiesel does not require any modifications to diesel engines.

This study addresses the limited availability of reviews specifically focusing on rubber seed oil (RSO) biodiesel in the existing literature. Only two review papers have been published on rubber seed oil-based biodiesel, one in 2016 by Onoji et al. [23] and another in 2018 by Ulfah et al. [24]. Compared to these previous studies, the present write-up offers a more comprehensive and up-to-date review of RSO biodiesel.

In addition to examining the updated research papers on RSO biodiesel, this review specifically highlights recent studies on the performance of rubber seed oil biodiesel in diesel engines. This aspect sets it apart from previous reviews and provides a unique contribution to the field.

The novelty of this review lies in its extensive coverage of various aspects related to rubber seed oil biodiesel. It surpasses a mere analysis of the fuel qualities of RSO biodiesel reported over the years by delving into its performance in diesel engines. Moreover, the review encompasses other crucial factors such as extraction techniques for rubber seed oil, biodiesel production techniques, factors influencing the transesterification of rubber seed oil, and the challenges associated with transesterifying non-edible oils. By addressing these diverse aspects, the study offers a holistic understanding of the potential and challenges associated with rubber seed oil-based biodiesel.

Furthermore, the review explores future prospects for utilizing rubber seed oil as a feedstock for biodiesel production. It sheds light on ongoing research efforts and identifies areas that require further investigation to optimize the production process and enhance the overall performance and sustainability of rubber seed oil biodiesel. By incorporating these novel aspects, the review provides a comprehensive and up-to-date overview of rubber seed oil-based biodiesel, making it an invaluable resource for researchers, policymakers, and industry stakeholders involved in the development and implementation of alternative fuels and renewable energy.

Rubber Seed Oils (Rubber Seed Oil-Based Biodiesel)

Rubber seeds have an average weight ranging from 3 to 5 grams, with the kernel constituting approximately 40% of the weight, the shell accounting for around 35%, and moisture making up the remaining 25% [25]. The kernel of a rubber seed contains oil, which has an oil content ranging between 35 and 38 percent, while the seed cake recovery is approximately 57 to 62 percent [25]. Rubber seeds yield two primary products: rubber oil and rubber seed cake. In the rubber belt of southern Nigeria, the rubber seeds typically mature and separate from the seed pod during the short dry season occurring between August and September [26]. The seeds can be processed to produce two different products: oil and cake [26]. The oil consists of 17 to 22% saturated fatty acids and 17 to 82% unsaturated fatty acids. It is semi-drying and has a yellowish color. Currently, the oil does not have any edible applications. However, it can be used in certain non-edible applications as a partial substitute for imported linseed oil due to its similar characteristics [26]. Among the three methods for extracting rubber seed oil—solvent extraction, expeller extraction, and rotary extraction—the rotary extraction method is the most commonly employed [27]. The quality of the kernel, the degree of drying, and the quantity of molasses used during processing all affect the amount of oil and cake that can be obtained. The soap manufacturing industry utilizes rubber seed oil as one of its raw materials [27].

Rubber seed oil extraction methods

To extract rubber seed oil, the seeds undergo a series of preparation steps. Initially, the seeds are dried, crushed, and milled to reduce their size for conditioning. The milled seeds, also known as meal, are then subjected to scorching in a gas-heated rotary drier at temperatures of 60-70°C for 10-20 minutes [26]. There are two primary methods used for extracting rubber seed oil: chemical extraction and mechanical extraction. Chemical extraction involves the use of solvents to dissolve the oil from the seeds, while mechanical extraction involves pressing or grinding the seeds to extract the oil. Both methods have their own advantages and limitations.

Rubber seed oil has qualities that make it a potential partial substitute for linseed oil [26]. It has been reported that the oil contains a notable amount of free fatty acids (FFAs), with approximately 80% of the fatty acids being unsaturated (such as oleic, linoleic, and linolenic acids) and around 20% being saturated (such as palmitic and stearic acids) [28].

Overall, the available extraction techniques for rubber seed oil (RSO) include chemical extraction using solvents and mechanical extraction methods. These methods enable the recovery of RSO for various applications.

Conventional method (mechanical and chemical)

The extraction of oil from rubber seeds commonly involves two traditional methods: chemical extraction and mechanical extraction. Each method has its own characteristics and considerations. The mechanical extraction process, although associated with a high initial cost and limited yield, is known for producing high-quality oil [29-30]. Traditional mechanical screw press oil expellers have been deemed relatively inefficient, leaving approximately 8-14% of the available oil in the deoiled cake [31]. To improve oil recovery, Singh and Bargale (2000) recommended the use of double-stage compression expellers [32]. Pre-treating the seeds and employing a heated screw press head have been shown to enhance oil recovery, particularly when applying higher pressure [33].

Chemical extraction, on the other hand, involves the use of organic solvents like n-hexane to extract the oil from the seeds [17-18]. However, co-extraction of undesired oil constituents can result in lower oil quality. The yield of the extraction process can be influenced by several factors, including the boiling point of the solvent, solvent-to-seed ratio, milled seed size, moisture content, drying, and extraction duration [34]. For instance, Gimbun et al. (2012) reported the extraction of rubber seed oil from 100 grams of powdered seeds using 250 ml of n-hexane under specific conditions of 4 hours at 60°C [35]. The hexane is subsequently evaporated from the oil/hexane mixture using vacuum and a temperature of 60°C. Both mechanical and chemical extraction methods have their advantages and drawbacks, and the choice of method depends on factors such as desired oil quality, extraction efficiency, and cost considerations.

Gas-assisted mechanical expression method

A novel approach called Gas Assisted Mechanical Expression (GAME) involves dissolving supercritical CO₂ fluid into powdered rubber seeds and compressing them with a screw [36]. This method allows for lower-pressure application while achieving higher oil yields without compromising quality [36]. In the GAME process, CO₂ is dissolved in the oil present in the seeds before pressing. After equilibration, the oil/CO₂ mixture is extracted from the seeds. Studies conducted on cocoa have shown that during pressing, the dissolved CO₂ displaces some of the oil [37]. Comparative results have demonstrated that the liquid content of conventional press cakes and GAME press cakes is the same at the same effective mechanical pressure (absolute mechanical pressure minus the actual CO₂ pressure) [37]. However, the oil content of the GAME press cake is reduced by the same amount due to the saturation of the liquid with CO₂, which can reach up to 30 wt% CO₂. The magnitude of this effect increases with greater CO₂ solubility in the oil [38]. Additionally, the presence of dissolved CO₂ significantly reduces the viscosity of the oil, leading to an accelerated pressing process. After pressing, the depressurization step facilitates the removal of CO₂ from the cake and oil, and additional oil is extracted from the cake through entrainment in the gas flow [38].

The GAME method offers several advantages [38].:

• Higher oil yields can be achieved with reduced pressure requirements compared to conventional mechanical pressing (50 MPa vs. 100 MPa).
• Lower CO₂ pressure is required compared to supercritical extraction methods (10 MPa vs. 45-70 MPa).
• Significantly less CO₂ is needed compared to supercritical extraction (approximately 1 kg of CO₂ per kg of oil vs. 100 kg of CO₂ per kg of oil).
• The resulting products are nearly free of solvent, eliminating potential negative health effects on consumers.
• CO₂, under the conditions used, has been found to have a sterilizing effect.

The GAME approach presents a promising alternative for efficient rubber seed oil extraction, offering improved yields and reduced solvent usage compared to traditional methods.

Ultrasonic-assisted method (Rubber Seed Oil-Based Biodiesel)

Ultrasonic extraction, also known as ultrasonic-assisted extraction (UAE), is a highly effective method for releasing oils from seeds, kernels, and fruits. This technique utilizes ultrasound waves to induce rapid movement of the solvent, resulting in accelerated extraction processes and increased mass transfer rates [39]. The application of ultrasound creates small bubbles that undergo rapid formation and collapse, generating intense energy, pressure, and mechanical shear forces. These forces contribute to the breakdown of cell tissues in the seeds, facilitating the extraction of oil into the solvent [40].

One of the key advantages of ultrasonic extraction is its ability to achieve quick extraction times while minimizing solvent usage. In a conventional ultrasonic-assisted procedure, a known quantity of milled seed powder is placed in an Erlenmeyer flask along with an extraction solvent. The flask is then submerged in a sonication bath, where the ultrasonic waves are applied to initiate the extraction process. Once the extraction is complete, the extract is collected from the vessel and filtered using filter paper under vacuum conditions. To further refine the extract, drying chemicals such as anhydrous sodium can be employed to remove any remaining moisture from the seed filtrate [39].

Ultrasonic extraction offers several advantages over traditional extraction methods. It provides faster extraction times, allowing for more efficient processing and increased productivity. The intense cavitation and mechanical forces induced by ultrasound enhance the release of oils, resulting in higher extraction yields. Furthermore, this method requires minimal solvent usage, reducing both the environmental impact and solvent costs. The ability to perform extractions at ambient or lower temperatures also helps to preserve the integrity and quality of the extracted oils, as heat-sensitive compounds are less likely to degrade. Overall, ultrasonic extraction is a versatile and efficient technique for obtaining oils from various plant sources, making it a valuable tool in the field of oil extraction.

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Microwave-assisted method (Rubber Seed Oil-Based Biodiesel)

Microwave-assisted extraction (MAE) utilizes electromagnetic energy in the form of microwaves, which have frequencies ranging from 300 MHz to 300 GHz. These waves penetrate biomaterials and interact with molecules, leading to the generation of heat [41]. The selective interaction of microwaves with free water molecules in the material results in localized heating, causing a rapid temperature increase that can lead to the boiling of water. The pressure generated from the rapid expansion of boiling water can eventually rupture the walls of the cells [42]. Lipids, which have a low specific heat, are particularly susceptible to the effects of microwaves, making them more susceptible to damage [43]. This can result in the formation of permanent pores in the seeds, leading to increased oil output [44].

One of the main advantages of microwave-assisted extraction is its efficiency in heating. MAE has been shown to significantly shorten extraction times compared to conventional methods, while also reducing the environmental impact by generating less CO₂ and requiring less energy [45-46]. This technique greatly enhances the effectiveness of the extraction process [46]. Additionally, the brief exposure to microwaves helps protect the extracted chemicals from degradation processes [47]. The use of continuous microwave systems has demonstrated higher oil yields, improved oil quality, and enhanced extraction rates, making it a viable option for a wide range of oil-bearing seeds [48]. Therefore, microwave-assisted extraction is considered a feasible method for obtaining oil from the seeds of Pongamia pinnata. Pongamia pinnata, a tree species widely distributed in southern India, produces non-edible oil. Due to the increasing demand for fuels and the strain on edible oils, Pongamia pinnata has gained importance as a sustainable feedstock for biodiesel production. It offers several advantages, including the ability to grow in non-fertile and waste lands, making it an attractive option for biodiesel production [49].

In a nut-shell, microwave-assisted extraction (MAE) is an efficient and environmentally friendly method that can be employed to extract oil from various oil-bearing seeds. Its use in the extraction of oil from non-edible sources like Pongamia pinnata holds promise for sustainable biodiesel production, addressing the demand for alternative fuels while minimizing the strain on edible oil resources.

Aqueous Extraction Processing (Rubber Seed Oil-Based Biodiesel)

Aqueous extractions (AEP) offer several advantages over solvent extractions, primarily because water can be used as a solvent instead of more harmful organic solvents like hexane [13]. In solvent extractions, the oil in the seed substrate dissolves in the organic solvent, which is then evaporated to recover the oil. AEP allows for the separation of various fractions from the oil, including solid residue, protein-rich skim, lipid-rich cream, and free oil [50]. However, demulsification of the cream is often necessary to release the free oil.

While the extraction yield is not always the best predictor of recovered free oil, AEP has been shown to achieve competitive recovery yields of up to 96% in several research studies [49, 51]. To maximize oil recovery, pre-treatments are commonly employed before aqueous extractions. These pre-treatments aim to break down or soften the seed matrix. For example, roasting seeds can enhance oil release by breaking down the cell walls of the substrate, resulting in a higher oil extraction yield [52]. In one study, a 35% (w/w) oil extraction yield was achieved for wild almonds using the ideal roasting temperature and duration [51].

Flaking and extrusion can also be employed as pre-treatments to enhance cell breakdown, allowing for improved water penetration and the release of chemicals. Extruded full-fat soybean flakes (68%) demonstrated higher oil extraction yields compared to untreated soybean flakes (60%) during aqueous extraction [53]. Certain pre-treatments have been shown to specifically enhance free oil recovery. For instance, pretreating flaxseed kernels with 0.3 M citric acid and drying them at 70 °C for 1 hour before aqueous extraction resulted in a thinner cream layer and increased free oil yields from 19% to 83%. The acid treatment modified protein characteristics, leading to the coalescence of oil bodies and a decrease in the size of protein bodies, which contributed to the significant improvement in free oil recovery [54]. It’s worth noting that aqueous oil extraction can be carried out using various tools. For example, processing sunflower seeds in a blender instead of a twin-screw extruder resulted in a 35% higher oil yield [55].

Overall, aqueous extractions provide a more environmentally friendly alternative to organic solvent extractions, and with appropriate pre-treatments and optimization, they can achieve competitive oil recovery yields.

Aqueous enzymatic method (Rubber Seed Oil-Based Biodiesel)

Enzymes are highly effective as extractive media due to their specificity and efficiency. They can break down the cell walls of seed kernels, facilitating the release of oil. This enzymatic approach is considered environmentally friendly, and cost-effective, and allows for the conversion of the de-oiled cake into livestock feed without the use of solvents [34].

Enzymes offer several benefits when added to aqueous oil extraction. The cell walls of the cotyledon, composed of cellulose, hemicellulose, lignin, and pectin, present a challenge in releasing oil [34]. To overcome this, specific enzymes such as carbohydrates (e.g., cellulase, hemicellulase, and pectinase) can be used to break down the cell wall and enhance oil release. Protease enzymes can hydrolyze proteins in the cell membrane, thereby improving the efficiency of seed extraction [56-57]. Enzyme treatment is advantageous as it does not leave behind solvent residues, operates at low temperatures, and has minimal environmental impact [34, 56-57]. Life cycle analyses (LCA) conducted on enzymatic processes in the food, feed, and pharmaceutical industries have indicated reduced contributions to global warming, acidification, eutrophication, ozone generation, and energy consumption compared to other methods [58] (Jegannathan & Nielsen, 2009). Although enzymes can be costly, higher extraction yields or enzyme recycling can help offset the expenses [56, 59].

Optimizing extraction parameters is crucial for achieving optimal yields with enzymatic processes. Various factors, such as the type of enzyme (proteases vs. carbohydrases), pH of the enzymes, and their interaction with seed proteins, need to be considered. It is essential to set the pH of the enzymatic process far from the isoelectric pH (pI) of the seed proteins. Proteins tend to be insoluble at their pI, which can hinder oil extraction [34, 60-62]. Temperature is another critical factor when using enzymes, with the optimal range for enzymatic hydrolysis typically falling between 45 to 55 °C. Excessive temperatures can result in a loss of enzymatic activity, while low temperatures can slow down the rate of oil extraction and enzymatic reactions [34, 60].

In summary, enzymes offer specificity, efficiency, and environmental benefits in aqueous oil extraction. By breaking down cell walls and enhancing oil release, enzymes provide an effective and sustainable approach to extracting oil from seeds. Optimization of parameters such as enzyme type, pH, and temperature is necessary to achieve optimal extraction yields.

Physicochemical properties of rubber seed oil

The physicochemical properties of rubber seed oil (RSO) have been extensively studied and reported by various researchers [16]. These properties provide valuable insights into the potential industrial applications of RSO, particularly as a feedstock for biodiesel production and other uses.

The fatty acid composition of RSO is one of the key aspects investigated in the literature. Stearic acid (C18:0), palmitic acid (C16:0), oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3) are the most commonly reported fatty acids in RSO [63-64]. These fatty acids are often present in varying proportions, and there may also be smaller concentrations of other fatty acids. It is worth noting that RSO is typically rich in unsaturated fatty acids, particularly polyunsaturated acids [64]. The specific values reported by researchers may vary slightly due to differences in analysis methodologies, rubber tree origins, and extraction processes. However, it is not uncommon for various non-edible vegetable oils described in the literature to exhibit similar fatty acid compositions [65].

Understanding the physicochemical properties of RSO is crucial for assessing its suitability for different applications. These properties include parameters such as viscosity, density, iodine value, saponification value, acid value, and oxidative stability. Viscosity determines the flow characteristics of RSO and affects its usability in various processes. Density provides information about the oil’s weight and volume relationship. The iodine value indicates the degree of unsaturation, which affects the oil’s reactivity and stability. The saponification value reflects the average molecular weight of fatty acids and is used to assess the oil’s potential for soap production. The acid value measures the free fatty acid content, indicating the oil’s freshness and potential for hydrolytic rancidity. Oxidative stability is crucial in determining the oil’s resistance to oxidation, which impacts its shelf life and suitability for various applications.

While the specific physicochemical properties of RSO can vary depending on factors such as geographical location, extraction methods, and rubber tree varieties, research findings suggest that RSO shares similarities with other non-edible vegetable oils in terms of these properties. These properties collectively contribute to the characterization and potential utilization of RSO as an industrial feedstock.

In summary, the physicochemical properties of rubber seed oil, including its fatty acid composition and other relevant parameters, have been extensively studied. The presence of unsaturated fatty acids, particularly polyunsaturated acids, is a notable feature of RSO. These properties play a crucial role in determining the suitability of RSO for biodiesel production and other industrial applications. While slight variations in reported values may exist, RSO exhibits characteristics similar to other non-edible vegetable oils described in the literature.

Biodiesel production methods (Rubber Seed Oil-Based Biodiesel)

The manufacturing of biodiesel can be accomplished through various methods such as transesterification, pyrolysis, and water emulsion, among other techniques. As part of the biodiesel process, pre-treatment is essential for treating the feedstocks prior to their conversion into biodiesel. The main objectives of biodiesel production are to enhance the bio-oil’s volatility and reduce its viscosity.

Pretreatment is an essential step in the biodiesel production process that involves treating feedstocks before their conversion into biodiesel. This involves a series of phases aimed at eliminating substances that can negatively impact biodiesel production, including water, gums, suspended particles, polymers, and particularly free fatty acids (FFAs). Water, when present during alkaline transesterification, can lead to the formation of excessive soap concentrations, react with the alkaline catalyst sodium methylate to produce methanol and sodium hydroxide, and shift the equilibrium reaction towards hydrolysis in the presence of acid catalysis. The formation of soaps can cause equipment blockage and downtime. One approach to pretreatment involves combining caustic soda with FFAs, but this can result in significant yield reductions. However, acid pretreatment eliminates the formation of soaps, leading to minimal yield loss. Polymers, gums, and particles present in oil feedstocks can have detrimental effects on the biodiesel production process, such as catalyst degradation and interference with phase separation of oil/glycerol phases. The biodiesel industry worldwide has adopted various pretreatment techniques, including liquid acid treatment (esterification of FFAs with a liquid acid catalyst), distillation (FFA removal through distillation), blending (mixing low-FFA feedstocks with high-FFA feedstocks), glycerolysis (reaction of glycerol with FFAs), acid esterification with solid catalysts (reducing FFA levels through ion exchange), and FFA removal methods [66].

Transesterification (Rubber Seed Oil-Based Biodiesel)

Triglycerides, commonly referred to as fats, are an essential component of oils and serve as valuable feedstocks in the production of biodiesel. Biodiesel is a renewable and environmentally friendly alternative to petroleum diesel, offering various advantages such as reduced greenhouse gas emissions and lower levels of air pollutants. The transesterification process, which involves the conversion of triglycerides into biodiesel, is a widely adopted method on an industrial scale worldwide [70].

One of the key benefits of transesterified biodiesel is its significantly lower viscosity compared to petroleum diesel. The lower viscosity of biodiesel allows for better fuel flow and combustion characteristics, making it suitable for use in diesel engines without the need for significant modifications. This characteristic enables biodiesel to seamlessly replace petroleum diesel and contribute to a more sustainable transportation sector [70].

The production of biodiesel primarily focuses on the transesterification of triglycerides, which constitute a major portion of vegetable oils and animal fats. Transesterification involves the reaction of triglycerides with alcohol, typically methanol, in the presence of a catalyst. Methanol is commonly chosen as the preferred alcohol due to its cost-effectiveness, making methyl esters the most prevalent type of esters produced during transesterification [70].

In the transesterification process, catalysts play a crucial role in facilitating the reaction between triglycerides and alcohol. The catalysts can be categorized into three main types: bases, acids, and enzymes. Each catalyst type has its advantages and disadvantages, and the selection depends on factors such as reaction conditions, feedstock characteristics, and desired biodiesel quality. Bases, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), are commonly used in alkaline transesterification, which is the most prevalent method in commercial biodiesel production. Acids, such as sulfuric acid (H2SO4) or hydrochloric acid (HCl), are utilized in acid-catalyzed transesterification processes. Enzymes, on the other hand, offer the advantage of operating at lower temperatures and pressures, reducing energy requirements, and enabling milder reaction conditions [70]. Rubber Seed Oil-Based Biodiesel

Transesterification reactions can be carried out at either low temperatures (typically around 25-40°C) or high temperatures (around 60-70°C). Low-temperature transesterification is often preferred due to its milder reaction conditions and lower energy consumption [71]. However, high-temperature transesterification can be employed to accelerate the reaction rate and increase biodiesel production throughput. The choice of reaction temperature depends on factors such as the specific catalyst used, the desired reaction rate, and the nature of the feedstock [70].

During the transesterification process, glycerol is produced as a byproduct. Glycerol, also known as glycerin, is a valuable compound with various applications in industries such as pharmaceuticals, cosmetics, and food [72]. The separation and purification of glycerol from the biodiesel product are crucial steps to obtain high-purity glycerol for further utilization. Efficient glycerol separation is essential not only for economic reasons but also to ensure the quality and purity of the final biodiesel product [71].

To illustrate the overall process of producing biodiesel through the transesterification reaction, Scheme 1 provides a simplified equation and depicts the key steps involved [72]. The triglycerides react with methanol in the presence of a catalyst, resulting in the formation of methyl esters (biodiesel) and glycerol. The biodiesel product can then be subjected to additional purification steps, such as washing to remove any residual catalyst or impurities, to meet the required quality standards [70][71].

In general, the production of biodiesel through transesterification of triglycerides offers a sustainable and environmentally friendly alternative to petroleum diesel. The utilization of triglycerides found in oils as feedstocks allows for the generation of biodiesel with significantly lower viscosity, making it compatible with existing diesel engine infrastructure. The transesterification process, catalyzed by bases, acids, or enzymes, can be performed at either low or high temperatures, offering flexibility in process design. The production of methyl esters as the primary type of esters, with methanol being the preferred alcohol, ensures cost-effectiveness in large-scale biodiesel production. Furthermore, glycerol, a valuable byproduct, can be separated and utilized in various industries. Through ongoing research and technological advancements, the biodiesel industry continues to evolve, contributing to a more sustainable and cleaner future for the transportation and energy sectors

Scheme 1: Transesterification of vegetable oil for biodiesel production [72]

During the transesterification process, various intermediates are formed, including di- and mono-acyl-glycerol. The relationship between the conversion of alkyl esters and the intermediate products over time is illustrated qualitatively in the provided graphic [73]. Equation 1 provides a detailed description of the production of fatty acid methyl esters (FAMEs) biodiesel through transesterification. The reaction involves four steps, starting with the mixing of a catalyst, typically a strong base like NaOH or KOH, with the alcohol [74].

To create the catalyst, methanol is combined with a potent base such as sodium or potassium hydroxide. In this process, the base dissociates into Na⁺ and OH^– ions. The OH- ion removes the hydrogen from methanol, forming water and leaving CH₃O^– available for the reaction [74]. It is ideal for methanol to be completely dry to avoid any unwanted reactions. When the OH- ion interacts with the H⁺ ion, water is formed. Free fatty acids, which are fatty acids not present in triglyceride form, are prone to unintentional side reactions leading to the formation of soap [74].

In enzymatic transesterification, alcohol, typically methanol, acts as a catalyst in the presence of lipases. However, lipases are expensive, slow, and yield lower conversion compared to chemical catalysts [75-76]. Once the catalyst is prepared, the triglyceride reacts with three moles of methanol. Therefore, additional methanol needs to be added to ensure a complete reaction. The CH₃ group of methanol interacts with the free fatty acid, resulting in the formation of fatty acid methyl ester while the three connected carbons with hydrogen react with OH- ions [77].

Different catalysts are employed in transesterification, and here are brief descriptions of a few common types:

Base-Catalyzed Processes (Rubber Seed Oil-Based Biodiesel)

Base-catalyzed transesterification in plant oil processing is typically faster than acid-catalyzed processes. This, along with the less corrosive nature of base catalysts compared to acidic compounds, makes them the preferred choice in industrial applications [78]. Scheme 2 illustrates the mechanism for the base-catalyzed transesterification of vegetable oils. In the first stage (Eq. 1), the base reacts with the alcohol to form an alkoxide and a protonated catalyst. The alkoxide then attacks the carbonyl group of the triglyceride, leading to the formation of an alkyl ester and the corresponding anion of the diglyceride as a tetrahedral intermediate (Eq. 2 and 3). The catalyst is regenerated by deprotonating it (Eq. 4), enabling it to interact with a new molecule of alcohol and initiate another catalytic cycle. This mechanism also applies to the transformation of diglycerides and monoglycerides, resulting in a mixture of alkyl esters and glycerol. Alkaline metal alkoxides, such as CH₃ONa for methanolysis, are highly effective catalysts, providing excellent yields (>98%) in a short reaction time (30 min) even at low molar concentrations (0.5 mol%) [79]. However, their requirement for anhydrous conditions makes them incompatible with common industrial processes. Alkaline metal hydroxides (KOH and NaOH) are less expensive catalysts and can achieve similar high conversions of vegetable oils by increasing the catalyst concentration to 1 or 2 mol% [75]. However, even in an alcohol/oil mixture devoid of water, the interaction between the hydroxide and alcohol produces some water, which can lead to the hydrolysis of esters and the formation of soap (Scheme 2). This undesirable saponification reaction reduces ester yields and complicates the recovery of glycerol due to the formation of emulsions [80]. Potassium carbonate, when used at a concentration of 2 or 3 mol%, can yield significant amounts of fatty acid alkyl esters while minimizing soap production. This may be attributed to the formation of bicarbonate instead of water (Scheme 8), which does not hydrolyze the esters [81].

In summary, base-catalyzed transesterification processes are favored in plant oil processing due to their faster reaction rates and the lower corrosiveness of base catalysts compared to acids. The mechanism involves the interaction between the base catalyst and alcohol, leading to the formation of alkoxides and protonated catalysts. These species then react with triglycerides, diglycerides, and monoglycerides to produce alkyl esters and glycerol. Alkaline metal alkoxides offer high yields but require anhydrous conditions, while alkaline metal hydroxides are more cost-effective and can achieve similar conversions with higher catalyst concentrations. However, the presence of water in the system can result in ester hydrolysis and soap formation, reducing ester yields and complicating glycerol recovery. The use of potassium carbonate can mitigate soap production by forming bicarbonate instead of water. The choice of catalyst and reaction conditions depends on the specific requirements of the plant oil transesterification process.

Scheme 2. Mechanism of the base-catalyzed transesterification of vegetable oils [78]

Acid-Catalyzed Processes (Rubber Seed Oil-Based Biodiesel)

Brønsted acids, particularly sulfonic and sulfuric acids, are commonly preferred as catalysts for the transesterification process [82]. These catalysts can yield high conversions of vegetable oils to alkyl esters, but the reaction times are typically longer, often exceeding three hours, and require elevated temperatures above 100 °C [82]. The molar ratio of alcohol to vegetable oil is a critical factor that influences the transesterification reaction. The presence of an excess amount of alcohol favors the formation of the desired products [78]. However, an excessive amount of alcohol can complicate the recovery of glycerol. Therefore, the appropriate alcohol-to-oil ratio needs to be determined empirically, considering the specific procedure and requirements of the transesterification process. Rubber Seed Oil-Based Biodiesel

Scheme 3 illustrates the mechanism of acid-catalyzed transesterification of vegetable oils, focusing on the transesterification of monoglycerides, but the mechanism can be extended to include di- and triglycerides as well. The process begins with the protonation of the ester’s carbonyl group, leading to the formation of the carbocation II. Subsequently, the alcohol acts as a nucleophile, attacking the carbocation II and forming the tetrahedral intermediate III. This intermediate then reacts with glycerol, resulting in the formation of the new ester IV and regenerating the catalyst H+. It should be noted that the reaction between the carbocation II and the water present in the reaction mixture can lead to the formation of carboxylic acids. To prevent the competitive synthesis of carboxylic acids, which can reduce the yields of alkyl esters, it is recommended to carry out acid-catalyzed transesterification in the absence of water [78].

In general, Brønsted acids, particularly sulfonic and sulfuric acids, are commonly employed as catalysts for acid-catalyzed transesterification. These catalysts can yield high alkyl ester conversions, although the reaction times are typically longer and higher temperatures are required. The molar ratio of alcohol to vegetable oil is an important consideration to ensure the desired product formation while avoiding complications in glycerol recovery. Scheme 3 demonstrates the acid-catalyzed transesterification mechanism, highlighting the formation of carbocations and tetrahedral intermediates. The presence of water in the reaction mixture can lead to the formation of carboxylic acids, which can reduce the yields of alkyl esters. Therefore, performing acid-catalyzed transesterification in the absence of water is recommended to maximize the production of alkyl esters.

Scheme 3. Mechanism of the acid-catalyzed transesterification of vegetable oils [78]

Lipase-Catalyzed Processes (Rubber Seed Oil-Based Biodiesel)

It has been discovered that lipases, which are enzymes, can serve as catalysts in transesterification and esterification reactions, as well as other enzymatic reactions such as the hydrolysis of glycerol, alcoholysis, and acidolysis [83]. Hydrolytic enzymes, including lipases, are widely utilized in organic synthesis due to their availability and ease of handling [84]. These enzymes exhibit reasonable stability, do not require coenzymes, and are often tolerant to organic solvents. Their ability to perform regio- and enantioselective synthesis makes them valuable tools [85]. However, compared to base-catalyzed reactions, enzyme-catalyzed transesterification processes still face challenges in terms of reaction yields and durations. Both extracellular and intracellular lipases have been explored for the efficient transesterification of triglycerides [86]. To overcome some of the limitations, further research into enzyme-catalyzed transesterification processes is necessary. Key aspects of these investigations involve optimizing reaction parameters such as solvent, temperature, pH, and the type of microorganism producing the enzyme to develop characteristics suitable for industrial application [87]. Figure 1 provides an overview of the general process for enzymatic transesterification in biodiesel synthesis.

In summary, lipases are enzymes that can act as catalysts in transesterification and esterification reactions. Their use in organic synthesis, including biodiesel production (Rubber Seed Oil-Based Biodiesel), is advantageous due to their availability, stability, and regio- and enantioselective properties. However, compared to base-catalyzed reactions, enzyme-catalyzed transesterification processes still require optimization to improve reaction yields and durations. Further research is being conducted to explore the potential of extracellular and intracellular lipases for efficient triglyceride transesterification. Adjusting reaction parameters and optimizing enzymatic systems are important steps toward developing enzyme-catalyzed transesterification processes suitable for industrial applications.

Fig.  1: General biodiesel production process of enzyme transesterification [83]

Ultrasound-assisted technique (Rubber Seed Oil-Based Biodiesel)

Ultrasound refers to the process of transmitting an oscillating sound pressure wave at a frequency higher than the upper limit of human perception [88]. The frequency range of ultrasound typically falls between 20 kHz and 1 MHz, corresponding to liquid acoustic wavelengths of approximately 100-0.15 mm. Ultrasound has been found to accelerate mass transfer and chemical reactions, leading to faster reaction times and the potential for using less expensive reagents [88]. When sound propagates through a liquid, it generates both escalation waves and compression waves, resulting in the formation of an aerosol composed of solvent, solute vapor, and previously dissolved gases [83]. The formation of a microemulsion induced by ultrasonic cavitation leads to high rates of mass transfer, causing bubbles to grow and subsequently recompress. This phenomenon has been utilized as a means to overcome the challenges associated with the conventional transesterification process, which often suffers from slow batch reaction rates and time-consuming phase separation [83].

In a nutshell, ultrasound involves the transmission of high-frequency sound waves beyond the range of human perception. When applied to liquids, ultrasound accelerates mass transfer and chemical reactions, resulting in faster reaction times and potential cost savings in terms of reagents. The propagation of sound through liquids leads to the generation of escalation and compression waves, which create an aerosol comprising a solvent, solute vapor, and previously dissolved gases. The development of a microemulsion through ultrasonic cavitation enhances mass transfer rates, allowing bubbles to grow and recompress. The use of ultrasound has been proposed as a solution to address the challenges associated with slow batch reaction rates and time-consuming phase separation in traditional transesterification processes.

Microwave-assisted technique (Rubber Seed Oil-Based Biodiesel)

Microwave irradiation is widely utilized in many organic syntheses. Microwaves are electromagnetic waves with frequencies ranging from 0.3 GHz to 300 GHz. The primary mechanisms involved in microwave heating are ionic conduction and dipolar polarization. Dipolar polarization refers to the realignment of polar molecules, such as methanol and water, under an ever-changing electric field. This realignment counters the forces between the molecules, and when the molecules are not in phase with the applied field, they collide and release absorbed energy as heat. In ionic conduction, heat is generated as the motion of ions within a material slows, changes direction, and collides with each other in an oscillating electromagnetic field. However, the high frequency of microwaves causes the field to oscillate rapidly, resulting in polar molecules ceasing their motion due to their short response time and strong intermolecular interactions. As a result, microwaves at a frequency of 2.45 GHz are used in residential and laboratory synthesis because they do not produce heat. Rubber Seed Oil-Based Biodiesel

Microwave irradiation finds applications in various fields, including organic synthesis. It selectively accelerates chemical reactions by preferentially heating polar molecules, while nonpolar molecules remain unaffected by the microwave dielectric loss. This technique offers several advantages, such as faster reactions, improved product purity, increased yields, and enhanced energy efficiency. Extensive research has been conducted on microwave-assisted transesterification processes for biodiesel production from vegetable oils. Comparisons have shown that microwave-assisted methods are more energy-efficient, have less impact on enzymes, and result in higher biodiesel yields compared to conventional heating methods. Solid base catalysts like magnesium and calcium oxides, as well as strontium oxide, have been explored due to their accessibility, affordability, and noncorrosive properties. Strontium oxide, being insoluble in methanol, has gained interest as a heterogeneous catalyst among alkaline-earth metal oxides. Remarkable results have been achieved with a yield of 95% in just 30 minutes at a relatively moderate temperature of approximately 65 °C. Additionally, the use of diverse zinc and aluminum oxides as heterogeneous catalysts has led to high biodiesel conversion rates (98.3%) and glycerol purity (over 98%). Hydrotalcite, a catalyst with adjustable acidic and basic properties, has been extensively studied for biodiesel production, yielding high conversion rates (87.3%) from vegetable oils. Another promising catalyst, Eu2O3, a solid super base heterogeneous catalyst, has demonstrated excellent performance in achieving high yields (95%) of biodiesel. Alumina, when loaded with various compounds, has shown improved yield (95%) in vegetable oil biodiesel production. Zeolites have recently emerged as potential catalysts, achieving a conversion yield of 94.6% when used in transesterification processes. Furthermore, a supercritical process conducted under high temperature and pressure, without the need for a catalyst, allows for biodiesel production with advantages such as improved phase solubility, rapid reaction rates, and easy separation and purification of products. This process remains effective even in the presence of moisture and free fatty acids.

In addition to microwave-assisted methods, contemporary techniques and various catalysts are employed for biodiesel production from vegetable oils. These include the supercritical process, hydrotalcite/layered double hydroxide (LDH)-derived catalysts, solid superbase catalysts, alumina loaded with different compounds, ultrasound-assisted methods, and microwave-assisted methods.

Factors influencing the transesterification of RSO

When conducting this study, several criteria were taken into account to examine the factors influencing the transesterification of RSO for biodiesel production.

Free fatty acid (FFA) content of RSO (Rubber Seed Oil-Based Biodiesel)

Rubber seed oil (RSO) is known to have a high free fatty acid (FFA) content, ranging from 23.471 to 45 wt%, as mentioned in several studies [16, 49, 96]. The presence of high FFA levels in RSO can lead to the formation of undesirable by-products like soap during the transesterification process. This can cause issues with downstream processing and the production of biodiesel (Rubber Seed Oil-Based Biodiesel).

To address this, it is necessary to reduce the FFA content of crude RSO before the transesterification reaction takes place. One common method involves esterifying the oil with alcohol using a homogeneous acid catalyst like H₂SO₄. By doing so, the FFA levels can be reduced to less than 1 weight percent. While temperature and reaction time are important factors in the transesterification process, catalyst concentration and the ratio of alcohol to oil have a greater influence on the reduction of FFA content [98]. Optimizing these parameters can help achieve efficient conversion of RSO into biodiesel while minimizing the formation of soap and other unwanted by-products.

It’s worth noting that the specific conditions and requirements for the transesterification process may vary depending on the research or industry practices. Therefore, it’s always advisable to refer to the relevant studies or consult with experts in the field for detailed and up-to-date information.

Alcohol/oil molar ratio

The molar ratio of alcohol to oil is a crucial parameter that significantly affects the production of biodiesel. The rate at which triglycerides are converted to fatty acid methyl esters (FAME) is negatively impacted by a low molar ratio of alcohol to oil. On the other hand, a higher molar ratio may lead to a lower yield and more challenging separation, which can increase production costs and affect the overall biodiesel yield [97].

Since the transesterification process is reversible, an excess amount of alcohol is required to drive the reaction toward the production of biodiesel. However, an excessive amount of alcohol, particularly methanol, can lead to the emulsification of glycerol and biodiesel due to the polar hydroxyl group in methanol. This makes the reverse reaction, where glycerol and ester reconnect, more favorable and reduces the biodiesel yield [99].

To achieve a high biodiesel yield, typically around 98% (w/w), a methanol-to-oil molar ratio of approximately 6:1 is often recommended when using alkaline catalysts. This molar ratio is considered sufficient to facilitate the formation of fatty acid methyl esters and glycerol. Musa et al. (2016) suggested a molar ratio of 6:1 for methanol and 9:1 for ethanol in their study on the impact of alcohol-to-oil ratios and the type of alcohol in the transesterification process for biodiesel production [76].

It has been reported that the production of biodiesel increases linearly with an increase in the molar ratio of alcohol to oil. However, after reaching a certain point, the biodiesel yield starts to decline. Moreover, a higher molar ratio can make the separation of glycerol and methanol more challenging. In the transesterification of RSO, excess alcohol is used to drive the reaction forward. Studies have indicated that a maximum biodiesel yield of 75-96.8% (wt.%) can be achieved using a 6:1 molar ratio of alcohol to oil when using alkaline homogeneous catalysts like KOH [100].

According to Ahmad et al. (2014), the alcohol-to-oil molar ratio is the most important factor in the synthesis of RSO biodiesel, up to a certain point. Beyond that point, increasing the methanol concentration does not significantly impact the biodiesel yield and may lead to higher downstream processing requirements [101].

It is important to note that the optimal molar ratio of alcohol to oil may vary depending on the specific feedstock, catalyst, and process conditions. Thus, it is advisable to consult relevant studies and experts in the field for specific recommendations and to consider the characteristics of the particular biodiesel production system being used in Rubber Seed Oil-Based Biodiesel

Catalyst concentration (Rubber Seed Oil-Based Biodiesel)

In the production of biodiesel, various catalysts such as alkali, acid, or enzyme catalysts are commonly used to achieve desired yields [102]. However, excessive use of catalysts can lead to the formation of emulsions, which can increase viscosity and make the separation of biodiesel more challenging. This can also promote saponification, a process that significantly reduces biodiesel production in the end [102].

Among the alkali catalysts, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are widely employed in biodiesel production. In recent years, there has been increasing interest in the use of heterogeneous catalysts, which offer the advantage of recyclability and reusability [103]. Researchers have explored the effectiveness of heterogeneous catalysts in producing biodiesel from rubber seed oil (RSO).

Gimbun et al. investigated the use of a calcium oxide (CaO) catalyst derived from activated cement clinker for RSO biodiesel production [103]. They found that a catalyst concentration of 6% (w/w) resulted in the highest conversion rate of 92.3% [103]. Another study by Zamberi and Ani (2016) focused on the conversion of RSO to biodiesel using a CaO catalyst derived from used cockle shells. They determined that a catalyst concentration of 9% (w/w) provided a maximum yield of 88.06%, beyond which the yield started to decline [104].

Typically, the yield of RSO biodiesel increases with an increase in catalyst concentration up to an optimal level. The optimization process involves determining the appropriate catalyst weight percentage, calcination temperature, and time [97]. The physical appearance of RSO biodiesel can be influenced by the concentration of the catalyst. A higher catalyst concentration may result in a darker coloration of the biodiesel [97, 105].

It’s important to consider that the choice of catalyst and its optimal concentration can depend on factors such as the specific reaction conditions, feedstock characteristics, and desired biodiesel properties. Further research and experimentation are necessary to determine the most suitable catalyst and concentration for a particular biodiesel production process.

Reaction temperature (Rubber Seed Oil-Based Biodiesel)

The reaction temperature plays a significant role in the yield of biodiesel and the kinetics of the transesterification process. Generally, increasing the temperature of the reaction leads to a faster reaction rate and higher yield [97]. The higher temperature helps in thinning the oil by reducing its viscosity, which enhances the mixing of oil and alcohol and facilitates the separation of glycerol from biodiesel. However, it’s important to note that exceeding the permissible temperature range can result in a significant decrease in biodiesel yield [97].

When the reaction temperature surpasses the recommended range, side reactions can occur at a faster rate compared to the transesterification reaction. For example, the saponification of triglycerides, which is accelerated by high temperatures, and the hydrolysis of methyl esters of fatty acids into corresponding acid and alcohol can take place. These side reactions can lead to a decrease in biodiesel yield [99, 101].

In a study conducted by Gimbun et al. (2013) on the conversion of RSO to biodiesel, a temperature range between 40°C and 70°C was examined while keeping other variables constant [103]. It was observed that higher temperatures improved the effectiveness of transesterification and enhanced RSO conversion. For RSO transesterification using a limestone-based catalyst, 65°C was found to be the optimal temperature. Beyond 65°C, the RSO conversion was not significantly affected. However, a slight decrease in conversion was observed when the temperature reached 70°C. This can be attributed to the fact that methanol, being a component of the reaction mixture, evaporates at a temperature higher than its boiling point (64.7°C), resulting in an unfavorable oil-to-methanol ratio that can hinder the reaction process.

It’s crucial to carefully control the reaction temperature within the appropriate range to achieve optimal biodiesel yield without promoting excessive side reactions. The specific temperature range and its impact on the transesterification process can vary depending on the catalyst, feedstock, and other reaction conditions. Thus, it is advisable to conduct experimental trials and consult the relevant literature to determine the most suitable temperature for a given biodiesel production process.

Barrier of transesterification process for non-edible oils

Non-edible oils often have a high concentration of free fatty acids, typically ranging from 2.53% to 22% on a weight basis. When it comes to producing biodiesel, alkaline transesterification is not practical for oils with a high level of free fatty acids [106]. This is because alkaline transesterification leads to the formation of soap, requires a larger amount of catalyst, and reduces the efficiency of the catalyst. Soap formation increases viscosity, leading to the production of gels and foams, which hinders the purification of biodiesel from glycerol [75]. To address this challenge, various techniques have been employed for biodiesel production from non-edible oils with a high free fatty acid content. These techniques include two- or three-stage reactions, acid- and alkaline-catalyzed esterification and transesterification, enzymatic processes, and supercritical methanol [107].

In an enzymatic process, the water content in the raw material does not affect the low-temperature reaction. The lipase reaction takes place at the contact between the aqueous and oil phases, resulting in the production of high-purity alkyl esters that are easy to separate [108]. Supercritical transesterification, conducted at high temperatures and pressures, can tolerate a high percentage of water in the feedstock. This is because the catalyst used in the process is deactivated, and the reaction time is shortened to a few minutes [109-111].

These alternative techniques offer potential solutions for the production of biodiesel from non-edible oils with high free fatty acid content. Each method has its own advantages and considerations, and the choice of technique depends on factors such as the specific feedstock, available resources, and desired biodiesel quality. Further research and development are ongoing to improve the efficiency and viability of these techniques for large-scale biodiesel production.

Current overview of rubber seed oil biodiesel synthesized over the years

Numerous studies have focused on the production of biodiesel from feedstocks with high free fatty acid (FFA) content and its fuel-quality characteristics [4, 112]. It has been observed that the conventional alkaline catalyst transesterification technique used commercially is not suitable for transesterifying feedstocks with high FFA levels. The interaction between FFAs and alkaline catalysts leads to the formation of soap, which hinders the separation of glycerin and ester. To address this issue, Ramadhas et al. (2005) proposed a two-step transesterification process [8].

In the first stage of the process, acid-catalyzed transesterification is employed to reduce the FFA level of the oil to below 2%. The byproducts from the first step are then converted to mono-esters and glycerol through alkaline catalyst transesterification. The study investigated the influence of various factors such as the alcohol-to-oil molar ratio, catalyst quantity, reaction temperature, and reaction time in each step. It was noted that excess sulfuric acid can darken the substance. Furthermore, the molar ratio of alcohol to oil had a significant impact on the conversion efficiency. The alkaline-catalyzed esterification process achieved higher conversion rates within 30 minutes when using a molar ratio of 6:1. The most favorable reaction temperature for ester conversion was found to be 45°C. Biodiesel produced through this two-step esterification process exhibits a viscosity similar to that of diesel. Although biodiesel has a slightly lower calorific value compared to diesel (approximately 130°C), it has a higher flash point. Additionally, this two-step esterification process utilizing inexpensive, unrefined, non-edible oils helps reduce the overall cost of biodiesel production. The investigation suggests that biodiesel derived from raw rubber seed oil is a promising substitute for diesel. However, further studies are needed to examine the long-term engine performance, wear characteristics, and other fuel properties of engines powered by biodiesel. In conclusion, the two-step transesterification process described offers a viable solution for converting high FFA oils into esters, mitigating soap formation, and improving biodiesel production efficiency.

In a related investigation, Sugebo et al. (2021) conducted a study to enhance the oil yield from rubber seeds, analyze the oil’s physicochemical properties, and produce biodiesel using the optimized oil [113]. They employed a solvent extraction technique with a core composite design of the response surface methodology to extract the oil. The biodiesel was created through a two-step transesterification process using an acid-base catalyst and a 6:1 molar ratio of methanol to oil. This process was carried out at 60 °C for 90 minutes. Gas chromatography-mass spectrometry was used to determine the fatty acid composition of the biodiesel. By employing a solvent-to-solute ratio of 9:1 and an extraction time of 8 hours at 95 °C, they achieved the highest oil yield of 61.3 wt%. The favorable physicochemical properties of the oil resulted in a maximum biodiesel yield of 81.55 wt%, meeting the ASTM6751 and EN590 standards for biodiesel fuel quality. The synthetic biodiesel comprised 83.4% unsaturated and 16.1% saturated fatty acids. However, the presence of unsaturated fatty acids may decrease the fuel’s oxidation stability. These findings indicate that rubber seed could be a significant feedstock for biodiesel production.

In another study by Le et al. (2018), a novel technique using FAMEs as a co-solvent for transesterification in biodiesel manufacturing was developed [114]. By using 34% of the oil’s FAMEs as a co-solvent, they successfully transesterified the crude rubber seed oil (RSO) after esterification. The process resulted in high-quality biodiesel (99.2%) at 40 °C within 30 minutes, using a MeOH/oil molar ratio of 4.5:1 and 1 wt% KOH. The physicochemical characteristics of the biodiesel from RSO met the EN 14214 and JIS K2390 standards. This approach using FAMEs as a co-solvent offers several advantages, including reduced solvent consumption, faster reaction rate, and the ability to modify the physicochemical properties of biodiesel. This procedure is similar to the production of market biodiesel, which is often blended with other types of FAMEs.

Kawashima et al. (2008; 2009) emphasized the advantages of using heterogeneous base catalysts in biodiesel production, including reusability, non-corrosiveness, tolerance to water and free fatty acids (FFAs) in the feedstock, increased biodiesel yield and purity, simpler glycerol purification, and easy separation from the biodiesel product [115-116]. Calcium oxide (CaO) is one of the commonly used heterogeneous base catalysts for vegetable oil transesterification [117]. Its usage offers several benefits such as enhanced activity, mild reaction conditions, reusability, and low cost [118]. However, using nanopowdered CaO poses challenges due to its difficult availability and energy-intensive production process. Moreover, catalyst recovery and separation become difficult with nanoparticles. Gimbun et al. (2012) proposed the use of a limestone-based catalyst for the transesterification of high-free fatty acid (FFA) rubber seed oil (RSO) [35]. They activated clinker, which is pre-calcinated limestone, by continuously stirring it with methanol at reflux. The catalyst’s mineral composition was examined using internal X-ray diffraction and X-ray fluorescence (XRF). They obtained rubber seed oil through both microwave and soxhlet extraction using hexane as the solvent. The FFA content and fatty acid methyl ester content (GC-MS) were determined using gas chromatography-mass spectrometry. The study demonstrated efficient conversion of high FFA rubber seed oil to biodiesel, with a conversion rate of up to 96.9%. The catalyst used in this study showed no adverse effects from moisture and free fatty acids and could be easily regenerated without significant loss of activity. The best conversion rate of 96.9% was achieved using a catalyst activated at 700°C, with a catalyst loading of 5 weight percent, a methanol to oil molar ratio of 5:1, a reaction temperature of 65°C, and a reaction time of 4 hours. The resulting biodiesel met the specifications of the American standard test technique (ASTM D6751). Similarly, Omorogbe et al. (2013) explored various catalysts, including sodium hydroxide, sodium metal, sulphuric acid, phosphoric acid, clay acid, and alkaline, for the transesterification of crude and refined rubber seed oil (RSO) to produce biodiesel [119]. They evaluated the biodiesel yield, physicochemical composition, and fuel characteristics. The methyl ester yield from crude RSO was lowest with sodium hydroxide as the catalyst (15%), while the highest yield was obtained from refined RSO using sodium metal as the catalyst (92.1%). Transesterification improved the fuel qualities of the oil, resulting in increased fuel potential and decreased viscosity and free fatty acid values. Other fuel characteristics also met the ASTM criteria.

Pulungan et al. (2021) converted rubber seed oil into biodiesel using a heterogeneous catalyst based on natural zeolite [120]. The process involved calcining and activating natural zeolite (ZAA) to create an active catalyst. Wet impregnation of ZAA with metal oxides (PbO, ZnO, and ZrO₂) was carried out to form a bifunctional catalyst. The catalyst activity test was conducted at 60 °C for 60 minutes using 5% (w/w) catalysts. The activation process and ZrO₂ loading increased catalyst crystallinity, while PbO and ZnO loading decreased it. The modification of the catalyst components resulted in lower aluminum levels due to dealumination and reduced impurities. The specific surface area of the catalysts decreased after impregnation, but the overall pore volume increased. The loaded metal oxides improved biodiesel conversion and reduced FFA concentration. The ZrO₂/ZAA catalyst exhibited the highest performance, converting FFA to methyl ester at a rate of 86.22% and achieving an overall conversion rate of 58.10%. The biodiesel properties included a density of 0.880 g/cm3, a water content of 0.092%, and FFA content of 1.081%. These results demonstrate the potential of rubber seed oil-based biodiesel catalyzed by a bifunctional catalyst as a renewable fuel source for the future.

The four research works presented different approaches to address the challenges associated with biodiesel production from rubber seed oil. Ramadan et al. (2005) proposed a two-step transesterification process to mitigate soap formation, while Sugebo et al. (2021) optimized the oil extraction and biodiesel production parameters. Le et al. (2018) introduced a co-solvent technique, and Kawashima et al. (2008; 2009) explored the use of heterogeneous base catalysts. Pulungan et al. (2021) focused on the conversion of rubber seed oil using a bifunctional catalyst.

These studies demonstrated that biodiesel production from rubber seed oil is feasible and can meet relevant fuel standards. However, each approach had its own advantages and limitations. Ramadan et al. (2005) emphasized the utilization of inexpensive, non-edible oils, but further research on engine performance and wear characteristics is needed. Sugebo et al. (2021) achieved optimal oil yield and biodiesel production, but the presence of unsaturated fatty acids may affect the fuel’s oxidation stability. Le et al. (2018) introduced a co-solvent technique that offers advantages in solvent consumption and reaction rate, but the impact on other fuel properties should be investigated. Kawashima et al. (2008; 2009) highlighted the benefits of heterogeneous base catalysts, particularly CaO, but challenges with nano powdered CaO and catalyst recovery remain. Pulungan et al. (2021) explored a promising catalyst based on natural zeolite, but further investigation is required to assess its long-term performance and stability.

In addition to traditional chemical techniques, the use of lipase-catalyzed transesterification for biodiesel production has been gaining attention [23]. Lipases offer advantages such as simultaneous transesterification and esterification, easy separation of the product from the enzyme and glycerol, and reduced inhibition rates. Unlike chemical catalysts, lipases are not sensitive to most free fatty acids (FFA). A recent study introduced Pseudomonas aeruginosa BUP2, a strain known for its high lipase production [121]. Lipases, a subclass of esterases, are produced by various organisms including bacteria, fungi, yeast, mammals, and plants. They catalyze the breakdown of lipids into glycerol and long-chain fatty acids and can participate in hydrolysis, esterification, inter-esterification, and transesterification processes. Microbial lipases are commonly used in industry due to their diverse catalytic activities, ease of genetic manipulation, high yield, ability to proliferate in low-cost media, and lack of seasonal variations [123]. Common genera of lipase-producing bacteria and fungi include Pseudomonas, Bacillus, Serratia, Alcaligenes, Aspergillus, Penicillium, and Candida [124].

Panichikkal et al. (2018) studied the characteristics of rubber seed oil (RSO) and its potential use in lipase-catalyzed transesterification for biodiesel production [125]. They focused on the lipase-producing bacterium Pseudomonas aeruginosa strain BUP2. The study optimized various parameters using Response Surface Methodology (RSM) and Box-Behnken Design (BBD), including the oil-methanol ratio, enzyme unit, reaction temperature, and reaction duration. Under the optimized conditions of lipase (750 U), methanol ratio (1:10), temperature (45 °C), and reaction time (4 hours), a biodiesel yield of 99.52% was achieved. The fuel characteristics of the biodiesel produced under these conditions met the ASTM D6751 and EN 14214 standards. This study highlights the significant potential of utilizing lipase-catalyzed transesterification as an alternative method for biodiesel production using valuable raw feedstocks [125].

Lipase-catalyzed transesterification for rubber seed oil biodiesel production offers several advantages over conventional methods. One of the significant advantages is the ability of lipases to simultaneously catalyze both the transesterification of triglycerides and the esterification of free fatty acids. Unlike chemical catalysts, lipases can efficiently convert both triglycerides and free fatty acids into biodiesel, resulting in higher yields [23]. Additionally, lipases are tolerant to the presence of free fatty acids in the feedstock, eliminating the need for additional pre-treatment steps to remove them. This is particularly advantageous when working with high-FFA feedstocks like rubber seed oil [121].

Another advantage is the ease of product separation in lipase-catalyzed transesterification. Lipases enable the easier separation of the biodiesel product from the enzyme and glycerol. The enzyme can be readily recovered and reused, reducing the overall cost and environmental impact of the process. Furthermore, lipases exhibit lower rates of inhibition compared to chemical catalysts, leading to improved efficiency and kinetics of the transesterification reaction. This allows the reaction to proceed at a faster rate, potentially reducing the required reaction time [123].

However, there are also some disadvantages associated with lipase-catalyzed transesterification. The cost of acquiring lipases and maintaining their activity during the transesterification process can be higher compared to chemical catalysts. Lipases are biocatalysts that need to be obtained from biological sources or produced through recombinant techniques. Additionally, lipase activity is influenced by various process conditions such as temperature, pH, and water content. Optimal conditions must be maintained to ensure high catalytic efficiency, which may require additional control and monitoring during the reaction [124].

Furthermore, lipase-catalyzed transesterification generally exhibits slower reaction kinetics compared to chemical catalysts. This may result in longer reaction times or the need for higher enzyme loadings to achieve the desired conversion levels. Process optimization is crucial for lipase-catalyzed transesterification, as various parameters such as enzyme concentration, oil-to-methanol ratio, temperature, and reaction time need to be optimized to achieve optimal yields. This optimization process can be time-consuming and complex [123-124].

In summary, lipase-catalyzed transesterification offers advantages such as simultaneous conversion of triglycerides and free fatty acids, tolerance to FFAs, and easier product separation. However, it also presents challenges related to enzyme cost, process conditions, reaction kinetics, and process optimization. These factors should be carefully considered when evaluating the suitability of lipase-catalyzed transesterification compared to conventional methods for rubber seed oil biodiesel production.

Fig. 2: Action of lipase; RSO with methanol in the presence of lipase to form fatty acid methyl esters and glycerol [125]

Table 1:  Fuel properties of biodiesel [125]

The primary concerns in the synthesis of methyl esters for biodiesel are their low-temperature characteristics and oxidation stability, which are directly influenced by the chemical composition of the feedstock used. Feedstocks with high concentrations of fatty acids, such as palm oil methyl esters (POME), consist mainly of saturated methyl esters (SME) that exhibit poor low-temperature characteristics and tend to form crystals in cold climates, impeding smooth fuel flow during ignition [7]. In contrast, rubber seed oil methyl esters (RSOME) have a higher concentration of polyunsaturated methyl esters, including double bonds, and a lower proportion of SME. The presence and location of double bonds determine the susceptibility of unsaturated methyl esters to autoxidation. Poor oxidation stability of biodiesel results in decreased viscosity, acid value, and peroxide value, which can lead to engine malfunctions [126]. Previous studies have shown that the characteristics of biodiesel can be significantly improved by blending various methyl esters [127].

To enhance the low-temperature characteristics and oxidation stability of POME, Bokhari et al. (2014) conducted blending experiments with RSOME at different ratios [128]. The post-blend products underwent testing according to international biodiesel standards EN 14214 and ASTM D671. Kinetic parameters were calculated for the base transesterification reaction of POME and RSOME. The inclusion of RSOME, which contains a substantial amount of unsaturated fatty acids and is derived from non-edible sources, helped improve the low-temperature characteristics of the resulting fuel. The blend of POMEs and RSOMEs at varying volumetric ratios led to enhanced low-temperature qualities. POMEs exhibited superior oxidation stability with an induction period (IP) of 25.52 hours, compared to RSOMEs’ 3-hour IP. On the other hand, RSOMEs demonstrated favorable performance in terms of low-temperature properties. GC analysis revealed a significant improvement in low-temperature characteristics due to the reduced saturation of methyl esters in the post-blend mixture. The optimal blend ratio of POMEs to RSOMEs was found to be 20:80, as it provided acceptable levels of both low-temperature characteristics and oxidation stability. All blends met the international biodiesel standards set by EN and ASTM (Table 2). The transesterification kinetics of both POME and RSOME followed pseudo-first-order kinetics, with activation energies of 33.2 kJ/mol and 43.4 kJ/mol, respectively. The frequency factors for POME and RSOME were determined to be 2.4 x 10³ min^-1 and 1.3 x 10³ min^-1, respectively.

These findings demonstrate that blending POME with RSOME can effectively address the challenges of low-temperature characteristics and oxidation stability in biodiesel production. The inclusion of RSOME, with its higher proportion of unsaturated methyl esters, improves the fuel’s low-temperature properties while maintaining acceptable oxidation stability. By optimizing the blend ratio, the resulting biodiesel meets international standards and exhibits desirable characteristics for efficient and reliable engine performance.

Table 2: Post-blended biodiesel fuel properties [128]

Currently, a small percentage of rubber seeds are utilized in the breeding process of rubber plants, while the majority of seeds are left unused and discarded. This presents a waste management challenge for rubber seed oil (RSO) millers, as the extraction of oil from seeds generates rubber seed shells (RSSs) as byproducts. Onoji et al. (2017) investigated the potential of discarded rubber seed shells (RSS) as a solid base catalyst for transesterification of esterified rubber seed oil (RSO) to biodiesel [129].

The researchers characterized the catalyst using various techniques such as TGA, XRF, XRD, SEM, and N2 adsorption/desorption analysis (BET). To determine the influence of process factors (reaction time, methanol/oil ratio, and catalyst loading) on biodiesel production, experiments were designed using central composite design (CCD). The Response Surface Methodology (RSM) technique was employed to optimize the process, and a quadratic model with a statistically significant F-value of 12.38 and a p-value of 0.05 was developed.

The optimized conditions identified by RSM were a reaction time of 60 minutes, a methanol/oil ratio of 0.20 vol/vol, and a catalyst loading of 2.2 grams. Under these conditions, the maximum biodiesel yield was determined to be 83.11%, which was empirically verified as 83.06 ± 0.013%. The catalyst’s reusability was evaluated, and it was found that the biodiesel production remained above 80% even after the fourth cycle of use. The leached Ca2+ ion content in the biodiesel was measured to be 3.26 mg/kg (ppm).

The oxidation stability of the biodiesel was evaluated to be 7.8 hours, and its ester concentration, determined by gas chromatography, was found to be 96.7%. The described biodiesel met the requirements of ASTM D 6751 and EN 14214 (Table 3). These findings suggest that modern diesel engines can run on biodiesel derived from rubber seed shell oil using a waste rubber seed catalyst, without requiring any significant technological adjustments.

The study highlights the potential of utilizing discarded rubber seed shells as a solid base catalyst for biodiesel production. This approach not only addresses the waste management challenge associated with rubber seed oil extraction but also yields biodiesel that meets international standards. By optimizing the process conditions, the researchers achieved high biodiesel yields, reusability of the catalyst, and satisfactory oxidation stability and ester concentration. This suggests that biodiesel derived from rubber seed shell oil can be a viable alternative fuel for diesel engines without necessitating significant modifications to existing technology.

Table 3. Fuel Properties of Biodiesel from Rubber Seed Oil [129]

In the synthesis of biodiesel, Ahmad et al. (2014) utilized non-edible rubber seed oil (RSO) with a high free fatty acid (FFA) concentration of 45% [101]. The process involved two steps: acid esterification and base transesterification. Acid esterification was employed to lower the FFA value, followed by base transesterification. Parametric optimization of both stages was carried out using the response surface methodology (RSM), and the yield of biodiesel was evaluated using gas chromatography. The transformation of fatty acids into methyl esters was confirmed using Fourier Transform Infra-Red (FTIR) spectroscopy. The qualities of the RSO fatty acid methyl ester (FAME) fuel were examined based on the ASTM D6751 and EN 14214 standards, as shown in Table 4. The viscosity of the synthesized RSO biodiesel met the requirement, indicating that no additional modifications were necessary. The higher flash point of RSO methyl ester compared to diesel contributes to its safer storage. Due to the presence of more unsaturated fatty acids than saturated fatty acids, RSO FAME exhibited favorable low-temperature characteristics.

Oxidation stability is a crucial fuel characteristic that affects storage and performance. The obtained oxidation stability and Cold Filter Plugging Point (CFPP) of RSO biodiesel demonstrated results comparable to other published research and met recognized standards. Flashpoint (FP) is essential for biodiesel as an alternative fuel, as a higher FP reduces the risk of flammability during storage. Kinematic viscosity is a key parameter to consider when selecting biodiesel, as it directly relates to the type and degree of saturation of fatty acids. Higher levels of unsaturated fatty acids lead to reduced viscosity. The rubber seed oil biodiesel in this study met the requirements of ASTM D6751 and EN14214, with a kinematic viscosity of 3.89 mm2/s. Melvin et al. (2011) also observed that the kinematic viscosity of rubber seed oil-based biodiesel was lower than that of rubber seed oil [130].

Cold flow characteristics of biodiesel are typically determined by cloud point, pour point, and cold filter plugging point. The degree of saturation, length of the carbon chain, orientation of double bonds, and fatty acid composition influence these properties. An increase in carbon atom count raises the melting point of fatty acids, while unsaturation decreases the melting point. RSO FAME displayed a cloud point of 3.2°C, a pour point of -2°C, and a CFPP of 0°C, indicating improved cold flow properties due to a higher content of unsaturated fatty acids.

The oxidation stability of RSO FAME was evaluated using the Rancimate method outlined in EN 14112. Oxidation stability is crucial for fuel storage and performance, as poor oxidation stability can lead to increased viscosity, gumming, and deposition of undesirable particles during storage. The oxidation stability value for RSO FAME, 8.54 hours, exceeded previous investigations on rubber seed oil biodiesel [101], and it met the standards specified by ASTM D6751 and EN 14214.

In summary, Ahmad et al. (2014) demonstrated that RSO FAME can be synthesized with satisfactory qualities. It exhibited acceptable viscosity, improved cold flow characteristics due to higher unsaturated fatty acid content, and met the standards for oxidation stability. These findings highlight the potential of rubber seed oil biodiesel as a viable alternative fuel that can be utilized without significant modifications to existing diesel engines.

Table 4: Fuel Properties of RSO Biodiesel [101]

Trirahayu et al. (2022) aimed to simulate the transesterification reaction for biodiesel production from rubber seed oil (RSO) using methanol and a heterogeneous catalyst [97]. The simulation was conducted using ASPEN Hysys v11. To prevent soap production, acid-based catalyzed esterification was chosen. Soap formation can significantly reduce biodiesel yield. The simulation results indicated that 1146 L/h of biodiesel could be produced from an RSO feed rate of 1100 L/h, with a methanol to oil molar ratio of 1:6. Methanol recovery was implemented, and approximately 95% of the excess methanol could be regenerated.

According to the simulation findings, the produced biodiesel exhibited qualities that make it compatible with modern diesel engines. The study compared the biodiesel product qualities to those of previous studies, although the researchers had limited access to analytical tools, resulting in a smaller number of properties being compared (Table 4). Overall, the biodiesel properties obtained in this study were found to be comparable to other laboratory-scale biodiesel synthesis studies using RSO (Table 4). With the exception of viscosity, the biodiesel attributes fell within the ranges specified in ASTM D 6751 and EN 14214 standards (Table 4).

Additionally, economic studies demonstrated the potential of this technology and its favorable investment criteria, indicating its viability for large-scale biodiesel production.

Table 4. Product properties obtained from this study in comparison with other products from the literature [97]

Performance of Rubber Seed Oil Biodiesel on Diesel Engine

The performance and emissions of biodiesel in diesel engines have been extensively studied by researchers to ensure compliance with standards and assess its feasibility as a fuel source. The use of vegetable oils as feedstocks for biodiesel production and their utilization in compression ignition engines were first investigated by Rudolph Diesel in 1893 [41]. Subsequent developments and commercial uses of biodiesel from various feedstocks have been observed worldwide, including the use of peanut oil-based biodiesel in French diesel engines in 1900 and the introduction of soybean oil-based vehicles by Henry Ford in 1941 [136-137].

In recent years, studies have focused on evaluating the efficiency and emissions of biodiesel blends in diesel engines using different feedstocks. Biodiesel blends, such as B10, B20, and B30, have been tested, and Pongamia oil has been successfully converted into biodiesel with high yields [135-136]. Comparisons with regular diesel have shown that B30 fuel may exhibit lower brake thermal efficiency (BTE) and higher brake specific fuel consumption (BSFC), but it also leads to reduced emissions of carbon monoxide (CO), total hydrocarbons (THC), and smoke. However, NOx emissions were found to be higher [139]. Blends of biodiesel made from used cooking oil have also been evaluated, showing lower BTE but improved NOx emissions compared to standard diesel [140]. Various blends of biodiesel produced from used frying oil have demonstrated favorable engine performance characteristics, such as improved brake thermal efficiency (BTE) and lower brake-specific fuel consumption (BSFC) [141]. Similarly, engine performance and emissions have been investigated for blends of biodiesel made from rubber seed oil (RSO), showing improvements in BTE with increasing biodiesel content, along with increased NOx emissions and decreased CO and THC emissions [140].

The use of biodiesel from non-edible oil-bearing seed trees, including rubber seed oil, has gained attention, especially in countries like Nigeria, where edible oil resources are limited. Rubber seed oil biodiesel has shown positive environmental benefits, such as reduced smoke opacity, exhaust gas temperature, and emissions of carbon, nitrogen, and sulfur oxides. Due to its sulfur-free and low aromatic components, rubber seed oil biodiesel does not produce sulfur oxides (SOx) and polycyclic aromatic hydrocarbons (PAHs) [142-143]. The brake power, torque, brake thermal efficiency (BTE), and brake-specific fuel consumption (BSFC) of rubber seed oil biodiesel have demonstrated favorable performance characteristics for its potential adoption as a substitute fuel. Rubber trees in Nigeria, specifically the NIG800 series, have shown high rubber production and abundant seed availability, making them a promising feedstock for biodiesel production [144].

To assess the emission characteristics and engine performance of rubber seed oil biodiesel, Onoji et al. (2020) evaluated a TD202 diesel test engine running on biodiesel, diesel, and their blends [144]. The results showed that biodiesel and its blends had higher BSFC compared to diesel due to the lower heating value of biodiesel. Consequently, brake power, brake thermal efficiency (BTE), and torque were lower for biodiesel and its blends. However, the emissions of pollutants such as carbon monoxide (CO), total hydrocarbons (THCs), and smoke were significantly reduced with biodiesel and its blends. This reduction can be attributed to the absence of aromatic molecules, lower carbon content, and higher oxygen concentration in biodiesel. While carbon dioxide (CO2) emissions increased slightly due to the lower energy content of biodiesel, it was partially offset by the carbon dioxide absorption capability of plants. Further investigation is necessary to understand the combustion characteristics of biodiesel in the TD202 diesel engine, including ignition point, heat release rate, and crank angle analysis [144].

Generally, numerous studies have examined the performance and emission characteristics of biodiesel in diesel engines using various feedstocks. Rubber seed oil biodiesel has demonstrated promising environmental benefits, reduced emissions, and favorable performance characteristics. Its utilization as a viable substitute fuel source shows great potential, especially in regions where edible oil resources are limited. Further research is needed to explore and optimize the combustion characteristics of rubber seed oil biodiesel to enhance its efficiency and emissions performance in diesel engines.

Figure 2. Variation of brake-specific fuel consumption with speed for diesel and biodiesel blends [144]

Figure 3. Variation of brake thermal efficiency with speed for diesel and biodiesel blends [144]

Figure 4. Variation of engine torque with speed for diesel and biodiesel blends [144]

In a study conducted by Patil and Patil (2017), the combustion, performance, and emission characteristics of refined biodiesel, specifically rubber seed oil methyl ester (ROME), with the partial addition of n-butanol (butanol) were investigated using a single-cylinder, four-stroke diesel engine operating at a constant speed of 1500 rpm [145]. The comparison was made between neat ROME, neat diesel, and various blends of butanol-ROME at different load conditions.

The results showed that at full load conditions, economically and environmentally favorable characteristics were observed. However, as the percentage of butanol in the ROME blend increased from 5% to 20%, performance metrics such as brake-specific fuel consumption (BSFC) increased by 17%, and brake thermal efficiency (BTE) decreased by 14% (Figure 6). Neat diesel exhibited lower BSFC (0.25 kg/kWh) compared to neat ROME (0.30 kg/kW/h) and butanol-ROME blends (approximately 0.325 kg/kW/h). On the other hand, the BTE of neat ROME (35%) was more promising than that of the tested fuels, including neat diesel (32%) and butanol-ROME blends (approximately 30%). Therefore, in terms of performance characteristics, butanol-ROME blends were found to be less effective compared to neat diesel or neat ROME.

In terms of emissions, all tested fuels showed carbon monoxide (CO) emissions below 0.1% and hydrocarbon (HC) emissions below 35 ppm at full load conditions. It was observed that as the percentage of butanol in ROME increased from 5% to 20%, nitrogen oxide (NOx) emissions reduced by 10% at full load. While butanol-ROME blends emitted more NOx compared to neat ROME, the emissions were still lower than those of neat diesel (approximately 280 ppm). Therefore, from an emission perspective, butanol-ROME blends were more effective than neat diesel but less effective than neat ROME. Importantly, both neat ROME and butanol-ROME blends complied with the safe limits for NOx, CO, and HC emissions set by the EURO-6 standards.

Summarily, the study found that the tested butanol-ROME blends, when compared to neat diesel, exhibited lower performance characteristics and emissions. Neat ROME showed more promising performance and emission characteristics. Further research could explore the combined effects of variable compression ratio (VCR) and exhaust gas recirculation (EGR) in existing diesel engines to improve performance and emissions.

Figure 5. Schematic diagram of diesel engine setup [145]

Figure 6. Variation of brake-specific fuel consumption with load [145] (Patil & Patil, 2017).

The use of heterogeneous base catalysts in the synthesis of biodiesel has gained attention as an alternative to homogeneous catalysts due to their recyclability and affordability. Sai Bharadwaj et al. (2021) evaluated the effectiveness and emissions of biodiesel produced from rubber seed oil using a heterogeneous catalyst in a diesel engine [146]. The study found that as the brake mean effective pressure increased, the brake thermal efficiency gradually increased, and the brake-specific fuel consumption marginally decreased for all tested blends (B10, B20, and B30). The highest brake thermal efficiency was achieved by the B10 blend at 73.26%, which was close to the value obtained from petroleum diesel. The emissions of carbon monoxide, hydrocarbons, nitrogen oxide, and carbon dioxide also decreased as the brake mean effective pressure increased for each blend, supporting the notion that rubber seed oil-based biodiesel is a viable alternative to regular diesel.

The stability of rubber seed methyl ester when using antioxidants has also been investigated. Adam et al. (2017) synthesized rubber seed methyl ester using a two-step procedure and evaluated the effects of antioxidants on its fuel qualities and engine performance [149]. The study found that TBHQ had the greatest capacity to promote the stability of the biodiesel, followed by BHA, DPPD, and NPPD. The addition of antioxidants reduced the heat release rate, nitrogen oxide emissions, and maximum in-cylinder pressure compared to plain diesel. However, carbon monoxide and hydrocarbon emissions increased with the addition of antioxidants. Overall, the study concluded that antioxidant-treated rubber seed methyl ester blends can be used in diesel engines without requiring any additional modifications.

The use of heterogeneous catalysts and antioxidants in the production and stabilization of rubber seed oil-based biodiesel has shown promising results in terms of performance, emissions, and stability. These approaches offer environmentally friendly and cost-effective alternatives for biodiesel synthesis and utilization in diesel engines.

Figure 7. Variation of the BP with respect to engine speed [149]

Figure 8. Variation of HRR at 2000 rpm at full load [149]

Figure 9. Variation of NO emission with respect to engine speed [149]

Figure 10. Variation of cylinder pressure at 2000 rpm at full load [149].

The study conducted by Ramadhas et al. (2004) focused on converting unrefined rubber seed oil into biodiesel and evaluating its performance as a fuel for diesel engines [150]. The alkaline-catalyzed esterification process was not suitable for high-free fatty acid (FFA) oils like rubber seed oil. However, through a two-step esterification process, the researchers successfully transformed crude rubber seed oil into biodiesel with properties similar to diesel fuel, including density and viscosity. It was found that biodiesel had a slightly lower calorific value but a higher flash point compared to diesel. Blending biodiesel with lower ester concentrations can improve its performance. The key characteristics of rubber seed oil biodiesel were found to be comparable to diesel, suggesting its potential as a fuel or performance-enhancing additive in compression ignition engines.

The study also examined the performance and emission characteristics of biodiesel-diesel blends in compression ignition engines. Lower biodiesel blend proportions were found to increase thermal efficiency, with the B10 blend significantly improving brake thermal efficiency by approximately 3% at rated load settings. Additionally, using B10 resulted in lower emissions and brake-specific fuel consumption. Increasing the biodiesel blend content led to a reduction in smoke density in the exhaust gases, while the exhaust gas temperature increased. However, it should be noted that higher biodiesel blends may result in increased nitrogen oxide (NOx) emissions due to the temperature-dependent nature of NOx formation. The experimental findings indicated that rubber seed oil methyl esters can be used effectively in existing diesel engines without requiring any modifications. The use of biodiesel as a partial replacement for diesel fuel offers several advantages, including increased agricultural productivity, reduced fuel supply uncertainty, enhanced farmer independence, and a significant contribution to reducing air pollution. By utilizing rubber seed oil biodiesel, these benefits can be achieved, thereby promoting sustainable and environmentally friendly energy sources.

In Adam’s study (2014), the engine performance and emission characteristics of an indirect injection (IDI) diesel engine running on biodiesel made from a blend of crude rubber seed oil were investigated [151]. The 50:50 blend of crude rubber seed oil biodiesel and diesel fuel was chosen to achieve cost reduction and improve fuel properties. The torque, brake specific fuel consumption (BSFC), brake thermal efficiency (BTE), as well as CO and NOx emissions of the engine were evaluated for neat diesel, B5, B10, and B20 blends. The results showed that the torque obtained at the rated engine speed of 2500 rpm for neat diesel, B5, B10, and B20 blends were 87 Nm, 86 Nm, 85.3 Nm, and 85 Nm, respectively. The torque for all blends was slightly lower compared to neat diesel, ranging from 0% to 5% less. The BTE values for B5, B10, and B20 were 27.58%, 28.52%, and 26.45%, respectively, while neat diesel had a BTE of 26.99%. The BSFC decreased with lower blend ratios and increased proportionally as the blend ratio increased. As the blend ratio increased, the CO emissions decreased, while the exhaust gas temperature and NOx levels increased.

In another study by Khalil et al. (2016), equal blend percentages of rubber seed oil and palm oil were used to produce biodiesel, aiming to improve fuel quality and reduce costs [152]. Response surface methodology (RSM) was employed to analyze the effects of various factors on transesterification and optimize the biodiesel yield. The thermophysical characteristics of the biodiesel were investigated, and the performance and emissions of an unmodified IDI diesel engine were studied under both partial and full load conditions. The results showed that compared to diesel fuel, the torque and brake mean effective pressure (BMEP) of the engine decreased by 1.1% and 1%, respectively. The power and brake thermal efficiency (BTE) were both 1.1% lower, while the full-load brake-specific fuel consumption (BSFC) was 1.4% higher. The CO emissions were reduced by 2%, while NOx, CO2, and exhaust temperature showed average increases of 1.1%, 1.2%, and 1.1%, respectively.

These studies highlight the impact of biodiesel blends on engine performance and emissions. While some parameters may show slight decreases or increases compared to diesel fuel, overall, biodiesel blends can be used effectively in diesel engines, providing a viable alternative with potential environmental benefits.

Future Prospects of Rubber Seed Oil-Based Biodiesel and Its Performance on Diesel Engines

The future prospects of rubber seed oil-based biodiesel and its performance on diesel engines look promising due to ongoing technological advancements. Researchers are exploring innovative methods to enhance the efficiency of biodiesel production from rubber seed oil [153]. These advancements include the development of more efficient catalysts, novel extraction techniques, and improved transesterification processes. These technological improvements are expected to increase the overall yield and quality of rubber seed oil-based biodiesel, making it a more viable alternative to conventional diesel fuel [154].

Future research and development efforts will focus on improving the performance of rubber seed oil-based biodiesel in diesel engines. This includes addressing concerns related to fuel stability, lubricity, cold flow properties, and engine compatibility [155]. The formulation of additives and the optimization of fuel blends are areas of interest to enhance the performance and durability of rubber seed oil-based biodiesel. Advanced engine technologies, such as common rail fuel injection systems and exhaust gas recirculation, are also expected to improve the combustion efficiency and emissions characteristics of biodiesel fuels [156].

The future prospects of rubber seed oil-based biodiesel heavily rely on the availability and sustainability of the feedstock. Rubber trees are cultivated extensively in many tropical regions, providing a reliable and abundant source of raw material. Continued research and development in optimizing rubber tree cultivation and improving seed yield will ensure a sustainable supply of rubber seed oil for biodiesel production. Additionally, efforts are being made to utilize waste rubber seeds and by-products, reducing waste and maximizing resource utilization in the biodiesel production process [23].

Rubber seed oil-based biodiesel offers significant environmental benefits compared to conventional diesel fuel. It has lower carbon dioxide (CO₂), sulfur, and particulate matter emissions, contributing to reduced greenhouse gas emissions and improved air quality. The future prospects of rubber seed oil-based biodiesel include its potential role in reducing dependence on fossil fuels and mitigating climate change. Moreover, the biodiesel industry can provide economic opportunities, job creation, and rural development through the cultivation of rubber trees and the establishment of biodiesel production facilities [101].

Government policies and regulations promoting the use of renewable energy sources, such as biodiesel, will play a crucial role in shaping the future prospects of rubber seed oil-based biodiesel. Supportive policies, such as tax incentives, subsidies, and renewable energy mandates, can encourage market penetration and create a favorable business environment for biodiesel producers. The increasing demand for cleaner fuels and the growing awareness of environmental sustainability are expected to drive the adoption of rubber seed oil-based biodiesel in the transportation sector [105].

In a nutshell, the future prospects of rubber seed oil-based biodiesel and its performance on diesel engines are promising. Technological advancements, engine compatibility improvements, sustainable feedstock availability, environmental benefits, policy support, and market penetration are all factors contributing to the positive outlook for this renewable fuel source. Continued research, development, and collaboration among scientists, engineers, policymakers, and industry stakeholders will be essential in realizing the full potential of rubber seed oil-based biodiesel as a sustainable and economically viable alternative to conventional diesel fuel.

Challenges (Rubber Seed Oil-Based Biodiesel)

Rubber seed oil-based biodiesel has gained significant attention as an alternative fuel source due to its potential to reduce dependence on fossil fuels and mitigate environmental impacts. However, the adoption of this biodiesel faces various challenges that hinder its widespread implementation. This article aims to provide a comprehensive overview of the challenges associated with rubber seed oil-based biodiesel and its performance on diesel engines. By addressing these challenges, researchers, policymakers, and industry stakeholders can work towards overcoming barriers and realizing the full potential of this promising biofuel.

1. Feedstock Availability and Sustainability: One of the key challenges in utilizing rubber seed oil for biodiesel production is the availability and sustainability of the feedstock. Rubber trees require specific climatic conditions and extensive land areas, limiting the geographical regions where rubber cultivation is viable [157]. Additionally, competing uses for rubber seeds, such as animal feed and traditional applications, can impact the availability of the feedstock for biodiesel production. Ensuring a sustainable supply chain and implementing sound agricultural practices are crucial to overcome these challenges [158].
2. Extraction and Conversion Techniques: The extraction and conversion techniques used in rubber seed oil-based biodiesel production also pose challenges. The extraction process requires efficient and cost-effective methods to obtain high-quality oil from rubber seeds. Various extraction methods, such as mechanical pressing and solvent extraction, have been employed, each with its advantages and drawbacks [159]. Additionally, the conversion of rubber seed oil into biodiesel involves transesterification, which necessitates the use of catalysts and optimal reaction conditions. Research efforts are ongoing to enhance the efficiency of these processes and develop innovative techniques [160].

• Fuel Properties and Engine Performance: The fuel properties of rubber seed oil-based biodiesel can differ from conventional diesel fuel, which can influence engine performance [161]. Biodiesel may have higher viscosity, lower calorific value, and different cetane numbers compared to petroleum diesel, leading to challenges in engine combustion, fuel atomization, and emissions control [162]. Modifying the fuel properties through blending with conventional diesel or additives can help overcome these challenges, but careful optimization is required to ensure compatibility with engine systems and compliance with emission standards.

1. Storage and Stability: Rubber seed oil-based biodiesel tends to exhibit lower oxidative stability and higher susceptibility to degradation compared to petroleum diesel [163]. Factors such as moisture, oxygen, and temperature fluctuations can lead to increased oxidation, resulting in fuel degradation, formation of sediments, and reduced storage stability. Stabilization techniques, such as antioxidant additives and proper storage conditions, are essential to improve the shelf life and maintain fuel quality [164-165].
2. Economic Viability and Market Acceptance: The economic viability and market acceptance of rubber seed oil-based biodiesel are crucial for its large-scale adoption. The production costs associated with feedstock cultivation, extraction, conversion, and purification can affect the competitiveness of biodiesel against conventional diesel fuel [166-167]. Additionally, market dynamics, including fuel pricing, policy support, and consumer acceptance, influence the demand and market penetration of this alternative fuel [168]. Implementing favorable policies, incentivizing investments, and raising awareness about the environmental benefits of rubber seed oil-based biodiesel can help overcome these economic and market-related challenges.

Rubber seed oil-based biodiesel holds significant potential as a renewable and sustainable alternative to conventional diesel fuel. However, several challenges must be addressed to facilitate its widespread implementation. Overcoming feedstock availability issues, improving extraction and conversion techniques, optimizing fuel properties, ensuring storage stability, and enhancing economic viability and market acceptance are key areas that require further research, technological advancements, and policy support. By addressing these challenges, rubber seed oil-based biodiesel can contribute to a greener and more sustainable future in the transportation sector.

Conclusion

This review provides an overview of the current state of rubber seed oil (RSO) biodiesel as a renewable fuel source. RSO biodiesel is derived from rubber seed plants, and it offers promising potential for sustainable energy production. Various extraction techniques are employed to obtain oil from rubber seeds, followed by a transesterification process to convert the oil into biodiesel, known as Fatty Acid Methyl Ester (FAME). Similar to traditional petrodiesel, the transesterification process enhances the properties of RSO biodiesel, including viscosity, flash point, calorific value, and cetane number. In terms of engine compatibility, internal combustion engines (ICEs) can run on RSO biodiesel without requiring significant modifications. Several studies have been conducted to evaluate the performance of engines using biodiesel derived from rubber seed oil, as well as blends of biodiesel and petroleum diesel. These studies have demonstrated that the engine performance metrics are comparable to those of engines running on traditional petroleum-based fuels. This suggests that in a scenario where conventional fossil fuels become scarce in the future, biodiesel derived from rubber seed oil can effectively meet the demand for fuel. Moreover, the use of RSO biodiesel contributes to lower emissions from engines, aligning with the environmental standards established by various nations. This indicates that biodiesel can be a viable solution for reducing the environmental impact of transportation and meeting sustainability goals. There is still ample room for further research in this field, particularly exploring alternative feedstock options for biodiesel production. Investigating and identifying new and unique seeds that can be utilized to produce biodiesel can contribute to expanding the range of available feedstocks and enhancing the overall sustainability and efficiency of biodiesel production processes . Overall, the current advancements in RSO biodiesel demonstrate its potential as a renewable and environmentally friendly fuel source that can serve as a substitute for conventional petroleum-based fuels. Continued research and innovation in this area will further enhance the development and application of biodiesel, paving the way for a more sustainable and energy-independent future.

Declarations (Rubber Seed Oil-Based Biodiesel)

Ethics approval and consent to participate

Not applicable

Consent for publication

Each author in this manuscript has given permission for this work to be published.

Availability of data and material

Not applicable

Competing interests

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Funding

Not applicable

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