Study on Biodiesel Production: Feedstock Evolution, Catalyst Selection, and Influencing Factors Analysis

Abstract

As fossil fuel depletion and environmental pollution become increasingly severe, biodiesel has emerged as a promising renewable alternative to conventional diesel due to its biodegradability, low sulfur emissions, and high combustion efficiency. This paper provides a comprehensive review of the evolution of biodiesel feedstocks, major production technologies, and key factors influencing production efficiency and fuel quality. It traces the development of feedstocks from first-generation edible oils, second-generation non-edible oils and waste fats, to third-generation microalgal oils and fourth-generation biofuels based on synthetic biology, with a comparative analysis of their respective advantages and limitations. Various production technologies such as transesterification, direct esterification, supercritical alcohol methods, and enzyme-catalyzed transesterification are examined in terms of reaction mechanisms, process conditions, and applicability. The effects of critical process parameters including the alcohol-to-oil molar ratio, reaction time, and temperature on biodiesel yield and quality are discussed in detail. Particular attention is given to the role of catalysts, including both homogeneous and heterogeneous types, in enhancing conversion efficiency. In addition, life cycle assessment (LCA) is briefly considered to evaluate the environmental impact and sustainability of biodiesel production. This review serves as a valuable reference for improving biodiesel production technologies, advancing sustainable feedstock development, and promoting the commercial application of biodiesel.

1. Introduction

Global energy demand remains primarily dependent on conventional fossil fuels. With the continuous growth of energy consumption and the decline of fossil fuel reserves, energy security has become an increasingly prominent issue [1]. As shown in Figure 1, coal, oil, and natural gas together account for more than three-quarters of the global energy supply [2]. In addition to the rapid depletion of these resources, the combustion of fossil fuels emits large quantities of pollutants and greenhouse gases, including carbon dioxide (CO₂), hydrocarbons (HCs), carbon monoxide (CO), nitrogen oxide (NOₓ), and particulate matter (PM), which exacerbate global climate change and lead to severe environmental pollution [3,4]. According to projections by the Intergovernmental Panel on Climate Change (IPCC), if global greenhouse gas emissions continue to rise, the average global temperature is expected to increase by more than 2 °C by 2050, posing serious threats to ecosystems and human society [5]. In light of the gradual depletion of fossil fuel reserves and their adverse environmental impacts, the development of renewable, clean, and efficient alternative fuels has become a critical focus of global energy research [6]. Governments and research institutions worldwide are actively promoting the development of renewable energy sources, such as solar, wind, hydropower, geothermal, and biomass energy, which are gradually becoming key components of the future energy mix [7].

Against this background, biodiesel has emerged as a promising renewable fuel due to its wide availability, environmental friendliness, biodegradability, low sulfur emissions, and high combustion efficiency, showing great potential as a substitute for conventional diesel [8,9,10]. Biodiesel not only reduces dependence on fossil fuels but also effectively lowers greenhouse gas emissions. Moreover, it can be directly used in existing diesel engines without requiring significant modifications, making it one of the most promising clean alternatives to diesel fuel [11]. Several studies have explored the use of biodiesel blends and nanoparticle additives to enhance engine performance and reduce emissions. Gopinath Dhamodaran et al. [12] investigated Margosa biodiesel–diesel blends enhanced with 100 ppm of CuO/Fe₃O₄/g-C₃N₄ nanocomposites, reporting a maximum brake thermal efficiency (BTE) of 30.54% and a 7.8% decrease in brake specific fuel consumption compared to diesel. Emission levels of HC, CO, NO_x, and smoke were also reduced by 12.4%, 9.6%, 13.5%, and 15.3%, respectively. Similarly, Edgar N. Tec-Caamal et al. [13] examined biodiesel blends derived from waste cooking oil and karanja oil, incorporating CeO₂ nanoparticles and HHO gas. Although the unmodified B20 blend showed slightly lower BTE than diesel, the addition of these enhancers significantly improved efficiency and reduced CO and HC emissions by 35% and 30%, respectively, despite a 14% rise in NO_x emissions. In another study, Akash-Deep et al. [14] evaluated castor oil-based biodiesel blends in a compression ignition engine. The B20 blend demonstrated the highest BTE and lowest exhaust gas temperature, reflecting efficient energy utilization, while B10 and B20 exhibited lower emissions compared to diesel and other blend ratios. Furthermore, Ameren Kondaiah et al. [15] synthesized castor biodiesel via transesterification and observed significantly higher flash and fire points than diesel, though with a lower calorific value. Engine tests showed that castor biodiesel blends generated greater peak pressures and achieved BTE values only marginally 0.37% below those of diesel.

Globally, governments are actively promoting the production and utilization of biodiesel by introducing various incentive policies, such as the European Union’s Renewable Energy Directive (RED), the United States Renewable Fuel Standard (RFS), and India’s biodiesel promotion program, all of which have contributed to the expansion of the biodiesel market. The global biodiesel market has been growing rapidly. Statistics show that global biodiesel production was only 15,000 barrels in 2000, but increased to 289,000 barrels by 2020, and is projected to reach 41.4 billion liters by 2025 [16]. Among them, the European Union and the United States are the largest producers and consumers of biodiesel, while emerging economies such as India, China, and Brazil are also experiencing a continuous increase in biodiesel demand.

Biodiesel is typically produced through a transesterification reaction, in which vegetable oils, animal fats, or waste oils react with alcohols (usually methanol or ethanol) in the presence of a catalyst to generate fatty acid methyl esters (FAMEs) and glycerol [17]. This process is influenced by various factors, including the composition of raw materials, the type of catalyst, reaction conditions, and post-treatment procedures [18]. Among these factors, the choice of feedstock plays a crucial role in determining production costs, fuel quality, and sustainability. Traditional biodiesel feedstocks mainly consist of vegetable oils such as soybean oil, palm oil, and rapeseed oil. However, their large-scale application may lead to food supply shortages and the occupation of agricultural land, as these oils compete with food crops [19,20]. In recent years, researchers have shifted their attention to more economical and sustainable feedstocks, such as waste cooking oil, microalgal oil, animal fats, and non-edible vegetable oils [21,22]. Waste oils have attracted considerable interest due to their low cost and environmental benefits. However, their high impurity content and elevated acid value can lead to soap formation as a by-product during the transesterification process. Therefore, pretreatment steps such as deacidification, dehydration, and impurity removal are often required. Microalgal oil, as an efficient biomass energy source, not only offers a remarkably high oil yield but also absorbs carbon dioxide (CO₂) through photosynthesis, thereby reducing greenhouse gas emissions. Additionally, it can utilize industrial exhaust gases as a growth medium, making it one of the most promising biodiesel feedstocks for the future [23,24]. Furthermore, the use of oils extracted from woody plants (e.g., jatropha oil, neem oil, and rubber seed oil), marine organisms, and agricultural waste has emerged as a new research direction aimed at further reducing biodiesel production costs and enhancing sustainability [25,26].

The choice of catalyst plays a decisive role in the transesterification reaction, directly affecting the conversion rate, reaction time, energy consumption, and the quality of the resulting biodiesel. Currently, commonly used catalysts include homogeneous catalysts (such as alkaline catalysts NaOH and KOH, and acid catalysts H₂SO₄ and HCl), heterogeneous catalysts (such as metal oxides and solid acid–base catalysts), and enzymatic catalysts (such as lipases) [27]. Alkaline catalysts are widely employed in industrial production due to their fast reaction rates and low cost. However, they are less suitable for feedstocks with high free fatty acid (FFA) content, as they tend to cause saponification reactions, which hinder the separation and purification of biodiesel [28]. In contrast, acid catalysts exhibit better adaptability to high-FFA feedstocks and can effectively reduce the formation of soap by-products. Nonetheless, they typically have slower reaction rates and are highly corrosive, posing challenges for long-term equipment operation [29].

Heterogeneous catalysts have attracted increasing attention due to their reusability and ease of separation [30,31]. Among them, metal oxide catalysts such as TiO₂, ZnO, and Al₂O₃ demonstrate significant potential in biodiesel synthesis owing to their high catalytic activity and thermal stability [32]. In recent years, the study of nanocatalysts has gained momentum. Due to their large specific surface area and high catalytic efficiency, nanocatalysts can promote transesterification under relatively mild temperature and pressure conditions, thereby improving production efficiency and reducing the formation of by-products [33]. For example, TiO₂ and Al₂O₃ nanoparticles can achieve efficient catalysis under moderate reaction conditions, reduce energy consumption, and increase biodiesel yield.

In addition, biocatalysts such as lipases are considered environmentally friendly catalytic alternatives due to their ability to catalyze reactions efficiently at low temperatures while avoiding saponification. However, the high cost and limited operational stability of enzymatic catalysts remain major barriers to their industrial application. As a result, improving the stability and reusability of enzyme-based catalysis has become a key focus of current research [34].

The production of biodiesel is influenced by multiple factors, including the alcohol-to-oil molar ratio, reaction temperature, catalyst concentration, stirring speed, and reaction time [35,36]. Among these, the alcohol-to-oil molar ratio is a key parameter affecting the equilibrium of the transesterification reaction. Typically, the amount of alcohol used exceeds the theoretical stoichiometric ratio to improve the yield of fatty acid methyl esters. However, excessive alcohol increases the cost of subsequent recovery and refining processes [37]. Increasing the reaction temperature helps reduce the viscosity of the oil and accelerates the transesterification reaction, but excessively high temperatures may lead to alcohol evaporation losses and a higher probability of side reactions [38]. The choice of catalyst concentration requires a balance between enhancing the reaction rate and avoiding saponification; an excessive amount of catalyst may degrade product quality and increase post-treatment costs. Stirring speed is also critical to reaction efficiency. Proper stirring improves the contact between reactants at the phase interface and enhances mass transfer, but overly high stirring speeds may cause emulsification, complicating the separation of biodiesel and glycerol [39].

In recent years, the introduction of novel techniques, such as ultrasound-assisted transesterification, microwave heating, and supercritical fluid methods, has effectively shortened reaction times, enhanced reaction rates, and reduced energy consumption. For instance, ultrasound-assisted technology improves the mixing of reactants through cavitation, thereby enhancing catalytic efficiency [40]. Microwave heating enables rapid heating of the reaction system and increases catalyst activity [41,42]. The supercritical fluid method facilitates efficient transesterification without the need for a catalyst, thereby reducing catalyst residues and subsequent separation costs [43].

This study focuses on the key aspects of biodiesel production, including a systematic analysis of the evolution of biodiesel feedstocks, the selection and catalytic performance of various types of catalysts, the main influencing factors in the transesterification process, and recent advances in the optimization of biodiesel production technologies. By systematically reviewing the production technology of biodiesel, theoretical support is provided for the development of biodiesel, and a more efficient, environmentally friendly, and economically feasible production mode is achieved.

2. Evolution of Biodiesel Feedstocks

Biodiesel feedstocks play a crucial role in determining its economic feasibility, environmental sustainability, and fuel performance. With the growing global demand for renewable fuels, biodiesel feedstocks have evolved from first-generation to fourth-generation sources. Each generation of feedstock has been developed with the aim of increasing biodiesel yield, reducing dependence on food crops, and improving production efficiency and environmental sustainability. Based on their sources and sustainability characteristics, biodiesel feedstocks are generally classified into four generations: first-generation (edible oils), second-generation (non-edible oils and waste oils), third-generation (microalgal oils), and fourth-generation (synthetic biodiesel) [44]. These feedstocks offer distinct advantages in biodiesel production, while also presenting various technical and economic challenges. Figure 2 and Figure 3 illustrate the evolution of biodiesel feedstocks, products, and production technologies from the first to the fourth generation.

2.1. First-Generation Feedstock

First-generation biodiesel feedstocks are primarily derived from edible vegetable oils, such as soybean oil, palm oil, rapeseed oil, sunflower oil, and peanut oil [46]. Due to their high oil content, high conversion efficiency, and stable supply, these oils were widely used in the early stages of biodiesel production. However, the reliance on specific vegetable oils varies across regions due to differences in climate conditions, agricultural development, and policy support. For instance, in North America, soybean oil serves as the main feedstock for biodiesel production, supported by highly mechanized soybean cultivation and favorable government policies [47]. In contrast, Southeast Asian countries, such as Indonesia and Malaysia, rely heavily on palm oil due to its high yield per unit area and low production cost, making it a major source of biodiesel for the global market [48]. In Europe, countries such as Germany and France primarily use rapeseed oil because of its high content of unsaturated fatty acids, which provides good low-temperature fluidity and makes it suitable for cold climate conditions [49,50]. Additionally, although sunflower oil and peanut oil account for a relatively small share of the global biodiesel market, they still maintain a certain market presence in countries such as Ukraine, Russia, China, and some African nations.

However, despite the environmental advantages of first-generation biodiesel, its large-scale application has raised significant concerns regarding sustainability, particularly in terms of food security, environmental pollution, and resource consumption. First, as biodiesel production depends on edible vegetable oils, a substantial amount of agricultural land is allocated to oil crop cultivation, directly competing with food production. This “food-versus-fuel” issue has contributed to rising food prices, with particularly severe impacts on resource-limited developing countries [51]. Second, in order to meet market demand, some countries and regions have expanded biodiesel feedstock plantations, leading to large-scale deforestation of tropical rainforests. This not only damages ecosystems but also increases carbon emissions, potentially resulting in a carbon footprint for biodiesel that is even higher than that of conventional fossil diesel [52]. In addition, the production of first-generation biodiesel involves the intensive use of fertilizers and pesticides, which may cause soil degradation and water eutrophication, increasing the risk of environmental pollution. Therefore, although first-generation biodiesel remains the dominant source of biodiesel production today, its sustainability has been widely questioned, driving the research and development of second-generation biodiesel feedstocks.

2.2. Second-Generation Feedstock

To avoid the competition between biodiesel production and food supply associated with first-generation feedstocks, researchers have developed second-generation biodiesel feedstocks, consisting mainly of non-edible vegetable oils and waste oils [53]. These feedstocks include non-edible oils such as jatropha oil (Jatropha curcas), castor oil, neem oil, and rubber seed oil, as well as waste cooking oil, gutter oil, and animal fats, including tallow, lard, and fish oil [54].

Jatropha oil is widely regarded as one of the most representative feedstocks among second-generation biodiesel sources. Compared to first-generation feedstocks, jatropha oil offers several advantages, particularly in terms of oil yield and land adaptability [55]. Jatropha can grow on non-arable, marginal, and even arid or barren lands, requiring minimal soil conditions and avoiding competition with food crops for arable land, thus mitigating the “food-versus-fuel” issue [56]. Its non-edible nature makes it an ideal alternative resource for sustainable development. Moreover, jatropha fruits have a high oil content, and the oil can be efficiently extracted through mechanical pressing or solvent extraction, providing a substantial supply of feedstock with considerable development potential [57].

However, the practical application of jatropha oil still faces several challenges. First, the oil yield is closely related to the plant’s genetic traits, cultivation practices, and growing environment, resulting in significant regional variations in oil productivity. Second, the entire jatropha plant contains toxic compounds, such as phorbol esters and saponins, which may pose risks to human health and agricultural ecosystems during cultivation, harvesting, and oil processing. Moreover, residual toxins can affect the value of by-products; for instance, jatropha is not suitable for use as animal feed, thus requiring careful assessment of its ecological and environmental risks.

Castor oil is another important non-edible oil resource, derived from the seeds of the castor plant. Due to its unique fatty acid composition, particularly its high ricinoleic acid content (up to approximately 90%), castor oil has been widely used in industrial applications, including lubricants, coatings, plasticizers, and the pharmaceutical and chemical industries [58]. In recent years, castor oil has also been applied in biodiesel production. However, its physical properties, such as high viscosity and high density, limit its direct use in internal combustion engines. In particular, castor oil exhibits poor fluidity at low temperatures and is prone to gelling, which adversely affects fuel spray and atomization performance [59]. Therefore, to improve its suitability for diesel engine applications, castor oil is often blended with diesel, bio-alcohols, or other low-viscosity fuels, or chemically modified through transesterification and other techniques to optimize its fuel properties.

Waste oils, such as used cooking oil, gutter oil, and industrial waste oil, are among the most attractive feedstocks for second-generation biodiesel production [60]. The greatest advantages of these oils lie in their wide availability, low cost, and significant value for resource recycling. Their utilization not only helps reduce the overall production cost of biodiesel but also alleviates the environmental pollution caused by the disposal of urban waste oils, offering notable environmental and social benefits [61]. However, waste oils are generally of poor quality, often characterized by high contents of free fatty acids (FFAs), moisture, and solid impurities [62]. These contaminants can seriously interfere with the subsequent transesterification process, leading to catalyst deactivation, increased saponification, and reduced ester yield. Therefore, rigorous pretreatment of waste oils is essential before entering the transesterification stage. This typically includes acid-catalyzed esterification, neutralization and deacidification, dehydration, and impurity filtration. Although these processes improve conversion rates and reduce by-product formation, they also increase process complexity and operational costs to some extent.

Overall, second-generation biodiesel feedstocks offer significant improvements over first-generation feedstocks in terms of resource utilization, environmental friendliness, and social acceptability, making them particularly suitable for the current green energy transition driven by sustainable development goals. However, the large-scale commercialization of biodiesel at the global level remains constrained by limitations in feedstock supply, which are affected by factors such as raw material collection, cultivation scale, and regional policies.

2.3. Third-Generation Feedstock

Third-generation biodiesel feedstocks are primarily derived from microalgae, as well as certain marine organisms such as macroalgae, oil-producing bacteria, and fungi. These feedstocks are widely regarded as the next generation of sustainable fuel sources following first-generation (edible vegetable oils) and second-generation (non-edible vegetable oils and waste oils) biodiesel feedstocks. Among them, microalgae are considered the most promising third-generation biodiesel feedstock due to their unique physiological characteristics and excellent oil-producing capacity [63].

Compared with conventional terrestrial plants, microalgae exhibit significantly higher lipid accumulation capabilities, with some species containing lipid levels ranging from 40% to 80% of their dry weight—far exceeding those of common oil crops such as soybean and rapeseed. In addition, microalgae have extremely rapid growth rates and short life cycles, allowing for daily harvesting or even continuous cultivation. As a result, their annual oil yield per unit area is substantially higher than that of traditional energy crops. Reports indicate that certain high-yield microalgae, under optimized cultivation conditions, can achieve a theoretical annual oil yield of up to 90,000 L per hectare, which is approximately 30 times greater than that of soybean.

Microalgae cultivation does not rely on arable land and can be carried out on non-agricultural land, saline-alkali soil, water bodies, or even in industrial wastewater, greatly expanding its application scenarios and avoiding spatial competition with food production [64]. Moreover, during cultivation, microalgae absorb large amounts of CO₂ through photosynthesis, effectively reducing greenhouse gas emissions and contributing positively to mitigating global warming and achieving carbon neutrality targets [65]. Some studies have further indicated that utilizing nitrogen- and phosphorus-rich municipal or industrial wastewater as a culture medium not only enables the resourceful treatment of wastewater but also significantly reduces the cost of microalgae cultivation, thereby enhancing the synergistic benefits of both biofuel production and environmental management [66,67].

Despite the considerable theoretical and environmental advantages of microalgae, their practical application still faces significant technical and economic challenges. Currently, the two main microalgae cultivation methods are the open pond system and the closed photobioreactor (PBR) [68]. The open pond system features a simple structure and relatively low investment costs, making it suitable for large-scale primary cultivation. However, it is highly susceptible to external contamination, climate fluctuations, and water evaporation, often resulting in low microalgae density and limited lipid productivity [69,70]. In contrast, photobioreactors offer higher light utilization efficiency and better contamination control, enabling high-density continuous cultivation. Nevertheless, their construction and operational costs are considerably higher, and the system requires complex technical management, which has so far limited their widespread commercial application [71,72].

Following microalgae cultivation, lipid extraction becomes a critical step that directly affects both the yield and cost of biodiesel production. Traditional extraction methods primarily rely on organic solvents such as hexane and ethanol. However, these methods are associated with challenges such as difficulty in solvent recovery, high energy consumption, and environmental pollution. In recent years, researchers have explored a variety of novel extraction techniques, including supercritical CO₂ extraction, microwave-assisted extraction, ultrasonic disruption, and enzyme-assisted cell wall degradation, with the aim of improving extraction efficiency and reducing energy consumption [73,74,75]. Nevertheless, most of these techniques remain at the laboratory or pilot scale and have yet to be developed into stable, cost-effective industrial processes. In particular, the drying, cell disruption, and extraction stages account for a large share of total energy consumption, representing one of the major bottlenecks hindering the commercial viability of microalgae-based biodiesel.

Therefore, the future development of microalgae-based biodiesel needs to focus on two key aspects. The first is to cultivate high-lipid-yielding and stress-resistant microalgae strains through genetic engineering, domestication, or physiological regulation, thereby improving the cultivation efficiency per unit area. The second is to optimize the cultivation systems and lipid extraction processes to reduce overall energy consumption and production costs, making industrial production economically feasible. In addition, the comprehensive utilization of microalgal biomass, such as using residual algal biomass for biogas, animal feed, or fertilizer production, is also regarded as an important strategy to enhance both the economic efficiency and environmental benefits of the entire system.

2.4. Fourth-Generation Feedstock

The development of fourth-generation biodiesel feedstocks is based on advanced biotechnologies such as synthetic biology and genetic engineering. The core concept is to artificially design microbial cell factories and reconstruct their metabolic pathways to enable the efficient biosynthesis of target fuel molecules. In contrast to the first three generations of biodiesel, which rely on natural lipid sources, fourth-generation biodiesel adopts a bottom-up approach. It is no longer limited to the extraction of oils from plants, animals, or algae but utilizes precise metabolic regulation to directly convert carbon sources, such as glucose, hydrolysates of cellulose, or even industrially emitted carbon dioxide, into structurally controlled fuel molecules. These molecules include fatty acid esters, hydrocarbons, and isoprene-based biodiesel substitutes [76].

Common host strains used in current research include engineered Escherichia coli, yeast, and cyanobacteria. These microorganisms can acquire lipid-producing capabilities through the introduction of exogenous genes or the reconstruction of endogenous genetic pathways. For example, by inserting key enzyme genes such as wax ester synthase or aldehyde deformylating oxygenase (ADO), microorganisms can be engineered to convert fatty acid metabolites into esters or hydrocarbon fuels. In addition, some engineered cyanobacteria are capable of synthesizing hydrocarbon fuels under light and carbon dioxide conditions through photosynthetic pathways, enabling an integrated process of carbon capture and fuel synthesis. This approach holds significant potential for reducing greenhouse gas emissions [77].

The greatest advantage of fourth-generation biodiesel lies in the flexibility of its feedstock sources and the tunability of its product properties. On the one hand, its production does not rely on agricultural resources and can utilize non-food raw materials or even industrial waste as substrates, helping to alleviate the combined pressures of energy demand, food security, and environmental sustainability. On the other hand, the molecular structure of synthetic biodiesel can be precisely engineered through metabolic modification to produce fuels with superior combustion properties [78]. For instance, by adjusting carbon chain length, branching structure, or the position of functional groups, it is possible to synthesize biodiesel with lower viscosity, higher cetane number, and better low-temperature fluidity, thereby enhancing its compatibility with diesel engines and improving thermal efficiency.

Although fourth-generation biodiesel exhibits excellent sustainability and technical potential in theory, its commercialization still faces multiple challenges. First, the currently developed engineered microbial strains often suffer from insufficient stability, low yields, and the generation of undesirable by-products during the continuous and efficient synthesis of target fuel molecules. Further metabolic regulation and systems biology analysis are needed to optimize their physiological performance. Second, there are still engineering bottlenecks in the construction of large-scale microbial fermentation systems. Factors such as the high cost of culture media, difficulties in product separation and purification, and high energy consumption all limit the economic feasibility of the process. In addition, some synthetic fuels have low secretion efficiency, leading to intracellular accumulation of products, which may inhibit cell growth or even cause cytotoxicity. Therefore, the development of efficient secretion and recovery mechanisms is urgently needed [79].

At present, fourth-generation biodiesel remains at the laboratory or pilot scale. However, with the advancement of precise gene editing tools such as CRISPR/Cas, automated high-throughput screening platforms, artificial metabolic network modeling, and intelligent regulation systems, this field is progressing rapidly.

3. Production Method of Biodiesel

Biodiesel can be produced through various chemical or biochemical pathways, depending on factors such as the type of feedstock, desired fuel quality, economic considerations, and environmental impact. Among these methods, transesterification is the most widely adopted due to its simple process and high conversion efficiency. However, to overcome certain limitations, alternative methods such as direct esterification, supercritical alcohol methods, and enzyme-catalyzed transesterification have also been increasingly studied and applied.

Table 1. Comparison of common biodiesel production methods.

Type	Applicable Feedstocks	Reaction Conditions	Reaction Time	Advantages	Disadvantages	Refs.
Transesterification	Low FFA oils (FFA < 2%)	60–70 °C, atmospheric pressure	30–60 min	Mature technology, high conversion efficiency, scalable	Soap formation with high FFA; pretreatment often required	[80,81,82]
Direct Esterification	High-FFA oils	60–80 °C, atmospheric pressure	Several hours	Effectively reduces acid value; suitable for pretreatment	Slow reaction; equipment corrosion risk	[83,84,85]
Supercritical Alcohol Process	All types of oils, including high FFA	240–350 °C, >80 bar (high temperature/ pressure)	Minutes to 1 h	Catalyst-free; no soap formation; fast reaction	High energy consumption; complex and costly equipment; safety risks	[86,87,88]
Enzymatic Transesterification	All types of oils, including waste oils	30–60 °C, atmospheric pressure	8–24 h	Mild, eco-friendly, fewer by-products, easy separation	High enzyme cost; sensitive to inhibition; limited reusability	[89,90]

provides a detailed list of the advantages and disadvantages of different production processes.

3.1. Transesterification Reaction

Transesterification is currently the most widely used method for industrial biodiesel production. Its core reaction involves the conversion of triglyceride molecules with low-molecular-weight alcohols, typically methanol or ethanol, in the presence of a catalyst to produce fatty acid alkyl esters, with glycerol as a by-product [80]. This reaction is essentially reversible and theoretically requires one mole of triglyceride to react completely with three moles of alcohol. However, in practical applications, an excess of alcohol is often used (with an alcohol-to-oil molar ratio ranging from 6:1 to 12:1) to drive the reaction towards ester formation and improve the yield [2]. According to the type of catalyst used, transesterification can be classified into alkali-catalyzed, acid-catalyzed, and enzyme-catalyzed (or other heterogeneous catalyzed) reactions. Among these, alkali-catalyzed transesterification is the most common. Catalysts such as sodium hydroxide, potassium hydroxide, or sodium methoxide are typically used, and the reaction can be completed within 1 to 2 h at 60 to 65 °C under atmospheric pressure. This method offers advantages such as fast reaction rates, low cost, and mature technology [82]. However, it requires high feedstock purity, especially with regard to free fatty acid (FFA) content, which must be kept below 1%. Otherwise, saponification may occur, leading to the formation of soap by-products, reduced esterification efficiency, and increased separation costs [83].

In contrast, acid-catalyzed transesterification, using catalysts such as sulfuric acid or hydrochloric acid, is more suitable for feedstocks with high-FFA content. It can simultaneously promote both transesterification and esterification, effectively avoiding saponification. Although its reaction rate is slower and the reaction time is longer (typically 4 to 10 h), it is often employed as a pretreatment step before alkali catalysis to reduce the acid value. In recent years, to enhance environmental friendliness and reaction selectivity, various heterogeneous catalysts, such as solid bases, solid acids, and metal oxides, have been developed. These catalysts are reusable, water-tolerant, and easily separable, making them suitable for continuous production processes [91].

The transesterification reaction mechanism involves a three-step sequence, in which triglycerides are first converted into diglycerides, then into monoglycerides, and finally into fatty acid esters and glycerol. The reaction rate is influenced by multiple factors, including reaction temperature, alcohol-to-oil molar ratio, catalyst concentration, reaction time, and stirring speed [92]. In order to improve conversion efficiency, process intensification techniques such as ultrasound-assisted methods, microwave heating, microreactors, and continuous flow reactors have recently been introduced into industrial production, making transesterification more efficient, environmentally friendly, and adaptable to various feedstocks [93,94].

3.2. Direct Esterification Reaction

Direct esterification is a biodiesel production technique commonly used for the pretreatment of feedstocks with high free fatty acid (FFA) content, particularly suitable for non-refined oils such as waste cooking oil, gutter oil, and animal fats [84]. In this process, FFAs react with low-molecular-weight alcohols, mainly methanol, in the presence of an acidic catalyst to produce fatty acid alkyl esters (biodiesel) and water as a by-product. The reaction is typically carried out at 50 to 70 °C under atmospheric pressure, and common catalysts include concentrated sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), or other strong acids. Since this reaction is reversible, the water generated as a by-product inhibits the forward reaction. Therefore, dehydration techniques, such as the addition of molecular sieves, water-adsorbing agents, or vacuum treatment during the reaction, are often employed to remove water and improve conversion efficiency [95].

The main advantage of direct esterification is its ability to effectively reduce the acid value of the feedstock, which helps prevent saponification in the subsequent alkali-catalyzed transesterification step. This improves the adaptability and yield of the overall biodiesel production process. For this reason, direct esterification is typically used in industrial applications as a pretreatment step for high-FFA feedstocks, converting FFAs into esters before entering the transesterification stage [85]. Compared to transesterification, direct esterification is less sensitive to the triglyceride content of the feedstock, has a more stable reaction system, and can tolerate feedstocks with higher water content. This makes it particularly suitable for the valorization of low-quality oils [86]. However, its main drawbacks include a relatively slow reaction rate, the corrosive nature of acid catalysts, the need for careful post-treatment, and the presence of unconverted lipid components in the reaction product. Additionally, challenges remain in catalyst recovery and process continuity. To improve its industrial performance, recent research has focused on the use of solid acid catalysts, such as sulfonated carbon materials, heteropolyacids, and zeolite-based catalysts, to achieve a greener and more efficient esterification process.

3.3. Supercritical Alcohol Process

The supercritical alcohol process is an emerging biodiesel production technology that does not require the use of catalysts. It is suitable for a wide range of feedstocks, including those with high acid value, high water content, or without pretreatment. The principle of this method is to heat alcohol, usually methanol, to its supercritical state under high temperature and high pressure. Under these conditions, the physical properties of methanol change significantly, enhancing its miscibility and reactivity with oils, and enabling the simultaneous occurrence of transesterification and esterification reactions with high efficiency [87]. Under typical supercritical conditions, with temperatures ranging from 240 to 350 °C and pressures between 20 and 35 MPa [88], the dielectric constant of methanol decreases markedly, while its viscosity and surface tension are significantly reduced. This transforms the reaction system from a two-phase mixture into a single-phase system, greatly accelerating the reaction rate. Moreover, no alkali or acid catalysts are required, effectively avoiding catalyst residues, soap formation, and the complexities of subsequent separation processes [89].

This process exhibits excellent adaptability to various feedstocks, allowing for the simultaneous conversion of free fatty acids and triglycerides. It is especially suitable for the direct conversion of low-quality raw materials such as waste oils, gutter oils, and algal oils, making it a promising route for the valorization of inferior oil resources [96]. However, the supercritical alcohol process requires harsh operating conditions, as it must be performed under high temperature and high pressure. This imposes stringent demands on the pressure resistance, corrosion resistance, and sealing performance of the reaction equipment, leading to high initial investment costs and energy consumption. Additionally, during long-term operation, issues such as equipment fatigue, difficulties in reaction control, and the complexity of methanol recovery systems may arise [97]. Currently, industrial applications are still at the pilot and small-scale demonstration stages. Some studies have also proposed the use of supercritical ethanol or mixed alcohol systems instead of pure methanol to improve reaction flexibility and product properties. Overall, the supercritical alcohol process, as a highly efficient, green, and catalyst-free biodiesel production technology, shows great potential for processing complex feedstocks. Despite the remaining challenges in economic feasibility and engineering implementation, it is expected to become one of the key development directions for the biodiesel industry in the future.

3.4. Enzymatic Transesterification

Enzymatic transesterification is a green, environmentally friendly, and highly selective method for biodiesel production. Using lipase as a catalyst, this process promotes the transesterification reaction between oils and alcohols under mild conditions, resulting in the formation of fatty acid alkyl esters (FAMEs) and glycerol. Compared with conventional acid and alkali catalysis, enzymatic transesterification can efficiently catalyze both the transesterification of triglycerides and the esterification of free fatty acids without triggering saponification. This makes it particularly suitable for handling high-acid-value, unrefined, or water-containing feedstocks, such as waste cooking oil, animal fats, and microalgal oils [90]. The reaction is typically carried out at temperatures between 34 and 45 °C under atmospheric pressure. The molar ratio of alcohol-to-oil is usually optimized according to enzyme activity and feedstock properties, generally ranging from 3:1 to 6:1 [91]. As enzymatic reactions are sensitive to environmental temperature, pH, and alcohol concentration, excessive alcohol may inhibit enzyme activity. Therefore, the reaction system needs to be carefully controlled to maintain high conversion efficiency.

Lipases are available in both free and immobilized forms. Among them, immobilized lipases, such as Novozym 435 and Lipozyme TLIM, have become the core materials for industrial exploration due to their good reusability, high stability, and ease of separation from the reaction system. The main advantages of enzymatic transesterification include mild reaction conditions, high product purity, simplified processes without the need for neutralization or catalyst separation, strong adaptability to high-FFA feedstocks, and the production of high-quality glycerol as a by-product [98]. These features make it particularly suitable for producing high-quality biodiesel and fatty acid ester products for pharmaceutical and cosmetic applications.

However, several technical and economic challenges still hinder its commercialization. The high cost of enzymes, relatively long reaction times, susceptibility to alcohol-induced enzyme deactivation, and limited enzyme stability and reusability are key obstacles to large-scale applications [99]. To overcome these limitations, recent research has focused on developing engineered lipases with high activity, alcohol tolerance, and thermal stability, optimizing immobilization carriers and techniques, and employing multiphase systems or unconventional solvents, such as ionic liquids and deep eutectic solvents, to improve the reaction environment. In addition, the use of ultrasound or microwave-assisted techniques has been explored to enhance reaction rates [100]. Furthermore, novel approaches such as continuous flow enzymatic reactors and membrane separation coupled enzymatic systems have provided feasible solutions for the industrialization of enzymatic biodiesel production.

4. Factors Affecting the Production and Quality of Biodiesel

The production efficiency of biodiesel and the quality of the final fuel are significantly influenced by various process parameters. Among them, the alcohol-to-oil molar ratio, reaction time, and reaction temperature are key factors that determine the efficiency of the transesterification reaction, as well as the yield, purity, and fuel properties of the resulting biodiesel.

Table 2. The influence of different reaction conditions on the yield of biodiesel.

Feedstock	Molar Ratio	Catalyst	Temperature (°C)	Reaction Time	Yield (%)	Refs.
Chicken fat	15:1	CaO/CuFe2O4 nanoparticles	70 °C	4 h	94.52%	[101]
Cotton oil	13:1	Solid basic heterogeneous catalysts	120 °C	6 h	95%	[102]
Jatropha oil	12:1	Functionalized magnetic solid acid catalysts	90 °C	4 h	97.39%	[103]
Tall oil	15:1	H2SO4	55 °C	1 h	96.76%	[104]
Waste cooking oil	9:1	NaOH	40 °C	2 h	98.22%	[105]
Soybean oil	6:1	CH3KO	80 °C	15 min	91%	[106]
Palm oil	10:1	ZnO-silver nanoparticles	80 °C	1 h	97%	[107]
Waste cooking oil	90:1	H3Mo12O40P	190 °C	4 h	94.5%	[108]
Waste shark liver oil	10:1	H2SO4	60 °C	6.5 h	99%	[109]
Castor oil	12:1	transesterification of castor oil using benzimidazolium-based Brønsted acid ionic liquid	70 °C	24 h	59%	[110]

records the yields of several common biodiesel feedstocks under different reaction conditions.

4.1. Molar Ratio

The alcohol-to-oil molar ratio is one of the key process parameters influencing biodiesel yield and reaction kinetics. It is defined as the molar ratio between the alcohol, typically methanol or ethanol, and the oil feedstock, such as vegetable oils or animal fats. This parameter not only directly affects the efficiency of fatty acid methyl ester (FAME) formation, but also influences the purity of the glycerol by-product and the overall economic feasibility of the process.

From a theoretical perspective, the stoichiometric molar ratio for the transesterification reaction is 3:1 [111]. However, using only the theoretical ratio often results in incomplete reactions, leading to low conversion rates and significant amounts of residual monoglycerides (MAGs), diglycerides (DAGs), or unreacted triglycerides in the product. Therefore, in practical production, a molar ratio higher than the theoretical value is commonly employed to drive the reaction toward the formation of the desired products and to improve the final FAME yield [112].

Most studies have shown that, in alkali-catalyzed systems such as those using NaOH, KOH, or sodium methoxide, an alcohol-to-oil molar ratio between 6:1 and 12:1 can achieve high conversion rates. For example, some studies have reported that a 6:1 molar ratio can achieve over 90% conversion within 60 min, while increasing the ratio to 9:1 can result in near-complete conversion in a shorter time. However, the optimal molar ratio varies depending on the type of feedstock. For instance, waste cooking oil, animal fats, or high-acid-value oils containing elevated levels of free fatty acids (FFAs) are more susceptible to side reactions such as saponification. In such cases, a higher alcohol dosage, typically 9:1 or even 12:1, is often required to compensate for the unfavorable reaction kinetics and ensure efficient transesterification [113].

It is worth noting that although increasing the alcohol-to-oil molar ratio can enhance biodiesel yield, excessive alcohol usage also presents certain drawbacks. First, an excess of alcohol can make the separation of glycerol and methyl esters more difficult during post-processing, as glycerol has higher solubility in alcohol-rich environments. This affects both the purity of the final product and the efficiency of glycerol recovery. Second, the excess alcohol must be recovered and reused through processes such as distillation, which increases energy consumption and places additional demand on equipment, thereby affecting the overall economic viability of the process. Moreover, excessive alcohol may also inhibit catalyst activity and reduce its reusability efficiency [114].

In practical applications, the type of alcohol used also has a significant impact on the optimization of the molar ratio. Methanol is widely used due to its strong polarity, low boiling point, and low cost. Ethanol, while offering better renewability and lower toxicity, has poorer compatibility with oils and generally requires a higher molar ratio to achieve comparable conversion rates. In recent years, researchers have also explored the use of higher alcohols such as isopropanol and butanol. However, due to their limited reactivity and miscibility, these alcohols are currently used mainly in small-scale studies under specific conditions [115].

In summary, the optimization of the alcohol-to-oil molar ratio requires a comprehensive consideration of multiple factors, including the type of feedstock, the choice of catalyst, the type of alcohol, and the reaction conditions. In industrial-scale production, greater attention should be given to balancing raw material costs, alcohol recovery systems, energy consumption, and equipment investment to achieve both technical and economic optimization.

4.2. Time of Reaction

Reaction time is an important process parameter that affects the efficiency of the transesterification reaction and the final yield of biodiesel. Its optimization is essential for achieving a production process that is efficient, cost-effective, and sustainable. In the transesterification reaction, triglycerides react with alcohol in the presence of a catalyst to produce fatty acid methyl esters (FAMEs) and glycerol. The duration of the reaction not only determines the extent of conversion, but also influences the likelihood of side reactions and the overall energy consumption level [116].

In general, appropriately extending the reaction time helps improve the conversion of triglycerides, especially during the initial stage of the reaction when the reaction rate is relatively fast. However, when the reaction time exceeds a certain threshold, the improvement in yield tends to slow down as the system gradually reaches equilibrium, making further extension of the reaction time less beneficial [117]. In addition, when the feedstock contains a high level of free fatty acids (FFAs), an excessively long reaction time may induce side reactions such as saponification, increasing the difficulty in subsequent separation processes and negatively affecting product quality and process stability [118].

Under alkali-catalyzed conditions, using catalysts such as NaOH, KOH, or sodium methoxide, the transesterification reaction can typically be completed within a relatively short time due to the high catalytic efficiency. Studies have shown that at a reaction temperature of around 60 °C, a reaction time from 30 to 60 min is sufficient to achieve a conversion rate of over 90%. In contrast, acid-catalyzed systems, such as those using sulfuric acid or phosphoric acid, generally require longer reaction times, typically ranging from 2 to 6 h or more, especially when processing high-FFA or high-viscosity feedstocks. Moreover, when enzymatic or solid catalysts are used as alternatives, the reaction mechanism and mass transfer conditions differ, often resulting in even longer reaction times [119].

The optimal selection of reaction time is influenced by multiple factors, including but not limited to the type and concentration of the catalyst, the physicochemical properties of the feedstock, reaction temperature, alcohol-to-oil molar ratio, and mixing conditions. For example, the use of highly active catalysts can accelerate the reaction, thereby reducing the required reaction time. Operating at elevated temperatures can also enhance the reaction rate, although careful consideration must be given to the risk of alcohol evaporation and its impact on the overall reaction time.

In industrial production, the proper control of reaction time not only affects the production cycle but also has a direct impact on energy consumption, equipment utilization, and overall economic performance. An excessively short reaction time may lead to incomplete conversion, compromising product quality, while an overly long reaction time increases energy consumption and reduces production efficiency.

4.3. Temperature of Reaction

Reaction temperature is one of the key parameters influencing the reaction rate and conversion efficiency of the transesterification process in biodiesel production. Temperature not only directly promotes reaction kinetics but also affects mass transfer behavior, the compatibility of reactants, catalyst activity, and the properties of the final product [120]. Properly increasing the reaction temperature typically helps accelerate the reaction rate, shorten the reaction time, and improve biodiesel yield. However, improper temperature control may lead to alcohol evaporation, catalyst deactivation, or the occurrence of undesirable side reactions, and may even pose safety risks to equipment operation.

In alkali-catalyzed transesterification reactions, the commonly used reaction temperature ranges from 50 °C to 65 °C, which is close to the boiling point of methanol (64.7 °C). Within this temperature range, the miscibility between methanol and oils is significantly improved, and the viscosity of the reactants is reduced, which enhances mass transfer and the contact between the oil and alcohol phases, thereby facilitating the transesterification process [121]. Studies have shown that under moderate catalyst concentration and an appropriate alcohol-to-oil molar ratio, conducting the reaction at around 60 °C can achieve a fatty acid methyl ester conversion rate of over 90% within 30 to 60 min. However, if the temperature exceeds the boiling point of methanol, methanol vaporization becomes significant, leading to an increase in system pressure. This not only causes raw material losses but also affects reaction controllability and poses safety risks to the equipment [122]. Therefore, in alkali-catalyzed systems, excessively high temperatures should be avoided. It is common practice to use sealed reactors or pressurized systems to maintain solvent stability during the reaction.

In contrast, acid-catalyzed transesterification exhibits stronger adaptability, especially for feedstocks with high free fatty acid (FFA) content, such as waste cooking oil, animal fats, or industrial by-product oils. However, acid catalysts, such as sulfuric acid and phosphoric acid, have lower catalytic activity and are more dependent on temperature [123]. Typically, higher temperatures ranging from 70 °C to 100 °C or above are required, and the reaction needs to proceed for several hours to achieve a satisfactory conversion rate. Under high-temperature conditions, the viscosity of the oil decreases and molecular motion is intensified, which promotes the reaction. Nevertheless, it is essential to control alcohol evaporation and ensure the pressure tolerance of the equipment to maintain system stability.

In addition, when enzymatic catalysts such as lipases or solid catalysts such as metal oxides and heteropolyacids are used, the selection of reaction temperature becomes even more critical. The catalytic activity of these catalysts is optimal within a specific temperature range. Excessively high temperatures may cause enzyme deactivation or structural degradation of solid catalysts, while excessively low temperatures may lead to slow reaction rates and prolonged reaction times, ultimately affecting process efficiency [124]. For example, in lipase-catalyzed reactions, the optimal reaction temperature is usually between 35 °C and 50 °C, and mild conditions need to be maintained for an extended period to ensure a satisfactory conversion rate [125].

In recent years, some studies have proposed the use of auxiliary energy techniques, such as microwave heating, ultrasonic irradiation, and infrared radiation, to improve the flexibility and efficiency of temperature control. For instance, microwave-assisted heating can rapidly raise the reaction temperature in a short time, forming localized “hot spots” that enhance molecular collisions between reactants, thereby increasing conversion rates and shortening reaction times.

5. Catalysts

In most current biodiesel production processes, catalysts play a vital role by significantly increasing reaction rates and reducing reaction time. Although supercritical transesterification can proceed without the use of catalysts, it requires high temperatures, high pressures, and extended reaction durations, which substantially increase production costs. Especially in commercial applications, the use of catalysts is indispensable. Among the transesterification processes currently in use, the most commonly employed catalyst types include homogeneous catalysts and heterogeneous catalysts.

5.1. Homogeneous Catalyst

Homogeneous catalysts have been extensively utilized in industrial scale biodiesel production due to their high catalytic activity, low cost, and ease of operation. As these catalysts exist in the same phase as the reactants, typically the liquid phase, they promote better miscibility and molecular interaction, which enhances the reaction rate and reduces processing time. In transesterification, the key step in biodiesel synthesis, the use of homogeneous catalysts significantly improves both conversion efficiency and reaction kinetics. Based on their chemical characteristics, homogeneous catalysts are generally categorized into two main types: alkaline catalysts and acidic catalysts.

Common alkaline homogeneous catalysts include sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium methoxide (CH₃ONa). These catalysts exhibit excellent catalytic activity under atmospheric pressure at temperatures between 60 and 65 °C, typically completing the transesterification reaction within 30 to 60 min, with fatty acid methyl ester (FAME) yields exceeding 90 percent. In addition to their high efficiency, these catalysts are inexpensive and easy to operate and control, making them the most widely used catalytic system in current biodiesel production. However, their performance is highly dependent on feedstock quality, especially the free fatty acid (FFA) content, which must be maintained below 1 percent to avoid saponification reactions with the alkaline catalyst, resulting in the formation of soap by-products [126]. This not only reduces catalytic efficiency but also complicates phase separation, thereby increasing downstream purification costs [127]. Moreover, soap formation can lead to catalyst deactivation and emulsion formation in the reaction mixture, which can adversely affect the stability of continuous industrial operations [128].

In contrast, acidic homogeneous catalysts such as sulfuric acid (H₂SO₄) and hydrochloric acid (HCl) exhibit slower reaction rates and relatively lower catalytic activity, but they offer distinct advantages when processing feedstocks with high free fatty acid (FFA) content, such as waste cooking oil and animal fats. Acid catalysis can facilitate not only transesterification but also simultaneous esterification, converting FFAs into biodiesel and thereby effectively preventing saponification side reactions and improving overall conversion efficiency [129,130]. However, acid-catalyzed reactions typically require higher catalyst loading and longer reaction times, generally ranging from 4 to 10 h. These conditions impose more stringent requirements on the corrosion resistance of equipment, increasing the risk of material degradation and operational safety issues. Additionally, neutralization steps are required after the reaction to remove residual acidic species, which adds to process complexity and operating costs. As a result, acidic homogeneous catalysts are primarily used in pretreatment stages, particularly for reducing the acid value of high-FFA feedstocks before undergoing alkali-catalyzed transesterification, thus ensuring the efficiency and reliability of the subsequent reaction.

Shurooq T. Al-Humairi et al. [131] proposed an integrated foam column and reactive extraction system using the cationic surfactant CTAB to enrich and disrupt Chlorella vulgaris cells. Without requiring a drying step, the system achieved a FAME yield of 97 percent using KOH as a catalyst under optimized conditions of a methanol-to-lipid molar ratio of 1000 to 1 at 60 °C for 10 min. This method demonstrated strong tolerance to high water and FFA contents, significantly reducing energy consumption and simplifying the process. In another study, Ramachandran Kasirajan [44] employed a two-step approach involving acid-catalyzed esterification followed by base-catalyzed transesterification to convert crude oil containing 2.7 percent FFA. After sulfuric acid pretreatment, the optimized conditions (methanol-to-oil molar ratio of 9 to 1, 1 wt% KOH, 500 rpm, 40 min, 65 °C) yielded 99.2 percent biodiesel meeting ASTM standards. Similarly, Majid Mohadesi et al. [132] produced biodiesel from waste cooking oil (WCO) using a semi-industrial microreactor. After acid pretreatment to reduce FFA, they applied response surface methodology (RSM) to optimize key variables, achieving a FAME purity of 98.26 percent under 62.4 °C, 9.4 to 1 methanol-to-oil ratio, and 1.16 wt% KOH. Although homogeneous catalysts are typically difficult to recover, Vincenzo Benessere et al. [133] developed a novel zinc-based supported homogeneous catalyst that achieved over 90 percent FAME yield at 160 °C within 2 h for various acidic oils. The catalyst was easily recoverable, reusable over multiple cycles without significant loss of activity, and exhibited minimal metal leaching. Additionally, Chia-Hung Su [134] evaluated several homogeneous acid catalysts, demonstrating that among sulfuric, nitric, and hydrochloric acids, only hydrochloric acid could be fully retained in the methanol phase post-reaction, enabling effective recovery and reuse. Under optimized RSM conditions (76.67 °C, methanol-to-FFA molar ratio of 7.92, 0.54 M catalyst concentration, and 103.57 min reaction time), a maximum conversion of 98.19 percent was achieved. The study further confirmed hydrochloric acid’s catalytic stability and tolerance under high-moisture conditions, reinforcing its practical applicability in biodiesel systems.

5.2. Heterogeneous Catalyst

Heterogeneous catalysts offer several advantages over homogeneous systems, particularly in terms of reusability, ease of product separation, and environmental compatibility. These catalysts exist in a different phase from the reactants, typically as solids, and can be readily separated from the reaction mixture by simple filtration. With the growing demand for continuous production processes and reduced wastewater generation, heterogeneous catalysts have attracted increasing attention in the field of biodiesel synthesis.

Heterogeneous catalysts are generally classified into three categories: alkaline, acidic, and bifunctional catalysts. Common alkaline heterogeneous catalysts include metal oxides such as calcium oxide (CaO), magnesium oxide (MgO), and barium oxide (BaO), as well as alkali and alkaline earth metal carbonates such as potassium carbonate (K₂CO₃), sodium carbonate (Na₂CO₃), and calcium carbonate (CaCO₃). These catalysts exhibit high catalytic activity and are particularly suitable for feedstocks with low free fatty acid (FFA) content.

Acidic heterogeneous catalysts, such as sulfonated carbon materials, heteropoly acids, and ion exchange resins, are more suitable for feedstocks with high free fatty acid (FFA) content. These catalysts are capable of simultaneously catalyzing both esterification and transesterification reactions and exhibit greater resistance to deactivation by water and FFAs. However, they typically require longer reaction times and higher operating temperatures [135].

Recent research has focused on the development of nanostructured and bifunctional heterogeneous catalysts that combine both acidic and basic active sites to enhance catalytic efficiency and adaptability. Materials such as titanium dioxide (TiO₂), zinc oxide (ZnO), and zirconium dioxide (ZrO₂) have attracted considerable attention due to their high surface area and thermal stability, enabling efficient catalysis under relatively mild conditions [136,137]. Additionally, supporting solid catalysts on suitable carriers can improve their mechanical strength and facilitate recovery and reuse.

According to the findings of Shaheen et al. [138], metal oxide catalysts such as calcium oxide (CaO), magnesium oxide (MgO), and titanium dioxide (TiO₂) exhibit high ester yields, reusability, and relatively low cost, while also improving the purity of glycerol as a by-product. However, these alkaline catalysts are sensitive to the free fatty acid (FFA) content in the feedstock. When the FFA level exceeds 2 percent, saponification reactions may occur, leading to catalyst deactivation through the leaching of active sites, which ultimately affects catalytic efficiency and stability. Elsie Bet-Moushoul et al. [139] developed a CaO-based heterogeneous catalyst loaded with gold nanoparticles (CaO–AuNPs) for biodiesel synthesis. Their results demonstrated superior biodiesel quality compared to conventional catalysts and excellent reusability, with the catalyst maintaining high activity after more than 10 consecutive cycles. Similarly, Binta Hadi Jume et al. [140] synthesized a GO@ZrO₂–SrO nanocomposite catalyst that exhibited outstanding reusability. The study further reported fast mass transfer rates, low synthesis cost, and high catalytic efficiency, achieving a biodiesel yield of 91 percent under a methanol-to-oil molar ratio of 4 to 1, at 120 °C for 90 min. Bishwajit Changmai et al. [141] developed a magnetically responsive nano alkaline catalyst that could be efficiently recovered and reused via an external magnetic field. After nine cycles, the catalyst retained both high activity and structural integrity. Its unique core–shell solid structure also enhanced overall performance and operational stability, yielding up to 98 percent biodiesel under the tested conditions. Additionally, Alex Tangy et al. [142] emphasized in their review the high oxidative activity of SrO catalysts as a key factor contributing to their effectiveness in biodiesel production. Despite these promising advancements, heterogeneous catalysts still face challenges such as metal leaching, reduced catalytic activity after multiple cycles, and relatively high synthesis costs. These limitations must be addressed before their widespread commercialization can be achieved [143].

6. Life Cycle Assessment

Life cycle assessment (LCA) encompasses the entire biodiesel production process—from raw material acquisition, production, and processing to final combustion—allowing for a systematic quantification of key indicators such as carbon emissions, energy input, and environmental impact. Compared to conventional fossil diesel, biodiesel offers significant greenhouse gas (GHG) reduction benefits over its full life cycle, particularly due to carbon sequestration during feedstock cultivation and the reuse of waste-based resources, which enhance its environmental performance.

During the feedstock cultivation phase, energy crops such as microalgae and jatropha absorb large amounts of CO₂ through photosynthesis, effectively offsetting the GHG emissions generated during combustion [144]. Microalgae-based systems, in particular, not only possess strong carbon capture capabilities but can also utilize industrial flue gases as carbon sources [145], further improving carbon mitigation efficiency. The use of second-generation feedstocks, such as waste cooking oil and animal fats, can also reduce feedstock costs while preventing the environmental pollution caused by improper waste disposal, thereby optimizing the environmental profile of the entire life cycle.

However, the preprocessing of raw materials, catalyst preparation, transesterification reaction, product purification, and transportation still require substantial energy input and contribute to carbon emissions. For instance, processes such as supercritical alcohol transesterification, which operate under high-pressure and high-temperature conditions, or those requiring excess alcohol recovery, can significantly increase energy consumption and equipment costs. Additionally, the production and recovery of certain catalysts (e.g., sulfuric acid, ionic liquids) may pose potential environmental burdens. Therefore, optimizing process design and improving resource efficiency are essential to reducing the life cycle environmental impact.

To achieve sustainability goals, optimizing each stage of the biodiesel supply chain is crucial. Strategies such as improving alcohol recovery, using low-toxicity and recyclable catalysts, employing ultrasound or microwave-assisted technologies to reduce energy use, and implementing waste heat recovery systems can all enhance life cycle efficiency [146].

7. Conclusions

As a renewable and environmentally friendly fuel, biodiesel has demonstrated substantial potential in addressing the challenges posed by fossil fuel depletion and climate change. Over the past decades, significant progress has been made in the development of biodiesel feedstocks, production technologies, and catalytic systems, all aimed at improving fuel performance, economic viability, and environmental sustainability.

The evolution of biodiesel feedstocks from first-generation edible oils to fourth-generation synthetic biofuels has reflected a clear trend toward sustainability, resource diversification, and land-use efficiency. First-generation feedstocks, although widely utilized, raise concerns about food security and ecological degradation. In contrast, second-generation feedstocks such as non-edible oils and waste fats offer more sustainable alternatives but are constrained by regional availability and quality variability. Third-generation feedstocks, particularly microalgae, offer promising advantages in oil yield and carbon capture, yet still face considerable technical and economic barriers. Fourth-generation synthetic feedstocks, enabled by advances in synthetic biology and metabolic engineering, represent a highly flexible and sustainable direction, but remain in the experimental phase due to limitations in microbial productivity and system scalability.

From a production perspective, transesterification remains the most widely adopted method due to its simplicity and high efficiency. Nevertheless, alternative approaches such as direct esterification, supercritical alcohol processes, and enzymatic transesterification have demonstrated their unique advantages in processing high-FFA or low-quality feedstocks, and in achieving greener, more selective, and less energy-intensive conversion pathways. The choice of production method is highly dependent on feedstock type, process economics, and desired fuel quality.

Catalysts continue to play a critical role in optimizing biodiesel synthesis. Homogeneous catalysts, especially alkaline types, dominate industrial practice due to their low cost and high activity, but suffer from issues related to soap formation and post-treatment complexity. Heterogeneous catalysts, particularly nanostructured and bifunctional types, offer improved reusability and environmental compatibility, though challenges such as leaching and deactivation remain. Enzymatic catalysts, while attractive for their selectivity and mild operating conditions, require further advancements in cost reduction and operational stability to enable industrial adoption. Furthermore, key reaction parameters including alcohol-to-oil molar ratio, reaction time, and temperature must be carefully optimized to maximize yield and minimize energy input.

Life cycle assessment (LCA) results indicate that biodiesel, particularly when derived from waste-based or microalgal sources, significantly reduces greenhouse gas emissions compared to petroleum diesel. However, energy consumption and environmental burdens associated with catalyst use, alcohol recovery, and process heating must be addressed through integrated design, improved recovery systems, and cleaner catalytic alternatives.

Looking ahead, the sustainable advancement of biodiesel will rely on the development of low-cost and environmentally friendly feedstocks, green and efficient catalytic systems, and advanced process intensification technologies. Particular attention should be given to microalgae-based and synthetic biodiesel, which offer high lipid yields, carbon sequestration potential, and flexibility in feedstock sourcing. Breakthroughs in strain engineering, cultivation systems, and energy-efficient lipid extraction will be essential to make microalgal biodiesel commercially viable. At the same time, the design of robust, recyclable catalysts such as nanostructured metal oxides, bifunctional solids, and immobilized enzymes will help improve reaction efficiency while minimizing environmental impacts. Process intensification methods, including ultrasound- and microwave-assisted transesterification and membrane-integrated systems, are expected to reduce energy consumption and enhance scalability. Furthermore, the full valorization of biomass residues and the integration of biodiesel production with carbon capture, wastewater treatment, and renewable energy systems will promote a more circular and sustainable biofuel economy.