Efficient and Sustainable Biodiesel Production via Transesterification: Catalysts and Operating Conditions

Abstract

Biodiesel has received tremendous attention as a sustainable energy source. This review presents an overview of various catalysts utilized in biodiesel production and compares their potential for producing biodiesel. Presented here are the excellent features of the various catalysts while highlighting their drawbacks. For instance, production of biodiesel with homogeneous base catalysts is easy but it can only be used with refined oils having low levels of free fatty acid (FFAs). When homogeneous acid is used in esterification, it causes reactor corrosion. Water and FFAs do not affect heterogeneous acid catalysts. Thus, transesterification of triglycerides into biodiesel and converting FFAs into biodiesel through esterification can be catalyzed more efficiently using a heterogeneous acid catalyst. Biocatalysts are also being used to produce biodiesel from oils with high FFAs. However, heterogeneous acid catalysts and biocatalysts are not suitable for industrial application due to serious mass transfer limitations. Biodiesel yield and conversion were compared over various catalysts in this paper. Also presented are the effects of different reaction parameters on biodiesel yield over different catalysts. The correct interplay of factors like reaction temperature, time, alcohol-to-oil molar ratio, and catalyst loading produces optimal process conditions that give the highest biodiesel yield.

1. Introduction

The increased demand for fossil fuels and the need for a sustainable environment have become areas of great global concern. The world depends on the availability of fuels for its burgeoning population, which in turn desires a better quality of life. The yearly world consumption of primary energy is about 12.5 billion toe (ton of oil equivalent; 1 toe = 41.87 GJ), and with the growing population, the price of crude oil will increase and directly affect the global economy [1,2]. At present, petroleum-based fuels account for about 85% of primary energy consumption, 58% of which is utilized in the transport sector [3,4]. The mass utilization of non-renewable energy sources has raised concern for the option of renewable energies because of the detrimental environmental consequences, which incorporate global warming and serious climate changes [5,6]. The combustion of fossil-derived fuels produces approximately 98% of carbon emissions [7]. If the world continues to use non-sustainable energy sources for energy generation, it will not be wrong to assume there will be a time when this world will not be an ideal place to live. Therefore, in order to preserve our environment for future generations, it is necessary to take some measures in curtailing our demand for fossil fuels.

Sustainable development is the key issue, which has attracted significant attention and concern over the last few decades. Researchers are interested in developing potential renewable energy sources because of the declining fossil fuel reservoirs with an increase in crude oil prices and relatively rapid growth in the global population. For long-term environmental and economic growth, transitioning from energy dominated by fossil fuel to sustainable technologies based on renewable energy sources is now essential. This transition is not only challenging for researchers, but has economic and environmental implications [8]. The diversification of energy sources by transitioning from fossil-fuel-dominated energy to an energy portfolio covering a variety of renewable fuels would meet global energy demand [9]. Biodiesel has gotten a lot of attention as a green fuel because of its clean combustion characteristics and its sustainable sources [10,11]. In addition to being biodegradable, it is free of sulfur, has lower exhaust emission, and is free of aromatics [11]. Biodiesel production is a rapidly increasing biofuel technology since it provides a set of environmental advantages over traditional fossil fuels. Biofuel is employed as a first-generation fuel. According to a survey published by Zion Market Research, the global biofuel market is estimated to be USD 219 billion in 2022 [12]. Biodiesel is obtained from natural fats and oils and could be used in place of conventional fossil-based diesel without modifying the current diesel engines. Biodiesel offers several advantages over petroleum diesel, as illustrated in Figure 1.

Biodiesel is known for its higher lubricity compared to conventional diesel. Lubricity is crucial for reducing friction and wear in fuel injection systems, which is particularly beneficial for modern engines with high-pressure injection systems. The oxygen content in biodiesel enhances its lubricating properties, thereby protecting engine components and extending their lifespan. Regarding cetane number, which measures the combustion quality of diesel fuel, biodiesel typically has a higher cetane number than petroleum diesel. A higher cetane number means that biodiesel ignites more readily, leading to smoother engine operation, reduced noise, and lower emissions of unburned hydrocarbons and particulate matter [13]. Biodiesel also has positive effects on air quality and environmental sustainability. Due to its oxygen content, biodiesel burns more completely than fossil diesel, resulting in reduced emissions of carbon monoxide (CO), particulate matter (PM), and unburned hydrocarbons (HC). Additionally, biodiesel has lower sulfur content, leading to decreased sulfur oxide (SO_x) emissions. Biodiesel is biodegradable and non-toxic, which reduces environmental impact in case of spills. Its use also lowers net carbon dioxide (CO₂) emissions, as the CO₂ released during combustion is offset by the CO₂ absorbed by the plants used for biodiesel production, contributing to a lower overall carbon footprint.

To produce biodiesel from fats and oils, four distinct methods are used: pyrolysis, hydrocarbon blending dilution, transesterification, and emulsification [9]. The transesterification method is mainly used to produce biodiesel from different kinds of fats and oils worldwide because the transesterification method produces fuel that is highly compatible with the diesel engines that are currently in use [8]. The ultimate effect of transesterification is to bring the oil’s viscosity down to the level of fossil diesel fuel [10]. Transesterification is a method of producing biodiesel that involves the reaction of plant-based triglycerides, recycled oil waste, or animal fats with alcohol in the presence of an effective catalyst to generate alkyl esters (biodiesel), with glycerol produced as a byproduct (Figure 2a) [14,15]. In general, three sequential reversible reaction steps occur during this transesterification reaction. First, diglyceride is formed through the conversion of triglyceride to diglyceride, and then diglyceride is converted to monoglyceride. The process of converting monoglyceride to glycerol follows. At each step of the reaction, biodiesel (a fatty acid methyl ester, FAME) is produced. Therefore, the method produces a total of three fatty acid methyl esters [14]. Figure 2b illustrates the complete chemical reaction used to produce biodiesel. In general, for triglyceride transesterification reaction, methanol is the favored alcohol because it is less costly and can react quickly with triglycerides owing to its advantages, which include high polarity and being the alcohol with the shortest alkyl chain [16]. The removal of glycerol is crucial for ensuring high-quality biodiesel. One common method is settling, which allows the biodiesel and glycerol to separate naturally over time due to their different densities, with the denser glycerol settling at the bottom and being removed by decantation. Centrifugation accelerates this separation process by using centrifugal force to effectively separate the biodiesel from the glycerol based on their density differences [17]. Filtration techniques can also be employed to remove impurities, including glycerol, from the biodiesel and can be used in combination with other separation methods [18]. Additionally, chemical treatment involves using chemical agents, such as acids or bases, to react with glycerol and facilitate its removal by altering its properties for easier separation [18,19]. These methods collectively ensure the effective removal of glycerol, resulting in high-quality biodiesel suitable for use.

2. Biodiesel Production Feedstock

A wide variety of feedstock can be used to produce biodiesel. The feedstock cost account for approximately 75% of the total biodiesel production costs [20]. The main issues surrounding biodiesel commercialization are feedstock prices and production costs [21]. Thus, in order to achieve low biodiesel production costs, it is important to choose suitable feedstock that are fairly cheap to produce biodiesel.

Biodiesel feedstock comes in a variety of forms: non-edible oil, edible oil, algal lipid, and waste cooking oil (Figure 3) [22,23,24,25]. Currently, almost all biodiesel production begins with edible oil as a feedstock because it is widely available from the established agricultural sector. In Malaysia, Europe, and the United States, edible oils are the main feedstock utilized in the production of biodiesel. However, extensive use of edible oil in biodiesel production can put the entire world at risk, in which the slightest planning error can have a significant impact on food supply and the economy [1]. The limited agricultural land typically available for growing food would be compromised when the fuel economy and the food economy are both in competition for the same resources. Based on these circumstances, the production of biodiesel from edible food crop feedstocks can cause serious food supply issues in developing countries due to competition for the same resources [26].

Non-edible oils like Neem, Jatropha, Karanja, Castor, and Mahua have been identified as suitable feedstock for producing biodiesel [27]. Non-edible oil biodiesel may be deemed to have good prospects such as acceptable fuel quality, excellent engine efficiency, and desirable exhaust emission properties [28]. But biodiesel produced from non-edible oil incorporates both favorable and unfavorable attributes. Biodiesel is produced using non-edible oil as a feedstock because of its ready availability, lower aromatic content, biodegradability, and lower sulfur content [29]. However, non-edible oil is unfavorable as biodiesel feedstock due to the higher reactivity of its unsaturated fatty acid chains, lower volatility, higher viscosity, and carbon residue [29]. Micro algae are becoming increasingly popular as a feedstock for biodiesel production across the globe [30]. They have become a key feedstock for producing biodiesel because of its various benefits, including higher oil productivity, rapid growth rate than conventional crops, which are of course augmented by limited available land [30].

Waste cooking oil (WCO) could be used as a biodiesel feedstock instead of edible oil. WCO is released directly into the waterway or accumulated in the ground in most cafeterias, hotels, and other service industries [31]. Producing biodiesel from WCO presents three possible solutions: waste management, environmental, and economic [32]. It is environmentally friendly to produce biodiesel from WCO because it recycles used oil and also provides sustainable energy with lower emissions. It also reduces waste management costs. By utilizing WCO, the expense of producing biodiesel could be decreased by up to 70% [7]. Also, waste disposal concerns could be solved by producing biodiesel from WCO.

3. Types of Catalysts

As mentioned previously, transesterification of vegetable oil, animal fat, and recycled waste oil with alcohol in the presence of a suitable catalyst produces methyl ester of long-chain fatty acid (biodiesel), with glycerol as a byproduct [33]. During the transesterification reaction, a catalyst plays a key role in providing an active site for the reactants and reduces the reaction’s total activation energy. In general, production of biodiesel involves the use of different kinds of catalysts harnessed for the transesterification of triglycerides. These include homogeneous and heterogeneous catalysts, as well as biocatalysts (Figure 4).

3.1. Biocatalysts

Catalysts that are derived from living organisms are known as biocatalysts and are often referred to as enzyme catalysts. Almost all biochemical reactions taking place in living cells are initiated by these catalysts. Each enzyme has a unique three-dimensional structure that corresponds to the reactant’s shape. In the study of biodiesel production, biocatalysts have lately become increasingly crucial [34]. The use of enzyme catalysts aims to address the problems posed by the use of synthetic catalysts, including a high amount of energy demands, feedstock pretreatment requirement, high costs, and complex procedures. Biocatalyst studies demonstrate an environmentally sustainable strategy for the production of biodiesel employing renewable material and polymer under moderate reaction conditions [34]. Typically, the enzyme normally utilized as biocatalyst is immobilized on inert materials that enable it to withstand the change in temperature and moisture content. Covalent bonding, cross-linking, and encapsulation were among the immobilization techniques documented in the preparation of biocatalysts [35]. However, in the typical transesterification reaction, the use of polar molecules like water and methanol limits the enzyme’s catalytic performance. This is alleviated via the stepwise addition of an organic solvent such as ethanol [36]. All biocatalysts do not require harsh conditions to function; however, strict hydration control is necessary since they are water-sensitive (they can be activated with small amount of water) [37]. The catalytic potential of enzymatic catalysts has been demonstrated in the transesterification process used to make biodiesel that employ a class of enzymes referred to as lipases, which are obtained from plants, microorganisms, and animals [38,39]. Lipases may be isolated from various bacterial species, including Rhizopus oryzae, Rhizomucor miehei, Pseudomonas fluorescens, Expansum lipase, and Pseudomonas cepacian [40]. Lipases are capable of catalyzing both the transesterification of triglycerides and hydrolysis under moderate conditions, making them suitable for biodiesel production [41,42,43]. Their special features, such as enantioselectivity, regioselectivity, and specificity, enable them to catalyze reactions at moderate pressures and temperatures while minimizing byproduct formation [44,45]. Enzymatic biodiesel is often promoted as a more environmentally friendly alternative to conventional biodiesel production methods due to its reduced energy consumption and lower production of chemical byproducts [42].

In the last few decades, the advantages associated with enzymatic transesterification, especially those using lipases, have made many researchers shift attention from chemical transesterification [40]. Several biocatalysts for transesterification have been reported in the literature over the last few decades. For instance, Shah et al. [36] examined the performance of immobilized Pseudomonas cepacia in the production of jatropha curcas oil biodiesel. It was observed that a 98% biodiesel yield was obtained using the enzyme at a temperature of 50 °C, 5 wt.%, 4:1 MREO in 8 h. Another study presented the use of immobilized Burkholderia cepacia for the production of jatropha curcas oil biodiesel, which under optimal conditions achieved a maximum biodiesel yield of 100% with 10:1 MREO, 5.25 g catalyst loading, and 35 °C in 24 h [37]. The catalytic transesterification of WCO over a pancreatic lipase catalyst was examined by Jayaraman et al. [46]. The optimum oil conversion using this catalyst was 88% with 3:1 MRMO, 1.5 wt.% catalyst amount, and 60 °C in 4 h. Marin-Suarez et al. [47] utilized Novozym 435 lipase as biocatalyst for transesterification of residual fish oil using ethanol. The authors reported that the Novozym 435 lipase catalyst showed excellent transesterification performance, where a maximum biodiesel yield of 82.91% was obtained for the reaction conditions of 50 wt.% catalyst and 36:1 MREO at 35 °C in 8 h. Choi et al. [48] used rice bran lipase as biocatalyst for the production of rice bran oil biodiesel. The authors found that the biocatalyst provided a yield of 83.4% with 3 wt.% catalyst and 6:1 MRMO at 40 °C in 12 days. Another research focused on the usage of Lipozyme Thermomyces lanuginose IM in the transesterification of corn oil, which under optimal process conditions gave a maximum yield of 98.95% with 6:1 MREO, 2.8 wt.% catalyst amount, and 35 °C in 12 h [49]. Arumugam et al. [50] transesterified Calophyllum inophyllum using Rhizopus oryzae lipase. They found that the Rhizopus oryzae lipase catalyst showed excellent transesterification performance, where a biodiesel yield of 92% was obtained using the following conditions: 20 wt.% catalyst amount, 35 °C reaction temperature, and 12:1 MRMO in 72 h. Li et al. [51] also produced biodiesel from Pistacia chinensis bge seed oil using Rhizopus oryzae lipase catalyst. The authors found that the Rhizopus oryzae lipase catalyst showed good performance in the transesterification of Pistacia chinensis bge seed oil and provided a maximum biodiesel yield of 94% at a temperature of 37 °C in 60 h, 5:1 MRMO, and 7 wt.% enzyme dosages. Caballero et al. [52] transesterified sunflower oil over immobilized Pig pancreatic lipase biocatalyst using ethanol under reaction conditions of temperature T= 60 °C, time t = 19 h, 0.1 wt.% of catalyst, and 2:1 MREO, giving a maximum biodiesel conversion of 57.2%. In another study, Rhizomucor miehei lipase was utilized in the biodiesel production from triolein, which under optimal process conditions (5.0 wt.% catalyst, 3:1 MREO, and 40 °C in 96 h) achieved maximum biodiesel yield of 77% [53]. Elsewhere, Kamini et al. [54] utilized Cryptococcus spp. S-2, as biocatalyst for transesterification of rice bran oil in the presence of methanol. The Cryptococcus spp. S-2 catalyst showed excellent transesterification performance, giving a maximum biodiesel yield of 80.2% under the following conditions: temperature T= 30 °C, time t = 120 h, and 4:1 MRMO. Martin Mittelbach [55] examined the catalytic performance of Pseudomonas lipase for the production of biodiesel from sunflower oil in the presence of methanol. The author found that a maximum biodiesel yield of 80% was obtained following the addition of oleic acid to sunflower oil using Pseudomonas lipase enzyme under the optimum process conditions (time t = 5 h, 10:1 MRMO, and temperature T= 45 °C). Alonazi et al. [56] transesterified coffee oil over CaCO₃- immobilized Staphylococcus aureus and Bacillus stearothermophilus biocatalyst using methanol under reaction conditions of temperature T= 55 °C, time t = 12 h, 10 wt.% of enzyme catalyst, and 4:1 MRMO; a maximum biodiesel conversion of 97.66% was obtained using this combination of immobilized lipases.

Table 1. Some studies on biodiesel production using biocatalysts.

Catalyst	Feedstock	Reaction Parameters (Time, Temperature, Catalyst Amount, and Alcohol to Oil Molar Ratio)	Performance	Ref.
Novozym 435 lipase	Residual fish oil	8 h, 35 °C, 50 wt.%, 36:1	Yield = 82.91%	[47]
Pseudomonas cepacian	Jatropha curcas	8 h, 50 °C, 4:1	Yield = 98%	[36]
Pancreatic lipase	Waste cooking oil	4 h, 60 °C, 1.5 wt.%, 3:1	Conversion = 88%	[46]
Burkholderia cepacia	Jatropha curcas	24 h, 35 °C, 5.25 g, 10:1	Yield = 100%	[37]
Rice bran lipase	Rice bran oil	288 h, 40 °C, 3 wt.%, 6:1	Yield = 83.4%	[48]
Lipozyme Thermomyces lanuginose IM	Corn oil	12 h, 35 °C, 2.8 wt.%, 6:1	Yield = 98.95%	[49]
Rhizopus oryzae lipase	Calophyllum inophyllum	72 h, 35 °C, 20 wt.%, 12:1	Yield = 92%	[50]
Rhizopus oryzae lipase	Pistacia chinensis seed oil	60 h, 37 °C, 7 wt.% 5:1	Yield = 94%	[51]
Pig pancreatic lipase	Sunflower oil	19 h, 60 °C, 0.1 wt.%, 2:1	Yield = 57.2%	[52]
Rhizomucor miehei lipase	Triolein	96 h, 40 °C, 5 wt.%, 3:1	Yield = 77%	[53]
Cryptococcus spp. S-2	Rice bran oil	120 h, 30 °C, 4:1	Yield = 80.2%	[54]
Pseudomonas lipase	Sunflower oil	5 h, 45 °C, 10:1	Yield = 80%	[55]
Staphylococcus aureus and Bacillus stearothermophilus	Coffee oil	12 h, 55 °C, 14 wt.%, 3:1	Conversion = 97.66%	[56]

depicts a comparison of various biocatalysts utilized in several research studies, in terms of biodiesel yield, process conditions, and feedstock.

3.1.1. Homogeneous Catalysts Utilized in Biodiesel Production

Conventionally, homogeneous catalysts are employed in biodiesel production from various oils. For industrial biodiesel production, the most commonly utilized catalyst is a homogeneous catalyst, as it is easy to use and the biodiesel product is obtained in less time. Homogeneous catalysts, which generally may be liquid or gaseous, and reactants are both in the same phase [57,58]. Homogeneous catalysts are classified into two categories: acidic catalysts and alkaline catalysts [59].

3.1.2. Homogeneous Base Catalysts

For triglyceride transesterification, homogeneous base catalysts are typically utilized. The base catalysts are metal-based oxide and alkaline liquids such as potassium methoxide, sodium methoxide, carbonates, sodium hydroxide, and barium hydroxide [60]. The catalytic activity of homogenous base catalysts is very high in transesterification [60]. Homogeneous base catalysts are currently the most widely used method for large-scale biodiesel production. This method offers several advantages, including high catalytic activity and the ability to achieve complete conversion of triglycerides to biodiesel [61]. Due to these benefits, homogeneous base catalysts have become the preferred choice in industrial settings, significantly contributing to the efficiency and scalability of biodiesel production processes [61]. Metallic hydroxide is often used as homogenous base catalysts in transesterification because of lower prices, but they usually have lower activity in transesterification than alkoxide [62]. In transesterification reactions, alkoxide species are highly basic and have a high activity which then attacks the triglyceride carbonyl group to form long-chain fatty acid (biodiesel) and glycerol [7].

Many researchers have studied the homogeneous base-catalyzed transesterification reactions of triglycerides. In 2008, Rashid et al. [63] used homogeneous base catalyst potassium hydroxide (KOH) to produce rapeseed oil biodiesel. The authors discovered that potassium hydroxide can achieve a yield of 96% with a 1 wt.% catalyst loading. Phan et al. showed that potassium hydroxide can produce promising results with maximum biodiesel yields of up to 90% at a 0.75 wt.% catalyst concentration [64]. However, when the catalyst amount was raised to 1.5 wt.% potassium hydroxide, only 75% of biodiesel yield was achieved. This declining trend is explained by saponification, which happens when catalyst loading is high, resulting in a lower biodiesel yield and increases reactant viscosity. The performance of different KOH catalyst loading in the transesterification of croton megalocarpus oil was examined by Kafuku et al. [65]. They observed that the use of a 0.50 wt.% catalyst loading produced a fairly low biodiesel conversion (50%). However, the biodiesel conversion increased significantly and reached a maximum value of 88% at a catalyst amount of 1.0 wt.%. Furthermore, they discovered that increasing the catalyst loading above the optimal catalyst concentration of 1.0 wt.% reduced biodiesel conversion, and soap was discovered in greater quantities due to excess catalysts that favor the saponification process.

Sivakumar et al. [66] carried out research to optimize biodiesel yield using KOH catalyst. Their results showed that 1.2 wt.% KOH catalyst at 75 °C can be considered as an optimum condition for transesterification of waste scum. A yield of 96.7% was achieved for these process conditions in 30 min. KOH catalyst was utilized by Dueso et al. in the catalytic transesterification of sunflower oil [67]. Under the reaction conditions of 60 °C temperature, 1 wt.% catalyst loading and 6:1 molar ratio of methanol to oil (MRMO), a biodiesel yield of 98.6% was obtained in 3 h with the KOH catalyst.

Transesterification of Elaeagnus angustifolia seed oil in methanol utilizing potassium methoxide catalyst was investigated by Kamran et al. [68]. At reaction conditions of temperature T= 60 °C, time t = 60 min, 1.0 wt.% of catalyst, and 9:1 MRMO, the optimal biodiesel yield of 95% was achieved. Also, in the transesterification of rice bran using sodium hydroxide (NaOH) catalyst, 72.8% yield was obtained at 0.9 wt.% NaOH catalyst, a temperature of 60 °C in 2 h at 6:1 MRMO [69].

When compared to homogeneous acid-catalyzed processes, the homogeneous base-catalyzed process with sodium methoxide and potassium methoxide catalyst and edible oils like sunflower, soybean, and canola is four thousand times faster than that of homogenous acid-catalyzed reaction [62]. The faster reaction rate is due to the higher efficiency of base catalysts in transesterification reactions, particularly with edible oils. Base catalysts enhance the reaction rate by facilitating the formation of reactive intermediates more rapidly, which accelerates the overall transesterification process compared to acid catalysts [62]. The base catalyst may facilitate a more efficient reaction pathway or lower activation energy compared to the acid catalyst, leading to a faster reaction rate. Unfortunately, this approach has a few drawbacks, including the fact that the catalyst is not recycled, and must be removed from the product streams [58]. The catalyst can also react with oil impurities to produce soap, especially free fatty acids [59]. The problem of soap production during base-catalyzed transesterification is a recognized challenge. To address this issue, several strategies can be employed. First, feedstocks with high free fatty acid (FFA) content are often pre-treated with acid catalysts. This pre-treatment converts FFAs into esters, thereby reducing the potential for soap formation in the subsequent base-catalyzed transesterification [70]. Additionally, the use of advanced base catalysts, which are designed to tolerate higher levels of FFAs, can effectively minimize soap production by enhancing the efficiency of the transesterification process and reducing undesired side reactions [71]. Optimizing reaction conditions, such as temperature, catalyst concentration, and the methanol-to-oil ratio, also helps in reducing soap formation [72]. Finally, post-reaction treatments, including washing the biodiesel with water or using acid washes, can remove soap and other impurities from the final biodiesel product, ensuring its purity and quality [61]. These strategies collectively address the issue of soap production and improve the overall efficiency of the transesterification process. The effective use of homogeneous base catalysts is confined to refined oils containing a FFAs of less than 0.5 weight percent [59]. In addition, these catalysts are removed after the reaction is complete by washing the biodiesel with water, which could lead to the loss of alkyl esters, consumption of energy, and the production of significant quantities of wastewater [73]. Therefore, it is important to explore the possibility of replacing homogeneous base catalysts with some other suitable transesterification reaction catalysts in order to establish a low-cost and environmentally friendly biodiesel production method.

3.1.3. Homogeneous Acid Catalysts

The homogeneous acid-catalyzed transesterification process offers a considerable advantage over homogeneous base-catalyzed transesterification because presence of FFAs in the feedstock has no effect on the acid catalyst and both esterification and transesterification can be catalyzed at the same time [74]. The most widely used catalysts for acid-catalyzed transesterification are sulfuric acids, hydrochloric acids, and sulfonic acids [74]. Homogeneous acid-catalysts produce very high alkyl ester yields. However, compared to base-catalyzed transesterification reactions, the reactions are slower, making the process costly as a result of high energy consumption [75]. Acid-catalyzed transesterification reaction has the same problems of separation as the base-catalyzed process.

To optimize biodiesel production, several researchers have made attempts to investigate the homogeneous acid-catalyzed transesterification of triglycerides. Freedman et al. [76] conducted extensive research on the catalytic activity of sulfuric acid (H₂SO₄) in the production of soybean oil biodiesel. They demonstrated that a biodiesel conversion of >90% can be obtained with a 1 mol % catalyst and 30:1 MRMO for 69 h. Using sulfuric acid and hydrochloric acid catalysts, biodiesel was produced from castor oil by Meneghetti et al. [77]. The authors found that biodiesel can be produced by transesterifying castor oil using ethanol or methanol. Using either ethanol or methanol, they obtained almost similar biodiesel yields, but they noticed that when methanol was used, the transesterification process proceeded very fast. Crabbe et al. [78] transesterified crude palm oil with methanol over H₂SO₄ catalyst. The authors found that a biodiesel yield of 97% can be achieved using a 5 wt.% catalyst and 40:1 MRMO at 95 °C in 9 h. Similarly, Wang et al. [79] used an H₂SO₄ catalyst for WCO transesterification. The authors observed that the sulfuric acid catalyst shows enhanced transesterification activity and provided a maximum methyl ester conversion of 90% at a reaction temperature of 95 °C, 20:1 MRMO, and sulfuric acid loading of 4 wt.% in 10 h. The homogeneous acid-catalyzed biodiesel production process is effective to convert low-cost feedstock to biodiesel, but not so economical because large quantities of catalysts and a long reaction time are required to obtain a high biodiesel yield. Homogeneous acid-catalysts are preferable for low-cost feedstock due to their insensitivity to the presence of FFAs and water, while FFAs and water, if present in the feedstock, have an effect on homogeneous base catalysts. Therefore, it is very important to search for a catalyst that has the potential of overcoming issues related to homogeneous catalyst biodiesel production for sustainable energy processes.

Table 2. Some studies on biodiesel production utilizing homogeneous catalysts.

Catalyst	Feedstock	Reaction Parameters (Time, Temperature, Catalyst Amount, and Alcohol-to-Oil Molar Ratio)	Performance	Ref.
KOH	Croton megalocarpus oil	1 h, 60 °C, 1.0 wt.%, 30:1	Conversion = 88%	[65]
Potassium methoxide	Elaeagnus Angustifolia seed oil	1 h, 60 °C, 1 wt.%, 9:1	Yield = 95%	[68]
NaOH	Rice bran oil	2 h, 60 °C, 0.9 wt.%, 6:1	Yield = 72.8%	[69]
KOH	Waste scum	30 min, 75 °C, 1.2 wt.%, 6:1.	Yield = 96.7%	[66]
H2SO4	Soybean oil	69 h, 65 °C, 1 wt.%, 30:1	Conversion = >90%	[76]
H2SO4	Crude palm oil	9 h, 95 °C, 5 wt.%, 40:1	Yield = 97%	[78]
H2SO4	Used cooking oil	10 h, 95 °C, 4 wt.%, 20:1	Conversion = 90%	[79]

depicts a comparison of various homogeneous catalysts utilized by several research studies to convert different feedstock into biodiesel.

3.2. Heterogeneous Catalysts Used for Biodiesel Production

Heterogeneous catalysts are in different phases compared to the reactants and products [80]. Because of their availability and short reaction time, homogeneous catalysts are becoming more common in the biofuel industry [81]. However, when it comes to esterification and transesterification reactions, the use of a homogeneous catalyst causes many problems. For instance, when sulfuric acid is used in esterification, it causes reactor corrosion and high sulfur content in biodiesel products. For homogeneous base catalysts such as KOH and NaOH, product separation, liquid catalyst recovery, and product washing are required through the acid neutralization steps, which could lead to the production of a high amount of wastewater and higher energy consumption, thereby increasing the biodiesel production costs. It is expected that using heterogeneous catalysts as opposed to homogeneous catalysts will overcome the issues related to homogeneous catalysts. When compared to homogeneous catalysts, heterogeneous catalysts offer various benefits, including easy regeneration, elimination of product washing step, less corrosiveness, efficient conversion, environmental friendliness, and fewer disposal issues. Heterogeneous catalysts can handle multiple feedstocks in a single step [82]. However, the major problems of heterogeneous catalysis in biodiesel production include a higher ratio of alcohol to oil and higher temperature. Another disadvantage of heterogeneous catalysts is that they react with alcohol and oil to form three phases, which limits dispersion and hence reduces reaction rate [82]. The difficulty of mass transfer is resolved by the use of co-solvents such as heptane, ethanol, and tetrahydrofuran, which enables oil and alcohol miscibility, as a result of which the reaction rate is increased [83]. The use of catalyst supports is another way to overcome mass transfer difficulty [84]. The supports offer the active species increased pores and precise surface area in which they can attach and react with bulky triglycerides [84]. Heterogeneous catalysts are subdivided further into two categories: acidic catalysts and alkaline catalysts [80].

3.2.1. Heterogeneous Base Catalysts

Heterogeneous base catalysts with alkaline properties on their surface are excellent candidates to substitute a homogeneous base catalyst for the transesterification process. Heterogeneous base catalysts have the tremendous potential to address the major challenges encountered in the biodiesel production industry. These catalysts offer several advantages, which include easy regeneration, environmental friendliness, and fewer disposal issues [85]. Furthermore, during heterogeneous base-catalyzed transesterification reactions, only a small amount of wastewater is produced. Heterogeneous base catalysts are typically more resistant to FFAs and water in the economical feedstock, which includes waste cooking oil and fat as compared to homogeneous base catalysts [86]. Longer catalyst life, renewability, selectivity, and higher activity can all be achieved with heterogeneous base catalysts [87]. Figure 5 illustrates the mechanism of base-catalyzed transesterification. Reactions involving heterogeneous bases proceed through interactions between the Lewis or Brønsted basic sites of the catalyst and a monohydric alcohol, typically methanol or ethanol. The resulting alkoxide mixture then reacts with the triglyceride esters in the oil, ultimately producing biodiesel and glycerol through a series of steps [88]. Heterogeneous base catalysts are further subdivided into six groups. Supported alkali earth metal or supported alkaline earth metal, hydrotalcite, zeolite, metal oxide, mixed metal oxide, and non-oxide are the six groups of heterogeneous base catalysts [89].

Several heterogeneous base catalysts for biodiesel production have been documented in the literature over the last few decades. Liu et al. [90] transesterified soybean oil using strontium oxide (SrO) catalyst. The authors found that a biodiesel yield of 95.0% can be obtained with 12:1 MRMO and 3 wt.% catalyst at 65 °C for 30 min. Moreover, after ten cycles of reusing strontium oxide catalyst, the biodiesel yield was only reduced marginally. Kim et al. [91] transesterified soybean oil with methanol over Na/NaOH/_γAl₂O₃ catalyst. The biodiesel yield achieved was 94% at 1 wt.% catalyst loading and 9:1 MRMO at 60 °C after 2 h. Liu et al. [87] documented the use of calcium oxide (CaO) catalyst in the production of biodiesel. The specific surface area (m²/g) of the catalyst was determined using the Brunner–Emmett–Teller (BET) technique. The total surface area of a solid catalyst was measured by BET surface area analyzers. A powdered analyte was suspended in an inert gaseous bath, and the surface adsorption of gas molecules was measured. The CaO catalyst BET surface area was 0.56 m²/g. At a process condition of T= 65 °C, t = 3 h, 8 wt.% of CaO catalyst, and 12:1 MRMO, a maximum biodiesel yield of 95% was achieved. They also investigated the comparative activity of calcium oxide with K₂CO₃/_γAl₂O₃ and KF/_γAl₂O₃ solid catalyst. They found that the CaO catalyst retained its activity after repeated use for 20 cycles and the yield remained unaffected, while the biodiesel yield was reduced when K₂CO₃/_γAl₂O₃ and KF/_γAl₂O₃ solid catalysts are subsequently reused for 20 cycles. This is due to the dissolution of alkali metal compounds in alcohol, which decreases the catalytic activity, and thus reduces the yield in the subsequent reaction.

The activity of a CaO catalyst in biodiesel production from sunflower oil was investigated by Granados et al. [92]. They demonstrated that with a catalyst amount and temperature of 1.0 wt.% and 60 °C, respectively, and 13:1 MRMO in 90 min, a biodiesel yield of 94% was achieved. The BET pore size diameter of the CaO catalyst was 30 nm and the surface area of the CaO catalyst was 32 m²/g. During their investigation, they found that the catalyst’s active site was poisoned by atmospheric gases (H₂O and CO₂). Consequently, the catalyst was subjected to high-temperature activation treatment before reaction to enhance its activity. Even though the key poison species, the carbonate group, was removed from the surface by this treatment, active species leaching was also observed. But the amount of leaching did not result in a substantial decrease in activity and the catalyst retained its activity after repeated use for eight cycles [59]. However, biodiesel conversion reduced from 94% in the first cycle to 80% in the second cycle, and the yield remained unaffected after that. Veljkovic et al. [93] studied the kinetics of calcium oxide catalyst in the transesterification of sunflower oil using methanol. A biodiesel yield of 98% was achieved with 1 wt.% catalyst content at 60 °C, and 6:1 MRMO in 2 h reaction time. Kaur et al. [94] used a lithium-doped calcium oxide (Li/CaO) catalyst in the production of jatropha and karanja oil biodiesel. X-ray diffraction, BET, and transmission electron microscopy were used to characterize the catalyst. Under optimized reaction conditions of 12:1 MRMO and 5 wt.% catalyst at 65 ℃, the catalyst produced the highest conversion of >99% in 1 h and 2 h reaction time for karanja and jatropha oil, respectively. Kouzu et al. [95] transesterified soybean oil utilizing CaO catalyst and methanol. The authors found that the catalyst shows excellent transesterification performance, where a biodiesel yield of 93% was obtained with 0.78 g of catalyst, a reaction time of 1 h, and a mixing speed of 500 rpm.

Mootabadi et al. [96] evaluated the performance of different catalysts in the production of palm oil biodiesel: calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO). They recorded biodiesel yields of 95.2% (for BaO), 95.2% (for SrO), and 77.3% (for CaO) using a 3 wt.% catalyst in 1 h. The catalysts activity order was BaO > SrO > CaO. In another related study, Salamatinia et al. [97] studied the performance of BaO and SrO in the production of palm oil biodiesel. Biodiesel yields of 69.44% and 66.93% were achieved using 1 wt.% catalyst loading at 50 °C, and 9:1 MRMO in 30 min reaction time for BaO and SrO, respectively. They reported that the optimized biodiesel yields obtained with MRMO of 9:1 and catalyst loading of 3.0 wt.% in 50 min for SrO and BaO were >94% and 95.17%, respectively. Laskar et al. [98] employed waste snail shells as a heterogeneous base catalyst in the production of soybean oil biodiesel. This approach reduces the overall production costs and proves environmentally friendliness. The waste snail shells’ main component calcium carbonate (CaCO₃) is transformed into calcium oxide (CaO) catalyst by calcination in air for 5 h at 900 °C [98]. They obtained 98% biodiesel yield from soybean oil using 6:1 MRMO at 28 °C and 3.5 wt.% catalyst amount in 7 h reaction time. Moreover, the catalyst retained its activity after reuse for eight reaction cycles. Also, Viriya-empikul et al. [99] used waste eggshells as a heterogeneous catalyst in biodiesel production from palm olein oil. The waste eggshells major component CaCO₃ is transformed into CaO catalyst by calcination in the air for five hours at 800 °C. With a 10 wt.% catalyst and 18:1 MRMO, a biodiesel yield of >90% was achieved at 60 °C for 2 h. Nair et al. [100] investigated the use of clamshell as a catalyst in the transesterification of waste frying oil for biodiesel production. The CaO catalyst, which was synthesized from clamshell successfully transesterified waste frying oil, and >89% of biodiesel yield was obtained with 6.03:1 MRMO at 60 °C and 3 wt.% catalyst for 3 h. Birla et al. [101] reported an 87.28% biodiesel yield from WCO with 8.45:1 MRMO, 2 wt.% catalyst loading at 60 °C in 7 h using a heterogeneous base catalyst such as snail shells.

Xie et al. [102] transesterified soybean oil using NaX Zeolite/KOH heterogeneous solid base catalyst. The catalyst showed good transesterification performance, where a maximum biodiesel yield of 85.6% was obtained with 10:1 MRMO and 3.0 wt.% of catalyst loading at 65 °C for 8 h. In the transesterification of soybean oil with methanol, Fabbri et al. [103] employed a Na₂PEG (300), obtained by treating PEG 300 (polyethylene glycol of molecular weigh 300 Da) with methanolic sodium methoxide solution, as a base catalyst in the production of soybean oil biodiesel with dimethyl carbonate. According to these authors, the catalyst provided a conversion of approximately 99% at 5 wt.% catalyst amounts and 90 °C reaction temperature after 5 h reaction time. Elsewhere, Tan et al. [104] transesterified used cooking oil over chicken eggshells and ostrich eggshells as a catalyst. A biodiesel yield of 94% and 96% were obtained with calcined chicken eggshells and calcined ostrich eggshells, respectively, using 1.5 wt.% catalyst loading at 65 °C, and 12:1 MRMO in 2 h reaction time. Also, Xu et al. [105] investigated the catalytic performance of KF/Zn(Al)O catalyst in making biodiesel from soybean oil. The synthesized catalyst is more reactive than that of Zn-Al hydrotalcite. In their study, the reaction was conducted for 3 h with 3 wt.% catalyst and 6:1 MRMO at 65 °C. A biodiesel yield of about 95% was achieved with these conditions within 3 h.

Navajas et al. [106] studied Mg-Al hydrotalcite catalytic activity in the transesterification of sunflower oil. The co-precipitation method was used to produce the catalyst with a Mg:Al molar ratio of 5. They obtained a 92% biodiesel yield at 60 °C temperature, 48:1 MRMO, and 2 wt.% catalyst loading for 24 h. They attributed the Mg-Al hydrotalcite catalytic activity to the existing basic sites at the crystal edge. Trakarnpruk et al. [107] reported a high biodiesel yield by utilizing a K-loaded Mg-Al hydrotalcite catalyst in palm oil biodiesel production. The authors conducted the reaction with 7.0 wt.% catalyst and 30:1 MRMO at 100 °C for 7 h. A biodiesel yield of approximately 86.6% was obtained with these conditions. Hernandez et al. [108] transesterified sunflower oil and WCO over Na-Mg-Al hydrotalcite catalyst. The optimum MRMO was 15:1 for sunflower oil and 9:1 for WCO. The catalyst produces the highest yield of 88% and 67% for sunflower oil and WCO, respectively, at 60 °C for 8 h using a 7 wt.% catalyst (hydrotalcite impregnated with 5% of sodium for sunflower oil and 10% of sodium for WCO). Yan et al. [109] used Zn₃La₁(Lanthanum modified ZnO) for biodiesel production from WCO. A yield of 93.7% was reported with 3 wt.% catalyst and 26:1 MRMO at 200 °C for 3 h. Agarwal et al. [110] transesterified WCO with KOH/Al₂O₃ catalyst. They reported that the catalyst showed excellent transesterification performance, where a yield of 96.8% was obtained with catalytic loading of 15 wt.% and 9:1 MRMO at 70 °C for 2 h. Moreover, when KOH/Al₂O₃ catalyst was subsequently reused for three cycles, the catalyst resulted in a reasonable conversion rate (83%). Dehkordi et al. [111] reported a 92.1% biodiesel yield from WCO with 10.0 wt.% catalyst and 30:1 MRMO at 65 °C in 2 h using CaO-ZrO₂ catalyst. They observed that the stability of CaO-ZrO₂ catalyst declined as the Ca/Zr molar ratio increased, such that high stability was attained with a catalyst prepared with a low concentration of CaO. Xie et al. [112] investigated the catalytic activity of a Fe₃O₄@HKUST-ABIL composite in producing biodiesel from soybean oil. The core–shell structured Fe₃O₄@HKUST-ABIL catalyst was prepared utilizing layer-by-layer assembly method (Figure 6). The highest biodiesel yield of 92.30% was achieved with a catalyst loading of 1.2 wt.% and 30:1 MRMO at 65 °C for 3 h. There is no significant change in the catalytic activity of the Fe₃O₄@HKUST-ABIL composite after five reaction cycles.

Similarly, Xie et al. [113] transesterified soybean oil over zeolitic imidazolate framework ZIF-90 with organic guanidine (ZIF-90-Gua) (Figure 7). The solid catalyst has long-term catalytic activity because guanidine is covalently bound to the support. The catalytic activity of the ZIF-90-Gua towards transesterification was evaluated under the optimum reaction conditions of T= 65 °C, t = 6 h, 1 wt.% of ZIF-90-Gua catalyst, and 15:1 MRMO, and a maximum biodiesel yield of 95.40% was achieved. The ZIF-90-Gua catalyst was easily recovered by filtration and reused five times without substantial loss of catalytic activity, indicating that it has great potential to be used as an efficient catalyst for biodiesel production.

Abdelmigeed et al. [114] reported NaOH/magnetized ZIF-8 catalyst in synthesizing biodiesel from vegetable oil. A 99.80% vegetable oil yield was achieved with an MRMO of 21:1 and a catalyst loading of 3 wt.% at 65 °C for 1 h. Moreover, ASTM standard testing methods indicated that the produced biodiesel under optimal reaction conditions can be used as sustainable biodiesel fuel. In addition, the experimental data was well-fitted by the pseudo-second order kinetic model, as revealed by the kinetic study. Saeedi et al. [115] synthesized a sodium–zeolite imidazolate framework (ZIF-8) doped with potassium (KNa/ZIF-8) via sol–gel processing and investigated catalyst activity in the production of biodiesel from soybean oil. The addition of potassium to Na/ZIF-8 was aimed at enhancing its basicity, thereby leading to an improved catalytic performance in the production of biodiesel from soybean oil with methanol. The results show that KNa/ZIF-8 exhibited outstanding performance due to the highest number of basic sites in this sample and high surface area (around 1195 m²/g). The highest soybean oil yield of 98% was obtained using the KNa/ZIF-8 catalyst with potassium loading of 0.08% under an optimal reaction condition of MRMO of 10:1 and catalyst loading of 0.012 wt.% at 100 °C for 3.5 h. The catalyst demonstrated high stability and there was no significant loss in catalytic activity after reused for three cycles. Li et al. [116] prepared strontium oxide supported on MIL-100(Fe) catalyst [SrO-MIL-100(Fe)] using an in situ titration method (ST) and a mechanical mixing method (MM), and evaluated their performance in making palm oil biodiesel. The results show that MM-SrO exhibits the excellent activity, where a maximum biodiesel yield of 96.19% was achieved with an MRMO of 12:1 and a catalyst loading of 8 wt.% at 65 °C for 30 min. Moreover, when SrO-MIL-100(Fe)] catalyst was subsequently reused for three cycles, the catalyst resulted in a reasonable biodiesel yield (82.49%). Yang et al. [117] synthesized metal–organic framework-derived Mg-Zn hybrid catalysts (MgO@Zn-MOF) and investigated their performance in making biodiesel from soybean oil. As shown in Figure 8, Mg-Zn hybrid nanocatalysts were successfully synthesized under air and argon. The optimized reaction was performed with 1 wt.% catalyst and 3:1 MRMO at 210 °C for 2 h, and a maximum yield of 73.30% of the feedstock was obtained.

Fazaeli et al. [118] employed ZIF-8@GO doped with potassium and sodium (KNa/ZIF-8@GO) for the biodiesel transesterification process. The biodiesel yield reached 98% at 100 °C temperature, 18:1 MRMO, and 8 wt.% catalyst loading for 8 h. Furthermore, when the KNa/ZIF-8@GO catalyst was subsequently reused for three cycles, there was no appreciable loss in the catalytic activity of the KNa/ZIF-8@GO catalyst. Jamil et al. [119] transesterified WCO containing a high amount of FFA using Cu- and Ca-based metal–organic framework catalysts (Figure 9). The author found that a biodiesel yield of 84.5% can be obtained with 20:1 MRMO and 1 wt.% catalyst at 60 °C for 30 min. Moreover, after three cycles of reusing Cu- and Ca-based MOF catalysts, the biodiesel yield was reduced up to 7%.

In view of the literature presented above, it may therefore be argued that the heterogeneous base-catalyzed method is an efficient method of producing biodiesel with low cost and negligible environmental consequences. However, this process is sensitive to the FFA composition of feedstock used to produce biodiesel. Hence, the inability of heterogeneous base catalysts to withstand high FFAs of the economical feedstock under mild reaction conditions is the main limitation associated with their development.

Table 3. Some studies on biodiesel production utilizing heterogeneous base catalysts.

Catalyst	Feedstock	Reaction Parameters (Reaction Time, Temperature, Catalyst Amount, and Alcohol to Oil Molar Ratio)	Performance	Ref.
SrO	Soyabean oil	30 min, 65 °C, 3 wt.%, 12:1	Yield = 95%	[90]
Na/NaOH/γAl2O3	Soyabean oil	2 h, 60 °C, 1 wt.%, 9:1	Yield = 94%	[91]
CaO	Soyabean oil	3 h, 65 °C, 8 wt.%, 12:1	Yield = 95%	[87]
CaO	Sunflower oil	90 min, 60 °C, 1 wt.%, 13:1	Yield = 94%	[92]
CaO	Sunflower oil	2 h, 60 °C, 1 wt.%, 6:1	Yield = 98%	[93]
Snail shell	Waste cooking oil	7 h, 60 °C, 2 wt.%, 8.45:1	Yield = 87.28%	[101]
BaO	Palm oil	1 h, 50 °C, 3 wt.%, 9:1	Yield = 95.2%	[96]
SrO	Palm oil	1 h, 50 °C, 3 wt.%, 9:1	Yield = 95.2%	[96]
CaO	Palm oil	1 h, 50 °C, 3 wt.%, 12:1	Yield = 77.3%	[96]
BaO	Palm oil	60 min, 50 °C, 3 wt.%, 9:1	Yield = 95.17%	[97]
SrO	Palm oil	60 min, 50 °C, 3 wt.%, 9:1	Yield = >94%	[97]
Waste snail shells	Soyabean oil	7 h, 50 °C, 3 wt.%, 6:1	Yield = 98%	[98]
Waste eggshells (CaO)	Palm olein oil	2 h, 60 °C, 10.0 wt.%, 12:1	Yield = >90%	[99]
KOH/Al2O3	Waste cooking oil	2 h, 70 °C, 15 wt.%, 9:1	Yield = 96.8%	[110]
Li/CaO	Jatropha oil karanja oil	2 h, 65 °C, 5 wt.%, 12:1 1 h, 65 °C, 5 wt.%, 12:1	Conversion = >99 Conversion = >99	[94]
Ostrich eggshells	Used cooking oil	2 h, 65 °C, 1.5 wt.%, 12:1	Yield = 96%	[104]
Chicken-eggshells	Used cooking oil	2 h, 65 °C, 1.5 wt.%, 12:1	Yield = 94%	[104]
NaX Zeolite/KOH	Soyabean oil	8 h, 65 °C, 3 wt.%, 10:1	Yield = 85.6%	[102]
Na2PEG (300)	Soyabean oil	5 h, 70 °C, 6 wt.%, 30:1	Conversion = 99%	[103]
KF/Zn(Al)O	Soyabean	3 h, 65 °C, 3 wt.%, 6:1	Yield = 95%	[105]
Mg-Al hydrotalcite	Sunflower oil	24 h, 60 °C, 2 wt.%, 48:1	Yield = 92%	[106]
K-loaded Mg-Al hydrotalcite	Palm oil	7 h, 100 °C, 7 wt.%, 30:1	Yield = 86.6%	[107]
Na-Mg-Al hydrotalcite (HT-Na)	Waste cooking oil	8 h, 60 °C, 7 wt.%, 9:1	Yield = 67%	[108]
Zn3La1(Lanthanum-modified ZnO)	Waste cooking oil	3 h, 200 °C, 3 wt.%, 26:1	Yield = 93.7%	[109]
CaO (Clamshell)	Waste frying oil	3 h, 60 °C, 3 wt.%, 6.03:1	Yield = >89	[100]
CaO-ZrO2	Waste cooking oil	2 h, 65 °C, 10 wt.%, 30:1	Yield = 92.1%	[111]
Fe3O4@HKUST-ABIL	Soybean oil	3 h, 65 °C, 1.2 wt.%, 30:1	Yield = 92.30%	[112]
ZIF-90-Gua	Soybean oil	6 h, 65 °C, 1 wt.%, 15:1	Yield = 95.40%	[113]
NaOH/magnetized ZIF-8	Vegetable oil	1 h, 65 °C, 3 wt.%, 21:1	Yield = 99.80%	[114]
KNa/ZIF-8	Soybean oil	3.5 h, 100 °C, 0.012 wt.%, 10:1	Yield = 98%	[115]
SrO-MIL-100(Fe)	Palm oil	0.5 h, 65 °C, 8 wt.%, 12:1	Yield= 96.19%	[116]
MgO@Zn-MOF	Soybean oil	2 h, 210 °C, 1 wt.%, 3:1	Yield = 73.30%84	[117]
KNa/ZIF-8@GO	Soybean oil	8 h, 100 °C, 8 wt.%, 18:1	Yield = 98%	[118]
Cu- and Ca-based MOF	Waste cooking oil	0.5 h, 60 °C, 1 wt.%, 20:1	Yield = 84.5%	[119]

shows a comparison of different heterogeneous base catalysts utilized by several research studies, in terms of biodiesel yield, process conditions, and feedstock.

3.2.2. Heterogeneous Acid-Catalysts

Heterogeneous acid catalysts have an enormous capacity to carry out both esterification of free fatty acid and transesterification of triglycerides simultaneously using low-cost feedstock, thus lowering the cost of producing biodiesel. Heterogeneous acid catalysts can usually withstand more extreme operating conditions and cause less corrosion problems compared to homogeneous acid catalysts [120]. These catalysts have many acid sites of various Brønsted or Lewis acidity strengths. Biodiesel is usually produced from feedstock with high FFAs using heterogeneous acid catalysts. However, the benefits of heterogeneous acid catalysts include easy separation from the finished product and their being recyclable for repeated use. Therefore, heterogeneous acid catalysts offer a more economical path to the biodiesel production process [121,122]. Nevertheless, to achieve good conversion yield, higher temperatures, higher catalyst loading, and longer reaction times are required for heterogeneous acidic catalysts [123]. Also, in some cases, soap formation is observed using heterogeneous acid catalysts, and purification is required in most cases to ensure the appropriate fuel quality. In addition, it is important to avoid poisoning, deactivation, and leaching of acid sites. Low-cost feedstocks, for example, waste cooking oils, have been described as promising feedstocks with the potential to decrease the cost of production, and thus enhance biodiesel production economic viability. Heterogeneous acid catalysts are usually non-sensitive to water and free fatty acid. Thus, free fatty acid esterification and triglycerides transesterification into biodiesel can be catalyzed more efficiently with a solid acid catalyst. In addition, the acid-catalyzed process is cost-effective and can also address waste disposal problems because heterogeneous acid catalysts can produce biodiesel directly from low-cost feedstocks [124]. In general, heterogeneous acid catalysts provide positively charged acid sites to which the FFAs or triglycerides in the oil adsorb. The carbonyl oxygen in the FFAs or triglycerides interacts with the Lewis acid sites (L*) on the catalyst surface, forming a carbocation that initiates the transesterification reaction between methanol and the adsorbed triglycerides, as shown in Figure 10 [125]. Nucleophiles, such as methanol, resulting from the deprotonation of the hydroxyl group, attack the electrophilic carbon. This leads to a rearrangement step, producing an intermediate that eliminates water molecules and forms methyl ester [61].

The most effective catalysts from all the numerous areas of heterogeneous catalysis are metal oxide-based catalysts because mesoporous or porous metal oxides allow many catalytic processes. The importance of catalysts based on metal oxides lies in their successful catalysis of various processes, e.g., catalysis of acid–base, oxidation reactions, conversion of biomass, and photocatalysis. Due to their vast number of active acid sites, some metal oxides have higher catalytic activity, thus increasing the catalytic surface area. These properties improve the yield of reactions and reduce reaction times [126]. Most of these metal oxide-based catalysts originate from transition metals groups, such as titanium oxides (TiO₂), zirconium oxides (ZrO₂), tin oxides (SnO₂), tungsten oxides (WO₃), iron oxides (Fe₂O₄), and zinc oxides (ZnO). Among all these acid catalysts, sulfonated zirconia, tungsten–zirconia, and CaO–zirconia are suitable for transesterifying triglycerides into alkyl esters [127]. A suitable heterogeneous acid catalyst for transesterification reactions should have features including hydrophobic surface, interconnected large pores, and strong Brønsted/Lewis acid properties [128].

A myriad of studies on the utilization of the heterogeneous acid catalyst in biodiesel production have been conducted. Jitputti et al. [129] showed that sulfated zirconia (SO₄²⁻ /ZrO₂) can produce promising results with biodiesel yields of up to 86.3% and 90.3%, with crude coconut and palm kernel oils, respectively. However, biodiesel yields were only 48% (crude coconut oil) and 63% (palm kernel oil) when unsulfated zirconia was used as a catalyst instead of sulfated zirconia. The results showed that sulfated zirconia demonstrated the highest catalytic conversion (86.3% for coconut oil and 90.3% for palm kernel oil) in optimized conditions of 200 °C temperature, 6:1 MRMO, and 1.0 wt.% catalyst for 4 h. This essentially demonstrates that the main factor in achieving high triglyceride conversion is the modification of metal oxide surface acidity. Ibrahim et al. [130] explored the esterification of waste cooking oil using ZrO₂ loaded on various supports (TiO₂, SiO₂, Fe₂O₃, and Al₂O₃). The hybrid sol–gel auto combustion approach was used to synthesize the catalyst. The esterification process was conducted with 120:1 MRMO and 0.1 wt.% catalyst at 120 °C for 3 h. Optimal biodiesel conversion (48.6%) was achieved with a ZrO₂/SiO₂ catalyst within 3 h. Park et al. [131] performed esterification of WCO with ZrO₂, in which tungstate (WO₃) was integrated into ZrO₂ instead of sulfuric acid (H₂SO₄). WO₃/ZrO₂ was found to be more stable than SO₄²⁻/ZrO₂, thus preventing acid sites from leaching into the reaction media. The authors discovered that after 2 h at 75 °C, 93% of FFAs conversion was achieved using 9:1 MRMO. Despite the high acidity of sulfated zirconia, during liquid-phase transesterification reaction, it is known to suffer substantial deactivation due to sulfate leaching. By dissolving the freshly prepared SO₄²⁻/ZrO₂ catalyst in water, catalyst leaching has been tested [132]. Hydrolysis of the sulfate groups to H₂SO₄ and HSO₄⁻ rapidly lowers the pH of the suspension. This will induce transesterification through homogeneous acid catalysis, thereby obstructing heterogeneous catalytic activity. SO₄²⁻/ZrO₂ was prepared using chlorosulfonic acid, HSO₃Cl, rather than impregnation with H₂SO₄. The prepared catalyst (SO₄²⁻/ZrO₂) exhibited a very high catalytic activity and no leaching of sulfates was noted. Chlorosulfonic acid is a very dangerous chemical, and exposure for a very short period may lead to death or serious trauma [133].

The esterification of methanol-induced FFA using SO₄²⁻/SnO₂-ZrO catalyst was conducted by Enascuta et al. [134]. They reported that the esterification process involves Brønsted and Lewis acid sites. A biodiesel yield of 82.58% was achieved using the following conditions: 5.0 wt.% of catalyst and 6.5:1 of molar ratio of ethanol to oil (MREO) at 78 °C for 6 h. A number of new SO₄²/ZrO₂-based catalysts were obtained from MCM-41 silica-supported calcined zirconium sulfate (ZrX-MCM-41, where X represents zirconia sulfate precursor weight percentage) have been studied by Jiménez et al. in the production of biodiesel using sunflower oil having high water and FFA content [135]. They reported that optimized ethyl esters yield of 91.5% was obtained with 12:1 MREO and 14.6 wt.% of catalyst at 200 °C in 6 h. In addition, as-prepared catalysts are stable (without sulfate species leaching) with consistent catalytic activity after three catalytic cycles. Elsewhere, Gopinath et al. [136] used mesoporous sulfated Zr-KIT-6 to produce biodiesel from oleic acid and jatropha oil. The catalyst was synthesized using a hydrothermal process. Sulfated Zr-KIT-6 catalyst is produced by loading various amount of zirconia into a fixed gel in an acidic medium and then sulfating it (see Figure 11). The authors found that biodiesel conversions of 85% and 96% were achieved from jatropha oil and oleic acid, respectively, with 4 wt.% catalyst and 20:1 MRMO at 120 °C in 6 h.

Fatimah et al. [137] conducted transesterification of soybean oil with Zr₂O-supported bamboo leaf ash (Zr₂O/BLA) as a catalyst. The catalyst was synthesized using the impregnation technique and the catalyst had a 20 wt.% metal content. It is noted that the Zr₂O/BLA catalyst showed excellent transesterification performance, where biodiesel yield of 89.99% was achieved under optimal process condition (12 wt.% of catalyst and 15:1 MRMO at 50 °C for 0.5 h). However, they obtained a yield of 95.99% after 1 h of reaction. Wang et al. [138] applied a zirconium-based carbonaceous (Zr-SO₃H@CMC) catalyst to produce biodiesel from oleic acid. In this work, 99.1% biodiesel yield was obtained at 90 °C for 2h, using 5.0 wt.% catalyst amount, and 20:1 MRMO.

Guldhe et al. [139] evaluated the catalytic performance of tungstated zirconia (WO₃/ZrO₂) in the transesterification of S. Obliquus lipids with methanol. The Fourier transform infrared spectroscopy characterization of the WO₃/ZrO₂ indicated that Brønsted and Lewis acid sites are present. Transesterification of S. Obliquus lipids was carried out with 15 wt.% catalyst loading and 12:1 MRMO at 100 °C for 3 h. The maximum biodiesel conversion under this condition was 94.58%. Also, Sun et al. [140] explored the transesterification of sunflower oil using ZrO₂ loaded on La₂O₃ support (ZrO₂/La₂O₃) synthesized by wet impregnation, in the production of biodiesel (Figure 12). The authors reported optimum biodiesel conversion of 84.9% with 5 wt.% catalyst loading and 30:1 MRMO at 200 °C for 5 h.

Malhotra et al. [141] used Sodium/ZnO-loaded SBA-15 in the production of virgin cotton seed oil biodiesel. This catalyst was synthesized under atmospheric conditions by the wet impregnation method. Cotton seed oil is converted to biodiesel via transesterification process using the following conditions: 12 wt.% catalyst amount, 65 °C temperature, and 24:1 MRMO in 4 h. A yield of 98% was obtained under these conditions. After reuse for five reaction cycles, the catalyst maintained activity and no substantial reduction in catalytic performance was noted [141]. Elsewhere, the application of manganese-doped zinc oxide was reported by Baskar et al. [142] to produce biodiesel using Mahua oil. It is noticed that 97% yield was achieved with 8 wt.% catalyst amounts and 7:1 MRMO at 50 °C in 50 min. Another study from the same research group focused on using nickel-doped zinc oxide in castor oil transesterification (Figure 13) [143]. They reported optimal biodiesel yield of 95.20% in 60 min for nickel-doped zinc oxide catalyst, at 11 wt.% catalyst loading, 8:1 MRMO at 55 °C.

Borah et al. [144] investigated transesterification of Mesua ferrea oil with cobalt-doped zinc oxide catalyst. The optimized reaction was conducted with 2.5 wt.% catalyst and 9:1 MRMO at 60 °C for 3 h, and a maximum conversion of 98.03% of the feedstock was obtained. Soltani et al. [145] explored the transesterification of palm fatty acid distillate using a mesoporous SO₃H-ZnAl₄O₃ catalyst. A microwave irradiation heating system was used to synthesize the catalyst. They reported that the SO₃H-ZnAl₄O₃ catalyst showed excellent esterification performance, where an optimal yield of 94.59% was achieved with 1.5 wt.% of catalyst and 9:1 MRMO at 60 °C in 20 min. AlSharifi et al. [146] used lithium/zinc composite supported by waste chicken bone, synthesized by the wet impregnation method, to produce biodiesel from waste canola oil. The authors reported that the optimum biodiesel conversion of 98% using different metal ratios and reaction conditions of 18:1MRMO and 4.0 wt.% catalyst loading at 60 °C in 3.5 h.

Tin oxide (SnO₂) is a semiconductor material with a broad bandgap (approximately 3.7 eV). Generally, it is possible to obtain SnO₂ in mesostructured form by hydrolyzing inorganic precursors in the presence of surfactants as a structure director [147]. During calcination, most of the SnO₂ mesostructure collapses because they are not stable. An anionic surfactant with sulfate, such as that used in the preparation of mesoporous sulfated tin oxide, SO₄²⁻/SnO₂, has been proposed to improve the stability of the mesostructure [148]. The sulfate groups are isolated on the SnO₂ surface, which led to the stabilization of the mesostructure walls and ameliorates the stability. Additionally, SO₄²⁻/SnO₂ has catalytic transesterification reaction potential because it is a strong super acid. In octanoic acid esterification at temperatures below 140 °C with methanol, SO₄²⁻/SnO₂ showed superior catalytic activity to SO₄²⁻/ZrO₂ [147]. Notwithstanding, the utilization of sulfated tin oxide in the biodiesel process is still very limited. The limited study carried out on sulfated tin oxide could be attributed to its complex method of preparation because it is difficult to achieve oxide gels from its salt in contrast with sulfated zirconia, which can easily be prepared [149]. Kafuku et al. [20] applied the silica-supported SO₄²⁻/SnO₂ (SO₄²⁻/SnO₂-SiO₂) in the production of Moringa oleifera biodiesel. The optimized reaction was performed with 3 wt.% catalyst and 19.5:1 MRMO at 150 °C for 2.5 h, and a maximum yield of 84% of the feed stock was obtained. Similarly, Kafuku et al. [150] used SO₄²⁻/SnO₂-SiO₂ for transesterification of jatropha curcas. They reported an optimum methyl ester yield of 97% for a SO₄²⁻/SnO₂-SiO₂ catalyst and reaction conditions of 3.0 wt.% catalyst and 15:1 MRMO at 180 °C for 2 h.

Titanium dioxide (TiO₂) has attracted considerable interest in biodiesel production because of its acidic properties. Furthermore, the sulfuric group on the surface of TiO₂ would increase the catalyst strength [151]. He et al. [152] transesterified cottonseed oil with methanol using SO₄²⁻/ZrO₂ and SO₄²/TiO₂ catalysts. It was found that SO₄²/TiO₂ could achieve a maximum biodiesel yield of >90% compared to SO₄²⁻/ZrO₂, which could only achieve a yield of 85%. It was suggested that SO₄²/TiO₂ reactivity could be improved by adding silica SiO₂, to obtain SO₄²/TiO₂–SiO₂ [153]. The catalyst surface area increased when SO₄²/TiO₂ was loaded onto SiO₂ (from 99 m²/g to 258 m²/g). However, when compared to homogeneous catalysts temperatures between 50 and 100 °C, the temperature of the reaction is still high. Also, Gardy et al. [154] utilized TiO₂/PrSO₃H catalyst to produce WCO biodiesel and reported a 98.3% biodiesel yield with 2.5 wt.% catalyst and a 15:1 MRMO at 60 °C for 9 h. They reported that the catalyst was active in making biodiesel. They found that the catalyst retained activity, and there was no substantial loss in the activity after four cycles of reuse. In another work, Kaur et al. [155] studied a tungsten-supported TiO₂/SiO₂ catalyst in the transesterification of waste cottonseed oil. The sol–gel method was used to synthesize the catalyst. They reported an optimized biodiesel yield of 98% in 4 h at 5 wt.% catalyst loading and 30:1 MRMO at 65 °C reaction temperature.

Similarly, Dai et al. [156] examined the activity of Li₂TiO₃ on soybean oil treatment. The solid-state method was used to synthesize the catalyst, which involved the mixing and grinding of titanium oxide (TiO₂) and lithium carbonate (Li₂CO₃), after which a two-hour calcination at 800 °C was performed. The authors stated that optimum biodiesel conversion of 98.5% was attained with 6 wt.% catalyst and 24:1 MRMO at 65 °C in 2 h. To prepare a solid Li/TiO₂-based catalyst, a wet impregnation approach was employed. The authors reported that at 5.0 wt.% catalyst, 55 °C temperature, and 24:1 MRMO, they could achieve a yield of 98% in 3 h.

Furthermore, in oil refining and shape-selective petrochemical industries, zeolites are widely used as catalyst. Zeolite’s efficient catalytic activity is owing to its pore size, ion exchange, and chemical structure characteristics [62]. The zeolites framework structures contain equal-sized molecular pores and channels capable of absorbing molecules that fit within them while larger molecules are prevented. Zeolites have specific properties as a catalyst, such as shape selectivity, strong Brønsted acidity, the ability to maintain electro-neutrality, etc. [60]. However, due to the small pore size of zeolites, diffusion of reactant molecules into zeolite active sites may be a process constraint. In biodiesel production, zeolites such as H-USY, H-BETA, ZSM5, H-MOR have been discovered to have poor catalytic performance. This is because of the internal diffusion constraints of the larger reactant molecule into zeolites micropores. Thus, only the zeolite external surface catalyzes transesterification. The diffusion of bulky triglycerides molecules is severely limited; small-pore zeolites are not appropriate for transesterification [157,158]. Thus, zeolites with large pores are suitable heterogeneous acid catalysts.

In the production of castor oil biodiesel, Li et al. [159] employed Li/NaY as a catalyst. The Li/NaY was synthesized using the co-precipitation method. In the co-precipitation method, LiCO₃ and NaY zeolite support were mixed for some minutes and calcined at 750 °C for 4 h (Figure 14). They reported that an optimized oil yield of 98.6% was achieved at 3 wt.% catalyst, 18:1 MREO, at 75 °C in 2 h. Transesterification of oleic acid over zeolite Y with a Si/Al ratio of 3.1 was previously reported [160]. An optimal conversion of 85% was attained at 5 wt.% of Zeolite Y catalyst prepared from kaolin, 70 °C temperature, and 6:1 MREO in 1 h.

Yusuf et al. [161] prepared a series of desilicated zeolite-supported ZrO₂ catalyst [ZrO₂/ZSM-5(0.1–0.3 M)] via wet impregnation approach and they examined their catalytic activity in making biodiesel from soybean oil (Figure 15a). The as-prepared ZrO₂/ZSM-5(0.2 M) catalyst exhibited better catalytic activity in the transesterification of soybean oil with methanol, compared to ZrO₂/ZSM-5 catalyst, due to high surface area, large pore volume, and high dispersion of ZrO₂ on the desilicated zeolite support with improved acidity and high number of active sites. It was found that optimized oil conversion of 97.8% was achieved at 1 wt.% desilicated zeolite-supported ZrO₂ catalyst, 16:1 MRMO, and 200 °C temperature in 4 h. Moreover, the ZrO₂/ZSM-5(0.2 M) catalyst exhibited excellent stability and reusability, giving a biodiesel conversion of 93.8% after three cycles of reuse. A plausible mechanism was proposed for the transesterification reaction over the ZrO₂/ZSM-5(0.2 M) catalyst (Figure 15b).

Another solid acid based on beta zeolite was synthesized via wet impregnation method and named as SO₄²⁻/ZnO-β-zeolite and was used for simultaneous transesterification and esterification of waste cooking oil to biodiesel [162]. The highest WCO conversion of 96.9% was achieved with 3 wt.% SO₄²⁻/ZnO-β-zeolite catalyst, 15:1 MRMO, and 200 °C temperature in 8 h. Furthermore, the evaluation of the catalyst stability and reusability revealed that the catalyst demonstrated good stability, maintaining biodiesel conversion of >80% after three synthesis cycles. Kinetic study showed that the reaction of WCO to biodiesel followed first-order kinetics (with 38.58 kJ/mol activation energy), allowing proposition of a plausible mechanism for the reactions (Figure 16).

In the production of acidic soybean oil, Hou et al. [163] employed Mo/Ce/TiO₂ composites as a catalyst. They reported that Mo/Ce/TiO₂ catalyst exhibited better catalytic activity in the transesterification-esterfication of acidic soybean oil with methanol, due to the synergism of Brønsted and Lewis acid sites. They reported that an optimized oil yield of 93.8% was achieved at 5 wt.% catalyst, 30:1 MREO, at 140 °C in 8 h. In another study, Abati et al. [164] synthesize Al₂O₃ biobased heterogeneous catalysts using wet impregnation method. It was found that optimized oil conversion of 94.23% was achieved at 1 wt.% catalyst, and 90 °C temperature in 2 h. Elsewhere, the application of ZSM-5 was reported by Sathiyamoorthi et al. [165] to produce biodiesel using Jatropha curcas oil and a microchannel reactor. It is noticed that 99.7% yield was achieved with 10 wt.% catalyst amounts at 60 °C in 35 s. Another clay-based plasma-treated Co₃O₄/Kaolin catalyst was synthesized via the wet impregnation method and was used for the simultaneous transesterification and esterification of low-cost oil to biodiesel [166]. The highest oil conversion of 98.7% was achieved with 10 wt.% Co/Kaolin(400)-P(1000) catalyst, 20:1 MRMO, and 110 °C temperature in 3 h. Furthermore, the evaluation of the catalyst stability and reusability revealed that the catalyst demonstrated excellent stability, maintaining biodiesel conversion of 98.7% after five synthesis cycles. Elsewhere, Saadiah et al. [167] looked at the catalytic performance of CaO-zeolite/Fe₃O₄ catalyst in the synthesis of biodiesel from used cooking oil using conventional and magnetic reactors (Figure 17). According to the authors, the synthesized catalyst demonstrated excellent activity due to high magnetization, acid properties, and substantial surface area. In the magnetic reactor, the reaction was conducted for 300 min with 4 wt.% catalyst dosage and 5:1 MRMO at 55 °C. A maximum biodiesel yield of 96.91% was achieved with these conditions within 300 min. In addition, the CaO-zeolite/Fe₃O₄ catalyst exhibited excellent reusability, giving a biodiesel yield of 92.8% in the fourth run.

The most widely employed heterogeneous acid catalysts in liquid phase transesterification and esterification reaction are sulfonic ion exchange resins (Nafion resins, EBD resins, and Amberlyst resins) [168,169,170,171]. Sulfonic ion exchange resin has a cross-linking polymeric component where sulfonic groups bonded protons are the active sites of the transesterification and esterification process [172]. Furthermore, the swelling properties of sulfonic ion exchange resin influence their catalytic activity because swelling capacity limits reactant access to the acid site and therefore influences their activity [173]. Nafion SAC-13, EDS-100, and Amberlytes-15 are the most prevalent types of acidic ion exchange resins, and several researchers have made use of them [168,169,174]. The performance of these catalysts was observed to be good in free fatty acid esterification but poor in transesterification [175]. The performance of Amberlyst-15 as a catalyst for making biodiesel from sunflower oil was investigated by Vicente et al. [176]. It is observed that the Amberlyst-15 catalyst showed poor activity (0.7% oil conversion) with 6:1 MRMO and 1 wt.% catalyst at 60 °C for 8 h. Dos Reis et al. [177] utilized an Amberlytes-15 catalyst for biodiesel synthesis using Babassu coconut oil as substrate. Perhaps due to the high content of shorter-chain fatty acids in Babassu coconut oil and high MRMO of 300:1, Amberlytes-15 catalyst showed excellent transesterification performance, where oil conversion of 74% was obtained with 300:1 MRMO at 60 °C in 8 h. Therefore, if Amberlyte-15 is to be used, reaction temperature must be increased to 140–200 °C to achieve a faster reaction rate [171]. Amberlyte-15 and most of the ion exchange resin have poor thermal stability at a temperature above 150 °C. Therefore, this stability issue limits their applications.

Recently, biomass conversion-related reactions including transesterification and esterification have made substantial use of cellulose-derived solid acids. Among them, sulfonic (SO₃-H)-bearing cellulose-derived solid acids materials have drawn special attention [178]. These materials could incorporate a significant amount of hydrophobic molecules into the bulk of carbon because the density of the hydrophobic functional group bound to carbon sheets is very high. Such incorporation offers excellent accessibility to the sulfonic group, resulting in high catalytic efficiency. Hara [178] investigated the catalytic activity of SO₃-H-bearing carbon material in the production of oleic acid biodiesel. With a 0.307 g catalyst and 95 ° C temperature in 4 h, the optimal biodiesel yield using the SO₃-H-bearing carbon material catalyst was 99.9%. The authors found that the catalyst is stable and maintains similar catalytic activity after ten reaction cycles. Zong et al. [179] utilized sulfonated D-glucose-derived sugar for transesterification of palmitic acid and oleic acid. They found that a biodiesel yield of 95% was obtained with 5 wt.% catalyst and 10:1 MRMO at 80 °C for 5 h.

To date, there are few studies on the use of sulfonated carbon-based catalyst in the transesterification of feedstock containing triglycerides, typically from used cooking oil. Lou et al. [180] utilized sulfonated carbon-based catalyst produced from sucrose, cellulose, and starch in the transesterification of WCO using methanol. They claimed that the carbon-based catalyst produced from starch has higher acid sites, higher pore size, and higher pore volume which allowed reactants to access the SO₃-H sites. Furthermore, the optimum used cooking oil conversion using the carbon-based catalyst produced from starch was 92% with 30:1 MRMO and 10 wt.% catalyst at 80 °C in 8 h. Also, there was an insignificant change in performance of the catalyst even after fifty cycles, proving it to be a very stable catalyst.

By sulfonation of carbonized vegetable oil asphalt, Shu’s group developed a carbon-based catalyst and the catalyst was made up of a flexible carbon-based framework comprising sulfonic acid groups with polycyclic aromatic hydrocarbons that are highly dispersed [181]. They found that the carbon-based catalyst exhibited improved activity for the transesterification of WCO and provided an optimum conversion of 80.5% and 94.8% for triglycerides and FFA, respectively, with 0.2 wt.% catalyst and 16.8:1 MRMO at 220 °C in 4.5 h. They explained that optimum conversion exhibited by the catalyst is due to the hydrophobicity of the carbon sheet which prevented the hydroxyl group from being hydrated and bonded –SO₃H hydrophilic groups that increased the contact of methanol with the protonated carboxylic group of triglycerides. The development of an effective carbon-based acid catalyst for biodiesel production at mild reaction conditions remains a challenge. Nonetheless, carbon-based catalysts need to be further improved, including transesterification reaction conditions and improvement on catalyst synthesis.

In the field of heterogeneous acid catalysts, heteropolyacids (HPAs) also have significant potential for transesterification of waste vegetable oils. HPAs are mostly a type of solid acid with a well-defined structure that consists of early transition metal-oxygen anion clusters. HPAs are considered to be a more environmentally friendly and cost-effective alternative to traditional acid catalysts because of their high stability, high mobility of proton, strong Brønsted acidity, and non-toxicity [182]. The most widely used HPA happens to be heteropoly tungstate (H₃PW₁₂O₄₀), which has demonstrated outstanding performance in acid-catalyzed reaction including transesterification and esterification [183]. Cao et al. [184] evaluated the acidic activity of H₃PW₁₂O₄₀·6H₂O (PW12), for the transesterification of WCO. A biodiesel conversion of 87% was attained with 4.0 wt.% catalyst and 70:1 MRMO at 65 °C in 14 h. The authors found that the H₃PW₁₂O₄₀·6H₂O catalyst had a high FFA tolerance, and its catalytic activity was maintained after five catalytic cycles. However, the application of this catalyst on an industrial scale is restricted by the long reaction time and the high MRMO. In addition, if H₃PW₁₂O₄₀·6H₂O catalyst is to be used, the reaction temperature must not be more than 65 °C to achieve the stable catalytic performance [184]. They claimed that the transesterification must not be performed at a higher temperature because waste cooking oil contains a number of undesirable compounds that may induce side reactions. An attempt has been made to produce heteropolyacids with increased acidity by incorporating Lewis acid into them, resulting in heteropolyacids with Brønsted and Lewis acid sites [185]. Zhang et al. [185] transesterified used cooking oil over a Zr_0.7H_0.2PW₁₂O₄₀ (ZrHPW) catalyst. They found that the Zr_0.7H_0.2PW₁₂O₄₀ catalyst exhibited high acidity capacity compared to the original H₃PW₁₂O₄₀ catalyst. The investigated Zr_0.7H_0.2PW₁₂O₄₀ catalyst displays unique properties and yielded an optimum biodiesel conversion of 98.8% with 2.1 wt.% catalyst and 20:1 MRMO at 65 °C in 8 h. In addition, after five catalytic cycles, a 95% yield was obtained. Elsewhere, another author used H₃PW₁₂O₄₀/Nb₂O₅ catalyst in the transesterification process, which under optimum conditions produced a methyl ester yield of 92% with 18:1 MRMO and 3 wt.% catalyst at 200 °C in 20 h [186]. Brahmkhatri et al. [187] esterified lauric acid with H₃PW₁₂O₄₀-supported MCM-41 (Figure 18). They reported that butyl laurate yield reached 95% with 0.2 g of H₃PW₁₂O₄₀ loaded on MCM-41 supports catalyst and 1:2 oil to butanol ratio at 90 °C in 3h.

Kulkarni et al. [188] utilized H₃PW₁₂O₄₀ loaded on various supports (Al₂O₃, ZrO₂, SiO₂, and activated carbon) in the production of canola oil biodiesel, where the biodiesel yield reached 90% with 9:1 MRMO and 3.0 wt.% catalyst (H₃PW₁₂O₄₀ loaded on ZrO₂ supports) at 200 °C in 10 h. Alcañiz -Monge et al. [189] reported biodiesel production from esterification of palmitic acid using an activated carbon fiber-supported H₃PW₁₂O₄₀ (H₃PW₁₂O₄₀/ACF) catalyst. The authors obtained a biodiesel conversion of 89% with a 97:1 MRMO at 40 °C in 6 h.

Metal–organic framework-based acid catalysts have been extensively explored for the esterification or transesterification of edible/inedible oils to biodiesel [190,191]. Zhang et al. [192] synthesized nickel salt Keggin-type heteropolyacid on Zr(IV)-based metal–organic frameworks (NiHSiW/UiO-66) through hydrothermal method and investigated their catalytic performance in the production of biodiesel from oleic acid. The NiHSiW/UiO-66 catalyst was utilized as effective solid acid in the esterification of oleic acid to biodiesel with an optimal biodiesel conversion of 86.7% with 6 wt.% catalyst loading and 18:1 MRMO at 160 °C in 3 h. The high specific surface area of the NiHSiW/UiO-66 catalyst, as well as the synergistic effects of NiHSiW salts and the UiO-66 matrix, contributed to its exceptional activity. It was found that the NiHSiW/UiO-66 catalyst is relatively stable with around 50% biodiesel conversion after eight-times used. Similarly, Zhang et al. [193] explored the esterification of oleic acid using a Fe-BTC framework composite. A microwave irradiation heating system was used to synthesize the catalyst. They reported that the catalyst showed excellent esterification performance, where an optimal conversion of 72.3% was achieved with 10 wt.% catalyst loading and 16:1 MRMO at 160 °C in 3 h. Another solid acid based on metal–organic framework was synthesized via impregnation method and named as Sn₁.₅PW/Cu-BTC, and was used for the esterification of oleic acid to biodiesel with 87.7% conversion [194]. Amouhadi et al. [195] used the solvothermal method for preparing a heterogeneous MnO₂@Mn(btc) catalyst for the esterification of oleic acid with biodiesel yield of 98% using the following reaction conditions: 5 wt.% catalyst loading and 30:1 MRMO at 200 °C for 5 h. Qian et al. [196] synthesized a series of MOF-808-R catalysts (R represents the volume ratio of N,N-dimethylformamide (DMF) to formic acid, ranging from 3/1 to 1/3) via solvothermal method and investigated their catalytic activity in microalgal lipids to FAME (Figure 19a). The as-prepared MOF-808-3/1 catalyst shows enhanced catalytic activity with a conversion efficiency of 96.24%. According to the authors, the increase in catalytic activity of MOF-808-3/1 catalyst is due to increased acidity, enhanced hierarchical porosity, and increased pore size from 1.63 nm to 5.34 nm. Moreover, the MOF-808-3/1 catalyst exhibited excellent reusability with a biodiesel conversion of 93.78% after six cycles. Through density functional theory, it was confirmed that the exposed Zr⁴⁺ sites serve as the active centers for the reaction, and a plausible reaction route was proposed (Figure 19b).

Taddeo et al. [197] prepared three different types of MOF-5 (T-MOF-5, C-MOF-5, and B-MOF-5, where T, C, and B represent tetragonal, cubic, and blend respectively), characterized and then investigated their catalytic activity in the production waste cooking oil biodiesel through simultaneous transesterification and esterification reaction. It was found that B-MOF-5 catalyst is suitable for the simultaneous transesterification and esterification of waste oil with a biodiesel yield of 70% using the following reaction conditions: 5 wt.% catalyst loading and 20:1 MRMO at 150 °C for 3 h. Elsewhere, Li et al. [198] reported the use of a biomass-derived hydrophobic metal–organic framework (FDCA/SA-Hf) for the esterification of oleic acid (Figure 20). Under optimum reaction conditions of 4.1 wt.% catalyst and 19:1 MRMO at 49 °C for 9.5 h, the oleic acid yield obtained was 98.6%. Moreover, after six runs, the biodiesel yield was 90% due to the high hydrophobicity and stability. It is noted that the [Hf-O-CH₃] bond species observed in the presence of water is stabilized by the presence of a hydrophobic alkyl chain (SA) grafted on the acidic sites. This hydrophobic chain prevents the aggregation and adsorption of water at the acidic sites, which otherwise would lead to hydrolysis. As a result, the stability of the [Hf-O-CH₃] bond species is maintained despite the presence of water.

Oghabi et al. [199] used MOF nanocatalyst for the conversion of oleic acid in the presence of methanol. Their results suggested high capability of the developed catalyst with a biodiesel conversion of 95% under reaction conditions of 3 wt.% catalyst dosage and 10:1 MRMO at 160 °C in 6 h. Also, the developed nanocatalyst was highly reusable, with a biodiesel conversion of 87% after six sequence cycles. Narenji-Sani et al. [200] studied biodiesel production from free fatty acids using ZIF-like grafted H₆P₂W₁₈O₆₂ catalyst (Figure 21). They reported the highest conversion of 92% at 3 wt.% catalyst loading and 60:1 MRMO at 80 °C in 4 h.

Zhang et al. [201] prepared ZrSiW/Fe-BTC and ZrSiW/UiO-66 catalysts via hydrothermal method and investigated their catalytic performance in the production of biodiesel from oleic acid. In comparison with ZrSiW/Fe-BTC, the ZrSiW/UiO-66 catalyst exhibited better catalytic performance in the conversion of oleic acid to biodiesel in the presence of methanol, due to its high surface area, large pore size, and high acidity. They reported an oleic acid conversion of 98% using 0.24 g ZrSiW/UiO-66 catalyst, 20:1 MRMO, and 160 °C temperature in 4 h. Moreover, the catalyst showed excellent stability and reusability, with a biodiesel conversion of 88.9% after six consecutive runs. The authors also proposed a plausible mechanism for the esterification reaction involved (Figure 22).

Similarly, Pangestu et al. [202] employed a CuBTc-MOF catalyst prepared through solvothermal method for transesterification of palm oil. An optimum biodiesel yield of 91% was reported for CuBTc-MOF catalyst and reaction conditions of 0.04 g catalyst and 5:1 MRMO at 60 °C for 4 h. The plausible mechanism for transesterification reaction was proposed by the authors (Figure 23). They also reported that CuBTc-MOF catalyst is reusable over several cycles with no significant loss in activity.

Yusuf et al. [203] prepared CuO/UiO-66 metal–organic frameworks via a wet impregnation approach and examined their catalytic activity in making biodiesel from waste cooking oil. The as-prepared 7%CuO/UiO-66 catalyst exhibited better catalytic activity in the transesterification of waste cooking oil with methanol, due to the synergistic effect between UiO-66 and copper oxide in the composite. It was found that optimized oil conversion of 90.1% was achieved at 6 wt.% 7%CuO/UiO-66 catalyst, 20:1 MRMO, and 160 °C temperature in 4 h. Moreover, the 7%CuO/UiO-66 catalyst exhibited excellent stability and reusability, giving a biodiesel conversion of 80% after three cycles of reuse.

Table 4. Some studies on biodiesel production using heterogeneous acid catalysts.

Catalyst	Feedstock	Reaction Parameters (Reaction Time, Temperature, Catalyst Amount, and MRMO)	Performance	Ref.
SO42−/ZrO2	Palm kernel oil Coconut oil	4 h, 200 °C, 1 wt.%, 6:1	Yield = 90.3% Yield = 86.3%	[129]
ZrO2 loaded on different supports (TiO2, SiO2, Fe2O3, and Al2O3)	Waste cooking oil	3 h, 120 °C, 0.1 wt.%, 120:1	Conversion = 48.6%	[130]
WO3/ZrO2	Waste vegetable oil	1 h, 75 °C, 4 wt.%, 9:1	Conversion = 93%	[131]
SO42−/ZrO2-MCM-41	Sunflower oil	5 h, 200 °C, 14.6 wt.%, 14:1	Yield = 91.5%	[135]
Sulfated Zr-KIT-6	Oleic acid	6 h, 120 °C, 4 wt.%, 20:1	Conversion = 96%	[136]
WO3/ZrO2	S. Obliquus lipids	3 h, 100 °C, 15 wt.%, 12:1	Conversion = 94.58%	[139]
Zr2O-supported bamboo leaf ash (Zr2O/BLA)	Soybean oil	30 min, 50 °C, 12 wt.%, 15:1	Yield = 89.99%	[137]
Zirconium-based carbonaceous	Oleic acid	2 h, 90 °C, 5 wt.%, 20:1	Yield = 99.1%	[138]
ZrO2/La2O3	Sunflower oil	5 h, 200 °C, 5 wt.%, 30:1	Yield = 84.9%	[140]
Na/ZnO-loaded SBA-15	Cotton seed oil	4 h, 65 °C, 12 wt.%, 24:1	Yield = 98%	[141]
Manganese-doped ZnO	Mahua oil	50 min, 50 °C, 8 wt.%, 7:1	Yield = 97%	[142]
Nickel-doped ZnO	Castor oil	1 h, 55 °C, 11 wt.%, 8:1	Yield = 95.20%	[143]
Cobalt-doped ZnO	Mesua ferrea oil	3 h, 60 °C, 2.5 wt.%, 9:1	Conversion = 98.03%	[144]
SO3H-ZnAl4O3	Palm fatty acid	20 min, 60 °C, 1.5 wt.%, 9:1	Yield = 94.59%	[145]
Lithium/Zinc composite-supported waste chicken bone	Waste canola oil	3.5 h, 60 °C, 4.0 wt.%, 18:1	Conversion = 98%	[146]
SO42−/SnO2-SiO2	Jatropha curcas	2 h, 180 °C, 3 wt.%, 15:1	Yield = 97%	[150]
SO42−/TiO2	Cottonseed oil	8 h, 230 °C, 2 wt.%, 12:1	Yield = >90%	[152]
SO42−/TiO2–SiO2	Cottonseed oil	6 h, 200 °C, 3 wt.%, 9:1	Conversion = 92%	[153]
TiO2/PrSO3H	Waste cooking oil	9 h, 60 °C, 2.5 wt.%, 15:1	Yield = 98.3%	[154]
20-W/TiO2/SiO2	Waste Cottonseed oil	4 h, 65 °C, 5 wt.%, 30:1	Yield = 98%	[155]
Li2TiO3	Soybean oil	2 h, 65 °C, 6 wt.%, 24:1	Conversion = 98.5%	[156]
Li/NaY zeolite	Castor oil	2 h, 75 °C, 3 wt.%, 18:1	Yield = 98.6%	[159]
Zeolite Y	Oleic acid	1 h, 70 °C, 5 wt.%, 6:1	Conversion = 85%	[160]
ZrO2/ZSM-5(0.2 M)	Soybean oil	4 h, 200 °C, 1 wt.%, 16:1	Conversion = 97.8%	[161]
SO42−/ZnO-β-zeolite	Waste cooking oil	8 h, 200 °C, 3 wt.%, 15:1	Conversion = 96.9%	[162]
CaO-zeolite/Fe3O4	Used cooking oil	5 h, 55 °C, 4 wt.%, 5:1	Yield = 96.91%	[167]
Amberlyst-15	Babassu coconut oil	8 h, 60 °C, 300:1	Conversion = 74%	[177]
SO3-H-bearing carbon material	Oleic acid	4 h, 95 °C, 0.307 g	Yield = 99.9%	[178]
Sulfonated vegetable oil asphalt	Waste cooking oil	4.5 h, 220 °C, 0.2 wt.%, 16.8:1	Conversion = 94.8%	[181]
Sulfonated D-glucose derived sugar	Palmitic acid and oleic acid	5 h, 80 °C, 5 wt.%, 10:1	Yield = 95%	[179]
Sulfonated carbon-based catalyst derived from starch	Waste cooking oil	8 h, 80 °C, 10 wt.%, 30:1	Yield = 92%	[180]
H3PW12O40·6H2O (PW12)	Waste cooking oil	14 h, 65 °C, 4 wt.%, 70:1	Conversion = 87%	[184]
Zr0.7H0.2PW12O40	Waste cooking oil	8 h, 65 °C, 2.1 wt.%, 20:1	Conversion = 98.8%	[185]
H3PW12O40/Nb2O5	Used cooking oil	20 h, 200 °C, 3 wt.%, 18:1	Yield = 92%	[186]
H3PW12O40 loaded on ZrO2	Canola oil	10 h, 200 °C, 3 wt.%, 9:1	Yield = 90%	[188]
H3PW12O40/ACF	Palmitic acid	6 h, 40 °C, 1.1 wt.%, 97:1	Conversion = 89%	[189]
H3PW12O40-supported MCM-41	Lauric acid	3 h, 90 °C. 2:1	Yield = 95%	[187]
NiHSiW/UiO-66	Oleic acid	3 h, 160 °C, 6 wt.%, 18:1	Conversion = 86.7%	[192]
Fe-BTC frameworks	Oleic acid	3 h, 160 °C, 10 wt.%, 16:1	Conversion = 72.3%	[193]
MnO2@Mn(btc)	Oleic acid	5 h, 200 °C, 5 wt.%, 30:1	Conversion = 98%	[195]
Blend-MOF-5	Waste cooking oil	3 h, 150 °C, 5 wt.%, 20:1	Yield = 70%	[197]
FDCA/SA-Hf	Oleic acid	9.5 h, 49 °C, 5 wt.%, 19:1	Yield = 98.6%	[198]
ZIF-like grafted H6P2W18O62	Free fatty acids	4 h, 80 °C, 3 wt.%, 60:1	Conversion = 92%	[200]

Comparison of different heterogeneous acid catalysts used in several research studies to convert different feedstock into biodiesel.

4. Process Conditions/Parameters for Biodiesel Production

Transesterification reactions occur under various reaction process conditions, including reaction time, molar ratio of alcohol to oil, reaction temperature, and catalyst amount, all of which have a significant effect on biodiesel yield. To achieve a complete reaction and high biodiesel yield that meets regulatory specifications, these conditions have to be optimized during the transesterification reaction. The effects of these reaction conditions are explained in detail in the sections that follow, as depicted in Figure 24.

4.1. Effect of Methanol to Oil Molar Ratio (MRMO)

MRMO is vital in both transesterification and esterification reaction of triglycerides. Excess methanol is necessary to achieve high biodiesel yield during the reversible esterification of high FFA triglycerides to biodiesel [204]. When triglyceride interacts with the active site of the catalyst, it is converted into fatty acid methyl ester (FAME). At the carbonyl group, triglycerides are protonated to form carbocation, that could undergo a reaction to produce an ester [205]. However, excess methanol is flooded into the active sites of the catalyst, preventing formation of protonated triglycerides. During methanolysis, excess methanol is problematic during the separation of glycerol and ester layer because of the solubility of glycerol layer in the ester layer. Methanol functions as an emulsifier due to its polar nature, aiding emulsion formation [56]. Nevertheless, it is believed that using a large amount of methanol improved the interaction between triglycerides and catalyst and enhanced biodiesel yield [206].

Several attempts have been made to investigate the optimum MRMO required to obtain higher biodiesel yield. Li et al. [207] investigated the effect of process parameters on the biodiesel yield using MRMO in the range of 10:1 to 30:1. An optimal biodiesel yield of 90% was obtained using 20:1 MRMO at 65 °C with 3 wt.% catalyst amounts. Furthermore, a significant decrease in biodiesel yield was observed when changing the MRMO from 20:1 to 30:1. Hence, for economics and ease of methanol regeneration, 20:1 MRMO was adopted. Atapour et al. [208] found that a 90.8% biodiesel yield was obtained using 7:1 MRMO. The authors further observed that the biodiesel yield decreased by increasing the MRMO from 7:1 to 11:1 for the given transesterification reaction.

Amani et al. [209] investigated the impact of the MRMO on waste cooking palm oil biodiesel production. A biodiesel yield of 93% was obtained from the transesterification using 14:1 MRMO. Furthermore, when the MRMO was raised from 14:1 to 18:1, the biodiesel yield decreased. Similarly, Hoque et al. [210] explored the impact of methanol on biodiesel yield during WCO transesterification. They reported that maximum biodiesel yield was obtained with a 6:1 MRMO in 2 h. Birla et al. [101] utilized a heterogeneous catalyst obtained from snail shells to examine the effect of different variables on the production of biodiesel. An optimum biodiesel yield of 87.28% was obtained using 2.0 wt.% catalyst and optimal MRMO of 8.45:1 in 7 h. Furthermore, the authors observed no increase in biodiesel yield while the MRMO was increased beyond the optimum.

Phan et al. [64] studied the effect of methanol on biodiesel yield during transesterification of WCO using KOH catalyst. The authors found that methyl ester yield increased as MRMO increased from 5:1 to 8:1, and an optimum yield of 90% was obtained using 8:1 MRMO after about 90 min. They observed that biodiesel yield decreased when MRMO increased beyond 9:1 under the same process conditions. The authors further explained that the decrease in product yield was because separation of glycerol becomes difficult at higher MRMO and the yield of methyl ester reduced since a part of the byproduct, which is glycerol, remains in the biodiesel phase.

Patil et al. [211] investigated the effect of MRMO on biodiesel yield from WCO. They found that biodiesel yield increased with MRMO. However, once the optimal MRMO of 9:1 was reached, the effect became insignificant, and the yield remained essentially unchanged. Similarly, the influence of methanol amount on biodiesel conversion from transesterification reaction of castor and linseed oils was investigated by Varma et al. [212]. They found that biodiesel yield increased with MRMO and maximum biodiesel yield was realized at an optimal MRMO of 40:1 from transesterification of each feedstock. Further, biodiesel yield remained constant when the MRMO increased from 40:1 to 70:1. Song et al. [213] studied the effect of methanol amount on biodiesel conversion from transesterification reaction of RBD (refined, bleached, and deodorized) palm oil. The results showed that biodiesel yield increased with increasing MRMO and maximum biodiesel yield was attained at an optimum MRMO of 30:1 from transesterification of the feedstock. Further, it was observed that biodiesel yield remained constant when MRMO increased beyond the optimum. He et al. [214] investigated the impact of methanol/oil molar ratio on biodiesel production from soybean oil. The authors claimed that biodiesel yield increased as the MRMO increased, and maximum yield was obtained with MRMO of 40:1. Moreover, the authors further reported that no substantial increase in the biodiesel yield was observed when MRMO increased beyond the optimal molar ratio (40:1). Using a 12:1 molar ratio of methanol to vegetable oil, Kataria et al. [215] reported the maximum biodiesel yield of 98.5%. However, they observed a reduction in biodiesel yield beyond the optimum ratio due to excess methanol, resulting in the production of excess glycerol that could potentially block the active sites in the catalyst. Kant et al. [216] also reported similar results, achieving a conversion of 75.3% when using MRMO of 25:1. However, increasing the MRMO beyond the optimal ratio led to a decrease in the conversion. Furthermore, it was noted that an excess methanol had the potential to deactivate enzymatic catalyst, thereby affecting the overall biodiesel conversion [217].

In principle, it is generally concluded that the MRMO is one of the most important parameters that influence biodiesel yield during transesterification of oil to biodiesel. During transesterification of triglycerides to biodiesel, excess alcohol is required to push the reversible reaction process to the product side in order to increase reaction rate and biodiesel yield. However, as some studies have shown, using too much alcohol slowed the formation of the biodiesel products due to dilution of the system [218]. The use of excess amounts of alcohol also poses problems in processing, equipment cost, and energy consumption for methanol recovery [218,219]. Moreover, because of the increased solubility of glycerol in the ester phase, a considerable amount of methanol interferes with glycerol separation [220]. When glycerol is present in an ester solution, it aids in the shift of the reversible reaction process to the reactant’s side, reducing the ester yield [221]. Therefore, for transesterification of the selected feedstock, MRMO must be optimized in order to reduce biodiesel production cost and obtain the highest biodiesel yield possible.

4.2. Effect of Temperature

Another important process parameter that influences biodiesel yield and reaction rate during transesterification reaction is the temperature [48,49]. In principle, the rate of reaction increases as the temperature increases. The transesterification reaction can occur at various temperatures; however, a higher temperature is favorable, as it pushes the reaction to the product side due to its endothermic nature [222]. The use of high temperature reduces oil viscosities and mass transfer problems between the phases, which in turn increases biodiesel yield, reduces reaction time, and increases the rate of reaction [40,46].

Several experiments have been carried out to investigate the effect of temperature on biodiesel yield in order to achieve optimum temperature for the transesterification reaction. Kusdiana et al. [223] investigated the optimum reaction temperature to obtain higher biodiesel yield using supercritical methanol. They found that the conversion increases as the reaction temperature rises from 200 to 400 ℃. The authors noted that after 4 min at 350 °C, rapeseed oil was completely converted to biodiesel, yielding 95.0 wt.% biodiesel. However, the rapeseed oil conversion decreased as the temperature was increased above 350 °C. This is because biodiesel thermal decomposition takes place at a temperature above 350 °C. Varma et al. [212] reported similar findings. According to these authors, the conversion of linseed oil and castor oil to biodiesel increased with an increase in temperature, but at a temperature above 350 °C, thermal decomposition of biodiesel was observed. In addition, they also found that conversion of palm oil and groundnut oil to biodiesel increased with temperature, and biodiesel yield reached a maximum at 350 °C after 40 min. Miao et al. [224] explored the role of reaction temperature on biodiesel yield. They used methanol to transesterify soybean oil at temperatures ranging from 100 to 200 °C. A high biodiesel yield of 98.4% was recorded by the authors at 120 °C, which was considered to be the reaction’s optimal temperature. In the presence of an alkaline catalyst, Phan et al. evaluated the impact of temperature on the methyl ester yield utilizing used cooking oil as a feedstock [64]. They reported that methyl ester content increased as the temperature was increased from 30 to 50 °C, and the methyl ester content slightly decreased as the temperature was increased up to 70 °C because both saponification and transesterification reactions were enhanced at high temperatures. Lam et al. [225] evaluated the transesterification of WCO over a SO₄²⁻/SnO₂–SiO₂ catalyst to produce biodiesel. They found that temperature significantly influences biodiesel yield. The biodiesel yield increased with temperature, with an optimal yield of 81.4% obtained at 150 °C after 1 h. Furthermore, as stated by Saka et al. [226], at a higher temperature, typically around 400 °C, thermal decomposition of triglycerides occurred. Bahar et al. [227] reported high biodiesel yield when the reaction was conducted at room temperature. However, it was observed that beyond optimal temperature, there is a decrease in biodiesel yield. This decrease was attributed to the necessity to keeping the reaction temperature within the boiling point of methanol to prevent the evaporation of methanol. Gunay et al. [228] observed that high temperature promoted saponification reaction, which in turn decrease the biodiesel yield. Therefore, it is essential to optimize the reaction temperature to achieve a high biodiesel yield.

4.3. Effect of Reaction Time

Reaction time plays a crucial role in determining the reaction kinetics of oil conversion to biodiesel under supercritical conditions [229]. The conversion of triglycerides to biodiesel was observed to increase with time in the transesterification of triglycerides [229]. This was attributed to an increase in the reaction rate constant based on the Arrhenius equation. Also, owing to the mass transfer limitation between the oil–methanol catalyst, the conversion rate increases with reaction time. Nevertheless, exceeding the optimum reaction time can result in lower biodiesel yield due to backward transesterification reaction, which in turn reduces the ester content [230,231].

The reaction time is one of the important factors that influence triglyceride conversion to biodiesel; therefore, various attempts have been made to examine the impact of reaction time on biodiesel yield and to determine the optimal time for the transesterification reaction. Rashid et al. [63] studied the influence of reaction time on biodiesel production. The authors reported gradual increases in ester yield with time, obtaining a maximum biodiesel yield of 97.81% in about 70 min. Similarly, Lam et al. [225] investigated the influence of biodiesel yield during transesterification of WCO in the presence of SO₄²⁻/SnO₂ catalyst using a methanol–ethanol mixture. They reported that a maximum biodiesel yield of 81.40% was obtained with an MREO of 9:6:1, 6.0 wt.% catalyst, reaction temperature of 150 °C, and 60 min reaction time. In addition, the reaction temperature has a direct impact on the reaction time, where a higher reaction temperature assists in the reduction of reaction time for biodiesel production and vice versa [232,233]. The influence of temperature on reaction time during the production of used cooking oil biodiesel was evaluated by Fu et al. [234]. The authors observed that at 65 °C, 80% biodiesel yield was obtained in 11 h reaction time, while at 120 °C, the same biodiesel yield was achieved within just 2 h. Furthermore, the biodiesel yield increased to 95.2% when the reaction at 120 °C was continued for 5 h. Similarly, Kapilakarn et al. [235] obtained a fatty acid methyl ester (FAME) yield of 97% in 50 min at 55 °C, whereas the same FAME yield was obtained in only 20 min at 70 °C.

Thus, it can be inferred that reaction time is crucial in the production of biodiesel from triglyceride transesterification reactions. Since the transesterification reaction is reversible, reaction time must be enhanced to prevent backward reaction and achieve the maximum biodiesel yield for sustainable energy production.

4.4. Effect of Catalyst Loading

A catalyst is believed to increase the rate of a reaction by lowering the overall reaction activation energy. Therefore, the presence of catalysts in the conversion of triglycerides to biodiesel speeds up the reaction process by reducing the required activation energy. Typically, the reaction will be initiated with small catalyst loading, and it will be gradually increased, while other conditions are kept constant, until an optimum biodiesel yield is obtained. Bhatti et al. [236] reported a high biodiesel yield on chicken tallow at 40 °C when H₂SO₄ catalyst loading was increased from 1 to 3 g. Biodiesel yield decreased when H₂SO₄ catalyst loading was increased above 2.5 g at the same temperature. Dependence of biodiesel yield on catalyst loading differs slightly at other temperatures. Similarly, in the presence of methanol, Amani et al. [237] evaluated the impact of catalyst loading on biodiesel yield. The authors reported a maximum biodiesel yield of 90% using 3 wt.% catalyst loading in 3 h. However, it was observed that further increasing the catalyst loading to 4 wt.% lowered the biodiesel yield from 90% to 83%. It must also be mentioned that low catalyst amounts can lead to methanol solubility in low-cost feedstock such as waste cooking oil [238]. Santya et al. [239] reported that the optimal catalyst loading in the production of biodiesel from WCO was 1.5 wt.%. It was observed that increasing the catalyst loading beyond the optimum level resulted in a decrease in biodiesel conversion [240]. They identified mass transfer limitations as the primary reason for the decline in biodiesel conversion. They observed that increasing the catalyst loading beyond the optimum level (optimum level: 4.5 wt.%) resulted in a decrease in conversion rate. Therefore, the interdependence of reaction parameters necessitates their optimization, which should be tailored to the specific feedstocks and catalyst employed for the production of biodiesel [241].

5. Conclusions and Future Outlook

Among the various biodiesel production processes from oils and fats, transesterification is the most appealing process because transesterification produces fuel that is highly compatible with the diesel engines that are currently in use. The transesterification process is designed to reduce the viscosity of fats and natural oils. Although microemulsion of oils reduces viscosity, there are still issues with engine performance. This review presents a discussion of the biodiesel production process and various catalysts used in laboratory-scale biodiesel production to date. The following conclusions may be drawn from this review.

1. The feedstock used in biodiesel production is an essential parameter to consider when estimating the overall cost of biodiesel production. Therefore, many studies of various non-food crops feedstock, including non-edible oils, algal lipid, and WCO, have been carried out to reduce the cost of biodiesel production, and also to prevent competition between food and fuel for the same resources.

2. A base homogeneous catalysts process requires mild reaction conditions, has high biodiesel yield, is inexpensive, and has a rapid reaction rate. Nevertheless, the conversion is only effective with refined oils because the catalyst is sensitive to the FFA content of the oil, and produces glycerol and soap as byproducts. As a result, a substantial amount of water is required during the purification process.

3. The acid homogeneous catalyst is suitable for low-cost oil feedstock and it is insensitive to free fatty acid content of oils. Nevertheless, there are a few drawbacks to this process, such as slow reaction rate, corrosion problems, and difficulty in separation of catalyst from the products.

4. A heterogeneous catalyst offers several benefits over a homogeneous catalyst, including fewer disposal issues, elimination of product washing step, less corrosiveness, efficient conversion, faster reaction rate, environmental friendliness, mild reaction conditions, and easy regeneration. However, some issues are associated with heterogeneous catalysts compared with homogeneous catalysts; these include expensive catalyst preparation, being time-consuming, and a mass transfer problem.

5. The biocatalyst process requires a lower reaction temperature. However, this process has a few drawbacks such as slow reaction rate and the high cost of enzyme preparation. Furthermore, the enzyme activity is reduced due to glycerin solubility in biodiesel. The adaptability of the enzyme and further improvement of the current process must be studied in order to reduce cost and improve the reaction rate.

Overall, future work should focus on addressing the following aspects:

• Development of economically viable heterogeneous catalysts with both high activity and selectivity suitable for use in the industry;
• Investigating novel catalyst supports characterized by a selective surface area and an interconnected system of suitable pore sizes;
• Further investigation into biomass or waste materials as catalyst sources in order to develop new catalysts, reduce the cost of biodiesel and enhance the sustainability of commercially available solid catalysts;
• Improving hydrotalcite-based catalyst preparation routes and treatment steps in order to facilitate their transition from laboratory-scale applications to industrial-scale utilization;
• Continuing research and exploration of industrial enzymatic biodiesel production as a prospective and sustainable option for the future;
• Careful design of one-step catalyst preparation routes, avoiding complex multistep grafting methods, to enhance the interaction between acid sites and the catalyst framework structure, thus preventing the leakage of active phases;
• Research focused on investigating the phase behavior of mixtures and enhancing product purification processes;
• The technical viability of the integrated biodiesel–glycerol processing plant should be reevaluated by conducting a techno-economic analysis using real-world experimental and pilot-scale data.