Lewis Acid-Base Site-Assisted In Situ Transesterification Catalysis to Produce Biodiesel

Abstract

Biodiesel, a potent replacement for petroleum diesel, is derived from fatty acids in biomass through transesterification, which is renewable, non-toxic, and biodegradable and is a powerful replacement for petroleum diesel. Lewis acid has been proven effective for esterification and transesterification. The Lewis base enhances the electrophilic and nucleophilic properties of the molecules that bind to it, leading to the remarkable versatility of the Lewis base catalytic reaction. Many studies have shown that Lewis acid/base catalyzed in situ transesterification is a fast and environmentally friendly method for producing biodiesel. The utilization of Lewis acid-base sites to catalyze transesterification has been shown to enhance their efficiency and utilization of acid-base active sites. This review explores biodiesel production by different catalysts using Lewis acid-base sites, the conditions for catalytic transesterification, the effects of different reaction parameters on biodiesel production, and the biodiesel production process.

1. Introduction

Biodiesel, a sustainable energy source, has always been regarded as a potential substitute for oil. Transesterification is the primary method for producing biodiesel from natural biomass fats. Biodiesel production typically consists of two parts: lipid extraction and transesterification; transesterification usually involves a chemical reaction between lipids and methanol or ethanol [1]. However, there are more efficient ways to produce biodiesel than this method, as additional steps are required. The ester exchange process consists of two parts: lipid extraction and ester exchange. Situester exchange simplifies the lipid extraction step and enables efficient synthesis of biodiesel [2]. In producing lipids from plants, wastes, and other biomass, selecting dried biomass is beneficial for pretreatment [3,4] and breaking down biomass into small molecules as much as possible. For example, lipids in microalgae are concentrated in the cell [5]. The cell wall needs to be broken to release lipids. Pretreatment brings higher energy consumption, such as heating and mechanical shredding, which hinders the economic development of biodiesel. In situ transesterification can directly use wet biomass, such as microalgae salvaged from water, to produce biodiesel through a one-pot process, significantly improving production efficiency and reducing the energy consumption of drying microalgae [6]. In situ transesterification involves strong acids, such as sulfuric acid and nitric acid. It is not only accessible for the production equipment, but the excess acid in the oil also must be removed to make the resulting biodiesel up to the standard [7]. Lewis acid has higher reaction activity than traditional strong acids and can be transesterified under mild conditions [8].

Alkaline catalysts are also essential catalysts for transesterification, as they are easily obtained and inexpensive [9]. Basic hydroxide and methanol used in commercial biodiesel plants are efficient and short-lived. Studies have shown that compared to acidic catalysts, homogeneous basic catalysts such as basic hydroxide and basic methanol (NaOCH₃ and KOCH₃), the transesterification process can be shortened to one-third or even shorter than the original time, and extremely high conversion efficiency has been demonstrated within a few minutes of the initial reaction. However, in the production of biodiesel, the free high-oleic acid (FFA) environment may lead to soap formation. When the FFAs content exceeds 2 wt%, a soap formation reaction will occur in the system to form soap [10,11]. In order to make biodiesel comply with EN 14214 [12] and other standards, biodiesel must be purified to remove soluble impurities such as soap residual catalysts, methanol, and glycerol [13]. Dry and wet cleaning methods are commonly used in commerce to purify crude biodiesel. Dry cleaning uses ion exchange resin or magnesium silicate powder to remove impurities. The wet cleaning method washes with water repeatedly until the colorless biodiesel wastewater is obtained. The amount of wastewater in the wet cleaning method is 0.2–3.0 times the volume of raw biodiesel [14,15]. This demonstrates that this highly polluting biodiesel production method is unacceptable in the future. The study of heterogeneous alkaline catalysts reduces the shortage of catalysts [16]. It is said that the alkaline catalyst has vigorous catalytic activity because it has Lewis basic sites [17], and the surface positions of anionic oxides and hydroxides can serve as Lewis bases. Their strength will depend on the properties of cations and their local environment. The Lewis basic sites of many different catalysts can catalyze transesterification [18,19,20]. Lewis acid-base sites are widely used in heterogeneous catalysts [21], while double-active sites with Lewis acid-base sites are also used to enhance the conversion of compounds [22]. This review summarizes the current advanced Lewis acid/base and double active site catalysts and how they are used in in situ transesterification to produce biodiesel. Finally, we address the current limitations and challenges of these catalysts and outline effective and practical optimization conditions to enhance catalytic effectiveness.

There have been many critical reviews on the use of Lewis acid sites and Lewis basic sites to develop biodiesel catalysts [23,24,25,26,27]. In addition, some reviews of research progress focus on biomass conversion into biodiesel through in situ transesterification [1,28]. Some reviews [29,30] focused on the performance of deep eutectic solvents (DES) and ionic liquids (ILs) in biodiesel production, pointing out that the esterification and transesterification processes can be carried out simultaneously in a one-pot process. This paper aims to summarize the progress of Lewis acid-base catalysts for the synthesis of triglycerides through simultaneous FFA esterification and transesterification. Compared with existing literature reviews on Lewis acid/base catalysts, this paper systematically compares Lewis acid/base catalysts specifically used for biodiesel transesterification and Lewis acid-base bifunctional catalysts. In addition, comments are made based on the characteristics, advantages, and disadvantages of different catalysts, focusing on optimizing transesterification reaction conditions to improve catalyst conversion efficiency and the ability to produce biodiesel sustainably, which are of great significance to further industrial production. This review of existing research is believed to provide enough scientific value to guide researchers to further study more efficient biodiesel catalysts.

2. Transesterification of Biodiesel

2.1. Fatty Acid Composition and Properties of Biodiesel

Biodiesel is a mixture of monoalkyl esters obtained through the reaction of lipids (triglycerides) with short-chain alcohols in the presence of catalysts [1,23]. Triglycerides are one of the main components of biomass oil. In biomass, triglycerides are mainly composed of long carbon chains of C₈-C₂₄ linear fatty acids with simple structures and low oxygen content [31]. However, there are other lipids related to fatty acid ester bonds in their composition, such as phospholipids, glycolipids, and sphingolipids, all of which are saponified lipids. In the presence of excess methanol or ethanol, triates produce fatty acid methyl esters (FAME) through transesterification reactions that can be catalyzed by acids or bases. In addition, there are a lot of free fatty acids (FFAs) in natural biomass oil [32]. Under ideal conditions, FAME content in biodiesel is 100%, but there are impurities in biodiesel caused by low conversion rate and difficult separation [33], such as glycerol, monoglyceride, and diglyceride. Both fresh wet biomass and recycled biological waste contain a significant amount of FFAs, and FFAs can also hinder the transesterification reaction [34]. Usually, homogeneous or heterogeneous catalysts are used for esterification to reduce FFA content to less than 2% before transesterification [35]. The traditional catalyst for the esterification of FFAs to FAME has a high reaction temperature and wastes much energy [36,37]. Currently, some new catalysts are changing this situation [38]. Compared to other lipids, such as phospholipids, which cost more to remove, FFAs can be used as an acidic catalyst in an acidic environment containing FFAs, and supercritical methanol can overcome the mass transfer barrier between methanol and the oil phase in a unique environment (heated to 250 degrees Celsius or even higher). FFAs react with methanol through esterification to form FAME and water relatively faster than transesterification [39]. Some studies have shown that it is possible to convert saponified lipids into FAME using polar solvents, which usually require the use of enzyme assistance and other methods [40].

2.2. Research Status of Acid-Base Catalyzed Transesterification

Transesterification is a reversible reaction in which alkyl groups of triglycerides are exchanged with alcohol, also known as alcoholysis. Transesterification is a reversible reaction that involves exchanging alkyl groups of triglycerides with alcohols, also known as alcoholysis [41]. For instance, in Figure 1, the oil (triglycerides) reacts with methanol to form ditriglycerides and methyl esters, and the reaction continues to form monotriglycerides and methyl esters. Monoglycerides react with methanol to form glycerol and methyl ester.

Under ideal conditions, 1 mol triglyceride reacts with 3 mol alcohol to form 3 mol FAME and 1 mol glycerol. However, in the case of improper control of the conditions, there will be a side reaction to the transesterification reaction, resulting in the oil being saponified with alkali. In the fatty acid spectrum of biodiesel, medium- and long-chain fatty acids (C₁₂-C₂₄) account for a high proportion [42,43]. Acid catalysts, in the presence of acidic catalysts, esterify FFAs with methanol to form methyl esters; acidic catalysts are suitable for catalyzing raw materials with high-acid raw materials [44]. The most classical homogeneous acid catalysts include sulfuric acid, phosphoric acid, and para-toluenesulfonic acid. The reaction vessel was filled with a 1:10 volume of methanol, and then chloroform, hexane, or petroleum ether were added to the container for the extraction of FAME after transesterification. The reaction mixture was heated above 90 °C for 0.5–3 h, and then the extraction layer containing FAME was centrifuged to obtain biodiesel [45]. Taking the sulfonic acid group as an example, the connection between -SO₃H and the ester group weakens the strength of the carbon-oxygen bond. During the transesterification process, free protons (H) released by methanol replace the -SO₃H group and bond with -OH to form water molecules. During the transesterification reaction, they produce glycerol [46].

When an alkali catalyst is used, FFAs neutralization with triglycerides occurs in the presence of water. Alkali catalysts and alcohol produce alkali alkoxide and water, which is beneficial for hydrolysis and saponification. Some studies [47] suggest that the level of moisture in bio-oil also affects the choice of catalysts. If there is more water, it is suitable for acid-catalyzed transesterification. If the moisture is low (less than 3.0%), alkali-catalyzed transesterification is recommended. Alkali catalysis consists of three consecutive reversible reactions. In the reaction sequence, a triglyceride molecule is converted to diglyceride, monoglyceride, and finally glycerol [23]. In the industry, basic catalysts are more commonly used than acid catalysts due to their higher activity and lower process temperature than acid-catalyzed transesterification [48]. To use homogeneous alkali catalysts, the pH value needs to be neutralized to extract crude biodiesel more efficiently. The pH value of the mixture and the methanol content determine the extraction efficiency [49]. The extraction methods for crude biodiesel after transesterification include wet cleaning, dry cleaning, and membrane extraction [50]. Heterogeneous solid-base catalysts can easily be separated from the reaction mixture without the need for water [49]. They are easy to regenerate and less corrosive, resulting in safer and cheaper products, so currently, heterogeneous basic catalysts, including solid bases, are widely used [51,52], metal-supported solid bases [53], metal oxide-supported solid bases [54], basic catalysts based on metal organic frameworks (MOF) [27], and more.

3. Lewis Acidic Catalyst

Lewis acids are substances that lack electron pairs in the valence orbitals and therefore can accept electrons. Due to the presence of electron-deficient metal centers, they activate electron-rich substrates [55]. The higher acidity of the Lewis acid catalyst is conducive to improving the conversion rate of transesterification. In the acid transesterification reaction catalyzed by Lewis acid, the oxygen connection of the acid site with the carbonyl group of triglyceride/FFAs increases the electrophilicity of adjacent carbon atoms, making them more vulnerable to nucleophilic attack [56]. It goes through four steps: (1) forming carbonyl carbon cationic electrophilic reagents by Lewis acid; (2) nucleophilic attack of alcohols on carbon cations and the formation of tetrahedral intermediates; (3) proton transfer and breakdown of intermediates; and (4) formation of FAMEs and the regeneration of catalysts. The key factor is the formation of a strong electrophilic reagent between Lewis acid and triglyceride. This paper summarizes the different types of Lewis acidic catalysts and evaluates their catalytic effects.

Table 1. Comparison of different types of Lewis acid catalysts.

Type	Catalyst	Reaction Conditions	Performance	Comments	Ref.
Metal salt	ZnCl2	The molar ratio of catalyst to oil is 0.15 mol/mol, and the reaction is carried out at 150 °C for 1 h	76.7% by weight	It has esterification ability and reduces FFAs by 60% at 150 °C. This catalyst has a low conversion efficiency compared to mature catalysts.	[57]
Metal salt	AlCl3	Using 5.1 MPa CO2 as cosolvent mass ratio of biomass to ethanol of 1:20, 4 wt% AlCl3, 250 °C, 90 min	Over 90% conversion rate	The conversion rate of FFAs is 96%. AlCl3 plays a role in flocculating and suspending biomass in the reaction system, expanding the use of the catalyst, but the reaction conditions are too high	[57]
Metal salt	ZnLa, ZnPa, ZnSt and ZnOl	100 °C, 2 h, 500 rpm	The triglyceride conversion rate is higher than 99%, and the FAME yield is higher than 84%	They have lower mass transfer resistance than heterogeneous catalysts and can be easily separated from the reaction medium without activation or cleaning treatment	[58]
Metal oxide	Tin oxide (II)	When the molar ratio of methyl acetate to oil is over 40, 483 K, the reaction time is 4 h	90% FAME yield	The reaction system requires water and the reaction time is long	[59]
Metal oxide	MoO3/γ-Al2O3	The catalyst to oil ratio is 1:20; 373 K, 2–16 h,	The maximum conversion rate is 95%	Catalyst preparation is complex, requires long reaction times, and there is a conversion bottleneck in optimizing the transesterification process	[60]
Metal oxide	WO3/SMP	The 2 wt% WO3 loading, 4.5 wt% catalyst amount, 9:1 methanol to oil molar ratio, 45 min reaction time, and 343 K reaction temperature	96% biodiesel product	The excellent effect of the catalyst comes from the high Lewis acid sites, and the particle gap of the catalyst also plays a key role	[61]
Carbon nanotubes	Al3+-SO42−/MWCNTs	The mass ratio of catalyst to reactant is 0.9 wt%, and the molar ratio of methanol to oleic acid is 12:1, after 7 h	The conversion rate exceeds 95%	The carbon-carbon bond structure of MWCNTs is stable and suitable for loading rich Lewis acid sites. The pore structure and the existence of surface functional groups of MWCNTs enable them to effectively adsorb and immobilize the active components of the catalyst, improving the dispersibility and stability of the catalyst	[62]
Metal organic frameworks	MIL-101 (Cr)-SO3H	Microwave irradiation for 20 min	93% conversion rate	MOFs have a very high specific surface area and adjustable pore size, which provides a large number of active sites and good material transport channels for catalytic reactions, thereby improving catalytic efficiency. Microwave irradiation reduces the reaction time significantly compared to heating.	[63]
ILs	The ionic liquid [DDPA] [Tos]	The amount of catalyst was 8% w/v, the ratio of oil to methanol was 1:2 v/v, the reaction temperature was 65 °C, and the reaction time was 4 h	75% Biodiesel Yield	The catalyst is easy to prepare, has high recyclability, and remains low in reaction rate and final conversion	[64]
DES	The p-toluene sulfonate eutectic solvent	The molar ratio of methanol to oil was 8:1, the reaction temperature was 110 °C, and the reaction time was 2 h	The ester exchange rate can reach 98.66 ± 0.17%	It has the characteristics of ionic liquids and a higher reaction rate	[65]

shows the research results of different Lewis acidic catalysts.

3.1. Research Status of Lewis Acidic Catalyst

AlCl₃ and ZnCl₂ can be directly mixed with biomass oil and react, so they can be used as homogeneous Lewis acid catalysts [56]. These catalysts are inexpensive and easy to prepare. However, the reaction rate of these catalysts is very slow. Abraham Casas et al. carried out transesterification of sunflower oil with a ZnCl₂ catalyst/oil ratio of 0.15 mol/mol at 150 °C for one hour, and the methyl ester content was only 76.7% by weight. In addition to the ester exchange reaction, an esterification reaction was also carried out at the same time. After 1 h of reaction at 150 °C, the FFAs in the reactant decreased by 60%. It has been proven that the Lewis acid catalyst has two catalytic abilities of esterification and transesterification, and biomass raw materials containing more fatty acids, such as waste vegetable oil, can be used. One way to improve the transesterification efficiency of AlCl₃ and ZnCl₂ is to use cosolvent. Nestor et al. [66] use Tetrahydrofuran (THF) as a cosolvent. The addition of THF reduces the mass transfer problems commonly encountered in heterogeneous systems. In the same phase of AlCl₃ and triglyceride, the activation of carbonyl carbon is more favorable, resulting in higher conversion. Guan et al. [56] used CO₂ as cosolvent; AlCl₃ was dissolved in ethanol and 5 MPa CO₂ cosolvent, and the conversion reached more than 90% within 1 h at 180 °C. It has been found that AlCl₃ can also be used as a flocculant to collect microalgae and other biomass from natural water bodies or sewers, which will combine upstream and downstream operations. We drew a schematic diagram of the reactor as shown in Figure 2.

Divalent cations are catalysts for transesterification and esterification. Some reports use divalent metal acetate as a transesterification catalyst and metal carboxylates such as acetate and stearate as catalysts [58,67,68]. Some carboxylic acids, such as zinc laurate, zinc palmitate, zinc stearate, etc., are soluble in the reaction medium at 100 °C and rapidly recrystallize at room temperature, reducing the mass transfer resistance during the reaction process and separating them from the reaction medium. The ease of recovery from reactant media makes it a unique advantage.

3.2. Heterogeneous Lewis Acidic Catalysis

Lewis acid has the advantage of reducing equipment corrosion, particularly heterogeneous catalysts, solid Lewis acid, and immobilized catalysts, which not only ensure the catalyst’s recycling in the reaction but also help maintain its long-term activity of the catalyst [55,69]. Common solid Lewis acid catalysts such as metal oxides, molecular sieves, ion exchange resins, etc. Vinicius et al. [70] studied the effects of several metal oxide catalysts on esterification yield under different reaction conditions. Metal oxides, especially alumina and tin oxide, have great potential for producing biodiesel with conversion rates of 80–90%.

Sulfated metal oxide is an effective acid heterogeneous catalyst. Rodrigo Zunta Raia et al. [71] prepared biodiesel from jatropha curcas oil through simultaneous esterification and transesterification with sulphated zirconia. The sample with the highest acidity had the highest catalytic activity, resulting in an ester yield of 59.4%. Du et al. [72] is doped with tin oxide through hydrothermal synthesis of sulfated silica. SiO₂ can be highly dispersed in sulfated tin oxides, which contributes to the formation of strong acidic sites in sulfated tin oxides. The catalytic experiments show that tin oxide doped with sulfated silica has higher activity than traditional tin sulfate in the transesterification of triglyceride with methanol. However, the issues of poor stability and low catalytic efficiency of sulfur metal oxides have not been fully resolved. Zn/Sn catalysts supported by modified metal oxides with catalytic activity have potential prospects. Li et al. [73] studied the interaction between Zn and ZrO on a zirconia-supported zinc catalyst. The introduction of Zn species can significantly improve the acidity of the catalyst. Compared to the catalytic activity of different Zn-supported ZrO₂, it was found that the 7% Zn/ZrO⁺₂ catalyst had the best catalytic activity for biodiesel and had good selectivity. The Lewis acid catalyst is immobilized to form a heterogeneous catalyst, effectively eliminating or significantly reducing the use of harmful substances and their production in the chemical reaction process [74]. Polyoxometalates (POMs) are anionic clusters composed of various metal oxides, which have significant functions in the catalytic field. By introducing Lewis acid sites on the backbone of POMs, these compounds can play a catalytic role in esterification reactions. To improve catalytic efficiency, POMs are often dispersed on various solid supports, including silica, zirconia, carbon materials, titania, alumina, and porous silica. This dispersion method can effectively expand the surface area of POMs, thereby increasing the number of catalytically active sites and thereby improving catalytic performance [75]. Mirit et al. [69] successfully immobilized Lewis acids into a sol-gel matrix and immobilized Lewis acids (BF₃ and AlCl₃) in a silica matrix as catalysts, allowing their reuse and reuse in continuous biodiesel production. M.A.A. Aziz et al. [61] prepared WO₃ supported on silica mesoporous-macroparticle catalyst and studied the effect of WO₃ loading on FAME catalytic activity. Under optimal reaction conditions determined by the response surface methodology (RSM), the biodiesel conversion rate was 96%. The catalyst exhibits excellent catalytic performance, which is attributed to the high Lewis acid site content and pores within and between the catalyst particles. This structure provides efficient transport for reactants and products and significantly improves the efficiency of the entire catalytic reaction. Carbon nanotubes (CNTs), with their high thermal conductivity, accessibility to the active phase, good chemical stability, and high specific surface area in corrosive media, hold great promise as catalyst carriers. The wall defect structure of multi-walled carbon nanotubes (MWCNT) contributes to forming more Lewis acid active sites [62,76,77]. The preparation of sodium oxide impregnated on CNTs by Mohd et al. [78] and as a heterogeneous catalyst for transesterification of waste edible oil with the positive metal ion Na (cation) having Lewis acidity enhances the electronegativity of oxygen O⁺²⁻(anion) adjacent oxides, improving the alkaline strength of the catalyst. Shu et al. [62] prepared the solid Lewis acid catalyst, Al³⁺-SO₄²⁻/MWCNTs. The acid sites are primarily made up of Lewis acids. During the reaction process, the catalytic temperature is reduced, and the quality of the catalyst and reactants is relatively low. The conversion rate of FAME can reach 95%. Metal organic frameworks (MOFs) are a particular type of coordination polymer composed of metal ions and organic ligands [27]. MOFs have large porosity, uniform pore size, controllable functional groups, and structural durability, making them an ideal choice for synthetic transesterification/esterification catalysts. Hasan et al. [63] used sulfonic acid functional ligands to hydrothermally synthesize highly porous and acidic MOFs, namely MIL-101 (Cr)-SO₃H. Under microwave irradiation, it only took 20 min for MIL-101 (Cr)-SO₃H to achieve a 93% yield of methyl oleate. The catalyst can also be used under heating, but it takes 10 h to achieve the same yield. After thermal filtration, the catalyst’s yield slightly decreases before the third operation, demonstrating its reusability.

3.3. Lewis Acidic Ionic Liquids and Deep Eutectic Solvents

As a developing material, ILs have garnered attention due to their unique characteristics. Their thermal stability and recyclability are their main highlights. Introducing adjustable Lewis acid/Brønsted acid sites to ionic liquids enhances the catalytic activity and selectivity of ILs [79]. ILs play an essential role as catalysts in converting biomass oil to biodiesel, especially when they are designed to contain Lewis and Brønsted acid sites. The advantages of this method, including ease of operation, low corrosiveness, recyclability, high yield, low cost, no saponification process, and reduced waste generation [80], offer a promising outlook for the future of renewable energy.

Panchal et al. [63] synthesized ILs, 3-(N, N-dimethyldocecyl ammonium) propane sulfonic acid p-toluene sulfonate ([DDPA] [Tos]), for the transesterification of soybean oil into biodiesel. Under optimal conditions, the yield of biodiesel reaches 75%. The new ILs can be reused at least 6 times, and the conversion rate will not be significantly reduced. After 6 runs, the catalyst’s recovery rate reaches 68%, showing stable reusability. Han et al. [81] synthesized Brønsted-Lewis acidic ILs composed of [SO₃H-pmim] Cl and Sn (II), which was used in the catalytic transesterification of soybean oil with methanol to prepare biodiesel. The coexistence of Brønsted acid and Lewis acid also ensures the enhancement of catalytic activity. The presence of Sn²⁺ in the catalyst system plays a crucial role, as it tends to combine with the carbonyl oxygen of unsaturated fatty acids (showing strong electronegativity) to form intermediate complexes. This process promotes the effective interaction between the carbonyl carbon and methanol oxygen atoms, thereby enhancing the catalytic activity. At the same time, the existence of Brønsted acid proton (H⁺) may couple with carbonyl oxygen, resulting in a decrease in negative charge, which is beneficial to the release of Sn²⁺ and the formation of biodiesel. Xie et al. [82] modified Polyoxometallate (POM) hydrochloric acid with a Keggin structure using UiO-66-2COOH, a MOF material and then coordinated the modified POM anion with ILs cations with functional groups. Finally, the sulfonated acidic ILs were integrated into the prepared POM/UiO-66-2COOH composite structure. The prepared AILs/POM/UiO-66-2COOH solid catalyst has a high surface area and acidity. When the ratio of methanol to oil is 35:1, and the amount of catalyst is 10 wt%, for 6 h, the reaction temperature is 110 °C, and oil conversion is 95.8%. The intense interaction between SO₃H and POM can effectively prevent the loss of active components from the MOF carrier, ensuring the stability of the active components of the solid acid catalyst. This stability provides a sense of reassurance and confidence in the reliability of the catalyst. In addition, composite catalysts containing HPW and sulfonic acid groups showed better recycling performance. Hence, the strong interaction between HPW and -SO₃H groups not only increases the acidity of the catalyst but also prevents the loss of active substances from the MOF carrier.

DES, a type of IL, is a promising green solvent with unique properties. Their low cost, simplicity of preparation process, non-toxicity, and biodegradability make them a superior alternative to ILs [83]. DES has the potential to compensate for the lack of traditional liquid acid and solid superacid. They can be prepared by mildly mixing the hydrogen-bonded acceptor (HBA) and hydrogen-bonded donor (HBD), creating a wide range of hydrogen-bonded connections. Typically, DESs are composed of two or more components with a melting point lower than any single component [84]. The strong hydrogen bond acceptability and high acid strength of these chlorides lead to the formation of an extensive hydrogen bond network in DES. This network efficiently contacts biomass and promotes the deconstruction of biomass structure through intensely competitive hydrogen bond interaction [85]. Figure 3 shows the process of directly producing biodiesel from biomass.

Liu et al. [65] prepared DES with ChCl and p-toluenesulfonic acid as HBA and HBD. DES, composed of choline chloride and p-toluenesulfonic acid, not only has catalytic activity similar to that of the original p-toluene sulfonic acid but also has good solvent performance. In particular, the p-toluenesulfonate DES group can be easily separated from the reaction system. Under the conditions of mixing p-toluenesulfonic acid and ChCl in a molar ratio of 1:3, using a certain amount of DES as a catalyst, the molar ratio of methanol to lipids is 8:1, the reaction temperature is set to 110 °C, and the reaction lasts for 2 h. The conversion rate of the transesterification reaction can reach 98.66%. Ngatcha et al. [86] prepared ChCl-ZnCl₂ DES and ChCl-SnCl₂ DES. The study of the geometric structure of DES is a result of collaborative research. It reveals that the bond length of Sn-Cl is higher, and the charges of Zn and Sn atoms are 0.567~0.668 and 0.878~0.883 e, respectively. Despite being the metal center farthest from the choline part, it exhibits the most vigorous acidity, making ChCl-SnCl₂ DES more acidic. Alam et al. [87] proposed a hypoeutectic solvent using Lewis acid (ChCl-CrCl₃·6H₂O). The FAME content of the biomass treated by Brønsted acid DES and the catalyst at 120 °C, 5% methanol solution, and 5 mL solvent volume reached 39.86 mg/g.

3.4. The Research Gaps and Directions of Lewis Acidic Catalysts

The research on Lewis acid catalysts used for transesterification dates back more than 10 years [66] and provides reasonably sufficient research results. However, there are still some unresolved problems and new research perspectives. The auxiliary means of transesterification are insufficient. Islam et al. [55] studied the use of ultrasonic-assisted solid Lewis acid catalysis to convert FAME content greater than 99% in only 10 to 60 min. In the field of biomass pretreatment, microwave, ultrasound, and other physical means to assist chemical treatment have been widely used. There are relatively few studies on combining catalysts and physical means in the field of transesterification. In current research, using vegetable oils such as rapeseed and soybean oil as samples for transesterification has much efficiency in testing the catalyst. They are considering that commercially available vegetable oils go through multiple processes to remove impurities; using vegetable oils to prepare biodiesel is expensive. In the study of Alam et al. [87], microalgae were used to directly catalyze biodiesel, which we believe is in line with the future biodiesel production method. The studies on ILs and DES are insufficient. Ionic liquids and DES containing strong Lewis acid or Brønsted acid sites have been emphasized as a new class of environmentally friendly solvents and catalysts, showing green and effective catalytic potential in biodiesel synthesis through transesterification.

4. Lewis Basic Catalyst

Homogeneous base catalysts have been widely used in the esterification process of biodiesel. However, there are some limitations in their practical use, such as difficulty in separating the catalyst from the product after the reaction, formation of corrosive wastewater, and saponification problems that quickly occur during the reaction process, limiting their more comprehensive application. Using the Lewis basic site, the catalyst loaded with the Lewis basic site has the function of transesterification catalysis, which has more advantages than a homogeneous strong base catalyst in recycling and reducing waste pollution [52,88]. The Lewis alkaline site is fixed by new materials such as MOFs to form a solid alkaline catalyst, which can enhance the catalytic effect of the Lewis alkaline site, improve the catalyst’s stability, and increase the number of reuses of the catalyst [89,90,91,92,93]. Using homogeneous basic catalysts requires secondary processing and purification steps to isolate the catalyst from the reaction product. Conversely, heterogeneous catalysts are easily removed from the reaction mixture, making the purification step easier.

4.1. Research Status of Lewis Basic Catalysis

The influence of the Lewis basic site on the transesterification catalytic activity of the material is complex, mainly due to the location of the basic site and the crystal structure of the catalyst [94]. The CO₂ chemisorption spectra of the catalysts were analyzed by CO₂-temperature programmed desorption (CO₂-TPD) to distinguish the contribution of alkaline sites with different strength characteristics [95]. The basic sites of catalysts come from different sources. We have identified several main types of basic sites, including solid alkaline metal catalysts, alkaline earth metal oxide, siloxyl anion, Al₂O₃, SiO₂, and hydrotalcite doped with alkali metals and alkali metal nanomaterial.

4.2. A Solid Alkali Metal Catalyst

Cui et al. [96] using KF/γ-Al₂O₃ catalyst prepared by KF and spherical γ-Al₂O₃ calcined in air at 600 °C, the molar ratio of methanol to oil was 12:1, the amount of catalyst was 4%, and the temperature was 65 °C. The biodiesel with the best performance was obtained. Giovanni et al. [97] studied the transesterification of triglycerides catalyzed by NaAlO₂ (SA) to produce biodiesel. There are weak and strong basic sites on the catalyst’s surface. Compared to other alkali metal catalysts, the performance of SA is significantly better; although the difference in the number of bases is not significant, it is generally believed that the higher the basicity, the higher the yield of biodiesel, indicating that SA has a more robust alkaline site. In Singh et al. [98], phosphotungstic acid (TPA) on graphene oxide (GO) using the impregnation process, and a series of potassium-supported TPA/GO (K/TPA/GO) catalysts were prepared through a wet impregnation process. K impregnation increases the number of active sites (alkalinity), thereby increasing activity. Metal ions (TPA) on the surface of mixed metal oxides may lead to weak acid sites. K and TPA are applied to GO to create basic and acidic sites, respectively. In the high FFA transesterification procedure, the two steps of esterification and transesterification occur simultaneously on the catalyst surface. These findings have the potential to revolutionize the field of biodiesel production. Although some of these solid alkali metal catalysts doped with Lewis basic sites have been studied earlier, they fully demonstrate the catalytic characteristics of the basic Lewis sites, with shorter reaction times (less than 4 h) and lower reaction conditions (below 80 °C). Of course, there are also unresolved problems in the research. For example, the catalyst cannot transesterify in an environment with high FFAs. As FFAs increase, the conversion efficiency of FAME will still decrease. At the same time, most research is still in theory.

4.3. Alkaline Earth Metal Oxide Catalyst

Solid bases contain a variety of catalysts, such as alkaline earth metal oxides. CaO, MgO, SrO, and BaO are all catalysts with significant catalytic effects. Alkaline earth metal oxides are common solid alkaline catalysts that are easy to prepare due to their availability, non-toxicity, reusability, low cost, and high concentration of surface alkaline sites that provide activity [99]. We compared these studies and found that the catalytic effect of alkaline earth metal oxides is first directly related to the active metal composition. The oxides of Na and K are usually more basic than other alkaline earth metal oxides [100,101]. Cabrera et al. [102] incorporated Ce into the ZnAl hydrotalcite structure and modified its basicity, using it as a catalyst for soybean oil transesterification. MgAl-Ce (X) hydrotalcite is activated at high temperatures to produce intermediate and strong alkaline sites, which can be transesterified at 67 °C. The yield of FAME is 90%. Wang et al. [103] have synthesized an innovative, walnut-shaped composite catalyst derived from industrial by-products, incorporating calcium oxide and cancrinite. This novel catalyst, with its unique shape and composition, has the potential to significantly improve the efficiency of the transesterification process. Analysis via CO₂ temperature-programmed desorption (CO₂-TPD) techniques revealed that CO₂ release is attributed to various basic sites with differing strengths: weak, moderate, and strong. These sites are associated with hydroxyl groups (-OH), the O²⁻ anion within the Ca²⁺-O²⁻ linkage, and the less coordinated O²⁻ anion. The temperature at which pyrolysis occurs is a pivotal factor that affects the configuration of calcium within the catalyst, ultimately affecting its alkalinity and the strength of its basic properties. The catalyst’s elevated basic potency and intensity confer superior catalytic efficacy in the transesterification process involving methanol. Under optimal conditions, a temperature of 70 degrees Celsius, a duration of 3 h, a catalyst concentration of 6.0 weight percent, and a methanol-to-oil molar ratio of 10:1, FAME can yield 95%. Singh et al. [104] synthesized barium zirconate through a coprecipitation method for transesterification. Despite its low specific surface area, barium zirconate has high activity under suitable reaction conditions. The total alkalinity of barium zirconate is 1.21 mmol/g. The Lewis base on mixed metal oxides is the main reason for the alkaline strength of the catalyst. Under optimized transesterification conditions, the FAME conversion was 98.79 ± 0.5%. What’s particularly promising is that the barium zirconate catalyst was recovered and recycled 9 times without any significant activity loss. Alongside the prepared catalysts, those derived from natural rocks are potential alkaline earth metal-based catalysts for transesterification. Wang et al. [105] selected åkermanite (Ca₂MgSi₂O₇) from silicate ores for biodiesel transesterification. The CO₂-TPD data indicate that the åkermanite basic site’s concentration reaches 0.8822 mmol per gram. When the catalyst’s mass percentage is set at 20%, with a methanol-to-oil molar ratio of 10:1, and the reaction proceeds at a temperature of 190 degrees Celsius over 6 h, the FAME conversion rate impressively reaches 99%. Even after 16 consecutive catalytic cycles, the conversion rate remains robust at 85%, demonstrating the catalyst’s remarkable durability and stability.

SiO₂ with alkaline sites has been proven to be an attractive catalyst for transesterification. By hybridizing the structure and morphology of SiO₂, materials with different phases, tissue levels, and morphologies are controlled to achieve the transesterification of alcohols [106]. The catalytic activity of silica is due to the siloxyl groups (≡SiO-) formed in its structure, which promote the deprotonation of methanol and initiate transesterification [107]. Zapelini et al. [18] used cetyltrimethyl ammonium bromide (CTAB) and three linear alcohols to prepare different silica. The silica at the basic site is synthesized from cetyltrimethylammonium cations (CTA⁺) in the presence of ethanol, 1-propanol, or 1-butanol to synthesize hybrid silica in the form ≡SiO-CTA⁺. The catalytic activity is related to the external catalyst site on the outer surface of the particles because surfactants block the intermediate pore of the hybrid silica at ≡SiO-CTA⁺. The samples with larger median diameters of macropores have higher catalytic activity because it is easier to approach the alkaline catalytic sites in these cases. Using n-butanol is the best choice for preparing surfactants containing hybrid silica, which has more accessible siloxyl sites. Zapelini et al. studied the influence of different alcohols on the preparation of catalysts, proving that the preparation of catalysts is a crucial factor affecting catalytic efficiency. From the process of preparing different alkaline earth metal oxide catalysts, we believe that the basicity of the catalyst raw material itself is the most important, which ensures the necessary basic sites for the transesterification reaction. Secondly, the calculation temperature, molar ratio, and transesterification conditions during the preparation process of the catalyst also affect the number of basic sites of the catalyst and the contact area with reactants, which constitute the main factors affecting transesterification efficiency.

4.4. Lewis Alkaline Sites Loaded on Hydrotalcite

Layered Double Hydroxide (LDH) is the umbrella term for Hydrotalcite (HT) and Hydrotalcite-Like Compounds (HTLCs) [108]. HTLCs, with their broad specific surface area and heightened surface reactivity, garnered escalating interest in applications in anion exchange, adsorption, and catalytic processes. During the roasting process, HTLCs undergo a dehydration and de-ionization process, creating these mixed oxides through thermal decomposition. This transformation enhances the metal dispersion, specific surface area, and Lewis base characteristics of oxides. HTLCs can easily form mixed metal oxides with a high specific surface area, showing Lewis alkaline sites [109,110].

The calcination of hydrotalcite forms mixed oxides, which not only have basic properties but also have attractive properties such as high thermal stability and high surface area. The rehydration treatment of calcined solids can reconstruct the structure of hydrotalcite and completely transform the oxide into the reconstructed hydrotalcite phase, which can improve the solid’s thermal stability and significantly increase the catalyst’s basicity. Dahdah et al. [111] synthesized Mg-Al hydrotalcite with a Mg/Al molar ratio of 3 by the co-precipitation method under pH = 9.5~10 and temperature of 60 °C. Then, the solids were calcined at different temperatures (400–600 °C) and rehydrated. The calculated Mg₆Al₂ solids in this study are significantly inefficient regarding transesterification reaction rates. When using the rehydration Mg₆Al₂ catalyst, noticeable improvements were observed. The CO₂-TPD results showed that more robust basic sites were found in the rehydration catalyst compared to calcined competitors. These basic sites play a crucial role in catalyzing transesterification by converting methanol into adsorbed methoxy ions, which attack carbonyl carbon in triglyceride molecules. The basicity of HT materials can be adjusted according to the properties of cations, compensating for ions, and activation temperatures. Liu et al. [112] prepared solid basic catalysts by preparing Zn-Al aluminum-aluminum talc at different temperatures. They studied the correlation between the basicity of the solid base and catalytic activity. The basic hydroxyl groups on the surface of zinc aluminum oxide samples are formed through decomposition and absorption of water. Cationic vacancies can be created by incorporating Al³⁺ cations into the ZnO skeleton. For Zn-Al oxides fired at high temperatures, these vacancies may exchange with Zn²⁺ cations on the surface, creating surface-isolated O²⁻anions. Mⁿ⁺-O²⁻ paired and isolated O²⁻anions are the primary basic sites. The SBA-15 porous structure is a frequently employed scaffold for crafting solid catalysts, renowned for its customizable structural attributes that facilitate distinctive characteristics. Its orderly architecture, robust hydrothermal endurance, expansive surface area, volume, and well-regulated pore size distribution position it as a superior candidate for dispersing metal active centers [113]. The SBA-15 supported catalyst, with its high specific surface area, large porosity, and regular channel structure. The SBA-15 catalyst has been successfully used in biofuels, such as vegetable oil hydrogenation, hinting at its promising future applications in the field of energy [114]. Marimuthu Prabu et al. prepared catalytically active Mg/Al/Zn-HT (HT/SBA-15 nanocomposites) doped with SBA-15. The origin of weak and strong basic sites is attributed to OH- and O²⁻, respectively, which can be called Brønsted and Lewis basic sites. The supported alkaline earth metal oxide catalyst on BEA zeolite studied by Szkudlarek et al. has the highest triglyceride conversion rate of 90.5% and the FAME yield of 94.6% [115]. HTLC catalysts loaded with Lewis base sites have high activity, basicity, and specific surface area values, especially in terms of recovery and stability compared to alkaline earth metal oxides. Compared to some HTLC catalysts containing Lewis acid sites and Lewis basic sites and multi-functional magnetic acid-base catalysts, which can improve recovery capacity [116,117], there is still a large development space, and it is a promising development direction to load more catalytic function sites and acid-base amphiprotic sites in the zeolite.

4.5. Alkaline Metal-Based Nanomaterials

Mesoporous nanomaterials have made significant progress and are widely used in biomedicine, the petrochemical industry, and catalysis. Nanomaterials can be prepared with various properties and sizes through different synthesis methods [118]. Nanocatalysts have attracted particular attention to biodiesel production because nanoparticles have higher catalytic activity due to their smaller size and uniform dispersion ability [119]. Metal oxide-based catalysts play a crucial role in transesterification catalysis. The key to their catalytic effect lies in the metal elements that bring an alkaline site. Nano-catalyst carriers can address critical issues such as the leaching of active metal materials into products and their reusability [120,121]. Silva et al. [122] have conducted extensive research on various carbon nitride-based nanomaterials (CNs). Their findings indicate that these CNs have vital sites facilitating the transesterification process, effectively functioning as alkaline catalysts. The presence of these basic sites can be attributed to the incorporation of metallic elements into the catalysts’ composition. Chen et al. [123] have successfully synthesized porous Mg-Al-O composite nanoparticles through an aerosol-assisted process. Adding Al₂O₃ to the composite structure significantly enhances the availability of catalytic sites within the nanomaterial framework. Compared to the direct solid-phase thermal decomposition precursor method, the number of sites obtained by the aerosol-based method is significantly increased due to an increase in the specific surface area of MgO dispersion.

4.6. The Research Gaps and Directions of Lewis Base Catalysts

The use of homogeneous and heterogeneous bases for biodiesel transesterification is an up-and-coming method, and most current studies use heterogeneous catalysts to support Lewis alkaline sites. However, traditional homogeneous base catalysts still have a high proportion of use. For example, the catalytic efficiency of KOH reaches more than 90%, which is higher than 79% of ZnAl hydrotalcite supported by Lewis base sites [124]. Heterogeneous catalysts still need to improve the number of basic sites, and the supporting carrier should also be carefully considered. The above studies have shown that increasing the density of basic sites and the contact area with reactants is the key to improving catalytic efficiency. The process of improving the efficiency of these two catalysts is still under study. In addition, we have noticed that research on alkaline ILs is developing rapidly [125]. Similar to acidic ILs, they can convert raw materials into biodiesel directly, and ILs are homogeneous and suitable for use as auxiliary means such as microwaves and ultrasound.

5. Lewis Acid-Base Bifunctional Catalyst

Lewis acid/base catalysts have unique characteristics. Over the years, researchers have been working to overcome the catalyst’s limitations to produce biodiesel with significant economic benefits. For instance, due to the issue of soap formation in the production of biodiesel with alkaline catalysts and high FFA raw materials, the reaction conditions for acidic catalysts require higher temperatures, and high FFA interference in situ transesterification of vegetable oils affects catalytic efficiency, so a new catalyst is needed to promote both FFA esterification and triglyceride transesterification. The basic characteristics of catalysts containing double Lewis acid-base sites include simultaneous esterification and transesterification ability (Figure 4). They can be modified according to FFAs in different biodiesel feedstocks to achieve the goal of high efficiency and low cost.

5.1. Metal Oxide Catalysts Containing Lewis Acid-Base Sites

CaO has been extensively studied as a catalyst in biodiesel production due to its high basicity, biocompatibility, and friendliness to the environment. This metal oxide catalyst can be obtained from natural limestone. CaO reacts with CO₂ and H₂O in the air to form CaCO₃ and Ca(OH)₂, respectively, which reduces the activity of the catalyst. Because CaO is the most common heterogeneous alkaline catalyst, combining CaO with other metal oxides (such as ZnO and MgO) [126] can enhance the stability of CaO-based catalysts. Impregnating NiO in CaO can enhance the stability of the catalyst and prepare catalysts with acid-base dual functional sites. NiO stabilizes CaO by inhibiting the reaction of CaO with CO₂ and H₂O to form calcite and Ca(OH)₂ [127]. The acid-base catalyst with an acid-base function was prepared by impregnating the CaO catalyst with NiO. The results show that NiO can catalyze the esterification of fatty acids to methyl ester. Due to the low surface area of CaO, the increase in calcination temperature will increase the alkalinity but will reduce the surface area, loading a bifunctional acid-base nanocatalyst through acid modification, increasing pore volume and specific surface area, adding acid-base sites, and improving biodiesel bifunctional performance to increase the output of biodiesel. Ghasemi et al. made significant progress in research on bifunctional catalysts equipped with acidic sites. First, it showed that in terms of the conversion of FFAs, 99% of FFAs were converted, the conversion of triglycerides reached 73.4%, and the total FAME conversion reached 93.4% [128]. Lanthanum has a unique electronic structure, which is used to receive and lose electrons, so it has dual characteristics of acid and base. FFA interferes with the alkali catalyst, and adding lanthanum enhances transesterification’s catalytic ability under high FFA [129]. Lin et al. prepared a Halloysite nanotube functionalized with La-Ca bimetallic oxides as a novel transesterification catalyst. FFA in the reaction system was easy to attack calcium sites, where lanthanum was protected to catalyze the transesterification of triglycerides. The incorporation of lanthanum promoted the adsorption of methanol, promoting transesterification. This catalyst solves the problem of the poor stability of traditional calcium oxide. It still has an efficiency of 88.7% after being repeated five times. It is suitable for use in environments with high FFAs. In environments exceeding 5 wt%, efficiency reaches 94.9% [130]. Lee et al. prepared a bifunctional acid-based CaO-La₂O₃ catalyst; the highest yield of biodiesel is 98.76%. The co-precipitation of La and Ca promoted the dispersion of CaO, increasing the acid and basic sites on the surface. That is to say, the unique synergism between acid-base sites from Ca-O-La, Ca-O-Ca-O, and La-O-La-O bonds can carry out esterification-transesterification at the same time [131]. Vanadium-based catalysts contain Lewis acid and Brønsted acid sites, which play a crucial role in many catalytic processes [132,133]. V₂O₅ is a promising metal oxide that can be used to develop bifunctional catalysts with CaO. Mulyatun et al. studied several kinds of over-doped V₂O₅ to develop acid-base bifunctional group CaO-based catalysts. V₂O₅ can provide Lewis and Brønsted acid sites for acid-base heterogeneous catalyst systems based on CaO. The TPD-NH₃ analysis results show that the V₂O₅-CaO catalyst has more acid sites and the most robust acid strength, which is especially beneficial for catalytic esterification. V₂O₅ contributes Lewis and Brønsted acidity, of which Brønsted acid is responsible for catalytic transesterification. Lewis acids help accelerate the esterification reaction, and the CaO component acts as a Lewis base site to activate the transformation of triglycerides to FAME [134]. The impregnation of W-Mo catalyst into powdered CaO; Ca-O provides a suitable alkaline site for the conversion of triglycerides to FAME, while W³⁺ and Mo⁶⁺ are considered to be acidic active centers and can esterify a small amount of free fatty acids. Compared to other metal-supported CaO catalysts, bimetallic oxides also have a higher specific surface area. Due to the high specific surface area contribution from MoO₃ and WO₃, the highest biodiesel yield of the catalyst is 96.2% [135]. It can be seen from the above studies that the acid-base bifunctional site improves catalyst stability by introducing other metal elements into the alkali metal oxides and mainly increases the conversion function of FFAs, which alleviates the problems that the Lewis alkaline site cannot achieve a particularly high conversion effect and is easy to be interfered with by the environment when catalyzed bio-oil conversion.

5.2. Heteropoly Acid Catalysts with Acid-Base Bifunctional Groups

Boric acid-based catalysts have high Brønsted acidity. Boric acid-based catalysts with metal ions (such as H₇BW₁₁TiO₄₀) produce Lewis acid sites as a synergistic catalyst for esterification [136,137]. Wang et al. successfully prepared a series of new acid-base dual-functional metal boron catalysts using a sol-gel method. Under optimal reaction conditions, the catalysts have high acidity and basic sites. Jatropha jatropha oil can be obtained from one pot. The high biodiesel yield is 96.0% [138]. Heteropoly acid (HPAs), especially 12-tungstophosphoric acid (H₃PW₁₂O₄₀, HPW), with a Keggin structure, is recognized as an economical and environmentally friendly heterogeneous acid catalyst due to its good thermal stability, low corrosion, and high acid strength. Cobalt-based zeolite imidazolium skeleton (ZIF-67) has strong Lewis acid-base active sites due to its special Co-N coordination, which can achieve synergistic catalysis with HPW. Cheng et al. synthesized a new bifunctional heterogeneous catalyst between HPW and ZIF-67, which can effectively catalyze the conversion of triglycerides in microalgae to biodiesel. The terminal W=O group at HPW interacts with the N-terminal of ZIF-67 through a covalent W-O-N bond. The recyclability of the catalyst is enhanced. The released synergistic unsaturated Co cations and N-terminal enhanced the Lewis acid-base properties of the catalyst. They promoted the conversion of microalgae lipids to fatty acid methyl esters, and the catalytic conversion was as high as 98.5% [139]. Lee et al. fabricated a series of triazole heteropolyacid nanocomposites utilizing 1,2,4-triazole and 12-tungstophosphoric acid (PWA) as precursors, employing a precipitation method. These nanocomposites were subsequently applied in the transesterification of rapeseed oil. The optimized formulation of triazole-PWA nanocomposites exhibited commendable structural stability, an expansive surface area, dual functionality, moderate acidity, and pronounced alkalinity. The authors conducted a comparative analysis of the catalytic conversion efficiency between the composite catalyst and its components, triazole and PWA. The results indicated that the composite catalyst outperformed its constituent elements. This superior catalytic performance is hypothesized to stem from the synergistic effect of bifunctional catalysis, where both acidic and alkaline sites are concurrently enhanced [140].

5.3. Sulfated Solid Acid-Base Amphoteric Catalyst

Solid acid catalysts offer a significant advantage in biodiesel production by eliminating the need for a washing process, simplifying the overall procedure. Their ease of separation from the reaction medium facilitates recovery and reuse and minimizes the risk of acid-induced corrosion and environmental pollution. The versatility of solid acid catalysts is further enhanced by the variety of methods available for their synthesis, with sulfonation being a prominent technique. By meticulously controlling the sulfonation parameters, such as duration, temperature, and the solid-liquid ratio, it is possible to tailor the catalyst’s acidity and the number of active sites, thereby enabling the straightforward preparation of catalysts that meet specific requirements with ease [141]. The sulfonic acid group introduction method generally chooses the contact between concentrated H₂SO₄ and carbon to introduce a single-bond SO₃H group. The relationship between the catalytic yield of FAME and the concentration of the single-bond SO₃H group has been proved. The yield of FAME increases with the increase in the sulfonic group content on the catalyst’s surface, which is related to the way of introducing single-bond SO₃H groups [142,143]. Sulfation is also suitable for loading other catalysts to form more acid sites and higher acidity to catalyze the transesterification of triglycerides under high FFAs, such as those supported on metal oxides [144]. Because the solid acid catalyst requires a high catalyst loading and the reaction time is extended, try to load the sulfonic acid group to the basic metal oxide. The MgO/MgSO₄ catalyst prepared by Akash has a sulfur content of up to 13.65%, and the best biodiesel yield predicted by RSM is 98.8% [145]. Shobhana-Gnanaserkhar et al. added alkaline metal oxide, CeO₂, to sulfated activated carbon as an alkaline accelerator to adsorb a large number of sulfur ions to form strong acid sites to achieve simultaneous esterification and transesterification [146]. At 90 °C, with a methanol to oil ratio of 12:1, a catalyst loading of 3 wt%, and a reaction time of 1 h, 93% of FFAs were converted. The high catalytic activity was also attributed to the catalyst’s large pore size, which made the diffusion of FFAs and triglycerides more significant, so they could react with CeO₂ and Ce₂O₃ in the catalyst’s pores.

5.4. Magnetic Functionalized Double Lewis Acid-Base Sites

Compared to other metals, iron has many advantages, such as low cost, non-toxicity, and environmental protection. However, magnetic particles often gather into large clusters, which hinders the good dispersion of magnetic catalysts in the reaction mixture and reduces the catalyst’s recovery efficiency. Magnetic particles encapsulated on the catalyst carrier can create a magnetic multi-functional catalyst carrier, which is an effective way to enhance its chemical stability and prevent aggregation [147]. Some studies have shown that the catalytic effect of doping Fe₃O₄ magnetic nanoparticles on the transesterification catalyst is better than that of the original catalyst, and it is easy to separate and reduce the catalyst leaching into the mixture. Nurul et al. mixed Fe₃O₄ particles into the zeolite-supported CaO catalyst; the catalyst can effectively process the transesterification of the waste oil with high FFA content, resulting in a large amount of biodiesel. The catalyst has ferromagnetic properties, and the saturation magnetization of 31.759 emu/g makes the separation much easier after the reaction [148]. The CaO-zeolite/Fe₃O₄ catalyst is both alkaline and acidic. It is suitable for the transesterification of raw materials with high FFA content. The maximum yield of biodiesel reached 93.64% after 240 min at 55 °C, which was 30–50 °C lower than other magnetic catalysts and had the exact FAME conversion [149,150,151].

Fe₂O₃ catalyst has been used in biodiesel production and as a substitute for acid-base catalyst in transesterification, which makes the biodiesel production process more cost-effective. Because its low-toxic nanoparticles can be used to immobilize biological enzymes, they have good adaptability and adjustability under different technological processes [152,153]. The compound Fe₂O₃ is known to enhance the presence of strong acid sites, which are particularly effective for the esterification of FFAs in biodiesel production. Acting as a Lewis acid, it facilitates the adsorption of carboxyl groups, thereby improving the conversion of waste cooking oil into biodiesel. Zahra Mansoorsamaei et al. have developed a novel biochar-supported Fe₂O₃/Fe₂K₆O₅ catalyst. This catalyst demonstrated significant magnetization values of 2.098 and 5.906 emu/g under a magnetic field of 14,000 Oe, indicating its potential for facile recovery using an external magnetic field [154]. The CaO discussed in the previous article has the advantages of an alkaline active site, easy availability, wide source, low cost, and highly catalytic activity, and is also widely used in the synthesis of biodiesel. However, under transesterification, a small amount of soluble matter will be leached from the CaO catalyst [155]. Although Fe₂O₃ has a specific enhancement and stability effect, it is still insufficient. Doping different metal elements in CaO can enhance its structural stability. Xia et al. prepared co-doped Fe₂O₃-CaO nanocatalysts. Co was doped into the Fe₂O₃ lattice to transform α-Fe₂O₃ crystals into γ-Fe₂O₃ crystals. The catalyst has stronger magnetic separation strength. Acid-base bifunctional catalysts can effectively esterify FFAs and react with methanol, providing a synergistic acid-base catalytic transesterification effect for biodiesel production [156]. Fe₂O₄ has a high application value in nano-magnetic materials. The magnetic hysteresis loop shows that Fe₂O₄ nanocomposites are ferrimagnetic, and nano-catalyst carriers can enhance the strength of basic sites in them [157,158]. Alkaline oxide ZnFe₂O₄ carried out a synergistic catalytic process by esterification of FFA at the acidic site and transesterifying triglyceride at the alkaline site. Wang et al. studied the synthesis of a magnetic acid-base amphoteric nanoparticle catalyst Zn8@Fe-C400. The yield of Jatropha curcas biodiesel reached 100.0%. The yield of biodiesel was more than 94.3% after 10 cycles. No significant macroscopic saponification was observed [159].

Co-MOF materials include several strong acid-base sites, and Co pyrolysis forms Co nanoparticles that are partially oxidized to Co₃O₄. This cobalt oxide gives the catalyst strong magnetism, and the catalyst can be magnetically separated from the mixture and reused. Guo et al. synthesized bifunctional magnetic acid-base catalysts with different Co and N coordination through the thermal decomposition of ZIF-67 at high temperatures in minutes. The acid-base site allows its FAME conversion rate to reach 96% [160].

5.5. The Research Gaps and Directions of Lewis Acid-Base Catalysts

The catalytic activity of Lewis acid-base dual-functional site catalysis is reflected in the balance of acid and base sites. The number of catalyst loading sites in the study is determined by the amount of catalyst and the calcination temperature. Generally, strong Lewis acid sites and strong Lewis basic sites can accelerate the transesterification reaction. Zhu et al. research proved that excessive La doping in Sr-La/wollastonite catalysts can also reduce the basicity of the catalyst, thus adversely affecting biodiesel production [161]. Kingkam [162] pointed out in his research that the balance of the amount of acid-base sites is the key to controlling catalytic activity and forming side reactions. Its catalyst creates new weak basic sites and has higher reaction efficiency than strong basic sites. The number and balance of active sites in bifunctional catalysts are still worthy of in-depth study. As biocatalysts, enzymes catalyze reactions much faster than traditional chemical catalysts, which helps improve biodiesel production efficiency. There are also many studies on enzyme-assisted catalysts for transesterification [163,164,165]. The Lewis acid-base dual-functional catalyst under study has low reaction temperatures and other conditions that can be used to immobilize the enzyme on the catalyst surface to enhance the catalytic effect.

6. Conclusions

Since biodiesel uses agricultural waste, grain, waste edible oil, etc., with a high free fatty acid content, there are still obstacles in the transesterification process. Fortunately, unlike traditional acid or base-catalyzed transesterification, catalysts suitable for catalyzing biodiesel under high FFA conditions are constantly being studied and discovered, providing a way for green, efficient, and economical biodiesel production. Compared with the direct use of metal chlorides as Lewis acid catalysts, immobilized catalysts are a more popular development trend. Immobilized Lewis acid catalysts will help reduce the formation of more harmful pollutants. Using metal oxides to configure Lewis acid sites or selecting more advanced inorganic carriers, such as pom and CNTs, can significantly increase the acid strength of the catalyst and the number of Lewis acid sites. ILs and DES are more conducive to catalyst recovery and maintenance of homogeneous catalyst activity. They form strong forces through electrostatic forces, van der Waals forces between ion pairs, and hydrogen bonds between HBD and HBA, which affect the reaction medium’s solubility, viscosity, and conductivity and improve catalytic activity.

At Lewis basic catalysts, we introduced new catalysts such as alkali metals, alkaline earth metals, alkaline metal-doped hydrotalcite, and alkali metal nanomaterials. These catalysts overcome the shortcomings of saponification pollution and the difficulty in recycling traditional alkaline catalysts.

The Lewis acid-base bifunctional catalyst is a research integration of previous Lewis acid-base catalysts. It hopes to achieve higher efficiency and multifunctionality by developing catalysts with dual sites. The acidic and basic sites in the composite catalyst improve catalytic efficiency and reduce the interference of free fatty acids on ester exchange. Introducing catalyst loading sites increases the contact area between the catalyst and the reactant. Magnetic catalysts are more accessible to recycle. Many advantages make studying catalysts containing Lewis acid and base sites more valuable.