# An Overview of Biodiesel Production via Heterogeneous
Catalysts: Synthesis, Current Advances, and Challenges

Maya Yaghi, Sandra Chidiac, Sary Awad, Youssef El Rayess, Nancy Zgheib

► To cite this version:

Maya Yaghi, Sandra Chidiac, Sary Awad, Youssef El Rayess, Nancy Zgheib. An Overview of Biodiesel Production via Heterogeneous Catalysts: Synthesis, Current Advances, and Challenges. Clean Technologies, 2025, 7, 10.3390/cleantechnol7030062. [http://dx.doi.org/10.3390/cleantechnol7030062](http://dx.doi.org/10.3390/cleantechnol7030062)

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Review

# An Overview of Biodiesel Production via Heterogeneous
Catalysts: Synthesis, Current Advances, and Challenges

Maya Yaghi <fnref n="1" />,<fnref n="2" />, Sandra Chidiac <fnref n="3" />, Sary Awad <fnref n="2" />, Youssef El Rayess <fnref n="3" />,* and Nancy Zgheib <fnref n="1" />,*

## Abstract

Biodiesel, a renewable and environmentally friendly alternative to fossil fuels, has attracted significant attention due to its potential to reduce greenhouse gas emissions. However, high production costs and complex processing remain challenges. Heterogeneous catalysts have shown promise in overcoming these barriers by offering benefits, such as easy separation, reusability, low-cost raw materials, and the ability to reduce reaction times and energy consumption. This review evaluates key classes of heterogeneous catalysts, such as metal oxides, ion exchange resins, and zeolites, and their performance in transesterification and esterification processes. It highlights the importance of catalyst preparation methods, textural properties, including surface area, pore volume, and pore size, activation techniques, and critical operational parameters, like the methanol-to-oil ratio, temperature, time, catalyst loading, and reusability. The analysis reveals that catalysts supported on high surface area materials often achieve higher biodiesel yields, while metal oxides derived from natural sources provide cost-effective and sustainable options. Challenges, such as catalyst deactivation, sensitivity to feedstock composition, and variability in performance, are discussed. Overall, the findings underscore the potential of heterogeneous catalysts to enhance biodiesel production efficiency, although further optimization and standardized evaluation protocols are necessary for their broader industrial application.

**Keywords:** biodiesel; heterogeneous base catalyst; heterogeneous acid catalysts; alkaline metal earth oxides; zeolites; heteropoly acid

## 1. Introduction

The world's rapid population growth, urbanization, and industrial growth are ever-increasing global energy demand. This demand continues to be largely met by conventional fossil fuels, such as coal, oil, and natural gas [1]. However, concerns about the depletion of fossil fuel reserves, environmental degradation, and the acceleration of greenhouse gas emissions and climate change are prompting an urgent search for sustainable and renewable alternatives [2,3]. Moreover, the rising cost and demand for petro-diesel add further momentum to this transition.

Among the various renewable energy options, biodiesel has emerged as a viable substitute for fossil diesel, offering significant environmental and performance advantages [4]. Unlike diesel—which is a mixture of hydrocarbons derived from petroleum—biodiesel consists of mono-alkyl esters of long-chain fatty acids produced from plant oils, animal fats,

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check for
updates

Academic Editor: Constantinos

V. Chrysikopoulos

Received: 16 May 2025

Revised: 9 July 2025

Accepted: 11 July 2025

Published: 15 July 2025

**Citation:** Yaghi, M.; Chidiac, S.; Awad, S.; El Rayess, Y.; Zgheib, N. An Overview of Biodiesel Production via Heterogeneous Catalysts: Synthesis, Current Advances, and Challenges. *Clean Technol.* **2025**, *7*, 62.
https://doi.org/10.3390/cleantechnol7030062

**Copyright:** © 2025 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Department of Chemical Engineering, School of Engineering, Holy Spirit University of Kaslik, Jounieh BP 446, Lebanon; mayayaghi@net.usek.edu.lb</footnote>

<footnote n="2">² IMT-Atlantique, GEPEA UMR-CNRS 6144, 44300 Nantes, France; sary.awad@imt-atlantique.fr</footnote>

<footnote n="3">³ Department of Agriculture and Food Engineering, School of Engineering, Holy Spirit University of Kaslik, Jounieh BP 446, Lebanon; sandra.chidiac@usek.edu.lb</footnote>

<footnote>* Correspondence: youssefrayess@usek.edu.lb (Y.E.R.); nancyzgheib@usek.edu.lb (N.Z.)</footnote>
and other lipid-rich feedstocks [5,6]. Biodiesel has become increasingly popular in recent years as a fuel substitute for diesel engines due to its unique benefits, which include being environmentally friendly, biodegradable, and less toxic than fossil fuel [7,8]. Additionally, biodiesel is characterized by better lubricity, a higher flash point, lower sulfur content, and higher oxygen content than conventional diesel fuels [9]. The properties of biodiesel include being water-immiscible and having a boiling point between 182 and 338 °C, a vapor pressure of less than 2 mmHg, and a specific gravity that fluctuates between 0.86 and 0.90 [8]. In addition, biodiesel has a vapor density less than 1 g/L, a high cetane number, a high octane number, low net emissions, low viscosity, and a lack of aromatic and sulfur content, contributing to its great combustion efficiency and extended life in diesel engines [10–12].

Biodiesel can be synthesized from a wide range of feedstocks, including animal fats, such as beef tallow, vegetable oils, such as Jatropha, soybean, sunflower, palm, castor seed, cottonseed, rapeseed, canola, olive, Macuba, etc., waste cooking oil (WCO), and microalgae. Transesterification is a chemical reaction in which triglycerides react with alcohol in the presence of a catalyst. It is the most widely used method for biodiesel production due to its simplicity and high conversion efficiency [2]. The process involves a nucleophilic reaction between alcohol and edible or inedible oils, such as triglycerides (TG), resulting in the formation of fatty acid methyl esters (FAME) and glycerol as a byproduct [13]. The transesterification process involves three consecutive reversible reactions: (i) triglyceride to diglyceride conversion, (ii) diglyceride to monoglyceride conversion, and (iii) monoglyceride to glycerol conversion. Every conversion stage results in the formation of an ester; therefore, one TG molecule yields three ester molecules. This process, however, is sensitive to the quality of the feedstock, especially the content of free fatty acids (FFAs), which require pre-treatment through esterification. Depending on the FFA content, a single-pot strategy combining transesterification and esterification may be used to improve cost efficiency and reduce processing time [14,15].

Oils and fats do not dissolve in alcohol, which prevents the formation of a uniform mixture. As a result, the interaction between the reactants is limited, leading to a slower reaction rate and reduced overall efficiency. To overcome this challenge, catalysts are introduced to enhance the reaction kinetics. They work by lowering the activation energy, offering an alternative reaction pathway, and increasing the contact surface area between the immiscible reactants. Catalysts used in biodiesel production are generally classified into two main types: homogeneous and heterogeneous. In homogeneous catalysis, the catalyst is in the same phase as the reactants, with both typically being in the liquid phase. This allows for efficient mixing and faster reaction rates but makes the separation and reuse of the catalyst more challenging. On the other hand, heterogeneous catalysts are in a different phase, usually solid, while the reactants are liquid [16]. This difference simplifies catalyst recovery, reduces purification steps, and lowers waste. The choice between these two types of catalysts has a direct influence on the reaction rate, product yield, cost efficiency, and scalability of the process. Heterogeneous catalysts have attracted growing interest in recent years due to their reusability, ease of separation, and lower environmental impact.

In this context, the central research question addressed is as follows: How do the physicochemical properties of heterogeneous catalysts influence the efficiency, yield, and sustainability of biodiesel production? Therefore, this review aims to explore the synthesis of biodiesel using heterogeneous acid and base catalysts while also providing a comparative overview of homogeneous systems. It critically examines how the physicochemical properties of these catalysts impact biodiesel yield and discusses their practical advantages and limitations in the context of sustainable and efficient fuel production.
## 2. Homogeneous Catalysts

Homogeneous catalysts are widely used in the traditional methods of biodiesel production because of their quick and easy reaction, which provides high amounts of biodiesel. The classification of homogeneous catalysts is shown in Figure 1, and they can be categorized into two groups: acid and base catalysts [15].

<figure><img src="image_3.png" /><figcaption>Figure 1. Different types of homogeneous and heterogeneous catalysts.</figcaption></figure>

### 2.1. *Homogeneous Acid Catalysts*

Waste cooking oils with high levels of free fatty acids (FFAs) cannot be efficiently converted into biodiesel using an alkaline catalyst, as the FFAs react to form soap. This soap formation hinders the separation of esters, glycerin, and water. To address the issues caused by liquid base catalysts, liquid acid-catalyzed transesterification is recommended. Common acids used as catalysts in transesterification include ferric sulfate, hydrochloric acid, sulfuric acid, and organic sulfonic acids [17]. Under acidic conditions ($H^+$), the carbonyl group of the triglyceride is protonated, creating a positively charged carbon species. This activated carbonyl carbon then reacts with methanol, a nucleophile, to form a tetrahedral intermediate. The departure of the glycerol moiety, facilitated by a proton transfer, allows for easier formation of the methyl ester [15]. However, acid catalysts generally catalyze reactions more slowly than homogeneous base catalysts, requiring longer reaction times, higher temperatures, larger volumes of alcohol, and more catalysts to achieve higher biodiesel yields. As a result, acid catalysts are less commonly used for single-step biodiesel production and are less suitable for commercial applications [18].

### 2.2. *Homogeneous Base Catalysts*

Homogeneous base catalysts can function at low temperatures and atmospheric pressures and produce high yields quickly; therefore, they are more readily available, simpler to use, and more economical [15,16,19]. Homogeneous base catalysts have faster kinetics at relatively moderate reaction conditions and are typically chosen over acidic catalysis. According to reports, the reaction occurs approximately 4000 times faster in the presence of alkaline catalysts compared to acidic catalysts. Because they are inexpensive, react quickly,
and produce a large amount of biodiesel under mild reaction conditions, homogeneous alkali catalysts are widely used [20].

Several homogeneous base catalysts, such as KOH, NaOH, and NaOCH3, have been used to date for the production of FAME. Excellent catalytic activities, including low reaction times and high biodiesel yields, were demonstrated using NaOH and KOH as catalysts during the manufacture of biodiesel at room temperature and pressure. But, there are several limitations to this process, namely, the formation of water as a byproduct, which lowers the output of biodiesel. Apart from potassium and sodium hydroxide, sodium and potassium methoxide perform better in biodiesel production because no water is produced during these procedures. The transesterification of vegetable oils with high FFA levels (>2 wt%) cannot be effectively conducted with alkaline catalysts due to the formation of soap. Homogeneous base catalysts are more appropriate for refined vegetable oils with low FFA concentrations (less than 0.5 wt% to less than 2 wt%) [13].

Despite being relatively cheap and reliable, homogeneous catalysts' wide use is limited by four factors: (1) their toxic character; (2) the lack of catalyst recycling; (3) saponification issues; and (4) difficulties with catalyst separation. Biodiesel synthesized using homogeneous catalysts therefore needs to be purified to get rid of soaps and other impurities, which increases operating costs and produces a lot of wastewater [21].

### 3. Heterogeneous Catalysts

#### 3.1. General Properties of Heterogeneous Catalysts

Heterogeneous catalysts produce biodiesel from low-quality oil with high FFA and elevated water content [13]. The exploration of a large variety of heterogeneous catalysts demonstrated excellent catalytic capabilities. Compared to homogeneous catalysts, heterogeneous catalysts, as mentioned in Figure 1, offer several advantages, such as eliminating the drawbacks presented by homogeneous catalysts, ease of separation, recyclability, and the ability to be reused. Additionally, solid catalysts require less energy, are less corrosive, and are generally less harmful to the environment. Using these catalysts offers several advantages, including minimal wastewater generation during the process. Moreover, they facilitate easier extraction of glycerol from the final mixture (glycerol, biodiesel, and catalyst) and result in high-quality glycerol [13,19]. The catalyst's efficacy is influenced by its physical properties, including its surface area, particle size, morphology, porosity, shape, and mechanical integrity [22]. These properties are examined using several techniques, such as XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), SEM (scanning electron microscopy), BET (Brunauer–Emmett–Teller), TGA (thermogravimetric analysis), and the Hammett basicity method. The heterogeneous catalysts are the main focus of this review. The following sections will include a list of the most important acid and base heterogeneous catalysts.

#### 3.2. Heterogeneous Acid Catalysts

Heterogeneous acid catalysts possess two types of acid sites: Bronsted acid sites, wherein a solid acid catalyst can provide a proton (H⁺ addition) to facilitate simultaneous esterification and transesterification, and Lewis acid sites, wherein the catalyst may receive an electron pair [9,17]. Heterogeneous acid catalysts present many benefits, including reduced corrosion and the need for washing, as well as their insensitivity to moisture and FFA concentration [23].

Solid heterogeneous acid catalysts that are frequently used include heteropoly acid (HPA), mixed metal oxides, supported acid catalysts, sulfated waste catalysts, and ion exchange resins [24]. The studies conducted using various types of heterogeneous acid catalysts are presented in Table 1.
### 3.2.1. Heteropoly Acid Catalysts (HPA)

HPA catalysts have been extensively investigated as they are highly effective in biodiesel production. They surpass other homogeneous and heterogeneous catalysts due to their unique properties. These include strong acidity, high redox properties, good thermal stability, ease of separation, good reusability, fewer side products, reduced waste generation, non-toxicity, and ease of handling. Additionally, in both esterification and transesterification processes, HPAs are insensitive to excessive FFAs and water formation [24,25]. Nevertheless, the main disadvantage of HPAs is their limited surface area (less than 10 m²/g). This limitation can be addressed by dispersing the particles on a solid substrate with a large surface area, such as a heterogeneous catalyst with superior thermal stability, high recyclability, and an extensive surface area [26,27].

The catalytic activity of HPAs in biodiesel production arises primarily from their strong Brønsted and Lewis acid sites. In the transesterification reaction, the HPA catalysts can facilitate the nucleophilic attack of alcohol molecules on the triglyceride molecules, breaking the ester bonds and producing FAMEs and glycerol. The high acidity of HPAs promotes the esterification of free fatty acids (FFAs) present in oils, which is a crucial step when dealing with feedstocks that contain high amounts of FFAs, such as waste oils or low-quality feedstocks.

HPA catalysts can be synthesized using several methods, the most common being the acidification procedure combined with ether extraction; however, this approach presents safety concerns due to the use of toxic substances. While the ion exchange process offers a high level of safety, it requires prolonged production cycles. Although the impregnation method is straightforward, it exhibits limitations in catalyst reusability and suffers from a reduction in catalytic activity. The sol–gel technique is primarily used for nanomaterial-based catalysts and can be conducted at low to moderate temperatures; however, this method involves lengthy production times and poses safety risks [25].

HPA catalysts are made of a XO₄ central tetrahedron group surrounded by octahedral metal oxygen. The most common type of HPA is Keggin heteropoly acid with the general formula H₃XM₁₂O₄₀, where X is the heteroatom, such as P⁵⁺ and Si⁴⁺, and M is the addendum atom, such as Mo⁶⁺ and W⁶⁺. The most frequently used Keggin structures are phosphomolybdic acid, phosphotungstic acid, silicomolybdic acid, and silicotungstic acid. The other type of HPA is heteropoly anions of the Well-Dawson type with a general structure of (X⁺)₂M₁₈O₆₂, where X⁺ can be fluorine, phosphorus, sulfur, or arsenic and M may be Mo⁶⁺ or W⁶⁺ [28,29]. HPAs with a Keggin structure are preferred due to their higher thermal stability, their higher acid strength, and their easier synthesis compared to other types of HPAs [30].

Nevertheless, HPAs exhibit high solubility in polar solvents, and the esterification and transesterification reactions catalyzed by these HPAs typically occur in homogeneous rather than heterogeneous systems. As a result, separating the HPAs from the reaction medium for subsequent catalytic cycles requires time-consuming processes, like extraction or distillation. Moreover, the specific surface area of bulk HPAs is very low (less than 5 m²/g), which restricts their catalytic activity due to the limited number of available acid sites [31].

To overcome these drawbacks, two strategies have been employed. The first approach involves supporting the Keggin HPAs on high surface area solid matrices, such as mesoporous silica materials, e.g., MCM-41 (Mobil Composition of Matter No.41), MCM-48, and SBA-15 (Santa Barbara Amorphous 15), zirconium, zeolites, activated carbon, and silica. The second strategy consists of converting the Keggin HPAs to solid salts, which can be used as heterogeneous catalysts. This approach allows for the production of mate-
rials with reasonable surface areas and porosity and that are insoluble in polar solvents. Cesium-exchanged Keggin heteropolyacid salts are among the most used catalysts.

Bento et al. [32] evaluated the esterification and transesterification of lipid-rich fungal biomass with ethanol using the H₃PMo impregnated in Al₂O₃ as a catalyst. A yield of 96.6% biodiesel was achieved using 15 wt% of catalyst and a 120:1 ethanol-to-oil ratio at 200 °C for 6 h. The findings indicated that under the specified temperature and reaction time used, increasing catalyst loading and the ethanol to oil molar ratio resulted in greater conversion rates. The authors demonstrated that 95.6% biodiesel yield was achieved while dropping the ethanol to oil ratio to 90:1 and the catalyst percentage to 10 wt.%, and these settings are optimal when reducing the cost of production is considered.

Guo et al. [33] proposed a new strategy to improve HPAs’ implication in biodiesel production by synthesizing H₆PV₃MoW₈O₄₀/AC-Ag (activated carbon) catalyst for the transesterification of soybean oil with methanol. Using a 30:1 methanol-to-oil molar ratio, a reaction time of 10 h, a temperature of 140 °C, and an 8 wt.% catalyst dose, a biodiesel yield of 91.3% was achieved. Proton exchange with Ag was identified as the cause of the high dispersion of H₆PV₃MoW₈O₄₀ on the AC support. This interaction strengthened the bond between the support and the HPAs, boosting its catalytic stability. The catalyst demonstrated strong resistance to water and FFAs, which are commonly found in low-quality oils and readily recyclable in five cycles.

Tungstophosphoric acid (TPA) catalyst comprises an important class of supported catalysts; being non-toxic, recoverable, and reusable, these catalysts make the biodiesel production process both economically and environmentally viable. Other significant benefits of using this catalytic system include simplified post-reaction processing, short reaction times, high product yields, and easy catalyst handling [34]. Because TPA is a strong acid, it increases the protonation of several catalysts by causing multiple groups to interact electrostatically [27]. TPA may be utilized as a loaded, supported, anchored, or exchanged catalyst.

Pithadia and Patel [35] synthesized n-butyl levulinate using an innovative and sustainable heterogeneous catalyst made of 12-tungstophosphoric acid supported on zeolitic MCM-22, achieving 68% conversion. The catalyst could be reused up to six times without a significant reduction in conversion efficiency. The immobilization of 12-TPA on Zr support for the esterification of palmitic acid with methanol was investigated by Alcañiz-Monge et al. [36]. By using the sol–gel process and hydrothermal treatment, the catalyst was created with large porosity, which resulted in improved thermal and chemical stability under the esterification reaction, high conversions rate of 95%, and reduced HPA leaching in comparison to other supported TPAs.

The catalytic capabilities of TPA supported on bentonite clay catalysts show great promise. Khan et al. [37] showed that 90% of used frying oil was transformed into biodiesel during the transesterification process utilizing TPA impregnated on raw bentonite using a 1:10 oil-to-methanol ratio with 0.7 g of catalyst at a reaction temperature of 85 °C for 4.5 h. Six successful runs with the catalyst were performed again without a drop in the catalytic activity.

### 3.2.2. Supported Acid Catalyst on the Basis of Support

Acid catalysts on supports are primarily used to enhance the catalyst’s function by establishing electronic interactions between the support and the active phase. The support structure, typically porous, is crucial for catalytic performance, as it facilitates the efficient diffusion of reactants and products, resulting in higher activity. The size, type, and density of these pores directly impact catalytic efficiency. For optimal performance, careful control over catalyst dispersion during preparation is essential to securely anchor the catalyst.
Commonly used supports include metal oxides, such as aluminum oxide, magnesium oxide, silica gel, titanium oxide, aluminosilicates, zeolites, sulfonated metal oxides, and zirconium dioxide (ZrO₂).

## Aluminum-Supported Acid Catalysts

Aluminum oxide is widely used as a support in catalytic processes, both in its non-porous and porous forms, due to its high specific surface area, large pore volume, and large pore size. The transesterification of palm kernel oil with methanol using an aluminum phosphate solid acid catalyst has shown significant variations in biodiesel yield based on the catalyst's form. Under certain conditions (200 °C, oil-to-methanol ratio of 1:5, 10 g of catalyst, and a reaction time of 5 h), biodiesel yields reached 69% with a crystalline catalyst, while only 63% was achieved with an amorphous form [38].

This catalyst was synthesized by co-precipitating aluminum nitrate and orthophosphoric acid, followed by calcination at 400 °C (673 K) for 3 h for the crystalline form, while the amorphous form was left uncalcined [39]. When the crystalline catalyst was shaped into 3 mm cylindrical pellets, the biodiesel yield significantly increased to 90.1%. This outcome underscores the impact of the catalyst's shape on its efficiency, as the cylindrical form likely enhances surface area accessibility and mass transfer, allowing for a more effective interaction between reactants and active sites, thereby optimizing the catalytic process.

## Silicate-Supported Acid Catalysts

Sulfonic-acid-functionalized mesoporous silica-based solid acid catalysts with different morphologies are widely used for biodiesel production. Distinct morphologies, such as spherical and cubic shapes, were synthesized by Hegde et al. [40] and tested in biodiesel production through the esterification of oleic acid with methanol. The study revealed that the catalyst's morphology strongly affects catalytic efficiency and that the cubic-shaped catalyst outperformed its spherical counterpart. Optimal conditions, such as catalyst concentration, acid-to-alcohol ratio, loading, temperature, and time, yielded maximum conversion, with the cubic catalyst demonstrating stability across three reuse cycles.

Silicates, including mesoporous silica synthesized with agents like cetyltrimethylammonium bromide (CDAB), Pluronic L64, and P123, are commonly used as supports for catalysts in biodiesel production. These silica-based catalysts can incorporate sulfonic acid groups, with higher surface acidity correlating to increased catalytic activity and biodiesel yield. Biodiesel production was tested using CDAB-SO₃H-C₁₂, SBA-15-SO₃H-L64, and SBA-15-SO₃H-P123 catalysts under optimal conditions. Among them, SBA-15-SO₃H-P123, possessing the highest surface acidity, achieved 85% biodiesel yield with a reaction time of 20 h, while CDAB-SO₃H-C12, with the lowest surface acidity, resulted in a substantially lower yield of 55% [41].

## Zinc-Oxide-Supported Acid Catalysts

Zinc oxide is also employed as a support for acidic catalysts in the transesterification process for biodiesel production. Active solid acid catalysts, specifically sulfate-modified zinc oxide (SO₄²⁻-ZnO and SO₄²⁻/ZnO), were synthesized and characterized to evaluate their effectiveness in the transesterification of soybean oil with methanol for biodiesel production [42]. The catalyst preparation methods showed a significant influence on catalytic activity; SO₄²⁻-ZnO, prepared through coprecipitation, demonstrated superior catalytic performance compared to SO₄²⁻/ZnO, which was prepared through impregnation. This enhanced activity is attributed to the greater incorporation of sulfonate groups into the zinc oxide structure during coprecipitation, resulting in more active acid sites. Under mild conditions (65 °C, methanol-to-oil molar ratio of six, and 4 wt% catalyst loading), the
$SO_4^{2-}$-ZnO catalyst achieved a promising fatty acid methyl ester (FAME) yield of 80.19% within 4 h, underscoring its potential for biodiesel production.

In related studies, zinc oxide has also served as a support for acidic catalysts in biodiesel transesterification. For instance, using ZnO-supported sulfuric acid (ZnO/$SO_4^{2-}$) as a catalyst in palm oil transesterification with alcohol resulted in a biodiesel yield of 90.3% in just 1 h under optimal conditions (oil-to-alcohol molar ratio of 1:6). Furthermore, a zinc-oxide-supported strontium nitrate catalyst achieved a high biodiesel yield of 94.7% in soybean oil transesterification with a 4 h reaction time, 65 °C temperature, and 1:12 oil-to-alcohol molar ratio [43]. These findings emphasize that the catalyst preparation method and choice of support material critically impact catalytic performance, demonstrating that optimized preparation can significantly enhance biodiesel yields in transesterification reactions.

## Zirconium-Oxide-Supported Acid Catalysts

Zirconia is a well-known and widely used catalyst in various industrial applications due to its strong acidic sites and thermal stability. Its large pores, capable of accommodating large fatty acid molecules, help mitigate diffusional limitations [3]. Additionally, zirconium oxide serves as an efficient support for various catalysts because of its large surface area. Recently, sulfated zirconia ($SO_4$/ZrO₂) has gained global recognition as a solid acid catalyst for biodiesel production, mainly due to its high acid strength, which enhances the transesterification process. For example, Evangelista et al. [44] employed zirconium-doped SBA-15 (Zr-SBA-15 and $SO_4^{2-}$/Zr-SBA-15) as a catalyst for biodiesel production. The inclusion of zirconium reduced the mesoporous structure, leading to a decrease in surface area and microporosity. Under optimal reaction conditions—5 wt% catalyst (relative to oleic acid), a methanol acid molar ratio of 20:1, an 8 h reaction time, and a temperature of 65 °C—the best catalytic performance was achieved with $SO_4^{2-}$/Zr-SBA-15, yielding 80.7%.

Zirconium oxides also have various applications, such as in the production of tubular carbon membranes (TCM) and water-resistant zirconium sulfate tetrahydrate [Zr($SO_4$)₂·4H₂O]. These materials are synthesized by impregnating TCM with different amounts of zirconium sulfate, which makes them effective heterogeneous acid catalysts for esterifying acidified oil with methanol. The catalytic activity of 11.4 wt% zirconium sulfate/TCM is 1.8 times greater than that of bulk zirconium sulfate, owing to the high specific surface area of TCM [45]. In this formulation, zirconium sulfate (ZS) is uniformly dispersed on TCM, with a higher concentration on the membrane's surface due to diffusion effects. The catalytic performance of ZS/TCM was further validated in a tubular catalytic membrane reactor (TCMR), where the biodiesel produced from acidified oil met ASTM and GB fuel standards.

Recent research has also focused on glycerol esterification with acetic acid using Zr-modified hierarchical mordenite catalysts [46]. Zr-modified hierarchical mordenite, prepared through HF and NH₄F etching and further modified via incipient wetness impregnation to optimize surface acidity, demonstrated superior catalytic performance compared to unmodified zeolite substrates. This improvement is attributed to an optimal ratio of strong Brönsted and Lewis acid sites, which enhances active site accessibility. The inclusion of zirconium increased the catalytic turnover rate compared to unmodified zeolite substrates thanks to the abundance of Brönsted and Lewis acid sites. Using 2 g of glycerol, 0.1 g of catalyst, and a glycerol-to-acetic acid weight ratio of 1:10, the process achieved glycerol conversion of 90.6% at 100 °C over three hours. The catalysts exhibited excellent selectivity towards triacetylglycerol and high catalytic activity in the esterification reaction.

In contrast, titanium oxide (TiO₂) has also been explored as an alternative material for heterogeneous catalysis due to its high surface area, which helps stabilize catalysts within its mesoporous structure. Titanium oxysulphate sulfuric acid complex hydrate (TiOSH)
was employed to pre-treat low-grade crude palm oil (LGCPO) with high free fatty acid (FFA) content. A catalyst dosage of 7 wt% and a methanol-to-oil molar ratio of 10:1 for 120 min successfully reduced the initial FFA content of 11.3% to below 3% [47].

### 3.2.3. Sulfated Waste Catalysts

The use of sulfonated heterogeneous catalysts made from renewable resources to increase the sustainability and economic viability of biodiesel production has drawn a lot of attention lately [48,49]. Large-scale industrial waste, ash from plants and trees, shells, bones, and natural materials like clays are examples of these renewable resources. They are divided into four categories according to where they come from: shells, bones, plants or tree ash, and natural sources. These materials are very effective and recyclable catalysts because they have a high density of acid sites and strong acidity when functionalized with sulfonic acid groups. During the esterification stage of biodiesel synthesis, their exceptional catalytic performance is especially noticeable, providing a viable option for the production of sustainable energy [50,51]

The potential use of Brazilian açaí berry seeds, a copious and polluting waste, as a novel source for the production of heterogeneous acid catalysts employed in the transesterification of WCO with low FFA was investigated by Zavarize and de Oliveira [52]. When combined with KOH, açaí seeds exhibit excellent thermal behavior, which may have facilitated the creation of activated carbon. This carbonic base's chemical and physical characteristics gave it superior catalytic qualities and aided in the adhesion of sulfonic groups. A totally random designed experiment was used to examine the yield of activated carbon and catalyst, as well as the density of $SO_3H$ groups and total acidity. Optimized catalytic tests resulted in biodiesel production with a yield of 89.10% and 11 cycles of catalyst reuse.

Sulfonated biochar was used by Li et al. to conduct the simultaneous esterification of FFAs and the transesterification of cooking oil [53]. In that study, a molar ratio of 20:1 methanol to cooking oil was used, with a reaction time of 15 h and a temperature of 100 °C, resulting in an 88% yield. After five cycles, the yield of methyl esters dropped from 88% to 80% due to the leaching of the $-SO_3H$ functional groups.

A sulfonated sago pith waste (SPW) was successfully produced by Zailan et al. [54] to esterify palm fatty acid distillate (PFAD), and a high conversion rate of 93.03% was achieved. The ideal conditions for preparing the sulfonated SPW catalyst were a carbonization temperature of 200 °C, a 1:20 mass ratio of dry SPW to sulfuric acid, and a sulfonation time of 5 min.

Kader et al. [49] sulfonated calcined waste chicken and cow bones individually using sulfuric acid ($H_2SO_4$). Under optimal reaction conditions, the sulfated bones demonstrated excellent catalytic activity for the esterification of PFAD, achieving a yield of over 80%. The sulfated bones exhibited various acidic sites, ranging from weak sites associated with the presence of -OH groups to moderate and strong sites linked to the generated $-SO_3H$ groups. The acidity of the prepared materials was assessed using ammonia temperature-programmed desorption (NH$_3$-TPD). To study the effect of catalyst weight on the esterification of palm oil, the following conditions were applied: 3-5 wt% catalyst, a 20:1 molar ratio of methanol to PFAD, a reaction time of 180 min, and a reaction temperature of 70 °C. The initial use of a small amount of catalyst did not result in any significant conversion. The conversion of biodiesel increased considerably when the catalyst dose was raised from 3 to 6 wt%. It was found that optimal catalyst loading was at 5 wt%. When catalyst loading was increased to 6 wt%, conversion sharply dropped, resulting in thick slurries that made it challenging to adequately stir the mixture.
For the purpose of producing glycerol-free biodiesel, two environmental waste products, PFAD and spent bleaching clay (SBC) from the palm oil refining industry, were used as inexpensive feedstock and catalytic support, respectively. The methylating agent was dimethyl carbonate (DMC), which is thought to be an acceptable environmentally friendly substitute for methanol in the production of glycerol-free biodiesel. After calcination and sulphonation, a heterogeneous acid catalyst produced from SBC was used for the reactions. At ideal reaction conditions consisting of 10 wt% of catalyst, a reaction time of 4 h, a reaction temperature of 100 °C, and a PFAD/DMC molar ratio of 1:10, the highest conversion efficiency of 93.18% was achieved [55]. Biodiesel production using bamboo, wheat straw, and peanut shell biochar was determined by Zhang et al. in a different study [56]. The biochar was produced using a two-step carbonization/sulfonation process. Under optimum esterification conditions, including 10 wt% catalyst loading, an 8:1 methanol to oleic acid molar ratio, and a temperature of 75 °C for eight hours, high biodiesel yields were achieved. The yields were 98.0% for bamboo, 97.4% for wheat straw, and 96.8% for peanut shell. The catalyst's conversion of oleic acid fell to around 80% after four cycles. Karmakar et al. [57] used Mesuaferrea linn (MFL) seed shells to convert castor oil with high FFA content into biodiesel. They studied the esterification reaction using methanol and recorded an FFA conversion of 92% at 60 °C for 1.5 h with 7 wt.% catalyst. While using a catalyst derived from waste cork biochar, Bhatia et al. [58] converted WCO into biodiesel. Waste cork was prepared through pyrolysis at 600 °C and activation with H₂SO₄. The decrease in surface area and pore size resulted from the oxidation reaction between sulfonic functional groups and carbon and the activation of cork biochar was due to the filling of the pores with -SO₃H groups. The transesterification reaction resulted in a high yield of 98% when the following conditions were used: an alcohol/oil molar ratio of 25:1, catalyst loading of 1.5% w/v, and a temperature of 65 °C.

Falowo et al. [59] produced biodiesel from discarded vegetable oil using the ash of ripe and unripe plantain peels. Given the high acid value of the WCO, it was pre-treated with 3% (v/v) H₂SO₄ through esterification, reducing its acidity from 5 mg KOH/g to 1 mg KOH/g. The optimal transesterification conditions were a catalyst concentration of 0.5 wt%, a methanol/WCO molar ratio of 6:1, a reaction temperature of 45 °C, and a reaction time of 45 min, achieving a biodiesel yield of 97.96%

Yaakouby et al. [60] investigated biodiesel production from PFAD over a sulfonated natural hydroxyapatite (HAPSS-SO₄) catalyst obtained from sardine scales. They demonstrated that the catalyst, which was mainly composed of calcium sulfate, exhibits an excellent catalytic performance for esterification, while FFAs were strongly adsorbed and activated at Ca sites. Under optimal reaction conditions of a 15:1 methanol to PFAD molar ratio, 3 wt% catalyst loading, and a 3 h reaction time at 70 °C, the catalyst achieved a high PFAD esterification rate of 96.75%. Additionally, the catalyst showed excellent stability and was successfully regenerated through three cycles.

### 3.2.4. Ion Exchange Resin

Ion exchange resin catalysts are also part of heterogeneous acid catalysts. By enhancing the adsorption capacity and surface activity of mesoporous materials, the ion exchange resin enhances their surface properties and adds new functionality for the manufacturing of biodiesel [61]. The resin catalyst group includes non-porous Amberlyst resins, Nafion resins, and the sulphonic ion exchange resin. These catalysts often have strong acidic activity and are highly effective in the two-step esterification process. However, the swelling characteristic of these catalysts often causes alterations in the resin's surface area and pore structure, which adversely impact catalytic activity and efficiency [44].
The most common ion exchange resin to produce biodiesel is Amberlyst-15. Pasa et al. [62] explored Amberlyst-15 in the synthesis of ethyl ester from Macuba oil at optimum conditions of 130 °C, an ethanol-to-oil molar ratio of 9:1, and 16 wt% of catalyst. A biodiesel yield of 89.1% was achieved using these conditions. Maçaira et al. [63] investigated the use of Nafion resin (SAC-13) for biodiesel production using a solvent mixture of supercritical methanol and carbon dioxide in a continuous reactor. A biodiesel yield of 88% was achieved at a reaction temperature of 200 °C within 2 min. Various types of cation exchange resins are available, including Purolite, NKC-9, D61, and others. Using the purolite CT 275 as a catalyst, Rodrigues et al. [64] investigated the transesterification of Macuba oil. A biodiesel yield of 93% was achieved using an 8.6:1 ethanol to oil ratio and a reaction temperature of 85 °C after a reaction time of 9 h. Furthermore, the catalyst remained stable without treatment for ten consecutive cycles.

### 3.2.5. Mixed Metal Oxides

Metals and their oxides are constantly being combined to develop new catalysts. By utilizing the synergy between various components, these mixed metal oxides improve catalytic performance and successfully address issues heterogeneous catalysts frequently face. This method can increase the catalytic sites' activity or enhance their surface area, pore size, magnetic separability, and structural stability, among other characteristics. Additionally, they might strengthen resistance to inexpensive, low-quality feedstocks like leftover cooking oil. "Mixed-metal-oxide catalysts" are catalysts composed of two or more metal oxides. In contrast to single metal oxides, the main goal of the construction of mixed metal oxide catalysts was to increase stability, surface area, and basic or acid strength. For this, several solid catalysts that are stable, reusable, and incredibly effective were created [65].

Ferric molybdate, $Fe_2(MoO_4)_3$, for instance, was synthesized by Al Kahlaway et al. [66] as nanoparticles for the catalytic conversion of oleic acid and the production of biodiesel. At 70 °C, a 90.5% yield was obtained with a 9:1 alcohol/oleic molar ratio and 3 wt% of catalyst. The catalyst demonstrated fair capacity for recycling as it was reused up to four cycles, showing a drop in oleic acid conversion by 3%, and this trend continued until the sixth cycle.

After preparing the magnetic $Fe_3O_4/SiO_2$ composites, which consist of silica on the shell and iron oxides in the core, Xie and Wang [12] copolymerized the Brønsted acidic ionic liquid (IL), 1-vinyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate. This was achieved through radical grafting to immobilize the polymeric acidic IL onto the magnetic support. Characterization results confirmed that the polymeric acidic IL was successfully anchored on the magnetic support, leading to the formation of an ideal core-shell structured $Fe_3O_4/SiO_2$ support with strong magnetic responsiveness. With the help of the complementary properties of the magnetic homogeneous support, the catalyst demonstrated remarkable catalytic activity and recycling potential, ultimately reaching an 86% biodiesel yield after five cycles. The catalyst was used effectively for both the transesterification of soybean oil and the esterification of FFAs, making it an effective solid acid catalyst for one-pot biodiesel production from low-quality feedstocks. Similarly, Ding et al. [67] converted oleic acid to methyl and ethyl oleate, achieving a yield of 94.8 and 92.1%, respectively, using a magnetically recyclable $Fe_3O_4@SiO_2@PIL$ catalyst. Using 9.5 wt.% loading, the catalyst was recycled up to six times, resulting in a drop in ethanol esterification from 92.1 to 89.2% after repeated use six times. Using the hydrothermal technique, Zhang et al. [68] created a catalyst from MIL-100(Fe) encapsulated by $H_4SiW$ and investigated its catalytic activity for the esterification of lauric acid. The catalyst $H_4SiW/MIL-100(Fe)$ was successfully produced and had an approximately 300 nm sized cube-like structure.
The ideal lauric acid conversion of 80.3% was attained by employing a lauric acid to methanol ratio of 1:12, a temperature of 160 °C, and a reaction period of three hours.

The catalyst maintained conversion higher than 60% when it was reused up until the eleventh cycle. Changmai et al. [69] investigated the use of Fe₃O₄@SiO₂-SO₃H core@shell nanoparticulate for the esterification and transesterification of Jatropha curcas oil.

Through a sequential procedure that included coprecipitation, coating, and functionalization, the catalyst was effectively created. First, magnetite (FeO₄) was created using the coprecipitation method, which was followed by silica coating and sulfonic acid functionalization.

The catalyst displayed an acid density of 0.76 mmol/g, a magnetic saturation of 30.94 emu/g, a pore size of 3.48 nm, and a surface area of 32.88 m²/g. Transesterification produced 98% conversion with 8 wt.% of the catalyst, 3.5 h reaction time, and a 9:1 methanol/oil molar ratio. Because of its efficient one-pot regeneration of active sites, the solid acid catalyst showed outstanding stability, reusability, and magnetic separability. It also performed consistently in JCO methyl transesterification and esterification over 10 cycles. The leaching and the methylation of sulfonic acid were the causes of the decrease in conversion efficiency.

The esterification of OA was carried out by Varão et al. [70] using MnFe₂O₄ and CoFe₂O₄ ferrite nanoparticles coated with sulfonated lignin (SL). Acetyl sulfate, a sulfonating agent, was used with sugarcane bagasse lignin to produce SL. Different reactant proportions were used to produce two different types of SL, the solids SL5 and SL7.5, which were then coated on cobalt and manganese magnetic nanoparticles (MNPs). A conversion of approximately 80% of OA into FAME was achieved using CoFe₂O₄-SL5 and MnFe₂O₄·SL7.5 under 6 h and 100 °C. It was observed that MNPs with SL with the highest specific surface area (SBET) exhibited better catalytic yield, suggesting that the surface area is just as crucial as the -SO₃H group content. The material with the largest SBET enhances the accessibility of oleic acid to acidic sites, thereby promoting the esterification reaction.

Metal-organic frameworks (MOFs) are porous materials made of molecular building pieces joined by strong coordination interactions, such as inorganic clusters or metal ions and organic linkers. Amouhadi et al. [71] studied the esterification of oleic acid using a heterogeneous MnO₂@Mn(btc) catalyst that was created using the solvothermal process. The catalyst with 15 wt% MnO₂@Mn(btc) loading demonstrated exceptional catalytic activity and durability, achieving a 98% biodiesel yield over at least five cycles under moderate conditions. This was achieved with a 12:1 ethanol-to-oleic acid molar ratio, 3 wt% catalyst, a reaction temperature of 100 °C, and a reaction time of 12 h.

Al-Jaberi et al. [72] performed the esterification of PFAD utilizing a heterogeneous catalyst made of manganese-nickel doped on sulfated zirconia catalyst MnO-NiO-SO₄²⁻/ZrO₂. Low-grade palm oil contains water, impurities, and free fatty acids, making it difficult to produce biodiesel using a catalyst in a single run. The experiment yielded a high conversion of 97.7% after PFAD was esterified using a normal conventional reflux reactor for three hours at 70 °C, a 15:1 methanol:oil ratio, and 3 wt.% catalyst loading.

Likewise, in order to convert low-quality oils, Wang et al. [73] synthesized a new bifunctional acid catalyst, Molybdenum and zirconium oxides, supported on KIT-6 silica MoO₃/ZrO₂/KIT-6. Under ideal reaction conditions, this catalyst was able to convert 92.7% of the oil. Because the Brønsted and Lewis acid sites work in concert, this catalyst demonstrated high activities for both transesterification of triglycerides and esterification of free fatty acids (FFAs) at the same time. This allowed for the one-pot heterogeneous production of biodiesel using low-quality oil as feedstock.

Vieira et al. [74] synthesized HZSM-5 zeolite catalyst coated with sulfated lanthanum oxide for the conversion of oleic acid to biodiesel. According to the results, a temperature
of 100 °C, a methanol to oleic acid molar ratio of 10:1, and a 10 wt% of catalyst quantity required 7 h to complete the conversion of oleic acid.

Biodiesel conversion of wild mustard seed oil was performed using lanthanum tita-
nium dioxide (LaTiO₃) nanoparticles as the catalyst. The highest biodiesel rate obtained
was 92.21% while using a reaction temperature of 80 °C for 60 min, and the catalyst kept its
activity for at least five runs (90% yield after five runs) [75].

A new heterogeneous catalyst composite CuS-FeS/SiO₂ made of silica from rice husks was employed to transesterify palm oil with methanol. CuS-FeS/SiO₂ showed outstanding surface area and thermal stability. It had an ability to stimulate the esterification process, which was attributed to higher levels of acidity at the active site, which made it easier for the methanol and FFA carbonyl moieties to interact. Using a conventional reflux system, esterification conversion of over 98% was achieved with the following reaction parameters: a methanol: palm oil molar ratio equal to 15:1, a catalyst concentration of 2 wt%, a reaction temperature of 70 °C, and a reaction duration of 180 min. The catalyst showed a high reusability profile, with potency maintained across five reaction cycles [76].
<table><caption>**Table 1.** Literature review of biodiesel production processes using acid heterogeneous catalysis.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15" style="text-align: left; font-weight: bold;">Heteropoly Acids</td></tr><tr><td rowspan="3">Oleic acid</td><td rowspan="3">Esterification with methanol</td><td>TPA/MCM-41</td><td></td><td></td><td></td><td></td><td rowspan="3">Impregnation<br>T<sub>calcination MCM-41</sub>=555 °C for 5 h<br>T<sub>calcination SBA-15</sub>=500 °C for 6 h</td><td rowspan="3">40 °C<br>2-4 h</td><td rowspan="3">40:1</td><td rowspan="3">0.1 g</td><td rowspan="3">TPA/MCM-41<br>C = 100%<br>TPA/SBA-15 C = 98%</td><td rowspan="3">4</td><td rowspan="3">[77]</td></tr><tr><td>360</td><td>0.5</td><td>3.01</td><td>1.41 acidity</td></tr><tr><td>TPA/SBA-15</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td>714</td><td>1.12</td><td>6.20</td><td>1.82 acidity</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Water, cooking oil</td><td rowspan="2">Transesterification with ethanol</td><td>12-tungstophosphoric acid (TPA) supported on MCM-48</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 550 °C for 6 h</td><td rowspan="2">60 °C<br>8 h</td><td rowspan="2">-</td><td rowspan="2">0.1 g</td><td rowspan="2">C = 95%</td><td rowspan="2">4</td><td rowspan="2">[78]</td></tr><tr><td>286</td><td>0.38</td><td>-</td><td>1.53 acidity</td></tr><tr><td rowspan="2">Levulinic acid (LA)</td><td rowspan="2">Esterification with butanol</td><td>12-tungstophosphoric acid and zeolitic support, MCM-22</td><td></td><td></td><td></td><td></td><td rowspan="2">Wet impregnation</td><td rowspan="2">90 °C<br>8 h</td><td rowspan="2">2:1</td><td rowspan="2">1.86</td><td rowspan="2">C = 68%</td><td rowspan="2">6</td><td rowspan="2">[35]</td></tr><tr><td>258</td><td>0.25</td><td>5.31</td><td>-</td></tr><tr><td rowspan="3">Glycerol</td><td rowspan="3">Esterification with methanol</td><td>12-tungstophosphoric anchored to MCM-41</td><td></td><td></td><td></td><td></td><td rowspan="3">MCM-41 calcination at 550 °C for 5 h<br>ZrO<sub>2</sub> incipient impregnation</td><td rowspan="3">100 °C<br>6 h</td><td rowspan="3">6:1</td><td rowspan="3">0.15</td><td rowspan="3">TPA/MCM-41 C = 87%<br>TPA/ZrO<sub>2</sub>C = 80%</td><td rowspan="3">4</td><td rowspan="3">[79]</td></tr><tr><td>360</td><td>-</td><td>-</td><td>0.08 acidity</td></tr><tr><td>12-tungstophosphoric anchored to ZrO<sub>2</sub></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td>146</td><td>-</td><td>-</td><td>0.08 acidity</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Palmitic acid</td><td rowspan="2">Esterification with methanol</td><td>12-tungstophosphoric heteropoly acid over zirconia</td><td></td><td></td><td></td><td></td><td rowspan="2">Sol-gel method<br>Hydrothermal treatment</td><td rowspan="2">60 °C<br>-</td><td rowspan="2">-</td><td rowspan="2">30</td><td rowspan="2">C = 95%</td><td rowspan="2">5</td><td rowspan="2">[36]</td></tr><tr><td>365</td><td>0.33</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td>12-tungstosilicicacid anchored to SBA-15</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 300 °C for 3 h</td><td rowspan="2">60 °C<br>8 h</td><td rowspan="2">20:1</td><td rowspan="2">30</td><td rowspan="2">Y = 89.7%</td><td rowspan="2">5</td><td rowspan="2">[80]</td></tr><tr><td>537</td><td>0.594</td><td>5.322</td><td>-</td></tr><tr><td rowspan="2">Spent frying oil</td><td rowspan="2">Esterification with methanol</td><td>TPA/bentonite</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 500 °C for 5 h</td><td rowspan="2">85 °C<br>4.5 h</td><td rowspan="2">10:1</td><td rowspan="2">10</td><td rowspan="2">Y = 96%</td><td rowspan="2">6</td><td rowspan="2">[37]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>4.5 acidity</td></tr><tr><td rowspan="2">Wild olive oil</td><td rowspan="2">Transesterification with methanol</td><td>TPA/Cr-Al</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 120 °C for 12 h</td><td rowspan="2">80 °C<br>5 h</td><td rowspan="2">21:1</td><td rowspan="2">4</td><td rowspan="2">Y = 93%</td><td rowspan="2">5</td><td rowspan="2">[81]</td></tr><tr><td>56.168</td><td>0.014</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td>TPA@C-NiZr-MOF</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 300 °C for 180 min</td><td rowspan="2">140 °C<br>4 h</td><td rowspan="2">20:1</td><td rowspan="2">0.15 g</td><td rowspan="2">C = 91.9%</td><td rowspan="2">6</td><td rowspan="2">[82]</td></tr><tr><td>441</td><td>0.3</td><td>2.72</td><td>8.82 acidity</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td>Picolinic-acid-modified 12-tungstophosphoric acid</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br>Calcination at 300 °C for 180 min</td><td rowspan="2">80 °C<br>5 h</td><td rowspan="2">10:1</td><td rowspan="2">7</td><td rowspan="2">C = 100%</td><td rowspan="2">4</td><td rowspan="2">[83]</td></tr><tr><td>27.6</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">One-pot fungal biomass</td><td rowspan="2">Esterification and transesterification with ethanol</td><td>H<sub>3</sub>PMo/Al<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation</td><td rowspan="2">200 °C<br>6 h</td><td rowspan="2">120:1</td><td rowspan="2">15</td><td rowspan="2">C = 96.6%</td><td rowspan="2">N/A</td><td rowspan="2">[84]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Molasses, vinasse</td><td rowspan="2">Esterification and transesterification with ethanol</td><td>H<sub>3</sub>PMo/Al<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation</td><td rowspan="2">200 °C<br>6 h</td><td rowspan="2">120:1</td><td rowspan="2">-</td><td rowspan="2">C > 95%</td><td rowspan="2">N/A</td><td rowspan="2">[85]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Oleic acid, rapeseed oil</td><td rowspan="2">Esterification with methanol</td><td>H<sub>3</sub>PMo<sub>12</sub>O<sub>40</sub>@EB-COF</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation</td><td rowspan="2">70 °C<br>8 h</td><td rowspan="2">15:1</td><td rowspan="2">10</td><td rowspan="2">C = 85%</td><td rowspan="2">6</td><td rowspan="2">[86]</td></tr><tr><td>24.7</td><td>0.0698</td><td>-</td><td>-</td></tr></tbody></table>
<table><caption>Table 1. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15"><strong>Heteropoly Acids</strong></td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td>H₆PV₃MoW₈O₄₀/AC-Ag<br>309.8</td><td>0.23</td><td>2.98</td><td>-</td><td>Impregnation<br>Calcination at 300 °C for 3 h</td><td>140 °C</td><td>10 h</td><td>30:1</td><td>8</td><td>C = 91.3%</td><td>5</td><td>[33]</td></tr><tr><td colspan="15"><strong>Supported Acid Catalyst on the Basis of Support</strong></td></tr><tr><td>Palm oil</td><td>Esterification and transesterification with methanol</td><td>Al-SBA-15 (stable silica)<br>844-938</td><td>1.284-<br>1.292</td><td>5.5-6.1</td><td>-</td><td>Sol-gel method<br>Calcination at 550 °C for 6 h</td><td>215 °C</td><td>4 h</td><td>30:1</td><td>15</td><td>Y = 40-87%</td><td>N/A</td><td>[87]</td></tr><tr><td>Jatropha oil</td><td>Esterification and transesterification with methanol</td><td>Monometallic catalysts (Ce/Al-MCM-41 and Zr/Al-MCM-41) and bimetallic catalyst (Ce-Zr/Al-MCM-41)<br>261</td><td>0.2</td><td>4.3</td><td>-</td><td>Impregnation/co-impregnation<br>Calcination at 500 °C for 5 h</td><td>90 °C</td><td>4 h</td><td>6:1</td><td>5</td><td>C = 93%</td><td>N/A</td><td>[88]</td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td>SBA-15 silica<br>325</td><td>0.52</td><td>5.56</td><td>1.91-2.95</td><td>Calcination 823 K for 6 h</td><td>70 °C</td><td>12 h</td><td>15:1</td><td>5</td><td>C = 91.7%</td><td>5</td><td>[89]</td></tr><tr><td>2-aryl benzimidazoles and benzothiazoles</td><td>Transesterification with ethanol</td><td>SBA-15-SO₃H<br>568</td><td>0.23</td><td>2.04</td><td>-</td><td>Hydrothermal treatment at 100 °C for 48 h<br>Calcination at 450 °C for 8 h</td><td>Room</td><td>24 h</td><td>1:1</td><td>0.05</td><td>C = 100%<br>Y = 70-85%</td><td>5</td><td>[90]</td></tr><tr><td>Waste cooking oil</td><td>Esterification with methanol</td><td>S-TiO₂/SBA-15<br>733.98</td><td>0.09</td><td>5.10</td><td>0.1 acidity</td><td>Impregnation<br>Calcination at 540 °C</td><td>200 °C</td><td>30 min</td><td>15:1</td><td>1</td><td>Y = 94.96%</td><td>3</td><td>[91]</td></tr><tr><td>Municipal sewage sludge</td><td>Transesterification and esterification with methanol</td><td>Zr-SBA-15<br>531</td><td>1.41</td><td>12.8</td><td>-</td><td>Hydrothermal process at 130 °C for 24 h<br>Calcination at 450 °C for 5 h</td><td>209 °C</td><td>3 h</td><td>0.25 g/mL</td><td>15.5</td><td>Y > 90%</td><td>N/A</td><td>[92]</td></tr><tr><td rowspan="3">Oleic acid</td><td rowspan="3">Esterification with methanol</td><td>Zr-SBA-15<br>675.6</td><td>0.84</td><td>6.37</td><td>1.44 acidity</td><td>Zr-SBA-15<br>impregnation<br>Calcination at 600 °C for 6 h<br>SO₄²⁻/Zr-SBA-15</td><td>65 °C</td><td>8 h</td><td>20:1</td><td>5</td><td>Y = 80.7%</td><td>N/A</td><td>[44]</td></tr><tr><td>SO₄²⁻/Zr-SBA-15<br>525.3</td><td>0.80</td><td>6.37</td><td>1.97 acidity</td><td>Hotplate at approximately 65 °C under constant stirring<br>Oven drying at 110 °C for 15 h</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>[Zr(SO₄)₂·H₂O]/tubular carbon membranes (ZS/TCM)<br>109</td><td>0.21</td><td>7.12</td><td>-</td><td>Impregnation</td><td>60 °C</td><td>30 min</td><td>6:1</td><td>-</td><td>C = 99.9%</td><td>5</td><td>[45]</td></tr><tr><td>Neem oil</td><td>Esterification and transesterification with methanol</td><td>Sulfated zirconia<br>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 600 °C for 5 h<br>-</td><td>65 °C<br>-</td><td>2 h<br>-</td><td>9:1</td><td>1</td><td>C = 95%</td><td>N/A</td><td>[93]</td></tr><tr><td>Cerbera odollam (sea mango) oil</td><td>Transesterification with methanol</td><td>Sulfated zirconia alumina and montmorillonite KSF<br>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 400 °C for 2.5 h<br>-</td><td>64.7 °C</td><td>1 h</td><td>6:1</td><td>0.1</td><td>Y = 83.8%</td><td>N/A</td><td>[94]</td></tr><tr><td rowspan="2">Fatty acid</td><td rowspan="2">Esterification with methanol</td><td>Sulfated lanthanum oxide (SLO)<br>SLO/HZSM-5<br>217</td><td>0.1</td><td>2.5</td><td>-</td><td>Impregnation<br>Calcination at 400 °C for 3 h<br>-</td><td>100 °C</td><td>4 h</td><td>20:1</td><td>10</td><td>SLO/HZSM-5<br>C = 100%</td><td>N/A</td><td>[95]</td></tr></tbody></table>
<table><caption>Table 1. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15" style="text-align: left; font-weight: bold;">Supported Acid Catalyst on the Basis of Support</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Esterification with methanol and glycerol</td><td colspan="4">Lanthanum-supported sulfated zirconia</td><td rowspan="2">Calcination at 600 °C</td><td rowspan="2">60/150 °C</td><td rowspan="2">3 h</td><td rowspan="2">10:1</td><td rowspan="2">3</td><td rowspan="2">C > 95%</td><td rowspan="2">N/A</td><td rowspan="2">[96]</td></tr><tr><td>98</td><td>0.09</td><td>-</td></tr><tr><td rowspan="2">Palm fatty acid distillate</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">SO<sub>3</sub>H-GO@TiO<sub>2</sub></td><td rowspan="2">Microwave-assisted hydrothermal acid treatment</td><td rowspan="2">70 °C</td><td rowspan="2">40 min</td><td rowspan="2">9:1</td><td rowspan="2">3</td><td rowspan="2">Y = 96.73%</td><td rowspan="2">10</td><td rowspan="2">[97]</td></tr><tr><td>611</td><td>0.16</td><td>7.25</td><td>3.30 acidity</td></tr><tr><td colspan="15" style="text-align: left; font-weight: bold;">Mixed Oxides</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td colspan="4">Zirconia-alumina nanocatalyst</td><td rowspan="2">Impregnation Calcination at 550 °C for 4 h</td><td rowspan="2">90 °C</td><td rowspan="2">4 h</td><td rowspan="2">9:1</td><td rowspan="2">3</td><td rowspan="2">C = 91.6%</td><td rowspan="2">7</td><td rowspan="2">[98]</td></tr><tr><td>14.35</td><td>0.0017</td><td>4.9</td><td>-</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with butanol and ethanol</td><td colspan="4">Fe(III)-based MOF (MIL-53)</td><td rowspan="2">Prepared with composites Ultrasound</td><td rowspan="2">Room T°</td><td rowspan="2">15 min</td><td rowspan="2">16:1</td><td rowspan="2">30</td><td>C = 96% (ethanol)<br/>C = 98% (butanol)</td><td rowspan="2">5</td><td rowspan="2">[99]</td></tr><tr><td>569</td><td>127</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with ethanol</td><td colspan="4">Fe<sub>2</sub>(MoO<sub>4</sub>)<sub>3</sub></td><td rowspan="2">Sol-gel method Calcination at 500 °C for 2 h</td><td rowspan="2">70 °C</td><td rowspan="2">-</td><td rowspan="2">9:1</td><td rowspan="2">3</td><td rowspan="2">C = 92.50%</td><td rowspan="2">6</td><td rowspan="2">[66]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Transesterification and esterification with methanol</td><td colspan="4">Fe<sub>3</sub>O<sub>4</sub>/SiO<sub>2</sub></td><td rowspan="2">Hydrothermal method Sol-gel method</td><td rowspan="2">120 °C</td><td rowspan="2">6 h</td><td rowspan="2">35:1</td><td rowspan="2">9</td><td rowspan="2">C = 93.3%</td><td rowspan="2">5</td><td rowspan="2">[12]</td></tr><tr><td>58.99</td><td>0.14</td><td>9.1</td><td>1.89 acidity</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with ethanol</td><td colspan="4">Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub></td><td rowspan="2">N/A</td><td rowspan="2">90 °C</td><td rowspan="2">4 h</td><td rowspan="2">11.5:1</td><td rowspan="2">9.5</td><td>C = 92.10%<br/>Y = 92.1%</td><td rowspan="2">6</td><td rowspan="2">[67]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Fatty acids, non-food oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>-SO<sub>3</sub>H</td><td rowspan="2">Sol-gel method Calcination at 1000 °C for 5 h</td><td rowspan="2">65 °C</td><td rowspan="2">4 h</td><td rowspan="2">20:1</td><td rowspan="2">4</td><td rowspan="2">Y = 97.8%</td><td rowspan="2">5</td><td rowspan="2">[68]</td></tr><tr><td>4.5</td><td>-</td><td>8.9</td><td>0.0444 basicity<br/>0.020 acidity</td></tr><tr><td rowspan="2">Jatropha curcas oil</td><td rowspan="2">Esterification and transesterification with methanol</td><td colspan="4">Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>-SO<sub>3</sub>H core</td><td rowspan="2">Co-precipitation Coating Functionalization</td><td rowspan="2">80 °C</td><td rowspan="2">3.5 h</td><td rowspan="2">9:1</td><td rowspan="2">8</td><td rowspan="2">Y = 98%<br/>C = 98.7%</td><td rowspan="2">10</td><td rowspan="2">[100]</td></tr><tr><td>32.88</td><td>0.07</td><td>3.48</td><td>0.76 acidity</td></tr><tr><td rowspan="2">Palm oil</td><td rowspan="2">Esterification with methanol</td><td colspan="4">Ferric alginate</td><td rowspan="2">N/A</td><td rowspan="2">60 °C</td><td rowspan="2">3 h</td><td rowspan="2">16:1</td><td rowspan="2">2</td><td rowspan="2">C = 98%</td><td rowspan="2">N/A</td><td rowspan="2">[101]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Palm oil</td><td rowspan="2">Esterification and transesterification with methanol</td><td colspan="4">Ferric hydrogen</td><td rowspan="2">N/A</td><td rowspan="2">205 °C</td><td rowspan="2">4 h</td><td rowspan="2">15:1</td><td rowspan="2">1</td><td rowspan="2">Y = 94.5%</td><td rowspan="2">5</td><td rowspan="2">[102]</td></tr><tr><td>4.9</td><td>-</td><td>34.4</td><td>10.5 acidity</td></tr><tr><td rowspan="2">Mesua ferrea oil</td><td rowspan="2">Esterification and transesterification with methanol</td><td colspan="4">Co-doped ZnO</td><td rowspan="2">N/A</td><td rowspan="2">60 °C</td><td rowspan="2">3 h</td><td rowspan="2">9:1</td><td rowspan="2">2.5</td><td rowspan="2">C = 98.03%</td><td rowspan="2">4</td><td rowspan="2">[103]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>0.91</td></tr><tr><td rowspan="3">Oleic acid</td><td rowspan="3">Esterification with methanol</td><td colspan="4">CoFe<sub>2</sub>O<sub>4</sub>-SL5</td><td rowspan="3">Co-precipitation</td><td rowspan="3">100 °C</td><td rowspan="3">6 h</td><td rowspan="3">10:1</td><td rowspan="3">5</td><td>C = 79.5%<br/>C = 78.5%</td><td rowspan="3">N/A</td><td rowspan="3">[70]</td></tr><tr><td>41.47</td><td>0.129</td><td>9.25</td><td>-</td></tr><tr><td>34.41</td><td>0.1128</td><td>9.39</td><td>1.8 acidity</td></tr></tbody></table>
<table><caption>Table 1. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15" style="text-align: left; font-weight: bold;">Mixed Oxides</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">MnO-NiO-SO₄²⁻/ZrO₂</td><td>Wet impregnation<br/>Calcination at 600 °C for 3 h</td><td>70 °C</td><td>3 h</td><td>15:1</td><td>3</td><td>C = 97%</td><td>5</td><td>[72]</td></tr><tr><td>Water, cooking oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Sr/ZrO₂, Mg/ZrO₂, Ca/ZrO₂, and Ba/ZrO₂</td><td>Wet impregnation<br/>Calcination at 650 °C for 5 h</td><td>115.5 °C</td><td>169 min</td><td>29:1</td><td>2.7</td><td>Y = 79.70%</td><td>N/A</td><td>[104]</td></tr><tr><td rowspan="6">Soybean oil, corn oil</td><td rowspan="6">Transesterification and esterification with methanol</td><td colspan="4">ZnO-SiO₂/ZrO₂</td><td rowspan="6">Sol-gel method</td><td rowspan="6">190 °C</td><td rowspan="6">1.15 h</td><td rowspan="6">9.4:1</td><td rowspan="6">-</td><td rowspan="6">C > 90%<br/>Y > 99% using<br/>two-stage packed bed<br/>reactor</td><td rowspan="6">N/A</td><td rowspan="6">[105]</td></tr><tr><td colspan="4">15</td></tr><tr><td colspan="4">ZnO-TiO₂-Nd₂O₃/ZrO₂</td></tr><tr><td colspan="4">15</td></tr><tr><td colspan="4">ZnO-SiO₂-Yb₂O₃/ZrO₂</td></tr><tr><td colspan="4">15</td></tr><tr><td colspan="4">ZnO-Yb₂O₃/ZrO₂</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>N/A</td><td>Esterification with glycerol</td><td colspan="4">Zr-modified hierarchical mordenite</td><td>Incipient wetness impregnation<br/>Calcination at 450 °C</td><td>100 °C</td><td>3 h</td><td>10:1</td><td>0.1 g</td><td>Glycerol C = 90.6%</td><td>3</td><td>[46]</td></tr><tr><td>Brown grease</td><td>Esterification with methanol</td><td colspan="4">ZrO₂</td><td>Impregnation</td><td>200 °C</td><td>2 h</td><td>5.2:1</td><td>0.8 g</td><td>C = 78%</td><td>N/A</td><td>[106]</td></tr><tr><td>Palmitic acid</td><td>Esterification with methanol</td><td colspan="4">ZrO₂-TiO₂</td><td>Wet chemical deposition<br/>precipitation method<br/>Ultrasound</td><td>100 °C</td><td>5 h</td><td>20:1</td><td>5</td><td>Y > 85%</td><td>5</td><td>[107]</td></tr><tr><td>Palm oil</td><td>Transesterification with methanol</td><td colspan="4">TiO₂-ZnO</td><td>Calcination at 400 °C for 3 h<br/>Calcination at 400 °C for 3 h</td><td>60 °C</td><td>5 h</td><td>6:1</td><td>200 mg</td><td>C = 98%<br/>Y = 92%</td><td>N/A</td><td>[108]</td></tr><tr><td>Waste soybean oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Titania-silica (S_TSC)</td><td>Impregnation<br/>Calcination at 450-800 °C for 2 h</td><td>120 °C</td><td>3 h</td><td>20:1</td><td>10</td><td>C = 93.8%</td><td>3</td><td>[109]</td></tr><tr><td>Low-grade palm oil</td><td>Esterification with methanol</td><td colspan="4">Titanium oxysulphate sulphuric acid<br/>5-sulfosalicylic acid dihydrate</td><td>N/A</td><td>60 °C</td><td>1 h</td><td>8:1</td><td>1.5</td><td>Y = 90%</td><td>N/A</td><td>[47]</td></tr><tr><td>Jatropha curcas, crude oil</td><td>Esterification with methanol</td><td colspan="4">Mesoporous Ti-Mo Bi metal oxide</td><td>Calcination at 600 °C for 5 h</td><td>180 °C</td><td>2 h</td><td>20:1</td><td>3</td><td>Y = 95%<br/>C = 87.8%</td><td>5</td><td>[110]</td></tr></tbody></table>
<table><caption>Table 1. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15" style="background-color: #cccccc; font-weight: bold;">Sulphated Waste</td></tr><tr><td>Waste cooking oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Sulfonating pyrolyzed rice husk with concentrated sulfuric acid (RHC)</td><td>N/A</td><td>110 °C</td><td>15 h</td><td>20:1</td><td>5</td><td>Y = 87.57%<br>C = 98.17%</td><td>N/A</td><td>[53]</td></tr><tr><td>Malaysian palm fatty acid distillate</td><td>Esterification and transesterification with methanol</td><td colspan="4">Rice husk bioderived silica-supported Cu₂S-FeS</td><td>Co-precipitation<br>Calcination at 600 °C for 6 h</td><td>70 °C</td><td>3 h</td><td>15:1</td><td>2</td><td>C = 98%</td><td>5</td><td>[76]</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">Sulphonated multi-walled carbon nanotubes (s-MWCNTs)</td><td>Purification<br>Sulfonation</td><td>170 °C</td><td>2 h</td><td>20:1</td><td>3</td><td>Y = 93.5%</td><td>5</td><td>[111]</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">Sulphonated sago pith waste</td><td>Carbonization<br>Sulfonation</td><td>70 °C</td><td>90 min</td><td>20:1</td><td>2.5</td><td>Y = 99.34%<br>C = 94.03%</td><td>N/A</td><td>[54]</td></tr><tr><td rowspan="4">Palmitic oil</td><td rowspan="4">Esterification with methanol</td><td colspan="4">Sulfate (SIL-1)</td><td rowspan="4">Impregnation</td><td rowspan="4">65 °C</td><td rowspan="4">8 h</td><td rowspan="4">9:1</td><td rowspan="4">15</td><td rowspan="4">Y &gt; 98%</td><td rowspan="4">5</td><td rowspan="4">[112]</td></tr><tr><td colspan="4">215.6 0.384 6.89 -</td></tr><tr><td colspan="4">Trifluoromethanesulfonate (SIL-2)</td></tr><tr><td colspan="4">202 0.365 6.81 -</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td colspan="4">Dihydrogen (SIL-3)</td><td rowspan="2">Sulfonation</td><td rowspan="2">85 °C</td><td rowspan="2">3 h</td><td rowspan="2">7:1</td><td rowspan="2">12</td><td rowspan="2">C = 96%</td><td rowspan="2">4</td><td rowspan="2">[113]</td></tr><tr><td colspan="4">162.5 0.269 6.57 -</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">Sulfonated-activated carbon from bamboo</td><td>Carbonization<br>Sulfonation</td><td>85 °C</td><td>5 h</td><td>5:1</td><td>3 g</td><td>Y = 92%<br>C = 97.4%</td><td>N/A</td><td>[114]</td></tr><tr><td>Castor oil</td><td>Esterification with methanol</td><td colspan="4">Sulfonated beet pulp</td><td>N/A</td><td>50 °C</td><td>1 h</td><td>20:1</td><td>1</td><td>C = 90.83%</td><td>N/A</td><td>[115]</td></tr><tr><td rowspan="2">Palm fatty acid distillate</td><td rowspan="2">Esterification with methanol</td><td colspan="4">Chicken bone</td><td rowspan="2">Microwave irradiation<br>Calcination at 900 °C for 5 h</td><td rowspan="2">70 °C</td><td rowspan="2">3 h</td><td rowspan="2">20:1</td><td rowspan="2">5</td><td>Chicken bone Y = 80.8%<br>C = 98.2%<br>Cow bone<br>Y = 81.5%<br>C = 97.7%</td><td rowspan="2">N/A</td><td rowspan="2">[49]</td></tr><tr><td colspan="4">0.3436 0.00374 5.6928 3.4 acidity<br>0.5635 0.005 5.7253 -</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">Sulfonated glucose</td><td>Carbonization<br>Sulfonation</td><td>75 °C</td><td>2 h</td><td>10:1</td><td>2.5</td><td>Y = 91.41%<br>C = 92%</td><td>6</td><td>[116]</td></tr><tr><td>Palm fatty acid distillate</td><td>Esterification with methanol</td><td colspan="4">Sulfonated glucose-derived acid</td><td>Carbonization</td><td>75 °C</td><td>2 h</td><td>10:1</td><td>2.5</td><td>Y = 92.3%<br>C = 95.4%</td><td>6</td><td>[117]</td></tr></tbody></table>
<table><caption>Table 1. *Cont.*</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel (Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15"><strong>Sulphated Waste</strong></td></tr><tr><td>Acidified oil</td><td>Esterification with methanol</td><td colspan="4">Sulfonated Polyethersulfone (SPES)/Polyethersulfone (PES)</td><td>Sulfonation</td><td>65 °C</td><td>5 h</td><td>13.5:1</td><td>-</td><td>C = 97.60%</td><td>5</td><td>[118]</td></tr><tr><td>Tallow fat and canola oil</td><td>Esterification with methanol</td><td colspan="4">Sulfonated polymer waste</td><td>Sulfonation</td><td>75 °C</td><td>2.5 h</td><td>-</td><td>-</td><td>C = 91%<br>Y = 99%</td><td>N/A</td><td>[119]</td></tr><tr><td>Olive pomace oil</td><td>Esterification with methanol</td><td colspan="4">Olive pomace activated carbon</td><td>Sulfonation<br>Pyrolysis<br>Steam activation</td><td>60 °C</td><td>5 h</td><td>9:1</td><td>20%</td><td>Y = 97%</td><td>-</td><td>[48]</td></tr><tr><td>Palm oil</td><td>Esterification with methanol</td><td colspan="4">Spent bleaching clay</td><td>Calcination at 600 °C<br>Sulfonation<br>Reflux method</td><td>100 °C</td><td>4 h</td><td>10:1</td><td>10</td><td>C = 93.18%</td><td>3</td><td>[55]</td></tr><tr><td colspan="15"><strong>Ion Exchange Resin</strong></td></tr><tr><td>Chlorella protothecoides<br>Scenedesmus obliquus<br>microalga</td><td>Esterification with methanol</td><td colspan="4">Amberlyst-15</td><td>Co-precipitation<br>Calcination at 650 °C for 5 h</td><td>120 °C</td><td>1 h</td><td>5:1</td><td>2.5</td><td>C > 90%</td><td>N/A</td><td>[120]</td></tr><tr><td>Acrocomia aculeata<br>(Macaúba) crude oil</td><td>Transesterification with ethanol</td><td colspan="4">Amberlyst-15</td><td>N/A</td><td>130 °C</td><td>-</td><td>9:1</td><td>16</td><td>Y = 89.10%<br>C = 86%</td><td>N/A</td><td>[62]</td></tr><tr><td>Macauba pulp oil</td><td>Transesterification with ethanol</td><td colspan="4">Cation exchange resin: Purolite® CT275</td><td>N/A</td><td>85 °C</td><td>9 h</td><td>8.6:1</td><td>30.4</td><td>C = 93%</td><td>10</td><td>[64]</td></tr><tr><td rowspan="3">Waste frying oil</td><td rowspan="3">Esterification with methanol</td><td colspan="4">Cation exchange resin: NKC-9</td><td rowspan="3">N/A</td><td rowspan="3">65 °C</td><td rowspan="3">3 h</td><td rowspan="3">3:1</td><td rowspan="3">18</td><td rowspan="3">C = 90%</td><td rowspan="3">10</td><td rowspan="3">[121]</td></tr><tr><td colspan="4">Cation exchange resin: D61</td></tr><tr><td colspan="4">Purolite CT275DR</td></tr><tr><td rowspan="3">Rapeseed oil</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">Purolite CT169DR</td><td rowspan="3">N/A</td><td rowspan="3">140 °C</td><td rowspan="3">8 h</td><td rowspan="3">27.7 mol/mol</td><td rowspan="3">10</td><td rowspan="3">Y = 16.5–55%</td><td rowspan="3">N/A</td><td rowspan="3">[122]</td></tr><tr><td colspan="4">Purolite D5081</td></tr><tr><td colspan="4">Putolite® CT-269 ion exchange resin</td></tr><tr><td>N. gaditana<br>microalga oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Putolite® CT-269 ion exchange resin</td><td>N/A</td><td>95 °C</td><td>-</td><td>33:1</td><td>-</td><td>C = 81.6%</td><td>N/A</td><td>[123]</td></tr><tr><td>Used cooking oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Purolite D5081</td><td>N/A</td><td>56 °C</td><td>-</td><td>18:1</td><td>9</td><td>C = 92%</td><td>N/A</td><td>[124]</td></tr></tbody></table>
<table><caption>Table 1. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel<br>(Y = Yield, C = Conversion)</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Time</th><th>Ratio of Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="15">Ion Exchange Resin</td></tr><tr><td>Waste cooking oil</td><td>Esterification with ethanol</td><td>NKC-9 ion-exchange resin and H-beta zeolite</td><td>-</td><td>-</td><td>-</td><td>-</td><td>N/A</td><td>80 °C</td><td>6 h</td><td>3:1</td><td>15</td><td>C = 98.40%</td><td>5</td><td>[125]</td></tr><tr><td>Oil feedstock from waste fried oil</td><td>Esterification with methanol</td><td>NKC-9 resin</td><td>-</td><td>-</td><td>-</td><td>-</td><td>N/A</td><td>65 °C</td><td>500 h</td><td>2.8:1</td><td>-</td><td>C = 98%</td><td>N/A</td><td>[126]</td></tr><tr><td>Sunflower oil</td><td>Transesterification with methanol</td><td>Solid acid (SAC-13)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>N/A</td><td>200 °C</td><td>2 min</td><td>25:1</td><td>9 g</td><td>C = 88%</td><td>N/A</td><td>[63]</td></tr></tbody></table>
### 3.2.6. Heterogeneous Base Catalysts

In recent years, extensive research has focused on heterogeneous base catalysts for biodiesel production. These catalysts were preferred over acid catalysts in transesterification processes due to their strong basic sites and high activity [9]. However, heterogeneous base catalysts are only suitable for biodiesel feedstocks with low FFA content; otherwise, they may react with FFAs through the saponification reaction. This reduces the amount of biodiesel produced by making the process of separating biodiesel from glycerol laborious, due to their tiny surface area and hence their low resistance to atmospheric carbon dioxide [127]. These catalysts generally require less reaction time and less process temperatures and are more active than acid catalysts [18]. In the following (Table 2), we will present and discuss the most often utilized heterogeneous base catalysts, including alkaline earth metal oxides, alkali metal salts, and mixed metal oxides catalysts.

### 3.2.7. Alkaline Earth Metal Oxides

It is commonly recognized that alkaline earth metal oxides (AMO), including calcium oxide (CaO), magnesium oxide (MgO), strontium oxide (SrO), and barium oxide (BaO), are basic solid catalysts. These catalysts are readily available, affordable, non-corrosive, and recyclable. Following a thermal pre-treatment, AMOs become active for transesterification, with their alkalinities determining the main catalytic activity. However, high FFA content and water adsorption at the surface sites may poison basic sites, deactivating these catalysts. As a result, the side reactions associated with saponification cause the activity to diminish with each reuse. Various investigations have established the following sequence for the basic strength of alkaline earth oxides, which has been linked to the catalytic activity: BaO > SrO > CaO > MgO [128,129]. CaO-based catalysts have been identified as one of the most often utilized heterogeneous catalysts in the manufacture of biodiesel among alkaline earth metals due to its affordable cost, poor solubility in methanol, low toxicity, and abundant supply from natural resources [130]. Biodiesel production and quality are affected by the chemical characteristics of CaO, such as the calcium content, catalyst activation, basicity strength, thermal stability, reusability, and catalyst leaching, as well as the physical characteristics, such as the catalyst's surface area, particle size and shape, surface morphology, pore radius and volume, and mechanical strength [131]. On the other hand, the catalyst may become inactive when exposed to air, as the CaO surface's basic sites may become poisoned. However, calcination at high temperatures in a furnace can restore its activity. Calcium glyceroxide could occur during transesterification processes, along with adverse reactions like saponification, and it may be difficult to fully separate CaO due to its solubility in a methanol-glycerol mixture [132].

CaO heterogeneous catalyst can be produced from commercial sources through the calcination (600–1000 °C) of minerals or different types of wastes, such as natural bio-waste materials like eggshell, snail shell, ashes, bones, etc., and it can be used for the production of biodiesel in its various forms, including pure CaO, mixed oxides CaO, natural CaO origins, and supported and loaded CaO catalysts [133–135].

The production of biodiesel from transesterification and/or esterification reactions with methanol using pure CaO as a heterogeneous catalyst can be performed using various types of oils, such as soybean oil, sunflower oil, palm oil, *Zanthoxylum bungeanum* seed oil (ZSO), silk cotton seed oil, beef tallow, animal fats, microalgae lipids, and Jatropha curcas oil. Biodiesel production is accomplished by employing pure CaO catalyst that comes from commercial dolomite or lime sources. Thermal decomposition, or calcination, is applied to the catalyst as a pre-treatment to break down the material and increase the number of active sites at particular temperatures, which influences the production of methyl ester [136,137].
When combined with an artificial neural network (ANN) and a genetic algorithm, the microwave-assisted transesterification process (MATP) using CaO catalyst proved to be more energy-efficient (23% less energy) than the conventional transesterification process. It also resulted in accelerated reaction rates, reduced particle size, increased surface area, and increased catalyst activity at optimized conditions of 8:1 methanol-to-oil ratio, 0.3 wt% catalyst, and a reaction time of 1.9 min. Using this technology the biodiesel yield reached 97.4%, and the catalyst was successfully reused for ten consecutive runs [138].

Olubunmi et al. [139] investigated the optimization and kinetic analysis of biodiesel production using a CaO solid catalyst with beef tallow as the feedstock. They conducted a two-step transesterification process, where the oil sample was pre-treated through esterification with sulfuric acid. Under optimal conditions, the synthesized catalyst achieved a maximum FAME conversion of 72%. These conditions included 7.1% CaO, a methanol-to-oil ratio of 9:1, a reaction temperature of 60 °C, and a reaction time of 96 min. The acid-catalyzed esterification of beef tallow led to an optimal reduction in FFA to 0.6.

CaO in the form of dolomite beads could be used for the transesterification reaction and showed excellent activity and ability to maintain strength and shape throughout the reaction. The surface area and pore volume of the catalyst decreased as the inorganic binder (pseudo boehmite sol) amount increased. A study performed by Woo et al. [140] showed that the dolomite catalyst prepared with 20 wt% of inorganic binder resulted in a high biodiesel yield of 92% when a reaction temperature of 65 °C for 2 h of reaction time was used.

It is worth mentioning that according to the stated studies using pure CaO catalysts, high biodiesel yields are achieved at relatively low temperatures ranging between 60 and 80 °C. The reaction mechanism of CaO-catalyzed transesterification has been documented in several studies presented in Figure 2.

Two reaction stages compose the mechanism of the reaction in the oil transesterification reaction with CaO catalyst: first, the adsorption of the reactants on the catalyst's surface, followed by a reaction between the reactants. To guarantee an appropriate adsorption response and an easy diffusion process, a heterogeneous catalyst should possess important characteristics like a large specific surface area and organized distribution of many pores. The second stage consists of the product desorbing from the catalyst's surface [20,141].

As previously mentioned, CaO is an alkaline earth oxide. The oxygen anion that is present on the surface of CaO is the one that is responsible for the catalytic ability in the transesterification reaction [142]. Strong Lewis basicity is produced by connected oxygen anions on the surface of CaO, and this has a significant impact on the catalytic activity during the transesterification reaction. The creation of basic sites on the oxide phase of the CaO surface is crucial for the removal of protons from organic compounds, which sets off a basic catalytic process. This leads to the formation of the methoxide anion (CH₃O-) with greater catalytic activity through the retraction of the proton from methanol (step (1), Figure 2), which is the initial stage of the transesterification process caused by the presence of basic active sites in the CaO catalyst [143]. This process, referred to as oil methanolysis, creates methanol adsorptive sites where O-H bonds easily break to produce hydrogen cations and methoxide anions [144]. Next, in the following steps, triglyceride molecules are converted into diglycerides, monoglycerides, fatty acid methyl esters (FAME), and glycerol [20]. In step (2), a tetrahedral intermediate forms when the methoxide anion attacks the carbonyl carbon of the triglyceride molecule. After the intermediate rearranges itself, one FAME and one diglyceride anion are produced, as shown in step (3) in Figure 2 [145]. In step (4), the proton from the methoxide cation attracts the diglyceride anion, resulting in the formation of diglyceride and the regeneration of the CaO catalyst's active site. This process repeats until CH₃O- attacks the remaining two carbonyl carbons, producing
glycerol as a byproduct. It is important to note that the reaction involving monoglycerides, which ultimately produces biodiesel and glycerol, is considerably slower than the earlier stages of the transesterification process involving triglycerides and diglycerides with methanol [132,143].

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Mechanism of pure CaO catalyst in transesterification: (1) formation of methoxide anion; (2) methoxide anion attacks the triglyceride, leading to the formation of alkoxy carbonyl (tetrahedral intermediate); (3) tetrahedral rearranged into a more stable form (adapted from [145]).</figcaption></figure>

In order to commercialize CaO as a heterogeneous base catalyst, the dissolution of active species in glycerol-methanol and biodiesel-glycerol-methanol mixtures must be minimized. In addition, the moisture and FFA content in oils should be considered critical parameters in the reaction, as CaO is very sensitive to the purity of the oil [146]. Based on the following studies, mixed CaO showed improved catalytic properties when compared to pure CaO, and it is one of the most commonly used solid heterogeneous catalysts. In the mixture, the number of basic sites is higher, and the stability of the catalyst is superior to the pure CaO.

Authors have reported the use of CaO supported on alumina, metal oxides (SnO, MgO), and mesoporous silicas, such as SBA-15, zeolites, graphite oxides, and activated carbon. These studies generally demonstrated that supported CaO showed better catalytic activity than unsupported material. This is explained by the positive impact of the support's chemical and textural characteristics on the dispersion of CaO nanoparticles [147].

CaO can be mixed with metal oxides derived from an alumina carrier. Meng et al. [148] studied the effect of the calcination temperature of the Ca/Al composite oxide, which ranged from 120 °C to 1000 °C, on the transesterification reaction of rapeseed oil with
methanol. When the calcination temperature is 600 °C, the surface area and the porosity of the catalyst decreased, yielding 94% of FAME. Combining CaO with hydrotalcite-like materials (layered double hydroxides (LDH)) is also possible. When soybean oil was transesterified with methanol using Ca/Al layered double oxide (LDO) and CaFeAl/LDO catalysts, the latter showed the greatest catalytic activity and stability, generating a 90% yield of FAME. Additionally, because this catalyst is insoluble in methanol, it could be separated easily and reused up to eight times [149]. CaO can be mixed with a single metal oxide, such as tin oxide (SnO₂) or zinc oxide (ZnO). CaO/SnO₂ catalyst has a gel-like structure and demonstrates high catalytic activity, alternating between acid and base sites and offering high yields (89.5%) with decreased sensitivity to FFAs [150].

Furthermore, CaO can be mixed with earth metal oxides, such as MgO. Korbag and Korbag [151] synthesized an alkaline earth metal oxide (CaO/MgO) catalyst, which yielded an optimum biodiesel rate of 99% at a reaction temperature of 60 °C, exhibiting strong catalytic activity, too. Maleki et al. [152] synthesized CaO-La₂O₃ mixed oxides from the lanthanide group through co-precipitation coupled with lithium impregnation. Optimal biodiesel conversion of 96.3% was obtained using 1wt% of Li/Ca-La. Calcination performed at 70 °C for 2 h showed high basic strength with high catalytic activity and reuse for up to four cycles. Finally, CaO can be combined with metal nickel oxide (NiO) and niobium oxide (NdO). The use of CaO-NiO and CaO-Nd₂O₃ in the transesterification reaction of Jatropha oil showed high catalytic activity, yielding 86.3% and 82.2% for CaO-NiO and CaO-Nd₂O₃, respectively. The catalyst was easily separated from the product and recycled for six runs [146].

Given their high Ca content, natural materials, such as ashes from pineapple [153], rice husk [154], and Tectona grandis leaves [155], eggshells [122], animal bones [156], golden apple snails [157], and crab shells [158], are thought to be a novel and promising source of CaO. These materials showed excellent stability during the transesterification process; moreover, they are biodegradable, non-toxic, renewable, and safe to use and store [159]. These renewable sources are readily available and low-cost and show great potential in biodiesel production [133]. The calcination process is essential for regenerating the catalyst and facilitating the morphological changes and conversion of CaCO₃ to CaO, which in turn enhances the basic strength, surface area, and catalytic activity. The calcination temperatures used to activate the catalyst and increase the FAME yield to more than 90% were quite similar in all studies. These temperatures ranged from 700 °C [155,160] to 800 °C [154,156,157,161–164]) and 900 °C [101,165–170]).

De Barros et al. [153] calcinated the catalyst from pineapple leaves first at a temperature of 600 °C for 2 h, followed by a temperature of 900 °C for 1 h to prevent CO₂ chemisorption from producing small losses. The components found in pineapple leaves showed a modest concentration of soluble alkaline (0.39 mmol/g) and an ash content of 6%. The catalyst's activity decreased at the fourth run after being steady for three consecutive runs because of the impregnation of organic compounds into the pores (conversion equal to 98.92% after the first cycle, 97.6% after the second cycle, and 94.3% after the third cycle).

Tectona grandis leaves were utilized by Gohain et al. [155] as catalysts for the transesterification of WCO. The calcination process thermally activates the metal carbonate salt, which subsequently activates the metal oxide, prevents potassium leaching, and enhances the catalyst's reusability for up to five cycles. The leaves were calcined at 700 °C, resulting in a biodiesel yield of 95%. Likewise, Putra et al. [160] developed an affordable heterogeneous catalyst, CaO/SiO₂, from peat clay and eggshell waste. The authors demonstrated that the calcination of clay at 700 °C increased the surface area of the catalyst and the concentration of active sites and enhanced the biodiesel yield from WCO to up to 91%.
However, calcination temperatures above 1000 °C could deteriorate the catalytic activity and reduce the surface area.

Ngaini et al. [153] showed that the surface area of the rice husk catalyst could be improved through recalcination at a temperature of 800 °C for 16 h. The catalyst successfully achieved a high yield of 97% FAME from palm oil and remained effective for up to five reuse cycles. De Oliveira et al. [163], investigated the calcination of eggshells at 800 °C for 24 h and demonstrated a sharp increase in pore size (from 3.16 to 4.15 nm) and pore volume due to CO₂ removal, leading to a change in the catalyst’s structure. The transesterification of frying residual oil using these catalysts leads to a FAME yield of 97.7%.

Gollakota et al. [168], Proença et al. [170], Rahman et al. [169], and Wei et al. [165] reported that high calcination temperatures of 900 °C will cause the catalyst to have an undefined, irregular morphology due to the aggregation of Ca particles on the external surface of the catalyst, causing a blockage and leading to a non-porous structure. Kirubakaran and Arul Mozhi Selvan [167], Gollakota et al. [168], Boey et al. [101], Rahman et al. [169], and Birla et al. [166], respectively, recommended the use of the following calcination temperatures for the complete conversion of CaCO₃ to CaO: 900 °C for eggshells, >700 °C for crab shells, and 880 °C for snail shells. All of the listed studies demonstrated a FAME yield higher than 90%, except for Kirubakaran and Arul Mozhi Selvan [167], who indicated a yield of 85% and maintained activity for five successive runs. Crab shells were highly reusable up to 11 times with high conversion rates [101], and eggshells used for biodiesel production from vegetable oil were recycled 13 times with no significant loss [165].

Moreover, it is important to note that all of the listed studies adopted an optimum transesterification reaction temperature between 60 °C and 65 °C for at least 2 h for a biodiesel yield approximately higher than 90%, with the exception of De Barros et al. [153], who achieved a 98.92% biodiesel conversion rate at 60 °C in just 30 min. Because calcined pineapple leaves displayed great catalytic activity, a short response time, which ranged from 30 to 180 min during the study, was sufficient to obtain high conversion rates.

Loaded and supported CaO catalysts are an important group of heterogeneous catalysts used in the transesterification of triglycerides to produce biodiesel. Pure CaO typically has a low surface area, so increasing this surface area is essential. As a result, researchers have explored integrating a catalyst support onto the surface of CaO to enhance its surface area and improve its catalytic performance. The chemical and textural properties of the carrier positively influence the dispersion of CaO particles. In this arrangement, the carrier stabilizes the active species on its surface, with CaO acting as the catalytically active component. The catalytic activity of CaO is mainly driven by several oxygen anions with low coordination numbers. By modifying the coordination of Ca²⁺ and O²⁻ on the catalyst’s surface, the number and strength of basic sites are altered, which in turn affects the catalytic performance [142]. Additionally, using a support or carrier improves the stabilizing properties of the CaO catalyst [142] and reduces leaching [39].

Different approaches were used to develop supported and loaded CaO catalysts, such as precipitation, co-precipitation, impregnation, and wet impregnation followed by a calcination process, to activate the catalyst. Usually, the calcination temperature has an impact on the quantity of surface oxygen. CaO-loaded catalysts, typically formed in the upper particles, aid in separating the catalyst from the reaction mixture [171].

A CaO-loaded microcapsule catalyst was synthesized using a co-extrusion method by encapsulating CaO particles with butanol-modified alginate as the shell material. These microcapsules were employed in the transesterification of rapeseed oil under optimal conditions, including a methanol to oil ratio of 8:1 and 20 wt% of CaO microcapsules at a reaction temperature of 65 °C for 4 h. The biodiesel yields obtained from the first, second, third, and fourth cycles were 95.5%, 97.7%, 94.5%, and 80.9%, respectively. It is important
to note that Ca²⁺ leaching into the reaction mixture and the FAME phase were decreased through the encapsulation of CaO powder [172].

Regarding CaO supported on alumina, Elias et al. [173] used this type of catalyst to produce biodiesel from both waste palm oil (WPO) and waste sunflower oil (WSO), resulting in a biodiesel yield of 89% for WPO and 98% for WCO. The catalyst was calcined at 750 °C for 6 h. The synthesis of the catalyst via the co-precipitation method using NaOH increased the surface area of CaO/Al₂O₃, leading to an increase in FAME conversion. In addition, the presence of Al₂O₃ enhanced the catalytic activity and demonstrated that a prolonged transesterification reaction would decrease biodiesel production.

The application of zinc-doped calcium oxide (Zn/CaO) catalyst in waste cotton seed oil transesterification was effectively investigated by Kataria et al. [174]. In their work, the wet impregnation approach was used to generate the nanocrystalline catalyst, which was then calcined at 700 °C for 4 h. WCO was used as a feedstock to perform the transesterification with the following conditions: a methanol to oil ratio of 12:1, 5 wt% of zinc-doped calcium oxide, a reaction temperature ranging between 60 and 65 °C, and a reaction time of 1.4 h. Methanol was used in excess not only to accelerate the transesterification process but also to help clear product molecules from the catalyst's surface, regenerating the active sites.

Zeolites have been widely investigated as a carrier in biodiesel production due to their characteristic shape selectivity and acidic sites [175]. A novel catalyst based on CaO obtained from leftover chicken eggshells supported on pure cracninite zeolitic material was synthesized by Pavlović et al. [176]. The catalyst was synthesized through ultrasound-assisted impregnation and then calcined at a temperature range of 450 to 600 °C for 4 h. The generated catalyst had evenly distributed CaO active sites and maintained its original zeolite structure. The ultrasound technique shortened the synthesis time and energy, and the synthesized catalyst demonstrated notable characteristics, including uniform, consistently sized, and distributed pores, high basic strength, and well-dispersed active species. When sunflower oil was esterified at 60 °C for two hours with 4 wt% catalyst and a methanol to oil molar ratio of 12:1, 96.5% of FAME content was achieved.

MgO is a commonly utilized, economical, and non-toxic material [177]. Given its low cost and multifunctional qualities, MgO is an important material that has been utilized extensively in many different sectors and has outstanding catalytic activity [178]. MgO catalyst has the lowest basic strength compared to the other earth metal oxides (8.2 ≥ H ≥ 6.8) [129]. Using microwave (MW) irradiation, Abdel Salam et al. [179] produced a metal-organic framework, Mg-MOF: Mg₃(BDC)₃(H₂O)₂. They subsequently utilized the same microwave to produce biodiesel from oleic acid. High conversion of 97% was achieved with the following reaction parameters: a methanol to oil molar ratio of 15:1, catalyst loading of 0.15 wt%, MW power of 150 W, and a reaction duration of 8 min. Ultrasonic waves were used to prepare the catalyst, and the trapped solvent's molecules were desorbed at 220 °C under the vacuum conditions. Due to the hygroscopic nature of the Mg-MOF, water molecules entirely replaced methanol molecules throughout the heating process at 220 °C, followed by exposure to ambient air. The alkali-free, calcined MgAl hydrotalcite (MgAl HT) catalyst was created by Tajuddin [180] and employed in the transesterification of WCO into biodiesel. The synthesized catalyst produced the highest percentage yield of biodiesel of 87.23% when 5 wt% of the catalyst was used with a MgAl ratio of 3:1.

SrO and BaO both have higher basic strength, as mentioned earlier (17.2 ≥ H ≥ 15), compared to other AMOs; therefore, they have greater catalytic activity [129].

Banerjee et al. [181] examined the transesterification of WCO with methanol using Sr-Ce-based mixed metal oxides. Catalyst synthesis was carried out through gel combustion route. A biodiesel yield of 99.5% was achieved using the ideal circumstances for this
reaction, which are as follows: 2 wt% of catalyst, a 1:14 oil to methanol ratio, 120 min reaction duration, 65 °C reaction temperature, and 700 rpm stirring speed. Reusability research was also carried out, and the results showed that the catalyst could be readily renewed for four runs.

Using a new acid-base bifunctional catalyst (SrO-ZnO/Al₂O₃), Al-Saadi et al. [182] studied the influence of the metal oxide's composition on the transesterification reaction of corn oil and the esterification reaction of oleic acid. Under optimal reaction conditions of a 5:1 ethanol to maize oil molar ratio, 70 °C reaction temperature, and 6 h of reaction time, the conversion for esterification reaction achieved was 71.4%. The transesterification reaction produced a high yield of biodiesel, reaching 95.1% at 70 °C after three hours of reaction time, with a maize oil to ethanol ratio of 1:10. In both situations, catalyst loading of 10 wt% was employed. The catalyst showed acid-base sites; however, because Sr and Zn leached from the catalytic surface, the recovered catalyst only produced 19% biodiesel.

SrO, when combined with TiO₂, serves as a widely used mixed oxide heterogeneous catalyst for biodiesel production. A 98% FAME yield was achieved from the transesterification of waste cooking oil using Sr-Ti mixed metal oxides with a 4:1 atomic ratio (providing maximum basic sites), 1 wt% catalyst loading, and a methanol to oil molar ratio of 11:1 within 80 min of reaction time at a temperature of 65 °C. Using Sr-Ti mixed metal oxide catalysts, 83% FAME conversion was attained after eight cycles [183]. In a similar study, Li et al. [184] reported that under optimal reaction conditions of 170 °C with a methanol to oil ratio of 15:1 and a reaction time of 3 h, the use of a mesoporous SrTiO₃ catalyst in the transesterification of palm oil resulted in a 93.14% FAME yield. In studies involving BaO, the response surface methodology, based on a central composite design, was used to optimize the transesterification of corn oil. A methanol to oil molar ratio of 11.32:1 and a catalyst amount of 3.6 wt% were found to be statistically significant at the 95% confidence level for the transesterification of corn oil with methanol using Ba(OH)₂ and dimethyl ether as a co-solvent. The reaction time, however, had no impact on the conversion. Under optimal conditions, a biodiesel yield of 99.15% was achieved in two hours using diethyl ether and Ba(OH)₂ as the catalyst [185]. Using ultrasound-assisted transesterification, Saha and Goud [186] examined the transesterification of Karanja oil utilizing Ba(OH)₂·8H₂O as a heterogeneous base catalyst. They also examined the impact of the catalyst's concentration, reaction duration, reaction temperature, and reactant molar ratio on the biodiesel's production. According to reports, the following ideal operating parameters can lead to a maximum conversion of 84%: a 9:1 methanol to oil molar ratio, 0.5 wt% of catalyst loading, and 90 min of reaction time. The study also showed that the catalyst is capable of catalyzing the transesterification process at a low temperature. Singh et al. [2] reported that lipids from the Anabaena Pasteur Culture Collection of Cyanobacteria (PCC 7120) microalgae could be extracted and converted into biodiesel using Ba impregnated on TiO₄ as a catalyst. The extraction of Anabaena oil from microalgae was performed using a solvothermal microwave technique. Barium impregnated on TiO₄ catalyst was used to facilitate the transesterification of methanol, yielding 98.41% of biodiesel. FAME conversion achieved through successive use of the Ba₂TiO₄ catalyst reached 81.07% after six cycles.

A variety of metal-based oxides, including alkali metals, have been utilized as catalysts in the transesterification of oils. Alkaline metal salts, such as potassium, sodium, and lithium, are basic solid catalysts that can be mixed or supported on materials like Al₂O₃, SiO₂, La₂O₃, SnO₂, CeO₂, and TiO₂. Additionally, solid catalysts derived from biomass, such as diatomic earth, animal bones, seashells, and eggshells, have been proposed as more environmentally friendly alternatives. This broad range of catalysts allows for the selection of the most appropriate system based on factors like the type and quality of the oil, its availability, and the economic conditions of the biorefinery or rural plant [129].
### 3.2.8. Alkali Metal Salts

Potassium is a common heterogeneous catalyst used in the manufacturing of biodiesel. The catalyst is less expensive and therefore utilized in the manufacturing of biodiesel. During the transesterification process of vegetable oils, the activity of heterogeneous potassium catalysts is largely dependent on the fundamental strength of their specific surface area and pore volume. Nevertheless, a number of researchers have noted serious issues with potassium leaking from the catalysts during transesterification processes [19,187].

Niju et al. [188] used the pseudo stem of bananas, Poovan banana pseudostem (PBPS), to transesterify Madhuca indica oil with methanol. Using a methanol to oil ratio of 14.9, a catalyst percentage of 5.9 wt%, and a process duration of 2.96 h, the biodiesel conversion rate achieved was 98.8%. According to the study, PBPS calcination produced a novel catalyst that may be utilized as a solid catalyst for biodiesel generation that is both extremely effective and sustainable.

A solid base catalyst consisting of 15 wt% potassium iodide (KI) placed on mesoporous silica was employed by Samart et al. [189] for the transesterification of soybean oil. The optimal reaction conditions included a methanol to oil ratio of 16:1, 5 wt% catalyst, a reaction temperature of 70 °C, and a reaction time of 8 h, resulting in 90% conversion. When KI solution was impregnated on mesoporous silica at a concentration of 15 wt% using an initial wetness impregnation method, the maximal activity of the catalyst was achieved.

With a biodiesel output of 97.8%, Naeem et al. [190] recently reported using KOH/corkcob-derived activated carbon bifunctional catalyst for biodiesel synthesis. They succeeded in raising the specific surface area (SSA) of corn cobs from 211 m²g⁻¹ to 877 m²g⁻¹ by chemically treating them with H₂SO₄ at an acid to biomass ratio of 5:1 followed by neutralization and pyrolysis at 600 °C for 4 h. Even though a high methanol to oil ratio of 18:1 was needed, the catalyst enabled the simultaneous esterification and transesterification of biodiesel from WCO at an impressively low temperature of 45 °C. Upon reuse, the total conversion dropped significantly due to the leaching of KOH; however, it maintained good stability and conversion after five cycles towards the esterification of FFA.

In order to determine the ideal reaction temperature for the transesterification of castor oil to biodiesel, Roy et al. [191] synthesized a catalyst named potassium (K) promoted lanthanum oxide (La₂O₃). They adjusted the temperature between 45 and 75 °C using 1 wt% of catalyst loading, a stirrer speed of 400 rpm, and a methanol to oil ratio of 16:1 over a duration of three hours. As the reaction temperature increased from 45 to 65 °C, the authors showed that FAME conversion increased from 55 to 84%. However, beyond 65 °C, the conversion efficiency declined. Because 65 °C is near methanol's boiling point of 64.7 °C, it represents the threshold temperature for achieving high conversion efficiency in the reaction.

Goudarzi and Izadbakhsh [192] achieved 97% biodiesel yield using K/SnO₂ as catalyst for the transesterification of a mix of palm, sunflower, and soybean oil. Solid Sn(OH)₄ was used as catalyst support, and the catalyst was prepared via impregnation using different molar ratios of potassium to tin (1–4). The transesterification reaction was conducted at 65 °C with a methanol to oil molar ratio of 12:1, 3 wt% catalyst, and a reaction time of 1.5 h using a K:Sn molar ratio of 2:1. When compared to homogeneous KOH and heterogeneous K/γ-Al₂O₃, K/SnO₂ catalyst was found to be superior in terms of leaching stability and requiring less washing during the methyl ester phase.

Concerning lithium catalysts, Anastopoulos et al. [193] employed LiNO₃/CaO as a heterogeneous catalyst for the transesterification of sunflower oil and used frying oil, achieving yields of 97.8% for sunflower oil and 96.7% for used frying oil. This was accomplished under a methanol to oil molar ratio of 12:1, a reaction time of 2 h, a catalyst amount
of 3.5 wt%, and an agitation speed of 600 rpm. Oils with a low FFA and water content can be converted using the catalyst 3.5 wt% of LiNO₃/CaO at least three times.

Sodium in heterogeneous catalysts can be used in different forms, including Na-based geopolymers, Na-alginate, and Na-bentonite, supported and impregnated forms of sodium on different carriers.

Botti et al. [194] assessed the catalytic activity of potassium- and sodium-based
geopolymers for the transesterification of soybean oil and methanol. The catalysts were
heat-treated in a broad range of temperatures (110 to 700 °C). The sodium-based formula-
tion showed smaller SSA and higher mean pore sizes (6.34–32.62 m²/g; 17 nm) than the
potassium-based formulation (28.64–62.54 m²/g; 9 nm). Sodium formulations displayed
a greater degree of shrinkage following heat treatment, resulting in a decrease in surface
area. However, during the transesterification of soybean oil, Na-based geopolymers out-
performed K-based geopolymers and resulted in FAME contents of 85.1 and 89.9% for
samples treated at 500 and 300 °C, respectively. The results were mainly attributed to
Na-geopolymers’ larger average pore size, which allowed for a more efficient mass transfer
rate between reactants and catalytic sites.

According to recently published work, a natural bifunctional catalyst was synthesized from alginate to catalyze the simultaneous esterification and transesterification of waste cooking oil [195]. The catalyst was reused up to 25 times and under the following conditions: a methanol-to-oil ratio of 25:1, 3 wt% of catalyst loading, and 10 min of sonication at 750 W. There was no discernible decrease in the regenerated catalytic activity of the catalyst. The authors demonstrated that conversion increased along with reaction time, but they also demonstrated that prolonged usage of high-power sonication might have a negative impact on conversion. The optimal parameters of the catalyst synthesis leading to a maximum yield conversion of 97% were as follows: a crosslinking time of 0.5 h, a CaCl₂ concentration of 1% *w/v*, a sodium alginate to κ-carrageenan mass ratio of 1.5:1, and a sodium bentonite to κ-carrageenan mass ratio of 4:1. NaNO₃ impregnated on SiAl was utilized for the transesterification of rapeseed oil with methanol. FAME yield exceeded 99% when using the following conditions: 5 wt% of catalyst, an agitation speed of 700 rpm, a methanol/oil ratio of 9:1, and a reaction temperature of 65 °C while using an impregnation degree of NaNO₃/SiAl equal to 20/1, where the number 20 is expressed in mmol Na·g⁻¹ [196]

In 2021, Naeem et al. [197] demonstrated how several factors affect the amount of biodiesel that is ultimately produced from waste frying sunflower oil using Na-SiO₂@TiO₂ catalyst. The catalyst was identified using several techniques, such as SEM, FTIR, XPS, and BET, which revealed a high surface area of 107.26 m²/g. Na hydroxide impregnation was performed to increase the surface area of the catalyst. The following ideal working settings were used: a reaction temperature of 65 °C, a molar ratio of methanol/oil equal to 25:1, a catalyst concentration of 2.5 wt%, and contact time of 2 h. These conditions produced an excellent FAME yield of 98%. In addition to this, the authors observed that after recycling the catalyst five times, the yield of biodiesel synthesis was more than 80%.

3.2.9. Mixed Metal Oxides

Jamil et al. [198] studied metal-organic framework (MOF) catalysts based on calcium and copper for esterification. In that study, Cu-MOF served as an acid catalyst, while Ca-MOF was used as an alkali catalyst. Both Cu-MOF and Ca-MOF were synthesized through hydrothermal and solvothermal techniques. When used individually, Cu-MOF and Ca-MOF achieved yields of 78.3% and 78%, respectively. However, when combined, they produced a yield of 85%. The catalysts were recovered and reused three times without significant loss in activity. CoFe₂O₄@graphene oxide (GO) and CoFe₂O₄ heterogeneous nanocatalysts were synthesized using the ultrasonic method and applied in biodiesel
production from waste edible oil (WEO) via a two-step transesterification process. The results indicated that CoFe₂O₄@GO had a higher number of basic sites (0.911 mmol/g) compared to CoFe₂O₄ (0.306 mmol/g), which resulted in a higher biodiesel yield of 98.17%, compared to 91.64% for CoFe₂O₄. This was achieved under optimal conditions: a reaction time of 55.75 min, a temperature of 64.75 °C, a methanol-to-oil ratio of 16.5:1, and a catalyst concentration of 5.22 wt%. Additionally, biodiesel conversion of 98.03% was reached in 3 h at 60 °C with a 2.5 wt% catalyst loading and a 9:1 methanol-to-oil molar ratio [199].

Nanocatalytic technology is now seen as a promising solution for addressing several bottleneck issues in biodiesel production, as it offers higher selectivity due to the nano-sized pores on the surface of the catalysts [200]. Zinc oxide (ZnO) nanoparticles, with their hexagonal wurtzite structure, provide better transparency, oxygen vacancy, and stronger affinity for polar substrates. These properties can be enhanced by varying the dopant concentrations [201,202]. Raj et al. [203] demonstrated the production of biodiesel from Nannochloropsis oculata microalgae with a high yield of 87.5% using a Mn/ZnO@PEG (Polyethylene glycol) nanocatalyst. After five reuse cycles, the biodiesel yield decreased slightly to 85.7%, showing good reusability of the nanocatalyst.

In another study, Baskar et al. [200] used a Ni-doped ZnO nanocomposite for biodiesel production from edible castor oil, achieving a high yield of 95.2%. The yield increased from 85% to 95% when the temperature was raised from 40 °C to 50 °C, suggesting an endothermic transesterification mechanism. However, high temperatures negatively impacted biodiesel production by reducing the polarity of methanol during the reaction. Zn-doped CaO nanocatalysts were also used for biodiesel production from Calophyllum inophyllum oil, yielding 91.95% biodiesel under optimal conditions: a methanol-to-oil ratio of 9.66:1, 5% catalyst concentration (w/v), a reaction time of 81.31 min, and a temperature of 56.71 °C. The study highlighted the importance of high biodiesel conversion rates and low feedstock costs and emphasized the reuse of the solvent and the catalyst to reduce operational costs [93].

Singh et al. [204] highlighted the role of zeolite in enhancing the catalytic activity of ZnO/zeolite catalysts and reducing metal ion leaching during biodiesel production from Jatropha oil with high FFA content. The optimal conditions were a temperature of 200 °C, a methanol-to-oil molar ratio of 30:1, a reaction time of 1 h, and 1 wt% catalyst loading, achieving a biodiesel yield of 93.8%. Supporting active sites with zeolite reduced ZnO leaching from 1283.15 ppm to 127.52 ppm, and PbO/zeolite showed a reduction in PbO leaching from 3400 ppm to 9.29 ppm.

Kataria et al. [174] investigated zinc-doped calcium oxide for the transesterification of waste cooking oil (WCO). Using a 12:1 methanol-to-oil ratio, 5 wt% zinc-doped calcium oxide, and reaction conditions of 60–65 °C for 1.4 h, a high biodiesel yield of 98.5% was obtained. The study also explored the use of biodiesel blends in compression ignition engines (CI), assessing engine performance under various loads, injection pressures, and compression ratios.

Kalavathy and Baskar [205] utilized silica-doped ZnO as a heterogeneous nanocatalyst for the transesterification of Ulva lactuca oil. The highest biodiesel yield of 97.43% was achieved under optimal conditions: 8 wt% catalyst, a 9:1 methanol-to-oil ratio, 55 °C, and a reaction time of 50 min. For FAME production from waste frying oil (WFO), a new electrocatalytic system based on ZnAl-layered double hydroxide (LDH)@SiO₂ was studied. The addition of SiO₂ improved the catalyst's basicity and facilitated methoxide formation via nucleophilic attack on the triglyceride carbonyl. This system achieved 98.4% FAME yield after 90 min at 25 °C using 2 wt% ZnAl-LDH@SiO₂ and a 6:1 methanol-to-WFO molar ratio. The catalyst exhibited excellent stability after five cycles and performed effectively even with WFO containing 3 wt% FFA.
<table><caption>**Table 2.** Literature review of biodiesel production processes using basic heterogeneous catalysis.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Alkaline earth metal oxides</td></tr><tr><td>Zanthoxylum bungeanum seed oil (ZSO)</td><td>Transesterification with methanol</td><td>Commercial CaO</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 900 °C for 1.5 h</td><td>65 °C</td><td>2 h 45</td><td>11.69:1</td><td>2.52</td><td>C > 96%</td><td>N/A</td><td>[206]</td></tr><tr><td>Silk cotton seed oil</td><td>Transesterification with methanol</td><td>Commercial CaO</td><td>20.29</td><td>0.029</td><td>3.117</td><td>-</td><td>N/A</td><td>-</td><td>114 s</td><td>18:1</td><td>0.3</td><td>Y = 97.40%</td><td>10</td><td>[138]</td></tr><tr><td>Sunflower oil</td><td>Transesterification with methanol</td><td>Commercial CaO</td><td>4.6</td><td>0.01</td><td>14.6</td><td>-</td><td>Calcination at 500 °C and 900 °C for 1.5–5.5 h</td><td>100 °C</td><td>5.5 h</td><td>6:1</td><td>1 mass%</td><td>C = 91%</td><td>N/A</td><td>[207]</td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td>Commercial CaO</td><td>7.114</td><td>0.0607</td><td>-</td><td>-</td><td>Calcination at 900 °C for 3 h</td><td>62 °C</td><td>-</td><td>10:1</td><td>6</td><td>Y = 89.36%</td><td>N/A</td><td>[208]</td></tr><tr><td>Jatropha oil</td><td>Transesterification with methanol</td><td>Dolomite-bead-PB-20 (CaMg(CO₃)₂)</td><td>57</td><td>0.43</td><td>58</td><td>-</td><td>Calcination at 550 °C</td><td>65 °C</td><td>2 h</td><td>10:1</td><td>1</td><td>N/A</td><td>2</td><td>[140]</td></tr><tr><td>Rapeseed oil</td><td>Transesterification with methanol</td><td>Ca/Al composite oxide-based alkaline</td><td>5.14</td><td>-</td><td>-</td><td>-</td><td>Calcination at 120 °C, 400 °C, 600 °C, 800 °C, and 1000 °C for 8 h</td><td>65 °C</td><td>3 h</td><td>15:1</td><td>6</td><td>Y = 94%</td><td>N/A</td><td>[148]</td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td>CaFeAl/LDO (layered double oxide)</td><td>117</td><td>-</td><td>-</td><td>-</td><td>Co-precipitation T<sub>calcination</sub> = 750 °C for 3 h</td><td>60 °C</td><td>1 h</td><td>12:1</td><td>6</td><td>Y = 90%</td><td>8</td><td>[149]</td></tr><tr><td>Babassu oil (Attalea speciosa)</td><td>Transesterification with methanol</td><td>CaO/SnO₂</td><td>7.80</td><td>0.064</td><td>33</td><td>-</td><td>Calcination at 650 °C for 5 h</td><td>54.1 °C</td><td>2 h</td><td>10:1</td><td>6</td><td>Y = 89.58%</td><td>N/A</td><td>[150]</td></tr><tr><td>Olive, sunflower, corn oil</td><td>Transesterification with methanol</td><td>CaO–MgO</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 600 °C for 2 h</td><td>60 °C</td><td>1–6 h</td><td>6:1</td><td>2</td><td>C = 99%</td><td>N/A</td><td>[151]</td></tr><tr><td>Sunflower oil, soybean oil</td><td>Transesterification with methanol</td><td>Calcium zincate</td><td>76.7</td><td>0.144</td><td>4.3</td><td>-</td><td>Calcination at 800 °C</td><td>60 °C</td><td>45 min</td><td>12:1</td><td>3</td><td>Y > 90%</td><td>3</td><td>[209]</td></tr><tr><td>Canola oil</td><td>Transesterification with methanol</td><td>CaO-La₂O₃ (commercial CaO)</td><td>18.23</td><td>-</td><td>80–120</td><td>-</td><td>Co-precipitation Wet impregnation T<sub>calcination</sub> = 700 °C for 6 h</td><td>65 °C</td><td>2.5 h</td><td>15:1</td><td>5</td><td>Y = 96.30%</td><td>5</td><td>[152]</td></tr><tr><td rowspan="3">Jatropha Curcas oil</td><td rowspan="3">Transesterification with methanol</td><td>CaO-NiO</td><td>7.2</td><td>-</td><td>34.7</td><td>6.32 basicity</td><td rowspan="3">Co-precipitation Calcination T = 900 °C for 6 h</td><td rowspan="3">65 °C</td><td rowspan="3">6 h</td><td rowspan="3">15:1</td><td rowspan="3">5</td><td rowspan="3">C > 80%</td><td rowspan="3">6</td><td rowspan="3">[146]</td></tr><tr><td>CaO-Nd₂O₃</td><td>8</td><td>-</td><td>62.3</td><td>4.09 basicity</td></tr><tr><td>Pineapple leaves ash</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td>Pineapple leaves ash</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 600 °C for 2 h and 900 °C for 1 h</td><td>60 °C</td><td>30 min</td><td>40:1</td><td>4</td><td>C > 98%</td><td>4</td><td>[153]</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="5">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #e0e0e0; font-weight: bold;">Alkaline earth metal oxides</td></tr><tr><td rowspan="6">Palm oil</td><td rowspan="6">Esterification saponification with methanol</td><td colspan="4">Rice husk ash (RHA)</td><td rowspan="6">Calcination at 800 °C for 16 h</td><td rowspan="6">65 °C</td><td rowspan="6">1 h</td><td rowspan="6">5:1</td><td rowspan="6">5</td><td rowspan="6">Y > 97%</td><td rowspan="6">5</td><td rowspan="6">[154]</td></tr><tr><td colspan="4">1.914 0.0005161 - -</td></tr><tr><td colspan="4">Activated RHA (ARHA)</td></tr><tr><td colspan="4">18.947 0.008109 - -</td></tr><tr><td colspan="4">Huskcatbase</td></tr><tr><td colspan="4">14.493 0.004985 - -</td></tr><tr><td colspan="4">Huskcatacid</td></tr><tr><td colspan="4">7.362 0.002726 - -</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Esterification and transesterification with methanol</td><td colspan="4">Tectona grandis leaves ash</td><td rowspan="2">Calcination at 700 °C for 4 h</td><td rowspan="2">Room T°</td><td rowspan="2">3 h</td><td rowspan="2">6:1</td><td rowspan="2">2.5</td><td rowspan="2">C = 100%</td><td rowspan="2">5</td><td rowspan="2">[155]</td></tr><tr><td colspan="4">116.833 0.185 11.221 -</td></tr><tr><td rowspan="3">Crude palm oil</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">CaO/CaCO₃ + bottom ash</td><td rowspan="3">Calcination at 900 °C for 5 h</td><td rowspan="3">60 °C</td><td rowspan="3">3 h</td><td rowspan="3">12:1</td><td rowspan="3">2</td><td rowspan="3">C = 94.48%</td><td rowspan="3">N/A</td><td rowspan="3">[210]</td></tr><tr><td colspan="4">1.133 - 2-50 -</td></tr><tr><td colspan="4">CaO/CaCO₃ + fly ash</td></tr><tr><td colspan="4">1.719 - 2-50 -</td></tr><tr><td rowspan="2">Fish oil</td><td rowspan="2">Esterification with methanol, ethanol, and isopropanol</td><td colspan="4">Calcinated eggshell and copper oxide ([CaCu(OCH₃)₂])</td><td rowspan="2">Calcination at 800 °C for 6 h</td><td rowspan="2">65 °C<br>80 °C<br>85 °C</td><td rowspan="2">1.5 h</td><td rowspan="2">16:1</td><td rowspan="2">3</td><td rowspan="2">Y = 93%</td><td rowspan="2">N/A</td><td rowspan="2">[211]</td></tr><tr><td colspan="4">- - >100 -</td></tr><tr><td rowspan="2">Palm oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Calcined milled animal bone</td><td rowspan="2">Calcination at 800 °C</td><td rowspan="2">65 °C</td><td rowspan="2">4 h</td><td rowspan="2">18:1</td><td rowspan="2">20</td><td rowspan="2">C = 96.78%</td><td rowspan="2">5</td><td rowspan="2">[156]</td></tr><tr><td colspan="4">88.3 - - -</td></tr><tr><td rowspan="2">Macauba oil</td><td rowspan="2">Transesterification-esterification with methanol</td><td colspan="4">Eggshell</td><td rowspan="2">Calcination at 900 °C for 3.5 h</td><td rowspan="2">80 °C</td><td rowspan="2">1 h</td><td rowspan="2">10:1</td><td rowspan="2">15</td><td rowspan="2">C = 91.60%</td><td rowspan="2">N/A</td><td rowspan="2">[170]</td></tr><tr><td colspan="4">4 0.03 27.7 -</td></tr><tr><td rowspan="4">Frying residual oil</td><td rowspan="4">Transesterification with methanol</td><td colspan="4">Eggshell calcined and enriched with glycerin (ECEG)</td><td rowspan="4">Calcination<br>Wet impregnation<br>10 °C/min until 150 °C for 120 min<br>10 °C/min until 150 °C for 240 min<br>10 °C/min until 800 °C for 240 min</td><td rowspan="4">63 °C</td><td rowspan="4">3 h</td><td rowspan="4">6:1</td><td rowspan="4">15</td><td rowspan="4">Oil mass 15%<br>Y = 97.39%<br>Oil mass 5%<br>Y = 96.97%<br>Oil mass 3%<br>Y = 97.75%<br>Oil mass 1%<br>Y = 92.96%</td><td rowspan="4">4</td><td rowspan="4">[163]</td></tr><tr><td colspan="4">1.09 - 4.15 -</td></tr><tr><td colspan="4">Eggshell</td></tr><tr><td colspan="4">0.10806 0.001217 5.66399 -</td></tr><tr><td rowspan="2">Waste chicken fat</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Eggshell</td><td rowspan="2">Calcination at 900 °C for 4 h</td><td rowspan="2">57.5 °C</td><td rowspan="2">5 h</td><td rowspan="2">13:1</td><td rowspan="2">8.5</td><td rowspan="2">Y = 90.41%<br>C = 92.9%</td><td rowspan="2">5</td><td rowspan="2">[167]</td></tr><tr><td colspan="4">0.10806 0.001217 5.66399 -</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Alkaline earth metal oxides</td></tr><tr><td rowspan="6">Waste cooking oil</td><td rowspan="6">Transesterification with methanol</td><td colspan="4">Supported eggshell AEC-10</td><td rowspan="6">Calcination at 900 °C for 2 h<br>Wet impregnation</td><td rowspan="6">65 °C</td><td rowspan="6">3 h</td><td rowspan="6">12:1</td><td rowspan="6">10</td><td rowspan="6">C = &gt;95%</td><td rowspan="6">N/A</td><td rowspan="6">[168]</td></tr><tr><td>23.48</td><td>0.1</td><td>20.4</td><td>-</td></tr><tr><td colspan="4">Supported eggshell AEC-20</td></tr><tr><td>10.36</td><td>0.02</td><td>10.8</td><td>-</td></tr><tr><td colspan="4">Supported eggshell AEC-30</td></tr><tr><td>1.63</td><td>0.009</td><td>22.3</td><td>-</td></tr><tr><td rowspan="2">Palm oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Waste mud crab (Scylla serrata) shell</td><td rowspan="2">Calcination at 900 °C for 2 h</td><td rowspan="2">65 °C</td><td rowspan="2">3 h</td><td rowspan="2">0.5:1</td><td rowspan="2">5</td><td rowspan="2">C = 98.80%</td><td rowspan="2">11</td><td rowspan="2">[101]</td></tr><tr><td>13</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="4">Palm olein oil</td><td rowspan="4">Transesterification with methanol</td><td colspan="4">Waste shells of egg</td><td rowspan="4">Calcination at 800 °C for 4 h</td><td rowspan="4">60 °C</td><td rowspan="4">2 h</td><td rowspan="4">18:1</td><td rowspan="4">10</td><td rowspan="4">Y = &gt;90%</td><td rowspan="4">N/A</td><td rowspan="4">[157]</td></tr><tr><td>1.1</td><td>0.005</td><td>-</td><td>-</td></tr><tr><td colspan="4">Golden apple snail</td></tr><tr><td>0.9</td><td>0.004</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Rapeseed oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Meretrix venus</td><td rowspan="2">Co-extrusion</td><td rowspan="2">65 °C</td><td rowspan="2">4 h</td><td rowspan="2">8:1</td><td rowspan="2">20</td><td rowspan="2">1st cycle Y = 95.5%<br>2nd cycle Y = 97.7%<br>3rd cycle Y = 94.5%<br>4th cycle Y = 80.9%</td><td rowspan="2">3</td><td rowspan="2">[172]</td></tr><tr><td>4.26</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Refined, bleached,<br>and deodorized palm<br>oil (RBD palm oil)</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">CaO/γ-Al₂O₃</td><td rowspan="2">Calcination at 718 °C for 5 h<br>Impregnation</td><td rowspan="2">60 °C</td><td rowspan="2">3 h</td><td rowspan="2">15:1</td><td rowspan="2">9</td><td rowspan="2">C = 86.38%<br>Y = 79.32%</td><td rowspan="2">N/A</td><td rowspan="2">[212]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Canola oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">CaO with K₂CO₃</td><td rowspan="2">Calcination 499.85 °C for 3 h<br>Impregnation with 3–10% of K₂CO₃</td><td rowspan="2">65 °C</td><td rowspan="2">8 h</td><td rowspan="2">9:1</td><td rowspan="2">-</td><td rowspan="2">Y = 97.67%</td><td rowspan="2">5</td><td rowspan="2">[213]</td></tr><tr><td>10.24–14.65</td><td>-</td><td>2–300</td><td>0.88–1.64<br>basicity</td></tr><tr><td>Waste palm oil<br>(WPO),<br>waste sunflower oil<br>(WSO)</td><td>Esterification and<br>transesterification<br>with methanol</td><td colspan="4">CaO/Al₂O₃</td><td rowspan="2">Co-precipitation<br>Dissolution<br>Calcination at 750 °C for 6 h</td><td rowspan="2">65 °C</td><td rowspan="2">4 h</td><td rowspan="2">9:1</td><td rowspan="2">-</td><td rowspan="2">WPO Y = 89%<br>WCO Y = 98%</td><td rowspan="2">2</td><td rowspan="2">[173]</td></tr><tr><td></td><td></td><td>8.5683</td><td>0.045008</td><td>210.1157</td><td>-</td></tr><tr><td rowspan="2">Waste frying<br>sunflower oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">CaO/ZnFe₂O₄</td><td rowspan="2">Co-precipitation<br>Calcination at 800 °C for<br>2.5 h</td><td rowspan="2">65 °C</td><td rowspan="2">3 h</td><td rowspan="2">12:1</td><td rowspan="2">6</td><td rowspan="2">Y = 98%</td><td rowspan="2">5</td><td rowspan="2">[214]</td></tr><tr><td>141–198</td><td>0.39–0.65</td><td>1.5–25</td><td>-</td></tr><tr><td rowspan="2">Sunflower oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">CaO/ZM (zeolitic material)</td><td rowspan="2">Ultrasound-assisted<br>impregnation<br>Calcination at 450–600 °C for<br>6 h</td><td rowspan="2">60 °C</td><td rowspan="2">2 h</td><td rowspan="2">12:1</td><td rowspan="2">4</td><td rowspan="2">C = 96.50%</td><td rowspan="2">N/A</td><td rowspan="2">[176]</td></tr><tr><td>22.6</td><td>2.162</td><td>-</td><td>0–0.8645<br>basicity</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Alkaline earth metal oxides</td></tr><tr><td rowspan="3">Palm oil</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">CaO-loaded unimodal porous silica (CaO/U)</td><td rowspan="3">Calcination at 800 °C for 4 h<br>Impregnation with calcium nitrate tetrahydrate</td><td rowspan="3">60 °C</td><td rowspan="3">6 h</td><td rowspan="3">12:1</td><td rowspan="3">5</td><td rowspan="3">1st cycle Y = 94.15%<br>5th cycle Y = 88.87%</td><td rowspan="3">5</td><td rowspan="3">[215]</td></tr><tr><td colspan="4">22–371</td></tr><tr><td colspan="4">CaO-loaded bimodal porous silica (CaO/B)</td></tr><tr><td></td><td></td><td colspan="4">24–415</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Cotton seed oil (CSO), waste frying oil (WFO)</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Sr(NO₃)₂ on CaO</td><td rowspan="2">Calcination at 750 °C for 6 h<br>Impregnation</td><td rowspan="2">30–80 °C</td><td rowspan="2">2 h</td><td rowspan="2">12:1</td><td rowspan="2">3.5</td><td rowspan="2">CSO C = 97.3%<br>UFO C = 96.7%</td><td rowspan="2">3</td><td rowspan="2">[216]</td></tr><tr><td colspan="4">90</td></tr><tr><td>Oleic acid</td><td>Esterification with methanol</td><td colspan="4">Metal-organic framework Mg-MOF</td><td>N/A</td><td>70 °C</td><td>3 h</td><td>15:1</td><td>0.15</td><td>C = 97%</td><td>5</td><td>[179]</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Mg-Al hydrotalcite</td><td rowspan="2">Calcination at 450 °C for 3 h</td><td rowspan="2">65 °C</td><td rowspan="2">24 h</td><td rowspan="2">30:1</td><td rowspan="2">5</td><td rowspan="2">Y = 87.23%</td><td rowspan="2">N/A</td><td rowspan="2">[180]</td></tr><tr><td colspan="4">-</td></tr><tr><td rowspan="2">Stearic acid monoethanolamide</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">MgAl-layered double hydroxide (MgAl-LDH)</td><td rowspan="2">Calcination at 500 °C for 8 h<br>Rehydration</td><td rowspan="2">109 °C</td><td rowspan="2">4 h</td><td rowspan="2">1.1:1</td><td rowspan="2">5</td><td rowspan="2">C = 87%</td><td rowspan="2">N/A</td><td rowspan="2">[217]</td></tr><tr><td colspan="4">44</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Strontium zirconate</td><td rowspan="2">Calcination at 900 °C for 1 h</td><td rowspan="2">60 °C</td><td rowspan="2">3 h</td><td rowspan="2">12:1</td><td rowspan="2">3</td><td rowspan="2">Y = 98%</td><td rowspan="2">N/A</td><td rowspan="2">[218]</td></tr><tr><td colspan="4">-</td></tr><tr><td rowspan="2">Cotton seed oil (CSO)<br>Waste frying oil (WFO)</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Sr(NO₃)₂ on CaO</td><td rowspan="2">Impregnation<br>Calcination at 750 °C for 6 h</td><td rowspan="2">60 °C</td><td rowspan="2">2 h</td><td rowspan="2">12:1</td><td rowspan="2">3.5</td><td rowspan="2">CSO C = 97.3%<br>UFO C = 96.7%</td><td rowspan="2">3</td><td rowspan="2">[193]</td></tr><tr><td colspan="4">90</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Sr₃Al₂O₆</td><td rowspan="2">Calcination to up to 1200 °C</td><td rowspan="2">60 °C</td><td rowspan="2">61 min</td><td rowspan="2">25:1</td><td rowspan="2">1.3</td><td rowspan="2">C = 95.70%</td><td rowspan="2">4</td><td rowspan="2">[219]</td></tr><tr><td colspan="4">4.355</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Sr-Ce</td><td rowspan="2">Gel combustion<br>Calcination at 900 °C for 4 h</td><td rowspan="2">65 °C</td><td rowspan="2">2 h</td><td rowspan="2">14:1</td><td rowspan="2">2</td><td rowspan="2">C = 99.5%</td><td rowspan="2">4</td><td rowspan="2">[181]</td></tr><tr><td colspan="4">66</td></tr><tr><td rowspan="2">Corn oil and oleic acid</td><td rowspan="2">Esterification and transesterification with ethanol</td><td colspan="4">SrO-ZnO/Al₂O₃</td><td rowspan="2">Calcination at 900 °C for 6 h</td><td rowspan="2">70 °C</td><td rowspan="2">6 h</td><td rowspan="2">5:1</td><td rowspan="2">10</td><td rowspan="2">C = 95.1%</td><td rowspan="2">N/A</td><td rowspan="2">[182]</td></tr><tr><td colspan="4">3.515</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">Sr-Ti mixed metal oxide</td><td rowspan="2">Calcination at 880 °C for 8 h</td><td rowspan="2">65 °C</td><td rowspan="2">80 min</td><td rowspan="2">11:1</td><td rowspan="2">1</td><td rowspan="2">C = 97.90%</td><td rowspan="2">8</td><td rowspan="2">[183]</td></tr><tr><td colspan="4">43.6</td></tr><tr><td rowspan="2">Palm oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">SrTiO₃</td><td rowspan="2">Sol-gel method<br>Calcination at 1050 °C for 4 h</td><td rowspan="2">170 °C</td><td rowspan="2">3 h</td><td rowspan="2">15:1</td><td rowspan="2">6</td><td rowspan="2">Y = 93.14%</td><td rowspan="2">3</td><td rowspan="2">[53]</td></tr><tr><td colspan="4">13.07</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="text-align: left; font-weight: bold;">Alkaline earth metal oxides</td></tr><tr><td rowspan="6">Olive oil</td><td rowspan="6">Transesterification with methanol</td><td>SrO</td><td></td><td></td><td></td><td></td><td rowspan="6">Calcination at 900 °C for 5 h</td><td rowspan="6">45 °C</td><td rowspan="6">30 min</td><td rowspan="6">1:6</td><td rowspan="6">3.23 SrO<br>3.14 SrO/SiO<sub>2</sub></td><td rowspan="6">C = &gt;80%</td><td rowspan="6">4</td><td rowspan="6">[220]</td></tr><tr><td>4.743</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SrO/CaO</td><td></td><td></td><td></td></tr><tr><td>2.272</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SrO/SiO<sub>2</sub></td><td></td><td></td><td></td></tr><tr><td>1.966</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Corn oil</td><td rowspan="2">Transesterification with methanol</td><td>Ba(OH)<sub>2</sub></td><td></td><td></td><td></td><td></td><td rowspan="2">Calcination</td><td rowspan="2">-</td><td rowspan="2">118 min</td><td rowspan="2">11:32</td><td rowspan="2">3.6</td><td rowspan="2">Y = 99.15%</td><td rowspan="2">4</td><td rowspan="2">[185]</td></tr><tr><td>1.24</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Karanja oil</td><td>Transesterification with methanol</td><td>Ba(OH)<sub>2</sub>·8H<sub>2</sub>O</td><td></td><td></td><td></td><td></td><td>N/A</td><td>30 °C</td><td>1.5 h</td><td>9:1</td><td>0.5</td><td>C = 84%</td><td>N/A</td><td>[186]</td></tr><tr><td>Microalgae Anabaena PCC 7120</td><td>Transesterification with methanol</td><td>Ba<sub>2</sub>TiO<sub>4</sub></td><td></td><td></td><td></td><td></td><td>Wet impregnation<br>Calcination at 800 °C for 4 h</td><td>60 °C</td><td>4 h</td><td>15:1</td><td>3.5</td><td>C = 98.41%</td><td>6</td><td>[221]</td></tr><tr><td colspan="14" style="text-align: left; font-weight: bold;">Alkali metal salts</td></tr><tr><td>Safflower oil</td><td>Esterification with methanol</td><td>Potassium Titanate</td><td></td><td></td><td></td><td></td><td>Hydrothermal treatment at 200 °C for 20 h<br>Calcination</td><td>50 °C</td><td>1 h</td><td>1:1</td><td>3</td><td>Y = approx. 100%</td><td>3</td><td>[84]</td></tr><tr><td>Bauhinia monandra seed oil</td><td>Esterification and transesterification with methanol</td><td>Banana peels</td><td></td><td></td><td></td><td></td><td>Calcination at 700 °C for 4 h</td><td>65 °C</td><td>69.02 min</td><td>7.6:1</td><td>2.75</td><td>C = 98.5%</td><td>N/A</td><td>[222]</td></tr><tr><td rowspan="2">Madhuca indica</td><td rowspan="2">Transesterification with methanol</td><td>Banana pseudo stem</td><td></td><td></td><td></td><td></td><td>Calcination at 700 °C for 4 h</td><td>65 °C</td><td>178.1 min</td><td>14.9:1</td><td>5.9</td><td>C = 98.8%</td><td>3</td><td>[188]</td></tr><tr><td>4.580</td><td>0.006</td><td>2.245</td><td>-</td><td></td><td>Impregnation<br>Calcination at 500 °C for 3 h</td><td>50 °C</td><td>40 min</td><td>12:1</td><td>2</td><td>C = 95%</td><td>N/A</td><td>[223]</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Transesterification with methanol</td><td>KF/c-Al<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td>Impregnation<br>Calcination at 500 °C for 3 h</td><td>65 °C</td><td>8 h</td><td>15:1</td><td>3</td><td>C = 97.7%</td><td>N/A</td><td>[224]</td></tr><tr><td>41.7</td><td>-</td><td>7-40</td><td>1.68 basicity</td><td></td><td>Impregnation<br>Calcination at 600 °C for 3 h</td><td>70 °C</td><td>8 h</td><td>5 wt%</td><td>15</td><td>C = 90.09%</td><td>N/A</td><td>[189]</td></tr><tr><td rowspan="2">Jatropha oil</td><td rowspan="2">Transesterification with methanol</td><td>KI/mesoporous silica</td><td></td><td></td><td></td><td></td><td>Impregnation<br>Calcination at 500 °C for 4 h</td><td>70 °C</td><td>6 h</td><td>12:1</td><td>6</td><td>C &gt; 84%</td><td>3</td><td>[225]</td></tr><tr><td>801</td><td>1.05</td><td>5.23</td><td>-</td><td></td><td>Impregnation<br>Calcination at 500 °C for 4 h</td><td>70 °C</td><td>6 h</td><td>12:1</td><td>6</td><td>C &gt; 84%</td><td>3</td><td>[225]</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td>KOH/corncob-derived activated carbon</td><td></td><td></td><td></td><td></td><td>Wet impregnation<br>Calcination at 450 °C for 2 h</td><td>45 °C</td><td>1 h</td><td>18:1</td><td>1</td><td>Y = 97.80%<br>C = 92%</td><td>2</td><td>[190]</td></tr><tr><td>627</td><td>0.637</td><td>-</td><td>9.903 basicity</td><td></td><td>Sol-gel method<br>Combustion method<br>Calcination 900 °C for 5 h</td><td>65 °C</td><td>2.5 h</td><td>16:1</td><td>2</td><td>C = 97.5%</td><td>5</td><td>[191]</td></tr><tr><td>Castor seed oil</td><td>Transesterification with methanol</td><td>K-promoted La<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td>Combustion method<br>Calcination 900 °C for 5 h</td><td>65 °C</td><td>2.5 h</td><td>16:1</td><td>2</td><td>C = 97.5%</td><td>5</td><td>[191]</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Alkali metal salts</td></tr><tr><td rowspan="2">Palm, sunflower, and soybean oil</td><td rowspan="2">Transesterification with methanol</td><td>K/SnO₂</td><td></td><td></td><td></td><td></td><td rowspan="2">Calcination at 700 °C for 5 h</td><td rowspan="2">65 °C</td><td rowspan="2">1.5 h</td><td rowspan="2">12:1</td><td rowspan="2">3</td><td rowspan="2">Y = 97.5%</td><td rowspan="2">N/A</td><td rowspan="2">[192]</td></tr><tr><td>19.7</td><td>0.074</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Transesterification with methanol</td><td>K₂CO₃ supported on MgO</td><td></td><td></td><td></td><td></td><td rowspan="2">Calcination at 600 °C for 3 h</td><td rowspan="2">70 °C</td><td rowspan="2">45 min</td><td rowspan="2">4:1</td><td rowspan="2">1.3</td><td rowspan="2">Y = 99%</td><td rowspan="2">6</td><td rowspan="2">[226]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td>K₃PO₄</td><td></td><td></td><td></td><td></td><td rowspan="2">N/A</td><td rowspan="2">50 °C</td><td rowspan="2">1.5 h</td><td rowspan="2">6:1</td><td rowspan="2">3</td><td rowspan="2">Y = 92%</td><td rowspan="2">4</td><td rowspan="2">[227]</td></tr><tr><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Esterification and transesterification with ethanol</td><td>K₃PO₄/seashell</td><td></td><td></td><td></td><td></td><td rowspan="2">Calcination 800 °C for 4 h</td><td rowspan="2">60 °C</td><td rowspan="2">5 h</td><td rowspan="2">12:1</td><td rowspan="2">5–25</td><td rowspan="2">Y = 95%</td><td rowspan="2">N/A</td><td rowspan="2">[228]</td></tr><tr><td>22.5</td><td>8.5</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Sunflower oil (SO), used frying oil (UFO)</td><td rowspan="2">Transesterification with methanol</td><td>LiNO₃/CaO</td><td></td><td></td><td></td><td></td><td rowspan="2">Calcination at 750 °C for 6 h</td><td rowspan="2">60 °C</td><td rowspan="2">2 h</td><td rowspan="2">12:1</td><td rowspan="2">3.5</td><td rowspan="2">SO C = 97.8%<br/>UFO C = 96.7%</td><td rowspan="2">6</td><td rowspan="2">[193]</td></tr><tr><td>90</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="3">Soybean oil</td><td rowspan="3">Transesterification with methanol</td><td>Sodium geopolymer powder</td><td></td><td></td><td></td><td></td><td rowspan="3">Heat treatment at 110–700 °C</td><td rowspan="3">-</td><td rowspan="3">-</td><td rowspan="3">-</td><td rowspan="3">-</td><td rowspan="3">Y = 89.9%</td><td rowspan="3">N/A</td><td rowspan="3">[194]</td></tr><tr><td>6.34–32.62</td><td>-</td><td>17</td><td>-</td></tr><tr><td>Potassium geopolymer powder</td><td></td><td></td><td></td></tr><tr><td></td><td></td><td>28.64–62.54</td><td>-</td><td>9</td><td>-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Sunflower oil</td><td rowspan="2">Transesterification with methanol</td><td>Sodium titanate nanotubes</td><td></td><td></td><td></td><td></td><td rowspan="2">N/A</td><td rowspan="2">80 °C</td><td rowspan="2">2 h</td><td rowspan="2">40:1</td><td rowspan="2">1.5</td><td rowspan="2">C = 95.9%</td><td rowspan="2">3</td><td rowspan="2">[229]</td></tr><tr><td>120</td><td>0.29</td><td>5.37</td><td>-</td></tr><tr><td rowspan="2">Mixture of soybean and sunflower oils</td><td rowspan="2">Esterification and transesterification with methanol</td><td>Mixture of sodium alginate, k-carrageenan, and sodium bentonite.</td><td></td><td></td><td></td><td></td><td rowspan="2">N/A</td><td rowspan="2">65 °C</td><td rowspan="2">0.5 h</td><td rowspan="2">21:1</td><td rowspan="2">3</td><td rowspan="2">C = 97%</td><td rowspan="2">25</td><td rowspan="2">[195]</td></tr><tr><td>151</td><td>-</td><td>1.5mm</td><td>1.07 acidity<br/>5.84 basicity</td></tr><tr><td rowspan="2">Rapeseed oil</td><td rowspan="2">Transesterification with methanol</td><td>NaNO₃/SiAl</td><td></td><td></td><td></td><td></td><td rowspan="2">Impregnation<br/>Calcination at 600 °C for 25 h</td><td rowspan="2">65 °C</td><td rowspan="2">3 h</td><td rowspan="2">9:1</td><td rowspan="2">5</td><td rowspan="2">C > 99%</td><td rowspan="2">3</td><td rowspan="2">[196]</td></tr><tr><td>0.410–2.406</td><td>-</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Wild olive oil</td><td rowspan="2">Transesterification with methanol</td><td>Na-SiO₂@CeO₂</td><td></td><td></td><td></td><td></td><td rowspan="2">Wet impregnation<br/>Calcination at 500 °C for 5 h</td><td rowspan="2">65 °C</td><td rowspan="2">2 h</td><td rowspan="2">10:1</td><td rowspan="2">2.5</td><td rowspan="2">Y = 97%</td><td rowspan="2">5</td><td rowspan="2">[230]</td></tr><tr><td>88.4</td><td>0.0122</td><td>2.15</td><td>-</td></tr><tr><td rowspan="2">Waste cooking oil</td><td rowspan="2">Transesterification with methanol</td><td>Na-SiO₂@TiO₂</td><td></td><td></td><td></td><td></td><td rowspan="2">Wet impregnation<br/>Ultrasound<br/>Calcination at 500 °C for 5 h</td><td rowspan="2">65 °C</td><td rowspan="2">2 h</td><td rowspan="2">25:1</td><td rowspan="2">2.5</td><td rowspan="2">Y = 98%</td><td rowspan="2">5</td><td rowspan="2">[190]</td></tr><tr><td>107.26</td><td>0.133</td><td>6.16</td><td>-</td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Mixed metal oxides</td></tr><tr><td rowspan="3">Nannochloropsis oculate microalga</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">Al₂O₃-supported CaO and MgO</td><td rowspan="3">Sol-gel method<br/>Calcination at 500 °C for 6 h</td><td rowspan="3">50 °C</td><td rowspan="3">4 h</td><td rowspan="3">30:1</td><td rowspan="3">2</td><td rowspan="3">Y = 97.5%</td><td rowspan="3">2</td><td rowspan="3">[231]</td></tr><tr><td colspan="4">Al₂O₃</td></tr><tr><td colspan="4">119</td></tr><tr><td rowspan="5">Calophyllum inophyllum oil</td><td rowspan="5">Esterification with methanol</td><td colspan="4">SnO</td><td rowspan="5">Co-precipitation</td><td rowspan="5">160 °C</td><td rowspan="5">1 h</td><td rowspan="5">4 g of methanol</td><td rowspan="5">0.1</td><td rowspan="5">Y = 89%</td><td rowspan="5">10</td><td rowspan="5">[232]</td></tr><tr><td colspan="4">15</td></tr><tr><td colspan="4">(Al₂O₃)₈(SnO₂)</td></tr><tr><td colspan="4">22</td></tr><tr><td colspan="4">(Al₂O₃)₈(ZnO₂)</td></tr><tr><td colspan="4">33</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with methanol</td><td colspan="4">γ-Al₂O₃</td><td rowspan="2">Calcination at 450 °C for 4 h</td><td rowspan="2">275 °C</td><td rowspan="2">1 min</td><td rowspan="2">20:1</td><td rowspan="2">-</td><td rowspan="2">C = 90%</td><td rowspan="2">N/A</td><td rowspan="2">[233]</td></tr><tr><td colspan="4">85.98</td></tr><tr><td rowspan="6">Palm kernel oil and coconut oil</td><td rowspan="6">Transesterification with methanol</td><td colspan="4">LiNO₃/Al₂O₃</td><td rowspan="6">Incipient wetness impregnation<br/>Sol-gel method<br/>Calcination at 450 °C, 550 °C, 650 °C</td><td rowspan="6">60 °C</td><td rowspan="6">3 h</td><td rowspan="6">65:1</td><td rowspan="6">10-15-20</td><td rowspan="6">Palm oil C = 94%<br/>Coconut oil C = 99.8%</td><td rowspan="6">N/A</td><td rowspan="6">[234]</td></tr><tr><td colspan="4">NaNO₃/Al₂O₃ KNO₃/Al₂O₃</td></tr><tr><td colspan="4">-</td></tr><tr><td colspan="4">Ca(NO₃)₂/Al₂O₃</td></tr><tr><td colspan="4">-</td></tr><tr><td colspan="4">Mg(NO₃)₂/Al₂O₃</td></tr><tr><td colspan="4">-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Waste cooking oil</td><td rowspan="3">Esterification and transesterification with methanol</td><td colspan="4">Cu-MOF</td><td rowspan="3">Solvothermal and hydrothermal methods</td><td rowspan="3">60 °C</td><td rowspan="3">1 h</td><td rowspan="3">20:1</td><td rowspan="3">1 g/100 mL</td><td rowspan="3">Y = 85%</td><td rowspan="3">3</td><td rowspan="3">[198]</td></tr><tr><td colspan="4">118</td></tr><tr><td colspan="4">0.087</td></tr><tr><td colspan="4">Ca-MOF</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td colspan="4">101</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 2. Cont.</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="13"><strong>Mixed metal oxides</strong></td></tr><tr><td>Waste cooking oil</td><td>Transesterification with methanol</td><td colspan="4">Copper-doped zinc oxide nanocomposite (CZO)</td><td>Calcination 500 °C for 2 h</td><td>55 °C</td><td>50 min</td><td>8:1</td><td>12</td><td>Y = 97.71%</td><td>5</td><td>[235]</td></tr><tr><td>Oleic acid</td><td>Esterification with methanol</td><td colspan="4">Copper(II)-alginate beads</td><td>N/A</td><td>70 °C</td><td>3 h</td><td>10:1</td><td>250 mg</td><td>C = 71.80%</td><td>N/A</td><td>[236]</td></tr><tr><td rowspan="3">Waste edible oil</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">CoFe₂O₄</td><td rowspan="3">Ultrasound</td><td rowspan="3">64.75 °C</td><td rowspan="3">55.75 min</td><td rowspan="3">16.05:1</td><td rowspan="3">5.22</td><td>Y = 91.64%</td><td rowspan="3">5</td><td rowspan="3">[199]</td></tr><tr><td colspan="4">CoFe₂O₄@GO</td><td>Y = 98.17%</td></tr><tr><td colspan="4">- 0.164 12.61 0.91 basicity</td><td></td></tr><tr><td>Phoenix dactylifera L. kernel oil</td><td>Esterification and transesterification with methanol</td><td colspan="4">Mn@MgO-ZrO₂</td><td>Co-precipitation Impregnation Calcination at 650 °C for 4 h</td><td>90 °C</td><td>4 h</td><td>15:1</td><td>3</td><td>Y = 96.4%</td><td>6</td><td>[237]</td></tr><tr><td>Soybean oil</td><td>Transesterification and esterification with methanol</td><td colspan="4">MnO and TiO</td><td>N/A</td><td>260 °C</td><td>0.35 h</td><td>30:1</td><td>28.1g</td><td>Y = 96–99%</td><td>N/A</td><td>[238]</td></tr><tr><td rowspan="2">Oleic acid</td><td rowspan="2">Esterification with ethanol</td><td colspan="4">MnO₂@Mn (btc)</td><td rowspan="2">N/A</td><td rowspan="2">100 °C</td><td rowspan="2">12 h</td><td rowspan="2">12:1</td><td rowspan="2">3</td><td rowspan="2">Y = 98%</td><td rowspan="2">5</td><td rowspan="2">[71]</td></tr><tr><td colspan="4">10.867 0.039 - -</td></tr><tr><td rowspan="3">Nannochloropsis oculate oil</td><td rowspan="3">Transesterification with methanol</td><td colspan="4">Mn-ZnO capped with Polyethylene</td><td rowspan="3">Precipitation method Calcination at 700 °C for 3 h.</td><td rowspan="3">60 °C</td><td rowspan="3">4 h</td><td rowspan="3">15:1</td><td rowspan="3">3.5</td><td rowspan="3">Y = 87.5%</td><td rowspan="3">4</td><td rowspan="3">[203]</td></tr><tr><td colspan="4">Glycerol (PEG)</td></tr><tr><td colspan="4">- - - -</td></tr><tr><td>Soybean oil</td><td>Transesterification with methanol</td><td colspan="4">MoO₃/ZrO₂/KIT-6</td><td>Calcination 500 °C for 6 h</td><td>130 °C</td><td>12 h</td><td>20:1</td><td>12</td><td>C = 92.7%</td><td>5</td><td>[15]</td></tr><tr><td>Castor oil</td><td>Transesterification with methanol</td><td colspan="4">Ni-doped ZnO</td><td>Calcination at 800 °C for 3 h</td><td>55 °C</td><td>1 h</td><td>8:1</td><td>11</td><td>Y = 95.2%</td><td>3</td><td>[200]</td></tr><tr><td>Glycerol carbonate</td><td>Transesterification with methanol</td><td colspan="4">Ni/CaO</td><td>Calcination at 800 °C for 4 h</td><td>90 °C</td><td>1.5 h</td><td>3:1</td><td>3</td><td>C = 99.2%<br>Y = 94.02%</td><td>5</td><td>[239]</td></tr><tr><td rowspan="2">Soybean oil</td><td rowspan="2">Esterification and transesterification with methanol</td><td colspan="4">Ni₀.₅Zn₀.₅Fe₂O₄</td><td rowspan="2">Sol-gel method</td><td rowspan="2">180 °C</td><td rowspan="2">1 h</td><td rowspan="2">12–15:1</td><td rowspan="2">2–3</td><td rowspan="2">C = 99.54%</td><td rowspan="2">3</td><td rowspan="2">[202]</td></tr><tr><td colspan="4">65.289 0.167 4.27 -</td></tr><tr><td>Calophyllum inophyllum oil</td><td>Transesterification with methanol</td><td colspan="4">Zn/CaO</td><td>N/A</td><td>56.71 °C</td><td>81.31 min</td><td>9.66:1</td><td>5</td><td>C = 91.95%</td><td>N/A</td><td>[93]</td></tr><tr><td rowspan="2">Waste frying oil</td><td rowspan="2">Transesterification with methanol</td><td colspan="4">ZnAl-LDH@SiO₂</td><td rowspan="2">Titration method</td><td rowspan="2">25 °C</td><td rowspan="2">1.5 h</td><td rowspan="2">6:1</td><td rowspan="2">3.3</td><td rowspan="2">Y = 98.4%</td><td rowspan="2">5</td><td rowspan="2">[240]</td></tr><tr><td colspan="4">- - - -</td></tr></tbody></table>
<table><caption>Table 2. *Cont.*</caption><thead><tr><th rowspan="2">Raw Material</th><th rowspan="2">Reaction</th><th colspan="4">Catalyst</th><th rowspan="2">Preparation Methods</th><th colspan="4">Reaction Parameters</th><th rowspan="2">Biodiesel Yield</th><th rowspan="2">Reuse</th><th rowspan="2">Reference</th></tr><tr><th>Surface Area (m²/g)</th><th>Volume (cm³/g)</th><th>Pore Size (nm)</th><th>Acidity/Basicity (mmol/g)</th><th>T°</th><th>Reaction Time</th><th>Ratio Alcohol:Oil</th><th>Catalyst wt%</th></tr></thead><tbody><tr><td colspan="14" style="background-color: #cccccc; font-weight: bold;">Mixed metal oxides</td></tr><tr><td>Waste coconut oil</td><td>Pre-esterification</td><td>ZnO/CuO</td><td>-</td><td>-</td><td>-</td><td>-</td><td>Calcination at 300 °C for 3 h<br>Wet impregnation with zinc sulphate</td><td>55 °C</td><td>113 min</td><td>10.5:1</td><td>1.66</td><td>C = 90.26%</td><td>4</td><td>[241]</td></tr><tr><td>Jatropha curcas crude oil</td><td>Esterification and transesterification with methanol</td><td>ZnO/SiO₂</td><td>160</td><td>-</td><td>-</td><td>-</td><td>Impregnation<br>Calcination at 500 °C for 12 h</td><td>60 °C</td><td>20 min</td><td>12:1</td><td>2</td><td>C = 96%</td><td>10</td><td>[242]</td></tr><tr><td>Jatropha oil</td><td>Esterification with methanol</td><td>ZnO/zeolite, PbO/zeolite, and MgO</td><td>422.50</td><td>0.2655</td><td>2.593</td><td>-</td><td>Hydrothermal impregnation<br>precipitation (HIP) method</td><td>60–65 °C</td><td>1.4 h</td><td>12:1</td><td>5</td><td>Y = 98.5%</td><td>4</td><td>[174]</td></tr><tr><td>Chlorella vulgaris</td><td>Esterification and transesterification with methanol</td><td>SiC/NaOH-GO</td><td>1.60</td><td>-</td><td>27.1</td><td>-</td><td>Impregnation<br>Calcination at 400 °C for 5 h</td><td>85 °C</td><td>5 min</td><td>48:1</td><td>4</td><td>Transesterification<br>Y = 96%<br>Esterification<br>Y = 92%</td><td>4</td><td>[243]</td></tr></tbody></table>
### 3.3. Influence of Physico-Chemical Properties on Biodiesel Yield

Physical properties, such as surface area and pore size, impact the catalyst's overall effectiveness and consequently affect biodiesel yield [244]. A catalyst with a large surface area can create more active sites for reactants to adhere to the surface, hence decreasing reaction time and enhancing FAME production [20].

The calcination process and temperature directly affect the surface area of the catalyst. For example, the calcination of rice husk at 800 °C enhanced the catalyst's surface area and resulted in a FAME yield higher than 97% [154]. Similarly, Putra et al. [245] demonstrated that a calcination temperature of 700 °C resulted in an increased surface area of the waste eggshell and improved the biodiesel output.

The catalyst's pore size must be larger than the triglyceride molecules to allow effective access and interaction [20]. A well-designed pore structure can store large FFA molecules, facilitate methanol migration, and reduce diffusional limitations [3]. Feyzi et al. [245] observed that the pore diameter, pore volume, and surface area of the MgO-La₂O₃ catalyst increased with rising calcination temperatures up to a certain point. However, when calcination temperatures exceeded 600 °C, these parameters were negatively affected. These physical properties are determined through the N₂ adsorption-desorption method [100], the BET method for surface area [246] and the Barrett-Joyner (BJH) method for mean pore diameter and pore volume [20,247]. Alcañiz-Monge et al. [36], developed a catalyst with larger pores by using the sol-gel method and hydrothermal treatment, which led to the catalyst's chemical and thermal stability as well as its high catalytic activity and conversion rates.

### 3.4. An Analytical Review of Variability and Its Implications

Although the biodiesel yields reported in the literature are generally high, often ranging from 80% to over 98%, considerable variability is observed across different studies, even among those employing similar classes of heterogeneous catalysts. This variability can be attributed to several interrelated factors. One major source of variation lies in the composition of the feedstock. Differences in FFA content, moisture levels, and the structural profile of triglycerides can significantly influence catalytic activity. Catalysts possessing strong Brønsted acidity tend to perform more effectively with high-FFA feedstocks, whereas basic catalysts often yield lower conversions due to saponification reactions. Another influential factor is the method of catalyst preparation. Variations in synthesis techniques, such as impregnation versus sol-gel processes, along with differences in calcination temperatures and the choice of support materials (e.g., SBA-15, MCM-41, or ZrO₂), can markedly affect catalyst properties, including surface area, acidity, and thermal stability. The reaction conditions themselves also contribute to variability. Parameters like the alcohol-to-oil molar ratio, reaction temperature, duration, and catalyst dosage can all alter the conversion efficiency. While excess alcohol may enhance reaction equilibrium, it can also dilute the catalyst's activity or lead to increased processing costs. Nonetheless, general performance trends were assessed qualitatively across comparable studies. For instance, TPA-supported catalysts, such as TPA/SBA-15 and TPA/MCM-41, consistently achieved yields above 90%, largely influenced by the surface area and porosity of the support materials. In contrast, sulfonated biochar-based catalysts demonstrated a wider yield range (80-96%), which appeared to depend heavily on the degree of sulfonation and the nature of the raw feedstock. For heterogeneous basic catalysts, clear qualitative trends also emerge. Metal oxides, such as CaO and MgO, generally demonstrate high catalytic activity when derived from calcined natural sources like eggshells or dolomite, frequently achieving biodiesel yields above 90% under optimized conditions. However, their performance is often sensitive to feedstock FFA content due to saponification, which can reduce yield. Furthermore, catalyst
preparation methods, including calcination temperature and particle size control, critically affect their surface basicity and hence their activity. Supported basic catalysts, such as CaO dispersed on mesoporous supports (e.g., SBA-15 or alumina), show improved stability and reusability, with yields consistently in the 85–95% range. Reaction parameters, such as the methanol-to-oil ratio and temperature, are similarly decisive; optimal conditions typically involve molar ratios of 9:1 to 12:1 and reaction temperatures between 60 °C and 70 °C. Despite this, the reported variability remains notable, often arising from differences in catalyst recycling protocols and feedstock variability. Overall, while both acidic and basic heterogeneous catalysts demonstrate strong potential for efficient biodiesel production, their performance variability is intricately linked to feedstock composition, catalyst preparation, and reaction conditions.

Despite the wealth of research on biodiesel production using heterogeneous catalysts, a major limitation remains the inconsistency in reporting experimental details and statistical variability. Many studies do not provide standard deviations, error margins, or the number of replicates, which limits the reliability and comparability of reported yields and catalyst efficiencies. Notably, the majority of the research relies on response surface methodology (RSM) for process optimization. While RSM is a powerful tool for identifying optimal conditions, it often does not include or report statistical measures, such as standard deviations or replicate numbers, in its output, further contributing to the lack of data transparency and reproducibility. To address this problem, the development and adoption of standardized protocols for catalyst synthesis, characterization, and biodiesel production assays are recommended. These should include mandatory reporting of key performance indicators alongside statistical descriptors to enable meaningful cross-study comparisons and meta-analyses. Establishing such protocols would significantly advance the robustness and credibility of research in this field.

## 4. Advantages and Disadvantages of the Different Types of Heterogeneous Catalysts

The selected catalysts are compared by outlining the advantages and disadvantages of each type, as shown in Table 3 below.

<table><caption>**Table 3.** Advantages and disadvantages of heterogeneous catalysts.</caption><thead><tr><th>Advantages</th><th>Disadvantages</th><th>Reference</th></tr></thead><tbody><tr><td><strong>HPA catalysts</strong></td><td></td><td></td></tr><tr><td>Chemical stability<br>Economically and environmentally attractive<br>Convenient synthesis<br>Easily separated, thus improving reusability<br>HPAs on acceptable support increase the surface area and thermal stability of the catalyst<br>Supported HPAs increase active sites' accessibility and limit the formation of bulky residues<br>High mechanical and thermal stabilities, easy handling, nontoxicity, high reactivity and recyclability<br>Strong acidity, high redox properties, good thermal stability, easy separation, good reusability, less side products, less waste generation, non-toxicity, and easy handling<br>Limited surface area</td><td>The requirement of using more demanding reaction conditions<br>Leaching of active catalyst from the supporting material<br>Deactivation of the catalytic sites<br>The recyclability of HPA can be challenging<br>Leaching of the catalyst, leading to a decrease in the activity<br>Acidification procedure coupled with ether extraction could lower the yield of some HPAs<br>Substituted HPA catalysts can have low target product yield and low selectivity due to the resistance of mass transfer<br>Some HPAs require a high dosage and lower activity</td><td>[24–27,34,37]</td></tr></tbody></table>
<table><caption>Table 3. *Cont.*</caption><thead><tr><th>Advantages</th><th>Disadvantages</th><th>Reference</th></tr></thead><tbody><tr><td colspan="3"><strong>Supported acid catalyst on the basis of support</strong></td></tr><tr><td>Minimal waste<br>Ease of product separation<br>The occurrence of transesterification and esterification processes simultaneously<br>Insensitive to water and FFA content in the feedstock</td><td>Lower catalytic activity<br>Catalyst neutralization required<br>Additional step required for the removal of the catalyst</td><td>[8,248]</td></tr><tr><td colspan="3"><strong>Ion exchange resin</strong></td></tr><tr><td>Possibility of continuous or repeated use without renewal<br>Easy removal through decantation or otherwise</td><td>Lack of stability<br>Low exchange capacity<br>Reduced acid resistance<br>Low thermal stability</td><td>[249]</td></tr><tr><td colspan="3"><strong>Sulphated waste catalysts</strong></td></tr><tr><td>High stability<br>High acidity<br>Good catalytic efficiency<br>Excellent activity<br>Recycled several times</td><td>High amounts of inputs and high energy demand<br>High catalyst production cost and difficulty industrializing</td><td>[51,250]</td></tr><tr><td colspan="3"><strong>Alkali metal salt catalysts</strong></td></tr><tr><td>Less corrosive<br>High reaction rate<br>Less expensive<br>Simpler to separate than water during the transesterification procedure<br>Good morphology<br>Easy separation<br>Easy recovery<br>Environmental acceptance</td><td>Leaching of the catalyst<br>High loss of active catalyst<br>Lower activity<br>Lack of stability</td><td>[19,129,187]</td></tr><tr><td colspan="3"><strong>Alkaline earth metal oxide catalysts</strong></td></tr><tr><td>These catalysts are readily available, affordable, non-corrosive, and recyclable<br>Some of these catalysts have higher basic strength and therefore higher catalytic activity<br>Highly stable<br>Easily separated</td><td>FFA and water adsorption may poison basic sites, deactivating the catalyst<br>Low surface of some catalysts<br>Soap formation</td><td>[128,251,252]</td></tr><tr><td colspan="3"><strong>Mixed metal oxide catalysts</strong></td></tr><tr><td>Highly active<br>Easy separation<br>Effective catalysis of different reactions: acid-base catalysis, oxidation reactions, photocatalysis, or biomass conversion<br>Many potential applications<br>Low cost<br>Highly stable<br>Environmental benignity</td><td>Mixed metal oxides are much more complex than metal-based catalysts<br>High mass and heat transfer resistance and small contact areas<br>Insufficient utilization, quick deactivation, difficult regeneration</td><td>[3,65,201,253]</td></tr></tbody></table>
## 5. Conclusions

This review provides a comprehensive comparison of heterogeneous catalysts used for biodiesel production, with an emphasis on catalyst structure-performance relationships, preparation methods, and reaction conditions. While heterogeneous catalysis offers undeniable advantages over homogeneous systems, such as reusability, ease of separation, and lower environmental impact, the performance and practicality of different catalyst classes vary significantly.

Among the reviewed categories, heteropoly acids (HPAs) especially Keggin-type HPAs supported on mesoporous materials consistently demonstrated high biodiesel yields (>90%) and strong acid functionality. However, their solubility in polar solvents and low surface area in unsupported forms limit their long-term reusability. Sulfonated biochar-based catalysts, derived from agricultural waste and biomass, show great promise for sustainable and cost-effective catalysis. Their performance is strongly influenced by the precursor material and sulfonation degree, with biodiesel yields ranging from 80% to 96%. Despite their biodegradability and abundance, leaching of functional groups remains a challenge over multiple cycles.

Mixed metal oxides such as Fe-, Zr-, and Ti-based systems exhibited high thermal stability and dual-function acid-base properties, making them suitable for both esterification and transesterification, especially with low-quality or high-FFA feedstocks. Their versatility and recyclability are notable advantages, though some systems require high synthesis temperatures or involve expensive precursors. Ion exchange resins, particularly Amberlyst-15 and Purolite variants, offer fast reaction kinetics under mild conditions and excellent reusability (up to 10 cycles), but are limited by pore structure degradation and swelling behavior in certain solvent systems. Heterogeneous base catalysts, such as CaO, MgO, and supported alkali metal oxides, also demonstrated high catalytic activity, particularly in transesterification of low-FFA feedstocks. They offer advantages such as fast reaction rates and low cost, but their application is limited by sensitivity to moisture and FFAs, which can lead to soap formation and reduced biodiesel yield. Compared to acid catalysts, base catalysts generally achieve higher conversion efficiencies under milder conditions but require refined feedstocks to maintain effectiveness.

This review also revealed a significant variation in catalyst performance metrics due to differences in synthesis protocols, feedstock composition, and reporting standards. One of the key gaps addressed by this work is the lack of systematic comparison between catalyst classes based not only on reaction efficiency but also on sustainability, reusability, and adaptability to real-world feedstocks. For instance, although some catalysts demonstrated exceptional yields in controlled laboratory environments, their dependence on high-cost materials, complex synthesis routes, or sensitivity to impurities constrain their industrial relevance.

In conclusion, this article contributes to the field by systematically classifying and evaluating a wide range of heterogeneous catalysts, identifying their strengths, limitations, and application niches. It underscores the importance of tailoring catalyst design to feedstock quality and process scalability. Future research should focus on: (i) developing multifunctional and bifunctional catalysts which can perform both esterification and transesterification reactions in a single step, (ii) better exploring emerging catalyst families such as sulfonated biomass-derived carbons and bifunctional metal oxides, (iii) improving catalyst durability, (iv) optimizing green synthesis methods using plant extracts, marine biomass and enzymatic methods, (v) investigating catalyst shaping, structured reactors and process intensification enabling industrial-scale application and (vi) implementing data-driven approach matching specific catalyst types to various feedstocks to support the development of efficient, economically viable, and industrially scalable biodiesel production systems.
**Author Contributions:** Conceptualization, Y.E.R. and N.Z.; methodology, M.Y., S.C., Y.E.R. and N.Z.; investigation, M.Y. and S.C.; writing—original draft preparation, M.Y. and S.C.; writing—review and editing, Y.E.R., N.Z. and S.A.; visualization, Y.E.R.; supervision, Y.E.R., N.Z. and S.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Authors are very thankful for the Higher Center of Research of USEK for their support to this project.

**Conflicts of Interest:** The authors declare no conflicts of interest.

## Abbreviations

The following abbreviations are used in this manuscript:

<table><tr><td>AC</td><td>Activated Carbon</td></tr><tr><td>AMO</td><td>Alkaline Earth Metal Oxides</td></tr><tr><td>ANN</td><td>Artificial Neural Network</td></tr><tr><td>BET</td><td>Brunauer–Emmett–Teller</td></tr><tr><td>BJH</td><td>Barrett–Joyner</td></tr><tr><td>CELO</td><td>Crude Euphorbia lathyris seed oil</td></tr><tr><td>CI</td><td>Compression Ignition</td></tr><tr><td>CPO</td><td>Ceiba Pentandra Oil</td></tr><tr><td>CPO</td><td>Crude Palm Oil</td></tr><tr><td>DMC</td><td>Dimethyl Carbonate</td></tr><tr><td>FAME</td><td>Fatty Acid Methyl Ester</td></tr><tr><td>FFA</td><td>Free Fatty Acid</td></tr><tr><td>GO</td><td>Graphene Oxide</td></tr><tr><td>HPA</td><td>Heteropoly Acid</td></tr><tr><td>IL</td><td>Ionic Liquid</td></tr><tr><td>KI</td><td>Potassium Iodide</td></tr><tr><td>LDH</td><td>Layered Double Hydroxide</td></tr><tr><td>LDO</td><td>Layered Double Oxide</td></tr><tr><td>LGCPO</td><td>Low-Grade Crude Palm Oil</td></tr><tr><td>MATP</td><td>Microwave-Assisted Transesterification Process</td></tr><tr><td>MCM</td><td>Mobil Composition of Matter</td></tr><tr><td>MFL</td><td>Mesuaferrea Linn</td></tr><tr><td>MNP</td><td>Magnetic Nanoparticles</td></tr><tr><td>MOF</td><td>Metal–Organic Framework</td></tr><tr><td>MW</td><td>Microwave</td></tr><tr><td>NBPA</td><td>Nendran Banan Peduncle Ash</td></tr><tr><td>OA</td><td>Oleic Acid</td></tr><tr><td>PBPS</td><td>Poovan Banan Pseudostem</td></tr><tr><td>PCC</td><td>Pasteur Culture Collection of Cyanobacteria</td></tr><tr><td>PEG</td><td>Polyethylene Glycols</td></tr><tr><td>PFAD</td><td>Palm Fatty Acid Distillate</td></tr><tr><td>SBA</td><td>Santa Barbara Amorphous</td></tr><tr><td>SBC</td><td>Spent Bleaching Clay</td></tr><tr><td>SEM</td><td>Scanning Electron Microscopy</td></tr><tr><td>SL</td><td>Sulfonated Lignin</td></tr><tr><td>SPW</td><td>Sulfonated Sago Pith Waste</td></tr><tr><td>SSA</td><td>Specific Surface Area</td></tr><tr><td>TCM</td><td>Tubular Carbon Membrane</td></tr><tr><td>TG</td><td>Triglyceride</td></tr><tr><td>TGA</td><td>Thermogravimetric Analysis</td></tr></table>
<table><tr><td>TPA</td><td>Tungstophosphoric Acid</td></tr><tr><td>WCO</td><td>Waste Cooking Oil</td></tr><tr><td>WEO</td><td>Waste Edible Oil</td></tr><tr><td>WPO</td><td>Waste Palm Oil</td></tr><tr><td>WSO</td><td>Waste Sunflower Oil</td></tr><tr><td>Wt</td><td>Weight</td></tr><tr><td>w/v</td><td>Weight in Volume</td></tr><tr><td>w/w</td><td>Weigh in Weight</td></tr><tr><td>XPS</td><td>X-Ray Photoelectron Spectroscopy</td></tr><tr><td>XRD</td><td>X-Ray Diffraction</td></tr><tr><td>ZSO</td><td>Zanthoxylum bungeanum Seed Oil</td></tr></table>

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