State of the art of biodiesel production processes: a review of the heterogeneous catalyst

Abstract

A broadened focus on energy, the fast growing value of petroleum oil, harmful atmospheric emissions because of the evolution of greenhouse gases, natural contamination, and quick reduction approaches to obtain fossil fuels are critical factors in the search for alternative energy sources. The need for developing renewable energy sources with fewer environmental effects is increasing because of the problems caused by the extensive use of fossil fuels. Currently, creating energy from low-carbon origins and introducing eco-friendly modern technology are the main targets of researchers in the field. Biodiesel has been identified as an alternative renewable liquid fuel source that can be derived through thermal cracking, esterification and transesterification of different triglycerides. Among these processes, the most popular and convenient technique for biodiesel production is the transesterification of triglyceride with the help of a suitable alcohol and a catalyst. Many scientists have introduced different types of catalysts to optimize the reaction conditions and the biodiesel production yields. Catalyst selection involves the determination of the water content and the free fatty acids in the oil. Homogeneous base catalysts provide faster reaction rates than homogeneous acid catalysts. Recently researchers have paid attention to heterogeneous catalysts because of their high activity, high selectivity, catalyst recovery, reusability, easy separation from the products, and water tolerance properties. Biocatalysts present significant advantages in terms of environmental issues over conventional alkali-catalyzed processes. This review article focuses on various technologies used for biodiesel production, as well as the benefits and limitations of the different types of catalysts in the relevant production technologies. We also conduct a comparative study of biocatalysts and homogeneous and heterogeneous catalysts in biodiesel production technologies at the laboratory scale, as well as their industrial applications.

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1. Introduction

The constantly growing expense of traditional fossil fuels and the related natural effects of their use are significant concerns worldwide. The use of fuel increases daily given the high demand for energy. This demand is mostly met by fossil fuels such as coal, petroleum oils, and natural gas. Renewable sources are the most preferred alternative energy sources. Energy can be extracted from fossil fuels economically as well as in large quantities. Thus, scientists worldwide are working to enhance the development of the fuel mileage and the emission quality of internal combustion (IC) engines. Researchers have also concentrated on investigating alternative energy sources that can be utilized in IC engines without demonstrative changes in vehicle design. Low carbon sources are preferred for the production of energy and represent an eco-friendly green technology.

Considering the increase in global population, additional resources are needed to deliver energy for human consumption. To fulfill additional demands, researchers are considering renewable energy. The term biodiesel implies the substitution of traditional energy sources with an inexhaustible liquid fuel, which can be obtained from triglycerides and supplements the additional requirements of conventional petroleum diesel.¹ Biodiesel has recently become a popular research field because of its renewability, biodegradability, nontoxicity, and carbon neutrality. The transesterification process proceeds by adding triglyceride to methanol, ethanol, or any desirable alcohol, which is used to develop biodiesel.^2–4 Biodiesel can also be produced domestically from vegetable oils, animal fats, micro and macro algal oil⁵ or used cooking oil. The biodiesel produced can be treated as a cleaner-burning substitute for conventional petroleum diesel. Biodiesel contains mono alkyl esters of long chain fatty acids⁶ as well as methyl esters. These methyl esters include plant seed oils, animal fats, or even waste cooking oils and are produced by transesterification with methanol. Acids, bases, and enzymes catalyze the transesterification reactions. Heterogeneous catalysts are promising materials for the synthesis of biodiesel from different feedstocks.⁷ Biodiesel can fulfill the requirements for additional energy. Biodiesel is an interesting product because of its natural advantages and production from renewable assets. Unfortunately, high costs and the constrained accessibility of fat and oil resources limit the wider use of this alternative energy source. Biodiesel cost may be viewed from two aspects: the cost of the raw materials (fats and oils) and the processing cost. The expense of the crude materials represents 60–75% of the cost of aggregate biodiesel fuel.⁸ Utilization of waste cooking oil may significantly reduce the cost of the raw materials but also reduces the fuel quality.⁹ Studies are expected to discover a less expensive approach to utilize used cooking oils to produce biodiesel fuel. A continuous transesterification procedure may allow the reduction of the production cost associated with biodiesel. Feedstock obtainability, types, conversion technology, catalyst use, and process cost contribute to the total biodiesel production expenses.⁵

This review provides knowledge on the different biodiesel production technologies using different catalytic and non-catalytic processes. The use of biocatalysts to synthesize biodiesel is also discussed, and a comparative study of the conventional biodiesel production processes is performed. In addition to the catalyst preparation, the roles and effects of the different categories of heterogeneous and homogeneous catalysts on the esterification process of biodiesel production are described. This study also describes the advantages of non-catalytic biodiesel production processes over catalytic processes.

2. Biodiesel and its production processes

Several techniques are available to obtain biodiesel from different feedstocks. For example, transesterification via radio frequency, microwave, ultra-sonication, alcohol reflex temperatures, and alcohol supercritical temperatures are possible. All biodiesel production techniques can be classified as pyrolysis or thermal cracking, esterification, or transesterification processes.

Petro diesel, widely known as diesel, is obtained through the fractional distillation of crude oil. Petro diesel contains hydrocarbon molecules that range in size from 8 to 21 carbon atoms. An ordinary petro diesel containing 16 carbon atoms is presented in Fig. 1(a). A petro diesel molecule is composed of a pure hydrocarbon, that is, a molecule containing only hydrogen and carbon, with no oxygen molecules. Thus, after proper burning in atmospheric air, only CO₂ and H₂O are released from this molecule. Sometimes, hydrogen sulfide (H₂S) is also produced because of the presence of sulfur (S) in diesel. Typically, biodiesels contain long chain carbon molecules with hydrogen atoms, similar to petro diesel, with an additional ester functional group (–COOR). Biodiesel with 17 or 16 carbons with an ester group is illustrated in Fig. 1(b). Vegetable oil also typically contains long rows of carbon and hydrogen atoms with ester functional groups. Vegetable oil molecules are almost three times larger than normal diesel molecules. This large-sized structure is known as a triglyceride. The atomic size and structure of vegetable oil make it gel in cold weather, which means its direct use in engines is difficult. The ordinary atomic structure of vegetable oil is shown in Fig. 1(c).

Fig. 1 Molecular structure of (a) petroleum diesel (b) biodiesel and (c) vegetable oil.

Triglycerides are initially reduced to diglycerides, which are then reduced to monoglycerides. Monoglycerides are finally reduced to fatty acid esters. The gradual reaction mechanism producing the monoglycerides from triglycerides¹⁰ or vegetable oils is shown in Fig. 2, where R represents an alkyl group; R¹, R², and R³ are the fatty acid chains; and k₁, k₂, k₃, k₄, k₅, and k₆ represent the catalysts.

Fig. 2 Step by step triglyceride to monoglyceride production of vegetable oils.

Biodiesel can be produced in single or two-step reactions depending on the feedstock quality, for example, the water content and if they contain FFAs. Higher FFA content indicates a more acidic feedstock. Crude oil with a high acid value is first esterified with an acid catalyst and then transesterified with a suitable base catalyst. However, a large amount of wastewater is associated with this technique when a homogeneous catalyst used; thus, the process presents some harm to the environment. Laboratory-scale heterogeneous and bio-catalytic processes can minimize this problem. A flow diagram of the laboratory-scale or fixed-bed or single-step transesterification biodiesel production process is presented in Fig. 3(a). Researchers have attempted to utilize not only the main oil sources (such as seeds) but also the dry waste shell of the seeds. Other value-added products associated with biodiesel production have attracted research attention.

Fig. 3 (a) Biodiesel production flow chart for laboratory-scale processes (single step). (b) Biodiesel production flow chart for industrial scale processes (continuous).

Maiti et al.¹¹ introduced an integrated system where power was generated from dry jatropha seed shells through gasification. Then, utilizing the gasified producer gas, power was generated with the help of a producer gas engine. This electric power was partially utilized as an energy source in different steps of the integrated biodiesel production system from Jatropha curcas seeds. The relevant steps include screw pressing, oil refining, transesterification, glycerol purification, and soap making. The authors found that 8 h of continuous operation of the gasification process with a 64.8% efficiency can generate 10 kW of captive power with a 24.5% efficiency; here, the producer gas heating value was considered to be 5.2 MJ m⁻³ and the calorific value of the empty shells was 17.2 MJ kg⁻¹. Ghosh et al.¹² proposed an integrated process to produce oil-bearing Chlorella variabilis for lipid extrication utilizing a by-product of jatropha methyl ester production. Later, Ghosh et al.¹³ described an improved and integrated process to prepare fatty acid methyl ester (FAME) from whole seeds of J. curcas with the least energy use and zero effluent discharge; crude glycerol utilization was also integrated in this work. An integrated continuous Jatropha curcas biodiesel production process with its other value added product recovery techniques is represented in Fig. 3(b).

The industrially produced pure biodiesel is not exactly the same as petroleum diesel, with a small variation in some properties. The properties of petroleum diesel and the produced biodiesel according to the American Society for Testing and Materials and the European Standards are shown in Table 1.

Table 1 Comparison of the properties of biodiesel and petroleum diesel based on the ASTM and European Standards (EN)

Property	Units	Diesel		Biodiesel (B100)
		ASTM D975	EN 590	ASTM D6751	EN 14214
Density @ 15 °C (59 °F)	kg m^−3	850	835	800–900	860–900
Kinematic viscosity @ 40 °C	mm^2 s^−1	1.3–4.1	3.5	1.9–6.0	3.5–5.0
Lower heating value	kJ kg^−1	—	43 000	—	—
Cetane no.	—	40–55	53	48–65	Min. 51
Specific gravity @ 15 °C	g cm^−3	0.85	—	0.88	—
Carbon	% mass	87	—	77	—
Hydrogen	% mass	13	—	12	—
Oxygen, by dif.	% mass	0	0	11	—
Sulfur content	ppm	500	Max. 10 (mg kg^−1)	Max. 0.05	Max. 0.0010
Boiling point	°C	180 to 340	—	315 to 350	—
Flash point	°C	60 to 80	Min. 55	100 to 170	Min. 120
Cloud point	°C	−15 to 5	−5	−3 to 12	—
Pour point	°C	−35 to −15	—	−15 to 16	—
Cold flow plugging point	°C	—	—	Max. +5	—
Lubricity (HFRR)	μm	300–600	—	Max. 300	—
Water content	mg kg^−1	—	—	—	Max. 500
Acid value	mg KOH per g	—	—	Max. 0.80	Max. 0.50

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2.1. Prior to biodiesel production

2.1.1. Pyrolysis (thermal cracking) of oils. Pyrolysis is the shifting or transformation of one substance to another by employing heat. A catalyst is introduced into the process to minimize the conversion time. In other words, pyrolysis refers to the transformation of one organic material into another by thermal decomposition with the presence of a desired catalyst and the absence of air or oxygen.¹⁴ Vegetable oil, animal fats, natural fatty acids, or the methyl ester of fatty acids may be used as pyrolysis material. The transformation of vegetable oil and animal fat into a feedstock capable of producing biodiesel is a potential technology using a thermal cracking reaction. This innovation is particularly encouraging in areas where the hydro preparing industry is entrenched because this innovation is fundamentally the same as that of traditional petroleum refining.¹⁵ Dissimilar lower hydrocarbons are obtained by the pyrolysis of vegetable oil, which could be used as fuel. The properties of treated fuel made via pyrolytic syntheses and derived from vegetable oil are very similar to those of diesel fuel. Thus, numerous scientists have stated that this fuel is a suitable alternative for diesel.^15–18 Based on its operating conditions, the pyrolysis process can be classified¹⁹ as conventional pyrolysis, fast pyrolysis, or flash pyrolysis. The mechanism of the thermal decomposition of triglycerides is prone to complexity because of the various possible reactions and chemical structures, which depend upon the reaction conditions or pathway. A simple systematic reaction mechanism of the thermal decomposition of triglycerides is presented in Fig. 4.^18,20 Scientists have performed experiments on soybean, palm, and castor oils to determine the optimum distillation temperature, thereby obtaining fuel properties similar to those of petroleum-based fuels.¹⁹ Compared to the other conventional processes with respect to the yield, the equipment used for thermal cracking and pyrolysis is costly. By contrast, this process sometimes produces low-value materials and more gasoline than diesel fuel.²¹

Fig. 4 The mechanism of the thermal decomposition of triglycerides.

Nevertheless, the pyrolysis process requires high temperatures ranging from approximately 300 °C to 500 °C and product characterization is difficult because of the differences in the reaction pathway and the products acquired from the reactants.¹⁴ Lima et al.¹⁸ employed zeolite as a catalyst during the pyrolysis of soybean oil and found that the reaction temperature was approximately 400 °C under N₂ flow. The obtained products were olefins, paraffin, carboxylic acids, and aldehydes. Şensöz et al.²² inspected the effect of particle size on the pyrolysis of rapeseed. By changing the particle size of rapeseed over the range of 0.224–1.8 mm, they observed that the yields of the products are not dependent on the particle size. More than 30 compounds were detected from the pyrolysis of macauba fruit, and the amount of the main products diminished by increasing the pyrolysis temperature.²³

2.2. Esterification and transesterification processes

An esterification reaction is one where an ester is produced from one or two other organic substances. The most common method to produce esters is by chemically reacting an organic acid (carboxylic acid) and an alcohol (methanol) with the help of an acid catalyst. The general esterification reaction is shown in Fig. 5, where R represents small alkyl groups and R¹ denotes fatty acid chains.

Fig. 5 General esterification reaction.

Esterification can be performed on vegetable oils or animal fats (triglycerides) with methanol or ethanol (short chain alcohols) to produce biodiesel, especially where considerable quantities of free fatty acids (FFAs) are present. These include byproducts of waste oils, non-edible oils, animal oils, and refined vegetable oils. Such oils possess considerable quantities of saturated fatty acids, specifically stearic acid (IUPAC name: octadecanoic acid), which contains 18 carbon atoms. In some cases, the homogenous acid-catalyzed reaction is not viable because it may produce corrosion and environmental problems. By contrast, heterogeneous reactions do not show corrosive behavior. In addition, heterogeneous reactions are easier to use for splitting products, diminishing wastewater quantities, and lowering process instrumentation, expenses, time, and environmental effects. Thus, the heterogeneous acid-catalyzed reaction is preferred for esterification reactions. Such catalysis plays a significant role in producing cleaner and more profitable biodiesel by esterification. Thus chemical processes employing heterogeneous catalysts are the most acceptable to researchers for creating biodiesel by esterification.²⁴

Creating biodiesel from the transesterification reaction with the help of a catalyst is a highly favored method. The transesterification reaction for biodiesel production can be performed using various methods and is broadly described as the addition of an alcohol (generally methanol or ethanol) to lipids (vegetable oil, algal oil or animal fats) in the presence of a catalyst (acid or base).²⁵ Outlines of the transesterification reactions for fatty acid methyl esters (FAMEs) and fatty acid ethyl esters (FAEEs) are shown in Fig. 6(a) and (b), respectively, where R¹, R², and R³ represent mixtures of long fatty acid chains.²⁶

Fig. 6 Chemistry of the transesterification reaction of triglycerides for biodiesel production: using (a) FAMEs, and (b) FAEEs.

2.2.1. Microwave-assisted transesterification technology. Microwave irradiation or microwave-assisted transesterification is a technique where an electromagnetic microwave is used to heat up the system. Electromagnetic wave frequencies ranging from 0.3 GHz to 300 GHz, which are relatively low in the electromagnetic spectrum, are used to produce energy.²⁷ During microwave irradiation, the bonds are neither formed nor broken. However, the energy is rapidly transferred to the sample. A microwave is integrated into the process after the mixture (methanol, feedstock, and catalyst) chamber. Utilization of electromagnetic microwaves has drawn significant attention^28–31 because these waves present some advantages over conventional heating in transesterification. A simple schematic of the microwave-assisted transesterification process²⁹ is shown in Fig. 7. A faster and uniform heat distribution, higher yields of a cleaner product, less energy consumption, and the reduction of the catalyst to methanol ratio are among the attractive features of this technique.

Fig. 7 Microwave-assisted transesterification process.

2.2.2. Ultrasonic technology. Ultrasonic technologies are typically used in a variety of biological and chemical reactions to enhance the yield within a shorter reaction time. Ultrasonic technologies are an effective method to enhance the mass transfer rate between immiscible liquid–liquid phases within a heterogeneous catalyst.³² Generally, the audible range of human beings lies between 16 and 18 kHz,³³ whereas the ultrasonic sound range lies between 20 kHz and 100 MHz. The principle of ultrasound action in biodiesel production is primarily based on the emulsification of immiscible liquid reactants by micro-turbulence generated by the radial motion of cavitation bubbles. Molecules in the medium continuously vibrate and create cavities by compressing and stretching the molecular spacing of the medium, which is developed by a high-frequency sound wave. As a result, micro fine bubbles are formed by the sudden expansion and violent collapse, generating energy for chemical and mechanical effects.³⁴ This process allows a short reaction time and high yield because of the cavitation of the liquid–liquid immiscible system.³² Maneechakr et al.³⁵ stated that ultrasonic-assisted transesterification shortens the reaction time and minimizes the molar ratio of alcohol to oil, and reduces the energy consumption compared with the conventional mechanical stirring method. According to Lee et al.³⁶ ultrasonic irradiation reduces reaction times by at least 30 min compared with the conventional methods and produces the highest biodiesel yields.

3. Catalysts used for biodiesel production

At present, biodiesel is produced using soybean oil in the U.S.; rapeseed, sunflower, or soybean oils in the EU; and palm oil in Southeast Asia. Food versus fuel concerns have prompted the investigation of non-consumable oil feedstocks. The top Asian countries in biodiesel generation, such as the Philippines and Malaysia, also exploit edible oil (coconut and palm oil, respectively). Other edible raw materials, such as sunflower, peanut, camelina and linseed oil, are also utilized by some developing countries. The breakdown of crude materials used for biodiesel generation worldwide is 84% rapeseed oil, 13% sunflower oil, 1% palm oil and 2% others. The principal crude material utilized in India is jatropha, a non-edible oil.¹⁸ Determination of which oil is used for biodiesel generation depends on its availability, characteristics, and price.^17,18 Some edible and non-edible feedstocks for biodiesel production and their properties are listed in Tables 2 and 3.

Table 2 Feedstocks and their oil yield for biodiesel production

Crops		Algae
Name	Oil yield (%)	Species	Oil yield (%)
Babassu oil (Attalea speciosa)	60–70	Botryococcus bruanii	25–75
Borage oil (Borago officinalis)	20	Chlorella sp.	28–32
Camelina oil (C. sativa)	38–40	Crypthecodinium cohni	20
Castor oil (Ricinus communis)	45–50	Cylindrotheca sp.	16–37
Cuphea oil (Cuphea viscosissima)	25–43	Nitzschia sp.	45–47
Hemp oil (Cannabis sativa)	33	Phaeodactylum tricornutu	20–30
Jatropha oil (J. curcas)	45	Schizochytrium sp.	50–77
Jojoba oil (Simmondsia chinensis)	44	Tetraselmis suecica	15–23
Karanja oil (P. pinnata)	27–39	Isochrysis galbana	30–41
Linseed oil (Linum usitatissimum)	37–42	Pavlova lutheri	35.5
Neem oil (Azadirachta indica)	40–50	Nannochloropsis sp.	31–68

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Table 3 Some edible and non-edible sources (feedstocks) for biodiesel production and their properties

	Feedstock	Density, g cm^−3	Flash point, °C	Acid value, mg KOH per g	Heating value, MJ kg^−1	Viscosity, mm^2 s^−1 at 40 °C	Cloud point, °C	Peroxide value	Pour point, °C	Cetane no.	References
Edible oils	Soybean	0.910	254.0	0.20	39.60	32.60	2	44.5	−12.2	48.00	20,37–39
	Rapeseed	0.910	246.0	2.92	39.70	37.00	—	30.2	—	—	35, 36 and 40
	Sunflower	0.920	274.0	0.15	39.60	33.90	—	10.7	—	—	35, 36 and 40
	Palm	0.920	267.0	0.10	39.90	36.00	12	—	13	61.15	35, 40 and 41
	Peanut	0.900	271.0	3.00	39.80	39.60	—	82.7	—	—	35, 36 and 40
	Corn	0.910	277.0	0.11	39.50	—	—	18.4	—	—	35 and 36
	Cotton	0.910	234.0	—	39.50	—	—	64.8	—	—	35
	Moringa	0.859	176.0	0.19	40.11	5.074	21	—	19	67.07	42 and 43
	Calophyllum	0.877	162.5	0.30	39.51	5.538	12	—	13	57.30	42, 44 and 45
	Coconut	0.860	118.5	0.11	38.30	3.144	1	—	−4	59.00	42 and 46
	Aphanamixis polystachya	0.873	188.5	0.45	39.96	4.718	8	—	8	—	42, 45 and 47
	Rice bran	0.868	174.5	0.59	39.96	5.366	0	—	−3	73.60	42, 45 and 48
	Neem	0.868	120	0.65	39.81	3.700	9	—	2	48–53	45, 48 and 49
	Sesame	0.884	208.5	0.30	39.99	4.399	1	—	1	50.48	50 and 51
Non edible oils	Jatropha curcas	0.920	225.0	28.0	38.50	—	—	—	10	57.10	35 and 42
	Pongamina pinnata	0.910	205.0	5.06	34.00	—	—	—	—	—	35
	Palanga	0.900	221.0	44.0	39.25	—	—	—	—	—	35
	Tallow	0.920	—	—	40.05	—	—	—	—	—	35
	Poultry	0.900	—	—	39.40	—	—	—	—	—	35
	Used cooking oil	0.900	—	2.50	—	—	—	—	—	—	35

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Several catalysts are associated with biodiesel production technology. Based on previous reviews^5,52–55 catalysts can be classified as homogeneous catalysts, heterogeneous catalysts, and biocatalysts; these catalyst types, including their sub-types are listed in Fig. 8.

Fig. 8 Catalysts used in biodiesel production technology.

3.1. Homogeneous catalysts

Homogeneous catalysis involves a sequence of reactions that includes a catalyst from the same phase as the reactants. The term phase in this article refers to solid, liquid and gas. Most often, a homogeneous catalyst is dissolved or co-dissolved in the solvent with all the reactants. Sodium hydroxide (NaOH) or potassium hydroxide (KOH) are currently the most popular homogeneous catalysts for biodiesel production.⁵⁶ However, some researchers suggest that these homogeneous base catalysts are only suitable for feedstocks with a low FFA content. If the FFA content is larger than 6 wt%, the base catalyst process is unsuitable for biodiesel synthesis.⁵⁷ Thus, some scientists recommend that the FFA content should be less than 2 wt%.^21,58–60 This homogeneous base catalyst is gradually gaining popularity in industrial biodiesel production for the following reasons:

(a) A low reaction temperature is needed to synthesize biodiesel at atmospheric pressure.

(b) A high biodiesel yield could be possible at optimal conditions.

(c) Widely available and economical.

(d) Handling the reactants and catalyst is easier than one solid or a combination.

Although homogeneous catalysts present several advantages, they also feature some drawbacks. The limitations of using a homogeneous catalyst for biodiesel synthesis are listed below:

(a) Water generation throughout the acid esterification retards the process. Proper care should be taken to remove water via evaporation or chemical drying, which adds expenses to biodiesel production.

(b) Although instances of reusing catalysts have been reported, this method is almost never witnessed if ever completed over a production scale because of the associated expenditures.

(c) Corrosive nature of the catalysts concerned. The preferred acid (H₂SO₄) and base (SMO) catalysts presently employed in biodiesel synthesis are corrosive and must be handled conservatively.

3.2. Heterogeneous catalysts

Catalysts with a phase or state different from that of the reactants are considered to be heterogeneous catalysts. A heterogeneous catalyst is often a practical material that regularly creates active sites with its reactants under the reaction atmosphere. The application of a heterogeneous catalyst will result in simpler and less costly separation processes, as well as additional capital and energy costs. The disadvantages of heterogeneous catalysts include elevated temperatures and higher oil to alcohol ratios than those required in the homogeneous catalytic procedure. Some of these catalysts have demonstrated good performance even under the reaction conditions employed for homogeneous catalysts.^61,62 The separation, purification, and reusability of the catalyst are among the more attractive features of the heterogeneous catalytic process. Carbonates and hydrocarbonates of alkaline metals, alkaline metal oxides, alkaline metal hydroxide anionic resins, and basic zeolites may be used as heterogeneous catalysts.

Dossin et al.⁶³ demonstrated the heterogeneous catalyst transesterification process employing MgO as a catalyst. About 100 000 tons of biodiesel were generated every year by this process from a triolin feedstock. The remarkable heterogeneous procedure known as the Esterfip-H technology, which was developed by the Institute Français du Petrole (IFP) in 2006, delivers 160 000 tons of biodiesel every year.⁶⁴ The role and utilization of base and acid heterogeneous catalysts for biodiesel synthesis has been broadly described in the literature.

3.2.1. Acid heterogeneous catalysts for biodiesel production. Acid catalysts have a corrosive but less toxic effect and create few environmental problems.⁶⁵ This type of catalyst is more popular for both esterification and transesterification reactions when producing biodiesel from low quality feedstock.⁶⁶ Transesterification is catalyzed by Bronsted acids. The most ordinarily employed acid catalyst is H₂SO₄. Broadly, heterogeneous acid catalysts can be divided into two types: the low temperature type and the high temperature type;⁶⁷ catalysts of these types show better activities at low and high temperatures, respectively. While these catalysts provide exceptional returns in alkyl esters, their rates of reaction are moderate, demanding reaction times longer than 3 h and temperatures higher than 100 °C to achieve complete transformation.⁶⁸ The basic mechanism of an acid catalyst is shown in Fig. 9. Investigations using a heterogeneous acid catalyst to produce biodiesel are discussed in the next paragraph.

Fig. 9 Basic reaction mechanism of an acid catalytic process.

3.2.1.1. ZrO₂ as an acid heterogeneous catalyst. Zirconium dioxide (ZrO₂), sometimes referred to as zirconia, is used as an acid heterogeneous catalyst by many researchers. This substance has a strong surface acidity. Thus, many investigators often choose ZrO₂, sulfated ZrO₂,⁶⁹ and other modified metal oxides with sulfated ZrO₂ as acid heterogeneous catalysts.⁷⁰

According to Kiss et al.,⁷¹ sulfated zirconia (SO₄²⁻/ZrO₂) shows the best performance as a heterogeneous catalyst for esterification among zeolites, ion exchange resins, and mixed metal oxides. Park et al.⁷² successfully performed the transesterification of vegetable oil to convert FFA to FAME using sulfated zirconia (SO₄²⁻/ZrO₂) and tungstated zirconia (WO₃/ZrO₂). However, tungstated zirconia (WO₃/ZrO₂) requires a longer reaction time. Approximately 140 h of reaction time and a 75 °C reaction temperature are needed to achieve only 65% conversion.

Implementation of tungstated zirconia-alumina (WZA) and sulfated zirconia-alumina (SZA) was assessed by Furuta et al.⁷³ during the transesterification of soybean oil with methanol. The reaction was performed under atmospheric pressure and at 200–300 °C with the help of a fixed bed reactor. The investigators showed that WZA has higher activity than SZA but did not elaborate on the reason behind this phenomenon.

Jitputti et al.⁷⁴ utilized both sulfated zirconia (SO₄²⁻/ZrO₂) and unsulfated zirconia (ZrO₂) for the transesterification of palm kernel oil and crude coconut oil with the help of methanol. They reported 90.3% and 86.3% methyl ester yields from palm kernel oil and crude coconut oil, respectively, while using sulfated zirconia. Only a 64.5% palm kernel yield and a 49.3% crude coconut oil yield was possible in the case of unsulfated zirconia. These results clearly indicate that a slight change in the metal oxide surface activity is a major parameter influencing the yield of methyl ester.

3.2.1.2. Cation-exchange resins as heterogeneous catalysts. Many researchers have extensively used cation-exchange resins for biodiesel production at the laboratory scale; this resin is unsuitable for industrial applications. Liang et al.⁷⁵ described an optimal operational condition to convert soybean oil to biodiesel with the help of chloroaluminate ([Et₃NH]Cl–AlCl₃). 5 g of soybean oil, 2.33 g of methanol, a reaction time of 9 h, and a reaction temperature of 70 °C were recorded as the optimal conditions for a 98.5% biodiesel yield. The main advantages of the use of such a catalyst are low cost, high yield of biodiesel production, reusability of catalyst, no need for saponification, and simplicity of operation.

Some investigators have attempted to use a poly(DVB) resin sulfated with H₂SO₄, Amberlyst-35 (Rohm & Haas),⁷⁶ Amberlyst-15 (Rohm & Haas),⁷⁶ Amberlyst 15 DRY⁷⁷ and Nafion SAC-13 (ref. 78) as sulfonic acid ionic exchange resins. Limited functionality with a significantly high ratio of oil to alcohol is one of the major shortcomings of this type of catalyst. By using an organically functionalized acid catalyst, the key shortcomings of the aforementioned catalysts, including leaching and low surface areas, could be solved.⁷⁹

3.2.1.3. Solid heteropoly acids as heterogeneous catalysts. Heteropoly acid (HPA) catalysts are another low temperature heterogeneous catalyst utilized by Chai et al.⁸⁰ for high-quality biodiesel production. Narasimharao et al.⁸¹ studied insoluble HPA salts for solid acid esterification and transesterification processes; the general formula of these salts is y Cs_xH_3−xPW₁₂O₄₀, where x varies from 0.9 to 3. A Cs_2.5H_0.5PW₁₂O₄₀ catalyst could be used in simultaneous esterification and transesterification reactions without losing activity and selectivity. Hamad et al.⁸² introduced H₃PW₁₂O₄₀/SiO₂ and Cs₂HPW₁₂O₄₀ as heterogeneous HPAs for the transesterification of rapeseed oil with the help of ethanol. The strengths of the acid sites are measured in terms of the heating value.

Zhang et al.⁶⁵ studied the maximum yield of FAMEs using Cs_2.5H_0.5PW₁₂O₄₀ as a heterogeneous HPA for microwave-assisted transesterification to produce biodiesel from yellow horn (Xanthoceras sorbifolia Bunge) oil. The purpose of this study was to optimize the reaction temperature, reaction time, methanol to oil molar ratio, amount of catalyst, and catalyst reusability cycle. An approximately 96.22% FAME yield was achieved under optimal conditions. A reaction temperature of 70 °C, reaction time of 10 min, methanol to oil molar ratio of 12 : 1, and catalyst loading of 1% wt were recorded as optimal conditions and the catalyst could be reused a minimum of nine times.

3.2.1.4. Zeolites as acid heterogeneous catalysts. Zeolites are crystalline solid structures made of silicon, aluminum, and oxygen that frame a structure with holes and channels inside where cations, water, and/or little atoms may dwell. Zeolites have unique properties as catalysts, including acidic strength and shape selectivity. The catalytic properties vary because of the porous structure. The internal crystal structure and surface properties of a zeolite can suit a wide assortment of cations, such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ that ascribe to its simple nature. Approximately 191 exceptional zeolite frameworks have been identified.⁸³ About 40 naturally occurring zeolite frameworks are known. An improved zeolite (using La) called zeolite beta (La/zeolite beta) was studied by Shu et al.⁸⁴ as a heterogeneous acid catalyst for the methanolysis of soybean oil. This catalyst was synthesized using an ion exchange process through a suspension of zeolite beta in a lanthanum nitrate La(NO₃)₃ aqueous solution under high mixing at ambient temperature for 3 h and drying at 100 °C for 24 h. Then, the catalyst is calcined at 250 °C for 4 h. The author recorded only a 48.9 wt% triglyceride conversion using this catalyst. Karmee et al.⁸⁵ studied Hβ-zeolite, montmorillonite K-10, and ZnO as heterogeneous catalysts for the transesterification of non-edible Pongammia pinnata under reaction conditions of a 1 : 10 oil to methanol ratio, 0.115 wt% catalytic loading, 120 °C temperature, and 24 h reaction time. Yields of 59%, 47%, and 83%, respectively, were recorded. Suppes et al.⁸⁶ examined a biodiesel synthesis process where zeolites were used as the prospective acid heterogeneous catalyst. They used several types of zeolites and metals to examine the potential of zeolites during the transesterification of soybean oil possessing a 2.6 wt% FFA content. Xie et al. studied⁴ NaX zeolites (Si/Al = 1.23) as heterogeneous catalysts after improving the strength from <9.3 to 15.0–18.4. A yield of 85.6% was recorded under a 10% catalytic loading, 125 °C reaction temperature, and 2 h of reaction time. Ramos et al.⁸⁷ used zeolites (mordenite, beta, and X) to convert crude sunflower oil to FAME and recorded a 93.5–95.1% FAME yield at 60 °C. However, the time required to prepare the catalyst was relatively long because the process required heating at 500 °C for 10 h, drying at 120 °C for 12 h, and calcining at 550 °C for 15 h.

Zeolites can sometimes be used as a base heterogeneous catalyst. Marchetti et al.⁸⁸ used basic NaY zeolites and VO_X over ultrastable acidic Y zeolites as heterogeneous catalysts for the conversion of FFA to FEME. At 300 °C, the conversion times of FFA oil to biodiesel using these zeolites were only 10 min and 50 min, respectively. During the reaction, the presence of H₂O initially accelerated the reaction. However, when the reaction proceeded further, the H₂O decelerated the reaction. Thus, the overall biodiesel conversion rate was hampered.

Though acid heterogeneous catalysts are promising to biodiesel production technology, they also have some drawbacks for the production process. The advantages and disadvantages of acid heterogeneous catalysts are discussed in Table 4.

Table 4 Benefits and limitations of acid heterogeneous catalysts for biodiesel production

Catalyst used	Reaction parameters	Benefits and limitations	Ref.
Tungstated zirconia (WO3/ZrO2)	Reaction time 140 h, reaction temperature 75 °C and yield 65%	Long reaction times	66
Sulfated zirconia (SO4^2−/ZrO2)	Palm kernel oil yield 90.3%, coconut oil yield 86.3% under optimal conditions	Shorter reaction time than tungstated zirconia	66 and 79
Unsulfated zirconia (ZrO2)	Palm kernel oil yield 64.5%, coconut oil yield 49.3% under optimal conditions	Very poor biodiesel yield compared to others	79
Tungstated zircona-alumina (WZA)	Atmospheric pressure, reaction temperature 200–300 °C, with fixed bed reactor	Relatively higher activity than sulfated zirconia-alumina	67
Sulfated zirconia-alumina (SZA)	Atmospheric pressure, reaction temperature 200–300 °C, with fixed bed reactor	Relatively lower activity than tungstated zircona-alumina	67
Chloroaluminate ([Et3NH]Cl–AlCl3)	Soybean oil yield 98.5%, reaction time 9 h, reaction temperature 70 °C	Reaction time is quite high though yield is good	69
Heterogeneous HPA (Cs2.5H0.5PW12O40)	Reaction temperature 70 °C, reaction time 10 min, 12 : 1 methanol to oil ratio, catalyst loading 1% wt from yellow horn	Minimum time required with a high oil to alcohol ratio and the catalyst can be reused nine times	65
Sulfonic acid ionic exchange resins	—	High oil to alcohol ratio is needed	76​–78

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3.2.2. Base heterogeneous catalysts for biodiesel production. Currently, several solid base catalysts have been developed for biodiesel production, such as basic zeolites, alkaline earth metal oxides, and hydrotalcites. The most general catalysts, which are used as base heterogeneous catalysts, are the K/γ-Al₂O₃ catalyst,⁸⁹ the HTiO₂ hydrotalcite catalyst,⁹⁰ Ca and Zn mixed oxides,⁹¹ the calcium oxide (CaO) catalyst,^92,93 the MgO catalyst, Al₂O₃ supported CaO & MgO catalysts,³ the Li₄SiO₄ catalyst,⁹⁴ the Na₂SiO₃ catalyst,^95,96 alkaline earth metal oxides,⁹⁷ KF/Ca–Al,⁹⁸ basic zeolites, and alkali metal loaded alumina.⁹⁹ In addition, alkaline earth steel oxides, particularly CaO, have attracted much attention owing to their relatively higher strength, significantly low solubility in methanol, and synthesis from affordable resources such as limestone and calcium hydroxide.^100,101 Researchers recommend base heterogeneous-catalyzed transesterification for the following reasons: simplification of biodiesel production, simplification of the purification process, reduction in wastewater amount, lowering of the process and equipment costs, and reductions in environmental impact.^67,102 In addition to the ease of catalyst restoration, the action of the heterogeneous alkali catalyst may resemble that of its homogeneous counterpart under similar running conditions.¹⁰³ Transformation of the crude oil to biodiesel occurs in several steps. The systematic reaction mechanism of a simple base catalytic reaction is shown in Fig. 10.

Fig. 10 The reaction mechanism of base catalysis process.¹⁰⁴

3.2.2.1. CaO as a base heterogeneous catalyst. CaO is the most commonly exploited alkaline earth metal oxide for transesterification. FAME yields of nearly 98% were initially reported.¹⁰⁵ The ability to reuse the catalyst is a major topic of concern. The modification of CaO to a compound with an organic metallic nature, for example, Ca(OCH₃)₂ and Ca(C₃H₇O₃)₂, allows investigation of the reutilization function. Collected works on biodiesel state that an approximately 93% biodiesel yield may be obtained from a 20-cycle reaction. 95% Ca(C₃H₇O₃)₂/CaCO₃ has also been determined to be a capable heterogeneous catalyst with a reusability of 5 cycles and a good FAME yield.¹⁰⁶

The mechanics of transesterification introduced by Lam et al.⁵⁶ used CaO as a heterogeneous base catalyst. CaO was reacted with FFAs, and a certain amount of the catalyst was transformed into Ca soap by rejoining with the FFAs, causing limited catalyst recovery. As a basic standard of biodiesel, the concentration of the mineral matter should be less than 200 ppm. Kouzu et al.¹⁰⁷ determined a Ca concentration of 3065 ppm in the reaction products, thus exceeding this standard.

Some investigators noted that soluble matter can be removed by CaO throughout transesterification. CaO slightly dissolves in methanol, thus transforming into soluble calcium diglyceroxide, where CaO reacts with glycerin during the transesterification of soybean oil with methanol.^107​–109 Activated CaO is used to study the function of H₂O and CO₂ in the loss of catalytic performance in the presence of O₂ during the transesterification of sunflower oils.¹¹⁰ In the above studies, CaO was quickly hydrated and carbonated in the air. Activated CaO was affected because of the surface activity, as well as the absorption of CO₂ and H₂O on the surface area. If CaO is heated to 700 °C to eliminate the carbonate groups from the surface, then the catalytic action of CaO might be restored. However, filtering or removing the catalyst from the product was noticeable in the transesterification reaction. Considering its low solubility in methanol, CaO results in a high basic strength and fewer ecological effects. Furthermore, CaO can be produced from economical resources such as limestone and Ca(OH)₂.

KOH can be used for alternative of CaO. But the advantages of CaO over KOH include lower price, lower solubility, higher basicity, and easier handling. In actual practice, the reaction rate is unsatisfactory during the transesterification for relatively low activity.^109,111 However, reusable activity can be enhanced by washing¹⁰⁹ and improving the thermal activation treatment.¹⁰⁵ Being nano sized, CaO provides effective catalyst activity because of its high surface area. A surface structure of a metal oxide is presented in Fig. 11.

Fig. 11 Surface structure of metal oxides comparing its acidic and basic sites.

Biodiesel production from sunflower oil using a heterogeneous base catalyst (CaO) was conducted by Vujicic et al.¹¹² Experiments were accomplished using a commercial bench stirred tank reactor. The experiment was conducted using unchanged factors such as a reactor volume of 2 dm³, a stirring speed of 200 rpm, a 6 : 1 methanol to oil ratio, and 1 wt% of CaO catalyst loading. Temperature (60–120 °C), pressure (1–15 bar), and reaction time (1.5–5.5 h) effected the ester yields. The optimal transformation to methyl ester (almost 91%) was observed at 100 °C, although a positive effect of pressure (up to 10 bar) was observed at 80 °C. The founding of basic sites depends on the catalyst activation in air, which occurs at 900 °C. Catalyst corpuscle coalescence occurred during the reaction, which provided a gum-like construction and meaningful catalyst deactivation.

CaO was produced using a simple and flexible technique¹¹³ to raise the activity and to improve the properties of the calcined calcites through the hydration–dehydration method. This process prepares CaO that is extremely fit for biodiesel production. Newly prepared CaO has a relatively higher surface area and more basic sites than CaO obtained from the decomposition of calcite. With calcined calcite, the methyl ester content was enhanced to 93.9 wt% from 75.5 wt%. This study, developed through new hydration, delivers different important information on the influence of water on the properties and activities of CaO, and ensures a thermal disintegration technique for calcined calcite.

Bai et al.¹¹⁴ established a novel morphology for producing CaO simply and at low cost. The morphology produced possesses high catalytic activity in the transesterification reaction for biodiesel. A CaO microsphere with tiny holes was obtained by calcining a spherical CaCO₃ precursor prepared simply through the reaction of CaCl₂ and Na₂CO₃. During the transesterification of soybean oil, the CaO microsphere was employed, which possesses outstanding catalytic capability. An approximately 98.72% FAME yield was obtained.

3.2.2.2. BaO as a heterogeneous catalyst. Mootabadi et al.⁹⁷ conducted a palm oil transesterification experiment with the aid of alkaline earth metal oxide catalysts such as CaO, SrO, and BaO. The experiment was performed to observe the result of varying the reaction time (10–60 min), alcohol to palm oil molar ratio (3 : 1–15 : 1), and catalyst loading (0.5–3%) by creating 20 kHz ultrasonic cavitation and fluctuating ultrasonic amplitudes (25–100%). The natures of the catalysts were mostly dependent on their basic strengths. The activity ranking of the catalysts was CaO < SrO < BaO. In ideal circumstances, a 95% yield was reached with formal stirring within 60 minutes. Moreover, yields accomplished within 60 min for CaO, SrO, and BaO were 5.5–77.3%, 48.2–95.2% and 67.3–95.2%, respectively. Ultrasonic irradiation at a 50% amplitude was estimated as optimal, and the physical variations of the catalysts can be effectively explained after the ultrasonic-assisted reaction. The major reason for this activity drop for the recycled catalyst, dissolution, was investigated, especially for the BaO catalyst.
3.2.2.3. MgO as a base heterogeneous catalyst. Among the alkali earth materials, MgO is one of the widely used materials for transesterification.⁷⁰ López et al.¹¹⁵ studied MgO used as a base heterogeneous catalyst where an 18% yield of feedstock was achieved at a calcination temperature of 600 °C. The low surface area of the catalyst means that the reaction takes almost 8 h. MgO is used to analyze the catalytic activity to produce biodiesel. An experiment by Di Serio et al.^116,117 that used a 12 : 1 methanol to oil molar ratio and 5.0 wt% of the catalyst (MgO) gave a 92% biodiesel yield within 1 h. Another experiment stated¹¹⁸ that in a batch reactor, MgO worked efficiently and 500 tons of biodiesel production was achieved by transesterification at ambient temperature. In batch reactor biodiesel production, cost is minimal because of the temperature. Some researchers^116,119 stated that in supercritical conditions (300 °C) and at a high methanol to oil molar ratio (39.6 : 1), a MgO catalyst gives a 91% FAME yield.
3.2.2.4. SrO as a base heterogeneous catalyst. In solid base catalysis, although Ca and Mg are the more extensively used alkaline earth materials, strontium oxide (SrO) has also been used in biodiesel production. Using CO₂ temperature programmed desorption, pure SrO was confirmed to have the maximum basic strength,¹²⁰ which is comparable to that of BaO (26.5 < H). In addition, SrO has an inferior surface area with respect to MgO and CaO.¹²¹ Only a few experiments have been conducted using SrO as a heterogeneous catalyst for the transesterification of biodiesel production.

Zabeti et al.¹²² discussed the appropriateness of using SrO as a catalyst for the transesterification reaction. Liu et al.¹²³ found that in the reaction medium, SrO acts as an extremely active and soluble metal oxide. However, vegetable oil, methyl esters and SrO remain insoluble in methanol, thus SrO becomes a suitable heterogeneous catalyst for transesterification. After 30 min at 65 °C with an alcohol to oil molar ratio of 12 wt% and a 3 wt% catalyst loading, 90% yields of methyl esters were accomplished during the transesterification of soybean oil. However, the specific surface area of the catalyst was as low as 1.05 m² g⁻¹.

Salamatinia et al.¹²⁴ investigated the heterogeneous transesterification of palm oil for biodiesel production using an ultrasonic process. Briefly, response surface methodology was applied to optimize the biodiesel production process with the help of two (BaO and SrO) alkaline earth metal oxide catalysts. To optimize production, four variable factors were considered. Reaction time (10–60 min), alcohol to oil molar ratio (3 : 1–15 : 1), catalyst loading (0.5–3.0 wt%), and ultrasonic amplitude (25–100%) were included as the optimized factors. The mathematical frameworks and the steps of the process were established. The frameworks were able to correctly predict the biodiesel yield with less than 5% error for the SrO and BaO catalysts. The high activity of a catalyst mostly depends on its basic strength. Ultrasound was found to significantly enhance the process by decreasing the reaction time by almost 50 min, and a 2.8 wt% catalyst loading was used for creating biodiesel yields of more than 95%. The best results were given by a 9 : 1 alcohol to oil ratio and a ∼70% and ∼80% ultrasonic amplitude for the BaO and SrO catalysts, respectively.

3.2.2.5. Boron group oxides (Al₂O₃) supported upon CaO and MgO. Boron group components, especially aluminum and Al₂O₃, are widely utilized stacked with different other metal oxides, halides, nitrates, and alloys.^3,89,123,125 Boron and aluminum oxides are the most commonly used catalysts for mixed metal oxide production among the boron group elements. To achieve a decent yield of biodiesel production, different forms of oxides such as Al₂O₃, γ-Al₂O₃, and Al₂O₃ supported upon CaO and MgO are used as heterogeneous base catalysts. Xu et al.¹²⁶ used a mesoporous polyoxometalate–tantalum pentoxide composite catalyst, H₃PW₁₂O₄₀/Ta₂O₅, which was prepared using a one-step sol–gel hydrothermal method in the presence of a triblock copolymer surfactant.
3.2.2.6. Biodiesel production with carbon group catalysts. Formulating a carbon-based heterogeneous catalyst is simple, and is profitable for biodiesel production when used as a catalyst. Shu et al.¹²⁷ used the sulfonation of carbonized vegetable oil asphalt as a solid catalyst to produce biodiesel via the transesterification of vegetable oil. This catalyst can be used in both the esterification and the transesterification processes, while waste vegetable oil with large amounts of FFAs is present in the feedstock. The maximum conversion of triglyceride and FFA reached 80.5% to 94.8% after 4.5 h at 220 °C when using a methanol to oil ratio of 16.8 : 1 and 0.2 wt% of catalyst.

Dehkhoda et al.¹²⁸ studied the transesterification reaction of palm oil for biodiesel production, where KOH/AC was used as a heterogeneous catalyst. In their study, operating conditions were 70 °C, the molar ratio of alcohol to oil was 15 : 1, catalyst loading was 5 wt%, and reaction time was 15 h. An approximately 94% biodiesel yield was achieved, and the catalyst could be reused up to three times. Through the incineration of commercial grade sugar, a carbon catalyst was prepared and studied by Toda et al.¹²⁹ Only a 50% oil to ester conversion could be achieved from these carbon structures at the end of its first cycle, although catalytic activity remained unchanged.

Lou et al.¹³⁰ reported the arrangements of carbohydrate-derived catalysts from D-glucose, sucrose, cellulose, and different sorts of starches. The synergistic and textural properties of the arranged catalysts were examined at points of interest, and the starch-based catalyst was found to have the best reactant execution. The carbohydrate-derived catalysts displayed considerably higher synergistic effects for both esterification and transesterification in contrast with the two ordinary strong solid acid catalysts, sulfated zirconia (S-ZrO₂) and niobic acid (Nb₂O₅·nH₂O). The carbohydrate-derived catalysts also gave a significantly upgraded yield of methyl esters when converting waste cooking oils containing 27.8 wt% high FFAs to biodiesel. In addition, under the maximum response condition, the starch-based catalyst held a significantly high extent (approximately 93%) of its unique synergistic action after 50 cycles of progressive reuse and showed exceptional operational dependability. The carbohydrate-derived catalysts, particularly the starch-based catalyst, were exceptionally compelling, recyclable, eco-friendly, and apposite for the creation of biodiesel from waste oils containing high FFAs.

Faria et al.¹³¹ utilized tetramethyl guanidine on a silica gel surface¹³² as a base catalyst. ¹³C and ²⁹Si atomic attractive reverberation points of interest are in concurrence with the proposed structure. The investigator stated that the biodiesel yield was approximately 86.73% and the reaction time was approximately 3 h. In addition, the catalyst recovery was complete; almost 62% after the 9^th cycle of catalyst reuse.

3.2.2.7. Waste materials based heterogeneous catalysts. Mainly calcium-enriched waste materials are used as catalyst synthesis sources, with mollusc shell, eggshell, and bones being the most common. Researchers examined a number of systems to remove misused atoms and change useable catalysis with high cost viability this useable atoms recovery deals with high cost. CaO obtained from these waste materials could have potential for the biodiesel era. Boey et al.¹³³ reported that waste shells, which is an issue of CaO, can be utilized as a catalyst for the transesterification of palm olein into methyl esters. Analysis showed that the fundamental segment of the shell was CaCO₃, which changed into CaO when heated over 700 °C for 2 h. The economically produced catalyst affected the transesterification for biodiesel production, similar to laboratory CaO. Ideal conditions were found at a 0.5 : 1 methanol to oil mass ratio, a 5 wt% catalyst loading, a stirring speed of 500 rpm, and a reaction temperature of 65 °C. Reusability measurements established that the arranged catalyst could be recycled up to 11times. Factual examination was performed utilizing a central composite design to assess the effect of the parameters on biodiesel quality. Chakraborty et al.¹³⁴ obtained CaO from waste eggshells, which was considered as a viable catalyst for transesterification at 65 °C, with an oil/alcohol ratio of 1 : 9, and a catalyst loading of 10 wt%. An approximately 97% to 98% FAME yield was achieved, and the catalyst could be reused up to 17 times. Viriya-empikul et al.¹³⁵ studied the transesterification reaction of palm olein oil for biodiesel production, where waste eggshell, golden apples, and Meretrix venus were used as a waste base for a solid heterogeneous catalyst. In their study, operating conditions were 60 °C, the molecular ratio of alcohol to oil was 18 : 1, the catalyst loading was 10 wt%, and the reaction time was 1 h. The study produced 97%, 83%, and 78% biodiesel yields, respectively. Effective waste administration and a waste to vitality transformation can facilitate biodiesel generation utilizing eggshells.

Du et al.¹³⁶ studied the biodiesel production from soybean oil where waste eggshells were used as a heterogeneous catalyst. Operating conditions were as follows: temperature was 70 °C, molar ratio of alcohol to oil was 6.9 : 1, catalyst loading was 5 wt%, and reaction time was 5 h. An approximately 97.73% biodiesel yield was achieved, and the catalyst could be reused up to 6 times. Alternate specialists, Nakatani et al.¹³⁷ inspected the transesterification of soybean oil catalyzed by calcined shellfish shells.

3.2.2.8. Alkali metal inserted complexes as heterogeneous catalysts. Xie et al.⁴ studied the transesterification reaction of soybean oil for biodiesel production using NaX zeolites loaded with 10% KOH as a heterogeneous catalyst. In their study, the operating conditions were 65 °C, a molar ratio of alcohol to oil of 10 : 1, a catalyst loading of 3 wt%, and a reaction time of 8 h. An approximately 85.6% biodiesel yield was achieved. Fabbri et al.¹³⁸ studied the transesterification reaction of soybean oil for biodiesel production using Na₂PEG (300), a dimethyl carbonate, as a solid heterogeneous catalyst. In their study, the operating conditions were 70 °C, a molar ratio of alcohol to oil of 30 : 1, a catalyst loading of 6 wt%, and a reaction time of 5 h. An approximately 99% biodiesel yield was achieved.

Kondamudi et al.¹³⁹ incorporated an extraordinary bifunctional catalyst Quntinite-3T (Q-3T) for biodiesel generation from waste vegetable oils, restaurant oil, and poultry fat. These oils picked up mechanical vitality compared with costly sustenance-based vegetable oils. This bifunctional heterogeneous catalyst simultaneously changes FFAs and triglycerides (TGs) into biodiesel. Q-3T is obtained from a sodium source (Na-Q-3T) and ammonium (NH₄-Q-3T) sources using the sol–gel process and is characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). The catalyst was tried for soy, canola, espresso, and waste vegetable oils with variable measures of FFAs (0–30 wt%). The catalyst effectively changed both FFAs and TGs in a solitary step bunch reactor.

3.2.2.9. Transition metal oxides and derivatives as heterogeneous catalysts. Many transition metals of the periodic table exist. Some transition metals and their oxides are chosen as base heterogeneous catalysts by researchers for biodiesel production. ZnO, TiO₂,¹⁴⁰ TiO₂/SO₄²⁻ and ZrO₂/SO₄²⁻,¹¹⁹ ZnO, and ZrO₂ (ref. 74, 141 and 142) are the most commonly used heterogeneous catalysts. The action of ZrO functionalized with TiO was likewise investigated by López et al.¹⁴³ Nakagaki et al.¹⁴⁴ engineered sodium molybdate (Na₂MoO₄) and utilized it as a heterogeneous catalyst for the methanolysis of soybean oil. Transesterification responses occurred under moderately gentle conditions: obliging low temperatures, short reaction times, and normal pressure. In their study, the produced methyl ester was characterized using reverse-phase and size-exclusion chromatography and hydrogen-nuclear magnetic resonance spectroscopy. Transesterification responses occurred under moderately gentle conditions: low temperatures, short reaction times and normal pressure. The methyl esters combined were described using opposite stage and size-exclusion chromatography and hydrogen-nuclear magnetic resonance spectroscopy. The transesterification response of the triglycerides with methanol was exceptionally effective with methyl ester yields higher than 95%. The molybdenum(VI) complex appeared to have high Lewis acidity and most certainly act on alcohol O–H bonds, prompting transition states with strongly nucleophilic characteristics. The catalyst was effortlessly recouped and after being washed, demonstrated recyclability for an alternate synergistic response with comparative movement.

Yoo et al.¹⁴⁰ recommended ZnO as the best catalyst during the transesterification of rapeseed oil using transition metal oxides (ZnO, TiO₂, and ZrO₂) as heterogeneous catalysts because of their high activity. Operating conditions include a molar ratio of methanol to oil 40 : 1, a catalyst loading of 1 wt%, and a reaction time of 10 min. Brito et al.¹⁴⁵ studied transesterification and esterification reactions to obtain methyl ester using two series of complexes with the general formula M(n-butoxide)_4−x(maltolate)_x, where M = Ti or Zr and x = 0–4, as catalysts. Mixes containing different ratios of maltolate and n-butoxide ligands were produced from the response of maltol (3-hydroxy-2-methyl-4-pyrone) and n-butoxide metal forerunners. All structures containing maltolate as ligand, were exceptionally productive as catalysts in esterification, mostly those containing zirconium. By contrast, these catalysts showed exceptionally poor activity for transesterification.

da Silva et al.¹⁴⁶ studied the transesterification reaction of soybean and babassu oils for biodiesel production where Cu(II) and Co(II) were used as catalysts. The catalysts were characterized using infrared, atomic absorption, and thermogravimetry, and the biodiesels were characterized using infrared, NMR, CG, thermogravimetry, and physical chemistry analysis. In their study, the maximum adsorption values reported for the Cu(II) and Co(II) cations were 1.584 mg g⁻¹ and 1.260 mg g⁻¹, respectively, after 180 minutes. However, the conversion of oil to biodiesel was better when Co(II) was adsorbed on chitosan. Krohn et al.¹⁴⁷ considered the generation of algal biodiesel from Dunaliella tertiolecta, Nannochloropsis oculata, wild freshwater microalga, and macroalga lipids utilizing a highly effective continuous catalytic Mcgyan® process. The heterogeneous catalytic procedure utilizes supercritical methanol and permeable titania microspheres in a settled bunk reactor to catalyze the synchronous transesterification and esterification of triacylglycerides and FFAs, separately, into biodiesel. Yields of up to 85% of alkyl esters were obtained from triglycerides and FFAs, as measured by a 300 MHz HNMR spectroscope.

3.2.2.10. Some mixed metal oxides and derivatives as heterogeneous catalysts. Xu et al.¹²⁶ studied the transesterification reaction using KF/Zn(Al)O as a catalyst. The prepared KF/Zn(Al)O catalyst had an activity higher than that of Zn–Al hydrotalcites, such as Zn(Al)O, KF, KF/γ-Al₂O₃, and KF/ZnO compounds. This catalyst was especially viable when operating conditions were 65 °C, a molar ratio of methanol to oil of 6 : 1, KF/Zn(Al)O catalyst loading of 3 wt%, and a reaction time of 3 h. An approximately 95% biodiesel yield was achieved. The high action was credited to the establishment of new stages KF and KOH, and the effect of Zn(Al)O.

Wang et al.¹⁴⁸ produced MgO–MgAl₂O₄ by utilizing γ-Al₂O₃, and this MgAl₂O₄ composite was used as heterogeneous catalyst. This gave a more noteworthy biodiesel yield compared with a MgO/MgAl₂O₄/γ-Al₂O₃ material with the comparable loading of magnesium arranged by an ordinary impregnation process. The improved catalytic action of the prior material could be attributed to its higher basicity, specific surface area, pore volume, and size. Wen et al.¹⁴⁹ studied the transesterification of soybean oil with methanol for biodiesel production. Kalsilite (KAlSiO₄) was used as a heterogeneous catalyst. A method called co-precipitation was introduced to produce kalsilite and add lithium for property improvement as well as catalytic performance improvement for the transesterification of soybean oil. Comparatively low catalytic activity was shown by kalsilite. However, a small amount of lithium nitrate added using the impregnation method significantly boosted the catalytic activity. With operating conditions of a reaction temperature of 120 °C, a 3.84 cSt kinematic viscosity, and 2.3 wt% of lithium modified kalsilite, an almost 100% biodiesel yield was achieved.

3.2.2.11. Hydrotalcite metal oxides as heterogeneous catalysts. Georgogianni et al.¹⁵⁰ studied the transesterification of rapeseed oil for biodiesel production where an alkaline catalyst was used as either a homogeneous or heterogeneous catalyst. In this study, NaOH was used as a homogeneous catalyst, Mg MCM-41, Mg–Al hydrotalcite, and K⁺ impregnated zirconia were used as heterogeneous catalysts, and a 24 kHz ultrasonication frequency and 600 rpm stirring speed were maintained. Heterogeneous catalyst criteria depend on the porosity and surface basicity of the catalyst, and the catalytic activity depends on the basic strength of the catalyst. An approximately 97% biodiesel yield can be achieved with Mg–Al, showing the highest activity. Increasing the amount of potassium cations with the catalyst increases the activity of ZrO₂ in transesterification. The involvement of the ultrasonication frequency enhanced the transesterification reaction rather than the involvement of mechanical stirring. By performing filtration, the heterogeneous catalyst can be easily separated from the reaction mixture after the reaction.

Glišić et al.¹⁵¹ studied various systems for developing FAME production at higher temperatures and pressures with or without catalysts. His aim was to find the optimum way to produce biodiesel with minimum energy consumption and improve life cycle energy efficiency. Energy consumption (MJ kg⁻¹ FAME) depends on the degree of conversion of triglycerides. Energy consumption will be 25% lesser if the degree of conversion increased from 97 wt% to complete conversion. Other meaningful decreases in energy consumption might be gained at subcritical conditions by using an appropriate catalyst. This study exposed that if the heterogeneous process of biodiesel synthesis is acknowledged at subcritical conditions, then a further decrease in energy consumption is possible.

A comparative assessment, in terms of yield, reaction conditions and reusability of different promising heterogeneous catalysts utilized by several researchers has been listed in Table 5.

Table 5 Comparative assessment of promising heterogeneous catalysts with different feedstocks for biodiesel production

Catalyst name	Feedstock	Reaction conditions				Yield (v/v%)	Ref.
		Temp. (°C)	Time	Loading in wt%	Molar alcohol to oil ratio
Tungstated zirconia WO3/ZrO2	Vegetable oil	75	140 h	—	—	65	Kiss et al.^71
Sulphated zirconia (SO4^2−/ZrO2)	Palm kernel	—	—	—	—	90.3	Jitputti et al.^74
Unsulfated zirconia (ZrO2)	Coconut oil	—	—	—	—	86.3	Jitputti et al.^74
Chloroaluminate ([Et3NH]Cl–AlCl3)	Soybean oil	70	9 h	—	0.5 : 1	98.5	Liang et al.^75
Cs2.5H0.5PW12O40	Yellow horn oil	70	10 min	1%	12 : 1	96.22	Zhang et al.^65
La/zeolite beta	Soybean oil	100	3 h			48.9	Shu et al.^84
Hβ-zeolite	Pongamia pinnata	120	24 h	0.12%	10 : 1	59	Karmee et al.^85
Montmorillonite K-10	Pongamia pinnata	120	24 h	0.12%	11 : 1	47	Karmee et al.^85
ZnO	Pongamia pinnata	120	24 h	0.12%	12 : 1	83	Karmee et al.^85
NaX zeolites (Si/Al = 1.23)	Pongamia pinnata	125	2 h	10%	—	85.6	Xie et al.^4
Zeolites	Sunflower oil	60	—	—	—	93.5–95.1	Ramos et al.^87
CaO	Sunflower oil	100	1.5–5.5 h	1%	6 : 1	91	Vujicic et al.^112
CaO	Palm oil	—	1 h	0.5–5%	3 : 1–15 : 1	48.2–95.2	Mootabadi et al.^97
SrO	Palm oil	—	1 h	—	—	67.3–95.2	Mootabadi et al.^97
BaO	Palm oil	—	1 h	—	—	55–77.3	Mootabadi et al.^97
MgO	Vegetable oil	600	8 h	—	—	18	López et al.^115
MgO	Vegetable oil	—		5%	12 : 1	92	Di Serio et al.^116
BaO & SrO	Vegetable oil	—	10–60 min	0.5–3%	3 : 1–15 : 1	95%	Salamatinia et al.^124
SrO	Vegetable oil	65	30 min	3%	12 : 1	90	Liu et al.^123
KOH/AC	Palm oil	70	15 h	5%	15 : 1	94	Dehkhoda et al.^128
Silica gel	Vegetable oil	—	3 h	—	—	—	Faria et al.^131
CaO from eggshell	Palm olein	65		10%	9 : 1	97.98	Chakraborty et al.^134
CaO from eggshell	Palm olein	60	1 h	10%	18 : 1	97	Viriya-empikul et al.^135
Golden apple	Palm olein	60	1 h	10%	18 : 1	83	Viriya-empikul et al.^135
Meretrix venus	Palm olein	60	1 h	10%	18 : 1	78	Viriya-empikul et al.^135
CaO from eggshell	Soybean oil	70	5 h	5%	6.9 : 1	97.73	Du et al.^136
NaX zeolites loaded with 10% KOH		65	8 h	3%	10 : 1	85.6	Xie et al.^4
Dimethyl carbonate	Soybean oil	70	5 h	6%	30 : 1	99	Fabbri et al.^138
KF/Zn(Al)O	Vegetable oil	65	3 h	3%	6 : 1	95	Wang et al.^148
Kalsilite (KAlSiO4)	Soybean oil	120	—	2.30%	—	100	Wen et al.^149

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3.3. Biocatalysts

Biocatalysts are catalysts obtained from a living organism and promote chemical reactions without being affected themselves. Biocatalysts are also called enzymes or enzyme catalysts. These catalysts initiate or modify nearly all biochemical reactions in living cells. Each enzyme possesses individual three-dimensional patterns that suit the shape of the reactants. Biocatalysts have recently become progressively critical in the examination of biodiesel generation. These catalysts are hypothesized to beat synthetic catalysts.¹⁵² The search for an environmentally friendly methodology for biodiesel synthesis was explored utilizing proteins as catalysts. Typically, the difficulties confronted with conventional catalysts such as feedstock pretreatment, catalyst elimination, wastewater treatment, and the requirements for large amounts of energy are addressed in enzyme-catalyzed transesterification reactions. Biocatalysis utilizes an accumulation of enzymes known as lipases generated through microorganisms, animals, and plants.^153,154 Lipases could be separated from different sources of bacterial species, such as Pseudomonas fluorescens, Pseudomonas cepacia, Rhizomucor miehei, Rhizopus oryzae, Candida rugosa, Thermomyces lanuginosus, and Candida antarctica.¹⁵⁵

Biocatalysts could be classified into a range of major varieties as follows. (a) Microbes: a microscopic organism such as yeast and other anaerobic bacteria, archaea, bacteria, fungi, viruses, and microbial mergers. (b) Lipases: the most widely used class of enzymes in organic synthesis, lipases are preferred widely because of their better stability compared with other biocatalysts. Lipase can be classified into extracellular and intracellular lipases.¹⁵⁴ Extracellular lipases are mainly obtained from live-producing microorganism broth through purification. Major extracellular microorganisms are Mucor miehei, R. oryzae, C. antarctica, and P. cepacia. Intracellular lipases are present inside the cell or in the cell-producing wall. In most cases, intracellular lipases are found in the immobilized form. (c) Proteases: enzymes that break down proteins. Proteases can be found in animals, plants, bacteria, archaea, and viruses, for example, TEV protease and trypsinogen. (d) Cellulases: enzymes that break down cellulose. (e) Amylases: enzymes that break down starch into simple sugars.

Researchers choose enzyme transesterification because of the advantages over the chemical catalyzed transesterification process. Easy product removal, moderate process temperature (35–45 °C), zero by-products, and the reusability of the catalysts are the main features of using this type of catalyst. However, enzymatic reactions are insensitive to FFAs and the water content of the feedstock. Some investigations conducted by researchers using lipase-catalyzed transesterification of different feedstocks to produce biodiesel are presented in Table 6. Different types of catalysts used in the biodiesel production process are listed in Table 7.

Table 6 Transesterification of different feedstocks using biocatalysts

Enzyme	Feedstock	Alcohol	Temperature (°C)	Time (h)	Yield (%)	Ref.
IM B. cepacia lipase	Palm oil	Methanol	30	72	100	156
Lipozyme IM60	Tallow oil	Primary alcohols	45	5	94.8–98.5	157
Lipozyme IM-20	Mowrah oil	Alcohols (C4–C18)	60	6	86.8–99.2	158
E. aerogenes lipase	Jatropha oil	Methanol	55	48	94	159
Novozym® 435	Soybean oil	Methyl acetate	40	14	92	160
Lipozyme IM-77	Soybean oil	Methanol	—	—	92.2	161
R. oryzae lipase	Plant oil	Methanol	—	—	90	162
P. expansum lipase	Corn oil	Methanol	40	24	86	163
Cryptococcus spp. S-2	Rice bran oil	Methanol	30	96	80	164
Pseudomonas lipase	Sunflower oil	Methanol	45	5	79	165
PS 30 lipase	Palm oil	Ethanol	40	8	72	166

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Table 7 Relative advantages and disadvantages of different catalysts involved in biodiesel production technology

Catalyst type	Advantages	Disadvantages	Ref.
Homogeneous	• Effectively active on metal atoms	• Hazardous for the environment compared to heterogeneous catalysts	56 and 167
	• Very fast reaction rate	• Hydroscopic nature (NaOH, KOH)
	• Reaction can occur under mild conditions thus relatively less energy is required for esterification and transesterification	• Low quality glycerol produced thus requires a lengthy distillation process for purification
	• Relatively cheap and available (NaOH and KOH)	• Homogeneous base catalysts are sensitive to the FFA content present in the oil
	• Preferred method for low-grade feedstock	• Soap forms in the case of higher (2 wt%) FFA content in the oil, thus the biodiesel conversion rate is reduced
	• Sometimes esterification and transesterification occur instantaneously	• Purification of biodiesel from the product is relatively difficult and requires a huge amount of water
		• Poisoning occurs when the catalyst is exposed to ambient air
		• Acid homogeneous catalysts are very harmful, very corrosive to the reactor and pipeline and require careful handling
Heterogeneous	• The separation of the glycerol and catalyst from biodiesel is much easier	• Converts triglycerides at a relatively slower rate	56
	• Not mixed with ethanol or methanol thus separation is easier	• Complicated catalyst synthesis procedures leads to higher cost
	• Because of large pore size, the diffusion problem is minimized	• Effectively active only on surface atoms
	• High catalytic stability against leaching and poisoning
	• Easy separation of the catalyst from the product
	• Economic because of its reusable nature
Biocatalyst	• Tolerate free fatty acids and water content	• Could not be commercialized for the production of biodiesel due to the long residence time and high cost	19 and 56
	• Easy purification of biodiesel and glycol	• High cost
	• Environmentally friendly and does not produces volatile organic compounds	• Long process time due to very slow reaction rate
	• High possibility to reuse and regenerate the catalyst	• Sensitive to alcohol, normally methanol, that can deactivated the enzyme
	• Only a simple purification step is required
	• Transesterification can be carried out at low reaction temperatures, even lower than those for a homogeneous base catalyst

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4. Non-catalytic biodiesel production

Several techniques are involved in biodiesel synthesis aside from a catalytic response, such as the purification of esters, supercritical transesterification, BIOX co-solvent transesterification, and catalyst separation. Considering the advantages over catalytic biodiesel synthesis, as well as the environmental impact, supercritical transesterification, and BIOX co-solvent transesterification processes are receiving popularity.¹⁶⁸

4.1. Supercritical alcohol transesterification

Biodiesel can be produced by the transesterification reaction under supercritical conditions in methanol (SC MeOH) without using any catalyst.^104,169 Forward reaction rates are as high as 50–95%, and forward reactions occur within the initial 10 minutes.^170,171 However, this reaction needs higher temperatures and pressures, approximately 250–400 °C and 1200 psi,^172,173 respectively. For this reason, this reaction consumes more energy and increases the production cost. Supercritical methanol, ethanol, propanol, and butanol have been demonstrated to be involved in probably the best procedures for the transesterification of triglycerides without the involvement of a catalyst. Supercritical alcohol techniques are a non-catalytic method for biodiesel synthesis, where high pressures and high temperatures sustain the transesterification reaction.¹⁷⁴ Initially, conversion is significantly fast because of the reaction conditions, which are high temperatures, high pressures, and high ratios of alcohol to oil. An approximately 50–95% conversion occurs within the first 10 minutes but temperature ranges are quite high (250–400 °C). During the transesterification of vegetable oil, an approximately 1 : 6 to 1 : 40 oil/alcohol molar ratio is used under supercritical alcohol conditions.¹⁰⁹

The major difficulties or shortcomings of using supercritical alcohol are high temperatures (250–400 °C), high pressures (200–400 bar), and⁶⁴ high alcohol to oil ratios,¹⁷⁵ usually at 41 : 1, which increases the biodiesel production cost. In the supercritical alcohol transesterification process, the presence of small amounts of water does not hamper the oil to biodiesel conversion.^104,176,177 Moreover, the presence of water accelerates the formation of the methyl ester and the esterification of FFA in one stage. The reaction took 4 min at 250–400 °C and 35–60 MPa. Iijima et al.¹⁷⁸ suggested a supercritical condition where reaction conditions were a reaction temperature of 643–773 K, a reaction pressure of 20–60 MPa, and a reaction time of only 4–12 min for the production of biodiesel without a glycerin by-product.

The supercritical transesterification procedure involves high temperature and pressure conditions that enhance phase solubility, reduce mass transfer impediments, provide higher conversion rates, and provide easier steps for separation and purification. Table 8 presents the supercritical transesterification of vegetable oil with corresponding reaction conditions.¹⁷⁹

Table 8 Reaction parameters and corresponding yields of supercritical alcohol processes

Reaction parameter	Unit	Methanol	Ethanol	1-Propanol	1-Butanol	1-Octanol
Temperature	°C	239.2	243.2	264.2	287.2	385
Pressure	MPa	8.09	6.38	5.06	4.9	2.86
Yield after 10 min	Mass%	98	79	81	80	—
Yield after 30 min	Mass%	98	88	85	75	—

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4.2. BIOX co-solvent transesterification

BIOX (co-solvent) transesterification is a new method proposed by Canadian professor David Boocock from the University of Toronto. Owing to the lower solubility of methanol in oil, the rate of transformation of the oil directly into the ester is quite slow. This problem could be minimized by introducing a co-solvent, which can mix with the methanol and oil. Tetrahydrofuran (THF) is a type of co-solvent with a boiling point very close to that of methanol; thus, it requires a very low operating temperature of 30 °C. The continuous BIOX co-solvent process takes less than 90 min near ambient temperature and at atmospheric pressure. FFA and triglycerides both converted in a single phase in two steps.¹⁸⁰ By improving the solubility of the alcohol in triglyceride using the co-solvent, the slow reaction rate could be improved. The reaction time becomes 5–10 min and, aside from catalyst residues, is minimized to only one phase (ester or the glycerol phase).

Demirbas¹⁸¹ studied THF as a co-solvent with methanol to form a single phase. After finishing the reaction, the biodiesel glycerol phase was clean. The alcohol and THF co-solvent were both recovered in a single step. Nevertheless, because of the probable hazards and toxicity of the co-solvents, the solvent needs to be entirely removed from the glycerol phase and the biodiesel phase; in addition, the end products must be water-free.¹⁸² Using a co-solvent such as tetrahydrofuran as well as methyl tertiary butyl ether significantly accelerates methanolysis. In spite of this, similar to one-phase butanolysis, one-phase methanolysis initially displays a rapid development of the ester, but then drastically slows.¹⁸²

The restoration of excessive alcohol is challenging in the case of the BIOX co-solvent approach because the boiling point for the THF co-solvent is quite close to that of methanol.¹⁸³ The particular remarkable benefit of the BIOX co-solvent technique is that it employs inert, recoverable co-solvents in a single pass reaction that normally requires only seconds at ambient temperature and pressure, and no catalyst residues appear in both the biodiesel phase and the glycerol phase.¹⁸³ This technique can be used with crude vegetable oils, waste cooking oils, and animal fats. Table 9 presents a comparative study between catalytic and non-catalytic transesterification biodiesel production processes.

Table 9 Advantages and disadvantages of using non-catalytic transesterification over conventional transesterification^a

	Advantages	Disadvantages
a Ref. 180​–183
Non-catalytic method	• Less water is produced as a by product and sometimes the presence of water accelerates the conversion rate	• More energy is required by the reaction step especially in the heating step as a high power consumption is involved
	• Simpler purification steps involved	• High temperature and pressure required
	• Simpler separation steps involved	• High alcohol to oil ratio is needed
	• High quality glycerin is generated as a by product	• Relatively lower production yield than for the conventional method
	• Environmentally friendly as a smaller amount of chemicals are used	• Need to take more care over the production process as it involves a higher pressure and temperature
	• Less time required
	• Low quality feedstock could be transformed easily into biodiesel
Conventional method	• Lower power consumption regarding heating	• Higher process cost
	• Higher yield is possible	• Greater time is required than for the non-conventional method
	• Relatively lower temperature and pressure required	• Cost involved with catalyst loading
		• Preparation of catalyst is quite complex

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5. Conclusion

Among the several biodiesel synthesis processes from natural oils and fats, transesterification is currently the most attractive approach because transesterification is essentially a successive response. The motivation behind the methodology is to reduce the viscosity of the oil or fat. Although mixing oils and different solvents and microemulsions of vegetable oils reduces the viscosity, problems of engine performance (i.e., lubricating oil contamination, carbon decomposition) still exist. Involving specialized focal points in both materials science and reactor design is crucial if biodiesel is to remain a vital participant in the renewable energy sector in the 21^st century.

The targets of this study were to review distinctive biodiesel generation techniques (both catalytic and non-catalytic) and the utilization of heterogeneous catalysts in biodiesel production to date. In view of the audit, the accompanying conclusions are:

• A base homogeneous catalyst process has a rapid reaction rate, high yield, needs mild reaction conditions, and has a lower energy consumption; it is insensitive to water content, easy to obtain, and has a low cost. However, this process is sensitive to FFA content (>2 wt%) in the oil and forms soap and glycerol as byproducts, thereby needing excess water during purification.

• Acid homogeneous catalysts are insensitive to both FFAs and water content in oil and are suitable for low-grade oil, which has a high acid value. However, some problems are associated with this catalyst compared with the heterogeneous catalyst, including a relatively slow reaction rate, corrosive nature, catalyst separation from the product, reusability of catalyst, and soap formation.

• A heterogeneous catalyst has some advantages over a homogeneous catalyst, including easy separation, simple recovery techniques, and reusability of the catalyst from the product. In addition, this catalyst has faster reaction rate (base) and milder reaction conditions are required compared to the homogeneous process. Sometimes, catalyst preparation is expensive and unavailable. This catalyst is still a long way from industrial applications because its assessment has only been completed in stirred batch reactors. Only a few studies have been conducted on persistent procedures utilizing pressed bed stream reactors.

• A relatively lower reaction temperature is needed for biocatalytic processes compared with that for both homogeneous acid and base catalytic processes. The major limitation is the preparation cost of the enzymes and the reaction rate, and this method is the slowest among the processes. Furthermore, the catalyst is more expensive than that for both the homogeneous acid and base catalytic processes. In addition, the low solubility of glycerin in biodiesel reduces the enzyme activity. Further development of the existing process, enzyme flexibility, and adaptability must be studied, which can lower the cost and improve the conversion rate.

• Non-catalytic supercritical alcohol procedures need harsh reaction conditions (temperature, pressure, and methanol to oil ratio). However, non-catalytic supercritical alcohol transesterification takes less time, and FFAs completely convert into the ester. In addition, the BIOX co-solvent process is a more acceptable process to minimize the low solubility problem of methanol in oils.

Nomenclature and abbreviations

IC	Internal combustion
CI	Compression ignition
SI	Spark ignition
HC	Hydrocarbon
ASTM	American Society for Testing and Materials
EN	European Standard
FAME	Fatty acid methyl ester
FAEE	Fatty acid ethyl ester
FFA	Free fatty acid
SC MeOH	Supercritical condition in presence of methanol
psi	Pound per square inch
HPAs	Heteropolyacids
MAT	Microwave-assisted transesterification
kHz	Kilo hertz
MJ kg^−1	Mega joule per kilogram
°C	Degree celsius
RSM	Response surface methodology
TGs	Triglycerides
DGs	Diglycerides
MGs	Monoglycerides
IUPAC	International Union of Pure and Applied Chemistry
XRD	X-ray diffraction
SEM	Scanning electron microscopy
HR-TEM	High resolution transmission electron microscopy
TEV	Tobacco etch virus
SMO	Sodium methylate

Acknowledgements

The authors would like to thank University of Malaya for financial support through High Impact Research grant titled: Clean Diesel Technology for Military and Civilian Transport Vehicles having grant number UM.C/HIR/MOHE/ENG/07 and Program Rakan Penyelidikan Universiti Malaya (PRPUM) (Grant no. CG054-2013).

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