Recent Developments in Catalysts for Biodiesel Production Applications

Abstract

The world’s most urgent problem today is the quick depletion of energy resources, which necessitates research into alternative energy sources in order to meet the world’s explosive growth in energy demand. Among other renewable energy sources, biodiesel holds promise for meeting energy demand at a low cost through a variety of processes. In the biodiesel industry, sophisticated catalysts have recently grown in popularity for their ability to activate esterification and transesterification processes. The goal of this chapter is to give a general overview of catalyst developments, including their benefits and drawbacks in the biodiesel production process. In particular, we present a comparison of various homogeneous and heterogeneous catalysts. We found that nanocatalysts hold the most promise for the production of biodiesel.

1. Introduction

The increased global energy demand and the pursuit of environmentally friendly technology drive researchers toward alternative energy sources [1]. Currently, crude oil (35%), coal (29%), natural gas (24%), nuclear energy (7%), and renewable energy (5%) account for the majority of global energy consumption from fossil fuels [2]. This increased use of fossil fuels contributes to the global collapse of fossil fuels, air pollution, and global warming. Furthermore, it is predicted that all fossil fuel sources will be depleted by 2050 [3]. As a result, scholars have been motivated to find out renewable energy sources as clean energy alternatives, such as solar, wind, tidal, and geothermal energy, as well as biomass derived energy, to overcome these energy limitation [4]. Numerous innovative ideas can improve renewable energy technologies and provide sustainable methods to meet rising energy demand in a clean environment. In this regard, biodiesel is a potential fuel with a lower use of fossil fuels [5].

Furthermore, biodiesel offers the same performance as engine stability as petroleum diesel fuel, is nonflammable and nontoxic, reduces tailpipe emissions, visible smoke, and noxious fumes, and is safe for use in all conventional diesel engines [6]. It is produced from mono-alkyl esters of long-chain fatty acids through transesterification of vegetable oil by using catalysts as shown in Figure 1. It is renewable, nontoxic, biodegradable, and environmentally friendly and can be used in compression-ignition (diesel) engines with little or no modification due to its adjustable physical and chemical properties. It is a renewable energy source made from various oil sources, such as animal fat, canola, mustard, soybean, and sunflower, waste oils, such as waste edible or nonedible oil, waste cooking oil, and microbial oil.

Figure 1.

Transesterification reactions of glycerides with alcohol to get methyl esters [7, 8].

Figure 2 shows various sources of feedstock for biodiesel production. Furthermore, essential microorganisms in biodiesel production are classified into four types: bacteria, fungi, microalgae, and yeasts [7]. Yeasts include Rhodotorula graminis [8], Candida tropicalis, and Yarrowia lipolytica [9]. Fungi also include Coniochaeta hoffmannii, [10] Alternaria alternata, Cladosporium cladosporioides, Epicoccum nigrum, Ffusarium oxysporum, Aspergillus parasiticus, and Emericella nidulans [11]. Chlorella minutissima [12], Scenedesmus obliquus, and Desmodesmus spp. [13] are among the microalgae. Furthermore, bacteria such as Serratia sp. and Bacillus sp. were used [14, 15, 16, 17]. Additionally, biodiesel is an ethyl ester derived from plant oil, microbial oil, animal oil, and the disposal of edible oil [16, 18].

Figure 2.

Different feedstocks for biodiesel production.

2. Biodiesel production methods

Generally, there are many ways for biodiesel production. Recently, the most commonly used methods are esterification and transesterification. According to recent reports, esterification stands for the reaction of FFAs and alcohol to make FAAEs, and water is released, whereas transesterification stands for the reaction of triglycerides or triacylglycerols (TAGs) with alcohol to make FAAE and glycerol is produced as a by-product [16, 18]. In terms of reaction rate, esterification process is faster than transesterification process, for the reason that of it is short step reaction as shown in Figure 1 [19, 20].

3. Catalysts for biodiesel production

Recently, catalysts have drawn a lot of attention for their applications in a range of fields, including materials science, bioremediation, food, photonic crystals, cosmetics, medication delivery, and food [19]. Nanomaterial-based catalysts have enormous promise for reducing biodiesel production costs and hastening transesterification processes [21]. The transesterification process is researched mostly through the employment of various types of nanocatalysts. The core, the surface, and the shell are the three layers that make up a nanoparticle, which is not a straightforward molecule. The NP’s central region is known as the core, and its surface layer is made up of small molecules [8, 20, 22].

This chapter aims to overview recent developments in catalyst research and the types used in this process and their application to the production of biodiesel. Then, we point out the limitations of nanocatalysts in the production of biodiesel.

3.1 Biocatalysts (Enzyme)

Generally, either catalytic or non-catalytic processes are employed in the production of biodiesel. Among catalytic techniques, biocatalysts can be a successful way to make biodiesel in a sustainable way. Biocatalytic technologies for making biodiesel are in urgent demand to mitigate greenhouse gas emissions from conventional diesel or fossil fuels. The transesterification process in the production of biodiesel is usually catalyzed by lipases with superior biochemical and physiological features. In total, 70–95% of ethanol and methanol are produced by bacterial and fungal lipases [22]. Biodiesel is typically made of fatty acid alkyl esters, which are either mono-alkyl esters of fatty acid methyl esters or fatty acid ethyl esters, depending on the alcohol employed in the synthesis as shown in Figures 1 and 3.

Figure 3.

Biodiesel production from feedstock: A. one-step process; B. two-step process [7, 8].

Furthermore, the problems brought on by alkali and acid-catalyzed procedures are minimized by the biocatalytic biodiesel synthesis process. Since enzymes can esterify low-quality feedstock with a high concentration of free fatty acids, using enzyme catalysts has a number of economic and environmental advantages, such as the production of pure and highly valuable glycerol, the generation of the least amount of wastewater, the use of mild reaction conditions, and the absence of soap formation (FFA). Because they are simple to separate from the reaction mixture and have a generally lower chance of contamination, this production process is uncomplicated and has a low energy consumption [23, 24]. Recent developments in biomaterial catalysts, including cellulose, cellobiose, glucosidase, laccase, and xylanase, increase the effectiveness and durability of catalytic processes. Since nano biocatalysts may easily be reused and recovered by a continuous and large-scale process, using biocatalysts makes biocatalyst recovery and reusability easy (Figure 4) [24, 25].

Figure 4.

The various types of catalysts.

3.2 Homogeneous catalysts

The most common biodiesel productions process, such as esterification, ester hydrolysis, and transesterification, have all been studied by using both acid and base homogeneous catalysts. A chemical in a like phase of the reaction structure catalyzes a series of reactions in production kinetics. The homogeneous catalyst is the most popular catalyst used because it is easy to react, good conversion rate, and very fast to complete the reaction in the synthesis of biodiesel. In order to dissolve homogeneous catalysts, the solvent is normally in the same phase as all of the reactants. However, the fundamental limitation it is problematic for reuse, and sometimes recycling this catalyst is very expensive [25, 26].

3.2.1 Homogeneous alkaline (base) catalysts

These catalysts are mostly alkaline liquids with high activity in transesterification reaction of low free fatty acid (FFA) [25, 26]. The extensively used catalysts, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), potassium methoxide (CH₃OK), and carbonates (Table 1) [27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43]. The conservative method for producing biodiesel from pure vegetable oils is the transesterification process using alkaline catalysts. For example, three moles of alcohol and one mole of triglyceride undergo this reaction, producing one mole of glycerol and three moles of fatty esters. The best alcohol-to-triglyceride ratio was a six-to-one give-up, which was 98%. Alkali metal methoxides are more energetic than alkali metal hydroxides, for the production of biodiesel, but its drawback is the significant amount of FFA remains as soap, it requests an extra catalyst and the catalyst is lost to the soap [44, 45]. In addition, deactivation, slow reaction rates, and saponification are the main drawback of alkaline catalysts, difficult to discern the product from biodiesel in the reaction system have greater than 2.5% FFA concentration result unwanted saponification, leads to a loss in enzymatic activity and requires extra energy to solve various technical issues [46, 47].

Catalyst	Feedstock	Yield/Conversion (%)	Ref
MgO	Moringa oleifera seeds oil	93.69	[27]
CaO-MgO	Jatropha curcas oil	90	[28]
CaO-ZnO	Sunflower oil	>.90	[29]
CaO-La2O3	Soybean oil	94.3	[30]
KF-CaO-Fe3O4	Stillingia oil	>95	[31]
KOH/AL2O3	Palm oil	91.07	[32]
Li/ZnO-Fe3O4	Rapeseed oil	99.8	[33]
KF/CaO	Tallow seed oil	96.8	[34]
NaAlO2/g-Al2O3	Palm oil	97.65	[35]
25%MoO3/B-ZSM-5	Oleic acid	98	[36]
CaO/CuFe2O4	Chicken fat	94.52	[37]
KOH/Fe3O4@Al2O3	Canola oil	98.8	[38]
MgO/MgFe2O4	Sunflower oil	91.2	[39]
Cr/Ca/g-Al2O3	Cooking oil	92.79	[40]
$\gamma$-Al2O3/KI	Palm oil	98	[41]
Ca/$\gamma$-Al2O3	Corn oil	87.89	[42]
Cs/Al/Fe3O4	Sunflower oil	95	[43]

Table 1.

Various nanocatalysts are used for biodiesel production from different sources [27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43].

3.2.2 Homogeneous acid catalyst

The homogenous acid catalysts, such as hydrochloric acid (HCl), sulfonic(R-SO₃H), and sulfuric acids (H₂SO4), as well as Brønsted acids are commonly used catalysts for both the esterification and transesterification process (Table 1). These catalysts increase the yield of alkyl esters and lesser the cost of the feedstock, making the process more cost-effective. However, the process is less appealing since it requires high temperatures, operates more slowly, causes corrosion, and has higher purifying and separation expenses. Less rate reactions are involved in the transesterification processes catalyzed by acids. However, due to the slower reaction rates compared to alkali-catalyzed reactions and the higher energy demands, the procedure is economically difficult [48, 49]. Therefore, homogeneous acid catalysis is applicable for both transesterification and esterification reaction processes [50, 51]. For example, p-toluene sulphonic acid catalyst and the Amberlyst-35 sulphonic acid catalyst were employed in series at 5% concentration in a specific application to extract the ester from vegetable refining waste. There are various alcohols, such ethanol, butanol, and methanol, for esterification reactions producing >90% biodiesel [52].

3.3 Heterogeneous catalysts

These groups of catalysts are involved in a different state of product and reactants and also, they are noncorrosive, as well as easily separable from the products. These catalysts offer certain exceptional qualities. Furthermore, heterogeneous catalysts (solid) have empathetic nature with little harm to the environment. It is crucial to develop some suitable heterogeneous catalysts for the manufacture of biodiesel from affordable feedstocks. These heterogeneous catalysts are further subdivided into basic and acidic catalysts [51, 52].

3.3.1 Heterogeneous base catalysts

Heterogeneous base catalysts overcome a number of obstacles, including saponification, which prevents glycerol from separating from the methyl ester layer. Heterogeneous base catalysts overcome a number of obstacles, including saponification, which prevents glycerol from separating from the methyl ester surface. In contrast, these catalysts have promising advantages such as environmental friendliness, reduced waste material harm, non-corrosiveness, selectivity, tolerance of high FFA and moisture contents, promoting simple recovery, reusability, low cost, and green process. They can also be modified to increase activity, selectivity, and catalyst lifetime [53, 54]. Base alkali earth metal oxides, such as BeO, MgO, CaO, SrO, BaO, and RaO [55, 56], transition metal oxides [57], mixed metal oxides, such as CaTiO₃, CaMnO₃, Ca₂Fe2O₅, CaZrO₃, and CaO-CeO₂, ion exchange resins, and alkali metal compounds based on alumina are the most useful [58, 59, 60, 61]. These solid-base catalysts, including CaO, MgO, SrO, KNO₃/Al₂O₃, K₂CO₃/Al₂O₃, KF/Al₂O₃, Li/CaO, and KF/ZnO, applicable for transesterification [62, 63]. The basic hydrotalcite of Mg/Al, Li/Al, anion exchange resins, base zeolites, hydrotalcite, calcium carbonate rock, Li/CaO, MgO/KOH, and Na/NaOH/-Al₂O₃ [64, 65, 66, 67]. The oxide catalysts exhibit high yields and stability in the transesterification process [68, 69].

3.3.2 Heterogeneous acid catalysts

These catalysts are less toxic, corrosive, and cause less environmental issues [70]. They have a variety of acidic sites with varying levels of Lewis acidity. Despite offering encouraging results under modest reaction circumstances, they react much more slowly than solid base catalysts. These kinds of catalysts have further conditions like a high catalyst loading, high temperature, and prolonged reaction time [71]. Additionally, solid acid catalysts support the simultaneous transesterification and esterification of oils with high FFA contents to produce biodiesel. For example, solid-acid catalysts with organo-sulfonic groups, such as Nafion and Amberlyst, are used to speed up the esterification of fatty acids. In the transesterification reaction, another mesostructured catalyst modified by sulphonic acid is employed, leading to conversion rates as high as 100% [56, 72, 73, 74].

3.3.3 Heterogeneous nanocatalysts

These catalysts are known to improve the rate of transesterification reaction by removing unwanted processes and unnecessary reaction yield. These catalysts advance simple recovery, reusability, and a cost-effective friendly process [75]. In addition, these catalysts exhibit a number of advantages, such as enduring high FFA and moisture content, essential in certain insensitive high temperature and pressure. The cost-effective heterogeneous catalysts help out to diminish the overall cost of biodiesel production. These catalysts may be made to have a maximum yield of a reaction product by altering the number of atoms, surface functionality, and elemental composition, and they also have an efficient surface area, high stability, and higher resistance to saponification [76, 77, 78, 79, 80]. There are variety of techniques, including vacuum deposition, self-propagating high-temperature synthesis, evaporation, coprecipitation, electrochemical deposition, microwave combustion, hydrothermal, solvothermal, impregnation, and sol–gel technology [56, 81]. These catalysts are formed from nanoparticles with less than 100 nm variety of sizes and morphologies. They demonstrate critical advantages for both heterogeneous and homogeneous catalysts in terms of activity, selectivity, efficiency, and reusability [82, 83]. Here, biodiesel nanocatalysts are divided into magnetic and nonmagnetic categories.

3.3.3.1 Magnetic nanocatalysts

These catalysts can aid in the reaction without the need for centrifugation or ultra-filtration. It is a powerful tool for the rapid separation of catalysts from reaction systems, offering an alternative to time-consuming, solvent-intensive, and energy-intensive separation procedures while sustaining catalytic activity for successive cycles. They are suitable for inexpensive feedstocks [84, 85]. Among the number of magnetic catalyst Fe₃O₄, Cao/Fe₃O₄,Ca(OH)₂/Fe₃O₄,Cs/Al/Fe₃O₄,KF/CaO-Fe₃O₄,Fe₃O₄@SiO₂,MgO/MgFe₂O₄, and Ca/Fe₃O₄@SiO₂ are few of the magnetic catalyst that have recently been developed and used to make biodiesel [86, 87, 88, 89]. For example, A. Ali et al. reported the use of the CaO-Fe₃O₄ magnetic catalyst in the generation of biodiesel from palm seed oil. In addition, cadmium oxide and tin oxide magnetic nanocatalysts have been employed for esterification, transesterification, and hydrolysis reactions of soybean oil [90]. Furthermore, more active catalysts Fe₃O₄ and Fe₃O₄@SiO₂ (MNPs) have been used as fundamental recyclable catalysts achieving 96% yield production. The catalyst ZnO/BiFeO₃ was also a promising catalyst for the generation of biodiesel from canola oil and yields of 95.43 and 95.02% in the first and second cycles, respectively [90, 91]. Another example, SnO produces 84% yield of esterification without loss, at 200°C after 1 h reaction. Therefore, magnetic catalyst show fast, clean, superior stability, and recyclability.

3.3.3.2 Nonmagnetic nanocatalysts

The ability to reuse the catalysts allows for lower-cost biodiesel production in a fixed-bed reactor. There are number of nonmagnetic nanocatalysts, such as hydrotalcite, metal oxides, sulfated oxides, zeolites, and zirconia, which are frequently employed in the manufacture of biodiesel [44, 77, 92, 93, 94, 95]. Molina also used ZnO nanorods to produce biodiesel from olive oil and realized that their catalytic performance yield of 94.8% was slightly higher than the regular ZnO yield of 91.4% [81, 96]. Borah et al. reported a maximum production of 98.03% at a methanol/oil ratio of 9 at 60°C for 3 hours with 2.5 wt% Co/ZnO catalysts for biodiesel [34]. Zhang et al. improved by using the surface modification of the NaAlO₂/c-Al₂O₃ with the M/O of 20.79:1, 10.89 wt% catalysts, and at 64.72°C to achieve the highest biodiesel production, which was 97.65% [97]. Alkaline earth metal compounds, in particular, Ca-enclose nanomaterials, have the potential to be nonmagnetic catalysts for the transesterification of biodiesel. The most widely used nonmagnetic catalysts include MgO/TiO₂ [98], Mg-Al hydrotalcite [99], KF/CaO [100], Mg/Al [101], Li/CaO [102], MgO [40], metal–organic frameworks (MOFs) [97, 103], Nanozeolites al. [104], potassium-doped zeolite [27], and hydrotalcite. The implementation of nanocatalysts gets much attention due to vast surface area strong catalytic performance, and appropriate charge transport channel.

4. Advantages and disadvantages of different catalysts

Finally, Table 2 highlights the advantages and disadvantages of various catalysts. From this point of view, a homogeneous catalyst has been thoroughly examined, and drawbacks have been discussed in the literature. However, there is currently a lot of study being done in the relatively new field of heterogeneous catalysts. The literature has noted a number of benefits and drawbacks for catalysts, as summarized in Table 2.

Types of catalyst	Advantages	Disadvantages	Ref.
Homogenous base catalysts	Simple to use, require less time to achieve a complete reaction high selectivity, turnover frequency, reaction effortless optimization of activity, dissolves quickly.	Containing significant amounts of FFA, cannot be converted into biodiesel completely, remains as soap in vast quantities, required additional amount of catalyst and catalyst lost, sensitivity toward moisture and free fatty acids.	[105, 106, 107]
Homogenous acid catalysts	Offers very high yield, optimum reaction conditions, the process economically confronting, the increased energy requirements.	Slow process, it requires high temperature, it causes corrosion (reactors, pipes, vessels, etc.), other environmental issues, product separation, and purification costs make it a less-attractive process.	[49, 50, 51, 52]
Heterogeneous acid catalysts	Ease of separation. Catalysts can be reused.	High cost of synthesizing catalyst. Higher alcohol-to-oil molar ratios. High catalyst concentrations. Longer reaction time. May undergo deactivation.	[44, 45, 46]
Heterogeneous base catalysts	Ease of purification of the product. Effluent generation minimized. Catalysts can be reused.	High cost of synthesizing catalyst. Leaching of active sites may occur.	[53, 54, 55, 105, 106, 107]
Magnetic nanocatalysts	More active. Suitable for biodiesel production from low-cost feedstocks. Reused without loss in the activity. Cost and energy savings, particularly for high-temperature catalytic reactions. Maintain stability after mechanical, physical, and chemical modification.	Poor dispersion abilities. High cost of synthesis material Limitations in scale-up production processes. Mobility dependent on environment compatibilities. Limited availability for the catalytic process.	[56, 81, 82, 83, 84, 85, 86, 87]
Nonmagnetic nanocatalysts	Lower-cost production. Operate in a fixed-bed reactor. More suitable for the process.	Takes a long time to proceed with the reaction.	[88, 89, 90, 91, 92, 93, 103]
Biocatalyst	Ease of separation. Mild reaction condition. Produces high-purity product. Insensitive to FFA and water content. In the oil	High cost for catalyst synthesis. Sensitive to methanol, causing deactivation of the enzyme. Very slow reaction rate.	[23, 24]
Nanocatalyst	High activity and stability. High reusability. Mild reaction condition.	High cost for catalyst synthesis.	[2, 56, 73, 74, 75, 76, 79, 80]

Table 2.

Different group of catalysts for biodiesel production.

5. Conclusion

The development of various catalysts has recently received a lot of attention due to their high operating catalytic effectiveness. Numerous catalysts have been studied for their ability to improve biodiesel production performance. Among the various catalysts, homogeneous and heterogeneous are the better choices for biodiesel production, and their reaction is primarily dependent on catalytic systems. As a result, heterogeneous catalysts surpass homogeneous catalysts in terms of ease of separation, simplicity, and reusability. To overcome limitations such as stability, recyclability, durability, and aggregation, the production of highly efficient catalysts necessitates additional innovative strategies. The development of extremely active and selective catalysts, as well as their economic viability for industrial use, is an issue that must be addressed. The development of extremely active and selective catalysts that are also economically viable for use in biodiesel production is priority must be addressed.

Acknowledgments

The authors acknowledge Intech open access for the invitation and constructive comments.

Conflict of interest

The authors declare no conflict of interest.