Comparison of various solid catalysts for transesterification reaction.
Abstract
Biodiesel can be produced using domestic resources like straight vegetable oil, animal fats, and waste cooking oil. Its use, instead of conventional diesel, contributes to the reduction of CO2 emissions. The production of biodiesel through transesterification (TE) reactions requires adequate catalysts to speed up the reactions. The classical methods of biodiesel production were conducted using homogeneous catalysts, which have drawbacks such as high flammability, toxicity, corrosion, byproducts like soap and glycerol, and a high wastewater output. Recently, various types of heterogeneous catalysts and continuous reactors have been invented for the production of biodiesel. As a result, the initial choice of catalysts is crucial. However, it is also affected by the amount of free fatty acids in a given sample of oil. In addition, most of the catalysts are not suitable for large-scale industrial applications due to their high cost. Bifunctional heterogeneous catalysts are widely applicable and have a rich history of facilitating energy-efficient, selective molecular transformations, and contributing to chemical manufacturing processes like biodiesel. This chapter underlines the use of bifunctional heterogeneous catalysts for biodiesel production using low-cost feedstock. Furthermore, it examines the sustainability of catalysts and low-cost feedstock for large-scale biodiesel production. Finally, the chapter indicates a further perspective of biodiesel as an alternative fuel using low-cost feedstock and recommends a sustainable bifunctional heterogeneous catalyst for biodiesel production.
Keywords
- energy
- biodiesel
- bifunctional heterogeneous catalysts
- transesterification
- low-cost feedstock
1. Introduction
Because of the energy and global warming crisis, the development of renewable energy has been focused on worldwide [1]. Fossil fuel is the single largest energy source, representing 88% of all total world energy consumption [2]. The U.S. energy information administration, in its international energy outlook 2016 report, indicated that the world’s total energy consumption is significantly increasing [3]. However, numerous studies showed that the combustion of non-renewable fossil fuels contributes approximately 52% of CO2 emissions, which is the major source of greenhouse gases [4]. Nowadays, various renewable resources such as wind, geothermal, solar, wave energy, and biofuel are considered as alternative fossil fuels [5, 6]. Among these alternative fuels, biodiesel is being promoted as a supplementary fuel for diesel engines. The major benefits of biodiesel are its renewability, biodegradability, and ability to blend with other energy sources compared to other alternative fuels [7]. Even though biodiesel is a good alternative to petroleum diesel in various aspects, it is always jeopardized by the high cost of feedstock and the absence of economically and technically viable technology for its efficient production from feedstock [8, 9]. Biodiesel is also known as fatty acid methyl ester and is obtained by the transesterification reaction of methanol and vegetable oil in the presence of a suitable homogeneous or heterogeneous catalyst [10, 11]. Nowadays, various heterogeneous alkali catalysts such as zeolite, alkali earth metal oxides, KF/Al2O3, sodium aluminate etc. were developed for biodiesel production [9]. However, due to the expensive cost of catalyst synthesis, only selective heterogeneous catalysts were utilized in the industry [12].
Bifunctional catalysts facilitate the esterification of free fatty acids (FFAs) into alkyl esters alongside the transesterification (TE) reaction, which allows for the use of waste vegetable oils with high water and FFA contents for biodiesel production [13]. Under ideal reaction circumstances, acid-base bifunctional mixed-metal oxide catalysts produced biodiesel with a conversion rate of almost 100% [14]. For sustainable biomass upgrading, bifunctional heterogenous catalysts have the advantage of combining multiple catalytic processes in a single vessel. Additionally, a catalyst with an acid and base active phase will convert high FFA (>3%) feedstock in a single-step reaction through a simultaneous TE and esterification reaction process [15]. Mesoporous morphology dramatically enhanced the number of active sites, which helps to increase overall biodiesel production. Additionally, it is readily recyclable by straightforward washing and drying to remove adsorbed materials, sustaining activity for numerous cycles without showing any signs of metal leaching [16]. In order to employ waste vegetable oils with higher moisture and FFA levels for biodiesel generation, bifunctional solid catalysts enable the esterification of free fatty acids (FFA) into alkyl esters through the TE reaction [17]. Bifunctional heterogeneous catalysis is based on the fundamental idea that two different kinds of active sites cooperate to carry out a surface-catalyzed reaction [18]. Such two kinds of locations are frequently anticipated to catalyze several fundamental steps inside an overall reaction [19, 20]. Bifunctional materials could be recycled repeatedly, with only a slight deactivation that was attributed to the leaching of the various active metals during TE and poisoning by strongly adsorbed organics [8]. Before discussing the usage of comparable types of materials in biomass refining reactions, we highlight two important types of materials that are utilized as bifunctional catalysts in this section [21, 22]. Because they have enough acid sites on the surfaces with different strengths of Lewis acidity, Nafion-NR50, WO3-ZrO2, and SO4-ZrO2 are used for TE instead of other strong acid catalysts like hydrochloric, sulfuric, or phosphoric acid, hetero-poly acid inseminated on various supports (zirconia, silica, activated carbon, and alumina), and hydrochloric, sulfuric, or phosphoric acid [21]. Vegetable oil derived from edible plants, including palm, soybean, and sunflower oil are common feedstock in biodiesel production [23]. Currently, biodiesel production using edible vegetable oil as a raw material was the main cause of increased global food market prices [9]. Another issue related to edible oil bioenergy is the potential depletion of biomass resources as a result of intensive agricultural practices used in crop cultivation [4]. Numerous studies identified feedstock prices as the most important factor influencing the economic viability of the biodiesel market, accounting for up to 70–95% of total biodiesel production costs [15]. As a result, in order supply marketable biodiesel, the cost of the raw materials must be a key parameter [24]. The prominent alternate solution were using non-edible, low-cost feed stocks resource as a row material to investigate biodiesel [24, 25, 26]. Even though numerous study conducted on the production of biodiesel. The processing technologies are not yet commercially available, however, its predicted to enter the market within the next few years [27].
2. Biodiesel as alternative fuel
Biodiesel is extremely biodegradable and has a low toxicity level [23]. It emits nearly no aromatic chemicals or other chemical pollutants that are harmful to the environment [28]. When the entire life-cycle is evaluated (including cultivation, oil production, and oil conversion to biodiesel), it has a modest net contribution of carbon dioxide; and its production may be decentralized, it has tremendous potential for improving rural economies [29]. In comparison to diesel fuel, biodiesel emits no sulfur, less carbon monoxide, fewer particulates, less smoke and hydrocarbons, and more oxygen. More free oxygen results in complete combustion and lower emissions [30]. Currently, many countries use renewable energy like biodiesel (Figure 1).
3. Production of biodiesel from different feedstocks
One of the advantages of producing biodiesel as an alternative fuel lies in its wide range of available feedstock [33]. The feedstock for biodiesel can be different from one country to another depending on their geographical locations and agricultural practices [30, 33]. The best feedstock must be chosen to guarantee minimal manufacturing costs. The ideal characteristics of a biodiesel feedstock include high oil content, ideal FFA composition, affordable agricultural resources, predictable growth and harvesting seasons, constant seed maturation rates, and a potential market for agricultural by-products [30, 34]. Currently, both edible and non-edible oils are used to produce biodiesel; however, using edible oils on a large scale poses a significant risk to the world [32, 35]. The use of biodiesel feedstock varies from country to country since it depends on the availability and the cost of biodiesel production were depicted in Figure 2.
4. Transesterification (TE)
A chemical process called TE turns TG and alcohol into alkyl esters and glycerol. It is a useful method for converting oil and fat feedstock, which chemically mimics petroleum diesel, into biodiesel. This method converts oils (TG) to low-viscosity alkyl esters, which are similar to diesel fuel [36]. This material can, therefore, be used in current petroleum-based diesel engines without needing to be modified because it has properties similar to those of petroleum-based diesel fuel. Reactants are frequently combined in TE, a reversible reaction when they are heated. But if a catalyst is added, the reaction will proceed more quickly [15]. The simplest chemical reaction for TE of TG is presented in Figure 3.
TE of oil and animal fats with a sufficient catalyst is a useful procedure for biodiesel production [37]. Several chemical catalysts are being utilized for TE of oil. However, these chemicals are expensive, scarce, poisonous, and ineffective. TE is typically catalyzed chemically, as in base catalyzed TE and acid-catalyzed TE, or via enzyme catalysts, as in lipase-catalyzed TEs [38]. When alcohol, often methanol, is used in non-catalyzed TE, there is no need for a catalyst because the alcohol is used in supercritical conditions, when the alcohol is at a temperature and pressure above its critical point and there is no separation between the liquid and gas phases [39]. In the supercritical state, the dielectric constant of alcohol is decreased so that the two-phase formation of vegetable oil/alcohol mixture is not encountered and only a single phase is found favoring the reaction [40]. Each TE process necessitates a unique feedstock. Some esterification procedures are more advantageous than others, at least in terms of manufacturing costs, waste creation, productivity, and so on [40, 41].
5. Catalyst for transesterification
The transesterification of oil can be catalytic, non-catalytic, or enzymatic. Catalytic transesterification of TG to fatty acid methyl ester was a major strategy for increasing biodiesel yield because the catalyst accelerates the rate at which the chemical reaction approaches equilibrium without becoming permanently involved [7]. Even when only a small amount of a catalyst is used, it affect the transesterification rate. The selectivity of catalysts, which are not consumed and many applications have been developed [42].
5.1 Classification of catalyst
Catalysts may be classified generally according to their physical state, their chemical nature, or the nature of the reaction that they catalyze (Figure 4). Catalysts used for biodiesel production are categorized into two types: heterogeneous catalysts and homogenous catalysts.
5.1.1 Heterogeneous catalyst
In the heterogeneous TE reaction [8], a glyceride combines with an alcohol in the presence of a heterogeneous catalyst to produce fatty acid alkyl esters (biodiesel) and glycerol. The oxides of base supported on a large surface area, such as calcium oxide (CaO), magnesium oxide (MgO), and titanium dioxide, are commonly employed as heterogeneous catalysts in TE reactions [43, 44]. CaO is preferred as a catalyst because of its high activity, longer lifespan, and lack of consumption during the reaction [35]. Heterogeneous catalysts are understood to enhance the TE process by avoiding the extra processing costs associated with homogeneous catalysis and minimizing pollutant production (Table 1). Heterogeneous catalysts facilitate facile recovery, reusability, and a low-cost green process [15]. Efficient and low-cost heterogeneous catalysts help to reduce overall biodiesel production costs [25]. Heterogeneous catalysts are essential in difficult environments such as high temperatures and pressures. Such catalysts are easy to recover from the reaction mixture, can tolerate aqueous treatment stages, and can be modified to provide high activity, selectivity, and longer catalyst lifetimes. Several recent studies have focused on the technological and economic viability of producing biodiesel by heterogeneous acid-catalyzed TE [76]. As a result, the acidic catalytic reaction is very appealing for biodiesel production; however, acid catalysts exhibit lower catalytic performance in TE processes compared to basic catalysts, and heterogeneous solid catalysts were easily removed from the products in laboratory conditions. The water-washing and neutralizing processes were restricted [8].
Optimization condition | ||||||
---|---|---|---|---|---|---|
Catalyst | MEOH: oil | Temperature (°C) | Catalyst (%wt.) | Time (h) | Conversion (%) | Reference |
CaO from eggshell | 6:1 | 65 | 4 | 3 | 75.85 | [25] |
KOH load Snail shell | 9:1 | 65 | 6 | 3.5 | 96 | [37] |
KF/CaO | 8:1 | 65 | 5 | 2 | 95 | [6] |
Pyrolyzed rice husk | 20:1 | 110 | 5 | 3 | 98.17 | [45] |
Li loaded eggshell | 10:1 | 5 | 4 | 94 | [46] | |
TiO2–ZnO | 6:1 | 60 | 200 mg | 5 | 92.2 | [36] |
Scallop shell | 6:1 | 65 | 5 | - | 86 | [47] |
KBr loaded eggshell | 12:1 | 65 | 3 | 82.48 | [48] | |
KI loaded oyster shell | 6:1 | 60 | 3.5 | 85 | [49] | |
SBA-15, AlSBA-15, with K2CO3, K2SiO3 and Kac | 9:1 | 60 | 30 | 2.30 | 95 | [50] |
Snail shell modıfıed with TiO2–ZnO | 6:1 | 65 | 3 | 3 | 90-95 | [44] |
Activated Carbon with K2CO3 | 15:1 | 65 | 5 | 5 | 85.1% | [51] |
6:1 | 65 | 6 | 3 | 98 | [52] | |
CaO/Fe3O4 | 15:1 | 65 | 1.3 | 2 | 95 | [53] |
Li/MgO, KOH/MgO | 12:1 | 60 | 9 | 2 | 93.9 | [54] |
Mg-Al hydrotalcite | 4:1 | 45 | 1 | 1.5 | 95.2 | [55] |
Dolomite, CaMgO and CaZnO | 6:1 | 67.5 | 3 | 3 | 91.78 | [56] |
KF/Ca-Al hydrotalcite | 12:1 5 | 65 | 5 | 5 | 97.98 | [57] |
CaO/mesoporous silica | 16:1 | 60 | 5 | 8 | 95.2 | [58] |
KNO3/CaO-MgO | 6:1 | 52.5 | 0.9 | 3 | 78 | [59] |
CaO/Al2O3 | 12:1 | 65 | 100.54 | 5 | 98.64 | [60] |
Nano- KF/Al2O3 | 8:1 | 65 | 5 | 2 | 98.8 | [61] |
Biochar CaO/Al2O3 | 18:1 | 65 | 3 | 3.25 | 98.3 | [62] |
CaO/dolomite | 6:1 | 65 | 5 | 3 | 90 | [63] |
MgO/MgAl2O4 | 12:1 | 110 | 3 | 3 | 95 | [64] |
S2O82−/ZrO2 | 20:1 | 110 | 3 | 4 | 100 | [65] |
Sulfated zirconia | 20:1 | 150 | 3 | 6 | 100 | [66] |
SO3H-ZnAl2O4 | 5:1 | 200 | 1 | 3 | 74 | [67] |
HZSM-5 | 9:1 | 120 | 1 | 1 | 94.7 | [68] |
Carbon cryogel | 20:1 | 65 | 5 | 5 | 91.3 | [69] |
TiSBA-15-Me- PrSO3H | 90:1 | 65 | 5 | 9 | 71 | [70] |
Cs-Na2ZrO3 | 30:1 | 65 | 1 | 0.25 | 98.8 | [71] |
40K/PA-550 | 12:1 | 65 | 5 | 3.5 | 97 | [31] |
35KOH/ZSM5 | 12:1 | 60 | 18 | 8 | 93.8 | [72] |
Sodium silicate | 12:1 | 65 | 2.5 | 0.5 | 97 | [73] |
MgO/MgAl2O4 | 12:1 | 110 | 3 | 3 | - | [74] |
La/Mn oxide | 12:1 | 180 | 3 | 1.5 | - | [75] |
5.1.2 Homogenous catalyst
Homogeneous chemical catalysts have good selectivity, high turnover frequency, high reaction rate, and easy activity adjustment [26]. Homogeneous chemical catalysts such as NaOH, CH3ONa, and KOH were the most commonly used alkali catalysts (Table 1). Because of its high quality and low cost, homogeneous catalysts such as NaOH was used in transesterification; additionally, a small amount is required compared to KOH [42]. Currently, most of the heterogynous catalyst are not effective for the production of biodiesel which leads to hydrolysis or saponification of the fay acid methyl ester [7]. The resulting soap decreases the biodiesel yield and complexes the separation process. As a result, a two-step TE with acid first and alkali second was proposed [77]. The initial acid-based esterification efficiently reduces the oil’s FFA content and prepares the oil for alkali catalysis.
5.2 Heterogeneous catalyst for biodiesel production
Many investigations have been conducted into the synthesis of heterogeneous catalysts in order to alleviate the issues associated with homogeneous catalysts in biodiesel synthesis. The literature has numerous reports on heterogeneous catalysts (acidic, basic, and enzymatic) for biodiesel synthesis. Alkali metal oxides and derivatives, as well as alkaline earth metal oxides [78], derived waste material-based heterogeneous catalysts [44], are examples of these ion exchange resins and sulfated oxides.
5.3 Heterogeneous catalyst from solid waste
Heterogynous catalysts synthesized from waste material play a crucial role to reduce organic pollutant elimination. Conversion of waste biomasses to catalysts helps to improve environmental sustainability and renewable energy production such as biodiesel. CaO derived from seashells, chicken egg shells, and crab shells has been identified as an effective heterogeneous catalyst for biodiesel production (Tables 1 and 2). Those certain shells were reached in calcium carbonate (95%), with the rest being organic materials and other substances like MgCO3, phosphate, and trace metals [81]. Activated catalysts derived from calcined mud crab shells and waste cockle shells reacted with a 3 h reaction rate using palm oil, yielding 98.2% and 99% biodiesel, respectively [37]. Moreover, 98.5% FAME was derived in the TE reactions of palm oil using river snail shells as catalysts (>800°C). With 90% producing biodiesel, CaO leaching is also revealed to be the primary cause of catalytic activity. The CaO impregnated in deionized KF produces 85% biodiesel yield from soybean oil in 4 h with a catalyst concentration of 3.5% wt., with 1:6 oil-to-methanol molar ratio and a reaction temperature of 60°C [82].
Catalyst types | Examples | Advantages | Disadvantages | |
---|---|---|---|---|
Homogeneous | Alkali | NaOH, KOH |
|
|
Acid | H2SO4, HCl, HF |
|
| |
Heterogeneous | Alkali | CaO, SrO, MgO, mixed oxide, and hydrotalcite |
|
|
Acid | ZrO, TiO, ZnO, ion exchange resin, sulfonic modified Meso structured silica. |
|
|
5.4 Acid-catalyzed transesterification
Heterogeneous acid catalysts are less corrosive and harmful than homogeneous acid catalysts and cause fewer environmental problems [83]. These catalysts have a wide range of acidic sites (Figure 5) with varying degrees of Brønsted or Lewis acidity. While these catalysts show promising performance under mild reaction conditions, they react much more slowly than solid-base catalysts [19]. Furthermore, this type of catalyst requires a large catalytic loading, high temperature, and a long reaction time.
5.5 Base-catalyzed transesterification
The most widely utilized approach in the industry is the employment of alkaline catalysts in the TE of waste cooking oil. Because of their lower cost, metallic hydroxides are commonly utilized as catalysts (Figure 6). However, they have less activity than alkoxide [35]. According to reports, the pace of a base-catalyzed reaction is 4000 times faster than that of an acid-catalyzed reaction [83]. The most widely employed catalysts are potassium hydroxide (KOH) and sodium hydroxide (NaOH), both of which are highly sensitive to the quality of the reaction being impacted by the presence of water and free fatty acids [9]. In an alkaline environment, the presence of water may undergo saponification rather than esterification. Furthermore, the free fatty acids can react with the alkaline catalyst to produce soap and water [40]. Saponification not only depletes the catalyst but also results in the production of emulsions that impede biodiesel separation, recovery, and purification.
6. Bifunctional heterogeneous catalyst
Bifunctional heterogeneous catalysts are highly preferred by the chemical industry for both financial and environmental reasons, provided that they are available in the required quantity at a reasonable price and exhibit notable operational advantages over the corresponding homogeneous system [18]. Only a few examples of the use of heterogeneous multifunctional catalysts have been documented, and the majority of them, especially those containing transition metals, have issues with product contamination and limited reusability [20]. Additionally, they frequently require excessive amounts of expensive reagents or other additives, making them unsuitable for the mass production of extensive organic compounds [86]. Bifunctional catalysts that contain both supported acid and metal sites may encourage the transesterification reaction (Table 1). There are two types of bifunctional heterogeneous catalysts. i. Bimetallic: One of the most popular methods of regulating the catalytic performance of supported metal catalysts is to control the composition of bimetallic catalysts [21]. The intention of adding one metal to another is frequently to change the activity of the original metal rather than to establish a new catalytic function. The assumption that the modifying metal is absent from the surface layer limits its ability to directly produce a bifunctional effect in many of the situations where it is dominant [87]. Studies of these bifunctional (or “cooperative”) catalysts have attracted a lot of attention in the scientific community [11]. There was a significant amount of interest in biodiesel production based on numerous reports in the literature on bifunctional heterogeneous catalysts. However, efficient and effective resources should be used to synthesize those catalysts. Furthermore, for large-scale industrial applications, optimization parameters such as reaction rate and effective transesterification should be required. In addition to focusing on efficient oil-to-biodiesel conversion, waste material utilization as a bifunctional heterogynous catalyst should be required to address the current associated problem of environmental pollution. As a result, there has been a lot of research done on bifunctional heterogeneous catalysts. In general, the use of bifunctional (or multifunctional) catalysts is a big step in the right direction for catalysis, especially for reactions that necessitate various kinds of intermediates as part of an overall reaction (Table 1).
7. Current status and future research direction
The main source of greenhouse gases is CO2 emissions, which account for around 52% of emissions. Therefore, in order to reduce the greenhouse effect, alternative energy sources or technology must be developed. Alternative fossil fuels include a variety of renewable resources like wind, geothermal, solar, wave energy, and biofuel. Low efficiency and a high initial maintenance cost are a few downsides. Given the different difficulties of using renewable energy, biodiesel is a possible replacement for diesel made from petroleum. Due to its superior performance, biodegradability, non-toxic nature, lack of hazards, carbon neutrality, low pollution, environmental friendliness, low flammability, superior transportation, longer storage time, and low CO production, biodiesel is significant. On the other hand, it is crucial to produce biodiesel from non-edible sources as well as from edible sources. For the manufacturing of biodiesel, it is best practice to transesterify vegetable oil and animal fats using the appropriate catalyst. Currently, the TE of oil uses numerous chemical catalysts. However, those substances are expensive, scarce, hazardous, and ineffective. To address the current difficulties, it is therefore very important to produce alternative energy sources (biodiesel) from deep-frying oil and heterogeneous catalysts from solid waste. The bifunctional heterogynous catalyst has been the subject of numerous investigations and optimizations. To create an effective and efficient biodiesel yield, the bifunctional heterogynous catalyst must still be produced and studied. Additionally, intensification reactors will call for kinetics studies, optimization parameters, and cost-effective TE in the presence of a bifunctional heterogynous catalyst, which may result in the creation of a workable replacement process for current industrial units. Furthermore, for efficient biodiesel synthesis and the minimization of harmful environmental consequences connected with waste materials, bifunctional heterogynous catalysts made from a variety of environmentally favorable materials would be needed. Therefore, more research should be conducted to produce biodiesel from low-cost feedstock using effective bifunctional heterogeneous catalysts. This requires particularly extensive research to demonstrate their potential for use in the synthesis of industrial biodiesel.
8. Conclusion
Currently, there are numerous types of catalysts that have been investigated for transesterification. However, most of them are expansive, ineffective, and produce a lower yield of biodiesel. To overcome these problems, a bifunctional heterogynous catalyst was a viable alternative for producing high biodiesel yields. As a result, numerous studies on bifunctional heterogeneous catalysts should be conducted in order to improve biodiesel yields and reduce environmental pollution. A bifunctional heterogeneous catalyst can simultaneously esterify free fatty acids and transesterify triglycerides in oil. Using edible oil as a feedstock is not economical and is a major concern for biodiesel production. Therefore, research should be conducted on biomass waste as an alternative feedstock for the production of biodiesel. The chapter summarizes recent research advances in the development of bifunctional heterogynous catalysts for cost-effective biodiesel production. To prepare a better yield of biodiesel, future researchers should be focused on low-grade feedstock and bifunctional heterogeneous catalysts as an effective new route to replacing diesel fuel as a viable energy source in the near future.
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