2.1 Homogeneous catalysts in biodiesel production

The homogenous catalysts have been investigated for esterification, ester hydrolysis, and transesterification. The homogeneous catalysts are divided into acid and base catalysts. The usual alkali homogenous catalysts for biodiesel production are often NaOH, KOH, NaOCH3, and KOCH3. The studies indicated that the alkali catalysts could speed up the transesterification reaction of low free fatty acid (FFA) much more than acid catalysts, whereas, in the case of the oils containing high FFA or high water content, the homogenous acid catalysts are preferred (Banković-Ilić et al., 2017; Salimi and Hosseini, 2019).

Transesterification by alkali catalysts is the conventional route for biodiesel production from pure vegetable oils. In the biodiesel reaction, three moles of alcohol react with one mole of triglyceride to produce glycerol (1 mol) and fatty esters (3 mol). The optimum ratio of alcohol to triglyceride was 6:1to reach a biodiesel yield of 98 %. Some studies indicated that alkali metal methoxides (methoxide salts) are more active than alkali metal hydroxides for biodiesel production (Gardy et al., 2019; Felizardo et al., 2006; Marris, 2006). The yield of biodiesel is reduced and separation gets difficult when more than 0.5 wt% of FFA exists in the feedstocks due to soap formation. Also, for the feedstocks containing high water (more than 6 wt%), the yield of biodiesel production decreased, because of the sensitivity of the alkaline catalysts towards moisture and free fatty acids. The existence of water in feedstock decreases the yield of biodiesel and is needed to use more catalysts during the process. So, the reduction of the water in feedstock by pretreating FFA is needed for the transesterification reaction catalyzed by alkaline catalysts. Furthermore, the solubility of the catalyst makes it difficult to separate the catalyst from the products (Gardy et al., 2019; Zhang et al., 2014).

Besides, the acid catalysts used in the esterification process are HCl, H₂SO₄, H₃PO₄, organic sulfonic acids (R-SO₃H), and boron trifluoride (BF₃) (Samios et al., 2009; Moushoul et al., 2016). Acid catalysts can produce biodiesel from.

cheaper feedstocks, making the process more economical although it requires high temperature, takes place more slowly, and causes corrosion (reactors, pipes, vessels, etc) and other environmental issues; product separation and purification costs make it a less-attractive process. The transesterification reactions catalyzed by acids have less rate than the alkaline-catalyzed reactions (Koh and Mohd Ghazi, 2011). In these reactions, it is required expensive equipment to overcome the corrosion by acid (Gerpen, 2005; Lee, 2014; Lee et al., 2014). The disadvantages of homogeneous acid catalysts are that high concentration acidic catalysts are needed and the reaction is carried out at a high temperature (greater than100 °C). These conditions cause to form ether and partially burn the oil and finally reduce FAME yield (Gardy et al., 2019; Canakci and Van Gerpen, 2001). Furthermore, there are also some limitations such as separation of catalysts, sensitivity to water, FFAs in the feed and soap formation and needing to neutralize the catalyst.

2.2 Heterogeneous catalysts in biodiesel production

Heterogeneous catalysts have some excellent properties to be used as potential catalysts for biodiesel production. They are easily separated from the products and are noncorrosive. Also, they have an environmentally friendly nature and can be regenerated to reuse. Some suitable heterogeneous catalysts should be developed for biodiesel production from cheap feedstocks. Heterogeneous catalysts are prepared by various methods such as co-precipitation, sol–gel auto-combustion, vapour deposition, electrochemical methods, etc.

The heterogeneous catalysts are also divided into the base and acid catalysts.

2.3 Esterification by enzymes

The enzymes, the biological catalysts, can handle the transesterification reaction of vegetable oils or animal fats. Lipase enzymes are the main enzymes for this purpose. The enzymes are used to convert the raw materials containing high FFAs into FAME. The Enzymes for the production of biodiesels have some advantages and disadvantages. The advantages of enzymes are that they catalyze the reaction under mild conditions. Also, they are applicable for feedstocks with high FFA content and no saponification occurs during purification and neutralization issues (Shahid and Jamal, 2011). The biodiesel production by the enzyme catalysts is expensive and the reaction needs a long time and the reuse of the catalysts is limited. The yield of biodiesel obtained from the enzymatic reaction is relatively low, making it less economical (Zuliani et al., 2018).

3.1 Nonmagneticnanocatalysts

The ability to regenerate and reuse the solid catalysts makes it possible to lower-cost production of biodiesel and opportunities to operate in a fixed bed reactor. The mass transfer resistance is a common problem in heterogeneous catalysts and takes a long time to proceed with the reaction with acceptable efficiency. These drawbacks limit the application of bulk solid catalysts for industrial applications. The attempt to produce the catalyst with a small particle size and high active surface reduces the mass transfer resistance and helps to increase the yield of biodiesel production (Akia et al., 2014; biofueljournal, n.d.). There was an indirect relationship between particle size and the active surface of the catalyst (Banković-Ilić et al., 2017). A solution to solve the problem of the mass transfer resistance in catalysts is to reduce the particle size in the nanoscale and the development of nanocatalysts. Nanocatalysts usually have a high specific surface area (greater than50 m2 g⁻¹). Nanocatalysts due to having a large surface area resulted in a large number of catalytic sites and therefore high activity and causes to be frequently used in biofuel production (Banković-Ilić et al., 2017). The common nanocatalysts, used in biodiesel production are metal oxides, sulfated oxides, hydrotalcite, zeolites, zirconia, etc (Amini et al., 2013; Han and Guan, 2009; Boz et al., 2009; Feyzi and Shahbazia, 2015; Faria et al., 2009; Tamilmagan et al., 2015). Molina produced biodiesel from olive oils using ZnO nanorods and found that the catalytic performance of the ZnO nanorods (yield 94.8 %) was slightly better than that of the conventional ZnO (yield 91.4 %) (Akia et al., 2014; biofueljournal, n.d.; Molina, 2013).

Madhuvilakku and Piraman produced biodiesel from palm oil by ZnO and TiO2-ZnO nanocatalysts. The insertion of Ti ions in the Zinc oxide lattice led to make the defects in the oxide structure and promotes catalytic activity (Amini et al., 2013; Madhuvilakku and Piraman, 2013).

Borah et al., (Borah et al., 2019) used Mesua ferrea oil for biodiesel production using Co/ZnO nanocatalyst. They reported the maximum biodiesel yield of 98.03 % at methanol/oil of 9 at 60 °C for 3 h with 2.5 wt% catalysts. Zhang et al. optimized the process of biodiesel production over NaAlO2/γ-Al2O3 nanocatalyst by response surface methodology (RSM) (Zhang et al., 2020). The maximum biodiesel yield of 97.65 % was achieved at the M/O of 20.79:1, 10.89 wt% catalysts at 64.72 °C. The yield dropped to 52.16 % after three cycles because of active site leaching.

Baskar et al. (Baskar et al., 2018) produced biodiesel from castor oil with high free fatty acid using Ni-doped ZnO nanocatalyst. The response surface methodology (RSM) predicted 95.20 % at M/O of 8, 11 % wt.% catalyst at 55 °C for 60 min. The catalyst was able to be reused for 3 cycles.

As stated before, in the production of biodiesel the base catalysts exhibit more activity than acid catalysts and are more suitable for the process. So, the basic catalysts are mostly reviewed. Nayebzadeh et al. (Nayebzadeh et al., 2019) reported KOH/Ca12Al14O33-C nanocatalyst for the production of biodiesel from canola oil. The nanocatalyst was a carbonated calcium aluminate and was synthesized by the microwave combustion method.

Ibrahim et al used Na2O/CNT catalyst for efficient biodiesel production from waste oil (Ibrahim et al., 2020). The catalyst was prepared by impregnation of Na2O on the CNTs and calcination at 500 °C for 3 h. The mechanism of the reaction was proposed. In this mechanism, Na + cations act as Lewis acids and oxygen as the Brønsted bases and enhance the activity of the catalyst. (Ibrahim et al., 2020). More than 97 % biodiesel yield resulted from the reaction under conditions of M/O of 20: 1, 3 wt% catalysts at 65 °C for 3 h.

Among the catalysts used for biodiesel production from waste oil, called first-generation biodiesel catalysts, the alkaline earth metal compounds, especially Ca-containing nanomaterials are promising catalysts for the transesterification of oils.

Mguni et al. (Mguni et al., 2012) reported the application of MgO/TiO₂ nanocatalyst for the transesterification of sunflower oil. They investigated three MgO loadings, i.e. 10, 20, 30 wt%, where the related conversions were 225 °C were 84, 91, and 95 %, respectively. The testing of these catalysts at 150 °C resulted in 15, 35, and 42 % conversion for 10, 20, and 30 wt% MgO, respectively. So, they concluded that at 225 °C, the catalyst contained 20 wt% MgO exhibited the highest activity.

Obadiah et al. (Obadiah et al., 2012) used Pongamia oil for the transesterification by calcined Mg-Al hydrotalcite and the process was optimized to get 90.8 % biodiesel yield.

The potassium bitartrate on zirconia support catalyzed the reaction of soybean oil and methanol to get 98.03 % biodiesel yield with M/O of 16 at 60 °C for 2 h (Qiu et al., 2011). Wen et al. (Wen et al., 2010) prepared KF/CaO nanocatalyst and used it for biodiesel production from Chinese tallow seed oil. The yield was 96.8 % for the M/O of 12 and 4 wt% of catalyst at 65 °C for 2.5 h. Deng et al. (Deng et al., 2011) reported the application of hydrotalcite with an Mg/Al molar ratio of 3:1 as a promising catalyst to obtain biodiesel from Jatropha oil (95.2 % yield) under ultrasonic conditions.

Kaur and Ali reported the biodiesel production from Karanja and Jatropha oils using Li/CaO nanocatalyst and got a 99 % yield for Karanja oil and Jatropha oil transesterifications after 1 h and 2 h, respectively (Kaur and Ali, 2011).

The production of 99 % biodiesel yield from soybean oil by nano CaO at room temperature was reported by Reddy et al. (Reddy et al., 2006). The nano CaO in different forms were studied and found that due to the higher surface area, small crystallite size and defects exhibit high yields of transesterification of soybean oil. Also, nano MgO has been studied for biodiesel synthesis from soybean oil by Wang and Yang at both supercritical and subcritical temperatures (Wang and Yang, 2007). They notices the catalyst activity was low under normal temperatures. At 60 °C, the methyl ester yield was only about 3 % after 3 h in the presence of 3 wt% of nano- MgO. The transesterification of triglycerides with methanol under more moderate conditions led to observing a 99.04 % yield in 12 min.

Metal-organic frameworks (MOFs) are high surface area and stable materials with adjustable tunnels that could be used as catalysts and nanocatalyst support in biodiesel production. Zhang et al. reported that the nickel salts of Keggin-type hetero-polyacids encapsulated into a metal–organic framework (UiO-66) hybrid nanocatalysts exhibited excellent activity and reusability for esterification of oleic acid or lauric acid with methanol over silicotungstic acid (Zhang et al., 2020; Zhang et al., 2019). Besides, they reported the synthesis of Sn_1.5PW/Cu-BTC by a simple impregnation method and its application as a novel nanocatalyst for producing biodiesel from oleic acid (OA) through esterification. The conversion of oleic acid was 87.7 % under optimum reaction conditions. The nanocatalyst was reused seven times, and after three uses, the OA conversion remained 80 % (Zhang et al., 2020).

Nano zeolites are another kind of nanocatalysts used in biodiesel production. Nanocrystalline zeolites have acid sites, shape selectivity, and high surface area, which are also used in biodiesel production on an industrial scale. De Vasconcellos et al. (de Vasconcellos et al., 2018) added (3-aminopropyl) trimethoxysilane (APTMS) to functionalize the nano zeolites with glutaraldehyde. They studied the activity of the enzyme–nano zeolite complexes in ethanolysis transesterification of microalgae oil to fatty acid ethyl esters (FAEEs) and demonstrate the role of the complex. The lipase-nano zeolite resulted in around 93 % FAEEs yield and kept its activity after five consecutive cycles.

Al-Ani et al. (Al-Ani et al., 2018) reported the application of mesoporous basic Faujasite-type catalysts in biodiesel production and promising catalytic results were obtained.

Potassium doped zeolite imidazolate framework used in biodiesel process from soybean oil by transesterification process (Saeedi et al., 2016). The conversion of 98 % resulted in a methanol/oil molar ratio of 10:1 for 3.5 h over the Na/ZIF-8 nanocatalyst. The nanocatalyst was recycled and reused three times. There are many papers about the application of nano zeolites in biodiesel productions (Kim et al., 2018; Al-Ani et al., 2019; Amalia et al., 2019).

Hydrotalcites are another kind of nanomaterials, used as nanocatalysts or catalyst support in biodiesel production (Chelladurai and Rajamanickam, 2014). The production of biodiesel in the presence of hydrotalcite can be carried out at ambient temperature. The application of nano-Zn-Mg-Al hydrotalcite as solid base catalysts for the production of biodiesel from neem oil through transesterification was proposed by Chelladurai and Rajamanickam (Chelladurai and Rajamanickam, 2014). Besides, the application of Mg/Al oxides derived from hydrotalcite was reported for transesterification of Jatropha oil with a 95.2 % yield of biodiesel by Dias et al. (Dias et al., 2012). The application of Mg-Al nano hydrotalcite for the methanolysis of neem oil was reported by Manivannan and Karthikeyan (Manivannan and Karthikeyan, 2013). In their study, the effect of the various reaction parameters was investigated and resulted that the reaction temperature played important role in the resulting yield of biodiesel. The maximum yield of 84 % was reported at a reaction temperature of 65 ^◦C. More references about the application of hydrotalcite for biodiesel production are found in the literature (Gao et al., 2010; Sinha and Murugavelh, 2016; Prabu et al., 2019; Woodford et al., 2012).

3.2 Magnetic nanocatalysts

Recently, nanomagnetic catalysts were used for biodiesel production and are considered suitable for biodiesel production from cheap feedstocks (Mokhatr and El-Faramawy, 2021; Mokhatr et al., 2020). The magnetic properties can facilitate the separation of the catalyst from the reaction media, without needing centrifugal techniques and ultrafiltration. They maintained the catalytic activity for several cycles.

A wide range of magnetic nanomaterials have been recently synthesized and used for biodiesel production from cheap feedstocks, some of them reported below. Most of the magnetic nanocatalysts used in the process are in the form of a core/shell, where the Fe₃O₄ or other magnetic materials are the core. Table 1 presents the reaction details of some papers and some of them are described below.

Cadmium oxide and tin oxide nanocatalysts supported by magnetic material have been used in the esterification, transesterification, and hydrolysis reaction of soybean oil (Alves et al., 2014). In the esterification assisted by magnetic SnO, 84 % yield was achieved, at 200 °C after 1 h reaction. The catalyst was reused four times without loss in the activity. Mortadha A. Ali et al. reported the application of CaO-Fe₃O₄ as a nanomagnetic catalyst for biodiesel production from Date palm seed oil. The highest biodiesel yield was 69.7 % under the conditions of (65 °C reaction temperature, 300 min reaction time, 20 methanol/oil molar ratio, and 10 wt% of CaO/Fe₃O₄ catalyst loading) (Ali et al., 2017).

Liu et al. (Liu et al., 2010), reported the preparation of CaO/Fe₃O₄ nanocatalyst for methyl ester production and got 99 % and 95 % yield for the reaction using Ca (OH)₂ /Fe₃O₄ (1:7) after 4 h and 80 min, respectively.

Hu et al. (Hu et al., (201l)) reported the application of Stillingia oil for biodiesel production using KF/CaO-Fe₃O₄. The maximum activity resulted in 25 wt% KF/5 wt% Fe₃O₄; calcined at 600 °C for 3 h. Feyzi et al. (Feyzi et al., 2013) used Cs/Al/Fe₃O₄ nanocatalyst for biodiesel production from sunflower oil. The optimal catalyst showed high activity and a 94.8 % biodiesel yield resulted. The presented examples indicated that the nanocatalysts with a high surface area are more active in esterification than transesterification and hydrolysis reactions. In another paper, Tang et al (Tang et al., 2012) synthesized a magnetic Ca/Al/Fe3O4 composite by loading calcium aluminate onto Fe3O4 nanoparticles and reported its catalytic activity for preparing biodiesel with a biodiesel yield of 98.71 %. The composite kept its activity for 5 catalysis cycles.

Also, there is some paper about functionalized catalysts for biodiesel production. For example, Guanidine-functionalized Fe3O4 and Fe3O4@SiO2 nanoparticles as recyclable catalysts were applied in biodiesel production (Chen et al., 2018). Kanimozhi and Perinbam (Peng et al., 2020) reported the use of an extracellular lipase immobilized onto amino silane/Fe3O4 nanoparticles in biodiesel production from waste cooking oil. Wang et al. (Kanimozhi and Perinbam, 2013) studied glyceryl trioleate conversion to biodiesel over sulfamic and sulfonic silica-coated crystalline Fe/Fe3O4. They found that the sulfamic acid-functionalized nanocatalysts showed higher activity than the sulfonic acid-functionalized nanocatalysts. The above-mentioned examples could reveal the superiority of heterogeneous nanocatalysts compared to other traditional catalysts. The application of magnetic nanocatalysts due to their easy separation and reusability is important for the production of biodiesel.

Except for Fe₃O₄, some other magnetic materials have been used as a core of the core/shell catalysts. Some examples of these nanocatalysts are described below. Alaei and colleagues reported the application of magnetic MgO/MgFe2O4 nanocatalyst, synthesized via combustion method for biofuel production. The optimal conditions resulted in M/O of 12 at 110 °C and 4 wt% catalysts for 4 h, where a maximum biodiesel yield of 91.2 % was obtained (Alaei et al., 2018). Dantes et al. (Dantas et al., 2020) synthesized magnetic Ni0.5Zn0.5Fe2O4 and used it in the reactions of biodiesel production from soybean oil by both methyl and ethyl routes. The superior activity in the esterification reaction with conversions of 99.54 ± 0.16 % and 99.38 ± 0.18 % on the methyl and ethyl route, respectively. The nanocatalysts showed stability for 3 cycles. The maximum conversion for the transesterification reaction was 14 %. Seffati and co-workers (Seffati et al., 2019) reported biodiesel production by chicken fat with CaO/CuFe2O4 nanocatalyst with a yield of 94.52 % at the M/O of 15:1 at 70 °C and catalyst content of 3 % for 4 h. Some magnetic perovskites have also been studied in developing a magnetic catalyst for biodiesel production. For example, Salimi and Hosseini (Salimi and Hosseini, 2019) reported the magnetic ZnO/BiFeO3 nanocatalyst as a promising catalyst for biodiesel production from canola oil and found that under the optimum conditions, the biodiesel yields were 95.43 and 95.02 % in the first and second cycle, respectively. The optimum conditions were obtained at a catalyst dosage of 4 wt%, the molar ratio of methanol/canola oil of 15:1, and a reaction temperature of 65 °C.

Feyzi and Norouzi (Feyzi and Norouzi, 2016) reported the synthesis of a magnetic Ca/Fe3O4@SiO2 nanocatalyst using a combination of sol–gel and incipient wetness impregnation methods and used in the production of biodiesel. The nanocatalyst showed very effective activity at optimum conditions with a yield of biodiesel of 97 %.