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Conventionally, transesterification reactions are alkali catalyzed. Alkaline catalysts, such as sodium hydroxide, sodium methoxide, potassium hydroxide, and potassium methoxide, are more effective and most commonly used for BD production [43, 94]. When compared with acid or other type of catalysts, basic ones show a high conversion under mild temperature conditions and in short reaction times [95]. For transesterification giving maximum yield, the alcohol should be free of moisture, and the FFA content of the oil should be <0.5% [96]. The absence of moisture in the transesterification reaction is important because according to the equation (as shown for methyl esters next), the hydrolysis of the formed alkyl esters to FFAs can occur.

Similarly, because triacylglycerols are also esters, the reaction of the triacylglycerols with water can form FFA.

The ester yields are lower with crude oil in the presence of gums and extraneous material. The parameters (60 °C reaction temperature and 6 : 1 methanol:oil molar ratio) have almost become a standard for methanol‐based transesterification. Other alcohols (ethanol and butanol) require higher temperatures (75 and 114 °C, respectively) for optimum conversion [21]. Alkoxides in solution with the corresponding alcohol have the advantage over hydroxides that the water forming reaction according to the equation

cannot occur in the reaction system, thus ensuring that the transesterification reaction system remains as water‐free as possible. This reaction, though, is the one forming the transesterification causing alkoxide when using NaOH or KOH as catalysts.

Effects, similar to those discussed earlier, were observed in studies on transesterification of beef tallow [15]. FFA and especially water should be kept as low as possible [97]. NaOH reportedly was more effective than the alkoxide [98]; however, this may have been a result of the reaction conditions. Mixing was significant due to the immiscibility of NaOH/MeOH with beef tallow, with smaller NaOH/MeOH droplets resulting in faster transesterification [15]. Ethanol is more soluble in beef tallow, which increased yield [99], an observation that should hold for other feedstocks as well.

Several studies reveal the use of both NaOH and KOH in the transesterification of rapeseed oil [100]. At 32 °C, transesterification was 99% completed in 4 h when using an alkaline catalyst (NaOH or NaOMe). At >60 °C, using an alcohol:oil molar ratio of at least 6 : 1 and fully refined oils, the reaction was completed in 1 h yielding methyl, ethyl, or butyl esters [100]. Current work on producing BD from waste frying oils employed KOH. With the reaction conducted at ambient pressure and temperature, conversion rates of 80–90% were achieved within 5 min, even when stoichiometric amounts of methanol were employed [101]. Within two transesterifications (with more MeOH/KOH steps added to the methyl esters after the first step), the ester yields were 99%. It was concluded that FFA content up to 3% in the feedstock did not affect the process negatively, and phosphatides up to 300 ppm phosphorus contents were acceptable. The resultant methyl ester met the quality requirements for Austrian and European BD without further treatment.

In another study, similar to previous work on the transesterification of soybean oil, it was concluded that KOH is more effective than NaOH in the transesterification of safflower oil of Turkish origin [102]. In this experiment, the optimal conditions offering 97.7% methyl ester yields were as follows: 1.0% KOH catalyst (by weight), 69 ± 1 °C reaction temperature, 7 : 1 alcohol:vegetable oil molar ratio, and 18 min reaction time. Depending upon the vegetable oil and its constituent FAs influencing FFA content, adjustments to the alcohol:oil molar ratio and the amount of catalyst may be required as was reported for the alkaline transesterification of Brassica carinata oil [103]. However, advantages of NaOH over KOH as a catalyst are that sodium hydroxide‐catalyzed transesterifications tend to be completed faster [104], and sodium hydroxide is cheaper. It was [34] reported that the esters yield is affected by methanol/oil molar ratio, catalyst type, and its concentration and reaction temperature. They observed that BD with the best properties was obtained using an optimum methanol/oil molar ratio (6 : 1), potassium hydroxide as catalyst (1%), and 65 °C reaction temperature.

Although the alkali hydroxides are the catalysts of choice for methanolysis, these cannot be applied in transesterifications with higher and secondary alcohols, as the reactivity between KOH or NaOH and the alcohol to form the respective alkoxide anion dramatically decreases with increasing chain length [43]. So, if higher or branched‐chain esters are to be produced by alkaline catalysis, only the use of pure sodium or potassium is feasible, although under much higher reaction temperatures than those sufficient for methanolysis. Table 1.2 depicts the homogeneous alkaline catalysts at different reaction conditions for transesterification process.

The reaction mechanism of alkali‐catalyzed transesterification has long been known. The actual catalytic species is the respective alcoholate anion (i.e. methoxide for methanolysis). Transesterification is started by a nucleophilic attack of the alkoxide ion on the carbonyl carbon atom of the triglyceride molecule, resulting in a tetrahedral intermediate. In a second step, this intermediate splits into the desired methyl ester and the anion of the diglyceride. The latter reacts with methanol to from a diglyceride molecule, which will analogously be converted into monoglyceride and glycerol, and a methoxide ion, which can start another catalytic cycle.

The optimum concentration of homogeneous alkaline catalysts ranges from 0.5 to 1.0% by weight of the oil [111]. A high amount of FFAs in the reaction mixture can partly be compensated by the addition of more catalyst [22]. However, it was reported that higher catalyst concentrations increase the solubility of the methyl esters in the glycerol phase, so that a significant amount of esters remains in the lower phase even after separation [112]. Therefore, several investigators suggested for calculating the optimum amount of KOH or NaOH necessary to facilitate transesterification and at the same time neutralize the acidity of the oils (Table 1.2).

In principle, transesterification is a reversible reaction, although in the production of vegetable oil alkyl esters, i.e. BD, the back reaction does not occur or is negligible largely because the glycerol formed is not miscible with the product, leading to a two‐phase system. The transesterification of soybean oil with methanol or 1‐butanol was reported to proceed [35] with pseudo‐first‐order or second‐order kinetics, depending on the molar ratio of alcohol to soybean oil (30 : 1 pseudo‐first order, 6 : 1 second order; NaOBu catalyst), whereas the reverse reaction was second order [65].

The methanolysis of sunflower oil at a molar ratio of methanol:sunflower oil of 3 : 1 was reported to begin with second‐order kinetics, but then the rate decreased due to the formation of glycerol [108]. A force reaction (a reaction in which all three positions of the triacylglycerol react virtually simultaneously to give three alkyl ester molecules and glycerol), originally proposed as part of the forward reaction, has shown that second‐order kinetics are not followed and miscibility phenomena [113] can play a significant role. The cause is that the vegetable oil starting material and methanol are not well miscible. The development of glycerol from triacylglycerols proceeds stepwise via the di‐ and monoacylglycerols, with an FA alkyl ester molecule being formed in each step. From the fact that diacylglycerols reach their maximum concentration before the monoacylglycerols, it was concluded that the last step, formation of glycerol from monoacylglycerols, proceeds more rapidly than the formation of monoacylglycerols from diacylglycerols [114].

The count of cosolvents such as tetrahydrofuran (THF) or methyl tert‐butyl ether (MTBE) for methanolysis reaction was reported to notably accelerate the methanolysis of vegetable oils as a result of solubilizing methanol in the oil to a rate comparable to that of the faster butanolysis [115, 116]. This is to prevail over the limited miscibility of alcohol and oil at the early reaction stage, creating a single phase. The procedure is applicable for use with other alcohols and for acid‐catalyzed pretreatment of high FFA feedstocks. Though, molar ratios of alcohol:oil and other parameters are affected by the addition of the cosolvents. Here, some extra complexity also occurs due to recovering and recycling the cosolvent. This can be minimized by choosing a cosolvent with a boiling point near that of the alcohol being used. However, there may be some hazards associated with its most common cosolvents, THF and MTBE.

Table 1.2 Homogeneous catalysts and reaction conditions used for alkaline transesterification.

Catalyst type	Examples	Reaction conditions	Oils and fats	Alcohol	Esters yield	References
Alkali metals (dissolved in alcohol)	AlCl₃ · 6H₂O	Alcohol:oil = 10 : 1, T = 72 °C, t = 2 h, 1.5 wt% catalyst loading	Waste oil	Methanol	94%	[105]
Alkali metal alcoholates and hydroxide	KOH	Alcohol:oil = 9 : 1, T = 70 °C, t = 1 h, catalyst loading = 1.0 wt%	Waste cooking oil	Methanol	98.2%	[106]
	KOH	Alcohol:oil = 20.39 wt%, T = 57.1 °C, t = 54.1 min, catalyst loading = 0.4 wt%	Black mustard	Methanol	97.3%	[107]
	NaOH	Alcohol:oil = 10:1, T = 65 °C, t = 1.5 h, catalyst loading = 1.5 wt%	Waste cooking oil	Methanol	88.1	[108]
	CH₃ONa	Alcohol:oil = 3.37:1, T = 60 °C, t = 1 h, catalyst loading = 0.5 wt%	Sunflower oil	Methanol	99.7	[109]
	CH₃OK	Alcohol:oil = 5 : 1, T = 86 °C, t = 1.5 h, catalyst loading = 2 wt%	Thevetia peruviana seed oil	Dimethyl carbonate	97.1	[110]

Nevertheless, the traditional homogeneous catalysis offers a series of advantages; its major disadvantage is the fact that homogeneous catalysts cannot be reused. Moreover, catalyst residues have to be removed from the ester product, usually necessitating several washing steps, which increases production costs. Thus, there have been various attempts at simplifying product purification by applying heterogeneous catalysts, which can be recovered by decantation or filtration or are alternatively used in a fixed‐bed catalyst arrangement. The most frequently cited heterogeneous alkaline catalysts are alkali metal and alkaline earth metal carbonates and oxides. For the production of biofuels in tropical countries, Vargas et al. [117] recommended utilizing the ashes of oil crop waste (e.g. coconut fibers, shells, and husks) as catalysts. Such natural catalysts are rich in carbonates and potassium oxide and have shown considerable activity in transesterifications of coconut oil with methanol and water‐free ethanol. Some studies reveal the use of heterogeneous catalysts for transesterification of vegetable oils [118, 119]. No heterogeneous catalysts are commercially feasible in the 45–65 °C range. Some may be feasible at 100–150 °C; however, reactor residence times are more than 4 h, involving large amounts of catalysts. At temperature higher than 100–150 °C, the high pressures needed to keep the methanol in the liquid phase can significantly increase equipment costs [16].

The application of calcium carbonate may seem particularly promising, as it is a readily available, low‐cost substance. Moreover, Ho et al. reported that this catalyst showed no decrease in activity even after several weeks of utilization, and the spent calcium carbonate could easily be disposed of in cement kilns [120]. However, the high reaction temperatures and pressures and the high alcohol volumes required in this technology are likely to prevent its commercial applications. The alkali and alkaline earth metals as a catalyst are also in practice for transesterification of vegetable oils. Arzamendi et al. [121] investigated the methanolysis of refined sunflower oil with a series of catalysts consisting of alkaline and alkaline earth metals. Abdelhady et al. studied the activity of activated CaO as a heterogeneous catalyst in the production of BD by transesterification of sunflower oil with methanol [122]. In another study, Riso et al. investigated the performance of calcium methoxide as a solid base catalyst, and it was observed that 98% BD yields within 2 h [94]. However, drawbacks as associated with heterogeneous catalyst are reported for alkali metal or alkaline earth metal salts of carboxylic acids. The use of strong basic ion‐exchange resins as catalysts, on the other hand, is limited by their low stability at temperatures higher than 40 °C and by the fact that FFAs in the feedstock neutralize the catalysts even in low concentrations. Finally, glycerol released during the transesterification process has a strong affinity to polymeric resin material, which can result in complete impermeability of the catalysts [9].

Other possibilities for accelerating the transesterification are microwave [123] or ultrasonic [28] irradiation. Further fundamental materials, such as alkylguanidines, which were anchored to or entrapped in various supporting materials such as polystyrene and zeolite [124], also catalyze transesterification. Such schemes may provide for easier catalyst recovery and reuse. A review article on various transesterification strategies [125] suggested replacing conventional sodium and potassium compounds by guanidines, such as TBD (l,5,7‐triazabicyclo[4.4.0]dec‐5‐ene). These compounds enable high conversion under comparatively mild reaction conditions like conventional alkaline catalysts, while they will not cause the formation of soaps. Moreover, it was found that guanidines can be fixed on organic polymers, such as modified polystyrene, or can be entrapped in a SiO, sol–gel matrix, which facilitates heterogeneous catalysis and thus enables the repeated use of the catalyst preparation. However, guanidines tend to leach from the carrier, so that the activity of the fixed catalysts markedly decreases in repeated use.