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 |
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