of BD by transesterification has been the focus of many research studies. Several reviews on the production of BD by transesterification have been published [7189–92]. Usually, transesterification can proceed by base or acid catalysis. However, in homogeneous catalysis, alkali catalysis (sodium or potassium hydroxide; or the corresponding alkoxides) is a much more rapid process than acid catalysis [92, 93].
1.5 Variables Affecting Transesterification Reaction
The process of transesterification is affected by various variables depending upon the reaction conditions employed. The most pertinent variables for this kind of reaction are described as follows:
Catalyst type and concentration
Molar ratio of alcohol to vegetable oil
Reaction temperature
Agitation intensity
Reaction time
Water and FFA contents
1.6 Alkaline‐Catalyzed Transesterification
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
