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]
