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.
1.7 Acid‐Catalyzed Transesterification
Acid‐catalyzed transesterification offers the advantage of esterifying FFAs contained in the fats and oils and is therefore especially suited for the transesterification of highly acidic fatty materials, such as palm oil or waste edible oils. Used cooking oils typically contain 2–7% FFA, and animal fats contain 5–30% FFA. A few very low quality feedstocks, such as trap grease, can approach 100% FFA level. Also, acid‐catalyzed transesterification enables the production of long‐ or branched‐chain esters, which pose considerable difficulty in alkaline transesterification because the FFA react with the catalyst to form soap and water [126] as shown:
Up to 5% FFA, the reaction can still be catalyzed with an alkali catalyst, but additional catalyst must be added to compensate for that lost to soap. The soap produced during the reaction is either removed with the glycerol or washed out during the water wash. When the FFA level is >5%, the soap inhibits separation of the glycerol from the methyl esters and contributes to emulsion formation during the water wash. Intended for these cases, an acid catalyst such as sulfuric acid can be used to esterify the FFA to methyl esters as shown in the following reaction:
This process can be used as a pretreatment to convert the FFA to methyl esters, thereby reducing the FFA level. In that case, the low FFA pretreated oil can be transesterified with an alkali catalyst to convert the triglycerides to methyl esters [127]. As depicted in the reaction, water is produced and, if it accumulates, it can stop the reaction well before completion. It was projected to allow the alcohol to separate from the pretreated oil or fat after the reaction. Exclusion of this alcohol also removes the water formed by the esterification reaction and permits for a second step of esterification; alternatively, one may proceed directly to alkali‐catalyzed transesterification. It is important to note that the methanol–water mixture will also contain some dissolved oil and FFA that should be recovered and reprocessed. The pretreatment by an acidic ion‐exchange resin has also been described [128]. It was revealed [129, 130] that acid‐catalyzed esterification can be used to produce BD from low‐grade by‐products of the oil refining industry such as soap stock.
Acid catalysts provide high yields, but the transesterification reaction is slow than that by alkali catalysis and requires higher temperatures. The most common acids used are phosphoric, hydrochloric, sulfonic, and sulfuric acids. They are also employed in pretreatment steps, to esterify the FFA, prior to basic catalyzed reaction. Nevertheless, acid catalysis is also affected by the presence of water that inhibits the reaction. Transesterification happens at a faster rate in the presence of an alkaline catalyst than in the presence of the same amount of acid catalyst [131].
The typical reaction conditions for homogeneous acid‐catalyzed methanolysis are temperatures of up to 100 °C and pressures of up to five bars in order to keep the alcohol liquid [132]. A further disadvantage of acid catalysis – probably prompted by the higher reaction temperatures – is an increased formation of unwanted secondary products, such as dialkyl ethers or glycerol ethers [76]. Finally, in contrast to alkaline reactions, the presence of water in the reaction mixture proves absolutely detrimental for acid catalysis. Fonseca et al. reported