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 that the addition of 0.5% water to a mixture comprising soybean oil, methanol, and sulfuric acid reduced ester conversion from 95% to below 90% [127]. At a water content of 5%, ester conversion decreased to only 5.6%. It should also be noted that water released during esterification of FFA might inhibit further reaction, so that very acidic raw materials might give moderate conversion even in acid‐catalyzed alcoholysis. Shu et al. investigated the effect of reaction variables such as feed composition, temperature, and rate of mixing on the kinetics of the acid‐catalyzed transesterification of waste frying oils. The optimal yield of 99% was achieved after a 4 h reaction [133].
For acid‐catalyzed transesterification, the concentrated sulfuric acid is the most frequently used catalytic substance. Its advantages are its low price and its hygroscopicity, which is important for the esterification of FFAs, removing released water from the reaction mixture. Drawbacks include its corrosiveness, its tendency to attack double bonds in unsaturated FAs, and the fact that concentrated H2SO4 may cause dark coloring in the ester product [65]. Besides, also the use of various sulfonic acids as homogeneous catalysts is reported. These substances have lower catalytic activity than mineral acids. However, they pose fewer problems in handling and do not attack double bonds within the starting material.
1.8 Enzymatic‐Catalyzed Transesterification
Although at present BD is successfully produced chemically, there are several associated problems such as glycerol recovery and the need to use refined oils and fats as primary feedstocks [127]. The use of lipases from various microorganisms is becoming important in BD production. Lipases are enzymes that catalyze both the hydrolytic cleavage and the synthesis of ester bonds in glycerol esters. The disadvantages of using chemical catalysts can be overcome by using lipases as the catalysts for ester synthesis [134]. Advantages mentioned for lipase catalysis over chemical methods in the production of simple alkyl esters include the ability to esterify both acylglycerol linked and FFA in one step, the production of a glycerol side stream with minimal water content and with little or no inorganic material, and catalyst reuse. Other advantages include the occurrence of transesterification under mild temperature, pressure, and pH conditions; neither the ester product nor the glycerol phase has to be purified from basic catalyst residues or soaps. This means that phase separation is easier, high quality glycerol can be obtained as a by‐product, and environmental problems due to alkaline wastewater are eliminated [135]. Moreover, both the transesterification of triglycerides and the esterification of FFAs occur in one process step. Consequently, also highly acidic fatty materials, such as palm oil or waste oils, can be used without pretreatment [136]. Finally, many lipases show considerable activity in catalyzing transesterifications with long‐ or branched‐chain alcohols, which can hardly be converted to FA esters in the presence of conventional alkaline catalysts.
Early work on the application of enzymes for BD synthesis was conducted using sunflower oil as the feedstock [137] and various lipases to perform alcoholysis reactions in petroleum ether. From the tested lipases, only three were found to catalyze alcoholysis with an immobilized lipase preparation of a Pseudomonas sp. offering the maximum ester yields. Maximum conversion (99%) was obtained with ethane, and when the reaction was repeated without solvent, only 3% product was produced with methanol as alcohol, whereas with absolute ethanol and 96% ethanol and 1‐butanol, the ester yields were ranged between 70 and 82%, respectively. Reactions by a progression of homologous alcohols showed that reaction rates, with or without the addition of water, increased with increasing chain length of the alcohol. For methanol, the highest conversion was obtained without the addition of water, but for other alcohols the addition of water increased the esterification rate two to five times.
Pedro et al. reported the lipase‐catalyzed alcoholysis of low erucic acid rapeseed oil without organic solvent in a stirred batch reactor. The best
