The conversion of component TGs to simple alkyl esters (transesterification) with various alcohols reduces the high viscosity of oils and fats (see also Figure 1). Base catalysis of the transesterification with reagents such as sodium hydroxide is preferred over acid catalysis because the former is more rapid (74). Transesterification is a reversible reaction. The transesterification of soybean oil with methanol or 1-butanol proceeded 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) while the reverse reaction was second order (75).

Methyl esters are the most “popular” esters for several reasons. One reason is the low price of methanol compared to other alcohols. Generally, esters have significantly lower viscosities than the parent oils and fats (Tables III and IV). Accordingly, they improve the injection process and ensure better atomization of the fuel in the combustion chamber. The effect of the possible polymerization reaction is also decreased. The advantages of alkyl esters were noted early in studies on the use of sunflower oil and its esters as DF (29-31). Another advantage of the esters is possibly more benign emissions, for example, with the removal of glycerol (which is separated from the esters) the formation of undesirable acrolein may be avoided, as discussed above. These reasons as well as ease and rapidity of the process are responsible for the popularity of the transesterification method for reducing the viscosity-related problems of vegetable oils. The popularity of methyl esters has contributed to the term “biodiesel” now usually referring to vegetable oil esters and not neat vegetable oils.

In the early studies on sunflower esters, no transesterification method was reported (29-31). Another early study used H2S04 as the transesterification catalyst (76). It was then shown, however, that in homogeneous catalysis, alkali catalysis is a much more rapid process than acid catalysis in the transesterification reaction (74, 77). At 32°C, transesterification was 99% complete in 4 h when using an alkaline catalyst (NaOH or NaOMe). At 60 °С and a molar ratio alcohokoil of at least 6:1 and with fully refined oils, the reaction was complete in 1 h to give methyl, ethyl, or butyl esters. The reaction parameters investigated were molar ratio of alcohol to vegetable oil, type of catalyst (alkaline vs. acidic), temperature, reaction time, degree of refinement of the vegetable oil, and effect of the presence of moisture and free fatty acid. Although the crude oils could be transesterified, ester yields were reduced because of gums and extraneous material present in the crude oils.

Besides sodium hydroxide and sodium methoxide, potassium hydroxide is another common transesterification catalyst. Both NaOH and KOH were used in early work on the transesterification of rapeseed oil (78). Recent work on producing biodiesel (suitable for waste frying oils) employed KOH. With the reaction conducted at ambient pressure and temperature, conversion rates of 80 to 90% were achieved within 5 minutes, even when stoichiometric amounts of methanol were employed (79). In two steps, the ester yields are 99%. It was concluded that even a free fatty acid content of up to 3% in the feedstock did not affect the process negatively and phosphatides up to 300 ppm phosphorus were acceptable. The resulting methyl ester met the quality requirements for Austrian and European biodiesel without further treatment. In a study similar to previous work on the transesterification of soybean oil (74, 77), it was concluded that KOH is preferable to NaOH in the transesterification of safflower oil of Turkish origin (80). The optimal conditions were given as 1 wt-% KOH at 69±1 °С with a 7:1 alcohol: vegetable oil molar ratio to give 97.7% methyl ester yield in 18 minutes.

Patents. Most patents dealing with transesterification emphasize the engineering improvement of the process. Using patented procedures, a transesterification process permitting the recovery of all byproducts such as glycerol and fatty acids has been described (81). The use of alkaline catalysts is also preferred on the technical scale, as is documented by patents using sodium hydroxide, sodium methoxide, and potassium hydroxide (82-85). Different esters of C9.24 fatty acids were prepared with A^Q — or Fe203- containing catalysts (86). A sulfonated ion exchange catalyst was preferred as catalyst in the esterification of free fatty acids (87).

Other procedures. Besides the methods discussed here, other catalysts have been applied in transesterification reactions (88). Some recently studied variations of the above methods as applied to biodiesel preparation are briefly discussed here.

Methyl and ethyl esters of palm and coconut oils were produced by alcoholysis of raw or refined oils using boiler ashes, H2S04 and KOH as catalysts (89). Fuel yields > 90% were obtained using alcohols with low moisture content and EtOH-H20 azeotrope.

Instead of using the extracted oil as starting material for transesterification, sunflower seed oils were transesterified in situ using macerated seeds with methanol in the presence of H2S04 (90). Higher yields were obtained than from transesterification of the extracted oils. Moisture in the seeds reduced the yield of methyl esters. The cloud points of the in situ prepared esters appear slightly lower than those prepared by conventional methods.

Another study (97) reported the synthesis of methyl or ethyl esters with 90% yield by reacting palm and coconut oil from the press cake and oil mill and refinery waste with MeOH or EtOH in the presence of easily available catalysts such as ashes of the waste of these two oilseeds (fibers, shell, husk), lime, zeolites, etc. Similarly, it was reported that the methanolysis of vegetable oils is catalyzed by ashes from the combustion of plant wastes such as coconut shells or fibers of a palm tree that contain K2C03 or Na2C03 as catalyst (92). Thus the methanolysis of palm oil by refluxing 2 h with MeOH in the presence of coconut shell ash gave 96-98% methyl esters containing only 0.8-1.0% soap. The ethanolysis of vegetable oils over the readily accessible ash catalysts gave lower yields and less pure esters than the methanolysis.

Several catalysts (CaO, K2C03, Na2C03, Fe^, MeONa, NaA102, Zn, Cu, Sn, Pb, ZnO, and Dowex 2X8 (anion exchange resin)) were tested (mainly at 60-63 °С) for catalytic activity in the transesterification of low-erucic rapeseed oil with MeOH (93). The best catalyst was CaO on MgO. At 200°C and 68 atm, the anion exchange resin produced substantial amounts of fatty methyl esters and straight-chain hydrocarbons.

An enzymatic transesterification method utilizing lipases and methanol, ethanol, 2-propanol, and 2-methyl-1-propanol as alcohols gave alkyl esters of fatty acids (94, 95). This method eliminates product isolation and waste disposal problems.

Analysis of Transesterification Products. Hardly any chemical reaction, including transesterification, ever proceeds to completion. Therefore, the transesterified product, biodiesel, contains other materials. There are unreacted TGs and residual alcohol present as well as partially reacted mono — and diglycerides and glycerol co-product.

Glyceride mixtures were analyzed by TLC / FID (thin-layer chromatography / flame ionization detection) (96), which was also used in the studies on the variables affecting the yields of fatty esters from transesterified vegetable oils (74). Analysis of reaction mixtures by capillary GC determined esters, triglycerides, diglycerides and monoglycerides in one run (97). Free glycerol was determined in transesterified vegetable oils (98) Besides analyzing esters for sterols (99-101), which are often minor components in vegetable oils, and different glycerides (102-103), recently the previous GC method (97) was extended to include analysis of glycerol in one GC run (104). In both papers (97, 104), the hydroxy groups of the glycerides and glycerol were derivatized by silylation with Af-methyl-Af-trimethylsilyltrifluoroacetamide. A simultaneous analysis of methanol and glycerol was recently reported (105).

Other authors, using GC to determine the conversion of TGs to methyl esters, gave a correlation between the bound glycerol content determined by TLC/FID and the acyl conversion determined by GC (106). Glycerol has also been detected by high- performance liquid chromatography (HPLC) using pulsed amperometric detection, which offers the advantage of being more sensitive than refractometry and also suitable for detection of small amounts of glycerol for which GC may not be suitable (107).

Recently, an alternative method for determining the methyl ester content based on viscosity measurements, which agreed well with GC determinations, was reported (108). The method is reportedly more rapid than GC and therefore especially suitable for process control.