Goering et al. [24] have suggested that vegetable oils are too viscous for prolonged use in direct-injected diesel engines, which has led to poor fuel atomization and inefficient mixing with air, contributing to incomplete combustion. These chemical and physical properties caused vegetable oils to accumulate and remain as charred deposits when they contacted engine cylinder walls. The problem of charring and deposits of oils on the injector and cylinder wall can be overcome by better esterification of the oil to reduce the viscosity and remove glycerol.

Acid-catalyzed alcoholysis of triglycerides (TG) can be used to produce alkyl esters for a variety of traditional applications and for potentially large markets in the biodiesel fuel industry [26]. It can overcome some of the shortcomings of traditional base catalysis for producing alkyl esters. A significant disadvantage of base catalysts is their inability to esterify free fatty acids (FFA). These FFA are present at about 0.3 wt% in refined soybean oil and at significantly higher concentrations in waste greases, due to hydrolysis of the oil with water to produce FFA. The FFA react with soluble bases to form soaps through the saponification reaction mechanism. The soap forms emulsions and makes recovery of methyl esters (ME) difficult. Saponification consumes the base catalyst and reduces product yields. The use of alkaline catalysts requires that the oil reagent be dry and contain less than about 0.3 wt% FFA [27, 28].

Acid catalysts can handle large amounts of FFA and are commonly used to esterify FFA in fat or oil feedstock prior to base-catalyzed FFA alcoholysis to ME [29]. Though it solves FFA problems, it adds additional reaction and cleanup steps that increase batch times, catalyst cost, and waste generation.

Generally, acid-catalyzed methanolysis of TG is carried out at tem­peratures at or below that of methanol reflux (65°C). Using sulfuric acid catalysis under reflux conditions, Harrington and D’Arcy-Evans [30] first explored the feasibility of in situ transesterification, using homog­enized whole sunflower seeds as a substrate. Using reflux conditions, a 560-fold molar excess of methanol and a 12-fold molar excess of sulfu­ric acid relative to the number of moles of triacylglycerol (TAG) were used. They observed ester production, with yields up to 20% greater than in the transesterification of preextracted oil, and suggested that this was an effect of the water content of the seeds, an increased extractability of some seed lipids under acidic conditions, and also the transesterifica­tion of seed-hull lipids.

Stern et al. [31] have developed a process to prepare ethyl esters for use as a diesel fuel substitute from various vegetable oils using hydrated ethyl alcohol and crude vegetable oil, with sulfuric acid as a catalyst. Ethyl ester of 98% purity with a very low acidity has been reported.

Schwab et al. [32] have compared acid and base catalysts and con­firmed that, although base catalysts performed well at lower tempera­tures, acid catalysis requires higher temperatures. Liu [33] has compared the influence of acid and base catalysts on yield and purity of the product, and suggested that an acid catalyst is more effective for alcoholysis if the vegetable oil contains more than 1% FFA.

Goff et al. [34] have conducted acid-catalyzed alcoholysis of soybean oil using sulfuric, hydrochloric, formic, acetic, and nitric acids, which were evaluated at 0.1 and 1 wt% loadings at temperatures of 100°C and 120°C in sealed ampoules, and observed sulfuric acid was effective. Kinetic studies at 100°C with 0.5 wt% sulfuric acid catalyst and 9 times methanol stoichiometry provided more than 99 wt% conversion of TG in 8 h, and with less than 0.8 wt% FFA concentration in less than 4 h (see Fig. 6.12).

Base catalysts are generally preferred to acid catalysts because they lead to faster reactions [35]. Base catalysts generally used in transes­terification reactions are NaOH, KOH, and their alkoxides. KOH is pre­ferred to other bases because the end reaction mixture can be neutralized with phosphoric acid, which produces potassium phosphate, a well-known fertilizer [36].

Darnoko et al. [37] explained transesterification of palm oil with methanol and KOH as a catalyst by the following three-step reaction sequence:

Knothe et al. [38] have reported optimal conditions of a 1 wt% KOH catalyst at 69°C and 7:1 alcohol—vegetable oil molar ratio gave 97.7% conversions in 18 min, when KOH was used with high-purity feedstocks.

Freedman et al. [39] have studied transesterification of sunflower oil and soybean oil with the reaction variables (a) molar ratio of alcohol to vegetable oil, (b) type of alcohol (methanol, ethanol, and t-butanol), (c) type of catalyst (acidic and alkali), and (d) reaction temperature (60°C, 45°C, and 32°C). They have suggested that esterification was 90-98% com­pleted at the respective molar ratio of methanol to sunflower oil 4:1 and 6:1. All three alcohols produced high yields of esters. Alkaline catalysts were
generally much more effective than acid catalysts. The reaction was performed successfully at both 45°C and 60°C in 4 h, with the production of 97% of ME.

Kruclen et al. [40] have presented a process for conversion of a high — melting point palm oil fraction into ethyl esters, which could be used as a diesel fuel substitute. The amount of catalyst used (KOH) was 0.1-1%, and the reaction was completed rapidly at 80°C with yields of 80-94%, depend­ing on the concentration of catalysts. The specific gravity of ethyl ester varied from 0.847 to 0.864 with kinematic viscosity of 4.4-4.6 cSt at 40°C.

Gelbard et al. [41] have determined the yield of transesterification of rapeseed oil with methanol and base by 1H-NMR (nuclear magnetic resonance) spectroscopy. The relevant signals chosen for integration are those of methoxy groups in ME at 3.7 ppm (parts per million) (sin­glet) and of the a-carbonyl methylene groups present in all fatty ester derivatives at 2.3 ppm. The latter appears as a triplet, so accurate meas­urements require good separation of this multiple at 2.1 ppm, which is related to allylic protons.

Chadha et al. [42] have studied base-catalyzed transesterification of monoglycerides from pongamia oil. They separated monoglyceride frac­tions (MG) by column chromatography and then characterized the frac­tions by 1H-NMR spectroscopy in deuterated chloroform (CDCl3) and tetramethylsilane (TMS) (see Fig. 6.13). They explain that 1- or 2-MG are positional isomers. Consequently, in 1-MG, the methylene protons at

C-1 and C-3 are magnetically nonequivalent, due to four double doublets, which are observed in the spectra. But 2-MG, on the other hand, are symmetrical, and C-1 and C-3 methylene protons are magnetically equiva­lent and appear as a multiplate.