A transesterificação de óleos vegetais com metanol, bem como as principais aplicações de ésteres metílicos de ácidos graxos, são revisadas. São descritos os aspectos gerais desse processo e a aplicabilidade de diferentes tipos de catalisadores (ácidos, hidróxidos, alcóxidos e carbonatos de metais alcalinos, enzimas e bases não-iônicas, como aminas, amidinas, guanidinas e triamino(imino)fosforanos). Enfoque especial é dado às guanidinas, que podem ser facilmente heterogeneizadas em polímeros orgânicos. No entanto, as bases ancoradas lixiviam dos polímeros. Novas estratégias para a obtenção de guanidinas heterogeneizadas mais resistentes à lixiviação são propostas. Finalmente, são descritas várias aplicações para ésteres de ácidos graxos, obtidos por transesterificação de óleos vegetais.The transesterification of vegetable oils with methanol as well as the main uses of the fatty acid methyl esters are reviewed. The general aspects of this process and the applicability of different types of catalysts (acids, alkaline metal hydroxides, alkoxides and carbonates, enzymes and non-ionic bases, such as amines, amidines, guanidines and triamino(imino)phosphoranes) are described. Special attention is given to guanidines, which can be easily heterogenized on organic polymers. However, the anchored catalysts show leaching problems. New strategies to obtain non-leaching guanidine-containing catalysts are proposed. Finally, several applications of fatty acid esters, obtained by transesterification of vegetable oils, are described.

The formation of fatty acid methyl esters by transesterification with methanol can be monitored by 1H nuclear magnetic resonance spectroscopy; this accurate determination is simpler than chromatographic methods. JAOCS72, 1239-1241 (1995).

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Transesterification of soybean oil with methanol has been carried out in the presence of heterogenized alkylguanidines as catalysts. The alkylguanidines were anchored to modified polystyrene or siliceous MCM-41, encapsulated in the supercages of zeolite Y, or entrapped in SiO 2 sol-gel matrices. The catalytic activity of these catalysts was compared with that of their homogeneous counterparts, showing that the yields of methyl esters obtained in the homogeneous phase can be obtained with the guanidines anchored to the supports after longer reaction times. The catalysts prepared by immobilization of alkylguanidines in microporous systems showed diffusion restrictions for the vegetable oil as well as the low stability of the inorganic framework.For a long time, transesterification of vegetable oils with short-chain alcohols (e.g., methanol and ethanol) has been the method of choice for preparing fatty acid esters which, together with related alcohols, represent important oleochemical intermediates for further syntheses (1,2). Furthermore, these esters can be employed directly in diesel engines as an alternative fuel, without modification (3,4). The transesterification process can be accomplished by using several compounds as catalysts (2). Recently, we found that alkylguanidines (strong organic bases) in the homogeneous phase are very efficient catalysts for the methanolysis (5) and glycerolysis (6) of triglycerides. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) is the most active alkylguanidine, furnishing high yields of methyl esters or monoglycerides with short reaction times. However, when heterogenized on different substituted polystyrenes (PS), TBD showed slightly lower catalytic activity than its homogeneous analog, reaching the same high yields only after prolonged reaction times (7). In order to find more efficient heterogeneous catalysts to carry out the methanolysis of vegetable oils, we decided to investigate the behavior of 1,2,3-tricyclohexylguanidine (TCG, a trisubstituted alkylguanidine) encapsulated in zeolite Y (8), as well as TCG and TBD anchored onto the surface of MCM-41, as catalysts for this process. TBD has already shown good catalytic activity in Michael additions (9) and in the transesterification of ethyl propionate with n-butanol (10). The results obtained with these new catalytic systems were compared to those observed with TCG and TBD in the homogeneous phase, as well as supported on modified polystyrenes. EXPERIMENTAL PROCEDURESGuanidines. TBD > 98% (Fluka, Buchs, Switzerland) was used as purchased. TCG was synthesized according to our previously described method (5) from 2.0 g (9.7 meq) of 1,3-dicyclohexylcarbodiimide and 2.0 g (20.2 meq) of cyclohexylamine in 20 mL of tert-butanol. The mixture was stirred under a nitrogen atmosphere at 100°C for 19 h. The solvent was evaporated and the product recrystallized from tert-butanol. This produced a yield of 3.1 g (75%) with a melting point of 94-95°C. It was calculated that the crystallized product would consist of 74.7% C, 11.5% H, and 13.8%...

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Glycerolysis of methyl stearate and tristearin has been carried out in the presence of alkylguanidines-strong nonionic bases-as catalysts. When applied at 10 mol%, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,2,3-tricyclohexylguanidine, and 1,3-dicyclohexyl-2-n-octylguanidine give monoglycerides in more than 90% selectivity, in a maximum of 6 h reaction time. JAOCS 75, 755-756 (1998).