Alkylguanidine‐catalyzed heterogeneous transesterification of soybean oil

Abstract

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%...

Highly efficient carbene and polycarbene catalysis of the transesterification reaction

“…Among them special attention was paid to guanidine derivatives (see also [35][36][37][38] ), which having basicities close to carbenes do not form soaps during the reaction and their catalytic efficiencies are close to those for sodium hydroxide and methoxide (for 1,5,7-triazabicyclo[4.4.0]dec-5-ene TBD TON is no more than 270 at 70 C, 1 mol% of a catalyst, the molar ratio of rapeseed oil : methanol is 1:2.3). As can be seen from the data obtained in the present work carbenes are considerably more efficient (TON 2667 at room temperature, 0,1 mol% of a catalyst, the molar ratio of sunflower oil : methanol is 1 :20).…”

Acidic Mesoporous Silica for the Catalytic Conversion of Fatty Acids in Beef Tallow

“…[1][2][3][4][5][6] However, alkyl ester production has been limited due to the inability to compete economically because of high raw material and production costs. To improve the economic outlook of alkyl esters, the selection of raw materials, reaction conditions, and the use of efficient catalysts become critical.…”

Alcoholysis of triacylglycerols by heterogeneous catalysis

“…Activity of non-ionic bases as catalysts for the transesterification of vegetable oils was studied by Schuchardt et al [10][11][12]. Amino acids of high basicity catalyse the homogeneous alcoholysis as the investigations of the authors have shown.…”