Kinetic parameters of a homogeneous transmethylation of soybean oil

The kinetic parameters of a truly homogeneous base-catalyzed transmethylation of soybean oil at four temperatures from 23 to 50 degrees C have been determined for the first time. The addition of oxolane, as well as a high methanol/oil molar ratio (27:1), was used to achieve and maintain a monophasic system throughout the reactions. Second order rate constants were determined in terms of the concentrations of the mechanistic reactants, which were the glycerides and methoxide ions (and not methanol). Doubling the methoxide concentration increased the reaction rate twofold. At 23 degrees C the rate constants for the conversion of triglycerides to diglycerides, diglycerides to monoglycerides, and monoglycerides to glycerol were 6.3, 15.3 and 13.0 L mol(-1) min(-1), respectively. These increased to 54.2, 136 and 139 L mol(-1) min(-1) at 50 degrees C. These latter values (adjusted to 60 degrees C) were 65, 770 and 190 times larger, respectively, than values cited previously (and adjusted to the methoxide basis) for a transmethylation of Pongamia oil, which was also claimed to be homogeneous. Activation energies for all three steps were the same (63 kJ mol(-1)). Pre-exponential factors showed that in diglycerides and triglycerides the second and third ester chains sterically hindered the attack of methoxide ions on the ester carbonyl groups.