Kinetic and theoretical study of the conversion reactions of methyl oleate with glycerol on MgO

Abstract The reaction kinetics of the liquid-phase synthesis of glyceryl monooleates (monoglycerides, MG) and dioleates (diglycerides, DG) from methyl oleate (a fatty acid methyl ester, FAME) and glycerol (Gly) was studied on MgO. The reaction occurs in consecutive steps, with the MG formation by glycerolysis of FAME being the first step, followed by the transesterification of MG with FAME toward DG. The reaction system was investigated at different temperatures and reactant ratios. MG yields of up to 62% were obtained at 503 K using a Gly/FAME molar ratio of 4.5. A molecular modeling based on DFT calculations was used to study the Gly and FAME adsorptions and the reaction mechanism toward MG on a MgO (100) surface. Results showed that the adsorption of hydrophilic Gly on MgO is much stronger than that of hydrophobic FAME. Two transitions states and one tetrahedral intermediate participate in the reaction pathway toward MG. Based on the DFT results, a five-step heterogeneous Langmuir-Hinshelwood-Hougen-Watson mechanism and kinetic model were postulated considering that the reaction occurs between an adsorbed Gly molecule and a FAME molecule in the liquid phase. After preliminary discrimination based on initial rates, the surface reaction steps toward MG and DG were taken as rate-limiting. The model predicts the consecutive formation of DG after glycerolysis of FAME toward MG. In addition, the model anticipates that the MG/DG ratio will increase with temperature in agreement with the catalytic results and that the only relevant adsorption enthalpy is that of Gly. The kinetic parameters derived from regression of the experimental data at different temperatures satisfactorily describe also the catalytic results obtained at different Gly/FAME ratios.