Monoglyceride synthesis by glycerolysis of methyl oleate on MgO: catalytic and theoretical investigation of the active site

C. FERRETTI, S. FUENTE, R. FERULLO, N. CASTELLANI, C. APESTEGUÍA AND I. DI COSIMO

Detroit, Michigan

Congreso; 22nd North American Catalysis Society Meeting; 2011

The North American Catalysis Society

During biodiesel synthesis by oil or fat transesterification, glycerol (Gly) is obtained as the main co-product, representing 10 % of the biodiesel production. Nowadays, glycerol surplus is becoming a matter of environmental and economic concern and conversion of Gly into high value-added chemicals or fuels is needed. Monoglyceride (MG) synthesis by glycerolysis of fatty acid methyl esters (FAME), is an interesting option to transform this biomass-derived compound into fine chemicals. In this work, the glycerolysis of oleic acid (C18:1) methyl ester was studied using MgO. The chemical nature of the base site for promoting the MG synthesis was investigated using several characterization techniques and Density Functional Theory (DFT) calculations. In order to get insight into the chemical nature of the basic sites responsible for glycerolysis activity, a set of MgO catalyst treated at different Tcalc was prepared and characterized.  By FTIR of CO2, all the MgO catalysts showed different CO2 adsorption species on surface sites having the following base strength order: unidentate carbonate species (U.C.) formed on strongly basic O3c-2 or O4c-2 anions located in corners or edges > bidentate carbonate species (B.C.) formed on medium- strength Mg5c2+-O5c2- pairs  located on terraces > bicarbonate species on weak OH- groups. The higher band intensity of U.C. species suggests that the MgO surface contains mainly strong base sites in a number that decreases upon increasing Tcalc. Based on the identification of the surface oxygen species by FTIR, the contribution of these species was determined by deconvolution of the CO2 TPD curves. The effect of Tcalc on the total number of base sites (nb) and on the contribution of each surface species is presented showing that the strong base sites are affected by the increase of Tcalc whereas the other surface species remain unchanged. Glyceryl monooleate (MG) yields as high as 70% can be obtained at 493 K on MgO treated at different Tcalc with 30% DG and no TG formation. The initial catalytic activity of MgO for the glycerolysis reaction was affected by Tcalc in a similar way to the base sites indicating that the reaction mainly takes place on strong base sites, such as the coordinatively unsaturated oxygen anions present in corners or edges (O3c-2 or O4c-2). Preliminary DFT calculations reveal that Gly chemisorbs non-dissociatively on terrace sites whereas dissociative chemisorption of the O-H bond occurs on strong base sites (edges and corners) in agreement with the results presented here. The O-H bond breaking is a necessary step of the glycerolysis reaction mechanism. Gly chemisorption through the primary OH groups is favored over that of the secondary OH, in line with the catalytic results showing that the major MG isomer is á-MG (» 62% selectivity). Besides, calculations suggest that Gly adsorbs more strongly than FAME on MgO. Results suggest that the reaction i) is preferentially promoted by low-coordination oxygen anions where Gly primary OH groups dissociate; ii) probably follows an Eley-Rideal mechanism with negligible FAME adsorption.