"Basic catalysis on MgO: generation, characterization and catalytic properties of active sites"

J.I. DI COSIMO; V.K. DÍEZ; C.A. FERRETTI; C.R. APESTEGUÍA

Catalysis

RSC Publishing

Año: 2014; p. 1 - 28

The generation, characterization and catalytic properties of MgO active sites were studied. MgO samples stabilized at different temperatures were used to control the distribution of surface base sites; specifically, MgO was calcined at 673 K, 773 K and 873 K (samples MgO-673, MgO-773 and MgO-873). The nature, density and strength of MgO base sites were characterized by temperature-programmed desorption of CO2 and infrared spectroscopy after CO2 adsorption at 298 K and sequential evacuation at increasing temperatures. MgO samples contained surface sites of strong (low coordination O2 anions), medium (oxygen in Mg2þ-O2 pairs) and weak (OH groups) basicity. The density of strong basic sites was predominant on MgO-673. The increase of the calcination temperature drastically decreased the density of strong base sites and to a lesser extent that of weak OH groups, while slightly increased that of medium-strength base sites. The catalytic properties of MgO samples were proved for the aldol condensation of citral with acetone to yield pseudoionone, the hydrogen transfer reaction of mesityl oxide with 2-propanol to obtain the unsaturated alcohol 4-methyl-3-penten-2ol, and the synthesis of monoglycerides via the transesterification of methyl oleate with glycerol. The effect of calcination temperature on the MgO catalytic properties depended on the basicity requirements for the rate-limiting step of the base-catalyzed reaction. The activity for both the aldol condensation of citral with acetone and the glycerolysis of methyl oleate diminished with the MgO calcination temperature because these reactions were essentially promoted on strongly basic O2 sites. In contrast, the synthesis of 4-methyl-3-penten-2ol by the hydrogen transfer reduction of mesityl oxide with 2-propanol increased with calcination temperature because the reaction intermediate was formed on medium-strength Mg2þ-O2 pair basic sites. Additional information on the role played by the MgO active sites on the kinetics of base-catalyzed reactions was obtained by performing molecular modeling studies on our MgO catalysts using Density Functional Theory (DFT) for the glycerolysis of methyl oleate, an unsaturated fatty acid methyl ester (FAME). The molecular modeling of glycerol and FAME adsorptions was carried out using terrace, edge and corner sites for representing the MgO (100) surface. In agreement with catalytic results, calculations predicted that dissociative chemisorption of glycerol with O?H bond breaking occurs only on strong base sites (edge sites) whereas nondissociative adsorption takes place on medium-strength base sites such as those of terrace sites. Results also indicated that glycerol was more strongly adsorbed than FAME. The glycerol/FAME reaction would proceed then through a mechanism in which the most relevant adsorption step is that of glycerol.