Effect of MgO activation conditions on its catalytic properties for base-catalyzed reactions

• Published in: Catalysis today 2011-09, Vol.173 (1), p.21-27
• Main Authors: Díez, V.K., Ferretti, C.A., Torresi, P.A., Apesteguía, C.R., Di Cosimo, J.I.
• Format: Article
• Language: English
• Subjects: acetone; active sites; adsorption; Aldol condensation; anions; Base-catalyzed reactions; Catalysis; catalytic activity; Chemistry; citral; condensation; crystal structure; desorption; Exact sciences and technology; General and physical chemistry; glycerol; hydrogen; Hydrogen transfer; infrared spectroscopy; isopropyl alcohol; MgO activation; monoacylglycerols; Monoglycerides; oleic acid; oxygen; Surface physical chemistry; temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; transesterification
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2011.02.060

[Display omitted] ► The MgO basicity may be regulated by controlling the solid activation conditions. ► The density of strong base sites on MgO decreases with the calcination temperature. ► The site density of medium-strength basicity increases with calcination temperature. ► The MgO catalytic properties are modified by increasing the calcination temperature. The effect of the MgO calcination temperature on its basicity and catalytic properties was studied. Three MgO samples calcined at 673, 773 and 873 K (samples MgO-673, MgO-773 and MgO-873) were characterized by different physical and spectroscopic techniques. The surface base properties were probed by temperature-programmed desorption of CO 2 and infrared spectroscopy after CO 2 adsorption at 298 K and sequential evacuation at increasing temperatures. The dimensions of face-centered cubic unit cell for MgO samples decreased while crystallinity and mean crystallite size increased with calcination temperature. MgO samples contained surface sites of strong (low coordination O 2− anions), medium (oxygen in Mg 2+–O 2− pairs) and weak (OH − groups) basicity. The density of strong basic sites was predominant on MgO-673, but decreased with the calcination temperature together with the density of OH − groups; on the contrary, the density of Mg 2+–O 2− pair sites increased with calcination temperature. The catalytic properties of MgO samples were explored for the cross-aldol condensation of citral with acetone to obtain pseudoionones (PS), the transesterification of methyl oleate with glycerol to yield monoglycerides (MG), and the gas-phase hydrogen transfer reaction of mesityl oxide with 2-propanol to form 4-methyl-3-penten-2ol (UOL). The initial PS and MG formation rates decreased with calcination temperature following a trend similar to the density of strong basic sites which suggested that the rate limiting steps for both reactions involve coordinatively unsaturated O 2− active sites. In contrast, the initial UOL formation rate in mesityl oxide/2-propanol reaction increased with MgO calcination temperature following the same trend as medium-strength basic sites, thereby indicating that Mg 2+–O 2− pairs promote the formation of the six-atom cyclic intermediate needed in the Meerwein–Ponndorf–Verley mechanism.