Unsupported and silica-supported perovskite-type lanthanum manganite and lanthanum ferrite in the conversion of ethanol

Abstract

The acid-base properties of perovskite-type LaMnO3 and LaFeO3 catalysts were adjustable by immobilizing them on silica. Unsupported LaMnO3 and LaFeO3 contained majorly basic sites on their surfaces. Dispersing LaMnO3 and LaFeO3 on silica suppressed their basicities and improved their acidities due to the increased concentrations of tetravalent B-site cations. Catalytic performances of ethanol conversion reflected the acid-base properties of bulk and silica-supported LaMnO3 and LaFeO3. Under differential analysis conditions, unsupported perovskites were active in base-catalyzed reactions such as reverse aldolization and Tishchenko reaction, while silica-supported perovskites were active in aldolization and dehydration. Furthermore, a higher aldolization activity of LaMnO3/SiO2 than that of LaFeO3/SiO2 was discovered. The different aldolization activities were attributed to excess mobile oxygen on the surface of LaMnO3/SiO2, forming aldolization-active Lewis acid (Mn4+)-base (nonstoichiometric oxygen) pairs. In contrast, the Lewis acid-base pair was absent in LaFeO3/SiO2, of which oxygen vacancies were enriched with. Oxygen nonstoichiometry was responsible for the varying aldolization activities of silica-supported LaMnO3 and LaFeO3 catalysts.