The effect of MgO and preparation techniques of the FeMnOδ/MgO–Al2O3 catalyst used for the vapor phase oxidation of cyclohexane

Abstract

Magnesia modified alumina with varying contents of MgO was synthesized by the impregnation technique and used as support for the synthesis of supported iron–manganese oxide catalysts. The effect of synthesis techniques such as incipient wetness impregnation, wet impregnation, and sequential impregnation of iron and manganese over the modified supports was investigated in the vapor phase selective oxidation of cyclohexane. The addition of MgO resulted in MgAl₂O₄ spinel formation, which altered the acidic–basic properties and modified the interaction of Fe–Mn metal oxides with alumina. The iron oxides and manganese oxides were highly dispersed in the catalysts synthesized by the sequential impregnation (SI-I) method, whereas the iron oxide forms a highly crystalline α-Fe₂O₃ phase when prepared by the SI-II method. The catalyst prepared by the SI-I method (Mn addition followed by Fe) exhibited the highest activity and yield of KA oil and remained stable for more than 10 h. The formation of adsorbed cyclohexanolate (and phenolate) species was observed as intermediate species during the in situ DRIFTS studies for the reaction. Also, the catalytic performance was analysed by varying the catalyst weight. The increasing weight of the catalyst in the reactor affected the percent conversion of cyclohexane and the product distribution significantly.