The Influence Of Acid/Base Modification For MnO_x On Their Catalytic Oxidation Performance

Mn ions have a lot of valence states. Meanwhile, the same kind of manganese oxide also has several crystal structures. Thus, the manganese oxides have various structures, which have been used as oxidation catalyst widely. In heterogeneous catalysis, the micro-environment surrounding the active sites has an important influence on the catalytic activity. This paper aims to change the acid and alkaline properties of the manganese oxides, and to improve the catalytic performance of manganese oxide.In this paper, the manganese oxides were prepared by two methods:the precipitation method and the direct calcination method. Different amounts of H3PO4 or KOH were impregnated on the manganese oxides to obtain acid/base modified catalyst. The prepared catalysts were characterized by XRD, TPR, N2 adsorption and desorption, XRF and acid-base titration. Acid modified manganese oxide catalysts remained in the orthorhombic Mn2O3 phase while the acid amount increased. But the acid modified manganese oxide catalysts were difficult to be reduced, and the surface area decreased than their precursor. Alkali modification makes the crystalline phase transit gradually from orthorhombic Mn2O3 to a-MnO2, and which is easier to be reduced.The acid/base modified catalysts were used as catalyst for the oxidation of tetralin, cyclohexane, ethylbenzene, cyclohexene, benzaldehyde under oxygen atmosphere. The result showed that the acid modified catalyst had an adverse effect on the oxidation of ntetralin, cyclohexane and ethylbenzene. And this is associated with the fact that the acid modified catalyst is more difficult to be reduced. However, the acid modified catalyst had a favorable effect on the oxidation of cyclohexene and benzaldehyde because these substrates are easier to be adsorbed on acid sites. The base modified catalyst had a beneficial effect for all the studied substrates. And the probably expalanation is that alkali modification made the crystalline phase Mn2O2 transit to a-MnO2, which is easier to be reduced.