Modified manganase oxide octahedral molecular sieves for oxidation and consecutive oxidation - acid reactions

Abstract

The research was focused on improvement of the catalytic activity of octahedral manganese oxide molecular sieve (OMS-2) in oxidation and consecutive oxidation-acid reactions. For oxidation reaction, OMS-2 was modified through incorporation of metals and ion-exchange. Sulphation was applied in order to create the acid sites on Ti-OMS-2 sample which was proven to have the highest oxidative properties and tested for consecutive oxidation-acidic reaction. Ti, Fe, Co and Cu were incorporated into the OMS-2 framework by isomorphous substitution to form Ti-OMS-2, Fe-OMS-2, Co-OMS-2 and Cu-OMS-2, respectively. A new method to synthesize Ti-OMS-2 with high Ti/Mn ratio was applied. Titanium incorporated OMS-2 was successfully synthesized without addition of manganese (II) solution which was normally necessary to synthesize metal substituted OMS-2. Ion-exchange was carried out in order to replace K+ ion in the tunnel structure of OMS-2 framework by H+ ions using concentrated HNO3. Sulphation was done by impregnation of certain amount of H2SO4 in different solvent. The characterization results show that Ti-OMS-2 exhibited a significantly higher Lewis acidity compared to the un-incorporated one. The physicochemical properties-catalytic activity of the modified OMS-2 catalyst was studied in the oxidation of cyclohexane, cylohexene and styrene, and also in consecutive transformation of 1-octene to 1,2-octanediol. Oxidation of cyclohexane with TBHP as oxidant on transition metal substituted OMS-2 showed that Ti-OMS-2 with high titanium content gave the highest conversion, which may be due to the presence Ti sites in the framework and nonframework. For metal incorporated in the framework, there is a correlation of ionic radii of metal substituted with conversion of cyclohexane. An increase in ionic radii of metal substituted OMS-2 increased the conversion of cyclohexane. This correlation may be due to the increase in the Lewis acidity in the metal incorporated OMS-2. The study on the catalytic activity of H-exchanged catalysts in oxidation of cyclohexane showed an increase in conversion of cyclohexane after ion-exchange. For further investigation, Ti-OMS-2 with high Ti/Mn ratio was used in oxidation of cyclohexene and styrene using TBHP as oxidant. The results showed that both titanium sites in framework and non-framework increased the activity of OMS-2 in the oxidation of cyclohexene. However, it was observed that only non-framework titanium species induced a synergetic effect that enhanced the oxidation of styrene. There is a correlation between Ti site location in Ti-OMS-2 catalyst with activation of C-H and C=C bonds. Ti sites in the framework only played role in C-H bond activation whereas Ti site non-framework enhanced the catalytic activity for both types of bond activation. SO4 ²¯/Ti-OMS-2 was proven to be active for consecutive transformation of 1-octene to 1,2-octanediol. However, it was confirmed that Brönsted acid sites did not exist in the sample. The success of the consecutive reactions may be due to the generation of Brönsted acid from hydrolysis of water on the Lewis acid sites of SO4 2-/Ti-OMS-2 sample.