"NANO POROUS MANGANESE OXIDE WITH CRYPTOMELANE STRUCTURE PREPARATION, CHARACTERIZATION AND CATALYTIC ACTIVITIES''

Abstract

Manganese oxide octahedral molecular sieves (K- OMS-2) cryptomelane type (KMn8O16) with tunnel structured was found to be very active and are promising catalysts in the oxidation processes. K-OMS-2 samples are prepared via different methods such as; reflux using MnSO4.H2O, MnCl2.4H2O and Mn (CH3COO)2. 4H2O as precursors (acidic medium), oxidation with H2O2 (basic medium), reduction (neutral medium), solvent free method, in addition to the sono- chemistry one. Different techniques are applied to investigate the physico-chemical characteristics of the prepared K-OMS-2 samples as; x-ray powder diffraction analysis (XRD), Fourier transformer infra-red spectroscopy, high Raman spectroscopy, thermal analysis, resolution transmission electron microscopy (HRTEM), nitrogen adsorption-desorption programmed reduction. The XRD results established that, the prepared K- OMS-2 samples using reflux, reduction and solvent free methods were mono-phasic cryptomelane-type with nano- rods structure. Other oxide phases are formed within the preparation of K-OMS-2 in oxidation and sono-chemistry xvi isotherms and temperature

Abstract methods, which are shown to be significantly, affect the cryptomelane purity. Temperature-programmed experiments represent that the mobility and reactivity of the oxygen species are higher in K-OMS-2 sample prepared via the reduction method compared with the other ones. The performance of the prepared OMS-2 samples was studied towards the oxidative dehydrogenation of cyclohexane at reaction temperature varying between 300- 400oC in a flow system operated under atmospheric pressure with cyclohexane flow rate 3 ml/h. The results reveal that the converted products are mainly composed of gaseous products (C1-C3 and CO2) and liquid products which include oxygenated components (cyclohexanol, cyclohexanone and methanol), dehydrogenated (cyclohexene and benzene) and alkylated aromatic products (toluene, ethylbenzene, xylene, cumene and trimethylbenzene), in addition to water molecules formation. The prepared K-OMS-2 samples (pure cryptomelane material) on using the reduction route (K- OMS-2N) have higher catalytic activity and it is the most selective catalyst towards the oxidative dehydrogenation of cyclohexane. xvii