Synthesis, characterization and catalytic activity of cryptomelane/montmorillonite composites

Abstract Manganese oxides are considered to be highly effective catalysts for the oxidation of volatile organic compounds (VOCs). A series of MnO2/montmorillonite composites was prepared to test their catalytic activity in the combustion of model compounds - toluene and ethanol. Several different types of montmorillonites (Mt), referred to as STx-1b, SWy-2, SAz-2, and Kunipia- F, were used to prepare the composites. The MnO2/Mt composites were synthetized in a weight ratio of 1:1, using the reaction between KMnO4 and HCl for the generation of the MnO2 component. Advanced techniques including XRD, SEM/EDS, IR spectroscopy, N2 adsorption/desorption, and H2 TPR were used for physico-chemical characterization of obtained materials. The MnO2 component precipitated as cryptomelane, the tunnel α-MnO2 structure. In catalytic tests, all composite samples were more active than the reference MnO2. The performance of composites in toluene oxidation was correlated with their reducibility. In the case of ethanol, it is suggested that catalytic activity is additionally affected by differences in the distribution of potassium between the Mt and the cryptomelane caused by the different cation exchange capacity (CEC) of montmorillonites. Lower CEC of clay mineral leads to a higher share of potassium in MnO2 enhancing thereby its basicity, which is favorable for ethanol combustion.