The Tremendous Catalytic Activities of the Cryptomelane-Type Manganese Oxide Octahedral Molecular Sieve Prepared Without Calcination Process for Degradation of Methylene Blue

Abstract

The cryptomelane-type manganese oxides octahedral molecular sieve with different crystalline phases have been successfully synthesized and characterized. The oxides were synthesized using precipitation method through the redox reaction between KMnO4 and C6H8O7 with a mole ratio of 5:1 by the variation of flow rates (1, 2, and 3 mL/min). The prepared catalysts were characterized by XRD to determine their phase structures, purity, and crystallinity. The XRD results indicated that the samples without heat treatment (calcination) displayed the poor tunnel structures of cryptomelane-type manganese oxides with low intensities and broad peaks. Upon the thermal treatment at 600°C for 4 hours, the much more crystalline phases of similar cryptomelane structures were obtained. The high flow rate results in the more crystalline cryptomelane for calcined samples, whereas the flow rate has no substantial influence on the crystallinity of uncalcined samples. The catalytic activities of both oxides were evaluated for the degradation of methylene blue (MB) with H2O2 as an oxidant. The catalytic degradation of MB greatly enhanced using the uncalcined cryptomelane compared to calcined cryptomelane. The high flow rate has a positive impact on the degradation of MB for calcined samples, but the effect of flow rate results in the higher degradation of MB until a certain flow rate. The 98.4% degradation of MB was achieved by uncalcined samples under the optimum condition.