Synthesis of Cryptomelane-Type Manganese Oxides by Microwave Heating

Abstract

Cryptomelane was prepared by the reaction of potassium permanganate and maleic acid. A black gel was initially formed by stirring the mixture of the two reagents for 70 min at room temperature. The gel was split into two parts and calcined in a microwave oven and in a conventional oven. The products were characterized by XRD, TGA, and IR. In microwave syntheses, cryptomelane emerged at 320 °C. Cryptomelane can be obtained only above 370 °C in conventional syntheses. As the temperature continued to rise, another phase with a main XRD peak at 2.72 A occurred which is due to bixbyite (a CaF2 type structure material). The formation of bixbyite usually takes place at temperatures above 600 °C in conventional syntheses. By microwave treatment bixbyite was obtained at a temperature as low as 470 °C, and the amount of bixbyite increased as the calcination temperature was increased on the basis of ratios of relative intensities of XRD peaks of bixbyite and cryptomelane. Calcination time had a similar effect on the...