Synthesis of Cryptomelane-Type Manganese Oxides by Microwave Heating

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 Å occurred which is due to bixbyite (a CaF 2 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 phase transformation. At 425 °C well-ordered cryptomelane was formed after microwave treatment for 10 min, and bixbyite was detected after 3 h, while at the same temperature cryptomelane was obtained with weak and broad XRD peaks after conventional heating for 20 min. Cryptomelane was the only product with no bixbyite even after heating the sample for 10 h in a conventional oven at 425 °C. These data indicate that microwave heating may have the ability to accelerate the formation of crystalline material and specific phase transformations.