SYNTHESIS OF CRYPTOMELANE-TYPE MANGANESE OXIDES BY MICROWAVE-HEATING

Cryptomelane was prepared by the reaction of potassium permanganate an d maleic acid. A black gel was initially formed by stirring the mixtur e of the two reagents for 70 min at room temperature. The gel was spli t into two parts and calcined in a microwave oven and in a conventiona l oven. The products were characterized by XRD, TGA, and IR. In microw ave syntheses, cryptomelane emerged at 320 degrees C. Cryptomelane can be obtained only above 370 degrees C in conventional syntheses. As th e temperature continued to rise, another phase with a main XRD peak at 2.72 Angstrom occurred which is due to bixbyite (a CaF2 type structur e material). The formation of bixbyite usually takes place at temperat ures above 600 degrees C in conventional syntheses. By microwave treat ment bixbyite was obtained at a temperature as low as 470 degrees C, a nd 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 o n the phase transformation. At 425 degrees C well-ordered cryptomelane was formed after microwave treatment for 10 min, and bixbyite was det ected after 3 h, while at the same temperature cryptomelane was obtain ed with weak and broad XRD peaks after conventional heating for 20 min . Cryptomelane was the only product with no bixbyite even after heatin g the sample for 10 h in a conventional oven at 425 degrees C. These d ata indicate that microwave heating may have the ability to accelerate the formation of crystalline material and specific phase transformati ons.