KINETIC AND MECHANISTIC STUDY OF THE NONISOTHERMAL DECOMPOSITION OF MANGANESE(II) ACETATE TETRAHYDRATE

The non-isothermal decomposition of manganese(II) acetate tetrahydrate was studied between ambient temperature and 500 degrees C by means of thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The dehydration process was found to take place in two consecutive steps. The first step commences early (approximate to 20 degrees C) while the second step is accompanied by melting (at 120 degrees C) with the evolution of acetic acid. Careful investigation of the course of the decomposition revealed that two reaction intermediates are formed. The first was identified as acetyl manganese acetate, Mn(CH3COO)(2) . COCH3, at approximate to 120 degrees C, which is believed to form during the second dehydration step. The second intermediate compound was identified as manganese acetate hydroxide, Mn(CH3COO)(2) . OH, formed near 155 degrees C in an exothermic process. This is followed by the decomposition of the hydroxide intermediate to form MnO as a solid product, with possible melting. Then, catalytic decomposition of the anion takes place, with the formation of initial gaseous products on the surface of MnO. In air, however, Mn3O4 is formed as a residual solid product. IR spectroscopy, X-ray diffractometry and scanning electron microscopy (SEM) were used to identify and investigate the solid decomposition products. Gas chromatography (GC) was used to identify the volatile gaseous products as acetone, acetic acid, CO, CO2 and trace amounts of acetaldehyde. Methane and isobutene were formed at high temperatures, i.e. at 290 degrees C. The kinetic parameters (activation energy Delta E, and frequency factor ln A), and the thermodynamic parameters (enthalpy change Delta H, heat capacity C-p, and the entropy change Delta S), were calculated for the dehydration and the decomposition processes.