Kinetic and mechanistic study of the non-isothermal decomposition of manganese(II) acetate tetrahydrate

Abstract

The non-isothermal decomposition of manganese(II) acetate tetrahydrate was studied between ambient temperature and 500°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 (≈20°C) while the second step is accompanied by melting (at 120°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(CH 3COO) 2 · COCH 3, at ≈120°C, which is believed to form during the second dehydration step. The second intermediate compound was identified as manganese acetate hydroxide, Mn(CH 3COO) 2 · OH, formed near 155°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, Mn 3O 4 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, CO 2 and trace amounts of acetaldehyde. Methane and isobutene were formed at high temperatures, i.e. at 290°C. The kinetic parameters (activation energy Δ E, and frequency factor ln A), and the thermodynamic parameters (enthalpy change Δ H, heat capacity C p , and the entropy change Δ S), were calculated for the dehydration and the decomposition processes.