Thermal analysis of solvatomorphic decakis (dimethylammonium) dihydrogendodecatungstate hydrates

Abstract

This study aims to describe the thermal decomposition of two solvatomorphs of decakis(dimethylammonium) dihydrogendodecatungstate ((Me2NH2)10H2W12O42·10H2O and 11 H2O) under inert and oxidizing atmospheres. Thermal studies have been done by TG-MS, TG-DSC-MS, XRD and IR methods in both synthetic air and helium atmospheres. The general characteristics of thermal decomposition are similar for both solvatomorphs. Minor differences could be observed in the resolution and shifting of the decomposition peak temperatures depending on the heating rate or atmosphere used. The first step of decomposition is endothermic in both atmospheres and involves 2 and 5 water molecule elimination with ~ 150 and ~ 120 °C peak temperatures for the decahydrate and undecahydrate, respectively. The elimination of further water and dimethylamine was observed with increasing the temperature, as well as the disruption of the lattice of compounds. Until 300 °C, these processes are endothermic in both atmospheres, and the further decomposition processes at higher temperatures are left endothermic in helium, but become exothermic in synthetic air atmosphere. In helium atmosphere, above 350 °C, a solid-phase quasi-intramolecular redox reaction takes place when the dimethylamine degradation products react with the W=O bonds with formation of oxidative coupling products of the organic fragments and reduced tungsten oxide with WO~2.93 composition. In synthetic air, above 350 °C, burning of organic fragments takes place, there are no oxidative coupling products and reduced tungsten oxide formation, and the end product of decomposition is monoclinic WO3.