Thermal decomposition kinetics of deep eutectic solvent (DES) based on choline chloride and magnesium chloride hexahydrate: New details on the reaction mechanism and enthalpy–entropy compensation (EEC)

Abstract

In recent years, deep eutectic solvents (DESs) have attracted considerable attention, and they have been applied in many fields, such as dissolution and separation, electrochemistry, materials preparation, reaction, and catalysis. In this paper, a detailed thermal decomposition mechanism of DES-type II (consisting choline chloride (ChCl) and magnesium chloride hexahydrate (MgCl2·6H2O) in a molar ratio 2:1 (MgCl2·6H2O-[Ch]Cl)) was explained, using thermal analysis techniques. Physicochemical clarification of overall thermal decomposition mechanism and the influence of enthalpy–entropy compensation (EEC) on reactions mechanism emerging are presented for the first time, in favor of this DES type. In the kinetic analysis of the decomposition process, two approaches were used: model-free (inverse) and model-based (direct) methods. It was found that thermodynamic principles in the form of EEC are the source of kinetic compensation effect (KCE) during MgCl2·6H2O-[Ch]Cl thermal decomposition, as a consequence of the effects of molecular interactions. Key phenomenon in the complex multiple step process represents a parallel dehydration steps of MgCl2·6H2O in DES, leading to formation of intermediates, such as [MgCl1(H2O)5]1+ and [MgCl2(H2O)4]. It was established that formation of final products (Mg(OH)2 and MgOHCl) requires a higher expenditure of energy to overcome a high potential barrier, where reaction system compensates this energy via hydrogen bonding disruption. This was confirmed by the identification of a specific ‘oscillator’, flagged as H–O–H···Cl hydrogen bond donating system of the energy (“heat bath”). All kinetic parameters and mechanisms of individual reaction steps were confirmed by numerical optimization of the process and modulated dynamic predictions.