Cinética de decomposição de líquidos iônicos dicatiônicos com ânions derivados de ácidos carboxílicos

Abstract

This work describes the synthesis, structural characterization, and thermal stability of eight dicationic ionic liquids (DILs) containing the cations 1,4-bis(3-methylimidazolium-1-yl)butane ([Bis(C4MIM)]2+) and 1,10-bis(3- methylimidazolium-1-yl)decane ([Bis(C10MIM)]2+) with the anions butyrate (C3COO-), pentanoate (C4COO-), hexanoate (C5COO-) and heptanoate (C6COO-) and two monocationic ionic liquids (MILs) containing the cation 3-methyl-1-butylimidazolium (C4MIM+) and the anions pentanoate and hexanoate. The thermal characterization of these ILs by TGA showed that DILs with the cation [Bis(C4MIM)]2+ are more stable than DILs with the cation [Bis(C10MIM)]2+ and the MILs, the latter being similar. Only for the DILs with the cation [Bis(C4MIM)]2+ an increase in the anion alkyl chain lead to an increase in thermal stability. DSC analysis (-80 to 130 ° C) showed that the DILs with the cation [Bis(C4MIM)]2+ and the MILs are amorphous solids, while most of the DILs with the cation [Bis(C10MIM)]2+are crystalline solids. The decomposition kinetics of the DILs [bis(C4MIM)][C3COO]2, [Bis(C4MIM)][C6COO]2 and [Bis(C10MIM)][C3COO]2 were analyzed by isothermal and isoconversional methodologies. The isoconversional methodologies were the Friedman, Osawa-Flynn-Wall (OFW) and Kissinger-Akahira-Sunose (KAS). The results of the isothermal analyses indicated that the zero-order reaction model best describes the experimental data. Also, the Ea values indicate that the [Bis(C4MIM)][C3COO]2 has a higher decomposition rate than the others, indicating that the velocity of decomposition is proportional to the molar mass of the ILs. The IL [Bis(C4MIM)][C6COO]2 presented two well-defined decomposition steps in isoconversional analyzes. For this methodology, it was also found that the thermal stability of the ILs is proportional to their molar masses. Ea values varied with conversion indicating that the decomposition of these DILs occurs in several steps, except for the first decomposition step of IL [Bis(C4MIM)][C6COO]2. The lnA value for the first decomposition step of this IL was determined by the compensation effect and the f(α) function follows a similar trend to the theoretical diffusion models. The decomposition products of the isothermal analysis of the DILs [bis(C4MIM)][C3COO]2 and [bis(C10MIM)][C3COO]2 were analyzed by 1H RMN and ESI-MS. In turn, non-isothermal decomposition products were analyzed using TGA-FTIR. The data showed that the DIL [bis(C4MIM)][C3COO]2 decomposes by SN2, E2, and heterocyclic carbenes formation reactions. For the DIL [bis(C10MIM)][C3COO]2 it was not possible to identify the mechanisms of isothermal decomposition. In the case of non-isothermal decomposition, esters were form as products, suggesting that the decomposition occurs by SN2 reactions