Thermophysical Properties of Ionic Liquids with 1-Butyl-3-methylimidazolium Cation
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ICAL THERMODYNAMICS AND THERMOCHEMISTRY
Thermophysical Properties of Ionic Liquids with 1-Butyl-3-methylimidazolium Cation T. E. Skrebetsa, N. V. Shkaevaa,*, and D. S. Kosyakova a Northern
(Arctic) Federal University, Arkhangelsk, 63002 Russia *e-mail: [email protected]
Received November 28, 2019; revised January 9, 2020; accepted January 21, 2020
Abstract—Thermogravimetric analysis (TGA) is used to study the thermophysical properties of three ionic liquids with 1-butyl-3-methylimidazolium cation. The temperatures and enthalpies of melting and thermal decomposition are determined, and the IR spectra of gaseous products of thermal decomposition are studied. The relationship between the structure of compounds and their thermochemical characteristics is considered. Keywords: ionic liquids, thermophysical properties, thermogravimetric analysis, thermal stability, decomposition temperature DOI: 10.1134/S0036024420090265
INTRODUCTION Much attention is now being given to studying ionic liquids as solvents of a special kind. Their unique properties (relatively low melting points, their specific features as solvents in various reactions, the ease of separation of the dissolved compounds from the solvent) are of great interest and attract great attention of many researchers. Among other things, ionic liquids are considered promising media for the technological processes of processing plant materials and studying the properties of their individual components (e.g., cellulose, lignin, and ligno-carbohydrate complexes) [1–6]. Ionic liquids based on substituted imidazolium cations are normally used for these purposes. Such processes almost always have to be conducted at elevated temperatures, so characterizing such thermophysical properties of ionic liquids as specific heat, the temperatures and heats of phase transitions, and especially the nature of the processes of thermal decomposition of these compounds is of great importance. The variety of ways of using ionic liquids determines the interest in their thermophysical properties and the study of their thermal stability [7–11]. In [7], the thermal stability of a number of ionic liquids (phosphates and triflates) based on 1-alkyl-3methylimidazolium cations was studied via TGA in air. The authors questioned the importance of the results obtained with relatively quick heating of a sample (10–20 K/min), since this produced a difference in the results with respect to the temperature at which decomposition starts. In their opinion, such experiments give reproducible results only with identical
sample weights. The thermal instability of 1-alkyl-3methylimidazolium phosphates in air at temperatures above 473 K was demonstrated. It was found that an admixture of silicon dioxide accelerates the decomposition of ionic liquids. In addition, the authors investigated the effect a crucible’s material has on experimental results and showed that crucibles made of silver and alumina have the least effect. It was shown in [8] that imidazolium cations are more thermally stable than tetraalkyl
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