Thermal Properties of Cyano-Based Ionic Liquids Pablo Navarro, Marcos Larriba, Ester Rojo, Julia ́ n García,* and Francisco Rodríguez Department of Chemical Engineering, Complutense University of Madrid, E-28040 Madrid, Spain ABSTRACT: Nowadays, extraction of aromatics from aromatic/aliphatic mixtures is being investigated using cyano-based ionic liquids (ILs) as a new green alternative to currently used conventional organic extraction solvents, such as sulfolane. In this process, the maximum operation temperature (MOT) of the IL is a decisive property to know. Thus, thermal behavior of ILs is a target issue to study. The MOTs of cyano-based ILs 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]), 1- butyl-3-methylimidazolium dicyanamide ([bmim][DCA]), 1-ethyl-3-methylimidazo- lium thiocyanate ([emim][SCN]), 1-butyl-3-methylimidazolium thiocyanate ([bmim][SCN]), and 1-ethyl-3-methylimidazolium tricyanomethanide ([emim]- [TCM]) have been determined using dynamic and isothermal thermogravimetric analyses. In addition, specific heats from (296.2 to 372.2) K of all ILs included in this work have been also measured using differential scanning calorimetry (DSC). The MOT for [emim][TCM] was the highest, whereas the MOT for [emim][DCA], [bmim][DCA], and [bmim][SCN] were a little lower, the [emim][SCN] MOT being the lowest found. Specific heats of all ILs analyzed were higher than that of sulfolane. ■ INTRODUCTION Ionic liquids (ILs) are a current alternative to volatile organic compounds (VOCs), because of their properties, specifically their negligible vapor pressure. 1 In the liquid-liquid extraction of aromatics from aromatic/aliphatic mixtures the most promising ILs employed are cyano-based ILs, which have shown good extractive properties (both selectivity and aromatic distribution ratio) in comparison with sulfolane. 2-4 In addition to the extractive properties, other IL properties have to be determined to confirm their potential uses as solvents for aromatic extraction. This way, one of the more relevant IL properties is the maximum operation temperature (MOT), which is defined as the maximum temperature that a substance can support without decomposition. The common way to determine the thermal properties of a substance is a non-isothermal thermogravimetric analysis (TGA) known as dynamic analysis. 5-7 A dynamic analysis consists of a quick temperature ramp during a known short time. In dynamic analyses, the MOT is assumed to be the onset temperature, which is the cross point between the tangent straight lines to the TGA curve before and after decomposition started. However, this claim is not altogether correct, since the onset temperature is a function of evaporation and decom- position processes, and also because its value is also completely dependent on the heating rate used. Furthermore, the onset temperature has been experimentally determined to be an overestimated MOT, because dynamic analyses are performed for a short time. 5,8-10 On the other hand, isothermal TGA is made at a constant temperature for longer times. 5,8-10 This method produces a real MOT, close to the run time required by the application studied. Isothermal analyses for very long periods of time are needed when a good precision is required for the MOT. Nevertheless, prediction of long-term stability of ionic liquids at elevated temperatures by means of non- isothermal TGA is also possible. Seeberger et al. 8 outlined non- isothermal TGA method to estimate a MOT. Thus, the aim of this work has been to determine MOTs of the cyano-based ILs 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]), 1-butyl-3-methylimidazolium dicyanamide ([bmim][DCA]), 1-ethyl-3-methylimidazolium thiocyanate ([emim][SCN]), 1-butyl-3-methylimidazolium thiocyanate ([bmim][SCN]), and 1-ethyl-3-methylimidazolium tricyano- methanide ([emim][TCM]). All the ILs investigated here have proven to have both selectivity and aromatic distribution ratios that make them promising ILs in the liquid-liquid extraction of aromatics. 2-4 Dynamic and isothermal experiments by TGA have been carried out in this work. Specifically, dynamic decomposition parameters (onset temperature, T 10 % , T 50 % , and ashes remained at critical temperatures), the influence of different heating rates on dynamic values, and the ILs dependence on time through isothermal conditions have been studied. In addition, the Seeberger et al. model has been applied in order to estimate the behavior of each IL at extremely long times from nonisothermal TGA. Finally, the specific heats of all ILs from (296.2 to 372.2) K, necessary properties from a process design point of view, have been determined as well. ■ EXPERIMENTAL SECTION Chemicals. [emim][DCA], [bmim][DCA], [emim][SCN], [bmim][SCN], and [emim][TCM] ILs were purchased from Iolitec GmbH with mass purities higher than 0.98, whereas halides are less than 0.02 and water content is no more than 0.002, in mass fraction. To prevent water absorption, all ILs were stored in a desiccator and handed inside a glovebox under Received: February 11, 2013 Accepted: June 25, 2013 Published: July 9, 2013 Article pubs.acs.org/jced © 2013 American Chemical Society 2187 dx.doi.org/10.1021/je400140n | J. Chem. Eng. Data 2013, 58, 2187-2193