Thermal hazard and decomposition kinetics of 1-butyl-2,3-dimethylimidazolium nitrate via TGA/DSC and FTIR
Introduction
Ionic liquids (ILs) are generally considered to be molten liquid salts formed by the combination of an organic cation and an organic or inorganic anion at room temperature with a melting point below 373.00 K (Michael, 2010; Rogers and Seddon, 2003). During the past century, ILs have been frequently used as functional material and media for different applications because of their favorable properties such as high heat capacity, thermal energy storage density, negligible vapor pressure and low melting point (Liu et al., 2018). Therefore, they have been widely used in high-temperature lubricants, reaction catalysts, separators, gas adsorbents, batteries with high discharge capacity, and even in bioactive field (Asensio-Delgado et al., 2020; Gomez et al., 2020; Lee et al., 2020; Rather et al., 2020; Woo et al., 2020; Yoon et al., 2020). To solve the problem of correct and safe application of ILs at different operating temperature, it is necessary to further study the thermal stability and thermal decomposition products of ILs at high temperature.
With the development of chemical process technology, more processes are now required to be carried out at high temperature. This has led to higher requirements for the thermal stability of ILs that act as special functional material. In other words, the ILs must be stable and reliable under high-temperature conditions (Jiang et al., 2020; Smiglak et al., 2006). Compared to other solvents, ILs have a wider and more stable liquid temperature range in which they are traditionally regarded as a thermal stable molten salts (Huang et al., 2018; Li et al., 2020). However, not all ILs can exhibit reliable thermal stability at specific temperatures (Fox et al., 2003). For example, Liu et al. experimentally demonstrated that the flammability of two imidazole nitrates is caused by their decomposition to produce combustible substances (Liu and Zhang, 2019). Liaw et al. suggested that traditional research has underestimated the fire and explosion hazards of ILs and recommended the use of flash point data for the characterization of these properties (Liaw et al., 2012). Cao and Mu characterized the thermal stability of ILs based on the thermogravimetric analysis (TGA) experiments and divided the ILs into five grades according to their Tonset values (Cao and Mu, 2014). In addition, ([NO3])--containing ILs are often used as energetic materials due to the high energy of their functional groups. If the IL system fails or operates in an incorrect mode, it may cause an explosion and other serious safety issues (Drake et al., 2007; Ueda et al., 1999; Wellens et al., 2013). Therefore, there is an urgent need to study the thermal hazard parameters of nitric acid ILs in different atmospheres. The safe use of ILs in industry is impossible without the investigations of the relevant basic properties of these compounds.
To date, there are no reports on the detailed decomposition mechanism and basic characteristic parameters of the IL 1-butyl-2,3-dimethylimidazolium nitrate ([Bmmim][NO3]). Therefore, in this article, we aim to investigate the thermal stability of 1-butyl-2,3-dimethylimidazolium nitrate in different atmospheres (nitrogen/air) and the hazards of its gas products using auto-ignition temperature testing (AIT), simultaneous application of thermogravimetry and differential scanning calorimetry (TGA/DSC), and micro continuous closed-mouth flash point testing and thermogravimetric analysis coupled with fourier transformed infrared spectrometer (TGA-FTIR). The results obtained in this work provide important guidance for the application of ILs in actual process production and the improvement of the library of the basic parameters of ILs.
Section snippets
Materials
1-butyl-2,3-dimethylimidazolium nitrate ([Bmmim][NO3]) ionic liquids (99% mass purity) was supplied by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (China). To reduce the influence of impurity water on the experimental results, the samples were stored in a vacuum drying oven at 50 °C for one week prior to the experiment. The detailed chemical information for [Bmmim][NO3] is presented in Table 1 and Fig. 1.
Auto-ignition temperature testing
Following the ASTM E659-78 test standard, an AITTA 551 auto-ignition
Auto-ignition temperature testing
As explained above, when a certain amount of the sample is injected into the flask with a pipette, a hot flame is observed or the temperature increases at least 50 °C within 10 min, the sample is regarded as flammable at this temperature, and the ignition delay time is recorded (ASTM, 2005). Fig. 2 shows the preheating temperatures of [Bmmim][NO3] obtained in the auto-ignition temperature test results plotted (circles) versus the [Bmmim][NO3] sample volume. If the test sample has no flame, or
Conclusions
In this work, auto-ignition temperature measurements, flash point analysis, TGA/DSC and TGA-FTIR were employed to study the thermal hazard of [Bmmim][NO3] based on the standard test methods. The main results are as follows:
- (1)
Using multiple experiment performed under the same experimental conditions, we obtain the auto-ignition temperature of 341.00 ± 21.60 °C, and the corresponding ignition delay time of 8.60 s, while the flash point temperature is 305.70 ± 9.30 °C.
- (2)
At five different heating rates
Author contributions
Conceptualization: Jianwen Meng, Yong Pan; Funding acquisition: Yong Pan; Methodology: Jianwen Meng, Yong Pan and Jiajia Jiang; Experiment: Jianwen Meng and Zheng Ran; Validation: Jianwen Meng and Jiajia Jiang; Data curation: Zheng Ran and Yongheng Li; Mechanism analysis: Jianwen Meng and Yong Pan; Writing - Original Draft: Jianwen Meng and Yong Pan; Writing – review & editing: Yong Pan, Qingguo Wang and Juncheng Jiang.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the National Natural Science Foundation of China (No.51974165) and the Natural Science Foundation for Distinguished Young Scholars of Jiangsu Province of China (BK20190036). Yong Pan acknowledged the sponsorship of Qing Lan Project. Yongheng Li acknowledged the sponsorship of Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. SJCX20_0376).
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