Theoretical and experimental studies on the thermal decomposition of 1-butyl-3-methylimidazolium dibutyl phosphate
Introduction
Ionic liquids (ILs) are a kind of molten salts that are liquid over a wide temperature range, which have numerous attractive properties, such as benign miscibility, negligibly low vapor pressure and sound thermal stability (Rogers, 2003; Sheldon, 2012). Known as designer solvents, ILs can be tailored by appropriate selection of cations, anions and substituents in order to meet specific requirements (Binks et al., 2018; Maton et al., 2013). Nowadays, ILs with different characteristics and function have been applied in various fields. Wilkes (2004) concluded the solvent properties particularly important to catalytic reaction chemistry and proposed that ILs are good solvents for catalytic reactions. Jalili et al. (2009) researched the solubility of hydrogen sulfide in ILs, indicating that ILs can be used for the separation of hydrogen sulfide and carbon dioxide. Sonnier et al. (2016) found that phosphonium ionic liquid significantly reduced flammability without further addition of flame retardants and phosphorus modifies pyrolysis pathway, promotes charring and may act as flame inhibitor.
The application of ILs as flame retardants becomes a new tendency because of ILs’ excellent thermal stability and non-flammability. The flame-retardant tests verified that 1-butyl-3-methylimidazolium dibutyl phosphate ([Bmim][DBP]) effectively improved the flame resistance of epoxy resin (Jian et al., 2017; Jiang et al., 2017). Accordingly, in the paper, researches on the [Bmim][DBP] were carried out. [Bmim][DBP], its cation is an imidazolium salt with a curing function, and the anion is a kind of phosphate that have a flame retarding function. The imidazolium salt constituting the cation can be used as a curing agent, and has the characteristics of low dosage, long trial period, low volatility and low toxicity (Jian et al., 2017). Simultaneously, owing to the characteristics of high flame retardant efficiency, excellent smoke suppression effect, low toxicity and low corrosion, tributyl phosphate which constitutes the anionic structure has become a common halogen-free flame retardant (Chen et al., 2016). The structural formula of [Bmim][DBP] is shown below in Fig. 1.
Adapted to the application background of rapid development of science and technology, thousands of ILs have been synthesized, and most of them have realized their commercial applications. However, ILs may be decomposed to produce flammable gases if operate at an improper process temperature, which may lead to serious hazard problems (Smiglak et al., 2012; Wellens et al., 2013). Therefore, before applying [Bmim][DBP] to specific fields, it is necessary to analyze its thermal stability and the pyrolysis products. A few researchers have explored the thermal stability of ILs. Liu and Zhang (2019) assessed the thermal stability of 1,3-dimethylimidazolium nitrate, a versatile and novel solvent for the petrochemical industry. Chen et al. (2017) determined the flammability characteristics of 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Fredlake et al. (2004) found that the thermal stability increases with increasing anion size, and heat capacities increase with temperature and increasing number of atoms in the IL.
Therefore, the thermal stability and related performance analysis of ILs, especially the correlation between the thermal stability and application performance of ILs, are essential. By analyzing the thermal stability of ILs, on one hand, the results can provide theoretical basis for the judgment of decomposition temperature and the operation temperature interval. On the other hand, the characterization and analysis of pyrolysis products at high temperatures can help to analyze the possible mechanism of reaction and even show the potential environmental hazards and toxicities (Binks et al., 2018).
In this paper, the gas-phase and solid-phase pyrolysis products of [Bmim][DBP] were analyzed by GC-MS and FTIR, respectively. Meanwhile, the theoretical bond energies of major bonds in [Bmim][DBP] were calculated by Gaussian 16 software. Consequently, the pyrolysis products of [Bmim][DBP] were studied in detail in this work, providing a theoretical reference for its application. Besides, from the perspective of pyrolysis behavior and pyrolysis products, the feasibility of [Bmim][DBP] as a flame retardant for flammable materials was initially explored.
Section snippets
Material
[Bmim][DBP] with a minimum purity of 96.0%, was purchased commercially from Shanghai Maclean Biochemical Technology Co., Ltd (Shanghai, China). During the experiment, the chemicals used in this study were applied without any further purification or evaporation.
Computational methods
In order to reveal the thermal decomposition of [Bmim][DBP] and its flame-retardant mechanism, all calculations have been performed with the version of Gaussian 16 program package (Frisch et al., 2016). Visualizations for all calculated
Theoretical calculation results
Extensive theoretical and experimental studies have shown that hydrogen-bonding interaction is fundamental to the structures and properties of ILs and promote thermal stability enhancement of ILs (Hunt, 2007; Kohagen et al., 2011; Skarmoutsos et al., 2012; Zhang et al., 2020). Considering the possible hydrogen-bonding interaction in the ionic liquids, dispersion was added separately, so the 6–311++G(d,p) basis set was employed (KUMAR et al., 2016). The structure of [Bmim][DBP] is presented in
Conclusions
The process of thermal decomposition of [Bmim][DBP] was investigated theoretically and experimentally in this paper. The feasibility of [Bmim][DBP] as a flame retardant for flammable materials was initially explored. Hydrogen-bonding interaction is fundamental to the structures and properties of ILs and do promote the thermal stability enhancement of ILs. Three possible hydrogen bonds were confirmed by theoretical analysis.
Bond energies of major bonds of [Bmim][DBP] were predicted at
CRediT authorship contribution statement
Shunchao Li: Conceptualization, Software, Writing - original draft, Writing - review & editing. Huichun Jiang: Resources, Writing - original draft. Min Hua: Project administration, Funding acquisition. Xuhai Pan: Supervision, Funding acquisition. Hangchen Li: Software, Resources. Xinxin Guo: Visualization. Han Zhang: Investigation.
Declaration of interests
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.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No.51704171), Six Talent Peaks Project of Jiangsu (No. 2014-XCL-010) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We are grateful to the High-Performance Computing Center of Nanjing Tech University for supporting the computational resources.
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