Article
Hydrogen bond promoted thermal stability enhancement of acetate based ionic liquid

https://doi.org/10.1016/j.cjche.2020.02.019Get rights and content

Highlights

  • T5% of [C8C1Im][OAc]-based mixtures improved with adding [C4C1Im][NTf2].

  • Molecular interactions changed from π+–π+ to anion-π with adding [C4C1Im][NTf2].

  • Thermal stability of IL mixtures enhanced due to H-bonds formation.

Abstract

Acetate-based imidazolium ionic liquids (ILs) are of great importance and widely applied in biomass processing and engineering but under stability issue due to the structure self–rearrangement induced by C2–H deprotonation, by which the IL based biomass processing will be challenging. Herein, we demonstrated that the thermal stability of normal acetate-based imidazolim [C8C1Im][OAc] could be significantly improved by changing its cation and anion environment with the presence of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL ([C4C1Im][NTf2]). When the molar fraction of [C8C1Im][OAc] was 0.3, the thermal stability of [C8C1Im][OAc] could be significantly improved (ΔT5%dec = + 43 °C). Detailed information obtained from thermal gravimetric analysis (TGA) and nuclear magnetic resonance (NMR) revealed that the addition of [C4C1Im][NTf2] played a significant role in enhancing the thermal stability of [C8C1Im][OAc]. It was proposed that the formation of an anion–π+ structure network between [C8C1Im][OAc] and [C4C1Im][NTf2] via strong hydrogen bond interactions greatly affects the environment of hydrogen atom in the imidazolium ring of each IL.

Introduction

Ionic liquids (ILs), which consist entirely of organic cations and inorganic anions, are also named “designer solvents.” In recent years, ILs have been widely used in chemistry and chemical engineering fields, including carbon dioxide capture [[1], [2], [3]] and conversion [[4], [5], [6]], extraction [7,8] and membrane separation [9,10], synthesis and catalysis [11,12], electrochemistry [13,14], and biomass processes [[15], [16], [17], [18], [19], [20], [21], [22]], because of their unique qualities. For example, imidazolium acetate-based ILs exhibit great efficiency in biomass processing in terms of cellulose solubilization and lignin removal [[23], [24], [25], [26]]. However, experiments also indicated a potential thermal stability issue related to the acetate-based ILs in practical applications, especially in the field of biomass pyrolysis processing. From the viewpoint of chemical engineering and scale-up processing, improving the thermal stability of ILs remains challenging, and increasing the thermal stability of acetate-based ILs is also worthy of attention. Increasing the recovery of ILs contributes to improving the economic efficiency [27], and enhancing the thermal stability of ILs contributes to the recovery of ILs, thereby improving the process economics. Nevertheless, designing a novel IL with good stability is a long-term trip, and a stable IL is urgently needed in various fields. Considering that ILs can be tuned, their thermal stability can be improved through the addition of another substance. For example, the addition of another organic solvent may enhance the thermal stability but usually results in increased volatility and contamination. Moreover, the mechanisms are unclear. Inspired by our previous work in which a stable IL system of [Bmim]X/[Bmim]OH (X = Cl, BF4, and PF6,) was developed substituting for unstable [Bmim]OH for the hydrolysis of ethylene carbonate to ethylene glycol [28], and the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1Im][NTf2]) has also some applications in biomass processing [29], we proposed that the addition of [C4C1Im][NTf2] with high thermal stability and application in biomass processing could enhance the thermal stability of acetate-based ILs.

However, strong interactions in ILs exist, such as Coulomb force, hydrogen bonding, and Vander Waals' force. Among them, hydrogen bonding has a significant effect on the physicochemical properties of ILs [[30], [31], [32], [33]]. The competitive and cooperative characteristics among hydrogen bonding and π-type interactions in a typical imidazolium-oxalatoborate IL were demonstrated by the way of ab initio molecular dynamics simulations [34]. The C2–H of imidazole ring shows a certain degree of influence and is relatively easy to remove on account of its self-rearrangement. It can form hydrogen bonds with some nucleophilic anions, and the ability and intensity of forming hydrogen bonds are related to the nucleophilicity of anions. Furthermore, the interactions in ILs have a great influence on their properties, especially on the thermal stability. However, the effect of interactions between ILs on their thermal stability has rarely been reported. Thus, we used nuclear magnetic resonance (NMR) to further verify the hydrogen bond interactions in the binary IL system in terms of the enhancement of thermal stability of IL, and then derive important factors affecting the thermal stability of acetate–based ILs.

In this work, we selected the ILs 1-octyl-3-methylimidazolium acetate ([C8C1Im][OAc]) and [C4C1Im][NTf2] as the research objects, and the interaction mechanism between them was studied after the addition of different dosages of [C4C1Im][NTf2] to [C8C1Im][OAc] by thermal gravimetric analysis (TGA) and NMR. On the basis of the results of detailed experiments, it could be concluded that the thermal stability of [C8C1Im][OAc] was significantly improved with the presence of [C4C1Im][NTf2]. This improvement is probably because the short alkyl chain of [C4C1Im][NTf2] and the [OAc] with strong nucleophilicity would form strong hydrogen bonding with [C4C1Im]+; that is to say, the hydrogen bond strength of the binary system was enhanced due to the addition of [C4C1Im][NTf2]. Furthermore, the stacking structure of the mixture would change from π+–π+ accumulation to anion–π+ accumulation, which also influences the hydrogen bonding interaction of the binary system. For all of the above reasons, the thermal stability of [C8C1Im][OAc] was improved.

Section snippets

Materials

ILs, i.e. [C4C1Im][NTf2] and [C8C1Im][OAc], with purity of > 98 wt% were purchased from Lanzhou Zhongke Kateke Industry & Trade Co., Ltd. The ILs are stored in a desiccator with silica gel as desiccant. The ILs are always handled inside a glove box under an inert atmosphere of dry nitrogen. Table S1 shows a brief description of each chemical. All chemicals were used as received without purification after purchasing.

Mixtures of ILs

A certain concentration interval of {[C4C1Im][NTf2] + [C8C1Im][OAc]} binary

Thermal stability

Two parameters, Tstart and Tpeak, could be obtained from scan TGA. Tstart is the temperature at which the decomposition of the sample just starts. Tstart Tpeak is the temperature at which the sample has maximum degradation, which could be obtained from the peak in the DTG curves [35].

As shown in Fig.1(a), it can be seen that [C8C1im][OAc] IL begins to decompose below 150 °C (Tstart = 73 °C), and continue to decompose at 244 (Tpeak1) and 289 °C (Tpeak2) respectively. The decomposition happened

Conclusions

We demonstrated that the enrichment in [C4C1Im][NTf2] improved the thermal stability of [C8C1Im][OAc]. The T5% (the temperature of decomposing at 5%) of [C8C1Im][OAc] was increased drastically to 43 °C when the molar fraction of [C8C1Im][OAc] was 0.3 in the binary IL system. The thermal stability of the binary system [C8C1Im][OAc] and [C4C1Im][NTf2] was studied systematically by TG and NMR. The synergistic role of hydrogen bond and electrostatic interactions is proposed as the main reason for

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21776026), the Liaoning Revitalization Talents Program (XLYC1902037) and the start-up research funding of Beijing Institute of Technology (3160011181808).

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