Hydrogen Bonds in Imidazolium-Based Ionic Liquids with -NH₂: ab initio and Molecular Dynamics Study

Guangren Yu,^†,°́Suojiang Zhang ^†,*

^†Research Laboratory of Green Chemistry and Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100080, P. R. China

^°́Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China

^* Corresponding author, Tel./Fax: +86-10-8262-7080, E-mail: sjzhang@home.ipe.ac.cn

It is one of the typical applications using ionic liquids with functional -NH₂, for example 1-(3-aminoethyl)-3-methylimidazolium hexafluorophosphate ([aemim][PF₆]) and 1-(3-aminopropyl)-3-butylimidazolium tetrafluoroborate ([apbim][BF₄]), to substitute traditional aqueous alkanolamine solutions in the fixation and purification of CO₂. Unfortunately, such ionic liquids have undesirable high viscosity, about 2~3 order in magnitude higher than other conventional imidazolium-based ionic liquids, which limits their eventual use in large-scale CO₂ scrubbing applications. Moreover, the mechanism study on the CO₂ fixation in such ionic liquids has not been carried out thoroughly except a proposed "overall" reaction (J. Am. Chem. Soc. 2002, 124, 926, see to the left scheme). In this work, the microstructure and hydrogen bond characteristics in the above two ionic liquids were systematically investigated by using ab initio and molecular dynamics methods in order to design functional ionic liquids for capturing CO₂.

The results showed that (1) the hydrogen bonds are formed between the F-site on anions and the H-site on -NH₂ of cations, along with the well-known H-site attached to C2 on imidazolium ring (see Figure 1); (2) the anion fragments potentially serve as Lewis-base catalysts in the fixing of CO₂ (see Figure 2); and (3) the stronger amino-associated hydrogen bonds is one of the main factors causing high viscosity. Based on the above understandings, a type of new functional ionic liquids with lower viscosity is expected to be designed and synthesized in the near future by rationally preventing the hydrogen bonds.

Acknowledgment

This work was financially supported by National Natural Science Foundation of China (20436050).

 

(a)	(b)

Figure 1. Hydrogen bonds in the case of [aemim][PF₆]: (a) ionic pair structure in the gas phase calculated at MP2/6-31G*; (b) site-site radial distribution functions between H atom on [aemim]⁺ and F atom on [PF₆]^- from a MD simulation.

Figure 2. Proposed mechanism for the role of anion in CO₂ fixation.