(502f) Hydrogen Bonds in Imidazolium-Based Ionic Liquids with –Nh2: Ab Initio and Molecular Dynamics Study

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

Guangren Yu,^†,°ìSuojiang
Zhang ^†,*

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

^°ìGraduate 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).

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.