Angewandte
Correspondence
Chemie
Ionic Liquids II
Reply to the Correspondence on “Preorganization and
Cooperation for Highly Efficient and Reversible Capture
of Low-Concentration CO2 by Ionic Liquids”
carbon capture · carbon dioxide · ionic liquids · water
W
e would like to thank Prof. Dupont and co-workers[1] for
their interest in our Communication,[2] and appreciate their
concerns regarding the development of CO2 capture by ionic
liquids (ILs).[3] In a previous study,[2] we had reported
a preorganized imide-based IL [P4442][Suc] with the tri-n-
butylethylphosphonium cation and the succinimide anion for
highly efficient and reversible capture of low-concentration
CO2 through cooperative interactions between CO2 and
multiple active sites in the anion. In this reply to the
Correspondence of Dupont and co-workers, we show that
the water content is crucial to the preparation of the IL and its
performance in the capture of CO2.
Scheme 1. Effect of water on the preparation of [P4442][Suc].
First, the preparation procedures of [P4442][Suc] described
in two different publications were compared. It is well known
that neutral succinimide (Suc) contains an imide group (-CO-
NH-CO-), and its treatment with mild bases in the presence of
water yields the corresponding salt of succinamic acid through
a ring-opening reaction.[4] Therefore, in our work, [P4442][Suc]
was prepared through a neutralization reaction between Suc
and a solution of phosphonium hydroxide ([P4442][OH]) in
anhydrous ethanol (Scheme 1). In this way, only a small
amount of Suc was hydrolyzed to produce succinamic acid. It
amounts to only 3.2 mol% in our prepared IL. However, we
were surprised to find that the “IL” given by Dupont et al.[1]
was prepared through neutralization of [P4442][OH] and Suc in
water/ethanol (10:1), where many more succinamate anions
would be produced in the “IL” (calculated to be about
1
18.1 mol% from their H NMR spectra (Figure S2). There-
fore, it is very difficult to remove the water in the “IL”, and
the “IL” easily absorbs water owing to the production of an
amino acid anion (Scheme 1).
In order to investigate whether the addition of water to
our IL can restart the basic hydrolysis of succinimide anions
([Suc]), we added different amounts of water to our IL (water
contents: 3.3, 8.8, and 17.6 wt%). It can be seen from Figure 1
that the intensity of the new resonance at about d = 3.5 ppm
1
has been calculated from the H NMR spectrum of the IL in
Figure S1 of this Reply that the succinamate anion by-product
1
[*] Y. Huang, Dr. G. Cui, Dr. Y. Zhao, Dr. H. Wang, Z. Li, Prof. J. Wang
Henan Key Laboratory of Green Chemistry
Collaborative Innovation Center of Henan Province for Green
Manufacturing of Fine Chemicals
in the H NMR spectra, which can be assigned to the H2O
hydrogen atoms, increased with an increasing amount of
added water in [P4442][Suc]. No other new resonances or
obvious changes in the peak intensities and positions were
observed, indicating that there is almost no change in the
composition and structure of the IL during the addition of
H2O. In other words, the added water in the IL does not
restart the basic hydrolysis, thus the succinamate anion may
only be produced in the neutralization reaction.
Next, FTIR spectra of the IL from different sources were
compared. It is clear from Figure 2a that there are significant
differences at wavenumbers from 3000 to 3750 cmÀ1, which
were ascribed to the hydrogen-bonded OH stretching vibra-
tion of H2O,[5] and the water content of the “IL” prepared by
Dupont and co-workers was much higher than that of our IL.
To estimate the content of water in the “IL”, FTIR spectra of
our [P4442][Suc] samples prepared by addition of water (water
Key Laboratory of Green Chemical Media and Reactions
Ministry of Education, School of Chemistry and Chemical Engineer-
ing, Henan Normal University
Xinxiang, Henan 453007 (China)
E-mail: chemcgk@163.com
Dr. G. Cui, Prof. S. Dai
Chemical Sciences Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
Prof. S. Dai
Department of Chemistry, University of Tennessee
Knoxville, TN 37996 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
2
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2018, 57, 2 – 6
These are not the final page numbers!