Correspondence on “Preorganization and Cooperation for Highly Efficient and Reversible Capture of Low-Concentration CO2 by Ionic Liquids”

Full text of DOI:10.1002/anie.201712252

Angewandte
Correspondence
Chemie
International Edition: DOI: 10.1002/anie.201712252
German Edition: DOI: 10.1002/ange.201712252
Ionic Liquids I
Correspondence on “Preorganization and Cooperation
for Highly Efficient and Reversible Capture of Low-
Concentration CO by Ionic Liquids”
2
Marcileia Zanatta, Nathalia M. Simon, Francisco P. dos Santos, Marta C. Corvo,
Eurico J. Cabrita, and Jairton Dupont*
acid–base chemistry · carbon dioxide activation ·
carbon dioxide capture · ionic liquids ·
NMR spectroscopy
In memory of Faruk Nome
D
uring the last decade, carbon dioxide capture has grown
into a blooming field of research. In several cases, different
[
1,2]
types of ionic liquids (ILs) have been developed for CO
capture, particularly ILs containing basic anions. In search
2
[
3]
of ILs with superior CO capture capacity, a recent Commu-
2
nication published in this journal reported the use of imide-
based ILs with an impressive gravimetric CO capacity (up to
2
[
4]
2
2 wt%), namely tri-n-butylethylphosphononium ([P4442])-
Scheme 1. Proposed CO absorption mechanism by [P4442][Suc].
2
based ILs associated with diacetamide ([DAA]) and succini-
[
4]
mide ([Suc]) anions. The mechanism proposed on the basis
of a combination of quantum-chemical calculations, FT-IR,
spectrum of freshly prepared [P4442][Suc] (see the Supporting
Information and Figure S1 for details). Attached proton test
(APT) analysis indicated that signals corresponding to the
1
3
and C NMR investigations, involves cooperative interac-
tions between CO and multiple active sites of the preorgan-
ized anion (Scheme 1).
byproduct are constituted by two C=O and two CH groups
(Figures 1a and S2). The H NMR spectra also show two
2
2
1
We would like to note that, based on the provided
multiplets at d = 2.01 and 2.13 ppm corresponding to two
spectroscopic data, activation of CO by the anion is very
distinct CH moieties, together with a singlet at 8.11 ppm
2
2
unlikely to occur as proposed in Scheme 1.
possibly corresponding to NH (Figure S3). Further NMR
2
1
3
1
The first doubt arises from the C{ H} NMR spectra
analysis, heteronuclear multiple bond correlation (HMBC),
and heteronuclear single-quantum correlation (HSQC) ex-
periments (Figures S4 and S5) clearly show that these signals
are due to the 4-amino-4-oxobutanoate (anion from succi-
namic acid) resulting from the basic hydrolysis of the
pyrrolidine-2,5-dione (SucH) during the neutralization of
[P4442][OH]. Indeed, simple treatment of SucH with aqueous
[
4]
(
Figure 4 of the Communication), which show a second set
of signals at d = 34.4, 35.4, 174.8, and 176.4 ppm. These peaks
do not correspond to the cation or the anion, indicating the
presence of a second compound (around 35% as estimated by
1
3
C NMR analysis (see Figure S1 of this work), indicating that
the employed IL was contaminated. We have found the same
1
3
1
1
13
1
set of signals reported by the authors in the C{ H} NMR
NaOH affords 4-amino-4-oxobutanoate with H and C{ H}
NMR spectra identical to those of a commercial available
sample (see Figure 1b and Figure S6). Moreover, the pres-
ence of [Suc] and 4-amino-4-oxobutanoate anions was con-
firmed by ESI-MS analysis of the IL prepared from the
reaction of [P4442][OH] with SucH (Figure S19).
[*] Dr. M. Zanatta, Dr. N. M. Simon, Prof. F. P. dos Santos,
Prof. J. Dupont
Institute of Chemistry—Universidade Federal do Rio Grande do Sul
Av. Bento GonÅalves, 9500 Porto Alegre 91501-970 RS (Brazil)
E-mail: Jairton.dupont@ufrgs.br
The second doubt is related to the “strongest” experi-
mental evidence provided, which was based on the interpre-
Dr. M. C. Corvo
I3NCENIMAT, Dep. CiÞncia dos Materiais
Fac. CiÞncias e Tecnologia, UNL
1
3
1
tation of the C{ H} NMR spectra after interaction of
P4442][Suc] with 0.4 mol CO . The new signal at d =
[
2
2829-516 Caparica (Portugal)
1
58.9 ppm, and the shifts from d = 193.7 ppm (C=O) and
Prof. E. J. Cabrita
UCIBIO-REQUIMTE, Dep. Quimica
Fac. CiÞncias e Tecnologia, UNL
d = 32.8 ppm (CH ) of the neat “IL” to d = 189.8 ppm and d =
2
3
2.0 ppm, respectively, was attributed to the N–CO inter-
2
action. The further shifts to d = 182.0, 159.2, and 30.4 ppm
observed after absorption of another 1.25 mol of CO was
2829-516 Caparica (Portugal)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201712252.
2
[
5]
attributed to the cooperative N–CO –O interactions.
signal at d = 158–162 ppm in the C NMR spectra of ILs
A
2
1
3
2
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Chemie
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3
1
Figure 1. C{ H} NMR (126 MHz) spectra: a) APT of [P4442][Suc] obtained from [P4442][OH]+SucH in [D ]DMSO capillary; b) SucH+NaOH in
6
D O; c) [P ][Suc]+CO bubbled for 30 min in [D ]DMSO; d) [P ][HCO ] in D O; e) [P ][HCO ]+SucH in D O (after heating and exposure to
2
4442
2
6
4442
3
2
4442
3
2
reduced pressure); f) [P4442][HCO ]+SucH+CO bubbled for 30 min in D O.
3
2
2
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Chemie
containing basic anions is usually observed in the presence of
Hence the reported interaction of [P4442][Suc] containing
CO and is generally assigned to the carbonate- or carbamate-
water with CO2 affords [P4442][HCO ], and this mixture
2
3
type carbonyl groups, formed by the “strong” interaction of
(contaminated with approximately 35% of 4-amino-4-oxo-
butanoate associated with the [P4442] cation) may generate
a buffer-like solution that is responsible for the observed high
[5–17]
the N- and/or O-containing anions with CO (Scheme S1).
2
At this point, we would like to note that a shift of around
1
3
1
1
59 ppm is also the chemical shift ( C{ H} NMR spectra) of
CO sorption capacity, akin to that already observed for other
2
[
12,18,19]
[21]
ILs associated with bicarbonate anions
ing dissolved bicarbonate. Moreover, it has already been
demonstrated that the formation of bicarbonate, resulting
from the reaction of CO2 with the confined water in
imidazolium ILs associated with basic imidazolate and
or ILs contain-
ILs containing basic anions.
[
20]
In summary, the experimental evidence provided in the
Communication together with our data, and that reported by
[
21]
others, clearly show that the interaction of CO with ILs
2
containing basic anions (invariably contaminated with “con-
fined” water) produces mixtures containing bicarbonate
(another basic anion), generating a buffer-like solution.
Simple acid–base chemistry (buffer-like solutions) is hence
one of the basic models to be used to explain and to design ILs
with superior CO2 capture capacities. A strong base or
superbase is by definition a substance that has a high affinity
for protons and hence is quite unlikely to perform a nucleo-
philic attack on CO2.
[
22]
[12]
acetate anions,
or wet amino acid containing ILs,
is
responsible for a resonance observed around d = 159 ppm in
1
3
the C spectrum. Another important point in the reported
1
3
1
C{ H} NMR spectra is that the relative intensity of the peak
at d = 159 ppm does not change with increasing levels of CO2
from 0.4 to 1.65 mol per mol of the IL, Figure 4 of the
(
Communication) and apparently this intensity (integrals are
not provided) is not consistent with the proposed structure
containing one and two CO units per IL pair (Scheme 1).
2
Indeed, after bubbling CO for 30 minutes, we achieved
2
a relative intensity of 0.74 for the peak at d = 159 ppm Acknowledgements
1
3
1
compared to one [P4442][Suc] ion pair (determined by C{ H}
NMR analysis, see Figures 1c and S7–S9), which is more
consistent to the formation of a different compound.
Moreover, the reported chemical shift for the C=O and
Thanks are due to the following Brazilian agencies for partial
financial support: CAPES, CNPq, FAPERGS (16/2551-0000),
FAPESP (2015/08541-6), and INCT-Catalise.
CH moieties of the [Suc] anion after interaction with CO is
2
2
very close to that of SucH (the protonated anion, d =
1
79.6 ppm (CO) and d = 29.6 ppm (CH ) in [D ]DMSO; see Conflict of interest
2
6
Figure S10).
Although the authors assumed that the water content
determined by Karl Fischer titration) in the used IL was less
The authors declare no conflict of interest.
(
than 0.1 wt%, it is well known that it is extremely difficult to
prepare and especially to manipulate ILs without further
water absorption, particularly in the case of hydrophilic
[
23]
systems. Hence, it is most probable that the peak observed
1
3
1
at around d = 159 ppm in the C{ H} NMR spectra of
P4442][Suc] after exposure to CO is due to the formation of
bicarbonate. Indeed, the C{ H} NMR spectra (Figure 1d) of
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Version of record online: && &&
,
&&&&
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Correspondence
Ionic Liquids I
The preorganization and cooperation
mechanism of imide-based ionic liquids
reported in a recent Communication was
evocated to rationalize the extremely high
M. Zanatta, N. M. Simon,
F. P. dos Santos, M. C. Corvo,
E. J. Cabrita, J. Dupont*
&&&& — &&&&
gravimetric CO capture displayed by
2
these fluids. An analysis of the reported
spectroscopic evidences together with
additional experiments led to the propo-
sition of an alternative, simpler, and
feasible mechanism involving the forma-
tion of bicarbonate.
Correspondence on “Preorganization and
Cooperation for Highly Efficient and
Reversible Capture of Low-Concentration
CO by Ionic Liquids”
2
6
www.angewandte.org
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2018, 57, 2 – 6
These are not the final page numbers!