Enhanced CO2 uptake by intramolecular proton transfer reactions in amino-functionalized pyridine-based ILs

Article abstract of DOI:10.1039/c7cc01796k

This work presents a new strategy for the promotion of CO₂ uptake by an intramolecular proton transfer reaction in amino functionalized hydroxypyridine based anions.

Full text of DOI:10.1039/c7cc01796k

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Enhanced CO2 uptake by intramolecular proton transfer reactions
in amino-functionalized pyridine-based ILs
Mingguang Pan,ab R. Vijayaraghavan,b Fengling Zhou,b Mega Kar,b Haoran Li,a Congmin Wang*a
and Douglas R. MacFarlane*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
This work presents a new strategy for the promotion of CO2 rather than the carbamate.5c Maier and co-workers
uptake by an intramolecular proton transfer reaction in amino demonstrated that CO2 absorption by an amino-functionalized
functionalized hydroxypyridine based anions.
IL at the surface is far higher than that in the bulk at low
pressures, because carbamic acid is the dominating species in
the near-surface region while only carbamate is formed in the
bulk.5e More recently, Dai et al.5f provided a strategy to
activate an carboxylate group in amino acid ILs by introducing
an electron-withdrawing site into amino acid anions to achieve
multi-molar absorption of CO2.
Soaring carbon dioxide (CO2) emissions are triggering a series
of undesirable environmental issues, including climate change
and ocean acidification.1 The establishment of novel strategies
to selectively and efficiently absorb CO2 from post-combustion
flue gas is essential to mitigate this CO2 crisis. Ionic liquids (ILs),
especially functionalized ILs, have emerged as potential high
performing solvents for CO2 capture and separation by taking
advantage of their unique properties including negligible
vapour pressure, high thermal stability, wide liquid
temperature range, and excellent CO2 affinity.2
Re-visiting eqs. (1) and (2), the reason for the maximal
absorption capacity of 0.5 mol(CO /mol(IL) in the amino-
)
2
functionalized IL3 is that the generated carbamic acid reacts
with a further amine group in a simple proton transfer reaction.
This intermolecular proton transfer process also produces a
strong and dense hydrogen-bonded network, resulting in
increasing viscosity upon the CO2 uptake.6 One popular
approach is to control the reaction of CO2 with the amine
group to form the carbamic acid as the dominating species
rather than carbamate.5c Towards this end, our strategy
Inspired by the high reactivity in CO2 with aqueous amines,
Davis’s group first reported an amino-functionalized IL by
tethering an amine group on an imidazolium cation to
chemisorb CO2 in a stoichiometry of one mole CO2 to two
moles IL,3 as shown in eqs. (1) and (2). This considerably
exceeded the pure physical CO2 solubility in conventional ILs at
low pressure which typically is only around 0.035 mole
fraction.4
described here is to introduce an amine group and
a
deprotonated hydroxide group (O-) into an anion based on a
pyridine motif. Once CO2 reacts with the amine group to form
the carbamic acid, the proton on the carbamic acid can
transfer to the O- site. As a result, this intra-molecular proton
transfer reaction contributes to enhanced absorption of CO2
and controllable viscosity change.
To date, substantial efforts have been devoted to
developing novel amino-functionalized ILs for improving the
absorption performance of CO2, such as introducing an amino
group into the anion or adding more amino groups to the
cation or/and anion.5 For example, Brennecke et al. designed
amino-functionalized ILs by tethering an amino group onto the
anion in the vicinity of a carboxylate group to capture an
equimolar amount of CO2 through forming the carbamic acid
Thus, in the present work, a series of amino-functionalized
ILs based on anions containing amino- and deprotonated
hydroxyl- groups on a pyridine motif were prepared from the
acid-base
neutralization
between
amino-substituted
hydroxypyridines and ethanol solutions of phosphonium
hydroxide, which were obtained by an anion-exchange
method.7 The structures of these basic ILs were characterized
by NMR spectra, FTIR methods, and ESI-MS analysis (see the
Supporting Information).
These amino-functionalized ILs were investigated for CO2
capture. As shown in Table 1 and Figure 1, equimolar uptake of
CO2 as well as fast absorption kinetics by these ILs were
achieved. For example, [P66614][2-NH2-3-O-Py] exhibits a capac-
a.Department of Chemistry, Zhejiang University, Hangzhou, 310027, China. E-mail:
chewcm@zju.edu.cn
b.Department of Chemistry, Monash University, Clayton, VIC 3800, Australia. E-
mail: doug.macfarlane@monash.edu
† Electronic Supplementary Information (ESI) available: Experimental details and
more spectroscopic data. See DOI: 10.1039/x0xx00000x
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O
N
[P66614][2-NH2-3-O-Py]
[P66614][2-NH2-4-O-Py]
0.98(7.27)
0.90(6.68)
0.87(6.46)
0.99(7.35)
1567
1110
1223
1405
0.92217
0.91428
0.91939
0.92086
9.84
10.69
11.38
11.38
O
H2N
N
O
N
NH2
NH2
[P66614][2-NH2-6-O-Py]
[P66614][3-NH2-4-O-Py]
2-NH2-3-O-Py
2-NH2-6-O-Py
2-NH2-4-O-Py
O
[a] Reaction conditions: CO2 absorbed at 30 oC and 1 bar. [b] The molar capacity of
CO2 (mol CO2 per mole of IL, mol/mol), the value in brackets was gravimetric
capacity of CO2 (10-2 gram CO2 per gram of IL, 10-2 g/g or wt.%). [c] Determined at
30 ˚C. [d] Values for basic O- site given in H2O.
C4H9
P
C6H13
P
NH2
C4H9
C6H13
C4H9
C14H29
C6H13
N
C4H9
P66614
3-NH2-4-O-Py
P4444
Scheme 1 Structures of the anions and cations used in creating
amino-functionalized ILs.
ity of 0.98 mol CO2 per mole of IL at 30 ˚C and its absorption
time was only 25 min (Figure 1). Interestingly, the viscosity of
[P66614][2-NH2-3-O-Py] dramatically decreased from 1567 cP to
938 cP after CO2 uptake. As shown by Dai and co workers in a
similar system, the structural change on CO2 uptake, creating
greater anion asymmetry and disorder, may accelerate the
rotational and translational dynamics of IL, thus resulting in
the lower viscosity after CO2 uptake. The availability of an
intra-molecular hydrogen bond in the anion may also assist in
avoiding extensive inter-molecular H-bonding that often
occurs on CO2 uptake.8 Thanks to this viscosity decrease of
around 40 %, the absorption rate of [P66614][2-NH2-3-O-Py] is
self-promoting during the uptake process, which is very helpful
to practical application.
Figure 1 CO2 absorption by amino-functionalized ILs as a function of time at
30 ˚C: [P66614][2-NH2-3-O-Py] (■), [P66614][2-NH2-4-O-Py] (●), [P66614][2-NH2-
6-O-Py] (▲), [P66614][3-NH2-4-O-Py] (▼).
To find out the reason why these amino-functionalized ILs
can exhibit an equimolar absorption capacity of CO2, FT-IR and
NMR spectroscopy were employed as powerful means to gain
insight into the CO2 absorption mechanism. Compared with
neat [P66614][2-NH2-3-O-Py], a new peak at 1647 cm-1 (Figure S1)
attributable to a carbamate group stretch appears during CO2
uptake. The peak at 1524 cm-1 for [P66614][2-NH2-3-O-Py]
attributable to the C-O- stretch disappeared after CO2 uptake.
Instead, a new strong peak at 1325 cm-1 attributable to the C-
OH stretch appeared. Furthermore, a new peak at 3422 cm-1
attributable to the hydroxide group on the aromatic ring is
generated. Thus, the FT-IR data indicate that an intramolecular
proton transfer reactions occurs from the intermediate
carbamic acid to the negatively charged oxygen site after CO2
capture. Physical absorption of CO2 in the IL was also observed
at 2335 cm-1. Similarly, a robust signal at 157.5 ppm in 13C
NMR spectra is produced, which can be assigned to the
carbamate carbon (Figure 2a). A very small peak was observed
at 158.9 ppm, attributable to a carbonate carbon. No-
deuterium 13C NMR spectrocopy was further used to enhance
the signal of the carbonate carbon (Figure 2b).9 As expected,
an obvious peak at 159.8 ppm was seen that could be
attributed to a carbonate carbon.
These results indicate that there may be two different
reaction sites during the absorption process, the CO2 being
bound to either the amine, forming a carbamate, or the O- site,
forming a carbonate. Furthermore, according to the 1H NMR
spectra, the molar ratio of two different products after CO2
uptake was around 3:1 (Figures S2 and S3). Thus, the reaction
pathways in [P66614][2-NH2-3-O-Py] during the CO2 capture can
be proposed as shown in Scheme 2.
Variation of the relative positions of the negative oxygen
site and the amino group on the pyridine ring could
significantly affect the absorption capacity of CO2, originating
from the competing relationship of intramolecular and
intermolecular proton transfer reactions. For example,
[P66614][2-NH2-6-O-Py] absorbed a lower capacity of CO2 than
[P66614][3-NH2-4-O-Py], with absorption capacities of 0.87 and
0.99 mol/mol respectively (Figure S4 and S5). This is because
of the formation of zwitterions in [P66614][2-NH2-6-O-Py] during
the uptake process, originating from the inter-molecular
proton transfer reaction. As revealed by the FT-IR data of
[P66614][3-NH2-4-O-Py], a new peak at 1735 cm-1 emerged after
CO2 uptake, indicating that strong intramolecular hydrogen
bonds were formed in this case (Figure S6).10 The peak at 1588
cm-1 attributable to the C-O- stretch disappeared and a new
peak at 1325 cm-1 attributable to the C-OH stretch appeared
after CO2 uptake. Meanwhile, a peak attributable to the
Table 1. The effect of amino-functionalized ionic liquids on CO2 absorption[a]
vsicosity, density and pKa value.
,
Absorption
Capacity[b]
Viscosity[c]
(cP)
Density[c]
(g·cm-3
Ionic liquid
pKa[d]
)
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hydroxide group on the aromatic ring is generated at 3387 cm- high molar capacity of CO2 of nearly 0.80 mol/mol even at 80
1
.
˚C, indicating a robust chemical binding with CO2.
Absorption and desorption cycles were conducted to
observe the reversibility of IL during the capture process
(Figure S8). The absorption capacity remained around 0.90
mol/mol after 6 cycles, and during desorption less than 0.10
mol/mol remained under N2 bubbling at 80 ˚C for one hour,
indicating good reversibility of [P66614][2-NH2-3-O-Py]. Reversi-
bility was also confirmed by FT-IR of [P66614][2-NH2-3-O-Py]
after cycling, the spectra being almost the same as the fresh IL
except for an addition peak at 1636 cm-1 attributable to
retained CO2 (Figure S9).
Considering that water is inevitably involved in flue gas, it is
necessary to know the absorption performance of CO2 of
these amino-functionalized ILs in the presence of water. It was
found that addition of a small amount of water (around 0.4 to
0.5 wt.%) to [P66614][2-NH2-3-O-Py] does not affect the CO2
uptake, but when the water content increased to ~2.0 wt.%,
the absorption capacity decreased, presumably due to
reprotonation of the anion (Figure S10).5d
In order to increase the gravimetric capacity of CO2, an
effective approach is to shorten the alkyl chain length of the
cation in order to lower its molar mass. Thus,
tetrabutylphosphonium [P4444
] cation was also selected.
Compared with [P66614][2-NH2-3-O-Py], replacement of the
cation by [P4444] would increase the gravimetric capacity of CO2
from 7.3 to 9.4 wt.%. On the contrary, the molar absorption
capacity decreased from 0.98 to 0.79 mol/mol, which was
attributed to more facile intermolecular proton transfer
(Figure S11) due to the smaller size of the cation and therefore
enhanced interaction between anions. This result
demonstrates that although the phosphonium cations are not
able to chemisorb CO2, they play an important role in tuning
the reaction mode of functionalized anions with CO2.11
Figure 2 (a) 13C NMR spectra (100 MHz, room temperature, DMSO-d6)
of [P66614][2-NH2-3-O-Py] before (black) and after (red) CO2 uptake; (b)
No-deuterium 13C NMR spectra (100 MHz, room temperature) of
[P66614][2-NH2-3-O-Py] before (black) and after (red) CO2 capture.
Conclusions
We have developed a new strategy for high capacity CO2
capture by introducing an amino group into a basic, phenolic
anion to generate a series of amino-functionalized anion ILs.
When CO2 reacts with the amino group to form the carbamic
acid, intramolecular proton transfer can occur to the negative
oxygen group in the anion. As a result, an improved absorption
capacity of CO2 and fast absorption kinetics were achieved.
Interestingly, the viscosity of [P66614][2-NH2-3-O-Py] decreased
by around 40 % during the uptake process, indicating self-
promoted absorption kinetics. Furthermore, these amino-
functionalized ILs exhibit robust, water-tolerant, and
reverisible CO2 absorption. Replacement of the [P66614] cation
by [P4444] exhibits lower capacity of CO2, demonstrating the
important role of the cation in tuning the reaction mode of
these functionalized anions with CO2. We hope this work will
bring some inspiration for researchers to design smart
absorption systems for CO2 capture.
Scheme 2 Plausible reaction pathways in the IL [P66614][2-NH2-3-O-Py]
with CO2
These results demonstrat that intramolecular proton transfer
occurs in [P66614][3-NH2-4-O-Py] after CO2 absorption.
The effect of temperature on CO2 absorption was also
studied. As seen in Figure S7, [P66614][2-NH2-3-O-Py] exhibited
Prof. Wang acknowledges the support of the National Key
Basic Research Program of China (2015CB251401) and the
National Natural Science Foundation of China (21322602). Dr.
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Pan is supported by a scholarship from the State Scholarship
Fund of China. DRM is grateful to the Australian Research
Council for support under the Australian Laureate Program.
References
1
2
S. J. Davis, K. Caldeira and H. D. Mattews, Science, 2010, 329, 1330.
(a) L. A. Banchard, D. Hancu, E. J. Beckman and J. F. Brennecke,
Nature, 1999, 399, 28; (b) J. Dupont, R. F. de Souza and P. A. Z.
Suarez, Chem. Rev., 2002, 102, 3667; (c) J. L. Anderson, J. K. Dixon
and J. F. Brennecke, Acc. Chem. Res., 2007, 40, 1208; (d) X. Zhang, X.
Zhang, H. Dong, Z. Zhao, S. Zhang and Y. Huang, Energy Eviron. Sci.,
2012, 5, 6668; (e) M. Smiglak, J. M. Pringle, X. Lu, L. Han, S. Zhang, H.
,
Gao, D. R. MacFarlane and R. D. Rogers, Chem. Commun., 2014, 50
9228.
3
E. D. Bates, R. D. Mayton, I. Ntail and J. H. Davis, J. Am. Chem. Soc.
2002, 124, 926
J. Huang and T. Rüther, Aust. J. Chem., 2009, 62, 298.
(a) J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen and X. Lv, Chem. –
Eur. J., 2006, 12, 4021; (b) Y. Zhang, S. Zhang, X. Lu, Q. Zhou, W. Fan
and X. Zhang, Chem. – Eur. J., 2009, 15, 3003; (c) B. E. Gurkan, J. C.
de la Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich, E. A. Price, W.
F. Schneider and J. F. Brennecke, J. Am. Chem. Soc., 2010, 132, 2116;
(d) B. F. Goodrich, J. C. de la Fuente, B. E. Gurkan, Z. K. Lopez, E. A.
4
5
Price, Y. Huang and J. F. Brennecke, J. Phys. Chem. B, 2011, 115
,
9140; (e) I. Niedermaier, M. Bahlmann, C. Papp, C. Kolbeck, W. Wei,
S. K. Calderón, M. Grabau, P. S. Schulz, P. Wasserscheid, H.-P.
Steinrück and F. Maier, J. Am. Chem. Soc., 2014, 136, 436; (f) F.-F.
Chen, K. Huang, Y. Zhou, Z.-Q. Tian, X. Zhu, D.-J. Tao, D.-E. Jiang and
S. Dai, Angew. Chem., Int. Ed., 2016, 55, 7166.
6
7
K. E. Gutowski and E. J. Maginn, J. Am. Chem. Soc., 2008, 130, 14690.
(a) K. Fukumoto, M. Yoshizawa and H. W. Ohno, J. Am. Chem. Soc.,
2005, 127, 2398; (b) C. Wang, H. Luo, D.-E. Jiang, H. Li and S. Dai,
Angew. Chem., Int. Ed., 2010, 49, 5978.
8
9
(a) C. Wang, X. Luo, H. Luo, D. Jiang, H. Li and S. Dai, Angew. Chem.,
Int. Ed., 2011, 50, 4918; (b) A. Li, Z. Tian, T. Yan, D.-E. Jiang and S. Dai,
J. Phys. Chem. B, 2014, 118, 14880.
M. Pan, N. Cao, W. Lin, X. Luo, K. Chen, S. Che, H. Li and C. Wang,
ChemSusChem, 2016,
10 (a) X. Luo, F. Ding, W. Lin, Y. Qi, H. Li and C. Wang, J. Phys. Chem.
Lett., 2014, , 381; (b) X. Luo, X. Fan, G. Shi, H. Li and C. Wang, J.
9, 2351.
5
Phys. Chem. B, 2016, 120, 2807.
11 S. Saravanamurugan, A. J. Kunov-Kruse, R. Fehrmann and A. Riisager,
ChemSusChem, 2014, 7, 897.
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