Probing the effect of electron donation on CO2 absorbing 1,2,3-triazolide ionic liquids

Article abstract of DOI:10.1039/c3ra47097k

Development of the next generation materials for effective separation of gases is required to address various issues in energy and environmental applications. Ionic liquids (ILs) are among the most promising material types. To overcome the many hurdles in

Full text of DOI:10.1039/c3ra47097k

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Probing the effect of electron donation on CO2
absorbing 1,2,3-triazolide ionic liquids†
ab
Cite this: RSC Adv., 2014, 4, 12748
Wei Shi,ab Erik Albenze,ab Victor A. Kusuma,a
*
Robert L. Thompson,
David Hopkinson,a Krishnan Damodaran,c Anita S. Lee,d John R. Kitchin,d
a
ae
*
David R. Luebke and Hunaid Nulwala
Development of the next generation materials for effective separation of gases is required to address various
issues in energy and environmental applications. Ionic liquids (ILs) are among the most promising material
types. To overcome the many hurdles in making a new class of materials technologically applicable, it is
necessary to identify, access, and scale up a range of representative substances. In this work, CO2 reactive
triazolide ILs were synthesized and characterized with the aim of developing a deeper understanding of
how structural changes affect the overall properties of these substances. It was found that substituents on
the anion play a crucial role in dictating the physical properties for CO2 capture. Depending upon the
anion substituent, CO2 capacities between 0.07 and 0.4 mol CO2 per mol IL were observed. It was found
that less sterically-hindered anions and anions containing electron donating groups were more reactive
towards CO2. Detailed spectroscopic, CO2 absorption, rheological, and simulation studies were carried
out to understand the nature and influence of these substituents. The effect of water content was also
evaluated, and it was found that water had an unexpected impact on the properties of these materials,
resulting in an increased viscosity, but little change in the CO2 reactivity.
Received 27th November 2013
Accepted 17th February 2014
DOI: 10.1039/c3ra47097k
www.rsc.org/advances
In the case of CO2-reactive ILs, the presence of hydrogen atoms
on the anion which are available for hydrogen bonding is sus-
Introduction
pected of leading to high viscosities through the formation of salt-
bridges.18 It was shown that this salt-bridge formation can be
avoided by developing aprotic heterocyclic anions (AHA) which
lack the necessary proton to form salt bridges as reported by
Wang.11,19,20 These AHA ILs may then be used to efficiently and
reversibly capture CO2 in equimolar stoichiometry without the
increase in viscosity.21,22 The new ILs can be prepared from simple
azoles and can be tuned for stability, CO2 absorption enthalpy,
and CO2 capacity based on the choice of the azole.11,12,23–25
The pKa for a given 1,2,3-triazole typically falls within the
range useful in carbon capture, with the pKa of 1,2,3-triazole
being 13.9. Another benet of this class of AHA ILs is the ease of
access to substituted 1,2,3-triazoles which can be assembled by
a facile Cu(I) catalyzed click reaction (Scheme 1) resulting in a
region-specic 1,4-disubstituted triazole.13,15 The ability to
evaluate a variety of functional groups at specic locations on
the triazole ring associated with this chemistry is highly valu-
able, allowing evaluation of both steric and electronic effects on
the properties of the nal IL. A set of substituted triazolide
based AHA ILs containing the tetrabutyl phosphonium cation
were synthesized. The anions prepared are illustrated in Fig. 1.
The synthetic details for these ILs were reported previously.10
In this combined experimental and computational study, we
show that by varying the electronic nature of the substituent at
the 4-position on the triazole ring, stabilization of the negative
In addition of their novel physical properties, the diversity and
adaptability of ionic liquids (ILs) makes them attractive to
materials scientists seeking to create the ideal material for a
particular application.1 The requirements for CO2 capture
solvents in current and advanced fossil-based power systems
are considerable.2–8 If a truly superior material is to be located,
the thermal stability, low ammability, high CO2 solubility, and
extremely low vapor pressure which are characteristic of many
IL types will be important, but the ability to tailor the structure
and properties of the nal substance to optimize them for the
application will be indispensable.9–15 CO2-reactive ILs will be the
focus of this study since they are well-suited to address the most
immediate need in global climate change mitigation, capture of
CO2 from existing power plants.16,17
aU.S. Dept. Of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Rd.,
Pittsburgh, PA 15236, USA. E-mail: robert.thompson@contr.netl.doe.gov; Tel:
1-412-386-4953
bURS Corporation, P.O. Box 618, South Park, PA 15129, USA
cUniversity of Pittsburgh, Department of Chemistry, Pittsburgh, PA 15260, USA
dCarnegie Mellon University, Department of Chemical Engineering, Pittsburgh, PA
15213, USA
eCarnegie Mellon University, Department of Chemistry, Pittsburgh, PA 15213, USA.
E-mail: hnulwala@andrew.cmu.edu; Tel: +1-412-386-7343
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47097k
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respectively. The acquisition range was 100–3000 m/z with a
scanning rate of 1.03 spectra per s. Each sample was run for 4
min in both positive and negative polarities. MeOH blanks were
run between injections to decrease the amount of carryover that
may occur. The chromatograms were integrated and extracted for
MS spectra to determine the masses of the cations and anions.
FTIR spectroscopy
Variable pressure FTIR experiments performed were conducted
on a Nicolet Nexus 6700 ESP spectrometer (Thermo Fisher
Scientic, Waltham, MA), equipped with a wide-band mercury
cadmium telluride (MCT) detector cooled by liquid N2. Low
pressure infrared studies were performed in a stainless steel
vacuum chamber equipped with a turbo-molecular pump,
roughing pump, precision leak valve, and differentially pumped
optical windows (KBr). System pressure was measured using
an ionization gauge. The system base pressure was in the
10ꢁ7 mbar range aer an overnight evacuation. The sample was
manipulated inside the chamber with an XYZ translation stage
and a rotation stage. Samples were prepared on CaF2 windows
using a Janis Research (Wilmington, MA) copper sample holder,
accommodating one blank and one sample window.
Scheme 1
charge on the resulting anion can be affected, having a signif-
icant impact on the overall properties.
Experimental
General information
The 1H, 13C, 19F, and 31P NMR spectroscopic analyses were
completed using Bruker Avance III 300 or 400 MHz spectrom-
eter. Fourier transform infrared (FTIR) spectra were collected on
a Nicolet Spectrum 100 with an attenuated total reectance
(ATR) apparatus. Density measurements of the ILs were per-
formed using a Micromeritics Accupyc II 1340 pycnometer.
Water content was determined using a Metrohm 860 Karl Fisher
(KF) Thermoprep titration unit, equipped with an 831 KF
Coulometer; IL samples which had undergone KF titration were
treated as water-free samples. Viscosity measurements were
made at ambient temperatures (22–24 ꢀC) using a Rheosense
Inc. mVisc unit and 400 mL pipettes.
CO2 absorption
Gas solubility measurements were performed using a PCT-Pro
2000 apparatus from Setaram Inc. The PCT-Pro 2000 is a Sie-
vert's apparatus which determines gas solubility in a sample by
charging a sealed sample chamber of known volume with a
known quantity of CO2. The CO2-charged sample chamber is
isolated from the rest of the system and the pressure drop in the
chamber is measured. The drop in pressure due to CO2
absorption into the liquid is then measured, and the quantity of
CO2 absorbed into the liquid is determined from an equation of
state listed in the NIST Standard Reference Database 23.26,27
For all tests, between 0.2 g and 0.4 g of sample was loaded
into the sample chamber. The sample was held under a
dynamic vacuum for 4 hours prior to starting a test. During this
Thermogravimetric analysis
Thermal decomposition studies were performed on a Mettler
Toledo TGA/DSC 1 STARe System, equipped with a GC 200 gas
controller. Thermal decompositions were conducted under N2
purge from 22 ꢀC to 500 ꢀC at a ramp rate of 10 ꢀC per minute.
The onset of thermal decomposition (Tonset) was evaluated as
the temperature at which the signal baseline began to drop to a
value exceeding that observed in the preceding baseline noise.
ꢀ
evacuation, the sample was heated to 30 C and was stirred at
300 rpm. Aer evacuation, testing was initiated by dosing the
sample chamber with a known amount of CO2, and the sample
was allowed to equilibrate for at least 4 hours. Dosing was
repeated to obtain an isotherm over a pressure range of 0 to ꢀ10
bar. Throughout all tests, the sample was maintained at 30 C
and was stirred at 300 rpm.
CO2 absorption was also measured using a Thermo Scientic
Thermax 500 TGA. A sample volume of 50 cm3 was used with a
Mass spectrometry analysis
Mass spectrometry (MS) analysis was performed using an Agilent constant temperature of 40 ꢀC and pressure of 0.9 bar. The
1290 LC system and 6520 Q-TOF (quadrupole time of ight) sample was exposed to pure N2 under a constant ow rate of
detector (Agilent Technology). A 95% MeOH–5% H2O mixture 0.1 SLPM for several days until it reached an equilibrium mass.
was used as the mobile phase. A 0.1 mL sample was injected using An initial decrease in mass was observed due to evaporation of
an autosampler and introduced into the chromatograph using water from the sample and the surface of the sample bucket.
direct infusion (no column) with a mobile phase ow rate of Next the feed gas was switched to pure CO2 at a ow rate of 0.1
0.4 mL minꢁ1. An ESI (electrospray ionization) source was used to SLPM and was allowed approximately one day to reach equi-
introduce the samples into the Q-TOF with the following settings: librium as CO2 absorbed into the sample. The difference
fragmentor voltage ¼ 175 V, skimmer voltage ¼ 65 V, drying gas between these two equilibrium masses was considered to be the
ow rate and temperatures of 12 L minꢁ1 and 300 ꢀC, amount of CO2 absorbed by the sample.
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Table 1 Physical properties of ILs 1–5
Theoretical calculations
Ab initio gas-phase calculations were performed at the B3LYP/6-
311++g(d,p) level of theory to calculate CO2 interactions with
anions on different binding sites of the same anions. The
interaction energy was calculated as the energy difference
between the products and the reactants (CO2 and anions) cor-
responding to the respective optimized structures. Vibrational
frequency calculations were performed to conrm minimum
stable energy structures for both the reactants and the products.
Reaction enthalpies of CO2 with anions were also calculated at
298 K. The reaction enthalpy includes the total electronic
energy, zero point energy, thermal corrections to 298 K using
the standard ideal gas statistical mechanics formulation, and
PV (RT) energy. The charges for atoms of the anions were
obtained from the CHELPG protocol;28 all calculations were
performed using the Gaussian 09 program.29 The water inter-
actions with the anions were calculated in a similar fashion to
CO2. In the case of water, we used counterpoise correction in
both the structural optimization and frequency calculations.
FW,
Density,
g mLꢁ1
Tonset
,
wt% H2O
(by KF)a
mol% H2O
(by KF)a
IL
g molꢁ1
ꢀC
1
2
3
4
5
403.58
411.65
383.59
413.70
704.61
0.9836
0.9324
0.9290
0.9400
1.2141
238
185
151
227
263
1.06
2.35
0.914
1.04
0.973
23.7
53.7
19.5
23.9
38.0
a
Water content aer drying to constant weight at 50 ꢀC.
Table 2 CO2 absorption capacities of ILs 1–5
mol CO2 per mol IL
absorbeda
q PR3, ꢀ
(ref. 33)
d H5,
ppm
IL
1
2
3
4
5
0.10
0.40
0.07
0.30
0.23
145ꢀ (Ph)
7.59
7.23
7.12
7.16
7.36
132ꢀ (nBu)
182ꢀ (tBu)
180ꢀ (CH2tBu)
132ꢀ (nBu)
a
CO2 absorptions as determined at 30 ꢀC and 1 atm.
Materials
Unless otherwise stated, ACS reagent grade chemicals and
solvents were obtained from Aldrich and used as received.
Tetrabutyl phosphonium hydroxide (40% aqueous solution)
was obtained from TCI America and was used as received.
groups at the 4-position. Silyl- and uorinated ether-substituted
triazoles (4, 5) were also included to observe the effect of
hydrophobic substituents on the IL.
All ve 1,2,3-triazoles were converted into ILs via a dehy-
dration reaction with aqueous tetrabutyl phosphonium
hydroxide. The density, water content, and onset of thermal
decomposition of the resulting ILs were measured and the
results are listed in Table 2. The densities of all of the ILs except
for the uorinated 5 were approximately 1 g mLꢁ1. All ILs were
Syntheses of ionic liquids (1–5)
The details for the preparation of the ILs studied in this work,
which are shown in Fig. 1 are described in the ESI,† and were
published previously.10
ꢀ
vacuum dried to constant weight at 50 C, but KF titration of
Results and discussion
these samples showed that signicant moisture was retained in
the vacuum dried ILs as shown in Table 1.
The structures of the triazoles prepared are shown in Fig. 1.
Triazole targets were chosen to include electron-withdrawing
(phenyl – 1) and electron-donating (hexyl and tert-butyl – 2, 3)
Thermal decomposition analysis30 of ILs 1–5 showed that ILs
with electron donating groups on the triazole ring began to
decompose at lower temperatures than those with electron
withdrawing groups (Table 1). Alkyl groups on 2 and 3 cause the
ILs to begin thermal decomposition at a substantially lower
temperature than 1 and 5, presumably due to more efficient
electron donation to the anion. The thermal decomposition
temperatures under N2 as measured by TGA for 1–5 were found
to be comparable with results reported by Wang for both [P66614]-
[imidazolide] and [P66614][pyrazolide].11
The results for the CO2 solubility experiments are summa-
rized in Fig. 2 and Table 2, and the data for these experiments is
listed in ESI Tables 2 and 10.† The CO2 capacity at 1 bar and
ꢀ
30 C was shown to vary from 0.07 mol CO2 per mol IL in the
case of 3 to as high as 0.40 mol CO2 per mol IL in the case of 2.
For comparison, the amount of CO2 physisorbed by [hmim]-
[Tf2N] under identical conditions is 0.032 mol CO2 per mol
IL.31,32 CO2 absorption experiments were also performed by TGA
for 1 under comparable conditions, but the long equilibration
times required by the TGA (1–2 days) and the ability of the
Sievert's apparatus to stir samples led us to not pursue the TGA
for further studies (ESI Table 10†).
Fig.
1 Tetrabutyl phosphonium 4-substituted-1,2,3-triazolide IL
anions 1–5 prepared and studied in this work.
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Fig. 3 Viscosity of ILs 1–5, as determined at ambient temperature. ILs
1–5 were tested wet and dry, both with and without CO2 added. Samples
labeled a (black) are wet (dried to constant weight), b (hatched) are wet +
CO2, c (white) are dry (KF titrated), and d (grey) are dry + CO2.
Fig. 2 CO2 solubility in ILs 1–5 and [hmim][Tf2N] at 30 ꢀC as deter-
mined by Sievert's apparatus. Filled squares indicate data for [hmim]
[Tf2N], circles for 1, upwards triangles for 2, downwards triangles for 3,
diamonds for 4, and leftwards triangles for 5.
upeld 0.1–0.4 ppm for all ve triazoles aer conversion to ILs,
indicating that the electronic environment of the azole ring was
From the results obtained, it can be inferred that the steric more electron-rich aer converting to anionic form.
bulk of the substituent combined with their electron donating The viscosities of the 1,2,3-triazolide ILs were determined in
or withdrawing nature plays a signicant role in CO2 solubility. both wet and dry samples, both before and aer exposure to CO2;
In the cases of 2 and 5, the linear geometry of the hexyl- and these results are illustrated in Fig. 3 at room temperature (see data
uoroether-groups exhibit less steric crowding adjacent to the in ESI Tables 4 and 5†) The water content of each sample is shown
azole ring, leaving the incoming CO2 a greater amount of free in Table 1. Samples listed as dry were further treated by subjecting
space in which to react. In the case of 3, the branched nature of them to KF titration and thus contain less than 100 ppm H2O
ꢀ
the tert-butyl group maximizes the steric crowding near the (detection limit of KF titration at 120 C). Both wet and dry IL
azole ring and prevents the CO2 from having an easy approach samples 1–5 were later exposed to 1 bar CO2 by evacuating them
during reaction. IL 4 (TMSCH2 – electron donating group) has a on a Schlenk line and backlling with CO2 for 1 h while heating at
greater CO2 capacity than 1 (Ph – electron withdrawing group) 50 ꢀC, then cooling to room temperature.
suggesting that while steric crowding is an important factor in
determining CO2 capacity, electronic effects have a much larger from 1357 cP (2) to 8275 cP (5), and did not show any substantial
inuence on the CO2 reaction. change in viscosity aer exposure to CO2 (see Fig. 3). The dried
The wet IL samples were relatively high in viscosity, ranging
One method for quantifying the relative steric crowding near IL samples were signicantly lower in viscosity, ranging from
the triazolide ring is the angular spread of the phosphine cone 202 cP (4) to 880 cP (1), but unexpectedly increased signicantly
angle in comparable trialkyl phosphines (see Table 2).33 The in viscosity aer CO2 exposure. Comparison of wet and dry IL
cone angle is smaller for straight chain substituents (132ꢀ for 2 viscosities suggests that water molecules were able to interact
and 5), indicating less steric crowding, and grows larger for the strongly with the anions to form a hydrogen-bonding network,
bulkier, highly-branched substituents (180–182ꢀ for 3 and 4). which leads to high viscosities for the wet ILs (compare exper-
The cone angle for phenyl rings lies approximately half way imental water content values in Table 1 with viscosities in ESI
between the linear and branched alkyl groups (145ꢀ for 1). All Tables 4 and 5†).34,35 Thus, when CO2 was added to the wet ILs,
these observations are consistent with the assertion that sterics which already have a hydrogen bonding network in place, no
as well as electronics play a role in CO2 absorption.
further effect on the viscosity was observed. When CO2 was
The relative effect of electron donating and withdrawing added to the dry ILs, where no network previously existed, CO2
substitution at the 4-position is also observed in the 1H NMR by absorption caused viscosities to increase by increasing the
monitoring the chemical shi of the triazole ring proton at the 5- overall molecular weight of the anion, despite the lack of
position. The shis in the 1,2,3-triazolides with electron donating hydrogen bonding sites on the azole rings. It is also important
groups (ILs 2, 3, and 4 with shis of 7.12–7.23 ppm in CDCl3) are to note that when exposed to CO2 under absolutely dry condi-
observed upeld from those of the ILs with electron withdrawing tions the reaction product was a solid for 2 and 5.
groups, (ILs 1 and 5 with shis of 7.36–7.59 ppm), reecting the
A comparison of the CO2 solubility in wet and dry samples of 2
less shielded environment for the protons in the latter cases. and 3 is shown in Fig. 4. The data (ESI Table 6†) show that there
Related effects are also reected in the relative chemical shis for was no discernible difference in CO2 absorption capacity in the
the H5 proton in the parent 1H-1,2,3-triazoles and in the 1,2,3- presence of water with concentrations as high as 54 mole percent.
triazolide ILs, listed in ESI Table 3.† The H5 NMR signal shied Although water tends to increase IL viscosity at water contents
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Table 3 13C NMR chemical shifts before and after CO2 exposure for
neat ILs, 1–5a
Sample
d CO2, ppm
d C4, ppm
d C5, ppm
1
—
141.3
141.3
140.6
140.6
150.9
150.9
135.6
135.6
138.5
138.5
133.9
133.1
125.0
124.9
122.9
122.9
126.1
126.1
126.2
128.3
1 + 13CO2
158.1, 154.7
—
158.0, 154.6
—
157.9, 155.0
—
158.3, 155.0
—
159.0, 155.6, 153.6
2
2 + 13CO2
3
3 + 13CO2
4
4 + 13CO2
5
5 + 13CO2
a
Chemical shis are normalized to untreated C4.
Table 4 Mass spectral peaks for selected ions for methanol solutions
of ILs 1–5a
Fig. 4 CO2 absorption by wet and dry IL samples 2 and 3 at 30 ꢀC as
determined by Sievert's apparatus. Filled circles indicate data for wet 2
(vacuum dried to constant weight at 50 ꢀC), squares indicate data for
dry 2 (KF titrated after vacuum drying), triangles for wet 3, and dia-
monds for dry 3.
Sample
C+
Aꢁ
(2A + H)ꢁ
(2A + C)ꢁ
1
2
3
4
5
Calc.
Expt.
Calc.
Expt.
Calc.
Expt.
Calc.
Expt.
Calc.
Expt.
259.43
259.26
259.43
259.26
259.43
259.26
259.43
259.26
259.43
259.26
144.15
144.07
152.22
152.12
124.16
124.09
154.27
154.10
444.17
444.04
289.31
nd
547.73
547.37
563.87
563.49
507.75
507.43
567.97
nd
305.45
305.24
249.33
249.20
309.55
nd
889.35
889.13
1147.77
1147.33
a
C ¼ IL cation, A ¼ IL anion, nd ¼ not detected.
absence of water the CO2 directly reacts with triazolide anion
resulting in CO2 adduct product. In the presence of water the
reaction is perceived to be a combination of reactions including
direct triazolide reaction to CO2. The water can react with tri-
azolide anion to form neutral triazole and OHꢁ. The OH anion
reaction with CO2 would result in formation of [P4444] [HCO3].
The bicarbonate (pKa ¼ 10.3)25 could further react with triazole
to form the triazolide–CO2 adduct.
The 13C NMR spectra of neat 1,2,3-triazolide ILs were all
collected before and aer exposure to 13CO2. The two 1,2,3-tri-
azolide ring carbons, labelled C4 for the substituted site and C5
Fig. 5 Proposed CO2 absorption mechanism by ILs under wet and dry for the protonated site, along with the new peaks attributed to
addition of 13CO2, are listed in Table 3 (see ESI Fig. 12–16†). As
conditions.
the spectra were recorded in neat solutions, there were no
external reference peaks with which to compare, so all chemical
between 20 and 54 mole percent, the agitation caused by the shis were normalized to the C4 peak in the untreated IL
stirring in the CO2 absorption was adequate to overcome diffusion samples. Extra 13C peaks were observed in the neat NMR spectra
limitations, preventing any diminishing effect the moisture might of 3 before and aer 13CO2 exposure, although they were not
exert on the CO2 uptake through mass transport inhibition.
seen in the solution spectra. These may arise from the forma-
In the presence of water, we suspect that CO2 uptake tion of anionic dimers (see below – Table 4); although theoret-
happens via a different mechanism. Our proposed mechanism ical studies have predicted that the protonated triazoles may
is shown in Fig. 5 for CO2 absorption in both the presence and have multiple tautomer structures accessible in solution.36,37
absence of water. Fig. 5 shows that bicarbonate formation may These extra peaks may also be caused by some fraction of the
result in very similar CO2 uptake by utilizing a separate reaction phosphonium cation being converted to ylide-form which could
pathway yielding approximately the same CO2 content. In the also be reactive towards CO2.38,39
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The chemical shis recorded for the ring carbons did not tend
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to change aer CO2 exposure, with only 5 showing a slight
downeld shi of 2 ppm. However, two new signals were observed
for 1 through 4, and three new signals were observed for 5 aer
exposure to CO2. In every case, the new signals appeared between
153 and 159 ppm, which would be consistent with the formation
of carbamate functional groups. The relative intensities of the new
peaks are also consistent with their coming from an isotopically-
enriched source, rather than the non-enriched IL signals. The
new carbamate 13C NMR signals (153–159 ppm) for all ve
13CO2-treated AHA IL samples disappear upon dissolving them in
d6-DMSO. Free 13CO2 signal was not observed at 125 ppm, sug-
gesting that CO2 is not being physisorbed to any measureable
extent in these IL samples but is only being chemisorbed.
The mass spectra of 1,2,3-triazolide IL solutions in methanol
were all obtained, and the results are listed in Table 4. In all ve
cases, the molecular ion for the cations were observed in posi-
tive mode, and the molecular ion for the triazolide anions were
observed in negative mode, conrming the identities of all
products. In addition to the molecular ions, evidence of dimeric
Fig. 7 1 in 2 mL water after 1 week under ambient conditions.
anion species was observed for all ILs except 4, which yielded a evacuated to 10ꢁ6 mbar, suggesting that, like other azolides, the
molecular ion signal. The reactive ring proton at C5 is the most CO2 adsorption is reversible for triazolide ILs.11 Similar spectra
likely site for any side reactions which may be occurring, for 1, 3, and 4 (ESI Fig. 1–3†) in addition to the Raman spectra of
although the phosphonium a-methylene groups may also untreated 1–4 (ESI Fig. 8–11†) were also collected.
provide a reaction site.38,39
Samples of dry 1–5 were treated with H2O in an effort to
These materials were further characterized by FTIR studies. probe their water stability. All of the triazolide ILs failed to
Samples of ILs 1–5 were deposited as neat lms on CaF2 and dissolve in an excess of H2O, even aer 1 week. IL 4 was initially
were observed by transmission FTIR under varying pressures of insoluble, but appeared to decompose aer 2 days to give a
CO2. ILs 1–4 were rst evacuated (<10ꢁ6 mbar), then dosed with purplish solution. The remaining ILs were stable and immis-
10 and 100 mbar CO2, and new features were observed to appear cible in H2O for the duration of the experiment. Examination of
in the carbamate regions of the spectrum (1100–1200, 1600– the 1H NMR spectra of 1–5 in D2O failed to show any peaks
1800 cmꢁ1). It was not possible to prepare a sufficiently thin besides those expected for the ILs, suggesting that the small
layer of 5 to obtain useful FTIR spectra. The spectra obtained for amount of material that is dissolved is stable in water. A sample
2 are shown in Fig. 6, and new features can be seen to appear of 1 (0.5 g) which had been sitting in water (5 mL) for 1 week at
aer 10 mbar CO2 at 1754, 1628, and 1280 cmꢁ1 (see arrows). ambient temperature is shown in Fig. 7, in which the insoluble
These features disappeared when the sample was again IL can be seen at the lower le portion of the vial. Similar
pictures of 2–5 are shown in ESI Fig. 4–7.†
Dissolving dry samples of 1–5 in D2O for NMR analysis was
also attempted, but failed to show any signal, suggesting that
the ILs are insoluble in aqueous media. When sufficient CD3OD
was added to the D2O solution to permit 1H NMR signals to be
observed, no changes from the starting IL NMR signals could be
detected. This lack of change implies that, in addition to being
insoluble, 1, 2, 3 and 5 do not tend to react with water; however,
they are hygroscopic in nature and take up signicant amounts
of water.
Computational studies of the 1,2,3-triazolide system were
used to calculate the expected anionic charge for the atoms in
the azolide rings that were synthesized and the unsubstituted
1,2,3-triazolide anion. The results of these studies are illus-
trated in Fig. 8, and the charges are tabulated in ESI Table 7.†
Due to the symmetry of the unsubstituted 1,2,3-triazolide anion,
the charges on N1 and N3 are identical and roughly twice as
large as that expected for the central N2 atom. In the case of the
asymmetric substituted 1,2,3-triazolide anions, the largest
negative charge tended to be found at N1 and the smallest at N2;
N3 was the largest negative charge for 2 and 5. It was expected
Fig. 6 Variable pressure FTIR spectra of 2 under vacuum and after
exposure to 10 and 100 mbar CO2 (carbamate bands indicated by
arrows).
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well as in the amount of bend angle induced in the reacting CO2
˚
˚
molecule. The N–C bond distance at N1 is 1.61 A, or 0.06 A
shorter than the distance at N2; the O–C–O bond angle when
bonded to N1 is 137ꢀ, or 3ꢀ less linear than when bonded at N2.
When the electronic effects of substitution at the 4-position
are accounted for, the relative differences in interaction ener-
gies with both CO2 and H2O change considerably. In the phenyl
anion of 1, CO2 was found to have interaction energy at N1 of
ꢁ36 kJ molꢁ1, and H2O was found to have interaction energy of
ꢁ61 kJ molꢁ1. In the hexyl-substituted anion of 2, CO2 was
found to have interaction energy at N1 of ꢁ51 kJ molꢁ1, and H2O
was found to have interaction energy of ꢁ66 kJ molꢁ1. The effect
of replacing an electron withdrawing substituent (1 – phenyl) at
the 4-position of the azole ring with an electron donating group
(2 – hexyl) was to strengthen the interaction for both CO2 and
H2O, but the relative difference in energy was greater for CO2
than it was for H2O. This same trend is observed in all
substituted versions of the 1,2,3-triazolide anion studied (1–5).
These interactions are consistent with the expectation that
the electron withdrawal of the phenyl group in 1 pulls negative
charge away from the triazolide N atoms, rendering them less
able to chemically interact with CO2 or H2O. Conversely, the
electron donation from the hexyl group in 2 increases the
negative charges on the triazolide N atoms and renders them
more active towards both CO2 and H2O.
Fig. 8 Calculated CHELPG charges on N1 through N3 atoms for anions
in ILs 1–5. C atoms are shown in grey, H in white, N in blue, Si in yellow,
F in aqua, and O in red.
that the N atom with the largest negative charge would be the
most energetically favorable location for interaction with either
CO2 or H2O. This result for the anions diverged from theoretical
work performed on the protonated 1,2,3-triazoles, where the N2
tautomer tended to be the most stable.36,37
Computational studies of the reaction of the 4-substituted
1,2,3-triazolide anions with CO2 and H2O were also conducted.
The results of these studies suggest that H2O interacts more
strongly than CO2. In all cases, the N1 atom was the site of the
strongest interaction for both CO2 and H2O. For CO2, the N2 and
N3 interaction energies varied from 60% to 90% of that found
for the N1 interaction. For H2O, interaction energies tended not
to vary much between N atoms. The calculated values for the
interaction with CO2 and H2O are listed in ESI Tables 8 and 9,†
respectively. An illustration of the interactions dealt with in
these computations is shown in Fig. 9 for the anion of 2 inter-
acting with CO2 and H2O, respectively. In the case of 2, N3 had a
larger negative charge than N1, but interaction energy with CO2
and H2O were still the greatest with N1.
Conclusions
This study highlights the importance of selecting the right
substituent and points to the fact that the reactivity can be altered
based upon the electronic nature and steric bulk of the substitu-
ents. In this work, we have systematically studied the effects of
electron donating/withdrawing groups and steric hindrance on the
CO2 uptake behaviour and interactions with water of triazolide-
based AHA ILs. The absorption capacities of the materials in this
study, 0.07–0.40 mol CO2 per mol IL, were highly dependent upon
the nature of side groups present. We found that electron donating
groups are generally more desirable for CO2 uptake.
Characterization of the CO2-reacted products by 13C NMR
showed the formation of multiple isomers of carbamates. While
the AHA ILs were not water soluble, the presence of even small
amounts of moisture in the ILs exerted a large inuence on both
the viscosity and physical state of the IL aer CO2 absorption,
indicating that caution must be taken when comparing physical
properties of different ILs. Computational studies of the triazolide
anions suggest that they should be more reactive towards water
than CO2 and that the N1 atom of the azole ring should provide
the most reactive site for CO2. However, the presence of water
does not alter the overall CO2 uptake. One explanation may be
that in the presence of water the CO2 reaction happens via an
alternate mechanism involving bicarbonate formation.
In the unsubstituted 1,2,3-triazolide anion, the interaction
energy with CO2 was found to be ꢁ52 kJ molꢁ1 at N1, whereas
the interaction with H2O was ꢁ67 kJ molꢁ1. In the absence of
any azole ring substitution, H2O interacts more strongly than
CO2. These interactions are also reected in the shorter bond
distances calculated between CO2 and the N1 to N3 atoms, as
Acknowledgements
This project was funded by the Department of Energy, National
Energy Technology Laboratory, an agency of the United
States Government, through a support contract with URS
Fig. 9 Calculated interactions of anion of 2 with CO2 and H2O. The C
atoms are shown in grey, H in white, N in blue, and O in red.
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