Properties of alkylbenzimidazoles for CO2 and SO2 capture and comparisons to ionic liquids

Article abstract of DOI:10.1007/s11426-012-4661-3

To date, few reports have been concerned with the physical properties of the liquid phases of imidazoles and benzimidazolespotential starting materials for a great number of ionic liquids. Prior research has indicated that alkylimidazole solvents exhibit different, and potentially advantageous physical properties, when compared to corresponding imidazolium-based ionic liquids. Given that even the most fundamental physical properties of alkylimidazole solvents have only recently been reported, there is still a lack of data for other relevant imidazole derivatives, including benzimidazoles. In this work, we have synthesized a series of eight 1-n-alkylbenzimidazoles, with chain lengths ranging from ethyl to dodecyl, all of which exist as neat liquids at ambient temperature. Their densities and viscosities have been determined as functions of both temperature and molecular weight. Alkylbenzimidazoles have been found to exhibit viscosities that are more similar to imidazolium-based ILs than alkylimidazoles, owed to a large contribution to viscosity from the presence of a fused ring system. Solubilities of CO₂ and SO₂, two species of concern in the emission of coal-fired power generation, were determined for selected alkylbenzimidazoles to understand what effects a fused ring system might have on gas solubility. For both gases, alkylbenzimidazoles were determined to experience physical, non-chemically reactive, interactions. The solubility of CO₂ in alkylbenzimidazoles is 10%-30% less than observed for corresponding ILs and alkylimidazoles. 1-butylbenzimidazole was found to readily absorb at least 0.333 gram SO₂ per gram at low pressure and ambient temperature, which could be readily desorbed under an N2 flush, a behavior more similar to imidazolium-based ILs than alkylimidazoles. Thus, we find that as solvents for gas separations, benzimidazoles share characteristics with both ILs and alkylimidazoles. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-012-4661-3

SCIENCE CHINA
Chemistry
•
·
ARTICLES •
SPECIAL ISSUE · Ionic Liquid and Green Chemistry
August 2012 Vol.55 No.8: 1638–1647
doi: 10.1007/s11426-012-4661-3
Properties of alkylbenzimidazoles for CO and SO capture and
2
2
comparisons to ionic liquids
1
1
2,3
SHANNON Matthew S. , HINDMAN Michelle S. , DANIELSEN Scott. P. O. ,
2
,4
1
1*
TEDSTONE Jason M. , GILMORE Ricky D. & BARA Jason E.
1
Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, USA
2
NSF-REU Site: Engineering Solutions for Clean Energy Generation, Storage and Consumption, Department of Chemical & Biological
Engineering, University of Alabama, Tuscaloosa, USA
3
Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, USA
4
Department of Chemical & Biomolecular Engineering, Clemson University, Clemson, USA
Received February 16, 2012; accepted April 22, 2012; published online June 25, 2012
To date, few reports have been concerned with the physical properties of the liquid phases of imidazoles and benzimidazoles-
potential starting materials for a great number of ionic liquids. Prior research has indicated that alkylimidazole solvents exhibit
different, and potentially advantageous physical properties, when compared to corresponding imidazolium-based ionic liquids.
Given that even the most fundamental physical properties of alkylimidazole solvents have only recently been reported, there is
still a lack of data for other relevant imidazole derivatives, including benzimidazoles. In this work, we have synthesized a se-
ries of eight 1-n-alkylbenzimidazoles, with chain lengths ranging from ethyl to dodecyl, all of which exist as neat liquids at
ambient temperature. Their densities and viscosities have been determined as functions of both temperature and molecular
weight. Alkylbenzimidazoles have been found to exhibit viscosities that are more similar to imidazolium-based ILs than al-
2 2
kylimidazoles, owed to a large contribution to viscosity from the presence of a fused ring system. Solubilities of CO and SO ,
two species of concern in the emission of coal-fired power generation, were determined for selected alkylbenzimidazoles to
understand what effects a fused ring system might have on gas solubility. For both gases, alkylbenzimidazoles were deter-
mined to experience physical, non-chemically reactive, interactions. The solubility of CO
less than observed for corresponding ILs and alkylimidazoles. 1-butylbenzimidazole was found to readily absorb at least 0.333
gram SO per gram at low pressure and ambient temperature, which could be readily desorbed under an N flush, a behavior
2
in alkylbenzimidazoles is 10%–30%
2
2
more similar to imidazolium-based ILs than alkylimidazoles. Thus, we find that as solvents for gas separations, benzimidazoles
share characteristics with both ILs and alkylimidazoles.
2 2
benzimidazole, imidazole, ionic liquids, carbon dioxide (CO ) capture, sulfur dioxide (SO )
1
Introduction
[3–5], energetic materials [6–8], cellulose processing [9]
and polymer science [10–16], to name but a few. Benzim-
idazolium salts (Figure 1(b)), while possessing the same
modularity as their imidazolium counterparts, have rarely
been explored for the same applications that have been
proposed for imidazolium-based ILs. This might be at-
tributable to the much higher melting points of benzimidaz-
olium salts than imidazolium salts [17]. However, benzim-
idazolium salts have found great utility as frameworks for
Imidazolium salts (Figure 1(a)) have been the dominant
motif for the development of ionic liquids (ILs) and have
been explored in a diverse and ever-growing number of
2
applications, including CO capture [1, 2], electrochemistry
*
Corresponding author (email: jbara@eng.ua.edu)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
1639
N-heterocyclic carbenes (NHCs) for catalysis [18–20], as
liquid crystals (LCs) [21] and antimicrobials [22]. Benzo-
bis(imidazolium) or “Janus” benzimidazolium salts (Figure
1
(c)) have also been of interest for their unique fluorescence
and as sensors [23, 24]. Poly(benzimidazolium) salts can be
formed form the alkylation of poly(benzimidazole) and used
as anion conductors [25]. However, to our knowledge, there
are no reports on the use of benzimidazolium ILs (e.g.
Figure 2 Structures of (a) imidazoles and (b) benzimidazoles.
1
-ethyl-3-methylbenzimidazolium tetrafluoroborate) as bulk
To further this area of research, we have explored the prop-
erties of some relatively simple benzimidazole derivatives.
Herein, we report on density, viscosity and solubilities of
liquids, even as related to characterizations of physical
properties such as density or viscosity. Thus, although an
important building block in many applications [26], the im-
pact of the versatile and readily available benzimidazole
moiety on physical and thermodynamic properties has not
been quantified.
2 2
carbon dioxide (CO ) and sulfur dioxide (SO ) in
1-n-alkylbenzimidazoles (Figure 2(b)) and provide compar-
isons to alkylimidazoles and imidazolium-based ILs.
Density and viscosity were found to be strongly related to
the length of the alkyl chain and temperature, and highly
correlated functions were developed as a means of modeling
these properties. Solubility of CO
2
was also found to be
correlated to alkyl chain length and temperature, although
the 1-n-alkylbenzimidazole solvents appear to have some-
Although the characterization of physical properties for
imidazolium-based ILs has now been a priority of many
research efforts for more than a decade [27], there has only
recently been an interest in developing a similar under-
standing of the physical properties of the starting materials
for imidazolium-based ionic liquids (ILs), such as alkylim-
idazoles (Figure 2(a)) [28–32]. Fundamentally, alkylimid-
azoles can be functionalized in similar manners to ILs to
control their physical [28–30, 32, 33] and/or chemical [34]
and/or biological [35] properties. As there are likely quad-
what less of an affinity for CO
Interestingly, although 0.33 grams of SO
2
than alkylimidazoles or ILs.
could be absorbed
2
per gram of 1-butylbenzimidazole, this was not attributable
to a chemical reaction that was previously observed for al-
kylimidazoles [28]. The properties and behaviors of 1-n-
alkylbenzimidazoles thus appear to have commonalities
with both alkylimidazoles and imidazolium-based ILs.
15
18
rillions (10 ) if not quintillions (10 ) of possible ILs [36],
encompassing a number of different species, it may be
assumed that the number of possible imidazole derivatives
6
(
and mixtures) could easily exceed 10 (if not orders of
2
Materials and Method
magnitudegreater).
Imidazoles and benzimidazoles (Figure 2(b)) are known
for their antimicrobial, antifungal, and medicinal properties
and thus have numerous applications in the pharmaceutical
industry [26, 35, 37, 38]. In addition to these vital uses, im-
idazoles are an important class of building blocks for syn-
thetic organic chemistry [10, 39]. Furthermore, virtually all
imidazolium salts originate from imidazoles, and the prop-
erties of ILs are better understood via comparisons to imid-
azoles [30, 40]. Imidazoles have also found industrial utility
in separations, as the BASIL™ (biphasic acid scavenging
using ionic liquids) process directly used 1-methylimidazole
as an acid (H ) scavenger to form 1-methylimidazolium
chloride and enabled significant improvements in the man-
ufacture of alkoxyphenylphosphines [41, 42].
Developing understandings of neutral, imidazole-based
molecules can be keys to advancing IL-based technologies.
2
.1 Materials
All chemicals were purchased from Sigma-Aldrich (Mil-
waukee, WI USA) and were used as received without fur-
ther purification. CO and anhydrous SO were purchased
2 2
from AirGas (Radnor, PA USA) at a minimum of 99.98%
purity.
2
.2 Synthesis of 1-n-alkylbenzimidazoles
Sodium benzimidazolate (NaBenz) was first produced via
the neutralization of benzimidazole with NaOH in THF
(Scheme 1(a)) and subsequent drying. NaBenz was then
used to produce a series of 1-n-alkylbenzimidazoles (1–8)
via reaction with a corresponding 1-bromoalkane (Scheme
1(b)). This synthetic method is based on our prior work with
a variety of N-functionalized imidazoles [43].
+
2
.2.1 Procedure for preparing sodium benzimidazolate
(
NaBenz)
Benzimidazole (200.00 g, 1.693 mol) was dissolved in re-
fluxing THF (500 mL). NaOH (74.49 g, 1.862 mol) was
added and observed to gradually disappear as the reaction
was stirred for 16 hours at 65 °C. The reaction was stopped,
and while warm (~40 °C), filtered through a thin layer of
Figure 1 General structures of (a) imidazolium salts, (b) benzimidazoli-
um salts and (c) benzobis(imidazolium) salts.

1640
Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
mmol) and 1-bromopropane (23.79 g, 17.60 mL, 193 mmol)
in a manner similar to that employed for 1. Yield = 21.81 g
1
(
(
79.4%). H NMR (500 MHz, DMSO) δ 8.22 (s, 1H), 7.62
dd, J = 24.2, 7.9 Hz, 2H), 7.36–7.09 (m, 2H), 4.20 (t, J =
7
.0 Hz, 2H), 1.91–1.66 (m, 2H), 0.84 (t, J = 7.4 Hz, 3H).
1
H NMR consistent with published data [46, 47].
2
.2.2.3 Synthesis of 1-butylbenzimidazole (3)
Scheme 1 Synthesis of (a) NaBenz precursor and (b) subsequent for-
mation of 1-n-alkylimidazoles (1–8).
Compound 3 was produced from NaBenz (50.00 g, 357
mmol) and 1-bromobutane (44.01 g, 34.57 mL, 321 mmol)
in a manner similar to that employed for 1. H NMR (500
MHz, DMSO) δ 8.22 (s, 1H), 7.62 (dd, J = 28.1, 7.9 Hz,
1
2 3
basic Al O to remove the small amount of insoluble mate-
rial present. The solvent was removed via rotary evapora-
tion until a viscous oil remained, which was then spread
onto a large area Pyrex® glass tray and placed in a vacuum
oven set to 80 °C. After several minutes, the viscous oil
underwent a transition to an off-white solid. At this time,
the product was briefly removed from the oven and the solid
crushed to produce a fine powder. The powdered product
was returned to the vacuum oven for 16 hours at 120 °C.
2
H), 7.36–7.02 (m, 2H), 4.23 (t, J = 7.1 Hz, 2H), 1.91–1.62
1
(
m, 2H), 1.40–1.08 (m, 2H), 0.88 (t, J = 7.4 Hz, 3H). H
NMR consistent with published data [48, 49].
2.2.2.4 Synthesis of 1-pentylbenzimidazole (4)
Compound 4 was produced from NaBenz (30.00 g,
2
1
14 mmol) and 1-bromopentane (29.22 g, 23.93 mL,
93 mmol) in a manner similar to that employed for 1.
2
07.633 g of the product containing NaBenz and residual
1
Yield = 24.72 g (76.6%). H NMR (500 MHz, DMSO) δ
1
THF and NaOH were collected. By H NMR analysis, about
mol% THF remained in the NaBenz product based on the
8
2
2
.22 (s, 1H), 7.61 (dd, J = 27.0, 7.9 Hz, 2H), 7.22 (dt, J =
8.0, 7.4 Hz, 2H), 4.23 (t, J = 7.1 Hz, 2H), 1.91–1.62 (m,
4
expected chemical shifts of THF in DMSO-d6 (multiplets at
1
H), 1.47–1.03 (m, 4H), 0.83 (t, J = 7.2 Hz, 3H). H NMR
δ 3.60 and 1.76) [44]. The NaBenz product was used with-
consistent with published data [50].
1
out further purification. An image of this H NMR spectra is
1
provided as Supporting Information. H NMR (500 MHz,
2
.2.2.5 Synthesis of 1-hexylbenzimidazole (5)
DMSO-d6) δ 7.93–7.69 (m, 1H), 7.38 (ddt, J = 6.1, 3.2, 1.7
Hz, 2H), 6.80 (ddt, J = 5.9, 3.0, 1.4 Hz, 2H), 2.54–2.47 (m,
Compound 5 was produced from NaBenz (50.00 g, 357
mmol) and 1-bromohexane (53.06 g, 45.12 mL, 321 mmol)
in a manner similar to that employed for 1. Yield = 51.33 g
1
2
2
H).
1
(
(
78.9%). H NMR (500 MHz, DMSO) δ 8.22 (s, 1H), 7.64
d, J = 7.9 Hz, 1H), 7.58 (d, J = 7.9 Hz, 1H), 7.28–7.12 (m,
.2.2 Synthesis of1-n-alkylbenzimidazoles (1–8)
.2.2.1 Synthesis of 1-ethylbenzimidazole (1)
2H), 4.23 (t, J = 7.1 Hz, 2H), 1.83–1.69 (m, 2H), 1.41–1.08
1
(
m, 6H), 0.82 (t, J = 6.9 Hz, 3H). H NMR consistent with
NaBenz (50.00 g, 357 mmol) was dissolved in 500 mL THF
in a 1000 mL round bottom flask. Bromoethane (35.01 g,
published data [51].
2
3.98 mL, 321 mmol) was added and a solid precipitate was
2
.2.2.6 Synthesis of 1-octylbenzimidazole (6)
observed soon thereafter. A reflux condenser was attached
and the reaction allowed to proceed overnight at 65 °C
while stirring. After this time, the reaction was cooled, the
solids filtered and the THF removed via rotary evaporation.
The product was extracted into 400 mL of a 50:50 (V:V)
mixture of EtOAc and hexanes. The solution was then dried
Compound 6 was produced from NaBenz (50.00 g, 357
mmol) and 1-bromooctane (62.05 g, 55.9 mL, 321 mmol) in
a manner similar to that employed for 1. Yield = 53.44 g
1
(
(
72.2%). H NMR (500 MHz, DMSO) δ 8.21 (s, 1H), 7.61
dd, J = 31.0, 7.9 Hz, 2H), 7.32–7.15 (m, 2H), 4.22 (t, J =
over MgSO
through a plug of basic Al
4
, mixed with activated carbon and filtered
, which was then washed with
7.1 Hz, 2H), 1.85–1.70 (m, 2H), 1.51–1.01 (m, 10H), 0.82 (t,
J = 6.9 Hz, 3H).
2
O
3
an additional 200 mL of a the EtOAc/hexanes mixture.
The filtrate was reduced via rotary evaporation and the
product dried under vacuum lines to produce 1 as a pale
yellow oil. Yield = 25.885 g (55.1%). H NMR (500 MHz,
DMSO) δ 8.24 (s, 1H), 7.62 (dd, J = 28.1, 7.9 Hz, 2H), 7.22
2
.2.2.7 Synthesis of 1-decylbenzimidazole (7)
Compound 7 was produced from NaBenz (50.00 g, 357
mmol) and 1-bromodecane (71.07 g, 66.48 mL, 321 mmol)
in a manner similar to that employed for 1. Yield = 56.64 g
(68.2%). H NMR (500 MHz, DMSO) δ 8.21 (s, 1H), 7.61
(dd, J = 28.9, 7.9 Hz, 2H), 7.36–7.09 (m, 2H), 4.22 (t, J =
1
1
(
=
dt, J = 27.7, 7.4 Hz, 2H), 4.27 (q, J = 7.3 Hz, 2H), 1.41 (t, J
7.3 Hz, 3H). H NMR consistent with published data [45].
1
7
.1 Hz, 2H), 1.87–1.66 (m, 2H), 1.42–1.03 (m, 14H), 0.84 (t,
1
2
.2.2.2 Synthesis of 1-n-propylbenzimidazole (2)
J = 6.9 Hz, 3H). H NMR consistent with published data [50,
Compound 2 was produced from NaBenz (30.00 g, 214
52].

Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
1641
2
.2.2.8 Synthesis of 1-dodecylbenzimidazole (8)
< 1 torr) and very small (~0.1%) compared to the partial
pressure of the gas.
Compound 8 was produced from NaBenz (50.00 g, 357
mmol) and 1-bromododecane (77.86 g, 74.87 mL, 321
mmol) in a manner similar to that employed for 1. Yield =
2
.6 SO
2
Solubility measurements
in 1-butylbenzimidazole was measured in
1
6
1
4
1
4.52 g (70.1%). H NMR (500 MHz, DMSO) δ 8.21 (s,
H), 7.61 (dd, J = 33.0, 7.9 Hz, 2H), 7.36–7.07 (m, 2H),
.22 (t, J = 7.1 Hz, 2H), 1.87–1.64 (m, 2H), 1.46–0.99 (m,
Solubility of SO
2
a different manner than that of CO
5.00 g, 28.7 mmol) was added to a 50 mL round bottom
flask with a stir bar. The flask was secured with a clamp,
suspended over a stir-plate, and stirring initiated. SO gas at
1 psig was bubbled via a 1/16″ outside diameter stainless
2
. 1-Butylbenzimidazole
1
(
8H), 0.84 (t, J = 6.9 Hz, 3H). H NMR consistent with
published data [49].
2
~
2.3 Density measurements
steel tube into the stirring mixture. The mass of the flask
and its contents, as well as the color of the liquid were rec-
orded at several time intervals over a period of 20 minutes.
Density values for each 1-n-alkylbenzimidazole were ob-
o
o
tained over the range of 20.00–80.00 C at 10.00 C incre-
ments using a Mettler Toledo DM45 DeltaRange density
meter, using the same methodology as in our previous work
After this time, SO
2
was desorbed from the liquid via a
flush with N for 40 minutes, and the mass of the flask con-
2
tents and color of the liquid were observed to return to their
original value/state. The error associated with the measure-
ment of the mass of the flask and its contents is ±0.0005 g,
which is the uncertainty associated with the resolution of
the balance used.
[
0
30]. The uncertainty associated with this measurement is ±
.00005 g cm .
3
2
.4 Viscosity measurements
Viscosity data were obtained for each 1-n-alkylbenzimida-
zole using a Brookfield DV-II+ Pro viscometer, under the
same methodology as in our previous work, with the appro-
priate sized spindle (i.e. ULA) for relatively low viscosity
liquids [28, 30]. Measurements were taken over the range of
3
Results and discussion
3
.1 Densities of 1-n-alkylbenzimidazoles
The measured density values for 1-n-alkylbenzmidazoles
over the temperature range of 20–80 °C are presented in
Table 1. For each compound, density was observed to de-
crease linearly with increasing temperature. Also, across the
entire group of 1-n-alkylbenzimidazoles, density decreased
as the n-alkyl substituent length increased, which is similar
2
5
0.00–80.00 °C, with 5.00 °C increments from 20.00–
0.00 °C and 10.00 °C increments from 50.00–80.00 °C.
The temperature of the jacketed sample chamber was con-
trolled via the Brook field TC-602P circulating bath. The
viscometer accuracy is ±1% of the reading for torque meas-
urement with a repeatability of ±0.2% of the reading.
to observations across families of [C mim][X] ILs as the
n
length of the “C ” chain increases. Decreased density can be
n
2
.5 CO
2
Solubility measurements
in each 1-n-alkylbenzimidazole were
rationalized as the dilution of the polar imidazole ring (or
ions in the case of ILs) within the bulk by increasing the
volume fraction of the less dense hydrocarbon chains. A sur-
face plot of density against temperature and the contribution
of the alkyl chain for the 1-n-alkylbenzimidazoles is shown in
Figure 1. As in our previous work, the “molecular weight
parameter” (R′), a dimensionless value relating the molecular
weight of the side chain to the molecular weight of the entire
molecule, was used as means ofquantifying the impact of
then-alkyl substituent. R′ is calculated according to eq. (1)
and values for each compound are presented in Table 1.
Solubilities of CO
2
measured gravimetrically using equipment and methodolo-
gies described in our previous works [28–30]. Experiments
were conducted at temperatures of 30, 45, 60, and 75 °C (as
controlled by an oil bath). An initial charge of gas was fed
at 30 °C until the pressure equilibrated at ~5 atm. The re-
spective errors associated with H and S were calculated
based upon propagation of error of the experimental param-
eters (i.e. pressure, temperature, volumes, mass, etc.), in
which all errors associated with our instrumentation are
known. In this method, both the solubility and molecular
weight of the gas are factors that influence the overall error.
M (alkylchain)
W
R' 
(1)
Measurements for CO
2
solubility exhibit typical experi-
M (molecule)
W
mental errors of less than ~4%, similar to our previous re-
sults for 1-n-alkylimidazoles. Based on published vapor
pressure data and our previous measurements for
Similar to the results obtained for our characterizations
of 1-n-alkylimidazoles [30], a surface plot of density is ap-
proximately planar with respect to temperature and R′ (Fig-
ure 3). However, a 7%–10% increase in density is observed
when comparing 1-n-alkylbenzimidazoles to 1-n-alkylimi-
dazoles over the same temperature range. 1-n-Alkylbenzimi-
1
1
-n-alkylimidazoles [28–32], the vapor pressure of the
-n-alkylbenzimidazole compounds can be assumed as neg-
ligible under the experimental temperature and pressure
conditions, as it is low (~5 mm Hg maximum and typically

1642
Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
Table 1 Densitiesof 1-n-alkylbenzimidazoles (1–8)

3
Density (g cm ) at given temperature
Compound

1
M
W
(g mol )
R′
20 °C
30 °C
40 °C
50 °C
60 °C
70 °C
80 °C
1
, ethyl
146.19
160.22
174.24
188.27
202.30
230.35
258.40
286.45
0.199
0.269
0.328
0.378
0.421
0.492
0.547
0.591
1.09399
1.06565
1.04381
1.02438
1.00943
0.98502
0.96606
0.95587
1.08626
1.05799
1.03630
1.01694
1.00210
0.97794
0.95911
0.94913
1.07859
1.05039
1.02884
1.00936
0.99484
0.97092
0.95221
0.94243
1.07093
1.04282
1.02141
1.00221
0.98760
0.96391
0.94534
0.93569
1.06327
1.03525
1.01401
0.99485
0.98037
0.95693
0.93849
0.92885
1.0556
1.04792
1.02009
0.99918
0.98012
0.96589
0.94300
0.92477
0.91483
2
, propyl
1.02769
1.00660
0.98750
0.97315
0.94996
0.93163
0.92183
3
, butyl
4
, pentyl
5
, hexyl
6
, octyl
7
, decyl
8
, dodecyl
dazoles are less dense than many ILs, although this is likely
due to the absence of elements other than C, H and N and/or
fluorinated groups [27, 53–56].
The data presented in Table 1 and Figure 3 could be fit
with a high degree of accuracy to the same form of equation
we applied to our density measurements of 1-n-alkylimi-
correlated to the length of the n-alkyl substituent, with 1-do-
decylbenzimidazole observed to be ~3.5× more viscous
between than 1-ethylbenzimidazole at 20 °C, although the
magnitude of this difference reduced to ∼2× at 80 °C.
The temperature dependence of viscosity for each com-
pound can be shown to follow the Litovitz Model (eq. (3)),
which we have shown provides a very accurate model of the
viscosities of 1-n-alkylimidazoles, and has also been suc-
cessfully applied to imidazolium-based ILs [53]. Using the
data in Table 3, we obtained A and B coefficients for com-
pounds 1–8 via regression using Matlab, which are pre-
dazoles (eq. (2)). A surface regression of the data in Figure
2
3
0
performed with Matlab provides an excellent fit (R =
4
3
.9968) to eq. (2), with constants A = 7.253×10 g cm
1
3
3
K , B = 0.3479 g cm , and C = 1.3710 g cm , where T is
temperature in Kelvin and R′ is the dimensionless molecular
weight parameter for a given molecule (Table 1). Thus, this
relatively simple model provides the ability to accurately
2
sented along with goodness of fit values (R ) in Table 4.

B 
T 
describe density at
a
given temperature for 1-n-
(T)  Aexp
(3)


3

alkylbenzimidazoles with side chains as large as dodecyl.

(T, R')  AT  BR' C
(2)
Figure 4 presents a surface plot of the viscosity data in
Table 2 with respect to temperature and R′. As can be seen
in Figure 4 a relatively smooth surface exists between R′
values of 0.269–0.547, which corresponds to the range en-
compassed by 1-propylbenzimidazole and 1-decylbenz-
imidazole. Sharper changes in viscosity occur at the edges
of the plot, corresponding to 1-ethylbenzimidazole and
3
.2 Viscosities of 1-n-alkylbenzimidazoles
The measured viscosity values for 1-n-alkylbenzimidazoles
over the temperature range of 20–80 °C are presented in
Table 2. Viscosities of the eight 1-n-benzalkylimidazole
compounds examined were <100 cP. For each compound,
the viscosity was observed to decrease in a nonlinear fash-
ion with increasing temperature. Viscosity was strongly
1
-dodecylbenzimidazole. Attempts to fit the entire surface
to a 2-parameter (T, R′) Litovitz Model, although success-
fully applied in our previous work with 1-n-alkylimidazoles,
were unsuccessful for 1-n-alkylbenzimidazoles. Thus, a
Figure 3 Three-dimensional plot of 1-n-alkylbenzimidazole densities as
related to temperature and R′.
Figure 4 Three-dimensional plot of 1-n-alkylbenzimidazole viscosities as
related to temperature and R′.

Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
1643
Table 3 Viscosities of 1-n-alkylbenzimidazoles (1–8)
Viscosity (cP) at given temperature
Compound
ethyl
, propyl
20 °C
27.09
49.01
49.49
52.79
60.44
68.10
71.13
96.34
25 °C
20.67
35.82
38.34
40.91
45.37
50.51
54.12
70.19
30 °C
16.03
26.51
28.19
30.35
33.89
38.04
41.02
52.55
35 °C
12.73
20.06
21.50
23.25
25.72
29.25
31.71
40.07
40 °C
10.32
15.62
16.71
18.00
19.98
23.00
24.97
31.06
45 °C
8.50
50 °C
7.18
60 °C
5.31
70 °C
4.13
5.15
5.45
5.91
6.64
7.46
8.17
9.54
80 °C
1
3.29
3.99
4.15
4.54
4.91
5.67
6.20
7.14
2
12.37
13.23
14.36
15.86
18.20
19.95
24.54
10.05
10.77
11.60
13.87
14.83
16.24
19.75
7.01
3
, butyl
7.47
4
, pentyl
8.05
5
, hexyl
8.85
6
, octyl
10.26
11.22
13.36
7
, decyl
8
, dodecyl
Table 4 Empirical constants and quality of fit for eq. (3) as applied to
viscosities of 1-n-alkylbenzimidazoles (1–8)
in governing molecular motion, while the viscosities of the
latter are much more sensitive to the influence of alkyl
chain length. This effect of increasing viscosity with in-
creasing chain length is also observed for homologous se-
ries of ILs that vary only in the length of the alkyl chain [27,

8
3
2
Compound
, ethyl
, propyl
A (cP)
0.188
0.129
0.139
0.155
0.168
0.195
0.227
0.212
B × 10 (K )
1.24
R
1
0.9977
0.9979
0.9989
0.9989
0.9988
0.9994
0.9997
0.9996
2
1.48
3
, butyl
1.48
5
3, 57–60].
4
, pentyl
1.47
Figure 5 presents a graphical illustration of the relative
5
, hexyl
1.48
viscosity ranges exhibited by ILs, 1-n-alkylimidazoles and
-n-alkylbenzimidazoles. As is apparent, 1-n-alkylbenzimi-
6
, octyl
1.47
1
7
, decyl
1.45
dazoles possess viscosities that mostly exceed those exhib-
ited by 1-n-alkylimidazoles, yet fall near the lower range of
those exhibited by 1-n-alkyl-3-methylimidazolium ILs. As a
point of reference, 1-n-alkylbenzimidazoles tend to overlap
the region occupied by 1-ethyl-3-methylimidazolium salts
8
, dodecyl
1.54
single equation for viscosity cannot be applied across the
entire series.
However, in examination of Table 4, it appears that for
the range of compounds that fall between 1-propylbenzim-
idazole (2) and 1-decylbenzimidazole (7), a single model
can be developed, as the constant ‘A’ is correlated R′ and B
(e.g. [C
2 2 2
mim][Tf N], [C mim][dca], etc.). The underlying
data points used to form this graph can be viewed in the
Supporting Information from our prior work [30].
8
3
is virtually independent of R′, as its value is 1.47×10 K for
3
.3 Solubility of CO
2
each of the compounds. We have found that the relationship
2
between A and R′ is well-described (R = 0.9896) by eq. (4),
Henry’s constants (H) and volumetric solubilities (S) of CO
2
with coefficients of 0.722 cP and 2.0445 (dimensionless).
A  0.0722exp(2.0445 R')
(4)
The substitution of eq. (4) into eq. (3) yields eq. (5), which
can be used to describe the viscosity (cP) of 1-n-alkylben-
zimidazoles ranging from 1-propylbenzimidazole to 1-
2
decyl-benzimidazole, with an R = 0.9934.
8

1.4710 

(T, R)  0.0722exp 2.0445 R'
(5)

3


T

Eq. (5) thus presents a useful and accurate model for es-
timating the viscosity of a range of 1-n-alkylbenzimidazoles
as a function of temperature and the contribution of the al-
kyl chain. This model is somewhat more simplistic than that
found for describing the viscosity of 1-n-alkylimidazoles,
although the 1-n-alkylimidazoles were much less viscous.
This may be due to the fact that viscosities of the
Figure 5 Graphical comparison of reported viscosity ranges of 1-n-alkyl-
imidazoles (lower, blue); 1-n-alkylbenzimidazoles (this work)(middle,
grey); and 1-n-alkyl-3-methylimidazolium ILs (upper, red). Modified with
permission from Shannon MS, Bara JE. Properties of alkylimidazoles as
solvents for CO capture and comparisons to imidazolium-based ionic
2
liquids. Ind Eng Chem Res, 2011, 50(14): 8665–8677. Copyright 2011
1
-n-alkylbenzimidazoles span a relatively smaller range of
magnitudes than 1-n-alkylimidazoles, where the large fused
ring system in the former plays a much more influential role
American Chemical Society.

1644
Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
in 1-n-alkylbenzimidazoles at temperatures between
Also shown in Table 3, for the convenience of the reader,
are Henry’s constants, which describe the solubility of CO
2
3
0–75 °C are presented in Table 3.
The data presented are largely indicative of physical sol-
in terms of mole fraction at a given temperature. However,
as with the same discussion in our previous work, this ob-
servation and assessment of using Henry’s constants (i.e.
mole basis) for gas solubility comparisons among varying
substituent chain lengths is not appropriate. It can be seen
that although the Henry’s constants (i.e. mole fraction of
ubility behavior, rather than any type of chemical reaction
between CO and the benzimidazole molecules. This be-
2
havior is consistent with what has been observed for al-
kylimidazoles and most ILs, indicating that each of these
classes of compounds would be most useful under high
pressures or high concentrations of CO
pressure applications (i.e. post-combustion CO
At any given temperature, 1-ethylbenzimidazole showed
the highest CO solubility per volume, as extension of the
alkyl chain serves to diminish CO solubility, as has been
observed for alkylimidazoles and imidazolium-based ILs [2,
9, 30, 61]. The values reported are lower than those re-
ported for alkylimidazoles and imidazolium-based ILs [2,
9, 30], indicating that the six-membered aromatic ring
present within benzimidazoles has a negative effect on
CO
affinity. A similar effect has been observed for ben-
zyl-functionalized imidazolium-based ILs [2, 62], although
interactions between CO and the electrons were expected
to increase CO solubility. One possible explanation is that
the benzimidazole solvent (and by extension, ILs with ben-
zyl groups) possess less free volume in which CO can dis-
solve. This is supported by the fact that improved separation
of CO /N has been observed in ionic liquid-based mem-
branes where these groups are present [1, 62, 63]. We have
not measured comparative data for CO solubility in
-benzylimidazole due to the fact it is a crystalline solid
2
, rather than in low
CO
2
) are similar for each molecule at a given temperature,
this is primarily due to an increase in solvent molecular
weight (i.e. fewer moles of solvent) rather than increased
2
capture).
2
2
affinity of CO for the solvent.
2
3
.5 Solubility of SO2
2
Experimental results for SO
2
absorption in 1-butylbenzim-
2
idazole are presented below in Table 6, with Figure 6 also
provided as visual evidence to support our observations of
the color change associated with SO absorption. A larger
2
2
version of Figure 6 is provided as Supporting Information.
The choice of 1-butylbenzimidazole was based on its rela-
tively low viscosity amongst this group of compounds and
the expectation that its vapor pressure is lower than that of
1-ethylbenzimidazole. It is likely that any of the 1-n-
alkylbenzimidazole compounds would produce similar re-
sults. It should be noted that the small difference in mass
recorded between the initial and final readings is slightly
less than zero. This is almost certainly due to the loss of
small quantities of 1-butylbenzimidazole on the stainless
steel tubing that was submerged into the liquid to introduce
2
2
2
2
2
2
1
o
with a melting point of ~70 C [43, 47, 64].
Table 5 Solubility of CO
2
in several 1-n-alkylbenzimidazoles at partial pressures of ~5 atm and temperatures between 30 and 75 °C expressed as both
Henry’s constants (H(atm)) and volumetric solubilities (S)
o
a)
Compound
, ethyl
Temperature ( C)
30
H (atm)
96
+/
2
4
5
7
2
4
6
8
3
6
12
22
S
+/
1
1.80
1.36
1.03
0.82
1.60
1.14
0.83
0.70
1.30
0.86
0.55
0.38
0.05
0.04
0.03
0.03
0.04
0.04
0.03
0.03
0.05
0.04
0.03
0.03
4
6
7
5
0
5
126
160
202
88
123
160
200
89
3
, butyl
30
4
6
7
5
0
5
5
, hexyl
30
4
6
7
5
0
5
133
203
298
3
3
1
1
a) S [=] (cm gas (STP)) (cm solvent) atm ; uncertainty represents +/ one standard deviation.
Table 6 Experimental data for absorption of SO
2
in 5.00 g (28.7 mmol) 1-butylbenzimidazole at 25 °C (3)
m (g SO mol SO
2
mol SO (mol

2
2
Time (min)
Flask mass (g)
1
Observation
absorbed)
absorbed
0.00000
0.00536
0.01541
0.02775
0.00028
1-butylbenzimidazole)
0
(start SO
5
2
)
98.865
99.208
99.851
100.641
98.847
0.000
0.00000
pale yellow
bright yellow
0.343
0.18676
1
0
0.986
0.53688
yellow-orange
2
0 (stop SO
2
, start N
2
flush)
1.776
0.96703
dark orange, more viscous
pale yellow
6
0 (stop N
2
flush)
0.018
0.009801

Shannon MS, et al. Sci China Chem August (2012) Vol.55 No.8
1645
imidazolium-based ILs. CO
2
solubility, however, is some-
what negatively impacted by the presence of the benzimid-
azole moiety, as measured values were less than those ob-
tained for 1-n-alkylimidazoles and many imidazolium-based
ILs. Further research is needed as to why the presence of
benzene rings in both neutral and charged (i.e. ILs) mole-
cules hinders CO
was demonstrated to exhibit physical, rather than chemical
interactions with SO , a property more in line with imidazo-
lium-based ILs than alkylimidazoles.
-n-Alkylbenzimidazoles can be viewed to have hybrid
2
dissolution. Finally, 1-butylbenzimi dazole
2
1
properties of ILs and alkylimidazoles. It is our hope that this
initial study on the properties of benzimidazoles in the liquid
state will bring greater attention to this versatile class of
molecules in identifying applications and gain a more thor-
ough understanding of imidazolium/benzimidazolium ILs.
Figure 6 Progressive darkening from essentially colorless to pale yellow
to orange in 1-butylbenzimidazole (3) upon exposure to SO and subse-
(bot-
2
quent return to an essentially colorless liquid upon flushing with N
tom right).
2
SO
2
.
This work was supported by ION Engineering, LLC; United States De-
partment of Energy-National Energy Technology Laboratory
The data in Table 6 indicate that SO
rapidly absorbed at room temperature and low pressure (~1
psig) to at least a 1:1 (mol:mol) ratio with 1-butylbenzim-
idazole. Furthermore, SO
temperature using only an N
observed that 1-hexylimidazole was also capable of absorb-
ing at least 1 mole of SO per mole 1-hexylimidazole under
similar conditions, a portion of that SO was not readily
desorbed from 1-hexylimidazole. SO and 1-hexylimid-
2
can be readily and
(DE-FE00005799); and the National Science Foundation Research Expe-
riences for Undergraduates Program (EEC-1062705) is gratefully
acknowledged. Additional support from the University of Alabama Re-
2
can be easily desorbed at room
search Grants Committee is also gratefully acknowledged. The authors
2
flush. While we previously
1
thank Ken Belmore of the University of Alabama for acquisition of
NMR spectra.
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