Chinese Journal of Chemical Engineering 23 (2015) 1728–1732
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Chinese Journal of Chemical Engineering
journal homepage: www.elsevier.com/locate/CJChE
Materials and Product Engineering
Synthesis and characterization of switchable ionic compound based on
DBU, CH3OH, and CO2☆
Yuan Xie 1,2, Richard Parnas 1,3, Bin Liang 1,2, Yingying Liu 2, Chuandong Tao 1,2, Houfang Lu 1,2,⁎
1
2
3
College of Chemical Engineering, Sichuan University, Chengdu 610065, China
Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu 610065, China
Institute of Materials Science, Univ. of Connecticut, Storrs CT 06269-3136, USA
a r t i c l e
i n f o
Article history:
Received 10 February 2015
Received in revised form 13 July 2015
Accepted 25 July 2015
Available online 14 August 2015
Keywords:
DBU
Switchable ionic compounds
Synthesis
a b s t r a c t
Switchable ionic compounds have wide applications in chemical processes. A switchable ionic compound based
on 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU), CH3OH and CO2 was synthesized and characterized. DBU/
CH3OH/CO2 ionic compound was prepared in the presence of excess methanol, and then the excess methanol
was removed by reduced pressure distillation in CO2 atmosphere. The product yield (100%) reached the theoretical maximum for the first time. Its structure was identified by NMR, FT-IR, and XRD. The crystal product shows 8
strong peaks in XRD at 2θ values of 16.0547°, 16.4308°, 16.7651°, 18.8714°, 19.2140°, 21.9471°, 22.0780°, and
25.5661°. Its decomposition onset temperature (53 °C) was affirmed by TGA, which is lower than its melting
point. And its ionic switch point was measured by conductivity.
© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction
Almost every chemical process uses several steps, such as reaction,
extraction, separation, and purification. In most cases, the solvent
required in a particular step is different from the one required in the
next step. Therefore, the choice of solvents is very important in a
consecutive process. A common practice is to remove the solvent after
each step and then add a new solvent in the next step in order to
match the process operation, which may increase the cost and environmental impact or decrease the yield [1,2]. Therefore, it is interesting and
meaningful to find solvents that can be used in several consecutive
reaction and separation steps. The properties of such solvents must
often be altered, preferably under mild conditions, to proceed from
step to step.
Currently, a smart type of switchable solvent was proposed by
Jessop et al. [1]. The solvent properties can be reversibly changed
under mild conditions by reacting with an alcohol and an acid gas.
This kind of switchable solvent, mainly containing amine, amidine or
guanidine, is capable of transformation from a mixture of molecular
compounds to an ionic liquid at room temperature. The ionic liquid
can be converted back to the mixture of molecular compounds by
bubbling inert gas through the liquid or by heating to accelerate the
transformation.
☆ Supported by the Doctoral Fund of Ministry of Education of China (20130181130006),
the Key Program of National Natural Science Foundation of China (21336008), and the
National Natural Science Foundation of China (21476150).
⁎ Corresponding author.
E-mail address: [email protected] (H. Lu).
Owing to their good properties, such as the alkalinity, catalysis,
solubility, and change of polarity, the switchable solvents can be used
in multistep reaction processes. For example, using 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) to extract a nonpolar organic compound,
the compound can be easily separated by reacting DBU with alcohol
and CO2 to form a polar ionic compound. Finally, the solvent of DBU
and alcohol is recovered by bubbling inert gases to release CO2 at room
temperature or by heating to encourage thermal decomposition [1,3].
Zhang reported a CO2-based switchable system for pre-activation of
microcrystalline cellulose in catalytic hydrolysis to glucose [4]. The CO2/
organic base/solvent system can dissolve or swell lignocellulosic
biomass. The processed microcrystalline cellulose or lignocellulosic
biomass was more prone to catalytic hydrolysis. Cao et al. used DBU as
an organic base to catalyze transesterification of soybean oil and microbial lipid to produce biodiesel [5], in which byproduct glycerol was
extracted from biodiesel by bubbling CO2 into the mixture of products.
It is energy efficient and low cost compared with the washing and
distillation operations used in commercial biodiesel production [6].
DBU, when used as a solvent and organic catalyst in biodiesel
production, can be removed from the main product by phase separation
instead of distillation. For industrial application, the physiochemical
properties of the ionic form of DBU/CH3OH/CO2 in the biodiesel and
glycerol phases are necessary fundamental data. Synthesizing and characterization of pure DBU/CH3OH/CO2 are the primary focus of this work.
Reports on the synthesis of new switchable solvents are rapidly
increasing [7]. The best known switchable solvents, the DBU-alkyl
carbonate salts, are usually synthesized by 1:1 DBU-alcohol mixture,
in which the polarity and melting point of the ionic form can be finely
tuned by the chain length of the alcohol and using shorter alcohols to
http://dx.doi.org/10.1016/j.cjche.2015.08.005
1004-9541/© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
Y. Xie et al. / Chinese Journal of Chemical Engineering 23 (2015) 1728–1732
get a greater difference between the polarities of the ionic and neutral
forms of the solvent [8]. The DBU-alkyl carbonate salts also can formed
with other acid gases such as COS, CS2, or SO2 [9].
For the synthesized ionic compound based on DBU, CH3OH, and CO2,
Cao et al. found that the product molar ratio of DBU:CH3OH:CO2 is
about 1:1:0.74 rather than the theoretical value 1:1:1. Their synthetic
method was to bubble CO2 through DBU with equivalent methanol [5].
A solid product formed in only 8 min and the absorption efficiency of
0.74 (CO2 mol)·(DBU mol)−1 was achieved. The absorbing efficiency
did not increase even prolonging the absorbing time. The reaction did
not go to completion mainly because the solid was generated so fast
that some clathrate compounds were produced and/or the methanol
was volatilized when CO2 was bubbled through the reactant. Jessop
also reported the methyl carbonate salt of DBU as a stable white solid
that can be isolated by bubbling CO2 through a THF solution containing
equal amounts of methanol and DBU [10], but the compound synthesized by this method was contaminated by the solvent and needed
reprocessing which is also very difficult. So it inevitably reduces the
yield and increases the cost. And the residual THF may decrease the accuracy in the research of physiochemical properties of DBU/CH3OH/CO2.
Our work here reports the preparation and characterization of reversible
nonpolar-to-polar ionic compounds based on DBU, CH3OH, and CO2. Due
to the new synthetic method, using surplus CH3OH and a CO2 protective
atmosphere, the product ratio of DBU:CH3OH:CO2 matched the theoretical ratio of 1:1:1. The pure stoichiometric ionic compound can be used
as a switchable chemical in different systems to study its properties
and influences on extraction, dissolution, reaction, and separation, etc.
1729
3. Results and Discussion
DBU/CH3OH/CO2, a white solid product, was synthesized by a new
method (Experimental Section). Three parallel experiments were conducted and the yields reached to the theoretical yield, 100%. As shown
in Table 1, the molar ratio of DBU, CH3OH, and CO2 is reached to 1:1:1,
unlike 1:1:0.74 in the literature [5]. Compared with the method in the literature [10], the pure ionic compound was obtained without further purification and the yield was significantly higher. The result indicates that
reactant DBU completely converted to ionic compound following Fig. 1.
The molecular structure was also confirmed by NMR and FTIR results.
Table 1
The yield of solid product DBU/CH3OH/CO2
DBU/g
CH3OH/g
DBU/CH3OH/CO2/g
The yield of
DBU/CH3OH/CO2/%
The molar ratio of
DBU, CH3OH, and
CO2
10.0154
10.0273
10.0805
6.4369
6.5940
6.3629
15.0456
15.0605
15.1435
100
100
100
1:1:1
1:1:1
1:1:1
2. Experiment Section
2.1. Materials
2.2. Preparation and characterization of ionic compound (DBU/CH3OH/CO2)
The DBU/CH3OH/CO2 was prepared via bubbling CO2 through a 1:3
molar ratio solution of dried DBU and CH3OH. A tube was inserted and
CO2 bubbled through the DBU/CH3OH liquid at a constant rate for 2 h
to make sure that the reaction was completed. In the bubbling operation, the solution was stirred with a magnetic rotor and the temperature
was kept at 30 °C. The unreacted methanol after the reaction was removed by vacuum distillation under CO2 protective atmosphere. The
temperature and absolute pressure of vacuum distillation were 39 °C
and 0.08 MPa, respectively. A solid compound was obtained after removing the surplus methanol. As the mole number of DBU was
known, the mass of the solid compound is in accordance with the
mole ratio of 1:1:1 of DBU, CH3OH, and CO2, so get the 1:1:1 theoretical
ratio compound.
Synthesized ionic compound (DBU/CH3OH/CO2) was characterized
by means of nuclear magnetic resonance spectroscopy (NMR, Bruker
AV 400) to confirm the presence of carbonate salts. 13C NMR spectra
of samples in DMSO-d6 were internally referenced to DMSO at
39.52 ppm and 1H NMR spectra of samples in DMSO-d6 were internally
referenced to DMSO at 2.50 ppm.
The compound was also characterized by FT-IR spectroscopy (Perkin
Elmer L1600300) with thin KBr disk prepared under high pressure, and
by XRD (X'Pert Pro MPD).
Thermogravimetric analysis (TGA, EXSTAR 6000) was performed to
determine the decomposition onset of the salt in an air atmosphere and
the scan rate was 10 °C per minute.
Conductivity measurements were performed at 30 °C to prove the
ionic change and the proposed reaction.
Although an ionic solid product with the stoichiometric ratio of 1:1:1
was obtained under the above experiment condition, it can be well
dissolved in CH3OH.
3.1. Characterization of DBU/CH3OH/CO2
The structure of the DBU/CH3OH/CO2 was confirmed by NMR and
FT-IR and the decomposition temperature of DBU/CH3OH/CO2 was
determined by TG.
3.2. TG curve of DBU/CH3OH/CO2
The crystal data and structure of the solid state DBU/CH3OH/CO2 had
been reported by crystallography [8]. But the XRD spectrum (shown in
Fig. 2) of the solid state DBU/CH3OH/CO2 was not found in the PDF data
4000
XRD intensity
1,8-Diazabicyclo[5.4.0] undec-7-ene (DBU) (N99%) was distilled
under vacuum over CaH2 and then dried with 4 Å molecular sieves.
CH3OH (N99%) was dried with 4 Å molecular sieves. CO2 gas (N 99%)
was dried by a silica column.
Fig. 1. Reversible chemical absorption of CO2 by CH3OH and DBU.
2000
0
12
15
18
21
24
27
ο
2 θ /( )
Fig. 2. The XRD spectrum of the ionic compound of DBU/CH3OH/CO2.
30
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Y. Xie et al. / Chinese Journal of Chemical Engineering 23 (2015) 1728–1732
base. From the spectrum, one can see the compound has obvious X-ray
diffraction peaks. The eight strong peaks were 16.0547°, 16.4308°,
16.7651°, 18.8714°, 19.2140°, 21.9471°, 22.0780°, and 25.5661° respectively. So it should have a certain melting/freezing point.
The decomposition point was detected with TGA (see Fig. 3). The
results show that the decomposition onset temperature of DBU/
CH3OH/CO2 ionic compound is 53 °C, while its melting point as reported
in literature is close to 60 °C [8]. It means that the ionic compound
decomposed before it melted. This property makes the ionic switch
easy by decomposing DBU/CH3OH/CO2 and releasing CO2 at a mild
temperature.
100
53 °C
TG /%
80
hydroxyl of methanol was not found in 1H NMR (Fig. 4). It may be because the methanol has a proton exchange with DMSO-d6. For the product, a broad peak at 8.787 ppm appeared. It belongs to the N–H proton
of DBU, which shows DBU/CH3OH/CO2 was created. Furthermore, from
the 1H NMR spectra (Fig. 4(a)), the peak areas at each chemical shift are
in the proper ratios of the associated protons within the product, which
suggests that the product is at high purity. From the 13C NMR spectra,
the position of the peaks supports this supposition. The spectrum of
the DBU-CH3OH solution is a combination of the spectra of DBU and
CH3OH. After bubbling CO2 through the solution, however, a new carbon signal was observed at δ = 156.64, which is assigned to the carbonyl carbon of the formed carbonate. Moreover, the shift from 160.04 to
164.29 shown by the signals of the DBU carbons close to the protonation
site (C-7, downfield shift) was expected. Similar NMR spectra of DBU
alkylcarbonate salts (alkyl = ethyl, propyl, butyl, hexyl, octyl, decyl)
had been reported by Jessop [8]. Cao et al. detected their synthesized
product by 13C NMR and found more peaks than the carbon atom numbers in DBU/CH3OH/CO2 ionic liquid [5]. Those unaccounted for peaks
indicate the likely presence of some impurities in their product.
60
3.4. FT-IR spectrum of DBU, DBU/CH3OH, and DBU/CH3OH/CO2 systems
40
20
0
20
40
60
80
100
120
140
160
180
200
Temperature / °C
Fig. 3. The TGA curve of the ionic compound of DBU/CH3OH/CO2.
3.3. Characterization by 1H NMR and 13C NMR spectroscopy
NMR spectra were collected to characterize DBU/CH3OH (1:1)
solution and (DBU/CH3OH/CO2) crystal samples. The 1H NMR and 13C
NMR spectra of samples are shown in Figs. 4 and 5 and the spectral
parameters are listed in Tables 2 and 3, respectively. The atoms of
reactant and product are numbered according to Fig. 6(a) and (b).
NMR results show that both DBU/CH3OH and DBU/CH3OH/CO2
samples without any impurity substances. The hydrogen atom of
In order to make sure the functional group of the product formed as
described in Fig. 1, the FT-IR spectrum of DBU, DBU/CH3OH, and DBU/
CH3OH/CO2 systems are compared in Fig. 7.
The FT-IR spectra are shown for (a) DBU, (b) the mixture of DBUCH3OH (1:1), and the product (c) DBU/CH3OH/CO2 (1:1:1). Compared
to DBU, the spectrum of DBU/CH3OH exhibits an absorption peak at
1046 cm− 1 that belongs to the C–O vibration of CH3OH. But it is not
found in the spectrum of DBU/CH3OH/CO2. That indicates that methanol
was completely associated with the ionic molecule. On the other hand,
two bands at 1106 cm− 1 and 1207 cm−1 are observed owing to the
symmetric and asymmetric C–O–C stretching vibrations, respectively.
DBU and DBU/CH3OH show a peak at 1614 cm− 1, attributed to the
C = N ring stretching vibration. After reacting with CO2 the peak shifts
and becomes weaker at 1588 cm−1. There also appears a strong absorption at 1648 cm−1 and a weak peak at 1325 cm−1 which belong to C =
O and C–O of the (O = C–O−)-R group stretching absorption bands, respectively. Similar absorption peak of DBU, CH3(CH2)5OH, and CO2 was
also reported [8], but due to the overlap of ν(C = N) and ν(C = O) in the
same region, there is a strong band at 1648 cm− 1. The bands at
Fig. 4. (a) The 1H NMR spectra of DBU/CH3OH/CO2 (1:1:1), (b) DBU/CH3OH (1:1) using internal DMSO-d6 as a chemical shift reference.
Y. Xie et al. / Chinese Journal of Chemical Engineering 23 (2015) 1728–1732
1731
Fig. 5. (a) The 13C NMR spectra of a neat sample of DBU/CH3OH/CO2(1:1:1), (b) DBU/CH3OH(1:1) using internal DMSO-d6 as a chemical shift reference.
Table 2
1
H NMR shifts and signal assignments in spectra of DBU/CH3OH (1:1) and DBU/CH3OH/
CO2 (1:1:1). Shifts are referred to an internal standard, DMSO-d6 (δ = 2.50)
DBU:CH3OH (1:1)
DBU:CH3OH:CO2 (1:1:1)
H①
nH
δ
H②
nH
δ
DBU 3, 4, 5
DBU 10
DBU 6
DBU 9
DBU 2, 11
CH3OH
6
2
2
2
4
3
1.472–1.574
1.663
2.244
3.062
3.062–3.166
3.049
DBU 3, 4, 5
DBU 10
DBU 6
DBU 9, CH3OH 13
DBU 11
DBU 2
DBU 8
6
2
2
5
2
2
1
1.580–1.640
1.840
2.650
3.166–3.246
3.406
3.460
8.787
①
②
Atom numbering shown in Fig. 6(a).
Atom numbering shown in Fig. 6(b).
2924 cm−1 and 2852 cm−1 are from the C–H stretching vibration in the
ring and from the alcohol and are therefore also seen even after
bubbling CO2. The N–H stretching of DBU/CH3OH/CO2 at 3117 cm− 1
and 3240 cm− 1 confirm protonation of the DBU, consistent with the
1
H NMR and 13C NMR spectroscopy data. However, there is a broad
band of DBU from 3375 cm−1 to 3225 cm−1, which was caused by the
trace water in the thin KBr disk prepared under high pressure in the
IR testing process. Similar bands were observed in both spectra of the
Fig. 6. (a) Numbering of the carbon atoms in DBU and CH3OH. (b) Numbering of carbon
atoms in the product of DBU, CH3OH, and CO2.
Table 3
13
C NMR shifts and signal assignments in spectra of DBU/CH3OH (1:1) and DBU/CH3OH/
CO2 (1:1:1). Shifts are referred to an internal standard, DMSO-d6 (δ = 39.50)
DBU:CH3OH:CO2 (1:1:1)
δ
C②
δ
DBU 10
DBU 4
DBU 5
DBU 3
DBU 6
DBU 9
DBU 11
DBU 2
DBU 7
CH3OH
22.21
25.72
28.04
29.05
36.10
43.36
47.47
51.78
160.04
48.32
DBU 10
DBU 4
DBU 5
DBU 3
DBU 6
DBU 9
DBU 11
DBU 2
DBU 7
CH3-O− 13
–O–COO− 12
19.59
23.97
26.44
28.44
31.87
47.70
51.18
52.84
164.29
48.50
156.64
①
②
Atom numbering shown in Fig. 6(a).
Atom numbering shown in Fig. 6(b).
(c)
C=O
Transmittance
DBU:CH3OH (1:1)
C①
C=N
N-H
C=N
(b)
C-O
(a)
C=N
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber /cm-1
Fig. 7. FT-IR spectra of (a) DBU, (b) DBU/CH3OH (1:1), and (c) the product of DBU/CH3OH/
CO2.
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Y. Xie et al. / Chinese Journal of Chemical Engineering 23 (2015) 1728–1732
DBU/CH3OH and the DBU/CH3OH/CO2, but the band of DBU/CH3OH was
stronger and wider as expected.
3.5. The ionic change of DBU/CH3OH/CO2
Conductivity measurement was adopted to measure the ionic
change for DBU/CH3OH/CO2, and the data are shown in Table 4.
Table 4
The conductivity of different pure substances and mixtures (30 °C)
Sample
Conductivity/μs · cm−1
DBU
CH3OH
DBU/CH3OH (1:3)
DBU/CH3OH(1:3)/CO2①
0
5.823
330.0
5163
①
The molar ratio of DBU and CH3OH was 1:3 react with CO2 for 2 h.
Ionic change which may greatly change its polarity is the key property for the switchable polarity solvent being used in extraction and
separation applications. The conductivities of DBU (0 μs · cm−1) and
CH3OH (5.823 μs · cm−1) at 30 °C are very low, and their 1:3 mixture
is also low. However, after bubbling CO2, the ionic liquid of DBU/
CH3OH/CO2 in CH3OH has a high conductivity of 5163 μs · cm−1. The
literature also reported the great conductivity and polarity change of a
neat, equimolar DBU, and 1-hexanol mixture from neutral to ionic,
and from nonpolar to polar at room temperature while CO2 was bubbled
through the liquid [8]. This result indicates the change from nonionic
(low conductivity) to ionic (high conductivity), and CO2 can be used
as a trigger for switching the DBU from molecular to ionic form. N2
can switch the ionic form back to the molecular compound.
3.6. The proposed reaction of DBU, CH3OH, and CO2
Since the conductivity of the equal volume mixture of DBU and
CH3OH is 330.0 μs · cm−1, which is much higher than the conductivity
of either DBU (0 μs · cm−1) or CH3OH (5.823 μs · cm−1), a 2-step chemical reaction sequence as shown in Fig. 8 is proposed.
According to Fig. 8, DBU and CH3 OH are in equilibrium with
DBUH+ and CH3O−, which is consistent with the literature of Medina
et al. [11] and Li et al. [12]. Medina et al. described DBU proton extraction from H2O and Li claimed DBU obtained a proton from sulfonyl
semicarbazide. After bubbling CO2 through the mixture of DBU and
CH3OH(1:3), the conductivity was 5163 μs · cm − 1 , indicating a
much higher ionic content than the DBU/CH3OH mixture. Therefore,
the second step of Fig. 8 shows the formation of the final ionic
product. A similar reaction was proposed by Mikkola et al. [2]
between DBU, glycerol, and CO 2 and Liu et al. [13], between DBU,
fluoroalcohols, and CO2.
Fig. 8. Proposed reaction for the formation of [DBUH]+[CH3OOCO]−.
4. Conclusions
1:1:1 of DBU/CH3OH/CO2 was prepared in excess CH3OH and the
solid ionic compound was obtained by removing the excess methanol
in CO2 atmosphere. NMR spectra show that DBU, CH3OH, and CO2
reacted in stoichiometric ratio; FT-IR spectra indicated that N–H, C–O,
and C = O bonds formed in the DBU/CH3OH/CO2 compound; TGA
curve detected its decomposition onset temperature being 53 °C.
Conductivity measurements confirmed the ionic switch process of the
switchable compound during the reaction process.
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