Research Article pubs.acs.org/journal/ascecg
DBU-Based Protic Ionic Liquids for CO2 Capture Xiao Zhu,*,† Mengling Song,† and Yingjie Xu‡ †
School of Chemistry and Chemical Engineering, Qufu Normal University, Jingxuan West Road, Qufu 273165, People’s Republic of China ‡ Department of Chemistry, Shaoxing University, Huancheng West Road, Shaoxing 312000, People’s Republic of China S Supporting Information *
ABSTRACT: The applications of the novel anion-functionalized protic ionic liquid (ILs), prepared from superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with imidazole, in the CO2 absorption have been investigated. It has been detected that this ionic liquid can reversibly capture about 1 mol of CO2 per mole ionic liquid. In addition, the influence of temperature, pressure, water, and substituent of anions has been uncovered. The capture of CO2 was significantly affected by the substituents in imidazole-based anion, suggesting that electric-charge distribution in imidazole ring system can play an important role in determining the reaction of ILs with CO2.
KEYWORDS: DBU, Ionic liquids, CO2 absorption, Substituents
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conversion,10 and so on. Since Brennecke et al.11 focused on the solubility measurements of CO2 in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) at 298.2 K and pressure up to 40 MPa, more attention has been paid to the solvation of CO2 in ILs, just the physical mechanism without chemical reactions.12,13 To improve the absorption capacity of CO2, in 2002, Davis and co-workers14 proposed a new strategy for the chemical absorption of CO2 under ambient pressure by the amino-functionalized ILs. Since then, numerous efforts have been made to study the effects of task-specific ionic liquids (TSILs) on the CO2 capture, and various functionalized ILs were synthesized, which have been used to capture15−26 or convert CO2.27−33 Although many kinds of ILs have already been reported, much work still needs to be done on account of the numerous species of ILs. Also, more observations about the behavior of different ILs in CO2 absorption remain to be uncovered. 1,8Diazabicyclo[5.4.0]undec-7-ene (DBU), as the traditional superbase, has been used in many organic reactions due to its strong alkaline nature.34,35 In recent years, some research about DBU-based ILs has been carried out to study their properties36,37 and applications.38−41 In 2016, Wang et al.25 demonstrated a new method for efficient synthesis of 3(2H)furanones triggered by CO2 using a series of base-functionalized ionic liquids, including DBU imidazole ([DBUH+][Im−]), composed by different cations and anions, which addressed the importance of the basicity of the anion. Consequently, to
INTRODUCTION As we all know, the greenhouse effect, giving rise to great climatic anomaly, is one of the most serious environmental problems from which worldwide humans are suffering. The main reason could be the excessive emission of greenhouse gases, such as carbon dioxide (CO2), arising from the combustion of coal and petrol.1,2 On the other hand, CO2 can be regarded as a cheap, nontoxic, and infinite natural resource,3 which can be converted to other energy sources and many chemical products. Accordingly, turning this “waste gas” into a valuable treasure is undoubtedly meaningful for environment, climate, and economy. To this end, carbon dioxide capture and storage (CCS) must be the fundamental procedure. Although aqueous amines are the most common approach to absorb CO2 gas in industrial field at present,4 there have been several unavoidable disadvantages inherently. For instance, the concurrent loss of the volatile amines and the uptake of water into the gas stream could lead to intensive energy consumption, cost increases, and equipment corrosion problems.5,6 Therefore, it is extremely necessary to seek more ideal sorbents with favorable performance, such as minimal regeneration energy, long-term stability, and high selectivity for CO2. After decades of developments, ionic liquids (ILs) have become a characteristic and abundant research topic, which has attracted more attention due to their outstanding properties such as negligible vapor pressure, high thermal stability, wide liquid range, and properties, adjustable according to different combinations of cations and anions. Also, numerous investigations have reported the applications of ILs in many fields, such as catalysts,7organic synthesis,8 electrochemistry,9 biomass © 2017 American Chemical Society
Received: June 8, 2017 Revised: July 23, 2017 Published: August 2, 2017 8192
DOI: 10.1021/acssuschemeng.7b01839 ACS Sustainable Chem. Eng. 2017, 5, 8192−8198
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ACS Sustainable Chemistry & Engineering
Figure 1. Structure of [DBUH+][RIm−] ionic liquids.
further enrich the understanding about the behavior of DBUbased ILs on CO2 capture, in this Article, a series of DBU-based ILs prepared from DBU and different imidazole-based anions were synthesized. Because much research42 has concluded that anions can play a major role in determining the properties of ILs, 10 kinds of substituted imidazole-based anions were selected to prepare different ILs (see Figure 1). The effects of these ILs on the CO2 absorption have been investigated and compared.
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Scheme 1. Experimental Diagram for CO2 Absorption and Desorptiona
EXPERIMENTAL SECTION
DBU, imidazole (Im), 2-methylimidazole, 2-ethylimidazole, 2propylimidazole, 2-isopropylimidazole, 2-ethyl-4-methylimidazole, 2nitroimidazole, 2-phenylimidazole, 4-methylimidazole, and 4-nitroimidazole were obtained in the highest purity grade possible, and were used as received unless otherwise stated. The ILs were synthesized via neutralization of an appropriate superbase DBU with different substituted imidazole, which has very weak acidity of N−H, that have been used to prepare other protic ILs in our previous work.43−46 After synthetic reaction, all of the IL samples were dried at 60 °C for 24 h before use in the vacuum drying oven (DZF-6020, Shanghai Jing Hong Laboratory Instrument Co., Ltd.), in which the vacuum degree is lower than 133 Pa. The rotary vane vacuum pump (VOP-100, Shanghai Yiheng Instruments Co., Ltd.) is used to supply the limit pressure of 6.7 × 10−2 Pa. 1 H NMR spectra and 13C NMR spectra were recorded on a Bruker spectrometer (500 MHz) in CDCl3. FTIR spectra were recorded on a Nicolet 470 FT-IR spectrometer. Ionic conductivity was determined by a DDS-307 conductivity instrument. Glass transition and decomposition temperatures were measured with a DSC-60 m from SHIMADZU and TGA 2100 series of TA Instrument (sensitivity of 0.2 μg) with a heating rate of 10 °C/min, respectively. The physical and chemical properties (such as decomposition temperature and electronic conductivity) and 1H NMR and 13C NMR data were determined separately and concluded in the Supporting Information. Absorption of CO2. As shown in Scheme 1, a typical process of CO2 absorption was as follows: CO2 (1) of atmospheric pressure was bubbled through about 2.0 g of ionic liquid in a closed glass container (8) with an inner diameter of 15 mm, and the flow rate was controlled at about 2 L/min using rotary flow meter (5). The glass container under magnetic stirring (7) was partly immersed in a circulation water bath (9) at desirable temperature. The standard uncertainty of temperature u(T) = ±0.1 °C. At regular intervals, the glass container was disconnected from CO2 supply and located in the electronic balance with an accuracy of ±0.0001 g to determine the weight, which could remain unchanged when absorption of CO2 achieved equilibrium. The standard uncertainty of molar ratio of CO2 and ionic liquid u(mole ratio of CO2 to IL) = ±0.03. To avoid the mass loss of ionic liquid and to ensure accuracy during weight measurements, the glass container should be wiped dry outside and then weighed together with its two gas tubes (11 and 12). Desorption of CO2. After an absorption process, the ionic liquid has been saturated with CO2. CO2 gas then was turned off before N2 (2) was bubbled through the CO2−IL system (8) at 80 °C with about
a
(1) CO2 gas cylinder; (2) N2 gas cylinder; (3 and 4) valves; (5 and 6) rotary flow meter; (7) magnetic stirrer; (8) glass container with ionic liquid; (9) water bath; and (10) off-gas aborption by NaOH.
0.06 L/min flow rate controlled by a rotary flow meter (6). At regular intervals, the glass container was disconnected from N2 supply and located in the electronic balance with an accuracy of ±0.0001 g to determine the weight, which could remain unchanged when all of the CO2 decomposed from the CO2−IL system. To avoid the mass loss of ionic liquid and to ensure accuracy during weight measurements, the glass container should be wiped dry outside and then weighed together with its two gas tubes (11 and 12). One absorption−desorption cycle can be denoted that CO2 was first absorbed into ionic liquids until they reached absorption equilibrium, which was followed by desorption of CO2 from the CO2−IL system in the presence of N2. Effect of Pressure on the Absorption. The effect of pressure on absorption of [DBUH+][Im−] was carried out at 40 °C. CO2 and N2 gases were mixed through a T-shape glass pipe and then bubbled through [DBUH+][Im−] in a glass container. To provide different pressure of CO2, the flow rates of CO2 and N2 were controlled separately by two rotary flow meters. For example, the mixture of CO2 with 0.006 L/min and N2 with 0.054 L/min can supply 0.1 bar of CO2. The standard uncertainty of CO2 pressure u(pCO2) = ±0.02 bar. At regular intervals, the glass container was disconnected from gas supply and located in the electronic balance with an accuracy of ±0.0001 g to determine the weight, which could remain unchanged when absorption of CO2 achieved equilibrium. Effect of Water on the Absorption. The effect of water on CO2 absorption of [DBUH+][Im−] was carried out at ambient pressure and 40 °C. First, N2 was bubbled through water in a glass container with about 0.06 L/min flow rate, which can drive water vapor at this temperature into the [DBUH+][Im−] in another glass container. The amount of water absorbed by [DBUH+][Im−] was determined at regular intervals by an electronic balance with an accuracy of ±0.0001 g. Next, CO2 was bubbled through [DBUH+][Im−] saturated with water vapor with an about 0.06 L/min flow rate. The mass increment was also determined at regular intervals so as to obtain the CO2 absorption of [DBUH+][Im−] in the presence of water. 8193
DOI: 10.1021/acssuschemeng.7b01839 ACS Sustainable Chem. Eng. 2017, 5, 8192−8198
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ACS Sustainable Chemistry & Engineering Effect of N2 Volume Flow Rate on the Desorption. The absorption of CO2 was first completed in [DBUH+][Im−] at 40 °C and a constant volume flow rate of CO2 (0.04 L/min). Subsequently, CO2 was desorbed from the [DBUH+][Im−]−CO2 system at 80 °C and different N2 volume flow rates (0.02 and 0.04 L/min). The change of mass was also determined at regular intervals to calculate the mole ratio of CO2 to [DBUH+][Im−] in the absorption and desorption process.
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RESULTS AND DISCUSSION The physical properties, involving decomposition temperature and electronic conductivity, have been measured and shown in Table S1. Although most of these ILs were liquid at room temperature with low melting point and moderate ion conductivity, [DBUH+][4-NitrIm−] and [DBUH+][2-NitrIm−] were obtained as waxy solids (see Figure S33), which can melt near 50 and 100 °C, respectively. Because [DBUH+][Im−] was regarded as a template substance, the behavior of [DBUH+][Im−] to capture CO2 has been first examined. As shown in Figure 2, the absorption
Figure 3. CO2 capture in [DBUH+][Im−] at 40 °C and at different volume flow rates (red ●, 1 L/min; ■, 2 L/min; blue ▲, 3 L/min).
Figure 4. CO2 capture in [DBUH+][Im−] at different pressures.
the absorption process was also slightly accelerated, which has been detected in other ILs.47 For example, the CO2 absorption capacity could reach about 0.8 mol CO2/mol IL at 0.9 bar as compared to 0.4 mol CO2 /mol IL at 0.1 bar. This phenomenon may be attributed to a significant decrease in the physical absorption of [DBUH+][Im−] at low partial pressure. Meanwhile, the effect of water on CO2 absorption of [DBUH+][Im−] was also investigated and shown in Figure 5. Obviously, water can remarkably affect the rate of CO2 absorption, especially at the beginning of the absorption process. Moreover, the CO2 absorption capacity of [DBUH+][Im−] decreased greatly in the presence of water.
Figure 2. CO2 capture in [DBUH+][Im−] at 2 L/min and at different temperatures (■, 25 °C; red ●, 30 °C; blue ▲, 35 °C; green▼, 40 °C; pink left-facing ◀, 45 °C).
ratio was very low in the beginning, but the capacity could reach close to 1 mol of CO2 per mole IL after long time reaction, which could indicate that [DBUH+][Im−] ionic liquid could adsorb CO2 efficiently with high capacity and rate. Furthermore, temperature could play a very important role in the capture process. From 25 to 40 °C, absorption capacity and rate increased with temperature increasing. Instead, absorption capacity behaved lowest at 45 °C, which indicated that desorption could be very quick and dominate at this temperature. It can be found clearly that the best behavior of CO2 absorption was at 40 °C, which probably resulted from the synergistic effects of action of mass transfer (diffusion rate), reaction dynamics, and reaction thermodynamics. Therefore, the absorption at different volume flow rates of CO2 from 1 to 3 L/min was also performed at 40 °C as shown in Figure 3. The CO2 absorption capacity increased with the volume flow rate of gas, and the highest CO2 absorption rate was found at 2 L/min, probably due to the coupling effects of reaction and transport. Because CO2 frequently coexists with other gases in the industrial process, it was necessary to study the influence of pressure on the CO2 absorption. Accordingly, N2 has been mixed with CO2 to supply different partial pressures of CO2. As illustrated in Figure 4, with partial pressure of CO2 increasing, the CO2 absorption capacity of [DBUH+][Im−] increased and
Figure 5. Effect of water on CO2 absorption of [DBUH+][Im−] at 40 °C. 8194
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Figure 6. Scanning TGA results for [DBUH+][Im−] and [DBUH+][ImCO2−] from room temperature with a 10 °C/min temperature ramping rate to 750 °C.
The stability of the CO2-captured [DBUH+][Im−] was further studied by TGA scanning shown in Figure 6. [DBUH+][Im−] and [DBUH+][Im CO2−] could lose the weight of 3.5% and 14.3%, respectively, when the temperature reached 84 °C, indicating that the release of CO2 was almost complete (based on the absorption experiment data, maximal mass ratio of CO2 and IL was about 18%). To find the influence of the substituents of imidazole-based anions in CO2 capture, all of these substitutive [DBUH+][Im−] ILs, except [DBUH+][4-NitrIm−] and [DBUH+][2-NitrIm−], which were solid, were employed to capture CO2 at 40 °C, and the absorption curves have been illustrated in Figure 7. It can
reaction with CO2. As shown in Figure 7, the phenyl ring in [DBUH+][2-PhIm−] could behave with a strong electronwithdrawing effect, which could lead to its lowest CO2 capacities. Second, steric hindrance also can greatly impact on the processes of CO2 capture. Obviously, 2-substituted ILs can give rise to remarkable steric hindrance for interactions of CO2 with N1, which could ultimately lead to their lower CO2 capacities than 4-substituted (b). Moreover, due to the synergistic effect of 2-substituted and 4-substituted, multisubstituted ILs could obtain poorer capture results than any other corresponding monosubstituted ILs. It is also interesting to uncover the absorption mechanism of [DBUH+][Im−], which have been investigated through IR and NMR spectra. As shown in Figure 8, the FT-IR spectra of the
Figure 8. Infrared spectra of [DBUH+][Im−] before and after the absorption of CO2: pure [DBUH+][Im−] (in black) and [DBUH+][Im CO2−] (in red).
Figure 7. CO2 capture in different [DBUH+][RIm−] with a constant volume flow rate of CO2 (2 L/min) at 40 °C.
[DBUH+][Im CO2−] contained a new peak near 1700 cm−1 as compared to [DBUH+][Im−], which could be attributed to a carbamate (CO) stretch. Furthermore, as shown in Figure 9, the 13C NMR spectrum of the CO2-absorbed [DBUH+][Im−] sample shows a new resonance signal at 163.5 ppm as compared to the CO2-free [DBUH+][Im−], which was in agreement with observation of Wang et al.48 and can be attributed to carbamate carbonyl carbon. As a typical superbase, proton affinities of DBU were so high that its protonated conjugate acid (DBUH+) cannot be deprotonated by hydroxide ion. Therefore, proton acceptors could provide a thermodynamic driving force for CO2 capture, which is based on the reaction of anion and CO2. Accordingly, based on the previous reports47,49,50 and the observed products, the CO2 absorption mechanism by [DBUH+][Im−] was presented in Scheme 2, which indicated the formation of carbamate. In addition, the cyclic absorptions of CO2 have been carried out as illustrated in Figure 10, which exhibited the effect of time on the release of CO2 under heating for the [DBUH+][Im CO2−] system. Obviously, the release of CO2 proceeded rapidly
be detected that (1) unsubstituted imidazole-superbase ionic liquid [DBUH+][Im−] showed very high CO2 absorption quantity, while the other [DBUH+][RIm−] was lower; (2) in the case of the same substituent, such as [DBUH+][4-MeIm−] and [DBUH+][2-MeIm−], the capacity of 2-substituted ILs was lower than 4-substituted in anion; (3) as for the same position, for instance, position 2, different substituent style could induce different influence and the capacity of ILs with alkyl was higher than with phenyl; and (4) ILs with multisubstituted anion could achieve lower absorption capacity as compared to the corresponding monosubstituted. The above observations could be explained preliminarily as follows. First, negative-ion fragmentations of deprotonated imidazole have a Π 5 6 conjugated system, and the largest electric-charge density must be in the N1, which has very strong reaction activity. When C2−H or/and C3−H in anion was/were substituted by R (R is the electron-donating group or electron-withdrawing group), the electric-charge density of N1 decreased because electric charge has been scattered, which could weaken the 8195
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Figure 9. 13C NMR spectra of [DBUH+][Im−] before and after CO2 absorption.
Scheme 2. CO2 Absorption Mechanism by [DBUH+][Im−]
Finally, the effects of N2 volume flow rate on desorption of CO2 have also been revealed and shown in Figure 11. As
Figure 10. Absorption and desorption of CO2 by [DBUH+][Im−] ionic liquid: 1a, the first absorption at 40 °C; 1b, the first release at 80 °C; 2a, the second absorption at 40 °C; 2b, the second release at 80 °C; 3a, the third absorption at 40 °C; and 3b, the third release at 80 °C.
Figure 11. CO2 desorption in [DBUH+][Im−] at different volume flow rates of N2.
at a temperature of 80 °C, and was essentially completed within 10 min. It also can be revealed in Figure 10 that CO2 absorption into and release from the [DBUH+][Im−] can be repeatedly recycled with only a slight loss of absorption capability, indicating that such IL can behave satisfying reutilization and effective CO2 absorption−desorption function.
compared to the desorption process at 0.04 L/min of N2, the speed of CO2 desorption at 0.02 L/min of N2 was reduced slightly, indicating that the pressure of N2 could also be an important factor in CO2 desorption. 8196
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CONCLUSIONS In summary, we have developed a new class of ILs containing DBU as cation and substituted imidazole as anions. [DBUH+][Im−] can behave remarkably efficient for the CO2 capture and excellent reutilization function. Moreover, it has been found that the capture of CO2 was significantly affected by the substituent in imidazole-based anion under ambient conditions. Interestingly, not only electron-withdrawing groups but also the electron-donating groups could induce the lower capacity of CO2 capture, which was different from our common views. In addition, multisubstituted anions could get the lowest CO2 capture than any other corresponding monosubstituted ones. Such observations could enrich our knowledge about the applications of ILs in CO2 absorption. Furthermore, these novel anion-functionalized protic ionic liquids can provide a potential alternative for CO2 capture in industry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01839. Physical properties (involving decomposition temperature and electronic conductivity), 1H NMR and 13C NMR spectra, TGA, and DSC thermogram profile of ILs involved in this Article (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiao Zhu: 0000-0002-2166-3984 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 21206085, 21403059, and 21406140) and the Scientific Research Program for Undergraduates of Qufu Normal University (no. 2016042).
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DOI: 10.1021/acssuschemeng.7b01839 ACS Sustainable Chem. Eng. 2017, 5, 8192−8198
