Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Supported Ionic Liquid Membranes with Dual-Site Interaction Mechanism for Efficient Separation of CO2 Xiaomin Zhang, Wenjie Xiong, Zhuoheng Tu, Lingling Peng, Youting Wu,* and Xingbang Hu* Separation Engineering Research Center, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, 163 Xianlin Road, Qixia District, Nanjing, Jiangsu Province 210023, People’s Republic of China
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ABSTRACT: Selective separation of CO2 from flue gas conforms to the criterion of sustainable society. Herein, a series of imidazolium-based phenolate ionic liquids (ILs) that have dual-site interaction centers to isolate CO2 from N2 by supported ionic liquid membranes (SILMs) is designed and prepared. Density, viscosity, and CO2 solubility in these ILs were measured. The effects of the electron-withdrawing or electron-donating ability of the substituents on the anion, the operation temperature and partial pressure on the permeability of CO2, and the ideal selectivity of CO2/N2 were investigated systematically. High permeability (up to 2540 barrers) and selectivity of CO2/N2 (up to 127) are achieved in 1-butyl-3-methylimidazolium phenolate ([bmim][PhO]) containing 15 wt % H2O under humidified condition. A novel facilitated transport mechanism, transfer of CO2 from carbene to phenolated anion, is proposed based on NMR, FT-IR, and theoretical calculation results. The new pathway is believed to offer an alternative opportunity for designing novel CO2 separation materials. KEYWORDS: Ionic liquid, Support ionic liquid membranes, Facilitated transport, Separation, Carbon dioxide
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and proton transfer.8 They also designed various azolate and phenolate ILs with enhanced CO2 absorption capacity.9−12 The combination of ILs with membrane separation technology has generated new approaches for the removal of CO2.13 Among them, supported ionic liquid membranes (SILMs) represent the most primitive and active method.14,15 Normal ILs were usually employed to prepare SILMs in the early literature to separate CO2.15−17 In recent years, in order to enhance the separation performance of SILMs, various functionalized ILs, including amine-functionalized ILs and carboxylate-based ILs, have also been proposed for the facilitated transport of CO2. For example, Matsuyama et al. reported an amine-terminated ILs as CO2-facilitated transport carrier.18 The prepared SILMs showed an elevated CO2/CH4 selectivity (more than 50) at room temperature and 0.1 bar. Subsequently, the same group designed amino acid ILs-based facilitated transport membranes for the separation of CO2 from N2 at high temperatures (>80 °C) under dry condition.19 Our group designed dicarboxylate-functionalized ILs and costefficient amine-functionalized protic ILs for the facilitated transport separation of CO2.20,21 Rieger et al. reported a promising phenolated-functionalized polymeric membrane material to realize the facilitated transport separation of
INTRODUCTION
Energy-efficient separation of carbon dioxide (CO2) from flue gas is a challenging task of environmental significance as well as industrial application. The concentration of conventional CO2 separation has mainly focused on the aqueous alkanolamine solutions-based absorption process.1 Although well established, this method suffers from a variety of intrinsic disadvantages, including high energy requirement for solvent desorption, corrosiveness, and decomposition of amine solution. Membrane separation technology represents one of the most ecofriendly, cost-effective, and processable technologies for gas separation.2 However, most membrane materials have to face the problem of balancing between permeability and selectivity.3,4 As green solvents, ionic liquids (ILs) have been screened as promising candidates for CO2 separation because of many attractive properties including high thermal stability, designable structure, and nonvolatile nature.5 As a pioneering work, Davis et.al first reported the amino-decorated ILs for efficient capture of CO2 with a 2:1 stoichiometry.6 Brennecke et.al designed the anion-functionalized amine acid ILs with equimolar CO2 capture.7 Recently, Wang et.al reported a novel strategy through the variety of basicity and steric hindrance of amino-functionalized ILs with 1:2 stoichiometry, which showed that one amine could combine two CO2 through the formation of an intramolecular hydrogen bond © XXXX American Chemical Society
Received: March 21, 2019 Revised: April 25, 2019
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DOI: 10.1021/acssuschemeng.9b01604 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Chemical structures of ionic liquids
CO2.22 It is well known that good solubility and easy desorption are the most important factors that influence the facilitated transport ability. 1,3-Dialkylimidazolium cations paired with basic anions have been found to be good absorbents for CO2, since they can react with CO2 to form carbene−CO2 adducts.23 However, the interaction of carbene and CO2 is very strong, which makes the release of CO2 quite difficult.24−26 Thus, use of these basic imidazolium ILs with high absorption capacity for facilitated transport of CO2 is full of challenges. Herein, we develop a novel facilitated transport membrane for the separation of CO2 which contains a series of imidazolium cations paired with phenolated anions. An easy desorption process is achieved through the transfer of CO2 from carbene to phenolated anion. Extremely high CO2 permeability and excellent gas pair ideal selectivity are realized. NMR, FT-IR, and theoretical calculations are used to systematically demonstrate the reversible reaction mechanism of IL with CO2.
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Determination of Physical Properties. The densities and viscosities were measured on Anton Paar DMA 5000 type automatic densitometer with a precision of 0.00001 g/cm3 and Brookfield LVDV- II+Pro viscometer with an uncertainty of ±1%, respectively. Preparation of Supported Ionic Liquid Membranes. The impregnation method was used for preparation of SILMs. Briefly, the pristine PES membrane was soaked in ILs (or the ILs solution containing 15 wt % H2O) at room temperature for 12 h to saturate the blank substrate. Then excess ILs (ILs solution) on the surface of the membrane was wiped away by using two blank PES supports before gas permeability measurements. Procedure for Gas Permeability Measurements. The gas permeability measurements were performed as in our previous work.20,21 Briefly, the permeability measurement of a single gas was measured with a stainless steel dual-chamber cell with feed and permeate side volumes of 582 and 242 mL, respectively. Two absolute pressure transducers of 2‰ precision and 6.0 bar scale connected with a computer were calibrated and used for the experiment. In each measurement the feed and permeate chambers were charged with N2 of ambient pressure as the balanced gas. Then, only in the feed chamber, the desired solute gas was loaded, resulting in a partial pressure between the feed side and the permeate side. The single gas permeability of N2 and CO2 was calculated using eqs 1 and 2
EXPERIMENTAL SECTION
Materials. CO2 (purity > 99.99%), N2 (purity > 99.99%), and CH4 (purity > 99.99%) were purchased from Nanjing Tianze Gas Co., Ltd. (China). Shanghai Chengjie Chemical Co., Ltd. provided the 1-butyl-3-methylimidazolium bromide (bmimBr, 99 wt %). The PES porous support was supplied by Beijing Membrane Corp. Silver oxide (Ag2O, 99.7 wt %) was bought from Aladdin Chemical Reagent CO., Ltd. Phenol (PhO, 98 wt %), 2-fluorophenol (2-F-PhO, 98 wt %), and 2-methylphenol (2-CH3-PhO, 98 wt %) were all purchased from Adamas Reagent Co., Ltd. 1-Butyl-2,3-dimethylimidazolium bromide (bmmimBr) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. 3-Fluorophenol, 4-fluorophenol, and 2methoxyphenol were purchased from Aladdin Industrial Corp. Preparation of ILs. The dual-site ILs were prepared by a one-pot method. For example, 10 g of [bmim][Br] was dissolved in an ethanol and H2O mixture (v/v 3:1). Then 1.05 equiv of Ag2O was added to the solution under dark condition, which was stirred for 72 h. Excess phenol was added in the slurry and stirred at room temperature for another 12 h. Subsequently, the AgBr precipitate was filtrated, and the solvent was distilled off at 70 °C under reduced pressure. Diethyl ether was used to wash the product five times. Finally, the product was purified on a Schlenk line at 60 °C and 0.15 mbar for 12 h until no bubbles evolved. For comparison, 1-butyl-2,3-dimethylimidazolium phenolate ([bmmim][PhO]) was also synthesized by the same procedure. The chemical structures of seven ILs are shown in Scheme 1. Characterization of ILs. The chemical structures of these ILs were confirmed through FT-IR and NMR spectra. FT-IR spectra were carried out on a Nicolet iS50 infrared spectrometer. NMR spectra were conducted on a Bruker DPX 400 MHz spectrometer, and d6DMSO and tetramethylsilane (TMS) were added in as solvent and internal standard, respectively. Quantitative 13C NMR were carried out on a 600 MHz spectrometer with CDCl3 in the capillary tube as internal standard solvent to investigate the reaction mechanism.
P=
L V dp2 Δp ART dt
Δp = p1 − p2
(1) (2)
where P is the permeability coefficient (barrer), V is the volume of the downstream chamber (cm3) of the cell, L is the thickness of the PES support (cm), A refers to the effective area (cm2), R is the ideal gas constant (J·mol−1·K−1), and T is the thermodynamic temperature (K). p1 and p2 represent the pressure of the feed side and permeate dp side, respectively. 2 represents the increasing rate of gas pressure in dt the permeate side. The ideal selectivity (S) was calculated from the ratio of the single gas permeability as follows S=
Pi Pj
(3)
For measurements in humidified condition, the equilibrated gas (N2) was saturated with water vapor. All measurements were performed in triplicate, and average values are presented. Determination of CO2 Absorption. The gas solubility measurements are the same as those in our previous work.27 More details can be found in the SI.
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RESULTS AND DISCUSSION Physical Properties. The viscosity and density are two important fundamental properties of ILs. The temperature dependence of the viscosity and density of these ILs was determined and is shown in Figures S1 and S2. It is well known that low viscosity favors mass transfer in gas separation. The viscosities follow the sequence of [bmim][PhO] < [bmim][3F-PhO] < [bmim][4-F-PhO] < [bmim][2-CH3-PhO]
