Article pubs.acs.org/IECR
Reversible Ionic Liquid Stabilized Carbamic Acids: A Pathway Toward Enhanced CO2 Capture Jackson R. Switzer,† Amy L. Ethier,† Kyle M. Flack,‡ Elizabeth J. Biddinger,† Leslie Gelbaum,‡ Pamela Pollet,‡ Charles A. Eckert,†,‡ and Charles L. Liotta*,†,‡ †
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States ABSTRACT: Capturing CO2 from flue gas streams under near ambient conditionse.g. coal-fired power plantshas traditionally involved the use of aqueous alkanol amine solutions. Aqueous solvent-based processes are energy intensive, as solvent regeneration can be costly. With growing concern over climate change, alternative CO2 capture technologies that are energy and cost efficient are required. We have established that nonaqueous silylamines can be used to efficiently and reversibly capture and release CO2 via the formation of reversible ionic liquids. We now report their unique, enhanced CO2 uptake at room temperature under 1 atm of CO2, as silylamines exhibit CO2 capture capacities greater than that expected from the conventional stoichiometry of a 2:1 amine to CO2 mole ratio. Experimental evidence is presented supporting the formation of a carbamic acid species in equilibrium with an ionic liquid network of ammonium−carbamate ion pairs to give a 3:2 amine to CO2 mole ratio. This is the f irst report of the stabilization of carbamic acid by reversible ionic liquids. Stabilization of carbamic acid leads to a significant increase in CO2 capacity (30% on average) over conventional amine solutions for CO2 capture.
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INTRODUCTION Traditional ionic liquids have been used in CO2 capture utilizing physical absorption to capture CO2 via weak van der Waal’s forces of interaction. However, at low (0.1 bar) to atmospheric pressures of CO2, they typically exhibit limited CO2 capacities.1 Alternatively, amine-functionalized ionic liquids (task-specific ionic liquids) have been investigated for their ability to capture CO2 through chemical reaction as well as physical absorption. The addition of a CO2-reactive amine group dramatically improves CO 2 capacity at low to atmospheric pressures of CO2.2−4 It has been reported that reversible ionic liquidsa class of switchable solvents containing amine functionalitiesreact with CO2 under solvent-free (nonaqueous) conditions to form ammonium−carbamate ion pairs.6,7 Reversible ionic liquids are designed so that upon reaction with CO2, the resulting network of ionic species remains a liquid at room temperature. Key to their applicability as solvent systems for CO2 capture is that upon heating, the ionic liquid can be easily and quantitatively reversed back to the starting amine for reuse.5 In contrast to amine-functionalized ionic liquids, reversible ionic liquids are ionic only after CO2 capture, eliminating some of the processing concerns associated with the elevated viscosity of ionic liquids.1 In fact, CO2 is reacted with a nonionic amine, initiating a solvent switch to form the reversible ionic liquid species (i.e., ammonium−carbamate ion pairs). Once the reaction is complete, the reversible ionic liquid is then capable of physically absorbing additional CO2.5 Several examples of reversible ionic liquids have been reported.5−11 Work in our laboratories has focused primarily on silylated amines, as incorporation of a silicon atom has proven beneficial in maintaining liquid properties in the ionic © 2013 American Chemical Society
state. Three of the silylamines we recently developed for CO2 capture are 3-(aminopropyl) triethylsilane, 3-(aminopropyl) tripropylsilane, and 3-(aminopropyl) trihexylsilane (Chart 1). Chart 1. Three Silylamines Developed for CO2 Capture 3(Aminopropyl) Triethylsilane, 3-(Aminopropyl) Tripropylsilane, and 3-(Aminopropyl) Trihexylsilane
Each contains a primary amine tethered to a trialkylsilane by a propyl linker. Several properties relevant to their application for CO2 capture, including reversal temperature, enthalpy of regeneration, and reversible ionic liquid viscosity, were recently reported.5 It is commonly presumed in the literature that the equilibrium reaction of a primary amine with CO2 in traditional organic solvents gives a 2:1 amine to CO2 mole ratio, forming only ammonium−carbamate ion pairs.12−16 Some experimental evidence, however, has been reported which supports the formation of carbamic acids in dipolar aprotic solvents such as DMSO and in reversible ionic liquids.17−20 Herein, we report gravimetric and spectroscopic (quantitative 13C NMR and FTIR) results that provide experimental evidence of a more Received: Revised: Accepted: Published: 13159
June 14, 2013 August 2, 2013 August 13, 2013 August 13, 2013 dx.doi.org/10.1021/ie4018836 | Ind. Eng. Chem. Res. 2013, 52, 13159−13163
Industrial & Engineering Chemistry Research
Article
Figure 1. ATR-FTIR spectra of (a) 3-(aminopropyl) tripropylsilane and (b) 3-(aminopropyl) tripropylsilane reversible ionic liquid after reaction with 1 bar of CO2 at 25 °C.
system). Weight measurements were made before and after CO2 introduction, to calculate the gravimetric CO2 uptake. The average CO 2 capacity measured gravimetrically in the quantitative NMR experiments was 0.62 ± 0.01 moles of CO2 per mole of amine. It is assumed that the slightly lower CO2 uptake observed in the NMR experiments compared to the gravimetric capacities reported is a result of mass transfer limitations in the NMR tube. To conduct quantitative 13C NMR, three important experimental changes are needed over standard 13C NMR. They include (1) a full 90° pulse, (2) extended delay time, and (3) proton decoupling only during the acquisition period. The first is to ensure complete excitation of the nucleus. The second is to allow full relaxation after the pulse is applied. To determine the appropriate delay time, an inversion recovery T1 experiment was performed. For 3-(aminopropyl) tripropylsilane in its reversible ionic liquid form, the longest T1 time comes from the carbonyl carbon (162.73 ppm) at 4.027 s. It is generally recommended that the delay time used be 5 times the longest T1 time. As such, for all of the quantitative experiments performed, a delay time of 20 s was used. The last experimental change (3) is done to avoid a false and disproportional signal buildup during proton decoupling as a result of the Nuclear Overhauser Effect. The protocol is named inverse gated decoupling. To increase the signal-to-noise ratio, 2000 scans were performed for each experiment over a period of 12 h. Attenuated Total ReflectanceFourier Transform Infrared (ATR-FTIR). ATR-FTIR spectroscopy was used to measure quantitatively the solubility of CO2 in the reversible ionic liquid. ATR-FTIR allows quantitative differentiation between chemically reacted and the physically absorbed CO2. Chemically reacted CO2 is represented by the N−H stretch of the ammonium cation from 2600 to 3100 cm−1, the asymmetric CO2− stretch of the carbamate anion at 1575 cm−1, and the carbonyl stretch of the carbamic acid at 1700 cm−1. Physically absorbed CO2 is represented by the asymmetric stretch of CO2 at 2330 cm−1. For each of the reversible ionic liquids studies, the CO2 mole fraction as a function of pressure was determined from the ATR-FTIR spectra. Then assuming an ideal vapor, the Henry’s law constants were calculated by using the equation
complex equilibrium in which carbamic acid is formed and stabilized by a liquid ammonium carbamate ionic network.
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MATERIALS AND METHODS Reversible Ionic Liquid Preparation. The silylamines (3(aminopropyl) triethylsilane, 3-(aminopropyl) tripropylsilane, and 3-(aminopropyl) trihexyl silane) were prepared using a one-step hydrosilylation reaction. The corresponding silane was coupled with allylamine using 2 mol % platinum divinyltetramethyldisiloxane (Pt-DVDS) in refluxing toluene to give an 89−96% yield of the corresponding reversible ionic liquid.5 The silylamines discussed in this manuscript are depicted in Chart 1. Gravimetric Uptake Experiments. For the gravimetric uptake experiments, approximately 1 mL of silylamine was added to an argon-purged scintillation vial at 25 °C. CO2 at 1 bar was introduced until weight uptake was determined to be constant. The increase in weight observed from the starting amine to the reversible ionic liquid state formed after exposure to CO2 was assumed to be representative of the CO2 uptake of the solvent system. Experiments were conducted in triplicate. The elimination of water from the system was critical to the above experiments since the presence of water in the silylamine could lead to additional CO2 uptake via formation of bicarbonate. Additionally, any water absorbed during the experiment could lead to a false attribution of the weight uptake solely to CO2 absorbance. Karl Fischer analysis indicated that 3-(aminopropyl) tripropylsilane contained on average 0.12 wt % water after synthesis and distillation. (This amount of water is insignificant compared to the CO2 weight uptake observed.) All of the silylamines were stored under nitrogen in a glovebox until use. During the gravimetric experiments, care was taken to keep the silylamines under an atmosphere of argon until CO2 was introduced. The CO2 employed was SFC grade, certified to contain less than
