Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
pubs.acs.org/JPCB
Encapsulation of Ionic Liquids with an Aprotic Heterocyclic Anion (AHA-IL) for CO2 Capture: Preserving the Favorable Thermodynamics and Enhancing the Kinetics of Absorption Cristian Moya,† Noelia Alonso-Morales,† Juan de Riva,† Oscar Morales-Collazo,‡ Joan F. Brennecke,‡ and Jose Palomar*,† †
Sección de Ingeniería Química (Dpto. Química Física Aplicada), Universidad Autónoma de Madrid, 28049 Madrid, Spain McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712-1589, United States
‡
S Supporting Information *
ABSTRACT: The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P66614][2-CNPyr]), for CO2 capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas−liquid equilibrium isotherms of CO2−AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO2 solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO2 chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO2 absorption rate is masstransfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO2 capture. The newly synthesized AHA-ENIL material was evaluated as a CO2 sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO2 absorption capacity of the AHA-IL but drastically enhance the CO2 absorption rate because of the increased gas− liquid surface contact area achieved by solvent encapsulation.
1. INTRODUCTION The interest in the application of ionic liquids (ILs) to capture CO2 from flue gas has experienced a rapid growth over the last years.1−4 The favorable properties of ILs as solvents (negligible vapor pressure, high thermal and chemical stabilities, etc.) can be adequately tuned by carefully choosing the cation or the anion that forms the IL. Although ILs have very good CO2 selectivity over other compounds,4,5 the physical solubility of CO2 in ILs is still too low at typical working pressures for practical application in flue gas absorption, where CO2 partial pressure is less than 0.15 bar.6 This has encouraged the development of different task-specific ILs (TSILs) that possess the ability to chemically react with CO2,7−9 increasing the amount of CO2 captured at low pressures.10 ILs with aprotic heterocyclic anions (AHA-ILs)11 are a promising family of ILs, in which the CO2 is able to bond with the anion by reversible carboxylation with a N-heterocyclic carbine.12 This reaction presents a 1:1 stoichiometry, and the equilibrium greatly favors the formation of the product, even at CO2 partial pressures lower than 1 bar.13 This high molar absorption at low pressure, in addition to a low enthalpy of reaction (−40 to −55 kJ/mol14) compared to that for © XXXX American Chemical Society
conventional CO2 capture by amines (−84 kJ/mol by ethylenediamine15), can potentially reduce the energy requirements associated with the regeneration step in a CO2 capture process. One of the main advantages of this family of AHA-ILs over other TSILs is that the viscosity of the media remains practically unchanged after reaction with CO2, whereas it greatly increases for other chemical absorbing ILs after their exposure to CO2.16,17 Among AHA-ILs, those based on the 2cyanopyrrolide anion ([2-CNPyr]) are among the best candidates for CO2 capture because of their high capacity and high reaction rates compared to those of conventional amine systems.18 In addition, the presence of water does not imply unfavorable effects on the CO2 absorption capacity of [2CNPyr]-based AHA-ILs,14 whereas in other TSILs, the CO2 chemical absorption extent decreases in the presence of water, as those IL solvents are based on carboxylate groups.19 It should be noted, however, that the presence of water causes an incremental increase in the viscosity of CO2−AHA-IL media, Received: December 9, 2017 Revised: February 14, 2018 Published: February 14, 2018 A
DOI: 10.1021/acs.jpcb.7b12137 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
the absorption phenomena. We note that lower-viscosity ILs with this anion have been developed,20 but we use a more viscous IL with longer alkyl chains on the cation to more clearly demonstrate the impact of encapsulation. First, CO2 absorption isotherms of the neat AHA-IL were measured, as we have done previously over a smaller pressure range,11 using a gravimetric high-pressure sorption analyzer at 298, 313, and 333 K, obtaining estimations of Henry’s law and reaction equilibrium constants to discern the role of the physical absorption in the CO2 capture process. Then, to undertake an in-depth analysis of the chemical reaction, kinetic absorption measurements were carried out using the attenuated total reflection (ATR) technique in conjunction with Fourier transform infrared spectroscopy (FTIR) in a gas−liquid Golden Gate reactor, at 298, 313, and 333 K and pressures from 0.1 to 5 bar. By following the characteristic vibrational signals of reactants and products with this in operando technique, the time-dependent concentration profiles of the different species involved in the chemical reaction were obtained. A chemical absorption model was applied to estimate the reaction kinetic constants and the mass transfer kinetic parameter (diffusion coefficients), obtaining quantitative evidence of the controlling step in the overall absorption process. Finally, with the objective of enhancing the mass transfer rates, ENIL material of AHA-IL was prepared with a 65% w/w content. The morphology, porous structure, chemical composition, and thermal stability of the synthesized AHA-ENIL were characterized. Then, the role of AHA-IL encapsulation in the CO2 capture performance was evaluated with equilibrium and time-dependent sorption gravimetric measurements, comparing the results to those obtained with the neat IL. The regeneration of the exhausted AHA-ENIL sorbent was analyzed by pressure and temperature swing and stripping desorption. Finally, the stability of the prepared AHA-ENIL material in the CO2 capture operation was tested over successive sorption−desorption cycles.
which is related to the hydrogen bonding between water and the reaction product.14 Regarding the selection of the counterion for AHA-ILs, tetraalkylphosphonium cations present good properties for CO2 capture applications because they confer high thermal and chemical stabilities to the absorbent, being possible to select the length of the alky chains for properly tuning the physical properties of the AHA-IL.20,21 In contrast, imidazolium-based cations lead to a competing reaction between the cation and the anion with CO2, which results in the reprotonation of the anion and a reduced stability of the IL.22,23 However, the chemistry of CO2 capture with phosphonium AHA-ILs can be complicated at higher temperatures by the formation of a phosphonium ylide, which can also react with CO2.2323 Despite the high CO2 absorption capacity, the viscosity of AHA-IL absorbents is still high compared to that of conventional solvents (between 100 and 500 cP at 298 K, depending on the particular AHA-IL20,21,24). These viscosity values are likely to produce serious mass transfer limitations in the absorption phenomena. In fact, it has been demonstrated that absorption rates are a limiting factor in CO2 capture systems based on ILs25−28 and, as a consequence, both thermodynamic and kinetic criteria should be considered in the IL selection for a CO2 absorption process.29−31 To enhance the mass transport properties of CO2−AHA-IL absorption media, increasing temperature implies higher diffusivity of CO2 in ILs,32 but at the expense of unfavorable thermodynamic effects (lower CO2 solubility and conversion).11 Gurkan et al.18 were able to perform kinetic constant estimations of the CO2 + AHA-IL reaction using highly diluted solutions of the AHA-IL in tetraglyme. This was necessary to eliminate mass transfer resistances to measure actual reaction rates. In general, dilution with cosolvents of lower viscosity may overcome the mass transfer limitations of the fluid. Also, the use of a supported ionic liquid phase (SILP)33,34 has been proposed as an alternative to increase the mass transfer rates of CO2 absorption in ILs.35−38 SILP systems are usually prepared by spreading the IL solvent over an inorganic porous support, which increases the gas−liquid interfacial area and, consequently, the mass transport rates of CO2 absorption in IL.36 The limitation of SILP materials is normally their reduced capacity, which is related to the low IL content (usually less than 30% by mass).37,38 As an alternative, the concept of encapsulated ionic liquids (ENILs)39−43 has recently emerged. Our group has developed new ENIL materials44 incorporating a large amount of IL (up to 80% w/w), prepared using hollow carbon submicrospheres45 (Ccap) with a diameter of 350−500 nm. This ENIL material, with solid appearance, allows discretizing the IL fluid in the form of submicrodrops, drastically increasing the gas−liquid contact area. An ENIL sorbent was evaluated for NH3 physical capture,46 demonstrating that this material maintains the high solvent capacity of ILs while greatly enhancing the kinetics of the absorption process. In addition, ENILs prepared with an acetate-based IL were successfully used for CO2 chemical capture,47 showing clear improvements in the absorption rates (2 orders of magnitude higher kinetic constants). Use of ENILs allowed easy regeneration of the exhausted sorbent, showing a fully reversible CO2 absorption process for acetate-based ILs, which are severely limited by the mass transfer rate because of the high viscosity of these systems. In this work, trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P66614][2-CNPyr]) is evaluated for CO2 capture, considering both the thermodynamic and kinetic aspects of
2. EXPERIMENTAL SECTION 2.1. Materials. IL Synthesis. Trihexyl(tetradecyl)phosphonium bromide (95% purity) was purchased from Strem Chemicals, 1H-pyrrole-2-carbonitrile (99% purity) and Amberlite IRN78 (Nuclear grade) were purchased from Alfa Aesar, and anhydrous toluene (99.8% purity) was purchased from EDM and used without further purification. Carbon Submicrocapsule (Ccap) Synthesis. Phenol (99%), paraformaldehyde (95−100%), aluminum trichloride (95− 100%), ammonia (38%), absolute ethanol, and hydrofluoric acid (48%) were supplied by Panreac. Tetraethylorthosilicate (98%, TEOS) and octadecyltrimethoxysilane (90%, C18TMS) were supplied by Sigma-Aldrich. Carbon dioxide, CO2, and nitrogen, N2, were supplied by Praxair, Inc., with a minimum purity of 99.999%. 2.2. Synthesis of AHA-IL (Trihexyl(tetradecyl)phosphonium 2-cyanopyrrol-1-ide). In a round bottom flask (500 mL), a solution of trihexyl(tetradecyl)phosphonium bromide ([P66614][Br]) (10 g, 17.74 mmol) in methanol (200 mL) was treated with Amberlite IRN78 (53 g) and mixed until no residual halide precipitation was observed by the AgNO3 test to obtain trihexyl(tetradecyl)phosphonium hydroxide ([P66614][OH]). The mixture was filtered and treated with 1H-pyrrole-2-carbonitrile (1.634 g, 17.74 mmol) and stirred for 2 days. Methanol and other volatiles were removed at 323.15 K under reduced pressure (10 mbar). Complete removal of volatiles was confirmed by 1H NMR. The water byproduct was B
DOI: 10.1021/acs.jpcb.7b12137 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
atmosphere of nitrogen within the range of 283−800 K at a heating rate of 10 K/min. 2.5. Gravimetric CO2 Sorption Experiments. CO2 sorption experiments were performed in a gravimetric highpressure analyzer (ISOSORP GAS LP-flow, Rubotherm). It consists of a magnetic suspension microbalance (MSB) that has a measuring load of 0−10 ± 0.01 mg. The apparatus has a working pressure range of 10−6 to 30 ± 0.01 bar at temperatures from ambient up to 403 ± 0.1 K. The detailed description of the absorption measurement procedure used with the magnetic suspension balance (MSB) can be found elsewhere.29,30 The buoyancy effect was corrected by previous measurements carried out with an inert gas (N2), varying the pressure from 1 to 20 bar at constant temperature, waiting for weight stabilization at each pressure step. These preliminary experiments provided accurate determinations of the loaded mass and the sample volume, by plotting the registered mass as a function of the gas density inside the MSB.29 The AHA-IL or AHA-ENIL sample (350 mg) loaded in the MSB is degassed to remove water and volatile impurities for 12 h (or until the mass remained constant) under vacuum (10−6 bar) at 333 K, estimating low water content in the material prior to CO2 absorption (
