Hybrid Ionic Liquid Capsules for Rapid CO2 Capture | Industrial

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Hybrid Ionic Liquid Capsules for Rapid CO Capture Qianwen Huang, Qinmo Luo, Yifei Wang, Emily B. Pentzer, and Burcu Gurkan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00314 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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Industrial & Engineering Chemistry Research

Ionic liquids encapsulated with polyurea and graphene oxide demonstrate improved CO2 capacity and absorption rates. 61x39mm (150 x 150 DPI)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Hybrid Ionic Liquid Capsules for Rapid CO2 Capture Qianwen Huang,a Qinmo Luo,b Yifei Wang, b Emily Pentzer,*,b Burcu Gurkan*,a

a

Department of Chemical Engineering Biomolecular Engineering, Case Western

Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States

b

Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue,

Cleveland, Ohio 44106, United States.

KEYWORDS. Encapsulated ionic liquids, emulsion polymerization, carbon dioxide separation, imidazolium

ABSTRACT. The kinetics of CO2 absorption by ionic liquids (ILs) were enhanced by the use of hybrid capsules composed of a core of IL and shell of polyurea and alkylated graphene oxide (GO). These composite structures were synthesized using a Pickering


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emulsion as a template and capsules of two different ILs were prepared. The contribution of the encapsulated IL on the CO2 absorption of the capsules is consistent with agitated neat IL, but with improved mass transfer rate of absorption across different pressures. This novel materials design allows for CO2 to be absorbed significantly faster compared to neat IL and provides insight into improved carbon capture technologies.

INTRODUCTION Carbon capture is a pressing need in modern society since the primary energy source continues to be fossil fuel burning which generates CO2 waste, a greenhouse gas. There are various technologies for CO2 capture such as absorption1, adsorption2, membrane separations3, and electrochemical separations4; each method takes advantage of a high CO2-affinity solvent or a functional material.5,6 Absorption-based technologies that utilize a CO2-absorbing fluid such as aqueous amines7 have been used in the utility sector, specifically in post-combustion flue gas treatment.8 However, CO2 capture with aqueous amines is an energy intensive process9 since it requires elevated temperatures for the desorption of CO2 and the recycle of the absorbing fluid. Ionic liquids (ILs) have been


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investigated as alternative solvents to aqueous amines as they present unique properties that are well suited for CO2 capture, such as high CO2 solubility, negligible volatility, wide liquidus range, and tuneable reaction enthalpy with CO2.10–13 The main challenge to utilizing ILs for CO2 capture is the high viscosity which results in severe mass transfer limitations,14 in addition to the cumbersome handling of liquids compared to solids on the small scale and in closed systems in terms of system complexity and maintenance (e.g., for applications requiring CO2 filtration in spacecraft and submarines). To overcome these challenges, ILs have been loaded onto porous solid supports such as fumed silica,15 carbon,16 and polymeric membranes3,17 which removes the necessity of pumping the absorbing liquid in a conventional absorber-desorber separation unit. Encapsulated ILs have been reported as effective gas uptake materials18–22 and have also found utility beyond gas separations such as electrode materials,23 microreactors,24,25 compartmentalized catalysis,26,27 and structures for extractions.28–32 In gas separations, encapsulation of IL can increase the active surface area of the IL, improving the kinetics of uptake compared to bulk (i.e., neat) ILs. Parlomar et al.19 prepared carbon


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capsule shells from phenol-formaldehyde resin and then soaked the capsules in an IL solution to impregnate them with imidazolium-based ILs. The obtained capsules showed a drastic increase in the rate of NH3 gas uptake compared to the IL itself. With a similar material, Lemus et al. demonstrated the capture-and-release of NH3,18 as well as CO2,22 as did Moya et al.20 Encapsulation of an IL containing an aprotic heterocyclic anion was also demonstrated, and thermodynamically favorable CO2 uptake under high pressure with enhanced mass transfer rate was shown.21 Recently, ILs have been confined in a laminated graphene oxide (GO) membrane33 primarily to improve high-pressure stability against leaching of the IL out of the support during CO2 separation. These reports illustrate that the encapsulation of ILs is promising for overcoming current limitations for their use in CO2 separations, and that facile and scalable access to such systems are required. In this paper, we report encapsulated ILs, synthesized via one-pot synthetic, benign and scalable route with a permeable polymer composite shell for CO2 capture, illustrating enhanced mass transfer in gas uptake compared to agitated and unagitated neat IL. The distinction in these encapsulated ILs compared to previous studies is the polyurea shell


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that keeps GO reinforcement sheets in place while providing permeability to CO2. Aditionally, here we show the effective uptake at low CO2 pressure (0 – 1.3 bar), with enhanced mass transfer rate. These novel materials are prepared from Pickering emulsion templates using alkylated GO nanosheets as surfactant and interfacial polymerization for stabilization. We illustrate this approach can be used to prepare capsules of [EMIM][TFSI] and [HMIM][TFSI] with large surface area formed by the polymer framework, for enhanced CO2 uptake at improved rates compared to still or agitated IL; an unmet need in CO2 separation technologies.

EXPERIMENTAL Materials 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], 99%) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][TFSI], 99%) were purchased from Iolitec (Tuscaloosa, Alabama). Graphite flakes, sulfuric acid (H2SO4), hydrogen

peroxide

(H2O2),

toluene,

N,N-dimethylformamide (DMF), potassium


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permanganate (KMnO4), and 1-octadecylamine (97%) were purchased from Alfa Aesar. 1,6-diisocyanatohexane (HDI, 99%) and 1,2-ethylene diamine (EDA, >98%) were purchased from Acros Organics. Heptane and dodecane were purchased from Fisher. Instrumentation Centrifugation was accomplished with Eppendorf 5804. Fourier Transform infrared (FTIR) spectroscopy was performed using an Agilent Cary 630 FTIR in ATR mode. The hand-held emulsifier used was from BioSpec Products, Model 985370. Optical microscopy images were taken using an AmScope M150C microscope with AmScope MU500-CK 5.0 MP USB microscope camera. 1H NMR measurements were collected using a Bruker Ascend III HD 500 MHz. Nova NanoLab 200 FEG-SEM/FIB under secondary electron scattering mode. Preparation of alkylated GO (C18-GO) Graphene oxide (GO) was prepared by a modified Hummer’s method as reported in the literature,34 and described in detail in our previous work.35 Briefly, natural graphite (1.0 g) was oxidized in concentrated H2SO4 (134 mL) at room temperature with KMnO4 (1.0 g, 0.063 mol) for 24 h; equimolar KMnO4 was repeatedly added every 24 h, a total of 4


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batches (4 g in total). Then, the mixture was diluted with ice–water (0.70 L) and aqueous H2O2 was added carefully until the pink color disappeared. Solid GO was collected by centrifugation, and washed repeatedly with 2-propanol until the supernatant was neutral. GO was dried under reduced pressure at room temperature. To prepare alkylated GO, GO (100 mg) and octadecylamine (1.0 g) were suspended in DMF (100 mL) at 50 °C until a dark precipitate formed. This precipitate was isolated by centrifugation and then redispersed in toluene (100 mL) with an additional of octadecylamine (2.0 g). The reaction vessel was loosely capped and stirred overnight at 50 °C. The product, a dark brown solid, was isolated by centrifugation, then washed with toluene and octane thoroughly and dried under reduced pressure. Preparation of IL capsules The preparation of IL capsules was accomplished similarly to our previous report23,32 For [EMIM][TFSI] capsules, the oil phase consisted of C18-GO in heptane (5 mL, 1 mg/mL standardized by parental GO), and the IL phase was a mixture of [EMIM][TFSI] (1.00 mL) and ethylenediamine (0.090 mL, 1.33 mol). These two solutions were combined in a vial and agitated by three cycles of emulsifying (20 s each) with a hand-held emulsifier at


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highest energy level, with 15 s rests between. Then, heptane (0.50 mL) was added to dilute the prepared emulsion. Next, a heptane solution of HDI (1.33 mol/mL, 1.25 ml) was added dropwise while gently swirling the vial by hand. The mixture was left unagitated for 72 h, and the capsules isolated by gravity filtration and washed thoroughly with hexanes. The capsules were dispersed in hexanes (100 mL) and propylamine (5 mL) added, then the solution allowed to stand for 3 h to quench any residual isocyanate groups. Of note, if the propyl amine step is not performed the capsules aggregate and cross-link, and cannot be re-dispersed. The solid particles were collected by gravity filtration and washed with hexanes to neutral pH (as determined by litmus test), then dried under reduced pressure to yield a light brown powder. The same procedure was followed for the preparation of [HMIM][TFSI] capsules, except: 1) 0.4 mg/mL C18-GO was used as oil phase; 2) the volumes of IL, EDA, and HDI were halved (these changes were made to account for higher viscosity of the IL).35 Isolation of Capsule Shell


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Capsule shell is isolated by extracting IL from capsules for 3 times with acetone washing followed by centrifugation. Each time the supernatant was discarded, and after the last wash, the obtained solid pellet was dried under vacuum. Analytical Methods To prepare samples for optical microscopy, the capsules were dispersed in dodecane, and then the suspension was drop cast onto a glass slide. Pressing of capsules was accomplished by placing capsules between two glass slides, then apply pressure manually, followed by separating two glass slides, which are ready for optical microscopy imaging. To quantify the content of IL in the capsules, 20 mg of capsules were mixed with an acetone-d6 solution containing mesitylene as internal standard (0.8 mL of 0.047 M). The extracted solution was then analyzed by 1H NMR spectroscopy and the wt% IL determined by the relative integration of signals for the IL and mesitylene. Samples for analysis by SEM were prepared by spreading isolated capsules on double sided tape, followed by blowing away loose particles with a nitrogen gas stream. CO2 Measurements


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The CO2 uptake capacities were measured by CO2 absorption isotherm in IL-based materials, including GO-IL capsules (2-3 g), neat ILs (same IL equivalent as the IL content of the corresponding capsule), and GO-IL capsule shells (same equivalent mass as the shell content of the corresponding capsule), using a Micrometritics TriStar II analyzer (pressure range 0 to 950 mmHg, resolution

Hybrid Ionic Liquid Capsules for Rapid CO2 Capture | Industrial

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