Energy Fuels 2010, 24, 5797–5804 Published on Web 10/04/2010
: DOI:10.1021/ef101027s
Polymerized Ionic Liquid Sorbents for CO2 Separation Azadeh Samadi, Ruben K. Kemmerlin, and Scott M. Husson* Department of Chemical and Biomolecular Engineering, Clemson University, 127 Earle Hall, Clemson, South Carolina 29634, United States Received August 5, 2010. Revised Manuscript Received September 10, 2010
This contribution describes a method to prepare adsorbent materials for recovery of carbon dioxide from mixed gases. The sorbents comprise a base support material and a covalently attached nanolayer coating of polymerized ionic liquid (pIL), poly([2-(methylacryloyloxy)ethyl]trimethylammonium chloride) (poly[(META)þCl-]). Initial measurements were made to determine the thickness evolution of the pIL nanolayers, using a model substrate designed to mimic the structure of the support material. Thereafter, the pIL was grafted from the surface of the support material using surface-initiated atom transfer radical polymerization. Subsequent ion exchange was done to replace Cl- anions with other anions to study the impact of pIL anion on CO2 adsorption; X-ray photoelectron spectroscopy results confirmed that the ion-exchange reactions were quantitative. Anions that were tested were BF4-, CH3SO3-, and CF3SO3-. Carbon dioxide and nitrogen adsorption isotherms were measured from 0 to 2.5 bar at temperatures from 20 to 60 C using a Wicke-Kallenbach cell. Measurements with N2 showed no measurable adsorption at the highest pressure studied. Measurements on unmodified support material also showed no CO2 uptake. CO2 adsorbed to pIL-modified substrates with linear isotherms over the pressure range studied. Using the adsorption isotherm data at low pressures, Henry’s constants and heats of adsorption were measured. From these values, it was found that anions play a marginal role in CO2 adsorption performance by these trimethylammonium-based pIL sorbents. Despite their high CO2/N2 selectivity, the capacity values of the neat pIL materials were modest. Even for a relatively high CO2 partial pressure of 0.78 bar, representing a pressure-swing operation used to process flue gas with 13 mol % CO2 at 6 bar, the poly[(META)þCF3SO3-] pIL had a capacity of 2 mmol/g on the basis of per mass of pIL. Opportunities exist for improving sorbent performance by changing polymer surface chemistry, and the synthesis methodology presented in this study can be applied to prepare a wide range of functional materials and yield new materials for CO2 separations. research. Coal-fired power plants are the most likely source of CO2 for storage.1 In addition to reducing greenhouse gases, CO2 byproduct can be used in enhanced oil recovery (EOR) projects to increase oil production from existing reservoirs.2 Currently in the U.S., over 1500 miles of pipeline transport 48 MMT of CO2 per year to oil fields in Texas and New Mexico for EOR.1 CO2 also is being evaluated as a solvent to liberate kerogen from oil shale.3 These factors make the capture of CO2 from flue gas streams of high interest to the global scientific community. Efficient, economical CO2 separation from these streams is paramount in the consideration of any future capture technology. CO2 can be recovered from flue gas emissions using conventional alkanolamine absorption processes; however, these processes have disadvantages that include high energy cost associated with solvent regeneration, as well as solvent replacement cost, and corrosion problems.4-6 Thus, improved technologies are needed for lower cost CO2 capture. Carbon dioxide was shown to have high solubility in ionic liquids (ILs) by Blanchard et al.7 over a decade ago. The extent
Introduction Fossil fuel combustion power plants dominate the world energy production and account for a high percentage of overall energy-related CO2 emissions. In the U.S., for example, the electric power sector accounted for 40.6% of all energy-related CO2 emissions in 2008, with estimated emission of 2360 million metric tons (MMT) of CO2.1 CO2 emissions related to energy use in mature economies of member countries of the Organization for Economic Cooperation and Development (OECD) were 47% of the world total, with the remaining 53% from non-OECD countries.1 Energy-related CO2 emissions by China, a non-OECD country, are projected to grow from 21% of global emissions in 2006 to 29% in 2030, accounting for 51% of the projected increase in world emissions over the period.1 Emissions from all non-OECD economies are projected to grow by 2.2% per year from 2006 to 2030.1 Thus, the industrialization of developing nations will increase the world demand for energy and associated emissions of greenhouse gases. The increase in global atmospheric carbon dioxide concentrations and the possibility of future constraints on greenhouse gas emissions has accelerated carbon dioxide capture and storage
(2) Hite, J. R. J. Petrol. Technol. 2005, 28-29. (3) Parkinson, G. Chem. Eng. Prog. 2006, 7-10. (4) Ho, M. T.; Allinson, G. W.; Wiley, D. E. Ind. Eng. Chem. Res. 2008, 47, 4883–4890. (5) Aaron, D.; Tsouris, C. Sep. Sci. Technol. 2005, 40, 321–348. (6) Tsouris, C.; Aaron, D. S.; Williams, K. A. Environ. Sci. Technol. 2010, 44, 4042–4045. (7) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 28–29.
*To whom correspondence should be addressed. E-mail: shusson@ clemson.edu. Tel.: þ1 (864) 656-4502. Fax: þ1 (864) 656-0784. (1) Emissions of Greenhouse Gases in the United States 2008; U.S. Energy Information Administration, Office of Integrated Analysis and Forecasting; U.S. Department of Energy: Washington, DC, 2009. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5797–5804
: DOI:10.1021/ef101027s
Samadi et al.
of CO2 solubility in an IL depends on its cation, anion, and substituents.8-10 Because of the relatively higher solubility of CO2 than methane or nitrogen in room temperature ionic liquids (RTILs), these solvents have been proposed for CO2/ CH4 and CO2/N2 separations using pressure-swing absorption.11,12 The primary drawback of separation using RTILs is that the volume of the RTIL solvent required is proportional to the volume of the gas to be processed and inversely proportional to the concentration of sorptive.11,13 Therefore, there would be a large volume of RTIL required to separate CO2 at low concentrations from large volume flue gas streams. RTIL cost currently prohibits this technique for large-scale commercial usage. Supported ionic liquid membranes (SILMs) have been proposed as an alternative platform for CO2 separation to avoid using a large volume of RTIL. SILMs, which are porous membranes filled with an RTIL, have been used at the laboratory scale for CO2 separations.14-17 One potential weakness of SILMs is that the ionic liquid is held in the membrane pores by capillary forces such that, if the transmembrane pressure is high enough, the ionic liquid may be pushed out of the membrane. Though, Scovazzo and coworkers17 have demonstrated long-term (>106 days) stability in continuous operation of SILMs for CO2/CH4 and CO2/N2 separations at low pressure (99%, Aldrich), copper(I) chloride (>99.995%, Aldrich), copper(II) chloride (99.99%, Aldrich), ethanol (99.5%, Aldrich), glycidyl methacrylate (97%, Aldrich), HPLC water (Aldrich), hydrogen peroxide (30% in water, VWR), methanol (99.8%, Aldrich), methyl ethyl ketone (MEK, 98%, Baker), [2-(methylacryloyloxy)ethyl]trimethylammonium chloride ((META)þCl-, 75% solution in water), triethylamine (g99.5%, Aldrich), and sulfuric acid (95-98%, EMD). Silicon wafers (Silicon Quest International) with a crystal orientation of Æ1-0-0æ were diced into 1 cm 3 cm sample sizes and used as substrates for nanolayer growth studies. Prior to use, the silicon substrates were cleaned by sonication in deionized water for 30 min, treated with a 3:1 mixture by volume of concentrated H2SO4 and H2O2 (Caution: This mixture reacts violently with organic compounds. Minimal volumes should be used with appropriate gloves, goggles, and a face shield for protection) for 1 h at approximately 70 C, and then rinsed thoroughly with HPLC water. Surface-Initiated Atom Transfer Radical Polymerization (ATRP). RC substrates were soaked in water for 15 min, rinsed with methanol, and dried. Direct immobilization of the initiator precursor, 2-BIB, was used to incorporate ATRP initiator groups onto the RC substrate surface. RC substrates were brought in contact with a solution of 3 mM 2-BIB and 3 mM triethylamine in anhydrous THF (solvent) for 2 h. A volume of approximately 60 mL was used for each sample. After reaction, the initiator-functionalized RC substrates were washed thoroughly with HPLC water and methanol. Poly[(META)þCl-] nanolayers were grown from the initiator-functionalized substrates by ATRP. For the ATRP reaction, a mixture of two parts by mass solvent (80:20 (v/v) methanolwater) and one part by mass monomer was used. Cu(I)Cl, Cu(II)Cl2, and bipy were added to the mixture in the following molar proportions: [(META)þCl-]/[Cu(I)Cl]/[bipy]/[Cu(II)Cl2] = 100:2:5:0.1 and 100:2:5:0.2. The mixture was degassed using three freeze-pump-thaw cycles. The reaction mixture was then transferred into an oxygen-free glovebox, and the initiatorfunctionalized substrates were put in the polymerization solution for different times. After polymerization, the substrate with grafted poly[(META)þCl-] was washed with HPLC water and methanol and dried in a stream of nitrogen. Ion exchange was done to replace Cl- anions with other anions. Anions that were tested were BF4-, CH3SO3-, and CF3SO3-. A known mass of the sodium salt (14.5 g of NaBF4, 15.6 g of NaSO3CH3, or 22.7 g of NaSO3CF3) was dissolved in HPLC water (150 mL) and brought in contact with the poly[(META)þCl-]-modified substrates at room temperature overnight to carry out the ion exchange reaction. The resulting pILs are named according to the new anion: poly[(META)þBF4-], poly[(META)þCH3SO3-], and poly[(META)þCF3SO3-]. Gravimetric measurements were done to determine the mass of pIL grafted from the cellulose substrate for each polymerization time. To ensure accurate measurements, each substrate was placed in a vacuum desiccator for 48 h to remove all moisture prior to initial weighing. It was determined that 48 h in the desiccator was sufficient to dry the substrate completely. The desiccator was transferred into a water free (
