Enhanced CO2 Absoprtion of Poly(ionic liquid)s

CO2 Absorption of Polymers of Ammonium-Based Ionic Liquid Monomers Jianbin Tang,1,2 Huadong Tang,1 Weilin Sun,2 Maciej Radosz1 and Youqing Shen*1 1 Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, WY 82071; 2Department of Polymer Materials and Engineering, Zhejiang University, Hangzhou 310027, China as initiator. 1H NMR and element analysis indicted pure poly(ionic liquid)s were obtain. CH3 CH3 H2 C H2 C C C O NaBF4 C NaBF4 C O O O Introduction Experimental Synthesis of ionic liquid monomers (Scheme 1): [MAETA][BF4] and [VBTA][BF4] Water in [(2-methacryloxy)ethyl]trimethyl ammonium chloride (75% wt) solution was removed at 40˚C under high vacuum. Dried [(2-methacryloxy)ethyl]-trimethyl ammonium chloride and NaBF4 were mixed in acetonitrile. The mixture was stirred overnight. The precipitated chloride salt was removed by filtration. The filtrate was concentrated and then poured in ether to precipitate out the product. The white crystal product was washed by ether and dried under vacuum. The synthesis of [VBTA][BF4] was similar. Synthesis of poly(ionic liquid)s: Ionic liquid monomer (3 g), AIBN (30 mg) in 3 ml of DMF were charged into a tube, tightly sealed, and degassed. The tube was immersed in an oil bath at 60 ˚C for 6 h. After polymerization, the solution of polymer was poured in methanol to precipitate out the product. The products were washed by ether and dried under vacuum at 100 ˚C. Measurements: Ionic liquid monomers and poly(ionic liquid)s were characterized by 1H NMR on a Bruker Advance DRX-400 spectrometer using dimethylsulfoxide (DMSO-d6) as solvent and element analysis (Midwest Microlab, LLC ). SEM was conducted on a Scanning Electron Microscope (Philips 505). The BET surface area of the powder was determined by nitrogen adsorption (Tristar 3000, Micromeritics Instruments Corp). The CO2 absorption of the poly(ionic liquid) was measured using a CAHN 1000 Electrobalance.. The buoyancy effects in these measurements were corrected according to literature.[5] The system was validated by measuring the CO2 absorption of an ionic liquid, 1-n-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF4]). The measured CO2 absorption capacity of [bmim][BF4] was identical to that reported.[4] N Cl- N BF4- N Cl- BF4- [VBTA][BF4] [MAETA][BF4] Scheme 1 Synthesis of ionic liquid monomers. The CO2 CO2 absorption kinetics of poly(ionic liquid)s: absorption kinetic of poly(ionic liquid)s and their monomers are shown in Figure 1. There was no weight increase when the two monomers exposed to CO2, which can be ascribed to their crystalline structures. When P[VBTA][BF4] or P[MAETA][BF4] exposed to CO2, their weights increased very fast. It took only 30 min to reach the equilibrium. The polymer (1.0 g) could gain 13.6 mg (7.4 mol %) and 15.3 mg/g (8.4 mol %) for P[VBTA][BF4] and P[MAETA][BF4], respectively. 10 8 CO2 Mole (%) We found that the polymers from ammonium based ionic liquids had very high CO2 absorption capacity, and could be a new kind of materials for CO2 capture and separation. N 6 P[VBTA][BF4] P[MAETA][BF4] 4 [VBTA][BF4] 2 [MAETA][BF4] 0 0 20 40 60 Time (min) Figure 1 CO2 absorption of ionic liquid polymers and their monomers (592.3 mmHg CO2, 22 °C). CO2 absorption and desorption cycles and selectivity of gas absorption: Repeated CO2 absorption and desorption cycles of P[MAETA][BF4] were tested by filling the chambers with CO2 and then vacuuming (Figure 2). Both absorption and desorption of P[MAETA][BF4] were very fast. It took only about 30 min to take up CO2 or to have a complete desorption of CO2. The desorption was complete, suggesting that the absorption/desorption was reversibly. No change in sorption/desorption kinetics and sorption capacity was observed after the four cycles. 8 CO2 Mole (%) The topic of global warming as a result of increased atomospheric CO2 concentration is becoming the most important environmental issue that the world faces today.[1] The capture and separation of CO2, especially CO2 from large point source, for example, fossil-fuel-fired electrical power–generation plants is critical to stabilize the atomospheric CO2 concentration.[2] The existing commercial CO2 capture facilities are based on the wet scrubbing process using aqueous alkanolamine solutions. It had disadvantages of energy intensive (i.e. high energy penalty), amine loss and degradation, release of volatile organic compounds, and equipment corrosion.[1] Recently, ionic liquids were proposed as non-volatile, and reversible absorbents for CO2 separation because CO2 is remarkably soluble in ionic liquids. However, the high viscosity limits their eventual use in large scale gas scrubbing applications.[3] 6 4 2 0 Results and discussion Synthesis of poly(ionic liquid)s: The two monomers were synthesized from corresponding chlorides using ion exchange reaction with NaBF4. The chloride salts cannot dissolve in acetonitrile, but the ionic liquids monomers with BF4 anion can dissolve in acetonitrile, acetone, DMF, DMSO. P[MAETA][BF4] and P[VBTA][BF] were prepared from the two ionic liquid monomers by free radical polymerization using AIBN 0 100 200 300 Time (min) 400 500 Figure 2 Cycles of CO2 absorption (592.3 mmHg CO2, 22 °C) and desorption by vacuuming of P[MAETA][BF4]. The absorption of the polymers is very selective, as shown in Figure 3. There was no weight increase when the polymers were exposed to N2 under the same conditions, indicative of that they selectively absorbed CO2. In N2/CO2 mixed gas, only CO2 was absorbed. CO2 Mole (%) 8 6 CO2 4 N2 2 0 0 20 40 60 80 Time (min) Figure 3 The selectivity of gas absorption (592.3 mmHg, 22 °C) of P[VBIB][BF4]. Conclusions The ammonium based poly(ionic liquid)s had high CO2 absorption capacity: 8.5 mole % and 7.4 mole %. for P[MAETA][BF4] and P[VBTA][BF4], respectively. The CO2 absorption and desorption are reversibly and selectively over N2. Acknowledgement: We thank the State of Wyoming (EORI project) and the University of Wyoming for financial support. References [1] White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. J. Air & Waste Manage. Assoc. 2003, 53: 645- 715. [2] Herzog, H.; Drake, E.; Adams, E. CO2 Capture, Reuse and Storage Technologies for Mitigating Global Climate Change. Report No. DOE/DE-AF22-96PC01257, U.S. Department of Energy: Pittsburgh, PA, 1999. [3] Eleanor D. Bates, Rebecca D. Mayton, James H. Davis, Jr. et al. J. Am. Chem. Soc. 2002,124, 926- 927. [4] Cesar Cadena, L. Anthony, Edward J. Maginn et al, J. Am. Chem. Soc. 2004,126, 5300- 5308 [5] Michael D. Macedonia, Darrin D. Moore, and Edward J. Maginn, Langmuir 2000, 16, 3823-3834