16654
J. Phys. Chem. B 2008, 112, 16654–16663
Physical and Chemical Absorptions of Carbon Dioxide in Room-Temperature Ionic Liquids A. Yokozeki,*,† Mark B. Shiflett,‡ Christopher P. Junk,‡ Liane M. Grieco,‡ and Thomas Foo‡ DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880 and DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880 ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: October 14, 2008
Gaseous solubilities of carbon dioxide (CO2) in 18 room-temperature ionic liquids (RTILs) have been measured at an isothermal condition (about 298 K) using a gravimetric microbalance. The observed pressure-temperaturecomposition (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been successfully applied for our previous works. Henry’s law constants have been obtained from the observed (PTx) data directly and/or from the EOS correlation. Ten RTILs among the present ionic liquids results in the physical absorption, and eight RTILs show the chemical absorption. The classification of whether the absorption is the physical or chemical type is based on the excess Gibbs and enthalpy functions as well as the magnitude of the Henry’s constant. In the chemical absorption cases, the ideal association model has been applied in order to interpret those excess thermodynamic functions. Then, two types of the chemical associations (AB and AB2, where A is CO2 and B is RTIL) have been observed with the heat of complex formations of about -11 (for AB) and from -27 to -37 (for AB2) kJ · mol-1, respectively. Introduction For the past several years, worldwide research on thermodynamic and transport properties of room-temperature ionic liquids (RTILs) and their mixtures with various chemicals has been conducted for possible applications of this new class of compounds.1-3 Among others, one of the promising applications with RTILs is a nonvolatile (so-called “green”) solvent for capturing unwanted compounds such as CO2, SO2, H2S, etc. in the exhaust gas stream of power plants. Although there are commercial organic solvents to capture these flue gases at the present, new RTIL solvents may provide viable and more environmentally friendly alternatives. The capture and sequestration of carbon dioxide in the power plant are urgently needed in order to reduce anthropogenic CO2 accumulations in the atmosphere.4 As often stated elsewhere, effectively capturing acid or sour gases from exhaust gases requires very strong absorption because of the relatively small partial pressures (e.g., 5-15 % v/v of CO2 at atmospheric pressure) of these gases in the gas stream. By strong absorption, we mean that the gas absorption needs to be “chemical” absorption (or reversible chemical complex formation), instead of the simple “physical” absorption (or no chemical reactions), which would be practical for highpressure gas absorption (e.g., CO2 partial pressure >525 kPa 5). It is well-known that CO2 possesses relatively high solubility in RTILs; for example, compared with the case of hydrocarbons.6 However, most reported cases for CO2 seem to be the physical absorption.6-17 To the best of our knowledge, a very high solubility of CO2 in RTIL with the chemical absorption is only observed in the case of the CO2 + [bmim][Ac] system.4,18,19 In this report, we have examined CO2 solubilities in 18 RTILs and identified seven more chemical absorption cases in addition to the known [bmim][Ac] system. The RTILs in this study are nine commercially available RTILs: 1-ethyl-3-methylimidazolium trifluoroacetate [emim][T* To whom correspondence should be addressed. E-mail: akimichi.
[email protected]. † DuPont Fluoroproducts Laboratory. ‡ DuPont Central Research and Development.
FA], 1-ethyl-3-methylimidazolium acetate [emim][Ac], 1-butyl3-methylimidazolium trifluoroacetate [bmim][TFA], 1-butyl-3methylimidazolium acetate [bmim][Ac], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [emim][Tf2N], 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6], 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4], 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate [hmim][FAP], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N]. In addition, nine new RTILs were synthesized to evaluate the interaction with carbon dioxide: 1-butyl-3-methylimidazolium 1,1,2,2tetrafluoroethanesulfonate [bmim][TFES], 1-butyl-3-methylimidazolium propionate [bmim][PRO], 1-butyl-3-methylimidazolium isobutyrate [bmim][ISB], 1-butyl-3-methylimidazolium trimethylacetate [bmim][TMA], tetrabutylphosphonium formate [TBP][FOR], 1-butyl-3-methylimidazolium levulinate [bmim][LEV], 1-butyl-3-methylimidazolium succinamate [bmim][SUC], bis(1-butyl-3-methylimidazolium) iminodiacetate [bmim]2[IDA], and 1-butyl-3-methylimidazolium iminoacetic acid acetate [bmim][IAAc]. Figure 1 provides the chemical name, abbreviation, and structure of the 18 ionic liquids studied. The binary systems of CO2 + [bmim][Ac], [bmim][PF6], [bmim][BF4], and [hmim][Tf2N] have been studied previously by us,8,10,19 but solubility measurements have been repeated here in order to compare the solubility with the present new RTILs at the same experimental condition. Solubility (PTx: pressure-temperature-composition) data have been collected at an isothermal condition of about 298 K, using a gravimetric microbalance method.10 The observed PTx data show both physical (10 systems) and chemical (8 systems) absorption cases and have been successfully correlated with an equation-of-state (EOS) model.10,20-22 These solubility behaviors have been further examined using the thermodynamic excess functions, as well as Henry’s law constants. In the strong absorption cases, the excess functions from the EOS correlation are interpreted in terms of a chemical association model.23-28 Experimental Details and Results Materials. Carbon dioxide (purity >99.99%, CAS No. 124-38-9) was purchased from MG Industries (Philadelphia,
10.1021/jp805784u CCC: $40.75 2008 American Chemical Society Published on Web 12/03/2008
Absorption of Carbon Dioxide in Ionic Liquids
J. Phys. Chem. B, Vol. 112, No. 51, 2008 16655
Figure 1. Chemical structures of the present ionic liquids and their abbreviations.
PA). The [emim][Ac] (assay g95%, C8H14N2O2, CAS No. 143314-17-4, Lot and Filling code S25819 14804B38), [bmim][Ac] (assay g95%, C10H18N2O2, CAS No. 284049-75-8, Lot and Filling code S25803 444041302), [bmim][PF6] (assay g96%, C8H15F6N2P, Lot and Filling code 1242554 15005226, CAS registry No. 174501-64-5), [bmim][BF4] (assay g97%, C8H15F4N2B, Lot and Filling code 1142017 31205158, CAS registry No. 174501-65-6) were obtained from Fluka/Aldrich (Buchs, Switzerland). The [emim][TFA] (assay g95%, C8H11F3N2O2, CAS No. 174899-65-1, Lot S4934747 814), [bmim][TFA] (assay g95%, C10H15F3N2O2, CAS No. 17489994-6, Lot EQ508558 616), [hmim][Tf2N] (assay g99%, C12H19N3F6O4S2, CAS No. 382150-50-7, Lot EQ500831 642), and [hmim][FAP] (assay g99%, C16H19N2F18P, CAS No. 713512-19-7, Lot S4872378-733) were obtained from EMD Chemicals, Inc. (Gibbstown, New Jersey). The ionic liquid [emim][Tf2N] (EMIIm, electrochemical grade, assay g99.5%, C8H11F6N3O4S2, Lot and Catalog No. 259095 IL-201-20-E, CAS registry No. 174899-82-2) was purchased from Covalent Associates Inc. (Woburn, MA).
Nine RTILs were prepared according to the methods described in the Supporting Information of this article. The cation containing precursor salts (purity >99%) were obtained from Fluka/Aldrich and were used to synthesize the corresponding RTILs using available anion precursors for this study. The molecular structure was verified by nuclear magnetic resonance (NMR). 19F NMR and 1H NMR spectra were recorded on a Bruker model DRX-400 spectrometer at 376.8937 and 400.550 MHz, respectively. The purity of all RTILs synthesized in the present work was estimated to be at least better than 95%. Experimental Method and Results. Both the commercially available and synthesized ionic liquids were dried by filling a borosilicate glass tube with about 5-10 g of the ionic liquid and pulling a coarse vacuum with a diaphragm pump (Pfeiffer, model MVP055-3, Nashua, NH) followed by further evacuation using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4 × 10-7 kPa while simultaneously heating and stirring the ionic liquid at a temperature of about 348 K for 5 days. The final water content was measured by Karl Fischer titration (Aqua-Star C3000, solutions AquaStar Coulomat C and A) and all ionic liquids
16656 J. Phys. Chem. B, Vol. 112, No. 51, 2008 contained less than 1 × 10-3 mass fraction of water. This water content in mass fraction translates the purity of all RTILs into better than about 98 mol %, after the evacuation treatment, and any other volatile impurities should have been removed in this treatment. This purity level of RTILs is sufficient for the present purpose (i.e., CO2 solubility measurements), although other measurements such as viscosity, thermal and electric conductivity, etc., may require much higher purities. The liquid density for each RTIL was measured at 298.15 K, and several samples were measured at higher temperatures (323.15, 348.15, and 373.15 K) using a pycnometer (Micromeritics AccuPyc 1330 with a 1 cm3 measuring cup). The uncertainty in the density measurement was ( 0.001 g cm-3. The ionic liquids were loaded into the pycnometer in a nitrogen-purged glovebox to minimize exposure to moisture. The density data are provided in Table S1 of the Supporting Information. Detailed descriptions of experimental equipment and procedures for the VLE are given in our previous reports;8,10,20 therefore, only the basic experimental techniques and measurement uncertainties are presented here. The gas solubility (VLE) measurements were made using a gravimetric microbalance10 (Hiden Isochema Ltd., IGA 003, Warrington, United Kingdom). A molecular sieve trap was installed to remove trace amounts of water from the CO2. Initially, about 65 mg of the RTIL was loaded into the sample container and heated to 348.15 K under a vacuum of about 10-9 MPa for 10 h to remove any trace amounts of water or other volatile impurities. The sample temperature was measured with a type K thermocouple with an accuracy of ( 0.1 K. The thermocouple was calibrated using a standard platinum resistance thermometer (SPRT model 5699, Hart Scientific, American Fork, UT, range 73-933 K) and readout (Blackstack model 1560 with SPRT module 2560). The Blackstack instrument and SPRT are a certified secondary temperature standard with a NIST traceable accuracy to (0.005 K. A single isotherm at about 298.15 K was measured for each RTIL. Pressures from 10-2 to 2.0 MPa were measured using a piezo-resistive strain gauge (Druck, model PDCR4010) with an accuracy of (0.8 kPa. The Druck pressure transducer was calibrated against a Paroscientific Model 760-6K (Redmond, WA) pressure transducer (range 0-41.5 MPa, serial No. 62724). This instrument is also a NIST certified secondary pressure standard with a traceable accuracy of 0.008% of full scale. The upper pressure limit of the microbalance reactor was 2.0 MPa, and several isobars up to 2.0 MPa (0.01, 0.05, 0.1, 0.4, 0.7, 1.0, 1.3, 1.5, and 2.0 MPa) were measured in the present study. In our previous reports,8,10 to ensure sufficient time for VLE, each T,P condition was maintained for a minimum of 3 h with a maximum time of 8 h. In this work, we found that 8 h was not sufficient to reach equilibrium at the present isothermal condition (about 298 K). Therefore, a maximum of 20 h was set for all sample measurements. The instrumental uncertainties in T and P are within ( 0.1 K and ( 0.8 kPa, respectively. These uncertainties do not cause any significant errors in the gas solubility measurement. The total uncertainties in the solubility data due to both random and systematic errors have been estimated to be less than 0.006 mole fraction at given T and P. Another large source of uncertainty in the present solubility experiments is due to the buoyancy correction in the data analysis. Analysis of the buoyancy effects requires an accurate measurement of the liquid density and CO2 gas density,10 which was calculated from a highly accurate correlation.29 Liquid density data of the present RTILs in Table S1 (Supporting Information) were used to obtain the corrected solubility data. The present experimental solubility (PTx) data are summarized in Tables 1-3. Observed isothermal PTx data at about 298.15 K will be
Yokozeki et al. TABLE 1: Experimental Solubility (PTx) Data for CO2 and Ionic Liquids CO2 (1) + [emim][TFA] (2)
CO2 (1) + [bmim][TFA] (2)
T/K
P/MPa
100x1
T/K
P/MPa
100x1
298.1 298.1 298.1 298.1 298.1 298.1 298.1 298.1 298.1
0.0100 0.0498 0.0998 0.3999 0.6997 0.9999 1.2997 1.4998 1.9996
0.1 0.9 1.8 6.8 11.5 16.0 20.1 22.6 28.2
298.1 298.1 298.1 298.0 298.2 298.1 298.1 298.1 298.1
0.0105 0.0504 0.1001 0.3999 0.6992 0.9996 1.3004 1.4996 1.9996
0.5 1.3 2.2 7.5 12.3 17.2 21.3 23.9 30.1
CO2 (1) + [emim][Ac] (2)
CO2 (1) + [bmim][Ac] (2)
T/K
P/MPa
100x1
T/K
P/MPa
100x1
298.1 298.1 298.1 298.1 298.1 298.1 298.1 298.1 298.1
0.0100 0.0499 0.1000 0.3996 0.6995 0.9996 1.2998 1.4997 1.9998
18.9 24.6 26.7 31.3 34.0 36.2 38.4 39.8 42.8
298.1 298.0 298.2 298.1 298.1 298.1 298.3 298.1 298.1
0.0102 0.0503 0.1003 0.3994 0.7001 0.9996 1.3002 1.5000 1.9994
16.2 25.1 27.5 32.6 35.7 38.3 40.6 42.0 45.5
CO2 (1) + [emim][Tf2N] (2)
CO2 (1) + [hmim][Tf2N] (2)
T/K
P/MPa
100x1
T/K
P/MPa
100x1
298.2 298.1 298.0 297.9 298.2 298.2 298.1 298.1 298.1
0.0100 0.0500 0.0998 0.3998 0.6998 0.9997 1.2999 1.4995 1.9998
0.4 1.5 3.0 10.7 17.4 23.4 28.7 31.7 39.0
297.4 297.4 297.3 297.4 297.3 297.4 297.4 297.3 297.3
0.0091 0.0487 0.0983 0.3943 0.6922 0.9890 1.2848 1.4820 1.9748
0.6 2.0 3.7 12.6 20.3 27.0 32.6 36.0 43.3
analyzed in the following subsections using our equation-of-state (EOS) model. Data Analyses Equation-Of-State (EOS) Model. As used in our previous works, we have employed a generic RK (Redlich-Kwong) type of cubic EOS, which is written in the following form:10,20-22
P)
RT a(T) V-b V(V + b)
a(T) ) 0.427480
(1)
R2T2c R(T) Pc
(2)
RTc Pc
(3)
b ) 0.08664
The temperature-dependent part of the a parameter in the EOS for pure compounds is modeled by the following empirical form: e3
R(T) )
∑ βk(1/Tr - Tr)k, (Tr ≡ T/Tc)
k)0
(4)
Absorption of Carbon Dioxide in Ionic Liquids
J. Phys. Chem. B, Vol. 112, No. 51, 2008 16657
TABLE 2: Experimental Solubility (PTx) Data for CO2 and Ionic Liquids
TABLE 3: Experimental Solubility (PTx) Data for CO2 and Ionic Liquids
CO2 (1) + [bmim][PF6] (2)
CO2 (1) + [bmim][BF4] (2)
CO2 (1) + [hmim][FAP] (2)
T/K
P/MPa
100x1
T/K
P/MPa
100x1
T/K
P/MPa
100x1
T/K
P/MPa
100x1
298.0 298.1 298.2 298.0 298.0 298.1 298.1 298.1 298.1
0.0105 0.0504 0.1004 0.3998 0.7003 0.9997 1.2996 1.5003 1.9997
0.1 0.9 1.9 7.0 11.8 16.1 20.2 22.8 28.6
298.2 298.2 298.1 298.2 298.2 298.0 298.2 298.0 298.1
0.0100 0.0500 0.1001 0.3996 0.7002 0.9997 1.3002 1.5001 2.0002
