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Carbon Dioxide Solubility in Polymerized Ionic Liquids Containing Ammonium and Imidazolium Cations from Magnetic Suspension Balance: P[VBTMA][BF4] and P[VBMI][BF4] Andre Blasig, Jianbin Tang, Xudong Hu, Sugata P. Tan, Youqing Shen, and Maciej Radosz* Soft Materials Laboratory, Department of Chemical & Petroleum Engineering, UniVersity of Wyoming, Laramie, Wyoming 82071-3295
Based on sorption experiments up to 180 bar and 75 °C, the CO2 solubility in an ammonium-type ionic polymer, such as poly(p-vinylbenzyltrimethyl ammonium tetrafluoroborate), is found to be much higher than that in an imidazolium-type ionic polymer, such as poly(1-(p-vinylbenzyl)-3-methyl-imidazolium tetrafluoroborate), which points to a cation-type effect on the CO2 sorption in polymerized ionic liquids. These two polymers remain glassy in the temperature and pressure ranges studied in this work. The CO2 solubility increases with decreasing temperature and with increasing pressure, but it reaches a limiting value, beyond which it is insensitive to pressure. Introduction
Table 1. Polymers Used in This Study
CO2 sorption in glassy polymers causes swelling1-8 and reduces the glass transition temperature.9-11 Most reported experimental sorption data have been obtained at low or moderate pressures, where the CO2 solubility is relatively low and the polymer swelling and glass transition depression can be neglected.12-22 The solubility data at elevated pressures are scarce. Hu et al.23 reported the transport of CO2 in thin films made of polymerized ionic liquids, referred to as ionic polymers, grafted with polyethylene glycol in the context of CO2 membrane applications. The clue for that work was that ionic liquids themselves are known to have high thermal stability and high CO2 solubility.24-29 We have previously reported the high CO2 solubility in ammonium type ionic polymer at low pressures (30 bar). After allowing CO2 to diffuse into the polymer for 30 min, the DSC temperature ramp is started at a heating rate of 10 °C/min. The polymer density method is tested against the literature data for polysulfone and found to be accurate to (3%. Table 2 lists the properties of polymers; the glass transition temperature
(Tg) and the density at 25 °C were reported previously30 for PSF and P[VBTMA][BF4] and measured in this work for P[VBMI][BF4]. Sorption Measurements. A magnetic suspension balance (MSB) from Rubotherm Pra¨zisionsmesstechnik GmbH, rated up to 500 bar and 150 °C, is used to measure the CO2 solubilities in each of the two polymers. A detailed description of the MSB procedure used in this work was reported elsewhere.30 In brief, the sorption chamber is flooded with CO2 from a gas cylinder and its pressure is controlled with a cylinder regulator at low pressures (e30 bar) or with a syringe pump (Isco, model 260D) at high pressures (>30 bar). The mass of the CO2 absorbed by the polymer is determined from the increase of the electromagnetic force needed to maintain the sample in suspension. Eventually, the equilibrium sorption (the solubility) is reached and the weight of the sample stops increasing. At this stage of polymer saturation with CO2, the weight reading from the microbalance at pressure P and temperature T is recorded as ws(P,T). The mass of the gas dissolved in the polymer, wfl(P,T), is calculated using the following equation:
wfl(P,T) ) ws(P,T) - ws(Vac,T) + Ff l(P,T)‚(Vr-b + Vp + Vsw) (1) where Ffl (P,T) is the density of the gas at P and T, Vr-b and Vp are the volumes of the rod-basket assembly and of the original polymer, respectively, and Vsw is the increase of the original polymer volume due to swelling. The last term of the above equation, Ffl(P,T)‚(Vr-b + Vp + Vsw), represents the buoyancy force. The volume of the rod-basket assembly, Vr-b is determined from a buoyancy experiment. When no sample is charged into the MSB basket, eq 1 simplifies to
0 ) ws(P,T) - ws(vac,T) + Ff l(P,T)‚Vr-b
(2)
The volume of the rod-basket assembly is determined by estimating the density of the gas, Ffl (P,T), and measuring the weights, ws(P,T) and ws(vac,T). The CO2 density, Ffl (P,T), is calculated from the Bender EOS.37 The volume of the pure polymer sample, prior to swelling due to sorption, Vp, is calculated from the mass and density of the sample. Since the polymer volume change due to swelling, Vsw, is set to zero in this work, the solubility of CO2 in the polymer is treated as the apparent solubility, rather than an absolute solubility, according to the following equation
(P,T) ) ws(P,T) - ws(vac,T) + wapparent fl Ff l(P,T)‚(Vr-b + Vp) (3) Results and Discussion CO2 in P[VBTMA][BF4]. The glass transition temperature has been measured for the CO2-P[VBTMA][BF4] system up to a CO2pressure of 70 bar. These data are shown in Figure 1. The glass transition temperature goes from 236 °C at ambient pressure to 221 °C at 70 bar; the polymer should, therefore, remain glassy at pressures greater than 70 bar, but it is not known at what pressures it becomes rubbery. The solubilities of CO2 in P[VBTMA][BF4] at 25, 50, and 75 °C and up to 180 bar are presented in Figure 2. At low pressures only, the solubility data can be represented by a simple
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Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007
Figure 1. P[VBTMA][BF4] glass transition temperature versus CO2 pressure.
Figure 2. Experimental points and dual-mode sorption curves for the CO2 apparent solubility in P[VBTMA][BF4] at 25, 50, and 75 °C.
correlation referred to as a dual-mode sorption model applicable to glassy polymers as follows:
C ) kD‚P +
C′H‚b‚P 1 + b‚P
(4)
where C is the gas apparent solubility in mL (STP)/mL polymer calculated using the weight from eq 3 and the density value of polymer in Table 2 and the density value of carbon dioxide at STP, where STP is the standard temperature and pressure (0 °C and 1 atm), P is pressure in atm, kD is Henry’s law solubility coefficient, C′H is the Langmuir sorption capacity constant, and b is the affinity constant for the Langmuir mode. For each isotherm, all three parameters (kD, C′H, and b) are regressed by fitting to our experimental CO2-P[VBTMA][BF4] solubility data at low pressures ( 15 bar), the solubility continues to increase with pressure up to a point, then levels off and shows little change with pressure. This behavior, consistent with typical phase diagrams of binary mixtures containing a compressible gas and a large-molecular-weight component, is example of type V according to the classification by Van Konynenburg and Scott.33 In order to explain it, a simple but realistic phase diagram with three isotherms, T1 < T2 < T3, is shown in Figure 3. This P-x diagram is calculated from statistical associating fluid theory (SAFT)38,39 in this work for a model solvent (propane) and a model long-chain molecule (tetracontane), for which there are
Figure 3. A typical phase diagram for a binary mixture containing a compressible solvent (propane) and a long chain molecule (tetracontane); calculated in this work from statistical associating fluid theory (SAFT).38,39
Table 3. Parameters for the Dual-Mode Sorption Model for P[VBTMA][BF4] (P < 15 Bar)
polymer-gas system P[VBTMA][BF4]-CO2
temp [°C]
kD [cm3 (STP)/ (cm3‚bar)]
C′H [cm3 (STP)/ cm3]
b [bar-1]
25 50 75
1.45 0.9 0.85
22 15 6.5
0.39 0.3 0.1
accurate data taken by Luszczyk.40 This quantitative phase diagram is qualitatively illustrated with a corresponding P-T projection shown in Figure 4.34 In Figure 3, we have three-phase lines at P1 and P2 shown as vertical dotted lines, which correspond to the points on the solid VLL curve in Figure 4. The phases in equilibrium at P1 and P2 are the polymer-rich liquid at the bottom and the gas-rich liquid and vapor at the top. The lower solid branches in Figure 3 are the gas-in-mixture solubility curves that correspond to the CO2 solubility curves in Figure 2. The three-phase pressures, P1 and P2, correspond to the “kink” of the solubility curves in Figure 3. At a higher temperature, T3, the gas-rich liquid-phase disappears, which leads to two coexisting phases, vapor and polymer-rich liquid. At T3, therefore, the solubility kink disappears.
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Figure 4. A qualitative P-T phase diagram that corresponds to Figure 3. Figure 7. CO2 solubility in PSF, P[VBTMA][BF4], and P[VBMI][BF4] at 35 °C as a function of pressure. Table 4. Limiting CO2 Apparent Solubility at Higher Pressures, Clim, for P[VBTMA][BF4] polymer-gas system P[VBTMA][BF4]-CO2
temp [°C]
Clim [cm3 (STP)/cm3]
25 50 75
170 162 133
Table 5. Parameters for the Dual-Mode Sorption Model for P[VBMI][BF4] (P < 15 Bar) polymer-gas system
temp [°C]
kD [cm3 (STP)/ (cm3‚bar)]
C′H [cm3 (STP)/cm3]
b [bar-1]
25 50 75
0.85 0.65 0.55
12.5 8.5 4
0.2 0.15 0.05
Figure 5. P[VBMI][BF4] glass transition temperature versus CO2 pressure. P[VBMI][BF4]-CO2
Table 6. Limiting CO2 Apparent Solubility at Higher Pressures, Clim for P[VBMI][BF4] polymer-gas system P[VBMI][BF4]-CO2
Figure 6. Experimental points and dual-mode sorption curves of the CO2 apparent solubility in P[VBMI][BF4] at 25, 50, and 75 °C.
Figures 3 and 4 illustrate a general trend that is not unique to tetracontane in propane: as the temperature increases, the solubility kink moves to higher pressures but lower solubilities; at high temperatures, the solubility kink disappears. In our case, the solubility kink corresponds to a limiting apparent solubility, Clim, as listed in Table 4 for CO2 in P[VBTMA][BF4]. The solubility does not change much with pressure at this limiting value. CO2 in P[VBMI][BF4]. The glass transition temperature for the CO2-P[VBMI][BF4] system shown in Figure 5 shifts from 116 °C at ambient pressure to 86 °C at 65 bar. Since the glass transition temperature at 65 bar is quite low (86 °C), the polymer should become rubbery at pressures somewhat greater than 70 bar, but it is not known exactly at what pressure it becomes rubbery. The apparent solubilities of CO2 in P[VBMI][BF4] at 25, 50, and 75 °C and up to 180 bar are presented in Figure 6. The
temp [°C]
Clim [cm3 (STP)/cm3]
25 50 75
149 137 111
dual-mode sorption fit is used again to represent the solubility data at low pressures (
