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Thermochemistry of Alkyl Pyridinium Bromide Ionic Liquids: Calorimetric Measurements and Calculations† Bo Tong,† Qing-Shan Liu,‡ Zhi-Cheng Tan,*,‡,§ and Urs Welz-Biermann*,‡ Department of Chemistry, School of Chemical Engineering, Dalian UniVersity of Technology, Dalian 116023, China, and China Ionic Liquid Laboratory and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed: May 21, 2009; ReVised Manuscript ReceiVed: June 22, 2009
Two ionic liquids, 1-ethylpyridinium bromide (EPBr) and 1-propylpyridinium bromide (PPBr), were prepared and the structures were characterized by 1H NMR. The thermodynamic properties of EPBr and PPBr were studied with adiabatic calorimetry (AC) and thermogravimatric analysis (TG-DTG). The heat capacity was precisely measured in the temperature range from 78 to 410 K by means of a fully automated adiabatic calorimeter. For EPBr, the melting temperature, enthalpy, and entropy of solid-liquid phase transition were determined to be 391.31 ( 0.28 K, 12.77 ( 0.09 kJ · mol-1, and 32.63 ( 0.22 J · K-1 · mol-1, respectively, and for PPBr they were 342.83 ( 0.69 K, 10.97 ( 0.05 kJ · mol-1, and 32.00 ( 0.10 J · K-1 · mol-1, respectively. The thermodynamic functions (HT0 - H0298.15) and (ST0 - S0298.15) were derived from the heat capacity data in the experimental temperature range with an interval of 5 K. The thermostablility of the compounds was further studied by TGA measurements. The phase change behavior and thermodynamic properties were compared and estimated in a series of alkyl pyridinium bromide ionic liquids. Results indicate that EPBr has higher melting and decomposition temperature, as well as phase transition enthalpy and entropy but lower heat capacity than PPBr due to their different molecular structures. 1. Introduction Ionic liquids are attracting increasing attention in many fields including organic chemistry,1-4 electrochemistry,2,5,6 catalysis,7-9 physical chemistry, and engineering10-14 with their special physical and chemical properties, such as low vapor pressure, low inflammability, high inherent conductivities, thermal stability, liquidity over a wide temperature range, easy recycling, and being a good solvent for a wide variety of organic and inorganic chemical compounds. The physicochemical properties of an ionic liquid vary greatly depending on the molecular structure, for example, miscibility with water and organic solvents, melting point, and viscosity.15-17 Besides, ionic liquids are “designable” as structural modifications in both the cation and anion permit the possibility to design task-specific applications when the ionic liquid contains a specific functionality covalently incorporated in either the cation or anion.4 Alkyl pyridinium bromide ionic liquids, which are easy to synthesize and purify, are one of them and show good perspective in the applications of extraction and separation processes, synthetic chemistry, catalysis, and materials science.18-20 However, most scientists focus on the synthesis and application of ionic liquids while few researchers put their efforts on the fundamental thermodynamic studies.10,20-23 As far as we know, the thermodynamic properties of ionic liquids, such as heat capacity Cp,m, glass transition temperature Tg, melting temperature Tm, thermal decomposition † Part of the special issue “Green Chemistry in Energy Production Symposium”. * To whom correspondence should be addressed. E-mail: (Z.-C.T.) tzc@ dicp.ac.cn; (U.W.-B.)
[email protected]. † Dalian University of Technology. ‡ China Ionic Liquid Laboratory, Chinese Academy of Sciences. § Thermochemistry Laboratory, Chinese Academy of Sciences.
temperature Tdecomp, enthalpy, and entropy of phase transitions are important properties that reflect the structures and stabilities of compounds but were rarely reported until now. In the present study, two ionic liquids 1-ethylpyridinium bromide (EPBr, CAS NO. 1906-79-2) and 1-propylpyridinium bromide (PPBr, CAS NO. 873-71-2) were prepared and the structures were characterized by 1H NMR. The thermodynamic properties of EPBr and PPBr were studied with adiabatic calorimetry (AC) and thermogravimatric analysis (TG-DTG). The phase change behavior and thermodynamic properties were compared and estimated in the series alkyl pyridinium bromide ionic liquids, which were very important in the industry and application of alkyl pyridinium bromide ionic liquids. 2. Experimental Section 2.1. Materials. All reagents were of commercial origin with purities >99.5%. 1-Bromopropane (AR grade, Sinopharm Chemical Regent Co., China), pyridine, and 1-bromoethane (AR grade, Tianjin Damao Chemical Reagent Co., China) were distilled before use. After absorbing the water by molecular sieves, ethyl acetate (AR grade, Tianjin Kemiou Chemical Reagent Co., China) and acetonitrile (AR grade, Tianjin Fengchuan Chemical Reagent Co., China) were distilled and used in the synthesis process. 2.2. Preparation of EPBr and PPBr ILs. Pyridine (1 mol) was placed in a 500 mL round-bottomed flask and stirred, and 1-bromoethane or 1-bromopropane (1.1 mol) was added dropwise into the flask at 70 °C. A slight excess of the 1-bromoethyl or 1-bromopropane was used to guarantee the consumption of pyridine. Ethyl acetate (80 mL) was added to reduce the viscosity of the mixture, which was left to stir under reflux at 70 °C for 48 h. The halide salt separated as a second phase
10.1021/jp9047538 2010 American Chemical Society Published on Web 07/17/2009
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TABLE 1: Experimental Molar Heat Capacities of EPBr and PPBr T K
Cp,m -1
J · K · mol
T -1
K
Cp,m -1
J · K · mol
T -1
K
Cp,m
T
Cp,m
-1
EPBr 112.825 114.310 115.777 117.254 118.706 120.141 122.451 125.457 128.389 131.317 134.260 137.187 140.095 142.990 145.862 148.691 151.509 154.275 157.015 159.741 162.457 165.147
89.92 90.61 91.29 91.99 92.68 93.38 94.51 96.01 97.50 99.03 100.6 102.2 103.8 105.4 107.0 108.6 110.2 111.9 113.5 115.1 116.8 118.5
229.327 230.295 233.046 235.996 239.019 242.710 246.377 249.319 252.303 255.273 258.223 261.164 264.144 267.165 270.171 273.161 276.142 279.102 282.050 285.031 288.045 291.028
PPBr 109.263 110.443 111.612 112.773 113.931 115.078 116.220 117.354 118.482 119.602 121.367 123.762 126.132 128.476 130.800 133.098 135.379 137.636 139.877 142.099 144.303 146.491 148.663
101.9 102.3 102.7 103.7 104.5 105.1 105.4 106.1 107.0 108.0 109.6 111.6 113.2 114.7 116.1 117.4 118.5 119.5 120.7 121.8 123.0 124.2 125.5
209.166 212.232 215.267 218.270 221.226 224.186 227.228 230.236 233.079 235.879 238.705 242.272 245.916 248.905 251.877 254.857 257.855 260.823 263.802 266.790 269.785 272.790 275.759
J · K · mol
77.657 78.910 80.835 82.735 84.596 86.427 88.247 90.064 91.831 93.600 95.319 97.043 98.712 100.356 102.007 103.584 105.159 106.748 108.294 109.824 111.334
76.37 76.76 77.37 77.98 78.61 79.23 79.87 80.53 81.18 81.84 82.50 83.18 83.84 84.51 85.19 85.85 86.52 87.20 87.88 88.56 89.24
167.830 170.492 173.132 175.742 178.330 180.889 184.399 188.087 190.997 193.889 196.797 199.725 202.635 205.562 208.506 211.429 214.368 217.323 220.256 223.219 226.300
120.1 121.8 123.5 125.1 126.8 128.5 130.8 133.2 135.2 137.1 139.1 141.1 143.1 145.1 147.2 149.2 151.3 153.4 155.5 157.6 159.8
293.984 296.941 299.903 302.846 305.833 308.861 311.872 314.867 317.847 320.809 323.753 326.680 329.637 332.620 335.578 338.514 341.443 344.374 347.145 350.716 353.930
211.1 213.4 215.8 218.2 220.6 223.0 225.5 227.9 230.4 232.8 235.3 237.7 239.7 241.7 244.3 246.8 248.3 250.9 253.4 255.1 257.2
78.023 79.011 80.509 81.987 83.448 84.877 86.291 87.683 89.059 90.419 91.759 93.086 94.399 95.699 96.985 98.259 99.522 100.773 102.016 103.246 104.467 105.680 106.884 108.077
81.93 82.58 83.51 84.05 85.09 85.97 87.20 88.42 89.68 90.18 90.61 91.64 92.72 93.54 94.26 95.12 95.89 96.75 97.24 97.66 98.77 99.93 100.3 100.8
150.821 152.964 155.094 157.212 159.316 161.411 163.494 165.562 167.612 169.627 171.595 173.525 175.436 177.336 179.229 181.811 184.960 187.964 190.976 193.999 197.005 200.025 203.068 206.105
126.5 127.4 128.5 129.6 131.0 132.2 133.4 135.0 137.0 140.7 146.1 150.0 152.2 153.9 155.2 157.4 159.3 160.7 162.2 163.8 165.7 167.7 169.2 170.4
278.727 281.695 284.662 287.618 290.563 293.520 296.504 299.478 302.426 305.368 308.308 311.222 314.133 317.045 319.893 322.596 325.158 327.559 329.799 331.870 333.775 335.515 337.098 338.540
242.3 246.8 251.8 257.7 264.1 270.1 277.0 286.4 295.6 306.0 318.8 333.0 350.3 371.4 396.1 425.8 460.4 500.2 547.5 602.5 666.6 740.5 826.3 921.4
from the ethyl acetate. Excess of ethyl acetate were removed by decantation. The following equations (1 and 2) show the reaction scheme:
The products were recrystallized from acetonitrile. The volume of acetonitrile used for the recrystallization was approximately half that of the halide salt. Acetonitrile was then decanted after crystallization; this step was repeated twice. After the third cycle, the remaining acetonitrile and 1-bromoethane
K
-1
T
-1
J · K · mol
-1
K
Cp,m -1
T
Cp,m
K
J · K · mol-1
162.0 162.7 164.7 166.9 169.1 171.8 174.5 176.7 179.0 181.2 183.4 185.6 187.9 190.2 192.5 194.8 197.1 199.4 201.7 204.0 206.4 208.7
356.934 359.924 362.891 365.830 368.779 371.726 374.657 377.584 381.896 384.471 386.772 388.702 390.114 391.059 393.468 397.309 400.484 403.532 406.667 409.650
259.4 263.8 268.3 274.9 283.7 294.8 311.8 336.2 383.3 518.9 880.6 1385 2426 2913 368.3 290.8 292.6 295.6 298.0 300.3
172.4 174.1 176.7 179.2 182.0 184.1 186.9 189.0 192.1 193.9 196.9 199.5 202.6 205.8 208.6 211.8 215.2 218.3 221.6 225.1 229.3 233.3 238.0
339.842 341.032 342.146 343.563 345.633 348.293 351.393 354.574 357.586 360.598 363.600 366.595 369.613 372.656 376.372 379.412 382.441 385.470 388.488 391.504 394.600 397.710
1029 1133 1156 753.9 464.8 353.8 301.7 302.4 303.4 304.5 305.8 306.9 307.8 308.6 310.4 311.2 312.5 313.9 315.1 316.3 317.4 318.5
J · K · mol
-1
-1
or 1-bromopropane were removed under reduced pressure using a rotary evaporator at 70 °C, and the bromide salt was finally dried in high vacuum at 70 °C. 2.3. 1H NMR of EPBr and PPBr ILs. The 1H NMR spectra were recorded on a Bruker-400 Hz spectrometer and chemical shifts were reported in parts per million using DMSO as a solvent. 2.4. Adiabatic Calorimetry (AC). Heat capacity measurements were carried out in a high-precision automated adiabatic calorimeter24-26 which was established by Thermochemistry Laboratory of Dalian Institute of Chemical Physics, Chinese Academy of Sciences in PR China. To verify the reliability of the adiabatic calorimeter, the molar heat capacities for Standard Reference Material 720, Synthetic Sapphire (R-Al2O3) were measured. The deviations of our experimental results from the recommended values by NIST27 were within (0.1% in the temperature range of 80-390 K.
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Figure 1. Experimental molar heat capacity Cp,m as a function of temperature. (a) EPBr; (b) PPBr; (c) comparison from 300 to 400 K.
2.5. Thermogravimetic Analysis (TGA). The TG measurements of the sample were carried out by a thermogravimetric analyzer (Model: TGA/ SDTA 851e, Mettler Toledo, Switzerland) under N2 with a flow rate of 40 mL · min-1 at the heating rate of 10 K · min-1 from 300 to 580 K, respectively. The sample mass of 7.45 mg (EPBr) and 6.37 mg (PPBr) were filled into alumina crucible without pressing, respectively. 3. Results and Discussion 3.1. 1H NMR of EPBr and PPBr ILs. The 1H NMR spectra δH (400 MHz, DMSO) of two ILs are listed in Table A of the Supporting Information. Analysis of EPBr and PPBr by 1H NMR resulting in spectra (see Figure A of the Supporting Information) is in good agreement with the literature28 and does not find impurities. 3.2. Low-Temperature Heat Capacity. Experimental molar heat capacities of two ILs measured by the adiabatic calorimeter over the experimental temperature range are listed in Table 1 and plotted in Figure 1, respectively. From Figure 1a (EPBr), a smoothed curve with no endothermic or exothermic peaks was observed from the liquid nitrogen
temperature to 380 K, which indicated that the sample was thermostable in this temperature range. From 380 to 400 K, a sharply endothermic peak corresponding to a melting process was observed with the peak temperature 391.31 K. The melting process was repeated twice and the melting temperature was determined to be 391.31 ( 0.28 K according to the three experimental results. The values of experimental heat capacities can be fitted to the following polynomial equations with least-squares method.29 Before the fusion (8-380 K) 0 Cp,m /J · K-1 · mol-1 ) 160.770 + 120.380x + 43.911x2 -
74.730x3 - 119.630x4 + 78.756x5 - 118.390x6
(3)
After the fusion (395-410 K) 0 Cp,m /J · K-1 · mol-1 ) 294.630 + 5.947x
(4)
where x is the reduced temperature, x ) [T - (Tmax + Tmin)/ 2]/[(Tmax - Tmin)/2]; T is the experimental temperature; and
Thermochemistry of Alkyl Pyridinium Bromide Ionic Liquids
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TABLE 2: Smoothed Heat Capacities and Thermodynamic Functions of EPBr and PPBr T K 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290 295 298.15 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400 405 410
HT - H298.15
Cp,m -1
J · K · mol 78.73 78.71 79.54 81.03 83.04 85.40 88.03 90.81 93.70 96.62 99.54 102.4 105.3 108.1 110.9 113.6 116.4 119.1 121.8 124.6 127.4 130.3 133.3 136.4 139.6 143.0 146.4 150.0 153.8 157.6 161.6 165.6 169.7 173.9 178.1 182.3 186.5 190.6 194.7 198.7 202.5 206.2 209.8 213.3 215.3 216.6 219.7 222.9 225.9 229.1 232.3 235.8 239.6 244.0 249.0 255.0 262.2 270.9 281.4 294.1 309.4 327.8 / 288.7 292.6 296.6 300.6
-1
kJ · mol
-1
EPBr -30.09 -29.69 -29.30 -28.90 -28.49 -28.07 -27.63 -27.19 -26.73 -26.25 -25.76 -25.25 -24.73 -24.20 -23.65 -23.09 -22.52 -21.93 -21.33 -20.71 -20.08 -19.44 -18.78 -18.10 -17.41 -16.71 -15.98 -15.24 -14.48 -13.70 -12.91 -12.09 -11.25 -10.39 -9.511 -8.610 -7.688 -6.745 -5.781 -4.798 -3.795 -2.773 -1.733 -0.675 0.000 0.400 1.490 2.597 3.719 4.856 6.010 7.180 8.368 9.577 10.81 12.07 13.36 14.69 16.07 17.51 19.02 20.61 phase transition / 33.16 34.61 36.09 37.58
ST - S298.15 -1
J · K · mol
-1
-162.1 -157.4 -152.9 -148.5 -144.2 -140.0 -136.0 -132.0 -128.1 -124.2 -120.4 -116.6 -112.9 -109.2 -105.5 -101.9 -98.22 -94.62 -91.03 -87.46 -83.90 -80.36 -76.83 -73.30 -69.79 -66.28 -62.77 -59.27 -55.76 -52.25 -48.73 -45.21 -41.68 -38.14 -34.59 -31.04 -27.47 -23.89 -20.30 -16.71 -13.10 -9.496 -5.886 -2.275 0.000 1.335 4.943 8.545 12.14 15.73 19.32 22.90 26.48 30.07 33.67 37.29 40.96 44.68 48.47 52.38 56.42 60.65 / 92.83 96.49 100.2 103.8
HT - H298.15
Cp,m -1
J · K · mol 83.06 86.46 89.79 92.97 96.03 99.04 102.1 105.2 108.5 111.8 115.1 118.2 121.1 123.7 126.0 128.3 131.1 135.3 / 155.7 158.6 161.4 164.3 167.3 170.4 173.6 177.0 180.6 184.5 188.6 192.9 197.4 202.1 207.1 212.3 217.7 223.5 229.7 236.4 243.8 252.3 262.2 273.9 282.4 288.0 305.2 326.5 352.9 / / / / / / 302.5 304.4 306.3 307.9 309.6 311.5 313.7 315.7 317.5 319.5
-1
kJ · mol
-1
PPBr -34.44 -34.02 -33.58 -33.12 -32.65 -32.16 -31.66 -31.14 -30.61 -30.06 -29.49 -28.91 -28.31 -27.70 -27.07 -26.44 -25.79 -25.12 glass transition / -23.96 -23.18 -22.38 -21.56 -20.73 -19.89 -19.03 -18.15 -17.26 -16.35 -15.41 -14.46 -13.49 -12.49 -11.46 -10.42 -9.341 -8.238 -7.105 -5.940 -4.740 -3.500 -2.215 -0.876 0.000 0.528 2.009 3.587 5.283 phase transition / / / / / / 16.98 18.50 20.02 21.56 23.10 24.65 26.22 27.79 29.37 30.97
ST - S298.15 J · K-1 · mol-1 -183.7 -178.6 -173.5 -168.6 -163.7 -159.0 -154.3 -149.7 -145.1 -140.6 -136.2 -131.8 -127.4 -123.1 -118.9 -114.7 -110.6 -106.5 / -99.99 -95.66 -91.38 -87.15 -82.96 -78.79 -74.65 -70.53 -66.42 -62.31 -58.21 -54.11 -50.00 -45.88 -41.75 -37.60 -33.43 -29.22 -24.99 -20.72 -16.39 -12.00 -7.532 -2.953 0.000 1.764 6.661 11.79 17.22 / / / / / / 51.22 55.46 59.67 63.85 68.00 72.11 76.20 80.26 84.29 88.30
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Figure 2. TG-DTG curves under high purity nitrogen. w is the mass loss in percent. (a) EPBr; (b) PPBr; (c) DTG curves of two ILs.
Tmax and Tmin are the upper and lower limit in the temperature region, respectively. The correlation coefficient of the fitting r2 ) 0.9981 and 0.9955, corresponding to eq 3 and 4, respectively. However, from Figure 1b (PPBr), an endothermic step corresponding to a glass transition occurred at the glass transition temperature Tg ) 171.595 K. A sharply endothermic peak corresponding to a melting process was observed with the peak temperature 342.83 K. The melting process was repeated twice and the melting temperature was determined to be 342.83 ( 0.69 K according to the two experimental results. Similarly, the values of experimental heat capacities were fitted to the following polynomial equations with least-squares method. Before the glass transition (78-165 K), 0 Cp,m /J · K-1 · mol-1 ) 109.490 + 28.801x + 1.571x2 -
9.155x - 10.402x + 7.121x - 7.845x 3
4
5
6
(5)
After fusion (355-400 K) 0 Cp,m /J · K-1 · mol-1 ) 310.530 + 8.712x + 3.097x2 -
0.713x3 - 6.183x4 + 0.471x5 + 3.554x6 (7) The correlation coefficient of the fitting is r2 ) 0.9996, 0.9999, and 0.9995 corresponding to eqs 5, 6, and 7, respectively. 3.3. Thermodynamic Function. The thermodynamic func0 0 ) and (ST0 - S298.15 ) of the two ILs relative to tions (HT0 - H298.15 the reference temperature 298.15 K were calculated in the experimental temperature range with an interval of 5 K, using the polynomial equations of heat capacity and thermodynamic relationships as follows For EPB, before melting 0 HT0 - H298.15 )
After the glass transition and before fusion (180-315 K)
ST0 0 Cp,m /J · K-1 · mol-1 3
) 204.590 + 66.960x + 19.987x + 2
4.091x + 19.909x4 + 27.504x5 - 9.812x6 (6)
After melting
-
0 S298.15
)
T 0 Cp,m (s)dT ∫298.15
(8)
0 Cp,m (s) dT 298.15 T
(9)
∫
T
Thermochemistry of Alkyl Pyridinium Bromide Ionic Liquids 0 HT0 - H298.15 )
T 0 0 Cp,m (s)dT + ∆fusHm + ∫298.15 T 0 ∫T Cp,m(l)dT i
f
ST0
-
0 S298.15
)
∫298.15 Ti
[ ]
(10)
0 0 ∆fusHm Cp,m (s) dT + + T Tm
∫TT f
[ ]
0 Cp,m (l) dT (11) T
where Ti is the temperature at which the solid-liquid phase transition started; Tf is the temperature at which the solid-liquid 0 is the standard molar enthalpy phase transition ended; ∆fusHm of fusion; Tm is the temperature of solid-liquid phase transition. For PPBr, the calculation of thermodynamic functions is the same with EPBr before and after the melting. Moreover, the glass transition was included in the calculation. 0 , ST0 The standard thermodynamic functions, HT0 - H298.15 0 S298.15 of the two ILs, are listed in Table 2. 3.4. The Thermostability Tested by TG-DTG. The TGDTG curves shown in Figure 2 indicated that the mass loss of EPBr was completed in a single step. The sample keeps thermostable below 470 K. It begins to lose weight at about 480 K, reaches the maximum rate of weight loss at 541.229 K and completely loses its weight when the temperature reaches 575 K. Similar one-step decomposition process occurs for PPBr beginning at about 460 K and finishing at about 570 K while the peak temperature of decomposition is 536.021K. 3.5. Comparison and Estimation of Thermodynamic Properties for Alkyl Pridinium Bromide Ionic Liquids. According to experimental data in Sections 3.3-3.4, the thermodynamic properties as well as the structure of EPBr and PPBr were compared and estimated as follows: (a) Molar heat capacity Cp,m(EPBr) < Cp,m(PPBr) reveals that EPBr has lower lattice energy than PPBr in low temperature due to shorter carbon chain in pyridinium cation of EPBr. (b) Melting temperature Tm(EPBr) > Tm(PPBr) and phase transition enthalpy ∆Hm (EPBr) > ∆Hm(PPBr) are possibly because of the fact that the H-π bond effects of pyridinium cation played the major role in ionic compounds.30 The more carbonyl group added in the cation, the more the steric hindrance strengthened, which results in a decrease of the melting temperature and enthalpy. (c) Thermal decomposition temperature Tdecomp(EPBr) > Tdecomp(PPBr) indicates that EPBr is more thermostable than PPBr which is favorable in practical applications for EPBr. Conclusions Two ionic liquids 1-ethylpyridinium bromide (EPBr) and 1-propylpyridinium bromide (PPBr) were prepared and characterized. The structure and purity were verified by 1H NMR. The thermodynamic properties of EPBr and PPBr were studied with adiabatic calorimetry and thermogravimatric analysis. The phase change behavior and thermodynamic properties were compared and estimated in the series of alkyl pyridinium
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