Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization

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Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of Alkoxy Derivatives of Imidazole and Benzimidazole Inna V. Garist† and Sergey P. Verevkin‡,* Department of Physical Chemistry, University of Rostock, Dr-Lorenz-Weg 1, D-18059, Rostock, Germany

Artemiy A. Samarov Chemical Department, Samara State Technical University, Samara 443100, Galaktionovskaya 141, Russia

Jason E. Bara* and Michelle S. Hindman Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama, United States 35487-0203

Scott P. O. Danielsen§ NSF-REU Site: Engineering Solutions for Clean Energy Generation, Storage and Consumption, Department of Chemical & Biological Engineering, University of Alabama, Tuscaloosa, Alabama, United States 35487-0203 S Supporting Information *

ABSTRACT: The design of physical solvents for applications such as CO2 capture has been an important research area as great emphasis is being placed on reducing greenhouse gas emissions. In gas treating with physical solvents, one of the most important solvent properties needed for efficient process operation is a low vapor pressure (≪100 Pa) at ambient temperature. We have identified alkoxy-functionalized imidazoles and benzimidazoles as candidates that can meet this criterion. Vapor pressures of alkoxy derivates of imidazole and benzimidazole have been determined as a function of temperature by the transpiration method. From these data, the molar enthalpies of vaporization (Δgl Hm) were calculated. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent speciesdimethyl ethers of ethylene glycol, diethylene glycol, and triethylene glycol. We observe that for species with the same number of atoms in the side chain [e.g., 1butylimidazole and 1-(2-methoxyethyl)-imidazole], replacing every third methylene group with an ether oxygen reduces vapor pressure by 50−75%.

1. INTRODUCTION In the design of new solvents and materials for gas separations applications (e.g., CO2 capture, natural gas sweetening), polar molecules with low vapor pressures are advantageous from the standpoint of efficient process design.1 Polar molecules are more likely to favor high CO2 physical (i.e., nonreactive) solubility and selectivity,2 while reducing solvent vapor pressure can aid in minimizing volatile losses to the gas stream. One class of solvent that meets these criteria is dimethyl ethers of poly(ethylene glycol) (DMPEG), which is commercially utilized in the Selexol process for various gas treating (CO2 and H2S) applications.3,4 Ionic liquids (ILs) have also been identified as candidates with low vapor pressures that are capable of removing CO2 and/or H2S.5,6 While many of the early ILs were built with alkyl substituents (e.g., 1-butyl-3methylimidazolium cations),7 ILs can be readily tailored to resemble DMPEG. Bara and co-workers first identified the advantages of appending ether groups to an IL in increasing the CO2 selectivity of IL solvents and membranes.5,8−12 However, as bulk fluids, most ILs are much more viscous (and more costly) than DMPEG and other common organic solvents © 2012 American Chemical Society

typically used in gas separation processes. This may limit their applicability within the conventional absorber-stripper process configuration. Although ILs have now been studied in many applications for over a decade, it has only been more recently that imidazolesprecursors for many of these ILshave been explored for the same applications.13−17 We have previously reported on several thermophysical properties of alkylimidazoles and alkybenzimidazoles,13−20 finding that while they are more volatile than ILs, they also possess much lower viscosities (1−2 orders of magnitude) and in some cases, improved CO2 solubility and unique interactions with SO2.21 We have also observed that the vapor pressure and other thermophysical properties of these molecules have very strong dependencies on the length of the substituent attached to the ring.13−18,20 However, for acid gas separations (e.g., removal of CO2 and/or SO2 and/or H2S), solvents with alkoxyReceived: Revised: Accepted: Published: 15517

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triple-ζ valence potential (TZVP) basis set29 with the Becke and Perdew (b-p)30,31 functional at the density functional theory (DFT) level. All COSMO files were generated at the TZVP level of theory, and vapor pressures and enthalpies of vaporization were predicted using the “Vapor Pressure” module in COSMOTherm (version C21_0111_a, COSMOlogic GmbH, Leverkusen, Germany), consistent with other published works that have utilized this software to predict thermophysical properties.27,32−36

based substituents will most likely be preferable to those with alkyl-based substituents. Thus, we now focus on the properties of N-alkoxy substituted imidazoles and benzimidazoles. Specifically, we aim to quantify the effect of the ether group on vapor pressure in both classes of molecules, and this work investigates the relationship between vaporization enthalpy and structure of N-alkoxy substituted imidazoles and benzimidazoles presented in Figure 1. Furthermore, we compare the

3. RESULTS AND DISCUSSION 3.1. Vapor Pressure and Enthalpies of Vaporization. Vapor pressures of N-alkoxy substituted imidazoles and benzimidazoles measured in this work and enthalpies of vaporization (Table 1) were treated with eqs 2 and 3, respectively,

Figure 1. Structures of N-alkoxy substituted imidazoles (left) and benzimidazoles (right) studied in this work.

R ·ln pi = a +

results to corresponding alkylimidazoles and present the results of a computational model for the vapor pressures of N-alkoxy substituted imidazoles generated using COSMOTherm.

Δgl Hm(T ) = −b + Δgl Cp·T

2. EXPERIMENTAL SECTION 2.1. Materials. N-alkoxy substituted imidazoles and benzimidazoles (PEGn-imidazole and (PEGn-benzimidazole, respectively) were prepared according to refs 8 and 22, and were further purified by multiple fractional distillations. 1H NMR data were consistent with published values,8,22−24 and our 1H NMR data and spectra images for N-alkoxy substituted benzimidazoles are provided as Supporting Information. For each sample, the degree of purity was determined by gas chromatography (GC) (Hewlett-Packard 5890 Series II) with flame ionization detector. No impurities greater than 5 × 10−4 mass fraction were detected in samples used in the study. 2.2. Vapor Pressure Measurements. Vapor pressures of N-alkoxy substituted imidazoles and benzimidazoles were determined via transpiration25,26 in a stream of saturated N2. Approximately 0.5 g of the compound of interest was charged together with 1 mm diameter glass beads into a thermostatted U-shaped saturator having a length of 20 cm and a diameter of 0.5 cm. N2 (1−5 L h−1) at constant temperature (±0.1 K) was passed through the U-tube, and the saturated with the sample gas was completely condensed in the cold trap within a definite time. A soap bubble flowmeter was used to measure the rate of N2 flow, which was optimized to achieve saturation equilibrium of the transporting gas at each temperature condition examined. The amount of the collected sample was measured by GC analysis with an external calibration standard (hydrocarbon nCnH2n+2). Assuming Daltoǹs Law of partial pressures is valid for the N2 stream saturated with the substance i of interest, values of pi were calculated via eq 1: pi = mi ·R ·Ta /V ·Mi ; V = VN2 + Vi ; (VN2 ≫ Vi )

⎛T ⎞ b + Δlg Cp·ln⎜ ⎟ T ⎝ T0 ⎠

(2) (3)

where pi is vapor pressure; a and b are adjustable parameters (Table 1); T0 is an arbitrarily selected reference temperature (T0 = 298.15 K in this work); and Δgl Cp is the difference between the molar heat capacities of the gaseous and the liquid phases. Values of Δgl Cp (Table 2) were calculated from the isobaric molar heat capacities (C1p) of N-alkoxy substituted imidazoles and benzimidazoles via the group contribution method of Acree and Chickos.37−39 Both the experimental and calculation procedures were verified against known measurements of vapor pressures for n-alcohols.25 It has been confirmed that vapor pressures derived from the transpiration method were generally reliable (±1−3%) and their accuracy was primarily governed by the reproducibility of the GC analysis. The effect of temperature fluctuations (±0.1 K) and carrier gas volume measurements (±0.001 dm3) were examined as contributors to the combined uncertainty of the vapor pressure but were calculated to be negligible. The estimated uncertainty in the pressure measurements for N-alkoxy substituted imidazoles and benzimidazoles was at the level of 0.5 to 3%. In order to assess the uncertainty in the enthalpies of vaporization, the experimental data were fit to the linear equation ln(pi) = f (T−1) using the method of least-squares. Uncertainty in the vaporization enthalpy was assumed to be identical to the deviation of experimental ln(pi) values calculated from the linear correlation. 3.2. Correlation of Vaporization Enthalpies. Vapor pressures of N-alkoxy substituted imidazoles and benzimidazoles (see Figure 1) have been measured for the first time. Their enthalpies of vaporization Δgl Hm have been determined from the temperature dependence of the vapor pressure according to eq 3. All results are listed in Table 1. A valuable check for internal consistency of the experimental vaporization enthalpies is a comparison of the Δgl Hm (298.15 K) values of N-alkoxy substituted imidazoles and benzimidazoles listed in the Table 2 with the Δgl Hm (298.15 K) values of ethylene glycol dimethyl ether (36.8 kJ·mol−1),40 diethylene glycol dimethyl ether (51.2 kJ·mol−1),41 and triethylene glycol dimethyl ether (64.7 kJ·mol−1).42 This comparison is presented in Figure 2. As illustrated by Figure 2, vaporization enthalpies of these systematic N-alkoxy substituted imidazoles and benzimidazoles are compared against analogous aliphatic ethers with

(1)

where R is the universal gas constant; mi is the mass of the transported compound, Mi is the molar mass of the compound, and Vi, its volume contribution to the gaseous phase. VN2 is the volume of the carrier gas, and Ta is the temperature of the soap bubble meter. The volume of the carrier gas VN2 was determined from the flow rate and the time measurement. 2.3. COSMOTherm Simulations. Using the same approach as our previous work,27 optimized structures of each alkoxy-functionalized imidazole and benzimidazole compound were developed using TURBOMOLE,28 applying the 15518

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Table 1. Experimental Vapor Pressures and Enthalpies of Vaporization for N-Alkoxy Substituted Imidazoles and Benzimidazoles Measured by the Transpiration Method Ta (K)

mb (mg)

V(N2)c (dm3)

flow of N2 (dm3·h−1)

1-(2-methoxyethyl)-imidazole 321.62 94672.19 78.1 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ R (R ·T /K) R ⎝ 298.15 ⎠ 312.0 315.0 317.0 318.0 321.0 321.0 323.9 326.8 328.5 330.8 334.8 338.4 338.8 341.7 341.8 344.4 346.7 350.0 352.7 352.7 356.0 359.9 363.9 368.0 371.9 375.9

pd (Pa)

(PEG1-imidazole);Δgl Hm

(pexp − pcalc) (Pa)

σe (%)

(298.15 K) = (71.39 ± 0.30) kJ·mol

6.03 4.82 5.96 5.22 4.82 7.78 1.32 1.98 8.96 4.34 4.82 9.75 1.90 3.00 12.10 4.32 4.80 12.90 3.20 4.80 15.26 2.40 4.80 18.89 1.77 4.82 22.49 1.98 1.98 28.18 0.660 1.98 36.17 0.938 2.17 44.31 0.792 1.98 44.78 0.757 2.07 56.39 0.757 2.07 59.94 0.613 2.17 71.73 0.551 2.07 86.15 0.541 2.17 101.17 0.594 1.98 123.49 0.495 1.98 120.80 0.253 1.01 152.34 0.253 1.01 196.89 0.253 1.01 258.87 0.253 1.01 323.54 0.253 1.01 401.64 0.253 1.01 514.33 1-(2-(2-methoxyethoxy)ethyl)-imidazole (PEG2-imidazole); Δgl Hm (298.15 K) 360.53 114713.4 102.0 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R (R ·T /K) R

1.51 1.67 0.49 1.74 0.95 2.29 2.01 1.87 1.67 2.30 0.98 1.75 1.46 1.76 1.87 1.85 1.95 2.30 3.02 2.46 1.62 2.09 2.75 3.44 4.27 5.47

0.15 0.08 0.28 0.09 0.06 0.09 0.10 0.10 −0.27 0.11 0.56 0.11 −0.36 0.13 −0.82 0.14 0.03 0.16 1.41 0.19 0.26 0.23 −2.53 0.27 −3.32 0.27 −2.75 0.33 0.59 0.35 0.46 0.41 2.62 0.48 −2.64 0.56 −0.40 0.67 −3.09 0.65 −0.49 0.81 0.80 1.03 8.63 1.34 6.04 1.67 2.91 2.06 15.38 2.62 = (84.30 ± 0.52) kJ·mol−1

1.49 5.93 4.45 3.68 −0.11 2.02 9.41 4.45 3.16 0.05 2.60 6.52 4.45 5.87 0.06 1.57 4.82 4.45 4.78 0.06 2.45 5.19 4.45 6.82 0.02 1.84 2.96 4.45 8.97 −0.07 3.35 6.08 4.45 8.12 0.06 2.99 4.08 4.45 10.79 −0.05 1.94 2.22 4.45 12.58 0.20 2.98 2.96 4.45 14.79 −0.40 2.41 2.15 4.45 16.52 0.07 2.77 2.15 4.45 18.96 −0.42 3.12 1.85 4.45 24.79 0.88 1.79 0.93 3.28 28.30 −0.88 2.29 1.11 4.45 30.29 1.11 2.68 1.11 4.45 35.44 −0.03 2.54 1.56 4.45 24.05 −0.70 4.37 1.63 4.45 39.40 0.48 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-imidazole (PEG3-imidazole);Δgl Hm (298.15 K) =(97.43 ± 0.25) 404.51 135201.21 126.7 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R (R ·T /K) R 342.0 339.6 347.3 344.7 349.3 353.0 351.5 355.4 357.2 360.0 361.1 363.4 366.4 369.3 369.3 372.2 366.9 373.6

323.2 328.2 333.2 338.3 348.2

0.47 0.65 0.42 0.66 0.84

98.99 81.56 30.91 28.76 14.42

5.15 5.15 5.15 5.15 5.15

0.06 0.09 0.16 0.26 0.66

15519

Δgl Hm(kJ·mol−1)

−1

0.00 0.00 0.00 0.00 0.00

70.31 70.07 69.92 69.84 69.60 69.61 69.38 69.15 69.01 68.83 68.53 68.24 68.21 67.98 67.98 67.77 67.59 67.34 67.13 67.13 66.87 66.56 66.25 65.94 65.63 65.32

0.07 0.07 0.08 0.07 0.08 0.09 0.09 0.10 0.11 0.12 0.13 0.14 0.17 0.19 0.20 0.23 0.17 0.25 kJ·mol−1

79.83 80.08 79.29 79.56 79.09 78.71 78.87 78.47 78.28 78.00 77.89 77.65 77.35 77.05 77.05 76.75 77.29 76.61

0.05 0.05 0.05 0.05 0.05

94.26 93.62 92.99 92.34 91.09

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Table 1. continued Ta (K)

mb (mg)

353.2 358.2 363.2 368.2 373.9 376.3 378.0 383.1 388.2

0.70 0.71 0.72 0.79 3.32 0.86 2.77 6.08 2.55

V(N2)c (dm3)

flow of N2 (dm3·h−1)

7.73 5.15 3.52 2.58 6.85 1.55 4.11 6.40 1.83 1-(2-methoxyethyl)-benzimidazole 369.39 121797.95 100.7 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R (R ·T /K) R

pd (Pa)

(pexp − pcalc) (Pa)

5.15 1.04 5.15 1.57 5.15 2.34 5.15 3.51 5.48 5.51 5.15 6.35 5.48 7.66 5.48 10.81 5.48 15.85 (PEG1-benzimidazole);Δgl Hm (298.15 K) =

325.6 329.6 332.3 333.6 337.1 341.4 343.2 343.2 344.7 348.3 351.6 355.2 355.3 358.2 359.1 360.6 363.1 363.1 363.2 364.1 366.2 366.6 367.1 367.1 370.1 370.1 372.1 373.1 374.1 376.1 376.1

0.34 23.43 2.00 0.20 0.47 23.12 4.97 0.28 0.52 18.32 5.00 0.39 0.35 11.93 4.97 0.41 0.35 8.29 4.97 0.58 0.51 8.03 4.82 0.89 1.30 16.57 5.00 1.09 0.99 13.32 5.00 1.04 0.52 6.02 4.82 1.20 0.29 2.44 4.88 1.68 0.58 3.85 4.82 2.09 0.73 3.79 4.84 2.66 0.78 3.91 5.00 2.79 0.47 1.78 3.06 3.65 0.60 2.18 4.84 3.84 0.78 2.50 5.00 4.35 0.60 1.50 5.00 5.61 0.63 1.58 5.00 5.57 1.93 4.84 4.84 5.56 0.73 1.73 3.06 5.86 0.67 1.33 5.00 6.99 0.63 1.25 5.00 7.06 0.90 1.60 3.01 7.79 0.91 1.69 4.84 7.52 0.71 1.07 3.06 9.23 1.26 1.94 4.84 9.04 0.66 0.85 3.01 10.86 1.06 1.25 5.00 11.80 1.41 1.61 4.84 12.20 0.70 0.68 1.57 14.35 1.34 1.25 5.00 14.96 1-(2-(2-methoxyethoxy)ethyl)-benzimidazole (PEG2-benzimidazole); Δgl Hm (298.15 394.26 134645.24 125.1 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R (R ·T /K) R 328.3 333.2 337.3 343.2 348.2 353.2 363.2 363.2 368.2 368.2 372.2 373.2 375.2 376.2

0.29 0.45 0.53 0.40 0.38 0.49 0.62 0.41 0.79 0.48 0.47 0.55 0.46 0.45

97.43 88.91 71.04 30.56 18.52 15.28 8.34 5.56 7.04 4.17 2.96 3.24 2.32 2.08

5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56 5.56

0.03 0.06 0.09 0.15 0.24 0.37 0.84 0.85 1.28 1.31 1.79 1.91 2.25 2.45 15520

σe (%)

Δgl Hm(kJ·mol−1)

0.01 0.06 −0.01 0.06 −0.04 0.06 −0.04 0.07 0.00 0.08 −0.25 0.08 0.18 0.09 0.00 0.10 0.41 0.13 (91.77 ± 0.39) kJ·mol−1

90.46 89.82 89.19 88.56 87.83 87.53 87.31 86.67 86.02

0.01 −0.01 0.01 −0.01 −0.01 0.01 0.05 0.00 0.02 0.06 −0.05 −0.23 −0.12 −0.04 −0.14 −0.15 0.17 0.10 0.09 −0.04 0.09 −0.09 0.37 0.09 −0.05 −0.26 0.09 0.21 −0.24 0.04 0.62 K) = (97.35 ± 0.33)

0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.08 0.08 0.08 0.08 0.08 0.09 0.09 0.09 0.10 0.10 0.10 0.11 0.11 0.12 0.12 kJ·mol−1

89.02 88.61 88.34 88.21 87.86 87.42 87.24 87.24 87.09 86.73 86.40 86.03 86.02 85.73 85.63 85.48 85.24 85.23 85.23 85.13 84.93 84.88 84.83 84.83 84.53 84.53 84.33 84.22 84.13 83.93 83.93

0.00 0.00 0.00 0.00 0.00 0.00 −0.03 −0.02 −0.02 0.01 0.03 0.01 0.04 0.07

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.06 0.06 0.06 0.06 0.06

93.58 92.97 92.46 91.72 91.09 90.47 89.22 89.21 88.59 88.59 88.09 87.96 87.71 87.59

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Table 1. continued Ta (K)

mb (mg)

V(N2)c (dm3)

flow of N2 (dm3·h−1)

1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-benzimidazole 413.62 146097.99 149.4 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R (R ·T /K) R 353.4 358.5 368.8 372.9 378.0 383.1 398.5 403.5 408.7

0.31 0.52 0.56 0.12 0.49 0.59 0.54 1.10 0.50

59.05 63.95 29.34 4.39 12.44 10.24 3.29 4.76 1.54

pd (Pa)

(pexp − pcalc) (Pa)

(PEG3-benzimidazole);Δgl Hm

4.39 4.39 4.39 4.39 4.39 4.39 4.39 4.39 4.39

0.05 0.08 0.17 0.25 0.36 0.53 1.51 2.13 3.00

Δgl Hm(kJ·mol−1)

σe (%) −1

(298.15 K) =(101.55 ± 0.37) kJ·mol

0.00 0.00 0.00 0.01 0.00 0.00 −0.03 −0.01 0.04

0.05 0.05 0.05 0.05 0.05 0.05 0.06 0.06 0.07

93.31 92.55 91.01 90.39 89.63 88.87 86.57 85.82 85.05

a Saturation temperature. bMass of transferred sample condensed at T = 243 K. cVolume of nitrogen used to transfer mass m of sample. dVapor pressure at temperature T, calculated from m and the residual vapor pressure at the cooling temperature T = 243 K. eThe uncertainty in the pressure measurements estimated by the expression σ(p/Pa)) = 0.005(p/Pa) + 0.05 recommended in ref 44.

Table 2. Compilation of Data on Enthalpies of Vaporization Δgl Hm at Tav and at 298.15 K for N-Alkoxy Substituted Imidazoles and Benzimidazoles compound

T-range (K)

1-(2-methoxyethyl)-imidazole 1-(2-(2-methoxyethoxy)ethyl)-imidazole 1-(2-(2-(2-methoxyethoxy)ethoxy)-ethyl)-imidazole 1-(2-methoxyethyl)-benzimidazole 1-(2-(2-methoxyethoxy)ethyl)-benzimidazole 1-(2-(2-(2-methoxyethoxy)ethoxy)-ethyl)-benzimidazole

312.0−375.9 339.6−373.6 323.2−388.2 325.6−376.1 328.3−376.2 353.4−408.7

Δgl Hm (Tav)(kJ·mol−1)

Clp(−Δgl Cp)a(J·mol)−1·K−1

± ± ± ± ± ±

259.5(78.1) 353.1(102.4) 446.7(126.7) 346.7(100.7) 440.3(125.1) 533.9(149.4)

68.1 78.4 90.4 86.4 90.6 89.3

0.3 0.5 0.3 0.4 0.3 0.4

Δgl Hm (298.15 K)b(kJ·mol−1) 71.4 84.3 97.4 91.8.4 97.4 101.6

± ± ± ± ± ±

0.3 0.5 0.3 0.4 0.3 0.4

Δgl Cp is the molar heat capacity difference between liquid and gaseous phases (see text); Δlp is the molar heat capacities of liquid at constant pressure. bDerived using eqs 2 and 3 with the molar heat capacity difference Δgl Cp. a

alkylimidazoles with the same number of atoms in the substituent group (i.e., chain length of seven atoms heptylimidazole and PEG2-imidazole). Figures 3 and 4 present data for the available correlations for imidazoles and benzimidazoles based on this work and previously published data.15 As illustrated by Figures 3 and 4, the replacement of every third carbon atom in the main chain with an ether oxygen results in vapor pressure reductions of ∼75% at the lower end of the temperature range, while the difference is still ∼50% at

Figure 2. Comparison of vaporization enthalpies Δgl Hm (298.15 K) in kJ·mol−1 of N-alkoxy substituted imidazoles and benzimidazoles derived in this work with those for the aliphatic ethers available in the literature (see text).

the same incrementally varied molecular structures. The data for the two classes of compounds are in excellent agreement with a correlation coefficient of 0.996, and confirming the reliability of the experimental results measured in this work. 3.3. Comparison Between Experimentally Determined Vapor Pressures of Alkoxy- and Alkyl-Functionalized Imidazoles and Benzimidazoles. Another important correlation in the study of these alkoxy-functionalized imidazoles is the change in vapor pressure relative to

Figure 3. Comparison of vapor pressures for analogues with seven and ten atoms in the substituent chain: heptylimidazole and PEG2imidazole (red); decylimidazole and PEG3-imidazole (blue). 15521

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Figure 6. Comparison of experimental data to COSMOTherm simulation for each of the N-alkoxy benzimidazole compounds.

Figure 4. Comparison of vapor pressures for analogues with four atoms in the substituent chain: butylbenzimidazole and PEG1benzimidazole.

In this work, we have also calculated vaporization enthalpies of alkoxy-functionalized imidazoles and benzimidazoles using the COSMOTherm, and the results are collected in Table 3.

the high end of the range. The reduction in vapor pressure is likely due to increased intra- and intermolecular interactions between the polar ethers groups and the imidazole ring. Such interactions are not available in the alkyl-functionalized analogues. 3.4. Comparison to COSMOTherm Calculation of Vapor Pressure. As COSMOTherm is a rapid and powerful tool for estimating the thermophysical properties of compounds, the correlation of experimental results with COSMOTherm simulations can help guide the design of new molecules as well as provide estimated data for engineering processes simulations. Plots of the experimental data (points) and the COSMOTherm calculation of vapor pressure (lines) are shown for alkoxy-functionalized imidazoles and benzimidazoles in Figures 5 and 6, respectively.

Table 3. Comparison of Experimental Values and COSMOTherm Calculations for Enthalpy of Vaporization at Lowest Temperature Examined compound 1-(2-methoxyethyl)imidazole 1-(2-(2-methoxyethoxy) ethyl)-imidazole 1-(2-(2-(2-methoxyethoxy) ethoxy)-ethyl)-imidazole 1-(2-methoxyethyl)benzimidazole 1-(2-(2-methoxyethoxy) ethyl)-benzimidazole 1-(2-(2-(2-methoxyethoxy) ethoxy)-ethyl)benzimidazole

T (K)

Δgl Hm exp.

Δgl Hm COSMO

Δ (exp.− COSMO)

312

70.31

62.75

7.56

340

80.08

74.81

5.27

6.58

323

94.26

90.39

3.87

4.10

326

89.02

74.94

328

93.58

88.98

4.60

4.91

353

93.31

100.28

−6.97

−7.47

14.1

% difference 10.8

15.8

Interestingly, COSMOtherm tends to consistently underestimate heat of vaporization in comparison to the experimental data except for PEG3-benzimidazole (Table 3). In contrast, in our prior work with chlorobenzenes and chlorophenols, we observed that COSMOTherm can either overestimate or underestimate the enthalpy of vaporization.43 However, the differences between the experimentally determined values and the computational values for those compounds were on average smaller than for these N-alkoxy imidazole and benzimidazole compounds.43 While the compounds examined in that work and the imidazole/benzimidazole compounds share commonalities in that both are based on ring systems with π bonds, the method of structural variation applied is quite different. The earlier work examined substitution patterns on the ring itself, while here we have incrementally extended the length of the substituent. Thus, for this class of compounds, we observe better agreement between vapor pressure data when the length of the substituent is small, yet the quality of the prediction for enthalpy of vaporization tends to improve when the alkoxy chain is extended. As a conclusion, in spite of the significant differences with the experiment the COSMOTherm calculations on the alkoxy-functionalized imidazoles and benzimida-

Figure 5. Comparison of experimental data to COSMOTherm simulation for each of the N-alkoxy imidazole compounds.

As can be seen from the plots, the quality of the COSMOTherm calculation is most accurate for the smallest members of the series, and the magnitude of the deviation becomes larger as the number of ether groups is extended. With the exception of PEG1-benzimidazole, COSMOTherm consistently underpredicts vapor pressure by approximately 5− 15%. 15522

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Industrial & Engineering Chemistry Research zoles are able to provide the data of technical quality, sufficient for the practical application in chemical engineering and as initial estimate for design purposes. While the design of new solvents, especially ILs, for CO2 capture and other separations applications has largely focused on the use of molecules with alkyl substituents, it appears that alkoxy-functionalized solvents are the much more promising candidates when reductions in vapor pressure are sought. We have also observed better performances for CO2 separations using alkoxy-functionalized ILs and we are currently studying the performances of these alkoxyimidazoles for CO2/CH4 separations similar to our prior work with alkylimidazoles.13

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR data for alkoxy-functionalized imidazoles and benzimidazoles. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work has been supported by the German Science Foundation (DFG) in the frame of the priority program SPP 1191 “Ionic Liquids”. Additional support for this work provided by ION Engineering, LLC; United States Department of Energy − National Energy Technology Laboratory (DEFE00005799); and the National Science Foundation Research Experiences for Undergraduates Program (EEC-1062705) is gratefully acknowledged. The authors also wish to thank Ken Belmore of the University of Alabama for acquisition of 1H NMR spectra.

4. CONCLUSIONS The dependence of vapor pressure on temperature and the number of ether repeat units has been characterized for three imidazole and three benzimidazole derivatives. For both sets of compounds, vapor pressure was observed to decrease with increasing temperature and increasing the number of ether linkages. An internal consistency check with analogous glymes validated the experimental data. COSMOTherm was employed to simulate the vapor pressure behavior of these compounds over the experimental temperature range. It was found that COSMO calculations tended to underpredict enthalpy of vaporization by ∼5−15%, while also tending to underpredict vapor pressure. The best agreements between experimental and simulated vapor pressures were observed for the smallest members of each series (i.e., PEG1). Comparisons between alkoxy- and alkyl-functionalized imidazoles/benzimidazoles reveals that the inclusion of ethers has a large impact on reducing vapor pressure for structural analogues, with 50−75% reductions observed at a given temperature. These results can provide useful guidelines for the design of solvents for applications such as CO2 capture, as ether groups can serve to reduce overall solvent vapor pressure while simultaneously improving CO2 selectivity relative to alkyl-based analogues.2,8,12

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49-381-498-6508; fax: +49-381-498-6524); e-mail: [email protected] (S.P.V.); Phone: +1-205-3486836; fax: +1-205-348-7558); e-mail: [email protected] (J.E.B.). Present Addresses

‡ Faculty of Interdisciplinary Research, Department “Science and Technology of Life, Light and Matter”, University of Rostock, Rostock, Germany. § Home institution: Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States 19104−6315.

Notes

The authors declare no competing financial interest. † On leave from Mogilev State University of Food Technologies, Mogilev, Belarus. 15523

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