Article pubs.acs.org/IECR
COSMOTherm as a Tool for Estimating the Thermophysical Properties of Alkylimidazoles as Solvents for CO2 Separations Jason E. Bara,†,* Joshua D. Moon,†,‡ Kristofer R. Reclusado,‡,§ and John W. Whitley† †
Department of Chemical and Biological Engineering and ‡NSF-REU Site: Engineering Solutions for Clean Energy Generation, Storage and Consumption, Department of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, Alabama 35487-0203, United States § Department of Chemical Engineering, Villanova University, Villanova, Pennsylvania 19085, United States ABSTRACT: The imidazole core is a versatile building block for a variety of materials including pharmaceuticals, ionic liquids, and polymers. While the thermophysical properties of ionic liquids and their application as solvents for CO2 separations have been the focus of a broad research effort for more than a decade, studies on properties and applications of imidazoles (from which many ILs are synthesized) have begun only recently. Similar to ILs, the physical and chemical properties of imidazoles can also be tuned via molecular design strategies, resulting in a vast array of possible structures. This immense experimental space necessitates the use of a rapid means of predicting thermophysical properties to guide the design of future solvents and gain fundamental insights into structure−property relationships. To this end, we have employed COSMOTherm as a means of rapidly screening properties of alkylimidazoles relevant to CO2 separation processes and comparing these calculated values to experimental results. Results indicate that COSMOTherm is an effective and accurate tool for predicting the properties of smaller alkylimidazoles (e.g., 1-methylimidazole), but in many cases is much less accurate for the larger alkylimidazoles.
1. INTRODUCTION Interest in novel solvents for gas treating applications such as CO2 capture and natural gas sweetening has been a growing research area due to emphasis on reducing global emissions of greenhouse gases (GHGs) and the desire for clean energy sources.1−3 Among the nonaqueous solvents proposed, ionic liquids (ILs) have been at the forefront due to their tunable structures and negligible volatility,4−7 the latter of which can be highly advantageous in minimizing solvent losses and reducing the need for recovery equipment such as condensers.8,9 However, when compared to conventional organic solvents already commercialized for industrial gas treating,10−12 ILs may possess some disadvantages such as increased viscosity, multiple degradation pathways, and higher solvent costs.13−15 Neutral nonaqueous solvents and their combinations with ILs to absorb CO2 through physical and/or chemical mechanisms have also been explored.16−28 Research in ILs as solvents for CO2 removal and other diverse applications has resulted in the characterization of perhaps thousands of compounds and the compilation of large data sets of thermophysical properties with associated mathematical models.15,29−32 Because the number of potential ILs is so vast (∼106 “simple” ILs),33 there is a significant need to employ computational methods to screen and predict IL properties rather than empirically examine every individual IL.34−38 The COSMOTherm software package has become a widely used framework that has been employed by our group and others as a means of rapidly examining the thermophysical properties of existing and theoretical ILs. For many ILs, COSMO calculations have been shown to provide highly accurate estimations of density and molar volume (±1%) and estimations (±50%) for CO2 solubility in the form of Henry’s Constants.39−43 COSMOTherm has also been similarly applied © 2013 American Chemical Society
to studies of small organic molecules and oligomers for CO2 absorption.26,44−46 Despite the large efforts in the experimental and computational studies of ILs for CO2 removal, there has been little work devoted to imidazoles, the molecules from which imidazoliumbased ILs are synthesized (Figure 1). We have recently begun
Figure 1. General structure of (a) N-functionalized imidazole and (b) 1,3-difunctionalized imidazolium salt (i.e., imidazolium-based IL).
to study imidazoles as a novel class of solvents for CO2 capture.13,18,47−49 The versatility of the imidazole platform and its amenability to synthetic tailoring is one of the key factors that give rise to the tunable properties of ILs.50,51 We have observed that the properties of imidazoles such as density, viscosity, and gas solubility are strongly dependent on the nature of the substituent appended to the ring.13,18,47−49 Other groups have demonstrated the systematic influence of imidazole structure on properties such as vapor pressure and pKa.52−57 Although much of our prior work has focused exclusively 1-nalkylimidazoles (Figure 1a, R = n-alkyl), it is apparent that there are a large number of potential imidazole-based solvents to study, especially when functionalization at one or more of the three carbon positions in the imidazole ring is considered.57 Received: Revised: Accepted: Published: 5498
January 8, 2013 March 15, 2013 March 19, 2013 March 19, 2013 dx.doi.org/10.1021/ie400094h | Ind. Eng. Chem. Res. 2013, 52, 5498−5506
Industrial & Engineering Chemistry Research
Article
chain”) conformers were considered for each compound. C30 features a new TZVPD-FINE level that could improve the agreement of calculations to experimental data (at the expense of potentially longer calculation times). While we did not choose to explore this option at this time, we will consider it in future works.
Thus, to identify additional imidazole-based solvents with desirable combinations of physical and chemical properties, we see a need to apply COSMOTherm calculations to the existing data for 1-n-alkylimidazoles so as to determine the accuracy and validity of this approach as a means of obtaining design guidelines for solvents for CO2 capture and other engineering applications. Additionally, the structure−property relationships obtained for imidazoles may be potentially correlated to physical and chemical properties of analogous ILs. Herein, we present the results of our studies employing COSMOTherm to simulate some of the thermophysical properties of 1-nalkylimidazoles relevant to CO2 capture and other gas processing applications and provide comparisons to published experimental data.
3. RESULTS AND DISCUSSION 3.1. Density and Viscosity. Tables 1 and 2 present our prior experimental results for density and viscosity at 25 °C alongside values calculated by COSMOTherm.49 The percent difference (%d) between the experimental value (expt and the COSMO prediction is calculated according to eq 1. %d = 100
2. COMPUTATIONAL METHODS A series of 11 1-n-alkylimidazoles (Figure 2) was selected for use within COSMOTherm (COSMOlogic GmbH, Leverkusen,
expt − COSMO expt
(1)
For density (ρ), the results for each species were found to be within ±1% with the exception of 1-methylimidazole and 1ethylimidazole, which were underestimated by COSMO on the order of 1.1−1.4%. Both C-21 and C-30 tend to underestimate the densities of smaller members of this group and overestimate the larger members. Overall, COSMO calculations do appear to provide a reliable prediction of density for 1-n-alkylimidazoles at 25 °C, with an average absolute percent deviation (AAPD) of 0.557% from C-21 and 0.565% from C-30. This level of accuracy is consistent with the quality of results obtained from molecular dynamics simulations for alkylimidazoles48 and for ILs using COSMOTherm.38,39 The uncertainty in the density meter used for the experimental measurements was ± 0.1 kg m−3 at each temperature. However, the viscosity (μ) values obtained from COSMO do not result in such uniform agreement. For 1-octylimidazole and smaller, the absolute difference between the experimental and simulated values is less than 0.840 cP for each species, although the relative difference to the experimental value can be large as 22.9%, and with the exception of 1-octylimidazole (which is slightly underestimated), COSMO tends to slightly overestimate viscosities within this group. However, for molecules larger than 1-octylimidazole, much larger disagreements are observed in both the absolute and relative differences. For 1octylimidazole and larger, COSMO consistently overestimates viscosities and with increasing error as the chain length is extended. C-21 and C-30 are capable of estimating the viscosities of 1-octylimidazole and smaller to within 1 cP, which provides a very useful tool for engineering approximations. However, when the entire range of compounds is
Figure 2. Structures of 1-n-alkylimidazoles considered in this manuscript.
Germany) based on the availability of comparative experimental data.13,49,55−57 This study was initiated using COSMOTherm version C21_0111_a, however during the course of our studies, COSMOTherm version C30_1301 was released. Thus we will also compare the results obtained with each version, herein referred to as C-21 and C-30, respectively. Densities, viscosities, molar volumes, COSMO volumes, and pKa values were obtained for each molecule at 298.15 K using the respective modules within COSMOTherm. For selected molecules, vapor pressures and solubility of CO2 with varying temperature were also calculated. COSMO files were developed by our group with optimized structures of 1-n-alkylimidazoles and their respective protonated forms (for pKa calculations) developed with TURBOMOLE,58 using the triple-ζ valence potential (TZVP) basis set59 with the Becke and Perdew (bp)60,61 functional at the density functional theory (DFT) level. All COSMO calculations were performed at the TZVP level of theory, consistent with our previous work62 and other published works on ILs that have utilized COSMOtherm.35,40−42,63 Only the fully extended (i.e., “straight
Table 1. Comparison of Experimental Data with COSMO Calculations for Density (ρ) of 1-n-Alkylimidazoles at 25 °C. Experimental Data from Shannon & Bara.49
a
compound
MW (g mol−1)
ρ expt (kg m−3)
ρ C-21 (kg m−3)
% d ρ C-21
ρ C-30 (kg m−3)
% d ρ C-30
1-methylimidazole 1-ethylimidazole 1-propylimidazole 1-butylimidazole 1-pentylimidazole 1-hexylimidazole 1-octylimidazole 1-decylimidazole 1-dodecylimidazole 1-tetradecylimidazolea
82.104 96.131 110.158 124.184 138.211 152.238 180.291 208.344 236.398 264.451
1030.85 990.15 968.75 947.40 935.00 926.10 907.90 897.95 891.25 886.00
1016.93 978.62 963.18 948.81 934.52 929.51 915.37 904.62 892.47 884.18
1.344 1.159 0.572 −0.148 0.051 −0.367 −0.820 −0.740 −0.137 0.205
1016.3 978.07 962.68 948.38 934.16 929.20 915.16 904.48 892.40 884.16
1.411 1.220 0.627 −0.103 0.090 −0.335 −0.800 −0.728 −0.129 0.208
Properties of supercooled liquid at 25 °C. 5499
dx.doi.org/10.1021/ie400094h | Ind. Eng. Chem. Res. 2013, 52, 5498−5506
Industrial & Engineering Chemistry Research
Article
Table 2. Comparison of Experimental Data with COSMO Calculations for Viscosity (μ) of 1-n-Alkylimidazoles at 25 °C. Experimental Data from Shannon & Bara.49
a
compound
μ expt (cP)
μ C-21 (cP)
% d μ C-21
μ C-30 (cP)
% d μ C-30
1-methylimidazole 1-ethylimidazole 1-propylimidazole 1-butylimidazole 1-pentylimidazole 1-hexylimidazole 1-octylimidazole 1-decylimidazole 1-dodecylimidazole 1-tetradecylimidazolea
1.77 2.04 2.81 3.47 4.49 5.07 7.77 10.8 15.0 20.3
1.49 1.75 2.17 2.81 3.65 4.78 8.18 14.2 24.4 42.6
15.9 14.4 22.9 19.1 18.6 5.66 −5.27 −31.1 −63.0 −110
1.37 1.60 1.99 2.57 3.35 4.38 7.51 13.0 22.6 39.5
22.8 21.5 29.3 25.8 25.4 13.6 3.39 −20.5 −50.4 −94.4
Properties of supercooled liquid at 25 °C.
considered, an AAPD of 23.8% is observed for C-21 and 30.7% for C-30. The uncertainty in the viscometer used for the experimental measurements was ±1% at each temperature. 3.2. Vapor Pressure and Enthalpy of Vaporization. Vapor pressures (p*) and enthalpies of vaporization (ΔHvap) of 1-n-alkylimidazoles and other imidazole derivatives have recently been determined in a series of works by Verevkin and co-workers. 52−54,56 Both p* and ΔH vap for 1-nalkylimidazoles were observed to be strongly correlated to the length of the alkyl substituent, that is, as the chain becomes larger, vapor pressure is suppressed while enthalpy of vaporization increases. As many current and novel physical and chemical solvents for CO2 capture have very low volatilities (or in the case of ILs, virtually no volatility) when compared to conventional organic solvents and water, precise measurements of their vapor pressure can prove to be challenging. As vapor pressure is an important consideration for gas processing applications,8−11 and is related to solvent loss and recovery, there is an inherent need to understand and model solvent volatility even though experimental data may not be available. COSMOTherm presents a potentially rapid and powerful methodology to meet this need. Figure 3 panels a and b compare the experimental data of Verevkin and co-workers55,56 with COSMO predictions of vapor pressure for six different 1-nalkylimidazole compounds, using C-21 and C-30, respectively. The maximum uncertainty in the experimental vapor pressure data was ∼3% of the measured value, and typically
