Theoretical Screening of Ionic Liquid Solvents for Carbon Capture

Abstract

[a] With global warming established as a critical problem, it is extremely important to cut down on emissions of CO 2 into the atmosphere. Prior to sequestration, it is necessary to separate the CO 2 from its emission source, for example, flue gas in a coal-fired power plant. The few coal plants with commercial CO 2 capture capability all use processes based on chemical absorption with a monoethanolamine (MEA) solvent. Unfortunately, MEA is a nonselective solvent prone to degradation and equipment corrosion, and mandates large equipment sizes, thereby increasing costs.Ionic liquids (ILs) [1] constitute an alternative solvent system and offer distinct advantages over traditional solvents such as MEA, some of which include: (1) high chemical stability; (2) low corrosion; (3) almost zero vapor pressure (i.e., "green"); (4) supportable on membranes; [2] and (5) a huge library of anion and cation choices, which can be potentially optimized for CO 2 solubility and selectivity.Over the last few years several ILs have been experimentally demonstrated to be efficient solvents for CO 2 . [3][4][5][6][7][8] This data provides useful trends that can be used to optimize the choice of ILs for CO 2 capture. However, each new experiment costs time and money and is often hindered because a specific IL may not be readily available. Thus, it is highly desirable to have a computational/theoretical tool that can quickly and accurately compute CO 2 solubility in any solvent (as a function of pressure and temperature). Atomic-level simulations, either molecular dynamics or binding-energy calculations, can provide useful insights into the interactions of CO 2 with the cation and the anion. [9][10][11] However, an accurate computation of solubility in such complex fluids faces many challenges, including accurate force-field development, clever Monte Carlo moves, and very long simulation times required for good statistical averaging.For a fast exploration, design, and screening of effective solvents it is highly desirable to adopt a general-purpose thermodynamic approach that computes the chemical potential of a solute (CO 2 in this case) in any solvent at an arbitrary dilution. A widely used method is the "conductor-like screening model for real solvents" (COSMO-RS), [12,13] which uses the statistical distribution (histogram) of the surface charge density of individual molecules, called the s profile, to derive an expression for the ensemble-averaged Gibbs free energy of an interacting system of molecules (solute + solvent at specified mole fractions) in the condensed (liquid) phase. From this a pseudochemical potential (m) of each species (i.e., the Gibbs free energy per molecule without the mixing entropy contribution) is obtained. If the pseudo-chemical potential of a solute molecule at a temperature T in a solution containing a mole-fraction x of the solute is m solution A C H T U N G T R E N N U N G (x, T), and that in the solute's own liquid environment is m self (T), then assuming ideal mixing law we have under equilibrium (k...

New materials for methane capture from dilute and medium-concentration sources

“…When the solute is dissolving from the gas phase, one needs to relate the quantity m * self to the chemical potential in the gas phase, which can be expressed as a function of the gas fugacity 15 . However, because our liquid exploration involved a fixed pressure (1 bar) and a fixed temperature (300 K), we found it more convenient to use m * self as a fitting parameter so as to match the experimental solubility of all relevant gases, that is, CH 4 (refs 9,16), N 2 (refs 17,18) 4 , N 2 and CO 2, respectively, to be used with COSMO-RS-computed m solvent under the parameter settings described in the Methods section.…”

“…The 10 ILs screened in Figures 1 and 2 (filled symbols) include cationic classes imidazolium, ammonium, phosphonium and pyridinium, and anions BF 4 , PF 6 and Tf 2 N (refs 15,20). We find that the highest CH 4 solubility in these classes of ILs occurs in the [ammonium][Tf 2 N] systems with a Henry's constant of 0.005 mol kg À 1 bar À 1 or less, that is, almost an order of magnitude smaller than that in pentane.…”

COSMO-RS as a tool for property prediction of IL mixtures—A review

“…In a recent COSMO-RS screening study of Henry's constants of CO 2 in 408 ILs, Zhang et al reported an improved CO 2 capture capability for ILs that are based on the [(C 2 F 5 ) 3 PF 3 ] anion [40]. A new model that allows for a good quantitative prediction of CO 2 solubilities in ILs has been proposed by Maiti [41]. The model is a combination of COSMO-RS with an equation of state (EoS) and needs two additional, solvent and pressure independent, fit parameters.…”

Design and screening of ionic liquids for C2H2/C2H4 separation by COSMO‐RS and experiments

“…Recently, it has been used to predict the activity coefficients, 26,27 gas solubilities [28][29][30] and separation selectivity of various gases in ILs, such as CO 2 /N 2 , 31 C 2 H 4 /C 2 H 6 32 and H 2 S/CO 2 /CH 4 /C 2 H 6, 33 but there are no relevant studies on C 2 H 2 /C 2 H 4 separation.…”