Enhanced Gas Absorption in the Ionic Liquid 1‑<i>n</i>‑Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide ([hmim][Tf₂N]) Confined in Silica Slit Pores: A Molecular Simulation Study

Two-dimensional <i>NP_xyT</i> and isostress-osmotic (<i>N</i>₂<i>P</i>_<i>xy</i><i>Tf</i>₁) Monte Carlo simulations were used to compute the density and gas absorption properties of the ionic liquid (IL) 1-<i>n</i>-hexyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­amide ([hmim]­[Tf₂N]) confined in silica slit pores (25–45 Å). Self-diffusivity values for both gas and IL were calculated from <i>NVE</i> molecular dynamics simulations using both smooth and atomistic potential models for silica. The simulations showed that the molar volume of [hmim]­[Tf₂N] confined in 25–45-Å silica slit pores is 12–31% larger than that of the bulk IL at 313–573 K and 1 bar. The amounts of CO₂, H₂, and N₂ absorbed in the confined IL are 1.1–3 times larger than those in the bulk IL because of the larger molar volume of the confined IL compared to the bulk IL. The CO₂, N₂, and H₂ molecules are generally absorbed close to the silica wall where the IL density is very low. This arrangement causes the self-diffusivities of these gases in the confined IL to be 2–8 times larger than those in the bulk IL at 298–573 K. The solubilities of water in the confined and bulk ILs are similar, which is likely due to strong water interactions with [hmim]­[Tf₂N] through hydrogen bonding, so that the molar volume of the confined IL plays a less important role in determining the H₂O solubility. Water molecules are largely absorbed in the IL-rich region rather than close to the silica wall. The self-diffusivities of water correlate with those of the confined IL. The confined IL exhibits self-diffusivities larger than those of the bulk IL at lower temperatures, but smaller than those of the bulk IL at higher temperatures. The findings from our simulations are consistent with available experimental data for similar confined IL systems

Molecular Simulations and Experimental Studies of Solubility and Diffusivity for Pure and Mixed Gases of H₂, CO₂, and Ar Absorbed in the Ionic Liquid 1-<i>n</i>-Hexyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)amide ([hmim][Tf₂N])

Classical molecular dynamics and Monte Carlo simulations are used to calculate the self-diffusivity and solubility of pure and mixed CO2, H2, and Ar gases absorbed in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]). Overall, the computed absorption isotherms, Henry’s law constants, and partial molar enthalpies for pure H2 agree well with the experimental data obtained by Maurer et al. [J. Chem. Eng. Data 2006, 51, 1364] and the experimental values determined in this work. However, the agreement is poor between the simulations and the experimental data by Noble et al. [Ind. Eng. Chem. Res. 2008, 47, 3453] and Costa Gomes [J. Chem. Eng. Data 2007, 52, 472] at high temperatures. The computed H2 permeability values are in good agreement with the experimental data at 313 K obtained by Luebke et al. [J. Membr. Sci. 2007, 298, 41; ibid, 2008, 322, 28], but about three times larger than the experimental value at 573 K from the same group. Our computed H2 solubilities using different H2 potential models have similar values and solute polarizations were found to have a negligible effect on the predicted gas solubilities for both the H2 and Ar. The interaction between H2 and the ionic liquid is weak, about three times smaller than between the ionic liquid and Ar and six times smaller than that of CO2 with the ionic liquid, results that are consistent with a decreasing solubility from CO2 to Ar and to H2. The molar volume of the ionic liquid was found to be the determining factor for the H2 solubility. For mixed H2 and Ar gases, the solubilities for both solutes decrease compared to the respective pure gas solubilities. For mixed gases of CO2 and H2, the solubility selectivity of CO2 over H2 decreases from about 30 at 313 K to about 3 at 573 K. For the permeability, the simulated values for CO2 in [hmim][Tf2N] are about 20−60% different than the experimental data by Luebke et al. [J. Membr. Sci. 2008, 322, 28]

Molecular Simulation and Experimental Study of CO₂ Absorption in Ionic Liquid Reverse Micelle

The structure and dynamics for CO₂ absorption in ionic liquid reverse micelle (ILRM) were studied using molecular simulations. The ILRM consisted of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]­[BF₄]) ionic liquid (IL) as the micelle core, the benzylhexadecyldimethylammonium ([BHD]⁺) chloride ([Cl]⁻) was the cationic surfactant, and benzene was used as the continuous solvent phase in this study. The diffusivity values of this ILRM system were also experimentally determined. Simulations indicate that there is ion exchange between the IL anion ([BF₄]⁻) and the surfactant anion ([Cl]⁻). It was also found that the [bmim]­[BF₄] IL exhibits small local density at the interface region between the IL core and the [BHD]⁺ surfactant cation layer, which leads to a smaller density for the [bmim]­[BF₄] IL inside the reverse micelle (RM) compared with the neat IL. These simulation findings are consistent with experimental results. Both our simulations and experimental results show that [bmim]­[BF₄] inside the RM diffuses 5–26 times faster than the neat IL, which is partly due to the fast <i>particle</i> diffusion for the ILRM nanodroplet (IL and surfactant) as a whole in benzene solvent compared with neat [bmim]­[BF₄] diffusion. Additionally, it was found that [bmim]­[BF₄] IL solved in benzene diffuses 2 orders of magnitude faster than the neat IL. Lastly, simulations show that CO₂ molecules are absorbed in four different regions of the ILRM system, that is, (I) in the IL inner core, (II) in the [BHD]⁺ surfactant cation layer, (III) at the interface between the [BHD]⁺ surfactant cation layer and benzene solvent, and (IV) in the benzene solvent. The CO₂ solubility was found to decrease in the order II > III ∼ IV > I, while the CO₂ diffusivity and permeability decrease in the following order: IV > III > II > I

Critical Assessment of CO₂ Solubility in Volatile Solvents at 298.15 K

Fifteen different low molar mass compounds are assessed as CO2 solvents based on bubble-point loci on the solvent-rich end (0.6 to 1.0 solvent wt fraction) of the CO2-solvent pressure−composition diagram at 298.15 K. Four of the five best solvents (in descending order of solvent strength on a mass fraction CO2 dissolved basis), acetone, methyl acetate, 1,4-dioxane, and 2-methoxyethyl acetate, are oxygen-rich, low molar mass species possessing one or more oxygen atoms in carbonyl, ether, and/or acetate groups that can interact favorably with CO2 via Lewis acid/Lewis base interactions. Methanol, a very low molar mass solvent, is comparable to 1,4-dioxane in solvent strength. The remaining solvents, in descending order of solvent strength on a mass basis, include 2-nitropropane, N,N-dimethylacetamide, acetylacetone, 1-nitropropane, iso-octane, 2-(2-butoxyethoxy)ethyl acetate, N-formylmorpholine, propylene carbonate, 2-butoxyethyl acetate, and N-tert-butylformamide. When compared on a molar basis, each of the six best CO2 solvents, 2-(2-butoxyethoxy)ethyl acetate, methyl acetate, 2-methoxyethyl acetate, 1,4-dioxane, acetone, and acetyl acetone, is rich in CO2-philic ether or carbonyl oxygen atoms. Methanol, which possesses a CO2-phobic hydroxyl group, is the worst CO2 solvent. COSMOtherm accurately predicted the relative solvent strengths of eight of the solvents that contain carbonyl, acetate, ether, and carbonate groups. However, COSMOtherm was not able to predict the correct ordering of solvents possessing hydroxyl, nitro-, amide, secondary amine, and tertiary amine groups. This important failure of the COSMOtherm approach for these molecules is apparently due to problems with the COSMO-RS parametrization

Development of a Conceptual Process for Selective CO₂ Capture from Fuel Gas Streams Using [hmim][Tf₂N] Ionic Liquid as a Physical Solvent

The ionic liquid (IL) [hmim]­[Tf₂N] was used as a physical solvent in an Aspen Plus simulation, employing the Peng–Robinson Equation of State (PR-EOS) with Boston–Mathias (BM) α-function and standard mixing rules, to develop a conceptual process for CO₂ capture from a shifted (undergone the water–gas shift reaction) warm fuel gas stream produced from Pittsburgh #8 coal for a 400 MWe IGCC power plant. The physical properties of the IL, including density, viscosity, surface tension, vapor pressure, and heat capacity were obtained from literature and modeled as a function of temperature. Also, available experimental solubility values for CO₂, H₂, H₂S, CO, and CH₄ in this IL were compiled, and their binary interaction parameters (δ_<i>ij</i> and <i>l</i>_<i>ij</i>) were optimized and correlated as functions of temperature. The Span–Wager EOS was also employed to generate CO₂ solubilities in [hmim]­[Tf₂N] at high pressures (up to 10 MPa) and temperatures (up to 510 K). The conceptual process developed consists of four adiabatic absorbers (2.4 m inner diameter (ID), 30 m high) arranged in parallel and packed with Plastic Pall Rings of 0.025 m for CO₂ capture; 3 flash drums arranged in series for solvent (IL) regeneration with the pressure-swing option; and a pressure-intercooling system for separating and pumping CO₂ up to 153 bar to the sequestration sites. The compositions of all process streams, CO₂ capture efficiency, and net power were calculated using the Aspen Plus simulator. The results showed that, based on the composition of the inlet gas stream to the absorbers, 95.12 mol % of CO₂ was captured and sent to sequestration sites; 98.37 mol % of H₂ was separated and sent to turbines; and the solvent exhibited a minimum loss of 1.23 mol %. These results indicate that the [hmim]­[Tf₂N] IL could be used as a physical solvent for CO₂ capture from warm shifted fuel gas streams with high efficiency

Nuclear Spin Relaxation and Molecular Interactions of a Novel Triazolium-Based Ionic Liquid

Nuclear spin relaxation, small-angle X-ray scattering (SAXS), and electrospray ionization mass spectrometry (ESI-MS) techniques are used to determine supramolecular arrangement of 3-methyl-1-octyl-4-phenyl-1H-triazol-1,2,3-ium bis­(trifluoromethanesulfonyl)­imide [OMPhTz]­[Tf₂N], an example of a triazolium-based ionic liquid. The results obtained showed first-order thermodynamic dependence for nuclear spin relaxation of the anion. First-order relaxation dependence is interpreted as through-bond dipolar relaxation. Greater than first-order dependence was found in the aliphatic protons, aromatic carbons (including nearest neighbors), and carbons at the end of the aliphatic tail. Greater than first order thermodynamic dependence of spin relaxation rates is interpreted as relaxation resulting from at least one mechanism additional to through-bond dipolar relaxation. In rigid portions of the cation, an additional spin relaxation mechanism is attributed to anisotropic effects, while greater than first order thermodynamic dependence of the octyl side chain’s spin relaxation rates is attributed to cation–cation interactions. Little interaction between the anion and the cation was observed by spin relaxation studies or by ESI-MS. No extended supramolecular structure was observed in this study, which was further supported by MS and SAXS. nuclear Overhauser enhancement (NOE) factors are used in conjunction with spin–lattice relaxation time (<i>T</i>₁) measurements to calculate rotational correlation times for C–H bonds (the time it takes for the vector represented by the bond between the two atoms to rotate by one radian). The rotational correlation times are used to represent segmental reorientation dynamics of the cation. A combination of techniques is used to determine the segmental interactions and dynamics of this example of a triazolium-based ionic liquid

Development of a Conceptual Process for Selective Capture of CO₂ from Fuel Gas Streams Using Two TEGO Ionic Liquids as Physical Solvents

Two ionic liquids (ILs), TEGO IL K5 and TEGO IL P51P, were used as physical solvents to develop a conceptual process for CO₂ capture from a shifted warm fuel gas stream produced from Pittsburgh no. 8 coal for a 400 MWe power plant. The physical properties of the two ILs and the solubilities of CO₂, H₂, N₂, and H₂S in the TEGO IL K5 solvent, as well as those of CO₂ and H₂ in the TEGO IL P51P solvent, were measured in our laboratories at pressures up to 30 bar and temperatures from 300 to 500 K. The Peng–Robinson equation-of-state (P-R EOS) with Boston–Mathias (BM) α function and standard mixing rules was used in the development of the process, and the solubility data were used to obtain the binary interaction parameters (δ_<i>ij</i> and <i>l</i>_<i>ij</i>) between the shifted gas constituents and the two ILs. The binary interaction parameters were then correlated as functions of temperature. The conceptual process consists of four identical adiabatic packed-bed absorbers (4.5 m i.d., 27 m height, packed with 0.0254 m plastic Pall Rings) arranged in parallel for CO₂ capture, three flash drums arranged in series for solvent regeneration,and two pressure/intercooling systems for separating and pumping CO₂ to sequestration sites. The compositions of all process streams, CO₂ capture efficiency, and net power were calculated using Aspen Plus for the two solvents. The results showed that TEGO IL K5 and TEGO IL P51P were able to capture 91.28% and 90.59% of CO₂ in the fuel gas stream, respectively

Toward a Materials Genome Approach for Ionic Liquids: Synthesis Guided by <i>Ab Initio</i> Property Maps

The Materials Genome Approach (MGA) aims to accelerate development of new materials by incorporating computational and data-driven approaches to reduce the cost of identification of optimal structures for a given application. Here, we use the MGA to guide the synthesis of triazolium-based ionic liquids (ILs). Our approach involves an IL property-mapping tool, which merges combinatorial structure enumeration, descriptor-based structure representation and sampling, and property prediction using molecular simulations. The simulated properties such as density, diffusivity, and gas solubility obtained for a selected set of representative ILs were used to build neural network models and map properties for all enumerated species. Herein, a family of ILs based on ca. 200 000 triazolium-based cations paired with the bis­(trifluoromethanesulfonyl)­amide anion was investigated using our MGA. Fourteen representative ILs spreading the entire range of predicted properties were subsequently synthesized and then characterized confirming the predicted density, diffusivity, and CO₂ Henry’s Law coefficient. Moreover, the property (CO₂, CH₄, and N₂ solubility) trends associated with exchange of the bis­(trifluoromethanesulfonyl)­amide anion with one of 32 other anions were explored and quantified