Tuning the Adsorption Interactions of Imidazole Derivatives with Specific Metal Cations

In this work, we report a computational study of the interactions between metal cations and imidazole derivatives in the gas phase. We first performed a systematic assessment of various density functionals and basis sets for predicting the binding energies between metal cations and the imidazoles. We find that the M11L functional in combination with the 6-311++G­(d,p) basis set provides the best compromise between accuracy and computational cost with our metal···imidazole complexes. We then evaluated the binding of a series of metal cations, including Li⁺, Na⁺, K⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ba²⁺, Hg²⁺, and Pb²⁺, with several substituted imidazole derivatives. We find that electron-donating groups increase the metal-binding energy, whereas electron-withdrawing groups decrease the metal-binding energy. Furthermore, the binding energy trends can be rationalized by the hardness of the metal cations and imidazole derivatives, providing a quick way to estimate the metal···imidazole binding strength. This insight can enable efficient screening protocols for identifying effective imidazole-based solvents and membranes for metal adsorption and provide a framework for understanding metal···imidazole interactions in biological systems

Electrostatic Potential within the Free Volume Space of Imidazole-Based Solvents: Insights into Gas Absorption Selectivity

In this work, a variety of molecular simulation tools are used to help characterize the selective absorption of CO₂ and CH₄ in imidazole-based solvents. We focus our efforts on a series of 1-<i>n</i>-alkyl-2-methyl-imidazoles and ether-functionalized imidazoles, over a temperature range from 293 to 353 K, and we perform detailed analysis of the free volume. We find that the electrostatic potential within the solvent free volume cavities provides a useful indication of the selective absorption of CO₂ and CH₄. The electrostatic potential calculation is significantly faster than the direct calculation of the chemical potential, and tests with the 1-<i>n</i>-alkyl-2-methyl-imidazoles and the ether-functionalized imidazoles indicate that this may be a useful screening tool for other solvents

Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes

Polyimides are at the forefront of advanced membrane materials for CO₂ capture and gas-purification processes. Recently, ionic polyimides (i-PIs) have been reported as a new class of condensation polymers that combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO₂ and CH₄ adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C₄mim]­[Tf₂N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations is used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials. Our results highlight the polymer relaxation process and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain constant while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO₂ molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. However, the CH₄ molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials as well as provide additional guidance for the interpretation of the simulated adsorption systems

Evaluation of Alkylimidazoles as Physical Solvents for CO₂/CH₄ Separation

1-<i>n</i>-Alkylimidazoles are a class of tunable solvents with low volatility and low viscosities. Although imidazoles have been known for some time in the pharmaceutical industry, and as convenient precursors for synthesizing imidazolium-based ionic liquids (ILs), only recently have they been given consideration in some of the same solvent-based separations applications that ILs have been studied for, such as post-combustion CO₂ capture and natural gas treating. “Sweetening”, the removal of CO₂, H₂S, and other “acid” gases from natural gas (CH₄), is an existing industrial application where low volatility, low viscosity physical solvents are already applied successfully and economically at large scale. Physical solvents are also used for syngas cleanup and in the emerging application of pre-combustion CO₂ capture. Given the similarities in physical properties between 1-<i>n</i>-alkylimidazoles, and physical solvents currently used in industrial gas treating, the 1-<i>n</i>-alkylimidazole class of solvents warrants further investigation. Solubilities of CO₂ and CH₄ in a series of 1-<i>n</i>-alkylimidazoles were measured under conditions relevant to the use of physical solvents for natural gas treating: ∼5 atm partial pressure of CO₂ and temperatures of 30–75 °C. Solubilities of CO₂ and CH₄ were found to be strongly dependent on temperature, with the solubility of each gas in all solvents diminishing with increasing temperature, although CO₂ exhibited a stronger temperature dependence than CH₄. Ideal CO₂/CH₄ solubility selectivities were also more favorable at lower temperatures in 1-<i>n</i>-alkylimidazole solvents with shorter chain lengths. CO₂ solubility decreased with increasing chain length, while CH₄ solubility exhibited a maximum in 1-hexylimidazole. The solubility trends observed with temperature and chain length can be explained through calculation of solution enthalpies and solvent fractional free volume as approximated from van der Waals volumes as calculated via atomic contributions. Of the solvents examined, 1-methylimidazole displays the most favorable CO₂ solubility and CO₂/CH₄ selectivity, and has the lowest viscosity. A comparison of 1-methylimidazole to commercially used solvents reveals similar physical properties and the potential for use in industrial gas processing. Imidazolium-based ILs are also compared, although they appear less favorable for use within established process schemes given their higher viscosities and reduced capacity for CO₂

Synthesis and Properties of 2‑Halo-1,3-diether-propanes: Diversifying the Range of Functionality in Glycerol-Derived Compounds

Synthesis of value-added chemicals from glycerol derivatives has been of real interest due to the excess volumes of glycerol resulting from biofuel production. Previously, we have demonstrated the controlled synthesis of symmetric and asymmetric 1,3-diether-2-propanol compounds bearing glycerol skeletons, which, in addition to potential applications as CO2 capture solvents, are also versatile intermediates for a number of further chemical transformations. Here, we demonstrate the conversion of these compounds to corresponding 2-halo-1,3-diethers as a means of further diversifying the range and properties of glycerol-derived compounds. Thermophysical properties of these compounds (density, molar volume, and viscosity) were measured over a temperature range of 20–80 °C. The experimental work was augmented by theoretical calculations of density, viscosity, vapor pressure, and dipole moment for each of the synthsized compounds as well as additional species with similar structures that have not yet been synthesized. The data obtained in this work provide a useful guide for valorization of glycerol in the form of new solvents and building blocks for value-added chemicals

Properties and Performance of Ether-Functionalized Imidazoles as Physical Solvents for CO₂ Separations

Previously, we investigated 1-<i>n</i>-alkylimidazoles as low viscosity, low vapor pressure physical solvents for CO₂/CH₄ separation and noted a decrease in performance as the length of the <i>n</i>-alkyl chain was extended. Here, we examine imidazoles featuring oligo­(ethylene glycol) substituents (“PEG_<i>n</i>-imidazoles”) as an opportunity to improve upon the separation performance of this class of molecules. In the current work, we have characterized the density and the viscosity of PEG_<i>n</i>-imidazoles over the temperature range 20–80 °C. PEG_<i>n</i>-imidazoles are slightly more viscous than 1-<i>n</i>-alkylimidazoles but still fall below 20 cP. Ideal gas solubilities of CO₂ and CH₄ were measured in PEG_<i>n</i>-imidazoles at gas partial pressures of ∼5 bar and temperatures of 25–70 °C. Solubilities of CO₂ and CH₄ were both found to decrease with increasing temperature, with a stronger dependence for CO₂. However, better CO₂/CH₄ selectivity was achieved in PEG_<i>n</i>-imidazoles at lower operating temperatures than was observed for 1-<i>n</i>-alkylimidazoles. Physical properties and gas separation performances were correlated with fractional free volume calculated via COSMOtherm, as well as solubility parameters. The results show trends of decreased FFV when polar ether groups comprise the substituent, and that CO₂ solubility and solubility selectivity for CO₂/CH₄ are improved compared to their nonpolar, hydrocarbon-based analogues

Experimental Densities and Calculated Fractional Free Volumes of Ionic Liquids with Tri- and Tetra-substituted Imidazolium Cations

Although it has been estimated that there are at least 1 million ionic liquids (ILs) that are accessible using commercially available starting materials, a great portion of the ILs that have been experimentally synthesized, characterized, and studied in a variety of applications are built around the relatively simple 1-<i>n</i>-alkyl-3-methylimidazolium ([C_nmim]) cation motif. Yet, there is no fundamental limitation or reason as to why tri- or tetra-functionalized imidazolium cations have received far less attention. Scant physical property data exist for just a few trifunctionalized imidazolium-based ILs and there is virtually no data on tetra-functionalized ILs. Thus, there are a broad experimental spaces on the “map” of ILs that are largely unexplored. We have sought to make an initial expedition into these “uncharted waters” and have synthesized imidazolium-based ILs with one more functional group(s) at the C(2), C(4), and/or C(5) positions of the imidazolium ring (as well as N(1) and N(3)). This manuscript reports the synthesis and experimental densities of these tri- and tetra-functionalized ILs as well as calculated densities and fractional free volumes from COSMOTherm. To the best of our knowledge, this is the first report of any detailed experimental measurements or computational studies relating to ILs with substitutions at the C(4) and C(5) positions

3D Printed Block Copolymer Nanostructures

The emergence of 3D printing has dramatically advanced the availability of tangible molecular and extended solid models. Interestingly, there are few nanostructure models available both commercially and through other do-it-yourself approaches such as 3D printing. This is unfortunate given the importance of nanotechnology in science today. In this work, we have filled part of this gap by designing and 3D printing several block copolymer (BCP) nanostructure morphologies. We used a variety of methods including manually drawing the files within 3D computer design software, using equations with mathematical graphing software, and developing a programming script to convert self-consistent field theory (SCFT) structure data into a 3D printable file. Conversion of SCF data into 3D printable structures may find broader applicability beyond creating BCP nanostructures as SCF calculations are used in a variety of geometric computations. All methods reported herein produced tangible 3D prints of approximately equal quality. These tangible models will be useful for educators, students, and researchers in polymer science and nanotechnology

Structure–Property Relationships in Ionic Liquids: A Study of the Influence of N(1) Ether and C(2) Methyl Substituents on the Vaporization Enthalpies of Imidazolium-Based Ionic Liquids

In this work, the QCM and TGA methods were used concurrently to study the two alkoxy-substituted ionic liquid (IL) series: 1-[oligo­(ethylene glycol)]-3-methylimidazolium bis­(triflamide) ([P_<i>x</i>mim]­[NTf₂]) and 1-[oligo­(ethylene glycol)]-2,3-dimethylimidazolium bis­(triflamide) ([P_<i>x</i>mmim]­[NTf₂]). For comparison, enthalpies of vaporization measured at elevated temperatures were adjusted to the reference temperature 298 K and tested for consistency. It was found that the vaporization enthalpies of the alkoxy-substituted ILs are significantly lower than those of the analogous ILs with the alkyl-substituted cation. This is in contrast to molecular solvents, for which alkoxy groups are typically observed to increase vaporization enthalpy relative to those of the hydrocarbon analogues. Two useful group contributions for the quick estimation of vaporization enthalpies of various alkoxy-substituted IL cations (e.g., imidazolium, ammonium, pyridinium) are recommended based on the findings of this work

Building Blocks for Ionic Liquids: Vapor Pressures and Vaporization Enthalpies of N‑Functionalized Imidazoles with Branched and Cycloalkyl Substituents

The imidazole structure offers a versatile means of developing molecules with controlled/tunable physicochemical properties that have significant utility in many applications and can be further derivatized to form ionic liquids. In the literature, the vast majority of studies on structure–property relationships in these types of molecules are devoted to linear (e.g., <i>n</i>-alkyl) substituents. However, imidazoles with branched or cycloalkyl groups are equally accessible through convenient synthetic methods – yet there are essentially no reports on the physical properties of such compounds in the literature. Here, the absolute vapor pressures of branched and cycloalkyl derivatives of imidazole have been determined as a function of temperature by the transpiration method. The standard molar enthalpies of vaporization were derived from the temperature dependences of vapor pressures. The measured data sets were successfully checked for internal consistency by comparison with vaporization enthalpies of the parent species, and a group contribution method is put forth by which the vaporization enthalpies of imidazoles, and imidazolium-based ILs, with alkyl groups in any configuration can be rapidly predicted