Molecular analysis of selective gas adsorption within composites of ionic polyimides and ionic liquids as gas separation membranes

Preparation and gas separation performance of polyimide membranes endcapped with ionic liquid-type structures

A set of ionic liquid capped polyimide membranes was prepared using 1-aminoethyl-3-methylimidazolium hexafluorophosphate (IL1) and 1-aminopropylimidazolium bis(trifluoromethylsulfonyl)imine (IL2) as the terminal groups. The products’ molecular weights, mechanical properties, and separation permeability (CO2/CH4) were investigated. For CO2/CH4 separation, the selectivity of the ionic liquid capped polyimide membranes was higher than that of noncapped ones. Among them, the membrane synthesized by 4.4′- diaminodiphenyl ether and 4.4′-(hexafluoroisopropyl) diphthalic anhydride (6FDA) as monomer, with IL1 as terminal group, displayed the best selectivity. Its permeability was 7.47 Barrer and selectivity 102.42, which exceeded the 1991 Robeson curve. Polyimide membranes capped by ionic liquid showed high gas selectivity and good gas permeability as well as good physical and chemical properties. Consequently, it can be concluded that introducing an ionic liquid structure to polyimide chains could make attractive membrane materials for various gas separation and related applications.

Understanding Gas Solubility of Pure Component and Binary Mixtures within Multivalent Ionic Liquids from Molecular Simulations

Understanding the molecular-level solubility of CO₂ and its mixtures is essential to the progress of gas-treating technologies. Herein, we use grand canonical Monte Carlo simulations to study the single-component gas absorption of SO₂, N₂, CH₄, and H₂ and binary mixtures of CO₂/SO₂, CO₂/N₂, CO₂/CH₄, and CO₂/H₂ of varying mole fractions within multivalent ionic liquids (ILs). Our results highlight the importance of the free volume effect and the anion effect when interpreting the absorption behavior of these mixtures, similar to the behavior of CO₂ found in our previous study (Phys. Chem. Chem. Phys. 2020, 22, 20618-20633). The deviation of gas solubility between the pure component absorption versus the binary absorption, as well as the solubility selectivity, highlights the importance of the relative affinity of gas species within a mixture to the different anions. The absorption selectivity within a specific IL system can be predicted based on the relative gas affinity to the anion.

Solubility Behavior of CO2 in Ionic Liquids Based on Ionic Polarity Index Analyses

Ionic liquids (ILs) can serve as effective CO₂ solvents with an appropriate selection of different anions and cations. However, due to the large library of potential IL compositions, rapid screening methods are needed for characterizing and ranking the expected properties. We have recently proposed the ionic polarity index (IPI) parameter, effectively connecting volume-based approaches and electrostatic potential analyses and providing a single metric that can potentially be used to rapidly screen for desirable IL properties. In this work, the corresponding anion and cation IPIs are used to generate correlations with respect to the CO₂ volumetric solubility in ILs. The relationships are generally applicable to groups of ILs within a homologous ion series, and this can be particularly valuable for prescreening different ion pairings for maximizing gas solvation performance.

Molecular insight into the anion effect and free volume effect of CO2 solubility in multivalent ionic liquids

For many years, experimental and theoretical studies have investigated the solubility of CO2 in a variety of ionic liquids (ILs), but the overarching absorption mechanism is still unclear. Currently, two different factors are believed to dominate the absorption performance: (a) the fractional free volume (FFV) accessible for absorption; and (b) the nature of the CO2 interactions with the anion species. The FFV is often more influential than the specific choice of the anion, but neither mechanism provides a complete picture. Herein, we have attempted to decouple these mechanisms in order to provide a more definitive molecular-level perspective of CO2 absorption in IL solvents. We simulate a series of nine different multivalent ILs comprised of imidazolium cations and sulfonate/sulfonimide anions tethered to benzene rings, along with a comprehensive analysis of the CO2 absorption and underlying molecular-level features. We find that the CO2 solubility has a very strong, linear correlation with respect to FFV, but only when comparisons are constrained to a common anion species. The choice of anion results in a fundamental remapping of the correlation between CO2 solubility and FFV. Overall, the free volume effect dominates in the ILs with smaller FFV values, while the choice of anion becomes more important in the systems with larger FFVs. Our proposed mechanistic map is intended to provide a more consistent framework for guiding further IL design for gas absorption applications.

Choline-Based Ionic Liquids-Incorporated IRMOF-1 for H2S/CH4 Capture: Insight from Molecular Dynamics Simulation

The removal of H2S and CH4 from natural gas is crucial as H2S causes environmental contamination, corrodes the gas stream pipelines, and decreases the feedstock for industrial productions. Many scientific researches have shown that the metal-organic framework (MOF)/ionic liquids (ILs) have great potential as alternative adsorbents to capture H2S. In this work, molecular dynamics (MD) simulation was carried out to determine the stability of ILs/IRMOF-1 as well as to study the solubility of H2S and CH4 gases in this ILs/IRMOF-1 hybrid material. Three choline-based ILs were incorporated into IRMOF-1 with different ratios of 0.4, 0.8, and 1.2% w/w, respectively, in which the most stable choline-based ILs/IRMOF-1 composite was analysed for H2S/CH4 solubility selectivity. Among the three choline-based ILs/IRMOF-1, [Chl] [SCN]/IRMOF-1 shows the most stable incorporation. However, the increment of ILs loaded in the IRMOF-1 significantly reduced the stability of the hybrid due to the crowding effect. Solvation free energy was then computed to determine the solubility of H2S and CH4 in the [Chl] [SCN]/IRMOF-1. H2S showed higher solubility compared to CH4, where its solubility declined with the increase of choline-based IL loading.

Molecular Transport Behavior of CO2 in Ionic Polyimides and Ionic Liquid Composite Membrane Materials

Ionic polyimides (i-PI) are a new class of polymer materials that are very promising for CO₂ capture membranes, and recent experimental studies have demonstrated their enhanced separation performance with the addition of imidazolium-based ionic liquids (ILs). However, there is very little known about the molecular-level interactions in these systems, which give rise to interesting gas adsorption and diffusion characteristics. In this study, we use a combination of Monte Carlo and molecular dynamics simulations to analyze the equilibrium and transport properties of CO₂ molecules in the i-PI and i-PI + IL composite materials. The addition of several different common ILs are modeled, which have a plasticization effect on the i-PI, lowering the glass transition temperature (T_g). The solubility of CO₂ strongly correlates with the T_g, but the diffusion demonstrates more unpredictable behavior. At low concentrations, the IL has a blocking effect, leading to reduced diffusion rates. However, as the IL surpasses a threshold value, the relationship is inverted and the IL has a facilitating effect on the gas transport. This behavior is attributed to the simultaneous contributions of the increased i-PI plasticization at higher IL concentrations (facilitating gas hopping rates from cavity-to-cavity) and the increased IL continuity throughout the system, enabling more favorable transport pathways for CO₂ diffusion.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.

On the Performance of Confined Deep Eutectic Solvents and Ionic Liquids for Separations of Carbon Dioxide from Methane: Molecular Dynamics Simulations

Classical molecular dynamics simulations were used to investigate the performance of slit graphite and titania (rutile) pores of 5.2 nm in width, partially and completely filled with deep eutectic solvents (DESs) or ionic liquids (ILs), for gas separations of a carbon dioxide-methane mixture of 5:95 molar ratio and temperatures and pressures on the order of 318 K and 100 bar, respectively. The DESs studied were ethaline and levuline (1:2 molar mixtures of choline chloride with ethylene glycol or levulinic acid), and the IL considered was 1- n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim⁺][NTf₂^-]. The performance of these systems in terms of solubility selectivity, diffusion selectivity, and permselectivity was compared against the performance of the bulk solvents (which could also be viewed as a model system for the micrometer-sized pores of a supported IL or DES membrane) and against carbon and rutile pores without preadsorbed solvent. The best performance in terms of permselectivity is obtained for bulk levuline and by rutile pores fully filled by ethaline, followed by graphite pores filled by ethaline and the IL. Empty rutile pores have the largest value of solubility selectivity, followed by bulk ethaline and rutile pores completely filled by the IL. The largest values of diffusivity selectivity were observed for bulk levuline, followed by ethaline completely filling a rutile nanopore and a graphite nanopore completely filled with the IL. These observations are rationalized by examining local density profiles and interaction energies among the different entities in our systems. In general, systems of nanopores fully filled by solvents, as well as the bulk solvents, have larger permselectivities than pores partially filled by the IL or the DESs. Drops of 2-3 orders of magnitude are observed in the gas diffusivity in pores filled with solvents with respect to systems of empty pores, which may be problematic if gas permeation is mainly controlled by diffusion. However, if adsorption dominates the gas permeation within the membrane, our results suggest that systems of levuline in the micrometer-sized pores of a supported DES membrane or ethaline confined in the rutile nanopores of a supported DES phase material might represent promising systems for gas separation.