Novel imidazolium-based room-temperature ionic liquids (RTILs) with one, two, or three oligo(ethylene glycol) substituents were synthesized. Solubilities and ideal solubility selectivities of CO2, ...N2, and CH4 at low pressure (1 atm) in these RTILs were determined using a pressure decay technique. Comparison to corresponding alkyl analogues of these RTILs reveals similar levels of CO2 solubility but lower solubilities of N2 and CH4. As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30−75% higher ideal solubility selectivities for CO2/N2 and CO2/CH4.
Polyimides and ionic liquids (ILs) are two classes of materials that have been widely studied as gas separation membranes, each demonstrating respective advantages and limitations. Both polyimides ...and ILs are amenable to modification/functionalization based on selection of the requisite precursors. However, there have been but a handful of reports considering how polyimides and ILs could be integrated to obtain fundamentally new materials with synergistic properties. In this manuscript, we demonstrate a new and versatile way to synthesize polyimides with imidazolium cations directly located within the polymer backbone to form polyimide–ionene hybrids, or “ionic polyimides”. Our strategy for synthesizing ionic polyimides does not require the use of amino-functionalized ILs. Instead, the imidization reaction occurs prior to polymerization in the formation of an imidazole-functionalized diimide monomer. This monomer is then reacted via step-growth (condensation) polymerization with p-dichloroxylene via Menshutkin reactions, simultaneously linking the monomers and creating the ionic components. The resultant ionic polyimide is amenable to thermal processing (e.g., extrusion, melt-pressing) and capable of forming thin films. Upon soaking thin films of the ionic polyimide in a widely used IL, 1-butyl-3-methylimidazolium bistriflimide (C4mimTf2N), a stoichiometric absorption of the IL into the ionic polyimide was observed, forming an ionic polyimide + IL composite. The gas separation performances of ionic polyimide and ionic polyimide + IL composite membranes were studied with respect to CO2, N2, CH4, and H2. The neat ionic polyimide exhibits low permeability to CO2 and H2 (∼0.9 and ∼1.6 barrers, respectively) and very low permeability to N2 and CH4 (∼0.03 barrers for both). For the ionic polyimide + IL composite, the permeabilities of CO2, N2, and CH4 increase by 1800–2700%, while H2 permeability only increased by ∼200%. The large increases in permeability for CO2, N2, and CH4 are due to greatly increased gas diffusivity through the material, with gas solubility essentially unchanged with the IL present. The ionic polyimide and ionic polyimide + IL composite were characterized using a number of techniques. Most interestingly, X-ray diffractometry (XRD) of the films reveals that the ionic polyimide + IL composite displays a sharp peak, indicating that the ionic polyimide may experience supramolecular assembly around the IL. Although the performances of these first ionic polyimide and ionic polyimide + IL composite membranes fall short of Robeson’s Upper Bounds, this work provides a strong foundation on which ionic polyimide materials with more sophisticated structural elements can be developed to understand the structure–property relationships underlying the ionic polyimide platform and ultimately produce high-performance gas separation membranes.
Polymeric membranes either containing, or built from, ionic liquids (ILs) are of great interest for enhanced CO2/light gas separation due to the stronger affinity of ILs toward quadrupolar CO2 ...molecules and hence high CO2 solubility selectivity. Herein, we report the development of a series of four novel anionic poly(IL)-IL composite membranes via a photopolymerization method for effective CO2 separation. Interestingly, these are the first examples of anionic poly(IL)-IL composite systems in which the poly(IL) component has delocalized sulfonimide anions pendant from the polymer backbone with imidazolium cations as “free” counterions. Two types of photopolymerizable methacryloxy-based IL monomers (MILs) with highly delocalized anions (−SO2–N(−)–SO2–CF3 and −SO2–N(−)–SO2–C7H7) and mobile imidazolium (C2 mim+) countercations were successfully synthesized and photopolymerized with two distinct amounts of free IL containing the same structural cation (C2 mimTf2N) and 20 wt % PEGDA cross-linker to serve as a composite matrix. The structure–property relationships of the four newly developed anionic poly(IL)-IL composite membranes were extensively characterized by thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction. All of the newly developed anionic poly(IL)-IL composite membranes exhibited superior CO2/CH4 and CO2/N2 selectivities together with moderate CO2/H2 selectivity and reasonable CO2 permeabilities. The membrane with an optimal composition and polymer architecture (MIL-C7H7/PEGDA(20%)/IL(1 equiv)) reaches the 2008 Robeson upper bound limit of CO2/CH4 due to the simultaneous improvement in permeability and selectivity (CO2 permeability ∼20 barrer and αCO2/CH4 ∼119). This study provides a promising strategy to explore the benefits of anionic poly(IL)-IL composites to separate CO2 from flue gas, natural gas, and syngas streams and open up new possibilities in polymer membrane design with strong candidate materials for practical applications.
Li–O2 batteries are actively being investigated due to their high theoretical energy density (∼11 000 Wh kg−1), which would compete with gasoline energy per Kg in electric vehicles. Linear glymes are ...the most appealing electrolytes for Li–O2 batteries due to their large electrochemical window, stability against radicals and solubility of Li+ metal ions. However, all these superior properties are tarnished by their high toxicity. Herein, a greener glyme derived from bio-sourced glycerol (1,2,3-trimethoxypropane (TMP)), is proposed for the first time as a solvent in an electrolyte for Li–O2 batteries. TMP performance has been compared to its toxic linear isomer, diglyme, and most popular tetraglyme as a liquid electrolyte and gel polymer electrolyte (GPE) membranes. GPEs were based on a mix of mono-, di- and tri- functional acrylates, cured simultaneously within a liquid electrolyte mix (1 M LiTFSI in the plasticizers) by UV-photopolymerisation. GPE-TMP based membranes showed a high ionic conductivity (2.33 × 10−3 S cm−1 at 25 °C), directly comparable to the other glymes. Moreover, this remarkable conductivity was very close to the liquid TMP-based electrolyte (3.59 × 10−3 S cm−1 at 25 °C). When used as electrolytes in lithium symmetrical cells, the GPE-TMP electrolyte enhanced the polarisation when compared to the liquid TMP-based cells, especially at higher rates (<0.6 V observed at ±1 mA cm−2). Performance in Li–O2 cells showed that GPE-TMP electrolytes achieved a discharge capacity as high as 2.75 mA h cm−2 (3819 mA h g−1), ahead of GPE-diglyme or GPE-tetraglyme electrolytes (2.34 and 2.09 mA h cm−2, respectively). When cycled, cells using TMP-based electrolyte had a similar capacity retention than the ones with tetraglyme, confirming the potential use of TMP as solvent/plasticizer in electrolytes for Li–O2 cells.
The use of ionic liquids (ILs) for CO
2
capture and the removal of acid gases from natural gas and other industrial processes has been one of the foremost research applications for this unique class ...of non-volatile solvents. However, most of the most broadly studied ILs lack sufficient capacities for CO
2
and other acid gases such as H
2
S, SO
2
, etc. to be viewed as viable replacements for aqueous amine technologies which have been used industrially for acid gas removal for nearly a century. Furthermore, many of the most well-known ILs are too viscous to be used within conventional process equipment and are likely too costly for use at large scales. As the negligible vapor pressure of ILs is an attractive property for gas separations, it is desirable to find new ILs with improved properties that can be synthesized from lower cost starting materials and/or natural products. Recently, new reactive and reversible IL solvents have emerged in efforts to improve upon the CO
2
capacity, physical properties and costs of IL-based gas separation technologies. In this review, we detail the differences between these novel approaches and the standard crop of ILs that have been reported in the literature. The various strategies that have been employed to develop these materials for energy-related separation applications will be examined, with an emphasis on how chemistry and physical properties relate to the demands of efficient chemical process engineering. Where applicable, comparisons to conventional (i.e., aqueous amine) solvents will be made so as provide baselines to commercial technologies. Finally, we introduce the concept of imidazoles and imidazole-amine hybrid solvents as another tunable platform for the removal of CO
2
, SO
2
, and H
2
S.
Reports of imidazoliumionic liquids (ILs) with at least one benzylic substituent bound to the imidazolium cation are far less common than the ubiquitous 1-alkyl-3-methylimidazolium ILs (C n mimX). ...Yet, there is significant motivation to determine structure–property relationships for imidazolium-based ILs with at least one benzylic substituent. Not only are these ILs just as straightforward to synthesize as C n mimX ILs, but ILs with benzylic substituents are also representative segments of poly(ILs) and ionenes where imidazolium cations with benzylic groups are commonly found. This report focuses on the effects of benzyl and methylbenzyl substituents on the density and viscosity of a series of imidazolium cations paired with bis(trifluoromethyl)sulfonylimide (more commonly known as bistriflimide) (Tf2N−) anions. Furthermore, the solubility of CO2 in 1-benzyl-3-methylimidazolium bistriflimide BnmimTf2N (CAS: 433337-24-7) was measured at 303.15, 318.15, 333.15, and 348.15 K at pressures in the range of 1–9 bar.
This work introduces a series of vinyl-imidazolium-based polyelectrolyte composites, which were structurally modified via impregnation with multivalent imidazolium-benzene ionic liquids (ILs) or ...crosslinked with novel cationic crosslinkers which possess internal imidazolium cations and vinylimidazolium cations at the periphery. A set of eight C
vimTf
N-based membranes were prepared via UV-initiated free radical polymerization, including four composites containing di-, tri-, tetra-, and hexa-imidazolium benzene ILs and four crosslinked derivatives which utilized tri- and tetra- vinylimidazolium benzene crosslinking agents. Structural and functional characterizations were performed, and pure gas permeation data were collected to better understand the effects of "free" ILs dispersed in the polymeric matrix versus integrated ionic crosslinks on the transport behaviors of these thin films. These imidazolium PIL:IL composites exhibited moderately high CO
permeabilities (~20-40 Barrer), a 4-7× increase relative to corresponding neat PIL, with excellent selectivities against N
or CH
The addition of imidazolium-benzene fillers with increased imidazolium content were shown to correspondingly enhance CO
solubility (di- < tri- < tetra- < hexa-), with the C
vimTf
N: Hexa(Im
)Benz Tf
N composite showing the highest CO
permeability (P
= 38.4 Barrer), while maintaining modest selectivities (α
= 20.2, α
= 23.6). Additionally, these metrics were similarly improved with the integration of more ionic content bonded to the polymeric matrix; increased P
with increased wt% of the tri- and tetra-vinylimidazolium benzene crosslinking agent was observed. This study demonstrates the intriguing interactions and effects of ionic additives or crosslinkers within a PIL matrix, revealing the potential for the tuning of the properties and transport behaviors of ionic polymers using ionic liquid-inspired small molecules.
Ionenes, polymers with ionic groups incorporated directly within the backbone are a highly versatile class of materials, although they have received much less attention than polyelectrolytes which ...have ionic groups pendant from the polymer backbone. By designing ionenes that incorporate robust properties of poly(ether ether ketone) (PEEK), we have achieved new imidazolium-containing PEEK–ionene architectures that create opportunities for enhanced CO2 separation membranes. To achieve these materials, an new imidazole-functionalized PEEK oligomer (ImK(EEK)2KIm) was synthesized through facile and straightforward routes. This molecule was then polymerized via condensation reactions with two different aromatic (α,α′-dibromo-p-xylene) and aliphatic (dibromohexyl containing-bis(imidazolium)hexane dibromide salt) comonomers and exchanged with bistriflimide (Tf2N) anion to obtain two unique PEEK–ionenes containing distinct PEEK and ionic segments: p(ImK(EEK)2Im-p-xylTf2N2) and p(ImK(EEK)2(ImC6)3Tf2N4). While the neat PEEK–ionenes exhibited high molecular weight but were not able to form high-quality films, adding stoichiometric amounts of “free” IL (1-methyl-3-butylimidazolium bistriflimide, C4mImTf2N), greatly improved the flexibility and processability of the resultant membranes. Further, the structure–property relationships of bulk PEEK–ionenes and corresponding composites were extensively characterized by different analytical techniques (thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, and solid-state NMR). The gas separation properties were investigated, with the PEEK–ionenes + IL composites exhibiting CO2 permeabilities of 14–94 barrer and good CO2/N2 permselectivities of 26–35, indicating that new designs of segmented ionenes and composites with ILs a promising material design strategy for developing gas separation membranes.
Cross-linkable gemini room temperature ionic liquids (GRTILs) were synthesized and photo-cross-linked into thin films. The resultant polymer membranes were tested for their permeabilities to CO
2, N
...2, CH
4 and H
2. Permeabilities for each gas were found to be much lower when compared to previously reported poly(RTIL) membranes, mainly as a result of highly restricted diffusion. Separation factors were similar to previously studied poly(RTIL) membranes. CH
4 and N
2 fluxes were small enough to consider these membranes as “barrier” films to the transport of those gases. Poly(GRTILs) may have use in applications where flow of those gases is not desirable.
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