Ionic Liquids for Carbon Dioxide Separation on Membranes

Reducing carbon dioxide (CO2) emissions is not only ecologist s dream. Re-use of this gas can be beneficial for many industries where it is an important substrate. Since in the majority of the cases CO2 leaves the industrial site mixed with other gases, it has to be purified before it is captured. One promising method involves the use of membranes incorporating ionic liquids (ILs) that readily absorb CO2. Although several well-performing ILs have been tested for this application, the search for an industrially relevant one is still ongoing. More importantly, the structure-performance relationship of the ILs as well as the dissolution mechanism of CO2 in ILs is not yet fully understood.This PhD thesis focused on the design and synthesis of ILs for the separation of CO2 from N2 or CH4 on Supported Ionic Liquid Membranes (SILMs) and IL-polymer blend membranes, as well as on establishing the structure-performance relation-ship of the novel compounds in CO2 separation.A study of a series of tri(ethylene glycol)-functionalized ILs based on different cationic cores showed that cations influence CO2 separation to a similar extent as anions. Gas separation selectivities of these monocationic compounds were on average two times lower than those of their dicationic analogues, due to the lower permeances of N2 and CH4 through the IL layer andmore interaction sites for CO2 in the latter. Similarly to the tri(ethylene glycol), the nitrile group was repeatedly reported to be very efficient in improving CO2 separations. Therefore, both moieties were combined into the pyrrolidinium and imidazolium ILs and used to prepare membranes. These ILs exhibited ca. 2.3 times higher CO2/N2 and CO2/CH4 gas separation selectivities than analogous ILs functionalized only with a glycol chain. In-situ FTIR-ATR spectroscopy was used to study the solubility of CO2,IL swelling and the interactions of CO2 with the nitrile group. The difunctionalized ILs were found to interact stronger with CO2 than the glycol-functionalized ILs.The widespread use of such highly functionalized ILs is restricted by the elaborate synthesis and the difficulties in purification, which lead to low yields. To overcome these problems and to propose a new approach to CO2 separation, metal-containing ILs were designed and evaluated for membrane performance. The compounds were composed of complex cations and bis(trifluoromethylsulfonyl)imide anions. In each cation, six imidazole ligands functionalized with a nitrile or an oligo(ethylene oxide) group were coordinated to a d-block central metal ion. Crystal structures of the nitrile-containing ILs were obtained. In order to explore the potential ofthe direct CO2-metal chemical binding, analogous ILs but with four or five ligands were examined for gas separation performance and showed selectivities similar to the hexacoordinate ILs. The membranes prepared from blends of these ILs and polymers, provided moderate selectivities.status: publishe

Dissolution and reaction of steviol glycosides in ionic liquids: a preliminary study

Poster presented by Justyna Kotlarskastatus: publishe

Fluorine-functionalized ionic liquids with high oxygen solubility

Eight fluorine-functionalized ionic liquids were synthesized and the oxygen solubility was compared to commercial ionic liquids without the extra fluorinated chain. The concentration of dissolved oxygen increased with the fluorine content of the alkyl chain, which can be attached either to the cation or the anion. This approach maintains the freedom to design an ionic liquid for a specific application, while at the same time the oxygen solubility is increased.status: publishe

Design of ionic liquids for separation of carbon dioxide from nitrogen and methane supported by ionic liquid membranes

Poster presented by Sandra Hojniakstatus: publishe

Separation of Carbon Dioxide from Nitrogen or Methane by Supported Ionic Liquid Membranes (SILMs): Influence of the Cation Charge of the Ionic Liquid

Supported ionic liquid membranes (SILMs) are promising tools for the separation of carbon dioxide from other gases. In this paper, new imidazolium, pyrrolidinium, piperidinium, and morpholinium ionic liquids with a triethylene glycol side chain and tosylate anions, as well as their symmetrical dicationic analogues, have been synthesized and incorporated into SILMs. The selectivities for CO2/N2 and CO2/CH4 separations have been measured. The selectivities exhibited by the dicationic ionic liquids are up to two times higher than the values of the corresponding monocationic ionic liquids. Quantum chemical calculations have been used to investigate the difference in the interaction of carbon dioxide with monocationic and dicationic ionic liquids. The reason for the increased gas separation selectivity of the dicationic ionic liquids is two-fold: (1) a decrease in permeance of nitrogen and methane through the ionic liquid layer, presumably due to their less favorable interactions with the gases, while the permeance of carbon dioxide is reduced much less; (2) an increase in the number of interaction sites for the interactions with the quadrupolar carbon dioxide molecules in the dicationic ionic liquids, compared to the monocationic analogues.status: publishe

Highly Selective Separation of Carbon Dioxide from Nitrogen and Methane by Nitrile/Glycol-Difunctionalized Ionic Liquids in Supported Ionic Liquid Membranes (SILMs)

Novel difunctionalized ionic liquids (ILs) containing a triethylene glycol monomethyl ether chain and a nitrile group on a pyrrolidinium or imidazolium cation have been synthesized and incorporated into supported ionic liquid membranes (SILMs). These ILs exhibit ca. 2.3 times higher CO2/N2 and CO2/CH4 gas separation selectivities than analogous ILs functionalized only with a glycol chain. Although the glycol moiety ensures room temperature liquidity of the pyrrolidinium and imidazolium ILs, the two classes of ILs benefit from the presence of a nitrile group in different ways. The difunctionalized pyrrolidinium ILs exhibit an increase in CO2 permeance, whereas the permeances of the contaminant gases rise negligibly, resulting in high gas separation selectivities. In the imidazolium ILs, the presence of a nitrile group does not always increase the CO2 permeance nor does it increase the CO2 solubility, as showed in situ by the ATR-FTIR spectroscopic method. High selectivity of these ILs is caused by the considerably reduced permeances of N2 and CH4, most likely due to the ability of the −CN group to reject the nonpolar contaminant gases. Apart from the CO2 solubility, IL–CO2 interactions and IL swelling were studied with the in situ ATR-FTIR spectroscopy. Different strengths of the IL–CO2 interactions were found to be the major difference between the two classes of ILs. The difunctionalized ILs interacted stronger with CO2 than the glycol-functionalized ILs, as manifested in the smaller bandwidths of the bending mode band of CO2 for the latter.status: publishe

Separation of Carbon Dioxide from Nitrogen or Methane by Supported Ionic Liquid Membranes (SILMs): Influence of the Cation Charge of the Ionic Liquid

Supported ionic liquid membranes (SILMs) are promising tools for the separation of carbon dioxide from other gases. In this paper, new imidazolium, pyrrolidinium, piperidinium, and morpholinium ionic liquids with a triethylene glycol side chain and tosylate anions, as well as their symmetrical dicationic analogues, have been synthesized and incorporated into SILMs. The selectivities for CO₂/N₂ and CO₂/CH₄ separations have been measured. The selectivities exhibited by the dicationic ionic liquids are up to two times higher than the values of the corresponding monocationic ionic liquids. Quantum chemical calculations have been used to investigate the difference in the interaction of carbon dioxide with monocationic and dicationic ionic liquids. The reason for the increased gas separation selectivity of the dicationic ionic liquids is two-fold: (1) a decrease in permeance of nitrogen and methane through the ionic liquid layer, presumably due to their less favorable interactions with the gases, while the permeance of carbon dioxide is reduced much less; (2) an increase in the number of interaction sites for the interactions with the quadrupolar carbon dioxide molecules in the dicationic ionic liquids, compared to the monocationic analogues

Highly Selective Separation of Carbon Dioxide from Nitrogen and Methane by Nitrile/Glycol-Difunctionalized Ionic Liquids in Supported Ionic Liquid Membranes (SILMs)

Novel difunctionalized ionic liquids (ILs) containing a triethylene glycol monomethyl ether chain and a nitrile group on a pyrrolidinium or imidazolium cation have been synthesized and incorporated into supported ionic liquid membranes (SILMs). These ILs exhibit ca. 2.3 times higher CO₂/N₂ and CO₂/CH₄ gas separation selectivities than analogous ILs functionalized only with a glycol chain. Although the glycol moiety ensures room temperature liquidity of the pyrrolidinium and imidazolium ILs, the two classes of ILs benefit from the presence of a nitrile group in different ways. The difunctionalized pyrrolidinium ILs exhibit an increase in CO₂ permeance, whereas the permeances of the contaminant gases rise negligibly, resulting in high gas separation selectivities. In the imidazolium ILs, the presence of a nitrile group does not always increase the CO₂ permeance nor does it increase the CO₂ solubility, as showed in situ by the ATR-FTIR spectroscopic method. High selectivity of these ILs is caused by the considerably reduced permeances of N₂ and CH₄, most likely due to the ability of the −CN group to reject the nonpolar contaminant gases. Apart from the CO₂ solubility, IL–CO₂ interactions and IL swelling were studied with the in situ ATR-FTIR spectroscopy. Different strengths of the IL–CO₂ interactions were found to be the major difference between the two classes of ILs. The difunctionalized ILs interacted stronger with CO₂ than the glycol-functionalized ILs, as manifested in the smaller bandwidths of the bending mode band of CO₂ for the latter