Supported Ionic Liquid Membranes SILMs

Dispersing the IL as a thin film on a membrane support yields materials that can have improved separation properties [43]. Recent reviews highlight the successful application of these supported ionic Hquid membrane (SILM) materials in organic extraction processes [44, 45] as well as analytical [46-52] and electrochemical applications [53-56]. 

A variety of membrane materials have been tested for SILM preparation, including, among others, polymeric Nylon (hydrophilic polyamide) [57] and PTFE (hydrophobic polytetrafluoroethylene) [58], PES (hydrophiHc polyethersuUbne) [59, 

Several groups have studied the gas transport and separation mechanism via SILM materials in recent years [68, 71, 72]. The quahty of the separating compound i from compound j is described by the selectivity a j according to Eq. (21.2). 

The ideal permeabiHty P of a given species i is given for each compound and each SILM material, and can be calculated from the steady-state flux through the membrane of thickness I and the pressure drop dp across the membrane according to Eq. (21.3). 

The permeance Pu,i for a substance i is defined as the ratio of the permeability P related to the membrane thickness I according to Eq. (21.4). 

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Table 7.3 shows a classification of the liquid membranes on the basis of the configuration and module types employed in gas separation. The liquid membranes can be divided in three main classes (i) supported liquid membrane (SLM), (ii) bulk liquid membrane (BLM), and (iii) supported ionic liquid membrane (SILM). 

Park Y-I, Kim B-S, Byun Y-H et al (2009) Preparation of supported ionic liquid membranes (SILMs) for the removal of acidic gases from crude natural gas. Desalination 236 342-348... 

Supported ionic liquid membranes (SILMs) have been recently brought into focus due to their unique properties that can prevent the loss of solvent by evaporation. Ionic liquids (ILs) are organic salts with negligible vapor pressures. They are thermally robust with a wide temperature range in the liquid state, for example, up to 300°C, compared to 100°C for water, and their polarity and hydrophilicity/ hydrophobicity can be tuned by a suitable combination of cation and anion. Noble et al. have extensively studied SILMs for CO2 separation, including the gelation of SILMs. Excellent CO2 separation performance has been reported [127-129],... 

Since the concept of supported ionic liquid membranes (SILMs) is quite similar to that of conventional SLMs, it seems logical that the same membrane configurations used in the latter will be useful when ILs are used as carriers in supported membrane operation. Between these different configurations, we can find the following. 

The supported ionic liquid membranes (SILMs) were prepared by depositing the ionic liquids [hmim][Tf2N] and [H2NC3H6mim][Tf2N] on top of cross-linked nylon support discs in a shallow glass container. A sufficient amount of ionic liquid to cover the membrane was used. The membrane was allowed to absorb the ionic liquid for at least 4 hours, and then the SILMs were removed from container and blotted dry. Further details concerning this procedure were published earlier [19]. 

Membrane processes offer another attractive option to apply ionic liquids in separation processes. Two general approaches have been reported the use of ionic liquids as bulk ionic liquid membranes (BILM) [131,132] or as supported ionic liquid membranes (SILM) [133, 134]. In the former case, the ionic liquid acts as selective separation layer in the latter case, the membrane supports the liquid separation layer and separates two conpartments, resulting in a very efficient ionic liquid use and separation unit. Reported exanples for SILMs include reactions with in situ product removal [135], extraction of bioorganic substances [136,137] and removal of CO2 and SO2 from gas streams [138,139]. SILMs have also been successfully... 

SLMs are perous membranes with the peres saturated with a solvent mixture. SLMs suffer significant solvent loss due to volatilization when conventional solvents are employed as supported liquid. The used of ILs as the immobilized phase within the pores of the membranes is the improving in the membrane stability and their performance do not dep>end of the water paesence (a. Scovazzo et al., 2009). Supported ionic liquid membranes (SILMs) increase the efficiency and selectivity of separation resp>ect to non-supp>orted liquid membranes because the higher area of contact IL-gases. 

The use of supported ionic liquid membranes in different fields of application has received growing attention during last decade. One of the most studied applications of SILMs is the selective separation of organic compounds. The first example was reported by Branco et al. [18], who studied the selective transport of 1,4-dioxane,... 

The prepared membranes SILMs showed high CO2 permeability (744 barrer) and CO2/He selectivity of 8.6. Furthermore, the stabihty of the membranes at higher temperature (125 °C) approached the range of interest in the capture of CO2 from coal gasification plants. However, higher temperatures could not be reached mainly due to the support failure rather than any effect on the ionic liquid. 

In the last decade, supported liquid membranes based on ionic liquids (SILMs) have been successfully applied in separation and purification of organic compounds, involved in the synthesis of pharmaceutical and fine chemicals, (alcohol, esters, organic acids and amino acids) and mixed gases [16-25],... 

The immobilization method was also found to have influence on the membrane stability. A comparative study of the preparation of SILMs by two different methods, under pressure and vacuum were reported by Hemandez-Femandez et al. [26]. They used the ionic liquids, [bmim+][Cl"], [bmim ][BF ], [bmim ][PF "] and [bmim llNTf ] as liquid phase supported on a nylon membrane. Small losses of ionic liquid were observed after 7 days of operation when the ionic liquid was immobilized under pressure in a diffusion cell using n-hexane/n-hexane as surrounding phases. However, the losses of IL were higher when immobilization was carried out under vacuum, especially with the most viscous ionic liquids ([bmim+] [PF ] and [bmim+][CT]). This behaviour was explained by the fact that the higher viscosity of ILs makes difflcult their penetration into the middle of the deeper pores of the membrane, and therefore, the ionic liquid was mainly immobilized on the most external layer of the membrane, and consequently, the immobilized ionic liquid is more easily removed during operation. 

The resulting SILMs were stable under assayed conditions. These authors highlighted the importance of the consideration of two main possible effects on the performance and stability of SILMs in water mediums (a) the loss of ionic liquids phase from the supporting membrane to the adjacent aqueous solutions by dissolution/ emulsification and (b) the formation of water microenvironments inside the ionic liquids phase, which constitute new, non-selective environments for solute transport, leading to a deterioration of the SLM performance and selectivity. 

Temperature stability is important for some applications of SILMs in gas separation, such as capture of CO from coal gasification plants. Ilcovich et al. [25] analysed the stability of a SILM based on [hmim+lfNTfj ] supported on a polysulphone organic membranes in the selective separation of CO from He at high temperature. This membrane was found to be stable up to 125 C, the failure of the membranes above that temperature being attributable to support failure rather than any effect on the ionic liquid. Recently, Myers et al. [32] reported operation of [hmim ][NTfj ] supported on nylon membranes up to 300 C. It was found that permeability in this [hmim [NTfj ] membrane increased with temperature while the selectivity decreased. 

SO.,"] < [MeSO.,"] [40]. These results were quite encouraging and suggested that these SLMs based on ILs could be used for the selective separation of the organic esters from the reaction mixture. SILMs can also be used for the separation of aromatic hydrocarbons from aliphatic hydrocarbons. In this context, the selective separation of benzene, toluene and p-xylene from n-heptane was achieved using SILMs based on [bmim ][PE "], [hmim+][PFg ], [omim+][PFg ] and [Et MeMoEtN [TfjN"] supported on a polyvinyhdene fluoride manbrane [9]. It was found that aromahc hydrocarbons were successfully transported through the membrane based on these ionic liquids, and the maximum selectivity to n-heptane was when benzene used in the aromatic permeation and [bmim+][PFg ] was taken in the hquid membrane phase. 

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