Supported ionic liquid membranes stability

Supported Ionic Liquid Membranes Preparation, Stability and Applications... 

Here, a review on supported ionic liquid membrane technology including issues such as methods of preparation and characterization, stability, transport mechanisms and applications is presented. 

Fortunate R, Afonso CAM, Benavente J, Rodriguez-CasteUon E, Crespo IG (2005) Stability of supported ionic liquid membranes as studied by x-ray photoelectron spectroscopy. 1 Membr Sci 256 216-223... 

F.J. Hemandez-Femandez, A.P. de los Rfos, F. Tomas-Alonso et al.. Understanding the influence of the ionic liquid composition and the surrounding phase nature on the stability of supported ionic liquid membranes. AfC/iJ J. 58 (2012) 583-590. 

Fortunate, R Afonso, CAM Benavente, J Rodriguez-Castellon, E Crespo, J. Stability of supported ionic liquid membranes as studied by X-ray photoeleetron spectroscopy. Journal of Membrane Science, 2005, 256, 216-223. 

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. 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

Fortunato, R. et al.. Supported liquid membranes using ionic liquids study of stability and transport mechanism, /. Membr. Sci., 242,197, 2004. 

The ability of calixarenes to bind large metal ions with high kinetic stability is important in the search for complexants for radionuclides such as Cs (ti/2 = 30.2 yr) and Sr (ti/2 = 65 d) from the reprocessing of exhausted nuclear fuel. There has been considerable interest in caesium-complexed calix[4]-bis-crowns as selective Cs-carriers. Transport isotherms of trace level Cs through supported liquid membranes containing calix[4]-bis-crowns have been determined as a function of the ionic concentration of the aqueous feeder solutions, and l,3-calix[4]-bis-o-benzo-crown-6 appears to be much more efficient in decontamination than mixtures of crown ethers and acidic exchangers, especially in highly acidic media. " ... 

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. 

The liquid property of the API-ILs seems to be one solution to overcome the disadvantages of limited solubility, low bioavailability, variable polymorphs, and limited membrane transport, but in the same time may also present challenges related to their preparation, handling, and the need for special devices for dehvery. Recently, we showed that a supported ionic Hquid phase (SILP) strategy [17] not only can be successfully applied to API-ILs, but also provides an easier way to handle and dose these liquid APIs with additional advantages such as improved thermal stability and rapid and complete leaching from the solid support... 

Depending on their structure, symmetric membranes can be catalogued as porous and non-porous or dense (polymeric swollen-network), while asymmetric membranes for desalting applications (NF and RO, basically) consist of a dense and thin active layer and a thick porous sublayer for mechanical stability (usually an UF membrane). Moreover, supported liquid membranes (SLMs), and aetivated membranes (AMs), which basically consist in the immobilization of speeifie agents (organic solvent, carrier or ionic liquid at room temperature) in the pores/structure of a support membrane, have been developed for selective separation of valuable/contaminant compoimds [8-11]. 

Wang, B., Lin, J., Wu, F. and Peng, Y, Stability and selectivity of supported liquid membranes with ionic liquids for the separation of organic liquids by vapor permeation, Ind. Eng. Chem. Res. 47, 8355-8360 (2008). 

Calix[4]-W5-crowns 1-7 are used as selective cesium-carriers in supported liquid membranes (SLMs). Application of the D esi diffusional model allows the transport isotherms of trace level Cs through SLMs (containing calix[4]-6/5-crowns) to be determined as a function of the ionic concentration of the aqueous feed solutions. Compound 5 appears to be much more efficient than mixtures of crown ethers and acidic exchangers, especially in very acidic media. Decontamination factors greater than 20 are obtained in the treatment of synthetic acidic radioactive wastes. Permeability coefficient measurements are conducted for repetitive transport experiments in order to determine the SMLs stability with time. Very good results (over 50 days of stability) and high decontamination yields are observed with l,3-calfac[4]-Aw-crowns 5 and 6. 

To conclude, based on the uniqueness of ionic liquids, appropriate ionic liquid immobilization strategies can be used to separate REs. The levextrel resin, supported liquid membranes, and sol-gel materials containing ILs and extractants have high stability, capacity, and selectivity for the RE separation, demmistrating the unique properties of ILs. It will be more useful in RE concentration from very dilute RE solutions and the purification of high-purity REs. 

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