SILMs membranes

In 2009 Scovazzo published a significant paper where literature data obtained using the proposed model to predict gas solubility and gas permeability was summarized, along with adding new data, on the SILMs membranes permeabilities and selectivities for the gas pairs CO2/N2, CO2/CH4, O2/N2, ethylene/ethane, propylene/propane, 1-butene/butane, and 1,3-butadiene/butane, with the object as to serve as guide for future researches in this area. 

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). 

In an other work, the SILMs have been employed for the selective separation of CO2 from He [16]. The CO2/He gas pair is useful, as more safely tested surrogate, for CO2/H2 which will be important in green house gas abatement from advanced coal gasification plants. The SILMs were made by soaking two different porous membranes (polysuhbne and polyethersulfone, 0.2 pm pore diameter) in the ionic hquid [hmim] [Tf N]. 

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. 

Scovazzo P (2009) Determination of the upper limits, benchmarks, and critical properties for gas separations using stabilized room temperature ionic liquid membranes (SILMs) for the purpose of guiding future research. J Membr Sci 343 199-211... 

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... 

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],... 

Scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) analysis could be used to physico-chemical characterization of SILMs [26]. This technique allows the characterization of the membrane surface morphology and the examination of the global chemical composition of the membranes and the distribution of the ILs within pores. Figure 11.2 shows examples of SEM micrographs of a plain nylon membrane and supported liquid membranes based on [bmim ][PF ] prepared by using the pressure method [26]. 

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 stability of a SILM based on [bmim ][BF ] supported in a nylon membrane has been also analysed in other organic solvents, such as n-hexane//eri-butyl methyl ether and n-hexane/dimethyl sulphoxide [29]. The SEM-EDX study of the membranes after continnons operation showed that the stability of the supported liquid membrane increases with the decrease of the polarity of the solvent used. 

The stability of SILMs based on l-n-alkyl-3-melhylimidazolium hexafluorophos-phate (n=4,8,10) supported on poly vinylidene fluoride (PVDF) membrane towards contacting aqueous phase has been also analysed by several authors [4, 17, 30]. 

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 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,... 

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. 

Other interesting field of apphcation of supported ionic hquid membranes is the separation of mixed gases. Since SILMs may have the potential for industrial applications, specifically, low-pressure systans such as the treatment of bio-methane from anaerobic digesters and CO capture from flue gases, much effort and many resources have been extended on developing new SILMs [24,41 3]. In this context. 

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. 

Gelation of SILMs, in which a gel is formed to avoid the flowing of the IL away from the gelled membrane. 

One of the most studied applications of SILMs is the selective separation of organic compounds. The first example was reported by Branco et al. [71], who studied the selective transport of 1,4-dioxane, 1-propanol, 1-butanol, cyclohexa-nol, cyclohexanone, morpholine, and methyhnorpholine as a model of seven-component mixture of representative organic compounds. For that, four ILs based on the l- -alkyl-3 methylimidazolium combined with the anion hexafluoro-phosphate or tetrafluoroborate, immobilized in different organic polymeric membranes, were used. The use of the IL [bmim+][PFg ] immobilized in a PVDF membrane allowed an extremely highly selective transport of secondary amines over tertiary amines (up to a 55 1 ratio). 

SILMs have also been successfully applied to the selective separation of bisphenol A from aqueous solution [70], For these experiments, PVDF membrane was used as a support medium, and ILs based on different cations such as phospho-nium, imidazolium, ammonium, and pyridinium were used as liquid phase. Obtained results proved that permeation of bisphenol A from feed phase to receiving phase was 44% for [TBP][PF6] followed by 39.88% for [OMIM][PF6]. A maximum of 62% permeation was obtained for [TBP][PF6] with pH control. 

Other recent application of SILMs based on IL was for removal of persistent organic pollutants (dioxins) [100]. These authors demonstrated the potential of using ceramic membranes with immobilized ILs for the removal of dioxins from high-temperature vapor sources. All membranes assayed were stable at temperatures up to 200°C. 

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