Metal separation using supported liquid membranes

The solubilities of the various gases in [BMIM][PFg] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and CO2 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub CO2 from stack gases composed of N2 and O2. Since we know of no measurements of H2S, SO, or NO solubility in [BMIM][PFg], we do not loiow if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

Ho, W.S. and Wang, B., Inventors Commodore Separation Technologies Inc., Assinee. Combined supported liquid membrane/stripping dispersion process for removal and recovery of metals Dialkyl monothiophosphoric acids and their use as extractants, US Patent... 

Promising results are shown by recently developed integrated SLM-ELM [84, 85] systems. These techniques are known as supported liquid membrane with strip dispersion (SLMSD), pseudo-emulsion-based hollow fiber strip dispersion (PEHFSD), emulsion pertraction technology (EPP), and strip dispersion hybrid Hquid membrane (SDHLM). AH techniques are the same the organic phase (carrier, dissolved in diluent) and back extraction aqueous phase are emulsified before injection into the module and can be separated at the module outlet. The difference is only in the type of the SLM contactors hoUow fiber or flat sheet and in the Hquid membrane (carrier) composition. These techniques have been successfuUy demonstrated for the removal and recovery of metals from wastewaters. Nevertheless, the techniques stiU need to be tested in specific apphcations to evaluate the suitabUity of the technology for commercial use. 

The solubility of different gases in the various ILs, as discussed above, suggests that ILs should be excellent candidates for a wide variety of industrially important gas separations. There is also the possibility of doing higher temperature gas separations due to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric... 

Indeed, the tuneable properties of ILs associated with their environment-friendly perception have increased their investigation as alternative reaction media to replace traditional organic solvents in organic synthesis [11-13], catalytic reactions [12-16], electrochemical applications [17-19], biochemistry [20-24], and material engineering [25], It has also been reported that ILs can be used in extraction and as liquid phase in supported liquid membranes (SLMs) for the separation and recovery of organic compounds, metals, and gases [26-31]. 

Various substances such as amino acids, organic acids, NaOH, NaCl, carbon dioxide, oxygen, metals, and various ions, such as Cd(II), Cu(II), Co(II), and Fe(III), can be separated by using suitable carrier agents in liquid or solid composite membranes. Liquid membranes behave like double liquid-liquid extraction systems where the usage of organic solvent is minimized. Such devices are generally prepared as bulk liquid, emulsion liquid, and supported liquid membranes. 

Extensive studies including both inner-sphere and outer-sphere complexation of cations were performed with lasa-locid A, which is a small natural ionophore containing a salicylic acid fragment (Figure 1). The ability of lasalocid to form neutral outer-sphere complexes with species like Co(NH3)(5 +, Cr(bpy)3 ", Pt(bpy)(NH3)2 " " allows one to use it as an ionophore for the membrane transport (including chiroselective transport) of such species. The lasalocid ionophore also was shown to be an efficient carrier for toxic water-soluble metal cations such as Pb + and Cd + across artificial flat-sheet-supported liquid membranes, which represent a potential system for separation of these cations. 

The use of 13a in the extraction process or in the transport through supported liquid membranes (SLMs) allows to recover more than 98% of the cesium cation present in solution, making this derivative extremely attractive for declassification of nuclear wastes. Ligand 13a was dso used for the selective detection of cesium in ISEs and ISFETs with very high selectivity and low detection limit. Very recently, we anchored calix[4]arene-crown-5 and -crown-6 derivatives on silica-gel via hydrosilanization and we were able to separate by chromatography potassium or cesium fi"om other alkali metal ions with high efficiency. ... 

Abstract. The synthesis of 1,2- and l,3-calix[4]-Z w-crowns, double calix[4]arenes and double calixcrowns have been shown to depend on the reaction conditions (nature of the base, structure of the ditosylates, and the stoichiometry of the reactants). The 1,3-altemate conformation of the 1,3-calix[4]- w-crowns was shown to be favourable to the selective complexation of cesium cation. The observed Na /Cs selectivity was exploited in separation processes using them as carriers in transport through supported liquid membranes (SLMs). The best Na "/Cs selectivity (1/45 000) was observed for the naphthyl derivative 7. Calix(aza)crowns and 1,3-calix[4]-/ w-(aza)-crowns were also produced through the preliminary formation of the Schiff base-calixarenes, which were further hydrogenated. The syntheses consisted of the 1,3-selective alkylation of calixarenes followed by cyclization into a 1,3-bridged calixarene or by the direct 1,3-capping of the calixarene with appropriate ditosylates. Soft metal complexation by these ligands is also presented. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

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