Separations with calixarenes

Bunzli, J.-C.G., Besanjon, F., Ihringer, F., 2000. Bimetallic lanthanide supramolecular edifices with calixarenes. In Lumetta, G.J., Rogers, R.D., Gopalan, A. (Eds.), Calixarenes for Separations, vol. 757. American Chemical Society, Washington, DC, pp. 179-194 (chapter 14). 

For conformational isomers (conformers), whether they can be isolated as separate species is mainly a question of the energy barrier. This means that only a small difference may exist between chiral and nonchiral compounds, which is best illustrated by atropisomers of biaryl compounds. We therefore discuss in the following sections some interesting examples for chiral conformations found with calixarenes. 

The effect of the presence of crown ethers in the eluents [7,33], or in the stationary phases [34-37] on the effectiveness of separation of alkali metals on cation exchangers has been investigated. Higher selectivity of Na separation was observed on silica resins impregnated with calixarene [38]. 

Supramolecular dyad molecules are also available by utilizing host-guest chemistry. Konishi et al. reported that improvement of electron-transfer efficiencies could be attained in the electron-transfer systems composed of donors and fuUerenes connected with calixarene, in which calixarene could capture the electron donor [136]. It is interesting to note that back electron transfer in these systems obeyed second order kinetics, indicating that the generated radical ions are solvated separately after electron transfer by deforming the supramolecular. 

In this chapter selected examples of metal ion separations with polymeric macrocycles such as crown ethers, calixarenes, resorcinarenes, calixcrowns and cyclodextrins, reported in recent literature, are presented. Particularly, the use of those polymers in separation processes such as ion flotation, solvent extraction as well as transport across liquid and polymer membranes is shown. First, selected examples of crown ether polymers variety cross linked as metal ion carriers are described, then selectivity species by donor sites bonding and coordination are characterized. 

After the separation of the actinides from the high-level waste, it is desirable to remove certain other fission products from the nuclear wastes. Some Cs and Sr are low-charged cations that react well with macro-cyclic ligands (e.g., crown ethers, calixarenes). Research to synthesize and investigate the properties of macrocyclic ligands for application in nuclear waste treatment has been an active effort internationally. Some of the results obtained are discussed in section 12.7. 

Single-element Separation Extraction of Cs + ion is fairly difficult due to the small charge density of the atomic surface. Thus, calix-crowns were preferentially used for the extraction, because they trap Cs + ion not only by coordinating with the crown ring, but also by interaction with the n-electrons of the phenyl rings of the calixarene (382, 383). On the other hand, many reports appeared concerning extraction of Sr2+ from acidic solutions by crown ethers (384). 

In mixtures with the hydrophobic dicarbollide anion, the extraction of Ca, Sr, and Ba by calixarenes with oxygen donor atoms improves, allowing the separation from alkali ions.131 The competition by alkali ions is lower than in crown ether extraction systems. Separation factors S with CA2 in nitrobenzene are log S = 7 (Ca), 5.5 (Sr), and 5.4 (Ba). 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

In order to find compounds able to perform an efficient lanthanide/actinide separation, the Twente group prepared two cavitands (Cvl and Cv2) functionalized with CMPO groups and two cavitands (Cv3 and Cv4) with carbamoylmethylphosphonate (CMP) groups192193 (see Section 4.7). Distribution ratios displayed by CMPO cavitands are much lower than those found for the calixarene counterpart. This important decrease of extracting ability of cavitand is probably due to the presence of a carbon atom between the benzene unit and the nitrogen atom, causing N-protonation below pH 2. Furthermore, the Am/Eu selectivity is less than that of CMPO-calix[4]... 

As was shown first by Rais and Tachimori,31 a synergistic effect was also observed for mixtures of CDD with heterocyclic compounds. This direction was also actively investigated in the last years. Mixtures of CCD with N-tridentate heterocyclic compounds,32 diamides of dipicolinic acid (DPA),33-35 and calixarenes with heterocyclic reaction centers36-38 were studied for extraction. For the most part, extraction of MAs and separation of MAs from lanthanides were the center of attention in these works. But, it should be mentioned that all solvents with CCD also extract cesium. 

Recently the separation of enantiomers by RIfS and SPR using calixarenes with chiral amide residues was demonstrated [19]. Chirasil-Calix (Fig. 7) is well known from capillary GC as a stationary-phase material because of its good thermal and long-term stability. The separation of amino acid derivatives and lactic esters was widely studied [23]. 

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