Chelating Resins

Ion exchangers incorporating chelating groups have higher selectivity among cations than conventional strong or weak acid resins. (In one sense, adsorption of a di- or trivalent cation by any resin is a chelation process, since the ion is coordinated by two or more ligands linked by a chain of covalent bonds, but the term chelating resin is more usually reserved for resins containing discrete chelating groups, each attached to a single monomer unit. A given cation may be coordinated by one or more of these chelating units.) Technical aspects of their use were outlined by Waitz in 1979. A review and a monograph emphasizing analytical applications were published by Warshawsky and Marhol, respectively, in 1982. A comprehensive review by Sahni and Reedijk appeared in 1984, and a shorter update by Beauvais and Alexandratos in 1998.

Resins with an enormous variety of chelating ligands have been synthesized in the laboratory, but few have been manufactured commercially. Essentially any complexing agent used in analytical chemistry can be coupled to a resin, although the challenge to the synthetic chemist is to avoid sacrificing ligand groups such as thiol or primary amino groups in the coupling reaction. Many published syntheses have been too complex to be economic on an industrial scale or have yielded resins of low stability, but new resin structures continue to be reported. In this review we can consider only a few illustrative examples.

Table 1 Selectivities of strong and weak acid cation exchangers3

Ion

Na +

K +

Mg2 +

Ca2 +

Zn2 +

Nl2 +

Cu2 +

Pb2 +

K

1.00

1.5

4.3

11

4.8

6.1

5.9

39

K

1.00

14

98

38

550

a Rational selectivities Kof a sulfonated polystyrene resin (8% cross-linked); molal selectivities K of a methacrylic acid resin (5% cross-linked, degree of ionization 0.85, background electrolyte 1 mol L~1 NaNO3), relative to Na#.

a Rational selectivities Kof a sulfonated polystyrene resin (8% cross-linked); molal selectivities K of a methacrylic acid resin (5% cross-linked, degree of ionization 0.85, background electrolyte 1 mol L~1 NaNO3), relative to Na#.

Table 2 Some commercial chelating resins

Chelating group

Examples

Amidoxime

Duolite ES 346a, Diaion CR-50b

Aminophosphonate

Duolite ES 467, Lewatit OC 1060c

Iminodiacetate

Dowex A-1d, Diaion CR-10,

Duolite ES 466, Lewatit TP 208

Diphosphonate

Diphonixe

Bis(2-picolyl)amine

Dow 3N (Dowex XFS 4195)

2-Picolyl-2'-

Dow 2N (Dowex XFS 43084)

hydroxypropylamine

Oligoamine

Diaion CR-20, Lewatit E 304,

Sumichelate MC10f

aRohm and Haas; b Mitsubishi Chemical; c Bayer; dDow Chemical; eEichrom Industries; 'Sumitomo Chemical.

aRohm and Haas; b Mitsubishi Chemical; c Bayer; dDow Chemical; eEichrom Industries; 'Sumitomo Chemical.

Early chelating resins based on phenolic polymers were largely displaced by styrene-divinylbenzene resins, functionalized via chloromethylation, because of their greater stability. More recently, acrylic polymers have gained ground, with glycidyl methacrylate being a favoured monomer because ligands are easily coupled by reaction with its epoxide group, although possible hydrolysis of the ester link at extremes of pH limits the applicability of these resins. Sherrington, Driessen and their co-workers have used this strategy to prepare a suite of chelating resins containing nitrogen heterocycles. Similarly, Chanda and Rempel have coupled a wide range of chelating groups to polyben-zimidazole beads activated with epichlorohydrin.

Table 2 lists some commercially available chelat-ing resins. The order of preference for iminodiacetate resins is typically:

Univalent cations: Ag > Li > Na > K > Rb > Cs Divalent cations: Hg > U(VI) > Cu > Pb > Ni

> Ca > Mg > Ba > Sr Trivalent cations: Cr > In > Fe > Ce > Al > La

Structures for the iminodiacetate and amino-phosphonate groups are shown in Figure 1. Some selectivity coefficients (no definition given) for iminodiacetate resins, recalculated from Waitz, are collected in Table 3.

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