Organic Resins

These are the most widely used of exchangers. They are made by addition polymerization processes to

Figure 4 Breakthrough curves. (A) Favourable equilibrium, KA> 1, shape of profile constant throughout the bed. (B) Un-

favourable equilibrium, spread out with time. permission.)

K£<1, edge of profile becomes more (Reproduced from Harland, 1994, with produce resins capable of cation and anion exchange. There is much on-going research devoted to devising synthetic routes to new resins aimed at the refinement of their capabilities, but the bulk of commercial production follows well-established routes.

Polystyrene resins Ethenylbenzene (styrene) readily forms an addition polymer with divinylbenzene (DVB) when initiated by a benzoyl peroxide catalyst. The polymerization process can be controlled to produce resins with various degrees of cross-linking as robust, spherical, beads. The ability to vary the extent of cross-linking increases the range of possible applications by altering the physical and chemical nature of the beads. In addition the production process can be moderated to give beads of closely controlled particle size distribution, a requirement for the industrial use of resins in large columns. Subsequent treatment of the styrene-DVB copolymer beads can introduce ion exchange properties. If the beads are treated with hot sulfuric acid the aromatic ring systems will become sulfonated, thereby introducing the sulfonic acid functional group (-SO3H) into the resin. When the treated resins are then washed with sodium hydroxide or sodium chloride, the sodium form of the resin (R) is produced, namely:

The sodium form is used as a strong acid cation exchanger, the sodium ion being the ion for which the resin has least selectivity.

Anion functionality can be introduced by a two-step process. The first step involves a chloromethyla-tion using a Friedel-Crafts reaction between the copolymer and chloromethoxymethane with an aluminium chloride catalyst. The second step is to react the chloromethyl groups (-CH2Cl), introduced into the styrene moities, with an aliphatic amine. If this is trimethylamine,(CH3)3N, then the functional group produced on the resin is R—CH2N(CH3)+ Cl , and the resin is said to be a Type I strongly basic anion exchanger. The use of dimethylethanolamine [(CH3)3(C2H4OH)N] to react with the chloromethyl groups yields a resin with the functional group R—CH2N(CH3)2(C2H4OH) + CP, which is a Type II strong base anion exchanger. When methylamine, or dimethylamine, are used weakly basic resins are obtained, with the respective functional groups R—CH2NH(CH3) and R—C^N(CH3^.

Acrylic resins DVB forms polymers suited to ion exchange with materials other than styrene. The most commonly used are its copolymers with propenoic (acrylic) monomers. The use of methylpropenoic acid gives a weakly basic cation exchange resin (R—C(CH3)COOH). Substituted propenoic acid monomers, propenonitriles (acetonitriles), and alkyl propenoates (acrylic esters) have all been used to make weakly basic resins. The acrylic matrix can also play host to anion functionality. Incorporation of dimethylaminopropylamine (DMAPA) produces a weak base resin, while the employment of a subsequent chloromethylation step converts this to a strong base functionality. Acrylic resins can be used to develop a material with simultaneous properties of a weak and strong base. These are called bifunctional anion exchangers. The equivalent bifunctional cation exchanger is not now commercially available, although products of this sort have been marketed in the past. The acrylic resins have advantageous kinetic and equilibrium properties over the styrene resins when organic ions are being exchanged.

Selective resins The resins described above have been developed as nonselective exchangers, where the aim is to reduce the ionic content of an aqueous media to a minimum, such as is required in the 'polishing' of industrial boiler waters to reduce corrosion.

The flexibility offered by the skill of the synthetic organic chemist facilitates the introduction of specific groups into the polymer matrix to give the resulting exchanger the ability to take up an ion, or a group of ions, in preference to other ions. An example of this is the incorporation of the iminodiacetate group (—CH2N(CH2COO~)2) in a styrene-based matrix, which is then able to scavenge Fe, Ni, Cu, Co, Ca, Mg cations with the exclusion of other ions present. The iminodiacetate group is then described as a selective ionogenic group; further examples of these are given in Table 1.

Resins of this sort are continually being developed for specialist applications. The example in Table 1 of the use of a phenolic ionogenic group to pick up caesium has arisen from the nuclear industry. In this case a phenolformaldehyde copolymer is used to meet the temperature and radiation stability needs of that industry.

The interaction between a selective ionogenic group and a cation probably will not be strictly ionic. Often there has been a deliberate intent to induce chelating effects to achieve the desired selectivity. If this has happened, then the rate-controlling step for progress of cations into the resin is likely to be the formation of a chemical bond, as mentioned earlier, rather than a diffusion process. When the cation would not be expected to form strong chelate bonds with the ionogenic group, such as the caesium cation mentioned above, then the nature of the rate-determining step is less clearly defined. If a thermodynamic approach to a specific exchange process is wanted these facts must be considered. Clearly a true chelating process will not be reversible and the theories of ion exchange, which are reliant on the application of reversible thermodynamics, cannot be invoked.

This introduces a grey area into the study of the uptake of ions onto a substrate supposedly capable of ion exchange. The problem often arises in the study of inorganic ion exchange materials — particularly oxides and hydroxides when uptake is pH-dependent,

Table 1 Examples of ionogenic groups and their selectivity

Matrix

Group

Selectivity

Styrene-DVB Styrene-DVB Styrene-DVB Styrene-DVB Styrene-DVB Phenol-formaldehyde

Iminodiacetate — CH2— N(CH2COO~)2 Aminophosphonate — CH2— NH(CH2PO3)2~ Thiol; thiocarbamide —SH;—CH2—SC(NH)NH2 N-Methylglucamine —CH2N(CH3)[(CHOH)4CH2OH] Benzyltriethylammonium —C6H4N(C2H5)3+ Phenol: phenol-methylenesulfonate —C6H3(OH), —C6H2(OH)CH2SO3

Fe, Ni, Co, Cu, Ca, Mg Pb, Cu, Zn, UO2+, Ca, Mg Pt, Pb, Au, Hg B (as boric acid)

no3-

Cs

and surface deposition of metal oxides and salts can occur. In many cases workers have found that the use of Freundlich isotherms (or similar treatments) can be successfully used to describe ion uptake.

Resin structures The traditional resins made as described above have internal structures created by the entanglement of their constituent polymer chains. The amount of entanglement can be varied by controlling the extent to which the chains are cross-linked. When water is present, the beads swell and the interior of the resin beads resembles a gel electrolyte, with the ingoing ion able to diffuse through regions of gel to reach the ionogenic groups. The ions migrate along pathways between the linked polymer chains that are close in dimension to the size of hydrated ions (cations or anions). This means that the porosity that they represent can be described as microporous. It is not visible even under a scanning electron microscope, as illustrated in Figure 5, and cannot be estimated by the standard methods of porosity determination, such as nitrogen BET or porosimeter measurements.

The tightly packed nature of these gel-type resins increases the chance of micropore blockage in applications where naturally occurring high molecular weight organic molecules (e.g. humic and fulvic acids) are present in water. This organic fouling was present in the earlier anion exchangers and led to the development of a new type of resin with more open internal structures. This was achieved by two routes, the sol and nonsol route.

In the sol method a solvent capable of solvating the copolymer is introduced into the polymerization pro cess. If the cross-linking is high (about 7-13%), pockets of solvent arise between regions of dense hydrocarbon chains. When the solvent is subsequently removed by distillation, these pockets are retained as distinct pores held by the rigidity arising from the cross-linking. In the nonsol method the organic solvent does not function as a solvent for the copolymer, but acts as a diluent causing localized regions of copolymer to form. These regions become porous when the diluent is removed.

These resins are termed macroporous, and the extent of their regions of porosity can be readily measured by porosity techniques and are visible in scanning electron micrographs (see Figure 6). Some literature describes them as macroreticulate because the pores they contain cover a much wider pore size distribution than the conventional International Union of Pure and Applied Chemistry (IUPAC) definition of a macroporous material. The IUPAC definition is traditionally related to inorganic materials where a macropore is one of greater than 50 nm in width. Figure 7 illustrates the envisaged pore structure of a macroporous resin.

Macroporous resins are commercially available with acrylic and styrene skeletons, both cation and anion, carrying all types of functional groups. Their successful development has spawned two other major uses of acrylic and styrene resins that need highly porous media to function properly. These are the employment of resins as catalysts, and their use in the separation and purification of vitamins and antibiotics. Although these are of high industrial significance, they fall outside the intent of this article and will not be considered further.

Figure 5 Scanning electron micrograph of the internal surface of a gel resin. Magnification x 17000. (University of Manchester Electron Microscopy Unit, courtesy of Hoechst Celanese Corporation.)
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