Strong Acid Cation Exchangers

The most important strong acid cation exchangers are those of arylsulfonic acid type.

Polycondensation structures of this type can be obtained as follows:

1. By the sulfonation of a phenol followed by the condensation of the sulfonated product with formaldehyde.

2. By the sulfonation of a preformed phenolformaldehyde three-dimensional network.

In the first method, the addition of unsulfonated phenol to provide the trifunctionality is essential. The structures created are illustrated in Figure 1.

A method for the synthesis of sulfonated condensation exchangers in bead form has been developed using organic solvents as dispersion media. This is an alternative to the grinding of bulk polymers as previously mentioned.

Most commercially available strong acid cation exchangers are those based on styrene-DVB copolymers with different morphologies of their three-dimensional networks. These products have higher capacities and better durabilities than their polycondensation predecessors. The common method for the production of these structures consists in sul-fonation of the styrene-DVB copolymers with sul-fonation agents such as sulfuric acid, sulfur trioxide, oleum or chlorosulfonic acid.

From the point of view of the mechanism, the sulfonation is an electrophilic substitution into an aromatic ring whereby the -SO3H group is attached in the para-position and a double sulfonation is probably impossible because of steric hindrance due to the polymer chain.

During the sulfonation reactions, crosslinking side reactions take place independent of the sulfonating agent, however chlorosulfonic acid apparently leads to the most crosslinks.

Side crosslinks are due to the inter-chain sulfone bridges that appear by reaction between the already attached -SO3H groups and the unreacted aromatic rings. Intra-chain sulfone bridges also can appear. The chemical structure of a sulfonated styrene-DVB copolymer is illustrated in Figure 2.

The pre-swelling with organic solvents of the copolymer beads before sulfonation reduces the number of sulfone bridges.

Addition to the styrene-DVB mixture of small amounts of a polar monomer, such as acrylonitrile, vinylpyridine, etc., improves the physical properties of the resultant ion exchanger - especially its resistance to osmotic shock because of the more uniform sulfonation reaction.

Sulfonations with the agents previously mentioned show some differences. Thus, reaction with sul-furic acid used the acid itself as a reaction medium hence a considerable excess of reagent is required. Reactions with chlorosulfonic acid or sulfur trioxide may be performed in an organic solvent, thus they need only a small excess of reagent over the stoichiometric quantities.

Sulfonations with the latter reagents take place at lower temperatures than with sulfuric acid which requires a temperature at about 100°C.

Post-sulfonation treatment of the sulfonated products is important to maintain whole beads. This can be achieved by the prevention of the changes that determine swelling, called 'osmotic shock', which leads to the disintegration of the beads. The gradual addition of water, or aqueous electrolyte solutions, decreases this deleterious effect.

Macroporous copolymer beads, because of their large internal surface areas, have a higher reactivity towards the sulfonation agents. They also require much lower quantities of organic swelling solvent, and are less susceptible to degradation by osmotic shock, both during preparation and in subsequent usage. In addition, they have a higher oxidation stability than the sulfonated structures of the gel type.

Strong acid cation gel-type exchangers have received major attention because of their utility in

Figure 2 Chemical structure of sulfonated styrene-DVB copolymer-based cation exchanger.

water softening which is their principal use. The equivalent macroporous structures can also be used as catalysts for certain reactions, particularly in non-aqueous media, instead of sulfuric and toluene-4-sul-fonic acids. The resin catalysts show some advantages compared to low molecular weight acids, such as in their regeneration and potential reuse.

Because of their very high acidity, the aryl-SO3H groups are fully ionized throughout the pH domain of aqueous solutions. The very low preference of the sulfonic-type cation exchanger for the H ion requires the use of large quantities of mineral acids for its regeneration to the H form after the exhaustion cycle, especially in water treatment processes.

The strong acid exchanger in its H form participates in ion exchange reactions with bases like NaOH and with alkaline or neutral salts. The latter reaction is called 'salt-splitting'.

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