Strong Base Anion Exchangers

Strong base anion exchangers are known only as polymerization products. Those with quaternary ammonium groups are the most common commercially available exchangers. Their preparation is performed by the chloromethylation of gel- or macroporous-type styrene-DVB copolymers in bead form, followed by the amination of the chloromethylated copolymers with trimethylamine or dimethylethanolamine leading to the so-called strong base anion exchangers of Types I and II, respectively. Their chemical structures are shown in Figure 4.

Usually the chloromethylation is carried out with monochloromethyl ether, in the presence of a Lewis acid (ZnCl2, AlCl3, SnCl4, etc.) as catalyst. The reaction takes place under mild conditions: temperature about 50°C and reaction times of 5-8 h. Generally, the -CH2Cl groups are attached to over 90% of the para-positions of the styrene aromatic rings, following the chloromethylation of the mono alkylbenzene derivatives.

The main chloromethylation reaction is usually accompanied by a side alkylation reaction between pre-attached -CH2Cl groups and non-functionalized aromatic rings. Such a side reaction determines interchain and/or intra-chain methylene bridges that decrease the amount of -CH2Cl groups as well as the swelling capacity of the chloromethylated product. The latter aspect is especially prevalent in the case of gel-type copolymers. In most cases, the styrene-DVB macroporous networks show a reduction of their specific area and of the volume of their pores after chloromethylation, but an increase of the average diameter of the pores can be observed.

The use of a large excess of chloromethyl methyl ether or mixtures of chloroform or carbon tetrachloride with the halogenated ether reduces the side reaction.

An alternative route to obtain the chloromethylated styrene-DVB network is via the free-radical polymerization of chloromethylstyrene (vinylbenzyl chloride) with divinylbenzene. The first monomer is a 60 : 40 mixture of meta-: para-isomers.

The chemical structures of the two crosslinked polystyrene-based chloromethylated compounds or products are illustrated in Figures 5A and B. From these two figures one can see that the chloromethyl-styrene-DVB copolymer (Figure 5A) has a more homogeneous chemical structure than the chloro-methylated styrene-DVB copolymer (Figure 5B), but the former structure has the drawback of a much higher cost. For this reason chloromethylated styrene-DVB copolymers are chosen as the precursors to polystyrene-based anion exchangers.

Aminations of the chloromethylated styrene-DVB copolymers with trimethylamine and dimethyl-ethanolamine take place easily, because the benzylic chlorine structure has a very high reactivity towards these nucleophilic reagents. Amination is performed

Figure 3 Some methods for the preparation of weak acid cation exchangers.

in organic or aqueous media, at a temperature of about 40-50°C, and reaction times of 6-8 h. It must also be mentioned that amination with the two amines, in contrast to the chloromethylation reaction, does not lead to crosslinking side reactions. When the reactions are carried out in water, a side reaction can occur at a very low level from the hydrolysis of a small number of -CH2Cl groups.

The chemical structures of strong base anion exchangers of Types I and II are not very stable in alkaline media because of the well-known Hofmann degradation, a property of quaternary ammonium compounds; the Type II displays a lower stability in alkaline media than Type I.

Hofmann degradation of the two structures takes place according to Figure 6. The degradation can lead to both loss of exchange capacity (routes A and A' in Figure 6) and the appearance of a weak base capacity caused by the presence of tertiary amine groups (B, B' and C in Figure 6).

Strong base anion exchangers have a lower thermal stability than the cation exchangers.

Other commercially available strong base exchangers are those formed with an acrylic matrix. They are usually made in bead form by free-radical polymerization of 3-dimethylaminopropyl methac-rylamide with DVB followed by a quaternization reaction of the copolymer with alkyl halides as shown

Figure 4 Classical structures of the structural units of Type I and Type II strong base anion exchangers.

in Figure 7. For the quatemization, gel- or macropor-ous-type copolymers can be used.

Generally, the acrylic strong base anion exchangers have a lower stability to hydrolysis, especially under acid or alkaline conditions, compared to the polystyrene-based exchangers. The hydrolysis becomes more significant when the spacer between the amide group and the quaternary group decreases in size. Thus, the product with a spacer of only one methylene group between the two functional groups has hydrolytic instability. The same phenomenon occurs in the anion exchanger prepared from 3-dimethylaminopropyl methacrylate, CH2=C(CH3)COO(CH2)3N(CH3)2, instead of the amide monomer.

In addition to the strong base anion exchangers previously presented as commercially available products, other specialized strong base exchangers are known.

In an effort to develop anion exchangers with preference for the NO3" anion over the SO2~ anion (an important factor for nitrate removal from potable water which invariably contains sulfate), the design of such a structure was conceived. It is the reaction product of the chloromethylated styrene-DVB copolymer with triethylamine, and can be described as a strong base anion exchanger of Type III.

Gel or macroporous 4-vinylpyridine-DVB co-polymers are the precursors for strong base

Figure 5 The two crosslinked polystyrene-based chloromethylated structures.

Figure 6 Hofmann degradation of Type I and Type II strong base anion exchangers.

CH2CH2OH

Figure 6 Hofmann degradation of Type I and Type II strong base anion exchangers.

exchangers. These exchangers are made by the well-known quaternization reaction with alkyl halides as shown in Figure 8.

The synthesis of this category of anion exchangers takes place by a single chemical transformation step which avoids crosslinking side reactions. However, these exchangers cannot be utilized in many fields of application because of their very low chemical stability in alkaline media.

Ion exchangers with benzyltrialkylphosphonium groups, especially benzyltri-n-butylphosphonium halide can be made. These structures are not used in ion exchange processes but have special applications as phase-transfer catalysts. For the improvement of their properties, structures with a spacer larger than one methylene group between the aromatic ring and the phosphonium group have been synthesized. Figure 9 shows the phosphonium-type structures and their preparative routes.

Commercially available exchangers of Types I and II are fully ionized in the whole pH domain of the aqueous medium, like the strong acid ones. The Type I exchanger is such a strong base that a considerable quantity of NaOH is required for its regeneration in the OH form, while the Type II exchanger, a weaker base, requires less. This aspect is an advantage of the Type II structure over Type I.

The strong base anion exchangers in their OH form react with both strong and weak acids. With the latter, the strong base anion exchanger of Type I is more effective than Type II. Because of this situation, Type I exchangers are used for soluble and

Figure 7 Preparation of an acrylic strong base anion exchanger.

colloidal silica removal from natural waters. For removal of the colloidal silica only, Type I strong base anion exchangers with special macroporous structures are effective.

Certain Type I strong base exchangers in their Cl form are used for adsorption of ionic organic compounds and are called 'scavenger' ion exchangers.

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