Synthesis of Anion Exchange Resin (IV)
3. Synthesis of weakly basic anion exchange resin
Chloromethylstyrene divinylbenzene copolymer can be aminated with many amines, and the aminated product with primary amine and secondary amine is weak base anion exchange resin.
The hydration ability of weak base group is slightly poor, and the expansion in alkaline solution is also small, so the amination reaction can be faster. Especially, the macroporous chloromethylated copolymer can be aminated at a higher temperature (40~60 ℃) to obtain a weak basic anion resin with good strength. However, the swelling agent has a great influence on the properties of aminated products. For example, when dichloroethane is used for full swelling and dimethylamine is used for amination, the tertiary amine resin obtained is relatively loose in structure, high in water content, good in dynamic performance, but small in volume exchange capacity. If amination is carried out under the condition of no swelling agent or poor swelling, although the degree of amination can be very high, its kinetic performance is quite different from the former. This is shown in the slow speed of ion exchange and high water consumption. For macroporous anion exchange resin, its pore structure is also significantly affected.
During the synthesis of weak base anion resin, the weak base group generated may further react with unreacted chloromethyl group to generate higher amino group, thus generating additional crosslinking and reducing the exchange capacity of the resin. Amination with dimethylamine will also produce considerable strong base exchange groups. For example, chloromethylated copolymer containing 8% DVB is aminated with dimethylamine to obtain 4.02mmol/g of total exchange capacity, including 0.67mmol/g of strong base exchange capacity. Sometimes higher. In order to minimize the generation of strong reducing groups, the copolymer should be fully swelled during amination, and the concentration of amine in the medium should be increased. If dioxane is used as the medium and anhydrous dimethylamine is used for amination at 0 ℃ for 4~10 days (DVB2%~7.5%), the tertiary amine resin obtained does not contain strong base groups, and the measured weak base exchange capacity is consistent with that calculated by chlorine content. It shows that the tertiary amino group does not react with other chloromethyl groups.
The synthesis of primary amine weakly basic resin is special. The early primary amine resin was prepared by nitration and reduction of cross-linked polystyrene, and its performance was not ideal. There are two methods to synthesize primary amine resin from chloromethyl cross-linked polystyrene: one is amination with phthalimide, and then hydrolysis; Second, it is obtained by amination with hexamethylenetetramine and decomposition with hydrochloric acid. The amination of hexamethylenetetramine can be carried out at 30 ℃ in the presence of chloroform or dichloroethane. After 10 hours, the six times imprinted tetramine group on the resin is decomposed with 1:3 concentrated hydrochloric acid ethanol solution, and the primary amine resin with exchange capacity greater than 4.5 mmol/g can be obtained.
Chloromethylated crosslinked polystyrene undergoes amination reaction with guanidine under swelling and anhydrous conditions to obtain a medium strong alkaline anion resin with special performance. In terms of ion exchange performance, it can decompose neutral salts like strong alkaline resin; And regeneration can be carried out with ammonia water, just like weakly alkaline resin.
In addition, polyamine resin with proper proportion of strong base and weak base groups, which is synthesized by step amination or mixed aminating agent, also shows excellent performance in some applications.
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