Synthesis of Anion Exchange Resin (I)
Anion exchange resins produced in industry are mostly made of styrene divinylbenzene copolymer spheres as the skeleton, and introduced quaternary ammonium groups or other amine groups through chloromethylation and amination. Its varieties and specifications are far more than those of cation exchange resin. Therefore, it is widely used and one of the main strains of ion exchange resin. The basic synthesis route is:
1. Chloromethylation of cross-linked polystyrene
Benzyl halide is very active. The introduction of chloromethyl into the benzene ring has become a very useful intermediate, which can easily be made into a series of anion exchange resins.
Chloromethylation on benzene ring is a special case of Friedel Crafts reaction, which can be carried out with hydrochloric acid under the action of catalyst by using polyformaldehyde, formaldehyde aqueous solution, formaldehyde dimethyl acetal, formaldehyde ethylene glycol, chloromethyl ether, dichloromethyl ether, etc. Chloromethyl ether is not only a highly active chloromethylation reagent, but also a good swelling agent of styrene divinylbenzene copolymer. Although it has great irritation and toxicity to human respiratory system, it is still widely used.
Industrial chloromethyl ether contains a small amount of dichloromethyl ether and excessive HCl. The content of HCl in chloromethyl ether purified by distillation is reduced, sometimes its reactivity is reduced. During chloromethylation, the consumption of chloromethyl ether is only a small part, and most of it is used as swelling agent or medium. The common gel type anion exchange resin requires 2.5 times the amount of methyl chloride to make it fully swollen and able to be stirred. When chloromethylation is carried out in low crosslinking copolymerization or macroporous copolymer, more chloromethyl ether is required. In order to save chloromethyl ether, sometimes appropriate solvents can be selected and mixed with chloromethyl ether, such as dichloroethane, dichloromethane, tetrachloroethylene, etc., but the addition of these solvents will slow down the rate of chloromethylation.
There are many catalysts for Friedel Crafts reaction. In chloromethylation of cross-linked polystyrene with chloromethyl chloride, the most commonly used catalysts are ZnCl2, AICl3, SnCl4, etc. The order of catalytic activity of some chlorides is:
AlCl3>SbCl5>FeCl3,>SnCl4>TiCl4>ZnCl2
In industry, ZnCl2 with lower activity is used, and the amount is about 60% of the copolymer. It has the advantage of mild catalysis and fewer side reactions to produce new cross-linking structures. AlCl3 and SnCl4 can accelerate the side reaction. In chloromethylation of polystyrene, the cross-linking of linear polymer chains can lead to the appearance of gel in a few hours or even minutes. The addition of solvent reduces the concentration of chloromethyl ether and increases the proportion of side reaction. The existence of side reaction will cause additional crosslinking of cross-linked polystyrene, which will cause the change of the performance of the final product - anion exchange resin.
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