Synthesis of Anion Exchange Resin (III)

Synthesis of Anion Exchange Resin (III)

  • 2022-10-26

2. Synthesis of strong basic anion exchange resin

Among anion exchange resins, the yield of quaternary ammonium based strong alkaline resin is the largest. The main variety is gel type resin containing one N+(CH3) 3, with a crosslinking degree of 7 or 8, called strong base type I. Another ion exchange resin containing one N+(CH3) 2CH2CH2OH group is called strong base II. Strong basic anion exchange resin can remove all anions in water, including silicate and organic acid, and is mainly used for the preparation of pure water. From the perspective of application, this kind of ion exchange resin is required to have higher volume exchange capacity, better ion exchange kinetic performance, appropriate particle size distribution and higher mechanical strength. In addition, there are some gel type resins with low crosslinking degree (4% DVB) and macroporous strong basic anion exchange resins with high crosslinking degree (8%~12% DVB) for different purposes.

Crosslinked polystyrene spheres with different structures are chloromethylated and aminated with trimethylamine to obtain strong base I resin. The reaction can be carried out smoothly at about 30 ℃. However, it is necessary to add appropriate solvents, such as benzene, ethane, carbon tetrachloride, dioxane, acetone, ethanol, etc. Otherwise, not only the amination speed is slow, but also the crosslinking side reaction may occur.

In actual production, trimethylamine aqueous solution or trimethylamine hydrochloride can be used. When aminating with trimethylamine hydrochloride, slowly add NaOH solution to the system to gradually free trimethylamine from rice for reaction, which can be completed in about 16h.

In the same way, the strong base II anion exchange resin can be obtained by amination with dimethylethanolamine.

The crosslinking degree has a great influence on the performance of gel type strong basic anion exchange resin. The low cross-linked resin has a large swelling degree in water and a large mesh, which is conducive to the exchange of larger organic ions. However, the volume exchange capacity of this resin is small and its strength is poor. In particular, the volume change during transformation (such as OH ->C1-1) is large, which will affect the operation process and service life. When the crosslinking is too high, the structure of the resin is too tight, which often reduces the degree of chloromethylation and the exchange capacity, and the exchange rate and exchange capacity of organic ions will be significantly reduced.

The amination process has great influence on the strength of gel resin. In order to prevent the resin from cracking, an appropriate amount of NaCl is generally added to the amination medium. Trimethylamine or (NaOH) shall also be added step by step (or in batches), and slowly diluted with water after reaction to wash away residual trimethylamine.

In addition to trimethylamine, triethylamine, tribulamine, dimethylpropanolamine, methyldiethanolamine, dioctyl ethanolamine, etc. can also be used for quaternization of chloromethylated crosslinked polystyrene. However, due to the large volume of these amines, the amination reaction is difficult to complete, and the ion exchange performance of the aminated products will inevitably be affected by the group steric hindrance, so they have not been used in actual production. Commercial resins Amberlite IRA-400, Dowex-1 and domestic 201 are all products aminated with trimethylamine. Amberlite IRA-410, Dowex-2 and domestic 202 are type II resins aminated with dimethylethanol.

The amination method of macroporous resin is similar to that of gel resin. The rate of amination reaction (feeding rate) can be faster, and even if NaCl is not added in the reaction medium, the sphere will not be broken.

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