Synthesis of Strong Acid Cation Exchange Resin (III)
4. Swelling agent
In industry, dichloroethane is used as swelling agent to prepare strong acid cation exchange resin, but this is not the best choice. Dichloroethane cannot be miscible with sulfuric acid, and the diffusion of sulfuric acid into the resin becomes the control factor of sulfonation rate. Dimethyl sulfoxide can be miscible with sulfuric acid, which is conducive to the diffusion of sulfuric acid into the copolymer and maintains a high concentration in the copolymer, thus significantly accelerating the sulfonation rate. If the crosslinking agent is 8% m-DVB and the reaction takes 9 h, the exchange capacity of the copolymer with dimethyl sulfoxide or dichloroethane as the swelling agent is 4.69 meg/g and 2.08 meq/g respectively. It seems that the dielectric constant of the swelling agent and the homogeneity of the swelling agent sulfuric acid system are one of the factors affecting the sulfonation rate.
Sulfonation (1h, 100 ℃) is carried out without swelling agent. The reaction is only carried out on the surface of the sphere (A), and the exchange capacity of cation exchange resin is not very high. This is mainly because the sulfonating agent (96% HSO) is difficult to diffuse to the interior of the sphere quickly. Of course, if the reaction time is extended, the sulfonation reaction will also gradually move towards the interior of the sphere. When the sphere is swollen with dichloroethane, even if the reaction takes place at room temperature for 1h, a considerable amount of sulfonic acid group (B) will be introduced into the sphere.
The sulfonation reaction of macroporous copolymers is fast. The influence of the swelling agent is much smaller. The swelling agent will not only affect the speed of the functional base of the copolymer, but also affect the performance of the product. This is a feature of polymer reaction, and similar situations will be encountered in the following description.
5. Effect of polar monomer on sulfonation rate of copolymer
The sulfonation property of the copolymer can be greatly improved by adding a small amount of polar monomers. The existence of polar monomer chain in the copolymer improves the affinity of polymer chain and acid. At the same time, the activation energy of sulfonation reaction was significantly reduced by "neighbor group effect". Therefore, the sulfonation reaction speed is greatly improved. In this case, the sulfonation process is controlled by the reaction rate rather than the diffusion rate.
It can be seen from the above description that the sulfonation of cross-linked copolymer spheres is much more complex than that of small molecule aromatic hydrocarbons. The requirements for sulfonated product - strong acid cation exchange resin are also various, including higher exchange capacity, better dynamic performance, higher mechanical strength and swelling performance. These are all related to the sulfonation process.
After sulfonated, crosslinked polystyrene changed from lipophilicity to strong hydrophilicity. The cation exchange resin must be diluted slowly with water and gradually transited from concentrated sulfuric acid to water, otherwise, the cation exchange resin will be broken due to rapid swelling. In actual production, the residual sulfuric acid concentration after sulfonation is generally about 86%, at this time, the sulfuric acid contained in the cationic resin ball is only about 37%. When the cation exchange resin contacts with more than 40% sulfuric acid solution, the heat release is very small. However, if it contacts with dilute sulfuric acid, the heat release will increase sharply with the decrease of sulfuric acid concentration. Therefore, when the concentration of sulfuric acid outside the cation resin drops below 40%, it is necessary to dilute it carefully to ensure that the sphere will not break. On the other hand, the swelling degree of cationic resin will gradually increase in the process of sulfuric acid dilution. In order to prevent the cation exchange resin from swelling too quickly, it is also necessary to slowly dilute the sulfuric acid. The research shows that during the dilution process, the content of sulfuric acid solution in the sulfonic acid resin sphere and the corresponding resin volume gradually decrease. When the concentration of sulfuric acid is lower than 67%, the hydration of sulfonic acid group increases, and the volume of cation resin expands rapidly. This means that the dilution rate should be slowed down after the concentration of sulfuric acid drops to 67%, otherwise the ball of cation exchange resin will break due to rapid expansion of resin.
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