Synthesis of Strong Acid Cation Exchange Resin (I)
Strong acid cation exchange resin is made by sulfonation reaction, introducing sulfonic group into the benzene ring of cross-linked polystyrene.
The sulfonation of aromatic rings is a typical electrophilic substitution reaction. In addition to concentrated sulfuric acid and chlorosulfonic acid, sulfur trioxide and oleum can also be used as sulfonating agents. Therefore, in addition to our most commonly used concentrated sulfuric acid, chlorosulfonic acid, sulfur trioxide and oleum can also be used as sulfonating agents to prepare strong acid cation exchange resin. The sulfonation of small aromatic rings is easy to carry out, and the position of sulfonic groups introduced into aromatic rings is related to the reaction temperature. The higher the temperature is, the larger the proportion of para sulfonated products is, and the higher the exchange capacity of strong acid cation exchange resin is. However, sulfonation of benzene ring is a reversible reaction and theoretical yield cannot be obtained. If benzene is sulfonated with concentrated sulfuric acid, and the reaction is carried out at 100 ℃ and 200 ℃, the yield of benzene sulfonic acid is 72% and 78% respectively. The sulfonation of styrene divinylbenzene copolymer spheres is generally carried out with 93% industrial acid at 70~80 ℃. As the copolymer has a compact three-dimensional network structure, its sulfonation is a heterogeneous reaction. Even in the case of using solvents (such as dichloroethane), the sulfonation speed is often slow.
The main factors affecting the sulfonation reaction of copolymer spheres are:
1. Structure of copolymer
Sulfuric acid, a commonly used sulfonation reagent, cannot swell the non-polar styrene divinylbenzene copolymer and enter into the copolymer for sulfonation reaction. Therefore, the copolymer is generally expanded with dichloroethane in advance. Even so, the compactness of the copolymer structure still has a great influence on the sulfonation reaction. It can be imagined that the higher the degree of crosslinking, the smaller the rate of sulfonation reaction. The activation energy increases with the degree of crosslinking, indicating that the sulfonation reaction is a diffusion controlled process. The sulfonation reaction itself is fast, and the 2% DVB copolymer can reach the reaction equilibrium after one hour. However, for 8% DVB copolymer sphere, due to the slow diffusion of sulfuric acid into the sphere, it takes four hours to reach a similar reaction degree.
The structure of macroporous resin is conducive to the diffusion of sulfuric acid. The sulfonation reaction is also fast. For example, when toluene is used as porogen (FX = 0.5), the sulfonation rate of spheres containing DVB 16% and 32% is faster than that of gel copolymers containing 8% DVB. The sulfonation rate of the copolymer with non good solvent as porogen is between the above-mentioned toluene porogen and gel type copolymer spheres.
The effect of the difference in copolymer structure caused by crosslinking degree and pore forming agent on sulfonation rate is more significant at lower sulfonation temperature.
English
