(111b) A Study On Sulfonated Cross-Linked Chitosan Resin As Solid Acid Catalyst
Strong acid resins, e.g. Nafion and sulfonated polystyrene are very effective solid acidic catalysts in esterification reaction, ester exchange reaction, etherification reaction, etc., while the high cost limits their extensive application. Sulfonated chitosan has a very similar nature with Nafion and chitosan has the advantages in natural reserves and price, consequently, a lot of efforts have been tried on sulfoanted chitosan as an alternative strong acid resin. However, the thermostability and acidity of sulfonated chitosan prepared by the direct sulfonation of chitosan are not satisfactory. In this study, chitosan was firstly cross-linked by glutaraldehyde before being sulfonated to improve its thermal and acid stability; Moreover, silica particles were added in the molding process to enhance its acidity. The cross-linked chitosan sulfate resin shows good stability and catalytic activity. The DTA-TGA analysis reveals that the effective temperature of the resin is up to 190 ºC, at which most organic esterification or etherification reactions can occurs smoothly. The sulfonation ratio of the cross-linked chitosan is about 35% and the retention rate of the sulfonic groups is more than 75% under the endurable temperature for 5h. The addition of silica can increase the sulfonation rate of cross-linked chitosan and improve the stability of the sulfonic groups. The BET specific surface of the resin is around 730m2/g, its sulfonic groups can be fully used since the encapsulation of sulfonic groups can be avoided. Further studies shows that cross-linked chitosan sulfate resin can be used as acidic catalyst for esterification reactions such as synthetizing ethyl nipagin ester and butyl acetate. The esterification ratio can reach up to 96% and the resin can be repeatedly used for more than 5 times.