(20a) Preparation and Characterization of Sulfonated Crosslinked Chitosan Resin As Solid Acid Catalysts

 Sulfonated crosslinked chitosan resin (SCCR) was synthesized by sulfonating crosslinked chitosan resin (CCR) heterogeneously at -5℃ and with concentrated H₂SO₄ as the sulfonation agent. The properties and application of SCCR as solid acid catalysts were studied and compared with chitosan sulfate (CHS), which was prepared by sulfonating chitosan homogenously in acetic acid with dilute H₂SO₄ as the sulfonation agent. The results demonstrated that the sulfur content and acidic amount of SCCR are significantly increased compared to those of CHS, and the acidic sites in the SCCR including C₆-O and C₂-N sulfate groups are all weak acidic sites. CCRs of higher crosslinking degrees have a higher sulfonation rate of the C₆ secondary hydroxyl groups. High-temperature treatment verified that the crosslinking can improve the thermostability of both SCCR backbone and its acidic groups, and a higher crosslinking degree leads to a better thermostability. The catalytic activity and repeatability of the SCCR and CHS as solid acid catalysts were explored and compared by studying their application in the synthesis of tributyl citrate from citric acid and butanol. The results showed that SCCR is a better catalyst than CHS, the esterification rate of citrate acid can keep above 92% by repeatedly using the SCCR (theoretical crosslinking degree = 100%) for 6 times.