Crosslinked chitosan resins(CCRs)with different crosslinking degrees were synthesized by using the inverse emulsion method and glutaraldehyde as crosslinking agent

and then sulfonated at -5℃ with concentrated H2SO4 as sulfonation agent.The properties and application of obtained sulfonated crosslinked chitosan resins(SCCRs)as solid acidic catalysts were studied.IR analysis showed that sulfonation of CCR occurred at both C2 amino groups and C6 secondary hydroxyl groups.Sulfur content measurements demonstrated that the higher the crosslinking degree of the SCCR

the larger the sulfonation rate of its C6 hydroxyl groups.The acidic sites in SCCR including C6-O and C2-N sulfate groups were all weak acidic sites.The acid content of SCCR was much higher than chitosan sulfate reported in the literature

which was prepared by direct sulfonation of chitosan without pre-crosslinking.High-temperature treatment verified that crosslinking improved the thermostability of SCCR.The SCCR with a higher crosslinking degree had better mass retention rate as well as larger sulfur retention rate.The catalytic activity and repeatability of SCCR as solid acidic catalysts were verified by testing its application in the synthesis of tributyl citrate from citric acid and butanol.The results showed that esterification rate of citrate acid could reach 94.8% when SCCR with a 100% crosslinking degree was used

and esterification rate could keep above 92% by repeated use of the SCCR for 6 times.Comparison with chitosan sulfate demonstrated that SCCR was a much better solid acidic catalyst and had a promising application prospect.