Application, Characterization And Preparation Of A Solid Acid Resin Based On Chitosan

Nowadays, the resin-based solid acid that most commonly used is macro porousstrong acid exchange resin, the catalytic performance of which is mainly determined bythe active groups, the exchange capacity, the pore structure and the extreme workingtemperature of the resin. There are many advantages in using resin catalysis to displacetraditional acid catalyst but the high cost limited its widespread application. In contrast,chitosan is much cheaper to obtain and many research has been carried out to develop asolid acid catalyst using chitosan as the matrix. Chitin is a kind of natural polymermaterial of biological polysaccharide that has a large quantity in stock and regenerationin the nature. As the deacelation product of chitin, chitosan has a good reaction activityto generate a variety of derivatives which make it a potential material to prepare aneconomic and environment-friendly catalyst of resin-based solid acid.In previous study, chitosan sulfate which can be also called sulfonated chitosan (SC)that generated from chitosan and dilute sulphuric acid has applied in a variety ofesterification as a solid acid catalyst. At the time it played catalytic effect thedisadvantage also existed in the weak thermal stability, the low strength rigid and thelong reaction time, which need to be further improved. The cross-linking reaction of theamino between chitosan molecules can enhance the thermal stability and rigidity whilethe sulfuric acid esterification reaction of the hydroxyl can introduce an acid group.Thus sulfonated cross-linked chitosan resin (SCCR) prepared by cross-linking andsuffocation of chitosan has the possibility to be a resin-based solid acid catalyst.In the chitosan molecules, amino is the group not only for cross-linking reactionbut also combining with sulfate. So the cross-linking degree has an impact on acidconsumption and thermal stability of the SCCR and then affects the catalytic properties.Therefore, comparative study on the degree of cross-linking should be implemented.Firstly, cross-linked chitosan resin (CCR) with different degrees of cross-linking wassynthesized and characterized and then SCCR was synthesized under optimized reactionconditions and the characterization focusing on acid amount and thermal stability wastaken after that. At last, the effect of SCCR as an acid catalytic was investigated in twospecific esterification reactions. The main conclusions are as follows.The cross-linked chitosan resin with the degree of cross-linking from20%to100%was synthesized by inverse suspension-cross-linking method with CaCO3as the progenic agent and glutaraldehyde as the cross-linking agent and the average conversionratio of cross-linking reaction is89%. FTIR illustrates that amino groups in chitosanhave participated in the formations of both the Schiff base (N=C) and the imide linkage(–CONH–) during the synthesis. In the acidic, neutral and alkaline solution, the swellingratio of resins declined with the increase of cross-linking degree and rose with theincrease of H+concentration in the solution.The sulfonated cross-linked chitosan resin was synthesized under the optimalreaction conditions and FTIR analysis confirmed that the characteristic peaks ofsulfonated groups at1230-1250cm-1(S=0) and800cm-1(C-O-S) appeared in themolecular structure of resins. The acid tests showed that the acidic groups contained insulfonated cross-linked chitosan resins are weak acids and the amount of acid are4.57,4.34,4.29,4.21and4.17mmol/g according to the20%,40%,60%,80%and100%cross-linking degree respectively. Compared to the3.13mmol/g of the chitosan sulfate,the acid capacity of SCCR increased more than30%.The temperature stability of the sulfonated cross-linked chitosan resin waspositively correlated with the degree of cross-linking and analysis of TG/DTG and DSCdemonstrated that the average decomposition temperatures for SCCR with cross-linkingdegree of20%,40%,60%,80%and100%are214.7℃，218.3℃，220.8℃，222.6℃，226.3℃respectively, which are higher than the203.3℃of chitosan sulfate. Moreover,the thermal stability of SCCR allows its utilization below120℃and resins with higherdegree of cross-linking is more stable at higher temperatures.SCCR with cross-linking degree of100%was used as a solid acid catalyst forsynthesis of tributyl citrate (TBC) reacting from citric acid and butane. Theesterification rate of the reaction reached about90%and maintained at87%after beingreused five times, which is24%higher than the chitosan sulfate. The esterification ofisobutyl acetate from acetic acid and isobutene was also catalyzed by SCCR and thecatalytic activity remained98%of the first time after reused five times. Sulfonatedcross-linked chitosan resin is a kind of solid acid catalyst with a stable catalyticefficiency and can be well reused.