Preparation, Structure And Characterization Of Natural Polymers Based Organic And Carbon Aerogels

The nanostructured natural polymers based organic and carbon aerogels were synthesized by a sol-gel technique.The preparation process and the microstructures of aerogels were studied,and the effects of preparation parameters,as well as the adsorption properties of the prepared aerogels were investigated.Based on the experiments,the formation mechanisms of the natural polymers based aerogels and their carbon aerogels were proposed. This study enriched the basic theory and applications of organic aerogels and carbon aerogels prepared on the basis of the sol-gel process.1.Chitosan-based aerogels with high adsorption performanceNew natural polymer-based aerogels,cross-linked chitosan-based aerogels, were prepared by the sol-gel route with glutaradehyde,glyoxal,and formaldehyde as cross-linkers.The alcogels were dried by supercritical carbon dioxide(CO₂) fluid extraction.The resulting materials were characterized using scanning electron microscopy(SEM),nitrogen adsorption/desorption analysis,and Fourier transform infrared(FT-IR) spectroscopy.The Brunauer-Emmett-Teller(BET) analyses indicate that the largest surface areas for the chitosan-glutaraldehyde/-glyoxal and -formaldehyde aerogels are 569,707,and 845 m²/g,respectively.We believe that the N₂ adsorption-desorption hysteresis is a permanent type.Every aerogel sample was tested for 3 times repeatedly for the N₂ adsorption-desorption experiment;the adsorption-desorption hysteresis curves and the PSD curves are same.This indicates the mesopores are stable.IR spectra of chitosan and cross-linked chitosan-based aerogels indicate that the primary amine peak at 1653 cm^-1 decreased as the chitosan was crosslinked, while a new peak for C=N amine appeared at 1653-1656 cm^-1.The peak at 1602 cm^-1 disappeared in the aerogels due to the loss of free amines, indicating a Schiff-base amine functionality.The key factor to obtaining stable aerogels with a high specific surface area is the full replacement of the solvent with absolute ethanol.If the solvent is not fully replaced by absolute ethanol,only a broken wet-gel will be produced.The decomposition of the chitosan-based aerogel is observed from ca.220 to 450℃,and the profiles of the TG curves for both non-cross-linked and cross-linked materials are similar. It was also found that carbon aerogels could not be produced after calcining the chitosanbased aerogels under a flow of nitrogen because the thermal degradation of the main chains in chitosan and chitosan-based aerogels during heating destroyed the network structure.Furthermore,the adsorption of the anionic surfactant sodium dodecylbenzene-sulfonate(SDBS) from aqueous solution by the materials was investigated.The aerogels exhibit high adsorption capability,can remove SDBS from acidic aqueous solutions,and have potential applications in controlling SDBS pollution.2.Starch derived carbon aerogels with high-performance for sorption of cationic dyesA synthetic strategy to prepare CAs by the carbonization of starch aerogels from its corresponding wet-gels through the ambient pressure drying process. The textural characteristics of CAs could be tailored by adjusting the carbonization temperature and activation time.Thus the technique of CA synthesis is relatively simple and with low cost for the short gelation time and ambient pressure drying.To prepare CAs,the esculent and soluble starch was dissolved into water to form the wet-gels,after the water in the gels was exchanged by acetone,the acetone gels were dried at ambient pressure to obtain the white starch aerogels,which were carbonized in N₂ flow.SEM images show that the starch aerogels exhibit a fibrillar solid network with a diameter of 30-40 nm and CAs consist of flakes with a side-length of 60-120μm as the starch aerogels were carbonized at 850℃for 2.0 h.When the magnification is 200,the carbon aerogels keep the same shape after activating with CO₂.SEM image of ACAs show high mesoporosity,obviously. However,SEM image of CAs don't show mesoporous structure.The N₂ adsorption measurements indicate that the specific surface area of CAs is 137 m²/g and the average pore size is 1.54 nm,only few mesopores are observed according to the BET pore size distribution(PSD).However,the specific surface areas of starch aerogel(calculated using BET model) was 103 m²/g and PSD ranged from 3 nm to 80 nm,typical of mesoporous and macroporous solids.Further experiments show that the suitable activation conditions could increase the specific surface areas and pore volume of CAs,which increased with the activate time prolonging.After activations the CAs contained both micropores and mesopores,its average micropore size is 0.89 nm,and all the mesopore size are in the range of 2-10 nm whatever the activation time is, which reveals that the activation process creates new micropore and mesopore structures.For example,the largest specific surface areas of CAs increased to 1571 m²/g after being activated at 800℃for 7.0 h under CO₂ flow.The pH value of solution considerably affected the adsorption amounts of basic dyes on the CAs.For example,as the pH value of dye solution is 10 and dye's solution is 500 ppm,the absorption amount is the largest,those for CV,MV and MB are respectively 1515,1423 and 1181 mg/g.3.Synthesis,characterization and application of chitosan/gelatin/genipin aerogelsNew natural polymer-based aerogels,cross-linked chitosan aerogels and chitosan-gelatin aerogels,were prepared by the sol-gel route with genipin as cross-linkers.The alcogels were dried by supercritical carbon dioxide(CO₂) fluid extraction.The resulting materials were characterized using scanning electron microscopy(SEM),nitrogen adsorption/desorption analysis.The Brunauer-Emmett-Teller(BET) analyses indicate that the largest surface areas for the natural polymers based aerogels are 567 m²/g.The key factor to obtaining stable aerogels with a high specific surface areas is the full replacement of the solvent with absolute ethanol.If the solvent is not fully replaced by absolute ethanol,only a broken wet-gel will be produced.It was also found that carbon aerogels could not be produced after calcining the chitosan based aerogels under a flow of nitrogen because the thermal degradation of the main chains in chitosan and chitosan-based aerogels during heating destroyed the network structure.Furthermore,the adsorption of the anionic dyes sunset yellow and tartrazine from aqueous solution by the materials was investigated.The aerogels exhibit high adsorption capability, can remove sunset yellow and tartrazine from acidic aqueous solutions,and have potential applications in controlling anionic dyes pollution.4.Synthesis,characterization and application of carrageenan based organic and carbon aerogelsCarrageenan based organic cyrogels and aerogels were prepared by the sol-gel route and freeze drying and supercritical carbon dioxide(CO₂) drying from its corresponding wet-gels,and carrageenan based carbon aerogels and cyrogels were prepared by the carbonization of carrageenan aerogels and cyrogels,respectively.The textural characteristics of carrageenan based carbon aerogels and cyrogels could be tailored by change dry method.Thus the technique of CA synthesis is relatively simple and with low cost for the short gelation time and freeze drying.To prepare CAs,the esculent and soluble carrageenan was dissolved into water to form the wet-gels,after the water in the gels was exchanged by alcohol,the alcogels were dried at supercritical carbon dioxide(CO₂) or freeeze to obtain the white carrageenan aerogels and cyrogels,which were carbonized in N₂ flow at 1000℃.SEM images show that the carrageenan aerogeis exhibit a fibrillar solid network with a diameter of 20 nm and carbon aerogels and cyrogels consist of carbon particles.SEM image of aerogels and cyrogels show high mesoporosity, obviously.However,SEM image of carbon aerogels don't show microporous structure.The N₂ adsorption measurements indicate that the largest specific surface area of organic aerogels is 346 m²/g and the average pore size is 2.98 nm.However,the specific surface area of carbon cyrogels(calculated using BET model) was 1185 m²/g and the average pore size are 0.85 and 3.9,typical of mesoporous and microporous solids.The pH value of solution considerably affected the adsorption amounts of basic dyes on the CAs.For example,as the pH value of dye solution is 10 and dye's solution is 500 ppm,the absorption amount is the largest,those for CV,MV are respectively 2207,1725 mg/g at pH 10.