Preparation And Properties Study Of Organic Aerogels Based On Lignin

Organic aerogels have some execellent properties such as their special porous structure, larger specific surface area, nanoscale pore, nanoscale framework and non-crystalline structure, which are favourable to enhance the surface adsorption potential. The polymerization of resorcinol with formaldehyde catalyzed by alkali catalyst result in the formation of organic aerogel via a sol-gel process, acid aging process, and supercritical fluid drying. The aerogel is widely used in the field of mechanics, acoustics, electricity, calorifics and optics. Thongh the potential uses of aerogel is widely, the costs of manufacturing are high, and the synthesis cycle is long. It is different to promote the commercialization of aerogels, address this problem, this article discusses the research work that using the low hydrolysis lignin and resorcinol to prepare the organic aerogel.In the paper, the enzymed lignin, resorcinol and formaldehyde was as a precursor, the NaOH was as a catalyst. The lignin-based RF organic aerogels, prepared by the sol-gel method, was of nano-porous structure. We studied the microstructure and properties of the aerogel, including preparation of aerogels, the characterization of microstructure. In order to further reduce the gel time, the ultrasonic treatment is introduced in the experiment. The concentration of reactant, catalyst ratio and lignin content were influenced on the density, drying shrinkage and pore structure of the gel. The Surface Area and Porosity Analyzer. Scanning electron microscopy (SEM), Tranmission electron micrographs, X-ray photoelectron spectroscopy (XPS), Thermal analytical balance (TG), infrared spectroscopy (FT-IR) were used to research the characteristics of the gel microstructure and chemical composition. The detail of the research process and relevant conclusions are as follows:(1) When the temperature is under certain, the gel time increased with the lignin content and the reactant concentration. If the reactant concentration is too low or the catalyst ratio is too high, the aerogel could not be obtained. In the experiment, the minimum reactant concentration is 5% and the maximum catalyst ratio is 200. In general, the performance of gel was the best when the maximum addition of lignin (L%)50%, the catalyst ratio(LR/C) 75, and reactant concentration of 15%. The density and volume of gel drying shrinkage is mainly affected by the concentration of reactants, the ratio of the catalyst and lignin content. RF aerogels obtained by the density of lignin-based range (0.244g/cm3-0.728g/cm3), gel drying shrinkage ranging from (19.1%-63.6%).(2) Though the analysis the surface area and pore size of the organic aerogel, the results showed that the adsorption isotherm is similar like the type V isotherms. It indicated that lignin-based RF aerogel was the network-based porous materials dominated in both holes and surrounded by open pores characterized of a large hole. While the lignin content is of 10%, the adsorption isotherms seem to be more close to the type IV. In the medium pressure area, the isotherm rised slowly and close to the level. It means that the aerogel own the considerable microporous characteristics. When the lignin content of 50%, specific surface area of the lignin-based gel was 191.43m2/g, average pore size is 13.67nm, and the specific surface area of RF gels 552.24 m2/g, average pore size of 5.89nm. According to the BJH pore size distribution of lignin-based RF organic aerogels and RF organic aerogels, the pore size distribution of the RF aerogel is narrow, and pore diameter is mainly in the context of 3-8nm. When the lignin content is of 50%, the range of pore size become larger, the number of macroporous increased, and the aperture is also concentrated in the 10-20nm range. Though the scanning electron microscopy and transmission electron microscopy of the gels, the lignin reacted with formaldehyde replaced the resorcinol in some extent. The reactant polymerized each other and cross-linked together. Finally, it formed the aerogel with a continuous three-dimensional network structure and microscopic features.(3) The gel and raw materials analyzed by the FT-IR and XPS, and then sub-peak fitting Cis spectrum. C4 correspond to the peak aera increased with lignin content. It indicated the reactants formed the ester bond between each other, the peak area of C2, C1, corresponding to the crosslinks C-O-C,-CH2-were also large. It'is consistent with the result of infrared spectra.(4) The gels and material analyzed by the thermal analytical balance, the result is that organic aerogels obtained under different conditions have similar thermal decomposition characteristics. Before 200℃, the weight loss rate of RF aerogel is slightly higher than the lignin-based gels; 400-600℃, the weight loss rate of the gel obtained the maximum, the weight loss rate is about 30%; in the end of thermal decomposition, the weight loss rate of organic aerogel is slightly higher than the RF aerogel.