Pretreatment Of Cellulose To Hydrogel Followed By Acid Hydrolysis Producing Glucose Under Microwave Radiation

Since the 21 st century,some problems become increasingly serious, such as the shortage of fossil energy and environmental pollution. Cellulose, the most abundant biomass in nature,has attracted extensive attention. Transformation of cellulose to value-added chemicals and green energies is being considered as a promising alternative to fossil fuels. Generally, the first utilization step is hydrolysis of cellulose to glucose. However, due to a robust crystal structure of the natural polymer with a high chemical stability, the hydrolysis processes are difficult and inefficient. Thus, it has great significance and application value to the appropriate pretreatment of cellulose, which aim to increase the efficiency of hydrolysis. In this study, NaOH/Urea swelling system and NaOH swelling- then HCl regeneration system(NaOH/HCl) were developed for the pretreatment of cellulose to hydrogel, followed by acid hydrolysis producing glucose under microwave radiation. The development of the pretreatment systems, the change of the structure and properties of cellulose, the microwave-assisted hydrolysis, the analysis of hydrolysate and the hydrolysis mechanism of hydrogel are discussed in details. The following results are achieved in this dissertation from the above experimental research:(1) After the pretreatment of α-cellulose by NaOH/Urea swelling system, the crystallization of cellulose I was mainly converted to amorphous cellulose with a minor amount of cellulose II. It also showed an increasing of particles size, a porous structure and a marked decreasing of crystallinity of hydrogel. Furthermore, some amorphous cellulose might be realigned and converted to cellulose II during the drying of hydrogel. The cellulose hydrogel was hydrolyzed by 0.06 M H2SO4 for 10 min under microwave radiation. The yield of glucose was 30.1%, much higher than 7.18% of original α-cellulose. The ozone treatment of cellulose solution before regeneration significantly enhanced the hydrolysis efficiency and glucose yield. Moreover, the hydrogel was regenerated by different styles also can influence the hydrolysis reaction.(2) The hydrolysis of original α-cellulose is only performed in the amorphous area, butnot the crystalline area. While the hydrogel is amorphous in structure with low density, all parts can be hydrolyzed by the dilute acid. The chain in cellulose hydrogel are relatively free approached, making that the H+ ions can diffuse easily in the hydrogel and access the β-1, 4glycosidic bonds, which ensure a more efficient hydrolysis. In fact, water is an effective microwave energy absorber in comparison with cellulose. Due to the hydrogel consist of water and cellulose, the activation of cellulose could be enhanced under microwave irradiation by the adsorbed water in the hydrogel through interaction with neighboring groups.(3) To overcome the drawbacks of NaOH/Urea system, which needs high concentration of urea and a large amount of water during the process of regeneration, NaOH/HCl system was further developed in the study. The hydrogel was hydrolyzed at 160℃ for 10 min under microwave radiation. The cellulose conversion and glucose yield were 54.1% and 36.7%,respectively. The reaction time, temperature and H+ ions concentration can markedly influence the yield and selectivity of glucose. For the hydrogel, when the reaction time being20 min, the glucose yield reached 80.9%. When the temperature increased to 180℃, the glucose yield and selectivity dramatically decreased to 26.9% and 32.6%, respectively. The glucose yield increased nearly linearly with the increase of H+ ions concentration.(4) Pretreatment of cellulose by NaOH swelling and then HCl regenerating can make H+ions and NaCl diffuse into the cellulose hydrogel before the hydrolysis reaction. Due to H+ions to react with β-1, 4 glycosidic bonds as well as the strong ability of Cl- to disrupt the massive hydrogen bond, the hydrolysis rate was greatly enhanced. In addition, the presence of NaCl in hydrogel networks also can inhibit amorphous cellulose converting to cellulose Ⅱ in the process of drying.(5) Both NaOH/Urea system and NaOH/HCl system are effective for many kinds of cellulose and fiber products, which have the high crystallinity and the high degree of polymerization, such as microcrystalline cellulose, filter paper, ramie fibre and absorbent cotton.