Preparation Of Cellulose Acetate From Rice Straw

The shortage of resources and environmental pollutions have become increasingly prominent with the rapid development of industrialization. Much attention has been focused on developing renewable resources all over the world. Straw is a kind of abundant renewable resource derived from agricultural wastes. Preparation of chemicals from straw is not only favorable to ease the shortage of resources but also to reduce environmental pollution, which is consistent with the requirements of sustainable development strategy. In this work, cellulose diacetate(CDA) was prepared by using rice straw as raw material. The main contents are as following:Crude cellulose was isolated from rice straw by pretreatment with dilute alkaline and acid solutions successively, and it was further converted into microcrystalline cellulose in dilute hydrochloric acid. Toluene-ethanol mixture was employed to remove waxy and soluble impurity from rice straw, the dilute alkali/H2O2 and dilute acid solution were then used in rice straw pretreatmen. Variables such as extraction time, concentration of alkaline solution, the time of acid treatment and the size of rice straw were investigated. The structure of cellulose samples was characterized by infrared spectrometer(IR) and X-ray diffractometer(XRD), and the morphology was observed by scanning electron microscope(SEM). The results showed that most of lignin and hemicellulose were removed in dilute alkali solution, and a small amount of residual lignin and hemicellulose were further removed in dilute acid solution. Cellulose content of 87 wt% in the product was obtained with 5 wt% KOH/H2O2 and pH value 3.5 of acetic acid. The removal ratio of lignin and hemicellulose reached 95% and 94%, respectively. Then 95 wt% cellulose content in microcrystalline was obtained when the crude cellulose was stirred at 70 oC for 1 h in the presence of 6 wt% hydrochloric solution.The preparation of CDA was catalyzed by phosphotungstic acid with rice straw microcrystalline cellulose as raw material was investigated. The effect of variables,including phosphotungstic acid strength, the amount of phosphotungstic acid, reaction time and the amount of acetic anhydride, on the yield and the degree of substitution(DS value)were also investigated. Cellulose acetate with DS value of 2.24 and yield of 53% was obtained with molar ratio of glucose unit: phosphotungstic acid: acetic acid: acetic anhydride = 1:0.05:0.8:4, in dichloromethane for 10 h under reflux. The structure of microcrystalline cellulose and cellulose acetate were confirmed by IR and XRD, and the DS value of cellulose acetate was also calculated via 1H nuclear magnetic resonance(1H NMR). The thermal properties were determined by thermal analysis including thermogravimetry analysis(TGA) and differential scanning calorimetry(DSC). The results revealed that the thermal stability and thermoplastic of cellulose acetate was enhanced compared with those of microcrystalline cellulose, and the thermal stability and thermoplastic of the production improved with increase in the degree of substitution.The catalytic properties of Amberlyst 15 were further investigated for the acetylationof microcrystalline cellulose derived from rice straw. Both the fresh and recovered catalysts were characterized by means of IR, XRD, TGA, DSC, and SEM. The results show that the catalytic activity of Amberlyst 15 was similar with that of phosphotungstic acid but with a smaller amount. Cellulose acetate with DS value of 2.35 and yield of 51%was obtained with glucose units: acetic acid: acetic anhydride = 1:0.8:6(molar ratio),Amberlyst 15: glucose unit = 0.75(mass ratio), in dichloromethane for 10 h under reflux.Amberlyst 15 was recovered and reused without further treatment. It was found that the catalytic performance of Amberlyst 15 almost kept constant during four recycles.