Study On The Process Of Converting Rice Straw Hemicellulose Into Xylose And Xylitol

Compared with the cellulose and lignin components in biomass,hemicellulose was far less utilized.Xylan is the main component in hemicellulose.Xylan can be hydrolyzed into xylose and then converted into xylitol under certain conditions.Because of unique physiological properties,xylose and xylitol have been widely used in food and medicine.It is an important high-value-added utilization for hemicellulose converting into xylose and xylitol.In this work,Xylose and xylitol produced from rice straw hemicellulose have been studied in detail and the maain works are as follows:Firstly,based on the research on separating the main components of rice straw in environment friendly way from steam exploded rice straw,the microwave-assisted acid hydrolysis method and boiling method were adopted to separate hemicellulose.In microwave-assisted acid hydrolysis,with the conditions of microwave power 700 W,0.6 mol/L of sulfuric acid,100℃,20 h,the content of hemicellulose was reduced to only 0.59%in the residuals.After boiled in water twice,with 1 h for each,the hemicellulose was 2.99%in the residuals.Secondly,taking the aqueous solution obtained from cooking the steam exploded rice straw as starting material which contains most of the hemicellulose.In microwave-assisted acid hydrolysis,with the conditions of microwave power 700 W,0.3 mol/L of sulfuric acid,100℃,10 h,the conversion rate of hemicellulose to xylose was 90.83%.Then purified hydrolysate with D301 and 732 resin,when the volume ratio of neutralized liquid to resin was 1:1,the purified solution was more clear than before,if only purified with D301 anion resin,the loss rate of xylose was 10.73%,then purified with 732 cation resin,the loss rate of xylose was 16.62%.With the volume ratio of neutralized liquid to resin changed from 1:2 to 1:4,the purified solution was more and more clear,the loss rate of xylose increased from 21.68%to 24.82%,the required amount of distilled water to leaching resin from two times the volume of neutralized liquid increased to four times the volume of neutralized liquid.Thirdly,the xylose was purified by chromatography.The best eluents were proved to be methanol:ethyl acetate=1:120（v/v）and methanol:ethyl acetate=1:15（v/v）.After solid-liquid separation,the purity of xylose was 79.48%in the solid part and 53.42%in the liquid part.Taking the dry steam exploded rice straw as a benchmark,the hydrolytic yield of xylose was 9.49%,the yield of the xylose（solid）was 4.61%and the yield of the xylose（liquid）was 1.45%.Finally,converting xylose into xylitol with Sodium borohydride（NaBH₄）alkaline solution was conducted.When taking the commercial xylose as starting material,with the molar ratio of NaBH₄ to xylose of 0.5:1,the conversion rate was 94.23%,the mass fraction of xylitol was 78.87%in the product.When the molar ratio of NaBH₄ to xylose increased from 0.5:1 to 0.8:1,the conversion rate increased slightly,but the mass fraction of xylitol reduced from 78.87%to 68.79%.After different treatment of the aqueous solution from cooking the steam exploded rice straw were converted into xylose and then into xylitol with NaBH₄ alkaline solution.When the ratio of neutralized liquid to resin was 1:4,the purified xylose solution was continued to react with NaBH₄ alkaline solution,the conversion rate of 94.73%was obtained,which was almost equal to the conversion rate when taking the commercial xylose as starting material.