Studies On The Structural Changes And The Mechanism Of Corn Stalk Hemicelluloses During The Active Oxygen Cooking Processes

China is an agricultural country and lots of agricultural straws are produced per year.Most of them are backfilled to the field and burnt in the field, while small part is used for thefeed and raw material of pulp. With the decrease of fossil energy reserve and enhancement ofenvironmental protection consciousness, many research focus on the renewable biomass tofind an alternative energy resources for the fossil energy. Generally speaking, there are twoways to utilize the lignocellulosic biomass: direct conversion and indirect conversion. Thelignocellulosic biomass can be converted into fuels, bio-oil by pyrolysis, gasification, andliquidation in the direct conversion process. If want to get the high-value-added chemicalsfrom the biomass, it is necessary to pretreat the biomass into hemicelluloses, cellulose, andlignin before carry through the conversion. Pretreatment can increase the percent conversionof reactant, and is in favor of the isolation and purification of products. Therefore, it is a keyquestion to find an efficient and clean pretreatment process for the biorefinery. And our groupdeveloped a novel biomass pretreatment process, active oxygen cooking process. In thepresent study, corn stalk is treated by the active oxygen, and the structural changes and themechanism of corn stalk hemicelluloses during the active oxygen cooking process are studied.The active oxygen cooking process of corn stalk was performed on the optimumconditions, temperature165°C, initial oxygen pressure1.0MPa, peroxide dosage3%（basedon the oven dry raw material）, ratio of solid to liquid1:6, MgO dosage15%（based on theoven dry raw material）. Isolation and characterization the hemicelluloses from the rawmaterial, pulp, and yellow liquor showed that the structure of corn hemicelluloses wereL-arabino-D-xylan, and the4-O-methyl-glucuronic acids and acetyl groups also attached tothe backbone of hemicellulose and the glycosidic bonds were broken during cooking process,and the acetyl groups were also hydrolyzed.For further understand the hemicelluloses in the yellow liquor, the hemicelluloses weresequentially precipitated with one, two, three, five, and seven volume of ethanol. The resultsshowed that the weight-average molecular weight （MW） of hemicelluloses precipitated by onevolume of ethanol was24824g/mol, but it accounted for5.31%of total hemicelluloses. Andthe MWof hemicelluloses precipitated with two volume ethanol was only4574g/mol, the rest of hemicelluloses fractions presented low-molecular-weight （c.a.2500g/mol）. With increaseof the volume of ethanol, the impurities in the hemicelluloses fractions increased. In the finalresidue, there was29.61%solid content, which mainly composed of degradation products oflignin.To study the effects of chemicals added in the cooking process, several cookingprocesses were designed. After cooking, it found that enough dosage MgO can preventcarbonizing of raw material during the active oxygen cooking process, and Mg（OH）₂showedthe some protective effect to the raw material and can replace MgO. However, the cellulosewas seriously degraded when MgO was replaced by NaOH in the active oxygen cookingprocess, even show good cooking effect. In this study, we also found that the oxygen played aleading role in the cooking process to remove the lignin and hemicelluloses, the effect ofperoxide can be ignored when both oxygen and peroxide were added. The structure ofhemicelluloses obtained from cooking liquor indicated that the hemicelluloses removedduring the cooking process possessed more side chains than that in the pulp.In the study of effect of incubation time on the cooking effect, it found that67.86%hemicelluloses and75.60%lignin were removed from the raw material after get the desiredtemperature half an hour. In the rest of incubation time, the removal ratio of lignin andhemicelluloses was very low. The MWof hemicelluloses decreased from19643g/mol to5471g/mol due to the break of glycosidic bonds during the cooking process. And the HexA alsowas detected in pulp.Based on the analysis of structural changes of hemicelluloses, some mechanisms abouthemicellulosic remove and structural changes were presented. The alkalinity of MgO in thecooking liquor and protective function of Mg²⁺were first condition of active oxygen appliedin the cooking process. Hot-water extraction and basic hydrolysis were the primarily route ofremoving hemicelluloses, and the remove of lignin also can accelerated the remove ofhemicelluloses. Moreover, alkaline degradation of hemicelluloses backbone also caused theglycosidic bounds break and produced some low molecular substances.