【作者】 李辉勇；

【导师】 刘又年；

【作者基本信息】 中南大学 ， 应用化学， 2012， 博士

【摘要】 开发以木质纤维素为原料的生物质能是解决当前人类社会所面临的能源与环境问题的主要途径之一。全世界稻谷的产量仅次于小麦与玉米,稻草秸秆是主要的木质纤维素原料,目前稻草秸秆主要是作为燃料或还田肥料,利用效率低。在本论文中,分别研究了NaOH预处理、H2O2/NaOH预处理、O3/NaOH预处理三种化学方法对稻草秸秆组分、酶水解糖化及表观结构的影响,分析探讨了稻草秸秆的三种化学预处理机理,并对木质素的臭氧氧化降解动力学及机理进行了研究。研究结果表明,三种化学预处理方法均可降低稻草秸秆中木质素的含量,提高纤维素的含量,并可明显促进稻草秸秆酶解糖化。预处理后稻草秸秆的酶解糖化率得到明显提高,其中稻草秸秆的O3/NaOH预处理效果最好。在相同酶解条件下(pH5.0、每单位底物加酶量为31.2mg/g、酶水解温度45℃、酶水解时间120h), NaOH预处理(2%NaOH、固液比1：12、处理温度30℃、处理时间24h)、H2O2/NaOH预处理(2%NaOH、2%H2O2、固液比1：12、处理温度40℃、处理时间24h,最优处理条件)、O3/NaOH预处理(在2%NaOH、固液比1：12、温度30℃条件下处理时间20h后,再通入03处理4h),处理后的稻草秸秆的糖化率分别为74.90%、83.23%、92.57%、53.53%。预处理前后稻草秸秆的扫描电镜观察表明：未处理稻草秸秆的外表面整齐地被硅质突起、硅细胞、栓细胞所覆盖；NaOH预处理后的稻草秸秆外表面覆盖物部分被除掉；H2O2/NaOH预处理后的稻草秸秆外表面覆盖物已完全除掉,下层机械组织充分暴露；O3/NaOH预处理后的稻草秸秆外表面覆盖物已被完全除掉,下层机械组织间物质也部分失去,纤维素结构暴露。未处理稻草秸秆的内表面覆盖着一层较厚的硅质与腊质,髓腔薄壁细胞基本不可见；NaOH预处理与H2O2/NaOH预处理后的稻草秸秆内表面中硅质与腊质已除去,髓腔薄壁细胞外露且其中的物质基本上已失去；而经过O3/NaOH预处理后的内表面中硅质与腊质已除完全除去,髓腔薄壁细胞外露且细胞内物质已完全除去,细胞壁较松弛,产生的孔隙更大。因而在增大酶解时有效比表面积、加快酶解速率方面,O3/NaOH预处理优于H2O2/NaOH预处理,H2O2/NaOH预处理优于NaOH预处理,处理后的稻草秸秆明显优于未处理。稻草秸秆的NaOH预处理机理与H2O2/NaOH预处理机理相似,都是在于NaOH使以醚键相连的木质素结构单元断开,高分子量的木质素分解为小分子量的木质素而溶解在溶液中。同时,NaOH使连接木质素分子与半纤维素分子间的酯键因皂化作用断开,半纤维素分子与纤维素分子间的氢键强度减小,从而使稻草秸秆发生膨胀,体积增大。差别在于H202促进了其它物质的氧化溶出,并不能使木质素分子开环氧化降解。O3/NaOH预处理机理在于臭氧使木质素开环氧化降解,同时除纤维素以外的其它物质也同时被氧化降解。这与臭氧的氧化能力强于过氧化氢有关。预处理前后稻草秸秆成分含量的变化及酶水解糖化率表明,原料中木质素含量并不是影响其酶解糖化的决定因素,预处理前后有效比表面积的变化对其酶解糖化也有非常重要的影响。动力学研究表明,碱性条件下木质素的臭氧氧化降解为一级反应,反应的活化能在45kJ/mol左右,其氧化降解在于羟基自由基氧化作用,氧化降解过程主要经过了由高分子量木质素分子到小分子量的基本结构单元,到苯丙烷基本结构单元侧链氧化为苯甲酸,再到苯环被开环氧化为羧酸和醛,最后被氧化为乙酸、更多还原

【Abstract】 Development of biomass energy which is prepared by lignocellulose is the main way to solve the problems of energy and environment. The methods of pretreatment of rice straw were studied in this paper.The composition, enzyme hydrolysis saccharification and the surface morphology of rice straw were studied after the pretreatment of rice straw by NaOH,H2O2/NaOH and03/NaOH. The mechanism of pretreatment was discussed, and the kinetics of ozone oxidization on lignin was investigated as well.Pretreatment was evaluated by the conversion of rice straw hemicellulose and cellulose to reducing sugars, the results showed that all the pretreatment dropped the content of lignin, increased the content of cellulose and accelerated the enzymatic saccharification of rice straw. The enzymatic scarification ratio of the pretreated straw was improved obviously. The best pretreatment effect was obtained by03/NaOH. Under the same enzymatic hydrolysis conditions(using31.2g enzyme preparation per gram of substrate,45℃, pH5.0,120h), by NaOH pretreatment(sodium hydroxide2.0%, solid-to-liquid ratio1:12, temperature30℃, reaction time24h), H2O2/NaOH pretreatment (sodium hydroxide2.0%, hydrogen peroxide2.0%, solid-to-liquid ratio1:12, temperature40℃, reaction time24h, the optimal pretreatment condition),03/NaOH pretreatment (after the milled rice straw was mixed with2.0%NaOH solution and shaken by an incubator at140rpm at30℃for20h, ozone pretreatment was performed by continuously sparging ozone through the suspension for4h) and untreatment, the enzymatic scarification ratios of rice straw were74.90%、83.23%、92.57%、53.53%, respectively.The results of SEM images of the rice straw show that the outer surface of the untreated rice straw is coated tidily with silica cells, suberized cells and silica swelling. However the silica cells, suberized cells and silica swelling which coated on the surface of rice straw have been removed partly by NaOH pretreatment, removed entirely by H2O2/NaOH and03/NaOH pretreatment. The inner substances of thin-walled cells were removed partly by NaOH pretreatment, removed entirely by and03/NaOH pretreatment. The cell walls became loosened and the porosity was increased since the inner substances of the thin-walled cells were completely lost, thus03/NaOH, H2O2/NaOH, NaOH pretreatment is much better than untreated on the increasing of the effect surface of enzymatic hydrolysis and accelerating of enzymatic hydrolysis velocity. The method of03/NaOH pretreatment shows the best effect among those pretreatments.There is no big difference on the mechanism of NaOH and H2O2/NaOH pretreatment, either bond of the lignin was broken down by NaOH, high molecular weight lignin is decomposed into small molecular weight lignin and dissolved in solution. At the same time, ester bond which contacted lignin and hemicellulose was broken due to saponification by NaOH, thus thehydrogen bond strength decreased between hemicellulose and cellulose, and so that the rice straw expensed, the difference between those two methods was that H2O2promoted the solve of materials instead of opening epoxidation degradation of lignin. While O3make lignin open epoxidation degradation, meanwhile, other materials were also oxidation, due to the oxidation of O3is much stronger than that of H2O2.The component contents change and enzyme hydrolysis sacchrification rate of rice straw before and after the pretreatment show that the contents of lignin is not the decisive factor to the enzyme hydrolysis saccharification, the change of effective specific surface area of the rice straw have very important influence after pretreatment.The decomposition kinetics results show that the ozone oxidation degradation of lignin was first order reaction under the condition of alkaline, and its reaction activation energy was about45kJ/mol, the mechanism of lignin oxidative degradation was hydroxyl free radical oxidation, during the oxidative degradation, the lignin was take place from high molecular to small molecular weight, from benzene propane to benzoic acid by oxidation to carboxylic acids and aldehydes, and finally to acetic acid, carbon dioxide and water by oxidation.更多还原