【作者】 樊永明；

【导师】 谢益民； 张志毅；

【作者基本信息】 北京林业大学 ， 林产化学加工工程， 2007， 博士

【摘要】 由于植物纤维原料各组分在化学药品中的溶解性以及反应性能各不相同，从而可以采用溶剂将各组分分离。甲酸对木素具有良好的溶解能力，可在温和的条件下进行蒸煮。木素脱除、碳水化合物降解是制浆中发生的最为主要的反应，对它们的研究是制浆机理研究中最为重要的部分。本论文以麦草磨木木素（MWL）和木素碳水化合物复合体（LCC）对甲酸脱木素化学机理进行了研究，以过氧化氢和硫酸作为助剂，进行了进行了甲酸（F）蒸煮、甲酸-过氧化氢法（FP）蒸煮、甲酸—过氧化氢—硫酸（FPS）蒸煮以及甲酸—硫酸（FS）等组合进行了蒸煮处理结果对比，运用元素分析，凝胶色谱分析（GPC），傅立叶变换红外光谱（FTIR），核磁共振(¹H-NMR，¹³C-NMR)，色质联用（GC-MS）等仪器分析手段，比较了处理前后MWL和LCC的化学变化，研究了蒸煮过程中麦草脱木素化学反应机理、木素脱除的历程，主要结果如下：¹H-NMR和¹³C-NMR、GPC以及元素分析结果表明，麦草木素为GSH型木素结构，结构单元实验式为C₉H_8.35O_3.61（OCH₃）_1.342，相对于木材原料木素，麦草木素中侧链不饱和双键较少，羧基和醛基含量较低，酚羟基占麦草木素总羟基的27％，麦草木素大分子由30个左右C₉单元所组成。F、FP、FPS处理后分子量分布宽度增加，木素分子间发生聚合，处理后木素大分子中C₉单元数达到50左右，三种处理过程中降解和缩合反应均同时存在。F、FP、FPS处理后的酚羟基增加到原来的1.6～2.8倍。F处理后木素中氢、氧的含量减少，甲氧基含量增加，醇羟基减少，侧链的不饱和度增加。FP、FPS方法处理侧链有氧化现象，木素中有新的非共轭羰基生成，共轭羰基减少。CGC/MS分析发现，蒸煮后木素侧链断裂产物中，既有C_β—C_γ、C_α—C_β断裂也有Ar-C_α间断裂。蒸煮过程中发生的化学反应有：导致侧链不饱和度增加的羟基脱水反应；木素的甲酰化以及酯化反应；使侧链C_γ消除、侧链不饱和度增加的Prins逆反应；导致酚羟基大幅度增加的部分芳醚键断裂反应；使木素中羰基含量增加的Prilezhaev环氧化反应。此外还发现苯环上的活性氧亲电取代反应使木素中酚羟基数量增加、木素缩合反应导致木素分子量提高。对LCC的研究发现，LCC中所含的碳水化合物主要为木糖和葡萄糖。LCC甲酸处理后所得水不溶组分中含有紫丁香基、愈疮木基以及对羟苯基结构木素，木素之间以β-O-4结构、β-β结构、5-5结构、β-1结构和β-5结构等方式相互联结，木素中含有α-C=O结构、α-β不饱和结构等。LCC中糖类与木素之间以酯键、醚键形式以及缩醛结构方式联结，其中碳水化合物中主要是木聚糖，所联结的位置为C_α居多。LCC中的酯类结构、β-β缩合结构以及部分β-O-4结构对甲酸十分稳定。甲酸处理后LCC中应含有较多的紫丁香基木素以及其与碳水化合物形成的酯键联结结构。甲酸处理后LCC中仍然含有相当数量的β-O-4结构、酯化的α-C、葡萄糖、木糖等结构或成分。甲酸处理后LCC会被甲酰化。磨木木素与木糖共同在甲酸中进行处理时木糖可与木素之间产生新的结合。蒸煮前期所脱除的木素中含有较多数量的酚羟基和共轭结构羰基，在预处理阶段以及蒸煮初期溶出的木素中在结构上愈疮木基结构木素占多数。蒸煮后期溶出的木素非共轭羰基结构较高，同时含有更多的酚醚键结构，所脱除的木素中愈疮木基木素与紫丁香基木素的溶出数量相近，其中含有较多的甲酸脂类结构，化学反应对木素溶解起到重要作用。木聚糖对酸不稳定，原料中大部分木聚糖蒸煮后降解，浆中所残留的木聚糖相对较少，原料中阿拉伯糖几乎全部转移至黑液中。甲酸中加入过氧化氢后对碳水化合物具有一定保护作用。通过对蒸煮历程研究发现，木素脱除速率可分为三个阶段：在大量脱木素阶段，20min左右得率降至60％左右，脱出木素总量的70％左右；在缓和脱木素阶段，得率降至50％左右，30-40min之内脱除木素总量的20％左右；在残余脱木素阶段，得率降至45％左右，木素脱除不超过木素总量的10％。所考察的四种方法中，总脱木素速率为FPS＞FP＞F/FS。碳水化合物降解速率也分为三个阶段。在第一阶段（初期碳水化合物迅速降解阶段），23-35min内碳水化合物降解速率最快，溶出量为大约为总溶出量的80％左右。第二阶段（中期碳水化合物缓和降解阶段），在32-42min内碳水化合物溶出量为总溶出量的10％左右。第三阶段（后期碳水化合物缓慢降解阶段）各方法碳水化合物脱除量均不超过10％。碳水化合物总降解速率为FS＞F＞FP/FPS。通过采用平板模型对蒸煮传质过程进行数学描述以及实验验证，提出了蒸煮过程三个阶段的传质速控模型：蒸煮反应初期外扩散速率为速控步骤；反应中期内扩散速率成为速控步骤，反应后期化学反应速率为速控步骤。通过对蒸煮过程中碳水化合物以及木素随时间变化规律进行研究，发现F、FP、FPS方法的脱木素选择性均在1.5以上，蒸煮过程中木素脱除比例（对总木素）一直高于碳水化合物溶出速率（对碳水化合物总量）。总体脱木素选择性的大小顺序为FPS＞FP＞F＞FS。加入过氧化氢可增加脱木素选择性和脱木素量，加入硫酸可加速蒸煮进程。FP法可在维持较高的总体脱木素选择性的前提下在较高温度下蒸煮，FPS法则应在相对较低温度下进行蒸煮。蒸煮过程动力学研究发现，F、FP、FPS三种方法的脱木素过程均为表观1级反应，活化能的大小顺序为Ea_F＞Ea_FP＞Ea_FPS，加入过氧化氢和硫酸后可使脱木素反应更易于进行。F、FP、FPS三种蒸煮方法的碳水化合物降解过程均为表观1级反应过程，三个蒸煮方法中碳水化合物降解活化能差异很小（5％以内）。升高温度有利于甲酸蒸煮过程中木素的脱除，但FPS法蒸煮温度不宜过高。化学热力学分析结果发现，蒸煮过程中熵增加，宏观上为吸热过程，蒸煮过程可以自发进行，但永远不可能进行到底。更多还原

【Abstract】 The non-wood fiber materials will still play a very important role in the foreseeable future in the pulp and paper industry of China. The conventional pulping technology of the annual plants caused serious environmental problems due to the water pollution. It will be of practical significance to develop a renovative pulping technology of non-wood materials to China paper industry.Due to the different dissolvability in formic acid, the components in wheat straw can be separated with the solvet. Compared to cellulose, lignin can be readily dissolved in formic acid and, therefore, it can be easily applied to the separation of lignin from the raw materials. The volatility of formic acid is good enough to be simply evaporated and recycled in the loop.The delignification and degradation of carbohydrates are considered the most important reactions in the cooking process, In this paper, the milled wheat straw lignin （MWL） and lignin carbohydrates complex （LCC） was employed in the study of delignification mechanism. The wheat straw pulping was conducted with aqueous formic acid （F）, formic acid-hydrogen peroxide （FP）, formic acid-hydrogen peroxide-sulfuric acid and formic acid-sulfuric acid （FS）. The element analysis, gel permeation chromatography （GPC）, FTIR, proton nuclear magnetic resonance （¹H -NMR）, carbon nuclear magnetic resonance (¹³C -NMR) and GC/MS are also used in the structure analysis. in wheat straw before and after the aforesaid treatment. The mechanisms of delignification and carbohydrates degradation, chemical reaction kinetics were studied in this research. The following results are obtained:According to the ¹H -NMR and ¹³C -NMR results, it can be found that the lignin in wheat straw is GSH type. Based on the element analysis results, the empirical formula of lignin is C₉H_8.35O_3.61（OCH₃）_1.342. The element of oxygen content in wheat straw is rather high, and the degree of unsaturation is low. The lignin molecular of wheat straw consist about 30 of C₉ units.The reactions of degradation and condensation of lignin occurred simultaneously in the process of the F, FP and FPS. The distribution of the lignin molecular weight becomes wider than MWL after F, FP,FPS treatments, the number of C₉ unit in lignin macromolecue can be as high as 50. There exists a little amount of carboxyl and aldehyde groups in the original lignin. The phenolic hydroxyl is about 27% of the total hydroxyl group contents and can increase readily to 1.6-2.8 times of the initial phenolic hydroxyl concentration after F, FP and FPS treatment. After the F treatment the element of hydrogen and oxygen and aliphatic hydroxyl groups decreased, the degree of unsaturation increased; after FP and FPS treatment, the oxidation reaction occurred in the side chains, the conjugated carbonyl decreased. Thefragmentations of the side chains include the C_β-C_γ、 C_α-C_β and Ar-C_α.The chemical reactions in the cooking process are: esterification reactions of the hydroxyl group with formic acid, which may cause the increase of the unconjugated carboxyl groups; the Baeyer-Villiger reactions, the oxidation of the side chains; reverse Prins reactions, resulting in the C_γ elimination and the increase of the degree of unsaturation in side chains; the electrophilic substitution of aromatic hydogen with the HO⁺, resulting in the increase of the phenolic hydroxyl groups; the Prilezhaev epoxidation reaction, leading to the increment of the carbonyl groups and esterification in the side chains; the condensation of lignin, in the result of the increment of molecular weight of lignin.The carbohydrates in LCC are mainly xylan, the bond type in lignin include the β-O-4, β-β, 5-5, β-1 and β-5. The α-C=O and unsaterated α-β structure can also be found in the lignin of LCC. The linkage between lignin and carbohydrates include ester bond, ether bond as well as acetal structure.The ester bond and the β-O-4 structures in LCC are stable in formic acid treatment. The LCC can be esterified.The new bonding can be found after the co-treatment of MWL and xylose with formic acid.There exist three stages in the delignification process according to the rate. In the first delignification stage, about 70% of the total lignin was delignified in about 20 min. In the second stage about 20% of the total lignin was removed in 30-40 min; in the third stage the residual lignin, less than 10% of the total lignin was removed in the following cooking time. In the four process tested, the sequence of the delignification rate is FPS>FP>F and FS.The carbohydrates degradation rate can also be divided into three stages. In the first stage（fast degradation stage）, about 80% of the total carbohydrates degraded are removed in about 23-35 min; the second stage（mild degradation stage）, there are only about 10% of the total carbohydrates removed are eliminated in 32-42 min; in the third stage the carbohydrates eliminated are less than 10% of the totally removed from the straw. In the four process tested, the sequence of the carbohydrates degradation rate is FS>F>FP and FPS.the results showed that the coefficients of delignification selectivity of F,FP,FPS are all above 1.5. The delignification rate is higher than the degradation of carbohydrates. The delignification selectivity of the whole process is FPS>FP>F>FS. The addition of hydrogen peroxide can increase both the delignification selectivity and the amount of delignification. The cooking process can be accelerated by the addition of small amount of sulfuric acid. The FP process can be carried out at high temperature with a very good delignification selectivity. FPS should be conducted under lower temperature to avoid the mass degradation of carbohydrates.The kinetics analysis of the cooking process showed that the delignification reactions in the three observed process of F, FP, FPS are all in the first order, and the active energy are E_aF>E_aFP>E_aFPS. The addition of the peroxide and sulfuric acid facilitates the delignification process. The carbohydrates degradation reactions in the three process observed are all in the first order too, but the active energy are almost the same in different process with the variations of 5%. The rising of the temperature is favorite forthe delignification in the F process while the FPS should not conducted under a high temperature due to the carbohydrates degradation. The chemical thermodynamics analysis showed that the cooking process is a endothermic process. Due to the entropy production in the cooking process the reaction can occurr spontaneously. In the first cooking stage, the out-side diffusion is the controlling process, in the second cooking stage the in-side diffusion is the controlling process, and in the final cooking stage the chemical reactions are the controlling process.更多还原