生物质酸预处理过程假木素的来源、生成机理及其过程控制与分离

Processing of lignocellulosics to ethanol consists of four major unit operations: pretreatment, hydrolysis, fermentation, and product separation/purification. Pretreatment is required to alter the biomass macroscopic and microscopic size and structure as well as its submicroscopic chemical composition and structure so that hydrolysis of the carbohydrate fraction to monomeric sugars can be achieved more rapidly and with greater yields. Acid pretreatment, as a promising and competitive pretreatment technology, has many defects such as energy-intensive, high cost, and formation of byproducts inhibitory to the subsequent hydrolysis and fermentation processes. Pseudo-lignin is a key factor limiting acid pretreatment efficiency. In this project, the dual-wavelength K-ratios ultraviolet spectrophotometry is established to determine the content of pseudo-lignin. The concentration distribution of hemicellulose in the cell wall is in-situ quantitatively measured by immunohistochemistry and confocal laser scanning microscopy. The reaction of pseudo-lignin formation is simulated with the model compounds during the acid pretreatment of biomass, and the reaction mechanism of pseudo-lignin formation and the chemical structure of pseudo-lignin are analyzed by the isotope labeling method and the density functional theory (DFT) based on the first-principle calculations. The additives for acid pretreatment are screened and synthesized guided by the reaction mechanism of pseudo-lignin formation and the above simulation results, and the acid pretreatment P-factor is proposed by borrowing from cooking H-factor concept to optimize and control the acid pretreatment process. The two-liquid phase reaction and extraction technology for the acid pretreatment of biomass is developed by coupling chemical reactions and chemical separation process, and the separation technology of pseudo-lignin by liquid-liquid displacement is proposed using the concept of displacement cooking for reference to separate and remove pseudo-lignin in the organic phase. The project attempts to achieve a more fundamental understanding on the origination, formation mechanism and chemical structure characteristics of pseudo-lignin, and to develop the new process control and separation technology for pseudo-lignin so that the high-efficiency acid pretreatment is established to promote the commercialization of fuel ethanol production. Through these studies, the theoretical and technical support can be afforded for the separation and utilization of biomass, the production of dissolving pulp, the development of platform chemicals and biomass materials.

生物燃料乙醇生产过程中，酸预处理是最具前景和竞争力的预处理方式，但存在着耗能大、成本高、产生抑制物等问题，其中，假木素是制约酸预处理效果的关键因素。本项目探索用双波长系数倍率法紫外分光光度计测定假木素含量，用免疫细胞化学标记法和激光扫描共聚焦显微镜原位定量测量半纤维素在细胞壁中的浓度分布，揭示假木素的生成来源及其与半纤维素降解溶出的关系；应用稳定同位素位标记法和基于第一性原理的密度泛函理论（DFT）计算方法研究假木素的生成反应，阐明假木素的结构特征及其生成机理；建立“酸预处理P-因子”，筛选或合成酸预处理助剂，优化控制酸处理过程；建立双液相反应萃取酸预处理技术和液液置换分离技术，耦合化学反应和化工分离过程去除假木素。本项目力图突破假木素制约生物质酸预处理效果的瓶颈问题，促进燃料乙醇的商业化生产，并为生物质原料的分离与利用、溶解浆生产、生物质平台化学品和生物质新材料开发提供理论与技术支撑。

生物燃料乙醇生产过程中，酸预处理是最具前景和竞争力的预处理方式，但存在着耗能大、成本高、产生抑制物等问题，其中，假木素是制约酸预处理效果的关键因素。为此，本项目从探索假木素产生的碳源出发，分析水解体系及离子形式对假木素产生的影响，开发预水解过程中假木素产生的抑制助剂，揭示助剂抑制假木素产生的途径及机理。研究结果表明：木质纤维原料中半纤维素是假木素产生的主要碳源，糠醛是假木素产生过程中的最主要的中间体，木质素的存在会加速假木质素的产生。水解体系中氢离子及酸根类型共同决定聚糖的水解、糠醛的产生及假木质素的形成，其中氯离子能促进聚糖分解，糠醛的产生，同时可以稳定糠醛，减少假木质素的产生。在水及稀酸水解体系中，加入一定量的氯化钠，不仅可以促进聚糖的分解，糠醛的产生，还能抑制假木质素的产生。除氯化钠之外，Nb2O5也有抑制水水解过程中假木素产生的作用。Nb2O5在稀酸水解及水水解过程中的作用机理不同。在稀酸水解过程中，Nb2O5能促进糠醛的产生，而在水水解过程中则能抑制糠醛产生。除了在水解过程中添加助剂，置换水解不仅可以在同等半纤维素移除率的基础上，提高木寡糖的提取率，缩短水解时间，还能极大的减少水解产物继续发生分解、缩合等反应，减少假木质素的产生；注意的是，严格控制置换水解工艺是获得最佳效率前提。本项目的相关研究成果为生物质原料的分离与利用、溶解浆生产、生物质平台化学品和生物质新材料开发提供理论与技术支撑。