木质纤维废物低温梯级水热转化的异步溶解与水解机理研究

Because of the global fuel issues, ethanol production from crop stalks has caught much attention. Hydrothermal conversion for biomass waste, which can break the lignocellulosic structure and hydrolyze cellulose into fermentable sugars, is considered as an efficient technology for pretreatment and hydrolysis. However, early hydrothermal technologies represented disadvantages of high temperatures and high energy consumption; and could lead to synchronous dissolution and hydrolysis of cellulose, resulting in the decomposition and loss of target products. Based on the differences on molecular structures and chemical bonds of cellulose, hemicellulose and lignin, as well as the corresponding energy demands for their dissolution and hydrolysis, a multilevel low-temperature hydrothermal technology is proposed in this project to dissolve and hydrolyze lignocellulosic waste asynchronously. The research focuses are theoretical mechanisms of asynchronous dissolution and hydrolysis of cellulose, as well as asynchronous dissolution and interaction laws of cellulose, hemicellulose and lignin. The crystallinity, degrees of polymerization, and molecular structures of major components under low-temperature hydrothermal conditions will be investigated, together with the breaking law of the hydrogen and covalent bonds. The kinetics for surface reaction will thus be analyzed, and the mechanisms and critical parameters of asynchronous dissolution and hydrolysis will be determined. Subsequently, the multilevel low-temperature hydrothermal process will be optimized according to the theoretical results, to achieve dissolution of amorphous components, hydorgen bond breaking of crystalline components, and glycosidic bond breaking of dissolved components. The multilevel low-temperature hydrothermal technology for asynchronous dissolution and hydrolysis of lignocellulose can avoid noneffective energy consumption and reduce product loss by step controlling dissolution and hydrolysis processes. The results of this project are supposed to provide important theoretical basis for ethanol production from lignocellulosic waste.

秸秆的乙醇化技术已备受关注，水热转化能破坏木质纤维结构并水解生成可发酵糖，是一种高效的预处理和水解技术。但早期水热技术表现了高温高能耗的缺陷，且各组分溶解水解同时进行，导致产物分解损失。本项目根据纤维素、半纤维素和木质素的分子结构特性和化学键差异，及其溶解水解反应对能量的不同需求，提出低温梯级水热转化实现木质纤维废物的异步溶解与水解，重点针对其中纤维素异步溶解水解的理论机理，及纤维素与半纤维素、木质素异步溶解机制和相互影响开展研究。通过考察各组分在低温水热条件下结晶性、聚合度与分子结构变化，以及化学键断裂规律，分析其溶解反应动力学，明确各组分异步溶解水解机制和突破条件；进而优化梯级低温水热工艺，先后进行无定形组分溶解、结晶组分的氢键破坏、以及溶解组分中糖苷键的断开，实现各组分异步溶解与水解，不但能避免无效能量损耗，而且能分步控制溶解水解过程，减少产物损失，为生物乙醇技术的突破奠定基础。

针对早期水热转化各组分同时溶解水解导致可发酵糖等目标产物损失和抑制物质积累的缺陷，本研究根据纤维素、半纤维素和木质素分子结构差异，提出了木质纤维废物异步水热溶解水解工艺。通过研究秸秆中纤维素、半纤维素等组分在低温梯级水热条件下的结晶性与聚合度变化、微观结构及化学键变化规律，获得了纤维素异步溶解水解机制，明确了其溶解表面反应动力学规律，揭示了纤维素、半纤维素等组分独立溶解和水解机理；进而根据突破条件优化了半连续式低温梯级水热反应工艺，使木质素、半纤维素和纤维素在各自适宜的低温水热条件下异步溶解和水解以提高可发酵糖产率，为生物乙醇技术的突破和应用奠定了理论和实践基础。. 研究表明，190-220℃间随温度升高和时间延长（5min至40min），秸秆的结晶度从43.1%增至72.5%，FT-IR证明了半纤维素在这一过程中逐渐溶解，而纤维素特征峰均得以保留。对于纤维素的溶解反应，其结晶性和聚合度在240℃以上显著降低，分子间和分子内氢键也遭到破坏，而糖苷键直至260℃仍然存在。因此，无定形的木质素和半纤维素，以及结晶性的纤维素，能够依次在190℃和240℃发生溶解。反应动力学研究表明，由于溶解速率很大以及不同组分的差异，190℃以上的条件下，木质纤维废物的溶解反应不符合表面反应速率方程。而纤维素溶解反应速率常数在实验条件内未发生突变，其活化能计算结果为189.8±8.6 kJ/mol。研究从化学结构证实了木质纤维废物依次进行无定形组分溶解、结晶组分破坏和糖苷键断开的反应规律。在此基础上，利用开发的半连续式水热反应装置，在压力4.0MPa—5.0MPa和停留时间30min条件下，分别在195℃—205℃、245℃—250℃和280℃反应温度下，依次实现木质纤维废物中的木质素和半纤维素的溶解、纤维素溶解以及纤维素水解，并通过结晶性、化学键和化学组分分析，明确了不同组分独立溶解水解的反应机理，获得了相对较高的可发酵糖产率（13.11%、13.66%），明确了玉米秸秆的多级水热溶解和水解工艺的优化条件。项目研究完成了全部研究内容，达到了预期目标，解决了关键科学问题，取得了相应的研究成果，在生物质废物资源化领域做出了应有的科学贡献。

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