纤维素结构调控快速热解产物选择性的机制研究

Although enzymatic and acid hydrolysis have received most of attention, a little-explored alternative method to obtain fermentable sugars is fast pyrolysis. Fast pyrolysis can depolymerize cellulose to levoglucosan at very short residence time without the use of enzymes or catalysts. Several microorganisms have been identified that can use levoglucosan as substrate. Pyrolysis has a lower cost than that of biochemical pathways and is a promising process to obtain fermentable sugars. Low yield of levoglucosan from biomass is potential drawback to this process.The structure of cellulose can significantly influence the distribution of fast pyrolysis products. A deeper understanding the mechanisms of how the composition influence the selectivity of pyrolysis product is required to enhance levoglucosan yield during pyrolysis. In this study, ionic liquid, dilute sulfuric acid, ball milling are used to change the structure (crystallinity, degree of polymerization and specific surface area) of microcrystalline cellulose. The pyrolysis reaction kinetics of cellulose before and after pretreatment are studied, and the pyrolysis mechanism is analyzed. The hydrogen transfer and the changes of radicals during pyrolysis of cellulose are characteriaed and the formation pathways of levoglucosan are synchronous investigated at a molecular level. Coupling different fast pyrolysis conditions, the influence of structure on the formation of levoglucosan is studied. The key factors that restrict the selectivity of the target product during the pyrolysis process are also defined. Overall, the research of this project would provide a theoretical basis to further improve the yield of levoglucosan from cellulose and lay foundation for later microbial fermentation to produce energy and chemicals from lignocellulose.

纤维素快速热解可以快速经济地制备左旋葡聚糖，从而为微生物发酵生产能源和化学品提供持续合理的原料。木质纤维素转化为左旋葡聚糖的选择性低是该方法存在的主要问题。纤维素的结构可以显著影响快速热解产物的分布。本项目采用离子液体、稀硫酸、球磨三种预处理手段联合处理微晶纤维素，定向改变纤维素的结构（结晶度、聚合度及比表面积）。研究预处理前后纤维素的热解反应动力学，分析热解机理；检测热解过程中氢迁移和自由基的变化，从分子水平阐明生物质的结构对热解产物形成的调控机制；耦合不同的快速热解条件，探索纤维素的结构对快速热解产物分布的影响，明确热解过程中制约目标产物选择性的关键因素。该研究对进一步提高纤维素快速热解转化为左旋葡聚糖的选择性提供理论指导，为微生物更高效地以生物质为原料生产能源和化学品奠定基础。

纤维素快速热解可以快速经济地制备左旋葡聚糖，为微生物发酵生产能源和化学品提供持续合理的原料。木质纤维素转化为左旋葡聚糖的选择性低是该工艺存在的关键问题。纤维素的结构可以显著影响快速热解产物分布。本项目采用离子液体、稀硫酸、球磨、粗甘油预处理定向改变纤维素的结构（结晶度、聚合度及比表面积），探讨预处理前后纤维素的热解反应动力学，分析热解机理；耦合不同的快速热解条件，研究纤维素的结构对快速热解产物分布的影响，明确热解过程中制约目标产物选择性的关键因素。结晶纤维素由于高度有序的紧密结构，具有良好的热稳定性，需要更高的反应活化能，热解反应更集中。球磨和离子液体预处理后，纤维素的结晶度降低，分子结构松散无序，热稳定性降低，因此整体活化能降低，结构崩溃引起的剧烈反应得到缓解。在快速热解过程中，高结晶度的纤维素有利于快速热解糖化，在400°C下，左旋葡萄糖产率达到64.3%。该研究为进一步提高纤维素快速热解转化为左旋葡聚糖的选择性提供理论指导，为微生物更高效地以生物质为原料生产能源和化学品奠定基础。