基于密度泛函理论的纤维素模型物亚临界水热降解机理研究

Due to the increasing shortage of fossil resources and the deterioration of global climate, the energy utilization of cellulosic biomass decomposing into high value-added chemicals is becoming a bright development orientation for scientific research and industrial application. Cellulose, its hydrolysate microcrystalline cellulose and complete hydrolyzed product as well as fructose have been investigating its decomposition behavior. However, limited by the complexity of reaction and the difficulties in the determination of reaction intermediates for the decomposition reaction of cellulose and its hydrolyzed products, the mechanisms and the influences of experimental conditions on the mechanisms are not yet clear. Based on the previous research of our group, this project will begin with the experimental results about products distributions, and then investigate the transformation products, decomposition products and reaction pathways of cellulose models in neutral and acidic subcritical water to explain the transformation and decomposition behaviors of cellulose model at the molecular-electronic level by employing the density functional theory method, in which solvation effect is described by the short-range action. Combined with experimental results, the theoretical results can further reveal the decomposition rule of cellulose models in subcritical water and the essential influence of pH on decomposition mechanism and product distribution, and therefore theoretical basis of hydrolysis decomposition reaction of cellulose in subcritical water will be obtained, which promotes the further efficient use of cellulose fibers and its products and provide theoretical guidance for the realization of cellulose-based polygeneration technology.

由于化石资源的日益匮乏和全球气候的恶化，纤维素类生物质降解转化为高附加值化学品的能源化利用成为科研开发与工业应用的一个重要发展方向，纤维素及其水解产物微晶纤维素、完全水解产物葡萄糖以及果糖都被作为原料研究其降解行为。然而受限于反应的复杂性和反应中间体测定的难度，纤维素及其水解产物的降解反应机理及实验条件对机理的本质影响尚不明晰。基于课题组前期研究，本项目拟以实验的产物分布结果为依据，借助密度泛函理论，通过近程作用描述溶剂化作用，研究纤维素模型物在中性及酸性亚临界水中的转化形式以及降解产物和降解反应途径，从分子-电子水平阐述纤维素模型物的转化及降解行为。再结合实验结果，进而揭示纤维素模型物在亚临界水中降解过程的反应规律和pH值变化对其降解机理及产物分布的本质影响，最终形成纤维素水热降解反应的理论基础，促进纤维素纤维及其产品的进一步有效利用，同时为实现纤维素基多联产技术提供理论指导。

本项目借助密度泛函理论，以实验的产物分布结果为依据，研究了纤维二糖在中性及酸性亚临界水中降解生成葡萄糖、果糖、1,6-脱水葡萄糖、5-羟甲基糠醛、乙酰丙酸、糠醛、1,2,4-苯三酚、甘油醛、二羟基丙酮、丙酮醛、乳酸、赤藓糖、乙醇醛等产物的反应路径及相关热力学数据。进一步分析各基元反应，揭示H+在不同类型基元反应中促进或抑制作用的本质原因，主要研究内容、重要结果和关键数据等如下：.1）醛-烯醇异构为亲核反应，C-H首先发生断裂并决定了反应活化能。α-H活化程度较低的酮或π-π共轭的醛结构，酸性条件下为亲电反应。H+首先进攻C=O形成C-OH。H+的存在能够降低活化能。.2）烯醇-醛酮异构为亲核反应，O-H首先发生断裂， 随后C-H的形成决定了反应活化能。酸性条件下1,1-烯二醇异构化为羧酸反应是亲电反应。H+首先形成C-H并决定了反应活化能。H+的存在能够降低活化能。.3）C-O-C键断裂或形成的开环闭环异构化反应，环O原子上H+的得失为过渡态，决定了反应活化能。酸性条件下H+对活化能影响较小。.4）中性条件下脱水反应中H2O的形成决定于被脱去H+的活性。当被脱去H+活性较低时，为亲电反应， H2O优先水解出H+与羟基形成吸附态H2O；当被脱去H+活性较高时反应为亲核反应，C(O)-H优先发生断裂。酸性条件下亲核反应居多，H+优先与羟基作用形成H2O，因而H+的存在能够降低活化能。H+的存在对脱水反应的促进或抑制作用取决于碳链结构。.5）对于水合反应，主要包括C-C水合和C-O水合两类。C=C水合时，C-H优先趋于成键并形成过渡态；C-O-C开环水合或中性条件下C=O水合时， H2O分子优先吸附并趋于解离H+而形成过渡态；酸性条件下部分酮基C=O水合时，H+具有较强亲电性而优先与C=O形成C-OH，然后H2O吸附并趋于解离而形成过渡态。H+的存在对水合反应的促进或抑制作用取决于碳链结构。.6）酸性逆羟醛缩合反应所需活化能低于碱性逆羟醛缩合反应。酸性逆羟醛缩合的过渡态为C-C断裂的同时O-H的断裂，而碱性逆羟醛缩合反应的过渡态则是C-C断裂时C-H的形成。H+的存在能够降低活化能。